Thickening is the first stage in separating water from the concentrate. In the ore dressing and beneficiation process, the concentrate sludge from the flotation is fed to the center of a large thickening tank cast in concrete. In thickening, the concentrate sludge is sorted into two products. The water in the sludge rises to the surface. It is almost free of solids and exits the tank via the overflow edge. The mass that sinks to the bottom of the tank consists of the solid substance in the concentrate sludge, meaning the valuable ore granules. The valuable thickened mass is transferred with rakes towards the central cone at the bottom of the tank, from where it is pumped to the next stage of the concentration process, filtration.

The Dorr thickener was developed in 1906 by J.V.N. Dorr. It includes a large tank made of concrete with equipment. The sludge is fed down a separate chute to a feeder in the middle of the tank. The clarified liquid flows into the overflow chute and from there exits the thickener. The solids descend and settle, are pushed by the rakes towards the center of the tank and exit through the bottom cone into the outlet piping.