Methods for Beneficiation of Ash and Slag Waste from Coal-Fired Thermal Power Plants and Ways for Their Commercial Use (a Review)

Ash and slag waste (ASW) from coal-fired thermal power plants (TPPs), the amounts of which make several tens or even hundreds of millions of tons per annum, require allocation of large land areas for storing them. This waste is a source of pollution emitted into the atmosphere and it poisons the aqueous medium and soil. Ash and slag waste consists primarily of powderlike material containing a large quantity of unburned carbon (5–25%), magnetic materials (5–20%), and alumino-silicate components. All these components are a valuable raw material for industry, especially silicon and aluminum oxides, which can be used in the production of construction materials. For this purpose, ash must be preliminarily subjected to beneficiation using physical and physicochemical methods. The article presents an analysis of dry and wet ash beneficiation methods with a view to increase the volumes of using ASW produced from domestic power plants. For achieving higher strength of concretes, mechanical activation is carried out, which relates to dry ASW beneficiation methods. To obtain an alumino-silicate product containing 1.5–4.5% of carbon, such dry processing method as electrostatic separation is used. By subjecting dry ash to combined magnetic and electrostatic separation methods, it is possible to obtain an ash and a conducting product with a high concentration of carbon. For Russia, wet ASW beneficiation methods are of the greatest importance, because wet ash and slag removal is used at the majority of TPPs. Hydraulic classification of waste for obtaining products containing particles of different sizes is one of the wet beneficiation process stages. By using the froth flotation method, it is possible to obtain a concentrate containing 65–70% of carbon, a material that can serve as boiler fuel. The froth flotation is regarded as a necessary method for integrated utilization of ASW. Wet magnetic and electrostatic separation are the basic methods for producing magnetic concentrate containing 62–65% of iron. The article presents methods for separating hollow microspheres from AWS, which are a valuable product for aggregates used in the civil construction industry. The article also considers integrated ash beneficiation methods for obtaining four basic products (the figures in parentheses indicate the content of the relevant element): hollow microspheres, carbon product (64.5% of carbon), magnetic product (62% of iron), and alumino-silicate product (no more than 3% of carbon).