A cenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a byproduct of coal combustion at thermal power plants. On average 0.5% of the fly ash generated in a coal fired power station boiler are Cenospheres.
When fly ash is wet discharged in to ash ponds/lagoons cenospheres float on the water surface, get finally air borne and cause dust pollution.
The floaters are collected from the water surface, dried and graded by screening into particle sizes between 150 to 300 micron.
Picture of Cenospheres
The end-product is used as a filler in compounds and paints as well as a component in oilwell cement.