Block caving is an inexpensive bulk mining method where the ore is drawn by gravity and which is suitable for ore bodies which are low grade and are consistent in their grades. Depending on how the ore is broken there are three block caving systems 1) The Grizzly system for fine material, 2) the slusher system for coarser material and 3) the LHD system for very coarse material.
Full gravity (grizzly system): The components of the grizzly system are haulage level, transfer raises, grizzly level, finger raises, under cut level. The haulage and grizzly levels are driven across the ore block and can be driven simultaneously. Finger raises are driven to the undercut level from the grizzly level and transfer raises are driven from the haulage level to the grizzly level with a loading chute at the haulage level and a grizzly rail at the grizzly level. Once the undercut is blasted the broken ore flows through these raises, is sized by the grizzly rail and then into transfer chutes and loaded into rail cars. The oversized material at the grizzly is broken by a sledge hammer or by secondary blasting.
Slusher drift system: The components of this system are haulage level, slusher level above the haulage level, finger raises, and the undercut level. Haulage drifts are driven across the block to be mined while the slusher drifts are driven at right angles to the haulage level. Finger raises are then driven from the slusher level to the undercut level. The block above the undercut level is longholed and blasted to start the caving process.
LHD (Rubber tired) System: This system consists of the haulage level, ore transfer raises, the production level, draw point entries, draw cones to the undercut level, and undercut drifts. The undercut is longholed and blasted to promote caving. At the draw point large pieces of ore can be handled due to the wide draw cones which are broken up into small pieces by small drills and explosives. There should be sufficient spacing between the haulage levels and the production levels to accommodate enough storage in the transfer raises so that loading of the trains is not delayed waiting for the LHDs
Detailed Definition:
Detailed Definition:
Ore properties: A typical ore body should have a fairly large lateral extent with well distributed mineralization, with fairly weak or fairly strong rock with enough fractures in different orientations to allow the rock mass to break into small pieces under gravity to pass through the draw holes into the production drifts. The horizontal area of the mining block being mined is usually around 90 m.
Cavability determination: The next step after determining the ore body size is the cavalibity of the blocks being mined. For this step the frequency and orientation of the fracture sets must be determined. The standard method of determining the number of fracture sets is by evaluating the Rock Quality Designation (RQD) of the cores obtained from diamond drilling. The RQD only acts as a preliminary guide for determining cavalibilty, but it gives little indication on the orientation or the occurrence of all the fracture sets. An exploratory drift has to be driven in order to make a final assessment on the cavability of the blocks.
Two vertical fracture sets at approximately 90 deg orientation to each other and a third approximately horizontal set will form the rectangular blocks when the fractures separate. The spacing of the sets determines the size of the blocks at the draw points. The composition of the fracture and the joint fill material aid in determining the ease with which the rock mass fractures or if the fill material is structurally stronger or weaker than the rock mass. Other important features to be identified are faults, dykes, and waste capping which are incorporated in rock mass classification systems based on which the ease or difficulty of caving is determined.
MINING SYSTEM SELECTION AND DESIGN
Two vertical fracture sets at approximately 90 deg orientation to each other and a third approximately horizontal set will form the rectangular blocks when the fractures separate. The spacing of the sets determines the size of the blocks at the draw points. The composition of the fracture and the joint fill material aid in determining the ease with which the rock mass fractures or if the fill material is structurally stronger or weaker than the rock mass. Other important features to be identified are faults, dykes, and waste capping which are incorporated in rock mass classification systems based on which the ease or difficulty of caving is determined.
MINING SYSTEM SELECTION AND DESIGN
After the cavability of the ore has been determined the next step is to determine the mining system to be employed. This chiefly depends on the size of broken material arriving at the draw point but other factors like cost of labour, availability and cost of equipment, economics, mine site location, technical expertise in labour etc are also taken into account.
From previous studies it has been observed that finer ore size necessitates narrow draw point spacing and coarse ore size demands wider spacing. For fine material the gravity system is preferred, for slightly coarser material the slusher system is used and for very coarse material the LHD system should be considered.
From previous studies it has been observed that finer ore size necessitates narrow draw point spacing and coarse ore size demands wider spacing. For fine material the gravity system is preferred, for slightly coarser material the slusher system is used and for very coarse material the LHD system should be considered.
