• The hazardous gases produced in mines depend on the mining method and the minerals being mined.
• Toxic, suffocating, and flammable gases can occur naturally in mines and minerals or be produced by various operations.
• Metal mines produce different, and often more toxic, gases than coal mines.

Mining is one of the world’s most hazardous occupations. Ill health and loss of life due to exposure to toxic gases and fire and explosions due to flammable gases, especially in underground mines, are unfortunately common occurrences in the industry. Safety managers must be prepared for gas-related emergencies, but they must also know which gases pose a risk in their mines. This article covers the common gases found depending on the type and method of mining and the materials mined.

Mine Gases

The main gases in mines that should be monitored can be toxic, flammable, or suffocating:

• Toxic gases: Common mine toxins include carbon monoxide (CO) and nitrogen oxides (NOx), formed as byproducts of incomplete fossil-fuel combustion in machinery, and hydrogen sulfide, which occurs naturally. Sulfur Dioxide (SO₂) is also released because it is used in mining or produced as a byproduct.
• Flammable gases: Gases formed by operations or occurring naturally, such as CO, methane (CH4), and hydrogen (H2), can form flammable and explosive mixtures with air. At sufficient concentrations, CO can spontaneously ignite without an ignition source if temperatures get high enough.
• Suffocating gases: In underground and other confined spaces with little ventilation, some gases are lethal because they displace oxygen and suffocate people. These are CO2, nitrogen (N2), and methane.
• Oxygen: This gas is necessary for breathing and survival. Miners must monitor its concentration in the air to ensure it does not fall below 19.5 percent. Normal oxygen levels are 20.9 percent.

Some gases can be toxic and flammable, increasing the risks they pose to miners.

Mining Operations

Mining for essential resources includes various operations at the mine site, such as quarrying, well operations, crushing, washing, screening, flotation, and other preparations.

The hazardous gases produced depend on the types of operations and conditions created by the mining type, leading to specific dangers called damps after the German “Dampf ” (vapors). For example,

• Blasting in surface mining releases carbon monoxide, or white damp.
• Methane, or firedamp, forms from the natural decomposition of organic matter in underground coal seams and can pose a fire and explosion risk.
• Hydrogen sulfide, or stink damp, also forms from natural organic matter decomposition at the bottom of water bodies.
• Carbon dioxide formed by machinery use creates black damp because there is not enough oxygen to support a burning candle or human breathing.
• Afterdamp is the mixture of gases formed after a mine explosion or fire.

Beyond quarrying, mine processing involves many toxins and can release toxic gases that must be monitored.

• Chemicals in processing: Operations like processing and refineries may use chemicals such as hydrochloric acid, sulfuric acid, nitric acid, and sodium hydroxide to dissolve ores, and their fumes must be monitored.
• Flotation chemicals: Several chemicals, compressed gases, and solvents are used and must be monitored.
• Byproducts: Mineral ore processing can produce toxic gases such as sulfur dioxide, carbon monoxide (CO), nitrogen oxides (NOx), and hydrogen sulfide (H2S), which must be monitored.

Hazardous gases can also depend on the materials mined because of chemical agents used in processing or mineral dust.  Therefore, it is necessary to know the type of mining method that is used.

Types of Mines

Mines can be surface, underground, in-situ, dredging, and artisanal and small-scale mines (ASM); see Table 1. The choice of mining method depends on where a resource is found, and more than one option can be used, so gas risks for a single mineral can differ by mining type.

Mined materials include iron and ferro-alloy metals (nickel, titanium, etc.), non-ferrous metals (aluminum, lead, lithium, tin, zinc, etc.), precious metals (gold, silver, and platinum), minerals (salt, potash, gypsum, asbestos, etc.), and fossil fuels (coal, natural gas, petroleum, etc.)

Figure 1: Types of mines, modified from EcoServants. (Credits: https://ecoservantsproject.org/the-different-types-of-mining-and-their-environmental-impacts/)

1. Surface mines

Surface mines can be open-pit, strip, mountaintop removal (MTR), and placer types used to mine coal, copper, iron, and other metals.

Hazards here result from detonating explosives during blasting, which releases nitrogen (N), CO, nitric oxide (NO), and nitrogen dioxide (NO2) when detonation does not occur properly.

• Nitric oxide: NO oxidizes to NO2, forming large, toxic orange clouds.
• Carbon monoxide: CO is not a problem in surface mines but is a problem in trench mines where it accumulates at the bottom of pits. The gas can seep into the ground and collect in underground confined spaces.
• Methane: Methane in coal seams exposed by mining can be released and ignited during blasting; it is also found in coal recovery tunnels and chutes.

2. Underground mines

Underground mines go deep into the earth for coal, salt, potash, copper, and gold.

• Common gases: Some gases occur naturally, like methane and hydrogen sulfide, while others, like CO and CO2, are produced by operations.
• Oxygen: Insufficient oxygen is also dangerous in underground mines because toxic and other gases can build up.
• Fire risks: Accumulation of gases such as CO, H2S, and methane increases fire and explosion risks in underground mines.

Blasting rock and using heavy machinery release gases that can get trapped if ventilation is insufficient to remove them to the surface.  The deeper the mines, the higher the risk of gas buildup hazards.

Underground mines require hard-rock blasting, drilling, construction, and barriers that constrict working spaces, which can increase gas risks.

3. In situ Solution Mining

During in situ mining, solutions are injected into the earth to dissolve target minerals and pumped back to the land surface for recovery.

