Process For Extracting Values from Lithium Slag

Abstract

A process for extracting values from lithium slag comprising: (a) hydrothermally treating lithium slag with an aqueous solution of an alkaline compound at selected temperature and duration; (b) performing an ion exchange step on the alkaline treated lithium slag; and (c) recovering values selected from the group consisting of aluminium compounds, silicon compounds and compounds containing silicon and aluminium.

Claims

1. A process for extracting values from lithium slag comprising: (a) hydrothermally treating lithium slag with an aqueous solution of an alkaline compound at selected temperature and selected duration; (b) performing an ion exchange step on alkaline treated lithium slag; and (c) recovering values selected from the group consisting of aluminium compounds, silicon compounds and compounds containing silicon and aluminium.

2. The process of claim 1, wherein the alkaline compound (AC) is a strongly alkaline compound selected from the group consisting of strongly alkaline compounds of sodium or potassium including caustic soda, potassium hydroxide, sodium carbonate and potassium carbonate.

3. The process of claim 1, wherein the lithium slag to AC weight by weight ratio is in the range of about 1:0.1 to about 1:2.

4. The process of claim 3, wherein said selected temperature is higher than about 60 C., 90 C.

5. The process of claim 4, wherein solids density of lithium slag in the alkaline aqueous solution is up to about 50%.

6. The process of claim 2, wherein the hydrothermal treatment solubilises small amounts of both alumina and silica as silicates with a greater proportion of silica than alumina being solubilised.

7. The process of claim 6, wherein solubilised silicates are precipitated in a precipitation step.

8. The process of claim 7, wherein the precipitation step allows regeneration of the alkaline compound selected for the hydrothermal step and the selected alkaline compound is recycled to the hydrothermal treatment step.

9. The process of claim 2, wherein a solid/liquid separation step follows the hydrothermal treatment with the alkaline compound, the separated solid residue then being subjected to an acid leaching step.

10. The process of claim 9, wherein the acid leaching step involves hydrochloric acid to form aluminium chloride hexahydrate in an acid leachate.

11. The process of claim 1, wherein the ion exchange step is conducted by contacting an aqueous solution of an ammonium compound with the alkaline treated residue.

12. The process of claim 1, wherein ion exchanged residue is roasted prior to the acid leaching step under conditions effective to remove all moisture and part of the ammonia where used for ion exchange.

13. The process of claim 9, wherein reactive silica from the acid extraction residue is redissolved by alkaline leaching.

14. The process of claim 13, wherein the silica is precipitated from solution by lowering the pH of the solution.

15. The process of claim 9, wherein aluminium trichloride hexahydrate is precipitated from the acid leachate using a gaseous precipitant.

16. The process of claim 15, wherein aluminium trichloride hexahydrate is converted to alumina or high purity alumina (HPA) through a further calcining step at temperatures of between about 700 C. and 1600 C.

17. The process of claim 1, wherein, prior to step (a), the lithium slag is beneficiated in at least one process selected from the group consisting of washing with acid to remove impurities, magnetic separation and particle sizing adjustment to optimise the hydrothermal treatment step.

18. The process of claim 17, wherein particle sizing is adjusted to less than 100 microns.

19. The process of claim 11, wherein said ammonium compound is selected from the group consisting of ammonium hydroxide and ammonium carbonate.

20. The process of claim 15, wherein said acid gas is hydrochloric acid gas.

21. The process of claim 18, wherein particle sizing is adjusted to less than 50 microns.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] The process for extracting values from lithium slag may be more fully understood from the following description of preferred but non-limiting embodiments made with reference to the FIGURE showing a flow diagram for the process.

