0191 -9067/81 /030259-11 $02.00/0 Copyright ® 1981 SUNS AT Energy Council PREPARATION OF CELL FEED MATERIALS FOR ALUMINUM AND MAGNESIUM PRODUCTION JAMES A. BARCLAY Division of Mineral Resources Technology Bureau of Mines U.S. Department of the Interior Washington, DC 20241 Abstract—Magnesium chloride for electrolysis to magnesium in a fused-salt bath is made by reacting seawater with milk of lime to precipitate magnesium hydroxide, separating the latter from the barren seawater, dissolving the hydroxide in hydrochloric acid, and crystallizing and drying the compound MgCI2-1.5H2O. Solar evaporation of Great Salt Lake brine, followed by spray drying, also is practiced. Dolomite may be calcined and ground to a fine powder to be mixed with ferrosilicon for reduction to magnesium in vacuum retorts by a nonelectrolytic process. Alumina for smelting to aluminum in Hall-Heroult cells is prepared from bauxite by pressure digestion with strong NaOH solution, thickening and filtration to separate a waste “red mud,” and crystallization of A12O3-3H2O in the presence of seed crystals. The A12O3-3H2O then is calcined to A12O3. The Bureau of Mines and other research on the recovery of alumina from nonbauxitic aluminous materials is discussed, and possible applications of acid leaching, alkaline sinter-caustic leaching, and carbochlorination processes to lunar materials are briefly evaluated. CELL FEEDS FOR MAGNESIUM PRODUCTION Magnesium is produced in the United States mainly by the fused-salt electrolysis of magnesium chloride obtained from seawater or brines, although some also is made by metallothermic reduction of magnesium oxide obtained from dolomite. A block flowsheet for the latter process is shown in Fig. 1. Cell feed preparation in this case involves nothing more than calcining the dolomite and pulverizing it to a fine powder. Magnesium chloride for the electrolytic process may be obtained from seawater or from lake or well brines. For all practical purposes, the sea is an inexhaustible source. If 90.7 million tonnes of magnesium per year was extracted from the sea for 1 million years, the concentration of magnesium in seawater would only drop from 0.13% to 0.12%. The Dow process for economic recovery of magnesium chloride from a solution as dilute as seawater is an excellent example of chemical engineering ingenuity, as shown in Fig. 2. The Freeport, Texas, plant of Dow Chemical Company originally used oyster shells dredged from Galveston Bay as the source of lime, as shown by the solid lines at the upper left in the figure. An alternative processing route is shown by the dashed line, where dolomite is calcined to furnish the lime. This technique has the advantage of recovering the MgO content of the dolomite along with that precipitated from the seawater.
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