Engineering Controls Database
Silica Dust Exposure Control in Industrial Sand Processing Operations.
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The operations in the case report supplies washed sand mainly to the glass industry but also for other industrial uses. Reports from three sites were used to create this summary. Site A: In the first factory, dredges are used to mine sand from flooded areas under the topsoil, typically reaching depths of 50 feet. The sand/water slurry is pumped to a scalper to remove rocks and slime from the slurry. It is then pumped to holding tanks. In the process building, the sand is sized by settling (coarse fraction removed), dewatered (clay removed), passed through a scrubber, again dewatered by two parallel screws, and then passed through a flotation circuit (iron impurities removed). A ball milling step can be added to the process for size reduction. One operator and one helper work in the processing building. After processing, the wet sand is conveyed outside and stockpiled in the open. A front-end loader moves the sand to a covered belt conveyor, which transports it 1,000 feet to the sand drying and loading area. Damp sand discharges from the covered belt conveyor into a bucket elevator which transports the sand to the dryer. The sand is dried, screened, and transported by a second bucket elevator to a screw conveyor which empties into one of three concrete silos. Sand can be loaded directly from the silos through one of three uncontrolled (non-ventilated) loading spouts into railroad hopper cars or one uncontrolled loading spout into trucks. Sand is also transported by screw conveyor from the bottom of the silos to a third bucket elevator which empties into two elevated hopper bins each equipped with a controlled (ventilated) loading spout. One spout is used for finish filling of railroad cars and the other spout is used for filling trucks. (Typically, the uncontrolled spouts are used to pre-fill railroad cars and the controlled spouts are used for the final filling of railroad cars and the complete filling of trucks. The uncontrolled spout is normally not used to fill trucks.) Total railroad and truck shipments represent approximately equal tonnage. The truck loading area consists of a bulk sand hopper bin, an uncontrolled loading spout (not in use), a controlled enclosed-type retractable spout, truck scales, and an enclosed control room. One operator spends the shift filling several different types of trucks at this station. (The same operator also fills railroad cars.) The job sequence is as follows: An empty truck stops on the truck scales beneath the loading spout. The loader operator, from within an elevated control room, lowers the spout either into the open truck, down to the hatch on a hopper truck, or down to the center rib of cross ribbed truck and starts the sand to flow. In open trucks, the spout remains a few inches above the top of the sand pile in the truck and is retracted as the sand pile rises, When the sand pile reaches the desired height, the sand flow is stopped, the truck moves forward a few feet, the spout is lowered, and loading resumes. This process is repeated until the truck is filled, with the load being distributed throughout the truck. For hopper trucks, the spout remains a few inches above the open hatch. This permits visual observation from the control room into the truck, so as to reduce the risk of spillage from overfilling. As the truck fills, the sand flow is stopped, the spout raised, the truck moved forward, the spout lowered down to the next open hatch, and loading resumed until the truck is filled. For trucks with ribs (no longitudinal rib) the spout is usually lowered between the ribs and filled like the open truck. When the sand reaches the desired height in the truck, flow is stopped, the spout retracted above the ribs, and the truck moved forward. The spout is lowered again and loading resumed. For trucks with a longitudinal rib, the spout is lowered down to this rib and the sand flows over this rib into the truck. As the truck fills, sand flow is stopped to allow the truck to move forward, and resumed until the truck is filled. For open top trucks, a tarp is stretched over the trailer and tied down before leaving the site. It takes approximately 5 minutes to load 50,000 pounds (500 cubic feet) of sand (an average load for most trucks). The railroad car loading area consists of three uncontrolled loading spouts, a bulk sand storage hopper bin, a controlled enclosed-type retractable spout with horizontal travel capability, scales, and an enclosed control room. The same operator that fills trucks also fills several different types of railroad hopper cars at this station. The job sequence is as follows: Prior to filling, the hatches are opened; the cars inspected and if needed, washed out; and then positioned alongside the silos. Preliminary filling is done from an uncontrolled spout (a flexible hose with a metal pipe fitted with a gate) directly from one of the three silos. The spout is positioned over the open hatch and sand flows by gravity into the car. When the portion of the car under the open hatch is filled, the flow is stopped, the car moved forward, the hatch just filled through is closed, and the flow resumed into the next open hatch. It takes approximately 45 minutes to load 100 tons of sand from the uncontrolled spout. When