SODIUM CHLORIDE PROCESSING OF NICKEL SULPHIDE ORE OR CONCENTRATES

FIELD OF THE INVENTION The present invention relates to a new hydrometallurgical method for recovering nickel from nickel sulphide bearing raw materials, such as sulphide concentrate or ore. In general, the process involves the oxidative atmospheric pressure leaching of nickel sulphide concentrate or ore using a sodium chloride/hydrochloric acid based leach media.

BACKGROUND OF THE INVENTION

The world nickel resources are divided into two major categories, sulphide ore and oxidised ore (laterite ore). The conventional exploitation of nickel sulphide ore is essentially a pyrometallurgical process, where the mined ore is then finely ground, and the nickel sulphide minerals concentrated by froth flotation to produce a nickel concentrate. The concentrate is then treated further by smelting and reduction to produce a nickel bearing matte, which contains also copper, cobalt, and iron. The matte is then refined by known hydrometallurgical processes, which might include oxidative leaching or pressure leaching, followed by impurity removal and hydrogen reduction or electrowinning.

A drawback of the smelting process is the generation of sulphur dioxide, which has to be treated in an acid plant to produce sulphuric acid, a product that is not always easy to dispose of from the smelter location. Losses of nickel and cobalt into smelter slag are significant, and there can be problems in dealing with some of the minor elements in concentrates, such as magnesium and arsenic.

A number of other hydrometallurgical routes for processing nickel sulphide concentrates have been discussed in the literature, generally relying on grinding, or fine grinding of the concentrate, followed by oxidative

pressure leaching of the sulphide to produce sulphuric acid for the leach process.

Biological treatment of nickel sulphides has also been described, where bacterially assisted leaching is followed by solution purification, metal separation, and electrowinning of nickel. The long residence times required for this type of process has resulted in extremely large reactors for the leach stage, and the process has therefore not achieved commercial success to date due to the large capital requirements.

The proprietary "Activox" process, which is described for example in

EP patent 1 303 641 , relies on an extremely fine grind of the nickel concentrate followed by high pressure oxidative leaching to extract the nickel into a sulphate solution, followed by known impurity removal steps and recovery of the metallic nickel.

The hydrometallurgical processes described above generally have the disadvantage that much of the sulphur content of the sulphide is oxidised to sulphuric acid, with high costs of reagents for neutralisation, and generation of large amounts of waste, such as ammonium sulphate or gypsum, to dispose of. As would be appreciated these two factors combine to make these processes commercially unattractive due to the high costs involved.

WO patent application 96/41029, "Chloride assisted hydro- metallurgical extraction of nickel and cobalt from sulphide ores", teaches the pressure oxidation leaching of nickel and cobalt from sulphide ores in the presence of oxygen, and an acid solution containing halide, copper, and sulphate ions. The leach slurry is treated by solids separation and solution purification, precipitation of mixed nickel and cobalt hydroxide, re-leaching of the precipitate in ammoniacal solution, followed by solvent extraction for metal separation and electrowinning of the metals. The process suffers from

similar limitations to the other sulphate based hydrometallurgical processes described above.

This invention aims to overcome some of the limitations of the existing processes by providing a low cost chlorine/chloride based process for nickel sulphide concentrate treatment.

The discussion of documents, acts, materials, devices, 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 before the priority date of each claim of this application.

SUMMARY OF THE INVENTION

In general, the present invention provides a hydrometallurgical method to recover nickel from nickel sulphide bearing raw materials, such as nickel sulphide concentrate or ore. The method is based on oxidative leaching of the sulphides with air or oxygen, sodium chloride solution and hydrochloric acid at atmospheric pressure. The process is applicable to all nickel sulphide concentrates, but is particularly suitable where the concentrate is difficult to treat by the conventional smelting methods.

In general, the method is aimed at recovery of nickel from sulphidic nickel concentrates, but it also encompasses recovery of the cobalt and copper, which are frequently found in nickel sulphide flotation concentrates.

