Method of preparing fumed SiO.sub.2

#There exist aqueous solutions of H.sub.2 SiF.sub.6, such as waste streams from processing of ores or minerals that contain silicon and fluorine values, which would be useful in chemical processes, such as in preparing fumed SiO.sub.2 and HF, if the H.sub.2 SiF.sub.6 could be purified to a constant quality by distillation. Attempts to distill water or other low-boiling material from relatively weak solutions of H.sub.2 SiF.sub.6, in order to concentrate or purify the H.sub.2 SiF.sub.6, cause decomposition and create unwanted forms of SiO.sub.2. Attempts to distill H.sub.2 SiF.sub.6 away from higher boiling impurities, such as H.sub.3 PO.sub.4 or H.sub.2 SO.sub.4, also cause decomposition and create unwanted forms of SiO.sub.2. The SiO.sub.2 so formed has no significant commercial value, thus the commercial significance of H.sub.2 SiF.sub.6 is substantially negated by the formation of relatively useless forms of grainy-type SiO.sub.2. If the aqueous H.sub.2 SiF.sub.6 can be converted to a relatively constant quality and concentration, it can be used in processes which form commercially significant products, such as being combusted to make fumed SiO.sub.2 and HF. Fumed SiO.sub.2 is a form of SiO.sub.2 which has good commercial value.

#There exists then, a need for converting aqueous solutions of H.sub.2 SiF.sub.6, especially those solutions which are not of constant quality or concentration, to solutions which can be distilled to relatively constant quality and concentration without encountering the decomposition of H.sub.2 SiF.sub.6 and without creating unwanted forms of SiO.sub.2.

#As further background to certain embodiments of the present invention, there are a number of references which teach the burning of silicon fluoride compounds, such as SiF.sub.4 and H.sub.2 SiF.sub.6, by injecting the silicon fluoride compounds into a combusted mixture of fuel and oxygen in order to prepare fumed SiO.sub.2. The oxygen is generally supplied as air and, conveniently, the fuel may be hydrogen. The fuel also may be an easily combusted hydrocarbon such as an aliphatic hydrocarbon having from 1 to about 12 carbon atoms or mixtures of such hydrocarbons. Representative patents for such formation of fumed SiO.sub.2 are, for example, U.S. Pat. Nos. 2,819,151; 3,203,759; 3,110,562 and others. It is highly desirable that the feed stream of H.sub.2 SiF.sub.6 be of substantially constant quality and concentration in order that the process not be erratic, especially since such processes are normally expected to be performed continuously.

#An aqueous waste stream from a mineral processing plant, containing about 25% H.sub.2 SiF.sub.6 and a small percent of heavies (mostly H.sub.2 SO.sub.4 and H.sub.2 PO.sub.4), is fed to a distillation column in an attempt to distill off water and form a more concentrated aqueous solution of H.sub.2 SiF.sub.6. It is soon found that a grainy form of SiO.sub.2 is being formed and is plugging the overhead condenser and lines and aqueous H.sub.2 SiF.sub.6 is coming over. This is evidence that some of the H.sub.2 SiF.sub.6 is undergoing decomposition to SiF.sub.4 and HF, and the SiF.sub.4 is reacting with water in the overhead to give SiO.sub.2.

#The same waste stream, with about 10% HF added to it, is fed to the distillation column. Excess water and HF distill over leaving an azeotrope containing about 36% H.sub.2 SiF.sub.6, about 10% HF and about 54% water which boils at about 116.degree. C. No decomposition of the H.sub.2 SiF.sub.6 is found and no SiOf.sub.2 is formed. The ternary azeotrope is then distilled from the higher boiling heavies and during the distillation of the azeotrope, no decomposition is found and formation of unwanted SiO.sub.2 is avoided.

#The azeotrope is fed into the flame in a combustion chamber where a mixture of hydrogen and air is burned. The hot gases from the combustion chamber are immediately passed through a solids separator where the fumed SiO.sub.2 product is collected and removed. The HF and any other fluoride values present in the gases which have passed through the separator are cycled back to the aqueous waste stream for further use in an azeotropic mixture.