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Home , ANFO, Explosive

Practical methods to control losses and reduce and levels 1n• m1ne•

Ammonia and nitrate G.f. RfVfY and 94% .ANFO Relatively small concentrations readily dissolves in water, releasing o f ammonia in wat'er are very detri­ G.F. Revey, member S!.IE, is president Geotek &Assoc iates, both a mmo nia and nitrate. Emul­ me ntal to fish, particularly most Inc. Highlands Hanth. CO. SMf preprint95·2G. SUE Annual sion and watergel-based explosives trout species. The toxicity of ammo­ Meeting, March G-9.1995, Denver. CO.Driginal manuscript febru· also contain a large amount of am­ nia varies with pH and . al'{ 1995. Revised 111anuscript October 1995. Discussion of this monium nitrate and other oxidizing Researchers have found that, at peer·mi!Wed and approved paper is invited and IDUSI be subliil· salts that can leach n itrates to lower temperature and pH, the tox­ le~ indUDiitate. prior to OtL31.1996. ground water. The rate at which ni­ icity o f free ammonia increases trates leach from different explo­ (Wiber et al., 1991). In aqueous solu­ sives varies dramatically, based on tions. ammonia exists in two forms: the 's composition. In tests free ammonia, which carries no conducted at the ICI E xplosives ionic charge (NH3) , and ammonium, Abstract Technical Center in McMasterville, which carries a positive charge Most commercial explosives con­ Que bec, Canad a, n itrate leaching (NH~ · ). The free ammonia is the tain from 70% to 94% (by weight) rates were established fo r the fo l­ more toxic of the two. The US E nvi­ ammonium nitrate. When some of the lowing: ronmental Protection Agency's explosives end up in shot rock and ore, (EPA) ambient water quality crite­ through either spillage or incomplete • standard ANFO, rion is 0.02 mg/L free ammonia. For , ammonia and can • W R "water resistant" ANFO United States mines, NPD ES per­ leadz into the ground water. In recent (with additives to inhibit the in­ mits commonly include a limit of lO years, state andfederal regulators have gress of water), mg/L total ammonia as N in end-of­ applied more stringent water-quality • -sensitive wate rgel, pipe effluents. The E PA drinking­ standards, particularly at new mines and water criterion for nitrate as and development projects. • detonator-sensitive emulsion. (NC\·-N) is 10 mg/L. In When conditions pemzit, warm-blooded animals, nitrate can bulk ANFO, a mixture of ammonium Table 1 shows the cumulative be reduced to nitrites in the ­ nitrate andfuel oil, is the e.lplosive of nitra te leached from these explo­ trointestinal tract. The nitrite choice. ANFO is less costly than other sives a t d ifferent exposure hou rs reaches the bloodstream where it explosives but it dissolves readily in (Watson, 1991). reacts directly with hemoglobin to water. Several case histories in the produce methaemoglobin that im­ United States and Canada show a As expected, the emulsion did pairs transport. clear connection between uncontrolled not release nitrates as readily as the losses of bulk ~plosives and high ni­ ANFO or watergel explosive. The Ammonia and nitrate sources trate levels in mine effluents. leaching rate for emulsion explo ­ Many mines have learned that Mining companies have tried sev­ sives is much lower, because the am­ there is a direct re lationshi p be­ eral approaches to reduce ammonia monium nitrate is conta ined in an tween the ammonia and nitrate lev­ and nitrate levels in groundwater. aqueous phase that is surrounded by els in water and the amount of These approaches ra.ngefrom control­ an oil (or oil and wax:) p hase. undetonated explosives in the rock ling explosive losses to treating the H ence, when water contacts through which the water flows. Most mine effluents. In this paper, control­ undetonated emulsions, the ammo­ commercial blasting agents contain ling e.qJlosive losses is addressed. For nium nitrate is protected by the rela­ from 70% to 94% ammonium ni­ both packaged and bulk explosives, tively impervious oil and wax trate. ANFO, the most commonly guidelines to limit losses during stor­ matrix. Despite their very slow used blasting agent, is usually a mix­ age, handling and rtse are described in leaching rates, emu lsions (when ture o f 6% #2 diesel (DFO) detaiL given enough water exposure time)

