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Process for preparing ultra-white alumina trihydrate

Image Number 3 for United States Patent #5342485.

A process for preparing an ultra-white alumina trihydrate having a G.E. brightness of at least about 98%. The product is alumina trihydrate that has been rehydrated with coprecipitation, and the crystal phase of the product is primarily gibbsite in thin platelet form having a ratio of thickness to diameter of about 0.2 to about 0.3. The product is prepared by flash activation of a white initial alumina trihydrate, addition of the activated material to a purified sodium aluminate liquor having an alumina to caustic ratio of about 0.5, and rehydrating and precipitating the alumina at ambient temperature under agitation for about 48 hours, followed by washing and drying of the resulting precipitate. The material is especially useful as a filler or coating in papermaking, and it can be substituted for a portion of titanium dioxide pigment in the ratio of 1:1, and can be combined with titanium dioxide as a pigment mixture, either in slurry form or in dry form. The pigment mixture is added to the papermill machine chest to enhance optical properties beyond those achieved by either pigment alone.

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