Recovery process

Abstract

A process for the recovery of lithium from lithium bearing mica rich minerals, the process comprising passing an ore containing one or more lithium bearing mica rich minerals to at least one pre-treatment step, passing the pre-treated ore to an acid leach step thereby producing a pregnant leach solution, subjecting the pregnant leach solution to a series of process steps in which one or more impurity metals are removed, and recovering lithium as a lithium containing salt product.

Claims

1. A process for the recovery of lithium from lithium bearing mica rich minerals, the process comprising passing an ore containing one or more lithium bearing mica rich minerals to at least one pre-treatment step, passing the pre-treated ore to a leach step in which concentrated sulfuric acid is added under atmospheric conditions thereby producing a leach slurry that is in turn passed to a solid liquid separation step, the separation step producing a leach residue and a pregnant leach solution, subjecting the pregnant leach solution to a series of process steps in which one or more impurity metals are removed, including precipitating alunite in a low pH impurity removal step, and recovering lithium as a lithium containing salt product.

2. The process according to claim 1, wherein the lithium containing salt is Li.sub.2CO.sub.3.

3. The process according to claim 1, wherein the process further includes producing additional products, wherein the additional products include one or more of a potassium containing salt, a rubidium containing salt, a cesium containing salt, a fluorine salt, aluminium as a salt or alumina, and a silica containing product.

4. The process according to claim 3, wherein the fluorine salt comprises a mixture of aluminium fluoride and alumina.

5. The process according to claim 1, wherein the mica rich minerals include lepidolite and/or zinnwaldite.

6. The process according to claim 1, wherein the pre-treatment step comprises one or both of a concentration step and a milling step.

7. The process according to claim 6, wherein the milling step produces a product having a particle size of <P.sub.80 150 micron.

8. The process according to claim 1, wherein the leach residue contains silica.

9. The process according to claim 8, by which a silicate product is also recovered from the acid leach residue, the process comprising the following additional method steps: (i) Leaching the leach residue that contains silica with sodium hydroxide solution to produce a leach slurry; and (ii) Filtering and washing the leach slurry to produce a leach residue and a filtrate, the filtrate containing sodium silicate.

10. The process according to claim 1, wherein the leach step is conducted at a temperature of between 90 and 130 C.

11. The process according to claim 10, wherein the leach step is conducted at a temperature of at or about 120 C.

12. The process according to claim 1, wherein the leach step is carried out with an excess of H.sub.2SO.sub.4 providing a free acid concentration of greater than about 50 g/L H.sub.2SO.sub.4.

13. The process according to claim 12, wherein a total sulfate concentration during the leach step is close to a saturation limit of the slurry at the leaching temperature.

14. The process according to claim 1, wherein the leach step has a retention time of between about 6 to 18 hours.

15. The process according to claim 1, wherein the leach step has a retention time of about 12 hours.

16. The process according to claim 1, wherein selective crystallization is utilized to precipitate mixed monovalent alum salts from the pregnant leach solution by cooling with or without evaporation providing a crystallization slurry.

17. The process according to claim 16, wherein the mixed monovalent alum salts are separated from the crystallization slurry from alum crystallization by filtration or decantation, forming a filtrate, the filtrate containing greater than about 95% of the lithium contained from the initial ore containing one or more lithium bearing mica rich minerals.

18. The process according to claim 16, wherein the monovalent alum salts are separated from the crystallization slurry after alum crystallization by filtration or decantation, forming a lithium containing filtrate, the filtrate containing greater than about 95% of the lithium contained from the initial ore containing one or more lithium bearing mica rich minerals, and impurities present in the lithium containing filtrate are removed by precipitation through the addition of an acid in a low pH impurity removal step.

19. The process according to claim 18, wherein solids from the low pH impurity removal step are washed with water to recover entrained lithium.

20. The process according to claim 18, wherein from the low pH impurity removal step filtrate is passed to a high pH impurity removal step, in which impurity base metals, including either manganese or magnesium, are precipitated through the addition of a base.

21. The process according to claim 18, wherein from the low pH impurity removal step filtrate is passed to a high pH impurity removal step, in which impurity base metals, including either manganese or magnesium, are precipitated through the addition of a base, and calcium is precipitated from a filtered product of the high pH impurity removal step by the addition of a monovalent carbonate salt.

