New method for reducing sulfate content by direct production of lithium carbonate from lithium sulfate and sodium (potassium) carbonate

Abstract

From the lithium sulfate purification completed liquid-liquid and sodium (potassium) carbonate purification completed liquid heat precipitation process output crude lithium carbonate, to output fine lithium carbonate wet product, there are already removed silicon, aluminum, iron, magnesium, calcium, heavy metal method mostly unchanged, but can be used in the present application 2 "pre-deposition supplementary impurity removal" auxiliary, detection method is not changed; using the present application 1 "reverse feeding, no circulating mother liquor" and 3 "high efficiency desorption", can make industrial grade lithium sulfate to 0.03%, the main content increases to 2.5N, battery grade sulfate decreases to 0.010%-0.008%, the main content is stable to 3N or even touch 3.5N-4N limit position. "High efficiency desorption" by'micro-temperature heat precipitation and hot washing' (105-115 degrees Celsius) and 'high temperature strong desorption' (140-160-170 degrees Celsius) release the deeply wrapped impurities such as sulfate in the core position of peritectic, then by "hydrocyclone separation" liquid phase effectively taken away. Heat precipitation changes to small crystal and moves the heat aging time to 'high temperature strong desorption'.

Description

[0001] This application is a divisional application of Chinese invention patent application No. 201910942746.7, entitled "A Novel Method for Direct Production of Lithium Carbonate from Lithium Sulfate and Sodium (Potassium) Carbonate to Reduce Sulfate Content". The applicant is Dai Ailin. Technical Field

[0002] This invention relates to a method for producing lithium salts. Specifically, this invention relates to a novel method for reducing the sulfate content in lithium carbonate obtained by the thermal precipitation reaction of lithium sulfate solution and sodium (potassium) carbonate solution. More specifically, this invention relates to a method for significantly reducing the sulfate content in various grades of lithium carbonate produced directly from lithium ores such as spodumene, lepidolite, lepidolite, carbonate-type salt lake lithium minerals, phosphogypsum, petalite, and carbonate sedimentary rock lithium minerals via the sulfuric acid process, sulfate process, or other sulfide process (i.e., without conversion through the more expensive lithium hydroxide). Similar fields that can be naturally extended to the content of this invention, particularly item 3 "efficient desorption," are described in paragraph

[0083] ; to avoid excessive complexity, this specification only provides a detailed description using the spodumene-sulfuric acid process as an example. Background Technology

[0003] At the beginning of the last century, industrial-grade lithium carbonate produced in Europe typically contained 0.70-0.80 wt% sulfate, equivalent to 1.035%-1.183% sodium sulfate, with an arithmetic average of 1.109%. This was significantly higher than that of many other water-insoluble and slightly soluble carbonate products.

[0004] In the 1940s and 50s, the former Lithium of America invented the spodumene sulfate process for lithium carbonate and built a plant in Bismarck, North Carolina with an annual output of 9,000 tons. The sulfuric acid content of its industrial-grade lithium carbonate was much lower than that of early European products, reaching 0.35% for Grade 1 and 0.50% for Grade 2. However, it was still considered too high for industries such as mid-to-high-end lithium-containing glass.

[0005] The production of medium- to high-grade lithium-containing glasses, such as microcrystalline glass, requires lithium carbonate with a sulfate content as low as 0.20% (Corning Glass standard). This is then purified using the Truste Method, also known as the carbonation method. This method involves injecting carbon dioxide into a lithium carbonate slurry prepared with 20 times its volume of deionized water, causing the lithium carbonate to acidify (sometimes called hydrogenate) into lithium bicarbonate with a water solubility of 5%. The sodium sulfate impurity is diluted in a large amount of water, and then the lithium bicarbonate is decomposed by heating, removing the carbon dioxide. In this low-sulfate environment, lithium carbonate is redeprecipitated, achieving the goal of reducing the sulfate content to 0.20%. However, this purification process is lengthy, requires significant equipment investment, and greatly increases costs.

[0006] The chemical composition of industrial-grade lithium carbonate as specified in the current Chinese national standard GB / T11075-2013 is shown in Table 1 below: Table 1

[0007] The moisture content of the product should meet the requirements of Table 2 below: Table 2

[0008] The chemical composition of battery-grade lithium carbonate as specified in the current Chinese nonferrous metals industry standard YS / T582-2013 is shown in Table 3 below: Table 3

[0009] As can be seen from the lithium carbonate chemical composition tables specified in GB / T11075-2013 and YS / T582-2013, the sulfate content in industrial grade 0, 1, and 2 (corresponding to Li₂CO₃-0, Li₂CO₃-1, and Li₂CO₃-2 in the above tables) lithium carbonate is 1-2 orders of magnitude higher than that of other impurities such as Fe, Ca, Mg, and Cl. For battery grade lithium carbonate, this difference is even greater, being 2-3 orders of magnitude higher than that of Mg, Ca, Fe, Zn, Cu, Pb, Si, Al, Mn, Ni, and Cl. Clearly, this is because reducing the sulfate content is technically more difficult than reducing the content of other impurities. It should be noted that it took about 40 years to reduce the sulfate content of industrial lithium carbonate produced by the earliest sulfuric acid (salt) process in Europe from 0.7%-0.8% to 0.50%-0.35% for the original American Lithium Company's sulfuric acid process industrial grade lithium carbonate; and more than 100 years to reduce it to 0.20% for ordinary industrial grade and 0.08% for battery grade (which is essentially also a type of industrial grade), which shows how difficult it is.

[0010] Chinese invention patent application CN107915240A (publication date: April 17, 2018) discloses a method for producing battery-grade lithium carbonate using the sulfuric acid process. It employs "cyclic leaching," which effectively increases the concentration of the leached lithium sulfate solution; and "EDTA complexation of calcium and magnesium for lithium precipitation," which effectively reduces the content of calcium and magnesium impurities in the crude lithium carbonate obtained from the thermal precipitation reaction. A remaining problem is that the contents of sulfate and sodium impurities are still very high, at 0.08% and 0.025% respectively, which is detrimental to improving the quality of lithium batteries.

[0011] III. Summary of the Invention Unless otherwise stated, all percentages or proportions mentioned in this application are by mass percentages. This invention uses the spodumene-sulfuric acid process as an example to illustrate its contents. It should be understood that this should not be construed as a limitation on the scope of protection of this invention. All technologies implemented based on the contents of this invention are covered within the scope of protection intended by this application.

[0012] The technical problem to be solved by this invention is: (1) Based on the existing production technology and product standard YS / T582-2013 for directly manufacturing battery-grade lithium carbonate by thermal precipitation process of lithium sulfate purification solution and sodium (potassium) carbonate purification solution, further innovate some processes to significantly reduce the content of impurity sulfate to 0.010%-0.008%, and at the same time slightly reduce the content of other impurities, so that the main content of battery-grade lithium carbonate can stably reach 3N level. Under optimized conditions, some products reach 3.5N level, and some products approach, eventually or even reach 4N level. The inventors of this application believe that the limit of the main content value of lithium carbonate directly produced by thermal precipitation method of lithium sulfate solution and sodium (potassium) carbonate solution may be 4N.

[0013] (2) Based on the existing production technology and product standard GB / T11079-2013 for manufacturing industrial-grade lithium carbonate using thermal precipitation of lithium sulfate purification solution and sodium (potassium) carbonate purification solution, further innovations are made to significantly reduce the sulfate content of impurities to a lower "new zero grade" of 0.03%, while reducing the content of sodium and other impurities, increasing the main content to 99.50%; the sulfate content of "new first grade" is 0.10%, and the main content is increased to 99.35%. Other lower grades of industrial lithium carbonate are no longer considered, because once the three contents of this invention are fully implemented, the sulfate content in the produced industrial lithium carbonate will no longer be in the thousands, but in the tens of thousands.

[0014] The solution to the technical problem of this invention is achieved through its three inventive components: 1. "Reverse feeding, no mother liquor circulation"; 2. "Pre-precipitation supplementation and impurity removal"; 3. "High-efficiency desorption", in two stages to realize the objectives described in paragraphs

[0012] -

[0013] . The following paragraphs

[0015] -

[0069] will provide a more detailed explanation using the example of direct lithium carbonate production via the spodumene-sulfuric acid process: The first and third items of this invention are essential technologies, used from the initial process of obtaining crude lithium carbonate through the thermal precipitation reaction of purified lithium sulfate solution and purified sodium (potassium) carbonate solution, until obtaining various grades of refined lithium carbonate wet products; existing technologies for drying, pulverizing, and packaging refined lithium carbonate remain unchanged; existing technologies for removing impurities such as silicon, iron, aluminum, magnesium, calcium, heavy metals, and magnetic metals before the thermal precipitation process remain essentially unchanged; all detection methods remain unchanged. The second item of this invention is an optional technology, used before the thermal precipitation reaction, mainly for the production of industrial-grade lithium carbonate, and can also be used for battery-grade lithium carbonate when necessary.