the railroad car is nearly full, flow is stopped, all hatches except the last one filled through have been closed, and the car is moved forward under the controlled spout. The operator, standing on an open platform overlooking the top of the car, positions the spout over the open hatch and lowers it to the hatch. Then, the operator, from the enclosed control room at ground level, adds the last of the sand to complete the car's load. The spout is retracted, the hatch closed, and a front-end loader moves the car to a pickup area. It usually takes less than a minute to top off each car using the controlled spout. The front-end loader operator operates an enclosed cab rubber-tired loader at different locations throughout the facility. This worker may transfer damp sand from the stockpile to the covered belt conveyor, move empty and filled railroad cars, and whatever else may be needed. The major hazard associated with this operation is inhalation of crystalline silica during certain dry operations (loading and filling, screening). Dust sources were located near the screen, the top of the dryer, at the bucket elevator discharge into the screw conveyor at the top of the silos, and on top of the hopper bin above the truck scales. Short-term real-time measurements identified work practices that could affect dust exposure in the truck loading area. Real-time measurements show that the proper use of the spout can greatly reduce the amount of dust generated during truck filling. The type of truck being filled is a major factor in the effectiveness of the spout. Trucks with longitudinal ribs defeat the purpose of the loading spout by not allowing it to be lowered past the ribs to the bottom of the truck. As a result, sand flows over the rib, is dispersed, and entraps air as it free falls into the truck, thus increasing the amount of dust generated. This can also be a factor in cross rib-type trucks when the spout is positioned over a rib. When the spout is lowered to the bottom of the truck and maintained near the top of the sand pile as it discharges into the truck, the free fall distance through open air is reduced to a few inches, thus reducing the amount of air entrapped and the resultant dust generated. When loading hopper trucks, sand free falls up to 8 feet inside an enclosed container which contains most of the dust generated. Both uncontrolled (nonventilated) spouts and controlled (ventilated) spouts are used at this operation. A comparison of the two types of spouts indicates that potential dust exposures during loading operations could be reduced by 90% with the elimination of the nonventilated spouts. During the sampling visit, the highest area concentration (0.77 mg/m3 respirable quartz) occurred on the catwalk on top of a railroad hopper car during filling using an uncontrolled loading spout, indicating the need for revision of this operation. In the sand screening area, one of three samples exceeded the NIOSH REL (0.07 mg/m3); none exceeded the MSHA PEL. Site B: The factory making up site 2 no longer contains sand suitable for use in glass manufacture; instead foundry and multipurpose sands of a wide range of sizes are produced. Sand is shipped from the plant via truck (rather than by truck and rail) in bulk or in bags. Sand is dredged from ponds and pumped to a holding tank. The sand is washed, then classified according to size by a flume, which transfers the sized product into storage piles. Sand from storage is moved by front-end loader to one of two somewhat different drying plants. Dryer #l is a fuel oil-fired rotary kiln equipped with a wet scrubber. Sand from dryer #l is classified by one or more of 12 vibrating screens, discharged to bins located below the screens and above the loading area. A blending site is also located beneath the bins adjacent to truck loading. Sands are blended by the manual opening of slide gates, allowing one or more size sands to fall onto a belt conveyor. An infinite range of sand sizes in the final product are possible. The blended sand is conveyed to hoppers and loaded into trucks by gravity feed. The blending and loading operation at dryer #l is a major source of sand (quartz) emissions at this plant. Dryer #2 consists of a series of three fluidized bed dryers discharging to one of two vibrating screens. Screened sand is conveyed to one of five silos and loaded into trucks using a ventilated loading spout, enclosed in a separate, drive-through enclosure. Alternatively, dried sand can be conveyed to the screening operations located by dryer #l. Air from the fluidized beds is cleaned in a bag house, and the waste dust pneumatically transported to a dust disposal station. (A ventilated loading spout is used at the disposal station). Although dust exposure is unlikely in the wet processing areas in this plant, the drying and loading operations are areas of potential overexposure in this and other plants within the industry. The bulk loading area in dryer plant #l is particularly troublesome. Unvented truck loading, conveyor transfer points, and blending points are located in a small, confined area. Site C: The plant produces foundry sand, glass sand, and 230 mesh ground sand for pigments. Foundry sand is a newer product and has been produced only since 1987. Sand is shipped from the plant in bulk via truck or rail. The plant employs 48 workers and operates on three shifts, but is