Mineralogical Influence on Leaching Behaviour of Steelmaking Slags Steelmaking Behaviourof Leaching on Influence Mineralogical Engström Fredrik

ISSN: 1402-1544 ISBN 978-91-7439-XXX-X Se i listan och fyll i siffror där kryssen är

DOCTORAL T H E SI S Fredrik Engström Mineralogical Influence on Leaching Behaviour of Steelmaking Slags Steelmaking Behaviourof Leaching on Influence Mineralogical Engström Fredrik

Department of Chemical Engineering and Geoscience Division of Minerals and Metallurgical Engineering Mineralogical Influence on Leaching ISSN: 1402-1544 ISBN 978-91-7439-197-8

Luleå University of Technology 2010 Behaviour of Steelmaking Slags A Laboratory Investigation

Fredrik Engström A Laboratory Investigation Laboratory A

Mineralogical influence on leaching behaviour of steelmaking slags

A Laboratory Investigation

Fredrik Engström

Doctoral Thesis

Luleå University of Technology Department of Chemical Engineering and Geoscience Division of Minerals and Metallurgical Engineering SE-97187 Luleå Sweden

2010

Printed by Universitetstryckeriet, Luleå 2010

ISSN: 1402-1544 ISBN 978-91-7439-197-8 Luleå 2010 www.ltu.se LIST OF PAPERS

This thesis is based on the following papers:

I. Dirk Durinck, Fredrik Engström, Sander Arnout, Jeroen Heulens, Peter Tom Jones, Bo Björkman, Bart Blanpain and Patrick Wollants: Hot stage processing of metallurgical slags. Resources, Conservation and Recycling (2008), Vol 52, No 10, p 1121-1131.

II. Mia Tossavainen, Fredrik Engström, Qixing Yang, Nourreddine Menad, Margareta Lidström Larsson and Bo Björkman: Characteristics of steel slag under different cooling conditions. Waste Management (2007), Vol 27, No 10, p 1335-1344.

III. Fredrik Engström, Daniel Adolfsson, Qixing Yang, Caisa Samuelsson and Bo Björkman: Crystallization behaviour of some steelmaking slags. Steel Research International (2010), Vol 81, No 5, p 362-371.

IV. Fredrik Engström, Margareta Lidström Larsson, Caisa Samuelsson, Åke Sandström, Ryan Robinson and Bo Björkman: Leaching behaviour of aged steel slags. Submitted (Resources, Conservation and Recycling) 2010.

V. Fredrik Engström, Daniel Adolfsson, Caisa Samuelsson, Åke Sandström and Bo Björkman: A study of the solubility of pure slag minerals. Submitted (Minerals Engineering) 2010.

I VI. Sina Mostaghel, Fredrik Engström, Caisa Samuelsson and Bo Björkman: Stability of spinels in a high basicity EAF slag. Proceedings of 6th European Slag Conference, October 20-22, 2010, Madrid, Spain.

F. Engström’s contribution to the papers:

• Practical experiments.

• Responsible for characterization of materials obtained during experimental work using XRD, SEM and leaching.

• Interpretation of leaching data and correlation to mineralogical phases.

• Thermodynamic calculations.

• Responsible for writing Papers II-V and parts of Paper I.

• Supervising and contributing to the discussion of Paper VI.

II Related publications not included in this thesis:

VII. Mia Tossavainen, Fredrik Engström, Nourreddine Menad and Qixing Yang: Stability of modified steel slags. Proceedings of 4th European Slag Conference, June 20-21, 2005, Oulu, Finland.

VIII. Qixing Yang, Fredrik Engström, Mia Tossavainen and Daniel Adolfsson: Treatments of AOD slag to enhance recycling and resource conservation. Proceeding of Securing the Future, International Conference on Mining and the Environment, Metals and Energy Recovery, June 27-July 1, 2005, Skellefteå, Sweden.

IX. Qixing Yang, Fredrik Engström, Mia Tossavainen and Mingzhao He: AOD slag treatment to recover metal and to prevent slag dusting. Proceeding of the 7th Nordic-Japan Symposium on Science and Technology of Process Metallurgy, Jernkontoret, September 15-16, 2005, Stockholm, Sweden.

X. Qixing Yang, Lotta Nedar, Fredrik Engström and Mingzhao He: Treatments of AOD slag to produce aggregates for road construction. Proceeding of AISTech 2006, May 1-4, 2006, Cleveland, USA, Vol. 1, p 573-583.

XI. Qixing Yang, Björn Haase, Fredrik Engström and Anita Wedholm: Stabilization of EAF slag for use as construction material. Proceedings of REWAS 2008, Global Symposium on Recycling,

III Waste Treatment. Minerals, Metals & Materials Society, October 12- 15, 2008, Cancun, Mexico, p 49-54.

XII. Fredrik Engström, Margareta Lidström Larsson, Caisa Samuelsson and Bo Björkman: Ageing investigation of steel slags from electric arc furnace processes. Proceedings of REWAS 2008, Global Symposium on Recycling, Waste Treatment. Minerals, Metals & Materials Society, October 12-15, 2008, Cancun, Mexico, p 353- 358.

XIII. Qixing Yang, Fredrik Engström, Bo Björkman and Daniel Adolfsson: Modification study of a steel slag to prevent the slag disintegration after metal recovery and to enhance slag utilization. Proceedings of the VIII international conference on molten slags, fluxes and salts, January 18-21, 2009, Santiago, Chile, p 33-41.

XIV. Fredrik Engström, Caisa Samuelsson and Bo Björkman: Mineralogical influence of different cooling conditions on leaching behaviour of steelmaking slags. Proceedings of the 1st International Slag Valorisation Symposium, 6-7 April, 2009, Leuven, Belgium, p 67-80.

XV. Charlotte Andersson, Bo Björkman, Fredrik Engström, Sina Mostaghel and Caisa Samuelsson: The need for fundamental measurements for a sustainable extraction of metals. Seetharaman – Seminar 2010.

IV XVI. Daniel Adolfsson, Fredrik Engström, Ryan Robinson and Bo Björkman: Cementitious phases in ladle slag. Accepted for publication in Steel Research International, November 2010.

XVII. Daniel Adolfsson, Fredrik Engström, Ryan Robinson and Bo Björkman: Hydraulic properties of ladle slag. Submitted to Cement and Concrete Research, September 2010.

XVIII. Daniel Adolfsson, Ryan Robinson, Fredrik Engström and Bo Björkman: Hydraulic properties of mayenite. Submitted to Cement and Concrete Research, September 2010.

XIX. Chandra Sekhar Gahan, Jan-Eric Sundkvist, Fredrik Engström and Åke Sandström: Utilization of steel slags as neutralizing agents in biooxidation of a refractory gold concentrate and their influence on the subsequent cyanidation. Submitted to Resources, Conservation and Recycling, September 2010.

XX. Chandra Sekhar Gahan, Jan-Eric Sundkvist, Fredrik Engström and Åke Sandström: Comparative assessment of Industrial oxidic by- products as neutralising agents in biooxidation and their influence on gold recovery in subsequent cyanidation. Proceedings of the 11th International Seminar on Mineral Processing Technology MPT- 2010, December 15-17, 2010, Jamshedpur, India.

V VI ABSTRACT The Swedish steelmaking industry produces large amounts of by-products. In 2008, the total amount of slag produced reached approximately 1,300,000 metric tons, of which 20% was deposited. Due to its strength, durability and chemistry, steel slag is of interest in the field of construction, since it has similar or better qualities than ordinary ballast stone, which makes it a competitive construction material. However, some steel slags face an array of quality concerns that might hinder their use. These concerns generally involve the following physical and chemical properties:

• Volume expansion • Disintegration • Leaching of metals

By controlling and modifying process parameters during slag handling in liquid state, the physical and chemical properties of steel slags can be adequately modified to obtain a high-quality product for external application. The present work was undertaken as a research project within the Minerals and Metals Recycling Research Centre, MiMeR. The major goal of this work has been to investigate how different treatment methods including hot stage processing, cooling rates, ageing time and chemical composition influence the final properties of the slag. Analysis techniques used in this investigation include: thermodynamic calculations using FactsageTM, X-ray diffraction analyses (XRD), scanning electron microscopy (SEM), leaching tests (EN12457-2/3) and thermo-gravimetric analyses (TG).

VII The results from this study show that it is possible to control/change the properties of the final product by additions to the liquid slag, thereby changing the chemical composition, as well as by varying the rate and method of cooling. The mineralogical composition, the size of the crystals and the composition of some solid solutions are affected by the cooling rate. The solubility of elements such as chromium and molybdenum varies, probably due to their presence in different minerals. The reactivity of the investigated slag samples increases as the cooling rate increases.

When steel slags are aged, the leaching properties of the materials are changed. The total leachability and the pH decrease for all the investigated samples. All elements except magnesium decrease in leachability. As the slags are aged CaCO3 is formed on the slag surfaces. The degree of carbonation differs between different slags, due to the presence of different calcium-rich minerals in the slag. In order to form

CaCO3, the calcium-containing mineral must be dissolved. This means that the solubility of the calcium-containing mineral will affect the outcome of the carbonation. The rate of dissolution for six typical slag minerals was investigated in order to distinguish the difference in solubility between the different minerals. Acidic to alkaline pHs (4, 7 and 10) were selected to investigate the solubility of the minerals under conditions comparable to those prevailing in newly produced slags and the potential future pH values obtained under acid conditions. It can be concluded that all six minerals behave differently when dissolving and that the rate of dissolution is generally slower at higher pH. At pH 10, the solubility of merwinite, akermanite and gehlenite is considered slow. The dissolution of -Ca2SiO4 is not affected in the same way as the other minerals when the pH is changed.

VIII ACKNOWLEDGEMENTS First and foremost, I wish to thank my thesis advisors, Professor Bo Björkman, Associate Professor Caisa Samuelsson and Dr Qixing Yang for all your help and guidance during the course of my work. Dr Margareta Lidström Larsson, för att jag fick lära känna dig samt för den korta men lärorika tiden vi fick tillsammans inom området slagg, du är saknad.

I would also like to thank my colleagues and friends for putting up with me all these years. Danny P, Ryro, Ulla, Anaitich, Raggsockan, Challe and Secharo, you have all ‘now left the building’, so to speak. Åke B, Sempan, Biggish, Anders, Samuel, LILLKORVEN, U-tuff och sist men inte minst Muchtagellerna; without you, this work would have been considerably more difficult. Thanks also to Associate Professor Nourreddine Menad, now working at BRGM in France, for all the fruitful and entertaining discussions. To member companies in MiMeR, Jernkontoret – The Swedish Steel Producers Association (TO55 – Steel production residues), VINNOVA – The Swedish Governmental Agency for Innovation Systems, Mistra – The foundation for Strategic Environmental Research and CAMM – Centre for Advanced Mining & Metallurgy, I extend my sincere thanks for invaluable financial support and commitment. Without your help, this study would never have been possible.

Finally, I wish to thank my family. Linda, thanks for all your help and wonderful support. Thank you Moa and Emil, you mean everything to me. Johan, Mum and Dad, Thank you for helping to make me the person I am.

Fredrik Engström, December 2010, Luleå, Sweden

X CONTENTS

1 INTRODUCTION ...... 1

1.1 BACKGROUND...... 1 1.2 STEELMAKING SLAGS ...... 9 1.3 UTILIZATION OF STEELMAKING SLAGS ...... 12 1.4 CRYSTALLIZATION THEORY ...... 15 1.5 AIM AND SCOPE ...... 19 2 MATERIALS AND EXPERIMENTAL PROCEDURE ...... 21

2.1 MATERIAL ...... 21 2.2 EXPERIMENTAL PROCEDURE ...... 21 3 RESULTS AND DISCUSSION ...... 27

3.1 LITERATURE REVIEW - HOT STAGE MODIFICATION, PAPER I ...... 27 3.2 A MINERALOGICAL INTERPRETATION OF THE SOLIDIFICATION /SOLUBILITY OF STEEL SLAGS. PAPERS II & III ...... 28 3.3 AGEING INVESTIGATION OF EAF SLAGS, PAPER IV ...... 43 3.4 A STUDY OF THE SOLUBILITY OF PURE SLAG MINERALS, PAPER V ...... 49 3.5 STABILITY OF SPINELS IN HIGH-BASICITY EAF SLAG, PAPER VI...... 51 4 CONCLUDING DISCUSSION ...... 55 5 CONCLUSIONS ...... 63 6 FUTURE WORK ...... 65 7 REFERENCES ...... 67

XII 1 INTRODUCTION

1.1 BACKGROUND

Large amounts of by-products are produced by the Swedish steelmaking industry each year. In 2008, the total amount of slag produced reached approximately 1.3 million tonnes, corresponding to almost 0.3% of the total steel slag production worldwide the same year [1,2]. 35%, mostly blast furnace slag, was sold as external products and approximately 20% was used for landfilling. The remaining 45% was used internally at the steel plants. The landfilled amount is high in comparison to Europe overall. In 2006, only 7% of the slags produced was dumped, while approximately 80% was used in external applications [1]. There are several reasons for the low utilization of steel slags in Sweden compared to other European countries: high availability of good stone material, high availability of land for landfilling, and a high share of scrap-based and high-alloy steelmaking. In Sweden, a number of goals have been formulated in order to obtain a so-called good building environment, promoting sustainable management of land, water and other resources [3]. Among these criteria are:

• By 2010, extraction of natural gravel in the country will not exceed 12 million tonnes per year. • The total quantity of waste generated will not increase and maximum use will be made of its resource potential while minimizing health and environmental effects and associated risks.

To date, these criteria “goals” have not been reached. Due to its strength, durability and chemistry, steel slag could be considered in the field of construction, since the material provides similar and sometimes even better

1 properties than granite and flint gravel [4]. Using slag in construction would contribute to a reduction in the amount of landfilled material and, at the same time, preserve natural resources, fulfilling the environmental goal. However, the possibility of using slag is limited due to the lack of rules and guidelines regarding testing, assessment and use of slag in Sweden. The technical and environmental obstacles for not using some slags in construction include:

• Volumetric expansion • Disintegration • Leaching of metals

According to Monaco and Lu [5], the volumetric expansion is considered to be associated with the presence of free lime (CaO) and free periclase (MgO) in the solidified slag. Free lime and periclase react with moisture, resulting in an expansion due to the formation of Mg(OH)2 and Ca(OH)2 [6]. The understanding of these phenomena has led to the development of several treatment techniques, including steam and water treatment, in order to enhance the hydration [7].

It is well established that, upon cooling, pure dicalcium silicate undergoes several phase transformations from the high-temperature -Ca2SiO4 to the low-temperature -Ca2SiO4 polymorph, Figure 1. As the athermal, martensitic-like transformation of the monoclinic -polymorph to the orthorhombic -polymorph is accompanied by a volume expansion of about 12%, high internal stresses are built up in the slag during this transformation, finally causing the disintegration of the slag [8,9].

Figure 1: Phase transformation of pure Ca2SiO4 [10].

The polymorphic transformation of to -Ca2SiO4 is known to occur in some EAF (Electric Arc Furnace) and AOD slags (Argon Oxygen Decarburization) from stainless steel production depending on composition and the cooling rate [11]. In addition to dust problems at the slag yard, the usage of the disintegrating slag in external applications is limited due to the high amount of fines. One route to preventing slag disintegration is to inhibit the - to -transformation of Ca2SiO4 or completely avoid the presence of

Ca2SiO4. The first option was elaborated in 1986 by the development of a borate-based stabiliser for stainless steel slags [12]. Seki and co-workers [12] corroborated that by adding borates to a high-temperature slag,

Ca2SiO4 grains in the slag could be stabilized. The addition of only 0.2 wt%

3 of B2O3 was sufficient to prevent the disintegration of a slag with 51 wt%

CaO, 33 wt% SiO2 and 11 wt% MgO. Phosphorus also exhibits a stabilizing effect of Ca2SiO4. Experiments conducted in laboratory scale have shown promising results; however, larger amounts are required compared to borate additions [13]. Instead of using chemical additions, the - to - transformation of Ca2SiO4 can also be avoided by rapid cooling [14]. According to the laboratory experiments, the required slag cooling rate is about 5°C/s [15,16]. This stabilization method was further developed by showing, in laboratory scale, that a granulation process transforms a disintegrating slag into a slag product suitable for construction applications [17]. The second option to avert slag disintegration is by avoiding the presence of Ca2SiO4 by modifying the slag composition. Adding a relatively large amount of a SiO2 source seems to be the best way to avoid Ca2SiO4 precipitation. In laboratory experiments, AOD slag was stabilized with 12 wt% of waste glass, containing 70–75 wt% of SiO2 [18]. The cost of such an operation would be far lower than that of the borate additions. However, the limited heat content and heat conductivity of slags make it difficult to apply this method of dissolving large amounts of SiO2 in an industrial environment.

Apart from volume stability and disintegration, leaching of potentially hazardous compounds during reuse is another key issue in slag valorisation. The leaching from steel slags is generally characterized as a surface reaction, followed by a solid-solid diffusion process, in order to retain equilibrium in the materials [19]. It is therefore reasonable to believe that the rate of leaching decreases with time as the diffusion from the bulk of the solid slag to the surface is slow. Minimization of the surface area and/or formation of a less reactive surface layer on the slag can therefore

4 be assumed to decrease the leachability. One way of introducing such a layer is by letting the slag react with CO2 (g), forming calcium carbonates,

CaCO3. Research has shown that carbonation of alkaline solid material can lead to an improvement of their environmental qualities [20,21]. The mechanism behind this formation depends on several factors such as temperature, particle size, porosity and CO2 diffusivity [22-24]. According to Huijgen et al. [22], the diffusion of calcium through the solid slag matrix, towards the surface, appears to be the rate-determining step in carbonation, implying that the solubility of all calcium-containing mineral in the slag will affect the outcome of the carbonation of the slag surface. However, when it comes to leaching, the exact mechanisms still remain unclear. Therefore, a lot of effort is being put into a mineralogical interpretation of the leaching. High resolution techniques such as XANES (X-ray absorption near-edge structure) and WDS (wavelength dispersive spectroscopy) together with simulations of the leaching behaviour are today used in order to characterize the slag in detail, in order to identify all possible sources of release. The possibility to simulate the leaching behaviour from aged steel slags, using geochemical modelling has been investigated [25,26]. These types of simulations sometimes fail to fully describe these complex systems, mostly because available data (solubility/thermodynamic/sorption) for the minerals occurring in the slag systems often are incomplete or missing.

Chromium is the metal in the slag that is given the most attention when it comes to leaching. Several types of slag contain a significant fraction of oxidized chromium [27]. This is especially the case for ferrochromium and stainless steel slags. Nevertheless, chromium oxides are also encountered in regular steel slags and several types of non-ferrous slags. When these

5 slags are recycled, chromium comes in contact with the environment. Although Cr3+ is an essential nutrient for mammals, elevated levels are hazardous and may cause, for example, skin rashes. Cr6+ is carcinogenic; hence, its presence in ground water should be prevented [28,29].

One way to decrease Cr release from the slag is by incorporating the chromium into stable mineral phases during cooling of the slag. The leaching levels are believed to be limited substantially when chromium is contained in a spinel phase—AB2O4, with A being a bivalent cation and B a trivalent cation [30]. Based on this, Kühn and Mudersbach [31] related chromium leaching to the concentration of spinel-forming compounds in the slag. They performed a large number of extraction tests on industrial stainless steel electric arc furnace slags and derived an empirical formula, factor sp, to relate the overall slag composition to the chromium leaching:

Factor sp = 0.2MgO + 1.0Al2O3 +nFeOx 0.5Cr2O3

‘n’ is a number between 1 and 4, depending on the oxidation state of the slag. When factor sp is below 5, a high chromium leaching is observed. When factor sp is above 5, chromium leaching is low. In addition to enrichment of chromium in spinel phases, microprobe measurements performed on synthetic manufactured steel slags have shown that chromium can be enriched in numerous phases, e.g. bredigite

(Ca7MgSi4O16), merwinite (Ca3MgSi2O8) and wollastonite (CaSiO3) [32]. As in the case of carbonation, this means that the solubility of the different chromium-containing slag minerals must be known before the leaching of metals, e.g. chromium, can be explained.

6 Apart from what already has been discussed, very little is reported in the literature regarding the influence of cooling on the properties of slag, especially for steelmaking slags. As the solidification behaviour is highly dependent on the exact composition, results are not easily transferable to other slags. Ground granulated blast furnace slags (GGBS) are known to possess improved hydration reactivity compared to slowly cooled blast furnace slag, due to the formation of glass [33]. The formation of a glassy material depends on both the chemical composition and the cooling conditions. According to Daugherty et al. [34], glass was easier to produce, as the acidity of the slag increased for a series of synthetic slag compositions that was quenched and annealed. Ionescu et al. [35], [36] have shown how water quenching of steel slag results in products with a high content of glassy material. Silicate melts have high viscosity due to long molecule chains, and rearrangement into crystals only takes place slowly. If the cooling is rapid, the slag passes from a liquid state to a solid without development of a crystalline structure [37]. Glasses, such as granulated slags, can be regarded as super-cooled liquids having a very high viscosity.

Besides glass formation, controlling cooling conditions can be a means of affecting mineral transformation and, consequently, the solubility of 6+ elements like chromium. According to Lee and Nassarella [38], Cr is usually formed at lower temperatures. They suggest that by cooling the slag rapidly, the formation of Cr6+ will be limited due to the slower kinetics. Monaco and Lu [39] have reported variations in the composition of the wustite-type solid solution as well as a variation in crystal size when cooling differently. Despite extensive research efforts on these topics, a number of

7 questions remain. The leaching behaviour is still the primary cause for difficulties with valorisation.

8 1.2 STEELMAKING SLAGS Slag is formed through a chemical reaction of a flux and the gangue of an ore, the ash from a fuel, or with the impurities oxidized during the production and refining of a metal. In most iron- and steelmaking processes, the slag is in intimate contact with the liquid metal and chemical reactions readily occur between the slag and the metal. The main purpose of slag in processes such as basic oxygen furnace (BOF) and electric arc furnace (EAF) is to extract unwanted elements from the steel bath, help preventing metal oxidation and limit the heat losses from the steel [40].

1.2.1 BOF slag BOF slag is produced as a by-product when iron is converted into steel in the BOF converter through the injection of oxygen. The slag is produced by the addition of fluxes, such as limestone and dolomite, during the process. Unwanted elements such as carbon, silicon and phosphor are either oxidized to gases or chemically bonded into the slag. The most common minerals found in the BOF slag are listed in Table 1. All minerals may not necessarily be present in all slags of a given type.

Table 1: Most abundant minerals usually found in BOF slag.

Mineral name Structural formula

Larnite -Ca2SiO4

Srebrodolskite Ca2Fe2O5

Tricalcium silicate Ca3SiO5 2+ 3+ Spinel (Fe,Mg,Mn,Al) Me Me 2O4 Wustite FeO Lime CaO Periclase MgO

9 1.2.2 EAF slag EAF slag is produced as a by-product during the melting of scrap, when processed in the electric arc furnace at temperatures above 1600°C. Phosphorus, sulphur, silicon and sometimes carbon are removed by lancing oxygen into the melt, forming a liquid oxide slag. In Sweden, two types of EAF slag are produced; carbon and high-alloyed steel. EAF slag which originates from the carbon steel manufacturing usually has a high content of iron and basicity B2 (CaO/SiO2) around 2.5, while the EAF slag generated from the production of high-alloyed steel might differ considerably. The most common minerals found in the two EAF slags are listed in Table 2.

Table 2: Most abundant minerals usually found in EAF slags. Electric arc furnace slag (carbon steel)

Mineral name Structural formula

Larnite -Ca2SiO4

Srebrodolskite Ca2Fe2O5

Brownmillerite Ca2(Al,Fe)O5 2+ 3+ Spinel (Fe,Mg,Mn,Al,Cr) Me Me 2O4 Wustite FeO Periclase MgO

Electric arc furnace slag (high alloy steel)

Mineral name Structural formula

Bredegite Ca14Mg2(SiO4)8

Merwinite Ca3MgSi2O8

Akermanite Ca2MgSi2O7

Gehlenite Ca2Al2SiO7

Cuspidine Ca4F2Si2O7 Periclase MgO 2+ 3+ Spinel (Fe,Mg,Mn,Al,Cr) Me Me 2O4

10 1.2.3 AOD slag In the manufacture of high-alloyed stainless steel the AOD (Argon Oxygen Decarburization) converter is often the first refining step after the electric arc furnace. By decreasing the oxygen content in the decarburization gas gradually, using inert gas (nitrogen and/or argon), a significant removal of carbon is achieved without extensive losses of chromium to the slag. The AOD slag often shows disintegrating properties due to the formation of -

Ca2SiO4. The most likely minerals to be found in the AOD slag are listed in Table 3.

Table 3: Most abundant minerals usually found in AOD slag.

Mineral name Structural formula

Ingesonite -Ca2SiO4

Larnite -Ca2SiO4

Merwinite Ca3MgSi2O8

Melilite (Ca,Na)2(Al,Mg,Fe)(Si,Al)2O7

Fluorite CaF2 2+ 3+ Spinel (Fe,Mg,Mn,Al,Cr) Me Me 2O4 Lime CaO Periclase MgO

1.2.4 Secondary metallurgical slag Secondary metallurgical slag is produced during the treatment of crude steel, and arises from the subsequent treatment of steel produced both in the BOF and EAF processes. Regardless of whether they originate in ore- or scrap-based steelmaking, secondary metallurgical slags often have a high content of CaO, SiO2, Al2O3 and MgO. The most common minerals found in the secondary metallurgical slag are listed in Table 4.

11 Table 4: Most abundant minerals usually found in secondary metallurgical slag.

Mineral name Structural formula

Ingesonite -Ca2SiO4

Larnite -Ca2SiO4

Bredegite Ca14Mg2(SiO4)8

Mayenite Ca12Al14O33

Tricalcium aluminate Ca3Al2O6

Cuspidine Ca4F2Si2O7 2+ 3+ Spinel (Fe,Mg,Mn,Al,Cr) Me Me 2O4 Lime CaO Periclase MgO

1.3 UTILIZATION OF STEELMAKING SLAGS An analysis of the focus areas in slag-related research in the periods 1996– 2000 and 2001–2004 indicates that slag recycling issues have gained importance in research, reflecting the increasing awareness of sustainable production and environmentally friendly processes [41,42], Within this research, the emphasis is primarily on finding and evaluating slag- containing products, such as:

• metallurgical fluxes • cement • aggregates for road and waterway construction • soil improvers and fertilizers

The usage of iron and steel slags in external applications such as road building and cement is nothing new. The earliest reports on the utilization of slags refer to Aristotle, who used slags as a medicament already around

12 350 BC [43]. During the following centuries, slag was mainly used as construction material. The Romans used slag in road construction about 2000 years ago. In 1813, the first road made of slag in modern times was built in England. However, slag has not only been used in road construction. In 1589, the Germans used slag from iron making when manufacturing cannonballs [44].

In Europe, the total slag production in 2006 reached approximately 45.5 million tonnes (blast furnace slag included). 80% of the slag produced was used in external applications such as road construction and manufacturing of cement, Figure 2.

Figure 2: Use of steel slags in Europe 2006: 45.5 million tonnes [1].

13 In Sweden today, by far the largest external application area for slag is within road construction. The physical and chemical properties of the steel slags often make them suitable for ballast in asphalt manufacturing. The highly basic steel slags easily react with the acid bitumen, forming a high- performance asphalt product. Tests performed by the Danish Road Directorate show an increase of 30% in durability when comparing normal mastic asphalt to asphalt made with EAF slag. The increase in porosity compared to ordinary stone material also helps to keep the road free of water, thereby reducing the risk for aquaplaning while at the same time lowering the noise generated by the rolling tires. Apart from road construction, steel slags may be suitable for applications such as reinforcement of riverbanks and in the manufacture of cement and fertilizer. As a fertilizer, converter lime combines many positive characteristics. Thanks to its mineral composition, it promotes plant growth. The immune system of the plants is especially strengthened by the high concentration of silica in the slag [45]. Apart from external recycling of steel slags, internal recycling is also applied. Tata Steel [46], Sidera [47] and Ferriere Nord [48] recycle their ladle slag back into the process as slag former. Apart from use as slag formers, ladle slag can also substitute some of the cement needed when manufacturing briquettes, which contributes to the reduction of CO2 emissions.

14 1.4 CRYSTALLIZATION THEORY As a new phase is formed in a system, the reaction is always associated with the Gibbs free energy of the system. When comparing Gibbs free energy of two mineral arrangements to determine which is the more stable, it is necessary to have a common basis for comparison. The widely adopted convention in chemistry is to compare Gibbs free energy levels based on the free energy of formation from the elements, Gf. The free energy of formation is the energy difference between the elements in their standard state and the elements chemically bonded to form a mineral at a certain temperature and/or pressure. The stability of a mineralogical arrangement may be expressed in terms of Gibbs free energy according to reaction (1).

= − G mineralogical reaction G f(products) G f(reactants) (1)

If, at a specific temperature – pressure condition, G reaction < 0, the product is more stable and the reaction may occur spontaneously. If G reaction > 0, then the reactants are more stable. The equilibrium condition is obtained if

G reaction = 0; thus, there will be no reaction taking place between the product and the reactants. All systems endeavour to minimize the total Gibbs free energy in terms of obtaining a stable state [49-51].

1.4.1 Nucleation Homogeneous nucleation is by definition the formation of nuclei within one phase. Conversely, where foreign elements are present, the result is heterogeneous nucleation, i.e. formation of crystals through more than one phase. In all chemical systems, the atoms are constantly moving, hence,

15 bumping into other atoms, and forming a variety of chemical combinations. Some combinations, called embryos, will have the structure and composition of a mineral that could crystallize from the melt through nucleation. Most embryos will be small, consisting of only a few atoms, while some will be larger. Whether these embryos will grow to form minerals or not depends both on the stability and the size of the embryo. If the new embryo has a higher Gibbs free energy of formation than the melt, the embryo will not have enough power to grow. Instead, the embryos will break apart and constituent atoms will return to the lower energy level represented by the melt. A stable nucleus which can grow further will only exist if the embryo reaches a certain critical radius, rc. If a spherical crystal shape is assumed, then the critical radius of a nucleus that becomes stable can be obtained from equation (2), where , Gv, G and Gf correspond to the surface energy per unit area, the volume energy, the surface energy and the free energy of formation [49-51].

