Hard-Rock gold Mining.

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Underground hard-rock mining encompasses a variety of techniques used to extract valuable minerals, predominantly metals and gems, from beneath the Earth’s surface. This contrasts with soft-rock mining, which targets softer minerals like salt, coal, and oil sands. In Gold Mining, accessing underground ore is achieved through declines (ramps), inclined vertical shafts, or adits (horizontal excavations).

By Kelapstick – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6286804

Mine Access: Declines, shafts, and adits serve as entry points into underground mines. Declines, often initiated from open cut mines, may require safety measures like galvanized steel culverts. Shafts, primarily for deep ore bodies, offer economical haulage compared to trucks. Adits are horizontal tunnels suited for near-horizontal ore bodies without the need for ramps or shafts.

Ore Access: Levels extend horizontally from declines or shafts to reach ore bodies, with stopes then excavated perpendicular or near-perpendicular to access the ore.

Planning and Preparation: Successful core drilling relies on meticulous planning and preparation. This involves thorough site assessments, securing permits, prioritizing safety, selecting appropriate equipment, establishing timelines, and coordinating logistics. These measures mitigate risks, ensure compliance, and enhance efficiency during drilling operations.

Understanding the intricacies of mine access, ore access, and the planning process is fundamental to the success and safety of underground hard-rock mining endeavors.

Development Mining and Production Mining in Underground Operations

Underground mining unfolds in two distinct phases: development mining and production mining. Development mining involves excavating non-valuable waste rock to establish access to the orebody. The process includes steps like muck removal, scaling for safety, support installation, face rock drilling, loading explosives, and blasting. The initial path for mining, known as the “Decline,” is crucial and requires pre-planning for various facilities.

Production mining, the subsequent phase, branches into long hole and short hole methods. Short hole mining, akin to development mining, takes place in ore-rich areas. Long hole mining, on the other hand, entails drilling holes between two excavations at different elevations, loading them with explosives, and extracting ore from the lower excavation. The distinction between these mining phases and methods highlights the intricacies of navigating the subterranean world for resource extraction.

The Vital Role of Ventilation in Underground Hard Rock Mining.

Ventilation emerges as a critical component in underground hard rock mining, serving multifaceted purposes. It is paramount for clearing hazardous gases and dust generated by activities like drilling, blasting, and diesel equipment operation. The ventilation system safeguards against various pollutants, including silica dust, NOx, diesel particulate, carbon monoxide, and naturally occurring gases like radon. Additionally, ventilation plays a pivotal role in managing underground temperatures, either cooling hot workplaces in deep mines or heating air in frigid locations before it enters the mine. Ventilation raises act as conduits for transferring fresh air from the surface to underground workspaces and can be adapted as emergency escape routes. Understanding the sources of heat in underground mines, such as virgin rock temperature, machinery, auto compression, fissure water, human body heat, and blasting, underscores the complexity of maintaining a safe and comfortable environment for mining personnel.

Ground Support in Underground Hard-Rock Mining: Upholding Stability Below the Surface

Area Ground Support:

Area ground support is vital for preventing major ground failure in underground mining. Three types of rock bolts are commonly used:

  1. Mechanical Bolts:
  • Point anchor bolts, or expansion shell bolts, are inserted into drilled holes and tightened, holding the rock together.
  • Considered temporary support due to corrosion susceptibility; not grouted.
  1. Grouted Bolts:
  • Resin-grouted rebar provides more support than point anchor bolts.
  • Polyester resin cartridges are installed in drilled holes, and rebar bolts are spun to mix and harden the resin.
  • Offers permanent support with a lifespan of 20–30 years.
  1. Friction Bolts:
  • Friction stabilizers (e.g., Split Set) are easy to install by hammering into drill holes.
  • Susceptible to corrosion; grouting improves resistance.
  • Swellex, similar to friction stabilizers, uses high-pressure water to expand the bolt diameter for rock support.

Local Ground Support:

Local ground support prevents smaller rocks from falling and includes:

  1. Welded Wire Mesh:
  • Metal screen with 10 cm x 10 cm openings held to the back using point anchor bolts or resin-grouted rebar.
  1. Shotcrete:
  • Fibre-reinforced spray-on concrete applied to the back and ribs.
  • Thickness ranges from 50 mm to 100 mm.
  1. Latex Membranes:
  • Sprayed on backs and ribs, similar to shotcrete, but in smaller amounts.

