6 Dynamic Process Models for Fine Grinding and Dispersing
221
below the experimental values. For limestone (Fig. 16a) there are two clear outliers
and no linear correlation could be found between S sim and S exp . The poorer agreement
of the results for limestone may be due to the fact that the collision energy distribution shows strong fluctuations in the range of the breakage energy distribution
values for limestone (see Fig. 15). In addition, the energy actually transferred to the
particles does not exactly correspond to the collision energy, which is a maximum
value. Therefore, it should be checked whether the simulated collision energies are
in principle too low, or whether multiple stressing is also an important factor in the
grinding of limestone.
4.2 Circulation Mode
Materials can be ground in four different modes. Therein, continuous passage mode
is most often used in industry. Circulation mode is an approach often used in laboratory scale to understand grinding processes, avoiding the usage of large amounts of
material. In 1996, Kwade developed a stress energy model that calculated the stress
energy based on the kinetic energy of the grinding beads [28]. After correlation with
the specific energy and the information of the final product size, it is possible to
obtain an optimum curve for specific energy input.
Circulation mode is often the favored method in laboratory scale to determine the
operation parameters for a certain final product size, but also in industry if very fine
particles have to be produced. Therefore, it is of great interest to identify the important
influencing parameters in circulation mode and correlate it towards one or multiple
passage mode, i.e. different continuous modes that are often applied in industry
for medium fine and coarse products like minerals, agrochemicals and ores. Kwade
introduced several parameters that influence the kinetic energy and one of those
parameters affecting the energy transfer coefficient is the viscosity of the product
suspension [29]. Thereby, the viscosity is dependent on the mass concentration,
particle size and fluid viscosity. If the fluid viscosity is not varied, but the mass
concentration, then beside viscosity also the capture probability is influenced.
Generally grinding in circulation mode results in broader particle size distributions compared to running in multiple passage mode. Especially the first circulations
result in broader distributions, but longer grinding times and with that higher number of cycles reduce the poly dispersity index PDI. However, operation in circulation
mode with a sufficient number of cycles delivers distinctly narrower residence time
distributions and, thus, particle size distribution than operating the mill in one passage
mode (Fig. 17) [30].
The grinding experiments were executed in two different types and sizes of mills
to consider mill specific effects: One was the LM4 IsaMill
TM from Netzsch, the other
one the PM-1 from Drais. The LM4 has an internal deflector wheel and its volume
is around 4.5 L. Six perforated discs have been used with a distance of 35 mm.
The PM-1 is smaller and its volume is around 700 mL. There are five perforated
discs installed. In the LM4 IsaMill
TM , the mass concentration was kept constant to
221
below the experimental values. For limestone (Fig. 16a) there are two clear outliers
and no linear correlation could be found between S sim and S exp . The poorer agreement
of the results for limestone may be due to the fact that the collision energy distribution shows strong fluctuations in the range of the breakage energy distribution
values for limestone (see Fig. 15). In addition, the energy actually transferred to the
particles does not exactly correspond to the collision energy, which is a maximum
value. Therefore, it should be checked whether the simulated collision energies are
in principle too low, or whether multiple stressing is also an important factor in the
grinding of limestone.
4.2 Circulation Mode
Materials can be ground in four different modes. Therein, continuous passage mode
is most often used in industry. Circulation mode is an approach often used in laboratory scale to understand grinding processes, avoiding the usage of large amounts of
material. In 1996, Kwade developed a stress energy model that calculated the stress
energy based on the kinetic energy of the grinding beads [28]. After correlation with
the specific energy and the information of the final product size, it is possible to
obtain an optimum curve for specific energy input.
Circulation mode is often the favored method in laboratory scale to determine the
operation parameters for a certain final product size, but also in industry if very fine
particles have to be produced. Therefore, it is of great interest to identify the important
influencing parameters in circulation mode and correlate it towards one or multiple
passage mode, i.e. different continuous modes that are often applied in industry
for medium fine and coarse products like minerals, agrochemicals and ores. Kwade
introduced several parameters that influence the kinetic energy and one of those
parameters affecting the energy transfer coefficient is the viscosity of the product
suspension [29]. Thereby, the viscosity is dependent on the mass concentration,
particle size and fluid viscosity. If the fluid viscosity is not varied, but the mass
concentration, then beside viscosity also the capture probability is influenced.
Generally grinding in circulation mode results in broader particle size distributions compared to running in multiple passage mode. Especially the first circulations
result in broader distributions, but longer grinding times and with that higher number of cycles reduce the poly dispersity index PDI. However, operation in circulation
mode with a sufficient number of cycles delivers distinctly narrower residence time
distributions and, thus, particle size distribution than operating the mill in one passage
mode (Fig. 17) [30].
The grinding experiments were executed in two different types and sizes of mills
to consider mill specific effects: One was the LM4 IsaMill
TM from Netzsch, the other
one the PM-1 from Drais. The LM4 has an internal deflector wheel and its volume
is around 4.5 L. Six perforated discs have been used with a distance of 35 mm.
The PM-1 is smaller and its volume is around 700 mL. There are five perforated
discs installed. In the LM4 IsaMill
TM , the mass concentration was kept constant to