The Cathodic Behavior of Aluminum from Pt/Al₂O₃ Catalysts in Molten LiF-AlF₃-CaF₂ and Implications for Metal Recovery from Spent Catalysts

Abstract

Electrochemical behaviour of aluminium ions from the Pt/γ-Al2O3 spent catalyst in the eutectic [LiF (63.6 wt.%)–AlF3 (36.4 wt.%)]eutectic−5 wt.% CaF2 melt was studied by the means of cyclic voltammetry, chronopotentiometry and chronoamperometry methods. Tungsten rod (diameter 2 mm) was used as a working electrode. The XRD method was used to study the composition of melt collected near the working electrode and the spent catalyst. The aluminium reduction kinetics was studied concerning varying parameters like spent catalyst content in the melt and the temperature. The reduction of Al3+ ions on the tungsten electrode changed from diffusion-controlled to quasi-reversible process. The charge transfer coefficient and the diffusion coefficient were calculated from the data obtained from the above-mentioned methods. The estimated diffusion activation energy was 117.85 kJ.mole−1.

Towards Understanding the Cathode Process Mechanism and Kinetics in Molten LiF–AlF3 during the Treatment of Spent Pt/Al2O3 Catalysts

“…In the upper left corners of CVs, the cathodic peak current densities i cp were plotted versus the square root of the sweep rate. For a diffusion-controlled process, this dependence should be linear, which is observed neither in the current study nor in previous publications [10]. Even in the case of linearity being observed, the experimental data often do not agree with theoretical predictions made with the Randles-Sevcik equation (the slope and the intercept are different):…”

“…The electrochemical measurements were performed in a three-electrode graphite cell heated by an electric furnace. The cell design and the experimental procedure were similar to the one in [10]. A tungsten (W) rod of 2 mm diameter with an active surface of 0.659 cm 2 was the working electrode.…”

“…The lifespan of catalysts varies between 3 to 5 years after which either the precious metals can be extracted from the spent catalysts or they can be regenerated [4]. Hydrometallurgical [5][6][7][8] and pyrometallurgical methods are extensively used at laboratory and industrial scales for the recovery of PGMs from the spent catalysts, while electrochemical techniques are infrequently used [9][10][11][12]. New methods and materials are reported annually.…”

“…Since the electrochemical process seems attractive for zero-waste and energy-efficient recovery of precious metals from spent catalysts, the understanding of aluminum reduction mechanism and kinetics becomes important to provide the recovery with high productivity. The cathode process of aluminum reduction on tungsten as a working electrode was previously studied in the LiF-AlF 3 -CaF 2 melt with the addition of various amounts of spent Pt/Al 2 O 3 catalyst [10]. 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 cm 2 •s -1 , which is extremely low for molten salts.…”

Anode Process on Platinum in Low Temperature KF-AlF3-Al2O3 Based Melts: An Update

“…[1][2][3] Significant progress has been made in the field of creating oxygen-evolving anodes and aluminum-wetted cathodes over the past two decades. [4][5][6][7][8][9] However, even the most corrosion-resistant materials cannot withstand the simultaneous effects of high temperature and aggressive electrolyte. The situation changes when the cryolite-alumina melt is replaced with a KF-AlF 3 -Al 2 O 3 based molten electrolyte offering an operating temperature in the range of 750 °C-850 °C.…”