Preliminary roasting at a moderate temperature is the first step in the hydrochloric acid process developed at the National Bureau of Standards for the production of aluminum from clay. The powdered clay is stirred by rabble arms (visible within the furnace door) as it passes downward in the triple-hearth Wedge furnace. Roasting increases the solubility of the alumina content so that it may be dissolved out with dilute hydrochloric acid and thus separated from the silicon, which remains as silica, insoluble in acid.
National Institute of Standards and Technology
National Institute of Standards and Technology