For many industrial applications of the phosphoric acid, for instance in the fodder and detergent industries, it is necessary to maintain the fluorine content of the phosphoric acid as low as possible. Moreover, a low fluorine content is required to prevent corrosion of components that are in contact with the phosphoric acid.
Several processes are known for reducing the fluorine content of the phosphoric acid. DE-AS 11 48 531, for example, describes a process in which hot combustion gases and water vapor are simultaneously added to a solution containing at least 48% P.sub.2 O.sub.5. A similar process is described in U.S. Pat. No. 2,977,196.
Another process is disclosed in DD-PS No. 30 306 in which prior to concentrating the phosphoric acid by heating, finely-distributed silicic acid is added to convert most of the fluorine to silicon tetrafluoride, which is separated in the boiling phase of the phosphoric acid.
Processes are also known in which SiO.sub.2 is added in the form of infusorial earth, water glass, diatomaceous earth, etc. prior to concentrating the phosphoric acid, thereby effecting a precipitation of the alkaline silicofluorides contained in the phosphoric acid and/or the formation of volatile silicofluoride (U.S. Pat. No. 3,193,351).
In another known process alkaline or alkaline earth compounds are added to the diluted and/or concentrated phosphoric acid, if necessary in conjunction with the addition of SiO.sub.2, thus achieving a precipitation of the resulting alkaline or alkaline earth silicofluorides.
When adding water glass, the reaction of the fluorides in the phosphoric acid or the hydrofluoric acid is such that silicofluorides are formed which precipitate with the alkali of the water glass and/or escape as gaseous SiF.sub.4 or H.sub.2 SiF.sub.6. This reaction produces an equilibrium of about 0.4% F. in a phosphoric acid with a P.sub.2 O.sub.5 concentration of 54%.
It is also known that phosphate rock has a certain SiO.sub.2 content. When the phosphate rock is digested by sulphuric acid, part of the Si compounds are dissolved in the phosphoric acid. When concentrating phosphoric acid which was obtained from phosphate rock containing SiO.sub.2, it is also possible to reduce the fluoride content by precipitating the alkaline or alkaline earth silicofluorides or by volatilizing the H.sub.2 SiF.sub.6 or SiF.sub.4 +H.sub.2 F.sub.2.
The known processes generally have a high energy and equipment demand and are not sufficient to obtain the low fluorine content required.