Fundamentals of Fine Coal Dewatering

The aim of this project was to develop a fundamental understanding of coal-water interactions and of the dewatering process that will form the basis for improving the dewatering behaviour of all black coals, not just problem coals.  The objectives of the project were:

• To develop a reproducible benchmark laboratory vacuum filtration apparatus for characterising the dewatering behaviour of fine coal slurries.  
• To relate microstructural and dewatering properties of fine coal filter cakes.  
• To quantify the extent of interaction of water with black coals and to assess the influence of coal properties on this interaction.

The following list summarises the most significant project findings and achievements.

For dewatering by means of vacuum filtration, it was demonstrated that the filter cake microstructure, via its permeability, controls:  

• the rate of cake formation, ie the throughput of the filtration process, and
• the desaturation rate ie the rate of moisture removal from the filter cake.  

The importance of cake microstructure on the overall performance of fine coal filtration implies that controlling cake microstructure should be the dominant strategy for optimising existing filtration equipment and designing more efficient, cost effective filtration technology for the future.  

Methods have been developed for simulating filter cake microstructure and predicting their permeability. Such techniques, which include a novel scheme for calculating the flow through porous media, will greatly assist future work on improving and controlling filter cake microstructure.  

The project has found that coal hydrophobicity does not affect cake formation nor the dewatering kinetics.  

However, both coal hydrophobocity and cake permeability influence the steady-state moisture of fine coal filter cakes, which is typically around 20%. Recent work indicates that steady-state cake moisture can decrease as cake permeability decreases. Experimental evidence also suggests that the steady-state cake moisture level depends on:  

• non-kinetic properties of cake microstructure, such as pore size distribution;
• the capacity of flocs to retain moisture, and
• the extent of intraparticle porosity.  

Nuclear magnetic resonance techniques have been developed to characterise moisture in fine coal beds and filter cakes in terms of its interaction with the coal surface. Information provided by these techniques can complement the outcomes of filter cake microstructure studies in devising ways of reducing cake moisture retention.  

Water micro-capillaries (which is mostly intraparticle) has been quantified by nuclear magnetic resonance and gravimetric sorption techniques and shown to be in the range 3-8% for bituminous coals. It is coal dependent and cannot be removed by conventional mechanical means.  

An Australian defacto standard vacuum filtration test was derived: the Single Leaf Filter Test (SLFT). This allows an operator to conduct both top and bottom-fed filtration tests reproducibly. The test monitors the complete filtration cycle by measuring on-line most filtration parameters of practical significance, and has served in a number of plant upgrade studies. The SLFT has potential as a practical tool for sizing full scale equipment. It has also been proposed as an Australian standard.

Outcomes

Nuclear magnetic resonance and gravimetric sorption techniques have been developed which can distinguish, quantify and characterise different forms of water in fine coal specimens. Thus non-freezing water, rapidly-relaxing water, monolayer water and equilibrium water uptake at a relative pressure of 0.9 (all of which relate largely to interparticle moisture) have been defined as measures of the extent of interaction of water with coal and values are compared for a wide range of bituminous coals. For all coals studied, the quantity of non-freezing water is similar to that of equilibrium water uptake at a relative pressure of 0.9mm and 3-5 times that of monlayer water, whereas the quantity of rapidly-relaxing water is about twice that of monolayer water.  

Oxidation of higher-rank bituminous coals, under conditions which produce an increase in the surface concentration of retained carbon-oxygen functional groups and hence a decrease in coal hydrophobicity, did not lead to any significant changes in the interaction of intraparticle water with these coals. No conclusive evidence that this decrease in hydrophobicity influences residual moisture levels in filter cakes was obtained.  

The interaction of water, particularly with higher-rank bituminous coals, can be enhanced significantly by inherent clay minerals, but is not sensitive to maceral composition.  

NMR relaxation techniques have been used to characterise the interaction of both intraparticle and interparticle water with fine coal. The interaction of water in interparticle voids and larger intraparticle pores with the coal surface was found to decrease with increasing water content (or extent of saturation) and with increase in the expected hydrophobicity of the coal, to increase with both decreasing particle size and the breadth of the size distribution, and to be lower for coals that are perceived to be easier to dewater.  

The average NMR relaxation time for interparticle water in both saturated and desaturated filter cakes, prepared from a low-volatile coal flotation concentrate, was found to correlate significantly with cake permeability, and hence cake structure. This indicates that NMR relaxation techniques can provide a measure of the ease of cake dewatering and that water remains in larger interparticle voids after dewatering of more permeable cakes. Flocculation of the flotation concentrate reduced the quantity of water interacting with this coal.

Recommendations for Future Work

NMR techniques developed for the study of fluids in oil-bearing source rocks have potential for providing information on the size distribution of water volumes remaining in desaturated cakes. One approach would be to generate a series of filter cakes with a range of saturation levels by means of capillary pressure experiments. Information on the distribution of moisture within these cakes, together with morphological analysis and modelling of the cake microstructure, could then be applied to the development of strategies to reduce the residual moisture levels in dewatered cakes. The effects of various additives could also be incorporated into such a study.  

Results obtained in this project suggest that a relationship exists between the interaction of interparticle water with the coal surface, as measured by NMR relaxation techniques, and the ease of dewatering of fine coal. Further work involving both NMR experiments and laboratory-scale dewatering tests on a range of bituminous coals is needed to verify this tentative finding.  

The interaction of water with a wider range of coals containing swelling clay species should be investigated by gravimetric sorption and other techniques. Areas of interest include ultrafine coal and tailings with a significant swelling clay content and particular coals for which dewatering of fines is poor due to the presence of these clays.  

Analytical techniques for quantifying the extent of interaction of water with coals, based on the physical properties of this water.  

Relationships between water quantities which measure this interaction and coal oxygen content. An NMR-based method for characterising and comparing the interaction of interparticle water with fine coals.