Crucible induction furnace provided with a preventive measure against low melting point metals

A crucible induction furnace is disclosed which is provided with a preventive measure against low melting point metals and having a crucible refractory within an induction coil in a barrel container, in which the crucible induction furnace comprises a coil protection member located between the crucible refractory and the induction coil and which includes at least one air-permeating portion. An air supply pipe is in communication with the air-permeating portion for supplying pressurized air to the crucible refractory. The air permeating portion is disposed to distribute pressurized air having a higher pressure adjacent to the bottom portion of the crucible refractory than to the upper portion of the crucible refractory.

BACKGROUND OF THE INVENTION 
The invention relates to a crucible induction furnace provided with a 
preventive measure against low melting point metals. 
There is a recent tendency for scraps or the like of galvanized steels and 
which are to be used in automobiles and washing machines to improve their 
corrosion resistance and which are to be melted in crucible induction 
furnaces. 
FIG. 6 is a sectional view showing a main portion of a conventional 
example. When a steel containing zinc is melted at about 1500.degree. C. 
in a crucible induction furnace 1 that is made up of a crucible refractory 
2 and an induction coil 3, the zinc 5 in the molten metal bath 4 is 
susceptible to permeation through the crucible refractory 2 due to the 
static pressure P.sub.S1 as shown in FIG. 6 , thus reaching the induction 
coil 3. As the reaching amount of zinc increases, the induction coil 3 may 
be burnt by the heated zinc 5 or, may, in the worst case, cause a hydrogen 
explosion due to its contact with water in a cooling coil. 
To detect such undesirable conditions while charging a molten metal into 
the furnace, a molten metal leakage sensor is arranged on the inner 
surface of the induction coil 3. Such a sensor is disclosed in, e.g., 
Japanese Utility Model Unexamined Publications Nos. 101792/1988 and 
182568/1987, and Japanese Utility Model Examined Publication No. 
7278/1983. 
The melting point of zinc is 420.degree. C. and its evaporating temperature 
is 920.degree. C., while the melting temperature of cast iron is about 
1500.degree. C. Thus, in the crucible refractory 2 having a porosity of 
about 20%, it is likely that zinc in a gaseous state initially and in a 
liquid state as it permeates through the crucible refractory will 
eventually reach the outer side of the crucible refractory. Despite the 
fact that the crucible refractory 2 maintains its integrity without molten 
steel flashing, the permeation of the zinc therethrough causes the molten 
metal leakage sensor to operate erroneously or it burns and impairs the 
insulation of the induction coil 3 so as to reduce the refractory life. 
In view of the above circumstances, a technique to check the permeation of 
low melting point metals is proposed in U.S. Pat. No. 4,989,218, in which 
gas passages consist of pipes providing holes and grooves inside the 
crucible refractory, and furthermore, consists of an additional lining of 
porous gas passages on the furnace wall, though not shown in the Figures. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a crucible induction furnace 
provided with a preventive measure against low melting point metals, in 
which the measure can block permeation of low melting point metals through 
a crucible refractory. 
The present invention is applied to a crucible induction furnace provided 
with a preventive measure against low melting point metals and having a 
crucible refractory within an induction coil in a barrel container, the 
crucible induction furnace comprising: a coil protection member being laid 
between the crucible refractory and the induction coil and including at 
least one air-permeating portion; and an air supply pipe being in 
communication with the air-permeating portion for supplying a pressurized 
air to the crucible refractory from the outside of the barrel container; 
wherein the air permeating portion is disposed to distribute the 
pressurized air so that the pressure becomes higher in proportion to that 
adjacent to the bottom portion of the crucible refractory than at the 
upper portion of the crucible refractory. 
A first aspect of the invention is applied to a crucible induction furnace 
provided with a preventive measure against low melting point metals, 
comprising: a lining member that is lined between a crucible refractory 
and an induction coil; an air-permeating member that is lined between the 
crucible refractory and the lining member; a porous member that is in 
direct communication with the gas-permeating member and disposed at a 
bottom portion of the crucible, the porous member having a porosity that 
is higher than a porosity of the air-permeating member; and an air supply 
pipe that is in communication with the porous member. 
A second aspect of the invention is applied to a crucible induction furnace 
provided with a preventive measure against low melting point metals, which 
is made up of a coil protection member and a crucible refractory in an 
induction coil. In such a crucible induction furnace, the induction coil 
and the coil protection member are air-permeable in the inward and outward 
direction; the crucible induction furnace is accommodated airtightly in a 
barrel container; and an air supply pipe for supplying outside air is 
connected to the barrel container. 
A third aspect of the invention is applied to a crucible induction furnace 
provided with a preventive measure against low melting point metals, which 
is made up of an induction coil and a crucible refractory. Such crucible 
induction furnace is accommodated airtightly in a bottom-closed barrel 
container with a lid that can be opened and closed, and an evacuating 
device is connected to an upper portion of the bottom-closed barrel 
container. 
