Abstract: Highly toxic metallurgical waste materials from copper smelting and/or refining operations are processed on a cyclic basis for recovering copper values therefrom while protecting the environment from contamination. The waste materials, usually smelter flue dust and/or refinery sludge containing copper values and principally arsenic, bismuth, lead, antimony, and cadmium, as toxic values, with or without added sulfuric acid, are reacted in an autoclave at an overpressure atmosphere of O.sub.2. The resulting solution, pregnant with copper and still containing a significant amount of arsenic, i.e. from about 0.5 to about 2.0 grams per liter, is subjected to copper cementation on metallic iron to recover copper and to provide iron values in the solution while substantially eliminating residual toxic constituents, surprisingly without the evolution of arsine gas. The copper-barren but iron-containing solution is then recycled to the leaching step.