[0015] By utilizing the techniques described in items 1 and 2 of this invention, along with existing techniques for removing impurities such as silicon, iron, aluminum, magnesium, calcium, heavy metals, and magnetic metals, various solid lithium ores, such as spodumene, lepidolite, primary lithium carbonate from carbonate-type salt lake ore (produced in the Zabuye Chaka, Longmucuo, and Jieze Chaka salt lakes in Tibet), lepidolite, lithium aluminum phosphate, petalite, and (in the future) carbonate-type sedimentary rock lithium ore from central Yunnan, can be purified into a purified liquid in the form of lithium sulfate. This purified liquid is then subjected to a thermal precipitation reaction with the purified sodium (potassium) carbonate liquid. In the directly produced lithium carbonate of various grades, the sulfate content is reduced to 0.20%-0.15% for industrial grade in the first step. For battery grade lithium carbonate, the thermal precipitation and hot washing operation utilizes part of the technique described in item 3 of this invention, 'micro-temperature thermal precipitation and hot washing' (see paragraphs

[0026] -

[0029] ,

[0039] for detailed explanation), which reduces the sulfate content to 0.08%.

[0016] The second step, by fully utilizing the "high-efficiency desorption" technology described in Section 3 of this invention, further significantly reduces the sulfate content of impurities to 0.03%-0.02% for industrial grade and 0.01%-0.008% for battery grade, as indicated in paragraphs

[0012] -

[0013] . Furthermore, due to the powerful desorption function of "high-efficiency desorption," in addition to further significantly reducing the sulfate content, it can also simultaneously reduce other water-soluble, slightly water-soluble, and water-insoluble impurities that are chemically adsorbed and deeply encapsulated within lithium carbonate particles. This will contribute to increasing the main content, enabling battery-grade and industrial-grade products to achieve the main content targets described in paragraphs

[0012] -

[0013] .

[0017] From this paragraph up to paragraph

[0068] , the contents of these three inventions will be further explained in detail, taking the direct production of lithium carbonate via the spodumene-sulfuric acid process as an example: "Reverse feeding, non-circulating mother liquor" was invented by the inventor of this application between 1978 and 1980 when he was in charge of the direct production of industrial-grade lithium carbonate with 0.22%-0.15% sulfate using the spodumene-sulfuric acid process at a chemical plant in Chengdu (see the attached document to this application). Specifically, it refers to: 1. Starting from the thermal precipitation of crude lithium carbonate from the purified lithium sulfate solution and the purified sodium carbonate solution, the classic spodumene-sulfuric acid process invented by the former Lithium Americas Company is applied, i.e., the attached... Figure 1 As shown, a saturated solution of Na2CO3 as a precipitant is added to a 20% Li2SO4 solution (this method can be called "forward feeding"). In the reverse direction, the purified lithium sulfate solution is added to the sodium carbonate purified solution at a temperature of 90-95 degrees Celsius under vigorous stirring, with appropriate dispersion at the feeding points, to precipitate coarse lithium carbonate particles with less chemical adsorption and less deep encapsulation of sulfate ions.

[0018] 2. Simultaneously, instead of following the classic operating process of the original American Lithium Company, which involved freezing the primary hot mother liquor from the hot centrifugation to 0°C to -15°C to crystallize sodium sulfate and then returning the secondary cold mother liquor to the acidification leaching process to recover lithium, a separate process route was adopted. This secondary cold mother liquor, containing 10-15 g / L of lithium (converted to lithium carbonate), was heated and concentrated until the initial formation of a sodium sulfate crystal film on the liquid surface (the mother liquor retains a slight excess of soda ash, and during the concentration process, crude carbon gradually precipitates out). Lithium carbonate), while hot, is filtered out to remove crude lithium carbonate, which is then returned to the acidification leaching process (or washed to industrial grade 2); the three-stage hot mother liquor from which crude lithium carbonate is filtered out is combined with the new primary hot mother liquor for lithium precipitation and then frozen to precipitate sodium sulfate, and the "cold precipitation of sodium sulfate - hot precipitation of crude lithium carbonate" operation is carried out alternately; or (this part has only been tested), after recovering lithium by precipitating lithium phosphate, lithium fluoride, and organic lithium acids (such as lithium stearate) from the secondary cold mother liquor (or even the primary hot mother liquor), the sodium sulfate is directly recovered by vacuum concentration.

[0019] "Non-circulating mother liquor" means that a large amount of lithium-containing sodium sulfate mother liquor is no longer returned to the leaching process. This minimizes the content of harmful sodium sulfate in the leached lithium sulfate solution system, further reducing the sulfate concentration in the thermally precipitated lithium carbonate reaction solution. This is combined with the beneficial technical effects of "reverse feeding" to reduce the chemical adsorption and deep encapsulation of sulfate (hereinafter referred to as "crystallization"). Furthermore, because the sodium sulfate concentration in the purified lithium sulfate solution is significantly reduced and the salt effect is decreased, the first-pass yield of thermally precipitated crude lithium carbonate is improved.

[0020] The beneficial technical effects of "reverse feeding and non-circulating mother liquor" are achieved with very low equipment and cost investment. Previously, there was no economically effective method in the field to reduce the sulfate content in lithium carbonate produced directly by the spodumene-sulfuric acid process to below 0.35%. Moreover, this invention is an essential supporting and foundational technology for the subsequent invention "efficient desorption," which is another creative improvement that further significantly reduces the sulfate content.

[0021] The second aspect of this invention, "pre-precipitation supplementation for impurity removal," is an optional method and has three uses: 1) If errors are discovered in the earlier leaching and sequential precipitation processes for removing impurities such as aluminum, iron, magnesium, calcium, and heavy metals only before the start of the thermal precipitation process, resulting in incomplete precipitation of some impurities, insufficient coagulation of colloidal particles, or filter cloth damage or improper placement causing filter penetration, and these impurity indicators in the lithium sulfate purification solution exceed the standards, then before the thermal precipitation process, a small amount of sodium carbonate purification solution can be slowly added to the lithium sulfate purification solution using a "forward feeding" method with stirring. Close visual inspection and turbidity meter observation are necessary until a white residue just appears in the lithium sulfate purification solution. When fine precipitates appear, feeding is stopped immediately. The lithium sulfate purified solution discharged from the sampling port and carefully filtered is tested. Once these impurity indicators are within acceptable limits, feeding is stopped (if they are still not within acceptable limits, a small amount of sodium carbonate purified solution is added until they are within acceptable limits). Stirring continues for more than 15 minutes to allow excess aluminum, iron, magnesium, certain heavy metal hydroxides, and calcium carbonate to fully coagulate and precipitate. Then, the solution is pumped out and filtered under micro-vacuum in a suction filter tank. The initial filtrate will definitely be turbid. It is pumped out and circulated for filtration until the filter cake successfully bridges and the filtrate sample is completely clear. At this point, filtrate circulation filtration is stopped. If the filter cake is observed to be fine and slippery (mainly magnesium hydroxide and aluminum) with a small amount of slightly coarse particles (lithium carbonate), it indicates that the aforementioned impurities have been purified to a good degree. The successfully filtered lithium sulfate purified solution is confirmed as the purified solution after further testing. Compared to returning the substandard lithium sulfate purification solution to the leaching process to remove these impurities again, the advantages and cost-effectiveness of this simple error correction and salvage method are obvious, especially for small manufacturers with poor equipment and management conditions.

[0022] After completing the "pre-precipitation replenishment and impurity removal", the production of industrial-grade lithium carbonate, the production of battery-grade lithium carbonate with current technical standards or higher technical standards in the future, must all be carried out by thermal precipitation in the manner of "reverse feeding" (and "non-circulating mother liquor").

[0023] 2) By combining the contents of the first and second items of this invention, and if necessary, the hot precipitation and hot agitation washing operation, supplemented by the 'micro-temperature hot precipitation and hot agitation washing' part of the third item of this invention, along with a number of existing impurity removal technologies, it is possible to achieve the industrial-grade lithium carbonate exceeding the 0.35% threshold and successfully produce products with sulfate content as low as 0.15% (analytical purity standard) or even 0.10% (i.e., the standard of premium lithium carbonate produced by the conversion of lithium hydroxide produced by the spodumene-lime method of Xinjiang lithium salt plant; currently, this is very close to the battery-grade standard).

[0024] 3) In the past, it was often observed that completely clear lithium sulfate purification solutions would flocculate and precipitate iron, aluminum, and magnesium hydroxides during the concentration process. This is consistent with the phenomenon recorded in the literature on the spodumene-sulfuric acid process for lithium carbonate published by the former Lithium Americas Corporation. This indicates that it is necessary to allow these colloidal impurities remaining in the lithium sulfate purification solution due to filtration to fully flocculate, co-precipitate, and remove them multiple times. Especially when producing battery-grade lithium carbonate (including other varieties of high-purity lithium carbonate), if a process of circulating leaching without concentrating lithium sulfate is used, colloidal impurities such as aluminum, iron, magnesium, and certain heavy metal hydroxides may not be sufficiently heated for a long time or the surface charge of the colloidal particles may not be eliminated, thus failing to fully coagulate and filtration. Therefore, a "pre-precipitation supplementary impurity removal" technique can be used to supplement and remove impurities in parallel before thermal precipitation of crude lithium carbonate.

