Electric Arc Furnace Operations And Working 4m6p6o

UNIT 6 ELECTRIC ARC FURNACE

Electric Arc Furnace Steel Making  Steel making in an electric arc furnace became commercial in the beginning of the last century.  The sizes of electric furnaces was initially small like 1-10 tons but now electric furnaces of much greater capacity are used.  In this presentation we will see the Design construction, working in EAF practice respectively.

DESIGN OF EAF  The temperatures of an EAF using carbon electrodes exceed 4000 c and hence steel making temperatures are easily achieved and maintained.  There are 2 types of EAF:1) Direct arc -> Current flows from the electrode to the charge and heat is transferred from the arc to the charge by radiation.

a) direct arc b) indirect arc c) submerged arc

2) Indirect arc -> Arc is struck between two carbon electrodes and heat is transferred to the charge by radiations.

Arc furnaces are of two different designs :1) The roof along with the electrodes swing clearly off the body to facilitate charging from top.

2) The roof is lifted a little and the furnace body moves to one side clearly off the roof to facilitate charging.

The furnace unit consists of the following parts :1) Furnace body :– Shell -> It is a welded or riveted steel plate construction. -> It has a spherical bottom. -> Spout is welded to the bottom and the main door is situated directly opposite the spout. -> Flat plate ring is used for . -> Top sealing ring is water cooled. Furnace shell used at Jindal steel plant

Refractory lining -> The furnace shell is lined from inside with suitable refractory. Hearth -> Next to the shell at the bottom and is followed by magnesite brickwork. -> Magnesite is prepared from Dolomite. Sidewall -> Cylindrical part of the shell extends vertically from slag line to the top of the shell. -> Magnesite, Dolomite are used to line the side wall. -> Thickness is in the range of 35 – 50 cms.

REFRACTORY LINING

Side Wall of EAF

Tap hole -> Generally a gap is left at the proper place while laying the bricks for making Tap hole. -> Usually used for tapping molten metal and slag. Spout -> The teeming ladle quantity fire bricks are used to line the spout next to the steel plate.

Tap hole and runner that allow liquid steel to flow from an EAF

2) Gears for furnace body movement :-> Furnace body is tilted 45 degrees for tapping side and 15 degrees for slag side. -> Tilting gear is either hydraulic or electric. -> Large furnaces are rotated in a horizontal plane as well, 30 degrees forward and 30 degrees backward movement gives nine positions of arcing. Hence charge melts more evenly. 3) Roof :-> High alumina[ 70-80%] bricks are used for roof lining. -> The roof fails because of splashing of iron oxide rich slag which corrode it at steelmaking temperature.

4) Electrode and its :Electrodes -> They are either of carbon or graphite and are capable of carrying current at density. -> Graphite is preferred to carbon as it has high conductivity. -> Their sizes vary from almost 1-3 m long. -> Cross section is 100-110 mm dia. -> The consumption varies with practice and its and is generally 3-6kg/t of steel made.

-> The furnace is provided with a vertical column to which horizontal arms are attached and the electrode is held by the arm in position.

Graphite electrodes

5) Transformers :-> Large transformers are required. -> Voltage 33 kV or more for primary. -> Secondary 60-300 volts. -> It is economical to have at least two furnaces operating side by side to improve electrical power factor. -> Circuit breakers are used. -> Capacity is 470-650 kVA per tonne. -> Hearth area capacity is 750-900 kVA/m^2 .

6) Power rating and consumption :-> Arc voltage and power factor decreases with increasing current in the circuit. -> Power consumption varies from 450-600 kWH/ton for melting. -> For refining is 150-400 kWH/ton. -> Ways in which total power is spent are Charge - 50-65% Electrical – 8-12% Water cooling – 3-5% Radiation losses – 20-30%

• Charging :-> It is carried out from top. -> A drop bottom basket is used for charging. -> Orange peel type design is replaced by Clamp-shell. -> Mobile charging machines are used for big furnaces.

EAF - Operation • The operation of an arc furnace depends on furnace design, raw materials, charge facilities, refining process, quality and type of product required. • The refining process used in an EAF can be classified as 1) Acid process 2) Basic process. ->oxidizing single slag, double slag ->reducing single slag ->double slag practice

General Outline of an EAF Heat  Although the details of a heat very much depends on the local operating conditions, a general outline of a heat is given.  The essential steps are :1) Preparation of charge. 2) Charging 3) Melt-down 4) Refining 5) Finishing and tapping of single slag heat 6) Slag-off and making reducing slag 7) Reducing period 8) Finishing and tapping of heat

Preparation of Charge • The lining is inspected for damage due to previous heat. • The eroded portion, generally the slag-metal boundary lines, tap hole, spout and damaged hearth parts are all repaired in hot condition. • Granular Dolmite or Magnesite is used for repairing damaged parts. • Fettling is done either manually or by use of machines.