22. The process according to claim 21, wherein lithium carbonate is precipitated by the addition of a monovalent carbonate salt to the filtered product of calcium precipitation.

23. The process according to claim 16, wherein the mixed monovalent alum salts are re-dissolved in water and passed to selective precipitation to precipitate Al(OH).sub.3.

24. The process according to claim 23, wherein precipitated Al(OH).sub.3 from selective precipitation is separated by filtration or decantation, whereby a resulting filtrate contains monovalent cation salts.

25. The process according to claim 23, wherein precipitated Al(OH).sub.3 from selective precipitation is separated by filtration or decantation, whereby a resulting filtrate contains rubidium and cesium sulfates that are further processed to provide formates.

26. The process according to claim 16, wherein fluoride is recovered from the lithium bearing mica rich minerals by way of the additional method steps: a) Selective crystallization of khademite from the filtrate arising from the separation of the monovalent alum salts from the crystallization slurry, producing a fluoride crystallization slurry; and b) Separation of the crystallized khademite from the fluoride crystallization slurry by filtration or decantation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The process of the present disclosure will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:

(2) FIGS. 1A and 1B comprise a flow sheet depicting a process for the recovery of lithium from mica rich minerals in accordance with the present disclosure, showing as one embodiment in particular a process for recovery of each of lithium, rubidium, cesium, potassium, fluorine and aluminium from lepidolite ore or concentrate by acid leach, alum crystallization, impurity removal, Li.sub.2CO.sub.3 recovery, Al(OH)s precipitation, khademite precipitation, and rubidium, cesium, potassium sulfate crystallization;

(3) FIG. 2 is a flow sheet depicting in detail the leaching stage of the process of FIG. 1;

(4) FIG. 3 is a flow sheet depicting how a silicate product is recovered from the acid leach residue of the process of FIGS. 1A and 1B; and

(5) FIGS. 4A and 4B comprise a flow sheet in accordance with a third embodiment of the present disclosure, being the recovery of a rubidium and cesium product.

BEST MODE(S) FOR CARRYING OUT THE DISCLOSURE

(6) The process of the present disclosure comprises a novel and improved combination of operating steps, one or more of which may have been used previously, in other combinations and for other purposes, in mineral processing and hydrometallurgical processes.

(7) In very general terms, in one embodiment of the present disclosure, a lithium containing mineral, lepidolite, is pre-concentrated, if required, by a mineral separation process, for example flotation. The lepidolite ore or concentrate is then subjected to a pre-treatment step comprising, for example, fine milling. The lithium, potassium, rubidium, cesium, fluorine and aluminium present in lepidolite are extracted by strong sulfuric acid leaching, producing a leach liquor containing lithium, potassium, rubidium, cesium, fluorine and aluminium and a leach residue containing silica. A majority of the potassium, rubidium, cesium, fluorine and aluminium present in the leach liquor are separated from lithium as mixed sulfate salts. Hydrometallurgical techniques such as selective precipitation and crystallization separate potassium, rubidium, cesium, fluorine and aluminium from the mixed sulfate salt into potentially saleable products, including, but not limited to, Al(OH).sub.3 and K.sub.2SO.sub.4, Rb.sub.2SO.sub.4, AlFSO.sub.4 and Cs.sub.2SO.sub.4. Lithium is separated from residual impurities, including, but not limited to, sulfuric acid, aluminium, iron, manganese, calcium, rubidium, cesium and potassium by hydrometallurgical techniques, such as selective precipitation and crystallization, to produce saleable Li.sub.2CO.sub.3.

(8) Lepidolite is a lilac-grey or rose coloured lithium phyllosilicate (mica group) mineral and a member of the polylithionite-trilithionite series. The standard chemical formula for Lepidolite is K(Li,Al).sub.3(Al,Si).sub.4O10(F,OH).sub.2, although it is understood this may vary. It occurs in granite pegmatites, high temperature quartz veins, greisens and granites. Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl. Lepidolite can contain up to 7.7% Li.sub.2O. The lepidolite in pegmatite bodies can be separated from the gangue minerals by flotation, or classification.

(9) It is envisaged that the processes of the present disclosure are applicable to any lithium bearing mica ores, such as lepidolite, but also including zinnwaldite. Zinnwaldite is a lithium containing silicate mineral in the mica group, generally light brown, grey or white in colour, and having the chemical formula KLiFeAl(AlSi.sub.3)O.sub.10(OH,F).sub.2.

