DC electric arc melting apparatus

Two DC electric arc furnaces are provided side by side. The both DC electric arc furnaces are connected by a duct. Each of two power source apparatuses is mounted near and associated with each of the DC electric arc furnaces. The anodes of the both power source apparatuses are mutually connected by an anode cable and the cathodes of the apparatuses by a cathode cable. The anodes and the cathodes of the both DC elecric furnaces are connected to the anode cable and the cathode cable, respectively, via a connection circuit adapted to supply electric power alternately to either of the DC electric arc furnaces. When a raw material is molten in one of the DC electric arc furnaces, electric power is supplied by the both power source apparatuses to this DC electric arc furance. The exhaust gas at a high temperature produced in this DC electric arc furnace is sent to the other DC electric arc furnace through the duct and preheats another raw material charged there beforehand.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a DC electric arc melting apparatus to melt 
various raw materials by electric arc heating. 
2. Description of the Prior Art 
In order to melt raw materials, a DC electric arc furnace, for example, is 
used. When the raw materials are molten in the DC electric arc furnace, 
exhaust gas at a high temperature is produced there. There is a technique 
to effectively utilize the heat of the exhaust gas. Namely, in addition to 
a DC electric arc furnace 11f including a power source apparatus 61, 
another DC electric arc furnace 12f is additionally provided and the both 
furnaces are connected by ducts 24f and 25f as shown in FIG. 4. The 
exhaust gas at a high temperature produced in the first DC electric arc 
furnace 11f in melting operation is sent to the second DC electric arc 
furnace 12f through the ducts 24f and 25f. In the second DC electric arc 
furnace 12f, a raw material charged beforehand is preheated by utilizing 
the heat of the exhaust gas. After the melting operation in the first DC 
electric arc furnace 11f is finished, the melting operation in the second 
DC electric arc furnace is performed. In this case, the power source 
apparatus 61 of the first DC electric arc furnace 11f is utilized as that 
for the second furnace. Namely, change-over switches 62 and 63 are 
interposed in a connection circuit between the power source apparatus 61 
and the first DC electric arc furnace 11f. These switches 62 and 63 are 
connected to the second DC electric arc furnace 12f by an anode cable 43f 
and a cathode cable 44f, respectively. Electric power is supplied from the 
power source apparatus 61 to the second DC electric arc furnace 12f by 
changing over the switches 62 and 63. The melting operation in the second 
DC electric arc furnace 12f can be performed with less electric energy 
since the raw material there has already been preheated. Again in this 
case, the exhaust gas at a high temperature produced in the second DC 
electric arc furnace 12f is sent into the first DC electric arc furnace 
11f through the ducts 25f and 24f and is used for the preheating there. 
The above mentioned technique to utilize the exhaust gas for the 
preheating is disclosed, for example, in a Japanese published unexamined 
patent application No. 1-167577. 
Now, the above mentioned both DC electric arc furnaces 11f and 12f are 
connected by the ducts 24f and 25f, each DC electric arc furnace is 
provided with a tap hole and a slagging door, and a furnace roof elevating 
and swinging apparatus is provided close to each electric arc furnace. 
Furthermore, spaces for works of tapping out molten metal and slag and 
repairing the furnace are necessary around each furnace. 
According to the technique shown in FIG. 4, however, a power source 
apparatus capable of supplying 100% of the operating power of any one of 
the DC electric arc furnaces as the power source apparatus 61. There 
appears a problem that such a power source apparatus is large-sized and 
therefore is difficult to be mounted around the furnace having the above 
mentioned various spatial restrictions. Moreover, the full current to 
operate the second DC electric arc furnace 12f flows through the anode 
cable 43f and the cathode cable 44f connecting the DC electric arc furnace 
12f to the switches 62 and 63, respectively and so these cables have to 
have a sufficiently large cross section to pass the full operating 
current. Cables having a large cross section are expensive. Furthermore, 
the work of laying the cables is difficult. Namely, the cables must be 
laid at places kept away from the above mentioned ducts 24f and 25f, the 
tap hole, the slagging door and the spaces for the above mentioned works. 
Consequently, the cables must be bent and be supported at various points. 
