Drill-ring, sink shaft, pattern, fragmented, water-resistant.

1. … are passageways sunk from the surface downwards or underground from one level to another.

2. The blasted rock must be well …, to suit the excavation equipment.

3. The drilling and charging … is similar to that of similar surface blasts.

4. The drilling is done with … which comprises a circular I-beam to which the machines are fixed.

5. The explosives used in shaft sinking must be very …

 

4) Quote the sentences in which these words and word combinations are used:

to be expose to, risky blasting jobs, to be well fragmented, the burden and spacing, emulsion explosives, specific charge, to minimize overbreak, remote control, primary reinforcement.

 

5) Answer the following questions:

1) What is a sink shaft and for what purposes is it sunk?

2) Why is shaft sinking one of the most difficult and risky blasting jobs?

3) What does the design of the cross section depend on?

4) Which are the most common drilling and blasting methods in shaft sinking?

5) What explosives are preferred in shaft sinking?

6) What techniques (methods) are used in shaft sinking?

 

6) Tell what you know about:

 

1) Shafts and sinking shafts as a whole.

2) Explosives used in shaft sinking.

3) Parallel hole drilling.

4) Full face boring of shafts.

5) About lift cage raising.

 

 

Mining in Steep Orebodies

 

 

Based on Gravity

The dimensions of mineral deposits vary greatly, from mas­sive formations stretching over several square kilometres, to half metre-wide quartz veins contain­ing some 20 gm/t gold.

In recovering the minerals, the miners prefer to leave hanging-wall and footwall waste rock intact. In the larger deposits, machines can operate in the drifts without problems. When the mineralization narrows to a few metres only, it can become self-defeating to excavate space for standard machines, because of dilution. For such situations, a selection of slim machines is available from Atlas Copco, capa­ble of mechanized mining in drifts from 2.0 m-wide. These include a face jumbo for narrow drifting, a similar longhole drilling rig, and a 2 cu m loader.

 

 

Sublevel Open Stoping

Sublevel open stoping (SLOS) is used fоr mining mineral deposits with: steep dip where the footwall inclina­tion exceeds the angle of repose; stable rock in both hanging wall and footwall; competent ore and host rock and; regular ore boundaries. SLOS recovers the ore in large open stopes, which are normally backfilled.

The orebody is divided into separ­ate stopes. between which ore sections are set aside for pillars to support the hanging wall. Pillars are normally shaped as vertical beams, across the orebody. Horizontal sections of ore are also left, as crown pillars.

Miners want the largest possible stopes, to obtain the highest mining efficiency, subject to the stability of the rock mass, which limits their design dimensions.

Sublevel drifts are located within the orebody, between the main levels, for longhole drilling of blast patterns. The drill pattern accurately specifies where the blastholes are collared, and the depth and angle of each hole.

Drawpoints are located below the stope, to enable safe mucking by LHD machines, which may tip into an adja­cent orepass, or into trucks or rail cars. The trough-shaped stope bottom is typical, with loading drifts at regular intervals. Nowadays, the loading level can be integrated with the undercut, and mucking out performed by a remote control LHD working in the open stope.

 

Figure 1. Sublevel open stoping layout.

 

Sublevel stoping requires a straight­forward shape of stopes and ore boundaries, within which only ore is drilled. In larger orebodies, modules of ore may be mined along strike, as primary and secondary stopes.

Bighole Stoping

Bighole stoping is an up-scaled variant of sublevel open stoping, using longer, larger-diameter DTH blastholes, rang­ing from 140 to 165 mm. Blast patterns are similar to SLOS, but with holes up to 100 m-long.

A pattern with 140 mm blastholes will break a rock slice 4.0 m thick, with 6.0 m toe spacing. DTH drilling is more accurate than tophammer drilling, allowing the vertical spacing between sublevels to be extended, from 40 m with SLOS mining, to 60 m with bighole stoping. However, the risk of damage to the rock structures has to be taken into account by the mine planners.

 

Cement Consolidated Fill

Deep-mined ore is generally too valu­able to leave as boundary pillars between open stopes. To minimize ore losses, mines have developed consolidated backfill techniques for open stopes. Vertical Crater Retreat, Sublevel Open Stoping and Big Hole Stoping.

