Dynamic simulation and control of the Bayer process. A review

In the Bayer process, used for the production of alumina from the bauxite ores, there are frequent disturbances and a large number of interacting processes which incorporate considerable dead time. The challenge to any alumina refinery is to maximize the production of alumina (plant now and yield) and minimize the energy costs per ton of alumina subject to constraints on liquor sodium oxalate concentration, levels of silica in liquor, digester ratio, mud washer dilution, and particle size distribution in precipitation. In addition, liquor inventories and the surge volume must be maintained within high and low limits. As a result, steady-state operation is difficult to attain, and the refinery, in general, does not run at its optimal operating conditions. There are, however, major economic incentives, typically in excess of several million dollars per year, to stabilize the operation of the plant, and cost competitiveness is one crucially important strategic factor in the alumina industry. Bayer process simulation and process control are two important tools available for use by management, and process engineers to evaluate and improve performance in their plants. This paper reviews the current simulation and control technologies in alumina refineries and the basis for the application of advanced control to various processes within a refinery.