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Title:
METHOD FOR THE TREATMENT OF ACIDIC LEACH LIQUORS
Document Type and Number:
WIPO Patent Application WO/2013/026093
Kind Code:
A1
Abstract:
A method (10) for the separation of multivalent species from monovalent species in an acidic leach solution (14). The method (14) comprises the steps of passing the acidic leach solution (14) through a membrane in a membrane treatment step (16) to separate a membrane permeate (20) and a membrane retentate (22). The method also comprises recovering the membrane permeate (20) containing the monovalent species; and collecting the membrane retentate containing the multivalent species.

Inventors:
MULLER, Brett David (Level 1, Homeric House442 Murray Stree, Perth Western Australia 6012, AU)
Application Number:
AU2012/000982
Publication Date:
February 28, 2013
Filing Date:
August 22, 2012
Export Citation:
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Assignee:
NEWAMU IP HOLDINGS PTY LTD (Level 1, Homeric House442 Murray Stree, Perth Western Australia 6012, AU)
MULLER, Brett David (Level 1, Homeric House442 Murray Stree, Perth Western Australia 6012, AU)
International Classes:
B01D61/00; C22B3/08; C22B23/00
Domestic Patent References:
Foreign References:
US6355175B1
CA2033545A1
Attorney, Agent or Firm:
JANET STEAD & ASSOCIATES PATENT AND TRADE MARK ATTORNEYS (PO Box 1649, West Perth, Western Australia 6872, AU)
Download PDF:
Claims:
Claims

1. A method for the separation of multivalent species from monovalent species in an acidic leach solution, the method comprising the steps of: i) passing the acidic leach solution through a membrane in a membrane treatment step to separate a membrane permeate and a membrane retentate;

ii) recovering the membrane permeate containing the monovalent species; and

iii) collecting the membrane retentate containing the multivalent species.

2. A method for the separation of multivalent species from monovalent species as defined in claim 1 , wherein the acidic leach solution is a leach solution from a metal recovery process.

3. A method for the separation of multivalent species from monovalent species as defined in claim 2, wherein the metal recovery process is a process for the recovery of nickel and/or cobalt from lateritic ore or concentrate.

4. A method for the separation of multivalent species from monovalent species as defined in claim 3, wherein the acidic solution is a sulphuric acid solution. < '.

5. A method for the separation of nickel and other valuable metals leached from lateritic ore using sulphuric acid, the method comprising the steps of: i) passing an acidic leach solution containing the nickel and other valuable metals through a membrane in a membrane treatment step to separate a membrane permeate and a membrane retentate; ii) recovering the membrane permeate containing the sulphuric acid; and,

iii) collecting the membrane retentate containing the nickel and other valuable metals for further processing.

6. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 5, wherein the membrane treatment step comprises nano-filtration. 7. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 6, wherein the membrane is a nano- filtration (NF) membrane.

8. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 7, wherein the NF membrane has high acid resistance and a Dalton cut off of around 200 to 300.

9. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 8, wherein the NF membranes is a Koch MPF-30 or GE Osmonics Desal DK membrane. 0. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 5, wherein the membrane treatment step may comprise reverse osmosis.

11. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 5, wherein the membrane treatment step comprises nano-filtration combined with reverse osmosis. 12. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 5, wherein the membrane treatment step comprises djffusion^dialysis^

13. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 5, wherein the method also comprises a pre-treatment step prior to the membrane treatment step, the pre-treatment step comprising the removal of residual tailings solids from the acidic leach solution.

14. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 13, wherein the pre-treatment step comprises clarification and/or filtration of the acid leach solution to form a treated acidic leach solution which is then passed to the membrane treatment step.

15. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 12, wherein subsequent to the membrane treatment step the collected membrane retentate is treated to remove iron and aluminium using a pressurised autoclave, and the solids precipitated in the autoclave are separated from the solution by means of a solid/liquid separation step.

16. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 15, wherein the resulting solution from the solid/liquid separation step s again treated by a membrane in a second membrane treatment step to recover some of the sulphuric acid.

17. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 16, wherein the membrane in the second membrane treatment step is a nano-filtration (NF) membrane. 8. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 17, wherein the NF membrane has high acid_resistance-and-a--Dalton~cut~of^ membrane permeate consists predominantly of water and sulphuric acid, which pass through the membrane as monovalent H+ and HS04" ions.

19. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 18, wherein the membrane retentate contains the remaining water, plus the majority of the metal sulphate solute species in a more concentrated stream.

20. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 12 or claim 18, wherein the membrane permeate is further treated in a separation step to separate the sulphuric acid from water.

21. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 20, wherein the separation step is carried out using a reverse osmosis (RO) membrane.

22. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 21 , wherein RO membranes are Hydranautics ESPA1 4040 and Dow Filmtec TW-30 membranes.

23. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 22, wherein the majority of the water typically passes through the RO membrane while the sulphuric acid is retained forming a more concentrated sulphuric acid.

24. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 23, wherein the more concentrated sulphuric acid is fed to an acid stream.

25. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 24, wherein the acid stream is returned to the leach circuit, reducing overall acid consumption. 26. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 23, wherein the water which passes through the RO membrane is used as a slightly acidic water source elsewhere in the nickel process flowsheet. 27. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 26, wherein the slightly acidic water source is used as wash water in the counter current decantation (CCD) circuit. 28. A method for the separation of nickel and other valuable metals leached from lateritic ore as defined in claim 15, wherein the membrane retentate containing the various metal species is further treated to recover the metals using usual metal recovery processing steps.

29. A method for the separation of multivalent metal species from sulphuric acid in a sulphuric acid leach solution, the method comprising: i) passing the sulphuric acid leach solution through a membrane in a membrane filtration step to separate a permeate and a retentate, the permeate comprising sulphuric acid and the retentate comprising a multivalent metal sulphate solution;

ii) recovering the permeate comprising sulphuric acid; and

iii) treating the retentate to remove iron and aluminium,

30. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the sulphuric acid leach solution is a leach solution from a metal recovery process. 31. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 30, wherein the metal recovery process is a process for the recovery of nickel and/or cobalt from lateritic ore or concentrate.

32. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the membrane in the membrane filtration step is a diffusion dialysis membrane.

33. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the membrane in the membrane filtration step is an NF membrane.

34. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the membrane filtration step comprises reverse osmosis.

35. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the iron and aluminium removal step is carried out using a pressurised autoclave to remove iron and aluminium sulphate from the retentate solution to form an autoclave treated solution.

36. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 29, wherein the iron and aluminium removal step is carried out at a temperature in the range of about 150-250°C. 37. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 36, wherein the iron and aluminium removal step is carried out at a temperature of about 200°C.

38. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 35, wherein the autoclave treated solution may be subjected to a solid liquid separation step.

39. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 38, wherein the solid liquid separation step involves thickening and/or filtration. 40. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 39, wherein the resulting solids free solution from the solid liquid separation step passes to a further membrane treatment step. 41. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 40, wherein the further membrane treatment step is effected by using an NF membrane.

42. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 41 , wherein further sulphuric acid is generated by passing the autoclave treated solution through the NF membrane in the further membrane treatment step.

43. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 36, wherein the resulting NF permeate comprises the monovalent species such as sulphuric acid, and the resulting NF retentate comprises the multivalent species.

44. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 43, wherein the NF permeate comprises water and solute species which pass through the pores in the membrane, and the NF retentate comprises water and solute species which do not pass through the membrane.

45. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 44, wherein the NF membrane rejects the majority of divalent and trivalent solute species while allowing monovalent species to pass through relatively unhindered.

46. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 45, wherein the NF permeate consists predominantly of water, and sulphuric acid, which pass .through the membrane as monovalent H+ and HS04" ions.

47. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 45, wherein the NF retentate contains the remaining water, plus the majority of the metal sulphate solute species.

48. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 46, wherein the NF permeate is further treated in a separation step to separate the sulphuric acid from water,

49. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 48, wherein the separation step is carried out using a reverse osmosis (RO) membrane.

50. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 49, wherein the RO membrane is a Hydranautics ESPA1 4040 or a Dow Filmtec T -30 membrane. 51. A method for the separation of multivalent metal species from sulphuric ac d as defined in claim 49, wherein the majority of the water typically passes through the RO membrane while the sulphuric acid is retained forming a more concentrated sulphuric acid solution.

52. A method for the separation of multivalent metal species from sulphuric acid as defined in claim 47, wherein the NF retentate containing the various metal species is further treated to recover the metals using usual metal recovery processing steps.

Description:
"METHOD FOR THE TREATMENT OF ACIDIC LEACH LIQUORS"

Field of the Invention

The present invention relates to a method for the treatment of acidic leach liquors obtained from a metal recovery process, such as in a leaching process for the recovery of nickel from lateritic ores. The method preferably also involves steps for the recovery of sulphuric acid from leach liquors. Background to the Invention

The most commonly used hydrometallurgical process route for the production of nickel involves the leaching of nickel bearing lateritic ore in sulphuric acid (H 2 S0 4 ) at either atmospheric or elevated pressure. The solid residue is separated from the leach liquor by counter current washing for disposal. The leach liquor contains substantial residual free acid, which is most commonly consumed by introducing limestone (CaC0 3 ) slurry to decrease solution acidity to approximately pH 4.

The neutralisation reaction produces solid gypsum waste (CaS0 4 * 2H 2 0) and liberates carbon dioxide gas (C0 2 ). For every ton of H 2 S0 neutralised, over a ton of CaC0 3 is consumed, producing 1.8 ton of solid CaS0 4 * 2H 2 0 waste and releasing 450 kg of C0 2 to the atmosphere. The costs associated with this include:

• The additional tailings storage costs

· the CaC0 3 reagent cost

• the cost of wasted H 2 S0 4 not being reacted during leaching

• the environmental cost of releasing C0 2 to the atmosphere. Therefore the disposal of the excess acidic solution is costly due to the techniques required for disposal and is of course wasteful since the excess acid is thrown away. The disposal of such solutions is also not ideal for the environment and can leave toxic residues. Therefore there is a need to employ this excess acid in a positive way, both by trying to recover any remaining nickel and/or other valuable metals such as cobalt in the excess acid solution, as well as by reusing the excess acid such as for the leaching stage. There have been many studies carried out investigating the behaviour of acidic aqueous solutions in methods using nano-filtration (NF) membranes and diffusion dialysis technology. Most of the work in this area has centred around the specific details of the membrane process rather than being focussed on any useful commercial application of these techniques. In the commercial field there has been some work carried out in relation to the use of membrane technology to treat effluent streams, but no commercially useful application of this technology to a metal recovery process and liquors derived from such a process. 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 According to one aspect of the present invention there is provided a method for the separation of multivalent species from monovalent species in an acidic leach solution, the method comprising the steps of: i) passing the acidic leach solution through a membrane in a membrane treatment step to separate a membrane permeate and a mejTibj¾ne_retentate; ii) recovering the membrane permeate containing the monovalent species; and

iii) collecting the membrane retentate containing the multivalent species. Typically the acidic leach solution is a leach solution from a metal recovery process. ' Preferably the metal recovery process is a process for the recovery of nickel and/or cobalt from lateritic ore or concentrate. Preferably the acidic solution is a sulphuric acid solution. According to another aspect of the present invention there is provided a method for the separation of nickel and other valuable metals leached from lateritic ore using sulphuric acid, the method comprising the steps of: i) passing an acidic leach solution containing the nickel and other valuable metals through a membrane in a membrane treatment step

, to separate a membrane permeate and a membrane retentate;

ii) recovering the membrane permeate containing the sulphuric acid; and,

iii) collecting the membrane retentate containing the nickel and other valuable metals for further processing.

