AU2007288123A1

AU2007288123A1 - Improved hydrometallurgical method for the extraction of nickel from laterite ores

Info

Publication number: AU2007288123A1
Application number: AU2007288123A
Authority: AU
Inventors: Michael Rodriguez
Original assignee: Murrin Murrin Operations Pty Ltd
Current assignee: Murrin Murrin Operations Pty Ltd
Priority date: 2006-08-23
Filing date: 2007-08-23
Publication date: 2008-02-28
Anticipated expiration: 2027-08-23
Also published as: WO2008022395A1; BRPI0714269A2; EP2054534A4; CA2659449A1; AU2007288123B2; EP2054534A1

Description

WO 2008/022395 PCT/AU2007/001213 -1 "Improved Hydrometallurgical Method for the Extraction of Nickel from Laterite Ores" Field of the Invention The present invention relates to an improved hydrometallurgical method for the 5 extraction of nickel from laterite ores. More particularly, the hydrometallurgical method of the present invention is intended to allow more efficient processing of all components of an ore body containing nickel. Background Art To date, hydrometallurgical methods for treating nickel laterite ores in Australia 10 have typically involved the use of elevated temperature and pressure. Such high pressure acid leach (HPAL) processes require specialised equipment resulting in substantial capital outlay, in addition to costly energy requirements. As a result, these leaching methods are not considered to be suitable for the treatment of low grade nickel ores. 15 Applicant's International Patent Application PCT/AU2006/001128 (WO2007/016737) discloses a low cost hydrometallurgical method for treating low grade nickel ore, involving a heap leach of the coarse ore fraction. Heap leaching is known to be a low cost leaching option although it raises a number of processing issues, in particular the significant dissolution of iron at lower 20 temperatures and pressures. As a result the pregnant leach liquor produced from a heap leach contains a significant amount of iron predominantly in the ferric (oxidised) form. The presence of ferric iron is undesirable in the recovery circuit and must be removed by pre-reduction or precipitation, generally as goethite, hematite or jarosite. Precipitation is brought about by raising the pH of the liquor 25 by adding a suitable neutralant. Pre-reduction is brought about by the reduction of the ferric form to ferrous through the addition of a reductant, for example hydrogen sulphide.

WO 2008/022395 PCT/AU2007/001213 -2 Hydrogen sulphide is in turn an undesirable reductant as it can be quite costly, it introduces handling issues, and the reduction of ferric to ferrous by hydrogen sulphide (Equation 1 below) also results in the formation of elemental sulphur, which ultimately contributes to scaling. 5 Fez (SO 4

)

3

+H

2 S -->2FeSO 4

+H

2 S0 4 +S (1) Furthermore, this reaction also results in the regeneration of sulphuric acid, which in the past has typically been neutralised by the addition of calcrete or limestone. Applicant's International Patent Application PCT/AU2007/000210 discloses a process able to substantially overcome these problems by precipitating the iron 10 from an atmospheric leach liquor as hematite using an autoclave. The chemical reaction for the precipitation of iron as hematite in an autoclave is given in Equation 2. Fe 2 (S0 4

)

3 +3H 2 0 - Fe 2 0 3 + 3H 2 S0 4 (2) Application PCT/AU2007/000210 also discussed the possibility of utilising the 15 regenerated acid by directing the overflow from a counter current decantation (CCD) circuit to a leaching circuit, before sending a final clarified leach liquor to the metal recovery circuit. However, the proportion of acid generated to iron reduced is approximately 3 to 1, respectively, which is still a substantial quantity of acid that eventually needs to be neutralised prior to the metal recovery circuit. 20 Applicant's International Patent Application PC/AU2007/000087 (WO/20007/087675) discloses a method for using sulphur dioxide as a reductant for ferric iron. This results in the formation of ferrous sulphate and sulphuric acid as shown in Equation 3. Fe 2 (S0 4

)

