Diaphragm cells/process cell efficiency

In a balanced plant, all the chemically treated brine flows through ion exchange, not just that equivalent to the membrane-cell production. This adds a cost to the process, because in a segregated operation only that brine corresponding to the membrane-cell production would be so treated. At the same time, it improves the quality of the brine fed to the diaphragm cells. This in turn improves the current efficiency of those cells, but little information is available in the literature, and it is not possible to quantify the phenomenon here. 

In any electrolytic process, where it is necessary to control the diffusion of the products of decomposition, the most vital point to life and efficiency of the cell is the diaphragm or more recently the membrane. This tough reality, which was met with crude short lived diaphragms at the turn of the century, has forced inventors to eventually produce more stabilized materia and ion specific membranes with parallel progress in cell efficiency and quality of the electrolysis products. This paper will review the various electrochemical processes using separators in 1900 and attempt to describe the improvements achieved during the following decades. 

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. 

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... 

The same process sometimes can be performed efficiently in cells either with or without diaphragms. Figures 19.16(e) and (f) are for making adiponitrile by reduction of acetonitrile. In the newer design, Figure 19.16(f), the flow rate of the electrolyte is high... 

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