Cathode Block Research Articles

The use of carbon-containing wastes of aluminum production in ferrous metallurgy

The article discusses the prospects of recycling the most massive wastes of aluminum production (namely, used cathode blocks of electrolyzers, gas treatment dust, gas treatment residue, and flotation tailing). It have been indicated the volumes of wasteaccumulation and a special attention has been focused on the need of their disposal with a view to improve the environmental conditions of the territories adjacent to the industrial zones. Specific characteristics of the generated waste have been determined, which indicate the possibility of their secondary use and transfer from waste to by-products.Existing technical solutions relevant to the issue have been reviewed, and the reasons preventing their implementation have been explained. The most promising methods of waste processing to be carried out successfully in the current economic conditions have been identified. Spent cathode blocks can be used at ferrous metallurgy enterprises (in blast furnaces and converters) as a substitute for expensive coke and fluorspar, and finely dispersed waste can be used at cement enterprises. The areas have been determined that in the future will significantly increase the volume of processing and the demand for these wastes of the aluminum industry in technological processes of ferrous metallurgy. The possibilities for cooperation between aluminum refineries and ferrous metallurgy enterprises, as well as other related industries, were emphasized in detail.

Numerical simulation of cathodic voltage drop in Hall-Héroult cell

Numerical simulations are widely used to predict cathode voltage drop in Hall-Héroult cells and to determine the qualifications needed for voltage savings. The aim of this study is to investigate the effect of change in the properties of cathode assembly materials with temperature, the effect of the copper insert in collector bar and effect of change in cathode design on the cathodic voltage drop, horizontal current density and potlife. Thermal and electrical measurements were performed on twenty cells to validate the model results. The results proved that resistivity of both collector bar and cathode block is considered the main property that controls the cathodic voltage drop in the cells. While the copper insert has a much higher potential for providing a lower cathode voltage, great effect on the horizontal current density in the cell and increases the expected potlife. However, the choice of cathode assembly materials must be compatible between cathodic voltage drop and horizontal current density. Therefore, these items have been examined for different cases to reach optimal conditions. The results show that select anthracitic with 30% graphite cathode blocks and adjust the resistivity of steel collector bars and cast iron, as well as insert copper in the steel collector bar, are effective in bringing down the cathodic voltage drop, lower the electro-erosion of the cathode block and the expected potlife will be increased.

Method of protection of cathode blocks of aluminium electrolysis cells

Cathode wear is one of the main lifetime limiting factors of an aluminium electrolysis cell. In the present work the composition of the wettable TiB2-based coating and the cost-effective method of its application to protect the most defenseless areas of the cell bottom are proposed in relation with the cathode blocks wear profile and failure mechanisms. The protective coating composition comprises a mixture of TiB2 powders with bimodal or multimodal grain size distribution (65-40 wt. %) and a water-soluble environmentally friendly binder (35-60 wt. %). The proposed TiB2-C protective coating is characterised by high adhesion to the cathode block material, as well as wear resistance, resistance to cracks initiation and their propagation, wettability by liquid aluminium. That leads to a contact electrical resistance decrease and an increase of a chemical resistance at the Al-TiB2-C interface. The developed method of the cathode blocks protection was tested in operating conditions on an industrial electrolysis cell. The expected lifetime of cathode blocks was increased at least for 4 mounts. The results are promising and show the efficiency of the proposed method for cathode blocks protection.

Cathodic Wear by Delamination of the Al4C3 Layer During Aluminium Electrolysis

In aluminium reduction cells, an electrochemical reaction occurs between the molten electrolyte film below the aluminium pad and the carbon cathode blocks, leading to the formation of an Al4C3 layer on the cathode blocks. The properties and role of this Al4C3 layer are therefore important for the aluminium production industry, as they could help increase the life expectancy of electrolysis cells and impact the resistive voltage losses. The purpose of this study is to gain a better understanding of the formation, growth and mechanical stability of the aluminium carbide layer formed on top of the cathode block. A reliable scenario describing both the mechanical and electrochemical behaviours of the Al4C3 layer is proposed. For different industrial graphitized cathode grades, a series of experiments were carried out in a bench-scale Hall-Heroult electrolysis cell and the Al4C3 layer formed on top of the cathode was characterized. Thereafter, the CALPHAD method was combined with density functional theory simulations to estimate the electrical and physical properties of Al4C3 together with the phase equilibria occurring at the interface between the carbide layer and the aluminium pad and the cathode blocks respectively. From these calculations, a scenario for carbide layer growth and mechanical stability was established.