C 3 S = + = D 4 r ()− T + 2 Gtot Gv G D Gf ()crystal Gf ()melt T 4 r (2) E 3 U

In Figure 2, equation (2) is plotted as a function of the radius. As can be noticed, the Gibbs free energy shows a maximum at a certain radius. Only at radius higher or equal to this value is there a driving force for nuclei to grow, the critical radius. The relation between critical radius and G is expressed as;

2 2 r = − = (3) c G f K T

16 where K and T correspond to a constant and the degree of undercooling.

Unstable Stable +

GV - Gibbs free energy energy - Gibbs free Gtot

rc Radius (r)

Figure 2: The Gibbs free energy of formation Gtot of crystal nuclei is the sum of the surface energy G and the volume energy Gv. Only nuclei larger than the critical radius rc are stable with respect to the melt.

For crystals to grow, it is necessary to heavily undercool in terms of reaching sufficient activation energy and thereby overcome the difficulties related to the surface energy. The consequence of equation (3) will therefore be that a strong undercooling (high T) provides a high driving force and a small critical radius, which results in a high probability for nucleation to occur [49-51].

1.4.2 Growth Once the nuclei are stable, the further growth is characterized by a gradual exchange of atoms at the surface of the nuclei. In order to predict the degree of solidification, both time and rate of growth must be considered.

17 Figure 3 schematically shows the temperature dependence between growth and nucleation.

Rate of growth Rate of nucleation

Nucleation

Growth

T0 T1 T2 (liquid slag) T Under cooling

Figure 3: The degree of reaction as a function of temperature.

T1 = low degree of undercooling. The nucleation will be the rate-determining step for the crystallization. A small amount of nuclei with high growth will result in few, but large, crystals.

T2 = high degree of undercooling. The growth will be the speed-dependent step in the crystallization. Many small nuclei with low growth rate result in numerous small crystals [49-51].

18 1.5 AIM AND SCOPE During the past 35 years, intensive research has been carried out related to the origin of the steel slags generated from the steel industry worldwide. This research has contributed to the development of new products and the conservation of natural resources. Despite the effort that has been invested, approximately 3.2 million tonnes of slag was still sent for deposit in Europe in 2006. In order to develop new applications for slags, the fundamental properties that determine the quality of the product must be further investigated. Most of the research conducted so far has focused on solving questions related to volume expansion and disintegration. This research has led to the development of new methods in order to prevent the formation of, as well as to treat, steel slags showing these properties. In recent years, questions dealing with leaching have attracted increased interest. So far, few studies have been published linking the leaching properties with variation in process parameters. Thus, the work conducted within this thesis presents a study regarding different types of steel slags and how the fundamental properties of the steel slag are influenced by variation in process parameters. The aim of this thesis was;

I. To investigate the possibility of modifying steel slags in hot stage. II. To determine how different cooling conditions influence the mineralogy and the leaching behaviour of some steelmaking slags. III. To determine the long-term stability of some EAF slags. IV. To determine how some of the individual minerals found in the slag behave when dissolving in aqueous media. V. To understand the chromium distribution in some EAF slags.

19 20 2 MATERIALS AND EXPERIMENTAL PROCEDURE

2.1 MATERIAL

The chemical compositions of the materials used in the different studies are shown in Table 5.

Table 5: The chemical composition of the material used. wt%

Material CaO SiO2 MgO Al2O3 Cr2O3 MnO FeO Fe2O3 Femet Paper II &III Ladle slag 42.5 14.2 12.6 22.9 0.3 0.2 0.5 1.1 0.4 BOF slag 45.0 11.1 9.6 1.9 0.1 3.1 10.7 10.9 2.3 EAF-slag 1 45.5 32.2 5.2 3.7 4.8 2.0 3.3 1.0 0.1 EAF-slag 2 38.8 14.1 3.9 6.7 2.7 5.0 5.6 20.3 0.6

Paper IV EAF slag 1 28.8 11.8 8.5 4.9 2.0 6.1 25.5 4.9 4.8 EAF slag 2 42.4 30.1 5.0 3.2 6.8 2.7 7.0 0.0 0.3 EAF slag 3 26.4 31.0 18.1 9.4 7.0 2.2 3.6 0.0 0.4

Paper V Mayenite 48.5 51.5 Merwinite 51.2 36.5 12.3 Akermanite 41.1 44.1 14.8 Gehlenite 40.9 21.9 37.2 Ingesonite 65.1 34.9 Tricalcium 62.3 37.7 aluminate

Paper VI EAF slag 36.7 14.0 11.2 6.0 3.2 5.1 10.9 10.3 0.6

2.2 EXPERIMENTAL PROCEDURE

2.2.1 Chemical analyses

The total composition of each material was analysed by Ovako Steel AB (Sweden) with inductively coupled plasma emission spectroscopy, ICP, and x-ray fluorescence spectroscopy, XRF. The content of Fe and Feoxid was

21 determined through titration. The analyses were done in duplicate and the results are presented as a mean value.

2.2.2 Specific surface area and density

The specific surface area was determined according to the BET-method with a Micromeretics Flowsorb 2300 and density was measured with a Micromeretics Multivolume Pycnometer 1305 on material prepared for leaching, <4 mm.

2.2.3 Glass analyses

The glass content was analysed by Scancem Research AB using optical microscopy according to ER 9103. A representative slag sample was ground and the fraction 32-40 m was used. In polarized light, optical isotropic grains, “glass”, have a different colour compared to crystalline anisotropic grains. If the grain contains more than 50% isotropic material it is identified as glass.

2.2.4 X-ray diffraction analyses

For XRD, all samples were prepared in a ringmill. The samples were analysed with a Siemens D5000 x-ray diffractometer, using copper K radiation. XRD patterns were recorded in the 2-theta range 10 to 90°, in 0.02°/step. Initially, XRD patterns were recorded by counting 1 s/step, and some samples were later rerun counting 3-8 s/step. The phase identification was made by reference patterns in an evaluation program supplied by the manufacturer of the equipment.

22 2.2.5 Scanning electron microscope investigation

The mineralogy of the slag samples was examined in a Philips XL 30 SEM using a beam operation voltage of 20 kV and spot size 6. Semi-quantitative and qualitative elemental analyses were performed with an energy dispersive spectrometer (EDS) fitted with an Everhart and Thornley detector behind a berylium window. Before mapping the texture and mineralogy of the fractured sample surface using the secondary electron (SE) image signal, the samples were sputter coated with a conductive layer of gold.

2.2.6 Leaching test

All the slag samples were crushed to a particle size of <4 mm and leached according to the one-stage batch test EN 12457-2 [52], except for two samples, ladle slag and granulated EAF slag 1 (Paper II), which were leached according to the two-stage batch test EN 12457-3 [53]. The filtrates were analysed by the laboratory Analytica AB (Sweden). The leaching tests were done in duplicate and the results are presented as a mean value.

2.2.7 Thermodynamic calculations in Factsage

Thermodynamic calculations were conducted using Factsage [54] using compound database FS53base.cdb, FToxid53base.cdb and solution database FToxid53soln.sda. FToxid-slag and FToxid-MeO were used. During calculation, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplications in the data set.

23 2.2.8 Titrations

Titration of synthetic manufactured slag minerals (paper V) was conducted using a Radiometer Copenhagen ABU 901 Autoburette attached to a

Radiometer Copenhagen PHM 290 pH meter. Assayed HNO3, 0.1 M was used as titrant. The choice of HNO3 was made in order to minimize possible formation of complexes. For each experiment, approximately 0.05 g of mineral was used together with 100 ml of deionized water (Milli-Q). The titration was performed on a sized fraction, 20-38 m. The pH electrode was calibrated before each experiment with adequate standard solution (pH 4, 7 or 10). The temperature in the reaction vessel was kept constant (25°C), using a water bath. A magnetic stirrer was used for mixing during titration and the stirring speed was kept constant throughout the titration. During titration, nitrogen was injected into the reaction vessel, protecting the system from CO2 and formation of carbonates.

2.2.9 Carbonation analyses

The degree of carbonation was measured using thermal decomposition (TGA-MS) according to the method described by Huijgen et al. [22]. TGA- MS analyses were performed in a thermo-gravimetric analysis system (Netzsch STA 409) coupled with a quadruple mass spectrometer (QMS). The samples were heated in alumina crucibles under an oxygen atmosphere at 20°C/min from 25 to 1000°C. Weight loss was measured by the TGA, while the gas was analysed for CO2 and H2O. The analyses were divided into three steps: Step (1) 25-105°C; step (2) 105-500°C; step (3) 500-1000°C. These steps represent (1) moisture, (2) organic elemental carbon and MgCO3 (if present) and (3) CaCO3 (inorganic carbon),

24 respectively. During heating, the samples were kept isothermally at 105, 500 and 1000°C for 15 min, giving enough time for the reactions to fully occur. The third weight loss from the TGA curve (m500-1000°C) was used to describe the calcium carbonate content.

25 26 3 RESULTS AND DISCUSSION This chapter briefly summarizes the six appended papers. The purpose, methods used, and the main conclusions are presented for each paper.

3.1 LITERATURE REVIEW - HOT STAGE MODIFICATION, PAPER I

When slag recycling issues are studied, the cold slag and its properties are generally considered to be fixed. The whole high-temperature slag treatment process, which results in the slag product, is too often ignored and disregarded. Even when the slag treatment process is considered to only begin at the moment of slag/metal separation to avoid making compromises towards metal or process quality, there is still considerable potential to influence the chemical composition and the mineralogy of the cold slag during the hot stage of slag processing.

Figure 5: General overview of the possible stages in slag processing. The stages of interest for this article are indicated by the dotted ellipse.

27 The aim of this review article is to give an overview of the scientific studies dedicated to the hot stage of slag processing, i.e. from the moment of slag/metal separation to complete cooling at the slag yard, Figure 5. Using in-depth case studies on Ca2SiO4-driven slag disintegration, chromium leaching and influence of microstructure, it is shown that the functional properties of the cooled slag can be significantly enhanced by small- or large-scale additions to the high-temperature slag and/or variations in the cooling path, even without interfering with the metallurgical process. The technology to implement such hot stage processing steps in an industrial environment is currently available. No innovative technological solutions are required. Rather, advances in hot stage slag processing seem to rely primarily on further unravelling the relationships between process, structure and properties. This knowledge is required to identify the critical process parameters for quality control. Moreover, it could even allow purposeful alteration of slag compositions and cooling paths to tailor the slag to a certain application.

3.2 A MINERALOGICAL INTERPRETATION OF THE SOLIDIFICATION /SOLUBILITY OF STEEL SLAGS. PAPERS II & III Four types of steel slags, a ladle slag, a BOF (basic oxygen furnace) slag and two different EAF (electric arc furnace) slags, were characterized and modified by semi-rapid cooling in crucibles and rapid cooling by water granulation. The aim of this study was to investigate the effect of different cooling conditions on the properties of slags with respect to their mineralogy, leaching and volume stability. Optical microscopy, X-ray diffraction, scanning electron microscope and a standard leaching test have been used for the investigation. According to both the XRD and the SEM analyses of the modified slag, it could be noted that there was a clear

28 difference in particle size distribution due to the different cooling conditions. A summary of the phases found in the slag samples can be seen in Table 6.

Table 6: A summary of phases identified in the slag samples. Ladle slag BOF-slag EAF-slag 1 EAF-slag 2 Original Rapid Original Semi-rapid Rapid Original Semi-rapid Rapid Original Semi-rapid Rapid Minerals:

Ca3SiO5 X

-Ca2SiO4 X

-Ca2SiO4 XXX XXX

-Ca2SiO4 XXX

Ca3Mg(SiO4)2 XXX

Ca2Fe2O5 XX

Ca2(Al,Fe)2O5 XXX

CaAl2SiO6 X

Ca2Al2SiO7 XX

Ca12Al14O33 X

Fe2O3 XXX (Fe,Mg,Mn)O XXX X XX MgO XX CaO (ss) XX Spinel (ss) XXX

Ladle slag Ladle slag is difficult to handle and store, due to its disintegrating properties. The XRD reveals that the ladle slag is the only one that becomes almost completely amorphous by granulation. The major mineral in the original slag is mayenite, Ca12Al14O33, followed in order by free MgO,

-Ca2SiO4, -Ca2SiO4 and Ca2Al2SiO7. The -form may undergo a phase transformation during cooling at 400-500°C to -form and the volume increase (>10%) causes a pulverization of the slag [5]. The formation of -

Ca2SiO4 is a plausible explanation for the disintegration. It was not possible to filter the leaching solution of the original slag, which might be due to cement-forming properties of the slag. One crystalline phase, undissolved MgO, was identified in the granulated ladle slag, Table 6. With SEM and

29 mapping of selected elements, two phases were identified: a matrix consisting mainly of calcium, silicon and aluminium (glass matrix) enclosing small fragments of MgO (1), see Figure 6. The MgO particles are well distributed in the matrix (2).

Figure 6: Scanning electron micrograph of rapidly cooled ladle slag. Dark fragments of (1) MgO in a matrix (2) with high content of calcium, silicon and alumina.

BOF slag The original BOF slag has high specific surface owing to high content of fines and pores compared to the granulated slag. According to the XRD results, Table 6, the major phase in the original BOF slag is larnite, -

Ca2SiO4. With SEM and mapping of selected elements, silicon and calcium were detected in the same phase, agreeing with the identification of larnite

30 as the major phase. Parts with high coexistence of iron, manganese and magnesium were also distinguished with SEM; possibly the (Mg,Fe,Mn)O solid solution also found with XRD. In the semi-rapidly cooled BOF slag, four crystalline phases were identified with XRD. A wustite-type solid solution containing magnesium, iron and manganese (Mg,Fe,Mn)O, - calcium silicate (-Ca2SiO4), calcium ferrite (Ca2Fe2O5) and a calcium, manganese oxide (Ca,Mn)O phase. All phases except for the (Ca,Mn)O were also detected with SEM and mapping, Figure 7, particle 1-3.

Figure 7: Scanning electron micrograph of the semi-rapidly cooled BOF- slag. (1) (Mg,Fe,Mn)O, (2) Calcium silicate, (3) Calcium ferrite.

In the rapidly cooled BOF slag, three crystalline phases were identified through XRD. A wustite-type solid solution containing magnesium, iron and manganese (Mg,Fe,Mn)O, tricalcium silicate and -Ca2SiO4. These phases

31 were also confirmed by SEM, phase 1-3, Figure 8, where phase 2 is tricalcium silicate (Ca3SiO5), i.e. the euhedral prismatic microphenochrysts which, according to Goldring and Juckes [55], are typical for Ca3SiO5 and phase 3, the matrix phase, crystallizing last, probably containing the -

Ca2SiO4 seen in XRD. All phases identified in the rapidly cooled BOF slag, agree with the thermodynamic calculation, Figure 9, i.e. indicating that the fast cooling enables the presence of metastable phases, such as Ca3SiO5 and -Ca2SiO4 at lower temperatures.

Figure 8: Scanning electron micrograph of the rapidly cooled BOF -slag. (1) (Mg,Fe,Mn) oxide, (2) Calcium silicate, (3) Matrix containing (Ca,Si,Ti,V,Mn,Fe) oxides.

As seen in Paper 2, Figure 3, the composition of the wustite-type solid solution is different when comparing the semi-rapidly and the rapidly cooled

32 BOF slag. According to the thermodynamic calculations, Figure 9, MgO is already present as crystals in the liquid slag at 1600ºC. The slight change in position which occurs in the diffractogram is explained by a higher concentration of MgO in the wustite-type solid solution. As the slag is cooled rapidly, neither FeO nor MnO has the same possibility of crystallizing and forming solid solution with MgO, due to i the later crystallization in comparison to MgO, Figure 9. The latter was further confirmed by the SEM analysis. According to semi-quantitative analyses, the solid solution contains 51 at% MgO, 42 at% FeO and 7 at% MnO, in the semi-rapidly cooled slag, while the solid solutions in the rapidly cooled slag were made up of 78 at% MgO, 16 at% FeO and 6 at% MnO. When a phase can form thermodynamically, the crystal size will depend on the temperature to which the crystals are exposed and the duration of the exposure. There was a significant difference in crystal size between the two modified BOF slags, Figure 7 and 8. The size of the crystals present in the semi-rapidly cooled slag varied between 40-200 m, indicating that these minerals have had more time to grow. In the rapidly cooled BOF slag, the variation in crystal size is larger compared to the semi-rapidly cooled BOF slag. The wustite-type solid solution (phase 1) and the tricalcium silicate (phase 2), Figure 8, have a crystal size varying between 20-100 m. The matrix (phase 3), Figure 8, has a much smaller grain size than the other two phases discussed. The smaller crystal size of this silicate matrix can thus be explained in terms of not having the same time to develop. Based on the thermodynamic calculations, it can be concluded that both the wustite-type solid solution and the tricalcium silicate were present in the liquid slag at the time the rapid cooling with water was carried out.

However, -Ca2SiO4 is expected to form during rapid cooling with water. The leaching of calcium and iron is reduced in the granulated BOF slag.

33 Iron is present in the matrix, as discussed above, and the leaching is very low in all three slag samples. Calcium, on the other hand, is also present in the major silicate phase, Ca3SiO5. The solubility of silicon is increased in the granulated slag compared to the original. The leaching result shows that the dissolution of the minor elements is not prevented by the rapid cooling procedure. Vanadium is most soluble in the granulated BOF slag, correlating to the silica leaching, indicating a more enhanced leaching from the fine-grained silicate matrix.

BOF-slag

Liquid slag Ca SiO (s) 60 3 5 CaO(s) FeO

γ − Ca2SiO4(s) MgO

α′− Ca2SiO4(s) MnO(s) 50 Liquid slag Ca2Fe2O5(s)

40 γ − α′− Ca2SiO4(s) Ca2SiO4(s)

30 Ca2Fe2O5(s) Ca3SiO5(s) FeO(s)

20 Phase distribution wt % distribution Phase

CaO(s)

10 MgO(s)

MnO(s) 0 25 203 380 558 735 913 1090 1268 1445 1623 1800 T(ºC)

Figure 9: Thermodynamic calculation of BOF slag using Factsage.

34 EAF slag 1 In the semi-rapidly cooled EAF slag three crystalline phases were identified with XRD (Table 6) i.e., a spinel containing magnesium and chromium

(magnesiochromite, MgCr2O4), calcium magnesium silicate (merwinite

Ca3Mg(SiO4)2) and -dicalcium silicate (-Ca2SiO4). Both merwinite (phase 1) and magnesiochromite (phase 3) were found with SEM as well, Figure 10. From Figure 10, it can be seen that manganese is present in the solid solution along with chromium and magnesium. Except for the three phases which were found with XRD, an additional phase was observed with SEM and mapping, i.e., a calcium alumina silicate phase (phase 2), see Figure 10. According to the thermodynamic calculation, Figure 11, only one phase of calcium alumina silicate exists in the system, namely Ca2Al2SiO7 (gehlenite). Gehlenite is thermodynamically formed below 1270ºC according to the calculations. The later crystallization of gehlenite agrees well with the texture of the semi-rapidly cooled EAF slag. As seen in Figure 10, both the merwinite and the spinel found have their own specific structures, characterized by sharp edges, while the gehlenite is located between the other two. According to the thermodynamic calculations in Figure 11 both merwinite and the spinel crystallize earlier than gehlenite, which explains the texture of the semi-rapidly cooled EAF slag. Two crystalline phases were identified in the rapidly cooled EAF slag with XRD, Table 6, both similar to those that were found in the semi-rapidly cooled EAF slag, merwinite and the spinel, containing magnesium and chromium. When comparing the diffractogram in Paper 2, Figure 3, a broadening of the peak width can be observed as a result of the rapid cooling with water. Suryanarayana and Grant [56] suggest that this may be caused by a decreased crystallite size.

Figure 10: Scanning electron micrograph and accompanied mapping of the semi-rapidly cooled EAF slag. (1) Calcium magnesium silicate, (2) Calcium alumina silicate, (3) Chromium containing spinel.

36 EAF-slag 1

Liquid slag Ca MgSi O (s) 60 3 2 8 α′− Ca2SiO4(s) FeCr2O4(s) ss-spinel α− Ca2SiO4(s) (MgO)(Cr2O3)(s) ss-spinel

Ca2Al2SiO7(s) MnAl2O4(s) ss-spinel 50 Ca3Si2O7(s) Liquid slag

40 Ca3MgSi2O8(s)

Ca3Si2O7(s)

α′− 20 Ca2SiO4(s) Phase distribution wt %

α− Ca2SiO4(s) 10 FeCr2O4(s) (MgO)(Cr2O3)(s)

Ca2Al2SiO7(s) MnAl O (s) 0 2 4 25 203 380 558 735 913 1090 1268 1445 1623 1800 T(ºC)

Figure 11: Thermodynamic calculation of EAF slag 1 using Factsage.

Furthermore, the differences in crystal size found between the two modified slags are significant (Figures 10 and 12). The spinel phase has the same size and texture in both materials, while the rest of the phases vary considerably, indicating that the spinels were crystallized already at the time the granulation started, which is also confirmed by the calculations, Figure 11. The large, well-defined merwinite and gehlenite crystals, which were found in the semi-rapidly cooled slag, were no longer present in the rapidly cooled EAF slag. Instead, a mixture of calcium, magnesium, alumina and silica was found (area 2), Figure 12. According to the theory on nucleation and growth, area 2 most likely consists of small merwinite and gehlenite crystals, due to the rapid cooling i.e., the rapid crystallization.

Figure 12: Scanning electron micrograph of rapidly cooled EAF slag. (1) Chromium containing spinel, (2) Complex of (Ca,Al,Si,Mg) oxides.

The content of calcium and silicon is high in the EAF slag 1, Table 5. The solubility of these two major elements, as well as aluminium, iron and magnesium, is shown in Paper 2, Table 3. The leachability is very low and varies in the three samples. The solubility of aluminium is reduced substantially in the semi-rapidly cooled and the granulated slag, which indicates that one of the matrix-forming phases is stable. On the other hand, the mobility of silica seems to increase when granulating. There does not seem to be any obvious correlation between the solubility of the major and the minor elements. The varying dissolution of the metals chromium, molybdenum and vanadium is more likely a result of the presence in different minerals. The solubility of chromium is very low, 20

38 ppm of the total chromium content, in all three samples. Vanadium, on the other hand, is most leachable in the semi-rapidly cooled slag.

EAF slag 2

The XRD analysis reveals that the slag is very complex and some phases have varying content of substituted ions. The identified main mineral is -

Ca2SiO4 in both the original slag and the two modifications. A wustite-type solid solution ((Fe,Mg,Mn)O), Ca2(Al,Fe)2O5 and Fe2O3 were also identified, Table 6. A broadening in the diffraction peaks, indicating smaller particle size, could be seen when cooling rapidly. Calcium, iron and silicon are the major elements in the matrix of the EAF slag 2. As can be seen in Appendix 2, Table 3, calcium, aluminium and iron have the lowest leachability in the granulated slag, while silicon as well as the minor elements chromium, molybdenum and vanadium have the lowest leaching in the semi-rapidly cooled slag.

3.2.1 GLASS FORMATION (Paper II)

Daugherty et al. [34] claim that an acid slag Mb(CaO+MgO/SiO2+Al2O3)<1 produces a glassy material more readily compared to a more basic slag when cooled rapidly. The investigated slags are considered to be basic Mb= (1.4-3.9) and should therefore mainly contain crystalline material. The measured glass content for the original and granulated slag is listed in Paper 2, Table 1. Both the EAF slag 1 and the ladle slag show significant changes in glass content. In order to determine the glass formation in the slag, it is not enough to look at the chemical analyses. It is also important to consider the chemical analyses of the remnant melt due to high- temperature crystallization. To better understand the glass formation, the

39 crystallization path and corresponding melt composition at equilibrium conditions were calculated using Factsage. The calculated Mb value is shown in Figure 13.

Figure 13: Glass forming tendency Mb as a function of liquid slag temperature.

According to the thermodynamic calculations, the MgO crystallization from the liquid ladle slag starts already at approximately 1800°C. Only ~38% of the total MgO content is present in the liquid slag at 1400°C. The remaining 60% has already been crystallized as pure MgO. This phenomenon can also be seen in Figure 6. When MgO crystallization takes place, the Mb- factor in the liquid material is changed from original 1.5 to 1.25 at 1400°C, influencing the glass forming properties in the material. Due to early crystallization of solid solution, spinel phases, the liquid slag composition of the EAF slag 1 is changed during cooling. Thermodynamically, the formation of spinel already starts at ~1950°C.

40 When the formation takes place, MgO reacts with chromium, forming magnesiochromite (MgCr2O4). The formation of spinel is increased as the temperature of the liquid slag decreases, resulting in a lower Mb factor. The

Mb ratio is decreased from 1.41 to 1.34. As seen in Figure 13, both the EAF slag 1 and the ladle slag tend to become more acid as the liquid slag temperature decreases, due to crystallization from the liquid slag. Neither the BOF slag nor the EAF slag 2 shows any tendency of forming glass when cooling rapidly, according to Appendix 2, Table 1. The high Mb value shows that the slag is also basic at low temperatures, Figure 13.

3.2.2 REACTIVITY, BOF AND EAF SLAG 1 (Paper III) The leaching from steel slags is generally characterized as a surface reaction, followed by a solid-solid diffusion process, in order to retain equilibrium in the materials [19]. A minimization of the surface area of the slag is therefore likely to enable a decrease in leachability. Leaching and specific surface data regarding these materials are listed in Paper 2, Table 1 and Table 3. The specific surface area data was unfortunately not measured in relation to the semi-rapidly cooled samples. However, since the original and semi-rapidly cooled samples were prepared for leaching in the same way (crushing < 4mm), it is assumed that these materials have a similar specific surface area. It has earlier been concluded that no distinct changes in the total leachability could be noticed when comparing the semi-rapidly cooled with the rapidly cooled materials. However, a decrease in the specific surface area was noted when the semi-rapidly cooled slag was compared against the rapidly cooled slag. To gain a better understanding of the reactivity with regard to the surface chemistry of the differently cooled slags, a reactivity factor was introduced and calculated according to equation (4). The

41 reactivity ratio of the rapidly and semi-rapidly cooled slags is given in Table 7.

Leached amount (mg) leached from (kg) dry material (mg/kg) = = = mg/m2 (4) Specific surface area m 2 /kg

Table 7: The - ratio of rapid and semi-rapidly cooled slag. BOF slag EAF-slag 1

(Rapid)/(Semi) (Rapid)/(Semi) Element Semi-rapid cooling Rapid cooling % Semi-rapid cooling Rapid cooling %

Ca 1,87E+00 9,86E+00 526% 2,90E-01 2,69E+00 928% Na 1,87E-03 2,89E-02 1543% 1,09E-03 3,09E-02 2824% S 4,62E-03 1,06E-01 2305% 2,55E-03 1,52E-02 596% Si 6,34E-03 2,98E-01 4694% 6,30E-02 7,78E-01 1235% Al 8,15E-03 7,76E-03 95% 2,30E-03 1,61E-02 701% Ba 1,75E-04 8,93E-04 510% 2,67E-04 4,05E-04 151% Cr 3,29E-06 1,74E-04 5284% 3,68E-04 5,50E-03 1494% Mn 4,23E-05 3,99E-05 94% 6,26E-06 5,23E-05 835% Mo 2,79E-05 3,11E-04 1117% 4,82E-05 3,84E-04 797% V 2,98E-04 3,67E-02 12317% 1,24E-03 1,79E-03 144%

As seen in Table 7, there are important differences in reactivity due to the different cooling conditions. Almost all elements become more reactive if treated with rapid cooling. The reactivity for silica is increased by ~ 4700% and ~1200%, respectively, and for chromium ~5300% and ~1500%, respectively, for the BOF and EAF slag 1. Unlike the blast furnace slag, which also becomes more reactive when cooled rapidly, due to the high content of amorphous phases, both the quenched BOF and EAF slag 1 have a low content of glass. Instead, two possible explanations for the increase in reactivity are the presence of metastable phases and the increased number of small crystals on the surface due to the rapid cooling with water.

42 3.3 AGEING INVESTIGATION OF EAF SLAGS, PAPER IV The aim of this work was to investigate how stable EAF slags are when aged, in an environment open to seasonable weather conditions, with respect to leaching and mineralogy. Three different EAF slags were used in this study. To guarantee a freshly produced and homogenous slag material, the samples were collected directly from the steel plant. All slag samples used in these experiments were crushed and sieved to a fraction of 1

During the 24 months of ageing, the materials were exposed to a large variation in both precipitation and temperature. All phases identified in the non-aged samples (0-month) using XRD, were also identified in the aged slag samples. In terms of stability, this means that no drastic change in mineralogy in the bulk of these materials occurs upon ageing. It can therefore be concluded that all reactions (leaching/changes in mineralogy) only take place on the surfaces. After ageing, the materials show large variations regarding the ability to leach. After 12 months of ageing, the electrical conductivity (total leachability) has decreased by a factor 3.4, 3.8 and 3.0 for EAF slag 1, EAF slag 2 and EAF slag 3, respectively. The EAF slags’ ability to influence (increase) the pH of the water they are exposed to also decreases with time. During the 24-month period that materials were exposed to weathering, pH decreased by approximately 1. A decrease in

43 pH may influence the solubility of specific elements differently. As an example, the solubility of Mg can be considered. According to the pourbaix diagrams shown in Figure 14, the dominant magnesium phase at pH 12 -6 (10 M) will be Mg(OH)2 (s). If the pH is lowered to 11, the dominant magnesium phase is instead Mg2+.

