Design and Manufacturing of an Mg Electrolytic Cell

In 2011 Valparaiso University was awarded a grant from the Department of Energy (DOE) ARPA-E program to produce magnesium (Mg) from magnesium oxide (MgO). The majority of Mg in the world is produced in China via the Pidgeon Process. The Pidgeon Process uses thermal reduction to produce Mg at high temperatures and is very harmful to the environment due to the burning of large amounts of fossil fuels. An alternative process to produce Mg is through the electrolysis of magnesium chloride. US Magnesium is the only North American producer of Mg that electrolyzes magnesium chloride. Our process uses both a thermal and electrical input for Mg production, providing energy savings, environmental benefits, and cost advantages. A laboratory setup is required to perform experiments to determine the feasibility of the proposed process along with developing optimum process parameters. Multiple cells have been manufactured and tested in the laboratory at Valparaiso University. All test cells have incorporated the following common components and operate in a similar manner. An electrical furnace provides thermal energy to the cell and a Gamry 3000 potentiostat provides current for the electrolysis process. A fluoride based electrolyte is contained within a graphite crucible which is surrounded by a stainless steel enclosure. The graphite crucible resists the corrosive effects of the electrolyte while the enclosure is used to seal the cell from atmosphere. Anodes and cathodes are immersed within the electrolyte along with MgO pellets. A shroud surrounds the cathode and piping provides an inert gas, Argon, to the system. Thermal energy is used to melt the electrolyte, provide energy for the reaction, and vaporize the Mg. Once the system is heated to approximately 1000 degrees C, current is supplied to the system and electrolysis occurs. The MgO is decomposed and Mg forms at the cathode as a liquid and floats to the top of the electrolyte where it vaporizes and exits the cell as a gas. Carbon dioxide forms at the anode and bubbles to the top of the electrolyte, also exiting the cell as a gas. Three cells were manufactured, and there are differences between these cells. The first cell was designed to work with a current of 1 amp. Our process was validated with this cell as Mg was produced at an acceptable efficiency of 85 percent. The purpose of the second cell was to produce more Mg, better approximating a commercial cell. Therefore, the second cell was operated with a current of 20 amps. Experimental testing revealed that this cell had design flaws. Components deformed under the high temperature, causing contact between the anode and the shroud, resulting in a short circuit. In addition, it was not possible to fill the cell with the required amount electrolyte, and the insulation between the anode busbar and the crucible became dislodged, creating another short circuit. These problems were addressed in the third cell, also designed to operate at 20 amps. A reservoir was added to the lid that can be filled with electrolyte and fed into the system. The components that deformed due to the high temperatures were addressed by adding structural strength and increasing the distance between adjacent components. Finally, a mechanical link was added to constrain the insulation between the anode busbar and the crucible, preventing short circuits