Equilibrium of the 2Al(l)+AlCl3(g)=3AlCl(g) Reaction in the Subhalide Process of Aluminium (Study of Extractive Metallurgy of Aluminium (1)) By Takeaki Kikuchi*, Toshio Kurosawa* and Testuo Yagihashi* Equilibriumconstants of a fundamental reaction of the aluminium subhalide process, 2Al(l)+AlCl3(g)= 3AlCl(g), were determined by the flow method using argon carrier between 1000℃ and 1250℃. As a result of this experiment, equilibrium constants and standard free energy were obtained by the following equation: The heat of formation and entropy of AlCl(g) obtained from the experimental data and other thermodynamic values were -22,250cal/mot and 48.7cal/mol respectively. By the use of the equilibrium constants, the reaction ratio of aluminium trichloride was calculated at a reduced pressure and in argon carrier, respectively. (Receivedmarch 10, 1964) I. Introduction trichloride supplied to the reaction zone were applied Aluminium is produced by means of the fused salt previously. In this investigation, the latter method electrolysis using alumina obtained mainly from rela- was selected to obtain the equilibrium constants. tively higher grade bauxite. However, another 1. Experimental apparatus extraction method called the Gross or subhalide process has recently been investigated, and some technological The apparatus used in this experiment is shown by or industrialization reports have already been the schematic diagram in Fig. 1. The apparatus con- sists of an argon purifier, aluminium trichloride evapora- published. In this process, crude aluminium alloy is produced tor, reaction tube, and condensing tube of aluminium by reduction of alumina bearing ores with carbonaceous trichloride gas. Argon in a bomb was purified and reducing material such as coke or charcoal in the measured by passing through concentrated sulphuric acid, calcium chloride, soda lime, phosphorus pentoxide first step, and then aluminium trichloride gas is alld magnesium chip hea七ed at 400℃. After then, it passed over the alloy heated at a high temperature to was fed to the aluminium trichloride evaporator form aluminium monochloride, and finally pure through a liquid paraffin flow meter. aluminium can be redeposited when monochloride is The evaporator constructed by transparent quartz cooled. tubing was immersed in a silicone oil thermostat and This process has a characteristic in the utilization heated. properly below 180℃, namely, the sublimation of subchloride of aluminium monochloride, and it will be significant if this process is developed industri- point of aluminium trichloride. Evaporated alu- minium trichloride gas was carried into the reaction ally. Several reports have been published in regard tube by argon. to the fundamental reaction of this process, viz., aluminium monochloride formation by the reaction of The reaction tube consisted of a quartz tube of aluminium with aluminium trichloride, but consider- 800mm long and 40mm in inner diameter sheathed internally with a high grade graphite liner (32mm in able differences can be observed in the results. Therefore, equilibrium constants of this reaction inner diameter, 500mm in length), and silica wool was were measured by the flow method in order to obtain packed between the tube and the liner. The reaction tube was heated in a Siliconit furnace reasonable values. Moreover, some thermodynamic where the temperature was maintaued within±5℃ relations concerning the subhalide process were at 1100℃ in the 8cm range. A calibrated Pt-PtRh evaluated from the obtained values. thermocouple was set upon the surface of the reaction II. Experimental Apparatus and Procedure (3) P. Gross, C.S. Campbell, P. J. C. Kent & D.L. Levi: For the purpose of obtaining equilibrium constants, A General Discussion on the Physical Chemistry of the spectral method(1)(2) measuring the absorption Procoss metallurgy, (1948), 206. intensity of formed aluminium monochloride and the (4) P. Weiss: Z. Erzbergbau u. Metallhuttenwesen, 3 flow method(3)-(7) determining the reacted aluminium (1950), 241. (5) K. E. Martin, A.S. Russell, & C.N. Cochran: J. Amer. *National Research Institute for Metals , Tokyo, Japan. Chem. Soc., 73 (1951), 1466. (1) L. M. Foster, A. S. Russell & C.N. Cochran: J. Amer. (6) Shimizu, Uchida, Oikawa & Ito: Keikinzoku, 20 Chem. Soc, 72 (1950), 2580. (1956), 9. (2) M. Heise, K. Wieland: Helvetica Chemica Acta, (7) Morinaga & Takahashi: Denki Kagakukai, 1963, 34 (1951), 2182. Spring Meeting Lecture. Trans. JIM 1964 Vol.5 Equilibrium of the 2Al(l)+AlCl3=3AlCl(g) Reaction in the Subhalide Process of Aluminium 123 Fig. 1 Experimental apparatus for determination of equilibrium constants by the flow method. tube just above the position of the inserted aluminium until it became constant at a desired temperature. Then boat, but no temperature difference was found between the aluminium trichloride evaporator was immersed the inner and outer positions of the reaction tube. in a silicone oil bath held at a proper temperature. Aluminium trichloride