FM&6JIP6 TKK- MK — Xf Thermodynamic analysis of dust sulphation reactions

Yongxiang Yang Ari Jokilaakso

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Helsinki University of Technology Publications in Materials Science and Metallurgy ^ _ Teknillisen korkeakoulun materiaalitekniikan ja metallurgian julkaisuja I KK-MK-^7 Helsinki University of Technology Publications in Materials Science and Metallurgy TKK-MK-27 Teknillisen korkeakoulun materiaalitekniikan ja metallurgian julkaisuja TKK-MK-27

Thermodynamic analysis of dust sulphation reactions

Yongxiang Yang, Ari Jokilaakso

Helsinki University of Technology Department of Materials Science and Rock Engineering Laboratory of Materials Processing and Powder Metallurgy Espoo 1997 Research Program SULA 2 - Energy in steel and metal production

Project SULA 2 - 302, Computer simulation of gas-solid reactions in the flash smelting process

Financing TEKES - Technology Development Centre, Finland

Distribution: Dr. Ari Jokilaakso. Helsinki University of Technology Laboratory of Materials Processing and Powder Metallurgy P.O. Box 6200 FIN-02015 HUT Tel. +358-0-451 2775 Fax.+358-0-451 2799 E-mail: [email protected]

ISBN 951-22-3850-0 ISSN 1455-2329

Pica-Set Oy Espoo 1997 DISCLAIMER

Portions of this document may be illegible electronic image products. Images are produced from the best available original document. ABSTRACT

Sulphation reactions of metal oxides with SO2 and 0% or SO3 play significant roles in sulphation roasting of sulphide and oxide minerals as well as in desulphurisation process of combustion gases. In metallurgical waste-heat boilers for sulphide smelting, the sulphation of the oxidic flue dust in the atmosphere containing sulphur oxides is an unavoidable process, and the sulphation reactions have to be guided in a controlled way in the proper parts of the gas handling equipment. In this report, some thermodynamic analyses were conducted for the oxide sulphation reactions in relation to sulphide smelting processes. The phase stability of Me-S-O systems especially for oxides - sulphates equilibrium was studied under different thermodynamic conditions of gas compositions and temperatures. The sulphate stability was analysed for an example of gas compositions in the copper flash smelter of Outokumpu Harjavalta Metals Oy, in relation to temperature. In the report, most of the information was from literature. Moreover, a number of thermodynamic computations were carried out with the HSC program, and the constructed phase stability diagrams were compared with those from the literature whenever possible. The maximum temperatures for stable sulphates under normal operating conditions of the waste-heat boilers in sulphide smelting processes were obtained. This report will serve as the basis for the kinetic studies of the sulphation reactions and the sulphation reaction modelling in pyrometallurgical processes. CONTENTS

1. INTRODUCTION...... 1

2. SULPHATION REACTIONS OF METAL OXIDES WITH SOz AND 020R S03... 3

2.1 Industrial application of oxide sulphation reactions ...... 3

2.2 C haracteristics of sulphatton reactions of solid oxides...... 4

2.3 The need for comprehensive thermodynamics , kinetics and reaction models

FOR DUST SULPHATTON...... 5

3. THERMODYNAMICS OF OXIDE - SULPHATE REACTIONS...... 6

3.1 General thermodynamic considerations ...... 6

3.2 Analysis of sulphate decomposition in waste -heat boiler environment ...... 8

3.3 Phase stability diagrams for Me-S-0 systems...... 10 3.3.1 Sulphation of copper oxides...... 11 3.3.2 Sulphation of nickel oxide...... 15 3.3.3 Sulphation of cobalt oxides...... 17 3.3.4 Sulphation of zinc oxide...... 20 3.3.5 Sulphation of lead oxide...... 22 3.3.6 Sulphation of iron and manganese oxides...... 23

3.4 Industrial observations ...... 25

4. SUMMARY...... 26

REFERENCES 27 1

1. INTRODUCTION

Pyrometallurgical processing of sulphide concentrates in non-ferrous metallurgical industry involves the production of metals, slags as well as high temperature off­ gases. The generated off-gas contains a significant amount of flue-dust, and it has to be treated properly and economically in order to meet increasingly strict environmental regulation and to minimise the production cost.

From operational and process point of view, the off-gas handling is as important as the metal production, and it is actually an integrated part of the whole smelter operation. The flue-gases usually leave the smelting furnace or a converter at about 1200 to 1400°C with high content of S02 and dust loading. They are normally cooled in a waste-heat boiler or in a cooling chamber to capture the dust and optionally to recover thermal energy. Waste-heat boilers have been widely used in copper smelting and in some other non-ferrous pyrometallurgical processes. For example, in Outokumpu flash melting process, the smelting furnaces or converters are always equipped with waste-heat boilers. The waste-heat boilers play a critical role in maintaining the continuous operation of the smelter.

As a by-product, smelting and converting operation generates notable amount of flue- dust. The dust, as part of the raw materials, amounts to approximately 1 - 10% of the total feed materials. It has to be captured and returned to the process or treated separately to recover the metal valuables. Dust generation not only reduce the smelting efficiency, but cause a lot of operational problems. Some common problems related the flue-dust in the gas-handling equipment are:

(1) build-ups in the uptake conjunction to the boiler or a cooling chamber, needing frequent cleaning; (2) slagging and fouling the heat transfer surfaces in the boilers and gas ducts, which causes substantial cleaning and dust removal work; (3) corrosion and erosion to the heat transfer surfaces and tubes; (4) decreasing heat transfer efficiency due to high resistance from dust layer, which also causes the more serious dust slagging, build-ups and fouling for the increased wall temperature.

Therefore, optimised furnace operation with minimised dust rate is one important goal for furnace and smelting development.

Under the operation condition of sulphide smelting, the off-gas contains very high S02 such as 10 - 70% in Outokumpu flash smelting process [Biswas and Davenport 1994], and small amount of 02. The flue-dust from the furnace is mainly in oxide forms. The oxide dust will undergo a series of physical and chemical processes along the gas cooling route in this atmospheric thermal environment. In addition to cooling, solidification or agglomeration, the sulphating reactions are unavoidable, which have to be controlled or directed in a preferred way. The metal sulphates normally have lower melting temperature, more sticky and much more corrosive than their parent oxides. The sulphating reaction is substantially exothermic, releasing large amount of heat. The released heat affects the gas cooling and thermal performance of the boiler 2 and subsequent dust capture equipment such as electrostatic precipitator. Because of the gas cooling and increased air in-leakage to the boiler, the dust sulphating is taking place wherever the thermodynamic and kinetic conditions are favourable.

