Solid Liquid Extraction

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8.7. Solid – liquid extraction 8.7.1. Theoretical background Solid – liquid extraction (or leaching) is the separation of a solid solute from a mixture of solids by dissolving it in a liquid phase. Basically, there are three components in leaching: solid solute, insoluble solids and solvent. In most cases, the diffusion of intra-particle soluble component(s) controls the extraction rate. Therefore the process is often called as diffusion extraction. Solid – liquid extraction is widely used in food (e.g. extraction of sugar from sugar cane or sugar beet; isolation of vegetable oils from different seeds) and pharmaceutical industries (e.g. extraction of active components from medicinal plants), and in hydrometallurgy (e.g. leaching of metal ions from ores). For mathematical modeling of leaching an ideal solid – liquid extraction is defined. The solvent added to the dry raw material is partly taken up by the solid material and the soluble ingredients are instantaneously dissolved. Often all the solute is already dissolved (e.g. sugar in sugar beet). The solution is then split into two parts. The external solution is called extract and the internal one raffinate (in the case of plant materials this is cell liquor). The simplifying assumptions are defined as follows: 1. For batch extraction the mass ratio of extract (B) and raffinate (A) does not change during extraction (liquid ratio: f  B A =constant). In a continuous system the mass flow rate ratio of extract and raffinate remains constant throughout the equipment. 2. The densities of solutions remain practically constant during extraction. 3. The solid matrix has no absorptive properties. Hence, after infinite time the concentrations in the extract and raffinate phases are the same. 4. The surface of solid material available for mass transfer does not change with time. 5. The concentration of the raffinate (cell

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