WO2007087675A1

WO2007087675A1 - Improved base metal recovery process from heap leaching

Info

Publication number: WO2007087675A1
Application number: PCT/AU2007/000087
Authority: WO
Inventors: Michael Rodriguez; Bruce Wedderburn; Brett Crossley
Original assignee: Murrin Murrin Operations Pty Ltd
Priority date: 2006-01-31
Filing date: 2007-01-31
Publication date: 2007-08-09
Also published as: BRPI0707373A2; AU2007211831B2; EP1984528A4; EP1984528A1; CA2640122A1; AU2007211831A1

Abstract

A process for the recovery of base metals from an oxide ore, comprising the steps of: e) forming at least one heap (40) of the oxide ore containing the base metals to be recovered; f) irrigating the at least one heap (40) of oxide ore with a leach solution (42) comprising sulphuric acid; g) collecting resulting pregnant leach solution (44,50) from the irrigated heap (40); and treating the pregnant leach solution (44,50) with a reducing gas stream to create a treated pregnant leach solution for recovery of required base metals. The process is applicable to treatment of laterite ores and the conversion of ferric ions to ferrous ions, with regeneration of sulphuric acid in the process, has benefits in terms of enhanced process efficiency and reduced operating costs.

Description

IMPROVED BASE METAL RECOVERY PROCESS FROM HEAP LEACHING FIELD OF THE INVENTION

The present invention relates to a process for improving the recovery of base metals using heap leaching. The process relates particularly, but not exclusively, to the recovery of nickel and cobalt from laterite ores. BACKGROUND TO THE INVENTION

Nickel is typically extracted from its oxide ores by the use of both pyrometallurgical and hydrometallurgical methods. Pyrometallurgical methods involve the smelting of ores for nickel matte production and smelting for ferronickel production. Hydrometallurgical methods include the Caron method of ammoniacal leaching and Freeport Sulphur method of pressure leaching with sulphuric acid. In Australia, nickel is extracted from its oxide ores using both the hydrometallurgical methods of the Caron process and the pressure leaching process. Unfortunately, most of these conventional methods are energy and capital cost intensive and are not suitable for low-grade nickel ores, such as nickel laterites. Therefore, the cost effective exploitation of low-grade oxide nickel and cobalt ores, in Australia and internationally, requires the development of new methods having a much lower energy cost than the existing methods. Heap leaching is a conventional hydrometallurgical method of economically extracting low grade ores and has been successfully used to recover metals such as copper, gold and nickel. Generally it involves forming heaps of raw ore and introducing leach solution such as sulphuric acid onto the top of the heap to percolate down through the heap. The effluent liquor is drained from the base of the heap and passes to a recovery plant where the metals are recovered. A problem hindering the heap leaching of nickel and cobalt containing laterite ores is the substantial clay component of these ores. Most clays have a detrimental effect on the percolation of the leach solution through the ore since the clay components of the ore can swell and close off voids leading to channelling of the leach solution.

In a previous invention of an associated company of the present applicant described in Australian provisional application 2005904274. the contents of which are hereby incorporated by reference, the associated company has reported the separation of the laterite ore into a fine fraction and a coarse fraction prior to heap leaching. The coarse fraction has a high percentage of clay material and is treated with an agglomeration step prior to being formed into a heap for heap leaching. The treatment by agglomeration assists in overcoming the problems associated with the presence of the clay particles. In the case of heap leaching of nickel laterites, the iron which is leached into solution is typically present in the form of greater than 80% ferric sulphate. The previously disclosed invention in Australian provisional patent application 2005904274 for heap leaching nickel laterites, describes the need to add hydrogen sulphide to reduce the. ferric sulphate to ferrous sulphate in the pregnant leach solution.

The addition of hydrogen sulphide has a number of disadvantages including that: (a) the reagent is expensive to produce, (b) the reagent can be dangerous to handle, and (c) the addition of hydrogen sulphide results in the formation of elemental sulphur which forms sulphur particulates and sulphur scale, and needs to be removed from the process, as given by the formula:

Fe₂(SO₄)S + H₂S → 2FeSO₄ + H₂SO₄ + S

The present invention aims to alleviate or at least partly alleviate one or more of the difficulties associated with the prior art.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere. SUMMARY OF THE INVENTION The present invention provides a process for the recovery of base metals from an oxide ore, comprising the steps of: a) forming at least one heap of the oxide ore containing the base metals to be recovered; b) irrigating the at least one heap of oxide ore with a leach solution comprising sulphuric acid; c) collecting resulting pregnant leach solution from the irrigated heap; and d) treating the pregnant leach solution with a reducing gas stream to create a treated pregnant leach solution for recovery of required base metals.

