Bleaching earth is made from naturally occurring minerals such as palygorskite, which is also known as attapulgite, sepiolite, bentonite, and other minerals that all belong to the aluminum silicate family. Bleaching earth, often called “fuller’s earth,” has been known and used for many years. The use of fuller’s earth for filtration and colour improvement of oils was identified and developed by W.B. Albright and H. Eckstein for N.K. Fairbanks & Company, Chicago, IL. Initially, in 1880, they developed a process to bleach cottonseed oil with fuller’s earth. Bleaching earths consist of natural clays, which have “bleaching activity,” and others that become active only after a specific treatment. The first activated bleaching earths were developed by Pfirschinger Mineralwerke, Kitzingen in 1907 and by Erdwerke Kronwinkl, Franz Schmidt & Co. in 1909.
The natural, neutral, or non-activated bleaching clays are derived from clay mineral deposits, which are merely dried, milled, and sieved to obtain a desired range of particle sizes. Natural weathering over a very long period has rendered the original mineral partly porous, and has given it some power of adsorption for pigments, soap, and so forth. Such material came into early use to cleanse fat from sheepskins (i.e. fulling); therefore, in England it became known as Fuller’s earth. This class of material is particularly mild in action, not promoting chemical changes in triglycerides, showing little tendency to split soap, and generally responding only modestly when bleached with acid to make it a more active adsorbent.
Throughout the world (with the exception of Antarctica), deposits of different forms of the clay mineral bentonite are found, usually near the surface and in seams up to a 15-feet thickness. The name “bentonite” is derived from the large deposits associated with Bentonite shale at Rock River, Montana. At least 85% of this material consists of forms of an aluminium silicate known as montmorillonite, which was identified as long ago as 1847 at Montmorillon, France. The activity of this form of bentonite changes little with acid leaching, and it is not the source of acid-activated bleaching earths.
A much more important variant, calcium montmorillonite or non-swelling bentonite [southern (United States) bentonite, Texas bentonite, etc.], lends itself very readily to leaching with mineral acid (HCl or H2SO4); its power of adsorption and catalysis is thereby dramatically increased.
Acid-activated clays were first produced in Germany around 1905; then several other countries around the world followed—this extension is continuing today. The approximate world production of 700,000 tons per year (2001) must be seen against a world production of total bentonite-type minerals of 6 million tons per year. The acid-activated clays are used in cleaning mineral oils as well as vegetable and animal fats; they are also used as catalysts in promoting organic reactions such as polymerization and isomerization, and may take the form of fixed beds or powdered additions.