Mining 101

Types of Mining

A mine can be described as any attempt to extract minerals or other materials from the ground. These materials include metals such as gold, copper, nickel, and aluminum, fuels such as coal, peat, oil, and gas, and industrial materials such as gravel, limestone, or quartz. The most common abandoned mines in the West are hardrock mines, which extracted copper, gold, iron ore, lead, molybdenum, phosphate rock, platinum, potash, silver, uranium, and zinc.

In general, there are three types of mining operations:

• Underground mining
• Surface mining
• In situ/Solution mining
• Surface Mining

Due to modern technology and the ability to move huge quantities of material, surface mining has become a more economically and technically feasible operation. Although there are several types of surface mining, open pit mining is the most common method used for the majority of metals in the U.S.

Open pit mining involves the removal of overburden with large excavators and hauling equipment. It is commonly used for low grade ores, where large amounts of materials must be processed to get enough of the target mineral to make it profitable. An open pit is usually widest at the top and has a series of levels, called benches, which get more towards the bottom.

Other types of large scale surface mining operations include strip mining and mountaintop removal. Placer mining involves removing target minerals from sand and gravel in a streambed or former stream channel using simple tools or mechanized dredging equipment.

Underground Mining

Underground mining involves tunneling into the earth to reach a deposit of valuable minerals. Although not utilized extensively by modern mining companies, underground mining was a very common practice during the opening and settling of the American West. It is still used today in situations where the metals are too deep to make surface mining economically feasible.

Underground mining used to involve sinking a shaft (vertical tunnel) or adit (horizontal tunnel) into bedrock using picks, shovels, and explosives. Modern underground mines are more highly mechanized and require little manual labor. After reaching valuable metal deposits, the ore is moved to the surface with conveyors or shuttles for processing.

Although underground mining usually results in minimal surface disturbance when compared to other types of mining, the resulting network of tunnels can stretch for many miles, at different levels below ground. The underground tunnels are supported with timber, concrete, and steel and often becomes get with groundwater, which reacts with oxygen and the metals, causing acidic mine drainage.

In Situ/Solution Mining

Solution mining, also called in situ, is used when mineral deposits are concentrated and too deep below the earth’s surface to make other mining methods feasible. This technology is often used for uranium and copper.

Instead of physically removing the ore body, solution mining involves a series of wells which pump chemical solvents directly into the ground to dissolve the ore. The solution is then pumped back to the surface through a different set of wells so that it can be processed. Although there is little surface disturbance involved in solution mining, it can pollute groundwater supplies due to the infiltration of the chemical solvents or dissolved metals.

Mineral Processing

Mineral processing is a series of physical, chemical and biological processes used to concentrate minerals and metals for human use. In general, there are three distinct stages in mineral processing.

• Comminution
• Separation
• Beneficiation

Comminution
The first step in mineral processing is usually crushing or grinding the ore. This step, called comminution, tends to make the valuable minerals more easily accessible. 

Separation
The second step in mineral processing is call separation. Often, the type of separation technique used at a mine site will have a large impact on the type of environmental problems found there. The following techniques are commonly used by the mining industry to separate the valuable minerals from the waste rock and other material.

• Gravity separation – The minerals are separated based upon their specific gravities with the help of water or air. A gold prospector swirling water and gravel in the bottom of a pan is using a simplified form of gravity separation. Often mercury is added to the mixture . It bonds with the gold, forming an amalgam. When the amalgam is heated, the mercury vaporizes and the gold stays intact. Due to the use of mercury, this process can be very detrimental to human health and the environment.
   
• Magnetic separation – The crushed ore is magnetized, sometimes by heating. resulting in the ability to separate iron minerals, like pyrite, from the more valuable minerals. The waste is almost pure pyrite, or fool’s gold, and is very reactive.
   
• Flotation – Reagents, which are soap-like chemicals, are added to the finely ground minerals. Waste rock, like silica-sand drop to the bottom of the vat . The valuable metals adhere to the bubbles and “float”. The bubbles with the metals can then be skimmed off of the top in a concentrated form.
   
• Chemical separation – This type of separation usually starts with dissolving  the minerals in water or oil. In a process called leaching, other chemicals, such as cyanide or sulfuric acid, are then added to the mixture and bond with the valuable metals. After the minerals are dissolved in the cyanide or sulfuric acid, the solution is collected. By passing an electric current through the solution or adding zinc or carbon, the metals are caused to precipitate, or resolidify in a concentrated form. A reverse process of precipitation is used causing the metal to come out of solution in a highly concentrated form. 
   
• Biological separation – Using bacteria to accelerate chemical reactions is new to the metal processing industry. Bacteria are increasingly being used to remove gold from sulfides or take metals from out of a solution.

Beneficiation
The final stage in mineral processing is beneficiation, or smelting. By heating the concentrated metals to high temperatures, leftover wastes and impurities are burned off.