Cobalt oxide was prepared from spent lithium ion batteries (LIBs) by reductive leaching, copper sulfide precipitation, cobalt oxalate precipitation and thermal decomposition. The cobalt rich non-magnetic 16 mesh fraction obtained from spent LIBs by mechanical separation was leached using 2 M H2SO4, 6 vol% H2O2, reaction temperature 60 degrees C, agitation speed 300 rpm, pulp density 100 g/L, reaction time 1 h. The leaching efficiency of cobalt was more than 99% and its concentration was 27.4 g/L. Copper was removed (99.9%) as CuS by precipitating with Na2S. The crystalline solid CoC2O4 center dot 2H(2)O selectively precipitated by treating the copper-free liquor with oxalic acid was calcined to produce crystalline Co3O4, of which primary average particle size was 340 nm. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.