Better Classifying Cyclones

Despite classifying cyclones being important coal preparation plant workhorses that effect fine coal particle mixture component separations, project C27012 did not identify any near term potential technological improvements on the horizon.

The Rong Classifying Cyclone Concept (RCCC) of the late 1990s/early 2000s (Rong and Napier-Munn, 2000) did however show significant improvement promise at the 100 mm diameter scale. At the time, this smart breakthrough was considered further, firstly again at 100 mm diameter pilot scale followed by similar pilot scale work with 200 mm diameter units. Testing then progressed with 480 mm diameter units at pilot and industrial (at Dartbrook CHPP) scales, before a final trial of 375 mm units at Integra CHPP. Unfortunately, the results from all this follow-up work were not sufficiently compelling for the RCCC to be progressed by the Australian Coal Preparation Industry at that time.  Instead, there was contemporaneous industry-wide acceptance and adoption of the alternative larger diameter classifying cyclone concept (LDCCC), proposed by CSIRO. Over the ensuing circa 20-year period, it became evident that desired small size cuts were difficult to achieve with the LDCCC. Most modern plants reverted to the use of clusters of conventional small diameter classifying cyclone (CSDCC) units.

The inherent design of such CSDCC units has not radically departed from those available prior to the introduction of the LDCCC units. Consequently, in light of its initial promise, it is considered timely to revisit the RCCC design and reconsider the potential benefits that it could bring to Australian coal preparation plants.

An initial cursory look concluded that the RCCC design retained its initial considerable merit and therefore the reason(s) for its failure to receive widespread industry acceptance must be established and rectified. On that basis, this short three-month paper-based study was undertaken. Its objective was to identify any technological reasons for the previous failure and, if warranted, suggest a way forward to provide credible evidence to the Australian coal preparation industry why the RCCC should be reconsidered.

This investigation identified that:

• The RCCC represents a significant advancement in classifying cyclone technology because of a number of features which These appear conceptually to have combined to reduce short circuiting, increase separating forces and induce superior internal slurry streamline flows.

• The original testing successfully demonstrated the concept at the 100 and 200 mm diameter pilot scales, but failed to impress during industrial testing at 480 mm and 375 mm scales.

• No inherent issues with the RCCC design were apparent, but significant configuration compromises, necessitated to attempt side-by-side industrial comparisons, may have negated fair RCCC performance from ensuing.

• A 600 mm diameter RCCC had been tested in a mineral processing plant in South Africa at feed pressures ranging from 60 kPa to 100 kPa. It was found that the best separation for the RCCC was achieved at a feed pressure between 70 kPa to 80 kPa. This finding was exactly the same as that derived for the coal-based study discussed in 3 (d) above. Consequently, for best efficiency, the RCCC should be operated at a relatively low feed pressure, such as 70-80 kPa.

• Severe project budgetary constraints prevented contemporaneous remedy of these situations and it is clear that significant short comings in the industrial scale testing program design and execution were major contributors to the less than satisfactory performance findings.

• Whilst the combined effects of these shortcomings appear to have condemned the RCCC technology to premature industrial rejection, fresh objective interrogation of the metallurgical results reveals significant promise remains.

• There were also significant non-technical contemporaneous deleterious issues prevailing that are considered major contributors to the lack of industry follow-up at that time.
• The relevant patents and other publications appear to provide all the relevant information to replicate the initial work.

On the basis of these findings, it is concluded that the RCCC retains its considerable merit, and that small changes to the way it is tested will likely lead to a successful industrial demonstration. It is also concluded that such work represents an excellent opportunity to resurrect the technology, capture the value from previous projects and suitably elevate the RCCC profile to the major betterment to the Australian coal preparation industry.

Moreover, that betterment to the Australian coal industry, in monetary terms, is much higher now than it was when the RCCC concept was originally conceived (turn of twenty-first century). This is because contemporary plant designs almost ubiquitously include three size splits with DMC, hindered bed settling devices and flotation circuits. Consequently, there are thousands, if not tens of thousands of individual classifying cyclones in the Australian coal preparation industry.

These are primarily used as an enabling preparation tool to facilitate efficient downstream processing in size- specific beneficiation units, whose sole purpose is to efficiently process those size fractions that report to classifying cyclone underflow (sands) and overflow (slimes). To function optimally, units such as spirals, teetered bed separators, reflux classifiers and the like, require a feed material that is devoid of contaminating slimes. Similarly, to function correctly, slimes-sized particle processes, for example conventional flotation machines, Jameson cells, Microcels and the like, require a feed material that is devoid of sands and other coarse size fractions.

It is apparent therefore that efficiency increases in classifying cyclone operation do not unto themselves lead to any potential plant yield increases, but they do facilitate commensurate yield increases from better performance from the downstream units.

The Australian coal industry has tended, with some success, to mitigate these losses by extending the effective size range that can be separated by the respective downstream processes. However, getting the size separation done correctly in the first place is always more effective.

Combined with the facts that more and more tonnes are processed annually and currently, coal prices are very high, the betterment to the industry of improved classifying cyclone efficiencies and performance is huge.