Theory And Technology Research Of Diaphragm Electrowinning Sheet-form Copper From Acidic Cupric Chloride Etchant Waste

During the etching process of producing PCB, the acidic etchant waste which contains significant amounts of cupric chloride is produced, and it will not only destroy the environment, but also lead to the great waste of metallic copper if it is discharged. Due to at some shortages such as only getting copper powder or sponge copper, lots of chlorine release during conventional electrowining process, a new technology of electrowining sheet-form metallic copper from acidic cupric chloride etchant waste in the diaphragm cell with cation exchange membrane was proposed in this paper. Copper-starting-sheet cathode and graphite anode were adopted in this process. The electrochemical behaviors of cathodic reaction, the copper electrodeposition mechanism and technical conditions were investigated.Thermodynamics analysis of copper chloride complexes was studied and the E-pH diagram of Cu(Ⅱ)-Cu(Ⅰ)-NH3-HCl-H2O system was drawed at first. The results showed that, cupric ions were mainly in the form of CuCl+ and cuprous ions were principally in the form of CuCl32- complex in the etching waste concentration used in the experimental.The electrochemical behaviors of copper electrodeposition in acid cupper chloride solution were investigated using cyclic voltammetry, linear polarization, potentiostatic current-time transient. It was indicated by studying the cathodic polarization behavior that copper reduction process was carried out through two steps at low overpotential, which included CuCl+ restoring to CuCl32- by getting one electron and CuCl32-converting to Cu by fetching another. Furthermore, CuCl+ could be deoxidated to Cu directly when the cathodic overpotential exceeded a certain value. Cathodic polarization curves under different additives concentration showed that the additives had the effect of increasing the cathodic polarization, which was conducive to the formation of compact sheet-form metallic copper. The exchange current density of copper electrodeposition was 7.53x10-3A/cm2, apparent activation energy of cathodic reaction was 13.804kJ/mol. Current-time transient curves under different applied potentials indicated that copper electrodeposition undergone three-dimensional growth mode. The electrocrystallization mechanism changed from progressive nucleation to instantaneous nucleation with the applied potential negative shift. The growth rate of nuclei K with the applied potential E satisfied the relationship:lgK=-1.74-3.65*E。Based on the mechanism analysis, technical conditions studies on electrowining sheet-form metallic copper from acidic cupric chloride etchant waste was carried out. The diaphragm cell was separated into big cathode chamber and small anode chamber with cation exchange membrane. The optimum conditions were obtained as follows:cathode current density 200A/m2, Cu2+ concentration range was about 40-20 g/L in catholyte, initial [cl] 5.5 mol/L in catholyte, bath temperature 35℃, polar distance 5 cm, [H+] 3.0 mol/L, additives 0.2 g/L. Under the optimum conditions, integrated experiments were obtained good results, the cathode current efficiency was about 87.68%, and average cell voltage was about 2.95V, the unit energy consumption was 2684 kWh/t per ton metal copper. Photographs and SEM images showed that compact sheet-form metallic copper were obtained and the purity of cathode copper was greater than 99.90%, it was in line with GB/T467-1997 cathode copper standards.