The formation of sulphates in flue-gases plays different role in different processes. Because of their high corrosiveness and low melting temperature, their generation and presence are not expected. For instance, in power plant boiler furnaces, the formation and condensation of sulphates from gas phase, especially alkaline sulphates (Na 2S04, K2S04) has very detrimental effect on the boiler tube and other heat transfer surfaces [Cen et al., 1994]. Even the S02 content in the combustion gases is very low (a few per cents) compared to that in sulphide smelting processes, the formation of S03 and sulphates is substantial enough to cause serious corrosion and fouling problems.

In copper smelting and other sulphide smelting processes, the formation of metal sulphates differs from those in power plant boiler or other combustion furnaces. The main sulphate components are heavy metals of copper, lead, zinc, nickel or cobalt. The sulphating environment is more supporting, favoured by high S02 or S03, oxygen and catalytic particulate (Fe203, CuO, ZnO etc.). The corrosion problem from those sulphates is not as serious as with alkaline sulphates, and the heat releasing from the sulphating reaction affect boiler operation significantly. As far as the waste-heat boiler is concerned in copper smelting process, the sulphating reactions are preferred to take place in the radiation section of the boiler, not in the convection section where there are convection tube-banks in dense clusters. It is evenprohibited to occur in the down stream electrostatic precipitators, which will cause overheating to the elements and steel structure. In the radiation section of the boiler, the sulphating reactions are expected to complete, and the sulphated dust is cooled well below the softening temperature before the dust travels to the convection section. Because the non- uniform thermal, chemical and flow conditions in the boiler, as well as the catalytic nature of the reactions, the sulphating reaction mechanism is very complicated.

In the current study, the thermodynamic conditions for the oxide sulphating reactions will be investigated both from the literature and from thermodynamic computations. The thermodynamic potential of sulphating reactions of concerned oxides will be analysed under practical operating conditions of the waste-heat boilers (range of temperature, S02, S03 and 02 partial pressures. The maximum stable temperatures of sulphates will be determined for various metals based on thermodynamic calculations. Due to the specific importance of the waste-heat boiler in the Outokumpu flash smelting process, the attention will be paid to the boilers of the flash smelting process. The current work will serve the studies of the sulphation kinetics and sulphation modelling in the next step. 3

2. SULPHATION REACTIONS OF METAL OXIDES WITH S02 AND 02 OR S03 2.1 Industrial application of oxide sulphation reactions

Sulphation reactions of metal oxides with SO? - O2 mixture or SO3 have a great industrial significance. The sulphation reactions are present in mainly four types of industrial processes:

(1) Sulphation roasting of sulphide minerals. (2) Sulphation roasting of oxide minerals, such as nickel laterite ores and deep ocean manganese nodules. (3) Removal of SO2 from stack gas in combustion processes with solid oxide absorbents. (4) Sulphation of oxide dust in the off-gas handling equipment in sulphide smelting/converting processes, e.g. in waste-heat boilers or electrostatic precipitators (ESPs).

Each type of above mentioned processes have different purposes or features of the sulphation reactions. During the sulphation roasting of sulphide minerals, the main purpose is to change the sulphide to water soluble metal sulphates, and to separate the main minerals from impurities or unwanted species. The sulphated minerals are subsequently extracted by hydrometallurgical processes. Because the starting minerals are sulphides, the sulphation starts after the sulphides change to oxides, which are intermediate solid products. The sulphation gases (SO2 or SO3) are a part of the oxidation products.

Sulphation of oxide minerals has similar aim to the sulphation of sulphides, but with different characteristics. The solid oxides are raw materials, and the sulphation gases have to be obtained externally, e.g. off-gas from combustion furnaces with minor amount of SO2 or SO3 or off-gas from a sulphide smelting furnace with high S02 content. The sulphates are normally the main products for hydrometallurgical metal recovery.

Desulphurisation of stack gases in boiler or other combustion furnaces has a totally different purpose. The environmental control of SO2 containing gas emission is the main reason. During the desulphurisation process, the SO2 in the off-gas (usually very dilute, a few thousands of ppm) reacts with some basic oxides (e.g. CaO, MgO, or ZnO) to form sulphates. The product sulphates have to be easily disposed or stored.

Sulphation of oxide dust in sulphide smelting has completely another industrial significance, compared to the above three types of processes. It is an instantaneous process in the sulphide smelter off-gas upon cooling. All the reactants (solid oxide and gaseous compounds SO2 or SO3) are by-products of the smelting process. In some parts of the equipment the sulphation is desired to be complete (e.g. radiation chamber of the boiler), while in some parts of the equipment the sulphation is to be avoided (e.g. convection chamber of the boiler and ESPs). Therefore, the sulphation reactions have to be controlled with adjusting operation conditions of the equipment. 4

2.2 Characteristics of suiphation reactions of solid oxides

(1) Suiphation reaction of metal oxides with SO2 and O2 can be expressed in the following equation (2-1):

MeO(s) + 802(g) + 1/202(g) = MeSO^s) (2-1) or a combination of equations (2) and (3):

MeO(s) + S03(g) = MeSO^s) (2-2)

S02(g)+l/202(g) = SOs(g) (2-3)

Experimental observations prove that many metal oxides can be sulphated directly following equation (2-1) with a gaseous mixture of S02 and O2 in the absence of S03, and no catalyst is needed. However, some oxides can not be sulphated through reaction (2-1) directly, and a catalyst (metal oxide or Pt metal) has to be used to assist the reaction to proceed. The catalyzation is to promote reaction (2-3) which provides the direct suiphation gas SO3. In this case, the suiphation indicates a strong catalytic nature. According to various suiphation systems, it was noticed that in the first case, the metal oxide itself is a catalyst for the formation of SO3, and the reaction (2-1) can proceed without external catalysts. For example, CuO, Fe2C>3, MnC>2, CaO, MgO can react with SO2 and O2 to form directly sulphates. If the metal oxide could not act as a catalyst for the formation of SO3, the oxide can hardly be sulphated directly by SO2 and O2 or the suiphation rate is very slow without external catalysts. The examples are CoO, ZnO and NiO which need external catalyst for suiphation with a mixture of SO2 and O2. From a broader point of view, the suiphation reaction (2-1) can be regarded as a catalytic reaction, self or external catalytic. In the presence of S03, the suiphation reaction (2-2) can always takes place and is always faster than reaction (2-1).

It could be concluded that reaction (2-2) is the real route of suiphation. Whether the presence of sulphur trioxide is necessary depends on the catalytic nature of the oxide to be sulphated for the formation of SO3 in reaction (2-3). Reaction (2-3) is a truly catalytic reaction, which always needs a catalyst. Self-catalysing oxides promote the formation of S03 and the formed SO3 sulphates the oxides. Effective catalysts for the SO3 formation are Pt metal, V2O5, Fe20s and CuO.

