One of the most difficult tasks conform the designer of an electrolytic cell for the production of aluminum is the design of the container itself, because:
- Molten fluoride bath at 960o C has both low viscosity and interfacial tension that allow easy penetration of porous construction materials.
- During the initial baking of a newly built cell, the different rates of thermal expansion of the construction materials cause dimensional changes that distort the vessel and can cause early failure by leaking electrolyte and molten aluminum to the cell wall.
- Throughout the life of the cell, chemical reactions-some beneficial and some detrimental-take place between the molten electrolyte and the construction.
There is no know commercially available material that can retain its insulating value and resist penetration and subsequent chemical attack by molten fluoride electrolyte at 950-1000oC. Therefore, the only way to retain the necessary insulation for an appreciable period of time is to create a container of frozen electrolyte on the surface of the lining at the sidewalls and below collector bar level in the bottom. This is done by controlling heat balance to create low-temperature is others at the proper location in the lining by a careful selection of insulation material.
Q = hArT.Q is the heat.
Thermal balance considerations are of prime importance in designing and operating a cell if the optimum temperature profile is to be maintained for any cell design. Current efficiency drops because of increased metal re-oxidation and increased sodium activity resulting from these changes.
Reactions During Baking
To achieve the desired isotherms and avoid cracking, necessary for long life, relined cells must be baked carefully under line load. To allow line load baking, petroleum coke is used as a resistor on each cathode block and contact is made by lowering the anodes to this coke layer. To avoid too rapid de-volatilization of the seam mix, resistor coke is not placed on seams. Crushed bath is spread along the carbonaceous sidelining blocks during baking to protect them from air oxidation.
Reactions During Startup
When the lining reaches operating temperature (950-980oC), molten electrolyte tapped from operating cells is poured into the cavity, anodes are lifted and electrolysis begins. Since steady state has not yet been reached, the lining is cool enough to stop any rapid penetration of the electrolyte by freezing it. Since cathode blocks are attacked by the electrolyte in the absence of aluminum, within 12-14 hours a layer of molten aluminum is poured into the cell to create a molten pad completely covering the cathode. If the metal is added too early it may penetrate the seams or peripheral gaps caused by baking shrinkage and greatly shorten cell life. Once electrolysis beings, Sodium penetrates the lining, causing expansion and closing the gaps. Since sodium penetration is relatively slow, the addition of metal should be delayed until sodium has adequately penetrated the carbon.
Reactions During Operation
During operation molten bath penetrates the carbon lining and insulation until it reaches the freeze isotherm, baked carbon has a porosity of 25-30 per- cent and penetration of the bath takes place largely through the pores. Initial penetration of the bottom carbon lining is by sodium. This results in a concentration of about 0.003 per cent sodium in the metal pad, varying with bath ration and current density. Penetration continues to the freeze isotherm, somewhat below 880oC, the eutectic temperature on the alkaline side of the system NaF-AIF3.
Thick layers of powdered alumina as insulation under the carbon block should be avoided as sodium aluminates forms where the freeze isotherm crosses this alumina, causing the alumina to expand at this localized zone. This expansion can be large enough that in combination with the block heaving, 45o angle shear.
Bench scale and limited commercial tests have demonstrated that graphite is less subject to bath penetration and subsequent swelling than the present carbon block linings. One of the main reasons that electrically claimed anthracite blocks are more resistant to this swelling is the high-temperature calcinations of the aggregate. Even with electrically claimed anthracite, properties of cathode blocks aggregate. Pith binder can only be carbonized at about 1200oC. This has led to a trend towards lining blocks that have been baked at higher temperatures in electric resistance furnaces. Since the maximum temperature in such furnaces is 2000-2400 oC, complete graphitization does not take place. Higher baking temperature permits carbon materials other than anthracite to be used in their manufacture.
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