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
