Apparatus for producing silicon carbide consisting mainly of .beta.-type crystal

.beta.-type silicon carbide having exceedingly high purity is stably and continuously produced in an apparatus comprising a vertical type reaction vessel having an inlet, a preheating zone, a heating zone, a cooling zone and a closable outlet which are sequentially communicated in this order in vertical direction, and a heat insulating layer composed essentially of fine powders of graphite and carbonaceous materials arranged on at least outside of the heating zone, the heating zone of the reaction vessel being made of graphite and having a heating means to indirectly and electrically heat charged materials, the preheating zone having a horizontal cross-sectional area larger than that of the heating zone at any level above an arbitrary position of the preheating zone.

BACKGROUND OF THE INVENTION 
The present invention relates to an industrial apparatus for cheaply and 
continuously producing silicon carbide of .beta.-type crystal of fine and 
high purity using silica and carbon as starting materials. Particularly, 
the present invention relates to an improvement of the apparatus described 
in our U.S. patent application Ser. No. 18,939. 
A particle of .beta.-type silicon carbide crystal has a rather roundish 
shape, so that particles of .beta.-type silicon carbide crystal are suited 
to a fine powdery abrasive for super precise grinding which affords faster 
grinding speed than conventional fine powdery abrasives without incurring 
deep scars on surfaces of works to be abraded. They have excellent filling 
property and oxidation resistance resulting also from their roundish 
particulate shape, so that they are suited to such uses as a filler and a 
coating material for improving an oxidation resistance of a heating 
element or a refractory made of silicon carbide and the like, particularly 
.beta.-type silicon carbide crystals of fine and high purity are suited to 
a starting material for sintered bodies such as a gas turbine element, a 
high temperature heat exchanger, a heat-resistant jig element, a jig 
element for treating melted materials, a part of high temperature furnace, 
a chemical equipment part and the like. 
A conventional method of industrially producing the silicon carbide has 
been effected using a publicly known classical discontinuous Acheson type 
electric furnace. Thus, a sealing of the electric furnace has been 
difficult. Therefore, it has many environmental, labor and sanitary 
problems and has not been an efficient and economical method. In addition, 
the conventional method of using the Acheson type furnace has drawbacks 
that it can obtain .beta.-type silicon carbide merely in a very small 
quantity as a by-product in producing .alpha.-type silicon carbide and 
that the .beta.-type silicon carbide contains .alpha.-type silicon carbide 
and other impurities in high percentage and cannot be mass produced at 
high yield percentage. 
There has hitherto been proposed many methods of continuously producing 
silicon carbide. For example, U.S. Pat. No. 2,178,773 Specification 
discloses a method wherein silicon carbide is continuously produced using 
a shaft kiln for a purpose of obtaining silicon carbide suitable for 
abrasives. In this method, in order to prevent a mixture of materials 
silica and carbon from agglomerating or solidifying in a cake shape due to 
melt of silica at a low temperature zone, the mixture of the materials is 
directly charged extremely little by little in a high temperature zone to 
completely react the materials in an upper part of the high temperature 
zone and the reacted materials are further heated to grow into large 
coarse crystals noticeable by a naked eye. However, this method of 
completing the SiC forming reaction at the high temperature zone and 
growing the crystals into large and coarse crystals is very difficult to 
practice, because the reaction products are sintered in the high 
temperature zone to form mutually agglomerated bodies or adhere on inner 
wall of the reaction vessel whereby a smooth transfer or descent thereof 
is disturbed. West German Pat. No. 1,186,447 discloses a method of 
continuously producing .beta.-type silicon carbide in an intermediate 
process using a shaft kiln, for a final purpose of obtaining .alpha.-type 
silicon carbide. This method uses silica sand coated with carbonaceous 
powders as a raw material and a special rection vessel having a gas vent 
hole at reaction zone, in order to prevent agglomeration of a mass of the 
material due to melt of the coated silica sand. However, the method has 
not taken into consideration a behaviour of SiO gas produced at the 
forming reaction of silicon carbide. Thus, the SiO gas is discharged in a 
great quantity from the gas vent hole arranged at the reaction vessel, 
therefore, not only quality and yield of the reaction product are 
deteriorated, but also heat efficiency is extremely lowered owing to 
discharges of heat of formation of the SiO gas and sensible heat of CO 
gas. Furthermore, a long period of stable operation cannot be expected, 
because the gas vent hole is clogged by the deposition of the SiO gas. 
