EP0117928B1 - Process for production of steel by the melt-down of iron-sponge in an electric-arc-furnace - Google Patents

Description

The invention relates to a method for producing steel by melting sponge iron in an electric arc furnace, the sponge iron being produced by direct reduction.

As is well known, difficulties arise when an electric arc furnace is only loaded with sponge iron. This is due to the low density and poor conductivity of the sponge iron. Nevertheless, the aim is to use mainly sponge iron in the arc furnace. Here it is advantageous to use the so-called swamp mode of operation, which consists in charging the sponge iron onto liquid, carbon-containing iron (hot metal) already in the furnace, the further supply of energy taking place essentially via the liquid bath.

In the swamp mode of operation, however, the arc furnace must still be completely emptied every two to three batches in order to repair the delivery. In this way, uninterrupted furnace operation with sump is not possible; rather, a sump must be generated again after each emptying, so that the advantages of this mode of operation can only be used incompletely if no liquid iron from another source, preferably the blast furnace, is available. However, it is precisely this that is not available in a sponge processing company.

The arc furnace operation is also inevitably associated with strongly fluctuating energy consumption due to its characteristic and, moreover, discontinuous mode of operation. These fluctuations extend both to the chronological sequence and to the absolute amount of the decrease in energy. An electrical network is required to connect an arc furnace because it is so strong that the reaction - due to furnace operation - does not exceed the maximum permissible limit values.

The object of the invention is to provide a method which makes it possible to enable the advantageous operation of the arc furnace with sump by ensuring that hot metal is sufficiently available and at the same time that the process sequence is as economical as possible.

The invention solves this problem in that the sponge iron is reacted on a bath of liquid, carbon-containing iron in the Uchtbogenofen, wherein the liquid, carbon-containing iron (hot metal) is also generated from sponge iron or pre-reduced ore in an electric reduction furnace, which is dependent on the is regulated by the electric arc furnace-induced load fluctuations so that a practically constant load on the electrical network results.

The process according to the invention thus achieves an overall effect by combining a process step in which the carbon-containing iron required for the sump in the arc furnace is obtained - and preferably from the same starting material as is used in the arc furnace - with the melting of the sponge iron in the arc furnace , which goes beyond the sum of the individual processes taking place in the process sections, because the load on the electrical network is largely evened out in a surprisingly simple manner at the same time as the melting process is improved.

Under the expression arc furnace. are to be understood as directly heated arc furnaces, in which the heating is carried out by electric arcs burning between the electrodes and the metallic insert or the steel bath (direct arc furnace). The term “electric reduction furnace” means furnaces in which the electrodes are either immersed in an open slag bath or in a standing Möller column and in which the energy conversion takes place preferably by means of resistance heating (submerged arc furnace). These furnaces are well suited for reduction work, even with an open slag bath. From sponge iron and added carbon carriers, they produce carbon-containing iron, which is used in the arc furnace as a sump. The electric reduction furnaces can be operated with variable power consumption.

In an advantageous further embodiment of the invention, it is proposed that the waste heat resulting from the direct reduction in the exhaust gas and the energy carriers occurring during or direct reduction and / or for the direct reduction are used to generate electrical energy to cover the energy requirements of the electric reduction furnace and arc furnace combination. Energy sources can be excess solid, carbon-containing materials or combustible gases which are produced in the direct reduction or excess which are produced in the production of the reducing medium for the direct reduction. flammable gases or solid, carbonaceous materials.

An advantageous embodiment consists in that the amount and the analysis of the carbon-containing iron used as the sump in the arc furnace are selected such that the total carbon balance is balanced during the charging of sponge iron in the arc furnace, the active power of the arc furnace being regulated in such a way that the arc furnace is in the thermal equilibrium necessary for melting iron sponge. Thermal equilibrium means that there is no overheating and no freezing.

Another advantageous embodiment consists in the fact that sponge iron with a lower degree of metallization is mainly used for the production of liquid, carbon-containing iron (hot metal) in the electric reduction furnace.