(10) In one form of the present disclosure the process comprises the method steps of: (i) Separation of the lithium containing mineral, lepidolite, from gangue minerals, such as quartz and feldspar, by froth flotation, if required, to produce a lepidolite concentrate; (ii) Fine milling the lepidolite concentrate; (iii) Leaching lepidolite in sufficient sulfuric acid solution under atmospheric conditions to enable the lithium, potassium, rubidium, cesium and aluminium to be converted to soluble sulfates and to also extract any fluorine present, forming a pregnant leach solution; (iv) Selective crystallization of the mixed monovalent alum salts including KAl(SO.sub.4).sub.2.12H.sub.2O, RbAl(SO.sub.4).sub.2.12H.sub.2O and CsAl(SO.sub.4).sub.2.12H.sub.2O by cooling with or without evaporation. The addition of K.sub.2SO.sub.4, Rb.sub.2SO.sub.4 and/or Cs.sub.2SO.sub.4 can be used to increase the recovery of Al if required; (v) Separation of the monovalent-alum salts from the liquor by filtration or decantation in which the resulting filtrate contains the large majority of the lithium contained from the initial lepidolite ore or concentrate; (vi) Impurities present in the lithium containing filtrate, such as H.sub.2SO.sub.4, aluminium, chromium and iron are removed by precipitation using a suitable base, such as limestone, lime or monovalent carbonate or hydroxide salts, but preferably limestone. The pH of the solution is increased by the addition of the base to allow for the neutralisation and precipitation of the impurities; (vii) Separation of the impurity metals and sulfate from the liquor by filtration or decantation in which the resulting filtrate contains the large majority of the lithium contained from the initial lepidolite ore or concentrate. The solids are washed with water to recover entrained lithium; (viii) Precipitation of impurity base metals, such as, but not limited to, manganese and magnesium, using a suitable base, such as lime or monovalent hydroxide salts, but preferably lime; (ix) Separation of the impurity metals and sulfate from the liquor by filtration or decantation in which the resulting filtrate contains the large majority of the lithium contained from the initial lepidolite ore or concentrate. The solids are washed with water to recover entrained lithium; (x) Precipitation of calcium ions by the addition of a monovalent carbonate salt, such as Li.sub.2CO.sub.3, Na.sub.2CO.sub.3 or K.sub.2CO.sub.3; (xi) Separation of the precipitated calcium salt from the liquor by filtration or decantation in which the resulting filtrate contains the large majority of the lithium contained from the initial lepidolite ore or concentrate; (xii) Precipitation of Li.sub.2CO.sub.3 by the addition of a monovalent carbonate salt, such as Na.sub.2CO.sub.3 or K.sub.2CO.sub.3. Separation of the lithium salt from the liquor by filtration or decantation; (xiii) Crystallization of monovalent sulfate salts from filtrate by salting out and/or evaporation; (xiv) The mixed monovalent alum salts are re-dissolved in water and subject to conventional hydrometallurgical techniques, such as, but not limited to selective precipitation using limestone, lime or monovalent carbonate or hydroxide salts to precipitate Al(OH).sub.3; (xv) Separation of precipitated Al(OH).sub.3 from the liquor by filtration or decantation in which the resulting filtrate contains monovalent cation salts; and (xvi) The monovalent cation salts are separated by selective crystallization. Rubidium and cesium sulfates can be further processed if required to form other salts, for example formates.

(11) Fluoride recovery can be achieved in the context of the process of the present disclosure also, utilising these additional method steps: c) Selective crystallization of khademite, a double salt of aluminium fluoride and aluminium sulfate, from the filtrate is recovered at step (v) below. Khademite is recovered by continued agitation of the liquor with the addition of khademite as seed to increase the rate of crystallization. d) Separation of the khademite from the liquor by filtration or decantation in which the resulting filtrate contains the large majority of the Li contained from the initial Li-mica ore or concentrate; e) Khademite can be refined further if required; f) Khademite can be re-dissolved in water and subject to precipitation of aluminium hydroxyl fluoride using Al(OH).sub.3 recovered from step (vii). The reaction of which is understood to proceed as follows:
6AlSO.sub.4F.sub.(aq)+Al(OH).sub.3(s).fwdarw.3AlF.sub.2OH.sub.(s)+2Al.sub.2(SO.sub.4).sub.3(aq) g) Separation of the aluminium hydroxyl fluoride from the liquor by filtration or decantation in which the resulting filtrate contains aluminium sulphate, which is directed to step (i). The addition of aluminium sulphate allows for an increased recovery of khademite; h) The aluminium hydroxyl fluoride is calcined in the temperature range of 350 C. to 600 C. to produce the aluminium fluoride aluminium oxide mixture; and i) Khademite can be roasted at >700 C. to produce a mixture of AlF.sub.3 and Al.sub.2O.sub.3.

