Many types of high emissivity coatings containing materials that absorb and re-radiate heat energy are often applied on refractories to enhance performance. Compounds containing chromium, zirconium and cerium are some of the materials used in these coatings. Commercial coatings with silicate and phosphate binder systems ensure satisfactory coating film strength in many industrial applications, in which operating temperatures are below 1100°C. However, these coatings are not effective at high temperatures or after a long service period because the emissivities of the materials decrease. Sometimes, there is even a loss of the high emissivity coatings themselves. Small differences in the thermal expansion coefficients of the substrate and the coating can also result in the bond between them shearing.
The EMISSHIELD family of coatings contains binders formulated in such a way that the bond strength between the substrate and the coating is optimum. These binder systems adhere to a range of ceramic and metallic substrates, which have diverse thermal expansion coefficients. EMISSHIELD coatings maintain emissivities from 0.85 to 0.92 at temperatures ranging from 800 to 1650°C. The emissivities do not vary with sustained service as the emissivity agents used in the coatings are mostly borides, which do not oxidize.
The binder systems are compatible with dense and lightweight refractories, carbon-containing refractories, refractory ceramic fibers, ferrous alloys as well as many non-ferrous metals. The binders permit the coating to move according to any dimensional changes of the substrate, thus preventing shearing of the bond between the substrate and the coating as a result of thermal expansion mismatch. In fact, the bond continues to develop strength with increasing temperature and time.
EMISSHIELD high emissivity coatings with silicate-based binder systems can be applied to refractory ceramic fiber (RCF) blanket and modules, dense refractories and insulating firebrick (IFB).
Silver Melting Furnaces
The high emissivity coating can be used on the alumina brick roof of an electric furnace used for melting silver. The coating, along with the silicate binder system, minimizes refractory wear caused by alkali attack and almost doubles the service life of the roof. It also results in even heating of the roof, practically eliminating the formation of hot spots.
Electric Arc Furnaces
Another area of successful application is electric arc furnace delta shapes. In AC and DC furnaces, the application of high emissivity coatings on delta port walls and the hot face results in doubled delta life, energy savings and thermal shock protection.
Figure 1. Precast delta with high emissivity coating after approximately 2/3 of its service life. The delta was put back into service, where it provided over 100 additional heats.
Cracking of the refractory is usually observed during overcoat repairs to the sidewall or glass tank block. Application of EMISSHIELD high emissivity coating to overcoat blocks significantly reduces cracking when subjected to molten glass contact.
Use of the EMISSHIELD high emissivity coating in the preheat section tunnel kiln refractories results in good fuel savings on account of burner turn down. Prior to application of the coating, the surface has to be prepared by removing reaction products, adhering materials and friable surfaces.
Figure 2. Tunnel kiln preheat section (right 2/3 of photo) after application of high emissivity coating. The textural differences between the new and used refractory brick surfaces are obvious.
IFBs in tunnel kiln afterburners can be coated with EMISSHIELD high emissivity coatings to reduce the fuel used for afterburner operation by about 25%. Similarly, spraying ladle covers used for drying out refractory castable-lined steel ladles with the high emissivity coating results in considerable energy savings.
EMISSHIELD high emissivity coatings formulated for use on RCF blankets/ modules can be sprayed in periodic kilns lined with RCF modules. By this, the heat up time and the cool-down time can each be reduced by about 35%, increasing the productivity of the kiln.
Application of EMISSHIELD high emissivity coating can provide a number of benefits such as energy savings, reduced wear and sustained service. The coatings stay firmly attached to refractory, RCF and metal substrates. The chief benefits of using the coatings on refractory linings, however, are reduced maintenance costs and greater furnace availability.
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.