METHOD OF LITHIUM SULFATE AND SODIUM (POTASSIUM) CARBONATE DIRECTLY PRODUCING LITHIUM CARBONATE AND REDUCING SULFATE RADICAL CONTENT

Abstract

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 radicals to 0.03%, increase the main content to 2.5N, reduce battery grade sulfate radicals 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 radicals 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.

Claims

1. A new method for substantially reducing the content of impurity sulfate radicals in lithium carbonate directly produced by lithium sulfate and sodium carbonate (potassium), wherein, compared with the conventional method for reducing the content of sulfate radicals only by multiple thermal stirring washings with deionized water, the method is characterized in that: taking a spodumene-sulfuric acid method for directly producing an industrial-grade lithium carbonate and a battery-grade lithium carbonate as an example, on the basis that most of the current techniques for removing impurities such as silicon, iron, aluminum, magnesium, calcium and heavy metals before the current thermal precipitation process of manufacturers remain unchanged, the current techniques for drying, crushing, metering and packaging a refined wet lithium carbonate remain unchanged, and the current various detection methods remain unchanged, combinations of the following three techniques of the invention are applied from a process of obtaining coarse lithium carbonate by a thermal precipitation reaction of a completely purified lithium sulfate solution with a completely purified sodium (potassium) carbonate solution to a process of obtaining the refined wet lithium carbonate: 1, an essential “reverse feeding without mother liquor circulation” technique; 2, an optional “supplementary impurity removal by pre-precipitation” technique; and 3, an essential “high-efficiency desorption” technique, so as to produce industrial zero-grade, industrial “new first-grade”, industrial “new zero-grade” and “new battery-grade” lithium carbonates with corresponding contents of sulfate radicals reaching 0.20%, 0.10%, 0.03% and 0.008%, respectively.

2. The “reverse feeding without mother liquor circulation” technique according to claim 1, wherein: the conventional operation of adding the completely purified sodium carbonate solution into the completely purified lithium sulfate solution in the thermal precipitation procedure is reversed, that is, the completely purified lithium sulfate solution is added into the completely purified sodium carbonate solution, so that the chemical adsorption and deep coating of sulfate radicals by lithium carbonate particles are substantially reduced; a primary hot mother liquor of coarse lithium carbonate-1 obtained by centrifuging-rinsing is treated in one of the following three ways, and not sent back to an acidification material leaching process: {circle around (1)} after the primary hot mother liquor is cooled to 0° C. to −15° C. for crystallization, and centrifuged to obtain mirabilite, another process route is developed: a secondary cold mother liquor is concentrated until a sodium sulfate crystallization film is exactly formed, and filtered while hot to obtain the coarse lithium carbonate which is precipitated again, and sent back to the acidification material leaching process, and a tertiary hot mother liquor is combined with the primary hot mother liquor for the crystallization of mirabilite . . . , as such, the operations of “cooling for precipitating mirabilite and heating for precipitating coarse lithium carbonate” are alternately performed; {circle around (2)} the secondary cold mother liquor is concentrated in vacuum to recover anhydrous sodium sulfate after the lithium is recovered therefrom by precipitating water-insoluble lithium salts such as lithium phosphate, lithium fluoride or lithium stearate; {circle around (3)} the primary hot mother liquor is directly concentrated in vacuum continuously to recover anhydrous sodium sulfate after water-insoluble lithium salts such as lithium phosphate, lithium fluoride or lithium stearate are recovered therefrom, so that the concentration of the sodium sulfate in a thermal precipitation reaction solution system is substantially reduced, the beneficial technical effect of reducing the content of sulfate radicals by the “reverse feeding” is superposed, and the first pass yield of the coarse lithium carbonate-1 is improved due to the reduction of the salt effect.