The work of bending and supporting the cables having a large cross section 
is extremely difficult since these cables are less flexible and heavy. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a DC electric arc 
melting apparatus wherein when a raw material is molten in a first DC 
electric arc furnace, another raw material in a second DC electric arc 
furnace can be preheated, at a place causing the least loss of heat and as 
close as possible to the first furnace, by making use of the exhaust gas 
at a high temperature produced in the first DC electric arc furnace. When 
such preheating is available, there is a usefulness that the melting 
operation of the preheated raw material in the second DC electric arc 
furnace can get off with less electric energy than that of a cool raw 
material. 
A second object of the present invention is to provide a DC electric arc 
melting apparatus wherein when electric power is supplied to the first DC 
electric arc furnace for the melting operation there, the electric power 
is supplied not only by the power source apparatus for the first DC 
electric arc furnace but also by the power source apparatus for the second 
DC electric arc furnace where the raw material is preheated but any 
electric power is not consumed. 
When not only the power source apparatus for the first DC electric arc 
furnace but also that for the second DC electric arc furnace can be 
utilized in this manner, there is an advantage that a cheap power source 
apparatus with an output equal to a half of the operating electric power 
of the first DC electric arc furnace suffices as the power source 
apparatus for the first DC electric arc furnace. 
A third object of the present invention is to provide a DC electric arc 
melting apparatus wherein when the raw material is molten in the second DC 
electric arc furnace, by contrast to the case of the above mentioned 
objects, as well, the raw material can be preheated in the first DC 
electric arc furnace by making use of the exhaust gas produced in the 
second DC electric arc furnace. The third object is as well to provide a 
DC electric arc melting apparatus wherein when electric power is supplied 
to the second DC electric arc furnace for the melting operation there, the 
electric power is supplied not only from the power source apparatus for 
the second DC electric arc furnace but also from the power source 
apparatus for the first DC electric arc furnace and, in addition, a power 
source apparatus with an output equal to a half of the operating electric 
power of the second DC electric arc furnace suffices as the power source 
apparatus for the second DC electric arc furnace. 
A fourth object of the present invention is to provide a DC electric arc 
melting apparatus wherein the power source apparatuses, provided for the 
first and second DC electric arc furnaces, respectively, are small-sized 
and can be mounted in a small empty space available around each of the DC 
electric arc furnaces. 
Either of the power source apparatuses for the first and second DC electric 
arc furnaces is small-sized since the power source apparatuses have only 
to have an output equal to a half of the operating electric power of the 
DC electric arc furnace as is mentioned above. Accordingly, there is an 
advantage that even though the use of the space around the DC electric arc 
furnace is restricted due to the above mentioned tap hole and slagging 
door and the spaces for the works, the small-sized power source 
apparatuses can be mounted in a small empty space apart from such members 
and the spaces for the works. In other words, this is a feature that the 
small-sized power source apparatus, mounted in such small empty space, do 
not interfere at all with the works around the furnaces. 
A fifth object of the present invention is to provide a DC electric arc 
melting apparatus wherein, among the power supply cables connecting the DC 
electric arc furnaces to the power source apparatuses, the anode cable 
connecting the anodes of the both power source apparatuses and the cathode 
cable connecting the cathodes of them can be both relatively thin. 
