AU2012297573A1

AU2012297573A1 - Method for the recovery of magnesium sulphate and production of magnesium oxide

Info

Publication number: AU2012297573A1
Application number: AU2012297573A
Authority: AU
Inventors: Brett David MULLER
Original assignee: MULLER BRETT; NEWAMU IP HOLDINGS Pty Ltd
Current assignee: NEWAMU IP HOLDINGS Pty Ltd
Priority date: 2011-08-16
Filing date: 2012-08-16
Publication date: 2013-05-09
Anticipated expiration: 2032-08-16
Also published as: WO2013023254A8; AU2012297573B2; AU2012297573A8; WO2013023254A1

Abstract

A method (10) for the recovery of solid magnesium sulphate from a process solution (12) containing dissolved magnesium sulphate. The method comprises concentrating the process solution (12) using membrane filtration (16) to form a concentrated process solution, and treating the concentrated process solution to precipitate solid magnesium sulphate.

Description

WO 2013/023254 PCT/AU2012/000980 1 "METHOD FOR THE RECOVERY OF MAGNESIUM SULPHATE AND PRODUCTION OF MAGNESIUM OXIDE" Field of the Invention The present invention relates to a method for the recovery of magnesium 5 sulphate from a process solution. The invention relates particularly although not exclusively to a process solution formed during a nickel extraction process such as a sulphuric acid leach of lateritic ore. The invention also describes a method for the conversion of magnesium sulphate to magnesium oxide. 10 Background to the Invention In current commercial practice, nickel is leached from lateritic ore using sulphuric acid, normally at high temperature and pressure. One of the most commonly used methods to purify the nickel and cobalt in the leach solution 15 involves a partial neutralisation step with limestone, to bring the pH of the solution to between 4 and 4.5. This precipitates the majority of the iron and aluminium, plus.some chromium and copper, from solution as hydroxides. The precipitate containing gypsum, plus hydroxides of iron, aluminium, copper and chromium, is separated from solution, washed, and discarded to 20 tailings impoundment. The remaining solution containing the nickel and cobalt is further neutralised by magnesia, to precipitate nickel cobalt mixed hydroxide product. Magnesia is used in this step instead of lime, as this avoids contamination 25 of the product by the precipitation of gypsum (which happens when lime is used due to the lower solubility of calcium sulphate as compared to magnesium sulphate). The solution still contains some nickel and cobalt, which is removed by further precipitation with lime in a second precipitation stage. The solids from this stage are washed and returned to leach to 30 recover the metal value.

WO 2013/023254 PCT/AU2012/000980 2 The solution leaving this second mixed hydroxide precipitation stage now contains largely manganese and magnesium sulphates. The manganese is removed by further increasing the pH with lime, and also by sparging air into 5 the precipitation tanks to encourage the oxidation of Mn(ll) to Mn(IV) and promote the precipitation of MnO 2 . Following the removal of manganese, the solution high in magnesium sulphate is discarded, either to a body of water such as an ocean, or to an evaporation pond where the crystallised magnesium sulphate remains as waste. 10 However it would be preferable economically and environmentally if the magnesium sulphate in this waste solution could be recycled and reused. Prior art methods to recover magnesium sulphate in these instances have been expensive to operate, often employing high temperatures and 15 pressures, and requiring sophisticated equipment and technology. In addition, since magnesium oxide is a chemical required in the metal recovery process it would be desirable if the magnesium sulphate could be converted to magnesium oxide. 20 References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere. Summary of the Invention The present invention provides a method for the recovery of solid 25 magnesium sulphate from a process solution containing dissolved magnesium sulphate, the method comprising: i) concentrating the process solution using membrane filtration to form a concentrated process solution; and 30 ii) treating the concentrated process solution to precipitate solid magnesium sulphate.