Carbon Cathode Wear in Aluminium Electrolysis Cells

Autopsies of six spent potlinings with different carbon cathode block grades, amperage regimes and cell designs were conducted at three separate smelters to reveal possible mechanisms causing cathode wear. The microstructure of the cathode samples from the autopsies was investigated by optical and electron microscopy and x-ray computed tomography, while the composition of the solid bath at the surface and in the interior pores was investigated by x-ray diffraction and electron microscopy. The present findings revealed that the cathode surface was characterized by a wear pattern resembling pitting corrosion, and it is discussed that the observed variations in the bath chemistry play a major role in the wear mechanism and the pitting of the surface. A hypothesis involving initiation and termination of the main reaction causing the cathode wear is proposed based on the effect of the consumption of aluminium fluoride in the molten bath layer between the carbon and the molten aluminium pad, resulting in partial solidification of the bath and spatial variation of the current density.

Separation and Recycling of Spent Carbon Cathode Blocks in the Aluminum Industry by the Vacuum Distillation Process

Aluminum is the second most produced metal after iron. China is the top producer of primary aluminum with a production capacity of 41,000 kt and an output in 2016 of 32,000 kt. A large amount of spent carbon cathode block (SCCB) is produced after electrolytic pot failure. SCCB consists of carbon, fluorides, alkali metals, carbides, nitrides, cyanides, and oxides, and is considered to be a hazardous material because it contains significant concentrations of toxic and soluble cyanides and fluorides. There is no economical and efficient process for the treatment of SCCB and is most commonly disposed in landfill. In this study, the vacuum distillation process (VDP) has been used to separate and recycle SCCB. The results show that Na3AlF6, NaF, and sodium metal can be effectively separated from SCCB by VDP, and the distillation ratio is above 80% at a distillation temperature of 1200°C. The carbon content in the distilled SCCB is above 91% and the impurities are mainly CaF2 and Al2O3.

Operational Problems of a Graphitized Cathodic Block Lining in Contemporary Aluminum Electrolyzers

A carbon lining in the form of cathode blocks with a different graphite content under electrolyzer conditions is subjected to mechanical wear as a result of rapid melt movement, and this is more significant than for amorphous carbon blocks. The problem of operating a graphitized cathode block lining is discussed. After performing an autopsy for cathode units the condition of some aluminum electrolyzer lining sections with blocks having a different degree of graphitization is studied. Reasons are examined and determined for premature electrolyzer breakdown with graphite-containing hearth blocks. Results are provided for temperature measurement of high-power electrolyzer cathode units.

Practical Application of the Concept of Separating the Zone of Overheating and Reducing the Stress Level in Cathode Block of an Aluminium Electrolytic Cell During Formation of Connection with the Steel Pin by Cast Iron Pouring

Abstract The article presents the concept of division of a carbon block into single cathode sections to reduce stresses that occur during thermal shock accompanying the operation of pouring cast iron into a shaped groove in which the current-carrying steel pin is mounted. The temperature field and the state of stress were determined by FEM. Experimental measurements were used to determine the core displacement during pouring operation. The calculations were verified by thermovision and photoelastic method. Based on the temperature field and stress field analysis, a method for the block division into sections has been proposed, resulting in a significant reduction of the stress level.

Analysis of the State of Stress in Cathode Block of an Aluminium Electrolytic Cell During Formation of Connection with the Steel Pin by Cast Iron Pouring

Abstract The article presents various aspects of the analysis of the state of stress in carbon block induced by the effect of temperature when a steel pin is connected to the block by pouring a shaped groove with cast iron. Changes in the temperature and state of stress in the block were examined by FEM (Abaqus program). The numerical model was validated by experimental measurements of pin deflection during pouring of the groove with cast iron.

