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Coal Preparation

Tailings Management Through Control Of Colloidal Interactions

Coal Preparation » Environmental Improvement

Published: July 11Project Number: C18039

Get ReportAuthor: Philip Ofori, Anh V Nguyen, Bruce Firth, Clint McNally | CSIRO Energy Technology & School of Chemical Engineering University of Queensland

Handling fines rejects, stabilising tailings dams with a high proportion of clay materials and the emerging problem of availability, quality and cost of water present significant challenges to the coal industry. Effective solutions for these challenges are needed and require an improved understanding of surface interactions forces between tailings particles which control the settling and consolidation of flocculated coal preparation plant (CPP) tailings. The thickened tailings approach provides a possible pathway for addressing some of the issues concerning tailings dam management, rehabilitation and water recovery. This project was undertaken to:

· Provide insights into the surface interaction forces between particles during coagulation, flocculation, settling and bed consolidation of coal preparation tailings in intensive thickening processes;

· Correlate the understanding of surface interaction forces with particle and floc settling rates and degree of bed consolidation during sedimentation; and

· Develop approaches to enhance and optimise CPP tailings thickening for demonstration at a pilot scale in a subsequent project.

Four samples from four coal preparation plants in the Bowen Basin and Hunter Valley, together with over 20 flocculant samples from three major international suppliers were obtained, characterised and examined in this project.

The flocculation tests were conducted under reproducible hydrodynamic conditions in batch mode. Flocculants were found that were effective in flocculating each of the four tailings samples and optimum dosages were identified with respect to settling rates.  Hydrodynamic conditions were found to have profound impact on floc settling rates and to a lesser extent sediment consolidation. Low mixing speeds produced large flocs with high settling rates, while very high mixing speed during flocculation resulted in irreversible floc degradation leading to much reduced settling rates with some additional sediment consolidation. The surface interaction forces between the coal tailings surfaces were measured and statistically characterised using atomic force microscopy (AFM). The AFM results showed that there was an optimum flocculant concentration for each tailings sample which produced the highest adhesive strength of surface interaction forces needed to resist floc breakage by external forces. However, the optimum concentrations that produced the highest adhesive forces differed from those required to produce optimum settling rates. The concentrations required to produce the highest adhesive forces in the AFM were much lower than those required to produce the highest settling rates in the sedimentation tests due to the large surface area of particles in the concentrated suspensions used in the settling tests compared just a few particles used in the atomic force measurements. Fractal analysis by laser scattering to examine the structures of flocculated aggregates showed that there were optimum mixing speeds and flocculant dosages to produce flocs with compact structure of fractal dimension greater than 2.

Application of shear to pre-sedimented flocs resulted in significant additional sediment consolidation. In this approach, flocculation and settling were partially decoupled from consolidation, allowing flocculation to be optimised for fast settling. Subsequent application of shear to the settled sediment aided in the removal of inter and intra-floc liquor to improve consolidation and sediment density. While the approach was simple to implement in batch mode, its implementation in a continuous system without restabilising the entire suspension could be a challenge. It is recommended that this approach be tested on a continuous pilot scale system to provide the basis for large scale testing.

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