Method for greatly reducing content of sulfate radicals in lithium carbonate of each level in spodumene sulfuric acid method

Abstract

The invention relates to a method for greatly reducing the content of sulfate radicals in all levels of lithium carbonate in a spodumene sulfuric acid method, which is characterized by comprising the following steps of: based on various impurity removal methods of the existing industrial-grade and battery-grade products, adopting a reverse charging and non-circulating mother liquor process; before formal operation of thermal precipitation, a measure of'pre-precipitation supplementation and impurity removal 'is adopted; during thermal precipitation operation, aging is not executed temporarily to pursuit large-grain coarse lithium carbonate crystals; the crude lithium carbonate is subjected to hot stirring washing and centrifugation with 3 times of deionized water once and then is subjected to powerful desorption treatment, most of sodium sulfate and other impurities are released, and fine lithium carbonate is obtained; drying and crushing; industrial-grade and battery-grade sulfate radicals are respectively reduced to 0.03% and 0.008% to the minimum, and the main contents are respectively increased to 99.5%, 99.95% and even 99.990%.

Description

[0001] This application claims the priority of a prior application with the patent application number 201810900977.7 and the invention title "Technology for Greatly Reducing the Sulfate Radical Content in Lithium Carbonate at All Levels by the Sulfuric Acid Method of Spodumene", which was filed with the State Intellectual Property Office of China on August 9, 2018. The full text of the prior application is incorporated herein by reference. Technical Field

[0002] The present invention relates to a method for producing lithium salts. In particular, the present invention relates to a method for greatly reducing the sulfate radical content in lithium carbonate at all levels by the sulfuric acid method of spodumene. Background Art

[0003] The demand for lithium batteries has begun to grow rapidly. It is very likely that due to the large-scale production of electric vehicles, there will be an explosive demand growth for high-purity lithium salts, especially battery-grade lithium carbonate.

[0004] Lithium carbonate is mainly extracted from spodumene, lepidolite and salt lake brine. The lithium resource reserves in China rank second in the world. The spodumene in Ganzi and Aba, Sichuan, the lepidolite in Yichun, Jiangxi, the lithium in the East and West Taijinar and Yiliping salt lakes in Qinghai, and the lithium in the Zabuye Qaka salt lake in Tibet all have huge reserves.

[0005] From the perspective of the resource composition, it is more appropriate to produce lithium chloride from the lithium brine in the salt lakes in Qinghai, China, and then electrolyze it into metallic lithium. However, due to the high similarity of the atomic structures of magnesium and lithium in high-magnesium brine, it is difficult to separate them, resulting in great technical difficulties in the development of lithium chloride. In addition, the natural conditions in the mining areas are very poor, and the progress has not been fast enough for many years. The lithium content in the two solid lithium ores, spodumene and lepidolite, is already very high compared to that in salt lake lithium. The lithium sulfate solution leached by the sulfuric acid method of spodumene and the sulfate method of lepidolite can now reach a saturated state without concentration, and it is more convenient to produce industrial-grade and battery-grade lithium carbonate. At present, in China, extracting lithium from ores still has technical advantages over extracting lithium from brine, and it is also relatively fast to expand production capacity.

[0006] Since the large-scale production technology for manufacturing industrial-grade and battery-grade lithium carbonate (2N-3N grade, 99.5%-99.9%) from lithium sulfate and soda ash in China is already relatively mature (please refer to Tianqi Lithium Industry Co., Ltd., Sichuan Province, Chinese invention patent CN107915240A, April 17, 2018, A Method for Producing Battery-Grade Lithium Carbonate by the Sulfuric Acid Method), compared with other enterprises in the world that produce lithium carbonate from lithium sulfate and soda ash, it already has quite advanced nature. Therefore, lithium carbonate produced by the sulfuric acid method of spodumene is still the preferred process for most current investors.

[0007] However, the major issue with the patented battery-grade lithium carbonate technical standard, YS / T582-2013, is the relatively high sulfate and sodium impurity contents, at 0.08% and 0.025%, respectively, which are 1-2 orders of magnitude higher than other impurities and make it stand out from the crowd. This standard is merely acceptable to the battery industry due to the current high price of 4N-grade high-purity lithium carbonate. If a relatively simple new technology solution could be developed at a relatively low cost to significantly reduce the sulfate content in the lithium ore-lithium sulfate-battery-grade lithium carbonate by another order of magnitude, allowing the updated battery-grade lithium carbonate to easily and stably surpass 3N grade, approaching or even reaching 4N, it would be a major technological breakthrough. This would undoubtedly benefit lithium batteries in terms of quality, lifespan, and reliability, potentially spurring the rapid development of new energy vehicles.

[0008] Regarding industrial lithium carbonate, certain industries, such as the mid- to high-end lithium glass manufacturing industry, are particularly troubled by the presence of sulfate as an impurity. This is because the high viscosity of hot glass melt makes it difficult for the tiny sulfur oxide bubbles produced by sulfate decomposition to float up and be removed completely, significantly impacting both glass quality and cost. The current industrial standard GB / T11075-2013 specifies a zero-grade sulfate content of 0.20%. While this matches the standard set by Corning Glass in the United States, a relatively simple new technical solution to reduce this content by an order of magnitude to 0.02% at a relatively low cost, creating a new zero-grade standard, would be a significant technological breakthrough and potentially enhance my country's competitiveness in exporting this product.

[0009] Here is a brief review of the production history of industrial-grade lithium carbonate in the world. It can be seen that the impurity sulfate ion in lithium carbonate produced by the spodumene sulfate process has always been too high. There are indeed special reasons and "excusable" circumstances:

[0010] At first, the industrial-grade lithium carbonate produced in Europe generally had an impurity sulfate content of 0.70-0.80%, which is equivalent to 1.035%-1.183% sodium sulfate, with an arithmetic average of 1.109%. This seems a bit exceptional compared to many inorganic salt products.