An advantageous embodiment consists in that the excess carbon-containing material separated from the discharge of the direct reduction with solid carbon-containing reducing agents is at least partially burned in a combustion unit with the addition of oxygen-containing gases, the hot combustion gases and the exhaust gas of the direct reduction are used to generate electrical energy, wherein the amount of electrical energy generated is controlled so that this corresponds at least to the maximum energy requirement of the arc furnace plus the minimum energy requirement of the electric reduction furnace, and that the energy not required by the arc furnace is converted in the electric reduction furnace. The excess carbonaceous material is completely burned if its quality is not suitable for use in the electric reduction furnace or if an addition is not required there. Good quality means that the ash and sulfur content is relatively low and the ash is basic. It is also possible to process the separated carbonaceous material and then to insert the good quality fraction into the electric reduction furnace and the poor quality fraction into the combustion. The minimum energy requirement of the electric reduction furnace is the holding power.

The sensible heat of the hot combustion gases and the exhaust gases from the direct reduction are used to generate steam, and the steam drives a generator to generate electricity via steam turbines. The hot combustion gases and the exhaust gases from the direct reduction are expediently conducted separately into separate steam generators and the steam flows into separate turbines. As a result, the turbine for the steam of the exhaust gas of the direct reduction can always be operated in the optimal range, and better utilization and control is possible. The amount of electrical energy generated must correspond to the maximum energy requirement of the electric arc furnace plus the minimum energy requirement of the electric reduction furnace. More electrical energy can also be generated for other purposes of one's own operation, but this additional generation is then not included in the regulation of the current distribution. The electrical energy is distributed in such a way that the energy requirement of the arc furnace is always "met", i.e. if it has high energy requirements, the electric reduction furnace receives less electrical energy, and when the arc furnace is switched off, the electric reduction furnace receives more energy.

The sponge iron is divided in such a way that the amount of carbon-containing iron (hot metal) required for steel production in the electric arc furnace is obtained in the electric reduction furnace.

After hot sieving, the sponge iron can be used hot in the smelting furnaces. The combustion of the excess carbonaceous material can take place in fluidized bed apparatuses or dust fires, such as e.g. B. cyclone firing.

A preferred embodiment is that the exhaust gas from the direct reduction is afterburned before use in electrical power generation. On the one hand, this means that the latent heat content of the exhaust gas is utilized and, on the other hand, uncontrolled combustion is avoided, especially with higher contents of combustible gaseous and solid components.

A preferred embodiment is that further combustible material is charged into the combustion unit. As a result, self-sufficient operation can be carried out even when the heat in the exhaust gas and the hot combustion gases of the excess carbonaceous material are too low.

A preferred embodiment is that the combustion unit is a circulating fluidized bed. The circulating fluidized bed works without a jump in the material density between the dense phase and the dust space above. The solids concentration decreases continuously from bottom to top.

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Such processes, which are particularly suitable for the combustion of the excess carbon-containing material, are described in DE-AS 25 39 546. US Pat. No. 4,165,717, DE-OS 26 24 302, US Pat. No. 4,111,158.

A preferred embodiment consists in that a flammable gas is generated by separate smoldering and / or partial gasification of solid, carbon-containing material, the combustible gas is used to generate electrical energy and the smoldered solid, carbon-containing material in the direct reduction and / or the electroreduction furnace and / or the combustion unit is used. Direct reduction is relieved by the use of carbon-containing material on the exhaust side and its throughput is increased. Since the exhaust gases from the direct reduction contain fewer combustible gaseous components, less electrical energy is generated by the exhaust gas, i. H. the non-controllable base load becomes smaller and the control possibility of the electrical energy generated by the combustion increases. Part of the carbonized material, or possibly everything, can be fed into the combustion unit, so that the quantity charged in the direct reduction is also largely flexible. The generation of electrical energy by the combustible gases is very flexible. Some of the flammable gas can also be used for other purposes in your own company.

A preferred embodiment is that the smoldering and / or partial gasification takes place in a circulating fluidized bed. The circulating fluidized bed is very suitable and flexible to operate. A particularly suitable method is described in EP-A-0 062 363. If the carbonized material from the gasification stage is used in the direct reduction, no charging takes place in the combustion stage.

A preferred embodiment consists of the fact that combustible gas is stored in a gas storage device and is removed when necessary to generate electrical energy. A very good flexibility is achieved through this storage, and reserves are also created especially for the start-up and shutdown operations.

A preferred embodiment is that the combustible gas is used to generate electrical energy using a gas turbine. A very fast regulation of the amount of energy generated is possible with a gas turbine.

A preferred embodiment is that baking coals are used in the circulating fluidized bed. This makes it possible to use these coals without additional effort, which cannot be used directly in direct reduction.

