Abstract
Pulp preconditioning using a stirred tank as a pretreatment process is vital to the flotation system, which can be used to improve the flotation efficiency of mineral particles. The kinetic energy that is dissipated in the stirred tank could strengthen the interaction process between mineral particles and flotation reagents to improve the flotation efficiency in the presence of the preconditioning. In this paper, the effect of the conditioning speed on the coal fly ash flotation was investigated using numerical simulations and conditioning-flotation tests. The large eddy simulation coupled with the Smagorinsky-Lilly subgrid model was employed to simulate the turbulence flow field in the stirred tank, which was equipped with a six blade Rushton turbine. The impeller rotation was modelled using the sliding mesh. The simulation results showed that the large eddy simulation (LES) well matched the previous experimental data. The turbulence characteristics, such as the mean velocity, turbulent kinetic energy, power consumption and instantaneous structures of trailing vortices were analysed in detail. The turbulent length scale (η) decreased as the rotation speed increased, and the minimum value of η was almost unchanged when the rotation speed was more than 1200 rpm. The conditioning-flotation tests of coal fly ash were conducted using different conditioning speeds. The results showed that the removal of unburned carbon was greatly improved due to the strengthened turbulence in the stirred tank, and the optimal results were obtained with an LOI of 3.32%, a yield of 78.69% and an RUC of 80.89% when the conditioning speed was 1200 rpm.
Highlights
Coal is the main energy source in China [1]
This paper focuses on the turbulent properties in the outflow generated by the Rushton turbine in a stirred tank and the conditioning-flotation tests of the flotation separation of unburned carbon from coal fly ash
The localaccording instantaneous mean velocity, the partly periodic velocity and partly the turbulent fluctuations to to their statistical averages using velocity fluctuations a fluctuations turbulently velocity ui should be decomposed into the mean, the periodic and resolved the partinofin their statistical averages using thethe
Summary
Coal is the main energy source in China [1]. The direct combustion of coal in thermal power plants has released many pollutants into the environment [2]. Coal fly ash (FA) is one of the main by-products of pulverized coal combustion [3]. The unburned carbon (UC) content in FA, which is evaluated using conventional loss on ignition (LOI), might be an obstacle for the utilization of FA in a variety of industries, especially for its use as a raw material in cement and concrete [4]. The recommended LOIs of the Class 1 and 2 fly ash types used in concrete should be less than 5% and. Froth flotation is still the main method for the separation of unburned carbon from FA [8,9]
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