Inductive heating device utilizing a heat insulator

An inductive heating device for heating a workpiece made of carbon has a heating coil disposed so as to surround the carbon workpiece. A first and second heat insulating layer is interposed between the workpiece and the heating coil, with the first layer being made of a carbon powder and the second layer being made of a carbon and silica powder mixture. A high frequency electrical source is used to drive the heating coil, which causes an eddy current to be induced in the workpiece, thus heating the workpiece. The first and second heat insulating layers prevent the surface of the workpiece from oxidizing and minimize heat loss from the workpiece during the heating thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an inductive heating device for heating 
workpieces at rest and, in particular, to a device for graphitizing 
carbon. 
2. Description of the Prior Art 
A conventional heating device for heating workpieces and, in particular, 
for graphitizing carbon is shown in FIG. 1. In FIG. 1, electrodes 2 are 
placed on opposite sides of pieces of carbon 21. A power source 6 supplies 
electric current to the electrodes 2 and the pieces of carbon 21. The 
current is passed between the electrodes 2 and, therefore, the current is 
also passed through the pieces of carbon 21. A first insulator 3, 
comprising carbon powder "A", is used to fill the spaces between the 
pieces of carbon 21, the carbon powder "A" facilitating the application of 
current to the pieces of carbon 21. A second insulator 4, comprising 
carbon powder "B", is used to cover the top of the electrodes 2 and the 
pieces of carbon 21 to insulate thermally the top of the heating device. A 
refractory member 5 is placed below the electrodes 2, the pieces of carbon 
21 and the carbon powder "A" to insulate thermally the bottom of the 
heating device. 
In the conventional heating device, in order to supply current uniformly to 
the pieces of carbon 21, the first insulator 3, comprising the carbon 
powder "A", is used to fill the gaps between the pieces of carbon 21. In 
this case, the spatial distribution of the current between the electrodes 
2 varies, the amount of variation depending on how well the gaps between 
the pieces of carbon 21 are filled with the carbon powder "A". In 
addition, a long period of time is required for applying current to 
adequately heat the pieces of carbon 21. Such a lengthy current 
application time results in excessive heat loss through the electrodes 2. 
For these reasons, the ratio of electric power applied by the power source 
6 to the sum of the surface areas of the pieces of carbon 21 to be heated 
(hereinafter referred to as "a surface power density") is, in general, set 
to about 3.5 W/cm.sup.2. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of the present invention is to provide 
an inductive heating device suitable for graphitization. The device has a 
heating coil which surrounds a workpiece to be heated and a heat 
insulating layer interposed between the workpiece to be heated and the 
heating coil. As current flows through the heating coil, an eddy current 
is induced in the peripheral portion of the workpiece to be heated, thus 
heating the workpiece. The heat insulating layer reduces the dissipation 
of heat from the workpiece being heated, and also prevents the surface of 
the workpiece from oxidizing. In a preferred embodiment, the heat 
insulating layer comprises two layers, the first being made of a carbon 
powder, and the second being made of a carbon and silica powder mixture. 
Using the device of the present invention, the workpiece can be heated to 
a temperature of 2,200.degree. C. or higher, in a short period of time, 
with a high surface power density.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the heating device of the present invention will be 
described below with reference to FIGS. 2 and 3. 
As shown in FIGS. 2 and 3, an inductive heating coil 22 is arranged around 
a workpiece 21 which is made of carbon. The space between the coil 22 and 
the workpiece 21 is filled with a heat insulating material 23, such as 
carbon powder "B". The coil 22, the workpiece 21 and the heat insulating 
material 23 are placed on a refractory member 24, made of refractory 
bricks or the like. The heating coil 22 is then connected to an AC power 
source (not shown). 
In this embodiment, the inductive heating coil 22 surrounds the workpiece 
21. As current flows in the heating coil 22, an eddy current is induced in 
the peripheral portion of the workpiece 21 to be heated. The permeation 
depth of the eddy current varies, depending on the geometry involved and 
the amount of current flowing in the heating coil 22. The core of the 
workpiece 21 is then heated by the conduction of heat from the peripheral 
portion of the workpiece. 
In order to prevent the dissipation of heat from the peripheral portion of 
the workpiece and to prevent the peripheral portion of the workpiece 21 
from oxidizing, the workpiece 21 is surrounded by the heat insulating 
material 23, such as carbon powder "B". Heating temperatures and saturated 
temperatures, as indicated in the graphical representations in FIGS. 4 and 
5, can be obtained using the above-described heating and insulating 
method. As can be seen, a surface power density of 7.0 W/cm.sup.2, or 
more. A surface power density of 7.0 W/cm.sup.2 is necessary for 
maintaining a heating temperature of 2,200.degree. C. With a surface power 
density in excess of 20 W/cm.sup.2, the heating speed is extremely high 
and, therefore, the workpiece may be cracked by thermal shock. 
