CA2663356A1

CA2663356A1 - Process for the indirect bioleaching of zinc sulphide

Info

Publication number: CA2663356A1
Application number: CA002663356A
Authority: CA
Inventors: Stefan Walters Robertson
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-09-21
Filing date: 2006-09-19
Publication date: 2007-03-29
Anticipated expiration: 2026-09-19
Also published as: CA2663356C; WO2007034413A3; PE20070729A1; WO2007034413A2

Abstract

A process for the indirect bioleaching of zinc sulphide is disclosed, and includes leaching, solvent extraction, electrowinning, and oxidant regeneration steps in a closed circuit. The leaching step utilizes ferric iron as an oxidant, the solvent extraction step utilizes di-2 ethylhexyl phosphoric acid (D2EHPA) as a solvent extraction reagent, the oxidant regeneration step includes the oxidation of ferrous iron to ferric iron in a ferric iron generator; and the process includes minimizing the loading of ferric iron onto the solvent extraction reagent in the solvent extraction step by means of one or more of the steps of minimizing the concentration of ferric iron in the leach stream and minimizing the oxidation of ferrous iron to ferric iron in the solvent extraction step.

Description

PROCESS FOR THE INDIRECT BIOLEACHING OF ZINC SULPHIDE

FIELD OF THE INVENTION

This invention relates to a process for the indirect bioleaching of zinc sulphide.
BACKGROUND TO THE INVENTION
Existing commercial routes for the processing of zinc sulphide concentrates include smelting, conventional roast/leaching, which accounts for 80% of commercial zinc production, and pressure leaching. Chloride based leaching systems have also been developed but have not yet progressed to commercial implementation due to complexity and operating difficulties.

Pressure leaching and roast/leach processes cannot handle low-grade zinc concentrates or mixed lead/zinc concentrates as primary feedstock. The conventional roast/leach process also has environmental problems, such as SO2 emissions, which require conversion to sulphuric acid. This is expensive and can be uneconomic due to overproduction of sulphuric acid in many parts of the world. The use of direct purification and electrowinning as used in the roast/leach process also generates toxic residues that typically contain cadmium and arsenic or antimony, which is expensive to treat. Direct purification and electrowinning circuits are also sensitive to certain impurities such as halogens. Solvent extraction (SX) has recently been employed for zinc recovery prior to electrowinning (EW) in cases where the feed contains high levels of chloride and to upgrade the zinc concentration where the leach solution contains concentrations too low for electrowinning, e.g. Skorpion Zinc refinery in Namibia (Sole et al., Hydrometallurgy 78 (2005) 52-78).

DIRECT BIOLEACHING

Processes for direct bioleaching of zinc concentrates have recently been developed and described in the patents by Basson et al. WO 01/18266 and Steemson et al. WO 94/28184 as an alternative to conventional roast-leaching. These direct bioleaching processes can treat low-grade zinc concentrates and do not generate any environmentally hazardous emissions or residues. The leach solution is purified by precipitation of all iron in the pregnant leach stream (PLS) followed by the recovery of zinc by solvent extraction and electrowinning. This eliminates the need for an expensive and complicated purification circuit.

Disadvantages of direct bioleaching include the complete conversion of sulphur to sulphuric acid in the bioleach, resulting in increased oxygen and neutralizing agent consumptions.

INDIRECT BIOLEACHING
In order to avoid the increased neutralization and aeration costs arising from the complete conversion of sulphur in the direct bioleaching of zinc sulphide, an indirect bioleaching process has been proposed which consists of a chemical (ferric) leaching step and a bacterial iron regeneration (FIG) step.
Leuking and Nesbitt (US Patent 5,827,701, 6,043,022, European Patent 0808910A2) described a biological ferric iron generator, which consists of an agitated tank sparged with an air/CO2 mixture and fed with a ferrous sulphate solution. The pH is controlled by addition of sulphuric acid or neutralizing agent. The cell density in the reactor is monitored with a particle size analyzer and is used to control the rate at which ferrous sulphate feed solution is fed.
Hearne et al. (Lead and Zinc Processing, The metallurgical society of CIM, 1998), developed a closed-circuit indirect bioleaching process for sphalerite (ZnS) concentrate consisting of a chemical leach and a ferric iron regeneration step using a packed bed column, followed by the recovery of zinc from the PLS by solvent extraction (SX) with a Zeneca developmental reagent DS 5846. This reagent is a substituted bis-dithiophosphoramide which selectively extracts zinc from iron (III) and iron (II).

