Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Title of Invention
PROCESS FOR PRODUCING REFINED METAL OR METALLOID
Technical Field
[0001] The present invention relates to process for producing a refined
metal or metalloid.
Background Art
[0002] Metallurgical grade silicon is produced by mixing carbon with
silica and performing reduction in an arc furnace. Trichlorosilane is
synthesized by the reaction of the metallurgical grade silicon with HCI,
and refined by rectification; subsequently, reduction is performed at a
high temperature using hydrogen; thus, semiconductor grade silicon is
produced. At present, as a principal raw material for silicon used for
solar cells, an off-grade product produced during the manufacturing of
the semiconductor grade silicon is used.
[0003] In the process for producing the semiconductor grade silicon,
silicon with extremely high purity can be produced, but the process is
expensive for the following reasons: the conversion rate from
trichlorosilane gas to silicon is low and a large amount of hydrogen is
needed to make equilibrium to silicon advantageous; a large amount of
non-reacted gas needs to be circulated and reused in order to increase
the conversion ratio; a variety of silane halides coexist in the
non-reacted trichlorosilane gas, and therefore separation by rectification
is needed again; a large amount of silicon tetrachloride that cannot be
reduced with hydrogen finally is produced, and the like.
[0004] On the other hand, solar cells receive attention as a potent
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solution for environmental issues such as increase in carbon dioxide gas
these days, and the demand for the solar cells has been remarkably
increased. For this reason, if only the off-grade product of the
semiconductor grade silicon is used for the raw material for the solar
cells, shortage of the raw material in future may be caused. Moreover,
because silicon for solar cells is expensive and the like, the solar cells at
present are still expensive. The price of electricity obtained by the
solar cell is several times that of the commercial electricity, and supply
of inexpensive silicon for solar cells has been desired.
[0005] Silicon can be also refined by electrolysis; in Patent Literature 1
and Patent Literature 2, electrolysis refinement using solid silicon as a
cathode is examined; in Patent Literature 3, electrolysis refinement
using fused silicon as a cathode is examined.
Citation List
Patent Literature
[0006] Patent Literature 1: W02008/115072
Patent Literature 2: U.S. Patent No. 3219561
Patent Literature 3: U.S. Patent No. 3254010
Summary of Invention
Technical Problem
[0007] In the electro-refining process of silicon, by the processes
disclosed in Patent Literature 1 and Patent Literature 2 in which
electrolysis is performed using solid silicon as the cathode, silicon
deposited on the cathode grows dendritically to cause a short circuit
between electrodes; for this reason, it is difficult to continue
electrolysis, and it is remarkably difficult to prevent an electrolytic bath
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from being trapped in a deposit. Moreover, in the process disclosed in
Patent Literature 3 in which electrolysis is performed using fused silicon
as the cathode, if the electrolysis temperature reaches approximately
1410 C or more that is the melting point of silicon, the reverse reaction
of reduced silicon is made to reduce the current efficiency of the
electrolysis, and choice of suitable furnace materials that can be used at
high temperature is limited; for such reasons, industrialization of the
process is difficult. For the same reason, with respect to
electro-refining of materials of metals and metalloids other than silicon
such as germanium, particularly materials whose melting point is
relatively high or that are difficult to be electrolyzed in an aqueous
solution, industrialization is difficult in most cases.
[0008] An object of the present invention is to provide a process for
producing a refined metal or metalloid in which an electrolysis
temperature can be lower than the melting point of a metal element or
metalloid element that is an object to be refined, and the dendritic
growth of the refined product and the trapping of an electrolytic bath in
the refined product can be suppressed.
Solution to Problem
[0009] A process for producing a refined metal or metalloid according
to the present invention comprises an electrolysis step, a withdrawal
step, a deposition step, and a recovery step.
[0010] In the electrolysis step, in an electrolytic bath set in a container
for an electrolysis in which a material comprising a metal element or
metalloid element and impurities is subjected to act as an anode, and an
alloy comprising the same metal element or metalloid element as the
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metal element or metalloid element contained in the anode and a
medium metal that does not substantially form a solid solution with the
metal element or metalloid element and having a complete solidification
temperature lower than the melting point of the metal element or
non-metal element is subjected to act as a cathode, electrolysis is
performed at an electrolysis temperature at which the alloy of the
cathode can be a liquid phase to move the metal element or metalloid
element in the anode to the alloy of the cathode.
The complete solidification temperature of an alloy is a
temperature corresponding to the lowest value of a liquidus in a
solid-liquid phase diagram of the alloy; at a temperature less than the
complete solidification temperature, the alloy cannot contain a liquid
phase.
That the medium metal that does not substantially form a solid
solution with the metal element or metalloid element means that the
solid solubility limit of the medium metal to the metal element or
metalloid element at the complete solidification temperature is not more
than 1 % by mass.
