PROCESS FOR PRODUCING REFINED NICKEL AND OTHER PRODUCTS FROM A

MIXED HYDROXIDE INTERMEDIATE

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a process for the refining of an impure nickel containing intermediate, to produce a refined nickel metal, preferably a solid metallic nickel powder. The process may also involve the recovery of a nickel and cobalt containing mixed sulphide precipitate, a copper precipitate and/or a zinc precipitate, whilst rejecting manganese and magnesium into the leach residue. In a preferred process sulphur is recovered as ammonium sulphate.

In particular, the process relates to the processing of a mixed hydroxide intermediate, preferably a nickel-cobalt-manganese-magnesium hydroxyl-sulphate intermediate, which may also contain zinc and copper. Such intermediates are also known as Mixed Hydroxide Product (or Precipitate) or "MHP" or as referred to herein as a "mixed hydroxide intermediate".

Description of the Related Art

Recovery of nickel as an intermediate product for subsequent refining to a metallic product is well established, with mixed sulphide mattes from smelting operations and mixed sulphide precipitates from leaching operations being common examples.

Precipitation of intermediates from sulphuric acid leaching liquors as hydroxides and hydroxy-sulphates has been under consideration over an extended time period, as discussed for example in U.S. Patents 1 ,091 ,545, USP 2,899,300 and USP 3,466,144 and Canadian Patent 618,826.

Nickel containing intermediates produced by addition of an alkali reagent are commonly referred to as mixed hydroxide precipitates (or products) or MHP, particularly when they contain significant quantities of cobalt and manganese and minor quantities of zinc and cobalt, as when they are derived from nickel laterite acid leach solutions or nickel sulphide acidic pressure oxidation leach solutions. In addition, if using magnesia as the precipitating agent, they typically contain some unreacted magnesium. Such mixed hydroxide intermediates typically contain 3% to 5% sulphur in the form of sulphate (9 to 15% sulphate), so can more properly be termed a mixed hydroxy-sulphate or basic sulphate.

U.S Patent 1 ,091 ,545 describes a process for producing a nickel hydrate product with magnesia, from a purified solution produced from acid leaching of nickel silicate ore.

Canadian Patent 618,826 describes a process that includes producing a nickel and cobalt intermediate product using an alkaline reagent, from a purified solution produced from pressure acid leaching of a lateritic nickel ore. The nickel and cobalt containing intermediate product may be re-leached in ammonia compounds.

U.S. Patent 2,899,300 includes a description for producing a mixed nickel/cobalt intermediate, using magnesia and provides an analysis, but does not give an indication of a suitable refining route.

U.S. Patent 3,466,144 describes a process which includes producing a nickel-cobalt- manganese containing intermediate product using an alkaline reagent, from a purified solution produced from pressure acid leaching of a lateritic nickel ore, followed by refining of the intermediate by leaching in ammonium sulphate with ammonia added. No information is given on the leaching conditions that may be employed.

Patent Application WO 01/6298 in the name of Anaconda Nickel Limited describes a process which includes leaching a MHP in ammoniacal ammonium sulphate solutions of approximately 500 to 650 g/L ammonium sulphate concentration. The nickel in solution is reasonably dilute, such that its concentration is increased by evaporation of part of the water from the solution. The solution then undergoes a nickel-cobalt separation by solvent extraction, before hydrogen reduction of separate nickel and cobalt solutions. A bleed of the nickel-free ammonium sulphate solution is taken and treated with lime, to form gypsum and magnesium sulphate to enable the ammonia to be recovered by distillation. Magnesium sulphate is removed from the leach circuit in this manner, as indicated by Figure 4 of the Anaconda specification. The process described suffers the disadvantages of a low nickel tenor, requiring steam for concentration. There is also the need to recover ammonia by a two-stage process, requiring lime and steam, so that magnesium can be bled from the leach circuit. Subsequently it has the disadvantage of being costly to operate.

U.S. Patents 5,855,858 and 6,383,460 in the name of Cominco Engineering Services Ltd. both describe processes that include leaching a nickel/cobalt hydroxide that also contains magnesium, copper and zinc having been removed in upstream process steps. The leaching of the mixed hydroxide is carried out in ammonium sulphate solution with no free ammonia and produces a leach solution with a recommended maximum nickel concentration 10 g/L, although a wider range "of about 3 to 25 g/L" is claimed. It is then followed by three solvent extraction (SX) circuits. In the first SX circuit cobalt is extracted. In the second SX circuit magnesium is extracted. In the third SX circuit, nickel is extracted, then stripped back off the organic reagent into sulphuric acid so that it may be concentrated into a form suitable for electrowinning.

