Filtering process using inorganic synthetically prepared filter sand

Claim 1

In a process for removing solids from a liquid wherein said liquid is passed through a bed of filter media consisting of discrete, inorganic, artifically produced filter sand having the same chemical composition, the improvement comprising passing said liquid over artifically produced filter sand having grain densities of between about 1.0 and 2.2 grams per cubic centimeter, and grain diameters of between about 0.3 and 10 mm, wherein the grain density decreases with increasing grain diameter, backwashing said artifically produced filter sand and settling said artifically produced filter sand with sand grain diameters distributed from the largest to the smallest in the direction of liquid passage, said discrete, inorganic artifically produced filter sand consists essentially of a preliminary product mixed with a powder selected from the group consisting of amorphous particles, crystalline materials, and mixtures thereof, granulated with addition of water, dried, expanded at a temperature up to about 400° C, melting the outside at a temperature of about 700° C, and cooling to form spheres, said preliminary product produced by: (a) forming a solution containing in parts by weight about 1 part sodium silicate, having a molar ratio of Na 2 O:SiO 2 of 1:2.5 to 1:4.5, 0.02 parts to 0.8 parts of inorganic fibrous silicate or finely powdered silicate and 2-10 parts of water; (b) adding means for producing foaming gas to the solution; and (c) heating the solution to about 140° - 360° C to drive off any free water phase.