The beneficiation of coal combustion ash

Abstract

AbstractPulverized coal combustion ash is an important source of strategic materials in the USA. Bottom ash is used as a source of aggregate for use in concrete and masonry units (blocks). It is processed primarily to improve its grading. A top size is removed and the finest sizes are removed via wet or dry screens. Pyrite and rock may also be present with the ash. These materials can be removed by spiral concentrators and jigs. In some cases high-quality/high-value lightweight aggregates are produced from stored bottom ash. Fly ash is used as a pozzolanic additive to Portland cement concrete. In addition to partially replacing the cement, it contributes substantially to the durability of the concrete. The advent of low-NOx burners and selective catalytic reduction (SCR) supported ammonia injection has altered the character of the fly ash, particularly the Class F, low-Ca type, by generally increasing the amount of unburned carbon. This contaminant adsorbs air-entrainment reagents and can decrease the resistance of the concrete to freeze-thaw damage. Over the past decade several technologies and approaches have been developed to remove the carbon from the fly ash, including: air classification; electrostatic separation; and fluidized-bed combustion. Other approaches such as microwave heating also show promise. Froth flotation has been successfully applied to wet ash. The amount of ash that is beneficiated has increased to a current level of about 1 million tons per year in the USA, which is expected to grow in time due to the need for predictable materials with constant characteristics. The primary environmental advantage of ash beneficiation is that it enables the use of combustion ash that would otherwise be disposed as waste. High-quality, consistent products can be generated, thus increasing the usefulness and acceptance of these processed products in both traditional and emerging markets. By doing so, the amount of ash that is utilized will be increased, thus reducing the amount of ash that is disposed, while conserving other resources such as aggregate and sand for other uses not applicable to combustion ash.