Anodic reaction, leaching

Continuous and semicontinuous electrochemical reactors are normally employed for effluent metal ion remediation, where the anode reaction is usually oxygen evolution from water [compare with Equation (26.4)]. After the metal contaminant is captured on the cathode, the cathode can be discarded, the collected metal can be resold, or the deposited metal can be chemically or elecfro-chemically etched into a small volume of a suitable leaching liquor (e.g., water) so as to increase its concentration substantially. 

Zinc is prepared electroanalytically from an acid solution of purified zinc sulphate, using aluminium sheets as cathodes and anodes of pure lead. The anodic reaction is the liberation of oxygen, and so the concentration of free sulphuric acid tends to increase continuously to counter this the electrolyte is circulated, and the more acid solutions are returned to the leaching process. 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

Copper may also be recovered from leach solutions electrolytically. Electrowinning requires the use of an insoluble anode such as hard lead, comparable to the liberator cell used for liquor purification in copper electrorefining. Consequently, there are net electrochemical reactions involved in electrowinning (Eqs. 13.20 and 13.22), as opposed to the situation with electrorefining, so that about 1.7 V are required for this step. This results in a much higher electrical power consumption of about 2.8 kWh/kg copper for electrowinning, compared to about 0.2 kWh/kg for electrorefining. 

The electrochemical conversions of solid compounds and materials that are in direct contact with electrolyte solutions or liquid electrolytes (ionic liquids), belong to the most widespread reactions in electrochemistry. Such conversions take place in a wide variety of circumstances, including the majority of primary and secondary batteries, in corrosion, in electrochemical machining, in electrochemical mineral leaching, in electrochemical refining (e.g., copper refining), and in electrochemical surface treatments (e.g., the anodization of aluminum). 

The purpose of carrying out the electrolysis in the FLUBOR divided diaphragm cell is to produce Pb cathodes in the cathodic compartment and to oxidise the ferrous ion to ferric ion in the anodic compartment, thereby regenerating the ferric fluoborate leaching solution. The electrolysis reactions are the following ... 

It should be emphasized that thermodynamically [73,74], Ru02 can oxidize to RuOJ or Ru04 (in alkaline solutions). Ru02 can be leached from the anode during the course of the chlorine evolution reaction via the loss of the unstable adsorbed intermediates or the oxidized surface oxides formed during the discharge of the chloride ions as described by the reaction schemes (13)-(15). 

Where ferric chloride is used for leaching and converted to ferrous iron, it can be regenerated by reaction of the solution (after lead chloride removal) with chlorine from the electrolytic cell. The can be done using a packed absorption tower irrigated with depleted solution in exchange with an npflow of gas from the anode compartment of the 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 leach solution can be purified by cementation with lead powder and can then be electrolysed to produce high purity lead at the cathode. Ferrous iron is oxidised at the anode of a compartmented cell in the same way as for the chloride system above. The advantage of this reaction is that electrode potentials are sufficiently low to avoid competing reactions, which either form oxygen or Pb02. This will mean that ordinary graphite can be used with long Ufe. 

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