WO2009039580A1

WO2009039580A1 - Stabilisation of metal sulfates

Info

Publication number: WO2009039580A1
Application number: PCT/AU2008/001431
Authority: WO
Inventors: Eric Girvan Roche; John Baines
Original assignee: BHP Billiton SSM Development Pty Ltd
Current assignee: BHP SSM Indonesia Holdings Pty Ltd
Priority date: 2007-09-26
Filing date: 2008-09-26
Publication date: 2009-04-02
Anticipated expiration: 2010-03-26

Abstract

A process for producing a stable metal sulfate product including the steps of : a) providing a source of metal sulfate crystals; b) adding a sub-stoichiometric amount of a stabilising compound to the metal sulfate crystals to form a paste or slurry like intermediate product; and c) allowing the paste or slurry like product to solidify.

Description

STABILISATION OF METAL SULFATES

The present invention relates to a process for stabilising metal sulfates that may be present in a brine solution in a metal recovery process. The invention is particularly applicable to the stabilising magnesium sulfate crystals, that may have been salted out of the final brine from a nickel and/or cobalt recovery process. It is also applicable to stabilising other metal sulfates that may be present in such solution, such as iron sulfate and manganese sulfate, or metal sulfate salts sourced from another source.

The stabilised metal sulfate product will be a solidified compound which will exhibit good stability to water and is dense and geotechnically stable.

Background of the Invention Laterite ores include both a high magnesium content saprolite component, and a low magnesium content limonite component. In commercial processes, such as the Cawse process in Western Australia, nickel and cobalt are recovered from laterite ore by high pressure acid leach processes where the nickel and cobalt are leached from the ore with sulfuric acid. Following the addition of magnesium oxide, the nickel and cobalt are recovered as a mixed nickel and cobalt hydroxide precipitate.

Other non-commercial processes have been described where a mixed hydroxide precipitate is produced following the addition of a neutralising agent in an atmospheric pressure acid leach, or a combination of high pressure and atmospheric pressure leach processes. An example of such a process is described by Liu in WO 03/093517. Further non-commercial processes have been described where the ore is treated in a heap leach process, for example in U.S. patent 6,312,500 in the name of BHP Minerals International Inc. and WO/AU2005/001360 and WO/AU2006/053376 each in the name of BHP Billiton SSM Technology Pty Ltd. During such nickel recovery processes, magnesium values contained in the saprolitic silicates of nickel containing laterite ores are generally discarded as waste. Other metals, for example magnesium, ferric iron, ferrous iron, aluminium, chromium and manganese may also be discarded, or metals from other sources such as any magnesium solubilised from magnesium oxide, that may be used in the process, is also discarded as waste. The dissolved metals generally report to brine associated with the refinery as metal sulfates or metal chloride brine.

Processes have been developed that aim to utilise the magnesium that is present in the magnesium sulfates of brine ponds so as to regenerate magnesium oxide for further use in nickel and cobalt recovery processes. For example, such processes have been published in PCT/AU2006/001983 and PCT/AU2006/001984, both in the name of BHP Billiton SSM Development Pty Ltd, the contents of each are incorporated herein by reference.

In arid regions, a solution that includes magnesium sulfate, for example the brine solution from a laterite ore leaching process, is bled from the circuit and stored in evaporation ponds where the magnesium precipitates due to evaporation. The water balances in these circuits are maintained with "fresh" make-up water. While this method is acceptable in arid regions it is not suitable for areas where there is a high net positive rainfall. Hence an alternative method for rejecting magnesium from solution and fixing it into an environmentally suitable product is required.

The current process for rejecting magnesium from solution in areas of high rainfall, is to increase the solution pH with lime to precipitate magnesium producing a residue containing gypsum and magnesium hydroxide. The lime consumption and disposal requirements add significantly to project costs and can result in the project becoming uneconomical. Hence alternative options for rejecting magnesium from solution need to be developed to improve the economics for processing tropical laterites/saprolites. The present invention aims to provide a process where magnesium sulfate in solution, for example magnesium sulfate that is the waste product of a nickel and/or cobalt recovery process, is converted to a stable, environmentally suitable end product

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.

Brief Description of the Invention

The present invention provides a process for the stabilisation of a crystallised metal sulfate product. The process is particularly applicable to stabilising magnesium sulfate crystals that have been recovered from the brine solution that is generally regarded as waste from nickel and/or recovery processes. It is also applicable to stabilising other sulfates that may be present, such as iron sulfate and manganese sulfate.

