Abstract: An improved process is described for producing titanium dioxide from ilmenite-type ore. The ore is digested with aqueous hydrogen fluoride, the resulting liquid mixture treated with a water-soluble oxidizing agent, and the ferric iron impurities, in the resulting digestion solution, at a pH between about 1.0 and 6.0, are extracted out with a mono-or dialkyl, or mono- or di-(alkylphenyl) orthophosphoric acid, such as di-(2-ethylhexyl) orthophosphoric acid. The remaining solution is treated with ammonium hydroxide to precipitate hydrated titanium dioxide which is then calcined to form pigmentary titanium dioxide. Alkali metal and ammonium salts of the organophosphoric acid are also useful in the process.

Abstract: An improved process is described for converting aqueous fluosilicic acid waste product into anhydrous hydrogen fluoride and finely divided silica in which aqueous fluosilicic acid is pyrolyzed to produce silica and a dilute aqueous solution of hydrogen fluoride and fluosilic acid. The resulting dilute solution is subjected to electrodialysis, thereby obtaining a hydrogen fluoride concentration greater than that which characterizes the boiling point ridge which divides the phase diagram of the system hydrogen fluoride/fluosilicic acid/water, i.e. HF/H.sub.2 SiF.sub.6 /H.sub.2 O, into two regions. Anhydrous hydrogen fluoride is recovered from the resulting composition by distillation.

Abstract: Removal of sulfur dioxide from an aqueous solution containing the same is effected by subjecting the solution to electrodialytic water splitting. In particular SO.sub.2 is removed from dilute gas streams by means of alkaline solution scrubbing, regeneration of the scrubbing solution and liberation of concentrated SO.sub.2 by means of a membrane water splitter. Waste sulfate produced in the process may be converted to sulfuric acid in a separate membrane water splitter or otherwise processed by conventional means.

Abstract: An improved process is described for recovering TiO.sub.2, useful as a white pigment, from ilmenite-type ores, including the steps of digesting the ore with aqueous hydrofluroic acid, separating out iron impurities from the resulting solution, precipitating hydrated titanium dioxide from the iron-free solution with ammonium hydroxide, and calcining the precipitate to obtain pigmentary TiO.sub.2, wherein the improvement comprises subjecting aqueous solutions of by-product ammonium fluoride, formed in the process, to electrodialytic water-splitting to form an aqueous solution of ammonium hydroxide and an aqueous solution of hydrogen fluoride and recycling said aqueous ammonium hydroxide to the precipitation step and said aqueous hydrogen fluoride to the digestion step.

Abstract: Sulfur dioxide is separated from an aqueous solution containing the same by subjecting the stream to electrodialytic water splitting. In particular a novel method for removing SO.sub.2 from dilute gas streams by means of alkaline solution scrubbing, regeneration of the scrubbing solution and liberation of concentrated SO.sub.2 effected by means of a two-compartment membrane water splitter is provided. Optionally, waste sulfate produced in the process may be converted to sulfuric acid in a separate membrane water splitter or otherwise processed by conventional means.