The membrane treatment step may comprise nano-filtration. Preferably the membrane is a nano-filtration (NF) membrane. Preferably the NF membrane has high acid resistance and a Daiton cut off of around 200 to 300. Examples of NF membranes commercially available and that may be suitable are: Koch MPF-30 and GE Osmonics Desal DK.

The membrane treatment step may comprise reverse osmosis. The membrane treatment step may comprise nano-filtration combined with reverse osmosis. The membrane treatment step may comprise diffusion dialysis. Preferably the method also comprises a pre-treatment step prior to the membrane treatment step, the pre-treatment step comprising the removal of residual tailings solids from the acidic leach solution. Preferably the pre- treatment step comprises clarification and/or filtration of the acid leach solution to form a treated acidic leach solution which is then passed to the membrane treatment step.

Preferably subsequent to the membrane treatment step the collected membrane retentate is treated to remove iron and aluminium using a pressurised autoclave, and the solids precipitated in the autoclave are separated from the solution by means of a solid/liquid separation step. Advantageously the resulting solution from the solid/liquid separation step is again treated by a membrane in a second membrane treatment step to recover some of the sulphuric acid. Preferably the membrane in the second membrane treatment step is a nano-filtration (NF) membrane.

Preferably the NF membrane has high acid resistance and a Dalton cut off of around 200 to 300, and the membrane permeate consists predominantly of water and sulphuric acid, which pass through the membrane as monovalent H + and HSCV ions. Preferably the membrane retentate contains the remaining water, plus the majority of the metal sulphate solute species in a more concentrated stream.

The membrane permeate may be further treated in a separation step to separate the sulphuric acid from water. Preferably the separation step is carried out using a reverse osmosis (RO) membrane. Typically RO membranes that may be suitable for use in this step are Hydranautics ESPA1 4040 and Dow Filmtec TW-30. The majority of the water typically passes through the RO membrane while the sulphuric acid is retained. In this way the sulphuric acid becomes more concent ed ~ andlTia ~ be " fed to an acid stream. The acid stream may be returned to the leach circuit, reducing overall acid consumption. The water which passes through the RO membrane may be used as a slightly acidic water source elsewhere in the nickel process flowsheet, possibly as wash water in the counter current decantation (CCD) circuit.

The membrane retentate containing the various metal species may be further treated to recover the metals using usual metal recovery processing steps. The benefit of treating the membrane retentate obtained in this way is that the retentate contains less free acid than the original liquor so that less calcium carbonate is required in a subsequent neutralisation step, thereby producing less CaS0 4 * 2H 2 0 waste and emitting less CO2 to the atmosphere.

According to a still further aspect of the present invention there is provided a method for the separation of multivalent metal species from sulphuric acid in a sulphuric acid leach solution, the method comprising the steps of: i) passing the sulphuric acid leach solution through a membrane in a membrane treatment step to separate a permeate and a retentate, the permeate comprising sulphuric acid and the retentate comprising a multivalent metal sulphate solution;

ii) recovering the permeate comprising sulphuric acid; and

iii) treating the retentate to remove iron and aluminium. Typically the sulphuric acid leach solution is a leach solution from a metal recovery process. Preferably the metal recovery process is a process for the recovery of nickel and/or cobalt from lateritic ore or concentrate.

The membrane in the membrane treatment step is preferably a diffusion dialysis membrane. The membrane in the membrane treatment step may be an NF membrane. The membrane treatment step may comprise reverse osmosis. Preferably the iron and aluminium removal step is carried out using a pressurised autoclave to remove iron and aluminium sulphate from the retentate solution to form an autoclave treated solution. Preferably the iron ' ' and aluminium removal step is carried out at a temperature in the range of about 150-250°C, and more preferably at about 200°C.

The discharge autoclave treated solution or slurry may be subjected to a solid liquid separation step. Preferably the solid liquid separation step involves thickening and/or filtration.

Preferably the resulting solids free solution from the solid liquid separation step passes to a further membrane treatment step such as by using an NF membrane.

