ES2644864T3 - Inert Anode Assembly - Google Patents

Description

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DESCRIPTION

Inert anode assembly.

Cross reference to a related request

The present application is a request for continuation in part of the US document serial number 10 / 056.915, filed on January 25, 2002. Priority is claimed under Article 35 U.S.C. 119 (e) based on U.S. Provisional Application No. 60 / 428,818 filed on November 25, 2002.

Field of the Invention

The present invention relates to structures and methods for protecting inert anodes and other electrodes and electrode support materials against degradation by a molten electrolytic bath based on cryolite, and HF / O2 and other gases generated in an electrolytic cell. The present invention also improves metal production, such as aluminum production, limiting contamination of the bath and metal and reducing thermal shock during initial preheating and placing anodes in electrolytic cells.

Background of the invention

Aluminum is conventionally produced by the electrolysis of alumina dissolved in molten electrolytes based on cryolite at temperatures between approximately 850 ° C and 1000 ° C; The process is known as the Hall-Heroult process. This process is well known and is described, for example, in patent specification No. 5,279,715 (The Camera et al.) A Hall-Heroult reduction cell typically comprises a steel shell having an insulating coating of refractory material, which in turn has a carbon coating that makes contact with molten constituents. The electrolyte is based on molten cryolite (NaaAlFa) that can contain a variety of additives such as LiF, CaF2, MgF2 or AlF3, and contains dissolved high purity alumina (A ^ Oa). The carbon coating has a useful life of three to eight years, or even less in adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum, as well as sodium intercalation, which causes swelling and deformation of the cathode carbon blocks. In addition, the penetration of the species of sodium, other substances contained in the cryolite, or the air leads to the formation of toxic compounds that include cyanides. The anodes are at least partially submerged in the bathroom and are subject to the same conditions.

The Hall process, although commercial today, has certain limitations, such as the requirement that the process operate at relatively high temperatures, typically approximately 970 ° C to 1000 ° C. High cell temperatures are necessary to achieve high solubility of alumina. At these temperatures, the electrolyte and molten aluminum react progressively with the mayonnaise of the ceramic or carbon materials, creating electrode erosion problems, which can cause cellular contamination and metal and electrolyte content. Therefore, it is generally thought that electrolytic constituents are adverse to the rest of the cell.

The electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature of 1000 ° C before metal productions can begin. The heating should be done gradually and uniformly to avoid thermal shock to the cell components which in turn can cause breakage or detachment. The heating operation minimizes thermal shock in the coating, electrodes and other structural assemblies joined after the introduction of the electrolyte and molten metal into the cell. The prior art carbon anodes can be placed in the electrolyte at room temperature, and heated by the energy of the cell at operating temperatures, at which time the nominal current of the anode will be reached.

The newer ceramic inert anodes have much longer lives, but both the anodes and their supports are prone to thermal shock and, therefore, generally need to be preheated in an oven or the like outside the electrolytic cell before insertion into the hot electrolyte Thermal shock / cracking can occur both during movement of the anodes in position and during placement in the molten salt. Thermal shock refers to the thermal gradient (positive or negative) through the anode that occurs during movement from the preheating furnace to the cell, and also after the insertion of the anodes into the molten salt. A thermal gradient as low as 50 ° C can cause cracking.

Various attempts have been made to introduce various particles into the inert anode or to cover them with various protective materials, but it is practically impossible to avoid some dissolution and, eventually, such attempts lead to a certain amount of contamination of the bath and the aluminum which

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it is produced. In an attempt to protect the electrodes in an electrolysis cell from thermal shock during start-up, US Patent Specification No. 4,265,717 (Wiltzius) indicated the protection of hollow cylindrical cathodes of TiB2 by inserting sparks of Aluminum alloy in the cathode cavity and also protecting the cathode with a metallic thermal dispersion jacket that has a heat insulating inner layer that comes into contact with the TiB2. Here, the thermal insulation layer was manufactured from expanded fibrous Chinese clay-clay (AhO3 ■ 2SiO2 ■ 2H2O), which is subsequently dissolved in the molten electrolyte, introducing Si. A refractory repair mass is described in US Patent No. 5,928,717 (Cherico et al.). Here, a powdered mixture of alumina, metal fuel such as magnesium, zirconium, chromium and more additive aluminum selected from aluminum fluoride, barium sulfate, cerium oxide or calcium fluoride with an oxygen stream is used, under pressure. , to contact and cure non-uniform and similar crystalline structures on the surface of the refractory used. However, this mainly refers to the repair and refractories already present that have come in contact with molten aluminum or molten glass.

