Recovery of cenospheres from solid waste produced by coal-fired power plants

Coal-fired power plants produce over 300 million tons of fly ash each year and this amount still increases. Recently, this severe source of environmental contaminants is considered as a raw material used to provide transition metals, rare-earth elements and cenospheres. Cenosphere is a hollow aluminosilicate spherical particle of diameter between 20 µm and 500 μm, which is one of those coal combustion byproducts that attracted attention of industrial and materials engineers due to their specific physical and chemical properties. Traditionally, cenospheres are utilized as construction materials component due to their high compressive and tensile strength. Novel applications, for example, as a catalyst carrier or as thermal barrier coating, result from their specific properties such as low bulk and apparent density, high chemical and thermal stability, and low thermal conductivity. The paper briefly summarizes the present state of knowledge on physical and chemical properties of cenospheres obtained from coal fly ash, the methods for their recovery, and their utilization. The novelty of this review is a detailed analysis of parameters specific to cenospheres, which distinguish them from other solid fly ash particles and opens new possibilities of their utilization. Particular attention was paid to magnetic cenospheres, which can be applied to the production of modern functional materials of high chemical and thermal stability and mechanical strength, combined with their magnetic properties, allowing their easy separation from gaseous or liquid phases.