A PROCESS FOR EXTRACTING METALS USING A SYSTEM COMPRISING CARBON DIOXIDE

TECHNICAL FIELD

[0001] The present invention relates to a process for extracting metals, in particular lithium, using a system comprising carbon dioxide.

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Selection of metal extraction processes is based on a number of considerations. First, process economics, observing both capital and operating costs, must be considered. Second, environmental considerations also have to be taken into account. Process economics is favoured by plant that is more efficient and takes up less space, this reducing capital costs. Reagent recycling is helpful in reducing operating costs. Reagent recycling, especially where the extraction process can be operated within a closed system, is also favourable for environmental considerations.

[0004] Taking lithium as an example, lithium can - apart from its extraction from brines - be extracted from a range of minerals. Such minerals include alumino-silicates such as spodumene but lithium can also be found in economically relevant quantities in micas and mica type minerals such as the phyllosilicate lepidolite (with approximate chemical formula K(Li,AI)3(AI, Si, Rb)40io(F,OH)2) and zinnwaldite (with approximate formula KLiFeAI(AISi3)Oio(OH,F)2) , which can also be considered a relative of siderophyllite or polylithionate. Some lithium clays may also be amenable to economic lithium extraction. Lithium extraction is considered a complex chemical process with the need for multiple unit operations including calcination, leaching, impurity removal and crystallisation, involving a number of crystallisation steps. Other metals require complex extraction, for example the distinct rare earths. [0005] In the lithium field, process economics drives consideration of the following: increasing extraction efficiency, producing value added products in addition to the lithium salt, reducing costs and combinations of these options.

[0006] It is against this background that the process of the present invention has been developed.

SUMMARY OF INVENTION

[0007] The present invention provides, in one embodiment, a process for extracting a metal from a metal containing material comprising contacting a process stream containing a species of the metal with a system comprising carbon dioxide to solubilise metals as carbonates or bicarbonates in the process stream. The species of the metal is, for example, the metal itself, a compound containing the metal including a mineral containing the metal. The metal may be extracted as a metallic compound. By ‘solubilise” is also included maintaining metals, including in ionic state, in solution.

[0008] The primary metallic compound of interest contains lithium with the metal containing material being a lithium containing material, optionally a lithium containing mineral. However, the process is not limited - in its broadest form to lithium containing materials. The process may be applied to extraction of metals such as nickel or other base metals. The process is most likely to be suitable for metals or metallic compounds that have traditionally been extracted using a carbonate route, involving - for example sodium carbonate leaching of the metal containing material.

[0009] The metal containing material - whether or not containing lithium - may include, without limitation, “run of mine” (ROM) material containing lithium, lithium mineral concentrates, lithium containing clays and lithium containing micas and lithium containing compounds formed naturally or during the process of extracting the metallic compound. The process stream may be selected, again without limitation, from the group consisting of ROM material containing the metal, metal containing concentrate, metal containing solid and/or metal containing solution or other materials containing economic quantities of the metal. Without limitation, lithium minerals which may be treated according to the process include pegmatite, lepidolite, zinnwaldite and other lithium mica minerals. Treatment of lithium minerals such as spodumene is also possible. Desirably, the mineral or mineral concentrate has a lithium oxide content above about 0.3 to 0.4 wt% U2O to utilise the optimised mass- and energy balanced flowsheet as described in embodiments below. Lower grade materials are, however, still able to be treated using a less preferred embodiment, as also described in this specification.

[0010] In one embodiment, the system comprising carbon dioxide is a system containing supercritical carbon dioxide or a system exposed to conditions conducive to the formation of supercritical carbon dioxide. Systems containing supercritical carbon dioxide may include, in particular, supercritical carbon dioxide alone or as a mixture containing supercritical carbon dioxide, with a mixture of supercritical carbon dioxide and subcritical water having been found useful in a key embodiment of the present process. In an alternative embodiment, the water phase is also utilised under supercritical conditions. By “supercritical” is intended a compound subjected to a pressure and a temperature higher than its critical point. In the supercritical region, a compound - in fluid state - has an intermediate behaviour between that of a liquid and a gas. Typically, supercritical fluids possess liquid-like densities, gas-like viscosities and diffusivities intermediate to that of a liquid and a gas. The extractive strength of a supercritical carbon dioxide and water system may be controlled by adjustment of temperature and CO2/H2O ratio. The system comprising supercritical carbon dioxide is desirably free of added oxidants or chelating agents. In any event, where lithium is of interest, it does not appear to be chelated by an agent such as EDTA.

