“My Expereince in metal mines in India”
EXPLOSIVES AND BLASTING IN MINES
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Today’s commercial explosives have resulted from a gradual evolution that began over 600 years ago. Black powder was first used in guns around the 14th centaury. But it was not until the 17th Centaury that this explosive began to replace fire setting as the principle method for loosening rock. As black powder became accepted in the mining Industry, the number of accidents increased and a need for safe explosives and initiating systems emerged.
Explosives have been the primary method of breaking and loosening rock since the introduction of black powder and largely through dedication to research and development in safety and quality, have evolved into today’s wide range of safe and cost-effective products.
When properly initiated, commercial explosive are rapidly converted into gases at high temperature and pressure. When confined by rock, expanding explosion gases result in extremely high strains in the rock. The energy released during detonation acts equally in all directions but as one would expect, tends to escape through any path(s) of least resistance. Therefore, blast holes should to charged and stemmed so that the gases are confined for sufficient time to provide optimum breakage, displacement and looseness of the blasted rock.
An explosive contains the following ingredients:
Oxidizer
An oxidizer is a chemical which provides oxygen for the reasons. Ammonium nitrate is b far the most common oxidiaser.
Fuel
Fuel reacts with oxygen to provide heat. Common fuels include fuel oil and aluminums powder.
Sensitizer
A sensiiser provides voids which acts as hot spots and at which reaction starts during detonation. Semsitisers are generally air or gas in the form of very small bubbles, sometimes encapsulated in glass micro balloons (GNBs).
In-hole delay systems provide a predetermined time interval between each down line initiation and detonation of the corresponding explosive charge. A suitable time delay ensues that the initiation signal has reached the detonators within each charge before it or adjacent charges begin to disrupt the surrounding rock mass. This minimizes the risk of downlines being physically damaged or cut off by ground moment during the balst, and permits the use of longer inter-row delays where required for improved blast performance.
In general it is recommended that all blast holes should contain two or more primers. This multipoint initiation provides security against disruption of the explosive column by round moment, of propagation failure within the charge because of water penetration, etc. Whether the primary delay in the charge is number N eg 600 ms the insurance delay will be number N1 ie. 650 ms to provide single point initiation under normal circumstances.
CHARGING
Charging and Measuring Blast holes
All blast holes should be cleaned immediately after the drilling to ensure they are clear and are drilled to the correct depth. This is particularly important in broken and joined ground which falls in easily. Blast holes should be plugged immediately after drilling to prevent water, rocks and drill cuttings from entering the hole. Drill cuttings and rock may need to be cleared from around the collar to prevent these from falling into the blast hole during charging
All blstholes must have their depth measured and recorded immediately before charging. Short blastholes can lead to over charging toe, and digging problems, while overcharged blast holes cause fly rock hazards, noise, and reduce the effectiveness of the explosives.
Blast holes which are too deep must to back filled to the correct depth to prevent excessive vibration and damage drilled are also caused by excessive blast hole depth on upper benches. Bore hole charging, any blocked blast holes should be cleared with a charging pole or steel bar. It may be necessary to bring a drill back to the blast to clear, or re-drill some blast holes. A torch or mirror is often useful to examine the blast hole for blockages, cavity or rough sections before charging.
POSITIONING THE PRIMER (S)
BOTTOM PRIMER
The bottom primer should be lowered until it just rests on the bottom of the blast holes. If ANFO or Energan explosives are being used, pull the primer up at least 1m. When charging Powergel, leave the primer on the bottom of the blast hole until after pumping of the charge has commenced then lift it out of any mud on the bottom. Secure the down line at the collar of the blast hole.
TOP PRIMER
The technique for positioning top primer depends on the explosive type being charged. In blast holes charged with ANFO or Energan Gold, top primers can be lowered into the blast hole before charging commences. Alternatively, the primer may be lowered to the correct location as the blast hole is being charged. Secure the down line at the collar, and ensure it cannot be pulled into the blast hole as charging continues.
EXCEL MILLISECOND (MS) DETONATORS
Excel MS detonators are a series of high-strength, non-electric detonators with millisecond (MS) delay intervals between consecutive delay numbers. They are manufactured with over coated Exel signal tube for improved reliability and performance. Each of these units consists of
· A length of pink Excel tube with an outside diameter of 3mm.
· A non-electric, millisecond-delay detonator (No 8 Star strength) for the optimum initiation of detonator-sensitive explosives: and
· A Colour-coded, plastic J-Hook stamped with the delay number.
ROCK PROPERTIES
Blasting results are usually affected more by rock properties than by the properties of he explosive used to blast the rock mass. Layers of different materials, weathering, bedding planes, joints and other discontinuities can present considerable challenges in blast design. In some instances these geological features allow significant amounts of explosion energy to be wasted rather than working on the rock.
Closely jointed rock needs a relatively low powder factor to achieve satisfactory fragmentation, displacement and muck pile looseness. Considerably higher powder or energy factors are required in tough, massive rock, In this case, explosion energy can wedge open and extend few pre-existing fractures, and a large number of new fractures must to created to achieve the required fragmentation. An explosive effectiveness varies with rock type. In sedimentary strata, such as shale or sandstone, a low energy explosive (eg ANFO) usually produces sufficient strain to cause adequate fragmentation. A higher energy explosive is required in stronger, denier rocks. Rock properties affect many aspects of the mining operation:
1. Selection of drilling equipment and explosives. Adequate fragmentation in massive rock may need rather than large diameter blast holes. In closely jointed rock, fine fragmentation is achieved easily, and muck pile looseness often becomes the main aim of blasting. This situation requires an explosive with high heavy energy.
2. Operational procedures. In closely jointed or weathered rock, blast holes drilled when the blast is fired. Continuous drilling along a bench is the usual practice. In some cases the pattern may have gaps aligning with the end of blast blocks.
3. Blast geometry and delay timing. Closer joints encourage the use of larger blasts hole diameters burden distances and blast hole spacing longer stemming columns and longer delays between dependent charges.
4. Undesirable side effects of blasting. Over break and instability potential of high walls and final pit walls increase with withering, joint frequency, orientation and water content of the rock mass.
Blast hole expansion phase
Shock energy is delivered during expansion of the blast hole up to the point of blast hole equilibrium. Fractures are created by the high compressive stress wave generated by the detonation.
Gas Expansion Phase
Gas energy is delivered during the gas expansion phase. This energy contributes to further fragmentation by extending fractures, and behave he broken rock.
Shock Energy
When an explosive charge detonates inside a blast hole, an intense compressive stress wave is transmitted into the rock mass. As the compressive stress wave generated by the explosive, passes through the rock mass, the redial compressive stress causes a complementary tensile stress to be generated. A dense crack network will be produced around the blast hole by the tensile stress and new cracks will start to form wherever the tensile stress exceeds the tensile strength of the rock. Spalling of the rock will also when the compressive stress wave strikes a free face or an open discontinuity in the rock and a tensile stress wave is reflected back towards the blast hole. Shock energy should not be mistaken for fragmentation Energy. Fragmentation also occurs during subsequent stages of rock breakage and is not solely a result of tensile failure of the rock structure.
Gas Energy
Cracks started by the tensile stress in the rock mass are extended by the thigh pressure explosion gases which flow into cracks and joints. Cracks open further and explosion gases start to heave the rock mass forward. The gas energy used during this phase of rock breakage is called Heave Energy. High pressure explosion gases eventually escape to the atmospheres at around 100 MPa (1000 atmospheres pressure an effective work on the total energy liberated to this point is called the Fragmentation Energy and is equal to the Relative Effective.
Shock Energy + Gas Energy = Fragmentation Energy and
Gas Energy = Heave Energy
Influence of Free Faces
Free faces and open joints play a major role in the rock breakage process. A boulder (with many free faces being popped using a small blast hole and a part-cartridge of Power- gel may satisfactorily broken at a powder factor of 0.05 kg/m3. In comparison, a tight narrow trench or ramp, with one free face, in the same rock may require a powder factor of over 0.7 kg/cm3 to give an adequate result. Free faces located at optimum distance from the blast hole enable the explosives energy to perform the greatest amount of work on the rock mass. A blast will be more efficient if it has two free faces rather than one.
TOO MUCH BURDEN
If the distance to reasonable free faces is too great (excessive burdens), much of the explosion energy will be dissipated in excessive crushing of the rock immediately around the blast hole and much more energy will be released in the form of vibration. Because of the lack of moment the extension of cracks by explosion gases cannot occur, and therefore, fragmentation and muck pile looseness suffer.
