Cathode Carbon Block Research Articles

Footprint of harmful substances in spent pot lining of aluminum reduction cell

Abstract Spent pot lining (SPL) from aluminum reduction cells is considered to be hazardous materials due to containing a large amount of soluble fluoride salts and trace toxic cyanides. The distribution of fluorides and cyanide in a 350 kA cell operated for 2396 days was analyzed and the footprint and corrosion mechanism of the harmful substances in SPL were also studied. It is found that the fluorides are mainly concentrated in the cathode carbon block and the layer of dry barrier under the cathodes, which is closely related to permeability of the cathodes and dry barrier the fluorides penetrate in. Cyanide has a low concentration in the cell center and a high concentration in the sidewall, which is positively related to the air amount entering into the areas in the cells.

Numerical Investigation on Effective Thermal Conductivity and Heat Transfer Characteristics in a Furnace for Treating Spent Cathode Carbon Blocks

The aluminum spent cathode carbon block (SCCB) is defined as a hazardous material because it contains soluble fluoride and toxic cyanide compounds. To make the SCCB harmless, it has been proposed to separate the carbon and fluoride salts using a high-temperature resistance furnace. To evaluate the temperature distribution and heat transfer of the furnace, the effective thermal conductivity of SCCB was calculated by numerical simulation, and the results were compared with fundamental structural models. After considering the radiative heat transfer, the effective thermal conductivity of SCCB increased as the temperature increased. When the high-temperature resistance furnace was heated for 10 h under two conditions, considering radiation at the monitoring point, the temperature was found to be 61°C higher than that without considering radiation. Moreover, the volatilization rate of fluoride salts of the two models differed by 15.6%. This indicates that the temperature distribution in the furnace is uniform when the radiation effect is included.

Analysis on thermal behavior of fluorides and cyanides for heat-treating spent cathode carbon blocks from aluminum smelters by TG/DSC-MS & ECSA®

This paper adopts a novel data-processing method of ECSA® based on TG/DSC-MS system to basically study the characteristics of release and conversion of fluorides and cyanides during heat treatment of the spent cathode carbon block (SCCB). All the experiments were conducted at 10K⋅min-1 heating rate and under Ar or Ar-O2 atmospheres. The results indicate that the release of fluorides was just a steady but slow phase transition process under both Ar and Ar-O2 atmospheres, which can be comparatively accelerated when the carbon material was burnt. The cyanides were effectively decomposed at high temperature and at Ar-O2 atmosphere, with around three quarters of the cyanides being converted to the N2 and nearly a quarter being to the NO. Finally, analysis on the flue gas composition indicates that it had a more complicated composition of CO2, N2, NO, NO2, HCN under Ar-O2 atmosphere but only had a composition of CO2 and NO under Ar atmosphere.

Detoxification of spent cathode carbon blocks from aluminum smelters by joint controlling temperature-vacuum process

The spent cathode carbon block (SCCB) from aluminum reduction cells is considered to be a hazardous material due to containing a large amount of soluble fluoride salts and toxic cyanides. The serious environment pollution and ecological destruction will be caused by being openpiled or buried of SCCB. A joint temperature-vacuum controlling process for treating SCCB is proposed in this paper. The thermodynamic analysis by FactSage 7.0, series of experimental investigation, and characteristics tests on the product materials by SEM and XRD were carried out. The factors including temperature, vacuum pressure and retention time that influence the SCCB’s detoxification effects were comprehensively investigated and discussed in detail. It was found out that the soluble fluoride content can be reduced to a level of 3.5 mg·L−1 while cyanide is completely decomposed. A fixed carbon content of 97.89% in the treated materials is obtained under the optimum temperature and vacuum controlling conditions.

