Process intensification on flotation separation of carbon and ash from coal gasification slag using turbulent eddy regulation

Due to the serious inclusion of carbon and ash and its fine particle size, the flotation separation of coal gasification slag is difficult, which restricts its resource utilization. Turbulent regulation is an effective way to strengthen the flotation recovery of fine-particle minerals. Turbulent small-scale eddies directly act on the movement of fine particles. With the help of vortex generators, turbulent eddy regulation was implemented to strengthen the char-ash flotation separation of coal gasification residue. Firstly, the CFD numerical simulation was used to calculate the flow field in the vortex mineralization tube, and the influence of different structures of vortex generators on the characteristic parameters of turbulence was analyzed. The coal gasification slag flotation performances under different flow field conditions were obtained through flotation experiments. On this basis, a stepped vortex flotation process compatible with the floatability of minerals was constructed. The results show that the span wise hairpin vortex, primary convective vortex and flow vortex pairs with opposite secondary rotation direction are formed behind each vortex generator when rectangular vortex generators are built in the mineralization tube. The strong interactions between vortices and vortices behind adjacent vortex generators and between vortices and mainstream significantly improve the turbulent kinetic energy, reduce the minimum vortex scale, and strengthen the collision between fine particles and bubbles. The inclination angle of the vortex generator has a significant effect on the turbulence characteristics. When the inclination angle of the vortex generator increases from 25° to 55°, the turbulent kinetic energy and turbulent energy dissipation rate are significantly increased. The average turbulent energy increases from 0.041 m²/s² to 0.142 m²/s² and the average minimum vortex size is reduced from 16.10 μm to 10.34 μm. A balanced vortex mineralization device with multiple rows of vortex generators with the same inclination is used to carry out carbon ash flotation separation test on coal gasification slag. The flotation recovery of different particle sizes in coal gasification slag shows that the finer the particle size is, the greater the turbulence kinetic energy is required and the smaller the vortex scale is. However, when the turbulence is too strong, the coarser particles may detach from bubble surface. Within the study scope, for coal gasification slag particles of-45 μm and 45-75 μm, the proper mean minimum vortex scale is 12.74 μm and 14.71 μm respectively, and the corresponding mean turbulent kinetic energy should not exceed 0.080 m²/s² and 0.056 m²/s² respectively. Vortex generators with different tilt angles are orderly arranged along the flow direction in the mineralized tube, and a stepped vortex flotation process with gradually weakened turbulence is constructed. The flotation test shows that the flotation index with combustible recovery of 89.99% and tailings loss on ignition of 4.66% can be obtained by using the stepped vortex flotation device for ash separation from coal gasification slag, which is better than the flotation performance of the equilibrium vortex device and the mechanical stirring flotation machine. Through the physical regulation of fluid environment, a new process strengthening method for carbon ash flotation separation from coal gasification slag is provided.