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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 > Automation to Control Silica Dust During Pallet Loading... | |||||||||||||||||||||
Automation to Control Silica Dust During Pallet Loading Process | |||||||||||||||||||||
Two recent Bureau studies evaluated workers dust workers dust exposures' in automated pallet loading processes. The first study involved a Bureau designed dust control system using a push-pull ventilation technique. The unit was shown to reduce workers' dust exposures by 76% during pallet loading at a ground silica operation. A hydraulic lift table was used to automate the bag stacking process; this kept the loading height constant throughout the entire pallet loading cycle. The second study evaluated different commercial automated pallet loading systems. These systems automatically stack the bags onto pallets, shrink wrap the pallets, and deliver the pallets to a fork lift pickup location. Results from a ground silica evaluation site indicated respirable dust samples taken were below the Threshold Limit Value (TLV) as established by the Mine Safety and Health Administration (MSHA).
The purpose of this report is to discuss two recent studies performed by the Bureau of Mines to measure the dust exposure of workers performing the bag stacking process at ground silica sand plants when using some type of automated system. Ground silica product is commonly packaged into 22.68 kg (50lb.) or 45.36 kg (100 lb.) bags onto pallets. These bags are then shipped to customers in either railcars or trailer trucks. Bag loading onto the pallet by workers is performed at either a stationary location and then taken by a fork lift to the transportation vehicle, or directly inside the vehicle using a flexible snake conveyor. This latter case was addressed by a Bureau study a few years ago using an exhaust ventilation dust control system which removed a significant amount of the respirable dust from the vehicle and thus reduced the exposure of the bag stacker (Cecala, 1987). The first case, which is addressed in this report, involves controlling dust exposures to workers when the pallet loading process is performed at a stationary location. This report describes two separate Bureau studies in this area. The first study dealt with the Bureau designed pallet loading dust control system for use at conventional pallet loading operations. This is a partially automated system that requires the worker to stack to a constant loading height for the entire pallet through the use of a hydraulic lift table. The worker stacks the bags to approximately knuckle height, since this is the best ergonomic height for workers. In addition, a push-pull ventilation system captures the dust generated or released during the bag stacking process. The intent with this partially automated system was to allow the worker to continue performing the bag stacking process, while removing the dust from the bag stacker's work area. The second study dealt with monitoring dust levels in and around a completely automated bag stacking system also at a ground silica sand operation. In this case, the worker is removed from the dust source. Totally automated bag stacking systems ahve been in use for a number of years in various commodity operations, but it was not until recently that they have been introduced in ground silica operations.
Both of the automated systems used in these two studies will be briefly described. The first study evaluated the Bureau's pallet loading dust control system. This system has two primary goals: 1) to capture and remove the dust generated from the bag stacking process and keep it away from the worker and; 2) to improve the bag loading action of the worker from an ergonomic design standpoint to reduce the biomechanical strain on workers. To capture and remove the dust generated from the bag stacking process, a design using a pushpull ventilation technique was incorporated. This design is based on a recommendation for tank ventilation by the American Conference of Governmental Industrial Hygienists (1980). The initial design for this application used a 0.12 m3/s (250 cfm), high-velocity slotted blower at one end of the pallet. The blower directs a stream of air across the top of the bags, which is captured by a 1.18 m3/s (2,500 cfm) exhaust system on the opposite side of the pallet. As the stream of air moves across the top of the bags, it entrains more air and dust and carries it into the exhaust system (Figure 1).  Figure 1. Use of smoke to provide visual indication of effectiveness of push-pull ventilation system. For the bag stacking operation, the worker normally catches the bag from the conveyor belt at a height varying anywhere from waist to chest height and then loads it onto the pallet. Ergonomically the optimum height to load bags onto a pallet is knuckle height, approximately (76 cm (30 inches) above the ground. To constantly achieve this ergonomic loading height, a hydraulic lift table is used which varies the height of the pallet after each layer of bags is loaded. The pallet loading dust control system is designed to work as follows: Three to four empty pallets are loaded onto the hydraulic lift table and the table is raised to the desired loading height. The bags are loaded by the stacker on layer at a time. The dust control ventilation system directs a high-velocity stream of air over the top of the bags being loaded. The airstream entrains any dust that is emitted or generated during the bag stacking process, and carries it into the exhaust system on the opposite side of the pallet. After each layer is completed, the pallet is hydraulically lowered the thickness of one bag, approximately 10.16 cm (4 inches). After the pallet is completely loaded, it is removed by a forklift and the cycle is repeated. The only critical feature of the ventilation system is that the height of the bags must remain beneath the blowing ventilation system. The second study evaluated the dust exposure to workers operating totally automated bag stacking systems. The case presented in this report involves a system at a ground silica sand facility. The unit at this operation totally automated the entire bag loading and bag stacking operations. Two Durant packers were used each having two fill nozzles per unit. These machines automatically delivered 22.68 kg bags to a Master Conveyor system that performs the automated bag stacking process. An incoming conveyor delivers the bags to an air table where they are aligned and positioned by mechanical arms (Figure 2). After a whole layer of bags is completed, the air table slides open and the bags are loaded onto the pallet. When the pallet is automatically shrink wrapped, then delivered to a roller conveyor where it slides down to a form lift pick-up location. There are numerous commercial units built by various manufacturers that are capable of performing this automated work process.  Figure 2. Mechanical arm positioning layer of bags to be stacked onto the pallet.
