Aluminum Smelting Cells Research Articles

Recovery of fluoride values from spent pot-lining: Precipitation of an aluminium hydroxyfluoride hydrate product

Treatment of spent pot-lining (SPL) from aluminium smelting cells by a two-stage leaching scheme comprising a water wash and an Al 3+ leach and fluoride recovery as an aluminium hydroxyfluoride product has been studied for extraction of fluoride and then recovery as smelter grade AlF 3. The NaF content of a −1.18 mm size fraction was removed by the water wash, while the more refractory Na 3AlF 6 and CaF 2 were removed by treatment with 0.34 M Al 3+ solution at 25 °C for 24 h, which yielded an overall fluoride extraction of 76–86 mol%. Mathematical modelling using experimental stability constant data was carried out to predict the effect of combining solutions and identify ways to manipulate the solution equilibria to maximise fluoride precipitation yields. The predictions were then tested experimentally. In the 4.5–5.5 pH range, selective precipitation of fluoride as an aluminium hydroxyfluoride hydrate product was achieved by neutralisation of the combined solutions with addition of 2 M NaOH solution. Higher pH values lead to the co-precipitation of hydrolysed sodium fluoroaluminates. Characterisation of precipitates using X-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectroscopy, thermogravimetric analysis, differential thermal analysis, aluminium and fluoride determination have pointed out both the AlF 2(OH)·1.4H 2O stoichiometry and possible thermal decomposition pathways to yield a dehydrated aluminium hydroxyfluoride product, AlF 2(OH), that could be used for smelter grade AlF 3 production. The kinetics of hydrolysis are such that nucleation dominates while particle growth is restricted. Techniques to allow slow hydrolysis are necessary to form smelter grade AlF 3.

A Case Study of Variation in Aluminum Smelting Cell Thermal State with Control Implications

The thermal state of an aluminum smelting cell is determined principally by the following parameters: electrolyte temperature and excess aluminum fluoride concentration. Despite the attempts to control them within a predetermined specification range, large variations are observed. The focus of the present study is to identify the causes of the variation and recommend the solutions to minimize it. The task was accomplished by monitoring an industrial operating cell. The Apollo root cause analysis technique has been used to identify the underlying causes of the variation. Smelter’s response of addition of AlF3 in order to control the variation was one of the primary causes. The AlF3 addition to the cell should be based on its mass balance requirement.

The multivariable model-based control of the non-alumina electrolyte variables in aluminum smelting cells

This paper investigates the application of multivariable model-based control to improve the regulatory control of electrolyte temperature, aluminum fluoride concentration, liquidus temperature, superheat, and electrolyte height. Also examined are therappropriateness of different control structures and the possible inclusion of recently developed sensors for alumina concentration and individual cell duct flowrate, temperature, and heat loss. For the smelter in this study, the maximum improvement possible with a multivariable model-based controller is predicted to be 30–40% reduction in standard deviation in electrolyte temperature, aluminum fluoride concentration, liquidus temperature, and superheat, and around half this for electrolyte height. Three control structures were found to be appropriate; all are different than the existing control structure, which was found to be suboptimal. Linear Quadratic Gaussian controllers were designed for each control structure and their predicted performance compared.

Wave Detection and Characterization from Current and Voltage Signals of an Aluminium Smelting Cell

In aluminium electrolysis cell there is a layer of 10-20 cm of liquid aluminium. On top of this there is a layer of a melted salt electrolyte with a number of carbon anode blocks partially submerged. The density difference of the two liquids is small and low frequency waves can occur in the interface. The waves cause variation in the current distribution between the individual anode carbon blocks. These variations are in this article analysed and visualised with MATLAB to show information of the temporal variations, frequencies and spatial modes of the waves.

Aluminium fluoride consumption and control in smelting cells

For optimum operation and performance of aluminium smelting cells, each technology has an ideal aluminium fluoride concentration for the solvent electrolyte. The actual value used is influenced by the alumina dissolution and varies because of different alumina‐feeding technologies. The aluminium fluoride is consumed within the cell and therefore, chemical analysis is performed as part of the process to determine the amount required for replenishment of the consumption. However the cell has a dynamic energy balance and that causes simultaneous variations in the distribution of solid and liquid electrolyte components, complicating the overall situation. Ultimately, it results in the chemical analysis varying. Thus, it becomes a major challenge to determine the amount of aluminium fluoride that needs to be added to replenish the consumption, whilst maintaining the cell at close to its optimum range.

The Effects of Petroleum Coke Properties on Carbon Anode Quality

Comprehensive bench-scale testing of the effects of calcined coke on the properties ofprebaked anodes for aluminum smelting cells has revealed correlations between coke and anode properties. Extensive measurements of the physical properties of coke as well as impurities, determinations of performance-indicative anode properties, and correlation by regression analyses provided statistically significant relationships which can generally be explained in terms of reasonable chemical and physical interactions.

