低白色度石灰石粒子への炭酸カルシウム被覆による高白色度化

概要

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Calcium carbonate powder with high value of whiteness parameter is widely used in various industrial applications such as paper, plastics, glass etc. The value of the whiteness parameter of the powder mainly depends on the whiteness of limestone used as raw material. White limestone is formed through lithological re-crystallization by geological thermal-metamorphism, thus, the reserves of white limestone are limited and regionally maldistributed in Japan. In turn, the production of calcium carbonate products with the high whiteness is viable only in some areas. The native carbon (eg. graphite) as well as sulfide and oxides species, blamed for giving the ordinary limestone a gray color, are not easy to be removed from limestone particles. With this in mind the authors are putting forward a process for production of white calcium carbonate by using ordinary gray colored limestone found in many areas throughout Japan. The process is carried out in two steps: 1. reducing the coloring substances, contained in limestone particles, by heating. 2. covering the surface of limestone particles, mainly calcite, with crystallized white calcium carbonate. The experimental results point out that the whiteness of calcium carbonate particles is considerably increased by using this method, suggesting that the ordinary gray colored limestone can be used to produce high whiteness calcium carbonate products.