closed on weekends. Operational areas include dredging, wet processing, drying and bulk loading, and maintenance. In addition, the plant has a small milling operation in which we were particularly interested. Sand is dredged from ponds 5 miles from the processing plant and pumped to holding tanks. The sand is then sent to the wet process area, where it is sized in centrifuges, washed in screw classifiers and scrubbers, cleaned of fines and clays using rake classifiers, and cleaned of iron by chemical flotation. The sand then passes through a hydrocyclone, is dewatered by vacuum filtration, and transferred to the damp storage tanks. The wet operations are performed under roof. The sand is next dried in a rotary kiln, classified by one or more vibrating screens, discharged to an elevator, and bulk loaded into either rail cars or trucks. Plant employees load only the rail cars, while the truck drivers load the trucks. Both ventilated and unventilated loading spouts are used. A portion of the sand from the screens is diverted to the milling process. One pebble mill is operational, while the other mills are permanently shut down. One worker is assigned to the mill and he spends most of his time in an isolated control room. As with Sites A and B, since the dredging and processing operations at Site C are performed wet, the greatest silica exposure hazard appears to occur in drying and loading. |
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| Overexposure to crystalline forms of silica causes the lung disease silicosis; symptoms include cough, shortness of breath, chest pain, weakness, and wheezing. Silicosis usually occurs after years of exposure, but may appear in a shorter time if exposure concentrations are very high. This latter form is referred to as rapidly-developing silicosis, and its etiology and pathology are not as well-understood. Silicosis is usually diagnosed through chest X-rays, occupational exposure histories, and pulmonary function tests. Since the dredging and processing operations in the sand mines are performed wet, the greatest silica exposure hazard would appear to occur in drying operations as well as moving and loading of dried product. | |
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Site A: In the processing plant, the operator works in an air-conditioned control room; and in the drying and loading areas workers spend considerable time in air-conditioned sheds. Roof fans provide dilution ventilation in the processing plant. No local exhaust was utilized. In the drying plant, air from the fluidized bed dryer is exhausted through a wet scrubber. This scrubber also exhausts the first bucket elevator housing. Since the supply fan for the dryer is not connected via a control system to the scrubber fan, the dryer operator is required to monitor a pressure gauge to keep the dryer under a negative gauge pressure and set the fan dampers accordingly. A bag house sitting atop one silo provides bin venting for the silos and exhaust for the final bucket elevator and the loading spouts. The loading spout exhaust is not connected directly to the bag house, but to the elevator housing, the elevator housing serving as an exhaust plenum. The loading spouts consist of a central flexible hose, approximately 12 inches in diameter, which serves as a conduit for the sand. It is surrounded by a larger hose which terminates in a funnel. The funnel can be lowered into a rail car or tank truck hatch, and provides a source of exhaust for air entrained/displaced by the loading process. For open truck loading, the funnel could function as a capture hood, although its effectiveness would diminish. The controlled spouts' exhaust ventilation rates are inadequate. The manufacturer of these spouts recommends an exhaust volume of 800 to 1,000 cfm for a sand loading capacity of 200 cfm (the facility’s sand loading capacity is about 100 cfm.). The measured air volume of the controlled spout for truck filling was 400 cfm and 220 cfm for the railroad car loading spout. A new duct system connecting these units directly to the existing dust collector was recommended for installation. It was recommended that the facility replace the uncontrolled spouts with a controlled enclosed-type spout. From air samples and real-time measurements, a controlled spout could reduce dust emissions by about 90%. This reduction could be even greater when the exhaust volume of the spout is increased to the manufacturer's recommendation and good operating practices are used. Controlled loading spouts for enclosed railroad cars are available and could be adapted to this operation. Good work practices during sand loading operations should be developed for using the controlled loading spouts. When filling open-type trucks, it is important to keep the spout discharge near the top of the accumulating sand pile, retracting the head of the spout as the truck fills so as to minimize dust generation. When filling any vehicle having ribs, trucks or coffin-lid-type railroad cars, the sand should never be allowed to flow from a spout over a rib into the vehicle. For trucks with ribs, the controlled spout can be lowered between the ribs to maintain the spout near the top of the sand pile. If the spout cannot be lowered between or past the ribs, the ribs should be removed prior to loading. For coffin-lid cars, care should be taken t o position the spout so as to avoid