Boiler slag is a coarse, granular, incombustible by-product of coal combustion and is thus classified as a coal combustion byproduct. It is angular, dense, and very hard. Boiler slag is only produced in a wet-bottom boiler, as this boiler has a special design that keeps bottom ash in a molten state until it is removed. Essentially, boiler slag is what bottom ash turns into in very specialized boilers. Boiler slag particles are generally between 0.5 and 5.0 mm in size and have a smooth surface. However, if gases are trapped in the slag as it is cooled it can have a somewhat porous texture. As well, boiler slag that comes from burning lignite or sub-bituminous coal is more porous than harder anthracite coals.
In wet-bottom boilers, bottom ash is kept in a molten state until it is actively removed. When this ash is taken from the boiler, it flows into quenching water that cools it rapidly. This quick cooling of the molten bottom ash causes it to crystallize immediately into a black, dense, glassy mass that fractures into sharp, angular particles. These particles are known as boiler slag, and can be crushed into different sizes for a variety of uses.
Coal ash - which includes boiler slag - can have a different chemical makeup depending on where the coal was mined. Broadly speaking, coal ash is a pollutant, and it contains acidic, toxic, and radioactive matter. This ash can contain lead, arsenic, mercury, cadmium, and uranium. The EPA found that significant exposure to components of coal ash increases a persons risk of developing cancer and other respiratory diseases. As well, storage lagoons can pollute groundwater and if ingested, the arsenic contaminated water increases a persons risk of developing cancer. Inhalation is not as much of an issue with boiler slag as it is much heavier than fly ash, but ingestion of boiler slag can have nervous system impacts, result in cognitive defects, developmental delays, and behavioral problems along with increasing a persons chance of developing lung disease, kidney disease, and gastrointestinal illness.
When fly ash, boiler slag, and bottom ash are disposed in lagoons, there are associated environmental effects. These wet lagoons can be an issue, because if they do not have proper liners to prevent leaking and leaching then groundwater contamination is much more common. Leaching is a process that occurs when coal ash is wet, and it simply means that the toxic components of the ash dissolve out and percolate through water. This groundwater contamination can be harmful to human heath if the groundwater is a source of drinking water.
The coarse, dense nature of boiler slag makes it beneficial for use as a wear-resistant component in surface coatings of roads. Smaller-sized boiler slag can be used as a blasting grit, and can be used for coating roofing shingles. Aside from these major uses, boiler slag can be re-purposed into a raw material for cement or can be spread onto icy roads to prevent slipping.
Since boiler slag is widely used but not widely produced, almost all of the boiler slag produced is re-purposed. In the United States in 2007, 1.5 million metric tons of boiler slag were re-used, about 80% of what was produced overall.
Boiler slag that is not to be recycled is discarded in landfills or storage lagoons. If the boiler slag goes to a storage lagoon, it is generally mixed with fly ash and referred to collectively as ponded ash. About 30% of all coal ash is disposed of when wet as ponded ash. This ash is potentially usable even after it is put in a storage lagoon. Because fly ash and boiler slag have different weights, the heavier boiler slag (or bottom ash, which is disposed of in essentially the same way) settles first and the fly ash remains suspended. This ponded ash can be reclaimed if the boiler slag is scraped up and dewatered. The more boiler slag that is in ponded ash, the easier it is to dewater and the more potential it has for reuse. This reclaimed ash can be used in embankment construction or as a filler.