[0011] The metal containing material may be subjected to calcination - which refers to heat treatment typically of minerals or mineral concentrates to effect structural changes (including, without limitation, phase changes, decrepitation or solid-state chemical transformations) - prior to contacting with the system containing carbon dioxide. Calcination may also refer to roasting which typically involves the thermal treatment of minerals in the presence of intentionally added reagents. Calcination temperature depends on the phase change to be induced in the metal containing material. For a lithium containing mineral, calcination may be conducted in the temperature range 700 to 1300°C, with temperatures between 900 to 1000°C typically applied to lithium containing minerals, such as lithium micas, rather than spodumene where clinker formation above 1100 to 1300°C may be problematic. The calcination residence time is again mineral dependent but would typically be selected in the range of 20 minutes to 4 hours. Preferably, no additional reagent or fluxing is required for calcination. [0012] The system containing carbon dioxide may include an inorganic reagent, preferably a carbonate such as sodium carbonate or potassium carbonate, in aqueous solution. Such a reagent may be provided, in part, by a recycle stream within the process. However, where a system containing supercritical carbon dioxide is used, lesser amounts of such carbonates are required than in prior processes due to the leaching capacity of the system containing supercritical carbon dioxide. A further advantage of this is that less sodium ions are introduced to the circuit, this simplifying impurity removal steps and crystallisation, for example where lithium carbonate or lithium hydroxide monohydrate production is concerned.

[0013] The process of extraction conveniently involves a pressure leaching step, conveniently involving an autoclave unit operation and conveniently involving countercurrent leach operation whether with a carbonate reagent (such as sodium carbonate or potassium carbonate) or, alternatively, a system containing supercritical carbon dioxide. One or a plurality of autoclaves may be used. Multiple autoclave operation allows the option of a counter-current leach that, in the case of lithium, allows for either the concentration of lithium in the discharge liquor; or, alternatively, allows for the substantially complete extraction of lithium from particularly refractory minerals. The latter case is different from the former because it would not be expected to provide a comparably high lithium concentration in the discharge liquor.

[0014] In some embodiments, pressure leaching - particularly where conducted in multiple autoclaves and where sodium carbonate pressure leaching for example is used - is accompanied by attrition, for example through ultrasonication or use of grinding media such as ceramic balls or steel balls. Such attrition allows contact of the leaching system - for example including supercritical CO2-H2O and sodium carbonate - with fresh mineral surfaces not occluded by leaching reaction products which may interfere with extraction. Further, in the case of lithium extraction, attrition disrupts and minimises the lithium re-absorption mechanism that may cause poor lithium extraction where no attrition is used.

[0015] In the case of lithium extraction, for example, and in particular where higher grade lithium ores are concerned, the process preferably includes a carbonation step involving contacting of carbon dioxide with the carbonate containing liquor from the leach step. Carbon dioxide is pressurised and desirably contacted with the carbonate containing liquor from the leach step by a process such as sparging. The carbon dioxide converts aqueous carbonate to bicarbonate (for example lithium carbonate to lithium bicarbonate though benefit may be achieved for any metal that forms a relatively insoluble carbonate provided that, upon bicarbonation, pH decreases sufficiently to disrupt its association with hydroxide) thereby allowing a greater solubility limit for aqueous lithium and more efficient extraction. If the metal concentration is below the solubility limit of, for example, the metal carbonate, no carbonation is required. In some embodiments, the carbonation could be conducted, at least partially, during a leaching step.

[0016] Following separation of leach residue from pregnant liquor, impurities - such as fluoride, boron and calcium - are removed. A desired metal compound may be extracted or metal recovered in further unit operations.

[0017] Where lithium carbonate and carbonation is concerned, the carbonated liquor - containing lithium bicarbonate - is subjected to a decarbonation step. This conveniently involves heating of the lithium bicarbonate containing solution to cause decarbonation of the lithium bicarbonate to produce a purified lithium carbonate. The carbon dioxide generated by decarbonation may be reused in carbonation or in the leach step. An evaporation step may be included, if necessary, for example where the lithium concentration in the solution has not exceeded the solubility limit of lithium carbonate as may be the case where the process is employed to treat low grade lithium ores. The preferred counter-current leaching step described above is expected to reduce evaporation duty at lower expected cost than would otherwise be required (in evaporation) in producing lithium carbonate providing a benefit of a more energy optimised leach circuit and overall process. This allows recovery or separation of lithium carbonate by filtration which may be economically achieved at a solids density of 10 w/w% and above.

