Retention and retreatment of ore in part of flow a line until it satisfies criteria for release. Used in comminution to reduce overgrinding by passing intermediate particles repeatedly through grinding systems, classifying the product and returning oversize. Used in concentration (e.g., rougher-scavenger-cleaner flotation) to retain a selected fraction of ore in circuit for retreatment (a middling), until it is either upgraded to rank as concentrate or sufficiently denuded of value to be rejected as tailing.