| 摘要 |
<p>In the production of a synthetic fluorspar of low silicon dioxide content wherein hexafluosilicic acid is added in an amount ranging from slightly below up to about stoichiometric amount to an aqueous calcium carbonate suspension at a temperature from about 0 DEG to 40 DEG C., and the calcium fluoride formed is separated off from the dilute silica sol and optionally washed out, the improvement which comprises reacting the calcium carbonate with the hexafluosilicic acid in such a way that the pH-value of the fully reacted solution amounts to between about 3.6 and 4.6, and adding a fast-reacting proton acceptor to the reaction mixture immediately before separation of the calcium fluoride in amount sufficient to raise the pH to about 5.0 to 6.4, preferably about 5.8 to 6.2. Suitable proton acceptors include the alkali metal and ammonium hydroxides carbonates and hydrogen carbonates or alkaline earth metal hydroxide. Anionic or non-ionic dispersants may also be added and the calcium fluoride is separated by decanting or centrifugation within about 1 second to 10 minutes, preferably about 4 to 10 seconds, after raising the pH. The separated calcium fluoride may be washed to remove silica sol therefrom and then again be treated to raise its pH before separation.</p> |
