Precipitation (or double-decomposition) Reaction

The fatty acid is first saponified in relatively large volume of heated water with an equimolar to a slight excess quantity of a strong alkali solution (sodium hydroxide, caustic potash solution, ammonia). The resulting alkali stearate is soluble in water. Particle size, surface area and particle morphology or shape are influenced by the concentration of reactants and precipitation rates. In general, the higher the dilution of the saponified fatty acid, the smaller the particles and the larger the surface will be. These properties determine the bulk (apparent) density, which is typically low for precipitated metallic stearates. The desired water-insoluble metallic stearate is subsequently produced by adding a metal salt solution (Ca or Zinc Chloride). The water-soluble salts such as sodium chloride or sodium sulphate, generated as by-products of the precipitation reaction, must be removed from the metallic stearate by filtration and washing. Finally, the resulting wet cake must be dried and the product deagglomerated or milled. This double-decomposition reaction typically produces very light, fine powders with a large surface area and a more platelet morphology. These types of metallic stearates are used in applications requiring fine particle size and high surface area for the best lubrication and release properties and where special emphasis is placed on good hydrophobic properties.