GB791993A - Improvements in or relating to filter aids and process of making the same - Google Patents

Abstract

A filter aid consists of the comminution product of an artificially-expanded multi-cellular perlitic mineral having an apparent bulk density of between 1 and 6 lbs./cu. ft., and containing preferably not more than 40 per cent by weight of particles smaller than 3 microns and not more than 20 per cent by weight of particles larger than 40 microns, at least 80 per cent by volume of the product having the property of subsiding completely in water and being substantially free of sealed or partiallysealed voids. The filter aid may be produced by expanding a perlitic mineral to produce a multi-cellular product having the above apparent bulk density and preparing from it, e.g. by crushing, a fragmental material whose particles are less than 149 microns in size. In one example, comminuted perlite is expanded at approximately 1700 DEG C. by being dropped directly into a flame and conveyed thence in a fraction of a second. A cooled air stream containing the particles is then fed directly into a high speed fan milling-blower and the milled product passed to a cyclone so regulated that the particle size of the product is less than 149 microns. In another example, cyclone fines from a perlite expansion operation designed to produce a major proportion of plaster or concrete aggregate (having an apparent bulk density of 7 to 15 lbs./cu. ft.) are milled and then classified so that the majority of the particles are between 2 and 40 microns (preferably more than 60 per cent by weight being in the range 3 to 10 microns). Alternatively, the fines may be subjected directly to air classification. Un-milled and un-classified fines may be used to form a loose filter bed per se. Volcanic ash expanded, comminuted and air-classified is another example of filter aid, but if the comminuting and air-classifying steps are omitted this material also may be used to form a loose filter bed.ALSO:A filter aid consists of the comminution product of an artificially-expanded multi-cellular perlitic mineral having an apparent bulk density of between 1 and 6 lbs./cu. ft., and containing preferably not more than 40 per cent by weight of particles smaller than 3 microns and not more than 20 per cent by weight of particles larger than 40 microns, at least 80 per cent by volume of the product having the property of subsiding completely in water and being substantially free of sealed or partially-sealed voids. The filter aid may be produced by expanding a perlitic mineral to produce a multi-cellular product having the above apparent bulk density and preparing from it, e.g., by crushing, a fragmental material whose particles are less than 149 microns in size. In one example comminuted perlite is expanded at approximately 1700 DEG C. by being dropped directly into a flame and conveyed thence in a fraction of a second. A cooled air stream containing the particles is then fed directly into a high speed fan milling-blower and the milled product passed to a cyclone so regulated that the particle size of the product is less than 149 microns. In another example cyclone fines from a perlite expansion operation designed to produce a major proportion of plaster or concrete aggregate (having an apparent bulk density of 7 to 15 lbs./cu. ft.) are milled and then classified so that the majority of the particles are between 2 and 40 microns (preferably more than 60 per cent by weight being in the range 3 to 10 microns). Alternatively the fines may be subjected directly to air classification. Un-milled and unclassified fines may be used to form a loose filter bed per se. Volcanic ash expanded, comminuted and air-classified is another example of filter aid, but if the comminuting and air-classifying steps are omitted this material also may be used to form a loose filter bed.