THE GRIZZLY SYSYEM
The grizzly system is best suited for weak ore which breaks finely and hence requires closely spaced draw points. It is also a favoured method when sophisticated equipment is not available and where labour is cheap.
HAULAGE LEVEL. The haulage or gathering level is driven about 18 m below the grizzly level to allow sufficient room for storage for loading the trains. The haulage level is supported by concrete to counteract the extreme stress conditions encountered. Haulage equipment dimensions and safety requirements determine the cross-sectional size.
RAISES. Transfer raises are driven from the haulage level to the grizzly with each transfer raise serving multiple grizzlies. The grizzly chamber spacing and the distance between the two levels determines the number of grizzlies that can be served by one transfer raise. The transfer raises are generally lined with timber or concrete to insure that the raise will last the life of the production from above. Chutes are usually installed at the bottom of the transfer raises and grizzly rails are installed at the top of the transfer raise for sizing the material passing down the raise. The cross-sectional size will be determined by the amount of ore storage required within the constraints of the ground conditions.
GRIZZLY DRIFTS. Grizzly drifts are driven to connect the top of the transfer raises and serve as manway access to the draw points and for distribution of fresh air to the grizzlies. The grizzly drifts are usually lined with concrete or timber for support. The grizzly drifts are usually small in size since no production equipment will be required to pass through. They are connected around the fringes of the ore block for ease of access between the drifts and for ventilation purposes.
DRAW (FINGER) RAISES. Finger raises are driven from the grizzly to the undercut level. Two raises are driven on opposite sides and at right angles to the grizzly drift and located at the top of the transfer raise. The distance between the grizzly and undercut level depends on how much pillar is required between the two levels. The raises may or may not be lined with concrete.
UNDERCUT DRIFTS. Undercut drifts connect the tops of the draw raises. The pillars formed during the driving of the drifts are later drilled and blasted to initiate caving. These drifts should be large enough to accommodate the drilling equipment used to drill the pillars and to provide expansion room for the blasted pillar rock.
VENTILATION. The ventilation of a grizzly system is fairly simple. Fresh air is passed through the haulage drifts and then up to the grizzly levels. Fresh air is circulated through the drifts.
SLUSHER SYSTEMS
Slushers are generally used when ore breaks up into medium coarse particles. This system generally more equipment intensive than the gravity system since it uses electric slushers. The scrapers are about 1.5 to 1.8 m in width
HAULAGE DRIFTS. Haulage drifts are driven on even centers across the block to be mined. The distance between haulages is a function of the slusher drift length and the draw point spacing. The size of drift is determined by the size of haulage equipment. The drifts are generally supported by concrete.
SLUSHER DRIFTS. Slusher drifts are usually driven directly above and at right angles to the haulage drifts. The slusher drift consists of a loading cut out directly above the haulage, a slusher cut out at the end opposite from the slusher drift, and the slusher drift. The size of opening is a function of the scraper size. The width of the finished slusher drift should be only 1 ft (0.3 m) greater than the width of the scraper thus preventing side spill as the loaded scraper is pulled to the draw hole and produce greater payloads for each scraper trip. The slusher cut out, loading cut out, and slusher drift are usually lined with concrete. The floor of the slusher drift should have rails installed as this provides a better sliding surface for the scraper and prevents gouging of the floor concrete. The drift should also have an upward grade to 5%. This will allow for water drainage toward the haulage and also will favour the loaded scraper for somewhat easier loading. The slusher drifts are usually staggered along the haulage. That is, every other slusher drift is driven at 180° from the previous slusher drift.
VENTILATION DRIFTS. Ventilation drifts are driven halfway between each pair of haulage drifts directly under the tail ends of the slusher drifts from two adjacent haulages. Small ventilation raises to the slusher drift above serve for exhausting contaminated air from the slusher drift. The ventilation drift is sized according to the amount of exhaust air that it will be required to handle. Ventilation drifts are not usually concreted but may require roof bolts and shot crete.
FINGER (DRAW) RAISES. Finger raises are driven at 90⁰ to the slusher drifts at an angle of approximately 45° above the horizontal. Finger raises are driven from both sides of the slusher drift, generally in opposite sets although they may be staggered from one side of the drift to the other. The size of the finger raise is determined by the size of material passing through the finger raise. Lining of the finger raise is desirable to prevent the raise from getting unstable from rock rubbing against rock during the production cycle.
