AU2002221333A1

AU2002221333A1 - Hydroxide solids enrichment by precipitate contact

Info

Publication number: AU2002221333A1
Application number: AU2002221333A
Authority: AU
Inventors: Cesare Giusue Ferron; Erin Maree Jones; Michael John Miller
Original assignee: QNI Technology Pty Ltd
Current assignee: QNI Technology Pty Ltd
Priority date: 2000-12-13
Filing date: 2001-12-13
Publication date: 2002-08-29
Anticipated expiration: 2021-12-13

Description

HYDROXIDE SOLIDS ENRICHMENT BY PRECIPITATE CONTACT

The present invention provides for a method of precipitating nickel and cobalt hydroxides from acidic solutions by the use of magnesium oxide as an alkaline agent to raise the solution pH coupled with a novel precipitate contact system. The precipitate contact system enhances the quality of the nickel and cobalt hydroxide solids recovered, while also fully utilising the neutralising potential of the magnesium oxide used initially.

Background of the invention

Magnesium oxide (MgO) is effective in the recovery of nickel and cobalt as hydroxides in hydrometallurgical processes treating acidic sulphate liquors containing nickel and cobalt and other soluble entities derived from pressure acid leaching or bio-leaching of nickel and/or cobalt containing ores and concentrates.

MgO is a relatively expensive reagent. The present invention aims to achieve a process with more efficient use of MgO and almost total recovery of nickel and cobalt in one treatment stage.

When the pH of acidic liquors containing soluble entities such as Ni, Co, Fe, Cu, Al and Mn sulphates is raised by the use of basic reagents such as NaOH, Ca(OH)₂ Mg(OH)₂ etc, the hydroxides (hydrated oxides) of the above elements will precipitate from solution in a sequence governed by their solubility products. This is a well documented fact and forms the basis of many precipitation processes practised in industry and in analytical chemistry, and is described for example by Jackson in "Hydrometallurgical Extraction and Reclamation", chapter 4 p 145.

The use of MgO as a base for this purpose has been described in prior art studies, for example US Patent 2,899,300 describes the use of MgO (reactive magnesia) either in a powder or slurry form to raise the pH to about 8.2 for a period of one hour to completely precipitate nickel and cobalt as the hydroxides together with approximately 35% the manganese present.

US Patent 3,466,144 also describes the use of MgO to raise the pH of the acid sulphate solution containing Ni and Co to at least 8.0 to precipitate nickel and cobalt and manganese, if present, from solution but at a temperature of 70°C to 90° C. No quantitative values are given in the description to assess the quality of the recovered mixed hydroxide precipitate so the presence of unreacted MgO is not disclosed.

Schiller and Khalafaller in Mining Engineering 2/1984 describe the use of MgO and its advantages over Ca(OH)₂ for the precipitation of metal from aqueous solution. Also described in this paper is the effect of ageing of an MgO slurry and its efficiency and the filterability of the resultant hydroxide product.

Australian patent 701829 describes the addition of a precalculated (based on stoichiometry) amount of MgO to an acid sulphate liquor containing nickel, cobalt and manganese ions to precipitate a majority of nickel and cobalt and a minority of the manganese present as hydroxides. This process is concerned with minimising the manganese content of the mixed nickel, cobalt hydroxide precipitate rather than total recovery of nickel and cobalt in one process stage. Consequently the process then requires further upward pH adjustment with Ca(OH)₂ to recover the remaining Ni, Co and Mn, which solids are recycled for further treatment to improve the overall recovery of Ni and Co. This patent also refers to a reaction time of at least 1 hour.

There are problems associated with the approach of patent 701829 as extra treatment capacity is required in the form of tank reactors, monitoring devices and different reagent storage and handling requirements. Also when valuable materials are recycled there is always the potential for losses to occur. The use of Ca(OH)₂ to strip the remaining Ni and Co from solution has the potential to form gypsum scale (CaSO₄) which is difficult to remove from processing equipment. In addition, manganese hydroxides precipitated together with nickel and cobalt hydroxides when aged together inhibit the redissolution of Ni and Co species in ammoniacal ammonium carbonate liquors.

