Recovery of valuable metals from a spent nickel–metal hydride battery: Selective chlorination roasting of an anodic active material with CCl4 gas

Abstract

A method for recovery of valuable metals from a spent nickel–metal hydride battery has been investigated. Mischmetal–nickel alloy (MmNi5), which acts as an anode active material, was separated from an electrode scrap mixture using a sedimentation process. The separated MmNi5 contains rare earth metals (14.5mass% Ce, 10.8mass% La, 4.5mass% Nd) and transition metals (49.2mass% Ni, 9.8mass% Co, 4.7mass% Mn). CCl4 gas was employed as a chlorination roasting agent to prevent the formation of rare earth oxychlorides. When MmNi5 was chlorinated at 673K, it was decomposed into rare earth chlorides and nickel alloy. The chlorinated sample was subsequently leached with distilled water to recover these materials. The leaching yields of La, Ce, and Nd reached 96.3, 77.4, and 80.1mass%, respectively, and 85.7mass% of the Ni and 87.1mass% of the Co could be recovered as Ni–Co alloy. The relatively lower recovery ratios of Ce and Nd can be attributed to the stability of Ce and Nd oxychlorides. When the chlorination temperature reached 773K, MmNi5 was completely chlorinated. At 973K and higher, a greenish precipitate was also observed in the low-temperature zone of the reaction tube. Because the vapor pressures of rare earth and transition metal chlorides differ from each other, the transition metal chlorides can be separated by vaporization. Their yields reached 94.6% (La), 95.3% (Ce), and 94.9% (Nd) in the chlorinated residue.