BACKGROUND TO THE INVENTION Figure 1 is a flowsheet of a known method of extracting cobalt from a nickel sulphate solution with a direct addition of ammonia to a cobalt solvent extraction phase.

A problem with this method is that the concentrated nickel sulphate solution with the direct addition of ammonia results in the formation of insoluble nickel ammonium sulphate double salts. Australian patent No. 667539 in the name of Outokumpu sets out to avoid the formation of this double salt by a two stage process involving : i) pre-neutralisation of a cationic extractant such as CYANEX 272 to form the ammonium salt ; and ii) pre-extraction or exchange of the CYANEX 272 ammonium salt with magnesium sulphate in an aqueous solution to form a CYANEX 272 magnesium salt which is contacted with an aqueous nickel sulphate solution in a solvent extraction circuit so as to extract nickel.

The applicants International patent application No.

PCT/AU98/00457 avoids the relatively expensive two stage pre-equilibration of Outokumpu by adding chemically

reactive magnesia, magnesium hydroxide, or magnesium carbonate to the cationic extractant without the pre- neutralisation step. However, if magnesia or magnesium pre-equilibrated extractant is used to avoid the formation of double salts this introduces magnesium ions which contaminate the final ammonium sulphate product.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method of pre-equilibrating a cationic solvent extractant, said method involving contacting the cationic extractant with a portion of a purified valuable metal stream wherein valuable metal ions of said portion of the purified stream are loaded on the cationic extractant to form a pre-equilibrated cationic extractant.

According to another aspect of the present invention there is provided a method of extracting impurity metal ions from an impure valuable metal sulphate stream in a solvent extraction circuit, said method comprising the steps of : i) contacting a cationic solvent extractant with a portion of a purified valuable metal sulphate stream wherein valuable metal ions of said portion of the purified stream are loaded on the cationic extractant to form a pre-equilibrated cationic extractant ; and ii) contacting the pre-equilibrated cationic extractant with the impure valuable metal sulphate stream in said solvent extraction circuit wherein the impurity metal ions exchange with the valuable metal ions whereby the pre-equilibrated cationic extractant is loaded with the impure metal ions and a raffinate of said extraction circuit is enriched in the valuable metal ions.

According to a further aspect of the present invention there is provided a method of pre-equilibrating a cationic solvent extractant, said method involving contacting the cationic extractant with an aqueous metal ion solution with the direct addition of an ammonia solution to effect loading of the metal ion on the cationic extractant to form a pre-equilibrated cationic extractant, the metal ion solution being selected or preconditioned to a predetermined concentration of the metal ions sufficient to avoid formation of insoluble ammonium/metal sulphate double salts.

Conventionally it is recognised that the direct addition of an ammonia solution to a metal ion solution is not appropriate insofar as it forms insoluble metal salts.

For example, it is understood that the direct addition of an ammonia solution to a concentrated nickel sulphate solution forms insoluble nickel double salts.

Preferably the metal ion solution is preconditioned by dilution with a diluent to achieve the predetermined concentration of metal ions. More preferably the diluent is water.

Generally the metal ion solution is a portion of a purified valuable metal sulphate stream that being a raffinate of an impurity solvent extraction circuit.

Typically the purified valuable metal stream is a purified nickel sulphate stream. More typically the impurity metal ions include cobalt, copper, zinc and/or manganese.

Preferably the purified nickel sulphate stream is the raffinate of the solvent extraction circuit. More preferably the impure valuable metal sulphate stream is a concentrated nickel sulphate liquor obtained by acid pressure leaching of a nickel-cobalt sulphide mineral such as a precipitate obtained during the processing of nickel lateritic ore, or nickel mattes.

Typically pre-equilibration of the cationic extractant with the nickel ions is effected with the direct addition of an ammonia solution. More typically the portion of the purified nickel sulphate stream is preconditioned by dilution with a diluent to achieve a predetermined concentration of nickel sufficient to avoid formation of insoluble nickel double salts. Generally the diluent is water.

Alternatively pre-equilibration of the cationic extractant is effected with the direct addition of magnesia or magnesium hydroxide. In this embodiment, unlike with the addition of an ammonia solution, insoluble nickel double salts do not form and there is no need to dilute the portion of the purified nickel sulphate stream.

Preferably the method of extracting impurity metal ions further comprises the step of stripping of an impurity loaded cationic extractant from the solvent extraction circuit with sulphuric acid to yield the cationic extractant for recycle to pre-equilibration. More preferably stripping of said impurity loaded extractant produces an aqueous impurity metal strip solution which is processed to recover one or more of said impurity metal ions, such as cobalt.

Typically the method of extracting impurity metal ions also comprises the step of electrowinning a high purity electrolyte of the nickel enriched raffinate to recover high purity nickel. Alternatively the nickel enriched raffinate or a derivative thereof is subjected to hydrogen reduction.

