ANFO, water gels, and emulsions are three common types of blasting agents used in mining, quarrying, and construction. They have distinct advantages and disadvantages, making them suitable for different applications.
ANFO (Ammonium Nitrate / Fuel Oil)
ANFO is a simple explosive mixture of porous ammonium nitrate prills and fuel oil (typically diesel).
Advantages
- Low cost: it’s the cheapest commercial explosive, making it extremely economical for large-scale operations.
- Safety: ANFO is very insensitive (it’s a blasting agent, not a high explosive), meaning it’s safe to manufacture, transport, and handle. It requires a primer or booster to detonate.
- Ease of use: it can be easily manufactured on-site and loaded in bulk by pouring or blowing it into dry boreholes.
- Good heave: it produces a large volume of gas upon detonation, which is effective for “heaving” or moving large amounts of softer rock.
Disadvantages
- No water resistance: ammonium nitrate is highly soluble in water. ANFO cannot be used in wet or damp boreholes without expensive and cumbersome protective liners, as water will dissolve the AN and render it non-explosive.
- Low bulk strength: it has a low density, meaning it has less energy per volume compared to water gels and emulsions. This requires larger diameter boreholes to achieve the same blasting power.
- Toxic fumes: if not mixed in the correct ratio, ANFO can produce significant amounts of toxic post-blast fumes, such as nitrogen oxides (NOx) and carbon monoxide (CO).
- Low detonation velocity: it has a relatively low velocity of detonation (VOD), making it less effective for shattering very hard rock.
Water Gels (Slurries)
Water gels (or slurries) are an older technology largely superseded by emulsions. They consist of an oxidizer (like ammonium nitrate), fuel, and a sensitizer suspended in a gelled aqueous solution.
Advantages
- Good water resistance: the gel matrix provides good protection against water, allowing it to be used in wet boreholes where ANFO would fail.
- Higher bulk strength: they are denser and have a higher velocity of detonation (VOD) than ANFO, providing more energy per volume for breaking harder rock.
- Adjustable sensitivity: their formulation can be adjusted to make them either cap-sensitive (detonated by a blasting cap) or require a booster.
- Safer than dynamite: they were developed as a safer, less toxic alternative to nitroglycerin-based dynamites.
Disadvantages
- Higher cost: they are more expensive to produce than ANFO.
- Storage stability: over time, the solid ingredients can settle out of the gel matrix, and the gel structure can break down, reducing its shelf life and performance.
- Less efficient than emulsions: they are generally considered less stable and less powerful than modern emulsion explosives.
- Temperature sensitivity: their performance and stability can be negatively affected by very low temperatures.
Emulsions
Emulsions are the most modern of the three. They consist of microscopic droplets of a water-based oxidizer solution suspended in a continuous fuel oil phase, stabilized by an emulsifying agent. This creates a “water-in-oil” structure.
Advantages
- Excellent water resistance: the oil-continuous phase makes emulsions virtually waterproof, making them ideal for blasting in very wet conditions.
- Highest bulk strength: they have the highest density and velocity of detonation (VOD) of the three, providing maximum energy for breaking the toughest rock.
- Excellent safety: emulsions are extremely safe. The base matrix is classified as an oxidizer and is not explosive until it is sensitized (usually with small gas bubbles, known as gassing) during the borehole loading process.
- Pumpable: their consistency allows them to be easily pumped into boreholes, fully filling the space (coupling) and maximizing energy transfer to the rock.
- Good fume quality: when properly formulated, they produce fewer toxic fumes than ANFO.
Disadvantages
- Highest cost: emulsions are the most expensive of the three on a per-pound or per-kilogram basis, though their high energy can make them more cost-effective overall.
- Requires booster: they are very insensitive and always require a powerful primer or booster to initiate detonation.
- Temperature sensitivity: they can be susceptible to crystallization at very low temperatures, which can affect their performance and sensitivity.
