WO2009039579A1

WO2009039579A1 - Acid recovery from metal sulfates

Info

Publication number: WO2009039579A1
Application number: PCT/AU2008/001430
Authority: WO
Inventors: Eric Girvan Roche; Jaidev Prasad
Original assignee: Bhp Billiton Ssm Development Pty Ltd
Priority date: 2007-09-26
Filing date: 2008-09-26
Publication date: 2009-04-02

Abstract

A process for the recovery of sulfuric acid from a metal sulfate in solution, including the steps of : a) providing a source of metal sulfate in solution; b) converting the metal sulfate in solution into a crystallised metal sulfate salt; c) calcining the crystallised metal sulfate salt to produce the corresponding metal oxide and sulfur dioxide gas; and d) generating sulfuric acid.

Description

ACID RECOVERY FROM METAL SULFATES

Introduction

The present invention relates to a process for generating sulfuric acid from metal sulfates. In particular, the present invention relates to a process where metal sulfates that are discarded as waste material following an acid leach process are used to generate a dilute sulfuric acid.

In one embodiment, the present invention aims to generate sulfuric acid from metal sulfates that report to brine ponds following the sulfuric acid leaching of nickel and/or cobalt containing laterite ores.

The process endeavours to utilise the metal sulfates by crystallising the metal sulfate to form a salt, and then drying and calcining the salt to produce the corresponding metal oxide and sulfur dioxide. Sulfuric acid can be generated from the sulfur dioxide produced as an off gas during the calcining process without an acid plant, while the metal oxide product may be beneficiated and sold separately as a valuable product, recirculated for use in the nickel and/or cobalt recovery process, or simply discarded.

A preferred aspect of the invention relates to the recovery of sulfuric acid from magnesium sulfate brines associated with acid heap leaching or atmospheric leaching of nickel and cobalt containing laterite ores, where the sulfuric acid can be recirculated back to the leach step in such processes.

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 a 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 ponds associated with the refinery as metal sulfates or metal chloride brine.

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.

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.

One feature of many nickel laterite acid leach processes is the onsite production of sulfuric acid from elemental sulfur using an acid plant. Typically, an acid plant provides by-product heat, in the form of steam, and sulfuric acid of concentration 98% w/w. The use of 98% sulfuric acid and steam to operate high pressure acid leach (HPAL) autoclaves means that both products are committed to the nickel leaching process.

However, heap and atmospheric processes, which operate at lower temperatures than HPAL, do not need the heat of dilution of the acid or the latent heat of the by-product steam to maintain operating temperatures. Dilute acid streams can be used for leaching nickel laterite ores in heap and atmospheric leaching without detriment to the process. Thus, a process which usefully recovers sulfate values in the waste brine, without an acid plant, while delivering sulfuric acid in dilute form to an atmospheric or heap leach, would have an economic advantage.

An acid plant represents a significant capital and operating cost and produces high strength acid and steam. In a heap leach process, in practice, the acid may be dilute, and only about 10%- 15% strength sulfuric acid is needed. Heap leaching also requires less steam than conventional high pressure acid leach processes. Approximately 500 kg/t of sulfuric acid is used to heap leach a nickel containing laterite ore. Much of this acid reports as a metal sulfate, particularly magnesium sulfate, in a brine after the recovery of nickel and cobalt. In wet inland locations, the disposal of metal sulfate brine is difficult.

A desired feature of the present invention is to develop a scheme where sulfuric acid is generated from the waste metal sulfates, particularly those metal sulfates which report to brine ponds following a nickel and/or cobalt containing ore leach process, without the need for an acid plant.

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 generation of sulfuric acid from metal sulfates. In one embodiment, the process includes the steps of generating sulfuric acid from the sulfur dioxide containing gases, which are produced by calcining crystallised metal sulfate salts.

In a preferred embodiment of the invention, the metal sulfate is sourced from the waste brine associated with the acid leaching of nickel and/or cobalt containing ores in a nickel and/or cobalt recovery process. However, the invention is not restricted to that source, and any metal sulfate in solution or in solid form may be utilised in the process of the invention.

In a preferred form of the invention, the process utilises the sulfur dioxide, which is contained within the off gases from the calcining step, together with recycled process water and oxygen, in a carbon catalysed absorption step to generate sulfuric acid without the need for an acid plant. The sulfuric acid produced by this process is dilute and may be suitable as a leach solution in a heap leach or atmospheric leach nickel and/or cobalt recovery process, and may be recirculated for use in the leach solution in such processes.

