In operations planning in an open pit mine an economic blocks model is generally used which attempts to determine which blocks will be extracted during each period. Lagrangean relaxation of the capacity constraints leads to a maximal flow problem on a multi period network. The best known method, proposed by Dagdeleen, is a heuristic method of decomposition in periods. We propose an algorithm based on the idea of Dagdeleen which yields an optimal solution provided that some conditions are met. When the solution is not optimal, it is an excellent lower bound. Numerical results show the substantial gain in time achieved by this algorithm as well as the quality of the solution.