Leaching—continued equipment

Modem manufacturing processes quench the roast by continuous discharge into the leach water held in tanks equipped with agitators. At this point the pH of the leach solution is adjusted to between 8 and 9 to precipitate aluminum and siHcon. The modem leaching operations are very rapid because no or htde lime is used. After separation of the ore residue and precipitated impurities using rotary vacuum filters, the cmde Hquid sodium chromate may need to be treated to remove vanadium, if present, in a separate operation. The ore residue and precipitants are either recycled or treated to reduce hexavalent chromium to Cr(III) before disposal. 

It is classification by contacting method that provides the two principal categories into which leaching equipment is divided (I) that in which the leaching is accomphshed oy percolation and (2) that in which particulate solids are dispersed into a hquid and subsequently separated from it. Each includes batch and continuous units. Materials which disintegrate during leaching are treated in equipment of the second class. 

Continuous Percolators Coarse sohds are also leached by percolation in moving-bed equipment, including single-deck and nmlti-deck rake classifiers, bucket-elevator contactors, and horizontal-belt conveyors. 

Dispersed-Solids Leacbing Equipment for leaching fine solids by dispersion and separation includes batch tanks agitated by rotating impellers or by air and a variety of continuous devices. 

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. 

Leaching has in the past been carried out mainly as a batch process although many continuous plants have also been developed. The type of equipment employed depends on the nature of the solid—whether it is granular or cellular and whether it is coarse or fine. The normal distinction between coarse and fine solids is that the former have sufficiently large settling velocities for them to be readily separable from the liquid, whereas the latter can be maintained in suspension with the aid of only a small amount of agitation. 

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. 

For the first step, 12 g. of ground coal and 120 ml. of alkaline solution were mixed and placed in a 300-ml. stainless steel autoclave equipped with a turbine agitator. The system was flushed with nitrogen and then heated to the desired temperature while the mixture was stirred continuously. After a period of treatment at constant temperature and pressure, the autoclave was cooled quickly, and the contents were filtered to recover the coal. The filter cake was washed with 400 ml. of distilled water, dried at 90 C for 4 hr., weighed, and analyzed for total sulfur and ash. A portion of the alkali-treated coal (usually 2.5-3.0 g) was leached for an additional... 

Continuous leaching can also be carried out on a laboratory scale, with one or more tanks (in series or in parallel) and with recycling of solids, if necessary. The difficulties that arise in long-term operation of these systems are usually due to wear on the equipment due to the action of the mineral particles, problems in maintaining complete suspension and homogeneity of the mineral, and accurate transfer of relatively small volumes of mineral suspensions from the feed tank to the... 

---