JPH0660794B2 - Preheating method for raw materials using exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention utilizes exhaust gas generated in an electric furnace for melting or refining (hereinafter, simply referred to as an electric furnace) to produce metal scrap, limestone, iron ore. The present invention relates to a method of preheating a solid raw material such as stone.

[Conventional technology]

For example, when a raw material such as metal scrap is charged into an electric furnace in a preheated state, energy such as electric power consumed for melting or refining is reduced by the preheating amount. Since the raw material is charged in the preheated state, the furnace condition is not significantly disturbed as compared with the charging of the cold material, so that a stable operation state can be obtained. In addition, recently, it has been considered to use a melting furnace or a refining furnace for finishing preheating of the raw material which is preheated in a bucket and preheat the raw material to a temperature as high as possible.

For this preheating of the raw materials, various methods have been conventionally proposed in which exhaust gas generated in an electric furnace is used as a heat source.
When this exhaust gas is used for preheating the raw material, the exhaust heat is effectively recovered. Also, the dust floating in the exhaust gas,
As the exhaust gas passes through the raw material layer, it is captured and removed by the raw material layer. However, the flow rate and temperature of the exhaust gas generated in the electric furnace are not constant, and if this is sent to the raw material preheating furnace as it is, the furnace pressure of the electric furnace will change due to fluctuations in the flow rate and temperature of the exhaust gas and the ventilation resistance of the raw material. It fluctuates and the operating condition becomes unstable.

Therefore, when a large amount of exhaust gas is generated in the electric furnace, a method has been developed in which only a part of the exhaust gas is used for preheating the raw material to control the internal pressure of the furnace so as not to adversely affect the operating state.

For example, when preheating the raw material only by the exhaust gas of the electric furnace during arc melting, in Japanese Patent Publication No. 58-10674,
In the middle of the exhaust gas duct from the arc furnace through the preheating furnace to the cooler and the exhaust fan, a bypass pipe bypassing the preheating furnace is provided, and when the temperature of the exhaust gas sent to the preheating furnace exceeds a set value, the bypass pipe The exhaust gas is passed to the cooler via the.

Further, in JP-A-57-33787, a flow rate adjusting valve is provided in the middle of the exhaust gas duct from the electric furnace to the cooler and the exhaust fan via the preheating furnace, and the furnace pressure of the electric furnace is kept constant by this flow rate adjusting valve. The method of maintaining the operating condition and stabilizing the operating condition is adopted.

[Problems to be Solved by the Invention]

When the exhaust gas is directly sent from the electric furnace to the cooler through the bypass pipe as in Japanese Patent Publication No. 58-10674, the temperature change in the cooler becomes large and the flow rate of the refrigerant such as water used for cooling the exhaust gas. Needs to be changed in a complicated manner. In addition, the amount of heat held by the exhaust gas cannot be efficiently recovered for preheating the raw material. Furthermore, since the filter effect due to the exhaust gas passing through the raw material layer cannot be obtained, a large amount of dust floats in the exhaust gas sent to the cooler,
There is a need for dust removal equipment such as a bag filter that is effective for dust concentrations that change over a wide range.

Further, as in Japanese Patent Laid-Open No. 57-33787, in a method of maintaining a constant furnace pressure by a flow rate control valve provided in the middle of an exhaust gas duct, when two or more electric furnaces are arranged, a preheating furnace is used. The fluctuation range of the flow rate and temperature of the exhaust gas sent in is large, the ventilation resistance when the exhaust gas passes through the raw material layer of the preheating furnace also changes, and the temperature of the exhaust gas at the exit side of the preheating furnace also changes, so the pressure of the exhaust fan rises. Ability also fluctuates. Due to these fluctuation factors, the accuracy of controlling the furnace pressure is reduced.
For this reason, the exhaust gas dust collection state of the electric furnace becomes unstable, and the environment inside the building deteriorates due to the dust generation from the inside of the furnace due to the rise of the pressure inside the furnace, and the infiltration air into the furnace due to the decrease of the pressure inside the furnace. Causes a decrease in thermal efficiency.

Therefore, the present invention, by sending the total amount of the exhaust gas generated in the electric furnace into the bucket under controlled flow rate conditions, effectively recover the heat amount of the exhaust gas as a heat source for raw material preheating, and under stable conditions. The purpose is to preheat the bucket.