Respirable dust concentrations were taken for all personnel at each operation, as well as taking a number of area samples in and around the stacking process. All worker samples during the first study, and a number of area samples for both studies were performed using RAM-1 dust monitors. The RAM-1 instrument is an instantaneous dust monitor that uses a light scattering device to measure respirable dust concentrations when used with a 10mm cyclone. This unit has been shown to closely correspond to gravimetric samples when calibrated to a specific dust type (Williams, 1994). All RAM-1 dust monitors were calibrated to the Supersil procduct, which is a ground silica product, before being taken into the field. All personnel in the second study were monitored using gravimetric sampling units to determine their respirable dust exposures (Figure 3). In addition, a number of area samples were taken which incorporated a sampling package of three gravimetric packages. For all gravimetric samples, Dupont P2000 sampling pumps were used and were calibrated to a flow rate of 1.7 liters/minute. Nylon Dorr-Oliver cyclones were used to classify the respirable fraction of dust which was deposited on DWS-3 membrane filters in field cassettes. These filters were pre and cost weighed on a analytical balance. These weights were used to determine the Time Weighted Average (TWA) dust concentrations for each of the samples. This was determined using the following equation: TWA = dust weight (milligrams/(xxx liters/minute)(run time-minutes)(0.00) m3/liter)]. This provides a respirable dust exposure level reading in mg/m3. A number of the samples were then analyzed for quartz content using the X-ray diffraction method, which was performed at the analytical laboratory at Pittsburgh Research Center.  Figure 3. Gravimetric sample unit on worker to determine his respirable dust exposure over the workday. The X-ray diffraction technique was performed using the following procedure. The dust samples were ashed in a low temperature radio-frequency asher. The samples were then ashed in an owygen plasma at approximately 350 watts for over a 2 hour time period. After this process, each residue is then deposited on a micron membrane filter and dried at room temperature for several hours. A Perkin-Elmer Model 1750 X-ray Diffraction Monitor was then used to determine the actual analysis of the sample. This unit is equipped with a fast recovery deuterated triglycine sulfate detector which is necessary for quartz analysis. Each personnel sample was analyzed for quartz as well as the middle TLV sample for the 3 sample gravimetric packages.
The first field analysis involved the evaluation of the Bureau' designed pallet loading dust control system. In this evaluation, one worker performed the entire work process. The operator would load bags with product from a double nozzle fluidized air bag fill machine. When the bags were full, he would remove them from the fill machine and stack them onto a pallet located behind him. After a pallet was fully stacked, a forklift was used to transport it to the warehouse area. Four RAM-1 dust monitors were positioned at various locations around the bag loading and stacking station. The cyclone for one RAM-1 monitor was located on the lapel of the bag stacker to monitor his exposure. The second and third cyclones were placed near the blower and exhaust units, respectively, on either end of the pallet. The fourth cyclone for the RAM-1 unit was placed back in the mill building to monitor background dust levels. The primary interest was to determine the worker's exposure from the lapel monitor. Because the worker left the bag filling and stacking location after the completion of each pallet to transport the pallet to the storage area using a fork lift, the analysis was performed on a pallet-by-pallet basis. Two different workers were monitored at this operation during the evaluation period. The effectiveness of the system was determined by comparing the dust levels obtained under normal operating conditions (system off) with those obtained with the pallet loading dust control system in operation. Table 1 shows the dust measurements at the four sample locations during this field evaluation. Considering the entire test period, the worker's dust exposure was reduced by 75.6% with the pallet loading dust control system based on the average system of concentration of 0.82 mg/m3 versus 0.20 mg/m3 with the system in operation. For the most part, the stacker and blower results were fairly consistent. The main reason the exhaust monitor location was somewhat higher was because of a bag leakage problem occurring from a poor bag seal at the back of some bags during the filling process was pulled across the exhaust monitor. The background monitor was used to determine dust levels in the mill building so as to not allow an external source to influence or bias the results of the evaluation. For the most part, the background dust concentration remained fairly constant, around 0.1 mg/m3. In a few cases, the concentration tended to rise, which again was mainly due to bag leakage. Dust liberated from bag leakage was not totally captured by the pallet loading dust control system since it occurred at the bag filling station. 
