Coal Preparation » General
The objective of the study was to undertake a review of seal, spray and wash water filtration technologies currently used in Australia and overseas and technologies with the potential to improve process water quality from a solids content perspective. This is relevant for water used for filter cloth spray water and other nozzle protection duties, froth wash and TBS up‐flow water, and pump seal water including seal water for liquid ring vacuum pumps.
Current common practice at Australian coal preparation plants is to either not filter those water streams at all or use one of a few filtration technologies, with strainers and semi or fully automatic backwash filters being most common. Fully automatic forced backwash filters with nozzles, suction scanners and/or mechanical screen cleaning are most suitable and cost effective for solids removal from process water for seal water or nozzle protection where a separation size of 50 - 500 μm is sufficient. Media filters, in particular the continuous sand filter (CWUF / Dynasand®) and the Fiber Media (FW) filter are higher in capex but separate down to smaller particle size and thus, should be considered if complete solids removal is required for example if process water contains mineral matter which otherwise is introduced into the product such as for floc makeup, flotation froth wash or TBS upwards water. However, media filters should be fed with < 200 mg/l solids water and thus, coarse screen filters might be required for pre‐separation if water solids content is significantly higher.
Process water within coal preparation plants requires certain water quality depending on the usage and detailed equipment requirements. The water quality and size separation is specific to the particular usage and cannot be generalised. Even a particular duty, such as filter cloth spray nozzles on HVBFs requires drastically different particle cut size depending on the type and aperture of nozzles used. However, the size range from 10 to 500 μm is being the focus for this study.
Methods and equipment for removal of suspended solids from process water is reviewed. Standard and common technologies, such as strainers or automatic screen backwashing filters are extensively used while niche or novel technologies, such as the FW special (fiber) media filter, the Baleen filter or pleated disc filters are recently introduced with limited installations.
Certain technologies, such as flotation, gravity clarification and to a certain extent centrifugal separation are less suitable for process water due to either limited separation (for example of low density material using gravity separation) or low suitability for low solids water duties. They are described in less detail compared to more suitable technologies and their limitations explained.
Automatic forced backflush screen filters with 'proper' screen cleaning using brushes and/or suction nozzles or 'scanners' with sequential, high velocity dirt removal are suitable from 50 μm upwards and most suited for 200 - 1,000 μm screen apertures. They are low cost equipment with limited maintenance requirements. Nevertheless, installations in coal prep plants were partly unsuccessful, most likely due to installation, commissioning or operational shortcomings, as lack of maintenance or improper backflush results in operational problems. They are mostly limited to non‐sticking, non‐organic and relatively coarse (>> 25 μm) dirt.
Extensive design variations exist for backflush filters, so that equipment selection has to consider a detailed review with particular emphasis on robustness and effective cleaning mechanisms with brushes and/or high velocity suction nozzles cleaning the screen sequential.
Above backflush filters have fixed screens, while variable screens that 'open up' during backwash are realized through pleated discs pushed together during filtration and released during backwash. This results in better media cleaning and makes them more suitable for organic material and separation down to 10 μm, while maintenance due to their many parts is reported problematic.
Media filters, either continuous sand or special media (FW) filters, are significantly higher in capex and require usually more maintenance, but have the capacity to remove solids and also sticky, organic material down to about 10 μm. While pressure media filters have not been successful in CHPPs (while reasons are unknown), continuous media filters are used in a few non‐process water duties within coal mines successfully. The FW fiber media filter has been successfully trialed on pilot scale and is proven in full scale for a few months (at time of writing) for generating high quality process water with < 5ppm solids. The FW filter, as typical for media filters, is sensitive to high feed solids and typically < 200 mg/l in the feed should be achieved. Media filters used in desalination usually have strainers or automatic screen filters installed prior, to remove coarse solids and reduce feed solids to above limit. Coarse solids are separated by media filters, but might not be purged during backwash so that they are most suitable for low solids duties with predominantly 10 - 500 μm particles.
Indicative costs ($/m3/h) are given for water filtration equipment suitable to provide a comparison. Following list provides technologies in order of its capital costs (equipment only) from low to high without consideration of cut size based on 150 m3/h nominal flow:
Cyclonic < Automatic screen filters < Pleated disc backwash filters < Media filters < Baleen Filter
These are indicative only, as each filter has a broad but different size separation range and flow rates vary drastically with changing cut or screen size.
Commercial examples of water filtration technologies, if available with performance and maintenance details, are given for CHPPs in Australia and overseas as well as related industries. While some usable performance details were gathered, it was not possible to establish exact performance failures in particular for technologies or equipment disused or abandoned.
Technologies used for water filtration in other industries such as cooling tower water, pool water and irrigation are discussed as most filters were developed for those high overall volume applications.
An e-newsletter has also been published for this project, highlighting its significance for the industry.