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Air Pollution Engineering Manual Second Edition Air & Waste Management ASSOCIATION Since 1907 Edited by Wayne T. Davis A Wiley-Interscience Publication JOHN WILEY & SONS, INC. New York Chichester Weinheim Brisbane Singapore Toronto MINERAL PRODUCTS INDUSTRY 715 28. Particulate Emission Tests Conducted on the Unit #2 Lime Kiln in Alabaster, Alabama, for Allied Products Corporation, Guardian Systems, Inc., Leeds, AL, October 1990. 29. Particulate Emission Tests Conducted on #1 Lime Kiln in Alabaster, Alabama, for Allied Products Corporation, Guardian Systems, Inc., Leeds, AL, October 1991. 30. Mass Emission Tests Conducted on the #2 Rotary Lime Kiln in Saginaw, Alabama, for SI Lime Company, Guardian Systems, Inc., Leeds, AL, October 1986. 31. Flue Gas Characterization Studies Conducted on the #4 Lime Kiln in Saginaw, Alabama, for Dravo Lime Company, Guardian Systems, Inc., Leeds, AL, July 1991. 32. R. D. Rovang, Trip Report, Paul Lime Company, Douglas, AZ, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, January 1973. 33. T. E. Eggleston, Air Pollution Emission Test, Bethlehem Mines Corporation, Annville, PA, EMB Test No. 74-LIM-l, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, August 1974. 34. Air Pollution Emission Test, Marblehead Lime Company, Gary, Indiana, Report No. 74-LIM-7, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, 1974. 35. Emissions Survey Conducted at ChemstarLime Company, Located in Bancroft, Idaho, American Environmental Testing Company, Inc., Spanish Fork, Utah, February 26, 1993. SAND AND GRAVEL PROCESSING John Richards and Brian Palm U.S. companies produce more than 900,000,000 tons of sand and gravel per year.12 These products are used as a raw material in the manufacturing of materials such as concrete and asphalt for both road and building construction. There are more than 8,000 sand and gravel facilities located throughout all 50 states. Because sand and gravel are high-volume, lowcost commodities, transportation is a significant part of the total delivered cost. Therefore, natural aggregate, including sand and gravel, is commonly used within 30-50 miles of the place of extraction. The industry providing sand and gravel products includes many vertically integrated companies that may also operate stone crushing plants, asphaltic concrete plants, or ready-mixed concrete plants. Sand and gravel is typically produced from unconsolidated deposits formed by the action of water in the form of rivers, lakes, and glaciers. Deposits are mined, washed to remove impurities such as clay, screened to generate the required size ranges, and stockpiled. In some cases, crushing oflarger gravel is necessary. Sand and gravel plants used screening units and other processing equipment similar to that described in the section on "Stone Crushing." sand and gravel are recovered from these underwater deposits. Plants processing these granular, noncohesive materials use a variety ofspecial excavation systems such as floating clamshell dredges, floating bucket ladder dredges, draglines, underwater wheel excavators, and suction pumps.3,4 In some facilities, submersible pumps with close-coupled drive units are used to optimize delivery of the excavated high-density slurry to the plant. In other plants, the material can be transported by barges or conveyors. The mining area where sand and gravel products are extracted from deposits at or near ground level is referred to as a pit.3 Depending on the location and characteristics ofthe deposit, sand and gravel are removed by power shovels, front-end loaders, or bulldozers equipped with rippers. Many of the pit deposits have inherently high moisture levels. The extracted sand and gravel are transported to the processing plant by means of trucks or conveyors. After recovery of the mined material from a stockpile, it is processed in a grizzly feeder for removal of oversized material. In some plants, the oversized material is conveyed to a stockpile for use as fill or sub-base material. In other cases, the oversized material is processed in one or more crushers. Each of the crushers is associated with a screen in a loop configuration to allow undersized material to bypass the crusher. The reduction ratios of the crushers are selected to minimize generation of fugitive dust and to provide adequate throughput. The undersized material from the grizzly feeder is conveyed to one or more sets ofvibrating inclined or vibrating horizontal screens for size classification of the material. The large material separated in this process step is stockpiled and sold as gravel. The small material passing through the screens is PROCESS DESCRIPTION Construction Sand and Gravel The general process for the preparation of sand and gravel is illustrated in Figure 1. Processing sand and gravel starts with the removal of material from naturally occurring deposits of unconsolidated granular material. Some of these deposits occur at or immediately below ground level at the bottoms of streams, lakes, and oceans. More than 300 million tons of Sand storage Figure 1. Example Flowchart of a Sand and Gravel Plant. 716 MINERAL PRODUCTS INDUSTRY washed to remove clay particles and organic material present with the sand. The sand is often further separated by size in a set of water classifiers and then dewatered. The sand is stockpiled or stored in bins. AIR EMISSIONS CHARACTERIZATION The main pollutant emission from sand and gravel and stone crushing operations is particulate matter. Since 1987, EPAhas regulated ambient particulate matter as PM10, particles having an aerodynamic diameter (equivalent diameter of a spherical particle with a density of 1.0 cm3) equal to or less than 10 gm. In 1997, EPA modified the PMio National Ambient Air Quality Standards (NAAQS) and added new NAAQS for PM2.5. This is particulate matter with an aerodynamic diameter equal to or less than 2.5 gm. Total suspended particulate (aerodynamic diameters up to approximately 45 gm) is no longer an EPA regulated air pollutant. Mineral particulate matter is formed