•A new type of dual punched rigid-flexible impeller was proposed.•Dual punched rigid-flexible impeller was optimized by CFD simulation.•Dual punched rigid-flexible impeller can improve the solid suspension quality.•Dual punched rigid-flexible impeller can reduce power consumption.The hydrodynamics of solid-liquid suspension process in a stirred tank with a dual rigid impeller, a dual rigid-flexible impeller, and a dual punched rigid-flexible impeller were investigated using computational fluid dynamics (CFD) simulation. A classical Eulerian-Eulerian approach coupled with standard k-ε turbulence model was employed to simulate solid-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, impeller speed, flexible connection piece width/length of dual rigid-flexible impeller, aperture size/ratio of dual punched rigid-flexible impeller, particle diameter, and liquid viscosity on the homogeneity degree of solid-liquid system were investigated. Results showed that the homogeneity degree of solid-liquid system increased with an increase in impeller speed. A long and wide flexible connection piece was conductive to solid particles suspension process. Larger particle diameter resulted in less homogenous distribution of solid particles. An increase in liquid viscosity was beneficial to maintain solid particles in suspension state. The optimum aperture ratio and aperture diameter were 12% and 8 mm, respectively, for solid particles suspension process. It was found that dual punched rigid-flexible impeller was more efficient in terms of solid particles suspension quality compared with dual rigid impeller and dual rigid-flexible impeller under the same power consumption.Dual punched rigid-flexible impeller consists of two rigid impellers and six silicone flexible connection pieces. There are a certain number of holes on the surface of the flexible connection pieces of dual punched rigid-flexible impeller. The flexible connection pieces are under the interaction of agitating shaft and flowing medium, which can cause a series of disturbances on the flowing medium and generate a series of high-speed jet flows. Dual punched rigid-flexible impeller can improve solid particles suspension degree compared with dual rigid impeller and dual rigid-flexible impeller under the same power consumption.Download high-res image (40KB)Download full-size image

•A new type of punched rigid-flexible impeller was proposed.•Punched rigid-flexible impeller can improve solid-liquid homogeneous degree.•Punched rigid-flexible impeller can reduce the power consumption.•Homogeneity ξ was calculated for solid-liquid mixing performance evaluation.Computational fluid dynamics (CFD) was used to investigate the hydrodynamics of solid-liquid suspension process in a stirred tank with rigid impellers, rigid-flexible impellers and punched rigid-flexible impellers. The effects of impeller type, impeller speed, flexible connection piece length, impeller spacing, particle size, and aperture size/ratio on the mixing quality were investigated. Results showed that the degree of solid-liquid homogeneity increased with an increase in impeller speed. A long flexible connection piece was conductive to solid particles suspension process. The solid particles could not obtain enough momentum to suspend to the upper region of stirred tank with small impeller spacing. Larger particle size resulted in less homogenous distribution of solid particles. The optimum aperture ratio and aperture diameter of punched rigid-flexible impeller were 12% and 8 mm, respectively, for solid particles suspension process. It was found that punched rigid-flexible impeller was more efficient in suspending solid particles compared with rigid impeller and rigid-flexible impeller at the same power consumption. In addition, less impeller power was consumed by punched rigid-flexible impeller compared with rigid impeller and rigid-flexible impeller at the same impeller speed.The punched rigid-flexible impeller consists of a rigid impeller and six flexible connection pieces. A certain number of holes exist on the surface of the rigid impeller blades. The flexible connection pieces were connected to the blades of rigid impeller. The flexible connection pieces can cause a series of disturbances on the flowing medium and transfer the impeller energy through the form of wave. A series of high-speed jet flows and a plurality of vortices can be generated in the flow field due to the existence of apertures. These conditions are beneficial to improve the solid particles suspension quality in the solid-liquid mixing process.Download high-res image (31KB)Download full-size image

•Segregated regions were observed through visualization experiment.•Largest Lyapunov exponent was used to characterize chaotic characteristics.•Laminar mixing intensification was achieved by punched rigid-flexible impeller.•The co-reverse mixing was much more efficient than normal mixing.Laminar mixing of non-Newtonian CMC solution was investigated experimentally in a stirred tank with a rigid impeller, a rigid impeller coupled with a chaotic motor, a rigid-flexible impeller, a rigid-flexible impeller coupled with a chaotic motor, a punched rigid-flexible impeller, and a punched rigid-flexible impeller coupled with a chaotic motor. Visualization experiment was used to reveal the presence of segregated regions, and observe the structure changes of segregated regions. The mixing performance was characterized by the largest Lyapunov exponent (LLE) and dimensionless mixing time (NTm). Results showed that punched rigid-flexible impeller could further enhance the value of LLE, namely, the chaotic extent of mixing system compared with rigid impeller and rigid-flexible impeller, and the NTm for punched rigid-flexible impeller was shorter than that for rigid impeller and rigid-flexible impeller at the same consumption per unit volume (Pv). The chaotic motor could further increase LLE value, and reduce the NTm on the basis of the normal motor. Moreover, the optimum aperture diameter and aperture ratio of punched rigid-flexible impeller were 8 mm and 12%, respectively, and the variable speed period of 10 s and constant speed period of 5 s of chaotic motor were particularly suitable for the laminar mixing process.Two doughnut rings have been formed above and below the rigid impeller with a normal motor. These regions remained segregated from the rest of the mixing system, and acid penetrated the segregated regions due to the diffusive mechanisms, not the convective flow mechanisms, to neutralize the base. The presence of the segregated regions would extent the mixing time. The punched rigid-flexible impeller coupled with a chaotic motor was more effective to eliminate segregated regions. Because punched rigid-flexible impeller could generate a series of high-speed jet flows in the rotation process, and co-reverse rotation of chaotic motor could generate aperiodic flows.Download high-res image (96KB)Download full-size image