Melt/freeze heat transfer measurements in cryolite-based electrolytes

The need to maintain a solid electrolyte ledge on the sidewalls of aluminum smelting cells has recently provoked interest in the heat transfer rate from the molten to the frozen phase at this interface. In this work an electronic voltage to frequency conversion technique was developed to record thermocouple signals inside a vertical, air cooled graphite cylinder rotating in molten cryolite. Precise measurement of the solid electrolyte which freezes around the internally-cooled cylinder allowed determination of the interfacial heat transfer coefficient under both free and forced convection conditions. Correlation of the local Nusselt numbers for heat transfer to the stationary cylinder yields an expression similar to that obtained for natural convection in a rectangular enclosure. However, rotation of the cylinder at peripheral velocities of 0.09 to 0.12 m s−1 caused forced convection to dominate in the cavity and increased the heat transfer coefficients by 34 to 45 pct, respectively. The data provide a basis on which to calculate the convective heat flow to the sidewall ledge for a range of hydrodynamic conditions in the cell.

Effect of convective heat transfer and phase change on the stability of aluminium smelting cells

AbstractThis paper focuses on two aspects of smelting operation that are highly dependent on the rate of heat transfer in the molten bath. As a consequence of the elevated temperature and the corrosive nature of the fluoride mixture, the cell sidewalls are designed so that some electrolyte will freeze onto them, protecting the carbon from erosion and at the same time insulating the cell against excessive heat loss. The thickness and dynamic variation of this frozen ledge are determined by the convective heat transfer from the bath to the freeze surface.Another situation where the rate of convective heat flow exerts a subtle influence is the dissolution of aluminium oxide powder in the bath. The localized feeding of large quantities of the powder causes transient electrolyte freezing on some of the added material, hindering its dissolution and causing the formation of an alumina sludge below the molten aluminium pad.

Ceramic materials for aluminum smelting cells

Ceramic materials for aluminum smelting cells

TiB2 Cathode Material: Application in Conventional VSS Cells

A low-cost proprietary TiB2 cathode material Prior to scale-up and application in full-size aluminum reduction cells, the original composition development work was conducted by Nagle and Buchta.8 has been developed which circumvents the problems previously experienced in using TiB2 in aluminum smelting cells, i.e., chemical instability, poor thermal shock and impact resistance, attachment problems, and high cost. During the period 1981–1983, the proprietary material was tested as a cathode lining in six vertical stud Soderberg (VSS) aluminum reduction cells at the Martin Marietta Aluminum plant, The Dalles, Oregon. The plant test has demonstrated that the material achieves excellent durability and longevity in commercial aluminum reduction cells, and can be utilized with no modification to conventional cell operating procedures. Current efficiency improvements and energy savings have been realized, and the rate of TiB2 loss indicates a potential 3–5 year life for the cathode material. Analysis of core samples taken from the experimental cathodes at The Dalles during a temporary shutdown confirmed that the cathode lining was functioning as predicted.

TiB 2-coated cathodes for aluminum smelting cells

Pipes consisting of coatings 700 μm thick of chemically vapor-deposited TiB 2 on carbon were produced and tested for corrosion and/or erosion resistance in a laboratory aluminum smelting cell for 100 h. The projected lifetime of the coating was approximately 30 days, a factor of 50 shorter than that projected by assuming uniform saturation dissolution of TiB 2 in molten aluminum. The corrosion and/or erosion as measured from scanning electron micrographs pointed to grain boundary attack with subsequent crystalline pull-out as the mechanism for the shorter than expected lifetimes.

Energy consumption of aluminium smelting cells containing solid wetted cathodes

Tests with 10 kA pilot cells show that substituting a wetted, dimensionally stable cathode for the turbulent metal cathode allows the anode-cathode distance (ACD) to be decreased, with an attendant voltage reduction of about 1 V, depending on current density. Since high current efficiency is retained, specific energy consumption is decreased by about 20%. These savings are not as great as anticipated, however, because the effective electrical resistivity of the electrolyte increases as the ACD is lowered. This effect is ascribed to an increased void fraction of anode gas in the interpolar space, which appears to be dependent on anode size.

Effects of anthracite calcination and formulation variables on properties of bench scale aluminum smelting cell cathodes

Several anthracites were calcined at temperatures ranging from about 1100 to 2300°C and bench scale cathodes fabricated, baked, and tested for electrical resistivity and expansion during electrolysis. Cathode electrical resistivity did not decrease much with increasing anthracite calcination temperature to about 1800°C but decreased sharply there-after. Cathode expansion during electrolysis was inversely proportional to anthracite calcination temperature to about 2000°C. Electrical resistivity generally decreased during test cell operation. A correlation between anthracite properties and cathode properties for anthracites calcined at 2100°C could not be made. With a commercial electrically calcined anthracite, the finer of two aggregate sizings resulted in cathodes with superior properties, and properties generally optimized at a binder level producing about a zero volume change during baking.

Effects of anthracite calcination and formulation variables on properties of bench scale aluminum smelting cell cathodes

Effects of anthracite calcination and formulation variables on properties of bench scale aluminum smelting cell cathodes

Predicting Current Efficiency of Industrial Aluminum Smelting Cells from Laboratory Measurements

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Percolation of Hall Bath through Carbon Potlining and Insulation

Samples of lining and insulation from a transverse cross-section of an aluminum smelting cell were examined by X-ray diffraction and differential thermal analysis. Potlining was penetrated by catholyte, with its composition changing to correspond with the isotherm passing through each point. Penetration was limited by the eutectic temperature. Air leakage into the cell caused Na{in2}CO{in3} and CaO formation near the sides.