Electrolysis diaphragms

This plant was the beginning of what is now the Chemical Park Delfzijl. The plants on the Chemical Park now employ 1400 people, and are owned by several chemical companies. One of the plants is the Akzo Nobel Base Chemicals Diaphragm Electrolysis Plant. 

The Diaphragm Electrolysis Plant (Fig. 14.1) is well integrated in the Chemical Park Delfzijl. The suppliers and customers product-stream amounts of the Diaphragm Electrolysis Plant are optimised on the basis of the production capacity of each plant. 

Fig. 14.1 Suppliers and cell-liquor clients of the Diaphragm Electrolysis Plant at Delfzijl, The Netherlands.

Solution-mined brine is used for salt production, and is also the feed for the diaphragm electrolysis and soda-ash production. The cell-liquor derived from the electrolysis is also used for soda-ash production after pre-carbonation. 

The Diaphragm Electrolysis Plant Delfzijl has a cellroom containing 184 OxyTech MDC-29 cells and an annual liquefaction capacity of 130 000 tons of chlorine. 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

Recycling the hypochlorite to the feed brine has provided an excellent possibility of eliminating completely the chlorate and bromate emissions of the chlorine destruction unit of a diaphragm electrolysis plant. The main advantage of the hypochlorite recycling and cathodic reduction procedure is the reduction of bromate to bromide. 

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

On a large scale, hydrogen peroxide is produced by the electrolysis of ammonium hydrogensulphate, using a platinum anode and a lead cathode separated by a diaphragm. The essential process occurring is ... 

Conversion of aqueous NaCl to Cl and NaOH is achieved in three types of electrolytic cells the diaphragm cell, the membrane cell, and the mercury cell. The distinguishing feature of these cells is the manner by which the electrolysis products are prevented from mixing with each other, thus ensuring generation of products having proper purity. 

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

Current Efficiency. Current efficiency for caustic production in diaphragm and membrane cells can be estimated from collection of a known amount of caustic over a period of time and from a knowledge of the number of coulombs of electricity passed during that time period. An alternative method involves analysis of the gases evolved during electrolysis and determining the anolyte composition. Material balance considerations (7) show the expression for the caustic efficiency for membrane cells to be... 

Caustic Soda. Diaphragm cell caustic is commercially purified by the DH process or the ammonia extraction method offered by PPG and OxyTech (see Fig. 38), essentially involving Hquid—Hquid extraction to reduce the salt and sodium chlorate content (86). Thus 50% caustic comes in contact with ammonia in a countercurrent fashion at 60°C and up to 2500 kPa (25 atm) pressure, the Hquid NH absorbing salt, chlorate, carbonate, water, and some caustic. The overflow from the reactor is stripped of NH, which is then concentrated and returned to the extraction process. The product, about 62% NaOH and devoid of impurities, is stripped free of NH, which is concentrated and recirculated. MetaUic impurities can be reduced to low concentrations by electrolysis employing porous cathodes. The caustic is then freed of Fe, Ni, Pb, and Cu ions, which are deposited on the cathode. 

Alkaline Chloride Electrolysis by the Diaphragm Process System Hooker, Uhde GmbH, Dortmund, Germany, 1985. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Nickel. Most nickel is also refined by electrolysis. Both copper and nickel dissolve at the potential required for anodic dissolution. To prevent plating of the dissolved copper at the cathode, a diaphragm cell is used, and the anolyte is circulated through a purification circuit before entering the cathodic compartment (see Nickel and nickel alloys). 

Other commercial cells designed for the electrolysis of fused sodium chloride iaclude the Danneel-Lon2a cell and the Seward cell, both used before World War I. The former had no diaphragm and the sodium was confined to the cathode 2one by salt curtains (ceramic walls) the latter utili2ed the contact-electrode principle, where the cathode was immersed only a few millimeters ia the electrolyte. The Ciba cell was used over a longer period of time. 

Manufacture. Most chlorate is manufactured by the electrolysis of sodium chloride solution in electrochemical cells without diaphragms. Potassium chloride can be electroly2ed for the direct production of potassium chlorate (35,36), but because sodium chlorate is so much more soluble (see Fig. 2), the production of the sodium salt is generally preferred. Potassium chlorate may be obtained from the sodium chlorate by a metathesis reaction with potassium chloride (37). 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

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