Preferably the cationic solvent extractant comprises a phosphinic acid. More preferably the phosphinic acid is bis (2, 4, 4 trimethylpentyl) phosphinic acid, or a derivative thereof, such as that commercially available as CYANEX 272. Generally the cationic extractant is diluted with another diluent such as kerosene.

BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a better understanding of the nature of the present invention, a preferred embodiment of a method of pre-equilibrating a cationic extractant and a method of extracting impurity metal ions from a valuable metal sulphate stream in a solvent extraction circuit will now be described, by way of example only, with reference to the following flowsheets in which : Figure 1 depicts a known method of extracting cobalt and other impurity metals from a nickel sulphate stream in a solvent extraction circuit ; and Figure 2 shows one embodiment of a method according to the invention of extracting cobalt and other impurity metals from a nickel sulphate stream in a solvent extraction circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in Figure 2 there is one embodiment of a method of extracting cobalt and other impurity metals from a

concentrated nickel sulphate solution by a solvent extraction process whereby a cationic solvent extractant is separately pre-equilibrated with a portion of a purified nickel sulphate solution in such a manner that it is loaded with nickel without precipitating insoluble nickel double salts. The nickel loaded extractant is then transferred to an impure cobalt nickel solution where the cobalt and certain other impurity metals exchange with nickel leaving a purified concentrated nickel sulphate solution suitable for hydrogen reduction or electrowinning. The cobalt loaded extractant is stripped with dilute sulphuric acid before being recycled whilst an aqueous cobalt strip solution is further processed to recover cobalt.

The process of this embodiment extracts cobalt and certain other impurities into a cationic extractant phase by exchanging with nickel and thus avoids the introduction of ammonia or alkali ions to the nickel solution. This direct addition of ammonia or alkali ions is required when the extractant is used directly in the known method without pre-treatment as shown in Figure 1. By avoiding the introduction of ammonia or other alkali ions to the nickel sulphate solution, both insoluble double salts of nickel are eliminated and the end solution, after hydrogen reduction of nickel with ammonia, is a pure solution of ammonium sulphate suitable for evaporation and recovery.

To avoid the formation of double salts whilst pre- equilibrating the extractant with nickel using the ammonia solution, the portion of the nickel solution is in this embodiment sufficiently diluted with water to a concentration of between 30 to 60 g/l nickel and nickel is

completely extracted by adjusting the pH. The diluted aqueous ammonium sulphate raffinate, preferably containing less than 200 ppm nickel, is evaporated or combined with a concentrated ammonium sulphate end solution after hydrogen reduction.

It should be appreciated that a variation of this pre- equilibration step includes the use of magnesia or magnesium hydroxide as an alkali to adjust the pH instead of ammonia. This addition of magnesia or magnesium hydroxide is taught in the applicant's International patent application No. PCT/AU98/00457. In this case the dilute aqueous magnesium sulphate raffinate is discarded.

There is no need to dilute the portion of the nickel sulphate as there is no risk of forming the insoluble nickel double salts. This also avoids the direct addition of magnesium ions to the nickel sulphate process stream and hence produces a pure ammonium sulphate end solution after nickel recovery.

Example A cationic organic extractant phase comprising of 0. 45M CYANEX 272 (16. 7% v/v) dissolved in Shellsol 2046 kerosene was contacted with a portion of a nickel sulphate solution remaining after extraction of cobalt at a temperature of 50°C. This nickel sulphate solution, which contained 67. 9 g/l Ni and <50 ppm Co was diluted with an equal volume of water, and contacted for 5 minutes with organic phase at a stirring speed of 1200 rpm whilst adding 12. Soi aqueous ammonia solution to maintain a constant pH. When the pH was maintained at pH 6, nickel was transferred from the aqueous phase into the organic phase which now contained

8. 33 g/l Ni. This loading represents about 620 of the stoichiometric loading capacity of the extractant. When the pH was further raised to pH 6. 3 with more ammonia addition, the organic phase was fully saturated with 13. 48 g/l Ni. The aqueous phase contained ammonium sulphate and a lower concentration of nickel that depended upon the O/A volume ratio employed, concentration of CYANEX 272 and final pH. In other tests an ammonium sulphate solution solution containing <200 ppm Ni was obtained for evaporation and recovery of ammonium sulphate crystals.

The following conditions and data are applicable to this pre-equilibration of CYANEX 272.

