Danish engineering company Danfoss, in support of engineering components provider BMG Electronics, has helped to assist DryTech International in its development of a unique wet screw feeder for use by a South African platinum mine in its processing plant and smelter, in Limpopo.
DryTech provides design solutions to thermal processing problems, developing a variety of complex thermal processes ranging from mineral concentrate flash dryers to high-temperature reduction kilns and continuous vacuum dryers.
DryTech developed the wet screw feeder for this plant owing to unusual characteristics of the material being processed.
The material is extremely sticky – similar in constitution to clay – and needed to be transferred into a dryer.
Also, the material was to be received not only from mines within the immediate area, but also from mines in the Mpumalanga province of South Africa, a distance of over 300 km away.
During this transit period, the plant had found that some composites dried out, while others remained wet.
From a mechanical design perspective, the gears required for the feeder would be large-scale, posing some practical challenges when it came to physically fitting into the unit synchronisation without the screws clashing.
Larger gears would also substantially increase the costs of the wet screw feeders. Ease of maintenance was a further requirement for the plant.
The throughput needed from the first wet screw feeder was between 30 t/h and 40 t/h, with 15 t/h required for the second feeder.
A trial machine was built by DryTech for testing, and a three-screw shaft solution was found to be the most effective.
While the three-screw shafts were mechanically linked to one another during the trial, the client favoured a solution with independent shafts, using the preferred Danfoss FC 302 22 kW units controlling a 22 kW motor or gearbox on each shaft, with electronic synchronisation between them.
This was a critical requirement, as all three shafts needed to rotate in a coordinated manner to help prevent mechanical damage.
This is owing to the fact that, should any of the shafts go out of synchronisation, even by a programmable amount, the machine must be stopped.
During normal operation, the load on each drive is relatively light – at less than 50% motor full load torque – but because of the material’s consistency, there are times when higher loads occur. It was for this reason that the 22 kW drives with 22 kW motors were put forward.
The motors are mounted above the gearbox driving each screw shaft, which, according to DryTech, means that the motor drives the gearbox via a belt drive.
The final application set up by BMG Electronics field service technician Juan Lerm comprises three screws, connected to the 22 kW motors and reducing gearboxes.
The master – or centre – motor is attached to a Danfoss FC 302 with a Profibus and encoder option.
The Profibus master is the plant control system controlling the operation and speed of this motor according to process requirements. The master motor is fitted with an encoder to increase its dynamic torque response, and the shaft of this motor has a reference encoder to measure the actual screw shaft rotational speed.