In the design of inert anodes for the production of aluminum or other metals, a matrix or assembly of uncoated inert anodes can be mounted on a refractory insulating cover molded below a metal plate, through which an electrical path is provided Continue from the cell. In this provision, shown in Figure 3 of US Patent Specification Nos. 6,551,489 B2 and 6,558,526 B2 (both of D'Astolfo Jr. et al.), It is necessary to provide protection of the metal plate and molten refractory. The problem, however, is that the majority of refractory materials are not able to withstand severe thermal shock and gradients found during preheating operations without cracking or resisting a certain amount of dissolution during cell operation. This design is expensive and requires a lot of assembly.

Aluminum electrolysis cells have historically used carbon anodes on a commercial scale. Energy consumption and the cost of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. The use of inert anodes instead of traditional carbon anodes makes it possible to use a highly productive cell design, thus reducing capital costs. Significant environmental benefits are also obtained because inert anodes do not essentially produce CO2 or CF4 emissions.

The inert anodes may be made of, for example, a ceramic metal ceramic cermet or a metal-containing material. Some examples of inert anode ceramic compositions are provided in US Patent Nos. 6,126,799; 6,217,739 B1; 6,372,119 B1; and 6,423,195 B1 (all of Ray et al., respectively), incorporated herein by reference. These anodes comprise a ceramic phase and can also comprise a metal phase. They are essentially free of voids and while they show low solubility and good dimensional stability, there is still some corrosion in the Hall cell baths at 1000 ° C.

In addition to the problems of thermal shock of electrodes and electrode support and other problems of erosion and cellular contamination, an improved, simplified and more cost-effective overall design of the electrode / electrode holder is needed.

Summary of the invention

One of the main objects of this invention is to protect anode electrodes from inert cermet and joined assemblies from thermal shock and chemical reagents. Another main object of the invention is to provide a simplified electrode assembly containing a minimum of materials, parts and contaminants. These and other objects are made by providing an electrolysis apparatus comprising a plurality of anodes, each year having a lower portion immersed in a molten electrolytic bath, in which a solid material is selected from the group consisting of alumina and cryolite, and mixtures thereof, together with a smaller effective amount, of about 5% by weight to 25% by weight of cementitious binder, coming into contact and circumscribing said solid material at least a higher portion of at least one of said anodes. The solid material can be applied by molding / casting, immersion, spraying or the like, and after dissolution in or very few impurities can be introduced into the molten bath.

The invention also provides an electrolysis apparatus comprising an inert anode system comprising at least one inert anode having a bottom part in contact with a molten salt bath, in which at least a top part of the inert anode contacts and is circumscribed by a solid material subject to the attack of gases from the bath, in which the solid material is selected from the group consisting of alumina and cryolite-alumina cement, which will be dissolved in the presence of the molten salt bath. The alumina cement material is preferably A ^ O3 at least 92% pure, insulating and very advantageously, highly resistant to temperature. The alumina-cryolite material is preferably from about 40% by weight to 80% by weight of cryolite, at least 2% by weight of alumina and from 5% by weight to 25% by weight of a high resistant cementitious material temperature. By "cryolite" refers to sodium aluminum fluoride that may contain various elements.

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alkaline and alkaline earth, such as calcium, magnesium, potassium, lithium and beryllium in various relationships, as well as the specific formula Na3AlF6. The alumina can also be used as the main component of 5% by weight to 15% by weight of heat resistant refractory cementitious material. The alumina-cement structure can be advantageously formulated from 50% by volume to 95% by volume density (i.e., having 5% by volume to 50% by volume porosity) allowing air inclusions to provide advantages of a preheating of more than 1000 ° C before insertion into the bathroom. The alumina can also contain up to 15% by weight of other oxides, such as, for example, CaO2; SiO2 and others, as well as the cement mentioned above.

