alkaline earths, less frequently alkali montmorillonite clays or siliceous rocks, used to decolorize various substances, mainly liquids, as well as to remove harmful and contaminating impurities. The use of bleaching clays is based on their capacity, either in the natural or activated state, to absorb pigments, slime, sediments, tars, and the like.
Bleaching clays consist mainly of clay minerals (montmorillonite, beidellite, and saponite) with admixtures of quartz, feldspar, biotite, pyroxene, and other mineral fragments. The predominant particle size is less than 0.01 mm. The clays are distinguished by a high content of water, two-thirds of which is evaporated at 110°C, and by highly pronounced sorption properties. Bleaching clays are found mainly in Cretaceous, Paleogene, and Neocene deposits in areas of volcanic activity. They are formed as a result of the hydrochemical alteration of volcanic rocks, such as tuff and ash.
Bleaching clays are used mainly in the refining and cracking of petroleum. In this process they act simultaneously as catalysts that accelerate the breakup of heavy hydrocarbons into gasoline and as adsorbents that retain harmful impurities on their surface.
In addition to the clays, tripoli and opoka are used in the purification of petroleum products, vegetable oils, fats, vinegar, wines, and fruit juices; the sugar industry uses mainly diatomites. Unlike the clays, tripoli and opoka do not acquire improved adsorption properties after activation by acids.
The wide use of bleaching clays in the manufacture of cloth prior to the 1830’s gave rise to the term “fuller’s earth.” Varieties of bleaching clays usually bear local names: kil (Crimea), gumbrin and askanit (Soviet Georgia), giliabi (bentonite; Azerbaijan), and floridin (USA).