Abstract: According to the invention, the grains of the hydrate slurry are separated by sedimentation where the dispersed bubbles of air are removed in course of the presizing, then a supplementary sizing is carried out. Preferably, a liquid free from hydrate grains is fed in to stream against the sinking grains. This liquid is advantageously the spent liquor of the hydrate filter. The apparatus according to the invention consists of a container with conical bottom, a feeding device, a slurry outlet stub and an overflow channel. The inlet stubs of the feeding device, however, are placed off-center and every stub is connected with a presizing equipment. This equipment is composed of a bundle of pipes and/or a pack of plates which are slop placed to geometrical axis: At the upper part of the presizing equipment there is a foam overflow apparatus. The bottom of each presizing equipment is connected with a common cone-frustum type deflector casing. The casing is concentric with the container.