How did this platinum mine reduce risk and cost using efficient motion control?

SOUTH AFRICA: BMG Electronics used Danfoss drives to support DryTech International in its development of a unique wet screw feeder for use in a platinum operation comprising mine, processing plant, and smelter. This solution improves reliability with elegant synchronization free of servo motion control, reduces costs, and eases maintenance time too.

Challenge: Unusual material requires unique solution

DryTech International developed the wet screw feeder for this plant to deal with unusual characteristics of the material being processed from platinum group metals (PGMs), chrome and base metals. The material is extremely sticky, with a consistency like clay, and needed to be transferred smoothly into a dryer. In addition, this material is received not only from mines within the immediate area in Limpopo province, but also from mines in the Mpumalanga province of South Africa, more than 300 km away. During transit, some composites dry out, while others remain 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 synchronization without the screws clashing. Larger gears would also substantially increase the costs of the wet screw feeders. Ease of maintenance was a further requirement by the plant. The throughput needed from the first wet screw feeder was between 30 and 40 tonnes per hour, with 15 tonnes per hour required for the second feeder.

Solution: Synchronized screw shafts

3D wet feeder model without guards	3D wet feeder model with guards	DryTech wet screw feeder setup using FC 302 drives

DryTech built a trial machine to test the application, and the trial showed that a three-screw shaft was the most effective solution. Although during the trial, the three screw shafts were mechanically linked to one other, the client favoured a solution with independent shafts, electronically synchronized using VLT® AutomationDrive FC 302 units controlling a 22 kW motor/gearbox on each shaft. This was a critical requirement, as all three independent shafts need to rotate in a coordinated manner to help prevent mechanical damage. For example, should one of the motors, gearboxes or shafts become overloaded and slow the motor down, it must be detected by an out-of-synchronization function and all screw shafts stopped. Once the problem is cleared and the application brought back manually, then the application automatically reverts back into the correct synchronized position at start-up, instead of having to return the shaft to a home position manually.

Tighter speed and operation control

The final application set up by Juan Lerm, Field Service Technician at BMG Electronics, comprises three screws connected to the 22 kW motors and reducing gearboxes. The master motor (center motor) is controlled by a VLT® AutomationDrive FC 302 with PROFIBUS® and encoder options.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.

The higher the pulses per revolution (PPR), the better the syncing, as the increments are closer to each other, thus increasing the resolution of the feedback.

“In this application we used 8192 PPR multiturn absolute encoders,” explains Mick Baugh, Electronics Manager, Electromechanical Division, BMG. "When the wet feeder needs to run, it gives a ‘start’ command to the master variable speed drive (VSD) via PROFIBUS®. The master VSD then gives the start signals to the follower VSDs.”

The two follower screws are connected to VLT® AutomationDrive FC 302 with a VLT® Synchronizing Controller MCO 350 option also with PROFIBUS® for monitoring purposes. The master encoder is connected to both the MCOs as a master reference, which means that the MCO 350 cards monitor the speed of the master shaft. The follower encoder is fitted on the screw shaft of the follower screws, and these encoders are connected to each respective MCO as a feedback signal.