In the production process, a light cyan or grayish black film sometimes appears on the surface of the anode. In severe cases, the voltage of the cell increases, the current decreases, and the current does not increase. This phenomenon indicates that the anode has been passivated, and then the bluntness occurs. What are the reasons for this?
(1) Current density is too large: When the anode current density is greater than 4 A/dm2, the alloy anode or single metal anode does not dissolve, oxygen is largely evolved on the anode surface, and the anode plate is black and inactive. In actual operation, although the current density has not changed, sometimes the anode rod is not cleaned well, resulting in some parts of the anode being poorly contacted and not conducting, which actually increases the current density of other parts of the anode and causes the anode to passivate. The phenomenon appears.
(2) Too little free sodium cyanide: On the alloy anode or copper anode, when copper dissolves, it reacts with free cyanide in the solution to produce copper cyanide ions. When the free sodium cyanide in the solution is insufficient, a black or light cyan passivation film is formed on the surface of the anode, so that the active surface of the anode is reduced, so that the anode current density is increased, the anode is passivated, and the anode surface generates divalent copper ions. At this point, the surface of the anode is light cyan, and the surface of the solution is light blue.
(3) too little free sodium hydroxide: In the alloy anode or tin anode, tin dissolves and reacts with free sodium hydroxide in solution to form stannate. When the free NaOH in the solution is insufficient, a black metastannic acid film is formed on the anode surface and adheres to the anode, so that the anode activation surface is reduced, thereby increasing the anode current density and causing the anode to passivate.