The proportion of oxidizers and fuels in the mix is critical to the performance of the explosive, and we say that the explosive must be oxygen balanced.

Oxygen Balance

It is important to achieve an oxygen balance within the explosive. This means there is exactly enough oxygen present to completely oxidize the contained fuel, but none left over to react with the contained nitrogen. We can calculate the proportion of ammonium nitrate and fuel oil to achieve an oxygen-balanced reaction. The decomposition of an oxygen-balanced ANFO is:

We can find the atomic weights for each of the elements from the periodic table.:

N = 14.006

H = 1.0078

N = 14.006

C = 12.009

And then, we can calculate the molecular weight of the ammonium nitrate and the fuel oil.

${\text{NH}}_{\text{4}}{\text{NO}}_{\text{3}}$: molecular weight = 80.05 g/mol

${\text{CH}}_2$: molecular weight = 14.03 g/mol

The ideal mixture, i.e., the oxygen-balanced mixture, consists of 3 moles of ammonium nitrate and one mole of fuel oil.

Next, we should find the weight ratio of ammonium nitrate to fuel oil.

For 3 moles of ${\text{NH}}_{\text{4}}{\text{NO}}_{\text{3}}$:  3*80.05 = 240.15 g

For 1 mole of ${\text{CH}}_2$: 1*14.03 = 14.03 g

The molecular weight of ANFO is, therefore: 240.15 + 14.03 = 254.18 g

The percent by weight is:

${\text{NH}}_{\text{4}}{\text{NO}}_{\text{3}}$:  (240.15 / 254.18) *100 = 94.5% by weight

${\text{CH}}_2$:  (14.03 / 254.18) * 100 = 5.5% by weight

So, now we know that when we mix the ammonium nitrate with the fuel oil, we need to do it in this proportion if we want an oxygen balanced reaction.

We can calculate the density of ANFO mixed to this ratio. Why we would we want to do that? If we know the weight per unit volume of the correct mix, then we can tell the blasters who make the mixture in the field to weigh one cup of the mixture to ensure the weight matches the specification weight. Note, I used “cup” as the volume measurement where one cup equals eight fluid ounces. You can use any unit, as long as you are consistent.

Let’s look at an example.

We will assume that the industrial-grade ammonium nitrate and fuel oil have the following densities: 53 lb/ft³ for the ammonium nitrate and 8.0 lb/gal for the fuel oil. How much ammonium nitrate will be required for each gallon of fuel oil to make an oxygen-balanced batch of ANFO? We will need to mix 2.6 ft³ of ammonium nitrate with that gallon of fuel oil. You should verify this result.

Continuing with this example: we know that the blasters will have a scale on the mixing truck and they will have a “cup”, which they will use to do a “cup density” check. In other words, they will fill the cup with their batch of mixed ANFO and weigh it. They will compare that weight to the weight that they have been given as the specified weight. If the blaster is using an 8-ounce container for the cup density check, what weight should be displayed on the scale?

You don’t have sufficient information to answer this question. What is the calculation that you need to do to answer the question, and what are you missing?

Although you know the weight of each ingredient in the mix, you don’t know the density of the mix. The manufacturer of the ammonium nitrate would be able to give you that information, or you could do a simple experiment in the lab to determine the density of that mix. For our purposes here, let’s assume that the density of this mix is 6.68 lb/gal. Therefore, the number that you need to give to your blaster is 0.42 lb.

The mixing equipment on the truck requires regular calibration, and even if calibrated, sometimes it can malfunction. Thus, it is recommended that blasters perform a cup-density check frequently, e.g., every 5 or so holes in a surface mine application.

We’ve looked at the ideal, i.e., oxygen-balanced mixture. Let’s look at two other cases in which we have too little or too much ammonium nitrate for the amount of fuel oil that has been added. We’ll choose a 92: 8 ratio for the first case and a 96.6: 3.4 ratio for the second case, I’ve picked these ratios so that we have an integer number of moles in our formula.

Thus, for the first case with a 92:8 ratio, we have:

And for the second case, with a 96.6 : 3.4 ratio, we have:

Let’s look at the right-hand side of the equations. In the case of ideal mix, the products of the reaction, in addition to the energy that is not shown, are water vapor, carbon dioxide and nitrogen. In the first case of too little oxidizer, i.e., the ammonium nitrate, we produce carbon monoxide, a deadly gas. In the second case of too much oxidizer, we produce NO. The gas, NO, changes to NO₂ when exposed to the atmosphere. NO₂ is a toxic, but unlike CO, which is colorless, NO₂ produces a bright yellow-orange cloud.

We are not going to calculate the volume of toxic gas produced, but be aware that it is significant. In addition to the production of toxic gas, the energy release is reduced.

The oxygen balance can be lost for varied reasons. The most common include:

• poor quality control at the batch mixing truck;
• incorrect formulation, for example by adding a sensitizer like aluminum, but not accounting for it in the addition of the fuel oil;
• water in the hole;
• lack of confinement;
• improper stemming;
• improper timing of adjacent holes, allowing the explosive to become unconfined by the time it detonates;
• open-air blasting;
• explosives at the collar of the hole (increases fume production, not fragmentation).

The whole point of ensuring an oxygen balance is to achieve the maximum energy from the reaction and to prevent the generation of toxic gases. Remember, however, that the production of large quantities of gas is key to the efficacy of the explosive to fragment rock. On average 700 - 1000 l of gases/kg of explosive are produced, and these are mostly benign – N, CO₂, and water. It is impractical to achieve a perfect mix in the field, shot after shot, and consequently, a small (<4%) toxic  component of about 3% CO and 1% NO₂, depending on the oxygen balance, is not unusual. The percentage of toxic gases will increase significantly as the oxygen balance deteriorates.

