Studies on carbon flotation from fly ash

• Complicated porous structures were observed on coarse carbon via SEM.
• Coarse carbon (> 25 μm) was difficult to be floated even with 4.5 kg/t diesel.
• Increase in CIS improves carbon recovery and separation efficiency of coarse carbon by 79% and 110%, respectively.
• Increase in CIS reduces the collector consumption by nearly 80%.

The separation of carbon from fly ash significantly improves the potential end-us of combustion byproducts. Froth flotation is the most effective separation technique for ultrafine material. Diesel dosages, conditioning time, and impeller rotating speeds' influences on carbon flotation performance were studied. From flotation products, + 25 and − 25 μm fractions were screened out to study their different flotation behaviors. At a given impeller speed, carbon recovery of +25 μm fraction increased significantly from 38.46% to 74.22% over a range of diesel dosage from 0.5 to 4.5 kg/t, while that of -25 μm fraction was relatively constant at 85% over the same diesel dosage range. Morphologies and microstructures of coarse and fine carbon were studied by using scanning electron microscope. Porous structures of coarse carbon were observed. It's difficult for diesel droplets of several microns to penetrate the pores of coarse carbon particles, which increases diesel consumption. Increasing impeller speeds from 1200 to 1600 rpm during conditioning improved the carbon recovery and separation efficiency of + 25 μm fraction by 79% and 110%, respectively, which significantly reduced diesel consumption. Flotation kinetic studies indicate that increasing impeller speeds during conditioning stage can improve the flotation rate constant and maximum carbon recovery for both + 25 and − 25 μm fractions.