Study on Small Grinding Media for Submicron Particle Production in Stirred Media Mills

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Author Last Name: 
Prof. Dr.-Ing. Jörg Schwedes, Dr.-Ing. Stefan Mende, Dipl.-Ing. Grit Mende
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Research Area: 
Size Reduction
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The investigations of the last five years have shown that a production of stable suspensions of hard ceramic materials with a median particle size smaller than 10 nm is possible by wet grinding in a stirred media mill. Nevertheless, no absolute limit of the grindability was yet reached.

By means of experimental results the influence of the electrostatic stabilization on different pH-values during the comminution of fused corundum on the grinding progress and the grinding media wear is discussed.

Results for comminution of fused corundum with grinding media of different diameters between 100 μm and 1300 μm and stirrer tip speeds between 6 m/s and 15 m/s are presented and discussed with regard to the grinding progress and the grinding media wear.

The particles in stirred media mills are stressed and ground between two grinding beads, one grinding bead and a wall or between a grinding bead and an agitator element in zones of high energy density [1, 2, 3]. Only for a desagglomeration or a desintegration of microorganisms shear forces are a further grinding mechanism.

During the approaching process of two grinding beads or a grinding bead and a wall the fluid between the collision partners will flow out of the gap. The smaller the product particles are, the better they will follow the fluid out of the active volume being the volume in which particles can be captured and stressed [4, 5].

With decreasing product particle sizes two opposite effects can be observed:

• While two grinding media approach each other, the fluid between them will be displaced. Wereby particles will follow the fluid flow out of the gap more easily the smaller the particles are. This may lead to media contacts without any grinding.

• On the other hand, with decreasing product particle size the absolute number of particles increases with the reduction factor to the power of three. Thus, a capture and stressing of more than one particle or a particle bed is possible.

The loss of kinetic energy during the collision is partly used for comminution. It has to be asked, how the energy is transferred to the feed particles. For that reason it is interesting to know how many particles are captured. According to the number of captured particles three cases can be distinguished [6]:

a) Only one particle is captured, which is stressed with the total energy (single particle stressing).

b) More than one particle is captured between two beads, all particles have contact to both beads during the stress event and all particles are stressed independent of each other. In this case at first that particle is captured, which has the largest size and/or which has the smallest distance to the connection line of both bead centers. This particle is stressed with the maximum energy. The particles, which are captured between the two beads after the first particle, are stressed with a considerably reduced energy. At the end of the stress event diverse single particle stressings with different intensities occur.

c) A particle bed is captured and stressed between two grinding beads.

Up to now it is not investigated how many particles are captured and stressed.

For a better understanding of the stress events in the sub micron particle size range, the particle motion in the displacement flow between a grinding chamber wall and an approaching grinding media was observed. Due to the small sizes of grinding beads and especially of product particles the product particle motion cannot be investigated optically in existing (real sized) stirred media mills. For that reason a model was developed. Parameters as volume concentration, approaching velocity and angle, model grinding bead diameter and material were varied.