sand flowing over the rib. (Hatch-type hopper vehicles do not have cross members and do not present this problem.) When filling hopper- type trucks, the controlled spout head should be lowered as close to the hatch as possible, yet that flow of sand can still be seen so that the fill can be stopped before overflow occurs. Also, all hatches on hopper-type vehicles should be kept closed except the one being used. Site B: A 25,000 cfm wet scrubber (in addition to the wet scrubber used for the rotary kiln) provides dust collection for the screens, bin room (sand bin covers were also added), and bulk loader conveyor in dryer plant #l. In dryer plant #2, the same bag house that exhausts the dryers provides local exhaust for the belt conveyor transfer points and the two vibrating screens. In both areas, air-conditioned (window units) control rooms provide isolation for part of the work shift. A ventilated loading spout is used for bulk loading in dryer plant #2. This spout is isolated in a separate building and an automatic sampling system is employed to minimize exposure in that operation. The loading spouts consist of a central flexible hose, approximately 12 inches in diameter which serves as a conduit for the sand. It is surrounded by a larger hose which terminates in a funnel. The funnel can be lowered into a rail car or tank truck hatch, and provides a source of exhaust for air entrained/displaced by the loading process. For open truck loading, the funnel could function as a capture hood, although its effectiveness would diminish. One company-owned dump truck has been retrofitted with a custom-designed load cover to close in an otherwise open truck. No local exhaust is provided in the bagging area; however, the bagging operator works in a filtered air shower, the "OASIS" system developed by the Bureau of Mines. No fresh makeup air is provided anywhere else in this building. In all three areas, centralized vacuum systems assist in housekeeping. HEPA filter equipped vacuum cleaners are available to clean operator control rooms in both dryer plants in Site B. Site C: The screens and conveyor belts in the dry processing area at this plant are covered and equipped with local exhaust ventilation. A central exhaust duct is fed by individual ducts from each of the screens. In general, the ventilation systems in the plant appeared to be well designed, following standard ventilation design practice, and appeared to be well maintained. Some of the loading spouts for truck loading are equipped with 12-inch diameter concentric ventilated ducts. These ventilated spouts are similar to those we observed at another sand plant, however, the exhaust air volumes of 800 to 1,600 cfm, reported to us by plant personnel, are much higher than the air volumes we measured at the other plant. Also, the site will not accept trucks for loading sand with the ribs up. (We observed during an in-depth survey at another facility that pouring sand over the ribs was a source of dust emissions.) For trucks with manhole openings, the spout must extend into the truck and a second truck lid must be open to provide makeup air during loading. The company has almost completed installation of a fully-automated unloading system with ventilated spouts for foundry sand. |
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| 171-12A; 171-12B; 171-13A; 171-14A; | |
| 212322 | |
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aerosol photometers aerosol photometers glass sand mining glass sand mining Industrial sand Industrial sand loading spouts loading spouts manual materials handling. manual materials handling. real-time monitoring real-time monitoring respirable dust respirable dust silicosis silicosis |
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Site A: Wet operations (dredging, washing, etc.) at the case study facility control dust by their nature. Dry operations, which presented the greatest opportunity for exposure, incorporate engineering controls which are effective by removing workers physically from dust-producing activities in rooms or sheds and by providing dilution ventilation. Exhaust air from the drying operation is wet-scrubbed. As two different methods are available for loading cars, this permitted a comparison of the two types of spouts which indicated that potential dust exposures during loading operations could be reduced by 90% with the elimination of the nonventilated spouts. Site B: The authors of the site report for this operation felt the excellent housekeeping and the local exhaust in the screening area of dryer plant #l, and the use of ventilated loading spouts in bulk loading and bag house dust disposal were particularly noteworthy. The plant was reported to have an apparently excellent health and safety program that had made significant progress in reducing dust exposures. Site C: The authors of this site report noted particularly the local exhaust ventilation of the dry processing area, the fact that ventilated loading spouts with healthy exhaust volumes were used in the loading operation (and that the newer, foundry sand operation would soon be using ventilated, automatic spouts exclusively), that for trucks with manhole lids, the spout had to extend into the truck with a second lid open (if available) for ventilation and that it was site policy not to load trucks with their ribs up because this was a known dust source. |