(2) The suiphation reactions are highly exothermic, which plays important role in heat balance of the process (e.g. in off-gas cooling systems).

(3) The suiphation reactions cause a large volume expansion of the solid product compared to the parent oxides. The molecular volume ratio of sulphate to oxide is from 2 to over 3.5. This causes the suiphation reaction kinetics and mechanism very complicated. Porosity of reaction particles or agglomerates is a very important factor for efficient suiphation in practice. Only specific reaction models can be utilised in modelling. 5

(4) Sulphation of many oxides form intermediate basic sulphates, which complicates sulphation reactions (CuO, ZnO, PbO have basic sulphates).

(5) Metal sulphates are stable at relatively low temperatures. Most of the sulphates decompose at elevated temperatures, without melting. However, PbS04 and some alkali (K, Na) sulphates have clear melting points, and the melting temperature is around 1000°C (PbS04 - 1170°C, K2S04 - 1069°C, Na 2SQ4 - 884°C). They can also form complex compounds with lower melting point (K2S04 - Na 2S04 - FeS04: 554°C). Those low melting sulphates are detrimental in boilers and gas handling system, and they cause significant high temperature corrosion to steel tubes.

2.3 The need for comprehensive thermodynamics, kinetics and reaction models for dust sulphation

The sulphation process is a complicated heterogeneous and catalytic reaction system. It is sensitive to temperature, gas composition, and especially the physical structure of the solid particles. A lot of industrial and scientific studies have been carried out for the first tree types of applications since the early 20th century, the sulphation roasting of sulphide and oxide minerals as well as the desulphurisation of combustion stack gases. Thermodynamic analysis on the sulphation possibility and proper temperature and gas environment have been reported by many researchers. Plenty of kinetic study results are available for many reaction systems. Various macroscopic gas - solid reaction models are reported for the process of S02 removal with oxide absorbents. However, for dust sulphation system, no studies were reported for the involved gas - particle system. No specific studies were reported even for microscopic sulphation reactions for dust sulphation.

For modelling the dust sulphation reactions in the sulphide smelter off-gases, the knowledge of thermodynamic limit for the formation of the sulphates and the stability of the formed sulphates are required, at the first place. In addition, both microscopic reaction kinetics/mechanism and the macroscopic gas - particle reaction models are needed. Examples from other systems are available, and similar treatment can be made. However, a lot of experimental work are required to obtain the basic kinetic data under the gas - particle system with proper temperature and atmospheric environment. Appropriate gas - solid reaction models have to be tested against the basic kinetic data. However, no ready information and data are available in the literature for the current research topic. This report tries to give answers for mostly needed thermodynamic questions in the sulphation system. 6

3. THERMODYNAMICS OF OXIDE - SULPHATE REACTIONS

3.1 General thermodynamic considerations

As was mentioned in Chapter 2, sulphation of metal oxides with SO2 and O2 or SO3 is a heterogeneous, exothermic, and in many cases catalytic reaction. The reactions are favoured by low temperatures at which the reactants (oxides) and the products (sulphates) are usually in solid states. In the atmosphere of SO2 and O2, the sulphation of metal oxides can be regarded to take place in two series reactions:

MeO(s) + S03 ^ MeS04(s) (3-1)

catalyst SO2+I/2O2 ^ SO3 (3-2) or in one reaction directly:

MeO(s) + SO2 + 1/202 ^ MeSQ4(s) (3-3)

Direct sulphation can take place for some oxides without the necessary pre-existence of SO3, and in this case the oxide to be sulphated acts as a self catalyst. However, many oxides require external catalysts to gain meaningful sulphating rate via the formation of SO3, and thus SO3 formation (3-2) becomes an important reaction for the sulphation. The above reactions are all reversible, and the product stability is sensitive to temperature and partial pressures of the gas components. In many real systems, sulphation takes place in several steps, and various basic sulphates may be formed before the last product MeSQ4. Table I shows the sulphation reactions of various oxides useful in pyrometallugical processes [Kellogg 1964, 1989].

Table I Chemical reactions of the oxide sulphation with SO3 or SO2 and O2

System Reactions Remarks Cobalt CoO + SO3 = C0SO4 I/3C03O4 + SO2 + I/3O2 = C0SO4 Copper 2CuO + S03 = CuOCuS04 CuOCuS04 + S03 = CuS04 1/2C u20 + S02 + 3/402 = CuS04 at low temperatures or high pso2 CU2O 4- SO2 4- I/2O2 — CU2SO4 fNagamori and Habashi 1974]. Iron l/3Fe203 + S03 = l/3Fe2(S04)3 l/2Fe203 + S02 + l/402 =FeSG4 at high temperatures and low po2 Lead 5PbO+S03 = PbS04-4Pb0 3/2(PbS04-4Pb0) +S03= 5/2(PbS04-2PbO) 2(PbS04-2Pb0) + S03= 3(PbS04-Pb0) PbS04PbO + SO3 = 2PbS04 Manganese I/3IS41I3O4 + SO3 4- I/3O2™ M11SO4 Nickel NiO + SO3 = NiS04 Zinc 3Zn0+S03 = Zn0-2ZnS04 Zn0-2ZnS04 + SO3 = 3ZnS04 Aluminium 1/3A1203 + S03 = 1/3A12(S04)3 Calcium CaO + S02 l/202 = CaS0 4 Magnesium MgO + SO3 = MgSG4 7

The stability of the sulphates can be analysed by studying the equilibrium decomposition pressure of the sulphates pS03, and compared with the real partial pressure of SO3 in the system. The real partial pressure of SO3 in the smelter off-gas is usually very low to measure, and its value can be evaluated with a calculated thermodynamically equilibrium value. This method will be used to study the sulphates stability at first in this chapter.

The thermodynamics of sulphate formation is usually analysed with stability or predominance area diagrams. The diagram shows the effect of wider gas compositions and directly related to the partial pressures of SO2 and 0%. Many studies included the sulphide phase as the T=const starting material due to its importance as raw materials for roasting or smelting, the system is normally discussed in Me-O-S three component system. As was well (a) At constant temperature described by Rosenqvist [1978], two types of equilibrium diagrams are usually used: (1) log p02 - log pS02 at constant temperature, and (2) log p02 - 1/T at constant partial pressure of SO2. They are schematically illustrated in Figure. 1. From the characteristics of the sulphating MeSO, reactions and in combination with Figure 1, the sulphation is favoured by higher partial pressures of oxygen and sulphur dioxide at constant temperature, and by lower temperature at constant partial pressure of sulphur dioxide due to high temp ]/y—► low temp exothermic nature. If the sulphate product forms solid solution to other components, (b) At constant pS02 its activity will become less than unity, and the sulphate product become more Figure 1. Phase stability diagrams of Me- stable. In the current study, attention is S-0 system [Rosenqvist 1978]. paid to the equilibria between the oxide phase and the sulphate phase and the stability of the sulphate phases.

