Industrially hydrogen fluoride is manufactured by the reaction of sulfuric acid with fluorspar (acid grade):

The small quantities of compounds such as carbonates or oxides (e.g. iron oxide) in the fluorspar also react with thesulfuric acid, necessitating a ca. 5-10% excess of sulfuric acid. dioxide reacts with the already formed hydrogen fluoride to form silicon tetrafluoride, thereby reducing the hydrogen fluoride yield. The above-mentioned impurities in fluorspar produce water and carbon dioxide in addition to silicon tetrafluoride produced from :

Since the reaction is endothermic the reaction is generally carried out in an indirectly heated rotary tube furnace (length up to 30 m, diameter up to 3 m) at high temperatures (temperature ca. 200 °C). The capacity of such indirectly heated furnaces can be more than 45 t HF per day. 3.8 t of calcium sulfate (anhydrite) is produced per ton of hydrogen fluoride.

The reaction of solid fluorspar with liquid sulfuric acid produces a solid (calcium sulfate) and a gaseous (hydrogen fluoride) product. The reaction passes through a paste-like phase. Many reactor modifications have been developed to attain an optimal reaction in this phase. The aim is thorough mixing of this phase, which is achieved by internal fittings in the rotary tube (e.g. paddles) or by kneading the reaction mixture in a kneader.

Upon leaving the furnace the hydrogen fluoride formed is scrubbed with concentrated sulfuric acid, which is subsequently utilized in the reaction with fluorspar. In this scrubbing the water in the hydrogen fluoride is removed by being bound by added oleum.

The Bayer process is represented as a flow sheet in Fig. 1.7-2.

The necessary process heat is supplied at several points in the process:
(1)preheating of fluorspar by hot gas
(2)preheating of sulfuric acid by heat exchange with the raw gaseous hydrogen fluoride
(3)addition of sulfur trioxide as oleum
(4)heating of the rotary tube furnace

Fluorspar preheated to 400 °C and preheated sulfuric acid are fed into a mixer, in which partial reaction occurs, the reaction being completed in the rotary tube furnace. The raw hydrogen tluoride, which in addition to air, silicon tetrafluoride, hydrogen, sulfur dioxide and carbon dioxide also contains calcium sulfate dust, is scrubbed with sulfuric acid. The scrubbing acid, after addition of oleum to bind the water, is reacted with tluorspar. Pure hydrogen fluoride (b.p. 19.9 °C) is obtained by multistage cooling of the raw hydrogen tluoride. If necessary, the hydrogen fluoride can be purified further by post-treatment steps e.g. distillation. The gas remaining after condensation is scrubbed with sulfuric acid to remove residual hydrogen fluoride. Afterwards the gas, which still contains silicon tetrafluoride, is scrubbed with hydrofluoric acid to form hexafluorosilicic acid:

Upon scrubbing with water, silicon dioxide is produced in addition to hexafluorosilicic acid:



Hexaluorosilicic acid can be widely utilized (manufacture of fluorides and hexafluorosilicates).

Buss has developed a widely operated variant of the fluorspar decomposition in which the pre-reaction is carried out in a continuously operating kneader, which is linked to an indirectly heated rotary tube furnace in which the reaction proceeds to completion. In a further variant (a process developed by DuPont) the necessary heat is supplied by reacting sulfur trioxide with water to sulfuric acid in the reactor (e.g. in a fluidized bed process). The heat of sulfuric acid formation thereby liberated provides much of the energy for the endothermic fluorspar decomposition.

The calcium sulfate leaving the rotary tube furnace (anhydrite) still contains excess sulfuric acid, which is neutralized with lime. After treatment this anhydrite can be utilized e.g. in the laying of cement floors, for infilling in mines or in the cement industry as a setting regulator.

The pyrolytic cracking of silicon tetrafluoride at high temperatures is a further way of utilizing the hexafluorosilicic acid byproduct:



This process has a certain importance in the manufacture of FLUOSIL, a silicon dioxide with a large specific surface area.