Brief description of the drawings

Figure 1 is a cross-sectional view of an example of an anode system with a plurality of anodes;

Figure 2, which best illustrates the invention, is a plan view, partially in section, of an anodic system with a plurality of anodes used for example in aluminum processing, where the anodes are joined and circumscribed by a solid block which comprises cryolite and / or alumina;

Figure 3 is a plan view, partially in section, similar to Figure 2, but with a spray or immersion application to provide material that also circumscribes the entire portion of the anodes, but not in block form; Y

Figure 4 is a plan view, partially in sections, of the system of Figures 2 and 3 after substantial contact with a molten salt bath, showing a partial dissolution of the circumscribed solid block.

Detailed description of preferred embodiments

Referring now to Figure 1, an electrolytic cell is shown comprising an inert anode system 10 in an electrolysis apparatus, used, for example, to produce aluminum, and comprises a top structure and a plurality of inert anodes 14 and 14 '. The upper structure may include a refractory 12 to which the inert anodes are joined through a plate 18. The refractory material may be a flat structure or, for example, the hollow box type structure shown, filled with insulation 28. The bolts Metal 16 can anchor the inert anodes to the refractory 12 and to a top metal plate, usually of steel 18 anchored to the refractory 12 by metal anchors 20 or the like. The entire inert anode system 12, 18 and 28 is attached to a massive metal support 22. The inert anode system can be quite large, the length 30 of the refractory being approximately 1 to 2 m (3 feet to 6 feet) , and the wall thickness 31 being from about 2 cm to 10 cm. The refractory 12 has an outer or outer side 24 as shown, and can have an inner side 26. The inside of the refractory 12 can be filled with layers of low density ceramic plates 28, as shown, or an insulating mat made of fibers ceramic, or other materials, or left hollow. As you can see, this type of system is quite complicated in construction.

The gases 32 of the molten salt bath 34 and anode 14, 14 'are very aggressive even for stainless steel, especially several gases in combination. The gases shown as circles (bubbles) 32 of the bath or the anodes 14 '(only the gas of the two external anodes are shown for simplicity) pass over the bath 34 as the gas flow arrows 36. The bath of Molten salts 34 usually used in the Hall process to produce aluminum is based on molten cryolite (such as NaF plus AIF3), at a weight ratio of NaF bath to AF3 in a range of approximately 1.0: 1 to 1 , 6: 1 and at a temperature usually from about 850 ° C to 1050 ° C, preferably from 950 ° C to 975 ° C. Additionally, bath additives can be added for various purposes. The inert anodes are not fully submerged in the molten bath, normally the upper edge of the anode is above the bath at a distance 38, usually about 5 cm to 30 cm, called the gas or steam space. The most commonly generated gases 32 include HF, AlF3, O2 and NaAlF4. A combination of HF and O2 is particularly corrosive to metals and ceramics, especially at temperatures of approximately 400 ° C. The oxygen is generated in the anodes according to the reaction:

2AhO3 (solution) + 12e "^ 4Al (liquid) + 3O2 (gas) (I)

and HF is generated from the bath according to reaction (II):

2AlF3 (solution) + 3H2O A ^ O3 (solution) + 6 HF (gas) (II).

The water source is the water chemically linked to the fusion quality alumina fed to the fusion cell. The temperature of the refractory 12 at points 13 where there could be contact of HF and O2 is approximately 700 ° C to 1000 ° C depending on the distance from the molten cryolite.

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With reference now to Figure 2, a simpler and preferred embodiment of the inert anode system 10 of this invention is shown assembled and, in the case shown, molded, before contact with the molten electrolyte. As can be seen, the system 10 also contains a plurality of inert anodes 14 and 14 ', and a circumscribed support material 12'. A fixed metal plate 18 is secured by a series of anchors 20 all retained by the massive metal support 22. Aqm, a drastically different anode is used that circumscribes the solid structure 12 ', hitherto not considered, which contacts the anodes 14 and 14 'at points 40 and 42 when the solid structure 12' is first molded, before insertion into an electrolysis apparatus. The comparison with figure 1 shows the simplicity of this new system.

