AU2011331911B2

AU2011331911B2 - Method for the leaching of a metal sulfide

Info

Publication number: AU2011331911B2
Application number: AU2011331911A
Authority: AU
Inventors: Daniel Helm; Ross Keating
Original assignee: Murrin Murrin Operations Pty Ltd
Current assignee: Murrin Murrin Operations Pty Ltd
Priority date: 2010-11-19
Filing date: 2011-11-18
Publication date: 2015-03-19
Anticipated expiration: 2031-11-18
Also published as: AU2011331911A1; CA2815563A1; WO2012065222A1; EP2640861A1; BR112013011808A2

Abstract

A method (10) for the leaching of a metal sulfide, the method characterised by the steps of: combining a metal sulfide (12) with water to form a slurry (13); directing the slurry (13) to an autoclave (14) for acid leaching at elevated temperature and pressure; adding an oxidant (16) to the autoclave (14); and adding a metal powder (18) to the autoclave (14), whereby the addition of the metal powder (18) reduces a residual acid concentration in a pregnant leach solution (20) that exits the autoclave (14).

Description

WO 2012/065222 PCT/AU2011/001487 1 "Method for the Leaching of a Metal Sulfide" TECHNICAL FIELD [0001]The present invention relates to a method for the leaching of a metal sulfide. More particularly, the method of the present invention is intended to improve the metal to acid ratio in the leach solution when leaching metal sulfides. BACKGROUND ART [00021 The leaching of metal sulfides is traditionally conducted as an oxidative leach in the presence of oxygen and acid. Oxidative leaching of metal sulfides will often generate some acid as a result of the oxidation of a number of sulfide species to their sulfate in the presence of free acid. This excess acid then needs to be neutralised prior to downstream recovery processes. This is achieved by the addition of a neutralising agent including, for example, ammonia or sodium hydroxide. Alternatively, ammonium sulfate or magnesium sulfate can be added. The addition of neutralising agents generates impurities, which can interfere with recovery processes. [0003] For example, in the treatment of nickel and cobalt sulfide, excess acid has to date been neutralised by the addition of ammonia. The resultant ammonium ion in the presence of sulfate ions can lead to the precipitation of nickel or cobalt double salts in a solvent extraction circuit downstream. This results in a loss of nickel and/or cobalt from precipitation and these precipitates also cause significant scaling and blockages in the plant which in turn can lead to downtime and significant loss of production. [0004] In order to avoid double salt formation, the leach liquor is diluted to ensure the ammonium sulfate concentration does not exceed solubility limits for the double salts. This also results in lower recoveries, as the productive capacity of a nickel refinery is generally proportional to the nickel concentration in the solution feeding to the nickel recovery circuit. [0005] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the WO 2012/065222 PCT/AU2011/001487 2 common general knowledge in Australia or elsewhere as at the priority date of the application. [00061 Throughout the specification it is understood that the term metal sulfide includes sulfide ores, sulfide concentrates and metal sulfide precipitates exiting a precipitation circuit. [0007] Throughout the specification and claims, 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. SUMMARY OF INVENTION [0008] In accordance with the present invention there is provided a method for the leaching of a metal sulfide, the method characterised by the steps of: combining a metal sulfide with water to form a slurry; directing the slurry to an autoclave for acid leaching at elevated temperature and pressure; adding an oxidant to the autoclave; and adding a metal powder to the autoclave, whereby the addition of the metal powder reduces a residual acid concentration in a pregnant leach solution (PLS) that exits the autoclave. [0009] Preferably, the metal in the metal powder is the same as the metal in the metal sulfide. [0010] More preferably, the metal powder is nickel powder. [0011] Preferably, the metal powder is added so as to achieve a residual acid concentration within in the range of about 4 to 12 g/L in the PLS exiting the autoclave (after flashing).

WO 2012/065222 PCT/AU2011/001487 3 [0012] More preferably, the metal powder is added so as to achieve a residual acid concentration within in the range of about 4 to 6 g/L, in the PLS (after flashing). [0013] The particle size of the metal powder is preferably finer than about 90% passing 200 pm. [0014] More preferably, the size of the metal powder is about 90% passing 40 pm. [00151 Preferably, the metal powder is sized as dry metal powder. [0016] The oxidant is preferably in the form of oxygen gas, or a mixture of oxygen in air. [0017] Preferably, the temperature of the slurry within the autoclave is within the range of about 165*C and 175*C. [0018] The pressure within the autoclave is preferably at least about 1100 kPa(g) at an operating temperature of 170 0 C. [0019] The partial pressure of oxygen within the autoclave is preferably at least about 400kPa(g). [0020] More preferably, the partial pressure of oxygen within the autoclave is at least about 600kPa(g). [0021] Preferably, the retention time of the slurry within the autoclave is at least about 40 minutes. [0022] More preferably, the retention time of the slurry within the autoclave is within the range of about 60 minutes to 90 minutes. [0023] In one form of the present invention the metal sulfide is nickel sulfide. [0024] Preferably, where the metal sulfide is nickel sulfide then the metal powder is in the form of nickel powder. Alternatively, the metal powder may be in the form of another metal powder, such as cobalt powder.

