Separation by Countercurrent Washing of Coal-Caustic Mixtures during Chemical Coal Cleaning

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Abstract
Leaching coal with molten sodium hydroxide at 370–390°C converts most of the sulfur and mineral components of the coal into soluble species. The unreacted caustic and soluble components are then separated from the cleaned coal by a series of washing and filtration steps. A laboratory-scale simulation of a 6-stage countercurrent washing and filtration procedure was performed on Illinois No. 6 and Kentucky No. 11 coal samples that had been leached with molten sodium hydroxide. The mass of filter cakes and filtrates during each wash cycle and the concentrations of all major components of the caustic solutions were determined in each process stream. The countercurrent washing procedure resulted in a relatively clean coal and a final filtrate with a caustic concentration close to the desired 50%. However, after several coals had been processed, filtration rates decreased markedly and the mass of filtrate recovered also decreased. This was due to a build-up of a precipitate, consisting predominantly of Na2CO3, on filter cakes in the middle stages of the countercurrent process step. Process modifications to avoid this build-up are proposed.