Abstract: An aperature is formed in the bottom wall of the launder. A downwardly extending container communicates with the aperture. A weir is positioned immediately downstream of the aperture. The container and weir combine to form a separator. In the launder, free water and solids contaminants settle and form a bottom layer of dirty water. The layer is diverted into the container through the aperture and the weir temporarily retains a small head of it. Temporary retention in the separator therefore generates a vertical fluid column comprising an upper layer of relatively clean froth and a lower layer of dirty water, said layers having an ascertainable interface. The elevation of the interface is monitored and contaminated fluid is withdrawn from the container as underflow at a rate controlled to keep the interface elevation substantially constant. The froth overflowing the weir is reduced in water and solids relative to the original froth issuing from the flotation vessel.