[0025] The third invention, "High-Efficiency Desorption," is a collective term for two technologies: "Powerful Desorption" and "Gyrocyclone Separation." It represents a powerful new technology that further reduces sulfate content to a maximum extent, moving beyond the new threshold of 0.20%-0.15%-0.10% for industrial-grade lithium carbonate and 0.08% for battery-grade lithium carbonate. This achieves the second-stage goal of reducing sulfate content to 0.03%-0.02% for industrial-grade lithium carbonate and 0.01%-0.008% for battery-grade lithium carbonate, while increasing the main content to 99.50% and 3.5N-4N, respectively.

[0026] "Powerful desorption" consists of two parts: "micro-lifting warm precipitation and hot stirring washing" and "medium-high temperature powerful desorption".

[0027] The "micro-temperature precipitation and hot washing" method is also an improvement based on the technical principles described in paragraphs

[0055] -

[0068] regarding the first and third items of this invention: During the entire process of removing peritectic sulfate ions through the hot precipitation, hot washing, and "powerful desorption" system, adsorption and desorption are in dynamic equilibrium. Adsorption is exothermic, and desorption is endothermic. Increasing the temperature shifts the equilibrium towards desorption (Le Chatelier's principle), which is beneficial for desorption. That is, the desorption effect is positively correlated with temperature and is a continuous process. Referring to the data from small experiment 1) in paragraph

[0031] , it can be approximately predicted that even a 10-degree Celsius increase in the hot precipitation and hot washing temperature will significantly increase the desorption of sulfate ions. Furthermore, the curvature of the lithium carbonate solubility-temperature curve is negative; increasing the temperature can improve its yield in a single pass.

[0028] The allowable pressure of most existing jacketed reactors is 0.6 MPa for the jacket and 0.2 MPa for the reactor interior. Therefore, without altering the existing main equipment, by simply changing some operating methods and parameters, thermal precipitation and thermal agitation can be performed to effectively reduce the sulfate content of crude lithium carbonate. This provides a better technical foundation for subsequent 'medium-high temperature strong desorption' operations, increasing the likelihood of producing 4N grade lithium carbonate with sulfate levels far below 0.008%, close to 0.005%-0.003% (50-30 ppm). The results of the small-scale test in paragraph

[0031] can be used to estimate this effect. Furthermore, this process improvement, due to the reduced sulfate content of the crude lithium carbonate entering the reactor, also reduces the consumption of expensive deionized water. Therefore, it is recommended to operate at 104.8 degrees Celsius (0.13 MPa), or 115.2 degrees Celsius (0.18 MPa), with a maximum of 120.2 degrees Celsius (0.20 MPa).

[0029] However, there is no need to excessively increase the temperature. Firstly, the technical effect of reducing sulfate in a high-concentration sulfate environment is still limited, and secondly, there is no need to replace it with a jacketed reactor with a higher allowable stress.

[0030] "High-Temperature Strong Desorption" refers to a key innovative invention where, for the portion of sulfate ions within the pericrystalline core of crude lithium carbonate particles that is difficult to reduce using conventional hot stirring and centrifugation methods, a significant increase in temperature intensifies the thermal motion of various molecules, ions, and atomic groups in the crude lithium carbonate-deionized water slurry system. This loosens the coordination bonds between sulfate ions and lithium ions within the pericrystalline core of the lithium carbonate particles, strongly promoting their detachment from the lithium carbonate particles and their release and dissolution in a large quantity of deionized water. A considerable portion of other water-soluble, slightly soluble, and water-insoluble impurities are also released, dissolved, or suspended in a large quantity of deionized water due to the intensified thermal motion. The underlying adsorption and desorption technology principles are detailed in paragraphs

[0055] -

[0068] ; detailed operational instructions are provided in paragraphs

[0040] -

[0041] .

[0031] The following two easily repeatable small-scale test results, obtained by the inventors of this application through gradually increasing pressure and temperature to achieve "strong desorption," demonstrate that this invention does indeed have a powerful desorption effect on sulfate ions within the pericrystalline structure of lithium carbonate particles: 1) Using a home cooking pressure cooker, add an appropriate amount of tap water. Pour into a stainless steel cup industrial-grade crude lithium carbonate (previously washed once with three times its volume of hot distilled water, reducing sulfate to below 0.40%) produced using a "reverse feeding, non-circulating mother liquor" process. Add another three times the volume of distilled water. Cover the stainless steel cup to prevent tap water contamination. Heat to 0.12 MPa (the maximum pressure of a home cooking pressure cooker, corresponding to a saturated steam pressure of approximately 105 degrees Celsius; at this point, the nozzle will emit a rapid steam jet). Perform static, forceful desorption and thermal aging for 1 hour. After natural cooling and depressurization, remove the liquid phase using a decantation method. Add one volume of distilled water and wash once more. Chemical analysis shows that the sulfate content of the refined lithium carbonate has decreased to below 0.15%, exceeding expectations.

[0032] 2) Then, a simple stainless steel hot press was used to increase the pressure and temperature for testing: First, the product was washed once with 3 times the amount of hot distilled water to reduce the sulfate content to 0.35%. Then, it was strongly desorbed and thermally aged for 1 hour with 6 times the amount of distilled water at a pressure of 0.4-0.6 MPa (corresponding to a saturated steam pressure of about 146-160 degrees Celsius). Then, it was washed once with 1 times the amount of distilled water. The chemical method showed that the sulfate content of industrial-grade refined lithium carbonate was reduced to 0.04-0.035%, which is a sharp drop and is lower than the current standard of 0.08% for battery-grade lithium carbonate sulfate. The beneficial technical effect is very obvious.

[0033] Based on the above pilot-scale test results, it can be predicted that by adding "strong desorption" and "cyclone separation" impurity removal technologies, lithium carbonate manufacturers using the spodumene-sulfuric acid process with existing processes, equipment, and management can further and significantly reduce sulfate content to 0.03%-0.02% for industrial-grade lithium carbonate; for more advanced manufacturers, it can be reduced to 0.008% for battery-grade lithium carbonate. This is because the "strong desorption" technology they rely on is entirely the same.

[0034] "Swirl separation" refers to a simple, low-investment, and easy-to-operate solid-liquid separation technology that can efficiently separate impurities such as sulfate ions dissolved and suspended in a large amount of deionized water after lithium carbonate particles have been removed through "strong desorption". It is superior to various solid-liquid separation technologies using filter cloth because most of the water-insoluble particulate impurities removed and suspended in a large amount of deionized water will be directly carried away by the rotating liquid phase; while using filter cloth to separate this solid-liquid phase will retain more of these water-insoluble particulate impurities, thus negating the beneficial effects of "strong desorption".

[0035] The following example, using the direct production of industrial-grade and battery-grade lithium carbonate via the spodumene-sulfuric acid process, further illustrates the solution to the technical problem of this invention in a systematic and complete manner: (1) For the direct production of industrial grade zero lithium carbonate and battery grade lithium carbonate by the thermal precipitation method using lithium sulfate solution and sodium carbonate solution (after applying the "high-efficiency desorption" technology, the industrial grade secondary and primary lithium carbonate have no value to mention), the existing technologies for removing impurities such as silicon, aluminum, iron, magnesium, calcium, heavy metals, and magnetic metals before the thermal precipitation process remain basically unchanged; the "precipitation supplementary impurity removal" technology of the present invention can also be selected for supplementation; the existing technologies for drying, pulverizing, metering, and packaging the wet fine lithium carbonate remain unchanged; all detection methods remain unchanged. Take Figure 1 as an example: Figure 1 shows a schematic process flow diagram of the traditional spodumene-sulfuric acid method for producing industrial grade lithium carbonate by the former American Lithium Corporation. The so-called "existing technologies for removing impurities such as silicon, aluminum, iron, magnesium, calcium, heavy metals, and magnetic metals before the thermal precipitation process" in this specification refers to the impurity removal technologies included in all processes before obtaining the "20% Li2SO4 solution" in Figure 1 .

[0036] (2) The sodium carbonate formula in the thermal precipitation process is excessive by 5%.

[0037] (3) The purified complete solutions of lithium sulfate and sodium carbonate must be subjected to thermal precipitation and subsequent related operations in the manner of "reverse feeding, non-circulating mother liquor". When necessary, the "precipitation supplementary impurity removal" technology is applied.

[0038] (4) Make a major modification to the operation of the thermal precipitation process, that is, temporarily do not pursue obtaining large-particle-size coarse lithium carbonate particles, and move the thermal aging time to the "strong desorption" process to complete together. This move is to make a technical preparation for the "strong desorption" to effectively release impurities such as sulfate contained in the peritectic formed in the initial stage of the thermal precipitation reaction. For details, please refer to the technical principle explanations for this improvement in paragraphs

[0065] -

[0068] .

