Process for improving rheology of clay slurries

Abstract

In the process for producing a refined kaolin clay pigment by forming a crude kaolin clay into an aqueous slurry, performing a particle size classification, and subjecting the slurry to reductive bleaching with an alkali metal hydrosulfite; a method is disclosed for reducing the viscosity of a slurried product including the refined pigment, which method comprises adding a source of barium ion to the said bleached slurry, to thereby precipitate at least the sulfate ion present in that slurry from oxidation of the hydrosulfite ion during the bleaching step. The barium ion may be derived, e.g., from barium carbonate, which is added while the slurry preferably includes at least a 50% solids content by weight. The bleached slurry may be one which has been partially purified by an initial flotation, and which includes at least 99% by weight of particles less than 5 microns E.S.D., with substantially 100% by weight of the particles having an E.S.D. less than 10 microns. In this instance, the slurry may also include sulfate ion resulting from acidification of the float cell underflow with sulfuric acid prior to bleaching, as well as further sulfate ion resulting from the addition of alum prior to the filtering step which preferably pecedes the barium ion addition. The barium ion serves to precipitate the sulfate ion derived from all of these sources. The barium ion may also be added to a kaolin slurry as part of a high solids processing sequence, wherein a classification and a bleaching step are each conducted at a solids content between 60 and 75%, and at an alkaline pH.