Furnace bottom structure of direct current electric furnace

A furnace bottom structure of a direct current electric furnace has at least one furnace bottom electrode and at least one gas injection tuyere provided in the furnace bottom refractory of the direct current electric furnace. Preferably, the furnace bottom electrodes and the gas injection tuyere are embedded in a substantially cylindrical refractory body with downwardly diverging tapered upper portion. The refractory body, the furnace bottom electrodes and the gas injection tuyere form an integral furnace bottom block which is detachably attached to the bottom of the electric furnace so as to close a bottom opening of the furnace.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a furnace bottom structure of a direct 
current electric furnace which melts and refines metals by arcs produced 
by a direct current. 
2. Description of the Related Art 
Conventionally, steel making arc furnaces are of alternating current type. 
In recent years, however, thyristors and diodes serving as large-capacity 
AC/DC converters are available and, for this reason, direct current 
electric furnaces are attracting attention. 
In producing a steel by a direct current electric furnace, scraps are 
loaded in the furnace and then electric power is supplied to the 
electrodes of the furnace thereby melting the scrap and refining the melt. 
In order to promote melting and refining, oxygen gas is supplied into the 
furnace through a lance which is inserted in a charging hole. Feeding of a 
lance into an electric furnace, however, requires a plurality of workers 
who are obliged to sustain heavy and laborious work at high air 
temperature. This is quite undesirable from the view point of safety. 
Apparatus has been proposed which can feed a lance by means of a truck. 
Such an apparatus requires a large installation space which restricts the 
working space in front of the furnace. 
Moreover, oxygen blowing through a lance alone cannot provide sufficient 
melting effect and does not provide uniform composition of the melt due to 
insufficient stirring effect. 
As an improvement in the above-described art which makes use of a lance, an 
electric furnace has been proposed in Japanese Patent Laid-Open No. 
57-60011 in which a plurality of tuyeres for blowing an oxidizing gas or 
an inert gas are disposed under the upper electlode on the furnace bottom 
along a circle concentric to the electrode. This improved electric 
furnace, however, suffers from a problem in that, during the supply of 
electric power to the upper electrode, electric current flows through the 
furnace shell to generate sparks between MgO-C type tuyere bricks and the 
tuyere so that the tuyere can become molten allowing leakage of gas. 
Consequently, the operation of the furnace becomes stable and dangerous. 
In order to overcome this problem, an electric furnace has been proposed 
in Japanese Patent Laid-Open No. 57-188980 in which the surface of an 
outer tube of a double tuyere is coated with an electrically insulating 
material while the double tuyere itself is grounded, thereby preventing 
generation of arc. This art, however, is intended to prevent generation of 
an arc only at the gap between the furnace bottom gas blowing tuyere and 
the tuyere bricks around the tuyere. 
All of this known arts is intended for overcoming problems encountered with 
the alternating current electric furnace, and no proposal has been made 
for elimination of the same problem in direct current furnaces. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a bottom 
block structure of a direct current electric furnace which enables oxygen 
injection to be done safely and securely at a low cost, thus ensuring a 
quick melting of scrap charged in the furnace and which can promote 
refining of the molten metal. 
To this end, according to the present invention, there is provided a bottom 
block structure of a direct current electric furnace in which a furnace 
bottom electrode and a gas injection tuyere are provided in the furnace 
bottom refractory of the furnace. 
The invention also provides a bottom block structure of a direct current 
electric furnace, comprising a furnace bottom block including a 
substantially cylindrical refractory body formed on a base plate and 
having a downwardly diverging tapered upper portion, and a furnace bottom 
electrode and a gas injection tuyere which are embedded in the refractory 
body, the furnace bottom block being detachably mounted on the bottom 
opening of the furnace. 
Thus, in the operation of the direct current electric furnace of the 
invention, oxygen gas or an inert gas is injected into the furnace through 
the gas injection tuyere provided on the furnace bottom, thus promoting 
melting of the scrap by an arc while accelerating refining reaction of the 
molten steel. 
In addition, detachable furnace bottom block allows easy renewal of the gas 
injection tuyere and furnace bottom electrode. 
Other objects, features and advantages of the present invention will become 
clear from the following description of the embodiments taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present, invention will be described with 
reference to the accompanying drawings. 
