Extraction, also known as solvent extraction or liquid-liquid extraction (to distinguish it from solid-liquid extraction, i.e., leaching), also known as extraction (commonly used in petroleum refining industry), is a kind of mass transfer separation process that uses liquid extractants to treat immiscible two-component or multi-component solutions to achieve component separation, and is a widely used unit operation. Common extractors include ultrasonic extractors, microwave extractors, and centrifugal extractors, which use the difference in solubility or partition coefficient of compounds in two immiscible (or slightly soluble) solvents to transfer compounds from one solvent to another.