3. The “high-efficiency desorption” technique according to claim 1, {circle around (1)} consisting of “strong desorption” and “hydrocyclone separation” techniques, wherein the “strong desorption” further consists of “thermal precipitation with small temperature increases and thermal stirring washing” and “medium-high temperature strong desorption” techniques; {circle around (2)} the “thermal precipitation with small temperature increases and thermal stirring washing” means that in cases where the current common jacketed reactor is still used for the thermal precipitation and thermal stirring washing operation, under the condition that the allowable pressure is 0.6 MPa for the jacket and 0.2 MPa for the reactor, a preferred range of 0.13-0.18-0.20 MPa (but not limited to a lower limit of 0.13 MPa) of saturated steam pressure in the reactor (corresponding to the temperature of 105-115-120° C. of the reaction solution) is selected for the thermal precipitation and thermal stirring washing (with 3 times, but not limited thereto, of deionized water added for the thermal stirring washing) to moderately reduce the content of sulfate radicals of the coarse lithium carbonate-1 and the coarse lithium carbonate-2, moderately improve the first pass yield of lithium and moderately reduce the amount of deionized water used in the thermal stirring washing; {circle around (3)} the “medium-high temperature strong desorption” refers to such a critical innovative technique that the coarse lithium carbonate-2 and deionized water with a mass that is 3-4-5-6 times (but not limited thereto) that of the coarse lithium carbonate-2 are added to a pressure reactor (taken as an example), or a reactor or a reaction pipeline, and the mixture is subjected to the strong desorption and thermal aging for 1 hour or more in a preferred range of 0.5-0.6-0.7-0.8-1.2 MPa (but not limited to thereto) of saturated stream pressure (corresponding to the temperature of 152-159-165-170-188° C.) under conditions of low-speed stirring, rolling, and constant low-speed movement of the solid phase of the reaction solution, so that part of sulfate radicals and most of the other water-soluble, slightly water-soluble and water-insoluble impurities, which are deeply coated in the core of the coarse lithium carbonate due to the chemical desorption in the initial stage of the thermal precipitation reaction and thus are extremely difficult to remove, are desorbed and released in a large amount of deionized water due to intensified thermal motion of various molecules, ions and atom groups in the solution system, and the lithium carbonate crystals with small particle size are recrystallized into large crystals with extremely low content of sulfate radicals in an environment with low concentration of sulfate radicals in the large amount of deionized water; {circle around (4)} and the “hydrocyclone separation” means that most of these water-soluble, slightly water-soluble and water-insoluble impurities separated from the lithium carbonate particles by the “medium-high temperature strong desorption” operation are brought out directly with the rotating flowing liquid phase, instead of being trapped in the refined lithium carbonate particles, as is the case with various solid-liquid separation techniques using filter cloth or filter elements.

4. The “thermal precipitation with small temperature increases and thermal stirring washing” technique according to claim 3, wherein the thermal precipitation operation is characterized in that: {circle around (1)} the completely purified sodium carbonate solution is added to a reactor for the thermal precipitation with small temperature increases and heated with the jacket opened; a manhole on the reactor is covered, and the reactor is fully sealed after air in the reactor is completely driven out; when the reactor is heated to a selected temperature such as 105-115-120° C. (but not limited to the low limit range of 105° C.), a stirrer is started and is kept working effectively, the completely purified lithium sulfate solution is rapidly sprayed in a mist form into the reaction through pressurized sprinklers arranged at multiples points (the feeding time is shortened by more than 50% compared with the conventional thermal precipitation process); the coarse lithium carbonate-1 with small particle size is obtained, and the thermal aging duration of crystals with large particle size obtained by the traditional process is moved backward to the “medium-high temperature strong desorption” process in the initial stage, refined lithium carbonate large crystals generated by recrystallization are obtained in an environment with a large amount of deionized water in the later stage, so that most of impurities such as sulfate radicals deeply coated in the core of the coarse lithium carbonate-1 particles due to the chemical adsorption in the early stage of the thermal precipitation reaction are released; after the feeding is finished, the reactor is de-pressurized and cooled, when the temperature of the reaction solution in the reactor is reduced to 90-95° C., the reaction solution is immediately discharged, centrifuged and rinsed to obtain the lithium carbonate-2; {circle around (1)} the equivalence ratio of sodium carbonate to lithium sulfate is 1.05:1.00.

5. The “thermal precipitation with small temperature increases and thermal stirring washing” technique according to claim 3, wherein the thermal stirring washing operation is characterized in that: the coarse lithium carbonate-1 is transferred while hot into a reactor for the thermal stirring washing with small temperature increases into which deionized water with selected mass multiples such as 3-4-5 times for the industrial-grade lithium carbonate and 5-6 times for the battery-grade carbonate (but not limited thereto) has been added, is heated to 90-95° C. with a stirrer started; a manhole on the reactor is covered, and the heating is continued; the reactor is sealed after air in the reactor is completely driven out, and subjected to the thermal stirring washing for 15 minutes after being heated to a selected temperature such as 105-115-120° C. (but not limited to the lower limit of 105° C.), and then de-pressurized and cooled to 95° C., after which the reaction solution is discharged, centrifuged and rinsed to obtain the industrial-grade coarse lithium carbonate-2 and battery-grade coarse lithium carbonate-2 with the content of sulfate radicals controlled to be 0.30%-0.20% and 0.15%-0.10%, respectively, for later use.

