Coal Preparation » Dewatering
The aim of the project which began in early 1994, was to investigate the shear coagulation/shear flocculation of fine coal, and its effects on the filtration rate and final moisture content of de-watered coal product. In the past, it has been difficult to float the coal in the ultrafine (slimes) fraction to produce a low-ash product, but this problem has been addressed by the introduction of new technologies such as the Jameson Cell and Microcel, which are able to make a low-ash product.
However, the product from the slimes flotation must then be dewatered if it is to be mixed with a coarser coal product for transportation and shipping. The ultrafines present special dewatering problems which are related simply to the particle size.
One way of increasing the mean particle size prior to filtration and dewatering, is through shear coagulation and/or shear flocculation. A polymer is used to neutralise the surface of the coal particles and the particles are brought into contact with each other by increasing the collision rate, in a controlled shear environment. Any loose open flocs which are formed are broken up by the shear. The result is a suspension of tightly knit aggregates which lead to a filter cake which is more permeable to flow of water.
Laboratory experiments have been conducted on both fine pulverised coal samples and flotation product from operating coal preparation plants. Pilot-scale continuous trials were also undertaken at two operating sites.
Particular objectives were:
- to investigate the effect of shear coagulation on the shape of the particle size distribution, especially the destination of the ultrafines, and whether or not they can be caused to attach to the coarser particles in the distribution;
- to investigate the filtration and dewatering characteristics of the aggregates formed by shear aggregation or coagulation in a filtration device;
- to investigate the possibility of improving the ability of existing dewatering devices such as disc filters, centrifuges and belt filters, designed for fine coal, to cope with added ultrafines;
- to investigate the effect of aggregates formed by shear coagulation, on the dustiness characteristics of the product.
Problems to be addressed were:
- increasing the rate of filtration of coal fines and ultrafines;
- reducing the total moisture content of fine coal filter cake;
- reducing handling and environmental problems caused by fine coal dust.
Shear coagulation has been shown convincingly to be a real effect. Using shear in the form of energy supplied by an impeller, it is possible to achieve higher filtration rates and lower residual moisture levels than with a coagulant alone.
Application of shear to a flotation product in the presence of coagulant leads to increases in filtration rate with increase in the shear rate (stirrer speed) and coagulation time, up to a point. Plant pilot-scale tests have shown that a net low energy input (0.44 kWh/t solids) is all that is required to observe the effect in practice.
A Rushton type (Disc style - high shear) impeller provided the best results for a given power/energy input.
The filtration rate of a flotation product from two operating coal preparation plants has been increased using shear coagulation.
It is possible to coagulate fine coal and ultrafines, to produce dense, compact, high-strength aggregates rather than loose flocs.
Conditioning time is found to be an important variable. When the particle size distribution is monitored as a function of time, it is found that the sub-10mm fraction can be minimised, and the mean particle size (Sauter mean diameter) can be increased by a factor of three or more.
The coagulant addition rates are relatively low. Success has been achieved at concentrations as low as 15g/tonne, dry solids basis.
The filter cake formed from shear coagulated coal appears to be quite strong. There is no evidence that the cake compresses as the filtration pressure increases. Large increases in filtration rate can be achieved without the need for hyperbaric filtration.
The filter cake total moisture from operating preparation plants has been decreased using shear coagulation. Alternatively, increased filter cake throughput could be obtained without an increase in filter cake total moisture.
The positive effects of shear coagulation/shear flocculation are sufficiently proven at laboratory and pilot demonstration scale such that the technique should be exploited by coal producers. Thus producers are sought to demonstrate the technique at full-scale.
Further research should concentrate on screening alternative types of reagents or binders which result in a greater expulsion of water from the aggregates.
The technique should be investigated for the enhancement of tailings thickening and filtration.
The specific benefits to coal producers are of two types:
- Where a washery already has some type of filter or dewatering machine in operation, such as a disc filter or a screen bowl centrifuge, the benefit will be the ability to handle an increased proportion of -75mm material in the feed.
- Where a plant is being considered for the recovery of ultrafine coal in the slimes fraction, an improved capability to make a product at an acceptable production rate and acceptable level of moisture.