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88 Corrosion Control Through Organic Coatings separating out heavy metals from the innocuous abrasive and paint binder. The approaches used are: • Physical separation • Burning off the innocuous parts • Acid extraction and then precipitation of the metals At the present time, none of these methods is feasible for the quantities or types of heavy abrasives used in maintenance coatings. They are described here for those wanting a general orientation in the area of lead-contaminated blasting debris. 5.2.1 PHYSICAL SEPARATION Methods involving physical separation depend on a difference between the physical properties (size, electromagnetics) of the abrasive and those of the paint debris. Sieving requires the abrasive particles to be different in size and electrostatic separation requires the particles to have a different response to an electric field. 5.2.1.1 Sieving Tapscott et al. [7] and Jermyn and Wichner [8] have investigated the possibility of separating paint particles from a plastic abrasive by sieving. The plastic abrasive media presumably has vastly different mechanical properties than those of the old paint and, upon impact, is not pulverized in the same way as the coating to be removed. The boundary used in these studies was 250 microns; material smaller than this was assumed to be hazardous waste (paint dust contaminated with heavy metals). The theory was fine, but the actual execution did not work so well. Photomicrographs showed that many extremely small particles, which the authors believe to be old paint, adhered to large plastic abrasive particles. In this case, sieving failed due to adhesive forces between the small paint particles and the larger abrasive media particles. A general problem with this technique is the comparative size of the hazardous and nonhazardous particulate. Depending on the abrasive used and the condition of the paint, they may break down into a similar range of particle sizes. In such cases, screening or sieving techniques cannot separate the waste into hazardous and nonhazardous components. 5.2.1.2 Electrostatic Separation Tapscott et al. [7] have also examined electrostatic separation of spent abrasive. In this process, spent plastic abrasive is injected into a high-voltage, direct-current electric field. Material separation depends on the attraction of the particles for the electric field. In theory, metal contaminants can be separated from nonmetal blasting debris. In practice, Tapscott and colleagues reported, the process sometimes produced fractions with heavier metal concentrations, but the separation was insufficient. Neither fraction could be treated as nonhazardous waste. In general, the results were erratic. © 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
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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