UNDERCUT DRIFTS. Undercut drifts are driven above the tops of the finger raises. Undercut drift sizing is usually determined by the size of equipment used for the long hole drilling. The elevation of the undercut level is determined by the amount of pillar desired between the slusher drifts and the cave bottom. The pillar should be adequate to provide for wear during the production period and for protection of the slusher drift if weight should occur.
LONGHOLE DRILLING. Long hole drilling is usually done in the shape of a fan with the bottom holes being drilled at a minimum angle of 45°. The size of the long holes and the amount of explosives required depend on the rock strength and these two will determine the spacing between the fans. No more than two or three drill fans in an undercut drift should be blasted at one time.
VENTILATION. Ventilation with a slusher system is somewhat more positive than with the grizzly system. An intake ventilation lateral can be driven around the fringes of the mining area or underneath the haulage level. Fresh air flows through the loading drifts and into the slusher drift. The air then passes through the slusher drift to the vent connection and into the exhaust ventilation drifts where it flows to a main vent lateral and then is discharged. The volume of air feeding each slusher drift can also be controlled by placing a vent box at each vent connection. By changing the size of vent box opening, the amount of air in each slusher drift is controlled. This system does require constant monitoring to be sure that various controls are in proper position, especially in areas of high secondary blasting.
LHD (Rubber tired) System
The LHD system is well suited for ore bodies that are not too fractured and which will cave in large pieces that would be large enough to be handled by mechanical equipments. Due to the system requiring less development per ton of ore and has a high production-output capability high productivity and efficiency that can be obtained.
HAULAGE LEVEL. The haulage levels are located well below the production level so that adequate storage is available in the centralized ore passes. This will provide sufficient loading capacity so that haulage trains are not directly dependent on LHDs for loading and larger-sized ore cars can be used. The size of the haulage drifts is dependent on the size of haulage equipment. Since the haulage level is located well below the caving level, the drifts are not usually subjected to unusual weight, and therefore standard support methods are adequate.
PRODUCTION LEVEL. Production drifts are driven on even centers across the ore body or production block. The spacing between production drifts is determined by draw point layout
and spacing. The size of the drifts depends on the size of production units being used. The larger the production unit, the greater the cross section required. The production drifts can be subjected to future heavy weight; therefore, good support is required. These drifts should be roof bolted and concreted or at least shotcreted.
DRAWPOINTS. Draw points are driven horizontally from the production drifts. They are driven usually about 45° to the production drift to facilitate entry of the LHD for production. The distance from the brow of the draw point to the opposite rib of the production drift must be adequate for the LHD to be at nearly zero degrees articulation when entering the muck pile. The height of the brow of the draw point must be high enough to allow the bucket of the LHD to raise its load but not so high that the broken rock will flood out into the production drift. The draw point is usually lined with concrete in order to maintain the proper size and also for support against the weight from undercutting and blasting.
UNDERCUT LEVEL. The undercut level is usually driven directly above the production drifts generally in the range of 15 m. The size of the undercut drifts is determined by the size of equipment used. These drifts are not generally supported unless rock conditions require some temporary support.
LONGHOLE DRILLING. Drilling of the undercut long holes and for the draw cones is done from the undercut level. Drilling the draw cones from the undercut level does require more drilling than if done from the production level, but the blasting sequence for the draw cones is much safer from this level. Blasting for the draw cones is usually done one or two rings at a time. When drilling is done from the bottom, once the first rings have been blasted, access to the remaining rings is difficult and hazardous.
Blasting of the draw cones immediately precedes blasting of the undercut. The cones should only precede the undercut blasting by one or two draw points. This way, the minimum amount of ground ahead of the cave line is opened and more support is offered to the weight that precedes the cave line. Blasting of the long holes is done two or three fans at a time. With this system, it is fairly easy to check for un blasted pillars. As with the other systems, the cave line should be advanced at some angle to the major workings to minimize the amount of weight transferred to the production level.
OREPASSES (STORAGE BINS). Ore passes are driven between the production and haulage levels for transferring the ore to the trains. The ore passes are spaced intermittently along the production drifts so they can be serviced by several draw points. Loading chutes are installed at the bottom of each ore pass and a dump pocket set up at the production level. Ore passes are generally placed in good quality rock and hence not usually lined. Raise boring machines are used to bore long ore passes.
