The world mineral industry is experiencing an unprecedented interest in nickel-cobalt extraction from laterite ores through high pressure acid leach (HPAL) and solvent extraction-electrowinning (SX-EW) processes. In WA, three nickel laterite projects have been commissioned.

These are the Cawse project of Centaur Mining Ltd, the Bulong project of Preston Resources and the Murrin Murrin project of Anaconda Nickel Ltd. The HPAL process for the three projects is very similar, however the down stream processes (including SX) differ substantially.

In the Murrin Murrin process (Motteram et al., 1996), a sulfide precipitation is used to separate the nickel, cobalt, copper and zinc from impurities such as calcium, magnesium and manganese which remain in the leach solution. After solids/liquid separation, the nickel, cobalt, copper and zinc are re-leached under pressure with acid. Further solution purification is needed to separate nickel and cobalt from copper and zinc. The cobalt is then separated from nickel by solvent extraction with Cyanex 272. The nickel and cobalt are recovered by reduction with hydrogen. The drawbacks of the Murrin Murrin process are: 'The separation of manganese from cobalt by sulphide precipitation is incomplete and causes problems in the downstream processes, The leaching of sulphides needs high pressure and high temperature, indicating high capital and operating costs.

The separation of other impurities such as copper and zinc from nickel and cobalt needs separate processes.

In the Cawse process (Manson et al., 1997), a hydroxide precipitation is used to separate the impurities such as calcium, magnesium and manganese (partly). After solids/liquid separation, the nickel, cobalt, copper and zinc are re-leached with an ammoniacal solution. Nickel and copper are separated from cobalt and zinc by solvent extraction with LIX84I. Further solution purification is needed to separate nickel from copper and cobalt from zinc.

The nickel is recovered by electrowinning while cobalt is precipitated as sulphide. The drawbacks of the Cawse process are: The use of ammoniacal leaching to separate manganese from cobalt results in complexity of the flowsheet and causes serious problems in the downstream processes, The reductive stripping of cobalt from organic extractant and the re-oximation of the organic extractant cause organic degradation (Kindred, 2000), which in turn results in crud formation, Ammonia is expensive and the scrubbing and recovery of ammonia are difficult, Cobalt product containing zinc is a semi-product, indicating revenue loss.

The Bulong process (Taylor and Cairns, 1997), uses a direct solvent extraction approach. Cobalt, copper, zinc and manganese are separated from nickel, calcium and magnesium by solvent extraction with Cyanex 272. The nickel in the raffinate is separated from calcium and magnesium by solvent extraction with Versatic 10 and then electrowon.

The solution containing cobalt, copper, zinc and manganese is subjected to sulphide precipitation, solids/liquid separation and acid pressure re-leach to separate cobalt, copper and zinc from manganese. The copper is eliminated from the solution by ion exchange and zinc by solvent extraction with D2EHPA. The cobalt is then recovered from

the purified solution by electrowinning. The drawbacks of the Bulong process are: Manganese is separated from cobalt by sulphide precipitation and other impurities are separated by different further processes (ion exchange and further SX), Gypsum precipitation occurs in both Cyanex 272 and Versatic 10 circuits, Aqueous feed solution with its original volume is treated twice-in both the Cyanex and Versatic 10 SX circuits. This requires larger circuits (higher capital expenditure) than if the valuable metal (nickel) were concentrated by extraction into the organic phase in the first circuit.

An object of the present invention is to provide a practical process for separating cobalt and nickel from impurities contained in leach solutions such as those derived from lateritic ores.

Accordingly, the present invention provides a method of separating cobalt and nickel from other elements contained in an aqueous leach solution, the method including the steps of: (a) adding a precipitating agent to the aqueous leach solution to precipitate elements including cobalt and nickel; (b) separating the precipitate from the treated aqueous leach solution; (c) leaching the precipitate with an acid to form a leach liquor ; and (d) subjecting the leach liquor to successive solvent extraction steps with an organophosphoric acid and an organophosphinic acid.

