Method and apparatus for preparing high-purity metallic silicon

High-yield preparation of high-purity metallic silicon at is performed by subjecting a stream of oxides of silicon (e.g. in an aerosol) to reaction heat in the presence of a mixture of a material of the group including silicon carbide and silicon dioxide; and a material of the group including carbon and carbon-containing substance. Preferably, silicon oxide produced by the reaction is scavenged from exhaust gas leaving the reaction chamber, re-condensed, and returned to the reaction chamber.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a method and apparatus for 
preparing high-purity metallic silicon, such as is used in solar cells. 
More particularly, the invention relates to a method and apparatus for 
efficiently and economically preparing high-purity metallic silicon from 
powdered silicon dioxide. 
Conventionally, metallic silicon or ferrosilicon has been prepared in arc 
furnaces from a mixture of silicon dioxide and carbon. In order to ensure 
proper ventilation and to improve the efficiency of the reduction reaction 
in the high temperature region of the furnace, it has been considered 
essential to use large grains or masses of silicon dioxide. 
On the other hand, there has recently been an increasing demand for 
metallic silicon of high purity, especially that having a purity higher 
than 99.999%. Such high-purity metallic silicon has numerous applications, 
among which are solar cells. Refined natural silica is commonly used as 
the source of silicon dioxide. This refined silica is generally powdered 
or granular with a grain size of less than several milimeters. Therefore, 
in order to use this refined silica as the source silicon dioxide in the 
conventional apparatus, an extra process step is needed to sufficiently 
increase the grain size of the refined silica. This obviously leads to 
higher production costs and may also lower the purity of the source 
material. 
In order to eliminate these problems in the prior art, an improved process 
for preparing high-purity metallic silicon has been proposed in Japanese 
Patent First Publication (Tokkai) Showa 57-11223. However, even the 
proposed improved process still requires that a part of the source silicon 
dioxide charge have a grain size of 3 to 12 milimeters. 
Another improved process of preparing high-purity metallic silicon has been 
disclosed in the Japanese Patent First Publication (Tokkai) Showa 
58-69713. In the disclosed process, the reaction between silica and carbon 
takes place in a high-temperature plasma jet which transports the 
resultant product onto a carbon layer. In this proposed process, a large 
amount of silicon carbide is created as a result of reaction with the 
carbon layer. The created silicon carbide tends to accumulate within the 
carbon layer and fill interstices between the carbon grains, which 
inhibits further reaction. Due to this defect, the process proposed in the 
Japanese Patent First Publication 58-69713 is still not useful in the 
manufacture of high-purity metallic silicon. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a novel 
process and apparatus for preparing high-purity metallic silicon which can 
use fine-grained silicon dioxide without further preparatory steps for 
increasing grain size. 
In order to accomplish the aforementioned and other objects, a process for 
preparing high-purity metallic silicon comprises a step of preparing a 
mixture of at least one of carbon and carbide and at least one of silica 
and silicon carbide, a step of charging the resulting mixture into an arc 
furnace, and directly injecting a material including silicon dioxide or 
silicon oxide into a high temperature region in which silica is reduced to 
create silicon dioxide. 
Directly injecting the silicon dioxide or silicon oxide material into the 
high-temperature region of the furnace charged with the aforementioned 
mixture induces a reaction between the silicon dioxide or silicon oxide 
and carbon or silicon carbide to create metallic silicon. 
An apparatus for implementing the aforementioned process, according to the 
present invention, includes a nozzle for blowing the silicon dioxide or 
silicon oxide. The nozzle has an end directed at the arcing region between 
a pair of electrodes of the arc furnace. 
