J. Cent. South Univ. (2012) 19: 1813−1816 DOI: 10.1007/s11771­012­1213­0

Carbothermic reduction of alumina with carbon in vacuum

YU Qing­chun(郁青春), YUAN Hai­bin(袁海滨), ZHU Fu­long(朱富龙), ZHANG Han(张晗), WANG Chen(王辰), LIU Da­chun(刘大春), YANG Bin(杨斌) National Engineering Laboratory of Vacuum Metallurgy, Key Laboratory of Breeding Base of Complex Nonferrous Metal Resources Clear Utilization in Yunnan Province, State Key Laboratory of Nonferrous Metal Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China © Central South University Press and Springer­Verlag Berlin Heidelberg 2012

Abstract: Carbothermic reduction alumina in vacuum was conducted, and the products were analysed by means of XRD and gas chromatography. Thermodynamic analysis shows that in vacuum the initial carbothermic reduction reaction temperature reduces compared with that under normal pressure, and the preferential order of products is Al4O4C, Al4C3, Al2OC, Al2O and Al. Experiment results show that the carbothermic reduction products of alumina are Al4O4C and Al4C3, and neither Al2OC, Al2O or Al was found. During the carbothermic reduction process, the reaction rate of Al2O3 and carbon decreases gradually with increasing time. Meanwhile, lower system pressure or higher temperature is beneficial to the carbothermic reduction of alumina process. Al4O4C is firstly formed in the carbothermic reaction, and then Al4C3 is formed in lower system pressure or at higher temperature.

Key words: alumina; carbothermic reduction; vacuum; aluminum

the formation of aluminum carbide and oxycarbide 1 Introduction byproducts complicates the carbothermic reduction and chlorination process. The thermodynamic study of Aluminum is currently produced industrially via the chemical reactions provides a basic understanding of the Hall­He´roult process by dissolving Al2O3 in fused process prior to designing suitable reaction experiments, NaF­AlF3 (cryolite) followed by direct current and provides a useful guideline for the selection of electrolysis. The main drawbacks of the electrolytic processing conditions [11−12]. production are its high energy consumption, the release Prior to experiments, it is essential to determine the of perfluorocarbons, and the high specific CO2­equiv feasibility of the chemical reactions, the nature and emissions. The greenhouse gas emission by the amount of the solid and gaseous species present in the electrolytic Al production contributes to 2.5% of the system. These can be determined from the calculation of world anthropogenic CO2­equiv emissions [1−3]. During the standard Gibbs free energy changes for chemical the past years, there were a lot of attempts to produce reactions and the standard free energy changes at a given metal aluminum by nonelectrolysis [4−7]. Carbothermic set of processing conditions such as reaction temperature, reduction and chlorination process [8−10] is one of the pressure and reactant concentration [13]. most effective ways of extracting aluminum in the The main purpose of this work was to investigate metallurgical industry. In this process, carbon and the carbothermic process in vacuum. The resultant aluminum chloride are used as reducing agent and samples were investigated by X­ray diffraction, and gas chlorinating agent, respectively, and minerals in the form chromatography analysis. of metal oxides react with them. The corresponding metal subchlorides formed and separated from minerals 2 Experimental in gaseous form are unstable at the temperature of operation and vaporize readily. Subchlorides can be 2.1 Experimental equipment condensed, separated, and further processed to get The schematic diagram of experimental setup is desired pure metals. shown in Fig. 1. A reaction zone, composed of cylinder However, during the carbothermic reduction stage, containing sample, is controlled by a temperature

Foundation item: Project(U0837604) supported by the Natural Science Foundation of Yunnan Province, China; Project(Jinchuan 201114) supported by the Pre Research Foundation of Jinchuan Group Ltd., China; Project(2011148) supported by the Analysis and Testing Funds of Kunming University of Science and Technology, China Received date: 2011−05−13; Accepted date: 2011−12−20 Corresponding author: YU Qing­chun, Associate Professor; Tel: +86−871−5114017; E­mail: [email protected] 1814 J. Cent. South Univ. (2012) 19: 1813−1816

T ∆ H 0 = ∆H 0 + ∆c dT (4) T 298 K ∫ 298 K p

T ∆c ∆S 0 = ∆S 0 + p dT (5) T 298K ∫ 298K T

0 0 0 ∆ H 298 = ( ∑ν i H 298 K ) P − (∑ ν i H 298 K ) R (6)

0 0 0 ∆ S 298 = ( ∑ν i S 298 K ) P − (∑ ν i S 298 K ) R (7) ∆ c = ( ν c ) − ( ν c ) (8) Fig. 1 Schematic diagram of experimental setup P ∑ i p P ∑ i p R Based on the reactions given in Table 1, the initial controller. The reactor was provided with a central reaction temperatures of these reactions have been removable tube that was permitted to insert a calculated using the thermodynamic data from Refs. thermocouple in order to know the temperature in the [14−16] under the condition of partial pressure of 100 Pa reaction zone by means of a digital thermometer. A zone and normal pressure. In the view of reaction kinetics, for the collection of the reaction products consisted of a dismissing carbon monoxide gas under the condition of box­like collecting plate, where most of the reaction low partial pressure gives advantages to chemical products were condensed, and a washing trapper for the equilibrium, which makes reaction move toward right collection of gas products. side. It can be found in Table 1 that in vacuum the initial 2.2 Materials and sample preparation reaction temperature reduces, which means that much Analytical grade of alumina and graphite were used less energy would be consumed. The relationships as the raw materials in experiments. The solid samples between Gibbs free energy change and temperature of used in the experiments were mixtures of Al2O3 and these reactions calculated under the condition of partial graphite with molar ratio of 1:4. pressure of 100 Pa are shown in Fig. 2.

