Lithopone and Zinc Sulfide Pigments

The lithopones are manufactured by mixing zinc sulfate and barium sulfide solutions  

The zinc salt solutions, which currently mainly come from zinc and copper smelters, contain impurities which have to be completely removed prior to use, because all heavy metals form colored sulfides. For the purification step the solution is oxidized with chlorine (pH 4), which precipitates most of the iron and manganese as the oxide-hydrate and part of the cobalt, nickel and cadmium present as their hydroxides. In the second step, the elements nobler than zinc (Ni, Co, Cd, In, Tl, Pb, Cu, Ag) are precipitated as their metals by adding zinc dust and the metals returned to the copper smelters for noble metal extraction. After purification the solutions are adjusted to a particular zinc content. Mixtures of zinc sulfate and zinc chloride solutions are used for manufacturing lithopone types with more than 30% ZnS. 

Barium sulfide solutions are obtained by reduction of low silica natural heavy spar with coke or coal in rotary kilns at ca. 1250°C and leaching with water. Since all the heavy metal sulfides remain in the residue, the filtered solution can be directly used for the precipitation of the zinc salt. 

Purification of the ZnS04/ZnCl2 solution by removal of foreign metals  

Light stability of lithopones increased by doping with 0.02 to 0.5% Co 

---