<Original Papers>Oxidative Ammonia Leaching of a Copper-Arsenic Deposit and the Oxidation of As (III)

The oxidative ammonia leaching of a copper-arsenic deposit containing Cu_3As, Cu, CuSO_4・5H_2O, Cu_2O and As_2O_3,a product from the purification of copper electrolyte, and the oxidation of As (III) to As (V) in aqueous ammoniacal solution by oxygen under pressure were investigated. The potential-pH diagram for the Cu-As-NH_3-H_2O system at 25℃ was constructed by using critically selected thermodynamic data. The diagram predicts that in ammonia teaching, copper and arsenic in the copper-arsenic deposit are extracted in the form of Cu(NH_3)_2^+ and AsO_2^- which oxidize to Cu(NH_3)_4^ and HAsO_4, respectively. All copper and arsenic in the copper-arsenic deposit were extracted in the presence of excess NH_3 as stable Cu(NH_3)_4^ at 25℃ and at an oxygen pressure of 2kg/cm^2. The leaching rate was enhanced by increasing the NH_3/Cu molar ratio, oxygen pressure and temperature. Addition of (NH_4)_2SO_4 as a buffer reagent also promoted the leaching reaction. During teaching, As (III) was oxidized to As (V) at 25℃ and at an oxygen pressure of 2kg/cm^2. In the absence of Cu (II), the oxidation of As (III) in aqueous ammoniacal solution by oxygen under pressure hardly proceeded even at 80℃. In the presence of Cu (II), the oxidation was not observed at 25℃, but was enhanced with an increase in temperature. The acceleration of oxidation at 25℃ was accomplished in the presence of Cu (I) or by the addition of metallic copper powder or cuprous oxide. The oxidation in aqueous ammoniacal solution was suppressed by the formation of arsenic (V)