WO 2013/023254 PCT/AU2012/000980 3 Typically the process solution is a process solution associated with leaching of a metal containing ore or concentrate. Typically the process solution is an effluent solution associated with a leach of nickel and/or cobalt 5 containing ore. The nickel and/or cobalt containing ore is preferably lateritic ore. Typically the process solution also comprises dissolved manganese. The manganese may be removed from the process solution by increasing the 10 pH. Typically the pH is increased using lime. Alternatively the pH may be increased by sparging the process solution with air to encourage the oxidation of Mn(II) to Mn(IV). Preferably membrane filtration comprises reverse osmosis. Membrane 15 filtration may comprise nano-filtration. ' Membrane filtration may comprise reverse osmosis and nano-filtration. Preferably the treatment step comprises evaporation of the concentrated process solution. Evaporation may be carried out using steam. Typically 20 evaporation produces a monohydrate salt of magnesium sulphate. By using membrane filtration to initially form a concentrated process solution, less steam is required in the evaporation step to form the solid magnesium sulphate. 25 In the case of evaporation, preferably a bleed stream is employed which assists in minimising precipitation of chlorides or fluorides from solution. Alternatively, the treatment step comprises refrigerative cooling of the concentrated process solution. Typically the refrigerative cooling takes place 30 at a temperature of about 2 to 12 0 C resulting in the formation of a heptahydrate salt of magnesium sulphate.

WO 2013/023254 PCT/AU2012/000980 4 Preferably the method further comprises the step of dehydrating the solid magnesium sulphate. Typically the solid magnesium sulphate is monohydrate or heptahydrate magnesium sulphate. Typically the solid magnesium sulphate is converted to anhydrous magnesium sulphate in the 5 dehydration step. Preferably solid magnesium sulphate, in the form of magnesium sulphate monohydrate or heptahydrate, is treated in the dehydration step by heating and dehydrating at temperatures greater than 360 0 C, which results in the formation of anhydrous magnesium sulphate. 10 The anhydrous magnesium sulphate may be further treated in a reduction step to form magnesium oxide. Preferably the reduction step takes place using indirect heat exchange with hot gas from a sulphur burner. The sulphur burner typically produces a feed gas that can be converted to sulphuric acid in a conventional sulphuric acid plant, following the removal 15 of some heat during the reduction of anhydrous magnesium sulphate to magnesium oxide. The temperature of the feed gas is preferably about 1100*C. Preferably magnesium sulphate is treated with elemental sulphur in the 20 reduction step to form magnesium oxide. The magnesium oxide can then be employed in the usual processing stages in the nickel and/or cobalt recovery as outlined above. The reaction typically proceeds according to the following chemistry: 25 4 MgSO 4 (s) + 1 S 2 (g) -> 4 MgO(s) + 6 S0 2 (g) Preferably the anhydrous magnesium sulphate is combined with solid 30 elemental sulphur. It is then heated by the hot sulphur burner gas indirectly in a counter current manner in the reduction step utilising a screw conveyor or similar device designed to provide good gas-to-solids contact, and to WO 2013/023254 PCT/AU2012/000980 5. provide a solids residence time of about 20-60 minutes. More preferably the solids residence time is about 40 minutes. The exit temperature for the hot solids and cooled exiting sulphur burner 5 gas is preferably about. 700-900*C. The resulting magnesium oxide product is preferably comprised of at least about 90% w/w magnesium oxide with the remaining amount being unreacted magnesium sulphate. The resulting magnesium oxide product typically is cooled and stored for later use in the mixed hydroxide precipitation, or as otherwise needed. 10 The particle size of the magnesium oxide may be reduced by milling if required. The cooled sulphur burner gas may be mixed with combustion products of a 15 secondary sulphur burner, with additional air to form a combined gas to combust any unreacted elemental sulphur and to achieve an 02 to S02 molar ratio of around 0.8-1.0. The combined gas is preferably cooled to a temperature of around 430 0 C, such as by using a waste heat boiler, where heat is recovered as high pressure steam. The S02 gas generated during 20 the reduction reaction can be collected as a separate, high purity S02 gas stream that can be used to produce liquefied SO 2 if required. The present invention further provides a method for the conversion of solid magnesium sulphate to magnesium oxide, the method comprising: 25 i) dehydrating solid magnesium -sulphate to form anhydrous magnesium sulphate; and ii) reducing the anhydrous magnesium sulphate to form magnesium oxide. 30 WO 2013/023254 PCT/AU2012/000980 6 Preferably the magnesium sulphate is converted to anhydrous magnesium sulphate in the dehydration step using a dryer or dehydrator. Typically the dehydration step is carried out at temperatures greater than 360 0 C. 5 Preferably the reduction step takes place using indirect heat exchange with hot gas from a sulphur burner. The sulphur burner typically produces a feed gas that can be converted to sulphuric acid in a conventional sulphuric acid plant, following the removal of some heat during the reduction of anhydrous magnesium sulphate to magnesium oxide. The temperature of the feed 10 gas is preferably about 1100*C. Alternatively, hot combustion gases from a carbon based fuel source, such as natural gas, diesel or fuel oil, can be used to provide indirect heating to the anhydrous magnesium sulphate and sulphur, for the reduction step 15 The present invention also provides a method for the preparation of magnesium oxide from a process solution containing dissolved magnesium sulphate, the method comprising: 20 i) concentrating the process solution using membrane filtration to form a concentrated process solution; ii) treating the concentrated process solution to precipitate solid magnesium sulphate; and iii) reducing the solid magnesium sulphate to form magnesium 25 oxide. Preferably the solid magnesium sulphate is. converted to anhydrous magnesium sulphate in a dehydration step prior to the reduction step. Preferably the reduction step takes place using indirect heat exchange with 30 hot gas from a sulphur burner.

