Process of refining sodium hexafluorosilicate containing gypsum

A process of refining a crude sodium hexafluorosilicate containing calcium sulfate as an impurity, comprising the steps of:

• (a) preparing an aqueous slurry of the crude sodium hexafluorosilicate;
• (b) adding an alkali metal compound, which can neutralize sulfuric acid and which is selected from the group consisting of sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate, potassium hydrogen carbonate and lithium hydroxide, to said slurry to form a solid phase consisting essentially of calcium fluoride and silicon dioxide through decomposition of sodium hexafluorosilicate and calcium sulfate accompanied by the formation of a water soluble sulfate by neutralization of sulfuric acid formed during said decomposition with said alkali metal compound;
• (c) separating said solid phase from the product of step (b);
• (d) making said solid phase react with an acid, which can form a water soluble calcium salt, in an aqueous solution comprising sodium ions;
• and
• (e) recovering solid phase sodium hexafluorosilicate from the product of step (d).