Plasma Extraction of Metals in Space - Juniper Publishers
Journal of Insights in Mining Science & Technology
Abstract
Extraction
of purified metals from extraterrestrial materials can be accomplished in
several ways, such as beneficiation, hydrogen reduction, recovery of spent rockets, direct melting of
iron meteorites, and plasma isotope separation. Presented here is a method for
multiple simultaneous extraction
of multiple metals and metalloids from regolith. Two patented approaches are
described which can operate on a planetary surface, or in the microgravity environment of orbit.
This approach to isotope separation applies to regolith fines but is
advantageously applied to the effluent
of a patented oxygen extraction method. In this way, a plurality of valuable
raw materials can be obtained with a single system, suitable for operation on the Moon or at the surface
of an asteroid. Silicon is of interest for studies of purity due to its
importance in photovoltaics. A silicon-aluminum
aerospace alloy can be produced directly, called Silumin, which has value in
construction of habitats and space craft in space. Silicon can also be combined with carbon to
form the wide-bandgap semiconductor SiC from which high-power and
radiation-tolerant power transistors
can be fabricated. Furthermore, this method lends itself to additive
manufacturing whereby specific shapes of purified metals can be formed directly from the plasma extraction
process.
Keywords: Extraterrestrial materials; Space solar
power; Gravity; Radiation; Vacuum; Asteroids; Ammonia
Introduction
In situ
resource utilization (ISRU) involves extraction of raw materials and the making of value-added
components from materials found
in space. Earth launch requires enormous energies
and will always be costly. Employing indigenous extraterrestrial
materials enables greater exploitation of space resources,
which can be used in an orbital economy, or delivered back to markets on the home world. One
lucrative prospect is to build
vast solar farms which beam power gigawatts of power
wirelessly to customers on the ground. Known as space solar power (SSP) [1,2] the economics at
large scale is highly favorable
if the photovoltaics, structural elements, wires, and power electronics can be fabricated from in
situ resources [3]. This study
explores a new method which is the subject of three US patents by the author [4-6].Space resources include sunlight, vacuum,
gases, variable gravity,
radiation, and solid matter in the form of asteroids and planetary bodies. Asteroids are grouped into
categories and may include: low
atomic weight volatiles such as water, methane, and
ammonia; carbon-rich amalgams with silicate-based rocks; stony iron bodies with substantial amounts
of relatively pure transition
metals; and rocky asteroids with mostly silicate-based
minerals containing other elements including, but not limited to: calcium, magnesium, aluminum,
and titanium [7-10]. Terrestrial
methods of mining and extraction generally rely on strong gravity and water. Without these,
mining and extraction in space
is difficult. Delivery of water into orbit is expensive, and production of high gravity (e.g. 9.8 m/s2)
requires a large centrifuge or
rocket accelerations, so that extractive techniques call for a thorough re-thinking for ISRU.Several extractive methods have been
developed for use in space,
such as oxygen for life support, water for rocket fuel, titanium for structures, ceramics and
refractories for building blocks
and roadways, and iron for railroads. This paper studies an energetic means by which multiple,
simultaneous streams of pure
metals can be extracted from the broken and shattered surface rock dust known as regolith.
Regolith covers all airless bodies
in space. Within this powdery tuff can be found a wide variety of chondrites, spherules, brecciated
minerals, and free iron. The
goal of this plasma extraction method is to heat such regolith enough to disaggregate into
individual atoms which can be
separated by mass. The energy penalty is high; however, the payoff is in a high yield of several
useful raw materials in a manner
well-suitable to further value-added operations.
Apparatus
High
temperature is an equalizer for regolith because at
sufficiently high temperatures, physical morphology is irrelevant. However, molten regolith is an
excellent solvent, and will
compromise most refractory crucibles. Liquid regolith simulant in yttria-stabilized zircon will
melt together into a Abstract Extraction of purified metals from
extraterrestrial materials can be accomplished in several ways, such as
beneficiation, hydrogen reduction, recovery
of spent rockets, direct melting of iron meteorites, and plasma isotope
separation. Presented here is a method for multiple simultaneous extraction of multiple metals and metalloids
from regolith. Two patented approaches are described which can operate on a
planetary surface, or in the
microgravity environment of orbit. This approach to isotope separation applies
to regolith fines but is advantageously applied to the effluent of a patented oxygen extraction
method. In this way, a plurality of valuable raw materials can be obtained with
a single system, suitable for
operation on the Moon or at the surface of an asteroid. Silicon is of interest
for studies of purity due to its importance in photovoltaics. A silicon-aluminum aerospace alloy can be
produced directly, called Silumin, which has value in construction of habitats
and space craft in space. Silicon
can also be combined with carbon to form the wide-bandgap semiconductor SiC
from which high-power and radiation-tolerant power transistors can be fabricated. Furthermore,
this method lends itself to additive manufacturing whereby specific shapes of
purified metals can be formed
directly from the plasma extraction process.Keywords:Extraterrestrial materials; Space solar
power; Gravity; Radiation; Vacuum; Asteroids; Ammonia puddle. The only refractory known to
withstand molten silicates is
thorium dioxide [11]. Thoria has the highest melting point of any metal oxide and has no transition
temperatures up to 2950 K
[12,13]. When heated to these temperatures, molten regolith will spontaneously devolve oxygen gas.
