A separator and process for separating particles from a gaseous mixture wherein the separator includes: at least one chamber comprising a zone (1) which forms a rectangular or square cross section for passage of the mixture circulating in a tube reactor (R) towards a turning zone (3) which rotates the mixture in a vertical plane through an angle of at most 360 degrees; an external wall (30) which defines the turn through an angle of 70 to 225 degrees; a deflection (10) which turns co-axially with the external wall (30) through an angle of at least 30 degrees and at most the angle through which the external wall turns plus 90 degrees; a gas outlet (4) which is coaxial with the turning zone; the separator is indirectly or otherwise connected to a secondary cyclone (12). It also includes a particle outlet (9) having a wall (32) connected to forming zone (1).