One embodiment consists of the fact that the excess carbon-containing material separated from the discharge of the direct reduction is used in the electric reduction furnace, additional energy sources are burned in a combustion unit with the addition of oxygen-containing gases, the hot combustion gases and the exhaust gas of the direct reduction are used to generate electrical energy , wherein the amount of electrical energy generated is controlled so that it corresponds at least to the maximum energy consumption of the arc furnace plus the minimum energy requirement of the electric reduction furnace, and that the energy not required by the arc furnace is converted in the electric reduction furnace. The separated excess carbon is completely added to the electric reduction furnace when this carbon is of good quality and is required in the electric reduction furnace.

A preferred embodiment is that the direct reduction is carried out in a rotary kiln. The coals used as reducing agents mostly contain higher levels of volatile components, such as. B. lignite, and have a high reactivity.

The invention is explained in more detail with the aid of figures.

As Figure 1 shows. is charged into the rotary kiln 1 iron ore 2 and reduced to sponge iron. The discharge material 3 is separated in a separation stage 4 into sponge iron 5 and excess carbon-containing material, of which a part 6a is passed into the electric reduction furnace 7 and the other part 6b into the circulating fluidized bed 8 and is burned by means of air 9. The hot combustion gas 10 is fed into the steam generator 11. A generator 13 is driven with the steam 12. The electrical energy generated is fed via line 14 to the electric reduction furnace 7 and the arc furnace 16. The exhaust gas 17 of the rotary kiln 1 is afterburned in a post-combustion chamber 18 with the addition of air 19. The hot gas 20 is fed into the steam generator 21. A generator 23 is driven with the steam 22. The electrical energy generated is fed into line 14 via line 24. The iron sponge 5 is charged in part 5a in the electric reduction furnace 7 and in part 5b in the arc furnace 16. The pig iron produced in the electric reduction furnace 7 is charged into the arc furnace 16, from which the steel 25 is drawn off. As much electrical energy as is required is always supplied to the arc furnace 16 via line 14a. The remaining electrical energy is fed into the electric reduction furnace 7 via line 14b.

The rotary kiln 1 can be operated with coal with a high proportion of volatile components. which are charged via 26 into the loading end and are partly blown into the discharge end by the blowing device 27. In this case, the exhaust gas 17 contains higher proportions of combustible gaseous components and the amount of electrical energy generated in 24 is correspondingly large.

In the circulating fluidized bed 28, coal 29 can additionally be smelted and partially burnt with gases 30 containing oxygen. The combustible gas 31 is burned in a gas turbine 32 that drives a generator 33. The electrical energy generated is fed into line 14 via line 34. The smoldered carbon-containing material is charged from the fluidized bed 28 via line 35 into the rotary kiln 1. In this case, no coal with a high proportion of volatile components is charged into the rotary kiln and the exhaust gas 17 contains only a small proportion of combustible, gaseous components. The amount of electrical energy generated in 24 is correspondingly less.

The generation of electrical energy can be increased by adding coal 36 to the fluidized bed 8. Part of the carbonized material from the fluidized bed 28 can be fed into the fluidized bed 8 via line 37.

Excess current which is constantly generated can be drawn off via line 40 for other consumers of the company.

Flammable gas is stored in the gas store 38 and removed if necessary. Flammable gas can be withdrawn for operation via line 39 if this amount is scheduled for generation.

Via line 41, ore and aggregates can be charged in the electric reduction furnace 7.

When operating the electric arc furnace, the liquid, carbon-containing iron produced in the electric reduction furnace is adjusted in terms of quantity and analysis - mainly carbon - in such a way that the total carbon balance is balanced when the sponge iron is charged. If, e.g. B. by a mistake in sponge iron production, sponge iron of a lower degree of metallization, z. B. instead of 92% only 85%, this can still be processed. However, the spongy iron is only used to charge the electric reduction furnace. It is therefore possible to operate the process with spongy iron with different degrees of metallization.

The steam generated in the steam generator 11 can also be passed to the steam generator 21 via line 12a.

FIG. 2 shows a typical load diagram for three arc furnaces and two electric reduction furnaces that work in a combined operation. The time in minutes is plotted on the x-axis and the active power in megawatts on the y-axis. The dotted curve shows the sum of the active powers of the electric reduction furnaces, the dashed curve shows the sum of the active powers of the arc furnaces and the solid curve shows the sum of the active powers of all melting furnaces. The diagram shows the course of typical work cycles. In particular, it can be seen that the total active power of all furnaces is relatively constant, although the arc furnaces have very large fluctuations in power consumption.