In the following sections of this chapter, the stability of sulphate phase and the affecting parameters such as temperature and partial pressures of the gases will be discussed for different systems concerned, based on the literature. With a clear image on how the operating parameters influence the product stability, dust sulphation reactions could be controlled and directed to take place in the desired places/devices in the gas handling system, when the sulphation kinetics are taken into account at the same time. 8

3.2 Analysis of sulphate decomposition in waste-heat boiler environment

As mentioned in Chapter 3.1, the sulphate-oxide equilibrium can be studied by analysing the decomposition pressure of the sulphates. For metal sulphates, the decomposition reaction can be written as a reverse reaction of the sulphation reaction (3-1):

MeSCMs) ^ MeO(s) + SO3 (3-4)

For the formation of the sulphates, the reaction is just the reverse reaction of (3-4). Under the thermodynamic equilibrium, the standard Gibbs energy of the system is a function of temperature and the decomposition pressure psos, by assuming the activity of both MeSO^s) and MeO(s) as unity:

AG° = - RT In K = RT In pS03 (3-5)

In a real system with a partial pressure of SO3 as PSo3> the Gibbs energy change of the system can be expressed as:

AG =AG° + RTln(Ps03)"1 = RT In (pso3/Pso3) (3-6)

It is obvious that if the real partial pressure of SO3 (Pso3) is greater than the equilibrium decomposition pressure of the sulphate (psos), the sulphate will be stable, otherwise the sulphate will decompose or the oxide will be stable.

In a system of the smelter off-gas which contains SO2 and O2 as in the waste-heat boiler environment for copper smelting, the real partial pressure of SO3 is determined by the equilibrium reaction between SO2 and SO3: catalyst SO2+I/2O2 ^ SO3 (3-7)

Psos can be calculated by assuming the thermodynamic equilibrium state of the system. For an off-gas composition similar to the current operating conditions as in Outokumpu Harjavalta Metals Oy for copper smelting, shown in Table I, the calculated equilibrium gas composition as function of temperature from 400 to 1200°C is shown in Figure 2. In the computation. Total amount of 100 mole different species was assumed. In Figure 2, only the equilibrium species of SO2 and SO3, O2, CO2, H2O and N2 are drawn, and other minor species such as H2SO4 are neglected in this figure due to very low amounts. Earlier studies on the equilibrium gas composition of the waste-heat boiler in Outokmpu copper flash smelting process showed similar results [Backman et al. 1986].

Figure 3 shows the comparison of the calculated equilibrium partial pressure of SO3 (Psos) for the given gas system shown in Table II with the decomposition pressure of the sulphates (psos) for variety of metal sulphates. 9

Table E Off-gas composition in the waste-heat boiler at Outokumpu Haijavalta Metals Oy for copper smelting [Kyto et al., 1993-1995]

Species SO, CO% H,0 o2 Nz Composition (% vol.) 40 2 3 2 53

Temperature (°C)

Figure 2. Equilibrium gas composition as a function of temperature for a given smelter off-gas shown in Table II.

SO;-40,000 ppm

Equilibi

SO,- 1,000 ppm

Equilibrium Pso, corresponds to a smelter off-gas:

400 500 600 700 800 900 1000 1100 1200 1300 1400 Temperature (°C)

Figure 3. Comparison of equilibrium S03 partial pressure (psos) in a smelter off-gas with the decomposition pressure of sulphates (Psoa). 10

From Figure 3 it can be seen that at the given gas composition shown in Table I, the equilibrium SO3 content decreases from about 40,000 ppm at 400°C to about 1000 ppm at 1400°C. If this estimated value of the SO3 partial pressure is achieved in the off-gas system, the maximum stable temperature of the sulphates are shown in Table II for the corresponding decomposition reactions. However, it should be noted that the formation of SO3 is normally below the equilibrium value due to the reaction kinetics which needs sufficiently high temperature and long residence time, which is particularly true at temperatures below about 600 - 650°C [Sarkar 1982, Piret and Spitz 1993]. Therefore, the real stable temperature of the sulphates may be lower than the values listed in Table n. Table II also lists the approximate maximum temperatures for the stability of sulphates or basic sulphates observed from log poz - log pso2 diagrams from literature and calculated with HSC software presented in this report bythe current authors, in a wider range of gas compositions for boiler operation (l-10%O2 and 10-70%SC>2). It can be seen that the stability temperatures are very close to the values estimated with decomposition pressure method for a typical gas composition of the boiler atmosphere (Table II).

Table III Stable temperatures for metal sulphates under waste-heat boiler atmosphere, calculated with HSC software.

Reaction Line No. in Max. stable T Approx. T for wide gas Fig. 23 (°C)X) composition (°C)2) 2CuS04=Cu0CuS04+S03 3 -735 -700 2C u0C uS04=C u0+S03 4 -790 -800 l/3Fe,(S04)3=l/3Fe,.03+ SO, 2 -645 -650 NiS04= NiO+SO, 5 -825 -800 CoS04= NiO+SO, 6 -890 -900 3ZnS04= Zn0-2ZnS04+S03 1 -480 -500 Zn0-2ZnS04= 3ZnO+SQ3 7 -995 -950 2PbS04= PbS04-PbO+SO, 8 -1095 -1000 Other basic lead sulphates -1200 (mixture)

1) Corresponding gas composition shown in Table I: 40% SO2 and 2%Oz. 2) For gas composition: 1-10%C>2 and 10-70%S02. The approximate temperatures are estimated roughly from the phase stability diagrams.

3.3 Phase stability diagrams for Me-S-0 systems

In this part of the review, sulphation thermodynamics of copper oxides will be discussed at first due to its relative importance in sulphide smelting process, and especially as the main component in the furnace flue dust of copper smelting. Moreover, sulphation of other oxides related to non-ferrous sulphide smelting will also be reviewed briefly, including nickel and cobalt, zinc and lead, as well as iron and manganese oxides. Comprehensive reviews on the thermodynamics of sulphation reactions for extractive metallurgy can be found in references [Ingraham and Kerby 1967, Kellogg 1964]. 11

3.3.1 Sulphation of copper oxides Sulphation of copper oxides draws a great attention, because of its relative importance in roasting of sulphide minerals, its presence in some oxide minerals (e.g. manganese nodules), its catalytic nature for SO2 conversion and removal from combustion stack gases. Various studies were reported in the 1960s and 1970s from metallurgical point of view. Some recent studies are more from the interests of SO2 removal processes. No public studies were found on the copper oxide sulphation as flue dust in off-gas handling systems. Copper has two oxides, cupric CuO and cuprous CU2O, and two sulphates, cupric CUSO4 and cuprous CU2SO4 and a basic sulphate CuO-CuSCU.