An Approach for Estimation of Cathode Voltage Drop in an Aluminum Reduction Cell with an Inclined Carbon Block and a Copper Insert

Numerical models of a cathode block assembly in a Hall–Heroult cell, comprising of liquid aluminium, carbon block, current collector, ramming paste and a copper insert were built and the finite element method simulations were carried out to model the cathode voltage drop (CVD), the current distribution and, the effect of geometrical parameters on the CVD. The objective of the study was to quantify the drop in the CVD for different cathode assembly design. Flat- and inclined-interface carbon block top-surface and a copper insert versus the conventional insert-free designs were simulated with a myriad of other geometrical parameters to optimise the design. The results informed about the optimum insert positioning to about 75 mm from the collector base and the energy saving possibilities due to reduction in the CVD with a cathode design with inclined-interface carbon block and copper insert in the collector bar.

Carbide Formation at a Carbon-Graphite Lining Cathode Surface Wettable with Aluminum

For protecting a cathode block surface and a whole electrolyzer lining it is proposed to use boron and titanium compounds in the form of an active substrate of their carbides (TiC and B4C) that under successive electrodeposition conditions with transformation and reaction transitions create a layer wettable with aluminum. Combined synthesis processes are studied for wettable coatings and the mechanism of carbide formation on carbon cathode surface layers. Possible processes of aluminum carbide formation with its transition into carboxide surface substances are suggested for discussion.

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.

Impact of aluminium pad and operation parameters on graphitised and graphitic cathodes expansion and bath penetration

Cathode sodium expansion and bath penetration are important properties to consider in the Hall–Héroult process. Rapoport test is the standard method for sodium expansion determination. However, there is no standard methodology for bath diffusion measurement. In this work, expansion measurements are achieved on laboratory scale alumina reduction cells for graphitic and graphitised cathode block grades. Cathode carrots were then extracted to measure bath penetration. Results revealed that the presence of an aluminium pad at the beginning of the experiment has reduced the sodium expansion and bath penetration by about 50%. Sodium concentration in the bath has increased the expansion rate and bath penetration depth, but not the expansion level, which is driven by the current density. This demonstrates that the startup conditions are crucial to the lifespan of the electrolysis cell.

Mathematical Modeling of Electrolyte Filtration through the Porous Cathode Blocks during Aluminum Electrolysis with Regard Interblock Seams

This article investigates electrolyte filtration in the bottom of the aluminum electrolyzer cathode device using the mathematical modeling. Penetration of molten electrolyte in the heat insulation part of the lining is one of the main reasons of electrolyzer premature shutdown, because it leads to bottom destruction and excessive heat loss. This problem is considered a two-phase filtration of incompressible immiscible liquids in an inhomogeneous non-deformable porous body. The verification of the model on the problem of water filtration pin a porous medium has confirmed its adequacy. With the help of the developed mathematical model the dynamics of the impregnation of the lining of the cathode and electrolyte device defined thermal balance baths. Research has identified the speed of penetration of the melt in the bottom of the bath during service of the electrolyzer.

Thermodynamic and thermochemical investigation of the deposits formed on the cathode surface of aluminum electrolysis cells

The efficiency of the Hall-Heroult process is influenced by the formation of solid or solid–liquid deposits on the cathode block surface. These deposits have a high electrical and thermal resistivity. Hence, they disturb the electrical current path at the aluminum and cathode block interface. The phase diagrams of deposits with a typical composition were calculated by thermodynamic equilibrium, and the results were confirmed by differential scanning calorimetry measurements. The ledge toe is mainly composed by Na3AlF6(ss), Al2O3(s), and liquid bath in industrial cells. The liquid fraction in the deposits at temperature lower than 933 °C is due to the presence of Na5Al3F14, Na2Ca3Al2F14, and NaCaAlF6 and depends on [Al2O3], [CaF2] and the cryolite ratio (CR). The side ledge in industrial cell contains liquid phase when the CR is lower than 2.8. The sludge contains Al2O3(s) and liquid bath. The ledge toe grows when the local temperature is lower than the solidification temperature of Na3AlF6.

