BLASTHOLE DRILLING IN OPEN PIT MINING 19 TALKING TECHNICALLY A complete range With the acquisition of Ingersoll- Rand’s Drilling Solutions, Baker Hughes Mining Tools (BHMT) and Thiessen Team businesses, Atlas Copco has a complete range of products to offer to large quarries and open pit mines. Much of the world’s mining output begins through drilling of holes with rotary drills. Ingersoll-Rand built air-powered rotary drills for many years prior to the introduction of their first fully hydrau- lic unit, the T4, in 1968. About rotary drills It is important to note that rotary drills are capable of two methods of drilling. The majority of the units operate as pure rotary drills, driving tricone or fixed-type bits. The fixed-type bits, such as claw or drag bits, have no mo- ving parts and cut through rock by shea- ring it. Thus, these bits are limited to the softest material. The other method utilized by rotary drill rigs is down-the- hole (DTH) drilling. High-pressure air compressors are used to provide com- pressed air through the drillstring to drive the DTH hammer (see illustration page 20). The primary difference between rotary drilling and other methods is the absence of percussion. In most rotary applications, the preferred bit is the tricone bit. Tricone bits rely on crush- ing and spalling the rock. This is accomplished through transferring downforce, known as pulldown, to the bit while rotating in order to drive the carbides into the rock as the three cones rotate around their respective axis. Rotation is provided by a hydraulic or electric motor-driven gearbox (called a rotary head) that moves up and down the tower via a feed system. Feed sys- tems utilize cables, chains or rack-and- pinion mechanisms driven by hydraulic cylinders, hydraulic motors or electric motors. The preference at Atlas Copco is to use cables for pulldown, as they are lightweight and inexpensive, and allow easier detection of wear to help avoid catastrophic failures. Atlas Copco’s largest drill, the Pit Viper 351E, operates on a blast pattern at an open pit copper mine. Rotary blasthole drills are the predominant method of drilling 9 inch (229 mm) diameter holes or greater. Putting rotary drilling into perspective Mining prosperity Atlas Copco offers a complete range of rotary as well as DTH and top- hammer drill rigs for most types of open pit mining and quarrying applications. But how do these technologies complement each other and how do drillers know which method to choose, and when?
Transcript
Blasthole Drilling in open pit Mining 19
TalkingTeChniCally
acompleterange
With the acquisition of Ingersoll-Rand’s Drilling Solutions, Baker Hughes Mining Tools (BHMT) and Thiessen Team businesses, Atlas Copco has a complete range of products to offer to large quarries and open pit mines. Much of the world’s mining output begins through drilling of holes with rotary
drills. Ingersoll-Rand built air-powered rotary drills for many years prior to the introduction of their first fully hydrau-lic unit, the T4, in 1968.
aboutrotarydrills
It is important to note that rotary drills are capable of two methods of drilling. The majority of the units operate as pure rotary drills, driving tricone or fixed-type bits. The fixed-type bits, such as claw or drag bits, have no mo- ving parts and cut through rock by shea- ring it. Thus, these bits are limited to the softest material. The other method utilized by rotary drill rigs is down-the-hole (DTH) drilling. High-pressure air compressors are used to provide com-pressed air through the drillstring to drive the DTH hammer (see illustration page 20). The primary difference between
rotary drilling and other methods is the absence of percussion. In most rotary applications, the preferred bit is the tricone bit. Tricone bits rely on crush-ing and spalling the rock. This is accomplished through transferring downforce, known as pulldown, to the bit while rotating in order to drive the carbides into the rock as the three cones rotate around their respective axis. Rotation is provided by a hydraulic or electric motor-driven gearbox (called a rotary head) that moves up and down the tower via a feed system. Feed sys-tems utilize cables, chains or rack-and-pinion mechanisms driven by hydraulic cylinders, hydraulic motors or electric motors. The preference at Atlas Copco is to use cables for pulldown, as they are lightweight and inexpensive, and allow easier detection of wear to help avoid catastrophic failures.
Atlas Copco’s largest drill, the Pit Viper 351E, operates on a blast pattern at an open pit copper mine. Rotary blasthole drills are the predominant method of drilling 9 inch (229 mm) diameter holes or greater.
Puttingrotarydrillingintoperspective
MiningprosperityAtlas Copco offers a complete range of rotary as well as DTH and top-hammer drill rigs for most types of open pit mining and quarrying applications. But how do these technologies complement each other and how do drillers know which method to choose, and when?
