JP3510367B2 - Operating method of electric smelting furnace with improved desulfurization ability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is capable of reducing power consumption and maintaining high desulfurization efficiency while maintaining desulfurization ability when recovering metals by melting and reducing raw materials mainly containing oxides in an electric smelting furnace. Regarding how to operate.

[0002]

2. Description of the Related Art In a smelter that uses a blast furnace, an electric furnace, a converter, etc., a large amount of metal components is contained in a raw material pretreatment step, when supplying a raw material into the smelting furnace, and when operating the smelting furnace. Dust is generated. In a factory equipped with a surface treatment line, a large amount of sludge is generated from the waste acid / waste liquid treatment process, water recycling facility, etc.
Oxide scale is generated in rolling lines, processing lines and the like. The treatment of dust, sludge, scale, etc. is roughly classified into (1) landfill and ocean dumping, (2) sale as raw material to non-ferrous metal manufacturers, and (3) recycled use as raw material in the factory. Above all, it is desirable to use it effectively as a raw material for metal smelting from the viewpoints of environmental problems, effective use of resources, processing costs and the like. Dust, sludge, scale, etc. are all oxides and hydroxides in the generation process, and are in the form of fine powder, and in some cases, contain a large amount of water. Therefore, it is difficult to handle and dispose of them, and various methods have been conventionally proposed as countermeasures.

For example, Japanese Unexamined Patent Publication No. 4-66625,
In Japanese Examined Patent Publication No. 4-61043 and Japanese Examined Patent Publication No. 4-77054, crude zinc oxide and reduced pellets are produced by mixing coke, lime and the like and treating the mixture with a reduction kiln. Since the obtained reduced pellets need to be further processed, this method is effective only in a factory equipped with a blast furnace, an electric furnace, and the like. Recently, a DC arc furnace method in which smelted dust that has been dried and pulverized is charged into an electric furnace equipped with a hollow electrode and treated by arc heat is used in some cases. However, in the DC arc furnace method, there is a problem of loss due to powder supply and secondary dust.

A method of recovering metal components contained in dust, sludge, scale, etc. as a molten metal or alloy has also been adopted. For example, in JP-A-1-247535 and JP-A-5-9529, iron-making dust and sludges are charged into a shaft furnace having tuyere, and hot air is blown from the tuyere to melt and reduce the value. You are getting metal. In JP-A-4-2734, molten pig iron is obtained by charging pellets into a vertical reduction furnace equipped with a preheating section and a reducing section, and then performing smelting reduction in the smelting furnace. These methods are premised on continuous operation, and there is a problem that productivity is reduced and production cost is increased due to operation stop due to trouble. Further, in Japanese Patent Laid-Open No. 3-197625, the slag composition and amount are adjusted according to changes in slag resistance, but there is room for improvement in control accuracy in operating with high efficiency. As a method with high operation flexibility, a resistance heating type electrode buried type electric furnace made of ore or the like as a raw material is used for manufacturing an alloy. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-212342 introduces a method for stable operation based on electric resistance indices such as resistance and reactance obtained from electrodes. Japanese Patent Publication No. 63-5671 discloses a method of estimating the electrode tip position from the electrode length and maintaining stable operation based on the estimation result.

[0005]

Dust, sludge, scale and the like contain various components depending on the generation process. When recovering valuable materials from various kinds of dust, sludge, scale, etc. as raw materials, quality control of raw materials is required. However, in actual operation, it is difficult to keep the quality of recovered valuable materials constant, because the amount of production, composition, etc. fluctuate due to changes in the operating conditions of the source. As a result, the processing cost tends to increase due to variations in the quality of the recovered metal, a decrease in productivity, variations in the energy intensity, and so on. In order to suppress the influence of fluctuations in the raw materials, it is necessary to perform detailed management regarding management of the raw material storage, composition management, moisture management, etc., leading to an increase in management costs. Moreover, a large amount of capital investment cost is required for that reason. Moreover, even if the raw materials are blended and smelted under perfect raw material management, they may be pulverized in the transportation process to reach the smelting furnace or turned into dust inside the smelting furnace. The pulverization or dusting causes a change in the concentration of a specific component, and also makes the raw material composition in the reaction region different from that in the compounding stage. Therefore, the operating condition becomes unstable, the refining reaction such as desulfurization does not proceed sufficiently, and it causes an increase in the electric power consumption rate.