Preferably, the reducing gas stream comprises sulphur dioxide. Preferably, the reducing gas stream comprises a mixture of sulphur dioxide and oxygen. Preferably the sulphur dioxide and the oxygen are in the ratio of about 1 :1.

More preferably, the reducing gas stream comprises a mixture of sulphur dioxide, oxygen and an inert gas such as nitrogen. More preferably, the pregnant leach solution is treated with the reducing gas stream in the presence of activated carbon or a similar adsorbent. Typically, the activated carbon is added to the pregnant leach solution prior to treatment of the solution with the reducing gas. The activated carbon is preferably in the form of granular activated carbon. The granular activated carbon is preferably of 6 x 12 mesh size. The pregnant leach solution may be contacted with activated carbon in an absorption column - operated in gas continuous mode - or a slurry reactor.

The leach solution, or leachant, employed for irrigating the at least one heap may include additional components to sulphuric acid. For example, the leachant could include sulphur dioxide or a salt from which sulphur dioxide may be derived, such as a metabisulphite salt.

The process may further comprise a beneficiation step which is employed prior to step a) to separate the ore into a substantially coarse fraction and a substantially fine fraction. The fine fraction is generally the undersize fraction of about a 2mm aperture screen; or, alternatively, may be the undersize fraction of about a 1 mm screen. The coarse ore fraction is generally the oversize reject from the screen. The at least one heap is preferably formed from the coarse are fraction. The coarse ore fraction is preferably agglomerated prior to forming the heap. Agglomeration preferably comprises wetting the ore to a moisture content of about 20% to 30%. The agglomeration generally also comprises wetting with a sulphuric acid containing solution and curing for about 1 to 20 days prior to irrigation of the heap. Sulphuric acid content of the wetting agent may vary from 01 wt% to 100 wt % (concentrated sulphuric acid). The leach solution applied to the heap may be a recirculated process solution such as a counter-current decantation overflow solution from a pressure leaching step. The counter-current decantation overflow solution preferably contains dilute sulphuric acid in the range of 10 to 50 g/l. This solution could also be employed in the wetting stage. I

Preferably, the oxide ore is a laterite ore containing nickel, cobalt and iron.

Typically, the iron in the ore is present in the form of ferric sulphate, and the reducing gas is applied at a concentration to convert at least a portion of the ferric sulphate to ferrous sulphate. Preferably, the reducing gas is applied at a concentration to convert all of the ferric sulphate to ferrous sulphate.

Preferably, the flow of the gas stream to the pregnant leach solution is adjusted so that the treated pregnant leach solution has a concentration of less than 2g/l ferric sulphate.

The treated pregnant leach solution is preferably passed to a neutralisation step prior to a base metal recovery step. In relation to this latter step, hydrogen sulphide is generally used to form metal sulphides in the metal recovery step which is conducted through selective precipitation of nickel and cobalt.

It is believed that the addition of the reducing gas to the pregnant leach solution assists in the breakdown of ferric sulphate to ferrous sulphate. It is also believed that the treatment of the pregnant leach solution with the reducing gas stream in the presence of activated carbon further improves the reaction kinetics for ferric sulphate to ferrous sulphate conversion. Utilisation of the reducing gas is thought to be improved with the addition of the activated carbon which may act as a catalyst. The activated carbon may also usefully remove iron intermediate species that could interfere with leaching/base metal recovery steps.

Thus, undesirable ferric ions may be removed from the pregnant leach solution while sulphuric acid is produced in the reduction reaction. Therefore, the reduction reaction has the benefit of removing ferric ions and at the same time generating sulphuric acid required in the leaching process in a useful regeneration step. A further benefit of the reduction reaction is that by removing ferric ions from the pregnant leach solution the amount of hydrogen sulphide required in the metal recovery step is reduced since hydrogen sulphide is not needed for the reduction of ferric ions.

The present invention further comprises a process for the recovery of base metals from an oxide ore, the process comprising the steps of: a) forming at least a first heap of the oxide ore; b) irrigating the first heap of oxide ore with a leach solution comprising sulphuric acid; c) collecting resulting first pregnant leach solution from the irrigated first heap; d) passing at least a portion of the first pregnant leach solution as an intermediate leach solution to a second heap of the oxide ore, the intermediate leach solution having been treated with a reducing gas mixture prior to being delivered to the second heap; and e) collecting resulting second pregnant leach solution from the second heap for recovery of required base metals.

