V-6

Mining and Beneficiation of Lunar Ores

RICHARD J. WILLIAMS, DAVID S. McKAY, DAVID GILES AND THEODORE E. BUNCH

The beneficiation of lunar plagioclase and ilmenite ores to feedstock grade permits a rapid growth of the space manufacturing economy by maximizing the production rate of metals and oxygen. A beneficiation scheme based on electrostatic and magnetic separation is preferred over conventional schemes but such a scheme cannot be completely modeled because beneficiation processes are empirical and because some properties of lunar minerals have not been measured. To meet anticipated shipping and processing needs, the peak lunar mining rate will exceed 1000 tons/hr by the fifth year of operation. Such capabilities will be best obtained by automated mining vehicles and conveyor systems rather than trucks. It may be possible to extract about 40 kg of volatiles (60 percent H₂O) by thermally processing the less than 20- ilmenite concentrate extracted from 130 tons of ilmenite ore. A thermodynamic analysis of an extraction process is presented.

INTRODUCTION

It has been proposed that lunar rocks and soils will be useful raw materials for manufacturing materials for use in space construction (refs. 1, 2). Many factors relating to the use of lunar materials as industrial feedstocks have been discussed previously (ref. 3). However, a major topic of considerable practical importance to the utilization of lunar resources, the beneficiation of ores, has not been fully discussed in view of recent work on lunar samples and in lunar science. This paper examines the potentials for the mining and beneficiation of lunar materials.

The beneficiation of ores to an industrial feedstock grade is always desirable before such material is chemically processed, both because it reduces the size and energy requirements of chemical processes and because it can significantly reduce the complexity of such processes while increasing their efficiency (ref. 4). Also, since beneficiation usually requires only differentiation of components by physical properties, it is a relatively low-energy process (refs. 5, 6).

The physical and chemical properties of lunar ores are described elsewhere (refs. 3, 6). (The use of the word "ore" here is somewhat unusual, but we are assuming that the economic benefit of the use of these materials is established.) The availability of lunar materials rich in plagioclase and ilmenite (ref. 6). The proposal of chemical processing schemes to obtain Al and SiO₂ from plagioclase and Fe and Ti from ilmenite (ref. 4). the desire to produce CaOMgO-Al₂O₃-SiO₂ fiberglasses (ref. 7) and the recognition that fine-grained (<20) ilmenite is a potential source of lunar H₂ (ref. 6) have determined the type and scale of mining and beneficiation schemes discussed here.

Two very simple mass-flow economics by which lunar materials might be converted into useful products are illustrated in figure 1. Mining is, of course, confined to the lunar surface. Shipping carries material from the lunar surface to a manufacturing facility located in space; shipping includes packaging materials for transport, launch from the lunar surface, and transport within space from the Moon to the manufacturing facility. Launching and transportation are assumed to use "mass drivers" (refs. 1, 2). At the manufacturing facility (labeled "process" in fig. 1), the ores are converted to useful products. Neither the shipping nor the processing portions of these models is considered in any detail here.

Although the locations of mining and manufacturing are defined, beneficiation can occur either after (fig. 1, route A) or before (fig. 1, route B) shipping. Beneficiation on the lunar surface is developed here because, for a given shipping capacity, the removal of waste by beneficiation before shipping allows a larger mass of products to be produced. A consequence of this choice is that the mining output is larger than it would be for the model A in figure 1. Note that if the manufacturing were also on the lunar surface, an even larger mass of products could be shipped from the lunar surface. Here we consider only model B in figure 1, where mining and beneficiation occur on the lunar surface; the results of other studies (refs. 1-4, 6, 8, 9) are used to set the scales of the lunar surface operations.

Fortunately, lunar soils are generally fine-grained (<100; ref. 3), and crushing is therefore unnecessary. However, disaggregation and size separation are preliminary steps in the beneficiation process. Subsequent beneficiation might involve the separation of a nonmagnetic phase from a weakly magnetic waste for plagioclase ore or of materials with different electrical properties and densities for ilmenite ores. We have proposed two schemes for doing this, one based on established available technology and the other on extrapolations of existing phenomena and technology.