The heating device and method described above is suitable for manufacturing 
silicon carbonate. 
In operation, when an alternating current is applied to the inductive 
heating coil 22 and the temperature of the workpiece 21 increases, heat 
flows from the workpiece 21 to the heating coil 22, the refractory member 
4, and the top portion 51. Therefore, a loss of heat occurs. In order to 
limit the loss of heat, the heat insulating material 23 is made of carbon 
powder, carbon black or charcoal powder. This type of heat insulating 
material can sufficiently withstand the highest heat treatment temperature 
(about 3,000.degree. C.) which is applied to the workpiece 21. 
However, when the temperature of the workpiece 21 exceeds about 
2,000.degree. C., the electrical resistance of the heat insulating 
material 23, which is in contact with the workpiece, is considerably 
decreased; e.g., the heat insulating material 23 becomes electrically 
conductive. As a result, an eddy current also flows in the heat insulating 
material 23, thus resulting in a loss of electrical power. Accordingly, 
when the workpiece 21 is made of carbon, the temperature rise is generally 
limited to 2,900.degree. C. with an electric power consumption of 6.0 KWH 
per kilogram of the sintered carbon product. 
In addition, after heat treatment, the workpiece must stand for a length of 
time sufficient to allow it to cool. However, since the workpiece 21 and 
the heat insulating material 23 have a low thermal conductivity, it takes 
five to ten days to cool the workpiece to about 400.degree. C., the 
highest temperature at which the graphitized carbon can be removed from 
the device without oxidizing in the air. 
To improve the utilization of the device of the present invention, further 
modifications can be made, as best illustrated in FIGS. 6 and 7. In the 
device of FIGS. 6 and 7, the heat insulating material 23 has been replaced 
by two heat insulating layers 23a and 23b. The first heat insulating layer 
23a is made of carbon powder packed to a thickness of 5 to 15 cm. The 
second heat insulating layer 23b is made of a mixture of carbon powder and 
silica powder, and this layer is dielectric. The second heat insulating 
layer 23b surrounds the first heat insulating layer 23a. A few concrete 
examples of the device of FIGS. 6 and 7 are described below: 
CONCRETE EXAMPLE 1 
In the inductive heating device of FIGS. 6 and 7 for graphitization, as 
described above, the first heat insulating layer 23a was made of carbon 
black and had a thickness of 10 cm. For the second heat insulating layer 
23b, 50 parts by weight of petroleum coke, consisting of 25% petroleum 
coke by weight having a grain size of less than 100 mesh, and 75% 
petroleum coke by weight having a grain size of 32 to 100 mesh, was 
uniformly mixed with 50 parts by weight of silica grain having a purity of 
more than 95% and a grain size of 32 to 100 mesh. The resulting mixture 
had an electrical resistivity of more than 1 .OMEGA.-cm under a pressure 
of 1 kg/cm.sup.2, and was packed into the device to an average thickness 
of 15 cm to provide the second heat insulating layer 23b. In a region 51 
above the workpiece 21 to be heated, the thickness of the first heat 
insulating layer 23a was 15 cm, and the thickness of the second heat 
insulating layer 23b was 30 cm. In a region 53 below the workpiece 21, the 
thickness of the first heat insulating layer 23a was 5 cm, and the 
thickness of the second heat insulating layer 23b was 30 cm. In the device 
thus constructed, when the workpiece, a sintered carbon product, was 
heated by connecting the inductive heating coil 22 to a high frequency 
electric source, the temperature of the workpiece 21 reached 3,000.degree. 
C., with the electric power per kilogram of the workpiece being 3.5 KWH. 
It took 72 hours to cool the workpiece to 400.degree. C. 
CONCRETE EXAMPLE 2 
The first heat insulating layer 23a was made of a heat insulating material, 
which was carbon powder having a grain size of less than 32 mesh. The 
other components were exactly the same as those of Concrete Example 1. A 
sintered carbon product workpiece was heated in the same manner as in 
Concrete Example 1. In this example, however, the temperature of the 
workpiece reached 3,000.degree. C. with 4 KWH electric power per kilogram 
of the workpiece, and it took 65 hours to cool the workpiece to 
400.degree. C. 
In the above-described Concrete Examples 1 and 2, the refractory cement 
used for the inductive heating coil 22 was not damaged at all. 
The Concrete Examples described above are provided only to aid in the 
understanding of the invention and, accordingly, the invention is not 
limited thereto or thereby. Furthermore, the application of the device is 
not limited only to the graphitization of carbon material. 
As described above, in the device of FIGS. 6 and 7, the first heat 
insulating layer 23a of carbon, and the second heat insulating layer 23b 
of carbon and silica, are provided in such a manner that the layer 23b 
surrounds the layer 23a. Therefore, the drawbacks accompanying the 
conventional heating device are eliminated, power consumption is 
considerably decreased, and the percentage of utilization of the device is 
increased.