Abstract

Liquid-solid extraction is complex at the fundamental level, which is masked by the apparent simplicity of industrial processes used to effect extraction. The structure of solids which have to be extracted is varied, and leads to a combination of diffusional and other mass-transfer processes which effect solvent contact and solute extraction. Other factors that determine extraction rates and hence equipment size include particle size, physical and chemical properties of the solvent, solubility dependences on temperature, and solvent agitation. Extractors rely on either percolation or agitation to give intimate contact between the solvent and the solids to be extracted. Early batch extractors have to some extent been superseded by continuous devices where the mode of solids transport offers the key to successful operation. The method of solids transport is used as a means of classifying the different equipment types. The design of an extraction system generally requires that careful consideration be given to safety and environmental factors, particularly with regard to solvent and dust loadings in the atmosphere of the working environment.