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Helsinki University of Technology Faculty of Process Engineering and Materials Science Department of Materials Science and Rock Engineering Laboratory of Metallurgy

Thermodynamic properties of chromium bearing slags and minerals

Research programme: SULA II Research project: Chemistry of Chromium Bearing Processes

Key words: chromium oxides, slag chromite, oxidation state, activities, phase relations, chromium recovery, FeCr, stainless steel

Vuorimiehentie 2 K FIN-02150 Espoo, Finland

DismaunoN of im document is unlimited ISSN 0785-5168 ISBN 951-22-3170-0 2B ABSTRACT

In this report, the thermodynamic properties of chromium bearing slags and minerals were reviewed based on the available information in the literature. It includes the ana ­ lysing methods for oxidation state of chromium in slags, oxidation state of chromium and activities of chromium oxides in slags and minerals. The phase diagrams of chro ­ mium oxide systems and chromium distributions between slag and metal phases are also covered in this review.

Concerning the analysing methods, it was found that most of the available approaches are limited to iron free slag systems and the ‘sample preparation is very sensitive to the analysing results. In silicate slags under reducing atmosphere, divalent and trivalent chromium co-exist in the slags. It is agreed that the fraction of divalent chromium to total chromium increases with higher temperature, lower slag basicity and oxygen po ­ tential. For the slags under oxidising atmosphere, trivalent, pentavalent and hexavalent states were reported to be stable. The activities of CrO and CrO, 5 were concluded to have positive deviation from ideal solution. Slag basicity has a positive effect and tem­ perature has a negative effect on the activities of chromium oxides.

The phase diagrams of the Cr-O, binary, and ternary chromium containing oxide sys­ tems have been examined systematically. The analysis shows that the data on the quar­ ternary and quinary systems are insufficient, and require further investigation. The most important features of the chromium containing silicate slags are the large miscibility gaps and the stability of the chromite spinel. The phase diagrams are affected signifi ­ cantly by system oxygen potential due to the different possible oxidation states of chromium. In addition, the factors influencing the chromium recovery in FeCr and stainless steel production processes were also inspected, including the effects of slag basicity and temperature as well as the compositions of the melt. The knowledge indi ­ cates that increasing slag basicity is very effective to increase the chromium recovery from slag. The recovery of chromium to the metal phase is enhanced by the presence of MgO and A1203 in the slag. TABLE OF CONTENTS

1 Introduction. 1

2 Oxidation states of chromium under different conditions ...... 2 2.1 Analysing method for chromium oxides in slags ...... 2 2.2 Oxidation state of chromium in slags...... 4 2.2.1 Chromium containing slags in high oxygen partial pressure (air) ..5 2.2.2 Chromium containing slags in low oxygen partial pressure ...... 5 2.3 Oxidation state of chromium in chromite solid solutions ...... 8 3 Activities of chromium oxides in slags...... 11 3.1 Activities of chromium oxides in literature under different conditions 11 3.2 Comparison of activities of chromium oxides in slags and discussion. 16 3.3 Activities in chromite solid solution ...... 18 4 Phase diagrams of chromium containing oxide systems...... 19 4.1 Cr-0 system...... 19 4.2 Binary oxide systems...... 20 4.2.1 Fe-Cr-0 system...... 20 4.2.2 Ca-Cr-0 system...... 21 4.2.3 Mg-Cr-0 system...... 23 4.2.4 Al-Cr-0 system...... 23 4.2.5 Si-Cr-0 system...... 23 4.3 Ternary oxide systems...... 24 4.3.1 Si-Ca-Cr-0 system...... 24 4.3.2 Si-Mg-Cr-0 system...... 26 4.3.3 Si-Al-Cr-0 system...... 27 4.3.4 Ca-Mg-Cr-0 system...... 27 4.3.5 Ca-AI-Cr-0 system...... 28 4.3.6 Mg-Al-Cr-0 system...... 29 4.3.7 Fe-Mg-Cr-0 system...... 29 4.3.8 Fe-Si-Cr-0 system...... 30 4.3.9 Fe-Al-Cr-0 system...... 31 4.4 Quaternary oxide systems...... 32 4.4.1 Ca-Mg-Si-Cr-0 system...... 32 4.5 Quinary oxide systems...... 4.5.1 Ca-Mg-Al-Si-Cr-0 system...... 4.6 Chromite raw materials...... 5 Distribution of chromium between slag and metal...... 39 5.1 Distribution of chromium between slag and FeCr...... 39 5.2 Distribution of chromium between slag and metal (steel)...... '...... 42

6 Summary...... 45

7 References 47 DISCLAIMER

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1 Introduction

The steadily growing consumption of stainless steel in the world is the impact for increasing the production of both ferrochromium and stainless steel. Also the R&D work in these fields has been greatly stimulated at present. From economic point of view, the optimisation of chromium yield in the related smelting and refining processes in the FeCr and stainless steel production industries is very important. A successful operation in a large extent depends on the chromium yield in the metal phase, i.e., chromium distribution controlled by the reactions between slag and metal phases. High chromium recovery would be beneficial in saving raw materials and energy as well as in reducing eventual chromium pollution from chromium containing slags and wastes. Thermodynamic approach can quantitatively define the effects of the various process variables on the chromium distribution between the slag and the metal, and further on the chromium yield. To carry out an effective thermodynamic evaluation of chromium distributions, thermodynamic properties in the slag phase are needed. The knowl ­ edge of the thermodynamic properties of slags including activities of chromium oxides and its phase relations will definitely help to evaluate the equilibrium distribution, to promote ther ­ modynamic process modelling, and further to better understand the process to achieve a higher recovery of chromium from the minerals.

Chromium, the fourth element in the first transition series of the periodic table, has five elec­ trons on the orbital 3d and one electron on 4s, thus exists in a variety of oxidation states. The behaviour of chromium oxides in metallurgical slags is, therefore, very complex due to the co-existence of multivalent chromium ions, the high melting point of chromium oxides con­ taining slags, the characteristics of chromium oxides volatilisation, and the sophisticated structures of chromite raw materials. Physically, Cr2+ colour is blue, Cr3+ colour is green, Cr6+ colour is yellow to red. These three chromium ions are identified to exist in the different raw materials under the different environmental conditions. In chromite minerals as well in py- rometallurgical processing, CrJ+ is the dominant valence. Cr203 is moderately stable with re­ spect to its constituent elements. For comparison, Cr203 is less readily reduced than FeO, CoO and NiO, but somewhat more readily reduced than MnO, and is more easily reduced than the very stable oxides Si02, A1203, CaO and MgO. Mainly because of the tremendous technological importance of iron and its compounds, the equilibrium thermodynamic rela­ tions pertaining to this element at high temperatures are well known. In contrast, quantitative data on the thermodynamics of chromium oxide in silicate melts are practically very few, and until recently phase-equilibrium data in systems containing chromium were available mostly at high oxygen pressures (air), under poorly defined reducing conditions or under certain lim­ ited experimental conditions, such as metallic chromium saturated system.

Even though volume of work can be found in the literature on the chromium properties in the metallurgical field, some fundamental nature and behaviour in general still need to be clari­ fied. In this report, the available information on the oxidation state of chromium, the activities of chromium oxides in the slags and the phase relations of chromium oxides with the inter­ ested other slag components have been surveyed and discussed. The aim was to provide a guided thermodynamic knowledge to the process modelling and control for ferrochromium and stainless steel production. 2

2 Oxidation states of chromium under different conditions

The oxidation states of chromium both in the natural or synthetic chromite solid solution and in the slag systems have been investigated since 1930 ’s (Korber and Oelsen, 1935). Mean ­ while, the technologies for determining the valences of chromium in different base matrix have been developed. Due to the specific difficulties involved in the analysis, there exist cer­ tain limitations of each analysing method, especially for the divalent chromium determination in silicate slags. In this chapter, a few articles relating to the analysis of divalent/trivalent chromium in silicate slag will be first summarised, then the oxidation state of chromium in slags and in the solid solutions will be reviewed and discussed.

2.1 Analysing method for chromium oxides in slags

In FeCr and stainless steel production conditions, Cr2~, Cr3+ and Cr6+ are present in the slags at different stages. All of the slags contains chromium (III), but normally none of them con ­ tains both chromium(II) and chromium (VI) in significant amount at the same time (Marston and Knight, 1977). Therefore, the constitution of each slag could be ascertained from the total chromium content and the amount of redox species present, that is chromium (II) or chro­ mium (VI). The analysing method for determining the total chromium amount in the slags is well established in the book “Standard Methods of Analysis ’’ of United Steel Companies Ltd., Sheffield in 1961. For determination of divalent chromium or hexavalent chromium in the slag, there exist several published articles devoted for such purpose in the last fifty years.

Cr2+ was referred as "the most powerful reductant used in the form of a standard solution in volumetric analyses" (Lingane and Pecsok, 1948). It means that the divalent chromium in the slag will be easily oxidised when the slag is dissolved into a solution. This, no doubt, will cause certain difficulties or error in the chemical analysis. Close and Tillman (1969) investi ­ gated the chromium oxide analysing method. They showed that problems arise when two or more redox systems occur together. In the chemical analysis of redox pairs, it is possible and highly probable, that a reduced species of one metal will react with an oxidised species of another when the slag sample is decomposed with acids. Another difficulty is in differentiat ­ ing the amounts of reduced or oxidised species of elements whose redox potentials are similar in aqueous solution.

McCoy and Philbrook (1958) determined divalent chromium amount in iron-free slags with a chemical analysing method by dissolving the slag in a sulfuric-hydrofluoric acid mixture containing a known amount of sodium vanadate (V03~), titrating the excess vanadate, and calculating the chromium equivalent of the oxidant consumed.

Frohberg et al. (1967) used a relatively weak oxidising agent, iron(III) chloride (FeCl3) to dis­ solve the iron-free slags in an inert Ar atmosphere. The iron and chromium will exchange electrons so that Cr(II) is oxidised to Cr(III) and the same amount of Fe(III) can be reduced to Fe(II), the reduced iron (II), corresponding to the amount of divalent chromium ion, can then be titrated with standard potassium permanganate solution using ferroin as the indicator. The additions of manganese (II) sulphate and orthophosphoric acid were required to prevent the oxidation of chloride ions. The smallest content of Cr(II) which was analysed was 0.1% with 0.025% accuracy. With the Cr(II) content more than 1%, the accuracy of the analysis is 0.05%.

Robison and Pehlke (1974) developed an analytical technique applied for the slag in which the total iron content should be less than the total chromium content. This is the case in stainless steel making slags. They dissolved the slags in the presence of a relatively strong oxidising agent (vanadate), the excess of which was determined by titration with standard iron (II) solution. The procedure was the following. Synthetic slags were crushed to -70 mesh. 0.5 g crushed slag sample were dissolved for 20 minutes in 10 ml of hot 0.6M H2S04 with 3 ml of HF and a known quantity of (V0 3~). As long as an excess of oxidising agent (V03“) is present, the analysing results will be the same when dissolution was accomplished under air or argon. The routine dissolution were done in air in platinum vessels. In hot 6M H2S04, the (V03~) was reduced approximately 1% per hour. After diluting to 200 ml with H20, a 10 ml aliquot was drawn. The remaining 190 ml of solution was acidified to 6M with H2S04, cooled in an ice bath, and titrated with 0.02N ferrous sulphate solution. This titration established the total number of moles of Cr2+ and Fe2+ in the original sample. The end point was detected potentiometrically and visually. Ferroin was used as a visual indicator. The 10 ml aliquot was analysed for iron (by assuming the iron was mainly existing as Fe2+) and for total chromium by atomic absorption spectrometry, and Cr2+ and Cr3+ in the slag were de­ termined by the difference. All titration involving the chromous ion was carried out under a protecting atmosphere of flowing deoxygenated nitrogen. The uncertainty of the slag analysis was about ±5wt% for the content of iron, and ±0.02wt% for the content of Cr2+.

Marston and Knight (1977) developed a chemical analysing method to determine divalent and hexavalent chromium in iron-free lime-alumina-silica slags. About 0.3g of slag was dissolved in 1 h in 50 ml of hydrochloric acid containing some hydrofluoric acid with excess of iron (III) for divalent chromium containing slags. On the other hand, ammonium iron(II) sulphate was used when a reducing agent was required and was present in excess during the dissolu­ tion of slags containing chromium (VI). The amount of iron(II) was determined by potenti- ometric titration with dichromate solution. Concerning the analysing procedures, they con­ cluded that the sample preparation is very critical for the Cr2+ containing slags. They ob­ served that when the slag was ground to fine powder several days before the analysis, the chromium (II) content was determined as 0.2-0.3%, whereas if the slag was ground just be­ fore the analysis, the results were 1.09%, 1.22%, and 1.43% for the fine powdered slag, less fine slag and coarse powder slag dissolved overnight and then titrated slowly, respectively. They also proved that the slow rate of titration was not as significant as the risk of oxidation in the preparation of the fine slag powder. The major reason for the variation can be the het ­ erogeneity of the coarse particles of the slag. At low levels of the chromium (II) or chromium (VI) contents, random errors could be very severe. For the slags with higher chromium con ­ tents, the accuracy could be improved.

