$9.$ Batch contact extractors are characterized by their $\_\_$ operation, where the solvent and feed are mixed and then allowed to settle for phase separation.

$10.$ Continuous contact extractors, such as the $\_\_$ extractor, are preferred for large$-$scale operations due to their ability to handle large volumes of feed continuously.

$11.$ The effectiveness of solvent extraction can be quantified by the $\_\_$ coefficient, which measures the distribution of a solute between two immiscible solvents.

$20.$ The environmental impact of solvent extraction can be mitigated by selecting solvents that are $\_\_,$ easily recoverable, and have a low toxicity profile.