[0021] Lithium slag, in the form of spodumene ore residue for example, is obtained as a waste by-product from lithium refining, for example following the spodumene leaching step which liberates substantially all lithium from the ore. The spodumene leaching step may involve sulphuric acid leaching. The lithium slag (which could for example include 68% SiO.sub.2 and 26% Al.sub.2O.sub.3) is first beneficiated as follows in step 1. The particle size of the lithium is adjusted through methods such as milling and/or other classification techniques to an average particle size being less than 100 microns, desirably less than 50 microns. Magnetic particles are removed through any magnetic separation technique.

[0022] The lithium slag particles of particle size less than 50 microns (for example less than 38 microns) are then suspended, at a solids density of about 30%, in an aqueous caustic alkaline (AC) solution in an agitated tank reactor in step 2. The lithium slag to AC weight by weight ratio of the slurry is maintained in the range about 1:0.1 to about 1:2 (at 3.75M NaOH), i.e strongly alkaline, to optimise conversion of lithium slag to value silicon and alumina compounds. At lower AC ratios or alkaline concentrations, longer reaction times may be required for sufficient aluminium extraction.

[0023] The nature of the aluminium and silicon compounds obtained from the hydrothermal treatment step is dependent on the temperature and the concentration of the alkaline solution. The alkaline treated lithium slag residue contains such a compound or compounds, desirably exhibiting ion exchange properties (for example zeolites A, X or P), that are expected to be obtained in acceptable yield at temperature of about 90 C. or higher and duration of about 12 hours, though it will be understood that the duration is not critical provided that the target value compounds are obtained. The process is optimised, as described above, to a desired aluminium extraction level, for example 85% extraction or higher.

[0024] Optionally, the hydrothermal treatment is conducted in two stages and tank reactors. The first aging stage is conducted at 50 C. for about 1 hour. The second hydrothermal treatment stage is conducted, with heating to 90 C., for about 7 to 10 hours. A single hydrothermal treatment stage, at say 90-95 C. may also be used as an alternative with expected similar results in terms of product quality.

[0025] Hydrothermal treatment solubilises small amounts of alumina but silica is solubilised to greater extent as sodium silicate, given that caustic is the selected alkaline compound for hydrothermal treatment.

[0026] After the alkaline treatment of lithium slag, and solid/liquid separation step 3, the process includes an ion exchange step 4, to remove the introduced sodium or potassium or any cation already in the alkaline leached mineral matrix that may influence the quality of target value products. The ion exchange step 4 is conducted by contacting an aqueous solution of a suitable compound, such as an ammonium compound, for example ammonium chloride, ammonium sulphate, ammonium nitrate, ammonium hydroxide or ammonium carbonate, with the alkaline treated lithium slag residue at concentration of say 2M, with the alkaline treated lithium slag residue. The alkaline treated lithium slag residue is recovered from ion exchange by a solid/liquid separation stage 3 such as filtration or thickening.

[0027] Referring to ion exchange step 4 once again, the ion exchange step may have duration 30 to 60 minutes at a volume that will allow sufficient ion exchange and impurity removal. The concentration and solid density can vary. If lower concentrations are used, the ion exchange process may need to be repeated to compensate for the ion exchange equilibrium. If high concentrations are used, it is possible that the ion exchange step may be performed only once or as a single step. The ion exchange step 4 could be done at slightly higher temperatures than room temperature, for example 40 or 50 C. A process where the residue is washed with ammonium chloride in a counter current fashion may further optimise the ion exchange step 4.

[0028] The solid ion exchanged residue is heated to remove part of the ammonia as well as adsorbed water. During the heating process, the zeolite may undergo structural change likely related to ammonia release, but not necessarily solely because of it. Moreover, as residual ammonia and internal moisture in the ion exchanged residue may be associated with silica gel formation during subsequent acid leach treatment, as described below, and consequential solid liquid separation difficulties, the solid ion exchanged residue is desirably roasted to remove excess ammonia and internal moisture. Such excess ammonia may also be recycled, for example as ammonium chloride by contacting with hydrochloric acid and reused in the ion exchange step 4. The focus on recycling and minimising wastage provides cost and environmental benefits for the ion exchange step, subsequent acid leach step 8 and the overall process.

