Haulage system optimization for underground mines A discrete event simulation and mixed integer programming approach

Abstract: In coming decades, many underground mines will operate at greater depths, which will affect many operational factors such as increased rock stress, longer haulage distances, and higher energy consumption, which potentially can generate lower production rates. The increased rock stresses may lead to smaller sized openings, further restricting the size of loading and hauling equipment that can be used. Longer hauling distances result in increased energy consumption for loading and hauling equipment, and in turn, high energy consumption increases heat and gas emissions for diesel equipment. Heat emission increases ventilation costs as large volumes of air must be circulated to cool diesel engines and simultaneously maintain adequate air quality for personnel. The research presented in this thesis was carried out to evaluate and analyze different haulage systems, including diesel and electric trucks, shafts, and belt conveyors. The aim was to determine how these various material-handling equipment may produce the desired production objectives and lead to lower energy costs. The net present value (NPV) of the mine plan at increasing mining rates and altered commodity prices was also analyzed. The method used was the combination of discrete event simulation and mixed integer programing. Discrete event simulation was used to estimate mine production for different haulage systems, and the results were used to compute appropriate mining costs for each hauling option. Mixed integer programming (MIP) was then used to generate the optimal production schedule and mine plan. The analysis showed that an increasing use of electric trucks will have positive effects on production improvement because electric trucks have shorter cycle times than their diesel counterparts. Therefore, electric trucks can make more cycles than diesel trucks in the same period of time. The analysis also showed that low-profile equipment will remain viable for haulage in high stress environments that result in smaller sized mine openings. In addition, when friction hoist systems are used, rope speed and skip payload play important roles in production improvement. With belt conveyors, production improvements can be obtained by increasing surcharge angle and running the belt at a low speed. For long hauls, the troughing angle should be increased and the belt operated at a higher speed. Energy costs increase with depth and are higher for diesel trucks compared with other haulage options. At 1000-meter depths and with current energy prices, energy costs for diesel trucks, electric trucks, belt conveyor, and shaft account for 62%, 54%, 25%, and 14% of the total haulage costs, respectively. These findings indicate that minimizing the usage of diesel engine machines will have greater benefits towards cost reductions in an era of increasing energy prices and greater mine depths. Diesel machines also have high heat and gas emissions, which increases operating costs particularly for deeper mines where heat emissions increase ventilation costs. Changes in mine plans based on changing commodity prices at a fixed mining rate resulted in an increase in the NPV from $96M to ultimately $755M for the studied case. An increase in mining rate from 300,000 to 450,000 tonnes raised the NPV to $773.45M. This finding indicates that even though an increase in mining rates increases costs, companies may find that pursuing such a course is beneficial at certain commodity prices, especially when the price is elevated. When the price falls, increasing mining rate may need a detail evaluation of other parameters such as grade, recovery, and investment changes. The evaluation showed that the method of combining discrete event simulation and mixed integer programming can yield a feasible solution and better understanding of the operational systems and reduce risks in selecting a system before it is implemented. This study provides mining companies an analysis of the use of underground haulage systems that can aid decision making.