[0011 ] In the withdrawal step, after the electrolysis step, a part or the
whole of the alloy of the cathode is withdrawn to an outside of the
container for the electrolysis. In the withdrawal step, the alloy of the
cathode in which the concentration of the metalloid element or metal
element is higher than that of the composition corresponding to the
complete solidification temperature of the alloy may be withdrawn to an
outside of the container for the electrolysis.
[0012] In the deposition step, the withdrawn alloy is cooled at a
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temperature higher than the complete solidification temperature and
lower than the electrolysis temperature to deposit the metal element or
metalloid element contained in the alloy.
[0013] In the recovery step, the metal element or metalloid element
deposited from the cooled alloy is recovered.
[0014] According to the present invention, because the alloy
comprising the metal element or metalloid element and the medium
metal and having the complete solidification temperature lower than the
melting point of the metal element or metalloid element is subjected to
act as the cathode, the electrolysis temperature needed to make the
cathode a liquid phase can be reduced compared to the case where a
single metal element or metalloid element is subjected to act as the
cathode. Moreover, because in the electrolysis step, the cathode is a
liquid phase, a short circuit between electrodes caused by dendritic
growth of the metal element or metalloid element and trapping of the
electrolytic bath in the refined product of the element or metalloid
element can be suppressed.
[0015] Because the medium metal does not substantially form the solid
solution with the metal element or metalloid element, the concentration
of the metal element or metalloid element in the alloy of the cathode
subjected to the electrolysis step and the withdrawal step and brought to
the deposition step is higher than that of the composition corresponding
to the complete solidification temperature; thereby, the metal element or
metalloid element contained in the alloy can be selectively deposited
with high purity by cooling. Thereby, the purity of the recovered metal
element or metalloid element can be sufficiently higher than that of the
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material that forms the anode, and the refined metal element or
metalloid element can be easily obtained.
[0016] The medium metal may have a eutectic point with the metal
element or metalloid element. In this case, the concentration of the
metal element or metalloid element in the alloy of the cathode subjected
to the electrolysis step and the withdrawal step and brought to the
deposition step is higher than that of the composition corresponding to
the eutectic point; thereby, the metal element or metalloid element
contained in the alloy can be selectively deposited with further higher
purity by cooling.
[0017] Here, it is preferable that the process further comprise a reuse
step of using a residue after the metal element or metalloid element
deposited is recovered from the cooled alloy of the cathode, as the
cathode in the electrolysis step. The residue means a remaining object,
and it may be liquid or solid.
[0018] Thereby, the residue in which the concentration of the metal
element or metalloid element is sufficiently reduced is reused as the
cathode in the electrolysis step; accordingly, movement of the metal
element or metalloid element from the anode to the cathode can be
continuously performed with efficiency.
[0019] It is preferable that the metal element or metalloid element be
silicon or germanium.
[0020] In the case where the metal element or metalloid element is
silicon or germanium, it is preferable that the alloy of the cathode
comprise one or more metal elements selected from the group consisting
of aluminum, silver, copper, and zinc.
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[0021] It is preferable that the material for the anode comprise one or
more metal elements selected from the group consisting of silver,
copper, tin, and lead.
[0022] It is preferable that the electrolytic bath comprise cryolite.
[0023] It is preferable that the electrolysis temperature be higher than
the complete solidification temperature and lower than the melting point
of the metal element or metalloid element.
Advantageous Effects of Invention
[0024] According to the present invention, a process for producing a
refined metal element or metalloid element in which an electrolysis
temperature can be lower than the melting point of the metal element or
metalloid element that is an object to be refined, and dendritic growth of
a refined product and trapping of an electrolytic bath in the refined
product can be suppressed is provided.
Brief Description of Drawings
[0025]
[Figure 1] Figure 1 is a solid-liquid phase diagram of a germanium-lead
system.
[Figure 2]- Figure 2 is a solid-liquid phase diagram of a
silicon-aluminum system.
Description of Embodiments
[0026] The process for producing a refined metal or metalloid
according to the present invention mainly comprises an electrolysis step,
an withdrawal step, a deposition step, a recovery step, and when
necessary, a reuse step.
[0027] (Metal element or metalloid element)
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In the present invention, a metal element or metalloid element
(hereinafter, referred to as an element to be refined in some cases) is an
object to be refined. The element to be refined is not particularly
limited. Examples of the metal element include alkaline earth metals
such as beryllium; first transition elements such as scandium, titanium,
and nickel; second transition elements such as zirconium and yttrium;
lanthanoids such as lanthanum, neodymium, europium, dysprosium,
rhenium, and. samarium; actinoids such as thorium, uranium, plutonium,
and americium; and third transition elements such as platinum.
[0028] Examples of the metalloid element include silicon, arsenic,
antimony, and germanium.
[0029] Among these metal elements or metalloid elements, silicon,
germanium, nickel, lanthanoids, and actinoids are preferable, and silicon
and germanium are particularly preferable, considering easiness in
recovering from the alloy of the cathode that is a liquid phase.
