Thermodynamics of Gold Dissolution Behavior in CaO-SiO 2 -Al 2 O 3 -MgO sat Slag System
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thalpy change for the dissolution of gold into the CaO-SiO2-Al2O3-MgOsat slag system was measured to be about 80 kJ/mol, indicating that the gold dissolution is exothermic. From the iso-Au solubility contours, the dominant factor affecting the gold dissolution behavior is the (CaO + MgO)/SiO2 ratio, whereas the influence of Al2O3 was negligible. Consequently, less basic slags and higher processing temperatures, in conjunction with a strongly reducing atmosphere, are recommended to increase gold recovery during pyro-processing of Au-containing e-wastes. DOI: 10.1007/s11663-015-0421-z Ó The Minerals, Metals & Materials Society and ASM International 2015
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INTRODUCTION
GOLD is a bright yellow, dense, soft, malleable, and ductile metal and has been a valuable and highly soughtafter precious metal for coinage jewelry and other arts since long before the beginning of recorded history.[1] Physically, gold is the most ductile of all metals, is a good conductor of heat and electricity, and reflects infrared radiation strongly.[1,2] Chemically, it does not react with water, dry or humid air, and most corrosive reagents, so these are the reasons why it is well suited for use in coins and jewelry and as a protective coating on other more reactive metals.[2] Because gold has good physicochemical properties, it plays an increasingly important role in industrial appli-
YUN SOON HAN, Graduate Student, and JOO HYUN PARK, Professor, are with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea. Contact e-mail: basicity@ hanyang.ac.kr DOUGLAS R. SWINBOURNE, Professor, is with the School of Civil and Chemical Engineering, RMIT University, GPO Box 2476V, Melbourne, VIC 3001, Australia. Manuscript submitted March 13, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
cations. For example, over 300 tons of gold are used annually in electronic components such as electroplated coatings on connectors and contacts in mobile phones, computers, and other electrical devices.[3] However, 75 pct of all gold ever produced has been extracted since 1910, and gold reserves in the world are becoming rapidly depleted.[4] The cost of gold production and its price in the market have increased rapidly, and interest in the recycling of gold-containing materials has naturally increased. The wastes of electronic equipments (e-wastes) contain large amounts of precious metals compared to their own respective ores, and therefore such wastes may be considered as a secondary source of valuable metals. For instance, the concentration of gold in natural ore is commonly between 0.5 and 15 g/ton-ore (0.5 to 15 ppm), while its concentration in electronic circuit boards is over 10 times higher, typically being about 150 ppm in expansion cards and over 10,000 ppm in central processing units, CPUs.[5] This is the reason why Au-containing materials such as e-wastes are highly important sources in the concept of ‘Urban Mining Technology.’[6] From the viewpoint of pyrometallurgical processing, the distribution ratio of gold between metal and s
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