[0006] These problems are caused especially by a special steel production plant,
This is a problem when the mixing ratio of dust and sludge is high in a small lot, high-mix type steel plant, a steel mill that uses a large amount of scrap as a raw material, and the like. In addition, even in a factory equipped with various types of surface treatment lines and a factory that treats various kinds of waste by consignment of multiple business partners, there are problems in management and treatment due to the wide variety of types and compositions. Occurs. In a system where non-ferrous metals such as Zn with high vapor pressure are by-produced, evaporated non-ferrous metals are deposited and adhere to the relatively low temperature furnace wall, etc. There is a case. The present invention has been devised in order to solve such a problem, by controlling the operating conditions based on the composition of the slag generated in the smelting using an electric smelting furnace, especially valuable metals The objective is to reduce the power consumption per unit by collecting at a rate, shortening the dissolution time and improving the dissolution processing capacity.

[0007]

In order to achieve the object, an operating method of the present invention is to dissolve and reduce an oxide raw material containing steelmaking dust and waste liquid sludge as main components in an electric smelting furnace to obtain a slag component. CaO + SiO ₂ as 40-70%
When the slag is produced and the Ni, Cr-containing alloy is recovered, the basicity (CaO / SiO ₂ ) of the slag is 1.7 to
2.6, Al ₂ O ₃ concentration is set to 8 to 20%, and the unit of coke is maintained in the range of 250 to 320 kg / ton-metal. At this time, the variable factors in the transfer process from the raw material blending stage to the melting zone in the electric smelting furnace are set in advance, the raw material blending ratio is determined in consideration of these variable factors, and the raw materials are supplied to the furnace. The blending ratio is adjusted at the raw material blending stage so that the slag composition described above is obtained according to the analysis result immediately before. Further, the specific conductivity of the molten slag is measured, the slag composition is estimated from the measured value, and when the slag component is supplied into the furnace according to the estimated composition, the control accuracy of the slag is further improved and a quick response can be taken. .

The present invention is an operation method suitable for an electric smelting furnace such as a blast furnace type electric furnace and a blast furnace type electric furnace, particularly for an electric furnace equipped with a soderberg type self-baking electrode. Further, when a shaft type electric furnace having a tuyere for blowing powdery raw material into the furnace is provided at the melting area level on the side surface of the furnace body, a remarkable effect is exhibited. The oxide raw materials mainly composed of steel dust, waste acid sludge, etc., which are the raw materials of the electric smelting furnace, include sludges generated in the refining furnaces and smelting furnaces such as sintering furnaces, blast furnaces, electric furnaces and converters. ，
There are oxides containing iron oxide such as polishing powder and grinding dust. In particular, it is suitable as a raw material that mainly contains by-products generated in a factory that produces steel containing Ni and chromium, such as stainless steel and special steel.

[0009]

[Operation] The slag in the electric smelting furnace targeted by the present invention is
CaO, MgO, SiO ₂ , Al ₂ O ₃ , FeO, Cr
It is a multi-component slag containing components such as ₂ O ₃ . this house,
The basicity (CaO / SiO ₂ ) is an important factor in refining reactions such as desulfurization. The basicity is 1. to enhance the refining reaction.
It is necessary to maintain the range of 7 to 2.6, but at that time, the amount of (CaO + SiO ₂ ) in the slag is 40 to 70%.
To If the amount of (CaO + SiO ₂ ) is less than 40%, 60% or more of other components that do not contribute much to the refining reaction are present, and the base has a maximum value of 2.6 in the range specified in the present invention. Ca that contributes to desulfurization even if the degree is adjusted
The O concentration becomes less than 22%. Therefore, sufficient desulfurization cannot be performed even by adjusting the basicity. vice versa,
When the amount of (CaO + SiO ₂ ) in the slag exceeds 70%, it means that the mixing ratio of the oxide raw material such as dust having a high CaO concentration or SiO ₂ concentration in the raw material increases. In this case, the absolute amount of oxide in the blended raw material increases, and it is necessary to add a large amount of reducing agent such as coke to that extent. As a result, it becomes impossible to operate with high power efficiency while maintaining the basic unit of coke in a low range. Therefore, in the present invention, as defined in the slag (CaO + SiO ₂₎ content in the range of 40% to 70%.

