Methods for recovering metals from metal-containing materials are provided. The metal-containing material comprises either Co and Li (e.g., an electrode material from a spent lithium ion battery) or Fe and Al (e.g., bauxite). The metal-containing material is exposed to a leaching solution comprising ammonium hydrogen oxalate, oxalic acid, or both, to provide a solid composed of either cobalt oxalate or iron oxalate, and a solution of either lithium oxalate or aluminum oxalate. The solid is processed to provide either cobalt oxide or iron oxide; the solution is processed to provide either a lithium precipitate or an aluminum precipitate, and a filtrate comprising an oxalate; and the filtrate comprising the oxalate is processed to recover ammonium hydrogen oxalate, oxalic acid, or both. The method further comprises repeating the digestion step with the recovered ammonium hydrogen oxalate, the recovered oxalic acid, or both.