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ANFO Explosive

1955
  • Melvin A. Cook
  • Robert Akre
Bulk preparation of ANFO explosive in mining operations, showcasing explosives engineering.

(generated image for illustration only)

ANFO (Ammonium Nitrate/Fuel Oil) is a widely used bulk industrial explosive. It consists of a simple mixture of porous ammonium nitrate (AN) prills, which act as the oxidizer, and a fuel oil (FO), typically diesel, which is the fuel. ANFO is classified as a blasting agent due to its relative insensitivity, requiring a booster charge to detonate. Its low cost makes it dominant in mining and construction.

ANFO’s significance lies in its combination of low cost, safety, and effectiveness, which revolutionized the commercial explosives industry. The mixture typically consists of 94% ammonium nitrate and 6% fuel oil by weight. This ratio is close to the stoichiometric oxygen balance for the reaction, maximizing the energy output. The ammonium nitrate used is in the form of small, porous spheres called prills, which readily absorb the fuel oil, ensuring an intimate mix of oxidizer and fuel. This intimacy is crucial for detonation performance. Because ANFO is a blasting agent rather than a high explosive, it is very insensitive to shock, friction, and heat. It cannot be reliably detonated by a standard blasting cap and requires a high-explosive ‘booster’ (like a cast pentolite or TNT charge) to initiate a stable detonation.

One of ANFO’s main disadvantages is its poor water resistance; ammonium nitrate is highly soluble in water, and the presence of water desensitizes the mixture and reduces its effectiveness. To combat this, water-resistant variants like emulsions, watergels, and heavy ANFO blends have been developed. The performance of ANFO, particularly its Velocity of Detonation (VoD), is highly dependent on its confinement and charge diameter. In unconfined or small-diameter boreholes, it performs poorly, but in large, well-confined boreholes typical of open-pit mining, its performance is excellent and cost-effective. Its relatively low VoD (2,500-4,500 m/s) produces a strong heaving effect, which is ideal for breaking and moving large volumes of rock.

UNESCO Nomenclature: 3305
– Chemical engineering

Type

Chemical Process

Disruption

Substancial

Usage

Widespread Use

Precursors

  • Haber-Bosch process for industrial production of ammonia, the precursor to ammonium nitrate
  • development of fractional distillation for producing fuel oils
  • understanding of oxidizer-fuel mixtures from black powder technology
  • post-wwii surplus of ammonium nitrate originally intended for fertilizer and munitions

Applications

  • large-scale surface and underground mining
  • quarrying for stone and aggregate
  • civil construction projects like road and tunnel building
  • avalanche control
  • as a component in more complex explosive blends (e.g., anfoal)

Patents:

  • US2703528A

Potential Innovations Ideas

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Related to: ANFO, ammonium nitrate, fuel oil, blasting agent, industrial explosive, mining, quarrying, booster, insensitivity, prills.

Historical Context

ANFO Explosive

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(if date is unknown or not relevant, e.g. "fluid mechanics", a rounded estimation of its notable emergence is provided)

Related Invention, Innovation & Technical Principles

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