TOO LITTLE BURDEN
In sufficient burden causes high pressure explosion gases to vent prematurely, resulting in air blast and fly rock
CORRECT BURDEN
Optimum free face blasting requires that the distances to free faces (which may include the top of the bench) are sufficient to contain the explosion gases but are short enough for shock induces cracking and spalling to occur. This forces explosion gases to do useful work fragmenting rock. The nature of the rock mass dictates correct blast placement. Even within the same area of the mine, changing rock charteries can affect the ideal blast geometry.
Buffer Blasting
In buffer blasting, broken rock from the previous blast lies against the solid rock face. The effective burden is therefore larger than the drilled burden. This increased burden (as soon by the blast hole charge), reduces the moment of blasted rock and some rock breakage processes cannot occur. As a result, fragmentation may not e significantly affected, but rock looseness and diggability are reduced. A higher powder factor is often needed in order to maintain productivity.
Effects of Rock Properties
The most important rock mass properties which affect blasting include:
1. rock strength, and intact rock properties;
2. rock mass structure (joints and/or bedding planes) and
3. rock mass variability.
Rock strength.
In-situ rock samples can be collected, examined and tested to quality the physical characterizes which have an important effect on blasting results.
Compressive/Tensile Strength
Dynamic compressive strength of rocks are very much greater than the dynamic tensile strengths, to tensile breakage accounts for the greater part of the new fracture surfaces created in a blast.
Dynamic Compressive Strength
The dynamic compressive strength of a rock may be up to 10 times its static compressive strength. Where the dynamic compressive strength of rock is low, a greater proportion of explosion energy is immediately around the blast hole. In rock with a high dynamic compressive strength, blast hole crushing is minimal (several millimeters) and a greater proportion of the explosive’s strain energy is utilized in generating crack networks.
Dynamic Tensile Strength
Value of tensile strength differs widely with the rock type.
Elasticity and Toughness
Some rocks are able to sustain considerable pressure and deformation before they crack and break. The elasticity of rock is comely described in terms of the Young’s Modulus (E). This is the ratio of stress or pressure applied relative to the amount of strain of deformation. Rocks with a low Young’s modulus deform more before they fall, absorbing more energy.
IN- HOLE DELAY SYSTEM
In hole delay systems prove a predetermined time interval between each downline initiation and detonation of the corresponding explosive charge. A suitable time delay ensures that the initiation signal time has reached the detonators within each charge before it or adjacent charges begin to disrupt the surrounding rock mass. This minimizes the risk of downlines being physically damaged or cut off by ground moment during the blast, and permits the use of longer inter-row delays where required for improved blast performance.
In general it is recommended that all blast holes should contain two or more primers. This multipoint initiation provides security against disruption of the explosive column provides security against disruption of the column by ground moment, or proGood fragmentation in Primary blasts-key to reduced overall costs
It is very important that enterprises engaged in excavation for mining or construction purposes consider the total cost before finalizing drilling and blasting parameters. It is often observed that mining and construction projects attempt to increase the power factor to reduce input cost without realizing explosives consumption and or using expanded drill patterns. Attempts are made to reduce blasting cost without realizing the negative effects of a poor blast on the efficiency of downstream operations.
Cascading effects of post blast poor fragmentation and tight muck piles are:
· Loading machines encounter frequent digging conditions in the muck pile.
· Poor fill factor of loading machine bucket.
· Severe wear and tear of loading machines.
· More frequent breakdowns – example: rope breaking. Bucket teeth breakage.
· Loading machines waste time segregating boulders: losses in productivity, dumpers remains idle, higher turnaround time – poor utilization of dumpers.
· Damage to dumper body caused by huge / heavy boulders.
· Big boulders reduce carrying capacity – low hauling efficiency / productivity.
· Boulder can roll off the dumper and fall on haul road – a potential safety hazard.
· Where crushing is involved, poor efficiency of crushers.
· Frequent jams in crusher chute.
· Severe wear and tear of crusher parts.
· Frequent breakdowns.
Increased costs of maintenance, spares and repairs (loaders, dumpers and crusher)
Higher energy cost and fuel cost (leading, hauling and crushing)
Above are the hidden costs, which get affected due to poor blasts results. More and more operators are now adopting the Mine to Mill concept, looking at the total cost of operations. Blasting, loading, hauling and crushing are inter-related variables in the total cost equation.
Development during the last decade – Electronic delay Detonator
The aspects explained above have prompted the development of the Electronic Delay Detonator. Advancement of the Electronic have made possible the use of miniaturized microchip outside or inside the detonator shell.
In Hole Delay system
In hole delay system provide a predetermined time interval between each downline initiation and detonation of the corresponding explosive charge. A suitable time delay ensures that the initiation signal has reached the detonators within each charge before it or adjacent charges begin to disrupt the surrounding rock mass. This minimizes the risk of downlines being physically damaged or cut off by ground moment during the blast, and permits the use of longer inter-row delays where required for improved blast performance.
In general it is recommended that all blast holes should contain two or more primers. This multipoint initiation provides security against disruption of the explosive column by ground moment, or water penetration, etc. where the primary delay in the charge is number ‘N” e.g. 600 ms the insurance delay will be number ‘N”1 ie 650 ms to provide single point initiation under normal circumstances.
POWDER FACTOR AND EXPLOSIVES DISTRIBUTION
The powder factor (PF) is the mass of explosive (in Kilogram) used to break each cubic meter of solid rock. If the powder factor is expressed in Kg/tones it is important to quantify the figure with a statement of material density. Powder factor can be over emphasized as a design criterion in blasting. Since powder factors are defined by the mass of the explosive rather than it’s different strength may not be possible. Therefore energy factor is a more useful design tool.
SLEEP TIME IN BLAST HOLES
Sleep time is defined as the time between charging and firing of a shot. The sleep time of an explosive is important because the explosive is often charged into unfavorable conditions. Unfavorable conditions such as heat, cold, humidity and water cause the explosives is due to water. Explosives must be charged carefully and fired at the earliest practicable time. In some operations, this could be weeks after charging, but load-and-shout firing eliminates a number of possible processes of detonation. As the sleep time is dependent many factors, an Orica technical services representative should be consulted if the blast has to sleep for a long period.
MAGAZINES
Storage of Explosive
The storage of explosives is governed by regulations, and a Magazine License is required to store explosives and detonators. When building a new magazine, the proposal (with the construction details and location) should be approved prior to construction, to ensure that a license is granted.
Location of Magazine
The site of an explosives magazine and the qualities to be stored are governed by minimum prescribed distances from houses, roads, railways, electric power lines and other magazines. In hilly areas, the best locations are these for which the hills protect houses and roads in the vicinity. In flat country, where natural protection (eg. Intervening ground, standing green timber) is not presents, artificial mounds or barricades may be erected. All sites must be cleared so that there is no danger from bush fires, and local fire authorities should be advised of the location of all magazines. Generally, the detonator magazine is conveniently located close to the explosives magazine, but at the legally prescribed distance from it.
Construction of magazines.
Magazines should be weather, fire and projectile resistant. Steel, concrete, brick or concrete block construction usually provides this protection. Where the quantity of explosives to be stored exceeds 250 kg. The magazine should have lightning protection.
Doors to the magazines must be provided with a tamper-proof locking device, to prevent theft. In some States construction of doors is tightly specified. An “EXPLOSIVES” sign and a “NO SMOKING” sign should be securely attached to the door. The licensed quantity and type of explosives should also be indicated, as required by law
All brass nails or screws in the interior of the magazine which can come into contact with the stored material should be counter sunk. There should be no interior lighting, unless it is of the type approved by Statutory Authorities.
Many explosives deteriorate rapidly when in contact with water or when stored under humid conditions. Magazines should be constructed so that water cannot enter.
Deterioration of some explosives is accelerated by extreme temperature variations, so magazines should be properly insulated and provided with a sun shade roof. Temperature variations are reduced by painting the outside walls and the roof of the magazine with a reflective colour. Ventilation should be provided to allow air to circulate freely through a magazine and reduce high temperatures and humidity. Long exposure to hot and humid conditions has a deteriorating effect of explosives products, which could indirectly lead to accidents. Cases should not be stacked directly against the walls of the magazine or restrict wall vents, and roofs should have adequate ventilators. All external vents should be screened to prevent the entrance of sparks and air-borne debris from bushfires. Dry, well ventilated and reasonably cool magazines help to increase the shelf life stability of explosive products.
Operation of Magazine
Explosive magazine should be operated in accordance with statutory Regulations. Some of the principle points to be observed in the operation of magazines are summarized blow:
· To prevent fires from reaching magazines, surrounding trees must be kept well trimmed and grass, weeds and undergrowth should be cut. The open ground within 6m of the magazine must be kept of combustible materials.