due to physical attrition of the sand and gravel during crushing, screening, conveying, and loading. Physical attrition processes inherently yield particles that have aerodynamic diameters that are predominately larger than 10 gm. Only a small fraction of the mineral particulate is in the PM10 size range. Due to the large amount of energy needed to subdivide particles by physical attrition, even less of the mineral particulate emissions from sand and gravel operations are in the PM2.6 size range. Particles formed by physical attrition have chemical compositions identical to the parent material undergoing size reduction. Natural crustal elements are the main constituents of mineral particulate emissions from both crushed stone and sand and gravel processing. These emissions are similar to particulate emitted by agricultural and construction operations and a variety of natural sources. Potential emission sources for particulate matter in a typical sand and gravel plant include the following: Haul roads Crushers Conveyor transfer points Screens Driers Product loading operations The extent of particulate emission from each of these sources depends, in part, on the material size distribution and the moisture level. Sand and gravel facilities handling wet material have essentially negligible emissions with the possible exception of sand driers. The available emissions data for stone crushing plant processing equipment are summarized in Table 1. These data have been compiled from Sections 11.19.16 and 11.19.26 of the U.S. EPA AP-42 Emission Factor Compilation. Sand and gravel and crushed stone are used in a variety of similar construction applications. Emission factors for crushed stone processing (AP-42 Section 11.19.2) may be used for sand and gravel processing because of the similarities in geology, extraction methods, and processing methods. Relatively few sand and gravel plants operate sand driers. Table 1 summarizes the limited data available concerning total particulate emissions. There are no data available concerning PM10 or PM2.5 emissions. Emission factor data are provided for two general operating conditions: uncontrolled and controlled. The term uncontrolled applies to processes that are operating without (l)wet suppression techniques, or (2) hood and air pollution control systems. Unless otherwise stated, the controlled emission factor data apply to a wet suppression equipment system. The term controlled applies to sand plants using conventional wet suppression systems or air pollution control equipment. For plants using wet suppression, the term controlled applies to stone moisture levels that are equal to or greater than 1.5% moisture. Additional information concerning the rationale for using the 1.5% value is provided in the section of this manual concerning stone crushing. The PM10 emission factor data for screening operations, crushers, and conveyor transfer points summarized in Table 1 have been compiled by EPA and the National Stone Association in a series of tests conducted over the 1991 to 1997 time period.7-15 These are included in Section 11.19.2 of EPA's AP-42 data compilation.6 Due to the lack of data T Table 1. Summary of Processing Equipment Emission Factors Source Operating Condition Total Particulate Matter as "TSP" (lbs/ton) PMio (lbs/ton) PM2.5 (lbs/ton) Sand drier Primary, secondary, or tertiary crushers Fines crushers Conveyor transfer points Uncontrolled Controlled, wet scrubber Controlled, fabric filter Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled 2.0 6 0.039 6 0.010 b ND ND ND ND ND ND ND ND ND 0.0024c 0.00059c 0.015 d 0.0021d 0.0014 d 0.000048 d ND ND ND ND 0.00009 c ND 0.00007 d ND 0.000013 d "Particulate emissions factors for sand and gravel operations handling wet materials are not included because emissions are considered negligible. ^Emission factors are based on the Section 11.19.1, U.S. EPA Publication AP-42, Fifth Edition, July 1995. Emissions data for crushers are based on the emission factors presented in Section 11.19.2 for stone crushing plant tertiary crushers. ^Emissions data for sand and gravel conveying are based on the emission factors presented in Section 11.19.2 for stone crushing plant conveyors. MINERAL PRODUCTS INDUSTRY 717 specifically applicable to sand and gravel operations, EPA has recommended in AP-42 that emissions from sand and gravel operations be based on the Section 11.19.2 data. The PM2.5 emission factor data16 have been obtained by the National Stone Association and have been provided to EPA for inclusion in AP-42. All the EPA and NSA sponsored tests have been conducted using state-of-the-art fugitive emission capture techniques and U.S. EPA Reference Method emission testing procedures.1718 The EPA and NSA sponsored tests included comprehensive quality assurance procedures and full documentation of process operating conditions. The initial set of PM10 tests was conducted using EPA Method 201A. Starting in 1996, tests for PM10 and PM2.5 were conducted using a draft emission testing procedure that has recently been designated as EPA Method 20IB. There are very limited total particulate matter data for sand and gravel operations. In the case of stone crushing plants, EPA5 has recommended the use of a multiplying factor of 2.1 to convert from the available PM10 data to corresponding total particulate matter data. A similar approach can be taken for sand and gravel operations. Emissions during the transport of wet material within sand and gravel facilities are negligible. Particulate emission factor data for haul roads at sand and gravel facilities can be estimated using Eq. 1 for PMi0 and Eq. 2 for PM2.5. These equations have been derived based on tests at three stone crushing plants.19,20 E = (s/3)0,8(M/2)-0,9 E = 0.25(s/3)'8(M/2)-0'9 (1) (2) Where E is the PM10 emissions from haul roads (lbs/VMT); s, the haul road silt content (%); and M, the haul road moisture content (%). These equations indicate that the variables affecting dust emissions from stone crushing plant haul roads are the silt content and the moisture content. Silt