[0018] Filtrate from a lithium carbonate filtration step is desirably recycled in varying proportions to the carbonation and/or filtration steps, pressure leaching step and any bleed step. The purpose of such recycling is to ensure optimum lithium extraction, through recycling of lithium to lithium extraction and lithium recovery stages, though this may not be required if a battery grade lithium carbonate is the preferred metallic compound product from the process. In such case, an optimised decarbonation and evaporation unit operation may be sufficient to provide desired quality of the lithium carbonate which may be dried and packaged, following any included washing step(s). WO 2023/081961 > g > PCT/AU2022/051316

[0019] If desired, separated lithium carbonate may be further purified. In one embodiment, the lithium carbonate may be purified by bicarbonation. Lithium carbonate is solubilised in aqueous solution, desirably including a recycle filtrate from a downstream decarbonation step and water, in the presence of carbon dioxide to form a lithium bicarbonate solution analogously with the carbonation step described above.

[0020] Following fine solids removal, if required, the lithium bicarbonate solution may be treated to remove impurities, conveniently by ion exchange and fine filtration. Lithium carbonate, desirably battery grade, in further purified form can then be generated by decarbonation involving heating to liberate carbon dioxide. The lithium carbonate is separated, conveniently by filtration. A portion or all of the filtrate may be returned to bicarbonation for lithium carbonate solubilisation; or to the leaching step. The carbon dioxide may be reused for the bicarbonation step.

[0021] Optionally, lithium carbonate may be treated - desirably with hydrated lime - to produce lithium hydroxide solution and limestone. Limestone may be separated as a saleable product from the process.

[0022] If necessary, the lithium hydroxide solution may be subjected to impurity removal for removal of cationic impurities, for example by ion exchange.

[0023] Lithium hydroxide monohydrate is recovered from the lithium hydroxide solution by crystallisation. Evaporative crystallisation is convenient and a single crystallisation step may provide a lithium hydroxide monohydrate of desired purity.

[0024] Advantages of processes as described above include faster reaction times than current processes which may reduce capital costs through reducing sizing of process vessels, especially where systems containing supercritical carbon dioxide are used. Operating cost savings, particularly on reagents, are available through the recycling of reagents. In cases of pressure leaching, for example of a lithium containing material with sodium carbonate, metal extractions are promoted where attrition is used in a multistage process. Further, where run of mine material can be economically treated directly, there is a potential substantial saving in plant costs by saving unit operations such as beneficiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further features of the present invention are more fully described in the following description of non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0026] Figure 1 is a: flowsheet for producing lithium carbonate and/or lithium hydroxide according to one embodiment of the present invention.

[0027] Figure 2 is a: flowsheet for producing lithium carbonate and/or lithium hydroxide according to a further embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] Referring to Figure 1 , a lithium mica mineral concentrate 5 - such as lepidolite or zinnwaldite - is treated in lithium extraction process 1000 for the recovery of lithium hydroxide 140 and/or lithium carbonate 240.

[0029] Lithium mineral 5 is calcined in calcination step 10, employing a natural gas fired calciner of type known in the lithium extraction art, to produce a material more amenable to lithium extraction. The calcination step 10 may be conducted at a temperature in the range 700-1300°C for a period of 20 minutes to 4 hours. No additional reagents are mixed with the lithium mica mineral for calcination.

[0030] Calcined material 12 is introduced to pressure leaching unit operation 20. The calcined material is added to an autoclave unit operation where the number of autoclaves may range from one to six. Carbon dioxide 52, 132 is introduced to pressurise the autoclave(s) at pressure above the critical pressure of 73 bar, though it will be appreciated that other pressures within the supercritical range may be adopted, and a temperature of higher than the critical temperature of 31 °C. That is, leaching is accomplished using supercritical carbon dioxide. In a preferred embodiment, the pressure is higher than 100 bar while the temperature is higher than 200°C. In other embodiments, other pressures and temperatures within the supercritical range, i.e. above the critical point for carbon dioxide (or, in other embodiments, the critical point of WO 2023/081961 > g > PCT/AU2022/051316 the system, such as a CO2-H2O system, including supercritical carbon dioxide) may be adopted. The system comprising supercritical carbon dioxide is free of added oxidants or chelating agents.