Marston and Argent (1988) also investigated the oxidation state of chromium in the silicate slags by spectroscopic methods. In this method, the slag sample is required to be vitreous, the thickness is in the range of 0.3 to 3 mm which can be achieved by grinding the slag beads. Transmission spectra were determined in the range of 200 - 2500 nm using a Beckmann spectrophotometer. The intense absorption of Cr(VI) prevented transmission measurements at some ultra-violet peaks, even with the 0.3 mm samples. By using the spectrophotometer in 4 reflection mode with powdered slag samples, the wavelengths of these peaks could be de­ termined, although the absorption could not be quantified. In addition, in order to compare the accuracy, the spectra of aqueous solutions containing Cr(II), Cr(III) and Cr(VI) were also determined. Part of the slag samples were analysed for total chromium which was expressed in terms of elemental Cr.

In addition, McCoy and Langerberg (1964) and Tsai (1981) applied an indirect method to determine the Cr2+/CC+ ratio in slags. Because of inherent difficulties associated with wet- chemical analysis for ratios of Cr2+ to Cr3+ in iron containing chromium slags, it was found more reliable to use indirect methods rather than chemical analysis to ascertain the state of oxidation of chromium in the slag. For the iron-chromium equilibrium reaction, x(FeO)+Cr=xFe+(CrO x ), the relations of chromium partition LCr, iron partition LFe between slag and metal phases and equilibrium constant K were expressed as LogL Cr=xlgL Fe+lgK. As Cr203 is far more stable than iron oxide relative to its constituent metal and oxygen, the oxy ­ gen pressures at which reduction of significant parts of Cr3+ to Cr2+ occurs are much lower than in the corresponding iron systems. In this way, x value can be determined by the equi­ librium thermodynamic data as well as chromium and iron partition ratio.

Six different methods have been described above to analyse different oxidation states of chromium in slags. Four of them are wet-chemical analysing methods, one spectroscopic method and one indirect method. The difficulties encountered in the wet-chemical analysing method are as follows:

(1) Sample preparation including sampling, grinding, as well as powder storing. The possible precipitation of crystals and formation of metal inclusions in the melt during sampling could cause uncertainties in the slag analysis. The ground slag powder size and the storing conditions are very crucial factors for the analysing results. (2) A complete dissolution of chromium containing slags without oxidising the chromous ion by atmosphere and/or by some other unwanted factors. (3) Interaction when two or more redox elements present at the same time. The situation is even more difficult if the redox potentials are similar in aqueous solution.

Most of these available methods are limited to iron free slag systems. Only the technique de­ veloped by Robison and Pehlke (1974) can be applied for the slags containing iron. However, the total iron content should be lower than total chromium content. The indirect method can be applied under the specific experimental conditions, it needs some additional analysis for the related equilibrium elements both in the slag phase and in the metal phase. By the com­ parison, a combined method of wet-chemical and spectroscopic analysis is recommended for determining the oxidation state of chromium in the slag.

2.2 Oxidation state of chromium in slags

Considering chromium, the slags in the stainless steel making process are subjected to both oxidising conditions as during carbon removal and reducing conditions as in the final refining stage. In the FeCr process, the slag is formed and prevails in reducing conditions. The transi ­ tional metallic ions like chromium in the slag will respond to such oxygen potential changes by changing the oxidation states. Chromium oxide containing slags have been investigated at 5 temperatures between 1400 °C and 1650 °C under different oxygen partial pressures ranging from a very strong reducing atmosphere to ambient air. One of the major concerns is the oxi­ dation state of chromium in slags. The results showed that the existing forms of chromium in silicate melts are divalent, trivalent, pentavalent or hexavalent, depending on temperature, oxygen partial pressure and slag compositions.

2.2.1 Chromium containing slags in high oxygen partial pressure (air)

Very complex phase relations exist in the chromium oxide slag system because of the multi­ valences of chromium. Ford and White (1949) observed that the main existing form of chromium in the CaO-chromium oxides system in air was hexavalent.

Glasser and Osborn (1958) studied the phase relations in the system of Ca0-Cr203-Si02 at temperatures of interest in ceramics from 1120 to 1810 °C, and found that in the lime rich part of the system, chromium was oxidised to higher valences in air. The existence of a pen ­ tavalent chromium compound, Ca3(Cr04)2, was ascertained. In the ternary system CaO-CrOx - Si02, the presence of appreciable percentages of Si02, making the systems "acid", coupled with the generally high liquidus temperatures favour the predominance of trivalent chromium. When the lime to silica ratio is greater than that of the orthosilicate, higher oxidation states of chromium appear in significant amounts, especially at lower temperatures, that is Cr03.

Recently, the chromium oxidation state in the CaO-chromium oxides system was further identified by Mirtic et al.(1992) in air at the molar ratios of Ca0/Cr203 > 3/1 at temperatures of 800 °C and 1000 °C, and trivalent chromium was found to be incorporated into the lattice of CaO at low chromium oxide content, while at a high chromium oxide concentration, pen ­ tavalent chromium was favoured.

In general, trivalent, pentavalent, and hexavalent are the major existing forms of chromium in slags under air atmosphere. The temperature and slag composition define the amount of each oxidation state of chromium in the slags.

2.2.2 Chromium containing slags in low oxygen partial pressure

The properties of chromium containing slags under reducing atmosphere have been investi­ gated for over six decades. In 1935, Korber and Oelsen studied Si02 saturated chromium containing FeO-MnO-Si0 2 slags which was equilibrated with Fe-Cr melt. When the chro­ mium content in the alloy was high, within 17%-60%, the divalent chromium was the main existing form in the slags in the temperature range of 1600 to 1640°C. The disproportionation of the lower oxide to Cr203 and chromium was found to occur to a greater or lesser extent, depending upon the cooling rate.

McCoy and Philbrook (1958) investigated the reduction behaviour of chromous oxide from Ca0-Si02-Al203 slags by carbon-saturated iron at the temperature range from 1500 to 1650°C, that means the oxygen partial pressure in the order of 10"13 atm. Under such condi­ 6 tions, the divalent chromium was observed to be the main existing form of chromium in the slags, about 89 to 99% of the total chromium content.

McCoy and Langerberg(1964) and Tsai(1981) observed that the chromium mainly existed in divalent state in the Si02 saturated slag in equilibrium with Fe-Cr-Si-0 metal melt at steel­ making temperatures. The extent of the oxidation state of chromium in slag was determined by measuring the distribution ratio of chromium and iron between slag and metal phases.

In 1964, Healy and Schottmiller published their work on the chromium oxide-silica system at low oxygen partial pressure. They studied CrO, 5-Cr0-Si02 slag in tungsten crucible under H2-H20 atmosphere at 1600-1750°C. A deep-blue chromous silicate, probably Cr2Si04, was observed at about 1400°C to 1500°C temperatures, and was characterised by its distinctive X- ray diffraction pattern. Under reduced oxygen partial pressures corresponding to steam- hydrogen ratios below 0.05, a liquidus phase area of Cr-Si-0 slag system at 1750 °C was roughly determined in the triangle region marked by the three compositions of 100 pet Si02, 100 pet “CrO”, and 70 pet “CrO”-30 pet CrO 15. The details of the phase systems with various temperatures and slag compositions will be introduced later in the chapter of phase relations of chromium containing slags.

Frohberg and Richter (1968) studied the equilibrium between Cr2+ and Cr3+ in CaO-Si02- CrOx slags by equilibrating them in platinum boats under the atmosphere of hydrogen-steam- argon corresponding to oxygen partial pressure ranging from 10"6 to 10"9 atm at the tempera ­ tures of 1600°C and 1650°C. The results showed that the ratio of Cr2+/CrJ+ increases with in ­ creasing temperature and decreasing oxygen partial pressure as well as decreasing the slag basicity from 1.5 to 1.3.

Rankin and Biswas (1975) compared the different available information in the literature, and concluded that the stable oxide of chromium in equilibrium with oxygen saturated Fe-Cr-0 melts at steelmaking temperatures is either Cr(III) oxide (Cr203 or Fe0-Cr203) or Cr(II)/Cr(III) mixed oxide (Cr304 or distorted spinel) rather than pure Cr(II) oxide (CrO). For the Ca0-Al203-Fe0-Cr0x -Si02 slag equilibrated with Fe-Cr-Si alloy, under Ar protecting atmosphere at 1600°C, divalent chromium was the main existing form. CrO, 07 is the average chromium oxide composition in these low oxygen partial pressure conditions. It is equivalent to 84%Cr 2+ and 16%CrJ+ in the slags. The chromium oxidation state was observed to be a function of FeO content of the slag and Si content of the metal. In silica saturated Cr203-Cr0- Si02 system under H2-H20 atmosphere, the following equilibrium was obtained: 2CrO + l/202 = Cr203, AG°170o °c= -38654+2500 cal, and a^os/acro^ = PO21/2xl.9xl0 4. This indi ­ cates that in the Si02-saturated system at 1700°C for oxygen partial pressures below about 2.8x1 0"9 atm, CrO is more stable than Cr203.

Maeda and Sano (1982) studied the chromium oxide reduction with solid carbon in CaO- Mg0-Al203-Si02-Cr0x slags under one atmosphere of CO at 1500 °C and 1650 °C. They concluded that chromium in the slag existed as CrO. They showed that for melts with a con­ stant %CaO/%Si02 ratio, the solubility of chromium oxide decreases with increasing A1203 content in the melt.

Morita and Sano (1988) investigated Mg0-Al203-Si02-Ca0-Cr0x slags saturated with Mg0.Cr203 at 1600°C under reducing conditions, and concluded that Cr2+/CrJ+ increased 7 with decreasing oxygen partial pressure down to 3.6xl0 "13 atm, in which conditions about 25% of the chromium existed in divalent state. With the addition of A1203 from 0 to 10 wt%, Cr2+/CrJ+ decreased slightly, but by further increasing A1203 from 10 to 25 wt%, Cr2+/CrJ+ increased significantly.

Pretorius and Muan (1992) examined oxidation state of chromium in CaO-Al203-CrOx -Si02 melts under oxygen partial pressures from 10"9 '36 to 10~12'50 at 1500 °C. They observed that under such reducing conditions chromium was predominantly present in the divalent state in silicate melts, and the Cr2+/CrJ+ ratio was little affected by the amount of A1203 present in the slag.

Xiao (1993) studied the equilibrium of chromium containing slags in metallic chromium crucible at 1500 °C, 1550 °C and 1600 °C temperatures. The oxidation state of chromium in Si02-CaO-CrOx slags was analysed by wet chemical method. The effects of temperature, slag basicity, as well as additions of MgO and A1203 were investigated. The results showed that the divalent chromium fraction increased with increasing temperature and lowering slag ba ­ sicity. By partially substituting MgO for CaO, the oxidation state of chromium in the slag did not change significantly. Increasing the A1203 content from 0 to 10 mol% resulted in a lower divalent chromium fraction at the slag basicity of 1.0 in mole ratio. A further increase in the A1203 content to 20 mol% did not cause any obvious change.

Pei and Wijk (1994) carried experimental work on slag/metal equilibrium with the slag com­ positions of typical chromite smelting process at 1550 - 1650 °C temperatures. The MgO saturated Ca0-Si02-Al203-Mg0-Cr0x slag and Ni-Cr alloy were contained in a magnesia crucible. The system oxygen partial pressure was controlled by steam-hydrogen ratio in the gas phase. They also concluded that the ratio of divalent chromium in proportion to total chromium content in the slag was increased by decreasing basicity, increasing temperature and decreasing oxygen partial pressure.

In general, in silicate slags in equilibrium with Fe-Cr-O(unsat-) melts or in silica saturated slag systems under “reducing ” conditions, chromium is present mainly as Cr(II) or CrO, par ­ ticularly at higher levels of chromium in the metal. Therefore, it can be concluded that under the “reducing ” conditions, both Cr(II) and Cr(III) are present in all slags, and an equilibrium exists between these two valences depending on the equilibrium oxygen partial pressure, slag composition and temperature. Several researchers in the literature found that the ratio of Cr2+/CrJ+ in the slag increased with higher temperature, lower oxygen partial pressure and lower slag basicity, but due to the difference of the analysis method, experimental conditions etc., the quantitative results are not in a good agreement. The available investigations on chromium oxidation states in different slag systems are compared and listed in Table 1. In order to further compare these available experimental data, some of the experimental results from different researchers are compared under specific conditions and drawn in Fig.l. 8

Table 1 Summary of the investigations on chromium oxidation state in different slag systems under different conditions

System investigated Temperature,*C (^CaO+^Mgo)/NSi02 P02, atm (CrOx ) in slag CdVTc, Analysing method Reference

Ca0-Si02-Al203-Cr0x Pco=1. graphite wet-chemical: McCoy and Philbrook in graphite crucible 1530-1545 0.61-1.25 or 1E-15 2.S-3.7 wt% 0.89-0.99 sulfuric-hydrofluoric (1958) in mole ratio acid+sodium vanadate

CaO-Si02-CrOx wet-chemical: FeCI3+HCl Frohberg and Richter

in H2-Steam-Ar 1600-1650 1.39-1.61 1E-6-1E-9 0.62-6.48 wt% 0.34-0.90 +H2S04+H3P04 dissolving (1968)

in mole ratio with KMn0 4 titration CaO-SiOrAyV'FeO" indirect method Rankin and Biswas -CrO, with Fe-Cr-Si in 1600 1.07-2.46 1E-13.8- 1E-16.6 0.50-14 wt% 0.60-0.92 x(FeO)+Cr=xFe+(CrO x ) (1978)

Al203 or Si02 crucible in mole ratio recalculated

Ca0-Si02-Al203-Mg0 wet-chemical:HCI+HF Morita and Sano

-CrO* with Mg0.Cr203 in 1600 2Mg0.SiO2 sat. 0.21 - 3.6E-13 0.5-8 wt% 0.0-0.25 dissolving with K2Cr20? (1988) air or reducing conditions titration