Table 1. Dust concentrations at monitor locations Figure 4 is a bar graph of the bag stacker dust exposure, with and without the system in operation, while bagging 30-Micron product. The worker's dust exposure was brought into an acceptable range through the use of the pallet loading dust control system.  Figure 4. Bag stacker's dust exposure with and without the pallet loading dust control system. The second field analysis involved determining respirable dust exposures of workers and dust concentrations in the surrounding area to an automated bag loading and stacking machine. This work process normally involved four workers in and around this automated equipment. Two workers (one worker for each bag loading unit) were required to maintain a supply of new bags, (unfilled), on the bag feed conveyor. One worker remained at the controls of the automated palletizer to maintain the proper working order of that unit. The fourth worker operated the fork lift to remove full pallets of bags and to position a pallet of empty bags near the fill machine for the workers to load the bag feed conveyor. The Time Weighted Average (TWA), the Threshold Limit Value (TLV), and the Percent Exposure can be seen for workers at this operation for the four days of the evaluation in Table 2. As one can see, there is only one case where a worker's dust exposure, (day #1; worker #2)., exceeded the allowable concentration. Statistically, this value falls out of the range of all other personnel samples taken at this operation using the 95% confidence interval. It is believed that the unit was probably dumped, and thus measured oversized dust particles.  Table 2. Personnel gravimetric dust samples. In addition to personnel samples, there were four area sampling units taken during this analysis:
At each sampling location, there was a package of 3 gravimetric samplers. Table 3 lists the TWA, TLV, and the Percent Exposure for the various samples taken in coordination with the gravimetric sampling packages. Only the middle sample was analyzed for quartz using the x-ray diffraction technique for each gravimetric sampling package. In all cases, for the 4 days of testing, these samples remained under the allowable respirable exposure level. The average TWA for the gravimetric sampling packages was 0.067 mg/m3 with the average percent exposure being 0.35%.  Table 3. Area gravimetric dust sampling packages. Due to the low concentrations during this evaluation. The information obtained using the Instantaneous RAM-1 dust units is not presented. There is some concern over the accuracy of these units at low dust concentrations recorded by the RAM-1 unit remained at very low concentrations and in many cases the value closely corresponded with those of the gravimetric samples.
Reducing the dust exposure of workers during the bag stacking process has been a concern to both the silica sand industry as well as the Mine Safety and Health Administration. In addition to the problems associated with the dust exposure to workers from bag stacking, another major concern is the amount of strain placed on the workers when stacking bags in the conventional fashion. This job function can be extremely strenuous on workers since it is not uncommon for a single worker to load over 36.287 kg (40 tons) of product in a work day. Both studies presented in this report address the dust control and worker fatigue factors in different ways. The first study deals with the Bureau designed pallet loading dust control system. The intent of this system was to reduce the physiological and biomechanical strain placed on the worker by having the worker load to the most ergonomic height for the entire pallet. This is achieved by constantly adjusting the pallet height using a hydraulic lift table after each layer of bags is loaded. Ergonomically the optimum height to load a bag onto a pallet is at knuckle height, approximately 71 to 76 cm (28 to 30 inches) above the ground. This allows the worker to continue performing the stacking process while reducing the strain placed on him. Since the worker continues to perform the stacking process with this system, it is necessary to protect the worker by removing the dust generated during this process. This was achieved by using a push-pull ventilation system which reduced the workers exposure at the ground silica bagging unit by 76%. The second study addressed the strain and the dust exposure of the worker by removing the worker from the process and having the function performed robotically. The location of this second evaluation was believed to be the first operation in the country to automatically load and stack the bags onto pallets at a ground silica sand operation. The production rates with the totally automated system at this facility seemed to be comparable with production rates when the function was performed manually, approximately 72,575 kg (80 tons/shift). On a dust control basis, the results of the evaluation are impressive from both personnel as well as area samples.
The Bureau has evaluated two different systems at ground silica sand operations to reduce the dust exposures of workers in and around the bag stacking process. The first system was the Bureau designed pallet loading dust control system which uses a push-pull ventilation technique to capture and carry the dust generated during this process into an exhaust ventilation system. This system was shown to reduce the dust exposure of the bag stacker by 76%. The system also allows the worker to constantly load bags to the most ergonomic height through the use of a hydraulic lift table which varies the pallet height after each layer of bags is loaded. It was also believed that the system would have a slight impact on increasing production because of less downtime between pallets and less worker fatigue. The second study involved evaluating the dust exposure of workers using a commercial system which totally automates the bag loading and stacking process. Four different worker functions were monitored along with 4 area sampling units in and around this system. All dust exposure readings, except one, remained at levels well below the allowable levels as established by MSHA standards. This system seemed to be very effective from a dust control as well as from a production standpoint.
Cecala, A.B., A. Covelli, and E.D. Thimons, 1987, "Reducing Dust Exposures of Workers During Bag Stacking in Enclosed Vehicles," Bureau of Mines IC 9148, 1987, 13pp. American Conference of Governmental Industrial Hygienists, 1980, "Industrial Ventilation - A Manual of Recommended Practice," Edwards Brothers Inc., 365pp. Wiliams, K.L., and R.J. Timko, 1984, "Performance Evaluation of a Real-Time Aerosol Monitor," Bureau of Mines IC 8968, 1984, 20pp. |
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