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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 Dust Exposure > Laboratory-Scale Testing | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| LABORATORY-SCALE TESTING | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The laboratory test results were used only to compare the different systems. Originally, the areas of interest were to be the dissipation of gas once the exhaust system was turned on and the baseline concentration. However, there was not substantial difference in the dissipation of gas or the decay rate from one system to another, so the baseline gas levels were used as the primary evaluation in comparing the system. The effectiveness of the different ventilation systems decreased in the following order: 1. Exhaust system over snake conveyor. 2. Exhaust system on floor near pallets. 3. Blowing and exhaust system (push-pull system). 4. Blowing system off back roof. 5. Blowing system (high or mid-height; left, center, or right). 6. Exhaust system near door (floor). The average baseline concentrations at the different sampling locations are shown in table 1 for each system evaluated, listed in order of decreasing effectiveness.
The exhaust system over the snake conveyor, (at the center of the car) was identified as the most effective technique and was therefore selected for further study. The methane release point was moved to the top of the pallets to represent dust coming from all over the pallet. To determine the effectiveness of pulling dust up and away from the bag stacker, the exhaust ventilation port was moved from being immediately above the front edge to 18 to 36 in past the front edge of the pallets. The capture efficiency increased with distance from slinger. Since the pallet base dimension is approximately 48 in square; 36 in was thought to be the greatest distance possible, in order to maintain the necessary clearance from the back wall on the first pallet.
The effectiveness of the exhaust ventilation system located over the snake conveyor was evaluated in an actual working environment at a mineral processing plant. An exhaust system was fabricated and installed over the snake conveyor and slinger. At this plant, both railcars and trailer trucks were loaded from the snake conveyor. To optimize the technique in the actual work environment, both types of vehicles (railcars and trailer trucks) were monitored for dust concentrations at various locations, with and without the ventilation system. The RAM-1 real-time aerosol dust monitors, built by GCA Corp.,4 Cambridge, MA, were used to evaluate respirable dust levels at various locations during loading of the enclosed vehicles. The monitors use a light-scattering device to measure the dust concentration of a sample drawn in from the environment through a 10-mm cyclone.5 Dust was monitored at four locations inside the enclosed vehicles (fig. 4): Location 1--On the lapel of the bag stacker (to give a direct reading of personal dust exposure while stacking bags onto pallets). Location 2--At the right side of the end of the slinger. This was the bag valve side. This location gave a direct reading of dust levels where the stacker catches the bags. Location 3--At the transfer point between the snake conveyor, approximately 8 ft back from the conveyor-slinger transfer point. This location gave measurement of the dust buildup inside the main portion of the enclosed vehicle. Location 4--Over the snake conveyor, approximately 8 ft back from the conveyor-slinger transfer point. This location gave a measurement of the dust buildup inside the main portion of the enclosed vehicles. The signal from the RAM-1 dust monitor was fed directly into a strip-chart recorder as a function of time, which provides the notation of the starting and finishing times. Any downtime associated with loading the vehicle was also noted and excluded from the dust calculations. Dust concentrations for each vehicle were calculated. A planimeter was used to calculate the area under the curve, which was then divided by the sampling time. This yielded dust concentration values in milligrams per cubic meter. The following factors were taken into account in comparing loading using the ventilation system with the conventional loading process (with no ventilation):
Three variations of the exhaust ventilation system on the snake conveyor were evaluated, in each case for a 1-week period. In each case, a 2,100-cfm fan located outside of the vehicle was used. Flexible tubing was attached to the fan and extended to the snake conveyor slinger transfer point. The systems differed as follows:
Table 2 gives the average respirable dust concentration with the system off and on at the various sample locations, and the percent dust reduction for loading an entire vehicle. The values were measured only during actual work periods, and therefore they were higher than normal for a worker's eight hour exposure level, that includes break periods and times when the system is not operating.