As explained above, an economical and industrial method of continuously 
producing silicon carbide consisting of fine .beta.-type crystal has not 
yet been known. However, we have proposed previously in the above U.S. 
patent application Ser. No. 18,939 an apparatus for producing fine silicon 
carbide, comprising a vertical type reaction vessel having an inlet for 
charging starting materials, a preheating zone, a heating zone, a cooling 
zone and a closable outlet for the product which are sequentially 
communicated in this order in vertical direction, the heating zone being 
made of a graphite cylinder and having an effective heating width of 
0.10-0.35 m, and a means for heating the starting materials in the heating 
zone by an electrically indirect heating, and further comprising a heat 
insulating layer composed of fine powders of graphite and/or carbon and 
arranged on at least outside of the heating zone, whereby the materials 
charged from the charging inlet are preheated while descending in the 
preheating zone and subsequently indirectly heated and reacted with each 
other thus forming silicon carbide in the heating zone by the heating 
means while descending continuously or intermittently by their own weight 
and the formed silicon carbide is cooled while descending in the cooling 
zone and discharged from the outlet. 
The above apparatus affords an extremely easy, efficient, stable and 
continuous operation for a long period of time when .beta.-type silicon 
carbide is produced by using a carbonaceous material having a relatively 
good reactivity, such as anthracite or the like. However, when a 
carbonaceous material having an extremely low ash content, for example, 
oil coke or the like is used for a purpose of obtaining .beta.-type 
silicon carbide containing especially little amount of impurities of a 
solid solution state, the carbonaceous material has poor reactivity to 
react with the SiO gas, so that an amount of the SiO gas to be discharged 
together with generated gases increases and the SiO gas deposits on inner 
wall surface of the preheating zone. The deposit on the inner wall surface 
of the preheating zone becomes a cause of an extremely undesirable 
phenomenon of preventing a smooth descent of the materials by their own 
weight, and the deposit is quite difficult to remove, so that a long 
period of stable and continuous operation has been difficult. 
SUMMARY OF THE INVENTION 
The present invention relates to an improvement of the above apparatus of 
our previous application, and an object of the present invention is to 
provide an apparatus for producing silicon carbide consisting mainly of 
.beta.-type crystal, which can stably operate for a long period of time 
without a hindrance due to the deposit of the SiO gas of obstructing the 
smooth descent of the materials by their own weight in the preheating 
zone. 
The present invention provides, for achieving the above object, an 
apparatus for producing silicon carbide consisting mainly of .beta.-type 
crystal, comprising a vertical type reaction vessel having an inlet for 
charging starting materials, a preheating zone, a heating zone, a cooling 
zone and a closable outlet for the product which are sequentially 
communicated in this order in vertical direction, the heating zone being 
made a graphite cylinder and having a means for heating the starting 
materials in the heating zone by an electrically indirect heating, and a 
heat insulating layer composed essentially of fine powders of graphite 
and/or carbonaceous material and arranged on at least outside of the 
heating zone, characterized in that a horizontal inner cross-sectional 
area of the preheating zone at any level above an arbitrary position of 
the preheating zone is larger than a horizontal inner cross-sectional area 
of the heating zone.

DETAILED EXPLANATION OF THE INVENTION 
Reaction of silica with carbon to form silicon carbide is expressed 
generally by the following equation (1). 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (gas) (1) 
However, it has been known that practical main forming mechanisms are a 
formation of SiO gas according to the following equation (2) and a 
reaction of the resulting SiO gas with carbon to form silicon carbide 
according to the following equation (3). 
EQU SiO.sub.2 +C.fwdarw.SiO (gas)+CO (gas) (2) 
EQU SiO (gas)+2C.fwdarw.SiC+CO (gas) (3) 
According to the apparatus of the present invention, starting materials 
consisting mainly of silica powders and carbon powders for producing 
.beta.-type silicon carbide are charged from the material-charging inlet 
to an upper part of the preheating zone, preheated while descending in the 
preheating zone and converted to SiC according to the above equations (2) 
and (3) in the heating zone while descending by their own weight. Gases 
produced at the time of the reactions and not reacted are discharged from 
an upper part of the preheating zone, and the reaction product is cooled 
while descending in the cooling zone and discharged for recovery from the 
product-discharging outlet arranged at a lower part of the reaction 
vessel. 