Preferably further sulphuric acid is generated by passing the autoclave treated solution or slurry through the NF membrane in the further membrane treatment step. The resulting NF permeate comprises the monovalent species such as sulphuric acid, and the resulting NF retentate comprises the multivalent species.

The NF permeate comprises water and solute species which pass through the pores in the membrane, and the NF retentate comprises water and solute species which do not pass through the membrane. The majority of the solute species produced during the leaching of lateritic ore are divalent and trivalent cations (including one or more of Ni 2+ , Co 2+ , Mg 2+ , Al 3+ , Mn 2+ , Cr 3+ ) charge balanced by sulphate (S0 4 2' ) and bisulphate (HS0 " ) anions. NF membranes reject the majority of divalent and trivalent solute species while allowing monovalent species to pass through relatively unhindered.

Thus the NF permeate consists predominantly of water, and sulphuric acid, which pass through the membrane as ' monovalent H + and HS0 4 " ions. The " NF retentate contains the remaining water, plus the majority of the metal sulphate solute species.

The NF permeate may be further treated in a separation step to separate the sulphuric acid from water. Preferably the separation step is carried out using a reverse osmosis (RO) membrane. Typically RO membranes that may be suitable for use in this step are Hydranautics ESPA1 4040 and Dow Filmtec TW-30. The majority of the water typically passes through the RO membrane while the sulphuric acid is retained. In this way the sulphuric acid becomes more concentrated and may be fed to an acid stream. The acid stream may. be returned to the leach circuit, reducing overall acid consumption. The water which passes through the RO membrane may be used as a slightly acidic water source elsewhere in the nickel process flowsheet, possibly as wash water in the counter current decantation (CCD) circuit.

The NF retentate containing the various metal species may be further treated to recover the metals using usual metal recovery processing steps. The benefit of treating the retentate obtained in this way is that the retentate contains less free acid than the original liquor so that less calcium carbonate is required in a subsequent neutralisation step, thereby producing less CaS04 * 2H 2 0 waste and emitting less CO2 to the atmosphere. 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 an embodiment of a method for the separation of multivalent species from monovalent species in an acidic leach solution, given by way of example only, with reference to the , accompanying drawings, in which:

Figure 1 is a flow diagram of an embodiment of a method for the separation of multivalent species from monovalent species in an acidic leach solution.

Detailed Description of Preferred Embodiments

A preferred embodiment of a method 10 for the separation of multivalent species from monovalent species (sulphuric acid) in an acidic leach solution in accordance with the invention, is illustrated in Figure 1. The method includes the recovery of sulphuric acid which may then be recycled for use in the metal recovery process.

The method 10 for the separation of multivalent species from monovalent species in an acidic leach solution 14 comprises the steps of passing the acidic leach solution 14 through a membrane in a membrane treatment step 16 to separate the permeate (water and solute species which pass through the pores in the membrane), namely, a sulphuric acid solution 20, and retentate (water and solute species which do not pass through the membrane) namely a less concentrated sulphuric acid solution containing metal sulphate species 22. The method then comprises recovering the sulphuric acid solution 20 and collecting the solution 22 containing mainly the multivalent species for further processing. The acidic leach solution 14 is typically a sulphuric acid solution containing divalent and trivalent metal sulphates leached from lateritie nickel ore 24 in a sulphuric acid leach step 12. The acidic leach solution 14 is produced by first leaching lateritic nickel ore 24 in a sulphuric acid leach step 12 followed by separating it from solid tailings residue 42 in a counter current decantation (CCD) 18 circuit

The membrane treatment step 16 typically comprises the use of diffusion dialysis equipment such as that marketed by Mech-Chem Associates, Inc.

The membrane treatment step 16 may comprise NF, but the NF permeate is preferably diluted with water to reduce the sulphuric acid concentration before introducing the solution to the iron and aluminium removal autoclave 44.

Diffusion dialysis is preferable in the membrane treatment step because:

• The concentration of sulphuric acid in the metal sulphate solution which does not pass through the membrane is lower than would be achieved by using a NF membrane. This increases the effectiveness of the pressurised autoclave 44 as used in the later precipitation of iron and aluminium.

