Guidance about underground air quality and ventilation

Aim of ventilation

Ventilation in an underground mine is of critical importance to the occupational health and safety of underground employees.

The atmosphere underground is limited and confined, and is thus readily reduced to a sub-standard (or even dangerous) condition if contaminants (e.g. gases, dusts, fumes) produced in the course of operations are not controlled, safely extracted or diluted to acceptable levels. Ventilation is used to extract and dilute contaminants.

Sources of toxic, explosive and asphyxiant gases

Gases in mines may be naturally occurring, the products of combustion, or fumes from vehicle exhaust emissions or activities such as blasting. They include methane and other hydrocarbons, carbon dioxide, carbon monoxide, oxides of nitrogen, sulphur dioxide, hydrogen sulphide, ammonia and radon.

Methane and other hydrocarbons encountered during drilling are a potential explosive risk and can displace oxygen.

Blast fumes contain large quantities of oxides of nitrogen and carbon monoxide. Oxygen levels can also be depleted. Adequate ventilation to remove contaminants and air testing is required before workers can return to the workplace.

The most common mining occurrences of spontaneous combustion are in coal mines. However, spontaneous combustion of sulphide ores can produce high levels of sulphur dioxide and carbon monoxide.

Underground fires caused by vehicles can be a significant source of fumes.

Ammonium nitrate?fuel oil explosive (ANFO) is a powerful oxidising agent and spills of ANFO prill can promote fire in contact with combustibles. Ammonia may be generated where ammonium nitrate comes in contact with cement used in grouting, strengthening stope filling or in shotcreting.

Rarely, hydrogen cyanide may be generated where tailings are used in stope fill.

Diesel emissions

The potential health effects from exposure to diesel exhaust are well documented and underground mines are required to monitor and control diesel emissions. In 2012, the International Agency for Research on Cancer (IARC) classified diesel exhaust emissions as carcinogenic to humans (Group 1).

Diesel exhaust particulates consist of elemental and organic carbon components as well as unburnt hydrocarbons and other solid material.

Gaseous components include carbon monoxide, carbon monoxide, oxides of nitrogen, sulphur dioxide and organic compounds.

Enclosed cabins can be an effective means of reducing exposures to diesel emissions, although their use will not be practical for all activities in an underground mine. Cabins should be air-conditioned and positively pressurised (using HEPA filters) to ensure operators are both comfortable and protected from exposure to diesel emissions.

The operator is only protected while in the cabin and if the cabin seals and air conditioner filters are well maintained. This includes checking seals, leak testing and maintaining filters.

Dusts

Where practicable, wet drilling should be used to minimise dust generation. Otherwise drilling machines are to be fitted with effective collection devices.

Sulphide minerals oxidise rapidly when broken and fresh surfaces are exposed to air. When dispersed as dust, sparks or heat flash from blasting can initiate an explosion. Disulphides that contain iron (e.g. chalcopyrite, pyrite) are the most susceptible to 'flash' initiation and explosive propagation.

The basis of effective ventilation of underground mines is the adequacy of the primary ventilation system. This is the total mine air volume flow generally through the major underground workings, normally involving splits into parallel circuits.

Factors that determine the total primary volume capacity (and pressure) requirements include:

• extent and depth of the mine
• complexity of workings, including stoping and extraction systems
• size of development openings
• equipment used.

Many deep and extensive underground mines tend to use series ventilation circuits. The main problem with series or parallel-series circuits is progressive contamination of the air by recirculation from secondary ventilation system returns. There is also an increased fire risk, with the fumes and smoke from any fire in the intake or any upstream section of the mine being carried into working sections downstream.

Secondary ventilation refers to the provision of ventilation to development ends, stopes and services facilities that constitute secondary circuits tapped off the primary circuit or main through flow of air. These headings are typically ‘dead end’ configurations, and may have multiple parallel headings where flow-through of air is not available.

The use of secondary ventilation fans and ducting is normally required, most commonly in a ‘forced air’ configuration, but pressure or exhaust overlap or total exhaust may also be used.

Ventilation plans for underground mines

The manager of an underground mine must ensure that a plan of the ventilation system at the mine is kept at the mine that shows the direction, course and volume of air currents, and the position of all air doors, stoppings, fans, regulators and ventilating devices, in the mine.

It is important to understand the influence of following factors in determining the effectiveness of the ventilation planning process:

• the mine plan and schedule
• mine design and method
• modelling of airflow
• development of ventilation plans and schedules
• diesel equipment
• contaminants – both naturally occurring and man-made.

Air sources

The manager of a mine must ensure that:

• the supply of air for any ventilating equipment used underground is obtained from the purest source available
• the recirculation of air within any secondary ventilation circuit is maintained at the minimum level that is practicable
• where practicable, contaminated return air from any secondary ventilation circuit is exhausted directly to the primary return air exhaust system.

Please refer to the following documents:

Mine sites are required to carry out a structured risk assessment of their occupational health hazards, and develop a risk-based hygiene management plan (RBHMP).