As Table 10.1 shows, there was a spectacular rise in consumption of ammonium nitrate explosives during the 1960s. This was the start of the ammonium nitrate-fuel oil (ANFO) era in industrial explosives, which is expected to continue well into the future.
Ammonium nitrate was made by J. R. Glauber in 1659 (DuPont 1980). Its explosive properties must have been known early because of its connection with sodium and potassium nitrates, but it was 200 years before they were recognized commercially. In 1867, Ohlsson and Norrbin in Sweden patented an ammonium nitrate-nitroglycerin mixture that when sensitized with fuels formed an explosive that was less powerful but cheaper than dynamite. Alfred Nobel purchased the patent and expanded his range of products by using ammonium nitrate in dynamites. It was soon used in other mixtures; for example, in France it was mixed with di-nitro-naphthalene, and cylinders of the mixture were coated with paraffin, which contained carbon and repelled water. Because it was a safe, inexpensive, and strong explosive, sales increased. The problem was that it was soluble in the water that often flowed into drill holes and destroyed its sensitivity. This problem had to be solved if ammonium nitrate explosive was to be used in wet environments. The simplest solution, which was patented by DuPont in 1934, proved to be the best: Package the ammonium nitrate fuel mixture with the detonator in sealed cans. After that, this explosive became widely used in quarries and open pits.
Ammonium nitrate was regarded as a safe explosive because it was difficult to detonate, so the tragic events of 1947 came as a terrible shock and showed what a devastating explosive it really was. In that year shiploads of bagged fertilizer-grade ammonium nitrate (FGAN) blew up, causing great destruction and loss of life in three locations: Texas City in the United States, Brest in France, and the Black Sea near Russia. These terrible
|
TABLE 10.1 Sales of explosives in the United States, 1915-1975, in thousands of pounds
Source: Adapted from DuPont 1980. |
accidents highlighted both the danger and the great potential of ammonium nitrate when used as a commercial explosive.
FGAN was cheap so mining engineers started to experiment with it to break rocks. Results were inconclusive, probably because the coarse crystals and grains that were available were harder to detonate than powder and the carbonaceous sensitizers may not have been mixed well. Tests continued and within a few years FGAN prills (2 mm in diameter) were being made that quickly absorbed the fuel oil used as a sensitizer. This led to producing ANFO explosives, which soon replaced dynamite in dry holes because it was safer, cheaper, and easier to handle. The speed of hole charging increased tenfold, because ANFO could be blown up into holes and compacted. ANFO needed a big kick for detonation and this was provided by a small plug of dynamite that was detonated using a blasting cap.
By the early 1960s, the advantage of ANFO in underground as well as surface blasting was clear:
Up holes were previously limited to 70 ft (20 m) because of the problem of hand tamping conventional explosives, but pneumatic charging of ANFO overcame this restriction and resulted in improved blasting, reduced handling and improved charging performance. (Davies 1967)
The problem of wet holes increased as mines grew deeper. The revolutionary step in solving this problem was to make use of the water rather than try to repel it. This was done by making a saturated solution of ammonium nitrate in water, dispersing more ammonium nitrate and sensitizer in the solution, and initiating the dispersion with a powerful booster. The concept of an ammonium nitrate-water-sensitizer mixture was created by Melvin Cook of the University of Utah and out of it came the new technology of water-based explosives—measurably improving mining blasting and size reduction.
Cook, discussed in Chapter 2, was a brilliant physical chemist who worked on explosives with DuPont and the U. S. government before and during World War II. We rank his work with ANFO alongside the work of the cyanidation and flotation engineers as the great technical achievements in the mining industry in the 20th century.
Cook’s early research at the University of Utah was about the theory of mineral flotation, but in 1952 he returned to explosives research after being appointed director of a project supported by the U. S. army. He understood well the potential and problems of ammonium nitrate because he worked on it with DuPont.
Cook’s greatest commercial explosives invention was formulated in December of 1956 while consulting for Iron Ore Company of Canada where he created a new blasting agent using an unusual mixture of ammonium nitrate, aluminium powder and water. (Khodorovskiy 2000)
This was a seminal discovery in mining and civil engineering, because the blasting agent was the first slurry explosive and it allowed the safe ammonium nitrate to be used conveniently in wet ground. This explosive consisted of an ammonium nitrate/water dispersion sensitized by aluminum powder that produced a large amount of heat when oxidized. Guar gum was used as a thickener to minimize segregation and leakage. Water — based explosives were immediately successful because of their low cost, high strength, safety, and convenience. Problems using them in large-diameter up holes were eventually solved by adding cross-linking agents that instantly increased their viscosities.
A great advantage of water-based explosives was that they could be produced in bulk and pumped, which reduced the charging time and more accurately prepared mixtures.
For example at one mine 140 tonnes were loaded in 11 shifts and at another mine 13 tonnes were loaded in one shift. This compares to typical ANFO loading rates of just 2 tonnes per shift. (Law et al. 2001).
The result was that by 1963 twice as much ammonium nitrate was being used for blasting in the United States as high explosives, and this increased to 10 times by 1975. An important factor in making stronger, cheaper, and safer explosives was the development of high-energy emulsions. These consisted of droplets of ammonium nitrate in water covered by oil and distributed in finely dispersed voids in gas bubbles that acted as hot spots to initiate the chemical reaction. Emulsions could be blended with ANFO on site to produce explosives with different detonation properties. A report by engineers at one mine noted that
In 1993 an up-hole emulsion based explosive using gas microballoon sensitization was introduced. This product was specifically formulated for use in up holes and trials showed that it could be retained in a 127mm vertical up hole. …theproduct was used routinely in up holes with diameters from 76 mm to 115 mm. (Law et al. 2001)
The change from dynamite to ANFO had advantages as far as health, safety, and the rate of hole charging were concerned but it was not without problems. When it was first used it was mixed on site with fuel oil, and the mixing was often less than thorough with adverse effects on blasting. Because of its solubility in water, many novel methods were devised in those days to keep water away from ammonium nitrate—one of the more interesting examples being at one open pit where rain fell for 2 days out of 3 and molasses was added to ANFO to repel water. This practice was not continued; perhaps the cure caused more trouble than the problem. Despite the teething problems, ANFO-based explosives were safe and efficient by 1990, and any required detonation properties could be obtained. The safety hazards, nausea, and headaches associated with nitroglycerin had been eliminated.
But better explosives by themselves were not enough to ensure that best results were obtained by blasting. It was also necessary to optimize drilling and initiation systems.