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MDMW-Diatomite03 | Particulates | Sintering

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Diatomite processing paper
  11/95Diatomite Processing11.22-1 11.22 Diatomite Processing 11.22.1 Process Description 1-2 Diatomite is a chalky, sedimentary rock consisting mainly of an accumulation of skeletonsremaining from prehistoric diatoms, which are single-celled, microscopic aquatic plants. The skeletonsare essentially amorphous hydrated or opaline silica occasionally with some alumina. Diatomite isprimarily used to filter food processing products such as beer, whiskey, and fruit juice, and to filterorganic liquids such as solvents and oils. Diatomite also is often used as a filler in paint, paper, asphaltproducts, and plastic. The six-digit Source Classification Code (SCC) for diatomite processing is3-05-026.Most diatomite deposits are found at or near the earth's surface and can be mined by open pitmethods or quarrying. Diatomite mining in the United States is all open pit, normally using somecombination of bulldozers, scraper-carriers, power shovels, and trucks to remove overburden and thecrude material. In most cases, fragmentation by drilling and blasting is not necessary. The crudediatomite is loaded on trucks and transported to the mill or to stockpiles. Figure 11.22-1 shows atypical process flow diagram for diatomite processing.The processing of uncalcined or natural-grade diatomite consists of crushing and drying. Crudediatomite commonly contains as much as 40 percent moisture, in many cases over 60 percent. Primarycrushing to aggregate size (normally done by a hammermill) is followed by simultaneous milling-drying, in which suspended particles of diatomite are carried in a stream of hot gases. Flash and rotarydryers are used to dry the material to a powder of approximately 15 percent moisture. Typical flashdryer operating temperatures range from 70 to 430C (150 to 800F). The suspended particlesexiting the dryer pass through a series of fans, cyclones, and separators to a baghouse. These sequentialoperations separate the powder into various sizes, remove waste impurities, and expel the absorbedwater. These natural-milled diatomite products are then bagged or handled in bulk without additionalprocessing.For filtration uses, natural grade diatomite is calcined by heat treatment in gas- or fuel oil-firedrotary calciners, with or without a fluxing agent. Typical calciner operating temperatures range from650 to 1200C (1200 to 2200F). For straight-calcined grades, the powder is heated in large rotarycalciners to the point of incipient fusion, and thus, in the strict technical sense, the process is one of sintering rather than calcining. The material exiting the kiln then is further milled and classified.Straight calcining is used for adjusting the particle size distribution for use as a medium flow rate filteraid. The product of straight calcining has a pink color from the oxidation of iron in the raw material,which is more intense with increasing iron oxide content.Further particle size adjustment is brought about by the addition of a flux, usually soda ash,before the calcining step. Added fluxing agent sinters the diatomite particles and increases the particlesize, thereby allowing increased flow rate during liquid filtration. The resulting products are called flux-calcined . Flux-calcining produces a white product, believed to be colored by the  MININGPRIMARY CRUSHINGMILLING/DRYINGCLASSIFICATIONNATURAL MILLED PRODUCTSCALCININGMILLINGCLASSIFICATIONFINAL PRODUCT SHIPPING 1121121 1 11.22-2EMISSION FACTORS11/95Figure 11.22-1. Typical process flow diagram for diatomite processing.  11/95 Diatomite Processing 11.22-3conversion of iron to complex sodium-aluminum-iron silicates rather than to the oxide. Furthermilling and classifying follow calcining.11.22.2 Emissions And Controls 1-2 The primary pollutant of concern in diatomite processing is particulate matter (PM) and PMless than 10 micrometers (PM-10). Particulate matter is emitted from crushing, drying, calcining,classifying, and materials handling and transfer operations. Emissions from dryers and calcinersinclude products of combustion, such as carbon monoxide (CO), carbon dioxide (CO), nitrogen oxides 2 (NO), and sulfur oxides (SO), in addition to filterable and condensible PM. Table 11.22-1 x x summarizes the results of a trace element analysis for one type of finished diatomite. These elementsmay constitute a portion of the PM emitted by the sources listed above.Wet scrubbers and fabric filters are the most commonly used devices to control emissions fromdiatomite dryers and calciners. No information is available on the type of emission controls used oncrushing, classifying, and materials handling and transfer operations.Because of a lack of available data, no emission factors for diatomite processing are presented.  TABLE 11.22-1. TRACE ELEMENT CONTENT OF FINISHED DIATOMITE 2 Element a ppm b Element ppmAntimony*Arsenic*BariumBeryllium*BismuthBoronBromineCadmium*CeriumCesiumChlorineChromium*Cobalt*CopperDysprosiumErbiumEuropiumFluorineGadoliniumGalliumGermaniumGoldHafniumHolmiumIndiumIodineIridiumLanthanumLead*LithiumLutetiumManganese*25301<0.5100202105400100540<1<0.5150<15<10<0.5<0.5<0.2<0.51<0.51021<0.260Mercury*MolybdenumNeodymiumNickel*NiobiumOsmiumPalladiumPlatinumPraseodymiumRheniumRhodiumRubidiumRutheniumSamariumScandiumSelenium*SilverStrontiumTantalumTelluriumTerbiumThalliumThoriumThuliumTinTungstenUraniumVanadiumYtterbiumYttriumZincZirconium0.35201205<0.5<1<22<0.5<0.510<122010<0.52020<2<0.2<0.550.2<1<0.55200<0.5100<1020 a Listed hazardous air pollutants indicated by an asterisk (*). b < indicates below detection limit.11.22-4 EMISSION FACTORS 11/95
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