BACKGROUND TO THE INVENTION Solvent extraction of nickel from a leach liquor using a cationic extractant, such as VERSATIC 10, requires neutralisation of the counter-proton to maintain the extraction pH at approximately 6.5.

2 RCOO H+ + Ni2+ t (RCOO )2Ni2+ + 2H+ In a conventional solvent extraction circuit the extraction pH is controlled by the addition of an alkali solution, such as a sodium hydroxide or ammonium hydroxide solution.

Milk of Magnesia and lime have been unsuccessfully trial led in a solvent extraction circuit for maintaining the extraction pH at the required level. The relatively fine solid particles of magnesia and lime combine with an aqueous and an organic phase in the solvent extraction circuit to form crud or an emulsion layer which usually collects at an interface between the aqueous and organic phases. This crud or emulsion layer hinders disengagement of the aqueous and organic phases increasing the phase disengagement time and can result in organic phase losses in the aqueous phase waste stream.

Australian Patent No. 667539 in the name of Outokumpu describes a method of forming a magnesium salt of a cationic extractant and preventing the formation of jarosite and ammonium and alkali based nickel double salts

during solvent extraction. The method involves pre-neutralisation of a cationic extractant such as CYANEX 272 with ammonium to form a CYANEX 272 ammonium salt and thereafter a pre-extraction or exchange of the CYANEX 272 salt with magnesium sulphate in an aqueous solution to produce a CYANEX 272 magnesium salt. The CYANEX 272 magnesium salt is then, in a solvent extraction circuit, contacted with an aqueous solution containing valuable metals such as cobalt and nickel so as to extract the metal. The nickel solution which is stripped from the solvent extractant is then electrowon. Although this method is said to avoid the formation of ammonium and alkali based nickel double salts, it is relatively involved and expensive including both pre-neutralisation and pre-extraction steps.

SUMMARY OF THE INVENTION An intention of the present invention is to provide a process for pre-equilibrating an acidic organic cationic extractant to be used in a solvent extraction circuit, said process being relatively inexpensive and having a minimal adverse effect on the solvent extraction circuit.

According to one aspect of the present invention there is provided a process for separating and recovering one or more metal cations from an aqueous mineral feed liquor, said process involving the steps of: (a) pre-equilibrating an acidic organic cationic extractant by adding chemically reactive magnesia, magnesium hydroxide, or magnesium carbonate to a kerosene solution of the acidic organic cationic extractant so as to form a pre-equilibrated magnesium salt of the solvent extractant; and (b) contacting the aqueous feed liquor with the pre-equilibrated magnesium salt of the solvent extractant in a solvent extraction circuit so as to effectively exchange a magnesium ion of the

pre-equilibrated magnesium salt with said one or more metal cations; and (c) stripping said metal cations with an acidic solution to produce an aqueous strip solution suitable for electrowinning, and regenerating the organic extractant.

According to another aspect of the present invention there is provided a process for pre-equilibration of an acidic organic cationic extractant suitable for use in a solvent extraction circuit, said process involving the step of: (a) adding chemically reactive magnesia, magnesium hydroxide, or magnesium carbonate to a kerosene solution of the acidic organic cationic extractant so as to form a pre-equilibrated magnesium salt of the solvent extractant which can then be used in the solvent extraction circuit for effective loading of one or more metal cations contained in an aqueous mineral feed liquor.

Conventional solvent extraction generally requires pH control to maintain the pH at a level which is effective in metal loading of an acidic organic cationic extractant.

According to the present invention pre-equilibration of the acidic organic cationic extractant in process step (a) eliminates the need for pH control required at step (b).

Typically, the magnesia, magnesium hydroxide, or magnesium carbonate is in a substantially dry condition when added to the acidic organic cationic extractant. Alternatively, the magnesia, magnesium hydroxide, or magnesium carbonate is slurried with just sufficient water prior to its addition to the acidic organic cationic extractant.

Dry magnesia, magnesium hydroxide, or magnesium carbonate is preferred to slurried magnesia, magnesium hydroxide, or magnesium carbonate which exhibits relatively high levels of unreacted or entrained magnesia, magnesium hydroxide, or

magnesium carbonate during pre-equilibration at process step (a). It is understood that this unreacted or entrained magnesia, magnesium hydroxide, or magnesium carbonate then results in the formation of crud when water is also present.