Steemson in a paper presented at the ALTA Metallurgical Conference (1999) in Perth describes a two stage nickel cobalt hydroxide precipitation circuit for use with both MgO and Ca(OH)₂ however due to cost considerations

Ca (OH)₂ was chosen despite disadvantages associated with the quality of the hydroxide precipitate recovered.

Therefore an improvement on the prior art with respect to manganese and magnesium content of the final precipitate would seek to reduce these contaminants to low levels while utilising as fully as possible MgO used in the process.

The above discussion of documents, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

The present invention aims to overcome or at least alleviate one or more of the difficulties associated with the prior art.

Summary of the invention

In one embodiment, the present invention provides a process for the recovery of nickel and cobalt from an acidic sulphate feed liquor containing nickel and cobalt, said process including the steps of:

(i) taking an acidic sulphate feed liquor and contacting that liquor with magnesium oxide to precipitate the nickel and cobalt as a mixed hydroxide precipitate; and (ii) contacting the mixed hydroxide precipitate with further acidic sulphate feed liquor to re-dissolve unreacted magnesium oxide in the mixed hydroxide precipitate, and precipitate additional nickel and cobalt as a mixed hydroxide precipitate. Prior to contact with magnesium oxide, the acidic sulphate feed liquor preferably undergoes a preliminary neutralization stage to raise the pH of the liquor and to remove some impurities. In a preferred form, essentially all of the feed liquor exiting the neutralization stage is directed to the mixed hydroxide precipitation stage. Alternatively, the feed liquor exiting the neutralization stage may be split into at least two fractions, preferably but not necessarily of approximately equal size. Accordingly, the present invention further resides in a process for the recovery of nickel and cobalt from an acidic sulphate feed liquor containing nickel and cobalt, said process including the steps of:

(i) taking the acidic sulphate feed liquor and dividing the liquor into two fractions;

(ii) contacting the first fraction with magnesium oxide to precipitate the nickel and cobalt as a mixed hydroxide precipitate; and

(iii) contacting the mixed hydroxide precipitate with the second fraction to re-dissolve unreacted magnesium oxide in the mixed hydroxide precipitate, and precipitate additional nickel and cobalt as a mixed hydroxide precipitate.

Typically, the process of the present invention is useful in a nickel and cobalt recovery processes where nickel and cobalt is recovered from a pressure acid leach process from lateritic ore. It is contemplated that the process of the present invention be may be integrated into such a recovery process, where nickel and cobalt is recovered as a mixed hydroxide precipitate from an acidic sulphate solution with the use of MgO, while utilizing as efficiently as possible the MgO used in the process. Detailed description of the invention

A nickel and cobalt recovery process integrating the process of the present invention is described with reference to Figure 1. It should be understood that Figure 1 is a flow sheet showing preferred process steps of a broader nickel and cobalt extraction process, and the process of the invention should not be considered to be limited thereto.

The process described in Figure 1 illustrates a process where essentially all the acidic sulphate feed liquor exiting the secondary neutralization step (17) is directed to a mixed hydroxide precipitation/wash stage. An alternative arrangement where the acidic sulphate feed liquor exiting the secondary neutralization step is divided into two fractions is illustrated in Figure 2. The preliminary and subsequent processing steps described with reference to Figure 1 are also applicable to the embodiment of the invention described with reference to Figure 2, although those steps are not specifically referred to in Figure 2.