(12) In accordance with a further embodiment of the present disclosure a silicate product may be recovered from the acid leach residue, utilising the following additional method steps: (iii) Silica rich residue is leached with sodium hydroxide solution to produce a leach slurry; and (iv) Filtering and washing the leach slurry to produce a leach residue depleted in amorphous silica and a leach liquor containing sodium silicate.

(13) The sodium hydroxide reacts exothermically with the amorphous silica present in the acid leach residue to form sodium silicate solution. Different grades or Na.sub.2O/SiO.sub.2 ratios can be produced by changing the sodium hydroxide concentration and or percent solids in the leach. The leaching kinetics are rapid reaching equilibrium within about 15 minutes. Higher extractions are achievable with higher temperatures. The sodium silicate product is ready for sale. It is understood that the production of sodium silicate from the acid leach residue in this manner is significantly less complicated than conventional sodium silicate production routes.

(14) Lepidolite ore or concentrate is treated in accordance with the present disclosure as shown in FIGS. 1A and 1B. The relative grades of the metals in lepidolite are described only by way of example, and the process of the present disclosure is expected to be able to treat any lepidolite bearing material and any lithium bearing mica, not dependent on grade.

(15) In FIGS. 1A and 1B show a flow sheet in accordance with the present disclosure and in which the embodiment depicted is particularly intended for the processing of lepidolite containing ore or concentrate 1 to recover lithium as Li.sub.2CO.sub.3 44 and potassium, rubidium and cesium as separate sulfate salts 19 and aluminium as Al(OH).sub.3 17.

(16) The lepidolite containing ore or concentrate 1 is passed to a milling step 50, with water 2, in which the ore or concentrate is milled to reduce the particle size, for example to <P.sub.80 150 micron and preferably to <P.sub.80 75 micron, and enable rapid dissolution of the contained lepidolite. The milled lepidolite slurry 3 is directed to a leach step 60 in which at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and cesium are extracted into solution forming a pregnant leach solution (PLS). Concentrated H.sub.2SO.sub.4 4 is added to the leach stage. The leach reactors employed in the leach step 60 are heated using steam 5 to allow for high metal extractions and relatively short retention time. The leach step 60 is conducted at a temperature between about 90 and 130 C., with a retention time of between about 6 and 24 hours.

(17) The leach slurry 6 is passed from the leach step 60 to a solid liquid separation step, for example a belt filter 70, which enables the leach slurry to be filtered at or near the leaching temperature. The filtration stage produces a PLS 9 containing the bulk of the extracted lithium, potassium, aluminium, rubidium, fluorine and cesium and a leach residue 8 with high silica content, which is washed with water 7. The wash filtrate can be combined with the PLS 9 and the leach residue 8 is either discarded, stockpiled for sale, or further processed to produce saleable silica containing products.

(18) The total sulfate concentration in the leach step 60 is such that it is close to the saturation limit of the solution at the leaching temperature. For example, this could be 6.0M S at >90 C. Under these conditions the Applicants have noted >90% metal extraction is achieved within 12 hours.

(19) The PLS 9 from the filter 70 is passed to a monovalent alum crystallization stage 80. The temperature is reduced by applying a negative pressure and/or indirectly cooling, for example using cooling water.

(20) A monovalent alum crystallization slurry 10 is passed through a solid liquid separation stage 90, for example a belt filter, which enables the solids and liquid to be separated at or close to the temperature of the crystallization stage, for example between about 5 C. to 400 C. The filtrate 21 is passed to the fluoride crystallization stage 140 and the solids are washed with water 11.

