Jörg Schwedes

Mechanical production and stabilization of submicron particles in stirred media mills

A joint research project between the Technical University of Braunschweig and the Technical University of Munchen investigates the possibilities for the production of stable product suspensions in a particle size range smaller than 100 nm. This paper shows the experimental setup which allows the measurement of the most important electrochemical properties and the analysis of the particle size distribution of the product suspension as well as an adjustment of the pH value for stabilization during the comminution process. Results for comminution of fused corundum with different grinding media materials and grinding media sizes are shown. In addition, results showing the influence of the electrostatic stabilization on the grinding progress are presented. Further, the rheology of the product suspension is examined depending on grinding progress and suspension stability.

Motion and stress intensity of grinding beads in a stirred media mill. Part 2: Stress intensity and its effect on comminution

Experimental investigations concerning the comminution of limestone in a stirred media mill have been carried out. The results show that the circumferential velocity of the stirrer discs as well as the density and the size of the grinding beads affect the specific energy consumption necessary to achieve the required product fineness. The influence of these three operating parameters on the comminution result can be fully described by the stress intensity of the grinding beads. Therefore, specific energy consumption and stress intensity of the grinding beads are the comprehensive influencing variables on the comminution of limestone in stirred media mills. It is expected that this relationship is also valid for the comminution of other materials in stirred media mills. For a fixed specific energy input, an optimum stress intensity exists, for which the finest product is achieved. With increasing specific energy input, and therefore increasing product fineness, the optimum stress intensity decreases. Since the specific energy is proportional to the product of stress intensity and stress frequency, the comminution result can also be correlated to the stress frequency and the stress intensity. With increasing stress intensity, the stress frequency required for a certain product fineness decreases.

Motion and stress intensity of grinding beads in a stirred media mill. Part 1: Energy density distribution and motion of single grinding beads

Abstract Numerical calculations concerning the flow field, the distribution of the specific energy and the motion of single grinding beads in the grinding chamber of a stirred media mill have been carried out. The calculations are based on steady-state laminar stirring of a Newtonian fluid without grinding media. The flow field of the stirred fluid generates a characteristic distribution of the specific energy. Two zones characterized by a high energy density exist. In these zones the local specific energy is larger than the mean specific energy which is obtained by dividing the total amount of energy dissipated in the grinding chamber by the net volume of the grinding chamber. One zone extends around the stirrer disc whereas the other is located at the grinding chamber wall. The vilume of these two zones is only about 10% of the net grinding chamber volume. Approximately 90% of the entire energy input is dissipated there. Single grinding beads that are exposed to a previously determined flow pattern tend to follow an almost stationary individual trajectory in the grinding chamber. The position of the trajectory depends on the ratio of bead-to-fluid density, the ratio of bead-to-disc radius and the Reynolds number which describes the operating conditions of the stirred media mill. The influence of these parameters can be described by the so-called motion index. Up to a critical value of the motion index, the single bead follows basically the fluid flow and passes through the two zones of high energy density.

Stress intensity in stirred media mills and its effect on specific energy requirement

The comminution of limestone and fused corundum in stirred media mills has been investigated regarding specific energy requirement. The experimental results show that the tip speed of stirrer discs as well as the density and the size of grinding media affect the specific energy required to achieve a certain product fineness. Moreover, for comminution of fused corundum, the Youngs modulus of grinding media and of product particles influences the specific energy requirement. The kinetic energy of grinding media is transferred from the grinding media to the product particles during each stress event. For product materials with a high Youngs modulus (e.g., fused corundum), the transferred energy strongly depends on the Youngs modulus of grinding media, whereas the transferred energy is nearly independent of the Youngs modulus of grinding media for weak product materials. The effect of the operating parameters tip speed of stirrer, grinding media size, grinding media density and Youngs modulus of grinding media on the comminution result can be described by the stress intensity. The stress intensity is proportional to the energy, which is transferred from the grinding media to the product particles during a stress event. For each stress intensity, a certain relation between product fineness and specific energy exists. Therefore, the comprehensive influencing parameters on comminution of limestone and fused corundum in stirred media mills are the stress intensity and the specific energy.

