Anode, chlor-alkali

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

Sodium was made from amalgam ia Germany duriag World War II (68). The only other commercial appHcation appears to be the Tekkosha process (74—76). In this method, preheated amalgam from a chlor—alkali cell is suppHed as anode to a second cell operating at 220—240°C. This cell has an electrolyte of fused sodium hydroxide, sodium iodide, and sodium cyanide and an iron cathode. Operating conditions are given ia Table 6. 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

When chlor-alkali electrolysis is conducted in an undivided cell with mild-steel cathode, the chlorine generated anodically will react with the alkali produced cathodically, and a solution of sodium hypochlorite NaClO is formed. Hypochlorite ions are readily oxidized at the anode to chlorate ions this is the basis for electrolytic chlorate production. Perchlorates can also be obtained electrochemically. 

Owing to the fact that nearly all the heat generated by this type of electrolyser has to be dissipated via the anolyte flow, for the full industrial-scale demonstration electrolyser with an element size 2.5 m2 it was decided to use the bubble jet system [3], which was successfully tested previously with the chlor-alkali method. For FIC1 electrolysis, which from the material side is optimised to an approximate operation temperature of 60°C, an intense vertical temperature-profile flattening is essential to reduce the external flow rates and to allow rather low anode-side inlet temperatures. The intensive vertical mixing with the bubble jet proved to be suitable for this purpose. 

However, the published corrosion rates of Ru from oxygen evolution are not reliable, as they forecast [50] Ru losses as high as 40 g cm-2 h, which is inconsistent with the anode lifetimes observed in commercial chlor-alkali cells. 

In chlor-alkali production, EMOS should be able to determine problems with both anode coatings and membranes. The literature is replete with examples of the effect of different impurities on membranes [2] and of the analysis of different problems using polarisation curves to determine their cause [3, 4]. These analysis techniques have been incorporated into the expert system in the form of approximations of the polarisation curves. Use is made of the familiar k-factor (see Equation 8.2) or the more accurate logarithmic form of this factor (Equation 8.3) ... 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

In the chlor-alkali industry titanium brings its properties to application as a material in activated metal anodes. In fact this is the major use of titanium in the chlor-alkali industry. 

The chlor-alkali cell in this diagram electrolyzes an aqueous solution of sodium chloride to produce chlorine gas, hydrogen gas, and aqueous sodium hydroxide. The asbestos diaphragm stops the chlorine gas produced at the anode from mixing with the hydrogen gas produced at the cathode. Sodium hydroxide solution is removed from the cell periodically, and fresh brine is added to the cell. 

Scott et al. [33] designed a DMFC with stainless steel mesh as the anode FF plate that was able to remove the carbon dioxide gas effectively. Later, the same research group was able to demonstrate that using similar meshes as DLs in the anode side also improved the overall gas removal [26,34] (wet-proofed CFP was used as the DL on the cathode side). These meshes were used on the anode side and were made out of catalyzed Ti because similar meshes have been used extensively as catalyzed electrodes in other industries, such as the chlor-alkali industry [26]. 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

The high overpotential for O2 evolution could be avoided if the reaction were replaced with a different anodic reaction. This replacement could in turn reduce AE, the minimum cell potential difference, which depends on the nature of the electrode reactions. Such a strategy has already been applied with success in the chlor-alkali industry, where the CI2-H2 couple (A = 1.35 V) has been replaced with CI2-O2 (A ri0.90 V) (O2 is reduced at the so-called air cathode). 

In chlor-alkali diaphragm cells, a diaphragm is employed to separate chlorine hberated at the anode from the sodium hydroxide and hydrogen generated at the cathode. Without a diaphragm, the sodium hydroxide formed will combine with chlorine to form sodium hypochlorite and chlorate. In many cells, asbestos diaphragms are used for such separation. Many types of diaphragm cells are available. 

Titanium metal is especially utilized in environments of wet chlorine gas and bleaching solutions, ie, in the chlor— alkali industry and the pulp and paper industries, where titanium is used as anodes for chlorine production, chlorine—caustic scmbbers, pulp washers, and Cl2, C102, and HC104 storage and piping equipment (see Alkali and chlorine products Paper Pulp). 

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