Ammonium NitrateBased Explosives Overview J.H. Heck / B.D. Northup Op3muck, LLC April 1, 2017
Ammonium Nitrate-‐Based Explosives Overview
J.H. Heck / B.D. Northup
Op3muck, LLC April 1, 2017
Agenda
• Basic Ammonium Nitrate Chemistry • Ammonium Nitrate Prills • ANFO • Ammonium Nitrate-‐Based Emulsion Explosives • ANFO / Emulsion Blends • Gassed Emulsions • ANFO/Emulsion Blend Addi3ves • Applica3ons • General Guidelines • Discussion
Basic Ammonium Nitrate Chemistry
• Ammonium Nitrate is created by the reac3on of ammonia and nitric acid
NH3 + HNO3 (concentrated) → NH4NO3 + H2O
Ammonium Nitrate Prills
• Ammonium nitrate blas3ng prills are crystallized spheres of solid AN coated with surfactants and clay – Par3cle size 0.83 to 2.3 mm – Porosi3es 8% to 12% – Densi3es 1.3 to 1.5 grams per cubic cen3meter
Prill tower ~ 200 feet tall
spray nozzles
droplets of supersaturated AN solution
crystallized AN
updraft of warm air
Explosive-grade prills are made in a prill tower in which a hot, supersaturate AN liquid (4-percent water)—along with other additives to achieve porosity—is dropped from spray nozzles at a height of 100 to 200 feet against an updraft of warm air. Droplets of the AN solution crystallize as they fall; the longer the droplets are suspended, the larger the prill diameters. The crystallized AN particles are then completely dried and coated with surfactants and clay to minimize porosity and protect the surface from absorbing water in preparation for fuel absorption. Porosities range from 8 to 12 percent, whereas solid-grain densities range from 1.3 to 1.5 g/cm3. Particle sizes range from 0.83 to 2.3 mm in diameter. By comparison to explosive-grade prills, agricultural-grade prills are less porous (3- to 5-percent porosity) and far more dense.
Ammonium Nitrate Prill Manufacture
ANFO
• ANFO is a mixture of 94% ammonium nitrate prills and 6% fuel oil – The oil is absorbed into the prill, crea3ng in3mate contact between oxidizer and fuel, allowing reliable detona3on.
– This mixture gives the op3mum oxygen balance for detona3on. AN alone has too many oxygen atoms. The carbon based fuel supplies carbon and hydrogen atoms to balance the equa3on.
NH4NO3 + CXHY → N2 +H2O + CO2 – Some CO (carbon monoxide) and NOX (nitrous oxides) are produced as well but the mixture is designed to minimize these undesirable compounds
Ammonium Nitrate-‐Based Emulsion Explosives
• Blas3ng emulsions, like ANFO, are composed of an oxidizer salt and oil and have other minor compounds to allow the emulsion to form and remain stable.
• They are liquids of varying viscosi3es, depending on the applica3on.
• Unlike ANFO, they do not dissolve in water and become desensi3zed
• Because they contain water, they have less energy per unit mass than ANFO (~680 vs 880 calories per gram)
Emulsions Emulsions Emulsions are “water-in-oil” mixes that were developed in the early 1960’s to improve the performance of water gels. They amount to hot solutions of oxidizer salts (ammonium nitrates [AN], calcium nitrates [CN] or sodium nitrates [SN]) mixed with oil and an emulsifying agent. The oil phase usually consists of diesel fuel and/or mineral oil that includes micro- balloons as sensitizers.
The oxidizer solution is broken up into small, micron- sized droplets, which form a discontinuous phase within the continuous oil phase.
The small size of the liquid-nitrated salt particles provides a large surface area-to-volume ratio that amounts to more fuel being placed in intimate contact with the oxidizer. This, in turn, allows for a very fast detonation rate and a powerful explosive.
Packaged in plastic tubes or bulk-loaded from trucks, emulsions exhibit properties as follows:
• Bulk density = 1.15 - 1.45 g/cm3 • Detonation velocity = 14,500 - 18,500 ft/sec • Detonation pressure = 100 - 120 kbar.
5 µ
=
liquid oxidizer (AN) surrounded by liquid fuels
and sensitizers
ANFO/Emulsion Blends
ANFO is oben mixed with varying percentages of emulsion explosive to vary the detona3on, handling and water resistance proper3es. • Ammonium nitrate prills have a rela3vely high density (~1.5
g/cc) but because they are spherical, there is ~ 45% void space in a borehole column of ANFO. The bulk loaded density of ANFO is thus ~ 0.82 g/cc.
• The addi3on of a liquid emulsion explosive fills in the void spaces between the prills, adding more pounds and energy to a given volume of borehole. This mixture does not have as much energy per unit weight as ANFO alone but increases the energy per foot of borehole.
ANFO/Emulsion Blends • When the volume of emulsion fills all the void space (~45%),
the less energe3c emulsion pushes the prills farther apart and the energy per unit volume begins to decrease.
• Although the energy per unit mass decreases with higher emulsion percentages, the detona3on velocity increases up to ~70% emulsion, then begins to decrease.
ANFO/Emulsion Blends (cont’d) • The op3mum blend percentage for a given site is based on a
number of factors, including: – ANFO and emulsion pricing – Water resistance required – Available loading equipment – Drilling cost – Ini3a3on cost
• Low percentages (10-‐15%) can be cost effec3ve in dry holes • If some water resistance is required, blends of 25-‐30% are
effec3ve. • For very wet holes, emulsion percentages of 50% and higher
are required. • The higher the percentage of emulsion in the blend, the less
energy is available per unit weight.