As will be understood to persons skilled in the

art of the invention, the aqueous leach solution may be the leach solution obtained from acid digestion of an ore or ore concentrate, or may be a leach solution that has been subjected to other procedures to remove undesired elements therefrom. For instance, the leach solution may be one that has been subjected to a preliminary iron precipitation step.

The precipitating step is intended to precipitate out the cobalt and nickel (and some other elements) as a salt at the given pH, which precipitates can be re-leached in an acid. The pH will be one at which other elements (cations) in the aqueous leach liquor are not precipitated.

Any precipitating agent that would effect precipitation of the cobalt and nickel as an appropriate salt at the given pH would be appropriate. The precipitate is preferably a hydroxide or a carbonate, however other salts such as oxylate salts could be envisaged. If the precipitate is a hydroxide or carbonate, suitable precipitating agents would include alkaline earth metal oxides (magnesia, calcium oxide etc) alkaline earth metal carbonates (magnesium carbonate, calcium carbonate) alkali oxides and alkali carbonates. Magnesia is the preferred precipitating agent.

The elements that are precipitated out as hydroxides or carbonates will include zinc, copper, nickel and cobalt, to the extent that these elements are present in the aqueous leach solution. Some manganese may also be precipitated.

Any suitable acid may be used to re-leach the precipitate, such as sulphuric acid, hydrochloric acid, nitric acid and so on. Sulphuric acid is preferred.

The precipitation step effects the removal of magnesium and calcium (to the extent that these elements are present in the aqueous leach solution), and some manganese, which elements remain in the aqueous leach solution.

If the leach liquor (ie the liquor obtained following the re-leaching step) is subjected to solvent extraction with organophosphoric acid first, the aqueous raffinate from the organophosphoric acid leaching stage is subjected to solvent extraction with the organophosphinic acid. Nickel may recovered from the raffinate by any suitable technique, such as electrowinning, and cobalt recovered from the aqueous strip liquor by any suitable technique, such as electrowinning.

If the leach liquor is subjected to solvent extraction with organophosphinic acid first, nickel is recovered from the raffinate of the organophosphinic acid extraction stage (eg by electrowinning), and the aqueous strip liquor from the organophosphinic acid extraction stage is subjected to solvent extraction with organophosphoric acid to produce a raffinate from which cobalt may be recovered, for example by precipitation. In the case of precipitation of the cobalt from the raffinate, this may be done by the addition of sodium hydroxide which will effect precipitation of the cobalt as cobalt hydroxide.

The loaded organic phase resulting from solvent extraction with organophosphoric acid may be scrubbed with an aqueous scrub solution containing manganese and copper.

This aqueous scrub solution is preferably obtained by conducting a selective strip on a scrubbed loaded organic solution produced in the scrubbing step, and recycling the manganese and copper containing aqueous solution back to the scrubbing stage to be used as the aqueous scrub solution. A spent aqueous scrub solution generated in the scrubbing step is then recycled to the leach solution to maximise the recovery of cobalt (and nickel). The selectively stripped organic solution would usually then be subjected to bulk shipping to remove a large proportion of

the cations remaining therein to be recycled for use as the organic phase in the initial extraction step.

When the organophosphoric acid extraction step is conducted prior to the organophosphinic acid extraction step, preferably the organophosphoric acid extraction step involves: I. contacting the leach liquor produced in step (c) with an organic solution comprising an organophosphoric acid and a modifier dissolved in an organic solvent to produce a loaded organic solution containing manganese and, to the extent that they are present, zinc and copper, and a some cobalt and nickel, and an aqueous raffinate solution containing most of the cobalt and nickel; II. scrubbing the loaded organic solution with an aqueous scrub solution containing manganese and copper to produce a scrubbed organic solution containing less cobalt and nickel and a spent aqueous scrub solution containing some cobalt and nickel, as well as manganese; IIIo recycling the spent aqueous scrub solution to step I; and IV. selectively stripping the scrubbed organic solution to remove a portion of the manganese and copper to form the aqueous scrub solution for use in step II.

