Process for preparing silica pigment

Compacted silica powder having a density above 15 lbs./cu. ft and size less than 150 mesh is produced by heating a mixture of water and precipitated silica having an average ultimate particle size below 0.05 micron to evaporate water and form agglomerates, grinding the agglomerates to less than 150 mesh, and compacting the ground material to at least 15 lbs./cu. ft. bulk density. An aqueous slurry of precipitated silica is used, where the silica contains more than 85% SiO2, 3-14 moles bound water/mole SiO2, and 10-15% free water, has surface area 50-300 sq. m./gm., and may contain 1 mole alkaline earth metal, zinc, magnesium, or aluminium oxide per 10-150 moles SiO2. The aqueous slurry may have previously dried or partially dried SiO2 added during drying, which is carried out at 100-800 DEG C., and may be accompanied by tumbling, until the product contains 10-15% free water. The dried SiO2 is cooled below 50 DEG C. ground to pass, e.g. 325 mesh. The ground SiO2 is compacted: (i) by pressing in a mould; (ii) by pouring into a paper bag, reinforced with cardboard, in a mould and compressing under 140-150 lbs./sq. in., with further amounts of SiO2 being added as necessary to fill the bag; (iii) by pressing into the container for transport; or (iv) by passage between a pair of rolls coated with natural or synthetic rubber having a Durometer hardness 20-80. The compacted SiO2 is readily broken up to powder which is readily dispersible in natural, or synthetic rubbers, of which a number are specified. Specifications 756,857 and 756,877 are referred to.