A method for producing a fluoride fluorescent material comprises: preparing fluoride particles having a composition containing at least one element or ion A selected from the group consisting of alkaline metal elements and NH.sub.4.sup.+, at least one element M selected from the group consisting of Group-4 elements and Group-14 elements, Mn.sup.4+, and F, in which a molar ratio of A in 1 mol of the composition is 2, a total molar ratio of M and Mn.sup.4+ is 1, a molar ratio of Mn.sup.4+ is in a range of more than 0 and less than 0.2, and a molar ratio of F is 6; subjecting the fluoride particles to a first heat treatment at a temperature of 500 C. or more in an inert gas atmosphere; washing the first heat-treated fluoride particles with a washing liquid; and bringing the washed fluoride particles into contact with a fluorine-containing substance and subjecting the resulting fluoride particles to a second heat treatment at a temperature of 400 C. or more.

A red phosphor that has optical characteristics and durability under high-temperature and high-humidity environments, and a method for producing the same. The red phosphor includes a Mn-activated complex fluoride represented by the following general formula (1) and bismuth:
A2MF6:Mn4+(1)
wherein A represents at least one alkali metal element selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, and M represents at least one tetravalent element selected from the group consisting of silicon, germanium, tin, titanium, zirconium and hafnium.

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).

A method for producing a fluoride fluorescent material comprises: preparing fluoride particles having a composition containing at least one element or ion A selected from the group consisting of alkaline metal elements and NH.sub.4.sup.+, at least one element M selected from the group consisting of Group-4 elements and Group-14 elements, Mn.sup.4+, and F, in which a molar ratio of A in 1 mol of the composition is 2, a total molar ratio of M and Mn.sup.4+ is 1, a molar ratio of Mn.sup.4+ is in a range of more than 0 and less than 0.2, and a molar ratio of F is 6; subjecting the fluoride particles to a first heat treatment at a temperature of 500? C. or more in an inert gas atmosphere; washing the first heat-treated fluoride particles with a washing liquid; and bringing the washed fluoride particles into contact with a fluorine-containing substance and subjecting the resulting fluoride particles to a second heat treatment at a temperature of 400? C. or more.

In particular, in alternative embodiments, the invention provides for a method to recover silicofluoride and phosphate species from wastewaters, or barometric condenser waters, that are typically utilized in wet-process phosphoric acid facilities. The species are recovered via a continuous ion exchange approach that allows for economic recovery of the materials and especially with the silicofluoride component allows for the production of valuable industrial materials such as hydrofluoric acid and other fluoride salts as well as industrial-grade precipitated silica materials. Return of the treated waste water to the phos-acid plant allows for optimization of reagent usage.

Provided is a method for producing a fluoride phosphor. The method includes: providing a first solution containing an element M.sup.1 containing at least one selected from the group consisting of group 13 elements, manganese, and fluorine, a second solution containing an element M.sup.2 containing at least one selected from the group consisting of group 4 elements and group 14 elements, and a third solution containing at least one selected from the group consisting of alkali metal elements; and adding the second solution and the third solution to the first solution at substantially the same time.

Provided are a liquid dispersion of fluoride particles, which has low viscosity and excellent dispersibility, and is suitable for producing an optical film such as an antireflection film; a method for producing the same; and an optical film using the same. The liquid dispersion of fluoride particles according to the present invention is that in which particles of a fluoride represented by the chemical formula A.sub.xCF.sub.y (wherein A represents sodium or potassium, C represents silicon or boron, x is 1 or 2, and y is 4 or 6) are dispersed in an aprotic organic solvent having a relative permittivity of 5 to 40, and the optical film according to the present invention is produced by using the liquid dispersion of fluoride particles.

In one aspect, a process for preparing a Mn.sup.4+ doped phosphor of Formula I is provided A.sub.x[MF.sub.y]:Mn.sup.4+ (I). The process includes combining a first aqueous solution including a source of Mn with a second solution including H.sub.2MF.sub.6 to form a third solution, and combining the third solution with a fourth solution including a source of A to form the Mn.sup.4+ doped phosphor, where A is Li, Na, K, Rb, Cs, or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Y, La, Nb, Ta, Bi, Gd, or a combination thereof; x is the absolute value of the charge of the [MF.sub.y] ion; and y is 5, 6 or 7. Methods, phosphors and devices are also provided.