Advantageously, in order to improve the quality of the grade of feed material, the nickel and cobalt containing raw materials undergo a beneficiation step. The function of the beneficiation step is to produce a high grade, high density under-flow for feeding into the pressure acid leach autoclave, with minimal loss of nickel and cobalt values. Beneficiation takes advantage of the ore mineralogy to concentrate the nickel and cobalt values into a concentrate. Nickel and cobalt tend to concentrate in fractions from the ore that are in the order of 30-70 microns, whereas the coarse size fractions are dominated by low grade siliceous waste material. The beneficiation process consists of scrubbing the crushed run of mine ore, then extracting the finer material containing the nickel and cobalt using screens, cyclones and attritioning stages.

The fine materials containing the nickel and cobalt values are collected as a slurry, then thickened to a manageable under-flow before being sent to a slurry storage stage ahead of the pressure acid leach circuit. Low grade rejects from the beneficiation stage are returned to the mine. The slurry from the beneficiation stage undergoes pressure acid leach to efficiently leach the nickel and cobalt from the beneficiated ore. In a typical process, nickel and cobalt is first leached from a laterite slurry (1) containing nickel and cobalt with sulphuric acid (3) at elevated pressures, to create an acid sulphate liquor containing both nickel and cobalt and numerous impurities. To achieve adequate rejection of iron and other impurities to the solid reject phase, high pressures (typically about 700 psig) and temperatures (typically about 270° C) are required. This may be conducted in titanium lined autoclave vessels (5). Carbon dioxide and steam are released as a by-product (7).

A residue releach stage (9) uses excess acid from the pressure acid leach stage to recover precipitated nickel and cobalt returned from recycle streams produced at later stages of the recovery process. High acid leach coming from the pressure acid leach reactors may leach the nickel and cobalt from solids produced following later acid neutralization steps (shown by recycle arrow (4)). The more refractory elements in the recycle streams, notably silica, alumina and haematite leach to a lesser extent than the nickel and cobalt, but these solids will generally pass unreacted through the residue releach and neutralization stage and may be separated as waste solids.

The acidic sulphate slurry may then undergo a primary neutralisation step, wherein the function of this step is to neutralise excess acid from the solutions discharged from the pressure acid leach and to efficiently remove iron, aluminium and other impurities from the slurry. This may be achieved with the injection of air and air/SO₂ mixtures (8) into the slurry, typically in a series of stirred tank reactors (6). An alkaline reagent, such as ground limestone slurry (2) may be used for pH control. Typically, the limestone is ground to about 75 microns.

This primary neutralisation step is typically conducted in three stages. In the first stage, the excess acid is removed by using an alkaline reagent such as ground limestone, aiming for a target pH of 1.8 to 2.0. Air may be injected into the nickel and cobalt containing slurry to sparge excess CO₂. In the second stage, a mixture of air and SO₂ (8) is injected into the nickel and cobalt containing slurry. Trace levels of ferric iron in solution catalyse the oxidation of SO₂ to form Caro's acid H₂SO₅, which then oxidises ferrous iron to ferric iron.

In the third stage, the ferric iron is precipitated by the addition of further ground limestone to bring the pH up to between 2.5 to 3.5. This simultaneously precipitates some of the aluminium and chromium from the slurry. The nickel and cobalt containing slurry and associated solids may then be treated in a counter current decantation circuit to remove low grade solids.

The counter current decantation circuit separates low grade solids from the nickel and cobalt containing slurry in a series of thickeners (10). Slurry and associated solids from the primary neutralisation stage are fed to the first thickener, while barren wash solutions (11) from a later manganese removal stage is added to the final thickener. This counter current flow of the liquor from the primary neutralisation stage and the wash solution from later stages provides efficient use of wash water, and also ensures that the tailings slurry contains only low levels of soluble nickel and cobalt, with minimal dilution of the product liquor. The final underflow, including iron residue becomes the final process tailings (12), while the product liquor containing the nickel and cobalt values, then undergoes a secondary neutralization step.