· Smoking or the carrying of matches is not allowed in or near the magazine.
· Only approved electrical lighting systems should be used in or on explosives magazines.
· When thunderstorm approaches, the magazine must be closed and all people must be withdrawn to a safe distance from the magazine until the storm has passed.
· The magazine floor should be cleaned and swept frequently, and the sweepings should be removed and destroyed.
· Explosives and detonating cord any spillage of ANFO must be carefully collected and destroyed. Other materials such as paint, timber, tools etc. must be stored in the explosives magazine.
· Non-electric and electric detonators, Enduradets, TLDs, Connectors MS connectors must be stored in a separate detonators magazine.
· Cases of explosives or detonators should NOT be opened in the magazine.
· Oldest stocks should be used first.
· Cases of explosives should be stored in neat stacks not more than 2m high, with the date of manufacture visible. They should not be stacked tightly against the walls. All magazines should be securely locked when unattended. If the lock becomes inoperable, it should be replaced immediately to ensure security.
· Each magazine should have a stock book showing the types of explosives stored and the quantities, dates and times at which they are received and issued and by whom. Regular stock takes should be made with shortfalls or theft immediately reported to appropriate authorities and the police.
· Where necessary, explosives should be removed from the magazine before any repairs are carried out. This includes all remnants and spillage of explosives. Empty cases should not be used to store anything but explosives, and cases that have contained explosives and destroyed by burning in the open air. They should never be burned in a store or fireplace. It is important to ensure that the cases are absolutely empty before igniting them. The burning must be carried out in a location such that neither injury nor damage will result in the event of an explosion. All persons involved in this work should proceed to a safe place immediately after the fire has been started.
RETURNING SURPLUS EXPLOSIVES
As soon as a blast has been charged all surplus explosives products should be carefully collected, packed into their respective carrying cases and returned to the main magazine. Surplus explosives and detonators should not be deposited in general store areas or left in mine vehicles. The intention might be to consume the excess explosives in the next blast, but there is always a possibility that they will be forgotten, stolen or cause an accident with risk of injury or death.
Note
· Check that the no primers contain detonations before replacing them into cases.
· Always check that explosives and detonators and cases are clean, dry and in good condition before loading them into the vehicle or returning them to magazines. Empty explosives cases must not be returned to or stored inside magazines.
· Remember to complete the magazine stock record books after returning explosives.
STORAGE OF AN (Ammonium Nitrate)
The storage of Ammonium Nitrate such as Nirpotil is controlled by Regulations. Store for AN should be considered of non-flammable materials, and no smoking, welding, naked flames, etc. should occur with in 6 meters of the store. Combustible or flammable materials should not be stored in or near the AN store. A high-capacity fire-fighting system should be available close to the AN store. If bagged AN is stored on wooden pallets, they should be made of hardwood and cleaned after use, as wood impregnated with AN will burn vigorously.
To avoid the caking of AN prills , into the AN store must be waterproof and well ventilated, and bags of prill should be unopened and undamaged. To prevent degradation by temperature cycling, AN prills should be protected from direct sunlight and should not be stored in direct contact with external walls.
Note:
· AN stores should be kept dry, with the inside temperature maintained below 35 C by providing adequate ventilation and suitable insulation.
· AN should be protected from direct sunlight.
· Never smoke, weld or allow naked flames within 6 m of an AN Store.
· Stacks of AN packages should be kept at least 1m from external walls and ceiling
· . Combustible or inflammable materials must not be stored in or near an AN Store.
· Mixed ANFO must be stored in an approved explosives magazine.
· To minimize waste and avoid contamination of the environment, AN stores should have a suitable effluent containment system that will collect all spills and “any run off”.
STORAGE OF EMULISON PHASE (EP)
Liquid Emulsion Phase (EP) is transported in bulk tankers and stored in bulk tanks, EP storage tanks should be adequately vented to prevent any pressure “build up” in the event of fire and to prevent collapse of the tank during loading and unloading. The EP transport pump must be fitted with cut-outs to prevent temperature rise caused by dry running. Dry running of an EP pump may cause an explosion.
TRANSPORTATION OF EXPLOSIVES
The conditions applicable to transporting explosives products depend on the “risk category” which is determined by the type and the quality being carried. At any mines, local rules have been developed to ensure the safe transportation of explosives under specific site conditions.
EXPLOSIVES TRANSPORTATION LICENSES
Explosives of different compatibility groups must be transported in separate compartments, sufficiently separated to prevent fire spreading.
Explosives to different compatibility groups must be transported in separated to prevent a fire spreading.
DOCUMENTATION FOR TRANSPORTING EXPLOSIVES
Proper shipping documentation must be completed and accompany each consignment of dangerous goods carried on a public road.
TRASPORTATION REQUREMENTS
A competent person must be appointed to have responsibility for ensuring that explosives products are transported safely, according to all statutory rules. No person under 21 years of age is permitted to be in charge of any vehicle caring explosives in mines with a ticket or licensing system, a separate license should be issued for people trained to transport explosives. Passengers or people not involved in the blasting operation should not be carried in on a vehicle loaded with explosives.
Vehicles
Explosives transport vehicle should be easy to drive and handle, and should be well maintained. All batteries, electrical wiring and exhaust should be remote from any part of the load. The non earthed terminal of the battery should be fitted with an easily accessible isolation switch and this circuit should be insulated from the body of the vehicle in the event of an accident.
Signs
Signs with the word “EXPLOSIVES” in 150 mm high, red letters on a white or yellow back ground should be fitted when carrying explosives. This sign should be visible from the front and rear of the vehicle. All signs must be removed from the vehicle when no explosives are being carried.
Fire Extinguisher
At least one fire extinguisher in good operating condition must be carried in the explosives vehicle. The size and type of extinguisher are specified in the code.
Detonators must not be carried with explosives unless they are effectively separated by a solid wooden partition or other approved material. Where detonators are carried in a separate bag, care should be taken that these are not left unattended or in contact with high temperatures or moving parts.
Avoiding Hazards
During transportation of explosives, there must be no smoking in or around the vehicle, and matches should be carried separately from explosives products. Vehicles containing explosives should never be left unattended, must never be overloaded, and must only contain sufficient explosives for the job at hand. Loaded explosives vehicles must never be taken to workshops or store areas and repairs must not commence on a loaded explosives vehicle. Flammable materials, batteries and items not necessary for the transportation of explosives must not be carried with the load.
Note
· A vehicle used to transport explosives must conform to Statutory Regulations. A well maintained vehicle should be used for transpiration. An “EXPLOSIVES” signs, panted in red letters on a while or yellow background, should be securely fitted to the vehicle. This sign must be clearly visible. The sign should be removed when no explosives are being carried.
· Although a two-way radio is not required by statutory regulations, a unit turned to the frequency used by mobile equipment at the mine is recommended.
· At some mine, the explosives transportation vehicle is fitted with a flashing red light to give an obvious indication that it is an explosives carrier. Vehicles carrying explosives must never be left unattended on taken into workshops, stores or refueling areas.
· Transportation of explosives on mine sites must done in accordance with relevant statutory regulations. Many mines have specific site conditions and local rules; on sites where they apply, these rules must be understood and obeyed.
· In most States, people under 21 years of age are not permitted to handle explosives or to dive a vehicle transporting explosives. Many mine issue a separate authorization to people who have been trained to transport explosives. Passengers or people not involved in the blasting operation should not be carried in or on a vehicle transporting explosives.
· Some statutory regulations or local Mine Manager’s rules may differ from these general guidelines.
DESTRUCTION
When properly stored and handled, explosives and detonators will remains in good condition for a considerable time. Explosives which have been subjected to moisture heat during long-term storage may deteriorate and need to be destroyed. The absorption of moisture is the most common cause of moisture us the most common of deterioration. Explosives may contain ingredients which absorb moisture if they are exposed to dampness or high humidity. Moisture generally reduces both the sensitivity and strength of explosives. Moisture is also a major causes of deterioration in detonators and safety fuse. Deteriorated detonators and some explosives are often more hazardous to use than these in good condition. Explosives which have become unfit for use, or which have deteriorated, should be promptly and carefully destroyed. Under no circumstances must these materials be disposed of by of by burying.
Appearance of Deteriorated Explosives
Different explosives exhibit different sings of deterioration. As different explosives deteriorate, their appearance and physical properties also change in various ways. These “symptoms” of deterioration are usually apparent when the products are carefully examined.
Large quantities of explosives or detonators should be destroyed only b experienced people.
