Dechlorination of brine

Whichever form of is used, the active agent will be the same. In solution, proton shifts between the different forms are rapid [217], and which form takes part in the reaction depends entirely upon the pH of the brine. Since secondary dechlorination of brine usually follows or accompanies the addition of alkali, the sulfite ion is the predominant form. Miron [218] reports that it is also the most active. The following equilibria apply ... 

Metal Catalysts. The goal in catalytic dechlorination of brine is the deoxygenation of the hypochlorite ion ... 

T.F. O Brien, Dechlorination of Brines for Membrane Cell Operation. In N.M. Prout and J.S. Momhouse (eds). Modem Chlor-Alkali Technology, vol. 4, Elsevier Applied Science, London (1990), p. 251. 

Activated Carbon. The primary use of activated carbon is in the dechlorination of brine, but it also sometimes serves as a filter medium, most frequently in mercury-cell caustic service. In any case, consumption after the initial charge is small. 

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. 

The design work to produce a commercial competitive product showed that lowering the temperature of brine to less than 50°C, which was necessary in the demonstration plant, added additional equipment and installation costs to the skids. A series of experiments were carried out on a new improved membrane element, developed for higher brine temperature. In this way we could feed dechlorinated brine to the skid without addition of a cooler. The results of these experiments are shown in Table 11.1. 

Dechlorination of the brine by sparging it with air was also necessary, sometimes also with a reductant such as hydrazine present, prior to sulfide treatment. Otherwise, precipitate reoxidation and resolution would occur (Eq. 8.51). 

Dechlorination of depleted brine is an important step in the operation of mercury cells since the dissolved chlorine affected the... 

Finally, we consider the membrane cells in Fig. 6.5. The electrode processes are the same as those in the diaphragm cells (Eqs. 1 and 2). Anolyte processing is quite similar to that practiced with mercury cells. We saw above in the discussion on brine treatment that membrane cells had stricter requirements. The same is true regarding dechlorination of the depleted brine. After vacuum dechlorination, the residual active chlorine content is high enough to damage the ion-exchange resin in the brine purification... 

Dechlorination of depleted brine treatment of diaphragm cell liquor automatic flushing of membrane cells upon loss of power... 

As noted, the acid used in dechlorination of depleted brine does not have to be of the highest purity. The dechlorinated brine will circulate back through the treatment system, and any impurity that is removed there can be tolerated at a reasonable concentration in the acid. This may allow the use of byproduct acid or of some of the acidic effluent from ion exchanger regeneration. 

We consider instead the continuous treatment of a sidestream of the brine. The most efficient process configuration couples the destruction of chlorate with dechlorination of the depleted brine. The excess HQ in the destruction reactor then becomes part of the total acid addition to the dechlorination process. Figure 7.104 is an illustration. In part (a), the base case, there is no deliberate destruction of chlorate. The acid produces a pH low enough to reverse the hydrolysis of dissolved chlorine by decomposing HCXTl ... 

B. Chlorinated Condensate. An important aspect of chlorine cooling is the composition of the condensate. The water removed from the gas, or in the case of direct cooling of the cooling water as well as that condensed, will be saturated with chlorine. This water is often a process waste, and it must be made innocuous before discharge. Usually, the bulk of the chlorine is stripped from the water after adding acid to reverse its hydrolysis. Hie basics of dechlorination of aqueous streams were covered in the chapter on brine treatment (Section 7.5.9). 

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

To overcome membrane scaling, the operating pH of the feed brine to the unit was lowered to a range between 4 and 7. A simple modification was made to the plant to control the pH of the plant feed brine by mixing acidic dechlorinated brine with alkaline dechlorinated brine. This modification has proven to be effective and no further membrane fouling has occurred over the last two years. 

Low running cost. The RNDS requires no brine purge and less chemical dosing. As the RNDS uses dechlorinated brine at pH2, additional HC1 is unnecessary, achieving minimal chemical consumption and loss of NaCl. The RNDS consumes a small quantity of caustic soda at desorption. However, compared with former processes, the consumption is almost the same, since the amount of caustic soda needed for neutralising depleted brine is decreased. 

In order to remove effectively iodide by RNDS , oxidation of iodide to iodate or periodate is necessary. Iodide is oxidised to iodate with excess chlorine. Through contact of dechlorinated brine with the ion-exchange resin containing zirconium hydroxide, the iodide is therefore removed from the brine. 

When the brine leaves the electrolyzer it has a lower content of chloride and contains also dissolved chlorine and a certain amount of hydrochlorous acid. Chlorine makes working conditions difficult, particularly when the salt is dissolved in open tanks, and attacks the iron part of the equipment. For this reason the brine must be dechlorinated prior to resaturation and simultaneously cooled. When saturation has been completed and the brine enters the electrolyzer, the temperature should not exceed 50° to 60 °C. In winter when the temperature has dropped below 50 °C the brine must be heated. 

This dechlorination method is not considerably applied in practice as it entails the consumption of a certain amount of alkali hydroxide and also all the dissolved chlorine is depreciated by conversion to chloride. Therefore, this method is sometimes used for removing the last remnants of chlorine from the brine which has previously been dechlorinated by evacuation and aeration instead of the method using sodium bisulphite and sulphur dioxide because in this case undesirable sulphate ions are formed in the brine. 

The flowsheet of the air-stripping process for bromine recovery from brines (including seawater brines) is shown in Fig. 5 [60]. The stock brine from a reservoir, mbted with H2SO4 and Clj, is directed to the top of the desorber. The bromine-free brine is collected at the bottom of the desorber, neutralized with thiosulfate and lime milk prior to disposal. Release of the chlorine/bromine air mixture from the top of the desorber is directed to the dechlorinating tower (1) where the mixture is treated with diluted FeBrj solution. The halogen exchange is described by the reaction... 

Tknd last but not least it must be stated that in the case of the mercury and the membrane process the depleted brine leaving the cells must be dechlorinated before resaturation, for instance by spraying it into a vacuum of 50-60 kPa. 

Depleted brine, dechlorinated and neutral to alkaline, containing 260 to 280 GPL NaCl, and at a temperature in the range of 50 to 60°C, is saturated by pumping it up through a bed of salt in dissolving tanks. Brine generally leaves the saturator hot and saturated to prevent crystallization downstream it is generally diluted with a small bypass stream of weak brine. 

Since the ion-exchange resin is attacked by trace amounts of dissolved chlorine, it is essential that the depleted brine from the anode compartment is dechlorinated to decrease the chlorine content to < 1 ppm prior to resaturation. 

In the second scheme, brine is fed to a rubber-lined steel tank packed with graphite along with caustic to ensure conversion of chlorine to H0C1 and/or OC1 . Alternately, aeration in a perforated plate tower may be employed to dechlorinate the depleted brine as shown by Yokota.35 36... 

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