پديدآورندگان :
Karimi Saeed Department of Metallurgy and Materials Engineering, Hamedan University of Technology, Hamedan, Iran , Shoghi Sima Department of Metallurgy and Materials Engineering, Hamedan University of Technology, Hamedan, Iran , Moridani Zahra Department of Metallurgy and Materials Engineering, Hamedan University of Technology, Hamedan, Iran
چكيده فارسي :
In this research, the feasibility of using graphite anodes in the zinc electrowinning process has been investigated. The cathode and electrolyte used are aluminum and acidic zinc sulfate, respectively. The efficiency of current density in electrowinning conditions was calculated by changing two parameters of temperature and potential in the range of 25-50 °C and 3.0-4.5 V. Also, the weight loss of graphite anode was evaluated in these conditions. According to the results, the current density efficiency has increased from 10.0% to 35.6% with increasing temperature from 25 to 40 °C, and after that, the current efficiency has decreased to 34.4% at a temperature of 45 °C and about 16.7% at a temperature of 50 °C. But in the investigation of weight loss of graphite anode, it was observed that it increased from 1 to 3 g in the range of 25-50 °C. This shows that the graphite mass loses by decomposing into carbon monoxide or carbon dioxide, and this becomes more intense with increasing temperature. But considering the potential in the range of 3 to 4.5 V and the constant temperature of 40 °C, it was observed that the efficiency of the current intensity has decreased from 32% in the potential of 3.0 V to about 22.7% in the potential of 4.5 V. This indicates that the excess potential causes its consumption in side reactions and reduces the flow efficiency. In addition, the increase in mass loss from 2.5 g at 3.0 V to about 7.0 g at 4.5 V is quite evident. In general, it can be said that the graphite anode is not suitable for zinc electrowinning, and the reason for this is low efficiency and the loss of graphite during electrolysis. Good anode material must have the following three properties: electrical conductivity, electrocatalysis and stability and good corrosion resistance [1]. Lead-based alloys are widely used as anode material in the electrowinning industries, such as zinc [2,3], manganese [4], copper [5]. In the zinc electrowinning, due to some problems with lead alloys such as high OER potential, lead pollution to cathode products, short circuit due to lead distortion and high consumption of noble metals (such as Ag) [6]. There is always interest in finding ways to improve anode performance and minimize corrosion and overpotential problems [7]. In this regard, no research on the feasibility of using graphite as anode in the zinc electrowinning has been investigated. This anode is widely used in the production of aluminum from molten salt, and the emission of CO and CO2 gases is mentioned as one of the problems of this process [8]. In this research, graphite as anode in the zinc electrolysis process was evaluated from the perspective of current density efficiency and graphite mass loss. The electrolysis process was carried out at a concentration of 40 g/l zinc and 120 g/l sulfuric acid for 4 h. The anode was made of graphite and the cathode was made of aluminum. Temperature parameters were evaluated in the range of 25-50 °C and voltage in the range of 3.0 to 4.5 V. In order to make sure that there is no concentration polarization, the solution was continuously stirred by a stirrer. Electrolysis was done with a power source (RN-3040-S). The temperature of the solution was continuously measured using a thermometer, and a temperature adjustment error of 2 °C was created. In order to evaluate the efficiency of the current intensity, the aluminum cathode was weighed before and after the electrolysis process. Also, the graphite anode was weighed before and after electrolysis using a laboratory scale with an accuracy of two digits. The results of current intensity efficiency and mass loss of graphite in the investigation of two parameters of temperature and applied potential of zinc electrolysis are shown in Figure 1. According to Figure 1a, it is clear that the current density efficiency increased from 10.0 % to 35.6 % as the temperature increased from 25 to 40 °C, and then the current efficiency decreased to 34.4 % at 45 °C and about 16.7 % at 50 °C. It seems that side reactions such as the emission of hydrogen gas on cathode and evolution of O2, CO and CO2 have caused a drop in efficiency. In the investigation of weight reduction of graphite anode, it was observed that it increased from 1 to 3 g in the range of 25-50 °C. This shows that the graphite loses weight by decomposing into CO and CO2, and this becomes more intense with an increasing temperature. But considering the potential in the range of 3 to 4.5 volts and the constant temperature of 40 CO and CO2 (Figure 1b), it was observed that the efficiency of the current intensity has decreased from 32 % in the potential of 3.0 V to about 22.7 % in the potential of 4.5 V. This indicates that the excess potential causes its consumption in side reactions and reduces the current efficiency. In addition, the increase in mass loss from 2.5 g at 3.0 V to about 7.0 g at 4.5 V is quite evident. In general, it can be said that graphite anode is not suitable for zinc electrowinning, and the reason for this is low efficiency and graphite losses during electrolysis.