20 Blasthole Drilling in open pit Mining
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Pulldown
Pulldown is the force generated by the feed system. The actual weight on bit,or bit load, is the pulldown plus any dead weight such as the rotary head, drill rods and cables.
Moreweightwithrotary
It only takes one look to see that the biggest DTH and tophammer drill rigs are very different than the biggest rotary blasthole rigs. In fact, the Pit Viper 351 rotary drill rig weighs in excess of nine times that of Atlas Copco's largest DTH hammer drill rig, the ROC L8. Yet the Pit Viper 351 is drilling a hole that is generally only twice the diameter. Take a typical medium formation tricone bit with a recommended maximum load- ing of 900 kg/cm of bit diameter (5000 lb per inch of diameter). With a 200 mm (7-7/8 in) bit, you could run about 18,000 kg (40,000 lb) of weight on the bit. The laws of physics dictate that for every action, there is an equal and opposite reaction, meaning that if you push on the ground with 18,000 kg (40,000 lb), the same force will push back on the unit. There-fore, the weight of the machine must be over 18,000 kg (40,000 lb) at the location of the drill string to avoid the machine “lifting off” the jacks. To achieve a stable platform through proper placement of the tracks and levelling jacks, the distribution of weight results in an overall machine weight that approaches or exceeds twice the bit load rating. This weight does add cost to the machine, but the size of the components also translates to long life. Even smaller rotary blasthole drills are built to run 30,000 hours of operation, and some of the large blasthole drills have clocked over 100,000 hours of use.
Rigdesign
With the exception of one model, the rubber-tire mounted T4BH, Atlas Copco’s rotary blasthole drills are mounted on excavator-style undercarriages. Power-ful hydraulic-drive systems allow the machine to tram over a variety of ground conditions, though rotary blasthole drills should always operate on firm, flat benches.
Principle:The hammer is situated down the hole in direct contact with the drill bit. The hammer piston strikes the drill bit, resulting in an efficient transmission of the impact energy and insignificant power losses with the hole depth. The method is widely used for drilling long holes, not only for blasting, but also for water wells, shallow gas and oil wells, and for geo-thermal wells. In mining it is also developed for sampling using the reverse circulation technique (RC drilling).
TONS
Principle:Rotation is provided by a hydraulic or electric motor driven gearbox, called a rotary head, that moves up and down the tower via a feed system, generating the pulldown required to give sufficient weight on the bit. Flushing of drill cuttings between the wall of the hole and the drill rods is normally done with compressed air.
The tower supports the drill string during drilling as well as the rotation head and feed system.
Down-The-holemethod Rotarydrillingmethod
Blasthole Drilling in open pit Mining 21
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The key component of a rotary blast- hole drill is the tower, which is some-times referred to as the derrick or mast. Atlas Copco towers are four main mem-ber, open front structures in which the rotary head slides up and down via a guide system. The length and weight of the tower ultimately dictates the size of the mainframe and undercarriage.
Most drilling functions are hydrauli-cally driven. Powering these hydraulic systems, along with the air compressor, is a diesel engine or electric motor. Most rotary drills are diesel powered for good mobility. Electric powered units offer some advantages such as lower power cost (in most areas), no diesel emissions, no refueling requirement and less maintenance. However, some operations are not setup with the pro-per electrical infrastructure or staffing to run electric units. Even when elec-tric power is available, many custom-ers avoid electric drills as the trailing cable used to provide power makes it harder to move the unit between holes or patterns. Generally, electric power
is preferred on large single-pass units used in major open pit metals mines where electric shovels are employed, though electric power is now available on smaller units such as the Atlas Copco Pit Viper 271, Pit Viper 275 and DML.
Theimportanceofair
A key parameter of rotary drilling is flushing the cuttings from the hole. Inmost rotary blasthole drills, cuttings are lifted between the wall of the hole and the drill rods by compressed air. Sufficient air volume is required to lift these cuttings. Many types of tricone bits have been developed to meet vari-ous drilling needs. Softer formation bits are built with long carbides with wide spacing on the face of the bit. This design yields large cuttings which increase drill speed and reduce dust. It is important to have sufficient clearance between the wall of the hole and the drill rods in order for such large cuttings to pass. If this clearance, known as an- nular area, is not sufficient, the cuttings
The drilling platform is supported by a crawler undercarriage except during drilling when it is raised up by hydraulic jacks.