Charging  In top charging furnaces, little scrap is kept in bottom so that it provides cushioning for heavy scrap and saves the hearth from damage.  Burnt lime and spar are added with charge to form early slag of required qualities.  If refining is to be carried out during melting then lumpy iron ore or mill scale is also added.  During repairs, a charge basket is kept ready.  The furnace body is cleared off the roof and basket is lowered by an overhead crane.  Generally 40% heavy scrap, 40% medium and 20% light scrap is charged into the furnace.

Melt-Down  After charging is over the roof is placed back in position, the electrodes are lowered manually, the arc is struck and electrodes are put on automatic control.  As the metal just below the arc melts and drips down, the electrode travels automatically further down to maintain a stable arc.  During melting the choice of secondary voltage at each stage must be proper to obtain high power factor.  Heat developed in the arc must be utilized by charge as much as possible or else excess heat will be reflected to furnace lining and roof, hence causing damage .

Refining  Refining continues even during melting.  Refining can be commenced by addition of ore.  At the end of melting the first thing is to control Phosphorus.  Phosphorus can be eliminated by keeping the slag oxidizing and basic in nature.  Since Phosphorus is prone to reversion at high temp. it must be eliminated before bath picks up temperature.  During refining bath samples are periodically taken out to assess the process of refining.  Lime and spar maybe added to keep the slag in shape.

• Once the chemistry of the bath and temperature are at the desired level the heat is ready to be de-oxidized and finished for tapping. • Bath temp. is measured by an optical, immersion or radiation pyrometer. • In double slag practice, the slag is allowed to form and flow out towards end of melting by tilting the furnace. • Second slag is then introduced to carry out desulphurisation and alloy additions.

A man taking bath samples for analysis

Finishing and Tapping of Single Slag Heat • The heat is tapped by opening the tap hole. • The furnace is tilted by a jerk to prevent the slag from flowing along with the metal. • There should be a minimum of slag in the ladle. • The ladle should be placed in a pit beneath the spout well ahead of tapping time. • Final de-oxidation by Fe-Si and Al, and other alloying additions are made in the ladle.

Heat tapped in a ladle

Slag Off and Making Reducing Slag  In a double slag practice at the end of the oxidizing period slag is removed from the furnace by tilting it on the back side.  Small additions are made of spar thins down the slag which flows over readily.  Reducing slag is formed by adding a fresh charge of lime and spar in which a little of sand maybe added to help form the slag.  Strongly reducing carbidic slag can be formed by putting coke on slag after it has melted.  Slag without coke or little coke is called lime slag [white in color] and that with high percentage of coke is called carbidic slag [grey in color].

• After reducing refining is over the bath is fully de-oxidized by additions of Fe-Si and Al. • The alloying additions may now be added based on the analysis of the bath. • However it is not beneficial to hold the metal under reducing slag for too long since the bath is quiescent and stratification of metal with respect to composition may take place. • In modern designs this problem is overcome by using an induction stirrer at the bottom of the furnace from outside.

Finishing and Tapping of Double Slag Heat • The finishing job is not so different from refining and metal is tapped as explained in tapping of single slag heat. • But since alloying additions are made in reducing conditions and enough time is available for slag-metal separation, the product is more clean. • Care should be taken to avoid non-metallic pick-up from runners, ladles, moulds, etc during teeming.

LATEST TRENDS IN EAF DESIGN AND OPERATION

Modern EAF–based steelmaking plants feature following main advantages  Relatively low investment costs compared to the integrated blast-furnace/oxygen steelmaking route.  Utilization of a wide range of raw materials such as scrap, DRI, HBI and hot metal.  Ability to fulfill a wide range of production targets and steel qualities to meet the specific market requirements.  Low operational and thus conversion costs, promoting a relatively short return on investment.

Two main problems faced by EAF in the 21st century:• Highest thermal loads, created by an electric power input of =1 MW/t.

• Increase in oxygen injected into a furnace.

What are the changes in Modern EAF worldwide to cure the problems? • Connection with the mechanical equipment.

• Water-cooled parts. • Oxygen and carbon-injection systems. • Measurement units. • Robotic systems.

The Modern era EAF includes the installation of following equipments for best operation results: • Automatic scrap bucket pre-positioning. • Door-cleaning robot. • -free temperature measurement of the steel bath. • Ultra-high-power oxygen and carbon injection technology with RCB (Refining Combined Burner). • Automatic tap control from the main EAF pulpit via video camera.

• Automatic slag detection during tapping with the IRIS system (InfraRed Identification System). • Tap-hole cleaning robot. • Crane equipped with two auxiliary hooks for quick electrode exchange. • Electrode stand with spray-water cooling.

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