Oxygen can be captured by
absorbents using a deLaval nozzle, drift tube, and expansion bell [14], as seen in the center of Figure
1. The effluent of oxygen extraction
is ballistic nanometer-sized reduced-oxygen minerals (e.g. SiO, FeO), and this stream is the input
to the isotope separation
apparatus [15,16].
Figure 2
shows a cross-section schematic of the combined oxygen
extraction “dust roaster” and the plasma metals extractor “isotope separator” in the same
configuration as Figure 1. Material enters
at the top left and exits at the gas expansion bell, and then at the metals collection receptacles (items
41). Operation of the oxygen
dust roaster is to first liquefy the regolith via heater elements exterior to the thoria containment
vessel. Apertures in the bottom
of the liquefaction vessel admit magma to a vertical (thoria) tube which is heated by
radiofrequency (rf ) energy to engage
the conductive liquid [17] as it falls in lunar gravity and raises the temperature to that of a vapor.
Note that in zero gravity this
portion would require a centrifugally rotating apparatus to provide the appropriate artificial gravity
(approximately 0.18 of earth’s).
Not all mineral species will be vaporized, so that oxides of calcium and magnesium will fall into a
sealed pit from which it can be
later extracted to make refractory bricks. The supersonic deLaval nozzle in the vertical tube provides
escape for vaporized materials,
which pass through a short drift tube to cool slightly. This cooling region is on the order of
several centimeters, during which
the sub-oxide minerals will begin to coalesce, like the exhaust plume of a solid-fuel rocket motor.
Oxygen liberated from the
minerals will experience expansion upon reaching the
gas collection bell and is directed to absorption collectors made from praesidium-cerium oxide. These
collectors must be periodically
detached to thermally release their load of pure oxygen
[18]. The coalescing minerals become ballistic, meaning they are no longer entrained by the gases,
and escape the gas collection
bell through an aperture. The velocity of this stream carries material into the isotope separator.
Note that the isotope separator
is designed to also accept regolith fines which have been beneficiated to sub-micron dimension
but without the preliminary step
of extracting the oxygen. In that case, an optional
impeller is provided to deliver the initial kinetic velocity to these particles (item 10 in Figure 2).
The isotope
separator works in a similar manner to a mass spectrometer,
wherein ionized elements are accelerated in a plasma
beam which is passed through a transverse electric or magnetic field (item 38 in Figure 2) to
deflect the ions. Lighter ions
are deflected further than heavier ions, so the result is a spectrum of elements (dotted lines 42 in Figure
2), with those having higher
atomic numbers clustered closer to the beam axis, and the low atomic number elements deflected
at larger and larger angles.
Because of this, the separation efficiency is greatest for metals such as silicon and aluminum. Hot
ions impinge on the collection
receptacles, which are optionally cooled to accrete amorphous masses of purified metals. The
metals must be separated from
the receptacle, which may be advantageously formed
from the refractory CaO and MgO mentioned above. Non-ionized material will not be deflected and
can be captured in receptacle
46. The workpiece receiving non-ionized material can be manipulated in orientation to produce
convex shapes of this slag.
Such items may find use as building or shielding materials for space settlements.
