Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF PRODUCING SILICON
SPECIFICATION
Field of the Invention
My present invention relates to a method of producing silicon from quartz
and, more particularly, to a two-stage method of producing silicon from the
reaction of qusrtz with carbon.
Backæround of the Invention
Reference may be had, with respect to the production of silicon, to my
U.S. Patents 4,364,974, 4,366,137 and 4,389,493 and to the prior German
applications and other patent documents referred to therein.
It is known from these publications and others that it is possible to
carry out a reaction between the silicon containing raw material, i.e. quartz
and briquets of a reducing agent consistin~ predominantly of carbon at &
suitable temperature to ultimately reduce the quartz to silicon.
German Patent l9 15 905 and German Patent 30 32 720 describe briquets
which are produced for this purpose.
The process is carried out in an electrical low-shaft furnace to which the
quartz is supplied in granular or particulate form together with br;quets of
the silicon-dioxide containing carbonaceous material so that the overall
reaction proceeds in accordance with the equation Si02 + 3C = SiC + 2CO.
This reaction produces silicon ca~bide as a result of an excess of carbon and
is carried out at an upper portion of the furnace at a temperature below
1600 C.
At a lower part of the furnace at a temperature above 1600 C and
preferably in the range of 1800 to 2000 C, liquid quartz is reduced with the
silicon carbide thus formed in accordance with the reaction scheme:
Si02 + 2SiC = 3Si + 2CO. The carbon monoxide gases are discharged from the
furnace and molten silicon can be tapped from the furnace.
The briquets of the reducing medium are generally produced by hot
briquetting and the raw material quartz which is used can be any of the
silicon dioxide materials which have commonly been used as a source of silicon
especially quartzite and quartz sand. For the production of the reducing
agent briquets, quartz sand is ~enerally employed.
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The term "hot briquetting" as used herein is intended to refer to
binderless briquetting in which the starting materials are heated to a
temperature of ~30 C and 540 C and are pressed to form the briquets as in
German Patent 19 15 905. ~owever, when briquets of a reducing aeent are
described in this applicstion, it will be understood that they can be
fabricated in other ways as well.
In the system of German Patent 30 32 720, the reduction-a~ent briquets
which are used have only a slight stoichiometric excess of carbon with respect
to the reaction SiO2 + 3C = SiC t 2CO.
Indeed, one of the goals of this system is to ma~imize the completeness of
the reaction without leavin~ si~nificant carbon excess so that the subsequent
reduction of the quartz can be effected in the liquid state with only SiC a5
the reductant.
Any carbon excess which is present is provided to compensate for the loss
of carbon by the reaction with o~ygen from sources other than the SiO2.
Such carbon is of course lost insofar as the reduction of SiO2 is concerned.
The reducing briquets, after the initial stage, thus consist practically
entirely of silicon carbide snd do not have any significant content of free
carbon.
~hile the method tescribed in this last-mentioned patent and in my
above-mentioned U.S. patents have generally been satisfactory, they can still
be improved with respect to the yield of silicon and the energy efficiency,
i.e. the energy consumption per unit of silicon produced.
Ob.iects of the Invention
It is, therefore, ths principal object of the present invention to provide
an improved method of producing silicon in a low-shaft electrical furnace in
accordance with the principles described whereby the energy consumption per
unit of silicon produced is markedly increased.
Another object of my invention is to provide a process of the class
described but with an increased yield of silicon.
Here described is a process for producing silicon in a low-shaft
electrical furnace to which quartz and reducing-agent briquets are supplied
and which effects an initial reduction at an upper portion of the furnace at a
temperature below 1600 C to transform silicon dio~ide in these briquets to
silicon carbide which forms a reducing agent for the quartz at a lower high
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temperature region of the furnace, operating at 1800 C to 2000 C and in
which the molten quartz reacts ~ith the silicon carbide to form molten silicon.
The reducin~-a~ent briquets which nre uged have a si~nificant excess of
carbon, i.e. an e~cess over stoichiometric requirements of more than 50% by
weight above that required for the reaction SiO2 ~ 3C = SiC + 2CO.
Furthermore, in this upper portion of the electrical furnace operatin~ at a
temperature below 1600 C, the briquets are transformed at least in part to
an activated carbon by a cokin~ operation so that the e~cess carbon of the
briquets assumes a cokelike structure which remains until the briquets enter
the liquid quartz phase so that the quartz in the lower part of the furnace is
reduced not only by the silicon carbide which is quantitatively produced in
the briquets, but also by the coke-structured activated carbon of the briquets.