6. The “medium-high temperature strong desorption” technique according to claim 3, wherein the operations are characterized in that: taking the use of a reactor of the medium-to-high-temperature strong desorption as an example, deionized water with selected mass multiples such as 5-6 times (but not limited thereto) of the coarse lithium carbonate-2 (for producing lithium carbonate with purity more than 99.950%, deionized water with the purity of more than 18 MΩ.Math.cm is adopted) is pumped into the reactor, and heated with the jacket opened, with a low-speed stirring started, and the coarse lithium carbonate-2 is added; the reactor is heated to a selected temperature such as 144-159° C. (0.4-0.6 MPa) for the industrial “new zero-grade” lithium carbonate and 165-170-180° C. (0.7-0.8 MPa) for the “new battery-grade” lithium carbonate (but not limited thereto), and subjected to the strong desorption and thermal aging for 1 hour or more under the conditions of the low-speed stirring and the low-speed motion state of a slurry solid phase, so that water-soluble impurities mainly comprising sodium sulfate, and other slightly water-soluble and water-insoluble impurities in the peritectic core of the lithium carbonate particles are released in deionized water, and a large number of lithium carbonate crystals with small particle size are recrystallized into large lithium carbonate crystals with the content of sulfate radicals of 0.03% for the industrial “new zero-grade” lithium carbonate and 0.008% for the “new battery-grade” lithium carbonate, respectively, in an environment with the concentration of sulfate radicals far lower than the thermal precipitation reaction concentration.

7. The “hydrocyclone separation” technique according to claim 3, wherein: after the content of residual sulfate radicals in the lithium carbonate in the desorption reactor is detected to be qualified in the “medium-high temperature strong desorption”, a heating valve is closed, with the low-speed stirring maintained, the reactor is de-pressurized and cooling water is slowly introduced for cooling, when the pressure in the reactor is reduced to 0.05-0.06 MPa (but not limited thereto), the stirring speed is increased until the slurry is under a strong stirring condition, and the slurry is pumped into a hydrocyclone separator with the speed controlled to continuously separate a liquid phase from a solid phase; the solid phase is centrifuged and rinsed (for the “new battery-grade” lithium carbonate, the thermal stirring washing is performed one more time if necessary) to obtain the refined wet lithium carbonate (after drying), and for the industrial “new zero-grade” lithium carbonate and the “new battery-grade” lithium carbonate, the content of sulfate radicals should be reduced to 0.03% and 0.008%, respectively; the separated liquid phase cannot be circularly used for the initial operation of the thermal precipitation of the coarse lithium carbonate, and should be sent back to the leaching process, or sent back to the leaching process after the filter cloth and equipment are cleaned, and only part of the separated liquid phase that has been subjected to the full coagulation of impurities and precise filtration is allowed to be mixed into deionized water for the thermal stirring washing of the coarse lithium carbonate-1 to produce the industrial-grade products, and is prohibited in the subsequent processes.

8. The “medium-high temperature strong desorption” and the “hydrocyclone separation” techniques according to claim 3, wherein: in cases where a pipeline desorber is used to automatically and continuously perform the “medium-high temperature strong desorption” operation, the pressure of the slurry is reduced to 0.05-0.06 MPa (but not limited thereto) through a de-pressurized storage tank with a stirrer and a cooling water jacket, and the slurry is pumped into the hydrocyclone separator for the separation operation with the speed controlled.