3

+SO

2 +2H 2 0 -+ 2FeSO 4 +2H 2 S0 4 (3) 25 However, the ratio of acid produced to the amount of ferric iron consumed is lower than the hematite reaction of Application PCT/AU2007/000210, resulting in less WO 2008/022395 PCT/AU2007/001213 -3 acid in the final leach liquor. This in turn requires less neutralising agent to be added prior to the metals recovery circuit. One object of the present invention is to provide an integrated process involving at least two heap leaching stages and at least one iron precipitation step wherein the 5 acid generated in the iron precipitation step is utilised, and the requirement for a neutralising agent is reduced. The disadvantages associated with acid generation in iron removal phases, the handling of waste residue and the requirement for neutralising agents in the treatment of nickel containing ores are proposed to be significantly reduced. A further object of the present invention is to provide a 10 process incorporating at least a single heap leach stage and an iron precipitation step, wherein an ILS is recirculated through the heap and the iron precipitation step functions to provide a PLS discharge containing acid. The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the 15 discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to 20 imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Throughout the specification, the terms "ore" and "ore material" are understood to refer to any one of ore, ore reject material, concentrate, waste rock or mill scats material. 25 The content of Applicant's prior International Patent Applications PCT/AU2006/01128, PCT/AU2007/000210 and PCT/AU2007/000087 are incorporated herein by reference.

WO 2008/022395 PCT/AU2007/001213 -4 Disclosure of the Invention In accordance with the present invention there is provided an improved hydrometallurgical method for the extraction of nickel from laterite ores, the method characterised by the method steps of: 5 i) Forming at least one primary heap from nickel laterite ore material; ii) Leaching the primary heap of step i) with a leaching agent to produce an intermediate leach solution (ILS); iii) Recirculating the ILS of step ii) to the or each primary heap whereby the concentration of nickel in solution approaches saturation, or a 10 steady state, to form a pregnant leach solution (PLS); iv) Passing at least a portion of the PLS produced in step iii) to an iron precipitation step in which iron is precipitated as hematite at elevated temperature and pressure, releasing acid; v) Directing the discharge from the iron precipitation step iv) to a solid 15 liquid separation stage to produce a waste residue and a clarified PLS containing acid; vi) Irrigating at least one secondary heap of nickel laterite ore material with the clarified PLS of step v) to facilitate the leaching of the or each secondary heap and produce a secondary ILS; 20 vii) Passing a stream of a reducing gas into the secondary ILS of step vi) to facilitate the reduction of ferric iron to ferrous iron; viii) Recirculating the ILS of step vi) and vii) over the or each secondary heap of step vi) to produce a secondary PLS; and ix) Directing the PLS from step viii) to a metals recovery circuit.

WO 2008/022395 PCT/AU2007/001213 -5 Preferably, at least a portion of the ore material of step i) is crushed and agglomerated prior to forming the or each heap. Preferably, the sulphuric acid added during agglomeration is in the form of a sulphuric acid solution or concentrated sulphuric acid. 5 The sulphuric acid solution preferably has an acid concentration of about 15 to 150 g/L. The sulphuric acid solution may be provided in the form of a CCD liquor. Preferably, the total acid addition during agglomeration is within the range of 0 to 150 kg/t. 10 More preferably, the total acid addition during agglomeration is about 50 kg/t. Preferably, the leaching agent comprises sulphuric acid, and/or recirculated leach liquor. The concentration of sulphuric acid in the leaching agent preferably falls within the range of 15 and 150 g/L, and within about 10 and 50 g/L in the recirculated leach 15 liquor. More preferably, the concentration of sulphuric acid is about 45 g/L, and the recirculated leach liquor has an acid concentration of around 27 g/L. The irrigation rate of the leaching agent is preferably within the range of 1 and 60 L/m 2 /hr. 20 More preferably, the irrigation rate is around 15 L/m 2 /hr. A bleed of the ILS of step ii) and iii) is preferably withdrawn to maintain solution solubility and/or water balance.

WO 2008/022395 PCT/AU2007/001213 -6 The concentration of sulphuric acid nickel and iron in the PLS of step iii) is preferably within the range of about 15 to 150 g/L, 4 to 8 g/L, and 15 to 50 g/L, respectively. More preferably, the concentration of iron in the PLS of step iii) is within the range of about 30 to 50 g/L. 5 The iron precipitation at step iv) is preferably facilitated through the use of an autoclave to precipitate iron as hematite. Hematite is preferably precipitated at a temperature within the range of 100 to 260 oC, and a pressure within the range of 100 and 4500 kPa. More preferably, the solution temperature of the iron precipitation step iv) is 10 maintained within the range of 120 to 260 oC, for example 240 oC, and the pressure is maintained within the range of 200 to 4500 kPa, for example 4500kPa. Preferably, ore material is added to the iron precipitation step iv) to facilitate the reduction of acid in the autoclave discharge, to substantially avoid the re dissolution of iron. 15 More preferably, ore material is added to the iron precipitation step iv) such that the total solids is within about 5% to 25% w/w. More preferably, the total solids is about 10% w/w. The concentration of acid in the discharge from the iron precipitation step iv) is preferably within the range of 10 to 90 g/L. More preferably, the concentration of 20 acid in the discharge from the iron precipitation step iv) does not exceed 75 g/L. The secondary ILS of step vi) is preferably continuously recirculated through the heap of step vi). Preferably, the reducing gas stream of step vii) is also contacted with the PLS solution of step viii).