Preferably, the reducing gas stream comprises sulphur dioxide. Preferably, the reducing gas stream comprises a mixture of sulphur dioxide and oxygen. Preferably the sulphur dioxide and the oxygen are in the ratio of about 1 :1. More preferably, the reducing gas stream comprises a mixture of sulphur dioxide, oxygen and nitrogen.

More preferably, the intermediate leach solution is treated with the reducing gas stream in the presence of activated carbon or a similar adsorbent. Typically, the activated carbon is added to the intermediate leach solution prior to treatment of the solution with the reducing gas. The activated carbon is preferably in the form of granular activated carbon. The granular activated carbon is preferably of 6 x 12 mesh size.

The leach solution applied to the first heap may be a recirculated process solution such as a counter-current decantation overflow solution from a pressure leaching step. The counter-current decantation overflow solution preferably contains dilute sulphuric acid in the range of 10 to 50 g/l. The counter-current decantation overflow solution is typically combined with the intermediate leach solution before delivery to the second heap, the combined solution containing sulphuric acid in the range of 20 to 60g/l.

Preferably, the oxide ore is a laterite ore containing nickel, cobalt and iron.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

The nature of the invention will be better understood from the following detailed description of several specific embodiments of the invention, given by way of example only, with reference to the accompanying drawing, in which:

Figure 1 is a flow diagram of a process for the recovery of nickel and cobalt from laterite ores according to embodiments of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In Figure 1 there is shown a flow diagram of a process 10 for the recovery of nickel and cobalt from oxide ores in the form of laterite ores.

The process 10 includes a run of mine laterite ore 12 which is fed to a beneficiation plant having a crushing stage 14 and a grinding stage 16. Process water 18 is added to the grinding stage 16 at about 90⁰C.

The plant further comprises a screening circuit 20 used for a two-step screening of the product of the grinding stage 16. The screens used in the screening circuit 20 are a 5 mm primary screen and a secondary 1.75 mm screen. The coarse ore fraction rejected in the screening process and referred to as mill "scats" is passed from the screening circuit 20 to an agglomeration step 22. The scats comprise a significant quantity of the clay component of the laterite ore. The agglomeration step 22 comprises adjusting the moisture content of the coarse ore fraction to between about 20% and 30%. The agglomeration step may further comprise applying concentrated sulphuric acid during agglomeration in the range of about 10 kg to 300 kg acid per tonne of coarse are, following by curing for about 1 .to 20 days prior to irrigation.

The screen undersize material from the grinding step 16 is passed to a pressure acid leaching step 26. A sulphuric acid lixiviant is used to extract nickel and cobalt from the fine ore fraction into solution at elevated temperature and pressure; producing a leach slurry comprising liquor and residue solids components. The leach slurry is passed to a liquid/solid separation step, for example a counter-current decantation ("CCD") step 28. The solids underflow for the CCD step 28 is passed to waste 30. Between 0 and 100% of the separated or clarified liquor from the CCD step 28 is passed via a tank 31 to a neutralisation step 32, and a nickel and cobalt recovery step 34. Calcrete, a calcareous material similar to limestone, is used in the neutralisation step 32 to neutralise the liquor to between about 0 and 3g/l of free acid. Hydrogen sulphide is used in the nickel and cobalt recovery step 34 to generate a mixed sulphide product 36 suitable for further recovery operations. Barren liquor 38 from the recovery step 34 is returned to the CCD step 28. The agglomerated coarse ore fraction 20 from the agglomeration step 22 is stacked into a first heap 40 and a leach solution 42 is applied to the top of the heap 40, referred to as irrigation. The leach solution 42 consists of a CCD overflow solution 42 drawn from tank 32. The leach solution 42 contains dilute sulphuric acid in the range of about 10 to 50 g/l, and usually between about 20 to 50g/l. The leach solution is at elevated temperature, usually in the range of about 30⁰C to 6O⁰C. The leach solution 42 is allowed to percolate downwards through the heap 40 in order to leach the nickel and cobalt from the ore; in particular from the clay component of the ore.

A pregnant leach solution in the form of an intermediate leach solution 44 is collected from the base of the first heap 40 in a first collection pond 46. A stream of reducing gas mixture in the form of a stream of sulphur dioxide 54 is passed into the leach solution 44. The treated pregnant leach solution 56 is then applied to a second heap 48. In a preferred form of the invention, activated carbon is added to, or contacted with the mixture of the pregnant leach solution 44 and reducing gas mixture. This step could be conducted using adsorption/absorption column(s) or tank reactor(s). The activated carbon is 30 added in the range of about 5g/l to 500 g/l.

