HYDROMETALLURGICAL PROCESS FOR RECOVERY OF METAL VALUES FROM SPENT NICKEL METAL HYDRIDE SECONDARY BATTERIES

A completely hydrometallurgical process has been developed for the rec overy of metal values such as cobalt, nickel and rare earths from spen t nickel-metal hydride (Ni-MH) secondary batteries. Effects of hydroch loric acid concentration, temperature, reaction time and solid-to-liqu id ratio on leaching of metals contained in the electrode materials of the batteries were studied. The optimal operating conditions were fou nd to be 3 M HCl at a temperature of 95 degrees C and a 3-h leach time , and it was possible to treat up to 5.5 g of scrap in 50 mi of acid s olution where the recoveries 100% of cobalt, over 96% of nickel and 99 % of rare earths were achieved. A typical chemical composition of the resulting leach liquor was approximately, in grams per liter, 23.4 Ni, 1.7 Co, 3.4 Fe, 0.72 Zn, 0.46 Al, 1.2 Mn, 4.2 La, 0.26 Ce, 0.82 Pr, 2 .6 Nd and 0.074 Sm, as well as 50 Cl. The pH of the solution was aroun d 1.2. The rare earth values can be readily recovered from the leach L iquor by the use of a solvent extraction circuit with 25% bis(2-ethylh exyl) phosphoric acid (D2EHPA) in kerosene, in which a two-stage count er-current extraction at an O:A ratio of 3:1 at an equilibrium pH of 2 .0, a single-stage cobalt scrubbing with 0.3 M hydrochloric acid at an O:A ratio of 22:1, and a stripping operation with 2.0 M hydrochloric acid in one contact at an O:A ratio of 5:1 are involved. A mixed rare earth oxide of over 99% purity was obtained by selective precipitation with oxalic acid, and calcination of the precipitate. The total yield of rare earths approached 98%. The cobalt and nickel in the raffinate are effectively separated by selective extraction of cobalt with 25% TOA in kerosene after concentration (up to [Cl-] approximate to 220 g l(-1)). Nearly complete recovery of cobalt can be achieved by using a three-stage counter-current extraction at an O:A ratio of 2:1, followe d by stripping with a dilute hydrochloric acid solution (pH 2.0) in a single stage at an O:A ratio of 4:1. Subsequently the cobalt in the st rip liquor and the nickel remained in the raffinate are separately rec overed as oxalates by the addition of ammonium oxalate. A pure cobalt product and a nickel oxalate with a purity close to 99.9% were obtaine d. The total recoveries of cobalt and nickel were found to be approx. 98% and 96%, respectively. (C) 1998 Published by Elsevier Science B.V. All rights reserved.