| In the modern zinc-electrowinning industry, the Pb-Ag(0.5wt%to1.0wt%) and Pb-Ag(0.2wt%to0.3wt%)-Ca(0.06wt%to0.1wt%) anode is widely used. The oxide layer will form on the surface of anode during polarization. The phase, microstructure, conductivity, porosity, adhesive force, and oxygen evolution activity of anodic oxide films are of considerable importance to the oxygen evolution overpotential and the work-durability of anodes during zinc electroextraction. To explore the formative characteristic of lead-base alloy anode during zinc electroextraction, the experimental systems for assessment of anodic oxidation layer were designed. The real time data of anodic oxide films is obtained by anodic polarization curves, quasi-stationary polarization (Tafel), electrochemical impedance spectroscopy techniques, SEM, XRD, and EDXS. The real-time status of anodic oxide films is evaluated using the parameters of peak potential, peak current, oxygen evolution overpotential, surface (exchange) current density, charge transfer resistance, roughness, phase and microstructure. The (semi-)quantitative evaluation of the formation, change and stability of anodic oxide films is achieved.Pb-0.8%Ag and Pb-0.3%Ag-0.06%Ca rolled aolly anode that is widely used in the zinc-electrowinning industry is selected for research subjects as well as Pb-0.3%Ag-0.6%Sb aolly that is developed in the laboratory room. Three kinds of the anode were deal with pre-film-plating. Then six experiments samples ware obtained. The six experiments samples is electrolyzed in four solution system for15days'continue polarization. The real time data of electrochemistry, phase and microstructure is tested upon reaching specific polarization time points (0,1,2,3,6,9,12, and15days). The formative characteristic of anodic oxide films with increasing polarization time is longitudinally analyzed. The impact of types and pre-film-plating on formative characteristic of anodic oxide films is transversely compared. The major research results are drawn as follows:(1)50g·L-1Zn2+;150g·L-1H2SO4. The formative characteristic of anodic oxide films changes with anode types and pre-film-plating. With increasing polarization time, the overpotential of oxygen evolution of six experiments samples mainly presented a declining trend.With increasing polarization time, the size of microscopic particles of Pb-0.8%Ag anodic oxide films first increased and then decreased. The main phase is transformed from ?-PbO2to ?-PbO2. After15days of polarization, the microstructure was mainly composed of tetragonal crystalline grains with very few cavities. The uniform, well-defined crystal grains and regularity is obtained. The overpotential of oxygen evolution is780mV(500A·m-2).With increasing polarization time, the main phase of coating Pb-0.8%Ag is ?-PbO2. The intensity of diffraction peak gradually increased. Both manganese oxide and leady oxide are observed on the anodic oxide films after15days polarization. The leady oxide was wrapped in manganese oxide. The overpotential of oxygen evolution is784mV(500A-m-2).With increasing polarization time, the size of microscopic particles of Pb-0.3%Ag-0.06%Ca anodic oxide films increased. The quantity of corrosion holes first increased and then decreased. The main phase is transformed from PbSO4to ?-PbO2. The anodic oxide consisted mainly of a large cauliflower-like crystal grains. The overpotential of oxygen evolution is771mV(500A·m-2).With increasing polarization time, the main phase of coating Pb-0.3%Ag-0.06%Ca anode is transformed from ?-PbO2to ?-PbO2. Both manganese oxide and leady oxide are observed on the anodic oxide films after15days polarization. The leady oxide was wrapped in manganese oxide. The manganese oxide is plate shaped with large size. The leady oxide was mainly composed of fine tetragonal crystalline grains. The overpotential of oxygen evolution is764mV(500A·m-2).The composition of Pb-0.3%Ag-0.6%Sb anodic oxide layer reaches a stable state after3d of polarization. The anodic oxide consisted mainly of a large cauliflower-like crystal grains with many deeper holes.The main phase is ?