An Archimedes screw plant for separating slurry includes a tubular sifting device; a slurry feeding device; a device for evacuating the liquid fraction; and a device for evacuating the substantially dry separated solid. The device includes an upper compaction chamber of the separated solid where a solid plug of product is formed. In addition, the tubular sifting device has an upper opening being provided with an auxiliary contrast element for forming the solid plug of product.