Method of production of anhydrous hydrogen fluoride

An improved process of producing anhydrous hydrogen fluoride from a fluorosilicic acid solution by dehydration and decomposition of said acid with sulphuric acid, which comprises the steps of:

• a. heating an aqueous fluorosilicic acid solution free of silica suspension and having a concentration of 30 to 50% to the boiling point and partially evaporating said solution,
• b. conveying the boiling fluorosilicic acid solution and its vapors into a closed reactor, containing sulphuric acid in such an amount that the concentration of sulphuric acid after the reaction will be 72-78%,
• c. decomposing said fluorosilicic acid in said reactor at 150.degree.-170.degree. C to form steam, gaseous hydrogen fluoride and silicon tetrafluoride and a solution of 72-78% sulphuric acid, which may contain residual hydrogen fluoride,
• d. passing gaseous silicon tetrafluoride through the sulphuric acid solution from step (c) to effect the liberation of any residual hydrogen fluoride from said solution,
• e. withdrawing a gaseous stream comprising steam, hydrogen fluoride and silicon tetrafluoride from said closed reactor in step (c) and mixing with the liberated hydrogen fluoride from step (d), partially drying said gaseous stream with sulphuric acid, and then introducing said sulphuric acid into said closed reactor of step (c),
• f. absorbing hydrogen fluoride from said partially dried gaseous stream from step (e) in sulphuric acid solution resulting in the formation of fluosulphonic acid and water and a partially dried gaseous stream containing silicon tetrafluoride,
• g. distilling and condensing hydrogen fluoride from the sulphuric acid solution of step (f) to obtain a product containing about 99.9% HF,
• h. returning the acid solution after distillation in step (g) for absorption of hydrogen fluoride in step (f) while maintaining the concentration of sulphuric acid in said solution by addition of oleum in step (f) to maintain a molar ratio of H.sub.2 O :
• H.sub.2 SO.sub.4 of from 0.25 to 0.5,
• i. absorbing the remaining part of the gaseous silicon tetrafluoride from step (f) into a fluorosilicic acid solution, thereby forming a more concentrated fluorosilicic acid solution and precipitating silica, and
• j. separating the precipitated silica from step (i), from said concentrated fluorosilicic acid solution by filtration and introducing the filtered fluorosilicic acid solution into step (a).