WO2010040231A1

WO2010040231A1 - Method of boron introduction in anodes for aluminium production

Info

Publication number: WO2010040231A1
Application number: PCT/CA2009/001446
Authority: WO
Inventors: Marie-Josée CHOLLIER
Original assignee: Alcan International Limited
Priority date: 2008-10-09
Filing date: 2009-10-09
Publication date: 2010-04-15
Also published as: AR073805A1; CA2682044A1; CA2641009A1

Abstract

A method for boron introduction into a carbon anode of an electrolytic cell for the production of aluminium in order to improve the resistance of the anode to deterioration during the electrolysis process via the attack of air and of oxidizing gases released at the anode and which comprises: baking a green carbon anode in a baking furnace, the green carbon anode being at least partially surrounded by packing coke having a boron containing compound in order to obtain a prebaked carbon anode that includes boron.

Description

METHOD OF BORON INTRODUCTION IN ANODES FOR ALUMINIUM PRODUCTION

TECHNICAL FIELD

This invention relates to a method of introducing boron into the anodes used in the production of aluminium. More particularly, it relates to a method of introducing boron into the anodes, during anode baking, a process in which packing coke is used to protect the anodes from oxidation.

PRIOR ART

Aluminium is commonly produced by the electrolysis of alumina dissolved in a bath of molten electrolyte based on cryolite at temperatures in the vicinity of 950⁰C (Hall-Heroult process). During electrolysis, prebaked anodes, composed of carbon, are consumed by electrochemical reactions by contact with the electrolyte and are oxidized by contact with air and/or any oxidizing gases present.

The prebaked anodes for the production of aluminium, i.e. the anodes used in the Hall-Heroult process, are obtained by molding a carbon paste and by baking at a temperature of around 1200⁰C. The baking is carried out in furnaces inside which air and combustion gases circulate. The anodes are completely embedded in a packing material, a granular or pulverulent material based on petroleum or metallurgical coke, also referred to as packing coke. The packing coke protects the anodes during baking against, inter alia, the oxidation that they could undergo due to the relatively high baking temperature.

As mentioned above, the anodes have a tendency to oxidize during the electrolysis of alumina, when they are in contact with air and/or oxidizing gases present, such as CO₂. Furthermore, the oxidation resistance of the anode and, particularly, of the portion of the anode which remains outside of the electrolyte bath, is important since this makes it possible to decrease anode consumption and to reduce the formation of carbon dust. Carbon dust is undesirable because it reduces efficiency and increases cell temperature. Several attempts aiming to protect anodes from oxidation have been tried. Inter alia, the use of boron compounds makes it possible to increase oxidation resistance. These attempts have demonstrated a beneficial effect on the reactivity of laboratory anodes and on plant anode consumption. For example, a gain ranging up to 6 wt% of anode consumption was observed during tests carried out with 0.5 wt% of boric acid (H₃BO₃) added directly to the carbon paste used in anode fabrication. However, this method of boron introduction may cause problems during the manufacture of the anodes, since the boron modifies the rheological properties of the paste and causes metal contamination. The application of a solution containing boric acid to an anode has also given good results. However, a strong odor coming from the product resulted in discomfort and potential industrial health issues for the operators. Boron has also been introduced into the anodes by dipping the latter in an aqueous solution of boric acid. SUMMARY OF THE INVENTION

A general aspect of the invention provides a carbon anode for an electrolytic cell having prebaked anodes for the production of aluminium, comprising boron introduced by gas-phase impregnation during baking by means of packing coke having a boron containing compound. According to a general aspect, a method is proposed for introducing boron into a carbon anode for the production of aluminium comprising: baking a green carbon anode in a baking furnace, the green carbon anode being at least partially surrounded by packing coke having a boron containing compound in order to obtain a prebaked carbon anode that includes boron. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a graph illustrating the variation in the weight of the anodes (kg) as a function of the type of anode.