A B Ca-H2O, 25°C Mg-H2O, 25°C 2.0 2.0

1.5 CaO2(s) 1.5

1.0 1.0

0.5 0.5

Ca[2+] Mg[ +] 0 0 2 CaOH[+]

E(volts) E(volts) Mg(OH)2(s) -0.5 -0.5

-1.0 -1.0

-1.5 -1.5 MgH2(s) CaH2(s) -2.0 -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

C D

Al-H2O, 25°C Cr-H2O, 25°C 2.0 2.0

1.5 1.5 HCrO4[-] CrO [ -] 1.0 1.0 4 2

0.5 0.5 Al[3+] Cr(OH)[2+] Al(OH)3(s) 0 0 AlO2[-] E(volts) E(volts) Cr2O3(s) -0.5 -0.5

Cr[2+] -1.0 -1.0

-1.5 -1.5 Cr(s)

-2.0 Al(s) -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

Figure 14. Pourbaix diagram (10-6 M) calculated in Factsage [54], A) Calcium, B) Magnesium, C) Aluminium and D) Chromium.

The leaching of chromium, Figure 15, as well as molybdenum (slightly) decreases for all three slag types when comparing to initial values (0 month). Magnesium is the only element that increases in leachability when ageing, Figure 16. The leachability of magnesium seems to accelerate as

44 the materials are aged. When comparing mineralogy vs. leaching, it is worth mentioning that, EAF slag 1 has approximately 1.8 times higher content of magnesium dissolved in its structure compared to EAF slag 2. However, concerning the leaching of magnesium, EAF slag 2 leaches 2.4 times more magnesium than EAF slag 1 after 24 months of ageing. The XRD analyses reveal that the magnesium in EAF slag 1 is mainly present in a solid solution containing iron and manganese ((Fe,Mg,Mn)O), while in

EAF slag 2 it is mainly in the form of merwinite (Ca3MgSi2O8). This means, in terms of solubility, that the solid solution containing magnesium is less reactive compared to merwinite at this pH range. Consequently, the amount of a certain element can never be correlated to the leaching without considering in what phase it is present.

Figure 15: Leaching of chromium as a function of ageing time for the different EAF slags.

Figure 16: Leaching of magnesium as a function of ageing time for the different EAF slags.

Both calcium and aluminium show similar leaching behaviour as chromium with respect to the duration of ageing. The calcium and aluminium leaching decreases by a factor of 3.0/9.0, 3.0/64.0 and 2.3/2.9 for EAF slag 1, EAF slag 2 and EAF slag 3, respectively, after 24 months. According to Figure 14, the dominant calcium and aluminium phase in the solution, at the

2+ − -6 corresponding pH (10-11.5), will be Ca and AlO2 at 10 M, respectively. The Ca2+ is also the dominating calcium phase throughout the whole pH

− range, while the dominating aluminium phase is changed from AlO2 to 3+ Al(OH)3 (s) and finally Al as the pH decreases.

46 3.3.1 Surface reactions

Calcium carbonate (CaCO3) was formed on the slag surfaces within 6-12 months of ageing. Thermodynamic calculations performed on these materials show that free lime does not exist and thereby cannot be the source of carbonation. These assumptions were also verified by XRD. The calcium source for the reaction is, instead, most likely correlated to the dissolution of calcium rich silicates present in the slag, which has earlier been observed by Huijgen and Comans [26]. They described this transformation as an aqueous mechanism of carbonation, and the reaction is known to occur in three steps:

+ → + ()+ 2− () CO 2 (g) H 2O(aq) 2H aq CO3 aq (5)

− + + ()→ 2+ ()+ + Ca silicate(s) 2H aq Ca aq silicate(s) H 2O(aq) (6)

2+ + 2− → ↓ Ca (aq) CO3 (aq) CaCO3 (s) (7)

2− In step one (5), the rain water is saturated with CO2, forming CO3 ions in solution. Step two (6) corresponds to the actual leaching of Ca-rich minerals from the slag; in the case of EAF slag 1, -Ca2SiO4 and

Ca3MgSi2O8; and for EAF slag 2, Ca2Al2SiO7 and Ca3MgSi2O8; and for EAF slag 3, CaMgSiO4 and Ca2MgSi2O7. Step three (7) includes a simultaneous reaction between the products from step one and two, leading to the precipitation of calcite on the slag surfaces. Experiments performed in laboratory scale have shown that step three (7) is considered to be very rapid [22]. Huijgen et al. [22] suggest that step two, the diffusion of calcium through the solid slag matrix towards the slag surface, appears to be the

47 rate-determining step in the carbonation. As has been discussed earlier, the leaching of an element, in this case calcium, cannot be based only on the chemical analyses. More specifically, the solubility of each calcium- containing slag mineral must be considered in order to explain the leaching behaviour and thereby the carbonation. The degree of carbonation was highest for EAF 1 (1.5 wt%) followed by EAF 2 (0.6 wt%) and EAF 3 (0.4 wt%). The solubility of the Ca-rich slag minerals found in these materials is not reported in the literature. However, the results from this study indicate that the slag minerals found in EAF slag 1 (-Ca2SiO4, Ca3MgSi2O8) dissolve easier than those found in EAF slag 3 (CaMgSiO4, Ca2MgSi2O7). Knowing that calcite mineral will be the dominating phase in water solutions in the range of pH 7-14, depending on Ca concentration, the precipitation of calcite on the surface of the slag will most likely continue as long as there are slag minerals rich in calcium still dissolving. However, as the calcite layer increases, the surfaces of the slag grains will become less reactive, due to the formation of an insoluble surface layer. The extent of the decrease in total leachability that actually can be explained by the carbonation is, however, hard to distinguish. In Figure 17 it can clearly be seen that the aged sample of EAF slag 1 has a smaller grain size and a more level surface structure as a result of the carbonation.

48 Figure 17. SEM micrographs (surface) of A) EAF 1, 0 month sample B) EAF 1, 24 month sample.

3.4 A STUDY OF THE SOLUBILITY OF PURE SLAG MINERALS, PAPER V The aim of this study was to take the understanding of the solubility/leaching that occurs from ordinary steel slags one step further, and actually investigate how individual slag minerals behave during dissolution. Since steel slags are a mixture of numerous types of minerals, the solubility of each mineral will affect the outcome of the leachability. Six common slag minerals, mayenite (Ca12Al14O33), merwinite (Ca3MgSi2O8), akermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), -dicalcium silicate (-

Ca2SiO4) and tricalcium aluminate (Ca3Al2O6) were synthesized and evaluated through titration using HNO3 at constant pH. Acid to alkaline pHs (4, 7 and 10) were selected to investigate the solubility of the minerals under conditions comparable to those prevailing in newly produced slags and one pH value, representing acid conditions. At pH 4, the dissolution of the investigated minerals is generally considered as being rapid with a high acid consumption. The rate of dissolution differs between the different minerals, where tricalcium aluminate dissolves at the fastest rate and gehlenite at the slowest. At pH 10, the typical pH of a leachate from a newly

49 produced steel slag, the dissolution is slower compared to pH 4 and with a lower total acid consumption. Merwinite, akermanite and gehlenite show a pronounced decrease in solubility at pH 10 compared to pH 4. In the case of carbonation, it has been shown that the dissolution of calcium from the slag will determine the outcome of the carbonation [22]. In comparison with the results from this study a steel slag having -dicalcium silicate as a main mineral would form carbonates both faster and to a greater extent compared to steel slags having merwinite as the main mineral. This means that an AOD slag containing a high amount of -dicalcium silicate would carbonate to a greater extent compared to an EAF slag from stainless steel production, containing a high amount of merwinite. To minimize the initial leaching of trace metals, often present as solid solutions in the main mineral of the slag, the solubility of the host mineral should be as low as possible, which is contradictory to the conditions needed for carbonation. As mentioned earlier, it has been shown [32] that chromium can be enriched in the merwinite phase. According to the results, the rate of merwinite dissolution at high pH is low but not negligible. This means that the solubility of merwinite in a merwinite-based slag system partly can explain some of the chromium leaching occurring from these slags. To achieve an appropriate material for usage in external applications with regard to leaching properties it can be concluded that an optimized slag system should include a mixture of stable minerals suitable for capturing metals together with easily dissolved calcium-rich minerals, contributing to the formation of a less reactive surface layer in the form of carbonates. Although several of the investigated minerals show similarities in oxidic composition, there is a big difference in the rate of dissolution between the minerals. It is therefore obvious that the crystal structure of the minerals plays an important role in the dissolution.

3.5 STABILITY OF SPINELS IN HIGH-BASICITY EAF SLAG, PAPER VI.

Different methods have been developed in order to bind the remaining chromium in stainless steel slags with a B2 (CaO/SiO2) around 1.4 into stable phases. Most of the existing work is focused on the addition of oxides to the liquid stainless steel slag in order to increase the amount of stable chromium-rich spinel phases. In this study the influences of three different already formed spinels, (Magnetite (FeFe2O4), Spinel (MgAl2O4) and Chromite ore), in three different amounts, on an EAF slag with high basicity (B2 around 2.6) from a low-alloyed steel production were investigated. The aim of this study was to investigate how stable chromium- rich spinel phases are in high-basicity EAF slags. After addition and re- melting followed by solidification the mineralogical properties of the samples were studied using SEM and XRD. Thermodynamic calculations conducted using Factsage were used in order to explain the results.

Slag with Fe2Fe2O4 and MgAl2O4 additives There was good agreement between the XRD and SEM analyses, suggesting that calcium silicate and the wustite-type solid solutions, (Mn,Fe,Mg)O, are the main phases of the investigated slag. All samples of these two groups show an identical structure, where no spinel could be detected. Cr is enriched in small spots, which are distributed evenly in the wustite-type solid solution.

Slag with Chromite ore additive XRD patterns of the chromite modified EAF slags are shown in Figure 18. In addition to the two main phases of calcium silicate and the wustite-type solid solution, presence of the spinel phase can be seen. The intensity of

51 the spinel peaks is increased by increasing the amount of the additive, which is an indication that more additive results in formation of more spinels.

2 1: Calcium Silicate 2: Wustite type solid 1 1 solution 3 2 2 3: Spinel 1 1 1:1 Ratio 3 1 1 1 1 2

1 1 3 2 2 1:0.5 Ratio 1 1 1 3 1 1 1 2 Intensity 1 1 2 2 1 1 3 1:0.25 Ratio 1 3 11 1 2

1 1 2 2 1 1 Ref. Sample 3 1 3 1 1 1

25 30 35 40 45 50 2-Theta-Scale Figure 18: XRD patterns of the samples of EAF slag/ chromite ore.

The main difference between the stainless steel slag and the EAF slag used in this experiment is the basicity (B2). In order to protect the furnace lining and achieve a foamy slag, some EAF slags are saturated with magnesium. The ternary phase diagram of CaO-SiO2-MgO shows that, by this saturation, the primary crystallization field of the slag would change from Merwinite, Ca3MgSi2O8 (in the stainless steel slag) to the Periclase, MgO, in the EAF slag used in the experiments. To clarify the reasons for why chromium-rich spinels were only found for the chromite ore modified slag samples, the binary phase diagram of MgO-Cr2O3 can be referred to, Figure 19.

Figure 19: Binary diagram of the MgO-MgCr2O4 system [57].

According to Figure 19, the MgO solid solution dissolves up to 8% chromium at 1600ºC before the MgCr2O4 solid solution can be formed. By using FeFe2O4 and MgAl2O4 as additives, chromium will dissolve in the wustite-type solid solution and no spinels can be detected. Using the chromite ore as the additive increases the total content of the chromium to a critical limit (above 8%) at which, according to Figure 19, the MgCr2O4 solid solution is in equilibrium with MgO solid solution. According to the current results, it can generally be concluded that formation of the Cr-rich spinels is suppressed as long as free unsaturated MgO is present in the slag.

53 54 4 CONCLUDING DISCUSSION

Summarizing the results from this thesis, it can be concluded that influencing the behaviour of an individual steel slag is far more complex than going to the supermarket and buying ingredients for the evening supper. All stages, from the first decision that is made within the steel shop, adding the slag formers to the furnace, to the treatment of the solidified slag at the slag yard, will influence the properties of the slag product being manufactured. From the conducted experiments it can be concluded that three major parameters will influence the properties of the slag, all of which may vary during slag handling, Figure 20.

Figure 20: Parameters influencing the properties of the slag product.

The composition of the steel slag is almost always optimized based on the steel being manufactured. An AOD slag from the manufacturing of stainless steel differs significantly in composition compared to a BOF slag from the refining of hot metal. Using the CaO-SiO2-MgO ternary phase diagram, Figure 21, it is fairly easy to understand that the composition of the slag plays an important role in determining the final properties of the slag product. As an example, the composition of four different steel slags studied within this thesis can be used (Figure 21, point A-D). In addition to variations in phase composition, it can also be seen that crystallization/melting interval differs significantly between the different slag

55 types. Even a small variation in the chemical composition may ultimately lead to changes of the characteristics of the final slag. The review article on hot stage modification of slag has shown that it is possible to make changes to the chemical composition without jeopardizing the quality of the steel.

Figure 21: Ternary phase diagram CaO-SiO2-MgO [58].

The experiments conducted within this thesis have shown the importance of the cooling condition. In addition to variation in mineralogical composition due to the formation of metastable phases when cooling rapid, variations in crystal size, phase composition (solid solution), glass content as well as

56 reactivity of the slag have been shown to be influenced by the cooling conditions. As in the case of chemical composition, small variations in cooling conditions may ultimately have a big impact on the final properties, especially when it comes to leaching. The increase in reactivity for the rapidly cooled slag samples is believed to depend on several factors. These factors are:

Oxidation: During the rapid cooling with water, oxidation on the surfaces may occur, and thereby the formation of soluble phases.

Surface structure: Fast cooling will result in an abrasive surface, due to the presence of smaller grains at the surface. An abrasive surface tends to be more reactive than a plane surface, due to the increase in vapour pressure that occurs over a convex surface.

Increased amount of Fast cooling will result in more grain grain boundary: boundaries due to the increase in the number of small crystals in the material. Diffusion reactions are known to occur easier and faster along these boundaries.

Metastable phases: Increased solubility with water due to the increased energy of the system.

57 When it comes to the material handling including crushing/grinding, metal separation and ageing, the results show that it is important to handle each slag separately and in agreement with the composition and application of interest. As an example, mayenite and tricalcium aluminate based steel slags (secondary metallurgical slags) suitable for use in cement applications should not be exposed to precipitation. If exposed to water, these slags will lose their hydraulic properties. Steel slags used as replacement for ordinary stone material in construction applications should be aged for at least six months before usage. When ageing, the surface of the steel slag is pacified according to Figure 22, meaning that a protection layer (barrier) is formed around the slag grain.

Figure 22: Schematic picture of influence of ageing.

58 The experimental results also show that it is important that all handling including crushing and sieving is conducted before the material begins to age. If aged slags are subjected to crushing, new surfaces will be exposed, meaning that pacification must be conducted once again in order to obtain the demanded properties.

As discussed and highlighted earlier, the leaching from these materials is still the primary cause of difficulties with valorisation. So, how can these findings be applied in such a way that the leaching occurring from these materials is minimized? In order to answer that question the term leaching must first be fully explained. According to the given theory, the leaching occurring from steel slags can be explained in terms of the sum of the dissolution of the minerals that are present in the slag. Since leaching is a question of slag/water reaction, some phases will dissolve both faster and to a greater extent compared to others. The study performed on synthetic slag minerals verifies this.

The leaching of trace metals such as chromium is often the reason for the slag not being used in external applications. The amount of trace metals is sometimes not sufficient for creating an own phase; instead, they will be distributed as solid solutions in other minerals. Despite the presence of the trace metal in “pure” mineral or as a solid solution it is then the solubility of the host mineral as well as the solubility of the metal itself that will determine the solubility of the trace elements, in this case chromium. In order to be able to control the leaching it is preferable that the element that is going to be stabilized is enriched in as few minerals as possible. There are two ways of stabilizing slags with respect to solubility of a certain metal.

59 The first way is to capture the metal into minerals that do not dissolve. The second way is to capture the soluble mineral in a non-soluble slag matrix.

So, in which slag minerals is chromium likely to be found? To be able to answer that question the valence of chromium in steel slags must first be known. According to the present literature, chromium will be available as Cr2+ and Cr3+ in the steel slag [59-60]. According to the current understanding, Cr2+ and Cr3+ ions occupy, as a rule, the octahedral sites in the crystal structure [61-62]. This means that enrichment of chromium (Cr2+/Cr3+) will only occur in slag minerals having octahedral sites present in crystal structures. As discussed earlier, this suggests that the crystal structure of the slag mineral mix is of greatest importance.

As an example, the leaching of chromium from low-basicity (B2=1.4, Figure

21 - C) and high-basicity (B2=2.5, Figure 21 - B) EAF slag can be used. In the low-basicity EAF slag the most common minerals found include merwinite, akermanite, gehlenite and solid solution spinel phases. At

B2=2.5, typical minerals found in the EAF slag is (alpha, beta, gamma) dicalcium silicate, wustite type solid solution, dicalcium ferrite and merwinite. For the low-basicity EAF slag, investigation has shown that chromium will be enriched in spinel-type solid solution and in the merwinite phase, while the chromium is primary crystallized in the wustite-type solid solution and secondary in spinel phases for the high-basicity EAF slag. All these minerals have octahedral sites in their crystal structure.

When it comes to the low-basicity EAF slag, studies have shown that the spinel phase is crystallized at high temperature and is considered as being insoluble. However, the merwinite is considered as being soluble

60 throughout the entire pH range, which will make chromium dissolve parallel to merwinite. Since merwinite is considered as a possible matrix mineral, Figure 10, enclosing other elements into its structure, it can be assumed that the leaching of chromium from these types of slags will continue as long as merwinite dissolves. For the high-basicity EAF slag the primary crystallization of chromium occurs within the MgO solid solution. In order to save refractory materials and create a foamy slag, these types of EAF slags are often saturated with MgO, meaning that solid particles of MgO will be present in the liquid slag. The crystallization of chromium will start in these particles and continue until equilibrium conditions are reached, thereafter forming spinel. As the solubility of chromium in the magnesium- based solid solution (MgO-ss) is influenced by the temperature, Figure 19, it is reasonable to assume that MgCr2O4 will further nucleate as the temperature decreases, dissolving the MgO-ss. However, since this transformation is a solid-solid phase transformation, occurring at “low” temperatures, the kinetics of the transformation are believed to be slow. A pure magnesium oxide phase enriched in chromium is not a desired phase when it comes to chromium solubility. Numerous studies have shown that pure magnesium oxide will hydrate and expand but also dissolve [5, 6]. However, there is a solution to this problem. Since these slags often have a high concentration of iron oxide it can be assumed that wustite will enter the MgO-ss as the temperature decreases (liquid-solid reaction) due to complete solubility in each other. Investigations performed within this thesis have shown that the composition of the wustite-type solid solution in slag will vary with the rate of cooling. A rapid cooling will result in an MgO-based wustite-type solid solution, while a slower cooling rate will promote the enrichment of iron oxide into the structure. Experiments (as yet not published) conducted on the MgO-FeO + 4 wt% Cr2O3 system have shown

61 the importance of the iron oxide on the leaching behaviour of chromium from the wustite-type solid solution, Figure 23. According to Figure 23, the leaching of chromium from the wustite-type solid solution decreases with an increasing amount of FeO. In terms of cooling, this means that a slower cooling rate is preferable when these types of phases are present. However, thermodynamic calculations performed on similar systems, Figure 8, have shown that the cooling should not be too slow. Below

1100°C, Ca2Fe2O5 is recrystallized, meaning that wustite will leave the MgO-based wustite-type solid solution, once again forming the reactive MgO phase.

Figure 23: Cr leaching as a function of (Mg:Fe)O composition.

A good rule of thumb is that all steel slags should be treated individually and in harmony with; 1) the steel product being manufactured and 2) the steel slag being manufactured from a valorization point of view. From the results presented in this thesis it is fairly easy to understand why.

62 5 CONCLUSIONS

From this study the following conclusions have been stated:

1. Based on the literature survey conducted within this thesis, it has been shown that it is possible to affect the final properties of the slag by additives to the slag in hot stage.

2. The final phase composition of the slag will depend on the temperature at which the cooling starts. By cooling the liquid slag very rapidly, there will not be sufficient time for the crystals to grow. The rapid cooling will also enable the possibility of metastable phases at low temperatures.

3. To control the properties of a certain slag, the method of cooling must be consistent from heat to heat.

4. Slags with higher Mb (CaO+MgO/SiO2+Al2O3) factor than one may form glass when cooling rapidly in water, depending on the chemical analyses of the remnant melt.

5. The leaching test performed on the differently cooled slag samples shows that the solubility of elements such as chromium, molybdenum and vanadium in most cases is very low. The leaching of chromium is not prevented by the rapid cooling with water. The reactivity with water expressed as mg/m2 is on the contrary increased when rapid cooling (water granulation) is performed.

63 6. The disintegrating ladle slag became stable with respect to disintegration after performing rapid cooling, due to the formation of glass.

7. When steel slags are aged the total leachability is dramatically lowered. It is therefore important that all slags are aged at least 6 months before being used in external applications.

8. All pre-treatment of materials for external applications should be completed before the ageing period starts. If the pre-treatment is conducted after ageing, new fresh surfaces will be liberated and the ageing must be performed once again.

9. The content of a certain element can never be directly correlated to the leaching without considering in what phase it is present and how that certain phase is behaving in the slag system.

10. The rate of dissolution is in general slower at high pH.

11. In high-basicity EAF slags chromium is primarily crystallized in magnesium-type solid solutions.

64 6 FUTURE WORK

From this work it can be concluded that steel slags are complex materials that must be treated individually. The composition, as well as the method of cooling, will affect the outcome of the products, both when it comes to short/long-time leaching properties as well as the functionality of the materials. In order to further develop the knowledge within this area the following work is suggested for the future:

• In order to understand the leaching of trace elements such as chromium and molybdenum in the slag, the distribution in different mineralogical phases of these elements must be distinguished. • Further synthesizing of pure slag minerals including solid solutions must be conducted in order to investigate the behaviour of each mineral. • Linking mechanical properties to mineralogy. • Implement the knowledge into the real processes to a greater extent. Develop a handbook for slag treatment.

The common goal for the future is that it will eventually be possible to produce innovative steel grades while at the same time producing world- leading slag products that will help us to reduce the consumption of virgin materials.

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76 Paper I

Hot stage processing of metallurgical slags

D. Durinck, F. Engström, S. Arnout, J. Heulens, P.T Jones, B. Björkman, B. Blanpain and P. Wollants

Resources, Conservation and Recycling (2008) Vol 52, p 1121- 1131

Resources, Conservation and Recycling 52 (2008) 1121–1131

Contents lists available at ScienceDirect

Resources, Conservation and Recycling

journal homepage: www.elsevier.com/locate/resconrec

Review Hot stage processing of metallurgical slags

Dirk Durinck a,∗, Fredrik Engström b, Sander Arnout a, Jeroen Heulens a, Peter Tom Jones a, Bo Björkman b, Bart Blanpain a, Patrick Wollants a a Department of Metallurgy and Materials Engineering, Katholieke University of Leuven, Kasteelpark Arenberg 44 bus 2450, B-3001 Heverlee, Leuven, Belgium b Department of Chemical Engineering and Geosciences, Luleå University of Technology, 971 87 Luleå, Sweden article info abstract

Article history: Slags are an indispensable tool for the pyrometallurgical industry to extract and purify metals at compet- Received 20 February 2008 itive prices. Large volumes are produced annually, leading to important economical and ecological issues Received in revised form 6 May 2008 regarding their afterlife. To maximise the recycling potential, slag processing has become an integral part Accepted 2 July 2008 of the valorisation chain. However, processing is often directed solely towards the cooled slag. In this Available online 9 August 2008 article, the authors present an overview of the scientiﬁc studies dedicated to the hot stage of slag processing, i.e. from the moment of slag/metal separation to complete cooling at the slag yard. Using in-depth Keywords: case studies on C S driven slag disintegration and chromium leaching, it is shown that the functional Metallurgy 2 Slag properties of the cooled slag can be signiﬁcantly enhanced by small or large scale additions to the high Recycling temperature slag and/or variations in the cooling path, even without interfering with the metallurgical Microstructure process. The technology to implement such hot stage processing steps in an industrial environment is Disintegration currently available. No innovative technological solutions are required. Rather, advances in hot stage slag Leaching processing seem to rely primarily on further unravelling the relationships between process, structure and properties. This knowledge is required to identify the critical process parameters for quality control. Moreover, it could even allow to consciously alter slag compositions and cooling paths to tailor the slag to a certain application. © 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 1122 2. Case study: disintegration of air-cooled slags ...... 1122 2.1. Problem ...... 1122 2.2. Cause...... 1123 2.3. Solution ...... 1123 2.4. Conclusion ...... 1125 3. Case study: chromium leaching ...... 1125 3.1. Problem ...... 1125 3.2. Cause...... 1125 3.3. Solution ...... 1125 3.4. Conclusion ...... 1126 4. Process–structure–property ...... 1126 4.1. Process–structure ...... 1126 4.2. Structure–volume stability ...... 1127 4.3. Structure–leaching behaviour ...... 1127 4.4. Structure–hydraulic properties ...... 1128 4.5. Structure–mechanical properties ...... 1128

∗ Corresponding author. Tel.: +32 16321213; fax: +32 16321991. E-mail address: [email protected] (D. Durinck).

4.6. Conclusion ...... 1128 5. Outlook ...... 1128 6. Conclusions ...... 1128 Acknowledgements ...... 1129 References ...... 1129

1. Introduction levels are sufficiently high, the slag may even be used as feed material for high temperature reduction processes or leaching processes In pyrometallurgical processes slags are used to prevent oxi- to recover the oxidised metals (Balcázar and Kühn, 1999; Gorai et al., dation of the metal through contact with air, to limit heat losses 2003; Ye et al., 2003; Jones, 2004; Zhang and Meadowcroft, 2004; through radiation and to remove impurities from the molten metal Fleischandler and Daum, 2006; Kumar et al., 2006). The remain- (Engh, 1992; Slag Atlas, 1995). After the liquid slag is separated from ing slag is disposed of by land filling or is recycled as a secondary the metal, it is cooled to ambient temperatures, either in a matter resource. of seconds using water cooling in a granulation or pelletisation pro- An analysis of the focus areas in slag related research in the cess or in a slow process using air-cooling (Hiltunen and Hiltunen, periods 1996–2000 and 2001–2004 (Boom et al., 2000, 2005) 2004; Mihok et al., 2006). indicates that slag recycling issues have gained importance in The cooled slag constitutes a high volume by-product of the research, reflecting the increasing awareness of sustainable produc- pyrometallurgical industry. By far the largest slag producer is the tion and environmentally friendly processes. Within this research, iron and steelmaking industry with about 400 million tons of slag the emphasis is primarily put on finding and evaluating slag- in 2006 (USGS, 2007). About two thirds of this slag originates from containing products, such as: blast furnace processes, while the remaining third comes from steelmaking operations. Considerably less slag is formed during • metallurgical fluxes (Danilov, 2003; Aminorroaya et al., 2004; speciality steel and ferrous alloy production. However, the amount Porisiensi, 2004; Memoli et al., 2007); of stainless steel slag, for example, still added up to 8.5 million tons • cement (Wan et al., 2004; Frias et al., 2006; Adolfsson et al., 2007; in 2006 (Shen et al., 2004; ISSF, 2007). In the same year, the copper Moura et al., 2007; Oner and Akyuz, 2007); industry was responsible for approximately 30 million tons of slag • aggregates for road and waterway construction (Manso et al., (Gorai et al., 2003; ICSG, 2007), making it the main non-ferrous slag 2004; Penpolcharoen, 2005; Pundhir et al., 2005; Zelic, 2005; producer. Aiban, 2006; Xue et al., 2006; Yuksel et al., 2006); To avoid landfill costs and ecological issues, the slags are usually • soil improvers and fertilisers (Lopez et al., 1996; Rex, 2000, 2002; processed into a secondary resource (Fig. 1). The first step in this Wang and Cai, 2006). slag treatment is often the recovery of valuable and hazardous metals from the slag (Shen and Forssberg, 2003). The metal fraction can Another focus area is the determination of the environmen- be recovered by mechanical and/or physical processes including tal compatibility (Barna et al., 2004; Leuven and Willems, 2004; crushing, grinding, magnetic separation and flotation operations. If Gomes and Pinto, 2006; Chaurand et al., 2007). However, when slag-recycling issues are studied, the cold slag and its functional properties are generally considered to be fixed. The whole high temperature slag treatment process, which in the end leads to the slag product, is too often ignored and disregarded. Even when the slag treatment process is considered to only begin at the moment of slag/metal separation to avoid making compromises towards metal or process quality, there is still a considerable potential to influence the functional properties of the cold slag during the hot stage of slag processing, as will be shown in this article. This con- sideration implies that, at least to a certain extent, the properties can be tailored to the requirements of the targeted application. In this article, the authors review the scientific studies dedicated to the hot stage of slag processing in order to show that the slag recycling potential can be enhanced by (a) post-process additions to the high temperature slag to change its composition – without interfering with the metallurgical process – and/or (b) variations in the cooling path. In the first two sections, case studies on slag disintegration and chromium leaching are presented to illustrate how slag engineering can lead to a better exploitation of the valorisation potential of the slag. In a third section, attention is drawn towards the recent work in the field. In the last section an outlook towards the future is given.

