Apparatus for mixing a material with a gas/vapour, the apparatus comprising: a passage defined by a wall and having an inlet and an outlet, wherein a process liquid comprising a material and a carrier liquid flows from said inlet to said outlet; and a nozzle for introducing a gas/vapour into said passage at a mixing zone, wherein the cross sectional profile of the passage at the mixing zone is polygonal and said nozzle is configured as a slit, wherein said slit runs in a direction substantially perpendicular to the direction of flow of said process liquid at said inlet such that said gas/vapour enters said passage in a single plane and wherein the cross sectional area of said passage at said mixing zone is smaller than the cross sectional area of said passage at said inlet and wherein said nozzle is positioned in said wall of said passage above said process liquid such that said gas/vapour flows downwards towards said process liquid; wherein said process liquid comprises a suspension of a solid in a liquid, a colloidal solution or suspension, or an emulsion; characterised in that: said nozzle is provided in a rotatable member, wherein said rotatable member is rotatable through an arc of at least 10° such that the angle between said direction of flow of said gas/vapour and said direction of flow of said process liquid can be varied; said nozzle has a nozzle profile comprising a de Laval convergent-divergent section for introducing said gas/vapour at supersonic velocity into said passage at said mixing zone. a passage defined by a wall and having an inlet and an outlet, wherein a process liquid comprising a material and a carrier liquid flows from said inlet to said outlet; and a nozzle for introducing a gas/vapour into said passage at a mixing zone, wherein the cross sectional profile of the passage at the mixing zone is polygonal and said nozzle is configured as a slit, wherein said slit runs in a direction substantially perpendicular to the direction of flow of said process liquid at said inlet such that said gas/vapour enters said passage in a single plane and wherein the cross sectional area of said passage at said mixing zone is smaller than the cross sectional area of said passage at said inlet and wherein said nozzle is positioned in said wall of said passage above said process liquid such that said gas/vapour flows downwards towards said process liquid; wherein said process liquid comprises a suspension of a solid in a liquid, a colloidal solution or suspension, or an emulsion; characterised in that: said nozzle is provided in a rotatable member, wherein said rotatable member is rotatable through an arc of at least 10° such that the angle between said direction of flow of said gas/vapour and said direction of flow of said process liquid can be varied; said nozzle has a nozzle profile comprising a de Laval convergent-divergent section for introducing said gas/vapour at supersonic velocity into said passage at said mixing zone. said nozzle is provided in a rotatable member, wherein said rotatable member is rotatable through an arc of at least 10° such that the angle between said direction of flow of said gas/vapour and said direction of flow of said process liquid can be varied; said nozzle has a nozzle profile comprising a de Laval convergent-divergent section for introducing said gas/vapour at supersonic velocity into said passage at said mixing zone.