Sandblasting is used to clean dirt, corrosion, paint or other coatings from a variety of surfaces. The clean grit should in most cases contain no hazardous properties. Common industries where sandblasting is applied include shipbuilding and maintenance, transportation bridge maintenance, and military operations. Abrasive blasting has been a concern for a number of years in regard to worker safety during the blasting process. Issues of concern include worker exposure to silica dust, extreme noise exposure, and mechanical and electrical hazards (NIOSH, 1976). An issue of less noticeable concern is the disposal of the used ABM. In light of stringent waste management regulations and heightened awareness of environmental contamination, additional focus has been placed on the management of this waste stream.
The problem presented with the used abrasive blast media is that it may contain materials from the cleaned surface which impart hazardous characteristics to the grit. Sandblasting is often used to remove paint from metal and other surfaces. Surface coatings with paint are often necessary to protect from deterioration in the environment, most notably the marine environment (ships and bridges are the prime example). These paints usually contain heavy metals which act as anti-fouling and anti-corrosion agents. When the metal surfaces are cleaned as part of routine maintenance and repainted, the metals in the paint become part of the waste ABM matrix. And indeed, heavy metals are the most common contaminants of ABM waste. The contamination of the abrasive results in potential restriction for disposal and recycling (Ovenden, 1990).
Although no specific regulations are in place for used sandblast grit waste, it is a solid waste, and as with any non-excluded solid waste the generator of the waste is responsible for determining if the waste possesses hazardous characteristics, and is thus a hazardous waste. This is therefore a necessary step in determining the disposal and reuse options available. Environmental regulations require that a Toxicity Characteristic Leaching Procedure (TCLP) test be performed to determine if the material is hazardous. If it is hazardous, the material must be managed accordingly. If not determined hazardous, the grit is a solid waste which must be disposed of properly.
There are many types of abrasive blasting media available. Sand is one of the most common blasting materials. Sand is the least expensive non-reusable media. Alternatives to sand abrasives include other mineral sands with no free silica, metal slag, and coal slag. Coal slag has been used frequently as a blasting material. Media of these types may not be reused in the abrasive process, but can be recycled into other materials (e.g. cement or concrete). Types of abrasive blasting media which are used more than one time include garnet, steel shot, and glass bead. These media may be screened and separated after used to capture reusable particles. Plastic blast media is reusable and versatile. It can be used in circumstances when “harder” materials may be too damaging to sensitive surfaces. One such application is the surface of jets and planes. Spent plastic media may also be recycled into other materials such as counter tops. Other materials which have been used as blasting materials include walnut shells, impregnated sponge, and dry ice.
One problem with the management of this waste stream is that it has often times gone unnoticed as a solid waste and the need for testing for hazardous characteristics has not been recognized. This results in part from the physical appearance of the waste. When silica sand has been used, the waste very much looks like sand and is therefore not easily recognized by some as a solid waste. This material would be simply spread around the property and treated as additional soil. As new safety regulations resulted in different types of ABM being used, the residue from these materials was more noticeable as a solid waste. An example of this is coal slag, which although is similar in physical character to sand, is black in color. Used ABM is also more recognizable than in the past because safety regulations often require the ABM to be contained and not used in open atmosphere conditions. This has resulted in waste that is now collected, which in the past was perhaps left to make its way into the environment.
When surveying Florida regulatory data for this search, there was not a large amount of information was found. Toxicity Characteristic Leaching Procedure (TCLP) tests performed for larger projects in the state, and total metals concentration for a few also, but little of this data has been correlated. Enough data was collected, however, to make a few generalizations. For the most part, the waste ABM surveyed for this report was non-hazardous (only 3% was hazardous). However, the heavy metal content of the waste was still large enough to limit recycling and disposal options. Lead and Arsenic were the two metals encountered which exceeded the risk-based standards set by EPA and FDEP the most. Question still remain regarding the true leachability of the heavy metals to the environment. The typical data encountered in the regulatory files did not perform leaching tests to determine possible groundwater input, but rather to test for hazardous characteristics.
The challenges of managing this waste stream stem from the fact that it is generally not hazardous, its appearance is soil-like, and disposal and reuse options are not always clearly outlined to either the regulators or the industry. Nonhazardous sandblast grit waste still needs to be disposed of in a sanitary landfill. A lined MSW landfill is normally considered to be the requirement but the possibility of using construction and demolition waste landfills has been raised by generators.
A number of recycling options are possible for the management of ABM. Spent blasting abrasives have been used as a feedstock material in the production of Portland Cement. (Salt et al. 1994, Brabrand and Loehr 1993) This recycling option is currently practiced by the Tampa Port Authority, along with three cement kilns throughout the state of Florida. Used ABM also has the potential to be used as aggregate in the production of Portland cement concrete and in the production of asphalt concrete for roadways. In such cases the material not only has to meet state and federal regulatory disposal questions, but would also has to meet the physical and chemical requirements of the manufacturing process. Other options for recycling include recovering some fraction of ABM for reuse, using as a clean fill material (if clean enough), and using as a drainage material in landfills or septic tanks.
The management of solid waste from abrasive blasting is an issue which will be more frequently encountered in the future. While existing guidelines and regulations are available, a single resource which spans such a wide array of information does not currently exist. Future work should concentrate on collecting and summarizing best management practices for ABM waste management in a format which could be used by the many industries which perform abrasive blasting, and the engineering and regulatory community.
Townsend, T. (1997). Disposal and reuse options for sandblasting grit. In Florida:
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Post time: Jun-30-2020