Magnesium, Its Manufacture and Alloys

Abstract

Magnesium, Its Manufacture and Alloys WILLIAM K. ZINSZER, Permanente Metals Corporation, Permanente, California Magnesium, although eighth in abundance among the elements, does not exist in the free state. It is found in abundance across the earth's surface in the form of magnesite, which is mainly magnesium carbonate and calcium carbonate; as brucite, the hydroxide of magnesium; as magnesium chloride in brines; as carnalite, a double salt of magnesium and potassium chlorides; as serpentine, magnesium silicate; and, in sea-water as a small percentage of magnesium chloride. It was first isolated by the Englishman, Sir Humphrey Davy, in 1860. As recently as the 1920's however, work with the metal had scarcely passed the laboratory stage. Due to the character of magnesium oxide and the salts of magnesium, various methods of reducing the metal have been developed. Each of the methods obviously has its distinct advantages and disadvantages; they may be classified as follows: electrolytic, carbo-thermic, and silo-thermic. The electrolytic process, developed in this country by the Dow Chemical Company, was initially developed by I. G. Farbenindustrie in Germany and improved by Magnesium Elektron, Ltd., in England. The Dow process, as it is referred to in this country, grew out of the development of a cheap raw material in the magnesium chloride liquor residue from the brine treatments designed to use this material at Midland, Michigan. Its reduction to an almost anhydrous condition using dry hydrogen chloride gas was discovered and a cell was then developed capable of electrolyzing the salt. At the Dow Freeport plant in Texas, magnesia is recovered from salt water using lime to precipitate the magnesia. This substance is then converted to hydrous magnesium chloride by the addition of hydrochloric acid; changed to the almost anhydrous condition and the resulting salt used as the electrolyte in the cell where further reduction to the metal takes place through electrolysis. The Basic Magnesium Company at Las Vegas, Nevada, uses the electrolytic process. The magnesite ore is calcined to a degree which leaves a highly reactive magnesium oxide. This oxide is pelletted with carbonaceous material selected to give a high degree of porosity, and fed into chlorinating furnaces in which the magnesium oxide is converted to absolutely anhydrous Mg C12, the electrolyte for the cells. The electric current melts and decomposes the salt in the presence of the carbonaceous material, liberating chlorine gas at the anode. The residue from the impurity in the ore is filtered and recirculated until the metal floats to the surface at the cathode where it is dipped out. The carbo-thermic process used by Permanente Metals Corporation at Permanente, California (also called the Hansgirg process) is one of the most widely discussed processes at the present time. The process which uses carbon as a reducing agent will probably be a cheap method of extraction when completely developed since it employs the simple reaction: MgO+C+XA-Mg+CO. This reaction is reversible and requires very special equipment to achieve the temperature of 2200? necessary to permit reduction toward the right. Very