A fourth aspect of the invention is applied to a crucible induction furnace 
provided with a preventive measure against low melting point metals, in 
which the depth of a molten metal in the crucible is set to a value from 
1.0 to 0.3 times the inner diameter of the crucible. 
In the first aspect of the invention, although the porosity of the 
air-permeating member is not high as a result of the furnace building 
viewpoint, the air-permeating member is so arranged as to communicate with 
the air supply pipe through the porous member whose porosity is higher 
than its porosity. Therefore, the air permeability between the 
air-permeating member and the air supply pipe can be improved. Because the 
porous member is disposed at the bottom portion of the crucible 
refractory, the air supply pressure applied to the air-permeating member 
becomes larger at the bottom than at the bath surface, and this tendency 
is provided with a well balance to the static pressure of the molten 
liquid which is high in proportion to adjacent to the bottom of the bath 
and derives the permeability of the low melting point metals. As a result, 
the permeation blocking force, being proper, so as to prevent the air 
being wasted from around the bath surface by an excessive air supply 
pressure. The lining member out of the air-permeating member serves to 
prevent the supplied air from escaping toward the induction coil side. 
In the second aspect of the invention, the air supplied into the airtight 
bottom-closed barrel container acts on the crucible refractory by passing 
through the induction coil and the coil protection members, both 
permeating the air inward and outward through small openings or the like 
to thereby block the permeation of gases and liquids of low melting point 
metals. 
In the third aspect of the invention, the bottom-closed barrel container 
having a lid is evacuated by the evacuating device. Therefore, the low 
melting point and low evaporating point metals are evaporated and 
evacuated, thereby preventing their permeation through the crucible 
refractory. 
In the fourth aspect of the invention (see FIGS. 4 and 6), the ratio of the 
depth of the molten metal in the bath to a total bath amount, i.e., the 
static pressure at the furnace bottom is set to a value from 1.0 to 0.3 
times to the inner diameter of the crucible. This brings about a reduction 
of 1/1.3 to 1/5.3 compared with the conventional ratio that ranges from 
1.3 to 1.6 times the inner diameter of the crucible, thereby contributing 
to reducing the permeation of low melting point metals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a sectional view showing a half of a first embodiment of the 
invention; FIG. 2 is a sectional view showing a half of a second 
embodiment; FIG. 3 is a sectional view of a third embodiment; FIG. 4 is a 
sectional view showing a main portion of a fourth embodiment; and FIG. 5 
is an electric efficiency curve in heating versus a ratio of dimensions. 
The parts and components designated by the same reference numerals in the 
conventional example and in the figures have substantially the same 
functions, and their descriptions will in some cases be omitted. FIG. 5 is 
relevant to FIG. 4 to FIG. 6 which show a conventional example. 
In FIG. 1, an induction coil 3 is disposed inside of a yoke 7 that is 
supported within a furnace frame 6. Between the induction coil 3 and a 
crucible refractory 2 are interposed two layers, the two layers being 
worked by special materials. Namely, the outside layer in the two layers 
is a lining member 8 made of coil cement which is comparatively dense, and 
the inside layer in the two layers is an air-permeating member 9 made of 
asbestos or glass fiber which is air-permeable and thermally resistant. At 
the bottom portion of the air-permeating member 9, a porous member 10 made 
of a porous brick is arranged along the entire periphery or along a 
partial arc, so that the porous member 10 comes in contact with the 
air-permeating member 9 on a large surface, making both members 
communicable through the air. The air-permeating member 9 communicates 
with an air supply pipe 11. 
The air-permeating member 9, though air-permeable, is also relatively dense 
as it is a material for building the crucible refractory 2. Since it is so 
arranged that the air-permeating member 9 communicates with the air supply 
pipe 11 through the porous member 10 that is more porous than the member 
9, the air-permeating member 9 allows the gas pressure to act on the 
crucible refractory 2 by supplying the air, N.sub.2 gas, or Ar gas from 
the air supply pipe 11. While the presence of the lining member 8 serves 
to check the gas from leaking toward the induction coil side to a minimum 
possible level, the air-permeating member 9, being relatively dense and 
having a large resistance to the air, causes a larger gas pressure to be 
applied near a lower portion of the crucible refractory 2. That is, there 
exists such a relationship as P.sub.1 &gt;P.sub.2 &gt;P.sub.3 &gt;P.sub.4 in FIG. 
1. This tendency in pressure distribution matches the tendency in the 
static pressure distribution in a molten metal bath 4, and serves to 
effectively block the permeation of zinc 5 gas and liquid. As a result, 
the erroneous operation of a molten metal leakage sensor (not shown) and 
burning of the induction coil 3 due to the permeation of low melting point 
metals such as zinc are eliminated, and the crucible refractory 2 life is 
enhanced. 