As ammonium nitrate is “hygroscopic” prilled AN and ANFO readily absorb moisture from their surroundings. This may cause them to become noticeably damp to the touch, and as the prills tend to soften and stick together they will not flow as easily. Temperature cycling also causes prills to disintegrate into fine particles, as the AN crystais expand and contract.
AN or ANFO ptilld that have crumbled into fine particles may eventually agglomerate (“cake together”) and form hard lumps as they dry. If enough to form a” sludge”, and eventually dissolve completely.
Appearance of Deteriorated Emulsion Explosives
Emulsion explosives gradually become firmer and less sensitive to initiation as the age. The characteristic soft texture becomes hard as the emulsion crystallizes, Packaged emulsions may initially from a firm “crust” inside the wrapper as the age. The only sure test of suitability for the use of emuisions is to conduct unconfined fire tests.
INFORMATION REQUIRED DURING THE BALST.
The performance of explosives can only be properly assessed by monitoring during blasts. Equipment and techniques are available to record blast dynamics and collect information on complex events which occur very rapidly during the explosive rock interaction process.
There are a number of tools available for blast analysis which are used on a regular basis in mines to obtain:
1. general qualitative information:
2. firing times of detonators;
3. VOD of explosives charges;
4. burden moment data;
5. information on energy losses; and
6. values of ground vibration and air blast.
General qualitative information
Qualitative information (simply obtained with the use of a video) that can be used to assist with blast design includes the flowing:
· observation of the initiation sequence;
· potential misfired blast holes;
· effectiveness of type and length of stemming;
· face moment – degree and location;
· sources of fly rock and air blast;
· origin of oversize rock fragments; and
· exposition gas fumes, indicating possible reduction in explosives performance (e.g., as a result of water contamination)
High speed filming (500 frames per second) can be used in special cases but it is expensive and requires interpretation. Even photographs taken using a standard hand-held camera, can give useful information and can form part of a blast record.
High-speed photography is often used to assess the reliability, accuracy and precision of the initiating system. In the hole VOD is one the most commonly measured characteristics of explosives. Several techniques are available for VOD measurement, including continuous and point-to point systems.
VODs Measurement by the Electrical Contact System
In-hole VOD is one of the most commonly measured characteristics of explosives. Several techniques are available for VOD measurement including continuous and point-to point systems. VOD should be used essentially as a quality control measure. The ectrical contact system also measures point to point in hole VODs, using light weight electric cable inside blast holes. Each probe consists of a pair of insulated wires which are short circulated when the explosive’s detonation front consumes them. A data recorder measures the time at which successive in hole probes are shorted together. Data is analyzed after the blast, to calculate the electrical contact system is inexpensive and is simple to use in dry blast holes. However, the system requires expensive recording and analysis equipment, does not produce an immediate result, and is not reliable in wet conditions because of current leakage problems inside the blast hole.
Video Recording
Video recording cameras provide a simple and inexpensive method of recording the general appearance of production blasts. Video recording enables a blast to be reviewed immediately after it has been fired and, as stated previously, can provide a range of useful information. The video camera should have a high speed electronic shutter, to ensure clear images. A fully charged battery and a long tape a helps to capture the blast, even if it takes longer to fire than expected. A tripod is need to keep the camera steady and for remote recording of the blast.
Burden Moment
High-speed photography enables an accurate assessment of the time delay between blast hole detonation and initial movement of the free face, plus the velocity at which the face moves burden velocity measurements are used to calibrate the Sabrox computer blasting model and can be used to compare the heave energy of different explosives under similar conditions. The time to first movement is used to determine the optimum delay required between dependent blast holes.
To measure front-row burden movement, large markers are placed in vertical rows on the free face of the blast. The most effective markers are empty 200 liters drums, which are usually placed adjacent to front-row blast holes. The high-speed camera is located at a safe distance in front of and above the blast, where burden movement and the surface initiating system can be viewed clearly. The location of the camera, each marker, and the collar of each front-row blast hole must be accurately established b surveying.
After the high speed film has been developed, it is analyzed using computer software to determine burden. Specific relationships between burden distance, burden velocity and time to first movement can be determined for any explosive in a particular rock type.
In general, velocity is inversely proportional to burden, whereas the time to first movement is directly proportional to the burden. The time to first movement can be used to determine the minimum delay required between depend on blast holes to ensure that the face is moving before the adjacent blast hole fires. The recommended minimum delay time is twice the time to first movement.
Power waver Radar
Orica Explosives had developed the Power wave radar as a new method of measuring face velocity and time to first movement. This patented system is a unique tool for blast performance assessment and works similar to poico radars. It is capable of measuring numerous objects all moving at different velocities. Traditional techniques for measuring face velocity, such as high-speed cameras, have invaluable in analyzing face motion. But they have drawbacks, results are not available for several days or weeks, and actual face velocities can be measured only where scale control exists on the face. Power wave radar can be used to monitor any face and the results can be analyzed immediately.
Face velocities from power wave radar are presented as a velocity distribution graph. This reflects the fact that, the blast, the blasted face does not travel at a single velocity, but different areas of the face travel at different velocities.
Ground Vibration and Air blast
It is common for measurements of ground and air vibrations to be made during blasting, to ensure compliance with statutory requirements. Although these measurements are sometimes essential from a public comfort and damage point of view, they provide little useful information for blast performance evaluation.
Fragmentation
One measure of blasting performance is the size distribution of rock fragments. Practical methods for distribution of rock fragmentation vary from simple visual estimates to more complex photographic techniques.
The ideal way to assess fragmentation is to pass all of the rock through a series of calibrated screens. This technique is suitable for small test blasts, but is not appropriate for production blasts in mines.
The most common method of assessing fragmentation is a simple visual estimate of fragments on the surface of the muck pile. This may adequate for detecting gross problems, but is too subjective for a thorough blast assessment program.
Muck pile Displacement
Muck pile position shapes, and looseness are important erasures of blast performance, because, because the efficiency and cost of subsequent operations. The heave performance of a blast is best described by:
1. The maximum throw, which is a measure of the horizontal disc placement;
2. The horizontal displacement of the centre of gravity, which is a measure for the average forward displacement and
3. The muck pile well which is a measure of the over all movement and loosening of the rock mass.
Hence parameters are usually quantified by surveying the muck pile after the blast, and comparing those to pre-blast measurements. Laser profiling equipment can be used to collect this information more quickly and safely than conventional surveying techniques.
Blast Damage
Blasting may produce unwanted damage, in the form of over break beyond the design limits of the blast.
Digging Equipment performance depends on the fragmentation, position, shapes and looseness of the muck pile. Productivity of draglines, shovels, excavators and front-end loaders can be monitored to provide an overall measure of blast performance. As performance of this equipment depends on individual operators, measurements should be made over an extended period of time.
The functions of digging and loading equipment which can be monitored, to assist with blast performance assessment, are:
1. Bucket fill factors
2. overall productivity
3. time lost in handling oversize
4. Downtime for clean up, maintenance on buckets etc.
Measurement and recording these parameters are labour-intensive and time consuming as it is difficult to automate the collection of useful data. On-board monitoring equipment can now be used to record relevant information for production and maintenance planning, and is developing to a stage where it is possible to assess blast performance by measuring digging rates.
Blast Records
Effective blast performance assessment can be achieved only if adequate information is collected before, during and after the blast. To understand the factors which produce different results, including unwanted side-effects, information must be recorded for later analysis.
Simple report sheets can be used to record the essential information, to avoid any misunderstanding or reliance on memory alone. The relevant information for each operation depends to some extend on local conditions and requirements, but some factors are common to all blasting operations. Once a recording system has been established, it is relatively simple to develop it into a comprehensive data base, using a computer to store, manipulate and report relevant information. Such systems allow input of downstream actives such as diggability and could calculate a bottom line cost.
A video tape of each blast is simple and powerful recorded image that can easily be tagged and kept for future reference.
ENVIORONMENTAL EFFECTS OF BLASTING
Complaints following blasting are usually caused by an excess of noise, air blast, ground vibration and/or fly rock. In many cases, excess air blast is the causes of the complaint, even though the complaint may have in fact listed ground vibration.
· The generation of noise, air blast and ground vibration:
· Technique for estimating vibration levels;
· Current standards relating to damage and human comfort
· The total number of blast holes
· The type of explosives (i.e. bulk strength, VOD (etc)
Ground Vibration Part of the energy released in blast is transmitted through the surrounding rock in the form of a transport pressure pulse (vibration) having duration up to several times the actual blast duration. When the wave passes a point in the ground, a particle at that point is subject to motion. While the motion might to be random in character, individual elements of it resemble harmonic motion being defined in terms of frequency, amplitude of particle displacement, velocity or acceleration.
