EP0701635A1 - Treating prebaked carbon anodes for aluminium production - Google Patents
Treating prebaked carbon anodes for aluminium productionInfo
- Publication number
- EP0701635A1 EP0701635A1 EP94915259A EP94915259A EP0701635A1 EP 0701635 A1 EP0701635 A1 EP 0701635A1 EP 94915259 A EP94915259 A EP 94915259A EP 94915259 A EP94915259 A EP 94915259A EP 0701635 A1 EP0701635 A1 EP 0701635A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- anode
- boron
- cell
- carbon
- solution
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Definitions
- This invention relates to components, in particular prebaked carbon anodes and sidewalls, of electrolytic cells for the production of aluminium especially by the electrolysis of alumina in a molten fluoride electrolyte such as cryolite, and is particularly concerned with improving the resistance to oxidation of the sides and top of prebaked anodes and of sidewalls which are exposed to air and oxidising gases during cell operation.
- Background Art
- Aluminium is produced conventionally by the Hall- Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950°C.
- the anodes are usually prebaked carbon blocks that are consumed by the electrochemical reaction, corroded by contact with the electrolyte and disintegrated by the evolved oxidising gases .
- Prebaked anodes for aluminium production are made of a matrix of petroleum coke with pitch as binder. Their production involves various phases including preparing and treating the starting materials, mixing, forming and calcining at high temperature, followed by securing the current supply member by rodding.
- aluminium involves a complex reaction summarised by the relationship: AI2O3 + C > Al + CO2
- the real anode consumption is 40-50% greater, and amounts to about 20% of the production cost of the aluminium.
- Prebaked carbon anodes contain metallic impurities originating from the starting materials, which impurities undesirably influence the anode consumption.
- V, Fe, S and especially Na exert a catalytic activity influencing the anode oxidation reaction, favourising the attack by O 2 .
- phosphorous as phosphate or phosphoric acid
- phosphorous-based treating agents such as that described in US Patent 4 439 491 have not been successful as oxygen inhibitors for prebaked carbon anodes used for aluminium production.
- AIF3 has been proposed as additive on account of the fact that it is non-polluting to the bath. A reduction of the carbon consumption is obtained, but is attributed to the fact that AIF3 vapours reduce the differential reaction between coke and pitch, so the available saving is small because there is no reduction of the main oxidation.
- Other compounds such as AICI3 in an amount of 1-3%, or Si ⁇ 2 as H 2 Si ⁇ 3 in an amount of 0.2 to 1%, have also been tried, but without giving satisfactory results.
- Another proposed protective coating consists of alumina, but this has the disadvantage of creating a thermal insulation around the anode, leading to local overheating and acceleration of the oxidation process.
- US Patent 3 852 107 describes spraying a coating 0.5 to 5mm thick onto a pre-heated anode, the spray mixture comprising a matrix of a boron compound and a refractory filler such as a carbide.
- DE-A-28 09 295 described coating a carbon body such as a prebaked anode for aluminium-production, by using a solution of ammonium pentaborate or ammonium tetraborate to produce a glassy coating of anhydrous boric acid (B 2 O3) .
- Such coatings initially reduce the reactivity of the anode surface with oxygen, but the effect is short-lived and, once the coating has been worn away, is lost .
- Such coatings remain on an external surface of the anode and can easily be mechanically damaged during transport of the anode and its installation in the cell. Also, such coatings are not perfectly impervious to gas, and cannot protect the anode from oxidation.
- An object of the invention is to improve the resistance to oxidation of a preformed carbon anode or a cell sidewall for aluminium production by the incorporation of boron without the inherent drawbacks of the known proposals .
- the invention provides a method of treating a cell component, in particular a prebaked carbon-based anode or sidewall, of an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte such as cryolite, to improve the resistance thereof to deterioration during operation of the cell by the attack of air and oxidising gases released at the anode, using boron in acceptable amounts in the surface parts exposed in use to oxidising gases.
- the method according to the invention comprises treating the anode or other component in a boron-containing solution to intake the boron-containing solution to a selected depth over parts of the surface to be protected, this selected depth being in the range 1-lOcm, preferably at least 1.5 cm and at most about 5 cm, preferably still at least about 2 cm and at most about 4cm.
- the impregnation treatment of the invention provides a protected layer of one or several centimeters, wherein the boron penetrates in the pores into which oxidising air/gas enter.
- the treatment applies in particular to prebaked carbon anodes which are liable to be subject to mechanical damage of the external layer during transport.
- damage to the outer surface is not detrimental to the protection against oxidation, due to the thickness of the impregnation which provides a long-lasting protective effect as the anode wears away slowly during use.
- the impregnation treatment applies also to the cell sidewalls, particularly the upper part of the cell sidewall that is exposed to air and the effect of oxidising gases during use, as well as the lower part exposed to carbo- oxidation reactions with CO2 at the surface of the sidewall submerged in the electrolyte.