The ability to carry long drill rods up to 70 feet provides more time for drilling.
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will be ground between the wall of the hole and the rods or by the bit itself (called regrinding) until they are small enough to exit the hole. This results in excess dust and accelerated wear on the bit and drill rod.
Bailingvelocity
A traditional rule-of-thumb is a mini-mum of 1,525 m3/min (5000 cfm) of uphole velocity, the speed at which air exits the hole. The actual amount of air required will vary widely based on the density of the material and the size of the cuttings. Dense cuttings as found in iron ore mines will settle much quicker than lightweight overburden in coal mines and thus need more air coming up the hole to lift them; 1,525 m3/min (5000 cfm) may not be enough. However, har-der material is generally drilled with hard formation bits that utilize shorter cutting structures, thus yielding smaller chips. Conversely, some soft material can be drilled effectively with only 915 m3/min (3000 cfm) uphole velocity. Unfortunately, many operations have tried to increase uphole velocity by in-creasing the diameter of the drill rod. This is obviously much easier than get-ting a larger air compressor by retrofit-ting or purchasing a new machine. In some conditions, this strategy works, but more often, the reduced annular area results in increased wear and dust, and the drill rate may even drop. Whatever the application, it is critical to have pro-per bailing air.
Dustcontrol
A necessary evil created by the air com- pressor in drilling operations is the ge- neration of dust. To control the dust, the area surrounding the hole is en-closed by a dust hood. Dust hoods are sealed on the sides by dust curtains, and where the rod comes through the deck by a rod wiper or dust seal. A dust control system must be used in con-junction with the dust hood and cur-tains. The two most popular types of dust control are dry dust collectors and water injection. Dust collectors are es- sentially large vacuum cleaners that pull the dust away from the dust hood and run it through a collection of filter
Rotary drilling with tricone bits is the most cost efficient method for large hole diameters.
Large diameter holes produced by rotary drills, such as this Pit Viper 275, yield blast patterns with wider burden and spacing, resulting in fewer holes drilled.
To control the dust, the area around the hole is surrounded by a dust hood.
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elements. Water injection systems inject a fine amount of water into the air stream. Water injection is the more effective solution for ensuring dust is minimized, but the introduction of water into the hole can slow down the drilling process by increasing the den-sity of the cuttings at the bottom of the hole that the air compressor must move. Water injection systems require fre-quent refilling of the water tanks, and in freezing conditions, elaborate heat-ing systems must be used. Dust collec-tors offer a productivity advantage, but they can become plugged if not turned off when wet material is encountered. This is particularly a problem if the wet material freezes in the system.
Whenrotaryisbetter
Every drilling application is different, so we cannot say that there are parti-cular breakpoints where you should transition between drilling methods.
Generally, drilling below 152 mm (6 in) is best accomplished with tophammer units. Above this diameter, it is typi-cally done with a rotary rig, although tophammer units are doing some of this work effectively with the introduc- tion of larger platforms and more pow- erful rock drills. For harder material, say above 100 MPa (15,000 psi), uncon- fined compressive strength (UCS), DTH is often faster than pure rotary drilling if provided there is enough air pressure on board. Simply looking at our product range (see above) gives an indication of which methods are com-monly used for the different diameters found in construction and mining.
There are certain limitations im- posed on each method of drilling. With tophammer percussive drills, the po- wer of the rock drill itself limits the ability to transmit adequate force to larger diameter bits, especially at dee-per depths when percussive energy is successively reduced with each new rod
connection. Down-the-hole (DTH) tools solve this energy loss problem, but their maximum hole diameter is limited by the volume of air. To build the air pressure that translates directly to impact energy, a certain volume of air is required. Take for example a Secoroc QL80 203 mm (8 in) DTH hammer that is designed to operate at 25 bar (350 psi). Even with our largest high pressure compressor 686 41 m3/min (1,450 cfm), the pres-sure will only build to 23 bar (325 psi), thus providing less impact energy. In real terms, each blow of the piston is about 45 kg (100 lb) less than it is de- signed for. In some cases, this method will still outperform rotary drilling. For most large diameter blasthole drilling, there is simply not enough air on-board for a DTH to be as cost effective as rotary drilling with a tri-cone bit. Rotary drilling is still the pre-dominant method of drilling 230 mm (9 in) diameter or greater. This is driven primarily by the current limitations of
Rotary drilling with tricone bits is the most cost efficient method for large hole diameters.