[0011] In the 1950s, the former Lithium of America Company in the United States developed the spodumene sulfate process for producing lithium carbonate. The impurity sulfate content in its industrial-grade lithium carbonate standards was lower than that of early European products, with first-grade products at 0.35% and second-grade products at 0.50%, which was still relatively high. Figure 2It is the sulfate radical decline curve during the hot washing process with deionized water of the crude lithium carbonate first thermally precipitated by the former American Lithium Corporation process in 1978 at the factory where the inventors of this application once worked. It clearly shows that when the sulfate radical rapidly drops to 0.50%, it is extremely difficult to further drop significantly, which verifies that it is inevitable and appropriate to determine the sulfate radicals of industrial grade I and II lithium carbonate by this process to be 0.50% and 0.35% respectively.

[0012] When certain industries such as mid - to - high - grade lithium glass require low - sulfate - radical products (0.20%, namely the so - called Corning Glass Works standard), then the Truste Method, that is, the carbonization method, is used for purification. In this method, carbon dioxide is pressed into the lithium carbonate water slurry, making lithium carbonate acidified (some call it hydrogenated) into lithium bicarbonate with a solubility of 5% in water, diluting sodium sulfate impurities in a large amount of water, and then heating and decomposing lithium bicarbonate to expel carbon dioxide and re - precipitate lithium carbonate to achieve the goal of reducing the sulfate radical to 0.20%. Although this purification process has a long flow, huge equipment investment, and much higher costs, it is still the classic process for producing Corning - specification industrial - grade, 3N, 4N, and 5N - grade high - purity lithium carbonate using lithium carbonate, including using lithium hydroxide.

[0013] In the opinion of the inventors of this application, the key points of Tianqi Lithium Industry's invention patent CN107915240A are "circular leaching", "EDTA complexing calcium and magnesium for lithium precipitation", and the set of optimal operating parameters for each process starting from the leaching process, which are statistically integrated during long - term research, development, and production. As described in Paragraph

[0006] , the inventors of this application believe that the above - mentioned patent technology for lithium spodumene sulfuric acid method lithium carbonate of Tianqi Lithium Industry can be further improved with the goal of a sulfate radical content of 0.008% and a main content of 4N grade. Of course, this requires the enrichment of new breakthrough technologies. Summary of the Invention

[0014] After relatively detailed theoretical analysis and several experiments, the inventors of this application have found an innovative measure that does not require adding a large amount of equipment but only requires modifying part of the process flow, which can achieve the goal of reducing the sulfate radical in industrial - grade lithium carbonate by the lithium spodumene sulfuric acid method to 0.20% with low investment and low cost. This innovative measure is to significantly change the classic process of the former American Lithium Corporation from the process of thermally precipitating crude lithium carbonate from the lithium sulfate purification liquid and the soda ash purification liquid (see Figure 1 , the process of thermally precipitating crude lithium carbonate from the lithium sulfate purification liquid and the soda ash purification liquid corresponds to Figure 1 in which "20% Li2SO4 solution" and "saturated solution of Na2CO3" are mixed to "precipitate Li2CO3") to a "reverse feeding, non - recycling mother liquor" process.

[0015] As a result, our factory achieved an average sulfate content of 0.22% across multiple batches, with a minimum of 0.15%. This was the initial phase of process innovation, with trial production using a small-scale, semi-mechanical, semi-manual process, using rudimentary equipment and site conditions, limited staff quality, and limited management expertise. We were confident that, once improvements were made to equipment, site, and production management, we could reduce sulfate content to 0.10% using this new process, matching the premium lithium carbonate produced by the Xinjiang Lithium Salt Plant using the lithium hydroxide carbonization process. Our factory processed a specialized lithium carbonate for a certain electronic tube factory, and after blending 10 tons of product, the yield reached 0.25%, resulting in a total lithium yield of approximately 80%. We successfully delivered the following year, earning a processing fee of 10,000 yuan per ton, generating a profit.

[0016] The "reverse feeding, non-circulating mother liquor" process means: 1. Starting from the time when the purified lithium sulfate saturated liquid and soda ash solution are thermally precipitated with lithium carbonate, the soda ash solution of the original American Lithium Company's classic operating process is added to the lithium sulfate solution (which can be called "forward feeding"). The above-mentioned "forward feeding" procedure is reversed, and the lithium sulfate solution is gradually added to the strongly stirred soda ash solution at a temperature of 90-95 degrees Celsius in a slow and dispersed manner to precipitate crude lithium carbonate.

[0017] 2. The crude lithium carbonate was separated using the SS-800 three-column centrifuge commonly used by small businesses at the time. After the primary hot sodium sulfate mother liquor was frozen to below 0 degrees Celsius to crystallize out the mirabilite, the classic American Lithium Company process of returning the secondary cold mother liquor to the acidification material leaching process to recover lithium was no longer followed. Instead, a different process path was used. The secondary cold mother liquor containing up to 15-18 grams per liter of lithium carbonate was heated and concentrated until a sodium sulfate crystal film began to form on the liquid surface (during the concentration process, a slight excess of soda ash remained in the mother liquor, and crude lithium carbonate was gradually precipitated out by heat). The crude lithium carbonate was filtered out while hot and returned to the acidification material leaching process or, after accumulating a certain amount, was separately washed and purified into an industrial secondary product. The tertiary hot mother liquor from which the crude lithium carbonate was filtered out was merged while hot into the new primary hot mother liquor for lithium precipitation and frozen to precipitate the mirabilite, and the "cold precipitation of mirabilite-hot precipitation of crude lithium carbonate" operation was carried out alternately.