However, Hearne's process was abandoned because of the cost of the Zeneca SX reagent, which has since been taken off the market.

Aragones (US Patent 5,462,720) described an indirect bioleaching process for copper concentrate consisting of a chemical leaching step in a heap/pile or a two-stage stirred tank leach with addition of Ag catalyst if the feed is chalcopyrite. Aragones' FIG consists of a packed bed or biological contactors.
A portion of leach underflow solids is recycled back to the leach, and the settler overflow is recycled to the FIG for iron (III) regeneration. Cu is recovered by SX from a PLS bleed stream.

Carranza, Garcia, Pereda, Iglesias, Palencia and Romero [Hydrometallurgy, 24 (1990), pp.67-76; Microbiology Reviews, 11 (1993), pp.129-138;
Hydrometallurgy, 44 (1997), pp.29-42; Hydrometallurgy, 23 (1990) pp.191-202; Hydrometallurgy, 48 (1998) pp. 101-112; Spanish Patent 2009104-8803370 (1989); and Hydrometallurgy, 49 (1998) pp. 75-86] developed a similar indirect bioleach process for a mixed Cu/Zn concentrate, utilizing a packed-bed column FIG. Cu and Zn are recovered by SX from a PLS bleed stream.

In both the processes of Aragones and Carranza et al above, details of the SX
are not given, and raffinate iron and acid are not recycled to the FIG, such that the iron and acid balance is not optimal. Zinc is not extracted by SX in a closed circuit, but is removed from a bleed stream, where complete iron precipitation is carried out, followed by recovery of zinc and then Cu by SX.
The iron lost in the bleed is replaced by feeding Fe2(SO4)3 to the FIG, which may make the process uneconomical.
Van Staden (PCT Patent Application W02005/005672) developed an indirect bioleaching process for Cu concentrates, which includes a ferric leaching step consisting of a stirred-tank cascade, and a ferric iron generator (FIG) consisting of a stirred-tank cascade or fluidized-bed column. Iron precipitation is possible in the fluidized bed, which would otherwise inhibit and block the bacterial film if a packed bed was used. Excess iron introduced into the circuit is hydrolysed in the FIG, in order to maintain an optimal iron and acid balance.
The overall circuit is acid-neutral, and self-regulating in terms of the soluble iron inventory. The build-up of zinc is controlled with a bleed stream, since it is not possible to recover zinc selectively from the PLS with commercial SX
reagents such as D2EHPA, due to the co-loading of iron (III). Cu, on the other hand, can be extracted selectively from iron (III) with commercial SX
reagents.
The circuit is therefore limited in the concentration of zinc in the feed that can be tolerated.

ZINC SOLVENT EXTRACTION (Zn SX) A number of patents exist for processes to recover zinc from aqueous liquors using D2EHPA (di-2 ethylhexyl phosphoric acid) [Hydrometallurgy 3 (4) 327-342 (1978), Chem.Engg.Res. and Design 61 62-66 (1983), US Pat Nos.
3989607, 3441372, 4053552, 4124462, 4401531, 4423012, 4552629, 4618428, 5135652, CA Patents 1083830, 1098719, 1198290] or with substituted phosphinic acids (US Pat Nos. 3966569, 4721605). Typical conditions are pH 2-3, 20-30% D2EHPA in diluent, 30-40 C, O/A 3:1. The loaded organic is stripped with sulphuric acid.

These processes are however not suited to recover zinc from the PLS in a closed-circuit indirect bioleach process, since they require upfront removal of iron from the process stream. This is necessary since iron(III) loads preferentially onto the D2EHPA, using up the SX capacity and reducing the pH so that zinc cannot be recovered by the D2EHPA. It is also necessary to strip the D2EHPA with HCI to remove the build-up of iron(III). The iron present 5 as iron(III) is therefore lost to the circuit, and cannot be recycled.