The horizontal inner cross-sectional area below the arbitrary position of 
the preheating zone varies stepwise and/or continuously. For instance, the 
horizontal inner cross-sectional area can be shaped to have a partial 
longitudinal cross-section as shown in FIGS. 2, 3 and 4 and can assume a 
circular, tetragonal or elliptical form. FIG. 2 illustrates a shape 
wherein an upper part of the preheating zone is enlarged stepwise. FIG. 3 
illustrates a shape wherein an upper part of the preheating zone is 
enlarged to a funnel-like shape and a further upper part thereof has a 
vertical wall. FIG. 4 illustrates a shape wherein an upper part of the 
preheating zone is enlarged to a funnel-like shape up to its upper end. It 
should be noted that, in addition to the preheating zones having the 
longitudinal cross-sections as shown in FIGS. 2-4, any preheating zone 
having a larger horizontal inner cross-sectional area at at least one 
level of the preheating zone than that of the heating zone can prevent the 
hindrance by the deposit of the SiO gas of obstructing the descent of the 
materials by their own weight in the preheating zone. 
Thus, the most important matter in the apparatus of the present invention 
is that a horizontal inner cross-sectional area of the preheating zone at 
any level above the arbitrary position of the preheating zone is so formed 
that it is larger than a horizontal inner cross-sectional area of the 
heating zone. 
The apparatus for producing silicon carbide consisting mainly of 
.beta.-type crystal according to the present invention has a structure to 
prevent issue of the gases produced from a lower part of the reaction 
vessel and to pass the CO gas produced at the time of forming the silicon 
carbide through a layer of the materials charged or filled in the 
preheating zone thereby to preheat the charged materials and then through 
an upper part of the preheating zone for discharge, as mentioned above. 
Therefore, among the intermediate gaseous product SiO in the forming 
reaction of the silicon carbide, a portion of the SiO gas which could not 
be converted to SiC reaches together with the rising CO gas to the 
preheating zone and produces an adhesive deposit at a low temperature area 
of the preheating zone. Therefore, when a carbonaceous material of a low 
ash content such as oil coke, pitch coke, etc. is used, a usage of an 
apparatus like the apparatus of our aforementioned U.S. patent application 
Ser. No. 18,939 is not preferable wherein the cylinder forming the 
preheating zone is constituted from a vertical wall similarly as that of 
the heating zone. This is because the carbonaceous material having a low 
ash content is poor in reactivity and hence among the SiO gas produced in 
the heating zone the portion of SiO gas which could not be converted to 
SiC is considerably increased to also increase an amount of the deposit in 
the preheating zone. However, because the apparatus of our aforementioned 
U.S. patent application Ser. No. 18,939 has a structure that the cylinder 
forming the preheating zone is constructed from a vertical wall similarly 
as that of the heating zone, the apparatus is liable to suffer from an 
adverse influence of the adhesive deposit of the SiO gas, and thus not 
only the materials are mutually adhered to form agglomerates, but also the 
produced gases passing through the materials-filled preheating zone are 
deflected and risen along an inner wall surface of the cylinder 
constituting the preheating zone, so that a large amount of deposits are 
formed and adhered on the inner wall surface to reduce the horizontal 
inner cross-sectional area of the cylinder forming the preheating zone, 
whereby the smooth descent of the materials by their own weight which is 
most important for producing .beta.-type silicon carbide is considerably 
obstructed and a long period of stable and continuous operation of the 
apparatus becomes difficult. 
On the contrary, according to the apparatus of the present invention, a 
horizontal inner cross-sectional area of the cylinder constituting the 
preheating zone at any level above the arbitrary position of the 
preheating zone is so formed that it is larger than a horizontal inner 
cross-sectional area of the heating zone, so that the produced gases 
passing through the preheating zone are dispersed above the arbitrary 
position and a flow rate of the produced gases passing through the 
preheating zone per unit horizontal inner cross-sectional area of the 
preheating zone is decreased, whereby the deposit from the SiO gas is 
dispersed and the adverse influence of the adhesive deposit over the 
descending property of the materials by their own weight is lessened. In 
addition, a major portion of the produced gases generated in the heating 
zone and deflected and risen along the inner wall surface of the reaction 
vessel rises through the filled materials layer above the arbitrary 
position in the preheating zone, so that an amount of the gases rising 
along the inner wall surface of the preheating zone is decreased to 
substantially eliminate the forming and adhesion of the deposit on the 
inner wall surface of the preheating zone. Therefore, smooth deposit of 
the materials by their own weight can be ensured and a long period of 
stable and continuous operation of the apparatus can be maintained. 
