Proposed Extraction Schemes
In the following section, a variety of plausible processing technologies are described. In general it may be said that these technologies are derived from a limited amount of terrestrial experience and are totally untested in application to the extraction of nonterrestrial resources. Work must be initiated in the near future to sort out these and other proposals, for it is the experience of the metallurgical industry that such processes require much development and testing, at bench-top and pilot-plant levels, before production facilities are achieved. Development times of 10 to 25 years have been experienced, and many false steps have been taken. For example, an Alcoa plant for extraction of aluminum metal from anorthite by carbochlorination and electrolysis of aluminum chloride was constructed at a cost of $25 million, only to be shut down after a short operating time because of technical problems.
Many process schemes have been proposed for recovering a variety of metal products and volatiles from a variety of lunar feedstocks. A collection of pyrometallurgical, electrometallurgical, and hydrometallurgical approaches have been proposed, with varying amounts of research and engineering data to support them. The participants in this study looked at scenarios for the development of space resources in the next 25 years. The materials subgroup of study participants reached the consensus that, within this timeframe, oxygen production from lunar resources will be the major objective of the space program. The participants also assessed what metals recovery technologies can be implemented and suggested what time would be required to develop more complex metal processing technologies.
Recovery of Meteoritic Iron
History and thermodynamics teach us that the most appropriate metal to recover first is iron. The most basic approach to recovering iron on the Moon is to process the meteoritic iron found in lunar soils. Small chunks of meteoritic iron have been found in lunar samples, but most metal exists in the form of micron-size particles, particles encapsulated by or attached to silicates or glasses. The first processing effort should be to concentrate this elemental iron by mineral beneficiation techniques (such as magnetic separation). Current knowledge does not suggest that regions of elevated metal content can be found, but additional information on the abundance and ease of separation of the metal is needed. In addition, trace elements in the iron must be identified. Alloying elements or solutes can have a profound effect on the iron's mechanical properties. The iron and steel industry has probably never looked at iron alloys of lunar composition, because carbon and other solutes are always introduced in the smelting process.
The utilization of meteoritic iron is suggested as a first approach because it does not require the energy-intensive and chemically complex step of extraction. However, a process for separating the metal from adhering silicate minerals must be developed. The recovered lunar iron can either be processed directly into parts by powder metallurgical techniques or be melted at about 1550°C and cast into ingots for wrought products.
Even though this is the simplest process for producing lunar iron and the technologies for beneficiation, melting, and pressing or casting and forming are fairly well developed on Earth, the application of these technologies in the lunar environment will present many challenges.
Processing of Lunar Ilmenite
The next simplest recovering iron is ilmenite (FeTiO3). process for to reduce it from For the reaction
FeTiO3 + H2 = Fe° + TiO2 + H2O
the free energy at 1000°C is 5.86 kcal/mole. This gives a very low equilibrium constant Kc = 0.103 at 1000°C. Thus, the reduction of ilmenite to reduced iron plus TiO2 is not strongly favored thermodynamically and may not proceed to completion. Furthermore, the reaction will not proceed to the right unless H2O (g) is removed from the system. Thus, if thermodynamically favorable, the rate of reaction must be determined. .
Another ilmenite reduction scheme based on a commercial process has been suggested for the Moon (Rao et al. 1979). In it the ilmenite is reacted with carbon to reduce FeO to Fe°. The iron from the ilmenite is chlorinated at 800°C in a fluidized bed reactor while the TiO2 remains unchanged.
The FeCl3 gas is condensed and could be reacted with oxygen gas at 300-350°C in a second fluidized bed to produce Fe203. The Fe203 could then be reduced with either carbon or hydrogen gas below 1 OOO°C to produce low-carbon steel or iron. The CO or H2O formed would be recycled to recover the oxygen. Alternatively, the FeCl3 could be reduced directly to metallic iron with hydrogen at 700°C. The hydrogen chloride formed as a byproduct would be recycled. The flow diagram for this conceptual process is shown in figure 37.
The residue in this process is TiO2. which can be further processed to recover titanium metal. Since titanium forms a highly stable oxide, it cannot be reduced with carbon or hydrogen. It can, however, be reduced with calcium metal. A process has been developed to perform this reduction by pelletizing TiO2 and calcium metal powders and heating at 925-9500 C for several hours. The CaO is then preferentially dissolved by acid leaching. Disadvantages here are that the acid and water must be recycled and that water is not available at the site. Calcium metal could be provided from processing of anorthite for aluminum.
The most likely end product of ilmenite processing is a partially reduced mixture of Fe, FeO, TiO2, and other impurities. There are several possible options for using this type of product.
The simplest is not to try to recover the iron but rather to use the material as is to form" cermet" blocks for construction. Assuming that 50- to 200-pm particles are produced from the fluidized bed reactor, sintering may be successful at temperatures just below the melting point of iron. To evaluate this option further, considerable test work on pressing (using, for example, a hot isostatic press) should be done on simulated residues. If this option proves unsuccessful, the iron-titania residue may simply be stockpiled until such time as more advanced processing technology is developed for the lunar site.
If iron recovery is required and the chlorination process does not prove feasible, simply melting the iron out of the residue may successfully produce a crude iron alloy suitable for structural uses. Alternatively, a low-temperature carbonyl process could be utilized to extract the iron.
Other Proposed Processes
For lunar processes to be successfully developed, certain guidelines must be kept in mind.
These include the following:
The NASA SP-428 paper (Rao et al. 1979) was written by recognized metallurgical experts who did a competent job of assessing the available thermodynamic and kinetic literature for several aluminum, titanium, iron, magnesium, and oxygen extraction processes. Their analysis of the research done and needed may stand up to scientific scrutiny by their peers. None of their candidate processes, however, has been sufficiently tested to provide the data needed for process plant design by a competent engineering company.
A number of other processes have been referred to at this workshop. These include acid leaching, alkali leaching, fluorination, electrolysis, basalt vaporization, plasma smelting, and sulfide processing. Some of these processes can be summarily dismissed for such reasons as requiring large amounts of water or of a nonlunar reductant, impracticality of recycling, or requiring extraordinary amounts of energy. For those few which may warrant less cursory evaluation, the basic scientific data have not yet been provided.
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