2004 - Advanced EAF Technologies by VAI FUCHS

Hubert Knapp, Michel Hein VAI FUCHS

Abstracts

Electric Arc Furnace Technology in the 21st century is confronted by an extreme challenge. New developments in electric steel making, published by this article, show options to reduce consumption values and increase productivity of Electric Arc Furnaces. The gain in efficiency not only helps to reduce the costs, however, helps to meet modern environmental issues set by the World Convention at Kobe. This paper concentrates on conventional EAF Technology, electric power input, recent developments in furnace design as well as modern oxygen technology.

Introduction

EAF Technology has gained increasing importance in steel industry within the last 10 years. The aims are not only to decrease consumption values, however to increase productivity, using more active electrical power or support melting by auxiliary media, i.e. oxygen, carbon and natural gas, with the demand of more accuracy and higher yields. In addition the main metallic charge materials are changing to less density. Due to the lack of scrap in quantity and specific quality, tendencies are observed, that more DRI, Hot Metal or other substitutes for scrap are charged to meet the demands with respect to final steel composition with low trace elements.

VAl FUCHS, founded in 1969 at Legelshurst, Germany, is active in the design and construction of equipment for steel mills since more than 35 years and was driven by this challenge. World wide more than 90 Electric Arc Furnaces [EAF] and more than 100 Ladle Furnaces [LF] were constructed or revamped according to most modern design criteria with excellent results. A number of our developments are showing high acceptance in steel industry. This paper shall concentrate on new equipment ideas and melt shop layouts, which seem to be of interest for steel industry in India.

Current Conducting Electrode Arms [CCEA@]

The first current conducting electrode arms were developed and fabricated by VAl FUCHS in 1984 and are successfully in operation ever since. More than 300 such installations are in operation world wide today for applications in AC or DC technology of up to 120 MW.

The following advantages can be derived by this development:

• Smallest pitch circle possible.
• The arm itself fabricated from copper cladded steel.
• The thickness of the copper cladding according to the current density.
• The arm totally water cooled.
• Only one insulation between stool and mast head, dust free pre-arranged in the shop. . Clamping device with cup springs and hydraulic cylinder inside conductor.
• No magnetism inside conductor, insulation not necessary.
• High force to clamp electrodes, optimising availability and minimum maintenance.
• High regulation speeds with excellent active power input profiles.
• Power distribution equal on all three phases, adapted via secondary bus bar system.

RCB Injection Technology

The higher the furnace shell, the more pre-heating of the scrap is possible during melting and the more combustion reactions support the performance of the EAF. VAl FUCHS, therefore, concentrated on a combination of auxiliary burners in combination with new injection technologies the so-called Refining Combined Burner [RCB] Modules, shown in Figure 6 with its possibilities. Starting on the left hand side, the RCB can be used as auxiliary burner, using natural gas or LPG during melt down. As soon as the scrap underneath the RCB is melted, supersonic oxygen lancing can be started via the same RCB automatic by remote control from the pulpit.

For this purpose the RCB is mounted by means of a backpack panel, into the sidewall. The RCB is inclined at a defined angle to the steel bath. The backpack panel, fabricated from copper, is cooled by means of high-speed water. This backpack panel ensures, that scrap does not block the burner mouth, increasing the availability of the RCB.

The risk of back firing of the burner is minimised. By this method refractory erosion in the sidewall is minimised. The burner mouth is protected from splashes of steel and slag by a low fire mode. A combined injection with carbon powder on the right hand side is possible. Since no oxygen manipulator is necessary anymore, the slag door can be kept close and false air ingress is minimised, reducing overall energy losses. Operation within multiple reaction areas around the furnace shell are possible with positive effects on heat distribution inside the furnace as well as decarburisation rates of more than 150 kg C/h/m2 of bath surface. Oxygen thus is injected fully automated by remote control from the pulpit and in reproducible process steps.

Trials were undertaken with a slightly modified RCS injecting Carbon into stainless steel with very good results. Similar, it is possible to inject any other fine grain material (Le. Lime, FeSi, dust etc.) using condensed stream transportation methods.

First trials with a contact free measurement of the steel temperature via a RCS module show, that temperature in steel can be measured in an EAF contact free by means of remote control, minimising time losses and costs for the probes including more secure operations.

Use of DRI and Hot Metal

With increasing prices for scrap, especially with the demand on low trace elements in the final product, the use of Direct Reduced Iron (DRr) and Hot Metal is increasingly considered in Electric Steelmaking.

DRI needs more processing energy due to the final reduction of FeO as well as the amount of gangue. The carbon content in DRI has to be adapted to the degree of Metallization. In case of any surplus in carbon, oxygen can be injected very successfully. DRI preferentially is charged into the EAF via continuous feeding in rates adapted to power input for immediate melting, since the reduction of the FeO would create unexpected boiling reactions otherwise. More and more High Temp DRI directly from the DRI process is used, to minimise power-on time and thus reducing the overall energy requirements. This could be demonstrated at HYLSA, Monterrey, operating a Finger Shaft Furnace *. As an indication the electrical energy consumption is reduced by 50 to 60 kWh/t steel by the use of HighTemp DRI in case the temperature of the DRI is 400°C to 500°C. Definitely the DRI transportation systems have to be adapted respectively.

Hot Metal, to be charged into a EAF, needs specific charging technologies. Due to safety reasons hot metal shall not be charged via the open roof into an EAF equipped with water cooled side wall panels. Therefore a hot metal robot was developed by VAI FUCHS. N this case the hot metal is charged from the tapping side via a special launder into the furnace at a rate f up to 5t/min. decarburisation thus can be started quite early during the process and oxygen flow rates of up to 150 Nm3/min with multiple point injection were applied successfully ino the Finger Shaft Furnace with up to 40% hot metal.

In each case of a new plant or investment in an existing plant it is strongly recommended to ask for a respective evaluation and pre-study.

Operations today depend on modern electrode regulation systems, one of which was developed by VAl FUCHS & Vatron, called ArCOS. Further more the electric power supply asks for Flicker values created by the EAF below 1, which is reached by a dynamic compensation. The stability of the power supply can be improved by so-called reactors, which are integrated in the primary circuit or directly into the EAF transformer.

Summary

Electric Arc Furnace designs of the 21 st century are challenged by highest thermal loads, created by an electric power input of =1 MW/t and an increase in oxygen injected into a furnace. The aim is to reach heat cycle times of less than 40 minutes, producing more than 40 heats per day. This only can be reached with equipment, which is capable to cope with this challenge and easy to be maintained. VAl FUCHS, Legelshurst, Germany, since 20 years in the construction of Electric Arc Furnaces and Ladle Furnaces, has not only developed furnace constructions but also new oxygen technologies with excellent performance issues reaching highest productivity. The RCS technology with its wide variability from burner mode to oxygen injection without consumable lances and operated by remote control from the pulpit is one of the leading issues in EAF steel making