The basicity (CaO / SiO ₂ ) of slag is
The optimum range is 1.7 to 2.6. When the basicity is in this range, the electrical conductivity or resistance of the slag will be optimal,
Resistance heating efficiency improves. Further, in the current-controlled electric smelting furnace, the immersion amount of the electrode is optimized, and the rate of heat loss due to excessive rise does not increase. Therefore, the reduction reaction also proceeds sufficiently, and the addition amount of the reducing agent such as coke can be minimized. The basicity of the slag has the relationship shown in FIG. 1 with the specific electric conductivity. As the basicity increases, the specific conductivity increases, that is, the resistance value tends to decrease. If the basicity is higher than 2.6, effective resistance heating by slag cannot be obtained as described above. Further, since the resistance value of the slag is reduced, current easily flows in the furnace, and in a current control type furnace, the electrodes tend to float and heat loss increases. As a result, as shown in FIG. 2, the power consumption rate remarkably increases. On the other hand, at basicities below 1.7,
As shown in FIG. 1 in terms of resistance heating, the power consumption rate decreases, but the desulfurization reaction does not proceed sufficiently, so that [% S] after tapping significantly increases. That is, in order to accelerate the desulfurization reaction, it is necessary to set the basicity to 1.7 or more. In general, higher basicity is more advantageous for desulfurization reaction. In particular, since desulfurization ability is important in an electric smelting furnace using a raw material such as a binder having a high S content and waste acid sludge, the optimum range of basicity is 1.7 to 2 in consideration of resistance heating efficiency. Set to 6.

Al ₂ O ₃ is a component that affects the specific electric conductivity of the slag. In order to further increase the efficiency of resistance heating, it is necessary to maintain the Al ₂ O ₃ concentration in the slag within the range of 8 to 20%. When the Al ₂ O ₃ concentration is in this range, as shown in FIG. 3, the power consumption rate can be lowered at each basicity. Al ₂ O ₃ concentration is 2
If it exceeds 0%, the resistance of the slag becomes too large and the immersion of the electrode becomes too deep. As a result, arcing is likely to occur, the immersion of the electrodes fluctuates irregularly, and the furnace condition becomes unstable. On the contrary, when the Al ₂ O ₃ concentration does not reach 8%,
The resistance heating efficiency cannot be expected to improve while maintaining the desulfurization ability within the adjustment range of the basicity CaO / SiO ₂ = 1.7 to 2.6. That is, if desulfurization is prioritized, the power consumption rate will increase. From the above description, the (CaO + SiO ₂ ) concentration is 40 to 70%,
When the slag composition is adjusted so that the basicity (CaO / SiO ₂ ) is in the range of 1.7 to 2.6 and the Al ₂ O ₃ concentration is in the range of 8 to 20%, the power consumption is reduced. Also, it is possible to operate with satisfaction of both desulfurization ability.

[0012] Under such a raw material composition, the unit coke unit is maintained in the range of 250 to 320 kg / ton-metal. As shown in FIG. 4, the basic unit of coke is the basic unit of electric power and Cr ₂ O in the slag after tapping, which is an index of reducing ability.
₃ Affects concentration. Unit of coke is 250 kg /
If it is less than ton-metal, the amount of coke required for the reduction reaction of the oxide is insufficient, and as shown in FIG. 4, the reducing ability is reduced due to the rapid increase of (% Cr ₂ O ₃ ). On the contrary, the residual coke becomes excessive when the coke consumption rate exceeds 320 kg / ton-metal. Coke
Since it is a substance with high electrical conductivity, the internal resistance decreases as the residual amount increases. Therefore, as in the case of the basicity of the slag, the current easily flows in the furnace, and in the current control type furnace, the electrodes tend to float and the heat loss increases.
As a result, as shown in FIG. 4, the power efficiency sharply deteriorates and the power consumption rate rises.