WO 2012/065222 PCT/AU2011/001487 4 BRIEF DESCRIPTION OF THE DRAWINGS [0025] The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a method for leaching a metal sulfide; and Figure 2 is graph showing the reduction in ammonium sulfate concentration in the leach solution exiting the sulfide leach after impurity removal; and Figure 3 is a graph showing the increase in grade of the PLS after metal powder addition. BEST MODE FOR CARRYING OUT THE INVENTION [0026] In Figure 1 there is shown a method 10 for the leaching of metal sulfide ore or concentrate, for example nickel sulfide concentrate. A nickel sulfide ore or concentrate 12 is mixed with water to produce a slurry 13 and directed to an autoclave 14. An oxidant 16, for example oxygen gas or a mixture of air and oxygen, is introduced to the autoclave 14 to effect oxidative leaching of the nickel sulfide, and is maintained at a partial pressure within the autoclave of at least about 400kPa, for example at least about 600kPa (gauge). The overall pressure in the autoclave is maintained at about at least 11OOkPa (gauge) within the range of 165*C and 175*C, for example about 170 0 C. This results in the conversion of sulfide to sulfate. However, particularly in the case of nickel/cobalt mixed sulfide precipitates, the solids portion in the slurry often contains elemental sulfur and sulfide sulphur beyond the stoichiometric ratio. The Applicant has determined that some of the excess sulfide may be present in species such as Ni 2

S

3 , C0 2 S3 and the like. [0027] During an oxidative leach, excess sulfur and non stoichiometric sulfide is converted to sulfuric acid: S' + H 2 0+ 2 -- H 2 S0 4 Ni 2

S

3 + H 2 0 +t 102 -+ 2NiSO 4 + H 2 S0 4 WO 2012/065222 PCT/AU2011/001487 5 (0028] Further, the ore or concentrate may also contain impurities, for example iron in the form of iron(ll) sulfide, which also generates sulfuric acid under oxidative leach conditions: 2FeS+2H 2 0+9/202 -+ Fe 2 03 + 2H 2

SO

4 [00291A metal powder 18, for example nickel powder, having a maximum particle size of about 90% passing 200 pm, for example about 90% passing 40 pm (sized as dry metal powder), is added to the autoclave to neutralise the excess acid formed. The metal powder is added in accordance with the proportion of iron sulfide and the S:Me ratio in the autoclave feed material, where Me consists of the metals (other than Fe) that can form sulfides, for example NI, Co, Cu, Zn. The reaction is as follows:

H

2 SO4+ Ni* + 2 O2- NiSO 4 +H20 [0030] The slurry then remains in the autoclave for a retention time of at least about 40 minutes, for example about 60 to 90 minutes, after the metal powder 18 addition. The metal powder 18 addition is controlled such that the resultant slurry exiting the autoclave 14 is a nickel-containing pregnant leach solution (PLS) 20 with a low acid content of between about 4 to 12 g/L (after flashing), for example within the range of about 4 to 6g/L, which can then -be directed to a downstream recovery circuit 22. Some acidity in the slurry 13 and PLS 20 is desirable to avoid precipitation of target metals in the form of hydroxides. Any acid not neutralised by the matal powder is eventually converted to ammonium sulfate, by neutralisation with ammonia in an impurity removal circuit (not shown) as follows:

H

2

SO

4 + 2NH 3 -+ (NH) 2 So4 [0031] In Figure 2 there is shown a plot of the ammonium sulfate concentration in the PLS 20 after impurity removal. That is, once the sulfide leach is complete the PLS 20 proceeds to iron and copper removal steps (not shown). The impurity removal converts all iron in solution to ferric form. Thereafter, it requires the majority of the acid to be neutralised in order to precipitate the ferric iron as ferric hydroxide. Copper is subsequently removed through metathesis with a mixed sulfide of cobalt and nickel. Thus, ammonia is added to neutralise the majority of acid remaining in the PLS 20. It is clear from Figure 2 that after the addition of metal powder 18 into the autoclave 14 (at WO 2012/065222 PCT/AU2011/001487 6 the point indicated by the arrow) the downstream ammonium sulfate concentration drops substantially, and becomes much more stable than it is prior to metal powder 18 addition. The quantity of metal powder 18 to be added will depend on the acid generating species present in the feed slurry. The amount of metal powder 18 added will be calculated to provide the desired residual acid concentration in the PLS (after flashing). [0032] The addition of metal powder is envisaged to provide a number of benefits. In the first instance it reduces the acid content and importantly, significantly minimises the requirement for ammonia addition. (0033] Further, where the metal powder is the same as the target metal (e.g. nickel), which has been recycled from the downstream recovery processes, it increases the grade of the PLS exiting the autoclave, as evidenced by the trends observed in Figure 3 (Ni powder added between Samples 39710 and 39800). The recycling of target metal powder from downstream processes for use in the method 10 improves the final product quality. The improved final product quality can be attributed to the effective "re processing" of the metal powder when it is recirculated for use in the method 10. [00341 It is envisaged that pressures in the autoclave higher than 11 OOkPa are advantageous but the specific pressures employed will depend upon the construction and pressure constraints of the individual autoclave. [0035] Using prior art leaching methods, the nickel to cobalt ratio must be carefully monitored as ores with a low nickel:cobalt (for example about 8:1) ratio will result in a decrease of throughput as the amount of ore that-can be treated will often be limited volumetrically downstream. Alternatively, costs will be significantly higher to accommodate higher volumes, as higher cobalt concentrations leads to increased ammonium sulfate concentration downstream. It is understood that the use of metal powder in the method for the treatment of a metal suffide of the present invention provides greater flexibility with regard to the ores that can be treated when compared with prior art methods. [0036] Further, processes involving an atmospheric laterite leaching step followed by sulfide leaching step require careful monitoring of iron levels in the solution exiting the atmospheric leach. Increased iron concentrations in the atmospheric leach solutions WO 2012/065222 PCT/AU2011/001487 7 will result in increased iron levels in the mixed sulfide precipitated and, consequently, greater iron concentrations entering the sulfide leach. It is envisaged that the addition of metal powder to the sulfide leach allows for the refinement of mixed sulfide concentrates having high iron content, without increasing the ammonia neutralisation requirement downstream. [00371 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. A method for the leaching of a metal sulfide, the method characterised by the steps of: combining a metal sulfide with water to form a slurry; directing the slurry to an autoclave for acid leaching at elevated temperature and pressure; adding an oxidant to the autoclave; and adding a metal powder to the autoclave, whereby the addition of the metal powder reduces a residual acid concentration in a pregnant leach solution (PLS) that exits the autoclave.

2. A method according to claim 1, wherein the metal in the metal powder is the same as the metal in the metal sulfide.

3. A method according to claim 1 or 2, wherein the metal powder is nickel powder.

4. A method according to any one of the preceding claims, wherein the metal powder is added so as to achieve a residual acid concentration within in the range of about 4 to 12 g/L in the PLS exiting the autoclave (after flashing).

5. A method according to claim 4, wherein the metal powder is added so as to achieve a residual acid concentration within in the range of about 4 to 6 g/L, in the PLS (after flashing).

6. A method according to any one of the preceding claims, wherein the particle size of the metal powder is finer than about 90% passing 200 pm.

7. A method according to claim 6, wherein the size of the metal powder. is about 90% passing 40 pm. WO 2012/065222 PCT/AU2011/001487 9

8. A method according to any claim 6 or 7, wherein the metal powder is sized as dry metal powder.

9. A method according to any one of the preceding claims, wherein the oxidant is in the form of oxygen gas, or a mixture of oxygen in air.

10. A method according to any one of the preceding claims, wherein the temperature of the slurry within the autoclave is within the range of about 165"C and 175*C.

11. A method according to any one of the preceding claims, wherein the pressure within the autoclave is at least about 1100 kPa(g) at an operating temperature of 1700C.

12. A method according to any one of the preceding claims, wherein the partial pressure of oxygen within the autoclave is at least about 400kPa(g).

13. A method according to claim 12, wherein the partial pressure of oxygen within the autoclave is at least about 600kPa(g).

14. A method according to any one of the preceding claims, wherein the slurry is retained within the autoclave for at least about 40 minutes.

15. A method according to claim 14, wherein the retention time of the slurry within the autoclave is within the range of about 60 minutes to 90 minutes.

16. A method according to any one of the preceding claims, wherein the metal sulfide is nickel sulfide.

17. A method according to claim 16, where the metal powder is in the form of nickel powder.

18. A method according to any one of the preceding claims, wherein the metal powder is cobalt powder.

19. A method for the leaching of a metal sulfide substantially as hereinbefore described with reference to the Figures.