[0039] (5) The operation method of thermal precipitation and thermal washing uses the operation method of the'micro-warmed thermal precipitation and thermal washing' technology: after adding the purified sodium carbonate solution to the reaction kettle, raise the temperature, cover the manhole of the reaction kettle, and after driving out the air in the kettle, close the reaction kettle; when the temperature rises to the selected value, start the stirrer and always maintain effective stirring, and the purified complete solution of lithium sulfate pumped in is pumped into the reaction in the form of spray through a pressurized shower nozzle. After the feeding is completed, the reaction kettle immediately starts to release pressure (the steam heat should be recovered through the connecting pipeline), and when the temperature of the material in the kettle drops to 95 °C, immediately discharge the material for centrifugal washing to obtain coarse lithium carbonate 1.

[0040] (6) While still hot, transfer the crude lithium carbonate 1 obtained from the thermal precipitation into a reactor containing a selected ratio of 3-4-5 times for industrial grade and 5-6 times for battery grade deionized water, heated to 95 degrees Celsius, and with the stirrer activated. Cover the reactor with the manhole, continue heating, and after the air inside the reactor is expelled, seal the reactor. Heat the reactor to the same position as the thermal precipitation reaction in section

[0039] , maintain hot stirring for 15 minutes, depressurize the reactor (preferably by connecting a pipeline to recover steam heat), cool it to 95 degrees Celsius, discharge the material, centrifuge and rinse it, so that the sulfate content of the industrial grade and battery grade crude lithium carbonate 2 is controlled to 0.30-0.20% and 0.15%-0.10%, respectively, for later use.

[0041] (7) Pump deionized water into the medium-high temperature strong desorption reactor at a multiple selected by weight of crude lithium carbonate 2, turn on low-speed stirring, add crude lithium carbonate 2, and heat to the selected position such as 159-170 degrees Celsius (saturated vapor pressure 0.6-0.8 MPa). Under the condition of low-speed stirring and maintaining the low-speed movement of the solid phase of the slurry, strong desorption and thermal aging are carried out for more than 1 hour to release the water-soluble impurities, slightly soluble impurities and insoluble impurities mainly composed of sodium sulfate in the pericrystalline core of lithium carbonate particles into the deionized water, and recrystallize into large crystals with extremely low sulfate content under the condition that the sulfate concentration is much lower than the concentration of the thermal precipitation reaction.

[0042] (8) After the sulfate content is detected to be up to standard, depressurize the desorption reactor (it must be connected to a pipeline for recovery and utilization of steam heat). When the pressure drops to 0.05-0.06 MPa, increase the stirring speed until the slurry is kept in a strong stirring state. Control the speed and pump the slurry into the hydrocyclone separator to continuously separate the liquid and solid phases. The liquid phase containing the released water (slightly) soluble and water-insoluble particulate impurities is returned to the leaching process to recover lithium. A portion can be used to clean the filter cloth and equipment. Only a portion that has been fully coagulated and finely filtered is allowed to be used for hot washing of crude lithium carbonate 1 to produce industrial-grade products. This process is prohibited in subsequent steps. Centrifuge and rinse the solid phase (if necessary, re-wash the battery-grade product once more) to obtain industrial-grade lithium carbonate wet product with sulfate content reduced to 0.03-0.02% and battery-grade refined lithium carbonate wet product with sulfate content reduced to below 0.008%.

[0043] (9) For automated and continuous 'medium-high temperature strong desorption' operation using a pipeline desorber, the slurry pressure is reduced to 0.05-0.06 MPa through a pressure-reducing tank with a stirrer and cooling water jacket. The speed is controlled and the slurry is pumped into the hydrocyclone separator for separation.

[0044] The endpoint determination method for "medium-high temperature strong desorption" operation is as follows: Sampling is performed through a specially designed continuous sampling port in the desorption vessel or apparatus. The liquid phase sulfate content is detected intermittently, multiple times, or continuously online. Based on this content data, the amount of crude lithium carbonate added and its sulfate content, and the amount of deionized water added, the programmable computer calculates the residual sulfate content of the solid phase lithium carbonate (dry basis). Once the standard is met, the desorption endpoint can be confirmed.

[0045] Special Note Regarding the Technical Parameters of "Powerful Desorption": The design and control of various operational parameters, such as the initial washing water volume and number of initial washes for crude lithium carbonate 1 in the "micro-extraction, warm precipitation, and hot stirring and washing" stage, the desorption water volume for crude lithium carbonate 2 in the "medium-high temperature powerful desorption" stage, the saturated steam pressure-temperature control index of the desorber, the stirring speed or desorber speed, the desorption and thermal aging time, etc., are normal and completely necessary technical means determined by factors such as the grade of lithium carbonate produced, order quality requirements, raw material composition characteristics, output and cost control, and safe production management. The parameters exemplified in the invention content and specific embodiments of this specification are a whole; they are not static, rigid parameters. In actual application, they can be flexibly adjusted and controlled, and therefore all are covered within the scope of protection intended by this invention.

[0046] For example (but not limited to), the amount of deionized water used: When producing industrial-grade lithium carbonate, the total amount of deionized water used in 'micro-heat precipitation and hot stirring washing', 'medium-high temperature strong desorption', and centrifugal rinsing should be 8-9 times that of the finished lithium carbonate product. This can be allocated according to specific circumstances in a ratio of 2.5:5:0.5 or 1.5×1.5:5.5:0.5. For battery-grade lithium carbonate, the total amount is recommended to be 9-10 times that of the finished product, allocated in a ratio of 2.5:6:0.5 or 1.5×1.5:6.5:0.5. These parameters are all covered within the scope of protection intended by this invention.

[0047] For example (but not limited to), the temperature-saturated vapor pressure inside the desorption vessel: although the desorption effect is positively correlated with temperature and pressure—the higher the pressure, the easier and more abundant the sulfate and other impurities are removed, and the shorter the removal time—the higher the equipment cost and maintenance expenses, and the more complex the enterprise management. Considering factors such as product quality requirements, technical effectiveness, investment amount, production capacity, cost, pressure vessel safety management, and the existing equipment status of various manufacturers, it is recommended that 0.5-0.6 MPa be used for the production of industrial grade 0 lithium carbonate, with no need to exceed it. Only for automated, continuous production using pipeline-type desorbers can exceed 0.8 MPa, although not limited, it seems unnecessary to exceed 1.0 MPa. For battery-grade lithium carbonate, it is recommended to use 0.7-0.8-1.0 MPa, also with no need to exceed it. However, for automated, continuous production using pipeline-type desorbers, it can exceed 1.0-1.2 MPa, although not limited, it seems unnecessary to exceed the current low-pressure / medium-pressure vessel limit of 1.6 MPa. These parameters are also covered within the scope of protection of this invention.

[0048] Therefore, in paragraphs

[0028] ,

[0031] -

[0032] ,

[0039] -

[0043] ,

[0046] ,

[0047] , and

[0051] -

[0052] , all the appropriately adjustable technical parameters related to the "highly efficient desorption" technology should be included within the scope of protection of this invention.

[0049] The liquid phase, consisting mainly of sodium sulfate, water-soluble impurities, slightly water-soluble impurities, other colloidal impurities, and other particulate water-insoluble impurities that have been separated from crude lithium carbonate particles during the desorption process, is separated by a hydrocyclone separator. This is a better solid-liquid separation equipment selection for large-scale industrial, automated, and continuous production of "medium- and high-temperature strong desorption". If a liquid-solid phase separation method with filter cloth (such as centrifuge separation) is used, many suspended particulate water-insoluble impurities will be mixed into the solid phase, which will greatly reduce the originally excellent impurity removal effect of "medium- and high-temperature strong desorption".

[0050] The desorption reactors and apparatus used for "medium-high temperature high-intensity desorption" can be configured in the following ways: 1) a pressure reactor with a low-speed stirring and heating / cooling jacket, or a standard equipment; or 2) a low-speed spherical or horizontal cylindrical desorber, or a standard equipment or a custom design; or 3) for manufacturers with large production capacity, a pipeline desorber is most suitable, or a custom design; 4) regardless of the type of desorption reactor or apparatus chosen, indirect heating and cooling methods must be used, and direct steam heating is not allowed to avoid contaminating the slurry.

[0051] For the internal surface structure of the desorption vessel and apparatus used in "medium-high temperature high-intensity desorption," titanium plates are preferred, but stainless steel plates of 0Cr18Ni9Ti or 0Cr18Mo2Ti are also acceptable. However, for battery-grade products, if stainless steel is desired, there is a strict restriction that the magnetic metal chromium content of the product must be less than or equal to 3 ppm. Therefore, a small pressure vessel with a pressure resistance of 1.6 MPa must be used. A long-term (recommended 100 hours or more) immersion test with lithium carbonate slurry in the vessel under a saturated vapor pressure of 0.8-1.0-1.2 MPa is required to test the chromium leaching amount. If the chromium content of lithium carbonate increases by 1 ppm after the immersion test compared to before immersion, then that batch of material cannot be used and another must be selected. Additionally, lithium sulfate obtained from fluorine (chlorine) raw materials such as fluorolithium mica must also undergo this immersion test. However, the purpose of the test is to determine the corrosiveness of fluorine (chlorine) to these two materials. If corrosion is observed, a composite steel plate lined with polytetrafluoroethylene should be used as the structural material.

