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Mass Flow Measurement Improves Froth Recovery in Gold Flotation Plants.

Byline: Andrew Okely, Product Manager, Outokumpu Technologies; and Graham Seal, Project Manager, AMS Tech

The concentration and separation of ores in gold and other mineral processing operations often requires the use of flotation cells that rely on precision froth handling for increased recoveries in roughing, scavenging, and cleaning applications. Accurate and, more importantly, repeatable air flow measurements are essential for the efficient operation of large flotation tank cells.

Outokumpu Technology, located in New South Wales, Australia, developed a new method of flotation cell automation called Frothmaster, which is designed to remove the subjectivity associated with describing the appearance of the flotation froth. The system acquires and processes digital images of the froth using a sophisticated image processor that runs on a custom-built computer system that measures the speed, size, and stability of the froth. These parameters, which indicate the overall process performance, are used to calculate new setpoints for the airflow, tank level, and reagent dosages.

Accurately controlling the air flow to the flotation cells requires a mass flow meter instead of the more traditional volumetric flow meter, which is ineffective because the volume of air in the flotation tank cannot be directly related to the volume of air flowing through the feed pipes, due to large variations in air temperature in response to smaller changes in slurry temperature.

Outokumpu selected thermal flow meters by Fluid Components International for the Frothmaster system. These thermal flow meters increase frothing performance by measuring an air flow that is closely related to bubble volume within the flotation tank.

Mass flowmeters are unaffected by temperature and pressure and offer exceptional accuracy and repeatability. The accuracy provides a level of control in many processes that improves the product and the yield. Thermal mass technology is low maintenance because there are no moving parts, which reduces the need for spares or plant down time and results in a lower total costs.

Studies performed on the Frothmaster system by Peter Bourke and Seppo Ronkainen, of Outokumpu Technologies, evaluated the efficiency of controlling the frothing speed setpoint, which can control the concentrate grade. Conventional wisdom says if the concentrate grade is too low, then the frothing speed setpoint should be decreased; if the concentrate grade is too high, then the frothing speed setpoint should be increased. If the frothing speed is too high, then decrease the airflow, the pulp levels, or the frother dosage; if the frothing speed is too low, then increase the airflow, the pulp levels, or the frother dosage.

For the desired concentrate grade, the Frothmaster system determines and adjusts the speed setpoints to their optimum levels. The results of the study showed improved yields and reduced reagent costs. The reagent consumption rate was down 7.1%, and the cost of increased instrumentation was recovered in less than a month.

In the harsh environment of mineral processing, where rugged conditions combine with strict process control and safety requirements, thermal mass flow meters have a long established record of performance and reliability. The flow meters rely on thermal dispersion sensing, or heat loss, of a sensor in a flow stream to measure mass flow directly and can be used for measuring gas flow in large pipes, ducts, or stacks.

Fluid Components' FlexMASSter ST98 flow meter can be used in other process industries including petrochemical, power generation, wastewater treatment, food, pharmaceuticals, and pulp and paper. Typical uses include monitoring airflow, natural gas, hydrocarbon gases, biogas from digesters and landfill, airflow balancing, nitrogen blanketing, compressed air, and other industrial gases. Suitable for rugged environments, the standard flow element operates within a wide range of temperature and pressure.
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Author:Okely, Andrew; Seal, Graham
Publication:E&MJ - Engineering & Mining Journal
Geographic Code:8AUST
Date:Oct 1, 2002
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