Diaphragm cells anode efficiency

Diaphragm or membrane cells are used in special cases where impurities are easily codeposited on the cathode, anode slimes are a problem, or the anolyte and catholyte have a different composition. A diaphragm cell can be made by enclosing either the anodes or the cathodes in a rigid bag. Enclosing anodes in a bag is an efficient method to collect anode slimes, but the anode compartments must be cleaned quite often. Enclosing cathodes in a bag requires purification of the anolyte in a... 

The saturated (about 300 g/L NaCl) pretreated brine is preheated to about 90°C and fed to the anode compartment as a broken stream, broken in order to avoid electrical losses. Preheating the brine both increases cell conductivity by increasing ion mobility with the lower solution viscosity, and decreases chlorine solubility in the brine. In this way less dissolved chlorine is carried through the diaphragm in the percolating brine, which contributes to higher cell current efficiencies and lower concentrations of impurities in the... 

Current density refers to the total current flow in kiloamperes divided by the anode electrode area in square meters, expressed as kA/m. High-current densities are desirable, particularly for electrochemical j rocesses, which yield unstable products. With current densities of 2-3 kA/m", electrolytic products of the diaphragm cell are rapidly moved from the sites of formation, which decreases side reactions and maximizes current efficiencies [14]. High-current densities, however, increase heat generation, anode wear, and the operating voltage so that lower current densities (and more cells) are better if the cells can be made cheaply. 

NaOH Concentration. The anodic cell efficiency decreases with increasing NaOH strength in diaphragm cells and with increased back-migration of OH in membrane cells. 

It is also possible to electrolyse a solution containing iron in the ferrous state in a compartmented-diaphragm cell. Ferrous iron will not interfere with the cathode reactions but will be oxidised to ferric iron in the anode compartment. To achieve this, feed solution enters the cathode compartment and electrolyte passes through the diaphragm, constructed of an inert fabric, to the anode compartment and exits the cell. The anolyte can then be reused for leaching. Any escape or return of ferric iron to the cathode compartment will result in its reduction to ferrous iron in preference to lead deposition, thus reducing process current efficiency for lead recovery. 

The diaphragm cell (Figure 3.2) is so called because the anode and cathode compartments are separated by an asbestos diaphragm. This is designed to prevent contamination of the anode compartment by the products formed at the cathode. In addition to the fundamental reactions occurring at the anode and cathode the following may take place, reducing the current efficiency of the process. 

Metal Anode Diaphragm Cathode Cell feed g/L Electrolyte, g/L Temperat ure, °C CeU voltage, V Cathode current density, A/m Energy requirement, kWh/kg Current efficiency, %... 

An account of cell features should make a reference to the diaphragm. The diaphragm used in some electrolytic processes is essentially constituted of a separator wall, though this allows the free passage of the electric current. It performs the important function of preventing the products of electrolysis formed at the anode from coming into contact with those formed at the cathode so as to avoid, as far as feasible, either secondary reactions which would lower the current efficiency, or contamination of the products which would diminish their value. 

Sodium orthoarsenate is also obtained electrolytically by the method described under calcium arsenate (p. 198). Yields up to 100 per cent, may be obtained 9 by employing a cell with a diaphragm between iron electrodes. The anolyte should contain sodium arsenite, or sodium hydroxide and arsenious oxide (equivalent to 150 g. As2Os per litre), and the catholyte sodium hydroxide (150 g. per litre). With a current density of 3 amps, per sq. dm. the current efficiency is 100 per cent. A solid crust of sodium arsenate forms around the anode. The process may be rendered continuous by circulating the anolyte and removing the precipitated arsenate. Iron or nickel electrodes are... 

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