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U.S. Department of Labor | |
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| Occupational Safety & Health Administration | ||||||
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| Safety and Health Topics > Mineral Processing Dust Control > Reducing Respirable Dust Levels | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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1Mining engineer. 2Physical scientist 3Mechanical engineer technician Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.
Introduction Background Testing equipment and procedures Evaluation site 1 Testing Results Evaluation site 2 Testing Results Discussion Conclusions Appendix.-Test Results
1. Respirable quartz exposure by occupation in mineral processing facilities for 1988-92 2. Schematic of B&BCD 3. Test layout of evaluation site 1 4. Gravimetric dust concentrations with and without B&BCD at evaluation site 1 5. Reduction in RAM-1 dust concentrations with B&BCD at evaluation site 1 6. Product removed from exterior of bags with and without the B&BCD at evaluation site 1 7. Test layout of evaluation site 2 8. Increase in dust concentrations inside B&BCD at evaluation site 2 9. Reduction in RAM-1 dust concentrations with B&BCD device at evaluation site 2 10. Dust liberated from squeezing process of palletizer unit at evaluation site 2 11. Product removed from exterior of bags with and without B&BCD at evaluation site 2
A-1. Gravimetric results at field evaluation site 1: Normal system A-2. Gravimetric results at field evaluation site 1: B&BCD A-3. Results of RAM-1 dust monitor at site a sample locations A-4. Plant 1: Vacuum testing of bags to determine reduction in product on outside of bags A-5. Site 2: Results of RAM-1 dust monitor at sample locations
The U.S. Bureau of Mines has designed and tested a system called the Bag and Belt Cleaner Device (B&BCD) to reduce dust levels in and around the bag conveying and stacking process. The device physically cleans either 22.7 kg (50 lb) or 45.4 kg (100 lb) paper bags by using a combination of bushes and air jets. It is completely self-contained and is kept under negative pressure by a baghouse to ensure that dust and product removed from the bags during cleaning does not flow into the work environment and contaminate workers. The bags travel through the device on a chain conveyor, which permits any product or dust cleaned from the bags to fall into a hopper at the bottom of the device and be recycled back into the process via a screw conveyor. Once exiting the B&BCD, the outside of the bags and the conveyor are essentially product and dust free. The B&BCD was evaluated at two mineral processing plants to determine reductions with the device in use. The results of both field evaluations showed that the amount of product removed from the outside of the bags varied from 77 to 93 pct.
The purpose of this study was to determine a cost-effective way to lower respirable dust levels in and around the bag-stacking function at mineral processing operations. In mineral processing facilities, the bag loading and stacking processes are the highest dust exposure job categories in the metal/nonmetal mining industry. Figure 1 shows the respirable quartz exposure level of different job classifications at mineral processing facilities; the highest worker exposures are for bag operators and bag stackers.4 The U.S. Bureau of Mines (USBM) has worked on several different research projects to reduce worker exposure during bag stacking. One study dealt with lowering dust concentrations when loading bags on wooden pallets within enclosed railcars or trailer trucks. A flexible snake conveyor expedited the stacking of bags inside these vehicles. Individuals working in the vehicles, referred to as "stackers," were exposed to extremely high respirable dust concentrations during this palletizing process. At times, respirable dust concentrations were 40 to 50 times higher than the worker's threshold limit value (TLV) for the workday. The dust generated inside these enclosed vehicles during the bag-stacking process was not exiting the vehicle or being diluted with fresh air. The USBM examined various methods to effectively ventilate these vehicles and thus lower respirable dust concentrations. The goal was to remove the dust generated during the bag-stacking process and to keep it from contaminating the workers while being exhausted from the vehicle. The most effective design was a system built onto the flexible snake conveyor that was retreated from the vehicle as it was being loaded. The inlet to the exhaust system extended beyond the loading area and pulled dust toward the front of the vehicle and away from the bag stackers. A small exhaust port was also located at the last transfer pint to capture the dust liberated at this locations. Respirable dust concentrations were reduced between 65 and 95 pct in and around the bag-stacking location using this system (1)5. The USBM also developed an effective method to lower dust levels at conventional pallet loading operations; those sites where the pallet loading process is always performed at the exact same location. When a pallet is fully loaded, a fork lift carries it away. The cycle is then repeated by positioning a new pallet in the same locations as the previous one. The USBM performed an in-depth laboratory analysis investigating various ventilation methods to minimize the bag stacker's dust exposure. The final design used a push-pull ventilation technique to capture dust generated during bag stacking. A low-volume, high-velocity blower system operating at approximately 4.3 m3/min (150 cfm) generated a stream of air over the top layer of bags on the pallet. As this air stream traveled across the top of the stacked bags, it entrained dust generated during the bag-stacking process. The exhaust ventilation system pulled approximately 70.8 m3/min (2,500 cfm) of air and dust through the exhaust hood. This exhaust air was filtered through a baghouse or another device before being discharged outside the mill. There was a 70-pct reduction in the bag stacker's dust exposure during laboratory testing and a 76-pct reduction at the first field evaluation. This system has proven itself to be effective and reliable from a dust control, ergonomic, and production standpoint (2). The previous research looked at specific types of pallet loading applications and investigated effective techniques to control the dust generated at these locations. The intent of the current research is to clean both the bags and the belt before they reach the bag-stacking location. This reduces the worker's respirable dust exposure regardless of the bag-stacking method used. When this work was initiated, an in-depth literature search was performed to determine existing research and technology in this area. There was a significant amount of work performed on conveyor cleaning techniques (3-5). This previous research was beneficial in evaluating different methods for cleaning the bags of product. This research was aimed at designing a system that would have wide application and could be used regardless of the stacking method or location. The B&BCD was designed to be placed in-line between the bag loading and the bag-stacking process. The system was able to handle paper bags between 22.7kg (50lb) and 45.4 kg (100 lb), but with minor modifications, it could probably handle most bag sizes. Product removed or cleaned from the bags and belt is collected in a hopper at the bottom of the device and recycled back into the process periodically via a screw conveyor. By removing the product and dust from the exterior of the bags and the conveyor belt, dust liberation is greatly reduced while the bags are transported to the bag-stacking location. The ultimate goal of this research was to improve the health of workers by reducing respirable dust concentrations during the bag-stacking process. Dust exposure is reduced for the bag stacker, other workers in and around the area, as well as the end user of the bagged material.
In an effort to clean the bags of product material as they move through the device, the B&BCD uses a combination of both stationary and rotating brushes, along with air nozzles. The B&BCD is 3.1m (10ft) long and is installed as part of the belt line. Figure 2 shows a schematic of the B&BCD. As bags enter the device, they travel through flexible plastic stripping doors into an air lock chamber. Inside this air lock is a stationary brush on a swing arm that starts the cleaning process on the front and top of each bag. The bag then travels through a second plastic stripping door, exits the air lock chamber and enters the main section of the device. Once in the main cleaning chamber, the bag travels under a rotating circular brush that further cleans the top of the bags. This brush rotates opposite to the travel direction of the bags, creating additional friction and improving the cleaning action. The sides of the bags are cleaned by a stationary brush positioned on each side of the chamber. An air nozzle was located at the end of each of these brushes. The bags should always travel through the device with the valve on the same side. The valve side of the bag is normally much more contaminated with product than the nonvalve side. This contamination occurs as product spews from the fill nozzle during the fill cycle. Because of this contamination, the bag valve side needs more air to clean the bag than the nonvalve side. On the nonvalve side of the bag, a flat fan airjet delivered approximately 0.08m3/min (3.0 scfm) of air. On the valve side of the bag, a plastic airjet nozzle delivered approximately 0.57 mg/min (20 scfm) of air. This nozzle effectively removes product from the another portion of the bag valve area. A dust cloud from the bag valve occurs each time a bag travels past the nozzle. On the nonvalve side of the bag, the high volume airjet nozzle was not cost-effective due to the additional expense of providing pneumatic air. An air filter cleans all compressed air. Nozzle air pressure is adjusted with a pressure regulator. The optimal pressure to operate these nozzles is approximately 276 kPa (40 psi). The last cleaning process involves the bag traveling over a rotating circular brush located beneath the bag. In this case, the brush rotation is in the same direction as the bag movement. Although the movement is in the same direction as the bags, there is acceptable cleaning action because the weight of each bag forces it down on the bristles of the rotating brush. The bag then exits the device by going through another air lock chamber, again having two sets of flexible plastic stripping doors. A chain conveyor is used for the entire length of the device to allow product removed from the bags to fall into the hopper. Product cleaned from the bags is recycled back into the process. Initially, a high pressure air stream released from the slotted device, known as an air knife, was located at the far side of each of the rotating brushes to provide additional cleaning and to keep the brushes clean. These air knives provided very little, additional cleaning over the rotating brushes and tended to place the device under positive pressure. Because of this, air knives are not recommended as part of the B&BCD. The B&BCD is a self-contained system with three hookup requirements. The first is 440 V, three-phase electrical power. The device is protected by a 30-A breaker and requires approximately of A of current during normal operation. The second requirement is compressed air. Compressed air powers two air jets and two pneumatic cylinders. The two air nozzles need approximately 0.65 m3/min of compressed air at 276 kPa. A minimum quantity of air is necessary to periodically change the spacing of the stationary side brushes for 22.7 or 45.4 kg bags. The last requirement is to provide an exhaust air volume of approximately 34 m3/min to keep the system at a negative pressure relative to the surrounding atmosphere. This prevents dust generated within the B&BCD from flowing out of the unit and contaminating the work environment. The final recommended design of the B&BCD cost between $9,000 and $10,000 to fabricate. Approximately one-third of this cost was attributed to the chain conveyor portion. The other two-thirds would be additional cost for the various techniques to clean the bags and belt and to contain the dust and material removed within the device. The 34 m3/min of exhaust air volume to a baghouse is not included in this cost. It is also estimated that fabrication time was approximately 120 employee-hours. 4Watts, W. F., Jr., and D. R. Parker. Quartz exposure Trends in Metal and Nonmetal Mining. International Report. U.S. Bureau of Mines. Twin Cities Research Center. 5Italic numbers in parentheses refer to items in the list of references preceding the figures |
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