A washer wherein raw coal and water is fed into the head of an inclined trough equipped with openings in the bottom for the discharge of rejects. There are three types of Rheolaveurs used in coal washing: (1) the sealed discharge type for coarse sizes, from which the reject falls against an upward current of water and is removed by an automatic gate that controls the feed to a drowned elevator; (2) a system of two, three, or four superimposed troughs for washing fine coal below about 1/2 in (1/3 cm). The troughs are equipped with several bottom discharge devices. The separation of the heavy shale from coal and middlings takes place progressively until finally the pure shale is discharged from the lowest trough, and (3) a system for washing slurry consisting usually of two troughs one above the other and equipped with a number of Rheo boxes of the open discharge type but designed to minimize the loss of coal with the fine shale.