Copper scrap metal leaching

The decomposition of the lower sulfides of the heavy metals and the recovery of the metal as soluble salts and of sulfur in the elemental form have been demonstrated for pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, molybdenite, and associated metals such as nickel and cobalt. Pyrite and chalcopyrite are higher sulfides and to be amenable to this treatment have to be thermally decomposed at 600-650 C prior to leaching. The reactions with nitric acid are exothermic, and are carried out below 1 atm and at around 100°C. In addition to the sulfides, this technique has been applied successfully to the extraction of nonferrous metals from partly oxidized sulfide ores, fayalite slags, copper scrap, and other intermediate products, such as residue from electrolytic zinc plats. 

Many hydrometallurgical processes or process steps are used to upgrade concentrates, process recycled scrap metal, or purify aqueous process steams. Examples are (I) the leaching of molybdenite concentrate to remove Knpurities ,ft (2) leaching of tungsten carbide and molybdenum scrap-, (3) removal of copper impurities in nickel anolyte by cementation on metallic ruckel and (4) various methods for treating nuclear fuel elements. 

Although most circuits are designed to achieve maximum recovery of the species of interest in a single pass, in certain flow sheets this may not be desirable. In the recovery of copper from scrap metal in an anunonia leach, for example, a relatively high concentration of copper in the raffinate is necessary for the optimal operation of the leaching circuit, while in other circuits the concentration of acid produced in the raffinate may be more critical than the corresponding metal extraction. 

Cementation involves precipitation of the copper by passage of the leach solution over scrap iron (Eq. 13.25). This requires about 1 hr of contact time and produces a finely divided form of the metal analyzing ca. 90% Cu (dry) by replacement of the dissolved copper by iron. In theory this should only require 0.879 kg of iron for each kilogram of copper obtained (Eq. 13.25). In practice there is an iron consumption of 1.3-3 kg/kg copper [44]. At times as much as 10% of U.S. production has been from cement copper. 

Different agents can be used for the reduction leaching of asbolane and heterogenite SO2, sodium sulphite, bisulphite or metabisulphite (Na2S205), FeS04, metallic iron scraps or powdery copper. 

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