Present Technology

The flow sheet for present technology is shown in figure 2(a). The ilmenite ore is a mature mare soil (ref. 6). The ore is first passed through a coarse sieve to remove fragments larger than 1 cm. After sieving, the material must pass through an airlock; it is then treated with an air cyclone that disaggregates the soil and removes the <20- particles. (No commercial, industrial-scale cyclones of this type exist.) These particles are separated into ilmenite and tailings by magnetic separators. The coarse fraction is separated for density on a pneumatic table and the dense fraction is magnetically refined to yield the ilmenite concentrate. All tailings and products must be transferred out through an airlock for shipment. Plagioclase ore is treated in much the same way, except that magnetic refinement is used to concentrate the plagioclase. In a practical system, several parallel systems will be needed to handle the volume of ore. The cyclones that both disaggregate and separate according to size require large volumes of high-velocity gas (ref. 5). The use of air as a fluid medium for the beneficiation of the ores requires that large amounts of material be transferred from the vacuum of the lunar surface into an atmosphere and then returned to that vacuum. To do this with large amounts of materials at high rates (up to 1000 tons/hr, see below) without significant losses of air (if only as adsorbed gases) is a formidable technical problem.

Advanced Technology

An advanced technology system was designed and analyzed (fig. 2(b)). The ilmenite ore is coarse screened. The ore is sized and disaggregated by an electrostatic device in which the soil is charged and then separated for size based on its trajectory in an electrostatic field under lunar gravity. Figure 3 is a schematic diagram of this device; r and are ranges for lunar materials (ref. 10), and e, for terrestrial minerals (ref. 5).

The device was scaled using lunar gravity and the principles of electrostatics. The primary velocity for particles is supplied by falling in the lunar gravity field; the charging electrodes must supply an electrostatic field gradient in the direction of flow. Separation is achieved by diverting the particle flow into a tangential stream and nullifying a portion of the gravity force with electrostatic force. The resulting particle trajectories should permit separation based on particle size. The feed will vary in all three fundamental parameters (r, , and e) and thus 100 percent efficiency of separation is not possible. Only r and are well known for the various materials that make up lunar soil; e is known only for bulk soils and rocks. The range of r is much greater than for and thus, if e is not the controlling parameter, size separation should be possible. The discharge surfaces are only schematic; a grounded metal conveyor may be needed. An interesting alternative might be to use electrostatic levitation of the particles as a "frictionless" transport mechanism throughout the rest of the system. (A laboratory device that uses electrostatic fields to separate lunar soils according to size is discussed in ref. 11. However, to our knowledge, such a device does not now exist as commercial equipment.) After sizing, particles >200 and those <20 are first subjected to magnetic separation in a weak field to remove about 50 percent of agglutinitic glasses and, finally, ilmenite is electrostatically refined from the remaining silicates in separate streams.

ALthough electrostatic separation of a light fraction from a dark fraction has been demonstrated on lunar material (ref .12), no quantitative data exist for lunar minerals from which to derive operating parameters and detailed sizes of equipment. We note that, in principle, the ilmenite could be beneficiated by a two-step magnetic separation process with fields of different intensities. However, since usual magnetic separators are somewhat heavier and consume more power than electrostatic systems (refs. 5, 13- 15 we prefer electrostatic systems>

Plagioclase ore is treated similarly except the electrostatic refinement is unnecessary and the <20- fraction is collected for use as mass-driver reaction mass and as shielding at the space manufacturing facility (SMF).

Results of Beneficiation

This advanced system is analyzed in figures 4 and 5 in terms of fractions, efficiencies, and ore grade. Table 1 presents the parameters used in the analysis. The fractions ( f ) and grade ( g ) are based on lunar soil data (refs. 3, 6), the efficiencies (e) are estimates. In the absence of specific data on the electrostatic and magnetic properties of lunar minerals, efficiencies based on these properties can only be estimated. We also note that terrestrial beneficiation operations are also largely empirical (ref. 5). It is obvious that primary data should be collected and bench tests conducted.

a_f = fractions in original ore (from petrologic data, refs. 6 and 10), e = efficiency of process (assumed), and g = grade of ore (defined to be the mass fraction of the desired mineral in the mined soil, ref. 6). Subscripts and superscripts refer to the process steps in figures 4 and 5.