DESIGNING BLASTS WHICH GENERATE MINIMUM NOISE AND AIRBLAST
The number of complaints generally increases with the noise produced and so operators should try to minimize noise generation.
Do’s
· Ensure that the stemming column is sufficient to contain the explosion gases
· Ensure that the burden on crest holes is adequate.
· Use in hole detonators for initiation.
· Cover any detonating cord trunklines with sufficient stemming material
Don’t:
· Don’t use detonating cord trunklines
· Don’t blast oversize. Use a mechanical breakage method.
If noise, airblast and flyrock are to be prevented the length of the stemming column must be at least 16 times and preferably 14 times the blasthole diameter (particularly where bulk explosives are being used). The stemming lengths in the front row, however, may need to be adjusted on a hole-by-hole basis to coincide with any large variations in burden between the crest and toe. Where the shape of the face is very irregular or indented, use of laser face profiling and bore track blasthole deviation measurement equipment will eliminate error in human judgment when designing the charge configuration for the crest holes.
If vertical blast hole are drilled to give a design toe burden adjacent to a highly inclined face, the burden near the top rock at this level then occurs so rapidly that the explosion gases jet of cracks in the amount of breakage of which they are capable. Because these venting gases are still highly energetic, they will create noise airblast and fly rock.
Where the angle of the blast hole appreciably greater than the face angle, the burden near floor level may become too small, and noise, air blast and fly rock could be created from the area just above the toe.
If too much noise is produced by detonating cord trunk lines, either the cord should be covered or another method of initiating the explosive in the blast hole should be used. The trunk lines can be covered with sand of fine screenings. To achieve significant reductions in noise levels, sand covering of 150 mm should be used. Where noise is a major problem detonating cord trunk lines should not generally be used.
SECONDARY BLASTING
Secondary blasting which is seldom used in surface mines, could generate complaints. Plaster shooting is a rapid and inexpensive method of breaking oversize boulders but is a source of considerable noise. Popping of boulders is also generally noisy and should there fore be avoided in sensitive areas. Very experienced shot firers may be able to pop boulders with minimal noise, however, even optimized charges will produce some noise. If large numbers of oversize boulders are short together, they should be fired using the maximum possible delay range of Exel MS, or by using TLDs to reduce the number of shorts firing at the one time. The ultimate solution, of course, is to ensure that the primary blast does not produce rocks that have to be broken further.
EFFECT OF WEATHER CONDITIONS
The propagation of air vibrations away from the blast depends on weather conditions. Temperature and wind are variable and therefore, identical blasts may produce quite different noise levels at a certain point.
Normally air temperature decreased at higher altitudes. These conditions result in sound waves being bent upwards and away from the earth’s surface. This situation is favorable for blasting. If the air becomes warmer with increasing altitude, the phenomenon is referred to as a temperature in inversion. In this case, air vibration waves are bent back towards the ground and produce high noise levels at the points of return. This situation is unfavourable for blasting. Noise problems tend to occur on days with still conditions. IN some instances, small pilot shots are fired before the main blast to determine acceptability of weather conditions; escalations; escalation factors must be recorded to eventually specify reasonable pilot shot levels.
FLY ROCK PROBLEM
Damage to life, equipment and building can be severe if fly rock becomes a problem. With good blasting practice and well supervised charging of blast holes, the chances of explosives fly rock are negligible. Fly rock occurs explosion energy is vented violently into the atmosphere and propels rocks in front of it. Fly rock can occur in either primary or secondary blasting. In primary blasting, fly rock occurs where the burden or stemming length is too small or where blast holes are initiated out of sequence. Drilling inaccuracies or incorrect blast hole angles may cause the burdens to be larger or smaller than those planned. If the burden is too large, poor breakage results and ground vibrations are higher. If the burden too small, noise, air blast and fly rock can occur. It is important, therefore, to ensure that the blast hole is correctly positioned. The extra time taken in correctly positioning and aligning the drill will prevent such problems.
Fly rock also occurs where the stemming column is too short. As a rule of thumb, the stemming length should be not less than two-thirds the burden distance. However, care must be taken to ensure that the stemming column is not too long as oversize collar rock may result.
Where ANFO (or other bulk blasting agent) is used in the upper part of the blast hole, care must be taken that the blast hole is not accidentally overcharged. The explosive column rise should be monitored during the charging operations. The probability of fly rock is increased where blast holes are initiated out of sequence and a charge is forced to create upwards. This possibility can be eliminated by carefully checking the initiation sequence before firing.
INTIATION TECHNIQUES
In most circumstances, blasts initiated with detonating cord require the use of delays between blast holes for optimum results. When choosing delay timing, the burden and spacing of the blast holes must be considered, and the possibility of down holes must be considered, and possibility of down line cut offs caused poor blast performance. Anzomex slider primers can be used where cutoffs are common and longer delays are required to improve blast performance.
COMPUTER MODELING
Computer based blasting models provide a scientific approach to predicting blast performance. They are used to quickly evaluate alternative blast designs, reducing the need for costly and time consuming full-scale trials. Orica Explosives scientists and engineers have successfully developed and used a number of computer models for blasting and explosives detonation applications since the late 1960’s. In recent years various Orica models have been combined into a single integrated blasting model known as Sabex (Scientific Approach to blasting Rock with Explosives). This model is now used within Orica for the prediction and assessment of blast performance in operations around the world.
SCINTIFIC APPROCH TO BLASTING ROCK EXPLOSIVES (SABREX)
Sabrex predicts blast performance in terms of:
1. fragmentation;
2. burden movement (heave)
3. muck pile profile; and
4. blast damage.
Sabrex models the blast process for the geometry specified by the blast process for the geometry specified by the blast design, and can also calculate the cost for each design.
Sabrex blasting model can be used to:
· determine the effects of changes in blast hole diameter, blast hole pattern, explosives type and delay allocation;
· determine the effect of deviation from design (e.g. incorrect blast hole location);
· determine the effect of different rock properties; and
· design a blasting technique to provide a specified result (e.g. improved fragmentation, reduced blast damage and a defined muck pile profile)
Sabrex has been successfully used in surface mines to investigate blasting problems and techniques at both existing and proposed mining operations. Sabrex computer blasting model consists of a set of integrated modules which interact.
DATA INPUTS
Explosive Characteristics
In model the interaction between the explosive and the rock, the following explosive properties are required:
· density;
· velocity of detonation (VOD);
· detonation pressure and pressure –volume relationships; and
· explosive energy resulting from detonation.
Explosives characteristics needed for Sabrex are obtained from Orica Explosives detonation codes. The pressure-volume data from IDeX and CPeX are used to calculate the partitioning of the explosion energy into shock energy and gas energy components, which are then used to predict fragmentation and heave. Fragmentation is related to total energy, whilst heave is related to gas energy.
ROCK PROPERTIES
Sabrex requires the following rock properties:
1. density
2. unconfined compressive strength;
3. tensile strength;
4. sonic velocities (P wave and S wave velocities)
5. dynamic Poisson’s ratio;
6. porosity; and
7. structural characteristics of the rock mass.
A dynamic Young’s Modulus determined from sonic velocities is preferred to a static Young’s Modulus.
BLAST GEOMETRY
The essential aspects of blast geometry are:
· diameter, inclination and length of blastholes;
· Blasthole Pattern, burden distance and blastholes spacing;
· Effective sub drilling or stand off and stemming length;
· Face angle, crest burden;
· Number of rows of blast holes; and
· Allocation of delays.
COSTING DATA
Drilling and explosives costs can be obtained from the sabrex data base or they can be entered separately. Explosives cost per tonne, initiating system cost per blast or hole and drilling cost per meter and used to determine the overall blasting costs for the blast design.
DATA OUTPUTS
Sabrex predicts blast performance by using four integrated computer programs or “modules” which are described below:
Kuz-ram
The Kuz-ram module predicts the overall fragment size distribution based on an algorithm which considers:
· Total explosion energy available;
· “Rock Quality”, based on rock strength and structure; and
· Blast geometry and explosive distribution.
Kuz-ram estimates the overall size distribution of rock fragments for the blast design specified; it incorporates actual field results, and specified; it incorporate actual field results, and has been validated in a large number of applications. Kuz-ram is most useful at operations in which measurements or estimates of fragmentation are available. Where minimal geological information is available and no detailed fragmentation analysis has been done, reasonable predictions of fragmentation are achievable.