As is conventionally well known, the overall reaction can be expressed in 
the following formula (1): 
EQU SiO.sub.2 +2C.fwdarw.Si+2CO (1) 
However, in fact, during the reaction expressed in the formula (1), it is 
believed that the following reactions occur concurrently: 
EQU SiO.sub.2 +C.fwdarw.SiO+CO (2) 
EQU SiO+2C.fwdarw.SiC+CO (3) 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (4) 
EQU SiO+C.fwdarw.Si+CO (5) 
EQU SiC+SiO.sub.2 .fwdarw.Si+SiO+CO (6) 
EQU Si+SiO.sub.2 .fwdarw.2SiO (7) 
EQU SiO+SiC.fwdarw.2Si+CO (8) 
When powdered silicon dioxide is used as a source material and charged in 
the furnace into which the foregoing reactions are to take place, a large 
amount of silicon oxide is created during heating of the material in the 
reaction expressed into the formula (2), since powdered materials 
generally have higher dissociation constants than massive materials. Since 
silicon oxide has a relatively high vapor pressure, it tends to escape 
from the furnace and so lower the yield. Furthermore, as illustrated in 
the formula (4), the remaining silicon dioxide reacts with carbon to form 
silicon carbide which tends to accumulate at the bottom of the furnace and 
so lower production efficiency. 
In the process according to the present invention, as set forth above, a 
mixture of carbon and/or carbide, such as pitch, or other organic 
compounds, and silicon carbide and/or silica is charged into the furnace. 
In addition, the powdered source silica is injected in aerosol form 
directly into the highest temperature region of the furnace. Injecting the 
silica into the high temperature region promotes the reactions represented 
by the formulae (6), (7) or (2) and so creates silicon and gaseous silicon 
oxide. The gaseous silicon oxide then reacts according to the formulae (3) 
and (8) with the carbon or silicon carbide in the mixture, which itself 
produces silicon carbide according to formula (4), to form silicon and/or 
silicon carbide. The silicon carbide thus created again reacts with the 
silica or silicon oxide injected into the arcing region of the furnace 
according to formula (6) and (8). 
As will be appreciated herefrom, the process according to the present 
invention can achieve a remarkably high yield in the process of 
preparation of high-purity metallic silicon. 
Furthermore, according to the present invention, by adjusting the quantity 
of the silica or silicon oxide source material injected into the furnace, 
the amount of silicon carbide in the bottom of the furnace needed to 
perform the reaction can be adjusted. This, in turn, means that by 
adjusting the quantity of the silica or silicon oxide source material 
appropriately, the quantity of silicon carbide accumulating in the bottom 
of the furnace can be controlled. This allows continuous, long-term 
operation of the furnace used for preparation of high-purity metallic 
silicon. 
When the mixture to be supplied is carbon or carbide and silicon carbide, 
the mixture ratio in mol is preferably equal to or greater than 1/2 in 
C/SiC. On the other hand, when the mixture is carbon or carbide and 
silica, the mixture mol-ratio is preferably equal to or more than 3.5 in 
C/SiC ratio. The aforementioned mixture ratios keep silicon losses equal 
to or less than 15%. Comparison with silicon losses in the conventional 
art will show that the yield of metallic silicon in this process is 
significantly improved. The yield achieved by the present invention can be 
further improved by collecting and re-injecting gaseous silicon oxide 
which would otherwise escape to the atmosphere. 
In addition, by supplying the mixture of the carbon or carbide and silicon 
oxide or carbon or carbide and silicon carbide, heat in the furnace, i.e. 
sensible heat of created gas, can be effectively utilized for heating the 
material, to reduce required reaction heat. This makes it easier to 
increase or hold the temperature at the arcing region and thus makes the 
silicon preparation process easier. Furthermore, this reduces the amount 
of gas generated in the furnace and so facilitates ventilation. 
In practice, the source silica and/or silicon oxide in powder state is 
injected into the arcing region of the furnace together with a carrier gas 
which should be a non-oxidizing gas, such as Ar, H.sub.2, N.sub.2 or the 
like. 