[0018] In fact, if our factory had been able to improve its laboratory and workshop facilities, train its personnel, and continue production, we could have achieved a sulfate content of 0.10% and a main content of over 99.0% using this "reverse feeding, no mother liquor recirculation" process innovation, supplemented by the "pre-precipitation and supplemental impurity removal" measures described below. The processes and operating parameters of the former American Lithium Company's ore calcination, acidification, leaching, and impurity removal processes were largely advanced and feasible.

[0019] The inventors of this application noticed the importance of the full flocculation, coprecipitation, and multiple removals of colloidal impurities during the operation of removing impurities such as silicon, aluminum, iron, magnesium, and calcium by the precipitation method. The experience obtained is that before the formal operation of hot precipitation of crude lithium carbonate, the measure of "precipitation supplement for impurity removal" is taken, and the effect is very good. The method is as follows: First, in the "forward feeding" mode, slowly add a small amount of soda solution to the purified lithium sulfate solution. When white fine precipitates can just be observed, stop feeding and continue stirring for more than about a quarter of an hour. Then pump it into a suction filtration barrel for micro-vacuum filtration. Initially, a large amount of precipitates will surely pass through the filter. The filtrate is pumped out for circulating filtration until the filter cake forms a bridge successfully and the filtrate sample is observed to be completely clear, and then the filtrate circulation is stopped, and it is only then confirmed as the purified finished solution. The filter cake is white, delicate, and slippery to the touch, and mixed with a small amount of slightly coarser particles (lithium carbonate), indicating that impurities such as silicon, aluminum, magnesium, and calcium have been purified well. After testing, as long as the quality of the soda is also good and the dissolution and purification operations are correct, the formal hot precipitation operation can be started.

[0020] With the matching of the two innovative processes of "reverse feeding and non-circulating mother liquor" and "precipitation supplement for impurity removal", after the operation of "precipitation supplement for impurity removal" is completed, to produce industrial grade II lithium carbonate, the hot precipitation operation can be continued in the "forward feeding" mode; if producing grade I or grade 0, operate in the "reverse feeding" mode.

[0021] In addition, the harm of passing through the filter is very great. Whether it is cylindrical suction filtration, pressure filtration, leaf filtration, or plate and frame pressure filtration, it is necessary to wait until the filter residue forms a bridge on the filter cloth before intercepting the formal filtrate. All the previous ones are returned for circulating filtration, otherwise the impurity removal often falls short. From the start of filtering the saturated lithium sulfate solution and soda solution to the end of centrifuging the refined lithium carbonate, synthetic fiber filter cloth is used instead of cotton fiber filter cloth to prevent cotton fiber from falling off and mixing into the finished product.

[0022] The inventors of this application successfully recovered lithium phosphate with relatively low solubility in water from the secondary cold mother liquor (see attachment Figure 3 ), in order to make the process of recovering sodium sulfate from this mother liquor simpler. However, because there was no market demand for lithium phosphate at that time, the actual production was all carried out according to the "cold crystallization of mirabilite - hot precipitation of crude lithium carbonate" scheme to recover lithium and sodium.

[0023] Now that there is a market demand for a variety of lithium products, it is completely possible to recover lithium phosphate from the secondary cold mother liquor after "hot precipitation of lithium and cold crystallization of nitrate" using phosphoric acid and sodium dihydrogen phosphate; recover lithium fluoride using hydrofluoric acid and sodium bifluoride; recover lithium stearate using sodium stearate, and then recover anhydrous sodium sulfate by the vacuum multi-effect evaporation continuous method. If recovering lithium phosphate (for lithium batteries), because its solubility in water is very small, the lithium loss carried away by anhydrous sodium sulfate is the lowest, and it is still a good choice. Attachment Figure 3 shows the solubility data of lithium phosphate, lithium fluoride, and lithium carbonate in water, showing a huge difference with each successive order of magnitude being larger, indicating that this scheme for recovering lithium and sodium is better.

[0024] A better recycling solution is as follows: In the area south of Chengdu, Sichuan Province, from Xinjin, Meishan, Pengshan to Hongya and Danling in Ya'an City, there is a concentrated production area of spodumene sulfate process lithium products, which also happens to be a concentrated area of huge amounts of glauberite and concentrated area of sodium sulfate manufacturers. The mother liquor of mirabilite after recycling lithium phosphate (due to the large salt effect, the content of metallic lithium is still 50 ppm or more. If there is ion exchange or membrane separation recycling technology, it is advisable to recycle it again) is supplied nearby to sodium sulfate manufacturers for use as the leaching solution for extracting glauberite, and a "win-win" result can be obtained: The mirabilite manufacturers can significantly reduce costs; the lithium salt manufacturers can avoid the huge investment in equipment and workshops for vacuum triple-effect concentration and reduce the energy consumption cost.

[0025] However, whether recycling by itself or supplying it to other factories for recycling sodium sulfate, when the lithium content in the mother liquor is enriched to a certain concentration, it is advisable to recycle the lithium again.

[0026] Analysis of the technical principles based on the process innovation of "reverse feeding and non-recycling mother liquor": 1. Among the impurity indicators of industrial-grade lithium carbonate national standard GB / T 11075-2013 and battery-grade lithium carbonate industry standard YS / T 582-2013, the contents of sulfate and sodium are almost 1-2 orders of magnitude higher than those of other impurities, indicating that they are difficult to remove.

[0027] 2. The essential reason should be related to the structural characteristics of lithium ions, which are prone to forming coordination bonds with oxygen-containing acid radicals containing silicon, carbon, and sulfur, that is, it is greatly related to the chemical adsorption of low sulfate and the formation of inclusion crystals during the thermal precipitation of crude lithium carbonate. Especially in the initial stage of thermal precipitation, the adsorbed sulfate will be deeply wrapped as the particles of crude lithium carbonate grow, causing the greatest harm. Although alkali metals and alkaline earth metal elements do not have as strong polarizability as transition elements, they can all act as central atoms to form coordination compounds (complexes) with coordinating atoms. Sulfate has two coordinating oxygen atoms and is prone to forming a coordination compound with a relatively large stability constant with lithium ions in lithium carbonate, and the same is true for carbonate and silicate.

