Effect of frother addition mode on gas dispersion and coal flotation in a downflow flotation column

•A novel surfactant addition mode of flotation was proposed to reduce the surfactant waste.•Surfactant vapor mode presented higher aeration rate, higher bubble rising velocity and higher combustible matter recovery.•Relationship between aeration rate and bubble rising velocity of different addition modes was discussed.•Relationship between concentrate yield and aeration rate of different addition modes was discussed. The traditional addition mode of frother as liquid and a novel mode as vapor were comparatively studied in presences of methyl isobutyl carbinol (MIBC) and sec-Octyl alcohol (2-octanol). Aeration rate and coal flotation were investigated in a downflow flotation column while bubble rising velocity was measured in a glass tank, wherein the aeration rate and bubble rising velocity were studied in a gas–liquid system. The comparison between these two frother addition modes shows that the vapor mode presented a faster bubble rising velocity, a larger aeration rate, a higher concentrate yield and a higher combustible matter recovery independently of frother sort. In addition, under the liquid mode MIBC presented a smaller aeration rate, a faster bubble rising velocity, a lower concentrate yield and a lower combustible matter recovery as compared with 2-octanol, while MIBC gave a smaller aeration rate, a roughly equal bubble rising velocity, a lower concentrate yield and a lower combustible matter recovery under the vapor mode. The aeration rate sharply decreased as the bubble rising velocity increased up to a critical point above which the aeration rate became almost constant, independent of frother addition mode. Furthermore, the concentrate yield sharply increased as the aeration rate increased up to a critical point above which the concentrate yield became gradual, independent of frother sort and addition mode. The MIBC and 2-octanol consumption was significantly reduced when the frother was added as vapor rather than liquid, and 2-octanol vapor presented the lowest concentration for achieving the concentrate yield of 60%.