Ca0-Si02-Al203-Cr0x wet-chemical: FeCI3+HF Pretorius et al. in H2+C02 in Mo or Pt 1500 0,03-1.2 IE-9.56-1E-1 2.4 3.79-23.15 mol% 0.79-0.98 +H2S04 dissolving with (1992) capsule in mole ratio K2Cr207 titration

Ca0-Mg0-Si02-AI203 wet-chemical: FeCl3+HCI Xiao Ul -CrOx in equilibrium 1500-1600 0.2-1.5 1E-13-1E-15 4.5wt%-saturation o +H2SO4+H3PO4 dissolving (1993)

with metallic Cr in Ar in mole ratio with KMn0 4 titration

Ca0-Mg0-Si02-Al203 wet-chemical: FeCI3+HCI Pei and Wijk

-CrOx with MgO saturation 1600-1650 1.74-2.41 5E-11 -4E-12 0.7 -4.75wt% 0.18-0.49 +H2S04+H3P04 dissolving (1994) in H20+H2 mixture in mole ratio with KMn0 4 titration

Due to the difference in experimental methods, experimental condition as well as the slag analysing methods, there seems to exist a great discrepancy in the oxidation state of chro­ mium in the slags. According ------* • ♦McCoy etai.,iS30*C. to the authors ’ own experi ­ = . - ♦ m 8 0 61 1.25

0.9 ♦ » - * _ a 0 ♦Fretoergel al.. - ‘C. ence, the chemical analysing < 1600 1650 # O O ^Morita etal.,1600'C, B=-2.0

0.8 O method has certain problems, « e ao ^Pretorius et al.,lS00*C, 0.7 • such as sample storing and •Xiao et al.,l600*C. B=-i .0 0 0 preparing methods greatly 0.6 * 0 0 0 OPeietal..1600‘C. 6=2.2-24 affecting the analysing result. 0.5 ** In addition, dissolving time o 08 0 0 0.4 69 depends very much on the 0

0.3 CCO slag compositions. Certain X 0 0.2 300 sample residue after dissolv ­ A A^ A ing may still contain undis ­ 0.1 A A A A A solved chromium phases. All A , these will certainly cause 6 -14 -12 -10 -8 -6 -4 -2 scatter in analysis results. No LogP0 2, atm doubt, further investigations Fig. 1 Summary of the results showing the effect of oxygen partial are still needed to confirm the pressure on the oxidation state of chromium in the effect of sample preparation, slags from different researchers analysing method etc..

2.3 Oxidation state of chromium in chromite solid solutions

The dominant phase present in ores used for the production of FeCr is a complex oxide solid solution with the spinel type structure. The general formula of this spinel may be written as 9

A2+B2J+04, where A2+ can be Fe, Cr, Mg, Zn or Mn, and B3+ can be Al, Fe, Cr or V. The composition of ores used in the steelmaking industry corresponds roughly to that of the iron- rich chromite, i.e., FeCr204(iron chromite), whereas a spinel resembling MgCr204 picro- chromite is the major constituent in ores used as refractory material.

As it is well known, all chromites appear to be normal spinels (Dunitz and Orgel, 1957). Cr3+ ions occupy only octahedral sites in the chromite spinel structure. But the temperature will have effect on the site preference via the site entropy. The entropy term will increase with temperature in a direction that favours lower co-ordination numbers. Cr2+ in crystalline phases is stabilised in distorted octahedral sites (Bums and Bums, 1975, Bums 1975) and has been found in natural minerals such as diopside, olivine, and pyroxenes (Mao et. al, 1972, Boyd, 1975). Divalent chromium is also found in tetrahedral sites in spinels (Ulmer and White, 1966, Greskovich and Stubican, 1966, Mao and Bell, 1975). Ulmer and White (1966) presented that chromous ions might also exist in the spinel solid solution FeCr204-MgCr204 that coexist with a metal phase at 1300 °C. Cr2+ has probably substituted for Fe2+ which was reduced to metallic iron during synthesis.

Greskovich and Stubican (1966) heated mixtures of MgO and 50-66 mol% Cr203 in a slightly reducing atmosphere at 1800 °C to 2000 °C, and got a new spinel type crystalline solution with the unit cell formula (Mg8 .x Crx 2+)lvCr16:,+VI032. Cr2- and CrJ+ ions are not in the same co-ordination sites in the structure. Bums (1975), Bums and Bums (1975) indicated in their studies that the divalent chromium in the area of deformed octoploid in the crystalline is sta­ ble. CrJ+ ions have a strong preference for octahedral sites in mineral structures and discrimi­ nate against tetrahedral sites, whereas Cr2+ ions favour distorted environments, according to crystal field theory. CrJ+ ions in octahedral co-ordination with oxygen and tetrahedral (Cr6’r04)2' ions predominate in minerals on the Earth, whereas lunar minerals contain Cr2+ and CrJ+ ions. Meteorites also contain additional rare sulphide and nitride minerals of chromium.

In 1975, Stubican and Greskovich investigated trivalent and divalent chromium ions in spinels. In air atmosphere, even at very high temperature, Cr203 does not dissolve into MgCr204 spinel. But under reducing atmosphere, quite big amount of Cr2+ can be located in sites of Mg2+ at the area of tetraploid in the spinel structure.

Mao and Bell (1975) studied crystal-field effect on the oxidation states of iron and chromium in spinel. They concluded that it was possible to detect and assign bands for tetrahedrally co­ ordinated Fe2+, Fe',+ and Cr2+ ions, and for octahedrally co-ordinated Fe2+ and CrJ+ ions in the spinels.

In 1978, Toker investigated the thermodynamic properties of FeCr204, and found that the composition of the spinel phase extending a considerable distance beyond stoichiometric iron chromite toward Cr304, indicating that a substantial fraction of chromium in these spinels is present in the divalent form.

Badie and Berjoan (1985) studied the effects of temperature and oxygen partial pressure on Cr203 vaporisation from synthetic FeCr204 (66.6 mol%Cr 203). By treatment in air, conden­ sates contained 89% Cr203, this value dropped to 40% Cr203 under Ar atmosphere. This proved that selective extraction of Cr203 by evaporation depends very much on the oxygen partial pressure. Crystal phases in residues and condensates were characterised by x-ray dif­ 10 fraction analysis. Thermo-gravimetric experiments were carried out for a study of their sta­ bility. At very high temperature under Ar atmosphere, Cr3+ was partially reduced to Cr2+ leading to a tetragonal spinel distorted phase.

In brief, chromium can exist both in divalent state and trivalent state in the chromite in the metallurgical system and processes, depending on its origin and the processing conditions. 11

3 Activities of chromium oxides in slags

A quantitative evaluation of the thermodynamic properties and roles of chromium oxide in silicate melts, and hence of the chromium distributions between slags and alloys, requires a knowledge of the activity-composition relations of chromium oxide in the slags. Although the activity measurements in chromium containing slags have been an interesting topic for over twenty years, the available activity data in the literature are still very limited, especially for the slag systems under FeCr and stainless steel production conditions.

3.1 Activities of chromium oxides in literature under different conditions

Mohanty and Kay (1975) determined the activities of Cr203 in CaF2-Ca0-Cr203 and CaF2- Al203-Cr203 systems at temperatures of 1450 - 1550 °C. The slags were in equilibrium with Pt-Cr alloys of known chromium activity under controlled oxygen partial pressure by CO and C02 mixture, and found that the activity of Cr203 decreased with the addition of CaO and A1203 in the slag systems. In the CaF2-Ca0-Cr203 system, when the concentration of CaO in slags was lower than about 20 wt%, Cr203 had a positive deviation from the ideal behaviour, and this deviation decreased with increasing temperature. With a higher CaO concentration, Cr203 had a negative deviation which increased with temperature. In the CaF2-Al203-Cr203 system, Cr203 had a positive deviation from ideal in all composition ranges, which decreased with temperature. The oxidation state of chromium in slags was not considered in their work. The representative results are shown in Figs.2-5.

-2.0 002

0-015

0 005

WT % CaO

Fig.2 Isoactivity curves of Cr203 in CaF2-Ca0-Cr203 Fig.3 Activity of Cr203 and temperature relations slags at 1550 °C (Mohanty and Kay, 1975) at CaF2/CaO=75/25 (Mohanty and Kay, 1975)

Rankin and Biswas (1975) observed that the activity coefficient of Cr203 is greater than the activity coefficient of CrO in silicate slags, and thus in the oxygen saturated Fe-Cr-0 silicate slag system, trivalent chromium oxide may attain an activity of one and separate as a new phase even though the divalent chromium is predominant in the slag. In slags of constant composition, i.e., constant Ca0/Si02 and %A1203, CrO obeys Henry ’s law. 12

6 •€> I450*C 6 ■© I450*C x x 1500’C 1500 °C 0-8 - A A 1550 °C

O 0.5

CD 0.4

0.2 -

WT% CaO WT % Al2 03

Fig.4 Effect of CaO on aCr203 in CaF2-Ca0-Cr20:, Fig.5 Effect of Al2Os on aCr203 in CaF2-Ca0-Cr203 slags with 5.0wt%Cr2O3 (Mohanty and Kay, 1975) slags with 1.0wt%Cr2O3 (Mohanty and Kay, 1975)

Finn and Wellbeloved (1980) measured the activities of Cr203 in a slag of 29 wt% MgO, 25 wt% A1203 and 46 wt% Si02 equilibrated with metallic chromium at about 1650 °C by the solid electrolyte method. Unfortunately, the oxidation state of chromium in the slag was not considered, and the experimental results are limited.

Maeda and Sano (1982) investigated the equilibrium concentration of chromium in the mol­ ten Ca0-Mg0-Al203-Si02 system with the existence of Cr3C2 and C under one atmosphere of CO at 1500 and 1650 °C. They concluded that CrO had an amphoteric behaviour. The ac­ tivity coefficient of CrO was derived, and shown in Figs. 6 and 7, and found to have similar behaviour to the activity coefficients of TiO, 5 and MnO.

o(AI,0,)=!0wt "/.

• ibid =20vrt7.

30 40 50 60 (Si02)wt%

LOG (CaO»Aiq 5)/Si02 (molar base J

Fig.6 Relationship of yrr0 and (CaO+AlO, 5)/Si02 ratio in Fig.7 Effect of (%Si02) on yCr0 in Ca0-Al203 Ca0-Al203-Si02 slag at 1500 °C (Maeda and Sano, 1982) -Si02 slag at 1500 °C (Maeda and Sano, 1982)

Muan (1985) studied slag and metal equilibrium of chromium containing system, and deter­ mined the activity of CrO in Ca-Cr-Si-0 system in equilibrium with Mo-Cr alloy under con- 13

trolled oxygen partial pressure by mixing C02 and H2 in proper proportions at 1500 °C. Comparisons were made with the analogous iron oxide containing system CaO-‘FeO’-Si02. The similarity in the behaviour of ‘CrO’ and ‘FeO’ in these systems was observed.

Morita et al.(l988) studied the solubility of MgCr204 in Ca0-Mg0-Al203-Si02 slags and determined the activities of chromium oxide in the slags at 1600°C under reducing conditions by equilibrating the slags with Ni-Cr alloys. They showed that for melts with a constant %CaO/%Si02 ratio, the solubility of chromium oxide decreased with increasing A1203 con ­ tent in the melt. The activity coefficient of CrO and Cr015 increased with increasing oxygen partial pressure and CaO content in the melts in similar slag systems, illustrated in Figs.8 and 9.

80 6 Poj=2.l1«10'9 atra - S 8 —4 — t —f CrO

log Po2Cotm)

Fig.8 Effect of oxygen partial pressure on yCrOI 5 Fig.9 Effect of CaO content on yCr0i 5 and yCl0 and yCl0 in 2MgO.Si02 saturated MgO-Si02-CrOx in 2Mg0.Si02 saturated MgO-Si02-CrOx melts at 1600 °C (Morita et al., 1988) melts at 1600 °C (Morita et al., 1988)

Wijngaarden (1988) attempted to determine the activity of CrO in CaO-Si02-CrOx slags equilibrated with metallic chromium at a slag basicity of NCa0/NSi02 = 0.73 by the solid elec­ trolyte method at 1400 °C. The results proved that the electrochemical technique can be suc­ cessfully applied to measure the activities of chromium oxides in slags.

Pretorius and Muan (1989, 1992) studied the activities of chromium oxides in CaO-MgO- Si02-Al203-Cr0x slags at 1500 °C by the gas equilibrium method. The slag was in equilib ­ rium with Pt-Cr alloys under the controlled oxygen partial pressures by the gas mixture of H2 and C02, with logPO, of -9.56, -11.5 and -12.5 respectively. The results showed that the ac­ tivity-composition relation of CrO had corresponding behaviour to other divalent metal ox­ ides such as FeO, NiO and MnO. The activity coefficient of divalent chromium oxide in­ creased with increasing slag basicity and decreasing oxygen partial pressure. The activity co­ efficient of trivalent chromium oxide decreased sharply when slag basicity increased but lev­ eled off at a basicity ratio of about 0.70. By increasing the amount of A1203 in the slag, the activity coefficient of CrO increased. The results are expressed in Figs.10-15. 14

a k>epo J=.ii.5o(o*Ai,o )) O log pO, =-11.50 (1091 Al,cy • togp0 2=-I1.50(2096Al,0,)

0.6 0.8 Wt% CaO N CrO Wt% SiO.

