High Temperature Roasting of Sulphide Concentrate and its Effect on the Type of Precipitate Formed.

Transcription

1 High Temperature Roasting of Sulphide Concentrate and its Effect on the Type of Precipitate Formed. A dissertation submitted to the School of Mines, Faculty of Engineering, Technikon Witwatersrand, Johannesburg, South Africa, for the fulfilment of the degree of MAGISTER TECHNOLOGIAE: EXTRACTION METALLURGY. By Fortunate Magagula Supervisor: Dr. A.F. Mulaba-Bafubiandi Department of Metallurgy, Technikon Witwatersrand, Johannesburg December 2002.

2 Declaration I Fortunate Magagula, hereby declare that this dissertation is my own unaided work. It is being submitted to the Technikon Witwatersrand for the degree MAGISTER TECHNOLOGIAE Extraction Metallurgy. It has not been submitted before by myself or any other person to any institution for any degree or examination. Author's signature Date20421 eiq ii

3 Acknowledgements I sincerely thank my supervisor, Dr Mulaba for his academic guidance and logistical support and for making this study a success. I would also like to thank the Technikon Research Committee for the financial support. I thank the University of the Witwatersrand for allowing me to use their facilities. I am so grateful to my colleagues for their moral support. I would like to extend my gratitude to my husband for loving and motivating me throughout this study. iii

4 Dedication To my husband who made everything possible and bearable. Thank you for your tireless support. iv

5 Abstract The most commonly used route in the hydrometallurgical extraction of zinc and copper is the roast-leach-electrowin process. During the roasting process, the concentrate is subjected to either relatively low temperatures (partial roasting) or high temperatures (to achieve dead roasting) to produce a calcine that will be leacheable to extract zinc and copper. The resulting calcine contains zinc and copper in a form of oxides (ZnO, CuO), sulphates (ZnSO4, CuSO4) and ferrites ((Zn,Cu 1-x, Mx)0Fe203) or Zn,CuFe2O4) in the case of partial roasting. In the case of dead roasting, mostly the oxide forms are produced but in most cases ferrites will form as well. The means of avoiding the ferrites completely have not yet been achieved. Attempts in the past had only been focusing on either partial roasting or dead roasting without actually finding the optimum roasting conditions to minimise the ferrite formation. In this study the main objective was to identify optimised conditions for roasting, i.e. the possibility of producing these ferrites in minimum amounts as compared to the targeted zinc/copper oxides. Optimised roasting conditions were achieved in this study on a Zinc-copper ore from Maranda mine, where the sulphur removal test was used to ensure a dead roasting. This was done by analysing the amount of sulphur remaining after each roasting condition. Characterisation of the calcine has been done using the XRD and the Mossbauer spectroscopy. More zinc oxide than zinc ferrite was obtained at conditions of 800 C for 3 hours as per the XRD analyses. The sulphur removal test however, showed a dead roasting at 900 C (2% remaining sulphur) and this is attributed to the inadequate (not designed as in industry) supply of oxygen by the laboratory furnaces used. The precipitation of iron from the three acids (HCI, H2SO4 and HNO3) was done using NH4OH and NaOH. The Mossbauer and XRD characterisation techniques were used, where the XRD characterisation showed different spectra of the precipitate attributing to different compounds. The results of the precipitates from the optimised roasting conditions are those precipitates that are not commonly found in industry. The effect of the acids and the cations showed goethite to be formed from H2SO4 and HNO3, with NH4+ and Na + respectively. The possibility of the selective leaching of the concentrate has been investigated. This eliminates the roasting process completely and thus provides a possibility of leaving the pyrite

6 (FeS2) in the residue and thus minimising the amount of iron to be handled. Selective leaching has been done using Mn0 2 and Na2S208 in the presence of H2SO4. It was observed that starting with Mn02 as an oxidising agent does not achieve good selective leaching results between the sphalerite and the chalcopyrite. It was however possible to preferable leach sphalerite over chalcopyrite with the use of Na 2S2O8 as a starting oxidising agent. So the choice of the oxidising agent plays a role in selectively leaching different minerals. The optimised roasting conditions at high temperatures resulted in some type of precipitates, (mohrite, ferrihydrite and akaganeite) that are not commonly formed in industry. Jarosite, which is the most common precipitate formed in industry, could not be precipitated. Goethite was also fcund to be present. vi

7 Table of Contents Declaration Acknowledgements Dedication ii iii iv Abstract Table of Contents vii List of Tables List of Figures List of Abbreviations xiii xvi CHAPTER 1 INTRODUCTION 17 CHAPTER 2 - LITERATURE REVIEW Introduction Iron in Sulphide ores Roasting of sulphide ores Partial roasting Dead roasting Leaching Conventional leaching of sulphide ores Selective leaching of sulphide ores Precipitation Hydrolysis of Iron in Aqueous Media Iron precipitate products Mossbauer 37 vii

8 Chapter 3 EXPERIMENTAL Research Method Materials Ore and its origin Reagents and Apparatus Flotation Reagents Leaching Reagents Apparatus Experimental Procedures Flotation procedures Roasting procedure Sulphur determination X Ray Diffraction (XRD) Mossbauer Effect Spectroscopy (MES) Leaching procedures Selective Leaching procedures 43 Chapter 4 RESULTS AND DISCUSSION Introduction Flotation Results Roasting Results Sulphur removal X-Ray Diffraction (XRD) Mossbauer Results Leaching Results Neutral leaching HCI Neutral leach H 2SO4 Neutral leach HNO3 Neutral leach CONCLUSION Selective leaching Conclusions 90 viii