[0030] (Electrolysis step)
In the electrolysis step, in an electrolytic bath set in a container
for an electrolysis in which a material comprising an element to be
refined and impurities is subjected to act as an anode, and an alloy
comprising the same element to be refined as the element to be refined
contained in the anode and a medium metal and having a complete
solidification temperature lower than the melting point of the element to
be refined (specifically, described later) is subjected to act as a cathode,
electrolysis is performed at an electrolysis temperature at which the
alloy of the cathode can be a liquid phase, thereby to move the element
to be refined in the anode to the alloy of the cathode; thus, an alloy in
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which the concentration of the element to be refined is higher than the
concentration of the element to be refined in the alloy composition
corresponding to the complete solidification temperature (specifically,
described later) is obtained on the cathode.
[0031 ] (Anode)
The material for the anode is a material comprising an element
to be refined and impurities, and has another aspect as a raw material for
refinement. The material for the anode may be those that are a solid
phase at the electrolysis temperature; it is preferable for easiness in the
electrolysis reaction that the material for the anode be those that are a
liquid phase at the electrolysis temperature.
[0032] The impurities contained in the material for the anode are
elements more noble than the element to be refined or elements less
noble than the element to be refined. In the case where the element to
be refined is silicon, examples of elements more noble than silicon
include silver and copper, and examples of elements less noble than
silicon include sodium and magnesium. The case of germanium is
similar to the case of silicon; examples of elements more noble than
germanium include silver and copper, and examples of elements less
noble than germanium include sodium and magnesium. The
concentration of the impurities is not particularly limited, and the
concentration is several tens ppm to several % based on the material for
the anode in a mass ratio, for example.
[0033] It is preferable that the material for the anode be an alloy of the
element to be refined and impurities different from the element to be
refined (hereinafter, referred to as an anode medium metal in some
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cases), and it is more preferable that the material for the anode be an
alloy having a eutectic point lower than the melting point of the element
to be refined. In this case, it is preferable that in the alloy, the vapor
pressure be low and stable. Moreover, it is preferable that the anode
medium metal be an element more noble than the element to be refined.
The element to be refined and anode medium metal can be properly
selected on the basis of the theoretical decomposition voltage according
to thermodynamic data, for example. Examples of the theoretical
decomposition voltage of each element are shown below. Calculation
of the theoretical decomposition voltage may be performed according to
a method in which the dissolved species of each element is determined
and the free energy to be produced is examined, or a method in which
estimation is performed on the basis of the free energy to be produced of
a metal compound such as a halide that is relatively easily available.
For example, if the theoretical decomposition voltage in a fluoride fused
salt is estimated on the basis of the free energy to be produced of each
metal fluoride, the theoretical decomposition voltage is calculated as
follows: 1.9 V for Cu, 2.8 V for Fe(II), 3.4 V for Ti(IV), 3.6 V for
Mn(II), 3.7 V for Si, 4.1 V for Al, 4.6 V for K, 4.6 V for Na, 4.7 V for
Mg, and 5.3 V for Ca. In the case of an element M other than Si, these
theoretical decomposition voltages are calculated on an assumption that
the reaction represented by the following formula progresses. The
activity is 1 in each case, and the temperature is 1000 C.
MF. (solid) -> M (solid) + x/2F2 (gas)
In the case of Si, the theoretical decomposition voltage is calculated on
an assumption that the reaction represented by the following formula
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progresses. The activity is 1 in each case, and the temperature is
1000 C.
SiF4 (gas) -> Si (solid) + 2F2 (gas)
[0034] In the case where the element to be refined is silicon, examples
of the anode medium metal include one or more elements selected from
the group consisting of copper, tin, silver, gold, mercury, and lead, and
considering cost and an influence on the environment, preferable are
one or more elements selected from copper, silver, tin, and lead. The
alloy of the anode may contain two or more anode medium metals.
[0035] Moreover, as the purity of the anode medium metal, preferable
is not less than 3 N, more preferable is not less than 5 N, and
particularly preferable is not less than 6 N. The metals sufficiently
more noble than silicon such as silver and copper and metals sufficiently
less noble than silicon such as sodium and magnesium are removed by
electrolysis; moreover, the metal used as the cathode medium metal,
which will be described later, is removed in the deposition step and does
not influence the purity of silicon; accordingly, the metal does not need
to be considered as the impurities of the anode medium metal.
[0036] (Cathode)
As the cathode, used is an alloy comprising the same element to
be refined as the element to be refined contained in the anode and a
medium metal different from the element to be refined (hereinafter,
referred to as a cathode medium metal in some cases) and having a
complete solidification temperature lower than the melting point of the
element to be refined. The cathode medium metal does not
substantially form a solid solution with the element to be refined.
