Hydrophobic flocculation and Box–Wilson experimental design for beneficiating fine coal

Abstract

The purpose of this study is twofold: first, to determine the appropriate bridging liquid, pH, stirring speed and dispersant for hydrophobic flocculation of coal. Kerosene, n-heptane, benzole and n-hexane were used as bridging liquids. The coal suspension could be flocculated better by kerosene compared to the other bridging liquids at natural pH value of coal. From the experiments carried out to determine the effect of stirring speed on the hydrophobic flocculation of coal suspension, it was found that a stirring speed of 750rpm was required to achieve the maximum combustible recovery and minimum ash content values of flocculated coal particles. The effects of sodium silicate, sodium hexametaphosphate, sodium phosphate and sodium pyrophosphate used as inorganic dispersants on the hydrophobic flocculation of coal were also studied. Sodium silicate was more effective than the other dispersants in the presence of kerosene. The best results were obtained with a concentration of 1g/dm3 sodium silicate and 4g/dm3 concentration of kerosene. It was determined that a clean coal having an ash content of 10.33% with a combustible recovery of 89.54%. The second fold of study is the Box–Wilson experimental design method and it was employed to evaluate the effects of important variables such as bridging liquid concentration (kerosene), dispersant (sodium silicate) concentration and stirring speed on the hydrophobic flocculation of coal. Response function coefficients were determined by the regression analysis of experimental data and the predictions were found to be in good agreement with the experimental results. Similar to the results of the first fold of experimental studies, the optimum kerosene concentration, sodium silicate concentration and stirring speed were determined as 4g/dm3, 1g/dm3 and 750rpm, respectively, when considering combustible recovery and ash content.