NASA developed and patented the EMISSHIELD high emissivity coating technology for thermal protection for orbiters. This technology was later licensed for use in industrial applications. Use of high emissivity coatings in steel plant applications, especially electric arc furnaces, appears to help in saving energy and maintenance costs.
For many decades, high emissivity coatings have been used in many high temperature industrial applications. However, they did not achieve widespread acceptance in steelmaking applications. The major reasons are a lack of understanding of emissivity concepts and the subsequent misapplication of these coatings. Moreover, many coating materials could not withstand the corrosive environments that are typical in steelmaking. The emissivity materials aged and also lost their high emissivity during prolonged use at high temperatures. Often, binder failure caused the coatings to peel off the substrate.
EMISSHIELD high emissivity coatings do not degrade during service. The binder’s adherence to the substrate is also strong. EMISSHIELD coatings adhere well to steel and all types of refractories. EMISSHIELD high temperature high emissivity coatings containing binders designed for steel substrates can be used in electric arc furnace (EAF) applications.
Applications in Electric Arc Furnaces
Water-Cooled Panel Roof
Generally, a water-cooled panel roof runs a certain number of heats after which maintenance is required for removing slag and scale and for repairing leaks. During operation, the roof is usually removed twice from service for repairing small leaks.
Before the application of the EMISSHIELD coating, slag is removed from the water-cooled panel roof mechanically. Scale is removed by grit blasting to reach bare metal. The roof is then washed using an alkaline detergent solution in deionised water for grease removal. The surface is dried and the coating is sprayed on the roof using a high volume, low pressure (HVLP) sprayer (Figure 1).
Figure 1. Application of high temperature high emissivity coating to a water panel EAF roof
In an EAF shop, temperature measurements of the cooling water were taken at the roof water output after the application of EMISSHIELD coating to the roof. The water temperature increased by only 2-5°F, against the 11-13°F temperature rise observed when the roof was uncoated. This reduced energy loss contributed to energy savings. More heat energy was available for heating the furnace charge, resulting in the arc time reduced by three minutes for each heat and considerable savings during the roof’s life. The coating strongly adhered to the roof and lasted hundreds of thermal cycles.
The service life of the roof almost doubled after coating, saving about 88% of maintenance costs. No interim repairs were necessary and this resulted in additional savings. Overall, the application of the high emissivity coating on the water panel EAF roof resulted in an impressive return on investment of about 10 to 1.
Precast Refractory Deltas
Precast refractory deltas in EAF shops will benefit from a coating with a different binder system, which is specially designed for dense high alumina refractories. The EMISSHIELD coating is applied to the electrode ports and the hot face. Figure 2 shows the hot face of a delta temporarily taken off a furnace after 2-1/2 weeks of service.
Figure 2. Magnesium-aluminate spinel delta with high emissivity coating after 2-1/2 week’s service
The benefit in coating precast deltas is not energy savings. The photo shows refractory wear and a complete removal of the coating on the hot face. The coating more likely absorbs energy and re-emits it, thus allowing the cold delta to heat up gradually during the first few heats. This could eliminate thermal shock and cracking, which would speed up refractory wear during later heats. Besides, the high emissivity coating provides furnace conditions that ensure even heating on hot faces, reducing thermal stresses.
Other EAF applications where using EMISSHIELD high emissivity coatings may be valuable include electrode arms and water-cooled upper sidewall panels in which heat exposure usually causes corrosion and metal warping.
The use of EMISSHIELD high emissivity coatings in electric furnaces results in great energy and maintenance savings that return the cost of these coatings several times. High emissivity coatings can be successfully applied to extend the service life of water- cooled panel roofs, refractories and shields.
This information has been sourced, reviewed and adapted from materials provided by ANH Refractories Europe.
For more information on this source please visit ANH Refractories Europe.