Downs electrolytic cells

Practically all of the sodium produced in the United States involves the use of the Downs electrolytic cell, which consists of a massive graphite anode surrounded by two or more iron cathodes. The electrolyte is an aqueous solution of sodium choloride. The NaCl salt is continuously added. As electric current is passed between the electrodes, chlorine gas is collected in a hood over the graphite anode and piped off to further processing for marketing. The electrolysis of sodium chloride proceeds as follows ... 

The first, and now obsolete, industrial processes for producing raw sodium metal were based on the carbon reduction of sodium carbonate or sodium hydroxide. The first industrial production of pure sodium metal was performed by molten-salt electrolysis of the pure sodium hydroxide, NaOH, in so-caUed Castner cells. Most modern processes for the production of sodium now involve molten-salt electrolysis of highly pure sodium chloride. Actually, since 1921, when the process was invented by J.C. Downs, the electrolysis has been performed in Downs electrolytic cells at the DuPont de Nemours Canadian facilities at Niagara Falls, Ontario, Canada. The electrolytic cell consists of four cylindrical anodes made of graphite surrounded at the bottom of the cell by steel cathodes, and a fine steel mesh acts as a separator between anodic and cathodic compartments. Each cell contains a batch of 8 tonnes of a molten-salt mixture with the following chemical composition NaCl (28 wt.%), CaCl (26 wL%), and BaClj (46 wt.%). 

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Zinc electrowinning takes place in an electrolytic cell and involves running an electric current from a lead-silver alloy anode through the aqueous zinc solution. This process charges the suspended zinc and forces it to deposit onto an aluminum cathode (a plate with an opposite charge) that is immersed in the solution. Every 24 to 48 h, each cell is shut down, the zinc-coated cathodes removed and rinsed, and the zinc mechanically stripped from the aluminum plates. The zinc concentrate is then melted and cast into ingots, and is often as high as 99.995% pure. 

Some metals are extracted in electrolytic cells. In section 11.3, you saw the extraction of sodium from molten sodium chloride in a Downs cell. Other reactive metals, including lithium, beryllium, magnesium, calcium, and radium, are also extracted industrially by the electrolysis of their molten chlorides. 

The Quantitative Aspects of Electrolysis activity in eChapter 18.13 walks you through the steps for calculating the mass of metal that can be produced by an electrolytic cell for a given time at a certain current. Write down each of the steps required for the problem given then use a similar procedure to determine the mass of copper that can be produced by passing a current of 18.0 A through a solution of copper(II) ions for 1.0 hour. 

The electrolytic cell is placed on a stand (Fig. 13) which is made of Alsimag 222 ceramic. The anode and cathode are connected to platinum wires that run down the inside of the stand and are attached to terminal posts near the base of the stand. 

With few exceptions the electrodes in electrolytic cells are arranged either vertically of horizontally. There is no exact rule as to which particular arrangement should be used as the position of the electrodes depends on the specific nature of the electrolytic process involved. Nevertheless, certain general rules may be laid down. 

It can be seen that the electrolytic cell must accept 11 679 cal. from the surroundings per each mole of decomposed water to keep a constant temperature (when working adiabatically the electrolytic cell would cool down). This heat is also covered by electric energy. Therefore, to achieve an isothermal decomposition of a mole of water a total amount of not only 56 693 cal. but 68 372 cal. in the form of electrical energy is necessary which corresponds to the minimum terminal voltage across the electrolytic cell ... 

Referring to a list of standard electrode potentials, such as in Table 8.3, one speaks of an electrochemical series, and the metals lower down in the se-ries(with positive electrode potentials) are called noble metals. Any combination of half-reactions in an electrochemical cell, which gives a nonzero E value, can be used as a galvanic cell (i.e., a battery). If the reaction is driven by an applied external potential, we speak of an electrolytic cell. Reduction takes place at the cathode and oxidation at the anode. The reduction reactions in Table 8.3 are ordered with increasing potential or pe values. The oxidant in reactions with latter pe (or E°) can oxidize a reductant at a lower pe (or ) and vice versa for example, combining half-reactions we obtain an overall redox reaction ... 

Unlike the Daniell and Leclanche cells, the lead-acid cell is rechargeable. So, when the battery runs down, you do not need to replace it. Instead, an electric current is applied in a direction opposite to that discussed above. As a result of the input of energy, the reactions are reversed. The cell is eventually restored to its charged state. During recharge, the cell functions as an electrolytic cell, which you will learn about in the next section. 

