Coal Preparation » Environmental Improvement
The coal industry recognises the significant challenges associated with the handling of fines rejects and with stabilising tailings dams with a high proportion of clay materials. Also the availability and cost of water is seen as an emerging problem. The industry wants to have better solutions than are currently available.
The main objective of this project was to examine the potential of paste technology as an effective and sustainable approach to tailings handling and water recovery. An approach that is increasingly being adopted in hard rock mineral processing plants for tailings treatment is paste technology. The Centre for Applied Energy Research, University of Kentucky (UKCAER) has been supported by the US Department of Energy (DoE) over the last few years to assess paste technology as an alternative approach to tailing enclosures in coal mining.
A partnership between CSIRO and UKCAER was established to evaluate paste thickening technology as applied to typical Australian tailings materials. In applying the technology to Australian tailings, the nature of the tailings was a major factor and required some changes to the reagent regime used to produce the best surface chemistry outcome. A comprehensive characterisation of the selected tailings materials was undertaken.
The investigation included laboratory and small pilot scale studies of the flocculated fine refuse solids for producing highly-thickened solids capable of disposal as a paste. The program included flocculant screening, batch settling tests for optimisation of flocculant dosage and feed solids concentration. This was followed by small scale, semi-batch and continuous paste thickening experiments and rheology tests to determine yield stress as a function of percent suspended solids of settled tailings. Two tailings samples were examined in this study, one sample from the Bowen Basin referred to in this discussion as Tailings Sample A, and another sample from the Hunter Valley referred to as Tailing Sample B.
The results of the study showed that for Tailings Sample A, Deep Cone thickening will be effective in producing a paste containing 50% solids from feed material of 15% solids. The flocculant requirement would be 150 g/t of a high molecular weight anionic polymer. The paste would have a yield stress of about 160 Pa, which is suitable in terms of pumpability and disposal as a paste.
The other tailings sample (Tailings Sample B) was found to be anomalous with respect to percent solids and ash. It contained a high percent solids and a high proportion of coal (55%) and was more difficult to flocculate even with both anionic and cationic flocculants as a majority of clays stayed in suspension. Thickening studies showed that about 42% solids could be achieved with this material. Vacuum filtration of the flocculated material produced only 51% solids, signifying some difficulty in dewatering this sample. More intensive pre-treatment may be required for this material. If a paste is required, then a processing step to recover some of the coal from the tailings may be required. Slurry containing high ash particles generally produces better thickened tailings.