Periodic reverse current

This smelting plant adopts a flash smelting furnace improved by MMS and the refining plant adopts high current density electrolysis using periodical reversing current. 

The current density i t) is the periodic function of a time, which for periodic reverse currents is given by [15] ... 

Popov KI, Maksimovic MD, Simic MS (1982) The effect of periodic reverse current on the surface rougneses of metal deposits and maximal deposition rate. Surf Technol 16 209-218... 

Popov KI, Maksimovic MD, Totovski d6 (1982) Fundamental aspects of pulsating current metal electrodeposition. VI the comparison of electrode surface roughening in pulsating current and periodic reverse current electrodeposition of metals. Surf Technol 17 125-129... 

Current density can be increased without impairing the quaUty of the copper by polishing the cathode surface by brief periodic current reversals (PCR). Reversed current electrolysis, first developed for electroplating, was tested in 1952 for copper refining. Although good results were obtained, no suitable electrical equipment for current reversal was available. The thyristor-controUed siUcon rectifier, introduced in the 1960s, provided a means for... 

Modern solutions fall mainly into three types (a) the plain cyanide bath which contains typically 20-25 g/1 of copper cyanide, 25-30 g/1 total sodium cyanide (6.2 g/1 free sodium cyanide), and is operated at 21-38 C and 110-160 A/m (b) the Rochelle copper bath to which is added 35-50g/1 of Rochelle salt and which is used at 66 C at up to 645 A/m and (c) the high-efficiency cyanide baths which may contain up to 125 g/1 of copper cyanide, 6-11 g/1 of free sodium or potassium cyanide, 15-30 g/1 of sodium or potassium hydroxide, and are operated at up to 6-9A/dm and 65-90 C. Most bright cyanide copper baths are of the high-efficiency type and, in addition, contain one or more of the many patented brightening and levelling agents available. Periodic reverse (p.r.) current is also sometimes used to produce smoother deposits. 

Typically, the electrodes are of lead dioxide on a titanium substrate in the form of horizontal perforated plates, usually 5-40 mm apart, depending on the conductivity of the liquid. A potential difference of 5-10 V may be applied to give current densities of the order of 100 A/m2. Frequently, the conductivity of the suspension itself is adequate, though it may be necessary to add ionic materials, such as sodium chloride or sulphuric acid. Electrode fouling can usually be prevented by periodically reversing the polarity of the electrodes. Occasionally, consumable iron or aluminium anodes may be used because the ions released into the suspension may then assist flocculation of the suspended solids. 

EDR is typically used for demineralization of brackish water, which often contains poorly soluble minerals such as calcium bicarbonate and calcium sulfate, as well as colloids such as humic and fulvic acids and iron hydroxides The periodic reversal of the direction of the electric current avoids scaling and fouling of the membranes by such substances. 

Raghava Rao et al. [89] selectively removed neutral salts contained in spent chromium tanning solutions to achieve a more efficient technique for recycling the unused chromium and process water. The electrodialysis unit contained Neosepta CL-25T and ACH-45T membranes. An application of 13-30 V to a 5 dm solution over a period of 5 - 6 h produced currents between 2 and 4 A 90% of the sodium chloride and 50% of the sodium sulfate were selectively removed with minimal transport of Cr(III) species across the membranes. Addition of EDTA to the spent liquor as well as periodic reversal of electrode polarities eliminated membrane fouling. 

In PCR, the forward to reverse period ratio is typically between 20/1 and 30/1. The PCR has two main effects. The current density may be increased without anode passivation. The reversal current depletes built up metal concentration within the anodic boundary layer. This helps in avoiding the precipitation of metal salts, which is one of the causes of anode passivation. On the cathode, thinning of the diffusion layer and selective removal of nodules during the reverse current phase result in smoother deposits. The major disadvantage of PCR is higher energy costs. 

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