Table 2 presents the calculated chemical composition of the ilmenite and plagioclase concentrates. These compositions were obtained by combining chemical analyses of the appropriate lunar minerals with those of the soil type from which the minerals would be extracted using the data on light matrix breccias for the plagioclase concentrate and those for high titanium basalts for the ilmenite (ref. 10). Enrichment factors (the weight ratio of the oxide in the concentrate to that in the appropriate bulk soil for FeO, TiO₂, and Al₂O₃) over ore are substantial but not spectacular. The primary effect of the beneficiation is shown in figure 6. The useful mass (hashed area) is derived from considerations of reaction mass, shielding mass, and processing mass needed at the SMF and is scaled to the low profile model for lunar mass-driver capability (ref.8). The dilution effect of useful mass by waste mass without beneficiation is obvious since useful mass is only 17 percent of mined mass.

MINING SCHEMES

Quantification of Mass Needs

The processes proposed for extracting Al from plagioclase (ref. 4) and Fe and Ti fro ilmenite (ref. 4) and for producing fiberglass from process residues and plagioclase concentrate (ref. 15) can be set into a system to produce feedstock that is 83 percent plagioclase concentrate and 17 percent ilmenite concentrate. The products of such processing are shown in table 3. It appears that the production of fiberglass will require about 4 percent of the total capacity; therefore, the data in table 3 are normalized to this factor. The residue, a rather complex silicate-oxide mixture, is assumed to be an adequate shielding material.

Furthermore, 20 percent of the shipped mass is allocated for use as reaction mass to move mass-driver tugs from L2 to the SMF. We assume that the mass for the first year (30,000 tons) will not be beneficiated or processed by our schemes, and that the residue from chemical processing will meet the shielding needs for the next year. The result is shown in figure 7, a system optimized to deliver maximum processible mass. Note the excess mass accumulates at the SMF during years 2 through 5 (the values are yearly mass budgets that are not cumulative) and that the 20-percent reaction mass is simply an assumption that has not been tested or optimized. After year 5, waste will simply accumulate at the SMF unless further growth occurs; this may present a problem if a static future is assumed.

The production from chemical processing is summarized in figure 8. The percentages are normalized to exclude residues. These production data and beneficiation analysis (figs. 4 and 5) can be back-calculated to scale the mining operations (fig. 9) and waste/useful mass data (fig. 6). Finally, the data are presented in numerical form in table 4. The mining rate we derived from a 3700/hr/yr work time, which is based on the lunar day/night cycle, a loss of one shift at the beginning for startup and one at the end of each day for shutdown, and 7.5-hr production for each 8-hr shift.* (*This working time could be increased by a factor of 2.2 if a source of continuous illumination and power were available; all rates would decrease by this factor. A potentially more severe problem, however, is the recovery of equipment from the extreme cold of the lunar night.)

The mining rates can now be converted to a rough map of the are to be mined (fig. 10a). The style and scale of mining changes after year 2 ; those for year 1 are small and can be handled by simple trucks. Table 5 gives estimates of the size, mass, and power of the processing modules. These values are based on data from references 5 and 12-14, but, in the absence of experimental data on beneficiation of lunar ores, they should considered no more than order-of-magnitude estimates. Masses could be significantly less on the Moon. The masses of conveyors, supports, and module housing were not considered in table 5 . Each module in table 5 is sized to accept over 220 tons/hr; they should meet the maximum mass flow need with about 10 percent excess capacity. The module in table 5b handles a <20- ilmenite fraction; only one should be needed even at maximum flows. Figure 10b summarizes a site layout and emplacement strategy. Although the separation of fine-grained ilmenite is primarily dictated by its potential as an H₂ ore, its separation does supplement ilmenite production by about 25 percent and thus its early emplacement is favored.

Figure 10(b) also illustrates another recommended part of a mining system - stockpiles. Space must be provided for stockpiles since they are essential so that, if a subsystem breaks down, the entire system will not be automatically crippled. The locations of the packaging plant and mass driver were not indicated in figure 10. Although we are certain that optimum sites for a mass driver and mining operations can be found and developed, we are not as confident that both will exist within a few hundred meters of each other. A connecting delivery system should be considered in any final base plans, but cannot be adequately quantified until site locations are defined. Finally, a problem implicit in figure 6 must be considered briefly. Considerable waste mass will be generated during these operations (3X10⁶ tons, i.e., ~3X10⁶ m³ /yr at maximum mining rates). Although conveniently located craters can be filled with wastes initially, back-filling the mine site is the only feasible long-term solution. We have not analyzed this problem fully.