By treating the pregnant leach solution 44 with sulphur dioxide, the ferric sulphate in this solution is reduced to ferrous sulphate generating sulphuric acid in the reaction (regeneration) step as follows:

Fe₂(SO₄)_S + SO₂ + 2H₂O → 2FeSO₄ + 2H₂SO₄

This is a beneficial step since ferric ions are removed, and 2 mols of sulphuric acid are generated for each mol of ferric sulphate converted. The generation of sulphuric acid in the reaction means that less sulphuric acid needs to be added to the leaching process contributing a saving in production costs. In addition, less hydrogen sulphide is required as a precipitant in the base metal recovery stage with consequential reduction in cost and hazard.

The stream of reducing gas may also be a mixture of different gases, for example a mixture of sulphur dioxide, nitrogen and oxygen. The choice of a suitable mixture will often depend on the availability of existing plant equipment, and using existing gas equipment would usually provide the advantage of further cost savings. For example, a reducing gas stream containing a mixture of sulphur dioxide, oxygen and nitrogen may be produced by taking a bleed stream from an existing sulphur burning acid plant, or alternatively a dedicated sulphur burning plant may be installed. Control over mixture composition may also enable control over the degree of reduction of ferric ions to ferrous ions in the process.

The introduction of the reducing gas mixture into the pregnant leach solution allows the sulphuric acid concentration to be adjusted, as required, simply by adjusting the flow of sulphur dioxide. For example, the leach solution 56 applied to the heap 48 is adjusted to between about 20 g/l to 60 g/l sulphuric acid, by means of regulating the sulphur dioxide addition and resultant conversion of ferric sulphate to ferrous sulphate. The second heap 48 is again formed of the agglomerated coarse ore fraction from the agglomeration step 22. One of the purposes of providing multiple 30 heaps is to neutralise the incoming free acid down to the range of about 0 to 20 g/land preferably down to about 0 to 10 g/l. This assists in reducing the neutralisation costs in the neutralisation step 32.

The final pregnant leach solution 50 is collected from a base of the heap 48 in pond 52. The pregnant leach solution 50 is returned to the existing process circuit by way of the tank 31 as shown in Figure 1. As an alternative to the addition of the gas stream at step 54, or in addition to the step 54, a reducing gas stream 58 may be added to the final pregnant leach solution 50 to reduce the concentration of ferric ions. This avoids the need to use hydrogen sulphide to reduce the ferric ions.

The heap leach pregnant leach solution 44 typically contains between about 3 g/l and 5 g/l nickel in solution and also between about 1 g/l and 25 g/l iron, mainly in the form of ferric sulphate. The ferric sulphate is typically reduced to less than 2 g/l with the addition of gas mixture containing sulphur dioxide, oxygen and nitrogen.

As an alternative to passing the pregnant leach solution 50 to the tank 31 , the leach solution 50 may be passed to the neutralisation step 32 and then to the base metal recovery step 34. A reducing gas stream 60 is added to the process line immediately prior to the neutralisation step 32 (in addition to the streams 56 and 58) to remove ferric ions immediately prior to neutralisation.

In the recovery step 34, the nickel and cobalt is recovered by treating the leach solution 50 with hydrogen sulphide gas at a partial pressure of between about 150 kPa and 400 kPa at a temperature of between about 8O⁰C and 120⁰C.

The residence time is between about 15 and 90 minutes. The precipitated mixed sulphide product contains between about 50 and 55 % nickel and between about

3 and 5 % cobalt. Since the bulk of the ferric sulphate has already been reduced, the requirement of hydrogen sulphide for reduction of the ferric ions is significantly reduced thereby improving the efficiency of the metal recovery process.

Barren liquor 38 from the recovery step 34 may be used to flush the/or each heap once those heaps are exhausted as a significant leach solution inventory may remain contained therein. The barren liquor 38 is passed to the CCD step 28 before being conveyed to the heaps.

The process 10 of the invention may be varied according to operational requirements. For example, more than two heaps may be employed. Stream 60 of reducing gas may take the place of streams 56 and/or 58 described above.

The effect of reducing the electro-potential (Eh) in the recirculating solutions by adding the reducing gas stream is that some minerals such as the manganese containing mineral asbolane, which contain both cobalt and nickel, would be rapidly leached. These minerals would, under typical heap leach conditions, be slow to leach due to the high oxidation potential of the heap leach solution.