Accordingly, in one embodiment, the present invention resides in a process for the recovery of sulfuric acid from a metal sulfate in solution, including the steps of: a) providing a source of metal sulfate in solution; b) converting the metal sulfate in solution into a crystallised metal sulfate salt; c) calcining the crystallised metal sulfate salt to produce the corresponding metal oxide and sulfur dioxide gas; and d) generating sulfuric acid.

The process of the present invention is particularly suitable for heap leaching or atmospheric pressure leaching of nickel and/or cobalt containing ores, as in general, heap leaching and atmospheric pressure leaching of such ores utilise dilute sulfuric acid rather than the high strength acid generally associated with high pressure atmospheric leaching of ores. The sulfuric acid that is generated from the process of this invention, would generally be dilute sulfuric acid, for example about 10%-15% strength, and such sulfuric acid that is generated through this process is suitable for use in either a heap leach or an atmospheric leach process. The sulfuric acid produced during this process may be up to 40%-50% strength in some circumstances.

In a preferred embodiment, the metal sulfate is sourced from part of a nickel and/or cobalt recovery process that utilises sulfuric acid to leach nickel and/or cobalt containing ores. In this embodiment, the sulfuric acid that is generated may be recirculated to the leach step in the nickel and/or cobalt recovery process.

Accordingly, in a further embodiment of the invention, the present invention resides in a process for the recovery of sulfuric acid 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 in solution that is derived from part of a nickel and cobalt recovery process that utilises sulfuric acid to leach nickel and cobalt containing ores; b) converting the metal sulfate in solution into a crystallised metal sulfate salt; c) calcining the crystallised metal sulfate salt to produce the corresponding metal oxide and sulfur dioxide; and d) regenerating sulfuric acid and recirculating that sulfuric acid for use in the leach step of the nickel and/or cobalt recovery process.

The metal of the metal sulfate, and the corresponding metal oxide that may be processed in the process of invention may be any metal sulfate and oxide. Typically, in a nickel and/or cobalt recovery process, the metal sulfate will be derived from minerals that are contained in a nickel containing ore and are subsequently leached during the sulfuric acid leach process. For example, if a nickel containing laterite ore is leached, the metals that are typically present in the leachate include magnesium, iron (II), iron (III), aluminium, chromium, manganese, nickel or cobalt, which metals will form sulfates in the subsequent process solutions. The process does however lend itself to processing metal sulfates that may be sourced from other areas apart from those sulfates produced as part of a nickel and/or cobalt recovery process.

In a preferred embodiment, the sulfur dioxide off-gas from the calcining process is combined with oxygen and water, for example recycled process water, to generate the sulfuric acid in an absorption step without the use of an acid plant. A make-up source of sulfur dioxide for the sulfuric acid generation may be used by combusting elemental sulfur in a sulfur burner.

The oxygen may be sourced as a result of higher air stoichiometry in the calciner or the sulfur burner, or added as dilution air after steam generation. Preferably, the recycled process water is sourced from one or more of raw brine, fresh water, the solution produced by crystallising out the metal sulfate, or an intermediate leach solution in the nickel and/or cobalt recovery process. In a particular embodiment, the process relates to the processing of magnesium sulfate as the majority of the other metals in the resultant process solution may have been disposed of during the nickel and/or cobalt recovery process. For example, the majority of the aluminium and iron is typically disposed of as hydroxides prior to nickel and cobalt recovery. The majority of the magnesium that is leached will generally remain and report to the brine as magnesium sulfate. Magnesium oxide is typically used in nickel and/or cobalt recovery processes as an alkali to promote hydroxide precipitation. Where magnesium oxide is used, the magnesium will remain in solution and may be discarded as magnesium sulfate to the brine.

A particular source of magnesium sulfate in nickel and/or cobalt processing, is from the naturally occurring magnesium in the nickel containing ore, particularly the leaching of magnesium minerals such as serpentine, which is often present in large amounts in saprolites. The naturally occurring magnesium will leach as magnesium sulfate following the addition of sulfuric acid. Furthermore, the naturally occurring magnesium will leach and report to the brine solution if a magnesium containing alkali, such as magnesium oxide, magnesium hydroxide, magnesium carbonate or dolomite is used for neutralisation purposes.