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The complete solidification temperature of an alloy is a
temperature corresponding to the lowest value of a liquidus in the
solid-liquid phase diagram of the alloy; at a temperature less than the
complete solidification temperature, the alloy cannot contain a liquid
phase.
That the cathode medium metal does not substantially form a
solid solution with the element to be refined means that the solid
solubility limit of the cathode medium metal to the element to be refined
at the complete solidification temperature is not more than I% by mass.
The cathode medium metal may have a eutectic point with the
element to be refined. Namely, an alloy of the cathode medium metal
and the metal to be refined may have the minimum value in the liquidus.
Moreover, it is preferable that in the alloy, vapor pressure be low
and stable.
[0037] In the case where the element to be refined is silicon, examples
of such a cathode medium metal include one or more members selected
from the group consisting of aluminum, copper, tin, gallium, indium,
silver, gold, mercury, and lead; among them, considering cost and an
influence on the environment, preferable are one or more members
selected from the group consisting of aluminum, silver, copper, and
zinc. The alloy may contain two or more cathode medium metals.
For example, a solid-liquid phase diagram of a germanium-lead
system in which the element to be refined is Ge and the cathode
medium metal is Pb is as shown in Figure 1, and the complete
solidification temperature in this system is 327 C that is the lowest
value in the liquidus A, i.e., a point B. The concentration of Ge
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(element to be refined) corresponding to the complete solidification
temperature in the alloy is 0 wt%.
Moreover, a solid-liquid phase diagram of a silicon-aluminum
system in which the element to be refined is Si and the cathode medium
metal is Al is as shown in Figure 2. The complete solidification
temperature in this system is 577 C that is a point C showing the lowest
value (relative minimum value) in the liquidus A; as the point C, a point
at which the liquidus has the minimum value is referred to as a eutectic
point. The concentration of Si in the composition corresponding to the
complete solidification temperature of the alloy is approximately 12.6
wt%, and the composition is a composition corresponding to the
eutectic point and also referred to as a eutectic composition. The alloy
system such as a silicon-silver system and a germanium-zinc system
shows a phase diagram having the same eutectic point as that in Figure
2.
[0038] Moreover, as the purity of the cathode medium metal, preferable
is not less than 3 N, more preferable is not less than 5 N, and
particularly preferable is not less than 6 N. Moreover, particularly, the
contents of P and B each are preferably not more than 0.5 ppm, more
preferably not more than 0.3 ppm, and particularly preferably not more
than 0.1 ppm based on the cathode medium metal.
[0039] The ratio of the element in the alloy of the cathode at the time of
starting electrolysis is not particularly limited, and a cathode medium
metal containing no element to be refined (containing the alloy) may be
used. However, when the withdrawal step is performed after the
electrolysis step is performed, the concentration of the element to be
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refined in the alloy of the cathode needs to be more than that of the
composition corresponding to the complete solidification temperature.
In the alloy having a eutectic point, the concentration of the
element to be refined at the time of withdrawal needs to be higher than
that of the composition corresponding to the eutectic point. In other
words, for example, in Figure 2, the concentration of Si that is the
element to be refined in the alloy at the time of withdrawal needs to be
more than 12.6 wt% that is the concentration of the eutectic
composition. Moreover, particularly, in order to recover the element to
be refined efficiently, it is preferable that at a determined electrolysis
temperature the concentration of the element to be refined in the alloy of
the cathode be increased by electrolysis until it is close to the saturated
concentration of the element to be refined that is the maximum
concentration of the element to be refined at which the alloy of the
cathode can exist as a single phase of a liquid phase.
[0040] (Electrolytic bath)
The electrolytic bath is not particularly limited as long as it is
those that can conduct ions of the element to be refined, and metal
halides are preferable. Examples of a metal element that forms a metal
halide include one or more elements selected from alkali metals,
alkaline earth metals, aluminum, zinc, and copper. Moreover,
examples of a halogen that forms a metal halide include one or more
element selected from the group consisting of fluorine, chlorine, and
bromine. Moreover, two or more of these metal halides may be mixed
and used. Examples of a mixture of metal halides include a mixture of
sodium fluoride and aluminium fluoride. As the electrolytic bath,
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more specifically, preferable are cryolite (3NaF=A1F3), calcium chloride,
and the like for industrial availability. These electrolytic baths are
used at fused state.
[0041 ] In the case where it is desired to increase the purity of the
element to be refined as an object to be refined, it is preferable that the
purity of the electrolytic bath be increased. From such a viewpoint, as
the purity of the electrolytic bath, preferable is not less than 3 N, more
preferable is not less than 5 N, and particularly preferable is not less
than 6 N. Moreover, particularly, in the case where silicon or
germanium is an object to be refined, the contents of P and B each are
preferably not more than 0.5 ppm, more preferably not more than 0.3
ppm, and particularly preferably not more than 0.1 ppm based on the
electrolytic bath.