[0052] Schemes using glass-lined inner walls for desorbers require prior material testing to determine the leaching levels of elements such as boron, aluminum, silicon, lead, and antimony in the glass lining under alkaline lithium carbonate slurry conditions, prolonged exposure (ideally over 100 hours), high temperature (saturated vapor pressure 0.8-1.0-1.2 MPa), and low-speed stirring. If any of the aforementioned elements, along with other alkali-soluble elements with limitations on battery-grade lithium carbonate impurities, leach and cause the material to fail to meet standards, the glass-lined inner wall material formulation should be rejected, and an alternative should be selected. Fluorine (chlorine)-containing raw materials, such as lithium sulfate obtained from fluorolithium mica, are not suitable for use in glass-lined inner wall reactors or equipment.

[0053] "Hydrocyclone separation" uses standardized equipment or a self-designed hydrocyclone separator, and the material selection method is the same as that used in the desorption vessel and apparatus used in the aforementioned "medium-high temperature strong desorption" for the inner surface structure of the material in contact with the material.

[0054] The overall beneficial effects of this invention are as follows: It significantly reduces the content of sulfate and other impurities in industrial-grade and battery-grade lithium carbonate produced directly from lithium sulfate solution and sodium (potassium) carbonate solution extracted from various lithium ores and sulfur-containing raw materials as described in paragraph

[0002] , while increasing the main content of these two types of lithium carbonate. This will greatly reduce and blur the original huge gaps in quality, cost, and selling price between industrial-grade and battery-grade lithium carbonate produced directly by the sulfuric acid method, sulfate method, and sulfur compound method, and high-purity lithium carbonate produced by various methods. It will also simplify future revisions to lithium carbonate technical standards. These beneficial effects are highly conducive to promoting the rapid development of high-end lithium industries such as lithium batteries, and extremely beneficial to the long-term parallel development of ore lithium salts and salt lake lithium salts.

[0055] The technical principles underlying both "reverse feeding, no mother liquor circulation" and "high-efficiency desorption" are analyzed from this section to section

[0068] as follows: Paragraphs

[0003] -

[0010] have already pointed out the long-standing problem of excessively high levels of sulfate impurities in lithium carbonate produced directly from lithium ore via the sulfuric acid and sulfate processes. The root cause is likely the structural characteristics of lithium ions, which readily form coordinate bonds with silicon, carbon, and sulfur-containing oxyacid anions. This facilitates the chemical adsorption of sulfate ions during the thermal precipitation of crude lithium carbonate, leading to encapsulation (peritecrystal formation). Especially in the early stages of thermal precipitation, the adsorbed sulfate ions grow with the crude lithium carbonate particles and may even become deeply encapsulated, making them extremely difficult to remove using existing thermal washing methods, posing the greatest risk. While alkali metals and alkaline earth metals are not as polar as transition elements, they can all act as central atoms to form complexes with coordinating atoms. Lithium atoms, in particular, have the smallest radius among all metals, which is conducive to forming complexes with a slightly higher stability constant. Sulfate ions have two coordinating oxygen atoms, which also facilitates the formation of complexes with lithium ions in lithium carbonate with a slightly higher stability constant, resulting in higher coordinate bond energies and stronger chemical adsorption (the same applies to carbonate and silicate ions).

[0056] According to Langmuir's theory of adsorption phenomena on solid surfaces in physical chemistry, at relatively high temperatures of 90-95 degrees Celsius, the physical adsorption force based on van der Waals forces is very weak during the precipitation and washing of crude lithium carbonate particles, while the desorption tendency is relatively large. Since sulfate ions have two coordinating oxygen atoms that can serve as coordination sites for complexes, the probability of forming sulfate complexes with slightly larger stability constants is high during the precipitation of crude lithium carbonate when the sulfate concentration is high. The adsorption of sulfate ions on the surface of crude lithium carbonate particles is mainly chemisorption, with lithium ions as the adsorbent and sulfate ions as the adsorbate. Other characteristics of chemisorption are: a) High selectivity. During thermal precipitation, lithium carbonate particles strongly adsorb both sulfate and carbonate ions. The probability and quantity of adsorption depend mainly on the concentration of the adsorbate, as the Fryndrich adsorption equation shows that the amount of adsorption increases with the increase of adsorbate concentration. b) Only monolayer adsorption occurs. This is because chemisorption is accomplished by the residual bonding forces of molecules on the surface of solid molecules, forming new chemical bonds with the adsorbate. Therefore, once the surface is saturated with adsorption, it will no longer adsorb adsorbates with the same charge to form a second adsorption layer. Furthermore, adsorption is exothermic and difficult to reverse, meaning desorption is difficult and requires endothermic absorption. This chemisorption also promotes the encapsulation of sulfate ions during crystal growth. Once sulfate ions are adsorbed onto lithium carbonate particles, they are difficult to desorb, and lithium carbonate molecules coordinated with the sulfate ion will be adsorbed onto the outside, forming an encapsulation of sulfate ions, i.e., peritectic crystals. This makes it difficult for conventional washing methods to desorb and remove sulfate ions from lithium carbonate particles, resulting in a high sulfate content, which is unavoidable.

[0057] From the perspectives of both adsorbate and adsorbent, production practice has shown that the former has a greater impact on the content of sulfate impurities.

[0058] Based on the above theoretical analysis, to reduce the content of sulfate ions, the most important thing is to minimize the concentration of sulfate ions in the adsorbate of the thermal precipitation reaction system. Secondly, it is necessary to adopt a "slow, hot, aged" operation method to obtain large-particle-size coarse lithium carbonate particles, thereby reducing sulfate ions in the peritectic (this is the theoretical basis for the first-stage technical measures for reducing sulfate ions mentioned above in this specification). In the second stage, it is necessary to find a relatively simple, low-cost, and powerful desorption technology to release the sulfate ions in the peritectic that are difficult to remove by existing hot stirring and centrifugation methods.

[0059] Based on the above understanding, the inventors of this application proposed a "reverse feeding, non-circulating mother liquor" technical solution. "Reverse feeding" is based on the principle that chemical adsorption simultaneously possesses selective adsorption, monolayer adsorption, and difficulty in desorption: In the initial feeding stage, the newly formed lithium carbonate microparticles are in an environment with high concentrations of carbonate and low concentrations of sulfate. Therefore, the probability of carbonate adsorbing on their surface is high, while the probability of sulfate adsorbing is low, with only a few areas adsorbing sulfate (and silicate). Due to the characteristics of monolayer adsorption, after the surface of the lithium carbonate particles is saturated with carbonate, it no longer adsorbs electronegative sulfate and carbonate. Because the adsorbed carbonate is not easily desorbed in reverse, it will quickly adsorb free electronegative lithium ions (followed by sodium ions), cross-adsorbing carbonate and lithium ions. The lithium carbonate particles can then rapidly grow in a lower sulfate concentration environment, significantly reducing the number of sulfate adsorbed compared to the "forward feeding" process.

[0060] The precipitated lithium carbonate particles adsorb a layer of carbonate ions, some of which adsorb sodium ions to form sodium carbonate. This doesn't cause major problems: firstly, these carbonate ions will chemically adsorb and react with lithium ions dissociated from the continuously added lithium sulfate, forming lithium carbonate with much lower solubility and stronger bonds. This causes the lithium carbonate particles to grow larger, while the sodium ions displaced and exchanged by the lithium ions added later in the thermal precipitation system will be adsorbed by sulfate ions in the reaction solution and transferred to the reaction solution. Secondly, sodium carbonate and lithium carbonate do not form a double salt, making them relatively easy to wash off during subsequent hot water stirring and washing. Of course, a small amount of sodium ions and sulfate ions will come close together to form sodium sulfate, which will be coated by the later-adsorbed lithium carbonate and difficult to wash off. Sometimes, it is found that the amount of sodium in the lithium carbonate product is slightly less than the equivalent amount of sulfate ions, indicating the presence of trace amounts of other metal elements, such as calcium sulfate, which are coated and even more difficult to wash off.

[0061] The "reverse feeding" process utilizes a high concentration of adsorbate carbonate ions to preemptively complex lithium ions in nascent lithium carbonate particles, preventing sulfate ions from extensively complexing and encapsulating the adsorbent lithium ions in the lithium carbonate particles, thus successfully reducing the sulfate content in the product. With "reverse feeding," crude lithium carbonate only needs to be added to deionized water at a ratio of 1:2-3 (mass ratio) and subjected to hot stirring and centrifugation three times to obtain a product with a sulfate content of 0.15%-0.20%. Each reaction yields 30 kg of refined lithium carbonate, requiring only 5 kg more soda ash than the original process. Part of this soda ash is mixed into the primary sodium sulfate hot mother liquor (partially added to the wash water), and a portion is automatically consumed during the subsequent concentration of the secondary cold mother liquor for "thermal precipitation of crude lithium carbonate," making it economically worthwhile.

