Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium

Definitions

• the present invention relates to a process for preheating a tank provided with anodes and cathodes for the production of aluminum by electrolysis.
• Aluminum is produced industrially by igneous electrolysis, that is to say by electrolysis of alumina in solution in a molten cryolite bath.
• This bath is contained in a tank comprising a steel box, which is coated internally with refractory and / or insulating materials, and a cathode assembly located at the bottom of the tank.
• Anodes made of carbonaceous material are partially immersed in the electrolysis bath.
• the electrolysis current which circulates in the electrolysis bath and the sheet of liquid aluminum via the anodes and cathode elements, operates the alumina reduction reactions and also makes it possible to maintain the bath. electrolysis at a temperature of the order of 950 ° C.
• the tanks are arranged in series and are subjected to a current of the same intensity.
• it is necessary to ensure the temperature rise of the tank which is initially cold. This is a delicate operation during which thermal shocks must be avoided.
• a tank requires a very large investment and has a lifespan typically between 3 and 7 years. It is therefore necessary to take all the precautions so as not to reduce the period of activity of the tank. For this, the temperature rise within the tank must be slow, typically 20 ° C per hour.
• a uniform layer of a conductive granular material is deposited between the anodes and the cathodes, this layer then authorizing a process for preheating the tank by resistance.
• the object of the present invention is to solve the drawbacks mentioned above, and for this purpose relates to a process for preheating a tank provided with anodes and cathodes for the production of aluminum by electrolysis, said process comprising a first step, before current supply to the tank, during which a layer of a conductive granulated material is deposited and then crushed between the anodes and the cathodes, characterized in that the conductive granulated material is based on graphite and in that the layer of granulated material conductor extends, after crushing, only on a part of the lower surface of each anode.
• the use of such a layer of conductive granulated material makes it possible to preheat the tank to the desired temperature in a reasonable period of time of the order of 60 hours, without using shunts having disadvantages in terms of safety. and productivity.
• the use of graphite on only part of the contact surface of each anode makes it possible to increase the resistance, and thus to accelerate the rise in temperature and to reduce the duration of the operation.
• this effect comes from the improvement in the reproducibility of the total resistance offered by the layer of conductive granulated material. Indeed, this resistance depends on the pressure exerted on the layer and the thickness of this layer.
• a well chosen surface / thickness pair will then make it possible to obtain a total resistance that is not very sensitive to variations in these parameters and will generate fewer hot spots on the cathodes.
• the arrangement of the granulated material makes it possible to adapt the resistance to obtain the most uniform heating profile possible. Indeed, the degree of freedom released by not covering the entire contact surface of each anode makes it possible to accentuate the heating of the parts which are most subject to thermal losses.
• the layer of conductive granulated material covers, after crushing, between 5 and 40%, typically 5 to 20%, of the lower surface of each anode.
• Said layer of carbonaceous material preferably also takes the form of studs.
• the deposition of the layer of conductive granular material is preferably carried out in the form of studs.
• the number of these is advantageously between 3 and 20, inclusive, and is typically between 4 and 8, inclusive.
• These studs can be aligned, but can also be staggered, or even asymmetrically.
• these studs can be of different sizes and have any general shape in section, in particular circular or oval.
• two or more studs can have a section of different size (corresponding to a different diameter in the case of studs with cricular section).
• each pad has an initial thickness, before crushing, of between 0.5 and 4 cm. After crushing, the thickness is typically between 0.5 and 3 cm. In a particularly advantageous manner, each pad has a thickness respectively, before crushing, of the order of 3 cm, and after crushing, of the order of 2 cm.
• the studs are produced using a template placed on the cathodes and comprising a plate provided with several orifices in each of which is introduced conductive granulated material.
• the graphite grains of the conductive granular material have a size of between 1 and 8 mm.
• This conductive granulated material, based on graphite can also comprise at least one other material capable of varying its resistivity, such as an under-calcined carbon material or alumina.
• the invention also relates to a process for preheating a tank by the production of aluminum, comprising the following steps: - formation of a layer of the conductive granulated material on a part of the surface of a cathode,
• each anode placed on the layer of granulated material, - establishing an electrical connection between the rod of each anode and the anode frame,
• each anode on the layer of granulated material causes the compression of this layer, which is generally crushed under the action of the weight of the anode assembly.
• Figure 1 is a sectional view of a tank after deposition of the conductive granulated material and crushing of the latter between the anodes and the cathodes.
• FIG. 2 is a top view of a template for depositing the studs within the tank.
• FIG. 3 is a cross-sectional view of the template shown in FIG. 2.
• Figure 4 is a view of a pad of conductive granulated material after removal of the template.
• a tank 1 for the production of aluminum by electrolysis typically comprises a metal box 2 internally lined with refractory materials 3, 4, cathodes 5 made of carbonaceous material, anode assemblies 6, an anode frame 7, means 8, such as covers, for recovering the effluents emitted by the tank 1 in operation, and means 9 for supplying the tank with alumina and / or AIF3.
• the anode assemblies 6 each comprise at least one anode (or anode block) 10 and a rod 1 1, the latter typically having a multipode 1 2 for fixing the anode 10.
• a first step is carried out during which studs 13 of a granulated material conductor 25 essentially based on graphite have been placed, then crushed between the cathodes 5 and the anodes 10. More precisely, the various studs 13 are placed discontinuously between the cathodes 5 and the lower surface (or "contact surface") 14 of each of the anodes 10. Each contact surface 14 is then partially in contact with the conductive granulated material 25.
• the latter is advantageously made using grains of which 90 to 95% have a particle size between 1 and 8 mm.
• These pads 13 are advantageously arranged so as to heat the periphery more than the center of each cathode 5 which is generally warmer. In operation, the parts close to the walls of the tank 1 can thus benefit from a more efficient rise in temperature.
• Tests were carried out on several Pechiney AP-30 tanks in which four studs similar to those described above were placed for each anode, the tanks being moreover fitted with graphitic cathode blocks. The tests were carried out at an intensity of 305 kA, the circuit being made without shunt by removing the elements which short-circuit the tank.
• a template 15 was used to position the studs 13 in the tank 1 before setting up the anode assemblies 6. More specifically, such a template 15 is produced in the form of a plate 16 comprising several aligned holes 17, which are four in number in this case.
• the plate 16 has a length of about 1.50 m, a width of 65 cm, and a thickness of 3 cm.
• the orifices 17 are substantially circular and have a diameter of the order of 20 cm.
• This plate 16 is firstly placed in the tank 1 in contact with a cathode 5.
• the orifices 17 are then filled with the aid of the conductive granulated material 25, and the plate 16 is finally removed.
• each pad 13 of conductive granulated material 25 flares slightly and turns into a conical trunk having a diameter of 20 to 24 cm at the base, and a diameter from 14 to 16 cm at the top. The conical trunks are then crushed under the weight of each anode assembly.
• thermocouples were inserted on the surface of the anode blocks as follows: three were inserted in the central corridor, two in each of the two lateral corridors, one at each of the two heads, and two at opposite angles.
• thermocouples located in the central corridor were in the range of 850 and 1000 ° C. All the other thermocouples were above the minimum targets, namely, more than 700 ° C in the heads, more than 600 ° C in the side aisles, and more than 500 ° C in the corners. In addition, no hot spots were apparent on the cathodes. Finally, at any time, the temperature rise in the central corridor was carried out at less than 30 ° C per hour.
• connection of the anode rods to the anode frame can be advantageously carried out using preheating hoses.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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• 2002
  • 2002-09-20 FR FR0211670A patent/FR2844811B1/fr not_active Expired - Fee Related
• 2003
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