Molten slag is granulated with water to produce a granulated-slag/water sry that is fed into the bottom of a horizontally extending foraminous drum. This drum has radially inwardly extending vanes and is rotated to lift up the granulated slag and separate it from the water, discharging the slag onto a conveyor belt for storage or further use. The water running out of the slag slurry is recirculated for use in the initial granulation step for the molten slag stream.