Table 6 presents an analysis of bottom scraper design in terms of load, size, speed, mining rate, and haul distance. The minimum pit separation that can be expected is 300 m refs. 6; we have used this value and the pit sizes (table 4) to compute minimum haul distance and thus to compute trucking needs for lunar mining (table 7). Such vehicles would require about 100 kW per vehicle, within the range of fuel cell systems of reasonable size and mass. However, it is obvious that, for large-scale mining, the vehicles and mass involved become very large and are probably not feasible even without considerations of haul road construction and maintenance.

TABLE 6 .-PARAMETERIZATION OF TRUCKING

c = capacity, tons

s = maximum speed, km/hr

L = one-way haul length, m

MR = mining rate tons/hr

f = duty cycle

N = number of trucks

x = ratio of loading and unloading speed to s

d = depth of cut

= length of cut, m

w = width of cut, m

= density

= c/(dw)

N = 2000 f (MR/cs)(xL + )

Since it is more likely that the pits will be several kilometers apart figure 10(a), refs. 6); trucks will probably not be a feasible option, and a conveyor system, such as that shown in figure 10(a), will be required. However, because of the general mobility and utility of trucks, several should be included in any base design. For comparison, an equivalent conveyor system is presented. The masses reported are for one belt system of equivalent length; power requirements will be about 100 kW/km. Finally, N represents the number of 50-ton capacity loaders necessary to load the conveyor, derived by extrapolating to zero-haul distance for the parameterization in (table 7).

An automated mining system of the required capacity could be developed: possibly a moderate-sized bucket-wheel excavator (ref. 19) that would feed a shiftable conveyor system (ref. 18). A commercial vehicle capable of handling 1000 tons/hr would have a terrestrial mass of 460 tons and would require about 715 kW. Since much of the mass is structural support, a lunar excavator would be less massive. Experience indicates that bucket wheels on the excavator would last about 150-200 hr (ref. 19) however, a longer lifetime could be expected in excavating loose soil. Replacement parts and facilities for refitting and retooling must be available near the mine site. Maintenance and repair requirements must be considered in any advanced mining and beneficiation models.

A mature mining operation would proceed as follows: a shiftable conveyor would parallel the strip to be mined (fig. 10(a)). The bucket-wheel excavator would mine a 50-m-wide strip parallel to the belt. After the strip had been mined to a depth of 2 m, the excavator would move to the other pit and follow the same pattern. The unworked pit is backfilled with tailing from beneficiation. The shiftable belt would be pulled 50 m radially into its new location by trucks (fig. 10(a)). Each strip should be mined in 12.5 24-hr days. The remaining 1.5 days of the lunation would be used to startup and shutdown the operation. It will take almost 10 years to mine the two 2-km pits shown in figure 10(a) at 1000 tons/hr. Stripping to depths greater than 2 m should be possible; however, we have used 2 in for analysis because regoliths of this depth can be expected at almost any location on the lunar surface.

Summary of Mining and Beneficiation Scenario

Table 8 summarizes the lunar mining and beneficiaon scenario and the mass and installed-power requirements. The mass and power estimates were presented earlier and are subject to the constraints as noted. To start the scenario, year -1 was chosen to display the predevelopment mapping required. A rational mining and beneficiation scenario cannot be developed without this step. Mining output for year 1 is too small to be analyzed in the present context: it is quite literally a dump-truck and shovel operation that can be handled by base construction vehicles.

TABLE 8 .-MINING AND BENEFICIATION SCENARIO

Year (lunar)	Activity	Mass, tons	Power, kW
-1	Detailed mapping and exploration of site	-	-
1	30,000 tons/yr, unbeneficiated Only coarse sievesa	2	2
	Mining by base construction Vehicles	10	100
2	717,000 tons/yr, beneficiated One plagioclase/ilmenite modulea	77	272
	Two 2-km permanent conveyor	40	400
	Two large truck systems	40	200
	One time ilmenite modulea	77	115
3	1,800,000 tons/yr beneficiated Two plagioclase/ilmenite modulesa	154	544
	Two shiftable conveyor systems	40	400
4	2,726,000 tons/yr, beneficiated One plagioclase/ilmenite modulea	77	272
	One bucket wheel excavatora	460	715
5	3,656,000 tons/yr, beneficiated One plagioclase/ilmenite modulea	77	272
-	-	1054	3492