Earlier studies indicate that CUSO4 is more stable than Fe2(S04>3 but less stable than ZnS&t, NiS04 and C0SO4 [Blanks 1961, Holappa 1970]. Figure 4 is an illustration of the predominance area diagram of sulphate-oxide equilibrium at 1000 K for Co, Cu, Fe, Ni and Zn systems [Holappa 1970]. CUSO4 starts to decompose to CuO-CuSC>4 at about 600°C, and a measured temperature of complete decomposition (the decomposition pressure reaches to 1 atm) is within 750 - 800°C, which is 802-805°C of the calculated value [Knacke et al. 1991, Barin 1993]. CuO CUSO4 starts to decompose at about 650°C, and it reaches a complete decomposition at temperatures between 850-900°C. The calculated complete decomposition temperature is 867 °C [Knacke et al. 1991]. CuO is unstable at high temperatures, and it decomposes to Cu20 at 1091°C. G12O is more stable at higher temperatures, its melting point is 1244°C, and it decomposes at over 2000 K [Knacke et al. 1991].

POSSIBLE SEPARATIONS AT IQOO "K BASED ONLY ON PRESSURES OF S02 AND <>2 °S0

3-IO Fe — Co I -10 3-10

LOG Po2

Figure 4. Stability diagram of sulphates - oxides systems at 1000 K [Holappa 1970].

Kellogg [1956]calculated the equilibrium conditions between cupric oxide and sulphates CuO - CuO CUSO4 - G1SO4. Blanks [1961] published experimental studies 12 on the sulphate - oxide equilibria for several metals including copper. Skeaff and Espelund [1973] studied the equilibrium of sulphates - oxides, including CUSO4 - CuO-.CuSC>4 at 600 - 800°C with EMF method. Turkdogan and Vinters [1977] published solid solutions in sulphation roasting system, including CuO-CuO CuS O4 mixtures with SO2 + O2 + N2 gases at 800 and 850°C.

Rosenqvist [1978] has summarised Cu-S-0 and Cu-Fe-S-O systems from the literature and his own work. Figure 5 shows the phase equilibria of Cu-S-0 system as functions of oxygen partial pressure and temperature. It includes the Cu20 - CuS04 and CuO - CuO CUSO4 - CUSO4 equilibrium. For the oxygen partial pressure of 0.01- 0.10 atm, the stabile phases should be changing from CU2O or CuO to CuO - CuO CuSO# at the temperature near 800°C. In his paper, the interaction of CuFe204 with CUSO4 was also mentioned in the context of preferential sulphating roasting. One important phenomenon he pointed out is the probable existence of a ternary eutectic melt between Q12S, CU2O and CUSO4 at about 400-450°C, from other works [Schenck et al. 1913; Lewis et al. 1949] and his own experiments. However, other work suggested that CU2SO4 may exist under these conditions [Nagamori and Habashi 1974]. It was indicated in this work that CU2SO4 exists at 300°C and above with different gas compositions. Over 400°C, stable CU2SO4 needs a partial pressure of S02 over 1 atm. Figure 6 shows the Cu-S-0 stability diagram at 350°C and the variation of CU2SO4 stability zone with temperature in relation to the gas compositions. However, further investigation of this low melting phenomenon was not found later in other research work. Anyway, care has to be taken under the boiler environment that the coexistence of Q12S, CU2O and CUSO4 is quite possible, and partial pressure of SO2 is quite high. Thus, formation of low melting eutectic melt is probable.

------Pc = 0.1 atm CuOVcR

Cu2S Vs. \ Melt

10 11 13 14 15

Figure 5. Phase stability diagram of Cu-S-0 system as functions of oxygen partial pressure and temperature [Rosenqvist 1978]. 13

- CuS

C uSO. •I.O-

Cu„0

CuO 11

U------‘------>------1------1------1------1------L.I..I ____ L-. I -20 -15 -|0 LOG p<^

700 * C 6 00* 500*

350*

-20 LOG

Figure 6 . Stability diagram of Cu-S-O systemat 350°C and variation of C11SO4 stability zone [Nagamori and Habashi 1974].

Figure 7 illustrates a phase equilibrium diagram of Cu-S-0 system at a constant temperature 700°C [Turkdogan 1973]. It shows the certain gas compositions required for sulphation of copper oxides. It also indicates that under normal roasting/smelting conditions, direct sulphation of copper sulphide to sulphate is not possible, and copper oxide and oxysulphate has to be first formed. In order to see the temperature effect on the phase relations of the oxide - sulphate systems, calculation was made with ESC software [Roine 1994] in the present study of the equilibrium between copper oxides and sulphates at temperatures between 300°C and 1200°C as functions of O2 and SO2 partial pressures, as show in Figure 8. The range of operating atmosphere for metallurgical waste-heat boilers is: po2=0.01-0.10 atm; pso2=0.10-0.70 atm, as shown in the shaded area. In the calculation, the sulphide phases are omitted due to their minor presence in the furnace dust of the flash smelting operation.

From Figure 8, the final stable product phases can be recognised under the operating conditions of the boiler if the starting phase is CU2O or CuO at temperatures over 1200°C in the beginning of the boiler. Below 800°C, the stable product phase should be CuO CUSO4 and G1SO4, and when the temperature is further dropped to below 700°C, the product should be G1SO4. This conclusion is in consistent with Figure 5. It can also be seen that the CU2SO4 phase cannot be formed at normal conditions except at very high SO2 or very low O2 partial pressures. 14

CuSO,

Figure 7. Phase equilibrium of Cu-S-0 system at 700°C [Turkdogan 1973]

700°C

CuSO,

Waate-heiat boiler - _' operating conditions' (Po,:0.01|-0.10 atm) (Pso^.10 - 0.70 atm)

Figure 8. Phase stability diagram of copper oxides and sulphates at different temperatures of 300 - 1200°C calculated with HSC software.

According to Figures 6 and 8, the direct formation of CuS04 from CuO is not possible, and the intermediate oxysulphate CuO-CuS04 has to be formed first. However, complete sulphation of €%() can go through CuO CuS04, CuzS04 or directly to CuS04 at different atmospheres. 15

Ingraham [1965] studied thermal decomposition of cupric sulphate and cupric oxysulphate at temperatures of 750 - 950 K. A three-dimensional predominance- volume diagram was created based on the experimental results. Compared to the predominance-area diagram at constant temperatures, the temperature dependence can be clearly shown, Figure 9. The temperature range expressed in 104/T is 800 - 950 K. If the solid solution is formed with the copper sulphates, the activity of the sulphates will be lower than unity. Thus, the equilibrium partial pressures of oxygen and sulphur dioxide will be lower, and the sulphate phase will be more stable. Turkdogan and Vinters [1977] found that CuS04 and NiS04 sulphated from CuO and NiO can both form solid solution with MnS04. However, the presence of MnSQ4 in the boiler dust is not likely the case. They also indicated the possibility of direct formation of CuS04 from CuO without intermediate CuO CuS04 due to the formation of the solution with MnS04.

log p0

Figure 9. Predominance-volume diagram of Cu-S-O systems at 800 - 950 K [ Ingraham 1965].

