TN 295 No. 9005 *- ^ o* • «S ; : > * f-g *5 »l!nL'» > o»* .isst.. A .S.'X&.X cp*.-jki..% > 4 \.^.\ o°* c ^ ;• ^ -^•.•^W, o* V" '©,. ♦•^•* ^ < <$> *o«o a ^> 'A/mA;#AM.A. : W ; .A ^^ •A\V2S.//'lo AT vv & v ^ *'7^t* a <> •«.■;*" A 6 v ^ .^t;t*' a c'*^»"i*"^o v ^o *^t*' a ^, * * ^ *o. /.-i-^i-X ,o°..^ait. o ./.-i^'\ 5>* -.5 -. * ^ :£w&>^ +M& z£*mx* ^& *£nm>s. ^^ ^0^ *bV" ^\. p %/ : Jill'- \f : '^% P ° ?/ ^^»° : ^ ' , ^%'' l/ aP-nK '*•.«' A v^?v v^v %^v v^-y v 1 l0 "^ h* •& A ^5°. ?• .n _» • :^ '•■ /°* : ^ A A v^ ^ \, A v <^ -o.»* G^ ^ ♦**?;?» A <^ -o.T* 0^ V *7f.T* A 200.00 1 .33 .29 .42 .32 21.87 .13 .06 .07 .07 >89 Bag conveyor to dock conveyor transfer point Bag room air intake (from loading dock area) 61 79 83 78 *RAM off scale. ro 2.0 E 55 en LU o o o h- C/) Z> Q Operator's lapel, 325 mesh Conventional system. FIGURE 7. - RAM strip chart showing dust levels using new nozzle and conventional nozzle. ''Dashed line is threshold limit value.) 13 The new filling hardware does use a time delay to clean the fill nozzle. The addition of a 7-s time delay did not have a significant effect on productivity. Figure 8 shows that an extra 7-s delay per machine is actually distributed over the fill time of four machines: Delay per bag = Time delay per machine Number of machines ^ = 1.75 s. Actual timing of the old and new hardware (table 5) confirms that the new hardware adds about 1.5 s to the bag-filling cycle on a four-station machine. Other produc- tivity considerations that were not quan- tified include reduced cleanup around the machine, reduced product loss, less bag- house dust loading, cleaner bags, and cleaner workers. Foster-Miller, Inc., is now marketing this new hardware and has applied for a patent. As testing of the system continues, some areas of wear and maintenance of equipment are being noted. A second unit recently installed correct- ed some of the inherent problems encoun- tered in the first unit. The performance and operation of the Foster-Miller bag- ging system will continue to be improved. TABLE 5. - Field time study of new self -cleaning nozzle Additional Product Actual time Additional time to grade, to fill 50 time per load truck mesh bags , sec bag, sec with 480 bags , min 120.... 70 1.4 11:12 180.... 83 1.6 13:17 325.... 76 1.5 12:00 Once product material is packaged, the objective is to prevent spillage and bag breakage. The design of the bag valve plays an important role in reducing spillage during bag handling. The basic bag valve consists of a 3- to 5-in tube of kraft paper inserted into the bag. This tube or sleeve receives the nozzle during the filling cycle and collapses when the bag falls onto the discharge conveyor. Product material is frequently Time to fill 4 bags with cleaning delay Time to fill 4 bags, no cleaning Total time added to cycle i TIME, arbitrary units FIGURE 8. - Sequence of machine operation showing effect of cleaning-time delay. trapped within this sleeve and, as the stiff kraft paper flexes during handling, product dribbles from the bag valve. A better bag valve uses a thin polyethylene tube in place of the kraft paper. This design still traps product within the tube, but the flexible plastic is less prone to flexing open and spillage is re- duced, although not eliminated. Foster Miller attempted to develop a mechanical bag valve seal that was com- patable with existing bag-manufacturing equipment (11). Their idea used a two- component sleeve where a stiff preformed plastic acted like a spring to draw a more flexible plastic sleeve closed. In shipment to the plant , it was found that the preformed plastic sleeve lost its shape and did not perform as designed. Champion International Corp. recently introduced a unique leakproof poly- ethylene bag valve (called "Sift Proof Valve"), which consists of a sealed tube with a slit facing the bottom of the bag (fig. 9). During filling, the polyeth- ylene valve opens to direct the flow of product downward to the bottom of the bag. The force of the product flow stretches the polyethylene during fill- ing. The stretched polyethylene then overlaps after the bag leaves the ma- chine. Preliminary reports suggest that the sleeve does help to reduce spillage. A patent has been issued on this development. 14 , v ;■ ■.,,■,■..:■.