Ian Wark Research Institute
ARC Special Research Centre
ARC Special Research Centre
Research Area: Chemical engineering, physical chemistry, interface science and surface chemistry
Degree: Honours
Supervisor: A/Prof Daniel Fornasiero
Description: Australia is the world's biggest coal exporter and coal, a key energy mineral, is Australia's largest export. The industry partner in this project, BM Alliance Coal Operations Pty Ltd (BMA) is a major producer of coking and steaming coal with more than one third of Australia's annual coal exports. BMA has seven mines that are located in the coal-rich Bowen basin in Queensland, with coal reserves estimated at more than 25,000 million tonnes.
BMA has reported relatively large variations in their coal flotation performance in and between the BMA plants. More importantly, intermittent poor coal quality causes major disruption in some plant flotation circuits, resulting in very low coal flotation performance.
Coal preparation or the cleaning of coal involves its selective separation from mineral matter. The latter is composed mainly of clay minerals, but pyrite is often present. While coal particles larger than 0.5 mm in diameter are treated by gravity separation methods, smaller coal particles, the topic of this project, are cleaned by flotation.
In froth flotation, air bubbles generated at the bottom of the flotation cells collide with particles in the pulp solution and rise with the attached particles to the froth at the top of the flotation cell, where the particles are then recovered. The selective separation of particles by flotation relies on the exploitation of sometimes quite subtle differences in hydrophobicity between the surfaces of coal and mineral particles, because normally only hydrophobic particles become attached to bubbles after their collision. The hydrophobicity of the particle, and hence the stability of the particle-bubble aggregate, has to be relatively strong, for turbulence in the stirred flotation cell may cause detachment. In flotation, the particle-bubble collision, attachment and detachment processes are all particle (and bubble) size and density dependent, resulting in the characteristic experimental flotation recovery (or flotation rate constant) versus particle size curve, with a maximum appearing at an intermediate particle diameter. As part of a major research effort dealing with the fundamental physics and chemistry of multiple bubble-particle collisions, within its ARC Special Research Centre program, the Chief Investigators have developed a robust, analytical rate constant prediction model. In the first application of this SRC work in a parallel applied investigation, the model has now been successfully applied to the flotation of chalcopyrite particles in a copper ore, under the turbulent conditions that exist within a Rushton turbine cell. This model predicts that the lower flotation of fine and coarse particles results mainly from their lower collision efficiency and lower stability efficiency (in the high turbulence of the flotation cell), respectively, with bubbles. This maximum in flotation shifts to larger or smaller particle sizes for minerals of lower or higher density.
This model has not been applied to coal flotation. Coal and the environments in which it is processed possess their own special properties, presenting a major challenge that we now address.
Generally, coal is naturally hydrophobic whilst most of the clay minerals present with coal, such as kaolinite, montmorillonite and illite, are hydrophilic. In practice, this, however, does not translate necessarily into high coal flotation and good rejection of clay minerals because coal may oxidise (or weather) and clay may compromise part of the coal particle as interstitial or slime coatings, which both result in decreased overall hydrophobicity. High clay (or ash) content in the flotation concentrate is generally due to clay being locked with coal in coarse particles and to fine clay particles reporting to the concentrate by entrainment. The proportion of clay increases with decreasing particle size, because of its friable nature. Better control of oil dispersion and interactions between oil droplet and coal particles has resulted in improved coal recovery and selectivity, and reduced oil consumption. In the presence of clay slime coatings, dispersants such as phosphate ions have been added to improve coal flotation. Selectivity is again the issue, for the dispersant should target the clay, rather than the coal particles.
Water quality has also been reported to influence coal flotation with the presence of multivalent ions such as Ca, Fe and Al or humic acid decreasing coal flotation performance.
There have been several attempts to predict coal flotation by relating coal hydrophobicity to coal characteristics such as its rank (carbon content), constituents (macerals) and ash content. The best, if rather old, prediction arises from the floatability index based on the proportion of floatable and non-floatable components present in the coal particles (surface component hypothesis). These predictions were limited by the non-availability at the time of surface spectroscopic techniques, and therefore it was assumed in these calculations that the surface species and their proportion were the same as those in the bulk.
Using a combination of surface chemistry, physics and process modelling, the aims are to:
- Establish benchmarks for optimising the flotation performance for selected coals;
- Determine the factors controlling the flotation performance of selected coals;
- Establish strategies to combat poor flotation and improve performance for problem flotation streams.
The research will expose students to a combination of surface chemistry, bubble-particle physics and process modelling. Graduates will be very well prepared for a PhD in any environment where interfaces are important.
