A brief analysis of the technological advancements in fluoride salt production for aluminum electrolysis in China

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A brief analysis of the technological progress in the production of fluoride salts for aluminum electrolysis in China. Fluoride salts used in aluminum electrolysis include aluminum fluoride, cryolite, calcium fluoride, magnesium fluoride, lithium fluoride, etc., among which aluminum fluoride and cryolite are the most commonly used. The consumption of aluminum fluoride salts is approximately 25 kilograms per ton. In 2003, China’s production of cryolite was 160,000 tons, and that of aluminum fluoride was 140,000 tons. The fluorides mentioned in this article mainly refer to these two products. I. Current Status of Fluoride Salt Production Technology for Aluminum Electrolysis in China 1. Production Processes At present, there are various production processes for fluoride salts in China. Representative processes for aluminum fluoride production include: hydrofluoric acid-wet process, hydrogen fluoride-anhydrous process, and fluorosilicic acid process ； The representative processes for the production of cryolite include the soda ash fluoraluminate process, the sodium fluosilicate process, and the clay brine process. 　　2. Sources of fluorine in the products 　In China’s 300,000 tons of fluoride salt products, 88% of the fluorine comes from fluorite, while 12% comes from by-products of phosphate fertilizers. 　　3. Product types and structure The main fluoride products in China include: hydrated aluminum fluoride, anhydrous aluminum fluoride, ordinary cryolite, and polymerized cryolite. 　　Hydrated aluminum fluoride accounts for 88% of the total production of aluminum fluoride, while anhydrous aluminum fluoride accounts for 12% ； Ordinary cryolite accounts for 75% of the total cryolite, while high-molecular cryolite accounts for 25%. II. Achievements in technological progress in fluoride salt production in China In the early 1950s, China’s first aluminum smelter – Fushun Aluminum Plant – adopted technology from the Soviet Union to establish the country’s first fluoride salt factory. Over the past 50 years, as China’s primary aluminum production has continued to increase and aluminum electrolysis technology has advanced, the production of fluoride salts in the country has involved the introduction, assimilation, and adoption of advanced foreign technologies, alongside continuous innovation of its own. This approach has enabled it to meet the evolving demands for fluoride salt products in aluminum electrolysis production. This is evident in the following two aspects: 1. Polymeric ratio cryolite and anhydrous aluminum fluoride products have been developed. The polymeric ratio of the cryolite products developed in China remains above 2.85, while that of ordinary cryolite is around 2.0. In addition, polymeric ratio cryolite features a higher melting point (20–30°C higher than that of ordinary cryolite) as well as lower burning loss, making it highly suitable for starting up new aluminum electrolysis cells, with significant advantages. 　　A. **Reduce the amount of soda ash used to cut production costs. In large-scale pre-baked cell production in China, the electrolyte molecular ratio is generally maintained at 2.2–2.4.** When starting a new cell, due to sodium absorption at the cathode, it is generally required that the electrolyte molecule ratio be around 3.0; if ordinary cryolite is used for cell startup, a large amount of soda ash must also be added to meet the process requirements for the electrolyte ； Using polymers instead of cryolite for starting up the process requires only a small amount of soda ash (typically 40% of the amount used with ordinary cryolite), which is sufficient to meet the process requirements and reduces production costs. 　　B. Using polymers instead of cryolite to initiate cell formation helps to create a well-structured cell wall, thereby extending the lifespan of the electrolyzer. When using ordinary cryolite to initiate operation, a large amount of soda ash must be added, which leads to localized accumulation of sodium during the filling of the electrolyzer. During startup, uneven absorption of sodium by the cathode can occur, easily causing deformation and breakage of the cathode carbon blocks ； Using polymers instead of cryolite to initiate the reaction prevents uneven sodium absorption at the cathode to the greatest extent possible, as little or no alkali is required. Furthermore, since polymers have a higher melting point than cryolite and melt more slowly, during the period of decrease in electrolysis temperature after the electrolyzer is started, an electrolyte with a higher initial crystallization point forms in the gaps of the cell lining, which facilitates the formation of a well-structured furnace wall. 　　C. Polymers have lower burning losses compared to cryolite, resulting in less volatilization loss of gaseous fluorine during the electrolysis process; **this improves the operating conditions in the electrolyzer and reduces fluorine pollution. In addition, our country also introduced the dry-process technology from the Swiss company Buss, and built an anhydrous aluminum fluoride production line at the Xiangxiang Aluminum Plant in the early 1990s. Anhydrous aluminum fluoride features a high main content, low moisture content, and a high bulk density, making it particularly suitable for reducing the molecular ratio of the electrolyte after the electrolyzer is started up. Compared to wet-process products, anhydrous aluminum fluoride has the following advantages in use: a) Its main content is over 90%, which is nearly 5 percentage points higher than that of wet-process products; it has a low impurity content, which reduces the amount of aluminum fluoride required and lowers production costs, thereby helping to improve the quality of primary aluminum. 　　b. The moisture content is low, at less than 1.0%, which is far lower than the 7.0% moisture level of wet-process products. When an anhydrous aluminum fluoride product is used in the electrolysis process, AlF3 hardly undergoes hydrolysis, resulting in a significantly higher proportion of usable active components compared to wet-process products. More importantly, it avoids the harsh operating conditions resulting from the hydrolysis of aluminum fluoride associated with wet-process products, which is beneficial for environmental protection. 　　