In order to adjust the composition to the target slag composition, the factors for varying the components in the conveying process from the raw material mixing stage to the melting zone in the electric smelting furnace are obtained in advance. By determining the blending ratio of the raw materials in consideration of those factors at the blending stage,
The target slag composition can be controlled. Factors of varying the components include the usage ratio of the powdery raw material, the particle size distribution, the water content, the cold strength after the assembly, and the hot strength when charged into the furnace. The components and proportions that decrease due to dusting (dusting) loss are empirically understood in relation to these factors. Therefore, it becomes possible to predict the final slag composition that incorporates these fluctuation factors. In some cases, when carrying out sampling and analysis during the transfer process, for example, from the kneading machine to immediately before the kneading machine or in the steps after the kneading machine, and when adjusting the raw material composition and charging conditions based on the analyzed values, the target A slag with a high degree of agreement with the composition is obtained. In any case, sampling and analyzing the charging raw material immediately before charging into the furnace is effective in adjusting the slag composition with high accuracy.

In order to adjust the slag composition more precisely, the specific electric conductivity of the slag in the furnace is measured in the electric smelting furnace in operation, and the slag component is adjusted so that the measured value matches the target value. If necessary, the slag forming agent will be supplied from outside the furnace. For example, since the relationship between the basicity and the specific conductivity is obtained in advance as shown in FIG. 1, when the Al ₂ O ₃ concentration is in a substantially constant range, the basicity (C
aO / SiO ₂ ) can be estimated. Therefore, CaO /
CaO amount or SiO ₂ amount required for adjusting the SiO ₂ is calculated. The specific conductivity of slag is measured outside the furnace by collecting the minimum amount of slag required for measurement with a slag sampler.
A commonly used circuit such as an AC bridge method is adopted as the specific conductivity measuring electrode. Alternatively, the specific conductivity of the slag in the furnace can be directly measured by inserting a probe for measuring electric conductivity from above the raw material layer or from tuyere of the side wall of the furnace.

Accurate slag control is achieved by measuring the temperature at the same time as measuring the specific conductivity. As the temperature measuring means used in this case, a thermocouple attached to the specific electric conductivity measuring electrode is preferable. Alternatively, a means for separately measuring only the temperature can be adopted. For example, it is effective to use a thermocouple with a refractory embedded type and a non-contact type temperature measuring device such as a radiation thermometer. Whichever method you use,
Even if the slag composition is the same, the specific conductivity changes with temperature, so it is desirable to measure the specific conductivity and temperature simultaneously in order to improve control accuracy. In particular, as a method of supplying a slag-forming agent for adjustment, when importance is attached to rapid effect, an injection method in which a powder-like slag-forming agent is mainly blown directly into the melting region is adopted. The injection method is
It also has the effect of stirring the melting zone and makes the current-carrying characteristics in the furnace uniform. In particular, in the case of a multi-electrode type, it is expected that the depth of immersion of the electrodes will be made uniform to reduce the power consumption rate and prevent uneven consumption of the electrodes. However, without being restricted by this, in an electric smelting furnace equipped with a hollow electrode, a powdery substance can be supplied to the melting region from the hollow electrode. Further, a method of supplying the lump-like slag-forming agent from above into the melting zone through a dedicated charging hole, or a method of charging simultaneously with the charging raw material is also effective.

Coke is supplied both internally and externally. As for the exterior method, as the raw material is supplied in a normal operation, additional material is added as the charged raw material level decreases.
Coke is used in various shapes such as powder, granules, and lumps, but it is adjusted to the optimum shape and particle size distribution according to the characteristics of the electric smelting furnace and the particle sizes of the main raw materials and other auxiliary raw materials. It As an interior method, after coking the oxide raw material together with a binder and the like in the optimum amount and particle size of coke,
Briquette, pellets, etc. are formed and dried if necessary.
It is desirable to perform heat treatment such as sintering or to cure the material for several days so as to ensure a certain level of strength before charging. By controlling the operating conditions as described above, the operating state between the taps becomes more stable, the power efficiency is improved, and the recovered metal also has excellent quality.

[0017]

[Examples] Oxide raw materials such as electric furnace dust, converter dust, and waste acid sludge generated in a steelmaking factory producing various stainless steels were used. The composition of each oxide raw material is shown in Table 1.
Since these oxide raw materials have a considerable range of components even in the same lot, the representative values and fluctuation ranges are also shown in Table 1. Each oxide raw material was dried, and coke as a reducing agent and a binder were mixed, and kneading and forming were performed. At this time, with respect to the metal production amount estimated from the raw material components, 50% was internally set as the target of 300 kg / ton-metal as a unit of coke, and the remaining 50% was prepared for external addition.

