Both Hall and Heioult selected molten cryolite (Na3 AIF6) as the electrolyte for the electrolysis of alumina because cryolite has uniquely high solubility for oxides although certain additives are beneficial, the major component of the electrolyte used today is still cryolite. The most common additives are aluminum fluoride and calcium fluoride. Some aluminium producers, lithium fluoride with lesser popularity, magnesium fluoride and sodium chloride are the ideal additive.
It should increase or at least not decrease alumina solubility, increase electrical conductivity, decrease density (to provide better separation between the aluminum and the molten salt) and decrease vapour pressure (to minimize vapour loss). It should not contain or produce ionic species with lower discharge potentials than aluminum (for the action) or oxygen (for the anions). As there is no ideal additive compromises are made/ Alumina solubility and electrical conductivity are often sacrificed or improved faraday efficiently.
Phase equilibrium in the Hall-Heroult electrolyte system has been investigated by thermal analysis (TA), differential thermal analysis (DTA) and a quenching technique. Both TA and the more sensitive DTA suffer from the sluggishness of some of the thermal transformations.
Liquids = Constant + 3(coefficient) (terms)
Electrode reactions in Hall - Herolut Cells
The chemical and electrochemical reactions taking place at or near the anode and cathode of a Hall-Heroult cell are of great practical and theoretical importance. Although not fully understood, in spite of extensive research, enough is know to form a reasonably consistent picture While the carbon lining of the cell is often referred to as the cathode, the true cathode, the true cathode is the molten aluminum that rests on if. Generally, the sides of the cell cavity are covered with frozen electrolyte that often contains undissolved alumina.
The cathode reaction is often expressed simplistically as:
Al3+3c = Al
However, as pointed out previously, no Al3+ cat ions are present in the bulk melt. Instead, aluminum is found in various anion complexes. The fact that the Na+ion is the principal current carrier has led some investigators to assume that sodium is the primary product discharged at the cathode and that aluminum is produced by a secondary reaction.
Al2O3(soln.)+C(s) = 2Al(1) +CO2(g)
Is about 0.22 V ore favorable than for producing sodium.
6NaF(soln.)+ Al2O3(soln.) + C(s) = 2 AlF3(soln.) + CO2(g) + 6 Na (g)
Over the range of compositions and temperatures used industrially. This difference was calculated by:
While three-electron transfer processes involving oxyfluoroaluminate ions are possible, more probable overall cathode reactions are.
This reaction would predominate near the cryokite ratio; this Reaction becomes more important as the NaF/AlF3 ratio is lowered until it becomes the major reaction in very acid melts.
Where CR= cryolite ration I= catholic current density (A/cm2)
It is not necessary to assume electrolytic deposition of sodium at the cathode to explain the sodium content of the aluminum. The equilibrium reaction.
The elementary anode reaction can be written as:
C + 202- = CO2 + 4e-
2Al2O2F42- + C = CO2 + 2Al2OF4 + 4e-
Equilibrium between Al2O2F22- and Al2OF62- would be restored by:
Al2 OF4 + Al2 OF62- = Al2 O2 F42- + 2 AlF3
It would quickly hydrolysis with moisture in the air to CO3 and HF. Calandra at all believes that reaction is responsible for the anode effect, to be discussed in detail later. By using high sweep rates they were able to determine the diffusion coefficient of the O2 - containing species. They interpreted its abnormally low value (0.7X10-6 cm2/s) as an indication that the anion dissociates before discharging.
The anode mechanism must also explain the large displacement from thermodynamic equilibrium. Oxygen reacting with carbon at cell temperature should equilibrate to about 99% CO and 1% CO2 Based on the volume of gas produced or net carob consumed, however, the primary product formed at the.
Discussion of alternate energy sources is beyond the scope of this work, but discussion of the energy requirements for aluminum smelting and what can be done to conserve energy warrants consideration. It gives a breakdown of the energy required to produce a kilogram of aluminum. The energy content of a carbon anode consists of two parts. First is the energy produced by consumption (burning) of the anode, which amounts to a heat of combustion of 1.88Mj/kg of carbon or about 0.84Mj/kg of aluminum produced? This energy lowers the electrical power required by the process. Second is the energy required to make and bake the anode?
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