WO 2008/022395 PCT/AU2007/001213 -7 The concentration of sulphuric acid, nickel and iron in the PLS of step viii) is preferably within the range of 15 to 150 g/L 4 to 8 g/L, and 10 to 50 g/L, respectively. Preferably, the reducing gas stream of step vii) comprises sulphur dioxide. 5 More preferably, the reducing gas stream of step vii) comprises a mixture of sulphur dioxide and oxygen. Still more preferably, the reducing gas stream of step vii) comprises a mixture of sulphur dioxide, oxygen and nitrogen. Preferably, the reducing gas stream is used in the presence of activated carbon. 10 Where activated carbon is added, it is preferably added within the range of about 5 to 500 g/L. Activated carbon may be added in the form of any one or more of granules, pellets or powder. In one form of the present invention the PLS of step ix) is neutralised with a neutralising agent prior to being directed to the metals recovery circuit. 15 The neutralisation agent preferably comprises any one or more of limestone, lime and calcrete. The acid concentration in the PLS directed to the metals recovery circuit is preferably within the range of about 0 to 5 g/L. Preferably, the nickel concentration of the PLS of step ix) is within the range of 4 20 to 8 g/L. In accordance with the present invention there is further provided an improved hydrometallurgical method for the extraction of nickel from laterite ores, the method characterised by the method steps of: i) Forming at least one primary heap from nickel laterite ore material; WO 2008/022395 PCT/AU2007/001213 -8 ii) Leaching the or each primary heap of step i) with a leaching agent to produce an intermediate leach solution (ILS); iii) Recirculating the ILS of step ii) to the or each primary heap at least once whereby the concentration of nickel in solution approaches 5 saturation, or a steady state to form a pregnant leach solution (PLS); iv) Passing at least a portion of the PLS produced in step iii) to an iron precipitation step in which iron is precipitated as hematite at elevated temperature and pressure, thereby releasing acid; and v) Directing the discharge from the iron precipitation of step iv) to a 10 solid liquid separation stage to produce a waste residue and a clarified PLS containing acid. Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which: 15 Figure 1 is a schematic representation of a flow sheet depicting a hydrometallurgical method for the extraction of nickel from laterite ore in accordance with the present invention. Best Mode(s) for Carrying Out the Invention In Figure 1 there is shown an improved hydrometallurgical method 10 for the 20 extraction of nickel from laterite ore material. In general terms, the method 10 utilises the acid produced from a high temperature and pressure hematite precipitation circuit to leach a secondary heap, which further utilises an SO 2 reduction circuit to facilitate the removal of ferric iron in preparation for metal recovery. 25 A laterite ore material 12 is passed to a crushing step 14 and proceeds as crushed ore 15 through an agglomeration circuit 16, in which agglomeration is WO 2008/022395 PCT/AU2007/001213 -9 achieved with sulphuric acid or a liquor produced from a counter current decantation ("CCD") circuit, the total acid addition during agglomeration being within the range of about 0 to 150 kg of acid per tonne, and more preferably about 50 kg per tonne. The agglomerated ore material 18 is stacked to form at least 5 one primary heap 20. A leaching agent 22 is percolated through the heap 20, to produce a primary intermediate leach solution ("ILS") 24, which exits the bottom of the heap 20. The leaching agent 22 contains at least sulphuric acid and water, such that the acid concentration is within the range of about 15 and 150 g/L, for example about 10 45 g/L. The leaching agent is irrigated through the heap at a rate within the range of 1 and 60 L/m 2 /hr, for example 15 L/m 2 /hr. The ILS 24 is recirculated through the heap 20 continuously until the concentration of nickel in the solution approaches saturation or steady state, thereby producing a pregnant leach solution ("PLS") 26. The recirculated 15 intermediate leach solution 24 has an acid concentration within the range of about 10 to 50 g/L acid, for example about 27 g/L. The rate of percolation is typically in the range of about 1 and 60 L/m 2 /hr, for example about 15 L/m 2 /hr. Typically, the concentration of acid, nickel and iron in the PLS 26 is within the range of about 15 to 150 g/L, 4 to 8 g/L and 15 to 50 g/L, respectively. Ideally, the concentration of 20 acid in the PLS 26 about 15g/L and iron within the range of about 30 to 50 g/L. A portion of the PLS 26 is bled off in order to maintain solution solubility and/or water balance. This portion of the PLS 26 is then passed to an iron precipitation step 28, comprising at least one autoclave 30. Additional ore material 29 is added directly to the autoclave 30, such that the total solids in the autoclave 30 is within 25 about 5 to 25% w/w, for example 10% wlw. It is envisaged that the ore material 29 may comprise ore material from the crushing step 14. The autoclave 30 operates at elevated temperature and pressure so as to facilitate iron precipitation in the form of hematite. The autoclave operates at a temperature of between about 100 and 260 OC, or 120 to 260 0C, for example about 240 oC. The pressure 30 of the autoclave is maintained between about 100 to 4500 kPa, or 200 to 4500 kPa, for example about 4500 kPa.