Typically, the acidic organic cationic extractant comprises a high molecular weight organic carboxylic, phosphoric, phosphonic, or phosphinic acid such as the proprietary products VERSATIC 10, D2EPHA, PC88A, or CYANEX 272 respectively. Alternatively, the acidic organic cationic extractant may comprise a dithiophosphoramide or dithiophosphinic acid such as the proprietary products ZENECA DS 6001 or CYANEX 301, respectively. Generally, the acidic organic cationic extractant is dissolved or diluted in kerosene to give solutions typically containing between 5 to 50% w/v of said extractant.

Typically, the magnesia, magnesium hydroxide, or magnesium carbonate is derived from a technical or commercial grade calcined magnesite or dolomite. Although the invention is not restricted to magnesia, magnesium hydroxide, or magnesium carbonate of a specific particle size distribution it will be appreciated that a relatively fine particle size is preferred so as to minimise the reaction time required to pre-equilibrate the acidic organic cationic extractant in process step (a).

Preferably, the step of adding magnesia, or magnesium hydroxide, or magnesium carbonate to the acidic organic cationic extractant involves adding a stoichiometric quantity of magnesia, magnesium hydroxide, or magnesium carbonate. A stoichiometric quantity of magnesia, magnesium hydroxide, or magnesium carbonate is considered to be that required to achieve the following reaction: 2R-H+ + MgO t R2Mg2+ + H2O

where R-H+ represents an acidic organic cationic extractant, MgO represents the equivalent reactive MgO content of basic magnesium minerals containing magnesia, magnesium hydroxide, or magnesium carbonate, and R-2Mg2+ represents a pre-equilibrated magnesium salt of the solvent extractant.

Typically, a quantity of between 30% to 150% of the stoichiometric requirement of magnesia, magnesium hydroxide, or magnesium carbonate is added to the acidic organic cationic extractant. More typically, the process further involves subsequent to step (a) filtering of the pre-equilibrated magnesium salt of the solvent extractant so as to remove unreacted magnesia, magnesium hydroxide, or magnesium carbonate prior to solvent extraction.

Although it was observed that a more complete reaction of the extractant with magnesia, magnesium hydroxide, or magnesium carbonate occurred with an amount of magnesia, magnesium hydroxide, or magnesium carbonate in excess of the stoichiometric requirement, this resulted in high filter demand, or the formation of a stable emulsion or crud when slurries were used. Alternatively, when excess magnesia, magnesium hydroxide, or magnesium carbonate slurry is added to the acidic organic cationic extractant, the addition of a phase modifier has been observed to suppress the formation of crud. In one such embodiment the phase modifier is an alcohol phase modifier such as isodecanol or iso-tridecanol.

Preferably, the step of adding magnesia, magnesium hydroxide, or magnesium carbonate is carried out with sub-stoichiometric levels to minimise the filter demand and crud formation.

Typically, the step of adding magnesia, magnesium hydroxide, or magnesium carbonate is conducted at a

temperature of between approximately ambient to 950C. More typically, the temperature of pre-equilibration is from between approximately SOC to 600C. Preferably, the step of adding magnesia, magnesium hydroxide, or magnesium carbonate is performed under conditions of agitation at a stirring rate of at least approximately 60 rpm.

Preferably, process step (a) involving the pre-equilibration of the acidic organic cationic extractant is relatively fast occurring in less than approximately five (5) minutes, typically when using finely ground magnesia, magnesium hydroxide, or magnesium carbonate at approximately 500C.

Typically, process step (b) involving loading of the pre-equilibrated magnesium salt of the solvent extractant in the solvent extraction circuit is also relatively fast, equilibrium being established in less than approximately two (2) minutes.

Typically, the mineral feed liquor is a metal sulphate leach liquor. In this example said one or more valuable metals loaded during solvent extraction in step (b) include nickel, cobalt, copper, zinc, manganese and/or iron.

BRIEF DESCRIPTION OF DRAWINGS In order to achieve a better understanding of the nature of the present invention several preferred embodiments of a process involving pre-equilibration of an acidic organic cationic extractant using magnesia, magnesium hydroxide, or magnesium carbonate will now be described in some detail, by way of example only, with reference to the following drawings/flowsheets in which: Figure 1 is a flowsheet illustrating an apparatus used for the pre-equilibration of a kerosene solution of VERSATIC 10 with substantially dry magnesia; Figure 2 is a flowsheet illustrating

pre-equilibration of VERSATIC 10 solution in kerosene with 100% excess dry magnesia followed by solvent extraction of a nickel sulphate feed and stripping of the nickel with sulphuric acid to provide barren organic which is recycled; Figure 3 is a flowsheet depicting 75% partial pre-equilibration of VERSATIC 10 solution in kerosene with dry magnesia followed by solvent extraction of a nickel sulphate feed and stripping of the nickel with sulphuric acid; and Figure 4 is a flowsheet showing 50% partial pre-equilibration of VERSATIC 10 solution in kerosene with dry magnesium hydroxide followed by solvent extraction of a nickel sulphate feed liquor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS It has been observed that pre-equilibration of an acidic organic cationic extractant with magnesium avoids the need for pH control in a solvent extraction circuit during the cation exchange reaction shown below, in this example involving the metal cation nickel.