Preferably, the briquets are provided so that the e~cess of carbon is less
than 90% by wei~ht and generally is about 80% by wei~ht.
I have found that optimal results can be obtained when the total e~cess
carbon of the briquets is sufficient by itself to reduce at least 50~ by
weight of the quartz in the aforementioned lower portion of the low-shaft
electrical furnace. This may be accomplished by properly proportioning the
various components of the burden or char~e of the ~urnace.
Obviously I need not utilize exclusively the high~carbon e~cess briquets
described and these briquets can be partly combined with briquets containing
lesser amounts of carbon provided there is a substantial proportion of the
sctivated carbon. In addition, I can make up part of the carbon requirements
by the use of a classical charge or burden which can contain, for e~ample, 3
metric tons of quartz, 0.4 metric ton of charcoal, 0.4 metric ton of peat
coke, 0.3 metric ton of petroleum coke and 0.5 metric ton of ash-free coal as
long as sufficient activated carbon is provided by the reducin~ briquets
described.
As has already been indicated, the manner and means for producing the
red~cing briquets is optional and any conventional method of equipment csn be
used for this purpose as long as the briquets are stable and can be introduced
in the described manner as part of the burden or charge of the low-shaft
electrical furnace, together with the quartz raw material.
In a preferred or best mode embodiment of the invention, however, hot
briquetting is used and the briquets are of eg~ or pillow shape having a
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volume of 15 to 60 cm3. The carbon content of the reducing briquets which
are to be fabricated by hot briquecting should preferably be predominantly
caking coal and can include comparatively inert carbon carriers such as
petroleum coke, anthracite, graphite, lignite co~e and pit coal or bituminous
coke.
When it is desired to convert the silicon which is for~ed to other end
products such as ferrosilicon and silicon-metal alloys of other compositions,
suitable metal-containing substances such as iron chips or ~ranulated iron may
be added.
The process is based upon my discovery that the activated carbon described
above can promote a si~nificantly higher degree of reduction of the silicon
dioxide in the lower portion of the furnace beyond that which would be
expected merely from the addition of an equivalent amount of carbon as part of
the burden and with a substantially reduced energy input.
More particularly in accordance with the invention there is provided, a
method for producing silicon in an electrical-shaft furnace comprising the
steps of:
(a) introducing into said furnsce reductant briquets containing silicon
dioxide and a carbonaceous reducin~ agent together with quartz, said
carbonaceous reducing agent being present within said briquets in more than
507~ and less than 90~ by weight excess of that stoichiomet~ically required for
the reaction SiO2 + 3C = SiC + 2C0 in said briquets;
(b) reacting at a temperature below about 1600C said briquets at an
upper portion of said furnace to transform the sllicon aioxide therein to
silicon carbide and produce an activated carbon with a coke structure within
said briquet, said briquets having an internal surface of between 20 and
230 m /g:
tc) reacting the silicon carbide thus produced with molten quartz at a
lower portion of said furnace at a temperature above 1600C to reduce the
quartz to siiicon; and
(d) reducing with said activated carbon a further amount of molten quartz
in said lower portion of said furnace.
Brief Description of the Drawin~
The above and other objects, features and advantages of the present
invention will become more readily apparent from the following description and
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examples, reference being made to the accompanyin~ drawing, the sole FIGURE of
which is a diagra~matic vertical section through a low-shaft electrical
furnace for carrying out the process embodying the present invention.
Specific ~escri~tion
In the drawin~ I have shown a low-shaft electrical furnace 10 which has an
upper portion 11 and a lower portion 12 located above the hearth 13 from which
the molten silicon can be tspped through a tapping hole 14.
The burden or charge is introduced at 15 and can consist of silicon
dioxide containing reductant briquets 16 which are formed by hot brlquetting
as previously dsscribed with a minimum of 50% excess oP carbon above the
stoichiometric quantity required for the conversion of all of the SiO2 of
these briquets into silicon carbide.
The temperature at the upper portion 11 of the furnace is above the coking
temperature of the carbon excess and is also above the temperature required
for activation of the reduction of the silicon d~oxide of the briquets to
silicon carbide in accordance with the equations presented earlier. The
coking ~ases and the carbon monoxide which are formed are discharged at 17.