(21) Washed monovalent alum 12 is dissolved in water 13 in an alum dissolution stage 100, producing an alum solution 14. Aluminium is precipitated from solution by the addition of limestone 15 in an aluminium precipitation stage 110. The precipitation slurry 16 is passed to a solid liquid separation stage 120, which separates the insoluble aluminium hydroxide 17 from the liquor 18. The liquor 18 contains the majority of the potassium, cesium and rubidium from the ore or concentrate and is subject to a selective crystallization step 130. A potassium sulfate product is recovered together with a mixed rubidium sulfate and cesium sulfate product, collectively indicated at 19. The mixed rubidium and cesium product can be further processed, if required, to produce other products, for example rubidium and cesium formates.

(22) The monovalent alum crystallization filtrate 21; exiting the filtration stage 90, contains the majority, typically >99%, of the contained lithium from the lepidolite ore or concentrate 1. It is subject to crystallization of a fluoride containing double salt in the fluoride crystallization stage 140 by mixing with the addition of khademite (AlFSO.sub.4.5H.sub.2O) as seed (not shown). The crystallization slurry 22 is passed to filtration, for example a solid liquid separation stage 150, which separates the khademite containing solids, being a fluoride intermediate 23, from a fluoride depleted liquor or filtrate 24. The khademite solids 23 can be further processed to produce saleable fluoride products if required.

(23) H.sub.2SO.sub.4 is neutralised and impurity elements, such as iron and aluminium, are precipitated from the fluoride crystallization filtrate 24 by the addition of limestone 25 and steam 26 in a low pH impurity removal stage 160. A slurry 27 from stage 160 is passed to solid liquid separation 170 and washing with water 29, and the impurity solids 30 are then discarded.

(24) The low pH impurity removal stage 160 operates under the following conditions. The precipitation of alunite (NaAl.sub.3(SO.sub.4).sub.2(OH).sub.6) is targeted, which allows the precipitation of fluoride from solution. Fluoride replaces OH in the chemical structure to form (NaAl.sub.3(SO.sub.4).sub.2(F).sub.6). The Applicants expect both alunites to be present. This requires the addition of monovalent cations such as sodium, potassium and the like. Potassium, rubidium and cesium can replace sodium in the alunite structure. Lithium does not form an alunite. Potassium, rubidium and cesium are present in the liquor from the mica. Sodium is present in the lithium precipitation filtrate (as sodium sulphate) which is used to prepare the reagents as slurry (limestone slurry or lime slurry). This allows for the addition of sodium in this stage of the process of the disclosure.

(25) Alunite precipitates at high temperature (>90 C.) and in the pH range of 2-3, preferably about 2.50. In tests the Applicants have consistently produced alunite and the fluorine concentration has dropped from 5 g/L to <2 g/L.

(26) It is desirable to remove fluoride in this stage as lithium may precipitate as lithium fluoride in the subsequent high pH impurity removal stage. Alunite also filters and dewaters well, so as well as capturing the fluoride, it is also easy to handle.

(27) The filtrate 28 from the low pH impurity removal stage 160, which contains the majority of the contained lithium from the lepidolite ore or concentrate 1, is passed to a high pH impurity removal stage 180. Lime 31 is used to precipitate impurity base metals such as manganese and magnesium. A slurry 32 from stage 180 is passed to a solid liquid separation step 190 and washing with water 33, from which the high pH impurity removal solids 36 are discarded.

(28) The filtrate 35 from the high pH impurity removal stage 180, which contains the majority of the contained lithium from the lepidolite ore or concentrate 1, is subjected to a calcium removal stage 200, which can be a combination of precipitation and ion exchange. Sodium carbonate solution 37 is used to precipitate calcium from solution as CaCO.sub.3 40. A slurry 38 from stage 200 is passed to a solid liquid separation step 210, from which the precipitated CaCO.sub.3 40 and residual lithium is recycled to the low pH impurity removal stage 160.

(29) The filtrate 39 from the calcium precipitation can be further cleaned of calcium by an ion exchange process (not shown), if required.

(30) The filtrate 39 from the calcium removal stage 200, which contains the majority of the contained lithium from the lepidolite ore or concentrate 1 and is low in impurities, is subject to the lithium recovery stage 220. If required, this solution is pre-concentrated by evaporation (not shown). Na.sub.2CO.sub.3 41 is added to the filtrate 39 to force the precipitation of Li.sub.2CO.sub.3 44. Reactors (not shown) employed in stage 220 are heated to allow for high lithium recovery, for example to about 90 C.