[0028] 3. In that case, it is necessary to analyze in a bit more detail the theory of adsorption and desorption on the surface of solid substances: According to the Langmuir theory of solid surface adsorption in physical chemistry, when precipitating and washing crude lithium carbonate particles at a relatively high temperature of 90 - 95 °C, the physical adsorption force based on van der Waals forces is very weak, while the desorption tendency is relatively large. Since there are two coordinated oxygen atoms in sulfate that can serve as ligand sites for complexes, when precipitating crude lithium carbonate, in the case of a high sulfate concentration, it is very likely to form a sulfate complex with a very large stability constant. The adsorption of sulfate on the surface of crude lithium carbonate particles is mainly chemical adsorption, with lithium ions as the adsorbent and sulfate as the adsorbate. Several other characteristics of chemical adsorption are as follows: a. It has a very high selectivity. During the thermal precipitation reaction, both the adsorption of sulfate and carbonate by lithium carbonate particles is very strong. Which one has a greater chance of being adsorbed and a larger amount adsorbed mainly depends on the concentration of the adsorbate because the Freundlich adsorption formula shows that the adsorption amount increases with the increase in the concentration of the adsorbate. b. Only monolayer adsorption occurs. This is because chemical adsorption is achieved by the residual bond force of the surface layer molecules of the solid to form new chemical bonds with the adsorbate. Therefore, after the surface is saturated with adsorption, it will no longer adsorb adsorbates with the same charge to form a second adsorption layer. c. Heat is released during adsorption and it is not easy to reverse, that is, desorption is very difficult and requires heat absorption. This kind of chemical adsorption also promotes the encapsulation of sulfate during the crystal growth process. Because once sulfate is adsorbed onto lithium carbonate particles and it is not easy to desorb, then lithium carbonate molecules coordinated with this sulfate will be adsorbed on the outside, forming an encapsulation of sulfate, that is, forming a peritectic. This makes it difficult to desorb and remove sulfate in lithium carbonate particles by conventional washing methods, and it is inevitable that its content will be on the high side.

[0029] 4. From the perspectives of these two factors, namely the adsorbate and the adsorbent, production practice has proved that the former has a greater impact on the content of impurity sulfate.

[0030] 5. Based on the above theoretical analysis, to reduce the content of impurity sulfate, the most important thing is to minimize the concentration of the adsorbate sulfate during the thermal precipitation reaction. Secondly, adopt the operation method of "slow, hot, and aging" to obtain large-sized crude lithium carbonate particles, so as to reduce the chemically adsorbed and encapsulated sulfate. The last measure is to seek a relatively simple, low-input, low-cost but powerful desorption new technology to release the sulfate that has been chemically adsorbed and deeply encapsulated and is difficult to remove by existing hot washing methods.

[0031] Naturally, the concept of "reverse feeding without recycling mother liquor" emerged. "Reverse feeding" is based on the principles of chemisorption, which includes selective adsorption, monolayer adsorption, and difficult desorption: At the initial stage of feeding, the newly formed tiny lithium carbonate particles are in an environment with a high concentration of carbonate ions and a low concentration of sulfate ions. Therefore, the probability of carbonate ions being adsorbed on their surface is high, while the probability of sulfate ions being adsorbed is low. Only a few sites adsorb sulfate ions (and silicate ions); due to the characteristics of monolayer adsorption, after the surface of lithium carbonate particles is saturated with adsorbed carbonate ions, they no longer adsorb electronegative sulfate and carbonate ions. Because the adsorbed carbonate ions are not easily desorbed reversely, they will quickly adsorb free electropositive lithium ions (followed by sodium ions), cross-adsorbing carbonate ions and lithium ions. As a result, lithium carbonate particles can grow rapidly in an environment with a low concentration of sulfate ions, and the amount of sulfate ions adsorbed is much less than that in the "forward feeding" process.

[0032] A part of the layer of carbonate ions adsorbed by the precipitated lithium carbonate particles will adsorb sodium ions and become sodium carbonate molecules, which does not cause major problems: First, these sodium carbonate molecules will undergo chemisorption and then chemical reactions with the lithium ions dissociated from the continuously added lithium sulfate, precipitating lithium carbonate with a much lower solubility than sodium carbonate, making the lithium carbonate particles larger. The precipitated sodium ions will be adsorbed by sulfate ions in the reaction solution and transferred to the solution; Second, sodium carbonate and lithium carbonate do not form double salts, and it is relatively easy to wash them off during the subsequent hot water stirring and washing process. Of course, a small amount of sodium ions will form sodium sulfate when approaching sulfate ions and be wrapped by the subsequently adsorbed lithium carbonate, making it difficult to wash off. The amount is slightly less than that of equivalent sulfate ions (because there are also trace amounts of other metal element sulfates wrapped), becoming the second largest impurity content.

[0033] The "reverse feeding" process borrows the high-concentration adsorbate carbonate ions in this way to preferentially complex the lithium ions in the newly formed lithium carbonate particles, preventing a large amount of sulfate ions from complexing with the adsorbed lithium ions in the lithium carbonate particles and being wrapped, successfully reducing the sulfate ion content in the product. After adopting "reverse feeding", the crude lithium carbonate only needs to be added with deionized water in a ratio of 1:2 - 3 and washed thermally 3 times to obtain a product with a sulfate ion content of 0.15% - 0.20%. Each time after a reaction, 30 kg of high-quality lithium carbonate is obtained, only 5 kg more soda ash is added than the original process, and part of it is mixed into the primary sodium sulfate hot mother liquor (part into the washing water). Then, when concentrating the secondary cold mother liquor to "thermally precipitate crude lithium carbonate", part of it will be automatically consumed, which is economically worthwhile.

