Anode/anolyte

Typical areas where titanium has found widespread industrial use in membrane technology are cells, anodes, anolyte headers, anolyte containers, filters, heat exchangers, chlorate removal systems and various parts of the brine system. 

The buffer system in cIEF is heterogeneous the buffer in the reservoir surrounding the cathode (cathoiyte) is of a high pH, frequently 20 mAf sodium hydroxide, whereas the buffer in the reservoir surrounding the anode (anolyte) is of a low pH, commonly 10-20 mM phosphoric acid. For narrow gradients (e.g., pH 6-8), weaker acids and bases such as glutamic acid and arginine, respectively, are used.42... 

As mentioned in Section 17.1, the anodic and cathodic compartments of an electrochemical cell can be separated by an ion-exchange membrane or a porous diaphragm. The division of a cell is often practiced in industrial processes, despite the additional costs, the need for additional seals and possible maintenance problems. A separator may indeed allow a more independent choice of anode/anolyte or cathode/catholyte, enable current eftkiency to be maintained due to the exclusion of redox shuttles and help to isolate electrode products or prevent the formation of explosive or toxic mixtures, for example H2-O2. However, if possible, undivided cells are preferred, as they lead to lower ohmic drops and to much simpler technologies. 

To achieve this pH, a diaphragm is needed to separate the electrolyte near the cathode (catholyte) and the electrolyte near the anode (anolyte). The anolyte is acidified during operation as water decomposes to oxygen and protons as the anodic reaction [2]. The need for a diaphragm adds to the operational complexity and difficulty of conventional electro winning cells. 

Small amounts of propionitrile and bis(cyanoethyl) ether are formed as by-products. The hydrogen ions are formed from water at the anode and pass to the cathode through a membrane. The catholyte that is continuously recirculated in the cell consists of a mixture of acrylonitrile, water, and a tetraalkylammonium salt the anolyte is recirculated aqueous sulfuric acid. A quantity of catholyte is continuously removed for recovery of adiponitrile and unreacted acrylonitrile the latter is fed back to the catholyte with fresh acrylonitrile. Oxygen that is produced at the anodes is vented and water is added to the circulating anolyte to replace the water that is lost through electrolysis. The operating temperature of the cell is ca 50—60°C. Current densities are 0.25-1.5 A/cm (see Electrochemical processing). 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

Fig. 25. OxyTech MGC electroly2er a, membrane b, anode assembly c, manifold spacer d, anolyte outlet e, catholyte outlet f, bulkhead g, brine inlet h, NaOH inlet i, insulating channel j, bulkhead insulator k, interface material 1, cathode assembly m, interceU bus n, tie rod o, current distributor p,...

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). 

According to U.S. Patent 2,966,493, the 2,3-bis-(3-pyridyl)-2,3-butanedlol used as the starting material may be prepared as follows. A solution of 1,430 g of 3-acetyl-pyridine in 7,042 ml of a 1 N aqueous solution of potassium hydroxide is placed into a cathode chamber containing a mercury cathode with a surface of 353 cm and is separated from an anode chamber by an Alundum membrane. As anode a platinum wire is used and the anolyte consists of a 1 N solution of aqueous potassium hydroxide which Is replenished from time to time. 

A similar situation arises when a vertical metal plate is partly immersed in an electrolyte solution (Fig. 1.48c), and owing to differential aeration the upper area of the plate will become cathodic and the lower area anodic. With time the anodic area extends upwards owing to the mixing of the anolyte and catholyte by convection and by the neutralisation of the alkali by absorption of atmospheric carbon dioxide. 

Thus if the cathodic and anodic sites are separated from one another by the geometry of the system and if the solution is relatively stagnant the pH of the anolyte will decrease whereas that of the catholyte will increase. 

Suzuki, T., Yambe, M. and Kitomura, Y., Composition of an Anolyte Within Pit Anode of Austenitic Stainless Steel in Chloride Solution , Corrosion, 29, 18 (1973)... 

Half-cell one half of an electrochemical cell, comprising one electrode (anode or cathode) and its immediate electrolyte (anolyte or catholyte). 

Occluded Cell a corrosion cell of a geometry that prevents intermingling of the anodic reaction products (anolyte) with the bulk solution, resulting in a decrease in pH of the anolyte shielded areas or pits, crevices or cracks in the surface of the metal are examples. 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

In order to provide for purification of the electrolyte, diaphragm cells are used to form separate anode and cathode compartments, and the anodes are encased in loose-fitting, open-weave bags to facilitate the removal of slime with the anodes. The anolyte is continuously taken out, purified and fed into the cathode compartments where nickel electrodeposits on the cathodes. A small hydrostatic head of purified electrolyte in the cathode compartment is maintained in order to prevent the diffusion of anolyte with its impurities into the cathode compartments. 

Mixing of the electrode products causes hydrolytic precipitation of the nickel and, after separation of the nickel hydroxide, the filtrate was returned to the cells. The sequence of the electrolytic purification steps is outlined in Figure 6.28. Nickel hydroxide slurry is first added to the anolyte for the purpose of raising the pH to 3.7 (2 H+ + Ni(OH) = Ni2+ + 2 H20), and iron(II) is oxidized by introducing chlorine. This causes hydrolytic precipitation of the iron(III) and corrects the nickel ion deficiency by the low anodic current efficiency. The iron(III) hydroxide is removed by filteration. The clarified solution is then treated with nickel carbonate and further chlorine to oxidize the cobalt(II) and allow its separation as cobalt(I II) hydroxide. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

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