Mineral excavation is one of the processes which requires the minimization of material handling cost. Cost-per-tonne evaluation of loading equipment involves the explanation of some key concepts. The electric rope shovel is a rather big equipment which uses winches, wire ropes, and electric engines to move its bucket forward and upward into the face of rock. Hydraulic excavator uses fluid pressure and cylinders to move its boom and bucket, usually it excavates towards itself. Both of these pieces of equipment play an important role in open pit mining but have very different economic aspects while working with highly compacted material.
Hard digging is a term which means challenging geological conditions in which the material is hard to dig because of the high strength of rock or insufficient fragmentation due to blasting process. In a mining operation, the fragmented state of ore greatly influences the performance of machine and the load on the loading equipment (Nikkhah et al., 2022). Poor fragmented rock increases the digging resistance and makes the machine work harder to load this material. As a result, the cycle of loading becomes longer and mechanical wear on the ground engaging tools is higher.
The main point where the cost-per-tonne of both machines differs is related to the capital expenditure (CapEx) and the operating expenditure (OpEx). Electric rope shovels have a rather high CapEx – in tens of millions of dollars but they can be used during up to 30 years of service life. Since this equipment uses electricity and does not have complicated hydraulics, it has rather low OpEx. Hydraulic excavators are much cheaper initially but their service life is 10-15 years only. This equipment has high hourly running cost due to the usage of diesel and complicated hydraulics.
The design of each machine determines the load on maintenance in case of hard digging. Hydraulic excavators are good at providing a high breakout force of loading in any direction and this quality makes them effective to use with poorly blasted muck piles (Edwards & Griffiths, 2000). But the hard load on hydraulic pumps and seals makes frequent replacement and stoppages necessary. Electric rope shovels cannot provide this selective articulation but they have rather strong mechanical construction and can resist heavy shock loading.
In the end, electric rope shovel wins in the practical calculation of cost-per-tonne in long-term hard digging situations. Despite the effectiveness of a hydraulic excavator in solving the problem of hard geological face with the help of high breakout force, this advantage results in higher hydraulic maintenance and high cost of fuel and, therefore, higher cost-per-tonne. In contrast, electric rope shovel maintains the constant rhythm of loading. Thanks to amortizing its high initial cost in millions of tonnes and high mechanical availability, the rope shovel provides the lowest cost-per-tonne.
To summarize, the choice between these two machines in hard digging conditions is connected to the size of the project. Even though hydraulic excavators are rather cheap initially and provide better selective breakout force, high wear rate and operating expenditures make their long-term cost-per-tonne higher than the one of electric rope shovel. Electric rope shovel has a high initial cost but compensates for it with great durability. For long-life open pit mines with high volume of hard rock, the rope shovel will give the most cost-effective solution.
References
Edwards, D. J., & Griffiths, I. J. (2000). Artificial intelligence approach to calculation of hydraulic excavator cycle time and output. Mining Technology, 109(1), 23–29. https://doi.org/10.1179/mnt.2000.109.1.23
Nikkhah, A., Vakylabad, A. B., Hassanzadeh, A., Niedoba, T., & Surowiak, A. (2022). An Evaluation on the Impact of Ore Fragmented by Blasting on Mining Performance. Minerals, 12(2), 258. https://doi.org/10.3390/min12020258
