Cathode Assembly for Molten Regolith Electrolysis
- Sunday, 01 May 2011
The production of oxygen and metals from off-Earth planetary materials (regolith or rocks) is an important focus area of technology development led by the In-Situ Resource Utilization (ISRU) Project. Over the past few years, oxygen production by direct electrolysis of lunar oxides in molten form has achieved major milestones, and is poised as a high-payoff technology for lunar and Martian missions.
Molten Regolith Electrolysis (MRE) has significant advantages: it can potentially extract all the oxygen contained in the lunar soil, being only limited by the chosen operating temperature and the resulting constraints on the reactor materials. It produces both gaseous oxygen at the anode, and metals in their molten form at the cathode, directly in one step, enabling their use with minimal processing on the lunar surface.
However, the full potential of the technology will be realized only if several critical obstacles are overcome. A major milestone was recently achieved when inert anodes capable of enabling the formation of oxygen on their surfaces, while remaining largely intact in the aggressive oxide melts, were developed and extensively tested.
The viability of the reactor for NASA planetary surface missions relies on proving the concept of a self-heating reactor; it uses the Joule heating from the electrolytic current to sustain its operating temperature at lower energy consumption, and enables sustained processing by self-protecting the reactor from the chemically aggressive oxides and ferroalloys. The concept is in use in the industrial production of aluminum at lower temperatures, and in a different chemistry than the MRE technology. The successful development of a self-heating reactor relies on a reliable cathode containing molten metal and the metal/oxide melt interface. The current state of the art in MRE is not satisfactory, and results in either the dissolution of the cathode in molten metal or the fracture of the containing crucible.
Innovative solutions are needed for the development of a cathode assembly capable of containment and removal of molten materials at regular intervals during continuous operations. Such materials contained in the cathode consist mainly of ferrosilicon alloys and molten silica-rich oxides of compositions found in lunar and Martian materials. The successful development of the critical cathode technology and insertion into a prototype reactor will allow the NASA project to advance toward prototyping for flight consideration.
Molten Regolith Electrolysis presents unique challenges. The reactor operates near 1600 oC to ensure that the iron produced at the cathode remains liquid. Lunar regolith consists of 10% iron oxide by weight, and is one of the first oxides to be reduced according to the electrochemical series. Maintaining iron in the liquid state enables its periodic removal from the furnace without removal of the cathode, and allows the use of techniques such as furnace tapping and siphoning in use in the metal production industry. This choice also brings some unique challenges because of the powerful ability of molten iron to dissolve and alloy with other metals and eroding refractory ceramics. Iron oxide has been the source of several failures of containment crucibles in the current project. The developed cathode assembly must provide both electrification and containment of the iron-rich molten metal pool, and operate without maintenance or replacement for periods of up to one year during space missions.