BOBCAT
The boabct module produces five empirically derived quantities which describe blast performance and rates each of these relative to a standard or reference blast. The five performance criteria are:
· Grade level factor (GLF)
· Column Fragmentation (CNF)
· Collar Block Factor (FRF)
· Heave Factor(HEF)
Bobcat accounts for the explosive distribution in the blast hole and separates the shock and heave effects of the explosive. Grade level factor, Column Fragmentation and collar Block Factor are indicators for fragmentation at grade level, adjuacent to the explosives column, and adjacent to the collar respectively. Fly-rock Factor indicates the relative amount of fly rock and Heave Factor indicates relative heave.
Bobct has been used and validated at many mine sites and useful where limited information is available.
HEAVE
The heave module divides the blast section into small blocks, each representing a small volume of the muck pile. The rock in front of each row of the blast holes is colored to allow the displacement of each row of blast holes to be seen. The heave module works by empirically simulating the muck pile as a mixture of fragments separated by the explosion gases. At this stage, the burden is a mixture of gas and rock.
The movement of the first layers of rock begins a process in which successive layers of rock are pushed by the gas pressure behind them. Heave velocities of the rock fragments are calculated, and the trajectory of each can be calculate from a knowledge of the horizontal and water vertical velocities. The module provides the following results:
· Heave velocities calculated at the toe, the midpoint of the explosive charge and the centre of the stemming column;
· A graphical representation of the muck pile profile; and
· A summery of the muck pile profile geometry.
DAMAGE PREDCTION
The damage module predicts rock damage behind and below a blast, represented as a graphical damage envelope. Predictions are made for free separate are:
1. extend of damage behind and below the blast hole due to crushing
2. back break at the collar due to cratering; and
3. grade separation, a line separating broken rock from undisturbed rock at grade level.
Predictions from the Damage module are useful for determining:
· The amount of effective sub drilling required;
· Minimum acceptable stemming length; and
· Stand-off distance required when blasting overburden.
MODEL CALIBRATION
Sabrex can be “fine turned” to produce more accurate predictions for operating mines, by measuring actual blast performance and comparing this to predictions. Field measurements are made before, during and after a blast to provide information which permits more effective calibration of the model for specific site conditions.
Techniques for blast performance evaluation are calibrated in the field using the following methods.
HIGH SPEED PHOTOGRAPHY
The rock Absorption Factor or the amount of heave energy which is absorbed by the rock mass is known in the modeling process. Actual burden velocity data gathered using high speed photography enables sabrex to calculate the Rock Absorption Factor for a particular rock type. This can then be used to calculate the post blast muck pile profile in the heave module.
FRAGMENTATION DISTRUBUTION
Fragmentation distributions measured in the field can be used for calibrating the Sabrex fragmentation modules to specific site conditions.
CRATER TESTING
In-situ crater tests provide information on the optimum depth of burial for a particular combination of explosives and rock type. Sabrex can be used to replicate the in situ tests by using laboratory rock property data to model the rock response to blasting. Where crater tests can be done, this is the most accurate method of calibrating Sabrex to specific site conditions.
BLAST DAMAGE VARIFICATION
Blast damage outside design limits can be quantified and compared to Sabrex. Severe back brake will be visible and measurable, whilst more subtle damage can be assessed by seismic refraction surveys before and after the blast.
MUCK PILE SURVEYS
A post blast survey of the muck pile provides data for plotting accurate cross-sections. This enables the muck pile profile, center of gravity, and angle of repose to be compared with Sabrex predictions.
DESIGNING BLASTS FOR MINIUM GROUND VIBRATIONS
Operations should ensure that the effective burden (burden at the instant of detonation of the charge) is not too large. If the burden is so great that the explosion gases find it difficult to fragment and used in rock movement is dissipated as strain waves and results in increased ground vibration levels.
Effective sub drill should be the minimum distance for which good blasting results are obtained, which is generally about 8-10 times the blast hole diameter. If more than this used, the additional charge produces energy which mainly results as ground vibrations.
Vibrations from each blat hole tend to reinforce one another unless positive steps are taken to minimize the additive effects of the individual waves. When choosing the firing order and delay timing, ground vibrations can be controlled by observing the following rules.
· Minimize the mass of explosives on each delay.
· Select delay intervals such that any blast hole in the second or a subsequent row can break out easily. If the delay between the rows is too short, the burden the effective rows is too short, the burden on one row will still be essentially in place when blast holes along the next row fire. These later-firing blast holes, there will not have an effective free face (other than the horizontal top of the bench) to which to shoot; the resulting effective burdens will cause relatively high vibration levels.
· Under severe conditions (or in operations where large-diameter blast holes are used) explosives charges can be decked and separately delayed. The top deck is generally the first to be fired. In such cases, it is essential that the stemming column between the charge decks is long enough to prevent the shock wave from the earlier firing charge desensitizing the lower deck charge before it is due to fire. This distance increases with the blat hole diameter and varies also with the type(s) of explosive, type of stemming, rock properties and wetness of the strata.
· It is preferable to initiate blasts from a free and rather than in the middle. This lengthens the blast duration and reduces the explosive energy dissipated in any given time interval.
· Use the hole detonators for the initiation of blast holes so that longer delays can be used between blast holes without introducing cut off problems. Longer delays will reduce the reinforcing effects of individual pressure pulses in the rock and increase the total duration of the blast leading to lower ground vibration levels.
· Sequence the blast so it fires in the direction away from a potential target site.
CONTRABLE VAIRABLES
1. Charge weight / delay
2. Charge weight per blast
3. Length of delay
4. Burden & spacing
5. Stemming length
6. Stemming Type
7. Charge length & diameter
8. Blasthole angle Orientation of blast face
9. Direction of initiation
10. Initiating system
11. Blast time
NON-CONTROLIABLE VARIABLES
1. General topography
2. Atmospheric conditions
3. Rock Type (particularly joint structure)
AIR BLAST AND NOISE
A disturbance in the surrounding a blast is created by the premature release of explosion gases from the premature release of explosion gases from the blast hole. This can occur through the stemming, pre-existing and / or blast induced tractures in the faced and by the forward movement of the rock mass itself. The air disturbance is propagated by a series of overpressures which decrease in intensity with increasing distance from the blast.
· The air pressure pulse (APP) due to the poison like effect as air is moved by direct rock displacement.
· The rock pressure pulse (RPP) due to the vertical component of the ground vibration traveling along the surface. This pulse is the first signal to reach the air blast transducer.
· The gas release pulse (GRP) results from blowouts (face busting and cratering)
· The stemming release pulse (SRP) due to rifting (stemming ejection).
· Air blast due to the initiation system products (such as detonating cord).
MEASUREMENT OF AIRBLAST
A typical air blast monitoring requires the following three components:
1. Tripod mounted detector which enables the detector to be placed in a stable poison approximately 1.5 m above the ground.
2. Air blast detector system which includes the microphone and associated signal conditioning amplifiers. The wind screen covers the microphone and is required to reduce the noise due to wind.
3. Data recorder and analyzer (ie, composer and appropriate software).
DESIGNING BLASTS WHICH GENERATE MINIMUM NOISE AND AIRBLAST
The number of complaints generally increases with the noise produced and so operatrors should always try to minimize noise generation.
Do’s
· Ensure that the stemming column is sufficient to contain the explosion the explosion gases.
· Ensure that the burden on crest holes is adequate.
· Use in hole detonators for initation.
· Cover any detonating cord trunk lines with sufficient stemming material.
Don’s:
· Don’t use detonating cord trunk lines.
· Don’t blast oversize. Use a mechanical breakage method.
IF noise, air blast and fly rock are to be prevented, the length of the stemming column must be at least 16 times and preferably about 24 times the blast holes diameter (particularly where bulk explosives are being used). The stemming lengths in the front row, however, may need to be adjusted on a hole-by-basis to coincide with any large variations in burden between the crest and toe. Where the shape of the face is very irregular or indented, use of laser face profiling and bore track blast hole deviation measurement equipment will eliminative error in human judgment when designing the charge configuration for the crest holes.
If vertical blat holes are drilled to give a design toe burden adjacent to a highly inclined face, the burden near the top of the charge tends to be too small. Fracturing rock level then occurs so rapidly that the explosion gases jets out of cracks in the face before they have been able create the amount of breakage of which they are capable. Because these venting gases are still highly energetic, they will create noise air blast and fly rock.
Where the angle of the blast hole is appreciably greater than the face angle, the burden near floor level may become too small, and noise, air blast and fly rock could be created from the area just the toe.
