WO2008034189A1

WO2008034189A1 - Improved process and plant for producing nickel

Info

Publication number: WO2008034189A1
Application number: PCT/AU2007/001398
Authority: WO
Inventors: Andrew Langham Gillies; Patrick Anthony Treasure
Original assignee: Metallica Minerals Ltd
Priority date: 2006-09-21
Filing date: 2007-09-21
Publication date: 2008-03-27
Also published as: AU2007299591A1

Abstract

A method for recovering nickel, a nickel compound or a mixed nickel and cobalt compound comprising: (a) providing a nickel-containing leach solution in which nickel is dissolved in a solvent; (b) subjecting the leach solution to nano-filtration or reverse osmosis to recover solvent therefrom and to form a nickel-containing solution having greater nickel concentration than the leach solution; (c) removing specific metal ions, such as copper and/or trivalent iron, that adversely impact subsequent nickel and cobalt recovery; and (d) treating the nickel-containing solution from step (c) above to recover nickel or a nickel compound.

Description

IMPROVED PROCESS AND PLANT FOR PRODUCING NICKEL

FIELD OF THE INVENTION

The present invention relates to a process and plant for producing nickel or a nickel compound. Suitably, the process and plant also produces cobalt or a cobalt compound.

BACKGROUND TO THE INVENTION

The worldwide demand for nickel is increasing greatly. As a result, the price of nickel on world markets is at historical highs and nickel producers are struggling to meet the demand for nickel.

As a result of these market forces, nickel producers are looking more and more toward the exploitation of nickel laterite deposits. In the past, the exploitation of laterite deposits has been both technically difficult and economically unattractive. However, recent advances in technology have made the exploitation of these deposits more feasible

It is an object of the present invention to provide a method and a plant for producing nickel or a nickel compound.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a method for recovering nickel, a nickel compound or a mixed nickel and cobalt compound comprising:

(a) providing a nickel-containing leach solution in which nickel is dissolved in a solvent;

(b) subjecting the leach solution to nano-filtration or reverse osmosis to recover solvent therefrom and to form a nickel-containing solution having greater nickel concentration than the leach solution;

(c) removing specific metal ions, such as copper and/or trivalent iron, that adversely impact subsequent nickel and cobalt recovery; and (d) treating the nickel-containing solution from step (c) above to recover nickel or a nickel compound.

The nickel-containing leach solution is provided in step (a) using any known leaching process. Examples include atmospheric leaching, heated atmospheric leaching, high temperature and high pressure leaching, vat leaching and heap leaching of either pelletised or run of mine (ROM) ore. An ore or concentrate may be treated in the leaching step to obtain the leach solution.

The leaching step may use an acid as a solvent. The acid may be a mineral acid, such as sulphuric acid or hydrochloric acid, with sulphuric acid frequently being used as a leachant in nickel processing.

Alternatively, other leachants may be used. For example, ammonium sulphate may be used as a leaching solution.

In one embodiment of the present invention, the leach solution containing dissolved nickel is obtained by heap leaching. In the heap leaching process, the ore or concentrate may be first pelletised by agglomeration. The pellets may be formed by mixing an ore with water and passing that mixture to a blending or pugging mill and then to a pelletising drum or other pelletiser. An acidic solution may be mixed with the water prior to the pugging stage. Alternatively, an acidic solution may be later mixed with the pellets.

The pellets are subsequently stacked into piles or heaps and the piles or heaps irrigated with the leach solvent. The leach solvent percolates through the heap and leaches nickel (and typically other metals as well) from the heap. A pregnant leach solution containing dissolved nickel (and typically other dissolved metals as well) is then recovered from the heap leaching step.

The leaching step may be used to dissolve nickel from an ore body or concentrate containing a laterite or oxide ore material. Alternatively, the ore body or concentrate may contain nickel in the form of nickel sulphides.

Nickel containing ores and concentrates will typically also contain cobalt. The step of forming the nickel-containing leach solution will normally also result in the dissolution of cobalt from the ore or concentrate. Other metals may also go into solution during the leaching step, particularly if an acid leach solution is used. For example, significant quantities of iron, magnesium or aluminium may be dissolved into solution during leaching. In some cases, it may be desirable to treat the leach solution to remove some of these other dissolved metals prior to treating the leach solution to recover nickel or a nickel compound therefrom. Such treatments to remove other dissolved metals may include selective precipitation, chemical reduction, solvent extraction or ion exchange. In most nickel plants, removal of undesired dissolved metals from the leach solution is focused upon removing dissolved iron from the leach solution. This is generally achieved through partial neutralisation of the leach solution to precipitate an iron hydroxide sludge. This process can result in a loss of nickel by entrapment in the precipitated phase.