U.S. Patent 6,171 ,564, also in the name of Cominco Engineering Services Ltd also describes a process which includes leaching a nickel/cobalt hydroxide, also including a second ammonia leaching stage, where the nickel/cobalt hydroxide is derived from processing a laterite ore and contains manganese.

Due to the dilution of the ammonium sulphate leach liquor with various wash water streams, water must be evaporated to increase the ammonium sulphate strength, so that it is suitable for recycling as fresh leach liquor.

This process clearly has a number of disadvantages. Separation of cobalt and magnesium from nickel requires two solvent extraction processes. Nickel must be concentrated to a level suitable for metal recovery by a third solvent extraction process. Water must be evaporated from the leach liquor so that it is at a suitable concentration for recycling to the nickel/cobalt hydroxide leach.

U.S. Patent 5,976,218 in the name of Henkel Corporation describes a process that includes leaching a nickel/cobalt hydroxide in an ammonia - ammonium carbonate solution, with a titratable ammonia level of between 40 and 100 g/L. The nickel tenor of this solution is around 10 g/L. It is followed by an oxidation step to precipitate manganese and ensure cobalt is in the three plus (3+) oxidation state, then a nickel solvent extraction step to separate nickel and cobalt and concentrate the nickel for subsequent electrowinning. Cobalt is recovered by selective precipitation or solvent extraction. Like the above described patents this process suffers disadvantages of having low nickel concentration, and requiring a nickel-cobalt separation by solvent extraction. Water balance is maintained by steam stripping to recover ammonia and carbon dioxide from a bleed stream.

Another variant of this process is described in U.S. Patent 6,701 ,314. In this variant, a reducing agent in the form of a mixed nickel-cobalt sulphide or hydroxylamine sulphate is added to an ammonia - ammonium carbonate leach solution to enhance dissolution of nickel and cobalt from a mixed hydroxide. Leach solution concentrations of up to 16 wt% ammonia, 12 wt% carbon dioxide and 1.5 wt% nickel (approximately 160 g/L ammonia, 120 g/L carbon dioxide and 15 g/L nickel) are claimed.

Descriptions of similar processes have been provided in the literature related to commercial adaptations of this flowsheet, including three by the same authors as U.S. Patent 5,976,218. A 2007 paper by Fittock, one of the inventors listed in U.S. Patent 6,701 ,314, describes the industrial application of this process variation, which includes construction of new refining capacity, nickel solution concentration of 23 g/L after MHP leaching, a nickel solvent extraction step to separate nickel and cobalt and concentrate the nickel, and subsequent nickel recovery by steam distillation to form basic nickel carbonate, which is then calcined and sintered to produce nickel oxide and nickel metal products.

Whilst these processes have been successful in producing refined nickel from mixed hydroxide intermediates, they suffer from a number of disadvantages in addition to those described above, including sulphate contamination of the leaching solution, and the requirement to build new refining capacity at high capital cost.

A number of published papers have included descriptions of leaching nickel/cobalt hydroxide in ammonia-ammonium sulphate solutions. A paper by Mason and Hawker presented at the ALTA 1998 Nickel Cobalt Conference provides a detailed description of pilot plant work on the Ramu Nickel / Cobalt project. The nickel/cobalt hydroxide was produced using lime, so is heavily contaminated with calcium sulphate (gypsum). The leach solution was described as containing 45 to 65 g/L free ammonia and 135 g/L ammonium sulphate. After leaching the solution contained 12 to 14 g/L nickel, with initially 1000 mg/L manganese, reducing to less than15 mg/L with aeration (oxidation with air). Similar to U.S. Patent 5,976,218, downstream process steps included an oxidation step to precipitate the remaining manganese and oxidise cobalt to the three plus (3+) oxidation state, nickel solvent extraction to separate nickel and cobalt and concentrate the nickel for subsequent electrowinning and cobalt and zinc recovery by precipitation. Extraction of nickel in the ammonia-ammonium sulphate leach was relatively modest at around 90%, requiring a further acid leaching treatment to increase recovery. Ammonia was recovered by a lime boil, a two stage process, requiring lime and steam, so that water could be bled from the leach circuit. This process has the disadvantages of the processes described previously and in addition requires an additional acid leaching step to achieve high nickel recovery. A paper by Steemson at the ALTA 1999 Nickel Cobalt Conference gives a comparison of various aspects of processing mixed hydroxide precipitates, with particular reference to the Ramu project discussed by Mason and Hawker. This paper gives typical mixed hydroxide assays based on using either a high quality commercial magnesia (MgO) or lime (either CaO or Ca(OH) ₂ ), as follows:

During the re-leach of the lime based mixed hydroxide precipitate in ammonia-ammonium sulphate solution, it was noted that 50 to 70% of the magnesium extracted and calcium reached saturation concentration in solution of 500 to 700 mg/L. In particular, the paper states that, "It is clear that magnesium buildup would be excessive if an ammonia / ammonium sulphate leachate were treating an MgO based nickel / cobalt hydroxide." This is a distinct disadvantage of this combination and process.

It is apparent that the above described processes each suffer from more than one of the following disadvantages:

1 . Nickel concentrations in the leach solution are less than 30 g/L, meaning further concentration steps are required for economic recovery of a final nickel product;

2. Initial manganese concentrations in solution are elevated, requiring further treatment to reduce them to manageable levels;

3. Magnesium is soluble in ammonium sulphate solution, which would lead to contamination of co-product ammonium sulphate crystal, resulting in the need to recover ammonia by other means;

4. Cobalt must be separated from nickel by a solvent extraction step; and

5. They are new flowsheets, requiring significant capital expenditure for new plant and equipment.

An ammonia based nickel refining process which has been used on nickel sulphide concentrates and nickel mattes is the Sherritt Gordon pressure leach - pressure reduction process. This process has been described in detail in a number of papers and in the book "The Winning of Nickel". In this process, nickel sulphide feed, containing nickel, cobalt, copper, zinc, iron and sulphur is oxidatively leached in an ammonia - ammonium sulphate solution at temperatures up to 100 °C and pressures up to 1000 kPag using ammonia, to form metal ammine complexes in an ammonium sulphate solution. Copper removal is combined with an ammonia removal step known as "copper boil". This adjusts the solution chemistry, so that cobalt does not substantially contaminate nickel produced in pressure reduction. Zinc does not reduce in pressure reduction, so is left in solution to be recovered along with cobalt and residual nickel as part of a mixed cobalt-nickel-zinc sulphide product.

In one variant of this process, as described for instance in U.S Patents U.S. 5,468,281 , U.S. 6,264,904, and U.S. 6,267,800, as well as a number of publications, mixed nickel-cobalt sulphides, undergo a conventional ammonia - ammonium sulphate leach, as described above, followed by nickel-cobalt separation by fractional crystallisation to produce a cobalt(lll) hexamine salt and a nickel solution. Nickel is then recovered via the copper boil, and subsequent conventional oxydrolysis and hydrogen pressure reduction steps. Cobalt is re-dissolved then recovered by hydrogen pressure reduction.

In another variant, this process has been modified to produce a leach solution with a nickel concentration feeding reduction in excess of 100 g/L.

To date the variants of the Sherritt Gordon process have not treated feeds high in manganese and magnesium, due to potential for contamination of refinery products, in particular manganese in nickel metal and magnesium in ammonium sulphate crystal.

U.S. Patent 3,816,098 (G.B. Patent 1 ,439,380) in the name of Sherritt Gordon Mines Limited describes a process to treat a partially refined basic nickel carbonate feed from which cobalt and copper have been removed prior to precipitation and containing 0.1 to 0.2% magnesium and 0.1 to 0.2% manganese. After leaching in ammonia - ammonium sulphate solution, 92.7% of the magnesium and 60% of the manganese were extracted. After solid-liquid separation and oxidation at >200 °C, the resulting solution contained 44.2 g/L nickel, 2.4 g/L magnesium and 2.0 g/L manganese. Nickel was reduced by hydrogen under pressure, to produce nickel metal powder containing 1 1 g/t magnesium and 16 g/t manganese. The build-up of magnesium and manganese was controlled by bleeding contaminated ammonium sulphate solution from the process. This patent is silent on methods by which the contaminated bleed solution is treated to dispose of the magnesium and manganese and recover the ammonium sulphate. Thus, this process too suffers from a number of disadvantages, including: 1 . The requirement to substantially remove magnesium, manganese, cobalt and copper ahead of the leaching process;

