Hot repair refractory materials for a crown come in several varieties based on the hot face material composition, the glass melt composition and the extent of the repair. In the case of a silica crown, it is common practice to apply a seal layer on a new construction. Even though this is not really considered a repair, it is still installed hot due to the large thermal expansion of the silica crown upon heat up. The seal layer consists of a monolithic material that is very thermal shock resistant and will only set under high temperature conditions. It is important to wait until the crown has completely expanded during heat up to apply this layer to ensure the monolithic layer will not crack or form a gap between the seal layer and the hot face material. This same type of monolithic material can also be used later in campaign as an actual...
Hot repair refractory materials for a crown come in several varieties based on the hot face material composition, the glass melt composition and the extent of the repair. In the case of a silica crown, it is common practice to apply a seal layer on a new construction. Even though this is not really considered a repair, it is still installed hot due to the large thermal expansion of the silica crown upon heat up. The seal layer consists of a monolithic material that is very thermal shock resistant and will only set under high temperature conditions. It is important to wait until the crown has completely expanded during heat up to apply this layer to ensure the monolithic layer will not crack or form a gap between the seal layer and the hot face material. This same type of monolithic material can also be used later in campaign as an actual repair material in the event that the entire crown is overcoated. The installation method recommended for this type of repair or the seal layer utilizes Shotkast® Installation equipment in order to minimize the time it takes to make the repair. This equipment enables an average rate of installation of 3.5-4.5 metric tons per hour.
In the event that the entire silica crown does not need a repair, but rather small patching of rat holes the Shotkast® Installation method is not necessary, nor efficient. For small holes, a one-component silica castable is sufficient. This material can be mixed with water in a bucket and poured over the damaged area. If the hole is too large to patch with a monolithic material, a combination of vitreous silica brick and the monolithic material can be used.
Other materials are available using the Shotkast® Installation method in order to provide solutions for the various crown materials used in glass melting. In the event of a high-alumina crown construction, a tabular alumina monolithic is recommended unless the melt is a boron containing glass, then a zircon monolithic is recommended due to its increased alkali resistance. Regardless of the material used, when overcoating an entire crown it is important to choose a refractory that can withstand the hot-face temperatures, resist alkali attack and be completely self-supporting since there is a high chance that the new monolithic material will become the hot-face material over time.
In regenerators, the crown can be repaired in the same manner as described in the section above. If the regenerator crown is constructed with a magnesite material, a zircon material is recommended for the seal layer and/or the hot repair material. If it is the regenerator wall that needs a repair, the installation method will be different compared to the Shotkast® Installation since this monolithic material will need to be applied on a vertical surface rather than a horizontal. In this case, gunning the refractory is effective since it provides a dense, homogeneous monolithic lining on a vertical surface without the use of forms. Gun mixes should wet up well, have as wide a water range as possible, and provide excellent coverage. A 70 per cent alumina gunning mix is recommended for repairing regenerator walls and/or stacks in a glass furnace due to its exceptional refractoriness, volume stability and high service temperature limits.
In soda-lime glass melters, the most common material used in superstructure applications is an alumina-zirconia-silica (AZS), either bonded or fused cast. As the furnace progresses through its campaign, large blocks may need to be replaced; most commonly burner blocks. Fused cast AZS has very poor thermal shock resistance, so either a bonded AZS material or a zirconia-
mullite material are best due to the increased thermal shock resistance. If the entire block does not need to be replaced, a zirconia-mullite heat-setting monolithic is applied as a patch. However, if the damaged section requires multiple block replacement or the damaged section is not readily accessible from the outside of the furnace, ceramic welding may be the best choice. Contacting a company that specializes in ceramic welding can help determine the best course of action for making the repair.
In nearly all glass melter furnaces, the sidewall blocks exhibit the most wear in the melter since it is in contact with the glass and the atmosphere of the furnace. Historically, in soda-lime glass furnaces the sidewall overcoats have been composed of an AZS material, either fused-cast or bonded. In the case of fused-cast AZS overcoats, extra-ordinary measures must be taken in order to install the block without cracking. Therefore, bonded AZS materials have been used in order to minimize the like-lihood of thermal shock during installation. However, since this is a highly corrosive application, the corrosion resistance is also a factor in choosing the refractory. Another option for more severe applications, like wool glass furnaces or soda-lime glass furnaces with a significant portion of the campaign remaining, a chrome-alumina refractory brick is recommended. Some glassmakers have had concerns about the chrome-alumina refractory causing colouring of the glass melt. It is theorized this has not been an issue in the case of overcoats since the amount of refractory in contact with the melt is so small in respect the entire melt volume. Toledo Engineering Company recently provided a case study of using this chrome-alumina refractory as an overcoat material with great success.
When choosing a refractory for hot repair applications it is important to consider material compatibility with the existing refractory, service temperature limit and depending on the application either alkali resistance or glass corrosion resistance. But, if the refractory is not resistant to thermal shock, then the rest of the properties will not be bene-ficial. Since the installation of these hot repair materials can be dangerous and strenuous at times, the cost of the installation can far outweigh the cost of the materials, therefore it is imperative to have experienced professionals perform the work with high quality refractory materials in order to minimize the risk and the time of the installation.
ANH Refractories is one of the world’s largest and most respected suppliers of refractory materials. With lineage that dates back over 100 years, ANH Refractories is the leading provider of refractory solutions to the global industrial market.
ANH Refractories operates more than 17 production facilities spread over three continents and manufactures materials under the world recog-nized brands of A.P. Green, North American Refractories Company, and Harbison-Walker Refractories Company.