MINING • JULY 1996 61 TABLE 1 FIGURE 1

Percentage of nitrates leached from impact test explosive* Time, hr ANFO WR ANFO WaterGel Emulsion 0.1 -25 1 >50 -25 6 24.6 0.6 144 > 75 1.2 * When 25% of the nitrates are dissolved, the explosive is probably no longer detonable.

can produce significant levels of nitrates and ammonia. By comparison, spilled ANFO will quickly dissolve in water and release all its ammonia and nitrates. If con­ tinuously spilled, the daily level of nitrates and ammonia released by any type of explosive that is exposed to wa­ ter will eventually become significant. The ammonium nitrate leaching rate for packaged explosives will vary based on the integrity of the package. However, this is usually a moot point, because packages of undetonated FIGURE 2 explosives are almost always ruptured by the violent rock movement within the blast. Despite being ruptured, Multiple priming to prevent rock the packaged explosive can often be recovered from shot movement cutoffs. rock. - (NG-) based will also leach ammonia and nitrates at varying rates based on their composition. In addition, NG-sensitized products are much more sensitive to shock impact; for this reason, as well as environmental concerns, the occurrence of unfired in shot rock should be prevented (Fig. 1). In underground metal mines, noxious levels of am­ monia gas often occur when undetonated explosives mix with alkaline water draining from l:t::menleu fill or grout­ ing operations (Joyce, 1992). This parallel ammonia problem is another reason to control explosive spills. The conclusion from this analysis is that losses of all types of explosives must be controlled, regardless of their composition or packaging.

Managing explosives losses or damaged bins, rail cars and transfer augers. Bulk­ There are several ways that undetonated explosives emulsion spills are often seen at storage-tank outlets and end up on the ground or in shot rock. First, sloppy han­ at pump-transfer areas. Maintenance employees are an dling, storage and loading practices may cause a signifi­ important part of a complete explosives-management cant amount of explosive spillage, particularly when bulk program. They should understand that all bins, tanks, explosives are used. Poor drilling and loading practices storage trailers and loading equipment should be regu­ can also cause significant amounts of explosives to re­ larly maintained to prevent explosives spills. Employees main unuc::Lunalc::u. Charges are often disrupted or torn wbo understand the importance of preventing explosive away by premature rock movement caused by earlier spills can greatly reduce their occurrence. However, no . Drill patterns, stemming or collar length, level of training will completely prevent all spills; so it is explosive selection, priming methods and delay timing important to develop spill containment and clean-up are the elements of blast design that can be adjusted to procedures. To contain spills, some surface mines have control charge cutoffs or failures. placed their bulk-explosive bins in concrete containment In a paper presented at the 1991 Northwest Mining tanks, or they constructed rock berms around tanks and Association convention in Spokane, WA, the authors dis­ bins. Explosive manufacturers can usually provide spill cussed water-monitoring case histories from three sepa­ clean-up recommendations for their products, and, in rate underground mines in Canada (Wiber et al., 1991). many areas. they can provide special mobile clean-up In all three cases, the levels of ammonia in mine water crew and equipment services. were lowered by at least 50% after rigorous explosive Blast-design considerations. For safety, environmen­ management programs were started. tal and economic reasons, blast designs should include Storage and handling controls. In both surface and measures that ensure complete detonation of all explo­ underground mining,ANFO and bulk-emulsion blasting sives. For this analysis, any charge or portion of a charge agents are often spilled during storage, transfer or load­ that fails for any reason is considered a misfire. Some ing. Bulk ANFO commonly spills out of poorly designed common causes of misfires, along with design practices

