The earliest method of synthetic soda ash was invented at the end of 18. In Lublan, France, mirabilite was reduced by limestone and coal at high temperature and carbonized to obtain coarse black ash mainly containing Na2C03. After leaching, evaporation, refining, recrystallization and drying, heavy soda ash with purity of about 97% was obtained. 186 1 year, Belgian E. Solvi independently invented soda ash and obtained a patent. Because the protection of technical secrets has not been widely used, it only started to break through in the United States in the 1920s. In particular, a famous chemical expert in China, Hou Zai 1932, published the book "The Manufacture of Soda Alkali", which was kept secret for 70 years, and Su Wei's method was made public. Hou He 19 .39- 1942 also established Hou Shi caustic soda process, and established a pilot plant in Sichuan. 1952 Dalian chemical plant established a joint soda production workshop. The "NA method" introduced by Asahi Glass Company in Japan is essentially a compromise method between soda and ammonia. The ratio of soda ash to ammonium chloride can be adjusted at will.