When the organophosphoric acid extraction step is conducted following the organophosphinic acid extraction step, preferably the organophosphoric acid extraction step involves: I. contacting the aqueous strip liquor from the organophosphinic acid extraction stage with an organic solution comprising an organophosphoric acid and a modifier dissolved in an organic solvent to produce a loaded organic solution containing manganese and, to the extent that they are present, zinc and copper, and

a some cobalt, and an aqueous raffinate solution containing most of the cobalt; II. scrubbing the loaded organic solution with an aqueous scrub solution containing manganese and copper to produce a scrubbed organic solution containing less cobalt and a spent aqueous scrub solution containing some cobalt, as well as manganese; III. recycling the spent aqueous scrub solution to step I; and IV. selectively stripping the scrubbed organic solution to remove a portion of the manganese and copper to form the aqueous scrub solution for use in step II.

Preferably, the organophosphoric acid solvent extraction step also includes the step of: V. bulk stripping the selectively stripped organic solution from step IV to remove a large proportion or almost all of the cations therein to regenerate the organic solution for recycling to step I.

In the most preferred embodiment of the invention, the organophosphoric acid is di-2-ethylhexyl phosphoric acid (D2EHPA), however it will be appreciated by persons skilled in the art that an organophosphoric acid having extraction characteristics similar to D2EHPA could be used. Organophosphoric acids have the formula (RO) 2PO2H, in which R represents an organic group. The two organic groups R, which may be the same or different, can be selected from optionally substituted branched, straight chained or cyclic alkyl, alkenyl or alkynyl groups.

Preferably the organic groups are fairly bulky, and have a minimum of 4 carbon atoms, more preferably from 6 to 18 carbon atoms. The organic groups may suitably be n-octyl, cyclooctyl or 2-ethylhexyl.

The modifier is any suitable modifier that improves separation of the organic and aqueous phases.

Suitable modifiers include 2-ethylhexanol, isodecanol and isotridecanol and tri n-butyl phosphate (TBP). TBP is the preferred modifier.

The organic solvent may be any suitable organic solvent for the organophosphoric acid that achieves good phase separation from the aqueous phase with the modifier.

Kerosene is the most common solvent/diluent used for this purpose due to its low cost and availability.

The scrubbed organic solution may be selectively stripped of manganese and copper by contacting it with an acid. Sulphuric acid is preferred due to its low cost, however other acids such as HC1 and HN03 could be used.

The selectively stripped organic solution may be bulk stripped of impurity elements by contacting the selectively stripped organic solution with a second acid. The second acid should be stronger than the first acid, and may therefore be the same acid at a higher concentration, or a stronger acid. Accordingly, when sulphuric acid is used as the first acid, a more concentrated sulphuric acid or hydrochloric acid may be used as the second acid to bulk strip the selectively stripped organic solution of the impurity elements. The bulk stripped organic solution may then be recycled to step I.

The range of the Mn and Cu in the aqueous scrub solution will depend on how much Co (and Ni) is to be scrubbed and the aqueous to organic (A/0) ratio.

Preferably, the mole ratio of (Mn+Cu)/ (Co+Ni) is at least 1.5 for complete scrubbing.

The amount of organophosphoric acid in the organic solution used in step I will depend on the concentration of the elements to be extracted and the A/0 ratio, however the amount would typically be in the range of from 3% to 35% v/v, with a preferred range of 5% to 25%.

At levels above 35% the organic solution will be too viscous resulting in lower extraction kinetics.

The amount of modifier in the organic solution used in step I will also vary. The range of modifier will typically be in the range of 3% to 15% v/v, with a preferred range of 5% to 10%.

Preferably, the pH of the aqueous phase is maintained in a range from 3.0 to 4.5 and more preferably 3.5 to 4.0 in step I of the organophosphoric acid extraction stage. The temperature is preferably maintained in the range of from 10°C to 60°C, more preferably from 20 to 40°C. Whilst temperatures as low as 10°C are achievable, a temperature lower than 15°C results in high viscosity. At temperatures higher than 60°C there is a risk of evaporation and degradation of the organic phase.

