Diatomite Crushing, Grinding and Drying
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The processing of uncalcined or natural-grade diatomite consists of crushing and drying. Crude diatomite commonly contains as significantly as 40 percent moisture, in many cases more than 60 percent. Primary crushing to aggregate size (usually completed 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 rotary dryers are utilized to dry the material to a powder of roughly 15 percent moisture. Common flash dryer operating temperatures range from 70 to 430 C (150 to 800 F). The suspended particles exiting the dryer pass through a series of fans, cyclones, and separators to a baghouse. These sequential operations separate the powder into different sizes, eliminate waste impurities, and expel the absorbed water. These natural-milled diatomite merchandise are then bagged or handled in bulk without having further processing.
For filtration uses, natural grade diatomite is calcined by heat therapy in gas- or fuel oil-fired rotary calciners, with or with out a fluxing agent. Common calciner operating temperatures range from 650 to 1200 C (1200 to 2200 F). For straight-calcined grades, the powder is heated in large rotary calciners to the point of incipient fusion, and thus, inside the strict technical sense, the approach is one of sintering instead of calcining. The material exiting the kiln then is further milled and classified.
Straight calcining is employed for adjusting the particle size distribution for use as a medium flow rate filter aid. The item of straight calcining has a pink color from the oxidation of iron within the raw material, which is more intense with increasing iron oxide content material.
Further particle size adjustment is brought about by the addition of a flux, normally soda ash, just before the calcining step. Added fluxing agent sinters the diatomite particles and increases the particle size, thereby permitting increased flow rate during liquid filtration. The resulting goods are named 鈥渇lux-calcined鈥. Flux-calcining produces a white product, believed to be colored by the conversion of iron to complex sodium-aluminum-iron silicates instead of to the oxide. Further milling and classifying follow calcining.