ANFO, Emulsion and Heavy ANFO blends - Useful explosive and blasting agent for modern day blasting
Useful Explosive & Blasting Agent for modern day Blasting Author: Needed for an explosionAmmonium Nitrate (AN) An is a weak explosive base, its explosion temperature being 1130 degree centigrade. 2NH4NO3 → 2N2 + 4H2O + O2 + 378 cal/g. It being oxygen positive, is often used as oxygen supplier in addition to being an explosive base. It forms the explosive base in ANFO (Ammonium Nitrate – Fuel Oil) explosives, which are now widely used.
ANFO Ammonium Nitrate / Fuel Oil Ammonium Nitrate Production Reaction of anhydrous ammonia gas and concentrated nitric acid. After the reaction the AN solution is about 83% concentration. Excess water is evaporated off to leave an AN concentration of 95 to 99.9%.
AN Production cont’d AN Solution is turned into prill. A small percentage of water is left in the solution when the prill is formed. When the prill is dried the water is removed and it leaves voids in the prill. This is where the diesel fuel goes when added to the prill.
AN Prill manufacturing Schematic VAPOURISED AMMONIA 83% - 86% AN solution AQUOUS NITRIC ACID Evaporator Industrial or Blasting grade Prilling Tower Prilled at 96 - 97% Melt Dryer Pre-Dryer Cooler To Storage Coater Screen Over-size & Fines to recycle
AN Prill manufacturing Blasting grade AN prills are made by spraying molten AN into a prilling tower. Droplets fall under carefully controlled cooling conditions. The AN solidifies while falling, taking on an approximately spherical shape of relatively uniform size. Prilling tower conditions must be such that will produce a "porous" prill that will absorb the proper amount of fuel oil (6 percent by weight). High density prills will not properly absorb the fuel oil and blasting performance will suffer
Properties of explosive-grade prill Properties important for explosive-grade AN prills are: Low clay content, Low moisture content, Free-flow sizing, Oil absorbency, Low particle density, Good friability and Non-caking consistency.
Temperature Cycling of AN AN responds to temperature changes by changing its crystalline structure. The phenomenon is called Cycling. The two temperature at which cycling occur under normal condition are 0 and 32 degree C. Therefore, product stored over four seasons or in tropical atmosphere undergo some degree of cycling.
Thermal and Moisture stability of AN In addition, AN is hygroscopic; can attract moisture from atmosphere and slowly dissolve itself. For this reason, blasting grade prills have some protective coating which offers some amount of water resistance. When AN prill has been exposed to both humidity and temperature, crystalline structure changes, this is the first cycle.
Thermal and Moisture stability of AN cont’d… After AN has undergone more than one cycle, the waterproof coating is broken; moisture deteriorates prills. Therefore, performance of cycled AN prills will not be consistent. Some AN prill manufacturers claim to have technology which gives Thermal stability to the prills, thereby deterioration is checked
Thermal and Moisture stability of AN cont’d…(Storage) The major problem associated with the storage of Ammonium Nitrate (AN) are hygroscopicity, thermal stability and caking. AN must be protected in storage from rain, high humidity, to prevent caking or in more extreme conditions, total breakdown of product due to phase transition because of change in storage temperature. Caking of AN is prevented by coating the finished product with inert dust and / or a surfactant and by adding an inorganic salt or nucleating agent (a stabilizer to prevent the prills breakdown – Nucleating agents are fine particulate material which when added to AN melt provide nuclei for crystal growth and prevent super-cooling which leads to growth of a small number of large crystals) to the melt prior to prilling.
General specifications of explosive- grade AN For good blasting-grade prills size distribution roughly between 6 and 20 mesh. Good blasting-grade prill is porous, which enables it to readily absorb and hold correct amount of fuel oil (about 5.7%). Oil is distributed throughout the prill particle, improving detonability of the ANFO mixture. The ability of oiled prill to be detonated depends on the density of prills. Dense prills often are not detonable, or if initiated, perform at very low rate of detonation.