The aqueous to organic ratio (A/O) in step I is most preferably 1: 1, but may lie in the range from 10: 1 to 1: 10, and preferably 1: 2 to 5: 1. The aqueous to organic ratio maintained in step II may lie within the range of from 1: 5 to 1: 200, but preferably it is in the range of 1: 5 to 1: 20, and most suitably it is from 1: 5 to 1: 10.

In the most preferred embodiment of the invention, the organophosphinic acid is di-2,4,4- trimethylpentyl phosphinic acid (eg Cyanex 272). However it will it will be appreciated by persons skilled in the art that any organophosphinic acid having extraction characteristics similar to or the same as di-2,4,4- trimethylpentyl phosphinic acid could be used.

Organophosphinic acids have the formula R2PO2H, in which R represents an organic group. The two organic groups R, which may be the same or different, can be selected from optionally substituted branched, straight chained or cyclic alkyl, alkenyl or alkynyl groups. Preferably the organic

groups are fairly bulky, and have a minimum of 4 carbon atoms, more preferably from 6 to 18 carbon atoms. The organic groups are preferably unsubstituted branched, straight chained or cyclic alkyl groups, and may suitably be n-octyl, cyclooctyl, 2-ethylhexyl or 2,4,4- trimethylpentyl.

The term"alkyl"used either alone or in a compound word such as"optionally substituted alkyl"or "optionally substituted cycloalkyl"denotes straight chain, branched or mono-or poly-cyclic alkyl, preferably C1-30 alkyl or cycloalkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isbutyl, sec-butyl, tert-butyl, amyl, isoamyl, sec- amyl, 1,2-dimethylpropyl, 1, 1-dimethylpropyl, hexyl, 4- methylpentyl, 1-methylpentyl, 2-methylpentyl, 3- methylpentyl, 1, 1-dimethylbutyl, 2,2-dimethylbutyl, 3,3- dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2- trimethylpropyl, 1,1,2-trimethylpropyl, heptyl, 5- methylhexyl, 1-methylhexyl, 2,2-dimethylpentyl, 3,3- dimethylpentyl, 4,4-dimetylpentyl, 1,2-dimethylpentyl, 1,3- dimethylpentyl, 1,4-dimethylpentyl, 1,2,3-trimethylbutyl, 1,1,2-trimethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6-or 7- methyloctyl, 1-, 2-, 3-, 4-or 5-ethylheptyl, 1-2-or 3- propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-and 8- methylnonyl, 1-, 2-, 3-, 4-, 5-or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6-or 7-ethylnonyl, 1-, 2-, 3-, 4-or 5-propyloctyl, 1-, 2-or 3-butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-or 8- ethyldecyl, 1-, 2-, 3-, 4-, 5-or 6-propylnonyl, 1-, 2-, 3- or 4-butyloctyl, 1-2-pentylheptyl and the like. Examples of cyclic alkyl include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl and cyclodecyl and the like. The alkyl may optionally be substituted by any non-deleterious

substituent.

In this specification"optionally substituted" means that a group may or may not be further substituted with one or more groups selected from alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, aryloxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, nitro, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, amino, alkylamino, dialkylamino, alkenylamino, alkynylamino, arylamino, diarylamino, benzylamino, dibenzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, diacylamino, acyloxy, alkylsulphonyloxy, arylsulphenyloxy, heterocyclyl, heterocycloxy, heterocyclamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy, mercapto, alkylthio, benzylthio, acylthio and the like.

Suitable optional substituents will be chosen on the basis that the organophosphinic or organophosphoric acid have the desired extraction characteristics, and the substituents do not react with any other component of the mixture under the given extraction conditions.

The term"halogen"denotes fluorine, chlorine, bromine or iodine.

The amount of organophosphinic acid in the organic solution used in the organophosphinic acid solvent extraction step will depend on the concentration of the elements to be extracted and the A/0 ratio, however the amount would typically be in the range of from 3% to 35% v/v, with a preferred range of 5% to 25%. At levels above 35% the organic solution will be too viscous resulting in lower extraction kinetics.