2. With regard to the sustainable development of the fluoride salts industry, bold efforts were made in the comprehensive utilization of fluorine resources in phosphate fertilizers, and new technologies were introduced. The process for producing cryolite using sodium fluosilicate was successfully developed, and a process for manufacturing aluminum fluoride via the fluosilicic acid method was adopted. At present, the vast majority of fluoride salt products worldwide are manufactured using fluorite as a raw material. The world’s reserves of fluorite (CaF2) amount to approximately 623 million tons, while China’s reserves are around 140 million tons. In 2003, China’s fluorspar production was 2.5 million tons. As an important strategic resource for the development of the nuclear energy industry, fluorite is highly valued by countries around the world due to its limited reserves; as a result, they place great emphasis on protecting these fluorite resources and even adopt policies of importing large amounts of fluorite for storage. China ranks third in the world in terms of fluorite reserves. However, as an important non-renewable strategic resource, China introduced policies in 2003 to stop issuing new permits for fluorite mining, imposed tariffs to restrict fluorite exports, and placed export controls on fluorite-derived products such as hydrofluoric acid (HF). At the same time, the country has been actively seeking new sources of fluorine. The fluorine content in phosphate rock varieties is about 2–4%. The world’s apatite reserves amount to about 60 billion tons, while China’s reserves are around 15 billion tons. It is evident that the fluorine resources associated with phosphate rocks are much larger in quantity than those in fluorspar; therefore, countries around the world attach great importance to the comprehensive utilization of fluorine resources in phosphate rocks. The United States is a major producer of phosphate rock, and 30% of the fluorosilicic acid produced as a by-product is used in the manufacture of fluoride salts for aluminum electrolysis. 　　It was against this backdrop that, in the early 1990s, phosphate fertilizer manufacturers such as those in Guixi, Jiangxi, Luzhai, Guangxi, and Hongfu, Guizhou, introduced four sets of production facilities for manufacturing aluminum fluoride from fluorosilicic acid from France and Germany, in order to address the issue of fluorine pollution through environmental protection measures. The apparatus for producing cryolite using the sodium fluosilicate method, developed by Jiaozuo Duofuduo Chemical Co., Ltd., was put into operation in 2000. 　　3. Granular anhydrous cryolite and aluminum fluoride products were successfully developed. To meet the needs of mechanized feeding in aluminum electrolysis abroad, and to address the problems caused by the decomposition of fluorides during the electrolysis process, which leads to the emission of gaseous fluorine, as well as pollution resulting from the loss of solid fluorine during feeding, China successfully developed granular anhydrous fluoride products in 2002. III. Problems in the production technology of fluorides in China Since fluoride production in China relies primarily on the hydrofluoric acid method, and hydrofluoric acid is highly corrosive and poses serious risks to human health, this approach makes the fluoride production process highly dependent on specific materials and advanced equipment, resulting in numerous problems. The main issues include: 1. Backward technological levels and severe pollution. The performance of fluoride salt products does not meet the development goals of China’s aluminum electrolysis industry, which aim for high quality, efficiency, low consumption, long service life, and environmental sustainability; 95% of fluoride salt manufacturers still use the production processes developed in the Soviet Union in the 1950s ； 60% of the production capacity is still devoted to wet-process aluminum fluoride and ordinary cryolite ； Process parameters are still determined based on experience; the degree of automation is low, fluoride pollution emissions are high, and it is difficult to address this issue. 　　2. The R&D capability is weak, and the pace of technological progress is slow. The pre-reactors and pre-reaction technologies, as well as the internal return material systems and related technologies that have long been used abroad for hydrofluoric acid production, have not yet been successfully developed. 　　3. Poor capability in new technologies for digestion and absorption. As early as the early 1990s, our country introduced four advanced production lines for aluminum fluoride using the fluorosilicic acid method; the facilities in Guixi, Jiangxi, and Jingxiang, Hubei, were shut down ； Production at Luzhai in Guangxi and Hongfu in Guizhou is intermittent, and the product quality is poor. IV. Development Trends in Fluoride Salt Production Technologies 1. A shift towards anhydrous aluminum fluoride products in product development and process innovation. At present, Jiaozuo Duofuduo Chemical Co., Ltd. is introducing technology from abroad to build a project for producing 30,000 tons of anhydrous aluminum fluoride. Once this production line is operational, compared to aluminum fluoride products currently available on the market, its main component concentration will be higher, reaching over 93%, while the moisture content can be kept below 0.5%. 　　2. Regarding the selection of fluorine sources, as fluorspar resources become scarce, focus will be placed on the comprehensive utilization technologies for fluorine resources derived as by-products of phosphate fertilizers, in order to improve promptly the process for producing aluminum fluoride using the fluorosilicic acid method. 　　3. Recently, the production of large-grained, low-moisture aluminum fluoride using fluorosilicic acid and bauxite under high temperature and pressure has become a topic of research interest. This post was last edited by yzhms on 2008-9-3 17:54]