(31) A slurry 42 from stage 220 is passed to a solid liquid separation step 230 and washing. A filtrate 43 from step 230 is directed to the sodium sulfate crystallization stage 240 to recover Na.sub.2SO.sub.4 45. The filtrate 46 from this stage is recycled to the low pH impurity removal stage 160.

(32) In FIG. 2 there is shown a flow sheet in accordance with the leaching stage 60 of the present disclosure. Like numerals denote like parts, steps or processes. The lepidolite containing ore or concentrate 1 is passed to the milling step 50 in which the ore or concentrate is milled, with water 2, to reduce the particle size and enable rapid dissolution of the contained lepidolite, as noted hereinabove. Alternatively, the milling step 50 is a dry milling process. The milled lepidolite 3 is directed to the first of four leach reactors in the leaching stage 60, for example a first leach reactor 61. In leach reactor 61 concentrated sulfuric acid 4 is added at a rate to provide the sulfate ions necessary to form sulfate salts of the relevant cations in the mica and as well as excess to enable a residual sulfuric acid concentration of >50 g/L in the leach liquor. That is, acid is generally added as a ratio control, relative to the feed rate. Steam 5 may also be added to ensure the target temperature of about 120 C. is achieved. The percent solids of the mica containing leach feed is also controlled to target a specific sulfur concentration of the final leach liquor.

(33) Leach slurry discharges from the first leach reactor 61 and enters a second leach reactor 71. Slurry then gravities through the second leach reactor 71 to a third leach reactor 81 and subsequently to a fourth leach reactor 91. The several leach reactors 61, 71, 81 and 91 are required to provide the necessary retention time to achieve adequate extraction of the valuable components from the mica and to minimise short circuiting of slurry to the solid liquid separation step 70. The retention time in the leaching stage 60 is less than about 18 hours, for example between 6 to 18 hours, and in one form about 12 hours. Steam may be added to each of the reactors 71, 81 and 91 also, if required to maintain the target temperature.

(34) The sulfuric acid concentration in the liquor can range from >500 g/L H.sub.2SO.sub.4, in particular exiting the earlier reactors, for example reactors 61 and 71, down to >50 g/L H.sub.2SO.sub.4 exiting the final reactor 91. The free acid concentration is dependent on the percent solids in the mica feed, and target sulfur concentration in the leach liquor, but is preferably >50 g/L.

(35) Slurry from the fourth reactor 91 is passed to the solid liquid separation step 70, which enables the leach slurry to be filtered at or near the leaching temperature. The filtration stage produces the PLS 9 containing the bulk of the extracted lithium, potassium, aluminium, rubidium, fluorine and cesium and a leach residue 8 with high silica content, which is washed with water. The wash filtrate can be combined with the PLS 9 and the leach residue 8 is either discarded, stockpiled for sale or further processed to produce saleable silica containing products.

(36) In FIG. 3 there is shown a flow sheet in accordance with a second embodiment of the present disclosure and the production of a sodium silicate product 51. Like numerals denote like parts, steps or processes. The lepidolite containing ore or concentrate 1 is passed to the milling step 50 in which the ore or concentrate is milled to reduce the particle size and enable rapid dissolution of the contained lepidolite as noted hereinabove. The milled lepidolite 3 is directed to the acid leach step 60 in which at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and cesium are extracted into solution forming a pregnant leach solution (PLS). Concentrated H.sub.2SO.sub.4 4 is added to the leach stage 60 with steam 5 as required.

(37) The acid leach slurry 6 is passed from the leach step 60 to the solid liquid separation step 70, for example a belt filter, which enables the leach slurry 6 to be filtered at or near the leaching temperature. The filtration stage produces an acid leach residue 8 with high silica content, which is washed with water 7 to remove entrained leach liquor.

(38) The acid leach residue 8 is directed to an alkaline leaching stage 240, in which at least a proportion of the contained silica is extracted into solution forming a sodium silicate liquor. Concentrated sodium hydroxide solution 47, for example 50% w/w, is added to the alkaline leach stage. The leaching stage 240 is conducted at atmospheric conditions and at an elevated temperature but less than the boiling point. The leach slurry 48 is passed to a solid liquid separation stage 180, which separates a saleable sodium silicate product 51 as a filtrate, from the insoluble residue 49. The insoluble residue 49 is washed with water 48 to recover entrained sodium silicate, which can be combined with the sodium silicate product 51 or recycled to the alkaline leach stage 240.

