Modified asbestos diaphragm

There have been various reports comparing costs of polymer-modified asbestos diaphragms and non-asbestos diaphragms. The non-asbestos diaphragm is not economical for low current density operations. At higher current densities, the life of the diaphragm should be substantially more than three years to be economically attractive, as evident from the cost comparison developed by PPG [85], and presented in Table 4.7.6. 

The current cell employs vertical DSA coated titanium anodes, vertical cathodes of woven wire mesh bolted to a perforated steel backplate, and a vacuum-deposited modified asbestos diaphragm. A single bipolar element may have 100 m of both anode and cathode active area. The anode of one element is connected to the cathode of the next by copper spring clips. This connection is immersed in the cell liquor during operation. Figures 33 B and 34 show these internal cell parts. 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

A wide variety of techniques and modifiers have been investigated to impart dimensional stability to asbestos diaphragms. The techniques examined include heat-treating the diaphragm at 500-600°C instead of 450°C, using crocidolite asbestos and mixing... 

OxyTech MDC-55 cell. Tlie modified cell is outfitted with the Modified Diaphragm and expandable anodes, whereas the. standard cell is outfitted with the standard asbestos diaphragm and the standard DSA anode. Conditions anolyte temperature of 93 °C, anolyte NaCI concentration of 250 g/L, and catholyte NaOH concentration of 130 g/L. 

The combination of the Modified Diaphragm and expandable DSA anodes reduces power consumption by 10-15% from that of regular asbestos diaphragms and standard, fixed DSA anodes [127]. Table 12 presents performance data for the two most common MDC cell sizes [125]. The OxyTech MDC-29 is shown in Fig. 41. The licensed chlorine capacity of OxyTech cells now exceeds 20 000 t/d. 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

S.3. Diaphragm Resistance. The calculation of the resistance across a separator, such as an asbestos or a polymeric diaphragm, is identical to the method employed to determine the resistance of an electrolyte between two parallel plates. However, the distance between the two faces of a separator is not equal to its thickness, as the liquid path in the separator is tortuous and the area is restricted by the finite porosity. Therefore, the 1/A term in the parallel plate configuration should be modified to read as [tortuosity x length]/[porosity x area] or as the ratio of tortuosity to porosity, which is called the MacMullin number. The MacMullin number, characterizing a... 

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