PURPOSE: To enhance recovery efficiency by a method wherein mud is allowed to fall through a net of a crasher chamber into a mud tank, while large blocks of mud are pressed and crashed and allowed to fall through a net of a foreign matter receiving hopper into the mud tank and the foreign matters are removed from the hopper.

CONSTITUTION: Mud fallen into a hopper 42 is allowed to fall into a crasher chamber 44 to crash it by shock, and soft mud is allowed to fall through nets on the bottom and side into a mud tank 50. Next, large blocks or hard blocks of mud such as blocks of clay are forced into a foreign matter receiving hopper 46 by a forcing means 48, following which feeding the mud into the hopper 42 and reciprocating motion of the means 48 is repeated, whereby the mud in the hopper 46 is crushed and fed downwardly through a net of a discharge port 46c into the tank 50. Foreign matters which can not be passed through the net gradually accumulate in the hopper 46 and when the hopper has been fully filled with the foreign matters, it is sensed by a sensor 46s. Further the hopper 46 is rotated and inclined by a hydraulic cylinder to discharge the foreign matters accumulated to a foreign matter discharge hopper 50A. As a result, a large block hard to be crashed is automatically crashed into small pieces and foreign matters can be selectively separated.