The mined-out stope may be back-tilled with de-slimed tailings, mixed with Portland cement and an accelera­tor in a pumpable slurry wilh 60 to 70% solid matter. This Cemented Hydraulic Fill (CHF) hardens into a solid block, replacing the ore.

Alternatively, a mix of crushed rock and cement slurry may be used to pro­duce Cemented Rock Fill (CRF). The rock may be recycled waste, or pro­duced in a surface quarry. It is crushed and gravitated underground through fill raises, from where it can be dis­tributed by mine trucks. The crushed rock is sprayed with cement slurry, while being deposited in the stope. Consolidated backfill allows the removal of pillars at a later stage.

 

Figure 2. Bighole sloping layout.

The primary stopes are mined first, then backfilled before recovery of the secondary stope blocks. These, in turn, are backfilled with a lower сement-tailing ratio, as the final backfill does not require the same strength as in primary stopes.

Backfilling with CHF/CRF, com­bined with a primary-secondary mining sequence, allows close to 100% recovery of orebodies.

 

Vertical Crater Retreat

Vertical Crater Retreat (VCR) applies to orebodies with steep dip and com­petent rock in both ore and host rock. Part of the blasted ore will remain in the stope over the production cycle, serving as temporary support.

VCR was originally developed by the Canadian mining company INCO, and uses the crater blasting technique of powerful explosives in large dia­meter holes. Concentrated spherical charges are used to excavate the ore in horizontal slices, from the stope bottom upwards. The ore gravitates to the stope floor draw points, and is removed by loaders. Each stope is cleaned out before CHF backfilling.

Development for VCR stoping consists of: a haulage drift along the orebody at the drawpoint level; drawpoint loading arrangement underneath the stope; an undercut; and an overcut access for drilling and charging.

The ore in a slope block is drilled from the overcut excavation using DTH rigs. Holes, mainly vertical, are drilled downward, breaking through into the undercut. Hole diameters vary from 140 to 165 mm, commonly spaced on a 4.0 m x 4.0 m grid.

From the overcut, powerful spherical charges are positioned by skilled crew in the lower section of the blast hole, at specified distances from the stope roof. The hole depth is measured, and it is stemmed at the correct height. Explosive charges are lowered down each hole and stemmed, usually to blast a 3.0 m slice of ore, which falls into the space below.

VCR charging is complex, and its techniques have to be mastered, in order to avoid damaging the surrounding rock.

 

Cut and Fill Stoping

Cut-and-fill mining is applied to mining steeply dipping orebodies, in strata with good to moderate stability, and a comparatively high-grade mineralization.

It provides better selectivity than SLOS and VCR mining, and preferred for orebodies with irregular shape and scattered mineralization where high grade sections can be mined separately, and low grade rock left in the stopes.

Cut-and-fill mining excavates the ore in horizontal slices, starting from bottom undercut, advancing upward.The ore is drilled, blasted, loaded an removed from the stope, which is then backfilled with deslimed sand tailings from the dressing plant, or waste rock carried in by LHD from development drives. The fill serves both to support stope walls, and as a working platform when mining the next slice.

Before filling, stope entries are barricaded and drainage tubes installed The stope is filled with sand to almost full height, and cement is mixed into the final pours, to provide a solid floor for mobile machines to operate.

 

Figure 3. VCR primary stoping

 

Development for cut-and-fill mining includes: a footwall haulage drive along the orebody at the main level; and undercut of the stope area, with drains for water; a spiral ramp in the footwall. with access drive to the undercut; and a raise connection to the level above, for ventilation and filling material.

The stope face appears as a wall, with an open slot at the bottom, above the fill. Breasting holes are drilled by a rig. charged and blasted, with the slot underneath providing swell space for the blasted rock.

The mineralization shows in the stope face, where it can be inspected by geologists. The drill pattern can be modified, to follow variations in ore boundaries. Sections with low grade can be left in place, or deposited in adjacent mined-out stope sections. Mining can divert from planned slope boundaries, and recover enclosures of mineral from the host rock.

The smooth fill surface and con­trolled fragmentation are ideal for LHD loaders, the standard vehicle for mucking and transport in cut-and-fill mines. Tramming distances from stope to orepass must be within convenient range, and the orepass may be con­structed within the stope using steel lining segments installed in advance of each sand layer.