■.;. ».J? % » . « ■■ ■ ■ ■ ■f%&j^*#**i^J^'*#^* ^ iii )to'i wt sfi&mm \\ ~ ' mL- 'fxnw- FIGURE 9. - New leakproof polyethylene valve. Bag breakage can be a substantial source of respirable dust, especially with fine product sizes. Use of stronger bags can reduce breakage and customer complaints while increasing productivity. The records of one company (table 6) in- dicate that the frequency of broken bags can be reduced when a new three-ply , 50- lb tension-free-dried paper (also called "free-dried" paper) is used instead of a standard three-ply, 60-lb natural kraft paper. Free-dried paper gets its in- creased strength and resiliency from the fact that it is allowed to stretch in two directions rather than one when being manufactured. Bag breakage was reduced by a factor of 12. Based on the record, this industrial sand plant now specifies this type paper and finds the cost to be comparable to the previous type. 15 TABLE 6. - Operators record of bag breakage using free-dried and natural kraft paper bags Paper Product Number of bags loaded Number of bags broken Breakage , pet Sand 8,540 7,080 4,420 2,250 2-530 5 30 1 15 29 0.06 Natural kraft.. Free-dried. .... Natural kraft.. • • • CLO •••••••• • ••QO»* •••••• Ground silica .4 .02 .7 .4 WATER USE Use of water in a bagged, dry product to control dust has not been widely prac- ticed in the past. However, its use has been reexamined as a means of reducing worker exposure and simplifying complex engineering and administrative controls that are often required. Naturally, wa- ter cannot be used (1) with materials that react chemically, (2) or in materi- als where its use produces handling prob- lems , or (3) where it is unacceptable to the customer. However, there appear to be a number of applications where wa- ter can be of benefit in reducing dust levels. Water can control dust two distinct ways. First is suppression, which works by wetting product materials and causing the dust to adhere to the product or other dust particles; this prevents it from becoeing airborne. The second is airborne capture, in which water droplets collide with dust particles in the air, increasing their size and causing them to drop from the airstream. Most of the following experiments using water were designed to suppress dust rather than to capture it from the air. Early work conducted in an industrial laboratory studied the effects on flowa- bility and dust suppression by adding various quantities of water to 325-mesh silica. All samples were uniformly blended in a zig-zag blender (continuous feed). The flowability of the silica was then graded on a scale of 1 to 5, with 5 being optimum. Samples were then dropped from a height of 4 ft onto a paper placed on a black surface. The resulting dust cloud was judged by a three-person panel and graded 1 to 5, with 5 being the least dusty. The results of this study are summarized in table 7. From a material- handling viewpoint, no more than 1 to 2 pet moisture should be added. It appears that some moisture can help the flowabil- ity of the sand by removing the static charge found in the totally dry sand. Dust suppression did not occur, however, until moisture rose over 3 pet. TABLE 7. - Effect of water on flowability and dustiness of 325-mesh ground silica Sam- Added water, Flowability, Dustiness, ple wt pet grade 1 grade 2 1 1 1 2 1 5 3 3 2 4 3 4 3 2 5 5 4 1 5 6 5 1 5 trades 1-5; 5 is best flow. 2 Grades 1-5; 5 is least dustiness. The recent Bureau of Mines contract with Martin Marietta Laboratories (7) studied wet collection-and-suppression systems on a roll crusher to belt trans- fer point in a crushed limestone plant. Although this plant was not producing dried mineral product, the results are applicable to dried minerals operation. Martin Marietta measured the dust control achieved using air-atomized water sprays at the crusher discharge and a simple hydraulic water spray at the crusher in- let. The results showed that the water added as a mist, after crushing, was 55 to 65 pet effective in collecting respi- rable size dust. Adding water to the ore before crushing and allowing it to mix 16 thoroughly within the crusher suppressed respirable dust 70 to 80 pet. Using both systems together the effectiveness in- creased 80 to 95 pet. Two facts of interest from this study are that mixing water with the ore in the crusher provides good dust suppression, and that even the best airborne dust cap- ture spray (air atomizing)^ did not have sufficient contact time with the dust to be most effective. In the past, researchers have tried various types of foam to suppress dust in coal mines ( 13-14) , spraying foam on top of the coal much like water spray. The results showed that foam was not much better than a good water spray under the test conditions. New interest in foam systems for minimal moisture addition to dried products, combined with manufactur- ers claims of superior dust control using foam, led the Bureau to test the effec- tiveness of foam at two industrial sand plants. These studies differ from previ- ous work in that the foam was thoroughly mixed into dried product materials. Results in table 8 show that as foam is mixed with 30-mesh glass sand from a transfer point to transfer point, its dust suppression effectiveness increases. Additional testing using more foam showed "For details on the comparison of spray nozzle effectiveness for airborne