(153d) Evaluating the Use of Tribocharging in Electrostatic Beneficiation of Lunar Simulant

In electrostatic beneficiation, fine powders are tribocharged by contact with materials of a different composition. Particles in the powder are charged with positive or negative polarities, depending upon their composition relative to the charging material. These charged particles can then undergo electrostatic separation in an electric field based upon their charge-to-mass ratio (q/m). The efficiency of separation is dependant upon the powder's bulk and surface composition, as well as the fineness of the powder. Triboelectric separation of coal from minerals, quartz from feldspar, phosphate rock from silica sand, and phosphorous and silica from iron ore have been successfully achieved in laboratory experiments and pilot plant studies. The potential application of this technology to Lunar regolith may enhance the ability to separate mineral components that could be used for structural materials on the earth's moon. The fine granular regolith and lunar environment are ideal for triboelectrification and electrostatic separation; the electric fields can be stronger in a vacuum, the lack of moisture prevents the regolith grains from sticking together, and the lower gravitational pull increases separation of the charged particles, all of which enhance mineral segregation. This paper will present currently funded research that is evaluating on a lab-scale the use of triboelectrification and electrostatic separation of minerals found in simulated Lunar regolith.

Lunar regolith is a powdery dust with a mean grain size range of 45-100um and characteristics similar to that of silty sand. It has low electrical conductivity and dielectric losses, permitting the accumulation of electrostatic charge. Lunar regolith has high concentrations of aluminum, silicon, calcium, magnesium, iron, manganese, sodium, and titanium oxides (unlike earth's soil). For this research, three different Lunar simulants; NASA JSC-1, Shimuzu Corporation FJS-1, and Arizona Lunar Simulant ALS, were used. They were each analyzed using X-ray Photoelectron Spectroscopy (XPS) and Secondary Electron Microscopy (SEM) to determine mineral surface composition, speciation, and size analysis. The information obtained from these techniques was utilized to help understand how different surface characteristics affect charge transfer during triboelectrification. Different grain size regimes (75-100 µm, 50?75 µm, and < 50 µm) were also examined during this investigation in an effort to find the optimum conditions for beneficiation.

An electrostatic separator was designed and tested by the Kennedy Space Center Electrostatics Laboratory for use in Martian environments. This separator consists of two electrically conducting plates, angled towards each other such that the spacing between them decreases linearly. This allows highly charged particles to be collected at the top and particles with lower charge or mobility to be collected near the bottom (higher E- field); providing for more optimum collection efficiency. High voltages of opposite polarity were applied to each plate to attract the oppositely charged particles. The separator was used on this project for beneficiating the three lunar simulants in both earth's atmosphere and under vacuum conditions.

Four static mixers of different materials were explored for the triboelectrification process; namely aluminum, copper, stainless steel, and polytetrafluoroethylene (PTFE). These materials were selected because they offered a wide variation in work functions amongst the materials (aluminum 4.53 eV, copper 5.11 eV, stainless steel 5.37 eV, and PTFE 5.8 eV). The difference in the work functions of each material influences the charge obtained by the simulant grains. The resulting charge-to-mass ratio was expected to lead to variations in mineral separation and allow for optimization of the process.

This paper will discuss the experimental design and results achieved to date on the use of triboelectrification to beneficiate three different lunar simulants in two unique environments. Specific details will include how surface characteristics between the different simulants and the variation in charge material affect the beneficiation of lunar minerals that may in the future be used to build a lunar outpost.