Fig. 10 aCr0 in CaO-CrOx -Si02 slags with different Fig. 11 yCl0 as a function of slag basicity and A1203 slag basicity at 1500°C(wt%CaO/wt%SiO2) content in Ca0-Al203-Cr0x -Si02 slags at 1500°C (Pretorius and Muan, 1992)

Si02

□ log pO,=-9.56 O log pO,=-l 1.50 A tog pO,=-IZ50 J

Wt% CaO

Fig. 12 Isoactivity curves of CrO in CaO-CrO,.-SiO, Fig. 13 Effect of slag basicity on YcroiV'Yao slags at 1500 °C, Po 2=10 atm in CaO-CrOx -SiO, slags at 1500°C (Pretorius and Muan, 1992)

O togpO J*-ll.50(l0*AL0 >) O tog pO.-l 1.50(10% ALOJ • tog p0 2=-i 1.50 (20% ALOO • togpO 2-11^0(20%Ai:o5 A tog p0 2=-tZ58 (10% AljOj) A tog p0 2«=-tZSS(IO% alo 5 A tog ^-=-12-58(20*. AIjOj)

Wt% CaO Wt% CaO Wt% SiO. Wt% SiO.

Fig. 14 Effect of slag basicity and Po 2 on yCr0 and yCr0!5 in Ca0-Al203-Cr0x -Si02 system at 1500 °C saturated by Cr203-Al203 sesquioxide solid solutions. The sesquioxides solid solutions contained about 4% and 10% A1203, respectively (Pretorius and Muan, 1992) 15

U '

0.6 0.8 Wt% CaO Wt% CaO Wt% SiO. Wt% SiO.

Fig. 15 Effect of slag basicity and Po 2 on yCr0 and yCrOI 5 in eskolaite saturated Ca0-Cr0x -Si02 system at 1500 °C (Pretorius and Muan, 1992)

mo&SxO* In Xiao and Holappa ’s (1993, 1995) study, activities of CrO and CrO, 5 in slags were determined by the elec­ tromotive force method in equilib­ rium with chromium saturated Cr-Ag alloy at 1600 °C. The studied slag compositions covered the liquidus sections in the CaO-Si02-MgO- Al203-CrOx multi-component slags from binary, ternary, quaternary to quinary systems. Based on the ex­ Fig. 16 Activities of CrO in CaO-Si02-CrO-CrO, 5 slags in equilibrium with Cr at 1600 °C, solid lines were calculated perimental results, the activities of and points were measured (Xiao and Holappa, 1995) chromium oxides were calculated by applying the regular solution model. The results in the quasi-ternary system are shown in Figs. 16 and 17. The effect of temperature was investigated from 1500 to 1600 °C, and the effects of slag basicity and additions of MgO and A1203 were investigated at 1600 °C, as shown in Figs. 18 to 19. mdtSiOz In addition, Pei and Wijk (1994) measured the activities of chromium oxides in Ca0-Si02-Al203-Mg0sat- CrOx slag at 1550 to 1650 °C. When the activity of oxygen in the Fe-Cr-0 system was increased, the activities of chromium oxides will correspond ­ ingly increase and an oxide of chromium will separate as a new phase when its activity in the system becomes unity. The separated oxide Fig.17 Activities of CrO, 5 in CaO-SiO2-CrO-Cr0,5 slags is likely to be the same as that which in equilibrium with Cr at 1600 °C, solid lines were calculated is in equilibrium with the pure Fe- and points were measured (Xiao and Holappa, 1995) 16

Cr-O(sat) alloy system. This means that the activities of CrO and Cr203 in the slag increase as the activity of oxygen is increased until one of the following will form: CrO, Cr203, Fe0.Cr203, or Cr304. The results also showed that the activity of CrO has a strong positive deviation from ideality. With increasing slag basicity, the activity coefficient of CrO will in­ crease significantly.

T-1873 K

0.8 -

0.6 ■- 0.6 ■■

O T-1773K

0.2 - 0.2 - • T-1873 K

mol % CrO,

Fig. 18 Effects of temperature and slag basicity on the activity of CrO in the Ca0-Si02-Cr0x system, standard states: pure liquid CrO (Xiao, 1993)

0.8 0.8 -■ ©oo

0.6 0.6 % O (MgO)=Omol% aCrO aCrO o. * 6o O(Al2O3)-0mon4 °»o 0(MgO)=IOmol%! 0.4 ■- 0.4 e (Al^op-IOmoM • (MgO)= 20mol% i • C • (Ayy-^OmoRi

* 9 0.2 • 0.2 - % Si01B 1 0 *) ° >W!V'1-0 (MgU)=NMg0/(iNc,0+NMg0+Nsio T) 0 (AI103)-NAiTch /(Nc»o+Nao 2+NAi!o 1)

20 40 60 80 20 40 60 mol % CrO, mo!%CrOv

Fig. 19 Effects of MgO and A1203 additions on the activity of CrO in the Ca0-Si02-Cr0x quasi-ternary slag system, standard states: pure liquid CrO (Xiao, 1993)

3.2 Comparison of activities of chromium oxides in slags and discussion

The activities obtained from different researchers under different experimental conditions have been listed and compared in Table 2. It seems that even though there are not so many available publications related to the activity determinations of chromium oxides, direct com­ paring of all these results is still very difficult due to the different experimental conditions: investigated slag compositions, temperatures, as well as equilibrium system oxygen partial 17 pressure. As an example, Investigation *oo Temperature Noo/NsiOi Method 1gP<>2 Fig. 20 shows a compari ­ O 1500 “C 1.0 emf 5-13.21 Present O 1600 “C 1.0 emf 5-12.05 son of the results from Present A 1600 "C 0.70 emf 5-12.05 WitoKaardenrni 1400 “C 0.73 5-1452 Pretori usfl91 1500 °C 1.0 eas eouflibrium -956.-1150.-12.50 Muan (1985), Muanf201 1500 °C 0.75 fas eotrilibrium "strocar redueins" Wijngaarden (1988), Pretorius et al. (1992) 0.8 ■- and Xiao (1993) under » o similar slag systems. A 0.6 • ■ / ° / o 0 general consistency of &CrO the activity values can be j ° 0.4 AT/ observed from the figure.

Temperature has a weak 0.2 - lowering effect on the : activity of CrO. Increas ­ ing the slag basicity will 10 20 30 40 50 increase the activity of mol%CrO x chromium oxides. Fig.20 Comparison of aCr0 in silicate slags from different researchers (Xiao, 1993)

Table 2. Summary of the literature on the activities of chromium oxides in slags.

System investigated Temperature,°C (Nca0+NMq0)/Nsi02 Cf/T* The results and comments Reference

CaF2-Ca0-Cr203 and Both CaO and Al203 decreased aCr203. Temperature effect was positive Mohanty and Kay CaF2-AI203-Cr203 with 1450-1550 - as Cr203 if CaF2/CaO=3, and negative if CaF2/CaO=4. Postive deviation (1975) Pt-Cr alloys from ideal in CaF2-Ai203-Cr203 and in CaF2-Ca0-Cr203 was observed if CaF2/CaO=4 and negative deviation if CaF2/CaO=3. Silicate slags in equilibrium The activity coeffident of Cr203 is greater than the activity coefficient of CrO. Rankin and Biswas with Fe-Cr-0 melts -1600 - CrO predominant With increasing slag basidty, the acticity coefficient of CrO will increase. (1975) (A review paper) At constant slag compositions, CrO shows the Henrian behaviour in slag. Mg0-Si02-Ai203-Cr203 with The activities of chromium oxide in the 29%Mg0-25%AI 203-46%Si02 slags Finn and Wellbeloved Fe-Cr alloy in Mo crucible 1650 0.94 as Cr203 containing 2, 4, 6% and pure Cr203 were measured respectively by EMF (1980) with partly alumina lining method. This is only trial work. No solid conclusion was obtained. Ca0-Si02-Al203-Mg0 CrO in the melts behaves amphoterically. In Ca0-AI2O3-Si02 system, Maeda and Sano -CrO, with solid carbon. 1500 0.69-1.45 CrO predominant activity coefficient of CrO will increase with Al203 where CrO behaves as (1982)

Cr3C2, and Pco-1 atm base. In the system, ac2O3-3.92x10' 16, acr0=3.79x1 O'3. CaO-Si02-CrO, Activity of CrO was determined by gas equilibrium method. in H2+C02 with MoCr alloy 1500 0.75-1.22 as CrO Activity of CrO has a positive deviation from the ideality. (1985) in equilibrium The activity-composition relations of CrO show similarity to those of FeO. Ca0-Si02-Al203-Mg0 Activity coefficients of CrO^s and CrO increase with increasing CaO Morita and Sano -CrO, with Mg0.Cr203 and 1600 2Mg0.Si02 sat. <0.24 content in the melts, and with increasing the system oxygen partial (1988) NiCr alloy at PO2=2.11x10" 9 pressure. CaO-Si02-CrOx with CrAg Activity of CrO in the slags was experimentally measured by EMF method, Wijngaarden alloy in equilibrium with 1400 0.73 as CrO activity of CrO shows a positive deviation from the ideal. (1988) metallic Cr crucible CaO-Si02-AI203-CrO, The activity coefficient of CrO increases with slag basicity and Al203 content Pretorius et al. in H2+C02 in equilibrium 1500 0.03-1.2 0.79 - 0.98 and with decreasing oxygen partial pressure, and the activity coefficient of (1992)

with PtCr alloy CrO,.5 decreases sharply with increasing slag basicity and levels off at B=0.7. Ca0-Mg0-Si02-Al203 The activities of CrO and Cr015 have a positive deviation from the ideal Xiao -CrO, in equilibrium 1500- 1600 0.2-1.5 0.15-1.0 solution in the studied slag systems. aCrOK will increase with slag basicity (1993) with metallic Cr in Ar and decrease with temperature. MgO and Al203 effects were also studied. Ca0-MgO-SiO2-AI203 The activity of CrO showed a positive deviation from ideality. The activity Pei and Wijk -CrO, with MgO saturation 1600-1650 1.74-2.41 0.18-0.49 coefficient of CrO increases with slag basicity. (1994) in H20+H2 mixture 3.3 Activities in chromite solid solution

In solid state study, Ban-ya et al.(1994) measured the activity of Fe0.Cr203 in Fe0.Cr203- Mg0.Cr203 spinel solid solution at 1150 - 1450 °C, the system was saturated with Cr203 and in equilibrium with AgFe alloy in the spinel crucible under the controlled CO- C02 gas ■ atmosphere. The solubility of Cr203 in the spinel phase was found to be negligibly small. The activities of constitu­ ents in Fe0.Cr203-Mg0.Cr203 spinel solid solution coexisted with Cr203 showed negative deviation from ideality. The results are shown in Fig.21.

In the similar way, Hino et al. (1995) meas­ ured the activities of Fe0.Cr203 in

Fe0.Cr203-Mg0.Cr203-Mg0.Al203 spinel Fig.21 Activities of constituents in Fe0.Cr2O3- solid solution at 1300 °C, and the results Mg0.Cr203 system at 1300°C (Ban-ya et al., 1994) also showed negative deviation from ideal­ ity. The changes of the activity of Fe0.Cr203 with the compositions were estimated by regu­ lar solution model and the comparing results are given in Fig.22, in which the activities of MgO.(Crx Al,.x )203 were calculated by Gibbs-Duhem integration. The calculated iso-activity curves of the constituents in the pseudo-ternary solid solution saturated with (Cr,Al)203 were calculated based on the regular solution model, and shown in Fig.23, where, (FeO)o 49 .(Cr2O3)0 5x , (MgO)0 5.(Cr2O3)05 and (MgO)0.47.(Al2O3)0 53 are the actual form of Fe0.Cr203, Mg0.Cr203 and Mg0.Al203. The calculated and observed results are in good agreement.

—O-FeOCrzOa - MgOCrzOs -D-FeOCrzOs-MgOTCnuAic^kOs (FeO)0 4g (Cr2O3)0 S1 —A—FeOCrzOa - MgO(Cro.sAifts)20j —♦-FeOCrzOa - MgO-(Crc^Mo»>203 -■-FeO-CrzOa - MgOAbOs

02 ,0.6

N Fe0 Cr203

Fig.22 Calculated and observed activities of Fe0.Cr203 Fig.23 Iso-activity curves in the (FeO)049 .(Cr2O3)0 5I- in Fe0.Cr203-Mg0.(Crx All.x )203 system saturated (MgO)0 5.(Cr2O3)0 5-(MgO)o,47.(Al203)0 53 system with (Cr,Al)203 at 1300°C (Hino et al., 1995) sat. with (Cr,Al)203 at 1300°C (Hino et al., 1995) 19

4 Phase diagrams of chromium containing oxide systems

Phase diagrams are an important tool in metallurgical research and development work as they give invaluable information on the phase relations and structures of the initial (raw) materials, the eventual intermediate products as well as the final products of the process chain. In com­ plex reaction systems phase diagrams also form a basis to realise and define the reaction mechanism, i.e., how a reaction proceeds. In this chapter, the chromium oxides related phase diagrams and phase relations will be introduced and discussed from simple to complex oxide systems.

4.1 Cr-O system

In order to better understand the role of chromium oxide in the metallurgical slags, it is nec ­ essary to be acquainted with the binary system of Cr-O. A part of the system is shown in Fig. 24. It seems that there is no stable intermediate crystalline phase between Cr and Cr203. The liquidus temperatures decrease sharply from about 2265°C of Cr203 melting point in air to 1705°C of the eutectic point due to the increase in divalent chromium content of the liquid oxide phase with decreasing oxygen potential. The oxygen partial pressure at the eutectic point is inferred be to of the order of 10"12 atm (Toker et ah, 1991).

