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 (Al2O3, 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.

Radioactive Elements
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 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.

Furnace Environment
The furnace environment can have a detrimental effect on refractories. If the furnace is processing metals other than steel or aluminum (e.g., certain compositions of brass, bronze or alloy steel), the refractory may retain lead, chrome or nickel – metals that are subject to stricter environmental regulations. In glass-melting furnaces and other vessels where sodium is a constituent, accumulation of sodium oxide can cause the refractory’s melting temperature to decline, thereby reducing its value as a recycling feedstock. Spent refractory ceramic fiber (RCF) materials can be a good source of relatively pure aluminosilicates, but these materials can represent a handling hazard due to the presence of crystalline silica, which could pose an inhalation risk.

Uses for Recycled Refractories
The most common re-use for spent refractory is “grog” or “aggregate” in new brick or tile. A certain percentage of ground, pre-fired material is frequently added to such products to reduce shrinkage and improve quality. Other construction uses for recycled refractories include: roofing granules, landscape material, soil stabilization, slag conditioner and fuel source (in the case of carbonaceous refractories).

In some instances, chrome-magnesite brick can be recycled as a feed source of chromium for the production of ferroalloys. Refractories used in high-temperature processing of precious metals (e.g., silver, gold, platinum) are quite often recycled to recover high-value materials that have penetrated into the lining.

Testing Before Shipping
If hazardous constituents (including radioactivity) in the spent refractory are possible, the furnace user should have samples tested before engaging in discussions with recyclers or landfills. Having a shipment rejected by the recipient can add significant unanticipated cost to what could otherwise be a smooth transaction.

From the opposite perspective, a furnace user shouldn’t assume that his spent refractory has only one available fate – disposal at a hazardous-waste landfill. Armed with favorable test results, the user may find that recycling is a much lower-cost option than disposal as a hazardous waste. In fact, materials rich in Al2O3 can even bring a substantial return at current market prices.

According to Dr. Manuel Dekermenjian, principal consultant at Environ and co-lecturer at USC with this columnist, “Recycling and re-use options for industrial materials, including spent refractory, are often overlooked despite their financial, regulatory and environmental payback potential. Knowledge of the material’s chemical composition is the most powerful tool in deciding the best fate for spent refractory.”IH