9 4.6 Precipitation results Procedure Results XRD characterisation of precipitates Mossbauer characterisation of precipitates. 102 Chapter 5 Conclusions Recommendations 105 References 106 Appendices 111 ix

10 List of Tables Table : Comparison of Iron Precipitation Processes 35 Table : Mineral and metal abundances in the Run Of Mine. 39 Table 4.2.1: Results showing mass recovery with respect to variation of modifiers types and ratios 46 Table 4.2.2: Table showing zinc and copper recoveries 47 Table : Percentage sulphur from varying roasting conditions. 50 Table : XRD spectral Intensities of phases at different... roasting conditions. 57 Table : Hyperfine interaction parameters of the components in the concentrate sample. 61 Table : Hyperfine interaction parameters components in calcines roasted for 2 hrs at different temperatures. 63 Table : Comparison of component abundances (2hrs) Table : Hyperfine interaction parameters of the components in the calcines, roasted for 4 hrs at different temperatures 67 Table : Comparison of component abundances (4 hrs).. 69 Table : Hyperfine interaction parameters of the spectral components in the spectrum of calcines roasted at 800 C for different durations. 70 Table : Comparison of component abundances (800 C). 72 x

11 Table : Table showing neutral leaching results 74 Table : Percentages of elements remaining after neutral leaching. 81 Table : Table : Table 4.5.1: Table 4.5.2: Table 4.5.3: Table 4.5.4: Dissolution of elements during hot acid leaching8l Table showing percentages of elements remaining after hot acid leaching. 82 Percentage Extraction in 5M H 2SO4 and 10% (w/v) Mn02 83 Percentage Extraction in 7M H 2SO4 and 10% (w/v) Mn Percentage Extraction in 5M H 2SO4 and 20% (w/v) Mn Percentage Extraction in 7M H 2SO4 and 20% (w/v) Mn02 85 Table 4.5.5: Percentage Extraction in 5M H 2SO4 and 10% (w/v) Na2S2O8. 86 Table 4.5.6: Table 4.5.7: Table 4.5.8: Table 4.5.9: Table : Percentage Extraction in 7M H 2SO4 and 10% (w/v) Na2S2O8. 87 Percentage Extraction in 5M H 2SO4 and 20% (w/v) Na2S2O8. 88 Percentage Extraction in 7M H 2SO4 and 20% (w/v) Na2S2O8. 88 Percentage Extraction of residue in 7M H 2SO4 and 20% (w/v) Mn Table summarising the amount of iron (in %) remaining from precipitation at 80 C and filtration after 24 hours. 94 xi

12 Table : Results summarising the amount of iron (in %) remaining from precipitation at 95 C and filtration after 24 hours 96 Table : Hyperfine interaction parameters of the spectral components in the spectrum of samples. 103 Table A: Hyperfine interaction parameters at room temperature of candidate Fe-bearing phases that may occur in the samples 111 Table B: Hyperfine interaction parameters at room temperature of candidate Fe-bearing phases that may occur in the precipitate samples 112 xii

13 List of Figures Figure : Schematic representation of the hydrolysisprecipitation process 30 Figure 4.2.1: Effect of conditioning time on mineral recoveries. 46 Figure : Percentage sulphur remaining at varying roasting conditions.observing the temperature effect 51 Figure : Percentage sulphur remaining at varying roasting conditions observing time effect 52 Figure : XRD graph showing the effect of roasting temperature on the formation of zinc oxide and zinc ferrite Figure : Effect of roasting temperature on the formation of zinc oxide and zinc ferrite at 800 C 55 Figure : XRD spectral Intensities of the zinc ferrite and... zinc oxide at 700 C. 58 Figure : Graph showing intensities of Zinc ferrite and zinc oxide (zincite) at 800 C. 60 Figure : Mossbauer spectr um of the concentrate from the zinc ore sample. 61 Figure : M6ssbauer spectra of calcines roasted for 2hrs at different temperatures (700 C, 800 C, and 900 C). 64 Figure : Graph showing the effect of temperature on the amount of phases formed during roasting for 2 hours. 66 Figure : Mossbauer spectra of the calcine samples, roasted for 4hrs at different temperatures 68

14 Figure : Mossbauer spectrum of calcine samples roasted at 800 C for different durations. 71 Figure : Dissolution of the metals in HCI showing different extraction rates. 75 Figure : Dissolution of the metals in H 2SO4 showing different extraction rates. 76 Figure : Dissolution of the metals in HNO 3 showing different extraction rates. 77 Figure : Dissolution of copper in the three leaching acids. 78 Figure : Dissolution of Zinc in the three leaching acids... showing the effect of each acid. 79 Figure : Dissolution of iron in the acids showing the effect of each acid. 80 Figure Figure Figure Figure Percentage Extraction in 5M H2SO4 and 10% (w/v) Mn Percentage Extraction in 7M H 2SO4 and 20% (w/v) Mn02 86 Percentage Extraction in 5M H 2SO4 and 10% (w/v) Na2S2O8. 87 Percentage Extraction in 7M H 2SO4 and 20% (w/v) Na2S2O8. 89 Figure : The comparison of HCI and H 2SO4 pregnant solutions in precipitating iron using NaOH. 98 Figure : The effect of H 2SO4 and HNO3 in the amount of iron remaining from precipitating with NaOH 100 Figure XRD spectra for precipitates where iron precipitation was optimum. 101 xiv

