Coal Preparation » Dewatering
In Australia vacuum filters (disc or horizontal belt) are the predominant technology used to dewater coal flotation concentrates. However the final cake moistures using this conventional approach are limited by the vacuum pressure that can be applied to the feed via the filter media, which is limited to -70 kPa. Pressure or hyperbaric filtration can deliver pressures of up to 600 kPa, which greatly enhances the filtration rate and the ultimate cake moisture that can be achieved.
The hyperbaric disc filter consists of a modified disc filter and feed trough, similar to the vacuum disc filter assemblies currently used in conventional flotation concentrate dewatering. The filter assembly, however, is contained within a pressure vessel, so that applied pressures of up to 600 kPa can be achieved to drive the dewatering process, and achieve surface moisture levels significantly less than conventional vacuum filtration. The pressure vessel incorporates an internal chain conveyor to continually transport the discharged cake to a twin gated discharge sluice. The twin gates operate in cycles to discharge the filtercake at atmospheric pressure from the vessel in a batch wise operation.
The standard hyperbaric disc filter design can be modified to include steam cabins in the final third of the caking drying zone. Introducing steam during the drying process has been proven to further reduce the final cake moistures above what is achievable using air only.
Recent laboratory assessments of the hyperbaric filter technology undertaken as part of feasibility study projects had indicated that significant reductions (greater than 15% absolute) in filtercake moistures were achievable compared to conventional vacuum filter technology.
Detailed Pilot Testing Program
The specific aim of this project was evaluate, through a pilot scale testing program, that the application of the high pressure air filtration and optionally supplemented with steam can achieve the dewatering potential indicated by the recent feasibility study laboratory assessments and a more recent pilot testing program undertaken in Germany.
The hyperbaric filter test rig (Hi-Bar) was supplied on a wet hire basis by Bokela GmbH. The site selected to host the pilot testing program was Glencore's Tahmoor Coal Handling and Preparation Plant (CHPP). Both the Tahmoor coal thickener underflow stream and a bulk flotation concentrate sample from Glencore's Oaky Creek CHPP were assessed as part of the research. Both feed types assessed were coking coal flotation concentrates with the Tahmoor coal thickener underflow being finer with a p50 of 0.045 mm, relative to the Oaky Creek flotation concentrate with a p50 of 0.090 mm.
During the testing program both the Tahmoor and Oaky Creek flotation concentrate feed types were evaluated over a wide range of filter process settings to determine the minimum filtercake moistures that can be achieved.
The research validated the main conclusion from previous technical reviews that the hyperbaric filter technology can be implemented to dewater coal flotation concentrates and target moisture levels not achievable using conventional dewatering technologies.
Through pilot scale testing it was proven that the hyperbaric filter can achieve filter cake free moisture levels as low as 16.6% (ar) and 13.9% (ar) for the finer (Tahmoor) and coarser (Oaky Creek) feed types respectively, These moisture levels were achieved using process air only.
When high pressure steam is used to supplement the drying process in the hyperbaric filter dewatering zone, then filter cake free moisture levels of 10.4% (ar) and 8.3% (ar) were achieved for the Tahmoor and Oaky Creek feed types respectively.
These attained filtercake free moisture levels represent significant reductions in flotation concentrate moistures compared to those conventional vacuum filtration technology can achieve. Benchmark Single Leaf Filter Testing (SLFT) on the Tahmoor feed and comparative site operational data from the Oaky Creek CHPP indicated that conventional vacuum filtration can achieve filtercake moisture levels of 30.6% (ar) and 26.0% (ar) respectively on similar feed types. This represents a moisture reduction potential over conventional vacuum filtration of at least 12% (absolute) using a hyperbaric filter running process air only and a moisture reduction potential of 18% (absolute) supplementing the drying process in the hyperbaric filter with steam.
When high pressure steam is used to supplement the drying process, the moist cake is subject to vessel pressures of up to 5 bar and saturated steam temperatures of approximately 150° C, for a cycle period of up to 2 minutes. The coking properties of the Tahmoor and Oaky Creek filter feed and filtercake samples were examined to determine if the high temperature steam had a deleterious effect on the filtercake product coking properties. The analysis indicated that there was a significant reduction in fluidity for the Tahmoor filtercake relative to the respective feed samples and a minor reduction in fluidity for the Oaky Creek filtercake.
Fluidity is very susceptible to surface oxidation when samples are stored prior to analysis, and the deterioration in fluidity of the Tahmoor filtercake samples and not the Oaky Creek samples potentially indicate that some degree of oxidation may have occurred between sample collection and fluidity testing. Thus the results were deemed inconclusive, and further investigation is warranted to confirm that the high temperature steam used in the steam filtration process does not adversely affect the coking properties fine coal filter feeds.
Benchmarking Equipment and Power Costs
A high level comparison of the relative filter and ancillary equipment costs and power usage costs for the various flotation concentrate filter dewatering technologies was undertaken to determine the commercial scale implementation costs of hyperbaric filtration relative to conventional vacuum filtration.
A typical 1500 t/h CHPP (standard DMC and spirals plant) implementing froth flotation and concentrate dewatering to recover fines at a projected yield of 50% was used as a basis for the comparison.
The base case conventional vacuum filtration option represented the lowest equipment cost case and had the lowest installed power requirement, $15000 per tonne (ad), and 4 kW per tonne (ad) of filter feed respectively. However the high projected free moisture from the vacuum filter dewatering will increase the overall CHPP product moisture by 3.6% (ar) and reduce the overall CHPP gross CV by approximately 250 kcal/kg (gar), and as such it is unlikely that this option will make the implementation economically viable.
For the hyperbaric filter, using process air only, the equipment cost relative to the vacuum filter dewatering option will approximately double to $34000 per tonne (ad), and increase the power consumption to 13 kW per tonne (ad). The net effect on the overall CHPP product moisture and gross CV will be by 1.6% (ar) increase and a 100 kcal/kg (gar) reduction respectively.
For the hyperbaric filter implementation using steam and air to drive the dewatering process will cause a three-fold increase in equipment cost to $42000 per tonne (ad), and increase the power consumption significantly to 60 kW per tonne (ad). The net effect on the overall CHPP product moisture and gross CV will be a 1.0% (ar) increase and a 70 kcal/kg (gar) reduction respectively.
Depending on the flexibility of the customer's product quality specifications, the hyperbaric filter with air only option, will likely be the most suitable implementation to dewater the flotation concentrate. This option represents a trade-off between the higher capital and energy requirements of the hyperbaric filter using steam, and reducing the overall product moisture to an acceptable range so that the implementation is economically viable.
An extension of this project has been approved. The objective of this extension work is use the pilot testing data from this stage, laboratory Filtratest data assessments from a range of flotation coal concentrates to be tested by Bokela in Germany, and data supplied by Bokela as part of their in-kind support for this extension project, to further characterise the hyperbaric filter performance relative to the input feed coal quality characteristics and the filter operating process conditions.