It is also advantageous to use a modifier in the

organophosphinic acid extraction stage as for the organophosphoric acid extraction stage. Again, TBP is preferred.

Preferably, the pH of the aqueous phase is maintained in a range from 5.0 to 6.0 in the organophosphinic acid solvent extraction circuit. The temperature is preferably maintained in the range of from 10°C to 60°C, more preferably from 20 to 40°C. Whilst temperatures as low as 10°C are achievable, a temperature lower than 15°C results in high viscosity. At temperatures higher than 60°C there is a risk of evaporation and degradation of the organic phase.

The aqueous to organic ratio (A/O) in the organophosphinic acid solvent extraction circuit is most preferably 1 : 1, but may lie in the range from 3: 1 to 1: 3.

The cobalt extracted into the loaded organic phase in the organophosphinic acid extraction circuit is stripped therefrom and either recovered (when following the organophosphoric acid extraction stage) or directed to the organophosphoric acid extraction stage. In the stripping stage, the A/O ratio may be from 1 : 2 up to 1: 200, and the pH from 2.0 to 4.0. Other details concerning the organophosphinic acid extraction stage are well within the knowledge and experience of persons in the art of the invention.

The steps outlined above can be conducted in combination with other solvent extraction steps and optionally one or more precipitation steps to separate certain elements from each other. For instance, it is common in the art of the invention for a preliminary iron precipitation step to be conducted to precipitate out iron to leave an aqueous leach solution containing the target elements. However, it is a major advantage of the process

of the present invention that no precipitation steps for the purpose of separating manganese from cobalt are required. Therefore, in a preferred embodiment of the invention, the process does not include a sulphide precipitation stage for separating cobalt from manganese.

It will be well understood to persons skilled in the art of the invention that scrubbing stages of the type well known in the art may be used for recovering elements even if the scrubbing stages are not specifically mentioned. The design of the optimum arrangement of scrubbing stages will depend on the specific aqueous leach solution and the elements desired to be recovered therefrom (and target percentage recovery levels).

The present invention also provides a plant for conducting the methods and processes described above (in which the organophosphoric acid extraction stage precedes the organophosphinic acid extraction stage), the plant including: -a hydroxide or carbonate precipitation tank in which the aqueous leach solution is contacted with a precipitating agent; -a solid liquid separator (such as a thickener) in which the aqueous leach liquor following the precipitation step is separated from the precipitate; -a re-leaching tank in which the precipitate is re- leached; -an organophosphoric acid solvent extraction contactor in which the re-leached liquor is contacted with the organophosphoric acid-containing organic solution; -a scrubbing contactor for scrubbing the loaded organic solution generated in solvent extraction contactor; -a stripping contactor in which the scrubbed organic solution generated in the scrubbing contactor is subjected to selective stripping to generate the aqueous scrub solution for use in the scrubbing contactor;

a scrubbed organic solution conduit for conducting the scrubbed organic solution to the stripping contactor; an aqueous scrub solution conduit for conducting the aqueous scrub solution from the stripping contactor to the scrubbing contactor; a spent aqueous scrub solution conduit for conducting the spent aqueous scrub solution from the scrubbing contactor to the solvent extraction contactor; and an organophosphinic acid solvent extraction circuit in which the aqueous raffinate solution generated in the organophosphoric acid solvent extraction contactor is extracted with an organophosphinic acid.

The present invention also provides a plant for conducting the methods and processes described above (in which the organophosphinic acid extraction stage precedes the organophosphoric acid extraction stage), the plant including: a hydroxide or carbonate precipitation tank in which the aqueous leach solution is contacted with a precipitating agent; a solid liquid separator (such as a thickener) in which the aqueous leach liquor following the precipitation step is separated from the precipitate; a re-leaching tank in which the precipitate is re- leached; an organophosphinic acid solvent extraction circuit in which the re-leached liquor generated in the re-leaching tank is extracted with an organophosphinic acid ; an organophosphoric acid solvent extraction contactor in which the aqueous strip liquor from the organophosphinic acid solvent extraction circuit is contacted with the organophosphoric acid-containing organic solution ; a scrubbing contactor for scrubbing the loaded organic solution generated in solvent extraction contactor; a stripping contactor in which the scrubbed organic solution generated in the scrubbing contactor is