Figure 3 shows, basically, the same design and the same circumscription result, as Figure 2, but the application of the solid structure 12 by means of immersion or spraying means where the solid structure 12 'will be completely filled between the inert anodes such as 14 and 14 '. Although it is not as uniform as an exterior structure, the application is cost-effective, it has the same purpose as a pure and uniform molding / casting operation shown in Figure 2, it is lighter and uses less material.

Figure 4 shows the system 10 of Figures 2 or 3 inserted in an electrolysis apparatus, as can be used to produce aluminum, where the molten cryolite 34 (comprising NaaAlFa) contacts the inert anodes 14 and 14 'and has dissolved a portion of the solid material reduced 12 'a distance 44 from the bottom of anodes 14 and 14' leaving a thickness of solid material remaining 46. The remaining thickness 46 can be from 30% to 80%, preferably from 40% to 70 % of the thickness of the original solid structure 48, shown in Figures 2 and 3. Figure 4 shows a remaining solid structure thickness of 50%, although for the submerged or sprayed coating the surface will be a bit rougher than the one shown and 3 to possibly 5 or more repetitions may be required to obtain the desired block type shape. A thickness of the remaining solid structure of less than 30% will weaken the entire inert anode system 10 and impair the insulating effect of solid material 12 '. A remaining solid structure thickness greater than about 80% will not provide enough anode surface to allow the cell to function properly. Over a certain steam space 38, the cryolite 34 of the bath will condense and solidify at the bottom of the solid structure 12 ', in a steady state operation, adding an additional solid structure as shown by the dotted lines.

In this invention, the entire refractory slab, insulating boards, protective coatings / coatings of external inert anode, all of which dissolved to some extent in the molten bath that causes impurities, are replaced with a block of alumina, preferably 95% 99% pure weight, or a bath + alumina material, both containing a binder cement, to provide the solid structure 12 'shown in Figures 2 and 3. Whether the surrounding alumina or the bath + alumina support 12 'is dissolved in the molten cryolite bath 34, is not damaged and no more than 0.5% by weight of impurities are added based on the weight of the molten bath, or preferably no impurity is added to the molten bath. This also dramatically simplifies the structure of the entire system 10, with considerable savings in time and costs. It also makes the alignment of the anode much less critical in the assembly process. This solid block material 12 'initially completely encloses anodes 14, 14' and bolts 16, and is suspended by hooks 50 of steel plate 18. The alumina content of the block is adjusted to allow the assembly to withstand temperatures of preheating. In addition, in the cryolite + alumina material, the weight ratio of the bath (NaF + AF3) is preferably from about 1.2 to 1.6 to withstand preheating temperatures. When the anode is adjusted, part of the solid material 12 'dissolves in the bath, exposing the lower part of the anode to electrolysis, while the upper part remains solid, like a natural crust, to provide insulation and smoke protection. This bark will grow and shrink as the anode rises and falls, providing continuous protection and isolation. When the system 10 is set in the molten bath 34, as shown in Figure 4, it automatically provides the only two materials that must be added to the bath: alumina and more bath to fill the gaps between anodes 14 and 14 '. Normally, commercial aluminum can have a maximum of about 0.3 to 0.65% impurities; where the permissible range of each impurity is approximately 0.1% to 0.6% Fe; from 0% to 0.05% Cu; from 0% to 0.05% of Zn; 0% to 0.05% Ni; and from 0% to 0.35% of Si. The use of alumina, A ^ O3 or the support of bath + alumina, but, in both cases, any cement material based on associated alumina, will allow the production of commercial grade aluminum.

The most complicated material composition containing the solid structure of bath + alumina 12 'will now be analyzed. The solid structure of bath + malleable alumina 12 'normally comprises from about 40% by weight to about 80% by weight, preferably from about 55% by weight to about 70% by weight of aluminum and sodium fluoride powder. ; from about 2% by weight to about 25% by weight, preferably from about 2% by weight to about 10% by weight of aluminum oxide powder