DETAILED DESCRIPTION The invention provides a method of treating anodes for the production of aluminium and, particularly, by electrolysis of alumina in molten fluorinated electrolytes, such as cryolite, in order to improve their resistance to oxidation by the attack of oxidizing gases, including air. In order to reduce anode consumption, boron is introduced into the anodes by gas-phase impregnation during the process for baking green anodes, before they are introduced into the electrolysis cells. More particularly, the boron is introduced via a packing coke treated with boric acid.

The boron may be introduced into the packing coke by impregnating the latter with a solution of boric acid (H₃BO₃), diluted in water, and by subsequently drying the impregnated packing coke before introduction into the baking furnace. The boron may also be introduced by adding, via mechanical mixing at ambient temperature, milled powder or granular particles of boric acid to the packing coke particles. For example, milled boric acid may be added to the packing coke particles. With the powder or particles, the majority of the packing coke particles are coated with boric acid before being introduced into the baking furnace.

The packing coke, before it is introduced into the baking furnace, may comprise approximately between 0.2 and 10 wt% of boric acid or of a boron containing compound. Preferably, the packing coke comprises between approximately 2 and 8 wt% of boric acid or of a boron containing compound and, most preferably, the packing coke comprises approximately 6 wt% of boric acid or of a boron containing compound.

In an alternative embodiment of the invention, the boron content is reduced as much as possible and may be reduced so that the packing coke comprises a minimum effective amount between approximately 0.2 and 2 wt% of boric acid or of a boron containing compound. At high contents of boric acid or of boron containing compound, it is expected that the packing coke may stick to the anodes during the baking procedure.

The packing coke treated with boric acid is positioned around green anodes, that is to say before anode baking, in a baking furnace. In one embodiment, the green anodes are completely covered with packing coke treated with boric acid. The anodes are subsequently baked in a similar manner to the standard anode baking procedure over several days with packing coke, that is to say, preheated, subjected to baking at a temperature of around 1200⁰C and cooled. It is appreciated that the temperatures of the various stages, their duration and the rate of increase of the temperature may vary depending on the baking furnace used and the intrinsic properties of the anodes.

Table 1 , below, indicates the results of the tests, carried out in a laboratory, that measure the reactivity to air and to CO₂ of core samples of anodes baked in the presence of packing coke treated with boric acid. The core samples of the control anode were baked in the presence of packing coke substantially free of boric acid. A person skilled in the art will appreciate that the expression "substantially free" of boric acid or of a boron containing compound is understood to mean packing coke in which boric acid or another selected boron containing compound has not been deliberately introduced. For example, and without being limiting, packing coke having a content of less than generally 0.1 wt% up to 0.2 wt% of boric acid or of a boron containing compound is substantially free of boric acid or of a boron containing compound. Similarly, an anode that is "substantially free" of boron means an anode in which boron has not been deliberately introduced. The core samples of the anodes (B) to (D) were baked in the presence of packing coke treated with a variable amount of boric acid (PP). For these core samples, the packing coke was introduced via mechanical mixing of boric acid powder and of packing coke. The core samples of the anode (E) were also baked in the presence of packing coke treated with boric acid. For these core samples, the boric acid was introduced into the packing coke by the addition of a solution of boric acid (150 grams per liter) per 500 grams (g) of packing coke (PS). The packing coke was dried after the boric acid treatment and before introduction into the baking furnace.

For the tests carried out, the packing coke included particles smaller than 13 millimeters (mm). More particularly, 38 wt% of the particles had a diameter varying between 8 and 13 mm, 56 wt% of the particles had a diameter varying between 5 and 8 mm and 6 wt% of the particles had a diameter of less than 5 mm.

The boric acid was approximately 99.8 wt% pure and around 5 wt% of the boric acid particles had a dimension greater than 420 micrometers (μm), 72 wt% of the particles had a dimension varying between 150 and 420 μm, 19 wt% of the particles had a dimension varying between 75 and 150 μm, 3 wt% of the particles had a dimension varying between 45 and 75 μm and 1 wt% of the particles had a dimension of less than 45 μm.