Methods
To study
plasma dissociation of sub-oxide minerals, the silicon
monoxide (SiO) molecule is most prevalent from regolith, and important because of the silicon it
contains. The dissociation energy
of SiO is 460 kJ per mole of the molecule. This value can be used to estimate the minimum power to
break apart molecules into
their constituent atoms as a function of beam flow
rate in moles/second. These atoms must then be ionized, and the ionization energy for silicon is 786
kJ/mol. The moving beam of
ionized elements represents a current, which, induced onto the stream to motivate the ions in the
direction intended, adds a
third component of electrical power for the most energy-intensive operations in the isotope
separator. For a reference case
of 11.5 g/sec of regolith (total), this requires a primary power of 139kW, with silicon extraction of
10.4 kg/hour.Separation of
isotopes goes by the quotient of electric charge
by ion mass, called the charge-mass ratio (q/m). The Lorentz force deflects ions from a
collimated beam, creating a geometrically
simple separation, either by radius in the case of a transverse magnetic field, or by angle in
the case of a transverse electric
field. Figure 3 shows the elements adjacent to silicon, along with their global average abundance on
the moon. The ionization energy
mentioned above must be imparted to each ion
for separation to occur, so the first ionization energy is also listed in Figure 3. Figure 3 does not tell
the entire story of isotope separation.
There can exist resonances wherein the second ionization
(q=+2) of an element twice as massive can coincide with the stream of a singly ionized lighter
element. For example, heavy iron
(Fe58) is 1.999 the mass of Si29; fortunately, the second ionization energy for iron is 1561.9
kJ/mol. In practice, the amount
of energy imparted by rf fields is to be carefully calibrated to avoid resonances.
Results
Three means
of obtaining pure elements are: a)
physical apertures, with the
drawback of buildupb) isolated receptaclesc) capturing
an entire spectrum of element and excising those
slivers of desired puritySilicon
is bracketed by phosphorus and aluminum. Both P and
Al are dopants in semiconductor silicon, p-type and n-type respectively, therefore silicon intended for
photovoltaics or integrated
circuits should be free of these elements. Assuming method (2) above, the results of q/m from a
transverse electric field are
shown in Figure 4. The left plot shows the separation by q/m “bins” for P, Si, and Al with per mol
ionization energy less than
1100 kJ. The right plot in Figure 4 shows what happens if ionization energy exceeds that of Fe and Ni
but remains lesser than the
(generally much greater) third ionization energy of elements three times as massive as Si. It is
seen that the purity of each
element depends on the overlap of similar isotope q/m ratios, as well as the fineness of the
aperture dividing one isotope
from the next. Another factor is the spread in the velocity distribution. Although this method of
plasma separation of isotopes
is not in equilibrium, tendencies in that direction will approach a Maxwell-Boltzmann
distribution of velocities, which
has an appreciable dispersion. This issue can be partly ameliorated by beam profile techniques, or
by velocity filter mechanisms.
Beam diameter is another parameter which will affect
purity because of parallax between ions emerging from different points across a beam cross
section. It can be appreciated that
a trade exists between yield and purity. Furthermore, self-shielding by the beam of material will raise
the energy required for full
dissociation and ionization. Thus, it can be further appreciated that there exists a trade space
between throughput and
efficiency.
Conclusion
For beam
ionizations below 1100 kJ/mol a clean separation of
silicon from other elements is possible. Pure aluminum is similarly accessible, as is the small amount
of phosphorus found in most
regolith samples. Together, these three elements can
become the basis for complete solar cells and rudimentary integrated circuits. A mining operation
producing these materials may
find customers willing to produce solar power and computer chips in space. One example is the solar
power satellite known as the
“tin can” [19], with the potential to provide gigawatts of power to paying terrestrial customers. By
combining the receptacles for
q/m ratios 3.55 and 3.75, in various degrees of overlap,
an aluminum alloy containing silicon (“Silumin”) can be produced. Silumin has been used for
aerospace applications and may
be of interest to customers building habitats or spacecraft on the moon or at orbital platforms. Heavier
elements such as iron, nickel
and cobalt will not separate well, but may be blended
in a fashion like the alloying of Silumin to make various steels and ferromagnetic metals of use in
rail launchers and circumpolar
railroads for remaining in constant sunlight while remaining on the moon. Further optimization
work is planned through
simulation and modeling tools to explore the trade spaces identified by this work.StatementThe
author declares no financial conflict of interest related to this work. Portions of this work were
presented at the International
Space Development Conference in Chicago, Illinois, US in 2010.References1.
Schubert PJ (2005) A Novel
Method for Element Beneficiation Applied to
Solar Panel Production. Space Exploration, Albuquerque, NM. 2. Schubert
PJ ((2006) Synergistic Construction Mechanisms for Habitats in Space Environs. International Space
Development Conference las Angeles
CA.3. Schubert PJ (2009) Energy and Mass Balance
for a Cislunar Architecture supporting
SSP. AIAA SPACE 09 Pasadena CA.
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