[0018]

[Table 1]

Example 1 In the raw material composition plan, using the representative values in Table 1, the CaO + SiO ₂ concentration of the slag was 68%,
Various raw materials were blended so that CaO / SiO ₂ was 2.55. As shown in Table 2, the mixing ratio of the raw material A was 20.
7%, the raw material B was 43.2%, the raw material C was 24.5%, and the raw material D was 11.6%. With this blending ratio, the CaO amount and the SiO ₂ amount are calculated from the representative values of the respective components, and CaO / SiO ₂ becomes 2.55. However, when the briquette just before being charged into the electric smelting furnace after being formed and cured was sampled and analyzed, it was found that
₂ O ₃ = 17.9% and CaO / SiO ₂ = 2.59. That is, the actual basicity is 0.
4 was a high value. Therefore, CaO /
In order to lower the SiO ₂ , the blending ratio of the raw material D with a large amount of SiO ₂ was increased to 14.4%, and the blending ratio of the raw material B was changed to 4%.
It decreased to 0.4%. The briquette thus prepared was supplied to an electric smelting furnace for smelting.

The melting time required 155 minutes, and the amount of metal and slag obtained was 0.63 in terms of the ratio to the charged raw materials, as shown in Table 4. Also, Ca in the slag
The O + SiO ₂ concentration was 68.5% at charge No.1. The power consumption rate was 2450 KWH / ton, and the [% S] in the tap metal was 0.015%. Even during the operation of this charge, the briquette in the hopper before charging was sampled and analyzed, and CaO / SiO ₂ was 2.35 as expected, but (% Al ₂ O ₃ ) was 1
The value was 9.4%, which was near the upper limit of the optimum range. Therefore, in the compounding process, the compounding ratio was set to 3 with the aim of diluting (% Al ₂ O ₃ ).
%, Raw material E is newly added, and the blending ratio of raw material B is 37.4%.
Reduced to%. The operation of Charge No. 102, in which this briquette was charged and melted, required a melting time of 150 minutes,
The amount of the obtained metal and slag was 0.65 in terms of charging raw material. The composition of slag after tapping is CaO / SiO.
₂ was 1.72 and (% Al ₂ O ₃ ) was 17.5%, which were in the proper ranges. As a result, the power consumption rate is 2400 KWH /
[% S] in the metal after tapping was 0.027% in tons.

As described above, the actual analytical value of the briquette is fed back to the raw material blending plan, and the operating method is adjusted to the optimum slag composition, that is, the specific electric conductivity, and continuously 3 charges (charge No. 103 to 105). Was operated. The adjustment of the raw material composition during that time is as shown in Table 2. CaO + SiO ₂ concentration was 67.4 between 5 charges.
〜68.5%, CaO / SiO ₂ ratio is 1.71-2.3
5, (% Al ₂ O ₃ ) could be contained in the range of 16.5 to 19.4%, respectively. The electric power consumption rate of each charge in this embodiment is 2390 to 24 as shown in Table 3.
It was 50 KWH / ton-metal. In addition, the ratio of (deposited metal + slag) amount / charged raw material between 5 charges is
It was maintained at a high level of 0.63 to 0.66. Regarding the desulfurization capacity, the sulfur concentration in the metal after tapping [%
S] was in the range of 0.015 to 0.029%, and was maintained at a level where there was no problem in using it as a raw material for stainless steel. The dissolution time is also shortened as compared with the comparative example described later. From these, it can be seen that the input power is effectively consumed as the melting energy.

Example 2 Similar to Example 1, various raw materials shown in Table 1 were blended and made into a slab, and after curing for several days, they were supplied to an electric smelting furnace. The raw material composition is the method of feeding back the analytical value as in Example 1, and the target Al ₂ O ₃ concentration is 10 to 1
It was adjusted within the range of 5%. Furthermore, during operation of the electric smelting furnace, the specific conductivity of the furnace slag is measured by inserting a probe-type electrode, and the basicity (CaO / SiO 2) estimated from the measured value is measured.
_The basicity was adjusted within the optimum range by supplying CaO or SiO ₂ depending on ₂ ). Further, the slag component was blown into the melting zone in the furnace so that the basicity did not fall below 1.7 and the basicity became as low as possible.

