The Management of Post-Blast Fumes

Post-blast fume is a product of combustion of ammonium nitrate based explosives from a blast. The products of combustion from a blast may include oxides of nitrogen, carbon monoxide, carbon dioxide, nitrogen and water vapour. Oxides of nitrogen and carbon monoxide are referred to as toxic fumes. Nitrogen dioxide is visible as a reddish brown colour; the others are not visible.

CHEMICAL REACTION

• Under ideal conditions the products of explosion are nitrogen gas, carbon dioxide and water: 3NH4NO3 + CH2 → 3N2 + CO2 + 7H2O .. (222 kJ/kg) Under these conditions, the VOD is at its peak and energy released is maximum
• If oxygen Negative: 2NH4NO3 + CH2 → 2N2 + CO + 5H2O .. (194 kJ/kg) This gives CO instead of CO2 & energy is lower
• If oxygen Positive: 5NH4NO3 + CH2 → 4N2 + 2NO + CO2 + 7H2O .. (143 kJ/kg) This gives N0x instead of CO2 & energy is lower

MAJOR CAUSES OF POST-BLAST FUMES

• Use of emulsion/ANFO formulation which is deficient in oil content or rich in ammonium nitrate content.
• Poor diesel absorption by ammonium nitrate prills.
• Water ingress in products as the product exceeds sleep time limits.
• Critical density of the products used.
• Poor loading (often hose handling) practices.
• Critical diameter of the blast holes.
• Product desensitisation by dynamic pressure desensitisation (DPD).
• Ground conditions such as presence of cavities, cracked ground.
• Chemical reaction of acidic soils and pyrite during/after detonation.
• Lack of proper confinement (stemming of blastholes).
• Presence of dynamic water within the blastholes.

EFFECTIVE WAYS TO REDUCE FORMATION OF POST-BLAST FUMES

• Use explosives products that are suited to blast conditions.
• Ensure sleep times are kept to the practicable minimum.
• Ensure the Mobile Processing Units and other equipment are calibrated.
• Always load the product using the appropriate techniques.
• Proper operation of the Mobile Processing Units and equipment.
• Ensure the gassing agent delivery rates are correct.
• Ensure effective stemming is in place.
• Monitor the amount of hydrocarbon (diesel) in the product.
• Keep ammonium nitrate dry.

Edition @16 March 2022 JR