© 2006 by Taylor & Francis Group, LLC

Abrasive Blasting and Heavy-Metal Contamination 89
5.2.2 LOW-TEMPERATURE ASHING (OXIDIZABLE ABRASIVE ONLY)
Low-temperature ashing (LTA) can be used on oxidizable blasting debris —
for example, plastic abrasive — to achieve a high degree of volume reduction
in the waste. Trials performed with this technique on plastic abrasive resulted
in a 95% reduction in the volume of solid waste. The ash remaining after
oxidation must be disposed of as hazardous waste, but the volume is dramatically
reduced [9].
LTA involves subjecting the spent abrasive to mild oxidation conditions at
moderately elevated temperatures. The process is relatively robust: it does not depend
on the mechanical properties of the waste, such as particle size, or on the pigments
found in it. It is suitable for abrasives that decompose — with significant solids
volume reduction — when subjected to temperatures of 500 to 600 C. Candidate
abrasives include plastic media, walnut shells, and wheat starch.
The low temperature range used in LTA is thought to be more likely to
completely contain hazardous components in the solid ash than is incineration at
high temperatures. This belief may be unrealistic, however, given that the combustion
products of paint debris mixed with plastic or agricultural abrasives are
likely to be very complex mixtures [8, 9]. Studies of the mixtures generated by
LTA of ground walnut shell abrasive identified at least 35 volatile organic compounds
(VOCs), including propanol, methyl acetate, several methoxyphenols and
other phenols, and a number of benzaldehyde and benzene compounds. In the
same studies, low-temperature ashing of an acrylic abrasive generated VOCs,
including alkanols, C 4-dioxane, and esters of methacrylic, alkanoic, pentenoic, and
acetic acids [8, 9].
LTA cannot be used for mineral or metallic abrasives, which are most commonly
used in heavy industrial blasting of steelwork. However, the lighter abrasives required
for cleaning aluminium are possible candidates for LTA. Further work would be
required to identify the VOCs generated by a particular abrasive medium before the
technique could be recommended.
5.2.3 ACID EXTRACTION AND DIGESTION
Acid extraction and digestion is a multistage process that involves extracting metal
contaminants from spent blasting debris into an acidic solution, separating the (solid)
spent debris from the solution, and then precipitating the metal contaminants as
metal salts. After this process, the blasting debris is considered decontaminated and
can be deposited in a landfill. The metals in the abrasive debris — now in the
precipitate — are still hazardous waste but are of greatly reduced volume.
Trials of this technique were performed by the U.S. Army on spent, contaminated
coal slag; mixed plastic; and glass bead abrasives. Various digestive processes and
acids were used, and leachable metal concentrations of lead, cadmium, and chromium
were measured using the TCLP method before and after the acid digestion.
The results were disappointing: the acid digestion processes removed only a fraction
of the total heavy metal contaminants in the abrasives [9]. Based on these results,
this technique does not appear to be promising for treating spent abrasive.
© 2006 by Taylor & Francis Group, LLC