What Is a Carbon Cathode?

A carbon cathode used in aluminum electrolysis refers to a carbon electrode in an aluminum electrolytic cell that is connected to the negative electrode of a power source. It is an important part of the aluminum electrolytic tank, including bottom carbon blocks, side carbon blocks, tamped carbon paste or carbon glue connected to the carbon blocks, and cathode steel rods. The carbon cathode for aluminum electrolysis is located at the bottom of the electrolytic cell, and the exterior is built with refractory material and reinforced with a steel shell.

Above the carbon cathode for aluminum electrolysis is an aluminum liquid and an electrolytic solution, and an electrolytic reaction for precipitating aluminum is performed on the interface between the aluminum cathode and the aluminum hafnium. The direct current flows out of the tank through the aluminum liquid, the bottom carbon block and the cathode steel bar. The carbon cathode is the most important structural part of the aluminum electrolytic tank. The material condition, installation quality and working condition of the carbon cathode for aluminum electrolysis have a great influence on the efficiency of aluminum electrolysis current and the power consumption of aluminum electrolysis. Carbon cathode voltage drop or furnace bottom voltage drop for aluminum electrolysis is generally above 400mV, which accounts for about 10% of the tank voltage. In aluminum electrolytic production, the cathode is deformed and fractured by the erosion and impact of molten salt and aluminum liquid, and various stresses. When it is severely damaged, it is necessary to stop the groove for overhaul. The normal service life of carbon cathode lining for aluminum electrolysis is 4 ~ 5a

Low-carbon anthracite, pitch coke, metallurgical coke, natural and artificial graphite, and coal pitch are the main raw materials for side-built cathode carbon blocks, side carbon blocks, tamped carbon paste, and carbon glue. These raw materials are first subjected to calcination treatment. Anthracite and pitch coke are calcined in a pot calciner or rotary kiln at a high temperature of 1473-1623K to improve its compactness and thermal stability and reduce its resistivity. Anthracite can also be calcined in an electric calciner, and the calcination temperature is generally 2073-2173K. Calcination can further improve the thermal stability and reduce the specific resistance of anthracite.

The calcined coke and the dehydrated metallurgical coke and graphite are crushed and pulverized, mixed according to a certain particle size ratio, and mixed with the binder coal pitch in an intermittent or continuous gripper to make a carbon paste.

The carbon green block is roasted in a closed or open multi-chamber ring roaster. China's small and medium-sized carbon plants often use an inverted flame kiln or a multi-chamber side-by-side roasting kiln, which has low thermal energy efficiency. The calcination temperature of the cathode carbon block is 1123-1423K, and the lower temperature can be used for the cathode carbon blocks of stone coke or semi-graphite coke. Graphitized or semi-graphitized cathode carbon blocks require higher firing temperatures, generally 1073-1123K. Roasting is to pyrolyze and coke the binder asphalt, so that the green body has higher mechanical strength, better thermal stability and higher conductivity. The firing curve and furnace loading operation have a great influence on the quality of the fired product.

The cathode steel rod has a rectangular cross-section, and a slot (dovetail groove) into which the steel rod is inserted is processed on the cathode carbon block according to the size of the selected steel rod. Pig iron casting or carbon paste tamping method to connect the cathode steel rod with the carbon block ^[2]

The installed carbon cathode must meet the following quality requirements: the bottom of the carbon tank is a seamless whole without voids and cracks; the conductivity must be high enough, and the steel rod has good contact points with the carbon block; it can resist electrolyte and aluminum Sufficient strength for erosion and abrasion ^[3]

During the start-up and operation of the aluminum electrolytic cell, the carbon cathode undergoes solution penetration, selective absorption of sodium, and the formation of aluminum carbide under the erosion and scouring of the electrolyte and aluminum, and the physical and chemical effects and thermal stress effects will cause the carbon cathode to gradually deform, Uplifts and cracks eventually lead to the shutdown of the electrolytic cell and overhaul. The signs that the carbon cathode lining is broken need to be overhauled are: the iron content in aluminum suddenly rises, the carbon cathode steel rod is melted by the aluminum liquid, the tank bottom voltage is significantly increased, the carbon cathode lining is swelled seriously, and the carbon cathode current distribution is severe Both, the deformation of the trough shell is severe and endangers normal operation.

When the carbon cathode lining is overhauled, the aluminum electrolytic tank is first powered off, all the electrolyte and aluminum liquid in the tank are removed, the old lining is completely removed, and the entire carbon cathode lining is built and installed in accordance with the construction requirements of the new electrolytic tank.

The service life of carbon cathode lining is generally 4-5a. The quality of carbon cathode materials, construction quality, baking start of aluminum electrolytic cell and quality of production management are the main factors affecting the service life of aluminum electrolytic cell. In order to extend the service life of the cathode lining and reduce the cathode voltage drop, TiB2 cathode coatings, side SiC brick layers, and research and development of new carbon cathode structural forms are being studied.