OBJECT OF THE INVENTION

It is an object of this invention to provide a process for the indirect bioleaching of zinc sulphide that at least partly overcomes the mentioned problems.
SUMMARY OF THE INVENTION

In accordance with this invention there is provided an indirect zinc sulphide bioleaching process which includes leaching, solvent extraction, electrowinning, and oxidant regeneration steps in a closed circuit;
the leaching step utilizing ferric iron as an oxidant, the solvent extraction step utilizing di-2 ethylhexyl phosphoric acid (D2EHPA) as a solvent extraction reagent, and the oxidant regeneration step including the oxidation of ferrous iron to ferric iron in a ferric iron generator;
the process including minimizing the loading of ferric iron onto the solvent extraction reagent in the solvent extraction step by means of one or more of the steps of minimizing the concentration of ferric iron in the leach stream and minimizing the oxidation of ferrous iron to ferric iron in the solvent extraction step.

There is further provided for the step of minimizing the concentration of ferric iron in the leach stream to include one or more of:
i. using a counter-current leaching stage and recycling solids in the leach stream; and ii. extracting ferric iron from the leach stream by means of a first solvent extraction step, and processing the extracted ferric iron further for reintroducing the processed ferric iron into the leach stream after the leach stream has been processed in at least one subsequent solvent extraction step.

There is still further provided for the step of minimizing the oxidation of ferrous iron to ferric iron in the solvent extraction step to include:
iii. using a pulsed column in the solvent extraction stage.

There is further provided for the process to preferably include steps i), ii) and iii).
As an alternative to step ii), iron (III) can be removed prior to solvent extraction by precipitation with limestone, slaked lime or zinc calcine, in which case it is necessary to replenish the iron lost in the precipitation step by the addition of fresh FeSO4 to the FIG.
There is also provided for the leaching step to include oxidising zinc sulphide by means of ferric iron to form a pregnant leach stream containing zinc sulphate and ferrous iron in solution according to chemical reaction (1):
ZnS + Fe2(SO4)3 ^ ZnSO4 + 2FeSO4 + S (1) There is also provided for the solvent extraction step to include treating the leach stream of reaction (1) with the D2EHPA reagent to form a reagent loaded with zinc according to chemical reaction (2):
2 RH + Zn2+ ^ R2Zn +2H+ (2) in which (R) represents the D2EHPA reagent.

There is further provided for the process to include stripping zinc from the loaded D2EHPA reagent produced by reaction (2) by means of an acid according to reaction (3):
R2Zn +2H+ ^ 2 RH + Zn2+ (3) There is still further provided for the process to include treating the solvent extraction raffinate in a ferric iron generator (FIG) to regenerate ferric iron from the ferrous iron according to reaction (4):

FeSO4 + 0.25 02 + 0.5 H2SO4 ^ 0.5 Fe2(SO4)3 + 0.5 H20 (4) There is further provided for the counter-current leaching of step (i) to be conducted in a stirred tank cascade, with each stirred tank associated with a settler.

There is also provided for zinc in the electrowinning step to be recovered in the form of zinc cathodes, and preferably for the step of stripping zinc from the loaded organic substance produced by reaction (2) to include the use of return electrolyte from the electrowinning step.

There is further provided for the ferric iron extracted in step ii) to be processed further by recovering the ferric iron from a solvent extraction reagent used in the first solvent extraction step, preferably a D2EHPA reagent, and for the ferric iron to be recovered by treating the loaded D2EHPA reagent of step (ii) with hydrochloric acid to strip any residual ferric iron which may have loaded onto the D2EHPA reagent during the solvent extraction step into a strip liquor, adding sulphuric acid to the strip liquor, distilling the strip liquor to recover hydrochloric acid and form a ferric sulphate and sulphuric acid solution, and recycling the ferric sulphate and sulphuric acid solution to the ferric iron generator (FIG).

There is also provided for the process to include treating the loaded D2EHPA
reagent produced by reaction (2) with hydrochloric acid to strip any residual ferric iron which may have loaded onto the D2EHPA reagent during the solvent extraction step into a strip liquor, adding sulphuric acid to the strip liquor, distilling the strip liquor to recover hydrochloric acid and form a ferric sulphate and sulphuric acid solution, and recycling the ferric sulphate and sulphuric acid solution to the ferric iron generator (FIG).
There is further provided for the counter-current leach process to include at least two leach stages to facilitate counter-current operation, but preferably three or more leach stages to also obtain better approximation to plug flow, and for each leach stage to have an associated settling stage with a settler.