In one embodiment of the invention, the metal sulfate may exist as a hydrated product whether that be as the sulfate as it exists in brine or sourced from elsewhere. In order to produce a solid component, the metal sulfate in solution may be crystallised. The metal sulfate may be crystallised by a conventional 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 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 the calcination step to evaporate much of the water producing a hot concentrated solution of the metal sulfates, which upon cooling, crystallise out of the solution as crystallised metal sulfates.

The crystallised metal sulfates may be dehydrated prior to calcining, to remove excess water. Preferably the crystals are dried to a monohydrate or anhydrous state prior to calcining to reduce water vapour in the sulfur dioxide gas stream.

The crystallised metal sulfate salts may then be calcined. Any suitable calciner operating at elevated temperature may be used for this purpose. Suitable calciners may include rotary kilns, multiple hearth roasters, fluid bed roasters and other calciners.

Coal is the preferred fuel for the calciner, however any suitable fuel may be used such as natural gas, methane, propane, carbon monoxide, hydrogen, elemental sulfur, biomass, waste paper or plastic, or liquid hydrocarbon fuels, depending on the availability of the appropriate fuel. Calcining the crystallised metal sulfate salts converts the metal sulfate to the corresponding metal oxide while sulfur dioxide is released in the off gases from the calciner.

When magnesium sulfate salts are processed, the resultant magnesium oxide produced during the calcination processes may be useful as an alkali for use in the nickel and cobalt recovery process, depending upon the reactivity of the magnesium oxide. Alternatively, other uses may be found for the magnesium oxide, and indeed other oxides that may result, including commercially on- selling the product, or the oxide may simply be disposed of as waste, depending on its quality. The calciner may also include a coal combustion chamber and an ash handling system in order to reduce contamination of the corresponding metal oxide, and enhance the reactivity of the metal oxide.

In a particular embodiment, the sulfur acid absorption step includes a step of injecting the sulfur dioxide/oxygen mix into a slurry of carbon particles in recycled process water. The carbon particles act as a catalyst for the process.

The process for generating the sulfuric acid would generally take place in a stirred tank reactor with a gas dispersion turbine. Alternatively, a scrubbing tower, or series of scrubbing towers may be used where carbon is used as packing material within the towers.

A thickener or other solid liquid separation step may be included if desired to recover and recycle the carbon to the acid generation step.

Brief Description of the Drawings

Figures 1 and 2 show possible flowsheets for the leaching of a nickel laterite ore.

Figure 3 shows a graph of the kinetics of acid generation comparing experimental use with water and a magnesium sulfate brine solution.

Detailed Description of the Invention It would be convenient to describe the detailed embodiments of the invention with reference to Figures 1 and 2. The possible flowsheets shown in these drawings are 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 for heap leaching of nickel laterite ore, while Figure 2 shows a similar flowsheet, however the process includes a cooling/recrystallisation step where acid produced from the absorption step is used in the crystallisation process, and/or heat produced by the gas from the calcination step reduces the water content by evaporation, concentrating the magnesium content and allowing the magnesium to crystallise naturally when the solution cools.

During a heap leach or atmospheric leach process (1 ), sulfuric acid is generally used in order to leach the nickel and/or cobalt containing ore. This sulfuric acid is generally dilute, for example 10%-15% strength is suitable. 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 leached 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 this particular process, the resultant brine solution that remains following the precipitation of both wanted and unwanted metals will retain metal sulfates.

Figures 1 and 2 show a process where magnesium sulfate remains in the brine solution following the precipitation of nickel and cobalt.

The metal sulfate upon crystallisation from the brine solution will generally form a hydrated salt. In the embodiment shown in Figure 1 , the process involves crystallising the hydrated magnesium sulfate (6). This can be achieved by a conventional evaporative crystallisation step.

Alternatively, crystallisation can be achieved by the use of sulfuric acid, which is added to the brine solution to salt out solid crystalline metal sulfate. Figure 2 shows an embodiment where sulfuric acid that has been produced in accordance with the process (8) is used for the crystallisation/salting step.

Further concentrated sulfuric acid may be added if desired, to dehydrate the magnesium sulfate crystals to produce a solid, substantially dehydrated crystalline metal sulfate product.

In a preferred form, the concentration of the acid used in the salting process is in excess of 100 g/L.