WO 2008/022395 PCT/AU2007/001213 -10 A discharge 32 from the iron precipitation step 28 has a acid concentration within the range of about 10 to 90 g/L, for example, 75g/L. The discharge 32 is directed to a solid liquid separation stage, for example a CCD circuit 34. A CCD overflow solution 36 is a clarified PLS. A CCD underflow 38 is a waste 5 residue and is sent to tails 40. The CCD overflow solution 36 is then used as a leaching agent irrigated through at least one secondary heap 42 formed from the ore material 15, to effect the leaching of nickel from that additional ore material. The ore material 15 directed to form the secondary heap 42 is again agglomerated in an agglomeration step 44 10 substantially similar to agglomeration step 16. A secondary ILS 46 is recirculated through the heap 42 continuously to produce a secondary PLS 48 containing sulphuric acid, nickel and iron in the range of about 15 to 150 g/L, 4 to 8 g/L and 10 to 50 g/L, respectively. A reducing gas stream 50 is passed through the secondary ILS 46 as it is 15 recirculated, so as to reduce ferric iron to ferrous iron therein. The PLS 48 is also subjected to the reducing gas stream 50. The reducing gas stream 50 contains, for example, sulphur dioxide. Alternately, it may contain sulphur dioxide and oxygen, or sulphur dioxide, oxygen and nitrogen. The reducing gas stream 50 may also be introduced to the secondary ILS 46 and PLS 48 in the presence of 20 granular activated carbon acting as a catalyst. The activated carbon, if added, is added in the range of about 5 to 500 g/L. It is envisaged that other forms of activated carbon, such as pellets and powder, may also be utilised. It is believed that the addition of reducing gas to the secondary ILS 46 and PLS 48 assists in the breakdown of ferric sulphate to ferrous sulphate. With regard to 25 the use of activated carbon it is believed that the reaction kinetics for ferric sulphate to ferrous sulphate conversion are improved. This is turn is believed to be as a result of improved utilisation of the reducing gas through the catalytic action of the activated carbon.