[0034] The principles of the "slow, stir, heat, age" operation in the process of hot precipitation of crude lithium carbonate are introduced as follows. As we all know, large lithium carbonate particles can be obtained through the above procedures, reducing the adsorption and encapsulation of sulfate radicals. The theories on which it is based are: 1. Langmuir theory, the smaller the surface of the adsorbent, that is, the larger the particle size, the less the adsorption capacity; 2. Kelvin formula, aging can cause small crystals to automatically transform into large crystals (the system free energy decreases and tends to be stable). During this transformation process, under the conditions of stirring and heating, some of the adsorbed and encapsulated sulfate radicals and sodium ions can be released into the reaction solution; however, in the early stage of the reaction, the sulfate radicals adsorbed by the primary lithium carbonate particles have been deeply encapsulated, and in the later stage of the reaction, the sulfate radical concentration in the reaction solution is already very high. In the dynamic reversible state of adsorption-desorption, the amount of sulfate radicals adsorbed and encapsulated in the lithium carbonate particles is still on the high side, and new technologies are still needed to break through and solve this problem; 3. Le Chatelier's principle, increasing the temperature is conducive to desorption.

[0035] In the initial stage of hot precipitation of lithium carbonate, especially when the feeding is relatively fast and the stirring is ineffective, the hot precipitate often has a very strong viscosity. The reasons are as follows: a. The four main ion concentrations of the two hot feed solutions of lithium sulfate and soda ash are all very high, and the reaction tendency is strong. The lithium ions of the primary lithium carbonate are easily coordinated with carbonate radicals, sulfate radicals, and silicate radicals to form complex salts. One layer of acid radicals outside the lithium ions, one layer of lithium ions outside the acid radicals, and another layer of acid radicals... quickly stick to each other to form a mass; these lithium ions will also stick to the inner wall of the glass-lined reaction tank or the stirrer composed of silicate. This is a rapid entropy increase process with a large driving force. However, as time goes by, the bonding mass relaxes and disintegrates due to the continuous adjustment of various chemical bonds inside, and the lithium carbonate particles inside the bonding mass are continuously precipitated and automatically grow into large crystals, and the sulfate radicals continuously combine with sodium ions and dissolve in hot water, and only a few bonding masses continue to stick to the wall or the stirrer (there are also those that do not stick). b. If the desilication of the lithium sulfate and soda ash solutions is ineffective, lithium silicate will be generated during hot precipitation. It has a very strong viscosity and will increase the self-adhesion force of lithium carbonate particles and is prone to caking after drying. The high-modulus (4-5 modulus, up to 8-9 modulus) liquid lithium silicate of the concrete sealer is very firm after construction drying and curing and is no longer afraid of long-term water immersion. This is because liquid lithium silicate has a characteristic that once dehydrated, it will never dissolve in water again, which is very different from water glass, that is, sodium silicate.

[0036] The operation of "non-circulating mother liquor" further reduces the sulfate radical concentration in the reaction solution of hot precipitation of lithium carbonate, making the beneficial effects of "reverse feeding" in reducing the adsorption and encapsulation of sulfate radicals superimposed; also, due to the reduction of the sodium sulfate effect, the primary yield of crude lithium carbonate is slightly increased, which is also a very important supporting innovation measure.

[0037] The above technical principles are also to clarify that to further reduce the sulfate radical and sodium content in lithium carbonate, these adsorption-desorption technical principles can still be continued to be relied on.

[0038] The technical problems to be solved by the present invention are as follows: 1. On the basis of the existing production technology for manufacturing industrial-grade lithium carbonate by the thermal precipitation process of lithium sulfate purification liquid and soda ash or potassium carbonate purification liquid and the product standard GB / T 11079-2013, some processes are innovated to greatly reduce the content of impurity sulfate radical to "new zero grade" of 0.03%, together with the reduced values of impurities such as sodium and chloride radicals, so that the main content increases to 99.50%; for "new first grade", the sulfate radical is 0.10% and the main content increases to 99.35%.

[0039] 2. On the basis of the existing production technology for manufacturing battery-grade lithium carbonate by the thermal precipitation process of lithium sulfate purification liquid and soda ash or potassium carbonate purification liquid and the product standard YS / T 582-2013, some processes are innovated to greatly reduce the content of impurity sulfate radical to 0.010%-0.008%, together with the simultaneous reduction of the contents of impurities such as sodium and chloride radicals, so that the main content of battery-grade lithium carbonate stably reaches 3N grade, and some products are strived to reach 3.5N and approach 4N grade. The inventor of the present application believes that the limit of the main content value of lithium carbonate produced by the thermal precipitation method of lithium sulfate solution and soda ash or potassium carbonate solution may be 4N.

[0040] The solutions of the present invention to solve its technical problems are as follows: 1. For the production of industrial first-grade and zero-grade lithium carbonate and battery-grade lithium carbonate by the thermal precipitation method of lithium sulfate and soda ash, the methods for removing impurities such as silicon, aluminum, iron, magnesium, calcium, heavy metals and magnetic metals before the thermal precipitation process are basically unchanged; if necessary, the "precipitation supplementary impurity removal" means of the present invention can be selected for supplementation.

[0041] 2. The soda ash formula is 105% by equivalent or slightly more.