In Examples 1 and 2 shown below, the results of the reduction in ferric ion concentrations of pregnant leach solutions having been treated with reducing gas mixtures according to the invention are provided. EXAMPLE 1

A first pregnant leach solution containing high iron levels in the form of ferric sulphate was treated with pure SO₂ gas to reduce the ferric sulphate to ferrous sulphate. The composition of the feed solutions is set out in Table 1 below: Table 1 : Composition of Pregnant Leach Solution 1.

Solution 1 was contacted with 100% SO₂ and this reduced the Eh of Solution 1 from 506mV to 422 mV (Pt-Ag/AgCI reference electrode) as the iron in ferric form was converted to ferrous sulphate. In addition, the free acid concentration increased from 19.2 g/l to 45.9 g/l as the ferric sulphate was converted to ferrous sulphate, as per the following reaction regenerating sulphuric acid.

Fe₂(SO₄)S + SO₂ + 2H₂O 2FeSO₄ + 2H₂SO₄

The composition of the resultant pregnant leach solution is set out in Table 2 below:

Table 2: Composition of Treated Pregnant Leach Solution 1.

EXAMPLE 2

A second pregnant leach solution containing high iron levels in the form of ferric sulphate was treated with a gas mixture of O₂, N₂ and SO₂ to reduce the ferric sulphate to ferrous sulphate. The composition of the feed solutions is set out in Table 3 below:

Table 3: Composition of Pregnant Leach Solution 2.

Solution 2 was contacted with 4% O₂, 79% N₂ and 17% SO₂ and this reduced the Eh of Solution 1 from 504mV to 490 mV (Pt-Ag/AgCI reference electrode) as the iron in ferric form was converted to ferrous sulphate. In addition the free acid concentration increased from 40.3 g/l to 44.9 g/l as the ferric sulphate was converted to ferrous sulphate, as per the following reaction.

Fe₂(SO₄)₃ + SO₂ + 2H₂O 2FeSO₄ + 2H₂SO₄

The composition of the resultant pregnant leach solution is set out in Table

4 below:

Table 4: Composition of Treated Pregnant Leach Solution 2.

The effect of reducing the electro-potential (Eh) in the recirculating solutions, through application of the SO₂ reducing gas, is that some minerals, such as asbolane, which contain both cobalt and nickel, would be rapidly leached. These minerals would, under typical heap leach conditions, be slow to leach due to the high oxidation potential of the heap leach solution. The use of a reducing gas containing sulphur dioxide, oxygen and nitrogen instead of hydrogen sulphide overcomes the disadvantages associated with the use of hydrogen sulphide. The reducing gas stream of the invention is significantly less expensive to produce, and the associated safety and health risks are less. Furthermore, the addition of sulphur dioxide, to reduce the ferric sulphate to ferrous sulphate, results in the formation of sulphuric acid which can be utilised in the heap leaching process, advantageously avoiding the formation of elemental sulphur.

In Example 3 shown below, activated carbon was added to a third pregnant leach solution which was then treated with a reducing gas mixture of sulphur dioxide according to the invention. The effect of the presence of activated carbon on the reduction of ferric to ferrous ions in the pregnant leach solution was measured as summarised in Table 6. EXAMPLE 3

A third pregnant leach solution containing high iron levels in the form of ferric sulphate was treated with pure SO₂ gas to reduce the ferric sulphate to ferrous sulphate. The composition of the third leach solution is set out in Table 5.

Table 5: Composition of Pregnant leach Solution 3.

Leach solution 3 was contacted with 100% SO₂ in two identical tests. In the first test, no activated carbon was added to the third pregnant leach solution and in the second test activated carbon was added to the leach solution. The activated carbon used was Haycarb (trade mark) of grade YAG of a granular form of 6x12 mesh.

The comparison of the two tests is shown below in Table 6 which summarises the conversion to ferric sulphate for each of the first and second tests.