The primary purpose of the secondary neutralisation step is to remove the remaining iron, aluminium and other impurities from the acidic sulphate feed liquor containing the nickel and cobalt that exits the counter current decantation circuit. This is generally achieved by increasing the pH of the liquor with finely ground limestone (16) and simultaneously oxidising the iron in solution using injected air (15). The reactions may be completed in a series of agitated vessels (14) with air sparging. Generally, iron, aluminium and gypsum are precipitated and the solids are collected in a clarifier (42). Some solids may be recirculated and returned to the first vessel of the secondary neutralization step to undergo further neutralisation which assists in solid precipitation, especially for gypsum. The remainder of the solids is returned to the residue releach circuit (9) by flow (4) to recover nickel and cobalt co-precipitates. The purified acidic sulphate feed liquor containing the nickel and cobalt is then sent to the mixed hydroxide precipitation/wash circuit.

It is a feature of the present invention to integrate a mixed hydroxide precipitation/wash circuit into the nickel and cobalt recovery process. In order to achieve precipitation of nickel and cobalt as hydroxides, the incoming acidic sulphate feed liquor (17) is contacted with magnesium oxide, preferably in a slurry form (18) to precipitate the nickel and cobalt as a mixed hydroxide precipitate. Figure 1 illustrates essentially all of the acidic sulphate feed liquor exiting the secondary neutralization step being transferred to the mixed hydroxide precipitate/wash circuit.

The acidic sulphate feed liquor geneally has a pH of approximately 4.0 to 5.0, preferably about 4.5 when exiting the secondary neutralization step. Upon contact with the magnesium oxide, the pH of the incoming feed liquor is raised to approximately 7.5 to 8.5, and most preferably about 8.2. At this pH, the nickel and cobalt and some manganese will precipitate as hydroxides together with some unreacted magnesium oxide to form a mixed hydroxide precipitate. The mixed hydroxide precipitate is collected as a solid or thickened slurry in a mixed hydroxide precipitate clarifier (20).

It is a further feature of the present invention to utilise the excess and unreacted magnesium oxide from the mixed hydroxide precipitate to achieve further precipitation of nickel and cobalt from the incoming acidic sulphate solution. This is achieved by subjecting the mixed hydroxide precipitate from clarifier (20) to a wash step (22) whereby the mixed hydroxide precipitate is contacted with further acidic sulphate feed liquor from the secondary neutralisation step. The incoming stream of the acidic sulphate feed liquor generally has a pH of about 4.5, which is sufficient to leach the magnesium oxide from the mixed hydroxide precipitate. Simultaneously, this will precipitate some additional nickel and cobalt from the incoming acidic sulphate liquor. Nickel precipitation has a buffering effect upon the pH, and the final pH of the mixed hydroxide wash stage will be in the range of from 6.5 to 7.5. This is dependent on the initial nickel tenor following from the secondary neutralisation step, and the level of magnesium oxide in the mixed hydroxide precipitate.

The solids remaining after the mixed hydroxide wash are collected in a clarifier (24), and the liquor is returned to the mixed hydroxide precipitation stage for precipitation of the bulk of the nickel and cobalt values with magnesium oxide slurry. The solids collected following the mixed hydroxide wash stage are filtered and washed to displace chloride contaminated liquor. After a final re-pulp wash, the solids are filtered in a pressure filter to achieve the lowest practical moisture level and to recover the nickel and cobalt as hydroxides.

Overflow liquor following the mixed hydroxide precipitation/wash is sent to a manganese removal stage to precipitate manganese from solution as an inert waste (26). This inert waste can be safely stored as long term tailings. The reactions preferably take place in a series of stirred reactors with a clarifier (41) after the final reaction vessel to collect the solids. Slaked lime is used to raise the pH to a nominal level of about 8.5 or higher. Air sparging is used to oxidize manganese, in which form it is more readily precipitated and forms more stable tailings solids.

Final solids collected in the manganese removal clarifier are sent to tailings, while the overflow from this step is split into two streams, one which serves as a wash water in the counter current decantation circuit (11), while the other is used as a bleed stream from the circuit to eliminate magnesium sulphate from the system (27).

Figure 2 is an example of how the mixed hydroxide precipitate/wash circuit can be operated to effect contact of the feed liquor and precipitated solids where the acidic sulphate feed liquor exiting the secondary neutralization step is split in a 1 :1 ratio distinct from Figure 1 , where essentially all of the acidic sulphate feed liquor is made to contact the mixed hydroxide precipitates in the mixed hydroxide wash/precipitate circuit.

The acidic sulphate feed liquor (31) containing Ni, Co, Mn and Mg typically of pH 4.5 is split into two fractions of approximately equal size. The first fraction (37) is reacted with MgO under pH control in a conventional neutralising process to precipitate maximum nickel and cobalt as well as manganese as hydroxides in reactor vessels (32), (33), (34) and thickener (35). The pH is controlled by the addition of MgO to reach a value of about 8.2. The precipitated thickened solids (42) are removed from the underflow of thickener (35) and brought into contact with the second fraction (38) of the incoming acidic feed liquor in vessel (36). The pH upon contact in this vessel is approximately 6.8 The overflow from vessel (36) is either sent to reactor (32) for further precipitation or discarded via stream (39). Variations of the initial split ratio can be effected to obtain the desired result from anywhere from a 1 :100 ratio to a 100:1 ratio. The underflow precipitate from reactor (36) passes to a belt filter (40) and the recovered Ni, Co hydroxide solids pass for further processing to recover nickel and cobalt.

The following example provides quantitative information on the quality of the solids recovered from reactor (36) compared with those removed from thickener (35) when the incoming feed liquor was split 1 :1.

The process of the present invention, which includes both a mixed hydroxide precipitation step and the step of washing the precipitate with further or a second fraction of the acidic sulphate feed liquor has several advantages over using a single magnesium oxide or magnesium oxide/calcium oxide precipitation step. A particular advantage is the magnesium content in the final product compared to a single stage precipitation route. In a single stage precipitation route there is no facility for recovering or removing the magnesium from the final mixed hydroxide precipitate. Excess magnesium oxide remains in the product and represents an inefficient use of a costly reagent, and also adds to expense in down stream processing and transport. By contacting the mixed hydroxide precipitate with a second or further fraction of the acidic sulphate feed liquor, the pH of the reaction mixture is lowered so as to remove excess or undissolved magnesium oxide. This is more readily achieved when the nickel and cobalt tenor of the feed liquor is higher and when the mixed hydroxide wash stage accepts essentially all of the feed liquor proceeding to the precipitation stage.

A further advantage is a lower manganese content in the final product compared to a single stage precipitation route recovering total nickel and cobalt. Manganese hydroxide is generally regarded as an undesirable contaminant in the hydroxide product. In the two part process of the present application, some manganese is precipitated in the process of recovering the nickel and cobalt in the first higher pH precipitation stage. However, the fraction of nickel and cobalt precipitated in the mixed hydroxide wash stage contains negligible manganese possibly due to the lower reaction pH. This serves to reduce the overall manganese level of the final hydroxide product.

A further advantage is that the final nickel and cobalt product has better filterability and product handling characteristics compared to those products precipitated from a single magnesium oxide precipitation. It has been observed that with a single magnesium hydroxide precipitation step, given the level of magnesium hydroxide that remains in the precipitate, the product tends to be slimy, difficult to filter resulting in the need for further additional filtration equipment, resulting in additional operating costs. Mixed hydroxide precipitates produced with high magnesium content were observed to aggregate whereas precipitates with low magnesium content following a secondary wash were free flowing solids with little or no tendency to aggregate in storage. A further advantage is the reduced reagent costs by eliminating the requirement for lime precipitation of nickel and cobalt residual from the solution.

Prior art processes which terminate the nickel and cobalt precipitation before completion separate the nickel and cobalt hydroxides from solution and precipitate the remainder of nickel and cobalt along with some of the manganese by using slaked lime. The impure hydroxide produced in this step is recycled to an earlier part the process where it is leached in the highly acidic conditions. The lime that is used in this process is an added process cost because the additional lime is effectively used to neutralise excess acid from the pressure acid leach discharge, whereas the same function can be much more economically performed using limestone or calcrete, which are cheaper reagents for this function.