2. Case study: disintegration of air-cooled slags

2.1. Problem

Fig. 1. General overview of the possible stages in slag processing. The stages of In the past, blast furnace ironmaking slags were known to interest for this article are indicated by the dotted ellipse. exhibit a peculiar disintegration upon air-cooling. During the ini- D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131 1123

ity ﬁeld of C2S in the CaO–MgO–SiO2–Al2O3 slag system, with an adjustment for the calcium sulphide content:

CaO + 0.8MgO ≤ 1.20SiO2 + 0.39Al2O3 + 1.75S

CaO ≤ 0.93SiO2 + 0.55Al2O3 + 1.75S

The oxides refer to their respective weight fractions. If the slag con- forms to either one of these conditions, the slag should not contain large amounts of C2S and, as a consequence, should not disintegrate. It is pertinent to point out that Parker and Ryder (1942) were aware that slags can contain a small amount of C2S without this causing the disintegration upon cooling. In an attempt to further improve the criteria, the conditions were tested against a large number of synthetic and industrial slags in a separate study (Gutt and Russel, 1972). However, the criteria proved to reliably predict the disintegration. Only at MgO levels of 10 wt% or more, stable slags have the tendency to be mistakenly labelled as disintegrating slags. In Fig. 3, the conditions are compared with a recently developed thermodynamic solidiﬁcation model for air-cooled slags (Durinck et al., 2008a,b). The bold lines show the compositional limits in the CaO–SiO2–MgO and CaO–SiO2–Al2O3 slag systems, while the contour lines indicate the simulated C2S level in the slag. Initially, these criteria were primarily used to keep stable and disintegrating blast furnace slags separate at the slag yard. This way, a certain grain size distribution could be guaranteed to the slag recycling company, while the disintegrated slag was landﬁlled. No early records were found of alterations to the slag composi-

Fig. 2. Phase transformations for pure C2S(Kim et al., 1992). tion in order to meet the conditions and obtain a stable slag. Over time, the composition of blast furnace gradually changed because tial solidification of the slag at the slag yard, slag pieces of several of metallurgical reasons, thereby moving out of the C2S precipita- centimetres in diameter and more were formed. However, at a cer- tion area and solving the disintegration problem. For electric arc tain temperature during further cooling, the pieces broke up and and ladle refining slags from (stainless) steelmaking, however, a disintegrated into a fine powder, causing severe dust issues in the solution for the disintegrating slags still had to be found. One path- surroundings of the meltshop. Furthermore, the slag could not be way is to prevent slag disintegration, either by inhibiting the ␤- valorised as an aggregate in construction applications – which is to ␥-transformation of C2S or by outright avoiding the presence of the typical use for air-cooled slag – and often had to be landfilled. C2S. In recent years, the problem for blast furnace slags has been elim- The first option was elaborated in 1986 by the development of inated by changes in slag compositions (Juckes, 2002). However, a borate-based stabiliser for stainless steel slag (Seki et al., 1986). a similar disintegration is still frequently observed for electric arc For pure C2S, it was previously demonstrated that borates stabilise and ladle refining slags from (stainless) steelmaking. Recently, a the higher temperature polymorphs to ambient temperatures by calcium–ferrite slag from a continuous copper converting process forming a solid solution (Ghosh et al., 1979; Ghose et al., 1983; Lai was shown to disintegrate as well (Bruckard et al., 2004). In this et al., 1992; Kim and Hong, 2004). The crystallographic mechanism 4− 3− case study, the causes and solutions for this disintegration are elab- is believed to be the partial replacement of SiO4 units by BO3 orated. units (Fletcher and Glasser, 1993). Because of the large difference in ionic radius between Si4+ and B3+, this replacement suppresses 2+ 4− 2.2. Cause the Ca migrations and SiO4 rotations required for the ␤-to␥- transformation, even with only 0.13 wt% of B2O3 (Ghose et al., 1983). It is well established that this kind of disintegration is driven Seki and co-workers corroborated that by adding borates to a · by the presence of dicalcium silicate (2CaO SiO2 or C2S) in the slag high temperature slag, C2S grains in the slag can also be stabilised (Juckes, 2002). This mineral undergoes several phase transforma- (see also Fig. 4). The addition of only 0.2 wt% of B2O3 was sufficient tions from one polymorph to another when the slag is cooled, as can to avoid the disintegration of a slag with 51 wt% CaO, 33 wt% SiO2 be seen in Fig. 2. As the athermal, martensitic-like transformation and 11wt% MgO. Recently, the present authors have measured the of the monoclinic ␤-polymorph to the orthorhombic ␥-polymorph borate distribution amongst the different phases in such stabilised (Thomas and Stephenson, 1978–1979) is accompanied by a volume slags, showing that only a small fraction of the added borates ends expansion of about 12% (Kim et al., 1992), high internal stresses are up in the C2S and contributes to the chemical stabilisation (Durinck built up in the slag during this transformation, finally causing the et al., 2008a,b). Therefore, the B2O3 level required for stabilisation disintegration of the slag. seems even lower when the C2S grains are enclosed in slags compared to free C2S grains. One of the drawbacks of the borates is that 2.3. Solution they have a negative effect on steel hardness and hot tearing during rolling or forging (Özmen and Inger, 2006). Therefore, they need After the link between crystallography and grain size distribu- to be added to the slag after slag–metal separation. Fortunately, tion was understood, solutions could be pursued. Already during the heat content of the slag is sufficient to melt and dissolve the the second world war, compositional limits for disintegrating slags stabiliser. However, measures to ensure a homogeneous distribu- were defined (Parker and Ryder, 1942) defined, based on the stabil- tion have to be taken. Considering its effectiveness and simplicity, 1124 D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131

Fig. 3. C2S in the slag systems CaO–SiO2–MgO (left) and CaO–SiO2–Al2O3 (right). The bold lines denote the compositional limits (Parker and Ryder, 1942). The surface plot indicates the C2S level in the slag simulated by a slag solidiﬁcation model (Durinck et al., 2008a,b).

borate stabilisation of air-cooled slags is implemented in several industrial practices, leading to a slag product that can be valorised as an aggregate for construction applications (Yang et al., 2006). It must be remarked that recently concerns have risen about the possible health effects of boron leaching from treated slags (Yoshinaga et al., 2001). Although boron is a essential trace element for humans (Nielsen, 2008), chronic exposure to higher levels can lead to loss of appetite, nausea, weight and decreased sexual activity (Devirian and Volpe, 2003). A tolerable level of boron in drinking water is considered to be 0.5 mg/l in a guideline issued by the World Health Organisation (WHO, 1998). Actual boron leaching from slags, however, has not been studied extensively. Extrapolation of results from leaching tests on coal ashes (Narukawa et al., 2003; Iwashita et al., 2005) would indicate that a signiﬁcant amount of the boron present in the slag will leach out. Apart from borates, other compounds also exhibit a stabilising effect on C2S(Ghosh et al., 1979; Ghose et al., 1983; Lai et al., 1992; Geiseler, 2000). Recently, a qualitative criterion based on ionic radius, ionic valence and crystallographic structure of the additive was developed (Lopatin and Chizhikova, 2007), which is capable of accurately predicting whether or not a compound will stabilise the ␤-polymorph. Regarding slag stabilisation, the effect of phosphorus additions was investigated in laboratory conditions (Yang et al., 2006). Satisfying results were obtained, but signiﬁcantly larger amounts are required compared to borate additions. Instead of using chemical additions, the ␤-to␥-transformation of C2S can also be avoided by rapid cooling (Chan et al., 1992). According to the laboratory experiments (Sakamoto, 1996, 2000), the required slag cooling rate is about 5 ◦C/s. This stabilisation method was further developed by showing on a laboratory scale that a granulation process transforms a disintegrating slag into a slag product suitable for construction applications (Yang et al., 2005). The second option to avert slag disintegration is by outright avoiding the presence of C2S by modifying the slag composition. However, in many cases such altered slags do not have the appropriate high temperature metallurgical properties. In stainless steelmaking, for example, C2S free, low basicity slags cause rapid Fig. 4. Effect of B2O3 additions to a C2S containing slag (Durinck et al., 2008a,b). Top: the untreated slag contains highly fractured ␥-C2S grains and disintegrates during degradation of magnesia based refractories (Jones et al., 1999) and cooling. Bottom: the treated slag contains stable ␤-C2S and did not disintegrate. high chromium losses (Guo et al., 2007). To avoid making such D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131 1125

oxides are also encountered in regular steel slags and several types of non-ferrous slags. When these slags are recycled, chromium comes in contact with the environment. Although Cr3+ is an essential nutrient for mammals, elevated levels are hazardous and may cause, for example, skin rashes. Cr6+ is carcinogenic, hence its presence in ground water should certainly be avoided (Hughes et al., 1994; Rowbotham et al., 2000).

3.2. Cause

The Gibbs free energy change of the oxidation reaction of metal- lic chromium exceeds that of other common elements in chromium containing alloys. Therefore, a significant fraction of the available chromium is oxidised during the metallurgical process and taken up by the slag. For example, the chromium oxide concentrations in electric arc furnace (EAF) slags from stainless steel production can Fig. 5. Process principle to dissolve large quantities of SiO2 in steelmaking slags be as high as 15 wt% (Vidacak et al., 2002). At high temperatures (Kühn and Behmenburg, 2000; Kühn et al., 2000). and under reducing conditions chromium is primarily present in the slag as Cr2+ and Cr3+ (De Villiers and Muan, 1992; Morita et compromises towards process and metal quality, the slag com- al., 2005). However, when during or even after cooling free lime 6+ position must be adjusted subsequent to slag–metal separation. and oxygen are available, Cr will form over time (Hattori et al., Although also other additives such as MgO are claimed to have a 1978; Lee and Nassaralla, 1998; Pillay, 2003). In contact with water, 3+ 6+ beneficial effect (Eriksson and Björkman, 2004), adding a relatively the Cr and Cr can be released from the solid slag by means of large amount of a SiO2 source seems to be the best way to avoid C2S a leaching process (vanderSloot et al., 1997; Hill, 2002). Especially precipitation. In laboratory experiments, stainless steel decarburi- the latter is highly soluble in water. sation slag was stabilised with 12 wt% of waste glass, containing 70–75 wt% of SiO2 (Sakamoto, 2001). The cost of such an operation 3.3. Solution would be far lower than that of the borate additions. However, the limited heat content and heat conductivity of slags make it One pathway to limit the effects of chromium on the environ- difficult to extrapolate this method of dissolving large amounts of ment is to remove chromium from contaminated waters (Mohan SiO2 in an industrial environment. However, solutions have been and Pittman, 2006). One of the intensively examined processing devised. routes is the immobilisation of chromium by invoking the pre- By co-injecting oxygen, a large amount of quartz sand (99% SiO2) cipitation of ettringite (Ca6Al2(SO4)3(OH)12 ·26H2O), in which the 2− 3+ can be dissolved in a basic oxygen furnace steelmaking slag (Kühn chromate (CrO4 ) and chromic (Cr ) ions can replace the sulphate 2− 3+ and Behmenburg, 2000; Kühn et al., 2000) (see Fig. 5). This way, the (SO4 ) and aluminium (Al ) ions (Rose et al., 2003; Zhang and presence of free lime and the associated volume instability prob- Reardon, 2003; Peysson et al., 2005; Chrysochoou and Dermatas, lems can be avoided. The injected oxygen reacts with FeO in the 2006; Saikia et al., 2006). slag to form Fe2O3, whilst generating the additional heat required Another option is to avoid water contamination by chromium for the sand dissolution. This method has been adopted by Thyssen release from the slag in the first place. The mixing of bivalent fer- Krupp Stahl AG for years. Recently, ArcelorMittal Ghent has also rous sulphate with the cooled slag has shown to reduce a part of the implemented a similar treatment process (Sichien, 2007). In the available Cr6+ to the less mobile Cr3+ (Geelhoed et al., 2003), which case of disintegrating slags containing little or no FeO, an alternative can lead to significantly reduced leaching levels in short term batch solution for the heat generation must be found. tests. Therefore, this technique is used in several steel shops. How- ever, it is not a long-term solution as the additions no longer inhibit 2+ 3+ 2.4. Conclusion the Cr release, once all the Fe has been oxidised to Fe . An alternative way to decrease Cr release from the slag is by This case study shows the importance of a sound understanding incorporating the chromium into stable mineral phases during of the link between slag structure (mineralogy and crystallogra- cooling of the slag (Fig. 6). The leaching levels are believed to phy) and functional properties (grain size distribution). By working be limited substantially when chromium is contained in a spinel out how the slag structure develops during solidification, compo- phase—AB2O4 with A being a bivalent cation and B a trivalent cation sitional limits can be imposed and the slag may be rendered into a (Kühn and Behmenburg, 2000; Kühn et al., 2000; Tossavainen and more reliable and higher value product. By specific manipulations Forssberg, 2000; Kühn and Mudersbach, 2004). using borate or silica additions or changes in the cooling path, the Based on this, Kühn and Mudersbach (2004) related chromium powdery slag, which can only be considered as a waste stream, can leaching to the concentration of spinel forming compounds in even be transformed into a valuable by-product, without interfer- the slag. They performed a large number of extraction tests on ing with the metallurgical process. industrial stainless steel electric arc furnace slags and derived an empirical formula factor sp to relate the overall slag composition to the chromium leaching: 3. Case study: chromium leaching

Factorsp = 0.2MgO + 1.0Al2O3 + nFeOx − 0.5Cr2O3 3.1. Problem ‘n’ is a number between 1 and 4, depending on the oxidation state of Several types of slag contain a signiﬁcant fraction of oxidised the slag. When factor sp is below 5 wt%, a high chromium leaching chromium (Potesser et al., 2005). This is especially the case for was observed. When factor sp is above 5, chromium leaching was ferrochromium and stainless steel slags. Nevertheless, chromium low (Fig. 7). 1126 D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131

ﬁed slag (vanderSloot et al., 1997), the latter should be minimised to reduce long term leaching (Mostbauer, 2003). As chromium is an expensive alloying element, there is also an important economic incentive. A multitude of laboratory and industrial studies have been performed to optimise the chromium recovery inside the metallurgical process (McCoy and Langenberg, 1964; Gustafsson and Vikman, 1992; Masucci et al., 1993; Görnerup and Lahiri, 1998a,b; Görnerup and Jacobsson, 1998; Durinck et al., 2007a,b; Guo et al., 2007), which allowed to identify the main control parameters. Firstly, thermodynamic conditions should promote the chromium reduction reaction. A relatively high concentration of reducing agents (Si, C, etc.) in the metal bath is desired, as this increases the driving force for the reduction according to Le Chatelier’s principle. Concurrently, a relatively high slag basicity is required, as this increases the activity coefﬁcient of the chromium oxide. Secondly, the slag practice should allow for intimate contact between metal and slag to create optimal kinetic conditions for the reduction of

Fig. 6. Cr-containing spinel particles in an air-cooled ferrochromium slag CrOx to Cr. When both kinetic and thermodynamic conditions are (Tanskanen and Makkonen, 2005). met, the chromium oxide levels are reduced signiﬁcantly.

Using laboratory furnace experiments they subsequently 3.4. Conclusion showed that the addition of MgO, Al2O3 or FeO to the high temperature slag decreases the level of chromium leaching. In cor- This case study shows that in some cases objectives for metallurgical and environmental slag engineering coincide. Due to respondence with the empirical formula, the effects of Al2O3 and FeO additions far exceeded the effect of MgO additions. When the this synergy, slag optimisation for recycling purposes can already start prior to slag/metal separation. Furthermore, it is shown Al2O3 level was increased from 4 to 15 wt%, Cr leaching decreased from 0.10 to 0.03 mg/l. An increase in the FeO level from 0.5 to 8 wt% that the functional properties of the cooled can be enhanced by led to a decrease in Cr leaching from 0.10 to 0.01 mg/l. When both high temperature manipulations. By inexpensive bauxite additions, were combined, the leaching level dropped below the detection chromium leaching levels can be decreased significantly. However, limit. it is also shown that the relationship between slag structure and the leaching behaviour is not yet completely understood, which opens For economical reasons, industrial tests focussed on Al2O3- possibilities for further improvements. containing bauxite additions. By raising the Al2O3 level from 5 to 12 wt%, Cr leaching dropped from 0.10 to 0.03 mg/l, although 4. Process–structure–property the Cr2O3 level in the slag exceeded 10 wt%. Once the industrial feasibility was proven, the relationship was implemented as a quality control system for the environmental compatibility of stainless The previous case studies have illustrated the beneficial effects steel slags at Outokumpu Tornio Works, Finland (Roininen et al., of slag engineering in the hot stage of processing on the functional 2005). However, it became apparent that the factor sp did not fully properties of the cooled slag. However, hardly any peer-reviewed describe chromium leaching. Industrial tests showed that for the publications on hot stage slag processing can be found. Liquid slags same slag rapid cooling leads to lower leaching levels, indicating are almost exclusively investigated with respect to their metallur- that the mechanisms of leaching and the effects of process alter- gical properties. When slag-recycling issues are studied, the cold ations are not yet fully understood. slag and its functional properties are in most cases considered to be A last, complementary pathway to limit the effects of Cr leach- fixed. However, it has been shown that, at least to a certain extent, ing from slag, is by minimising the available chromium in the slag. the properties can be tailored to the requirements of the targeted Although it is known that leaching levels in standing water are not application. To be able to steer the properties of the cold slag, it is directly governed by the overall chromium oxide level in the solidi- imperative not only to characterise the different aspects but also to understand the links between process parameters, microstructure and properties (Fig. 8). In the following paragraphs, the scientific articles on these relationships are discussed.

4.1. Process–structure

Several experimental studies have investigated the development of the microstructure during cooling. The experiments involve melting an industrial or synthetic slag in a laboratory furnace and then enforcing a certain cooling path. Air-cooling has received the most attention (Gatellier et al., 1993; Rocabois et al., 1997; Park, 2006; Durinck et al., 2007a,b). Typically, a linear cooling rate of 1 ◦C/min is imposed to match the total experimental cooling time – about 24 h – to the industrial conditions. The resulting slag microstructures proved to contain similar phases as industrial samples. Some studies have also investigated the effects of the cooling path on the microstructure. Recently, a new technique was developed based on the immersion of a water Fig. 7. Chromium leaching as a function of factor sp (Kühn and Mudersbach, 2004). cooled probe in a slag, which allows to study different cooling rates D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131 1127

uid phase at the moment when the viscosity of the liquid reaches 1013 Pa s (Tammann, 1922). At the corresponding temperature, i.e. the glass transition temperature, the mobility of the structural units in the liquid is insufficient to be arranged in a crystal. For slags, the glass transition temperature lies around 750 ◦C(Avramov et al., 2005). In order to avoid crystal nucleation and growth during cooling from the process temperature to the glass transition temperature, a high cooling rate is required (Daugherty et al., 1983; Ionescu et al., 1998; Ionescu et al., 2001). For slags, the critical cooling rate is situated between 0.1 and 5 ◦C/s, depending on the degree of polymerisation in the liquid. The glass transition temperature for several silicate glasses indicates that (Avramov et al., 2005) a high fraction of network formers (SiO2,Al2O3) increases the glass forming ability. A study on the glass formation in CaO–SiO2–Al2O3 slags (Heulens, 2007) confirmed that a higher basicity slag, i.e. containing less network formers, requires Fig. 8. Aspects and interrelationships in the research field of slag engineering for recycling purposes. higher cooling rates to obtain a fully amorphous slag. It was also established that the transition temperature increases with higher cooling rates. From a more pragmatic viewpoint, the glass form- in a single experiment (Verscheure et al., 2006; Campforts et al., ing compositions within several ternary slag systems were plotted 2007). Also, a time–temperature-transformation (TTT) diagram for (Richet et al., 2006), considering a industrially relevant cooling rate a 43 wt% CaO, 34 wt% SiO , 14 wt% Al O and 6 wt% MgO blast fur- ◦ 2 2 3 ofafew10 /s. It shows that at such cooling rates no glass is formed nace slag was constructed (Kashiwaya and Nakauchi, 2007). To in the CaO–SiO –FeO system, which forms the basis for carbon steel see the effects of accelerated cooling, four industrial air-cooled 2 slags. Therefore, such slags are not granulated in industrial practice. steel slags were subjected to crucible cooling and water granula- In the CaO–SiO –Al O system, on the other hand, glass is read- tion (Tossavainen et al., 2007). Despite these studies, the obtained 2 2 3 ily formed, indicating that blast furnace ironmaking slag can be results remain at best fragmentary with respect to composition. vitrified by granulation. As the solidification behaviour is highly dependent on the exact composition, results are not easily transferable to other slags. Some 4.2. Structure–volume stability headway has been made by the development of a slag solidification model for air-cooled slags (Durinck et al., 2007a,b). By using the Apart from the structure development during cooling, under- Scheil assumptions – equilibrium at solid/liquid interface, no diffu- standing how the properties are influenced by the microstructure sion in solids, rapid diffusion in liquids (Porter and Easterling, 1981) is crucial for the optimisation of the properties of the cold slag. – a good match between experimental and modelled slag mineral- This is, however, a difficult relationship to establish. As presented ogy was obtained for the considered slag compositions (Fig. 9). This in Section 2, the effect of C S on the particle size distribution is well allows to simulate the effect of minor changes to the composition 2 understood. It has also been proven that the presence of free CaO rather than performing a complete new experiment. However, as or MgO in the slag leads to a longer term volume instability due to kinetics change as a function of composition, drastic changes still the expansive hydration to Ca(OH) and Mg(OH) . This understand- have to be verified by experiments. 2 2 ing led to the development of several slag weathering treatments Glass formation in slags by means of granulation or pelletisation to achieve complete hydration before application of the slag (da has received attention as well. In general, all materials, regardless Silveira et al., 2005). Alternatively, processes have been devised and their particular composition, form glasses during cooling from a liq- implemented to avoid the presence of free CaO or MgO in the slag (Kühn and Behmenburg, 2000; Kühn et al., 2000; Sichien, 2007).

4.3. Structure–leaching behaviour

Apart from volume stability, leaching of potentially hazardous compounds during reuse is another key issue in slag valorisation. The leaching process from slags is generally characterized as a surface reaction. Therefore, the rate of leaching decreases with time (Kühn and Behmenburg, 2000; Kühn et al., 2000), as the diffusion from the bulk of the solid slag to the surface is slow. However, the exact mechanisms remain unclear. Therefore, a lot of effort is being put into a mineralogical interpretation of leaching. On the one hand, high-resolution techniques are used to characterise the slag microstructure in detail, in order to identify all possible sources of release. X-ray absorption near-edge structure (XANES) spectroscopy (Chaurand et al., 2007) and wavelength dispersive spectroscopy (WDS) (Drissen, 2007) were used to investigate where Cr and V leaching originates in steelmaking slags. On the other hand, the modelling of leaching processes is receiving increasingly more attention. Thermodynamic and kinetic considerations have been shown to Fig. 9. Comparison between the experimental and the simulated mineralogy of an be able to describe the actual leaching behaviour for similar air-cooled 48 wt% CaO, 37 wt% SiO2 and 15 wt% MgO slag (Durinck et al., 2007a,b). materials, such as cementious waste (Halim et al., 2005) and 1128 D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131

municipal solid waste incinerator fly ash (Van Herck et al., 2000). 5. Outlook Regarding slag materials, the leaching behaviour of trace elements from historical Cu slags (Parsons et al., 2001) and chromium ore During the last decennia, certain metallurgical slags, such as processing slags (Geelhoed et al., 2002) have been analysed exten- air-cooled and granulated blast furnace slags, have made an evo- sively. lution from an essentially costly stream to a valuable by-product From a more pragmatic viewpoint, possible slag treatments as a secondary resource. Governmental regulations are starting to to reduce leaching are being investigated. Rapid cooling with acknowledge this and in some cases the official status of by-product water may result in an amorphous slag, encapsulating the oxides has been granted. For example, from December 2005 onwards, the in the structure, and thereby lowering the total leaching abil- Ministry for Environment and Nature Protection, Agriculture and ity (Tossavainen and Forssberg, 2000; Tossavainen et al., 2007). Consumer Protection of the state of North Rhine-Westphalia offi- Experiments showed, however, that glass formation in the water- cially recognizes air-cooled and granulated blast furnace slags as granulated slag samples is insufficient to enclose the oxides and valuable products. prevent them from leaching. The differences between the origi- In the future it is to be expected that also other slag types nal slowly cooled and granulated slag samples were low. Further will undergo the evolution from waste to by-product, thereby investigation of the material showed that the reactivity at the sur- improving the industrial ecology of steelmaking and non-ferrous faces increased with water granulation. The cause for this increase production. A systematic reflection on the end properties of the is believed to be correlated with the oxidation that occurs on the cooled slag product, starting inside the metallurgical facilities, will surface, the increased amount of grain boundaries and the presence lead to a better control over the properties and will decrease their of metastable phases on the surface (Engström, 2007). variability. Moreover, by consciously altering slag compositions and Despite extensive research efforts on the leaching topic, lots of cooling paths, new fields of applications will be opened. For exam- questions remain. The leaching behaviour is still the primary cause ple, the production of paving stones or even table tops by slag for difficulties with valorisation. casting could be a promising application. On a longer term, metallurgical slag can evolve further from a secondary resource to a 4.4. Structure–hydraulic properties tailored resource. The fact that its properties can be adjusted to the wishes of the customer, could be a clear advantage over the The effects of the microstructure on the hydraulic properties of common primary resources. slags have been widely studied, since the chemical analyses of a slags can be very similar to that of an ordinary Portland cement. 6. Conclusions Especially blast furnace ironmaking slags have been successfully applied as cement replacement. It was shown that that the heat The case studies on slag disintegration and chromium leaching of hydration from an amorphous slag increased by 60% compared illustrate the potential benefits of a properly designed hot stage pro- to a fully crystalline material (Ionescu et al., 1998), which was cessing step on the properties of the cooled slag. In both cases the confirmed by several studies (Ionescu et al., 2001; Shij, 2004). Oxi- pursued functional properties, respectively grain size distribution dation of bivalent iron proved to enhance the cementious potential and chromium leaching, could be linked to the structure of the slag. even further (Murphy et al., 1997). Therefore, slag granulation pro- By changing compositional and cooling parameters, hot stage treat- cesses, which lead to an amorphous blast furnace slag, have been ments were developed to engineer the structure and, consequently, used in industry for decades. The index of hydraulicity links the the properties. hydraulic properties to the slag composition in wt% (Mihok et al., Technologically, solutions have been devised to implement 2006): these treatments at an industrial scale, in an economically viable fashion and without interfering with the metallurgical processes. CaO + 1.4MgO + Al O ih = 2 3 Small-scale additions, such as the B2O3 additions to avoid the SiO2 expansive C2S transformation, can be performed in the slag pot itself. Large-scale additions, such as the SiO additions to com- As can be seen, high alumina and magnesia levels in the slag are 2 pletely avoid C S formation, require a separate slag treatment recommended to increase the hydraulic properties of the slag. 2 facility to generate the required heat for dissolution. Cooling paths can be varied from rapid cooling in a matter of seconds using gran- 4.5. Structure–mechanical properties ulation or pelletisation processes to slow cooling in a matter of days at the slag yard or even in the slag pot itself. No studies have been found that link any of the mechanical prop- Based on the success in a number of isolated cases, the authors ␮ erties – Los Angeles impact value, -Deval abrasion value, Polished strongly suggest to further explore the possible effects of hot stage Stone value, etc. – to the microstructure. However, these mechan- processing on the cooled slag. It is believed that a conscious hot ical properties often determine the nature of the application in stage processing step can both increase slag utilisation rates and which the slag can be recycled. There is thus a clear need of research make higher value applications achievable. As future advances in in this field. the field do not seem to require innovative technological solutions, attention should primarily go to unravelling the relationships 4.6. Conclusion between process, structure and properties. This article has shown that for short and long-term volume stability these relationships At this point, the hot stage processing of metallurgical slags is a are already established. Regarding environmental and hydraulic fairly small research area. The scientific research has mainly focused properties, some initial work has been performed. However, a mul- on the development of the slag structure during cooling. The rela- titude of questions remain. Considering the mechanical properties tionships between structure and properties have been investigated of metallurgical slags, our knowledge is still a mostly blanc can- less thoroughly. Several aspects are not properly understood or have vas. To the best of the authors’ knowledge, none of the mechanical even been completely ignored. However, this knowledge is cru- properties has been investigated on a structural level. Nevertheless, cial for enhancing the valorisation potential of the metallurgical only when these properties are better understood, treatments can slags. be developed to improve them. D. Durinck et al. / Resources, Conservation and Recycling 52 (2008) 1121–1131 1129

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Paper II

Characteristics of steel slag under different cooling conditions

M. Tossavainen, F. Engström, Q. Yang, N. Menad, M. Lidström Larsson and B. Björkman

Waste Management, (2007), Vol 27, p 1335-1344

Waste Management 27 (2007) 1335–1344 www.elsevier.com/locate/wasman Technical paper Characteristics of steel slag under diﬀerent cooling conditions

M. Tossavainen a, F. Engstrom b,*, Q. Yang b, N. Menad b, M. Lidstrom Larsson b, B. Bjorkman b

a Division of Mineral Processing, Lulea˚ University of Technology, SE-971 87 Lulea˚, Sweden b Division of Process Metallurgy, Lulea˚ University of Technology, SE-971 87 Lulea˚, Sweden

Accepted 8 August 2006 Available online 26 September 2006

Abstract

Four types of steel slags, a ladle slag, a BOF (basic oxygen furnace) slag and two different EAF (electric arc furnace) slags, were characterized and modified by semi-rapid cooling in crucibles and rapid cooling by water granulation. The aim of this work was to investigate the effect of different cooling conditions on the properties of glassy slags with respect to their leaching and volume stability. Optical microscopy, X-ray diffraction, scanning electron microscope and a standard test leaching (prEN 12457-2/3) have been used for the investigation. The results show that the disintegrated ladle slag was made volume stable by water granulation, which consisted of 98% glass. How- ever EAF slag 1, EAF slag 2 and the BOF slag formed 17%, 1% and 1% glass, respectively. The leaching test showed that the glass-containing matrix did not prevent leaching of minor elements from the modified slags. The solubility of chromium, molybdenum and vanadium varied in the different modifications, probably due to their presence in different minerals and their different distributions. Ó 2006 Elsevier Ltd. All rights reserved.