ANFO/Emulsion Blends (cont’d) • At blend percentages from 0% (all ANFO) to ~40%, augured
delivery systems are typically used. • Blend percentages from 40% to 50% are not normally used
because of sensi3vity and handling problems (the mixture is very viscous).
• Blend percentages of 50% to 70% can be augured successfully.
• Blend percentages above 70% are pumped. • In standing water applica3ons, ANFO alone cannot be used as
it dissolves. • Even though they are heavier than water, augured blends can
“bridge over” in standing water, crea3ng pockets of water in the borehole and hindering or stopping the detona3on.
ANFO/Emulsion Blend ProperCes
ANFO/Emulsion Blend ProperCes (cont’d)
Gassed Emulsions • Chemically sensi3zed (“Gassed”) emulsions do not use ANFO. • They are manufactured in a plant and sensi3zed in the field by the
addi3on of chemical sensi3zers as they are loaded in the borehole. • The density can be varied by the addi3on of chemicals that react
together and produce very 3ny gas bubbles in the explosive column. In-‐hole densi3es typically vary from ~1.25 g/cc at the bojom of the hole to ~0.9 g/cc at the base of the stemming.
• The addi3on of gassing agents in the field must be closely controlled to ensure product consistency throughout the blast.
• Since the product expands with 3me, 3me consuming quality checks must be taken throughout the process.
• Gassed emulsion usage assumes that more energy is needed at the bojom than the top of the hole. This assump3on is not valid if the burdens are consistent along the borehole..
Gassed Emulsion ProperCes
ANFO/Emulsion Blend AddiCves • A number of less expensive energe3c material addi3ves have been tried
over the years to reduce blas3ng costs without adversely affec3ng performance. – Coal – Corn – Rice Hulls – Aluminum – Metal Filings – Wood Chips – Styrofoam – Sawdust
• Each of these materials either hindered proper detona3on or had storage or handling problems
• In recent years, the use of OS5, a specialized oil shale addi3ve without these drawbacks has shown excellent results when mixed with both ANFO and augured ANFO/emulsion blends – Energy = 2190 cal/g – Reduces poten3al for NOX gasses – About ⅓ cost of ANFO per pound
ApplicaCons The selec3on of the best product for any given rock forma3on is based on a number of factors: • Rock characteris3cs • Amount of water present • Loading equipment available • Product pricing • Site loading/crushing equipment
General Guidelines There are too many variables to make firm recommenda3ons on the op3mum product for any given rock forma3on. However, some general guidelines are useful: • In dry holes, in most rock types, ANFO is the most cost effec3ve
choice. The addi3on of low percentages of emulsion can boost the available energy per unit borehole length at a low addi3onal cost.
• Where there is occasional water or water in the bojoms of the boreholes, dewatering allows the use of a bojom load of a low percentage blend and a column load of ANFO from there up.
• Another op3on for the above case is to keep a supply of waterproof bags on the blend truck and load smaller diameter bags with a waterproof emulsion blend un3l the explosive column is out of the water. The remainder of the borehole can then be loaded with ANFO. In a 6 ½ inch borehole, a 1.3 g/cc blend in a 5 inch bag gives ~ the same pounds per foot as the full column diameter of ANFO.
General Guidelines (cont’d) • In holes that are slowly making water, dewatering the holes and
loading a water resistant blend (above 25%) is recommended. • For loca3ons where dewatering is imprac3cal or impossible, a
pumpable emulsion blend can be pumped in the bojom of the hole, displacing the water without danger of the explosive column “bridging over”.
• As a general rule, the more energy the explosive has per unit of weight, the more cost effec3ve it is. A mine is buying energy, not pounds, to break rock. It takes ~1.3 pounds of a straight emulsion or ~1.1 pounds of a 40% blend to provide the same energy as 1 pound of ANFO. A less energe3c but more dense explosive may be desirable if there are high drilling or ini3a3ng/priming costs or water condi3ons demand it.
Energy Equivalents per Pound • 20% Blend 835 cal/g 1.05 # emulsion = 1.0 # ANFO • 25% Blend 830 cal/g 1.06 # emulsion = 1.0 # ANFO • 30% Blend 820 cal/g 1.07 # emulsion = 1.0 # ANFO • 40% Blend 800 cal/g 1.10 # emulsion = 1.0 # ANFO • 50% Blend 780 cal/g 1.13 # emulsion = 1.0 # ANFO • 60% Blend 760 cal/g 1.16 # emulsion = 1.0 # ANFO • 70% Blend 740 cal/g 1.19 # emulsion = 1.0 # ANFO • 80% Blend 720 cal/g 1.22 # emulsion = 1.0 # ANFO • 100% Blend 680 cal/g 1.29 # emulsion = 1.0 # ANFO
Delivery Systems • ANFO
– Paper bags – Auger truck – Cement mixer
• 10%-‐70% emulsion blends – Auger truck
• 70%-‐100% emulsion blends – Pump Truck
• Chemically sensi3zed (“gassed”) emulsions – Specialized pump truck
Acknowledgement • Physical and chemical property data were taken from the
DYNO Explosives Engineer’s Guide in the DYNO iPhone app. • Other manufacturer’s literature show that their equivalent
products have similar physical and chemical proper3es.
Discussion