WO 2013/023254 PCT/AU2012/000980 7 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. Likewise the 5 word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention. Brief Description of the Drawings 10 The nature of the invention will be better understood from the following detailed description of several specific embodiments of a method for the recovery of solid magnesium sulphate from a process solution containing dissolved magnesium sulphate, and a method for the conversion of solid magnesium sulphate to magnesium oxide, given by way of example only, 15 with reference to the accompanying drawings, in which: Figure 1 is a flow diagram of the steps in one embodiment of a method for the recovery of solid magnesium sulphate from a process solution containing dissolved magnesium sulphate, followed by a further 20 reduction step to form magnesium oxide; Figure 2 is a graph showing flux versus feed magnesium concentration for RO filtration at 40 bar; Figure 3 is a graph showing gas composition and MgO conversion versus temperature, with sulphur addition; and, 25 Figure 4 is a graph showing gas composition and MgO conversion versus temperature, without sulphur addition.

WO 2013/023254 PCT/AU2012/000980 8 Detailed Description of Preferred Embodiments A preferred embodiment of a method 10 for the recovery of solid magnesium sulphate from a process solution 12 containing dissolved magnesium sulphate in accordance with the invention is illustrated in Figure 5 1. The embodiment also includes a method for the conversion of solid magnesium sulphate to magnesium oxide. The method comprises the steps of concentrating the process solution 12 in a concentration step 14 using membrane filtration 16 to form a concentrated 10 process solution and treating the concentrated process solution to precipitate solid magnesium sulphate in a treatment step. Typically the process solution is a process solution 12 associated with leaching of a metal containing ore or concentrate. Typically the process 15 solution 12 is an effluent solution associated with a leach of nickel and/or cobalt containing ore such as lateritic ore. The process solution 12 may also contain dissolved manganese which is preferably removed (not illustrated) prior to membrane filtration. The 20 manganese may be removed from the process solution by increasing the pH such as with lime or alternatively by sparging the process solution with air to encourage the oxidation of Mn(ll) to Mn(IV). Preferably membrane filtration 16 comprises reverse osmosis membrane 25 technology. Membrane filtration may comprise nano-filtration technology. Membrane filtration may comprise both reverse osmosis and nano-filtration. Preferably a relatively dilute process solution containing predominantly aqueous magnesium sulphate can be concentrated by removing some of the water through a reverse osmosis membrane. The recovered water 17 30 can be used elsewhere in the process.

