Chemical and Electrochemical Characterisation of Oxide/Hydroxide Impurities in the Electrolyte for Magnesium Production

Abstract

This work is part of a research program where the aim is to develop an

electroanalytical technique to determine the amount of dissolved oxide and hydroxide in industrial Mg electrolytes. The systems studied were mixtures of MgCl2 and NaCl, ranging from pure MgCl2 to melts containing 10 mole % MgCl2 / 90 mole % NaCl. To these melts, additions of MgO and MgOHCl were done at temperatures ranging from 475 to 850°C. Voltammetric measurements were performed before and after addition of MgO or MgOHCl and melt samples were taken for analysis. The quenched melt samples wereanalysed by carbothermal reduction analysis and the acid consumption method to obtain the O2- and OH- contents in the samples. Linear sweep voltammetry was performed with a sweep rate of 200 mV/sec in two potential regions to detect the concentration of dissolved MgOHCl and MgO. The experiments were performed inside a glove box having water and oxygen contents of, respectively, <1 ppm and <2 ppm. The results indicate that the rate of decomposition of MgOHCl increases with increasing temperature, as expected. In melts with high content of NaCl the underpotential deposition of sodium has a large influence on the uncertainty in the reading of the current density of MgOHCl reduction. Linear relations between MgO and MgOHCl concentrations and the peak current densities for the electrochemical reactions of the dissolved MgO and MgOHCl species in the melt, respectively, were observed. The diffusion coefficients of MgO and MgOHCl in the different melts were calculated. The diffusion coefficients decrease with increasing content of MgCl2. For MgOHCl the diffusion coefficient decreases from 5.6*10-5 cm2/sec in 20 mole % MgCl2 / 80 mole % NaCl to 2.1*10-5 cm2/sec in pure MgCl2, both measurements done at 800°C. For MgO the diffusion coefficient decreases from 6*10-5 cm2/sec in 41.5 mole % MgCl2 / 58.5 mole % NaCl to 0.8*10-5 cm2/sec in pure MgCl2, both measurements done at 730°C. The results show that it is possible to use cyclic voltammetry for quantitative analysis of MgOHCl in MgCl2 based melts.