9. The “medium-high temperature strong desorption” technique according to claim 3, wherein the equipment is characterized in that: the main body of the desorption equipment can be selected from a vertical pressure reactor with a stirring jacket, a low-rotating-speed spherical or horizontal cylindrical pressure reactor and a pipeline reactor, all adopting dividing-wall heating and cooling; the material of the part contacting the reaction solution (or whole) of the equipment, such as the inner wall or whole of a reactor, the outer wall or whole of a stirrer, and the whole or inner wall of a pipe fitting, is selected from 0Cr18Ni9Ti stainless steel, 0Cr18Mo2Ti stainless steel, glass lining, titanium material (reactor lining) and polytetrafluoroethylene (reactor lining); in cases where a reactor made of the 2 stainless steel materials is adopted to produce the battery-grade products, in addition to strictly controlling the content of fluorine in the purified lithium sulfate solution and the content of chlorine in the purified sodium (potassium) carbonate solution obtained from fluorine-containing lithium ores such as fluor-lepidolite, there is also a strict restriction of heavy metal impurities such as the magnetic metal chromium content of less than or equal to 3 ppm, so the coarse lithium carbonate slurry must be firstly soaked in a small pressure reactor with pressure resistance of 1.6 MPa at a saturated steam pressure of 0.8-1.0-1.2 MPa for a long time (100 hours or more are recommended) to detect the leaching amount thereof, if the heavy metal content of the lithium carbonate exceeds an acceptable level after the soaking test, the batch of materials is rejected, and another selection is needed; in cases where a reactor with the glass lining in the inner wall is adopted, a test with material is also needed firstly to detect the dissolution amount of elements such as boron, aluminum, silicon, lead and antimony in the glass lining under conditions of alkaline lithium carbonate slurry, long time (100 hours or more are recommended), high temperature (corresponding to a saturated steam pressure of 0.8-1.0-1.2 MPa) and low-speed stirring, if indexes of the impurities of the battery-grade lithium carbonate are unqualified, the formula comprising the glass lining in the inner wall is rejected, and another selection is needed; the material of part contacting the reaction solution of the “hydrocyclone separation” equipment selected is the same as that of the desorber.

10. The “high-efficiency desorption” technique according to claim 1, wherein: throughout the “thermal precipitation with small temperature increases and thermal stirring washing”, “medium-high temperature strong desorption” and ‘hydrocyclone separation” operations, among all the listed technical parameters, except that the typical temperature range of 90-95° C. of the conventional thermal precipitation and thermal stirring washing belongs to the prior art, other characteristic parameters such as the amount and purity of deionized water, control indexes of the saturated steam pressure-temperature of the reactor, control indexes of the content of sulfate radicals of the coarse lithium carbonate-1 and -2, rotating speed for stirring in the reactor, rotating speed of spherical and cylindrical horizontal desorbers, duration of the desorption and thermal aging, and hydrocyclone separation operation parameters are considered as a whole instead of rigid parameters, and can be adjusted reasonably and moderately within a certain range according to the grade of lithium carbonate to be produced, quality requirements of orders, component characteristics of raw materials, yield and cost control, safety production management and other factors; for example, for producing the industrial-grade lithium carbonate, the amount of deionized water can be distributed as appropriate according to a ratio of 2.5:5:0.5 or 1.5+1.5:5.5:0.5 for the thermal stirring washing with small temperature increases, the “medium-high temperature strong desorption” and the centrifuging-rinsing, and the total amount of the deionized water used in the three processes with mass multiples of 8-9 times of the finished lithium carbonate is sufficient; for producing the battery-grade lithium carbonate, the amount of deionized water can be distributed according to a ratio of 2.5:6:0.5 or 1.5+1.5:6.5:0.5, and the total amount of deionized water used in the three processes with mass multiples of 9-10 times of the finished lithium carbonate is sufficient; for another example, in the operation of the “thermal precipitation with small temperature increases and thermal stirring washing”, the control range of the saturated steam pressure-temperature parameter of a reactor is 104.8° C.-0.13 MPa or 115.2° C.-0.18 MPa and up to 120.2° C.-0.20 MPa, which is relatively suitable for current thermal precipitation main equipment of manufacturers and do not need to be replaced, in the operation of the “medium-high temperature strong desorption”, the temperature of 159-170° C. (0.6-0.8 MPa) is relatively suitable for tank and kettle main equipment, and in the automatic and continuous operation using a pipeline desorber, the control range can be more than 170° C. (0.8 MPa) to 180° C. (1.00 MPa) and up to 188° C. (1.20 MPa), with no need to exceed 200° C. (1.60 MPa) and entering the category of medium-pressure vessel management; therefore, the reasonably adjusted technical parameters are also included in the technical scope of the invention intended to protect.

