Rock drills
Rock drill is the term applied to all machines using air for drilling holes into rock by combined percussive and rotary action. The hole diameter is normally upto 100mm.
The rock drills are classified mainly as follows:
(1) Jack hammers (also called Sinkers)
(2) Drifters.
(3) Stopers.
(4) Wagon drills.
A jack hammer, so familiar to mine workers, is a hand-held and unmounted drill used for vertically downward drilling. It weighs from 15 to 25 kgf and is used for drilling upto a depth of 2 m (rarely 3 m); hole dia. is generally 30 to 37 mm and rarely 50 mm. In a few cases a jack hammer may be mounted on an air leg. Though ordinarily used for dry drilling, it can be adapted for wet drilling as well.
A drifter is a mounted drill, generally designed for horizontal drilling; It is heavier than the Jack hammer and is used in quarries and for tunnel driving. The widely used mounting is the column and arm and the drill may be used for wet as well as dry drilling. Its working is like a jack hammer.
A stoper is a drill for drilling upward and derives its name from its widespread use in mine slopes. It is used normally for wet drilling.
A wagon drill- is essentially a drifter type drill capable of movement up and down a vertical guide and mounted on a portable frame fitted with wheels. The hole dia. is from 50 to 100 mm and the depth drilled ranges from 3 to 15 m.,
Compressed air was the motive power for wagon drills till recently but now-a-days some wagon drills are operated by hydraulic power, as hydraulic power is more efficient than compressed air power.
Jack hammer drill
It is a compressed air operated drill to which air is supplied from external compressors through hose pipes at a pressure of about 6 kgf/ cm2. The drill weighs 15 to 25 kgf and drills holes of dia. 30 mm to38 mm (rarely upto 50 mm) up to 3 m depth. The drill rod is hexagonal in cross-section, suitably shaped at one end to form the shank and the other end is so shaped as to form a non-detachable single chisel bit with a tungsten carbide insert. Drill rods may also be equipped with detachable X type tungsten carbide drill bits. In a shift of 8 hrs, two workers who hold the drill can drill 60 holes, each 1.2 to 1.5 m deep in sand stone, laterite etc. When hand-held, the machine drills vertically downward holes only but if mounted on air legs, it may be used for drilling inclined holes. An oil bottle placed between the drill and the air receiver, and connected by hose pipes to both, provides lubrication to the drill when working. For dust suppression a jack hammer can be adapted to wet drilling by some modifications so that the drill cuttings mixed with water come out of the hole in the form of a sludge. The air consumption is generally 2-2.5 m3 of free air/min. (See Vol. 3 for more description).
Wagon drill
A compressed air operated drifter mounted on a mobile frame and capable of travel up and down a mast is known as Wagon drill. The frame is usually tyred wheel mounted though crawler chain mounting is provided in a few models. Tyred wheel mounted wagon drills can be pulled by the operator and his helper to the hole sites on a level ground. A wagon drill, as stated earlier, is used to drill holes of dia. varying from 50.mm to 100mm for depth of 3m to 15 m. The mast for the drifter is usually 3 m long providing for nearly, 3 m vertical travel of the latter. This travel is possible with the help of a comp. air driven feed motor through chain (known as chain feed). The drifter provides the rotary motion as well as the percussive action to the drill rods, and in turn, to the drill bit. The drill bit is detachable X type with tungsten carbide insert. Compressed air fed through the hollow drill rods blows away the cuttings to the surface. Total meterage drilled in an 8-hour shift is 60-70 m in rocks like sandstone, coal, etc. including the time spent on shifting the drill from hole to hole. The mast is capable of swivelling from vertical to a horizontal position and it can be kept fixed at any angle between the horizontal and the vertical, thereby facilitating vertical, horizontal or inclined drilling upto40°. The drill is not self propelling, and receives air from external compressor (Fig. 5.16).
The maximum air consumption is 8 to 19 m3/min. of free air at 6 kgf/cm2 including air blowing for drill bit of 60-75 mm dia.
Though a detachable X-bit is the drill bit on most of the wagon drills, some wagon drills used for 100 mm dia. holes use down-the hole hammers. Such down-the-hole hammers are used for larger dia. holes also and are described a little later
Down-the-hole hammers
In a large size wagon drill using a drifter a considerable portion of the drifter's energy is utilised in overcoming the inertia of the drill string making up the column of the drill rods and in rotating them. Such loss of the drill energy increases with depth. .This waste of energy is considerably reduced by the use of the down-the-hole hammer. The drill bit used may be a carset bit (a X-bit with little modification) or a button bit which is fitted in the hammer. The comp. air going down the hollow drill rods forces the piston which directly hammers the drill bit without any drill rod in-between. The number of blows is from 500 to 2400 per min. When using down-the hole hammer the drifter is replaced by a rotary head placed at the top of the drill string and driven by a built-in piston type air motor. The rotational speed of the drill rods is nearly 15-25 r.p.m. The rotary head is also used to tighten and loosen threaded joints on rods. The-up and down travel of the drill rods is by a chain feed. Fig. 5.17 shows down-the-hole hammer, type 100 ASS used on HALCO drills for holes of 100 mm to 125 mm dia. Its specifications: Outside dia. 89 mm, length without bit 94 cms; weight without bit 31 kg.