Formation of Deposits on the Cathode Surface of Aluminum Electrolysis Cells

The efficiency of electrolysis cells for aluminum production is reduced when deposits are formed on the cathode block surface. Overfeeding of alumina or excessive heat loss in industrial cells leads to the formation of highly resistive deposits. In this study, the chemical composition of sludge, ledge toe, and thin deposits was investigated at the bottom of both industrial and experimental electrolysis cells. The formation of deposits in laboratory experiments was demonstrated in acidic, neutral, and basic electrolytic bath. A gradient of chiolite (Na5Al3F14) and α-Al2O3 was observed in the deposits. The bath at the bottom of the experimental electrolysis cell had a higher cryolite ratio implying a higher liquidus temperature. The sludge formed at the bottom of the cell can lift the aluminum metal resulting in an important reduction of the contact surface between the aluminum and the cathode block. Moreover, the deposits disturb the current path and generate horizontal current components in the metal which enhance the motion and lower the current efficiency. A thin film of bath supersaturated in alumina was observed under the metal. This work provides clarification on the formation mechanisms of the various deposits responsible for the deterioration of the cathode surface.

Numerical Simulation of the Effect of Cathode Materials on the Displacement in a 350 KA Aluminum Reduction Cell

A three dimensional mathematical model was developed to calculate the displacement of a 350 kA cell with graphitized cathode during preheat and early operation, and the calculated results have been compared and discussed with the measurements. The displacement has been studied for three typical kinds of aluminum reduction cells (semi-graphitic, graphitic and graphitized) after 30 days start-up. First, the sodium concentration distribution after 30 days start-up in cathode carbon blocks was calculated by one FE model of a cathode block. And numerical simulation was utilized to study the displacement fields of the above three typical kinds of the cell, which took the stress caused by chemistry and temperature gradient into account. The results indicate that the value of the cell displacement decreases with time during 30 days start-up and reaches a constant value.

Optimization of the cathode block shape of an aluminum electrolysis cell

A finite element/optimization model was developed to determine the optimum shape of the cathode block of an AP-30 aluminum electrolysis cell. The optimum shape is the design that makes the electric current density on the surface of the cathode block as uniform as possible so as to simultaneously prolong its lifetime, to minimize its energy consumption and to minimize the cost of material. Results indicate that by making some changes in the design of the collector bar, the cathode voltage drop can be reduced by 93 mV and the lifetime of the cell can be increased by 20%.

Cryolitic vein XPS imaging from an industrial grade carbon

Aluminium carbide formation and subsequent dissolution is a generally accepted mechanism to explain the cathode wear in the Hall-Héroult industry. Carbide formation is thought to occur inside the cathode pores. Being dependent upon cathodic current density, it is believed to be an electrochemical process. It is also associated with the presence of cryolitic bath species involved in the dissolution of the carbide layer and thus enabling further carbide formation.X-Ray Photoelectron Spectroscopy imaging has the capacity to illustrate both the species chemistry and the spatial localization of such species. However, spectroscopic imaging requires long acquisition time. The value of mathematical treatment becomes obvious when applying multivariate data analysis on raw data. Principal Component Analysis (PCA) can substantially reduce the collection time without reducing the analysis quality of the data.In the current work, the C 1s photoelectron region was imaged and aluminium carbide (Al4C3), aluminium oxycarbide (Al4O4C and Al2OC), and graphite (C) were spatially tracked within the first millimetres under the cathode block surface. Preliminary work on differentiating between cryolite (Na3AlF6), alumina (Al2O3), and Al4C3 in the Al 2p narrow scan regions was conducted. More generally, the XPS analysis enabled further investigation of the chemical species present in bath penetrated veins of the carbon cathode.

Testing of Aluminium Carbide Formation in Hall-Heroult Electrolytic Cell

The trend in the aluminium smelting industry today is to operate cells with graphitized carbon cathode linings, increased current density and acidic bath chemistry. The resulting problem is an accelerated wear of graphitized cathode blocks, thought to be caused by formation and subsequent dissolution of Al4C3 at the cathode lining surface. The cycle of formation and subsequent dissolution Al4C3 is recognized as one of the most important mechanism causing pothole and surface wear, which results in limiting of the cell lifetime and loss efficiency. A special laboratory test method was developed to elucidate the mechanism of Al4C3 formation in electrolytic cell. The Al4C3 formation in the region between the carbon surface and aluminium as well as between the carbon surface and electrolytic bath has also been studied using X-ray diffraction, as well as optical and scanning electron microscopy. Solid Al4C3 layer was observed at the carbon surface. A possible mechanism which explains the presence of Al4C3 at the metal-bath interface is the transfer of dissolved carbide in the bath from metal-carbon interface.