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Surface Mining

Surface mining, including strip mining, open-pit mining and mountaintop removal mining, is a broad category of mining in which soil and rock overlying the mineral deposit are removed, in contrast to underground mining, in which the overlying rock is left in place, and the mineral is removed through shafts or tunnels.

The Main Difference between  Surface mining &  Underground mining is that :  Surface mining is suitable for large, low-grade ore deposits which occur below a thin layer of rock or sand. Underground mining is used for small, high-grade deposits covered with a thick soil or rock above the ore.

Surface mining began in the mid-16th century and is practiced throughout the world, although the majority of surface coal mining occurs in North America. It gained popularity throughout the 20th century, and surface mines now produce most of the coal mined in the United States.

In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden. Next, large machines, such as dragline excavators or bucket wheel excavators, extract the mineral.

Image above shows the methods of surface mining.

There are five main methods of surface mining, detailed below.

1. Strip mining

2. Open-pit mining

3.  Mountaintop removal

4.  Dredging

5.  Highwall mining

1.  Strip mining

"Strip mining" is the practice of mining a seam of mineral, by first removing a long strip of overlying soil and rock (the overburden); this activity is also referred to as "overburden removal". It is most commonly used to mine coal and lignite (brown coal). Strip mining is only practical when the ore body to be excavated is relatively near the surface. This type of mining uses some of the largest machines on earth, including bucket-wheel excavators which can move as much as 12,000 cubic metres of earth per hour.

There are two forms of strip mining. The more common method is "area stripping", which is used on fairly flat terrain, to extract deposits over a large area. As each long strip is excavated, the overburden is placed in the excavation produced by the previous strip.

"Contour mining" involves removing the overburden above the mineral seam near the outcrop in hilly terrain, where the mineral outcrop usually follows the contour of the land. Contour stripping is often followed by auger mining into the hillside, to remove more of the mineral. This method commonly leaves behind terraces in mountainsides.

2.  Open-pit mining

"Open-pit mining" refers to a method of extracting rock or minerals from the earth through their removal from an open pit or borrow. Although open-pit mining is sometimes mistakenly referred to as "strip mining", the two methods are different.

The El Chino mine located near Silver City, New Mexico is an open-pit copper mine.

3.  Mountaintop removal

"Mountaintop removal mining" (MTR) is a form of coal mining that mines coal seams beneath mountaintops by first removing the mountaintop overlying the coal seam. Explosives are used to break up the rock layers above the seam, which are then removed. Excess mining waste or "overburden" is dumped by large trucks into fills in nearby hollow or valley fills. MTR involves the mass restructuring of earth in order to reach the coal seam as deep as 400 feet (120 m) below the surface. Mountaintop removal replaces the original steep landscape with a much flatter topography. Economic development attempts on reclaimed mine sites include prisons such the Big Sandy Federal Penitentiary in Martin County, Kentucky, small town airports, golf courses such as Twisted Gun in Mingo County, West Virginia and Stonecrest Golf Course in Floyd County, Kentucky, as well as industrial scrubber sludge disposal sites, solid waste landfills, trailer parks, explosive manufacturers, and storage rental lockers.

The technique has been used increasingly in recent years in the Appalachian coal fields of West Virginia, Kentucky, Virginia and Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial.

4.  Dredging

"Dredging" is a method for mining below the water table. It is mostly associated with gold mining. Small dredges often use suction to bring the mined material up from the bottom of a water body. Historically, large-scale dredging often used a floating dredge, a barge-like vessel which scooped material up on a conveyor belt in front, removed the desirable component on board, and returned the unwanted material via another conveyor belt in back. In gravel-filled river valleys with shallow water tables, a floating dredge could work its way through the loose sediment in a pond of its own making.

5.  Highwall mining

Highwall mining is another form of mining sometimes conducted to recover additional coal adjacent to a surface mined area. The method evolved from auger mining but does not meet the definition of surface mining since it does not involve the removal of overburden to expose the coal seam.

CERB final report No. 2014-004 "Highwall Mining: Design Methodology, Safety, and Suitability" by Yi Luo characterizes it as a "relatively new semi-surface and semi-underground coal mining method that evolved from auger mining".

In Highwall mining, the coal seam is penetrated by a continuous miner propelled by a hydraulic Pushbeam Transfer Mechanism (PTM). A typical cycle includes sumping (launch-pushing forward) and shearing (raising and lowering the cutterhead boom to cut the entire height of the coal seam). As the coal recovery cycle continues, the cutterhead is progressively launched into the coal seam for 19.72 feet (6.01 m). 

