EXPLOSIVES he past decade and that ending a century earlier
Thad interesting parallels in commercial explosives.
The 1860’ssaw the birth of the dynamites that dominated the industry for a century. In the decade past, a new series of blasting agents attained such dominance in the industry as to indicate that they too may reign supreme for at least a century. Fundamental patents of t h e dynamites and the blasting cap issued in the 1860’s Those of the blasting agents and their boosters charac. terized the explosives patents of the 1960’8. The cartridgedetonator method of blasting had its birth with the cap-sensitive dynamites. Mechanized, bulk. on-site mixing and loading methods characterize the present era of far less sensitive blasting agents ang show promise also of outlasting the century. The term blasting agent is conventional for fuel. sensitized AN (ammonium nitrate) explosives where the fuel alone is nonexplosive and the (cap-insensitive)product requires powerful boosterina to detonate it. Like ammonium perchlorate (AP),similarly loosely regarded as nonexplosive, AN is itself explosive in the pure, granular form (Table I). Slurry explosives (11, 12, 18) are “fuel”-sensitized AN with or without such other oxidizers as sodium nitrate (SN), sodium perchlorate (Nap), and ammonium perchlorate in which the solid “fuel” and (the solid) part of the oxidizer(s) are dispersed in a continuous fluid L
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.
A SURVEY OF TECHNICAL ADVANCES
he recipient of re 1988 Eger Murphree ward in Indusia/ and kngineering Chemistry points out the immediate commercial itnportance gained by slurry explosives and slurry blasting agents by virtue of excellent reproducibility and ease of manufacture
.
..
I
medium, generally an aqueous solution with or without other polar solvents and often more 01‘ less aerated. The term blasting agent, from the history and intent of ita wage, implies much leas hazardous mplosives than those for which the explosives designation is used. For this reason slurry mplasives also have been dcsignataa blasting agents because even those in which the “fuel” ig ahighexphive are gcnedly lean d t i v e to all typa of initbtion and are less harzardous (by virtue of their water cantent) than other explosiven, including the relaGydy insensitive AN/FO (ammonium nitrate/€uel oil). For thc p r e m t discussion, because SE and SBA justif, separate dassifications anyway, convention will be oayed to slurries as slurry explosives (SE) when they are sensitized withexphiven [e.g., hinitrotoluene 0, mokeless powder (SP), high stw+-double or triple base--rmokeless powder (HSSP)]and slum, blastbgagents (SBA) when the fuel is not an d o s i v e e$., aluminum, sulfur, and solid hydrocarbon. Evolution of prrrmo9ium &ate/fud oil. W e slurry explosivca and slurry blwting agents are patented inventions, ammonium nitrate/fuel oil generated spontaneously outside the explosives industry-in fact, weg under an initial smng opposition by the industry. F w stagea ofthiawohtion may be cited: (1) The anmonium nitrate unique for ammonium nitratc/fuel oil [FGAN (fertilizer-grade ammonium
nitrate)] was developed in the early 1940’s as a fertilizer initially rejected for use in explosives as too coarse, high in moisture, and low in density. “Explosive-grade” ammonium nitrate is nonporous and nonfunctional in ammonium nitratelfuel oil except in very h e meah Sizes.
(2) The fertilizer-grade ammonium nitrate shipload explosion catastrophes in 1947 at Texas City and Brrst (France) left the lasting impreasion on the public, particularly mine operators then paying high prices for explosives, that ammonium nitrate is a low-cost, powerful explosive with great blaating potential somehow overlooked by the explosives industry. (3) Under this exroneous impression, mining engineem attempted (in the mid-1950’s) to use fatilizaF a d e ammonium nitrate directly as a blasting agent in large boreholes on the Iron Range of Northem Minnesota, not realizing that the product available to them
TABLE I. EXPLOSIVE SENSITIVITY O F A M M O N I U M NITRATE (AN) AND A M M O N I U M PERCHLORATE (AP) [Relative to 94/6 ammonium nitrate/fuel oil (AN/FO)] do, in.a
M.B.,
g/cc
Density, Coating
AN AP AP
None None None
AN
Petro-Ag (0.01%) Guhr None Guhr Petro-Ag None
AN AN AN/FO AN/FO AN/FO
M e s h size
+ +
ga
-65 200 -65 200 Slightly coarser f20
1.02 1.21 1.21
5 5 6
150 232 146
0.88
8
380
$20 +20 f20 +20 20
0.82 0.86 0.82 0.86 0.88
+
F-20 20 4 4 4
F 1500 35 5 10
a dc = critical diameter; M.B. = minimum booster-cast 50/50 pentolite or equivalent; F = failed; ( F - 2 0 : failed in 20-in. diameter charge).