is defined as material that has a maximum size of 74 pm and can pass through a 200-mesh screen. Silt and moisture inherent in the material are measured by techniques described in Appendix C of EPA's AP-42 emission factor compilation.4 The moisture content of the haul road depends on the frequency of watering, the water application rates, and the evaporation rates. The silt levels typically range from 5 to 10% by weight. The moisture levels are usually in the range of 4 to 8%. These equations do not apply to wintertime conditions, when there is snow or ice on the road surfaces. Equations 1 and 2 also do not apply to facilities using chemical dust suppressants on haul road surfaces. A comparison of Eqs. 1 and 2 with EPA's general unpaved road equation is provided in the section concerning stone crushing. Emissions from quarry haul roads are slightly lower than predicted by EPA's general equation provided in AP-42. Sand and gravel facilities using fossil-fuel-fired sand driers will have slight emissions of nitrogen oxides, sulfur oxides, and volatile organic compounds. The emission rate for nitrogen oxides is estimated at 0.031 pounds per ton of sand dried based on data in Section 11.19.1 of EPA's AP-42 compilation.5 The emission rate of sulfur dioxide can be estimated based n the sulfur content of the fuel. The sulfur dioxide emissions can range from near-negligible levels for natural gas and No. 2 fuel oil to modest emission rates for No. 6 oil. Emission factors for fossil fuels can be found in Section 1 of EPA's AP-42 compilation. The emission rates of volatile organic compounds for drier burners are very small. Limited data concerning the appropriate emission factors for VOC emissions are provided in Section 11.19.1 ofAP-42.6 AIR POLLUTION CONTROL MEASURES Sand and gravel operations handle inherently wet materials with negligible fugitive dust emissions. Additional air pollution control measures are not needed for these wet operations. The air pollution control measures for sand and gravel operations handling drier material from pits at or near ground level include (1) conventional wet suppression, (2) hooding with either wet scrubber or fabric filter control, (3) minimizing drop heights to storage piles, (4) covered conveyors, and (5) good plant housekeeping. The effectiveness of these techniques is similar to applications in the stone crushing industry discussed in another section of this publication. REFERENCES 1. D. Meyer, and S. Zelnak, Jr., "Structure and Economics." Chap ter 2 in The Aggregates Handbook, National Stone Association, Washington, D.C., 1992. 2. V. Tepordei, "Construction Sand and Gravel Statistical Com pendium," United States Geological Survey Minerals Information, USGS Reston, Virginia, May 5, 1997. 3. R. Archibald, "Extraction Principles." Chapter 7 in The Aggregates Handbook, National Stone Association, Washington, D.C., 1992. 4. Ohio Department of Natural Resources, Division of Geological Survey, "Sand and Gravel," GeoFacts No. 19, Columbus, Ohio, 1996. 5. U.S. EPA, "Sand and Gravel Processing," Section 11.19.1 of EPA, AP-42, Compilation of Emission Factors, July 1995. 6. U.S. EPA, "Sand and Gravel Processing," Section 11.19.2 of EPA, AP-42, Compilation ofEmission Factors, July 1995. 7. J. Richards, T. Brozell, and W. Kirk. "PM10 Emission Factors for a Stone Crushing Plant Deister Vibrating Screen," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D1-0055, Task 2.84, February 1992. 8. J. Richards, T. Brozell, and W. Kirk. "PM10 Emission Factors for a Stone Crushing Plant Tertiary Crusher," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D1-0055, Task 2.84, February 1992. 9. W. Kirk, T. Brozell, and J. Richards, "PMi0 Emission Factors for a Stone Crushing Plant Deister Vibrating Screen and Crusher," Report to the National Stone Association, December 1992. 10. T. Brozell, J. Richards, and W. Kirk. "PM10 Emission Factors for a Stone Crushing Plant Tertiary Crusher and Vibrating Screen," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D0-0122, December 1992. 11. T. Brozell, "PM10 Emission Factors for Two Transfer Points at a Granite Stone Crushing Plant," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-DO-0122, January 1994. 12. T. Brozell, "PM10 Emission Factors for a Stone Crushing Plant Transfer Point," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D0-0122, February 1993. 13. T. Brozell, and J. Richards. "PM10 Emission Factors for a Limestone Crushing Plant Vibrating Screen and Crusher, Bristol Tennessee," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D2-0163, July 1993. 14. T. Brozell, and J. Richards. "PM10 Emission Factors for a Limestone Crushing Plant Vibrating Screen and Crusher, 718 MINERAL PRODUCTS INDUSTRY Maryville Tennessee," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D2-0163, July 1993. 15. T. Brozell, T. Holder, and J. Richards, "Measurement of PMio & PM2.5 Emission Factors at a Stone Crushing Plant," Final Report to the National Stone Association, December 1996. 16. T. Brozell, "PM2.5 and PM10 Emission Factor Testing at Two Stone Crushing Plant Quarries," Presented at the Environmental and Safety Meeting, National Stone Association, October 1997. 17. J. Richards, and T. Brozell. "PM10 Emission Factors for Sloped, Vibrating Screens and Tertiary Crushers at Five Stone Crushing Plants," Paper presented at the National Stone Association Annual Meeting Concerning Health, Safety, and Environment, Atlanta, Georgia, November, 1993. 18. T. Brozell, "PM10 Emission Factors for a Haul Road at a Granite Stone Crushing Plant," Final report to the National Stone Association, December 1994. 19. J. Richards, and T. Brozell, "PM10, PM2.5, and PMi Emission Factors for Haul Roads at Two Stone Crushing Plants," Final report to the National Stone Association, Washington D.C., November 1995. 