WO 2008/022395 PCT/AU2007/001213 -11 Acid is generated through the reduction of ferric iron to ferrous iron in the ILS 46 and PLS 48, in the manner recited at Equation 3 hereinabove. A substantial proportion of any acid remaining in the secondary PLS 48 is neutralised by any one or more of calcrete, limestone or lime in a neutralisation step 52, such that the 5 final acid concentration in the secondary clarified PLS is within the range of 0 and 5 g/L. A final neutralised PLS 54 from the neutralisation step 52 is passed directly or indirectly to a metals recovery circuit in which value metals, including nickel, are recovered through any one or more of available methods. As can be seen from the above description, the improved hydrometallurgical 10 method of the present invention allows the utilisation of acid generated in the precipitation of iron as hematite, thereby almost doubling the.amount of ore that can be treated when compared with prior art processes. Further, the residue produced from the CCD circuits may be reduced by about 50 to 75% when compared with prior art processes. Still further, the amount of neutralising agent 15 required to treat the PLS before passing same to metals recovery may be significantly reduced compared with prior art processes. Whilst the above description relates primarily to the recovery of nickel from laterite ores, it is to be understood that additional metal values are likely to be extracted and available for recovery using known metals recovery techniques. One such 20 additional base metal, often associated with lateritic nickel is cobalt. It is envisaged that the primary and secondary heaps described hereinabove may be constituted by two or more heaps operating in the manner described. This is to be understood to cover a circumstance in which, for example, the primary heap is constituted by a series of two functionally distinct heaps between which an ILS 25 is passed. Further, it is to be understood to also encompass a situation in which multiple heaps operate in parallel. It is further to be understood that each heap may comprise a portion of a larger heap that is operationally distinct from other portions of that larger heap. It is still further envisaged that the ore material 12 may be separated into a fine 30 fraction and a coarse fraction. The fine fraction being processed using a high WO 2008/022395 PCT/AU2007/001213 -12 pressure acid leach (HPAL) circuit operating parallel to the primary heap leach of the coarse ore material. In such a circumstance it is understood that the discharge from the HPAL autoclave may be immediately passed to a CCD circuit and the overflow used to leach a secondary heap, or it may combined with the 5 hematite autoclave discharge 32 before proceeding to the CCD circuit 34. Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

1. An improved hydrometallurgical method for the extraction of nickel from laterite ores, the method characterised by the method steps of: i) Forming at least one primary heap from nickel laterite ore 5 material; ii) Leaching the primary heap of step i) with a leaching agent to produce an intermediate leach solution (ILS); iii) Recirculating the ILS of step ii) to the or each primary heap whereby the concentration of nickel in solution approaches 10 saturation, or a steady state, to form a pregnant leach solution (PLS); iv) Passing at least a portion of the PLS produced in step iii) to an iron precipitation step in which iron is precipitated as hematite at elevated temperature and pressure, releasing 15 acid; v) Directing the discharge from the iron precipitation step iv) to a solid liquid separation stage to produce a waste residue and a clarified PLS containing acid; vi) Irrigating at least one secondary heap of nickel laterite ore 20 material with the clarified PLS of step v) to facilitate the leaching of the or each secondary heap to produce a secondary ILS; vii) Passing a stream of a reducing gas into the secondary ILS of step vi) to facilitate the reduction of ferric iron to ferrous iron; WO 2008/022395 PCT/AU2007/001213 -14 viii) Recirculating the ILS of step vi) and vii) over the or each secondary hap of step vi) to produce a secondary PLS; and ix) Directing the PLS from step viii) to a metals recovery circuit.

2. A method according to claim 1, wherein at least a portion of the ore material of 5 step i) is crushed and agglomerated prior to forming the or each heap.

3. A method according to claim 2, wherein agglomeration of the ore material is achieved using concentrated sulphuric acid.

4. A method according to claim 2, wherein agglomeration of the ore material is achieved using a sulphuric acid solution. 10

5. A method according to claim 4, wherein the sulphuric acid solution has an acid concentration of about 15 to 150 g/L.

6. A method according to claim 4, wherein the sulphuric acid solution is provided in the form of a counter current decantation (CCD) liquor.

7. A method according to any one of claims 2 to 6, wherein the total acid addition 15 during agglomeration is within the range of about 0 to 150 kg/t.

8. A method according to any one of claims 2 to 7, wherein the total acid addition during agglomeration is about 50 kg/t.

9. A method according to any one of the preceding claims, wherein the leaching agent comprises sulphuric acid. 20

10. A method according to claim 9, wherein the concentration of sulphuric acid in the leaching agent is within the range of about 15 to 150 g/L.

11. A method according to claim 9 or 10, wherein the concentration of sulphuric acid in the leaching agent is about 45g/L. WO 2008/022395 PCT/AU2007/001213 -15

12. A method according to any one of claims 1 to 8, wherein the leaching agent comprises recirculated leach liquor.

13. A method according to claim 12, wherein the concentration of sulphuric acid in the recirculated leach liquor is within the range of about 10 to 50 g/L. 5

14. A method according to claim 11 or 12, wherein the concentration of sulphuric acid in the recirculated leach liquor has an acid concentration of about 27 g/L.

15. A method according to any one of the preceding claims, wherein the irrigation rate of the leaching agent is within the range of about 1 and 60 L/m 2 /hr.

16. A method according to any one of the preceding claims, wherein the irrigation 10 rate of the leaching agent is about 15 L/m 2 /hr.