[0031 ] In an embodiment involving a supercritical CO2-H2O system, the temperature and pressures would be higher in line with the higher critical point for water, i.e. 220 bar and 374°C.

[0032] The use of multiple autoclaves provides a counter-current leach option that allows for either the concentration of lithium in the final discharge liquor or, alternatively, allows for the substantially complete extraction from particularly refractory minerals such as lithium containing clays. The latter case is different from the former because it will not provide a comparably high lithium concentration in the discharge liquor.

[0033] In a preferred embodiment, a single autoclave is able to provide the required lithium extraction, however, for some minerals multiple autoclaves will be required. Another embodiment will thus require the use of an attrition mechanism such as in situ ultrasonication or grinding media such as ceramic balls or steel balls. Alternatively, a mill may be situated between each autoclave to provide the required material abrasion. The purpose of attrition, where included, is to disrupt passivation of the mineral surfaces by constituents precipitated or crystallised during the process or disrupt and minimise a lithium re-absorption mechanism that is responsible for poor lithium extraction where attrition is not used.

[0034] In the embodiment shown, slurry 22 from the autoclave leaching step 20 is passed to a carbonation step 30 where the slurry is cooled to below 40°C and may be sparged with carbon dioxide 64. This is required in cases where a lithium rich solution has reached the saturation or solubility limit of lithium carbonate, for example in the case of lithium micas and pegmatites. The carbon dioxide then converts aqueous carbonate to bicarbonate, thereby allowing a greater solubility limit for aqueous lithium. In this embodiment, the generated leach solution is passed to a filtration system, such as plate and frame or vacuum filter, to separate the lithium-rich liquor from the leach residue 34. The leach residue is washed with wash water 32 to improve lithium recovery through entraining lithium bicarbonate. The wash water 32 used to wash the leach residue for the recovery of entrained lithium may then be returned to the autoclave(s) in the leaching step 20 for use as a portion of the aqueous phase. WO 2023/081961 > g > PCT/AU2022/051316

[0035] In another embodiment, where the lithium concentration of the leach solution is below the solubility limit of lithium carbonate, such as in the case of most lithium containing clays, no carbonation is required and the generated solution may be passed directly to the filtration system, conveniently as above described.

[0036] Filtrate 36 from carbonation step 30 is then passed to an impurity removal step 40 for removal of fluoride 402, boron 404 and calcium 405 in respective impurity removal stages 401 , 403 and 404, for example and preferably by ion exchange. It will be understood that other impurities may be removed dependent on the lithium containing mineral treated in process 1000. A polishing filter may be included upstream of the impurity removal stages 401 , 403, 404 to remove any fine solids not separated in the previously described filtration step.

[0037] The polished filtrate 42 is directed to decarbonation/evaporation step 50 where it is heated to higher than 80°C to cause decarbonation of lithium carbonate to produce purified lithium carbonate and carbon dioxide 52. The carbon dioxide 52 generated by the decarbonation reaction is then re-used in the carbonation step 30 and/or the leach step 20.

[0038] To obtain a slurry density, from about 10 w/w% and above, suitable for recovery of lithium carbonate, evaporation may be carried out, if necessary, as part of decarbonation/evaporation step 50. This may be necessary for lower grade lithium minerals, such as some lithium clays, or in other cases where the lithium concentration in the solution does not exceed the solubility limit of lithium carbonate without concentration. Alternative lithium solution concentration techniques may be used subject to economics. Where evaporation is conducted, as preferred, the countercurrent leaching step described above is expected to reduce evaporation duty at lower expected cost than would otherwise be required (in evaporation) in producing lithium carbonate providing a benefit of a more energy optimised leach circuit and overall process.

[0039] The lithium carbonate slurry generated during the decarbonation/evaporation step 50 is preferably separated by either centrifuge, plate and frame or vacuum belt filters in solid-liquid separation step 60 with the selection depending on the design slurry density. [0040] The filtrate may be recycled in varying proportions to the carbonation/filtration step 30, the pressure leach step 20 and the bleed unit 310. Alternative embodiments may also include recycle streams directed to other parts of the process such as reagent make-up. The purpose of the recycle is to ensure optimum lithium extraction by recycling of lithium containing streams to lithium extraction and lithium recovery stages, minimising lithium losses.

