Abstract: In a coal gasification installation having a gasifier reactor and a water bath at the bottom of the gasifier reactor, a mixture of water and slag particles removed from the water bath is admitted under pressure into a container full of water and having a partition member for separating the heavier slag particles entrained to the bottom of the container from the lighter particles allowed on its opposite side to rise and float. A dam at the surface of the water in the container retains the floating particles while the clean water flows over the dam to an outlet. The floating particles which may be collected inside the container or entrained through a second outlet, are filtered and classified. Filtering is effected with a drum filter; and when several such filters are used in cascade, the slag particles are classified for recycling.