A process for reducing the concentration of one or more arsenic-containing compounds in an aqueous solution comprising at least one fluoroacid, which process comprises: (i) contacting the aqueous solution with an oxidising agent to produce one or more Asv-containing compounds; and (ii) removal of precipitated arsenic-containing compounds; wherein the process comprises a step (iii) the addition of an aqueous alkali solution or slurry, which may take place after step (i) and before step (ii) or after step (ii).

The present invention provides a nonaqueous electrolytic solution that provides an electric double layer capacitor having excellent durability. The nonaqueous electrolytic solution of the present invention is a nonaqueous electrolytic solution for electric double layer capacitors prepared by dissolving a quaternary ammonium salt as an electrolyte in a nonaqueous solvent, and the nonaqueous electrolytic solution has an alkali metal cation concentration of 0.1 to 30 ppm.

Provided are high purity tungsten pentachloride, and a method for obtaining such high purity tungsten pentachloride at a high yield and in an efficient manner. Tungsten pentachloride in which a total content of metal impurities excluding Sb, Ti, and As is less than 10 wtppm is obtained by uniformly mixing one or more types of reducing agents selected from Sb, Ti, and As and tungsten hexachloride at a molar ratio of 1.0:2.0 to 1.0:5.0 (reducing agent/WCl.sub.6 ratio) in an inert atmosphere to obtain a mixture, heating and reducing the mixture for 1 to 100 hours in a temperature range in which a chloride of tungsten and the reducing agent becomes a liquid phase to obtain a reduced product, heating the reduced product for 1 to 100 hours at 100 Pa or less and in a temperature range of 90 to 130° C., and performing reduced-pressure distillation thereto to obtain a reduced-pressure distilled product, heating and sublimating the reduced-pressure distilled product for 1 to 100 hours at 100 Pa or less and in a temperature range of 130 to 170° C., and performing sublimation purification of achieving precipitation at 70 to 120° C.

Disclosed is a process for producing a fluoride gas that can produces fluoride gases such as BF.sub.3, SiF.sub.4, GeF.sub.4, PF.sub.5 or AsF.sub.5 at a reduced production cost in a simple manner. The process is characterized in that a compound containing an atom, which, together with a fluorine atom, can form a polyatomic ion, is added to a hydrogen fluoride solution to produce the polyatomic ion in a hydrogen fluoride solution and to evolve a fluoride gas comprising the fluorine atom and the atom that, together with the fluorine atom, can form a polyatomic ion.

The present invention provides a nonaqueous electrolytic solution that provides an electric double layer capacitor having excellent durability. The nonaqueous electrolytic solution of the present invention is a nonaqueous electrolytic solution for electric double layer capacitors prepared by dissolving a quaternary ammonium salt as an electrolyte in a nonaqueous solvent, and the nonaqueous electrolytic solution has an alkali metal cation concentration of 0.1 to 30 ppm.

A process for reducing the concentration of one or more arsenic-containing compounds in an aqueous solution comprising at least one fluoroacid, which process comprises: (i) contacting the aqueous solution with an oxidising agent to produce one or more Asv-containing compounds; and (ii) removal of precipitated arsenic-containing compounds; wherein the process comprises a step (iii) the addition of an aqueous alkali solution or slurry, which may take place after step (i) and before step (ii) or after step (ii).

Provided are high purity tungsten pentachloride, and a method for obtaining such high purity tungsten pentachloride at a high yield and in an efficient manner. Tungsten pentachloride in which a total content of metal impurities excluding Sb, Ti, and As is less than 10 wtppm is obtained by uniformly mixing one or more types of reducing agents selected from Sb, Ti, and As and tungsten hexachloride at a molar ratio of 1.0:2.0 to 1.0:5.0 (reducing agent/WCl.sub.6 ratio) in an inert atmosphere to obtain a mixture, heating and reducing the mixture for 1 to 100 hours in a temperature range in which a chloride of tungsten and the reducing agent becomes a liquid phase to obtain a reduced product, heating the reduced product for 1 to 100 hours at 100 Pa or less and in a temperature range of 90 to 130 C., and performing reduced-pressure distillation thereto to obtain a reduced-pressure distilled product, heating and sublimating the reduced-pressure distilled product for 1 to 100 hours at 100 Pa or less and in a temperature range of 130 to 170 C., and performing sublimation purification of achieving precipitation at 70 to 120 C.

A process for purification of hydrofluoric acid reduces the content of heavy metals, including arsenic, to values lower than five parts per million, without using any chemicals and with an integrated design of hot and cold streams that provide low energy consumption. The process allows extraction of heavy metals, especially arsenic, with minimal waste generation and while maintaining an original oxidation state, which for the case of arsenic is +3, so that the residue can he converted into a product with commercial value, such as arsenious acid. The process includes operation of four systems, namely, a hydrofluoric acid purification system, an arsenic concentration system, a hot water system, and a cold water system. The extraction of heavy metals is performed by synchronized operations of these four systems.

The present invention relates to a method of treating lead anode slime having high fluorine and arsenic content, in particular to a method comprising smelting of the lead anode slime and cleaning the produced off gases in a one or more wet gas cleaning stages.