EP1436446A2

EP1436446A2 - Coating precursor and method for coating a substrate with a refractory layer

Info

Publication number: EP1436446A2
Application number: EP02785544A
Authority: EP
Inventors: Airy-Pierre Lamaze; Christian Barthelemy
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 2001-10-15
Filing date: 2002-10-11
Publication date: 2004-07-14
Also published as: FR2830856B1; CA2463656A1; FR2830856A1; WO2003033767A2; RU2004114883A; US20040197482A1; ZA200403289B; WO2003033767A3; RU2293797C2

Abstract

The invention concerns a coating precursor comprising a silicone resin, a metal compound and an organic solvent capable of dissolving said silicone and of suspending said metal compound, said silicone resin and said metal compound being capable of chemically reacting so as to produce a solid layer on a substrate after the organic solvent has evaporated and a cohesive refractory layer after a calcination process. The invention also concerns a method for coating a specific surface of a substrate with at least a refractory silicon-containing layer which consists in coating the substrate with a coating precursor of the invention, so as to form a raw layer and carrying out a heat treatment so as to calcine said raw layer and form a cohesive refractory layer. The invention enables to obtain a protective coating resistant to oxidizing environments, liquid metal or molten salt.

Description

COATING PRECURSOR AND METHOD FOR COATING A SUBSTRATE WITH A REFRACTORY LAYER

Field of the invention

The present invention relates to the protection of objects and materials intended for the production of aluminum by electrolysis in molten salt, in particular according to the Hall-Héroult process. It relates in particular to the protective coatings of said objects and materials.

State of the art

Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a bath based on molten cryolite, called electrolyte bath, in particular according to the well-known Hall-Héroult process. The electrolyte bath is typically contained in cells, called "electrolysis cells", comprising a steel box, which is coated internally with refractory and / or insulating materials, and a cathode assembly normally located at the bottom of the cell. The cathode assembly typically comprises precooked cathode blocks of carbonaceous material. Anodes are partially immersed in the electrolyte bath. The expression “electrolysis cell” normally designates the assembly comprising an electrolysis tank and one or more anodes.

Objects and materials that are used in the aluminum industry are often exposed to corrosive environments and subjected to high temperatures and significant thermal and mechanical stresses. This is the case in particular of the elements of an aluminum production cell by electrolysis which are exposed to the corrosive action of gaseous effluents (which may contain oxygen, carbon monoxide and / or fluorinated gases), from the liquid metal to very high temperature (typically up to around 1000 ° C) and / or a molten salt (typically molten cryolite). These elements include, in particular, the anodes, anode rods, internal coatings of the tanks, the brickwork and the cathode blocks.

Although the resistance of the materials commonly used in the aluminum industry is generally sufficient, there are certain applications or conditions for which an even greater resistance is sought. This is the case in particular when it is sought to reduce the wear of the cathodes containing graphite.

The Applicant has therefore sought means to increase the chemical, and possibly mechanical, resistance of the electrolytic cell elements.

Description of the invention

The subject of the invention is a coating precursor comprising a silicone resin (or organosiloxane), a mineral filler and an organic solvent capable of dissolving said resin and of suspending said mineral filler, said silicone resin and said mineral filler being capable of chemically reacting so as to produce a solid layer on a substrate after evaporation of the organic solvent and a cohesive refractory layer after a calcination operation.

Said precursor, which is typically in the form of a suspension or a slip, is preferably homogeneous. It is typically obtained by mixing the resin, the mineral filler and the organic solvent.

The silicone resin is a polysiloxane preferably comprising a proportion of OH groups, such as a polymethylsiloxane, a polydimethylsiloxane, a polymethylsilsesquioxane, or a mixture of these, comprising a proportion of OH groups substituted for the methyl groups. The Applicant has noted that the proportion of OH groups is preferably between approximately 0.5% and approximately 2%. Too low a proportion of OH groups does not confer a sufficient propensity to form a solid layer after evaporation of the solvent and with high cohesiveness after calcination. A very high proportion of OH groups can make polysiloxane difficult to produce at an acceptable cost. The silanol groups (Si-OH) are preferably stable in order to allow storage of the resin. These OH groups can be grafted to a polysiloxane by hydrolysis. The siloxane units of the polysiloxane according to the invention are advantageously, in whole or in part, tri- or quadri-functional.

The proportion of silicone resin in the precursor is typically between 5 and 30% by weight, and preferably between 7.5 and 20% by weight, in order to allow satisfactory ceramization of the coating during calcination. Excluding solvent, the proportion of silicone resin in the precursor is typically between 15 and 40% by weight.

The organic solvent is typically an apolar solvent, such as xylene or toluene. Xylene can be a mixture of different types of xylene, such as o and p. The proportion of solvent in the precursor is typically between 20 and 60% by weight, and more typically between 30% and 55% by weight.

