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Coal Preparation

Enhanced Dewatering of CHPP Tailings Through Modification of the Structure of Pre-sedimented Flocs

Coal Preparation » Dewatering

Published: January 14Project Number: C21054

Get ReportAuthor: Philip Ofori, Clint McNally and Bruce Firth & Anh Nguyen | CSIRO & University of Queensland

The effective management and disposal of coal preparation process effluents containing colloidal particulate matter, particularly fine tailings with clay minerals are important to the Australian coal industry from both environmental and water recovery perspectives. Tailings management solutions that produce drier end product would reduce environmental impact and increase water recycling, hence reduce water consumption. The thickened tailings or paste thickening approach could be a cost-effective tailings management pathway for the coal industry if properly tuned to the characteristics of coal preparation plant tailings. The technology produces tailings thickened to a paste-like mass which can be stored on the surface by itself or by co-disposal with coarse reject.

A previous study on bench scale (ACARP Project C18039) showed that modification of the floc network structure could assist in additional sediment consolidation and densification. This approach allows flocculation to be optimised for fast settling and then with subsequent application of shear to the settled sediment, promote the additional removal of liquor to increase aggregate density. A dryer product will thus be produced while still maintaining fast settling rates and hence further reduce the footprint and rehabilitation time of tailings storage facilities, as well as increase process water recovery for reuse.

The aim of this project was to examine the effectiveness of applied shear in modifying the structure of a pre-settled floc network as a means of producing thickened tailings of superior geotechnical properties. Detailed experimental study of the application of shear to modify the structure of strongly flocculated concentrated floc aggregates was undertaken. Mechanical shear was applied to pre-settled floc aggregates that were in various stages of consolidation. The shearing action altered the structure of the aggregates and also caused the breakup and re-aggregation into more compact aggregates, releasing trapped inter and intra-aggregate liquor to produce a denser network structure.

The study conducted using a continuous pilot scale thickening system showed that shear modification of the floc network structure was effective in producing significant additional sediment consolidation resulting in denser underflow product for the two tailings samples studied. The extent of additional consolidation varied between the tailings samples tested and appear to depend on the characteristics of the tailings such as particle size distribution and particle surface potential in water. The underflow solids concentration increase of 9 percentage points was achieved for one tailings sample and 6 percentage points for the other. The design of the mechanical shearing mechanisms and the position of applied shear were found to be important. Early indications are that it is more effective when applied soon after aggregate formation. The optimum rate of applied shear depended on the nature of the tailings and was different for the two tailings samples. Cryogenic scanning microscopy confirmed that the shearing breaks the flocculated aggregates into smaller aggregates and also reduces the spaces between the particles and aggregates. The spacing between floc aggregates were also found to be larger in the unsheared product than the sheared product implying that the sheared underflow product is more compacted than the unsheared product.

The optimum shear rates and intensities and position of the shear field to produce maximum extent of densification were identified for the two tailings samples. Based on the knowledge and results acquired in this study, implementation of the concepts on a large scale thickener is warranted with the aim of providing further valuable information and confidence in the data to support technology adoption decisions. The benefits of this technology are the ability to generate a denser underflow product while maintaining or increasing the throughput of the thickener. It also helps to produce consistent thickener underflow product that has been of concern in the industry. The approach could significantly reduce the volume and increase the density of the waste slurry produced consistent with the industry's push to deliver a more environmentally sustainable tailings management and water use efficiency practices.

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