A differential thermal analysis technique was used to study the carbothermic reduction of aluminum oxide a t reduced pressures in the temperature range 1700-2200" K. The reduction was found to proceed through the intermediate oxycarbide A1404C, identified by previous workers, to the aluminum carbide. The A140aC and another oxycarbide, AlZOC, were formed by a direct solid-solid reaction, rather than by formation of a gaseous aluminum suboxide and subsequent reaction with carbon as has been postulated.The carbon monoxide pressures over the following reactions were measured:Heats and free energies of reaction were found and standard heats and free energies of formation were calculated for AlzOC, A1404C, and A I L The values for A14C3 agreed with previously published results.The direct reaction did not occur. To account for the aluminum produced a t high temperatures, the reaction below was postulated:The calculated equilibrium pressure above this reaction agreed with experimental observations. ISTRODUCTIONIn the past few years, there has been a revival of interest in the production of aluminum by direct reduction (1, 2). Since a carbothermic reduction of aluminum oxide is involved in these processes, the present work was carried out to investigate the details of this reaction.Despite the industrial supremacy of the Hall-Heroult electrolytic process, there has been a sustained experimental interest in the aluminum-oxygen-carbon* system. Previous experiments on the system may be divided into two main groups. The first of these involved studies of the gas pressures existing above the system and paralleled the present investigation. The second, and more recent group, was a study of the compounds and phase relations occurring in the system, as determined by analyses of solidified melts of varying composition. In the latter group, a large number of compounds have been postulated by various workers (3, 4), but the existence of many of these has been disproved a t a later date. Foster, Long, and Hunter ( 5 ) have subjected the system to an extensive analysis in which two oxycarbides, AlpOC and A1404C, were found, as well as the known carbide,The existence of the two oxycarbides has been confirmed by Filonenko et al. (6, 7).* I n the text, the words graphite and carbon are used inte~changeably.
The rate of reaction ( a ) has been studied over the temperature range 1140-1460' C, by following the loss in weight of the reactants in high vacuum.( a T h e loss in weight of the reactants was found to be greater than that which can be accounted for by the above reaction. The excess weight loss was found to be due to a brownish, glassy deposit which appeared outside the reaction zone. This material was analyzed by X-ray diffraction and found to be an intimate mixture of Mg?SiOs and Si. Experiments conducted showed that both of these materials are not appreciably volatile a t 1350' C. This transfer of silicon and silicate is postulated to occur by the formation of gaseous SiO formed from thc reaction mixture. Based on this assumption a possible mechanism for this reaction is postulated.
The production of magnesium by the follo\\~ing reaction has been st~~clied: ( a )The rate of reaction ( a ) \\;as followed by continuous weight loss measurements of the reactants.Keeping the particle size of the reactar~ts ant1 size and shape of the charge constant, the following factors ivhicli affect the yield of n~agnesium \Ifere investigated: (1) temperature, from 1050-15fjO0 C ; (2) pressure, from less than 1 p to 261 m m of I-Ig; (3) catalysts, CaF2, BaF?, and MgI:?; and (-1) ferrosilico~~ gracle, from lS.iyO to I)6.Tcj/, Si.It was found that the yield illcreased by a factor 1.55 for a 50" increase in temperature over the temperature range investigated. !It temperatures belon~ 1300" C the yielcl falls linearly \\.it11 increase ill pressure, \\lhile a t temperatures above 1:100" C the 1-ield remains relatively constant until the pressure esceecls the eq~~ilibrium magnesium pressure. All fluoricle additions were found to increase the yielcl, CaFr being the 111ost efiective. The rate of the reaction \\?as also found to have a clirect relationship with the silicor~ activity.
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