Reducing temperatures and dust concentrations of fresh intake air in deep-level mines

Abstract

Some of the world's deepest mines are located in South Africa. Mines are progressively increasing in depth to obtain mineral deposits, with the deepest mine already reaching 3 891 km deep. As the mine is deepened and virgin rock temperatures (VRT) increase, the mine ventilation network must be expanded and optimised in order to provide safe working conditions, mandated and regulated by the Mine Health and Safety Act of South Africa.

Increased temperatures add to the heat absorbed by the intake air. The return air from the development ends mixes with the intake air of the closest crosscut with through-ventilation, heating the air and introducing additional airborne dust. The air temperature may not exceed a wet-bulb temperature (Twb) of 32.5 °C and a dry-bulb temperature (Tdb) of 37.0 °C. At the same time, the air velocity of working areas must be between 0.3 m/s and 8 m/s. The lower limit ensures sufficient airflow and the upper limit avoids increased airborne dust. The dust concentration occupational exposure limit (OEL) is 0.1 mg/m3. Properly executed ventilation control in the mine can alleviate the extra heat and dust added by development end areas.

The study aims to provide a method to mitigate fresh intake air mixing with hot return development air. The study's objectives are to 1) develop a method to decrease localised temperatures and dust concentrations in deep-level mines, 2) apply the method provided to improve the ventilation conditions, and 3) validate the simulation results and method after the optimal solution has been implemented.

The proposed method consists of five steps: identification of air mixing, creation of a ventilation simulation model, identification of possible solutions, development and evaluation of simulation results, and implementation and validation of solution.

The method was applied to two different mines, Mine A and Mine B, with problematic ventilation networks. Two case studies were evaluated where air mixing was identified at the deepest crosscut farthest from the shaft. Solutions to mitigate the air mixing were developed by creating calibrated ventilation simulation models with a mean absolute error (MAE) of < 5%. Thereafter a list of ventilation and cooling methods was created to mitigate localized heat and dust problems.

The provided decision tree diagram suggested the same mitigation strategies for both case studies namely, raisebore hole (RBH) return and unequipped ore pass return. The calibrated ventilation simulation models were used to virtually implement the possible solutions and create different scenarios. The simulation results of the scenarios were compared with an evaluation matrix, and the optimal solutions were chosen. The solution for case study 1 observed a 3 °C drop in Twb at the intake of the working crosscut. At the same time solution for case study 2 observed a 5 °C drop in Twb at the intake of the working crosscut. The respective simulated solutions yielded satisfactory results and were implemented. The validation of the simulated results was done by empirical data acquired after implementation.

The implemented solutions improved the conditions of the working crosscut by decreasing the Twb by 3 °C for case study 1 and 5 °C for case study 2. At the same time, preventing airborne dust created in the development end from entering the intake air stream for both case studies. Thus, case study 1 and case study 2 satisfied all the study objectives.