It is preferable that the horizontal inner cross-sectional area of the 
preheating zone at any level above the arbitrary position of the 
preheating zone is at least about 1.4 times of the horizontal inner 
cross-sectional area of the heating zone. If the ratio is smaller than 
about 1.4, the produced gases cannot be dispersed sufficiently and the 
flow rate of the produced gases rising along the inner wall surface of the 
preheating zone cannot be decreased considerably. While, if the ratio is 
too large, a thermal energy dissipated from an upper part of the 
preheating zone becomes uneconomically large and the material carbons are 
likely oxidized, particularly in an apparatus for producing silicon 
carbide which has an open upper structure, so that most satisfactory 
results can be obtained when the ratio is within a range of about 2.0-20. 
A height of the arbitrary position of the preheating zone is preferably not 
more than about 0.5 m from an upper end of the heating zone. If the height 
is larger than about 0.6 m, the produced gases are heat exchanged and 
cooled in the preheating zone at lower level than the arbitrary position, 
so that the unreacted or remaining SiO gas deposits already at high 
concentration before reaching the arbitrary position to obstruct the 
descent of the materials by their own weight whereby the advantageous 
effect which would be brought from the enlarging of the horizontal inner 
cross-sectional area of the preheating zone cannot be expected. Most 
favourable results can be obtained when the height is within a range of 
about 0.1-0.4 m. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be explained in more detail with 
reference to preferred embodiments shown in the attached drawings. 
Referring to FIG. 1, the apparatus of the present invention comprises an 
inlet 1 for charging starting materials 18, a preheating zone 2, a heating 
zone 3, a cooling zone 4 and a closable outlet 5 for the product which are 
sequentially communicated in this order in vertical direction to form a 
cylindrical reaction vessel 6, the heating zone 3 forming a part of the 
cylindrical reaction vessel 6 being made of graphite and having an 
electric resistive heating member 7 made of graphite and a heat reflection 
cylinder 8 made of graphite for indirectly and electrically heating the 
materials charged in the heating zone 3, and a heat insulating layer 9 
composed of fine powders of carbonaceous material and/or graphite arranged 
on at least outside of the heating zone 3. A horizontal inner 
cross-sectional area of the preheating zone 2 forming a part of the 
reaction vessel 6 is larger at any level above an arbitrary position of 
the preheating zone 2 than a horizontal inner cross-sectional area of the 
heating zone 3. Gaseous reaction products of other gases are discharged 
via outlet 17. 
It is essential that the cylinder constituting the heating zone 3 is made 
of a heat-resistant material such as graphite, because it is heated to an 
extremely high temperature. The cylinders constituting the preheating zone 
2 and the cooling zone 4 are not necessarily so high heat-resistant 
material as graphite. However, lower part of the preheating zone at lower 
level than the arbitrary position is preferably made of graphite, since it 
is often exposed to relatively high temperatures. The other parts of the 
apparatus can be made of heat-resistant bricks or amorphous refractories. 
Preferably, the effective heating width in the heating zone 3 is a length 
of within a range of about 0.10-0.35 m. If the effective heating width is 
larger than about 0.35 m, differences between reaction temperatures in the 
horizontal heating direction of the heating zone 3 become so large that a 
uniform reaction product can hardly by obtained, whereas, if the effective 
heating width is less than about 0.10 m, the charged materials are likely 
to bridge, so that the smooth descent of the charged materials by their 
own weight becomes difficult and the production of the silicon carbide 
becomes so small that the apparatus can scarcely be used industrially. 
The term "effective heating width" used herein means a twice of a distance 
between an inner wall surface of the cylinder forming the heating zone and 
a charged material therein positioned most distant from the inner wall 
surface on a same horizontal plane of the heating zone. 
As the indirectly electrically heating means, such means as shown in FIG. 1 
consisting of an electric resistive heating member 7 made of graphite and 
a heat reflection cylinder 8 made of graphite or the like, for example, is 
advantageously used. The heating member 7 can be any desired shape such as 
rod shape, U-shape, ribbon shape or other various shapes and is so 
arranged near around the heating zone 3 that a heat-generating portion of 
the heating member 7 corresponds to a vertical length of the heating zone 
3 while heating the materials charged in the heating zone 3 as uniformly 
as possible. A vertical length of the heat-generating portion of the 
heating member 7 is preferably within a range of about 0.3-1.5 m. If the 
length of the heat-generating portion is shorter than about 0.3 m, a 
uniform heating of the materials can scarcely be effected and a descending 
speed of the charged materials has to be slowed down and the productivity 
is decreased in order to effect the reaction of forming silicon carbide 
completely. While, if the length of the heat-generating portion is longer 
than about 1.5 m, once produced .beta.-type silicon carbide is further 
heated to a higher temperature and thus experiences crystal growth through 
decomposition and regeneration and converted to .alpha.-type silicon 
carbide which firmly adheres on the inner wall surface of the reaction 
vessel 6 or yields by itself an agglomerated body to render the discharge 
of the product considerably difficult. The heat reflection graphite 
cylinder 8 may form an inner wall of a chamber containing the heat 
insulating layer 9 as shown in FIG. 1, or an inner lining of the chamber. 