1. Introduction year, as compared to the 20.3 million tonnes produced in 2003 (SGU, 2004). Large quantities of materials are used in the construc- Due to its high strength, durability and chemistry, steel tion and maintenance of roads each year. In Sweden, the slag is a suitable material in the field of construction, and production of rock material (aggregates) in 2003 was 70 its use also contributes to a reduction in the amount of million tonnes, 50% of which was used for road making landfilled waste. Unfortunately, in spite of its potential in and 10% in the manufacture of concrete (SGU, 2004). In 2002, only 25% of the Swedish steel-slag production Sweden, two interim targets regarding the environmental (896 kt) was sold as external products (source: private com- quality objective ‘‘A Good Built Environment’’ have been munication with steel industry representatives). set, according to which, by 2010, the reused materials will This is due to the fact that in addition to the lack of rules represent at least 15% of the aggregate used and by 2005 and guidelines regarding testing, assessing and using slag in the landfilled waste will be reduced by at least 50% com- Sweden, the technical and environmental obstacles for pared to 1994. Gravel is used in concrete and according some slags in construction include low volume stability to the environmental quality objectives (A Good Built and leaching of elements. Other impediments are a long Environment, 2004), by 2010, the extraction of natural tradition and knowledge of using rock material and the fact gravel in Sweden shall not exceed 12 million tonnes per that in Sweden there are still quite good resources of high- quality rock material. The fear that some slags are environ- * Corresponding author. Tel.: +46 920 491388; fax: +46 920 491199. mentally hazardous is also something that has to be E-mail address: [email protected] (F. Engstrom). considered.

Rapid cooling by water granulation can result in an C . Electric arc furnace slag 1, high alloyed steel, EAF amorphous slag, encapsulating metals and oxides, and slag 1 thereby lowering the solubility of the heavy metals com- D . Electric arc furnace slag 2, low alloyed steel, EAF pared to rock material used for road making (Tossavainen slag 2 and Forssberg, 2000). The formation of a glassy material depends on both the chemical composition and the cooling The materials, except the disintegrated ladle slag, were conditions. According to Daugherty et al. (1983), glass was crushed with a jaw crusher, Retsch BB3, to <30–40 mm easier to produce, as the acidity of the slag increased for a before splitting into 1–1.5 kg sub-samples. series of synthetic slag compositions that was quenched and annealed. Ionescu et al. (1998, 2001) have shown 3. Methods for characterization how water quenching of steel slag results in products with a high content of glassy material. Silicate melts have high 3.1. Physico-chemical and mineralogical composition viscosity due to long molecule chains, and rearrangement into crystals only takes place slowly. If the cooling is rapid, The total composition of each material was analyzed by the slag passes from a liquid state to a solid without devel- Ovako Steel AB with inductively coupled plasma emission opment of a crystalline structure (Lea, 1983). Glasses, such spectroscopy, (ICP), and X-ray fluorescence spectroscopy, as granulated slags, can be regarded as super-cooled liquids (XRF). Titration was used for analysis of Fe and FeO having a very high viscosity. and infrared adsorption spectroscopy, IR, for carbon and By enhancing the amount of amorphous material in a sulphur. slag, the potential hydrating properties are increased and The specific surface area was determined according to the material can also be used in cement and concrete prod- the BET-method with a Micromeretics Flowsorb 2300 ucts of higher quality compared to conventional road-mak- and density was measured with a Micromeretics Multivol- ing materials (Murphy et al., 1997; Ionescu et al., 1998, ume Pycnometer 1305 on material prepared for leaching, 2001; Shij, 2004). For a disintegrated slag, use in concrete <4 mm. is particularly interesting, as grinding costs can be reduced. All of the slag samples were crushed to a particle-size of Besides glass formation, controlling cooling conditions <4 mm and leached according to the one-stage batch test can be a means of affecting mineral transformation and prEN 12457-2 (CEN, 2002a) except two samples, ladle slag consequently the solubility of elements like chromium. and granulated EAF slag 1, which were leached according Chemical compounds containing hexavalent chromium to the two-stage batch test prEN 12457-3 (CEN, 2002b). (Cr6+) are generally considered far more toxic than those The leaching tests were done in duplicate and the results containing the trivalent form (Cr3+)(Plunkett, 1976; Wind- are presented as a mean value. The filtrates were analyzed holz, 1976). According to Lee and Nassarella (1998),Cr6+ by the laboratory Analytica AB (Sweden). is usually formed at lower temperatures and a rapid cooling The mineralogy of the slag phases was studied on pol- reduces the formation by limiting the kinetics of the ished thin samples using scanning electron microscopy formation. (SEM), Philips XL 30, with energy dispersive analysis This paper presents a study regarding four different types (EDX). X-ray diffraction analysis (XRD) was performed of steelmaking slags; a ladle slag, a basic oxygen furnace on pulverized material using a Siemens D5000 automatic (BOF) slag and two types of electric arc furnace (EAF) slags, diffractometer with a step and continuous scanning modified by different cooling conditions. The aim was to device. Diffraction patterns were measured in a 2h range determine if rapid cooling by water granulation would result of 10–90° using (Cu Ka) radiation of 50 kV and 30 mA. in a glassy slag with improved properties regarding leaching The glass content was analyzed according to the ER- and volume stability. For EAF slag, the leaching of metals 9103 method (Scancem Research AB, Sweden) using opti- such as chromium and molybdenum is a concern. The qual- cal microscope. ities of the matrix of the modified slag were studied and the To be able to better understand the crystallization pro- possibility of encapsulating metals in a glassy matrix, and cedure and the formation of glass, thermodynamic calcula- thereby reducing the leaching, is discussed. tions were conducted using Factsage (Bale et al., 2002) version 5.4 using compound database FS53base.cdb, 2. Materials FToxid53base.cdb and solution database FToxid53- soln.sda. FToxid-slag and FToxid-MeO were selected as 2.1. Investigated steel making slags standard stable. During calculating, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplica- Representative samples (20–30 kg) of four different steel tions in the data set. slags were obtained from steelmaking companies in Sweden: 4. Modification trials

A . Ladle slag, ladle slag All slags except the ladle slag were modiﬁed in two ways B . Basic oxygen furnace slag, BOF slag for comparison with the original slags: M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344 1337

1. Re-melting and water-granulation (rapid cooling). 1600 2. Re-melting and cooling in the crucible (semi-rapid 1400 Semi rapid cooling). 1200 Rapid 1000 The ladle slag was only modiﬁed by re-melting and water-granulation. 800 600 4.1. Crucible systems 400

Furnace temperature, ˚C 200 Two different crucible systems for re-melting the materi- 0 als were developed in order to minimize reactions between 0426 the refractory material and the slag. A graphite crucible Time in hours system was used for the ladle slag with a low content of Fig. 2. Temperature profile for semi-rapid and rapid cooling. Fe-oxides and MgO crucible system for the two different types of EAF and BOF slags with high values of CaO/ SiO2 and high contents of Fe oxides. the granulation. The duration for the tapping and granula- The graphite crucible system is shown in Fig. 1. The tion was approximately a few seconds. outer crucible was made of refractory castable (MgO For re-melting the EAF- and BOF slags in the MgO cru- 80%). With a refractory cover, the system could be closed cible system, a thermocouple was placed above the slag and to minimize air intrusion and oxidation of the inner, graph- the heating rate was controlled to 4–6 °C/min. The time for ite crucible. melting the slag varied from 6 to 8 h. The water granula- The MgO crucible system consisted of an outer crucible tion of the slags re-melted in the MgO crucible system made of castable with 94% Al2O3, enclosing a graphite cru- was carried out in the same way as for the ladle slag. cible and an inner MgO crucible. A refractory cover was For the semi-rapid cooling, the re-melted slag was left to also placed on the top of the system to minimize air intru- cool in the MgO crucible. The cooling time from a temper- sion during slag melting and cooling. ature of 1600 °C to room temperature was estimated to be 5 h. The temperature changes for the experiments are 4.2. Modifications shown in Fig. 2.

The ladle slag re-melted in the graphite crucible system 5. Results became liquid within 1 h. For granulation, the liquid slag (1600 °C) was poured into the granulation head, as 5.1. Physical properties shown in Fig. 1. Water jets formed in the granulation head hit the pouring slag, and the generated slag granules were The re-melted slags, which were left to cool in the cruci- collected at the bottom of the water tank at the end of bles (semi-rapid cooling), resulted in large pieces that were

a b

Fig. 1. Graphite crucible system (a) and equipment for water granulation (b). 1338 M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344 crushed to <4 mm for leaching tests. The water-granulated des is high in both the BOF slag and the EAF slag 2, and material of the BOF slag, the EAF slag 1 and the EAF slag the amount of Al2O3 and MgO is high in the ladle slag. 2 consisted of granular particles, 2–4 mm. During the rapid The chromium content is higher in EAF slag 1 and sig- cooling process the ladle slag reacted with water to produce nificantly higher in EAF slag 2 than in the other two a volumetric stable, brittle and porous product. Table 1 slags. summarizes the compact density, the BET surface and The solubility of five major elements (Ca, Mg, Fe, Si, the results from the glass measuring test of the original Al) in the matrix and of three minor elements (Cr, Mo, and water granulated slag samples. From this table, it V), expressed as mg/kg of the element dissolved, is can be seen that the BET surface was reduced substantially shown in Table 3. The leaching of silicon is increased in the granular particles, mainly due to the reduction of the in all samples except for EAF slag 2 compared to semi amount of fines. rapid cooling, while the aluminium leaching is decreased when cooling rapidly. No significant changes in the 5.2. Physico-chemical and mineralogical characterization minor elements can be seen when cooling differently. The values reported by the laboratory are in many cases Chemical compositions of the four original slags are low and there was good agreement between duplicate shown in Table 2. It shows that the content of iron oxi- samples.

Table 1 The compact density (g/cm3), the BET-surface (m2/g) and the glass content (%) in the slag samples Sample Compact density BET-surface Glass content Original (g/cm3) Granulated (g/cm3) Original (m2/g) Granulated (m2/g) Original (%) Granulated (%) Ladle slag 3.03 2.76 0.75 0.81 18 98 BOF slag 3.53 3.65 2.35 0.21 7 1 EAF slag 1 3.25 3.34 2.23 0.17 2 17 EAF slag 2 3.59 3.77 1.23 0.59 4 1

Table 2 Chemical composition of the original slag samples Samples % ppm

Fe2O3 FeO Fe met. Al2O3 CaO MgO MnO SiO2 Cr Mo Zn Ni Cu K Na P Ti V Ladle slag 1.1 0.5 0.4 22.9 42.5 12.6 0.2 14.2 2700 280 370 70 20 80 <20 <50 840 280 BOF slag 10.9 10.7 2.3 1.9 45.0 9.6 3.1 11.1 506 39 37 25 8 220 <10 2270 8270 14800 EAF slag 1 1.0 3.3 0.1 3.7 45.5 5.2 2.0 32.2 32700 500 130 3180 140 590 150 <50 7910 310 EAF slag 2 20.3 5.6 0.6 6.7 38.8 3.9 5.0 14.1 26800 70 260 90 160 <20 <20 2000 2400 1700

Table 3 Results obtained from standard test leaching of investigated in mg/kg Slag sample Ca Mg Fe Si Al Cr Mo V Limit valuea 0.5 0.5 Ladle slag Rapid coolingc 1140 nd 0.37 15.6 298.5 0.08 0.008 0.2 BOF slag Originalb 7095 nd 0.14 4.9 2.63 0.03 0.21 0.3 Semi rapid coolingb 4405 nd 0.07 14.9 19.15 0.01 0.07 0.7 Rapid coolingb 2070 nd nd 62.5 1.6 0.04 0.07 7.7 EAF slag 1 Originalb 1145 nd 0.04 37.4 139 0.73 3.9 0.3 Semi rapid coolinga 646.5 2.2 nd 140.5 5.12 0.82 0.11 2.8 Rapid coolingc 457 4.34 nd 132.2 2.73 0.93 0.07 0.3 EAF slag 2 Originalb 1545 nd 0.171 3.49 636 5.8 0.8 0.3 Semi rapid coolingb 2505 nd 0.067 1.08 426 0.008 0.02 0.02 Rapid coolingb 684 nd 0.05 50.4 45.6 3.8 0.4 2.5 nd = not detected. a Limit value for inert landﬁll. b prEN 12457. c prEn 12457-3. M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344 1339

Fig. 3. XRD patterns of the investigated slags, with diﬀerent cooling conditions.

5.3. XRD pletely amorphous by granulation. The major mineral in the original slag is mayenite, Ca12Al14O33, followed in All investigated slag samples are basic Mb(CaO + MgO)/ order by free MgO. b-Ca2SiO4, c-Ca2SiO4 and Ca2Al2SiO7 (SiO2 +Al2O3) > 1, which according to Daugherty et al. were also identified. The b-form may undergo a phase (1983), result in mainly crystalline slags. The values of transformation during cooling at 400–500 °Ctoc-form Mb are 1.5, 3.9, 1.4 and 2.1 for ladle slag, BOF slag, EAF and the volume increase (>10%) causes a pulverization of slag 1 and EAF slag 2, respectively. The comparison of the slag (Monaco and Lu, 1996). The expansive c-Ca2SiO4 the XRD pattern of the original and the modified slags, is a plausible explanation for the disintegration. The leach- Fig. 3, shows that all samples, except the granulated ladle ing solution of the original slag was not possible to filter, slag, consist largely of crystalline material. It is important which might be due to cement-forming properties of the to note that the very complex composition makes the iden- slag. tification of phases difficult. The examination program for One crystalline phase was identified in the granulated XRD analysis gives a high probability of several phases. ladle slag: unassimilated MgO. With SEM and mapping The phases present in Fig. 3 are those that are likely to of selected elements, two phases were identified, a matrix be present according to other knowledge, e.g., from the consisting mainly of calcium, silicon and aluminium (glass SEM studies. matrix) enclosing small fragments of MgO (1), see Fig. 4. The MgO particles are well distributed in the matrix (2). 6. Discussion 6.1.2. BOF slag 6.1. A mineralogical interpretation of the solubility The original BOF slag has high BET surface because of a high content of fines and pores compared to the granu- The investigations with XRD were complemented with lated material. According to the XRD results, Fig. 3, the SEM studies in order to evaluate the impact of different major phase in the original BOF slag is larnite, b-Ca2SiO4. cooling methods on the matrix of the slags and the effect With SEM and mapping of selected elements, Fig. 5, silicon on solubility (leaching) of minor elements. and calcium coexist (particle 1) in the same phases, which agree with the findings of larnite as the major phase. Parts 6.1.1. Ladle slag with high co-existence of iron, manganese and magnesium The ladle slag is difficult to handle and store due to the (particle 3) were distinguished, possibly the (Fe, Mn, Mg)O disintegrating properties. The XRD graphs, Fig. 3, show solid solution also found in XRD, as well as pure MgO that the ladle slag is the only one that becomes almost com- grains (particle 2). 1340 M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344

Fig. 4. SEM picture of the water granulated ladle slag. Dark fragments of (1) MgO in a matrix and (2) with high content of calcium, silicon and aluminium (glass).

Fig. 5. SEM investigation of the original BOF slag: (1) calcium silicate, (2) MgO, and (3) fragment rich in iron, manganese and magnesium.

According to the XRD analysis, Fig. 3, the main mineral Ca2SiO4 and lime (Goldring and Juckes, 1997). Quick cool- in the granulated BOF slag is Ca3SiO5. This phase exists at ing (Luxań et al., 2000; Monaco and Lu, 1996), as well as high temperatures, and is liable to transform on cooling to presence of impurity ions (Ionescu et al., 1998), prevent M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344 1341 formation of Ca2SiO4. In our tests, the transformation has was identified as the main mineral in both the original slag probably taken place in the original and the semi-rapidly and the two modifications. c-Ca2SiO4 was only found in cooled slags. the original and the semi-rapid slags, which might explain With SEM and mapping, a calcium silicate phase was the high BET surface in these samples. observed as big crystals (particle 1) as well as fibre-shaped With SEM and mapping of selected elements, two particles in the granulated BOF slag, Fig. 6. The euhedral matrix-forming phases and a spinel phase were differenti- prismatic microphenochrysts, that according to Goldring ated in the semi-rapidly cooled EAF slag 1, see Fig. 7. and Juckes (1997) are typical for Ca3SiO5, were clearly dis- Two of the phases correlate with the XRD identification tinguished. MgO is present as small spherical particles (2) of merwinite (particle 3) and solid solution spinel phase distributed in the matrix that contains high content of cal- (particle 1), (Mg, Mn)(Cr, Al)2O4. The other matrix-form- cium and iron (area 3). ing phase contains aluminium (particle 2) and SEM results The leaching of calcium and iron is reduced in the gran- show co-existence with primarily silicon, calcium and ulated BOF slag, see Table 3. Iron is present in the matrix, oxygen. as discussed above, and the leaching is very low in all three The content of calcium and silicon is high in the EAF slag samples. Calcium, on the other hand, is also present in slag 1, Table 2. The solubility of these two major elements, the major silicate phase, Ca3SiO5. The solubility of silicon as well as aluminium, iron and magnesium, is shown in is increased in the granulated slag compared to the original. Table 3. The leachability is very low and varies in the three The leaching result shows that the dissolution of the minor modifications. The solubility of aluminium is reduced sub- elements is not prevented by the rapid cooling procedure, stantially in the semi-rapidly cooled and the granulated see Table 3. Vanadium is most soluble in the granulated slag, which indicates that one of the matrix-forming phases BOF slag, correlating to the silica leaching. is stable. On the other hand, the mobility of silica seems to increase when granulating. There does not seem to be any 6.1.3. EAF slag 1 obvious correlation between the solubility of the major and The XRD graphs of the EAF slag 1, Fig. 3, show that the minor elements. The varying dissolution of the metals the original and the two modifications contain a large pro- chromium, molybdenum and vanadium is more likely a portion of crystalline phases. Merwinite, Ca3Mg(SiO4)2 result of the presence in different minerals. The solubility

Fig. 6. SEM picture of the granulated BOF slag: (1) silicate, (2) MgO, and (3) matrix with high content of iron. 1342 M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344

Fig. 7. SEM picture of semi rapidly cooled EAF slag 1: spinel phase (1), Al–Ca–Si–O phase (2), and silicate phase (3). of chromium is very low, 20 ppm of the total chromium and granulated slag is listed in Table 1. Both the EAF slag content, in all three samples. Vanadium, on the other hand, 1 and the ladle slag show significant changes in glass con- is most leachable in the semi-rapidly cooled slag. tent. In order to determine the glass formation possibility in the slag, it is not enough to look at the chemical analyses. 6.1.4. EAF slag 2 It is also of importance to consider the chemical analyses of The XRD analysis, Fig. 3, shows that the slag is very the rest melt due to high temperature crystallization. To complex and some phases have varying contents of substi- better understand the glass formation, the crystallization tuted ions. The identified main mineral in the original slag path and corresponding melt composition at equilibrium and the two modifications is b-Ca2SiO4. A wustite-type conditions were calculated using Factsage 5.4, see Fig. 8. solid solution ((Fe, Mg, Mn)O), Ca2(Al, Fe)2O5 and According to the thermodynamic calculations, the MgO Fe2O3 were also identified. A broadening in the diffraction crystallization from the liquid ladle slag already starts at peaks, indicating smaller crystallite size, could be seen approximately 1800 °C. Only 38% of the total MgO con- when cooling rapidly. tent is present in the liquid slag at 1400 °C. The remaining Calcium, iron and silicon are the major elements in the 60% has already been crystallized as pure MgO. This phe- matrix of the EAF slag 2. As can be seen in Table 3, cal- nomenon can be seen in both Figs. 3 and 4. When MgO cium, aluminium and iron have the lowest leachability in crystallization takes place, the Mb in the liquid material the granulated slag, while silicon, as well as the minor ele- is changed from original 1.5 to 1.25 at 1400 °C, influencing ments chromium, molybdenum and vanadium are most the glass forming properties in the material. insoluble in the semi-rapidly cooled slag. A similar behav- Due to early crystallization of solid solution, spinel iour of the major elements takes place for the BOF slag and phases, the liquid slag composition of the EAF slag 1 is the EAF slag 1 as well, but the effect on the leaching of the changed during cooling. Thermodynamically, the forma- minor elements is different. tion of spinel already starts at 1950 °C. When the formation takes place, MgO is reacting with chromium, forming 6.2. Glass formation magnesiochromite (MgCr2O4). This formation lowers the Mb factor, lowering it as the temperature decreases and As mentioned earlier, Daugherty et al. (1983) claimed the formation of spinel increases. The Mb ratio is decreased 3 < that an acid slag Mb 1 produces a glassy material more form original 1.41 to 1.34. As seen in Fig. 8, both the EAF readily compared to a more basic slag when cooling rap- slag 1 and the ladle slag tend to become more acidic as the idly. However, the investigated slags are considered to be liquid slag temperature decreases. basic (1.4–3.9) and should therefore mainly contain crystal- Neither the BOF slag nor the EAF slag 2 slags show any line material. The measured glass content for the original tendency of forming glass when cooling rapidly, according M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344 1343

Fig. 8. Glass forming tendency Mb, as a function of liquid slag temperature.

to Table 1. The high Mb value indicates a basic slag, even at 5. To choose one appropriate modification method for all low temperatures, Fig. 8. slags is difficult, depending both on slag composition According to the previous results discussed, it can be and the product to be manufactured. concluded that different behaviour was observed in relation to the cooling conditions. In view of recycling, i.e., having a lot of by-products generated each year within the steel Acknowledgements industry, the knowledge gained from the trials will hope- fully facilitate the possible use of residues in new fields This work was financed by MiMeR (Minerals and Met- and applications. als Recycling Research Centre) and Vinnova. The authors thank the members of MiMeR for the opportunity to pres- 7. Conclusions ent the data and for fruitful discussions about the tests and results. From this investigation, the following conclusions are drawn: References

1. The mineralogical composition is complex for the evalu- A Good Built Environment, 2004-10-18. Available from: . ated slag samples. XRD reveals the presence of different Bale, C.W., Chartrand, P., Decterov, S.A., Eriksson, G., Hack, K., Ben kinds of calcium silicate in all samples. Mahfoud, R., Melançon, J., Pelton, A.D., Petersen, S., 2002. Fact sage 2. The results obtained from the test leaching show that the thermochemical software and databases. Calphad Journal 62, 189–228. solubility of elements such as chromium, molybdenum CEN, 2002a. Final draft prEN 12457-2. Characterization of waste- and vanadium for the different investigated slags is in leaching-compliance test of leaching of granular waste material and sludges – Part 2: one-stage batch test at a liquid to solid ration of 10 l/ most cases very low in percentage. On the other hand, kg for materials with particle size below 4 mm (with or without particle the differences between the original and modified sam- reduction). ples are low. The leaching of chromium is not prevented CEN, 2002b. Final draft prEN 12457-3. Characterization of waste- by cooling rapidly according to the Official Journal of leaching-compliance test of leaching of granular waste material and the European Communities (2003). sludges – Part 3: two-stage batch test at a liquid to solid ration of 2 l/kg and 8 l/kg for materials with high solid content and with particle size 3. Slags with a Mb factor higher than 1 may form glass below 4 mm (with or without particle reduction). when cooling rapidly, depending on chemical analyses Daugherty, K.E., Saad, B., Weirich, C., Eberendu, A., 1983. The glass of the smelt. Still, the formation of glass in the investi- content of slag and hydraulic activity. Silicates Industriels 4 (5), 107–110. gated granulated slag samples has not been sufficient Goldring, D.C., Juckes, L.M., 1997. Petrology and stability of steel slags. to enclose the heavy metals and prevent them from Ironmaking and Steelmaking 24 (6), 447–456. Ionescu, D., Meadowcroft, T.R., Barr, P.V., 1998. Hydration potential of leaching. high iron level glasses: criteria for the recycling of steel slag as a 4. The disintegration ladle slag became stable after per- portland cement additive. In: 1998 ICSTI/Ironmaking Conference forming rapid cooling, due to the formation of glass. Proceedings, pp. 1245–1254. 1344 M. Tossavainen et al. / Waste Management 27 (2007) 1335–1344

Ionescu, D., Meadowcroft, T.R., Barr, P.V., 2001. Early-age hydration Official Journal of the European Communities, 2003. Council Decision of kinetics of steel slags. Advances in Cement Research 13 (1), 21–30. 19 December 2002. Establishing criteria and procedures for acceptance Lea, F.M., 1983. The Chemistry of Cement and Concrete, third ed. of waste at landfills pursuant to Article 16 of and Annex II to Directive Edward Arnold, London, p. 459. 1999/31/EC, 16.1.2003. Lee, Y., Nassarella, C.L., 1998. Formation of hexavalent chromium by Plunkett, E.R., 1976. Handbook of Industrial Toxicology. Chemical reaction between slag and magnesite-chrome refractory. Metallurgical Publishing Co., New York, NY, pp. 108–109. and Materials Transactions B 29B, 405–410. SGU, Geological Survey of Sweden, 2004. Aggregates, Production and Luxań, M.P., Sotolongo, R., Sotolongo, F., Dorrego, F., Herrero, E., Resources 2003, Uppsala, Sweden. Per. Pupl. 2004:3, ISSN 0283-2038 2000. Characteristics of the slags produced in the fusion of scrap steel (in Swedish). by electric arc furnace. Concrete Research 30, 517–519. Shij, C., 2004. Steel slag – its production, processing, characteristics and Monaco, A., Lu, W.-K., 1996. The properties of steel slag aggregates and cementious properties. Journal of Materials in Civil Engineering, their dependence on the melt shop practiceSteelmaking Conference ASCE (May/June), 230–236. Proceedings, vol. 79. A publication of the Iron & Steel Society, Tossavainen, M., Forssberg, E., 2000. Studies of the leaching behaviour of Pittsburgh, pp. 701–711. rock material and slag used in road construction: a mineralogical Murphy, J.N., Meadowcroft, T.R., Barr, P.V., 1997. Enhancement of the interpretation. Steel Research 71 (11), 442–448. cementious properties of steelmaking slag. Canadian Metallurgical Windholz, M. (Ed.), 1976. The Merck Index. Merck, Rahway, NJ, Quarterly 36 (5), 315–331. p. 976. Paper III

Crystallization behaviour of some steelmaking slags

F. Engström, D. Adolfsson, Q. Yang, C. Samuelsson and B. Björkman

Steel Research International (2010), Vol 81, p 362-371

DOI: 10.1002/srin.200900154 steel research int. 81 (2010) No. 5

Crystallization Behaviour of some Steelmaking Slags

F. Engstro¨ m*, D. Adolfsson, Q. Yang, C. Samuelsson, and B. Bjo¨ rkman

Division of Process Metallurgy, Lulea˚ University of Technology, 971 87 Lulea˚, Sweden, [email protected] * Corresponding author

The present study was aimed at highlighting the ﬁnal properties of two different steelmaking slags which undergo different cooling rates. The experiments were conducted in laboratory scale using an induction furnace. One of the slags originates from an electric arc furnace (EAF) (high- alloyed) and the second slag from a basic oxygen furnace (BOF). The treatment of the slag included re-melting along with different cooling rates. The material collected from the tests was characterized through X-ray diffraction, scanning electron microscopy as well as thermodynamic calculations which were compared with experimental results, for conﬁrmation. The results indicate that both the EAF and BOF slags show increased reactivity with water, as well as a decrease in crystal size when rapid cooling is applied. The wu¨stite-type solid solution (Mg,Fe,Mn)O varies in composition depending on the cooling conditions. Metastable Ca3SiO5 was found in the rapidly- cooled BOF slag.

Keywords: steel slag, modiﬁcations, cooling rate, crystallization, matrix

Submitted on 19 October 2009, accepted on 25 January 2010

Introduction AOD slag (Argon Oxygen Decarburization) depending on the cooling rate [8]. However, that has not been reported for Large amounts of by-products are produced by the EAF slags [9]. Thomas and Stephenson [10] believe that Swedish steelmaking industry each year. In 2007, the total impurities in the EAF slag stabilize the metastable b- amount of slag produced reached approximately 1 500 000 Ca2SiO4 from disintegration. tonnes. Only 43%, mostly blast furnace slag, was sold as Very little has been reported in the literature regarding the external products, and approximately 35% was used for influence of cooling in relation to the properties of slag, landfilling [1]. These figures are very high in comparison to especially when it comes to steelmaking slags (EAF, BOF). other European countries. In Germany, only 7% of the steel Ground granulated blast-furnace slags (GGBS) are known to slags produced is dumped, while 93% is used for other possess improved hydration reactivity compared to slowly applications [2]. In Sweden, a number of goals have been cooled blast furnace slag, due to the formation of glass [11]. formulated in order to obtain a so-called good building However, nothing similar is reported for neither EAF nor environment [3]. Among these are the targets that (i) By BOF slags. Monaco and Lu [9] reported variations in the 2010 re-used material should represent at least 15% of the composition of the wu¨stite-type solid solution as well as a aggregates used; (ii) Landfill waste should be reused to an variation in crystal size when cooling differently. extent of at least 50% by 2005 compared to 1994. In order to gain a better understanding, and to be able to Due to its strength, durability and chemistry, steel slag develop future products, the influence of the cooling rate could be considered in the field of construction since the with regard to mineralogy and phase distribution was material provides similar properties as granite and flint investigated. The experiments were conducted on two gravel [2]. The use of steel slags may also contribute to a different steel slags which were exposed to different cooling reduction in the amount of landfilled waste. The possibility rates. Thereafter, different techniques have been used for of using slag is limited due to the lack of rules and guidelines the characterization; these included scanning electron regarding testing, assessment and use of slag in Sweden. The microscopy (SEM) provided with energy dispersive spectra technical and environmental obstacles for some slags in (EDS), X-ray powder diffraction (XRD) as well as construction include volumetric expansion, disintegration, thermodynamic calculations with Factsage 5.4. and leaching of metals. According to Monaco and Lu [4], the volumetric Experimental Procedure expansion is assumed to be associated with the presence of free lime and free periclase in the solidified slag. Free lime Materials. 20–30 kg representative samples of two and periclase react with moisture, resulting in an expansion different steel slags were provided by steelmaking compa- due to the formation of hydroxides [5]. nies in Sweden: Upon cooling, pure dicalcium silicate undergoes a phase – BOF slag transformation, from b-Ca2SiO4 to g-Ca2SiO4 at approx- – EAF slag, high-alloyed imately 763 K [6]. The latter results in a volume expansion of approximately 12 vol-% [7]. The polymorphic transforma- The materials were crushed with a jaw crusher, Retsch tion of b-Ca2SiO4 to g-Ca2SiO4 is known to occur in some BB3, to <30–40 mm before further splitting into 1–1.5 kg

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Table 1. Compact density, basicity B2 and chemical analysis of the raw materials used.