For the system of Cr-O at high oxygen pressures, the P-T phase diagram in Fig. 25 gives a general result. From this figure, it can be observed that at pressures higher than 2.5 kbar, Cr3Og and Cr205 will become unstable. The field labelled with “low Cr50]2” seems appeared, but it is almost impossible to draw boundaries to “high Cr50]2”. Cr02 is the stable phase to 1600 °C at 70 kbar oxygen pressure. When the pressure is increased, the stability area of Cr02 increases to both high and low temperatures, and at very high pressures Cr02 becomes the only stable oxide in the Cr-O system (White and Roy, 1975).

100,000c-

, LIQUID / TWO LIQUIDS 1 /

10,000 N____ Low Cr$ O, N------j°<,

1000 Cr}0« * LIQUID Cr, 0.

500 600 700 800 TEMPERATURE (°C)

Fig.24 Equilibrium relations in the system Cr-Cr203 Fig. 25 P-T diagram for Cr-O system at high in weight percent (Toker, Darken and Muan, 1991) oxygen pressure (White and Roy, 1975) Under lower oxygen partial pressures, Temperature, °C the P-T diagram is shown in Fig. 26. 1730 1700 1650 1600 1550 1500 According to the figures 25 and 26, the most stable oxide of chromium is Cr203. Not only is it stable under cer­ tain higher oxygen partial pressures at high temperatures, but also it is stable Liquid up to at least 1650°C in contact with - 1W oxide metallic chromium in the binary sys­ tem Cr-O. A liquid oxide which could be approximately expressed as CrO exists at the temperatures above 1665°C. The results show that there might be a small stability field of 4,8 4,9 5.0 5,1 5,2 5,3 5,4 5,5 5,6 5,7 Cr304 at the temperatures range from 1650 °C to 1705 °C. Fig. 26 P-T diagram for Cr-0 system at low oxygen partial pressures (Muan, 1985) 4.2 Binary oxide systems

4.2.1 Fe-Cr-O system

The coexisting phases in the Fe-Cr-0 system as a function of oxygen pressure and Fe/Cr ra- • tios of the condensed phases at steelmaking temperatures are shown in Figs.27 and 28.

IVPRESENT won* f AFTER NOLLE* ETAL .LIQUID1, OXIDE 1 SPINEL* UQUIQ OXIDE

.SPINEL ' t SESQUIOXlOE _| \ * i \ SPINEL SPINEL ♦ LIQUID ALLOT

SPINEL * LIQUID ALLOY

CfjOj * LIQUID ALLOY O2O3 + LIQUID ALLOY

LIQUID ALLOY SPINEL*LIQUID ALLOY LIQUID ALLOT

Fig. 27 Phase diagram of Fe-Cr-O at 1600 °C Fig. 28 Phase diagram of Fe-Cr-0 at 1700 °C (Toker, Darken and Muan, 1991) (Toker, Darken and Muan, 1991)

When a chromium containing slag is equilibrated with a FeCr alloy, the system oxygen po ­ tential is quite low, and significant proportions of the chromium in the oxide phase may pres ­ 21

ent in the divalent state. A particularly inter ­ 2300 esting phase in Fe-Cr-0 system is the spinel 2200 LIQUID iron chromite. The stability area of the spinel depends on the system oxygen potential, the composition ratio n Cr/(n Cr+n Fe), and the tem­ perature. The temperature effect is mainly due to the formation of the lower valent chromium at higher chromium to iron ratio. 1600 -■

The iron oxide-Cr 203 system is shown in SESQUIOXIDE Fig.29 at high temperatures in air. The dia­ 1400 gram illustrates the strong stabilising effect 1300 - 60 70 eo socy). of Cr203 on phases in which iron is present in the bivalent state. The phase relations in Fig. 29 Phase diagram of Fe304-Cr203 in air the system under reducing atmosphere, are (Muan, 1975) shown in Figs.30 and 31.

Fig.30 Cr2OrFeO„ system in equilibrium with metallic Fig.31 Cr203-Fe0I1 system under C02/H2 atmosphere Fe after Hoffmann (1965) (Slag Atlas, 1995, p.64) with P02=10"9 ~10"H atm (Riboud and Muan, 1964)

4.2.2 Ca-Cr-0 system

The phase relations in the CaO-chromium oxide system depend on the system oxygen poten ­ tial. The phase diagrams under air, intermediate reducing condition and in equilibrium with metallic chromium are shown in Figs. 32-35, respectively.

When chromium oxide is combined with CaO in air, the big amount of chromium will pres ­ ent in the oxidation states higher than three valent. In general, from all these figures, it can be seen that CaCr204 is the main compound formed by CaO and Cr203. In the temperature range of 776 to 1297 °C, a detailed phase diagram was recently studied and is shown in Fig. 32. It can be seen that in CaO rich area of the system, different chromate phases appear and contain hexavalent chromium (CaCr204), hexavalent and pentavalent mixed chromium (Ca3Cr30,3), pentavalent chromium (Ca3Cr208 ) and pentavalent and quartavalent mixed chromium 22

(Ca5Cr30]2) depending on the temperatures. With temperature increasing, the lower chro ­ mium valent is more stable.

Nc/Wc+V 2600

250C-'-\ 2400 2300 —2265, 2200 a-CaCr2CL *UQU!D /- 21300- 2000 / L-VUQUID 1900 LIQUID / j 1800 I [cc-CaG^O^- 2 1000 1700 a-CaCr2G,j Cr203 _ 1600 \ LIQUID | LIME t r 1500 \ 'I LIQUID \ CdsCrs024 1400 ^-CaCr204_ X uquid X i 1300 - 1200 1100 h/ LIQUID | 1 ie$l_ 1000 UME*Caq'9vrCr,0s°24 ______I 1 I {B-CoCiyOa i CrpOs CaO B 20 9Ca0Cr 203-4Cr03 fso C^03 WEIGHT % CoOCr203

Fig.32 Phase diagram of CaO-Chromium oxide Fig. 33 Phase diagram of CaO-Chromium oxide in air ( Kaiser et at., 1992) in air ( Ford and White, 1949)

INTERMEDIATE Cr-METAL 2600 2600 -

2400 LIQUID

2200 LIQUID

o 2000 - - 2000 - LIQUID

iZ 1800 LIME ♦ 0!-CeCr,0. LIME ♦ a-CoCfjO*

LIME ♦ /-CoCrjO, fi* ESK LIME » ^-CoCfjO, 1200 -

CflO 10 20 SO 40 50 CoO CfjO, CoO 10 20 SO 40 SO CoOCr.O, WEIGHT % WEIGHT %

Fig. 34 Phase diagram of Ca0-Cr203 under unspecified Fig.35 Phase diagram of CaO-Chromium oxide in reducing conditions (de Villers and Muan, 1992) equ. with metallic Cr(de Villers and Muan,1992)

The phase diagram showing the CaO-CrOx system in equilibrium with metallic chromium includes an “X” phase, which has an approximate (Ca0 4Cr0 6)Cr204 composition, a significant amount of chromium exists in the divalent oxidation state, and it partly replaces the Ca2+ site 23 in the compound. In the high chromium content region, the liquidus temperatures are lower in equilibrium with metallic chromium than in the intermediate oxygen potential atmosphere.

4.2.3 Mg-Cr-0 system

The particular interest in the phase diagram of Mg0-Cr203 is the picrochromite which is very stable until about 2350 °C high temperature. But locations of boundary lines at very high temperatures in this system are not very accurately known, the Fig.36 shows only the ap­ proximate relations .

2400

2800l <2800 2200

2700 2000 LIQUID

PICROCHROMITE. \ /Liquidx

R20 3 {SESQUIOXIOE SOLID SOLUTION) cc 2300 PER1CLASE <2200 LIQUID PICROCHROMITE^

2000 PERICLASE PICROCHROMITE PICROCHROMITE Cr2°3-

TWO SESOUIOXIDES

20 30 40 50 60 WEIGHT %

Fig.36 Phase diagram of Mg0-Cr203 Fig.37 Phase diagram of Al203-Cr203 . (Muan, 1975) (Muan, 1975)

4.2.4 Al-Cr-0 system

For the system of Al203-Cr203 with similar sesquioxide structure, a complete solid solution series forms between the two isomorphic end members at high temperatures, resulting in continuous slopes of the liquidus and solidus curves. A miscibility gap exists at temperatures below about 900 °C, as expressed in Fig.37.

4.2.5 Si-Cr-0 system

The oxygen potential has a strong effect on the equilibrium relations in the silica-chromium oxide system. Two phase diagrams of Si02-CrOx in air or in equilibrium with metallic chro­ mium are shown in Figs. 38 and 39, respectively. The most noteworthy -characters in the pic ­ tures is the very low mutual liquid solubility of chromium oxide and silica in air, and most of the chromium exists in trivalent state. For the system in equilibrium with metallic chromium, i.e. in lower oxygen partial pressure, there exists much larger mutual solubility. The liquidus 24 and solidus temperatures will be lowered about 300 °C. In such case, the chromium exists mainly in divalent oxidation state.

liquids Two liquids Liquid. -2250 Crist obalitc

E 1900 Eskolaite

Tridymitc + liquid - 1720 1700 -

Eskolaite + cristobalUc Eskolaite + tridymiv

Mass %

f ig.38 Phase diagram of Si02-Cr203 in air Fig.39 Phase diagram of Si02-CrO in contact with Cr (de Villiers and Muan, 1992) (de Villiers and Muan, 1992)

4.3 Ternary oxide systems

4.3.1 Si-Ca-Cr-O system

The phase relations of the CaO-Si02-CrOx system in air and in contact with metallic chro­ mium were determined respectively by Glasser and Osborn (1958), and de Villiers and Muan (1992), and shown in Figs. 40 and 41.

Si02

Gillespite

Fig. 40 Phase relations at liquidus temperatures in Fig. 41 Phase relations at liquidus temperatures in the system Ca0-Si02-Cr203 in air the system Ca0-Si02-Cr203 in equilibrium with Cr (Glasser and Osborn, 1958) (de Villiers and Muan, 1992) 25

In the phase diagram in air, the major part of the mixtures of compositions on the Si02 side of the join Ca2Si04-CaCr204 may be considered ternary because essentially all the chromium is present as CrJ . On the other hand, on the CaO side of the join Ca2Si04-CaCr204, a signifi­ cant amount of chromium in the solid solution between various calcium silicate phases and their calcium chromite analogues will present in higher valence, maybe in Cr6+, Cr3+ or even Cr4+. The compound Ca3Cr2Si30,2(uvarovite) occurs at subsolidus temperatures, decompos ­ ing at 1370°C to a-CaSi03 and Cr203. The trivalent oxide of chromium is very effective in increasing the liquidus temperature of silicate mixtures.

In the system in equilibrium with chromium, the presence of a primary field of uvarovite Ca3Cr2Si3012 is a conspicuous feature of the liquidus surface. The strong reducing condition has lowered the solidus surface sufficiently compared with that of air. The “X” phase (Ca0 4Cr0 6)Cr204 also occurs on the liquidus surface of CaO-chromium oxide.

The phase equilibrium along the join Ca- Si03-Cr203 in air is expressed in Fig.42,

The CaSi03-Cr203 join is binary except ______2200 under the conditions where two liquids exist. For example, at 1370 °C, 3 2 3 2 3 3 Ca Cr Si 0!2, Cr 0 and a-CaSi0 coex ­ Cr20, + L ist. For mixtures with the composition within CaSi03 and Ca3Cr2Si3012, the a- and P-CaSi03 inversion can be approached from two directions, a-CaSi03 converting $ CoS'Oj+CojC^SijOg to p-CaSi0 3 at temperatures below 1125 WEIGHT X °C, and with reverse at about 1125 °C. Figs.43 and 44 show the two isothermal Fig.42 Phase diagram of CaSi03-Cr203 in air sections of the system at 1350 °C and 1400 (Glasser and Osborn, 1958) °C, respectively.

Si02 SiOz

Tridymite > Uvarovite CrgOj

Tridymite + a. CoSiOj ♦ Cr203 -—a CaSi03 \ Ronkinite +Uvarovite Ca3Cr»Si30,2

Co3Si05 + CoO*y?CoCr £04 Co3Si05 + CoO + /? CaCr204

Fig.43 Isothermal section at 1350 °C of the system Fig.44 Isothermal section at 1400 °C of the system Ca0-Cr203-Si02 in air(Glasser and Osborn, 1958) Ca0-Cr203-Si.02 in air(Glasser and Osborn, 1958) 26

These two figures were meant to show the phase relations among the crystalline phase above and below the thermal stability limits of the ternary compound. Optical properties of the sili­ cate phases are affected by the chromium in the structures, even though the chromium amount may be very small. Dicalcium silicate crystals are in greenish-yellow colour, lighter in colour than the darker-green glass in which they may be embedded. Wollastonite crystals exhibited a faint bluish tint. With rankinite crystals, pale yellow or pink to deep purple pleo- chroic schemes appeared. Tridymite crystals were pale pink.