3.3.2 Sulphation of nickel oxide

NiS04 simply decomposes to NiO and SO3 without intermediate oxysulphate of nickel. Literature [Blanks, 1961] shows that NiS04 starts to decompose appreciably at temperatures of 750-800°C. The measured total equilibrium pressure of the gas mixture reaches one atm at temperatures of 850-900°C by different studies [Blanks, 1961]. The calculated complete decomposition temperature is 1152 K (879°C) [Knacke, 1991]. Compared to other metal sulphates, NiS04 is more stable than CuS04 and Fe2(S04)3 but less stable than CoS04. NiO has very high melting point (1955°C), and it is in solid state under normal pyrometallurgical conditions.

The thermodynamic data of nickel sulphate-oxide equilibrium were studied by Marchal [1925], Blanks [1961], Wohler and Flick [1934], and Warner [1964]. Warner [1964] correlated his own data with other three mentioned above, his recommended data was used by Kellogg [1964] in his review on sulphation equilibrium. Skeaff and Espelund [1973] showed good agreement of his own experimental data at 608 - 855 °C with the earlier work. 16

The first phase stability diagram was constructed by Ingraham [1966] in three dimensions, at temperatures of 1000 - 1150 K (727 - 877°C), as shown in Figure 10. It has been found that the NiSCU product layer formed during sulphation is very dense and diffusion of the sulphation gas through this product layer is very slow. According to Figure 10, at 1150 K (877°C), the stable phase is NiO under a wide range of pyrometallurgical operating conditions (psoz: 0.01-1.0 atm, poz: 0.1-0.10 atm). However, at a lower temperature of 1000 K (727°C), the stable phase should be NiSC>4. It can also be seen that at higher temperatures (e.g. 1150 K), oxidation of NiS to NiO has to go through the intermediate phase of M3S2 at atmospheric pressure. Makinen [1972] also studied the phase stability of Ni-S-0 system, and the stability diagram was constructed at temperatures of 627°C and 927°C by using earlier published data. The phase stability diagrams calculated later by Jacinto et al. [1983] showed more known sulphides of nickel, including NiSz and several non­ stoichiometry nickel sulphides, which were not included in earlier studies. An example of the stability diagram of Ni-S-0 system at 883 K (610°C) is shown in Figure 11.

LOG p.

LOG p Log p

Figure 10. Phase stability diagram of Ni-S- Figure 11. Predominance area diagram of O system at temperatures of 1000 - 1150 K N-S-0 system at 883 K [Jacinto et al. [Ingraham, 1966]. 1983].

The stability diagram of Ni-S-0 system at 800°C constructed by the current authors with HSC program is shown in Figure 12. A calculation of NiO-NiS04 equilibrium under different gas compositions at three temperatures by the authors is also shown in Figure 13. From Figure 13 it can be seen that under normal operating atmosphere of the boiler, N1SO4 becomes the stable phase at the temperatures below 800°C; above 800°C MSO4 will decompose to NiO. However, if only the NiO-NiS04 equilibrium is considered, all earlier studies give similar results. 17

Phase Stability Diagram at 800 C

NiS04

Figure 12. A predominance area diagram of N-S-0 Weste-heal boiler operating conations -0.10 atm) system at 800°C with ESC (Pkv Q.10-0.70 atm) software.

Phase Stability Diagram at 600 - 1000°C

1000BC Waste-heat boiler NiS04 operating conditions (POy-0.01-0.10 atm) (Pso^O.IO - 0.70 atm) Figure 13. A phase stability diagram of NiO- NiS04 equilibrium at 600, 800 and 1000°C with ESC software.

-12 -10 log p02(g)

3.3.3Sulphation of cobaltoxides.

Cobalt sulphate C0SO4 can decompose either to CoO or to C03O4, depending on oxygen pressure prevailing and temperature [Warner, 1964]. C0SO4 starts to decompose appreciably at about 700°C, and calculated decomposition temperature is 965°C [Knacke et al., 1991]. C0SO4 is the most stable sulphate compared to nickel, zinc, copper and iron sulphates, as shown in Figure 4 for the phase stability diagram of sulphate - oxide equilibrium, and also illustrated in Figure 14 for the total vapour pressure of the sulphates as a function of temperature [Palperi et al. 1971]. The decomposition of cobalt sulphate was also well studied by Blanks [1961]. Ingraham [1961] studied the thermochemistry of the Co-S-O system from 950 to 1200 K. The constructed phase stability diagram is shown in Figure 15 at two temperatures of 677°C (950 K) and 927°C (1200 K). 18

•CoSO,

*- 200.

700 800 TEMPERATURE *C

Figure 14.Total vapour pressure of the sulphates as function of temperature [Palperi et al. 1971].

o -4

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 LOG Po Figure 15. Phase stability diagram of Co-S-O system at 677 and 927°C [Ingraham 1961].

The constructed phase stability diagrams by the current authors with HSC software for the Co-S-O system is shown in Figures 16 and 17 at various temperatures, while Figure 15 is for oxide - sulphate equilibrium only.

From Figures 15 - 17, the same conclusion can be drawn that at about temperatures around and below 900°C, the cobalt sulphate will be a stable phase under the atmosphere of the waste-heat boiler operation. It can also be seen that C0SO4 will equilibrate with CoO at higher temperatures and lower oxygen partial pressure, and equilibrate with C03O4 at lower temperatures and higher oxygen partial pressure. More details about C03O4 and CoO equilibrium can be found elsewhere [Holappa 1970]. 19

Phase Stability Diagram at 800 -1000 C

Waste-heat boiler operating conditions (Po2:0.01 -0.10 atm) (Rso,:0.10 -0.70 atm)

CoS04 -

Co3S4

log p02(g)

Figure 16. Phase stability diagram of Co-S-0 system at 800 and 1000°C with ESC.

