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The strategy of recovering metals from scrap is, in general, much different from primary sources. One of the main differences between the treatment of scrap and that of primary sources is the fact that metals are frequently associated with other metals to form alloys in scrap, while metals occur in primary sources as oxides or sulfides.In this paper, factors affecting the dissolution behavior of metals from various alloy systems are reviewed and discussed. Specific examples are drawn from Au/Ag, Au/Cu, and Ag/Cu systems. The results of the dissolution of various metals from these alloys are reviewed and compared to the dissolution behavior of single metal systems in various lixiviants such as acids, cyanide and ammonia. It has been observed that the presence of other metals in alloys significantly affects the dissolution rate of the metal in question.The leaching behavior of metals from homogeneous alloys relies on the chemical interaction between atoms in the lattice of the alloys, while that from heterogeneous alloys is affected by galvanic interaction established in the solution. The manner in which the dissolution of a certain metal is influenced by surrounding metals is discussed in terms of the passive and noble nature of the metal in relation to the neighboring metals. [Copyright &y& Elsevier]

Rare earth elements (REEs) are of great importance to modern society and their reliable supply is a major concern of many industries that utilize them in metal alloys, semiconductors, electrical equipment, and defense equipment. REEs in the coal waste have been revealed to be an alternative resource for REEs production. In this study, the extraction, recovery, and upgrading of the REEs from coal waste has been realized with the bioleaching and precipitation processes. Reliable and sustainable acid and oxidant production from the oxidation of the pyrite with Acidithiobacillus ferrooxidans to generate acid for leaching were realized in this research. The acidified bioleaching solution was used to extract REEs from coal waste, with 13–14% yields for most REE elements (~72 h of leaching). However, recovery for longer duration tests was significant higher (varies from 40–60% for individual REEs). After extraction, precipitation and separation processes were designed with the aid of Visual Minteq calculations and modeling to concentrate the REEs. With the procedures designed in this research, a final REEs precipitate product containing 36.7% REEs was produced. [ABSTRACT FROM AUTHOR]