Underground hard-rock mining

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A 3D diagram of a modern underground mine with shaft access Underground 3D model.jpg
A 3D diagram of a modern underground mine with shaft access

Underground hard-rock mining refers to various underground mining techniques used to excavate "hard" minerals, usually those containing metals, [1] such as ore containing gold, silver, iron, copper, zinc, nickel, tin, and lead. It also involves the same techniques used to excavate ores of gems, such as diamonds and rubies. Soft-rock mining refers to the excavation of softer minerals, such as salt, coal, and oil sands.

Contents

Mine access

Underground access

Accessing underground ore can be achieved via a decline (ramp), inclined vertical shaft or adit.

Decline portal Wiluna underground.jpg
Decline portal

Declines are often started from the side of the high wall of an open cut mine when the ore body is of a payable grade sufficient to support an underground mining operation, but the strip ratio has become too great to support open cast extraction methods. They are also often built and maintained as an emergency safety access from the underground workings and a means of moving large equipment to the workings.

Ore access

Levels are excavated horizontally off the decline or shaft to access the ore body. Stopes are then excavated perpendicular (or near perpendicular) to the level into the ore.

Development mining vs. production mining

There are two principal phases of underground mining: development mining and production mining.

Development mining is composed of excavation almost entirely in (non-valuable) waste rock in order to gain access to the orebody. There are six steps in development mining: remove previously blasted material (muck out round), scaling (removing any unstable slabs of rock hanging from the roof and sidewalls to protect workers and equipment from damage), installing support or/and reinforcement using shotcrete or other supports, drill face rock, load explosives, and blast explosives. To start the mining, the first step is to make the path to go down. The path is defined as 'Decline' as describe above. Before the start of a decline, all pre-planning of the power facility, drilling arrangement, de-watering, ventilation and, muck withdrawal facilities are required. [2]

Production mining is further broken down into two methods, long hole and short hole. Short hole mining is similar to development mining, except that it occurs in ore. There are several methods of long hole mining. Typically, long hole mining requires two excavations within the ore at different elevations below surface (15 to 30 metres or 50 to 100 feet). Holes are drilled between the two excavations and loaded with explosives. The holes are blasted, and the ore is removed from the bottom excavation.[ citation needed ]

Ventilation

Door for directing ventilation in an old lead mine. The ore hopper at the front is not part of the ventilation. Smallcleugh door.jpg
Door for directing ventilation in an old lead mine. The ore hopper at the front is not part of the ventilation.

One of the most important aspects of underground hard rock mining is ventilation. Ventilation is the primary method of clearing hazardous gases and/or dust which are created from drilling and blasting activity (e.g., silica dust, NOx), diesel equipment (e.g., diesel particulate, carbon monoxide), or to protect against gases that are naturally emanating from the rock (e.g., radon gas). Ventilation is also used to manage underground temperatures for the workers. In deep, hot mines ventilation is used to cool the workplace; however, in very cold locations the air is heated to just above freezing before it enters the mine. Ventilation raises are typically used to transfer ventilation from surface to the workplaces, and can be modified for use as emergency escape routes. The primary sources of heat in underground hard rock mines are virgin rock temperature, machinery, auto compression, and fissure water. Other small contributing factors are human body heat and blasting.

Ground support

Some means of support is required in order to maintain the stability of the openings that are excavated. This support comes in two forms; local support and area support.

Area ground support

Area ground support is used to prevent major ground failure. Holes are drilled into the back (ceiling) and walls and a long steel rod (or rock bolt) is installed to hold the ground together. There are three categories of rock bolt, differentiated by how they engage the host rock. [3] They are:

Mechanical bolts

  • Point anchor bolts (or expansion shell bolts) are a common style of area ground support. A point anchor bolt is a metal bar between 20 mm – 25 mm in diameter, and between 1 m – 4 m long (the size is determined by the mine's engineering department). There is an expansion shell at the end of the bolt which is inserted into the hole. As the bolt is tightened by the installation drill the expansion shell expands and the bolt tightens holding the rock together. Mechanical bolts are considered temporary support as their lifespan is reduced by corrosion as they are not grouted. [3]

