Controlled recirculation of mine ventilation air: its effect on blast contaminant dissipation

Alexander, Nicholas Anthony
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A series of tests was undertaken on a recirculation scheme in a deep level gold mine to establish the effect of controlled recirculation of the mine ventilation air on blast contaminant dissipation. Clarification was needed as to whether the existing re-entry interval of three hours would have to be extended with the introduction of controlled recirculation. The re-entry interval is a time interval, after blasting, stipulated by the Inspector of Mines during which the workings are being cleared of blast contaminants and during which time no persons are permitted tc enter the workings. The fresh and recirculated air flow rates were varied and their effects on blast contaminant dissipation measured. Gas concentrations of the oxides of nitrogen (N0X ) and carbon monoxide were monitored continuously in the return air. Dust levels were monitored in the return air from two hours before the blast to four hours after the blast. Two gas models (mixed-volume and plug-flow) and residence time analysis were used to analyse the data. In all the tests, the critical blast contaminant for determining the re-entry interval was found to be NO*. In addition, the following parameters affected the re-entry interval; the amount of explosives ignited daily, the volume of the workings into which the NO* is dissipated by the ventilating air, the time taken for air to complete one circuit (the cycle time), leakage and short circuiting of air, and the fresh air flow rate. The recirculated air flow rate was found to have negligible effect on the re-entry interval.