Drying classification

Mechanical attrition is used to remove most of the spent binder. First, dry attrition or abrasion processes crush lumps to grain size. Mechanical abrasion is then used to separate the binder from the sand grains. Sometimes, sand is pneumatically propelled against a metal target plate. The impact of the sand on the plate scrubs off the clay and resin coating from the sand grains. Fines are separated and removed by dry classification. 

Dry Classification Method. The most widely accepted method for chrysotile fiber length characterization in the industry is the Quebec Standard lest (QS). 

V. Galperin, et al, Dry Classification of Phosphate Ore in Chilisai Phosphate Field, Chem Industry, 12, Moscow, 1988. 

Dry classification equipment uses a gas stream to convey the solids. The gas used most oflen is air, and for that reason the term air classifiers is often used to describe this type of equipment (see Fig. 4.10). Air classifiers evolved from two sources, the original simple expansion chamber and the Mumford and Moodie separator, patented in 1885. 

Calcium hydroxide is a product new to the market. There have been, in past, positive scientific reports of its usefulness. The benefits of calcium hydroxide over calcium carbonate are its functionality, particle shape (more spherical and thus less abrasive to the equipment) (Figure 2.18), its lower density (decreases the density of product and lowers the price), a refractive index closer to many polymers, and its lower cost (approximately half of the price of calcium carbonate). The manufacturing equipment includes an excitement chamber, metered conveying, pneumatic transportation, flash drying, classification, and silo storage. The manufacturer delivers product to customers by its own silo-trucks. 

There are two major appUcations for which these metallic media are primarily used the separation of solid particles by size, and the coarse screening of gas or liquid flows ahead of some finer processing stage. In the coarser, macrofiltration processes these are very versatile materials all of the dry classification (sieving, sifting) operations are covered here, as are almost all of the applications of filters for straining and coarse filtration, both of which rely upon the precise size and shape of the apertures in the mesh or sheet. 

The importance of hydrolysis potential, ie, whether moisture or water is present, is illustrated by the following example. In the normal dermal toxicity test, namely dry product on dry animal skin, sodium borohydride was found to be nontoxic under the classification of the Federal Hazardous Substances Act. Furthermore, it was not a skin sensitizer. But on moist skin, severe irritation and bums resulted. 

Finish removers are appHed by bmshing, spraying, troweling, flowing, or soaking. Removal is by water rinse, wipe and let dry, or solvent rinse. Removers may be neutral, basic, or acidic. The viscosity can vary from water thin, to a thick spray-on, to a paste trowel-on remover. The hazard classification, such as flammable or corrosive, is assigned by the U.S. Department of Transportation (DOT) for the hazardous materials contained in the remover. 

Lower flammability limits (LFL) are expressed as vol % in dry ambient air. No entry means ASHRAE Safety Classification (2) (see Fig. 3). NR = not rated. 

It is quite common ia the designs for fine classification to recontact the coarse stream transversely or ia counterflow with air before dischargiag it (see Fig. 9). This removes dry fine particles not removed ia the primary classification. That is, these particles are swept back iato the feed and given another chance to exit with the fine particles. Such an arrangement iacreases the overall sharpness iadex and reduces the overall apparent bypass. Another variation is to reenter the air from the sohd/gas separation of the coarse stream. 

The cooled reaction mass is extracted from the retort, cmshed and leached first with dilute mineral acid, and then with water to separate the tantalum powder from the salts. After drying and classification, the primary powder is ready for processing to sheet, rod, wire, or capacitor-grade powder. 

The ASTM Glassification. The ASTM classification system was adopted in 1938 as a standard means of specification. This system is used in the United States and in many other parts of the world, and is designated D388 in the ASTM Standards (18). The higher rank coals are specified by fixed carbon >69%, or for volatile matter <31%, on a dry, mineral-free basis. Lower rank coals are classified by calorific value on the moist, mineral-matter-free... 

---