The advantages of the invention are that the entire smelting process can be carried out independently of the performance of the available public supply network, that the operation takes place with minimal energy requirements per ton of steel, that the waste heat of the direct reduction producing the sponge iron is optimally used and that the excess carbon-containing material is used Material of the discharge of the direct reduction and possibly additionally used coal can be burned in an environmentally friendly manner by adding limestone with the accumulation of a landfillable CaS0 ₄ -containing residue.

Claims (15)

1. A process of making steel by melting sponge iron in an electric arc furnace, which sponge iron is produced by direct reduction characterized in that the sponge iron is reacted in an electric arc furnace on a pool of carbon-containing liquid iron, the carbon-containing liquid iron is produced also from sponge iron or from partly reduced ore in an electric reducing furnace, and in dependence on the fluctuations of the electrical load which are due to the operation of the electric arc furnace the operation of said electric reducing furnace is so controlled that the load on the electric power supply system is virtually stabilized.

2. A process according to claim 1, characterized in that the waste heat which becomes available in the exhaust gas as a result of the direct reduction and the energy carriers which are made available by and/or for the direct reduction are used to produce electric power to be supplied to the system comprising the electric reducing furnace and the electric arc furnace.

3. A process according to claim 1 or 2, characterized in that the quantity and analysis of the carbon-containing liquid iron charged to the electric arc furnace to form the hot metal pool therein are so selected that an overall carbon balance is obtained during the charging of sponge iron to the electric arc furnace, and the active power input to the electric arc furnace is so controlled that the thermal equilibrium required for the melting of sponge iron is maintained in the electric arc furnace.

4. A process according to any of claims 1 to 3, characterized in that sponge iron having a low degree of metallization is used mainly in the electric reducing furnace to produce carbon-containing liquid iron (hot metal).

5. A process according to any of claims 2 to 4, characterized in that surplus carbonaceous solids are separated from the solids produced by a direct reduction process with solid carbonaceous reducing agents, at least part of said surplus carbonaceous solids is burnt in a combustion furnace supplied with oxygen-containing gases, the hot flue gases produced by said combustion and the exhaust gas from the direct reduction stage are used to generate electric power at a controlled rate. which is at least as high as the sun from the highest power demand of the electric arc furnace plus the lowest power demand of the electric reduction furnace, and power which is not required by the electric arc furnace at a given time is consumed in the electric reducing furnace.

6. A process according to claim 5, characterised in that the exhaust gas from the direct reduction stage is afterburnt before it is used to generate electric power.

7. A process according to claim 5 or 6, characterized in that additional combustible material is supplied to the combustion furnace.

8. A process according to any of claims 5 to 7, characterized in that the combustion furnace comprises a circulating fluidized bed.

9. A process according to any of claims 5 to 8, characterized in that a combustible gas is produced in a separate step by a devolatilization and/or partial gasification of carbonaceous solids and is used to generate electric power, and the devolatilized carbonaceous solids are charged to the direct reduction stage and/or the electric reducing furnace and/or the combustion furnace.

10. A process according to claim 9, characterized in that the devolatilization and/or partial gasification is effected in a circulating fluidized bed.

11. A process according to claim 9 or 10, characterized in that combustible gas is stored in a gas holder and is taken therefrom for the generation of electric power in case of need.

12. A process according to any claims 5 to 11, characterized in that the combustible gas is used in a gas turbine for the generation of electric power.

13. A process according to claim 10, characterized in that caking coal is supplied to the circulating fluidized bed.

14. A process according to claim 2 or 3, characterized in that surplus carbonaceous solids which have been separated from the solids produced by the direct reduction are charged to the electric reducing furnace, additional energy carriers are burnt in a combustion furnace supplied with oxygen-containing gases, the hot flue gases and the exhaust gas from the direct reduction stage are used to generate electric power which is at least as high as the sum of the highest power demand of the electric arc furnace and the lowest power demand of the electric reducing furnace, and power which is not required in the electric arc furnace at a given time is consumed in the electric reducing furnace.

15. A process according to any claims 1 to 14, characterized in that the direct reduction is carried out in a rotary kiln.

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