In order to supply electric power to the first and second DC electric arc 
furnaces in the above mentioned manner, the anode cable connecting the 
anodes of the both power source apparatuses and the cathode cable 
connecting the cathodes of them are necessary. According to the present 
invention, however, when either the first or the second DC electric arc 
furnace is operated, half the operating electric power necessary for the 
operated DC electric arc furnace is supplied by the power source apparatus 
for the operated DC electric arc furnace and the other half is supplied by 
the power source apparatus for the other DC electric arc furnace through 
the above mentioned anode and cathode cables. Namely, only half the full 
current necessary for operating the DC electric arc furnaces flows in the 
anode and cathode cables. Consequently, these anode and cathode cables 
have only to have half the cross section necessary for passing the full 
operating current. Namely, the cables may be thin. Such thin cables are 
cheap and the work of laying the cables is easy.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIGS. 1 and 2, a first and a second DC electric arc furnaces 11 and 12 
are provided side by side. These DC electric furnaces 11 and 12 are 
located as close to each other as possible so that when the exhaust gas 
produced in one of the DC electric arc furnaces is sent to the other as is 
described hereinafter, the heat loss from the gas may be the least. Each 
DC electric arc furnace 11 or 12 includes a furnace body 13 and furnace 
roof 14 covering it. A bottom electrode (anode) 15 is mounted in a hearth 
bottom of the furnace body 13. The furnace body 13 is formed, at a part of 
the side wall thereof, with a protrusion 16 and is provided with a tap 
hole under the protrusion 16. Under the tap hole, ladle for taking out 
molten metal put on a truck is carried in or out in the direction shown by 
an arrow 17. The furnace body 13 is formed with a slagging door 18 at 
another part of the side wall of the body 13, for example, at the part 
opposite to the protrusion 16 as shown. The works of taking out slag and 
repairing the furnace are performed through the slagging door 18 and along 
the direction shown by an arrow 19. A top electrode (cathode) 21 is 
mounted in the furnace roof 14. The furnace roof 14 is moved to a retreat 
place shown by alternate long and two short dash lines or is put again 
onto the furnace body 13 in the direction shown by an arrow 22 by a well 
known furnace roof elevating and swinging apparatus not shown. 
One end of a preheating duct 24 or 25 is connected to the furnace roof 14 
of the DC electric arc furnace 11 or 12. The other ends of the preheating 
ducts 24 and 25 are connected to a combustion tower 26 provided between 
the DC electric arc furnaces 11 and 12 and the ducts 24 and 25 communicate 
with each other through the combustion tower 26. Each preheating duct 24 
or 25 is provided therein with a damper 27 or 28. One end of an exhaust 
duct 31 or 32 is connected to the protrusion 16 in the DC electric arc 
furnace 11 or 12. The other ends of these exhaust ducts 31 and 32 are 
connected to a common exhaust duct 33. The exhaust duct 33 is connected to 
a dust collector 35 via a blower 34. The exhaust duct 31 or 32 is provided 
therein with a damper 36 or 37. The combustion tower 26 is connected to 
the exhaust duct 33 by a by-pass duct 38 including therein a damper 39. 
Now an electric system is described. A power source apparatus 41 or 42 is 
mounted near each DC electric arc furnace 11 or 12. These power source 
apparatuses are those which are adapted to transform AC input power into 
DC output power with use of thyristors. Half the operating electric power 
of one DC electric arc furnace i.e. the DC electric furnace 11 or 12 is 
sufficient as the output of each power source apparatus 41 or 42. Anodes 
41a and 42a of the both power source apparatuses are connected to each 
other by an anode cable 43. Cathodes 41b and 42b are connected to each 
other by a cathode cable 44. As these cables 43 and 44, those cables are 
used which have a cross section sufficient for passing only half the full 
operating current of the DC electric arc furnace 11 or 12. The first DC 
electric arc furnace 11 and the second one 12 are connected to the cables 
43 and 44 through a connection circuit 45 which is adapted to supply 
electric power interchangeably to the DC electric furnace 11 or 12. In the 
present embodiment, the connection circuit includes three switches 46, 47 
and 48 for change-over operation. As the switch 46, a change-over switch 
is used and is located at the midway point between the both DC electric 
arc furnaces 11 and 12. On the other hand, simple making and breaking 
switches are used as the switches 47 and 48 which are located near the top 
electrodes 21 of the first and second DC electric arc furnaces 11 and 12, 
respectively. Numerals 49 through 52 represent connection cables 
connecting respective switches to the bottom electrodes 15, the anodes of 
the DC electric arc furnaces and to the top electrodes 21, the cathodes of 
the furnaces. As these cables 49 through 52, those cables are used which 
have a cross section sufficient for passing the operating full current. 
