Flow evaluation in colloidal mixers

Optimizing the process behavior of colloidal mixers using numerical simulations

Fig. 1 SEM image (magnification of 1,000) of bentonite with a w/s ratio of 5 after mixing in a colloidal mixer at a speed of 960 rpm.

Fig. 2 Top: Speed profile and moment measurement for a viscometer test run using the cylindrical geometry carried out for a CaCO₃ suspension with a w/s ratio of 0.6. Bottom: Analysis of the viscometric test to determine yield stress and viscosity for a CaCo₃ suspension with a w/s ratio of 0.6.

Fig. 3 Overview of measured yield stresses and viscosities using the viscometer with the cylindrical geometry for CaCO₃ suspensions with varying w/s ratios.

Fig. 4 Schematic view of the test mixer supplied by MAT Mischanlagentechnik GmbH equipped with pressure sensors and a plexiglass mixing vessel.

Fig. 5 Images of a paddle cycle taken with a high-speed camera at various speeds. Top: 960 rpm; center: 400 rpm; bottom: 100 rpm.

Fig. 6 Mean pressure values for one revolution with three paddle cycles in the colloidal mixer at 100 rpm for 30 liters of water.

Fig. 7 Measured power consumption of the colloidal mixer for various materials and filling levels.

Fig. 8 Vortex formation in a simulation of the original geometry of the colloidal mixer. The colored part shows the velocity distribution on the surface.

Fig. 9 Comparison between measured and calculated pressure curves over the circumference of the mixing chamber. A limestone dust suspension with a w/s ratio of 0.7 was used as the test material; the mixer was operated at a speed of 200 rpm. The peak pressure values calculated by the simulation could not be resolved by the measurements due to the delayed response of the sused sensors.

Fig. 10 Comparison of actual and calculated power consumption of the colloidal mixer for silica dust suspensions with various w/s ratios.

Fig. 11 Example showing some geometry variations of the tested mixing paddle. The image clearly shows the varied number of perforations and the change in their orientation, as well as the proportion of perforations.

Fig. 12 Analysis of mean pressure gradients in the simulation at various speeds and for different materials depending on the paddle geometry used.

Fig. 13 Particle distribution in a limestone dust suspension with a w/s ratio of 0.7. The diagram shows the isosurface for the volume concentration c_n = 1% at t = [0,5s; 1,0s; 2,0s; 5,0s].

Fig. 14 Comparison of simulated moments acting on the rotor at various speeds and for different materials depending on the paddle geometry used.

Colloidal mixers are used for the production of suspensions containing ultra-fine particles. Key requirements to be met include the uniform distribution of solid particles in the fluid phase and the decomposition of particles, i.e. the disintegration of lumps. Amongst other effects, particle decomposition is to achieve an increase in the reactive surface of the material. Construction industry applications of this process include the production of cement paste or bentonite suspensions. In order to optimize the particle decomposition and mixing behavior of colloidal mixers, various tests were carried out at IFF Weimar (Weimar Institute for Precast Technology and Construction) in collaboration with MAT Mischanlagentechnik GmbH, with the aim of optimizing the mixer and paddle geometry in order to improve decomposition efficiency. This article outlines the approach towards a holistic solution to this problem by combining experimental testing with numerical simulations.

Background

One of the key objectives of using colloidal mixers is to achieve a high degree of particle decomposition as a result of the mixing process. In the actual process, the degree of decomposition is very hard to evaluate and requires a sophisticated setup; in some cases, such an evaluation is not possible at all. Particle decomposition is difficult to capture visually, such as by a scanning electron microscope (SEM) (Fig. 1), because the physico-chemical processes that occur in the bentonite suspensions change the structure of the material. On the other hand, an indirect evaluation on...