How to Control Cabin Respirable Mine Dust Exposures and Comply with Occupational Exposure Limits (OEL) and International Best Practice for Mobile and Fixed Plant Equipment.
The harsh, hot and dust laden environment at mineral processing situations causes wear and tear of cabin components quickly. The installed protection factors often deteriorate rapidly once fine dust has entered the cabin / enclosure affecting seals, electrical components such as electric motors and fine dust build up that will eventually block airflow in [HVAC- Climate control] heat exchangers.
In the United States, there has been several studies conducted in the field by the National Institute for Occupational Safety and Health (NIOSH) with the clear objective of developing cost effective methods to improve filtration and cabin integrity in older machines in order to reduce high exposure levels of respirable silica dust and respirable mine dust. The research has demonstrated reduced exposure results in upgraded cabins with internal dust reduction factors between 65% to 99% when compared to the level of dust outside these cabins. In underground limestone mines “protection factor” levels were found to be as high as 1000 when installing (HEPA) high efficiency filters as a two-stage system.
In recent times, the actual exposure level to respirable dust that a mine operator is subjected to has become significant due to the resurgence of Black Lung disease amongst Queensland’s Coal Workers. The unfortunate findings by a government enquiry are that the disease never went away but remained undiagnosed due to several regulatory failures within the entire sector and a belief that the disease did not exist in Queensland. Coal Worker Pneumoconiosis or Black lung disease is a totally preventable disease when exposure to coal dust and silica are limited.
The Queensland’s Parliamentary Select Committee has finished their full report with 68 recommendations from the Inquiry into the re-identification of Coal Workers Pneumoconiosis.
The report has identified the NIOSH / US model as best practice for the committee with a recommendation to immediately lower occupational exposure limits (OEL) and effectively reduce current levels by half; in fact, these are the lowest exposure levels in Australia.
“Recommendation 19: An Occupational Exposure Limit (OEL) for respirable coal dust (including mixed mineral coal mine dust) should be set requiring duty holders to ensure a ‘coal worker’ is not exposed to atmosphere containing respirable dust exceeding an average concentration, calculated under AS 2985, equivalent to the following for an 8-hour period—
• for coal dust – 1.5mg/ m3 air, and
• for silica – 0.05mg/m3 air.
Section 89 of the Coal Mining Safety and Health Regulation 2001 should immediately be amended to give effect to this recommendation. Consideration should then be given to relocating the OEL provisions within the Coal Mining Safety and Health Act 1999.” [Report No. 2 55th Parliament Coal Workers’ Pneumoconiosis Select Committee May 2017]
The new exposure limits take into account years of research and analysis of mine workers in the US identifying respirable mine dust as the issue of most concern. Coal mine dust is made up of a mixture of substances: coal, silica and other elements like iron; Silica being the most toxic to human health. The definition of silica includes respirable dust particles with a diameter of <10 microns. Essentially our bodies do not have any defence mechanisms to expel these man-made particles. Respirable particles are produced by blasting, crushing, drilling, long wall mining and handling recently extracted mineral ores and its by-products. The latest findings for freshly produced respirable silica indicate there is a high level of toxicity.
Respirable Silica particulate at mine sites is highly reactive due to being electrostatically charged. Once in the body these highly reactive particles pierce lung tissue and will continue to damage surrounding lung tissue over time. Our defences will try unsuccessfully to break down these particles but the actual result is a cascade of enzymes that ultimately harms the lung tissue itself.
Breathe-Safe’s five key factors for maintaining and operating enclosed cabins and booths based on the findings by NIOSH.
(1) Ensure good cab enclosure integrity to achieve positive pressurisation against wind penetration into the enclosure. Studies show that significant improvements in cab protection factors were achieved when cab pressures exceeded 20 pascals [Cecala et al. 2005].
» Breathe-Safe Auto Pressure Control constantly adjusts to maintain 35 pascals of positive pressure.
(2) For respirable dust Breathe-Safe utilises High-Efficiency Particulate Arrestance (HEPA) filters on the pressurised intake air supply into the cab. Filter testing efficiency performance specifications used in the field were 95% to 99.97% on respirable-sized dusts. Laboratory experiments showed an order of magnitude increase in cab protection factors when using high-efficiency filter versus a 38% efficient filter on respirable-sized particles [NIOSH 2008].
» Breathe-Safe submits that standard cabin filters do not protect operators from respirable mine dust and cabins must be pressurised with HEPA filters installed.
» HEPA filters restrict airflow so it is not advised to just replace the original filters to HEPA. The entire system must be integrated with fresh air pressurisation, cabin sealing and HEPA filters to prevent respirable dust exposures.
(3) Use an efficient respirable dust recirculation filter (HEPA). Laboratory experiments showed an order of magnitude increase in cab protection factors on respirable-sized dusts as compared to using no recirculation filter [NIOSH 2008]. Some of the cab field demonstrations used recirculation filters that were HEPA 99.97% efficient. Laboratory testing also showed that the time for interior cab concentration to decrease and reach stability after the cab door is closed was cut by more than half when using the recirculation filter.
» Effective high protection factors are achieved with Breathe-Safe’s HEPA return air filtration.
(4) Minimize dust sources in the cab by using good housekeeping practices, such as periodically cleaning soiled cab floors, using a sweeping compound on the floor, or HEPA vacuuming dust from foot well and cloth seat [NIOSH 2001b]. Also, relocate heaters that are mounted near the floor. These units have been shown to blow air across soiled cab floors and increase dust levels inside the cab [NIOSH 2001a].
» Breathe-Safe is developing in-vehicle HEPA vacuum cleaner to effectively maintain a dust free cabin.
(5) Keep doors and windows closed during equipment operation. One study showed a ninefold increase in dust concentrations inside the cab when doors were frequently opened during the sampling period [Cecala et al. 2007].
» Breathe-Safe’s system data logging function records events such as loss of pressure and door/window events suggesting it may be useful to train operators’ to always keep doors closed.
» Breathe-Safe’s Digital Controller will display a warning light and sound to alert operator when a low-pressure event is detected. It can also alert mine site management if remote monitoring has been installed.
Summary: Compliance to Recommendation 19 means that several engineering controls will have to be put in place. Breathe-Safe can assist with installing a system that can effectively isolate a worker and effectively control respirable mine dust and other contaminants within enclosures or cabins. Below is a demonstration of a Breathe-Safe’s system that complies with recommendation 19 from the “CWP” Queensland Parliamentary Inquiry.
Breathe-Safe key features:
1. Cabin sealing – Excellent cabin integrity is required to maintain optimum positive pressure and prevent contamination from wind penetration.
2. Pre-cleaner – Expels up to 90% of airborne debris and extends the lifecycle of the fresh air HEPA filter.
3. HEPA filter Stage one – Removes respirable mine dust before pressuriser fan.
4. Pressuriser – Increases cabin pressure to push against external airborne particulate contamination.
5. HEPA filter Stage two – Return air filter: removes respirable dust from inside the cabin: from shoes, clothes and when opening door to enter cabin / enclosure. Note: Ideal location is close to feet (HVAC remains dust free).
6. Display panel to INFORM operator that PROTECTIVE system is working and when system requires maintenance.
A two stage HEPA system when installed will reduce maintenance / repair cost associated with HVAC – Climate Control as it remains dust free.