Further, Figure 2 shows an embodiment where the residual heat in the gas from the calcination step (1 1 ), which contains inert gases such as nitrogen and carbon dioxide, will raise the temperature of the magnesium sulfate brine during the absorption step (7) and remove a considerable amount of water vapour. This results in an increased magnesium concentration and removes water relatively cheaply, as it employs the specific heat of the gas which would otherwise be wasted. Cooling of the resulting hot saturated solution, to crystallise the metal sulfate salt thereby offers a cheaper alternative to an evaporative crystallisation step.

If desired, a soluble organic reagent may be added to the metal sulfate solution to lower the solubility of the metal sulfate salt.

The crystallised salts are separated from the process solution in a solid/liquid separation step. The solid crystallised salt may undergo a drying step to remove any water that may remain in excess. The drying step may be performed in any suitable commercial dryer, such as is described in U.S. patent 4,020,564. The disclosure in the U.S. patent is incorporated herein by reference. Other metal sulfates, such as iron or aluminium sulfate may be added to the process at this point.

The dried magnesium sulfate may be in the form of a monohydrate or anhydrous if desired. An advantage of drying to anhydrous magnesium sulfate is that less water vapour is included in the sulfur dioxide gas stream, and therefore less water is returned to the acidified process solution during the absorption step.

The crystallised metal sulfate salts are calcined (11 ). Any suitable calciner operating at elevated temperature may be used. Suitable calciners may include rotary kilns, multiple hearth roasters, fluid bed roasters and other calciners. The drying step may also take place in the same calciner.

Coal is the preferred fuel for the calciner, however any suitable fuel may be used such as natural gas, methane, propane, carbon monoxide, hydrogen, elemental sulfur, biomass, waste paper or plastic, or liquid hydrocarbon fuels.

In order to avoid contamination of the metal oxide with ash, if coal is used to power the calciner, the process may include a coal combustion chamber and an ash handling system to provide for higher purity metal oxide. This is particularly relevant if the magnesium oxide is to be used in other parts of the nickel and/or cobalt recovery process, for example the iron/aluminium precipitation, or the mixed hydroxide precipitation steps. The magnesium oxide may be suitably reactive for the mixed hydroxide precipitation step if contamination with ash from the coal is minimised.

Calcining the crystallised metal sulfate will produce the respective metal oxide together with sulfur dioxide in the off gases from the roasting process. In order to generate the sulfuric acid, the sulfur dioxide from the calcination step may be combined with oxygen and water in the absorption steps (7) and (12). The sulfur dioxide containing gas from the calcination step typically contains from 5% to 7% sulfur dioxide by volume. If excess air is used during the calcination process, there will be oxygen present in the gas. Additional air may be added to the gas stream to adjust the sulfur dioxide to oxygen ratio to an appropriate level for the absorption step.

Some makeup of sulfur dioxide may be required and that may be sourced from a simple sulfur burner. Combustion of sulfur in the sulfur burner may be achieved with any stoichiometry with air, oxygen, or oxygen enriched air. A preferred embodiment is to combust liquid elemental sulfur with sufficient air to afford an appropriate stoichiometry of oxygen for the absorption step.

The sulfur dioxide/oxygen gas mixture may be injected into a slurry of carbon particles suspended in the process solution produced in the solid/liquid separation step in the absorption process. In one embodiment, the sulfuric acid is generated in an absorption step by combining the sulfur dioxide and oxygen mix and injecting the mix into a slurry of carbon particles in recycled processed water (13). The carbon acts as the catalyst for the reaction.

The water may also be fresh water, or sourced from the brine solution itself, or an intermediate leachate that is available as part of the nickel and/or cobalt recovery process. The oxygen may be sourced as a result of higher air stoichiometry in the calciner, or in the sulfur burner, or can be added as dilution air after steam generation.

The sulfur dioxide/oxygen gas absorption step may be as described in a paper by Roizard, X, Py, C, and Midoux, in, "Kinetics of Sulfur Dioxide Oxidation in Slurries of Activated Carbon and Concentration Sulfuric Acid', Chem. Eng. ScL, 50 (13), 1995, BP 2069-2079. The disclosure of this paper is incorporated herein by reference.

The sulfur dioxide/oxygen gas absorption step may be best performed in a stirred tank reactor with a gas dispersion high shear turbine. The carbon catalyst in particulate form allows the sulfur dioxide and oxygen to react to produce sulfuric acid at commercially acceptable rates of reaction. Finely divided carbon increases the surface area catalysing the reaction.