If too much noise is produced by detonating cord trunk lines, either the cord should be covered or another method of initiating the explosive in the blast hole should be used. The trunk lines can be covered with sand or the screenings. To achieve significant reductions in noise levels, sand covering of 150mm should be used.
SECONDARY BLASTIG
Secondary blasting, which is seldom used in surface open pit mines, could generate complaints. Plater shooting is a rapid and inexpensive method of breaking over sizes boulders but a source considerable noise. Popping or boulders is also generally noisy and should therefore be avoided in sensitive areas. Very experienced shot fires may ever; even optimized charges will produce some noise. If large numbers o oversize boulders are shot together, they should be fired using the maximum possible delay range of Excel MS, or by using TLD to reduce the number of shots firing at the one time. The ultimate solution, of course, is to ensure that the primary blast does net produce rocks that have to be broken further.
EFFECT OF WEATHER CONDITIONS
The propagation of air vibrations away from the blast depends on weather conditions. Temperature and wind are variable and therefore, indexical blasts may produce quite deferent noise levels at a certain point.
Normally, air temperature decreases at higher altitudes. These conditions result in sound waves being bent upwards and away from the earth’s surface. This situation is favorable for blasting. If the air becomes warmer with increasing altitude, the phenomenon is referred to as a temperature inversion. In this case, air vibration waves are bent back towards the ground and produce high noise levels at the points of return. This situation is unfavorable for blasting. Noises problems tend to horizontal smoke plumes are generally visible on such days.
The effect of wind is directional and always results in increased noise levels downwind. Complaints can be reduced by blasting when winds are blowing away from residential areas (especially when the wind speed is high). Operators with severe noise problems should find out what relevant information can be obtained on a daily basis from the weather bureau and should then ensure that the available information is used to full advantage.
In some instances, small pilot shots are fired before the main blast to determine acceptability of weather conditions; escalation factors must be recorded to eventually reasonable pilot shot levels.
GENERAL CONSIDERATIONS
The frequency and severity of complaints can be reduced by observing the following further recommendations.
1. Fire large well designed blasts infrequently rather than a greater number of smaller blasts which have not received adequate planning and design effort. Nearby residents tend to be much more tolerant when blasts are fired twice per week instead of twice every day.
2. Keep a detailed record of:
· The place time date and location of the blast.
· The number of blast holes;
· The maximum charge mass per delay;
· The initiating system and delay allocation;
· The burden, spacing and stemming length;
· The diameter, length and angle of blast holes; and
· The weather conditions.
Blasts should be fired when people are busy with their daily tasks and when neighbors are experiencing high back ground noise. Advantage may be taken off traffic, lunch hour activity, the air blast from blasting.
FLY ROCK PROBLEM
Damage to life, equipment and buildings can be severe if fly rock becomes a problem. With good blasting practice and well supervised charging of blast holes, the chances of excessive fly rock are negligible.
Fly rock occurs where explosion energy is vented violently into the atmosphere and propels rocks in front of it. Fly rock can either primary or secondary blasting. In primary blasting, fly rock occurs where the burden or stemming length is too small or where blast holes are initiated out of sequence. Drilling inaccuracies or incorrect blast holes angles may cause the burdens to be larger or smaller than those planned. If the burden is too large, poor breakage results and ground vibrations are higher, if the burden is too small, noise, air blast and fly rock van occur. It is important, therefore, to ensure that the blast hole is correctly positioned. The extra time taken in correctly positioning and aligning the drill will prevent such problems.
Fly rock also occurs where the stemming column is too short. As a rule thumb, the stemming length should be not less than two-thirds the burden distance. If it is possible to use a longer stemming column, this is preferable. However, care must be taken long as oversize collar rock may result.
Where ANFO (or another bulk blasting agent) is used in the upper part of the blast hole, care must be taken that the blast hole is not accidentally overcharged. The explosive column rise should be monitored during the charging operations.
The probability of fly rock is increased where blast holes are initiated out of sequence and a charge is forced to crater upwards. This possibility can be eliminated by carefully checking the initiation sequence before firing.
THROW BLAST AND THROUGH—SEAM BLASTING
Throw blasting is surface coal mines can be defined as the controlled placement of overburden by drilling and blasting, to achieve the minimal overall over burden removal cost. Operating costs can be significantly reduced by using explosives to throw 20% to 30% of the overburden into its final position in the mined-out pit. Throw blasting is particularly suitable for dragline operations, and can be adopted for shovel and front end loader operations.
At operations using throw blasting, studies have indicated that significant overall economic benefits have been achieved, despite and increase in drilling and blasting costs. A full analysis of throw blasting should include:
· Increasing drilling blasting costs.
· Potential cost of coal damage or loss.
· Reduced amount of overburden to be handled / re-handle.
· Improved digging rates in looser muck piles
· Additional coal exposed.
· Potential for reduced capital investment.
· High wall stability.
The proportion of material thrown into its final position is usually expressed as a percentage of the total volume in situ before blasting. A higher proportion of the in situ material can be thrown into the final position it the strip is narrow and the overburden and coal seam are thick.
In the USA, it is not uncommon for surface coal mines to use design powder factors exceeding 1.0 kg/cubic. meter to throw up to 70% overburden into its final position. In Australia, powder factors of up to 0.6 kg/cubic. meter are currently used to achieve results of 20% to 30% thrown to spoil.
KEY THROW BLAST PARAMETERS ARE:
· The percentage throw and muck pile profile must suit mining equipment;
· The balance of fragmentation and muck pile looseness should maximize mining equipment productivity.
· Coal damage and coal loss must be minimized.
· Blast damage to the new high wall face must be controlled.
· The most effective combination of bulk explosives and initiating explosives should be used.
ROCK PROPERTIES
Rock properties are a major factor in throw blast design and performance. The rock type, water content, and frequency and location of discontinuities will all affected design parameters such as the overall energy requirements, burden and spacing and the spacing : burden which will vary from 1.25 to 2 depending on the rock structure and how it responds. It is important that the velocity be as uniform across the face as possible.
Throw blasting is often less effective in porous rocks, heavily laminated shales, and highly weathered clays. These materials usually require higher power factors than more elastic rocks such as sandstones, because they deform more easily and thus absorb more explosive energy. Throw blasting results are generally better if joints are cemented together or contain water, as energy is then more effectively transmitted through these discontinuities.
BLAST GEOMETRY
Blast hole diameter directly affects explosives distribution throughout the rock mass. Blast hole diameter has a significant influence on drilling and blasting productivity equipment. Where bench heights exceed 30 m and large draglines are used. Blast hole diameters of 270 mm and 311 mm are common. For lower bench heights and front end loaders, smaller blast hole diameters such as 187 mm are more appropriate. Strip width is usually determined b y the overall mine design and equipment used, and is usually inflexible. The pit width has a major influence on the throw achievable as wider strips result in lower percentage throw. The number of blast hole rows and design burdens is clearly influenced by strip width.
The choice of blast hole burdens is more significant than any other blast geometry parameter when throw blasting. Design burdens are commonly varied throughout the blast, with different distances for the front rows, the next few “throw” rows, the rows in the main body of the blast, and the back row. The front and throw rows have reduced burdens, to maximize forward movement. The rows in the body of the blast do not contribute to effective throw, and are designed to produce suitable fragmentation, muck pile looseness and muck pile shape. The back row has a reduced burden, to minimize damage to the new high wall.
FRONT ROW BURDEN
If the row does not detach itself evenly from the rock mass or has different momentum (ie. Large variations in face velocities) then the balance of the blast is effectively restricted or checked. This reduces throw and results in poor fragmentation and excessive final high walls in often used to give smooth faces with consistent front row burdens.
It is particularly important that the actual front row burden is accurately located to design. Where drilling layout or accuracy is not controlled, the efficiency of the blast will be reduced.
Burden velocities of 15 m/s to 25 m/s are adequate for successful throw. Approximately 20 m/s is commonly achieved with 5m to 7 m burdens on 270 mm diameter blast holes.
BURDEN FOR THROW ROWS
As the spacing between blast holes increases, the horizontal velocity component of the muck pile reduces at the midpoint between blast holes. Where excessive spacing is used, significant differential movement can be observed as the face is projected forward, and overall throw, will be reduced. The blast designer should assess the overall blast layout and consider a constant spacing for the entire blast. Use of a staggered blast layout is recommended, to optimize explosives distribution through out the rock mass.