17. A method according to any one of the preceding claims, wherein a bleed of the ILS of step ii) and iii) is withdrawn to maintain solution solubility and/or water balance.

18. A method according to any one of the preceding claims, wherein the 15 concentration of sulphuric acid, nickel and iron in the PLS of step iii) is within the range of about 15 to 150 g/L, 4 to 8 g/L and 15 to 50 g/L, respectively.

19. A method according to any one of the preceding claims, wherein the concentration of iron in the PLS of step iii) is within the range of about 30 to 50 g/L.

20 20. A method according to any one of the preceding claims, wherein the iron precipitation at step iv) is facilitated by the use of an autoclave to precipitate iron in the form of hematite.

21. A method according to any one of the preceding claims, wherein the hematite is precipitated at a temperature within the range of about 100 to 260 oC, and a 25 pressure within the range of about 100 and 4500 kPa. WO 2008/022395 PCT/AU2007/001213 -16

22. A method according to claim 20 or 21, wherein the hematite is precipitated at a temperature of about 240 'C, and a pressure of about 4500 kPa.

23. A method according to any one of the preceding claims, wherein ore material is added to the autoclave. 5

24. A method according to claim 23, wherein the quantity of ore material added to the autoclave is such that the total solids is within about 5 to 25% w/w.

25. A method according to claim 23 or 24, wherein the quantity of ore material added to the autoclave is such that the total solids is about 10% w/w.

26. A method according to any one of the preceding claims, wherein the 10 concentration of acid in the discharge from the iron precipitation step iv) is preferably within the range of about 10 to 90 g/L.

27. A method according to any one of the preceding claims, wherein the ILS of step vi) is preferably continuously recirculated through the heap of step vi).

28. A method according to any one of the preceding claims, wherein the reducing 15 gas stream of step vii) is also contacted with the PLS solution of step viii).

29. A method according to any one of the preceding claims, wherein the concentration of sulphuric acid, nickel and iron in the PLS of step viii) is preferably within the range of about 15 to 150 g/L, 4 to 8 g/L, and 10 to 50 g/L, respectively. 20

30. A method according to any one of the preceding claims, wherein the reducing gas stream of step vii) comprises sulphur dioxide.

31. A method according to any one of claims 1 to 29, wherein the reducing gas stream of step vii) comprises a mixture of sulphur dioxide and oxygen. WO 2008/022395 PCT/AU2007/001213 -17

32. A method according to any one of claims 1 to 29, wherein the reducing gas stream of step vii) comprises a mixture of sulphur dioxide, oxygen and nitrogen.

33. A method according to any one of the preceding claims, wherein the reducing 5 gas stream is used in the presence of activated carbon.

34. A method according to claim 33, wherein the quantity of activated carbon added is within the range of about 5 to 500 g/L.

35. A method according to any one of the preceding claims, wherein the PLS of step ix) is neutralised with a neutralising agent prior to being directed to the 10 metals recovery circuit.

36. A method according to claim 35, wherein the neutralisation agent comprises any one or more of limestone, lime, and calcrete.

37. A method according to any one of the preceding claims, wherein the concentration of nickel in the PLS of step ix) is within the range of about 4 to 15 8 g/L.

38. A method according to any one of the preceding claims, wherein the concentration of acid in the PLS directed to the metals recovery circuit is within the range of about 0 to 5 g/L.

39. An improved hydrometallurgical method for the extraction of nickel from 20 laterite ores, the method characterised by the method steps of: i) Forming at least one primary heap from nickel laterite ore material; ii) Leaching the or each primary heap of step i) with a leaching agent to produce an intermediate leach solution (ILS); WO 2008/022395 PCT/AU2007/001213 -18 iii) Recirculating the ILS of step ii) to the or each primary heap at least once whereby the concentration of nickel in solution approaches saturation, or a steady state to form a pregnant leach solution (PLS); iv) Passing at least a portion of the PLS produced in step iii) to an iron 5 precipitation step in which iron is precipitated as hematite at elevated temperature and pressure, thereby releasing acid; and v) Directing the discharge from the iron precipitation of step iv) to a solid liquid separation stage to produce a waste residue and a clarified PLS containing acid. 10

40. An improved hydrometallurgical method for the extraction of nickel from laterite ores, the method substantially as hereinbefore described with reference to Figure 1.