7.2.3: Classification of Common Industrial Explosives

7.2.3: Classification of Common Industrial Explosives

The U.S. Bureau of Alcohol, Tobacco, and Firearms (ATF) regulates different aspects of explosives manufacture and use. They classify explosives according to the following definitions:

High Explosive (HE): an explosive material that can be caused to detonate with a No. 8 blasting cap when unconfined; and

Blasting Agent (BA): a mixture consisting of a fuel and oxidizer, intended for blasting but otherwise not an explosive (cannot be detonated with a No. 8 blasting cap).

HE’s that can be detonated directly with a No. 8 cap are called cap-sensitive.

BA’s that cannot be detonated directly with a No. 8 cap are called cap-insensitive or non-cap-sensitive.

Low Explosive (LE): an explosive material that can be caused to deflagrate (burn) when unconfined.

These definitions are important, as the terms and the underlying concepts are in everyday use. However, don’t worry about "what is a #8 blasting cap…" just know what it means to be cap sensitive or cap insensitive.

Examples of products in these classes are:

• High Explosives
  • Dynamites
  • Gelatins
  • Semi-gelatins
  • Water gels & slurries and Emulsions
• Blasting Agents
  • Water gels & slurries and Emulsions
  • ANFO
  • Blends
• Low Explosives
  • Black powder

Dynamites are rarely used industrially today because of safety concerns. For that matter, there is little use for the gelatins, semi-gelatins, and binaries per se in mining applications. Water gels, slurries, emulsions and ANFO blends are the predominate explosives in use. Note, however, that water gels, slurries, and emulsions can be formulated to be cap sensitive. This is why I have shown them under high explosives and blasting agents.

The use of black powder in underground coal mines was outlawed decades ago in this country because it will ignite coal dust and methane mixtures, making it an explosion hazard in these mines. Unfortunately, you will find it in use in the mines of some lesser-developed countries. The safe alternative to black powder is a permissible explosive, although there is little demand for low explosives in modern mining operations. We’ll talk a little more about this near the end of this lesson.

The blasting agents are often categorized as dry and wet blasting agents.

Dry Blasting Agents

The dry blasting agents are the ANFO blends, and are not cap sensitive. ANFO has the following characteristics:

• the ratio of industrial-grade AN to No. 2 fuel oil is 94.5 : 5.5 % by weight;
• aluminum particles can be added, up to 6 % , by weight, to increase the energy (heat) output;
• it is available in bulk (most common) or packaged;
• it has poor water resistance;
• the critical diameter is approximately 4";
• the specific gravity is over the range of 0.75 - 0.95;
• the detonation velocity is approximately 15,000 fps.

The industrial-grade AN is normally provided as prills, which are uniform beads of a few millimeters in diameter. We haven’t defined some of these characteristics, such as critical diameter, but will do so shortly.

The poor water resistance of the ANFO blends is a serious drawback because water is present more often than not. Sometimes, several feet of water will accumulate quickly in a vertical hole, and other times, a small amount of water will seep into the hole after loading. Regardless, this creates a significant problem. Wet blasting agents were developed to have better water resistance, and contain more than 5% water by weight. Another important characteristic of the wet-blasting agents is their higher density, which translates into being able to load more energy in the hole.

Wet Blasting Agents

There are two major types of wet blasting agents: water gels & slurries and emulsions. Water gels and slurries are technically different, but in common usage, the two terms are used interchangeably.

• Water gels consist of an inorganic oxidizer such as ammonium nitrate with a gelling agent along with additional suspended oxidizers, fuel, stabilizers, and so on. They are a colloidal suspension of solid AN particles suspended in a liquid AN solution that is gelled using cross-linking agents (think Jell-O), containing up to 20% water. The explosive becomes slurry with the addition of suspended solids and, in fact, slurries are more common. However, in everyday usage, the words are used interchangeably and we will do so here as well. Other key features of slurries include:
  • they can be made cap sensitive, for example, adding up to 18% Al;
  • they are available in bulk or packaged;
  • they have good water resistance;
  • the critical diameter is < 1";
  • they have poor low-temperature performance;
  • their specific gravity ranges from 1.15 - 1.45;
  • the detonation velocity ranges from 14,500 fps - 18,500 fps.
• Emulsions consist of a two-liquid phase containing microscopic droplets of aqueous nitrates of salts (chiefly AN) dispersed in fuel oil, wax, or paraffin using an emulsifying agent (think Mayonnaise). Stabilizers, and so on are added to round out the mixture. Other key features of emulsions include:
  • emulsions are more efficient than slurries;
  • they are available in bulk or packaged;
  • they have excellent water resistance;
  • the critical diameter is < 1";
  • they have poor low-temperature performance;
  • their specific gravity ranges from 1.1 - 1.3;
  • the detonation velocity ranges from 14,500 fps - 18,500 fps.

Blends are a mix of dry ANFO and emulsion, and this mix is often known as heavy ANFO. ANFO is inexpensive and emulsions are expensive. The blend is designed to capture the advantages of an emulsion, but at a lower cost. Specifically, the addition of the emulsion will improve the water resistance of straight ANFO, and it increases the density of the explosive, which means that more energy can be loaded into the hole. The ratio of ANFO is emulsion will range from 80:20 to 20:80. As the percentage of emulsion increases, the desirable characteristics of water resistance and density increase, but then, so does the cost. In practice, you would work with application engineers from the manufacturer to achieve the balance that best matched your unique needs. Other key features of blends include:

• emulsions are more efficient than slurries;
• they are available in bulk or packaged;
• they have improved water resistance;
• the critical diameter varies (<4” but greater than 1”);
• they have poor low temperature performance;
• their specific gravity ranges from 1.15 - 1.3;
• the detonation velocity ranges from 16,500 fps - 17,500 fps.