Organic : 0. 45M (16. 7%v/v) CYANEX 272 in Shellsol 2046 pH Adjustment : 12. 5% Ammonia Pre-equil. Solution : Co SX Raffinate Vol Organic : 150 mL O/A : 1 Temperature : 50°C Agitation : 1200 rpm Contact Time : 5 minutes Aqueous, pg/mL PDT, Co Cu Zn Mn Co SX 0 4. 97 67900* 0 0 0 0 Raffinate Test 1 42 5. 97 60446* 0 0 0 0 Raffinate Test 2 47 6. 26 54188* 0 0 0 0 Raffinate # Phase disengagement time * Prior to Dilution Organic, g/mL Ni Co Cu Zn Mn Initial 0 1 0 2 0 Stripped Organic Test 1 Loaded 8330 9 0 29 0 Organic Test 2 Loaded 13480 10 0 43 0 Organic % Organic Pre-equilibration :

Ni Test 1 62 Test 2 100 The nickel pre-equilibrated CYANEX 272 phase was then contacted with an equal volume of an impure nickel sulphate solution containing 71. 98g/l Ni, 6. 50g/l Co, 11 ppm Cu, 5 ppm Zn and 11 ppm Mn at 50°C. After 5 minutes agitation at 1200 rpm, cobalt had transferred from the aqueous phase to the organic phase, whilst nickel transferred from the organic phase to the aqueous phase.

After one contact with CYANEX 272 loaded with 8. 33 g/l Ni, at a pH of 5. 25, 98. 7t Co was extracted, leaving 85 ppm Co in the aqueous phase and 76 ppm nickel remaining on the organic phase. When contacted with nickel saturated CYANEX 272 containing 13. 48 g/l Ni at a pH of 5. 75, 99. 4%- Co was extracted leaving 40 ppm Co in the aqueous phase and 138 ppm Ni on the organic phase. The organic phase also extracted the trace amounts of Cu, Zn and Mn. Thus the aqueous phase was purified from cobalt and other

impurities whilst becoming enriched with nickel, making the solution more suitable for nickel recovery by hydrogen reduction or electrowinning. The following conditions and data relate to this extraction of cobalt and other impurities from the impure nickel sulphate solution in a solvent extraction circuit utilising the nickel pre- equilibrated CYANEX 272.

Organic : 0. 45M (16. 7%v/v) CYANEX 272 in Shellsol 2046 pH Adjustment : Ni Pre-equilibrated Organic Aqueous Feed Solution : PLS (After Zn SX) Vol. Organic : 100 mL O/A : 1 Temperature : 50°C Agitation : 1200 rpm Contact Time : 5 min Test 1 Ni Pre-eq. Organic PDT, Equil. Ni Co Cu Zn Mn sec pH PLS 0 3.21 71980 6500 11 5 11 1st Contact 49 5.25 78100 85 0 0 0 2nd Contact 68 4.28 73370 5840 2 0 5 3it Contact 64 4. 24 72400 6750 5 0 7 Ni Pre-eq. 8330 0 29 0 Organic Co Loaded 76 7840 31 19 22 Organic

% Extraction : Ni* Co Cu Zn Mn l5t Contact-8. 5 98. 7 100. 0 100. 0 100. 0 2"d Contact-1. 9 10. 2 80. 7 96. 2 56. 6 3id Contact-0. 6-3. 8 61. 4 100. 0 35. 8 * Negative figures denote enrichment of Ni.

Test 2 Ni Pre-eq. Organic PDT, Equil. Ni Co Cu Zn Mn sec pH PLS 0 3. 21 71980 6500 11 5 11 1st Contact 200 5. 76 79720 40 0 0 0 2"ld Contact 140 4. 82 75590 3990 1 0 3 3 Contact 90 4. 70 72720 6760 3 0 6 Ni Pre-eq. 13480 10 0 43 0 Organic Co Loaded 138 9680 34 20 26 Organic % Extraction : Ni* Co Cu Zn Mn 1st Contact -10.8 99.4 100.0 100.0 100. 0 2nd Contact-5. 0 38. 6 89. 5 100. 0 74. 5 3rd Contact -1.0 -4.0 71.9 100.0 45.3 * Negative Figures denote enrichment of Ni.

Mass Balance : Ni Co Cu Zn Mn Unsaturated 1. 04 0. 87 0. 89-1. 57 0. 93 Organic Saturated Organic 1. 10 0. 90 0. 88-0. 67 0. 90

The present invention is not limited to the embodiments described above and numerous variations and modifications can be made to the method of pre-equilibrating a cationic extractant, and a method of extracting impurity metal ions from a valuable metal sulphate stream which still remain within the ambit of the present invention. For example, although the embodiment described relates to pre- equilibration of a cationic extractant with a portion or side stream of a purified nickel sulphate stream, the invention also extends to other valuable metal streams such as those including cobalt, copper, and zinc. The cationic solvent extractant is not limited to CYANEX 272 but rather extends to practically any cationic extractant which extracts an impurity metal such as cobalt at a lower pH than a valuable metal such as nickel and hence is able to exchange the impurity metal for the valuable metal after pre-equilibration.

The preceding example of the present invention is provided to illustrate a specific embodiment of the invention and is not intended to limit the scope of the method of the invention.