In the mining industry, flotation cells are used to separate valuable minerals from ordinary rocks. Raw material is crushed and then mixed with water and chemicals to create slurry. The slurry is fed into the flotation cells, where the entire mixture is aerated. The chemicals added to the slurry attach themselves to the minerals in the crushed rock and the aeration creates bubbles that the chemicals are drawn to. The bubbles carrying the chemicals and ore rise to the top of the liquid as foam, which flows over a wire wall into another flotation cell. The process is repeated as the ore passes from one vessel to the next until the desired concentration of ore is achieved.
The efficiency of the flotation process will be determined by the amount of control the operator has over the level of slurry in the tanks and the size of the bubbles created. If the bubbles are too small, they will not be buoyant enough to carry the minerals. If the bubbles are allowed to get too large, they will burst before they pass to the next flotation stage. Most flotation cells come in banks of three to eight vessels and because the process cascades from one level down to the next, an upset at one level will pass on to the next vessel.
Traditionally, mining companies have used ultrasonic transmitters to control the level in the vessels. The transducers are aimed at large floats placed inside large stilling wells. This has provided the mining operators with a cost effective method of monitoring the levels. However, if the float becomes damaged, coated or misaligned, the entire system must be shut down to correct the faulty float. Additionally, the range of float movement is very short and the accuracy of the ultrasonic device causes the system to become more of a step function than a smooth control. These problems have a big influence in the output of the cell.
An alternative to the ultrasonic and float method of measurement is the AT500 magnetostrictive transmitter. It takes a special float to suit this application, as it has to be capable of continuous travel in the presence of build-up on the probe. It must also be buoyant enough to float in the liquid, but heavy enough not to be affected by the agitation from the bubbles. By utilising a float with a larger internal diameter, the build-up on the probe could be avoided. The larger size of the float provided enough buoyancy, while the specific mass allowed the float to sink into the liquid without bouncing up and down with the agitation.
The AT500 equipped with the custom engineered float provided a cost effective alternative to the ultrasonic transmitter. The engineered float design provided more reliable and more consistent level control, resulting in improved process efficiency. By having a float attached to the probe, instead of floating freely in the liquid, a probe or float failure would not result in a process shutdown.
For more information contact Jacolize Goosen, ABB South Africa, +27 (0)10 202 5000, [email protected], www.abb.com/za
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