From its beginnings in the first decade of this century, flotation has gradually moved to a predominant role in mineral separation. There are several reasons for this, the continuing trend toward treatment of lower-grade and more finely disseminated ores being the major one and lately the re-treatment of tailings dam. Not only has this been true for sulphide ores, to which the flotation process was first applied commercially, but also for most non sulphide ores as well. A second factor has been the broad applicability of size to below 10mesh. Finally, more so than for any other separation process, flotation has almost no limitation in separating minerals.
Analysis of flotation is best approached through consideration of the overall process first and then of it mechanical components (Flotation Tanks + Mechanism). An ideal system will be a circuit in a steady state supplied at a constant rate with ore having constant initial properties, a fixed optimum size distribution, and its mineral surfaces pre-treated to yield optimum floatability. The feed would flow through the mechanical machines and circuits selected, designed and adjusted to give an optimum separation, resulting in a production of concentrate at a fixed rate and grade, and with an associated recovery which determined the overall metallurgical performances of the plant.
Real systems do not fulfill these ideal conditions, mainly because of the feed variation or disturbances. Before considering the disturbances to flotation specifically, it is important to emphasize the interlock between the grinding and the flotation circuit, not only with respect to particle size effects, but equally to flotation feed rate variations. The grinding circuit is usually designed to produce the optimum size distribution established in testing and given in the design criteria. When the product size alters from this optimum, control requires either changing feed tonnage to the circuit or changing product volume, with either causing changes in flotation feed rates.
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While grindability changes due to the variation in ore properties are disturbances to the griding circuit, they generate feed rate changes as disturbances to the flotation circuit. The variations in ore properties which affect flotation from those assumed in the design criteria must therefore necessarily include grindability changes.
This reflects important differences in flotation cell characteristics between the two processes. Grinding circuit are built and design with fixed total mill volumes and energy input, so the grinding intensity is not a controllable variable, instead grinding retention time is changes by variation of feed rates. In contrast, the flotation circuit is provided both with adjustable froth and pulp volume controls for variation of flotation intensity by aeration rate or hydrodynamic adjustment. Reagent levels and dosages provide a further means for intensity control.
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