WO 2013/023254 PCT/AU2012/000980 9 In the illustrated embodiment the treatment step further comprises evaporation 18 of the concentrated process solution. Evaporation may be carried out using steam 52 from a sulphuric acid plant 50 to heat the concentrated solution. Typically evaporation 18 produces a monohydrate 5 salt of magnesium sulphate 20 (one form of solid magnesium sulphate). By using membrane filtration 16 to initially form a concentrated process solution, less steam 52 is required in the evaporation step 18 to form the solid magnesium sulphate 20. By reducing the amount of steam required by the process, enough steam can be generated by the sulphuric acid plant 10 50 to avoid the need for supplementary steam production. Preferably a bleed stream 24 is employed during evaporation 18, which assists in minimising precipitation of chlorides or fluorides from solution. 15 Alternatively, the treatment step comprises refrigerative cooling (not shown in Figure 1) of the concentrated process solution (typically at a temperature of 2-12 *C) which results in the formation of a heptahydrate salt of magnesium sulphate, another form of solid magnesium sulphate. 20 Preferably the method 10 further comprises the step of dehydrating 22 the solid magnesium sulphate, such as by using a dryer and/or dehydrator 26. Typically the monohydrate salt of magnesium sulphate is in the form of a slurry 20 from the evaporation step 18, which ban be concentrated by using a centrifuge 30. The resulting centrate is then returned to the evaporation 25 step 18 and the solid magnesium sulphate is passed to the dryer/dehydrator 26. Typically the solid magnesium sulphate (monohydrate or heptahydrate magnesium sulphate) is converted to anhydrous magnesium sulphate 32 30 during the dehydration step 22 in the dryer/dehydrator 26. Preferably solid magnesium sulphate, in the form of magnesium sulphate monohydrate or heptahydrate, is treated in the dryer/dehydrator 26 by drying and/or WO 2013/023254 PCT/AU2012/000980 10 dehydrating at temperatures greater than 3600C which results in the formation of the anhydrous magnesium sulphate 32. The anhydrous magnesium sulphate 32 may be further treated in a 5 reduction step 34 by calcination to form magnesium oxide 36. Preferably the reduction step 34 takes place using indirect heat exchange with hot gas from a sulphur burner 38. The sulphur burner 38 typically produces a feed gas 46 that can be converted to sulphuric acid in the conventional sulphuric acid plant 50, following the removal of some heat during the reduction of 10 anhydrous magnesium sulphate to magnesium oxide. The temperature of the feed gas 46 is preferably about 1100C. The anhydrous magnesium sulphate 32 is treated with elemental sulphur in the reduction step 34 to form magnesium oxide 36. The magnesium oxide 15 36 can then be employed in the usual processing stages in the nickel and/or cobalt recovery as outlined above. The sulphur burner 38 is used to produce the feed gas 46 containing SO 2 and N 2 , plus enough elemental sulphur vapour to reduce the anhydrous 20 magnesium sulphate to magnesium oxide in the reduction step 34. The reduction reaction typically proceeds according to the following chemistry: 4 MgSO 4 (s) + 1 S 2 (g) -> 4 MgO(s) + 6 S0 2 (g) 25 Preferably the anhydrous magnesium sulphate 32 is combined with solid elemental sulphur. It is then heated by the hot sulphur burner gas indirectly in a counter current manner in the reduction step 34 utilising a screw conveyor or similar device designed to provide good gas-to-solids contact, and provide a solids residence time of about 20-60 minutes. More 30 preferably the solids residence time is about 40 minutes. The exit temperature for the hot solids and cooled exiting combined gas 40 is preferably about 700-9000C.