DML
ROC P55
CM 351
ECM 720
1"
ROC L630
ROC P65ROC L7CR
ROC L825
CM 785
Pit Viper 271
ECM 585
ECM 660ROC F9/F9CROC F9CR
ROC D9/D9RRC/D9CROC D7/D7RRC/D7CECM 590/592
ROC T15BVB 25
ECM 580CM 470
ROC D3ROC 203
DM25-SPROC L830
406mmPit Viper 351
381mm305mm127mm 152mm 178mm 330mm
CopperIron
16"15"13"25mm 51mm 76mm 102mm 356mm279mm
11"203mm 229mm
12"254mm
8" 9" 10"
Industrial minerals (Cement & Limestone)Gold
Coal
2" 3" 4" 5" 6" 7" 14"
Dimension Stone IndustryConstruction
Aggregate
DTH
COPROD
Tophammer
Tophammer / fully pneumatic
DTH / fully pneumatic
Rotary / DTH
Rotary
CM 765ROC L625
CM 348
Atlas Copco large rotary andDTH drill rigs are included
in this book. For information about the range of smaller surface rigs
visit www.surfacedrilling.comor contact your Atlas Copco
representative
ROC L740ROC F6BVB 25 DTHROC 203 DTH
Pit Viper 275 DM-M3
DM30T4BH
DML-SP
Pit Viper 235
DM45
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tophammer units and rig air systems. Tricone bits also become more cost ef-fective as the larger bits are equipped with larger bearings which in turn can handle higher loads. These higher loads translate to improved drill rates. An- other advantage of rotary rigs is the length of the drill rods that can be car-ried on board. Longer rods mean fewer connections. Smaller rotary blasthole machines utilize 9.1 meter (30 ft) length rods, while larger units are capable of running 10.7 meter (35 ft) or 12.2 meter (40 ft) rods. By comparison, topham-mer or DTH crawler drills use drill steel that is generally 6.1 meters (20 ft) or less in length. Further, some rotary rigs are large enough to handle a long tower that enables drilling of the entire bench height in a single-pass. At the largest open pit mines, rotary units are drilling 20 m (65 ft) deep holes in a single-pass to match the bench heights dictated by the large electric shovels that can dig a 17 m (55 ft) bench.
Productivityversuscost
Studies have shown that pure penetra-tion rate will increase linearly with increased pulldown. The same has also
been said of rotation speed. So why doesn’t every operation use more of each? Unfortunately, higher pulldown and rpm usually results in increased vibration and lower bit life. The vibra-tion causes increased wear-and-tear on the rig, but more importantly, it creates a very unpleasant environment for the operator. What invariably happens is that the operator reduces the weight or rpm until the vibration returns to a comfortable level. Some operations limit bit load and rpm even if there is no vibration in order to improve bit life. This is often the wrong strategy as the overall drilling cost per unit, also known as Total Drilling Cost (TDC), should be considered. TDC is calculated using the bit cost per meter/foot and the total rig cost per hour. The unit cost per hour includes labor, maintenance and po- wer, and possibly capital cost. The drilling speed really doesn’t impact this cost-per-hour figure. What it does impact though is the cost per unit produced (cost/meter/foot, cost/ton, etc…).
You generally want to push the rig harder to reduce the cost/foot, but there will be a point where the rig overloads the bits (see diagram).
largeversussmall
There are some drawbacks to rotary rigs. Smaller crawler rigs are more flexible with many advantages such as articulating and extendable booms and guides that allow drilling at many dif- ferent angles. Unlike crawler rigs, the components on rotary rigs are often not enclosed. They are mounted onto the frame in an open layout that makes them extremely easy to service. Large electric units normally have a machi-nery house to protect the electrical drivecomponents, and newer midrange sized blasthole units such as the Pit Viper 235 have the option of a machine enclosure. The general trend for 165 mm (6-1/2 in) or less is towards the smaller, more flex- ible units. However, many large scale quarries and small mines still favor the durability, life and simplicity of the larger rotary rigs for these small diam-eters. For the large scale open pit opera-tions that yield a high percentage of the total worldwide mineral production, it is anticipated that rotary drilling will remain the primary method for years to come.
