Open pit mine production scheduling can be formulated as an integer programming problem, which is well-known for demanding enormous storage capacity and a lot of calculation time. In order to avoid these problems, in this paper, 4-D network relaxation method is applied. The 4-D network relaxation method relaxes a few constraints with Lagrange multiplier method and allows us to apply the Lerchs-Grossman (LG) method, which gives the optimal solutions for the ultimate pit design of open pit mine, and find good suboptimal solution. However, previous studies reported the problem of huge calculation time. Therefore, we propose the algorithm to reduce calculation time. With the reduced calculation time, we examine the effect of price changes as a case study. In this study, we calculate the production schedules with three price scenarios; upward trend, constant and downward trend. A lot of price changes, which do not necessarily correspond to three scenarios, are applied onto the obtained schedules and calculate and examine the realized NPVs. The variance of realized NPVs in upper scenario is bigger than those of other two scenarios, but the probability of the realizing highest NPV is also highest. Therefore, it is better to schedule with upward trend, if mining companies can accept the risk. Furthermore, the reduced calculation time allows us to include capacity constraints not only for mill but also mining, this also means that the results become more realistic one.