15 Figure : Mossbauer spectrum of precipitates. 102 RV

16 List of Abbreviations XRD AAS MS WN mv CP ROM KV ma MES QS Bhf IS X-Ray Diffraction Atomic Absorption spectroscopy Mossbauer Spectroscopy Weight to volume Millivolts Chemical purity Runoff mine Kilovolts Milliamps Mossbauer Effect Spectroscopy Quadrupole splitting Magnetic field Isomer shift xvi

17 CHAPTER 1 INTRODUCTION 1.1 Aims and objectives The aim of this project was to optimise the roasting conditions i.e. to find the conditions that produce,minimal ferrites after the roasting process; to characterise precipitates that result from the pregnant solution after leaching of calcine from the optimised roasting conditions; to establish / find out the effect of high temperature roasting on the type of precipitate formed and to investigate the possibility of selective leaching. 1.2 Overview Roasting may be used to prepare sulphide concentrates for subsequent pyrometallurgical or hydrometallurgical operations. For pyrometallurgical processing, the usual purpose of roasting is to decrease the sulphur content to an optimum level for smelting to a matte. Partial (oxidizing) roasting is accomplished by controlling the access of air to the concentrate; a predetermined amount of sulphur is removed, and only part of the iron sulphide is oxidized, leaving the copper sulphide (for example) relatively unchanged. Total, or dead, roasting involves the complete oxidation of all sulphides, usually for a subsequent reduction process. (For hydrometallurgical extraction, roasting forms compounds that can be leached out.) Iron plays an important role in the production of non-ferrous metals. It is the fourth most common element in the composition of the earth after oxygen, silicon, and aluminium, and the second most common metal after aluminium. Mainly because of this abundance, iron may be present as an essential constituent of the ore or gangue, as a solid solution, or may be mixed with the ore in the form of various iron minerals (Ozberk and Minto, (1986)). Though it may be present as an essential constituent in other ores, it is also regarded as an impurity in many nonferrous metals. There have been a number of other processes on the subject of iron control, most of which were not practiced further because of their disadvantages. To name a few, Stein and Spink, (1990) made some developments for partial oxidation roasting of zinc concentrates, which afforded a solution to the ferrite problem. In this process, complete avoidance of zinc ferrite formation can be attained with resultant higher overall recoveries of zinc than are presently achieved via the 17

18 conventional dead-roast-leach-electrowinning process. However this results in some zinc being undigested in the leach residue and has to be recycled in the roasting circuit. In this study the behaviour of iron compounds in a concentrate is being observed as the concentrate is subjected to high temperature roasting. Roasting was done at different roasting conditions. This was to determine the ferrite formation and to find conditions that favour the zinc oxide formation over the ferrites. Conditions that promote ferrite formation over the zinc oxide must therefore be scrutinized to keep it as minimal as possible. Optimization of the roasting process has been investigated by studying the calcine using the XRD technique in conjunction with the sulphur removal test and M6ssbauer spectroscopy. The ore used was a zinc-copper ore from Maranda mine in the Murchison Greenstone belt in South Africa. As it will be stated in the literature review, studies have been made on partial roasting where the temperatures are kept low; this study focuses on the effect of high temperature roasting. The resulting calcine is leached and iron precipitated from the pregnant solution. The precipitated iron using different bases (NH4OH and NaOH) has been studied to see the effect of these cations. The possibility of selective leaching has also been studied as an alternative to the roast-leach route. The behaviour of individual sulphide minerals can aid in the understanding of selective leaching of a zinc-copper sulphide ore where pyrite exists. As it eliminates the roasting step, it thus reduces the energy consumption and also the purification step will be cheaper since less iron will have to be managed. Selective leaching has been done in this study using Mn02 and Na2S208 as oxidising agents in the presence of H2SO4. Though the optimum concentrations have not been established, the increase in concentrations has shown increase in the percentage extraction. The choice of the oxidising agent has been observed to play a role in selectively leaching each mineral over the other. Sphalerite was found to be leached first if Na2S2O8 was used first than when Mn02 was used. The thesis is divided into five chapters. The first chapter introduces the work, what it focuses on, its aims and problem statement. Chapter two gives the literature review. The main theme in chapter two is to outline what has been done previously in the field of iron removal and why the other approaches on the subject of iron control have been unsuccessful. In the third chapter, a methodology is outlined on the experiments conducted. This includes experiments from the as 18

19 received ore, how the liberation size was achieved, the concentration process, roasting experiments and the characterisation techniques of calcines, leaching experiments, iron precipitation experiments and the selective leaching experiments. Chapter four gives a detailed report on the results and their discussion. Some mini conclusions are stated at the end of some discussion of results. Chapter five gives a summary, conclusions and recommendations on the project. 1.3 Problem Statement Iron is invariably associated with most minerals. Its presence in significant amounts results in it locking a significant amount of the desired metal in a ferrite form, which is produced during roasting. The roasting process has to be optimised to minimise the production of ferrites. 19

