Abstract: This invention is concerned with an apparatus for concentrating a proppant fluid slurry. The concentration is carried out in a casing containing an axial elongated screen. Stationary angled vanes direct the slurry in a helical path in the space between the casing and the elongated tubular screen to avoid the problems of rotating members. The concentrating zone thus provided communicates with a mixing zone where baffles provide restricted flow between the edges of the baffles and the interior of the casing to give a mixing action, disperse slugs of slurry, and to avoid back flow of gas. The baffles are advantageously of resilient material. The mixing zone then leads to a foaming zone, followed by the outlet. It is advantageous to have inlet, concentrating zone, mixing zone, foaming zone and outlet for the foamed slurry in axial alignment and without interruptions so as to give a straight through flow.