Due to the possibility of a toxic component in a well-designed blast, certain precautions must be taken. The ventilation system in underground mines must be designed to dilute and carry away the gases from the blast, and no one should be allowed back into that part of the mine until sufficient time has passed to eliminate any gases from the blast. Some mines, and particularly smaller ones, make it a practice of blasting after the crew from the last shift of the day has exited the mine and the next shift won’t return until the following day. Other mines don’t have that luxury, and must ensure that a properly designed ventilation system is in place.

Fumes are normally less of a concern in a surface mine. However, it is important to note that fumes can be trapped in a pile of blasted rock, only to be liberated when the pile is loaded out. There have been a few fatalities from this, so this possibility is not to be ignored. The precaution of watering down the pile prior to loading is practiced both underground and in some surface applications. In addition to addressing a potential fume issue, the wetting action suppresses respirable dust.

A significant hazard can develop at surface mines if there is an excess production of NO₂ or CO. In the late 1990s, I was part of a team investigating a problem in the Powder River Basin, in which the large blasts were producing thick yellow clouds of NO_x – clouds that covered acres, and would drift for miles before dispersing! Fortunately, there were no reported ill effects and no fatalities, and that was lucky. My agency became involved when such clouds from a mine settled in a nearby town, near an elementary school filled with children. In the Basin, the problem was found to be water in the hole primarily, and loss of confinement as a secondary cause.

There was another illustrative problem that we investigated involving CO. These were trench-blasting applications, and there were a few fatalities due to CO poisoning. CO from the blast was trapped in the ground, but over a day or so, the gas migrated along a pipeline and entered a structure (basement), creating a toxic environment in the basement.

Fumes are an expected consequence when explosives are used. Proper procedures can help ensure an oxygen-balanced explosion, but will not guarantee a blast completely free of toxic fumes. Precautions, such as those mentioned here, must be implemented.

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.

The term Permissible Explosive can be traced back to early in the 20th century, when thousands of miners were being killed each year in the underground coal mines. Mine explosions caused many of those fatalities, and the mine explosions were set off by improper blasting practices. The U.S. Bureau of Mines conducted research to develop explosives that would not set off the mixtures of coal dust and methane that were typically found in the mines. Over the years, permissible came to mean two very important things: first, the explosive had been tested and certified by the USBM to meet the criteria; and second, the permissible explosive would be used according to a set of required practices. Without the latter, it is still possible to set off a mine explosion even though a certified-permissible explosive is used. Eventually, by the mid-20th century, mining laws mandated the use of permissible explosives and practices.

The migration from conventional to continuous mining practices in the coal mines has dramatically reduced the demand for permissible explosives in this country. Recently, around 900 metric tons of permissible explosives were sold in the U.S., whereas, in the mid-twentieth century, that number was closer to 60,000 metric tons! Nonetheless, it is commercially available and has application when it is necessary to blast in mines where methane could be present. Such applications would include: shooting the overlying strata for construction purposes in underground coal mines, e.g., for ventilation overcasts or increased headroom for belt drives/transfer points; and shooting large roof falls so that the rock can be loaded out and removed.

The permissible explosive must be used in accordance with the permissible practices. The key practices are as follows:

• qualified or certified persons, i.e., individuals meeting the requirements specified by the mining regulations, must conduct the blast.
• black powder, aluminum-cased detonators and safety fuses are prohibited;
• noncombustible stemming must be used;
• the minimum borehole spacing is 24” in coal and 18” in rock;
• no more than 20 boreholes are permitted to be fired in a round;
• no more than 3 lb of explosives can be loaded into the hole;
• the total delay period must be 1000 ms or less;
• the interval between delays must be at least 50 ms, but no more than 100 ms;
• the air must be tested for methane immediately before shots are fired, and the shot can be fired only if the concentration is less than 1.0 %;
• unconfined shooting is prohibited;
• all of the blasting circuits must be checked for continuity and resistance before the shot, using approved blasting galvanometer or blasting multi-meter.

There are many properties that define an explosive. Some are of most value to the engineers and scientists that formulate and test explosive products. Others are useful to mining engineers designing blast rounds. We’re going to focus on the latter, and such a list would include the following:

Properties of the explosive are key to the design of the blast round, and that will become clearer when we will talk about the design of blast rounds. However, properties of the rock are equally influential, and we’ll talk more about this in the future.

• Density: kg/m³.
• Weight strength: is a measure of the explosive energy, kcal/kg.
• Bulk strength: is the product of density and weight strength of the explosive; and is indicative of the explosive energy in the hole, kcal/m³.
• Critical diameter: the minimum diameter of the explosive column to sustain the detonation or deflagration. This will determine the size (diameter) of the hole that we drill.
• Critical density: the maximum density at which the explosion may not propagate. As holes become deeper, the concern is that the weight of the column will become so great that the density of the explosive near the bottom of the borehole will exceed the critical density.
• Sensitiveness: a measure of propagating ability. You will hear this term, but won’t use it in a calculation, although it may influence your decision to use boosters in the hole.
• Sensitivity: a measure of the energy required to initiate the explosive. For our purposes, we’re usually not interested in a quantitative measure of this property; but, rather, we need to know simply: is it or is it not cap-sensitive?
• Water resistance: a qualitative measure for our purposes.