FIGS. 1 and 2 show an embodiment of the direct current electric furnace of 
the present invention in which steel scraps are melted and refined under 
the supply of a direct current. The direct current electric furnace shown 
in FIGS. 1 and 2 have a furnace body 1, a furnace roof 2, and a furnace 
bottom portion 3 having a furnace bed refractory 6 sandwiched between the 
furnace bottom 4 and a bottom plate 5. Numeral 7 denotes an upper 
electrode made of graphite, while 8 denotes cylindrical steel furnace 
bottom electrodes which are embedded in the refractory 6 so as to be 
exposed in the upper surface of the refractory 6. In the illustrated 
embodiment, three furnace bottom electrodes 8 are disposed on a circle. 
Each electrode 8 has a diameter of 100 to 250 mm. A water cooling box 9 is 
provided to surround the portion of each furnace bottom electrode 8 
exposed to the outside of the furnace. Cooling water is supplied into the 
water cooling box through a water supply pipe 10 and is discharged through 
a drainage pipe 11 so as to cool the furnace bottom electrode 8, thus 
preventing melt-down of the electrode 8. The water supply pipe 10 serves 
also as a conductor for applying a voltage to the furnace bottom electrode 
8. A single double-walled tuyere 12, composed of an outer tube 13 and an 
inner tube 14, is embedded in the furnace bottom refractory 6 so as to 
project upright into the furnace from the exterior of the same and to open 
in the upper surface of the furnace bottom refractory 6 substantially at 
the surface, on which the furnace bottom electrodes 8 are disposed. 
Preferably, MgO-C type sleeve bricks are disposed around the furnace 
bottom electrodes 8 and the double-walled tuyere 12 so as to extend the 
life of the furnace bottom. The double-walled structure of the tuyere is 
intended for a cooling purpose. When an inert gas such as Ar gas is 
injected, there is no need for the cooling and, hence, a single-walled 
tuyere may be used. 
The operation of this embodiment is as follows. After charging the furnace 
body 1 of the direct current electric furnace with scraps, an upper 
electrode 7 made of graphite is inserted into the furnace through the 
furnace roof 2 and electric current power is applied through the upper 
electrode 7 and the furnace bottom electrodes 8 so that arcs are generated 
between the electrodes to produce heat which melts the scraps and refines 
the molten steel. Thus, the operation of the electric furnace itself is 
the same as that of known electric furnaces. 
During the melting and refining, oxygen gas is injected into the furnace 
body 1 through the inner tube 14. Propane gas is supplied through the gap 
between the outer tube 13 and the inner tube 14 of the double-walled 
tuyere 12, so as to cool the latter. Rates of supply of the oxygen gas and 
the propane gas are increased in the initial stage of the melting after 
the charging of the material or when the quantity of the scrap is large, 
for the purpose of enhancing the stirring effect. As a result, the 
velocity of upward flow of the steel melt is increased to enhance the 
exchange of heat with the surface of the steel melt, thus promoting 
melting of the scraps. 
Conversely, when the quantity of the scraps is small or when the melting 
has proceeded to some extent, the gas flow rates are decreased to optimize 
the operating condition. It is possible to efficiently operate the furnace 
by adjusting the gas supply rates in accordance with the quantity of the 
scrap in the initial stage of operation, while, in the refining stage, 
suitably controlling the flow rates of the propane gas and the oxygen gas 
in accordance with the contents of components such as C and temperature. 
The direct current electric furnace of the present invention has at least 
two furnace bottom electrodes and at least one gas injection tuyere which 
may be double-walled or single-walled. Thus, the combination of the 
furnace bottom electrodes and the tuyere shown in FIG. 2 is only 
illustrative and various other forms of combination are adoptable. For 
instance, in an arrangement shown in FIG. 3, a pair of bottom electrodes 8 
and a pair of tuyeres 12 are disposed alternatingly such that each tuyere 
12 is disposed at the same distance from both furnace bottom electrodes 8. 
In the arrangement shown in FIG. 4, three furnace bottom electrodes 3 and 
three tuyeres 12 are arranged alternatingly and evenly spaced. It is thus 
possible to adopt an optimum combination of the tuyere and furnace bottom 
electrode depending on conditions such as the capacity of the direct 
current electric furnace. 