The mineral filler is typically chosen from borides, carbides, nitrides and metal oxides or from borides, carbides and non-metal nitrides (such as boron nitrides and boron carbides (BC ,. ..)), or a combination or mixture thereof. Said mineral filler is advantageously chosen from metal compounds such as metal oxides, metal carbides, metal borides and metal nitrides, or a combination or a mixture of these. The mineral filler is preferably able to react chemically with the silicone resin so as to produce a solid layer after evaporation of the organic solvent and a refractory layer with high cohesiveness after calcination of said raw layer.

The metal compound is advantageously alumina, ZrO ₂ , ZrB ₂ , TiB ₂ or TiO ₂ or a combination or a mixture of these. The alumina is preferably a reactive calcined alpha alumina, called technical alumina, the hydration rate of which is very low (typically less than 1%, or even less than 0.5%). The proportion of mineral filler in the precursor is typically between 30% and 55% by weight. Too small a proportion leads to too fine a deposition and consequently requires the deposition of a large number of successive layers. Too large a proportion gives a precursor which is difficult to spread.

The mineral filler is preferably in the form of a fine powder, which makes it possible to obtain a fluid precursor and a uniform coating. It is typically added to the silicone resin / organic solvent mixture after a fine grinding operation. The particle size of the mineral filler powder is typically such that the grain size is between 0.05 μm and 5 μ.

The subject of the invention is also a method for coating a determined surface with a substrate of at least one refractory layer containing silicon in which: - the substrate is coated with a coating precursor according to the invention, so as to form a raw layer;

- Performing a heat treatment, called calcination, capable of causing the elimination of volatile matter, the calcination of said raw layer and the formation of a cohesive refractory layer.

The Applicant has observed that the method of the invention makes it possible to obtain a thin, resistant layer which is strongly adherent to the substrate which is resistant to liquid metal and / or to oxidation and which has a high cohesiveness.

The amount of said organic solvent is preferably such that all of the silicone resin is dissolved and that the solution obtained is capable of suspending the charge of mineral filler.

The coating precursor can be prepared in at least two operations: - a silicone resin is dissolved in an organic solvent, so as to obtain a silicone resin solution;

- the mineral filler is added to the silicone resin solution thus obtained. The coating of the substrate (which typically comprises the deposition and spreading of said precursor on the substrate) can be carried out by any known means. For example, the coating can be deposited by brushing (typically using a brush and / or a roller), by soaking, spraying or spraying (typically using a gun). The substrate can optionally be brought to a temperature above ambient before coating in order to promote the formation of a homogeneous deposit and the adhesion of the deposit by melting the resin.

The method according to the invention can also include complementary operations, such as preparing the parts of the surface of the substrate that it is desired to coat and / or drying the raw coating before the heat treatment. Said drying is used in particular to evaporate said organic solvent and to solidify, at least partially, the raw layer (so as to be able to handle the substrate without damaging the layer). The preparation of the surface of the substrate typically includes cleaning and / or degreasing (for example using acetone).

In certain applications, it may be advantageous to use a coating precursor further containing a wetting agent capable of promoting the formation of a thin layer. Said wetting agent is preferably a polyether silane, which promotes spreading of the coating on the substrate without preventing the ceramization of the refractory coating during the heat treatment. The chemical formula of said polyether silane is typically:

/ O - R CH ₃ - O - (CH ₂ - CH ₂ - O -) ₁₀ - CH ₂ - CH ₂ - CH ₂ - Si - O - R

\ O - R where R is an alkyl group, typically methyl.

Advantageously, the wetting agent also makes it possible to avoid or substantially delay the setting in solid of the precursor. The proportion of wetting agent in the precursor is typically between 1 and 5% by weight approximately, and preferably between 2 and 3% by weight, relative to the proportion of mineral filler. Relative to the total weight of the precursor, the proportion of wetting agent in the precursor is typically between 0.5 and 5%, and preferably between 1 and 3%, by weight.

The so-called calcination heat treatment comprises at least one step at an elevated temperature, which is typically between 800 and 1300 ° C., capable of transforming the raw layer into a refractory ceramic, which is advantageously in the vitreous state. The composition of the glassy phase typically comprises between 5 and 25% by weight of silica obtained from the resin (the remainder, typically 75 to 95% by weight, essentially consists of the mineral filler). The calcination temperature also depends on the substrate; for example, in the case of a metal substrate, it is advantageously lower than the softening temperature thereof. On the other hand, it is also preferable to use a calcination temperature higher than the temperature of use of the coated substrate. The heat treatment may include an intermediate step at a temperature between 200 and 600 ° C (typically between 200 and 250 ° C). This intermediate step is preferably capable of causing the crosslinking of the resin and, optionally, the decomposition of the latter before the "ceramization" (or final calcination) of the coating. In this case, it is possible, according to an advantageous variant of the invention, to continue the heat treatment of calcination in situ, that is to say when using the substrate at high temperature (this temperature preferably being higher than 650 ° C).

