1. Field of the Invention
This invention relates to a process for preparing hydrogen fluoride by electrodialysis of a fluoride solution. In particular, this invention relates to a process in which an aqueous fluoride solution such as a concentrated solution of ammonium fluoride, is electrodialyzed to produce hydrogen fluoride using an electrodialysis cell having an anode compartment with an anode immersed in an anolyte and a cathode compartment with a cathode immersed in a catholyte separated from one another by an inner electrodialysis compartment, said inner electrodialysis compartment bounded by two spaced cation exchange membranes.
2. Description of the Prior Art
Fluosilicic acid (H.sub.2 SiF.sub.6) and tetrafluorosilane (SiF.sub.4), which is easily hydrolyzed to fluosilicic acid, are by-products formed during the acidification of phosphate rock in the preparation of wet process phosphoric acid. These compounds can usually be recovered as aqueous scrubber by-products from attack tanks and evaporators of the phosphoric acid manufacturing facility. Unfortunately, fluosilicic acid is generally considered a waste material and is viewed as a disposal problem.
Hydrogen fluoride and hydrofluoric acid on the other hand, are key chemicals in almost all fluorinating processes and also find wide application as catalysts and reaction media in non-fluorinating processes. Recognizing this fact, the prior art has sought to develop various methods for producing hydrofluoric acid (hydrogen fluoride) from fluosilicic acid. In one process, fluosilicic acid is flame hydrolyzed to silica (SiO.sub.2), tetrafluorosilane, hydrogen fluoride and water vapor followed by selective solvent absorption of the hydrogen fluoride. It also is known to hydrolyze fluosilicic acid using a base such as an alkali metal hydroxide or ammonium hydroxide to produce an aqueous alkali metal or ammonium fluoride salt solution and solid silica. The silica is removed by filtration, the fluoride salt solution is concentrated by evaporation and hydrogen fluoride is produced by thermal decomposition of the fluoride salt or by its reaction with a strong mineral acid such as phosphoric acid or sulfuric acid. Due to the high energy requirements of these processes, however, neither has been commercialized.
It also is known to produce hydrogen fluoride from aqueous fluoride salt solutions, e.g., those produced by base hydrolysis of fluosilicic acid, using electrolytic treatment. Carlin U.S. Pat. No. 3,450,609, for example, describes an arrangement in which hydrogen fluoride is generated at the anode of an electrolytic cell using an alkali metal fluoride or ammonium fluoride solution as the catholyte and an electrically conductive anolyte of hydrofluoric acid separated from one another by an anion exchange membrane.
The prior art also discloses the use of electrodialytic water-splitting using bipolar membranes for producing hydrogen fluoride from alkali metal and ammonium fluoride solutions, e.g., see Chlanda et al. U.S. Pat. No. 3,787,304 and Nagasubramanian et al. U.S. Pat. No. 4,107,264. One disadvantage of this arrangement is that the significant pH gradient established across the bipolar membrane is very detrimental to the membrane's integrity and its useful life.
It is an object of the present invention to provide an improved process for producing hydrogen fluoride from aqueous streams containing alkali metal or ammonium fluoride using electrodialysis. The aqueous alkali metal or ammonium fluoride preferably is obtained from by-product fluorine compounds produced in the manufacture of wet process phosphoric acid.
This and other objects which will be apparent to those skilled in the art are achieved by the present invention.