A series of tests was undertaken on a recirculation scheme in a deepudlevel gold mine to establish the effect of controlled recirculation ofudthe mine ventilation air on blast contaminant dissipation.udClarification was needed as to whether the existing re-entry intervaludof three hours would have to be extended with the introduction ofudcontrolled recirculation. The re-entry interval is a time interval,udafter blasting, stipulated by the Inspector of Mines during which theudworkings are being cleared of blast contaminants and during which timeudno persons are permitted tc enter the workings.udThe fresh and recirculated air flow rates were varied and their effectsudon blast contaminant dissipation measured. Gas concentrations of theudoxides of nitrogen (N0X ) and carbon monoxide were monitoredudcontinuously in the return air. Dust levels were monitored in theudreturn air from two hours before the blast to four hours after theudblast.udTwo gas models (mixed-volume and plug-flow) and residence time analysisudwere used to analyse the data.udIn all the tests, the critical blast contaminant for determining theudre-entry interval was found to be NO*. In addition, the followingudparameters affected the re-entry interval; the amount of explosivesudignited daily, the volume of the workings into which the NO* isuddissipated by the ventilating air, the time taken for air to completeudone circuit (the cycle time), leakage and short circuiting of air, andudthe fresh air flow rate. The recirculated air flow rate was found toudhave negligible effect on the re-entry interval.
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