Air consumption at 7 kgf/cm2 is 5.5 m3/min.
(The largest well hole drills in India are at Kudremukh iron ore project — electric rotary drills for 310 mm dia. blast holes The matching equipment at Kudrcmukh has the capacities; electric shovels, 10.7 m3; production trucks, 108 te; front end loaders, 10 m3)
Hydraulic wagon drill
Some of the heavy duty wagon drills are powered by hydraulic pressure system. Fig. 5.18 shows a hydraulic wagon drill, model ROC 810 H manufactured by Atlas Copco (India) Ltd. It is equipped with a rock drill model COP 1038 HB manufactured by the same company. In the drill, compressed air is replaced by hydraulic pressure and the prime mover for the hydraulic power pack is an air cooled diesel engine. The absence of exhaust air results in a much lower noise level when compared with air-powered rock drill. It can drill holes of dia. 65 mm to 327 mm and can therefore be used as a well hole drill for 127 mm dia. holes for depth upto about 12 m. The hole is flushed with compressed air at 10 kgf/cm2. The rate of penetration in hard rock is generally 1 m/min using 90 mm dia bit. The rock drill 1038 HB is equipped with a hydraulically powered rotation motor whose speed can be varied from 0 to 200 r.p.m. and easily adapted to prevailing rock condition. The hydraulic system incorporates indicators which point out any fault or malfunction in the system, The boom system is operated by hydraulic pressure.
Automatic disc brakes contribute towards increased safety for the operation when travelling along steep inclines.
Well hole drill
This is usually a crawler mounted drill operated by a diesel engine or by an electric motor which is supplied power from an-external source through a trailing cable. It drills holes of 125 mm to 300 mm diameter, depth varying from 6 m to 18 m. It has a long mast, 3 m to 6 m, to accommodate the length of the drill rod. The mast is collapsible and the drill should not be moved over an, appreciable distance with the mast raised. The drills are of percussive as well as rotary type but the latter is common in coalmining areas. The drilling tool of rotary drill is a tricone bit on most of the drills but on the machines which are known as "down-the-hole percussive drills" (sometimes called "down-the-hole hammer drill"), the drilling tool is a cross bit (carset bit), or a button bit. In down-the-hole hammer drill the assembly of the drill bit and its short length pipe is called down-the-hole hammer (see fig. 5.17).
In the rotary drill the drill string is rotated by the prime mover through suitable gearing. The tricone bit attached at the end of the drill string is thus rotated and it is kept pressed against the rock by hydraulic or pneumatic pressure.
In down-the-hole percussive drill the rotation of the hollow drill rods is provided by a rotary head placed at-the top of the drill String and driven by a built—in air motor. The air motor is also used to tighten and loosen threaded joints on rods and bits. The up or down travel of the drill rods is by a chain, operated by a reversible piston type air motor (Chain Feed). A compressor mounted on a well hole drill helps to clean the hole as it is drilled. During drilling, the machine is levelled with the help of 3 hydraulic jacks. Normally twenty holes, each 9 m deep, can be drilled in one shift in sand stone, shale and coal. Only vertically downward drilling is possible on most models though holes 20° off vertical can be drilled by a few machines. On some machines the drill-rods and the tool are at one end and on others, in the middle of the machine. The latter arrangement is permissible where the burden of blast hole is large and the ground at the quarry edge strong enough to support the weight of the machine; but where this is not practicable drill rigs with the drill rods and tool at one end have to be used.
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A Well hole drill appears like a wagon drill suitable for large diam. holes, e.g. the wagon drill shown in fig. 5.18.
A rotary well hole drill can drill in a shift of 8 hours nearly 20^ holes, 200 mm dia. each 9 m deep, in sandstones, coal, shale and similar rocks.
Inclined drilling
Where the overburden consists of soft rock which can be conveniently removed by ripper and scraper-dozer combination an alternative to ripper is the method of drilling nearly horizontal blast_ holes and blasting them. Vertical (or nearly-vertical) blast holes have to be drilled where the overburden consists of hard rock like sandstone, laterite, etc.
30° off vertical may be considered to be the limit for inclined drilling of nearly-vertical holes on a bench. Larger angle increases the length of the hole, difficulties in charging it with explosives of fixed-shaped cartridges, proportion of stemmed section of the hole and gives face inclination unsuitable for travel of the shovel bucket.
