The coal floatation program’s introduction into the fine coal processing circuit at Stratford was brought about by the CHPP management team seeking ways to increase the yield to coking coal.
The first step, the introduction of a synthetic collector, proved to be a profitable decision that was then followed by the second and final step – the introduction of a custom-blended frother reagent.
The upgraded fines flotation circuit gave Stratford a boost in production (from 550 tonnes/hour to 610 tonnes/hour and even as high as 620 tonnes/hour available with favourable product) and encouraged further investigations as to how more output could be achieved.
Trial of new collector Part of the research included investigations into the chemical compounds being used in the processing circuits and looking at some alternatives.
Early in 2002, Stratford commenced a 12-month trial of the Nalco collector.
Throughout the trial, additive amounts were varied in an effort to find the optimum production levels. Ultimately, the mine made the decision to discard all product less than 38 microns and concentrate its efforts on larger particle sizes.
Described as highly selective, the synthetic collector used at Stratford is a blend of organic compounds that have proven to be effective in the separation of coal from any associated ash ingredients.
The manufacturer claims the collector’s active ingredients make it appropriate for use over a wide particle size range and particularly effective for coarse coal.
According to CHPP superintendent Tony James, the introduction of the new synthetic collector has meant some profitable benefits for the Stratford operation.
The average cake thickness off the end of the belt filter has increased from 12-15mm to 25-35mm – a 100% increase while maintaining acceptable moisture levels.
Using the compound, Stratford can now float the +125 micron coal that previous collector mediums had been unable to achieve. At Stratford, +125 micron coal has low ash content and can actually lower the overall product ash, allowing for increased tonnage from the dense medium circuit.
“And, with the change from the diesel collector to this new organic compound, working conditions inside the plant have improved markedly,” he said.
Frother compound introduction Part 2 of the flotation program was the introduction of the frother compound – a move that has continued the production and environmental improvements established by the collector.
Filter cake output has increased again and Stratford is now achieving an average cake thickness of 40 mm off the end of the belt – a further 14% increase since the frother and collector started working in tandem.
“It’s worth noting that those new figures have been achieved and maintained even with a reduction in the input feed tonnage to the fines processing circuit,” said Tony James.
“We have dropped the input feed back from 600 to 450 tph and still achieved improved yield figures – improvements that can be directly attributed to the flotation program.”
He said that the properties of the new frother allowed for increases in dosage, when required, without the attendant problems of foaming, pump cavitation, output drops and possible plant shutdown.
Stratford had experienced with other frothers. “We can now confidently increase the frother dosage level – which increases yield level – without suffering any residual problems throughout the plant,” he said.
The new frother, especially developed for use in flotation cells, produces smaller bubbles than other alcohol-based frothers. The manufacturer claims this smaller bubble size improves the coal/ash separation process and increases coal recovery by giving the fine coal more chances to attach to a rising bubble.
Oxidised coal Tony James said the coal flotation program had also achieved another significant breakthrough.
For the first time, the Stratford CHPP can float oxidised coal. “Oxidised coal is not a big part of our output but is, nonetheless, a vital part of the mine’s total output,” he said.
“By making the necessary adjustments to the dosage levels of the collector and frother, we can successfully process the oxidised coal through the fines circuit and significantly increase total mine output.”
While oxidised coal is not a commercial issue, the fact that the material was able to be processed showed the ability of the chemicals to be adapted to suit varying coal quality.
He added that Nalco had solved a separate problem associated with the oxidised coal. “Floating the oxidised coal reduces the loading on the (overloads) thickener. This overload, when coupled with the acidic properties of the coal, meant the flocculant wasn’t working as designed.”
“The company supplied us with a low pH flocculant that is very effective with oxidised coal.”
As far as Tony James is concerned, the flotation program has been a sound investment with real long-term corporate and personnel benefits.
He said the operation of the flotation and associated equipment in the prep plant has improved noticeably and the replacement of the diesel, used previously as a collector, has made an obvious difference to the air quality.
In addition, the higher flash points on the new collector and frother compounds have significantly reduced the risk of flotation related fires.
“The impact of the flotation program is translating into a better result where it counts the most – in our output tonnages,” said Tony James.