Cathode Process at the Electrolysis of KF-AlF 3 -Al 2 O 3 Melts and Suspensions

The kinetics of the cathode process on tungsten in KF-AlF 3 -Al 2 O 3 melts and suspensions was studied by means of voltammetry and galvanostatic polarization. The effects of the temperature, [KF]/[AlF 3 ] ratio, Al 2 O 3 content on the kinetic parameters of electrowinning of aluminum from the KF-AlF 3 -Al 2 O 3 melts and suspensions were determined. Introducing Al 2 O 3 slurry into the suspension and increasing its proportion make the diffusion of the electroactive ions to the cathode more difﬁcult. The parameters for the suspension electrolysis were chosen on the basis of electrochemical measurements. The possibility of aluminum production during the electrolysis of KF-AlF 3 -Al 2 O 3 suspension with 45 wt% Al 2 O 3 in the solid phase was demonstrated. the Issue on Progress Molten and Ionic

The electrolytic production of aluminum from low-temperature melts and suspensions is a very promising technique. [1][2][3][4][5] New electrode and construction materials can be used to permit a lowering of the electrolysis temperature by 150-200 • C. [6][7][8][9][10][11] This also serves to increase the average life of the electrolytic cell. Usage of suspensions with solid particles of Al 2 O 3 allows the maximum concentration of the electroactive ions to be maintained near the electrodes at the working temperature during the electrolysis.
The Al 2 O 3 content in the electrolytes of industrial electrolytic cells varies in the range of 2-4 wt%, whereas its solubility is about 8 wt%. [12][13][14] The low Al 2 O 3 content is mainly caused by the rapid phase transformation γ-Al 2 O 3 → α-Al 2 O 3 at temperatures above 850 • . [15][16][17] The latter form of alumina (α-Al 2 O 3 ) dissolves in a cryolite-alumina melt much more slowly than the former. Thus, an excess of Al 2 O 3 promotes the formation of crusts at the bottom of the electrolyzer. The lowering of the temperature reduces the rate of α-Al 2 O 3 formation and allows melts saturated with Al 2 O 3 to be used as well as the suspensions. [1][2][3][4][5] In this case, the rate of the electrochemical process (current density) is likely to be solely due to the Al 2 O 3 dissolution.
In terms of the anode process, the using KF-AlF 3 -Al 2 O 3 suspension for the electrolytic production of aluminum at lower temperature can reduce the chemical oxidation of carbon anodes and provide faster removal of anode gases from the denser electrolyte.
The present work is devoted to a determination of the kinetic features of the cathode process on tungsten in KF-AlF 3 -Al 2 O 3 melts and suspensions. The KF-AlF 3 -based molten systems were chosen due to the higher solubility of Al 2 O 3 at the investigated temperatures with other conditions remaining constant. [18][19][20] The results of the study supplement scarce data on the kinetics of aluminum electrowinning from KF-AlF 3 -Al 2 O 3 melts and suspensions. [21][22][23] Experimental Preparation of electrolytes.-The melts and suspensions were prepared from individual KF, AlF 3 (Chemically Pure Grade, OJSC "Vekton", Russia) and Al 2 O 3 (UC "RUSAL") compounds. The mixture of KF and AlF 3 having the required [KF]/[AlF 3 ] ratio was placed into the corundum crucible of the electrochemical cell and heated up to the working temperature by means of a resistance furnace. Then the prepared KF-AlF 3 mixtures were purified by means of potentiostatic electrolysis to remove residuals (electropositive in relation to z E-mail: suzdaltsev_av@mail.ru the aluminum impurities; 2 hours at a 0.2 V vs. the potential of aluminum electrode 24 ). Aluminium oxide was added into the purified KF-AlF 3 melts. Data on Al 2 O 3 solubility in the investigated melts and suspensions at the temperatures under study are presented in Table I. Suspensions having a 30% and 45% excess of alumina were chosen for experimental purposes based on the sedimentation effect: the study of cathode processes having a lower solid phase content is complicated by rapid sedimentation.
Electrochemical measurements.-Electrochemical measurements were carried out at temperatures of 670-800 • C in a threeelectrode corundum cell under air. Tungsten (W) rods (1 mm, immersion 10 mm) served as a working electrode. A graphite cylinder was used to form a counter electrode (working surface 12 cm 2 ). The potential of the working electrode was measured relative to the potential of the aluminum reference electrode. 24 AutoLab 320N and NOVA 1.11 potentiostat/galvanostat software solutions (MetrOhm, Netherlands) were used. The voltammograms were recorded at potential sweep rates from 0.01 to 0.5 V s −1 . Galvanostatic polarization curves were obtained by the fixation of the stationary potential at different cathode current impulses. In order to compensate the circuit resistance, both FRA and I-Interrupt procedures were used. The temperature was controlled and kept within ±2 • C by a Varta TP703 thermal regulator calibrated with a chromelalumel thermocouple and a USB-TC01 thermocouple module (National Instruments, USA).
Electrolysis test.-Electrolysis of the KF-AlF 3 -Al 2 O 3 suspension was performed at 715 • C in a corundum electrolytic cell under air. A vertical electrode arrangement was used. A tungsten plate (surface 2 × 50 cm 2 ), placed at the center of the corundum crucible with the KF-AlF 3 -Al 2 O 3 suspension, served as a cathode. Two graphite anodes (plates with the front surface of 25 cm 2 ) were located near the crucible wall. The anode-cathode distance was 2 cm. Al 2 O 3 was added to the electrolytic cell following the immersion of the electrodes. The voltage between the cathode and anodes was recorded during the electrolysis.  of the potassium reduction (K) at more negatively values than −0.5 V (vs. Al). There is one peak (Al ) of aluminum oxidation on the anode sides of CVs at 0.1. . . 0.2 V (vs. Al). The complicated form of anode peak can indicate different forms of reduced aluminum. 23 The rise of the working temperature from 715 to 800 • C multiplies the values of the anode and cathode currents on CVs by 1.5-2.0. At the same time the peak currents on CVs at 750 and 800 • C are closely matched. We assume that this is due to the reducing diffusion resistances with    The values of potentials (E pc ) and cathode peak current densities (j pc ) at different temperatures (T) and potential sweep rates (ν) for the aluminum electrowinning from the KF-AlF 3 -Al 2 O 3 melts and suspensions are listed in Tables II and III. Relationships j pc (ν 1/2 ) for all conditions are generally linear (Fig. 5). This means that the process under study is primarily limited by the diffusion. Some shifting of E pc with the ν rising indicates the quasi-reversibility of the process (Tables  II and III). Conversely, the increases in the solid Al 2 O 3 content in the KF-AlF 3 -Al 2 O 3(sat) suspensions made the diffusion more difficult. Numerically, the described effects can be analyzed from the diffusion coefficients (Table IV) (Table IV). These are close to the D values obtained for graphite and platinum electrodes in previous works. 21,22 The addition of the solid Al 2 O 3 and increases in its content in the KF-AlF 3 -Al 2 O 3(sat) suspensions lead to decreasing D values at the same temperature.   19,20 with temperature decreases. In addition, increases in the melt viscosity and the phase overvoltage should be taken into account. 2. The optimal temperature of the 1.3KF-AlF 3 -Al 2 O 3(sat) suspension electrolysis is 700-720 • C. This temperature supports the liquid state of aluminum in the electrolytic cell and the lower electrolyte overheating. 3. The optimal content of solid Al 2 O 3 in suspension is 45 wt%. Limit current densities for 1.3KF-AlF 3 -Al 2 O 3 suspensions with both 30% and 45 wt% Al 2 O 3 are practically equal. At the same time, the sedimentation ability of the latter is lower. This is conducive for better separation of the anolyte and catholyte.

