Bottom ash consists of ash particles from natural minerals found in coal. Minerals such as quartz, feldspar, calcite and clay minerals as kaolinite and illite.
In powder-fired power stations from time to time the pieces of melted and sintered ash particles fall to the bottom of the boiler, where they are collected. Hence the name boiler bottom ash or boiler sand. Through an opening in the boiler, the bottom ash falls into ash hoppers filled with water. Because of the contact with water, the hot particles granulate, resulting in a much smaller grain size. Then it is crushed. The bottom is transported to a precipitation basin hydraulically, after which it is stored outside. The material is fine grained and similar to sand with a predominantly angular grain shape. It largely consists of aluminium silicate compounds. From a mineralogical viewpoint it mainly consists of glass and to a lesser extent, quartz and feldspar. Bottom ash is inert and has low leaching characteristics as a result of which it is applicable without restrictions according to the Soil Quality Degree.