The toe of a bench can be removed by extra drilling of short length horizontal holes only in the toe and blasting them, or by resort to inclined drilling of the main (nearly vertical) blast holes. In vertical as well as inclined blast holes for the face, it is always essential to extend the hole slightly beyond the level of bench floor to secure proper fragmentation of toe if the hole-is terminating in hard rock.
Layout of a mechanised quarry
The layout of a quarry depends primarily on
(1) shape, size and dip of the deposit.
(2) proposed depth upto which mining activity is planned to extend,
(3) thickness of the overburden,
(4) surface topography,
(5) desired production,
(6) transport system for mineral and OB,
(7) arrangement for disposal of debris,
(8) type of mechanisation and finance available for it.
The layout of a quarry should depict the following:-
(1) Position of OB benches and mineral benches.
(2) Access to the benches and exit roads for the dumpers.
(3) Position of back filling area.
(4) Location of machinery which operates from one site over a long period such as a shovel, dragline, bucket wheel excavator, in relation to the benches.
Fig. 5.19 shows the layout of a mechanised quarry using dipper shovels, dumpers and well hole drills, the common equipment 'in most of our mines. The property is divided into areas along the strike, each area being nearly 100 to 300 m long. In the initial stages a box cut (trench) RK is made with the help of scraper, dozers or small capacity (2 m3) shovels to suit the depth of the trench. Such trench is essential for a shovel which stands on the floor of the quarry and operates on benches. Some amount of blasting may be necessary in making a trench in hard rocks. In the advanced stage of the quarry two Trenches RK and R|K] are in use, one for the OB and the other for the mineral which goes to the crushing plant. The OB bench and the mineral bench are usually parallel to each other and advance towards the quarriable limit of the mine.
In the figure, hi and hi are OB bench and mineral bench respectively. If the OB and mineral are of considerable thickness,, there may be two or more benchs for OB as well as for the mineral. In such cases ramps have to be provided between adjacent benches. Provision of two trenches, one for OB and the other for mineral, avoids congestion of dumpers on the haul roads. This layout can be modified to have a crusher and outgoing belt conveyor on the lower end of mineral trench. In-pit crusher with feeder breaker is the trend_ in open cast mines these days. Mineral is then transported by conveyor belts to coal handling plant or mineral processing plant. Some of the ntschanised opencast mines in South Eastern Coalfields Ltd., are now having in-pit crushers
A mobile crusher in a mechanised quarry receiving finishing touches in the manufacturing shop.
Courtesy of Simplex Engg. & Foundry Works Ltd., Bhilai.
Such layout of two trenches is not suitable it tne mineral is at much depth as formation of trenches would involve removal of large volume of earth and rock. Layout of one trench for both mineral and OB may then be adopted.
For deeper quarries switchback system of track layout is advantageous. Fig, 5.20b shows the tracks as they would appear in a vertical projection (exaggerated) for a locomotive-mine car combination of transport system. Numbers 0, 1, 2, 3, mark the levels of the floors. Thus to get from level 0 to working floor 3 a train must follow the route marked by the arrows. Switchbacks entail a big loss of time for shunting operations. Each switchback should be sufficiently long to hold a train and railway switches, or else to allow enough room for mine cars to turn round.
it shows a modified layout depicting the position of various coal and OB benches in a quarry (Shovel-Dumper Combination) at an advanced stage if OB is backfilled in the decoaled area. The seam is considered to be 18m thick dipping at 1 in 5.
The benches formed are sometimes designated as Bench No. 1, Bench No. 2, etc. or by the reduced level of the floor of the bench e.g. 500 metre bench, 510 metre bench, 520 metre bench, etc.
Pumps are installed in the quarry which has to be kept nearly dry for operation of shovel and dumpers. The usual arrangement is to have two or three pumps as a semi permanent installation fed by small portable pumps.
The area may be divided into strips of 45 to 60 m width along dip rise. A width of 45 to 60 m permits easy movement of dumpers, the minimum width required being 18 metres. Each strip provides a bench, which is equipped with ashovel and a well hole drill Daily progress of shovel in the strips L, N, is along dip rise direction, but the advance of the bench is along the strike when considered over a quarter or half year. The quarry excavation of course advances towards the dip to the quarriable limit.
The shovels S1 and S3 work on stone benches and the others (2 m3; work on coal benches. The overburden may be taken to dumping yard away from the quarry, or dumped in the decoaled area. Suitably graded roads, not steeper than I in 10, with rumps between the strips, are provided as shown in the figure. Shortage of shovels and ancillary equipment may require the management to work only a few benches at a time and not as many as shown in the figure.
it shows a spiral layout for dumper transport. As the haul road runs along the periphery of the quarry, excavation commences from the boundary of the quarry and proceeds downward forming one bench at a time. Such layout is an advantage in hill-top deposit which may be worked in horizontal slices..
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