2. Substantial dissolution of magnesium and manganese in the leach;

3. Nickel metal powder contamination with magnesium and manganese substantially higher than typical in hydrogen pressure reduction;

4. The requirement to bleed magnesium and manganese contaminated ammonium sulphate solution from the process.

The present invention provides a process for producing refined nickel metal free of manganese impurities from a mixed hydroxide intermediate that may generally contain significant quantities of magnesium and manganese. It also provides a process for producing copper sulphide, mixed cobalt-nickel-zinc sulphide and ammonium sulphate products, free from unacceptable levels of magnesium and manganese contamination.

The present invention aims to overcome one or more of the difficulties or disadvantages identified in the prior art documents.

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process to produce a refined nickel metal product from a mixed hydroxide intermediate. The process involves ammonia leaching a slurry containing the nickel and recovering the refined nickel product by direct hydrogen reduction of the resultant leach solution under pressure. In a preferred embodiment, other products may also be recovered from the leach solution, for example cobalt, zinc, copper and other nickel products from the residual nickel remaining in the leach solution, while also recovering ammonia as ammonium sulphate for reuse in the leach process, and/or as a saleable product. The process has the advantage of rejecting magnesium and manganese impurities from the liquor stream prior to the recovery of the metal values.

The mixed hydroxide intermediate product may be produced as an intermediate product in the processing of both nickel laterite and nickel sulphide materials including concentrates. It is also commonly referred to as a mixed hydroxide precipitate (or product), or MHP, but herein is referred to as a "mixed hydroxide intermediate".

The mixed hydroxide intermediate may be recovered from a sulphuric acid leaching liquor during the processing of a nickel laterite ore. For example, such intermediate products are produced when an alkali reagent such as magnesia or lime is used to raise the pH of an acidic sulphate solution to precipitate a nickel product as a mixed hydroxide intermediate. Such intermediates are also recovered from nickel sulphide acidic pressure oxidation leach solutions by the addition of a comparable alkali material.

Typically, a mixed hydroxide intermediate produced from processing a nickel laterite or nickel sulphide ore, contains the main element of interest, namely nickel, together with variable quantities of metals such as cobalt, zinc and copper as well as sulphur in the form of sulphates, and impurities such as manganese, magnesium, iron and calcium. Sodium and chlorine, in the form of chloride, may also be present.

The process of the present invention provides a process where a refined nickel metal product may be recovered from the mixed hydroxide intermediate by mixing the mixed hydroxide intermediate with a sulphide containing material having reductant properties, most preferably as ground nickel matte, or nickel containing sulphide or concentrate, which in general are impure nickel containing sulphide intermediate products from smelting, selective precipitation or concentration processes.

In the process of the present application, the combined sulphide containing reductant material / mixed hydroxide intermediate is formed into a slurry and leached under pressure with ammonia to dissolve any nickel, cobalt, copper and zinc while rejecting the majority of the manganese and magnesium. Ultimately, the nickel in the solution is treated, preferably with the addition of hydrogen at elevated temperature and pressure to reduce the nickel to a metallic nickel product, most preferably a metallic nickel powder. The copper, cobalt and zinc may also be recovered in additional sulphiding recovery stages.

Accordingly, in a first embodiment of the present invention there is provided a process for the recovery of a refined nickel metal product from a mixed hydroxide intermediate that has been produced from the processing of a nickel laterite or sulphide ore, the process including the steps of:

a) providing a mixed hydroxide intermediate and blending with a solution to form an intermediate slurry; b) mixing the mixed hydroxide intermediate slurry with a sulphide containing material, preferably ground nickel matte and/or nickel containing mixed sulphide, to form a combined slurry;

c) leaching the combined slurry with ammonia in an ammonia pressure leach step to dissolve the contained nickel, cobalt, zinc and any copper present into a leach solution, while maintaining the majority of the manganese and magnesium in a leach residue;

d) adding a reducing agent preferably hydrogen, to the leach solution under pressure to reduce the nickel to metallic nickel product; and

e) recovering the solid metallic nickel from the resultant slurry.

