Wet screening

Wet screening is being replaced in many laboratories by instmments that use a photosensor to record the diameter of each particle in a water suspension as that suspension flows past the sensor. 

After acid removal, scrap batteries are fed to a hammer mill in which they are ground to <5 cm particles. The ground components are fed to a conveyor and passed by a magnet to remove undesirable contamination. The lead scrap is then classified on a wet screen through which fine particles of lead sulfate and lead oxide pass, and the large oversize soHd particles are passed on to a hydrodynamic separator. The fine particles are settled to a thick slurry and the clarified washwater recirculated to the wet screen. 

Screening devices are used to make coarser separations that is, fine products having 95% passing ca 100-mm—50-p.m size. Dry-screening devices have a lower recommended size of ca 500 p.m. Wet-screening devices that produce 95% passing ca 500—50-p.m size are continually being improved. 

In another cocrystalHzation process, lime is mixed with 50% caustic and recycled filtrate and chlorinated to yield a slurry of calcium hypochlorite dihydrate and NaCl crystals that are separated in a hydrauHc classifier. The underflow is mixed with centrate mother Hquor and sent to a wet screen classifier the overflow is recycled to the hydroclone and the salt-rich bottoms are centrifuged. The centrate is recycled to the chlorinator and the salt used as feed to chloralkaH ceUs. The Ca(OCl)2-rich overheads from the hydroclone are centrifuged, the cake going to a dryer and the filtrate sent to the wet screen classifier (207). 

Additions of new flocculants after conventional thickening produce further dewatering of mineral slimes. A clay flocculated with polyacrylamides and rotated in a dmm can produce a growth of compact kaolin pellets (84), which can easily be wet-screened and dewatered. A device called a Dehydmm, which flocculates and pelletizes thickened sludges into round, 3-mm pellets, was developed for this purpose. Several units reported in commercial operation in Japan thicken fine refuse from coal-preparation plants. The product contains 50% moisture, compared with 3% soflds fed into the Dehydmm from the thickener underflow (85). In Poland, commercial use of the process to treat coal fines has been reported (86), and is said to compare favorably both economically and technically to thickening and vacuum filtration. 

Revolving screens inclined, cylindrical screens, rotated at low speeds (10-20 rpm). Used for the wet screening of relatively coarse material, but have now been largely replaced by vibrating screens. 

Preparing granulation solution 4. Wet massing 5. Wet screening 6. Drying 3. Roll compaction ... 

SatnpCe Preparation. Seventy grams of sample, previously calcined at 538°C, was dispersed in about 125 mL of sodium metaphosphate solution and "wet screened" using a sieve stack of 250, 270 and 325 mesh, 8-inch diameter sieves. The sieves were washed with water until the effluent was free of any particulate and then rinsed with acetone to de-water. After air drying, the contents of each sieve were transferred to a porcelain dish and the material was recalcined at 538°C. Appropriate weights taken during this procedure allowed for the determination of percent moisture in the starting material and sieve analysis relative to the appropriate sieve fractions. The total +325 mesh material was recombined and mixed for the test. 

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