Investigation of the Effect of Mechanical Mixing on Dewatering of Fine Coal Vacuum Filter Cakes

Horizontal vacuum belt filters provide a very efficient means of filtering fine coal slurries (nominally minus 0.5 mm). Depending on the nature of the coal processed, industry's best practice with this technology typically leads to cake discharge moisture in excess of 25%. Recent work (1) has permitted identification of filter cake microstructure as a major limiting factor for moisture removal that is distinct from coal properties. This result suggested that modifying and possibly controlling cake microstructure could offer potential for moisture reduction in vacuum filters. The paper discusses the findings of a laboratory study involving mechanical agitation of fine coal filter cakes during the drying phase using a helical ribbon mixer. The rational that led to the experimental approach used in this work is detailed. Test results are reported which confirm that alteration of cake microstructure during the drying phase of the vacuum filtration cycle has immediate and positive impact on cake drying. Moisture reductions up to 3% were measured with fine bituminous coal filter cakes. Analysis of the experimental results reveals that the effect of filter cake mixing on cake moisture reduction depends strongly on the time at which mixing is initiated during the drying cycle and the size distribution of the filter cake particles. The practical potential of the mixing process is also discussed