Field of the Invention

The present invention relates to a method for the recovery of magnesium 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.

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 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 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 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 recover the metal value. 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 spargin air into the precipitation tanks to encourage the oxidation of Mn(ll) to Mn(IV) and promote the precipitation of MnC>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.

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 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. 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 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

ii) treating the concentrated process solution to precipitate solid magnesium sulphate. 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 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 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(ll) to Mn(IV).

Preferably membrane filtration comprises reverse osmosis. Membrane 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 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. 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 at a temperature of about 2 to 12°C resulting in the formation of a heptahydrate salt of magnesium sulphate. 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 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°C, which results in the formation of anhydrous magnesium sulphate. 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 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 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:

4 MgS0 ₄ (s) + 1 S ₂ (g) -> 4 MgO(s) + 6 S0 ₂ (g)

Preferably the anhydrous magnesium sulphate 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 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. More preferably the solids residence time is about 40 minutes.

The exit temperature for the hot solids and cooled exiting sulphur burner 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.

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 secondary sulphur burner, with additional air to form a combined gas to combust any unreacted elemental sulphur and to achieve an 0 ₂ to S0 ₂ molar ratio of around 0.8-1.0. The combined gas is preferably cooled to a temperature of around 430°C, such as by using a waste heat boiler, where heat is recovered as high pressure steam. The SO2 gas generated during the reduction reaction can be collected as a separate, high purity S0 ₂ gas stream that can be used to produce liquefied S0 ₂ if required.

The present invention further provides a method for the conversion of solid magnesium sulphate to magnesium oxide, the method comprising: i) dehydrating solid magnesium sulphate to form anhydrous magnesium sulphate; and

ii) reducing the anhydrous magnesium sulphate to form magnesium oxide. 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°C. 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 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

The present invention also provides a method for the preparation of magnesium oxide 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;

ii) treating the concentrated process solution to precipitate solid magnesium sulphate; and

iii) reducing the solid magnesium sulphate to form magnesium 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 hot gas from a sulphur burner. 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 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 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, 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 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, '

Figure 4 is a graph showing gas composition and MgO conversion versus temperature, without sulphur addition. 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 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 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 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 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 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 can be used elsewhere in the process. 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 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 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. 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.

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 can be concentrated by using a centrifuge 30. The resulting centrate is then returned to the evaporation 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 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 dehydrating at temperatures greater than 360°C which results in the formation of the anhydrous magnesium sulphate 32.

The anhydrous magnesium sulphate 32 may be further treated in a 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 anhydrous magnesium sulphate to magnesium oxide. The temperature of the feed gas 46 is preferably about 1100°C.

The anhydrous magnesium sulphate 32 is treated with elemental sulphur in the reduction step 34 to form magnesium oxide 36. The magnesium oxide 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 S0 ₂ and N ₂ , plus enough elemental sulphur vapour to reduce the anhydrous magnesium sulphate to magnesium oxide in the reduction step 34. The reduction reaction typically proceeds according to the following chemistry:

4 MgS0 ₄ (s) + 1 S ₂ (g) -> 4 MgO(s) + 6 S0 ₂ (g) 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 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 700-900°C. • The resulting magnesium oxide product 36 is preferably comprised of at least about 90% w/w magnesium oxide with the remaining amount being uiireacted magnesium sulphate. The resulting magnesium oxide product 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 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.

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 O ₂ to S0 ₂ molar ratio of around 0.8-1.0. The combined gas 40 is preferably cooled to a temperature of around 350°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 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 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 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 nickel leaching step.

Test Results:

Water recovery from magnesium sulphate solution

The recovery of water from a neutral dilute magnesium sulphate solution 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 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 at the maximum magnesium feed concentration tested. Extrapolation of the results in Figure 2, magnesium concentrations of 30 g/L at 40 bar and 4 g/L at 50 to 60 bar are considered achievable.

Triple effect evaporation

The concentrated magnesium sulphate solution from the membrane filter is 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 (MgS0 ₄ *H ₂ 0). Crystals settle in the settling area of the effect and are discharged to the first effect centrifuge to separate the crystals from the saturated magnesium sulphate centrate. The first effect 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 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 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 rotary feeder that feeds the dehydrating screw heat exchanger.

To ensure monohydrate salt (MgS04 ^* H20). is produced the crystallisation must occur above ~ 70-75°C (Hogenboom, 1990).

Magnesium sulphate dehydration

The collected crystals from the triple effect centrifuges contain small amounts of unbound water, plus water of crystallisation. Evaporation of the unbound water occurs at < 110°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 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 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 supporting data.

Table 0-1 : Comparison of MgO production parameters with and without sulphur addition

This reduction in temperature for the magnesium sulphate reduction to magnesia, and thus energy input, is a key economic consideration for the invention.

Now that preferred embodiments of a method for the recovery of solid 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:

(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. (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 magnesium sulphate instead of coal or other carbon sources, allows this process to operate without generating C0 _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 wasted.

(v) The method of this invention avoids the need for potentially troublesome high pressure equipment such as . in a pressurised precipitation process.

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, 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.