Normally, 10 to 15 kg of used cathode carbon block is approximately produced through the production of 1 t of primary aluminum. The used cathode carbon block mainly consists of carbon, in which the fixed carbon content ranges from 65% – 85%. According to the positions of the electrolytic cell, there is difference in the content of each component. Since its composition contains certain amount of fluoride and cyanide, which are controlled in most of countries in the world and the environment permit shall be required and obtained for import and export. Our company has already obtained the Environmental Permit for its port of departure.
The comprehensive utilization of aluminum cell used cathode carbon block is not only concerning resource recovery but also required in terms of environmental protection. The main principle for the disposal of used cathode carbon blocks is hazard free on one hand and being recycled on the other. To sum up, there are three main aspects, including carbon recycle, electrolyte recycle and cyanide disposal. Some simple applications are recommended as follow:
Substitute of fuel and fluorite in blast furnace
At present, in the steel industry, limestone and fluorite are required to be used as fluxes, the function of which is to lower the melting point and viscosity of the slag. The carbon in the used lining can be exactly adopted as fuel to replace metallurgical coke. Fluorides can also be used as a flux to replace fluorite. Therefore, the used cathode carbon block is skillfully used as an additive, which kills two birds with one stone.
Combustion method is to crush the used carbon block and add additives such as fly ash and limestone etc. to control the combustion and decomposition condition of hazard substances, which both ensures the harmlessness and utilizes the heat energy of carbon material therein. 99.5% of cyanide can be decomposed at 300° C. When it is heated up to 400° C, 99.8% of cyanide can be decomposed, while 100% of cyanide can be decomposed at 700° C.
Supplementary fuel for cement manufacture
In the cement industry, two approaches for the recycle of used cathode lining are mainly considered. Firstly, the carbon component is used as fuel. Secondly, refractory materials can be used as substitute for raw materials. The composition of the cement is CaO－SiO2－Al2O3－Fe2O3.The cathode carbon can be used as supplementary fuel in the manufacture of cement, in which the alkali metal fluoride can be used as a catalyst in the sintering reaction of furnace charge. Therefore, the addition of the used cathode can lower the sintering temperature of the clinker and reduce the consumption of fuel. Al2O3 and silicon in the used cathode lining can be used as part of raw materials in the production process.
However, used cathode carbon block cannot be utilized by all of the cement plants. Because of its high sodium content, the used cathode carbon block is highly alkaline, which makes it impossible for the cement plants which produce low alkaline cement product to adopt this method.
Profile carbon block at the side of aluminum electrolytic cell.
During the production and operation of the aluminum electrolytic cell, the side and periphery of the cell requires larger specific resistance (for reducing horizontal current and increasing cathode-current density), good abrasion resistance (capable of withstanding the scrubbing impact of acidic electrolyte), and moderate thermal conductivity (for thermal insulation and dissipation as well as maintaining heat balance), as well as low expansion rate (to reduce the early damage rate of the carbon block at the side caused by sodium absorption).
Due to its high graphitization and sodium absorption saturation as well as its relatively high ash content, infiltrating certain amount (30-50%) of the used cathode can produce the carbon block profile at the side which has the characteristics of semi-graphitic carbon block, namely, larger specific resistance, good abrasion resistance and low sodium expansion rate and so on.