WO 2013/023254 PCT/AU2012/000980 11 The resulting magnesium oxide product 36 is preferably comprised of at least about 90% w/w magnesium oxide with the remaining amount being urireacted magnesium sulphate. The resulting magnesium oxide product 5 36 is typically cooled and stored for later use in the mixed hydroxide precipitation, or other application as required. The particle size of the magnesium oxide 36 may be reduced by milling if required. By recovering magnesium oxide 36 from waste solution according to the 10 invention, it may now be cost effective to replace lime used in other areas of the plant with recycled magnesium oxide,. such as for manganese removal, or secondary mixed hydroxide precipitation. This has the advantage of reducing the cost of the lime, but also reduces the amount of gypsum precipitate which must be stored with tailings. 15 The exiting combined gas 40 from the reduction step 34 may be mixed with combustion products of a secondary sulphur burner (not shown in Figure 1), with additional air to form a further combined gas to combust any unreacted elemental sulphur and to achieve an 02 to SO 2 molar ratio of around 0.8-1.0. 20 The combined gas 40 is preferably cooled to a temperature of around 350 0 C, such as by using a waste heat boiler 42, where heat is recovered as high pressure steam 44. The high pressure steam 44 is either used to heat a pressure acid leach 25 autoclave in the process, or to generate power by passing it through an expansion turbine, or as required at other points in the overall process. After exiting the waste heat boiler 42, dust is collected without introducing any water, either with a cyclone collector, large gravity settling chamber or a ceramic filter. The resulting clean gas 48 is then sent to the catalyst beds of 30 a conventional sulphuric acid plant 50, with integrated heat recovery to preheat the boiler feed water, and to superheat the boiler steam. Additionally, a heat recovery system is included in the absorber tower WO 2013/023254 PCT/AU2012/000980 12 design, to produce low to medium pressure steam 52 in a secondary boiler. This steam is required for the magnesium sulphate monohydrate evaporators in the evaporation step 18. The sulphuric acid produced by the sulphuric acid plant 50 is stored and used to leach the ore as required in the 5 nickel leaching step. Test Results: Water recovery from magnesium sulphate solution The recovery of water from a neutral dilute magnesium sulphate solution 10 was tested by feeding a dilute magnesium sulphate solution to the Sepa@ CFII test cell, fitted with the Koch TFC@ ULP reverse osmosis membrane. Permeate was collected in 100 mL increments, with the retentate recycled to the feed tank. The feed solution magnesium concentration thus increases as permeate is removed from the system. The performance of the 15 membrane can therefore be determined as a function of the magnesium concentration in the feed solution. The permeate flux observed for magnesium sulphate solution passing through the ULP membrane is shown in Figure 2 as a function of the feed magnesium concentration, at a pressure of 40 bar. The flux remained high 20 at the maximum magnesium feed concentration tested. Extrapolation of the results in Figure 2, magnesium concentrations of 30 g/L at 40 bar and, 40 g/L at 50 to 60 bar are considered achievable. Triple effect evaporation The concentrated magnesium sulphate solution from the membrane filter is 25 introduced into the first stage of a triple effect evaporator, mechanical vapour recompression unit or similar commercially available evaporator. Some of the magnesium sulphate crystallises in the first effect as the monohydrate salt (MgSO 4