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(AI2O3). The materials usually contain a lower effective amount of binder, usually from about 5% by weight to about 25% by weight; preferably from about 5% by weight to about 15% by weight of a cementitious material, preferably an alumina / cement based refractory cementitious material, preferably containing from about 65% by weight to 85% by weight of alumina ( A ^ O3) and 15% to 30% by weight of CaO. This cement material is a high temperature resistant material capable of withstanding temperatures from 800 ° C to 1200 ° C without degradation. In addition to alumina, the usual components may include, for example, CaO, SO2, Na2O and Fe2O3. Structure 12 'may also contain smaller amounts of NasA ^ F-M (natural cryolite). Water is added to the powder mixture to make a suspension and then approximately 10% by weight based on the entire powder mixture of the alumina-based cement material is added to join the bath + alumina material together. This bath + cement suspension material is then poured into a mold containing inert anodes 14, 14 'and hooks 50, followed by baking at approximately 125 ° C to 175 ° C for 10 hours to 15 hours to remove moisture . This provides a less porous structure, less resistant to temperature than the structure of alumina + purified cement, but is still preferred as chemically more similar to electrolyte.

The alumina material can be molded, cast, dipped or sprayed. It is essentially pure AI2O3 alone or mixed with a suitable alumina-based cementitious binder, with approximately 5% by weight to approximately 15% by weight of heat-resistant cementitious material, at high temperature (capable of withstanding temperatures of approximately 800 ° C to 1200 ° C without degradation).

Examples

An anode system was provided with a circumscribed solid material containing a mixture of cryolite, calcium aluminate cement and dispersant as described below.

Approximately 5,400 grams of calcium aluminate cement of 0.05-1.0 mm / grog were mixed with approximately 600 grams of calcium aluminate, 100 grams of Methocel (dispersant), 100 grams of a bentonite clay wetting agent, and 1200 grams of cryolite in -200 Hall bath which has a ratio of 0.90 to 1.50 (% sodium fluoride with respect to% aluminum fluoride) and then mix with 1000 grams to 7000 grams of water (on average, 3888 grams).

All solid ingredients were mixed, in a stainless steel container, for 2 to 5 minutes on a dry basis at low speeds. Water was slowly added to the mixed powders. The mixing process was periodically stopped to ensure that all ingredients were moist and uniformly dispersed or did not settle at the bottom of the mixing vessel.

The aqueous base mixture was then transferred to a container, to allow the anodes to be dipped by immersion with a layer of thickness up to 1/2 inch (1.27 cm) of the mixture. In the immersion coating process, the anodes were slowly lowered into the refractory coating of the mixture until completely submerged. The coating was allowed to equilibrate (that is, even in the area that was in immediate contact with the anodes). The anodes were then removed at a speed of approximately 12.5 cm / minute to allow at least a 0.6 cm thick layer of the bath block refractory to adhere to the surface of the anodes.

The anodes were suspended after a fixing device and a hot air dryer was used to accelerate drying of the bath block liner. Once the outer surface was dry to the touch, the anodes were submerged for the second and third layers, as required, for coating applications specified with the appropriate drying stage before the application of the next layer. Several more applications are required to obtain a complete block structure.

The anodes having the desired coating thickness were then placed in a preheating furnace and heated to approximately 960 ° C at a rate to prevent cracking of the anode and insulating coating. Once at a desired temperature, the coated anodes were removed from the heater and quickly transferred to a Hall cell with a loss of less than 10 ° C in temperature in less than 2 minutes necessary to transfer the anodes to the Hall cell.

After submerging in the Hall cell, the lining of the bath block dissolved to the bath line in less than 5 minutes. The dissolution of the bath block from the submerged portion of the anode allowed the current to flow for the production of aluminum metal. Importantly, the insulation of the dissolved bath block was of such a composition that it did not contaminate the metal or the cryolite used in the Hall cell. This provided a simple and economical compatible anode support, useful for the production of aluminum.

Having described the presently preferred embodiments, it should be understood that the invention may be

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otherwise incorporated within the scope of the appended claims. Aspects of the invention will now be described with reference to the following numbered clauses:

Clauses:

1. An electrolysis apparatus comprising a plurality of anodes, each year having a lower portion immersed in a molten electrolytic bath, in which a solid material is selected from the group consisting of alumina and cryolite, and mixtures thereof, together with an effective amount of cementitious binder, coming into contact and circumscribing said solid material at least a higher portion of at least one of said anodes.