In the operation of Charge No. 106, the initial specific electric conductivity was 1.2 Ω ^-1 / cm and the basicity was estimated to be 1.9. Since the basicity tended to gradually increase during the operation, the basicity target value was set to 1.8 and SiO ₂ was blown so as to lower the basicity by 0.1. The required amount of SiO ₂ was estimated from the estimated slag volume at that time. For example, in the middle stage of dissolution, it was estimated that about 2500 kg of slag was produced, of which 60% was estimated to be CaO + SiO ₂ . Therefore, when changing the specific conductivity at this time, about 28 kg of SiO ₂ calculated from FIG. 1 is calculated so that the basicity is decreased by 0.1 with respect to the increase of the specific conductivity of 0.1 Ω ⁻¹ / cm. Blew in. For the blowing, a method of blowing powdered SiO ₂ from the tuyere of the furnace wall to the molten region in the furnace using argon as a carrier gas was adopted. In this way, the change in basicity during charge No. 106 could be stabilized within the range of 1.8 to 1.95.

In Charge No. 107, the specific electric conductivity was 1.3 Ω ⁻¹ / cm at the initial stage of dissolution to 1.2 → 1.
0 → 0.80 Ω ⁻¹ / cm decreased, and the estimated value of CaO / SiO ₂ tended to decrease from 1.9 → 1.8 → 1.7. Therefore, for the initial estimated slag production amount of 1000 kg, about 22 kg of CaO necessary for increasing the basicity calculated from FIG. 1 by 0.1 is blown with the goal of CaO / SiO ₂ = 1.8. It is. Furthermore, since the basicity tended to decrease to 1.75 from the middle period to the latter period, the basicity was increased with respect to the estimated slag production amount of 4000 kg at this point in order to further increase the basicity by 0.07. It takes about 44 to increase by 0.07
Bubbling CaO of kg. In this way, the basicity could be stabilized in the range of 1.75 to 1.90 between the charges No. 107.

The operation method described above was continuously operated for 5 charges (charge No. 106 to 110), and tapping was carried out. 5
As shown in Table 3, the basicity between charges was at a lower level as compared with Example 1, and it was possible to control the ideal slag composition. In each charge in this case, the electric power consumption rate was 2380 to 2395 KWH / ton-metal, and it was possible to save only about 100 KWH / ton-metal compared to the comparative example described later. Also, regarding the desulfurization ability, the sulfur [% S] in the metal after tapping was as low as 0.019 to 0.023%, and it was maintained at a level without any problem when used as a raw material for stainless steel.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

Comparative Example 1 (Conventional Method) A briquette for an electric smelting furnace was formed by using raw materials generated in a stainless steel factory in the same operating condition as in Example 1. However, the representative value of each lot is used as an analysis value of various raw materials when the raw materials are mixed, and especially Ca as a slag component is used.
O, SiO ₂ and Al ₂ O ₃ were not considered. The target unit of coke is 300k for interior and exterior supply.
It was adjusted to around g / ton. After curing the compounded raw material in the same manner as in the example, it was supplied to the electric smelting furnace and continuously tapped for 6 charges (charge No. 121 to 126). In Comparative Example 1,
The method of feeding back the analytical value of the briquette during operation and controlling the raw material blending was not adopted. Table 5 shows the basicity and the Al ₂ O ₃ concentration obtained from the analysis of the discharged slag. As can be seen in Table 5, Charge No. 121
In the range of up to 123, the Al ₂ O ₃ concentration was in the proper range,
The basicity was out of the proper range. As a result, the electric power consumption per 6 charges is
Except for o.123, there were variations at a high level as compared to the examples of 2450 to 2750 KWH / ton-metal.
In addition, as the power efficiency deteriorates, the amount of raw material remaining in the furnace tends to increase, and as shown in Table 6, (metal pig iron +
The amount of slag / charged raw material ratio gradually decreased from 0.62 to 0.58. It can be seen that the melting time is also extended due to the deterioration of the power efficiency, which leads to a decrease in productivity.

