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and impurities such as bromine and iodine. Gaseous chlorine is transferred for use by pipeline or liquid chlorine is transported by rail car to offsite customers (DOW 1999). Rail cars and barges are generally purged with nitrogen to remove the chlorine and the chlorine is scrubbed in a tower with caustic. The resulting hypochlorite is treated with bisulfite and the effluent is treated for disposal. The dilute sodium hydroxide solution contains residual salt and must undergo an evaporative process to produce a product at a usable concentration. The greatest disadvantage to diaphragm cells is the large amount of salt that must be removed at this stage. The need for salt columns, centrifuges, cyclones, clarifiers, and filters increases the complexity, capital cost, and energy use in a diaphragm-based caustic plant (DOW 1999). Figure 6-5 shows the general arrangement for the concentration and production of the final sodium hydroxide product. This process concentrates the cell liquor from the diaphragm cell by evaporating water from the dilute caustic and separating the residual salt. The end result is a 50 percent sodium hydroxide solution with 1 percent residual sodium chloride. The advantage of diaphragm cells is that they operate at a lower voltage than mercury cells and use less electricity. The brine feedstock can also be less pure than that required by mercury or membrane cells. The mercury cell process (see Figure 6-6) uses two cells: an electrolyzer cell and a decomposer cell. The electrolyzer is essentially an electrolysis cell consisting of a large steel container shaped like a rectangular parallelogram with lined walls under a covering of flexible and anti-explosive rubber. A thin layer of mercury of about 3 millimeters in depth flows over the bottom of the steel container, serving as the cathode for the process. A saturated brine solution of about 25 percent NaCl by weight flows through the container above the mercury. The anode, consisting of titanium sheet coated with ruthenium oxide and titanium oxide, is incorporated into the cell cover and suspended horizontally in the brine solutions. The height of the anodes in the brine is adjusted to obtain an optimum distance from the mercury cathode. In the electrolyzer, chlorine evolves from the electrolytic decomposition of NaCl and moves upward through gas extraction slits in the cell cover. The chlorine gas is removed, purified, and sent to storage. Sodium ions are absorbed in the mercury layer, and the resulting sodium and mercury mixture (amalgam) is sent on to the decomposer cell. The decomposer is essentially a short-circuited galvanic cell and consists of a small cylindrical steel tower divided into two parts. The amalgam is semi-decomposed in the upper section, and the decomposition is then completed in the lower part. Graphite serves as the anode and amalgam serves as the cathode. The amalgam and water flowing through the cell come into direct contact with the graphite. In both parts of the tower the amalgam is decomposed by water with the formation of sodium hydroxide, the reformation of mercury, and the production of hydrogen gas. The mercury generated can be reused in the primary electrolytic cell. A relatively highly concentrated (50 percent) solution of sodium hydroxide is formed and can be used as it is or after it is further concentrated. The hydrogen gas is purified and used elsewhere in the plant. The depleted brine leaving the cell contains a high concentration of NaCl (21 to 22 percent by weight). Dissolved chlorine is removed from this solution, and it is resaturated with NaCl and purified for re-use. The high concentration of sodium hydroxide solution produced and the absence of residual salt are the major advantages of the mercury cell. No further evaporation or salt separation is needed to produce the finished product. However, mercury cells require higher voltage than both diaphragm and membrane 183PDF Image | The Chlor-Alkali Industry
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