Leaching selection

As can be seen from the list, there are many competing procedures for leaching, selective separation of the metals, and generation of products. Thus, the detailed design of a specific recovery process often involves a number of selected procedures, combined in unique ways. This chapter discusses processes containing a solvent extraction separation procedure. 

Inasmuch as the raw material involved in a leaching process is a heterogeneous mixture of a small quantity of the valuable component and a large quantity of the unwanted material, the development of chemical techniques to leach selectively only the desired or more valuable materials will be a major step forward in any hydrometallurgical processing. To... 

The program involved leaching selected industrial chemical process residuals with leachate derived from municipal refuse in a pilot-scale field facility measuring the concentration of the compounds which leached from the wastes and comparing these field concentrations to those in the EP and other candidate leaching tests. The Toxicity Characteristic Leaching Procedure or TCLP, is the outgrowth of these studies. 

This effect is referred to as pseudo-luminescence. In fact, the glass fibers are gradually leached selectively to the extent that ultimately only a naked silicon framework is left [814], [815], [816], [817]. 

Selective leaching. Selective leaching refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of unstabilized brass, whereby a weakened. 

Uranium Extraction from Ore Leach Liquors. Liquid—Hquid extraction is used as an alternative or as a sequel to ion exchange in the selective removal of uranium [7440-61-1] from ore leach Hquors (7,265,271). These Hquors differ from reprocessing feeds in that they are relatively dilute in uranium and only slightly radioactive, and contain sulfuric acid rather than nitric acid. 

Uranium ores are leached with dilute sulfuric acid or an alkaline carbonate [3812-32-6] solution. Hexavalent uranium forms anionic complexes, such as uranyl sulfate [56959-61-6], U02(S0 3, which are more selectively adsorbed by strong base anion exchangers than are other anions in the leach Hquors. Sulfate complexes are eluted with an acidified NaCl or ammonium nitrate [6484-52-2], NH NO, solution. Carbonate complexes are eluted with a neutral brine solution. Uranium is precipitated from the eluent and shipped to other locations for enrichment. Columnar recovery systems were popular in South Africa and Canada. Continuous resin-in-pulp (RIP) systems gained popularity in the United States since they eliminated a difficult and cosdy ore particle/leach hquor separation step. 

Leaching. Extracting a soluble metallic compound from an ore by selectively dissolving it in a suitable solvent. The solvent is usually recovered by precipitation of the metal or by other methods. 

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. 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

HydrometaHurgical Processes. The hydrometaHurgical treatments of oxide ores involve leaching with ammonia or with sulfuric acid. In the ammoniacal leaching process, the nickel oxide component of the ore first is reduced selectively. Then the ore is leached with ammonia which removes the nickel into solution, from which it is precipitated as nickel carbonate by heating. A nickel oxide product used in making steel is produced by roasting the carbonate. 

Containers should be clear in order to allow detection of foreign particles. Outer coverings minimize irradiation. Plugs used as stoppers are selected with care to prevent flaking into the contents and possible component leaching into the product. 

Increasingly, plastics are being used as parenteral packaging (qv) materials. Plastics such as poly(vinyl chloride), polyethylene, and polypropylene are employed. However, plastics may contain various additives that could leach into the product, such as plasticizers (qv) and antioxidants. PermeabiUty of plastics to oxygen, carbon dioxide, and water vapor must be tested in the selection of plastic containers. Furthermore, the plastic should withstand sterilization. Flaking of plastic particles should not occur and clarity necessary for inspection should be present. 

Ion Excha.nge, The recovery of uranium from leach solutions using ion exchange is a very important process (42). The uranium(VI) is selectively adsorbed to an anion-exchange resin as either the anionic sulfato or carbonato complexes. In carbonate solutions, the uranyl species is thought to be the tris carbonato complex, U02(C03) 3 [24646-13-7] and from sulfate solutions the anion is likely to be U02(S0 , where nis ) [56959-61-6] or 2 [27190-85-8], The uranium is eluted from the resin with a salt or acid solution of 1 AfMCl or MNO (M = H", Na", The sulfate solution is... 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Selectivity for a single metal of a group is the basis of a solvent extraction process for the recovery of copper (qv) from low concentration ore leach solutions containing high levels of iron (qv) and other interfering metals (16). 

A similar process has been devised by the U.S. Bureau of Mines (8) for extraction of nickel and cobalt from United States laterites. The reduction temperature is lowered to 525°C and the hoi ding time for the reaction is 15 minutes. An ammoniacal leach is also employed, but oxidation is controlled, resulting in high extraction of nickel and cobalt into solution. Mixers and settlers are added to separate and concentrate the metals in solution. Organic strippers are used to selectively remove the metals from the solution. The metals are then removed from the strippers. In the case of cobalt, spent cobalt electrolyte is used to separate the metal-containing solution and the stripper. MetaUic cobalt is then recovered by electrolysis from the solution. Using this method, 92.7 wt % nickel and 91.4 wt % cobalt have been economically extracted from domestic laterites containing 0.73 wt % nickel and 0.2 wt % cobalt (8). 

AH commercial copper extractants selectively extract copper from weakly acidic aqueous leach solutions by the general reaction... 

Fluxes. Fluxes, composed mostly of salts or oxides of metals, serve to protect underlying metal from the air. This prevents the formation of surface oxides that impede fusion and the formation of a strong solder joint. Fluxes may also act to selectively leach elements from the surface of the underlying metal. The result is a surface free of obstacles to fusion, and of a composition readily wetted by the solder. 

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