Coal Preparation » Fine Coal
The objective of this project was to quantify the value proposition of applying the Reflux Flotation Cell (RFC) to identified applications, utilising a broad range of feeds supplied by producers. Laboratory scale RFCs were used to obtain the data. The potential for complete recovery of liberated fine coal and complete washing of the product was investigated, as a function of the volumetric throughput. The purpose of this work was to provide a basis for understanding the value proposition of applying this technology as a “bolt‐on” or green‐field solution across the industry.
The Reflux Flotation Cell (RFC) is a low CAPEX flotation technology that has emerged over the past five years as a result of ACARP funding. The technology is robust, utilising a concentrated bubbly zone rather than a froth. This state is made possible by the presence of a lower system of inclined channels which increases the segregation of the bubbles from the downwards tailings flow. The arrangement permits greatly elevated processing rates (5‐10 fold, hence reductions in capital by a factor of 5, and in principle rapid pay‐back of less than 6 months), with very high carrying capacity. Downwards fluidization through the concentrated bubbly zone delivers almost complete cleaning of the product. This zone exhibits strong permeability in contrast to the very low permeability of froths. Process control is made robust through the absence of the froth zone, achieved through set point control of the volumetric flow rates entering and exiting the cell.
Although the above features of the technology have been demonstrated, this project focusses on the broadest range of feed materials to date, using the broadest range of processing methods, and process conditions. The system was first established at a laboratory scale, was validated at pilot scale, and pending the success of the first full‐scale trial in 2020, the technology will be available relatively soon. Thus, given the technology can be accessed, the findings from this project can be exploited by the industry.
The study developed a practical approach to optimising the interrelation between grade, recovery and throughput. A relatively low volumetric feed flux of 1 cm/s is used in conjunction with relatively strong fluidization in order to achieve complete cleaning, and hence establish the lowest possible product ash. The overflow liquid flux is maintained to ensure a gas volume fraction of 0.8 is maintained in the overflow to limit coalescence and loss in recovery. Recovery is then determined as a function of the gas flux, the aim being to determine the minimum required gas flux. By using the minimum, it is then easier to clean the product, and increase the throughput. The wash water flux is set at the minimum required to reach the target product ash. Finally, having established the necessary feed to gas flux ratio, the throughput can be progressively increased with this ratio preserved. While this systematic approach may appear long‐winded, it is also possible to operate using the typical values generated via this study, in a turn‐key fashion.
Often the sample quantity available for testing is limited. For this reason, a semi‐batch approach, requiring as little as 1 kg of feed, was established in order to quantify the potential separation performance of the RFC. The grade‐recovery curve produced from the RFC semibatch flotation method demonstrated an improved separation compared to the tree flotation method. Although the performance was not as good as for the continuous steady state method, the approach did perform better than that of the tree‐curve.
The introduction of robust flotation offering almost complete recovery and cleaning of the product at elevated throughput can deliver a multitude of benefits. Improved tailings management and water savings are expected with the increased in yield, reaching 50 % yield from some tailings feeds.