Extractor leaching

Batch Extractors. Coarse soHds are leached by percolation in fixed or moving-bed equipment. Both open and closed tanks (qv) having false bottoms are used, into which the soHds are dumped to a uniform depth and then treated with the solvent by percolation, immersion, or intermittent drainage methods. 

Tipping pan and horizontal filters are also used for leaching the modus operand of the Rotocel extractor resembles that of a tipping pan filter, although the details of its design differ slightly. 

Percolation In addition to being applied to ores and rock in place and by the simple technique of heap leaching (usually on verv large scale see Wadsworth, loc. cit.) percolation is carried out in batcJi tanks and in continuous or dump extractors (usually on smaller scale). 

The endless-belt percolator (Wakeman, loc. cit.) is similar in principle, but the successive feed, solvent spray, drainage, and dumping stations are hnearly rather than circulany disposed. Examples are the de Smet belt extractor (uncompartmented) and the Lurgi frame belt (compartmented), the latter being a kind of linear equivalent of the Rotocel. Horizontal-belt vacuum filters, which resemble endless-belt extractors, are sometimes used for leaching. 

Screw-Conveyor Extractors One type of continuous leaching equipment, employing the screw-conveyor principle, is strictly speaking neither a percolator nor a dispersed-solids extractor. Although it is often classed with percolators, there can be sufficient agitation of the solids during their conveyance by the screw that the action differs from an orthodox percolation. 

Process and Operating Conditions The major parameters that must be fixed or identified are the solvent to be used, the temperature, the terminal stream compositions and quantities, leaching cycle (batch or continuous), contact method, and specific extractor choice. 

The variety of extractors used in liquid-solid extraction is diverse, ranging from batchwise dump or heap leaching for the extraction of low grade ores to continuous countercurrent extractors to extract materials such as oilseeds and sugar beets where problems of solids transport have dominated equipment and development. 

The equipment needed is much simpler, so the overall cost of leaching is much lower. This can be of interest to routine laboratories with a limited budget, unable to afford a supercritical fluid extractor. 

Leaching is a unit operation where a solute molecule is removed from a solid using a fluid extractor. This is similar to liquid extraction, except that the solute to be removed comes from a solid rather than from a hquid as in the case of liquid extraction. Also, the fluid extractor may be a fluid or a gas. For example, poUutants can be leached out from solid wastes in a landhll as rain percolates down the heap. 

Leaching—A unit operation where the solute molecule is removed from a solid using a fluid extractor. 

Coarsely ground material is placed in the body of the extractor which may be jacketed for control of the extraction temperature. The packing must be even or the solvent flows preferentially through a limited volume of the bed and leaching is inefficient. In large extractors, channeling is prevented or reduced by horizontal, perforated plates placed at intervals in the bed these redistribute the percolating liquid. 

Fig. 5.8. (A) General scheme of a dynamic focused microwave-assisted extractor. (B) Experimental set-up used to integrate microwave-assisted extraction with the subsequent steps of the analytical process. (1) Leaching step CT controller, MO microwave oven, S sample, R condenser, WR water reservoir, TCPP two-channel piston pump, ER extract reservoir, SV switching valve. (2) Clean-up/preconcentration step M methanol, A air, B buffer, PP peristaltic pump, F filter, EL elution loop, MC mini-column, R retention direction, E elution direction, 1V1-1V3 injection valves, W waste. (3) Individual separation-detection step HPIV high-pressure injection valve, AC analytical column, DAD diode array detector, SR solvent reservoirs.

The glass was crushed, sieved, and leached for 24 hr with boiling distilled water in a Soxhlet-type extractor. Although there are variations, the values reported (Table X) would be reduced two to three orders of magnitude at ambient temperature, and the glasses would then have an acceptably low leach rate. Our program now is to determine the temperature dependence of leach rates and to correlate weight loss data with bulk leach rates based on specific elements. 

Several types of moving beds have been adopted for leaching. The Boilman extractor involves a chain of perforated baskets that moves downward at one side of the bucket... 

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