Show misfires in metal mine blasci blantin Bl astin ng BY SPENCER P L AND C LORIMER COLBURN numerous accidents are still being reported that have resulted from the delayed explosions in drill holes some of these accidents have been due to retarded burning of the fuse causing the charge to explode after the miners had returned to the face but the majority have been caused by the miners drilling or picking into unexploded explosives either in unexploded holes or in broken rock although the industry has been cautioned by the bierau of mines manufacturers of explosives and other agencies regarding the care that is necessary in the handling of explosives yet reports of these accidents are still being received and many of the accidents are fatal they occur in spite of the best efforts of the mine officials and men but many of those that have been reported could have been avoided if the following precautions had been observed misfires should not be approached even for the purpose of inspection until three hours have elapsed if fuse was used and ten minutes if electric detonators or electric igniters ig niters were used these are minimum recommendations it would be preferable to allow a longer time for fuses have been known to hang fire for twelve hours first determine cause of misfire the first inspection of the shot that has misfired should be to determine whether or not the cause be outside of the drill hole it may be that the fuse has not burned to the mouth of the drill hole or that the legs of the electric device electric detonator cr electric igniter or the leading wire is short circuited or open circuited outside the drill hole or that the blasting machine did not provide adequate current if the cause lies outside of the drill hole it may be remedied without disturbing the charge where a misfire has occurred in a hole containing an electric device the blasting machine should first be promptly disconnected and removed when a hole has misfired an effort should be made as soon as possible to have it fired it is dangerous to have about a mine holes loaded with explosives if it is not practical to fire the hole at once it is better to place a plug in the hole or mark it in some way so that the miners will be notified noil of the danger of it if little or no tamping material was used in the misfired hole an additional stick of explosive may be placed in it and fired which is almost sure to detonate the entire charge if there is no room in the hole for the firing of the additional stick of explosive another hole should be drilled near enough to the first so that a charge in the second hole will explode or expose the first explosives E engineer ng ineer bureau of mines and mining engineer bureau of mines charge and far enough from the first charge so there is no chance of the drill striking the charge where practical the holes should be parallel for short holes six feet or less the separating distance should be at least eighteen inches and if possible two feet or slightly more under these conditions extreme care should be exercised in mucking to make sure that the original charge exploded or to locate any part of the charge that did not explode it is recommended that before firing the second charge the fuse used with the detonator or the legs of the electric detonator in the first hole be anchored with wire or strong cord outside the drill hole so that if unexploded they may be removed more readily report and chart misfires in driving drifts or crosscuts cross cuts the work is usually continuously prosecuted by either one or two shifts the men are therefore going back to either their own face ar or to the face of the other shift when the rounds of holes are blasted the shots should be counted and a report made to the foreman as to whether all shots were fired the foreman or shift boss should inspect the face before the nex next t shift goes to work and inform the men regarding misfires misfires should also be recorded on a bulletin board situated where the miners can readily consult it before going to work many of the missed holes in drifting are lifters when they fail to go off the muck should be thrown back and the holes re shot in drifting therefore it is possible for the miners to be kept acquainted quain ted with the work of each other and accidents from missed holes avoided in back holes quite frequently one shot will shoot out other holes also and the danger is from unused explosives in the muck this fact should be carefully considered by the miners in drilling and firing holes in the back in a driller may be required to drill in one place one day and another place the next day the miner should count the holes as they blast and report to the shift boss whether they are all exploded if there has been a misfire the foreman should see that a plug is placed in the hole so marked as to give the direction of the hole and the date it misfired such a hole should be fired at the first opportunity in shaft sinking there are usually several men drilling at the same time in the bottom of the shaft if there has been a missed hole in blasting a careful notation should be made of it and a plug so placed that the men working in the bottom of the shaft will be always reminded of the danger of it in shaft sinking the working conditions are usually crowded and although the plug may be in the way and become broken it should be here re placed until the hole has been re shot and the danger eliminated misfired shots in shaft sinking are dangerous and should ie be fired at the earliest opportunity after being located how to reduce risk of missed holes A prevention is better than a cure A great many holes that misfire are the result of improper loading in order to reduce missed holes to a minimum the following recommendations should be followed 1 1 only unfrozen explosives should be used explosives should be inspected to observe that they have not deteriorated hy by improper storage or age 2 As a means of making more sure the detonation of explosives a high grade detonator never less than number six should be used 3 after a hole has been prepared to receive the charge it should be left until it Is time to blast then and then only should the hole be loaded and fired 4 in tamping care should be taken not to injure the fuse or electric wires if an electric detonator is used precaution should be taken to prevent permanent short circuit ing or open circuiting of the firing circuit 5 only wooden tamping bars should ie be used metal tamping bars injure fuse and legs of detonators more readily than wooden ones metal tamping bars are more likely to cause premature explosions from friction or sparks 6 in firing a series of holes electrically the circuit should be tested with a ga galvanometer V ano M before connecting to the blasting machine such testing should be made at a safe distance from the explosive charge 7 leading wires with broken insulation should not be used 8 A blasting machine of adequate capacity a should be used 9 only good quality fuse should be used the full the detonator should be attached to crim per should shou with a cap the cap be in a first class condition 10 in the preparation of primers electric detonator should be im embedded imbedded bedded in 0 61 o the cartridge and secured firm firmly y in place P from the that it can not become separated used with a detonator deto explosive if fuse is cart the fuse should not be imbedded embedded im bedded in the ridge but secured firmly in place holes boles by reason 11 misfires occur in wet and the fuse lu of no water proofing between by waterproof walterp detonator it is custom customary ary to on ted means of yellow soap or such paten bateni positions as cap seal or Seala kap to tc d do 3 blasting those who are called upon 0 t the he pu should be familiar with lo with eap exp e v the bureau of mines dealing 80 aa one of the best papers is bulletin balieu slid metal mine diners primer on explosives for quarrymen |