contain ~ 100 ppm hydrogen, implanted into the surfaces of the grains from the solar wind (4). The amount held varies with the nature of the substrate, and an ilmenite concentrate may yield much more hydrogen (or water) on heating than other constituents of the soil (5). Considerable attention has been given to the problems of obtaining an ilmenite concentrate for this purpose. If an ilmenite concentrate (or any other fraction) is made and subsequently heated, certainly the evolved gases should be saved. But for large-scale production of water, it would seem more reasonable to use raw, untreated soil. The heating of large volumes of a fine, powdery material in vacuum poses severe heat-transfer problems, but if a heat-exchange fluid could be used, these problems would be eliminated. Such a process was proposed by Wechsler et al. (10) in a paper published before Apollo; the proposal has since been largely ignored. Wechsler et al. proposed the use of pumped circulation of the evolved volatile materials themselves as a heat-exchange fluid (with an external heater), to both heat and pump the soil in a continuous countercurrent process. Heat loss via the effluent spent soil would be minimized by its transfer to new feed in a countercurrent fluidized-bed heat exchanger. Because lunar soils evolve not only hydrogen and water on heating but also CO2 and CO (11), both hydrogen and carbon could be recovered in such a process. Parallels with Terrestrial Pyrometallurgical Practice Various nonaqueous chemical processing procedures have been proposed to obtain specific materials from the lunar soils (e.g., 3, 12). Many of the flow sheets for these processes are based on theoretical considerations or adaptations of existing extractive pyrometallurgical practice, and quite obviously these will undergo considerable further changes when tried in a terrestrial pilot plant on simulated lunar materials. The literature on such practice is extensive and scattered, and information useful to lunar processing may be available but unrecognized in the literature on seemingly irrelevant processes. For example, the Torco "copper segregation process,” successfully used in Africa and elsewhere on a fairly large scale on oxidized copper ores, has some parallels, both in chemistry and technology, with some of the processes suggested for lunar "ores.” It involves a simultaneous chloridization, volatilization, transport and reduction of the copper (13). A considerable body of seemingly irrelevant terrestrial metallurgical technology may still be pertinent and should be drawn on for help in anticipating and hence avoiding otherwise unexpected and potentially serious problems. Most important, some of these problems may not appear in laboratory or small pilot plant operations. A good example from the steel industry is found in the study of blast furnace refractories. The iron blast furnace is a continuous process, in which ore, coke, and flux are charged in at the top and molten metal and slag are tapped out the bottom. The important parallel with some of the proposed lunar processes comes about because the fresh cold charge is heated (and in part reduced) by the effluent gases. This countercurrent flow is a common heat-saving feature of many pyrometallurgical processes. However, at the temperature of the hottest zone of the blast furnace, ~ 1650 °C, many seemingly nonvolatile constituents in the charge have significant vapor pressures and volatilize, at least in part. Zinc and alkalies are such substances. As a result of the countercurrent flow, however, the vaporized zinc and alkalies condense on the colder charge and are hence recycled. Even though very little alkali
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