[0042] 3. The purified liquid of lithium sulfate and soda ash is subjected to thermal precipitation and subsequent related operations in the manner of "reverse feeding without recycling mother liquor". The so-called "reverse feeding without recycling mother liquor" process means: 1) From the start of thermal precipitation of lithium carbonate from the purified saturated lithium sulfate solution and soda ash solution, reverse the original classic operation process of American Lithium Corporation of adding the soda ash solution to the lithium sulfate solution (which can be called "forward feeding"), that is, add the purified lithium sulfate completed liquid to the strongly stirred soda ash purified completed liquid at a temperature of 90-95 °C at an appropriate speed and in a dispersed manner to precipitate crude lithium carbonate; 2) Separate the crude lithium carbonate by a centrifuge. After freezing the primary hot mother liquor of sodium sulfate to below 0 °C to crystallize mirabilite, instead of following the original classic operation process of American Lithium Corporation (returning the secondary cold mother liquor to the acid leaching process of the feedstock to recover lithium), another process path is taken. Heat and concentrate this secondary cold mother liquor with a lithium carbonate content as high as 15-18 g / L until a sodium sulfate crystal film begins to form on the liquid surface (during the concentration process, a slightly excessive amount of soda ash is retained in the mother liquor, and crude lithium carbonate is gradually precipitated thermally). Filter out the crude lithium carbonate while it is hot, return it to the acid leaching process of the feedstock or accumulate it to a certain quantity and then separately wash and purify it into industrial grade II; The tertiary hot mother liquor after filtering out the crude lithium carbonate is merged into the new primary hot mother liquor for lithium precipitation while it is hot, and the operations of "cold crystallization of mirabilite - thermal precipitation of crude lithium carbonate" are carried out alternately.

[0043] 4. Make a major modification to the operation of the thermal precipitation process, that is, temporarily do not pursue obtaining large-particle-size crude lithium carbonate crystals, and complete the desorption steps described in paragraphs

[0059] and

[0060] after thermal aging. This is to reduce the deep encapsulation of impurities such as sulfate ions and sodium ions in a high-concentration environment, and do not care about the temporary adsorption of these impurities on the surface of the increased number of fine-particle-size lithium carbonate particles. They are relatively easy to be released during the primary thermal stirring, washing, and centrifugation operations and the desorption process in paragraphs

[0059] and

[0060] . For this purpose, the feeding speed of the purified lithium sulfate completed liquid can be appropriately increased, that is, cancel the aging time design part in the original feeding time, but the feeding device with dispersed and multi-point layout remains unchanged.

[0044] 5. First, thermally stir, wash, and centrifuge the crude lithium carbonate obtained by thermal precipitation once with 3 times its weight of deionized water to reduce the sulfate content of industrial grade and battery grade to below 0.40% and 0.30% respectively, and set aside for use.

[0045] 6. Under low-speed agitation, use the operation measures of increasing temperature, strongly desorbing, and performing heat preservation and aging to release most of the water-soluble impurities and other impurities represented by sodium sulfate that are deeply encapsulated into the water.

[0046] 7. Then, use a hydrocyclone to separate the liquid phase with more released fine-particle-sized water-insoluble impurities, and according to the turbidity of the liquid phase, obtain refined lithium carbonate by centrifugal rinsing or reheating and stirring for one more time. The original drying and pulverizing methods and equipment remain unchanged.

[0047] The beneficial effects of the present invention are as follows: With a relatively simple solution, less equipment investment, and lower costs, it can significantly reduce the contents of impurity sulfate and impurity sodium in industrial-grade and battery-grade lithium carbonate produced by the thermal precipitation process of lithium sulfate extracted from solid lithium ores such as spodumene and lepidolite and soda ash, and greatly improve the product quality of these two types of lithium carbonate. This can make full use of one of the advantageous resources in our country - solid lithium ores. It can help the rapid development of lithium battery vehicles with products of higher cost performance and consolidate the international status of our country's lithium salt industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Attached Figure 1 is a process flow schematic diagram of treating spodumene by the sulfuric acid method.

[0049] Attached Figure 2 is the washing curve of sulfate in the trial-produced product according to the process of American Lithium Corporation.

[0050] Attached Figure 3 are the solubility data of lithium phosphate, lithium fluoride, and lithium carbonate in water.

[0051] Attached Figure 4 is the sulfate reduction curve after implementing the method of this application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] Attached Figures 1-4 The detailed description is as follows. Attached Figure 1 is a process flow schematic diagram of the traditional sulfuric acid method for treating spodumene. Attached Figure 1 Source: "Chemistry and Technology of Lithium" by A. A. Ostroshko et al., published by China Industry Press, the first edition in Beijing in May 1965, page 160.

[0053] Attached Figure 2 is the washing curve of sulfate in the trial-produced product (i.e., the sulfate washing - reduction curve of the crude lithium carbonate produced by the thermal precipitation of the lithium carbonate process by the sulfuric acid method) according to the "forward feeding" process of American Lithium Corporation during the initial stage of small-scale production of lithium carbonate by the sulfuric acid method for spodumene presided over by the inventor from 1978 to 1979. This curve fully shows that the biggest shortcoming of this traditional process is the high content of impurity sulfate. The washing conditions are: crude Li2CO3: distilled water = 1:1.5, temperature 90 - 95 °C, stirring time 30 minutes, centrifugal drying at 1,300 revolutions per minute.

[0054] Attached Figure 3 are the solubility data of lithium phosphate, lithium fluoride, and lithium carbonate in water, showing a huge difference with an order of magnitude larger successively.

[0055] Attached Figure 4 is the decline curve of sulfate radical desorption by hot pressing during the impurity removal process of battery-grade lithium carbonate after implementing the method of this application. 1 represents one hot stirring and washing; 2 represents one hot pressing desorption.

[0056] The method for significantly reducing the sulfate radical content in lithium carbonate at all levels in the sulfuric acid method of spodumene of this application includes the following steps: 1. The crude lithium carbonate precipitated by heat is washed with 3 times deionized water at 90 - 95 °C, first stirred and washed once for about 15 minutes. After centrifugation, the sulfate radical of industrial-grade crude lithium carbonate is controlled below 0.40%, corresponding to new zero-grade and new first-grade products; for battery-grade, it is controlled below 0.30%.