[0062] The reason for the "slow, stirring, heating, and aging" operation in the thermal precipitation of crude lithium carbonate is to obtain large-diameter lithium carbonate particles to reduce the adsorption and encapsulation of sulfate ions. The underlying theories are: 1. Langmuir's theory: the smaller the surface area of ​​the adsorbent, i.e., the larger the particle size, the less adsorption is achieved; 2. Kelvin's formula: aging allows small crystals to automatically transform into large crystals (the system's free energy decreases and tends to stabilize). During this transformation, under stirring and heating conditions, some of the adsorbed and encapsulated sulfate and sodium ions can be released into the reaction solution. However, in the early stages of the reaction, the sulfate adsorbed by the nascent lithium carbonate particles is already deeply encapsulated. In the later stages of the reaction, the sulfate concentration in the reaction solution is already very high. Under the dynamic reversible state of adsorption-desorption, the amount of adsorbed and encapsulated sulfate ions in the lithium carbonate particles is still too high, requiring new technological breakthroughs to solve this problem; 3. Le Chatelier's principle: increasing the temperature is beneficial for desorption.

[0063] In the initial stage of thermal precipitation of lithium carbonate, especially when the feed is rapid and the stirring is weak, the precipitate often exhibits strong viscosity. The reasons are as follows: a) The concentrations of the four main ions in the lithium sulfate and soda ash solutions used in the thermal precipitation reaction are very high, resulting in a strong reaction tendency. The lithium ions in the nascent lithium carbonate readily coordinate with carbonate, sulfate, and silicate ions to form complex salts. A layer of acid ions surrounds the lithium ions, then another layer of lithium ions surrounds the acid ions, and so on, quickly forming a clump together. These lithium ions also adhere to the inner wall of the silicate-lined reaction vessel or the stirrer (as well as laboratory glassware and equipment). This is a rapid entropy increase process with a strong driving force. However, as time progresses, the clumps loosen and disintegrate due to the continuous adjustment of various internal chemical bonds. Lithium carbonate particles within the clumps continuously precipitate and automatically grow into large crystals, while sulfate ions continuously combine with sodium ions and dissolve in the hot water. Only a few or very few clumps remain adhered to the vessel wall or stirrer. b. If the desilication of lithium sulfate and soda ash solution is ineffective, lithium silicate will be formed during thermal precipitation. Lithium silicate is highly viscous, increasing the self-adhesive force of lithium carbonate particles and causing them to clump together after drying. Taking high-modulus (4-5 modulus, up to 8-9 modulus) liquid lithium silicate in concrete sealant as an example, it is very strong after application, drying, and curing, and is no longer susceptible to damage from long-term water immersion. This is because liquid lithium silicate has a unique characteristic: once dehydrated, it never re-dissolves in water, unlike water glass, i.e., sodium silicate.

[0064] In the early stages of thermal precipitation, lithium carbonate clumps, which is very harmful. Therefore, both lithium sulfate and soda ash purification solutions must undergo strict and effective desiliconization beforehand; they should also be fed in a spray-like manner at an appropriate speed through a reasonably arranged pressurized sprinkler inlet under strong and effective stirring; the inner wall of the stainless steel reaction vessel and the surface of the agitator should be smooth, free of scratches and spot welding slag to prevent lithium carbonate particles from sticking together.

[0065] The above technical principles also clarify that, using the "high-efficiency desorption" content of item 3 of this invention to further reduce the sulfate and sodium content in lithium carbonate still requires continued reliance on these adsorption-desorption technical principles. The technical principle of appropriately increasing the feeding speed of the lithium sulfate purification solution and postponing the thermal aging time in section

[0038] of the thermal precipitation process to obtain small-particle-size crude lithium carbonate is further explained: In the liquid-solid system, providing the conditions of "dilute, slow, stirring, hot, and aged" to obtain large-particle-size crystals aims to reduce the total surface area of ​​the crystals and reduce the adsorption of other harmful impurities on their surface. This universal technical principle is also applicable to crude lithium carbonate obtained from the thermal precipitation reaction of lithium sulfate solution and sodium carbonate solution. "Dilute" reduces the primary yield of lithium carbonate, which is too detrimental to production costs and should be omitted; "slow, stirring, hot, and aged" has been adopted in the first stage of reducing the sulfate content, i.e., reducing it to 0.20%-0.15%-0.10% for industrial grade and 0.08% for battery grade, and is effective and without error. However, to further significantly reduce sulfate content and achieve the second-stage goal of 0.03%-0.02% for industrial grade, 0.008% for battery grade, and even to produce 4N grade products, it is necessary to change the "slow" approach to "moderately faster" and shift the "Chen" operation position to obtain small-diameter crystals.

[0066] In the high-concentration environment of a thermal precipitation system, the sulfate and sodium ions, among other impurities, formed within the peritectic during the initial stage of thermal precipitation are located at the core of the large crystals and are extremely difficult to release by thermal agitation or even "strong desorption" (thermal agitation is essentially a form of desorption, only much weaker than "strong desorption"). It is precisely these sulfate ions that constitute the 0.08% content required for battery-grade specifications. This paper lays the technical groundwork for obtaining small-diameter crystals through thermal precipitation, which will allow us to solve this technical challenge in the subsequent "medium-high temperature strong desorption" process.

[0067] Reducing the crystal size, that is, minimizing the distance between the core and the outer surface of the crystal to a "shallow" level, allows the remaining sulfate ions and other impurities within the peritectic to be easily released during the "medium-high temperature strong desorption" process. Most of the small crystals can be significantly and completely broken down during this process, which is also known as thermal aging. They then recrystallize into larger crystals in a low-concentration impurity environment, maximizing the release of sulfate ions and other impurities. This technology improves the reliability of "medium-high temperature strong desorption" in significantly reducing sulfate ions, shortens pressure operation time, reduces pressure vessel volume, and lowers equipment investment. The seemingly contradictory approach of requiring large-diameter crystals in the first stage and small-diameter crystals in the second stage is simply due to the different sulfate ion occurrence states, necessitating different technical measures. The underlying principle remains the same despite the different circumstances.

[0068] It does not matter that the number of small-diameter lithium carbonate particles increases significantly or the total surface area increases significantly. The outer surface layer of small-diameter lithium carbonate particles temporarily adsorbs more sodium carbonate and impurities such as sulfate ions, because they are "buried" shallowly and are easily released during the initial hot washing and centrifugation of crude lithium carbonate and the "strong desorption" process. Attached Figure Description

[0069] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present invention, and are not intended to limit the present invention.

[0070] Figure 1 This is a schematic diagram of the spodumene-sulfuric acid process of the former Lithium Americas Corporation. Figure 2 It is the washing curve of sulfate ions in the trial production product according to the traditional process of the former American Lithium Company; Figure 3 These are the solubility data for lithium phosphate, lithium fluoride, and lithium carbonate in water; Figure 4 This is the curve showing the decrease in sulfate content of lithium carbonate after implementing the three technical components included in this application.

[0071] Appendix Figure 1-4 Detailed explanation is as follows: Appendix Figure 1 This is a schematic diagram of the traditional spodumene-sulfuric acid process for producing lithium carbonate, formerly employed by Lithium Americas. (Attached) Figure 1 From Ostroshko et al., Chemistry and Technology of Lithium, published by China Industry Press, first edition, Beijing, May 1965, page 160.

[0072] Appendix Figure 2 This application describes the washing curve of sulfate ions in the trial production of lithium carbonate using the spodumene-sulfuric acid process, which was overseen by the inventor in the early stages of small-scale production between 1978 and 1979-1980, following the "forward feeding" process of the former American Lithium Corporation. This curve clearly demonstrates that the biggest drawback of this traditional process is the high sulfate ion content. The washing conditions were: crude Li₂CO₃: distilled water = 1:1.5, temperature 90-95°C, stirring time 30 minutes, and centrifugation at 1300 rpm using an SS-800 tripod centrifuge.

[0073] Appendix Figure 3 The solubility data of lithium phosphate, lithium fluoride, and lithium carbonate in water show a huge difference of one order of magnitude, indicating that the lithium recovery rate is the highest when recovering lithium-containing sodium sulfate mother liquor by means of lithium phosphate.

[0074] Appendix Figure 4After implementing the "reverse feeding, non-circulating mother liquor" and "pre-precipitation supplementation and impurity removal" technologies of this invention, the sulfate reduction curves of industrial-grade and battery-grade crude lithium carbonate produced by thermal precipitation, after further impurity removal by "high-efficiency desorption," both show a precipitous drop. In the figure, letter A represents the "micro-temperature thermal precipitation and thermal stirring washing" stage, and letter B represents the "high-efficiency desorption" stage; the horizontal line represents the transfer of crude lithium carbonate 2 to the high-efficiency desorption reactor; the left curve represents industrial grade, and the right curve represents battery grade. Detailed Implementation

[0075] The following will further illustrate the present invention with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above-described content of the present invention are covered within the scope of protection intended by the present invention.

