1. Introduction
At present, natural minerals as fillers are widely used in coatings,
plastics, paper, ink and daily chemical industry [1–3]. The whiteness
is one of the most important properties of natural inorganic fillers.
Typically, carbon and iron depress the whiteness of kaolin, talc, pyrophyllite,
calcium carbonate and other natural minerals. Calcination at
high temperature is the best way to eliminate carbon. The iron is usually
removed from natural minerals by magnetic separation, acid
pickling, reduction, oxidation, flotation and other methods