It has been found that by adding a sub-stoichiometric amount of a stabilising compound, such as lime (slaked or unslaked lime) to crystallised metal sulfates, that a stable solid may be formed that is environmentally suitable for disposal. The stable product is rock-like, has low porosity and has substantially lower solubility in water than crystallised metal sulfates, and so can be readily used as landfill.

Accordingly, in one embodiment of the invention there is provided a process for producing a stable solid metal sulfate product including the steps of: a) providing a source of crystallised metal sulfate; b) adding a sub-stoichiometric amount of a stabilising compound to the crystallised metal sulfate to form a paste or slurry like intermediate product; and c) allowing the paste or slurry like product to solidify. The present invention is particularly applicable to providing a stable solid metal sulfate where the metal sulfate is derived from a nickel and/or cobalt recovery process although the process is not restricted to this embodiment. In a preferred embodiment, the metal sulfate may have been derived from the metal sulfates that are present in waste brine that result from a nickel and/or cobalt recovery process. The process is particularly applicable to processing waste brine that may result from the sulfuric acid leaching of nickel and/or cobalt containing laterite ores.

Accordingly, in the further embodiment of the invention, there is provided a process for producing a stable metal sulfate product from a metal sulfate waste material in a nickel and/or cobalt recovery process including the steps of: a) providing a source of metal sulfate crystals that have been derived from part of a nickel and/or cobalt recovery process; b) adding a sub-stoichiometric amount of a stabilising compound to the metal sulfate crystals to form a paste or slurry like intermediate product; and c) allowing the paste or slurry like product to solidify.

In a preferred embodiment, the metal sulfate crystals have been derived from the brine solution associated with a nickel and/or cobalt recovery process from the sulfuric acid leaching of nickel and/or cobalt containing laterite ores.

Preferably, the stabilising compound may be selected from calcium oxide, calcium carbonate, calcium hydroxide, magnesium oxide, magnesium carbonate, magnesium hydroxide, calcium silicate or sodium silicate, but calcium oxide and calcium hydroxide are preferred.

In a preferred embodiment, a small amount of water or brine may be added to the metal sulfate/stabilising compound mix to aid homogenous mixing, so that a paste or slurry like intermediate product is produced. The water may also be available from the hydrated form of the metal sulfate crystals. This paste or slurry like intermediate product may be pumped or transported to an empoundment such as a pit, where it is allowed to solidify.

The water or brine may be sourced from recycled process water and may include one or more of raw brine, fresh water, the solution produced by crystallising out the metal sulfate from a source of brine, or an intermediate leach solution in the nickel and/or cobalt recovery process.

The amount of stabilising compound added may be any suitable amount to form a paste or slurry like consistency, but preferably it is in an amount of 0.1 to 0.9, preferably 0.2 to 0.7 stoichiometric ratio with respect to the metal sulfate.

The source of the crystallised metal sulfate may be from any source, but in one preferred embodiment, it may be sourced by crystallising the metal sulfate that remains in the brine solution following a nickel and/or cobalt recovery process. The metal sulfate may exist in such solutions as a hydrated product.

Whereas the process may be applicable to processing a number of metal sulfates that may be present in such brine solutions, the process is particularly applicable to the processing of hydrated crystallised sulfates, such as monohydrated or heptahydrated magnesium sulfate, monohydrated or heptahydrated iron sulfate, or x hydrated manganese sulfates where x is between 1 and 6.

The metal sulfates may be crystallised by any manner, including an evaporative crystallisation step.

Alternatively, crystallisation may be achieved by using a concentrated sulfuric acid, which is added to the brine to salt out solid crystalline metal sulfate. Further concentrated sulfuric acid may be added to dehydrate the metal sulfate crystals to produce a solid, substantially dehydrated crystalline metal sulfate product. A detailed description of this process as applied to crystallising magnesium sulfate is published in application PCT/AU2006/001984 in the name of BHP Billiton SSM Development Pty Ltd, the entire content of which is incorporated herein by reference.

Further, the metal sulfate may be crystallised by using waste heat from other parts of a nickel and/or cobalt recovery process, in order to evaporate much of the water, producing a hot concentrated solution of the metal sulfate, which upon cooling, crystallises out of the solution as crystallised metal sulfates.

In one particular embodiment, the process of the present invention provides processing magnesium sulfate that is present in the brine solution following a nickel and/or cobalt recovery process. In this embodiment, crystallised magnesium sulfate, that has been sourced from a brine solution is mixed with a sub-stoichiometric amount of the stabilising compound together with an amount of water and/or brine to aid in homogenous mixing of the stabilising compound and magnesium sulfate crystals. This forms a slurry or paste like intermediate product. The resultant slurry or paste may be pumped to a pit or suitable empoundment and allowed to solidify in the pit to form a solid stable product.