In the preferred embodiment, the nozzle for injecting the silica or silicon 
carbide is made of carbon or silicon carbide. The injection end of the 
nozzle, which is subject to high temperatures, e.g. higher than 
2,000.degree. C., will tend to react with the injectant. However, after 
the nozzle has been reduced in length by the reaction to a given extent, 
the temperature at the point of injection will drop to the range of 
1,700.degree. to 1800.degree. C., and so the nozzle will cease to react 
with the injectant. Therefore, no significant effect will arise from the 
reaction between the nozzle and other reagents in the high temperature 
region. 
Preferably, the charge mixture is in granular form. Therefore, when the 
carbon or carbide and the silica making up the mixture are highly purified 
and thus in a fine powder state, sufficient grain size mass of those 
material is preferably pre-prepared by utilizing a a binder, such as 
sugar, phenolic plastic, starch, and the like. By enlarging the grain 
size, sufficient ventilation can be provided. 
Furthermore, by employing external heating means, the high temperature 
reaction region in the furnace can be expanded. This further improves the 
yield of the high-purity metallic silicon. It also improves the overall 
efficiency of the operation. In the preferred embodiment, external heating 
means may perform heating by way of high-frequency induction heating The 
external heating means may act on the peripheral walls of the furnace or 
directly on the charge, i.e. the mixture of carbon and/or carbide and 
silica and/or silicon carbide, to heat same to a temperature equal to or 
higher than 1,800.degree. C., and preferably to a temperature in excess of 
2,000.degree. C. 
The gaseous silicon oxide vented with the exhaust gas of the furnace may 
condense as the exhaust gas cools and thus can be collected by suitable 
means, such as a filter bag. However, due to the relatively small grain 
size of the condensed silicon oxide, the filter will tend to clog very 
easily. In order to solve this problem, the exhaust gas is injected into 
the stream of source silica to be injected into the furnace to cause 
condensation of the silicon oxide on the periphery of the silica, or 
otherwise the silicon dioxide is continuously introduced into the exhaust 
gas passage to induce condensation of the silicon oxide, whereby the 
silica grains serve as nuclei of condensation of the silicon oxide. The 
silicon oxide condensing on the silica can be easily collected by means of 
a cyclone separator and re-injected into the furnace. This would be much 
more convenient for effectively collecting or recovering the silicon oxide 
than the conventional process. In this case, the top of the furnace, 
through which the exhaust gas vents, is preferably held at a temperature 
equal to or higher than 1,700.degree. C. so as to prevent the gaseous 
silicon oxide from condensing thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
We refer now to the drawings, particularly to FIG. 1, which illustrates a 
major part of an arc furnace constituting the first embodiment of an 
apparatus for preparing high-purity metallic silicon and for implementing 
the preferred preparation process according to the present invention. A 
furnace body 10 is generally made of a graphitic refractory material and 
defines therein a reaction chamber 12. Upper and lower arc electrodes 14 
and 16 are inserted within the reaction chamber 12. The upper and lower 
arc electrodes 14 and 16 have arcing ends 14a and 16a opposing each other 
across a predetermined gap 18. The gap 18 between the arcing ends 14a and 
16a constitutes an arcing region. 
The upper electrode 14 is in the form of a hollow cylindrical shell and 
thus defines a path 20 through which silicon dioxide or silicon oxide 
suspended in aerosol form in a carrier gas flows. The carrier gas is a 
non-oxidizing gas, such as Ar, H.sub.2, N.sub.2 and the like. The path 20 
opens at the arcing region 18 to discharge the mixture of the carrier gas 
and the powdered silica, silicon dioxide and/or silicon oxide. 
On the other hand, as a burden in the furnace, a mixture 22 of carbon 
and/or carbide and silica and/or silicon carbide is charged within the 
arcing chamber 12. The burden mixture 22 has to fill at least the lower 
region of the furnace including the arcing region so that the silica, 
silicon dioxide and/or silicon oxide will react with the carbon and/or 
carbide in the burden mixture 22. The metallic silicon produced by the 
reaction collects in the bottom 24 of the reaction chamber 12 and flows 
out through an outlet 26 in the floor of the furnace body 10. 