Table 6: Ferric sulphate concentrations after test 1 and test 2 measured on the Pregnant Leach Solution (concentrations in mg/L)

Now that preferred embodiments of the process for the recovery of base metals have been described in detail, it will be apparent that the process provides a number of advantages over the prior art, including the following: i) Sulphuric acid is generated in the gas reduction step of the invention thereby reducing the amount of sulphuric acid that must be directly contributed to the process, and lowering overall production costs. ii) Less hydrogen sulphide is required in the overall recovery process due to the conversion of ferric ions to ferrous ions by the reducing gas stream of the invention. The reducing gas stream is cheaper to produce and has fewer associated health and safety risks than hydrogen sulphide. iii) The introduction of the reducing gas mixture into the leach solution allows the sulphuric acid concentration (and the ferric ion concentration) to be readily adjusted as required simply by adjusting the flow and concentration of the gas mixture. iv) The introduction of activated carbon to the pregnant leach solution together with the addition of the reducing gas mixture improves the ferric ion to ferrous ion conversion process thereby contributing to a reduction in overall operating costs.

It will be readily apparent to persons skilled in the relevant art that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, in the described embodiments only two heaps are shown. However, three or four heaps may be employed and the intermediate leach solution delivered to each heap may be treated with the stream of reducing gas prior to delivery to each successive heap. In addition, whilst the embodiments describe the use of granular activated carbon, other forms of activated carbon such as pellets and powder, may also be used. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.

Claims

CLAIMS:

1. A process for the recovery of base metals from an oxide ore, comprising the steps of: a) forming at least one heap of the oxide ore containing the base metals to be recovered; b) irrigating the at least one heap of oxide ore with a leach solution comprising sulphuric acid; c) collecting resulting pregnant leach solution from the irrigated heap; and d) treating the pregnant leach solution with a reducing gas stream to create a treated pregnant leach solution for recovery of required base metals.

2. The process of claim 1 wherein the pregnant leach solution is treated with the reducing gas stream in the presence of an adsorbent.

3. The process of claim 2 wherein the adsorbent is activated carbon, preferably granular activated carbon.

4. The process of claim 2 or 3 wherein the adsorbent is added to the pregnant leach solution prior to treatment of the solution with the reducing gas.

5. The process of any one of the preceding claims wherein the reducing gas stream comprises sulphur dioxide.

6. The process of claim 5 wherein the reducing gas stream comprises a mixture of sulphur dioxide and an oxidant.

7. The process of claim 6 wherein the oxidant is oxygen.

8. The process of any one of claims 4 to 7 wherein the reducing gas stream comprises a mixture of sulphur dioxide, oxidant and an inert gas.

9. The process of claim 8 wherein said inert gas is nitrogen.

10. The process of any one of the preceding claims further comprising an ore beneficiation step prior to step (a).

11. The process of claim 10 wherein the ore is separated into a substantially coarse fraction and a substantially fine fraction.

12. The process of claim 10 or 11 wherein said at least one heap is formed of the coarse fraction of beneficiated ore.

13. The process of claim 11 or 12 wherein the coarse ore fraction is agglomerated prior to forming the heap.

14. The process of claim 13 wherein agglomeration comprises wetting the ore to a moisture content of about 20% to 30% by weight.

15. The process of claim 14 wherein agglomeration comprises wetting with a sulphuric acid containing solution.

16. The process of claim 15 wherein agglomeration comprises curing for about 1 to 20 days prior to irrigation of the heap.

17. The process of any one of the preceding claims wherein the heap is irrigated with a recirculated process solution.

18. The process of any one of the preceding claims wherein the oxide ore is a laterite ore containing nickel, cobalt and iron.

19. The process of claim 17 or 18 wherein the pregnant leach solution contains iron in ferric form and the reducing gas is applied at a concentration to convert at least a portion of the ferric iron to ferrous iron, regenerating sulphuric acid for irrigating said at least one heap.

20. The process of claim 19 wherein the reducing gas is applied at a concentration to convert all of the ferric iron to ferrous iron.

21. The process of claim 19 or 20 wherein the flow of the reducing gas stream is adjusted so that the treated pregnant leach solution has a concentration of less than 2 g/l ferric sulphate.

22. The process of any one of the preceding claims wherein the treated pregnant leach solution is passed to a neutralization step prior to a base metal recovery step.

23. The process of any one of the preceding claims comprising the steps of: a) forming at least a first heap of the oxide ore; b) irrigating the first heap of oxide ore with a leach solution comprising sulphuric acid; c) collecting resulting first pregnant leach solution from the irrigated first heap; d) passing at least a portion of the first pregnant leach solution as an intermediate leach solution to a second heap of the oxide ore, the intermediate leach solution having been treated with a reducing gas mixture prior to being delivered to the second heap; and e) collecting resulting second pregnant leach solution from the second heap for recovery of required base metals.

24. The process of any one of claims 1 to 23 wherein the reducing gas stream is a bleed stream from a sulphur burning plant.