Phase Stability Diagram at 500 -1000 C

Waste-heat boiler operating conditions

(Pso^O.10 - 0.70 atm)

log p02(g)

Figure 17. Phase stability diagram of Co-S-O system at 500, 800 and 1000°C for sulphate-oxide equilibrium with ESC. 20

3.3.4 Sulphation of zinc oxide

Sulphation of zinc oxide has got great attention in sulphation roasting of zinc sulphide ore for hydrometallurgical production of zinc. ZnSC>4 has a crystal transformation at about 740°C (734 -742°C by different studies) from a phase to (3 phase with a large enthalpy change (endothermic). ZnS04 starts to decompose appreciably at about 700°C and completely at a calculated temperature of 941 °C to its basic sulphate Zn02ZnSC>4. Zn0-2ZnS04 then further decomposes to ZnO.

Ingraham and Kellogg [1963] the thermodynamic properties of zinc sulphate and basic sulphate as well as Zn-S-0 system. The phase stability diagram at 827° (1100 K) calculated from their experimentally obtained thermodynamic data is shown in Figure 18. It is also indicated in the diagram for the stable roast products under two gas compositions. At the atmosphere of waste-heat boiler operation (10-70 vol.% SO2, 1- 10 vol.% O2), the stable product should be basic zinc sulphate at 827°. Below this temperature, the stable product should be ZnSC>4.

Point A = Rooster Gas with 4% 0%, 10% SO£ Point 8 = Roaster Gas with 4% 02, 4% S02

Figure 18. Phase stability diagram of Zn-S-0 system at 827°C [Ingraham and Kellogg, 1963].

Figure 19 shows the calculated Zn-S-0 phase stability diagram at 800 and 1000°C with HSC software. Together with more calculations at different temperatures, it is shown that under boiler operating conditions, ZnO 2ZnS04 is the stable phase at below 1000°C, down to 600°C. At around 500°C, the stable phase is ZnS04, and at 500 - 600°C the stable phase should be the mixture of ZnS04 and ZnO 2ZnS04.

The complexity of the system lies in the coexistence of iron-containing system. The major concern in sulphation roasting of zinc sulphide concentrates is to minimise the formation of zinc ferrite ZnO FeiOg. Figure 20 shows the stability products for Zn-Fe- S-0 system as functions of temperature and oxygen partial pressure at pso2=l-0 atm [Rosenqvist 1978]. It can be seen that for P02 of 0.01-0.10 atm, zinc ferrite will not be stable below about 875°C. The stable phase should be FeaOg and ZnO 2ZnSC>4 or 21

ZnS04 below 875°C. Below about 760°C, Fe2(S04)3 start to form at po2=0.10 atm. In practice, the partial sulphation roasting takes place at about 870-900°C in order to get part of Zn0-2ZnS04 for hydrometallurgical production of zinc. However, small amount of ZnO Fe203 is normally formed.

Phase Stability Diagram at 800 and 1000 C

Wwte»heat boiler operating conditions (Ppr&OI -0.10 atm) {Pso,:0.1d -0.70 atm)

log p02(g)

Figure 19. Phase stability diagram for Zn-S-0 system at 800 and 1000°C calculated with HSC software.

838

Figure 20. stability products for Zn-Fe-S-O system as functions of temperature and oxygen partial pressure at pso2=1.0 atm [Rosenqvist 1978]. 22

3.3.5 Sulphation of lead oxide

PbSC>4 has a crystal transformation at 866 °C (1139 K) and the melting point is 1170°C (1443 K). The binary phase diagram of PbO - PbSC>4 system is well illustrated in Figure 21 [Kellogg and Basu, I960]. PbSC>4 forms three basic sulphates with PbO: PbS04-4PbO, PbS04 2PbO and PbS04 PbO. PbS04-4Pb0 decomposes at 895°C, and PbS04 2PbO and PbS04 PbO melt at 961 and 975°C, respectively. PbO has a melting point of 886°C. PbO can be further oxidised to Pb304 and PbOi at very high oxygen partial pressure and lower temperatures. PbO% decomposes to Pb304 at 307-344°C, and Pb304 decomposes to PbO at 628°C [Knacke et al. 1991].

Figure 21 Binary phase diagram of PbO - PbS04 system [Kellogg and Basu, I960].

PbS04 starts to decompose at about 800°C, and substantially at 900°C. Figure 22 shows the phase stability diagram of Pb-S-0 system at 827°C (1100 K) [Kellogg and Basu, I960]. Figure 23 is the phase stability diagram of Pb-S-0 system for lead sulphates - oxides equilibrium at 500, 900 and 1200°C calculated with ESC program.

Usuol Roaster Gas Compositions

Figure 22. Phase stability

diagram of Pb-S-0 ©\ PbS0 4-2Pb0 system at 827°C (1100 K) [Kellogg and Basu, I960].

-2.0 -

Log P„, aim. —— 23

Waste-heat boiler operating conditions (Po,:0.01 - 0.10 atm)

Pb02

log p02(g)

Figure 23. Phase stability diagram of Pb-S-O system at 1000 and 1200°C for oxides-sulphates equilibrium.

From Figure 23, it is evident that PbSC>4 is the stable phase at 827°C for the usual roaster gas compositions which are the same for boiler environment. The computation of the phase stability diagram with HSC indicates that in boiler environment, at 1200°C the stable phase is the mixture of the basic sulphates PbS04PbO and PbS04-2PbO in molten state. PbS044PbO decomposes at 895°C and is not stable at 1200°C. At 1000°C and below, the stable phase is already PbS04. It should be noted that PbO and its basic sulphates are low melting, and the liquidus temperature of PbO - PbS04-4Pb0 eutectic is only 835°C, as shown in Figure 21.

3.3.6Sulphation of iron and manganese oxides

Iron sulphides and oxides have their special merits for sulphide roasting and smelting, as they are substantially present in the raw materials and products for roasting and smelting operations. However, under normal operating conditions for highly oxidising roasting, iron sulphides are fully oxidised to FeaOg at temperatures above 800°C. Below 800°C, ferric sulphate may be stable. Figure 24 illustrates the phase stability diagram of Fe-S-0 system as functions of oxygen partial pressure and temperature at Pso2=latm [Rosenqvist 1978]. Ferric sulphate is not stable, and decomposes completely at the calculated temperature of 732°C (1005 K) [Knacke etal. 1991].

Sulphation of manganese oxide has got attention for recovering metals from ocean manganese nodules. MnS04 normally decomposes to Mns04, and in systems artificially maintained at very low or very high oxygen partial pressure it can decompose to MnO or Mn2C>3 [Kellogg, 1964]. Figure 25 shows the phase stability 24 diagram of Mn-S-O system at 700 and 1100 K (427 and 827°C) by Ingraham and Marier [1968]. The oxides-sulphate equilibrium of manganese is well described by Kellogg [1964] and Ingraham and Marier [1968]. Ingraham and Marier [1968] indicated that at 827°C the stable product should be MnaOg with 10% SO2 and 0%. At temperatures below about 800°C and a wider gas compositions, like in waste-heat boiler environment, MnSC>4 should be the stable product. It has been found that MnSC>4 is in solid state at least at 1000°C, but with two phase transformations at 460 and about 850°C [Skeaff and Espelund 1973]. It has been stated earlier in 3.3.1 that MnSC>4 may form solid solutions with CuS04 and N1SO4 which lower their activities and increase their stability.

l2=S!liq)'

p =1 atm'

13 14 15

Figure 24. Phase stability diagram of Fe-S-0 system at constant pso2=latm [Rosenqvist, 1978].