[0076] 1. Taking the direct production of lithium carbonate via the spodumene-sulfuric acid process as an example: When implementing the technical solution of this application, the existing impurity removal methods before the "micro-lifting warm precipitation and hot stirring washing" process remain unchanged in principle, but the "pre-precipitation supplementary impurity removal" technology of the second item of this invention can be selected; the "micro-lifting warm precipitation and hot stirring washing" process must adopt the "reverse feeding, non-circulating mother liquor" technology of the first item of this invention; the existing technologies for drying, crushing, metering, and packaging of refined lithium carbonate wet products remain unchanged; all detection methods remain unchanged; the endpoint judgment method of "strong desorption" in paragraph

[0044] only involves sampling measures and the calculation method of sulfate content in solid phase lithium carbonate, and does not involve changes in sulfate detection methods.

[0077] 2. When it is confirmed that the "pre-precipitation supplementary impurity removal" technology is needed, add lithium sulfate purification solution to the hot precipitation reactor, start stirring, and spray a small amount of sodium carbonate purification solution from the pressurized sprinkler nozzle at a medium speed in a mist. Once the liquid becomes turbid and white fine particles precipitate (when the iron content is high, it will have a yellow or red light), stop adding the material and continue stirring for a few minutes. Take a sample and filter it precisely, and test its iron, aluminum, magnesium, calcium and heavy metal content. If it has not yet met the standard, spray a small amount of sodium carbonate again and test again until it meets the standard.

[0078] 3. Begin filtering the qualified lithium sulfate purified solution. The initial filtrate is temporarily placed into a small turbid liquid tank (total volume is about 20% of the lithium sulfate purified solution volume) for circulation and filtration. The filtrate sample is tested again and if it meets the standard, it is considered that the filter cake bridging is successful and the purified solution is confirmed. Then, filtration continues (along with the lithium sulfate purified solution in the turbid liquid tank). Then, according to the process description of "reverse feeding, no circulation of mother liquor" in paragraphs

[0017] -

[0018] , and the method of "micro-lifting hot precipitation and hot stirring washing" technology described in paragraphs

[0031] -

[0032] ,

[0039] -

[0043] and

[0046] , crude lithium carbonate 1 is produced.

[0079] 4. For producers who do not need to use the "pre-precipitation supplementation and impurity removal" technology and directly follow the "reverse feeding, non-circulating mother liquor" process, the thermal precipitation is carried out in accordance with the technology described in paragraph

[0046] 'micro-lifting temperature thermal precipitation and thermal stirring and washing' to produce crude lithium carbonate 1. Because it is necessary to obtain small-diameter crude lithium carbonate particles, the feeding speed is doubled compared to before the technology of this invention was adopted.

[0080] 5. After feeding, depressurize to 95 degrees Celsius and immediately start centrifugation. Rinse with 90-95 degrees Celsius deionized water (the water volume, centrifuge speed, and centrifugation time need to be well controlled) so that the sulfate content of the discharged crude lithium carbonate is: 0.50%-0.40% for industrial grade and 0.30%-0.20% for battery grade. The primary sodium sulfate hot mother liquor is recycled to recover crude lithium carbonate or lithium phosphate and other lithium salts according to the method in the "non-circulating mother liquor" process instructions, and sodium sulfate or sodium sulfate is also recovered.

[0081] 6. Immediately transfer the crude lithium carbonate 1 while it is still hot to a reactor containing 3 times (industrial grade) or 4 times (battery grade) its weight of deionized water at a temperature of 90-95 degrees Celsius, with stirring already started. Cover the manhole, expel the air, and then seal the reactor. Continue to heat to the selected position, such as 115.2 degrees Celsius, and stir for 15 minutes. Depressurize, and after the temperature returns to 95 degrees Celsius, start discharging and centrifuging. The sulfate content of the initial washed crude lithium carbonate 2 should be controlled at 0.30%-0.20% for industrial grade and 0.15%-0.10% for battery grade.

[0082] 7. While still hot, transfer the initially washed crude lithium carbonate (2g) into a high-intensity desorption reactor that has been pumped with 3-4-5 times its weight (industrial grade) or 5-6 times its weight (battery grade) of deionized water (18 MΩ·cm purity, self-made, and preheated to 90-95°C for battery grade production) and has been started with low-speed stirring. Continue to raise the temperature to expel the air from the reactor, and then completely seal the reactor. For industrial grade production, raise the temperature to 144-159°C (saturated vapor pressure inside the reactor 0.4-0.6 MPa), and for battery grade production, raise the temperature to 165-170-180°C (saturated vapor pressure inside the reactor 0.7-0.8-1.0 MPa). Maintain low-speed stirring and temperature and pressure, and continue this 'medium-high temperature high-intensity desorption' and thermal aging process for more than 1 hour. During this period, a small amount of sample should be collected periodically through a specially designed sampling port. A large amount of liquid phase is rapidly tested for sulfate content (continuous online testing is preferred), and the residual sulfate content of lithium carbonate in the reactor is calculated accordingly. Once the target is met, the heating valve is closed, low-speed stirring is maintained, and cooling water is introduced to lower the temperature. When the pressure in the reactor has dropped to 0.05-0.06 MPa, the stirring speed is increased until the slurry is kept under strong stirring. The speed is controlled, and the slurry is pumped into a hydrocyclone separator to continuously separate the liquid and solid phases. The separated liquid phase contains water-slightly soluble impurities and particulate water-insoluble impurities adsorbed and encapsulated in the crude lithium carbonate. It cannot be recycled for use at the beginning of the thermal precipitation crude lithium carbonate process. It is returned to the leaching process or used to clean the filter cloth and equipment. Only a portion of the separated liquid, after sufficient coagulation of impurities and precise filtration, is allowed to be used in the deionized water of the thermally stirred crude lithium carbonate 1. After solid-phase centrifugation and rinsing, refined lithium carbonate wet product is obtained. The sulfate content of industrial grade should be 0.03%-0.02%, and the sulfate content of battery grade (after washing once more if necessary) is expected to be 0.010%-0.008%-0.005%, which reaches the limit value of 4N grade main content.

[0083] The third aspect of this invention, in addition to its application in the precipitation of lithium carbonate from lithium sulfate and sodium (potassium) carbonate solutions to significantly reduce the sulfate content of impurities, is also applicable to the following similar technical fields: the target product precipitated from two or more soluble inorganic substances through a precipitation reaction, where the core of the crystal (or particle) is chemically adsorbed and deeply encapsulated, making it difficult to remove by conventional washing methods. This product can be released in large quantities into a strongly desorbed liquid by moderately increasing the temperature and intensifying the thermal motion of its molecules, ions, and atomic groups, and then purified through hydrocyclone separation. Therefore, all of these are covered within the scope of this invention.

Claims

1. A method for reducing the sulfate content in the direct production of lithium carbonate from lithium sulfate and sodium (potassium) carbonate, characterized in that: Based on the classic spodumene-sulfuric acid process invented by the former American Lithium Corporation, as shown in Figure 1, improvements are made to it. The improvements are as follows: from the process of obtaining crude lithium carbonate by thermal precipitation reaction of lithium sulfate purified liquid and sodium (potassium) carbonate purified liquid to the process of obtaining refined lithium carbonate wet product, the following technologies are adopted: 1 "reverse feeding, no mother liquor circulation", 2 optional "pre-precipitation supplementation and impurity removal" and 3 "high-efficiency desorption". The "reverse feeding, non-circulating mother liquor" technology is characterized by the following: the traditional method of adding the original sodium carbonate purification solution from the hot precipitation process to the lithium sulfate purification solution is reversed, and the lithium sulfate purification solution is added to the soda ash purification solution; the primary hot mother liquor obtained by centrifugation and rinsing of crude lithium carbonate 1 is processed in the following three ways, without returning to the acidification leaching process, but instead: ① after cooling to 0-15 degrees Celsius for crystallization and centrifuging to remove sodium sulfate, the secondary cold mother liquor is concentrated until the sodium sulfate crystal film initially forms, and the crude lithium carbonate 1 that has precipitated again is filtered out while hot, and the tertiary hot mother liquor is combined with the crystallized sodium sulfate... and the "cold precipitation of sodium sulfate, hot precipitation of crude lithium carbonate" operation is carried out alternately, or ② after recovering lithium by precipitating lithium phosphate, lithium fluoride or lithium stearate from the secondary cold mother liquor, the sodium sulfate is recovered by multi-effect vacuum concentration, or ③ after recovering lithium phosphate, lithium fluoride or lithium stearate from the primary hot mother liquor, the sodium sulfate is directly recovered by multi-effect vacuum continuous concentration, and the mother liquor from centrifugation of sodium sulfate is combined with the primary hot mother liquor to recover its lithium; The "high-efficiency desorption" technology is characterized by its combination of "strong desorption" and "cyclone separation" technologies. "Strong desorption" is further comprised of two parts: "micro-heating and stirring" and "medium-high temperature strong desorption." "Micro-heating and stirring" refers to hot stirring of the jacketed reactor at a saturated vapor pressure of 0.13-0.20 MPa and a corresponding feed temperature of 105-120 degrees Celsius, with the allowable pressures in the jacket and reactor exceeding 0.6 MPa and 0.2 MPa respectively. Wash with 3 times its weight of deionized water; "Medium-high temperature strong desorption" refers to: the crude lithium carbonate obtained by "micro-extraction and warm stirring washing" is mixed with 3-6 times its weight of deionized water, placed in a pressure reactor, and desorbed and thermally aged for more than 1 hour under the conditions of saturated vapor pressure of 0.5-1.2 MPa, corresponding temperature of 152-188 degrees Celsius, and low-speed stirring; "Swirl separation" refers to: most of the sulfate ions adsorbed on the lithium carbonate particles obtained by "medium-high temperature strong desorption" are directly carried away by the rotating liquid phase; The "pre-precipitation supplementation and impurity removal" technology is characterized by the following steps: Lithium sulfate purification solution is added to the hot precipitation reactor, stirring is started, and at approximately 90 degrees Celsius, under close observation or online turbidity meter monitoring, a small amount of sodium carbonate purification solution is slowly added in a spray form from the nozzle of a pressurized shower head. Once the solution becomes turbid and white fine particles precipitate, the addition is stopped, and stirring continues for about 15 minutes. After precise filtration, the iron, aluminum, magnesium, calcium, and heavy metal contents are tested. If the content is not yet up to standard, a small amount of sodium carbonate purification solution is sprayed in again, and the test is repeated until the standard is met. The lithium sulfate purification solution that has met the standard is then filtered. The initial filtrate is temporarily placed in a small turbidity tank with a total volume of approximately 20% of the lithium sulfate purification solution volume for circulation and filtration. This process continues until the filtrate sample is tested again and meets the standard, at which point it is confirmed as the purified solution. The lithium sulfate purification solution in the turbidity tank is then filtered further to obtain the purified lithium sulfate solution.