2R-H+ + MgO t (RCOO )2Mg2 + H2O R2Mg2+ + Ni2+ t (RCOO )2Ni2+ + Mg2+ R H+ represents an acidic organic cationic extractant, and MgO represents the equivalent reactive MgO content of basic magnesium minerals containing magnesia, magnesium hydroxide, or magnesium carbonate.

According to various embodiments of the present invention magnesium was introduced into an acidic organic cationic extractant as magnesia (MgO, dry or slurried), magnesium hydroxide (slaked from magnesia), solid magnesium hydroxide Mg (OH)2, or on magnesium carbonate.

Various acidic organic cationic extractants including the following proprietary products, were pre-equilibrated with magnesium. It will be appreciated that this invention is not limited to particular acidic cationic extractants and that the following serve as examples.

VERSATIC 10 (Shell Chemicals Co.) is a mixture of synthetic, saturated carboxylic acid isomers, derived from a highly branched C10 structure.

R1, R2, and R3 are alkyl groups, of which at least one is methyl.

As a carboxylic acid, VERSATIC 10 exhibits low acidic strength due to alkyl substitution at the alpha position, but outstanding solubility in kerosene-type solvents. The physical/chemical characteristics of VERSATIC 10 are as follows: Molecular Weight: 175 Acid value (mg KOH/g): 321 Colour: Clear Water (wt%): 0.10 max.

Density at 200C (kg/l): 0.91 Viscosity (mm2/s): 45 (200C) 10 (50~C) Flash point (PMCC): 129"C ZENECA DS 6001 is a development solvent extraction reagent based on a tetra-substituted dithiophosphoramide structure.

R1 to R4 may be any of (substituted) alkyl, O-alkyl, aryl, and O-aryl.

CYANEX 301 is a dithiophosphinic acid solvent extraction reagent which is an isomer of DS 6001 having the following chemical structure.

where R1 and R2 are 2,4,4 trimethyl pentyl.

CYANEX 272 is another suitable organic cat ionic extractant with its active component being bis(2,4,4-trimethylpentyl) phosphinic acid represented by the following formula.

Since the active component of CYANEX 272 extractant is a phosphinic acid, metals are extracted through a cation exchange mechanism.

Molecular Weight: 290 Assay, Wt%: 85 Specific Gravity: 0.92 ( 2 4 OC ) Solubility in l6ug/mL (250C) water: Flash point: Closed Cup 1080C Appearance: Colourless to light amber liquid Although it will be appreciated that the invention is not limited to any particular Grade of magnesium, the following Grades of magnesia and magnesium hydroxide were trialled in the pre-equilibration of an acidic organic cationic extractant.

Magnesia (Technical Grade) Highly reactive calcined magnesite was produced under closely controlled conditions and then precision milled to superfine requirements to give an especially fine particle size distribution thereby achieving maximum area and high chemical reactivity.

MgO (F.I.B.): 96% Min.

CaO: 1.7% Max.

SiO2: 2.0% Max.

Fe203: 0.40% Max.

A1203: 0.40% Max.

B203: 0.005% Max.

LOI: 3.5% Max.

Tap Density (g/cc): 0.5 Min.

Magnesium Hydroxide (Laboratory Reagent Grade) Molecular Weight: 58.33 Assay: 95.0 - 100.5% Water-soluble matter: 2.0% Chloride (C1): 0.1% Sulphate (SO4): 0.5% Arsenic (As): 0.0004% Calcium (Ca): 0.0030% Iron (Fe): 0.05% Insoluble matter: 0.10% (in acetic acid)

Figure 1 illustrates a bench scale apparatus used for pre-equilibrating the acidic organic cationic extractant VERSATIC 10 with dry magnesia (MgO). The dry magnesia was fed through a screw feeder 10 to a jacketed reactor vessel 12 which contained the barren 1M solution of VERSATIC 10 in kerosene at a temperature of approximately 500C. The pre-equilibrated VERSATIC 10 was then pumped to a solvent extraction circuit with any unreacted magnesia being filtered before solvent extraction.