The heating graphite member 7 is positioned in a space defined by the 
cylindrical graphite heating zone 3 and the heat reflection graphite 
cylinder 8. The space is filled with a non-oxidizing gas such as argon, 
helium, nitrogen, carbon monoxide, hydrogen or the like introduced through 
an inlet 10 for charging the non-oxidizing gas, whereby consumption of the 
heating graphite member 7 due to oxidation resulting from invasion of air 
in the space is prevented. The heating member 7 is connected with an 
electric power source (not shown) via a guide electrode 11, a flexible 
conductor 12 and a buss bar 13. In this way, an electric current supplied 
by the guide electrode 11 is changed into heat at the heat-generating 
portion of the heating member 7, and a portion of the heat is transmitted 
directly to the cylinder constituting the heating zone 3 and another 
portion of the heat is transmitted after reflection by the reflection 
graphite cylinder 8 to the cylinder constituting the heating zone 3 and 
indirectly heat the charged materials by passing across the cylinder 
heating zone 3. 
When producing .beta.-type silicon carbide by using the apparatus of the 
present invention and by indirectly and electrically heating the charged 
materials at a desired temperature, a means for measuring a temperature of 
the outer wall or a temperature of the vicinity of the outer wall of the 
cylinder forming the heating zone 3 and regulating an applying electric 
power can be used as a device for controlling the reaction temperature. As 
a means for measuring the above-mentioned temperature, use may be made of 
an optical pyrometer, a radiation pyrometer, a dichroism thermometer or 
the like through a temperature-measuring pipe 14 one end of which being 
closed or left open, or a thermocouple such as tungsten-iridum or the like 
thermocouple for high temperature use. 
A heat insulating layer composed of fine powders of graphite and/or 
carbonaceous material is arranged at least around the heating zone 3, 
thereby to prevent dissipation of heat from the reaction vessel. The heat 
insulating layer 9 must be constituted from such a material that can 
withstand to a high temperature of about 2,000.degree. C. and does not 
react with graphite even in contact therewith and has an excellent heat 
insulating property at high temperature range. Therefore, fine powders of 
graphite and/or carbonaceous material are preferable as the heat 
insulating material. As the fine powders of graphite and/or carbonaceous 
material, use is made of, for example, finely ground various coke, natural 
or artificial graphite, roasted anthracite or other fine powdery 
carbonaceous materials or graphite materials such as lamp black, furnace 
black, acetylene black or the like. Particularly, carbon black and 
acetylene black are advantageous in the present invention in that they 
have small bulk density and superior heat insulating property at high 
temperature range. Preferably, the heat insulating layer has a thickness 
of within a range of about 0.1-1.5 m in horizontal direction. 
At outside of the heat insulating layer 9 is arranged an outer shell 15 
which can be prepared from a steel plate or a steel plate having a lining 
of heat insulating bricks 16 at its interior side. 
The outlet 5 for discharging the product must be a tightly closable 
product-discharging outlet, because of necessity of preventing an invasion 
of the atmosphere into the reaction vessel 6 through the outlet 5 and 
issue of the produced gases generated in the reaction vessel 6 to the 
atmosphere through the outlet 5 in case of discharging the reaction 
product. If the atmosphere invades into the reaction vessel 6 through the 
outlet 5, the reaction product is oxidized and its quality is 
deteriorated. While, if the produced gases issue to the atmosphere from 
the outlet 5, the deposition reaction of the intermediate product SiO gas 
contained in the product gases occurs at the cooling zone in the lower 
part of the reaction vessel 6. Therefore, not only the product quality is 
deteriorated, but also the products are agglomerated due to adhesive 
function of the deposit of the SiO gas and are scarcely discharged, when 
the issue of the produced gases is considerably large. 
The outlet 5 is preferably provided with a discharging means which can 
easily regulate the discharge rate of the reaction product. This is 
because that the descending speed of the materials and the product by 
their own weight in the heating zone 3 can be adjusted by regulating the 
discharge rate of the product, and a uniform product can easily and 
continuously be obtained by regulating the residence time of the materials 
and the product in the heating zone 3 to correspond to the reaction 
temperature thereby to cause the charged materials in the reaction vessel 
to react at an optimum condition for producing the .beta.-type silicon 
carbide. As the discharge means for regulating the discharge rate of the 
product, use is made of, for example, such device as a rotary type, a 
roller type, a table type, a screw type or other conventional type 
discharge device. 
