What are the main production routes for synthetic cryolite and their cost implications?

What are the main production routes for synthetic cryolite and their cost implications?
Synthetic cryolite production in China follows several routes. The fluorosilicic acid method captures fluorine from waste gases, reacting fluorosilicic acid with aluminum hydroxide and soda ash, with sub-variants including direct, ammonia, and ammonium fluoride-sodium aluminate processes. Carbonation involves passing carbon dioxide through sodium aluminate and sodium fluoride solutions to precipitate cryolite crystals. Another route recovers dilute hydrofluoric acid from aluminum smelting off-gases, reacting it with sodium aluminate. The alkali method, using soda ash, fluorite, and silica sand, sees limited industrial use. Additionally, electrolytic aluminum plants generate fluorine-containing waste residues containing over 30% cryolite, which can be regenerated into quality cryolite through specific treatments. Each route balances feedstock availability, energy costs, and product purity, with molecular ratio classification (high ratio 2.8-3.0 versus low ratio 1.8-2.2) determining suitability for different electrolytic cell operations.