The fly ash from coal combustion often has a high unburned-carbon content. This can make it unusable as a cement admixture, which is the major fly ash utilization market. As a result, high-carbon fly ashes are currently disposed of by landfilling, at sometimes considerable expense. In this paper, the use of froth flotation to remove the unburned carbon from fly ash is described. Three different fly ashes were studied that had loss-on-ignition (LOI) values of 6.79, 6.90, and 11.11 percent It was found that the use of an alkanolamide/oil mixture as a froth-conditioning reagent could greatly reduce the amount of collector needed by increasing the affinity of the collector for oxidized carbonaceous surfaces. For each of the three fly ashes, froth flotation reduced the LOI of the ash to less than 2 percent, which is well below the ASTM specification of no more than 6.0 percent LOI for Class F fly ash. Increasing carbon content of the ash tended to increase the quantities of reagents needed in flotation.
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