In the second embodiment shown in FIG. 2, a crucible induction furnace 20, 
which includes a crucible refractory 2, an induction coil 3, and a yoke 7, 
is accommodated in an airtight barrel container 12 having an air supply 
hole 11'. The induction coil 3 is provided with coil protection members. A 
coil protection member has small openings 13a interposed between the 
induction coil 3 and the crucible refractory 2. The induction coil 3 has 
small holes 3a for passing the pressurized gas through the coil. The coil 
protection members in this embodiment consist of: an air-permeating member 
9' made of, e.g., coil cement; an asbestos board 9a; and a mica board 13. 
The mica board 13 has the small openings 13a. The coil protection members 
are not limited to the above examples, but may be made of asbestos used in 
the first embodiment or the like which are well known. It should be noted 
here that the coil cement is used as the lining member 8 in the first 
embodiment and as the air-permeating member 9' in the second embodiment. 
This is because the gas pressure is applied to the entire surface of the 
induction coil 3 in the second embodiment and because the gas pressure 
undergoes a drastic reduction when applied to the thin and long extending 
air-permeating member made of asbestos of the like in the first 
embodiment. In short, it is because the function to be performed by the 
coil cement differs depending on the action of the gas pressure in each 
embodiment. There exists such a relationship as P.sub.1 =P.sub.2 . . . 
=P.sub.4 in FIG. 2. The barrel container 12 ensures airtightness at upper 
and lower surfaces A and B. It is preferable to increase the small 
openings on the mica board 13 in proportion to adjacent to the bottom 
portion. 
In the third embodiment shown in FIG. 3, a crucible induction furnace 30 is 
accommodated is an airtight fit in a barrel container 32 with lids 31a, 
31b using gaskets 32a, 32b or the like, and is connected to a evacuating 
device (not shown) through a duct 33a of the lower lid 31b and a flexible 
duct 33b. Its negative pressure is preferably set to from 400 to 650 Torr, 
and the recovered metals can be reused. A material 34 is charged by 
opening the upper lid 31a. 
By evacuating the barrel container with the lids using the evacuating 
device, low melting point and low evaporating point metals in steels or 
the like can be evaporated and evacuated, thereby preventing their 
permeation through the crucible refractory. 
With respect to the FIG. 5, in the fourth embodiment shown in FIG. 4, a 
comparison between the conventional example shown in FIG. 6 and this 
embodiment indicates that the ratio H.sub.0 /D.sub.0 of a height H.sub.0 
of a molten metal bath 4 to an inner diameter D.sub.0 of a crucible 
refractory 42 having an induction coil 43 is set to a value from 1.0 to 
0.3, and a static pressure P.sub.SO of the furnace bottom is limited to a 
low value with the bath 4 at the same level. 
In the conventional example shown in FIG. 6, it has conventionally been 
believed to have the ratio H.sub.1 /D.sub.1 set to 1.3 to 1.6 so that a 
vertically long cylinder-like form of the crucible is obtained. Although 
the static pressure P.sub.S1 is high and the permeating pressure of zinc 
is also high, the ratio H/D can be decreased by 1/1.3 to 1/5.3 if the 
crucible design is modified to the one shown in FIG. 4. 
Since it is generally assumed that electric efficiency in heating tends to 
decrease with a smaller H/D, the large H/D has been given as described 
above. However, the relationship between the ratio H/D and the electric 
efficiency is as shown in FIG. 5, which suggests that there will be no 
drastic reduction in the electric efficiency unless the ratio H/D is 
drastically decreased. It is understood from FIG. 5 that the H/D limit 
stands at about 0.3. 
According to this fourth embodiment, not only is the permeation of low 
melting point metals reduced, but also the large opening facilitates 
charging of the materials, thereby making it less likely to cause 
dangerous material bridging. 
The crucible induction furnace provided with a preventive measure against 
low melting point metals according to the first or second aspect of the 
invention keeps in check the permeation of the gases or liquids of low 
melting point metals such as zinc through the crucible refractory by 
applying pressure to the crucible refractory from its outer periphery, 
thereby not only preventing the molten metal leakage sensor from operating 
erroneously and the insulation of the induction coil from burning, but 
also allowing the crucible to be used for a long period of time with an 
extension of the interval between furnace re-buildings. 
The crucible induction furnace provided with a preventive measure against 
low melting point metals according to the third aspect of the invention, 
which comprises an induction coil and a crucible refractory, is 
accommodated airtightly in the bottom-closed barrel container with a lid 
that can be opened and closed, and the evacuating device is connected to 
an upper portion of the bottom-closed barrel container. Therefore, the 
evacuation of the bottom-closed barrel container with a lid by the 
evacuating device causes low melting point and low evaporating point 
metals in steels or the like to be evaporated and evacuated, thereby 
preventing their permeation through the crucible refractory. 
The crucible induction furnace provided with a preventive measure against 
low melting point metals according to the fourth aspect of the invention 
has its bath level within the crucible set to a value from 1.0 to 0.3 
times the inner diameter of the crucible. Therefore, the static pressure 
at the furnace bottom is reduced by 1/1.3 to 1/5.3 the conventional value, 
thereby contributing to preventing the permeation of the low melting point 
metals.