20. J. Richards, and T. Brozell. "A Comparison of NSA Quarry Haul Road Emission Factors with EPA's Unpaved Road Equation," Presented at the Environmental and Safety Meeting, National Stone Association, October 1997. STONE CRUSHING John Richards, Todd Brozell, and Brian Palm The Stone Crushing Industry in the United States produces more than 2.4 billion tons of stone per year. During the past 25 years, production has increased at an average annual rate of approximately 3%. There is considerable variability in the size of stone crushing operations. There are small portable plants with capacities of less than 100,000 tons per year and permanent plants with capacities ranging from 300,000 to more than 12,000,000 tons per year.1 A typical permanent facility has a capacity in the range of 300,000 to 800,000 tons per year.1 The stone products are used primarily in road construction, building construction, asphaltic concrete production, and cement production. The distance between the stone crushing plant and the point of stone use must be kept short because the transportation costs of stone are major components of the overall delivered cost. For this reason, the stone crushing industry is one of the most widely dispersed industries in the country with approximately 3,200 quarry operations located in 49 states. The most common types of stone produced include granite, limestone, dolomite, and sandstone. Other types of stone produced include quartzite, slate, marl, and shell.2 Of the various types of stone, granite, limestone, and dolomite account for a large majority of U.S. stone production.2 PROCESS DESCRIPTION The production of crushed stone starts with blasting to loosen rock from the quarry wall. Blasting is usually conducted between one and three times per week to provide sufficient inventory of fragmented stone for the processing equipment. Figure 1 shows a typical quarry with fragmented stone in the center right. Figure 1. Loading of Fragmented Stone in a Quarry. The fragmented stone is loaded into large-haul road trucks using a front-end loader, as shown in Figure 1, or hydraulic shovels. These trucks transport the stone to a grizzly feeder and a primary crusher. The distance from the loading point to the primary crusher is usually a fraction of a mile. The capacities of the haul road trucks range from 30 to 75 tons. The size of the rock ranges from several inches to more than 3 ft. Figure 2 is a general flowchart for a typical stone crushing plant. The material throughputs for this example plant are based on a typical overall production rate of 500 tons per hour, a level equivalent to an annual plant capacity of approximately 1,000,000 tons per year. The grizzly feeder and the primary crusher are usually located in or near the quarry pit to minimize haul truck transport time. Stone is reduced from a maximum size of approximately 3 ft to a maximum size of approximately 12 in. in the primary crusher. This stone is stored temporarily in a primary surge pile adjacent to the primary crusher. Stone recovered from the primary surge pile is transported by a conveyor to the primary screen. This vibrating, multideck screen separates the stone into size categories that include (1) oversized material exceeding 3 in., (2) moderately sized material between 1 and 3 in., and (3) undersized material smaller than approximately 1 in. The oversized material passes into one or more secondary crushers to reduce the stone size to a top size less than approximately 3 in. Both compression and impact crushers are used for secondary crushing. The moderately sized stone from the primary screen and the stone that is processed in the secondary crushers are combined and sent to the secondary screen. This is usually a vibrating inclined multideck screen. The screen mesh sizes on each of the two or three decks can be varied to produce streams of stone having the desired maximum sizes. The smallest stone falls through all the screen decks and often has a top size of approximately ^ in. diameter. The stone that falls through the initial deck but is too large to penetrate the lower decks is directed to several product storage piles or recirculates for additional size reduction. These products have top sizes that are usually 0.5 to 1 in. diameter. The oversized material from the secondary screen is transported by a conveyor to a surge bin feeding one or more tertiary crushers. All the crushed stone from the tertiary crushers is redirected back to the secondary MINERAL PRODUCTS INDUSTRY 719 and trees are often used to improve the appearance and to reduce noise. When a quarry site is no longer needed, the quarry pit is often converted into a lake for recreational use or for community water supplies. AIR EMISSIONS CHARACTERIZATION The only pollutant emission of concern from stone crushing operations is particulate matter. Since 1987, EPA has regulated ambient particulate matter as PMi0. In 1997, the EPA modified the PMio National Ambient Air Quality Standards (NAAQS) and added new NAAQS for PM2.5. Total suspended particulate (material less 45 pm) is no longer an EPA regulated air pollutant. Mineral particulate matter is formed due to physical attrition of the stone during crushing, screening, conveying, and loading. Physical attrition processes, such as the crusher shown in Figure 3, inherently yield particles that have aerodynamic diameters that are predominately larger than 10 pm. Only a small fraction of the mineral particulate is in the PM10 size range. Due to the large energies needed to subdivide particles by physical attrition, even less of the mineral particulate are in the PM2.5 size range. Particles formed by physical attrition have chemical compositions identical to the parent material undergoing size reduction. The main constituents of mineral particulate are natural crustal Figure 2. Example Plant Flowchart (Reprinted courtesy of the National Stone Association). screens. Material from the tertiary crusher is approximately 0.5 in. or less. In some plants, there is additional treatment of the fines (material less than ^ in. diameter) to produce manufactured sand. In these plants, one or more additional crusher-screening operations are used to further reduce the size of the rock. Manufactured sand is defined as material that will pass through a No. 8 sieve screen. Throughout the entire process, crushed rock of various size ranges is stockpiled awaiting shipment or further processing. Front-end loaders are often used for loading customer trucks. Conveyors are used for loading barges. At some large facilities, bins are used to facilitate loading customer trucks and rail cars. Stone crushing plants use several techniques to minimize their effects on surrounding communities. Landscaped berms Figure 3. Crusher. 