Interfacial wetting mechanisms of Al liquid on cathode carbon blocks of aluminum reduction cell for developing wettable cathode materials

Molecular dynamics simulation had been performed to investigate the wetting properties of Al droplets on amorphous carbon surfaces and graphite surfaces. The effects of temperature, graphitization of substrate and roughness were considered. Our results show that the contact angles can be improved effectively with the increased temperature. Furthermore, the Al droplet on graphite surface has a better wettability than that on amorphous carbon surface. Similarly, the contact angle will also reduce with the increased degree of graphitization and the wetting state will change from the transition wetting state to the Cassie state when the roughness increases. In addition to the contact angles on the rough surface, the remaining contact angles are less than 90°, which is different from the previous reports. The free energy and thermodynamic properties analysis were applied to character the solid-liquid interface properties and explain the wettability. Another interesting finding is that the study reveals the reason of the poor wettability between Al liquid and cathode carbon blocks of aluminum electrolytic cell. These findings improve our understandings of the wetting behaviors of Al droplets on cathode carbon block surfaces at the atomistic level, which is profitable to develop the wettable cathode materials for aluminum electrolysis.

Numerical Analysis in Mesoscopic Scale and Experimental Verification for Sodium Expansion Induced Stress of Cathode Carbon Blocks

Numerical Analysis in Mesoscopic Scale and Experimental Verification for Sodium Expansion Induced Stress of Cathode Carbon Blocks

Investigation and Choice of Materials for the Electrical Insulation of the Steel Bloom of an Aluminum Electrolyzer

The growth of the modern aluminum industry is characterized by a search for methods of increasing the capacity of electrolyzers, reducing emissions into the environment, and reducing energy consumption. In the production of aluminum, the specific electricity consumption can be reduced and current efficiency increased by decreasing the distance between the anode and cathode and at the same time reducing the horizontal currents in the molten aluminum. The horizontal component of the current density can be reduced by placing electrical insulation between the cathode carbon block and the bloom on the section ‘crust boundary – projection of the anode periphery on the bottom block’. Heatproof materials with the following properties are technologically most effective and economical for use as electrical insulation: good adhesion to steel, chemical resistance to cryolite-alumina melt, and quite low ultimate strength in compression. Low strength and stiffness are needed because during thermal expansion of the materials in the bottom block during the period of heating the tank to the working temperature with high resistance to compression there is a risk of the load increasing on the carbon cathode block and cracks forming in it. Moreover, such properties of the material will make it possible to decrease the risk of fracture of the cathode carbon block if the volume of the electric-insulation insert increases as a result of the electrolyte penetrating into it. The results of the investigations of the mechanical properties of some heatproof materials for use as electrical insulation of blooms in aluminum electrolyzers at high temperatures are presented in order to optimize the composition.

Recent Advances in Electrical Resistance Preheating of Aluminum Reduction Cells

There are two mainpreheating methods that are used nowadays for aluminum reduction cells. One is based on electrical resistance preheating with a thin bed of small coke and/or graphite particles between the anodes and the cathode carbon blocks. The other is flame preheating, where two or more gas or oil burners are used. Electrical resistance preheating is the oldest method, but is still frequently used by different aluminum producers. Many improvements have been made to this method by different companies over the last decade. In this paper, important points pertaining to the preparation and preheating of these cells, as well as measurements made during the preheating process and evaluation of the performance of the preheating, are illustrated. The preheating times of these cells were found to be between 36 h and 96 h for cell currents between 176 kA and 406 kA, while the resistance bed thickness was between 13 mm and 60 mm. The average cathode surface temperature at the end of the preheating was usually between 800°C and 950°C. The effect of the preheating methods on cell life is unclear and no quantifiable conclusions can be drawn. Some works carried out in the mathematical modeling area are also discussed. It is concluded that there is a need for more studies with real situations for preheated cells on the basis of actual measurements. The expected development in electrical resistance preheating of aluminum reduction cells is also summarized.

Numerical Simulation of Current Distribution in Cathode Carbon Block of an Aluminum Reduction Cell

Cathode carbon block wear is the main limiting factor for the lifetime of aluminum reduction cells. The wear rate is enhanced by current density. In this article, the current distribution at the surface of carbon block was calculated using a thermoelectric coupled model. Then the effects of effective length (l e), height of the cathode carbon block (h c), and width and height of the collector (w b and h b) on current distribution were investigated. The results show that l e has a great effect on the current distribution. With l e decreasing, the maximum current density increases rapidly and shifts toward the cell center. When the l e decreases from 1.67 m to 1.51 m, the maximum current density increases by 57.9%. Moreover, the maximum current density will be reduced with increasing h c or h b × w b. For h b × w b = 180 mm × 180 mm2, the maximum current density is reduced by 27.8%. However, increasing h c or h b × w b will decrease the temperature in the cathode carbon block. The results of this study may provide the database optimization of cell operation and design.