*H

2 0). Crystals settle in the settling area of the effect and are discharged to the first effect centrifuge to separate the 30 crystals from the saturated magnesium sulphate centrate. The first effect WO 2013/023254 PCT/AU2012/000980 13 centrifuge centrate, plus liquor from the first effect is pumped to the second effect. Further crystallisation of monohydrate magnesium sulphate occurs in the second effect, which settle in the settling area and are discharged to the 5 second effect centrifuge. Crystals are separated from the saturated magnesium sulphate centrate. The second effect centrifuge centrate, plus liquor from the second effect is pumped to the third effect. Further crystallisation of monohydrate magnesium sulphate occurs in the third effect, which settle in the settling area and are discharged to the third effect 10 centrifuge. Crystals are separated from the saturated magnesium sulphate centrate by the centrifuge. The centrate is pumped to either the second or third effect for further evaporation, with a small bleed pumped to tailings disposal to limit the build up of chlorides and fluorides. Crystals collected by the three centrifuges are discharged to a silo with a 15 rotary feeder that feeds the dehydrating screw heat exchanger. To ensure monohydrate salt (MgSO4*H20). is produced the crystallisation must occur above - 70-75 0 C (Hogenboom, 1990). Magnesium sulphate dehydration The collected crystals from the triple effect centrifuges contain small 20 amounts of unbound water, plus water of crystallisation. Evaporation of the unbound water occurs at < 110 0 C, but dehydration of the bound water requires heating the crystals to above 350*C (Emons, 1990). Magnesium sulphate reduction Without the addition of sulphur, the decomposition of magnesium sulphate .25 requires 998*C. In the presence of elemental sulphur, the reduction to magnesium oxide is thermodynamically spontaneously above 466*C. The selected reaction temperature range of 750-900*C is chosen to maximise the reaction kinetics, while also maximising the reactivity of the MgO product. The introduction of sulphur as a reducing gas lowers the required WO 2013/023254 PCT/AU2012/000980 14 temperature to achieve the magnesium sulphate reduction reaction, in turn improving the economics of the process. Table 0-1 shows the temperature required to reduce magnesium sulphate with and without sulphur gas present. Figures 3 and 4 in the accompanying drawings provide the 5 supporting data. Table 0-1: Comparison of MgO production parameters with and without sulphur addition Summary data With sulphur Without sulphur Reactions 2MgSO4 + S = 2MgO + 3SO2(g) 2MgSO4 = 2MgO + 2SO2(g) + 02(g) MgSO4 = MgO + S03(q) Spontaneous at > 4 66*C > 99 8*C Gas composition S02(g) 99.999 % v/v S02(g) 73-73 % v/v SO3(g) 0.001 % v/v S03(g) 7.86 % v/v 1__ _ 02(g)18.41% v/v Operating range 750-900*C >ioo*C 10 This reduction in temperature for the magnesium sulphate reduction to magnesia, arid thus energy input, is a key economic consideration for the invention. Now that preferred embodiments of a method for the recovery of solid 15 magnesium sulphate from a process solution containing dissolved magnesium sulphate, and a further reduction step to form magnesium oxide have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following: 20 (i) By using membrane filtration to pre-concentrate the magnesium sulphate process solution, less steam is required to evaporate the magnesium sulphate crystals, saving energy and cost.

WO 2013/023254 PCT/AU2012/000980 15 (ii) The use of a sulphur burner in the reduction step is beneficial because no carbon based fuels are required so that the greenhouse gas emissions are minimised. (iii) The use of sulphur as an energy source for the reduction of 5 magnesium sulphate instead of coal or other carbon sources, allows this process to operate without generating CO 2 . (iv) The low/medium pressure steam produced by the heat recovery section of the sulphuric acid plant can be used to evaporate the magnesium sulphate, so that the energy from this steam is not 10 wasted. (v) The method of this invention avoids the need for potentially troublesome high pressure equipment such as. in a pressurised precipitation process. 15 It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, the method has particular application in the recovery of nickel from lateritic ores, 20 but the method may be readily applicable to other process solutions high in magnesium sulphate such as from other metal recovery processes. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described. 25

Claims

1. A method for the recovery of solid magnesium sulphate from a process 5 solution containing dissolved magnesium sulphate, the method comprising: i) concentrating the process solution using membrane filtration to form a concentrated process solution; and ii) treating the concentrated process solution to precipitate solid 10 magnesium sulphate.

2. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein the process solution is a process solution associated with leaching of a metal-containing ore or concentrate. 15

3. A method for the recovery of solid magnesium sulphate as defined in claim 2, wherein the process solution is an effluent solution associated with a leach of nickel and/or cobalt containing ore. 20

4. A method for the recovery of solid magnesium sulphate as defined in claim 3, wherein nickel and/or cobalt containing ore is lateritic ore.

5. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein the process 'solution also comprises dissolved manganese. 25

6. A method for the recovery of solid magnesium sulphate as defined in claim 5, wherein the manganese may be removed from the process solution by increasing the pH. 30

7. A method for the recovery of solid magnesium sulphate as defined in claim 6, wherein the pH is increased using lime. WO 2013/023254 PCT/AU2012/000980 17

8. A method for the recovery of solid magnesium sulphate as defined in claim 6, wherein the pH is increased by sparging the process solution with air to encourage the oxidation of Mn(II) to Mn(IV). 5

9. A method for the recovery of solid magnesium- sulphate as defined in claim 1, wherein membrane filtration comprises reverse osmosis.

10. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein membrane filtration comprises nano-filtration. 10

11. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein membrane filtration comprises reverse osmosis and nano filtration. 15

12. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein the treatment step comprises evaporation of the concentrated process solution.

13. A method for the recovery of solid magnesium sulphate as defined in 20 claim 12, wherein evaporation may be carried out using steam.

14. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein evaporation produces a monohydrate salt of magnesium sulphate. 25

15. A method for the recovery of solid magnesium sulphate as defined in claim 12, wherein a bleed stream is employed during evaporation to assist in minimising precipitation of chlorides or fluorides from solution. 30

16. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein the treatment step comprises refrigerative cooling of the concentrated process solution. WO 2013/023254 PCT/AU2012/000980 18

17. A method for the recovery of solid magnesium sulphate as defined in claim 16, wherein the refrigerative cooling takes place at a temperature of about 2 to 12 0 C resulting in the formation of a heptahydrate salt of 5 magnesium sulphate.

18. A method for the recovery of solid magnesium sulphate as defined in claim 1, wherein the method further comprises the step of dehydrating the solid magnesium sulphate. 10

19. A method for the recovery of solid magnesium sulphate as defined in claim 18, wherein the solid magnesium sulphate is monohydrate or heptahydrate magnesium sulphate. 15

20. A method for the recovery of solid magnesium sulphate as defined in claim 19, wherein the solid magnesium sulphate is converted to anhydrous magnesium sulphate in the dehydration step.

21. A method for the recovery of solid magnesium sulphate as defined in 20 claim 19, wherein the solid magnesium sulphate is treated in the dehydration step by heating and dehydrating at temperatures greater than 3601C, which results in the formation of anhydrous magnesium sulphate.

22. A method for the recovery of solid magnesium sulphate as defined in 25 claim 21, wherein the anhydrous magnesium sulphate is further treated in a reduction step to form magnesium oxide.

23. A method for the recovery of solid magnesium sulphate as defined in claim 22, wherein the reduction step takes place using indirect heat 30 exchange with hot gas from a sulphur burner. WO 2013/023254 PCT/AU2012/000980 19

24. A method for the recovery of solid magnesium sulphate as defined in claim 23, wherein the sulphur burner typically produces a feed gas that can be converted to sulphuric acid in a conventional sulphuric acid plant, following the removal of some heat during the reduction of anhydrous 5 magnesium sulphate to magnesium oxide.

25. A method for the recovery of solid magnesium sulphate as defined in claim 24, wherein the temperature of the feed gas is preferably about 1100 0 C. 10

26. A method for the recovery of solid magnesium sulphate as defined in claim 22, wherein the magnesium sulphate is treated with elemental sulphur in the reduction step to form magnesium oxide. 15

27. A method for the recovery of solid magnesium sulphate as defined in claim 26, wherein the anhydrous magnesium sulphate is combined with elemental sulphur vapour.

28. A method for the recovery of solid magnesium sulphate as defined in 20 claim 27, wherein the anhydrous magnesium sulphate is combined in the reduction step with elemental sulphur vapour and SO 2 gas in a counter current manner utilising a screw conveyor or similar device designed to provide good gas-to-solids contact, and to provide a solids residence time of about 20-60 minutes. 25

29. A method for the recovery of solid magnesium sulphate as defined in claim 28, wherein the solids residence time is about 40 minutes.

