With mines and quarries are under pressure from two directions this is not good news.
Mining and Mineral Products
Good dust control is essential in meeting both environmental and occupational types of requirements.
Environmental Authorities: As suburban sprawl has reached many mines and quarries, people no longer accept a dusty environment. Environmental air quality authorities typically insist there be no visible dust emissions and limit total dust fallout beyond the boundary. Currently the EPA regulates Particulate Material at 35 µg/m3 for coarse particulate and 15 µg/m3 for fine particulate with the understanding that these limits may be much lower in "non-attainment" areas.
Occupational Health & Safety Authorities: Everyone understands employees should not work in a dusty environment. For dusts nototherwise regulated, the limit of dust concentration in the workplace is generally established as 15 mg/m3 for total dust and 5 mg/m3 for respirable dusts. If a dust has known hazardous properties, e.g. contains silica, then the allowable concentration is significantly reduced. As an example the current NIOSH guideline for crystalline silica sets a Permissible Exposure Limit (PEL) at only 0.05 mg/m3, to reduce potentially adverse health effects of silica dust exposure.
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GUIDELINES TO PRACTICAL DUST CONTROL
Years of experience in mining and quarry dust control has highlighted a number of significant points:
Good design and equipment selection is essential.
Anything marginal in either design or selection results in immediate or short term inadequacies in collector performance. The difference in selections between an effective and ineffective dust collector is often much narrower than in many other industries.
Everywhere that material is supposed to drop through air, dust is generated.
The most obvious dust generation points may seem to be conveyor transfer points and feed points from screens and crushers. But often, oversized feed-outs from screens or transfer of large product into bins will generate significant dust.
Budgetary constraints are important, so it is generally better to do some parts right than a lot of parts wrong.
DUST CONTROL IN SPECIFIC AREAS
The following is a list of some of the typical areas at mines and quarries generating dust that can be controlled by dust collection equipment:
- Rail or truck dumps
- Conveyor belt transfer points
- Silos or bin filling
- Rail, barge or truck loading
DUST COLLECTORS PHILOSOPY: Source or Central
Two basic strategies exist for applying dust control to mines and quarries; source collection (putting the dust collector as the source of the dust emission so collected dust can be deposited directly back into the process stream) or centrallized collection (putting the collector in a central location where dusty air is ducted to the collector and discharged as a separate process stream).
The main operating differences between these strategies are collector location and where the collected dust is sent. Source collection (placing the collector right at the source of the dust emission) returns dust to the initial process stream. Central systems generally remove dust from the original process stream and generate a separate process stream of collected dust. The desirability of each method depends mainly on the nature of the process and the relative merits of the collected dust being separated or incorporated in the final product.
Source collection operational advantages include:
- lower capital costs as less ducts, and fewer hoppers, screw conveyors or rotary valves are typically required.
- fewer maintenance problems as fewer ducts or hoppers are being used that can block or bridge. In addition, since there are fewer hoppers, there are no fewer hopper discharge devices that need to be maintained.
- a shutdown of one unit may result in temporary increased emissions in a single area, but will be unlikely to necessitate a complete plant shutdown.
Dust Collection Considerations
The following points need to be considered in the selection of central dust collectors for mining and quarrying applications:
- Mine and quarry dust is often abrasive and particle impact against filter media needs to be considered to avoid possible filter damage. This requires consideration of incoming air velocity, dust distribution, and air patterns within the collector.
- Dust removal and disposal requires careful consideration to avoid blockages, equipment abrasion, secondary dust emissions, and higher costs for maintenance and materials handling.
Source collection systems may be easier to deal with since hoppers and ducting are often not used. But when using source collection, the following should be considered:
- Since collectors are often much closer to the source of dust generation, be aware of any water or oil in the dust as this could create problems with filter medias.
- Filter abrasion typically is not an issue, however, if the source collector is being used as a bin vent, place the collector as far from the inlet as possible to avoid abrasion and/or unnecessarily high dust loading.
LEGISLATION (OTHER FEDERAL AND LOCAL LEGISLATION MAY APPLY)
- US EPA
Particulate Matter (PM):
- PM 2.5 = 15 µg/m3 (annual), and 35 µg/m3 (24-hour)
- PM 10 = 150 µg/m3 (24-hour)
- OSHA Regulations
- Directive CPL 03-00-007: National Emphasis Program - Crystalline Silica.
- 29 CFR 1910.1000 - Air Contaminants, NOTE: Twenty-five states, Puerto Rico and the Virgin Islands have OSHA-approved State Plans and have adopted their own standards and enforcement policies. For the most part, these States adopt standards that are identical to Federal OSHA. However, some States have adopted different standards applicable to this industry or may have different enforcement policies.
AP-42, Compilation of Air Pollutant Emission Factors, has been published since 1972 as the primary compilation of EPA's emission factor information. Chapter 11 focuses on the Mineral Products Industry with thirty-one separate sections covering processes from Hot Mix Asphalt Plants, to Abrasive Manufacturing.
Industrial Ventilation Manual
NIOSH Publication No. 76-179