The carbon will preferably be in particulate form to allow suspension by the agitator, so a thickener and underflow recycle is preferably included in order to recover and return carbon to the stirred tank reactor.

Alternatively, the absorption step may also take place in a scrubbing tower or towers whereby the solution is irrigated into the tower over pall-ring packing material. The gas mix is introduced at the base of each tower such that the gas flow is counter current to the solution flow. The pall-rings or packing materials are comprised, at least in part, of the carbon which is the catalyst.

The preferred case is for the packing material to be comprised entirely of carbon suitable for catalysing the reaction.

The sulfuric acid produced following the absorption step will generally be dilute, for example 10%- 15% strength, which makes it suitable for use in atmospheric and heap leach processes. The sulfuric acid may also find other uses in metal recovery processes, and may be used, for example in the salting out process of the crystallised metal sulfates in the current process.

The process chemistry involved may be summarised as follows:

Crystallisation:

MgSO₄ + 7H₂O -> MgSO₄.7H₂O

Drying:

MgSO₄.7H₂O -> MgSO₄-H₂O + 6H₂O

Calcination:

MgSO₄-H₂O + CO -> MgO + CO₂ + SO₂ + H₂O Absorption:

The process of the present application provides for an economic means to produce sulfuric acid which is suitable for use in a leach solution for either heap or atmospheric leaching of nickel and/or cobalt containing ores.

It is also anticipated that there will be significant capital cost savings with respect to avoiding the need to provide an acid plant for such processes. This of course is offset by the need to provide a dryer, calciner and infrastructure for the absorption steps in the current process, however it is anticipated that savings would be made with respect to the need for infrastructure.

Further, it is a particular benefit that a commercially useful product is recovered from a source of metal sulfates that would otherwise be at best, simply discarded as waste product. This also has the benefit of alleviating potential environmental concerns that could result by simply discarding the metal sulfates as waste product. There would be considerable capital and operating cost savings over conventional lime based precipitation of magnesium from magnesium sulfate containing brines.

In an additional benefit, water is removed using the waste heat of the calcination step. This reduces the cost in energy of removing water by evaporation, and maintaining a circuit water balance, should this be required. In this respect the crystallisation of magnesium sulfate also is made cheaper by removing the need for an expensive evaporative crystalliser.

A further advantage is that the process eliminates the need for concentrated acid, thereby saving on the cost of acid, or the cost of elemental sulfur used in the acid plant to produce concentrated acid.

A further advantage is that the metal oxide produced, particularly magnesium oxide, may be used as an alkali during the nickel and cobalt recovery process. Examples

The examples herein demonstrate the embodiment of the invention where acid is generated in a stirred tank reactor.

Example 1 - Comparative

A 3 L glass culture vessel was equipped with a 60mm Smith turbine agitator, filled with 2 L of water, placed on a hotplate and heated to 60 °C. Two stainless steel sparger tubes were used to deliver 0.5 L/min each of SO₂ and O₂ from gas bottles to a point just under the Smith impellor, which rotated at 300s^"1. Activated carbon 500 g (250 g/L) of was placed in the reaction vessel, and the acidity and aqueous SO₂ concentrations were monitored by titration. The experiment was allowed to continue for 19 hours. The results of the experiment are shown in Figure 3.

Example 2 - MgSO₄ brine

The conditions of Example 1 were repeated with a MgSO₄ brine solution containing 40 g/L Mg. The experiment was allowed to continue for 14 hours. The results of this experiment are shown in Figure 3.

The data in Figure 3 show that the kinetics of acid generation are pseudo-zero order, which signifies that oxygen solubility or the oxygen dissolution rate are rate determining. The rate of reaction is very similar between the water case (Example 1 ) and the MgSO₄ brine case (Example 2), which shows that magnesium sulfate does not inhibit the reaction. Furthermore the increase in acidity during both experiments has not inhibited the reaction kinetics. The concentration of dissolved SO₂ remained low throughout both experiments.

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 the recovery of sulfuric acid from a metal sulfate in solution, including the steps of: a) providing a source of metal sulfate in solution; b) converting the metal sulfate in solution into a crystallised metal sulfate salt; c) calcining the crystallised metal sulfate salt to produce the corresponding metal oxide and sulfur dioxide gas; and d) generating sulfuric acid.