VENTILATION. Ventilation requirements for the LHD system are significantly higher than for the other systems as a result of the high ventilation requirements for diesel equipment. Separate ventilation drifts usually driven underneath the production level are driven to bring fresh air to the working areas by means of raises driven from them to the production level. Small ventilation raises adjacent to each ore pass dump point pick up the contaminated air and take it to small exhaust vent drifts below the production level, and then to the main exhaust lateral, and finally discharge it to the surface.
GRIZZLY DRIFTS. Grizzly drifts are driven to connect the top of the transfer raises and serve as manway access to the draw points and for distribution of fresh air to the grizzlies. The grizzly drifts are usually lined with concrete or timber for support. The grizzly drifts are usually small in size since no production equipment will be required to pass through. They are connected around the fringes of the ore block for ease of access between the drifts and for ventilation purposes.
DRAW (FINGER) RAISES. Finger raises are driven from the grizzly to the undercut level. Two raises are driven on opposite sides and at right angles to the grizzly drift and located at the top of the transfer raise. The distance between the grizzly and undercut level depends on how much pillar is required between the two levels. The raises may or may not be lined with concrete.
UNDERCUT DRIFTS. Undercut drifts connect the tops of the draw raises. The pillars formed during the driving of the drifts are later drilled and blasted to initiate caving. These drifts should be large enough to accommodate the drilling equipment used to drill the pillars and to provide expansion room for the blasted pillar rock.
VENTILATION. The ventilation of a grizzly system is fairly simple. Fresh air is passed through the haulage drifts and then up to the grizzly levels. Fresh air is circulated through the drifts.
SLUSHER SYSTEMS
Slushers are generally used when ore breaks up into medium coarse particles. This system generally more equipment intensive than the gravity system since it uses electric slushers. The scrapers are about 1.5 to 1.8 m in width
HAULAGE DRIFTS. Haulage drifts are driven on even centers across the block to be mined. The distance between haulages is a function of the slusher drift length and the draw point spacing. The size of drift is determined by the size of haulage equipment. The drifts are generally supported by concrete.
SLUSHER DRIFTS. Slusher drifts are usually driven directly above and at right angles to the haulage drifts. The slusher drift consists of a loading cut out directly above the haulage, a slusher cut out at the end opposite from the slusher drift, and the slusher drift. The size of opening is a function of the scraper size. The width of the finished slusher drift should be only 1 ft (0.3 m) greater than the width of the scraper thus preventing side spill as the loaded scraper is pulled to the draw hole and produce greater payloads for each scraper trip. The slusher cut out, loading cut out, and slusher drift are usually lined with concrete. The floor of the slusher drift should have rails installed as this provides a better sliding surface for the scraper and prevents gouging of the floor concrete. The drift should also have an upward grade to 5%. This will allow for water drainage toward the haulage and also will favour the loaded scraper for somewhat easier loading. The slusher drifts are usually staggered along the haulage. That is, every other slusher drift is driven at 180° from the previous slusher drift.
VENTILATION DRIFTS. Ventilation drifts are driven halfway between each pair of haulage drifts directly under the tail ends of the slusher drifts from two adjacent haulages. Small ventilation raises to the slusher drift above serve for exhausting contaminated air from the slusher drift. The ventilation drift is sized according to the amount of exhaust air that it will be required to handle. Ventilation drifts are not usually concreted but may require roof bolts and shot crete.
FINGER (DRAW) RAISES. Finger raises are driven at 90⁰ to the slusher drifts at an angle of approximately 45° above the horizontal. Finger raises are driven from both sides of the slusher drift, generally in opposite sets although they may be staggered from one side of the drift to the other. The size of the finger raise is determined by the size of material passing through the finger raise. Lining of the finger raise is desirable to prevent the raise from getting unstable from rock rubbing against rock during the production cycle.
UNDERCUT DRIFTS. Undercut drifts are driven above the tops of the finger raises. Undercut drift sizing is usually determined by the size of equipment used for the long hole drilling. The elevation of the undercut level is determined by the amount of pillar desired between the slusher drifts and the cave bottom. The pillar should be adequate to provide for wear during the production period and for protection of the slusher drift if weight should occur.