There is a trend towards bench and stope, as at Mt Isa, Australia, and towards stope and paste fill, as at Zinkgruvan, Sweden.

 

Sublevel Caving

Sublevel caving (SLC) adapts to large orebodies. with sleep dip and continu­ity at depth. Sublevel footwall drills have to be stable, requiring occasional rockbolting only. The hangingwall has to fracture and collapse, following the cave, and subsidence of the ground surface above the orebody has to be tolerated.

Caving requires a rock mass where both orebody and host rock fracture under controlled conditions. As the mining removes rock without backfilling, the hanging wall keeps caving into the voids. Continued mining results in subsidence of the surface, where sinkholes may appear. Continuous caving is important, to avoid creation of cavities inside rock, where a sudden collapse could induce an inrush.

SLC extracts the ore through sub-levels, which are developed in the ore-body at regular vertical spacing. Each sublevel features a systematic layout with parallel drifts, along or across the orebody. In wide orebodies, sublevel drifts start from the footwall drive, and continue across to the hanging wall. In narrow orebodies, sublevel drifts branch off in both directions from a central crosscut drive.

Development to prepare SLC stopes is extensive, and mainly involves dri­ving multiple headings to prepare sub-levels. A ramp connection is needed to connect different sublevels, and to com­municate with main transport routes. Orepasses are also required, at strate­gic locations along sublevels, connect­ing to the main haulage level.

A section through the sublevel area will show drifts spread across the orebody, in a regular pattern, both in vertical and horizontal projections. The diamond shaped area, which can be traced above each drift, delineates the ore volume to be recovered from that drift.

Longhole rigs drill the ore section above the drift, in a fan-spread pattern, well ahead of production.

Blasting on each sublevel starts at the hanging wall, and mining retreats toward the footwall. Adjacent cross­cuts are mined at a similar pace, with upper sublevels maintained ahead of lower sublevels. to preserve the cave and avoid undermining.

Each longhole fan is blasted sepa­rately, and the ore fills the drawpoint. Mucking out by LUD continues until the waste dilution reaches the set limit. The LHD then moves to a freshly blasted crosscut, while the charging team prepares the next fan for blasting.

Sublevels are designed with tram­ming distances matched to particular sizes of LIID-loaders. Mucking out is, like the other procedures in sublevel caving, very efficient. and the loader can be kept in continuous operation. Waste dilution in SLC varies between 15% and 40%, and ore losses may be 15% to 25%, depending on local conditions.

Dilution is of less influence for ore-bodies with diffuse boundaries, where the host rock contains low-grade minerals. Similar rules apply to mag­netite ores, which are upgraded by simple magnetic separators. Sulphide ores, however, are refined by costly flotation processes, so dilution has to be closely controlled.

 

 

Figure 4. Cut-and-fill stope layout.

 

SLC is schematic, and repetitive, both in layout and working procedures. Development drifting, production drilling of long holes, charging, blast­ing and mucking out are all carried out separately, with work taking place at different levels simultaneously. There is always a place for the machines to work, making SLC a method which integrates mechanization into efficient ore production.

 

Block Caving

Block-caving is a large scale production mining method applicable to low' grade, massive orebodies with: large dimensions both vertically and horizontally; a rock mass that behaves properly, breaking into blocks of manageable size and a ground surface which is allowed to subside.

These rather unique conditions limit block-caving applications to special mineral deposits such as iron ore, low-grade copper and molybdenum mineralizations, and diamond-bearing kimberlite pipes.

 

Figure 5. Sublevel caving lavout

 

Block caving is based on gravity combined with internal rock stresses, to fracture and break the rock mass. The drilling and blasting required for ore production is minimal, while development volume is huge. Blocks of orebody may have areas of several thousands of square metres. Caving is induced by undercutting the block by blasting, destroying its ability to sup­port the overlying rock. Gravity forces, in the order of millions of tonnes, act to fracture the block. Continued pres­sure, and secondary blasting, break the rock into smaller pieces to pass the drawpoints, where the ore is handled by LHD-loaders or trains.

Figure 6. Block caving layout.

