There are several ways that coal ash can be recycled. As the coal is burned there is a relatively more volatile fraction known as “fly ash,” from which both silica and alumina may be obtained. Fly ash can also be converted into a substitute for Portland cement and used for building blocks in construction. Other relatively less volatile ash fractions known as bottom ash and boiler slag have uses as well and are much heavier. Overall though only about 40 percent of the total ash is utilized and a substantial fraction of the ash is placed in open, unlined ponds or pits. Occasionally the ash escapes and contaminates the environment. Either more ash needs to be used in other applications or safer methods for its storage need to be implemented.
Different fractions of ash may contain toxic heavy metals, often in much higher concentrations than are present in the original pulverized coal which is burned. These toxic metals and other rare elements present in both coal and residual ash may be extracted and purified. The other metals are strategic rare earths which have many applications often turn out to be present in concentrations as high or higher than are found in nature in ores. Thus, mining them from coal ash may turn out to be as effective and perhaps even more profitable than when trying to obtain them by direct mining. Strategic rare earths are key to certain sustainable technologies. Supplies of rare earths are presently very tight. Thus mining them from suitable coal ash samples may become cost effective. At the same time the more plentiful toxic metals may be removed and purified as well. As the other more common toxic metals are far from rare, there is less of a market for them, but they could be isolated and sold as well. Indeed, once all the toxic metals are removed from coal ash the remaining residues become relatively much less dangerous to the environment.
There are presently about 1100 coal ash sites nationwide to which the coal industry adds about 140 million tons of coal ash annually. Many sites contain ash which is contaminated with arsenic, lead, selenium and other toxic metals as well as cancer causing agents. People located within one mile of an unlined coal ash pond have a one in fifty risk of developing cancer. This is 2,000 times higher than what the EPA considers an acceptable lower risk rate.