[0042] In the present invention, alkali metal elements and alkaline earth
metal elements do not need to be considered as the impurities of the
electrolytic bath. In the electrolysis step, these elements are harder to
move to the cathode than silicon and germanium, and are hardly mixed
in the alloy of the cathode. Moreover, the metal used as the cathode
medium metal does not also need to be considered as the impurities of
the electrolytic bath.
[0043] In the case where a solid anode is used, according to the specific
gravity of the alloy of the cathode and that of the electrolytic bath, of the
cathode and the electrolytic bath, the one with a higher specific gravity
can be disposed in a relatively lower position than the one with a lower
specific gravity, and the anode can be disposed at a place away from the
cathode, or in the electrolytic bath, the anode and the cathode can be
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disposed at places away from each other in the horizontal direction.
Moreover, in the case where a liquid anode is used, in the electrolytic
bath, the anode and the cathode can be disposed spaced from each other
in the horizontal direction; or the same disposition as in the case of an
aluminum three-layer electro-refining process, namely, according to the
specific gravities of the liquids of the anode, the electrolytic bath, and
the cathode, in the order of the cathode, the electrolytic bath, and the
anode from the top or the opposite order of the anode, the electrolytic
bath, and the cathode from the top, these three elements can be disposed
such that the specific gravity of the element is higher at a lower
position. From the viewpoint of improvement in operationability,
reduction in the size of the reaction container, and uniform current
distribution, the same disposition as in the case of the aluminum
three-layer electro-refining process is preferable; in the case where
refinement of silicon is aimed, particularly preferable is that the
cathode, the electrolytic bath, and the anode are disposed in this order
from the top. In the present invention, the anode and the cathode are
disposed spaced from each other within the container for the
electrolysis, and the anode and the cathode act through the electrolytic
bath in the electrolysis step.
[0044] (Electrolysis condition)
The electrolysis temperature is set according to the composition
of the alloy of the cathode such that the alloy of the cathode is kept in a
liquid phase. It is preferable that the electrolysis temperature be higher
than the complete solidification temperature of the alloy of the cathode.
At a higher electrolysis temperature, the solubility of the element to be
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refined in the alloy of the cathode is improved; accordingly, a larger
amount of the element to be refined can be moved to the cathode and
recovered. It is preferable that the electrolysis temperature be lower
than the melting point of the element to be refined. If the electrolysis
temperature is less than the melting point of the element to be refined,
the current efficiency in the electrolysis is improved, and selection of
the material for the container for the electrolysis is easier.
[0045] For example, in the case where the element to be refined is
silicon and aluminum-silicon is used as the alloy of the cathode, the
complete solidification temperature of the alloy, i.e., the eutectic point is
577 C; accordingly, it is preferable that the electrolysis temperature be
set at a temperature higher than 577 C and lower than 1410 C that is the
melting point of silicon. For example, in the case where the element to
be refined is germanium and zinc-germanium is used as the alloy of the
cathode, the complete solidification temperature of the alloy, i.e., the
eutectic point is 398 C; accordingly, it is preferable that the electrolysis
temperature be set at a temperature higher than 398 C and lower than
958 C that is the melting point of germanium.
[0046] From the viewpoint of the yield of the element to be refined
during the electrolysis reaction, it is preferable that at a temperature of
not more than the melting point of the element to be refined the
electrolysis temperature be as high as possible.
In the case where the element to be refined is silicon, the
electrolysis temperature is preferably not less than 700 C, more
preferably not less than 900 C, and particularly preferably not less than
1100 C. Considering restrictions on the material for the container for
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the electrolysis or the like, it is preferable that the electrolysis
temperature be not more than 1300 C.
In the case where the element to be refined is germanium, the
electrolysis temperature is preferably not less than 500 C, more
preferably not less than 600 C, and particularly preferably not less than
700 C. Considering restrictions on the material for the container for
the electrolysis or the like, it is preferable that the electrolysis
temperature be not more than 900 C.
[0047] For example, in the case where the element to be refined is
silicon, aluminum is used as the cathode medium metal, pure aluminum
is used as the cathode, and the electrolysis step is started, the melting
point of aluminum is 660 C and the eutectic point of Al-Si is
approximately 580 C; accordingly, first, the electrolysis reaction is
started at a temperature not less than 660 C at which aluminum as the
cathode is a liquid phase. Then, silicon moves to the cathode to
produce Al-Si on the cathode as the electrolysis progresses; after that,
the electrolysis temperature can be reduced to 580 C because the alloy
can be a liquid phase at a temperature of not less than the eutectic point.
However, at a temperature lower than the eutectic point, the solid is
deposited to cause dendritic growth of silicon.
[0048] For example, in the case where the element to be refined is
germanium, zinc is used as the cathode medium metal, pure zinc is used
as the cathode, and the electrolysis step is started, the melting point of
zinc is 419 C and the eutectic point of Zn-Ge is approximately 398 C;
accordingly, first, the electrolysis reaction is started at a temperature of
not less than 419 C at which zinc as the cathode is a liquid phase.