•Largest Lyapunov exponent (LLE) was used to assess the mixing performance.•Relative standard deviation (RSD) was applied for solid suspension quality evaluation.•Double punched rigid-flexible impeller can improve solid-liquid homogeneous degree.•Chaotic motor can improve solid suspension quality with co-reverse periodic rotation.The mixing performance of solid-liquid system was investigated in a stirred tank with a double rigid impeller, a double rigid-flexible impeller, a double punched rigid-flexible impeller, and a double punched rigid-flexible impeller coupled with a chaotic motor. The effects of the impeller types, the flexible connection piece width/length, the aperture size/ratio, and the variable/constant speed periods were investigated. The mixing performance was characterized by the largest Lyapunov exponent (LLE) and axial local solid concentration profile. The results indicated that the double punched rigid-flexible impeller and chaotic motor could enhance the LLE value of solid-liquid mixing system on the basis of the double rigid-flexible impeller and traditional motor under the same power consumption. The variation in solid homogeneous degree was in good agreement with that of LLE. It was found that the double punched rigid-flexible impeller coupled with a chaotic motor could further improve the solid suspension quality and energy efficiency compared with the double rigid impeller or double rigid-flexible impeller coupled with a traditional motor in the solid-liquid mixing process.The double punched rigid-flexible impeller consists of two rigid impellers and six flexible connection pieces. A certain number of holes exist on the surface of the flexible connection pieces. The flexible connection pieces are under the interaction of agitating shaft and flowing medium, which can cause a series of disturbances on the flowing medium and transfer the impeller energy through the form of wave. With the existence of apertures, a series of high-speed jet flows and a plurality of vortices can be generated in the flow field. These conditions are beneficial to improve the mixing performance in the solid-liquid mixing process.Download high-res image (60KB)Download full-size image

Chloride in manganese ore adversely affects mineral extraction. The mechanisms and the factors that influence an alkali pretreatment to removal chlorine from manganese ore were explored to eliminate hazards posed by chlorine during the electrolysis of manganese. The results showed that sodium carbonate, when used as an alkaline additive, promoted the dissolution of insoluble chloride, enhanced the migration of chloride ions, and effectively stabilized Mn2+. The optimal conditions were a sodium carbonate concentration of 0.45 mol·L−1, a liquid-solid ratio of 5:1 mL·g−1, a reaction time of 1 h, and a temperature of 25°C. The chlorine removal efficiency was greater than 95%, and the ore grade (Mn) was increased by 2.7%.

•Mixing of two immiscible fluid in a mixer-settler was experimentally studied.•A double rigid-flexible combination impeller was designed with flexible piece.•The chaotic analysis of liquid–liquid mixing in a mixer-settler was addressed.•Results highlighted the reliable intensification effect of this novel impeller.•Multiple-body motion behavior of impeller intensified the chaotic mixing of fluid.Mixing is crucial in the dispersion of two immiscible fluids. The rational design of an impeller is necessary to form suitable flow conditions and improve fluid mixing efficiency. A double rigid-flexible combination impeller was designed by connecting the upper and lower rigid impeller blades with flexible pieces. Experimental measurements were performed in a laboratory-scale mixer-settler under different impeller types. The largest Lyapunov exponent (LLE) and multi-scale entropy (MSE) were investigated using Matlab. Results showed that the double rigid-flexible combination impeller enhanced liquid–liquid mixing in the mixer-settler through the multiple-body motion behavior triggered by the swings of flexible pieces. At the optimum mixing point of each impeller, the LLEs of the double impeller, double rigid combination impeller, and double rigid-flexible combination impeller were 0.018, 0.055, and 0.057, respectively. At 75 rpm, the MSE of the combination impellers was obviously greater than that of the double impeller, and the rigid-flexible combination impeller had larger MSE than the double rigid combination impeller. The mixing efficiency of the rigid-flexible combination impeller increased with increasing width and quantity of the flexible piece. The quantity of rigid blade slice also influenced the enhancement of mixing ability. The double rigid-flexible combination impeller intensified the chaotic mixing of the two-phase fluid by changing the flow field structure and energy dissipation mode, ultimately achieving an efficient-mixing operation.Formation process of double rigid-flexible combination impeller. The double rigid-flexible combination impeller was made of two Rushton disc turbine (RDT) rigid impellers fixed to the agitator shaft and six flexible connection piece, and the flexible pieces were connected to the upper and lower rigid impellers by bolt. The multiple-body motion behavior aroused by the vibration of flexible piece intensified the chaotic mixing.Download full-size image