A description will now be given of the reason why the numbers of the 
furnace bottom electrode and the gas injecting tuyere 12 are limited. 
In direct current electric furnace of a large scale, if only one furnace 
bottom electrode is used, it is impossible to supply the required large 
power and, in addition, the directivity of the arc is rendered unstable 
due to the supply of large electric current In contrast, when a plurality 
of furnace bottom electrodes are used, the current is shared by these 
electrodes. 
The gas injection tuyere is essential for injecting oxygen and Ar gas for 
the purpose of promoting melting and refining of steel. The number of the 
gas injection tuyeres may be determined in accordance with the number of 
the furnace bottom electrodes. It is, however, preferred that each of said 
furnace bottom electrode is symmetrically disposed in the furnace bottom 
and each gas injection tuyere is disposed between adjacent furnace bottom 
electrodes, at an equal distance from these furnace bottom electrodes. 
By using a plurality of electrodes in combination with gas injection 
tuyeres, it is possible to melt the scraps in a shorter time and to 
enhance the agitation effect during refining, thus shortening the 
operating time. Furthermore, automatic injection of the gas is made 
possible. 
FIG. 5 shows a different embodiment of the present invention. Numeral 8' 
designates a hollow cylindrical furnace bottom electrode which extends 
upright from the exterior to the interior of the furnace so as to be 
exposed in the upper surface of the refractory 6. Preferably, this furnace 
bottom electrode 8' has a wall thickness w ranging between 100 and 250 mm 
and inside diameter d ranging between and 1200 mm. 
The interior of the hollow cylindrical furnace bottom electrode 8' is 
charged with the refractory 6 in the center of which is embedded a single 
double-walled tuyere 12 composed of an outer tube 13 and an inner tube 14. 
The double-walled tuyere 12 extends from the exterior to the interior of 
the furnace so as to be exposed in the upper surface of the refractory 6. 
The embodiment shown in FIG. 5 is regarded as being a modification of the 
embodiment shown in FIGS. 1 and 2 obtained by substituting a single hollow 
cylindrical furnace bottom electrode 8' for the three cylindrical furnace 
bottom electrodes 8' shown in FIGS. 1 and 2. Thus, the effect produced by 
the combination of the single double-walled tuyere 12 and the hollow 
cylindrical furnace bottom electrode 8' is materially the same as that 
produced by the combination of the single double-walled tuyere and three 
furnace bottom electrodes used in the embodiment shown in FIGS. 1 and 2. 
The use of hollow cylindrical furnace bottom electrode in combination with 
the single central gas injection tuyere offers the following advantage. 
Firstly, it is to be pointed out that the structure for mounting the 
electrode and the nozzle becomes compact. In addition, the gas injected 
from the double-walled gas injection tuyere is expected to produce a 
certain cooling effect on the electrode. It is also expected that the 
electrode is protected by a mushroom generated in the nozzle portion. 
A description will now be given of a detachable furnace bottom block, with 
reference to FIGS. 6 and 7. The furnace bottom block 21 has a 
substantially cylindrical refractory 6 carried by a base plate 22 and 
having a downwardly diverging tapered portion. In the refractory 6 are 
embedded three furnace bottom electrodes 8 and a single double-walled 
tuyere 12. Thus, the refractory 6, furnace bottom electrodes 8 and the 
double-walled tuyere 12 form the furnace bottom block 21 which is an 
integral block 21. The block 21 is fitted in a furnace bottom opening 17 
provided in the furnace bottom portion 3 of the furnace body 1 and is 
detachably secured to the furnace bottom portion 3 by means of, for 
example, bolts. 
The furnace bottom portion 3 which is defined by the furnace bottom 4 of 
the furnace body 1, bottom plate 5 and the furnace bottom opening 17 is 
heat-insulated by means of refractory bricks 18. 
A description will now be given of the construction for mounting and 
demounting the furnace bottom block 21 in and from the furnace bottom 
opening 17, with specific reference to FIG. 8. 