As the indirectly electrically heating means, an indirect heating means may 
be mentioned consisting of an induction coil which inductively heat the 
graphite cylinder forming the heating zone and a heat insulating layer 
arranged between the induction coil and the graphite cylinder, in addition 
to the aforemtnioned heating means consisting of an electric resistive 
heating graphite member 7 and a heat reflection graphite cylinder 8. 
Hereinafter, a method of producing silicon carbide consisting mainly of 
.beta.-type crystal using the apparatus of the present invention will be 
explained. 
Materials consisting mainly of silica powders and carbon powders are mixed 
and shaped to a suitable shape and charged form the charging inlet of the 
reaction vessel into the preheating zone, and reacted to form SiC in the 
heating zone while descending by their own weight, and thereafter 
discharged from the product-discharging outlet, and produced gases are 
discharged from an upper part of the preheating zone so as to prevent 
deposition of SiO gas on the inner wall surface of the preheating zone 
thereby enabling a smooth descent of the materials by their own weight in 
the reaction vessel, whereby silicon carbide consisting mainly of 
.beta.-type crystal can be produced. 
Preferably, the starting materials used in the apparatus of the present 
invention are shaped to pellets having an average diameter of within a 
range of about 3-18 mm. This is because that the apparatus of the present 
invention has the structure that the horizontal inner cross-sectional area 
of the cylinder forming the preheating zone is narrowed at a lower part of 
the preheating zone, so that the charged materials form a bridge and their 
descent by their own weight is obstructed unless the materials have 
excellent flowability. If the average diameter of the pellets is smaller 
than about 3 mm, the materials have worse ventilation property and 
flowability and are liable to suffer the adverse influence of the deposit 
of the SiO gas. While, if the average diameter is larger than about 18 mm, 
an extremely prolonged time is required for completing the silicon 
carbide-forming reaction, so that the production operation becomes 
uneconomical. 
It is important that the shaped materials retain their originally shaped 
forms even when they are exposed to a high temperature or a thermal shock. 
For that purpose, a carbonaceous bonding agent is advantageously used 
which retain its bonding power even at a high temperature range. Shaped 
materials wherein the carbonaceous bonding agent is used have high 
compression strengths and are not degraded to powders and maintain the 
desired forms even in a high temperature reaction zone and retain their 
original forms, because the carbons in the bonding agent can react with 
the SiO gas in the produced gases to form silicon carbide even when the 
shaped materials are heated to yield the reaction product. Therefore, 
advantageous results can be obtained that scattering of the intermediate 
product SiO gas from the shaped materials can be decreased and an amount 
of the SiO gas which can be converted to SiC in the shaped materials 
increases, so that an amount of the SiO gas deposited in the preheating 
zone can be decreased. 
Illustrative advantageous carbonaceous bonding agents are coal tar, 
petroleum tar, wood tar, coal tar pitch, petroleum pitch, wood tar pitch, 
asphalt, molasses, phenol resins, ligninsulfonates and other materials 
having the similar effects. The carbonaceous bonding agent is preferably 
added to the mixture of silica powders and carbon powders in an amount of 
about 1.0-15.0% by weight calculated as solid relative to the mixture. 
In the production method using the apparatus according to the present 
invention, the pelletized mixed materials consisting mainly of silica 
powders and carbon powders can exclusively be used. However, it is also 
possible to incorporate and mix in the materials an additive such as wood 
pieces, wood chips, charcoal grains or the like, in order to decrease the 
influence of the deposit and improve the descent of the materials by their 
own weight, when producing .beta.-type silicon carbide by using 
carbonaceous material having an especially low reactivity. 
In the production method using the apparatus according to the present 
invention, silica powders of high purity obtained by pulverizing silica 
stones or silica sands, carbonaceous powders obtained by pulverizing a 
carbonaceous material having a low ash content, and a carbonaceous bonding 
agent may be used as starting materials. When reciping such starting 
materials, it is advantageous to select a mol ratio of C/SiO.sub.2 within 
a range of about 3.2-5.5. If the C/SiO.sub.2 mol ratio is smaller than 
about 3.2, a proportion of SiO gas which could not be reacted or converted 
to SiC to the whole SiO gas produced in the heating zone increases and a 
smooth descent of the materials by their own weight which is most 
important in continuously and stably producing silicaon carbide is often 
obstructed because of the adhesive deposit of the unreacted SiO gas in the 
preheating zone. While, if the C/SiO.sub.2 mol ratio is larger than about 
5.5, carbon powders which do not contribute to the silicon carbide-forming 
reaction are heated to a high temperature, so that heat efficiency is 
lowered and an expense required to the carbonaceous material becomes 
uneconomically large. 