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Then, germanium moves to the cathode to produce Zn-Ge on the
cathode as the electrolysis progresses; after that, the electrolysis
temperature can be reduced to 398 C because the alloy can be a liquid
phase at a temperature of not less than the eutectic point. However, at
a temperature lower than the eutectic point, the solid is deposited to
cause dendritic growth of germanium.
[0049] The atmosphere in the electrolysis step is not particularly
limited; the air or an inert gas is preferable, and it is more preferable for
progression of the electrolysis that no water, oxygen, and the like exist.
[0050] (Container for Electrolysis)
The material for the container for the electrolysis that
accommodates the electrolytic bath is not particularly limited; those that
do not react with the element to be refined and the electrolytic bath are
preferable; examples thereof include oxides, nitrides, carbides, and
carbonaceous materials. In the case where the element to be refined is
silicon, examples of oxides include silica, alumina, zirconia, titania, zinc
oxide, magnesia, and tin oxide; examples of nitrides include silicon
nitride and aluminum nitride, and also include those in which these
constituting elements are partially substituted by other element. For
example, a compound such as sialon comprising silicon, aluminum,
oxygen, and nitrogen can be used. Examples of carbides include SiC,
and examples of carbonaceous materials include graphite; those in
which these constituting elements are partially substituted by other
element can also be used. Further, similarly to the aluminum
electrolysis, a method in which the electrolytic bath is held by a
solidified electrolyte (for example, cryolite) may be used.
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The same holds true for the case where the element to be refined
is germanium.
[0051 ] (Withdrawal step)
In the withdrawal step, a part or the whole of the alloy of the
cathode subjected to the electrolysis as described above is withdrawn to
the outside of the container for the electrolysis. The withdrawal
process is not particularly limited; withdrawal may be performed in
batch or continuously.
[0052] (Deposition step)
In the deposition step, the alloy of the cathode withdrawn from
the container for the electrolysis is cooled at a temperature higher than
the complete solidification temperature and lower than the electrolysis
temperature to deposit the element to be refined contained in the
withdrawn alloy as a solid.
[0053] If the cooling temperature is not higher than the complete
solidification temperature of the alloy of the cathode, the medium metal
other than the element to be refined, namely, the cathode medium metal
is also deposited with the element to be refined; accordingly, it is
difficult to selectively recover only the target element to be refined from
the alloy of the cathode. Contrary to this, in the present invention, the
cooling temperature is higher than the complete solidification
temperature of the alloy of the cathode, and the concentration of the
element to be refined in the alloy of the cathode withdrawn from the
container for the electrolysis is higher than that of the composition
corresponding to the complete solidification temperature of the alloy;
accordingly, by cooling at the cooling temperature lower than the
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electrolysis temperature and higher than the complete solidification
temperature, the element to be refined can be selectively deposited from
the alloy of the cathode.
[0054] The upper limit of the cooling temperature is the electrolysis
temperature. The recoverable amount of the element to be refined
corresponds to the difference between the compositions in the liquidus
of the alloy corresponding to the difference between the electrolysis
temperature and the cooling temperature. Accordingly, in order to-
increase the recoverable amount of the element to be refined, it is
preferable that the difference between the electrolysis temperature and
the cooling temperature be larger.
[0055] In both the case where the element to be refined is silicon and
the case where the element to be refined is germanium, the difference
between the electrolysis temperature and the cooling temperature is
preferably not less than 100 C, more preferably not less than 200 C,
and still more preferably not less than 300 C. However, a larger
difference in the temperature leads to a larger thermal loss; for this
reason, in the case where the change of the composition in the liquidus
corresponding to the temperature is sufficient, the difference between
the electrolysis temperature and the cooling temperature may not be
large, and cooling can be performed with an economically optimal
difference of the temperature in an available range of the temperature.
[0056] The cooling temperature can be reduced to a temperature in the
vicinity of the complete solidification temperature of the alloy of the
cathode (for example, eutectic point); however, it is preferable from the
viewpoint of easy cooling operation that the cooling temperature be not
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less than the melting point of the cathode medium metal.
[0057] For example, in the case where the element to be refined is
silicon and an aluminum-silicon alloy is used as the alloy of the
cathode, at an electrolysis temperature of 1100 C, the cathode can keep
the state of the liquid phase until the concentration of silicon in the alloy
of the cathode reaches approximately 55% by mass at the maximum.
When the alloy is withdrawn to the outside of the container for the
electrolysis and cooled to 600 C, the concentration of silicon does not
become equilibrium unless the concentration of silicon is reduced to
15% by mass; accordingly, silicon corresponding to the difference of
40% by mass can be recovered as a solid.