A further advantage is that there is little potential for nickel and cobalt losses as the process of the invention avoids a need to recirculate a proportion of the nickel and cobalt precipitate to the pressure acid leach circuit, which is done in prior art processes to leach further nickel and cobalt from these solids. The process of the present invention eliminates the need to releach further nickel and cobalt using excess acid from the pressure acid leach discharge, and accordingly, eliminates the potential nickel and cobalt losses that could occur.

A further advantage is the lower recycle rates to the residue releach circuit, hence lowering circuit flow rate requirements to the bulk of the refining circuit. Residues recycled from a precipitation step using lime as used in the prior art, are returned as a slurry, bearing not only nickel, cobalt and manganese values, but also gypsum precipitated in the neutralisation step. This slurry increases the circuit volumetric and flow rate from the pressure acid leach discharge onwards, necessitating larger tankage and pumping requirements. These incremental capital and operating costs are avoided in the current mixed hydroxide precipitation/wash step of the process disclosed herein.

A further advantage is the lower sulphate and chloride levels in the final product by employment of higher pH conditions in the neutralisation step. During normal neutralisation of cobalt and nickel bearing liquors with magnesia, it is normal to experience sulphate and chloride contamination as co- precipitants from solution. This is believed to be due to the rapid precipitation of hydrated metal sulphate and chloride species from solution by employing a higher pH for the bulk of the metals precipitated. The process of the present invention reduces the substitution of sulphate and chloride species in the hydroxy complex, and hence the final hydroxide product.

Non-hydroxide species are undesirable components of the final product because of the added downstream costs which would include transport and undesirable reactions for downstream processing stages. For example, sulphate and chloride ions will leach in an ammonia/ammonium carbonate system, rendering the stoichiometric equivalent ammonia irrecoverable by conventional distillation methods.

The above description is intended to be illustrative of the preferred embodiments of the present invention. It should be understood by those skilled in the art, that many variations or alterations may be made without departing from the spirit or ambit of the invention.

Claims

Claims:

1. A process for the recovery of nickel and cobalt from an acidic sulphate feed liquor containing nickel and cobalt, said process including the steps of:

(i) taking an acidic sulphate feed liquor and contacting that liquor with magnesium oxide to precipitate the nickel and cobalt as a mixed hydroxide precipitate; and

(ii) contacting the mixed hydroxide precipitate with further acidic sulphate feed liquor to re-dissolve unreacted magnesium oxide in the mixed hydroxide precipitate, and precipitate additional nickel and cobalt as a mixed hydroxide precipitate.

2. A process according to claim 1 wherein, prior to the contact with magnesium oxide, the acidic sulphate feed liquor undergoes a neutralization stage, whereby an alkaline reagent is used to raise the pH of the feed liquor, and the feed liquor is subjected to an injection of air and/or an air/SO₂ mixture to remove impurities.

3. A process according to claim 2 wherein essentially all the acidic sulphate feed liquor exiting the neutralization stage is contacted with the magnesium oxide to precipitate the nickel and cobalt present in that feed liquor.

4. A process according to claim 1 wherein the mixed hydroxide precipitate is thickened in a clarifier prior to contact with the further acidic sulphate feed liquor.

5. A process according to claim 2, wherein the pH of the acidic sulphate feed liquor exiting the neutralization stage is approximately 4.0 to 5.0, and is raised to a pH of approximately 7.5 to 8.5 upon contact with the magnesium oxide.

6. A process according to claim 5 wherein the pH of the acidic sulphate feed liquor exiting the neutralization stage is about 4.5.

7. A process according to claim 5 wherein the pH of the acidic sulphate feed liquor is raised to about 8.2 upon contact with the magnesium oxide.