In the production method using the apparatus according to the present 
invention, a carbonaceous material having a low ash content and a poor 
reactivity can be used. In order to achieve the usage of such material, an 
improvement of reactivities of carbon powders with SiO.sub.2 as well as 
with the SiO gas by enlarging the surface area and surface activity of the 
carbonaceous material by finely grinding the carbonaceous material is 
important. Preferably, an average grain size of the carbonaceous material 
is not over than about 15 .mu.. 
Because impurities contained as ash content in the carbonaceous material 
used in the present invention have a tendency to be incorporated as a 
solid solution into the product .beta.-type silicon carbide, it is 
desirable to use carbon powders having a low ash content, preferably of 
not more than about 1%. Carbon powders satisfying the above condition, 
such as petroleum coke powders, pitch coke powders etc. can advantageously 
be used. 
When effecting the SiC-forming reaction in the heating zone of the 
apparatus of the present invention, the reaction temperature is maintained 
within a range of about 1,650-2,100.degree. C. If the reaction temperature 
is higher than about 2,100.degree. C., the formed .beta.-type silicon 
carbide crystals are grown and transited to .alpha.-type silicon carbide 
to result in a sintered mass, so that the reaction vessel is liable to be 
clogged and a continuous operation of the apparatus becomes difficult. 
While, if the reaction temperature is lower than about 1,650.degree. C., 
the reaction speed becomes so extremely low that the production becomes 
uneconomical. The reaction temperature is substantially correlated to the 
outer wall temperature of the graphite cylinder constituting the heating 
zone. Therefore, in order to maintain the reaction temperature within a 
range of about 1,650-2,100.degree. C., the outer wall temperature of the 
graphite cylinder constituting the heating zone is controlled within a 
range of about 1,750-2,350.degree. C. This is because the reaction 
temperature cannot be raised more than about 1,650.degree. C. if the outer 
wall temperature is lower than about 1,750.degree. C., while the reaction 
temperature rises to more than about 2,100.degree. C. if the outer wall 
temperature is higher than about 2,350.degree. C. 
In addition to the prevention of the flow deflection of the SiO gas on the 
inner wall surface in the preheating zone, preferably a level, i.e., 
height of the materials-filled layer above the arbitrary position of the 
preheating zone is maintained not over than about 0.6 m. If the production 
operation is effected maintaining the height at more than about 0.6 m, 
almost all of the SiO gas risen from the heating zone deposit on the 
surfaces of the materials and cannot be discharged to outside of the 
reaction system, so that a partial pressure of the SiO gas in the reaction 
vessel gradually increases and an amount of deposition of the SiO gas in 
the preheating zone also increases. As a result, smooth descent of the 
materials by their own weight is noticeably obstructed and a long period 
of stable and continuous operation becomes difficult. While, if the 
production operation is effected maintaining the height at too low level, 
the CO gas of high temperature is discharged without heat-exchanging 
sufficiently with the materials, and a thermal energy transferred from the 
reation vessel and dissipated in vain in the preheating zone increases, so 
that the heat efficiency is lowered and simultaneously an amount of the 
SiO gas discharged with the CO gas increases, therefore, heat loss and 
material loss become untolerably large and economical production of 
.beta.-type silicon carbide becomes difficult. Thus, the most favourable 
results can be obtained by selecting the height of the materials-filled 
layer within a range of about 0.1-0.5 m. 
The height of the materials-filled layer can be retained at a substantially 
constant level within the aforedescribed suitable range, or can be 
pulsated intermittently. 
Mass speed of the CO gas passing through the materials-filled layer above 
the arbitrary position of the preheating zone is preferably selected to 
not more than about 300 kg/m.sup.2 .multidot.hr. If the mass speed of the 
CO gas is more than about 300 kg/m.sub.2 .multidot.hr, an amount of the 
deposit per unit horizontal inner cross-sectional area at the SiO 
deposition region in the materials-filled layer increases along with the 
increase of the CO gas, so that a long period of smooth descent of the 
materials by their own weight cannot be maintained. While, if the mass 
speed of the CO gas is too low, output or production of silicon carbide 
per unit apparatus becomes uneconomically small. Therefore, the mass speed 
of the CO gas is preferably selected within a range of about 20-250 
kg/m.sup.2 .multidot.hr. 