Preferably, the metallic nickel product is a metallic nickel powder.

In a preferred embodiment of the process, the mixed hydroxide intermediate is washed with water, preferably desalinated water, or water containing low levels of dissolved salts, prior to forming the intermediate slurry, so as to substantially or partially remove any calcium, sodium and chloride that may be present. The washing particularly has the advantage of partially removing some magnesium and sulphate sulphur while converting the remaining magnesium to a less reactive hydroxide form.

The washed mixed hydroxide intermediate is then formed into an aqueous intermediate slurry. In a preferred embodiment, this step is achieved by the addition of ammonium sulphate at atmospheric pressure, either in the form of a solution or slurry dosed with ammonium sulphate crystals and preferably containing thiosulphates and polythionates, which enables partial reductive leaching of oxidised manganese, cobalt and nickel.

Preferably, the ammonium sulphate or at least a part of the ammonium sulphate used is recycled from the barren solution following the ammonia pressure leach step, while the ammonium sulphate crystals are recycled from a cobalt and zinc sulphiding recovery stage. The ammonium sulphate solution or slurry preferably has low titratable ammonia, in the range of 0 to 20g/L before addition of the mixed hydroxide intermediate.

The intermediate slurry is then mixed with a sulphide containing material, such as ground nickel matte or nickel containing mixed sulphide ahead the ammonia pressure leach step, or it may be mixed during the ammonia pressure leach step. The preferred ratio of mixed hydroxide to matte can vary over a wide range, depending largely on availability of feedstocks and economic considerations. Ratios of up to 1 part nickel in mixed hydroxide intermediate to 1 part nickel in matte or mixed sulphide are preferred, to avoid having to add external reductants, which can be expensive and make process chemistry more difficult to control.

The ammonia used in the ammonia pressure step is preferably in the form of a gas, liquid or aqueous solution. The pressure leach step is preferably conducted at a temperature of from 70 to 120°C and a pressure of from 600 to 1000 kPag in the presence of oxygen and carbon dioxide. This will produce a high concentration of nickel in the leach solution, typically 70 to 1 15 g/l. The manganese is oxidised to form an insoluble manganese containing residue. The carbon dioxide is absorbed into the ammonia solution and reacts with magnesium to form an insoluble basic magnesium carbonate compound, and to enhance magnesium and manganese precipitation. Preferably, reductants such as metal sulphides, thiosulphates or polythionates are also included into the ammonia pressure leach solution so as to maximise leaching of nickel and cobalt from the mixed hydroxide intermediate.

In a preferred embodiment, the leach slurry will undergo a solid/liquid separation step after this first stage of ammonia pressure leaching, where the leach solution is sent for downstream processing, while the partially leached solids are subjected to a second stage ammonia pressure leach. Air is preferably added to this second stage leach. After a further solid/liquid separation, the first and second stage leach solutions are combined for further processing.

Preferably a third stage ammonia pressure leach of the remaining solids is included with the leach solution added to that of the final and second stage leach.

The barren ammonia liquor from this stage may be recycled to the atmospheric leach stage in forming the intermediate slurry, or to the ammonia pressure leach step.

It is further preferred that the combined leach solutions containing nickel, cobalt, copper and zinc is first treated to remove copper from the solution before recovery of the nickel, cobalt and zinc. The copper may be separately precipitated from the solution in a copper boil process. In this process it is also preferred to adjust the titratable ammonia to nickel molar ratio in the leach solution to the range of 1.8 to 2.2 : 1 , most preferably around 2 : 1 , so as to ensure a minimal amount of cobalt is recovered as metal in the downstream nickel reduction step. Copper may also be removed from solution by other techniques such as precipitation with a suitable sulphiding reagent, such as hydrogen sulphide. Following the copper removal step, the leach solution preferably undergoes oxydrolysis where it is oxidised with air and heated to remove any remaining thiosulphate, polythionate and sulphamate compounds to avoid potential contamination of nickel and ammonium sulphate products. The leach solution then flows to the hydrogen reduction step where hydrogen is added to the solution under pressure at elevated temperature, so as to reduce the nickel to a nickel metal powder. Nickel may be recovered by powder/solution separation techniques and the nickel powder formed into other forms such as briquettes.

