When coal is burned in a boiler to produce energy, the incombustible materials constitute three components: fly ash, bottom ash and bottom slag. There is growing concern about the potential hazards of coal waste due to contamination with heavy metals such as chromium, lead and arsenic.
Fly ash is the light ‘dust’ that is exhausted from the burning process. Improvements over the past 20 years at coal fired facilities has allowed as much as 99% of this ash to be captured for re-use in other applications. Bottom ash is the unburned material and fine particulate that is not expelled in the flue.
Boiler slag is the molten material (ash and incombustible) that is sent to a quenching water bath. The quenching process immediately fractures the molten material producing a coarse, hard, black, angular, glassy material sometimes call black beauty. This coal slag is used for a variety of industrial applications including abrasive blasting. The concern is that once the coal slag is blasted, the process releases the heavy metals into the air or they get absorbed by ground water.
While coal slag (which contains minimal crystalline silica) has historically been considered to be a safe alternative to silica sand, the EPA is reviewing whether to classify fly ash and other coal burning wastes as a hazardous material. There is no need to wait for the EPA to make a final determination. There are many alternatives to coal slag for blasting such as crushed glass grit and other more specialty types of blasting media.