Process for purifying clay by selective flocculation

Claim 2

A process for producing a purified fine size coating grade clay pigment and a purified coarse size filler grade clay product from an impure sedimentary soft kaolin crude which contains colored titaniferous impurities which comprises forming a fluid alkaline Suspension of the crude clay without incorporating a polyanionic clay dispersing agent, removing grit from the suspension by sedimentation, increasing the pH of said suspension to a value in the range of about 9.5 to 11.5 by adding an alkali metal hydroxide compound unless the pH of the slip is already in said range, thereafter adding a polyanionic clay dispersing agent, reducing the pH of the suspension to a value in the range of 7.0 to 8.8 by adding a mineral acid, incorporating a synthetic organic weakly anionic partially hydrolyzed polyacrylamide flocculating agent, whereby clay particles are flocculated and impurities remain dispersed, separating the flocculated clay from the impurities, redispersing the flocculated clay, subjecting the redispersed flocculated clay to a hydraulic classification so as to separate the clay into a fine size coating clay fraction and a coarser size filler grade fraction and bleaching said fine size fraction of clay.

Claim 3

The process of claim 3 wherein said polyanionic clay dispersing agent is potassium silicate.

Claim 4

The process of claim 3 wherein said pH after addition of said alkali metal hydroxide is in the range of 10.0 to 10.5.

Claim 5

A process for producing a purified fine size coating grade pigment from an impure sedimentary soft kaolin crude which contains colored titaniferous impurities which comprises providing an aqueous pulp of the impure clay, without adding a polyanionic clay dispersing agent, adjusting the pH of said pulp to a value in the range of 9.5 to 11.5 by incorporating an alkali metal hydroxide, subjecting the alkaline pulp to hydraulic classification by sedimentation or centrifugation to recover a fine size fraction of clay in the form of an aqueous suspension, incorporating a polyanionic clay dispersing agent to deflocculate said suspension, decreasing pH to a value in the range of 7.0 to 8.8 by adding an acid, incorporating a synthetic organic weakly anionic partially hydrolyzed polyacrylamide as a flocculating agent, whereby clay particles are flocculated and impurities remain dispersed, separating the flocculated clay from the impurities, and bleaching the flocculated clay.

Claim 6

The process of claim 6 wherein said polyanionic clay dispersing agent is potassium silicate.

Claim 7

The process of claim 6 wherein said alkali metal hydroxide is sodium hydroxide and said polyanionic clay dispersing agent is potassium silicate.

Claim 8

The process of claim 6 wherein said pH after addition of said alkali hydroxide is in the range of 10.0 to 10.5.

Claim 9

The process of claim 6 wherein said clay contains a discrete iron oxide impurity and the pH of the pulp is reduced to a value above 8.5 by addition of said acid.