720 MINERAL PRODUCTS INDUSTRY elements. These emissions are similar to particulate emitted by agricultural and construction operations and a variety of natural sources. Potential emission points for particulate matter in a typical stone crushing plant include the following. The extent of particulate emission from each of these sources depends, in part, on stone size distribution and the moisture level of the small particulate matter. Quarry drilling and blasting Fragmented stone loading Haul roads Primary crushing Secondary crushing Conveyor transfer points Screening Tertiary crushing Fines crushing Product loading The available emissions data for stone crushing plant processing equipment are summarized in Table 1. Data are provided for two general operating conditions: uncontrolled and controlled. The term "uncontrolled" applies to processes that are operating without (1) wet suppression techniques, or (2) hood and air pollution control systems. The term "con trolled" applies to plants using conventional wet suppression systems, chemical dust suppressants, or air pollution control equipment. For plants using wet suppression, the term con trolled applies to stone moisture levels that are equal to or greater than 1.5% moisture. The 1.5% moisture level is an arbitrary value used to separate dry from wet conditions. This value continues to be used for consistency with EPA reports and emission factor compilations prepared in the 1970s and 1980s. Caution is needed in interpreting and using uncontrolled ("dry") emission factors. The potential for dust release from stone surfaces is moderate when the moisture levels are near zero and decreases rapidly as the moisture content increases to approximately the 1.0% to 1.5% range. The addition of even 0.1% moisture can have a significant beneficial impact on particulate emissions when stone moisture levels are in the 0.1 to 0.5% range. Due to the differences in particulate emissions as moisture decreases from 1.5%, it is difficult to assign a single emission factor value for the "dry" conditions. The PMio emission factor data for screening operations, tertiary crushers, fines crushers, and conveyor transfer points summarized in Table 1 have been compiled by EPA and the National Stone Association in a series of tests conducted from 1991 to 1997B~12 and are included in EPA's AP-42 data compilation.4 The PM2.5 emission factor data13 have been obtained by the National Stone Association and have been provided to EPA for later inclusion in AP-42. The EPA and NSA sponsored tests have been conducted using state-of-the-art fugitive emission capture techniques and U.S. EPA Reference Method emission testing procedures.14-16 The EPA and NSA sponsored tests included comprehensive quality-assurance procedures and full documentation of process operating conditions. The initial set of PM10 tests was conducted using EPA Method 201A. Starting in 1996, tests for PMm and PM2.5 were conducted using a draft emission testing procedure that has recently been designated as EPA Method 201B. There are no particulate emissions factor data available for blasting. These emissions are very difficult to measure due Table 1. Summary of Processing Equipment Emission Factors Source Operating Condition Total Particulate Matter as "TSP" (lbs/ton) PM10 (lbs/ton) PM2.5 (lbs/ton) Screening Fines screening Primary crushing Secondary crushing Tertiary crushing Fines crushing Conveyor transfer points Wet drilling (unfragmented stone) Truck loading (fragmented stone) Truck loading (crushed stone) Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled Uncontrolled Controlled Controlled Uncontrolled Controlled ND ND ND ND 0.0007 ND ND ND ND ND ND ND ND ND ND ND ND 0.015" 0.00084 0.071" 0.0021 ND6 ND6 ND6 ND6 0.0024 0.00059" 0.015" 0.0021" 0.0014" 0.000048 ND 0.00005c ND ND ND ND ND ND ND 0.00009c ND 0.00007c ND 0.000013" 0.000080" ND 0.000016" ND 0.00010" ND "Emission factors presented in the U.S. EPA Publication AP-42, Fifth Edition, July 1995. ^Emissions data for tertiary crushers used to estimate the maximum possible emissions from primary and secondary crushers. cEmission factor data presented in Ref. 13. MINERAL PRODUCTS INDUSTRY 721 primarily to the low blasting frequency and the very short duration of plume generation. Testing is also difficult due to the inability to contain the dispersing plume and MSHA safety requirements relating to the use of electrically powered instruments close to explosives. Despite the lack of emission factor data, blasting emissions are considered small due to the short duration of the plume. It is apparent that there are very limited total particulate matter data for stone crushing plant operations. EPA4 has recommended the use of a multiplying factor of 2.1 to convert from the available PMi0 data to corresponding total particulate matter data. Particulate emission factor data for quarry haul roads can be estimated using Eq. 1 for PMi0 and Eq. 2 for PM2.5. These equations have been derived based on tests at three stone crushing plants.1718 E = (s/3)'8(M/2)-0'9 (1) Figure 4. Road Clearance for a Haul Road Truck and Light Duty Truck. E = 0.25(s/3)a8(M/2)-'9 (2) where E is the PM10 emissions from haul roads (lbs/VMT); s, the haul road silt content (%); and M, the haul road moisture content (%). These equations indicate that the variables affecting dust emissions from stone crushing plant haul roads are the silt content and the moisture content. Silt is defined as material that has a maximum size of 74 gm and can pass through a 200-mesh sieve. Silt and moisture are measured by techniques described in Appendix C of EPA's AP-42 emission factor compilation.4 The moisture