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. For example, when a definite condition for b/a=1/3 is sought from Fig. 2, the following result can be

Fig. 5 Relation between the logarithm of apparent equilibrium constant and the AlCl3 flow rate at 1250℃. obtained: A: 0.0134mol/hr, AlCl3: 0.0402mol/hr: A: 0.0536mol/hr, AlCl3: 0.1608mol/hr, A: 0.1072mol/hr, AlCl3: 0.3216mol/hr and log K'p in each case can be obtained by intersec- tion of straight lines and a dotted line. Similarly, log K'p may be indicated in 1/3, 1/2, Fig. 2 Relation between the logarithm of apparent 2/3 and 1 of b/a in Figs. 2～5. equilibrium constants and the AlCl3 flow rate If the value of log K'p is extrapolated to the zero at 1000℃. rate of the total gas flow at constant b/a, the intersec- (8) B. N. Bhaduri & A. Fowler: Proc. Roy. Soc., A-145 tion on the ordinate correspondto the true equilibrium (1934), 321. value. Equilibrium of the 2Al(l)+AlCl3=3AlCl(g) Reaction in the Subhalide Process of Aluminium 125

Therefore, the total flow of a+b and long Kp' For instance, P. Weiss(4) used aluminium trichloride under the constant b/a condition obtained in the above- produced by the reaction of aluminium with chlorine mentioned way is plotted at each temperature in Fig. gas, and delivered it directly to the reaction zone. 6. In order to obtain the ture equilibrium constant In his experiment, equilibrium constants were obta- of 1000℃, for instance, the four stright lines relating ined by the aluminium trichloride reaction and ex- trapolation of the zero flow. However, these values are greatly different from the others, and it is also thought that this method has some difficulties in the experimental operation. A. S. Russell and K. E. Martin(5) passed aluminium trichloride gas over a series of aluminium particles. P. Gross(3), Shimizu and Uchida(6) supplied aluminium trichloride by an argon carrier into the aluminium melt or upon it at an atmospheric pressure. In the above experiments, the reaction was assumed to be in an equilibrium condition but there is some uncertainty in the determination of equilibrium. Morinaga and Takahashi(7) used the flow method similar to the present work, but they extrapolated the total flow of aluminium trichloride and argon to zero in spite of the inconstancy of b/a. L. M. Foster and A. S. Russell(1) obtained the value by combination of Fig. 6 Correlation between the flow rate of total spectral intensity of aluminium monochloride with gas, a+b versus log Kp' for a range of var- ious b/a and temperatures. thermodynamic values of relative materials. As shown in this figure, the values of the present work are similar to those of spectral and flow methods carried out by A. S. Russel(1) and Morinaga(7). For these reasons, it is thought that the values obtained in this experiment are fairly reasonable. 2. Thermodynamic consideration of this reaction The standard free energy change is then given by the equation (2)

The standard free energy change ΔG0T at each tem- perature is listed in Table 1.

Table 1 Standard free energy change accompanying the subhalide reaction, 2Al(l)+AlCl3=3AlCl, which is derived from equation (2). Fig. 7 Log Kp and 1/T obtained in the previous literature and the present work. to the logarithm of apparent equilibrium values and a+b were calculated in each b/a by least squares, and then four intersections corresponding to a+b= The change in heat content of this reaction is ΔH =66,060cal, viz., endothermic in monochloride forma- 0 are leveled. The straight lines obtained in each b/a are almost consisten, so that a dotted line is drawn tion and exothermic in decomposition, so that it in Fig. 6 to show the relation more clearly. The true becomes 1.42KWH/KgAl in electrical unit. equilibrium constants thus obtained at each tempera- The above value is important to consider the thermal ture can be adjusted by least squares as follows: balance in practical operation. As the subhalide process uses aluminium monochlo- (1) ride as the medium it is important to consider the ther- Fig. 7 shows the tore equilibrium values, the modynamic values of monochloride. equilibrium curve obtained from equation (1) and Therefore, the value obtained in this work was the values in the previous literature as a function of examined using other data. The relation between reciprocal temperature. heat content and entropy of this reaction may be It is very difficult to discuss the propriety of these values because of the different conditions in the selec- (9) J. D, Ans and E. Lax: Taschenboch fur Chemiker and Physiker 2 (1949), 328. tion of equilibrium temperature, determination of (10) P. Drossbach: Elektrochemie geschmolzener Salze. equilibrium evidence, combination of other thermodyn- 1938 amic values and experimental apparatus, etc. (11) H. H. Kellogg: Trans. AIME, 188 (1950), 862. 126 Takeaki Kikuchi, Toshio Kurosawa and Testuo Yagihashiwritten

