Bayer digestion maintenance optimisation with lazy constraints and Benders decomposition

This paper describes a maintenance scheduling model for digester banks. Digester banks are network-connected assets that lie on the critical path of the Bayer process, a chemical refinement process that converts bauxite ore into alumina. The banks require different maintenance activities at different due times. Furthermore, the maintenance schedule is subject to production-related constraints and resource limitations. Given the complexity of scheduling maintenance for large fleets of digester banks, a continuous-time, mixed-integer linear program is formulated to find the cost-minimising maintenance schedule that satisfies all required constraints. A solution approach that employs lazy constraints and Benders decomposition is proposed to solve the model. Unlike generic implementations of Benders decomposition, we show that the subproblems can be solved explicitly using a specialist algorithm. We solve the scheduling model for realistic scenarios involving two Bayer refineries based in Western Australia.