20 CHAPTER 2 - LITERATURE REVIEW 2.1 Introduction The most commonly used route in the zinc-copper industry is the roast-leach-electrowin process. The roasting conditions depend on industry preference and especially the advantages that each industry would prefer over the other. Partial roasting and dead roasting have been tried and are practiced by some industries. The presence and amount of iron in the concentrate also plays a major role in determining the choice of the roasting process where the products from each roasting process differ from each other. The cost of the iron removal process is dependent on the roasting process being used. Selective leaching is another alternative that if properly done, would eliminate the iron removal process. 2.2 Iron in Sulphide ores Zinc occurs in nature mainly as the sulphide (ZnS), which is mineralogically known as sphalerite. Various iron minerals generally accompany the occurrence of zinc in these sulphide deposits, (Elsherief, 1999). Conventional zinc concentrates i.e. beneficiated zinc ores; typically contain 5 10 percent iron. The iron commonly associated with the zinc concentrates can be present as either a replacement for zinc in sphalerite (ZnFeS2) or marmatite (Zn,Fe)S which is a variety of sphalerite or as separate minerals such as pyrite, pyrrhotite, or chalcopyrite. This makes the disposal of iron an integral part of the design and operation of zinc refineries, (Dutrizac, 1987). Deposits of metallic copper have been mined in several parts of the world. Currently, however, copper is found naturally as simple or complex sulphides or as compounds such as hydroxides or carbonate produced from sulphides by local weathering. There are some copper sulphides of economic importance that are associated with sulphides of Iron. They are chalcopyrite (CuFeS2) and Bornite (Cu5FeS4) with copper contents of 34.6% and 63.3% respectively, (Ozberk and Minto, 1986). The roasting of the concentrate produces some ferrites which at times are combined with the zinc in the case where the concentrate contains sphalerite and chalcopyrite. Sphalerite present in the ore usually reports in the concentrate (though in small amounts). 20

21 When leaching the calcine, besides copper, oxides of iron present in the ore are also leached. It is therefore necessary to exercise control over the amount and strength of the acid to be used for leaching to attain maximum copper and minimum iron extraction. When copper oxide minerals are leached, sulphuric acid has been found to be about five times the weight of the dissolved copper, (Ozberk and Minto, 1986) 2.3 Roasting of sulphide ores Partial roasting Stein and Spink, (1990) made some developments for partial oxidation roasting of zinc concentrates, which affords a solution to the ferrite problem. In their process, complete avoidance of zinc ferrite formation was attained with resultant higher overall recoveries of zinc than were presently achieved via the conventional dead-roast-leach-electrowinning process. In their process, the iron was maintained in its 2 + state throughout the roast by a controlled set of roast operating conditions. However this resulted in some zinc being undigested in the leach residue and had to be recycled in the roasting circuit. Since the zinc sulphide ore contained significant amounts of iron, there was a formation of zinc ferrite in the roasting of the concentrate. During the dead roasting of copper concentrates the following reactions occur: CuFeSO 4 + Heat Cu 2S + FeS + SO 2 ( ) Cu 2S > Cu 2O + SO 2 ( ) FeS + 02 > FeO +S02 ( ) During roasting of the zinc-copper concentrates the zinc is tied up in ferrites and also silicates according to the following reactions: Fe2O3 ZnO --> ZnO Fe2O3 ( ) SiO 2 + ZnO --> ZnO SiO 2 ( ) Chen and Cabri, (1993), studied the sulphation roasting in which the significant differences from the conventional dead roast are the roaster operating temperatures, the method of gas cooling and cleaning, the recycling of solutions to the roaster for thermal decomposition, also the absence of an iron removal stage such as jarosite or goethite. In this process the temperature is kept at 21

22 675 C and the following exothermic reactions can take place: 0.87ZnS FeS > 0.87ZnSO Fe AH (685 C) = Kcal ( ).0.87ZnS FeS > 0.29(ZnO 2ZnSO 4 )+ 0.64Fe AH (685 C) = Kcal 7) ( ZnO.2ZnSO 4 + Fe2O3 -+ ZnO.Fe ZnSO 4 AH (685 C) = 1.79 KCal ( ) CuFeS > CuSO Fe SO 2 AH (685 C) = KCal ( ) CuFeS > 0.5(CuO.CuSO 4 )+ 0.5Fe S0 2 AH (685 C) = KCal ( ) The highly exothermic character of the above reactions and the high oxidation states result form the heat being evolved from the reacting particles at a faster rate than it can be dissipated into the surrounding gas phase. This effect increases the temperature of the reacting particle over the bulk temperature of the bed (685 C) and hence the local displacement of the thermodynamic equilibrium from zinc sulphate to the oxy-sulphate (ZnO.2ZnSO4) can occur. This oxy-sulphate can react readily with hematite formed from the iron present in sphalerite and / or pyrite to produce zinc ferrite. Avoidance of the zinc ferrite would result in the production of zinc oxide. In the case of copper, it would results in the production of copper oxide. The roasted ore (calcine) would then be leached with H2SO4. The principal reactions occurring during the leaching would be: C U0 + H2SO4 Cu 2 + SO4- + H 2O ( ) ZnO + H 2SO 4 > Zn 2+ + SO4- + H 2O ( ) 22

23 Toho zinc's Annaka Refinery developed the Waelz process as a means of zinc refining, (Ozberk and Minto, 1986). The main feature of this process is that zinc is vaporised and recovered at the same stage and the clinker sold to cement companies as a source of iron. This eliminates the problem of iron residual disposal. One method that was tried by Van Niekerk and Begley, (1991) is the so-called residue fuming, which depends on decomposition of zinc ferrite and formation of zinc sulphate by roasting at high temperatures with sulphuric acid. The reaction is probably represented by the following equation ZnO.Fe H 2S0 4 > ZnSO 4 + Fe 2 (SO 4 ) 3 + 4H 20 ( ) The sulphate roasting process was developed in Akita refinery Materials Corporation, (Dutrizac, (1987)). In this process the iron in the zinc concentrate is removed as Fe203 to the final residue, which can be safely stockpiled without the possibility of the re-solution of the remaining heavy metals. Gula et al. (1992) developed an ion exchange process using diphonix resin for the control of iron from a cobalt solution to replace the bleed stream process. Another area of considerable activity is the development of a solvent extraction route for iron for integration into zinc hydrometallurgy to replace conventional iron removal by precipitation as jarosite, goethite or hematite. Solvent extraction has however not yet been successfully used commercially in the primary zinc industry (Dutrizac and Harris, 1996). Lakshmanan et al. (1992) studied the separation of iron from acid leach solutions containing zinc, by solvent extraction using N-alkyl hydroxamic acids. However, solvent extraction is being used commercially for iron removal, particularly when chloride hydrometallurgy is used. The extraction of copper from its sulphide ores also involves some iron removal processes. Copper sulphide concentrates generally contain valuable impurities such as silver, gold, platinum, selenium and tellurium, which because of the small quantities in which they report to iron-dominated residues, are totally uneconomical for hydrometallurgical recovery processes. 23