Theoretical and experimental investigation on particle and fluid motion in stirred media mills

Abstract The objective of this paper is to characterize fluid flow fields as well as grinding bead motion in stirred media mills. The fluid flow and grinding bead motion in media mills were visualized with a light-sheet technique thereby revealing information about fluid flow and grinding bead velocity distributions as well as zones of high differences of grinding bead velocities. The influence of fluid properties as well as stirrer geometry with respect to grinding bead properties on grinding bead motion was found to be significant.

Comminution of ceramics in stirred media mills and wear of grinding beads

Abstract Experimental investigations on the comminution of fused corundum and silicon carbide show that comminution and wear behaviour of ceramic grinding beads are affected by the structural constitution of grinding beads. For grinding beads composed of large primary particles, the achievable product fineness seems to be limited by the size of the primary particles. For the grinding beads composed of small primary particles, the product fineness is not limited by the size of the primary particles. The relation between specific energy requirement and product fineness is essentially affected by the amount of grinding bead wear. The wear of grinding beads is determined by the structure and the hardness of beads as well as by the hardness and the shape of the feed particles.

Consolidation and flow of cohesive bulk solids

Abstract The strength and the flow properties of bulk solids can be determined by performing shear tests. A shear test always consists of two parts, consolidation and measurement of strength. The measured strength depends on the way and degree of consolidation. During consolidation a stress history is impressed on the bulk solid sample which only vanishes after steady state flow. Since the consolidation is normally done nonhydrostatically, the strength will depend on the direction of stress application (anisotropy). Thus, the strength of a bulk solid sample strongly depends on its stress history and it can show anisotropic behaviour. These effects have to be considered when comparing available shear testers.

Energy distribution and particle trajectories in a grinding chamber of a stirred ball mill

Abstract In stirred ball mills, the specific energy input during comminution is the most important parameter influencing the product fineness. In order to get further information about the distribution of the local specific energy in the grinding chamber, the flow pattern in the chamber was calculated numerically. The calculations are restricted to a stationary operation of a stirred ball mill in the absence of grinding media when laminar stirring of a Newtonian fluid takes place. The calculations showed that there are two basically different operating ranges of the mill. They mainly differ in the flow pattern as well as in the spatial specific energy distribution. Therefore, the main difference is the location of the areas where the comminution is most successful. Furthermore individual spheric grinding beads were exposed to the determined flow patterns. The trajectory of small beads passes the regions of high energy densities in the grinding chamber, whereas larger beads tend to centrifuge.

Production of sub-micron particles by wet comminution in stirred media mills

A joined research project between the Technical University of Braunschweig and the Technical University of München investigates the possibilities for the production of stable product suspensions in a particle size range smaller than 100 nm. In this paper results for comminution of fused corundum showing the influence of the electrostatic stabilization at different pH-values on the grinding progress and the grinding media wear are presented. Furthermore, by means of experimental results the influence of the grinding media diameter and the stirrer tip speed is discussed.

Chapter 6 Wet Grinding in Stirred Media Mills

Publisher Summary This chapter discusses the design, physical fundamentals, grinding behavior, operating behavior and scale-up of stirred media mills. The chapter shows possibilities to develop new and optimize existing grinding processes with stirred media mills. As stirred media mills are operated usually wet, the chapter focuses on wet grinding. Grinding and dispersing in stirred media mills is a process, which is widely used in different industries such as chemical, ceramic, pharmaceutical, filler, ink and cosmetic industry. Stirred media mills belong to the group of mills with free movable grinding media. Stirred media mills are used for dispersion and deagglomeration processes as well as for true grinding of crystalline substances or the disintegration of microorganisms. With respect to the geometries of grinding chamber and agitator three different types of stirred media mills can be distinguished— (1) disc agitator, (2) pin-counter pin agitator, and (3) annular gap geometry. These mills are used for many applications in different industries and can be found especially where high product fineness is demanded.