5. Modelling of the overall plant
The overall plant can be modelled on the basis of the material
balances of the individual units and their connections,
with the need to take into account both the total
and the fraction mass flows. Thus, the speed of the
separating wheel chosen determines the maximum
particle size of the fines passing the classifier during
the closed-circuit operation of mill and classifier, simultaneously
resulting in the amount of the mass flow circulating
between classifier and mill (Fig. 11). As the mill
is operated with a nearly constant loading percentage, the
latter determines the feed rate to the plant from outside.
A low product fineness means that the circulating
mass flow is low and the plant throughput is high. A high
product fineness results in a high circulating mass flow
and a low plant throughput.
Figs. 12 and 13 show two operating conditions of the
closed-circuit grinding plant during cement clinker
grinding, which was already the basis for Fig. 3, to
demonstrate the operating range of the plant with respect
to throughput and product fineness. A relatively coarse
product with x80 = 48.1 m is made with a speed of
the separating wheel of 1150 rpm during the operating
condition A. The plant throughput amounts to 45 kg/h.
The recycle ratio, defined as the relationship between
mill throughput and plant throughput, amounts to 1.93.
A very fine product with x80 = 2.8 m is made during the
operating condition B with the nearly maximum speed
of the separating wheel of 10 000 rpm. The plant throughput
is decreased to 2.7 kg/h and the recycle ratio is
increased to 9.9.
Based on the experimentally determined data records of
various operating conditions, material-specific process
models of the closed-circuit grinding plant can be created
making it possible to precalculate arbitrary plant settings
for the materials tested. For example, the speed of the
separating wheel, as setting parameter, and the resulting
mass flows can be determined for a desired fineness
of a certain product. The other way round, the particle
size distribution of the product and the plant throughput
can be calculated if the speed of the separating wheel
is given.
The results of the simulation achieved with the process
model very well tally with the experimentally determined
data (Fig. 14). The particle size distributions of the real
products (black and green) are compared with the
simulated products (blue and red). However, it turned out
that the simulated distribution is not steady in some
sections and does not completely tally with the real
distribution. The conformity can be improved by a
further development of the algorithms on which the
process model is based, on the one hand. On the other
hand, the quality of the experimental data available for
the verification of the process model also influences the
quality of the simulation.
6. Final comment
It is not yet possible to apply the process described to materials
with unknown grindability properties. Therefore,
it is still justified to carry out investigations into the
grindability.