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2NaBr = 2Na + Br2 2Na + 2H2O = H2 + 2NaOH Br2 + H2O = HBr + HOBr (3.13) (3.14) (3.15) Bromine is also used as an etchant solution to recover gold from gold-plated materials, given that gold is an excellent electrically conductive material widely used in the electronics industry. Bahl and Leach filed two patents in 1978 and 1980 for bromide etching of gold. In 1978, they investigated the optimal ratio of bromine and sodium bromide, depending on the thickness of gold films (Bahl et al, 1978). They further investigated the methodologies to recover gold from the bromide etchants. They found that precipitation and decomposition could possibly efficiently recover gold by adding a base and heating, respectively (Bahl et al, 1980). In the late 1980s and early 1990s, the Great Lakes Chemical Corporation (Chemtura) devoted much effort to developing a series of stabilized bromine products, such as 1-chloro-3- bromo-5,5-dimethylhydrantoin (GEROBROM 3114), 1,3-dibromo-5,5-dimethylhydantoin (GEROBROM 5500) and GEROBROM 3400, to serve as the bromine source for precious metal leaching. The benefits of using the stabilized bromine are to reduce the loss of bromine, given the high vapor pressure of liquid bromine, and also to overcome the difficulty of handling liquid bromine. Sergent and Thanstrom presented the application of N-halohydantoin in the extraction of precious metals in U.S. Pat. 4,637,865 (Sergent et al., 1987). They proposed that 1,3-dibromo-5,5- dimethylhydantoin offered the high recovery of gold from several types of gold bearing ores, including carbonaceous and refractory ores. However, given the silver bromide precipitation on the surface of the gold plating materials which also contained silver, a relatively low recovery of gold was attained by using 1,3-dibromo-5,5-dimethylhydantoin. They also reported that electrowinning and electrodeposition were capable of recovering the precious metals from the pregnant leaching solution that contained N-halohydantoin. Sergent presented his work on the application of GEROBROM 3114 and GEROBROM 5500 for gold extraction in 1988 (Sergent, 1988). In his study, he reported that GEROBROM 5500 reacted with water to release free bromine and dimethylhydantoin. Both the bromine-based oxidizing reagents were capable of oxidizing and complexing gold to form tetrabromo gold ions (Sergent, 1988). An investigation was conducted by Dadgar in 1988 to compare cyanidation and bromine leaching of gold by using GEROBROM 3400 and cyanide (Dadgar, 1988). This study indicated that even though both GEROBROM 3400 and cyanide gave similar extractions (94-96%), the 31PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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