LITHOPONE

Lithopone is a mixed zinc sulfide-barium sulfate brilliant white pigment that contains about 30% zinc sulfide. The original light sensitiveness of this pigment has been mitigated by purification and by the addition of such agents as polythionates and cobalt sulfate.

Lithopone is manufactured by a process (Fig. 1) in which barium sulfide solution is prepared by reducing barite ore (BaSO ₄) with carbon and leaching the resulting mass.

Figure 1: Manufacture of lithopone.

Scrap zinc or concentrated zinc ores are dissolved in sulfuric acid, the solution is purified, and the two solutions are reacted. A heavy mixed precipitate results that is 28 to 30% zinc sulfide and 72 to 70% barium sulfate.

This precipitate is not suitable for a pigment until it is filtered, dried, crushed, heated to a high temperature, and quenched in cold water. The second heating in a muffle furnace at 725 C produces crystals of the right optical size.

Lithopone is used in water-based paints because of its excellent alkali resistance. It is also used as a whitener and reinforcing agent for rubber and as a filler and whitener for paper.

MAGNESIUM

Magnesium occurs in seawater and in ores such as dolomite (CaCO ₃ MgCO ₃), magnesite (MgCO ₃), and carnallite (MgCl ₂ KCl 6H ₂O).

Magnesium can be made by several methods (Fig. 1), but the most common method of manufacture is by the electrolytic process, as for example the electrolysis of magnesium chloride.

Figure 1: Process for the production of...