11. The “supplementary impurity removal by pre-precipitation” technique according to claim 1, wherein: the optional technique can be adopted in the following three situations: 1) if it is not found until the beginning of the thermal precipitation process that there is a mistake in the leaching operation or the operation of removing impurities such as aluminum, iron, magnesium, calcium, and heavy metals by successive precipitation in the early stage or the equipment is faulty, such that colloid particles formed of hydroxides of aluminum, iron, magnesium and certain heavy metals are not fully coagulated and thus incompletely precipitated, or the impurities pass through the filter since the filter cloth is damaged and improperly placed, or other impurity removal accidents occur, and the indexes of the impurities of the purified lithium sulfate solution are detected to be unqualified, then the technique can be adopted to perform efficient and convenient rescue; 2) the optional technique can be combined with the “reverse feeding without mother liquor circulation” to produce the current industrial zero-grade lithium carbonate; 3) the optional technique can be used in the production of the industrial “new first-grade”, industrial “new zero-grade”, current battery-grade and “new battery-grade” lithium carbonates (including other high-purity lithium carbonates); particularly, if a process of circular leaching without lithium sulfate concentration is adopted, the colloid impurities of the hydroxides of aluminum, iron, magnesium and certain heavy metals may not be heated for a long time or the surface charges of colloid particles may not be eliminated, and the impurities are not fully coagulated and co-precipitated, and thus may pass through the filter, as such, the technique can be adopted to perform efficient and convenient rescue.

12. The “supplementary impurity removal by pre-precipitation” technique according to claim 11, wherein the operations are characterized in that: the purified lithium sulfate solution is added into a thermal precipitation reactor, the stirring is started, a small amount of the completely purified sodium carbonate solution is sprayed in a mist form at a medium speed under the pressure of 0.05 MPa through pressurized sprinkler feeding ports under the condition of close observation or on-line detection using a turbidity meter at 90-95° C.; the feeding is stopped once the reaction solution become turbid followed by the precipitation of white fine substances (with yellow and red light when iron content substantially exceeds the acceptable level), and the stirring is continued for a few minutes; a sample is taken and precisely filtered, and then measured for the content of iron, aluminum, magnesium, calcium, heavy metals and silicon, if the content of impurities is unqualified, a small amount of completely purified sodium carbonate solution is sprayed into the reaction, and the sample is measured again until the content of impurities is qualified, after which the stirring is continued for another 15 minutes; the qualified purified lithium sulfate solution is filtered, and the initial filtrate is temporarily added into a small turbid solution tank (the total volume is about 20% of the volume of the purified lithium sulfate solution), and circularly filtered again until the filtrate sample is detected to be qualified, such that the filter cake is considered to be successfully bridged, and the filtrate is confirmed to be a completely purified solution, which is then filtered again together with the purified lithium sulfate solution in the turbid solution tank until all purified solution is converted into the completely purified lithium sulfate solution; if the residue has a fine texture with a few coarse particles (lithium carbonate) as detected by naked eyes, the operation results are very desirable.

13. The combinations of the three techniques of the invention according to claim 1, wherein: {circle around (1)} when the current industrial zero-grade lithium carbonate with 0.20% sulfate radicals needs to be produced, the techniques for removing impurities such as silicon, iron, aluminum, magnesium, calcium and heavy metals before the current thermal precipitation process remain basically unchanged, and all detection methods remain unchanged, and only a combination of the “reverse feeding without mother liquor circulation” and the “supplementary impurity removal by pre-precipitation” is sufficient; {circle around (2)} when the industrial “new first-grade” lithium carbonate with 0.10% sulfate radicals needs to be produced, a combination of the “reverse feeding without mother liquor circulation”, (or optional) “supplementary impurity removal by pre-precipitation” and the “thermal precipitation with small temperature increases and thermal stirring washing” is sufficient; {circle around (3)} when the industrial “new zero-grade” lithium carbonate with 0.03% sulfate radicals needs to be produced, on the basis of various impurity removal techniques for the current industrial-grade lithium carbonate, only a combination of the “reverse feeding without mother liquor circulation” and the “high-efficiency desorption”, in combination with the “supplementary impurity removal by pre-precipitation” if necessary, is sufficient; {circle around (4)} when the “new battery-grade lithium carbonate” with 0.01%-0.008% sulfate radicals needs to be produced, on the basis of various impurity removal techniques for the current battery-grade lithium carbonate, only a combination of the “reverse feeding without mother liquor circulation” and the “high-efficiency desorption”, in combination with the “supplementary impurity removal by pre-precipitation” if necessary (the operation parameters selected such as the amount of deionized water used, the temperature-pressure parameter, and the thermal aging duration are more stringent than those in the production of the industrial “new zero-grade” lithium carbonate), is sufficient.

