Extractive Metallurgy Indian Institute of Technology Kharagpur part 17

Extractive Metallurgy Indian Institute of Technology Kharagpur part 17

1. Principles of Extractive Metallurgy Lecture 17: Roasting: Pretreatment of Sulfide Concentrates 1

2. Concepts: 2 Stability diagrams of sulphides and oxides Extraction of Metal from their Sulphides Roasting and its purpose Roasting Reactions and Methods Furnaces uses for Roasting of Sulphides Furnaces for roasting of sulphide concentrate and their operation Mass and heat balance calculation in roasting Introduction to Predominance Area Diagram ( PAD) Construction of PAD using Thermodynamics Relations and Data Numerical Examples on Operating Parameters and Specific Roasting Product

3. 3 Keywords: Sulphides Ellingham Diagram Oxides Ellingham Diagram Standard Affinity Partial Roasting Total Roasting Chloridizing Roasting Roasting Furnaces Multiple Hearth Furnace Fluidized Bed Furnace Flash Furnace Heat Balance Predominance Area Diagram Cu-S-O Fe-S-O

4. https://doi.org/10.1007/s11085-019-09929-7 Oxidation of Metals (2019) 92:379–399   It shows the stability of the sulphides as a function of temperature or ease with which metal sulphides can be reduced. Reactions closer to the top of the diagram are the unstable sulphides. At the bottom of the diagram, sulphides are harder to reduce. Ellingham diagram for the selected metal sulphides

5. Metal Extraction from Sulphides Reducing a sulfide to metal is not practiced to any appreciable extent due to: 1) Carbon is useless as a reducing agent and if reduction is possible then CS 2 formed will create a disposal problem 2) Hydrogen is also useless as a reducing agent because the reduction is thermodynamically unfavorable. For the reaction MS+H 2 =M+H 2 S the equilibrium constant K 1 = P H2S /P H2 at 800°C is typically 2x10 -3 to 6 x10 -3 for Cu, Ni, Co and Fe sulfides. Lead sulfide can be reduced by hydrogen but at the temperature of reduction appreciable amounts of PbS volatilize. The yield of metal formed by the reaction: MS + H 2  M + H 2 S can be increased by shifting the equilibrium continuously to the right by removing H 2 S as soon as it is formed. CaO+H 2 S  CaS+H 2 O: K 2 = P H2O /P H2S at 800°C = 1.3 x 10 3 ; MS+H 2 +CaO  M+CaS+H 2 O, K = K 1 x K 2 =2.6 to 8 3) Reduction by metals is practiced to a limited extend; the reducing agent used is scrap iron (cheapest metal). The reaction can be represented by the equation: MS + Fe = M + FeS . The process is inefficient because the metal sulfide dissolves in the FeS formed. It is used for the reduction of rich stibnite ores or stibnite concentrates and to some extent galena.

6. Reactions closer to the top of the diagram are the most ‘noble’ metals (for example, gold and platinum), and their oxides are unstable and easily reduced. Moving towards the bottom of the diagram, metals become progressively more reactive and their oxides harder to reduce. Ellingham diagram for the selected Oxides Metals in Past Societies, Shadreck Chirikure ,2015, Springer, DOI: 10.1007/978-3-319-11641-9

7. Metal Extraction from Sulphides C can be used only for the reduction of oxides because its affinity for S 2 , Cl 2 and F 2 , is lower than that of the metals. The extractive metallurgy of sulphides consists generally of a combination of oxidation and other processes, either to obtain the metal directly, or the oxide, which will be reduced, or the soluble sulphate , which will be subjected to leaching and electrowinning . Predominance area diagrams for the stable constituents can be constructed using thermodynamic data under specified values of T, p O2 & p SO2  