A heating coil 28 is wound around the furnace body 10. The heating coil 28 
is designed to perform high-frequency induction heating. In the preferred 
embodiment, the heating coil 28 is arranged to heat the position of the 
burden mixture 22 above the arcing region to a temperature equal to or 
greater than 1,800.degree. C., and more preferably equal to or greater 
than 2,000.degree. C. 
In practice, direct-current electrical power is supplied to one of the 
upper and lower electrodes 14 and 16 to induce arcing across the arcing 
region 18. A mixture of carbon and silicon carbide in the form of pellets 
8 mm to 15 mm in diameter is used as the burden mixture 22. The pellets 
have a silicon carbide layer on the inside and a carbon layer on the 
outside. Silicon dioxide suspended in H.sub.2 gas serving as a carrier is 
injected into the arcing region 18. 
Under these conditions, high-purity metallic silicon was prepared both with 
and without external heating of the burden by high-frequency induction 
heating. The general conditions of operation of the apparatus are as 
follows: 
Molar C/SiC mixture ratio of is 1:1; 
rate of injection of silicon dioxide is 5 kg/h; 
rate of flow of H.sub.2 is 3N m.sup.3 /h; and 
electrical power is 100 kW/h. 
The inventive reduction processes were performed in the furnace of FIG. 1. 
The conventional reduction process used carbon pellets and silicon dioxide 
pellets and in a ratio of 2/1 (in mol). 
The following table I shows the results of the experimental reductions. 
TABLE I 
______________________________________ 
INVENTION INVENTION EXAMPLE 
(with (without FOR COM- 
external external ISON 
heating) heating) (conventional) 
______________________________________ 
Burden Pellet Pellet Pellet 
Configuration 
C/SiC 1/1 1/1 2/1 
(molar ratio) 
Pellet Supply 
7kg/h 5kg/h 7kg/h 
Amount 
SiO.sub.2 Blowing 
7kg/h 5kg/h -- 
Amount 
Carrier Gas 
3Nm.sup.3 /h 
3Nm.sup.3 /h 
-- 
(H.sub.2) 
Yield 95% 85% 54% 
Electricity 
19kW/kg-Si 20kW/kg-Si 35kW/kg-Si 
______________________________________ 
FIGS. 2 to 4 show second to fourth embodiments of the high-purity metallic 
silicon preparing apparatus according to the present invention. In FIGS. 2 
to 4, the elements having the same construction and same function as in 
the apparatus of FIG. 1 will be represented by the same reference 
numerals, and detailed description of the common elements will be omitted 
in order to avoid redundancy. 
In FIG. 2, an upper electrode 30 having an arcing end 30a opposes the 
arcing end 16a of the lower arc electrode 16. The upper arc electrode 30 
is in the form of a solid bar and inserted into the reaction chamber 
through the top of the furnace body 10. In order to inject the source 
silica, silicon dioxide or silicon oxide, a graphitic supply nozzle 32 
with a passage 34 for the powdered silica, silicon dioxide or silicon 
oxide suspended in the carrier gas extends through a vertical wall 36 of 
the furnace body 10 essentially horizontally so that its inner end opposes 
the arcing region 18 between the opposing faces of the upper and lower arc 
electrodes 30 and 16. 
In this arrangement, the upper solid electrode 30 will be stronger than the 
hollow cylindrical electrode of the first embodiment of FIG. 1. Therefore, 
despite the high temperatures in the reaction chamber, the upper electrode 
30 will be more durable for long-term use. 
In FIGS. 3, a pair of arc electrodes 40 and 42 are coaxially aligned 
horizontally. The arc electrodes 40 and 42 oppose each other at their 
inner ends across a given gap 44 which serves as the arcing region. A 
source supply nozzle 46 extends upwards through the floor 48 of the 
furnace body 10. 