content of the haul road depends on the frequency of watering, the water application rates, and the evaporation rates. The silt levels typically range from 5 to 10% by weight. The moisture levels are usually in the range of 4 to 8%. These equations do not apply to wintertime conditions when there is snow or ice on the road surfaces. Equations 1 and 2 also do not apply to facilities using chemical dust suppressants on haul road surfaces. Equation 1 estimates emissions of PMi0 that are lower than EPA's unpaved road equation included in Section 11.2 of AP42.4 The EPA equation was first published in 1974 and has evolved through seven different forms over a 21-year period. The version shown in Eq. 3 was included in the 1995 edition of AP-42. The road clearances designed into haul road trucks are considerably higher than the clearance ofautomobiles and light duty trucks, as illustrated in Figure 4. The combined effect of low vehicle speeds and high road clearances reduces the turbulence and air speeds under the vehicles where particulate can be retained. The different tread designs used on heavyduty off-road trucks also reduces the entrainment of dust and increases compaction of the road. AIR POLLUTION CONTROL MEASURES Control measures for reducing or eliminating PMi0 and PM2 5 emissions from quarries and processing plants include the following: Wet suppression Chemical suppressants Covering open operations to prevent dust entrainment by the wind Reducing the conveyor drop height Using hoods, industrial ventilation systems, and dust collectors (fabric filters or low static pressure drop wet scrubbers) on processes not amenable to wet suppression E = &5.9(s/12)(S/30)(W/3)'7(n;/4)05[(365 -p)/365] (3) where E is the haul road particulate emission factor (lbs/VMT); k, the particulate size multiplier, PMio (0.36) and TSP (0.80); s, the haul road silt content (%); S, the average vehicle speed (mph); W, the average vehicle weight (tons); w, the average number of wheels; and p, the number of days with at least 0.01 in. of rainfall. EPA's unpaved road equation was developed primarily for rural unpaved roads and iron and steel mine unpaved roads.419 Haul roads at stone crushing plants are considerably different from these unpaved road surfaces. Emissions are lower from quarry haul roads primarily because they are watered and compacted on a routine basis. Haul road trucks in stone quarries move at speeds of 15 to 25 mph, well below the speeds on public unpaved roads and level industrial unpaved roads. The wheels factor included in the general unpaved road equation is not applicable to stone crushing plant quarries. Table 2 lists typical operations in a rock quarry and processing plant and shows applicable air pollution control measures. Wet dust suppression is a common and effective method of controlling dust emissions.24 Emission factor tests conducted by EPA and NSA under controlled (wet suppression) and uncontrolled conditions indicated PMi0 emission suppression efficiencies of 90 to 95% for tertiary crushers, screening operations, and conveyor transfer points.14 The effectiveness of wet suppression has been enhanced during the past 10--15 years due to the development of improved atomizing nozzles and the use of wetting agents in some cases. Foaming agents have also been used to enhance PM1() emission suppression.20 Drilling and Blasting Quarries must drill and blast to obtain material in sizes that can be transferred and conveyed in an efficient manner to the 722 MINERAL PRODUCTS INDUSTRY Table 2. Particulate Emission Sources and Typical Control Measures for Rock Quarrying and Processing Operation or source Control options Drilling Blasting Loading of fragmented stone Haul roads Crushing Screening Conveyor transfer points Stockpiling Storage bins Windblown dust from roads and plant yard Loading product into railroad cars, trucks, and ships Liquid injection (water or water plus a wetting agent) Capturing and venting emissions to a control device Adoption of good blasting practices Wet suppression Wet suppression Treatment of roads with surface agents Paving customer haul roads Wet suppression Capturing and venting emissions to a control device Wet suppression Capturing and venting emissions to a control device Wet suppression Capturing and venting emissions to a control device Stacker conveyors (reduced drop heights) Water sprays at conveyor discharge Capturing and venting to a control device Wet suppression Surface-active agents Soil stabilization Paving Wet suppression primary crusher. Fugitive dust controls on drilling equipment can efficiently control emissions. These controls include wet suppression and dry collection. The wet suppression technique involves pumping water into the air blow back line leading down to the drill bit. The dry system uses a small hood and cyclone to capture the dust entrained during drilling. Since the late 1980s, there have been a number of significant improvements in blasting technology. These have been adopted to minimize vibration effects and nuisance dust problems. One of the most significant of these improvements has been the development and implementation of precision detonation techniques. These procedures involve the detonation of the explosive charges located in adjacent drilling holes with precisely timed millisecond delays. This technique results in a number of small detonations instead of one large blast. Precision detonation reduces vibration, noise, and dust generation. Blast control plugs and stemming materials are inert materials placed on top of the explosive charge to minimize fly rock, air blast, noise, and fugitive dust. Blast control plugs and stemming material oppose the explosive energy from vertical displacement and cause the explosive energy to break through the rock face.21,22,23 material being handled. Emissions of particulate matter are also minimized by carefully dumping the stone into the bed of the haul road truck. As shown in Figure 1, the drop height is kept very low. In addition to minimizing dust emissions, this practice reduces the maintenance requirements