Generally, the boric acid particles have a dimension less than 20 mesh (850 μm) and, preferably, the boric acid particles have a dimension less than 30 mesh (600 μm). Also, generally, the boric acid particles have a dimension greater than 270 mesh (53 μm) and, preferably, the boric acid particles have a dimension greater than 200 mesh (75 μm).

It is appreciated that the characteristics of the packing coke and of the boric acid may vary from those indicated above. Similarly, the concentration of boric acid for the introduction into the packing coke via a wet route may be different from that indicated above. The concentration of boric acid or of any other boron containing compound in the packing coke before baking may vary from those indicated above. In order to measure air oxidation, core samples of prebaked anodes, on exiting the baking furnace, were left in the open air whereas, in order to measure the CO2 reactivity, core samples of prebaked anodes were introduced into an environment containing almost exclusively CO₂. When exposed to air, the rate of oxidation of the various core samples of prebaked anodes was measured. It is observed that the rate of oxidation decreases when the core samples of prebaked anodes were baked with packing coke having a higher boric acid content, that is to say that the resulting core samples contained more boron. The lowest rate of oxidation was obtained for the core samples baked in the presence of packing coke in which boric acid had been introduced via a wet route, that is to say the core samples (E).

In table 1 , the "Dust" columns indicate the (weight) percentage of the original baked core samples which has come off as dust either following air oxidation or a reaction with CO₂. The "Residue" columns indicate the (weight) percentage of the original baked core samples remaining following air oxidation or a reaction with CO₂. Finally, the "Loss" columns indicate the (weight) percentage of the original baked core samples which have been oxidized and volatilized. The values of this column were obtained from the difference between the weight of the original baked core samples and the weight of the residual core samples and the dust.

When exposed to air or to CO₂, the residual percentage of the baked core samples containing boron (core samples (B) to (E)) is greater than the baked core samples substantially free of boron (A), that is to say that the reactivity of the anodes containing boron is lower than those substantially free of boron. While the air reactivity of the baked core samples decreased when they were baked surrounded by packing coke containing a higher amount of boric acid, the CO₂ reactivity of the core samples does not vary or varies very little as a function of the boric acid content of the packing coke. Thus, the laboratory tests have demonstrated that an anode baked in the presence of packing coke treated with 2 wt% boric acid, for example, has an oxidation rate six (6) times lower than that of the control anode, i.e. a reduction of 83 wt% in the air oxidation.

Tests were also carried out on an industrial scale. A section of an anode baking furnace was filled with packing coke treated with boric acid in order to obtain boron-treated anodes (BA). More particularly, 25 tons of packing coke were treated with boric acid. The resulting packing coke contained approximately 6 wt% of boron, in powder form, the particle dimension of which was less than 200 mesh (74 μm). Boric acid was introduced mechanically into the packing coke, that is to say via mechanical mixing. A section of the anode baking furnace was completely filled with the boron-treated packing coke. 128 anodes were baked with the treated packing coke.

Anodes substantially free of boric acid (REF) were baked in another section to avoid any boron contamination.

The boron-treated anodes (BA) and the reference anodes (REF) were placed randomly in a section of electrolytic cells also known as a potline. Anode consumption was evaluated from the weight of the cleaned spent anodes known as butts. Figure 1 shows the effect of boron on the variation in the weight of the anodes. The results show that the boron-treated prebaked anodes (BA) have a reduction of 7.5 kg in the consumption of carbon compared to the reference prebaked anodes (REF), which corresponds to a net drop in carbon consumption of 11.9 kg per ton of aluminium produced (2.6 wt%). Furthermore, no sticking of the anodes was observed during the anode baking process.

Table 1 : Results of the air and CO₂ reactivity tests of core samples of anodes baked in the presence of packing coke treated with boric acid.