62 JULY 1996 • M INING ENG INEERING FIGURE 3 and precompression failure in emulsion a nd watergel explosives. Many blasting incidents, often with severe Shielded blasthole burn cut for 45 damage to nearby structures, have been caused by to 50 mm (1.75 to 2 in.) jumbo propagating dynamite. In critical blasting areas, the rounds. propagation hazard is virtuall y eliminated by substitut­ 3' ------~ ing less sensitive explosives for dynamite. However, un­ der certain conditions, packaged emulsion and watergel @ explosives can fail when rock or gas p ressure from an adjacent charge squeezes them to a above their critical limit. This precompression or "dead-pressing" phenomenon is caused by several conditions or combi­ nations of conditions. When the ground is very seamy and wet, the magnitude of hole-to-hole shock is greatly increased. Shock also increases when the holes are very close together, which is always the case in underground tunnel and surface ditch blasting where the application demands tightly spaced holes. If precompression failures occur, one should try spacing the blastholes farther apart or switch to an explosive that can withstand higher pres­ sures. Tn tunnel rounds, the holes in the bum cut are usually spaced very closely together.1l1e hole-to-hole trans­ mitted to the charges in these holes can be reduced by placing unloaded relief holes bel\veen the loaded holes (Fig. 3). 18" Poor explosives choice. The type of explosive used can have a dramatic effect on overall losses. For instance, if bulk explosives are used instead of packaged explo­ sives, spillage losses will be relatively high. If bulk ANFO is used in wet holes, losses caused by complete fai lures or partial detonation will be high. At one underground that will prevent them, are given below: meta l mine in the northwest United States, their total Cutoffs. We cannot control ground conditions, but daily limit of nitrates in ground water is 45 kg (100 lb.) To we can control drill patterns, explosive loads and initia­ meet this limit, they can not tolerate any spillage, so they tion methods. In ground having weak seams or joints, the use only packaged explosives. Moreover, they use gasses and the shock from early firing charges c :;~n cause Magnafrac 3000, a special emulsion product with a dis­ premature movement of the rock containing adjacent, tinctive orange color that can be seen and removed from unfired holes. When the rock moves, it separates or cuts shot rock. off the explosive columns within it. The portions of the When conditions that cause very high ho le-to-hole columns that do not contain energized primers will mis­ shock p ressures exist, only explosives that can resist fire and contribute ammonia and nitrate to ground wa­ precompression should be used. ter. Many of these misfires can be prevented by using l oading controls. Without specific controls, mines us­ multiple in-hole delay primers (Fig. 2). ing bulk ANFO typically lose 2% to 5% to spillage and Some failures occur because there is too m uch delay blow back during pneumatic loading underground. Sur­ time between the adjacent holes or rows o f holes. In face auger-loading trucks with poo rly designed , or these situations, reducing the delay time between holes aimed, discharge hoses spill ANFO p rills o n to the can reduce cutoffs. Cutoffs and explosive losses o ften ground around hole collars during loading. Wind can also occur when de tonating cord down lines, and surface de­ add to losses by carrying some prill away from the hole lays are used in bench blasting. When is if the discharge hose is too high in the air. Blasters who used without a delay detonator in the hole, the ground are conscientious and aware can prevent most spills by swell that occurs when the first row of holes fires, can simply adjusting their loading practices. Moreover, when tear cord and delay connections before they fire, hence, spills do occur, they must know how to clean them up: causing multiple hole failures. This type of cutoff can be and they must understand the importance of doing it. prevented by using fully activated sequential timing For some loading applications, explosive makers are (FAST) systems that use relatively long in-hole delays in developing specialized equipmen t and products de­ combination with short surface delays. The goal with signed to reduce explosive losses. For instance, a pneu­ these systems is to have all of the in-hole initiators se­ matic ANFO-metering device that cuts ANFO losses by quentially energized before the first charges fire and 10% has been developed by the bulk- group rock starts moving. In very large shots that cannot be of ICJ Explosives Inc. This ..ANFO Miser" reduces blow­ fully ene rgized, one should try to have at least two rows loading losses by metering a preset amount of explosives energized behind the row that is firing. FAST sequential in to a blast hole, thus, preventing overloading. IC I Explo­ timing can be achieved with either no nelectric- or elec­ sives Inc. also developed Amex Ultra, a specially formu­ tric-initiation systems. lated ammonium nitrate and fue l prill that has virtually ?recompression failures. Several blasting problems no blow-back loss when pneumatically loaded in uudt:r­ can occur whe n hole-to-hole shock are too ground blasting operations. high. H igh blast-induced pressure in rock can cause sym.­ In many underground blasting applications, over­ pathetic detonation (propagation) of dynamite charges loading is the greatest cause of explosive losses. When

MINING EN G INEERING • JU LY 1996 6 3 FIGURE 4 FIGURE 5

Borehole sealing and double prim­ \ \ ing.

