PREPARATION AND PROPERTIES OF UNIFORMLY COATED INORGANIC COLLOIDAL PARTICLES .2. CHROMIUM HYDROUS OXIDE ON HEMATITE

Mixed or complex colloidal matter has been of interest in various applications, such as in fabrication of composite materials. For certain uses it maybe advantageous to deposit uniform layers of a given substance on the particles of another kind to achieve a modification of the surface charge characteristics and/or other properties (magnetic, optical, adsorptive, density, etc. ) For instance, several process have been developed for coating pigments. Spindle-type hematite particles of narrow size distribution were coated with uniform layers of chromium hydrous oxide by using a 'forced hydrolysis-precipitation' procedure. The coating process can be enhanced either by aging the chrom alum solution or by pretreating it with an addition of base. The subsequent heating of such solutions in the presence of hematite 'cores' yielded unjiformly coated particles if the available surface areas of the dispersed solids and the concentration of the chromium complex solutions were in appropriate ratios. Various properties of the coatedsolids are described.