In addition, Adendorff et al.(1992) de­ termined the crystal structure of Ca5Cr30]2 and Ca5Cr2Si012 based on the solidus phase relations of the system CaO-chromium oxide-Si0 2 in air, shown in Fig.45 (de Villiers et ah, 1987). From the structural analysis, it was concluded that to maintain charge balance in Ca5Cr3012, chromium must be tetravalent and hexavalent. This means that Si pre ­ dominates in tetrahedral site, which im­ plies that Cr ions in this site must be tet­ ravalent, in order to allow for isomor- phous substitution of Si4+. There is no CifCnOis Ca/CrO.h COO. Cr3+ in tetrahedral site. Fig.45 Solidus phase diagram of the system CaO-Cr oxide-Si0 2 in air (de Villiers et af, 1987)

4.3.2 Si-Mg-Cr-0 system

The liquidus phase diagram of the system Mg0-Si02-Cr203 in air is illustrated in Fig.46. From the diagram, it can be seen that the most noticeable features are the sweep of the two liquid regions crossing the diagram from the Cr203-Si02 side to the MgO-Si02 side, the large primary phase area of picrochromite MgCr204, the wedge of this primary phase area through the two liquid region, and the very restricted extent of the primary phase areas of magnesium silicates (forsterite and protoenstatite) adjacent to the MgO-Si02 join. It takes a rela­ tively small amount of Cr203 to saturate the silicate liquid with respect to the spinel phase. The thrust of the spinel Fig.46 Phase relations at liquidus. temperatures in MgO field through the two-liquid region -Si02-Cr203 in air after Keith (1954). (Muan, 1975) gives rise to a very complex sequence of appearance, disappearance, and reappearance of phases during crystallisation of certain mixtures in the system. In addition, the liquidus surface of the system in equilibrium with metallic chromium was also investigated, and collected in Slag Atlas, 1995, shown in Fig.47. 27

4.3.3 Si-Al-Cr-0 system

The phase diagram of the system Si02-Al203-Cr203 is shown in Fig.48. The main features are the two liquid region originating in the Cr203-Si02 side and closing as the A1203 content in­ creases, and the primary crystalline phase is the Al203-Cr203 solid solution (Muan, 1975).

Si02

0 a 100 K: CrO M: MgO

/

V \ ^

Fig.47 Liquidus surface in Mg0-Si02-Cr203 in Fig. 48 Phase diagram in the system Al203-Si02- equilibrium with Cr (Slag Atlas, 1995, p.141) Cr203 after Roeder et al(1968). (Muan, 1975)

4.3.4 Ca-Mg-Cr-0 system

The phase relations of the Ca0-Mg0-Cr203 system was investigated in neutral atmosphere by Panek (1976) and is illustrated in Fig.49. A solid solution with 97-100 wt% CaCr204 was formed in the CaCr204 end of the system. Periclase solid solution ap ­ peared in the high MgO range. There were invariant points at 37, 90 and 97 wt% CaCr204 at 2065 °C, and no liquid phase was formed in the system CaO- Mg0-CaCr204 below 2065 °C tempera ­ ture.

In 1988, Morita et al. studied the solubil­ ity of Cr203 in MgO-CaO melts in air, and found the solubility was in the range of 40 to 55 wt% of CrOx , illustrated in Fig.49 Phase diagram of MgO-CaCr204 Fig.50, in which more than 50% of (Panek, 1976) chromium was in hexavalent state. If the oxygen partial pressure was lowered from air to 10° atm, the liquid phase could not be ob­ served at any compositions. The existence of Cr6+ can promote the solubility of chromium oxide in the melt due to the low melting point of Cr03 of about 196 °C. 28

4.3.5 Ca-Al-Cr-0 system CaO

The system CaAl204-CaCr204 was studied by Ford and Rees (1958). The phase diagram is shown in Fig.51, the special feature in the diagram is the formation of an intennediate phase lOCaO.8 Al2O3.2CrO3.Cr2O] at a tem­ perature about 1400 °C. The chro ­ mium also exists with an oxidation state of Cr6C In 1992, Kaiser et al. (1992) investigated the system CaO- “CaCr204”-CaAl204 in air and under L : Liquid M : MgO C: CaO Cr Cr203 mildly reducing conditions. In the ?ig.50 Liquidus diagram of MgO-CaO-CrOx ternary system Ca-Al-Cr-0 in air, a hexava- system at 1600 °C in air (Morita et al.,1988) lent chromium bearing phase, the chrome-hauyne Ca4[(ALCr)6012](Cr04), appears, shown in Fig.52. Its solidus tempera ­ ture is higher than that of calcium chromate due to containing Cr6+. Be­ cause of tie-line relationships with chrome-hauyne, the compatibility regions containing low melting point of calcium chromate cover a consid­ Fig.51 Phase relations of Ca0.Cr203-Ca0.Al203 system erable proportion of the Ca-Al-Cr-0 (Ford and Rees, 1958) system with low solidus tempera ­ tures. The most noticeable feature of the system under a reducing atmosphere is the subdivision of the system into a calcium-rich and a low-calcium part by the Ca12Al14033-CaCr204 join. The liquidus phase relations of the ternary system are illustrated in Fig.53.

Fig.52 The system CaO-CaA l204-"CaCr204" isotherm at Fig.53 The system CaO-CaAfOATaCACV’ phase 1300 °C in air (Kaiser et al., 1992), (CaCr204) unstable diagram under H2/C02=1 (Kaiser et al., 1992) 6-12^7—Cai2Al[4033, C;A=Ca3AU06: CrHy= CaafALCrgO[2](CfO;) 29

It has seven solid primary phases and five ternary invariant points. The calcium chromate phases disappear whereas a new phase Ca6Al4Cr20]5 is stabilised. The calcium aluminate phases and their coexisting liquid phases dissolve only limited amount of Cr203. For exam ­ ple, at 1500 °C, liquid phase in equilibrium with solid a-CaCr204 phase contains 2.9mol% Cr203, and at 1700 °C, 13.3mol% Cr203.

4.3.6 Mg-Al-Cr-O system

The liquidus surface of the system Mg0-Al203-Cr203 is shown in Fig.54. At 1700 °C, the isothermal section for the system is expressed in Fig.55. The open circles in the figure repre ­ sent a single phase, and the black squares represent two phases. From the diagram, there was no detectable solubility of Cr203 in picro-chromite MgCr204 spinel even at 1700 °C tempera ­ ture, and this feature was due to the high octahedral field stabilisation energy of CrJ+ ion. The maximum solubility of A1203 in MgAl204 at 1700 °C was about 70 mol%.

2700-• AI2Oj —2600 •—*2400

V

; A SPINEL,

Fig.54 Isothermal curves of system Mg0-Al203-Cr,03 Fig.55 Isothermal section of system Mg0-Al203-Cr203 (Wilde and Rees, 1943) at 1700 °C (Stubican and Greskovich, 1975)

4.3.7 Fe-Mg-Cr-O system

For the system of MgO-iron oxide-Cr 203, a tetrahedron diagram by taking the variation of Fe0/Fe203 into account is shown in Fig.56. Woodhouse and White (1955) investigated the dissociation paths of mixtures in a plane of 30 mol% Cr203 at various temperatures by ther ­ mogravimetry method. The results are represented in Fig.57. From the figures, the conjuga ­ tion lines between coexisting sesquioxide and spinel phases are related to temperature, the stability field of spinel is not limited to the join corresponding to stoichiometric spinels, but extends into an area where the ratio of sesquioxides to divalent oxides, i.e., (Cr203+Fe203)/(MgO+FeO), is variable and in excess of one. The FeO to Fe203 ratio of the spinel phase increases rapidly with temperature at constant oxygen partial pressure (Muan, 1975).

At 1300 °C, the phase diagram of the system MgO-iron oxide-Cr 203 over a range of oxygen pressures of 0.2 to 10"11 atm was studied by Ulmer (1969) and is shown in Fig.58. It shows 30 that the spinel phase contains a certain amount of trivalent iron even under very strong reduc­ ing atmosphere.

62.4% FeO

SP + MW

MgO 70% Fe20- 70% MgO

MOL % MOL %

Fig.56 Tetrahedron Mg0-Fe0-Fe203-Cr203 system Fig.57 Phase relations in 30mol% Cr203 plane in air with a plane of 30mol% constant Cr203 in Mg0-Fe0-Fe203-Cr203 system (Woodhouse and White, 1955)

MgCr.O.

AIR or I0"1

SES0U10XIDE +SPINEL

-7 -6 SESQUIOX1DE

Fig.58 Phase diagram of MgO-iron oxide-Cr 203 at 1300 °C under different oxygen partial pressure (Ulmer, 1969)

4.3.8 Fe-Si-Cr-O system

The phase relations of the Fe0n -Si02-Cr203 system was constructed by Muan and Somiya (1960) and is expressed in Fig.59 in air and in Fig.60 in contact with metallic iron. From the diagram in air, it can be noted that the dominant chromite phase and the wedge shaped field of this phase are cutting through the two-liquid region. In the system equilibrated with metal­ lic iron, the homogeneous liquid regions at 1230 °C and 1400 °C are illustrated with dot and dash-dot lines (Slag Atlas, 1995, p.140). 31

Si02

0 a 100 w: Feo

■WCss f

/ so y / 00

Mass % FeO Mass % Cr203 —

Fig.59 Phase relations in Fe0x -Si07-Cr,03 in air Fig.60 Phase relations in FeOx -SiO,-Cr,0, in contact with Fe (Slag Atlas, 1995, p.140)

4.3.9 Fe-Al-Cr-0 system

The isothermal sections of the FeiOv-AhCh-Cr-oCh in air at 1250°C and 1500°C are shown in Figs.61 and 62, respectively.

ONE SESQUIOXIDE PHASE ONE SESQUIOXIDE PHASE

I. * SESOIOXIOE

Fig.61 Isothermal section in iron oxide-Al 203-Cr203 Fig.62 Isothermal section in iron oxide-Al 203-Cr203 in air at 1250°C(Muan and Somiya, 1959) in air at 1500°C(Muan and Somiya, 1959)

At 1250°C, all the iron is present in trivalent state, only sesquioxide phases exist in the sys­ tem. There is an extensive miscibility gap between Fe203 and A1203, when Cr203 is in­ creased, the gap will gradually close. When the temperature is increased to 1500°C, a spinel phase will form and become a dominant phase, and coexist with one sesquioxide solid solu­ tion. Figs. 63-6 5 show the phase relations of the system under different atmosphere. 32

Cr2°3 Cr203

0 O 100

-% 0 40 / FeOAI203 Mass % AU03 —- Mass % AUO.

Fjg.63 Liquidus surface in iron oxide-Al,0 3-Cr203 under Fig.64 Liquidus surface in iron oxide-Al 203-Cr203 Ar and 02 atmosphere after Rosenbach and Schmitz(1974) in air based on Muan and Somiya(1959) (Slag Atlas, 1995, p.107)

4.4 Quarternary oxide systems

4.4.1 Ca-Mg-Si-Cr-0 system FeO„

In the Ca0-Mg0-Cr203-Si02 system, a spinel solid solution is in equilib­ rium with the liquid. The solubility of chromium oxide (saturation con ­ centration), in equilibrium with a chromium-containing solid phase, decreases with increasing melt ba­ sicity and increasing oxygen poten­ tial in the system. The addition of MgCr204 to CaMgSi04 or Ca3MgSi208 will raise the liquidus temperature of the system. The phase Fig.65 Isothermal section in iron oxide-Al 203-Cr203 at 1550°C diagrams of MgCr204-CaMgSi04, under low oxygen partial pressure of 7.61x10' atm MgCr204-Ca3MgSi208 and a section (Jacob etal., 1987) of the MgO-Cr2Q3-CaSiO are shown in Figs.66, 67 and 68 respectively. The reaction invariant point is at 1490 °C. In CaO-CrOx - Si02 system the composition of the equilibrium solid phase (eskolaite) stays constant, whereas in the Ca0-Mg0-Cr203-Si02 system the composition of the solid phase (spinel) is dependent on the melt composition. The actual values for the chromium oxide solubility in the two systems are almost identical for siliceous melts. The concentration of Mg2+ in these spinels decreases with decreasing basicity and decreasing oxygen potential. The chromium content in the olivines increases with decreasing oxygen potential. Liquid 2200

Liquid, 2000 Sp ss+ Liquid Liquid 2000

Spinel + Liquid

Sp ss rMgO + Liquid

-1590 ± 1C

1400 CMS+MgO+ Liquid'

C,MS.

3CaO- MgO-2SiO

Fig.66 Phase diagram of MgCr204-CaMgSi04 Fig.67 Phase diagram of MgCr204-Ca3MgSi208 (El-Shahat and White, 1966) (El-Shahat and White, 1966)

CrA The spinel phase ((Mg,Cr)Cr204) is stable with silicate melts over a large range of melts compositions. As an approximation, MgCr204 the relative "strength" of the basicity of 2355 CaO and MgO are considered the same. MgO is known to be a somewhat weaker base than CaO. However, the molecular weight of MgO (40.31) is considerably lower than that of CaO (56.08). Therefore, it can be assumed that on a wt% basis the basicities of these two oxides are very 60 y 1502° CoSiO, similar. CoMgSi04

Fig.68 Phase relations of Mg0-Cr203-CaSi03 (El-Shahat and White, 1966) 4.5 Quinary oxide systems

4.5.1 Ca-Mg-Al-Si-Cr-0 system

El-Shahat and White (1964) investigated the system of MgAl204-MgCr204-Ca2Si04 and three sections through this ternary system: MgAl, gCrg 204-Ca2Si04, MgAl15Cr0 5O4-Ca2SiO4 and MgAlCr04-Ca2Si04. The phase diagrams of these three binary systems are expressed in Figs.69, 70 and 71, respectively. It can be easily seen that these three diagrams are very similar. At lower temperatures, the spinels and Ca2Si04 will form solid solutions in all three binary systems. It is evident that the binary eutectic temperatures increase with increasing the Cr203 content of the spinel.