(39) Varying the percent solids and the sodium hydroxide concentration, say between about 20 to 50% w/w, in the alkaline leaching stage 240 can produce a variety of sodium silicate grades.

(40) FIGS. 4A and 4B show a flow sheet in accordance with a third embodiment of the present disclosure, being the recovery of a potassium, rubidium and cesium product. Like numerals denote like parts, steps or processes. Washed monovalent alum 12 is dissolved in water 13. Aluminium is precipitated from solution by the addition of limestone 15 in the aluminium precipitation stage 110. The precipitation slurry is passed to the solid liquid separation stage 120, which separates the insoluble aluminium hydroxide 17 from the filtrate or liquor 18. The liquor 18 contains the majority of the potassium, cesium and rubidium from the ore or concentrate and is subject to selective crystallization discussed below.

(41) A potassium sulfate product is recovered by forced crystallization in the potassium sulfate crystalliser 130. A crystallization slurry 20 is passed to a solid liquid separation stage 250, where potassium sulfate 252 is separated from the liquor. A filtrate 254 is passed to a second crystallization stage 260 where the majority of the remaining potassium sulfate and some rubidium and cesium sulfate is crystallized. A slurry 256 from the crystallization stage 260 is passed to a solid liquid separation stage 270, where sulfate salts are separated from the liquor and recycled as recycle solids 258 to the alum dissolution stage 100 to recover those salts.

(42) A filtrate 272 from the mixed crystallization stage 260 is relatively free from potassium sulfate and contains the majority of the rubidium and cesium from the original ore or concentrate 1. The filtrate 272 is subject to crystallization to recover rubidium and cesium as sulfates in the Rb/Cs crystalliser 280. A resulting slurry 274 is passed to a solid liquid separation stage 290 where sulfate salts 276 are separated from a liquor or recycle filtrate 278, also recycled to the alum dissolution stage 100. The mixed rubidium and cesium product can be further processed, as required, to produce other products such as rubidium and cesium formates.

(43) As can be seen from the above description, the process of the present disclosure provides a novel and improved combination of operating steps, one or more of which may have been used commercially, in other combinations and for other purposes, in mineral processing and hydrometallurgical processes. The lithium containing mineral, lepidolite, is able to be pre-concentrated, if required, by a mineral separation process. The lithium, potassium, rubidium, cesium, fluoride and aluminium present in lepidolite are extracted by concentrated sulfuric acid leaching, producing a leach liquor containing lithium, potassium, rubidium, cesium, fluoride and aluminium and a leach residue containing silica. A majority of the aluminium, potassium, rubidium, cesium and fluorine present in the leach liquor are separated from lithium as mixed sulfate salts. Hydrometallurgical techniques including selective precipitation and crystallization are able to be implemented to separate aluminium, potassium, rubidium, cesium and fluorine from the mixed sulfate salt into potentially saleable products. Such saleable products include, but are not limited to, Al(OH).sub.3, AlF.sub.3 and K.sub.2SO.sub.4, Rb.sub.2SO.sub.4 and Cs.sub.2SO.sub.4. In the processes of the present disclosure lithium is separated from residual impurities, including, but not limited to, sulfuric acid, aluminium, iron, manganese, calcium, rubidium, cesium and potassium by hydrometallurgical techniques such as selective precipitation and crystallization to in turn produce saleable Li.sub.2CO.sub.3. Again as described, useful formate and silicate products may be obtained, as may useful rubidium and cesium products.

(44) The fluorine salts produced in accordance with the present disclosure typically comprise a mixture of aluminium fluoride and alumina. Such a mixture is envisaged to be suitable for use as a feed constituent in the electrolytic cells employed in aluminium production, rather than the currently used aluminium fluoride. The aluminium fluoride that is conventionally used is typically prepared by the reaction of aluminium hydroxide with hydrogen fluoride as the mixture added to electrolytic cells must be substantially free from impurities.

(45) It is envisaged that the feed to the leach at least contains lithium bearing mica but that other components may also be present, including spodumene ore and/or spent lithium battery products. It is further envisaged that the pretreatment step may include roasting.

(46) Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present disclosure.