METHOD FOR TREATING ALLOY

Provided is a method for obtaining a solution having a high concentration of nickel and/or cobalt from an alloy that contains copper as well as nickel and/or cobalt, in a waste lithium-ion battery. A method for treating an alloy comprises: a leaching step S for subjecting an alloy that contains copper as well as nickel and/or cobalt to a leaching treatment by using an acid solution in the presence of a sulfiding agent to obtain a leachate and a leaching residue; and a reduction step S for adding a reducing agent to a part of the obtained leachate and performing a reduction treatment to obtain a post-reduction solution and a reduction residue, wherein in the leaching step S, the leachate that has not been provided in the reduction treatment in the reduction step S, is repeatedly used as part or all of the acid solution added in the leaching treatment.