AN as blasting agent and AN based explosives
ANFO Characteristics ANFO Density ranges from 0.82 - 0.90 g/cc Oxygen Balanced @ 94.4% AN / 5.6% Fuel 3NH4NO3 + CH2 → 3N2 + 7H2O + CO2. Under fueling leads to: Greater sensitivity. Drastic loss of ENERGY. Orange smoke, inefficient reaction during detonation. Over fueling leads to: Loss of Energy. Effects of Fueling on Fume Production
Effects of Fueling on Fume Production
Effects of Fueling on Energy Output
ANFO and VOD ANFO VOD increases as the borehole diameter increases. Borehole Velocity of ANFO 16000 14000 V D(ft/sec) 12000 10000 8000 6000 O 4000 2000 0 1 2 3 4 5 6 7 8 9 10 11 12 Borehole Diameter (inches)
ANFO Characteristics The critical diameter of poured ANFO is about 2.0 – 2.5 inches. Under this it will not detonate. If it is loaded pneumatically the critical diameter can be increased to 1.0 inch. ANFO is considered a ‘Blasting Agent” This means it cannot be detonated with a #8 strength detonator when unconfined. It requires a primer assembly to be initiated.
ANFO Characteristics When ANFO detonates it does so inefficiently. This produces excess gases from the reaction. This is a benefit to blasting due to the rapid expansion of the gases out into the formation creating a “heave” effect. This heave effect allows for easier digging and better throw.
Fertilizer grade VS Explosive grade AN prills for ANFO Often Fertilizer grade AN prills are used as ANFO for blasting purpose. Blasting prill considered a porous prill which better distributes the fuel oil and results in much better performance on blasting job. Difference in properties of Fertilizer and Blasting prill:
ANFO Characteristics cont’d.. ANFO is not waterproof or even resistant. ANFO can deflagrate if a significant amount of water has damaged the ANFO. ANFO can fail to detonate if water has damaged enough ANFO in the hole. If the ANFO shoots “low order”, it shoots inefficiently and produces excess gases. Orange fumes!
Emulsion Water solutions of oxidizers in an oil medium (water-in- oil emulsion). A mixture of two or more immiscible liquids in which one is present as droplets, of microscopic size, distributed throughout the other. In other words, emulsion explosives are the intimate and homogenous mixture of two immiscible oxidiser and fuels phases, stabilized with the help of emulsifier. The internal phase is composed of solution of oxidiser salts e.g. Ammonium Nitrate etc. dispersed as microscopically fine droplets, which are surrounded by a continuous fuel phase. The emulsion, thus formed, is stabilized against liquid separation by an emulsifying agent.
Explosive Emulsion Anatomy Oil continuous, “external” phase (green) – 5 to 9% Mineral oils Diesel fuels Recycled oils Vegetable oils Aqueous salt discontinuous, “internal” phase (blue) – 91 to 95% Ammonium Nitrate Calcium Nitrate Sodium Nitrate Perchlorates
Explosive Emulsion Anatomy cont’d.. A bulking/gassing agent – for density control, is then dispersed thorough out the basic emulsion matrix. The gassing agent can either be ultra fine air bubbles or artificial bubbles from glass, resin or plastic. The bulking agents determine and control the sensitivity of emulsion products – whether emulsion is cap sensitive or booster sensitive. Since its micro cell is coated with an oily exterior, the emulsion has excellent water resistance property.
Characteristics of Explosive Emulsion The output energy of emulsion is very near to the calculated energy. Viscosity of emulsion can be varied by changing type of fuel oil used. Therefore emulsion explosives can be made in cartridge form of various sizes or can be used as bulk explosive in opencast as well as in underground workings. The ultra fine air bubbles or artificial air bubbles used as gassing agent acts as sensitiser.