-PbO2. The intensity of diffraction peak gradually increased. The overpotential of oxygen evolution is779mV(500A·m-2).With increasing polarization time, the main phase of coating Pb-0.3%Ag-0.6%Sb anode is changed as PbSO4??-PbO2??-PbO2. The manganese oxide is disappeared after15days polarization. The leady oxide was mainly composed of fine tetragonal crystalline grains. The overpotential of oxygen evolution is768mV(500A·m-2).(2)50g·L-1Zn2+;150g·L-1H2SO4;600mg·L-1Cl-?The composition of six anode sample reaches a stable state after6d of polarization. The major phase of the oxide layer is ?-PbO2no matter type of lead-based alloy and pre-film-plating. The oxide film is uniform. The uniform, well-defined crystal grains are disappeared. The oxygen evolution overpotential sway a little (712-730mV). It is demonstrated that high content Cl-decides the formation of anodic oxide films in acid zinc sulfate electrolyte system.(3)50g·L-1Zn2+;150g·L-1H2SO4;5g·L-1Mn2+?Both manganese oxide and leady oxide are observed no matter if pre-film-plating for Pb-0.8%Ag anode after15days polarization. Coarse grains and alternating phase are also observed. For Pb-0.8%Ag anode, the major phase of the oxide layer is ?-PbO2and oxygen evolution overpotential is653mV (500A-m-2). For coating Pb-0.8%Ag anode, the major phase of the oxide layer is ?-PbO2and oxygen evolution overpotential is674mV (500A·m-2).Both manganese oxide and leady oxide are observed no matter if pre-film-plating for Pb-0.3%Ag-0.06%Ca and Pb-0.3%Ag-0.6%Sb anode after15days polarization. The manganese oxide is outside. The leady oxide is inside. The distribution with clear layers is observed with many corrosion holes. For Pb-0.3%Ag-0.06%Ca and coating Pb-0.3%Ag-0.6%Sb anode, the major phase of the oxide layer is ?-PbO2and oxygen evolution overpotential is674mV (500A·m-2). The oxygen evolution overpotential of coating Pb-0.3%Ag-0.6%Sb is highest (0.701V at500A·m-2). The oxygen evolution overpotential of Pb-0.8%Ag and Pb-0.3%Ag-0.06%Ca anode is lowest (0.653V at500A·m-2).(4)50g·L-1Zn2+;150g·L-1H2SO4;600mg·L-1Cl-;5g·L-1Mn2+.The oxide layers are almost the same with uniform microstructure and less corrosion holes for three anode sample no matter if pre-film-plating. But the major phase of the oxide layer is different. The major phase are PbSO4for Pb-0.8%Ag and coating Pb-0.3%Ag-0.6%Sb. The major phase is ?-PbO2for Pb-0.3%Ag-0.06%Ca and coating Pb-0.8%Ag. The major phase are ?-PbO2for Pb-0.3%Ag-0.6%Sb.The oxygen evolution overpotential of Pb-0.3%Ag-0.6%Sb is lowest (633mV at500A·m-2). The oxygen evolution overpotential of Pb-0.8%Ag anode is highest (0.680V at500A·m-2).In summary, the formative characteristic of lead-base alloy anode during zinc electroextraction is controlled by the following three aspects:type of lead-based alloy; pre-film-plating process; the composition of electrolyte. The formative characteristic of anodic oxide films changes with anode types and pre-film-plating in pure acid zinc sulfate electrolyte system. The formative characteristic of anodic oxide films is decided by high content Cl-in acid zinc sulfate electrolyte system containing high Cl-. The effect of anode types and pre-film-plating is poor. The formative characteristic of anodic oxide films changes with anode types and pre-film-plating in acid zinc sulfate electrolyte system containing Mn2+. The oxide layers are almost the same with uniform microstructure and less corrosion holes for three anode sample no matter if pre-film-plating in acid zinc sulfate electrolyte system containing Cl-and Mn2+. But the major phase of the oxide layer is different. The formative characteristic is controlled by the three aspects:type of lead-based alloy, pre-film-plating process, the composition of electrolyte. |
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