[Means and Actions for Solving the Problems]

In order to achieve the object, the preheating method of the present invention introduces a solid raw material, which is preheated in a bucket, into an electric furnace for secondary preheating, and then melts or refines the preheated raw material in the electric furnace. At this time, two or more electric furnaces are arranged,
When a part of it is in the secondary preheating state, the rest of the electric furnace is placed in a melting or refining state, the exhaust gas generated in the plurality of electric furnaces is guided to the bucket via a duct leading to the bucket, and inside the duct. Based on the inlet pressure of the exhaust gas in the bucket, the number of revolutions of the exhaust fan provided in the exhaust duct connected to the outlet side of the bucket is controlled.

Hereinafter, the features of the present invention will be specifically described with reference to the drawings. In the following examples, stainless steel scrap was used as the raw material to be preheated, but the present invention is also applicable to other metal scraps, iron ores, various alloys, limestone, etc. Of course.

FIG. 1 shows an overall flow for explaining the raw material preheating according to the present invention. In the illustrated case, two electric furnaces are arranged and are alternately used for finishing preheating and melting.

Stainless steel scrap or the like is first charged as a preheating raw material 1 into a bucket 2 and is primarily preheated to a temperature of 200 to 300 ° C. Next, the preheating raw material 1 is charged into the electric furnace 3. The electric furnace 3 includes a heater 4 that heats the preheating raw material 1 by burning fuel. The preheating raw material 1 is secondarily preheated, that is, finish preheated in the electric furnace 3 to a temperature (about 700 ° C.) at which oxidation of valuable metals such as chromium does not become excessive. At this time, the electric furnace 5 is in a state of melting the preheated raw material. When the melting in the electric furnace 5 is completed, the electric furnace 3
The heater 4 is removed and the electrode 6 is attached.
Then, the melting of the raw materials is started in the electric furnace 3.

The electric furnaces 3 and 5 are provided with furnace pressure detectors 7a and 7b, respectively, and detect the furnace pressures of the electric furnaces 3 and 5. By comparing the detected value with a set value and adjusting the opening of the flow rate adjusting valves 8a, 8b so as to be the set value, the internal pressure of the electric furnaces 3, 5 can be maintained at the set value.

Exhaust gas generated in the electric furnace 3 and the electric furnace 5 is collected from the respective branch ducts 9a and 9b into the collecting duct 10 and sent to the bucket 2 in its entirety. An inlet valve 11 is provided in the middle of the collecting duct 10, and the pressure of the exhaust gas in front of the inlet valve 11 is detected by a pressure detector 12. The exhaust gas is sent from the upper side to the lower side into the bucket 2, preheats the raw material in the bucket 2, and then passes through the exhaust duct 13 in the lower portion to the exhaust fan.
Sent to 14. The pressure detector 12 may be provided behind the inlet valve 11.

Regarding the operating state of the electric furnace, not all of the furnaces are in the secondary preheating or melting state, and some of the furnaces include charging of raw materials, attachment / detachment of the heater 4 or the electrode 6, tapping, etc. There is also a state.
At such time, close the flow rate adjusting valve 8a or 8b of the furnace,
Exhaust gas is not drawn from the furnace. If neither furnace is in the secondary preheat or molten state, the inlet valve 11 and outlet valve
16 is closed, the bypass valve 18 is opened, and the exhaust fans 14 and 20 are operated at low speed.

The rotation speed of the exhaust fan 14 is controlled by the rotation speed controller 15. The rotation speed controller 15 adjusts the rotation speed of the exhaust fan 14 so that the exhaust gas pressure at the bucket entrance side detected by the pressure detector 12 described above becomes a set value. That is, when a large amount of exhaust gas is sent to the bucket 2 and the pressure rises, the rotation speed of the exhaust fan 14 is increased to suck a large amount of exhaust gas from the bucket 2 to make the pressure on the bucket inlet side constant. On the other hand, when a relatively small amount of exhaust gas is sent to the bucket 2 and the pressure drops, the rotation speed of the exhaust fan 14 is reduced to reduce the amount of exhaust gas suctioned.

Therefore, the entire amount of the exhaust gas generated in the electric furnace 3 and the electric furnace 5 is used for preheating the raw material in the bucket 2, and the exhaust heat is efficiently consumed for preheating the raw material.

Further, a bypass pipe 17 that does not pass through the bucket 2 is connected to the exhaust duct 13 from the collecting duct 10, and a bypass valve 18 is provided in the middle of the bypass pipe 17. This bypass pipe
17 is used during a preparation period such as when the preheating raw material 1 is refilled in the bucket 2. That is, during the preparation period, the inlet valve 11 and the outlet valve 16 are closed, the bypass valve 18 is opened, the exhaust gas is passed through the bypass pipe 17, and the bucket 2 is bypassed. Then, the exhaust gas that has passed through the exhaust fan 14 is removed by the dust collector 19 and then discharged into the atmosphere.