Impact of the Usage of a Slotted Cathode Carbon Block on Thermoelectric Field in an Aluminum Reduction Cell

The horizontal current in a metal pad of an aluminum reduction cell is critical because of its effect on the fluctuation of the metal pad. In this study, a novel cathode with a slotted cathode carbon block was proposed to decrease the horizontal current. The effects of the slotted cathode carbon block on the horizontal and vertical currents in the metal pad, cathode voltage, and temperature distribution in the cathode were calculated using the finite-element method. The results show that the slotted cathode carbon has great potential to decrease the horizontal current. When the length of slot b equals 400 mm, the maximum horizontal current density decreased by 50.4%. However, the cathode voltage in the cathode with the slotted cathode carbon was ~43 mV higher than that in a conventional cell, and the temperature in the slotted cathode carbon was slightly higher than that in a conventional carbon cell. Moreover, with increasing length of slot b, the maximum horizontal and vertical currents in the metal pad moved toward the cell center. The result of this study may provide the database in understanding the effect of the slotted cathode carbon on cell.

Study on Harmless and Resources Recovery Treatment Technology of Waste Cathode Carbon Blocks from Electrolytic Aluminum

The paper provides a brief introduction in the physical and chemical properties of waste aluminum cathodes, and its harmless treatment technology and resource comprehensive utilization technology. The main chemical composition of the waste cathode carbon blocks of electrolytic aluminum are carbon, fluorine, sodium, aluminum, calcium, silicon and etc. The main mineral components of waste cathode carbon include sodium fluoride, cryolite, calcium fluoride, silicate minerals and so on, therein the fluorine content is about 9.86%, and the content of cyanide ions (CN−) is 0.82 ppm, which are the most important reasons leading serious environmental hazards caused by the waste aluminum cathodes; and in which carbon has been graphitized mostly and the content of it is up to 58.56%, having a high recycling value. Using the flotation-chemical treatment process to recover fluorine and carbon, carbon powder was obtained with the carbon content of 88.00%, recovery of 95.90%; and the content of sodium and fluoride in sodium fluoride product are 49.33% and 38.53% respectively, the total recovery of fluorine in fluoride sodium and other products is 90.10%. It proves that the cyanide in the waste cathode carbon block of electrolytic aluminum has been completely decomposed. The process takes fully the factors of crushing, grinding, flotation, chemistry, waste gas absorption technology into account, and achieves the purpose of harmless treatment and comprehensive utilization of resources.

Simulation of thermal and sodium expansion stress in aluminum reduction cells

Two finite element(FE) models were built up for analysis of stress field in the lining of aluminum electrolysis cells. Distribution of sodium concentration in cathode carbon blocks was calculated by one FE model of a cathode block. Thermal stress field was calculated by the other slice model of the cell at the end of the heating-up. Then stresses coupling thermal and sodium expansion were considered after 30 d start-up. The results indicate that sodium penetrates to the bottom of the cathode block after 30 d start-up. The semi-graphitic carbon block has the largest stress at the thermal stage. After 30 d start-up the anthracitic carbon has the greatest sodium expansion stress and the graphitized carbon has the lowest sodium expansion stress. Sodium penetration can cause larger deformation and stress in the cathode carbon block than thermal expansion.

Review on Carbon Materials in Aluminium Smelting

The aluminium industry in Japan has faced the most serious problems since the oil crisis, especially by the steeply rising cost of electric power and carbon materials. Although smelters of our country hold fine techniques in the world with respect to pot operation and pot life, the shortage in resources has upset these predominances. Greater efforts, therefore, should be directed to energy conservation, material economization and labour saving.This report describes the recent topics on anode materials for aluminium smelting, such as coal tar pitch, petroleum pitch, pitch coke and petroleum coke. Solvent refined coal, which is a possible new source for anode material, is also introduced. Recent reports on Soderberg paste are reviewed in terms of the modification of binder, the role of fine coke in formulation and the effect of carbon black as an additive. Influences of uneven baking temperature of prebake anodes in a ring furnace upon the electrolytic operations are also reviewed. Various properties required for cathode carbon block and deterioration phenomena of carbon lining of the pots are described.