Abstract:

Faced with the increasing shortage of fluorspar，a strategic resource in China，an industrial route for preparation of high-purity aluminium fluoride from fluorosilicic acid，a by-product of phosphate fertilizer was developed.Fluorosilicic acid was used as raw material to synthesize sodium fluorosilicate，which was further dynamically decomposed to get silicon tetrafluoride.Silicon tetrafluoride was purified and then reacted with anhydrous aluminium chloride in vapour phase deposition to prepare high-purity aluminium fluoride.The reaction principle，process flow，key point control，product quality and raw material cost were also studied.The results showed that sodium fluorosilicate paste was dried by two-stage drying method，and the moisture content after drying was ≤0.05%（mass fraction）.Dynamic decomposition method was adopted for sodium fluorosilicate pyrolysis with decomposition rate >99%.The purity of crude silicon tetrafluoride gas after washing with concentrated sulfuric acid，molecular sieve adsorption and distilled purification could reach 99.5%.Aluminium chloride reacted with purified silicon tetrafluoride in gas state，and the purity of the product after reaction was >99.5%，which met the market demand in high-end field.This study could provide reference and guidance for the efficient and high-value utilization of low grade fluoride resources as by-product of phosphate fertilizer.

Key words: fluorosilicic acid, sodium fluorosilicate, vapor deposition, high purity aluminium fluoride