[0041] In one embodiment, if battery grade lithium carbonate is the preferred product, an optimised decarbonation/evaporation step 50 is sufficient to provide the desired quality. Product lithium carbonate may be dried and packaged.

[0042] As shown in Figure 1 , alternative schemes are available for the steps downstream of filtration step 60 though these schemes could be used in tandem where both battery grade lithium hydroxide monohydrate and lithium carbonate are desired products.

[0043] In the case of lithium hydroxide monohydrate being the required product, filtered lithium carbonate wet cake 62 from filtration step 60 is treated with hydrated lime 112 in liming step 110 at a temperature higher than 50°C to, following filtration, produce limestone 114 as a saleable product in addition to aqueous lithium hydroxide after filtration. The filtration is preferably carried out using a plate and frame filter or, alternatively, a combination of a settler and vacuum belt filter.

[0044] The lithium hydroxide solution or filtrate 114 may contain some cationic impurities such as calcium, magnesium and higher multivalent species because of reagent contamination. Therefore, a single or multi-stage ion exchange step 120 may be included to remove such impurities.

[0045] Purified lithium hydroxide solution 122 from ion exchange step 120 is subjected to evaporative crystallisation 130 to produce battery grade lithium hydroxide monohydrate. In a preferred embodiment, a single crystalliser is required to provide lithium hydroxide monohydrate (LHM) of desired purity. LHM crystals are then separated using a centrifuge with filtrate and wash liquor preferably being returned to liming step 110 or, alternatively as aqueous stream 132, to pressure leaching step 20. [0046] Crystallised LHM 134, in the form of a wet cake is then dried at a temperature higher than 40°C in drying and packaging step 140, preferably in a vacuum dryer to preserve crystal integrity. Dried LHM is then packaged as a dry material.

[0047] In an alternative scheme, if lithium carbonate is a desired product, the lithium carbonate wet cake 62A from filtration step 60 is directed to a bicarbonation step 210 for further purification. Bicarbonation step 210 involves solubilisation of the wet lithium carbonate cake. Solubilisation is with recycled liquor 231 from the downstream decarbonation stage 230 together with fresh water. The solubilisation liquor 231 is sparged with carbon dioxide 232 at a temperature lower than 40°C. The resulting lithium bicarbonate solution 233 is then passed through a polishing filter to remove any fine solid material.

[0048] The lithium bicarbonate solution 213 may contain some cationic impurities such as calcium, magnesium and higher multivalent species because of reagent contamination. Therefore, a single or multi-stage ion exchange step 220 may be included to remove such impurities.

[0049] As shown, in a preferred embodiment, battery grade lithium carbonate is generated from purified lithium bicarbonate solution 223 in decarbonation step 230. The purified lithium bicarbonate solution is heated to higher than 80°C, preferably higher than 90°C, to liberate carbon dioxide 232 and produce purified battery grade lithium carbonate which is separated by filtration. A portion or all of the filtrate, following separation of the lithium carbonate, may be returned as stream 231 to bicarbonation step 210 to solubilise the initial lithium carbonate wet cake 62A or, alternatively, a portion or all of the filtrate may be returned as stream 232 to autoclave leaching step 20. Carbon dioxide 232 generated by decarbonation may be re-used in bicarbonation step 210.

[0050] The battery grade lithium carbonate wet cake 234 is then directed to drying and packaging step 240 where it is dried at 80°C or higher. Dry lithium carbonate crystals are then packaged.

[0051] An alternative flowsheet is provided in Figure 2. Again, a lithium mica mineral concentrate 1050 - such as lepidolite or zinnwaldite - is treated in lithium extraction process 1200 for the recovery of lithium hydroxide 11400 and/or lithium carbonate 1240. [0052] Lithium mineral 1050 is calcined in calcination step 1100, employing a natural gas fired calciner of type known in the lithium extraction art, to produce a material more amenable to lithium extraction. The calcination step 10 may be conducted at a temperature in the range 700-1300°C for a period of 20 minutes to 4 hours. No additional reagents are mixed with the lithium mica mineral for calcination.

