Rotating-cylinder anode

Fig. 11. Schematic of an electrochemical cell and gold rotating-cylinder anode (RCA) for the indirect destruction of chlorinated organics using Co(III).

Several of these studies were conducted in an electrochemical batch reactor that had a rotating-cylinder anode (RCA). Since the anode was operated below the limiting current for mediator generation, relatively high coulombic efficiencies were achieved. Ion-exchange membranes were used to separate electrodes in Ag(II)-based processes, but were eliminated in processes based upon Co(III) and H2SO4. Rates of CO2 generation were measured and used to... 

Numerous treatability studies have been performed in small electrochemical batch reactors [13-15]. A typical batch reactor has a rotating cylinder anode (RCA) that is operated well below the limiting current for Ag(II) generation (Figure 4). The RCA enables the scientist or engineer to use a small apparatus to mimic mass-transport conditions in a pilot plant, without using massive flow-through electrochemical cells and pumps. 

Mediators are produced at the surface of the rotating cylinder anode (RCA) shown in Figure 4. The rate of generation, R, is proportional to the current density, i, and the surface area of the RCA. 

ECO cell — Electrochemical cell which is applied in wastewater treatment. A rotating cylinder employed as cathode and fixed anodes at the periphery are separated by a -> diaphragm. Small disks reaching from the separator up to close to the rotating cathode split the... 

Azobenzenes may be prepared by oxidation of hydrazobenzenes this may be made in an undivided cell by reduction of the nitro compound with constant current [178]. Additionally, azobenzenes may be obtained in low to fair yield by anodic oxidation of aromatic amines at a rotating platinum screen cylinder anode in aqueous DMF [179]. 

High mass-transport coefficients are obtained in cells with a rotating cylinder electrode (RCE) and a small gap between the anode and the cathode, Fig. 4(a). High rates of mass transport are experienced in the turbulent flow regime, so that RCE reactors allow metal deposition at high speed, even from dilute solutions. RCE reactors have been operated at a scale involving diameters from 5 to 100 cm, with rotation speeds from 100 to 1500 rpm and currents from 1 A to 10 kA [79], It... 

Figure 11. Illustration of two alternative designs for the rotating cylinder Hull (RCH) cell, which allows the study of non-uniform current distribution on the cathode, under controlled mass-transport conditions. A anode, C cathode, IC insulating cylinder. Reproduced from Ref. 150 with kind permission of Springer Science and Business Media, and with permission from Ref. 95, Copyright (1996) The Electrochemical Society.

A rotating cylinder electrode (RCE) with a cylindrical or pseudo-cylindrical counterelectrode around it essentially has the geometry of a parallel plate cell. The RCE was developed as a specialty tool for uniform fast electrodeposition in turbulent flow, and for the removal of metal ions from effluents with recovery of the metal in the form of a foil, flake, or powder [61-63]. In the first application, RCE cathodes became the major tool for silver removal from photographic fixer solutions in compact, high-rate units [64], and enabled the recycling of fixer and resale of more than 98 wt% of the silver. Typically, such cells had a stainless steel RCE cathodes of 10-20 cm diameter rotating at speeds up to 1400 rpm, stationary graphite anodes, and were operated at 50 A. 

Corrosion test methods can be divided into electrochemical and non-electrochemical methods. Among the electrochemical techniques that have been used successfully for corrosion prediction are potentiodynamic polarization scans, electrochemical impedance, corrosion current monitoring, controlled potential tests for cathodic and anodic protection, and the rotating cylinder electrode for studies of velocity effects [3i,32]. Though not literally a test, potential-pH (Pourbaix) diagrams have been used as road maps to help understand the results of other tests. 

In the majority of cases, metal is removed from the cathode at intervals, usually by manual peeling or scrapping the deposit. A rotating-cylinder cell for photographic silver recovery is shown in Fig. 7.6. Such cells are available with rotating cathodes (small-scale) or rotating anodes (on a larger scale). 

Industrially, the silver is recovered from either the wash water, or the bleach fix separately or from a mixture of the two using electrolysis employing a stainless steel cathode cylinder and an anode of stainless steel mesh. A typical wash solution composition contains silver (4 g L ), sodium thiosulphate (220 g L ), sodium bisulphite (22 g L ) and sodium ferric EDTA (4 g L ). At Coventry we have used a scaled down version of the industrial process employing 250 mL samples [46]. Electrolysis experiments were performed at ambient temperature with both wash and bleach fix solutions and in which the potential applied to the cathode and the speed of rotation of the cathode were varied. The sonic energy (30 W) was supplied by a 38 kHz bath. The results are given in Tab. 6.9. The table shows that the recovery of silver on sonication of the wash or bleach fix solutions is much improved especially if the electrode is rotated while ultrasound is applied. Yields with bleach fix (which contains ferric ions) are less since Fe and Ag compete for discharge (Eqs. 6.13 and 6.14). 

Abramson and King (22) found that etched Fe cylinders dissolved somewhat faster than cylinders which were polished with fine abrasive paper between runs. The following experiment with a Ni cylinder illustrates the effect of progressive roughening (23). A cylinder 1.9x2.52 cm, cut from a rolled anode bar, was rotated at U = 18, 000 cm /min in 400 ml of deaerated solution containing 1M HC1 and 0.05M FeClg. Weight losses in consecutive three-minute periods were ... 

The design of a SEM is shown in Fig. 2. It consists of the electronic gun (1) the Wehnelt cylinder (2) the anode (3) and beam alignment coils (4) on the top of the instrument. The condenser lenses (5) the aperture, and the objective lens (6) focus the beam onto the specimen that is mounted on the specimen holder (7). The latter one could be moved in X-, Y-, and Z-direc-tion within the specimen chamber. In addition, the sample could be moved by rotation. The arrangement to create the electronic beam is shown in Fig. 3. 

The anode is a hollow cylinder, typically about 10 cm in diameter, rotating at several thousand revolutions per minute in a vacuum of roughly lO Torr with a steady circulation of water at a typical flow rate of 20 liters per minute. Usually, a tuibomolecular pump is used for maintaining the very low pressure inside the tube. 

As the diagram in Figure 2.7 shows, the election beam strikes the generatrixes of the cylinder that constitutes the anode. Theiefoie, the position of these generatrixes must not change so as not to cause the X-ray beam to move. This condition is particularly severe, since it means that, despite the high flow of water, the anode s rotation must remain stable. 

The simplest electroforming process is that for manufacturing metal foil and the system is sketched in Fig. 8.2. The mandrel is a slowly rotating metal cylinder and the cell has a conforming anode so that the inter-electrode gap is constant. The anode may be dissolving pellets or rods or may be an inert electrode. The foil is separated from the mandrel by a knife system and is then passed through a roller system which keeps it taut. 

---