• Diffusion dialysis does not require elevated pressure to operate.

The membrane treatment step 16 may comprise reverse osmosis 26. Preferably the method 10 also comprises a pre-treatment step 28 prior to the membrane treatment step 16. The pre-treatment step 28 comprises the removal of residual solids from the acidic leach solution. Typically the pre- treatment step 28 comprises clarification and/or filtration of the acid leach solution 12 to form a treated solids free acidic leach solution which is then passed to the membrane treatment step 16. The sulphuric acid solution (permeate) 20 comprises chiefly water and solute species which pass through the pores in the membrane during the membrane treatment step, and the retained stream (retentate) 22 comprises mainly water and solute species which do not pass through the membrane. The majority of the solute species produced during the leaching of lateritic ore are divalent and trivalent cations charge balanced by sulphate (S0 4 2" ) and bisulphate (HSCV) anions. Membranes such as NF membranes reject the majority of divalent and trivalent solute species while allowing monovalent species to pass through relatively, unhindered.

.

Thus the sulphuric acid solution 20 consists predominantly of water and sulphuric acid, which pass through the membrane as monovalent H + and HSCV ions. The retained stream 22 contains water, plus the majority of the metal sulphate solute species. ~

The sulphuric acid solution 20 may be further treated using a reverse osmosis (RO) membrane 26 to separate the sulphuric acid 30 frbm water 32. Typically RO membranes that may be suitable for use in this step are commercially available products sold under the brand names of Hydranautics ESPA1 4040 and Dow Filmtec TW-30.

A large fraction of the water 32 typically passes through the RO membrane 26 while almost all of the sulphuric acid is retained in a more concentrated stream 30 than the feed 20 to the RO membrane 26. In this way the sulphuric acid may be recycled to an ore feed preparation step 36 or to the sulphuric acid leach step 34. By recycling the acid stream 30, overall acid consumption is reduced. The water 32 which passes through the RO membrane may be used as slightly acidic water source elsewhere in the nickel process flowsheet, possibly as supplementary wash water to the CCD circuit 18. The retained solution 22 from diffusion dialysis (retentate), containing the various metal species may be treated to remove iron and aluminium using a pressurised autoclave 44 at elevated temperature in the range of about 150- 250°C, preferably 200°C. Increasing the temperature in the autoclave reduces the solubility of both iron and aluminium, causing hematite and alunite solids to precipitate. This increases the amount of sulphuric acid in solution.

The solids 40 precipitated in the autoclave 44 are separated from the solution 46 by means of a solid/liquid separation step 45. Preferably the solid/liquid separation step 45 involves a thickener and filter. The liquid stream 46 from the solid/liquid separation step 45 is now low in both iron and aluminium content but contains a higher concentration of sulphuric acid than the autoclave feed solution 22. The resulting solution 46 can again be treated by a membrane in a second membrane treatment step 48 to recover some of the sulphuric acid.

Preferably the membrane employed in the second membrane treatment step 48 is an NF membrane. Examples of NF membranes commercially available and that may be suitable are: Koch MPF-30 and GE Osmonics Desal DK. Preferred NF membranes have high acid resistance and a Dalton cut off of around 200 to 300. The membrane permeate 50 is predominantly a sulphuric acid solution 50. The membrane retentate 52 is of similar concentration sulphuric acid to the membrane permeate 50, but contains most of the metal sulphate species in a more concentrated stream.

The membrane retentate 52 is further treated to recover the metals in usual metal recovery processing steps, such as a neutralisation step 38 using limestone, a mixed hydroxide precipitation (MHP) step 54 and manganese removal step 56. The membrane permeate 50 (sulphuric acid solution) may be further treated in a separation step to separate the sulphuric acid from water. Advantageously the separation step is carried out using the reverse osmosis (RO) membrane 26.

