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Avoiding high bottoms in brick-lined arc furnaces.

The electric arc furnace is one type of furnace that is used in the melting of ferrous metals. In an electric arc furnace the heat is produced by an arc struck between carbon or graphite electrodes to produce heat to melt the charge. Buildups in the bath area of refractory brick-lined electric arc furnaces are generally referred to as "high bottoms," and can result in a variety of problems for the operator.

Among the effects of high bottoms are: * There is less furnace volume available for scrap charge. * Increased difficulty in draining the furnace of metal and slag. * The buildup causes the heat and slag to be positioned higher in the furnace, thereby subjecting the walls and roof to greater radiant heat and subsequent accelerated deterioration of the refractory lining in these areas. *As the slag line moves up with the higher bottom, a greater refractory surface area is affected by the chemical slag attack. Therefore, zoning of the brick section may need to be expanded to accommodate for a now higher slag line. The slag or metal should not come in contact with the metal clad brick in the upper sidewalls.

High bottoms generally occur from one or a combination of ft following practices: melting a long run of high carbon heats, using excessive material when shovel patching bottom and banks between heats, excessive rebounds from gunite material normally applied to the slag line between heats) and a high tap hole will not allow the furnace to drain property. Avoiding High Bottoms

To help combat the resultant high bottom from running high carbon heats, the bottom can be brought down by making low carbon heats and dressing with SPAR, aluminum shot or fine ferrosilicon in small amounts. Proper maintenance of the bottom refractory, tap hole and spout can avoid many problems. The proper bottom level and contour must be maintained by visually observing the bottom, actual measurement or noting the heat level.

After a heat has been tapped, repairs to the bottom are started immediately to allow the new materials to sinter into place from the retained heat of the furnace. The furnace is tilted completely forward to drain away all remaining slag, then tilted backward to provide a better view of the bottom, The doors are opened to permit smoke and fumes to escape so the furnace can be inspected for damage on the banks, bottom, roof and around the tap hole or any other location where repairs may be needed. Small holes, which normally occur from poor initial ramming, poor drainage or water leaks, should be patched while they are still small and built up over a period of several heats.

A minimal amount of patching material should be applied to the slag line and bottom, excess material may cause a bottom build up.

Where deep hole patching is required the first consideration is to drain the furnace thoroughly. The hole can be patched with a shovel or ram type mix using the following procedure: Fill the hole with water, follow with patch material and cover the area with burnt lime. This type of patch will generally outlast surrounding bottom areas.

Gunnite repair programs vary widely. Variables that might be encountered are:

Skill level of crew

Type of material

Moisture content Tank and line pressures (normally 25 and 50 psi respectively) o Pipe length should be as close in size as possible o Time available to gun (tap to charge time) Repair schedules

Gunning can be used to cover bare shell areas, repair a bottom hole, dress the spout or pipe a taphole. All repair work should be performed on a regular basis. Delaying repairs usually results in increased down times as opposed to routine patching. Operating with a high bottom will destroy the roof and sidewall refractory linings of electric arc furnaces.

The electrical issue concerning high bottom problems can be divided into two major categories: power adaptation and adjustment of the regulation system. Although both are somewhat related in arc melting, each requires different manipulation during melting. Power Management

Electric power should be used in correlation with a well designed power profile tailored to the product made, charge material used, available power and type of furnace. A high bottom can result from a longer than necessary arc. Although a long arc is desirable during the initial heat, it should be reduced during the latter heating cycle to heat the metal more evenly. Continuing with a long arc after the scrap has been consumed into the melt, can create a top layer of hot metal at the desired temperature, while the bottom remains cold, creating (over a number of heats) a high bottom condition. High bottom conditions also have an adverse effect on water-cooled panels. To correct this problem, a well designed power profile that determines the arc length at different times during the melting process and the power taps usage is necessary. The type of charge material should be considered in designing an optimum power profile, as well as all electrical parameters. Regulator Adjustment

The regulator controls the arc in relation to current and voltage usage during melting. Electrodes move up and down as directed by an electrical feedback from the furnace power circuit. Excessive electrode movement during the latter part of the melting cycle can create the same bottom problem as excessive arc length mentioned above. A more stationary electrode will force more heat to the bottom of the furnace and can cause superheating, high electrode consumption, and a regarded meltdown inextreme cases. A finely tuned furnace regulator that will respond to the varying demands during heating is vital to optimum melting performance.

To avoid or minimize the occurrence of high furnace bottoms, proper selection of the hearth contour is of great importance. A well shaped hearth is nearly level from bank to bank with only slight protrusion of the banks. A large flat area will spread the initial molten steel evenly and prevent the bath level from rising too quickly. This allows the heavy scrap on the bottom to melt and, since the arc is near the bottom, create a hot bath. Remaining slag on the bottom will liquify and float to the top of the melt. Once a campaign with the proper hearth contour is underway, thorough slag draining will eliminate the most common cause of buildups. This is greatly aided by adding fluxes prior to tapping and leaving power on as long as possible prior to tap.

Excessive buildup of the lower furnace banks is another common cause for problems. The banks should be thick just below the slag line to provide a base for repair and prevent excessive amounts of gunning material from falling. ideally the contour between lining, slag line and bottom should be as close to vertical as possible, avoiding shelves and rolling scrap during meltdown.

Once a high bottom has been established, it can be attacked chemically, by power or heat. The power or heat applications are somewhat risky and should be done only when the buildup is very high. in the power approach, the furnace is tapped and the electrodes lowered until an arc is struck right on the bottom. A low tap setting is used and power remains on long enough for three holes to form in the build-up area.

Once liquid material has formed, spar is blown or shoveled into the pools and the furnace is drained. Material that remains usually will break up and is removed with the next heat. This approach, however, requires skill and extremely close supervision, otherwise there can be more damage than benefits. In general, the ounce of prevention through approach unnecessary.

The heat approach works in similar manner. A small charge is placed in the furnace and is deliberately overheated while holes are burned into the built-up area. Spar is added with the charge and during melting, again with the intent of breaking up the bottom build-up and tapping it out. Needless to say, the above skill and supervision requirements apply as well for this period if damage to the refractory is to be avoided.
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Title Annotation:buildup of contamination in the bottom of electric-arc furnaces can reduce performance
Author:Mihm, G.
Publication:Modern Casting
Date:Oct 1, 1990
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