Method for producing magnesium

#This invention relates to a carbothermic process for producing magnesium from magnesium oxide. More particularly, the present invention provides means for greatly decreasing the back-oxidation of magnesium vapor by carbon monoxide as the furnace gases are cooled, which has heretofore prevented successful commercial development of a carbothermic magnesium process.

#It has long been recognized among those skilled in the art that production of magnesium from magnesium oxide by reduction with carbon in an arc furnace is theoretically the most efficient and cheapest method for the commercial production of magnesium. In particular, it offers many advantages over metallothermic processes where a reducing agent such as silicon or aluminum requires an expensive preliminary operation requiring electric energy. In a carbothermic process, the magnesia-containing raw material is subjected to high temperature reduction by carbon in an arc furnace. The product initially produced comprises a vaporous composition which is theoretically approximately a 50--50 mixture of magnesium vapor and carbon monoxide gas. Thermodynamic calculations for the reaction show that the theoretical reaction equilibrium temperature at atmospheric pressure is about 1875.degree. C. and in order to carry out the reaction at a reasonable rate, it is has been found necessary to operate at temperatures on the order of 2000.degree. C. This thermodynamic constraint is undesirable since it causes extreme difficulty in cooling the furnace gases down to the equilibrium temperature of about 1875.degree. C., rapidly enough to prevent excessive back oxidation of magnesium by reaction with carbon monoxide to form magnesium oxide. The reaction in question, which is reversible as shown,

#Ti MgO+C.revreaction.Mg+CO

#FIG. 1 is a schematic view of one embodiment of this invention.

#FIG. 2 is a schematic view of second embodiment of this invention.

#As shown in FIG. 1, the process of this invention utilizes a furnace 10 having electrodes 12 which extends into the reaction bed 14 which is comprised mainly of magnesium oxide and a carbonaceous reducing material such a coke. The magnesium oxide and coke are introduced into the furnace 10 by means of conduit 16 which also is provided with means for closing the conduit to the atmosphere during reaction. The reaction bed 14 rests on hearth 18. When it is desired to initiate and maintain reaction in the reaction bed, electric arcs are generated between the electrodes 12 in order to effect reaction between magnesium oxide and the carbon to for magnesium vapor and carbon monoxide. During reaction, the vaporous composition comprising magnesium and carbon monoxide rises towards the top of the furnace and exits there from through opening 20. At the entrance 20, the vaporous composition generally is a temperature between about 2200.degree. C. and about 1900.degree. C. Thus, the temperature of the vaporous composition is within a sufficiently elevated range such that there is little or no back reaction between the vaporous magnesium and the carbon monoxide. Before the magnesium-carbon monoxide vaporous composition is cooled to a temperature below about 1900.degree. C., it is contacted with molten-magnesium which is heated to a temperature between about 800.degree. C. and 1100.degree. C., preferably between about 1000.degree. C. and about 1100.degree. C. in heater 22 and thereafter is introduced into opening 20 for intimate contact therein with the vaporous composition. In order to afford intimate contact between the liquid magnesium and the vaporous composition, the liquid magnesium is introduced as a spray. Furthermore, the liquid magnesium is introduced under conditions such that substantially all of the liquid magnesium is vaporized while minimizing the temperature to which the vaporized magnesium as subsequently heated by the vaporous composition of magnesium and carbon monoxide. By operating in this manner, substantial advantages are obtained as compared to the prior art proposals wherein cooling was effected with liquid magnesium in a relatively cooled conditions, e.g., 650.degree. C. to 670.degree. C. By operating in the manner described herein, the amount of heat extracted from the vaporous composition of magnesium and carbon monoxide is substantially equal to the heat of vaporization of the introduced liquid magnesium rather than relying solely upon reduction by transfer of sensible heat from the vaporous composition to the liquid magnesium. By utilizing the heat of vaporization to cool the vaporous composition as described herein, the vaporous composition removed from the furnace 10 is reduced in temperature to about the magnesium dew point, i.e., below about 1500.degree. C., preferably below about 1100.degree. C. and more preferably between about 1050.degree. C. and 1150.degree. C. substantially instantaneously. In contrast, prior art techniques utilizing low temperature liquid magnesium do not provide the desired substantial instantaneous reduction in temperature of the vaporous composition since these techniques rely upon a rise in the sensible heat of the liquid magnesium coolant which provides of far slower rate of heat extraction than the technique utilized in the present invention which relies upon the heat of vaporization of the liquid magnesium. Accordingly, the present invention provide a procedure wherein the temperature of the liquid magnesium is reduced from a high temperature range within which substantially no back reaction between carbon monoxide and magnesium is effected to a substantially lower temperature range wherein little or no back reaction of magnesium vapor occurs. The magnesium having a temperature within this lower temperature range then can be cooled at a slower rate utilizing transfer of sensible heat so as to condense and recover the magnesium.

#The vaporous composition resulting from the initial cooling step is passed from opening 20 through conduit 24 into scrubbing tower 26. Within scrubbing tower 26, the vaporous composition comprising magnesium and carbon monoxide is contacted with a molten flux composition for magnesium such as a conventional magnesium chloride composition.

#TABLE ______________________________________ Typical Flux Compositions (by weight)* Example 1 2 3 4 5 6 ______________________________________ MgCl.sub.2 22 10 -- 40 15 10 CaCl.sub.2 23 40 50 25 25 40 NaCl 54 30 30 20 40 20 KCl -- 20 20 15 20 30 alkaline earth chlorides 45 50 50 65 40 50 alkali metal chlorides 54 50 50 35 60 50 ______________________________________ *Other components, such as CaF.sub.2, LiF, or MgO may be present. See generally, for discussion of suitable flux compositions, Principles of Magnesium Technology, E. F. Ensley, ed., Pergammon Press (London, 1966), pp. 27-33, 76-78, 84-125. Note Flux #1 has a melting point below 400.degree. C. and high fluidity.