- the protective effect can be . enhanced by topcoating the impregnated sidewalls with a layer of refractory material e.g. particulate diboride in a colloidal carrier, such as titanium diboride in colloidal alumina, as described in WO 93/25731.
- a layer of refractory material e.g. particulate diboride in a colloidal carrier, such as titanium diboride in colloidal alumina, as described in WO 93/25731.
- the boron-containing solution comprises a boron compound such as B2O3, boric acid or tetraboric acid dissolved in a solvent preferably selected from methanol, ethylene glycol, glycerin, water containing at least one surface-active agent, and mixtures thereof.
- a solvent preferably selected from methanol, ethylene glycol, glycerin, water containing at least one surface-active agent, and mixtures thereof.
- the solution preferably contains 5 - 60 weight% of the boron compound in particular using a solution at a temperature in the range from 10° to 120°C, preferably 20°C to 80°C, these conditions ensuring excellent penetration of the solution into the porous carbon.
- solvents like methanol, ethylene glycol or glycerin are used at a temperature of about 80°C or above.
- solvents such as methanol, ethylene glycol and glycerin will be preferred, possibly with additives to enhance the solubility of the boron compound, and the treatment time may be extended to several hours.
- surfactant agents in particular tensio-active cationic agents are used.
- Anionic tensio-active agents can also be used. Such agents should be devoid of components that would undesirably contaminate the aluminium produced and components that promote oxidation of the carbon.
- These surface-active agents may possibly be present together with other solubility improving agents such as tartaric acid or citric acid, and the solution may be heated to improve and to speed up the impregnation of the anode.
- the anode can be treated by immersion in the boron- containing solution for about 2 minutes to 1 hour for a heated solution, followed by drying. Usually, a single impregnation suffices, but the impregnation and drying may be repeated until the treated anode surface is saturated with the boron compound.
- the treatment time depends principally on the exposed surface area of the anode and its porosity, as well as the temperature. It has been observed that prolonging the treatment does not significantly increase the boron concentration or the depth of penetration.
- Anodes are conveniently impregnated simply by dipping them into the solution, which can take place in ambient conditions, but the impregnation may be assisted by the application of a pressure differential, by applying pressure or a vacuum. Other ways of speeding up impregnation can also be used, such as the application of ultrasounds.
- the boron-containing solution impregnates the carbon anode to a depth of 1-lOcm, for example approximately 2 to 4 or 5cm, with a concentration of boron in the impregnated surface of the carbon anode in the range from 200 ⁇ pm to 0.35%, or possibly even higher. Even with the highest achievable levels of boron concentration, the problem of process contamination is avoided because the protective boron compounds are present only in the top and side surfaces of the anode needing protection, and only to a depth of several centimeters.
- the anode is usually made of petroleum coke and pitch, the anode having an open porosity in the range 5% to 30%, preferably from 5 to 20%.
- the porous material making up the anode may also be a composite carbon-based material comprising at least one further component such as refractory oxycompounds, in particular alumina. Examples of such materials are described in WO 93/25494 the contents whereof are incorporated herein by way of reference.
- the impregnation treatment of a pre-baked anode according to the invention is made after calcinining where the anode surface has highest porosity, so improving the penetration of the solution to a depth of one or several centimeters.
- Oxidation of the anodes increases with porosity.
- the intake of the boron-containing solution into the anode can monitored by checking the level of the solution, or simply by the time of immersion for a given solution and an anode of given porosity.
- the top and side surfaces of the anode can be immersed in the boron-containing solution simply by dipping the anode upside down in the solution. There is no need to treat the bottom of the anode where the electrochemical reaction takes place. In this way, only those parts of the anode which need protection are treated in a simple way, and the amount of boron in the anode (and hence in the aluminium produced) is minimized.
- the invention also concerns a prebaked carbon-based anode of an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte such as cryolite, wherein the top and side surfaces of the anode are impregnated to a depth of 1 to 10cm, usually 1.5cm to 5cm, preferably about 2 to 4cm, with a boron compound, to improve the resistance thereof to consumption during operation of the cell by air and oxidising gases released at the anode.
- the central part and the lower surface of the anode are essentially devoid of the boron-containing compound.
- Such an anode may be produced by the methods set out above, and may incorporate all of the features described in connection with the method.
- the invention also concerns an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte such as cryolite, comprising an anode or sidewall as set out above, the anode or sidewall being installed with the treated boron-containing surfaces of the anode in contact with air and oxidising gases released during operation of the cell.
- the concentration of the boron compound, in particular H3BO3 or B 2 O3, is important : greater concentrations provide a greater concentration gradient, favourising the kinetics of penetration of the solution into the porous anode. Solubility of the boron compounds can be increased by maintaining the solution at a suitable high temperature.
- Solvents with a low degree of inflammability are desirable. Should flammable solvents impregnate the carbon, this could lead to unwanted heat generation Sharinging oxidation of the carbon.
- solvents selected from methanol, ethylene glycol, glycerin and mixtures thereof at a temperature from 80° to 120°C, a concentration of 50-60 weight% of H3BO3 or B 2 O3 can be achieved in the solution, or about 20% when water with a surface-active agent is used as solvent .