Electrolysis of molten NaCI Just as electrolysis can decompose water into its elements, it also can separate molten sodium chloride into sodium metal and chlorine gas. This process, the only practical way to obtain elemental sodium, is carried out in a chamber called a Down s cell, as shown in Figure 21-18. The electrolyte in the cell is the molten sodium chloride itself. Remember that ionic compounds can conduct electricity only when their ions are free to move, such as when they are dissolved in water or are in the molten state. 

Mercury flows down the inclined base of the electrolytic cell (Fig. 1A). The base of the cell is electrically connected to the negative pole of the DC-supply. On the top of the mercury and flowing co-currently with it is a concentrated brine with a sodium content of ca. 310 g L-1 at the inlet. The brine must be purified thoroughly (see Sect. 5.2.3.4). Anodes are placed in the brine so there is a small gap between the anodes and the flowing mercury cathode. The anodes used nowadays are predominately of the DSA-type. They are constructed in form of parallel blades or rods, arranged in flow direction. The distance between these elements is needed for quick gas release. 

Electrolytic cells are used in industry for metal plating, and for purifying metals. For instance, pure sodium can be collected through electrolysis of sodium chloride solution in a Downs cell. The half reactions are as follows ... 

Downs cell Electrolytic cell used for the commercial electrolysis of molten sodium chloride to produce commercial-grade sodium. 

Several elements are produced commercially by electrolysis. In Sections 21-3 to 21-5, we described some electrolytic cells that produce sodium (the Downs cell), chlorine, hydrogen, and oxygen. Electrolysis of molten compounds is also the common method of obtaining other Group lA metals, HA metals (except barium), and aluminum (Section 22-3). Impure metals can also be refined electrolytically, as we will describe for copper in Section 22-8. 

A Hall-Heroult electrolytic cell Is used to produce aluminum metal. It is made of a steel shell lined with carbon that forms the cathode. Anodes of carbon hang down into the solution of aluminum oxide dissolved in cryolite. ... 

In an electrolytic cell, the passage of an electrical current initiates a redox reaction, e.g. in the Downs process (see Section 8.12 and Figure 10.1) for the manufacture of Na and CI2 (equation 7.4). 

In contrast to primary batteries, a secondary, or rechargeable, battery is recharged when it runs down by supplying electrical energy to reverse the cell reaction and re-form reactant. In other words, in this type of battery, the voltaic cells are periodically converted to electrolytic cells to restore nonequilibrium concentrations of the cell components. By far the most widely used secondary battery is the common car battery. Two newer types are the nickel-metal hydride battery and the lithium-ion battery. 

In addition to the Downs cell for sodium production and the chlor-alkali process for chlorine manufacture, industrial methods based on voltaic and electrolytic cells are used commonly to obtain metals and nonmetals from their ores or to purify them for later use. Here we focus on two key electrochemical processes. 

Understand the basis of an electrolytic cell describe the Downs cell for the production of Na, the chlor-alkali process and the importance of overvoltage for the production of CF, the electrorefining of Cu, and the use of cryolite in the production of Al know how water influences the products at the electrodes during electrolysis of aqueous salt solutions ( 21.7) (SP 21.8) (EPs 21.63-21.75,21.82)... 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

We shall consider first the flow of solution over a flat plate. In such systems, two forces will be acting upon the fluid. The first is the cause of the flow (that generated by a pump or a solution head) and is known as the inertial force. The second opposes the flow and results from viscous forces at the interface between the plate and the solution. Suppose we assume that the solution may be divided into elements, then the viscous force will initially cause that element next to the stationary plate to be retarded and later each element will be slowed down by that closer to the plate. Hence as the solution flows over the plate, a boundary layer of more slowly moving solution will develop, as shown in Fig. 1.11. In an electrolytic cell, the flat plate would normally be the electrode and therefore the formation of such boundary layers has particular importance. The electrode reaction takes place in the boundary layer in the presence of velocity gradients. 

The general technology may be illustrated by the example of sodium production in the Down s cell. The electrolyte is a molten mixture of sodium chloride (40%) and calcium chloride (60 wt %) requiring a process temperature of about 600°C. The principle of cell design is shown in Fig. 4.4, although more... 

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