Corrosion Control Through Organic Coatings Part 5 ppsx

85

5 Abrasive Blasting and

Heavy-Metal

Contamination

In the previous chapter, mention was made of the need to minimize spent abrasive

when blasting old coatings containing lead pigments. This chapter covers some

commonly used techniques to detect lead, chromium, and cadmium in spent abrasive

and methods for disposing of abrasive contaminated with lead-based paint (LBP)

chip or dust. Lead receives the most attention, both in this chapter and in the technical

literature. This is not surprising because the amount of lead in coatings still in service

dwarfs that of cadmium, barium, or chromium.

The growing body of literature on the treatment of lead-contaminated abrasive

seldom distinguishes between the various forms of lead found in old coatings,

although toxicology literature is careful to do so. Red lead (Pb3O4), for example, is

the most common lead pigment in old primers, and white lead (PbCO3 • Pb[OH]2)

is more commonly found in old topcoats. It is unknown whether or not these two

lead pigments will leach out at the same rate once they are in landfills. It is also

unknown whether they will respond to stabilization or immobilization treatments in

a similar manner. A great deal of research remains to be done in this area.

5.1 DETECTING CONTAMINATION

There are really two questions involved in detecting the presence of lead or other

heavy metals:

1. Does the old paint being removed contain heavy metals?

2. Will the lead leach out from a landfill?

The amount of a metal present in paint is not necessarily the amount that will

leach out when the contaminated blasting media and paint has been placed in a

landfill [1-3]. The rate at which a toxic metal leaches out depends on many factors.

At first, leaching comes from the surface of the paint particles. The initial rate,

therefore, depends most on the particle size of the pulverized paint. This in turn

depends on the condition of the paint to be removed, the type of abrasive used, and

the blasting process used [4]. Eventually, as the polymeric backbone of the paint

breaks down in a landfill, leaching comes from the bulk of the disintegrating paint

particles. The rate at which this happens depends more on the type of resin used in

formulating the paint and its chemistry in the environment of the landfill.

7278_C005.fm Page 85 Friday, February 3, 2006 12:38 PM

© 2006 by Taylor & Francis Group, LLC