(Digital Presentation) Effects of Oxide Ion and Sulfate Ion on Mg Metal Deposition in Molten Salt Electrolysis
Authors: Sasaki Tatsuya , Suehiro Riku , Morishige Taiki , Takenaka Toshihide , Matsumoto Toshiharu , Hirose Masanori , Nagayasu Katsushi 
Published: November 2022

With the growing demand for Mg, it is desirable to establish domestic Mg production in Japan. The author focuses on Mg production by molten salt electrolysis using MgCl2 extracted from concentrated seawater. The control of the contamination to the MgCl2 product should be a key issue, and also the influence of the contamination on the Mg production must be clarified. In this study, the effects of oxide ions and sulfate ions on Mg metal electrodeposition in molten salt have been investigated.

Molten MgCl2-NaCl-CaCl2 with trace CaF2, for the new IG method was used as the bath, and MgO (0~1.0mass%) or Na2SO4 (0.025~0.5mass%) was added to the bath. A Mo wire with BN cover was used as the working electrode, and a carbon rod was used as thecounter electrode. The reference electrode was a Ag/AgCl couple in the same bath in a mullite tube, and its potential was calibrated against the Mg deposition potential. Experiments were done at 700°C in an Ar atmosphere. The cathodic behavior investigated by cyclic voltammetry, and then potentio-static electrolysis was performed. Visual observation was carried out during the cyclic voltammetry and potentio-static electrolysis. The electrodeposits after rinsing was evaluated by XRD, XRF, SEM-EDX and ICP, and the cathodic current efficiency was calculated from the weight of the deposit and the quantity of electricity.

It was showed that the cathode reaction did not depend on MgO addition. Electrodeposits were obtained by potentio-static electrolysis regardless of MgO addition, and current efficiency was not significantly affected by MgO addition. The purity of Mg metal deposit was not influenced by the MgO addition in the bath, too; Mg metal above 99.9 mass% was obtained regardless of MgO addition, and the Na, K and Ca contents did not change evidently. It was suggested that the contamination of MgO to MgCl2 extracted from condensed seawater did not affect the Mg metal deposition directly. However, our previous work showed that the solubility of MgO in the bath was limited, so the MgO content in MgCl2 must be controlled.

The result of cyclic voltammetry showed that cathodic current peaks appeared around 1V (vs. Mg deposition potential, the same hereinafter) and 0.5V by the addition of Na2SO4, and these peak currents were increased as increasing the added amount. Since MgO and MoS2 were detected after the potentio-static electrolysis at 1.2V and 0.5V, respectively, the cathodic current around this potential region seemed not to correspond to Mg metal deposition. Magnesium metal was obtained by the potentio-static electrolysis below 0.0V even in the bath containing sulfate ion, but the current efficiency got worse rapidly as the added amount of Na2SO4 increased. The surface of Mg metal deposit became rough with Na2SO4 addition, which caused the deterioration in its purity. It was shown that the contamination of even a small amount of sulfate to MgCl2 gave a negative influence to Mg metal deposition, so the control of its contamination should be important in the Mg metal production from concentrated seawater.

This work was performed under the support of the commissioned work (JPNP14004) of the New Energy and Industrial Technology Development Organization (NEDO).