Electrodialysis salt from seawater

Extraction of salt from seawater occurs almost exclusively by solar evaporation in salt meadows, except in Japan where it is not possible for climatic reasons. Here electrodialysis is used to concentrate the seawater. 

Electrodialysis is used widely to desalinate brackish water, but this is by no means its only significant application. In Japan, which has no readily available natural salt brines, electrodialysis is used to concentrate salt from seawater. Tbe process is also used in the food industry to deionize cheese whey, and in a number of pollution-control applications. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Leading Examples Electrodialysis has its greatest use in removing salts from brackish water, where feed salinity is around 0.05-0.5 percent. For producing high-purity water, ED can economically reduce solute levels to extremely low levels as a hybrid process in combination with an ion-exchange bed. ED is not economical for the produc tion of potable water from seawater. Paradoxically, it is also used for the concentration of seawater from 3.5 to 20 percent salt. The concentration of monovalent ions and selective removal of divalent ions from seawater uses special membranes. This process is unique to Japan, where by law it is used to produce essentially all of its domestic table salt. ED is very widely used for deashing whey, where the desalted product is a useful food additive, especially for baby food. 

Food industry and medical supplies Edible salt production from seawater,23 demineralization of whey,24 recovery of amino acids from fermentation liquor,25 separation of amino acids,26 preparation of lactic acid27, gluconic acid28 amino acids,29 etc. from their salts, stabilization of grape juice30 and pre-treatment of wine,31 deacidification of sour orange juice,32 desalination of soups, desalination of soybean sauce,33 continuous fermentation in the presence of electrodialysis,34 de-ionization of sugar solution.35... 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Electrodialysis has the ability to concentrate salts to high levels with much less energy consumption than evaporation would require. This capability has been utilized in Japan to make edible salt by recovering NaCl from seawater and concentrating it to 20% before evaporation. The plants there are huge some have greater than 100 000 square meters of membrane. Salt recovered by electrodialysis in Kuwait is the raw material for a chlor-alkali plant there. Electrodialysis has also been used to concentrate salts in reverse osmosis brines [32]. 

The concentrates of salt solutions made by electrodialysis of seawater are suited as feed to the evaporators of salt manufacturing plants with considerable savings in overall energy requirements. Other applications also are based on the concentrating effects of electrodialysis, for instance, tenfold increases of concentrations of depleted streams from nickel and copper plating plants are made routinely. 

A flow scheme of one such seawater salt production plant is shown in Figure 10.15. A cogeneration unit produces the power required for the electrodialysis operation, which concentrates the salt in sea water to about 18-20 wt%. The waste stream from the power plant is then used to further concentrate the salt by evaporation. 

In electrodialysis, an applied electric field rather than a concentration gradient is used to draw ions across the membrane. Because it is faster than ordinary dialysis, electrodialysis is often used in biochemical analyses for purposes of fractionation, concentration, and desalting. Reverse osmosis (RO) is a process that uses semipermeable membranes to allow water permeation however, the membranes act as a barrier to the passage of dissolved and suspended particles. Typically, RO membranes are used to extract pure water from aqueous solutions of dissolved salts, such as seawater. The particle size cutoff is typically 0.0001 yum with driving pressures of 200 to 800 psi (1.4 to 5.5 MPa).61... 

About 1.4 million tons of sodium chloride (edible salt)/year has been produced in Japan since 1972 by the electrodialytic concentration of seawater. This technology has been exported to Korea, Taiwan and Kuwait. After seawater is concentrated up to about seven times or more its initial concentration by electrodialysis, an optimal concentration from an economic viewpoint, sodium chloride is produced by evaporating the concentrated solution. Table 6.4 shows a typical composition of the concentrated solution.51 In this process, monovalent cation and anion permselective ion exchange membranes have been used. This technology was developed in Japan to produce sodium chloride by the electro-... 

Seawater can also be concentrated by electrodialysis. An expenditure of about 150kWhrton of NaCl can bring the NaCl concentration up to 200gpl [20], This process has not found commercial application as an intermediate step in the production of large quantities of industrial salt. Where there is more value added on a smaller scale and not much competition from large producers, as in the production of table salt in Japan, it can operate successfully. 

Electrophoretic processes can remove waste waters from colloidal particles of loam, asbestos and even bacteria. Another method, electrodialysis can be used not only for desalination of seawater but in general for removal of different salts of a polluted water stream. For more information, see[l, 46-50]. 

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