100% magnesium pre-equilibration of 1M VERSATIC 10 is assumed to be approximately 12 g/L of magnesium. All tests were conducted with 1M (20% v/v) VERSATIC 10 and if phase modifiers were required a 10% v/v concentration was used.

Example 1 A 1M solution of VERSATIC 10 in kerosene was pre-equilibrated with excess magnesia and magnesium hydroxide in a dry and slurried form at temperatures of approximately 50 and 60"C.

In all cases the pre-equilibration reaction time was less than approximately two (2) minutes. The VERSATIC 10 extractant loading capacity for magnesium from slurried magnesium hydroxide (ie slaked MgO, 24 hours) at 600C was 10 g/L Mg compared to 11 and 12 g/L Mg achieved from slurried or dry magnesia, respectively. It was found that tests conducted with excess magnesium, in the form of magnesium hydroxide slurry, and unmodified VERSATIC 10 at a temperature of 50"C resulted in the formation of a VERSATIC-magnesium soap or third phase crud.

When 75% of the stoichiometric amount of magnesia was used in a dry or slurry form, the VERSATIC 10 loaded 8 g/L Mg and gave no crud.

Example 2 A solution of 1M VERSATIC 10 in kerosene was stirred with dry magnesia of various quantities at sub-stoichiometric levels and in excess of the theoretical stoichiometric quantity required for the following reaction in the temperature range 50 to 600C.

2 RCOO-H+ + Mg2+ o (RCOO-)2 Mg2+ + 2H+ With the addition of approximately 75% of the stoichiometric requirement of dry magnesium hydroxide, 90% of the magnesium available for pre-equilibration was extracted (ie approximately 8 g/L Mg loaded onto VERSATIC 10). Fast loading kinetics were again observed in all instances. With 50% dry magnesia, 90% of the added magnesia reacted in approximately 5 minutes to give 5 g/L Mg in the organic phase.

To obtain complete reaction with VERSATIC 10, excess magnesia was used. However, excess magnesia formed entrained solids which then resulted in filtering problems.

Therefore, partial pre-equilibration of dry magnesia (typically from between 50 to 75%) was preferred together with filtering of any unreacted magnesium hydroxide prior to the addition of an aqueous phase during solvent extraction. This reduced the likelihood of crud formation during solvent extraction.

Example 3 A solution of 1M VERSATIC 10 in kerosene was stirred with dry magnesia of both a General Purpose Reagent (GPR) Grade and a Technical Grade. The variations in magnesia quality had no significant effect on the loading capacity of VERSATIC 10 for magnesium. However, the kinetics of extraction depended on the surface area of the magnesia and its reactivity as measured by the time taken to neutralise a standard solution of citric acid.

Example 4 A solution of 1M VERSATIC 10 in kerosene was stirred with magnesia in a dry and slurried form with and without a phase modifier. VERSATIC 10 was modified with 10% v/v alcohol phase modifier, in this example Isodecanol and Iso-tridecanol, before contacting with dry or slurried magnesia. No significant variations were found in either the extraction capacity or the kinetics of magnesium pre-equilibration with the incorporation of alcohol phase modifiers to VERSATIC 10, in both dry and slurry magnesia addition. Greater than 85% magnesium pre-equilibration was achieved, with the exception of the VERSATIC/magnesia slurry system. It should be noted that the formation of an emulsion occurred with the addition of excess magnesia when no phase modifier was used. Fast kinetics of less than one minute were recorded for the tests.

Example 5 A solution of 1M VERSATIC 10 in kerosene was stirred with 75% stoichiometric dry magnesia at temperatures from 20 to 60"C and levels of agitation, ranging from 400 to 1200rpm.

The kinetics of magnesia loading on VERSATIC 10 were relatively fast at less than approximately two (2) minutes.

Furthermore, the loading capacity was not significantly affected by temperature or agitation with between 80 to 90% pre-equilibration of available magnesium being observed in all conditions.

Example 6 A test was carried out to determine the rate of nickel extraction or nickel loading capacity using VERSATIC 10 having been partially pre-equilibrated with 75% stoichiometric dry magnesium hydroxide and slurried magnesia It was observed that the kinetics of nickel extraction from magnesium pre-equilibrated VERSATIC 10 were fast at less than two (2) minutes.

The loading of nickel was about 18 g/L Ni. Slightly higher nickel loadings were observed in the presence of 10% isodecanol and iso-tridecanol phase modifiers, reflecting slightly higher magnesium uptake in pre-equilibration.