2. The method according to claim 1, wherein the "pre-precipitation supplementary impurity removal" technology is applied in the following three situations: 1) If it is discovered late, before the start of the thermal precipitation process, that there were errors in the previous leaching operation or the sequential precipitation method for removing aluminum, iron, magnesium, calcium, and heavy metals, resulting in insufficient coagulation and incomplete precipitation of colloidal particles formed by aluminum, iron, magnesium, and certain heavy metal hydroxides, or if the filter cloth is damaged or improperly placed, causing filter penetration, or if other impurity removal accidents occur, or if these impurity indicators in the lithium sulfate purification solution are detected to exceed the standard, then the "pre-precipitation supplementary impurity removal" technology can be adopted. 1) Highly efficient rescue; 2) For the production of industrial grade 0 lithium carbonate, only the first and second technologies are used, and the "micro-temperature heating precipitation and hot stirring washing" part of the third technology is added when necessary; 3) When producing battery grade lithium carbonate or other varieties of high-purity lithium carbonate, if a process of cyclic leaching without concentration of lithium sulfate is used, it is possible that colloidal impurities of aluminum, iron, magnesium, and certain heavy metal hydroxides may not be heated for a long time or the surface charge of colloidal particles may not be eliminated, and thus they may not be fully coagulated and co-precipitated, resulting in filter leakage. This can also be used before the formal hot precipitation operation to rescue the impurities.

3. The method according to claim 1, wherein the lithium-containing raw material for producing lithium sulfate is selected from: spodumene, lepidolite, primary lithium carbonate from carbonate-type salt lake lithium mines, lepidolite, phosphogypsum, petalite, or lithium battery waste.

4. The method according to claim 1, wherein during the thermal precipitation of crude lithium carbonate, the amount of sodium (potassium) carbonate in the feed is 5 mol more than the theoretical amount.

5. The method according to claim 1, wherein the "thermal precipitation" technology is characterized by: adding sodium carbonate to the thermal precipitation jacketed reactor to complete the purification solution, opening the jacket to raise the temperature, covering the reactor manhole, and after the air inside the reactor is expelled, the reactor is completely sealed; the temperature rises to 105-120 degrees Celsius, the stirrer is started and stirring is maintained continuously, and the lithium sulfate purification solution is pumped in the form of spray through multi-point arranged pressurized shower nozzles to carry out the thermal precipitation reaction; after the feeding is completed, the reactor immediately begins to depressurize and cool down, and when the temperature inside the reactor drops to 95 degrees Celsius, the material is immediately discharged for centrifugal washing to obtain crude lithium carbonate 1.

6. The method according to claim 5, wherein the "micro-heating and hot washing" technique is characterized by: transferring the crude lithium carbonate 1 obtained by hot precipitation into a hot washing reactor containing 3-5 times the amount of industrial grade and 5-6 times the amount of battery grade deionized water, heating it to 95 degrees Celsius, and starting the stirrer; covering the manhole; continuing to heat the reactor; after the air inside the reactor is exhausted, sealing the reactor completely; heating it to 105-120 degrees Celsius; maintaining hot washing for 15 minutes; depressurizing and cooling the reactor to 95 degrees Celsius; and then discharging the material for centrifugal washing to obtain crude lithium carbonate 2.

7. The method according to claim 6, wherein the 'medium-high temperature strong desorption' technology is characterized by: pumping deionized water in a selected multiple of the weight of crude lithium carbonate 2 into the medium-high temperature desorption reactor, opening the jacket to raise the temperature and starting low-speed stirring, adding crude lithium carbonate 2; raising the temperature to 152-188 degrees Celsius, maintaining low-speed stirring, keeping the slurry solid phase in a low-speed movement state, desorption and thermal aging for more than 1 hour, wherein the amount of deionized water used is as follows: for industrial grade lithium carbonate, 3-5 times the weight of crude lithium carbonate 2; for battery grade lithium carbonate, 5-6 times the weight of crude lithium carbonate 2.

8. The method according to claim 1, wherein the "cyclone separation" technology is characterized by: after the residual sulfate content of lithium carbonate in the desorption vessel is detected to meet the standard during the "medium-high temperature strong desorption" operation, the heating valve is closed, low-speed stirring is maintained, pressure is released and cooling water is introduced to cool down. When the pressure in the vessel drops to 0.05-0.06 MPa, the stirring speed is increased until the slurry is kept in a strong stirring state, and the slurry is pumped out into the cyclone separator to continuously separate the liquid and solid phases; the separated liquid phase is not recycled for the operation at the beginning of the thermal precipitation crude lithium carbonate process, but is returned to the leaching process for use or for cleaning filter cloth and equipment; only a portion of the separated liquid that has been fully coagulated and finely filtered is allowed to be used in the deionized water of the hot-stirred industrial grade crude lithium carbonate 1, but it is prohibited in subsequent processes; after the solid phase is centrifuged and washed, refined lithium carbonate wet product is obtained, with industrial grade sulfate content of 0.03%-0.02% and battery grade sulfate content as low as 0.010%-0.008%, and reaching the limit value of the main content 4N grade.

9. The method according to claim 1, wherein the "medium-high temperature strong desorption" technology and the "cyclone separation" technology are carried out automatically and continuously using a pipeline desorber.

10. The method according to claim 9, wherein the "medium-high temperature strong desorption" technology is characterized by: the main desorption device being a vertical pressure reactor with a stirring jacket, a low-speed spherical or cylindrical horizontal pressure reactor, or a pipeline reactor; all are indirect heating and cooling; the part in contact with the feed liquid is made of titanium, 0Cr18Ni9Ti stainless steel, 0Cr18Mo2Ti stainless steel, or glass enamel, but when using crude lithium carbonate produced from lithium ore containing halogen fluorine and chlorine, a polytetrafluoroethylene lining is used; if 0Cr18Ni9Ti stainless steel or 0Cr18Mo2Ti stainless steel is selected, because battery-grade products have a strict limit of magnetic metal chromium content less than or equal to 3ppm, a small pressure vessel with a pressure resistance of 1.6 MPa is first used to allow the carbon... The chromium leaching amount is tested by immersion in lithium carbonate slurry for more than 100 hours under saturated vapor pressure of 0.8-1.0-1.2 MPa in the reactor. If the chromium content of lithium carbonate increases by 1 ppm after immersion, the batch of material is rejected. Designers of inner wall enamelware must conduct a pre-concentration test to test the leaching amount of elements such as boron, aluminum, silicon, lead, and antimony in the enamelware under alkaline lithium carbonate slurry, for more than 100 hours, saturated vapor pressure of 0.8-1.2 MPa, and low-speed stirring conditions. If any of the aforementioned elements or other elements that are soluble in alkali and have limited impurity indicators for battery-grade lithium carbonate leaching are leached and cause the material to fail the test, the formulation of the inner wall enamelware material should be rejected. The material used for the part of the "cyclone separator" that comes into contact with the liquid is the same as that used in the desorption reactor and apparatus mentioned above.

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