ABSTRACT
Solvent extraction and solvent precipitation are commonly used either to recover valuable materials or to remove contaminants from aqueous or non-aqueous waste streams. Leaching is solvent extraction applied to solid feeds. They are usually physical processes whose selectivity is dependent on the relative solubilities of the feed components in a chosen solvent. For example, extraction with a liquid organic solvent such as kerosene, which is essentially insoluble in water, can be used to remove polar organic compounds such as phenols, organic acids, and ketones, etc., from industrial wastewaters. This makes the wastewater more suitable for disposal and at the same time recovers valuable chemicals for reuse or sale. The solvents used for extraction and solvent precipitation can be either conventional liquids below their critical point or supercritical fluids operating at conditions above the critical point of the pure solvent. Most, if not all, of the separations that can be made with supercritical fluid solvents can also be made with liquid solvents. However, in some specific applications there are enough operating benefits to favor supercritical fluids. Examples of these benefits are (1) lower operating temperature for heat-sensitive materials, 418(2) ease of solvent recovery by pressure reduction or temperature increase, and (3) faster phase separation due to lower viscosities and higher density differences between phases. The trend is toward increased use of supercritical fluid extraction solvents in recent years. However, their use still represents only a small fraction of the total solvent extraction operations in existence.