[0057] 2. According to the ratio of the crude lithium carbonate to be treated and deionized water of 1:6 - 7, add it to a stainless steel (0Cr 18 Ni9Ti) or a steel reaction kettle lined with titanium plate with low-speed stirring and a heating and cooling jacket, and carry out a strong desorption operation under a saturated steam pressure of 0.4 - 0.6 MPa (about 150 - 160 °C) with low-speed stirring and heat preservation for about 1 hour (including the aging time).

[0058] 3. For the slurry after completing the desorption operation, after the pressure in the kettle drops to 0.05 MPa, it is pressed into a hydrocyclone to separate the solid and liquid phases; if the turbidity of the liquid phase is high, it indicates that the water-insoluble impurities in the crude lithium carbonate are too high and the desorption effect is obvious, and it must be stirred and washed again before the obtained refined lithium carbonate is sent to the drying process; if the turbidity of the liquid phase is low, it can be directly continuously centrifuged and washed, and sent to the drying process. For battery-grade crude lithium carbonate, as long as the separation liquid is slightly turbid after desorption, it must be hot stirred and washed again before it can be recognized as refined lithium carbonate.

[0059] 4. Optionally, a low-speed spherical or cylindrical desorber can be used, with a material of 0Cr 18 Ni9Ti or titanium-lined composite steel plate.

[0060] 5. Optionally, various types of continuous external heat desorbers can be designed, with the same material as in paragraph

[0061] .

[0061] 6. The enamel lining design of the desorber inner wall needs to be pre-tested with materials, and the dissolution amounts of elements such as boron, aluminum, silicon, lead, and antimony in the enamel under alkaline slurry and long-time high-temperature stirring conditions need to be detected before making a choice.

[0062] The drying and pulverization processes can be completed using traditional equipment.

[0063] The static strong desorption bench test completed by the inventors of this application shows that for the thermally precipitated crude lithium carbonate used to produce industrial-grade lithium carbonate, first wash it with 3 times deionized water by hot stirring once until the sulfate radical drops to 0.35%, then add 6 times deionized water to a simple externally heated desorption heat aging device and keep it warm for 0.5 - 1 hour. After cooling, filter by suction and wash once, and the sulfate radical drops by one order of magnitude. Using the barium sulfate method (gravimetric method) for detection, almost no white precipitate of barium sulfate can be observed. After standing and aging for 24 hours, shake the Erlenmeyer flask, and only a very small amount of precipitate floats at the bottom.

[0064] Although the specification of this application takes the sulfuric acid method of lithium carbonate from spodumene as an example to illustrate the innovative processes of industrial-grade and battery-grade lithium carbonate, its application is not limited to this. It can actually include the cases where lithium sulfate obtained by all methods is thermally precipitated with soda ash to obtain crude lithium carbonate: the raw materials include various lithium-containing ores such as lepidolite, and the water-soluble anions combined with lithium also include chloride, nitrate, acetate and other organic acid radicals, and the alkali also includes potassium carbonate. Just for the production of insoluble lithium salts, most of the other low-content impurities other than sodium sulfate are also desorbed along with it. Expanding further, for any product obtained by the reaction and precipitation of two or more water-soluble raw materials, if there is chemical adsorption and deep encapsulation and the effect of removing impurities by conventional washing methods is limited, the method of the present invention can be used to greatly reduce the impurity content, so it is all included in the scope of the claims of this application.

[0065] In the invention content and specific implementation manners of this invention patent application, in order to illustrate "strong desorption" and "hydrocyclone separation" (collectively referred to as "efficient desorption"), the number of primary washing times of crude lithium carbonate and the ratio of deionized water, the ratio of deionized water for strong desorption, and operation parameters such as temperature, pressure, and duration, and the operation parameters of hydrocyclone separation are listed. However, this is only for illustrative purposes and is not limited to this. These parameters together form a whole to achieve the goal of further greatly reducing impurities such as sulfate radicals in lithium carbonate. However, according to the different requirements of different customers for product quality and the differences in specific conditions such as the manufacturer's equipment, these parameters need to be adjusted and can be adjusted, without and should not have subjective restrictions and third-party restrictions. As long as the method of increasing temperature for desorption and the hydrocyclone separation method are used to separate the solid and liquid phases to achieve the purpose of removing adsorbed and encapsulated impurities from insoluble and slightly soluble solid particles precipitated from the liquid, no matter how these parameters are adjusted, they are all included in the scope of the claims of this invention patent application.

Claims

1. A method for significantly reducing the sulfate content in lithium carbonate at all levels by the sulfuric acid method using spodumene, characterized in that, The method includes: based on various impurity removal methods for existing industrial-grade and battery-grade products, adopting the process of "reverse feeding without circulating mother liquor"; before the formal operation of thermal precipitation, adopting the measure of "precipitation for supplementary impurity removal"; during the thermal precipitation operation, temporarily not performing aging to pursue large-grained and coarse lithium carbonate crystals; after the crude lithium carbonate is subjected to 3-fold deionized water hot stirring, washing and centrifugation once, it is then subjected to strong desorption treatment to release most of the sodium sulfate and other impurities, obtaining refined lithium carbonate; drying and pulverizing; the sulfate radicals in industrial-grade and battery-grade products are respectively reduced to a minimum of 0.03% and 0.008%, and the main contents are respectively increased to 99.5% and 99.95% or even 99.990%.