8. Purpose of Roasting Roasting may be used to prepare sulfide concentrates to oxides for subsequent pyrometallurgical carbothermic reduction (smelting) operation. For pyrometallurgical processing, the purpose of roasting is also to decrease the sulfur content to an optimum level (partial roasting) for smelting to copper matte. In many modem processes, however, roasting is not a separate step, but is combined with matte smelting. For hydrometallurgical extraction, roasting forms water soluble compounds that can be leached out. The roasting process, which produces the so-called calcines, has several effects: Drying the concentrates Oxidizing a part of the iron present Controlling the sulfur content Partially removing volatile impurities, especially arsenic Preheating the calcined with added fluxes (silica & limestone)

9. Roasting Reactions Initially, sulfides such as pyrite and chalcopyrite decompose and generate sulfur vapor, which reacts with oxygen to form sulfur dioxide: The principal reactions, i.e., the formation of metal oxides, sulfur di/tri oxide, and metal sulfates are exothermic. In addition, there are secondary reactions such as the formation of basic sulfates, ferrites, magnetite and silicates, the last providing most of the slag in the subsequent smelting:

10. Roasting Methods Partial (oxidizing) roasting: A predetermined amount of sulfur is removed (800-850°C) and only part of the iron sulfide is oxidized as for the conventional way of extracting copper from sulfide concentrates (copper sulfide is relatively unchanged). The degree of roasting is determined by controlling the access of air. These conditions are important for the formation of a suitable matte. 2CuFeS 2 + (4-2x)O 2 = Cu 2 S + xFeS + (2-x) FeO + (3-x)SO 2 Total/dead roasting : is used for complete oxidation of all sulfides for a subsequent reduction process or for special hydrometallurgical operations. Sulfatizing roasting is carried out at 550-565°C to form sulfates. This method yields calcines well suited for hydrometallurgical treatment. The overall reaction for the sulfating roast can be considered a series of consecutive steps, such as:

11. Roasting Methods Chloridizing roasting , in which chlorine (usually in the form of NaCl , HCl /Cl 2 in presence of C) is added to the mix to convert the minerals of elements such as Ag, Cu and Pb into water-soluble chlorides: Chloridizing volatilization involves heating to ~1200°C in the presence of calcium chloride so that various metal chlorides and other volatile compounds can be separated. Decomposition of higher sulfides to lower sulfides , for example, 2MS 2 = MS + 0.5S 2 (g) Decomposition of sulfates to basic (oxy) sulfates Sulfation of metal oxides: Sulfide – sulfate interaction Sulfide –oxide interaction Reaction between product oxides or product and impurity oxides to form complex compounds such as ferrites and silicates

12. Roasters/Roasting Furnaces Multiple-hearth furnaces: produce pulverous calcines, the most widely used type until the middle of the 20 th century. Since then, they have been increasingly displaced by the fluidized bed roaster. Fluidized-bed reactors , the most modern type, also yield pulverous calcines. Their two key advantages are exploitation of waste heat and high productivity in consequence of favorable kinetics. Sintering (blast) roasters are used only if concentrates are to be processed in blast furnaces. The most important type is the Dwight-Lloyd sinter machine. Flash smelting furnaces combine roasting and smelting in the same unit. The average residence time of concentrate in the various roasters: Multiple-hearth furnace - a few hours Sintering apparatus - a few minutes Fluidized-bed reactor - a few seconds The roaster gases that are generated contain SO 2 (5-8 vol % for multiple- hearth furnaces 8-15 vol % for fluidized-bed reactors) which is generally converted to sulfuric acid in contact acid plants.

13. Multiple-hearth Furnace Tall cylindrical furnace is divided into compartments by ≥ 9 hearths having a central hole for the rotating drive shaft (fitted with arms and rabbles to move the material lying on the hearth) and a discharge hole near the wall. Each section of the hearth has a provision for air inlet, gas exit and burners (to preheat the furnace to working temperature while starting). The sulphide concentrate is fed from the top most hearth which drops on the next hearth through the discharge hole. Roasting is exothermic gas-solid reaction, no extra heat supply needed after start, larger exposed surface area increase roasting rate. The material keeps falling on the next hearth till it reaches the bottom hearth & discharged out of the furnace.