The process lends itself to the further benefit in that cobalt and zinc, in particular, may be recovered from the leach solution together with any residual nickel that may remain in the leach solution. In a preferred embodiment, a suitable sulphiding agent such as hydrogen sulphide, sodium hydrosulphide, sodium sulphide or ammonium sulphide is added to the leach solution following the step of the metallic nickel powder recovery, to recover the cobalt, zinc and any residual nickel as a mixed cobalt/nickel/zinc sulphide product. Alternatively, the zinc may advantageously be recovered in a two-stage sulphiding process where the zinc is recovered first in the sulphiding process as a zinc sulphide product, and then the cobalt and any residual nickel recovered in a second stage of the sulphiding process as a mixed nickel/cobalt sulphide product.

The process also has the advantage that any excess ammonium sulphate formed during this process, may be recovered by crystallisation of ammonium sulphate from the solution. Preferably, steam is used to evaporate the water to enable crystallisation. At least a portion of the recovered ammonium sulphate crystals may then be recycled, for use in the atmospheric leach step while forming the intermediate slurry, or the ammonia pressure leach step.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic flowsheet of the overall process of a preferred embodiment of the invention.

Figure 2 is a schematic flowsheet of the leaching phase of the process described in Figure 1. Figure 3 is a schematic flowsheet of the product recovery phase of the process described in Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Mixed hydroxide intermediate 100 is water washed 110 using desalinated water 120, to remove or partially remove contaminants. These include magnesium, calcium, sulphate, chloride, and sodium components. The water wash can consist of the washing of a filter cake on a filter, re-pulping of the mixed hydroxide intermediate in water to give a "water leach", a combination of these, or other techniques such as continuous counter-current decantation. Multiple stages of water washing have been found to be beneficial as has extended leaching times of up to several days. Multiple contacts are especially advantageous for calcium removal, as calcium will continue to leach out of the mixed hydroxide intermediate, constrained only by its solubility limit. An additional benefit of the water wash has been found to be the "fixing" of most of the remaining magnesium in the mixed hydroxide intermediate, so that it does not substantially dissolve in downstream ammonia leaching operations. The waste wash liquor 130 must be disposed of separately to other refinery liquors, so the water washing step is preferably carried out at the location where the mixed hydroxide intermediate is produced.

The washed mixed hydroxide intermediate 140 is initially leached 150 in an ammonium sulphate solution consisting of liquor recycled from the third stage pressure leach 160, dosed with ammonium sulphate crystals 170. This leach liquor 190 typically contains 0 to 20 g/L of titratable ammonia, 500 to 600 g/L of ammonium sulphate, and 3 to 6 g/L of thiosulphate and polythionates, enabling reductive leaching of oxidised manganese and cobalt. The ammonium sulphate crystal dosing 180 and mixed hydroxide intermediate leaching 150 may be carried out in a combined step. In addition to nickel and cobalt, a significant portion of the manganese goes into solution, while most of the magnesium does not dissolve. The resultant mixed hydroxide intermediate leach slurry 200 is forwarded to the first stage ammonia pressure leach 210 where it is mixed with ground nickel matte 220. Alternatively washed mixed hydroxide intermediate 145 may be directly blended with ground nickel matte 220 (Figure 2), or the nickel matte 205 in matte grinding step 215 as shown in Figure 1 , ahead of first stage ammonia pressure leach. Air 230 and ammonia 240 are added to leach the nickel matte - mixed hydroxide intermediate blend to produce a high nickel concentration in solution - typically 70 to 1 15 g/L, and oxidise manganese to form an insoluble manganese containing leach residue. Carbon dioxide, which is naturally in air at a concentration of 0.04% by volume or 0.06% by mass, is absorbed into the ammonia solution and reacts with magnesium to form an insoluble basic magnesium carbonate compound. Carbon dioxide can also be added to the leach to enhance magnesium precipitation.

After the first stage leach, the leach slurry 250 undergoes solid-liquid separation 260. The first stage leach liquor 270 is forwarded to downstream processes for further purification and nickel recovery 280. The partially leached solids 290 are subjected to a second stage ammonia pressure leach 300. Air 310 and ammonia 320 are again added to leach the partially leached solids to produce a high concentration nickel solution. If necessary, mixed hydroxide intermediate leach slurry 330 is added to increase the ammonium sulphate concentration to control the leach chemistry. After the second stage leach, the leach slurry 340 undergoes solid-liquid separation 350. The second stage leach liquor 360 is mixed with the first stage leach liquor for downstream processing 280.