For example, in the case where the element to be refined is
germanium and a zinc-germanium alloy is used as the alloy of the
cathode, at an electrolysis temperature of 800 C, the cathode can keep
the state of the liquid phase until the concentration of germanium in the
alloy of the cathode reaches 60% by mass at the maximum. When the
alloy is withdrawn to the outside of the container for the electrolysis and
cooled to 450 C, the concentration of germanium does not become
equilibrium unless the concentration of germanium is reduced to 12%
by mass; accordingly, germanium corresponding to the difference of
48% by mass can be recovered as a solid.
[0058] As the process for cooling the alloy of the cathode, a known
process can be used. Namely, examples of the process include a
process of keeping an withdrawn alloy of a cathode in a container kept
at a cooling temperature; and a process of keeping an withdrawn
cathode in a container kept at a temperature slightly higher than a
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cooling temperature, immersing a cooled body in which the cooling
temperature is set in the alloy of the cathode, and depositing an element
to be refined on the surface of the cooled body.
[0059] (Recovery step)
In the recovery step, a solid deposit of the element to be refined
is recovered from the alloy of the cathode cooled in the deposition step.
The recovering process is not particularly limited, and examples thereof
include filtration and centrifugation.
[0060] (Reuse step)
In the reuse step, a residue, after the element to be refined
deposited from the alloy of the cathode is recovered in the recovery
step, is used as the cathode in the electrolysis step.
[0061] According to such a process for producing a refined metal
element or metalloid element, i.e., an element to be refined, the element
to be refined and an element less noble than this are dissolved from the
material for the anode to the electrolytic bath, and the element to be
refined selectively moves from the electrolytic bath to the alloy of the
cathode and accumulates.
[0062] The alloy of the cathode in which the concentration of the
element to be refined is increased and that is a liquid phase is withdrawn
from the container for the electrolysis; in the deposition step, the
element to be refined is selectively deposited with high purity; in the
recovery step, the element to be refined deposited from the alloy of the
cathode is recovered.
[0063] In the electrolysis step, the alloy comprising the element to be
refined and the medium metal and having a complete solidification
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temperature lower than the melting point of the element to be refined is
used as the cathode; accordingly, the cathode can easily become a liquid
phase, and the electrolysis temperature can be lower than that in the
case where a single element to be refined is used as a cathode in a liquid
phase. For this reason, the electrolysis can be performed at a
temperature lower than the melting point of the element to be refined,
and energy load and load on the material for the container for the
electrolysis can be reduced compared to the case where a single element
to be refined is used as a cathode, and it is advantageous. Moreover,
because the cathode is a liquid phase, a stable electrode interface is
formed; when an alloy in which the concentration of the element to be
refined is higher than that in the composition corresponding to the
complete solidification temperature is obtained on the cathode, dendritic
growth of the element to be refined to cause a short circuit between
electrodes is suppressed, and trapping of the electrolytic bath in the
product of the element to be refined is suppressed.
[0064] Because the medium metal does not substantially form a solid
solution with the element to be refined, the concentration of the element
to be refined in the alloy of the cathode subjected to the electrolysis step
and the withdrawal step and brought to the deposition step is higher than
that of the composition corresponding to the complete solidification
temperature; thereby, the element to be refined contained in the cathode
can be selectively deposited with high purity by cooling. Thereby, the
purity of the recovered metal element or metalloid element can be
sufficiently higher than that of the material that forms the anode, and the
refined element to be refined can be easily obtained.
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[0065] In the reuse step, the residue, after the element to be refined
deposited from the cooled alloy of the cathode is recovered, is returned
to the cathode in the electrolysis step; thereby, the alloy in which the
concentration of the element to be refined is sufficiently reduced can be
reused as the cathode, and movement of the element to be refined from
the material for the anode to the cathode and increase in the
concentration can be continuously performed with high efficiency.
Accordingly, in the electrolysis step, the element to be refined does not
reach a saturated concentration to cause stagnation of the electrolysis,
and refinement can be continuously performed as long as the fluidity of
the alloy of the cathode can be maintained.
[0066] Moreover, it is preferable that the anode be an alloy that is a
liquid phase at the electrolysis temperature; thereby, the material serving
as the anode is easily added to the electrolytic bath properly, and the
electrolysis step is more easily continuously operated.
[0067] In the case where the element to be refined is silicon, according
to the present invention, more than 40% by mass of silicon can be
obtained in the electrolysis step based on the cathode (including the case
where aluminum is used as the cathode medium metal and pure
aluminum is used as the cathode at the time of starting the electrolysis
step), and for example, not less than 45% by mass of silicon can be
obtained in the recovery step. Moreover, in the case where the element
to be refined is germanium, more than 40% by mass of germanium can
be obtained in the electrolysis step based on the cathode (including the
case where zinc is used as the cathode medium metal and pure zinc is
used as the cathode at the time of starting the electrolysis step), and for
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example, more than 50% by mass of germanium can be obtained.
