Dynamic simulation of red mud washers used in aluminum industries
Creators: name: Ardila, Alexander
Publisher: name: RMIT University
Year: 2024

description: Clarifier-Thickener equipment is used in a wide range of continuous sedimentation and sludge thickening processes where solid particles from continuous inflow mixtures are separated from the liquid. In this operation, the concentration of solids increases due to settling, so that the formation of a thicker bed is inevitable with time. Under optimal operating conditions, it is always possible to obtain two discharges from these vessels: a highly concentrated suspension at the bottom (underflow), and a clarified liquid stream at the top of the equipment (overflow or effluent). <br>In the Bayer Process an insoluble sub-product is formed as a result of the digestion of the bauxite ore with caustic soda. This product is called “red mud” and it has to be continuously removed by settlers or thickeners/clarifiers. This project proposes the simulation of a continuous thickener/clarifier in order to predict the concentration profile and the height of the mud level (process controlled variable) as alternative to current measurement system, that contains long delay discrete sampling time(15 minutes each measurement). The simulation also, can provide an option of creating a knowledge base for off-line control. The project essentially involves two methods of simulation, namely mathematical modelling and neural networks. The model based applies a kinematic model to approximate the process behaviour using both, equipment and suspension characteristics. On the other hand, due to the large amount of historical data, neural network is proposed for system identification. The first method is based on the solution of a highly nonlinear model, based on kinematic modelling of sedimentation extended to flocculated suspensions.<br><br>This approach uses a conservative finite difference scheme of the upwind type for solving an initial boundary value problem (IBVP). The successful simulation of the equipment and further validation of the mathematical model are then achieved once the properties of the suspensions have been determined. These properties are related to flux batch settling and solid stress functions, whose parameters can be obtained experimentally.On-site testing of the characteristics of red mud was conducted at the Rio Tinto Yarwum Alumina Refinery in Queensland. The settling properties of the suspension were determined via batch settling. For measurement of the rheology properties, the vane technique was used employing a Haake VT 550 rheometer. The results of the simulation showed that the concentration profile and height of the heavy mud level can be determined via a steady state model for a given underflow concentration. These results, however, were not in good agreement with measured data. The second method of simulation involved the use of Rio Tinto Yarwun historical data to develop a neural model in order to obtain a relationship between process variables. This approach has the advantage that no mathematical model is needed. With this method, historical data (continuous data) are obtained and analysed, and daily averages of the variables involved in the process are calculated. Different network architectures are tested according to the washer process. Ultimately, two networks were developed to describe washer dynamics.