for the haul road trucks. Quarry Haul Roads Particulate emissions from quarry haul roads are controlled by the use of wet suppression and/or chemical dust suppressants. The water application rates for quarry haul roads are usually in the range of0.3 to 0.5 gallons per square yard. The frequency of water application is highly dependent on weather conditions and usually ranges from once every two to once every six hours of quarry operating time. The application frequency is adjusted as necessary to account for seasonal differences in the moisture evaporation rates. Water application is not needed during and immediately after rainfall. Water application is also not needed when there is snow cover on the road surface. Under these wintertime conditions, there is sufficient moisture inherently present on the road surface to minimize dust emissions. The types of chemical suppressants include both organic binders and hygroscopic materials. The organic binders partially seal the road surface to minimize reentrainment ofsilt particles. The hygroscopic materials, such as calcium chloride (CaCl2), retain moisture on the road surface and thereby minimize the release of particles. The application frequencies for chemical supressants range from monthly to quarterly depending on the specific characteristics of the additive. Diesel particulate are emitted from heavy-duty trucks, including the types used in stone quarries. These emissions are most common when the truck is shifting or when it is climbing the sloped surface of the quarry haul road. The rate of diesel emissions is primarily dependent on the design characteristics of the engine; however, normal maintenance of the vehicle engine can minimize these emissions. Primary Crusher Figure 5 shows a side view of a quarry haul truck dumping fragmented stone into a grizzly feeder in a quarry of a stone crushing plant. The inlet to the primary crusher is located to the left of the grizzly feeder. Because of the scale of the Loading Fragmented Stone PMio and PM2.s emissions from loading fragmented stone are inherently low due to the large particle size distribution of the Figure 5. Side View of a Grizzly Feeder and a Primary Crusher Showing Water Spraying During Truck Dumping. operation and the need for accessibility to the crusher opening, enclosure and venting to a control device are impractical. Water sprays, as shown in Figure 5, are used to wet the material and reduce dust emissions. Due to the large size of the rock, relatively small quantities of water are needed to suppress the dust emissions. Excessive quantities of water in winter are undesirable because they can cause freezing on the belt conveyor that transports the 12 inch topsize material to the primary surge pile. MINERAL PRODUCTS INDUSTRY 723 Secondary and Tertiary Crushing Secondary and tertiary crushers are used to produce stone products with the appropriate size distributions for use in road and building construction. Each of the crushers is selected and operated to provide the necessary reduction ratio. This is the ratio between the feed size and the crusher outlet material size. Crushers inherently generate less particulate matter when the reduction ratios are in the moderate range. By spreading the crushing requirements over both a secondary and tertiary crusher with a moderate reduction ratio, the overall emissions are lower than if a single crusher with a very high reduction ratio were used. There are two different categories of crushers used for secondary and tertiary duty. Compression crushers reduce stone sizes by forcing the feed material to pass through a fixed (but adjustable) area. Impact crushers reduce stone sizes by applying a high-speed impact force. Particulate control ofboth categories ofcrushers can usually be achieved by wet suppression systems located throughout the stone crushing plant. Moisture applied from the grizzly feeder and various points along the material path to the crushers reduces the potential for particulate emissions. Water sprays on the feed conveyors at the inlet ofthe crusher are used as part ofthe overall plant wet suppression system. All the water spray nozzles along the material flow path are not needed all the time. Excessive moisture does not provide any dust control benefits. Figure 6. Vibrating Inclined Screen. can be obtained by the plant-wide wet suppression system or with full enclosures with fabric filters or wet scrubbers. Screening and Conveyor Transfer Points The types of screening equipment used at stone crushing plants include (1) vibrating inclined, (2) stationary inclined, (3) vibrating grizzly feeder, (4) vibrating horizontal, and (5) rotary. The vibrating inclined screens similar to the unit shown in Figure 6 are the most common. These screens usually have two or more decks to separate the stone feed into three or more streams of different sizes. The size ranges in each of the material streams from the multideck vibrating inclined screen depend on the size of the openings in the wire mesh or rubber media. The ability to produce stones of the necessary size range also depends on the material throughput. Emissions from screens can be effectively controlled by the plant-wide wet suppression system. One part of this system is a set of nozzles located at the feed side of the screen. Screening operations can also be controlled by enclosures and air pollution control systems. Both fabric filters and lowpressure-drop wet scrubbers can be used as the particulate control system. Conveyor transfer points are locations where a stream of stone makes an abrupt change in direction or elevation as it is discharged from one conveyor to another. The potential for dust emissions is low because the drop heights are low. Emissions are usually minimized by partial enclosures that block the wind at the point of discharge. Control of particulate emissions Stockpiling and Crushed Stone Loading Dust emissions from stockpiles and stone loading operations are usually small. Dust emissions from stockpiling are not usually amenable to enclosure; therefore, wetting the material