Experimental investigation of the attachment of unburned carbon in coal fly ash to a stationary air bubble in aqueous solutions

Abstract Separating unburned carbon (UC) from coal fly ash (CFA) via froth flotation may be quantitatively characterized by the bubble–particle attachment angle (BPAA), susceptible to operational factors. This study established a novel experimental system to directly observe attachment behaviors between irregularly shaped UCs and a stationary air bubble. The independent and coupling effects of the main operational factors on the BPAA, including solution temperature, pH value, and collector dosage, were investigated via high-speed photography. The BPAA increased rapidly at the beginning of magnetic stirring for each factor, and then grew steadily when agitation time exceeded 180 s, gradually leveling off at its maximum value. The maximum BPAAs were 195°, 176°, and 169° at solution temperatures of 35, 30, and 25 °C, respectively. They were 169°, 144°, and 166° for pH values of 7, 9, and 11, respectively, and 169°, 189°, and 184° at collector dosages of 0, 1, and 2 mL/L, respectively. An overhigh dosage of collector kerosene resulted in oil agglomeration that hindered collision and adhesion. Response surface methodology (RSM) was adopted to analyze and optimize interactions between various factors, indicating that the predicted model that correlated the BPAA and the independent variables were in good agreement with the experimental data. The results show that the importance degree in BPAA decreases in a certain order. Moreover, several optimum solutions were obtained to achieve a maximum BPAA (~196°) under suitable conditions. The outcomes of this study strengthen bubble–particle attachment during the recovery of UCs from CFA by froth flotation, contributing to the comprehensive utilization of CFA.