Hereinafter, the present invention will be explained in more detail with 
reference to a preferred Working Example in comparison with a Comparative 
Working Example, which, however, should not be construed in any means as 
limitations of the present invention. 
In the Examples, all percentages are shown by weight basis unless otherwise 
specified. 
WORKING EXAMPLE .beta.-type silicon carbide is produced under the following 
conditions, using the apparatus of the present invention as shown in FIG. 
1 and having detailed measures or means as shown in the following Table 1. 
The preheating zone of the apparatus has a whole length of 0.8 m in 
vertical direction, and the horizontal inner cross-sectional area at a 
position of 0.25 m above the lower end of the preheating zone is 2.5 times 
of the horizontal inner cross-sectional area of the heating zone. 
TABLE 1 
______________________________________ 
Heating member: Graphite 
Indirectly electrically 
Reflection 
heating means cylinder: Graphite 
______________________________________ 
Product-discharging means 
Rotary valve 
Effective heating 0.24 m 
width of heating zone 
cylinder 
Whole vertical length 
0.8 m 
of heating zone 
Whole vertical length 
1.0 m 
of cooling zone 
Heat insulating material 
carbon black powder 
Thickness of heat 0.45 m 
insulating layer 
______________________________________ 
Silica stone powder having an SiO.sub.2 content of 99.7% and an average 
particle diameter of 150.mu., petroleum coke powders having a carbon 
content of 98.8% and an ash content of 0.3% and an average grain size of 
12.mu. and pitch powders having a carbon content of 50.4% and an average 
grain size of 20.mu. are reciped to a C/SiO.sub.2 mol ratio of 3.8 and a 
ratio of 7% of the carbonaceous materials powders to the silica stone 
powders, and mixed in a screw type mixer for 10 minutes. The resulting 
mixture is shaped to pellets having an average diameter of 10.3 mm on a 
pan type pelletizer and the shaped pellets are dried at a temperature of 
150.degree. C. for 1.5 hours. The dried pellets are charged in the 
apparatus from an upper part thereof and subjected to the SiC-forming 
reaction at a temperature of about 1,850.degree. C. by indirectly and 
electrically heating the materials while maintaining the height of the 
materials-filled layer (a height from the lower end of the enlarged 
portion of the preheating zone to the upper surface level of the charged 
materials) of 0.3 m in the preheating zone and the mass speed of the CO 
gas within a range of 200-220 kg/m.sup.2 .multidot.hr and allowing the 
materials to descend by their own weight in the heating zone having a 
controlled temperature by regulating the outer wall temperature of the 
graphite cylinder constituting the heating zone to 2,100.degree. C., and 
cooled in the cooling zone, and then continuously discharged from the 
outlet to obtain reaction product. The reaction product has a composition 
ratio of 74.6% of SiC, 2.3% of SiO.sub.2 and 23.1% of free carbons after 
removal of impurities. The reaction product is purified so as to remove 
remained silica and carbons, to obtain .beta.-type silicon carbide. The 
.beta.-type silicon carbide contains 0.2-0.3% of impurities other than 
free silica and free carbons. 
During and after a continuous 7 days operation under the above-described 
conditions, the descending property of the materials by their own weight 
is exceedingly good, so that a stable and continuous operation can be 
effected. 
COMATIVE WORKING EXAMPLE 
The same operation as that described in the above Working Example is 
effected using the production apparatus as shown in FIG. 5 having the 
detailed measures and means as shown in the preceding Table 1. 
After 9-10 hours from initiation of the charging of the materials, a 
phenomenon occurs that the descent of the materials by their own weight is 
retarded and the continuous operation is difficult. Then, the production 
apparatus is overhauled and studied to find out that a deposit of the SiO 
gas has firmly adhered on the inner wall surface of the graphite cylinder 
of the preheating zone and the horizontal inner cross-sectional area of 
the preheating zone has been narrowed to about half as much of the 
original horizontal inner cross-sectional area thereof. 
As apparent from the above explanations, the apparatus according to the 
present invention can stably and continuously produce silicon carbide 
consisting of .beta.-type crystal containing an exceptionally small 
content of impurities as a solid solution, so that the apparatus is 
industrially exceedingly useful. 
Although the present invention has been explained with reference to 
specific values and embodiments, it will of course be apparent to those 
skilled in the art that the present invention is not limited thereto and 
many variations and modifications are possible without departing from the 
broad aspect and scope of the present invention as defined in the appended 
claims.