fused-salt electrolysis of MgCl2; [2] silicothermic reduction of MgO; and [3] car- bothermic reduction of MgO. There are two distinct electrolytic processes in use: [1] the Dow seawater process using partially dehydrated cell feed, MgCl2-1.5H2O, and [2] the process developed in Germany by I. G. Farben which requires anhydrous MgCl2 as cell feed. The silicothermic process using ferrosilicon as the reductant, was developed in Canada as a commercially viable process by L. M. Pidgeon. It was a batch process using a relatively small externally-heated reactor; two different versions of it are now in use. The Magnetherm process developed in France uses a fluid-slag continuous reactor with a batch-type magnesium condenser. The Amati process, used at Bolzano, Italy, has not been described in the literature but is known to be a batch process using an internally-heated reactor much larger than the Pidgeon reactor. The carbothermic process was developed in Austria by Hansgirg; it is described only in the patent literature and in a Bureau of Mines report. This report presents a detailed analysis of operating variables and costs based on a World War II production plant. This process is no longer used any place in the world. All these processes were used in the USA and various other countries to meet the inordinately large demand for magnesium during World War II. Following the war, the demand dropped precipitously and competitive forces completely eliminated the use of thermal reduction processes in the USA for many years. The Dow SeawaterlElectrolytic Process A schematic flow diagram of the Dow seawater/electrolytic process is shown in Fig. 1 (3). The principal raw materials for this process are seawater, dolomite, salt, and fuels. A calcia-magnesia mixture is obtained from calcination of the dolomite, which is mixed with seawater to produce a filterable Mg(OH)2. The Mg(OH)2 is settled, filtered, and neutralized with HC1. The solution is evaporated and spray- dried to produce a cell feed of the approximate composition MgCl21.5H2O. The Dow cell is a steel pot which together with steel baffles close to counterelectrodes (anodes) is made the cathode for magnesium reduction. Consumable graphite anodes project through a cover into the cell and can be moved downward as they are consumed. The fused (700 °C) electrolyte is composed of 20% MgCl2, 20% CaCL, and 60% NaCl. The magnesium metal is collected at the top of the cell. Cathode current efficiency is 70% to 80% and the power requirement is about 18.5 kWh/kg (8.4 kWh/lb) of magnesium produced. Effluent gases are chlorine, HO, and water vapor which all go to HC1 makeup for neutralizing Mg(OH)2. The energy consumed is discussed in a later section. Anhydrous MgCl2 Electrolytic Process With the exception of Dow Chemical Company all other electrolytic magnesium producers use the I. G. Farben process or some modification of it. Figure 2 is a flow diagram of the process. The cell feed is anhydrous MgO, which greatly decreases the consumption of the graphite anodes. Simple drying does not produce anhydrous MgCl2 as is evident from the Dow Process, and MgO and MgOCl2 are formed before water is eliminated. I. G. Farben chlorinated MgO to obtain anhydrous MgCl2, and this process is presently used by Norsk Hydro. Briquetted MgO and carbon are chlorinated in electrically heated vertical shaped furnaces at 1100 to 1200 °C (2010 to 2190 °F), and the molten MgCL tapped from the bottom. Anhydrous MgCl2 may also be produced by chlorinating partially dehydrated
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