Английский язык для горных инженеров-механиков. Учебное пособие

top half of the bed, its picks cutting downwards-in the ideal case it easily deals with the now unsupported mid-seam anhydrite band.

All material cut from the face has to pass between the picks on the two drums which rotating in opposite sense, almost mesh together and behave like a two-roll crusher eliminating oversize lumps which would be unsuitable for the conveyor system. The machines while all of basically the same design are tailor-made to suit the height of the potash bed to be worked in each panel, the motor power being selected according to the work each drum is expected to do. Normally the shearer takes a 1.10 m cut off one face at each traverse and after completing one cut is flitted back to the other end to start another cut. As the retreat face progresses the inner wall is drilled and blasted, temporarily widening the space to accommodate the shearer as it is moved forward into position to make the next cut.

*MDPA–name of an enterprise.

LOCOMOTIVES

Locomotive haulage usually demands gentle gradients with slopes of about 1 – 20° and roadways of reasonable dimensions. Although they are not used extensively for the underground transportation of coal the use of locomotives for transporting men and materials is increasing.

In addition to running on conventional track, there are locomotives running on special forms of track to which they are captive, that is, unable to overturn or become derailed. On these locomotives the driving wheels have polyurethane tyres, which are forced against the track by hydraulic cylinder, extracting large tractive efforts from relatively light locomotives.

Locomotives are either diesel or battery powered. Strict precautions are necessary to ensure the safe operating of these locomotives underground. For example the regulations governing the use of diesel engines underground require that:

1.no air enters the engine without first being cleaned;

2.no exhaust gases are expelled without first being cooled and

3.no flames or sparks are emitted.

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These regulations are satisfied by firstly ensuring that the intake air flows through air filters and a flame-trap, which is fitted to prevent the transmission of flame and the ignition of an external inflammable atmosphere. Secondly, the exhaust gases are passed into a conditioner which absorbs some of the soluble noxious constituents and cools the gas before passing it through a second flame-trap. Thirdly, air drawn through the radiator for water cooling is mixed with the exhaust gases before being emitted to the atmosphere.

The development in underground locomotives includes a small battery powered vehicle operating on rubber tyres instead of the conventional steel wheel to give greater adhesion. The average speed of a locomotive is approximately 12 km/h, but a diesel locomotive, capable of speeds of 40 km/h is under development.

The most revolutionary development in underground transport is found in the use of the linear induction motor. Here the wheels are used merely for guiding the locomotive along the track. The tractive effort comes from the motors and through a reaction rail between the tracks.

PLACER MINING: DREDGING

The dredge may have been the first continuous mining machine invented; a crude device of this type was in use in the Netherlands in 1565. Dredging is the underwater excavation of a placer deposit, usually carried out from a floatingB vessel which may incorporate processing, and wastedisposal facilities. The body of water may be natural or man-made; depending on the sizes of the dredge and deposit, from 200 to 2000 gal/min (13 to 125 L/sec) of water may be required for both mining and processing waste disposal. Once popular in the United States, there are few placermining dredges operating there today (ехcept in one or two western states and Alaska), but 130 are still used in mining elsewhere in the world.

Dredges are classified as follows.

A. Mechanical

1.Bucket line (endless chain of buckets revolving along ladder)

2.Bucket-wheel suction (buckets discharge in suction pipeline)

3.Dipper (shovel, grapple, or dragline mounted on barge)

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B. Hydraulic

1.Suction (open intake suction line)

2.Cutterhead (excavation by rotating cutter on suction line)

The bucket-line dredge is the classical, continuous-excavating machine for poorly consolidated or loose material with some boulders; hence its early application to placers. A recent, more versatile development, the bucketwheel suction dredge, mounts buckets on a rotating wheel and discharge excavated material into a hydraulic conveyer. Dipper dredges have the disadvantages of all intermittent excavators but are able to dig tougher, more consolidated material with boulders. Hydraulic dredges were adapted to placer mining from channel excavating; the suction type is restricted to sand and gravel, while the cutterhead type can manage consolidated materials. Both hydraulic dredges have limited application for placer mining because of low heavy-metal recovery and boulder restrictions, although they offer more continuous excavation capability.