The solid-liquid phase relations when Ca2Si04 is added to the spinel components MgAl2G4 and MgCr204 are expressed in Fig.72. There are two main characteristics in this diagram. Firstly, the liquidus and solidus temperatures increase sharply with increasing MgCr204 to MgAl2Q4 ratio. Secondly, the solubility of the spinel component in the liquid coexisting in equilibrium with Ca2Si04 and spinel decreases with increasing MgCr204/MgAl204 ratio. 34

Later in 1966, El-Shahat and White studied the system MgAl204-MgCr204-CaMgSi04, es­ pecially three sections of the system: MgAl, 5Cr0 504-CaMgSi04, MgAlCr04-CaMgSi04 and MgAl0 5Cr, 504-CaMgSi04. The phase diagrams are shown in Figs.73-76, respectively. Similarly, the phase relations in the system MgAl204-MgCr204-Ca3MgSi208 and its section

2100 2100 " \ Liquid Liquid

1900 1900

x C2S + Liq.

1700 Spinel + Liq. Spinel + Li q. 1560 ±5'

1500 1500 -

Spinel + C2S ■+ Liq. Spinel + C-S Spinel + C2S

20 40 60 80

Fig.69 Phase diagram of MgAl, 8 Cr0 204-Ca2Si04 Fig.70 Phase diagram of MgAl, 5Cr0 504-Ca2Si04 section in MgAl204-MgCr204-Ca2Si04 ternary section in MgAl204-MgCr204-Ca2Si04 ternary system, c2S:2CaO.Si02 (El-Shahat and White, 1964) system, C2S:2Ca0.Si02(El-Shahat and White, 1964)

Ca2Si04

2100 - Liquid

1900

1700 Spinel + Liq. 1500'

1450 -PrP 1500 1500-5°

40 50 Co,SiO, 50 MgAl204 50 MgCr204

Fig.71 Phase diagram of MgAlCr04-Ca2Si04 Fig.72 The liquidus surface of the ternary section in MgAl204-MgCr204-Ca2Si04 ternary system MgAl204-MgCr204-Ca2S104 system, c2S:2Ca0.Si02 (El-Shahat and White, 1964) (El-Shahat and White, 1964)

MgAl, 5Cr0 504-Ca3MgSi208 were studied (El-Shahat and White, 1966). The results are shown in Figs.77, and 78. In general, from all these figures, the presence of Cr203 in the spinel will raise the initial melting temperature and lower the solubility of the spinel in the liquid silicate. 35

2200 2200

Liquid 2000 2000

Sp ss +Liquid 1760: 10'

Sp ss + MgO + Liquid

CMS+MgO +Liquid- Sp ss+MgO+Liq. Spss +CMS+MgO+Uq.- 1480'

1410*10° 1400 Sp ss+CMS+MgO + Li quid' Spss + CMS Sp ss+ CMS

MgAI^Cr^Q, 20 40 60 80 CoMgSiQ, 80 CoMgSiO,

Fig.73 Phase diagram of MgAl, 5Cr0 504-CaMgSi04 Fig.74 Phase diagram of MgAlCr04-CaMgSi04 section in MgAl204-MgCr204-CaMgSi04 system, section in MgAl204-MgCr204-CaMgSi04 system, C2S:2CaO..SiO,(EI-Shahat and White, 1966) C2S:2Ca0.Si02 (El-Shahat and White, 1966)

CoMgSiQ, 2200

Liquid

2000

1760*10*

MgO+Liq.

Sp ss + CMS♦MgO * Liq. • 1400 - Sp ss + CMS

Fig.75 Phase diagram of MgAlo.5Cr, 504-CaMgSi04 Fig.76 The liquidus surface of system section in MgAl204-MgCr204-CaMgSi04 system, MgAl204-MgCr204-CaMgSi04 c2S:2CaO.SiG2 (El-Shahat and White, 1966) (El-Shahat and White, 1966)

3CoO-MgO-2Si02 2200

2000 -

____ S£jnell.Ms22;Lia:__ 15251

1400 Spi nel ♦ CgS + Mer ^MgO + Liq. _

Mg0'Cr203 3CoO'MgO-2SiO ; 3Co0-Mg0-2Si0 2 3CoO-MgO-2Si02

Fig.77 Phase diagram of MgAl, 5Cr0 504-Ca3MgSi204 section Fig.78 The liquidus surface of the ternary in MgAl204-MgCr204-Ca3MgSi20s ternary system system MgAl204-MgCr204-Ca3MgSi208 C2S:2CaO.SiQ2, Menmerwinite (El-Shahat and White, 1966) (El-Shahat and White, 1966) 36

Noteworthy is the very low solubility of chromium oxide in basic melts saturated with a spinel phase, indicating that a very small amount of Cr203 is needed to saturate the liquid with spinel. The most significant feature of the addition of A1203 and/or MgO to the CaO- CrO-Si02 system is the dominance of sesquioxide(Al 203-Cr203 solid solutions) or spinel ((Mg,Cr)Cr204 and (Mg,Cr)(Al,Cr)204 solid solutions) as primary crystalline phases on the liquidus surface. This is in accordance with the well-known octahedral site preference of bi ­ valent chromium which causes this ion to be sbongly partitioned into sesquioxide and spinel structures in preference to the liquid phase. Because of the extensive use of oxides with the spinel structure in the steelmaking and ceramic industry, knowledge of the various reactions between spinel, liquid, and a metal phase, is of great interest.

In addition, in this multi-component (CMS2, system, the liquidus surface in the sub- n 000 * - - system CaO.Al9Oq.2SiO,- CaO.MgO.2SiO a /t~r\ rv n 2Mg0.Si02 with 0.2, 0.3 and 0.5 wt% of Mg0.Cr203 was studied by Onuma and Tohara (1983). The results are shown in Figs.79, 80 and 81.

For the system Ca0-Al203-Cr203-Si02- MgO at all oxygen potentials, a spinel phase is in equilibrium with the liquid over a large melt composition range. In 20 40 60 80 99.8 (M2S) basic melts, the equilibrium between Mass % M2$ spinel and liquid is limited by the forma­ Fig.79 Liquidus surface in system of Ca0.Al203.2Si02 - tion of an olivine phase. In acidic melts, CaO.MgO.2SiO2-MgO.Cr2O3-2MgO.SiO2 with at oxygen potentials of 10"9 '37 and 10"n '3° 0.2wt% of Mg0.Cr203 (Onuma and Tohara, 1983) atm, a Cr203-Al203 sesquioxide phase becomes a stable phase with the liquid. The addition of A1203 and MgO results in a substan­ tial decrease in the chromium solubility in basic melts (Pretorius, 1989).

99.7 20 40 60 80 99.7 0 20 40 60 80 99.5

Mass % M2S

Fig.80 Liquidus surface in system of Ca0.Al203.2Si02 Fig.81 Liquidus surface in system of Ca0.Al203.2Si02 -Ca0.Mg0.2Si02-Mg0.Cr203-2Mg0.Si02 with 0.3 -Ca0.Mg0.2Si02-Mg0.Cr203-2Mg0.Si02 with wt% of Mg0.Cr203 (Onuma and Tohara, 1983) 0.5wt% of Mg0.Cr203 (Onuma and Tohara, 1983) 37

4.6 Chromite raw materials

Natural chromite ore can be described with the aid of the following compounds: Fe304, Fe0.Cr203, Mg0.Cr203, Mg0.Al203 and (Cr,Al)203. These compounds form a complicated spinel structural solid solution. Among these components, magnetite Fe304 can be easily re­ duced in the process, for example, by carbon in the EAF furnace for FeCr production. The main concerned components in practice are the Fe0.Cr203-Mg0.Cr203-Mg0.Al203 pseudo-ternary spinel solid solutions coexist ­ ing with (Cr,Al)203 solid solution. Typical chromite compositions, world wide, can be • South Africa represented on the cross-section formed by O Turkish A Philippines Fe0.Cr203-Mg0.Cr203-Mg0.Al203 in the □ Indian Cr2°3 V Russian Fe0-Mg0-Cr203-Al203 quartemary dia­ 0 Commercial gram, as shown in Fig.82 (Hino et al., 1995). \ Chromium Pellet Therefore, chromite mineral can be ex­ pressed approximately as FeO ■ Cr203 (Fe,Mg)0.(Cr,Al)203 solid solution with spinel structure. Iron chromite (Fe0.Cr203) exists as the major component and picro- chromite (MgO.Cr203) as the second one in FeO ai 2o 3 the mineral. MgO • Al203

The phase relations of MgAl204 and MgO MgCr204 in the chrome are expressed in Fig.82 Average chromite composition in system Fig.83. In the diagram it is observed that Fe0-Mg0-Cr203-AI203 (Hino et al., 1995) there are four solid phases in the system Ca0-Mg0-Al203-Cr203-Si02 including chrome, periclase, monticellite and forsterite. The upper curve represents the change in liquidus temperature of the solid phases in equilibrium with vapour and liquid in the system. A portion of the solidus curve for these compositions and the temperature of the invariant points in end member systems of the series are given. From the diagram, the tempera ­ ture of the liquidus line changes more rapidly with composition near the alu­ mina end of the series. It should be in­ dicated that the six phase region in the Ca0-Mg0-Cr203-Si02 system contains about 2.5% of Mg0.Cr203, whereas the corresponding point in the CaO-MgO- Al203-Si02 system contains approxi ­ mately 15% of Mg0.Al203 (Berry, Al­ Fig. 83 Phase diagram of MgAl204 and MgCr204 in len and Snow, 1950). chrome (Berry et al. 1950) 38

Both Cr304 and distorted spinel were found to be stable only at high temperatures and they disproportionate on cooling according to the following reactions: 3Cr304 = 4Cr203 + Cr and 3(Fe0.Cr203-Cr304) = 3Fe0.Cr203 + 4Cr203+Cr. In ore and in chrome refractory materials, chromium occurs predominantly in spinel crystals with Fe2+ and Mg2+ as additional divalent ions in the structure and AT+ and FeJ as additional trivalent ions. With time, however, chromium oxide does not form a spinel structure, and other valence states of chromium may predominate in lime-chromium oxide mixtures.

In brief, about sixty phase relation diagrams were collected in this chapter to demonstrate the thermodynamic properties of chromium containing oxides systems. It seems that the phase relations of the metallurgical chromium oxide systems have been broadly investigated from monomeric Cr-0 system to multicomponent oxide systems of Fe, Ca, Mg, A1 and Si. The monometric, binary and ternary systems have been investigated systematically, mainly in air atmosphere, only some systems have been studied in equilibrium with metallic chromium. However, for the quarternary chromium containing oxide systems, only Ca-Mg-Si-Cr-0 sys­ tem was reported in the literature. The quinary Ca-Mg-Al-Si-Cr-0 system was only studied in air atmosphere, and the chromite spinel phase was the primary interest.

Based on the available information, the most important features of the liquidus surface of chromium, oxide containing silicate systems are the large liquid miscibility gaps and the dominance of the chromite spinel stability area. Subsolidus relations of special interest are those involving spinel phases. The oxygen potential of the gas phase is an important parame ­ ter in chromium oxide containing systems, because chromium may occur in different states of oxidation.

Obviously, the phase diagram information on chromium containing oxide systems are still not sufficient. A further study is needed especially in the systems under different oxygen par­ tial pressures. 39

5 Distribution of chromium between slag and metal

Ferrochromium and stainless steelmaking processes involve slag and metal phases in the melting and refining in EAF and AOD furnaces. A most essential requirement in these proc ­ esses is high chromium recovery from the slag to the metal phase. The slags generally con ­ tain CaO, MgO, A1203, Si02, CrOx and FeOy. The basic slag system for FeCr production is Mg0-Al203-Si02-Cr0x, and that for stainless steel production is CaO-Si02-CrOx . Chromium oxide can volatile at high temperatures, but any quantitative data is not available in the litera­ ture in order to judge the chromium oxide loss from the process by volatilisation. Usually, system atmosphere, temperature, composition of charge, size of charge, and type of furnace are all factors in chromium oxide loss. In general, the chromium loss from an open furnace is significant for runs of several hours duration at 1500 and 1600°C. For a closed furnace under similar conditions, however, the loss is considered to be negligible. In the chapter, the factors influencing chromium recovery from FeCr and AOD slags will be discussed based on litera­ ture, including FeO content, Si in metal, slag basicity, and temperature etc..

5.1 Distribution of chromium between slag and FeCr

In 1958, McCoy and Philbrook investigated the behaviour of chromium in slag-metal systems under carbon saturated reducing conditions, and discussed the chromium distribution between CdO/SO* slag and metal. In the system, the reduction of silica and chromium oxides from the slag occurred si­ multaneously, and the silica re­ duction in the slag was the con­ trolling reaction. The dependence of chromium distribution ratio on the silica -content of the slag at 1500 °C and 1600 °C are shown in Fig.84. The lime-silica ratios corresponding to the silica con ­ Fig.84 Chromium distribution ratio with silica content of the slag (McCoy and Philbrook, 1958) tents at constant alumina-lime ratio of 0.53 in weight were displayed on the upper abscissa scale of the figure. It seems that the chromium distribution ratio has a quite low value for lime-silica ratios above 1.2, but ris­ ing to a value of about 0.04 with a basicity 0.6. At lower temperature, 1500 °C, the ratio seems to be somewhat higher than that at 1600 °C. The reduction speed of silica was about 20 to 60 times slower than that of chromium oxide under the studied experimental conditions. It was concluded that the recovery of chromium in the metal can be nearly complete under strongly reducing conditions.