ANFO (AMONIUM NITRATE / FUEL OIL)
A number of carbonaceous materials can be used with ammonium nitrate (eg. Nitropril) to produce an explosive. However, fuel oil (distillate has proved itself to be the blast fuel oil (distillate) has proved itself to be best fuel. Distillate is readily available, relatively inexpensive and can be easily mixed with Nitropril AN to produce a uniform mix (ANFO) which is more sensitive and more reliable than mixtures of NItopril with powdered fuels. More volatile fuels (eg. Petrol, kerosene) can give greater sensitivity but offer no significant advantage in explosive strength. These fuels have lower flash pints which introduce the risk of a vapor exposition during mixing and charging.
Alternative to fuel oil include recycled engine and hydraulic oils and by-products from recycling other hydrocarbon based materials. These alternatives should not be introduced as a substitute to fuel oil until extensive compatibility test work has been completed to ensure optimum performance of explosive and evaluation of potential risks.
MIXING
ANFO is mixed on site at using 94% Nitropil AN to 6% fuel oil (by mass. Until normal conditions, correct mixing ensures a uniform distribution of the fuel throughout the AN oxidizer. This Process contributes manufacture of an explosive, and requires an appropriate license from the relevant statutory authority.
In large operations, quantities of correctly proportioned an mixed ANFO are charged quickly and efficiently by bulk ANFO mix trucks. The quantities of ANFO required for smaller operations can be mixed using a COXAN mixer.
Any machine used fro making should be designed to avoid the possibility of fractional heating, and any bearings or gears must be protected from spillage of AN or ANFO. Petrol engines cannot be used and electric-powered motors should be approved. Appropriate safety assessments of any mixing equipment must be conducted prior to use.
To provide an immediate visual indication of the distribution of fuel oil in the mix and to distinguish between a mixed ANFO and straight AN, the fuel oil is often colored with an oil-soluble dye. When added at the rate of 0.25% of fuel oil, it provides adequate colouration of ANFO mixtures. Deeper colour is achieved by adding more dye. Having recognized the cost effectiveness of using accurately blended consistent ANFO, most mine operators choose to purchase ANFO which has been mixed by a mobile manufacturing unit (MMU).
FUEL OIL CONTENT
Maximum explosive strength and oxygen balance minimizing fumes, are obtained with 5,7% (weight basis) distillate. With too much or too little distillate, energy yield falls off. Too much distillate also caused excess CO to be librated in the detonation gases, whereas insufficient distillate encourages the generation of greater volumes of oxides of nitrogen. The VOD is also highest at 5.7% distillate.
The appearance of distinctive reddish brown colored post destination fumes in an ANFO blast may indicate too little fuel oil in at least some of the ANFO. (These fumes can also appear where properly-mixed ANFO has blast hole water or has been inadequately primed).
DENSITY
The augured density of ANFO is normally about 0.82 g/cubic meter. Even in long vertical blast holes, ANFO is relatively incompressible and its density varies vary little, if at all, from the top to the bottom of the charge.
SENSITIVITY
Although ANFO mixtures are explosives, they are relatively insensitive and unless suitably primed will not detonate. The required size and type of primer depends upon blast holes diameter, confinement, AN particle size, charge density and other charge density and other factors. In general, the primer should have a very high VOD and should be completely embedded within the ANFO column. Detonating cords may de sensitizes, practically detonate or reliably detonate ANFO depending on conditions of use (e.g. blast hole diameter). The sensitivity of ANFO is also influenced by the percentage of fuel oil and the intimacy of contact between the oil and intimacy of contact between the relationship between fuel oil content and sensitivity for a typical Nitropril-based ANFO.
EFFECT OF CHANGE DIAMETER
The critical diameter of ANFO (ie. The diameter below which ANFO cannot undergo a stable detonation) is influenced by its confinement and charge density.
In an unconfined state (e.g., in a light plastic sausage), ANFO will not detonate effectively in diameters smaller than about 100mm. The blast hole diameter has a pronounced effect on the VOD of ANFO. As the diameter increases, so does the VOD. However, energy yield does not vary with the blast hole diameter, provided that thigh-order detonations are being achieved.
EFFECT OF PARTICLE SIZE
Sensitivity and VOD increase as the ANFO particle size becomes smaller (through either deliberate or uncontrolled breakdown of the AN prills). Where ANFO becomes very fine, a detonator-sensitive explosive may result. As the amount of fines in ANFO increases, the mix becomes more difficult to handle and does not flow as freely.
WATER RESISTANCE
Lack of water resistance is the major limitation and disadvantage of ANFO. AN is readily dissolved by water, and the addition of fuel oil does not improve the situation. The sensitivity strength and the VOD are all reduced by the absorption of water. ANFO which contains more than about 10% water usually fails to detonate. The longer ANFO is exposed to water, the greater the deterioration and the poorer the blast performance. Whatever there is a chance of even small quantities of blast hole water desensiting and rendering ANFO less effective, a water-resistant explosive should be used.
The experiences of mine operators provide ample warning that the blasting performance of ANFO cannot be relied upon in the presence of blast hole water. Keep ANFO dry is the rule to follow.
SAFETY AND ACCIDENT PREVENTION
Safe and cost efficient blasting requires all surface mine operators and supervisors to understand and follow correct procedures for handling and using explosives. Most mines have on site induction training to develop skills for specific jobs, including blasting specify the method, tools and equipment to be used for each job. These procedures, combined with local Mine rules and statutory regulations, are designed to maintain health and safety of all people working in the mining environment.
Blasting requires the use of special tools and equipment, which are usually subject to statutory regulations. All tools and equipment used for charging and firing explosives should be properly maintained, regularly checked and correctly used. These should be no improvisation or substitution, as this can cause information on some of the hazards associated with blasting with blasting. These are guidelines only and must not be used as a substitute for proper instruction and training. Brief information on some drilling and blasting accidents is also included, to emphasize the hazards of poor attitudes and unsafe work practices.
MISFIRES
In general, the term “misfire” refers to an explosive that fails to detonate after an attempt is made to initiate it. The modern explosives products and techniques has reduced both the rate and consequences of misfires, but they still represent a serious threat of injury to people, damage to equipment and costs to companies. Statutory Regulations and Standard relating to known to anyone likely to be involved with misfires. Orica Explosives Technical Service personal are available for assistance with handling misfires, investigating the cause, and preventing a recurrence. The best way to handle a misfire is to avoid having it happen in the first place.
DETECTION OF MISFIRES.
If a misfire does occur, it is important that it is detected and handled during the Shotfirer’s post blast inspection rather than during subsequent digging. Unless unfired detonating cord or signal tube is obvious on the surface, it is often difficult to be certain that blast holes have failed to explode in large overburden blasts. Nevertheless, it is important that the Shotfirer acts on any reasonable suspicion of misfire based on the overall appearance of the shot (i.e. if in doubt, assume that misfires exist) the appropriate action is a matter for experienced judgment. It may take the form of surveying the location of specific blast holes for subsequent “digging with care” warnings or attempting to recover the blast holes, depending on the degree of concern or suspicion.
During all muck pile removal operations, whether by shovel or dragline, operations should be constantly alert for unfired detonating cord or signal tube in the blasted ground.
HAZARDS
The almost universal use of non detonator-sensitive blasting agents in surface mines has greatly reduced the hazards of un detonated explosives in a muck pile. The introduction of in hole delays has effectively eliminated misfired blast holes resulting from down line cut-offs caused by ground moment during the blast. However, misfires can happen fro a variety of reasons, and should always be considered hazardous. The main potential danger relates to unfired initiating explosives in a muck pile, as these are more sensitive and may initiate a much larger but less sensitive blast hole charge.
Accidents arising from misfires can be avoided by understanding potential hazards and following standard safe working procedures. For example, it is dangerous to attempt to rectify a misfire too soon after blasting.
Where misfired blast holes can be refired, part of the explosives charge may have a reduced burden. Particular attention should be paid to the safe distance for guards and other personnel because of the increased risk of fly rock under these circumstances. If possible, additional burden may need to be placed on the explosive by using sand or other suitable material. If a misfired blast hole cannot be safely refired water or a combined water and air jet provide the safest means of excavating into a charge. Compressed air pressure alone should not be used.
Drilling a “reliving” hole adjacent to a misfired blast hole charge us presented as an option I most Standards and Regulations. This technique should be treated as an absolute last resort because of problems with drill hole deflection and displacement of unexposed charges in the muck pile. There is generally a hazard when redrilling blasted ground pattern, and accuracy in pattern layout is important.
Unfortunately, misfires are often only detected whilst digging the muckpile. Where detonating cord downlines have been used, the pres
under my guidance for the award of degree of Bachelor of Technology in Mining Engineering of National Institute of Technology, Rourkela. This report is a bonafide record of project work carried out by him under my guidance. This project work fulfills the requirement of the regulation regarding the nature and standard of work.