2. The method according to claim 1, wherein The process of "reverse feeding without circulating mother liquor" includes: reversing the way of adding the soda ash purification liquid in the thermal precipitation process of the lithium carbonate process by the sulfuric acid method of spodumene invented by American Lithium Corporation, that is, conversely dispersing and slowly adding the lithium sulfate purification liquid into the soda ash purification liquid, greatly reducing the chemical adsorption and encapsulation of sulfate radicals; after the primary hot mother liquor centrifuged to obtain crude lithium carbonate is cooled to about 0 °C for crystallization and centrifuged to obtain mirabilite, the secondary cold mother liquor no longer returns to the acidified material leaching process, but is concentrated until the sodium sulfate crystallization film begins to form, and the precipitated crude lithium carbonate is hot filtered out. The tertiary hot mother liquor is combined to crystallize mirabilite, and the operations of "cold precipitation of mirabilite and hot precipitation of crude lithium carbonate" are carried out alternately to further reduce the sodium sulfate concentration in the reaction solution during the thermal precipitation of crude lithium carbonate.

3. The method according to claim 1 or 2, characterized in that, When misoperations such as filtration through in the previous impurity removal and filtration process are discovered too late, or other impurity removal accidents occur, the measure of "precipitation for supplementary impurity removal" is adopted for efficient rescue, that is, before the formal thermal precipitation operation, under close observation, a small amount of lithium sulfate purification liquid is slowly added first. As soon as the reaction solution shows slight turbidity, stop adding the material and continue hot stirring for about a quarter of an hour, then filter. The filter residue is delicate with slightly coarser particles - that is, lithium carbonate, and that's it.

4. The method according to claim 1 or 2, characterized in that During the formal precipitation operation, temporarily do not pursue obtaining large-particle-size crude lithium carbonate particles, and the feeding speed can be appropriately increased to move the aging time backward.

5. The method according to claim 1 or 2, characterized in that: The crudely precipitated lithium carbonate is first thermally stirred and washed once with 3 times deionized water and then centrifuged, reducing the sulfate radicals of industrial grade and battery grade to below 0.40% and 0.30% respectively; then a reaction kettle made of 0Cr 18 Ni9Ti stainless steel or a composite plate lined with titanium plates is used to heat up with 6-7 times deionized water. The saturated steam pressure in the kettle is 0.4-0.6 MPa and the temperature is 150-160 °C for strong desorption and thermal aging for 1 hour, and most of the sodium sulfate and other impurities are released into the water.

6. The method according to claim 5, characterized in that When the pressure in the desorption kettle is reduced to 0.05 MPa, slowly press it into the hydrocyclone separator; according to the turbidity of the separated liquid phase, the lithium carbonate slurry is further purified by centrifugal washing or hot stirring, washing and centrifugation respectively to obtain refined lithium carbonate.

7. The method according to claim 5, characterized in that, Adopt a low-speed spherical or cylindrical desorber made of 0Cr 18 Ni9Ti or titanium-lined composite plate; or adopt a continuous external heating desorber; pre-detect the glass-lined design of the inner wall of the desorber with the feed, detect the dissolution amount of elements such as boron, aluminum, silicon, lead, and antimony in the glass lining under the conditions of alkaline slurry and long-time high-temperature stirring, and then make a choice.

8. A method for reducing the concentration of sulfate ions in a lithium-ion-containing solution, characterized in that, The method includes: 1) Gradually adding the lithium-ion-containing solution to the stirred soda ash solution at a temperature of 90 - 95 °C to precipitate crude lithium carbonate.

9. The method according to claim 8, characterized in that, Before the step 1), there is also a step of "precipitation for supplementary impurity removal", The step of "precipitation for supplementary impurity removal" includes: slowly adding the soda ash solution to the lithium-ion-containing solution, and stopping adding the material when white fine precipitates are observed. Continue stirring, pump the mixture into a suction filtration barrel for micro-vacuum filtration, pump out the filtrate for circulating filtration until the filter cake forms a bridge successfully and the filtrate sample is observed to be completely clear, then stop the filtrate circulation.

10. The method according to claim 8, characterized in that, After the step 1), it further includes: cooling the primary hot mother liquor from which crude lithium carbonate is centrifuged to about 0 °C for crystallization, centrifuging out mirabilite, and instead of returning the secondary cold mother liquor to the acidified material leaching process, concentrating it until the sodium sulfate crystallization film starts to form, hot filtering out the precipitated crude lithium carbonate, combining the tertiary hot mother liquor to crystallize mirabilite, and cross - performing the operations of "cold - precipitating mirabilite and hot - precipitating crude lithium carbonate" to further reduce the sodium sulfate concentration in the reaction solution during the hot precipitation of crude lithium carbonate.

11. The method according to claim 8, wherein When performing the operation of precipitating crude lithium carbonate in the step 1), instead of pursuing the acquisition of large - particle - size crude lithium carbonate particles, appropriately increase the feeding speed and shift the aging time backward.

12. The method according to claim 8, characterized in that, After the step 1), the precipitated crude lithium carbonate is first thermally stirred and washed once with 3 times deionized water and then centrifuged, and the sulfate radicals of industrial grade and battery grade are respectively reduced to below 0.40% and 0.30%; then a desorption kettle made of 0Cr 18 Ni9Ti stainless steel or a composite plate lined with titanium plates is used to heat up with 6-7 times deionized water, the saturated steam pressure in the kettle is 0.4-0.6 MPa, at 150-160 °C, and strong desorption and thermal aging are carried out for 1 hour.

13. The method according to claim 12, wherein When the pressure in the desorption kettle drops to 0.05 MPa, slowly press it into the hydrocyclone separator; according to the turbidity of the separated liquid phase, the lithium carbonate slurry is finally purified by centrifugal rinsing or hot - stirring and centrifuging respectively to obtain refined lithium carbonate.

14. The method according to claim 12, wherein Adopt a low-speed spherical or cylindrical desorber, with the material being 0Cr 18 Ni9Ti or titanium-lined composite plate; or adopt a continuous external heating desorber; pre-detect the glass-lined design plan of the desorber inner wall with the feed material, and detect the dissolution amount of elements such as boron, aluminum, silicon, lead, and antimony in the glass lining under the conditions of alkaline slurry and long-time high-temperature stirring. If the dissolution amount is large, do not use glass lining.

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