Cathodic reaction, leaching

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 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... 

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. 

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 ... 

Cathode design considerations include the conductivity of the active material its stability relative to various forms of carbon conductors volumetric changes of the reaction products particle size influenced by electrode thicknesses and rate requirements the use of processing aids such as polymers, surfactants, and rheology aids and how the active materials will interface with a current collector. Additionally, in the case of lithium cells, it is critical to conditicm the cathode components prior to cell assembly to remove moisture and volatile process aids which can leach into the organic electrolyte and then react with lithium metal. Such reactions are almost always detrimental to cell impedance and product shelf life. 

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. 

The salt bridge or a kind of ionic membrane suited to the leaching system is located between the anodic and cathodic compartments. The electrode reactions are as follows. 

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