Top Primer AN FO loaded above seal

Column Separation or Ejection in Slitted APEX ULTRA Poorly Tamped or Overloaded Holes ___- emulsion cartr idge seal

Water Level long-period delay are used to delay tunnel and other development rounds, the charges in the later firing hules are subjected to tremendous shock and gas pressure generated by the earlier fir ing charges. This Bottom Primer shock and pressure often tear away the rock around the collars of adjacent and yet unfired holes. Any explosive, whether in stick or bulk form, that is in this collar region ter will cause further ANFO loss when it wicks up the is cut off and ends up undetonated in the shot rock. To sifting ANFO into the main column. This problem is a control these losses, minimum open collar lengths should common cause of misfires in bench-blasting applications. be established for all underground blast loads, based on Holes should be sealed with a fully coupled and water­ geological conditions and application. ejection proof explosive before loading ANFO. A good seal can from hole collars will also cause explosive losses. Ejec­ be made by dropping a cut stick of an emulsion explo­ tion losses can be reduced by firmly tamping the cartridges sive, such as Apex Ultra, into the hole before pour load­ near the hole collar. However, never tamp the primer stick ­ ing ANFO (Fig. 5). this practice is dangerous and is prohibited by MSHA. When charge tamping will cause overloading, the explosive col­ Conclusion umn can be secured by some type of hole plug. To accu­ At all mines, significant reductions in the amount of rately implement good blast designs, operations must ammonia and nitrate in mine water can be achieved by have proper loading equipment and trained employees. developing an aggressive and ongoing explosive-management Loading equipment should be well maintained, and, in program. At three different mines in Canada, the implemen­ some cases, mines should consider using computerized tation of rigorous explosive-management programs re­ bulk-loading equipment that meters preset weights of duced ammonia levels by at least 50% in aU cases. explosives into blastholes (Fig. 4). As responsible stewards of our environment, mjning Sometimes during loading, packaged explosive col­ companies and explosive suppliers should work together umns are separated when rock chunks fall into the hole to establish products, loading equipment and training or when a cartridge becomes stuck. When this occurs, the programs aimed at lowering toxins in mine effluents. separated portion should be separately primed with the Explosive manufacturers, aware of these environmental same delay detonator used in the initial primer. Use of concerns, are developing new explosive products and the same delay will prevent one part of the separated loading equipment specifically designed to reduce explo­ charge from disrupting the other, and the desired firing sive losses. Excellent slide and video training programs sequence will be maintained. for explosive handlers are also available. Everyone Loading bulk ANFO into wet holes or letting ANFO should take action and work with their explosives sup­ "sleep" too long in wet or holes are also common plier to prevent ammonia and nitrate compliance prob­ causes of explosive loss. When water comes into contact lems. Mines that wait to act until the ammonia and with ANFO, it either dissolves it or wicks into it and nitrate limits have been exceeded will pay huge reme­ desensitizes it. In either case, the ANFO charge partially dial-control costs. a or completely fails to detonate. Tn underground develop­ ment rounds, static drill water should be blown out of References holes with compressed air before pneumatically loading Joyce, D.K., 1992, "Ammonia gas genera tion from ammonium ANFO. In top-loaded vertical holes, water-resistant nitrate in alkaline conditions,•· ICI Explosives Canada Report. cartridged or bulk explosives should be substituted for Wiber. M., et al. 1991. "Environmental aspects of explosives· ANFO. use," Northwest Mining Association Short Course Report, Spokane. In wet holes, packaged explosives are often not WA, December. loaded until out of water and the load is finished with Watson. C.G.• 1991. "Ammonium nitrate leaching fro m explo­ ANFO. If the hole is not sealed, the ANFO will sift past sives:· Internal ICY Explosives Report, Explosives Technical Center. the packaged ANFO and dissolve in the water. The wa- McMasterville. Quebec. Canada. February .

64 JULY 1996 • MINING ENGINEERING