Accordingly, in a first aspect of the present invention there is provided a method for the recovery of nickel from a nickel sulphide containing material including the steps of:

(a) providing a nickel sulphide containing material;

(b) oxidative leaching of the nickel sulphide containing material with a sodium chloride leach solution containing some cupric chloride and hydrochloric acid in an oxidising atmosphere and at atmospheric pressure to form a pregnant leach solution containing dissolved nickel; (c) treating the pregnant leach solution containing dissolved nickel to remove copper;

(d) purification of the nickel pregnant leach solution by the use of solvent extraction to remove cobalt, zinc and residual copper;

(e) recovering nickel from the pregnant leach solution to form a depleted leach solution; and

(f) electrolytically treating the depleted leach solution to recover chlorine, hydrogen, and sodium hydroxide.

The nickel sulphide containing material may be provided in any suitable form. It is preferred however that the nickel sulphide containing material is provided as fine particles. As such the method of providing a nickel sulphide containing material preferably includes the step of reducing the particle size of the nickel sulphide containing material to produce fine particles. Basically, the liberation of sulphide particles from the surrounding rock specifies the degree of grinding.

The nickel sulphide material contains also usually some copper. Certain amount of copper is preferable for nickel leaching but if the amount of copper in the raw material is higher than what is used assisting the leaching, the extra copper is recovered from recycling. If the raw material does not contain such an amount of copper it is preferable for the leaching to add some copper to the leaching stage. Copper can be added as copper concentrate, sludge or raw copper salt.

The oxidative leach is carried out at atmospheric pressure. In the preferred embodiment the oxidising atmosphere is air, oxygen, oxygen enriched air or a mixture thereof.

The leaching may be carried out at any suitable temperature but it is preferred that the leaching is carried out at a temperature of from 8O ⁰ C to 11O ⁰ C. The elevated leach temperature may be achieved in any of a number of ways but one way is to add steam to the leach vessel directly or indirectly.

All copper in the pregnant leach solution containing dissolved nickel is in the form of Cu(II) and the solution is treated to remove copper by before nickel recovery. A preferred method is to precipitate copper by adding alkali hydroxide or limestone to precipitate the cupric copper as copper hydroxy chloride. The material thus recovered is then re-subjected at least partly to the leaching stage to facilitate the efficient recycling of copper in the process. In this connection, the extra copper can be recovered from the process. Another method to separate copper from the pregnant leach solution is to feed the solution to solvent extraction for copper recovery.

In a further preferred embodiment the pregnant leach solution containing dissolved nickel is treated to remove zinc and cobalt. The zinc and cobalt may be removed by any technique well known in the art but are preferably removed by solvent extraction. If the extra copper is recovered by solvent extraction the copper extraction preferably takes place first and others after that.

The nickel may be recovered from the pregnant leach solution using any of a number of techniques well known in the art. It is preferably recovered by treatment of the pregnant leach solution containing dissolved nickel to precipitate nickel and to form a nickel depleted sodium chloride leach solution. In a particularly preferred embodiment the treatment involves addition of sodium hydroxide and the nickel is precipitated as nickel hydroxide.

Following recovery of the nickel in the form of nickel hydroxide it is preferred that the process includes the step of reducing the nickel hydroxide to metallic nickel. The reducing may be carried out in any way well known in the art but preferably includes contacting the nickel hydroxide with hydrogen gas.

In a preferred embodiment of the invention the process further includes removal of divalent metal ions from the nickel depleted leach solution. The removal of divalent metal ions of this type may be carried out by any method well known in the art but at least some of them are preferably removed by ion exchange.

The depleted leach solution is subjected to electrolytic treatment in chlorine alkali electrolysis to recover chlorine, hydrogen, and sodium hydroxide. These are re-used in the process of the invention thus providing significant cost savings.

