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Case Study

Mineral Processing - Concentrate Storage

Project Description

Molybdenum ore is ground and concentrated using a flotation process. The concentrate slurry collected is stored in a large agitated tank prior to dewatering.

The concentrator facility is old as is most of the equipment. Eventually the customer replaced the concentrate tanks with new taller tanks as well as the structural steel for the pipes, access and the support for the agitator. They also re-used the existing agitators (Denver Equipment, no longer in business) after the shaft was lengthened. After these changes were completed, there were vibrations throughout structural steel which raised a number of alarms.

Both of the solutions we provided worked very well and the vibration problems were eliminated.

MixPro® Challenge

Immediately after being contacted for advice, we obtained all the necessary dimensions, mechanical details of the existing agitator as well as the slurry parameters (weight percent solids, dry solids SG & particle size). We quickly identified that the lengthening of the shaft increased the bending forces and caused the natural frequency of the agitator shaft to come too close to the operating speed. This meant that the agitator forces were causing the shaft to resonate which had caused agitator failures in the past.

Diagram of basin and impeller

The agitators must uniformly suspend the slurry throughout the tanks, disperse the added oxygen, and uniformly distribute the activated carbon throughout the slurry. Not all the oxygen gas added into a slurry will be transferred for the various reactions which can be seen as bubbles breaking at the slurry surface. Less oxygen gas is needed to satisfy the reactions when vigorous agitation is used, though this requires more electrical power.

Recommended Solution

We provided two solutions to address the problem. One solution, (very low cost), was to reduce the operating speed of the agitator. This would move the operating speed further from the natural frequency and reduce the bend moment load. We were confident that the reduction in agitation intensity would still be more than adequate for the purpose of concentrate storage. The second solution was to provide a replacement shaft and impellers suitable for the new tank and the work required. We would also duplicate the Denver tapered coupling design. Since the customer has two concentrate tanks, they purchased one solution for each.

Project Success

Both of the solutions we provided worked very well and the vibration problems were eliminated. The slurry was properly agitated. Eventually a second replacement shaft and impeller was purchased.