24 Several hydrometallurgical processes have, however been developed to treat copper sulphide concentrates (Ozberk and Minto, 1986). In the roast-leach-electrowin process, iron is removed from the leach residue after neutralization. In the Arbiter Ammonium-oxygen leach process, iron is removed after leaching the concentrate with ammonia and oxygen and before the copper is electrowon, (Ozberk and Minto, 1986). The chloride leaching and copper electrowinning process involves two stages of iron removal. Iron present in the gangue minerals does not dissolve in a chloride medium and remains in the leach residue. Iron contained in chalcopyrite, marmatite and pyrrhotite, dissolves with copper and is removed from the spent electrolyte after electrowinning of copper in a diaphragm cell (Ozberk and Minto, 1986). A special mechanical aid leach was developed at Great Falls and at Eitrheim to dissolve the ferrites in the leach residues and to precipitate the iron as basic iron sulphate with calcine and ground lime-bearing rock, (Dutrizac, 1987). A method of residue treatment, called red roasting, which, is in fact, a sulphating roast, worked well, but the subsequent water leach to recover the zinc sulphate seriously affected the sulphate balance in the leaching circuit Dead roasting The calcine obtained by high temperature (800 C and above) roasting of the sulphide concentrate contains predominantly zinc oxide, minimal zinc sulphate, and about percent zinc ferrite. One of the problems of the hydrometallurgical methods of zinc extraction is the ferrite formation during the high temperature roasting (800 C and above) of the sulphide concentrate. The removal of iron was a major difficulty for the industry for many decades and was responsible for low overall zinc recoveries. The zinc ferrite locks in about 10 15% of the zinc originally present in the concentrate and is not dissolved in dilute acid and combines with residues. The efficient recovery of zinc metal requires rejection of iron residue in a form that minimises the zinc entrainment. Recovering zinc from these residues can be achieved by leaching with hot sulphuric acid, but under such 24

25 conditions a substantial portion of iron also dissolves. The reaction for the dissolution of zinc ferrite is shown below: ZnO Fe H 2SO 4 --> ZnSO 4 + Fe(SO 4 ) 3 + 4H20 ( ) The maximum leach temperature was restricted to below the melting point of the elemental sulphur (119 C) because the molten element sulphur, formed in the leach, coated the partially reacted metal sulphides and thus limited zinc extraction (Van Niekerk et al, 1991). The amount of zinc ferrite formed is directly proportional to the amount of iron present in the concentrate. This implies that the higher the iron level in the concentrate the more important and costly its removal is. The formation of zinc ferrite during roasting must be minimised to avoid unnecessary re-treatment of residues to recover the zinc. It is therefore important and necessary to optimize the roasting conditions that will result in less zinc ferrite formed. 2.4 Leaching Leaching, in general, is a process whereby a solid and a liquid chemically react and all part of the solid is dissolved as a soluble species in the liquid solvent. The leaching of sulphides can be conducted by a number of leaching methods. In this study, the conventional and the selective leaching were used. The hydrometallurgical processing of complex concentrates represents an ecologically attractive alternative with respect to classical pyrometallurgical technologies Conventional leaching of sulphide ores Conventional leaching is mostly done after the roasting process. The calcine is in the form of an oxide and thus easy to be leached in neutral sulphuric acid, which is normally used in industry. The leaching acid in this case is aimed to attack all the oxides. Some ferrites do not dissolve in the neutral leach and are therefore leached at rather intense conditions (high temperatures and stronger acid concentration). This also attacks the ferrites simultaneously Selective leaching of sulphide ores The overall recovery of the metal and the difficulty of separating it from impurity metals are generally governed by the efficiency selectivity of the leaching process (Lakshmanan et al, 1992). 25

26 Leaching is described as a heterogeneous (that is solid/liquid) reaction, as opposed to a homogeneous reaction with all the reactants in a single phase (e.g. all in solution as soluble species). Leaching can occur in a number of ways and various models or descriptions are used to describe these ways (Junea et al, 1996). There are basically two types of leaching reactions: those in which an oxidised metal compound is dissolved in a reagent solution: CuO H2SO4 CuSO4 + H2O ( ) Those in which the metal or compound must be oxidised during leaching: CuS + H2SO > CuSO4 + H 2O + S ( ) The oxidative leaching of metal sulphides is a complex process involving a number of possible chemical reactions in parallel and series. In addition, sulphide minerals display a variety of complex crystal structures, with replacement metal atoms often present and variations from the ideal stoichiometry. This diversity of character between sulphide minerals and even between samples of the same sulphide makes it almost impossible to predict accurately their behavior during oxidative leaching, (Junea et al. 1996). The overall leach can be limited by: Mass transport of reactants and products in solution In general, adequate transport of chemical species in solution can be achieved by suitable mixing equipment. Diffusion of reactants and products. Films of reaction products can often form on the surface of sulphide minerals undergoing oxidative leaching. The most common film forming materials are iron oxide, sulphur and insoluble sulphates. The formation of an iron oxide film is largely dependent on the ph value of the solution. With pyrite at a ph above 3.0, iron oxide will usually precipitate on the surface. Sulphide sulphur also occurs during oxidative leaching and can transform in a variety of oxidation states. The temperature affects the nature of the elemental sulphur. Below 118 C the sulphur is usually porous and the film does not inhibit the rate of oxidation. Above 118 C (which 26

