Leaching operation

Separating the soHd and Hquid phases in leaching operations, where a valuable material is contained in the Hquid phase, so its recovery is to be maximized. 

Leaching Chemistry. The purpose of the leaching operation is to dissolve the desired mineral and separate it from the gangue material. The reaction should be selective and fast, the solvent inexpensive or easHy regenerated. Several leaching agents are commonly used. 

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. 

AH stacks and vents attached to the process equipment must be protected to prevent environmental releases of hexavalent chromium. Electrostatic precipitators and baghouses are desirable on kiln and residue dryer stacks. Leaching operations should be hooded and stacks equipped with scmbbers (see Airpollution control methods). Recovered chromate values are returned to the leaching-water cycle. 

The economics of some mines frequentiy depend heavily on the revenues derived from leaching the waste to recover additional copper. This waste material, which has to be removed to uncover the ore, may be hauled to specially constmeted dumps, where the sulfides, the most common form of copper mineral, are oxidized and the leach solution can contact the waste material uniformly. Economics sometimes precludes optimization of dumps for leaching operations because of such factors as compaction, size distribution, and terrain (16). 

At the heart of a leaching plant design at any level—conceptual, pre-liminaiy, firm engineering, or whatever—is unit-operations and process design of the extraction unit or hne. The major aspects that are particular for the leaching operation are the selection of process and operating conditions and the sizing of the extrac tion equipment. 

The residence time of the cake-washing fluid within the cake is relatively short and is not normally considered useful for any kind of leaching operation. Therefore, it is assumed that all of the solute is in solution. 

In a leaching operation, the rate at which solute goes into solution is given by an equation of the form... 

Disposal of the residues of leaching operations can pose particular problems when they come into contact with natural waters, as this can eventually lead to the contamination of... 

The seepage and leaking of sulfuric acid solutions used in leaching can also produce liquid wastes however, this potential is offset by the copper producer s interest in collecting as much of the copper-bearing leachate as possible. Older operations generally do not have protective liners under the piles and experience some loss of leachate. New leaching operations use impermeable membranes to confine leach solutions and channel them to collection ponds. 

The tendency of gold(III) chlorocomplexes to adsorb on iron (hydrous) oxides that tend to form via in situ precipitation in acidic pressure leaching operations needs to be controlled to avoid undesirable metal losses. 

Goethite A process for removing iron from leach liquors from hydrometallurgical leaching operations. Used in recovering zinc from the residues of the electrolytic zinc process. See also Jarosite, Haematite. 

Three distinct processes are usually involved in leaching operations ... 

Elgin, J. C. Trans. Am. Inst. Chem. Eng. 32 (1936) 451. Graphical calculation of leaching operations. 

Determining the effects of impurity build-up on recycle of the extraction raffinate to the heap leaching operation, and on recycle in the extraction and electrowinning circuits ... 

The zinc circuit consists of a similar pH-controlled leach at 60°C under oxidizing conditions. Zinc fine dust with low copper content is leached with the copper barren raffinate and with part of the zinc raffinate. The zinc leaching operation is maintained at about pH 2 for most of the leaching time and then slowly raised to a final pH of 4.5, reducing the iron level to below 10 ppm. The leach solution is filtered and cleaned from impurity metals such as Cu, Ni, and Cd by an ordinary cementation procedure, again filtered and finally fed to a solvent extraction circuit. Zinc is extracted in three stages with... 

The possibility of using nitric acid as an oxidizing agent in a leaching operation is often prohibitive because many reagents used in solvent extraction are not stable in an oxidizing solution. By the AX method, however, a nitric acid (nitrate) leach solution can be converted to a weak sulfuric acid (sulfate) solution, well suited for a solvent extraction treatment. 

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