Lead chromate pigment with low soluble lead content - by pptn. in acid medium in presence of chloride and coating (NL150776)

摘要

Prodn. of a pigment contg. =50% PbCrO4 with 5 wt.% acid-soluble Pb (calc. as PbO) by combining aq. solns. contg. Pb2+ ions and CrO4= ions to form a ppte. contg PbCrO4 and maintaining an acid pH during pptn., then coating the ppte particles by depositing a coating material, adjusting to a pH 7, sepg. the ppte. and converting to the pigment. Pptn. is carried out without previous addn. of a buffer, in the presence of Cl- ions and opt. ions contg. Al, by mixing an aq. soln. contg. Pb2+ ions with an aq. soln. contg. CrO4=, SO4= and opt. MoO4= ions, an excess of Pb2+ ions being maintained during pptn., and the ppte. particles are given a coating contg. Si and Sb. The pigments combine a low acid-soluble Pb content with the desired tone and quality, comparable with those of pigment produced in the presence of an added buffer. The coating contg. Si and Sb greatly improves the resistance to light, weathering and industrial atmos.