was a slightly desensitized form distinctly different from the sensitized fertilizer-grade ammonium nitrate involved in the catastrophes. These early attempts were still not entirely unsuccessful owing to the explosive nature of ammonium nitrate in large boreholes and with multiple boostering with large dynamite charges. Doit-yourself efforts finally became successful when operators, after learning that fuel is needed to sensitize and strengthen ammonium nitrate, began to use about a gallon of fuel oil along with each bag of fertilizer-grade ammonium nitrate. (4) An important symposium held in October 1956 at the University of Minnesota brought together mining engineers and explosives scientists who then joined forces in the characterization, mechanization, and standardization of the ammonium nitrate/fuel oil system. Our contributions to the ammonium nitrate/fuel oil develop-
TABLE I I .
ment included much of these characterizations (7, 70,33) and the development of superior, economical boosters now standard worldwide for open-pit blasting (22, 23). The products that detonated in the accidental explosions of fertilizer-grade ammonium nitrate were coated (for anticaking) with nearly 1.0% wax (4) excellent for the intended purpose but unfortunately a good fuel-sensitizer for fertilizer-grade ammonium nitrate. By strange coincidence this amount of the coating gave fertilizer-grade ammonium nitrate a (sharp) maximum explosive sensitivity (26). Even the oxygen-balanced 94.5/5.5 ammonium nitrate/wax composition used extensively as a blasting agent, beginning 15 years earlier, is appreciably less sensitive. The relatively high sensitivity of the wax-coated fertilizer-grade ammonium nitrate, along with the large charge size factor-the shipload explosions involved 3000 to 4000 tons or more of fertilizer-grade ammonium nitrate in each case-explained how this fertilizer-grade ammonium nitrate was able to undergo sudden transition from deflagration to detonation in each of the shipload conflagrations. But it did not explain how the fires were initiated in the first place. The explanation developed in the litigation in which the author served as a plaintiff, investigator, and witness seemed conclusive : The fertilizer-grade ammonium nitrate was bagged and shipped hot (at 90" =k 20°), the paper bags often became ammonium nitrate-impregnated in exposure to moist and wet conditions and, while wax coatings do not lower the thermal stability of ammonium nitrate, Findlay and Rosebourne (29) had found 25 years earlier that cellulosic materials in intimate mixture with ammonium nitrate do so. In fact, the ammonium nitrate-paper combination was shown to self-heat in the very temperature range used in bagging and shipping the fertilizer-grade ammonium nitrate so that it often arrived at the seaport appreciably hotter than when it was bagged. Bag imbrittlement and even charring by self-
CLASSES O F WELL-CHARACTERIZED SLURRY EXPLOSIVES AND SLURRY BLASTING AGENTS Water, yo Designation0 % Sensitizer Oxidizerb Nominal Ranze SE-TNT 17-60 T N T AN, SN, BN, SC, Nap, C 15 8-40 SE-CB 15-35 CB AN, SN, C 15 12-1 6 20-60 S P SE-SP AN, SN, BY, SC, Nap, C 15 2-2oc SE-HSSP 20-60 HSSP AN, SN, C 15 2-20e SE-TNT/Al 5-25/0.5-40 TNT/AI AN, SN, Nap, C 15 10-30 SE-SP/A1 10-25/1-40 15 12-30 AN, SN, C SP/Al SBA-A1 0.1-40 AI AN, AN/SN, AN/NaP, NaP 15 6-30~ 0-12 fuel SBA-fuel 4.0-1 5 solid fuel AN, AN/SN, Nap, SC 15 3-1 6C a T N T , trinitrotoluene; CB, Composition B; S P , smokeless powder; H S S P , high strength smokeless powder; Fuel, various types-sulfur, gilsonite, other solid hydrocarbons, NH4-lignosulfonate, others. A N , ammonium nitrate; S N , sodium nitrate; E N , barium nitrate; N a p , sodium perchlorate; SC, sodium chlorate; C, combinations ( A N I S C incompatible). c L o w per cent requires water extenders, such as formamide, ethylene glycol, sugar, molasses.