With the method of introducing boron by gas-phase impregnation into the anodes by means of the packing coke described above, anode consumption is reduced.

The diffusion of boron, in the gaseous state, between the packing coke and the anodes is facilitated in the presence of fluorinated compounds.

Fluorinated compounds may originate from anode butts which contain aluminium fluoride and which are recycled into anodes to react with boric acid and form gaseous boron fluoride (BF₃ (g)) during baking.

It is appreciated that the boric acid in the packing coke, before its introduction into the baking furnace, may be replaced, completely or partly, by any other suitable boron containing compound. Therefore, the method of introducing the boron containing compound into the packing coke may be varied according to the nature of the boron containing compound introduced.

It goes without saying that the method and the boron-treated prebaked anodes described above are open to any modifications obvious to a person skilled in the art and that the invention is not restricted to the embodiments described above which are given purely by way of illustration.

Claims

1. Method for introducing boron into a carbon anode for the production of aluminium comprising: baking a green carbon anode in a baking furnace, the green carbon anode being at least partially surrounded by packing coke having a boron containing compound in order to obtain a prebaked carbon anode that includes boron.

2. Method according to Claim 1 , characterized in that the boron containing compound is boric acid (H₃BO₃).

3. Method according to either of Claims 1 and 2, which comprises introducing the boron containing compound into the packing coke via mechanical mixing.

4. Method according to Claim 3, characterized in that the boron containing compound is in the form of particles and the particle size is less than 20 mesh (850 μm).

5. Method according to Claim 3, characterized in that the boron containing compound is in the form of particles and the particle size is greater than 270 mesh (53 μm).

6. Method according to either of Claims 1 and 2, which comprises introducing the boron containing compound into the packing coke by immersion of the packing coke into a solution that includes boric acid; and drying the packing coke following the immersion.

7. Method according to Claim 2, characterized in that the packing coke, before baking, comprises approximately between 0.2 and 10 wt% of boric acid.

8. Method according to Claim 2, characterized in that the packing coke, before baking, comprises approximately between 0.2 and 2 wt% of boric acid.

9. Method according to Claim 2, characterized in that the packing coke, before baking, comprises approximately between 4 and 8 wt% of boric acid.

10. Method according to Claim 2, characterized in that the packing coke, before baking, comprises approximately 6 wt% of boric acid.

11. Method according to one of Claims 1 to 10, characterized in that the baking is carried out in the presence of fluorinated compounds.

12. Carbon anode for an electrolytic cell having prebaked anodes for the production of aluminium, comprising boron introduced by gas-phase impregnation during the baking by means of packing coke containing a boron compound.

13. Carbon anode according to Claim 12, characterized in that the boron containing compound is boric acid (H₃BO₃).

14. Carbon anode according to either of Claims 12 and 13, characterized in that the boron containing compound is introduced into the packing coke via mechanical mixing.

15. Carbon anode according to Claim 14, characterized in that the boron containing compound is in the form of particles.

16. Carbon anode according to Claim 15, characterized in that the particle size is less than 20 mesh (850 μm).

17. Carbon anode according to Claim 15, characterized in that the particle size is greater than 270 mesh (53 μm).

18. Carbon anode according to either of Claims 12 and 13, characterized in that the boron containing compound is introduced into the packing coke by immersion of the packing coke in a solution that includes boric acid and the packing coke is dried following the immersion, before the baking of the anode.

19. Carbon anode according to Claim 13, characterized in that the packing coke, before baking, comprises approximately between 0.2 and 10 wt% of boric acid.

20. Carbon anode according to Claim 13, characterized in that the packing coke, before baking, comprises approximately between 0.2 and 2 wt% of boric acid.

21. Carbon anode according to Claim 13, characterized in that the packing coke, before baking, comprises approximately between 4 and 8 wt% of boric acid.

22. Carbon anode according to Claim 13, characterized in that the packing coke, before baking, comprises approximately 6 wt% of boric acid.