Cathodic processes, aluminum

FIGURE 14.24 In the Hall process, aluminum oxide is dissolved in molten cryolite and the mixture is electrolyzed in a cell with carbon anodes and a steel cathode. The molten aluminum flows out of the bottom ot the cell. 

Figure 6.27 Anodic and cathodic processes involving aluminum.

P, y-Unsaturated esters (184) have been synthesized by a one-step electrochemical procedure from a-chloroesters (183) and aryl or vinyl halides (Scheme 73b) [294, 295]. This novel electroreductive cross-coupling method is based on the use of a Ni(II)(bpy) catalyst and a sacrificial aluminum anode in a one-compartment cell (Scheme 73). The whole cathodic process progresses at —1.2 V (SCE) (Scheme 73c),... 

In the modern version of this process, aluminum metal is obtained by electrolysis of aluminum oxide, which is refined from bauxite ore (AI2O3 2H2O). The aluminum oxide is dissolved at 1000°C in molten synthetic cryolite (Na3AlFg), another aluminum compound. The cell is lined with graphite, which forms the cathode for the reaction. Another set of graphite rods is immersed in the molten solution as an anode. The following half-reaction occurs at the cathode. 

While the metal or alloy electroless deposition reactions can be considered as cathodic processes, formation of oxides at the metallic surfaces without an external current source can be analyzed as anodic processes. This type of deposition can be illustrated in the example of chemical oxidation of aluminum in chromic acid solutions.9... 

Figure 20.22 The Hall-Heroult process operates at 900°C in smelters similar to this one. Note that carbon (graphite) serves as both the anode and the cathode. Recycled aluminum is often fed into the cell with the new aluminum. 

Aluminum is used as the anode and nichrome as the cathode. There is also available in the cell a catalytic amount of titanium for fixation purposes, in addition to naphthalene, which serves a purpose again in the reduction stage. The electrolyte is tetrabutylammonium chloride. Aluminum isopropoxide increases the over-all efliciency and turns this process into a catalytic one. This system starts with titanium tetraiso-propoxide. Reduction takes place, presumably again to the titanium (II) level. We have evidence from electrochemical experiments that titanium (II) is produced and involved in the fixation. Titanium (II), once formed, picks up N2 from the atmosphere and forms the complex, which then is available for reduction by sodium naphthalenide. Naphthalene is present in a catalytic amount and is reduced at the cathode to the radical anion. In this experiment, one can actually see it as a greenish color at the cathode. Naphthalenide reduces the No compound, producing the nitride. Normally, the reaction would stop at this point. We believe that in the electrochemical process, aluminum (III) abstracts nitride from titanium and forms aluminum nitride. This nitride transfer also can be observed in nonelectrolytic reactions. Thus, aluminum nitride is stored and ammonia is available at any time, merely by protonation. Both titanium and naphthalene are catalytic and permit operation of an over-all catalytic process. 

The deposition of copper on the surface of aluminum metal, as a consequence of its dissolution, is a cathodic process that can simply be presented by the reaction ... 

The reactor employed aluminum electrodes supplied by DC power. Chemical oxygen demand (COD), color, conductivity, pH and turbidity were monitored during the process. Aluminum dissolved at the anode, with water electrolyzed at the cathode to produce aluminum hydroxide and hydrogen gas. These reactions and hydroxide precipitation resulted in changes in solution pH during the process. Aluminum solubility was affected by solution pH, which was found to affect COD and color removal. 

Zinc is an element of group 12 of the periodic table, of which the outer electrons are 3d 4s and zinc is in the same group of elements, such as Cd and Hg. Zinc exists as 70 ppm in the Earth s crust and exists in similar amounts to Cr (100 ppm) and Ni (80ppm) [1]. Zinc is produced from sphalerite (ZnS) as ores. There are two processes, a dry process and a wet process, for the refinement of zinc. However, zinc is largely produced by the wet process. The wet process is equally to be said as an electrolysis process. The sphalerite contains lead, iron cadmium and copper, etc. besides zinc and, by a flotation separation of the ores, the zinc concentrate (50-55 % Zn) and the lead concentrate are separated. The zinc concentrate is burnt and put into a vessel and dissolved in an electrolysis foul solution. The zinc sulfate solution and concentrated mud (the common name is red mud) are separated by a filter. Cadmium and copper, etc., as impurities dissolve in the zinc sulfate solution. Then zinc dust is added to the solution to precipitate these impurities to form a clean solution of zinc sulfate to be electrolyzed. In the electrolysis, lead containing 1 % of silver is used as a cathode and aluminum is used as an anode, and zinc is produced on the cathode [2—4]. The properties of zinc are shown in Table 5.1. 

Seawater predominantly consists of about 3.5% of sodium chloride (NaCl) and many other ions. Chloride ions are very strong and could easily penetrate the passive film Thus, dissolution of the aluminium substrate occurs and results in corrosion. The adsorption of the corrosion inhibitor competes with anions such as chloride. By assuming that the corrosion inhibitor molecules preferentially react with Al - to form a precipitate of salt or complex on the surface of the aluminum substrate, the anodic and cathodic processes subsequently suppressed by inhibitor molecules. Thus, this result suggests that the protective film that was formed comprise aluminium hydroxide, oxide and salts or complexes of the corrosion inhibitor anions. 

The stability of electrolysis in a potassium cryolite-based electrolyte with low CR and quality of producing aluminum are determined by the anode and cathode processes, which affects voltage fluctuation and the... 

Manganese metal made by this process is 99.9% pure. It is in the form of irregular flakes (broken cathode deposits) about 3-mm thick, and because of its brittleness, has Httle use alone. Most of the electrolytic manganese that is used in the aluminum industry is ground to a fine size and compacted with granulated aluminum to form briquettes that typically contain 75% Mn and 25% Al. 

The cell for this process is unlike the cell for the electrolysis of aluminum which is made of carbon and also acts as the cathode. The cell for the fused-salt electrolysis is made of high temperature refractory oxide material because molten manganese readily dissolves carbon. The anode, like that for aluminum, is made of carbon. Cathode contact is made by water-cooled iron bars that are buried in the wall near the hearth of the refractory oxide cell. 

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Refractories in the Aluminum Industry. Carbon materials are used in the HaH-Heroult primary aluminum cell as anodes, cathodes, and sidewalls because of the need to withstand the corrosive action of the molten fluorides used in the process (see Aluminumand aluminum alloys). 

Production of one metric ton of molten aluminum requites about 500 kg of anode carbon and 7.5—10 kg of cathode blocks which is the largest industry usage of carbon materials. Aluminum smelters generally have an on-site carbon plant for anode production. Anode technology is focused on taw materials (petroleum coke and coal-tar pitch), processing techniques, and todding practices (74). 

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