"Method for the Precipitation of Nickel"

Field of the Invention

The present invention relates to a method for the precipitation of nickel. More particularly, the present invention is a hydrometallurgical method for the preferential precipitation of nickel and cobalt sulphides from solutions containing iron and/or chrome. Additionally, the method is intended to substantially avoid the formation of sulphide scale during the precipitation process.

Background Art

Nickel and cobalt are typically recovered from leach solutions by contacting the pregnant liquors with a suitable reductant such as hydrogen sulphide. It is known that iron and chrome will tend to co-precipitate as a sulphide under conventional hydrogen sulphide precipitation conditions. Such co-precipitation is undesirable for the detrimental effect on product quality and the demands placed on downstream processing of the mixed sulphide product.

In metallurgical circuits incorporating the high pressure acid leaching of nickel laterites, iron is most often rejected as a ferric oxyhydroxide (typically as a goethite) and as a hematite product . In some situations iron is also rejected as a jarosite product.

Unfortunately, the rejection of iron as a ferric oxyhydroxide results in significant co-precipitation of the valuable nickel and cobalt products. This either results in metallurgical losses or necessitates the reprocessing of the iron residue in order to recover the valuable nickel and cobalt.

Hematite is the most acceptable iron product for intermediate storage or disposal, because of its high thermodynamic stability, its high density (4.9 to 5.3 g/cm ³ ), its high iron content (60% - 70%) and its low adsorption of water and base metals.

However, the rejection of iron as a hematite product in the high pressure acid leaching processes used for nickel laterites necessitates the use of temperatures in the order of 25O ⁰ C and pressures in the order of 45 Bar. This process by its very nature involves capital intensive equipment which is itself expensive to maintain and has associated high operating costs.

The rejection of iron as jarosite, whilst resulting in lower losses of nickel and cobalt from the improved separation coefficients, is an expensive process as it typically requires the use of a suitable cation (such as ammonia) together with elevated temperatures and pressures.

In addition to the losses of nickel and cobalt associated with ferric oxyhydroxide and ferric hydroxide formation, the major problems of this process are the lower filtration rates, thickener settling characteristics and the thickener underflow densities achievable. Ferric hydroxide in particular, and to a lesser extent ferric oxyhydroxide, tends to have an open structure. This results in the incorporation of large amounts of base metals during settling. The solids produced also have a low density and low thickener underflow densities impact on both downstream processing equipment and the volume of tailings which needs to be disposed of.

In International Patent Application PCT/AU2003/001037 (WO 2004/016816) there is disclosed a process for preferential precipitation of nickel and cobalt from solutions containing iron through the addition of a reductant to reduce ferric ions to ferrous ions. This reaction generates acid which must be neutralised, before adding seed particles in the presence of further reductant to precipitate the nickel and cobalt. However, typically with this process significant scaling can occur, which adversely affects both the precipitation kinetics and recovery of the nickel and cobalt sulphide product.

The present method has as one objective thereof to substantially overcome the problem of scaling, whilst also providing the advantage that the incidence of iron sulphide co-precipitation is reduced, or to at least provide a useful alternative to prior art methods.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Further, reference to "seed surface area" or variations thereof will be understood as based on the assumption that the seed particles are perfect spheres at the D ₅₀ particle diameter.

Disclosure of the Invention

In accordance with the present invention there is provided a method for the recovery of nickel and cobalt from leach solutions in the presence of iron and/or chrome, the method comprising the steps of:

(i) adding a reductant to a leach solution containing nickel, cobalt and iron, such that any iron present as ferric sulphate is reduced to ferrous sulphate and/or any hexavalent chrome is reduced to trivalent chrome;

(ii) neutralising at least a portion of the free acid through addition of a neutralising agent;

(iii) further addition of the reducing agent to ensure all iron present remains in the ferrous form and/or any chrome remains in the trivalent form;

(iv) heating the solution prior to mixed sulphide precipitation;

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(v) adding a mixed sulphide seed and hydrogen sulphide to effect precipitation of the nickel and cobalt in the form of a mixed sulphide; and

(vi) maintaining this mixture in the presence of hydrogen sulphide for the required residence time to effect complete precipitation of the mixed sulphide product.

Preferably, the reductant in (i) and (iii) comprises one or more of hydrogen sulphide, sodium hydrogen sulphide, or sulphur dioxide.

After neutralisation, free acid concentration is preferably within the range of about 0.5 g/L to 3.5 g/L

The neutralising agent of step (ii) may comprise any one or more of limestone, lime and calcrete.

The reduction of step (i) preferably occurs at less than about 100 ⁰ C and ambient pressure.

Preferably, the resulting ferric sulphate concentration of the resulting solution of step (iii) is less than about 1 g/L.

Preferably, the oxidation potential of the solution resulting from step (i) through to (vi) is maintained between about 30OmV and 40OmV (measured against a Pt- Ag/AgCI reference electrode) to ensure no oxidation of ferrous sulphate to ferric sulphate occurs.

Preferably, the solution temperature is in the range of about 8O ⁰ C to 12O ⁰ C when sulphide seed is added.

Still preferably, the concentration of seed in solution is in the range of about 10g/L to 100g/L and the total seed surface area is between about 1 m ² /L and 10m ² /L.

