Production of chlor-alkali

Flow diagrams of a typical plant production of chlor-alkali indicating water use and wastewater generation are given in Figure 22.4. 

Results of raw waste load found in verification sampling of unit product of chlor-alkali are given in Table 22.9. 

Summary of Raw Waste Loadings Found in Verification Sampling of Unit Product of Chlor-Alkali (Mercury Cell and Diaphragm Cell Processes)... 

FIGU RE 22.5 General wastewater treatment process flow diagram at a mercury cell plant for the production of chlor-alkali. 

Statistical data on dry salt sales are available through 1994 (9). Dry salt includes salt produced as crystalline sodium chloride, but excludes salt in brine produced for production of chlor—alkali products and other chemicals. Table 7 gives United States dry salt sales for the period 1990—1994. 

Mercury has been known for over 4000 years. Its chemistry has many unusual aspects that have resulted in extensive usage by mankind. Mercury is the only common metal that is liquid at room temperature. It is also one of the very few elements that is monoatomic in its vapor phase at low temperatures, an indication of the weak Intermolecular forces that are present in mercury. The element exhibits surprisingly different chemistry from its congeners zinc and cadmium, a feature that is probably a consequence of the poor shielding by electrons in the completely filled 4f and 5d sublevels. The metal is used in a number of applications, including thermometers, amalgams, and production of chlor-alkali. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

Summary. Membrane cell processes have become important to modem technology to a great extent because of the development and utilization of perfluorinated membranes. The combination of metal anodes and the perfluorinated membranes has provided a modem revolution in the area of chlor-alkali production. 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

Discussions of chlor-alkali technology and production usually focus on the production of caustic soda from NaCl. The use of KCl to produce caustic potash, an application that accounts for only a few percent of chlorine production, is often ignored. The authors have made a conscious attempt not to do this. Therefore, this section discusses sources and recovery of KCl. The comprehensive discussion of the potash industry and the processing of ore to cormnercial forms of KCl by Zandon, Schoeld, and McManus [28] is the basis for much of what follows. 

Impurities in brine affect electrode reaction kinetics, cell performance, the condition of some cell components, and product quality. Treatment of brine to remove these impurities has always been an essential and economically significant part of chlor-alkali technology. The brine system typically has accounted for 15% or more of the total capital cost of a plant and 5-7% of its operating cost. The adoption of membrane cells has made brine specifications more stringent and increased the complexity and eost of the treatment process. Brine purification therefore is vital to good electrolyzer performance. This section considers the effects of various impurities in all types of electrolyzer and the fundamentals of the techniques used for their control. Section 7.5 covers the practical details of the various brine purification operations. 

The electrical hazards of AC supply and distribution also are those faced in industiy in general. We may consider the hazards from the primary supply through the area transformers as conventional. The DC side and the magnetic fields associated with high current are more characteristic of chlor-alkali production. These are discussed in Section 8.5.1. 

Abstract Ion-conducting materials are used as cell separators in electrolysis cells for the double purpose of carrying electric charges between electrodes and separating the products formed at each electrode. The purpose of this chapter is to provide an overview of chlor-alkali technology and associated cell separators. After a brief historical review of the chlor-alkali process, the main reaction characteristics (thermodynamics, cell reactions and kinetics) are detailed in Section 9.1. Main chlor-alkali technologies are described in Section 9.2. Main cell separators are described in Section 9.3 (diaphragm materials) and in Section 9.4 (membrane materials). Some improved electrolysis concepts are described in Section 9.5. 

But the impact which the aluminum industry makes on the ever-present problem of chlor-alkali balance in the world is substantial. The electrolysis of brine produces almost equal weights of chlorine and sodium hydroxide as co-products. 

The production capacity of chlor-alkali plants using the membrane process reached about 21 % of total world production capacity in 1995 and is predicted to increase to about 28% in 2001 [133]. 

Today the membrane process is the state of the art for producing chlorine and sodium hydroxide or potassium hydroxide. All new plants are using this technology. The production capacity of chlor-alkali plants using the membrane process reached about 21% of total world production capacity in 1995 and is predicted to increase to about 28% by 2001 (Table 19) [133]. The diaphragm cell capacity remains constant and there is a decline in mercury cell capacity. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

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