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AIR QUALITY CALCULATIONS Sand and Gravel Processing The processing of sand and gravel for a specific market involves the use of different combinations of washers, screens, and classifiers to segregate particle sizes; crushers to reduce oversized material; and storage and loading facilities. After being transported to the processing plant, the wet sand and gravel raw feed is stockpiled or emptied directly into a hopper,which typically is covered with a "grizzly" of parallel bars to screen out large cobbles and boulders. From the hopper,the material is transported to fixed or vibrating scalping screens by gravity,belt conveyors,hydraulic pump,or bucket elevators. The r scalping screens separate the oversize material from the smaller, marketable sizes. Oversize material may be used for erosion control,reclamation, or other uses,or it may be directed to a r., crusher for size reduction, to produce crushed aggregate, or to produce manufactured sands. Crushing generally is carried out in one or two stages,although three-stage crushing may also be performed. Following crushing, the material is returned to the screening operation for sizing. The material that passes through the scalping screen is fed into a battery of sizing screens, which generally consists of either horizontal or sloped, and either single or multideck,vibrating screens. r Rotating screens with water sprays are also used to process and wash wet sand and gravel. Screening separates the sand and gravel into different size ranges. Water is sprayed onto the material throughout the screening process. After screening,the sized gravel is transported to stockpiles, storage bins, or, in some cases,to crushers by belt conveyors,bucket elevators, or screw conveyors. r The sand is freed from clay and organic impurities by log washers or rotary scrubbers. After scrubbing,the sand typically is sized by water classification. Wet and dry screening is rarely used ` to size the sand. After classification,the sand is dewatered using screws, separatory cones,or hydroseparators. Material may also be rod milled to produce smaller sized fractions, although this ` practice is not common in the industry. After processing,the sand is transported to storage bins or stockpiles by belt conveyors,bucket elevators,or screw conveyors. .. Industrial sand and gravel typically are mined from open pits of naturally occurring quartz-rich sand and sandstone. Mining methods depend primarily on the degree of cementation of the rock. In some deposits,blasting is required to loosen the material prior to processing. The material may undergo primary crushing at the mine site before being transported to the processing plant. The mined rock is transported to the processing site and stockpiled. The material then is crushed. Depending on the degree of cementation, several stages of crushing may be required to achieve the desired size reduction. Gyratory crushers,jaw crushers,roll crushers, and impact mills are v used for primary and secondary crushing. After crushing, the size of the material is further reduced to 50 micrometers or smaller by grinding,using smooth rolls,media mills, autogenous Yr i.