We direct our attention mainly to bucket-line and bucket-wheel dredges because of their suitability for placer mining. In operation, a dredge “carries” its pond with it as it excavates the placer bank ahead while depositing waste behind. Material dug by a rotating bucket line is elevated on board the dredge; the bucket ladder digs in a vertical arc up the placer bank, then steps over with the aid of a spud and shore lines to a fresh face. The cut is about 1 ft (0.3 m) deep and 5 ft (1.5 m) wide. Dredges can excavate banks of about 50 ft (15 m) above water level to 160 ft (50 m) below.

With a bucket-line dredge, mineral processing is carried out on board. A tuil sluice disposes of fines while a stacker conveyor discharges coarser material. The output of a bucket-line may be estimated from its design specifications.

HAULAGE UNIT

We choose the truck as a haulage unit in open cast mines because it is used far more extensively than conveyors, scrapers, dozers, and other conveyances. As with the power shovel, ample performance data are available to assist in the selection procedure. The following considerations are important.

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1. Size rating. Truck capacity (live load) is usually measured on weight rather than volume basis to prevent overloading. Volume measure is struck capacity, based on loose (not bank) measure. Standard sizes vary among manufacturers, but the following ratings are representative. Trucks are divided somewhat arbitrarily into normal and giant sizes, reflecting extensive customizing in the upper range:

Normal sizes: 22, 30, 35,40,55,85,100,130 tons

(20, 27, 32, 36,50,77,90,117 tons) Giant sizes: 150,175, 200, 250, 300, 350 tons

(135, 158, 180,225, 270,315 tons) Our interest is mainly in the normal sizes.

2. Operating factors. These factors are similar to those for shovels, but some are expressed differently.

a)Working time. The same available time estimates are made for trucks as for shovels. Again, 50 min/hr and 7 hr/shift are used unless other conditions are specified.

b)Optimal number of swings. This is a function of the target area of the truck bed and the frequency of truck changes at the shovel. A proper

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UNIT V

HOISTING SYSTEM (METHODS)

It is with the hoisting system itself-these components of the hoist plant located in the hoist room-that engineering design is mainly needed. Three key factors govern hoist selection.

1.Production rate or tonnage to be hoisted per unit of time.

2.Depth of shaft

3.Number of levels to be accessed.

There are basically only two hoisting methods, plus some modifications, in use today: drum and friction. The drum hoist stores the rope not extended in the shaft. The friction-sheave hoist passes the rope (or ropes) over the drive wheel but does not store it. A minor method that has fallen in to disuse is reel hoisting, in which a single width of rope is wrapped in many layers. A new method devised in South Africa very deep shafts is multi-drum hoisting using multiple ropes.

Applications of the drum, friction-sheave and multidrum hoisting methods are summarized in Table 6.

Table 6. APPLICATIONS OF HOISTING METHODS

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Drum Hoisting. The simple drum system is a single cylindrical drum, plus attached electricals and mechanicals. With two skips, it is suitable only for single-level hoisting, typical of coal mining. For multilevel hoisting, a double-drum hoist is used, clutched to permit one skip or cage to stop while the other is in motion. With the drum method two skips or cages travel in balance; occasionally a counter weight replaces one conveyance (e.g., with a single drum to permit multilevel hoisting). Drum must be ground-mounted; idler sheaves are located in the headframe to support the hoist ropes other drum shapes that are employed to improve power characteristics are the conical and cylindroconical.

Friction-Sheave Hoisting. The friction sheave system operates on the principle that a hoist rope passing over a drive sheave is prevented from slipping even though there is a considerable difference in tension in the two segments of rope. The tread material on the sheave is usually polyurethane; using a proper lubricant with the rope improves its nonslipping qualities. Like the drum method, two conveyances (or one and a counterweight) are used in balance. Unlike the drum method, a tail rope is preferable. (It equalizes the rope weight and reduces slippage. Since the angle of wrap around to friction sheave also influences slippage, it can be increased by a deflection sheave or by ground mounting of the friction sheave). Intended for single level hoisting, the friction-sheave method must use a counterweight for multiple levels. Multiple (usually four) ropes are normally used: several ropes have better flexing qualities than a single rope and permit the use of a smaller sheave.

