Strategic open pit mine planning methodologies have traditionally broken up the production scheduling process into two steps. First, push-backs (or phases) are determined. Second, bench-phases are scheduled in time by defining which extracted material should be treated as ore, and which as waste, and deciding how extracted ore should be processed. Traditionally, this last step has been carried out using heuristics. In this article we describe a mixed integer programming (MIP) formulation with which this last step can be optimized, and compare it to Whittle's Milawa algorithm. Our results confirm that MIP techniques for this problem have come to the point where they can solve real-sized problem instances, computing solutions that are better both in terms of objective function and modelling detail.