subjected to selective stripping to generate the aqueous scrub solution for use in the scrubbing contactor; -a scrubbed organic solution conduit for conducting the scrubbed organic solution to the stripping contactor; -an aqueous scrub solution conduit for conducting the aqueous scrub solution from the stripping contactor to the scrubbing contactor; and -a spent aqueous scrub solution conduit for conducting the spent aqueous scrub solution from the scrubbing contactor to the solvent extraction contactor.

Preferably the plant further includes an organic solution recycle conduit for conducting the partially stripped scrubbed solution from the stripping contactor to the organophosphoric acid solvent extraction contactor.

More preferably, a bulk stripping contactor is located in this recycle conduit, in which the partially stripped scrubbed solution is stripped of remaining impurities before being conducted to the solvent extraction contactor.

As will be understood to persons skilled in the art of the invention, the solvent extraction contactor is preferably a countercurrent solvent extraction contactor.

Similarly, further scrubbing stages and vessels may be included in the plant.

The plants described above will also include the necessary conduits for passing the subject streams from one vessel, stage or circuit to the next. The organophosphinic acid solvent extraction circuit would include an organophosphinic acid solvent extraction contactor (preferably countercurrent), and possibly one or more scrubbing contactors, together with the appropriate conduits for passing the various streams between the vessels in the plant. The plants may also include a cobalt recovery stage, for instance a cobalt precipitation vessel or an electrolytic cell in which a cobalt electrowinning

process can be conducted. The arrangement of an appropriate plant including all of these elements is well within the skill of a person in the art of the invention now that the direction has been provided to conduct the process for recovering cobalt in the manner described above.

Some preferred embodiments of the invention are described in the accompanying examples and figures, in which: Figure 1 illustrates a flow chart for the one stage in the processes of the preferred embodiments of the invention, and represents schematically a part of the plants of the preferred embodiments of the invention; Figure 2 illustrates a schematic flow chart for one embodiment of the process of the invention, and represents schematically a plant for this embodiment of the invention; and Figure 3 illustrates a schematic flow chart for a second embodiment of the process of the invention, and represents schematically a plant for this embodiment of the invention.

Flow sheets for two alternative embodiments of the invention are illustrated in Figures 2 and 3. In these two figures, the boxes referring to"D2EHPA SX" (di-2- ethylhexyl phosphoric acid solvent extraction) should be understood to be a short-hand reference to all of the steps outlined in Figure 1.

The organophosphoric acid extraction stage which is a feature of the two processes of the preferred embodiments of the invention is described in detail in Figure 1. An aqueous leach solution 1 (which may be the stream exiting the"leach"box (the re-leach tank) in Figure 2 or the strip liquor exiting the"cyanex SX"box- the organophosphinic solvent extraction stage-in Figure

3) is fed into a countercurrent solvent extraction contactor 2 in which the aqueous leach solution 1 is contacted with an organic extractant solution 3 comprising a solution of an organophosphoric acid (D2EHPA) and a modifier (TBP) in an organic solvent (kerosene). The aqueous raffinate 4 from the solvent extraction contains most of the cobalt (and nickel, if present in the aqueous leach solution 1) and, to the extent that it is present, a large proportion of the magnesium, from which the cobalt (and nickel) may be recovered. However, it is noted'that if the organophosphinic acid solvent extraction stage is conducted prior to the organophosphoric acid extraction stage, as in the case of the Figure 3 process, nickel has already been separated from the aqueous leach solution, and the aqueous raffinate 4 from the organophosphoric extraction stage contains cobalt.

The loaded organic liquor 5 from the solvent extraction contactor 2 is scrubbed in a scrubbing contactor 6 with a scrub solution 7 containing manganese and copper.

