a process for the recovery of arsenic trioxide from acid solutions , particularly the aqueous acid effluent produced by a gas - washing process in the pyrometallurgy of copper sulfide ores is provided . generally , the process comprises concentrating the acid solution in one or more evaporators in series ; crystallizing arsenic contained in the concentrated solution as arsenic trioxide crystals ; filtering the crystallized solution to obtain a solid phase comprising impure arsenic trioxide crystals ; and purifying the solid phase to obtain a purified crystal product comprising arsenic trioxide . the process produces a commercially salable arsenic trioxide product without producing any solid or liquid residues subject to special handling or treatment regulations .