30. A method for the recovery of solid magnesium sulphate as defined in 30 claim 28, wherein the exit temperature for the hot solids and cooled exiting sulphur burner gas is preferably about 700-900 0 C. WO 2013/023254 PCT/AU2012/000980 20

31. A method for the recovery of solid magnesium sulphate as defined in claim 30, wherein the resulting magnesium oxide product is comprised of at least about 90% w/w magnesium oxide with the remaining amount being unreacted magnesium sulphate. 5

32. A method for the recovery of solid magnesium sulphate as defined in claim 31, wherein the resulting magnesium oxide product typically is cooled and stored for later use in the mixed hydroxide precipitation. 10

33. A method for the recovery of solid magnesium sulphate as defined in claim 26, wherein the particle size of the magnesium oxide is reduced by milling.

34. A method for the recovery of solid magnesium sulphate as defined in 15 claim 30, wherein the cooled sulphur burner gas is mixed with combustion products of a secondary sulphur burner, with additional air to form a combined gas to combust any unreacted elemental sulphur and to achieve an 02 to SO 2 molar ratio of around 0.8-1.0. 20

35. A method for the recovery of solid magnesium sulphate as defined in claim 34, wherein the combined gas is preferably cooled to a temperature of around 430 0 C.

36. A method for the recovery of solid magnesium sulphate as defined in 25 claim 35, wherein the combined gas is cooled using a waste heat boiler, where heat is recovered as high pressure steam.

37. A method for the recovery of solid magnesium sulphate as defined in claim 36, wherein the SO 2 gas generated during the reduction reaction can 30 be collected as a separate, high purity SO 2 gas stream that can be used to produce liquefied SO 2 . WO 2013/023254 PCT/AU2012/000980 21

38. A method for the conversion of solid magnesium sulphate to magnesium oxide, the method comprising: i) dehydrating solid magnesium sulphate to form anhydrous 5 magnesium sulphate; and ii) reducing the anhydrous magnesium sulphate to form magnesium oxide.

39. A method for the conversion of solid magnesium sulphate to 10 magnesium oxide as defined in claim 38, wherein the magnesium sulphate is converted to anhydrous magnesium sulphate in the dehydration step using a dryer or dehydrator.

40. A method for the conversion of solid magnesium sulphate to 15 magnesium oxide as defined in claim 39, wherein the dehydration step is carried out at temperatures greater than 360 0 C.

41. A method for the conversion of solid magnesium sulphate to magnesium oxide as defined in claim 38, wherein the reduction step takes 20 place using indirect heat exchange with hot gas from a sulphur burner.

42. A method for the conversion of solid magnesium sulphate to magnesium oxide as defined in claim 41, wherein the sulphur burner typically produces a feed gas that can be converted to sulphuric acid in a 25 conventional sulphuric acid plant, following the removal of some heat during the reduction of anhydrous magnesium sulphate to magnesium oxide.

43. A method for the conversion of solid magnesium sulphate to magnesium oxide as defined in claim 42, wherein the temperature of the 30 feed gas is preferably about 11000C. WO 2013/023254 PCT/AU2012/000980 22

44. A method for the conversion of solid magnesium sulphate to magnesium oxide as defined in claim 38, wherein hot combustion gases from a carbon based fuel source, such as natural gas, diesel or fuel oil, can be used to provide indirect heating to the anhydrous magnesium sulphate 5 and sulphur, for the reduction step.

45. A method for the preparation of magnesium oxide from a process solution containing dissolved magnesium sulphate, the method comprising: 10 i) concentrating the process solution using membrane filtration to form a concentrated process solution; ii) treating the concentrated process solution to precipitate solid magnesium sulphate; and iii) reducing the solid magnesium sulphate to form magnesium oxide. 15

46. A method for the preparation of magnesium oxide as defined in claim 45, wherein the solid magnesium sulphate is converted to anhydrous magnesium sulphate in a dehydration step prior to the reduction step. 20

47. A method for the preparation of magnesium oxide as defined in claim 46, wherein the reduction step takes place using indirect heat exchange with hot gas from a sulphur burner.