Continuous Extraction of Nickel from Superalloy Scraps Using Zinc Circulation
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INTRODUCTION
NICKEL (Ni)-based superalloys, which are mainly used in high-pressure turbine blades in aerospace and power generation industries,[1–3] exhibit excellent corrosion resistance and mechanical strength over a wide range of temperatures; however, they necessarily contain rare (scarce) and expensive metals such as rhenium (Re) and tantalum (Ta) for their high-temperature strength. Owing to the recent advances in the aircraft manufacturing and power industries, a rapid increase in the demand for Ni-based superalloys is anticipated.[1] As the production and usage of Ni-based superalloys increase, a large amount of impure scraps that cannot be reused is generated, and therefore, a simple and efficient method for recycling Ni along with Re and Ta from the scraps is highly preferable.[1–4] Some Ni-based superalloys can be recycled through remelting with virgin metals to produce primary superalloy ingots; however, the use of such remelted scraps in superalloy manufacturing makes quality control more difficult due to the high possibility of contamination by impurities.[2,4] Since very strict quality control is normally required for the production of turbine blades, only high-grade scraps containing negligible amounts of impurities can be reused using this method. Lower-grade scraps can potentially be utilized in the stainless steel production as a source of Ni, which does not allow the
RYOHEI YAGI is with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Contact e-mail: r-yagi@iis. u-tokyo.ac.jp TORU H. OKABE is with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Manuscript submitted October 27, 2016. Article published online February 27, 2017. 1494—VOLUME 48B, JUNE 2017
recovery or reuse of other elements such as Re and Ta.[1,2,4] In order to avoid losing valuable metals, various hydrometallurgical processes capable of separating the constituent elements of a superalloy via acid dissolution of scraps have been developed.[5–13] However, despite the applicability of these techniques to a broad variety of valuable metals, they are characterized by several disadvantages such as long processing times, large number of processing steps, and generation of toxic waste solutions. Moreover, the pulverization pretreatment procedure, which is usually required to effectively dissolve scraps in acid solutions, consumes large amounts of energy due to their high strength and refractory nature [m.p. 1593 K to 1723 K (1320 to 1450 °C)].[2,4] In order to establish an effective and environmentally sound recycling method capable of recovering Ni directly from large pieces of superalloy scraps without disposing valuable metals or generating waste solutions, the authors of the present study have developed a pyrometallurgical recovery technique utilizing molten Zn.[14] The advantages of using Zn as the extraction agent are as follows: (1) low melting point [693 K (420 °C)], (2) strong
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