Characterising long-term wet-stored fly ash following carbon and particle size separation

The paper describes a study carried out to characterise fly ash produced from hard-coal, following recovery from long-term pond and stockpile storage, and wet process separation. Three power station sites, representative of those in the United Kingdom, were considered and initial work examined the characteristics of recovered material from both types of storage area. Most of this had high loss-on-ignition (LOI) and was relatively coarse, with that in ponds generally having greater variability. Fly ash particles had reaction products on their surfaces and the presence of agglomerates was also noted. Methods including, pre-screening, column (hydraulic) separation, froth flotation and lamella hydraulic classifier were considered for processing. Three trials were carried out using various combinations, and it was possible to separate carbon and fly ash to give material groups ranging from carbon-rich to low LOI/high fineness, although fine fly ash particles (<5 µm) were distributed throughout these. Relatively minor changes in chemical composition and mineralogy of the separated materials were noted, except for particle groups with high LOI (reflecting fly ash dilution). Similar effects were generally obtained with both pond and stockpile storage. Mass balance calculations were made to quantify material distributions for the three processing trials carried out. Examples are given of yields achieved for potential end-uses from these, which indicate that optimisation to target particular fly ash properties should be possible.