cap- ture, see Bureau of Mines Technology News 150 (12) . dust reduction of 80 to 90 pet on three separate occasions at two different plants ( 15 ) that process whole grain sands. Limited work has been conducted on the applicability of foam in ground silicas. Although visual tests have in- dicated that foam has good potential, no quantitative studies have been conducted. Before foam can be considered as a dust control for whole-grain or ground silica, the following must be considered: • The foaming surfactant must be com- patible with the end uses of the product. Ultrapure grades of cer- tain products cannot tolerate even a few parts per million of surfactant. • Foam generators must be easy to con- trol and regulate. This is espe- cially critical if minimum moisture levels on the order of a few tenths of a percent are to be maintained. • Evaporation reduces the effective- ness of treatment. This is more likely to be a problem at high prod- uct temperatures. • Foam is relatively expensive. The average amount of surfactant per ton of sand treated was 0.012 gal. At a surfactant cost of $7.25/gal, the cost to treat each ton of sand is $0.09 (exclusive of capital and pow- er costs). Depending on usage, this number can vary from a low of $0.04/ ton to a high of $0.20/ton. TABLE 8. - Dust reductions as foam is mixed at transfer points Location and condition Av . of 4 filters, mg Std. dev. of 4 filters Dust reduction, pet Transfer point 1: 7.41 5.95 5.59 3.76 6.69 2.46 1.83 .87 .10 .05 .29 .10 1 ,o -, 19.7 Transfer point 2: J 1 oo n 32.7 Bulk loadout: J "1 « o 65.3 17 Foam appears to work by uniformly add- ing small amounts of water to the dried product material. The surfactant used to generate the foam has no measurable ef- fect on dust suppression, but is only necessary to increase the surface area of the water, thus distributing it uniformly into the product. Studies showed that the important dust- suppressing properties of foam are the addition of moisture, large water surface area, and good mixing with the dusty ma- terials. Since none of these properties is unique to foam, the Bureau decided to try fine mist sprays and steam for dust suppression ( 16 ) . Previous work by the Bureau with steam had focused on airborne collection rather than on suppression (17). The new experiments were conduct- ed at two industrial sand plants where an engine-cleaning-type generator made steam, which was added whole grain sand and mixed at a conveyor belt transfer point. Results showed that for an equiv- alent amount of water, steam was twice as effective as water in suppressing dust. Even with poor mixing and the preliminary nature of these tests, average dust re- ductions using steam were about 65 pet when adding 0.22 wt pet moisture. Steam will not contaminate the product material and is easily controlled; however, de- tailed engineering will be required to design a mixing system to prevent conden- sation and material buildup. The heat needed to make the steam also can be ex- pensive; estimates range from $0.06 to $0.08/ton of material. A use of water in bagging operations was tried at an industrial sand plant, where a company-designed air-atomized spray system was used to wet the nozzle area of the bag during filling and trans- port. The company also experimented with injecting water into a ground silica product as it flowed into the bag. The Bureau monitored the resulting dust re- ductions (18) . Generally, these methods reduced dust by about 50 pet, but of equal significance was the fact that wet- ting the bag valve during filling reduced dust levels by 50 pet inside rail cars where the bags were being palletized. Injection of water into the product was discontinued after the experiment because of customer complaints regarding frozen shipments of sand. Currently, the out- side surface of the bag is still being wetted. Foster-Miller, through Bureau of Mines contract ( 19 ) , compared the effectiveness of using electrically charged water sprays and uncharged water sprays for airborne capture of dust. Figure 10 shows that, regardless of the charge or type of dust, charged sprays were supe- rior to uncharged sprays. Charged water sprays were found to — • Be the best per unit use of water for airborne capture of dust. • Require a long residence the dust to capture it. time with o o o UJ o -z. o o (/> CO UJ o I0" 3 I 2 3 TIME, min FIGURE 10. - Effect of charged-water sprays on dust concentration in dust box. 18 • Need water free of suspended solids. • Not able to be used where static electrical sparks are a hazard. Using water for dust control for dried mineral products is still in the experimental stage, but results to date indicate that it is potentially an effective tool for the dust control engineer to consider. WORK PRACTICES AND HOUSEKEEPING The best engineering