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G. Fragnière et al.
1 Introduction
The importance of ultrafine grinding and dispersing has increased in various industries like chemical, pharmaceutical, ceramic, electronic and ore industry. For these
operations, stirred media mills are often used in which the particles are ground in
a suspension by relative velocities of grinding media down to the micrometer or
nanometer size. Especially expensive products and large installations require the
enhancement of models describing fine grinding in stirred media mills, for example
to decrease product quality fluctuations and to decrease the amount of wasted material. Grinding and dispersing in stirred media mills is often modeled by looking on the
mill operation parameters but neglecting the suspension properties. However, both
influence the kinetic energy of grinding media, the product transport of particles, and
the uneven axial distribution of grinding media. Next to grinding, these parameters
also influence the power consumption of the mill and the wear of mill and grinding
media. Through optimal adjustment of operation parameters, it is possible to reduce
the power consumption significantly, so that grinding processes are more economic
and work more environmental friendly.
Even though fine grinding in stirred media mills has been investigated for several years now, a model describing the effect of uneven grinding media distribution
was not considered so far. Furthermore, fluctuations of feed material and the effect
of start-up and shut-down processes are unknown contributions on product quality.
Using mill control might stabilize sensitive processes. Especially in circulation mode,
often used in pharmaceutical, chemical and paint industry, particle size distribution
and viscosity of the supplied suspension change dynamically during the grinding
process due to increasing particle interactions with decreasing particle size. Supplementary, in passage mode, coarse material can influence the grinding process if an
internal deflector wheel, which retains the grinding media in the grinding chamber,
is installed.
Incorporating stirred media mills in longer process chains, there is an increasing
demand for the prediction of material’s particle size distribution as function of operation time. In literature, most flow sheet models work with characteristic particle
size values and do not consider particle size distributions or the influence of operating parameters. Therefore, the description of entire particle size distributions in
dependency of geometry, operating parameters and processing mode is practically
impossible. However, there are models for dry comminution in ball mills, which
describe the breakage rate as a function of the size-dependent strength distribution
of the material and the distribution of stress energy [1, 2]. Additionally, it is possible to calculate the evolution of particle size distribution during comminution by
population balance modelling.
This contribution presents a process model for fine grinding in horizontally orientated stirred media mills for application in a dynamic flow sheet simulation. The
modelling approach is based on separating machine and material function, as well
as on considering grinding and transport phenomena (Fig. 1). In order to achieve
this, on the one hand the stress conditions in a “calibration mill” were investigated
G. Fragnière et al.
1 Introduction
The importance of ultrafine grinding and dispersing has increased in various industries like chemical, pharmaceutical, ceramic, electronic and ore industry. For these
operations, stirred media mills are often used in which the particles are ground in
a suspension by relative velocities of grinding media down to the micrometer or
nanometer size. Especially expensive products and large installations require the
enhancement of models describing fine grinding in stirred media mills, for example
to decrease product quality fluctuations and to decrease the amount of wasted material. Grinding and dispersing in stirred media mills is often modeled by looking on the
mill operation parameters but neglecting the suspension properties. However, both
influence the kinetic energy of grinding media, the product transport of particles, and
the uneven axial distribution of grinding media. Next to grinding, these parameters
also influence the power consumption of the mill and the wear of mill and grinding
media. Through optimal adjustment of operation parameters, it is possible to reduce
the power consumption significantly, so that grinding processes are more economic
and work more environmental friendly.
Even though fine grinding in stirred media mills has been investigated for several years now, a model describing the effect of uneven grinding media distribution
was not considered so far. Furthermore, fluctuations of feed material and the effect
of start-up and shut-down processes are unknown contributions on product quality.
Using mill control might stabilize sensitive processes. Especially in circulation mode,
often used in pharmaceutical, chemical and paint industry, particle size distribution
and viscosity of the supplied suspension change dynamically during the grinding
process due to increasing particle interactions with decreasing particle size. Supplementary, in passage mode, coarse material can influence the grinding process if an
internal deflector wheel, which retains the grinding media in the grinding chamber,
is installed.
Incorporating stirred media mills in longer process chains, there is an increasing
demand for the prediction of material’s particle size distribution as function of operation time. In literature, most flow sheet models work with characteristic particle
size values and do not consider particle size distributions or the influence of operating parameters. Therefore, the description of entire particle size distributions in
dependency of geometry, operating parameters and processing mode is practically
impossible. However, there are models for dry comminution in ball mills, which
describe the breakage rate as a function of the size-dependent strength distribution
of the material and the distribution of stress energy [1, 2]. Additionally, it is possible to calculate the evolution of particle size distribution during comminution by
population balance modelling.
This contribution presents a process model for fine grinding in horizontally orientated stirred media mills for application in a dynamic flow sheet simulation. The
modelling approach is based on separating machine and material function, as well
as on considering grinding and transport phenomena (Fig. 1). In order to achieve
this, on the one hand the stress conditions in a “calibration mill” were investigated