Development for block-caving applying conventional gravity flow requires an undercut, where the rock mass underneath the block is fractured by longhole blasting. Drawbells with finger raises are excavated beneath the undercut, to gather broken rock to the grizzly (picking hammer) level, where oversize boulders are caught, and broken by blasting or hydraulic hammer. A lower set of finger raises channels ore from the grizzlies to chutes for train loading on the main haulage level.

The intention is to maintain a steady draw from each block, and records are kept of volumes extracted from individual draw-points. It is often necessary to assist the rock mass fracturing, by longhole drilling and blasting in widely spaced patterns.

Drifts and other openings in the block caving area are excavated with minimum cross sections for man-entry. Still, heavy concrete lining and extensive rock bolting is necessary, to secure the integrity of mine drifts and drawpoint openings.

Where LHD loaders are used in the drawpoints, a ventilation level is added into development plans.

 

Shrinkage Stoping

In shrinkage stoping, traditionally a common mining method, ore is excavated in horizontal slices, starting from the stope bottom and advancing upwards. Part of the blasted ore is left in the stope, to serve as a working platform, and to give support to the stope walls.

Blasting swells the ore by about 50%, which means that a substantial amount has to be left in the stope until mining has reached the top section, following which final extraction can take place. Shrinkage stoping can be used for orebodies with: steep dips; comparatively stable ore and sidewall characteris­tics; regular ore boundaries; ore unaffected by storage (some sulphide ores oxidize, generating excessive heat).

The development consists of: haulage drift and crosscuts for muck­ing at stope bottom; establishment of drawpoints and undercut; and a raise from the haulage level passing through the undercut to the main level, providing access and ventilation to the working area.

Figure 6. Shrinkage stoping layout

 

Drilling and blasting are carried out as overhead stoping. The rough pile of blasted ore prevents the usage of mechanized equipment, making the method labour-intensive.

List of words

1. hanging wall – висячий бік
2. footwall – підошва виробки
3. drift – штрек
4. dilution – розубоження (розубожування) корисних копалин
5. drilling rig – буровий станок
6. dip – падіння
7. angle of repose – кут природного схилу
8. pillar – цілик
9. сollar – отвір бурової свердловини, забурювати свердловину
10. drill pattern – сітка разміщення бурових свердловин
11. mucking – прибирання відбитоі породи
12. stope bottom – підошва виробки
13. toe spacing – відстань між забоями бурових свердловин
14. cut and fill stoping – шарова система розробки із закладкою виробленого простору
15. long hole stoping – очисне виймання с відбійкою руди глибокими свердловинами
16. sublevel stoping – підповерхова розробка, система підповерхових штреків
17. shrinkage stoping – система розробки з магазинуванням
18. sublevel caving – підповерхове обвалення
19. crosscut – квершлаг
20. overcut – верхній вруб
21. undercut – нижній вруб

 

Exercises:

1) Give Ukrainian equivalents of the following words and word combinations:

 

To support hanging wall,drawpoints, adjacent orepasses, blast pattern,,sublevels, deep-mined ore, a mix of crushed rock and cement slurry, drawbell, grizzly level, haulage drive, finger raise,recovery of ore bodies;

 

2) Give English equivalents of the following words and word combinations:

 

Пiдошва забою, покрiвля забою, подрiблена порода, пiдготовчi роботи, рудоспуск,закладати= засипати, порода, що містить(основна порода),опускатися(про грунт), квершлаг, сили тяжіння, очисний забой з доставкою руди у вагонетках iз тяговим канатом, вiдстань пробiгу вагонеток, бремсберг(слiпий ствол)

 

3) Fill the blanks with the necessary words:

Sublevel drifts, pillars, backfilling drawpoints, hanging wall, sublevel caving

 

1. … … are located within the orebody, between the main levels, for longhole drilling of blast pattern.

2.The orebody is divided into separate stopes,between which ore sections are set aside for … to support the … ….

3. … are located below the slope, to enable safe mucking byLHD macines, which may tip into an adjacent orepass, ore into trucks or rail cars.

4. … with CHF/CRF, combined with a primary-secondary mining sequence, allows close to 100% recovery of orebodies.

5. … … adapts to large orebodies with steep dip and continuity at depth.

