Abstract
Electrochemical decomposition of spent catalyst dissolved in molten salts is a promising approach for the extraction of precious metals from them. This article reports the results of the study of aluminum electrowinning from the xLiF–(1-x)AlF3 melt (x = 0.64; 0.85) containing 0–5 wt.% of spent petroleum Pt/γ-Al2O3 catalyst on a tungsten electrode at 740–800 °C through cyclic voltammetry and chronoamperometry. The results evidence that the aluminum reduction in the LiF–AlF3 melts is a diffusion-controlled two-step process. Both one-electron and two-electron steps occur simultaneously at close (or same) potentials, which affect the cyclic voltammograms. The diffusion coefficients of electroactive species for the one-electron process were (2.20–6.50)∙10−6 cm2·s–1, and for the two-electron process, they were (0.15–2.20)−6 cm2·s−1. The numbers of electrons found from the chronoamperometry data were in the range from 1.06 to 1.90, indicating the variations of the partial current densities of the one- and two-electron processes. The 64LiF–36AlF3 melt with about 2.5 wt.% of the spent catalysts seems a better electrolyte for the catalyst treatment in terms of cathodic process and alumina solubility, and the range of temperatures from 780 to 800 °C is applicable. The mechanism of aluminum reduction from the studied melts seems complicated and deserves further study to find the optimal process parameters for aluminum reduction during the spent catalyst treatment and the primary metal production as well.
Highlights
The platinum group metals (PGMs) are extensively used in various fields such as the petroleum industry for improving gasoline quality [1], organic synthesis for cycloisomerization, in the hydrogen evolution reaction [2] and medicine [3]
The results indicated that the cathodic process had mixed kinetics with the diffusion coefficients of aluminum ions varying in the range (0.36–11.56)·10–8 cm2·s–1, which is extremely low for molten salts
According to Cyclic voltammetry (CV), in the 85LiF–15AlF3 melt, the diffusion coefficients were higher than those obtained in the 64LiF–36AlF3 melt, which may be the consequence of the high content of Li+ ions
Summary
The platinum group metals (PGMs) are extensively used in various fields such as the petroleum industry for improving gasoline quality [1], organic synthesis for cycloisomerization, in the hydrogen evolution reaction [2] and medicine [3]. Copper was recovered at the cathode with a current efficiency of 70%, and the PGMs (and other impurities) remained in the anode residue Another combination of pyrometallurgical and electrochemical methods was proposed in [20] where the process was performed at a much lower temperature between 700 and 850 ◦C. The initial tests [20] showed that concentration of Pt in the bipolar electrode was 100 times higher than in the refined aluminum after about 30 h of electrolysis In this technique, one can obtain an Al–Pt alloy with 10–20 wt.% of Pt, which should be further extracted by known hydrometallurgical methods [22] or by molten salt electrolysis. The cathode process of aluminum reduction on tungsten as a working electrode was previously studied in the LiF–AlF3–CaF2 melt with the addition of various amounts of spent Pt/Al2O3 catalyst [10]. The temperature of the cell was measured using a k-type thermocouple and maintained constant (±2 K) using the USB-TC01 thermocouple module
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