6.Explain Fused –salt processes, electro refining with example.

Electrorefining. Aluminum is the only metal that is electrorefined to any great extent in fused salts. The cell consists of a steel shell with a graphite-lined lower section and an upper section coated with alumina frozen out of the electrolyte. For this purpose a “three-layer” cell is used. The lowest layer is the anode, which consists of impure molten aluminum alloyed with copper to increase the layer’s density. The middle layer is the fused-salt electrolyte – a mixture of cryolite, BaF₂ , and alumina. The uppermost layer is pure aluminum, with a purity exceeding 99,99%. Electrical contact is made by graphite cathodes which are immersed in the floating molten aluminum cathode and by the graphite lining on the cell bottom, which is covered by the molten aluminum-copper anode.

Electrowining All primary aluminum extraction is performed by fused-salt electrolysis (see Aluminum production). In the Hall-Heroult process, alumina which has been converted from bauxite by the Bayer process is dissolved in molten cryolite (Na₃AlF₆). The electrolyte also contains CaF₂ and AlF₃. The electrolysis cell consists of a steel shell lined with refractory insulating bricks covered with graphite. The cathode is a pool of molten aluminium on the bottom of the cell. The anode is a carbon block immersed in the electrolyte from the top of the cell. Molten aluminum is produced at the cathode, the anode, which is consumed in the process, produces carbon dioxide. The overall cell reactions is: 2Al₂O₃ + 3C → 4Al + 3CO₂

At a typical cell operating temperature of 960⁰C the thermodynamic minimum decomposition voltage is 1.18V. To compensate for the resistances of the anode, cathode and electrolyte, as well as the anodic overvoltage, cells operate at the much higher applied potential of 4,0-4,5V. Aluminum is also produced industrially by the electrolysis of AlCl₃ in a solution of NaCl and LiCl. At 727⁰C the thermodynamic voltage requirement is 1,75 V, compared to the operating voltage of 3,3 V. The electrolysis cells employ bipolar carbon electrodes charged so that chlorine is evolved on the bottom surface and aluminum is deposited on the top surface of each electrode. This improves the space-time yield over that of the Hall-Heroult cell. Furthermore, the higher electrical conductance of the choloride electrolyte, the smaller anode-cathode separation, and the lower voltage drop across the electrodes make the electrolysis of AlCl₃ more energy-efficient than the Hall-Heroult proces.

7.Could you express Electrolysis cell, requirements of electrolysis cell. Basic consisting elements of the electrolytic cell are shown in the simplified scheme in Figure 1. Two electronic conductors – electrodes (anode and cathode), an ionic conductor(electrolyte) and an energy source (battery) are necessary to perform electrolysis. In the electrolysis cell electrode reactions are non-spontaneous, i.e. they are forced by an external source of energy. So, reduction of the metal ions to pure metal is enabled. For efficient electrolysis, the above elements have to satisfy some criteria. So, electrodes have to: be made by an appropriate material that promotes corresponding electrode reactions and exhibits good long-term stability and is environmental friendly\;be appropriately processed in order to remove electrode products easily. Cathodic edges should be rounded off to prevent dendritic growth;be appropriately arranged into the cell for better volume efficiency. The shorter cathode-anode distance, the lower the Ohmic drop in the cell and consequently the lower energy consumption. On the other side, grown deposit on the cathode may cause short cut if the electrodes are too close. The electrolyte should contain: – compound of the ion – participant in the electrode reaction; – compounds that provide optimal electrical conductivity, corresponding acidity/ alkality and/or complex forming ability; – corresponding additives for regulation of the surface tension, to optimize the diffusion potential, to provide better adhesion of the deposit over the cathodic substrate, to reach better optical and mechanical properties of the deposit; – in some cases – mechanical components, e.g. thin oil layer or inert plastic balls in order to reduce electrolyte evaporation; – additives for optimizing density and viscosity in the case of the molten salt electrolysis.The electrolysis cell has to be: – prepared by a material having good chemical, thermal and mechanical stability. Traditional materials are wood, glass, concrete, stainless metals/alloys, while the recent ones are various polymer materials or coatings;– equipped to shoulder electrodes (whose weight can reach even tens of tones) and to supply them with electricity; – equipped for heating/cooling, stirring/ circulation and filling/emptying the electrolyte;

8.Relate types of electrodes, which kind of criteria satisfy for electrodes and source of energy. Electrodes have to: – be made by an appropriate material that promotes corresponding electrode reactions and exhibits good long-term stability and is environmental friendly; – be appropriately processed in order to remove electrode products easily. Cathodic edges should be rounded off to prevent dendritic growth; – be appropriately arranged into the cell for better volume efficiency. The shorter cathode-anode distance, the lower the Ohmic drop in the cell and consequently the lower energy consumption. On the other side, grown deposit on the cathode may cause short cut if the electrodes are too close. The electrolyte should contain: – compound of the ion – participant in the electrode reaction; – compounds that provide optimal electrical conductivity, corresponding acidity/ alkality and/or complex forming ability; – corresponding additives for regulation of the surface tension, to optimize the diffusion potential, to provide better adhesion of the deposit over the cathodic substrate, to reach better optical and mechanical properties of the deposit; – in some cases – mechanical components, e.g. thin oil layer or inert plastic balls in order to reduce electrolyte evaporation; – additives for optimizing density and viscosity in the case of the molten salt electrolysis. The source of energy should be: – properly designed to provide voltage and current output high enough for performing electrolysis; – equipped with voltage regulation system; – long-term stable.An electrochemical reaction is a heterogeneous chemical process involving the transfer of charge to or from an electrode, generally a metal, carbon or a semiconductor. The charge transfer may be a cathodic process in which an otherwise stable species is reduced by the transfer of electrons from an electrode. Of course, electrochemistry is only possible in a cell which contains both an anode and a cathode and, to avoid the accumulation of net positive or net negative charge somewhere in the cell, the amount of reduction at the cathode and oxidation at the anode must be equal. Moreover, the necessity to maintain charge balance throughout the cell system has other important consequences:1. For electrolysis to occur, electrons must pass from the anode to the cathode through an external, electrical circuit interconnecting the two electrodes.2. There must be a mechanism for charge transport between the electrodes within the cell.