(220b) Understanding the Flow of Calcium Carbonate in Stirred Media Mills

Stirred
media mills are used extensively in industry for the fine grinding of minerals
such as calcium carbonate. For use in products such as paints and papers,
calcium carbonate with a d₈₀ of approximately 2 micrometers
is required. However, fine grinding has a low energy efficiency due to energy
dissipation as heat, noise or vibrations. Hence, there is potential for a lot
of improvements to be made by changing the design or operating parameters. One
way of determining which parameters to change is through the investigation of
flow patterns within the mill. Knowledge of the motion of grinding media and
calcium carbonate particles in different zones of the mill is indicative of the
grinding intensity in each zone.

Using
experimental methods, flow patterns in stirred media mills have previously been
visualised using positron emission particle tracking (PEPT). A grinding media
bead is irradiated and added to a batch vessel [1-2]. However, usually only one
grinding media bead is tracked, which means that experimental run times are
long and an average occupancy plot or velocity distribution is obtained.
Additionally, only the grinding media is tracked, so there is no information
about the motion or shear rates of the calcium carbonate slurry.

This
research looks at the potential use of particle image velocimetry (PIV) in
stirred media mills to overcome the disadvantages of PEPT and provide higher
resolution flow patterns. Using PIV, it is possible to look at the flow of either
the grinding media or smaller seeding particles which follow the flow of the mimic
fluid. The main challenge of using PIV is the requirement for a completely
transparent set-up. A Perspex vessel has
been produced, several mimic fluids have been tested for use in place of the
calcium carbonate slurry and glass beads have been used in place of the ceramic
grinding media. If the refractive index of the mimic fluid matches that of the
grinding media, the grinding media are not seen in PIV experiments, enabling
high quality images to be obtained.

Clove
oil, cedarwood oil and refractive index adapted silicone oil are Newtonian
fluids with refractive indices matching that of glass beads that could be used
to represent grinding media. Additionally, their viscosities cover a wide
range, enabling the effect of viscosity on flow patterns to be investigated. Differences
between the transparent and actual milling set-ups may lead to PIV data being invalid
for real mills. The main differences in the set-ups are the density of the grinding
media and the density, viscosity and friction coefficient of the slurry. All of
the transparent fluids have significantly lower densities than calcium
carbonate slurries. The frictional properties of the fluids are also different,
as has been confirmed using tribology techniques. The glass grinding media are
of similar density to Carbolite ceramic grinding
media. The effect of these parameters on the power draw of the mill have been
investigated. By using glycerol-water solutions of varying concentration in
place of the calcium carbonate slurries, it is possible to test fluids of
vastly different viscosities and friction factors by measuring the power draw
of the mill at a constant rotational speed.

To determine
the extent to which the selected mimic fluids are representative of a calcium
carbonate slurry, the viscosity of calcium carbonate slurries and how it
changes during grinding has been investigated. Slurry viscosity is of high
importance in grinding processes since it affects the amount of viscous energy
dissipation occurring during grinding media collisions. Due to the reduction in
particle size, viscosity changes throughout grinding, meaning that flow
patterns also change.

Calcium
carbonate slurries for use in the paint and paper industries have a very high
solids content (typically >70 wt%) and so viscosity has been measured using
a vane rheometer, since this is an effective way of eliminating wall slip [3]. Additionally,
the shear rate in the mill is unknown and varies throughout the mill, so the
viscosity was measured over a range of shear rates.

The
viscosity of calcium carbonate slurries with 72 wt% solids has been measured
using the vane rheometer, as is shown in figure 1. Since viscosity increases
during milling as particle size decreases, the viscosity was measured after
various amounts of energy had been consumed. The measurements were made at the temperature
that the mill had reached when the sample was taken.

Fig. 1.
Viscosity of calcium carbonate slurries after energy input by milling (energy
input is per tonne of calcium carbonate being ground)

The
viscosity of 72 wt% slurries during milling ranges from 0.01 Pa s to 0.4 Pa s.
By using a selection of transparent mimic fluids with different viscosities
within this range, the development of flow patterns over the course of a batch
grind could be determined.

By using
transparent mimic fluids with a refractive index matching that of glass beads,
PIV can be used to experimentally determine flow patterns of grinding beads or
slurry particles within stirred media mills. However, the differences between
the density, viscosity and friction coefficient of the fluids and grinding
media used in the transparent and actual milling set-ups mean that the results
are not directly related to a specific milling set-up. Despite this, PIV can
still provide valuable information about how flow patterns change with
parameters such as viscosity, grinding media size and amount and impeller
speed.

References

1.     Barley RW, Conway-Baker J, Pascoe RD, Kostuch J, McLoughlin
B, Parker DJ. (2002) Measurement of the motion of grinding media in a
vertically stirred mill using positron emission particle tracking (PEPT). Minerals
Engineering, 15, 53–59.

2.     Van Der Westhuizen AP, Govender I, Mainza AN, Rubenstein J. (2011)
Tracking the motion of media particles inside an IsaMill^TM using PEPT.
Minerals Engineering, 24(3–4), 195–204.

3.     Pierre,
A. Perrot, A. Histace, S. Gharsalli,
E.H. Kadri, A. (2017) Study on the Limitations of a
Vane Rheometer for Mineral
Suspensions Using Image Processing,
Rheologica Acta, 556, 351-367