Brief Description of the Drawings

Figure 1 shows a possible flowsheet for the leaching of a nickel or laterite ore. Figures 2 and 3 show the result of mixing monohydrated MgSO₄ crystals with CaO or Ca(OH)₂. Figures 4 to 9 show the result of hydrated MgSO₄ mixed with CaO or Ca(OH)₂ compared with monohydrated MgSO₄ mixed with CaO or Ca(OH)₂ and water.

Detailed Description of the Invention

It would be convenient to describe the detailed embodiment of the invention with reference to Figure 1. The possible flowsheet shown in this Figure is intended to be illustrative of the invention described, and the invention should not be considered to be limited thereto. Figure 1 illustrates a possible flowsheet where a nickel laterite ore is leached with sulfuric acid (1 ). The leachate will include dissolved metals from the ore, including dissolved magnesium, ferric iron, ferrous iron, aluminium, chromium and manganese together with the desired nickel and cobalt ions.

The pH of the leach liquor may be raised by the addition of a neutralising agent such as limestone (2) in order to precipitate out some of the unwanted products. Iron and aluminium will precipitate out and the iron and aluminium products are discarded as residue (3).

Nickel and cobalt may then be recovered by precipitating the nickel and cobalt as a mixed hydroxide product (for example by the addition of magnesium oxide) or as a mixed sulfide product (for example by the addition of hydrogen sulfide) (4) and (5).

If desired, by raising the pH of the leachate further, for example by the addition of further magnesium oxide, other impurities such as manganese will precipitate and be discarded as a manganese residue.

In the particular embodiment described, the resultant brine solution that remains following the precipitation of both wanted and unwanted metals will retain metal sulfates, in particular magnesium sulfates, but may also include other sulfates such as iron and manganese sulfates.

The magnesium sulfates are crystallised from the brine solution (6) and will generally form a hydrated salt, such as the magnesium sulfate heptahydrate (7) following a solid/liquid separation step to remove the solid magnesium sulfate salt from the resultant solution. The resultant solution (12) may be recycled to the nickel recovery process as required.

In order to stabilise the hydrated magnesium sulfate crystals, calcium oxide (8) is combined with the magnesium sulfate crystals together with additional water (9) if required, in order to form a paste or slurry like material (10). This paste or slurry like material may be transported, such as by pump, to long term storage (11 ), such as a clay-lined storage pit. In long term storage, the slurry or paste like composition will set to a rock-like product having low porosity and a substantially reduced solubility in water than the crystallised metal sulfates.

A perceived benefit of the process of the present invention is that the stabilised magnesium sulfate product may be used for other purposes, such as landfill, and is geotechnically stable. That is, it exhibits considerable strength, weight bearing ability and is resistant to extreme climatic events such as flooding, unlike for example disposal dams containing lime based precipitate sludges.

The present invention is described further in the following discussion of a part of the experimental work carried out by the applicant.

Solid Formation

The following mixtures were prepared:

Mono hydrated magnesium sulfate crystals (MgSO₄. H₂O) and:

• 0.3, 0.5 and 0.7 stoichiometric ratio (with respect to MgSO₄) addition of burnt lime (CaO) and hydrated lime (Ca(OH)₂) without additional water. • 0.5 stoichometric ratio (with respect to MgSO₄) addition of burnt lime and hydrated lime, plus additional water as required to make a paste

Hepta-hyd rated magnesium sulfate crystals (MgSO₄JH₂O) and:

• 0.3, 0.5 and 0.7 stoichiometric ratio (with respect to MgSO₄) addition of burnt lime (CaO) and hydrated lime (Ca(OH)₂) without additional water.

• 0.5 stoichiometric ratio (with respect to MgSO₄) addition of burnt lime and hydrated lime, plus additional water as required to make a paste

The ingredients were mixed with a mortar and pestle and placed into a mould if it appeared that the mixture might set. Dissolution Tests

Where a solid product was formed, a piece of the product was broken off and placed in excess water for a period of approximately 70 hours to test whether it re-dissolved.

Results

Solid Formation

MgSO₄.H₂O with lime - no water

Table 1 below presents the stoichiometric ratios of mixtures between lime powders (burnt and hydrated) and mono hydrated lime crystals for this section of work. Note that additional water was not added to create a paste.

Table 1 Lime mixtures with mono hydrated lime and comments on products

Without exception, mixing mono hydrated MgSO₄ crystals (kieserite), with CaO or Ca(OH)₂ (no additional water) did not cause a reaction and the mixture remained as a dry powder (Figure 2 and Figure 3).