Grouted bolts

  • Resin grouted rebar is used in areas which require more support than a point anchor bolt can give. The rebar used is of similar size as a point anchor bolt but does not have an expansion shell. Once the hole for the rebar is drilled, cartridges of polyester resin are installed in the hole. The rebar bolt is installed after the resin and spun by the installation drill. This opens the resin cartridge and mixes it. Once the resin hardens, the drill spinning tightens the rebar bolt holding the rock together. Resin grouted rebar is considered a permanent ground support with a lifespan of 20–30 years. [3]
  • Cable bolts are used to bind large masses of rock in the hanging wall and around large excavations. Cable bolts are much larger than standard rock bolts and rebar, usually between 6 and 25 metres long. Cable bolts are grouted with a cement grout. [3]

Friction bolts

  • Friction stabilizer (frequently called by the genericized trademark Split Set) are much easier to install than mechanical bolts or grouted bolts. The bolt is hammered into the drill hole, which has a smaller diameter than the bolt. Pressure from the bolt on the wall holds the rock together. Friction stabilizers are particularly susceptible to corrosion and rust from water unless they are grouted. Once grouted the friction increases by a factor of 3–4. [3]
  • Swellex is similar to Friction stabilizers, except the bolt diameter is smaller than the hole diameter. High pressure water is injected into the bolt to expand the bolt diameter to hold the rock together. Like the friction stabilizer, swellex is poorly protected from corrosion and rust. [3]

Local ground support

Local ground support is used to prevent smaller rocks from falling from the back and ribs. Not all excavations require local ground support.

Stope and retreat vs. stope and fill

Stope and retreat

Sub-Level Caving Subsidence reaches surface at the Ridgeway underground mine. Elura.png
Sub-Level Caving Subsidence reaches surface at the Ridgeway underground mine.

Using this method, mining is planned to extract rock from the stopes without filling the voids; this allows the wall rocks to cave in to the extracted stope after all the ore has been removed. The stope is then sealed to prevent access.

Stope and fill

Where large bulk ore bodies are to be mined at great depth, or where leaving pillars of ore is uneconomical, the open stope is filled with backfill, which can be a cement and rock mixture, a cement and sand mixture or a cement and tailings mixture. This method is popular as the refilled stopes provide support for the adjacent stopes, allowing total extraction of economic resources.

Methods

Schematic diagram of cut and fill mining Cut and fill schematic.png
Schematic diagram of cut and fill mining

The mining method selected is determined by the size, shape, orientation and type of orebody to be mined. The orebody can be narrow vein such as a gold mine in the Witwatersrand, the orebody can be massive similar to the Olympic Dam mine, South Australia, or Cadia-Ridgeway Mine, New South Wales. The width or size of the orebody is determined by the grade as well as the distribution of the ore. The dip of the orebody also has an influence on the mining method for example a narrow horizontal vein orebody will be mined by room and pillar or a longwall method whereas a vertical narrow vein orebody will be mined by an open stoping or cut and fill method. Further consideration is needed for the strength of the ore as well as the surrounding rock. An orebody hosted in strong self-supporting rock may be mined by an open stoping method and an orebody hosted in poor rock may need to be mined by a cut and fill method where the void is continuously filled as the ore is removed.

Selective mining methods

[7]

Bulk mining methods

Orebodies that do not cave readily are sometimes preconditioned by hydraulic fracturing, blasting, or by a combination of both. Hydraulic fracturing has been applied to preconditioning strong roof rock over coal longwall panels, and to inducing caving in both coal and hard rock mines.


Ore removal

In mines which use rubber-tired equipment for coarse ore removal, the ore (or "muck") is removed ("mucked out" or "bogged") from the stope using center articulated vehicles. These vehicles are referred to as "boggers" or LHD (Load, Haul, Dump machines). These pieces of equipment may operate using diesel engines or electric motors, and resemble a low-profile front end loader. Electrically powered LHD utilize trailing cables which are flexible and can be extended or retracted on a reel. [12]

In shallower mines the ore is then dumped into a truck to be hauled to the surface. In deeper mines, the ore is dumped down an ore pass (a vertical or near vertical excavation) where it falls to a collection level. On the collection level, it may receive primary crushing by a jaw or cone crusher, or by a rockbreaker. The ore is then moved by conveyor belts, trucks or occasionally trains to the shaft to be hoisted to the surface in buckets or skips and emptied into bins beneath the surface headframe for transport to the mill.

