"Hematite Precipitation"

Field of the Invention

The present invention relates to hematite precipitation. In particular, the present invention relates to a hydrometallurgical method for precipitating iron in the form of hematite from solutions containing nickel and cobalt, under atmospheric conditions. More particularly, the method of the present invention is intended to allow the precipitation of iron as hematite from solutions recovered from the processing of either nickel laterites and/or nickel sulphides.

Background Art

In hydrometallurgical nickel processing methods, iron is typically precipitated as a ferric oxyhydroxide (commonly a goethite) and as a hematite product from the high pressure acid leaching process for nickel laterites. In some situations iron is also rejected as a jarosite product.

Unfortunately the rejection of iron as a ferric oxyhydroxide also 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.

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 the use of capital intensive equipment. This equipment is also expensive to maintain and has associated high operating costs.

The jarosite process, whilst having lower losses of nickel and cobalt due to cleaner separation coefficients, can be an expensive process. The jarosite process typically requires the use of a suitable cation (such as Na ⁺ , K ⁺ or NH ₄ ⁺ ) together with elevated temperature and/or pressure.

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

Hematite is one of the most acceptable iron products for intermediate storage or disposal. This is due to 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.

Methods for the management of iron in hydrometallurgical systems have been discussed in the literature, see for example Queneau and Weir (1986), "Control of iron during hydrometallurgical processing of nickeliferous laterite ores", in Iron Control in Hydrometallurgy, Dutrizac, J. E. and Monhemius, A. J. Eds., Ellis Horwood Limited, Chichester, UK, pp. 76-105, and Whittington and Muir (2000), "Pressure Acid Leaching of Nickel Laterites: A Review", Min. Pro. Ext. Met. Rev., VoI 21 , pp527 - 600, at 558 - 565.

The method of the present invention has as one object thereof to overcome substantially the problems of the prior art, or to at least provide a useful alternative thereto.

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 to the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the present invention there is provided a hydrometallurgical method for precipitating iron in the form of hematite from leach solutions containing nickel, cobalt and iron, the method comprising the steps of:

(i) pre-neutralising the leach solution by the addition of a neutralising agent;

(ii) oxidising ferrous sulphate to ferric sulphate by sparging air or oxygen into the solution resulting from step (i) at an elevated temperature;

(iii) adjusting the free acid and pH of the solution resulting from step (ii) to levels suitable for hematite precipitation;

(iv) adding hematite seed to the solution resulting from step (iii) and maintaining the solution in these conditions for a residence time such that substantially complete conversion of ferric sulphate to hematite precipitate is achieved;

(v) filtering or thickening the solution from step (iv) to remove the hematite precipitate;

(vi) recycling a portion of the hematite of step (v) to step (iv) as the hematite seed; and

(vii) recovering nickel and cobalt from the filtrate solution.

Preferably, the oxidation of ferrous sulphate to ferric sulphate in step (ii) occurs at a temperature within the range of about 60 ⁰ C to 15O ⁰ C.

SiII preferably, the oxidation of ferrous sulphate to ferric sulphate in step (ii) occurs at a temperature within the range of about 6O ⁰ C to 100 ⁰ C.

The free acid concentration is preferably lower than about 10g/L.

The pH of the solution resulting from step (ii) is preferably maintained below about 2.5 to avoid precipitation of ferric oxyhydroxide.

Still further preferably, the solution temperature of step (iv) is maintained between about 60 ⁰ C and 15O ⁰ C during hematite precipitation.

Yet still further preferably, the solution temperature of step (iv) is maintained between about 6O ⁰ C and 100 ⁰ C during hematite precipitation.

Seed concentration preferably ranges between about 5g/L and 100g/L.

Preferably, air or oxygen is sparged into the solution of step (iv) during hematite precipitation.

The neutralising agent preferably comprises any one or more of calcrete, limestone or lime.

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 hydrometallurgical method for the precipitation of iron in the form of hematite from a leach solution containing nickel and cobalt 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 iron in the form of hematite from a leach solution containing nickel and cobalt. In this example the leach solution is the pregnant leach solution containing nickel, cobalt and iron resulting from the leaching of a nickel laterite or fraction thereof under atmospheric conditions.

As sulphuric acid is released when ferric sulphate is converted to hematite the free acid in the leach solution needs to be neutralised prior to hematite precipitation. The leach solution is subjected to a pre-neutralisation step 12 in which a neutralising agent 14, for example calcrete, limestone and/or lime, is added to lower the free acid concentration in solution to below about 10g/L whilst keeping the pH below about 2.5.

The solution resulting from pre-reduction is then heated to between about 60 and 15O ⁰ C, for example between about 60 and 100 ⁰ C, before undergoing an oxidation step 16 through exposure to an oxidising agent 18, for example air or oxygen. The majority of ferrous sulphate present is oxidised to ferric sulphate.

Whilst maintaining the solution temperature the solution proceeds to a precipitation stage 20 where hematite seed 22 is added in a concentration range of between about 5 and 100g/L. The quantity of seed added will not alter precipitation kinetics but the amount added can affect the final product composition. The addition of the hematite seed 22 promotes the formation of hematite over other forms of iron hydroxides. The amount of seed added is important in as much as too much seed is a waste from an installed capital and operating cost perspective. If too little seed is added the likelihood of iron precipitating as something other than hematite increases.

A residence time of between about 0.5 and 200 hours, for example between about 0.5 and 20 hours, is employed to effect complete conversion of ferric sulphate to hematite, and this residence time will vary depending on the operating conditions adopted. The product solution is then filtered or thickened 24 to remove the hematite from solution. A recycle stream 26 is then employed to return a portion of the hematite residue back to the precipitation circuit 20 as the hematite seed.

The filtrate or thickener overflow is then advanced to a nickel and cobalt recovery stage 26, in which the nickel and cobalt can be recovered using various known hydrometallurgical techniques including precipitation, solvent extraction or ion exchange.

The free acid is maintained in the range of about 0 to 10 g/L during the hematite precipitation stage 20. This can be achieved by adding a neutralising agent comprising calcrete, lime or limestone. Other neutralising agents including ammonia and caustic will also be suitable.

It is envisaged that it may also be beneficial to also sparge oxygen through the solution during the precipitation stage 20, thereby ensuring that ferrous iron is fully oxidised to ferric and enhancing the precipitation of hematite.

It is further envisaged that this method can be applied to leach solutions resulting from the leaching of nickel sulphide concentrates and nickel laterite ores.

As can be seen from the above description, the method of the present invention facilitates the precipitation of iron in the form of hematite from nickel containing hydrometallurgical process solutions ranging from atmospheric to medium pressure conditions, thereby constituting a very useful alternative to the methods of the prior art.