High purity synthetic fluorite, process for preparing the same and apparatus therefor

A process for preparing a synthetic fluorite CaF2 comprising the following steps:

• preparing a solution of NH4F, having a concentration comprised from 15 to 30% by weight, by basic hydrolysis, at a pH value comprised from 8.5 to 9.5, of H2SiF6 with an aqueous solution of NH3 having a concentration comprised from 10 to 25% by weight;
• filtering said solution of NH4F so as to yield a substantially silica-free aqueous solution of NH4F;
• treating said substantially silica-free aqueous solution of NH4F with calcium hydroxide in an excess amount comprised from 0.01 to 0.5%, relative to the stoichiometric amount, so as to yield a dispersion which is maintained under stirring for a time comprised from 10 to 60 minutes at a temperature comprised from 40 to 90° C.;
• filtering the latter solution so as to obtain the synthetic fluorite in the form of a slurry having a moisture content comprised from about 30% to about 50% by weight;
• subjecting said slurry to a drying-granulation step so as to yield an acid grade synthetic fluorite CaF2 in the form of granules with an average particle size greater than 1 mm for at least the 50% of the same.