1.3KF-AlF 3 -Al 2 O 3(sat)
Electrolysis of the 1.3KF-AlF 3 -Al 2 O 3 suspension with 45 wt% Al 2 O 3 in solid phase was carried out using the parameters: current -20 A; cathode current density -0.2 A cm −2 ; anode current density -0.4 A cm −2 ; anode-cathode distance -2 cm; voltage -4.5-6.0 V; temperature -715 • C. Photographs of the working electrolyzer as well as photos of the electrodes and solidified suspension after the electrolysis are presented in Fig. 9. Considerable gas evolution was observed on the anodes (including lateral sides) during the electrolysis (Fig. 9a). Composition of the anode gas was not determined in present work, but we assume that the CO 2 was the main anode product at a relatively high anode current density (more than 0.05-0.10 A cm −2 ). 12,30,31 Cathodic aluminum was formed near the W cathode in form of 2-8 mm droplets (Fig. 9e). The cathode was also well wetted by aluminum (Fig. 9b). Cathode current efficiency (applied quantity of electricity−48 A · h) was above than 50%. This value is comparable to the cathode current efficiencies obtained for other types of laboratory electrolyzers. 5,7,10 The results obtained indicate the possibility of the electrolytic production of aluminum during the electrolysis of the low-temperature KF-AlF 3 -Al 2 O 3 suspension with relatively high current efficiency. However, further study is needed, including research into the processes (particularly the kinetics of the anode process as well as the composition of anode gases) during electrolysis and optimization of the suspension composition and electrolysis parameters.     on the kinetic parameters of the aluminum electrowinning from the KF-AlF 3 -Al 2 O 3 melts and suspensions were studied both under stationary and non-stationary conditions.
It was shown that the introduction of Al 2 O 3 slurry and increases in its proportion in the suspension increases cathode potential (over potential) and decreases the limit current density of aluminum electrowinning under stationary conditions for all investigated temperatures and [KF]/[AlF 3 ] ratios. The diffusion coefficients of the electroactive ions at the cathode were estimated from the voltammograms. It was shown from both stationary and non-stationary measurements that the introduction of slurred Al 2 O 3 into the suspension and increases in its proportion impede the diffusion of the electroactive ions to the cathode.
The parameters for the suspension electrolysis were chosen on the basis of electrochemical measurements. The possibility of producing aluminum through the electrolysis of a KF-AlF 3 -Al 2 O 3 suspension with 45 wt% Al 2 O 3 in the solid phase was demonstrated.