Cost considerations are prevalent in open-pit mining activities when the availability of mineral resources depletes and the depth of the mines increases. In case of truck-and-shovel haulage, 45% to 60% of the total mining operation costs result from haulage costs. Deeper pits imply longer haul distances, longer cycle times, and larger number of trucks, resulting in higher diesel fuel consumption, higher maintenance costs, and increased greenhouse gas emissions. In-Pit Crushing and Conveying (IPCC) systems allow continuous haulage that uses conveyors as opposed to trucks for primary haulage. In addition, IPCC systems use electricity instead of diesel fuel to reduce unit operating costs and carbon emissions and to reduce the need for trucks. However, such advantages are accompanied by higher capital costs and altered pit shape which makes strategic planning very crucial.
There exist different types of IPCC systems depending on the requirement and mobility. In fixed systems, crushers are located out of the pit or on the final pit walls, whereas in semi-mobile IPCC (SMIPCC) system crushers are located on the active benches and are moved periodically after every six to eighteen months. In fully mobile IPCC (FMIPCC) systems, crushers are attached to shovels to eliminate the need for trucks. Semi-mobile IPCC configuration is a balanced approach because it maintains truck flexibility for shorter distances of benches to crushers and uses conveyors for longer vertical hauls. Financial calculations suggest that SMIPCC may reduce operating costs by 34% to 43% and the need for trucks by 20% to 60%, and lead to 28% life-of-mine net present cost savings.
The decision regarding the transition of a site from traditional trucking and loading to in-pit crushing and conveying is one of the major design issues. The development of transition strategies involves scenario modeling and discrete-event simulation, as it is necessary to predict the operation of the mine’s benches, maintenance of the equipment and road network. In the particular example of the copper-mine operation, scenario modeling revealed that the transition to the IPCC at a depth of 335 meters results in the lowest cumulative discounted cost, with an improvement of 17.6% compared to a pure truck-shovel system. It is also worth mentioning that the result is insensitive to changes in the discount rate.
There are certain restrictions and peculiarities of the mine design and operation in case of system implementation. For the proper alignment of the conveyor belt, it is necessary to create a flat profile of the ramps and special pockets, which can increase stripping ratio by 7%-8% to avoid intersections of the roads. Moreover, the usage of IPCC is conditioned by the amount of material flow, which should be at least 20 million tons annually and 15 minutes of hauling cycle duration.
Implementation of the IPCC system provides a reliable solution for controlling the growing cost of haulage and achieving environmental requirements. The success of such transition is based on the appropriate feasibility studies, which allow to find a balance between high investment costs and operating savings. The use of scenario modeling and detailed relocation schedule will enable to determine the optimal depth of transition and crushing plant location.