46
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
TABLE 1 1 1 . COMPARISONS OF TWO POWERFUL SLURRY EXPLOSIVES AND ONE SLURRY BLASTING AGENT W I T H TRITONAL A T CONSTANT VOLUME
(7-in. d X 12.5-in. 2 charges) SBASESETritonal A1 TNTIAI T N T / A l Oxidizer Oxidizer, % Fuel, T N T A1 Water Thickener Density, g/cc Relative weight Relative bulk strength (seismic method) Relative crater volume Relative air blast
None
AN/SN 49 None 35 15 1
1.45 1.1 0.93 1.22 0.99
AN/SN 39 20 25 15 1 1 .GO 1 .o
NaP 39 20 25 15 1 1.88 1.2
0.93 1.22 1.04
1.30 1.73 1.08
heating caused excessive spillage, rebagging, and contamination. Furthermore, several spontaneous fires in rail transit of the product had preceded the Texas City catastrophe. Immediately thereafter the wax coating was replaced by kieselguhr, a somewhat inferior anticaking agent but a safe one. I t was this (desensitized) guhr-coated fertilizer-grade ammonium nitrate that mine operators used in the .ammonium nitrate/fuel oil development. Slurry Explosives and Slurry Blasting Agents
The slurry blasting agent SBA-A1 (slurry blasting agent-aluminum) (Table 11) was discovered in 1956 (27), but the spontaneous aluminum-water reaction and difficulties of controlling critical physical factors influencing sensitization were not solved until about 1962, soon after which this series gained commercial prominence. Slurry explosives (SE-TNT), discovered in 1957 (ZO), gained immediate commercial importance by virtue of excellent reproducibility and ease of manufacture. Slurry explosive-trinitrotoluene (SE-TNT) is singularly useful in high pressure applications-e.g., deep well and underwater blasting. Slurry explosivesmokeless powder and slurry explosive-high strength smokeless powder (8, 24) were prominent in 196062, but their use has since diminished because they depend on a limited supply of government-surplus smokeless powder and high strength smokeless powder; primary smokeless powders are too expensive for commercial use. Slurry explosive-TNT/aluminum was first introduced in 1962 after the aluminum-water reaction became controllable (32). Slurry blasting agent-aluminum and slurry explosive-TNT/aluminum with high aluminum contents are the most powerful of all commercial explosives. Table I11 presents formulas, density, strength, constant volume cratering, and air blast comparisons of two slurry explosive-TNT/aluminum and one slurry blasting agent-aluminum compared with 80/20 tritonal, a powerful (military) cratering explosive and bomb
filler. Slurry blasting agent-aluminum (35’%) exhibited 22y0 greater cratering potential with only about as much explosive as tritonal. Slurry explosiveTNT/aluminum developed 22y0 greater cratering potential with about 93y0 as much explosive. The sodium perchlorate (Nap) slurry developed 73y0 more cratering potential with only 9% more explosive. Air blast differences were less spectacular but substantially in favor of slurry explosives and slurry blasting agent on the constant weight basis. Slurry explosive-TNT/aluminum has higher densities and bulk strengths at given A1 contents than the corresponding SBA-A1. Slurry blasting agent-fuel types ( 6 ) are the least powerful and lowest in density of the slurries but also have the lowest ingredient costs. Slurry explosives and slurry blasting agents have captured much of the open-pit blasting market because they are singularly applicable to very hard rock, as “bottom loads” in all types of rock, and under excessive water conditions. (Ammonium nitrate/fuel oil is not useful in water.) All-slurry blasting is rapidly gaining in importance over ammonium nitrate/fuel oil and bottomload slurry-top load ammonium nitrate/fuel oil blasting as operators check carefully all costs associated directly and indirectly with blasting. High loading densities and bulk strengths characteristic of slurry explosives and slurry blasting agents permit less drilling, higher blasting efficiencies, and better rock fragmentation. Savings are effected in labor, drilling, shoveling, hauling, crushing, and grinding. Scientific field-cost comparisons between slurry blasting agents and ammonium nitrate/fuel oil conducted in hard, medium, and even dry, soft rock where ammonium nitrate/fuel oil works best have generally favored the slurry-blasting-agent method (76, 27). Conventional in open-pit mining are large (9 f 3-in.) diameter, deep (45 f 15-ft) boreholes each loaded with 500 to 2000 lb of blasting agents. Hundreds of holes may be fired in a single blast usually engineered for best performance by sequential firing with scientifically controlled “msec delay” connectors with interlacing detonating fuse (“primacord”). Characterization of Ammonium Nitrate/Fuel Oil, Slurry Explosives, and Slurry Blasting Agents
