High Alumina Electric Arc Furnace Roof

High alumina brick by far is the most widely used product today for constructing electric arc furnace roofs. Of these bricks, those in the 70% alumina class (which retard slag corrosion, expand slightly at elevated temperatures, and resist thermal spalling) are the most popular. Such a brick is Rescal 70D.

As with upper sidewalls, it has become increasingly desirable in higher-powered furnaces to zone hot spots or arc erosion areas with higher purity alumina brick that exhibit greater refractoriness at extreme temperatures such as Duralite 80G, or basic brick such as Nucon 60. Fume port rings and/or electrode rings (depending upon the type of dust control equipment) are often installed with high strength, abrasion-resistant products such as Rescal 90XD. The delta section of the roof is generally constructed of a brick spoke and a rammed or cast high alumina fill, such as Vibrocast 85ULM. In small roofs where the use of brick spoke is impractical, the complete inner area is rammed or cast. There are many different brick shapes and combinations used to construct electric furnace roofs. The most popular design is the triple taper brick design.

Water-Cooled Electric Arc Furnace Roof

With water-cooled roofs, the only remaining portion of the refractory is in the delta section. To minimize downtime due to the curing of monolithic refractory delta materials installed in place, performed deltas are offered which are cured prior to shipment. Performed deltas are available in high alumina or alumina-chrome compositions.

Two-Shape, Triple Taper Brick Design

The two-shape, triple taper brick design is installed using only two shapes of special design that conform to the roof contour. Both shapes normally designated A and B are identical except that the B shape arch taper is greater. By using both shapes in combination, all annular rings for a given diameter furnace can be completed. 

General Installation Procedures for Electric Arc Furnace Roof

1. Check the rise and diameter dimensions on the roof form against those shown on engineering drawings.
2. Check roof ring fittings to ensure that the ring is set on form in the proper position in relation to the electrode and fume holes openings.
3. Check form for dry mortar or other foreign objects and remove them before starting.

With the roof ring in position and the form cleaned, the following steps should be taken:

1. Lay the first annular ring using the combinations designated. 
   • When laying two-shape roofs, exercise care to ensure all brick identification marks are at the top or cold face of the roof. Normally, two marks indicate an “A” shape and one mark, a “B” shape. 
   • When brick is laid with mortar, a thin joint is recommended. Make sure that the brick is completely covered on the vertical face that comes in contact with the previously laid brick and the back face which contacts the skew or previous annular rings. 
   • As each brick is laid, gently tap it with a hammer to ensure a tight fit. Use care to avoid deviation from the spherical axis. Any deviation should be corrected immediately by tapping the top or bottom of the brick.
   • As the ring is laid, three or four equally spaced “driving keys” should be left in the ring. This is accomplished by shimming the brick up from the roof form 1 ½” to 2”. Remove shims before starting the next annular ring. “Driving Keys” can be driven in place after each ring is completed or after the roof, including delta section, is completed.
2. Continue laying annular rings until the fume hole area is reached. At this point, the construction of the fume hole ring should begin. Fume hole rings can either be constructed on the roof form or prefabricated off the form and set in place. In either case:
   • Determine the combination of brick to be used from the engineering drawing.
   • Pre-assemble brick dries around the fume hole form to obtain the best fit.
   • Remove brick and reset in the same order using a dip or thinly troweled joint.
   • If necessary, cut a key brick to close the ring.
   • Band brick ring with steel strapping or heavy wire to snug joints.
   • If the roof rise makes laying the fume hole ring difficult, the ring should be pre-assembled off the form on the removable steel plate and set in position by crane or another lifting device.
3. With the fume hole in place, continue laying annular rings up to the last complete annular ring at the delta area. Note: if the areas between the fume hole ring and the annular ring are to be filled with monolithic material, leave sufficient space for the rammer or vibrator head.
4. After completion of the last full annular ring, electrode ring construction should begin using the same procedure as outlined for fume holes. 
5. Once the electrode rings are in place, the remainder of the partial annular rings and the spokes (if used) should be installed using the same bricklaying practices as an outline for the annular rings.
6. When all brick is in place, begin ramming or casting in the spaces between the electrode rings (delta section) and fume hold as follows:
   • Ramming:
     • Clean all foreign material from the area to be rammed.
     • Cover concrete form with a no permeable material to avoid moisture absorption from the ramming mix
     • Ram material at 45° angle to avoid horizontal laminations.
     • Use pneumatic rammers with steel wedge head
   • Casting:
     • Follow steps 1 and 2 as outlined for ramming
     • Mix castable using water additions as given in the bag instructions
     • Vibrate with a high frequency vibrator making sure that the material is thoroughly worked into all voids. iv. After casting, allow to air dry for approximately 24 hours before placing it on the furnace. During colder weather prevent any mixed material from freezing.

* This article is not a complete guide, contact your Resco Representative for a complete guide for your industry/application.