Underground mining (hard rock)
From Wikipedia, the free encyclopedia
Underground hard rock mining refers to various underground mining techniques used to excavate hard minerals such as those containing metals like gold, copper, zinc, nickel and lead or gems such as diamonds. In contrast soft rock mining refers to excavation of softer minerals such as coal, or oil sands.
Contents |
[edit] Mine Access
[edit] Underground Access
Accessing underground ore can be achieved via a decline (ramp) or vertical shaft.
- Declines can be a spiral tunnel which circles either the flank of the deposit or circles around the deposit. The decline begins with a box cut, which is the portal to the surface. Depending in the amount of overburden, and quality of bedrock a galvanized steel culvert may be required for safety purposes.
- Shafts are vertical excavations sunk adjacent to an ore body. Shafts are sunk for ore bodies where haulage to surface via truck is not economical. Shaft haulage is more economical than truck haulage at depth, and a mine may have both a decline and a ramp.
[edit] 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.
[edit] Development Mining vs. Production Mining
There are two principle types of mining, development mining and production mining.
Development mining is composed of excavation almost entirely in (non-valuable) waste rock. There are five steps in development mining, remove previously blasted material (muck out round), drill rock face, load explosives, blast explosives, support excavation.
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 different methods of long hole mining. Typically long hole mining requires two excavations within the ore at different elevations below surface, (15m-30m apart). Holes are drilled between the two excavations and loaded with explosives. The holes are blasted and the ore is removed from the bottom excavation.
[edit] Ventilation
One of the most important aspects of underground hard rock mining is ventilation. Ventilation is required to clear toxic fumes from blasting and removing exhaust fumes from diesel equipment. In deep hot mines ventilation is also required for cooling the workplace for miners. Ventilation raises are excavated to provide ventilation for the workplaces, and can be modified to be used as escape routes in case of emergency.
[edit] 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.
[edit] 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 metal bar (or rock bolt) is installed to hold the ground together. There are several different styles of area ground support.
[edit] Local Ground Support
Local ground support is used to prevent smaller rocks from falling from the back and walls. Not all excavations require local ground support.
- Welded Wire Mesh is a metal screen with 10 cm x 10 cm openings. It is held to the back using point anchor bolts or resin grouted rebar.
- Shotcrete is a spray on concrete which coats the back and walls preventing smaller rocks from falling. Shotcrete thickness can be between 50 mm-100 mm.
- Latex Membranes can be sprayed on the back and walls similar to shotcrete, but in smaller amounts.
[edit] Stope and Retreat vs. Stope and Fill
[edit] Stope and Retreat
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.
[edit] 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 can be filled with backfill, which can be 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.
[edit] Mining Methods
- Cut and Fill mining is a method of short hole mining used in narrow ore zones. An access ramp is driven off the main level to the bottom of the ore zone to be accessed. Using development mining techniques a drift is driven through the ore to the defined limit of mining. Upon completion the drift (or "cut") is filled back to the access ramp with the defined type of backfill, which may be either consolidated or unconsolidated. Another drift is driven on top of filled cut. This process continues until the top of the stope is reached.
- Drift and Fill is similar to cut and fill, except it is used in ore zones which are wider than the method of drifting will allow to be mined. In this case the first drift is developed in the ore, is backfilled using consolidated fill. The second drift is driven adjacent to the first drift. This carries on until the ore zone is mined out to its full width, at which time the second cut is started atop of the first cut.
- Room and Pillar mining : Room and pillar mining is commonly done in flat or gently dipping bedded ore bodies. Pillars are left in place in a regular pattern while the rooms are mined out. In many room and pillar mines, the pillars are taken out starting at the farthest point from the stope access, allowing the roof collapes and fill in the stope. This allows a greater recover as less ore is left behind in pillars.
- Block Caving is a form of stope and retreat, used to effect with large sized orebodies which are composed of hard, stable rock. The method works best with cylindrical, vertical orebodies, where the orebody can be dropped down into the stope, which is filled with waste, the whole process removing the ore from base upwards. The roof pillar, the rock which sits above the orebody, is either left in place or removed, depending on whether the deposit outcrops at surface.
[edit] Ore Removal
In mines which use rubber tired equipment for coarse ore removal, the ore is removed from the stope (referred to as "mucked out") using center articulated vehicles (referred to as boggers or LHD [short for Load, Haul, Dump]). These pieces of equipment may operate using diesel or electric engines and resemble a low-profile front end loader.
The ore is then dumped into a truck to be hauled to surface (in shallower mines). 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 via jaw crusher. The ore is then moved by Conveyor belts, trucks or occasionally trains to the shaft to be hoisted to surface in buckets or skips and emptied into bins beneath surface headframe for transport to the mill.
[edit] Trivia
- The deepest mines in the world are the TauTona (Western Deep Levels) and Savuka gold mines in the Witwatersrand region of South Africa, which are currently working at depths exceeding 3,700 meters. AngloGold plans to increase the maximum depth of these mines to 3,910m by 2009.
- The deepest hard rock mine in North America is Xstrata's Kidd Mine, which mines copper and zinc ore in Timmins, Ontario. Mining is actively occurring 8800 feet below surface. The shaft bottoms out at over 9600 feet down.
- The nickel ore at Greater Sudbury, Ontario is thought to have formed from a meteorite impact. See also Sudbury Basin.
- The deepest hard rock mines in Australia are the copper and zinc lead mines in Mount Isa, Queensland at 1,800 m.
- The deepest platinum-palladium mines in the world are on the Merensky Reef, in South Africa, with a resource of 203 million Troy ounces, currently worked to approximately 2,200 m depth.
- The harshest conditions for hard rock mining are in the Witwatersrand area of South Africa, where workers toil in temperatures of up to 45 °C (113 °F).However, massive refrigeration plants are used to bring the air temperature down to around 28 °C.
[edit] Hardrock mining terms
- stope and pillar
- longhole stoping
- benching
- breasting
- vertical crater retreat
- shrinkage stoping
- panel mining
[edit] See also
|
|
|
|---|---|
| Surface mining: | Open-pit mining, Quarrying, Strip mining, Placer mining, Mountaintop removal |
| Sub-surface mining: | Drift mining, Slope mining, Shaft mining, Hard rock mining, Borehole mining |