14. Fluidized Bed Furnace The fluidized-bed reactor where fine particles are suspended in hot air/O 2 , greater surface area are exposed and owing to intense stirring, the heat and mass exchanges are faster and roasting rate is very high. 2ZnS + 3O 2 = 2ZnO + 2SO 2 ΔH = -445 kJ/mole ZnS + 2O 2 = ZnSO 4 @500-600 o C ZnSO 4 + ZnS = 2ZnO + 2SO 2 @ 900 o C The roasting temperature kept usually 910−980°C (low sintering of the particles) by controlling the concentrate feed rate. The gas flow-rate is strong enough for the partial SO 2 pressure to be low, thus avoiding the formation of zinc sulfate. 2SO 2 + O 2 = 2SO 3 ; ZnO + SO 3 = ZnSO 4

15. Outokumpu Flash Smelting Furnace Concentrate burner mixes dry fines with oxygen bearing blast and directs the mixture in suspen-sion form into the reaction shaft. Most of the exothermic reaction between oxygen and sulphide particles take place in reaction shaft. In settler hearth , molten matte and slag droplets collect and form separate layers. An off-take removing SO 2 bearing gases from the furnace. + O 2 Tap holes near the bottom of the furnace for removing matte & slag periodically. The efficient operation of the furnace requires a good particle-gas suspension & steady feed material flow.

16. Zn-concentrate of composition ZnS 76%, FeS 17.6%, PbS 2.99%, SiO 2 3.41% is roasted with 15% excess amount of air in a fluidized bed reactor. 98% ZnS and FeS converted first to ZnO and Fe 2 O 3 and 75% of the total Fe charged forms ZnO.Fe 2 O 3 . Find the amount of calcine (roasted solid product) and flue formed for 1000kg concentrate. Find the calcine temperature if 15% of the heat input is lost. Assume that flue and calcine are in same (critical) temperature. Heat Balance in Roasting +2

17. Components Concentrate (Kg) Concentrate (Kg mol ) Calcine (Kg mol ) Stoichmt O 2 req (kg mol ) Flue (kg mol ) ZnS 760 760/(65.4+32)= 7.803 .02x7.803= 0.156 (7.647x1.5) + 2x3.5/2 = 14.9705 SO 2 = ( 6.897+ 0.75 ) + 2x (.75+.25 ) = 7.647 + 2 = 9.647 FeS 176 176/(56+32)=2 PbS 29.9 29.9/(207.2+32)=.125 Total Stoich O 2 = 14.9705 + .25 = 15.2205 O 2 = 15.2205 x.15 = 2.2831 SiO 2 34.1 34.1/(28+2x16) = .568 0.568 Fe 2 O 3. ZnO 2x.75/2= 0.75 N 2 = 15.2205 x 1.15 x 3.76 = 65.8134 Fe 2 O 3. 2x.25/2 = 0.25 ZnO 7.803x.98 -0.75 = 6.897 PbSO 4 0.125 2x.125 = .25 Heat generated from the reaction = 7.647 x 105950 + 292600 + .125 x 197000 + .75 x 4750 = 1130987.15 kcal = Heat input Heat in Flue and calcine = 1130987.15 x 0.85 = 961339.0775 kcal Sensible heat in flue considering critical temperature of T Kelvin = 9.647 x (H T – H 298 ) SO2 + 2.2831 x (H T – H 298 ) O2 + 65.8134 x (H T – H 298 ) N2 = 572.355T + 39.826x10 -3 T 2 -26.561x10 5 /T -183118.6021

18. Sensible heat in calcine= 0.156 x (H T – H 298 ) ZnS + 0.568 x (H T – H 298 ) SiO2 + 0.75 x (H T – H 298 ) Fe2O3.ZnO + 0.25 x (H T – H 298 ) Fe2O3 + 6.897 x (H T – H 298 ) ZnO + 0.125 x (H T – H 298 ) PbSO4 = 120.92521T + 13.65735 x 10 -3 T 2 + 15.27358 x 10 5 /T -41984.656 kcal Total heat in calcine and flue= 693.28021T+53.4833 x 10 -3 T 2 –11.28742 x 10 5 /T-225103.2581 kcal So, 693.28021T+53.4833 x 10 -3 T 2 –11.28742 x 10 5 /T-225103.2581 = 961339.0775 Or 693.28021T 2 -1186442.3356T + 53.4833 x 10 -3 T 3 –1128742 = 0 Omitting T 3 : 693.28021T 2 -1186442.3356T –1128742 =0 T = [1186442.3356 + (1186442.3356 2 + 4 x 693.28021x 1128742)0.5]/2x693.28021 T = 1712.297 K = 1439.3 o C

