Method of lithium sulfate and sodium (potassium) carbonate directly producing lithium carbonate and reducing sulfate radical content
Traditional methods for producing lithium carbonate involves thermal precipitation of a lithium sulfate purification liquid and a sodium (potassium) carbonate purification liquid to produce crude lithium carbonate to the production of a refined lithium carbonate wet product. The employment of “reverse feeding, non-circulating mother liquor”, “pre-precipitation supplementary impurity removal” and “high-efficiency desorption” can reduce industrial grade lithium carbonate sulfate anions to 0.03%, increase the main content to 2.5N, reduce battery grade sulfate anions to 0.008%, and stably increase the main content to 3N, or even reach the limit of 3.5N-4N. The high-efficiency desorption involves thermal precipitation with small temperature increases and thermal stirring washing, medium-high temperature strong desorption, and hydrocyclone separation. Impurities such as sulfate anions that are chemically adsorbed and encapsulated in the peritectic core of lithium carbonate particles can be released into deionized water, which are then effectively carried away by a hydrocyclone separation liquid phase.
1 . A method for preparing lithium carbonate from lithium sulfate and sodium carbonate or potassium carbonate, comprising the following steps:
S1: adding a 20 wt % lithium sulfate purified solution to a sodium carbonate or potassium carbonate purified solution at 90-95° C. to obtain crude lithium carbonate-1; separating the crude lithium carbonate-1 to obtain a primary hot mother liquor;
S2: cooling the primary hot mother liquor to −15° C. to crystallize mirabilite; separating the mirabilite by centrifugation to obtain a secondary cold mother liquor;
S3: concentrating the secondary cold mother liquor, to precipitate and separate crude lithium carbonate-1 again, and to obtain a tertiary hot mother liquor;
S4: combining the tertiary hot mother liquor with the primary hot mother liquor, and repeating steps S2-S3 to reduce the sulfate anion content in the liquid system;
S5: combining the crude lithium carbonate-1 obtained in steps S1 and S3, then transferring to deionized water which is 3 or 4 times the mass of the crude lithium carbonate-1 and has a temperature of 90-95° C.; heating to 115° C., stirring and washing for 15 minutes, then centrifuging and rinsing to obtain crude lithium carbonate-2;
S6: adding the crude lithium carbonate-2 obtained in step S5 to deionized water which is 3-6 times the mass of the crude lithium carbonate-2 and is in a desorption reactor; heating to 159-188° C., desorbing sulfate anions adsorbed on the crude lithium carbonate-2 into the deionized water under stirring, and performing thermal aging for not less than 1 hour, to obtain a thermally aged slurry; and
S7: adding the thermally aged slurry obtained in step S6 to a hydrocyclone to separate the liquid and solid phases.
2 . The method according to claim 1 , wherein in step S1: adding a saturated sodium carbonate or potassium carbonate purified solution into a reactor, heating to 105-120° C.; pumping the lithium sulfate purified solution which has been preheated to 90-95° C. into the reactor in a form of sprayed droplets through multi-point arranged showers to precipitate the crude lithium carbonate-1; then centrifuging to separate the crude lithium carbonate-1.
3 . The method according to claim 1 , wherein in step S1, the molar ratio of sodium carbonate or potassium carbonate to lithium sulfate is 105%.
4 . The method according to claim 1 , wherein before step S1: heating a 20 wt % lithium sulfate purified solution to 80-90° C.; under online detection by a turbidimeter, adding a small amount of sodium carbonate purified solution into the lithium sulfate purified solution; stopping addition immediately once the lithium sulfate purified solution becomes turbid and fine white, yellowish or reddish solids precipitate out; continuing to stir for 15 minutes and filtering, then detecting the contents of iron, aluminum, magnesium, calcium and heavy metals in the filtrate; if the contents are beyond the national standards, spraying a small amount of sodium carbonate purified solution again and re-detecting until the standards are met, then filtering.
5 . The method according to claim 1 , wherein the desorption reactor in step S6 is selected from a vertical cylindrical desorption reactor or a spherical desorption reactor equipped with a stirrer and a heating and cooling jacket; wherein the desorption reactor adopts a jacketed heating and cooling means, and direct steam heating is not allowed to avoid contaminating the content; and wherein the inner wall of the desorption reactor that comes into contact with the content is made of titanium, 0Cr18Ni9Ti stainless steel, 0Cr18Mo2Ti stainless steel, glass or polytetrafluoroethylene (PTFE).
6 . The method according to claim 1 , wherein in step S1, the 20 wt % lithium sulfate purified solution is derived from spodumene, lepidolite, primary lithium carbonate from carbonate salt lake lithium ore, zinnwaldite, amblygonite, petalite or lithium-containing waste.