In the next place, the operation of the DC electric arc melting apparatus 
is explained. First of all, the first DC electric arc furnace 11 is 
operated as follows. A raw material such as a scrap raw material or a 
reduced iron material is charged into the first DC electric arc furnace 
11. The work of charging the raw material is performed with the furnace 
roof 14 opened as is well known. The raw material to be charged may be 
either preheated or not. On the other hand, a raw material not preheated 
is charged into the second DC electric arc furnace 12. The switches 46 
through 48 are changed over beforehand to the states shown in the figures 
and the dampers 27, 28, 36, 37 and 39 are kept in the states shown in the 
figures. With these states, the both power source apparatuses 41 and 42 
are turned on. Then the DC electric power output by the both power source 
apparatuses 41 and 42 is supplied to the first DC electric arc furnace 11. 
An electric arc is struck by the supplied electric power in the first DC 
electric arc furnace 11 and the charged raw material is molten by the heat 
produced by the electric arc. While the melting operation in the first DC 
electric arc furnace 11 proceeds in this manner, exhaust gas at a high 
temperature is produced. The exhaust gas is sent to the second DC electric 
arc furnace 12 through the ducts 24 and 25. In the second DC electric arc 
furnace 12, the raw material charged beforehand is preheated by the 
supplied exhaust gas. The exhaust gas, which has been made use of for the 
preheating, is sent to the dust collector 35 through the ducts 32 and 33. 
In this case, the amounts of opening of the respective dampers are 
adjusted so that a suitable amount of the exhaust gas from the first DC 
electric arc furnace 11 may flow to the second DC electric arc furnace 12 
for the preheating and the rest of the exhaust gas may flow to the dust 
collector 35 without going to the second DC electric arc furnace 12. The 
above mentioned flow of the exhaust gas is formed, of course, by the 
operation of the blower 34. 
In the case of the melting operation in the first DC electric arc furnace 
11, the raw material may be additionally charged as occasion demands. 
After the melting operation in the first DC electric arc furnace 11 is 
finished, the both power source apparatuses 41 and 42 are turned off and 
the molten metal in the first DC electric arc furnace 11 is tapped out 
through the tapping hole. The first DC electric furnace 11 is repaired as 
occasion demands. 
After the operation in the first DC electric arc furnace 11 is finished in 
the above mentioned manner, the operation in the second DC electric arc 
furnace 12 is successively performed as follows in the same manner as in 
the case of the first DC electric arc furnace 11. A new raw material is 
first charged into the first DC electric arc furnace 11. The switches 46 
through 48 are changed over to the states opposite to those shown in the 
figures. The dampers 36 and 37 are turned to the states opposite to those 
shown in figures. With these states, the power source apparatuses 41 and 
42 are again turned on. Then the DC electric power output by the both 
power source apparatuses 41 and 42 is supplied to the second DC electric 
arc furnace 12, where the raw material is molten. The exhaust gas at a 
high temperature produced at this time is sent to the first DC electric 
arc furnace 11 through the ducts 25 and 24 and the raw material having 
been charged beforehand in the first DC electric arc furnace 11 is 
preheated by the exhaust gas. The exhaust gas, which has been made use of 
for the preheating, is sent to the dust collector 35 through the ducts 31 
and 33. 
When the raw material is molten in the second electric arc furnace 12, a 
less electric energy is sufficient for melting the raw material since the 
raw material has been preheated to a raised temperature. After the melting 
operation in the second DC electric arc furnace 12 is finished, the both 
power source apparatuses 41 and 42 are turned off and the molten metal in 
the second DC electric arc furnace 12 is tapped out. 
The above mentioned operations in the first DC electric arc furnace 11 and 
in the second one 12 are repeated alternately. 
In the next place, a different embodiment of the present invention is shown 
in FIG. 3. In this embodiment, a connection circuit between an anode cable 
43e and anodes 15e of first and second DC electric arc furnaces includes 
two switches 54 and 55 instead of the aforementioned change-over switch 
46. In this embodiment, when the first DC electric arc furnace 11e is 
operated, the switches 47e and 54 are made and the switches 48e and 55 are 
broken. On the other hand, when the second DC electric arc furnace 12e is 
operated, the switches 47e and 54 are broken and the switches 48e and 55 
are made. Those members in the present embodiment which are considered to 
be equivalent in view of construction and operation are given the same 
reference numerals but with an alphabet "e" as those of the corresponding 
members in the previous embodiment and the explanation of the members is 
not repeated.