4. Temperatures of separation (a) The magnetic properties of lunar soils and rocks have been the subject of numerous studies and the amenability of these materials to physical separation on the basis of magnetic properties is well established. The operation of magnets at lunar temperatures poses no obvious problem. Superconducting magnets require low temperatures, electromagnets are commonly operated in liquid nitrogen baths to reduce resistance, and iron return structures and permanent magnet properties are well known at low temperatures. Adams and McCord describe a method for separating agglutinates from bulk lunar soil by magnetic separation (12). The working fluid was ethanol in a pipette set in a Frantz Isodynamic Separator operated at 0.5 A. Although the field was not reported, experience with this type of separator indicates that the field should have been about 0.6 T ranging to lower values in the wider part of the pole gap. Their results from microscopic examination show that the fractions were at least 95% pure agglutinates or nonagglutinates depending on the product stream. Ilmenite, silicates, and dark breccia fragments were not recovered magnetically except when they occurred as impurities containing free iron. They quote several studies which show that these agglutinates contain metallic iron inclusions. These inclusions are as small as 40 A, which would place them in the single domain size range giving rise to a high susceptibility. Apparently, a magnetite (ferrimagnetic) structure has also been observed. Goldstein and Axon made a metallographic and electron probe study of the 284 “most magnetic” particles of four size fractions, > 74 to > 707 /zm in Apollo 16 soils. In these sizes, the original percentages of magnetically extracted metal ranged from 0.08 to 0.22 and did not include thoses particles < 74 p.m or those less magnetic. The origin of these particles is concluded to be liberation by temperature cycling and shock processes. A large number of these are free of associated minerals such as silica (13). A simple magnetic separator could collect such liberated metals easily. Romig and Goldstein estimate that there are 3.9 x IO10 metric tons of free iron alloy (with Ni, Co, P, S, and Cr) in the top 5 cm of lunar regolith. Larger amounts are thought to be available at greater depth and from crushed rock. They advocate powder metallurgy for small component fabrication on the lunar surface because of the environment and relatively low energy requirement (14). This technique might be used to form the magnet structures to be used for further collection. In addition to the free iron available in lunar soil, about half of the Al-rich anorthite occurs in soil particles with 10%-30% A12O3 (15). These occur to considerable depths, on the order of meters. A terrestrial ore similar to this is Wyoming anorthite in which Alcoa has an interest. This occurs as rock which would require crushing before beneficiation. There is also fine grained ilmenite which has a high helium content and might also contain hydrogen. Thus, this mineral, desirable for itself, could be considered a source of the cryogenic gases needed for the magnet system. For example, one ton of lunar soil containing 0.1 cm3/g (STP) would yield about 4 moles of helium or 0.13 liters of liquid at 4 K. In the case of hydrogen, in regions where soils contain 5% ilmenite and assuming 2000 ppm in the size range < 20 /xm, which accounts for about 10% of the ilmenite, one ton of soil would yield about 13 liters of liquid hydrogen stored at 20 K (16). The radiation darkening of optical glass products might be prevented by removal of color centers containing iron and titanium. Mackenzie has suggested the manufacturing of solar cell cover glasses and fiber optics for use in space and on Earth (16). Sorting out soil-derived glasses according to impurity content by physical means such as magnetic separation would be an efficient way to do this.
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