Chlorine, production without caustic

The subject of Sect. 5.2.1 to 5.2.4 was the electrochemical production of chlorine and of caustic alkali. It shall be noted in addition, that chlorine is obtained as a by product if metal chlorides like NaCl, MgCh, or others are electrolyzed in their molten form to produce the metals (see Chapter 4). From aqueous NaCl, caustic alkali and hydrochloric acid can be produced without generation of chlorine, using an electrodialytic separation process [35],... 

With the advent of dimensionally stable anodes and perfluorinated ion exchange membranes, over the past decade and a half, came an alternative method for the manufacture of chlorine and caustic soda. This new process produces a food grade product without pollution. 

The processes covered in this section do not produce elemental chlorine, nor are they truly sources of chlorine without caustic. Rather, they combine the two products to produce hypochlorites. Since these are valuable principally as sources of active chlorine and leave very little accompanying free caustic, they are treated in this chapter. 

PRODUCTION OF CAUSTIC WITHOUT CHLORINE 15.4.1. Causticization of Soda Ash... 

The electrolysis of sodium sulfate is a much-studied process for production of caustic soda without chlorine. When Na2S04 electrolyzes in a divided cell, oxygen and hydrogen form at the anode and cathode, leaving H+ and OH in the respective compartments. Dilute solutions of sulfuric acid and caustic soda form by the reactions... 

G. Faita, Caustic Soda without Chlorine Production, Seventh International Forum on Electrolysis, Clearwater, FL (1993). 

The process consumes a large quantity of electricity and energy costs are an important consideration for profitability. From the balanced equation, two moles of sodium chloride produce 2 moles of sodium hydroxide and 1 mole of chlorine gas. Don t confuse the molar ratio with the mass ratio. By using atomic masses, we can say that 117 g (2 moles x 58.5 g/mole) sodium chloride produces 80 g (2 moles x 40 g/mole) sodium hydroxide and 71 g chlorine (1 moles X 71 g/mole). Therefore, the mass ratio of caustic to chlorine is 80 to 71. The mass ratio is true for all mass units. A production of 80 pounds of caustic coproduces 71 pounds of chlorine. A plant consuming 117 tons of salt coproduces 80 tons of sodium hydroxide and 71 tons of chlorine. One of these is not produced without coproducing the other. This can be a problem in the marketplace when there is a demand for caustic but not chlorine or when there is a demand for chlorine but not caustic. 

Chlorine and sodium hydroxide production by the electrolysis of brine solutions necessarily locks the ratio of the two products to the theoretical ratio of the process [45, 46]. When the market for sodium hydroxide exceeds the market for chlorine, the causticization of sodium carbonate to sodium hydroxide (Section 7.3) may be used by some suppliers and consumers to supplement the available sodium hydroxide without producing large amounts of excess chlorine. Another expedient for large-scale chloralkali producers faced with this situation is to stimulate the chlorinated solvent or hydrochloric acid markets in an attempt to increase the consumption of chlorine to restore the balance. These measures are not usually rapid enough to respond over the short term unless the solvent plant is also operated by the chloralkali producer. 

---