Underground hard rock mining
refers to various underground mining techniques used to excavate hard
minerals such as those containing metals like gold
or gems such as diamonds
. In contrast soft rock mining refers to excavation of softer minerals such as coal
, or oil sands
Accessing underground ore
can be achieved via a decline (ramp), inclined 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 on the amount of overburden and quality of bedrock, a galvanized steel culvert may be required for safety purposes. They may also be started into the wall of an open cut mine.
- 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.
- Adits are horizontal excavations into the side of a hill or mountain. They are used for horizontal or near-horizontal ore bodies where there is no need for a ramp or shaft.
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.
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 principle 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 five steps in development mining: remove previously blasted material (muck out round), drill rock face, load explosives, blast explosives, and 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.
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.The main sources of heat in underground hard rock mines are virgin rock temperature, machinery, auto compression, and fissure water although other small factors contribute like people breathing, inefficiency of machinery, and blasting operations.
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 metal bar (or rock bolt
) is installed to hold the ground together. There are several different styles of area ground support.
- Point Anchor Bolts are a common style of area ground support. A point anchor bolt is a metal bar between 20mm-25mm in diameter, and between 1m-4m in length (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.
- 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 epoxy 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.
Local Ground Support
Local ground support is used to prevent
smaller rocks from falling from the backs and walls. Not all excavations require local ground support.
- Welded Wire Mesh is a metal screen with 10 cm x 10 cm (4 inch) openings. It is held to the backs using point anchor bolts or resin grouted rebar.
- Shotcrete is a spray on concrete which coats the backs and walls preventing smaller rocks from falling. Shotcrete thickness can be between 50 mm-100 mm.
- Latex Membranes can be sprayed on the backs and walls similar to shotcrete, but in smaller amounts.
Stope and Retreat vs. Stope and Fill
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.
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.
- 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 to collapse and fill in the stope. This allows a greater recovery as less ore is left behind in pillars.
- Block Caving such as is used at the Northparkes Mine in NSW, Australia, is used to effect with large sized orebodies which are typically composed of low-grade, friable ore. The method works best with cylindrical, vertical orebodies. Pre-production mining development work consists of driving accesses underneath the orebody This includes the formation of "drawbells" by undercutting and blasting. Initially, blasted ore is removed via the extraction level underneath the drawbells until a sufficient area of unsupported ore is formed that the orebody begins to fracture and cave on its own. The eventual aim of the block caving method is that the friable ore needs no blasting and continues to fracture and break up on its own, flowing down the drawbells to the extraction level, where it is removed from the ore chute mouths with loaders and sent off for processing. Eventually the fracturing will propagate to the surface, resulting in subsidence. One of the main hazards associated with block-caving is that fracturing can potentially stop before it reaches the surface unbeknownst to the people in control of the mine. If fracturing stops propagating upwards and extraction continues, a large void can be formed, resulting in the potential for a sudden and massive collapse and catastrophic windblast throughout the mine.
In mines which use rubber tired equipment for coarse ore removal, the ore is removed from the stope (referred to as "mucked out" or "bogged" ) 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.
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 surface.
- 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 (12,139 feet). AngloGold plans to increase the maximum depth of these mines to 3,910m (12,828 feet) 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 2,682m (8,800 feet) below surface. The shaft bottoms out at over 2,926m (9,600 feet) down.
- The deepest hard rock mines in Australia are the copper and zinc lead mines in Mount Isa, Queensland at 1,800 m (5,906 feet).
- 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 2,200m (7,218 feet) 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.
Hardrock mining terms
- stope and pillar
- longhole stoping
- vertical crater retreat
- shrinkage stoping
- panel mining