1100 eK 700 *K Mn SO*

LOG p Figure 25. Phase stability diagram of Mn-S-0 system at 427 and 827°C [Ingraham and Marier, 1968]. 25

3.4 Industrial observations

Various studies concerning the composition and mineralogy of the flue dust in the Outokumpu copper flash smelting process have been carried out inside Outokumpu Oy [Kyto, 1997]. It was found that the major phases of the flash smelting flue dust are CUSO4, Fe304 and CuFe02, and minor phases are Cu20, CuO, CuFe204, a-Fe 203, CuO-CuSCL, FeS04 and Si02. The dust from the duct to the boiler and the boiler contains also some amount of Cu2S and Cu. Other minor phases include ZnS04, PbS04. It was also found that the dust is not fully sulphated in the radiation and even in the convection section of the boiler. More complete sulphation may take place in the electrostatic precipitator.

Flue dust information from Kidd Creek copper smelter at Falconbridge using Mitsubishi continuous process was published by St. Eloi and Newman [1992] and by Evans et al. [1992]. Gases leaving the smelting furnace contain about 22% of S02 and a small amount of 02, which fall inside the gas composition of the current study. Both their smelting furnace and converting furnace are equipped with a waste-heat boiler, consisting of a radiation section and a convection section. Typical compositions of the dust samples from the radiation and convection sections of both smelting furnace and converting furnace boilers are shown in Table IV.

Table IV Mineralogical compositions of the boiler dusts from Kidd Creek copper smelter, division of Falconbridge Ltd [St. Eloi and Newman 1992].

SrBadiation C-Radiation

Ferrite 5-15% 10-18% PbS04 5-10% 4-7% CuS04 5-40% 1%

C u20 0-30% — CuO 10-30% 70% Other 0-5% 0-15%

S-Convection C-Convection

Ferrite 5-10% 5-15% PbS04 5-10% 2% Cu2S04 5-20% 40% CuS04 40-50% 10-20% Cu20 0-5%

CuO --— 30% Other 0-5% 0-5%

It can be seen that for the boiler of the smelting furnace, the dust still contains significant amount of copper oxides, and in the convection section the dust is almost all sulphated, especially for copper. For the boiler of the converting furnace, the dust from the radiation section is still highly oxidised, and in the convection section sulphation takes place but still with significant amount of cupric oxide. Iron is 26 essentially in ferrite form, and zinc and lead are all sulphated in both boilers already in the radiation sections.

According to Evans et al. [1992], air infiltration to the boiler significantly increase the dust sulphating degree. The dust deposit collected from the boiler walls indicates that copper essentially in oxide form from the furnace is sulphated to G1SO4 and CuO CUSO4, when there is air infiltration. Zinc and lead are basically in sulphate form, and their source are thought to be from vapour condensation. Zinc ferrite was observed in the deposits from both sections of the smelting furnace boiler. For the boiler in the converting furnace, the form of copper is mainly CuO based on the analysis of boiler wall deposit in the radiation section. On the wall of the convection section, cupric oxide is substantially sulphated to CuO CUSO4 and, but half of CuO remains unchanged.

4. SUMMARY

Formation of metal sulphates from their oxides depends very much on the temperature and the gas compositions. When the partial pressure of O2, SO2 or SO3 is higher than their thermodynamic equilibrium values at a given temperature, the sulphates will be stable, otherwise the sulphates or the basic sulphates will decompose to oxides. Lower temperature and higher partial pressure of the O2, SO2 or SO3 components all favour the formation of sulphates.

In this report, the thermodynamic conditions for stable sulphate phase are analysed with the phase stability diagrams at different temperatures as functions of partial pressures of O2 and SO2, specifically for the waste-heat boiler environment. Preferable operating conditions for the formation of sulphates can be observed from the diagrams for different Me-S-0 systems. As another approach, the decomposition pressure of the sulphates and the equilibrium partial pressure of SO3 for a given composition in the boiler atmosphere are calculated as function of temperature. Comparison of the decomposition partial pressure and the equilibrium partial pressure of SO3 results in the maximum stable temperatures for the sulphates or basic sulphates. However, the practical stable products depend very much on the sulphation kinetics, the residence time of the oxide particles under the thermodynamically feasible conditions. The kinetic topic will be dealt with in a separate report.

The following is a brief summary for the sulphates stable temperatures of different metals, under the waste-heat boiler environment: 1-10% O2, 10-70% SO2. The temperature of the process gas from smelting furnaces is normally from 1200 to 1400°C. The gas is usually cooled to about 600 - 800°C in the radiation section of the boiler. In the convection section, the gas is further cooled to about 300 - 400°C. Thermodynamically speaking, all the metal oxides discussed above will be sulphated in the convection section if not in the radiation section, provided that the particle residence time is long enough. In practice, most of the oxidic dust is sulphated before leaving the convection section of the boiler. Sometimes, part of the sulphation reactions happens, however, also in the electrostatic precipitator, which should be avoided in practice. For the waste-heat boiler, the sulphation reactions should be kept 27 minimum in the convection section, due to the possible low melting and sulphate mixtures. For different sulphates, FeiCSO^s is thermodynamically the most unstable one, and PbSC>4 is the most stable sulphate. Between the two, the stable order increases from CUSO4, ZnS04, MSO4, C0SO4 to MnSC>4. It can be seen that between the sulphate and oxide many metals (like zinc, lead and copper) form basic sulphates or oxysulphates, and they are normally more stable than their sulphates.

It should be noted that other low melting sulphates of alkali metals (e.g. K2SO4 and NaaSCU) have not been analysed in this report, due to their absence in most of the sulphidic smelting practice. However, if the sulphide raw materials contain even minor amounts of K or Na, they will be oxidised in the smelting furnaces due to their strong oxide stability, and their oxides will be more easily sulphated, compared to other non-ferrous metal oxides. For example, NaaSCU is unusually stable sulphate, and no direct measurements of its decomposition pressure are available [Kellogg, 1964], and it melts at 884°C. If the vanadium oxide is present, low melting melts of NaaO- V2O5CV2O4) may be formed (as low as 535°C for 5Na 20 V2O4 IIV2O5), which are very corrosive to the boiler tube-walls. They have much more significance in power plant boiler furnaces than in the sulphide smelting waste-heat boilers.

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