There is still further provided for in the case of the leach process including three leach stages, for the first leach stage to be associated with the first settling stage, for the second leach stage to be associated with the second settling stage, and for the third leach stage to be associated with the third settling stage.

There is also provided, for in the case of the leach process including three leach stages, for the leach slurry from the first leach stage to be fed to the first settling stage, for the leach slurry from the second leach stage to be fed to the second settling stage, and for the leach slurry from the third leach stage to be fed to the third settling stage.

There is also provided for the overflow of the third settling stage to be fed into the second leach stage, for the overflow of the second settling stage to be fed to the first leach stage, and for the overflow of the first settling stage to be fed to a solid/liquid separation stage and for separated solids to be fed to the first leach stage.

There is still further provided for underflow solids of the first settling stage to be fed to the second leach stage, for the underflow solids of the second settling stage to be fed to the third leach stage, and for the underflow solids of the third settling stage to be separated from the leach stream as residue.

There is still further provided for step (i) to include maintaining a large inventory of zinc sulphide in the first leach stage to retain reductive conditions and limit the concentration of ferric iron in the leach stream, and preferably for the inventory of zinc sulphide to be maintained by recycling at least a portion of the underflow solids from the settling stage associated with the first leach stage, which is the stage where the solids enters and the solution leaves the leach train, back into the first leach stage.

There is still further provided for the method to include a purification step which comprises bleeding a portion of the pregnant leach stream for zinc dust cementation and neutralisation to limit the accumulation of impurities in the leach stream.

There is further provided for the zinc sulphide feedstock to comprise a sphalerite feedstock, preferably a sphalerite concentrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way of example only and with reference to the following diagrams and tables in which:

Figure 1 is a flow diagram of an indirect bioleach process according to the invention;
Table 1 shows data relating to the FIG and leach conditions for the process described in Figure 1; and Table 2 shows data relating to a steady state mass balance for the process described in Figure 1 and Table 1.

DETAILED DESCRIPTION OF THE INVENTION

The current claim is for a closed-circuit indirect bioleach process for the treatment of sphalerite (ZnS) concentrate, utilizing commercially available SX
reagent D2EHPA for Zn recovery. The flowsheet is shown in Figure 1. The process consists of a chemical (ferric) leaching step (equation 1). The leach is carried out counter-currently in order to minimize the concentration of iron (III) in the PLS. A large inventory of ZnS is maintained in the first leach stage (the stage in which the solids are added and the PLS is removed) in order to maintain reductive conditions, which will limit the concentration of iron (III) in the PLS. This is done by recycling the underflow solids from the settler of the first leach stage back to the same stage.

ZnS + Fe2(SO4)3 -> ZnSO4 + 2FeSO4 + S (1) Zn is recovered from the PLS by solvent extraction with D2EHPA at the conditions described above, typically pH 2, 30% D2EHPA in C12-C13 diluent, 40 C, O/A 3:1. The concentration of D2EHPA can be increased to generate 5 more capacity if the PLS contains iron (III) in greater concentrations. The SX
is carried out in a pulsed column, to minimize the reoxidation of iron (II) to iron (III). Zinc is extracted preferentially from iron (II), but iron (III) loads preferentially onto the organic, and does not strip off at the same acid strength as the zinc:
2 RH+Zn2+-> R2Zn+2H+ (2) The loaded organic is stripped with sulphuric acid return electrolyte from the electrowinning unit operation.
R2Zn + 2H+ 2 RH + Zn2+ (3) In order to remove the build-up of iron (III) on the D2EHPA, the organic is stripped with HCI. H2SO4 from the return electrolyte is added to the HCI strip liquor, followed by distillation and recovery of the HCI, leaving behind a ferric sulphate and sulphuric acid solution, which is recycled to the FIG.

Alternatively, neutralization of the PLS with limestone and partial precipitation of iron (all iron (III)) is carried out prior to SX, in which case iron lost to the circuit is replenished by the addition of FeSO4 to the FIG.

The SX raffinate is fed to a bacterial ferric iron generator (FIG), consisting of a fluidized bed or stirred-tank cascade, which allows precipitation of iron without blocking the bacterial film, as would be the case in a packed bed column. The regenerated ferric iron solution is recycled to the leach.