Booetering. Ammonium nitrate/fuel oil, slurry explosives, and slurry blasting agents are mostly insensitive to commercial detonators and primacord and thus require powerful boosters for detonation. Properties most important in boosters include: (1) the detonation pressure [once only available from computations using the thermohydrodynamic theory (73) but today directly measurable by the “aquarium” method ( 2 5 ) ] , (2) booster shape [the highest booster efficiency occurs usually at a length/diameter (Z/d) ratio near unity], (3) VOL. 6 0
NO. 7 J U L Y 1 9 6 8
47
the ease of initiation and rate of velocity buildup (“highd e r ” detonation beginning right at the point of initiation is an egsential rquirement), and (4) safety. Figure 1 shows plots of pz against the M.B. (minimum booster) required to detonate two reference explosives-regular ammonium nitrate/fuel oil with coarse, porous, prilled ammonium nitrate and slurry explosive-TNT (2173with different boosters of I/d near unity. Procore boosters (23)were designed to maximize all factors 1 to 4. Factom 3 and 4 were simultaneouslye e d by using a relatively small, primacnd-smsitive charge of cast pentolite or PETN,that builds up suddenly to high order detonation, as a core for the main booster charge. (Rocore is a contraction for protected core.) Rocore boosters are the mast economical, efficient, and safest boostersin the modern industry. ksuming that, because it is an excellent ingredient for slurries, aluminum should also be a good ingredient for boostera, aluminized boosters have been promoted in the industry. Aluminum lowers the detonation pressure (brisance) and the boostaing efficiencyof all explosives owing to a relatively slow, initially endothermic reaction (to form A l 2 0 ) before the final highly exothermic reaction to Altos OCCUR (73). Thus tritonal and HBX @DX/TNT/Al/wax) are less effective than cast TNT and Composition B (RDX/TNT/wax) as boosters for slurry explosivur, slurry blasting agents, and ammonium nitrate/fuel oil. The same is true oflow density boosters. For example the 50/47/3 aluminum/ammonium nitrate/ fuel oil mixture has exceedingly high calorific value and tbus ermncouslyhas been considered to be a good booster for ammonium nitrate/fuel oil. T o show that it is not, when it was made cap-sensitive with fine (paint-grade) aluminum and (ball-milled) ammonium nitrate, it detonated the regular coarse ammonium nitrate/fuel oil with a minimum booster of 510 g compared with a minimum booster of only 140 g from the capsensitive 94/6 d u m nitrate/fuel oil also made with ballmilled ammonium nitrate and having about the same density. Whereas the tine-grained 96/6 ammonium nItrate/fuel oil detonated slurry wrplosiveTNT (21y0) at a minimum booster value of aluminum/ammonium nitrate/fuel oil 540, the 50/47/3 aluminum/ammodum nitratelfuel oil booster failed to detonate slurry explosiveTNT with 1000 g (minimum booster > 1000). Multiple boostering. A single Rocore booster on a primacord “downline” is quite adequate to detonate the entire charge of slurry explosives, slurry blasting agents, or ammonium nitrate/fuel oil at full s m & . Tkere are, however, possibilitiesof primacod cut-off and booster damage, and because the cost of boostaing is datively small anyway, operators usually “play d e ” by using two (sometimes four) boosters on one or two independent primacod downlines. 48
I N D V S T l l A L A N D ENGINEERING C H E M I S T R Y
Some claim that blasting is improved by the periodic spacing of highly aluminized slurry or ammonium nitrate/fuel oil. Multiple boostering (but not with aluminum boosters) may sometimes be helpful when the blasting agent is inferior and propagates only sluggishly or shows “fading” detonations, but it is useless for p r o p erly formulated blasting agents. In the use of overaluminized aluminum/ammonium nitrate/fuel oil and slurry blasting agent-aluminum boosters at various intervals in the borehole it has been claimed that the aluminum after detonation effectively distributes itself not just in the products of the aluminized charges but throughout the entire ammonium nitrate/fuel oil or slurry column and that the whole charge thus behaves as though it had been uniformly aluminized throughout. Diffusion in high density gases is much too slow and penetration of burning aluminum particles much too small for such an effect. Tests conducted both by the seismic method and in actual field blasting (Table IV)
TABLE IV. INFLUENCE OF ALUMINUM DISTRIBUTION ON STRENGTH OF ‘ALUMINIZED AMMONIUM NITRATE/FUEL OIL P A
L
I
I
9
I
~ i t , a t e / ~ Oil ~i 1.67 Kg
17/78/5 Aluminum/ Ammonium Nitrole/ Fuel O i l
50/47/3
urninurn/ Ammc Nitrale/Fusl
luniforml 5 Kg