Sample Analysis (mass%) Compact B2 (CaO/SiO2) density

3 FeO Fe2O3 Fe met. CaO MgO MnO SiO2 Al2O3 Cr2O3 MoO2 P2O5 TiO2 g/cm BOF slag 10.7 10.9 2.3 42.2 9.6 3.2 11.1 1.9 0.1 nd 0.5 1.4 3.5 3.8 EAF-slag 3.3 1.0 0.1 45.7 5.2 2.0 32.3 3.7 4.8 nd nd 1.3 3.3 1.4 nd ¼ not detected. sub-samples. Data of chemical analyses and compact Before cooling the slag (BOF and EAF), the samples were density of the materials used is listed in Table 1. re-melted in a laboratory induction furnace using an MgO crucible system, with an air atmosphere. The heating rate was Chemical analysis. The total composition of each set to approximately 4–6 K/minute. After approximately 6– material was analysed by Ovako Steel AB (Sweden) with 8 hours the slag reached the target temperature, 1873 K. inductively coupled plasma emission spectroscopy, ICP, and The semi-rapidly cooled slag was simply left to cool in the X-ray fluorescence spectroscopy, XRF. The content of Fe and MgO crucible. The time required for the slag to transform Feoxid was determined through titration. The analyses were from a liquid state to room temperature was determined to be done in duplicate, and the results are presented as a mean value. five hours. The cooling rate was approximately 0.3 K/s within the first hour and 0.04 K/s thereafter. Scanning electron microscopy (SEM). The mineral- In the case of granulation, the liquid slag was poured ogy of the slag samples was examined with a Philips XL 30 through a granulation head, where water jets dispatched the SEM using a beam operation voltage of 20 kV, and spot size 6. slag. The generated granules were collected at the bottom of Semi-quantitative and qualitative elemental analyses were the water tank. The estimated time from liquid to solid was performed with an energy dispersive spectrometer (EDS) only a few seconds, so the cooling rate was estimated to be fitted with an Everhart and Thornley detector behind a 500 K/s. The cooling profiles are shown in Figure 1. beryllium window. Before investigating the mineralogy of the For detailed description and figures, regarding the mod- sample using the secondary electron (SE) image signal, the ification and re-melting trials, see M. Tossavainen et al. [13]. samples were sputter coated with a conductive layer of gold. Results X-ray powder diffraction (XRD). All XRD samples were prepared in a ringmill for 2x30 seconds. Between the Physical and chemical properties. The re-melted slags grinding procedures, magnetic fractions were removed which were left to cool in the crucible (semi-rapid) resulted in through magnetic separation. The samples were analysed large pieces which were crushed to <4 mm to facilitate with a Siemens D5000 x-ray diffractometer, using copper Ka material handling during the characterization procedure. radiation. XRD patterns were recorded from 10 to 908 sin2u,in The granules obtained from the granulation had a spherical 0.028/step. Initially, XRD patterns were recorded by counting shape and a particle diameter of approximately 2–4 mm. 1 s/step, and at a further stage it was rerun with a counting of 8 The chemical analyses of the four modified slag types s/step and approximately 30–608 sin2u. The phase identifi- are shown in Table 2. Only minor changes in chemical cation was made by reference patterns in an evaluation composition in comparison to the input material, were program supplied by the manufacturer of the equipment. reported, see Table 1. Except for the variations in basicity (CaO/SiO2), it is also shown that the content of iron is higher Thermodynamic calculations. Thermodynamic calculations were made in Factsage 5.4 [12] using compound database FS53base.cdb, FToxid53base.cdb and solution database FToxid53soln.sda. FToxid-slagA and FToxid- MeO were used. During calculation, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplications in the data set. The formation of b-Ca2SiO4 (metastable) was ignored during these calculations.

Modiﬁcation. The importance of cooling was investigated with respect to the ﬁnal properties of the slag. Each slag was treated in two different ways before further comparison:

(1) Re-melting followed by cooling in crucible (semi-rapid cooling) (2) Re-meltingfollowedbywater-granulation(rapidcooling) Figure 1. Cooling proﬁle of the treated slag samples. www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 363 steel research int. 81 (2010) No. 5 Process Metallurgy

Table 2. Chemical analyses of the treated slag samples in mass%. XRD results. The investigated steel slags are character- þ þ > Element BOF slag EAF-slag ized as basic, i.e. (CaO MgO)/(SiO2 Al2O3) 1, which, according to Daugherty et al. [14], should result Semi rapid Rapid Semi rapid Rapid in the formation of crystalline phases during cooling. cooling cooling cooling cooling However, Tossavainen et al. [13] have shown that steel slags FeO 1.0 1.6 1.4 3.5 with a basicity >1 may have the ability to form glass during rapid cooling, due to changes that occur in the remaining Fe2O3 18.3 19.7 3.6 nd liquid slag during cooling. The XRD patterns of the Fe met. 2.8 1.8 0.4 0.2 differently treated slags are all given in Figure 2.The CaO 46.1 45.6 43.0 44.0 patterns reveal that the slags mainly consist of crystalline phases. MgO 10.0 9.9 7.1 6.5 According to Figure 2, four crystalline phases were MnO 3.1 3.1 2.4 2.3 identiﬁed in the semi-rapidly cooled BOF slag: a solid

SiO2 11.9 11.4 31.0 32.1 solution of magnesium, iron and manganese oxide (Mg,Fe,Mn)O, calcium ferrite (Ca2Fe2O5), calcium man- Al2O3 2.1 2.0 4.1 3.5 ganese oxide (Ca,Mn)O and b-dicalcium silicate (b- Cr2O3 nd nd 5.2 6.3 Ca2SiO4). In the rapidly cooled BOF slag, three crystalline phases were detected: tricalcium silicate (Ca3SiO5), a- MoO2 nd nd nd nd dicalcium silicate (a-Ca2SiO4) and a solid solution of P2O5 0.6 0.5 nd nd magnesium, iron and manganese oxide (Mg,Fe,Mn)O.

TiO2 1.3 1.4 1.3 1.1 Three phases were identiﬁed in the semi-rapidly cooled EAF slag, namely merwinite (Ca Mg(SiO ) ), magnesio- nd ¼ not detected. 3 4 2 chromite (MgCr2O4), and g-dicalcium silicate (g-Ca2SiO4). in the BOF slag than in the EAF slag, while the chromium In the rapidly cooled EAF slag, two crystalline phases were content is higher in the EAF slag. detected: merwinite and chromite.

Figure 2. XRD patterns of the treated slag samples.

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Figure 3. Results of the thermodynamic calculation.

Thermodynamic calculations. The result of the SEM. Through SEM and mapping, several combinations thermodynamic calculation is shown in Figure 3. The could be distinguished in the semi-rapidly cooled BOF slag, calculations are based on the following conditions: as shown in Figure 4, where three phases are marked. Phase 1 – Temperature interval: 298–2073 K refers to magnesium, iron and manganese oxide. Phase 2 – Pressure: 1 atm, constant represents calcium, silicon and oxygen, while in Phase 3, Phases formed at a level below 3 wt % were neglected in calcium, iron and oxygen coexist. Figure 3. In the rapidly cooled BOF slag, Figure 5, three phases were According to the results given in Figure 3, the first observed in the material: Phase 1, a magnesium, iron and crystalline phase to precipitate from the liquid BOF slag is manganese oxide; Phase 2, calcium, silica and oxygen, and MgO, followed by the crystallization of FeO and CaO. Phase 3, containing calcium, silica, titanium, vanadium, Tricalcium silicate, Ca3SiO5 is the first silicate to crystallize at manganese, iron and oxygen. According to SEM and mapping, approximately 1723 K. Below 1543 K, Ca3SiO5 is trans- it can be concluded that phase 2 consists of a higher amount 0 formed to a -Ca2SiO4 (an a-Ca2SiO4 polymorph) and CaO. of calcium than Phase 3. The first phase to be formed in the liquid EAF slag is the Through SEM and mapping, three phases were detected in magnesiochromite, which crystallizes above 2073 K. In the semi-rapidly cooled EAF slag, Figure 6. Phase 1 consists addition, MgCr2O4 is further transformed into chromite of calcium, magnesium and silica (a correlation was found). (FeCr2O4)at1543 K. Alpha dicalcium silicate, a- Phase 2 contains calcium, alumina and silica, whereas Phase Ca2SiO4, is the first silicate to be formed in the liquid EAF 3 is constituted by chromium, magnesium, manganese and slag at 1807 K. oxygen, along with some minor particles of metal iron. www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 365 steel research int. 81 (2010) No. 5 Process Metallurgy

Figure 4. Scanning electron micrograph and accompanied mapping of the semi-rapidly cooled BOF slag. (1) (Mg,Fe,Mn)oxide, (2) Calcium silicate, (3) Calcium ferrite.

Two phases were observed with SEM and mapping in the BOF slag. In the semi-rapidly cooled BOF slag, four rapidly cooled EAF slag, Figure 7: Phase 1, similar to Phase 3 crystalline phases were identified by XRD analyses: a in the semi-rapidly cooled EAF slag, consisting of chromium, wu¨stite-type solid solution containing magnesium, iron manganese, magnesium and oxygen, and Phase 2, calcium, and manganese (Mg,Fe,Mn)O, b-dicalcium silicate (b- magnesium, silica, alumina and oxygen. Ca2SiO4), calcium ferrite (Ca2Fe2O5), and a calcium, manganese oxide (Ca,Mn)O phase. All phases except for the (Ca,Mn)O were also detected with SEM and mapping, Discussion Figure 4, Particle 1–3. The (Mg,Fe,Mn)O is enclosed in the b-Ca2SiO4 structure, indicating an early crystallization, in As the characterization of the differently treated slags comparison to b-Ca2SiO4 and Ca2Fe2O5 .Nog-Ca2SiO4 progressed, it could be noted that there was a clear was found in the semi-rapidly cooled BOF slag, most likely difference in particle size distribution due to the different due to the stabilising effect of P2O5 [15]. cooling rates. The result obtained from the XRD, Figure 2, In the rapidly cooled BOF slag, three crystalline phases and SEM study, Figure 4–7, is in agreement with the were identified through XRD: a wu¨tite-type solid solution result obtained from the thermodynamic calculations, containing magnesium, iron and manganese (Mg,Fe,Mn)O, Figure 3. A summary of the phases found in the slag is tricalcium silicate and a-Ca2SiO4. These phases were also given in Table 3. confirmed by SEM, Phase 1–3, Figure 5, where Phase 2 is

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Figure 5. Scanning electron micrograph and accompanied mapping of the rapidly cooled BOF slag. (1) (Mg,Fe,Mn)oxide, (2) Calcium silicate, (3) Matrix containing (Ca,Si,Ti,V,Mn,Fe) oxides.

tricalcium silicate (Ca3SiO5) i.e. the euhedral prismatic crystallizing and forming solid solution with MgO, due to its microphenochrysts, that according to [16], are typical for later crystallization in comparison to MgO, Figure 3. The Ca3SiO5 and phase 3, the matrix phase, crystallizing last, latter was further confirmed by the SEM analyses. probably containing the a-Ca2SiO4 seen in XRD. All phases According to semi-quantitative analyses, the solid solution identified in the rapidly cooled BOF slag agree with the contains 51% MgO, 42% FeO and 7% MnO in the semi- thermodynamic calculation, Figure 3, indicating that the fast rapidly cooled slag, while the solid solutions in the rapidly cooling enables the presence of metastable phases, such as cooled slag consist of 78% MgO, 16% FeO and 6% MnO. Ca3SiO5 and a-Ca2SiO4 at lower temperatures. When a phase can form thermodynamically, the crystal As seen in Figure 2, the composition of the wustite-type size will depend on the temperature to which the crystals are solid solution is different when the semi-rapidly cooled slag exposed and the duration of the exposure. There was a is compared with the rapidly cooled BOF slag. According to significant difference in crystal size between the two the thermodynamic calculations, Figure 3, MgO is already modified BOF slags, Figure 4–5. The size of the crystals present as crystals in the liquid slag at 1873 K. The slight present in the semi-rapidly cooled slag varied between 40– change in position which occurs in the diffractogram is 200 mm, indicating that these minerals have had longer time explained in terms of having a higher concentration of MgO to grow. in the wustite-type solid solution. As the slag is cooled In the rapidly cooled BOF slag, the variation in crystal size rapidly, neither the FeO nor MnO has the same possibility of is more pronounced compared to the semi-rapidly cooled www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 367 steel research int. 81 (2010) No. 5 Process Metallurgy

Figure 6. Scanning electron micrograph and accompanied mapping of the semi-rapidly cooled EAF slag. (1) Calcium magnesium silicate, (2) Calcium alumina silicate, (3) Chromium containing spinel.

BOF slag. The wu¨stite-type solid solution (Phase 1) and the MgCr2O4), calcium magnesium silicate (merwinite tricalcium silicate (Phase 2), Figure 5, have a crystal size Ca3Mg(SiO4)2)andg-dicalcium silicate (g-Ca2SiO4). Both varying between 20–100 mm. The matrix (Phase 3), merwinite (Phase 1) and magnesiochromite (Phase 3) were Figure 5, has a much smaller size than the other two phases found with SEM as well (Figure 6). discussed. The smaller crystal size of the silicate matrix can From Figure 6, it is given that manganese is present in thus be explained in terms of not having the same time to the solid solution along with chromium and magnesium. develop. Based on the thermodynamic calculations, it can Besides the three phases that were found with XRD, an be concluded that both the wu¨stite-type solid solution and additional phase was observed with SEM and mapping; i.e., the tricalcium silicate were present in the liquid slag at the a calcium alumina silicate phase (Phase 2), see Figure 6. time the rapid cooling with water was carried out. However, According to the thermodynamic calculation (Figure 3), the a-Ca2SiO4 is expected to form as a result of the rapid only one phase of calcium alumina silicate exists in that cooling with water. A schematic figure of the reaction taking system; namely, Ca2Al2SiO7 (gehlenite). Gehlenite is place when the BOF slag is cooling is shown in Figure 8. thermodynamically formed below 1543 K, according to the calculations. The later crystallization of gehlenite is in EAF slag. In the semi-rapidly cooled EAF slag three conjunction with the observed texture in the semi-rapidly crystalline phases were identified with XRD: (Figure 2) a spinel cooled EAF slag. As seen in Figure 6, both merwinite and the containing magnesium and chromium (magnesiochromite, spinel found have a specific structure characterized by sharp

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Figure 7. Scanning electron micrograph and accompanied mapping of the rapidly cooled EAF slag. (1) Chromium containing spinel, (2) Matrix containing (Ca,Al,Si,Mg) oxides. edges, while the gehlenite is located between the other two. while the rest of the phases vary a lot, indicating that the According to the thermodynamic calculations in Figure 3, spinels were crystallized already at the time the granulation both merwinite and the spinel crystallize earlier than started, which in addition is confirmed by the calculations, gehlenite, which explains the texture obtained in the semi- see Figure 3. The large well defined merwinite and gehlenite rapidly cooled EAF slag. crystals which were found in the semi-rapidly cooled slag Two crystalline phases were identified in the rapidly were no longer present in the rapidly cooled EAF slag. cooled EAF slag with XRD (Figure 2) both similar to those Instead, a mixture of calcium, magnesium, alumina and that were found in the semi-rapidly cooled EAF slag; i.e., silica were found in area 2 (Phase 2), Figure 7. Based on the merwinite and the spinel, containing magnesium and theory of nucleation and growth [18, 19], it is suggested chromium. When comparing the peaks in Figure 2, a that area 2 consists of small merwinite and gehlenite crystals broadening of the peak width was observed as a result of the due to the rapid cooling i.e., the rapid crystallization. A rapid cooling with water. Suryanarayana and Grant [17] schematic illustration of the reactions taking place during suggest that this may be caused by a decreased crystallite cooling of the EAF slags is presented in Figure 8. size. Furthermore, the differences in crystal size found between Reactivity. According to Roman et al. [20], the leaching the two modified slags are significant, see Figures 6–7. The from steel slags is most likely characterized as a surface spinel phase has the same size and texture in both materials, reaction, followed by a solid-solid diffusion process, in order www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 369 steel research int. 81 (2010) No. 5 Process Metallurgy

Table 3. Summary of phases identiﬁed in slag samples. reactivity with regard to the surface chemistry of the Minerals BOF slag EAF slag differently cooled slags, a reactivity index a was introduced and calculated, according to (1). The reactivity index of the Semi-rapid Rapid Semi-rapid Rapid rapidly cooled, as well as the semi-rapidly cooled slags, is

Ca3SiO5 X given in Table 4. a-Ca2SiO4 X Leached amount b-Ca2SiO4 X Specific surface area ð Þ ð Þ mg leached from kg dry material 2 g-Ca2SiO4 X ¼ ¼ mg=m m2=kg Ca3Mg(SiO4)2 XX (1) Ca2Fe2O5 X As seen in Table 4, there are important differences in Ca2Al2SiO7 X reactivity due to the different cooling rates. Almost all (Fe,Mg,Mn)O X X elements become more reactive if treated with rapid cooling. CaO (ss) X The reactivity for silica is increased by 4700% and 1200%, respectively, and for chromium 5300% and Spinel (ss) X X 1500%, respectively, for the BOF and EAF slag. Unlike the blast furnace slag which also becomes more reactive when cooled rapidly due to the high content of amorphous to retain equilibrium in the materials. A minimization of the phases, Tossavainen et al. [13] have shown that both the surface area of the slag can therefore be assumed to enable a quenched BOF and EAF slag have a low content of glass. decrease in leachability. Tossavainen et al. [13] have earlier Instead, two possible explanations for the increase in presented leaching and specific surface data regarding these reactivity are the presence of metastable phases and the materials. The specific surface area data was not measured in increases in small crystals on the surface, according to the relation to the semi-rapidly cooled samples. However, since theory of crystallization and growth [18, 19]. the original and semi-rapidly cooled samples were prepared for leaching according to the same procedure (crushing Conclusions <4 mm), it is assumed that the materials have a similar specific surface area. From this study the following conclusions have been Tossavainen et al. [13] concluded that no distinct changes drawn: in the total leachability could be designated when comparing The final phase composition of the slag depends on the the semi-rapidly cooled slag with the rapidly cooled. In any temperature at which the cooling occurs. event, a decrease in the specific surface area was noted as By cooling the liquid slag very rapidly, there will not be the semi-rapidly cooled was compared with the rapidly sufficienttimeforthecrystalstogrow.Asaresult,crystalsof cooled slag. In order to gain a better understanding of the the slag will be much smaller, resulting in a more uniform

Figure 8. Schematic of reactions taking place during cooling of the slag.

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Table 4. The a- index of rapidly and semi-rapidly cooled slag. Element BOF slag EAF slag a (mg/m2) (Rapid)/(Semi) a (mg/m2) (Rapid)/(Semi) Semi rapid Rapid cooling % Semi rapid Rapid cooling % cooling cooling Ca 1.87Eþ00 9.86Eþ00 526% 2.90E-01 2.69Eþ00 928%

Na 1.87E-03 2.89E-02 1543% 1.09E-03 3.09E-02 2824% S 4.62E-03 1.06E-01 2305% 2.55E-03 1.52E-02 596% Si 6.34E-03 2.98E-01 4694% 6.30E-02 7.78E-01 1235% Al 8.15E-03 7.76E-03 95% 2.30E-03 1.61E-02 701% Ba 1.75E-04 8.93E-04 510% 2.67E-04 4.05E-04 151% Cr 3.29E-06 1.74E-04 5284% 3.68E-04 5.50E-03 1494%

Mn 4.23E-05 3.99E-05 94% 6.26E-06 5.23E-05 835% Mo 2.79E-05 3.11E-04 1117% 4.82E-05 3.84E-04 797%

composition. The rapid cooling also enables the formation metallurgical slags. Resources, Conservation and Recycling, 52 of metastable phases at low temperatures. (2008), No. 10, 1121–1131. [7] M. A. Bredig: Polymorphism of calcium Orthosilicate, Journal of A controlled cooling makes it possible to control the The American Ceramic Society, 33 (1948), No. 6, 188–192. properties of the slag material. [8] Q. Yang, L. Nedar, F. Engstrom and M. He: Treatments of AOD Slag In order to achieve a homogeneous slag product, the to Produce Aggregates for Road Construction, AISTech 2006 Pro- cooling must be controlled and kept constant. ceeding, Vol. 1, p 573–583. [9] A. Monaco and W.-K. Lu: The effect of cooling conditions on the The reactivity with water is increased when rapid cooling mineralogical characterization of steel slag, Proc Int Symp Res is applied. Conserv Environ Technol, Metall Ind, 1994 p 107–116. [10] G. H. Thomas and I. M. Stephenson: The Beta to Gamma Dicalcium Acknowledgements Silicate Phase Transformation and its significance on air cooled stability, Silicates Industriels, 43 (1978), No. 9, 195–200. This work was financed by MiMeR, Minerals and Metals [11] N. Tsuyuki and K. Koizumi: Granularity and Surface Structure Recycling Research Centre, through VINNOVA. The of Ground Granulated Blast-Furnace Slags, Journal of the Amer- ican Ceramic Society, 82 (1999), No. 8, 2188–92. authors wish to thank the members of MiMeR for the [12] C. W. Bale, P. Chartrand, S. A. Decterov, G. Eriksson, K. Hack, R. opportunity to work on this project and for all the fruitful Ben Mahfoud, J. Melançon, A. D. Pelton and S. Petersen: ‘‘FactSage discussions. Thermochemical Software and Databases’’, Calphad Journal, 62 (2002), 189–228. [13] M. Tossavainen, F. Engstrom, Q. Yang, N. Menad, M. Lidstrom References Larsson, and B. Bjorkman: Characteristics of steel slag under different cooling conditions, Waste Management, 27 (2006), [1] B. Haase: Overview of Residue Utilisation in Sweden: Focus on By- 1335–1344. products from Iron and Steel Industry, Proc. 1st Int. Slag Valorisation [14] K. E. Daugherty, B. Saad, C. Weirich, A. Eberendu: The glass Symposium, 6–7 April 2009, Leuven, pp 185–194. content of slag and hydraulic activity, Silicates Industriels, 48 [2] H. Motz, J. Geiseler: Products of steel slag an opportunity (1983), No. 4–5, 107–110. to save natural resources, Waste management, 21 (2001), 285– [15] Q. Yang, L. Nedar, F. Engstro¨m, M. He: Treatments of AOD slag to 293. produce aggregates for road construction. Ohio, USA: Aistech; 2006. [3] A Good Built Environment, 2004-10-18, available from http://miljo- [16] D. C. Goldring, L. M. Juckes: Petrology and stability of steel slags, mal.nu/english/english.php. Ironmaking and Steelmaking, 24 (1997), No. 6, 447–456. [4] A. Monaco, W.-K. Lu: The properties of steel slag aggregates and [17] C. Suryanarayana and M. Grant: X-Ray Diffraction A Practical their dependence on the melt shop practice, Steelmaking conference Approach, Plenum Press, New York, 1998, ISBN 0-306-45744-X. proceedings, Vol. 79, pp 701–711, A publication of the Iron & Steel [18] W. D. Nesse: Introduction to mineralogy, Oxford University Press, Society, (1996), Pittsburgh. New York Oxford, 2000, pp. 74–94, ISBN 0-19-510691-1. [5] L. M. Juckes: The volume stability of modern steelmaking slags, [19] T. Abel Engh: Principles of Metal Refining, Oxford University Press, Mineral Processing and Extractive Metallurgy (Trans. Inst. Min. Oxford, New York, Tokyo, 1992, ISBN 0-19-856337-X. Metall. C), 112 (2003), 177–197. [20] Q. Romera, M. Ku¨hn, ? Behmenburg and ? Capodilupo: Decreasing [6] D. Durinck, F. Engstro¨m, S. Arnout, J. Heulens, P. T. Jones, B. the scorification of chrome, Primary steelmaking, European Commis- Bjo¨rkman, B. Blanpain, P. Wollants: Hot stage processing of sion, Luxembourg, 2000, pp 39, ISBN 92-828-9627-0.

www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 371 Paper IV

Leaching behaviour of aged steel slags

F. Engström, M. Lidström Larsson, C. Samuelsson, Å. Sandström, R. Robinson and B. Björkman

Submitted 2010 (Resources, Conservation and Recycling)

LEACHING BEHAVIOUR OF AGED STEEL SLAGS

F. Engström1, M. Lidström Larsson2, C. Samuelsson, Å. Sandström, R. Robinson and B.

Björkman

Division of Minerals and Metallurgical Engineering

Luleå University of Technology

SE-971 87 Luleå

Sweden

1Corresponding author. [email protected], tel.;+46 920 491388; fax; +46 0920 491199 2 Deceased Abstract

Large amounts of slag are generated by the Swedish steel industry each year. The Electric

Arc Furnace (EAF) process generates about 200,000 tonnes of slag per annum, of which approximately 40% is deposited. An alternative to deposition is to use slag as road construction material. However, leaching of metals from the slag can be a reason to limit slag use in road construction. The aim of this work was to investigate how stable these materials are, with respect to leaching and mineralogy, when aged in an environment open to seasonal weather conditions. Three different EAF slags from domestic steel plants were used in this study. The materials were characterized after 0, 6, 12, 18 and 24 months to evaluate the ageing process. The analytical techniques that were used to evaluate the effect of ageing are scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and a standard test for leaching. The changes in ageing behaviour differ between the three materials. For all samples, the conductivity and pH decrease over time. The leaching of calcium, chromium and aluminium decreases over time, while the leaching of magnesium increases. CaCO3 was formed on slag surfaces, as CaO reacts with moisture and CO2 from the air.

Key words: steel slag, leaching, slag ageing, slag properties, carbonation

2 Introduction

The slag generation in the Swedish steel industry is about 1.3 million tonnes annually, of which more than 200,000 tonnes is generated from EAF (Electric Arc Furnace) processes during the refining of steel. Some of the slags are used in different applications, both as internal and external products, but some slags are still landfilled (Pålsson et al. 2010).

The reason for the low utilization in Sweden is mainly due to potential leaching of harmful elements, environmental legislation and competition from easily available virgin raw materials. Slag from scrap-based steelmaking is used for road construction elsewhere in Europe. In some respects, the slag may even be technically superior to virgin material in road construction. Due to its high strength and durability, steel slag is often a suitable material in the field of construction, replacing for example, gravel and rock (Motz and

Geiseler, 2001), thereby saving natural recourses. Besides the potential for leaching, other technical obstacles to using some slags in construction include:

• Swelling

• Disintegration

The swelling phenomenon is considered to be associated with the presence of free lime

(CaO) and free periclase (MgO) in the solidified slag (Monaco and Lu, 1996; Juckes,

2003). Free lime and periclase react with moisture, resulting in an expansion due to the formation of hydroxides.

Disintegration of slags is a result of the phase transformation that occurs upon cooling.

Pure dicalcium silicate undergoes a phase transformation from -Ca2SiO4 to -Ca2SiO4 at

3 approximately 500ºC, which results in a volume expansion of approximately 12-vol %, leading to disintegration (Bredig, 1950; Monaco and Lu, 1994; Durinck et al., 2008).

Slag has a long history of use in different applications and many efforts have been made to characterize, test and develop treatment techniques. Still, there is a gap of knowledge regarding the factors controlling the ability to leach, the long-term quality and control of properties of the final slag product. In earlier studies the possibility to simulate the leaching behaviour of aged steel slags, using geochemical modelling has been investigated (Apul et al., 2005; Huijgen and Comans, 2006). These types of simulations sometimes fail to fully describe these complex systems, mostly because available data

(solubility/thermodynamically/sorption) for the minerals occurring in the slag systems is often incomplete or missing. Beyond the potential for savings of natural resources and aggregates, steel slags, if treated and correctly used, possess characteristics for binding

CO2 by forming carbonates. According to Huijgen et al. (2005), the basic concept behind mineral carbonation is to imitate natural weathering processes in which calcium- and magnesium-containing minerals are converted into calcium and magnesium carbonates.

The maximum CO2 binding capacity is around 0.25 kg of CO2/kg steel slag on the basis of the total Ca content, which would be equivalent to a carbonate content of 20 wt% CO2

(Huijgen et al., (2005)). Factors such as temperature, particle size, porosity and CO2 diffusivity are known to influence the reaction rate of the carbonation (Huijgen et al.,

2005; Houst and Wittman, 1994; Malami and Kaloidas, 1994). In addition to advantages from a CO2 storage perspective, research has shown that carbonation of alkaline solid residues can lead to an improvement of their environmental qualities (Meima et al., 2002;

Reddy et al., 1994). In this study the main focus has been to investigate EAF slags as they

4 age naturally in an environment open to seasonal weather conditions. Leaching of metals from three different types of EAF slags, as well as the long-term stability regarding possible phase transformation, were studied.

5 Materials

Materials and experimental setup.

Over a period of two years, three different types of EAF slags from domestic steel plants were investigated regarding their long-term leaching behaviour and stability when exposed to seasonal weather conditions. To guarantee a freshly produced and homogenous slag material, the samples were collected directly from the steel plant in collaboration with the different Swedish slag producers. Representative samples, 10-20 kg, of each slag were collected from their respective producers.

A. Electric arc furnace slag 1, low alloyed steel EAF slag 1

B. Electric arc furnace slag 2, stainless steel EAF slag 2

C. Electric arc furnace slag 3, high alloyed steel EAF slag 3

All slag samples used in these experiments were crushed and sieved to a fraction of

Figure 1. Schematic picture of experimental setup.

The materials were sampled and characterized after 0, 6, 12, 18 and 24 months to evaluate the influence of ageing.

6 Methods for characterization

Physicochemical and mineralogical composition

The total composition of each material was analyzed by Ovako Steel AB with inductively coupled plasma emission spectroscopy (ICP) and X-ray fluorescence spectroscopy

(XRF). Titration was used for analysis of Fe and FeO and infrared adsorption spectroscopy, IR, for carbon and sulphur.

All the slag samples were leached according to the one-stage batch test EN 12457-2 (EN

12457-2, 2002). The leaching tests were done in duplicate and the results are presented as a mean value. The filtrates were analyzed by ALS (Sweden).

The mineralogy of the slag phases was studied on polished samples using a Philips XL 30 scanning electron microscope (SEM). Semi-quantitative and qualitative analyses were carried out using an energy dispersive spectrometer (EDS).

For X-ray diffraction analysis (XRD), all samples were prepared in a ringmill for 30 seconds. The samples were analyzed using a Siemens D5000 x-ray diffractometer, equipped with copper K radiation of 40kV and 40mA. The XRD pattern was recorded from 10 to 90° sin2 at 0.02°/step and 5s/step.

The grade of carbonation of the fresh and aged samples was measured using thermal decomposition (TGA-MS) according to the method described by Huijgen et al. (2005).

TGA-MS analyses were performed in a thermo-gravimetric analysis system (Netzsch

7 STA 409) coupled with a quadruple mass spectrometer (QMS). The samples were heated in alumina crucibles under an oxygen atmosphere at 20°C/min from 25 to 1000°C.

Weight loss was measured by the TGA, while the gas was analyzed for CO2 and H2O.

The analyses were divided into three steps: Step (1) 25-105°C; step (2) 105-500°C; step

(3) 500-1000°C. These steps represent (1) moisture, (2) organic elemental carbon and

MgCO3 (if present) and (3) CaCO3 (inorganic carbon), respectively. During heating, the samples were kept isothermally at 105, 500 and 1000°C for 15 min, giving enough time for the reactions to fully occur. The third weight loss from the TGA curve (m500-1000°C) was used to describe the calcium carbonate content.

Weather data regarding precipitation amount and average temperatures were obtained from the Swedish Meteorological and Hydrological Institute (SMHI).

Results

Physicochemical characterization

Chemical compositions of the three original slags are shown in Table 1. Calculated from

Table 1, CaO/SiO2 ratio is 2.4, 1.4 and 0.9 for EAF slag 1, EAF slag 2 and EAF slag 3, respectively. The content of iron oxide is highest in EAF slag 1, and the amount of MgO is highest in EAF slag 3. The chromium content is lowest in EAF slag 1.

Table 1. Chemical composition of the EAF slag samples.

8 The solubility of four major elements (Ca, Mg, Cr, Al) and one minor element (Mo), expressed as mg/kg of the element dissolved as a function of ageing time, together with pH and electrical conductivity, is shown in Figure 2. The leaching of calcium, chromium and aluminium decreased as the samples of all three slag aged, the leaching of molybdenum was more ore less constant (slightly decreasing), while the magnesium leaching increased over time. The pH as well as the electrical conductivity decreased.

There was good agreement with duplicate samples.

Figure 2. Results obtained from standard test leaching of investigated slag samples.

Mineralogical characterization

The XRD pattern of the original and aged slag samples are shown in Figure 3. All the samples consist largely of crystalline phases. According to Figure 3, four crystalline phases were identified in EAF slag 1: (1) a wustite type solid solution which, according to SEM and EDS analyses, contains a mixture of iron, magnesium and manganese, (2) - dicalcium silicate (-Ca2SiO4), (3) a spinel type solid solution which, according to SEM and EDS analyses, contains iron (FeFe2O4), and (4) merwinite (Ca3MgSi2O8). In EAF slag 2, three crystalline phases were detected: (1) gehlenite (Ca2Al2SiO7), (2) merwinite

(Ca3MgSi2O8) and (3) a spinel type solid solution which, according to SEM and EDS analyses, contains magnesium, manganese, chrome, iron and alumina

((Mg,Mn)(Cr,Fe,Al)2O4). Four phases were identified in EAF slag 3; namely, (1) monticellite (CaMgSiO4), (2) akermanite (Ca2MgSi2O7), (3) olivine ((Fe,Mg)2SiO4) and

9 (4) a spinel type solid solution which, according to SEM and EDS analyses, contains magnesium, manganese, chrome, iron and alumina ((Mg,Mn)(Cr,Fe,Al)2O4).

Figure 3. XRD patterns of the investigated slag.

After 6-12 months of ageing, a new mineral was formed on the surfaces of all three slag types. The new mineral is white and has a needle-like structure. XRD investigations of the slag samples reveal that calcium carbonate, CaCO3, has been formed on the surfaces,

Figure 4.

Figure 4. Optical micrographs and corresponding XRD pattern (white crystals) of EAF

1, 12-month sample.

TGA-MS

The results from the TGA-MS analyses are shown in Table 2. The weight loss due to calcination, m, is the difference in mass between the aged (24 months) and non-aged (0 months) samples at the given temperature range (500-1000°C) during the TGA-MS analyses. Using the method described by Huijgen et al. (2005), the weight changes in

Table 2 show that the degree of carbonation is highest for the aged (24 months) EAF slag

1 (1.54 wt %) followed by the aged EAF slag 2 (0.61 wt %) and the aged EAF slag 3

(0.31 wt %).

Table 2. TGA-MS results from the investigated EAF slag.

10 Temperature and precipitation

Figure 5 gives the precipitation and monthly average temperatures during the test period.

The total precipitation for the first and second year was 564 and 552 mm, respectively.

The highest and lowest temperatures measured during year one and two were 22.0/-

20.0°C and 22.0/-24.5°C, respectively.

Figure 5. Monthly average temperature and accumulative precipitation.

Discussion

A mineralogical interpretation of the solubility

The investigations with XRD and TGA were complimented with SEM studies in order to evaluate the impact of ageing on the matrix of the slags and the effect on leaching of elements.

EAF 1-3

During the 24 months of ageing, the materials were exposed to a large variation both in precipitation and temperature, as seen in Figure 5. All phases identified in the non-aged samples (0-month) using XRD, were also identified in the aged slag samples, Figure 3. In terms of stability, this means that no drastic change in mineralogy in the bulk of these materials occurs upon ageing. It can therefore be concluded that all reactions

(leaching/changes in mineralogy) only take place on the surfaces.

11 After ageing, the materials show large variations regarding leachability, according to

Figure 2. The electrical conductivity (total leachability) of the three different EAF slags shows a pronounced decrease after only the first six months. After 12 months of ageing, the electrical conductivity has decreased by a factor of 3.4, 3.8 and 3.0 for EAF slag 1,

EAF slag 2 and EAF slag 3, respectively. The EAF slags’ ability to influence (increase) the pH of the water to which they are exposed also decreases over time, according to

Figure 2. In the 24-month period during which the materials were exposed to weathering, pH decreased by approximately 1. A decrease in pH may influence the leaching of specific elements differently. As an example, the leachability of Mg can be considered.

According to the solubility diagrams shown in Figure 6, the dominant magnesium phase

-6 at pH 12 (10 M) will be Mg(OH)2 (s). If the pH is lowered to 11, the dominant magnesium phase is, instead, Mg2+.

When comparing to initial values (0 month), the leaching of chromium as well as molybdenum decreases (slightly) for all three slag types. As seen in Figure 2, the trend is more or less the same for all of the slags, even though the absolute values differ. For comparison, the values for inert landfill in Sweden, which are 0.5 mg/kg for both molybdenum and chromium, can be considered. Between 6-12 months, all slags are below this limit for chromium, while one of the slags, EAF slag 3, still is above the limit for molybdenum. As can be seen in Figure 6, the dominant phases of chromium in the pH

-2 range of these materials are Cr2O3(s) and CrO4 , depending on degree of oxidation.

Experiments conducted by Chaurand et al. (2007), have shown that chromium will be present as trivalent, and that its oxidation state in the slag does not change during water leaching and natural ageing of the solidified steel slag.

Figure 6. Pourbaix diagram (10-6 M) calculated in Factsage 6.1 (Bale et al. 2002). A)

Calcium, B) Magnesium, C) Aluminium and D) Chromium.

Both calcium and aluminium show similar leaching behaviour as chromium and molybdenum with respect to the duration of ageing, Figure 2. The calcium and aluminium leaching decrease by a factor of 3.0/9.0, 3.0/64.0 and 2.3/2.9 for EAF slag 1,

EAF slag 2 and EAF slag 3, respectively, after 24 months. According to Figure 6, the dominant calcium and aluminium phase in the solution, at the corresponding pH (Figure

2+ − -6 2+ 2), will be Ca and AlO2 at 10 M, respectively. The Ca is also the dominating calcium phase throughout the whole pH range, while the dominating aluminium phase is

− 3+ changed from AlO2 to Al(OH)3 (s) and finally Al as the pH decreases.

Magnesium is the only element that increases in leachability when ageing, according to

Figure 2. The leachability of magnesium seems to accelerate as the materials are aged.

This acceleration might be explained in terms of the decrease in pH and buffering capacity that occurs during ageing. As can be seen in Figure 6, non-soluble Mg(OH)2 will transform to soluble Mg2+ as the pH of the solution decreases, thereby accelerating the leachability of magnesium. The calcium, alumina and magnesium leaching are of greatest importance for understanding the leaching of EAF slags, because they often play an important role in the mineralogy.

When comparing mineralogy vs. leaching, it is worth mentioning that, according to Table

1, EAF slag 1 has approximately 1.8 times higher content of magnesium dissolved in its structure compared to EAF slag 2. However, concerning the leaching of magnesium, EAF

13 slag 2 leaches 2.4 times more magnesium than EAF slag 1 after 24 months of ageing. The same can be seen for chromium. EAF slags 2 and 3 have more or less the same chromium content. However, the leaching of chromium is 3.7 times higher from EAF 2 compared to

EAF 3, see Figure 2. Using magnesium as an example, the XRD analyses in Figure 3 reveal that the magnesium content in EAF slag 1 is mainly present in a solid solution containing iron and manganese ((Fe,Mg,Mn)O), while in EAF slag 2 it is mainly in the form of merwinite (Ca3MgSi2O8). This means, in terms of solubility, that the solid solution containing magnesium is less reactive compared to merwinite at this pH range.

Consequently, the amount of a certain element can never be correlated to the leaching without considering in what phase it is present.

Surface reactions

As Figure 4 reveals, calcium carbonate (CaCO3) was formed on the slag surfaces within

6-12 months of ageing. Thermodynamic calculations performed on these materials show that free lime does not exist and thereby cannot be the source of carbonation. These assumptions were also verified in XRD (Figure 3), where no free lime was identified.

The calcium source for the reaction is, instead, most likely correlated to the dissolution of calcium-rich silicates present in the slag, which has earlier been observed by Huijgen et al. (2006). They described this transformation as an aqueous mechanism of carbonation, and the reaction is known to occur in three steps:

+ → + ()+ 2− () CO 2 (g) H 2O(aq) 2H aq CO3 aq (1)

− + + ()→ 2+ ()+ + Ca silicate(s) 2H aq Ca aq silicate(s) H 2O(aq) (2)

14 2+ + 2− → ↓ Ca (aq) CO3 (aq) CaCO3 (s) (3)

2− In step one (1), the rainwater is saturated with CO2, forming CO3 ions in solution. Step two (2) corresponds to the actual leaching of Ca-rich minerals from the slag; in the case of EAF slag 1, -Ca2SiO4 and Ca3MgSi2O8; and for EAF slag 2, Ca2Al2SiO7 and

Ca3MgSi2O8; and for EAF slag 3, CaMgSiO4 and Ca2MgSi2O7. Step three (3) includes a simultaneous reaction between the products from step one and two, leading to the precipitation of calcite on the slag surfaces. Experiments performed in laboratory scale have shown that step three (3) is considered to be very rapid (Huijgen et al., 2005).

Huijgen et al. (2005), suggest that step two, the diffusion of calcium through the solid slag matrix towards the slag surface, appears to be the rate-determining step in the carbonation. As has been discussed earlier, the leaching of an element, in this case calcium, cannot be based only on the chemical analyses, Table 1. More specifically, the solubility of each calcium-containing slag mineral must be considered in order to explain the leaching behaviour and thereby the carbonation. In Table 2, it can be seen that the degree of carbonation when comparing the aged (24 month) against the non-aged (0 month) slag samples is highest for EAF slag 1 (1.54 wt%) followed by EAF 2 (0.61 wt%) and EAF 3 (0.38 wt%). The solubility of the Ca-rich slag minerals found in these materials is not reported in the literature. However, the results from this study indicate that the slag minerals found in EAF slag 1 (-Ca2SiO4, Ca3MgSi2O8) dissolve easier than those found in EAF slag 3 (CaMgSiO4, Ca2MgSi2O7).

Knowing that calcite mineral will be the dominating phase in water solutions in the range of pH 7-14, depending on Ca concentration, the precipitation of calcite on the surface of the slag will most likely continue as long as there are slag minerals rich in calcium still

15 dissolving. However, as the calcite layer increases, the surfaces of the slag grains will become less reactive, due to the formation of an insoluble surface layer. The extent of the decrease in total leachability that actually can be explained by the carbonation is, however, hard to distinguish. In Figure 7, it can clearly be seen that the aged sample of

EAF slag 1 has a smaller grain size and a more level surface structure as a result of the carbonation.

Figure 7. SEM micrographs (surface) of A) EAF 1, 0 month sample B) EAF 1, 24-month sample.

This study shows that the degree of carbonation (Table 2) that actually occurs during the

24 months of ageing differs greatly from the theoretically calculated values, in this case approximately 0.2-0.3 kg of CO2/kg of steel slag (17-23 wt% CO2). It can therefore be concluded that steel slags as a ballast material (course material) would not have any considerable carbon capturing capacity.

Beyond calcium carbonation, magnesium may also form carbonates, magnesite (MgCO3).

If a correlation is done in the same way as for calcium, the leaching (Figure 2) of magnesium would decrease as the degree of carbonation increases. As can be seen in

Figure 2, the leaching of magnesium instead increases as the pH decreases. In terms of carbonation, this means that magnesium has not been carbonated to the same extent as calcium. This was further verified when no magnesite was found during sample characterization. According to O’Conner et al. (2005), the carbonation of magnesium is

16 feasible but unlikely because of the relatively low content of magnesium compared to calcium in steel slags and the relatively low CO2 content in air. Typical pressures of CO2 needed for carbonating magnesium silicates via the aqueous carbonation route is a pCO2>100 bar and a long retention time.

In order to predict the long-term behaviour of materials such as steel slags, applied research such as this study must be complimented with fundamental studies in order to determine the properties of each separate mineral found in the slag.

17 Conclusions

Three different types of EAF slags have been evaluated with respect to their leaching properties when exposed to natural weather conditions during periods of up to 24 months.

From this investigation the following conclusions can be drawn:

1. When slag is aged its total leachability is dramatically lowered.

2. Steel slags should be aged at least 6 months before usage in external applications.

3. Slag material should be tested before use in external applications. A leaching test performed on a non-aged sample will not reflect how the material would behave after 6 month of ageing.

4. All pre-treatment of materials for external applications should be completed before the ageing period starts. If the pre-treatment is conducted after ageing, new fresh surfaces will be liberated and ageing process must be repeated.

5. The content of a certain element can never be directly correlated to the leaching without considering in what phase it is present and how that certain phase behaves in the slag system.

Clearly, the mineralogy of the surface changes as steel slags are aged. Some phases are dissolved (-Ca2SiO4, Ca3MgSi2O8),while new phases are formed (CaCO3). As slag is a complex material, each type of slag should be evaluated regarding its long-term properties before usage in external applications is considered.

18 Acknowledgements

We wish to thank MiMeR, VINNOVA and the Swedish Steel Producers’ Association, via

MISTRA and the Centre for Advanced Mining & Metallurgy (CAMM), for invaluable financial support and commitment. To our colleagues and company members, we extend our sincere thanks for their support and assistance.

19 References

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39, p 5736-5741.

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Melançon, J., Pelton, A.D. and Petersen, S., 2002. "FactSage Thermochemical Software and Databases", Calphad Journal, 62, p 189-228.

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Durinck, D., Engström, F., Arnout S., Heulens, J., Jones, P.T., Björkman, B., Blanpain,

B. and Wollants, P., 2008. Hot stage processing of metallurgical slags

Resources, Conservation and Recycling, 52 (10), p. 1121-1131.

20 EN 12457-2:2002., 2002. Characterisation of waste – leaching – compliance test for leaching of granular waste materials and sludges – Part 2: one stage batch test at a liquid to solid ratio of 10 l/kg for materials with particle size below 4 mm (without or with size reduction), European Committee for Standarization.

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1176.

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Huijgen, W.J.J. and Comans, R.N.J., 2006. Carbonation of steel slag for CO2 sequestration: leaching of products and reaction mechanisms. Environ. Sci. Technol.

40(8), 2790-2796.

Juckes, L.M., 2003. The volume stability of modern steelmaking slags, Mineral

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Malami, C.H. and Kaloidas, V., 1994. Carbonation and porosity of mortar specimens with pozzolanic and hydraulic cement admixtures. Cem. Concr. Res., 24, p 1444-1456.

21 Meima, J.A., Weijden van der, R.D., Eighmy, T.T. and Comans, R.N.J., 2002.

Carbonation processes in municipal solid waste incinerator bottom ash and their effect on leaching of copper and molybdenum. Appl. Geochem. 17(12), p 1503-1513.

Monaco, A. and Lu, W-K., 1994. The effect of cooling conditions on the mineralogical characterization of steel slag, Proc Int Symp Res Conserv Environ Technol Metall Ind, p

107-116.

Monaco, A. and Lu, W-K., 1996. The properties of steel slag aggregates and their dependence on the melt shop practice, Steelmaking conference proceedings, Vol. 79, p

701-711, A publication of the Iron & Steel Society, Pittsburgh.

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22 Reddy, K.J., Gloss, S.P. and Wang, L., 1994. Reaction of CO2 with alkaline solid wastes to reduce contaminant mobility. Water Res. 28(6), p 1377-1382.

23 Tables Table 1. Chemical composition of the EAF slag samples.

Samples CaO SiO2 MgO FeO MnO Al2O3 Cr2O3 Fe2O3 MoO3 TiO2 Femet

EAF 1 28.8 11.8 8.5 25.5 6.1 4.9 2.0 4.9 0.01 0.6 4.8

EAF 2 41.0 28.4 4.7 2.2 3.2 10.1 5.7 0.0 0.01 3.2 0.0 EAF 3 26.4 31.0 18.1 3.6 2.2 9.4 7.0 0.0 0.01 0.4 0.4

Table 2. TGA-MS results from the investigated EAF slag.

wt %

Samples m500-1000°C EAF 1 1.54 EAF 2 0.61 EAF 3 0.38

Figures

30 cm

Steel slag Net (mesh 0.5mm)

15 cm

Figure 1. Schematic picture of experimental setup.

Electrical conductivity pH

0 month 6 month 12 month 18 month 24 month 0 month 6 month 12 month 18 month 24 month

700 11.4

600 11.2 11 500 10.8 3 400 10.6

300 pH 10.4 s/cm 10.2 200 10 100 9.8 0 9.6 EAF 1 EAF 2 EAF 3 EAF 1 EAF 2 EAF 3

Ca leaching Mg leaching

0 month 6 month 12 month 18 month 24 month 0 month 6 month 12 month 18 month 24 month 700 25 600 20 500

400 15

300

mg/kg 10 mg/kg 200 5 100

0 0 EAF 1 EAF 2 EAF 3 EAF 1 EAF 2 EAF 3

Cr leaching Al leaching

0 month 6 month 12 month 18 month 24 month 0 month 6 month 12 month 18 month 24 month

10 100

1 10 mg/kg mg/kg 0.1 1

0.01 0.1 EAF 1 EAF 2 EAF 3 EAF 1 EAF 2 EAF 3

Mo leaching

0 month 6 month 12 month 18 month 24 month

0.1 mg/kg

0.01

0.001 EAF 1 EAF 2 EAF 3

Figure 2. Results obtained from standard test leaching of investigated slag samples.

Figure 3. XRD patterns of the investigated slag.

Figure 4. Optical micrographs and corresponding XRD pattern (white crystals) of EAF 1, 12-month sample.

20 1200

15 Avg. temp 1000 10 800 5 600 0 400 -5

Average temperature(°C) -10 200 precipitation (mm) precipitation Accumulative

-15 0 1 6 11 16 21 month

Figure 5. Monthly average temperature and accumulative precipitation.

A B Ca-H2O, 25 C Mg-H2O, 25 C 2.0 2.0

1.5 1.5 CaO2(s)

1.0 1.0

0.5 0.5

Ca[2+] 0 0 Mg[2+] E(volts) E(volts) Mg(OH) (s) CaOH[+] 2 -0.5 -0.5

-1.0 -1.0

-1.5 -1.5

MgH2(s) CaH2(s) -2.0 -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

C D Al-H2O, 25 C Cr-H2O, 25 C 2.0 2.0

1.5 1.5 HCrO 4[-]

CrO [ -] 1.0 1.0 4 2

0.5 0.5

Al[3+] Cr(OH)[2+] Al(OH)3(s) 0 0 AlO 2[-]

E(volts) E(volts) Cr2O3(s)

-0.5 -0.5

Cr[2+] -1.0 -1.0

-1.5 -1.5 Cr(s)

Al(s) -2.0 -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

Figure 6. Pourbaix diagram (10-6 M) calculated in Factsage 6.1 (Bale et al. 2002). A) Calcium, B) Magnesium, C) Aluminium and D) Chromium.

Figure 7. SEM micrographs (surface) of A) EAF 1, 0 month sample B) EAF 1, 24-month sample. Paper V

A study of the solubility of pure slag minerals

F. Engström, D. Adolfsson, C. Samuelsson, Å. Sandström and B. Björkman

Submitted 2010 (Minerals Engineering)

A STUDY OF THE SOLUBILITY OF PURE SLAG MINERALS

F. Engström1a, D. Adolfssonb, C. Samuelssona, Å. Sandströma and B. Björkmana a Division of Minerals and Metallurgical Engineering, Luleå University of Technology b SSAB EMEA,

SE-971 87 Luleå

Sweden

1Corresponding author. [email protected], tel.;+46 920 491388; fax; +46 0920 491199 Abstract

Large amounts of oxidic by-product are annually produced by the steel industry worldwide. By far the largest in volume is slag, generated from different stages of steel production. In order to avoid landfilling, steelmakers usually try to process the slag into useful resources that can be used externally. However, leaching of different metals can sometimes be a problem. Since steel slags are a mixture of numerous types of minerals, the solubility of each mineral will affect the outcome of the leachability. The aim of this study was to investigate how six common slag minerals behave during dissolution.

Mayenite (Ca12Al14O33), merwinite (Ca3MgSi2O8), akermanite (Ca2MgSi2O7), gehlenite

(Ca2Al2SiO7), -dicalcium silicate (-Ca2SiO4) and tricalcium aluminate (Ca3Al2O6) were synthesized and their dissolution was evaluated through titration using HNO3 at constant pH. Acidic to alkaline pHs (4, 7 and 10) were selected to investigate the solubility of the minerals under conditions comparable to those prevailing in newly produced slags, and one pH value, representing acid conditions. It can be concluded that all six minerals behave differently when dissolving and that the rate of dissolution is generally slower at higher pH values, which are normal in the case of steelmaking slags. At pH 10, the solubility of merwinite, akermanite and gehlenite is considered slow. The dissolution of

-Ca2SiO4 is not affected in the same way as the other minerals when the pH is changed.

Key words: slag, mineral, solubility, leaching

2 1. Introduction

In 2009, the annual production of crude steel worldwide reached the level of 1,223 million tonnes. For each tonne of crude steel manufactured, depending on process, approximately 200-500 kg of solid residue material, mainly in the form of oxides, is produced [1]. By far the largest in volume of these oxides is slag, generated in different stages of steel production. The main purpose of slags in pyrometallurgical processes, such as the basic oxygen furnace (BOF) and the electric arc furnace (EAF), is to extract unwanted elements/compounds from the steel bath, prevent metal oxidation and limit heat loss from the steel. Accumulation of minor elements in the slag often limits the possibility to reuse the slag within the process. In order to avoid landfilling, the steelmakers usually try to process the slag into useful materials. During the last 35 years, extensive work has been conducted in order to develop new slag products. Research has, for example, shown that due to its high strength and durability, steel slag is often a suitable material in various construction applications and can replace gravel and rock [2], thereby saving natural recourses. However, leaching of metals such as chromium and volume instability (volumetric expansion and disintegration) are technical and environmental properties of the slag that must be avoided [3-7].

The leaching from steel slags is characterized as a surface reaction, followed by a solid- solid diffusion process, in order to retain equilibrium in the materials [8]. Minimization of the surface area and/or formation of a less reactive surface layer on the slag can therefore be assumed to reduce leachability. This implies that slag disintegration will affect leachability, since the specific surface area increases as the slag is pulverised. One way of introducing a less reactive surface layer on the slag is by letting the slag react with CO2

3 (g), forming a protection layer of calcium carbonates, CaCO3, on the surface of the slag.

Research has shown that carbonation of alkaline solid material can lead to an improvement in leaching properties [9,10]. The mechanism behind the formation of

CaCO3 depends on several factors such as temperature, particle size, porosity and CO2 diffusivity [11-13]. The diffusion of calcium through the solid slag matrix, towards the surface, appears to be the rate-determining step in carbonation, implying that the solubility of the different calcium-containing minerals in the slag will affect the outcome of the carbonation of the slag surface [11]. The possibility to simulate the leaching behaviour from aged steel slags, using geochemical modelling has been investigated

[14,15]. These types of simulations sometimes fail to fully describe these complex systems, mostly because available data (solubility/thermodynamic/sorption) for the minerals occurring in the slag systems are often incomplete or missing. The possibility to use steel slags for neutralization of industrial effluents has been described in reference

[25,26]. The steel slags investigated had in general a high neutralization capacity, but the rate of dissolution was much slower compared to the reference material used, Ca(OH)2.

Microprobe measurements performed on synthetically manufactured steel slags have shown that chromium can be enriched in numerous phases e.g., spinels (AB2O4), bredigite (Ca7MgSi4O16), merwinite (Ca3MgSi2O8) and wollastonite (CaSiO3) [16]. As in the case of carbonation, this means that the solubility of the different chromium- containing slag minerals must be known before the leaching of metals e.g., chromium, can be explained completely. In order to gain a better understanding of the solubility of individual slag minerals, six common slag minerals, mayenite (Ca12Al14O33), merwinite

(Ca3MgSi2O8), akermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), -dicalcium silicate

4 (-Ca2SiO4) and tricalcium aluminate (Ca3Al2O6) were synthesized and their dissolution was evaluated through titration using HNO3 at constant pH. Acidic to alkaline pHs (4, 7 and 10) were selected to investigate the solubility of the minerals under conditions comparable to those prevailing in newly produced slags, and one pH value representing acid conditions.

5 2. Experimental procedure

2.1 Synthesis of slag minerals

Mayenite, merwinite, akermanite, gehlenite, -dicalcium silicate and tricalcium aluminate, all typical slag minerals, were synthesized in order to investigate the solubility of each mineral as a function of pH and time. Analytical grade calcium carbonate

(CaCO3), alumina (Al2O3), magnesium hydroxide (Mg(OH)2) and quartz (SiO2) were used for synthesis. For each mineral, the chemicals were stoichiometrically blended and formed into a pellet to achieve good contact between the particle surfaces. The parameters used for synthesizing the minerals are shown in Table 1. All six minerals were fired in air atmosphere once, except merwinite, which was fired twice, with grinding between firings.

Table 1. Parameters used in minerals synthesis.

2.2 X-ray diffraction

The purity of the synthetic minerals was confirmed by powder x-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer, equipped with copper K radiation. The

XRD pattern was recorded in the 2-theta range 10 to 90°, counting 1 sec/step.

2.3 Titration

In order to investigate the dissolution rate as a function of pH and time for each of the minerals, titration at constant pH 4, 7 and 10 was conducted using a Radiometer

Copenhagen ABU 901 Autoburette attached to a Radiometer Copenhagen PHM 290 pH

6 meter. Assayed HNO3, 0.1 M was used as titrant. The choice of HNO3 was made in order to minimize possible formation of complexes. For each experiment, 50 ± 2 mg of mineral was combined with 100 ml of deionized water (Milli-Q). Before the minerals were added to the reaction vessel, the pH was adjusted to the corresponding value of the titration (pH

4, 7 or 10) using 0.1 M NaOH and HNO3, respectively. The titration was performed on a sized fraction, 20-38 m. The sizing was carried out by grinding and sieving the minerals in ethanol. The pH electrode was calibrated before each experiment with adequate standard solution (pH 4, 7 or 10). The temperature in the reaction vessel was kept constant (25°C) using a water bath. A magnetic stirrer was used for mixing during titration and the stirring speed was kept constant throughout the titration. Nitrogen was injected in the reaction vessel, protecting the system from CO2 and concomitant formation of carbonates. The time of titration was 40 hours and the consumption of acid was logged every fifth minute.

7 3. Results

3.1 XRD

The XRD patterns of the synthetically manufactured slag minerals are shown in Figure 1.

The main peaks corresponding to the manufactured minerals are marked with a star in the diffractogram, Figure 1. As seen in the Figure 1, all six minerals are relatively pure with, in some cases, only small amounts of non-reacted chemicals present. In merwinite small amounts of non-reacted MgO were detected. In -dicalcium silicate minor amounts of non-reacted CaO were detected, and in tricalcium aluminate traces of CaAl2O4 were detected.

Figure 1: XRD pattern of synthetic minerals.

3.2 Titration

The titration curves of the synthetically manufactured minerals and their pH dependence are shown in Figure 2. The titration for each of the six minerals was repeated twice in order to optimize the system. According to Figure 2, the dissolution rate varies between the different minerals. The dissolution rate for all six investigated minerals also shows a strong dependency on pH.

Figure 2: Titration curves of synthetic minerals.