14. The specification of the present application is only intended to illustrate and explain the contents of the invention by taking the spodumene-sulfuric acid method as an example, which is still the largest scale production at present, and should not be construed as limiting the application scope of the disclosure; in fact, the “high-efficiency desorption” technique of the invention can be naturally extended to the following similar technical fields, in addition to being used to substantially reduce the content of impurity sulfate radicals in lithium carbonate precipitated from lithium sulfate solution and sodium (potassium) carbonate solution, which is suitable to be exemplified: efficiently removing the impurities that are difficult to remove by conventional washing methods due to chemical adsorption and deep coating in the core of the crystals (or particles) of the insoluble or slightly soluble target product separated out by the precipitation reaction of two or more soluble inorganic substances, and therefore, these technical fields are all included in the technical scope of the invention intended to be protected.

Description

IV. DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 is a schematic diagram of a process flow of a spodumene-sulfuric acid process of the former Lithium of America corporation;

[0077] FIG. 2 shows a washing curve of sulfate radicals of a pilot product according to the spodumene-sulfuric acid process of the former Lithium of America corporation;

[0078] FIG. 3 shows the solubility data for lithium phosphate, lithium fluoride, lithium carbonate in water; and

[0079] FIG. 4 shows the decline curves of the content of sulfate radicals in lithium carbonates after the implementation of the invention.

[0080] The drawings 1-4 are detailed as follows: FIG. 1 is a drawing from Aus Troche C., The Chemistry and Technology of Lithium [M]. Beijing: China Industrial Press, May 1965, first edition, page 160. Combining with the written part of this book, it can be clearly demonstrated that the thermal precipitation operation using the completely purified lithium sulfate and sodium carbonate solutions is performed according to the “forward feeding” process.

[0081] FIG. 2 shows a washing curve of sulfate radical of a pilot product according to the “forward feeding” procedure of the thermal precipitation in the spodumene-sulfuric acid process for producing lithium carbonate of the former Lithium of America corporation by the inventor of the present application in the initial stage of leading the small-scale industrial production by the spodumene-lithium sulfate process in 1978-79-80. The curve clearly shows that the content of sulfate radicals is extremely difficult to further reduce after it is reduced to about 0.35% by washing, fully indicating that the biggest shortcoming of this conventional process is the high content of sulfate impurity. The washing conditions are as follows: coarse lithium carbonate:distilled water=1:1.5, the temperature is 90-95° C., the stirring time is 30 minutes, and the SS-800 three-legged centrifuge is used for spin-drying at 1,000 rpm/min.

[0082] FIG. 3 shows the solubility data for lithium phosphate, lithium fluoride, and lithium carbonate in water, which presents great differences of an order of magnitude sequentially, indicating that lithium recovery from lithium-containing sodium sulfate mother liquor is highest in a lithium phosphate way.

[0083] FIG. 4 shows the decline curves of the content of sulfate radicals after the implementation of the “reverse feeding without mother liquor circulation”, “supplementary impurity removal by pre-precipitation” and “high-efficiency desorption” techniques of the invention, both showing a precipitous drop. In the figure, a character A represents the “thermal precipitation with small temperature increases” stage, and a character B represents the “thermal stirring washing with small temperature increases”, the “medium-high temperature strong desorption” and “hydrocyclone separation” stages; the horizontal line represents that the coarse lithium carbonate-1 of the product in the stage A is transferred to the operation in the stage B to produce refined lithium carbonate; the left curve is for the industrial-grade lithium carbonate and the right curve is for the battery-grade lithium carbonate.

V. DETAILED DESCRIPTION

[0084] The invention will be further described with reference to specific embodiments. It should be understood that the following examples are merely exemplary illustration and explanation of the invention, and should not be construed as limiting the protection scope of the invention. All techniques implemented based on the aforementioned content of the invention are included in the protection scope of the invention.

[0085] Taking the direct production of lithium carbonate by the spodumene-sulfuric acid process as an example: when the technical schemes of the invention are implemented, the current impurity removal method before the “thermal precipitation with small temperature increases and thermal stirring washing” procedure remains basically unchanged, and the “supplementary impurity removal by pre-precipitation” technique of the invention can be used for supplement; the “thermal precipitation with small temperature increases and thermal stirring washing” procedure must adopt the “reverse feeding without mother liquor circulation” technique; the current techniques for drying, crushing, metering and packaging the refined wet lithium carbonate remain unchanged; all detection methods remain unchanged; the method for determining the end point of the “medium-high temperature strong desorption” in paragraph [0046] only relates to sampling measures and a method for calculating the content of sulfate radicals in solid-phase lithium carbonate, and does not relate to the change of the detection method for sulfate radicals.