Scrubbed loaded organic solution 8 is selectively stripped with a first acid, a dilute sulphuric acid 9, in a stripping contactor 10 to form scrub solution 7, part of which is recycled to scrubbing contactor 6. Partially stripped organic extractant 11 is then stripped of all impurities with a second acid, dilute hydrochloric acid 12, in a bulk stripping contactor 13. Stripped organic solution 3 is recycled to extraction contactor 2.

Two alternative flow charts resulting from preferred embodiments of the invention are shown in Figures 2 and 3. Figure 2 shows a flow chart in which the organophosphoric acid is used in the first solvent extraction stage and the organophosphinic acid is used in the second solvent extraction stage. Figure 3 shows the reverse situation in which the organophosphinic acid is used in the first solvent extraction phase and the

organophosphoric acid is used in the second solvent extraction stage.

A preferred embodiment of the invention is described with reference to the accompanying example.

EXAMPLE 1 Separation of nickel and cobalt from manganese and other impurities, using a synthetic nickel laterite acid leach solution.

A. Hydroxide precipitation Magnesia is added to the aqueous leach liquor to precipitate out target elements cobalt and nickel as hydroxides, leaving behind the impurities magnesium and calcium (and some manganese) in the liquid phase.

Following the precipitation step, the solid liquid separation and releaching in sulphuric acid, the leach solution has the composition set out in Table 1.

Table 1 Composition of typical acid leach solution Concentration (g/L) Elements Before PPN After acid with MgO Leaching Ni 3. 0 60. 0 Co 0. 3 3. 0 Cl 0. 1 1. 0 Zn 0. 1 1. 0 Ca 0. 5 0. 1 Mn 2. 0 1. 0 Mg 3. 0 0. 1 NB Precipitate leached with acid containing 30g/l Ni (see Figures 2 and 3) 1. Extraction with an organophosphoric acid-containing organic solution.

The aim of the extraction is to extract all the zinc, copper and manganese (and calcium, to the extent that it is still present) from the aqueous leach solution ("PLS"

-plant leach solution) into the organophosphoric acid- containing solution and to minimise the extraction of cobalt (and nickel and magnesium, to the extent that they are present). This step is described in further detail below with reference to a leaching step conducted on a synthetic iron-free laterite leach solution containing 2.60 g/L Ni, 0.24 g/L Co, 0.27 g/L Zn, 0.52 g/L Ca, 0.09 g/L Cu, 1.87 g/L Mn and 2.89 g/L Mg.

The organic solution consisted of 12% di-2- ethylhexyl phosphoric acid (D2EHPA), 2.5% tri-n-butyl phosphate (TBP) and 85.5% kerosene (Shellsol 2046), all by volume. Four counter-current extraction stages were used at an A/O flowrate ratio of 1: 1. The pH in the four mixers was controlled at 3.7 using three pH controllers with the addition of ammonia solution (Fig 1). The results are shown in Table 1. All the zinc, calcium, manganese and copper were extracted in four stages. Some 12.6% cobalt, 20% magnesium and no nickel were co-extracted. The raffinate contained only cobalt, nickel and magnesium.

If the D2EHPA extraction described above had been preceded by the magnesia precipitation step described above, Mg (and Ca) would already have been removed (Figs 2 and 3), leaving only Co and Ni in the raffinate (Fig 2).

The Ni/Co raffinate could then proceed to a subsequent solvent extraction step where cobalt would be extracted from the aqueous solution by di-2,4,4-trimethylpentyl phosphinic acid (Cyanex 272), using a well known and proven commercial process. The cobalt and nickel could be then recovered by electrowinning from the Cyanex strip and raffinate solutions respectively (Fig 2), again using the known technology.

Alternatively, if the D2EHPA extraction described above had been preceded by the known magnesia precipitation step described above, and a known Cyanex 272 extraction, Ni

and Mg (and Ca) would already have been removed, leaving only Co in the D2EHPA raffinate, which could be recovered by hydroxide precipitation (Fig 3). Ni could be recovered by electrowinning from the Cyanex raffinate.