 

4) Quote the sentences in which these words and word combinations are used:

 

As crown pillars, trough- shaped stope, to minimize ore losses, de-slimed tailings, orebodies with irregular shape and scattered mineralization, breasting holes, induce an inrush, blocks of manageable size, internal rock stresses, heavy concrete lining and extensive rock bolting;

 

5) Answer the fallowing questions:

 

1) What kinds of mineral deposits are usually mined by open stoping?

2) What techniques have been developed to minimize ore losses?

3) To what kind of ore bodies does VCR apply?

4) Which method is preferred for orebodies with irregular shape and scattered mineralization?

5) What operations does development to prepare SLC involve?

6) What types of orebodies are mined by block caving?

 

6) Tell what you know about:

 

1) sublevel open stoping

2) cut and fill stoping

3) sublevel caving

4) block caving,

5) shrinkage stoping

 

Mineral Prospecting and Exploration

 

Finding Orebodies

For a geologist in the mining business, exploiting an ore-body is the easy part of the job. The hardest part is to find the ore-body and define it. But how do you find these accumulations of metallic minerals in the Earth's crust? The mining company has to ensure that an ore-body is eco­nomically viable, and needs a guarantee of ore production over a very long period of time, before it will engage in the heavy invest­ment required to set up a mining operation. Even after production starts, it is necessary to locate and delineate any extensions to the mineralization, and to look for new prospects that may replace the reserves being mined. Investigating extensions, and searching for new ore -bodies, are vital activities for the mining company.

Prospecting

 

Prospecting involves searching a dis­trict for minerals with a view to further operation. Exploration, while it sounds similar to prospecting, is the term used for systematic examination of a deposit. It is not easy to define the point where prospecting turns into exploration.

A geologist prospecting a district is looking for surface exposure of miner­als, by observing irregularities in colour, shape or rock composition. He uses a hammer, a magnifying glass and some other simple instruments to examine whatever seems to be of interest. His experience tells him where to look, to have the greatest chances of success. Sometimes he will stumble across ancient, shallow mine workings, which may be what led him to prospect that particular area in the first place.

Soil-covered ground is inaccessible to the prospector, whose first check would be to look for an outcrop of the mineralization. Where the ground cover comprises a shallow layer of alluviums, trenches can be dug across the mineralized area to expose the bedrock. A prospector will identify the discovery, measure both width and length, and calculate the mineralized area. Rock samples from trenches are sent to the laboratory for analysis. Even when minerals show on surface, determining any extension in depth is a matter of qualified guesswork. If the prospector's findings, and his theorizing about the probable existence of an ore-body are solid, the next step would be to explore the surrounding ground. Exploration is a term embracing geophysics, geochemistry, and also drilling into the ground for obtaining samples from any depth.

Geophysical Exploration

From surface, different geophysical methods are used to explore subsurface formations, based on the physical prop­erties of rock, and metal bearing miner­als such as magnetism, gravity, electrical conductivity, radioactivity, and sound velocity. Two or more geophysical methods are often combined in one survey, to acquire more reliable data. Results from the surveys are compiled, and matched with geological informa­tion from surface, and records from any core drilling, to decide if it is worth pro­ceeding with further exploration.

 

Surveys

 

Magnetic surveys measure variations in the Earth's magnetic field caused by magnetic properties of subsurface rock formations. In prospecting for metallic minerals, these techniques are particu­larly useful for locating magnetite, pyrrhotite and ilmenite.

Electromagnetic surveys are based on variations of electric conductivity in the rock mass. An electric conduc­tor is used to create a primary alternat­ing electromagnetic field. Induced currents produce a secondary field in the rock mass. The resultant field can be traced and measured, thus revealing the conductivity of the underground masses. Electromagnetic surveys are mainly used to map geo­logical structures, and to discover mineral deposits, such as sulphides containing copper or lead, magnetite, pyrite, graphite, and certain man­ganese minerals.

Electric surveys measure either the natural flow of electricity in the ground, or "galvanic" currents led into the ground and accurately con­trolled. Electrical surveys are used to locate mineral deposits at shallow depth, and map geological structures to determine the depth of overburden to bedrock, or to locate the ground-water table.