Hydrogen sulphide overpressure in step (v) and step (vi) is preferably maintained between about 10OkPa and 40OkPa in order to produce the mixed sulphide product.

Preferably, the residence time of step (vi) is between about 0.25 to 4 hours is employed to ensure complete precipitation of the mixed sulphide. Still preferably, the residence is between about 0.5 and 1.5 hours.

The concentration of nickel in the leach solution is preferably in the range of about 1 g/L to 50 g/L, and cobalt within about 0.1 g/L to 10 g/L.

Still preferably, the concentration of nickel is in the range of about 1 g/L to 10 g/L for a nickel laterite solution, or between about 10 g/L and 50 g/L for a nickel sulphide solution. Cobalt concentrations are preferably within the range of about 0.1 g/L to 2 g/L, and about 2 g/L to 10 g/L respectively.

Iron concentration in the leach solution is preferably in the range of about 0.5g/L to 15g/L.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawing, in which:-

Figure 1 is a diagrammatic representation of a flow sheet depicting a method for the precipitation of nickel and cobalt from leach solutions in the presence of iron in accordance with the present invention.

Best Mode(s) for Carrying Out the Invention

In Figure 1 there is shown a hydrometallurgical method 10 for precipitating nickel and cobalt from pregnant leach solutions, also containing iron, obtained from the high pressure acid leach of a nickel laterite ore. The nickel concentration is in the range of 1 g/L to 10 g/L. Cobalt concentrations are within the range of 0.1 g/L to 2 g/L. Iron concentration in the leach solution is in the range of about 0.5-15g/L.

Thθ method 10 of the present invention comprises passing such a pregnant leach solution to a pre-reduction step 12 in which hydrogen sulphide gas 13 is sparged through the solution at a temperature of less than 100 ⁰ C. Iron present as ferric sulphate (Fe ₂ (SO ₄ ) ₃ ) is reduced to ferrous sulphate (FeSO ₄ ) such that the resulting ferric concentration is less than 1 g/L.

The solution from the pre-reduction circuit 12 then undergoes neutralisation 14 using a calcrete slurry to reduce the free acid (FA) concentration to between about 0.5 and 3.5 g/L. It is understood that if the oxidation potential of the solution is not controlled, then ferric can form during neutralisation 14. Consequently, following neutralisation 14 a stream of hydrogen sulphide gas 15 is again passed through the solution from the pre-reduction circuit 12 in an additional reduction step 16, to ensure that the oxidation potential is within the range of about 300 to 400 mV (Pt-Ag/AgCI reference electrode), for example 350-380 mV.

A pre-heating step 18 raises the temperature of the solution from the reduction step 16 to between about 80 and 12O ⁰ C in preparation for a subsequent precipitation step 20. A mixed sulphide seed 21 in the range of 10 g/L to 100 g/L is introduced to the solution prior to the introduction of hydrogen sulphide gas 22.

The total seed surface area is between about 1m ² /L and 10m ² /L. Hydrogen sulphide gas 22 is introduced at an overpressure of 100-400 kPa to precipitate a mixed sulphide product 24. This is maintained for the duration of the residence time, between about 0.25 to 4 hours, for example 0.5 to 1.5 hours, in order to effect complete conversion to the mixed sulphide product 24.

The pre-heating step 18 allows the precipitation step 20 to occur within acceptable commercial parameters by increasing the kinetics of the precipitation reactions and also allows dissolved H ₂ S to be driven off.

Neutralisation may also be effected using any one of lime, limestone, ammonia or caustic.

The mixed sulphide precipitate contains nickel in the range of 50 to 55%, cobalt at 3 to 5% and iron at 1 to 3% wt/wt.

The method of the present invention will now be described with reference to an example that is to be understood as non-limiting.

Example 1 - Reduction of Scaling

Two feed solutions containing high iron levels were treated by pre-reduction with H ₂ S gas then passed through a neutralisation circuit to reduced residual free acid concentration to 1.4 g/L using calcrete slurry. The composition of the feed solutions is set out in Table 1 below:

Table 1: Composition of Mixed Sulphide Feed Solution 1.

Solution 1 proceeded directly to mixed sulphide precipitation, whilst the Eh of Solution 2 was first reduced to 350-380 mV (Pt-Ag/AgCI reference electrode) with H ₂ S to ensure all iron in ferric form was converted to ferrous before heating the solution in preparation for sulphide precipitation.

A further test was employed with Solution 2 in which a higher addition of mixed sulphide seed was added, refer to Table 2 below.

The results for scale formation during precipitation from each solution are given in Table 2. It is quite clear that the rate of scale growth is significantly reduced when

the second reduction step is incorporated into the flowsheet. This effect is enhanced by a further addition of seed to the solution.

Table 2: Effect on Scale Growth with Second Reduction Step and Seed Addition.

It is envisaged that the method of the present invention may be applied to the recovery of nickel and cobalt from nickel sulphide leach solutions. In such circumstances it is typical that the pregnant leach solution have a nickel concentration in the range of about 10 g/L to 50 g/L, and a cobalt concentration of about 2 g/L to 10 g/L. Again, iron concentration is typically in the range of about 0.5 to 15g/L

It is to be understood that the method of the present invention is equally applicable to leach solutions containing chrome.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.