Explosives & Initiation

Explosives release chemical energy in microseconds to break rock. Learn how detonation works, common mining explosives (ANFO, emulsions, heavy ANFO, packaged products), properties (density, velocity of detonation, energy, water resistance), oxygen balance, initiation systems (detonators, boosters, electronic delays) and safe handling.

Mining uses millions of tonnes of explosives a year, mostly ammonium nitrate–based. The right explosive, correctly primed and timed, gives good fragmentation, safe walls and low vibration. Poor choices cause misfires, fumes, flyrock and wasted money.

An analogy

An explosive is a fuel and oxygen already mixed together — like a campfire compressed into a split second, so fast that the gases hammer the rock apart.

Common explosives

ANFO: cheap, pourable, density 0.8–0.85, no water resistance, VOD ≈ 3500–4500 m/s. Emulsions: water-in-oil, water resistant, density 1.1–1.3, higher VOD. Heavy ANFO blends combine both. Packaged cartridges and boosters (cast pentolite) prime bulk products.

Oxygen balance and fumes

An explosive with zero oxygen balance produces mainly CO₂, H₂O and N₂. Too much fuel (negative OB) produces toxic CO; too little fuel (positive OB) produces NOx (orange fumes). Water-damaged ANFO also produces NOx.

Initiation systems

Detonator → booster → column charge. Shock-tube (non-electric) systems use surface and in-hole delays. Electronic detonators are programmable to 1 ms accuracy, improving fragmentation and vibration control. Detonating cord is used for presplits and trunklines.

Safety and regulations

Licensed storage (magazines), separation distances, transport rules, charging procedures, lightning protection, misfire handling and blast exclusion zones. Security against theft is legally required. Mobile manufacturing units (MMUs) mix bulk explosives at the hole.

Formulas

• Charge per metre: q_l = ρ_e × π d² / 4
• Mixture oxygen balance: OB_mix = Σ w_i OB_i
• ANFO: 94.5 % AN (OB +20 %) + 5.5 % fuel oil (OB ≈ −343 %) → OB ≈ 0

Worked example: ANFO oxygen balance

Given: 94.5 % AN (OB +20 %); 5.5 % fuel oil (OB −342.9 %)

Find: OB of ANFO.

• OB = 0.945 × 20 + 0.055 × (−342.9)
• = 18.9 − 18.86 = +0.04 %

Answer: ≈ 0 — balanced mixture

Worked example: Charge per metre

Given: 115 mm hole; ANFO 0.85 g/cm³

Find: kg/m.

• q = 850 × π × 0.115²/4 = 8.83 kg/m

Answer: ≈ 8.8 kg/m

Common mistakes

• Wrong: Loading ANFO in wet holes. — Right: Use emulsion or line holes — wet ANFO fails and makes NOx.
• Wrong: Too much fuel oil 'for power'. — Right: Excess fuel gives CO and wastes energy.

Applications

• Open-pit production: Bulk ANFO/emulsion from MMU trucks.
• Underground development: Pneumatically loaded ANFO or pumped emulsion.
• Wet holes: Emulsion or heavy ANFO for water resistance.

Key takeaways

• ANFO cheap; emulsion water resistant.
• q = ρ π d²/4.
• Zero oxygen balance minimises fumes.
• Electronic delays improve results.

Going further

• Detonation physics: Chapman–Jouguet theory, non-ideal detonation in ANFO.
• Energy partitioning: Shock vs gas energy for different rock types.
• Low-NOx products: Emulsion formulations reducing fume events.