Brine resaturation

An ideal membrane should transport as much water as possible without incurring losses in current efficiency. This would result in (1) increased NaCl concentration in the anolyte (hence, reduced brine resaturation requirements of the anolyte stream), and (2) decreased process soft water requirements on the catholyte side (for a desired caustic strength). Different water transport characteristics of individual membranes can offer the above process advantages. 

The subsections that follow give the outline of an approach to the commissioning of a typical membrane-cell plant with brine resaturation, primary brine treatment, secondary brine treatment, chlorine treatment, and caustic evaporation. 

C. Effects of Iodine. Iodide ftxtm the salt oxidizes within the membrane and anolyte compartment to iodate or periodate. In a brine resaturation circuit, there is thus a slow buildup of iodate, which can diffuse into the membrane and precipitate as crystalline paraperiodate salts. Once again, the consequence is an increase in voltage and a loss of current efficiency. 

Additionally, chlorine is physically dissolved in the anolyte. This amount and the chlorine, which is reversibly bonded in hypochlorite (reaction 5 and 7), can be removed after acidification by vacuum dechlorination. This is a precondition for brine resaturation in the anolyte circuit of amalgam and membrane process. 

Brine Resaturation. In older plants, the open vessels or pits used for storing the salt are also used as resaturators. The depleted brine from the cells is sprayed onto the salt and is saturated, the NaCl concentration reaching 310 - 315 g/L. Modern resaturators are closed vessels, to reduce environmental pollution [49], which could otherwise occur by the emission of a salt spray or mist. The weak brine is fed in at the base of the resaturator, and the saturated brine is drawn off at the top. If the flow rates of the brine and the continously added salt are chosen carefully, the differing dissolution rates of NaCl and CaS04 result in little calcium sulfate dissolving within the saturator [50]. Organic additives also reduce the dissolution rate of calcium sulfate [51]. The solubility (g per 100 g of H2O) of NaCl in water does not increase much with temperature (t, °C), whereas the solubility of KCl does ... 

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. 

The chlorine-free brine, still as a weak solution, can then be recirculated to the resaturator. Care should be taken in partial or staged conversions not to feed any diaphragm cell evaporated salt to membrane electrolysers as it may contain chromium and nickel from the evaporators, which are harmful to the membrane. 

A wide range of operating conditions and design philosophies affect mercury cell efficiency. For example, the fundamental distinction between a resaturation and a waste brine process influences the temperature and brine strength profile along the length of the cell and hence the overall efficiency. Another important factor is the quality of the brine. Impurities in the brine can cause base-plate deposits, which tend to reduce the anode/cathode gap. This gradual reduction in gap requires either manual or automatic adjustment and, eventually, the cell must be taken off-line and the thick mercury removed. 

Table 20.1 summarises the model errors from the validation trials and shows that the model is successful in predicting the steady-state condition of the plant. Errors in waste brine strength and temperature must be compared with the total change across the cell which is about 13% for brine strength and 40°C for temperature. This is because the plant is a waste brine process changes in brine temperature and strength are much smaller for a resaturation process. 

The deposition of hydrogen is also prevented by maintaining a high concentration of salt in the electrolyte this facilitates the discharge of alkali metal ions at a lower, i. e. less negative potential. For this reason saturated brine is fed into the electrolyzer and its rate of flow is so adjusted that on leaving the electrolyzer it still has a sufficiently high concentration the depleted brine is then purified, resaturated with solid salt and returned to the process. 

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. 

In both cases, the denuded mercury is pumped into the electrolysis cell again, as well as the brine, undergoing resaturation before. 

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. 

A more permanent and less troublesome alternative was simply to separate the diaphragm and mercury cell resaturation systems by installing an additional dissolver or by drilling additional brine wells. 

Sludges result from the pretreatment of resaturated brine for removal of impurities, and from brine to be discharged, which was occasionally necessary because of water buildup in the brine circuit. These sludges contain 8-15 mg/g (dry basis) mercury as a complex mixture of compounds. To recover the mercury, most of the water is removed, and then the sludge is resuspended in aqueous sodium hypochlorite. The hypochlorite oxidizes the sulfide and any elemental mercury present (Eqs. 8.46 and 8.51) in order to produce a concentrated aqueous stream of dissolved mercury salts. Insoluble components are then removed by filtration, and the solution is then returned to the brine circuit. When this reaches the electrolyzer, electrolytic reduction recovers the dissolved mercury present (Eq. 8.52). 

Saturated brine is fed to the anode chamber, chlorine gas produced by oxidation of the chloride ion, (Cl ) at the anode leaves the anode chamber as CI2 gas. The weak brine leaves the anode chamber for resaturation. Sodium ions, (Na ), and... 

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. 

The anolyte leaves the cell as depleted brine. This must be recycled to the cells to prevent a massive loss of salt, and it must be resaturated with salt to keep the current efficiency of the cells high. Technically, it is possible to operate with once-through brine. While losing salt, this option eliminates handling and treatment of the recycle stream. This practice was fairly common in the past but is almost unknown in today s mercurycell plants. The overriding issue is not the loss of salt but rather the escape of mercury from the system. 

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