[0053] Calcined material 1 102 is introduced to pressure leaching unit operation 1120. The calcined material 1102 is added to an autoclave unit operation where the number of autoclaves may range from one to six. A carbonate reagent, in one embodiment, an aqueous solution of sodium carbonate (also known as soda ash) is also added to the autoclave. The amount of sodium carbonate added to the autoclave is higher than stoichiometric when considering the amount of lithium in the feed ore. The temperature is 180°C or higher and the residence time is specific to the material being treated, for example, a specific mica might require 15 minutes while a pegmatite might require 30 minutes or longer. In a preferred embodiment, the stoichiometric excess of sodium carbonate to be added is 10% or less, and the reaction temperature is 230°C with a residence time of 15 minutes. A portion of the carbonate reagent may comprise recycled carbonate, in particular sodium and/or potassium carbonates, produced during the lithium extraction process.

[0054] The use of multiple autoclaves provides a counter-current leach option that allows for either the concentration of lithium in the final discharge liquor or, alternatively, allows for the substantially complete extraction from particularly refractory minerals. The latter case is different from the former because it will not provide a comparably high lithium concentration in the discharge liquor.

[0055] In a preferred embodiment, a single autoclave is able to provide the required lithium extraction, however, for some minerals multiple autoclaves will be required. Another embodiment will thus require the use of an attrition mechanism such as in situ ultrasonication or grinding media such as ceramic balls or steel balls. Alternatively, a mill may be situated between each autoclave to provide the required material abrasion. The purpose of attrition, where included, is to disrupt passivation of the mineral surfaces by constituents precipitated or crystallised during the process or a lithium re-absorption mechanism that is responsible for poor lithium extraction where attrition is not used. [0056] In the embodiment shown, slurry 1122 from the autoclave leaching step 1120 is passed to a carbonation step 1130 where the slurry is cooled to below 40°C and sparged with carbon dioxide 1152. This is required in cases where a lithium rich solution has reached the saturation or solubility limit of lithium carbonate. The carbon dioxide 1152 then converts aqueous carbonate to bicarbonate, thereby allowing a greater solubility limit for aqueous lithium. In this embodiment, the generated leach solution is passed to a filtration system, such as plate and frame or vacuum filters to separate the lithium-rich liquor from the leach residue 1134. The leach residue 1134 is washed with wash water 1320 to improve lithium recovery through entraining lithium bicarbonate. The wash water 1320 used to wash the leach residue for the recovery of entrained lithium may then be returned to the autoclave(s) in the leaching step 1120 for use as a portion of the aqueous phase.

[0057] In another embodiment, where the lithium concentration of the leach solution is below the solubility limit of lithium carbonate, no carbonation is required and the leach solution may be passed directly to the filtration system, conveniently as above described.

[0058] Filtrate 1136 from carbonation step 1130 is then passed to an impurity removal step 1140 for removal of fluoride 1402, boron 1404 and calcium 1405 in respective impurity removal stages 1401 , 1403 and 1404, for example and preferably by ion exchange. It will be understood that other impurities may be removed dependent on the lithium containing mineral treated in process 1200. A polishing filter may be included upstream of the impurity removal stages 1401 , 1403, 1404 to remove any fine solids not separated in the previously described filtration step.

[0059] The polished filtrate 1144 is directed to decarbonation/evaporation step 1150 where it is heated to higher than 80°C to cause decarbonation of lithium carbonate to produce purified lithium carbonate and carbon dioxide 1152. The carbon dioxide 1152 generated by the decarbonation reaction is then re-used in the carbonation step 1130 and/or the leach step 1120.

[0060] To obtain a slurry density, from about 10 w/w% and above, suitable for recovery of lithium carbonate, evaporation may be carried out, if necessary, as part of decarbonation/evaporation step 1150. This may be necessary for lower grade lithium minerals, such as lithium clays, or in other cases where the lithium concentration in the solution does not exceed the solubility limit of lithium carbonate without concentration.

Alternative lithium solution concentration techniques may be used.

[0061] The lithium carbonate slurry generated during the decarbonation/evaporation step 1150 is preferably separated by either centrifuge, plate and frame or vacuum belt filters in solid-liquid separation step 1160 with the selection depending on the design slurry density.

[0062] The filtrate may be recycled in varying proportions to the carbonation/filtration step 1130, the pressure leach step 1120 and the bleed unit 1310. Alternative embodiments may also include recycle streams directed to other parts of the process such as reagent make-up. The purpose of the recycle is to ensure optimum lithium extraction through recycling of lithium containing streams to lithium extraction or lithium recovery stages minimising lithium losses from the process.