In both flowsheet configurations, the nickel electrowinning spent electrolyte containing sulfuric acid is recycled to the hydroxide precipitate releach (Figs 2 and 3).

Table 2 Summary of semi-continuous extraction test results Metal Concentration (g/L) Total name In In loaded In extraction (%) feed organic raffinate Zn 0. 265 0. 255 0. 000 100. 00 Ca 0. 516 0. 605 0. 000 100. 00 Mn 1. 865 1. 775 0. 000 100. 00 Cu 0. 094 0. 089 0. 000 100. 00 Co 2400. 0310. 19612. 57 Ni 2. 597 0. 000 2. 535 0. 000 Mg 2. 893 0. 590 2. 130 20. 12 2. Scrubbing The aim of scrubbing is to scrub all of the co- extracted cobalt (and nickel, if present) and also magnesium if present, from the D2EHPA organic extraction solution to the aqueous scrub raffinate and to minimise the scrubbing of manganese, copper, calcium and zinc.

Two scrubbing stages were used with a combined solution of manganese (6.78 g/L) and copper (0.35 g/L) at an A/O flowrate ratio of 1: 5. Scrubbing results are listed in Table 3. The cobalt and nickel scrubbing efficiencies were 100% with no cobalt and nickel being left in the organic solution. This scrub raffinate containing 0.142 g/L

cobalt is recycled back to the feed. By doing this, the cobalt and nickel recovery approaches 100%.

Table 3 Summary of scrubbing test results using a combined Mn and Cu scrub solution Metal Concentration (g/L) Total name Scrub loaded scrubbed scrub scrub solution organic organic raffinate efficiency (I-,.) Zn 0.000 0. 255 0. 256 0. 000 0.00 Ca 0.035 0. 605 0. 610 0.054-0.82 Mn 6.781 1. 775 3. 095 0. 926-41.65 Cu 0.349 0. 089 0. 146 0.112-35.45 Co 0.000 0. 031 0. 000 0. 142 100.00 Ni 0.000 0. 000 0. 000 0. 001 100.00 Mg 0. 019 0. 590 0. 000 2. 372 100. 00 3. Selective stripping The aim of selective stripping is to generate a solution containing mainly manganese and copper for using as scrubbing solution in the previous stage.

One selective strip stage was used at an A/0 flowrate ratio of 1: 5 with 14.9 g/L sulphuric acid. The results are shown in Table 4. The strip raffinate contained 7.3 g/L manganese and 0.54 g/L copper, which was just slightly higher than the scrubbing solution used in the previous stage (6.78 g/L manganese and 0.35 g/L copper).

However this could be easily corrected by using slightly more dilute sulphuric acid for stripping, say 12 g/L.

4. Bulk stripping The aim of bulk stripping is to re-generate the organic D2EHPA solution by stripping all elements from the organic extractant.

Two bulk strip stages were used at an A/0 flowrate ratio of 1 : 5 with 37 g/L hydrochloric acid. The results are also shown in Table 4. All the zinc, calcium, copper and manganese were stripped. If no calcium is present in the feed (which is the case when the preliminary precipitation step has been conducted), sulphuric acid can be used for bulk stripping.

Table 4 Summary of semi-continuous stripping test results with an A/0 ratio of 1: 5 Metal Concentration (g/L) Total name Scrubbed Stripped strip strip organic organic 1 raffinate 1 efficiency (%) (Selective strip, 14.9 g/L H2SO4) Zn 0.256 0.255 0.022 1. 73 Ca 0.610 0405 0.280 33. 28 Mn 3.095 1.570 7.319 46.78 Cu 0. 146 0. 030 0. 540 78. 26 (Bulk strip, 36. 6 g/L HC1) Stripped Stripped Strip organic 1 organic 2 raffinate 2 Zn 0.2550.. 0001. 494100. 00 Ca 0.405 0.000 2.401 100.00 Mn 1.570 0.000 8.679 100. 00 Cu 0.030 0.000 0/159 100.00

It will be understood to persons skilled in the art of the invention that various modifications could be made to the preferred embodiments illustrated and described above without departing from the spirit and scope of the invention.