Mining Explosives: Types and Properties

Choosing mining explosives is one of the first decisions in any blast design. The same hole pattern can give good or poor fragmentation depending on the product, whether the hole is wet, and how the charge is initiated. Blast geometry, burden, spacing and timing are covered separately in drilling and blasting basics.

Low and high explosives

Explosives are classified first by how the reaction travels through them.

• Low explosives deflagrate: the reaction front moves by burning, slower than the speed of sound in the material. Black powder is the classic example, now used mainly for dimension stone and in safety fuse.
• High explosives detonate: a supersonic shock wave travels through the charge, sustained by the reaction directly behind it, giving very high pressure at the borehole wall.

High explosives are further divided by sensitivity. Under US regulations and IME definitions, a blasting agent is a mixture that cannot be detonated by a No. 8 test detonator when unconfined. ANFO and most bulk emulsions fall in this group, so they need a primer. Cap sensitive explosives, such as dynamite and packaged cap sensitive emulsions, can be fired directly by a detonator.

Types of mining explosives

ANFO

ANFO is porous ammonium nitrate prills coated with fuel oil. It is the lowest cost and most widely used bulk product in surface mines because it is safe to handle and easy to load. Poured density is about 0.8 to 0.85 g/cm³. Its velocity of detonation (VOD) depends strongly on hole diameter and confinement, rising from roughly 2500 m/s in small holes to around 4500 m/s in large diameter surface holes. Its weakness is water: ammonium nitrate dissolves, so ANFO has no water resistance.

Slurries and watergels

Watergels (slurries) are ammonium nitrate based products in a thickened water gel. The gel gives good water resistance and a density of about 1.1 to 1.3 g/cm³. They were the first widely used water resistant bulk product and have largely been replaced by emulsions.

Emulsions

An emulsion is a water in oil system: microscopic droplets of oxidiser solution are held in a thin continuous fuel phase. The fine structure gives intimate contact between oxidiser and fuel, which produces a high VOD, commonly about 4500 to 6000 m/s, and excellent water resistance. Density is adjusted with gassing or microballoons, usually within about 1.1 to 1.3 g/cm³, because an emulsion that is too dense becomes insensitive.

Heavy ANFO

Heavy ANFO blends emulsion with ANFO. The emulsion fills the voids between prills, raising density and bulk energy above ANFO. Blends with a low emulsion share behave much like a denser ANFO, while blends rich in emulsion become water resistant.

Dynamite

Dynamite is a nitroglycerine based, cap sensitive cartridge explosive with high density and strength. Because of its sensitivity and handling risks, it has been largely replaced by emulsion and watergel cartridges.

Boosters and primers

A primer is a cartridge or cast booster holding a detonator, placed in the column to start the main charge. Cast boosters made of pentolite or similar high VOD compositions have velocities above 7000 m/s and high detonation pressure, which reliably brings a non cap sensitive column such as ANFO to its full steady detonation.

Download diagram (WebP)

Initiation systems

• Safety fuse and plain detonator: a black powder core burns at a controlled rate and fires a plain detonator. Timing is poor, so it is now rare in mining.
• Electric detonators: a current heats a bridge wire that ignites a delay element and then the base charge. Circuits can be tested before firing, but the detonators are vulnerable to stray currents, static electricity, radio frequency energy and lightning.
• Shock tube (non electric, often called Nonel): a thin plastic tube coated inside with a small amount of reactive powder carries a low energy shock wave at about 2000 m/s to the detonator. It is immune to stray electrical energy.
• Electronic detonators: a microchip and capacitor replace the pyrotechnic delay element. Each detonator is programmed and fires only on a coded signal, giving delay accuracy to about 1 ms.

Key properties of mining explosives

Velocity of detonation

VOD is the speed at which the detonation front travels through the column, measured in m/s. A higher VOD gives a sharper shock and higher borehole pressure, suited to hard, massive rock. Lower VOD products give more heave, which suits softer or heavily jointed rock. Every explosive has a critical diameter below which detonation cannot be sustained, and VOD falls as the hole diameter approaches it.

Density and detonation pressure

Density sets how much explosive fits in a metre of hole and strongly affects detonation pressure, which is often estimated as

$$P_D \approx \frac{\rho_e D^2}{4}$$

For ANFO at 820 kg/m³ and 3500 m/s this gives about 2.5 GPa, while an emulsion at 1150 kg/m³ and 5000 m/s gives about 7.2 GPa.

Energy and strength

Energy is reported as relative weight strength (RWS) and relative bulk strength (RBS), both normally with ANFO set at 100. RBS compares energy per unit volume, so a dense emulsion can have a lower RWS than ANFO yet a higher RBS.

Water resistance

Water resistance is the ability to stay detonable after exposure to water. ANFO has none, watergels and heavy ANFO rich in emulsion are good, and emulsions are excellent.

Fume class

Detonation should produce mainly water vapour, carbon dioxide and nitrogen. A poorly balanced product, water contamination or poor confinement creates carbon monoxide and nitrogen oxides, the orange fumes seen after some ANFO blasts. IME fume classes rate the poisonous gases produced per 1.25 by 8 inch cartridge: Class 1 under 0.16 ft³, Class 2 0.16 to 0.33 ft³ and Class 3 0.33 to 0.67 ft³. Underground use needs good fume classes and adequate mine ventilation.

Values vary by manufacturer, diameter and confinement.

Worked example: charge mass for mining explosives

Key formulas for mining explosives

Selecting mining explosives in practice

Selection balances rock properties, hole conditions and cost. Hard, massive rock with high strength, assessed through rock mass classification, benefits from dense, high VOD products, at least in the bottom of the hole where the toe is hardest to break. Wet holes call for emulsion or water resistant heavy ANFO. Large open pit mines commonly load dry holes with ANFO and wet ones with emulsion or heavy ANFO.

Safe practice follows the magazine, transport, loading and misfire rules in MSHA 30 CFR Part 56 and 57 Subpart E and the IME Safety Library Publications: keep detonators apart from explosives until priming, never force a primer down a hole, check for stray current risk, clear the blast area, and wait the required time before inspecting for misfires. The wider risk controls are covered in mine hazards and controls.

Mining explosives interview and viva questions

1. What is the difference between deflagration and detonation? Deflagration is a subsonic burning reaction, as in black powder, while detonation is a supersonic shock wave sustained by the reaction behind it, as in ANFO and emulsions.
2. Why does ANFO need a booster? ANFO is a blasting agent that is not cap sensitive, so a high VOD booster with a detonator is used to bring the column to steady detonation.
3. Why are emulsions preferred in wet holes? Their oxidiser droplets are sealed in a continuous fuel phase, which gives excellent water resistance, and their density above 1.0 g/cm³ lets them displace water in the hole.
4. What is relative bulk strength? It is the energy per unit volume of an explosive compared with ANFO at 100, found by multiplying relative weight strength by the density ratio.
5. What causes orange fumes after a blast? Nitrogen oxides formed when the explosive is poorly balanced, contaminated with water or poorly confined, which is why fume class and good loading matter.