Hall-Heroult Cell

The purified aluminium oxide is then dissolved in molten cryolite (Na3AlF6). Cryolite, a mineral found naturally in Greenland, is used to reduce the working temperature of the Hall-Heroult cell from 2017°C (the melting point of pure aluminium oxide) to between 800 and 1000 °C. Therefore, the cryolite provides a considerable... 

Figure 5.6 The Hall—Heroult cell is used in industry to extract aluminium by electrolysis.

Hall-Heroult cell The electrolysis cell in which aluminium is extracted from purified bauxite dissolved in molten cryolite at 900°C. This cell has both a graphite anode and a graphite cathode. 

The inert electrode development work may need as much as 10-15 years before commercialization can be undertaken. If these efforts are met with success, we may be faced with a new cell design, radically different from the present Hall-Heroult cells. The goals may be... 

A current of 55,000 A is passed through a series of 100 Hall-Heroult cells for 24 hours. Calculate the maximum theoretical mass of aluminum that can be recovered. 

There have been sporadic attempts to produce aluminum by carbothermic reduction [3, 4]. In this approach, akin to the way iron oxides are reduced to iron in the iron blast furnace, the consumption of electrical energy is avoided or at least reduced. There have also been investigations of the production of aluminum by electrolysis of aluminum compounds other than the oxide (e.g. [5]). Some of these alternative electrolytic technologies have even reached a commercial scale [6] but the only method for aluminum production in industrial use today appears to be electrolysis in Hall-Heroult cells. Consequently, the present paper is confined to these cells. The literature on these cells is large. A recent search of the web of science with the subject Hall cell and similar subjects revealed 79 titles aluminum electrolysis yielded 109 publications. This number excludes papers published in the annual Light Metals volume of the Minerals Metals and Materials Society (TMS). Light Metals contains approximately forty papers each year on Hall cells. Consequently, the authors have made no attempt at a comprehensive examination of the literature on these topics. Rather we have included... 

Good descriptions of the production of aluminum can be found in the literature (Grjotheim etal. [7], Grjotheim and Welch [8], Grjotheim and Kvande [9], Burkin [10], and Peterson and Miller [11]). Referring to Fig. 2 [12], the first step in the production of aluminum from its ore ( bauxite ) is the selective leaching of the aluminum content (present as oxides/hy dr oxides of aluminum) into hot concentrated NaOH solution to form sodium aluminate in solution. After solution purification, very pure aluminum hydroxide is precipitated from the cooled, diluted solution by addition of seed particles to nucleate the precipitation. After solid-liquid separation the alumina is dried and calcined. These operations are the heart of the Bayer process and the alumina produced is shipped to a smelter where the alumina, dissolved in a molten salt electrolyte, is electrolyt-ically reduced to liquid aluminum in Hall- Heroult cells. This liquid aluminum,... 

Figure 3, from McGravie et al. [13] shows, in cross section, the three types of Hall-Heroult cell. The cross section of the... 

Fig. 3 Three types of Hall-Heroult cell in commercial use. The upper two use Soderberg anodes while the last uses prebaked anodes. From McGravie et al. [13].

The thermodynamics of the Hall-Heroult cell have recently been reexamined by Haupin [16] and by Haupin and Kvande [17]. In the latter paper, inert anodes producing oxygen are also treated. Among the useful results presented is a correlation for the activity of alumina in the electrolyte versus the relative oxide saturation (ROS), Fig. 5. 

Fig. 6 Overvoltages in a Hall-Heroult cell versus electrolyte alumina content. From the work of Haupin [16].

Fig. 8 Cell voltages and energy consumption for the Hall-Heroult cell, according to Haupin and Kvande [17],...

Hyland etal. [37] have recently reviewed sulfur and fluoride emissions (including particulate emissions) from Hall-Heroult cells and concluded that operational changes over the past few years, such as a tendency toward lower ratio (more volatile) electrolyte may have made emission control more difficult. 

Many effects of gas bubbles released at electrodes (on electrolyte flow, mass and heat transport, conduction, etc.) have been well studied in the past. A text with an extensive treatment of this topic is that of Hine [38]. However, in Hall-Heroult cells these effects are worthy of special mention because the relatively high current density, of the order of 1 A cm-2, and temperature make the volumetric gas evolution rate from the anode large. Furthermore, difficulties of measurement on actual cells mean less knowledge of these effects than in many other electrochemical cells. Finally, one effect of the bubble is to make the task difficult in reducing the enormous... 

Mathematical and Physical Modeling of Hall-Heroult Cells... 

