No longer just a buzzword, sustainability is a trend that will help provide the aggregates industry with the social license it needs to operate.

by Roy H. Grau III and Robert B. Krog

Increased awareness of a healthy work environment is common to all industries and the stone industry is no exception. The presence of operating diesel equipment, welding, stone production blasting, and silica dust in a working environment all contribute to air quality issues in underground stone mines. Particularly recognized is the exposure of workers to diesel particulate matter (DPM). Recent regulations call for a DPM standard of 160_TC ug/m³ (TC = Total Carbon). The National Institute for Occupational Safety and Health (NIOSH) has conducted research to improve the ventilation of these mines and consequently reduce the exposure of workers to harmful airborne contaminants. The results have shown that the ventilation of underground stone mines can be significantly enhanced using improved ventilation controls and techniques.

Large-opening stone mines present specialized ventilation challenges because their entries are much larger than in other types of mines. Most stone mines have entries about 40 feet wide and up to 30 feet high, with bench areas that may extend even higher. These large entries present the positive opportunity to move large air quantities at relatively low mine fan pressures resulting in lower fan horsepower and reduced operating costs. However, the difficulty in ventilating large-opening mines is that even with large air volumes, the air velocity is often less than 100 feet/minute. Higher ventilation velocities more quickly dilute and remove contaminants from the face areas. Higher ventilation airflow velocities can be achieved by effective ventilation planning and proper choice of ventilation fans.

Good mine planning is essential

Ventilation is improved significantly in underground stone mines by considering future ventilation needs in the mine planning process (Grau and Krog, 2008, Krog et al., 2004). By strategically selecting stopping designs, stopping and auxiliary fan locations, and stopping types, the mine ventilation efficiency (air quantity at face compared to total mine airflow) and, consequently, the air quality will be improved. Although an adequate large-opening mine ventilation system depends on many factors, preventing leakage between stoppings is a primary consideration. With reduced leakage, more of the airflow generated by the main mine fans reaches the faces to dilute and remove harmful contaminants.

The basic principles of mine ventilation planning include determining the air quantity that is needed to dilute and render harmless all contaminants, how to produce this airflow, and how to direct this airflow to the required mine locations. The necessary airflow quantity is highly dependent on the diesel equipment in use. Cleaner burning engines require less air to dilute diesel emissions and, consequently, require less production from the main mine fan. A mine with less stopping leakage also reduces the needed airflow from the main mine fan because a higher percentage of the airflow from the main fan reaches the face areas. In order to help mine operators determine how much airflow is necessary, NIOSH developed a user-friendly, stand-alone computer program called the “air quantity estimator” (Robertson et al., 2004). The program is available for download on the NIOSH mining Web site (www.cdc.gov/niosh/mining/products/product4.htm). The program uses diesel engine information from several sources to provide a starting point to estimate the air quantity required to dilute DPM contaminants to statutory levels in the main return of a mine.

Several studies performed by NIOSH in mines that produced from 1.0 million to 1.2 million tons of stone per year showed that air quantities of about 750,000 cfm were needed to comply with a 400_TC ug/m³ DPM concentration limit, although the necessary ventilation air quantity is highly dependent upon the diesel equipment in use and resultant ventilation efficiencies (Grau et al., 2002). In order to comply with the current DPM exposure limit of 160_TC ug/m³, the estimated required air flow would be greater.

Since haul trucks are the single highest source of DPM, confining the truck haulage to the return air courses is a good working practice and should be done whenever possible. Truck drivers’ exposures to DPM can be minimized by ensuring that truck cabs are equipped with positive filtration systems and the correct filters are used (Noll et al., 2008.)

Use the right fan

Generally, there are two types of fans used in underground stone mines: vane-axial and propeller fans. Vane-axial fans are designed to deliver air volumes at high static pressures. Propeller fans deliver large air quantities, but at much lower pressures. NIOSH observations have found that a total mine fan pressure of less than 0.75 inches (w.g.), not including shock losses, is common in drift portal stone mines and stone mines with slopes or declines operating at depths less than 100 feet or with shaft diameters greater than 15 feet. By comparison, coal mines have higher resistances, which typically range from 2 inches (w.g). to 10 inches (w.g.). The magnitude of the total mine fan pressures is an important factor when choosing what type of fan to use in a stone mine. Propeller fans are often the best choice for ventilating stone mines since these fans generate large air flows at lower static pressures, generally require less capital and lower operating costs, and have lower operating noise levels compared to vane-axial fans (Krog and Grau, 2006). Generally, second-hand vane-axial fans were readily available for sale and thus they are prevalent in many large-opening mines, but propeller fans should be considered in conditions such as those described above.