Importance

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Abstract

A method of making a composite pigment of precipitated calcium carbonate (PCC) and a silicon compound, in which the resulting composite pigment is endowed with an excellent combination of optical and mechanical properties. Among other things, a PCC/silicate composite pigment made according to this invention imparts increased bulk, light scattering power, porosity and printing properties to paper. The composite pigment material made according to this invention also is very competitive from the standpoint of its production cost. The method for making the composite pigment includes the step of introducing a soluble silicate compound into an aqueous medium containing a precipitate of calcium carbonate formed by carbonation of lime milk, and at a time when the calcium carbonate precipitation reaction has progressed to near completion. Then, an insoluble silicon compound is precipitated upon the precipitated calcium carbonate by carbonation of the reaction mixture in a manner in which the maximum temperature variation of the reaction mixture is kept less than 20.degree. C. The soluble silicate compound is added when the precipitation of the calcium carbonate has reached approximately 90% to less than 100% of completion. Further, the temperature of the reaction mixture during deposition of the silicon compound upon the precipitated calcium carbonate is at least 50.degree. C. or greater, and more preferably is kept between approximately 60 to 100.degree. C.

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First Claim