Abstract: An air classifier comprising a rotatable open-ended drum disposed with its axis horizontal or substantially so, for receiving material to be classified, means for creating a stream of air through the drum, means for introducing material to be classified into the drum at or near the upstream end thereof, means in the drum for conveying materially axially of the drum during rotation thereof, the conveying means being in the form of helical vanes of mutually opposite hand arranged in an upstream part of the drum to convey "heavies" in counter-current to the stream of air through the drum and in a downstream part of the drum being arranged to convey "lights" in the direction of the air stream, the two helical vanes being separated by an annular member which projects radially inwards from the inner periphery of the drum.