As in the second embodiment, the life-time of the electrodes can be 
extended by providing a separate source supply nozzle. 
In FIG. 4, a pair of electrodes 50 and 52 extend into the reaction chamber 
12. The electrodes 50 and 52 lie oblique to the vertical axis of the 
furnace body 10. The inner ends 50a and 52a oppose to each other 
indirectly to define an arcing region 54. A supply nozzle 56 leads toward 
the arcing region and injects the silica, silicon dioxide or silicon oxide 
into the arcing region 54. 
FIG. 5 shows the fifth embodiment of the high-purity metallic silicon 
preparing apparatus according to the invention. In this embodiment, the 
supply nozzle 60 projects into the reaction chamber 12 through the floor 
10a of the furnace body 10. Two arc electrodes 62 and 64 coaxially aligned 
horizontally and define the arcing region 18 between their opposing inner 
ends. This structure is essentially as described with reference to FIG. 3. 
As shown in FIG. 5, the supply nozzle 60 is connected to a reservoir 66 
filled with powdered silica, silicon dioxide and/or silicon oxide. The 
reservoir 66 is connected to a carrier gas supply tube 68 which supplies 
the carrier gas, such as H.sub.2. Therefore, the silica, silicon dioxide 
or silicon oxide is carried by the carrier gas through a supply passage 70 
and injected into the arcing region 18 through the supply nozzle 60. The 
reservoir 66 is also connected to the top of the reaction chamber 12 in 
the furnace body 10 through an exhaust tube 72 which introduces exhaust 
gas created in the reaction chamber and containing gaseous silicon oxide. 
Gaseous silicon oxide is cooled within the reservoir 66 and condenses onto 
the silica, silicon dioxide or silicon oxide particles. Therefore, the 
gaseous silicon oxide can be recirculated into the arcing region together 
with the silica, silicon dioxide or silicon oxide. The exhaust gas minus 
the silicon oxide component exits the reservoir 66 through an exhaust gas 
outlet 74. 
With this arrangement, since the gaseous silicon oxide can be effectively 
scavenged by recirculation through the reaction chamber 12 with the 
silica, silicon dioxide and/or silicon oxide, the preparation yield of the 
high-purity metallic silicon can be further improved. 
FIG. 6 shows a modification to the fifth embodiment of the apparatus. In 
this modification, the supply nozzle 60 (not shown in FIG. 6) is connected 
to a cyclone 76 through the supply passage 70. The cyclone 76 is also 
connected to the top of the reaction chamber through the exhaust tube 72. 
The exhaust tube 72 has an inlet 80 for aerosol silica, silicon dioxide 
and/or silicon oxide suspended in the carrier gas which mixes the latter 
with the exhaust gas into the cyclone. 
The cyclone 76 is also provided with the exhaust gas outlet 74 through 
which the exhaust gas minus the gaseous silicon oxide is vented. The 
cyclone 76 may also be provided with a supply control valve 82 for 
adjusting the amount of silica, silicon dioxide and/or silicon oxide to be 
injected into the arcing region 18 so as to control the amount of silicon 
carbide created so that all of the silicon carbide can participate in the 
reaction and that no silicon carbide will accumulate in the bottom of the 
furnace. 
Scavenging the gaseous silicon oxide created by the reaction in the furnace 
and recirculating same into the arcing region together with the silica, 
silicon dioxide and/or silicon oxide source, the yield can be further 
improved to approximately 99%. 
Therefore, the present invention fulfills all of the object and advantages 
sought therefor. 
While the present invention has been disclosed in terms of the preferred 
embodiments in order to facilitate better understanding of the invention, 
it should be appreciated that the invention can be embodied in various 
ways without departing from the principle of the invention. Therefore, the 
invention should be understood to include all possible embodiments and 
modifications to the shown embodiments which can be embodied without 
departing from the principles of the invention set out in the appended 
claims.