Particle geometry pigments

The surface area of a pigment, however, is not a definitive value it is controlled by factors such as the method of determination and the experimental parameters. Depending on the procedure, the experimental results may either reflect the exterior geometry of the pigment particles or also account for additional interior surface area, which is hard to define. Parameters such as the molecular size of the ad-... 

The zinc salt and BaS solutions are mixed thoroughly under controlled conditions (vessel geometry, temperature, pH, salt concentration, and stirring speed, see (a) in Fig. 20). The precipitated raw lithopone does not possess pigment properties. It is filtered off (b2) and dried (c) ca. 2 cm lumps of the material are calcined in a rotary kiln (d) directly heated with natural gas at 650-700 °C. Crystal growth is controlled by adding 1-2 wt% NaCl, 2 wt % Na2S04 and traces of Mg2 + (ca. 2000 ppm), and K+ (ca. 100-200 ppm). The temperature profile and residence time in the kiln are controlled to obtain ZnS with an optimum particle size of ca. 300 nm. 

Particle size is generally measured by sedigraph, which calculates a particle s size based on relative settling rate. Though this method assumes a spherical geometry (and very few papermaking minerals are spherical in nature), it is the most commonly used method of particle size measurement within the pigment industry. 

The publication B.H. Kaye, "Characterizing The Structure Of Fumed Pigments Using The Concepts Of Fractal Geometry", in press. Particle and Particle Systems Characterization contains many references leading into the specialist literature on Size Characterization. 

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