2D Classification of Coal: Refining the Segregation using the 2D Classifier

The goal of this project was to measure the size and density segregation of -2 mm coal powder that had been processed two times by the CSIRO 2D rotational classifier.

Stage 1: The project commenced in early March 2007, when CSIRO received six 44 gallon drums of coal slurry from Bulga Coal. The slurry was dewatered, dried and crushed, whereupon approximately 5 kg of coal powder was placed into the 2D classifier. It was found that the dry coal powder was too sticky to segregate in the optimal fashion. This non-optimal segregation pattern was primarily due to the -100 micron particles which tend to bond due to Van der Waal or electrostatic forces.

It was subsequently found, that removal of the -100 micron component did produce better segregation, provided that the 2D classifier was vibrated. In this case, the vibration was produced by repeated, rapid application of a rubber mallet to the 2D classifier rig. This effect arises, because the vibration breaks the bridging bonds in the fine powder, which, in turn, allows the powder to flow. We are currently investigating building a combined vibration/rotational classifier test rig, which will apply controllable vibration to the classifier. Unfortunately, that rig is not yet ready, so we proceeded with the research project by removing more of the fine powder component from the coal powder.

The coal powder segregated in the appropriate fashion when some 60% of the -250 micron size fraction was removed from the powder. Both size and density separation were observed, with most of the - 1 mm size fraction in the centre of the bed, while the + 1 mm size fraction was in the outer regions of the bed. Ash enrichments of around 30% were observed in the centre of the bed, while ash depletions of around 10 to 20% were found in the outer bed boundary region.

The Stage 1 results suggested that the rotational classifier will readily classify dry coal provided that some of the

- 250 micron particles are removed from the crushed coal. Alternatively, if the - 250 micron component is left unchanged then the classifier may still successfully process the coal powder, but the classifier system will require external vibration to produce freely flowing powder within the machine.

Stage 2: The object of Stage 2 was to, effectively, run the coal powders twice through the classifier to determine whether there was an improvement in size and density segregation. We obtained an improvement in size segregation, relative to the Stage 1 results, with a nearly perfect cut at around 600 microns, where it should be noted that there is nothing special about this 600 micron point. The actual cut size can be varied by increasing the size of the classifier and by changing the position of the sampling points within the granular bed.

As with the Stage 1 results, density segregation occurred within the 2D classifier. Further density segregation was evident when material was transferred from the 2D classifier into the smaller Petri Dish Classifier. The maximum enrichment in ash was obtained when material was transferred from the inner region of the 2D classifier into the Petri Dish Classifier and then sampled from the inner region of the Petri Dish Classifier. In terms of ash mass percentages, this processing pathway changed the initial ash percentage from ~ 26% to a final value of ~35%. The greatest depletion of ash was obtained by transferring material from the outer core of the 2D classifier through to the outer core of the Petri Dish classifier. This processing pathway changed the initial ash percentage from ~ 26% to a final value of ~21%.

These results suggest that, at least for our coal powder sample, the ash content within the coal is aggregated with the coal. So, it is possible that the ash is liberated when the coal is crushed. To obtain the desired 10% or lower ash content in the coal powder, the rotational classifier will require narrower size samples of the coal powder, where the coal powder will have to be as small as possible. We have shown in Stage 1 of this investigation that -100 micron powders are sticky and do not segregate as they do not produce a continuous avalanching flow within the classifier. To remedy this situation, it will be necessary to limit the powder size to around 250 micron and/or to add a vibrational system to the classifier as a means of improving the flow of powder within the classifier.