The bottom plate 5 is extended to form a reinforcement flange 19 around the 
lower end of the furnace bottom opening 17. At the same time, seats 20 are 
provided on the bottom plate 5. The base plate 22 of the furnace bottom 
block 21 is adapted to be fastened to the above-mentioned reinforcement 
flange 19 by means of bolts, 25 which are received in bolt holes 23, 24. 
Preferably, the base plate 22, which supports the furnace bottom block 21, 
is reinforced with a base-plate reinforcement flange 28 and ribs 29. 
Preferably, the bolts 25 are tightened through springs 26 acting between 
the heads of these bolts and the base plate 22 so that tightening force is 
stably exerted despite any thermal distortion around each bolt 25. 
The demounting of the furnace bottom block is conducted in the following 
manner. In order to separate the furnace bottom block 21 from the furnace 
bottom opening 17, separation jacks 27 are placed to act between the base 
plate 22 and the seats 20 provided on the bottom plate 5. Preferably, 
portable small jacks easy to handle are used as the separation jacks 27. A 
water supply pipe 10 for cooling the furnace bottom electrodes 8 is 
preferably connected by means of flanges 15 and adjacent portions of the 
flanges 15 consist of flexible electric conductors 16. 
A description will now be given of the operation for replacing the furnace 
bottom block 21, with reference to FIG. 9. 
A transfer car 30 is stationed beneath the furnace bottom block 21 and a 
lifting cylinder 31 is extended. Workers on a platform 32 then uncouples 
the flanges 15 so as to disconnect cables which lead to the furnace bottom 
electrode 8 and the gas injection tuyere 12, cooling pipes 10, 11 leading 
to the water cooling box 9 and the outer and inner tubes 13 and 14, and 
disconnect also cables for thermometer and so forth. At the same time, 
portable separation jacks 27 are set between the base plate 22 of the 
furnace bottom block 21 and the seats 20 provided on the bottom pate 5. 
Hydraulic lines leading to the separation jacks 27 are not shown. 
After completion of the above-described preparatory work, bolts 25 are 
loosened and removed to disconnect the base plate 22 of the furnace bottom 
block 22 from the reinforcement flange 19 around the bottom opening in the 
bottom plate 5. Subsequently, the separation jacks 27 are actuated so that 
the upper ends of these jacks 27 are pressed against the seats 20 on the 
bottom plate 5, so that the refractory 6 of the furnace bottom block 21 
with downwardly diverging tapered surface is separated from the refractory 
bricks on the surface, whereby the furnace bottom block 21 with the 
furnace bottom electrode 8 is extracted from the furnace bottom opening 17 
and seated on the transfer car 30. 
Then, the transfer car 30 is moved to a suitable place where any damaged 
portion of the refractory 6 is repaired and new furnace bottom electrodes 
8 and gas injection tuyere 12 are embedded. The furnace bottom block 21 
thus repaired is then brought to the position beneath the furnace by the 
transfer car 30. Then, the workers on the platform 32 operates the lifting 
cylinder 31 so that the furnace bottom block 21 on a support 33 is fitted 
in the furnace bottom opening 17. Then, the base plate 22 of the furnace 
bottom block 21 is fastened by means of bolts, 25 to the reinforcement 
flange 19 around the furnace bottom opening 17 in the bottom plate 5. 
Then. clearances between the refractory 6 of the furnace bottom block 21 
and the refractory bricks 18 on the furnace body are filled with mortar, 
thus completing the renewal of the furnace bottom block 21. 
As will be seen from the foregoing description, according to the present 
invention, it is possible to fully automatically control, from a remote 
control station, the operation for injecting oxygen through a tuyere 
provided on the bottom of a direct current electric furnace which melts 
scraps charged therein into steel melt and refines the steel melt. 
In addition, the combination of the furnace bottom electrodes and gas 
injection tuyere as specified by the present invention promotes melting of 
the scraps and enhances the agitation so as to increase the efficiency of 
the refining reaction. As a consequence, the time required for the melting 
and refining can be shortened. 
Furthermore, the invention allows an easy replacement of the furnace bottom 
block which includes the furnace bottom electrodes and tuyeres at a high 
efficiency with a comparatively simple apparatus. As a consequence, costs 
for installation and repair are reduced to attain a higher rate of 
operation of and a higher rate of operation of direct current electric 
furnace is attained.