In almost every underground metalliferous mine, blasting is an every day occurrence. Blasting takes place in both development and production, with considerable size variation in production blasts.
Before a firing, underground personnel are removed from the workings to a safe place underground or on the surface. Once the firing has been completed, the time that is allowed to elapse before personnel are allowed to return to the workings is known as the re-entry time. During this time it is calculated or at least expected that the concentrations of noxious gases have been diluted to safe levels by the mine ventilation.
This thesis attempts to establish whether various underground metalliferous mines around Australia have in place appropriate re-entry times. It briefly examines how mines reduce the exposure of blast fumes to personnel with their ventilation systems. Collected test data is examined to establish how well theoretical models of gas dilution represent the actual dilution of blast fumes. This data is also analysed to find the time at which it is safe to re-enter a blast area based on measured concentrations of carbon monoxide, nitric oxide and nitrogen dioxide.
Gas dilution decayed exponentially in all cases where a constant air quantity was supplied to blast void. In the case of development headings ventilated with ducting, there does not appear to be a correlation between the decay constant fitted to test data and the decay constant calculated from quantity and volume measurements of the blast area, assuming the void exists beyond end of the ducting. For stopes ventilated with primary ventilation, there appears to be a correlation between the calculated decay constant and the decay constant fitted to the data, but more test data would be required to confirm this.
Re-entry times can vary considerably whether they are based on STEL or TWA limits. If the TWA is used most development headings will have cleared in under 45 minutes, but may take over an hour if the ventilation is poor. For ventilated stopes, it may take up to 2 hours for gas concentrations to fall below the TWA.