In a particularly preferred embodiment the invention provides a method for the recovery of nickel from a nickel sulphide containing material including the steps of:

(a) providing a nickel sulphide containing material,

(b) oxidative leaching of the nickel sulphide containing material at atmospheric pressure with sodium chloride leach solution containing cupric chloride and hydrochloric acid in an oxidising atmosphere to form a pregnant leach solution containing dissolved nickel;

(c) treating the pregnant leach solution containing dissolved nickel to remove copper,

(d) subjecting the pregnant leach solution to solvent extraction to remove zinc and cobalt,

(e) treating the pregnant leach solution with sodium hydroxide to precipitate nickel from the pregnant leach solution as nickel hydroxide and to form a nickel depleted leach solution,

(f) removing divalent metal ions from the depleted leach solution, and (g) subjecting the depleted leach solution containing sodium chloride to electrolytic treatment in chlorine alkali electrolysis to recover chlorine, hydrogen, and sodium hydroxide.

LIST OF DRAWINGS Figure 1 presents a flow chart of one embodiment of the invention and

Figure 2 shows a flow chart of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for the recovery of nickel from a nickel sulphide containing material. Examples of suitable nickel sulphide containing material include a primary sulphide ore material, an ore concentrate obtained by froth flotation or from another extraction process, a tailings material containing residual nickel sulphide containing material.

Once the desired nickel sulphide containing material has been chosen it is then subjected to the further steps of the process of the invention. The nickel sulphide containing material may be in any suitable form for processing, but is required to be finely ground. Such a process may be carried out in any of a number of ways well known in the art but preferably involves grinding or comminution of the nickel sulphide containing material.

A preferred embodiment of process is summarised in Figure 1 and a discussion of this figure will serve to illustrate the invention. In the preferred embodiment, the nickel sulphide concentrate 1 is provided and then initially ground to an adequate fineness before addition to the leach stage.

The leach is preferably carried out at atmospheric pressure in a multistage counter current leach 2, with preferably at least two stages and with preferably a solids/liquid separation between stages. Each leach stage might typically be carried out in one or more agitated tanks. The leach liquor is preferably made up of sodium chloride solution such as sodium chloride solution from a chlorine alkali electrolysis cell 3 or hypersaline water. The most preferably the sodium chloride solution is a depleted solution of chlorine alkali electrolysis. Air, oxygen, or oxygen enriched air, are added to the leach to maintain the required oxidising conditions, and hydrochloric acid is added to maintain the required pH. Using of oxygen or oxygen enriched air is preferred since feeding of air results a large amount of nitrogen to be fed into the leaching reactors. It is preferably that the raw material contains some copper, which assists in the nickel leaching. It is expected that approximately 90% of the sulphide sulphur will be precipitated in the leach in elemental sulphur form, and will be disposed of in tailings, minimising sulphate take up in the leach solution.

After the last stage of the leaching 2 the solids are separated in a solids/liquid separation stage (not shown in figure) and the leach residue is disposed of the recycle. The solution from the last stage leaching is routed countercurrently to the earlier leaching stage. The sludge coming from the first leaching stage is forwarded to solids/liquid separation stage. Pregnant leach solution (PLS) comes from the first leaching stage.

The bulk of the pregnant leach solution is treated to remove all the cupric ions from the liquor in the Cu removal stage 5 by the addition of limestone or sodium hydroxide to precipitate copper hydroxy chloride (Cu2(OH) ₃ CI). This is separated and recycled at least partly to the leach stage 2. Sulphates especially CaSO ₄ that have possibly formed during leaching precipitate, too, and are recycled to the leaching stage. Soluble iron present in the pregnant leach solution co-precipitates with copper.

A portion of copper can also be removed from the cycle and recovered as a copper product if the amount of copper in the raw material is higher than what is adequate in the leaching stage 2. Another preferable way to the Cu removal of the pregnant leach solution is solvent extraction.

The pregnant leach solution is then preferably treated in a solvent extraction step 6 for further purification, where cobalt and zinc are extracted onto a suitable organic solvent. Cobalt and zinc are then water stripped from the organic solvent, and the cobalt recovered as either cobalt carbonate or cobalt sulphide by precipitation with either sodium carbonate or sodium sulphide respectively.