2.3 Experimental procedure Table 1 Initial reaction temperatures of carbothermic reduction Once the reagent solid sample was placed in the of alumina (K) reactor, cooling water and vacuum pump were opened, Reaction Normal Reaction 100 Pa and the reaction zone was heated as the pressure reached No. pressure 100 Pa. Vacuum degree is regulated by the corresponding control and measuring devices. The heating rate of (1) Al2O3+3C=2Al(l)+3CO(g) 1 775 2 288.07 system temperature was about 20 K/min. The system (2) 2Al2O3+9C=Al4C3+6CO(g) 1 714 2 099.45 temperature is kept constant for 140 min when it reached (3) Al2O3+2C=Al2O(g)+2CO(g) 1 743 2 376.67 1 693, 1 703, 1 753 and 1 853 K. Then, the heating (4) Al O +3C=Al OC+2CO(g) 1 720 2 306.27 device was turned off and vacuum degree was kept till 2 3 2 room temperature. (5) 2Al2O3+3C=Al4O4C+2CO(g) 1 689 2 306.27

(6) Al2O3+Al4C3=6Al(l)+3CO(g) 1 879 2 387.20 3 Results and discussion (7) Al4O4C+6C=Al4C3+4CO(g) 1 726 2 264.65

3.1 Thermodynamic analysis of carbothermic (8) Al4O4C+ Al4C3=8Al(l)+4CO(g) 1 889 2 345.26

reduction (9) 2Al2OC+3C=Al4C3+2CO(g) 1 701 2 302.75 During this solid−solid reaction, gaseous products are generated when the carbothermic reduction reaction According to Fig. 2, ΔG decreases sharply by takes place. In vacuum, gaseous products are removed increasing temperature. The initial temperatures of nine continuously through pump, which can accumulate the reactions are 1 775, 1 714, 1 743, 1 720, 1 689, 1 879, chemical reaction. Main formulas of thermodynamic 1 726, 1 889 and 1 704 K, respectively. The preferential calculations are as follows: order of products is Al4O4C>Al4C3>Al2OC>Al2O>Al 0 0 0 ∆GT = ∆HT −T ∆S T (1) during the carbothermic reduction of alumina. Aluminum generated by Al2O3­Al4C3 and Al4O4C­Al4C3 should be at ∆G = ∆G 0 + RT ln Q (2) T T p higher temperature. The temperature and system pressure p p in the furnace are important determinants of feasibility of g ν i g ν i where Q p = ( ) P /( ) R (3) p 0 p 0 reactions. J. Cent. South Univ. (2012) 19: 1813−1816 1815

Fig. 2 Gibbs free energy change versus temperature of Al2O3­C Fig. 4 XRD pattern of carbothermic reduction products at system at 100 Pa different temperatures

3.2 Effect of carbon monoxide change on carbon under that condition. The characteristic peaks of

carbothermic reduction Al4O4C and Al4C3 are found clearly over 1 703 K and The volume of carbon monoxide per 20 min and increases with increasing temperature. Meanwhile, the total volume of carbon monoxide were calculated, and diffraction intensities of carbon and alumina decrease the average production rate of carbon monoxide per with increasing temperature, which can be deduced that 20 min was determined. Figure 3 shows the average the reactions (2), (5) and (7) occur in the temperature production rate of carbon monoxide and total volume of range of 1 703−1 853 K. There is no diffraction peak of carbon monoxide. It can be found that production rate of Al2OC found by XRD analysis in the reaction product of carbon monoxide decreases, and total volume of carbon carbothermic process and no aluminum is collected in monoxide increases with increasing time. After about condensation tower of vacuum furnace. If there were

80 min, the production rate of carbon monoxide tends to Al2O gas, it would disproportionate into aluminum and be stable at about 0.02 L/min, and the volume of carbon alumina at low temperature and be found on the cooler. monoxide increases slowly at about 0.4 L per 20 min. According to the carbothermic reduction analysis, it can The carbon monoxide change means that reaction rate of be deduced that reactions (1), (3), (4), (6), (8) and (9) do alumina and carbon decreases gradually with increasing not occur between alumina and carbon at 40−150 Pa. time. The initial temperatures of reactions (2), (5) and (7) are 1 714, 1 689 and 1 726 K, respectively. According to the formula of X­ray quantitative analysis [17], the content of certain phase is proportional to its diffracted

intensity. That is to say, Al4O4C is firstly formed in the

carbothermic reaction, and then Al4C3 is formed at higher temperature. Thus, the diffraction intensity of Al4O4C is

stronger than that of Al4C3 with increasing temperature from 1 693 to 1 853 K. This analysis and experimental results are well agreed with QIU [18], who pointed out

that the first product Al4O4C is a stable compound in

thermodynamics, and another product Al4C3 is produced when reactant further reacts with carbon. Also,

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