Understanding and implementing both area and local ground support are crucial for maintaining stability and safety in the challenging environment of underground hard-rock mining.

Methods in Underground Hard-Rock Mining.

The choice of mining method in underground hard-rock mining is a strategic decision influenced by various factors related to the orebody. These factors include the size, shape, orientation, type, width, and dip of the orebody. Several mining methods are employed based on these considerations:

  1. Narrow Vein Mining:
  • Examples: Gold mines in the Witwatersrand.
  • Method: Room and pillar or longwall methods for narrow horizontal vein orebodies.
  1. Massive Orebody Mining:
  • Examples: Olympic Dam mine (South Australia), Cadia-Ridgeway Mine (New South Wales).
  • Method: Determined by orebody characteristics; may involve open stoping, cut and fill, or other methods.
  1. Orebody Width and Size:
  • Influenced by grade and ore distribution.
  • Width determines the appropriate mining method.
  1. Orebody Dip:
  • Horizontal vein orebodies: Room and pillar or longwall methods.
  • Vertical narrow vein orebodies: Open stoping or cut and fill methods.
  1. Rock Strength:
  • Strong, self-supporting rock: Open stoping method.
  • Poor rock: Cut and fill method, continuously filling voids as ore is extracted.

The selection of the optimal mining method involves a comprehensive assessment of orebody characteristics and surrounding rock conditions, ensuring efficient and safe extraction in the challenging underground environment.

Selective Mining Methods: Precision in Ore Extraction

Selective mining methods are employed in underground hard-rock mining to extract ore with precision, minimizing waste and dilution. Several techniques are utilized for this purpose:

  1. Cut and Fill Mining:
  • Applicability: Used in steeply dipping or irregular ore zones where long-hole methods are limited by hanging wall constraints.
  • Process: Ore is mined in horizontal or slightly inclined slices and filled with waste rock, sand, or tailings. Consolidation with concrete is optional.
  • Advantages: Expensive but selective, with low ore loss and dilution.
  1. Drift and Fill:
  • Similarity to Cut and Fill: Similar to cut and fill but employed in wider ore zones.
  • Procedure: Initial drift is developed in the ore and backfilled with consolidated fill. Subsequent drifts are driven adjacent to the first drift until the full width of the ore zone is mined.
  1. Shrinkage Stoping:
  • Applicability: Suitable for steeply dipping orebodies.
  • Process: After blasting, broken ore is left in the stope to support surrounding rock and serve as a working platform. Ore removal is gradual, allowing for drilling and blasting in successive slices. Stope is emptied after all ore is blasted.
  1. VRM/VCR (Vertical Retreat Mining/Vertical Crater Retreat):
  • Division of Mine: The mine is divided into vertical zones, typically around 50 meters deep, using open stoping with bottom-up mining.
  • Drilling and Blasting: Long-hole large-diameter holes are drilled vertically from the top into the ore body, and horizontal slices are blasted into an undercut.
  • Ore Retrieval: Ore retrieval is done from the bottom of the developed section, often using remote-controlled LHD machines.
  • Backfilling: Primary stopes are backfilled with cemented fill to support subsequent blasting in successive stopes. Side chambers are mined after the fill solidifies.

These selective mining methods showcase a strategic approach to ore extraction, balancing cost-effectiveness with minimal ore loss and dilution.

In underground hard-rock mining, rubber-tired equipment is used for coarse ore removal, employing center articulated vehicles like boggers or LHD machines. These machines, powered by diesel engines or electricity, resemble low-profile front-end loaders. Electrically operated LHDs use flexible trailing cables. The extracted ore, or “muck,” is then either loaded into trucks for surface hauling in shallower mines or deposited down an ore pass in deeper mines. In the latter case, the ore falls to a collection level, where it undergoes primary crushing through jaw or cone crushers or a rockbreaker.

From the collection level, the ore is transported using conveyor belts, trucks, or trains to the shaft for hoisting to the surface. In some scenarios, an underground primary crusher feeds ore directly to the surface via an inclined conveyor belt and incline shaft. Ore passes facilitate the movement of ore to the surface, with mining equipment accessing the ore body through a decline from the surface. This comprehensive process ensures efficient extraction and transportation of ore in the intricate underground mining environment.

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