decomposed from monochloride As evolution of aluminium trichloride gas was and unreacted trichloride were removed from another occurred after one minute of immersion, this was outlet, and condensed in the trap cooled with dry ice- chosen as the starting time of the flow method. Argon alcohol. was delivered into the evaporator at a predetermined flow rate, and the resultant gas mixture was supplied 2. Sample and measuring method to the reaction zone. The flow rate of argon was 5cc/min, 20cc/min, and 40cc/min respectively, and Aluminium and aluminium trichloride used in this aluminium trichloride evaporation was controlled by experiment were of high purity. Aluminium had a means of argon flow and bath temperature. The reac- purity of 99.92% and it contained 0.03%Si, 0.05 tion time was 2～3hr, but a longer reaction was con- Fe, and 0.005%Cu, asimpurities. Aluminium trichloride ducted in the case of low reaction temperature and synthesized by reaction of aluminium with chlorine slow argon flow in order to obtain a more-accurate was distilled and then sealed off into the evaporator resint. The reaction temperatures were 1000°, 1100℃, as fast as possible. Prior to the run, the evaporator 1200° and 1250℃. was connected with the reaction tube. Aluminium trichloride delivered into the reaction In order to maintain a high purity of aluminium zone reacted with aluminium heated in the uniform alumina boats of 98.8～99.2% Al2O3 and 0.3～0.4% temperature range, and aluminium monochloride SiO2 were used. The boat was made to fit the graphite formed in the hot zone was decomposed into metallic liner, and it was semicylindrical, 12mm in height, 23 aluminium and trichloride in the lower temperature mm in width and 70mm in length and 2mm in thick- zone. ness. To obtain a nearly constant surface area, alum- Aluminium was deposited in the form of small inium weighing 15～18g was used in all experiments. The measuring procedure was carried out in the particles on the surface of the graphite liner. After the lapse of a predetermined time, the silicone oil following way. The aluminium chip was set in the bath was removed out of the evaporator rapidly, and boat, and it was inserted in the reaction tube. Then cooled by dry ice-alcohol trap to cease the evolution the aluminium trichloride evaporator was connected of aluminium trichloride. Moreover, the reaction with one side of the reaction tube by the ball joint. tube was cooled by an electric fan. Then amounts of The amount of aluminium trichloride in the evaporator the evaporated aluminium trichloride and transported was 100～150g. After closing a cock of the evaporator aluminium were measured, and the equilibrium con- the whole system was evacuated and the leakage stants were obtained from the reaction ratio of alum- condition was then checked by the mercury man- inium trichloride. ometer. During this process, the aluminium trichloride III. Experimental Rresult and -Consideration evaporator was cooled in a dry ice-alcohol bath to 1. Calculation method of equilibrium constants prevent sublimation of aluminium trichloride. In the next place argon was delivered into the whole and experimental results system, and the reaction tube was heated rapidly The subhalide reaction may be denoted by the 124 Takeaki Kikuchi, Toshio Kurosawa and Testuo Yagihashi following equation according to the spectral studies carried by Bhaduri, et al.(8) and Foster et al.(1) Equilibrium constant Kp of the above reaction is described by the use of partial pressures, PAlCl3, PAlCl, and activity of aluminium denoted by aA1 To obtain these partial pressures, the reaction ratio a of aluminium trichloride can be written in the follow- ing relation If the total pressure at equilibrium, moll of alum- Fig. 3 Relation between the logarithm of apparent inium trichloride and argon supplied during the reac- equilibrium constant and the AlCl3 flow rate tion are taken to be P, a, b respectively, then mols of at 1100℃. trichloride, monochloride and argon can be denoted by a(1-a), 3aα, b respectively, and the total of mols may be represented as a(1+2a)+b. Therefore, partial pressures of PAlCl3 and PAlCl are Inserting these relations into the above Kp equation, and combining P and aA1 taken as unity due to atmospheric pressure and pure aluminium, the equilibrium constants can be expressed as follows: If constant b/a can be obtained in the gas mixture, the true equilibrium constants may be determined Fig. 4 Relation between the logarithm of apparent at each temperature by extrapolation of the low equilibrium constant and the AlCl3 flow rate total gas flow rate. Although b/a was difficult to be at 1200℃. made constant under the test conditions, constant b/a could be obtained by using the following method. Figs. 2～5 show a logarithm of apparent equilibrium constants, log K'p, on the ordinate and the alum- inium trichloride flow rate on the abscissa for a range of argon flows and temperatures, and the straight lines are drawn by least squares at each argon flow rate.