- Such solutions have desirable physio-chemical properties, providing excellent impregnation by immersion of an anode in about 2-60 minutes. In these conditions, treatment of an anode having a porosity of about 15-18% and a surface area of 2-3m 2 produces an impregnation to a depth of about 3-4 cm with a boron concentration of several hundreds of ppm.
- a surfactant such as those available under the tradenames NONIDET P 40 and
- SPAN 85 from Fluka
- GLUCOPON 225 DEHYPON LS
- QUAFIN LDM QUAFIN CT
- the treatment solution can first be prepared using metering means to mix the H 3 BO 3 or B 2 O 3 in the chosen solvent, in the desired proportions, in a container provided with a thermostatically-controlled heater and a mechanical mixer.
- the solution can then be heated to its use temperature in the range of 80° to 120°C, for example, and the hot solution transferred to a thermostatic vessel equipped with a level indicator.
- a boron salt is added to the solvent in a quantity sufficient to guarantee saturation of the solution upon heating, leaving a deposit of the undissolved salt in the bottom of the vessel.
- the anode to be treated is then dipped in the vessel, upside down, so its top and side surfaces are immersed in the hot solution. Immersion is continued for a set time, e.g. from 2-60 minutes, or until the level indicator has indicated a desired intake of the solution into the treated surfaces.
- the treated anode is then removed and dried.
- the vessel is then topped up to its initial level with hot solution from the container, ready for the treatment of another anode.
- the vapors produced in the described conditions are non toxic and can be freely released into the air without a need for costly treatment installations.
- the carbon consumption due to air-oxidation of anodes treated this way corresponds to about 12-15% with respect to the net consumption, which is comparable to what can be achieved with traditional aluminium protective coatings.
- the invention provides an excellent and long-lasting protective effect at much less cost and with less risk of imperfections in the protection than with aluminium coatings.
- the components of the treatment solution are inexpensive and are non-polluting both for the aluminium production process and for the environment.
- the method is simple to carry out, and the treated surfaces are uniformly impregnated with the boron compounds, leading to reliability in use because of uniform wear to the exposed surfaces of the anode or sidewall.
- boron acts as a "negative catalyst" it is possible to make the anode and sidewall from carbon powder containing a higher content of vanadium, thereby reducing the cost of raw materials .
Landscapes
- Electrolytic Production Of Metals (AREA)
Abstract
Description
Claims
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP94915259A EP0701635B1 (en) | 1993-06-02 | 1994-06-01 | Treating prebaked carbon anodes for aluminium production |
Applications Claiming Priority (8)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP93810401 | 1993-06-02 | ||
EP93810401 | 1993-06-02 | ||
EP93810545 | 1993-08-02 | ||
EP93810545 | 1993-08-02 | ||
US08/218,679 US5486278A (en) | 1993-06-02 | 1994-03-28 | Treating prebaked carbon components for aluminum production, the treated components thereof, and the components use in an electrolytic cell |
US218679 | 1994-03-28 | ||
EP94915259A EP0701635B1 (en) | 1993-06-02 | 1994-06-01 | Treating prebaked carbon anodes for aluminium production |
PCT/IB1994/000134 WO1994028200A1 (en) | 1993-06-02 | 1994-06-01 | Treating prebaked carbon anodes for aluminium production |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0701635A1 true EP0701635A1 (en) | 1996-03-20 |
EP0701635B1 EP0701635B1 (en) | 1998-05-06 |
Family
ID=27442733
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP94915259A Expired - Lifetime EP0701635B1 (en) | 1993-06-02 | 1994-06-01 | Treating prebaked carbon anodes for aluminium production |
Country Status (1)
Country | Link |
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EP (1) | EP0701635B1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN114016087A (en) * | 2021-10-28 | 2022-02-08 | 湖南国发控股有限公司 | Formula of efficient silicon-boron system impregnant and preparation and application processes thereof |
CN116815252A (en) * | 2023-06-29 | 2023-09-29 | 贵州晶垚无机材料有限公司 | Electrolytic prebaked anode anti-oxidation microcrystal protection slurry and preparation method thereof |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2021072548A1 (en) * | 2019-10-18 | 2021-04-22 | Laboratoire Cir Inc. | Process for drying anode coating |
-
1994
- 1994-06-01 EP EP94915259A patent/EP0701635B1/en not_active Expired - Lifetime
Non-Patent Citations (1)
Title |
---|
See references of WO9428200A1 * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN114016087A (en) * | 2021-10-28 | 2022-02-08 | 湖南国发控股有限公司 | Formula of efficient silicon-boron system impregnant and preparation and application processes thereof |
CN116815252A (en) * | 2023-06-29 | 2023-09-29 | 贵州晶垚无机材料有限公司 | Electrolytic prebaked anode anti-oxidation microcrystal protection slurry and preparation method thereof |
Also Published As
Publication number | Publication date |
---|---|
EP0701635B1 (en) | 1998-05-06 |
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