From its beginnings in the first decade of this century, flotation has gradually moved to a predominant role in mineral separation. Alain Kabemba, flotation process specialist at Delkor notes the major trend to treating lower- grade and more finely disseminated ores and lately the re-treatment of tailings. He also points to the broad applicability of size to below 10 μm.
Real systems do not fulfill ideal conditions, mainly because of feed variation or disturbances. "Before considering disturbances to flotation specifically," Kabemba says. "It is important to emphasise the interlock between grinding and flotation, 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.
"While grindability changes due to the variation in ore properties are disturbances to the grinding 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 machine characteristics between the two processes. Grinding circuits are built and designed with fixed total mill volumes and energy input, so the grinding intensity is not a controllable variable, instead grinding retention time is changed by variation of feed rates. In contrast, the flotation circuit is provided both with adjustable froth and pulp volume for variation of flotation intensity by aeration rate or hydrodynamic adjustment. Reagent levels and dosages provide a further means for intensity control."
One recent trend has been towards larger, metallurgical efficient and more cost effective machines. These depart from the simpler tank/mechanism combination towards design which segregates and directs flow and towards providing an external air supply for types which had been self aerating and towards the application of hydrodynamic principles to cell design, like the Delkor BQR range of flotation machines, initially the Bateman BQR Float Cells.
Bateman has steadily developed the BQR flotation cells which have been in application for the past 30 years, and with its acquisition of Delkor in 2008, decided to rebrand the equipment into the Delkor equipment range. Kabemba explains that BQR cell capacities range from 0.5 to 150 m3 currently installed and can be used in any application as roughers, scavengers and in cleaning and re- cleaning circuits.
The main objectives of the BQR design were to achieve the following core hydrodynamic functions:
Provide good contact between solid particles and air bubbles
Maintain a stable froth/pulp interface
Adequately suspend the solid particles in the slurry
Provide sufficient froth removal capacity
Provide adequate retention time to allow the desired recovery of valuable constituent. This led to the following benefits
Highest possible effective volume and reduced the froth travel distance
Improved metallurgical performances in terms of grade recovery and reduced capital and operating costs based on reduced fabrication material and ease of maintenance. Kabemba says: "there are not many differences in terms of design between BQR Flotation cells; however, from the BQR1000 upwards, the flotation cells have internal launders to maintain the design objectives and benefits highlighted.
"Operating variables, such as impeller speed, air rate, pulp and froth depths have to be adjustable over a sufficient range to provide optimum results with a given ore, grind and chemical treatment, but adjustment should not extend beyond the hydrodynamic regime in which good flotation is possible."
In the near future the BQR2000 (200 m3) and BQR3000 (300 m3) will be available to the market.Kabemba also explained: "Circular cells reduce the amount of dead volume when compared to square cells. This enables a much higher effective pulp volume, hence increasing the effective energy input into the flotation cell. In addition ‘tank type’ cells offer enhanced froth removal due to the uniform shape of the circular launders."
He concluded that "fully automated flotation cells are becoming more and more common with the aid of smart control and advances in software in the marketplace."