is most commonly used. The stockpiles usually develop a crusty surface that minimizes dust emissions from the stored stone products. The emissions that occur as the stone drops from the conveyor to the stockpile are minimized by keeping the drop heights as low as practical. Emissions of particulate matter during the loading of crushed stone into trucks, barges, or rail cars is minimized by maintaining proper moisture levels in the product stockpiles. The plant-wide wet suppression system provides most of the moisture necessary to control particulate emissions. If necessary, additional moisture can be sprayed on the stockpile to prevent excessive drying. Particulate emissions can also be reduced by minimizing the drop heights from the conveyor or front end loader to the truck, barge, or rail car being used to ship the product. Summary The overall impact of stone crushing operations on ambient levels of PM10 and PM2.5 is small. This is due (1) partially to the fact that relatively small quantities of mineral particulate 724 MINERAL PRODUCTS INDUSTRY are formed in these small size ranges because of the relatively low energy levels of the processing equipment and (2) partially to the effectiveness of conventional particulate control techniques. Recent upwind-downwind studies conducted using PM2.5 ambient monitoring instruments indicate that there is a near-negligible impact of stone crushing plant operations on ambient PM2.5 concentrations.25,26 REFERENCES 1. D. Meyer, and S. P. Zelnak, Jr., Chapter 2, "Structure and Economics," in The Aggregates Handbook, p. 2-2, National Stone Association, Washington D.C., 1991. 2. R. S. Huhta, Chapter 1, "Introduction to the Aggregate Industry," in The Aggregates Handbook, p. 1-4, National Stone Association, Washington D.C., 1991. 3. U.S. EPA, PM2.5 Criteria Document, U.S. EPA, July 17, 1997. 4. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, U.S. EPA, Research Triangle Park, N.C., July 1995. 5. J. Richards, T. Brozell, and W. Kirk. "PM10 Emission Factors for a Stone Crushing Plant Deister Vibrating Screen," Report to the U.S. EPA, Research Triangle Park, N.C., Contract No. 68-D1-0055, Task 2.84, February 1992. 6. J. Richards, T. Brozell, and W. Kirk. "PM10 Emission Factors for a Stone Crushing Plant Tertiary Crusher," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D1-0055, Task 2.84, February 1992. 7. W. Kirk, T. Brozell, and J. Richards, "PM10 Emission Factors for a Stone Crushing Plant Deister Vibrating Screen and Crusher," Report to the National Stone Association, December 1992. 8. T. Brozell, J. Richards, and W. Kirk, "PM10 Emission Factors for a Stone Crushing Plant Tertiary Crusher and Vibrating Screen," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D0-0122, December 1992. 9. T. Brozell, "PM10 Emission Factors for Two Transfer Points at a Granite Stone Crushing Plant," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D0-0122, January 1994. 10. T. Brozell, "PMjo Emission Factors for a Stone Crushing Plant Transfer Point," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D0-0122, February 1993. 11. T. Brozell, and J. Richards, "PM10 Emission Factors for a Limestone Crushing Plant Vibrating Screen and Crusher, Bristol, Tennessee," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D2-0163, July 1993. 12. T. Brozell, and J. Richards, "PM10 Emission Factors for a Limestone Crushing Plant Vibrating Screen and Crusher, Maryville, Tennessee," Report to the U.S. EPA, Research Triangle Park, NC, Contract No. 68-D2-0163, July 1993. 13. T. Brozell, T. Holder, and J. Richards, "Measurement of PM10 & PM2.5 Emission Factors at a Stone Crushing Plant," Final Report to the National Stone Association, December 1996. 14. J. Richards, and T. Brozell, "AP-42 and the Cooperative EPA-NSA Emission Factor Test Program," Paper presented at the NSA Environment, Safety, and Health Learning Forum, Nashville, Tennessee, October 22-24, 1995. 15. T. Brozell, "PM2.5 and PM10 Emission Factor Testing at Two Stone Crushing Plant Quarries," Presented at the Environmental and Safety Meeting, National Stone Association, October 1997. 16. J. Richards, and T. Brozell, "PM10 Emission Factors for Sloped, Vibrating Screens and Tertiary Crushers at Five Stone Crushing Plants," Paper presented at the National Stone Association Annual Meeting Concerning Health, Safety, and Environment, Atlanta, Georgia, November, 1993. 17. T. Brozell, "PM10 Emission Factors for a Haul Road at a Granite Stone Crushing Plant," Final report to the National Stone Association, December 1994. 18. J. Richards, and T. Brozell, "PM10, PM2.5, and PMj Emission Factors for Haul Roads at Two Stone Crushing Plants," Final report to the National Stone Association, Washington D.C., November 1995. 19. J. Richards, and T. Brozell, "A Comparison of NSA Quarry Haul Road Emission Factors with EPA's Unpaved Road Equation," Presented at the Environmental and Safety Meeting, National Stone Association, October 1997. 20. M. Kestner, "Using Suppressants to Control Dust Emissions," Parts I and II, Powder and Bulk Engineering, February 1989 and March 1989. 21. M. D. Wilkins, and P. N. Worsey, "Stemming Technique for Loading Angled Holes Charged with ANFO," Paper presented at the 1997 Meeting of the International Society of Explosive Engineers. 22. C. Deacon, P. Duniam, and M. Jones. "Improved Blast Control through the Use of Programmable Delay Detonators," Paper presented at the 1997 Meeting of the International Society of Explosive Engineers. 23. L. Workman, "Selecting the Right Explosives," Pit & Quarry, July 1997, pp. 43-45. 24. Midwest Research Institute, "Inspection Manual for PM10 Emissions from Paved/Unpaved Roads and Storage Piles," Final Report to the U.S. EPA, Office of Air Quality Planning and Standards, Stationary Source Compliance Division, EPA Contract No. 68-02-4463, October 1989. 25. J. Hayden, S. Whitt, J. Richards, T. Brozell, and B. Palm. "PM2.5 Ambient Air Monitoring at a Stone Crushing Plant," Aggregates Manager, March 1998, pp. 41-46. 26. J. Richards, "PM2.5 is Different," Stone Review, December-January 1999.