2. The electrolysis apparatus of clause 1, in which the anodes are inert anodes.

3. The electrolysis apparatus of clause 1, in which the electrolysis apparatus is an electrolytic cell used in the production of aluminum.

4. The electrolysis apparatus of clause 1 which also contains an upper metal plate.

5. The electrolysis apparatus of clause 1, wherein the solid material comprises from about 40% by weight to about 80% by weight of cryolite, from about 2% by weight to about 25% by weight of alumina and from about 5% by weight to about 25% by weight of cementitious binder material.

6. The electrolysis apparatus of clause 1, in which the solid material comprises alumina containing from 5% by weight to 15% by weight of cementitious binder material.

7. The electrolysis apparatus of clause 1, in which the solid material dissolves at temperatures of approximately 1000 ° C in the presence of a molten electrolytic bath based on cryolite.

8. The electrolysis apparatus of clause 1, in which the solid material dissolves to the extent that the thickness of the remaining solid material is 30% to 80% of the original thickness.

9. The electrolysis apparatus of clause 1, in which all of the at least one anode is circumscribed by the solid material.

10. An electrolysis apparatus comprising an inert anode system comprising at least one inert anode having a lower part in contact with a molten salt bath, in which at least an upper part of the inert anode contacts and is circumscribed by a solid material subject to the attack of gases of the bath, in which the solid material is selected from the group consisting of cement based on alumina and cryolite-alumina, which will be dissolved in the presence of the bath of molten salts.

11. The electrolysis apparatus of clause 10, where the solid material is from about 40% by weight to 80% by weight of cryolite, from about 2% by weight to 25% by weight of alumina and 5% by weight 25% weight by weight of a cementitious material.

12. The electrolysis apparatus of clause 10, in which the electrolysis apparatus is an electrolytic cell suitable for the production of aluminum.

13. The electrolysis apparatus of clause 10, in which the solid material dissolves to the extent that the thickness of the remaining solid material is 30% to 80% of the original thickness.

14. The electrolysis apparatus of clause 10, in which the solid material dissolves to the extent that the thickness of the remaining solid material is 40% to 70% of the original support thickness.

15. The electrolysis apparatus of clause 10, in which the cement material is a refractory cement based on alumina.

16. The electrolysis apparatus of clause 10, in which all of the at least one inert anode is circumscribed by the solid material.

17. The electrolysis apparatus of clause 10, where the solid material is applied by casting.

18. The electrolysis apparatus of clause 10, where the solid material is applied by spraying.

19. The electrolysis apparatus of clause 10, where the solid material is applied by immersion.

Claims (10)

5 10 fifteen twenty 25 30 35 1. An electrolysis apparatus comprising an inert anode system comprising at least one inert anode having a lower part in contact with a molten salt bath, in which at least an upper part of the inert anode contacts and is circumscribed by a solid material subject to the attack of gases from the bath, in which the solid material is selected from the group consisting of cement based on alumina and cryolite-alumina, which will be dissolved in the presence of the molten salt bath. 2. The electrolysis apparatus of claim 1, wherein the solid material is from about 40% by weight to 80% by weight of cryolite, from about 2% by weight to 25% by weight of alumina and 5% by weight. 25% weight by weight of a cementitious material. 3. The electrolysis apparatus of claim 1 or claim 2, wherein the electrolysis apparatus is an electrolytic cell suitable for the production of aluminum. 4. The electrolysis apparatus of any preceding claim, wherein the solid material dissolves to the extent that the thickness of the remaining solid material is 30% to 80% of the original thickness. 5. The electrolysis apparatus of any preceding claim, wherein the solid material dissolves to the extent that the thickness of the remaining solid material is 40% to 70% of the original support thickness. 6. The electrolysis apparatus of any of the preceding claims, wherein the cement material is an alumina-based refractory cement. 7. The electrolysis apparatus of any preceding claim, wherein all of the at least one inert anode is circumscribed by the solid material. 8. The electrolysis apparatus of any preceding claim, wherein the solid material is applied by casting. 9. The electrolysis apparatus of any preceding claim, wherein the solid material is applied by spraying. 10. The electrolysis apparatus of any preceding claim, wherein the solid material is applied by immersion.