[0086] Here, the specific embodiments will be described in combination with the 4 combinations of the three techniques of the invention described in paragraphs [0014-0015]:

[0087] {circle around (1)} The specific embodiment of producing the current industrial zero-grade lithium carbonate with 0.20% sulfate radicals according to the combination 1:

[0088] The “supplementary impurity removal by pre-precipitation” in paragraph [0022] is performed as follows:

[0089] A purified lithium sulfate solution is added into a thermal precipitation reactor, the stirring is started, and a small amount of purified sodium carbonate solution is sprayed in a mist form at a medium speed under the pressure of 0.05 MPa through pressurized sprinkler feeding ports; the feeding is stopped once the reaction solution become turbid followed by the precipitation of white fine substances (with yellow and red light when iron content substantially exceeds the acceptable level) as detected by naked eyes and a turbidity meter, and the stirring is continued for a few minutes; the sample is taken and precisely filtered, and then measured for the content of impurities such as iron, aluminum, magnesium, calcium, and heavy metals, and if the content of impurities is unqualified, a small amount of sodium carbonate is sprayed, and the sample is detected again until the content of impurities is qualified, after which the stirring is continued for another 15 min.

[0090] The qualified purified lithium sulfate solution is filtered, and the initial filtrate is temporarily added into a small turbid solution tank (the volume of which is about 20% of the volume of the purified lithium sulfate solution), and filtered again in a circular mode until the resulting filtrate is detected to be qualified, such that the filter cake is regarded as successfully bridged, and the filtrate is confirmed to be a completely purified solution.

[0091] Then, the specific operations of the “reverse feeding without mother liquor circulation” described in paragraphs [0018]-[0019] are performed to produce the current industrial zero-grade lithium carbonate with 0.20% sulfate radicals. The equivalence ratio of the sodium carbonate to the lithium sulfate should be 1.05.

[0092] For manufactures with advanced processes, equipment and instrument of production and detection, and management, on the basis of their current techniques, the current industrial zero-grade lithium carbonate can be simply produced by the “reverse feeding without mother liquor circulation” process with medium-low concentration of completely purified lithium sulfate solution with 10% or slightly more sulfate radicals without combination with the “supplementary impurity removal by pre-precipitation”.

[0093] {circle around (2)} The specific embodiment of producing the industrial “new first-grade” lithium carbonate according to the combination 2:

[0094] The “new first-grade” lithium carbonate with 0.10% sulfate radicals can be produced by applying the operations of the “reverse feeding without mother liquor circulation” process in paragraphs [0018]-[0019], the “thermal precipitation with small temperature increases and thermal stirring washing” operation of the “high-efficiency desorption” process in paragraphs [0041]-[0042], and optional operations of the “supplementary impurity removal by pre-precipitation” process in paragraph [0077] if necessary.

[0095] {circle around (3)} and {circle around (4)} The specific embodiment of producing the industrial “new zero-grade” and “new battery-grade” lithium carbonates according to the combinations 3 and 4:

[0096] According to the quality of the purified lithium sulfate solution, if the “supplementary impurity removal by pre-precipitation” is required, the purified lithium sulfate solution is purified supplementarily as per the embodiment described in paragraph [0077].

[0097] As described in paragraphs [0041]-[0042], the completely purified sodium carbonate solution is added to a reactor for the thermal precipitation with small temperature increases, and heated; a manhole on the reactor is covered, and the reactor is sealed after air in the reactor is completely driven out; when the reactor is heated to a selected temperature, such as 105° C. (0.13 MPa) for the industrial “new zero-grade” lithium sulfate and 118° C. (0.18 MPa) for the industrial “new battery-grade” lithium sulfate, the stirrer is started and is kept working effectively, the completely purified lithium sulfate solution is sprayed into the reaction in a mist form under a pressure of 0.1-0.3 MPa at a speed twice as fast as that of the original process for obtaining coarse lithium carbonate particles with large particle size through pressurized sprinklers arranged at multiple points, so as to firstly obtain the coarse lithium carbonate-1 with small particle size. Once the feeding is completed, the reactor is de-pressurized (a pipeline should be connected to recover steam heat), after which the reaction solution is immediately discharged, and centrifuged and rinsed to obtain the coarse lithium carbonate-1, so that the “thermal precipitation with small temperature increases” is completed. The coarse lithium carbonate-1 is immediately transferred while hot into a reactor for the thermal stirring washing with small temperature increases into which deionized water at 90-95° C. with a mass that is 3 times (for the industrial “new zero-grade” lithium carbonate) or 4 times (for the “new battery” lithium carbonate) the mass of the coarse lithium carbonate-1 has been added with the stirring started; a manhole on the reactor is covered, and the reactor is sealed after air is completely driven out, heated to a selected temperature such as 105° C. (0.13 MPa) for the industrial “new zero-grade” lithium carbonate and 120° C. (0.20 MPa) for the “new battery-grade” lithium carbonate, and subjected to the thermal stirring washing for 15 minutes, then de-pressurized, and after the reactor is cooled to 95° C., the reaction solution is discharged, centrifuged and rinsed to obtain the firstly washed coarse lithium carbonate-2 with the content of sulfate radicals reduced to 0.30%-0.20% for the industrial “new zero-grade” lithium carbonate and 0.15%-0.10% for the “new battery-grade” lithium carbonate for later use, so that the “thermal stirring washing with small temperature increases” operation is completed.

