Lithopone

History

Lithopone was discovered in the 1870s by DuPont. It was manufactured by Krebs Pigments and Chemical Company and other companies. The material came in different "seals", which varied in the content of zinc sulfide. Gold seal and Bronze seals contain 40-50% zinc sulfide, offering more hiding power and strength. Although its popularity peaked around 1920, approximately 223,352 tons were produced in 1990. It is mainly used in paints, putty, and in plastics.

Stability and darkening

Although barium sulfate is almost completely inert, zinc sulfide degrades upon exposure to UV light, leading to darkening of the pigment. The severity of this UV reaction is dependent on a combination of two factors; how much zinc sulfide makes up the pigments formulation, and its total accumulated UV exposure. Depending on these factors the pigment itself can vary in shade over time, ranging from pure white all the way to grey or even black. To suppress this effect, a dopant may be used, such as a small amount of cobalt salts, which would be added to the formulation. This process creates cobalt-doped zinc sulfide. The cobalt salts help to stabilize zinc sulfide so it will not have as severe a reaction to UV exposure.

Production

Lithopone is produced by coprecipitation of barium sulfate and zinc sulfide. Most commonly coprecipitation is effected by combining equimolar amounts of zinc sulfate and barium sulfide: : BaS + ZnSO4 → ZnS·BaSO4 This route affords a product that is 29.4 wt % ZnS and 70.6 wt % BaSO4. Variations exist, for example, more ZnS-rich materials are produced when zinc chloride is added to the mixture of zinc sulfate and barium sulfide.

Barium sulfide is produced by carbothermic reduction of barium sulfate. Zinc sulfate is obtained from a variety of zinc products, often waste, by treatment with sulfuric acid. John L. Mitchell created this method of calcination for production of lithopone in 1918 in hopes of increasing the quality of lithopone while reducing and controlling the formation of zinc oxide waste. Calcining is a thermal treatment process applied a solid material in order to cause thermal decomposition at below the melting point of the product. It begins with a zinc sulfate solution made by the reaction of sulfuric acid and zinc oxide then compressed to obtain the solution. A precipitation of raw materials: barium sulfide and zinc sulfate is dried then thoroughly grounded. The powder is then placed in an airtight furnace and heated from 200 to 950°C. The key is the lack of oxygen preventing the formation of zinc oxide from the desired zinc sulfide therefore maintaining the density and purity of lithopone .

ZnO + H2SO4 + 6 H2O → ZnSO4(H2O)7 ZnSO4 + BaS → ZnS + BaSO4

--

Safety

Lithopone is rather nontoxic, due to the compound's extreme insolubility. It has been used in medicine as a radiocontrast agent. Lithopone is allowed to be in contact with foodstuffs in the US and Europe.

References

Historical references

• .
• .
• .
• .

References

1. "Krebs Pigment & Chemical Company". [[DuPont]].
2. (1929). "Lithopone Composition and Process of Making Same".
3. Völz, Hans G. ''et al.'' "Pigments, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry, 2006 Wiley-VCH, Weinheim. {{doi. 10.1002/14356007.a20_243.pub2.