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Abstract

Precipitated amorphous silica useful for manufacturing microporous battery separators is prepared by establishing an aqueous solution of alkali metal silicate having an alkali metal oxide concentration of (a) from 2.1-2.6 grams per liter and a temperature of from about 179.degree. F. to 186.degree. F., or (b) from 5.6 to 7.2 grams per liter and a temperature of from 190.degree. to 198.degree. F.; adding from (a) 14.5 to 19 times or (b) 2 to 5 times respectively the amount of alkali metal silicate initially present in the initial aqueous solution simultaneously with acidifying agent, e.g., sulfuric acid, to the initial aqueous solution while maintaining the initial alkali metal oxide concentration therein substantially constant; reducing the pH of the resulting slurry to from 8 to 9; ageing the slurry at precipitation temperatures for from 15 to 90 minutes; reducing the pH of the aged slurry to from 3.8 to 4.7 and then recovering, e.g., separating, the silica from its entraining slurry, washing the recovered silica, and drying the washed precipitated silica. Optionally, the recovered and dried silica is milled after drying to produce a product having a median agglomerate particle size of from 6 to 15 micrometers.

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