In this example, the exhaust fan 20 is also provided behind the dust collector 19 to form two fans. However, only one exhaust fan may be provided in front of or behind the dust collector 19. The rotation speed control of the exhaust fans 14 and 20 may be performed on both or one of the exhaust fans 14 and 20.

In this way, all the exhaust gas generated in the electric furnace 3 and the electric furnace 5 is sent to the bucket 2. Therefore, the retained heat of the exhaust gas is efficiently consumed for bucket preheating, and the raw material can be preheated quickly. In addition, the bucket inlet pressure is kept constant, and fluctuations in the furnace pressure of the electric furnace are suppressed.As a result, dust inside the furnace causes deterioration of the building environment due to the rise of the furnace pressure, or the inside pressure of the furnace decreases as the furnace pressure decreases. It is possible to suppress a decrease in thermal efficiency due to invading air.

〔Example〕

Using the apparatus shown in FIG. 1, stainless steel scrap or the like is subjected to primary preheating in the bucket 2 and secondary preheating in the electric furnace 3,
In the electric furnace 5, when the secondary preheated scrap is melted, the average temperature generated in the electric furnace 3 and the electric furnace 5 is 300 ° C.
Of the exhaust gas was supplied to the bucket 2 while maintaining the pressure at the bucket inlet side to a water column of -200 mm.

The flow rate of this exhaust gas is 900 Nm ³ / min at maximum and 300 Nm ³ at minimum.
/ Min. When the entire amount of this exhaust gas was sent to the bucket 2 to preheat the preheating raw material 1, it was possible to preheat the scrap to an average temperature of 270 ° C in a preheating time of 60 minutes. On the other hand, as a comparative example, when the exhaust gas having a flow rate of 700 Nm ³ / min or more was flown to the dust collector 19 via the bypass pipe bypassing the bucket 2, the preheating raw material 1 was preheated, and the preheating time was 60 Average temperature of scrap 180 ° C in minutes
Met.

As described above, according to the present invention, a sufficient amount of heat is generated in the bucket 2.
Since it is supplied to the preheated raw material 1 inside, the preheated raw material 1 can be preheated to a high temperature by effectively utilizing the waste heat. As a result, compared with the method of releasing a part of the exhaust gas to the bypass pipe, Recovery amount improved by about 50%. In addition, fluctuations in the furnace pressure of the electric furnace were suppressed, and dust generation from the furnace and air entering the furnace could also be suppressed. Further, part of the dust floating in the exhaust gas was captured by the preheating raw material 1 in the bucket 2. Therefore, the exhaust gas sent from the exhaust fan 14 to the dust collector 19 also has a smaller dust content, and the load on the dust collector 19 is reduced.

〔The invention's effect〕

As described above, in the present invention, the entire amount of exhaust gas generated in the electric furnace is sent to the bucket and used for preheating the raw material. Therefore, the retained heat of the exhaust gas is effectively recovered for the raw material preheating, and the bucket preheating can be quickly performed and the preheating temperature can be increased. Further, fluctuations in the flow rate and temperature of the exhaust gas are absorbed by adjusting the rotation speed of the exhaust fan provided on the outlet side of the bucket, so the bucket inlet pressure is maintained at a substantially constant level and the raw material is supplied. Preheated. On the other hand, in the electric furnace, fluctuations in the furnace pressure are reduced, and the operating conditions can be stabilized. Further, since the exhaust fan is operated at a rotation speed required to maintain the furnace pressure constant, power consumption can be reduced.
Furthermore, since the entire amount of exhaust gas passes through the raw material layer in the bucket, the dust floating in the exhaust gas is captured by the raw material in the bucket, and the load on the dust collector is also reduced. In this way, according to the present invention, the efficiency of recovering the exhaust heat at the time of preheating the raw material can be improved, the operation of the electric furnace can be stabilized, and the exhaust gas can be effectively treated.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall flow from preheating to melting in which raw material preheating according to the present invention is incorporated.

Claims (1)

[Claims] 1. A solid raw material which is preheated in a bucket is charged into an electric furnace for secondary preheating, and when the preheated raw material is melted or refined in the electric furnace, two or more electric furnaces are used. Placed above, when a part of it is in the secondary preheating state, the rest of the electric furnace is placed in a melting or refining state, and the exhaust gas generated in the plurality of electric furnaces is guided to the bucket via a duct leading to the bucket. , Based on the bucket inlet side pressure of the exhaust gas in the duct, to control the rotation speed of the exhaust fan provided in the exhaust duct connected to the outlet side of the bucket of the raw material using the exhaust gas Preheating method.