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Abstract

A finely powdery magnesium hydroxide especially suitable as a flame-retardant filler for plastic compounds, in which the particles are provided optionally with a thin coating of a surfactant. The grain size of the magnesium hydroxide, measured by laser diffraction, is below 10 .mu.m. The median value of the grain size is greater than 0.8 .mu.m, and the maximum of the median value of the grain size is 3 .mu.m. The contents of water-soluble ionic impurities, viz. Ca.sup.++, Na.sup.+, K.sup.+, SO.sub.4 --, Cl.sup.-, of the magnesium hydroxide are below the following limits (in parts by weight): Ca.sup.++ <1000 ppm, Na.sup.+ <20 ppm, K.sup.+ <20 ppm, SO.sub.4 --<1500 ppm, Cl.sup.- <1000 ppm. The contents of Mn, Cu and Ni are below the following limits (in parts by weight): MnO<100 ppm, NiO<100 ppm, CuO<10 ppm. The process for preparing the magnesium hyroxide of this invention starts with a magnesium oxide obtained by spray roasting a magnesium chloride solution from which previously foreign matter has been removed, adding water thereto for hydration, whereupon the resulting magnesium hydroxide is removed by filtration and the filter cake material is subjected to a post-wash with water.

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First Claim