Uranium is the mineral usually associated with in situ mining hazards. Though uranium can be extracted by surface and underground mines, it is most often mined through the in situ method. This type of mining involves relatively less worker exposure to chemical risks, but hazardous encounters do occur.

Uranium is extracted as an ore that is dissolved in solutions to separate the mineral as uranium oxide called yellow cake. Tailings contain leftovers from mining and can include small amounts of radioactive materials.  Uranium is only slightly radioactive, but mining releases radon, radium-226, thorium-230, and their decay products into the air or transports them offsite through water.

• Radon: This radioactive inert gas is released in small amounts when uranium ore is mined and crushed. Radon levels are kept low through ventilation and use of fans.
• Radiation: Workers can also be exposed to alpha radiation from radon decay products, which is very toxic. Radium and thorium also emit alpha and gamma radiation as decay products, posing health risks.

4. Dredging

Some resources are excavated underwater by removing deposits from the bottom of water bodies, such as sand, gravel, and minerals. According to the International Association of Dredging Companies (IADC), hydrogen sulfide is the main risk in the dredging industry. It is toxic and flammable.

5. Artisanal and small-scale mines

Artisanal and small-scale mines (ASM) are informal, low-technology operations with little machinery, commonly used for gold and gemstones in developing countries.

One of the greatest chemical risks is mercury use in gold mining. Although mercury is a liquid, it forms colorless, odorless vapors at room temperature. ASM operations usually occur informally, with little or no personal protective gear, increasing exposure to hazardous gases.  Around 19 million miners suffer health effects from mercury exposure in ASM operations alone.

• Miners can also be exposed to mineral residues through inhalation or ingestion of chemicals like arsenic, copper, cobalt, lead, radon, silica, zinc, cyanide, and uranium. Gold mines use sodium cyanide to process the ore.
• Poor ventilation can also lead to insufficient oxygen in caves and galleries.

Materials Mined

The gases produced in mines depend on the materials being extracted, which determine the methods and chemicals used. Often, dust laden with the mined materials can also be a problem. Below are some gaseous hazards of common mined materials.

Coal mines

Methane danger is greater in coal mines than in metal mines. As a result, the risk of fires and explosions is higher in coal mines. Underground coal mines have the greatest explosion risk. Coal mines also have high levels of CO2, nitrogen, and CO. Oxygen deficiency can occur in blind ends or sealed areas.

Other gases that can be found, but only rarely, include hydrogen sulfide and hydrocarbons like ethane, gasoline, and kerosene.  If coal contains sulfur, fires and explosions can also produce sulfur dioxide in mine fumes.

Metal mines

In metal mines, methane release and explosion do occur, though not to the extent as in coal mines.

• Hydrogen: H2 is another explosive gas formed in metal mines.
• Carbon dioxide: CO2 can flow into metal mines and is also formed due to blasting.
• Sulfur products: High-sulfide metal mines can have sulfur dioxide, hydrogen sulfide, and SO3 in lethal levels due to blasting.
• Carbon monoxide: CO is a daily operational hazard from blasting and can accumulate in poorly ventilated spaces.
• Toxic ore dust: Some ores contain soluble toxic metals that become part of ore dust and are inhaled by miners, such as manganese and uranium.

Natural oil and gas mines

Several toxic and flammable gases are produced while mining for natural oil and gas.

• Hydrogen sulfide: The main hazard in natural oil and gas mining, hydrogen sulfide can be lethal in high concentrations, corrode metals, and form toxic gases when burned.
• Hydrocarbons: Petroleum gases contain toxins such as benzene and butane, as well as flammable and suffocating gases like methane. Workers’ exposure risk increases in areas like oil storage tanks, where lethal gas levels can accumulate.
• Diesel fumes: Diesel exhaust contains gases like CO, NOx, SOx, and CO2 from burning fossil fuel.
• Mercury vapor: Mercury is found naturally in oil and gas and can accumulate in pipes and other drilling equipment. Mercury vapor is a major hazard in the oil industry.

Salt mines

Salt and potash mines are prone to so-called “outbursts” that can “release millions of cubic feet of methane and fractured salt.” These outbursts are dangerous and can lead to explosions and the loss of miners’ lives. They can destroy equipment and even entire mines, or cause downtime. Methane occurs near the roof of caverns and must be factored into construction design.

Other gases found in salt mines include CO2, SO2, nitrogen, hydrogen, arsenic, carbonic acid, helium, and other hydrocarbons (ethane, propane, and butane).

Sand and gravel mines

Sand and gravel are extracted from surface mines and dredging, but they do generate hazardous gases. The most common gas during dredging is H2S. Exposure to diesel fumes such as CO2, CO, NOx, and SOx is also possible.

Monitoring the Air in Mines

Air in mines must be constantly monitored for hazardous gases that are present naturally or due to operations. Fixed and portable gas detectors can be used.

• Fixed gas monitors: These devices can be placed at levels where target gases exist to detect levels above permitted limits and automatically trigger alarms, shut down operations, and enable evacuations. These are suitable for underground mines.
• Portable gas detectors: These small instruments can be carried or worn by miners for spot checks in confined spaces to identify leaks and gas accumulation and provide alerts. These are suitable for all mine types.

Interscan offers fixed and portable devices for many of the common gases found in mines. It also offers intrinsically safe portable instruments for detecting explosive and flammable gases, providing additional protection for miners.

Contact us to find out more or schedule a demo of our fixed and portable gas detectors for your mines.

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