technology cannot keep plant dust levels low if the work- place is not kept clean and orderly. In much the same way as a strong safety program requires education and support from management, a strong housekeeping program requires the same commitment. The following work and housekeeping practices are essential to good dust control. Removing accumulations of dust from floors and ledges in the work area re- duces the amount of dust that can become airborne through vibration or wind cur- rents. The data in table 9 compare respirable dust from work area samples taken in a screen tower before and after cleanup. The cleanup reduces exposures by 75 pet of their bef ore-cleanup level. In this instance, the exposure is still high and would require the use of respirators. TABLE 9. ■ samples Effect of cleanup on dust Sampling period h. . Total respirable dust. mg/m 3 . . Respirable quartz. .mg/m 3 . . Before After clean- clean- up up 4.0 5.5 6.69 1.40 1.46 0.35 Use of brooms and shovels to clean up spills and broken bags must be avoided since large amounts of dust will be generated. Figure 11 shows the exposure of a bag machine operator when a worker on the floor below was using a broom to clean up. Vacuum systems represent the preferred method of cleanup. Although available performance data are limit- ed, most vacuums are centrally located systems that can service large areas throughout a plant. One satisfactory 4 6 TIME, min FIGURE 11. - Effect of broom sweeping on bag-machine operator's exposure. 10 19 system produces 600 SCFM at 8 in Hg. This system is used for general house- keeping, and to clean up equipment prior to maintenance operations. However, be- cause it is slow picking up broken bags of material, and a larger system is rec- ommended for this application. Many new plants are being designed for washdown. This design includes proper drainage and protection of the electrical system. In some cases, plants have been able to retrofit existing buildings for washdown. In cases where washdown is not possible, floors have been mopped or sprayed with mineral oil products to help capture and retain dust that collects. Dust exposures are often high where bags are manually handled, stacked, or palletized. When pallets are loaded into enclosed spaces such as box cars or en- closed trailers, exposure can be high. Box cars and trailers should be cleaned with a vacuum before loading. Spills and broken bags must be cleaned up promptly. When bags are palletized manually, work- ers should place the bags rather than throw them. Workers' practices in caring for their work clothes and personal cleanup after work deserve attention. Air hoses must not be used since such use resuspends dust in the air and is inherently danger- ous. Although use of company-supplied uniforms is not widespread in the indus- trial sand industry, it has been used as a part of a program of company relations and good housekeeping in some bagging industries. MSHA requires posting and the use of respirators in all work areas where res- pirable quartz exposures exceed the per- missible exposure level of 0.1 mg/m 3 . By rotation of work assignments, it is often possible to minimize workers potential exposure. Workers bagging or loading ground silica might rotate jobs with workers bagging whole-grain silica or performing other duties. An essential ingredient of good house- keeping and good work practices is an or- ganized program that starts at the high- est levels of company management and makes each level responsible for the one below it. Consequently, the plant mana- ger and shift supervisor bear primary responsiblility, but they must have the support of top management. Housekeeping duties must be performed on a timely basis. Spills and broken bags must be cleaned up right away. De- pending upon conditions, other areas may need to be cleaned daily, weekly, or even monthly. As a part of this regular housekeeping, the ventilation system must be maintained on a regular preventative maintenance schedule. Air velocities at pickup points should be checked on a reg- ularly scheduled basis just like other maintenance checks are made on such items as blowers, controls, materials-handling equipment, gates, valves, and vehicles. Leaks and obstructed ducts must be de- tected before system performance suffers. The dust in the air around the plant constitutes a significant proportion of the total dust exposure. Although ground silica is 100 pet quartz, the quartz con- tent of respirable dust samples taken around bagging operations is frequently less than 50 pet and sometimes as low as 10 or 20 pet; the remainder is background dust. Significant amounts of background dust can come from the discharge of ventila- tion systems, bag houses, and wet scrub- bers. Careful attention to the location of ventilation discharges and the dis- charge from dust collection equipment can help reduce background dust levels. In one instance, extending stack heights to 1-1/3 times the height of adjacent build- ings (fig. 12) placed the discharged dust above the natural turbulence that is de- veloped around and downwind from the building, background dust levels were significantly reduced. 