27 is close to the melting point of sulphur) the sulphur forms a very effective barrier against further rapid oxidation. In the case of chalcopyrite, inhibition of the oxidation by a film of sulphur can occur at a temperature below 118 C, especially in the sulphate system. c. Chemical reactions at the surface In systems with adequate mass transfer and where no surface films inhibit the leaching, the rate can be controlled by the heterogeneous reactions at the sulphide surface. Rates controlled by chemical reactions with relatively high activation energies are significantly enhanced by increasing temperature according to the Arrherius relationships: (Junea et al, 1996) Rate a K exp RT K is a constant E is activation energy R is the gas constant T is absolute temperature ( ) For many oxidative leaching systems, the rate will be more than double for every 10 C rise in temperature. Ferric sulphates can leach the sulphide according to the following mineral reaction. (Metallurgical test work and research report, Australia, 1989) MS + Fe(SO 4 )3 > MSO 4 + 2FeSO 4 + S ( ) Sulphide minerals display great variations in their response to leaching. Dutrizac and Palencia, (2002), studied the effect of the iron content in sphalerite on its rate of dissolution in ferric sulphate and ferric chloride media. They observed that the leaching rate increased in a linear manner with increasing iron content in both cases. A study was also done on the dissolution of chalcopyrite in ferric sulphate and ferric chloride media. Rates were observed to be faster in the chloride system. The activation energy was found to be about 42KJ/mol and 75 KJ/mol in the ferric chloride and ferric sulphate respectively. Leaching was found to be independent of the iron concentration in the sulphate system, as opposed to the leaching of sphalerite in the same media (Dutrizac, 1981). 27

28 A study on hydrochloric leaching of a complex zinc sulphide ore was investigated with the objective of obtaining selective dissolution. Only electrolysis under an applied potential of A300 mv permitted selective dissolution (Elsherief, 1999). Dissolution of chalcopyrite in a hydrochloric acid medium in the presence of manganese dioxide was studied. Chalcopyrite did not dissolve independently but underwent oxidative dissolution in the presence of manganese dioxide (Devi et al, 2000). Oxidative leaching of chalcopyrite with dissolved oxygen and / or with ferric ions was modeled and found to be promoted by high concentrations of ferrous ions in sulphuric acid solutions, containing cupric ions. The ferrous ions were released from chalcopyrite together with cupric ions during the leaching. This ferrous promoted chalcopyrite leaching and an auto-catalytic process and no supply of a promoter was necessary. This process could avoid running cost increases while improving copper extraction rates (Naoki et al, 2000). Godocikova et al. (2002) did a study on the structural and temperature sensitivity of chloride leaching of copper, lead and zinc from a complex CuPbZn sulphide concentrate, which was mechanically activated. They observed that mechanical activation influenced the leaching kinetics and recoveries of copper and zinc. Their suggested order of structural sensitivity was galena>chalcopyrite>sphalerite, in accordance with the temperature sensitivity. The understanding and application of the leaching behavior of individual sulphide minerals can be used to device and optimise more efficient and selective leaching processes for the sulphides in South African mining industry. This could be an alternative to the roast-leachelectrowin process and the problem of iron removal with costs being minimized. 2.5 Precipitation The precipitation process mostly governs the removal of iron in a filterable form. Precipitation commonly involves the addition of a solution containing a precipitating agent to an aqueous solution of the desired metal. The colloidal chemistry involved is quite complex, such that the 28

29 procedures to attain the desired gel structure in the precipitate have been developed through an empirical research (Rao, 1992 and Schwertmann, 1995). According to Schwertmann (1991) and Skoog et al, (1992), a precipitate forms through the process of nucleation and crystal growth, and the size of the particles formed is being determined by the rates at which these two processes occur. The initial step in the formation of a colloidal suspension is nucleation (Mullin, 1961). Further precipitation then involves a competition between additional nucleation and growth of the existing nuclei. The rate of nucleation has been shown to increase exponentially with increasing relative super saturation (Nielson, 1964). Saturation is the extent by which the solubility limit has been exceeded. The relative super saturation is given by the following equation (Nielson, 1964). Re lative Super Saturation = (2.5-1) Where Q is the concentration of the solid at any instant S is the equilibrium solubility. In contrast to the nucleation reaction, the rate, of particle growth is only moderately enhanced by a high relative super saturation. Thus, when the relative super saturation is high, nucleation is the major precipitation mechanism and this results in formation of a large number of small particles. When the relative super saturation is low, the particle growth predominates and a small number of large particles are produced. When the two mechanisms are present at almost equal rates, particles with a broad particle size distribution are produced Hydrolysis of Iron in Aqueous Media The study of the precipitation of a metal hydrous oxide, M203.xH2O, from solution is full of experimental and conceptual difficulties related to multiple reaction pathways that occur simultaneously and are difficult to separate. The hydrolysis of ferric ions can however be 29

30 described in a simpler approach as a series of hydrolytic polymerisation reactions involving deprotonation of the original hexa-aqua, Fe (H 20)63+ ion to form hydroxo- and oxo-species (Blesa, 1989). In general hydrolysis of ferric solutions is readily induced by addition of a base. Doumsa and de Bruyn. (1979) studied the hydrolysis and precipitation of ferric oxide and oxy-hydroxide from a Fe(NO3)3 solution. They elected Fe 3+ concentrations, temperature and ionic strength as important variables during the homogeneous based titration of the acidified solutions. Using titration methods coupled with optical density measurements, they distinguished between four stages in the hydrolysis-precipitation process. These are (i) hydrolysis to monomers and dimmers; (ii) reversible, rapid growth of small polymers; (iii) slow formation of large polymers; and (iv) precipitation of a solid phase. The schematic representation of these stages is shown in Figure OH Fe(OH) 2+ "4-111' Fe OH - FiVVH\e A nh + n/2 O H O H n + N e/ N e/ \1 \117 \ n/2 Figure : Schematic representation of the hydrolysis-precipitation process (Doumsa and de Bruyn, 1979). Doumsa and de Bruyn, (1979) investigated the role of chloride ions on the formation of Fe 3+ oxyhydroxides by homogenous injection of an alkali (NaOH) into acidified Fe 3+ solutions with Cl/Fe ratios varying between and found that chloride is an important constituent of the polynuclear particles formed during the early stages of titration (ph<2). With an increase in the amount of base added, this anion was found to be largely displaced by hydroxyl ions. 30

31 An investigation was done on the mechanism of formation of the Fe 3+ oxyhydroxides and oxides from the hydrolysis of Fe 3+ salt solution at elevated temperature (Music et al. 1994). The hydrolysis of Fe 3+ ions in nitrate and chloride solutions was found to commence with the formation of simple goethite complexes. This process was followed by the formation of polymeric species from the monomers. It has been shown that the goethite polymers in the nitrate solutions do not have nitrate ions in the polymer chain, whereas the polymers formed in chloride solutions contained some chloride ions in the place of the OH - ions. Murphy et al, (1976 a, b, c) observed that the nature of the anion does not affect the initial products of iron polymerisation, but affects the subsequent ageing process. The hydrolysis of iron from a solution containing sulphate anions has also been studied (Doumsa and de Bruyn, 1979). In this study it was noted that the SW, ± /Fe ratio in the solution had a marked effect on the titration curves obtained by plotting the ph of the solution against the amount of the base added. A similar study was done by Music et al, (1994) where he the formation of a (FeSO 4 )+ complex in a solution of ferric sulphate was observed to suppress the polymerisation process and the formation of oxyhydroxides and oxides. Instead basic Fe (III) sulphates were formed. Cornell et al, (1989) in their review, stated that the addition of sufficient base to give OH/Fe ratio greater than 3 immediately lead to precipitation of poorly ordered ferric hydroxide of which the degree of ordering depended on the method of preparation and the time of ageing. These precipitates resembled the mineral ferrihydride (Fe5HO8.4H20) and showed some similarity to the oxyhydroxide core of ferritin. Thermodynamic data indicated that ferrihydrite is unstable and with time transforms into more stable, crystalline oxides such as a-feo(oh) and a-fe203, which form via different mechanisms. Conditions that promote coagulation of particles of ferrihydrite favour formation of a-fe203, whereas the formation of a-feo(oh) proceeded and most readily at ph values that promoted dissolution of ferrihydrite. The master variable that governed the reaction products was the ph of the solution. As the a- Fe0(OH) nucleated and grew in the solution, it became more susceptible to the effect of solution variables than did a-fe203 which formed within a solid phase. Other factors that influenced the composition of the reaction products are temperature of ageing, suspension concentration, ionic strength and pre-treatment of either ferrihydrite or the ferric solution (Atkison et al, 1968 and Cornell et al, 1989). 31

32 The breakthrough for the precipitation of iron in a filterable form came in 1964 with the discovery of a method to precipitate iron in the form of jarosite. Other methods to precipitate iron from sulphate solutions soon followed and were put into practice. They are the goethite process, hematite process, conversion process, basic iron sulphate precipitation and the direct leaching of the sulphide concentrate. All these have permitted the recovery of 10-15% extra zinc, which was previously lost in leach residues with the ferrites. The dissolution of the ferrites in concentrated H2SO4 however leads to; (Murphy et.al. 1976a, b, c) An increase in the impurity level, especially iron and extensive purification would be required before electro winning; High acidity, which must be neutralised so that electro winning would be feasible. This led to the solution of precipitating the iron in a crystalline form easy to filter. This effect resulted in a poor dissolution of the zinc since only the zinc oxide fraction dissolved during leaching. It was known that ferrites would dissolve in strong acid solution at temperatures close to boiling point, but the combined dissolution of iron and zinc in sulphuric acid led to the problem of separating the two metals. Much research work followed in an attempt to precipitate the iron in a filterable form. The hydrolysis of iron (III) ions in aqueous solutions has received considerable attention through the years. A large number of studies have been devoted to the behaviour of iron oxide suspensions in solution in aqueous media. A review of the literature, which deals with both naturally occurring minerals and synthesised iron oxides, makes it clear that the method of preparation and subsequent treatment of the oxide may change drastically its surface activity Iron precipitate products Genin et al. (1991) studied the mechanism of oxidation of ferrous hydroxide in sulphated aqueous media and the importance of the initial ratio of the reactants. Using M6ssbauer spectroscopy, coupled with direct recording of the ph and the electrode potential, they concluded that the factor R, which is the ratio of the initial concentration of Fe 2+ and S0 42- to OW ions, has a remarkable influence on the end products as well as the initial products. Genin 32

