§ 4.2.3 Separating Minerals by Electrostatic Beneficiation
Minerals come in grains. For example, a scoop of lunar dirt will typically contain a number of minerals, but the different minerals will come in the form of different grains, each grain being a glob of mostly one particular mineral. Usually two or more different mineral grains will be fused together into one, which requires grinding the material in order to separate the grains. However, like sand on a beach, you often see free pure grains beside different free pure grains, or grains predominantly of one kind or another, depending upon origins.
The naturally pulverized lunar soil is like a fine sandy beach.
At the mine, it is easy to scoop up a mix of fine grains and separate pure grains of a particular mineral from the rest, and grains of predominantly one kind or another, using one or more of the following processes:
The material will be initially sieved by screens to separate grains by size. Optionally, the grains of each given size can be passed through the appropriately sized mechanical grinders and sieved again for uniformity.
The next step is to separate the mineral grains by a process called "electrostatic beneficiation", which means charging them with static electricity and separating them by passing them through an electric field, as pictured in the next figure.
An electrostatic beneficiator works because different minerals have different electrostatic affinities -- will absorb different amounts of charge depending upon their composition, and hence are deflected different amounts by an electric field. After grains are sieved by size, they are placed through a beneficiator. After a few passes through beneficiators, we have separated different minerals fairly well. (There's no change in physical or chemical identity; there's only separation of minerals.)
Beneficiators typically use free-fall of grains through electric fields. However, some beneficiators slide the grains down a ramp, and some put them across a rotating drum with a certain electrostatic charge so that grains of a certain affinity will stick to the drum and others will fall to the ground due to gravity or the centrifugal force. Thus, beneficiation separates minerals according to their electrostatic affinity, as well as their different densities (with gravity or the centrifugal force).
The grains are charged by any of the following methods: charging the screen that sieves them, or charging another surface which they slide over, or a diffuse electron beam as they fall. The charging method can depend upon which minerals we want to separate, since different minerals have different responses to different methods (and indeed to different temperatures, too).
The resultant material is collected in different bins whereby the enriched portion of the desired mineral is called the "concentrate" and the rest of the output is called the "gangue" or "tailings".
While on Earth we're usually interested in just one mineral and one bin, on the Moon we will often be interested in using more of the material. With an electrostatic beneficiator we could have multiple bins at the bottom, as the mineral stream will split up into multiple streams depending upon the degree of attraction or repulsion of each mineral.
Whereas electrostatic beneficiation is commonly used at mines on Earth, it world work even better in orbital space or on the Moon, dramatically so. The vacuum of space and the Moon means no air turbulence in the drop chamber. Air does not tolerate electric fields as well as vacuum, and in fact electric fields can be ten times stronger in vacuum. In space and on the Moon, there is no moisture to make grains stick together. Moisture also changes minerals' electric conductivity and reduces the differences between minerals, hence on Earth we often have to roast the material before beneficiation. The one sixth lunar gravity dramatically slows the fall of the material through the electric field, thereby greatly enhancing the separation. If we beneficiate minerals in orbit (e.g., asteroidal minerals), the centrifuges could create artificial gravity of any sensitivity, which would be superior to the Moon's surface as well.
Notably, the naturally fine lunar powder on the surface of the Moon was of keen interest in the early days, as grains would stick to things, and sometimes show levitational properties such as gliding when kicked. This is due to the very high electrostatic affinity of some of the grains. Indeed, lunar dust was a nuisance.
Experiments with simulated lunar soil have produced excellent results using beneficiators in a regular air environment. (Notably, there's also a lot of experience at Earth mines in separating the valuable mineral ilmenite, of particular interest and abundance on the Moon. Some engineering companies focus on ilmenite in their first lunar mission scenarios.)
Metal-producing minerals are not the only targets of beneficiation. Quick production of some kinds of simple glass products are also of interest.
For the Solar Power Satellite (SPS), the General Dynamics report states: "The presence of large quantities of fine glass particles in lunar regolith is particularly relevant to the recommended use of foamed glass as primary structure for the SPS solar array and antennas. Foamed glass is commercially manufactured from fine particles of ground glass by the addition of small quantities of foaming agents and the application of heat. Thus, beneficiation of lunar regolith to recover the large amounts of fine glass particles may permit the direct production of all of the foamed glass needed for the SPS with few or no intermediate steps required to prepare the glass for foaming."
Beneficiation could occur either at a central processing area on the lunar base or at the mine. As is often the case on Earth, locating the beneficiator at the lunar mine could significantly reduce hauling of ore and hence the cost of bigger haulers and more energy, but would require that the beneficiator be mobile. Some designs in the literature have a mobile beneficiator as part of the mobile excavation equipment whereby the waste is left behind in the same spot it was dug up, as landfill.
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