With the ever-decreasing quality of bauxite being processed in Bayer plants and the drive for increasing production, the stress on the mud circuit of modern plants is greater than ever. In some of our plants, we observed that tank life is often limited by a rapid increase in rake torque or erratic torque variation. Higher mud and sand load speed up the failure. A pilot scale thickener was built at Université du Québec à Chicoutimi, Québec, Canada to help understand this behavior. Several tests were run under various configurations and evidence was found that, with time, a dense kernel tends to develop in the center of the tank. This kernel is composed of mud at a high concentration of solids and of sand. The kernel tends to cause an increase in rake torque, erratic underflow solids and also underflow blockages. This eventually leads to premature stoppage of the tank operation. We tested several rake configurations, varied operating parameters and feed well configuration in order to limit the formation of this kernel. An improvement to the design of the feed system was tested and found to significantly improve the operation of units fed with a high fraction of coarse particles in the feed slurry. This paper presents results obtained in the pilot thickener and in a plant thickener.