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Diaphragm cells account for 71 percent of domestic production, mercury cells for 12 percent, and membrane cells for 16 percent, with other methods producing about 1 percent (CI 1999). Total production costs for using cells are a function of raw materials, energy, operating costs, and capital depreciation. Today, membrane cell technology is only a small factor in new capacity. Membrane cell technology requires a secondary brine treatment, disposal or recycling of spent anolyte, and has relatively high capital costs. Most diaphragm cell producers continue to rebuild their existing diaphragm cells, rather than convert to membranes. Chemical Reaction in Salt Electrolysis 2NaCl+2H2O 62NaOH + Cl2 + H2 Salt Water Sodium Chlorine Hydrogen Hydroxide Table 6-1. Characteristics of Various Chlorine/Sodium Hydroxide Electrolysis Cells Component Diaphragm Cell Mercury Cell Membrane Cell Cathode Steel/steel coating with Mercury flowing over steel Steel or nickel with a nickel nickel-based coating Anode Titanium with ruthenium and titanium oxide coatings; iridium oxide added to improve performance and extend life Titanium with ruthenium and titanium oxide coatings; iridium oxide added to improve performance and extend life Titanium with ruthenium and titanium oxide coatings; iridium oxide added to improve performance and extend life Diaphragm/ Asbestos and fibrous None Ion-exchange membrane Membrane Material polytetrafluoroethylene (fluorinated polymers) Cathode Product 10 to15% sodium hydroxide solution, containing 15 to17% salt (NaCl) (sent to evaporator for further processing); hydrogen gas Sodium amalgam (sent for further processing through a decomposer cell) 30-33% sodium hydroxide solution (sent to evaporator for further processing); hydrogen gas Anode Product Chlorine gas containing some oxygen, salt, water vapor, and sodium hydroxide Chlorine gas containing some oxygen, salt, and water vapor Chlorine gas containing some oxygen, salt, and water vapor Evaporator/Decom- position Product 50% sodium hydroxide solution containing 1% salt; solid salt from evaporator 50% sodium hydroxide solution; hydrogen gas 50% sodium hydroxide solution with very little salt Electricity Consumption 2,550 to 2,900 kWh/ton chlorine gas 3,250 to 3,450 kWh/ton chlorine gas 2,530 to 2,600 kWh/ton chlorine gas Sources: Sittig 1977, EPA 1990, EPA 1992b, EPA 1995b, DOW 1999. 179PDF Image | The Chlor-Alkali Industry
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