Mixing characteristics of two-stage compulsory stirred pulp-mixing and its influence on fine coal flotation

A two-stage compulsory stirred tank with a pitched-blade opening-type turbine was designed based on the process intensification of pulp pretreatment before fine coal flotation. The mixing characteristics of the stirred system were evaluated by investigating its flow dimensionless number, power dimensionless number, shear characteristics, cycle characteristics and mixing efficiency. Coal flotation experiment of anthracite coal from Yongcheng Mine in central China was performed by two-stage compulsory pulp-mixing. When the rotational speed goes up, the energy input and the mixing efficiency of the stirred system increase, the pulp cycle performance and shear performance by unit energy consumption decrease, but the decreasing trend of the latter is less than that of the former. At the same rotational speed, high shear characteristics and mixing efficiency as well as low circulation/shear ratio can be obtained with a large diameter impeller by unit energy consumption. A cyclone-static micro-bubble flotation column can achieve a better flotation index and a greater capacity at the mode of two-stage compulsory stirred pulp-mixing. A large diameter impeller and high rotational speed favor to recover hard-to-float coal particles. The recovery of coarse particles is improved by two-stage compulsory stirred pulp-mixing when the processing capacity in appropriate conditions, but in large capacity conditions, the fine-particle ash increases in flotation railings.