The intent of the laboratory-scale testing was to establish conditions that would be representative of actual field conditions and select the most effective system for field testing. The factor that was not simulated during laboratory-scale testing, but which proved to be significant during the field evaluation was the dust emitted from the bag valve at the snake conveyor-slinger transfer point. Any dust that was emitted at this point was drawn over the stacker to the exhaust ventilation inlet in front of the stacker (test 1). This can be seen from the railcar (290-mesh, 100-lb bag) and trailer truck (390-mesh, 100-lb bag) results. The dust reduction at the stacker and slinger locations was not nearly as good as the dust reduction at the conveyor location. Since the conveyor sample location was behind the transfer point dust source, it was not affected. The amount of dust liberated at the snake conveyor-slinger transfer location increased with 50-lb bags. The 50-lb bag results in test 1 showed no dust reduction at the stacker location and an increase in dust at the slinger location. The increase in dust measured at the slinger location can be attributed to the fact that the dust generated at this transfer location flowed directly over the slinger monitor as it was drawn to the exhaust ventilation system. Because of the dust at the bag valve side of the snake conveyor-slinger transfer point, a small exhaust port was extended to this location in test 2. The two 8-in-diam main exhaust lines were routed under the slinger because this would have been a more advantageous permanent location for the system. Visual observations and actual dust measurements both showed the small transfer point exhaust port to be effective in capturing the dust, with an average reduction of 55 pct. The main exhaust was not as effective as in the first system because the inlet underneath the slinger was not powerful enough to pull the dust from the pallet stacking location along the floor and away from the bag stacker position. The final and recommended design (test 3) incorporated the exhaust system extended over the pallets to capture the dust generated during bag loading. Visual observation showed that this system was very effective in capturing the dust that rose above the pallets during the stacking process. The small transfer point exhaust port was also used because it was effective in capturing the dust generated from the bag valve, which was shown to be a substantial dust contributor in test No. 1. The dust reductions achieved with this final version ranged from 65 to 95 pct at all sampling locations. These reductions are substantial, considering that a 2,100-cfm fan was used, which changed half the air in the railcar and trailer truck in 1 min. It is obvious that the use of a larger fan would increase the efficiency of the system. However, the added efficiency would have to be weighed against the accompanying increase in capital and operating costs. It is anticipated that a mineral processing plant, that installs a system similar to the one recommended here, as a permanent dust control technique, would run the exhaust into a baghouse dust collector. The only constraint on the system would be size of the tubing to be located under or over the snake conveyor.
The evaluated exhaust ventilation system required minimal capital and operating costs. The approximate cost of the system was as follows:
The only operating cost was the power needed for the 5-hp motor necessary to drive the fan. If the system is used in conjunction with a baghouse system, the incremental cost to operate the baghouse would also have to be considered. If a baghouse is not available to filter the dust pulled from the vehicles, some type of filtration system might be necessary. About 8 worker hours were required to set up the system. It required from 5 to 10 min to install the exhaust ventilation system into each enclosed vehicle before loading could begin during the field evaluations. Since there is always a down time between the loading of each vehicle, this did not affect production. As the vehicle was loaded and the snake conveyor was continually backed out of the vehicle, brackets and tubing were removed from on top of the snake conveyor to avoid a clearance problem with the mill building. For actual use in a plant, a more permanent installation could be developed. It did not appear in the Bureau's interest to pursue a more permanent installation or a deployment system since every plant would require its own design.
The exhaust ventilation system described in this report effectively lowers respirable dust concentrations when bags of product material are loaded directly into enclosed vehicles by workers. The final and recommended design exhausts about 2,000 cfm through a 12-in-diam tube located 3.5 ft in front of the slinger at a height of 6.5 ft. A small exhaust tube is used at the bag valve side of the snake conveyor-slinger transfer point to capture the dust generated at this location. When this system was used during loading, respirable dust reductions ranged from 65 to 95 pct in both railcars and trailer trucks. The system involves minimal equipment, installation, and operating costs, and can be modified by mineral processing plants for permanent installation at individual operations.
The technique of using a curtain arrangement to block off the half of a railcar that was not being loaded was evaluated. A brattice mining cloth was attached to two 5-ft-long pieces of 2- by 4-in lumber. Using extender poles to secure the 2 by 4's to the ceiling, a simple effective curtain barrier was installed in a matter of minutes. The curtain was designed to improve ventilation by preventing the airflow from traveling back into the half of the car that was not being loaded. A barrier curtain would only be applicable with blowing ventilation systems when loading railcars. Tests were performed comparing the effectiveness of different blowing ventilation systems with and without the curtain. The results showed that use of the curtain barriers produced no measurable difference in methane tracer gas concentrations. Table A-1 shows the average methane tracer gas concentration for the four sampling positions and the percent reduction in methane tracer gas levels with the curtain in place. There was a 4-pct increase in the gas concentration measured at the stacker location with the curtain in place when the results were averaged together, but it is believed that this increase was due mainly to sampling error.
4Reference to a specific manufacturer does not imply endorsement by the Bureau of Mines. 5Williams, K. L., and R. J. Timko. Performance Evaluation of a Real-Time Aerosol Monitor. BuMines IC 8968, 1984, 20 pp. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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