Although much of the mining and beneficiation operation is amenable to automation, it cannot be unmanned. An integral part of the operation will be to clear boulders and hazards and to survey the mining cuts so that the excavator can be used to its fullest. The basic device used will probably be a clamshell shovel and bulldozer derived from the base construction vehicles; it will probably be manned. Additional personnel will be required to maintain the machinery and to ensure quality control of the ore and concentrates. We estimate that at least 10 men/shift (30 total) without support personnel will be required, even with automation.

EXTRACTION OF VOLATILES FROM FINE-GRAINED ILMENITE

Much of the beneficiation effort will be expended in obtaining a fine-grained ilmenite concentrate because of its potential as a hydrogen ore (ref. 6). We performed a thermodynamic analysis of a device that could separate volatiles from the ore. Figure 11 illustrates a separation device and presents the equations that describe its physical behavior. Its chemical behavior is calculated from thermochemistry of C-O-H-S-N gases (ref. 20), assuming local equilibrium and the buffer conditions (ref. 20), given in Figure 11 . From gas release data and the boundary conditions in table 9, we computed the yields of volatiles. Such a system may offer a way to extract water from a lunar rock; it might be used either on the lunar surface or at the SMF to supplement water supplies.

TABLE 9 .-LUNAR GAS EXTRACTOR

Species	10X(g/T )	20X(g/T )
H2O (g)	2	70
H2	3	109
N2	24	765
CO	48	1,530
CO2	81	2,576
F	0.82	0.91
H2O ()	5,340	23,270
V1	2.83X105 cc	2.83X105 cc
V2	3.71X105 cc	1.06X107 cc

The device functions as follows: volume V₁, is filled with fine-grained ilmenite and connected to evacuated volume V₂ by a pipe. While V₂ is maintained at 300 K, V₁, is heated to 1000 K, perhaps by a solar concentrator. The temperatures are chosen so that liquid water can condense in V₂ and so that almost all the H₂ will be released from the powder in V₁, (the powder being not quite hot enough to sinter rapidly). The released volatiles react among themselves and with the powder to reduce some ilmenite to iron; the hydrogen and carbon are oxidized to water, carbon dioxide, and carbon monoxide. Sulfur reacts with iron to form FeS which reduces sulfur gas species to negligible levels. Nitrogen and helium are unreacted. The partial pressure of water is great enough to cause liquid water to form in V₂; this extraction of water from the vapor causes most of the hydrogen to react to water. After reaction, the device can then be cooled and liquid water and gases tapped from V₂.

The release of volatiles during mining and beneficiation might decrease the amounts available for extraction. However, this loss should be less than 0.1 percent of the total content (analysis from data given in ref. 22). Finally, the reduction of ilmenite may be kinetically inhibited (ref. 23). to such a degree that local equilibrium cannot be assumed (but this can only be tested experimentally).

SUMMARY AND CONCLUSIONS

The lunar mining operation described here is not enormous in comparison to large terrestrial operations. The mining rates are large enough to warrant the use of bucket-wheel excavators and conveyors. Such systems are commercially available for terrestrial use and are amenable to high degrees of automation. However, the need for maintenance and for clearing and surveying operations suggests that the installation will be manned.

The beneficiation of lunar plagioclase and ilmenite ores is possible with existing technology. However, more advanced techniques based on electrostatics and magnetics should be developed; these techniques require a determination of the properties of lunar minerals and benchtop testing of devices on lunar materials or their similants. Beneficiation produces a system which is maximized in terms of useful mass that reaches chemical processing. The beneficiation of lunar ores on the lunar surface is useful and possible, but a full analysis must await the results of laboratory studies.

The mining and beneficiation of lunar materials is neither an enormously large operation nor is it a small operation. Consequently, it is difficult to evaluate the sensitivity of economic models of nonterrestrial industries to these operations. Further study, particularly of other models and optimization, should be undertaken.

References

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Curator: Al Globus If you find any errors on this page contact Al Globus.

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