8. A process according to claim 1 wherein the pH of the mixed hydroxide precipitate is lowered to approximately 6.5 to 7.5 upon contact with the further acidic sulphate feed liquor.

9. A process for the recovery of nickel and cobalt from an acidic sulphate feed liquor containing nickel and cobalt, said process including the steps of: (i) taking the acidic sulphate feed liquor and dividing the liquor into two fractions;

10. A process according to claim 9 wherein prior to dividing the acidic sulphate feed liquor, the liquor undergoes a neutralization stage, whereby an alkaline reagent is used to raise the pH and the liquor is subjected with an injection of air and/or an air/ SO₂ mixtures to remove impurities.

11. A process according to claim 10 wherein the acidic sulphate feed liquor exiting the neutralization stage is divided into two fractions having a ratio of from

1 :100 to 100:1 to create a first and second fraction.

12. A process according to claim 10 wherein the acidic sulphate feed liquor exiting the neutralization stage is divided into two fractions of approximately equal proportions to create the first and second fraction.

13. A process according to claim 9, wherein the mixed hydroxide precipitate from the first fraction is thickened in a clarifier before contact with the second fraction.

14. A process according to claim 10 wherein the pH of the acidic sulphate feed liquor exiting the neutralization stage is approximately 4.0 to 5.0, and the pH of the first fraction is raised to a pH of approximately 7.5 to 8.5 upon contact with the magnesium oxide.

15. A process according to claim 14 wherein the pH of the acidic sulphate feed liquor exiting the neutralization stage is about 4.5.

16. A process according to claim 13 wherein the pH of the first fraction of the acidic sulphate feed liquor is raised to about 8.2 upon contact with the magnesium oxide.

17. A process according to claim 9 wherein the pH of the mixed hydroxide precipitate from the first fraction is lowered to approximately 6.5 to 7.5 upon contact with the second fraction of the acidic sulphate feed liquor.

18. A process according to claim 1 wherein the mixed hydroxide precipitate includes nickel hydroxide, cobalt hydroxide, manganese hydroxide and unreacted magnesium oxide.

19. A process according to claim 2 or 10 wherein the neutralization stage includes a primary neutralization step to remove iron, aluminium and other impurities from an acidic sulphate feed slurry by contacting the feed slurry with ground limestone to raise the pH of the slurry to 1.8 to 2.0 and injecting a mixture of air and SO₂ leading to oxidation of the iron to ferric iron.

20. A process according to claim 19 wherein the ferric iron is precipitated by addition of further limestone to raise the pH to about 2.5 to 3.5.

21. A process according to claim 19 wherein the acidic sulphate feed slurry undergoes a counter current decantation wash utilising wash solutions from a downstream stage, to separate low grade solids from the feed slurry following the primary neutralization step.

22. A process according to claim 21 wherein the overflow liquor from the counter current decantation wash is diverted to a secondary neutralisation step to remove any remaining iron, aluminium and other impurities by increasing the pH of the liquor to approximately 4.5, by the further addition of ground limestone, and oxidizing any remaining iron by the injection of air.

23. A process according to claim 22 wherein any solids following the secondary neutralization stage are returned to a residue releach circuit for further releaching to recover nickel and cobalt co-precipitated in the secondary neutralization step.

24. A process according to claim 1 or 9 wherein the mixed hydroxide precipitate following contact with the further or second acidic sulphate feed liquor is filtered and washed to displace chloride contaminated liquors, and filtered in a pressure filter to achieve the lowest practical moisture level.

25. A process according to anyone of the preceding claims wherein any overflow after collecting the mixed hydroxide precipitate is processed for manganese removal, including the steps of raising the pH of the solution to about 8.5 or higher by the addition of slaked lime and sparging with air to precipitate manganese as manganese oxide.

26. A process according to claim 1 substantially as hereinbefore described with reference to Figure 1.

27. A process according to claim 9 substantially as hereinbefore described with reference to Figure 2.