Coal Preparation » Dewatering
Efficient dewatering of coal is one of the highest priorities in production encountered by coal preparation engineers. The objectives may be to meet product specifications or environmental constraints or to facilitate the handling of coal and tailings. Reduction of fine coal moisture has become a major concern as the amount of fine coal increases together with pressures to process it. To significantly improve dewatering performance, a prerequisite is to understand the state of the art of coal dewatering practice. Therefore, a comprehensive study involving plant performance surveys has been conducted within the JKMRC/AMIRA Project P239A "Advances in Coal Preparation Technology". All sponsors' plants which are equipped with fine coal and/or tailings dewatering equipment were visited by JKMRC personnel.
Both equipment selection/operation and chemical practice can strongly influence dewatering performance for fine coal and for tailings.
A total of 27 sponsors' coal preparation plants, 12 equipment supply companies and 6 chemical companies were involved in various aspects of the study. The objectives were to:
- obtain detailed information dealing with dewatering practice and equipment performance from plant personnel
- gather information on new and recently developed dewatering equipment
- seek new dewatering agents
In this report, the emphasis is on fine coal dewatering. Unless noted otherwise, comments generally relate to fine coal (not tailings) dewatering. (The only devices observed for dewatering tailings were belt press filters and solid bowl centrifuges).
Disc and drum vacuum filters are still the most popularly used fine coal dewatering equipment, particularly in Queensland. Screen bowl centrifuges (SBC) rank second, and are the most commonly used machine in NSW. However, application of CMI fine coal centrifuges is increasing due to their lower cost, smaller space requirement and reasonable performance.
Moistures are usually 21-25 percent for filter cakes and 14-17 percent for centrifuge products. Screen bowl centrifuges are less efficient at capturing fine solids, but this can lead to a significant ash reduction in product.
For tailings dewatering, belt press filters are superior to solid bowl centrifuges in cake moisture and handleability, and flocculant consumption.
Within the dewatering equipment surveyed, pressure filtration is an effective technique to improve dewatering of fine coal, particularly for minus 0.1 mm material. Of the hyperbaric filters, the KHD Humboldt Wedag Hyperbar and Andritz HBF are most promising.
They are capable of reducing product moisture by 4-5 percent while increasing capacity by 100 to 200 percent in comparison with conventional vacuum filtration. Their disadvantages are higher capital cost, a more complicated structure, and possibly higher operating costs.
Other pressure filters, such as Larox PF, Python pinch press, also can be used in dewatering superfine (minus 0.1 mm) fine coal.
Apart from CMI HP-36B, there is another fine coal centrifuge (Siebtechnik or Wemco H-900) on the market. Both the machines are high G (500 G's) centrifuges, and have similar dewatering performance. Product moisture is around 12 percent with approximately 50 tph capacity in dewatering minus 0.5mm fine coal.
The phoenix belt filter press has the features of an improved pressure/ shear zone and low maintenance design. This machine may be a good choice for tailings dewatering.
No major developments in flocculants and coagulants have been identified. However, Quaker Chemical provides two chemical aids for coarse and fine coal dewatering respectively. Nalco's "Opticus™" and Allied Colloids' "Clarometer" flocculant dosing control systems can be applied to thickeners and filters
Pressure filtration is an effective technique for improving dewatering of fine coals since it overcomes the fundamental difficulty in vacuum filtration, producing more than one atmosphere (2-6 bar) of pressure drop across the filter cake.
Of the hyperbaric filters, the KHD Humboldt Wedag and Andritz HBF are the most promising equipment at present. This type of machine is capable of reducing product moisture by 4-5 percent while increasing capacity by 100-200 percent in comparison with conventional vacuum filtration. Their disadvantages are higher capital costs, a more complicated structure, and possibly higher operating costs.
The Ceramic PC capillary action positive pressure filter recently developed by Outokumpu uses ceramic plates as a filter medium within a high pressure vessel. This type of equipment may have the potential to further reduce the cake moisture if compered with the KHD and Antritz types, however, its capacity may be a matter for concern. Tests have not yet been conducted for fine coal.
The Larox PF pressure filter is a vertically constructed press with airblow drying under pressure. It achieves very low cake moisture, and requires little floor space. The Python pinch press, which is considered as the successor of the Charlestown's tube press filter, is not sensitive to feed solids concentration. For dewatering of superfine coal, the Andritz HBF hyperbaric filter, the Larox PF automatic pressure filter and also the Python pinch press may be the best options. The Phoenix belt filter press has an improved pressure/shear zone design and low maintenance requirements. This machine may be a good choice for tailings dewatering.
In comparison with conventional belt press filters, the Eimco expressor press is reported to maintain 10-50 times pressure and allow five times the cake dewatering time. It could be used to obtain a dryer cake or further reduce fine coal moisture form vacuum filtration or other dewaterng equipment.
There are three fine coal centrifuges on the market: CMI HP-36B, Siebtechnik and Wemco H-900. CMI fine coal centrifuges have achieved some acceptance in the Australian coal industry. Both the Siebtechnik H-900 and Wemco H-900 are high G (500 G's) centrifuges, and have similar dewatering performance.
The Eimco E-Duk feed dilution system appears to be useful in reducing flocculant consumption and increasing the capacities of tailings thickeners.
The rotary drum dryer developed by Babcock offers an option for fine coal drying. Operating costs of this dryer seem to be lower than for other thermal drying techniques.
Chemical Dewatering Aids
Few newly developed flocculants and coagulants have been identified from the chemical companies surveyed.
Pre-coagulation before flocculation has been strongly suggested to improve tailings dewatering.
Quaker Chemical now provides two chemical aids (Quadry 1000 and UCA 92.007) for coarse and fine coal dewatering respectively. Plant trials demonstrate that these chemical aids may offer good performance in terms of product moisture and throughput.