The 'Lincarb' Foaming Slag Processing
In Electric Arc Furnace Operations

1. Introduction

Linston's 'Lincarb' foaming slag systems are at present in use in the major electric arc furnace melting facilities within the United Kingdom.

The foaming slag practice has been adopted because of the potential benefits arising from its use.

1. Improved heat transfer from the arc to the metal.
2. Reductions in refractory wear and electrode wear.
3. Lower energy consumption per tonne of liquid steel.
4. Improved output rates.
5. Improved alloy oxide recovery.

2. Methods of foaming slags

In the 'Lincarb' based installations, a number of different methods of introducing the material into the slag is in use, depending on the limitations imposed by the furnace layouts and operations.

2.1 Equipment

In the U.K. installations a pneumatic unit (injector) is used to transfer the Lincarb Slag Foaming material from a Storage Hopper or Silo to the Furnace. This system has been found to provide the maximum flexibility for siting the unit, access for maintenance and cleaning, together with a low maintenance requirement.

The injector units are linked to the furnace by solid metal pipe work, protected at critical areas by insulating material to prevent damage from metal splashes, flames etc.

The Lincarb material is delivered from the solid metal pipe work into the Furnace via a Lance pipe. Where feasible the customers' own Oxygen Lances are adapted for the purpose to aid spares availability in the case of damage or pipe blockages.

2.2 Installation of Injection Delivery Pipes in the Furnace

The pipe introducing the Slag Foaming product (Carbon) into the Furnace can be sited in a number of different ways dependent on the individual furnaces and operational requirements.

The entry points into the Electric Arc Furnaces that have been used: -

1. Through a multiple pipe Lance Manipulator Unit (e.g. a Badischer unit) in tandem with other pipes for oxygen, silicon etc.
2. Through the Oxygen Lance Port, which has been enlarged for the purpose, to allow an extra pipe to convey the Carbon.
3. Through the Furnace door.
4. Through a specially constructed hole in the Furnace shell with a bricked area in the Furnace lining.

In general, it has been found that satisfactory foaming of the slag can be obtained by spraying the Lincarb material onto the surface of the slag. More effect can be attained by injecting the material into the slag by immersing the delivery pipe into the Furnace Slag to a depth of typically 100mm or greater (as measured in the flat bath condition).

2.3 Operation

There are different practices for foaming the Furnace slag, all are dependent upon the melting operations involved, for example, full Furnace steel making, melt only operations, steel type, slag carry over facilities etc.

Slag foaming using the Lincarb system has been most successfully employed in arc furnace plants, where a melt and tap practice is used, with the steel making operations, such as alloying, being carried out in the ladle.

The normal procedure is to melt the first basket, back-charge with the other baskets and commence the input of Lincarb for slag foaming when about 80-85% of the power required to melt the charge has been introduced. On certain high-powered furnaces slag foaming has also been carried out during the melting of the first basket to reduce sidewall damage when operating on long arcs and/or when uneven melting tends to take place.

The injection of Lincarb into the furnace is continuous after commencement to maintain the slag in foamed condition. Typical rates of injection are between 25-100kgs/minute, dependent on furnace operating parameters.

It has been found in practice that the foaming effect is enhanced if the Lincarb material is placed on, or into, the slag in an area where Oxygen is being infiltrated into the steel, for either refining purposes or as an additional practice to promote the slag foaming action.

2.4 Visual Appearance of the Slag

Arc Noise - the furnace crew judges the foam by assessing arc exposure measurement devices linked to automatic control units.

Furnace Sidewall Cooling Water Temperatures - By automated thermocouple measurement.

The use of the Lincarb injection system coupled to a melting process control computer enables the Furnace to be kept on its maximum transformer setting for a longer time, before having to reduce power, so getting faster temperature pick-up. In a similar manner on furnaces where the maximum power available can be used with no risk to the furnace, the use of foaming slags enhances temperature pick-up in the bath. The result, in both types of operation, is a faster output rate.

3. Material use

Works using the Lincarb system have found that the consumption of this product is 5-8kg per liquid steel tonne produced. The use of high carbon level material was found to give an extra benefit in terms of foaming performance. (Improved performance of petroleum coke based product over more standard metallurgical coke based product).

One works using the Lincarb system has found that the inclusion of approximately 10% of a Magnesite material aided both the formation and stability of the foaming behaviour in their slags.

5. Benefits

Monitored melting and refining trials at a number of electric arc steel plants have shown the following benefits can be obtained using slag foaming practices in carbon steel manufacture: -

1. Temperature rise rates in the metal increased by up to 50% for a given transformer tap setting.

2. Temperature rise rates in the metal per kWh power input increased by up to 50% for a given transformer tap setting.

3. Electric power savings of up to 30kWh per charged tonne have been obtained.

4. Frequency and severity of the wear of Furnace walls and particularly slag line areas has been reduced. Reductions of up to 20% in the number of repairs have been reported.

Due to the deoxidising effect of the Lincarb addition to the slag, the reversion of metallic oxides into the metal has been recorded, giving substantial cost benefits in down stream processes (e.g. AOD).