UNIT VI

OPEN PIT MINING AT BORON (MOJAVE DESERT)

Open pit mining at Boron commenced in 1956. Previously ore had been extracted by conventional underground room and pillar method. Overburden thickness varies between 60 m and 150 m and today the pit dimensions have grown to measure some 6000 ft from east to west. 5000 ft from north to south with pit bottom reaching a depth of 700 ft (250m). All

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ore and overburden is drilled and blasted using ANFO, and drill centers vary between 5.5m and 7.3 m, depending on optimum fragmentation of the ore. Benches are double-cut 22.5 m in height.

Loading of overburden is by three electric shovels, two fitted with 19 yd3 capacity bucket and third having a 15 yd3 bucket. Ore loading is by three 13 yd3 bucket front-end-loaders. All haulage is accomplished by a fleet of diesel-electric trucks: six of 154 t capacity and twelve 90 t capacity.

Because of the low specific gravity of the ore (1.6) and the consequent bulk handling problems, the 90t truck have been fitted with tailgates enabling them to carry loads of greater volume than usual. The current stripping ration is over 5:1 waste to ore. The total tonnage moved annually in the mining operation is regarded by the company as confidential information, but is probably of the order of 18Mt/y.

An in-pit crusher reduces the ore to less than 150 mm (6 in), after which the ore is taken out of the pit by an inclined (18°) long 42 in (1.066 m) wide belt conveyor to a surface stockpile. The belt, with a 600 hp (450kW) drive, runs at 600 ft/min and carries up to 1.090 t/h. The latest belting to be used is of a new type, manufactured with a single ply reinforcement of woven steel fabric, with good resistance to ripping and tearing.

A high speed loader fills 100 t capacity rail cars, and the product is transported to the storage and loading facilities.

THE ULAN COAL MINE EQUIPMENT

The Ulan coal mine, New South Wales Australia, is one of the most modern in the world in terms of equipment and techniques used. After prestripping operations to remove topsoil and vegetation in which cat dozers and 992 loaders are used, overburden is drilled, blasted and removed by dragline to expose the coal seam. Overburden drilling is accomplished with GD.100 drill which drills 311 mm diameter holes to a depth of 36 m; holes are drilled on a 12m x 12m pattern and charged with ANFO1. Ulan’s 8050 walking dragline has a boom length of 83.8 m, digging of 51.8 m and dumping height of 38.1 m. It is equipped with a 40 m capacity bucket. After overburden removal coal is then drilled, blasted and loaded by electric shovel into the hopper of an in-pit mobile crusher.

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The coal loading shovel with the capacity of 22.8 m3 delivers the blasted coal directly into the crusher's feed hopper. If the shovel is out of service, front-end loaders with 9.5 m3 buckets perform this duty.

The fully-mobile crusher-conveyor system is unique to coal mining in Australia. It has been founded to be very cost effective in comparison with truck haulage systems and has a maximum travelling speed of 800 m/h. Its design capacity is of 2100-2300 t/h crushed coal sized 200-300mm. Dimensions of the mobile crusher are length 51.8 m, width 13.9 m and height 14.6 m. It has a maximum climbing capacity of 10°, total operating weight of 540 t and power consumption of 921 kW.

Coal travels from the feed hopper to the roll crusher by apron feeder. Then it is discharged from the mobile crusher by belt conveyor onto a 30 m long belt wagon. The belt wagon has a maximum output of 2700 t/h; a belt width of 1600 mm, belt speed of 4.0 m/s and a slewing range of 360. Its slowing capacity provides the flexibility of conveying coal from the crusher boom conveyer to the transportable conveyer train each time the crusher is moved to a new position. The belt wagon discharges crushed coal onto a series of portable belt conveyers each with a length of 45 m.