The partially leached solids 370 are subjected to a third stage ammonia pressure leach 380. Air 390 and ammonia 400 are again added to leach any remaining nickel. Mixed hydroxide intermediate leach slurry 410 may again be added to increase the ammonium sulphate concentration. The third stage leach slurry 420 undergoes solid-liquid separation 430. The leach residue solids 440 may be further treated by filtration and washing to produce a final filter cake suitable for either disposal or further treatment for metals recovery. The third stage leach liquor 160 is forwarded to the ammonium sulphate crystal dosing tank 180.

Air may be added as separate streams to each pressure leaching stage as shown in Figure 2, or it may be added in a counter-current manner, such that the exhaust air from the Stage 3 leach becomes the feed air to Stage 2 leach 310 and the exhaust air from Stage 2 leach becomes the feed air for Stage 1 leach 230.

The combined first and second stage leach liquors 280 are forwarded to Copper Boil 510. Here copper is precipitated from solution as copper sulphide product 520, and titratable ammonia levels are adjusted, by the addition of steam 530 and sulphuric acid 540, to a ratio of around 2 moles of ammonia per mole of nickel, in preparation for nickel reduction. The reduction feed solution 550 is forwarded to Oxydrolysis 560 where it is oxidised with air 570 and heated with steam 580 to remove any remaining thiosulphate, polythionate and sulphamate compounds which would otherwise cause product contamination. The solution 590 then flows into Nickel Pressure Reduction 600, where hydrogen 610 is added in a batchwise manner to an agitated Autoclave to produce nickel metal powder. Nickel pressure reduction discharge 620 undergoes powder - solution separation 630. Metallic nickel powder product 640 may be transformed into other forms such as briquettes by drying and compacting in a conventional manner. The reduction end solution 650 is then treated with a sulphiding reagent such as hydrogen sulphide 660 in a mixed sulphide precipitation step 665 to produce a mixed cobalt-nickel-zinc sulphide product 670. The barren ammonium sulphate solution 680 passes to ammonium sulphate crystallisation 690. Steam 700 is used to evaporate the water for crystallisation. Crystalliser configurations can include multiple effect evaporators, vapour recompression or a combination of both. Ammonium sulphate crystal slurry 710 undergoes crystal - liquor separation or drying 720, with the liquor 730 returned to crystallisation. Ammonium sulphate crystal is directed to both product 740 and Ammonium Sulphate Crystal Dosing 750 of Third Stage Leach liquor ahead of mixed hydroxide intermediate leaching.

Whilst the steps from Copper Boil through to mixed sulphide precipitation are carried out in a conventional manner, a surprising aspect of this flowsheet is that the small quantity of manganese remaining in solution after the three stage pressure leach , remains in solution throughout the remaining process steps so does not contaminate copper sulphide, nickel metal or mixed cobalt-nickel-zinc sulphide products.

Examples

Example 1

Typical assays of the mixed hydroxide intermediate and nickel matte refinery feeds are shown in the table below. In addition, a typical precipitated nickel sulphide analysis is provided, as this is a feed that may partially or totally substitute the nickel matte.

Those skilled in the art would understand that the compositions of the feeds can vary over a significant range from those presented above.

Mixed hydroxide intermediates can be highly variable in composition. Iron, aluminium and silicon levels vary depending on the efficiency of upstream impurity removal processes. Manganese and cobalt levels vary depending on their concentration relative to nickel in the feed solution to the hydroxide precipitation step, which ultimately reflects on the ore or concentrate feed composition to the leaching process upstream of the mixed hydroxide precipitation step.

Nickel matte may have differing levels of iron content, depending on the extent to which iron is removed in the smelter converting process. Copper and cobalt levels can also vary significantly, depending on the concentrate feed to the smelter and the converting process used.

Mixed sulphide precipitates will have varying levels of cobalt, also depending on the on the ore or concentrate feed composition to the leaching process upstream of the mixed sulphide precipitation step.

Example 2

Typical analyses of the leach residue with nickel matte only and with combined feeds are shown in the following table. From the assays it is clear that manganese and magnesium from the mixed hydroxide intermediate are reporting to the leach residue.