FeSO4 +0.25 02 + 0.5H2SO4 -> 0.5 Fe2(SO4)3 + 0.5 H20 (4) EXAMPLE
Table 1 gives the stream conditions of a continuous leach and FIG for the treatment of sphalerite concentrate. The FIG was fed with ferrous sulphate solution, containing 30g/L Fe(II) and 7g/L Zn. The ferric iron generator consisted of a fluidised bed column containing activated carbon, inoculated with mesophile bacteria. Additional capacity was provided by three aerated stirred tank reactors. The columns and aerated stirred tanks were operated at pH 1 and 40 C. The redox potential in the column was 617mV w.r.t Ag/AgCI
and the redox in the high redox supply tank to the leach was 700mV w.r.t.
Ag/AgCI.

Sphalerite concentrate was slurried with water and OK nutrient solution to 20%
solids. The high redox solution and slurry were fed countercurrently to a 3-stage leach train with settlers for liquid-solid separation between each stage.
The redox potential exiting the leach train was 430mV, and the zinc concentration increased from 8 g/L in the high redox supply tank to 18.3 g/L
in the PLS holding tank. The leach reactors were operated at 40 C, and the pH
increased to 1.24 in the PLS. The effective solids concentration entering the leach was 1.58%, and the zinc extraction was above 95% over the leach train, as shown in Table 2, which gives a mass balance over the leach.
REFERENCES
FERRIC IRON GENERATOR (FIG) REFERENCES
1. ARAGONES, J.L., Process for biolixiviating copper sulfides by indirect contact with separation of effects, US Patent 5,462,720, 31 October 1995 2. CARRANZA, F., GARCIA, M.J., PALENCIA, I. AND PEREDA, J.
Selective Cyclic Bioleaching of a Copper-Zinc Sulphide Concentrate.
Hydrometallurgy, 24 (1990), pp.67-76 3. CARRANZA, F., IGLESIAS, N., ROMERO, R., PALENCIA, I., Kinetics improvement of high-grade sulphides bioleaching by effects separation.
Federation of European Microbiology Societies, Microbiology Reviews, 11 (1993), pp.129-138 4. CARRANZA, F., PALENCIA, AND ROMERO, R. Silver catalysed IBES
process: application to a Spanish copper - zinc sulphide concentrate.
Hydrometallurgy, 44 (1997), pp.29-42 5. FERRON, C.J.. Atmospheric leaching of zinc sulphide concentrates using regenerated ferric sulphate solutions. Lead-Zinc 2000 (eds.
Dutrizac, J.E., Gonzalez, D.M., Henke, D.M., James, S.E., Siegmund, A.H.-J., pp. 711-726 6. HEARNE, T.M., HAEGELE, R., BECK, R.D., Hydrometallurgical recovery of zinc from sulphide ores and concentrates, Zinc and Lead Processing, Eds. Dutrizac, J.E., Gonzalez, J.A., Bolton, G.L., Hancock, P. The Metallurgical Society of CIM, 1998 7. LUEKING, D.R., NESBITT, C.C., Method for the generation and use of ferric ions, US Patent 5,827,701, 6,043,022, European Patent 8. MULLER, HH, Evaluation of an alternative BFIG circuit for the indirect bioleaching of copper sulphide concentrates, MINTEK Technical Memorandum No 15804, MINTEK, Randburg, 30 March 2005.
9. PALENCIA, I., CARRANZA, F. and GARCIA, M.J., Leaching of a copper-zinc bulk sulphide concentrate using an aqueous ferric sulphate dilute solution in a semicontinuous system. Kinetics of dissolution of zinc.
Hydrometallurgy, 23 (1990) pp.191-202 10.PALENCIA, I. ROMERO, R., CARRANZA, F., Silver catalysed IBES
process: Application to a Spanish copper-zinc sulphide concentrate. Part 2. Biooxidation of the ferrous iron and catalyst recovery.
Hydrometallurgy, 48 (1998) pp. 101-112 11. PALENCIA, I., CARRANZA, F., BARRIGA, F., GARCIA, M.J., Process for the selective bioleaching of zinc from a mixed copper-zinc concentrate, Spanish Patent 2009104-8803370 (1989) 12. ROMERO, R., PALENCIA, I., CARRANZA, F., Silver catalysed IBES
process: Application to a Spanish copper-zinc sulphide concentrate. Part 3. Selection of the operational parameters for a continuous pilot plant.
Hydrometallurgy, 49 (1998) pp. 75-86 13. VAN STADEN, P.J., Oxidative Leach Process, Provisional South African Patent Application 2003/5433, 15 July 2003 DIRECT Zn BIOLEACH REFERENCES

14. BASSON, P., MILLER, D.M., DEW, D.W., NORTON, A., Recovery of zinc from zinc bearing sulphide minerals by bioleaching and electrowinning, International Patent WO 01/18266 Al, 15 March 2001 15.STEEMSON, M.L., SHEEHAN, G.J., WINBORNE, D.A., WONG, F.S., An Integrated bioleach/solvent extraction process for Zinc metal production from zinc concentrates, International patent WO 94/28184, 8 December SOLVENT EXTRACTION (SX) REFERENCES

16. BOATENG, D., Method for the solvent extraction of zinc, US Patent 5,135,652, 4 August 1992 17.CLITHEROE, J., SUDDERTH, R.B., Solvent Extraction of zinc from sulphite-bisulphite solution, US Patent 4,053,552, 11 October 1977 18. DUYVESTEYN, W., HOGSETT, R.F., Electrogalvanising utilizing primary and secondary zinc sources, US Patent 4,552,629, 12 November 1985 19.REYNOLDS, J.E., NICHOLAS, J.L., Manganese and zinc solvent extraction process, US Patent 4,423,012, 27 December 1983 20.SOLE, K.C., FEATHER, A.M., COLE, P.M., Solvent extraction in southern Africa: An update of some recent hydrometallurgical developments, Hydrometallurgy 78 (2005) 52-78

Claims

1. An indirect zinc sulphide bioleaching process which includes leaching, solvent extraction, electrowinning, and oxidant regeneration steps in a closed circuit;
the leaching step utilizing ferric iron as an oxidant, the solvent extraction step utilizing di-2 ethylhexyl phosphoric acid (D2EHPA) as a solvent extraction reagent, and the oxidant regeneration step including the oxidation of ferrous iron to ferric iron in a ferric iron generator; and the process including minimizing the loading of ferric iron onto the solvent extraction reagent in the solvent extraction step by means of one or more of the steps of minimizing the concentration of ferric iron in the leach stream through selective precipitation of iron (III) from the leach stream prior to solvent extraction, and minimizing the oxidation of ferrous iron to ferric iron in the solvent extraction step by using a pulsed column in the solvent extraction step.

2. A process as claimed in claim 1 in which the separation of iron (III) from iron (II) and zinc in the leach stream is performed by precipitation through neutralisation with limestone, slaked lime or zinc calcine, and replenishing of the iron by addition of ferrous sulphate to the FIG.

3. A process as claimed in claim 1 or claim 2 in which the step of minimizing the oxidation of ferrous iron to ferric iron in the solvent extraction step includes using a pulsed column in the solvent extraction step.

4. A process as claimed in any one of claims 1 to 3 which includes treating the loaded D2EHPA reagent produced in the solvent extraction step with hydrochloric acid to strip any residual ferric iron which may have loaded onto the D2EHPA reagent during the solvent extraction step into a strip liquor, adding sulphuric acid to the strip liquor, distilling the strip liquor to recover hydrochloric acid and form a ferric sulphate salt and sulphuric acid solution according to equation (1):

H2SO4 + HFeCl4 .fwdarw. 0.5 Fe2(SO4)3 + 4HCl (1) and recycling the ferric sulphate and sulphuric acid solution to the ferric iron generator (FIG).

5. A process as claimed in any one of claims 1 to 4 in which the solvent extraction step includes treating the leach stream of reaction (1) with the D2EHPA
reagent to form a reagent loaded with zinc according to chemical reaction (2):
2 RH + Zn2+ ~ R2Zn +2H+ (2) in which (R) represents the D2EHPA reagent.

6. A process as claimed in claim 5 which includes stripping zinc from the loaded D2EHPA reagent produced by reaction (2) by means of an acid according to reaction (3):
R2Zn +2H+ ~ 2 RH + Zn2+ (3)

7. A process as claimed in any one of claims 1 to 6 which includes treating the solvent extraction raffinate in a ferric iron generator (FIG) to regenerate ferric iron from the ferrous iron according to reaction (4):
FeSO4 + 0.25 O2 + 0.5 H2SO4 ~ 0.5 Fe2(SO4)3 + 0.5 H2O (4)

8. A process as claimed in any one of claims 2 to 7 in which the counter-current leaching of step (i) is conducted in a stirred tank cascade.

9. A process as claimed in any one of claims 1 to 8 in which zinc from the electrowinning step is recovered in the form of zinc cathodes.

10. A process as claimed in claim 9 in which the step of stripping zinc from the loaded organic substance produced by reaction (2) includes the use of return electrolyte from the electrowinning step.

11. A process as claimed in any one of claims 2 to 10 in which the ferric iron extracted in step ii) is processed further by recovering the ferric iron from a solvent extraction reagent used in the first solvent extraction step, preferably a D2EHPA reagent, and for the ferric iron to be recovered by treating the loaded D2EHPA reagent of step (ii) with hydrochloric acid to strip any residual ferric iron which may have loaded onto the D2EHPA reagent during the solvent extraction step into a strip liquor, adding sulphuric acid to the strip liquor, distilling the strip liquor to recover hydrochloric acid and form a ferric sulphate and sulphuric acid solution, and recycling the ferric sulphate and sulphuric acid solution to the ferric iron generator (FIG).

12. A process as claimed in any one of claims 5 to 11 which includes treating the loaded D2EHPA reagent produced by reaction (2) with hydrochloric acid to strip any residual ferric iron which may have loaded onto the D2EHPA reagent during the solvent extraction step into a strip liquor, adding sulphuric acid to the strip liquor, distilling the strip liquor to recover hydrochloric acid and form a ferric sulphate and sulphuric acid solution, and recycling the ferric sulphate and sulphuric acid solution to the ferric iron generator (FIG).

13. A process as claimed in any one of claims 2 to 12 in which the counter-current leach process includes two or more leach stages to facilitate counter-current operation.

14. A process as claimed in any one of claims 2 to 12 in which the counter-current leach process includes three leach stages to also obtain better approximation to plug flow.

15. A process as claimed in claim 13 or claim 14 in which each leach stage has an associated settling stage.

16. A process as claimed in claim 14 or claim 15 in which the first leach stage is associated with the first settling stage, the second leach stage is associated with the second settling stage, and the third leach stage is associated with the third settling stage.

17. A process as claimed in claim 16 in which the leach slurry from the third leach stage is fed to the third settling stage, the leach slurry from the second leach stage is fed to the second settling stage, and the leach slurry from the first leach stage is fed to the first settling stage.

18. A process as claimed in claim 16 or claim 17 in which the overflow of the third settling stage is fed into the second leach stage, the overflow of the second settling stage is fed to the first leach stage, and the overflow of the first settling stage is fed to a solid/liquid separation stage and separated solids are fed to the first leach stage.

19. A process as claimed in any one of claims 16 to 18 in which the underflow solids of the first settling stage are fed to the second leach stage, the underflow solids of the second settling stage are fed to the third leach stage, and the underflow solids of the third settling stage are separated from the leach stream as residue.

20. A process as claimed in any one of claims 13 to 19 in which step (i) includes maintaining a large inventory of zinc sulphide in the first leach stage to retain reductive conditions and limit the concentration of ferric iron in the leach stream.

21. A process as claimed in claim 20 in which the inventory of zinc sulphide is maintained by recycling at least a portion of the underflow solids from the settling stage associated with the first leach stage back into the first leach stage.

22. A process as claimed in any one of claims 1 to 21 which includes a purification step which comprises bleeding a portion of the pregnant leach stream for zinc dust cementation and neutralization to limit the accumulation of impurities in the leach stream.

23. A process as claimed in any one of claims 1 to 22 in which the zinc sulphide feedstock comprises a sphalerite feedstock

24. A process as claimed in any one of claims 1 to 22 in which the zinc sulphide feedstock comprises a sphalerite concentrate.

25. An indirect bioleaching process substantially as herein described and with reference to Figure 1.