Battery grade lithium carbonate is the key material of lithium battery, which is mainly obtained from sulfuric acid process of spodumene. The whole process is in acid and high sodium environment, there are problems such as acid mist pollution, equipment corrosion and unstable product quality in the production process. Compared with sulfuric acid process, alkali pressure leach process for producing battery grade lithium carbonate has the advantages of short process, good environmental protection, strong adaptability of raw materials and strong compatibility of sulfuric acid process. Through experiments and system optimization, a new alkali pressure leach process was established, it included transformation roasting, soda pressure leaching and twice carbonation-crystallization. In the 1960s and 1970s, China conducted phased research on alkali pressure leach process, carried out small-scale experiments, semi industrial experiments and continuous autoclave industrialization experiments, and obtained the main technical parameters. Since then, due to the popularization and application of sulfuric acid process, the research on alkali pressure leach process related technologies was interrupted. After 2012, China ENFI conducted systematic research and experimental verification on alkali pressure leach process, and developed a new alkali pressure leach process route with friendly environment and low cost. This paper introduced the reaction mechanism and process of producing battery grade lithium carbonate by alkali pressure leach process. The new alkali pressure leach process solved the problems existing in the previous alkali pressure leach process. First, the system did not discharge waste water, realizing the recycling and echelon utilization of water resources. It was estimated that the water consumption of per unit lithium carbonate product was less than 25 m3. Second, the energy consumption was reduced by adopting multi-stage flash evaporation, cold and hot flow echelon heat exchange and other technologies, and the steam consumption of per unit product was less than 15 t. This paper also analyzed the influencing factors of roasting, soda pressure leaching and carbonation-crystallization in alkali pressure leach process. Roasting was to calcine spodumene at high temperature, and α-spodumene was converted to more fusible β-spodumene. In this process, the temperature was an important factor affecting the crystal conversion rate. Due to different impurities, different Li2O content and particle size, spodumene in China and abroad would have different transformation temperatures, but the transformation temperature was basically controlled in the range of 1000~1150 ℃, with a narrow fluctuation range. In the soda pressure leaching process, with spodumene concentrate in Australia as raw material, it was concluded that alkali ratio, reaction temperature, reaction time and liquid-solid ratio were the four main factors. Alkali ratio had the greatest influence, and reaction time was the least. After more than 60 min, the change of reaction rate was not obvious. Carbonation-crystallization process repeated two times. First carbonation-crystallization process realized the separation of lithium and leaching residue to obtain crude lithium carbonate. In the secondary carbonation-crystallization process, the crude lithium carbonate was refined to remove soluble impurities to obtain battery grade lithium carbonate products. The carbonation process of Li2CO3 was a gas-liquid-solid three-phase reaction system. The dissolution of CO2 in aqueous solution and the mass transfer process of Li2CO3 were the key factors affecting the carbonation process. In production practice, the reaction rate was mainly improved by controlling the gas partial pressure of CO2, reaction temperature and the liquid-solid ratio of lithium carbonate to pure water. Li2CO3 crystallization process was a coupling process of Li2CO3 thermal decomposition and Li2CO3 recrystallization, with simple control and complex reaction kinetics. Temperature was the key factor to control the two reversible reactions of carbonation-crystallization process. In order to reduce the decomposition rate of Li2CO3 in the carbonation process, the solution temperature was usually controlled below 40 ℃. The reaction temperature in crystallization was generally greater than 90 ℃, which promoted the rapid decomposition of Li2CO3. The study showed that the higher the CO2 pressure, the shorter the carbonation reaction time. In order to reduce the loss of CO2, CO2 needed to be returned to the carbonation reactor for recycling, and the whole system was in a micro positive pressure state of 0.01~0.05 MPa in industry. And the liquid-solid ratio of lithium carbonate to pure water was larger, reaction time was also shorter. Combined with different working conditions of materials, selecting a large liquid-solid ratio was conducive to the control of lithium carbonate impurities and improve the product recovery. According to the above analysis, optimum control conditions for the production of battery grade lithium carbonate by alkali pressure leach process were as follows: the temperature of roasting process was 1000~1150 ℃. The alkali ratio was 1.9~2.1, the reaction temperature was 235 ℃, and the reaction time was 1 h. The carbonation temperature was lower than 40 ℃, the crystallization temperature was greater than 90 ℃, and the liquid-solid ratio for primary carbonation was 30:1~40:1, the liquid-solid ratio for secondary carbonation was 20:1~25:1. Theoretical research and experiments showed that alkali pressure leach process for the production of battery grade lithium carbonate had the advantages of excellent product indexes, good environmental protection effect, strong adaptability of raw materials and strong compatibility of sulfuric acid process, which was conducive to the upgrading and transformation of lithium salt industry. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.