Nickel is recovered from the pregnant leach solution as nickel hydroxide in a nickel precipitation step 7, where nickel is precipitated out of solution by the addition of sodium hydroxide. Sodium hydroxide is recycled from the chlorine alkali electrolysis 3 of this process.

The nickel hydroxide can then be converted to nickel metal by known methods in a metallic nickel reduction step 8 with a reductant. One preferred method is to use hydrogen as reductant, since hydrogen is one of the chemicals produced in the chlorine alkali electrolysis stage 3 of this process. However, using of the hydrogen formed in the electrolysis depends on the balance of the chemicals produced.

The nickel depleted leach solution still typically contains undesirable divalent metal impurities such as magnesium, lead, and traces of zinc, cobalt and nickel that cannot be fed to chlorine alkali electrolysis stage. The divalent metal species are typically removed in impurities removal stage 9. The removal takes place for example by absorption on an ion exchange resin, followed by stripping of the resin with a hydrochloric acid solution to produce a predominantly magnesium chloride solution containing also the

other impurities. The chloride solution may then be sent to evaporation ponds or for recovery of the solid material if required.

After the impurities removal stage the depleted solution, which is pure sodium chloride solution, passes to the chlorine alkali electrolysis stage 3, where a part of the sodium chloride is converted to sodium hydroxide at the cathode, releasing hydrogen gas, and releasing chlorine gas on the anode. A portion of both the hydrogen and the chlorine gas produced are then typically sent to a hydrochloric acid production plant 10 to produce the hydrochloric acid requirements for the concentrate leach stage 2.

This chlorine alkali electrolysis stage produces the sodium hydroxide required for the nickel hydroxide precipitation and possibly for copper hydroxy chloride precipitation. Gaseous chlorine and hydrogen are used for the hydrochloric acid production. The hydrogen is also used for the reduction of nickel hydroxide to nickel metal. Any imbalances, which should be small, in the production of these three raw materials from the chlorine alkali electrolysis stage, can be made up by direct purchase of the materials.

In an embodiment of the present invention, the sodium chloride solution from the electrolysis stage 3 is mixed with a recycled pregnant leach liquor stream before feeding to the last leaching stage, as well as any make up brine or hypersaline water required for makeup and this can be made in a simple reactor (not shown in figure).

In another embodiment, where the concentrate contains significant amounts of gold, silver, and/or platinum metal group (PGM) elements, gold and silver are in soluble form in a solution coming from the last leaching stage and is recovered in a known manner. The Au recovery step 4 to recover these may be introduced, as indicated in Fig. 2. The solution from which Au has been recovered is routed back to the earlier stage of the leaching 2. PGM elements that are in sparingly soluble form in the conditions

of the nickel leaching stage 2 are recovered by known methods from the leach residue.

The process described for recovery of nickel from a nickel sulphide bearing raw material or concentrate by employing a sodium chloride, copper(ll) chloride and hydrochloric acid leach has the following advantages over existing processes.

The process can be used on nickel sulphide concentrates, which are unsuitable for processing by the conventional smelting process. Especially concentrates in which Fe/Mg ratio is unfavourable for pyrometallurgical treatment can be preferably treated with the method of the present invention.

The high solubility of metal chlorides allows high recovery of the target metals. In addition the rapid kinetics of the process allows use of smaller reaction vessels than in sulphate based processes.

The use of the chlorine alkali electrolysis stage to treat the spent brine produces within the process the majority of the chemicals for leaching or precipitating required by the process.

The sodium chloride leach media allows the use of saline or hypersaline waters at least as make-up solution in addition to the sodium hydroxide produced in the chlorine alkali electrolysis, which may allow processing of the nickel sulphide concentrate in arid areas of the world where pure water might be unavailable.

The above description is intended to be illustrative of the preferred embodiments of the present invention. Variations to the invention without departing from the spirit or ambit described herein should also be considered to form part of the invention.