Example 3

Typical analyses of the copper sulphide product with nickel matte only and with combined feeds are shown in the following table. From the assays it is clear that manganese and magnesium from the mixed hydroxide intermediate are not reporting to the copper sulphide product. Copper Sulphide Product Analysis, dry basis

Cu, s, Mn, Mg, wt% wt% g/t g/t

Nickel Matte Only Feed 60 35 <1 <2

Combined Nickel Matte plus Mixed Hydroxide Intermediate 60 35 <1 <2

Example 4

Typical analyses of the nickel pressure reduction feed liquor with nickel matte only and with combined feeds are shown in the following table. Manganese has been substantially removed from solution at this point, while reporting to leach residue and not contaminating copper sulphide product. Magnesium has increased, but remains at least an order of magnitude below which it would dilute the nitrogen content of the ammonium sulphate product by any significant amount.

Example 5

Typical analyses of the nickel metal product with nickel matte only and with combined feeds are shown in the following table. From the assays it is clear that manganese and magnesium from the mixed hydroxide intermediate are not reporting to the nickel metal product.

Example 6

Typical analyses of the mixed nickel-cobalt-zinc sulphide product with nickel matte only and with combined feeds are shown in the following table. From the assays it is clear that zinc reports to this product, while that manganese and magnesium from the mixed hydroxide intermediate are not reporting to the mixed nickel-cobalt-zinc sulphide product.

Example 7

Typical analyses of the ammonium sulphate product with nickel matte only and with combined feeds are shown in the following table. From the assays it is clear that manganese and magnesium from the mixed hydroxide intermediate is not reporting to this product in excessive quantities.

The limits of mixed hydroxide intermediate addition depend primarily on economic drivers. As the level of cobalt in refinery feed increases, an equivalent mass of nickel is typically not reduced and reports to mixed sulphide. As a saleable product, the revenue received for the contained nickel in this stream is less than if sold as nickel metal. One variant to overcome this limitation is to add the cobalt hexamine precipitation process, as discussed for instance in U.S. Patent 5,468,281 , into the flowsheet. Zinc may reach an economic limit, as refiners of cobalt-nickel-zinc mixed sulphide typically have limits on zinc levels and/or apply penalties for high zinc feeds. This limitation may be overcome by producing separate zinc sulphide and mixed sulphide products by a two stage sulphiding process. The relative quantities of thiosulphates and polythionates that aid manganese and cobalt dissolution will reduce as the proportion of nickel feed from the mixed hydroxide intermediate increases, but this can be compensated for by the addition of other reductant species.

As the relative quantity of mixed hydroxide intermediate increases, the ability of the leach process to produce a nickel leach solution of high nickel tenor may become a limitation. This again is an economic rather than a technical limitation, with a wide range of nickel concentrations contemplated, for instance, US Patent 6,949,232 stating, "The leach solution produced will typically contain from 40 to 1 10 g/L nickel..."

Thus, within the constraints described above, there is no particular limitation to the relative quantity of mixed hydroxide that may be processed.

Advantages of processing the mixed hydroxide intermediate in this manner are:

1 . Nickel pressure reduction capacity is not compromised because the nickel concentration is maintained, due to the co-processing of feedstocks.

2. Capacity increases in the ammonia pressure leach are possible due to the lower air requirements for oxidising manganese in mixed hydroxide intermediate, relative to sulphur and iron in nickel sulphide matte. This also results in lower operating costs.

3. Refining costs are lowered due to lower sulphur levels in mixed hydroxide intermediate, relative to nickel matte. The typical sulphur to nickel mass ratio in mixed hydroxide intermediate is 1 to 12, whereas in nickel matte it is 1 to 3. This results in reduced ammonia make up requirements for producing ammonium sulphate. It also results in lower steam usage for crystallising and drying the ammonium sulphate so produced.

4. Copper & zinc do not have to be removed from the mixed hydroxide intermediate feedstock.

5. It is also contemplated that the manganese from the mixed hydroxide intermediate sequesters arsenic and selenium which may be present in the nickel sulphide matte, allowing reduced quantities of iron to be included in the refinery feed for sequestering these elements.

The invention described herein is susceptible to variations, modification and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.

Further patent applications may be filed in Australia or overseas on the basis of, or claiming priority from the present application. It is to be understood that the following provisionial claims are provided by way of example only and are not intended to limit the scope of what may be claimed in any such future application.

Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions.