Process for producing coated bismuth vanadate yellow pigments

A process for producing coated bismuth vanadate yellow pigments comprising:

• a) core particles of a yellow-coloring bismuth vanadate, said core particles having an external surface which is activated with a phosphate-containing compound;
• and
• b) a coating which is located on top of the activated external surface of the core particles, said coating comprising at least one layer of at least one material selected from the group consisting of aluminum phosphate, aluminum/zinc phosphate or silicon oxide;
• wherein at least 99 wt. % of the core particles have a diameter that is from 0.2 .mu.m to 3.5 .mu.m;
• comprising the following steps:
• (a) subjecting a reaction mixture comprising a suspension in nitric acid of bismuthyl nitrate and vanadium (V) oxide, or a suspension in nitric acid of bismuth (III) oxide and vanadium (V) oxide, to stirring to form bismuth vanadate particles;
• (b) terminating the stirring and holding the reaction mixture for a period of time;
• (c) filtering off the bismuth vanadate particles;
• (d) washing the bismuth vanadate particles;
• (e) dispersing the washed bismuth vanadate particles in a hot, phosphate-containing, aqueous solution to activate the surface of the particles;
• (f) filtering off the activated bismuth vanadate particles;
• (g) washing the activated bismuth vanadate particles;
• (h) dispersing the activated bismuth vanadate particles in water to form a suspension;
• (i) precipitating at least one material selected from the group consisting of hydrated aluminum phosphate, aluminum/zinc phosphate or silicon oxide onto the external surface of the activated bismuth vanadate particles in the suspension to form said coating;
• (j) filtering off the coated bismuth vanadate particles;
• and
• (k) washing and drying the coated bismuth vanadate particles, wherein said hot, phosphate-containing, aqueous solution is at a temperature of about 85.degree.-90.degree. C.