Measuring slurry level under foam
July 2001
Level Measurement & Control
Precious metal extraction processes have certainly evolved over time. The value placed on such commodities has allowed technology to improve the processes thus generating higher yields than ever before.
Extraction involves the slurry being pumped into flotation cells or vessels, together with special reagents, a regulated air supply and with agitation the slurry begins to froth, producing the 'mechanism' for extraction. The quantity of extraction is dependent on pH, flowrate, percentage base metal in the slurry and also the size of the air bubbles, the latter playing a vital role. Once some of these variables have been stabilised, the varying air bubble size results in a greater or lesser entrapment surface area. The foam layer with the entrapped metals is floated off through several stages resulting in a higher concentration for processing.
Critical to the entire principle is the control of the slurry level, hence the amount of foam produced and overflowed. Should this not be achieved within pre-defined limits, then the overflowed slurry will result in poorer yields.
Traditional level measurement featured various hybrids ranging from buoyancy transmitters, angular displacement transmitters to ultrasonic level transmitter using floats attached to some rather exotic mechanical devices to represent the level. This buoyancy type measurement is density-dependent, hence variations to the air supply result in level changes whilst in actuality the only variation is density. Further, due to turbulences, some of these mechanical systems endure great forces and at times, dependent on position, are rendered inoperable due to the restraint of movement. Product build-up on the floats and guide mechanism also result in erroneous measurements thus necessitating periodic maintenance. The proximity of the ultrasonic transducer relative to the agitator bearing sets also has been a source of problems. Some sets have been known to generate sporadic noises in the ultra-sound range resulting in false echoes for the transmitter.
The Krohne TDR level transmitter utilises a rod as a guide for the microwaves. This already guarantees immunity against noise, is independent of density variations, is less susceptible to build-up because of the reduced surface area, making it maintenance-free, and provides the correct level of the slurry under turbulent conditions. The rod can even be bent to a 90° angle to facilitate ease of mounting. Krohne believes that the BM100/102 with its patented proven technology, various successful site tests, option of two-wire at attractive price and reduced cost of ownership, has revolutionised this area of measurement.
Krohne
(011) 315 2685
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