Comparative Example 2: A briquette for an electric smelting furnace was assembled using the raw materials produced in a stainless steel factory in the same operating condition as in Example 1. However, when the raw materials were mixed, the analytical values of various raw materials were taken into consideration for the slag components of Al ₂ O ₃ , CaO and SiO ₂ , but CaO / SiO ₂ =
The compounding ratio was determined with the goal of 2.0 and Al ₂ O ₃ = 10 to 13%. In Comparative Example 2, the slag composition was adjusted by feeding back the analysis value of the briquette during the operation and controlling the blending ratio as in the case of the example. However, with regard to the coke consumption, the amount of external supply was consciously controlled, and it was adjusted to 330 kg / ton or more for 2 charges and 240 kg / ton or less for 2 charges. Supply various raw materials to the electric smelting furnace and charge 4 charges (charge No. 12
7-130) Continued tapping. In this case, the basicity and A
As shown in Table 5, the l ₂ O ₃ concentration could be maintained in an appropriate range for all four charges. However, in the case of charge Nos. 127 and 128 in which the basic unit of coke is lower than the appropriate range, as shown in Table 6, the (% Cr ₂ O ₃ ) of the tapped slag is higher than other charges, and the reducing ability is reduced. Was. In addition, the charge No.129,1 in which the basic unit of coke is higher than the proper range
In No. 30, the power consumption rate was 2470 to 2480 KWH / ton-metal, which was at a high level as compared with the examples.

Comparative Example 3: Briquettes for an electric smelting furnace were assembled using the raw materials generated in a stainless steel plant under the same operating conditions as in Example 1. When blending the raw materials, CaO
/ SiO ₂ = 2.0 and Al ₂ O ₃ = 11 to 13% were targeted, and the blending ratio was adjusted in the same manner as in Example 1. The amount of internal coke and the amount of external coke were controlled with the target of about 300 kg / ton of coke. Charge N
In o.140 and 141, the mixing ratio was selected so that the CaO + SiO ₂ concentration of the slag was less than 40%. As a result, as shown in Table 6, [% S] of the metal after tapping
Showed a high value. This indicates that the desulfurization reaction was insufficient due to the lack of the amount of CaO. Charge No. 1
42 and 143, the mixing ratio was selected so that the CaO + SiO ₂ concentration in the slag exceeded 90%. as a result,
As shown in Table 6, the (% Cr ₂ O
₃ ) The concentration was abnormally high compared to the examples. This indicates that the reduction reaction becomes insufficient when the CaO + SiO ₂ concentration exceeds 90%.

[0032]

[Table 5]

[0033]

[Table 6]

[0034]

As described above, in the present invention, the CaO / SiO ₂ ratio, (CaO + SiO ₂ ) concentration,
While maintaining high desulfurization ability by controlling the blending ratio so that the slag having an Al ₂ O ₃ concentration and the like in an appropriate range is formed in the electric smelting furnace and adjusting the coke basic unit to an appropriate range, The dissolution efficiency by resistance heating is improved. As a result, the electric power consumption rate is reduced, and the hot metal having a reduced [% S] to a low level that can be used as a raw material for stainless steel is produced with high productivity. Thus, according to the present invention, an electric arc furnace using scraps or the like as a raw material,
Even in the shaft type smelting furnace, it is possible to reduce the power consumption rate and stabilize the operation.

[Brief description of drawings]

[Figure 1] Relationship between basicity of slag and specific electric conductivity

Fig. 2 Effect of basicity of slag on power consumption and [% S] of post-tapping metal

[Fig. 3] Relationship between Al ₂ O ₃ concentration and power consumption rate

[Fig. 4] Effect of coke intensity on electric power intensity and Cr ₂ O ₃ concentration in slag after tapping

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Claims (3)

(57) [Claims] 1. An oxide raw material containing steelmaking dust and waste liquid sludge as main components is melted and reduced in an electric smelting furnace to produce slag having CaO + SiO ₂ of 40 to 70% as a slag component, and Ni, Cr. When recovering the containing alloy, the basicity (CaO / SiO ₂ ) of the slag is 1.7 to 2.6, Al
₂ O ₃ concentration is 8 to 20% and coke unit is 250
A method for operating an electric smelting furnace having an improved desulfurization ability to be maintained within the range of 320 kg / ton-metal. 2. A method of operating an electric smelting furnace in which, in the raw material mixing step, the raw material mixing ratio is adjusted so as to obtain the slag composition according to claim 1 in accordance with the analysis result immediately before the raw material is supplied into the furnace. 3. An electric smelting furnace equipped with a soderberg type self-baking electrode, the specific electric conductivity of the molten slag is measured, and the basicity (CaO / SiO ₂ ) of the slag is estimated from the measured value, and according to the estimation result. The operation method according to claim 1, wherein the slag component is supplied into the furnace by a slag component.