Dissolution of nickel hydroxide in ammoniacal aqueous solutions
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NICKEL-METAL HYDRIDE (Ni-MH) batteries have been used as a power source of hybrid electric vehicles (HEVs).[1] More than 200,000 HEVs equipped with Ni-MH batteries have already been sold since their commercialization in 1997. The total weight of Ni-MH battery modules mounted in a HEV is several tens of kilograms. Since the batteries have a finite lifetime, a considerable quantity of the spent batteries will be generated continuously in the near future. Recycling of the batteries at the end of their life is becoming an increasingly important issue due to the increased requirement for saving natural resources and from an economic point of view. The batteries contain the valuable metal resources Ni, Co, and rare-earth (RE) metals.[2–5] The positive electrode consists of Ni hydroxide as the active material and a porous Ni or Ni-plated steel substrate. The Ni hydroxide employed in commercialized batteries contains a small amount of Co or Zn as an additive.[6] The negative electrode is a hydrogenstorage alloy on a Ni-plated steel mesh substrate. The hydrogen-storage alloy is basically LaNi5, but Ni is partially substituted by Co, Mn, and Al, and La is substituted by mischmetal (denoted as Mm, a mixture of La, Ce, Nd, and Pr). In the batteries, the two electrodes are separated by a polypropylene insulating layer and sealed in a stainless steel or plastic case together with a KOH aqueous electrolyte. Some investigations on the separation and recovery of the valuable metals in the batteries have been reported.[2–5,7–14] The firm INMETCO, a subsidiary of INCO, Ltd., has treated the batteries with a pyrometallurgical process, where some metals such as Ni and Co are recovered.[14] However, pyrometallurgical processes have drawbacks in that they MASAO MIYAKE, Research Associate, and MASAFUMI MAEDA, Professor, are with the International Research Center for Sustainable Materials, Institute of Industrial Science, the University of Tokyo, Tokyo, 153-8505 Japan. Contact e-mail: [email protected] Manuscript submitted June 21, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
require a large amount of energy, and RE metals in the alloys are lost in slag. Hydrometallurgical processes have, therefore, been studied exclusively.[2–5,7–13] The methods are basically as follows. First, the batteries are mechanically crushed and dissolved in strong sulfuric or hydrochloric acid, and, then, the metal ions are separated by solvent extraction. Finally, single metals or metal salts are recovered by electrowinning or precipitation methods. However, these methods have complicated separation processes and consume a large amount of chemical agents, and, therefore, the recovery cost is rather high. We have proposed a new process, in which the hydrogenstorage alloy is separated and recovered without its decomposition.[13] From the battery scraps, the mixture comprising mainly the alloy and Ni hydroxide can be obtained by removing plastic materials, Fe substrates, and electrolytes through physical selection techniques such as flotation, magnetic
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