Refractory is a vital component in industrial heating furnaces. Regardless of whether its primary purpose is containment of material or heat, time and use eventually take their toll, and replacement becomes necessary. The cause could be any combination of mechanical damage, abrasion, erosion, corrosion, chemical degradation, thermal cycling, thermal shock and other forms of wear and tear.
Recycling of used refractory materials is currently more popular in Europe (where landfill space is scarce), but it is becoming more common in the U.S., especially at aluminum and steel processing facilities. There are three primary questions a furnace user must answer when deciding the fate of the spent refractory solids: (1) Is the material hazardous? (2) Does the refractory installer offer a recycling option? (3) Is recycling or landfilling the lowest-cost option?
Composition of Refractory
Many categories of refractory materials are not inherently hazardous. The majority of brick types (fireclay, alumina, silica) are comprised predominantly of earth minerals (AlO3, SiO2) and small quantities of oxide impurities (Na2O, K2O, CaO, MgO, Fe2O3, etc.). Exceptions include “basic brick” that contains significant concentrations of Cr2O3, which contains the hazardous heavy metal chromium. Castable and ceramic-fiber refractories are also typically comprised of alumina-silica blends.
On the other side of the spectrum, certain vacuum-furnace applications utilize linings comprised of graphite or other carbon-based refractories. Based solely on their elemental composition (i.e. absent any contaminants they have absorbed during use), many of these materials are non-hazardous and, therefore, suitable for both landfilling and recycling.
Somewhat surprisingly, trace concentrations of uranium and thorium may be a concern. Although natural concentrations of these metals in refractories are usually quite low (i.e. so low that radioactivity levels are essentially at background levels and therefore don’t exceed workplace safety thresholds), some spent refractories may be just “hot” enough to trigger rejection by a municipal or construction landfill. In such instances, recycling may be a more suitable option than landfilling, assuming human exposure concerns can be ruled out.