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~ phir of electrodes 18 extend into the furnace to pro~ide the heat
required for meltin~ the quartz which is introduced together with the briquets
as part of the burden so that at the lower portion 1~ or high tempernture bosh
of the furnace, the silicon carbide from the fully reacted briquets and the
e~cess activated carbon thereof can pass into the molten quartz and effect a
further reduction. The molten silicon collects at 13 as previously noted and
can be tapped from the furnace.
Specific Examples
E~ample l
For the production of about 600 metric tons of silicon, 1200 metric tons
of briquets were produced which were introduced into the electrical low-shaft
furnace together with an almost similar amount of lump quartz.
In the first step of the process, the briquets were produced from a
mixture consisting of:
30% caking coal,
32% petroleum coke, and
38~ quartz and (99.8% SiO2)
by the hot-briquetting process, i.e. the ~oal was used at temperatures of
about 500 C as 8 binder. The finished cooled briquets were egamined. They
contained
42(~ 0.4~% SiO2 and 52(~ 0.7~ Cfi .
A strength test showed that point pressure crushing strengths of 150 to
200 kg had been obtained, which are accounted for by the imbedding of the
inert substances such as petroleum coke and sand in the fused and resolidified
coal. Measurement of the internal surface of the briquets showed 0.5 to
l.0 m ~g, indicating that no significant reaction surface is present on
which heterogeneous reections between gases such as SiO and carbon can take
place at high conversion efficiency.
In 8 second step of the process, the briquets were introduced into an
electrical low-shaft furnace. Before introduction to the furnace the material
arising from abrasion and fracture during transport was separated by
screening. These fines amounted to less than 4~. This is a very good
result. Indeed, charcoal, peat, and coals are destroyed to a much higher
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degree. Values of more than 10~ are known.
A mi~ture of continuously fed lump quartz and briquets, statistically well
distributed by shakin~, was heated in the electrical low-shaft furnace and
brou~ht to reaction.
Considering the General reaction of the production of silicon
SiO2 ~ 2C = Si + 2C0,
one can see from the analysis o~ the briquets that carbon is present in
substantial e~cess and that only the further renction with the lump quartz
assures complete conversion. Since, however, before the formation of silicon,
silicon carbide arises accordinG to the e~uation
SiO2 + 3C = SiC + 2C0,
there remains the question, whether sufficient carbon is present in the
briquets for this reaction. Calculation shows that just less than twice as
much carbon (about 80% e~cess) is present as is required for the reaction.
That is a molar ratio of 1 : ~ to 1 : 6. This has been aimed for, to maintain
the coke-structures resulting from the hot-briquettin~, even if losses occur
by the reaction to silicon carbide.
This theory was tested by use of thermocouples to determine the
temperature zone at which the carbide reaction occurs. Samples were taken
from the hot material havin~ a temperature of between 1500 and 1600 C, and
showed clearly that the briquets still had their oriGinal shape, althou&h the
conversion between carbon and silicon already had started or was even complete.
Most of the briquets showed a white surPace, indicatin~ that reaction had
here occurred. Of more significance, however, was the measurement of the
internal surface of the cooled briquets, which showed a considerably increased
internal surface of between 20 and 230 m /g. The briquets were found to
consist essentially of SiC and activated carbon with the coke structure.
This leads to a considerable decrease of the SiO2 precipitate in the
~as-cleaning. Closely connected therewith are the enerGy consumption Pnd the
silicon yield. The consumption of electrical power in the furnace decreased
by about 14%, whereas the silicon yield increased by more than 20%. An
unexpected, but important economical advantaGe was the halving of the
consumption of the electrodes, which dropped from 128 kg/T. Si
to 59 k~/T. Si. The movements of the electrodes were minimized.
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Examole 2
In the production of ferrosilicon the situation is more favourable. Here
the lo~ses by the formation of SiO are less.
If one proceeds as described in the foregoing e~ample such that the input
of lump qusrtz and briquets rcmains in the ratio of 50 to 50, and scrap iron
is added so that a 75% silicon alloy results, the advanta~es of the briquets
are still clearly apparant, i.e. the consumption of electrisal power decreases
by 8~ and the yield of silicon increases by 12~.
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