19. The sulfide roasting is a complex process involving numerous reactions like: i ) decomposition of higher sulphides (MS 2 ) to lower sulphides (MS), ii) oxidation of sulphides (MS) to form oxides (MO) or sulfates (MSO 4 ), iii) oxidation of sulfur (S 2 ) to form its oxides (SO 2 , SO 3 ) iv) Sulfation of metal oxides (MO  MSO 4 ), v) Decomposition of sulfates to basic (oxy) sulfates [2MSO 4 (s) = MO.MSO 4 (s) + SO 3 (g)], vi) sulfide – sulfate interaction to produce MO & SO 2 , vii) sulfide –oxide interaction to produce M & SO 2 , viii) formation of sub-(2MO  M 2 O) or higher-oxides ( 2MO  M 2 O 3 ) etc. Relevant chemistry and necessary conditions for the formation and stability of different products in M─S─O system of roasting is treated by thermodynamics relationship/information . At constant temperature the stable (predominant) solid phases is represented in a two-dimensional diagram , using log p SO2 and log p O2 as co-ordinates, known as Kellogg Diagrams or Predominance Area Diagrams . Roasting and Predominance Area Diagram

20. Roasting and Predominance Area Diagram For 3 components (C=3) M─S─O system according to the phase rule (P = C − F + 2 ) the maximum number of phases (P) can co-exist at nonvariant (degree of freedom F= 0) cases is 5 ( P = 3 − 0 + 2 = 5 ), i.e., 4 condensed phases and 1 gaseous phase. If temperature is fixed, then P = 3 − 0 + 1 = 4 (3 condensed phases & 1 gaseous phase). The gas phase normally contains SO 2 and O 2 but some SO 3 and sulfur vapor ( S 2 ) may be present having 2 equilibrium relationships: S 2 + 2O 2 = 2SO 2 and 2SO 2 + O 2 = 2SO 3 . So, the composition of the gas mixture is defined by the partial pressures of two of the gaseous components. Thus in roasting at any fixed gas composition and temperature, the composition of the condensed phase also gets fixed . The stability domain of a phase can be represented in 3 dimensions with the coordinates T, p O2 , and p SO2 In 2-dimensional form, temperature is fixed at constant with log p O2 & log p SO2 as coordinates. It is also possible to plot logp O2 against T (or 1/T) for a given value of p SO2 .

21. All kind of roasting reactions can be written (or manipulated) in presence of O 2 and/or SO 2 and their G vs. T relation can be obtained, and equilibrium constant value (K) at a temperature (T) can be calculated using G = - RTlnK . From K for each of these roasting reactions (treating solids with unit activity), log p O2 and logp SO2 at which roasting products (calcine) are in equilibrium at a temperature can be determined. For dead roasting G vs. T plot for few roasting reactions log p O2 and logp SO2 linear relationship for equilibrium between two co-existing solids

22. Construction of Cu-S-O PAD at 1000K G = - RTlnK = -1.987x1000x2.303logK cal For Cu 2 O-CuO line: G = -8.95x1000 cal logK =8.95/4.576 =1.96 l og(1/p O2 0.5 ) = 1.96 - 0.5 log p O2 = 1.96 log p O2 = -3.92

23. https://www.crct.polymtl.ca/fact/factsage/PredSage.pdf Predominance area diagram of Cu-S-O system at 1000 , 1100 & 1200 K Predominance area diagram of Fe-S-O system at 1000K