In some cases the underground primary crusher feeds an inclined conveyor belt which delivers ore via an incline shaft direct to the surface. The ore is fed down ore passes, with mining equipment accessing the ore body via a decline from the surface.

Deepest mines

See also

References

  1. de la Vergne, Jack (2003). Hard Rock Miner's Handbook . Tempe/North Bay: McIntosh Engineering. p. 2. ISBN   0-9687006-1-6.
  2. Brazil, M. "Decline design in underground mines using constrained path optimisation" (PDF). math.uwaterloo.ca. Archived from the original (PDF) on 2010-11-24. Retrieved 19 Jun 2023.
  3. 1 2 3 4 5 6 Puhakka, Tulla (1997). Underground Drilling and Loading Handbook. Finland: Tamrock Corporation. pp. 153–170.
  4. 1 2 3 Puhakka, Tulla (1997). Underground Drilling and Loading Handbook. Finland: Tamrock Corporation. pp. 98–130.
  5. "The In-The-Hole Drill | Dirty Great Machines | Discovery Science". Archived from the original on 2017-02-02. Retrieved 2017-01-29.
  6. "Vale Inco's Creighton mine: Digging deeper by the day". Viewpoint (3): 2. 2008. Archived from the original on 2015-06-21. Vertical retreat mining (VRM) was introduced in the mid-1980s to replace the cut-and-fill mining method. The slot-slash mining method, a modified VRM, was introduced in the late 1990s and replaced the VRM mining.
  7. "Mining & Metallurgy 101". www.miningbasics.com. Archived from the original on 2011-12-06. Retrieved 2017-01-27.
  8. Fowler, J. C. W.; Hebblewhite, B. K. (2003). "Mining publication" (PDF). New South Wales. Archived (PDF) from the original on 2006-09-20. Retrieved 2007-05-30.
  9. Sjöberg, J., F. Perman, D. Lope Álvarez, B-M. Stöckel, K. Mäkitaavola, E. Storvall and T. Lavoie. "Deep sublevel cave mining and surface influence", in: Deep Mining 2017: Eighth International Conference on Deep and High Stress Mining (Perth, March 28–30, 2018). Wesseloo, J. (ed.), pp. 357 –372. Perth: Australian Centre for Geomechanics, Perth, ISBN 978-0-9924810-6-3, 2017.
  10. Ladinig, Tobias; Wagner, Horst; Karlsson, Wimmer; Grynienko, Michal (2022). "Raise Caving—A Hybrid Mining Method Addressing Current Deep Cave Mining Challenges". BHM Berg- und Hüttenmännische Monatshefte. 167 (4): 177–186. Bibcode:2022BHM...167..177L. doi: 10.1007/s00501-022-01217-3 .
  11. "LKAB utvecklar ny brytningsmetod – så går metoden raise caving till". SVT (in Swedish). 2021-06-08. Retrieved 2024-06-21.
  12. http://www.mineweb.com/archive/greGreener underground mining[ dead link ]
  13. "TauTona, Anglo Gold, South Africa". 2009. Archived from the original on 2019-05-12. Retrieved 2009-05-01.
  14. Godkin, David (1 February 2014). "Being safe is no accident". Canadian Mining Journal. Archived from the original on 19 July 2019. Retrieved 19 February 2020.
  15. "Home | Kidd Operations". Archived from the original on 2020-03-02. Retrieved 2020-02-19.
  16. "Agnico Eagle Mines Limited - Operations - Operations - LaRonde Complex". www.agnicoeagle.com. Archived from the original on 2022-02-01. Retrieved 2022-02-01.
  17. "Skalisty mine reaches design depth of 2,056 m below surface – Nornickel".
  18. "Mineral deposits: from their origin to their environmental impacts". Taylor & Francis. 1995. ISBN   978-9054105503.
  19. "Mt Isa". Larvotto Resources Limited. Retrieved 2025-03-10.

Further reading