As a competent solid product was not formed from this series of tests, its subsequent solubility was not tested. MgSO₄-H₂O with lime and water

Table 2 below shows the amount of water added to mixtures of 15g of MgSO₄-H₂O crystals and 50% stoichiometric ratio of lime powder (burnt and hydrated). As the water was added, comments were made on its consistency, and then more water was added until a paste was finally made. These final pastes were labelled (samples 15 and 16) and packed in a mould to wait and see if it set as a solid "concrete-like" product.

Table 2 Lime mixtures with mono hydrated lime and water, and comments on products

The paste formed from mixing MgSO₄-H₂O with hydrated and burnt lime both solidified to form a competent product. Pictures of these products are shown in Figure 4 and Figure 5.

Both of the solid products formed were quite hard, i.e. it took significant force to break off a chunk for dissolution testing, however a number of independent yet subjective judgements were consistent that the MgSO₄-H₂O / CaO / water mixture was stronger that the MgSO₄.H₂O / Ca(OH)₂ / water mixture. Close examination of the products suggest that the product made from mixing with burnt lime (CaO) is more consistent throughout and it is thought that this may be contributing to its physical competency MgSO₄-7H₂O with lime - no water

Table 3 below presents the stoichiometric ratios of mixtures between lime powders (burnt and hydrated) and hepta hydrated lime crystals. Note that additional water was not added to create a paste.

Table 3 Lime mixtures with hepta hydrated lime and comments on products

The hepta-hyd rated MgSO₄ crystals appear to have provided enough water to initially create a paste and then dry into a competent solid when mixed with both burnt and hydrated lime. This result was the same irrespective of whether burnt lime (CaO) or hydrated lime (Ca(OH)₂) was used. The solid products are shown in Figure 6 and Figure 7.

Upon close examination of the solid products formed without additional water, it did not appear as consistent as the product which was formed when mixing mono hydrated MgSO₄ with CaO and water. For the MgSO₄. H₂O/CaO/water mixture, the solid formed was quite consistent, however the MgSO₄.7H₂O/lime (no water) mixture, looked like crystals (probably MgSO₄) held together by a "cement". This is best demonstrated by comparing the photographs of sample 4 and sample15 shown in Figures 8 and 9.

It is proposed that the consistency of Sample 15 (Figure 8) has made it more physically competent (harder to break) than any of the other solids produced.

MgSO₄JH₂O with lime and water

Although tests were planned for this section, once a solid was made from mixing the heptahydrated magnesium sulfate crystals with both hydrated and0 burnt lime powders (no water) the tests planned with additional water were cancelled.

Dissolution Tests

A sub-sample of each of the solid products formed from the lime stabilisation5 tests as outlined above was then placed in water over a period of approximately 70 hours to test whether it re-dissolved, or whether it was stable in water. The samples tested and the dissolution results are presented in Table 4 below. 0 Table 4 Dissolution results for solids formed by lime addition to MgSO₄ crystals

%«

Solid Preparation

Percentage

Sample , Crystal _/mr Water Dissolved

MgSO₄crystal lLime Stoich. Number Preparation Addition hepta-hydrate Un-pulveπsed CaO 30% No 70% hepta-hydrate Un-pulveπsed CaO 50% No 58% hepta-hydrate Un-pulveπsed CaO 70% No 44% hepta-hydrate Pulverised CaO 30% No 62% hepta-hydrate Un-pulvensed Ca(OH)₂ 30% No 67% hepta-hydrate Un-pulveπsed Ca(OH)₂ 50% No 54% hepta-hydrate Un-pulveπsed Ca(OH)₂ 70% No 40%

15 mono-hydrate Un-pulveπsed CaO 50% Yes 49% 16 mono-hydrate Un-pulvensed Ca(OH)₂ 50% Yes 55% Table 4 shows that the solid products formed in the section on solid formation above only partially dissolved when placed in excess water over an extended test period. By contrast a similar test with unstabilised MgSO₄JH₂O shows that the material dissolves completely in approximately 1 minute.

General Discussion

The previously described results show that a "cement-like" solid product can be generated by mixing magnesium sulfate hydrate crystals with water and either burnt or hydrated lime. The water can be present as the hydrated form of the magnesium sulfate crystal, or can be separately added. Whilst this solid product can be generated using as little as 30% stoichiometric ratio of lime it is apparent that the solid product became less soluble as greater stoichiometric proportions of lime were added. It is postulated that that the reaction is a solid state version of the lime precipitation concept, where lime is added to saturated Mg solution to create magnesium hydroxide and gypsum via the following reaction:

2H₂O (I) + MgSO₄ (aq) + Ca(OH)₂ (s) -> Mg(OH)₂ (s) + CaSO₄.2H₂O (s)

Thus the decreased dissolution from higher stoichiometric lime additions is consistent with the observation that the products from this reaction are insoluble, whilst the un-reacted magnesium sulfate crystals remain soluble.