Maeda et al. (1981) studied chromium recovery from chromium-containing slags. They con­ cluded that mass transfer of chromium from the slag bulk to the interface controls the rate of chromium reduction. Chromium recovery from ferrochrome slags can reach 90-99% within a reasonable time. Most of the chromium left in high carbon ferrochrome slags was found to 40 exist as undissolved chromite and its reduction behaviour was markedly different from that of other slags. The addition of a small amount of silica could increase the reduction rate. The chromium recovered was as chromium carbide (Cr3C2) or Fe-Cr-C alloy.

R.H. Eric and Akyuzlu (1990) investigated the slag/metal equilibrium in FeCr production conditions, i.e., Si02-Ca0-Mg0-Al203-Cr0x -Fe0 slags and Cr-Fe-Si-C (sat.) alloys under Ar and CO atmosphere at 1500 °C and 1600 °C. They indicated that the chromium content of the slag decreases with increasing slag basicity, and this effect was stronger at relatively low ba ­ sicities, shown in Figs. 85, 86 and 87. It can be observed that the chromium content of the slag decreased rapidly as the basicity increased. Figs.88 to 90 display the equilibrium chromium contents in the slag compositions in the diagrams. The relationships of chromium partition with the iron partition in the slags are shown in Figs.91-93. In addition, the effect of FeO on the chromium oxide concentration in slag was also studied, the results are illustrated in Figs.94, 95 and 96. It was derived that the chromium content of the slag increased with in­ creasing FeO content due to the transfer of oxygen to the metal phase.

MgO Cap MgO - Cap

Fig.85 Effect of slag basicity on chromium content Fig:86 Effect of slag basicity on chromium content in in slag at 1500°C in Ar in contact with Fe-Cr-Si- slag at 1500°C in CO in contact with Fe-Cr-Si- C(saturated) alloy (Eric and Akyuzlu, 1990) C(saturated) alloy (Eric and Akyuzlu, 1990)

MgO * CaO SiO-

Fig.87 Effect of slag basicity on chromium content in Fig.88 Chromium concentration in FeCr slags slag at 1500°C in Ar in contact with Fe-Cr-Si- slag at 1500°C in CO in contact with Fe-Cr-Si- C(saturated) alloy (Eric and Akyuzlu, 1990) C(saturated) alloy (Eric and Akyuzlu, 1990) 41

SO,

0.172-0.012

/ ^

Fig.89 Chromium concentration in FeCr slags Fig.90 Chromium concentration in FeCr slags at 1500°C in CO in contact with Fe-Cr-Si- at 1600°C in CO in contact with Fe-Cr-Si- C(saturated) alloy (Eric and Akyuzlu, 1990) C(saturated) alloy (Eric and Akyuzlu, 1990)

- 1.75- Slope = 0,52

-2,00-

” - 2.50-

-2.00 -t.15 -1.50

Fig.91 Chromium partition ratio with iron partition Fig.92 Chromium partition ratio with iron partition ratio at 1500°C in Ar in contact with Fe-Cr-Si- ratio at 1500°C in CO in contact with Fe-Cr-Si- C(saturated) alloy(Eric and Akyuzlu, 1990) C(saturated) alloy(Eric and Akyuzlu, 1990)

-1.75 -

-2.00

-2.25

-2,50 -

-3.00 -3.00 -2.75 -2.50 -2.25 -2.00 -1,75 -1,50

log [

0.2 -

(FcO), «7« (FcO). Vs

Fig.95 Effect of FeO on chromium content in slags Fig.96 Effect of FeO on chromium content in slags at 1500°C in CO Ar in contact with Fe-Cr-Si- at 1600°C in CO Ar in contact with Fe-Cr-Si- C(saturated) alloy (Eric and Akyuzlu, 1990) C(saturated) alloy(Eric and Akyuzlu, 1990)

The temperature and slag compositions would have an effect on chromium partition. The re­ covery of chromium is enhanced by the presence of A1203 in the melt and increases with the A1203 content of the melt. The presence of MgO in the melts leads to the formation of a spinel phase which has a very low solubility in basic melts. The addition of MgO greatly en ­ hances the recovery of chromium in basic melts with a basicity ratio larger than 0.80. The re­ sults indicate that chromium recovery is further enhanced by the addition of A1203 to MgO- containing melts and that maximum recovery will be obtained for basic melts saturated with a (Mg,Cr)(Cr,Al)204 spinel solid solution. In general, increasing basicity and the addition of suitable oxides to a melt are effective ways of reducing the loss of chromium to the slag. The beneficial effect of increasing basicity, however, levels off when the basicity ratio exceeds a specific value.

5.2 Distribution of chromium between slag and metal (steel)

Robison and Pehlke (1974) carried out experimental work on the reduction kinetics of chro ­ mium oxide in a basic steelmaking slag (45wt% CaO, 35wt% Si02, 10 wt% MgO and 10 wt% A1203) by silicon dissolved in liquid iron under argon atmosphere at steelmaking tem­ peratures. The slag and metal were contained in zirconia crucibles. The typical reaction curves for chromium oxide reduction by silicon are shown in Fig.97. The solubility of triva- lent chromium in liquid slag as a function of equilibration temperature is expressed in Fig.98 in the range of 1550 to 1625 °C.

The experimental results showed a moderate effect of stirring on the reduction rate of biva ­ lent chromium oxide. The reaction was not limited by the rate of an interfacial chemical reac­ tion, but instead it was limited by mass transportation. Temperature had a significant effect on the reduction rate as well. The activation energy for the trivalent chromium reduction was 100 kcal/mole, and that for the divalent chromium reduction was 130 kcal/mole. 43

(5) Si 0% in Stag (2) Fe (6) Qr in Metot (7) Si “ " 1625- Total

1600-

(6) -

C7> - 34.0

1550-

15 30 45 60 75 90 105 120 TIME* MINUTES

% Cr' DISSOLVED IN SLAG Fig.97 Typical reaction curves for chromium oxide Fig.98 The solubility of trivalent chromium in reduction by silicon (Robison and Pehlke, 1974) liquid slag as a function of temperature (Robison and Pehlke, 1974)

Rankin and Biswas

(1975, 1978) re- ACID HEARTH STUDIES BASIC HEARTH STUDIES ported the chro ­ ' A. Grant et a/33 1610*(av) mium behaviour Basicity = 1.8 (av) B. Lapitskir*® and distribution in C. Pathy and Ward33 1686 ‘C slag-metal systems, Basicity = 1.2-2.0 and discussed the effect of FeO con ­ A. Korber and Oelsen 21 1600-1640‘C D. Tesche -’ tent of the slag, Si B. Schenck et a/39 1540-1740‘C 1590 ‘C. less than 1%C Basicity = 3.7 (av) C. Kojima and Sano 33 E. Bargone et a/* 3 content of the metal, 1650.1600. 1650‘C 1630-1690 ‘C slag basicity, and Basicity = 3.6 (av) temperature on the (% FeO) recovery of chro ­ Fig. 99 Chromium distribution ratio with FeO content of slag for mium from slags in acid and basic slags (Rankin and Biswas, 1975) stainless steelmak­ ing process. Fig.99 presents the chromium distribution ratio as a function of FeO content of the slag. It is clearly seen that chromium recovery is improved by low FeO content in slag, it means by lower oxygen potential in the system. The relations of chromium distribution ratio with Si content of metal are described in Fig. 100. Silicon content in the metal has a positive effect on the chromium recovery, which is quite reasonable because silicon is regularly used to reduce chromium oxide from decarburization period slags back to stainless steel melt in common AOD process. For the effect of slag basicity, from the well known industrial experi­ ence, chromium has a better recovery in basic slag system compared to that in the acid. For constant FeO content, chromium loss in the slag is decreased by increasing temperature at constant silicon levels. In addition, the chromium equilibrium in reduced slag-alloy systems was examined, the alloys containing 5-30% Cr and less than 2.5% Si, and the slags being CaO-Si02-AI203-CrOx -FeO with Ca0/Si02 between 0.27 and A1203, and A1203 content from zero to saturation. The iron and chromium contents of the slags were less than 2% Fe and 10% Cr. The experimental results of the variation of chromium partition ratio with iron parti ­ tion ratio and silica activity are expressed respectively in Figs. 101 and 102. The linear rela­ tionships were observed. 44

Korber and Oclsen, 1935: l600-i64U°C; Silica saturated. l.S-61% Cr; Mn0^; 6-27%Cr • Rankin and Biswas, 1978: I600°C; Basicity*!.?; 24-40%A!;O2; 4.5-3 l%Cr

log %Si Fig. 100 Chromium distribution ratio with Si Fig.101 Relations of chromium distribution ratio with iron content of metal (Rankin and Biswas, 1975) distribution ratio (Rankin and Biswas, 1978)

Group Co0/Si0 2 7.Al202 Group CoO/Si0 2 V.Atj0- -1-0 - I • 0-27-0-28 7 Ho m 0 36-0 49 0 _ Hb • 043" 11-u 043" 18-24 045-049 37-45

, __ Group Cc0/Si02 "4At20, Group CoO/Si 02 7.Al203 HI a m 0-62-077 1-78-2 20 25-38 - lb* 0-58-0-70 HIco 0-61-077 36-45 190-2-26 41-50

Fig. 102 Variation of chromium distribution ratio with silicon activity (Rankin and Biswas, 1978)

It can be concluded that increasing the basicity is an effective way of reducing the loss of chromium to the slag when the basicity is relatively low. The beneficial effect levels off sig­ nificantly, however, when the basicity exceeds 1.4. In the mean time, it must also be kept in mind that the slag volume in practical operations increases when lime is added to the slag to increase its basicity. Hence, a point will be reached where the actual loss of chromium to the slag may increase even though the concentration of chromium in the slag decreases due to the volume increase. 45

6 Summary

In the present report, a literature review was given on the thermodynamic properties of chromium bearing slags and raw materials. Based on the knowledge from the literature, a broad discussion was made concerning various topics, including the analysing methods for determining different chromium oxides in metallurgical slags, the oxidation state of chro ­ mium in slags under different atmospheres, and the activities of chromium oxides in slags and in chromite solid solution. In addition, the phase diagrams of chromium related oxide systems were collected, and the chromium distributions between slag and metal were analysed under FeCr and stainless steel production conditions. In brief, the following remarks can be sum­ marised.

Four wet-chemical analysing methods and one spectroscopic method were found in the litera­ ture for determining oxidation state of chromium in slags. Most of these analysing methods are limited to iron free systems. For the wet-chemical methods, there are certain difficulties such as sample preparation, slag dissolution and elements interaction. To get a more accurate analysing result, a combined method of wet-chemical and spectroscopic analysis is recom­ mended. Special attention should be paid to the sample preparation.

In silicate slags under reducing atmosphere, both divalent and trivalent chromium are present in all slags. It is agreed that the fraction of divalent chromium Cr2+/TCr increases with increas ­ ing temperature, lowering oxygen potential and decreasing slag basicity. However, due to the differences in studying and analysing methods, the quantitative results are scattered and need to be further investigated. For the slags under oxidising atmosphere, trivalent, pentavalent and hexavalent states were reported to be the possible existing forms of chromium. Higher oxida­ tion state is favoured by high CaO content and low temperature. Chromite ore used in FeCr production contains mainly divalent and trivalent chromium depending on its formation and environmental conditions.

Regarding to the activities of chromium oxides in slags, roughly ten researchers were found who measured activities of chromium oxides in slags under different conditions during the years 1975 to 1994. Direct comparison of these results is difficult. It is, however, clear that the activities of CrO and CrOj 5 have positive deviation from ideal solution. Slag basicity and A1203 content have a positive effect and temperature has a negative effect on the activities of chromium oxides in the slags. A few studies indicate that the activities of Fe0.Cr203 and MgO.Cr2Q3 constituents in chromite solid solution have negative deviation from ideality. The solubility of Cr203 in the chromite spinel phase is negligible.

Concerning the phase diagrams of chromium containing oxide systems, the available phase relations were collected from simple to multi-component oxide systems. Most of these phase diagrams were made in air atmosphere. The Cr-O, binary, and ternary systems have been studied systematically. The results on the quaternary and quinary systems are still not enough, and require to be further studied especially under different controlled oxygen partial pressures. According to the collected information on the phase relations, the most important features of the chromium containing silicate slags are the large miscibility gaps and the chromite spinel stability field. The phase diagrams are affected significantly by system oxy­ gen potential because of the different possible oxidation states of chromium. 46

Finally, the factors affecting the chromium recovery in FeCr and stainless steel production processes were also surveyed and discussed, including the effects of slag basicity and tem­ perature as well as FeO content in the slag and Si content in the metal melt. The recovery of chromium to the metal phase is enhanced by the presence of A1203 in the slag melt. The ad­ dition of MgO into the slag greatly improves the recovery of chromium with a basicity ratio higher than 0.80. Increasing slag basicity is a very effective way to increase the chromium recovery from slag, but the effect will level off when the basicity is higher than 1.4. The maximum recovery may be obtained for basic melts saturated with a (Mg,Cr)(Cr,Al)204 spinel solid solution. To minimise the chromium loss in slags, it is also important to control the slag volume in practice while decreasing the chromium oxides concentration in slags. 47

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