In order to move the 45 m long transportable conveyers to any desired location the mine uses two rubber-tyred transporter units. These 16-wheel machines, powered by a 10 kW diesel are 9.18 m long and 4.0 m wide. The machine weighs 28.000 kg and can carry a load of 70.000 kg; unloading speed on the flat is 20 km/h. Loaded is moved on the flat at 8 km/h, on the 5° incline at 3 km/h and on the 10° incline at 2 km/h. A versatile steering system allows the transporter to be maneuvered underneath the transportable conveyer. Another use of the transporter is to carry a bucket dragline completed with riggings.

The transportable conveyers, belt wagon conveyer and equipment on the crusher are controlled from the mobile crusher control room. This type of haulage system is considered to be most suited to thick coal seams or overburden removal.

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SUPPORTED METHODS

The supported class consists of those methods which require substantial amounts of artificial support to maintain stability in exploitation openings and systematic ground control throughout the mine. Supported methods are used when production openings will not remain standing during their active life and when major caving or subsidence to the surface cannot be tolerated. In other words, the supported class is employed when the other two categories of methods-unsupported and caving-are not applicable.

Where they can be utilized, pillars are the ultimate form of ground control, because they are in place and because they are capable of providing near-rigid support, in certain horizontal mining methods (e.g., room and pillar mining, stope and pillar mining), pillars are relied upon almost exclusively, supplemented by light artificial supports such as bolts and timber. In the vertical methods (which include practically all stoping), pillar maintain stability between stopes, but they are impractical and seldom employed within stopes because they interfere with mining operations. Heavy artificial support, therefore, generally have to be resorted to within the openings where the use of a supported methods is indicated.

Support systems for production workings are chosen to provide varying degrees of controlled wall closure and ground movement. Next to pillars, the most satisfactory form of support is backfill, which approaches 100% in its ability to support the superincumbent load without yielding. In certain instance, some yielding is acceptable and, in fact, preferable because artificial support cannot hold the superincumbent load. Heavy support systems of this type include timber stalls and cribs, timber or steel sets and trusses, and steel jacks, propers, arches, chocks, shields, and canopies. Timber is weaker and yields more than steel (sometimes a desirable feature) but is readily available, flexible, workable, easy to install, and economical.

In the design of artificial support systems for mining methods, an evaluation-preferably quantitative-of the load-carrying capacity of the natural rock structure is prerequisite. Rock mechanics tests are performed to evaluate the structural properties of the rock in situ. A useful though qualitative term to employ in classifying rock is competency. A competent rock is defined as

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rock which, because of its physical and geologic characteristics, is capable of sustaining openings without any heavy structural supports.

The supported class of mining methods in intended for application to rock ranging in competency from moderate to incompetent. There are three methods in this class.

1.Cut and fill stoping

2.Stull stoping

3.Square set stoping

Cut-and-ill and stull stoping are intended for moderately competent rock, while square set stoping is suitable for the least competent rock.

The supported methods have declined in use in the decades since World War II. Only cut and fill stoping lends itself to mechanization; consequently, costs of the other methods have risen relatively.

HYDRAULIC MINING

In hydraulic mining, a very high pressure jet of water, steady or pulsed, fragments consolidated mineral of rock in place. Thus it has application as a primary extraction or mining mechanism, although it is limited at present mainly to softer materials. When coupled with mechanical action (for mining or for cutting, drilling, or boring as well), then hydraulic attack becomes a secondary or supplemental technique.

Hydraulic mining utilizes the kinetic energy of a fluid jet to break and excavate material from the solid (in a completely hydraulic system, bulk transport occurs as well). Consequently, its effectiveness or cutting rate is primarily a function of nozzle (jet) size, flow rate, pressure, force, and power. Other important operating factors are stand off distance (range), the attack angle at which the jet stream impacts the face, and jet traverse rate. In most hydraulic systems, because of pump characteristics, pressure, and flow rate are not independent variables; in general, high flows are associated with low pressures, a combination suitable for the breaking or mining of soft materials. Results indicate that cutting rate is directly proportional to both flow rate as well as to force and kinetic energy.

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