Initiation of Explosive Emulsion When initiating shock wave applies to emulsion explosives the ultra fine air bubbles gets heated first and act as ‘Hot- spots’ in emulsion having very high temperature (about 1500 -1800 °C). At this high temperature the explosive reaction takes place. The resulting emulsion can serve as detonable matrices to carry solid fuels such as Aluminium powder, prilled AN etc.
Properties of Explosive Emulsion Emulsion explosives are much better water resistant than water gel slurry or ANFO; as oil phase is at outside i.e., water phase is enveloped within oil phase (Water – in – Oil emulsion). Emulsion explosives are much safer to handle, use and store as it is relatively insensitivity to detonation by friction, impact or fire. Therefore, it is much safer than NG based explosives and enhances safety standard of the workings. High VOD can be obtained. VOD depend upon the oxidixer droplet size (0.2 to 10 micron). Therefore, toughest rock conditions can be tackled very effectively and efficiently without compromising safety standard. Critical diameter of emulsion explosives again depends upon droplet size and sensitizer used. Because of the intimate mixture between oxidizer and fuel, emulsion explosives have higher energy than water gel slurries or ANFO and it matches with energy level of Nitroglycerine based explosives. Since Emulsion explosives are well oxygen-balanced, generates a minimum of noxious fumes and far less smoke.
Bulk Emulsion Characteristics Typical density of a sensitized emulsion runs about 1.20 g/cc. Loaded in diameters between 1and 12 inches. VOD increases with Borehole Diameter. Sensitivity/density can vary with the addition of microballoons or chemical gassification. Can be dead pressed when sensitized with microballoons. Can be classified as Oxidizer of Blasting Agent.
Heavy ANFO Blends Blends of ANFO and Emulsion This is an attempt to increase bulk density of ANFO and making the mixture a ‘repumpable’ consistency.
Heavy ANFO Blends Why blends are used? Increase the density of ANFO; hence, increase energy in the borehole Provide water resistance to ANFO Economic (price range between that of ANFO and emulsion) Reduce mining costs.
Heavy ANFO Blend Benefits Longer sleep times with heavy blends. Water resistance for ANFO in wet formations. Pattern expansions. Maximize energy in hole. Drill cost savings potential. More production per hole. Less shots per cut.
Heavy ANFO Blend Trucks Bulk trucks for loading of Heavy ANFOs are designed to blend components prior to loading and provide loading of emulsion or ANFO alone, or a combination of the two, in varying proportions.
Heavy ANFO Blend Densities .
Heavy ANFO Blend Experiment has shown that the performance of Heavy ANFO becomes sluggish as more emulsion is added unless the emulsion has been sensitized by gassing or microballoons. It appears that in hard-rock performance will suffer when there is more than 30 percent of un-sensitized emulsion in the mix. In softer formations greater percentages of un-sensitized product can usually be employed because suitable fragmentation of the rock depends to a greater degree on heave energy. The degree of non-ideal detonation introduced by the lack of sensitization means that a greater degree of the total energy is released as heave energy.
Bulk Heavy ANFO explosives A waterproof product is typically produced at 50 percent emulsion addition. However, to obtain a product that can be pumped reliably it is common to use a waterproof Heavy ANFO containing 60 to 70 percent emulsion. Such products should always be made with a sensitized emulsion, or performance will suffer.
Bulk Heavy ANFO cont’d.. When waterproof heavy ANFO blend is loaded into wet holes it should always be loaded from the bottom up. This is achieved using a bulk truck with a hose that can extend to the bottom of the blasthole. The product is pumped through the hose. The hose is retracted as loading proceeds, but is always kept in the explosive. The water rises on top of the advancing column of more dense explosive. Mixing does not occur if the loading is carefully performed.
Bulk Heavy ANFO cont’d.. When Heavy ANFO is augured into wet holes it spatters on impact with the water, and prill goes into the solution. Water is mixed into the explosive column. Bridging may occur with portions of the explosive column separated by a water gap. Since the gap sensitivity of these products is not large this may lead to the failure of a portion of the explosive column to detonate unless it happens to be primed on both sides of the water gap.
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