The principle of the preparation of anodized aluminum is the anodic effect (also known as anodization).
The anode effect is a unique phenomenon of molten salt electrolysis, and the performance of electrolytic aluminum production is even more obvious. When the anode effect occurs in production, the voltage of the electrolytic cell rises sharply, reaching 20-50V, sometimes even higher. Its occurrence has a great impact on the entire electrolysis series, reducing the current efficiency, affecting various technical indicators of electrolysis, and reducing the output and quality of aluminum, destroying the stable power supply of the entire electrolysis series. In terms of processing methods, there are nothing more than two: use effect rods (wood rods) to extinguish, or reduce the anode, increase the amount of aluminum oxide, in order to achieve the purpose of extinguishing the anode effect. No better treatment has been found yet.
The anode effect occurs because as the electrolysis process progresses, the oxygen-containing ions in the electrolyte gradually decrease. When it reaches a certain level, fluorine is precipitated and reacts with the anode carbon to generate carbon fluoride. The carbon fluoride is decomposed again. Fine carbon particles are precipitated. These carbon particles are attached to the surface of the anode, preventing the electrolyte from contacting the anode, making the electrolyte unable to wet the anode well, just like water can not wet the oiled surface, forming a gap between the electrolyte and the anode. A layer of poorly conductive gas film will increase the anode overvoltage and cause the anode effect. When new alumina is added, oxygen is precipitated on the anode again, and the oxygen reacts with the carbon powder to gradually make the surface of the anode quiet, the resistance decreases, and the electrolysis process becomes normal again.
In terms of production, when the anode effect occurs, the temperature of the electrolyte rises sharply from the normal value of 940°C to 955°C to 980°C to 990°C. The furnace side melts and becomes thinner, which increases the corrosion of the side carbon blocks. possibility. The sharp rise in voltage causes the series of currents to fluctuate and affects the output of the electrolytic cell. Increase in power consumption. The method of extinguishing the anode effect in production is to insert the effect rod (about 2 to 3 meters in diameter and 2 to 4 cm in diameter) into the aluminum liquid to burn the wood rod to remove the gas film on the bottom of the anode, and to clean the bottom of the anode, which is actually burning. The entire process of molten aluminum lasts about 3 to 5 minutes, and the electrochemical process of electrolysis is stopped at this time. This is the reason why electrolysis workers often say "the effect of time does not produce aluminum, and it also consumes electricity." As a result, serious losses of molten aluminum were caused. Take the 300KA intermediate blanking pre-baking tank as an example: the effect coefficient is 0.3 times per tank day, the effect time is 5 minutes, the current efficiency is 93%, and one anode effect produces less primary aluminum: 300×0.3355×5÷60=8.4kg, per ton of aluminum Power consumption increased by 158kwh. Most of this energy is converted into heat energy during production, which causes the temperature between the electrode pitches of the electrolytic cell to rise sharply, and then conducts to the periphery of the anode, which raises the temperature of the electrolytic cell and causes a large amount of volatilization of aluminum fluoride in the electrolyte. Therefore, the traditional anode effect method can no longer adapt to today's modern electrolytic cell production.
In the environmental aspect of aluminum electrolysis production, the anode effect is also accompanied by the production of PFCs (CF4·C2F6) which is destructive to the atmospheric ozone layer. Today's western developed countries have extremely strict environmental protection requirements for aluminum electrolysis.