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Electrolytic method for producing lithium carbonate: Electrolysis, lithium carbonate, production. (1) Electrolysis of lithium sulfate (or chloride) using a diaphragm method – taking the electrolysis of Li2SO4 as an example, the electrolysis of LiCl is roughly similar. Crude Li2CO3 is reacted with H2SO4 to produce Li2SO4. Following ion exchange treatment, a Li2SO4 solution is passed through cation exchange resin or chelating resin; during this contact process, multivalent cations such as calcium, magnesium, and iron in the Li2SO4 solution are adsorbed and removed, resulting in a high-purity Li2SO4 solution. A highly pure Li2SO4 solution is used as the anode solution, while a LiOH solution is used as the cathode solution; the two are separated by an ion-selective permeable membrane. The device is shown in Figure 1. During the electrolysis process, as shown in the figure, lithium ions pass through the cation-selective permeable membrane (such as perfluorosulfonic acid Rf2SO3H, perfluorocarboxylic acid membranes Rf2COOH, etc.) into the catholyte, whereas SO42- ions cannot pass through the membrane to enter the catholyte. This electrolysis process can proceed very completely, with high-purity LiOH being obtained on the cathode side, while H2SO4 is generated on the anode side; this H2SO4 can be reused. The high-purity LiOH solution obtained is carbonated with CO2 to yield Li2CO3, which is then purified and dried to produce refined Li2CO3 product. The mechanism is as follows: Cathode: 4Li++4H2O+4e=4LiOH+2H2↑ (1-5) Anode: 2H2O=O2↑+4H++4e (1-6) The Li2CO3 product obtained by this method has a high purity, and impurities such as calcium and magnesium, which are difficult to remove using other methods, can be reduced to even lower levels. However, in this method, the water-insoluble Li2CO3 reacts with the strong acid H2SO4, resulting in a large amount of SO42- ions in the anode solution. Even when a cation exchange membrane with high cation permeability is used, a significant amount of SO42- ions will end up in the LiOH solution obtained on the cathode side. When this LiOH aqueous solution is used as a raw material to produce Li2CO3, contamination by SO42- ions is inevitable. At the same time, a highly concentrated Li2SO4 with strong corrosivity is formed on the anode side, which requires expensive corrosion-resistant materials for the electrolyzer; moreover, the electricity consumption during electrolysis is high, and high demands are placed on the membrane as well. All these factors have constrained the industrialization of this method. Fig1-1: Installation diagram of diaphragm electrolysis. (2) Electrolysis of lithium bicarbonate using the diaphragm method: In a high-pressure resistant reaction tank, crude Li2CO3 is dispersed in water to form a slurry; while stirring thoroughly, high-pressure CO2 gas is introduced to react with Li2CO3 and produce LiHCO3. The concentration of LiHCO3 in the aqueous solution increases as the CO2 pressure rises, until Li2CO3 is completely dissolved. The resulting LiHCO3 aqueous solution was subjected to purification and removal of impurities in order to eliminate water-soluble humic substances and carbonates. The purified LiHCO3 solution is used as the anode, while water or a dilute aqueous LiOH solution is used as the cathode. The membrane in the middle is a cation exchange membrane formed by the copolymerization of cation exchange groups such as sulfonic acid or carboxylic acid groups with tetrafluoroethylene; the electrolysis device is similar to that in (1). During electrolysis, Li+ on the anode side passes through the cation exchange membrane to reach the cathode side, while HCO3- and other impurity anion ions cannot pass through the membrane and remain on the anode side. Li+ is electrolyzed on the cathode side through a membrane to produce LiOH: 4Li+ + 4H2O + 4e– → 4LiOH + 2H2↑ (1-7). On the anode side, HCO3– is electrolyzed to yield CO2, O2, and H2O: 4HCO3– → 4CO2↑ + O2↑ + 2H2O + 4e– (1-8). Then, the LiOH aqueous solution obtained from electrolysis is passed through a cation exchange resin to remove high-valent metal cations such as calcium, magnesium, and iron. In another reactor, an aqueous LiOH solution is reacted with CO2 to produce Li2CO3; the resulting Li2CO3 can be purified by recovery, washing, and drying to yield refined Li2CO3. Table 1-4 shows the impurity content in Li2CO3 products produced using two different cation exchange membranes. In this method, since toxic and corrosive CO2 gas is generated on the anode side, the device does not require expensive corrosion-resistant materials. The silicon impurity level in the high-purity Li2CO3 obtained is kept below 1×10^-4%, and the levels of other anionic impurities are also kept below 1×10^-4%. Alkali metal contamination can be eliminated, allowing the resulting product to be used in the electronics and optoelectronics industries. Table 1-4 Content of impurities in Li2CO3 product (%). Table 1-4: Percentage of impurities in the Li2CO3 product. Experimental examples: Cation exchange membrane, current efficiency; Silicon, Halogens, Sulfate, Nitrate. 1. Nafion-324: 64, 0.3×10-4, 1×10-4, 1×10-4, 1×10-4. 2. Nafion-901: 74, 0.2×10-4, 1×10-4, 1×10-4, 1×10-4