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Coal Preparation

Improving Solids Recovery and Moisture Reduction in Ultrafine Coal Dewatering

Coal Preparation » Dewatering

Published: August 20Project Number: C25018

Get ReportAuthor: Changzhi Bai, Hangil Park, Liguang Wang | University of Queensland

This report provides detailed information on fine coal dewatering in the screen bowl centrifuge (SBC). SBCs are used in some coal preparation plants in Australia to dewater the flotation concentrate, and the process often faces the problem of low solids recovery (e.g., 70%). This represents a loss of valuable fresh fine coal to effluents, loss of profit for Australian coal producers and the creation of a potential environmental liability with increased burden to tailings disposal.

The project aimed to improve ultrafine coal dewatering performance by enlarging particle size and focused on using chemical reagents to aggregate coal particles to improve the solids recovery of SBCs. It examined the potential of the chemical reagents for dewatering particles of different surface properties and size ranges.

A rapid chemical screening method using two benchtop centrifuges in series was developed and subsequently verified by pilot-scale SBC trials. The benchtop centrifuges can separately mimic the bowl section and screen section of a full-scale SBC. This newly developed chemical screening method is able to test reagents in a simple and fast way to greatly shorten the period of reagent trials.

With the benchtop centrifuges, 29 reagents were tested for their effectiveness in improving the solids recovery or reducing the product moisture content. Four batches of coal slurry samples collected from two mine sites were tested. The results suggested that:

  • Adding flocculants in the SBC feed can improve the solids recovery, especially when the feed contains a large proportion of ultrafine particles;
  • Anionic flocculants generally outperformed cationic and non-ionic flocculants;
  • Surfactants or inorganic salts were less effective in solids capture despite the fact that they could lower the product moisture content;
  • A given reagent would exhibit different levels of effectiveness for different SBC feeds, suggesting that the chemical scheme for a given site should be optimised based on its current feed properties to maximise and maintain the solids recovery.

The effect of the addition of coarse coals on the solids capture of the SBC operation was also investigated. The results indicated that adding more coarse particles in the feed would be beneficial as it enabled improvement of the solids recovery and reduction of the product moisture at the same time.

Recognising that the performance of an SBC is affected by multiple operating variables (e.g., feed volumetric flowrate, feed particle size distribution and solids content, G-force, particle residence time and pool depth) and that experimental work on centrifugal dewatering at pilot-scale or full-scale is difficult to carry out, a physical model was developed to predict the solids recovery of a screen bowl centrifuge. The filtrate stream out of the screen section often contains valuable coal and is recycled back to the feed inlet of the SBC, so the model focused on solids capture within the bowl section of the screen bowl centrifuge. The model was used to simulate the centrifugal dewatering with varying operational conditions, such as G-force, feed volumetric flowrate and feed solids content, and there was an excellent agreement between model predictions and experimental observations. The developed model has no adjustable parameters. It provides an advanced simulation tool that helps a fine coal dewatering centrifuge with improving the operational performance. The tool can be used to predict the solids recovery and cut size of full-scale screen bowl centrifuges with input parameters provided by the coal preparation plants. It can also provide guide to design, procurement, and flowsheeting of full-scale centrifuges.

The results of pilot-scale SBC trials confirmed that a higher solids loading would lead to a lower solids recovery, which lent support to the findings that were obtained using the newly developed physical model. Meanwhile, the performance of selected chemicals in the pilot-scale trials showed a trend similar to that of the benchtop tests, which validated the appropriateness of the newly developed benchtop experimental setup to mimic the dewatering operation of a full-scale SBC. Further analysis of the particle size distributions of the material streams found that the cut size of the SBC operation could be reduced by the addition of reagents.

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