24. Predominance Area Diagram From the 2-D ( at constant temperature ) PAD figure we infer that: 1. In single condensed phase region, the partial pressure of SO 2 and O 2 may be changed independently of each other ;  the system has two degrees of freedom (F = 2). 2. Along the equilibrium lines between two condensed phases, p SO2 and p O2 can be varies by satisfying particular relationship:  the system is univariant (F = 1). 3. At equilibrium where three condensed phases co-exist , p SO2 and p O2 are fixed (we cannot vary);  the system is nonvariant (F = 0). For fixed values of K 1 and K 2 , the relationship between logp SO2 and logp O 2 depends also on the partial pressures of S 2 and SO 3 . p S2 becomes large when p O2 is small and p SO2 is large, and p SO3 is large for large values of p SO2 and p O2 . When roasting is carried out in air, p SO2 + p O2 = 0.21 atm. At any selected temperature the gas mixture contains 4 components, but composition of the gas mixture is defined by the partial pressures of 2 of the components due to occurrence of 2 equilibrium reactions.

25. Example 1. Sphalerite ( ZnS ) is roasted at 750 o C (1023 K) and 1 atm pressure, i ) Calculate the partial pressure of SO 2 required to form ZnO∙ 2ZnSO 4 , if the gaseous mixture contains 1% O 2 . ii) Calculate the maximum partial pressure of SO 2 at which three phases Zn(l), ZnS (s) and ZnO (s) will coexist. Given: Solution: i ) At 1023 K, The roaster gas containing 16 % SO 2 and 1% O 2 will covert ZnO into ZnO . 2Zn SO 4 = 15.9 ~16% SO 2

26. Ex1. Solution Contd. Solution ii ) At 1023 K , for Similarly for reaction log1023 + 24.77 1023 = -88883.83638 = -1.987 1023 2.3 log     Or   Substituting log p O2 = −25 3675: logp SO2 = 18.98696+ 1.5 (-25.3675) = -19.0643 p SO2 = 10 -19.0643 = 8.62 10 -20 atm   So, Zn(l ), ZnS (s) and ZnO (s) will coexist at 750 o C at 8.62 10 -20 atm maximum partial pressure of SO 2  

27. 28 CONCLUSION Stability diagrams of sulphides at standard state indicates that no cheap reducing agent (C, H) is able to reduce sulphides to metals. Reduction by H 2 in non-standard condition or by Fe is possible but not economically viable. Pretreatment unit process, roasting, is used to convert sulfide concentrates to oxides for subsequent pyrometallurgical carbothermic reduction (smelting) operation. Roasting forms water soluble compounds for hydrometallurgical extraction by leaching. Roasting can be partial, total, chloridizing etc . Roasting being exothermic gas-solid reaction, no extra heat supply is needed after start, roasting rate is increased by exposing fresh surface to the reacting gas, which is done in multiple hearth and fluidized bed reactors.

28. 29 CONCLUSION In flash smelting furnace the heat evolve by roasting is done in a momentary reaction of preheated dry fine concentrates (and flux) with O 2 or O 2 -enriched preheated air so that calcine and flux become molten and molten matte and slag formed. Mass and heat balance for ZnS concentrate show that without burning any extra fuel, calcine temperature of ~1440 o C is possible while considering 15% excess air for roasting & 15% heat loss. At constant temperature the stable (predominant) solid phases in M-S-O roasting system is represented in a two-dimensional diagram, using log p SO2 & log p O2 as co-ordinates, known as Kellogg Diagrams or Predominance Area Diagrams . M-S-O ternary roasting system forms a different solid products; the operating parameter to have a desired roasting products like sulphate , oxides or partial roasting products etc can be obtained from PAD at a fixed temperature.

29. 30 References Physical chemistry of metallurgical processes , M. Shamsuddin , 2016, The Minerals, Metals & Materials Society and John Wiley & Sons, Inc. TREATISE ON PROCESS METALLURGY, VOL. 3 , Editor-in-Chief: SESHADRI SEETHARAMAN, 2014, Elsevier Ltd. Metallurgical Problems, 2 nd Ed., Allison Butts, 1943, McGraw-Hill Book Company Inc.