8 According to Table 2, the theoretical amount of 0.1 M HNO3 needed for complete dissolution of 50 mg of each mineral is 23.8, 12.2, 11.0, 18.2, 11.6 and 22.2 ml for mayenite, merwinite, akermanite, gehlenite, -dicalcium silicate and tricalcium aluminate, respectively, assuming that all elements dissolve. These theoretical values are based on, for aluminium the formation of Al3+, for calcium the formation of Ca2+ and for magnesium the formation of Mg2+ according to Eq. 1-3. The silica released from the silica-containing minerals dissolves in water according to Eq. 4, while aluminium at neutral pH values precipitates as hydroxide according to Eq. 5.

+ 2+ (1) CaO (s) + 2H Ca + H2O

+ 2+ (2) MgO (s) + 2H Mg + H2O

+ 3+ (3) Al2O3 (s) + 6H 2Al + 3 H2O

(4) SiO2 (s) + 2H2O Si(OH)4(aq)

3+ + (5) Al + 3H2O Al(OH)3(s) +3H

Table 2. Theoretical amount of acid needed for complete dissolution of calcium, magnesium and aluminium in the minerals.

Thus, dissolution of silica is a pH-independent reaction, while precipitation of aluminium hydroxide generates acid. At alkaline pH values the silica and aluminium react further with the release of more acidity (protons), Eq. 6-7.

- + (6) Si(OH)4(aq) SiO(OH)3 + H

- + (7) Al(OH)3(s) +H2O Al(OH)4 + H

9 4. Discussion

4.1 Comparison with the solubility of the pure oxide components

In order to further evaluate the impact of pH and time on the rate of dissolution of the individual synthetically manufactured slag minerals, predominance (pourbaix) diagrams of the end components (Ca, Mg, Al, Si) were used. The pourbaix diagram is an equilibrium diagram, reproducing the dominant phases of a certain element in an aqueous electrochemical system. Although a pourbaix diagram only reflects the most dominant phases in the system, it is a good tool for interpreting the dissolution of pure minerals.

The concentration used when calculating the pourbaix diagrams (10-6 M) was chosen to reflect the concentrations obtained in the initial stage of the dissolution of the six slag minerals.

Mayenite (Ca12Al14O33)

Mayenite is a common mineral found in ladle slags, produced during the refining of steel

[17,18]. The crystal structure of mayenite is known to be isometric-hextetrahedral [27].

According to [19-21], mayenite is considered hydraulic and is known to react rapidly with water. In Figure 2, it can be seen that the reactivity varies for the mayenite phase, depending on pH. Initially, the reactivity is similar at pH 4 and pH 7, while the initial reactivity tends to be lower at pH 10. At approximately 6 ml HNO3 consumed, the slope of the titration curve decreases at pH 4, while the reactivity at pH 7 stays intact and continues until the consumption of acid stops, at approximately 8.6 ml HNO3 consumed.

At pH 4, the reaction continues throughout the whole titration, consuming approximately

16.7 ml HNO3. At pH 10, the dissolution rate is divided into three different stages, the

10 first and the third stages having similar titration slopes, while the second stage has a slower dissolution rate. When the pH 10-titration stops consuming acid, the reaction has consumed approximately 7.9 ml HNO3, which can be compared with the theoretically calculated value, Table 2. The lower consumption of acid for pH 10 compared to pH 7 can partly be explained by the complex solubility of one of the elements in mayenite, namely aluminium. According to the pourbaix diagram shown in Figure 3C, the dominant aluminium species in solution depends on pH. At low pH Al3+ is the dominant species,

- while Al(OH)3(s) is the most stable species between pH 5-9 and Al(OH)4 above pH 9.

Based on the pourbaix diagrams, the only ion that is soluble to a greater extent at pH 7 is

2+ Ca , since the aluminium part forms solid Al(OH)3. The theoretical consumption of 0.1

M HNO3 to reach complete reaction with lime at pH 7 is 8.7 ml, which correlates well with the measured value. At pH 4, the initial slope of the titration curve is similar to the curve obtained at pH 7, suggesting that calcium is dissolved initially. At pH 10 the

2+ - reaction shows three different stages. According to Figure 3, Ca and Al(OH)4 are the dominating species, meaning that the system will generate protons through the formation

- of Al(OH)4 , Eq. 7. The shape of the pH 10 titration curve suggests that the first stage can be correlated to the dissolution of Ca2+, the second stage, the simultaneous dissolution of

2+ - 2+ Ca and Al(OH)4 and the third stage, dissolution of Ca . The change in slope occurring between stages two and three could be explained by the limited solubility of aluminium.

The differences in HNO3 consumption between pH 7 and pH 10 reflect the amount of

- Al(OH)4 formed.

11 Figure 3: Pourbaix diagrams (10-6 M) calculated in Factsage 6.1 [22]. A) Calcium, B)

Magnesium, C) Aluminium and D) Silicon.

Merwinite (Ca3MgSi2O8)

Merwinite is a mineral that is commonly found in steel slag arising from the production of stainless steel [17, 23]. The crystal structure of merwinite is known to be monoclinic- prismatic [27]. In Figure 2, it can be seen that the dissolution rate for the merwinite phase is pH-dependent. The dissolution is slightly faster at pH 4 than at pH 7 and significantly slower at pH 10. At pH 4 and pH 7, the system stops consuming acid after approximately five hours and with 11.5 ml HNO3 consumed, while at pH 10 the dissolution is continuous through the whole titration, with slow kinetics, consuming approximately 2.4 ml acid. The theoretically calculated values for the consumption of acid, Table 2, are in good agreement at pH 4 and pH 7, while a difference can be seen at pH 10. According to

Figure 3, merwinite has three active components at pH 10, namely calcium, magnesium and silicon. From a thermodynamic point of view, Figure 3, it is reasonable to believe that a slower dissolution of magnesium compared calcium and the simultaneous formation of protons, according to Eq. 6 is believed to explain the appearance of the pH

10 titration curve.

Akermanite (Ca2MgSi2O7)

Akermanite is a common mineral found in steel slag from the production of black and alloyed steel [17, 23]. The crystal structure of akermanite is known to be tetragonal- scalenohedral [27]. In Figure 2, it can be seen that the dissolution rate for the akermanite

12 phase is pH-dependent. In this case, the kinetics are considerably faster at pH 4 than at pH 7 and pH 10. At pH 4 and pH 7, the system stops consuming acid after approximately

10.7 ml HNO3 has been added while at pH 10 the reaction continues through the whole titration, consuming approximately 0.5 ml acid. The theoretically calculated values for the consumption of acid, Table 2, are in good agreement for pH 4 and pH 7, while a difference can be seen at pH 10. According to Figure 3, as in the case of merwinite, akermanite has three active components at pH 10, namely calcium, magnesium and silicon. In the case of akermanite, it is also reasonable to believe that it is the dissolution of magnesium and deprotonation of silica from the akermanite that will decide the dissolution rate. The hypothesis that the rate of dissolution for akermanite and merwinite is limited by the solubility of magnesium from the mineral is strengthened when the pH 7 titration curves of akermanite and merwinite are compared. Both minerals stop consuming HNO3 near the theoretical maximum acid consumption, cf. Table 2, meaning that the dissolution of silica is not affecting the solubility of the mineral to a greater extent. It can be understood from Figure 3 that at pH 7, there is a larger driving force for dissolution of calcium compared to magnesium. The concentration of magnesium is higher for akermanite compared to merwinite; it is therefore believed that the slower dissolution rate of akermanite compared to merwinite is a result of the magnesium dissolution.

Gehlenite (Ca2Al2SiO7)

Gehlenite is a typical slag mineral that is found in steel slag from the production of black and alloyed steel [17]. The crystal structure of gehlenite is known to be tetragonal-

13 scalenohedral [27]. Contrary to mayenite, gehlenite is not considered hydraulic [24]. In

Figure 2, it can be seen that the dissolution rate varies for the gehlenite phase, depending on pH. The dissolution rate is faster at pH 4 compared to pH 7 and pH 10. The volume of acid consumed was approximately 3.8 ml, 0.1 ml and 0.3 ml at pH 4, pH 7 and pH 10, respectively. Independent of pH, the solubility was very low in comparison with the theoretical calculated value, cf. Table 2. According to Figure 2, the volume of HNO3 consumed was slightly higher at pH 10 compared to pH 7, although the pH 10 system

- - should generate protons through the formation of Al(OH)4 and SiO(OH)3 , Eq. 6-7 and

Figure 3. This implies that aluminium and/or silicon bound in the crystal structure must be dissolved before the dissolution of the calcium contained in gehlenite can proceed.

-dicalcium silicate (-Ca2SiO4)

-dicalcium silicate is typically found in steel and ladle slags from the production of alloyed steel [17]. The crystal structure of -dicalcium silicate is known to be orthorhombic [16]. According to Figure 3, -dicalcium silicate has one active component in the pH ranges 4 and 7, namely calcium and two active components at pH 10. In Figure

2 it can be seen that the initial slope of the titration curve is similar at all three pH values investigated, meaning that the dissolution rate for -dicalcium silicate is not dependent on pH in the same way as the other investigated minerals. When the system stopped consuming acid the consumption of 0.1 M HNO3 was approximately 11.1 ml, 11.1 ml and 8.4 ml at pH 4, pH 7 and pH 10, respectively. The theoretically calculated maximum value for the acid consumption, Table 2, is in good agreement at pH 4 and pH 7, while a difference can be seen at pH 10. The low amount of acid consumed at pH 10 can be

14 explained by the simultaneous formation of protons due to the dissolution of silicon, Eq.

Tricalcium aluminate (Ca3Al2O6)

Tricalcium aluminate is a common mineral found in ladle slags produced during the refining of steel [17,18]. According to [19], tricalcium aluminate is considered to be hydraulic and the reaction with water is known to be rapid. In Figure 2, it can be seen that the pH dependence of the dissolution rate for the tricalcium aluminate phase varies in a similar way as mayenite. At pH 10, the rate of dissolution is divided into three different stages. According to Figure 2, the first stage has a steeper slope than the third stage, while the second stage (0.5-8 h) has a flatter slope. At the end of titration, the consumption of

0.1 M HNO3 is approximately 20.5 ml, 10.9 ml and 9.8 ml at pH 4, pH 7 and pH 10, respectively. These values differ with the theoretically calculated values, cf. Table 2. This can be interpreted in the same way as for mayenite. At pH 4, simultaneous dissolution of aluminium and calcium almost reaches the theoretical acid consumption. The theoretical consumption of 0.1 M HNO3 to reach complete reaction at pH 7, assuming that only calcium dissolves is, according to Table 2, 11.1 ml, which correlates well with the measured value, Figure 2. At pH 10, the reaction shows three different stages. According

2+ - to Figure 3, Ca and Al(OH)4 are the dominating species, implying that the system will

- generate protons through the formation of Al(OH)4 , Eq. 7. The shape of the pH 10 titration curve suggests that the first stage can be correlated to the dissolution of Ca2+, the

2+ - second stage to the dissolution of Ca and Al(OH)4 and during the last stage the dissolution of Ca2+. The change in slope occurring between stage two and three is

15 - probably due to the limited solubility of Al(OH)4 . The differences in HNO3 consumption

- between pH 7 and pH 10 reflect the amount of Al(OH)4 formed.

Although several of the investigated minerals show similarities in oxidic composition, they differ considerably with respect to the rate of dissolution. It is therefore obvious that the crystal structure of the minerals plays an important role in the dissolution. It is known from previous work [25,26] that the of dissolution of slags, kinetically, is a slow process.

As an example, at pH 3 it took 480 days for a BOF (Basic Oxygen Furnace) slag to reach equilibrium. In comparison to 480 days, 40 hours is a relatively short period. It is therefore believed that all of the investigated minerals would reach the theoretical consumption of acid if the time of titration were longer.

4.2 Comparison of the solubility of the different minerals.

The purpose of this study was to take the understanding of the solubility/leaching that occurs from ordinary steel slags one step further, and actually investigate how individual slag minerals behave during dissolution. The experience gained from these experiments is that all mineral behave differently regarding dissolution rate. A rapid acid consumption implies a high reactivity of the mineral, regardless of which components dissolve from the minerals e.g., CaO, MgO, Al2O3 and SiO2. At pH 4, the dissolution of the investigated minerals, is generally considered as being rapid with a high acid consumption. The rate of dissolution of the different minerals differs, where tricalcium aluminate dissolves at the fastest rate and gehlenite at the slowest. At pH 10, the typical pH of a leachate from a newly produced steel slag, the dissolution is slower compared to pH 4 and with a lower

16 total acid consumption. Merwinite, akermanite and gehlenite show a pronounced decrease in solubility at pH 10 compared to pH 4.

In the case of carbonation, it has been shown that the dissolution of calcium from the slag will determine the outcome of the carbonation [11]. In comparison with the results from this study, a steel slag having -dicalcium silicate as a main mineral, would form carbonates both faster and to a greater extent compared to steel slags having merwinite as the main mineral, Figure 2. This means that an AOD (Argon Oxygen Decarburization) slag containing a high amount of -dicalcium silicate would carbonate to a greater extent compared to an EAF (Electric Arc Furnace) slag from stainless steel production, containing a high amount of merwinite [17]. To minimize the initial leaching of trace metals, often present as solid solutions in the main mineral of the slag, the solubility of the host mineral should be as low as possible, which is contradictory to the conditions needed for carbonation. As mentioned earlier, it has been shown [16] that chromium can be enriched in the merwinite phase. According to Figure 2, the rate of merwinite dissolution at high pH is low but not negligible. This means that the solubility of merwinite in a merwinite-based slag system can partly explain some of the chromium leaching occurring from these slags. To achieve an appropriate material for usage in external applications with regard to leaching properties, it can be concluded that an optimized slag system should include a mixture of stable minerals suitable for capturing metals together with easily dissolved calcium-rich minerals, contributing to the formation of a less reactive surface layer in the form of carbonates.

17 5. Conclusions

The results discussed in this paper give valuable information on how different slag minerals behave during dissolution. The following conclusion can be made from this investigation:

1. All the six minerals investigated behave differently with respect to the rate of

dissolution.

2. The rate of dissolution is in general slower at high pH values, which are normal in

the case of steelmaking slags.

3. The crystal structure of the minerals in the slag plays a major role in determining

the kinetics of their dissolution.

4. In order to achieve a slag suitable for a specific application, a mixture of suitable

minerals should be chosen with respect to the application.

18 Acknowledgements

We wish to thank MiMeR, VINNOVA, the Swedish Steel Producers’ Association (TO

55) via MISTRA and the Centre for Advanced Mining & Metallurgy (CAMM) for invaluable financial support and commitment. We also extend our gratitude to colleagues and company members for their support and assistance.

19 References

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107-116.

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Conservation and Recycling, 52 (10), p 1121-1131.

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Carbonation processes in municipal solid waste incinerator bottom ash and their effect on leaching of copper and molybdenum. Appl. Geochem. 2002, 17(12), p 1503-1513.

[10] K. J. Reddy, S. P. Gloss and L. Wang., 1994. Reaction of CO2 with alkaline solid wastes to reduce contaminant mobility. Water Res. 1994, 28(6), p 1377-1382.

[11] W. J. J. Huijgen, G.J. Witkamp and R. N. J. Comans., 2005. Mineral CO2 sequestration by steel slag carbonation. Environ. Sci. Technol. 2005, 39(24), p 9676-

9682.

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1176.

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Simultaneous application of dissolution/precipitation and surface complexation/surface precipitation modelling to contaminant leaching, Environmental Science & Technology,

39, p 5736-5741.

[15] W. J. J. Huijgen and R. N. J. Comans., 2006. Carbonation of steel slag for CO2 sequestration: leaching of products and reaction mechanisms. Environ. Sci. Technol.

2006, 40(8), p 2790-2796.

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Promote Recycling, ISBN 978-94-6018-005-7.

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Björkman, 2006. Characteristics of steel slag under different cooling conditions, Waste management, 27, p 1335-1344.

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24 Tables Table 1. Parameters used in minerals synthesis.

Synthetic minerals Manufacturing Time. h Cooling temperature °C Mayenite 1400 24 Slow

Merwinite 1500 48 Slow Akermanite 1400 24 Slow

Gehlenite 1410 72 Slow -dicalcium silicate 1400 48 Slow Tricalcium aluminate 1250 48 Fast

Table 2. Theoretical amount of acid needed for complete dissolution of calcium, magnesium and aluminium in the minerals.

Synthetic minerals Volume of acid needed (ml) for neutralization of 50 mg of Total volume (ml) Ca2+ Mg+2 Al3+ Mayenite 8.66 - 15.15 23.81 Merwinite 9.12 3.04 - 12.16 Akermanite 7.34 3.66 - 11.00 Gehlenite 7.30 - 10.95 18.25 -dicalcium silicate 11.62 - - 11.62 Tricalcium aluminate 11.10 - 11.10 22.20

Figure 1. XRD patterns of synthetic minerals.

Figure 2 Titration curves of synthetic minerals.

A B Ca-H2O, 25 C Mg-H2O, 25 C 2.0 2.0

CaO (s) 1.5 2 1.5

1.0 1.0

0.5 0.5

Ca[2+]

Mg[2+] 0 CaOH[+] 0 E(volts) E(volts) Mg(OH)2(s)

-0.5 -0.5

-1.0 -1.0

-1.5 -1.5

MgH2(s) CaH2(s) -2.0 -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

C D Al-H2O, 25 C Si-H2O, 25 C 2.0 2.0

1.5 1.5

1.0 1.0

0.5 0.5

Al[3+]

Al(OH) 3(s)

0 0 H4SiO4(aq) Al(OH)4[-]

E(volts) E(volts) SiO(OH) 3[-]

-0.5 -0.5

-1.0 -1.0

SiH4(g) -1.5 -1.5

Al(s) -2.0 -2.0 4 6 8 10 12 14 4 6 8 10 12 14 pH pH

Figure 3: Pourbaix diagrams (10-6 M) calculated in Factsage 6.1 [22]. A) Calcium, B) Magnesium, C) Aluminium and D) Silicon. Paper VI

Stability of spinels in a high basicity EAF slag

S. Mostaghel, F. Engström, C. Samuelsson and B. Björkman

Proceedings of 6th European Slag Conference, October 19-22, 2010, Madrid, Spain

S. Mostaghel, F. Engström, C. Samuelsson, B. Björkman

STABILITY OF SPINELS IN A HIGH BASICITY EAF SLAG

Division of Extractive Metallurgy, Luleå University of Technology, Luleå, 971 87, Sweden

Abstract

Due to environmental regulations, steel producers, in Europe, are obliged to reduce the amount of landfilled material, which is mainly slag. By fulfilling technical and environmental criteria, slag can be used in civil engineering applications. One of the most important environmental considerations is the leaching behaviour of the slag, especially with respect to chromium. A considerable research effort has been devoted to decrease the leaching of chromium from the slags by forming stable spinel phases. Most of the existing work focused on spinel forming agents. In this paper, influences of three different already formed spinels, in three different amounts, on an EAF slag from a low alloyed steel production are investigated. After re-melting and solidification, mineralogical properties of the samples were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Only one of the additives resulted in formation of distinguishable chromium rich spinels, which could immobilize chromium. The results are discussed by comparison with stable phases at equilibrium based on thermodynamic calculations.

Keywords: Immobilization, Stabilization, Chromium, Electric Arc Furnace, Slag, Spinel

1 1. Introduction and background

Environmental regulations restrict industries to decrease the amount of landfill waste. As an example, Swedish parliament has established new laws to obtain a so called “good built Environment”. Accordingly:

• by 2010, at least 15% of the total aggregates must be re-use materials [1] • the landfill tax has increased over 70% within the last decade [2]

Currently, in Europe, iron and steel-making slag is utilised as a component of both cement and concrete [3-4]; it is also used in different products for building roads and pavements since it can improve the adhesion of asphalt to the particles and gives a harder end product [5].

Replacement of blast furnaces and LD converters with electric arc furnaces (EAF), over recent decades, has led to a drastic increase in the amount of EAF oxidizing slag or black slag. Due to usage of different composition of this slag, black slag has different properties than the slag from ore based steel making and stainless steel making [3].

Treatment of slag is important both from economical and environmental point of view. According to environmental considerations for use of the steel-making slags, the remaining chrome in the slag should be blocked in stable mineral phases to suppress its leaching [7]. A method has been developed to bind the remaining chrome content of the stainless steel slag in stable mineral phases by addition of spinel forming oxides to the liquid slag before, during, or after tapping. The effects of different spinel forming agents are described by a factor named “factor sp”; the formula below elaborates it. Higher values of factor sp (comparatively) mean a lower chrome leaching.

factor spopt. = 0.2×MgO + 1.0×Al2O3 + xopt. ×FeOn – 0.5×Cr2O3 [%-wt.]

(where xopt. depends from the oxidation state of the EAF-slag) [7].

For industrial purposes, addition of pure oxides to the slag is too expensive.

Therefore, MgO-, Al2O3-, and FeOn containing agents have been investigated and 2 they have proved their suitability as spinel forming materials to decrease the chrome leaching from stainless steel slag [7]. The efficiency of spinel forming increases in the order of MgO, Al2O3, and FeOn, respectively. It has been shown that spinel phase binding is strong enough to block chromium even for long periods of time [6].

In this paper, suitability of three different already formed spinels is investigated and their stability in a black slag is studied by scanning electron microscopy (SEM) and X- ray diffraction (XRD). Experimental results were then compared with thermodynamic calculations using FactSageTM 6.1. Earlier work established the validity of the sp factor for the stainless steel slag; in the current paper stability of spinels in a high basicity EAF slag is examined.

2. Materials and Methodology

A chromium containing EAF (black) slag with the chemical composition given in Table 1 was used in this study.

Comp. CaO MgO Al2O3 SiO2 Cr2O3 MnO Fe met. Fe2O3 FeO wt-% 36.7 11.2 6.0 14.0 3.2 5.1 0.6 10.3 10.9

Table 1: Chemical analysis of the EAF slag

Three different spinels were added to slag in three different fractions and the mixtures were re-melted and re-solidified. Utilized additives were:

1. Magnetite (FeFe2O4); a product from LKAB, Sweden (100% FeFe2O4). 2. Aluminium containing spinel; chemical metal based additive from Alfa Aesar,

Germany, (100% MgAl2O4). 3. Chromium containing spinel from chromite ore used in ferrochromium production, chemical composition of which is shown in Table 2.

Comp. Cr2O3 FeO SiO2 Al2O3 MgO CaO wt-% 29.5 12.6 9.1 22.9 20.0 0.6

Table 2: Chemical composition of the chromium containing spinel

The method that was used in this work in order to block chromium in stable minerals was replacement of the unbound chromium in slag with iron and aluminium in spinels

3 as stable trivalent chromium ion. If additives react appropriately with slag, and the replacement of elements accomplish successfully, the chromium will be stabilized in slag and its leachability will decrease considerably. In order to define the fraction of each additive in the mixtures, molar balance of the elements that are supposed to be substituted was the criteria. If desired amount of slag is mixed with stoichiometric amount of spinel required for such a substitution, the sample is called 1:1 ratio. If we decrease the amount of spinel to a half or a quarter, while slag is still in the same amount, the mixture is called 1:0.5 or 1:0.25 ratio, respectively. For each of the additives, three different ratios of slag/additive and a reference sample, which was merely slag without any additive, were prepared. Therefore, for each mixture, four samples were available, which is called a group of samples. All samples were placed in Magnesium crucibles. Re-melting was done in an induction furnace with Ar as the protection gas. In each run in the furnace, “one group of samples” was heated up to 1600C by 200C/h, and then it was kept at this temperature for half an hour. Molten materials were cooled and solidified slowly (inside the furnace) down to room temperature.

Mineralogical studies of the materials were done using Scanning electron Microscopy (SEM) and X-ray diffraction (XRD). SEM instrument was a Philips XL 30. Qualitative elemental analysis were performed with an energy dispersive spectrometer (EDS) fitted with an Everhart and Thornley detector behind a beryllium window.

Utilized XRD instrument was a Siemens D5000 X-ray diffractometer, copper K radiation with accelerating voltage of 20 kV. XRD patterns were recorded from 25 to 50° in 0.02° step by counting 8 s/step. The phase identification was made by the reference patterns in an evaluating program supplied by the instrument manufacturer.

To determine the stable solid phases formed upon cooling of the different slags, quantitative equilibrium calculations were made in FactSageTM 6.1, using compound database FS53base.cdb, FToxid53base.cdb and solution databases of FToxid53soln.sda. FToxid-slagA, FToxid-MeO and FToxid-SpinA. During calculation, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplications in the data set.

4 3. Results

3.1. Slag/FeFe2O4 and Slag/MgAl2O4 mixtures

There was a good agreement between the XRD and SEM analyses suggesting that calcium silicate and the wustite type solid solutions, (Mn,Fe,Mg)O, are the main phases of the investigated black slag. All samples of these two groups show an identical structure, where no spinel could be detected. Cr is enriched in small spots, which are distributed evenly in the wustite type solid solution. As an example, the

1:0.5 ratio of the slag/MgAl2O4 group is shown in Figure 1.

Figure 1: EDS mapping of the EAF slag/ MgAl2O4, 1:0.5 Ratio; Magnification ×800

3.2. Slag/chromium containing spinel

XRD patterns of the EAF slag/chromium containing spinel group are shown in Figure 2. In addition to two main phases of calcium silicate and the wustite type solid solution, presence of the spinel phase can be seen. The intensity of the spinel peaks are increased by increasing the amount of the additive, which is an indication that more additive results in formation of more spinels. In EDS mappings of the samples

5 of this group, the amount of chromium rich spinels increased steadily; therefore, only EDS mapping of the 1:0.25 and 1:1 samples are shown in Figures 3 and 4.

2 1: Calcium Silicate 2: Wustite type solid 1 1 solution 3 2 2 3: Spinel 1 1 1:1 Ratio 3 1 1 1 1 2

1 1 3 2 2 1:0.5 Ratio 1 1 1 3 1 1 1 2 Intensity 1 1 2 2 1 1 3 1:0.25 Ratio 1 3 11 1 2

1 1 2 2 1 1 Ref. Sample 3 1 3 1 1 1

25 30 35 40 45 50 2-Theta-Scale Figure 2: XRD patterns of the samples of EAF slag/chromium containing spinel group

Figure 3: EDS mapping of the EAF slag/Chromium containing spinel, 1:0.25 Ratio; Magnification ×800

Figure 4: EDS mapping of the EAF slag/Chromium containing spinel, 1:1 Ratio; Magnification ×800

4. Discussion

The main difference between the stainless steel slag and the EAF or black slag is the %CaO basicity (B2= ), which is resulted from different slag formers that are used in %SiO 2 the EAF. In fact, in order to protect the furnace lining and achieve a foamy slag, some EAF slags are saturated with magnesium. The ternary phase diagram of CaO,

SiO2, and MgO [8] shows that, by this saturation, the primary crystallization field of the slag would change from Merwinite, 3CaO.MgO.2SiO2 (in the stainless steel slag) to the Periclase, MgO, in the black slag. Considering this change, FeFe2O4 and

MgAl2O4 are not thermodynamically stable at high temperatures. In the current experiments, material heated up to 1600ºC; at such a high temperature the added spinels dissolved in the slag and no spinels could be detected for the solidified samples by SEM and XRD analyses. Here, the experimental results are in a good agreement with phase diagrams predictions.

7 Thermodynamic calculations (Figure 5) show that FeFe2O4 and MgAl2O4 are stable up to almost 1550ºC. As shown in the figure, above that temperature the total content of these spinels is zero, meaning that spinels are dissolved in the liquid slag.

Figure 5: Prediction of the thermodynamic calculations for the stability of different additives in the studied slag

According to the calculations, spinels should be formed once again during the cooling and solidification of the molten slag at equilibrium condition. The actual cooling rate in the present trials has been so fast that reaching equilibrium during cooling was not possible; thus, no spinels could be seen in the solidified samples. Earlier investigations by the authors of the current paper showed that by heat treatment of similar samples (holding the materials at 1350ºC for four days); there would be sufficient time for formation of the spinels.

The only stable additive between 1550ºC and 1600ºC is the chromium containing spinel. To clarify the reasons of this behaviour, the binary phase diagram of MgO-

Cr2O3 (Figure 6) can be referred to. According to the diagram, the MgO solid solution dissolves up to 8% chromium at 1600ºC before the MgCr2O4 solid solution can be formed. By using FeFe2O4 and MgAl2O4 as additives, chromium was dissolved in the wustite type solid solution and no spinels could be detected (Figure 1). Using the chromium containing spinel as the additive increases the total content of the chromium to a critical limit (above 8%) at which according to Figure 6 the MgCr2O4

8 solid solution is in equilibrium with MgO solid solution. This is the reason behind existence of the chromium rich spinels in the slag/chromium containing additive group (Figures 3 and 4).

Figure 6: Binary diagram of the MgO-MgCr2O4 system [9]

5. Conclusion

In magnesium saturated black slag, chromium is primarily crystallized in the wustite type solid solution. According to the current results it can be concluded that formation of the Cr-rich spinels are suppressed as long as free unsaturated MgO is present in the slag. In order to form stable chromium-rich spinels, the total chromium content of the system must be high enough (>8%) to reach equilibrium between MgO-based and MgCr2O4 solid solutions. Therefore, the only additive that may lead to formation of stable chromium rich spinels is the chromium containing spinel.

FeFe2O4 and MgAl2O4 are not thermodynamically stable at high temperatures and would be dissolved in the molten slag. Cooling rate of the material has an immense effect on the spinel formation; in another word, spinel formation is a kinetically controlled phenomenon. By an appropriate heat treatment of the slag (giving sufficient time to the system) even these two additives would cause the formation of spinels at lower temperatures.

9 References

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2) Ryan Robinson, “Studies in low temperature self-reduction of by-products from integrated iron and steelmaking”, Ph.D. thesis, Luleå University of Technology, ISSN: 1402-1544, 2008

3) J.M. Manso et al, “Durability of concrete made with EAF slag as aggregate”, Cement & Concrete Composites 28 (2006) 528–534 [3]

4) G.J. Osborne, “Durability of Portland blast-furnace slag cement concrete”, Cement and concrete composites, 21 (1999) 11-21 [4]

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