However dissolution of the un-reacted magnesium sulfate is physically impeded by the presence of the insoluble reaction products. The physical competency of the product is created by the amount and consistency of the Mg(OH)₂ and CaSO₄.2H₂O binding the left over magnesium sulfate crystals together.

This explains why the mixture of mono hydrated magnesium sulfate and lime without additional water does not create a solid product as there is insufficient water in the system, i.e. there must be enough water present to slake the lime (form Ca(OH)₂) and complete the reaction. In solid state system, the water is supplied by the hydrated magnesium sulfate crystals and minimal additional water is required. Alternatively, sufficient water can be naturally retained in the magnesium sulfate crystals, for example after separation by centrifuge or by filtration, to supply the small amount of water required.

Conclusions

• A cement-like product can be formed from both monohydrated and hepta hydrated MgSO₄ crystals using considerably less than a 100% stoichiometric ratio of lime

• The solid product can be generated using as little as 30% stoichiometric ratio of lime addition with respect to MgSO₄. It is postulated that the solid is created by binding un-reacted magnesium sulfate crystals with Mg(OH)₂ and gypsum that is formed from the reaction of slaked lime with MgSO₄ and water.

• The physical competency of the solid product is dependent upon the amount and consistency of reacted product in the solid. With optimum combination of ingredients the solid product is dense, exhibits low porosity, and is very hard. • For the stoichiometric ratios of lime addition described, the solid product exhibits very delayed solubility in water, with the amount redissolving decreasing with increased lime addition.

It is considered that emplacement of the paste or slurry, with hardening on standing, would allow the potentially very large blocks of stabilised metal sulfate to be stored for very long periods. This is analogous to the high stability of natural deposits of otherwise water soluble salts such as sodium chloride ('halite') and sodium carbonate ('trona'). Production of large stabilised blocks minimises the relative surface area of the material, which coupled with low porosity leads to higher bulk stability towards water. Emplacement of blocks of stabilised metal sulfate may also be further stabilised by capping or lining with clay or other water impervious material. The invention described herein is susceptible to variations, modifications 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.

Claims

CLAIMS:

1. A process for producing a stable metal sulfate product including the steps of: a) providing a source of metal sulfate crystals; b) adding a sub-stoichiometric amount of a stabilising compound to the metal sulfate crystals to form a paste or slurry like intermediate product; and c) allowing the paste or slurry like product to solidify.

2. A process for producing a stable metal sulfate product from a metal sulfate waste material in a nickel and/or cobalt recovery process, including the steps of: a) providing a source of metal sulfate crystals that have been derived from part of a nickel and/or cobalt recovery process; b) adding a sub-stoichiometric amount of a stabilising compound to the metal sulfate crystals to form a paste or slurry like intermediate product; and c) allowing the paste or slurry like product to solidify.

3. A process according to claim 2 wherein the source of metal sulfate crystals has been derived from the brine solution associated with a nickel and/or cobalt recovery process from leaching nickel and/or draft containing ores with sulfuric acid.

4. A process according to any one of the preceding claims wherein the metal sulfate crystals are hydrated crystals.

5. A process according to any one of the preceding claims wherein the metal sulfate crystals are magnesium sulfate monohydrate or heptahydrate; iron sulfate monohydrate or heptahydrate or manganese sulfate x hydrate where x is between 1 and 6.

6. A process according to claim 5 wherein the metal sulfate is magnesium sulfate monohydrate or heptahydrate.

7. A process according to claim 1 or 2 wherein the stabilising compound is selected from calcium oxide, calcium carbonate, calcium hydroxide, magnesium oxide, magnesium carbonate magnesium hydroxide, calcium silicate and sodium silicate.

8. A process according to claim 7 wherein the stabilising compound is calcium oxide or calcium hydroxide.

9. A process according to claim 1 or 2 wherein water and/or brine is included in the stabilising compound/metal sulfate mix in order to improve homogenous mixing of the mix.

10. A process according to claim 1 or 2 wherein the metal sulfate crystal/stabilising compound paste or slurry like intermediate product is pumped to an empoundment and allowed to solidify.

11. A process according to any one of the preceding claims wherein the amount of stabilising compound added is anywhere from 0.1 % to 0.9% stoichiometric ratio with respect to the metal sulfate.

12 A process according to claim 11 wherein the amount of stabilising compound is anywhere from 0.2% to 0.7% stoichiometric ratio with respect to the metal sulfate.