20 FIGURE 12. - Stacks to remove dust from plant dust=col lector discharge. Unpaved roads and worked out portions of pits are potential sources of back- ground dust. Unpaved roads can be sprayed with water and/or commercial dust treatment compounds. Water is an econom- ic solution only on temporary roads. The commercial treatment compounds are better in areas of moderate rainfall. Paved roads and parking areas may require the use of an ordinary street sweeper or automatic sprinkling system to keep them clean. A program of reclamation by planting may be used to avoid excessive pollution from runoff and will help con- trol potential dust during dry periods. SUMMARY Dust levels in the packaging areas of plants that process dried mineral products can be difficult to control. Three primary dust control techniques are available: (1) Ventilation of the area (both exhaust and clean makeup air) , (2) redesign of hardware to produce less dust, and (3) careful addition of water (under the proper circumstances). Administrative controls can be used to reduce a workers time in a dusty area and reinsure that the workplace is kept clean and orderly. The techniques summarized in this paper and described in detail in the references cited can be effective in reducing dust exposure in the bagging of industrial mineral products. REFERENCES 21 1. National Industrial Sand Associa- tion. Guidance and Solutions to Reducing Dust Levels in the Bagging of Whole Grain Silica Product. Silver Spring, MD, 1977, 35 pp. 2. U.S. Office of Management and Bud- get. Titles and Descriptions of Indus- tries. Part 1 in Standard Industrial Classification Manual. 1972, p. 40. 3. American Conference of Governmen- tal Industrial Hygienists. Industrial Ventilation. Edwards Brothers, Ann Arbor, MI, 16th ed. , 1980, pp. 6-1 to 6- 42. 4. U.S. Bureau of Mines. Dust Con- trol for Bag Filling Machines. Technol. News, No. 54, June 1983, 2 pp. 5. Thimons , E. D. , R. J. Bielicki, and F. N. Kissell. Using Sulfur Hexa- fluoride as a Gaseous Tracer To Study Ventilation Systems in Mines. BuMines RI 7916, 1974, 22 pp. 6. Vinson, R. P., J. C. Volkwein, and E. D. Thimons. SF 6 Tracer Gas Tests of Bagging-Machine Hood Enclosures. BuMines RI 8527, 1981, 10 pp. 7. Mody, V., W. Harris, R. Jukhete, D. Goldheim. Conveyor Belt Dust Control. Final report on BuMines Contract HOI 13007 with Martin Marietta Lab. , February 1984, 150 pp.; Richard Wilson, Twin Cities Research Center, BuMines, Minneapolis, MN. 8. U.S. Bureau of Mines. Air Curtain Provides Dust Protection. Technol. News, No. 21, Jan. 1976, 2 pp. 9. Volkwein, J. C. , S. J. Page, and E. D. Thimons. Canopy-Air Curtain Dust Reductions on a Gathering-Arm Loader. BuMines RI 8603, 1982, 9 pp. 10. Volkwein, J. C. Dust Control in Bagging Operations. Paper in Industrial Hygiene for Mining and Tunneling — Pro- ceedings of a Topical Symposium (Denver, CO., Nov. 6-7, 1978). ACGIH, Cincinnati, OH, 1979, pp. 51-57. 11. Muldoon, T. Private communica- tion, 1983; available upon request from T. Muldoon, Foster-Miller Inc., Waltham, MA, or J. C. Volkwein, BuMines, Pitts- burgh, PA. 12. U.S. Bureau of Mines. Dust Knock- down Performance of Water Spray Nozzles. Technol. News, No. 150, July 1982, 2 pp. 13. Hiltz, R. H. , and J. V. Fried. Using High Expansion Foam To Control Res- pirable Dust. Min. Congr. J., May 1973, pp. 54-60. 14. Seibel, R. J. Dust Control at a Transfer Point Using Foam on Water Sprays. BuMines TPR 97, May 1976, 12 pp. 15. Volkwein, J. C, A. B. Cecala, and E. D. Thimons. Adding Foam To Control Dust in Minerals Processing. BuMines RI 8808, 1983, 11 pp. 16. Cecala, A. B. Private communica- tion 1984; available upon request from A. B. Cecala, BuMines, Pittsburgh, PA. 17. Cheng, L. , E. E. Emmerling, and T. F. Tomb. Collection of Airborne Coal Dust by Steam. BuMines RI 7819, 1974, 13 pp. 18. Volkwein, J. C. , R. P. Vinson, and E. D. Thimons. Effectiveness of Three Water Spray Methods Used To Control Dust During Bagging. BuMines RI 8614, 1982, 9 PP. 19. McCoy, J., J. Melcher, J. Valen- tine, D. Monaghen, T. Muldoon, and J. Kelly. Evaluation of Charged Water Sprays for Dust Control. BuMines OFR 98- 83, 1983, 150 pp.; NTIS PB 83-210-476. *U.S. CPO: 1985-505-019/20,006 INT.-BU.O F MIN ES,PGH.,P A. 27883 d -dd n o > -•- -1 Q. p Ol. 3 n ft ~< ~+ (/) Q n c -1 _ . u- 3 3 □ a Q. 3 n n> 3 Q • (Q -•• z m D c > r~ O TJ ■a O 3D -< m O -< m 33 H 381 85 »* c in °s 3> s ° - m m z 2 > 7? 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