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| Safety and Health Topics > Mineral Processing Dust Control > Reducing Respirable Dust Concentrations at Mineral Processing Facilities Using Total Mill Ventilation Systems | |||||||||||||||||||||||||||||||||||||||||||||||
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1Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 2Project engineer, Canada Centre for Mineral and Energy Technology, Sydney, Nova Scotia, Canada 3Reserch supervisor, Pittsburgh Research Center
Abstract Background Introduction Testing equipment and procedures Mill 1 Testing Results Mill 2 Testing Results Tracer gas analysis Discussion Conclusions References
1. Increases in total mill dust levels as day or shift progresses and dust sources are not addressed using control techniques 2. Five dust monitoring locations at mill 1 3. Respirable dust levels at location 5 with and without total mill ventilation system 4. Effectiveness of total mill ventilation system at removing smoke from mill building 5. Six dust monitoring locations at mill 2 6. Percent reduction of repirable dust at each monitoring location for 2 days of testing for both two and four fans using RAM-1 devices 7. Temperature readings at three levels in mill building during day 1 of testing with and without total mill ventilation system 8. Worker taking gas sample for tracer gas analysis at exhaust fan 9. SF6 analysis - comparing four fans versus two fans for total mill ventilation system 10. SF6 analysis-effectiveness of total mill ventilation system with windows open versus closed 11. Ventilation profile curves
1. Dust reduction for gravimetric and RAM-1 instruments at five monitoring locations for both field evaluations at mill 1 2. Dust concentration and percent reduction for RAM-1 instruments at five monitoring locations at mill 2 3. Reduction of respirable dust levels with gravimetric samplers at three monitoring locations at mill 2
The U.S. Bureau of Mines has designed and evaluated total mill ventilation systems at two different mineral processing operations. Both systems have proven very effective at reducing respirable dust levels throughout the mill in a cost-effective manner. A 25,5000-cfm system installed at a clay processing mill provided approximately 10 air changes per hour. This system reduced respirable dust concentrations by approximately 40 pct throughout the mill building. The second evaluation was performed at a silica sand operation. Tests were performed with 50,000 and 100,000 cfm of ventilation to the mill building, corresponding to 17 and 34 air changes per hour. Average mill-wide respirable dust concentrations and increased visibility throughout the mills, they were also easy to install and required minimal maintenance. A total mill ventilation system provides a general purging of the mill air; the system should be viewed as a supplemental technique to assist other dust control systems in operation.
The U.S. Bureau of Mines has been performing research on methods of lowering the dust exposure of workers at mineral processing operations for a number of years. This research has mainly been directed at source control techniques in an effort to lower a worker's dust exposure at a particular job function. In some cases, this control technology provided a carryover effect in lowering the exposure of workers at other locations, but for the most part the technology had very little effect in lowering dust levels throughout an entire mill building. A recent Bureau study identified the various types of dust control techniques that were being used by the U.S. mining industry in coal preparation plants and mineral processing operations (1)4. This work briefly evaluated the advantages and disadvantages of the following techniques: ventilation, baghouse-type collectors, wet scrubbers, electrostatic precipitators, source control, sprays, good housekeeping, and personal protection devices. Looking specifically at mineral processing operations, the authors' experience has indicated that baghouse-type collectors are the most common technique used by the industry. At the other extreme is ventilation, which is given little consideration by the industry. Recently, the Bureau became aware of several mineral processing operations having problems with high overall mill dust levels. These operations were all using baghouse-type collectors to address their most serious dust contamination area. Although these major dust sources were being controlled, there were numerous minor dust sources that were not. These minor dust sources were causing dust levels in these operations to gradually increase as the day or shift progressed. In some cases, dust concentrations continually rose for the entire shift (figure 1, case A). In other cases, dust levels would stabilize at a certain level as natural ventilation provided a purging of the structure (figure 1, case B). These minor dust sources can be generated from numerous processes or events. Some sources are common throughout the industry, whereas others are site specific. Some common sources are product dropping off from the bottom of conveyor belts or being knocked off by the rollers on a conveyor line; product residue on walls, beams and equipment becoming airborne from the vibration of the processes and equipment operating within the mill; product on walkways and access area being generated as workers walk throughout the mill; product leakage from chutes or transfer points; leakage from dust control equipment; dust released or generated from improper housekeeping practices; and product released when inspecting screens or other milling processes when covers or lids are opened. Every effort should be made by plant managers to ensure that these dust sources listed, many of which are house keeping practices, are continually addressed to minimize their effects. A cost-effective supplemental dust control method to control a gradual increase in dust levels over the shift is to use a total mill ventilation system. During a Bureau survey of 25 coal prep plants, it was observed that those operations that effectively sued some type of general ventilation system had the lowest overall dust concentrations (2). One possible reason for the lack of general mill ventilation systems in the mineral processing industry is the shortage of published information addressing this. The Industrial Ventilation Manual is the primary reference source on general plant ventilation systems in the United States (3). Although this manual is excellent for engineers doing plan design work and is quoted by some authors as "the Bible" in this area (4), there is no specific information provided on the proper design of total mill ventilation systems. Eshelman (5) took the information provided in the Industrial Ventilation Manual on general and localized exhaust systems; expanded it, and noted the importance of mill ventilation systems, but did not provide plant engineers the needed information to implement the technique. Because of the lack of published information and the limited use of general ventilation systems in the industry, the Bureau decided to perform a short-term study in this area. This research effort was performed in an effort to improve the health and safety of men and women working in mineral processing operations throughout the nation.
The authors could not find published guidelines as to what ventilation volumes should be provided to mineral processing plants or mills. The Mine Safety and Health Administration (MSHA) uses 30 CFR 56.5005, which states that "control of employee exposure to harmful airborne contaminants shall be, insofar as feasible, by prevention of contamination, removal by exhaust ventilation is a possible technique to control employee exposure but does not give suggested guidelines for air volumes. The intent of the present study was to determine general guidelines and design criteria for operations that may be interested in implementing such a total mill ventilation system. Most mill building can be considered closed system, and thus any dust that is not being controlled within the structure will cause dust levels to gradually increase over the shift. A total mill ventilation system should be be designed to draw clean makeup air in near the base of the mill. This air provides general purging and may clear some dust- laden areas through the mill structure. This air should be discharged at or near the top of the structure, where it will not contaminate plant personnel working outside. In addition, thermodynamic effects from heat generated by mill equipment will product a chimney effect, thus assisting the basic flow pattern of this ventilation system. This technique should not present any environmental problems since the exit velocity and dust concentrations are relatively low. Without this exhaust system, dust can exit the building through open doors, windows, etc., and contaminate outside plant workers. This dust can also be recirculated back into the mill building, causing additional contamination problems. A total mill ventilation system is not a stand-alone technique, but is a supplemental technique to assist other localized dust control systems. |
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