Step (b) of the process of the present invention involves subjecting the leach solution to nano-filtration or reverse osmosis to recover solvent therefrom and to form a nickel-containing solution having greater concentration than the leach solution. In one embodiment, this step involves providing the leach solution under pressure to one or more membranes. Treated solution (permeate) and concentrate are recovered from this step. Suitably, the membrane is selected such that bivalent cations cannot pass through the membrane whereas monovalent cations can pass through the membrane. In this manner, the divalent metals that are dissolved in the leach solution (including nickel and cobalt) do not pass through the membrane and therefore form part of the concentrated reject stream. However, water and monovalent solvent ions can pass through the membrane, thereby recovering the solvent from the leach solution.

The recovered solvent from step (b) may be returned to the leaching step for reuse.

Step (b) of the process may involve pressurizing the leach solution and contacting the pressurized leach solution with the membrane in order to force the permeate to pass through the membrane.

The membrane may comprise a nano-membrane. An example of a commercially available membrane system that is suitable for use in step (b) of the present invention is available from Dynatec Systems, Inc., of Burlington, New Jersey, USA. - A -

Step (b) causes an increase in the concentration of nickel ions in solution, thereby forming a more concentrated nickel-containing solution. Recovery of solvent also occurs in step (b).

The nickel-containing solution recovered from step (b) may then be treated to remove specific metal ions from the solution, which specific metal ions are deleterious to the recovery of nickel and cobalt. The specific metal ions may comprise one or more of copper and/or trivalent iron ions. The solution may then be treated to recover nickel therefrom.

In one embodiment of the present invention, step (d), treating the solution containing nickel from step (c) to recover nickel or a nickel compound therefrom may comprise the steps of:

(e) subjecting the nickel-containing solution to ion exchange by contacting the nickel containing solution with an ion exchange resin to selectively remove nickel and cobalt from the solution;

(f) stripping nickel from the ion exchange resin to form a solution containing nickel; and

(g) recovering nickel or a nickel compound from the solution from step (f) above.

Suitably, the solution containing metal ions may be treated to remove iron therefrom or be subjected to an additional treatment step to reduce ferric ions to ferrous ions. If the solution also contains dissolved copper, it may be desirable to remove copper from the solution. This may be achieved, for example, by subsequently precipitating trace copper in the form of sulphides by use of sodium sulphide or hydrogen sulphide gas prior to the ion exchange step, in order to maintain purity in the final product. Desirably, the treatment to remove the copper from solution also acts as a final reduction to reduce any residual ferric ions to ferrous ions. This treatment may be carried out on either the pregnant leach solution or the concentrated metal ion solution leaving step (b). It is preferred that this step be carried out on the concentrated metal ion solution leaving step (b) as a lower volume of solution would need to be treated. The ion exchange treatment conducted in step (e) of the present invention may be carried out in an ion exchange processing plant such as one developed by Outokumpu Oy. This plant may use resin technology available from Purity Systems Inc., of Missoula, Montana, USA.

As mentioned above, cobalt is typically also leached from nickel-containing ores such that the leach solution contains both dissolved nickel and dissolved cobalt. Thus, steps (e), (f) and (g) above may comprise:

(e) subjecting the nickel and cobalt-containing solution to ion exchange by contacting the nickel and cobalt-containing solution with an ion exchange resin to selectively remove nickel and cobalt from the solution;

(f) stripping nickel from the ion exchange resin to form a solution containing nickel and stripping cobalt from the ion exchange resin to form a solution containing cobalt; and

(g) recovering nickel or a nickel compound and cobalt or a cobalt compound from the solutions from step (f) above.

For convenience and brevity of description, the present invention will hereinafter be described with reference to the recovery of nickel and cobalt or compounds thereof.

The ion exchange resin may suitably selectively adsorb nickel and cobalt from the solution. The thus-treated solution, which is depleted in nickel and cobalt, may be treated to recover further solvent therefrom and/or treated to remove other metals therefrom.