2. A process for the recovery of sulfuric acid 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 in solution that is derived from part of a nickel and/or cobalt recovery process that utilises sulfuric acid to leach nickel and/or cobalt containing ores; e) converting the metal sulfate in solution into a crystallised metal sulfate salt; f) calcining the crystallised metal sulfate salt to produce the corresponding metal oxide and sulfur dioxide; and g) regenerating sulfuric acid and recirculating the sulfuric acid for use in the nickel and/or cobalt recovery process.

3. A process according to claims 1 or 2 wherein sulfur dioxide in the off-gas of the calcining step is combined with oxygen and water to generate the sulfuric acid in an adsorption step.

4. A process according to claim 1 or 2 wherein the metal of the metal sulfate is magnesium, iron (II), iron (III), aluminium, chromium, manganese, nickel or cobalt

5. A process according to any one of claim 3 wherein the source of the metal sulfate is a brine solution.

6. A process according to claim 5 wherein the brine solution is produced in a nickel and/or cobalt recovery process wherein the nickel and/or cobalt recovery process includes the step of leaching a nickel and/or cobalt containing ore with sulfuric acid.

7. A process according to claims 1 or 2 wherein the metal sulfates are crystallised by evaporative crystallisation.

8. A process according to claims 1 or 2 wherein the metal sulfates are crystallised by adding sulfuric acid to salt out metal sulfate salts.

9. A process according to claim 8 wherein the sulfuric acid is sourced from the sulfuric acid generation step.

10. A process according to claims 1 or 2 wherein the metal sulfates are crystallised by using waste heat from the calcination step to remove water by evaporation, producing a hot concentrated solution of the metal sulfates, which on cooling crystallise out of the solution.

1 1. A process according to claims 1 or 2 wherein the crystallised metal sulfate is calcined in a calciner powered by coal, methane, natural gas, carbon monoxide or hydrogen.

12. A process according to claim 1 1 , wherein the calciner is powered by coal and provides a source of carbon monoxide.

13. A process according to claim 12 wherein the calciner includes a coal combustion chamber and an ash handling system to reduce ash contamination of the corresponding metal oxide.

14. A process according to claim 1 wherein the metal sulfate crystals are dehydrated to a monohydrate or anhydrous state prior to the calcining step.

15. A process according to claim 1 or 2 wherein the metal sulfate is magnesium sulfate, and the corresponding metal oxide is magnesium oxide.

16. A process according to claim15 wherein the magnesium oxide produced is sufficiently reactive to be recirculated into the nickel and/or cobalt recovery process, for use as an alkali in the precipitation of a nickel and/or cobalt hydroxide product, or for iron and/or aluminium precipitation.

17. A process according to claim 3 wherein a make-up source of sulfur dioxide for the sulfuric acid absorption step is produced by combusting elemental sulfur in a sulfur burner.

18. A process according to claim 3 wherein the oxygen is sourced as a result of the higher air stoichiometry in the calciner or the sulfur burner; or added as dilution air after steam generation.

19. A process according to any one of claims 3 to 7 wherein the sulfuric acid generation step includes the step of injecting the sulfur dioxide/oxygen mix into a slurry of carbon particles in recycled processed water.

20. A process according to claim 19 wherein the recycled process water is sourced from one or more of the raw brine, fresh water, a solution produced by crystallising out the metal sulfate, or an intermediate leach solution in the nickel and/or cobalt recovery process.

21. A process according to claims 19 or 20 wherein the carbon particles are used as a catalyst in the sulfuric acid generation process.

22. A process according to claims 1 or 2 wherein the sulfuric acid generation step is performed in a stirred tank reactor with a gas dispersion turbine.

23. A process according to claims 1 or 2 wherein a thickener and underflow recycle step is used in the sulfuric acid generation step to recover and return carbon to the stirred tank reactor.

24. A process according to claims 1 or 2 wherein the sulfuric acid generation step is performed in one or more scrubbing towers wherein the sulfur dioxide and oxygen are contacted with recycled process water and a carbon catalyst.

25. A process according to claim 24 whereby the carbon catalyst is in the form of a scrubbing tower packing material.

26. A process according to claim 2 wherein the nickel and/or cobalt recovery process is a heap leach or atmospheric leach process of a laterite ore with sulfuric acid as the leach solution.

27. A process according to claims 1 or 2 wherein the sulfuric acid generated is dilute sulfuric acid.