Gravimetric surveys measure small variations in the gravitational field caused by the pull of underlying rock masses. The variation in gravity may be caused by faults, anticlines, and salt domes that are often associ­ated with oil-bearing formations. Gravimetric surveys are also used to detect high-density minerals, like iron ore, pyrites and leadzinc mineralizations.

In regions where rock formations contain radioactive minerals, the intensity of radiation will be consider­ably higher than the normal back­ground level. Measuring radiation levels helps locate deposits containing uranium, thorium and other minerals associated with radioactive substances.

The seismic survey is based on variations of sound velocity experi­enced in different geological strata. The time is measured for sound to travel from a source on surface, through the underlying layers, and up again to one or more detectors placed at some distance on surface. The source of sound might be the blow of a sledgehammer, a heavy falling weight, a mechanical vibrator, or an explosive charge. Seismic surveys determine the quality of bedrock, and can locate the contact surface of geo­logical layers, or of a compact mineral deposit deep in the ground. Seismic surveys are also used to locate oil-bearing strata.

Geochemical surveying is another exploration technology featuring sev­eral specialities, the main one being to detect the presence of metals in the topsoil cover. By taking a large number of samples over an extended area, and analyzing the contents of each metal, regions of interest are identified. The area is then selected for more detailed studies.

 

Exploratory Drilling

 

For a driller, all other exploration methods are like beating about the bush. Drilling penetrates deep into the ground, and brings up samples of whatever it finds on its way. If there is any mineralization at given points far beneath the surface, drilling can give a straightforward answer, and can quan­tify its presence at that particular point.

There are two main methods of exploratory drilling. The most common, core drilling, yields a solid cylinder-shaped sample of the ground at an exact depth. Percussion drilling yields a crushed sample, comprising cuttings from a fairly well-determined depth in the hole. Beyond that, the drill hole itself can provide a complementary amount of information, particularly by logging using devices to detect physical anom­alies, similar to the geophysical surveys mentioned above.

Core drilling is also used to define the size and the exact borders of min­eralization during the lifetime of the mine. This is important for determin­ing ore grades being handled, and vital for calculating the mineral reserves that will keep the mine running in the future. A strategically-placed under­ground core drill may also probe for new orebodies in the neighbourhood.

 

Core Drilling

 

In 1863, the Swiss engineer M Lescot designed a tube with a diamond set face, for drilling in the Mount Cenis tunnel, where the rock was too hard for conventional tools. The intention was to explore rock quality ahead of the tunnel face, and warn miners of possible rock falls.

This was the accidental birth of core drilling, a technique now very widely used within the mining indus­try. Core drilling is carried out with special drill-rigs, using a hollow drill string with an impregnated diamond cutting bit to resist wear while drilling hard rock. The crown-shaped diamond bit cuts a cylindrical core of the rock, which is caught and retained in a double tube core-barrel.

A core-catcher is embedded in, or just above, the diamond bit, to make sure that the core does not fall out of the tube. In order to retrieve the core, the core-barrel is taken to surface, either by pulling up the complete drill string or, if the appropriate equipment is being used, by pulling up only the inner tube of the core-barrel with a spe­cial fishing device run inside the drill string at the end of a thin steel wire. The core is an intact sample of the underground geology, which can be examined thoroughly by the geologist to determine the exact nature of the rock and any mineralization. Samples of special interest are sent to a labora­tory for analysis to reveal any metal contents. Cores from exploration drilling are stored in special boxes, and kept in archives for a long period of time. Boxes are marked to identify from which hole, and at what depth, the sample was taken. The information gathered by core drilling is important, and represents substantial capital investment.

Traditionally, core drilling was a very arduous job, and developing new techniques, and more operator-friendly equipment, was very slow, and the cost per drilled metre was often pro­hibitive. Several techniques have been developed to reduce manual work, increase efficiency and cut the cost per drilled metre. Over the years, there have been developed thin walled core barrels, diamond impreg­nated bits, aluminium drill rods, fast rotating hydraulic rigs, mechanical rod handling, and, more recently, partly or totally computer-controlled rigs. Core drilling has always been the most pow­erful tool in mineral exploration. Now that it has become much cheaper, faster and easier, it is being used more widely.