33 et al, (1991), also concluded that the presence of certain anions like CO3 -, NOT, CI Br and SO4- has an influence, depending on the ph of the solution, and the oxidation mechanism of the ferrous hydroxide. Besides the jarosite form, which came to existence in 1964, other methods to precipitate iron from sulphate solution soon followed. Veille Montagne of Belgium developed the goethite process (Davey and Scott, 1976). Bryson et al. (1994) studied the batch precipitation of goethite from sulphate solution containing ferrous and zinc cations. In this process ferric ion is reduced to ferrous by adding zinc sulphide concentrate. Air was then injected in the hot solution at a ph to oxidise and precipitate crystalline ferric oxide hydroxide, a-feooh, known as goethite. Many of the parameters governing the removal of iron from leach liqueurs by the goethite process have been identified and investigated. Davey and Scott, (1976) studied the two variations of the process, namely, the Veille Montagne and the electrolytic zinc procedures. Both procedures were examined in sulphate and chloride liquors. They came up with some advantages of the goethite process over the other processes and the main ones being its superiority with respect to iron removal down to low levels and its ability to function without added alkali metals. The hematite process was developed independently by Dowa Mining Company of Japan and by Ruhr-Zinc of Germany (Dutrizac, 1987). In this process, iron is precipitated predominantly as iron oxide (Fe203) in the presence of oxygen at approximately 180 C and a total pressure of Mpa. The other process is the direct leach of zinc sulphide concentrate at an elevated temperature and in the presence of oxygen, which was developed by Sherritt Gordon (Van Niekerk and Begley, 1991). The last being the process in which iron is precipitated as a basic sulphate and was developed by Zincor in Springs (South Africa), (Van Niekerk and Begley, 1991). Murphy et al a, b, c), studied the ferric hydroxypoly-cations formed in partially neutralised ferric solutions. They studied the growth of precipitate phases in hydrolysed ferric chloride, ferric nitrate and ferric perchlorate solutions via electron microscopy and X-ray diffraction techniques. 33

34 Knight and Sylva, (1974) also studied the induction times before precipitation of iron oxides from nitrates, perchlorates and chloride over a wide range of initial OH/Fe ratios. Over the years much focus has been put on the chemistry of the hydrolysis of iron into a filterable form with much emphasis on the following properties of the precipitate: Filterable and washable, Must not incorporate sought after metal values, Other impurities should be controlled, Process should yield a precipitate that is ideally marketable, Stable final product. Gordon and Pickering, (1975) gave an account of the precipitation processes, the comparison being based on plant practice. Table below is a comparison of the processes which are most commonly used in South African companies with, some variations in the precipitation temperature. 34

35 Table : Comparison of Iron Precipitation Processes Gordon et al. (1975) Process Goethite Jarosite Hematite Operating Conditions ph < 1.5 Up to 2% H2SO4 Temperature C C -,,,, 200 C Anion Any SO4- only SO4 only Added required Product cations Nil Nat, K+, NH 4, 1 - (= R) Nil Compound formed a- and fl-feooh Rfe3(SO4)2(OH)6 a-fe203 (a-fe203) Cationic impurities Medium Low (apart from R) Low Anionic impurities Medium High Medium Filterability Very good Very good Very good Fe in filtrate < 1 g/l (often < 1 5 g/l 3 g/l 0.05 g/l) Studies have been conducted to define the precipitation parameter, to explain the structural relationship and to determine the extent of impurity incorporation. Most of these were carried over synthesised iron oxides. Current practice focuses on the elimination of iron from process streams via aqueous hydrolysis of ferric ions (Mullin, 1961). Davey and Scott (1976) and Dutrizac (1987) concluded that the conditions employed for the forms of iron to be precipitated from solution depend on the solution conditions. Murphy et al, (1976 a, b, c), observed the growth of precipitate phases in hydrolysed ferric nitrate, chloride and perchlorate solutions via electron microscopy and X-ray diffraction (XRD). 35

36 The Zincor basic ferric sulphate process is another method used to control iron. In this process iron is precipitated as an easily filterable basic sulphate, achieved by careful ph control as follows Fe(SO 4 ) 3 + 2Zn(OH)2 > Fe03.S03.H20 + 2ZnSO4 ( ) This process is used at Zincor in Springs, (Van Niekerk and Begley, (1991)) The residue sulphate roasting process is being carried out at the Akita Zinc Refinery of Mitsubishi Metal Corporation (Ozberk and Minto, (1986)). In this process, hot acid leach residue from the zinc plant is subjected to flotation, where silver is removed. The first stage flotation tailings are subjected to a sulphonation roast at C to decompose zinc ferrites as follows: ZnO.Fe203 + SO3 ---> ZnSO4 + Fe203 ( ) Zinc recovery from the roast is accomplished by leaching with a weak acid from the main zinc circuit, and iron is discarded as Fe203 in the final residue. The Grassroots Process was developed by Sherritt Gordon (Dutrizac, 1987). This process is used when an entire roaster section has to be replaced and involves two processes. The first process involves a neutralisation autoclave leach where most of the iron is precipitated, and the solution from this leach is then subjected to a further neutralisation step where more iron is precipitated. Solids from the first-stage are further leached for zinc recovery. The second pr6cess makes use of a high-acid pressure leach, which leaches zinc and iron. Ferric iron and concentrated zinc solution are then passed to a low-acid first-stage leach where iron is precipitated conventionally. An intensive study has also been conducted on the factors that affect the kinetics of nucleation and growth and purity of iron precipitates. Bryson and Teriele, (1994) concluded that contamination of the precipitate with zinc is shown to be approximately proportional to the zinc concentration in solution. Also, from measuring particle size distribution, he concluded that nucleation occurs as a result of weak outgrowths being dislodged from growing particles. As a 36