The benefit of treating the leach solution 14 in this way is that the retentate 52 contains less free acid and dissolved iron and aluminium than the original liquor. Therefore less calcium carbonate is required in the subsequent neutralisation step 38, thereby producing less gypsum waste 41 and emitting less C0 2 to the atmosphere.

Clearly by concentrating the process solutions according to the various steps in the method of the invention (for example in the membrane treatment steps 16, 26 and 48) the volumes of leach liquor to be processed are reduced thereby decreasing the required equipment size, the energy requirements, net water consumption and so on.

Test Results

Two stages of laboratory test work were completed to evaluate the performance of diffusion dialysis. A sample of leach solution was supplied by an industry partner. The leach solution is a water based solution containing nickel and cobalt (of about 4g/L nickel). The monovalent ions include sodium and potassium. The main divalent ions are nickel, cobalt, copper, zinc, iron(ll), manganese. The main trivalent ions are aluminium, iron(lll) and chrome(l!l). Total membrane operating time to date is three months.

The lab unit has a total active membrane area of 0.0929 m 2 and a linear flow path of 1.22 m. The unit has two feed solution reservoirs, for water and for the leach liquor. It is equipped with two diaphragm metering pumps to control the flow through the membrane stack for the water/recovered acid and the leach liquor/acid depletedjiquor. Jhe-performanee-oHhe ~ diffusion ' dialysis unit was evaluated for the leach liquor supplied. The effect of the following process variables were to be assessed:

• total flux

• relative flux

• feed liquor acid concentration

• magnesium sulphate concentration in the PLS and acid recovery stream.

A summary of the diffusion dialysis test results is shown in Table 1.

Table 1 : Summary of diffusion dialysis test work results The diffusion dialysis test rig was first operated continuously for a month on real plant PLS (Program 1), with no appreciable decrease in performance over the period of tests. The test rig was then operated for a second month of continuous operation on synthetic solutions (Program 2), again without decrease in performance over the period of the tests. The acid recovery performance was excellent, with over 90% of sulphuric acid recovery achievable, with only 1 - 4 % of metals permeating through the membranes when operating on real plant PLS. The second month of operation demonstrated that recycling magnesium sulphate solution to the water (acid recovery) side did not impact acid recovery significantly. This enables higher water recycle rates within the overall process flow sheet. The lower acid recovery in the synthetic tests compared to the plimt PLS tests at a given total flux rate demonstrates the inherent benefit of diffusion dialysis compared to other membrane systems. The synthetic tests were completed with lower total sulphate concentration in solution than the actual plant PLS samples. This demonstrates the benefit of high metal sulphates in solution providing increased driving force for sulphate through the membrane.

The selection of flux rate is a tradeoff between acid recovery and capital cost. The highest possible reject flux rate decreases the number of membrane cells required to process the PLS, but reduces the acid recovery. It is also desirable to operate at the lowest reclaim to reject flux ratio, as this minimises the amount of water addition to the process and increases the concentration of the recovered acid.

Now that preferred embodiments of a method for the separation of multivalent species from monovalent species (such sulphuric acid) in an acidic leach solution have been described in detail, as well as the further steps to recover sulphuric acid, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following:

(i) Neutralisation of excess acid by limestone generates vast quantities of carbon dioxide and gypsum waste. The preferred method reduces the environmental impact, both by reducing greenhouse gas emissions and the required tailings storage footprint. Any neutralised excess acid increases the net acid requirements for the nickel leach circuit. By returning the recovered acid to the leach circuit, the costs associated with the production of sulphuric acid are reduced.

Iron and aluminium sulphate can be partially precipitated prior to neutralisation and the acid generated by the precipitation reactions can be partially recovered. The recovered acid can be returned to the leach circuit, reducing the costs associated with the sulphuric acid production.

The net raw water requirements for the process plant are reduced. Depending on the plant location, decreasing water consumption may significantly reduce costs.

Membrane technology has advanced to the point where it can be considered for large-scale industrial processes.

It will be readily apparent to persons skilled in the relevant arts 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. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.