LONGHOLE DRILLING. Long hole drilling is usually done in the shape of a fan with the bottom holes being drilled at a minimum angle of 45°. The size of the long holes and the amount of explosives required depend on the rock strength and these two will determine the spacing between the fans. No more than two or three drill fans in an undercut drift should be blasted at one time.
VENTILATION. Ventilation with a slusher system is somewhat more positive than with the grizzly system. An intake ventilation lateral can be driven around the fringes of the mining area or underneath the haulage level. Fresh air flows through the loading drifts and into the slusher drift. The air then passes through the slusher drift to the vent connection and into the exhaust ventilation drifts where it flows to a main vent lateral and then is discharged. The volume of air feeding each slusher drift can also be controlled by placing a vent box at each vent connection. By changing the size of vent box opening, the amount of air in each slusher drift is controlled. This system does require constant monitoring to be sure that various controls are in proper position, especially in areas of high secondary blasting.
LHD (Rubber tired) System
The LHD system is well suited for ore bodies that are not too fractured and which will cave in large pieces that would be large enough to be handled by mechanical equipments. Due to the system requiring less development per ton of ore and has a high production-output capability high productivity and efficiency that can be obtained.
HAULAGE LEVEL. The haulage levels are located well below the production level so that adequate storage is available in the centralized ore passes. This will provide sufficient loading capacity so that haulage trains are not directly dependent on LHDs for loading and larger-sized ore cars can be used. The size of the haulage drifts is dependent on the size of haulage equipment. Since the haulage level is located well below the caving level, the drifts are not usually subjected to unusual weight, and therefore standard support methods are adequate.
PRODUCTION LEVEL. Production drifts are driven on even centers across the ore body or production block. The spacing between production drifts is determined by draw point layout
and spacing. The size of the drifts depends on the size of production units being used. The larger the production unit, the greater the cross section required. The production drifts can be subjected to future heavy weight; therefore, good support is required. These drifts should be roof bolted and concreted or at least shotcreted.
DRAWPOINTS. Draw points are driven horizontally from the production drifts. They are driven usually about 45° to the production drift to facilitate entry of the LHD for production. The distance from the brow of the draw point to the opposite rib of the production drift must be adequate for the LHD to be at nearly zero degrees articulation when entering the muck pile. The height of the brow of the draw point must be high enough to allow the bucket of the LHD to raise its load but not so high that the broken rock will flood out into the production drift. The draw point is usually lined with concrete in order to maintain the proper size and also for support against the weight from undercutting and blasting.
UNDERCUT LEVEL. The undercut level is usually driven directly above the production drifts generally in the range of 15 m. The size of the undercut drifts is determined by the size of equipment used. These drifts are not generally supported unless rock conditions require some temporary support.
LONGHOLE DRILLING. Drilling of the undercut long holes and for the draw cones is done from the undercut level. Drilling the draw cones from the undercut level does require more drilling than if done from the production level, but the blasting sequence for the draw cones is much safer from this level. Blasting for the draw cones is usually done one or two rings at a time. When drilling is done from the bottom, once the first rings have been blasted, access to the remaining rings is difficult and hazardous.
Blasting of the draw cones immediately precedes blasting of the undercut. The cones should only precede the undercut blasting by one or two draw points. This way, the minimum amount of ground ahead of the cave line is opened and more support is offered to the weight that precedes the cave line. Blasting of the long holes is done two or three fans at a time. With this system, it is fairly easy to check for un blasted pillars. As with the other systems, the cave line should be advanced at some angle to the major workings to minimize the amount of weight transferred to the production level.
OREPASSES (STORAGE BINS). Ore passes are driven between the production and haulage levels for transferring the ore to the trains. The ore passes are spaced intermittently along the production drifts so they can be serviced by several draw points. Loading chutes are installed at the bottom of each ore pass and a dump pocket set up at the production level. Ore passes are generally placed in good quality rock and hence not usually lined. Raise boring machines are used to bore long ore passes.
VENTILATION. Ventilation requirements for the LHD system are significantly higher than for the other systems as a result of the high ventilation requirements for diesel equipment. Separate ventilation drifts usually driven underneath the production level are driven to bring fresh air to the working areas by means of raises driven from them to the production level. Small ventilation raises adjacent to each ore pass dump point pick up the contaminated air and take it to small exhaust vent drifts below the production level, and then to the main exhaust lateral, and finally discharge it to the surface.
Aucun commentaire:
Enregistrer un commentaire