Accordingly, according to the present invention, the yields of the
obtained silicon and germanium are particularly high, and it is
economically advantageous. In the process according to the present
invention, the amount to be produced is controlled by the current.
[0068] The purity of the thus-obtained recovered product of the element
to be refined is extremely higher than that of the material for the anode
as the raw material, and the recovered product is suitably used for
electronic devices and spattering targets, and a raw material for silicon
for solar cells particularly in the case of silicon.
[0069] When necessary, the obtained recovered product of the element
to be refined is treated with an acid or an alkali in order to remove an
adhering residue of a metal component and a non-reacted metal
component, and further segregation such as directional solidification,
dissolution under high vacuum, and the like are performed; thereby,
impurity elements contained in the recovered product of the element to
be refined can be further reduced; in the case of silicon, it is particularly
preferable that the obtained polycrystalline silicon be subjected to
directional solidification to increase the purity.
[0070] In the present invention, silicon obtained in the case where the
element to be refined is silicon, for example, a solar cell using
polycrystalline silicon will be described.
[0071] Using silicon obtained according to the present invention, an
ingot is produced by a casting process or an electromagnetic casting
process. The conductivity type of a substrate in the solar cell is usually
a p type. For example, boron is added to silicon or aluminum is left at
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the time of refining silicon; thereby, a dopant can be introduced into
silicon to obtain a p type silicon. The ingot is sliced using an inner
diameter blade cutting, a multi-wire saw or the like. After slicing,
when necessary, the sliced surfaces are wrapped using free abrasive
grains; further, in order to remove the damaged layer, the wrapped ingot
slice is e.g., immersed in an etching solution such as hydrofluoric acid
to obtain a polycrystalline silicon substrate. In order to reduce optical
reflection loss on the surface of the polycrystalline silicon substrate, a
V -groove is mechanically formed on the surface using a dicing machine,
or a texture structure is formed by reactive ion etching or etching using
an acid, an alkali, or the like. Subsequently, a diffusion layer with an n
type dopant such as phosphorus and arsenic is formed on the light
receiving surface of the polycrystalline silicon substrate to obtain a p-n
junction. Further, a film layer of an oxide of TiO2 or the like is formed
on the surface of the diffusion layer, electrodes are provided in the
respective layers, and an antireflection film formed with MgF2 or the
like in order to reduce loss of optical energy caused by reflection is
provided; thus, a solar cell can be produced.
[0072] In the description above, the embodiment according to the
present invention has been described, but the embodiment according to
the present invention disclosed above is only an example, and the scope
of the present invention will not be limited to the embodiment. The
scope of the present invention is specified by the claims of the patent,
and includes all modifications equivalent to the description in the claims
of the patent in the meaning and the scope thereof.
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Examples
[0073] Examples will be shown in order to describe the present
invention more in detail, but the present invention will not be limited to
these.
[0074] (Example 1)
Aluminum, cryolite, and silica are placed into a graphite
crucible, and the crucible is set within an electric furnace having a
mullite furnace tube. Next, solid silicon containing impurities is
immersed in an electrolytic bath at 1100 C, and electrolysis is
performed using the solid silicon as the anode and aluminum in a liquid
phase provided on the bottom of the graphite crucible as the cathode.
[0075] After the electrolysis, the alloy of the cathode is recovered by
cooling. The obtained alloy is dissolved by hydrochloric acid; thereby,
refined silicon can be obtained. Moreover, the alloy of the cathode
after the electrolysis is withdrawn while at 1100 C, and kept at 700 C
for 3 hours to be partially deposited for solid-liquid separation; thereby,
a deposit in which the concentration of silicon is relatively high and a
liquid alloy in which the concentration of silicon is relatively low are
obtained, and refined silicon can be obtained. The liquid alloy
(residue) after the deposit (refined silicon) is separated can be returned
to the graphite crucible in the electrolysis furnace again to perform
electro-refining of silicon.
[0076] (Example 2)
An alloy of copper and silicon, cryolite, silica, calcium chloride,
barium chloride, and an alloy of aluminum and silicon are placed into a
magnesia crucible, and the crucible is set within an electric furnace
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having a mullite furnace tube. Next, electrolysis is performed at
1100 C using the alloy of copper and silicon as the anode and the alloy
of aluminum and silicon as the cathode.
After the electrolysis, the alloy of the cathode is recovered by
cooling. The obtained alloy of the cathode is dissolved by
hydrochloric acid; thereby, refined silicon can be obtained. Moreover,
the alloy after the electrolysis is withdrawn while at 1100 C, and kept at
700 C for 3 hours to be partially deposited for solid-liquid separation;
thereby, a deposit in which the concentration of silicon is relatively high
and a liquid alloy in which the concentration of silicon is relatively low
are obtained, and refined silicon can be obtained. The liquid alloy
(residue) after the deposit (refined silicon) is separated can be returned
to the magnesia crucible in the electrolysis furnace again to perform
electro-refining of silicon.
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