Coal Preparation » Fine Coal
The objective of this project was to provide a simple retrofittable device to enhance the flotation behaviour of poorly floating coals. A device capable of inducing cavitation in flotation feed generates very small "picobubbles" on the surface of coal particles, which have a greater affinity for attaching to larger bubbles than the coal surface itself, ie they act as a collector.
Various designs of cavitation device were assessed to determine which was the most applicable for our operation. A venturi design was identified as the best option, with its critical features being:
- the ability to set the throat length, and
- the ability to control the pressure recovery after cavitation.
A unit was designed and fabricated that matched the process capacity of the pilot scale mechanical flotation unit located at CSIRO's Catherine Hill Bay facility. A comprehensive test program using Catherine Hill Bay flotation feed with and without cavitation was undertaken. Variables were monitored, and a statistically based test program was undertaken to define critical variables and their possible interaction.
The cavitation device was also operated with a pilot scale Jameson cell and a Column flotation cell.
The results with mechanical cells showed that the use of a cavitation unit increased the yield of flotation product, while reducing the diesel collector dose by up to 50%. The statistical analysis of test runs showed that the collector dosing and use of cavitation unit were significant factors with respect to combustible recovery, while collector dose and feed rate were significant factors with respect to product ash. This analysis also showed that there was no interaction between factors, such as collector dose and cavitation.
The results for column flotation showed that increased combustible recovery can be achieved with use of cavitation when operated at low chemical dosing of collector compared to conventional high dose operations. However, our results also showed an increase in product ash for cavitated runs. The reason for this is unclear at this present time.
The results for tests with the Jameson cell were not encouraging. Cavitation actually decreased combustible recovery compared to conventional operation. It is thought that the use of a cavitation plate at the top of the downcomer may diminish the effectiveness of picobubbles formed in the cavitation device placed prior to the cavitation plate.
The effective use of a cavitation device at pilot scale has been demonstrated, and the results suggest that it would work at a larger scale. The issues facing installation of a cavitation unit into a flotation feed stream are mainly engineering concerns, such as where to place the device, is an extra sump and pump needed to ensure correct flow condition for the device, and can they be housed close to the flotation cells.
The results show that cavitation would have its most significant effect in reducing the use of collector. With a cost of collector such as diesel at 70 cents/litre, a 50% reduction in diesel use could save approximately $350,000 per annum for a plant processing 5 million tonnes per annum with 20% reporting to flotation.
Testing of candidate coals can now be done routinely with CSIRO's test facility at QCAT Brisbane, so selection of coals responsive to the technology would be a relatively simple process.