Do You Know The Symptoms Of Improperly Made ANFO? It Can Cause a Bad Blast.

Have you ever looked at a blast that followed the same pattern that is shot day after day, yet the seismograph is reading up to five times higher, the fragmentation is horrible and an orange-red cloud eerily hangs over the blast?

Why did this occur? You begin to talk with the blaster or the powder salesman and they keep offering the same answer, “geology.” Questioning how geology could cause these problems, you get the typical answers, such as mud seams and structures, one might even explain how drill hole deviation could have been worsened by the “new” structure that only occurred in the middle of the pattern.

What is not discussed is that, maybe, the ANFO used was not made to proper specification, and this caused the bad blast. The symptoms of improperly made ANFO can be:

  • Increase in ground vibration up to five times normal readings.
  • Increase in air overpressure levels.
  • Worse fragmentation with large boulders and minimal muckpile movement.
  • Holes rifling even though proper stemming was used.
  • Orange-red clouds of nitrogen oxides (NOxs) above the pattern.
  • No visible gasses, but large amount of carbon monoxides.

The mixture of ANFO (Figure 1) is very simple, blasting grade ammonium nitrate prills and some form of oil (often diesel oil). ANFO without a coloring will be white, but often times the oil is colored (red/pink or another color) to visually determine that there is oil on the prills.

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Figure 1 – ANFO.

An optimal ANFO should be 5.7 percent oil and 94.3 percent ammonium nitrate. Sometimes aluminum will be added to increase gas pressure and sensitivity. What surprises many, is that often the oil content is drastically different from what it is supposed to be.

In one mining operation where we began testing the oil content, the average oil range was around 3 percent and many other sites have varied between 2.5 to 8 percent oil. Why is this important? This oil creates an oxygen balance of the ANFO allowing for maximum gas pressure to break rock, in addition it changes the velocity of detonation, booster sensitivity and gas formation.

The graph below (Figure 2) shows how the oil content in ANFO changes these variables. Too little oil (under-oiling) causes:

  • Decreased energy.
  • Decreased velocity of detonation.
  • Easier detonation (lower booster size required).
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Figure 2 - ANFO Characteristics with Oiling.
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Figure 3 - ANFO Toxic Gas.

Too much oil (over-oiling) causes:

  • Decreased energy.
  • More difficult to detonate (larger booster size required).

What about the gasses produced by incorrect oil in the ANFO?

  • Too little oil in ANFO leads to formation of nitrous oxides (NOx) which leads to large orange-red clouds forming above the blast (Figure 3).
  • Too much oil leads to formation of carbon monoxides (CO) which has no visible color and no odor.

The next step is to develop an easy, fast and safe method to test ANFO to ensure that you are getting the proper oil ratio in your ANFO. This is actually a very simple test that can be done in minutes at any site with almost no investment in equipment. This test can be ran on every shot, every bag, randomly or only when a shot does not perform as expected and you want to be sure it is because of “geology” and not bad product.

Step 1: Take an empty, dry water bottle and take a sample of ANFO during loading (can take little bits at multiple points for increased accuracy).

Step 2: Weigh out 100 grams of this ANFO using a scale. PRO-TIP: After doing this one time, develop a little scoop that is the exact size as the 100 grams of ANFO.

Step 3: Place 100 grams of ANFO into an erlynmeyer flask with a long neck and is graduated with milliliter (mL) marks (Figure 4).

Step 4: Fill with water until the liquid level is in the neck of the flask, the amount of water is not important.

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Figure 4 - Erlenmeyer Flask

Step 5: Shake flask and dissolve the ANFO in water then let sit until oil rises to the top of water (15 to 30 minutes or less).

Step 6: Read the total amount of oil in mL that is sitting on top of water (Figure 5). The table below states the ideal range compared to the amount of ANFO used. One important note – some crystal habit modifier and surfactants may be present in oil layer as well, usually under 1 mL if present.

In general as the amount of ANFO dissolved increases the error in testing increases, therefore 100 grams is the recommended quantity for testing. If that oil layer range is outside of these limits, you know you have bad ANFO and the problem with the blasting is not geology, but the actual explosive.

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Figure 5 - Oil in Flask Neck

Amount of ANFO dissolved

Low Oil Range

High Oil Range

100 grams

5.5 mL

7 mL

200 grams

11 mL

14 mL

300 grams

16.5 mL

21 mL

400 grams

22 mL

28 mL

ANFO is the most powerful industrial explosive pound per pound and also the most economical. The major problem is the ANFO quality throughout the industry and a simple field test exists to determine exact oil percentages and protect your site from bad shots. In addition to proper testing, contracts should be written between product provider and product user to ensure that the liability for bad product does not fall on the user.

Dr. Calvin Konya is the president of Precision Blasting Services, and Anthony Konya is a project engineer for the company.