[0098] Lithium salts such as coarse lithium carbonate or lithium phosphate and mirabilite or anhydrous sodium sulfate are recovered from the primary sodium sulfate hot mother liquor generated in the “thermal precipitation with small temperature increases” process according to the operation of the “without mother liquor circulation” process.

[0099] Under the low-speed stirring, the coarse lithium carbonate-2 is transferred while hot into a reactor for the medium-high temperature strong desorption into which deionized water (for the battery-grade lithium carbonate, deionized water with the purity of 18 MΩ.Math.cm, self-made and preheated to 90-95° C. is adopted) with a mass that is 3-4-5 times (industrial-grade) or 5-6 times (battery-grade) that of the coarse lithium carbonate-2 has been pumped with the low-speed stirring started; the reactor is heated, and completely sealed after air in the reactor is driven out; when the industrial-grade lithium carbonate is to be produced, the reactor is heated to 144-159° C. (corresponding to the saturated steam pressure of 0.4-0.6 MPa in the reactor), and when the battery-grade lithium carbonate is to be produced, the reactor is heated to 165-170-180° C. (corresponding to the saturated steam pressure of 0.7-0.8-1.0 MPa in the reactor), and the “medium-high temperature strong desorption” and the thermal aging operation are continued for 1 hour or more with low-speed stirring, temperature and pressure maintained, so as to obtain lithium carbonate crystals with large particle size formed by recrystallization; during the process, a small amount of liquid phase is extruded out through a specially arranged sampling pipe opening at regular intervals of time for rapid detection of the content of sulfate radicals (continuous detection on-line is performed as far as possible), and the content of residual sulfate radicals in the lithium carbonate in the reactor is calculated accordingly; after the content is qualified, a heating valve is closed, the low-speed stirring is maintained, cooling water is slowly and carefully introduced into a jacket to cool the reactor, when the pressure in the reactor is reduced to 0.05-0.06 MPa, the stirring speed is increased until the slurry is under a strong stirring condition, and the slurry is pumped into a hydrocyclone separator with the speed controlled to continuously separate the liquid phase from the solid phase; the separated liquid phase contains slightly water soluble impurities and particulate water insoluble impurities which are adsorbed and coated in the coarse lithium carbonate, so that it cannot be circularly used for the initial operation of the thermal precipitation of the coarse lithium carbonate, and should be sent back to the leaching process, or sent back to the leaching process after the filter cloth and equipment are cleaned; only part of the separated liquid phase which is fully coagulated and subjected to precise filtration is allowed to be mixed into deionized water for the thermal stirring washing of the coarse lithium carbonate-1 to produce industrial-grade products, and prohibited in the subsequent processes. After the solid phase is subjected to the centrifuging-washing, the refined wet lithium carbonate is obtained. For the industrial “new first-grade” and “new zero-grade” lithium carbonates, the content of sulfate radicals should be 0.10% and 0.03%-0.02%, respectively, and for the “new battery-grade” lithium carbonate (subjected to the thermal stirring washing one more time if necessary), the content of sulfate radicals can be expected to reach 0.010%-0.008%-0.005%, or even can reach the limit of the main content of the 4N-grade lithium carbonate under the optimal conditions.

[0100] The “high-efficiency desorption” of the invention can be naturally extended to the following similar technical fields, in addition to being used to substantially reducing the content of impurity sulfate radicals in the lithium carbonate precipitated from the lithium sulfate solution and the sodium (potassium) carbonate solution as exemplified: efficiently removing the impurities that are difficult to remove by conventional washing methods due to chemical adsorption and deep coating in the core of the crystals (or particles) of the insoluble or slightly soluble target product separated out by the precipitation reaction of two or more soluble inorganic substances, and therefore, these are all included in the technical scope of the invention.