tion of pressure. From this figure it is estimated that the reaction ratio is relatively small when the total pressure of aluminium monochloride and trichlo- ride is 760mmHg, but a more effective yield can be Insertion of the standard free energy change obtained expected when the total pressure becomes smaller. in this experiment and

by J. D' Ans and E. Lax(9), and S298(AlCl3(g))=82 cal by P. Drossbach(10) in the above equation gives the heat content and entropy

Moreover, the standard free energy change in the aluminium monochloride formation due to the reac- tion of aluminium with chlorine is evaluated in the following way. Combining the equation (2) obtained Fig. 9 Temperature dependence of the reaction ratio of AlCl3 at various total pressures. in this work with that of aluminium trichloride formation given by H. H. Kellogg(11), Although the lower pressure leads to a more effective (3) reaction ratio, operational difficulties seem to be increased. Therefore, the atmospheric operation using an argon carrier is considered to be more useful (4) because of the simpler apparatus and operation. Fig. 10 shows the relation among reaction ratio, tem-

Therefore, the free energy function becomes (5)

The relation between (3) and (4) is shown in Fig. 8.

Fig. 10. Temperature dependence of the reaction ratio of AlCl3 at various mixing ratios of A to AlCl3.

perature, and b/a of A/AlCl3 mol ratio. The values of b/a=0, 10, 100, and 1000 correspond to the partial pressures of trichloride 1 atm, 69mm, 7.5mm and 0.76mm in the supplied gas, respectively, showing the increase of trichloride with decrease of the partial pressure. As shown in Fig. 10, the increasing tend- ency is more gentle than that of the previous condi- Fig. 8 Standard free energy change in the formation tion shown in Fig. 9. In practical operation, how- of AlCl and AlCl3. ever, it is necessary to consider not only the increase of the aluminium production rate but also the reac- As indicated in this figure, monochloride is more tion ratio of aluminium trichloride. stable at a higher temperature, and trichloride has IV. Summary an opposite tendency. It is necessary to consider various conditions in (1) Equilibrium constants of the subhalide reaction, the extraction of aluminium from crude aluminium 2Al(l)+AlCl3(g)=3AlCl(g), which is expected as the alloy. Therefore, the aluminium trichloride reacted new aluminium refining process were obtained by the was calculated under various conditions using the flow method. The relation among equilibrium con- values obtained in this work. stants, standard free energy and temperature was Fig. 9 shows the relation between the reaction ratio represented by of aluminium trichloride and temperature as a func- The Effect of Plastic Deformation on the CoerciveForce and Initial Permeability of Nickel Single Crystals 127

loride was evaluated as a function of reduced total pressure. In addition, the relation among reaction (2) Combining the obtained value with other ther- ratio of aluminium trichloride, A/AlCl3 mol ratio, modynamic data, the heat of formation and entropy and temperature was also obtained in the case of of aluminium monochloride was evaluated to be using an argon carrier. ΔHAlCl(g)=-22,250cal/mol and SAlCl(g)=48.7cal/mol, The authors wish to thank Dr. Koki Gunji of our respectively. Institute for his useful suggestion, and Mr. Chuji (3) By the use of equilibrium constants obtained in Muto for his help in this experiment. this experiment, the reaction ratio of abuminium trich-