Fine grinding in an air-swept ball mill working in closed circuit with a
forced vortex air classifier has been investigated first in continuous
laboratory and then in industrial scale. Results from the laboratory circuit
have shown significant influence of mill load (powder filling) on the
capacity of the circuit. In laboratory scale the mill load was measured and
controlled by a weighing platform placed under the complete mill set-up. The mill load of the industrial air-swept ball mill has till now been
controlled by measuring the power consumption of the mill motor. Thus,
it was necessary to operate far from the range of optimal mill filling. To
improve the circuit performance, alternative ways of mill load
measurement were investigated. An important factor was investment cost,
as many fine grinding mills operate in relatively small circuits that do not
wan-ant large investment. In this case the task was accomplished by using a piezoelectric strain
transducer, which detected strain changes in the mill shell. It was
installed in the midpoint of the shell, where the highest deformation was
obtained. As the transducer was on the top position, it indicated
compression, and on the bottom position it measured tension. By taking
the difference between the readings, it was possible to calculate total
strain variations, directly proportional to the mill load. The signal from
the transducer was sent to a control room via a cordless infrared
communication system, employing a microprocessor control card, which
could further regulate other operating parameters of the circuit according
to the software model. The installation required very low investment cost and provided
simplicity of application as well as stability and reliability of
characteristics under severe industrial condition. The measurement
system made it possible to optimise the mill load and thus increase the
production from 3.2 to 3.7 dh (15 per cent), and stabilise the circuit
operation.