The multiphysics and multiscale character of the important features of Hall-Heroult cell operation makes difficult laboratory scale experimentation that is relevant to industrial pot operations. For example, cell C E is influenced by the cell-scale flow of the metal and electrolyte, which is determined in turn by the magnetic field which depends on the entire cell current. CE also depends on the finer scale flow due to release of the carbon dioxide bubbles from the anodes. It is generally not possible to examine these two effects simultaneously in the laboratory. Also, the generally hostile environment inside Hall-Heroult cells makes experimentation difficult, and the high cost of modification of full-scale pots further complicates industrial trials. In this environment, numerical or mathematical modeling of pots would be expected to be a useful tool. 

Historically, the mathematical modeling of the Hall-Heroult cell became feasible in the 1970s when computers of sufficient power to examine many of the important phenomena were first available. In spite of a great deal of progress and much development effort, some critical issues remain in relating the output of the models to operational quantities of interest, such as noise and CE. In the following sections, each of the areas is discussed in more detail. 

Fig. 25 Comparison of the results of the mathematical model of Tabsh and coworkers for the thermal behavior of a Hall-Heroult cell with measurements [90].

Fig. 26 Comparison of the predictions of the comprehensive mathematical model of Tang et al. with measurements for a Hall-Heroult cell, during a period when 40 kg of aluminum fluoride was added to the electrolyte [91].

There has been much interest within the aluminum industry in advanced Hall-Heroult cells where the anode would be made of an inert material, rather than carbon. The anodic reaction is then merely the generation of oxygen, so the anode is not consumed. The... 

Fig. 30 The physical model of Banerjee and Evans for studying flow in Hall-Heroult cells driven by electromagnetic forces [97].

In the early 1980s Kaiser Aluminum investigated the use of titanium di-boride as a wettable cathode material for Hall-Heroult cells [47]. Similar investigations by Reynolds Metals Co. continued until that company s recent merger with Alcoa [107]. The Reynolds work, and earlier research and development by Martin Marietta Aluminum [108], involved TiB2-C composites. Approaches to the... 

This article has described the Hall-Heroult cell that is the mainstay of the aluminum industry throughout the world. Emphasis has been on the electrochemistry and electrochemical engineering that govern cell performance. The cell operation, electrolyte chemistry, thermodynamics, and electrode kinetics have been reviewed. Some complexities, notably the anode effect and the environmentally important fluoride emissions and anode gas bubbles and their effect on cell voltage, flow, and CE, have been examined. The incorporation of these phenomena, along with current distribution, magnetic fields, electromagnetically driven flow, heat and mass transport, and cell instability into mathematical models was summarized. 

Mention was made of physical modeling of Hall-Heroult cells and of the possibility of radical improvement in cell performance resulting from new materials for inert anodes and wettable cathodes. 

The electrolytic production of aluminum is carried out in Hall-Heroult cells that have changed little in nearly 100 years [39], The Hall-Heroult process operates at a high temperature (about 1250 K) and utilizes a molten salt electrolyte of alumina (AI2O3) and cryolite (Na3A102), with additives such as calcium fluoride and aluminum trifluoride. The cathode reaction is the reduction of AP+, with a consumable carbon anode. The overall reaction in the Hall-Heroult cell (shown schematically in Figure 26.15) is... 

The cells are strong steel boxes, lined with alumina (to act as a refractory), a thermal insulator, and carbon. The cathode is a liquid pool of aluminum that lies at the base of the cell, above a current collector consisting of a number of carbon blocks inlaid with steel bars. A frozen crust of electrolyte protects the cell housing from erosion. The cell has ports for the periodic addition of alumina through the crust, for the removal of A1 metal, and an extractor to vent anode gases (mainly CO2). As the carbon anode is consumed, it is lowered to maintain a constant anode/cathode gap (about 5 cm). In a typical plant for the production of 70,000 tons of A1 per year, 200 Hall-Heroult cells, each 3 m X 8 m in size with 15 m of anode area, are arranged in series. The operating current density is... 

FIGURE 26.15 Hall-Heroult cell for aluminum extraction process [30] (with kind permission from Springer Science and Business Media). 

A single Hall-Heroult cell (as shown in Fig. 18.22) produces about 1 ton of aluminum in 24 h. What current must be used to accomplish this ... 

ZrB2 is useful as a crucible material for metal melts because of its excellent corrosion resistance. It is also used in Hall-Heroult cells (for A1 production) as a cathode and in steel refining where it is used as thermowell tubes. 

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