Modeling and control of mill discharge pumps in the Aitik copper mine

Abstract: Boliden Aitik is the largest open-pit copper mine in Sweden, and during 2017, more than 39 million tonnes of ore was processed into concentrated copper, silver and gold. The distributed control system 800xA is provided by ABB, and this is what the operators use for process monitoring and control. Currently, the control of the mill discharge pumps operating to transport slurry from the mill circuit to the flotation system is not completely satisfactory. This leads to additional work for the operators, and can in worst case interfere with production.  This project has aimed to identify the problems with the existing control solution mainly from the operators and the engineers perspective, but also by looking at trend data. The discharge pump system has also been modeled, based on a combination of theoretical knowledge and real sensor data obtained from the process control system. The model has been implemented in a 800xA simulation environment, where alternative control solutions have been simulated and evaluated. The modeling procedure has been limited by the lack of pressure, density and flow sensors in slurry flows, and this has lead to an increased uncertainty, in particular for the dynamic characteristics of the pump system. The static characteristics of the discharge pumps have been modeled by regression analysis of a large amount of steady state data over several months, and the dynamic characteristics have been identified from a step response experiment. A cascade connection of proportional-integral (PI) controllers are used for pump control, where the primary controller uses parameter scheduling as a function of measured tank level. This leads to smooth control during normal mill circuit operation, which is a benefit for the flotation process, but it can also cause problems such as flooding, in particular at mill circuit starts or when switching between pumps. According to simulations, it seems like some modifications in the PI control should make it possible to use smooth control around the reference level, and to still avoid flooding at the situations that appear to be problematic today. The initial goal of this project was a live implementation of improved control, but this was not possible during the final weeks of this project. Instead, the simulation results should be used as support for future decision making and controller tuning.