Water consumption has become an important issue for the mineral processing industry. This situation has led to an increased effort for recovering the maximum amount of water in the solid-liquid separation process. Thickeners are designed to separate clear and reusable water from concentrate underflow. Conventional feedback control has not been effective in stabilizing the process operation when the characteristics of the feed changes over time, since process dynamics exhibits high non-linearity. This work presents an expert system for an industrial thickener. Main results are a reduction in flocculants consumption and a more stable operation, with few emergency conditions.