(169c) Process Function of Stirred Media Milling

The process function proposed by Rumpf describes the influence of mill type and mill design, operating parameters as well as feed properties on the disperse properties of the product particles, especially on the product fineness. The process function can be subdivided into the so-called machine and material functions. In case of a grinding process the machine function is determined by the type, frequency and energy of the stresses acting on the particles inside the mill whereas the material function is determined by the material and breakage properties of the feed particles, among others by the breakage and selection function.

In a stirred media mill the energy supplied to the grinding chamber is dissipated by friction inside the fluid and at the grinding chamber wall, by the displacement of the fluid between two approaching grinding media, by deformation of the grinding media and by stressing and breakage of the product particles. The stress energy and the so-called active specific energy which is the specific energy really transferred to the product particles depend on these physical processes inside the mill. Thus, the operating parameters determine substantially the machine function, especially the stress energy and the active specific energy. The active specific energy is the sum of the stress energies divided by the mass of the stressed product particles and, moreover, the specific energy input of the mill multiplied by an energy transfer factor.

By investigating the physical processes inside the mill the effect of the operating parameters on the stress energy and on the energy transfer factor was determined. Especially, the energy dissipation by displacement of the fluid between two approaching grinding media and by deformation by the grinding media will be discussed. The reduction in grinding media velocity due to the displacement of the fluid can be described by the Stokes-Number of the grinding media, which is determined by the size and the density of the grinding media and, particularly, by the viscosity of the displaced fluid or product suspension, respectively. The energy dissipation due to deformation of the grinding media depends on the young modulus of the grinding media compared to the one of the product material.

Grinding results show that the two main parameters of the process function, the stress energy and the active specific energy, can describe the effect of size, density and elasticity of grinding media, stirrer tip speed and viscosity on the product fineness. For each feed material a certain relationship between the active specific energy, the stress energy and the product fineness exists. If the stress energy is higher than the stress energy required for a particle, the influence of the feed material on this relationship can be described by a material factor "a".