Coal Preparation » Dewatering
The objective of the study was to undertake a review of tailings dewatering technologies currently used in Australia and Overseas and technologies with the potential to improve tailings dewatering either from a quality (moisture and handleability) or commercial (costs per tonne of tailings) perspective.
Findings: Current practise at Australian coal preparation plants is to thicken fine coal tailings with high-rate thickeners utilising high molecular weight polymeric flocculants. The thickener underflow is either dewatered further or disposed of. Disposal is either in fine tailings ponds or together with coarse reject by co-disposal. Commonly belt press filters are used if thickener underflow has to be dewatered further. Decanter centrifuges are no longer common, while membrane filter presses have just been introduced at Dartbrook due to the high clay content and the need to achieve consistent dry cakes for co-disposal.
Overseas, belt press filter and filter presses are utilised for coal tailings thickener underflow dewatering. Filter presses achieve lower cake moisture, have higher installation costs while the operating costs are higher for belt press filters due to the partly extensive requirement for flocculants. Only few installations of decanter centrifuges are known. However, the centripress, an advanced centrifuge achieves lower cake moisture compared to conventional decanter centrifuges.
Novel thickening technologies, such as deep bed or paste thickening (Deep ConeTM, Supaflo® and AKASET) and rake-less thickeners (Ultrasep, E-CatTM and others) are used in other industries and have good potential to replace conventional thickeners, if paste and/or high thickener underflow concentration is required. Rake-less thickeners have specific internals to improve floc formation, dilution and clarification. Deep bed or paste thickening is basically thickening with a high sediment bed to provide solids pressure for further compression and consolidation. Thus, improved control and higher rake torque are common in thickeners used for paste thickening. The height of deep bed thickeners is also extended to enable the required bed height. However, paste pumping requires significantly higher power and costs. Deep Cone technology will be tested on pilot scale in the US in 2005 for coal tailings. Ultrasep technology is used in Australia in other industries already.
Novel technologies which have potential for coal tailings dewatering include steel belt filter, geotextile dewatering, quick drying beds and the Kalgoorlie filter pipe: The steel belt filter utilises vacuum and mechanical pressure for dewatering and could be added with infra-red or other thermal drying technologies. Pilot scale trials on coal tailings were performed recently overseas. Geotextile dewatering utilise big filter tubes, filled and inflated by the slurry fed under pressure. Gravity drives the batch process which is very time intensive. This simple technology is currently tested and has potential for intermediate or special requirements. Quick drying beds are special drainage systems with air drying, which are introduced for biosolids. The Kalgoorlie filter pipe is a cross-flow filtration process in-situ of pipes during pumping. This technology provides thickening during pumping over distances and is tested in Australia for other tailings.
Indicative costs ($/tonne) are given for equipment in use and where cost data were available from supplier, users or from literature. A comparison of performance (cake moisture) versus throughput specific costs for the different technologies is given. Thickening has the lowest cost with highest product moisture. Deep bed thickening generates higher solids underflow at additional costs. Belt presses provide intermediate cake moisture at medium costs while membrane filter presses achieve lowest cake moisture but have highest costs, mainly through their high installation costs. Centrifuges and other technologies do not show a cost advantage compared to belt press filter, which achieve at least similar cake dryness. However, site specific and material specific factors might change those general assumptions. Major operating costs for most equipment, excluding filter presses for some coal tailings, is flocculant consumption.
Properties of coal tailings are discussed briefly with focus on handling, transport and consolidation. The impact of tailings characteristics, extracted from NERDDC Project C1295, on dewatering performance is summarises. The compressibility and its relevance to dewatering, based on compressive yield stress, is discussed together with the important effect of surface chemistry on clay containing coal tailings.
Commercial examples of dewatering technologies with performance and, if available, cost data is given for coal tailings in Australia and overseas as well as related industries. Performance data for technologies used in Australian plants are summarised including typical examples, historical data and details on the new membrane filter presses on coal tailings at Dartbrook. Extensive cost and cake moisture data is available for decanter, centripress, belt press and chamber filter press operation in Germany, which is presented. Costs for belt press filter and decanter are similar, while belt presses achieve lower moisture. Centripress moisture is similar to that of belt press product, however, at higher costs. Centipress costs are similar to costs of chamber filter presses, latter achieving lowest cake moisture. Published cost data from the US show lower costs for either belt press or chamber filter press depending on tonnage treated. Tailings dewatering in other countries is discussed to provide an overview and further performance and cost information for relevant technologies.
Technologies used in other industries -relevant for coal tailings- discussed are red mud thickening by deep bed and drum filtration in the alumina industry as well as various other tailings and clay-type or similar dewatering applications (phosphatic clay tailings dewatering, harbour dredge sludge, potash clay and kaolin).
Novel technologies currently under investigation or in R&D stage for coal tailings or used in other industries, are described to give initial criteria for evaluating suitability. A summary of current R&D relevant for coal tailings dewatering is given, including:
- Tailings handling by binder stabilisation, electro-dewatering and freeze-thawing
- Improved thickener technology, in particular AMIRA P266.
- Tailings thickening using dual polymer, paste thickening, novel hyperconcentrators and small diameter hydrocyclones.
- Combined dewatering (mechanical plus other drying mechanism, including steam, infra-red, heat, vibrating and microwave) and other novel technologies.
The report is concluded with a listing of R&D groups active in coal tailings dewatering related areas in Australia and overseas.