Then, the Pushbeam Transfer Mechanism (PTM) automatically inserts a 19.72-foot (6.01 m) long rectangular Pushbeam (Screw-Conveyor Segment) into the center section of the machine between the Powerhead and the cutterhead. The Pushbeam system can penetrate nearly 1,200 feet (366 m) into the coal seam.

One patented Highwall mining systems use augers enclosed inside the Pushbeam that prevent the mined coal from being contaminated by rock debris during the conveyance process. Using a video imaging and/or a gamma ray sensor and/or other Geo-Radar systems like a coal-rock interface detection sensor (CID), the operator can see ahead projection of the seam-rock interface and guide the continuous miner's progress. 

Highwall mining can produce thousands of tons of coal in contour-strip operations with narrow benches, previously mined areas, trench mine applications and steep-dip seams with controlled water-inflow pump system and/or a gas (inert) venting system.

Recovery with tunneling shape of Drives are much better than round Augering Holes, but the mapping of areas that have been developed by a Highwall miner are not mapped as rigorously as deep mined areas. 

Very little soil is displaced in contrast with mountain top removal; however a large amount of capital is required to operate and own a Highwall miner. But then this Highwall mining system is the innovative roadmap future potential and stay or being better competitive in the area of environmental friendly non mountain-top (overburden) removal operated by only 4 crew members.

Mapping of the outcrop as well as core hole data and samples taken during the bench making process are taken into account to best project the panels that the Highwall miner will cut. 

Obstacles that could be potentially damaged by subsidence and the natural contour of the Highwall are taken into account, and a surveyor points the Highwall miner in a line (Theoretical Survey Plot-Line) mostly perpendicular to the Highwall. 

Parallel lines represent the drive cut into the mountain (up to 1,200 feet (366 m) deep - 2015 records), without heading or corrective steering actuation on a navigation Azimuth during mining results in missing a portion of the coal seam and is a potential danger of cutting in pillars from previous mined drives due to horizontal drift (Roll) of the Pushbeam-Cuttermodule string.

Recently Highwall miners have penetrated more than 1200 feet (366 m) [2015 ongoing records] into the coal seam, and today's models are capable of going farther, with the support of gyro navigation and not limited anymore by the amount of cable stored on the machine. 

The maximum depth would be determined by the stress of further penetration and associated specific-power draw, ("Torsion and Tension" in Screw-Transporters String) but today's optimized Screw-Transporters Conveying Embodiments (called: Pushbeams) with Visual Product Development and Flow Simulation Behavior software "Discrete Element Modeling" (DEM) shows smart-drive extended penetrations are possible, even so under steep inclined angles from horizontal to more than 30 degree downhole.

In case of significant steep mining the new mining method phrase should be "Directional Mining" [Commonly Used Technologies as valuable synergy Directional Drilling and Directional Mining are categorized in "Surface to In-Seam" (SIS) Techniques], dry or wet, Dewatering is developed or Cutting & Dredging through Screw-Transporters are proactive in developing roadmap of the leading global Highwall mining engineering company.

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KGHM looking to sell two US copper assets

Poland’s state-run mining company, KGHM, plans to sell its Robinson and Carlota copper mines in the United States and reinvest the proceeds in its domestic operations, two banking sources told Reuters.

KGHM, one of Europe’s biggest copper and silver producers, became the owner of the US mines in 2012, when it bought rival Canadian Quadra FNX.

Copper, a highly efficient conductor of electricity and heat, is in high demand for use in renewable energy systems to generate power and in electric vehicles. New deposits are rare and increasingly difficult to recover.

The two US copper mines, in Nevada and Arizona, are valued at around $500 million and with strong copper prices they are expected to attract interest from North American private equity firms or smaller mining companies, one of the sources said.“Scenarios for individual assets are at various stages of advancement,” KGHM said in an emailed statement, adding it would continue to work on enhancing the value of its key assets.KGHM said the $500 million valuation given by the source “appears to be at odds with the company estimation,” declining to comment further.The company has been mining in Poland since the 1950s and it is 32% owned by the government.KGHM’s international expansion was questioned by the conservative Law and Justice (PiS) party, which took power in 2015.Since then, KGHM has put some of its foreign assets under review for potential sale and stressed it would focus mostly on investing in core and domestic assets, including copper and silver mines and copper smelters.Investors have also been keen for details on KGHM’s plans regarding Chilean Sierra Gorda mine, its flagship foreign asset, where it has a 55% stake, after its Japanese partner Sumitomo Metal Mining put its 45% on the block last October with the help of RBC Capital Markets.Sources at the time said KGHM would not take on the rest of the mine.In February, Chief Executive Marcin Chludzinski told Reuters that KGHM sees its stake in Sierra Gorda as “optimal.”