Improving Health and Safety at Surface and Underground Mines by Implementing Emerging Technologies Coupled With Geotechnical and Climatic Modeling
AuthorScalise, Kyle Austin
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Health and safety are an integral part of any mining operation – surface or underground. When deficiencies are identified, they usually lie with the factors that contribute to unsafe and unhealthy working conditions. The health and safety focus for modern underground mines involves improving the subsurface thermal climate through modeling and the use of waste heat. The health and safety focus for surface mines can relate to the slope stability and highwall management utilizing slope modeling and data analysis to develop improved blasting criteria.Section I. Improving the Subsurface Thermal ClimateUnderstanding the effects of various ventilation and climatic parameters on the working conditions and comfort in underground mines is critical for efficient ventilation system design, cost savings, and ensuring the health and safety of mine workers. To understand the effects that the thermal flywheel effect (TFE) has on the ventilation system design and the need for a potential cooling system, a comparison and analysis of the wet-bulb temperature and the dry-bulb temperatures of the ventilating air at the bottom of an intake shaft using two modeling software packages, VentsimTM and ClimsimTM is completed. The comparison shows the consequences and importance of taking into account the TFE when predicting the climatic conditions in future underground mines, especially when deciding on whether a cooling system should be implemented in order to provide adequate climatic conditions in the production stopes, dead-end development headings, and throughout the mine. VentsimTM is able to account for aspects of the TFE, while ClimsimTM does not. During summer, VentsimTM predicts the mine air wet bulb temperature to be 13.33°C and ClimsimTM predicts a wet bulb temperature of 29.11°C. This temperature difference is attributable to the TFE. Both software packages have their uses and are used within the mining industry. However, VentsimTM and ClimsimTM cannot take advantage of large volumes of ventilation and climatic data. Large volumes of ventilation and climatic data, including air volume, barometric pressure, dry bulb temperature and relative humidity were collected at active underground precious metal mines in Nevada, which allows for the determination of wet-bulb temperature (Tw) and other key parameters. Through the utilization of neural networks, the wet-bulb temperatures at the bottom of the intake shafts are predicted, while taking into account the “thermal flywheel effect” (TFE). The wet-bulb temperature is one of the most important climatic parameters to model and understand because it significantly affects the working conditions and the cooling capacity of the ventilating air. The accurate prediction of the wet-bulb temperatures at the bottom of intake shafts is critical when assessing the climatic conditions in future underground mines and deciding on whether a cooling system is needed to ensure safe working conditions throughout the mine. By utilizing accurate predictions of wet-bulb temperatures and other climatic parameters, mine personnel will be safer, and a more accurate ventilation design can be achieved, resulting in major cost savings for underground mines.The utilization of Nonlinear Autoregressive with Exogenous Input Neural Networks (NARXNN) makes it possible to accurately predict the wet-bulb temperature (Tw) of the mine air at the bottom of intake shafts at various Nevada mines based on surface Tw. Three data sets for different intake shafts at Nevada’s underground mines were analyzed using NARXNN. The results for the predicted data using closed loop step-ahead prediction showed acceptable error as well as definitive trends following of the collected data. The root-mean squared errors (RMSE=√MSE) for Shafts #1-3 were 0.57 °C, 0.33 °C and 0.61 °C, respectively. These errors are low and are good predictions of the actual underground environment. Without the accurate prediction using NARXNN it has been shown that when the thermal flywheel effect is not accounted for. The difference between simulated and measured air temperatures at the bottom of intake shafts can vary from 6°C to 10°C. The NARXNN also performed significantly better when compared to VentsimTM and ClimsimTM. When compared to the climatic data the NARXNN had an error of only 0.46°C while VentsimTM and ClimsimTM had errors of 7.94°C and 14.67°C, respectively.Many deep and highly mechanized underground metal mines require cooling. In many cases, appropriate working conditions might be fulfilled by ventilation alone, but some require the implementation of cooling systems. The most common cooling systems are bulk air cooling and localized cooling. These systems are effective, but they use considerable amounts of electricity. Ground source heat pumps (GSHP) can supplement existing cooling and heating systems, while geothermal systems can cool the mines in their entirety if there are significant sources of geothermal energy are present. A major advantage of thermal systems are their utilization of mine waste heat, flexibility and ability to be utilized on a large scale. GSHP and geothermal systems have yet to be integrated into mines on a small scale as presented within this work, but these systems have the ability to be integrated as complements to in-place cooling systems, thus reducing energy consumption. The cost for industrial electricity in Nevada for February of 2019 was 5.16 cents/kWh (U.S. Energy Information Administration, 2019). The low-end electricity generation capability for an ORC at 70°C is 3.3 kW. For one year, the energy produced from the ORC would be 28,908 kWh. If one ORC unit were to be implemented, based on the price of industrial electricity in Nevada with an energy output of 28,908 kWh, it would save the mine around $1,500 per year. It should be noted that this is only direct electricity savings and does not include the savings associated with reducing the waste heat rejected into the ventilating air. The savings from waste heat is much more complex because this system has not been implemented into a mining environment and the performance in unknown.Section II. Slope Stability and Highwall ManagementGeotechnical modeling of mines is crucial for understanding the many factors that can pose potential hazards or limit production opportunities. The process of back analyses for modeling is often used, but in many instances, the back analyses can be cumbersome if large data sets are used. While these models and back analyses may be accurate, these types of back analyses may not be optimal to be used at mines and by engineers because the process is so time-consuming. This paper introduces event-based back analyses, which utilizes prior events to create much more accurate models while eliminating large modeling time because the models are optimized from previous events. The completed analyses supply a more detailed insight of the potential dangers at Round Mountain as well as provide a better understanding of how risk results in missed opportunity. The more we understand about mining activities and processes through modeling, the safer and more productive mines will be.Monitoring open pit mine blasting vibrations is necessary to understand the impact that blasts have on the highwall. Assessing the performance of a mine blast from a geotechnical perspective typically requires analyzing ground motions, performing visual inspections and utilizing site-specific criteria, such as peak particle velocity and dominant frequency to rank the quality of the blast. This study compiles a combination of geophone data, which obtain peak particle velocity and dominant frequency of ground motions, radar, which observe slope velocity and current blasting practices to improve upon the blasting criteria present at Round Mountain. The blasting ground motion events that were collected for this interpretation are wall control techniques, trim blasting and modified production blasting. In addition to improved blasting criteria, a comparative assessment of trim blasting vs. modified production blasting is completed to better understand the ideal wall control technique to be used at Round Mountain. By compiling several sources of data to improve current blasting criteria, a safer geotechnical environment is created, which contributes to less downtime and more cost savings for Round Mountain. The results of this work describe a new integration of radar assessment with ground vibration monitoring to develop new blasting criteria for vibration sensitive locations.