What are airborne contaminants?
When a person breathes, inhalable particles suspended in the air enter their body. Most large particles are stopped in the nose, but smaller respirable particles reach the lungs.
Many dusts associated with mining contain substances that have little health effect when reasonably well controlled. These dusts are generically called dust ‘not otherwise specified’ (NOS). Other dusts can contain hazardous materials for which an exposure standard exists based on the material of concern.
One of the most common dusts encountered on mine sites is quartz (crystalline silica), which is a mineral found in many ore bodies. In the past, workers could develop silicosis, but an understanding of how to minimise exposure has meant there have been no new cases of silicosis in miners solely employed in Western Australian mines for over 40 years.
Several varieties of asbestiform minerals may be encountered during exploration activities for iron ore, base metals and gold and subsequent mining. Welding and abrasive blasting can also generate toxic dusts and fumes.
Where are they found?
People can be exposed to dust almost anywhere on a mine site, but activities such as drilling, blasting, hauling, stockpiling and crushing of ore have the potential to create unacceptable dust exposures if not controlled.
Wet ore or concentrates will reduce dust exposure, but people can be exposed to dust from dried spilled material, or generated from tailings storage facilities, product stockpiles and during product transfer.
Some metals that are mined or present as processing reagents are potentially toxic if inhaled or ingested, such as lead, arsenic and mercury. Hazardous gases and mists such as sulphur dioxide, hydrogen sulphide, hydrogen cyanide and ammonia can also be generated in process plants and refineries. Fibrous minerals such as crocidolite, chrysotile and grunerite can be encountered during exploration and mining.
Refer to classification of dusts is given in the Dusts not otherwise specified (dusts NOS) and occupational health issues position paper available from Australian Institute of Occupational Hygienists.
Diesel engines can emit diesel particulate matter (DPM), exhaust gases, including a wide range of organic vapours, and a small amount of metallic compounds, which are collectively referred to as diesel emissions
The potential short- and long-term health effects from exposure to diesel emissions are well documented. Exposure may occur in vehicle workshops and where diesel equipment is used in confined areas, such as underground, in ore and concentrate storage sheds or in large tanks.
Legionella pneumophila is a bacterial organism found in cooling towers, air conditioners and other water systems. In rare cases, the inhalation of contaminated aerosols may cause Legionnaires’ disease, which is a form of pneumonia caused by an acute bacterial infection of Legionella.
How can dusts and other airborne contaminants be managed?
Mineral exploration and blast hole drilling
Substantial dust can be generated during drilling operations, particularly if undertaken in dry conditions. The generation of dust needs to minimised as much as practicable.
Control measures include:
- fitting drills with a water injection or dust extraction system (or both) suitable to control or extract the dust at the hole during drilling
- where dust is being discharged through ducting, positioning the ducting so dust will not blow back on the operators or others working in the vicinity
- controlling dust at the source if using a cyclone sample collector and during the crushing and splitting of samples
- operating drill rigs from a well-sealed air-conditioned cabin
- regularly replacing filters or cleaning filtration devices and systems to prevent dust build-up.
Samplers and other operators who may be exposed to dust will require suitable personal protective equipment (PPE), which should be selected, worn and maintained in accordance with Australian Standards AS/NZS 1715 Selection, use and maintenance of respiratory protective equipment and AS/NZS 1716 Respiratory protective devices.
Australian Standards are available from Standards Australia
Loading and haulage
Dust needs to be controlled during the loading of broken material such as ore or waste into trucks, and on haulage roadways.
Controls can be provided through:
- wetting down of haul roads and blasted piles
- installing water sprays at dump pockets
- dust extraction at conveyor tipping points, crushers, and screens
- providing air-conditioned cabins in plant equipment.
For air-conditioned plant, the cabin doors, windows and any other access point such as cabling routes should be well sealed to prevent dust ingress.
Crushing, screening and processing
Dust needs to be effectively controlled at each crusher, mill and grinder, with dust control appliances fitted at the primary crusher feed hopper as well as secondary and tertiary crushers plus screens. Conveyor belt transfer points and stockpile tunnels may also require dust control measures.
Any spillage and dust build up on and around the plant and equipment needs to be monitored and removed as necessary. After processing, dust control will be required at stockpile stackers and reclaimers, and during loading and unloading operations (e.g. ship, train, road train).
Dust management is assisted by having wet process streams and dust extraction on transfer points.
Laboratory sample preparation
Most mine sites have a sample preparation and laboratory area, where crushing, screening and analysis of samples is routinely undertaken. Exposure to dusts (including silica) and hazardous reagents found in the laboratory needs to be controlled. Effective local ventilation is required for fume cupboards and sample preparation areas.
Personal monitoring may be required to confirm controls are effective, and biological monitoring may be required in areas such as assay laboratories using hazardous reagents, such as litharge (lead oxide).
Welding and cutting operations are common on mine sites, and generate metal and other fumes. Local extraction systems may be required to ensure fumes are not discharged into other work areas.
Abrasive blasting occurs at many mines. The abrasive agent used to clean equipment can generate considerable dust. Open-air abrasive blasting should be done away from working areas with appropriate dust control measures. The operators must be suitably provided with protective equipment with a clean air supply. There are limits on the amount of silica and other contaminants allowed in the abrasive material.
The aim is to reduce dust generation - PPE should be used as a last line of defence against exposure.
Atmospheric contaminants levels in workplaces at the mine must be maintained below the exposure standards and as low as practicable.
Where no specific exposure standard has been assigned and the substance is both of inherently low toxicity and free from toxic impurities, exposure to dusts should be maintained below 10 mg/m3, measured as inhalable dust (8 hour TWA).
TWA means time-weighted average. 8 hour TWA is the average value of exposure over the course of an 8-hour work shift.
Information on current exposure standards is available from Safe Work Australia’s Adopted national exposure standards for atmospheric contaminants in the occupational environment.
Any samples that exceed the exposure standard must be reported to the department as an exceedance.
Guidance on the interpretation of exposure standards for dusts and other airborne contaminants is available from Safe Work Australia.
A high impact function audit is available to assist in identifying the need for preventative measures.
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