[0029] The ion exchanged residue is separated and may be heated to say 350 C. for 1 hour or the temperature could be lower, say 250 C., but perhaps for 8 hours. It appears that a hardening of the structure of the zeolite occurs with the consequence that longer roasting times will lead to a decline in alumina extraction efficiency and shorter times will lead to silica gel formation under the same acid leaching conditions.

[0030] The ion exchanged residue is then subjected to an acid leaching step 5 in which the ion exchanged residue is re-slurried in hydrochloric acid with the object of producing a useful intermediate, aluminium trichloride hexahydrate. Process conditions, for example, involve 25 wt % HCl at room temperature and reaction duration one hour at a solids density of 10% to 25% depending on how well the gel formation is controlled. Higher solid densities are achievable where the gel formation is limited. Agitated tank reactor(s) are once again employed. At higher HCl concentrations the solubility of Al(H.sub.2O).sub.6Cl.sub.3 is reduced. At lower HCl concentrations, extraction may also be successful, although copious quantities of HCl will be needed to saturate the Al(H.sub.2O).sub.6Cl.sub.3 solution to precipitate the aluminium chloride hexahydrate out. Extraction may also occur at lower temperatures, for example at room temperature.

[0031] The acid leaching step 5 only requires hydrochloric acid in slight excess to stoichiometric amounts for reaction to form Al(H.sub.2O).sub.6Cl.sub.3. That is, just over 3 mole equivalents of HCl for every one mole equivalent of aluminium in the residue. Acid leachate is separated from the silica rich acid leached residue by filtration or centrifugation with both solid and liquid components being subjected to further processing steps.

[0032] The silica rich acid leached residue, separated in solid/liquid separation step 6, is subjected to an alkaline leaching step 8 to solubilise the silica to a sodium silicate solution which may then be treated and purified to precipitate reactive silica. The alkaline liquor from the alkaline hydrothermal treatment stage 2 could be used to redissolve the reactive silica from the acid extraction residue. The re-dissolution could include only reactive silica using mild conditions, for example 90 C. and a reaction time of about an hour. This should account for about 60-80 wt % of the silica in some lithium slag qualities. The remaining silica is mainly quartz that will require higher temperatures, for example 180 C. and increased pressure for silica solubilisation.

[0033] The sodium silicate solution may then be acidified, and silica precipitated through known processes in the silica production step 9 using an acid, for example sulphuric acid or hydrochloric acid, or CO.sub.2, at room temperature or under any other suitable conditions. The silica can then be washed and otherwise purified to the required purity, for example by adjusting the pH of the slurry to lower values to encourage the dissolution of impurities like sodium or potassium. Insolubles should be removed from the silicate solution before acidification with acids like HCl or H.sub.2SO.sub.4 for the lowering of pH until at least below 10 or even to as low as pH 2 in order to form precipitated silica.

[0034] To precipitate Al(H.sub.2O).sub.6Cl.sub.3 from the acid leachate from acid leaching step 5, the leachate is saturatedin precipitation stage 7with HCl gas through known methods and the mixture kept cool to afford the best conditions for precipitation due to the exothermic nature of the reaction. The purity of the Al(H.sub.2O).sub.6Cl.sub.3 may be improved upon by redissolution with water or dilute HCl and re-precipitation with HCl gas until the desired purity is reached. Washing of the product with 36% HCl could be included if proven to be desirable.

[0035] The process has significant potential for increasing profitability of lithium extraction operations by enabling treatment of previously low value, lithium slag, and using it as a feedstock to produce high purity alumina, high purity silica and a range of other compounds containing aluminium, silicon or both. At the same time, commercial benefits can be achieved by recycling reagents to minimise cost and substantially eliminate waste.

[0036] Modifications and variations to the process for extracting values from lithium slag may be apparent to skilled readers of this disclosure. Such modifications and variations are deemed within the scope of the present invention.