Abstract: An economical process for treating the acidic pond water to be used in the internal grinding circuit of a wet process phosphoric acid plant. Water is added to a quantity of a hydratable calcium compound, such as quicklime, to form a slaked lime slurry. A sufficient quantity of the slaked lime slurry is reacted with the acidic pond water to obtain a low fluoride solution having a fluoride content less than about 300 ppm and a pH in the range from about 2.0 to about 3.0, preferentially in the range 2.6-2.8, whereby calcium fluoride is precipitated. The precipitate bearing low fluoride solution is then clarified and the precipitate removed as underflow from the clarifier and transferred to the gypsum pond. In the preferred embodiment, the clarified low fluoride water is fed to the ball mill and is mixed with the phosphate rock being crushed.

Abstract: Magnesium and aluminum impurities may be removed from wet process phosphoric acid by adding a fluoride ion donating compound, such as hydrofluoric acid, for example, to thereby provide about four fluoride ions for each aluminum ion. Also, preferably, an aluminum ion donating compound, such as alum is added to the wet process acid to bring the aluminum ion to magnesium ion ratio up to about two to one to facilitate formation of crystalline compounds having the general stoichiometry MgAl.sub.2 F.sub.8. The treated filter grade acid is then further treated in a crystallizer to form large crystals which will readily precipitate so the purified liquid phase, low in magnesium and aluminum containing impurities, may be separated from the crystalline phase.