[0063] In one embodiment, if battery grade lithium carbonate is the preferred product, an optimised decarbonation/evaporation step 1150 is sufficient to provide the desired quality. Product lithium carbonate may be dried and packaged.

[0064] As shown in Figure 2, alternative schemes are available for the steps downstream of filtration step 1160 though these schemes could be used in tandem where both battery grade lithium hydroxide monohydrate and lithium carbonate are desired products.

[0065] In the case of lithium hydroxide monohydrate, filtered lithium carbonate wet cake 1164 from filtration step 1160 is treated with hydrated lime 1112 in liming step 11100 at a temperature higher than 50°C to, following filtration, produce limestone 1 1140 as a saleable product in addition to aqueous lithium hydroxide 11148 after filtration. The filtration is preferably carried out using a plate and frame filter or, alternatively, a combination of a settler and vacuum belt filter.

[0066] The lithium hydroxide solution or filtrate 11148 may contain some cationic impurities such as calcium, magnesium and higher multivalent species because of reagent contamination. Therefore, a single or multi-stage ion exchange step 11200 may be included to remove such impurities. [0067] Purified lithium hydroxide solution 11220 from ion exchange step 11200 is subjected to evaporative crystallisation 11300 to produce battery grade lithium hydroxide monohydrate. In a preferred embodiment, a single crystalliser is required to provide lithium hydroxide monohydrate (LHM) of desired purity. LHM crystals are then separated using a centrifuge with filtrate and wash liquor preferably being returned to liming step 11100 or, alternatively, to pressure leaching step 1120.

[0068] Crystallised LHM 11340, in the form of a wet cake, is then dried at a temperature higher than 40°C in drying and packaging step 11400, preferably in a vacuum dryer to preserve crystal integrity. Dried LHM is then packaged as a dry material.

[0069] In an alternative scheme, if lithium carbonate is a desired product, the lithium carbonate wet cake 1164A from filtration step 1160 is directed to a bicarbonation step 1210 for further purification. Bicarbonation step 1210 involves solubilisation of the wet lithium carbonate cake. Solubilisation is with recycled liquor 12318 from the downstream decarbonation stage 1230 together with fresh water. The solubilisation liquor 12318 is sparged with carbon dioxide 1232 at a temperature lower than 40°C. The resulting lithium bicarbonate solution is then passed through a polishing filter to remove any fine solid material.

[0070] The lithium bicarbonate solution 1213 or may contain some cationic impurities such as calcium, magnesium and higher multivalent species because of reagent contamination. Therefore, a single or multi-stage ion exchange step 1220 may be included to remove such impurities.

[0071] As shown, in a preferred embodiment, battery grade lithium carbonate is generated in decarbonation step 1230. The purified lithium bicarbonate solution 1223 is heated to higher than 80°C or, preferably, higher than 90°C to liberate carbon dioxide 1232 and produce purified battery grade lithium carbonate 1234 which is separated by filtration. A portion or all of the filtrate, following separation of the lithium carbonate, may be returned as stream 12318 to bicarbonation step 1210 to solubilise the initial lithium carbonate wet cake or, alternatively, a portion or all of the filtrate may be returned as stream 12320 to autoclave leaching step 1120. Carbon dioxide 1232 generated by decarbonation may be re-used in bicarbonation step 1210. [0072] The battery grade lithium carbonate wet cake 1234 is then directed to drying and packaging step 1240 where it is dried at 80°C or higher. Dry lithium carbonate crystals are then packaged.

[0073] Process scheme 1200 may achieve greater than 64% extraction of the lithium present in the lithium mica concentrate 1050.

[0074] In the embodiments described with reference to Figures 1 and 2, a lithium mica concentrate was treated. In other embodiments, a supercritical carbon dioxide system can be used to directly leach run of mine material. Under conditions, as described above, up to 55% of lithium present in the run of mine ore may be recovered which is greater than a scheme involving beneficiation by flotation and further steps including roasting of the flotation concentrate with sodium sulphate, gypsum and lime; and water leaching. Beneficiation allows for a 65% lithium recovery and the sulphate roast/leach steps (as above described) 80% lithium recovery providing an extraction of 52% (being the multiple of the beneficiation and leaching recoveries.

[0075] Modifications and variations to the process for extracting metals using a system comprising carbon dioxide may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present disclosure.

[0076] Throughout this 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