The ion exchange resin containing adsorbed nickel and cobalt is then treated to strip the nickel and cobalt therefrom. Suitably, this stripping step is conducted such that a cobalt-rich stream is removed from the loaded ion exchange resin, followed by removal of a nickel-rich stream from the ion exchange resin. Selective removal of cobalt and nickel from the loaded resin may take place by stripping the resin in two stages. Cobalt will typically be easier to strip from the resin than nickel so stage one may involve contacting the resin with a solvent of relatively lower concentration to remove most of the cobalt and only a small amount of the nickel from the resin, followed by treating the resin with a solvent of relatively higher concentration to strip most of the nickel and some of the cobalt. In this embodiment, a cobalt-rich stream and a nickel-rich stream are obtained.

The solutions recovered from stripping the loaded ion exchange resin may then be treated to recover nickel and cobalt therefrom. Alternatively, the stripping solutions containing nickel and cobalt may be treated to recover nickel-containing compounds and cobalt-containing compounds. Suitably, the streams are subjected to a crystallization step to recover crystals of a nickel compound and crystals of a cobalt compound. The nickel compound and the cobalt compound may then be subjected to further downstream processing to suit prevailing markets. For example, carbonates, sulphates, oxides and/or nickel metal or cobalt metal can be produced from the nickel and cobalt compounds using conventional technologies.

In a second aspect, the present invention provides a plant for recovering nickel, the plant comprising a leaching stage for producing a leach solution containing dissolved nickel; a nano-filtration or reverse osmosis stage to remove nickel from the leach solution, an ion exchange stage to remove nickel from the leach solution, and a further treatment stage for stripping nickel from the ion exchange resin and recovering nickel or a nickel compound therefrom.

The plant may further comprise combinations of one or more of the following:

a) a bulk reduction stage to convert ferric ions to ferrous ions

b) aseparate ion exchange process capable of selectively adsorbing ferric ions

c) a chemical precipitation for trace copper removal that also acts to reduce any remaining ferric ions to ferrous ions

d) a scavenging chemical reduction to ensure complete conversion of ferric to ferrous ions

The combinations of these processes may be varied according to the specific solution chemistry obtained from the heap leach solutions, These preparatory stages are all implemented prior to passing the leach solution to the nickel and cobalt ion exchange stage. Desirably, the stages are multiple due to the stringent purity specifications imposed by the market on sale of nickel and cobalt salts. Suitably, ion exchange is used to adsorb ferric ions and sulphide precipitation is used to precipitate copper ions. The ferric ions may be stripped from the ion exchange resin and produce a marketable ferric sulphate by-product by use of crystallisation from saturated strip solutions.

It is believed that the treatment of the solution to reduce ferric ions to ferrous ions is another novel feature of the invention. Thus, in a third aspect, the present invention provides a method for treating a solution containing dissolved nickel and/or cobalt and ferric ions to selectively remove nickel and/or cobalt from the solution comprising the steps of:

i) treating the solution to reduce ferric ions to ferrous ions or remove ferric ions;

ii) contacting the solution from (i) above with an ion exchange resin that selectively removes nickel and/or cobalt from solution but does not remove ferrous ions from solution; and

iii) separating the solution from the ion exchange resin

The solution may also contain dissolved copper and the method of the third aspect of the present invention may further comprise removing dissolved copper from the solution prior to step (ii). The step of removing the dissolved copper from solution may also be effective in reducing any trace or residual ferric ions to ferrous ions.

The dissolved copper may be removed from solution by converting the dissolved copper into copper sulphide, for example, by contacting the solution with sodium sulphide or with hydrogen sulphide. Advantageously, this also acts to reduce any residual ferric ions to ferrous ions.

Other features of a plant that can be used in various embodiments of the present invention will be described hereunder with reference to the accompanying drawings. The drawings also show various embodiments of a process in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a flow sheet of a plant and a process in accordance with an embodiment of the present invention;

Figure 2 shows a schematic diagram of the ion exchange part of the flow sheet shown in Figure 1 ;

Figure 3 shows a schematic diagram of the layout and operation of the ion exchange process in Figure 2; and

Figure 4 is a graph showing the solubility of various metal sulphate salts with respect to temperature.

DETAILED DESCRIPTION OF THE DRAWINGS

It is to be understood that the drawings are provided for the purpose of illustrating embodiments of the present invention. Thus, it will be appreciated that the invention should not be considered to be limited to the features as shown in the drawings.

Figure 1 shows a flow sheet of a process used for recovering nickel and cobalt compounds from an ore or concentrate containing nickel and cobalt. In Figure 1, a relatively dry ore, which may comprise a lateritic or oxide ore containing nickel and cobalt, is delivered by trucks 10 and blended at 12. The blended ore is reclaimed to a hinged grizzly 14 over a feed bin from where it is delivered to a sizer 16. The sizer may, for example, be set at a 35 mm gap. The sizer may be a Stamler sizer that is provided with self-cleaning engaging tools.

The sizer discharge is transferred via conveyor 18 to either a stacker conveyor 20 to build a crushed ore stockpile 22 for use during periods of agglomeration shutdown or to a pug mill 24 that discharges directly into an agglomeration drum 26. Water 28 is added to the pug mill feed to achieve a selected moisture level. For example, the moisture level in the pug mill may be approximately 20 - 25% by weight. The pug mill functions to ensure that adequate moisture is well dispersed throughout the ore prior to passing to the agglomeration step.

Acid may also be added to the pellets 26. For example, dilute acid additions of between 15% and 25% of the total acid required to achieve target nickel extraction may suitably be added to the pelletiser. This has been found to increase the leaching rate of the valuable metals.

The pellets from pelletiser 26 are transported by a series of conveyors 28 to a stacking conveyor 30 on the leach pad. The stack conveyor 30 is used to form stacks or heaps of the pellets. Some of the heaps are shown at 32, 34 and 36.

The flow sheet shown in Figure 1 utilises dilute sulphuric acid as a leachant. The sulphuric acid is delivered by delivery trucks 38 and stored in concentrated sulphuric acid tank 40. The sulphuric storage tank 40 has appropriate pumps and lines to transfer the acid to where it is required in the flow sheet. For example, line 42 transfers sulphuric acid to pelletiser 26.

The flow sheet shown in Figure 1 utilises heap leaching to form the pregnant leach solution. However, it will be appreciated that other leaching processes may be used in the process of the present invention. For example, the leaching process shown as part of the flow sheet of Figure 1 may be replaced by vat leaching. Further, although sulphuric acid has been described as the leachant used in the embodiment shown in fFigure 1, other leaching agents may also be used. Some examples of other leaching agents include hydrochloric acid and ammonium sulphate.

The pellet heaps 32, 34, 36 are leached with diluted sulphuric or other acidic solution, typically having a concentration of 40 grams to 80 grams per litre, using a counter-current technique, where ore older than say 30 pad days is irrigated with fresh sulphuric acid solution provided via line 44 and return solution pond 46. The newer ore heaps are irrigated with a proportion of the intermediate leach solution drained down from old ore panels. The intermediate leach solution is recovered in pond 48 and a proportion of the intermediate solution is returned via line 50 to irrigate the new heap 32.

The pregnant leach solution from the heap leaching is recovered in pond 51. The pregnant leach solution typically contains dissolved nickel and cobalt, as well as dissolved iron, magnesium and aluminium and small amounts of dissolved, copper, chromium, and manganese. At the completion of the leaching cycle, the spent ore is irrigated with water to remove the majority of the process acid. This acid-containing solution is recovered. The leached ore is then reclaimed to a field hopper. A slurry made of ground limestone and raw water is added at the hopper 54 and the resulting neutral spent ore slurry is hydraulically transferred to a residue pit/dam via line 56. Water from the surface of this storage facility may be recovered and returned to the reclaimed circuit.

Alternatively, the leached ore may be left in place on a permanent pad after rinsing and subsequent stages of leaching conducted on layers placed on top of this material.

The pregnant leach solution from pond 51 is passed through a sand filter 52 to remove large particulate material therefrom. The pregnant leach solution then passes via line 58, pump 60 and line 62 to a nano filtration plant 64. A booster pump 63 is used to pressurise the feed fluid to the required pressure for the nano filtration plant 64. The pregnant leach solution is circulated through the nano filtration membrane modules in nano filtration plant 64 at the design feed rate.

The nano filtration plant 64 is a reverse osmosis system. In nano filtration plant 64, the pregnant leach solution is separated into a permeate 66, which contains acid and water, and a reject stream 68. Reject stream 68 contains dissolved cobalt and nickel and other metals and represents a solution that has a greater nickel concentration (and a greater cobalt concentration) than the pregnant leach solution. Separation in the nano filtration plant 64 is achieved by virtue of the nano filtration membranes allowing monovalent cations and water to move through the membranes into the permeate stream 66, whilst not allowing the bivalent cations to permeate through the nano filtration membrane.

The nano filtration plant 64 may utilise technology available from Dynatec

Systems, Inc. Other nano filtration technologies may also be used. Indeed, the present invention envisages using any nano filtration or reverse osmosis system that enables recovery of acid from the pregnant leach solution and concentration of the dissolved nickel and cobalt into the rejectal concentrate stream 68. The concentrate stream 68 may then treated by any appropriate method to recover cobalt or nickel or to recover nickel compounds and cobalt compounds therefrom. In the embodiment of the invention shown in Fgure 1, the concentrate stream 68 is passed to an ion exchange plant 70. The ion exchange plant may, for example, be an Outokumpu ion exchange carousel plant, where in two contact stages, the nickel and cobalt are adsorbed onto the ion exchange resin. The resin is suitably selective against ferrous (Fe²⁺) ion, however, any ferric (Fe³⁺) ion or copper ion (Cu²⁺) will co-adsorb. If this becomes a significant problem in the process, the leach solution or concentrate may be treated to either remove ferric iron from solution (for example, via precipitation with lime or other alkaline material) or subject to a reduction step to bring the ferric iron back to ferrous irons, for example, by using iron filings. The removal of trace copper ions is then conducted by precipitation in their sulphide form by use of hydrogen sulphide gas or sodium sulphide solutions. This latter step also acts as a final reduction stage for any trace ferric ions remaining in the solution.

Figure 2 shows a schematic diagram of the entire process cycle for the ion exchange plant 70. The ion exchange plant 70 includes a number of individual ion exchange beds, some of which are shown at 72, 74 and 76. The actual physical layout of the ion exchange beds is shown in figure 3. As can be seen from Figure 3, the ion exchange beds are arranged in a circular layout and mounted on a carousel.

In operation of the ion exchange process, rinsed and regenerated ion exchange resin, for example as shown in bed 74, is contacted with the concentrate solution 68. The concentrate solution 68 passes sequentially through the resin beds in the adsorption stage 78. Depleted solution 80 is removed therefrom.

In the diagram shown in Figures 2 and 3, loaded adsorbent is schematically shown by the darker colouring in the resin beds whilst unloaded adsorbent or ion exchange resin is shown in the lighter grey colour.

When the resin bed 75 is fully loaded, it moves to the wash stage 82. In wash stage 82, the resin beds are washed with pure water.

After washing, the resin beds move to the regeneration stage 84. In regeneration stage 84, the loaded resin beds are stripped with sulphuric acid solution 86 to remove the adsorbed irons therefrom and to regenerate the ion exchange resin. This results in the formation of solutions containing nickel and cobalt that are removed via stream 88.

The carousel ion exchange plant 70 is a well developed package plant for ion exchange processing developed by Outukumpu. The plant enables minimum resin inventory and efficient continuous operation. The carousel resin plant 70 may utilise ion exchange resin from Purity Systems Inc. Suitable resins include those ion exchange resins sold by Purity Systems Inc. under their trade marks WP-I and WP-2 resins. These resins have been recycle tested for greater than 12,000 cycles with no observed loss in loading capacity.

The water used in the washing/elution circuit is desirably high quality water, as any impurities present will report to the products.

The raffinate 80 from the ion exchange plant 70 (which represents the concentrate stream 68 that has had a proportion, preferably most, of the nickel and cobalt removed therefrom) may be subjected to an optional nano filtration treatment 90 to recover additional acid and water for recycle via line 92 to solution pond 46. It will be appreciated that nano filtration 90 is an optional step in this process. The solution 94 from the nano filtration plant 90 may be mixed with lime 96 supplied via line 98 to cause precipitation of metal such as aluminium, chromium, manganese and iron. The resulting suspension or sludge may be removed from mixer 99 and sent via line 100 for storage in a residue pit/dam.

Careful control of the stripping of the nickel and cobalt values from the ion exchange resin in the ion exchange plant 70 enables stripping that is somewhat selective for cobalt and nickel, respectively, to occur from the ion exchange resin. For example, it has been found that cobalt is relatively easier to strip from the ion exchange resin than nickel. Therefore, a two stage stripping process may be used. In the first stage, a relatively smaller amount of a relatively lower concentration solvent may be passed through the loaded ion exchange resin to strip cobalt therefrom. Following that, a relatively larger amount of a relatively stronger solvent are used to strip nickel from the ion exchange resin. It will be understood that although this stripping process allows for selective stripping of cobalt and nickel, some cross-contamination will occur. Thus, a cobalt-rich stream and nickel-rich stream may be recovered from the ion exchange plant 70. However, the cobalt-rich stream will include some nickel and the nickel-rich stream is likely to include some cobalt.

In one example of the stripping process used in the ion exchange plant 70, the first stage may involve stripping cobalt by passing 1.6 bed volumes of approximately 7% sulphuric acid through the resin beds in the regeneration stage 84, followed by a second stage in which 2 - 5 bed volumes of approximately 14% sulphuric acid solution is passed through the resin beds in regeneration stage 84 to strip nickel from the resin beds.

During the first stage stripping, the solution 88 recovered from the ion exchange plant 70 is passed to a cobalt crystalisation stage. During the second stage stripping, the stripping solution 88 (which is nickel-rich) is passed to a nickel crystalisation step. For clarity, only one crystalliser 100 is shown in Figure 1. However, it will be understood that an additional crystalliser may also be used. The stripping desirably operates at elevated temperatures (circa 50 degrees Celsius) to allow high concentrations of nickel or cobalt in the strip solution.

Crystallisation occurs by simple cooling of the heated saturated solutions 88 leaving the ion exchange plant 70. In particular, heat exchanger 102 is used to cool solution 88 as it is fed into crystalliser 100. The crystalliser 100 may be any suitable proprietary crystalliser. One particularly suitable crystalliser may include a crystalliser cone that is positioned inside a conventionally shaped cylindrical tank, which becomes a cooling jacket. Chilled water may enter the tank tangentially at the bottom (where the volume between the tank and the cone is greatest) and circulate upwards through a diminishing volume in an effective counter-flow regime. This helps to ensure that there is at no stage a shock chilling zone, which would risk local uncontrolled crystallisation. The cone may be fitted with a central draft tube with a variable speed uplifting impeller that provides continuous internal recirculation. Feed solution may be dropped into the outer edge of the top of the cone and a baffled overflow may be located opposite the inlet feed.

The cone may discharge crystal mass through the bottom below the tank and feed an inclined variable speed flooded auger 104. The crystal mass is washed on a screen 106 with pure water 108, with washing volume balanced against water dehydration and evaporation losses.

The screen underflow 110 contains surplus acidic eluant as well as dissolved nickel and cobalt that did not crystallise. The screen underflow 110 is re-acidified with sulphuric acid via line 116 and then transferred via lines 112, 114 to the stripping module of the ion exchange plant 70.

Nickel and cobalt are crystallised as crystals of nickel sulphate and cobalt sulphate. The crystallisation of the metal sulphates may be assisted by the addition of make-up acid into the loaded strip solution prior to crystallising the sulphates. This further reduces the metal sulphate solubility.

The metal sulphate crystals are removed from screen 116 and transferred to conveyor 118 which subsequently transfers the metal sulphate crystals to crystal storage 120.

The downstream processing of the nickel sulphate and cobalt sulphate will be implemented to suit prevailing markets. Carbonates, oxides and/or nickel metal and cobalt metal can be produced from an aqueous sulphate solution using conventional technologies.

Selected crystallisation of nickel sulphate and cobalt sulphate may be based upon the simple principles of solubility limits as illustrated in Figure 4. The solubility behaviour is amplified by the control of the strip acid concentration as well as the selective stripping of cobalt and nickel from the loaded ion exchange resin.

Producing sulphate crystals of nickel enables a wide variety of processes to be applied to generate other salts without having to consume or neutralise the strong strip acid solutions. For example, to electrowin nickel to a metal powder product, the pH must be regulated to around 3 - 4 by neutralising the acid generated by the nickel reduction. Carbonate production can be driven in an aqueous sulphate environment maintained at slightly alkaline pH.

The person skilled in the art will understand that the present invention may be subject to variations and modifications other than those specifically described. It will understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

Claims:

1. A method for recovering nickel, a nickel compound or a mixed nickel and cobalt compound comprising:

(c) removing specific metal ions, such as copper and/or trivalent iron, that adversely impact subsequent nickel and cobalt recovery; and

(d) treating the nickel-containing solution from step (c) above to recover nickel or a nickel compound.

2. A method as claimed in claim 1 wherein the leach solution also contains cobalt.

3. A method as claimed in claim 2 wherein step (b) involves providing the leach solution under pressure to one or more membranes, said membrane being selected such that bivalent cations cannot pass through the membrane whereas monovalent cations can pass through the membrane such that divalent metals that are dissolved in the leach solution (including nickel and cobalt) do not pass through the membrane and therefore form part of a concentrated reject stream but water and monovalent solvent ions can pass through the membrane, thereby recovering the solvent from the leach solution.

4. A method as claimed in claim 3 wherein the solvent recovered from step (b) is returned to the leaching step for re-use.

5. A method as claimed in claim 1 wherein step (d) comprises the steps of:

(e) subjecting the nickel-containing solution to ion exchange by contacting the nickel containing solution with an ion exchange resin to selectively remove nickel and cobalt from the solution; (f) stripping nickel from the ion exchange resin to form a solution containing nickel; and

6. A method as claimed in claim 5 wherein the solution also contains cobalt and steps (e), (f) and (g) comprise:

7. A method as claimed in claim 6 wherein the ion exchange resin selectively adsorbs nickel and cobalt from the solution.

8. A method as claimed in claim 7 wherein treated solution from the ion exchange step (which is depleted in nickel and cobalt) is treated to recover further solvent therefrom and/or treated to remove other metals therefrom.

9. A method as claimed in claim 6 wherein the ion exchange resin containing adsorbed nickel and cobalt is treated to strip the nickel and cobalt therefrom such that a cobalt-rich stream is removed from the loaded ion exchange resin, followed by removal of a nickel-rich stream from the ion exchange resin.

10. A method as claimed in claim 9 wherein the solutions recovered from stripping the loaded ion exchange resin are treated to recover nickel and cobalt therefrom or treated to recover nickel-containing compounds and cobalt-containing compounds.

11. A method as claimed in claim 10 wherein the streams are subjected to a crystallization step to recover crystals of a nickel compound and crystals of a cobalt compound.

12. A method as claimed in claim 11 wherein the nickel compound and the cobalt compound is a carbonate, a sulphate, or an oxide.

13. A method as claimed in claim 1 wherein either the pregnant leach solution or the concentrated metal ion solution leaving step (b) is treated to reduce ferric ions to ferrous ions.

14. A method as claimed in claim 13 wherein the solution is treated such that copper is precipitated in the form of sulphides by use of sodium sulphide or hydrogen sulphide gas and any residual ferric ions are reduced to ferrous ions.

15. A plant for recovering nickel, the plant comprising a leaching stage for producing a leach solution containing dissolved nickel; a nano-fϊltration or reverse osmosis stage to remove nickel from the leach solution, an ion exchange stage to remove nickel from the leach solution, and a further treatment stage for stripping nickel from the ion exchange resin and recovering nickel or a nickel compound therefrom.

15. A plant as claimed in claim 14 further comprising combinations of one or more of the following:

e) a bulk reduction stage to convert ferric ions to ferrous ions

f) aseparate ion exchange process capable of selectively adsorbing ferric ions

g) a chemical precipitation for trace copper removal that also acts to reduce any remaining ferric ions to ferrous ions

h) a scavenging chemical reduction to ensure complete conversion of ferric to ferrous ions

16. A method for treating a solution containing dissolved nickel and/or cobalt and ferric ions to selectively remove nickel and/or cobalt from the solution comprising the steps of: i) treating the solution to reduce ferric ions to ferrous ions or remove ferric ions;

iii) separating the solution from the ion exchange resin

17. A method as claimed in claim 16 wherein the solution also contains dissolved copper and the method further comprises removing dissolved copper from the solution prior to step (ii).

18. A method as claimed in claim 17 wherein the dissolved copper is removed from solution by converting the dissolved copper into copper sulphide.