Abstract: Metallic copper is recovered from an ammoniacal copper sulfate solution by treating such solution with sulfur dioxide to form a precipitate of copper ammonium sulfite. The crystals are suspended in an acidic solution containing initially from 20 to 75 grams per liter of sulfuric acid, which preferably contains a substantial concentration (up to saturation) of ammonium sulfate, said solution containing from 40% to 70% by weight of such crystals, and the resulting suspension is heated at 140.degree.C. to 170.degree.C. and at 150 to 220 psi. until substantially all the copper ammonium sulfite has been converted to metallic copper. The metallic copper thereafter is separated from the residual solution.