Electrolysis of sea water

The electrolysis of saturated sodium chloride solution (brine) is the basis of a major industry. In countries where rock salt (sodium chloride) is found underground it is mined. In other countries it can be obtained by evaporation of sea water in large shallow lakes. Three very important substances are produced... 

Although a description of the technology of sea water electrolysis is beyond our scope, two major aspects of it may be briefly mentioned. 

The hydrogen production step of methane reforming could also be substituted by high-temperature electrolysis. The calculated balance here is 48 t/h of sea water plus 27 t/h of coal plus 15 t/h of air as input to yield 42 t/h of methanol. Excess hydrogen could be stored in tanks and retrieved for methanol synthesis, supposed the nuclear power needs to be used in peak periods [31]. 

The metal is now principally obtained in the U.S. by electrolysis of fused magnesium chloride derived from brines, wells, and sea water. 

Some years after Davy s death, Faraday examined the corrosion of cast iron in sea water and found that it corrodes faster near the water surface than deeper down. In 1834 he discovered the quantitative connection between corrosion weight loss and electric current. With this discovery he laid the scientific foundation of electrolysis and the principles of cathodic protection. 

As the corrosion rate, inclusive of local-cell corrosion, of a metal is related to electrode potential, usually by means of the Tafel equation and, of course, Faraday s second law of electrolysis, a necessary precursor to corrosion rate calculation is the assessment of electrode potential distribution on each metal in a system. In the absence of significant concentration variations in the electrolyte, a condition certainly satisfied in most practical sea-water systems, the exact prediction of electrode potential distribution at a given time involves the solution of the Laplace equation for the electrostatic potential (P) in the electrolyte at the position given by the three spatial coordinates (x, y, z). 

Magnesium is an important structural metal. It can be prepared through the sequence of steps precipitation of Mg42 from sea water by OH to form Mg(OH)2 conversion of Mg(OH)2 to MgCl2 electrolysis of molten MgCl2. 

Most Mg metal is manufactured by electrolysis of either anhydrous or partially hydrated MgCl2 from sea water. Addition of CaO to sea water precipitates Mg(OH)2,... 

Sodium chloride is found in salt beds, salt brines, and sea water throughout the world, and it is also mined is some locations. Consequently, sodium chloride is the source of numerous other sodium compounds. A large portion of the sodium chloride utilized is consumed in the production of sodium hydroxide (Eq. (11.23)). The production of sodium metal involves the electrolysis of the molten chloride, usually in the form of a eutectic mixture with calcium chloride. Sodium carbonate is an important material that is used in many ways such as making glass. It was formerly produced from NaCl by means of the Solvay process, in which the overall reaction is... 

Finally, examples of other somewhat unsuccessful alternative AF ideas based on e.g. in-situ sea water electrolysis, piezoelectric effects, ultrasounds and magnetic fields are summarised in Callow (1990), Swain (1998), Bertram (2000) and Yebra et al., (2004). 

One new way in which electrolysis could be used is by simply electrolyzing sea water to hydrogen and oxygen and then converting these to water in a fuel cell, which would supply the energy to decompose more water. Such enormous amounts of energy are used in this process that to be competitive, the over-all efficiency of the electrolysis fuel cell combination would have to be over 99.5%. It is doubtful that this could ever be attained. Other schemes of electrolysis in which sodium and chlorine are removed are subject to the same disadvantage. 

The recovery of important metals or their salts is possible by electrolysis in cells provided with ion-selective membranes, e.g. of uranium (71, 72, 73, 75), of magnesium from sea water (130), of iodine from iodide containing brines (158), of manganese (74). 

Chlorine is the most abundant of the halogens, especially in sea water, a ton of which contains in grams chlorine 15,000, bromine 97, iodine 0-17 (ratio 106 6,000 1). Its preparation depends on the discharge of its ion, either directly (i.e., electrolytically) or by oxidation. The older methods of oxidation (by manganese dioxide or by air in presence of certain catalysts) have now been replaced for technical purposes by the electrolysis of sodium chloride, which is primarily for the production of caustic soda, the chlorine being a by-product the chloroparaffins which are now so much used as solvents were developed to utilize this chlorine. 

Alternatively, the electrolysis of brine (sea water concentrated by evaporation) might be advantageous. It would be possible to control the anode potential for the production of Br2(/) (E0 = 1.08 V)to a value < 1.3 V so that the anode product is liquid bromine, not chlorine gas. The liquid Br2 for which there is no market would be rejected into the sea. Because the Br concentration is less than that of Cl-, measures to accommodate the cathode current density and avoid exceeding the limiting current for Br2 evolution would be necessary. 

Morizonoa, T., Watanabe, K., and Ohstsuka, K., Production of hydrogen by electrolysis with proton exchange membrane (PEM) using sea water and fundamental study of hybrid system with PV-ED-EC, Mem. Fac. Eng., 31, 213-218 (2002). 

Chlorine and sodium hydroxide are made by the electrolysis of brine using membrane cells. Conventional and improved membrane cell arrangements are described in U.S. 4,391,693, assigned to Dow Chemical. U.S. 4,470,889 (also to Dow) gives data on membrane materials and performance. What price of electricity is needed for it to be economical to produce chlorine from sea water (3.5 wt% NaCl) ... 

Extracting magnesium from sea water is an efficient and economical process. Sea water is mixed with lime, CaO, from oyster shells to form insoluble magnesium hydroxide, Mg(0H)2, which can be easily filtered out. Hydrochloric acid is added to the solid to form magnesium chloride. The electrolysis of molten magnesium chloride will produce pure magnesium metal. 

Preliminary studies on potential methods for the extraction of uranium from sea water took into consideration not only the extraction by solid sorbents, but also by solvent extraction, ion flotation, coprecipitation, and electrolysis. However, for a large-scale uranium recovery only the sorptive accumulation by use of a suitable solid sorbent seems to be feasible with regard to economic reasons and environmental impacts n9). 

Fluorine occurs in large quantities in the minerals fluorspar or fluorite, CaF2 cryolite, Na3AlFg, md fluorapatite, Ca5(P04)3F. It also occurs in small amounts in sea water, teeth, bones, and blood. F2 is such a strong oxidizing agent that it has not been produced by direct chemical oxidation of F ions. The pale yellow gas is prepared by electrolysis of a molten mixture of KF + HF, or KHF2, in a Monel metal cell. This must be done under anhydrous conditions because H2O is more readily oxidized than F. ... 

Chlorine occurs in Nature mainly as sodium chloride in sea water and in various inland salt lakes, and as solid deposits originating presumably from the prehistoric evaporation of salt lakes. Chlorine is prepared industrially, almost entirely by electrolysis of brine 2... 

One of the most common industrial methods for the production of sodium hydroxide depends on the electrolysis of brine in a diaphragm cell. The products of the electrolysis are chlorine, hydrogen, and cell liquor, which is a solution of sodium hydroxide and sodium chloride. A large fraction of the cost of commercial sodium hydroxide results from the concentration, separation, and purification of the alkali. The sodium hydroxide required in the sea water descaling process need... 

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