Coal Preparation » Fine Coal
The performance of flotation cells can be impaired by the occurrence of unfavourable hydrodynamic behaviours, such as channel flows, pulp circulation flows, dead volumes and excessive shear stress. In ACARP project C21051, hydrodynamic behaviours in both Microcel columns and Jameson cells were studied by using residence time distribution (RTD) tests combined with measurements of gas holdup distribution. Plant-scale investigations indicated that strong downward channel flows occurred along the axial direction in the Microcel columns studied, leading to large stagnant volumes of up to 70% of the total cell volume. In a plant-scale investigation of hydrodynamic behaviours in a large Jameson cell, it was found that the flow pattern was a type of perfect mixing (i.e. fast and intensive mixing) and a significant fraction of multiphase mixture exiting the downcomer could discharge from the tailings exit in a very short period of time. The intensive mixing could generate high shear stress, leading to the detachment of some particles from the particle-bubble aggregates, particularly for coarse and/or less hydrophobic particles.
The project objective was to improve the hydrodynamic behaviours of Microcel columns and Jameson cells, via:
· Designing and constructing simple and cost-effective modifications to reduce undesirable hydrodynamic behaviours in Microcel columns and Jameson cells;
· Carrying out plant-scale experiments to determine the effectiveness of modifications on Microcel columns performance; and
· Evaluating the performance of modification concepts for Jameson cells in pilot-scale tests.
Main Findings and Conclusions
The project has successfully completed the following tasks for improving the hydrodynamic behaviours of Microcel columns and Jameson cells:
· Development of simple and cost-effective modifications of slurry feed inlets and air sparger inlets to reduce undesirable hydrodynamic behaviours in Microcel columns;
· Assessment of the effectiveness of modified slurry feed inlets and two modifications of air sparger inlets in large scale Microcel columns;
· Performance evaluation of modified downcomer and aerosol addition of MIBC-water mixture for Jameson cells in pilot-scale tests.
The modification of air sparger inlets with extended length is a low-cost modification as only 6 of 12 original air sparger inlets are involved in the simple extension of length. Therefore, this cost-effective and reliable modification can be used without modifying the original slurry feed inlets to improve the performance of existing Microcel columns with a diameter of 4.8 metres and a height of 16 metres. It could be anticipated that the extended length in the range of 0.7 to 0.8 meters would give a further improvement. The use of air sparger inlets with extended length Microcel columns with a height of 9 meters would require further evaluation tests as the column height also plays an important role in determining the hydrodynamics behaviours of flotation columns.
The outputs from this project led to the following recommendations:
· More studies are required to develop an improved air sparging system which can provide sufficient mixing energy and at same time produce uniform gas holdup distribution in Microcel columns.
· Unbiased slurry feed and diesel collector distributions to parallel Microcel columns should be accomplished before evaluating any modification to improve flotation performance.
· Although tests on a 300mm Jameson cell have shown the potential of the modified downcomer, its performance needs to be thoroughly assessed against the unmodified downcomer on a scale-up cell under practical feed conditions at a mine site. Test data from these assessments is essential for industry to adopt this modification to the downcomer.
· Atomization with MIBC-water mixtures proved to mitigate the fire hazard by dilution and cooling in a pilot scale Jameson cell with a diameter of 300mm. It would be beneficial to examine the implementation methodology and effectiveness of this safe aerosol addition approach on a large scale Jameson cell at a mine site in reducing frother usage while maintaining or improving process performance.