Brine dechlorination

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. 

The depleted brine from the membrane and mercury cell processes carries dissolved chlorine. This brine is acidified to reduce the chlorine solubility, and then dechlorinated in a vacuum brine dechlorinator. The dechlorinated brine is returned to the brine wells for solution mining, or to the salt dissolver. If the membrane and diaphragm processes coexist at a given location, the dechlorinated brine is sent for re-saturation before being fed to the diaphragm cells. 

Mannig and Scherer [225] have discussed the role of hydrogen peroxide in the chlor-alkali industry, including its use in brine dechlorination. They cover handling of the peroxide, certain other applications such as the decolorizing of caustic soda, and the requirements of a dechlorination system. In particular, they comment on process control this is the subject of the following subsection. 

G.M. Forster, Brine Dechlorination by Activated Carbon, 29th Chlorine Institute Plant Operations Seminar, Tampa, FL (1986). 

There are three destinations for the depleted brine the brine feed header, the chlorate destruction reactor, and the brine dechlorinator. The last of these is the main process flow. 

Brine Dechlorination. Operation of the primary dechlorinator should be established next. It is not easily possible to simulate actual dechlorination. However, where vacuum dechlorination is used, it is important to establish that the vacuum and condenser systems operate properly. In the case of a conversion from mercury-cell technology, this may already be established. 

Brine Dechlorination. In the mercury and membrane processes, the depleted brine leaving the cells must be dechlorinated before resaturation. Further acidification with hydrochloric acid to pH 2-2.5 reduces the solubility of chlorine by shifting the equilibrium point of hydrolysis and inhibits the formation of hypochlorite and chlorate. Chlorine discharged in the anolyte tank prior to dechlorination may be fed into the chlorine system. The dissolved chlorine of the brine then is still 400 -1000 mg/L, depending on pH and temperature. The brine is passed down a packed column or sprayed into a vacuum of 50 - 60 kPa, which reduces the chlorine concentration in the brine to 10-30 mg/L. The vacuum is produced by steam jet or liquid-ring vacuum pump. The pure chlorine gas obtained is fed into the chlorine stream. 

Mercury cells are operated to maintain a 21—22 wt % NaCl concentration in the depleted brine and thus preserve good electrical conductivity. The depleted brine is dechlorinated and then resaturated with soHd salt prior to recycling back to the electroly2er. 

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. 

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... 

The new Kvaerner Chemetics process is low in both capital and operating costs and the effluent purge is significantly reduced. The process is based on a technique known as nanofiltration and the equipment is normally installed in a dechlorinated brine-side stream (see Fig. 11.1). 

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. 

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. 

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. 

Although the BDS resin is very stable, as with all ion-exchange resins, it is susceptible to oxidation by chlorine. For this reason, any residual chlorine in the brine after vacuum dechlorination should be reduced prior to BDS treatment. This is typically done with sodium bisulphite. The small additional sulphate load that results can easily be handled by the BDS system. 

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... 

Figure 5 Technological flowsheet for producing bromine fiom seawater brines by the air-stripping method 1. dechlorination column 2. bromine absorber 3. neutralizer 4. tank for dechlorinating solution 5. reducer 6. settling tank.

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. 

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). 

---