Example 7 A solution of 1M CYANEX 272 in kerosene was stirred at 600C for 20 minutes with 75% stoichiometric EMAG 45 magnesia which was found to have a surface area of 29.9 m2/g and a particle size range of 15 to 30 microns. After 2 minutes about 75% of the available magnesium had reacted with the CYANEX 272 and little further extraction took place over the next 20 minutes. Up to 90% of the available magnesium was extracted after the magnesia was further finely ground prior to reaction, or if the "as received" magnesia was vigorously stirred in an autoclave. However, results varied from 60 to 90% extraction. There was no significant effect of raising the temperature from 500C to 1000C or increasing the stirring intensity. When the same magnesia was used to pre-equilibrate VERSATIC 10 under similar conditions, the extraction efficiency was consistently around 90% after about 2 minutes.

Example 8 A solution of 0.6M ZENECA DS6001 reagent in kerosene was reacted with 75% stoichiometric EMAG 45 magnesia at 600C under similar conditions to tests with CYANEX 272 (see Example 7). With this reagent, only about 10% of the available magnesium was extracted, with no significant increase upon increasing reaction time or stirring. Up to 18% Mg extraction was achieved by raising the temperature to 1000C and by further grinding the magnesia in-situ in a ceramic ball mill. It appears that the extent of reaction is limited by the "weak acidity" of the functional group of this reagent.

Figure 2 shows that approximately 100% (12 g/L Mg) pre-equilibration was achieved with 100% excess dry magnesia powder addition. The calculated theoretical maximum nickel loading capacity of 30 g/L with 1M VERSATIC 10 was achieved at a temperature of 50"C with an extraction O/A of 1.0.

Figures 3 and 4 indicate a target of 75% and 50% magnesium pre-equilibration, with 68% and 40% pre-equilibration actually being achieved. The nickel extraction of 22.8 g/L (78%) and 15.1 g/L (50%) was found to be very close to the theoretical maximum extraction of 22 g/L and 15 g/L, respectively. This demonstrated that only the magnesium in the organic phase exchanged for nickel and no protons were exchanged.

It can be seen that nickel loading kinetics with magnesium pre-equilibrated VERSATIC 10 are fast, with equilibrium established in less than approximately two minutes.

Furthermore, it is believed that the VERSATIC 10 loading capacity for nickel is related to the amount of magnesium pre-equilibration which maintains the pH of extraction and the concentration of magnesium in the organic. The experimental loading capacity of nickel on 1M VERSATIC 10 corresponded to the theoretical calculations of 30 g/L Ni for 100%, 22 g/L for 75% and 15 g/L for 50% magnesium pre-equilibration. The amount of calcium remaining in the VERSATIC 10 organic phase increases significantly at pH of greater than approximately 6.9.

The addition of alcohol phase modifiers had insignificant effects on both the kinetics and loading capacity for magnesium pre-equilibration. However, the absence of a modifier could prompt the formation of an emulsion in conditions of excess magnesium. However, the addition of excess magnesium in the form of slurry is not recommended because of crud problems.

Now that several embodiments of the present invention have been described in some detail it will be apparent to those skilled in the relevant arts that the described process for pre-equilibration of an acidic organic cationic extractant has at least the following advantages: (i) magnesia, magnesium hydroxide, or magnesium carbonate are relatively inexpensive alkali substances suitable for pre-equilibration of an acidic organic cationic extractant; (ii) pre-equilibration of an acidic organic cationic extractant using magnesia, magnesium hydroxide, or magnesium carbonate has a minimal adverse effect on a solvent extraction circuit when carried out separately from the extraction step where water is mixed with the organic extractant; (iii) effective loading of metal cations by exchange with magnesium cation may be achieved in a solvent extraction circuit without the need for pH control; (iv) magnesium has a minimal adverse impact on the environment, magnesium sulphate being discharged as tailings; (v) the metal cation can be readily stripped with acid to provide an aqueous solution suitable for electrowinning, and regenerate organic cationic extractant for recycling.

The present invention is not limited to the embodiments described above and numerous variations and modifications can be made to the process for pre-equilibrating an acidic organic cat ionic extractant which still remain within the ambit of the present invention. For example, although the embodiments described relate to solvent extraction of a nickel sulphate leach liquor the invention also extends to cobalt, copper, zinc, or other sulphate leach liquors.

The preceding examples of the present invention are provided to illustrate specific embodiments of the invention and are not intended to limit the scope of the process of the invention.