The duration of the heat treatment is preferably such that it allows complete ceramization of the precursor. The rise in temperature is preferably slow enough to avoid cracking of the coating.

During the heat treatment, the organic compounds are removed (by evaporation and / or by decomposition), leaving a refractory solid on a surface of the substrate. This solid is for example formed from the metal originating from the compound of metal and silicon from the silicone resin. In the case of alumina, the Si-OH silanol groups of the polysiloxane seem to establish covalent bonds with the OH groups of the alumina, which bonds seem to transform into Si-O-Al bonds, with evolution of water, during heat treatment, to form an alumino-silicate, which is advantageously in the vitreous state. A similar mechanism could occur with metal compounds other than alumina.

The ambient atmosphere during the calcination treatment is advantageously non-oxidizing, in order to avoid in particular an oxidation of the substrate at the substrate / coating interface liable to cause decohesion between the substrate and the coating, or even the destruction of the substrate ( for example when it is made of graphite).

The final coating can comprise two or more successive layers, which can be applied by successive coatings and heat treatments, i.e. by successive coating / heat treatment sequences. In other words, the coating and calcination treatment operations of the layer are repeated for each elementary layer of the final coating. The successive layers may have a different composition, so as to give them different chemical and mechanical properties. This last variant makes it possible to adapt each layer to a local function, such as the adhesion to the substrate for the first layer, the mechanical resistance for the intermediate layers and the chemical resistance for the surface layer.

The substrate can be metal, refractory material or carbonaceous material, or a mixture or combination thereof. The substrate can be a molten salt electrolysis cell element for the production of aluminum.

The subject of the invention is also a molten salt electrolysis cell element for the production of aluminum, at least part of the surface of which comprises at least one refractory layer obtained by using said precursor or by using said coating process, which refractory layer is advantageously in the state vitreous, with or without a composition gradient in the direction perpendicular to the surface of the substrate.

The invention also relates to the use of said precursor or of said coating process for the protection of a material and / or of an electrolysis cell element in molten salt for the production of aluminum.

The molten salt electrolysis cell element for the production of aluminum can be metal, refractory material or carbonaceous material (such as graphite), or a mixture or combination thereof; it can be a particular object, in particular an anode made of carbonaceous material, an anode support element (such as an anode rod or an anodic log), an element or part of an electrolytic cell (such as '' a box or a box gunwale), an element for coating an electrolytic cell (such as a refractory brick or a brazing element), a cathode block made of carbon material or a mixture of carbon materials (such as a cathode block containing, in whole or in part, graphite). The substrate can be porous or non-porous.

The invention also relates to a molten salt electrolysis cell for the production of aluminum comprising at least one material and / or one element according to the invention.

testing

Trial 1

This test focused on graphite blocks of approximately 50 x 15 x 15 mm.

A slip was prepared with the following composition:

- mineral filler (a metal compound): 44.9% by weight of a TiB ₂ powder (reference Metabap 143) having a D50 of 1.7 μm;

- silicone resin: 14% by weight of a polymethylsiloxane MK from the company Wacker, which is a tri-functional resin with approximately 1% of OH groups. This resin was composed of approximately 80% of silica equivalent and 20% of methyl groups, which decompose at a temperature of the order of 450 ° C;

- organic solvent: 39.8% by weight of xylene;

- wetting agent: 1.35% by weight of polysilane Dynasylan ^® 4140 from Degussa-Hüls company (about 3% by weight relative to the quantity of TiB ₂ in all cases).

These proportions were such that the refractory coating obtained comprised about 80% by weight of equivalent of the metal compound and 20% by weight of equivalent of silica. The concentration of silicone resin in xylene was approximately 250 g / l.

The xylene was mixed so as to obtain a homogeneous mixture. The silicone resin was dissolved at room temperature in this organic solvent until a homogeneous solution was obtained. The wetting agent was then added to this solution. After a ripening time of 10 minutes, the charge was added to this solution and mixed (by stirring) until a homogeneous suspension was obtained.

Two graphite blocks (block 1 and block 2) were covered with a brush with two successive layers of the slip thus obtained. The blocks were dried at 100 ° C after each deposition.

Two other blocks of graphite (block 3 and block 4) were covered with a brush with two successive layers of the same slip. The blocks underwent a calcination operation at 900 ° C., under argon, after each deposition.

These four blocks (blocks n ° 1 to 4) and an uncoated control block (block n ° 5) underwent an oxidation resistance test consisting in bringing them to a temperature of 720 ° C in the presence of air for 48 hours. At the end of this test, the control block (n ° 5) was reduced to ash; blocks 1 and 2 had lost 70% of their weight and blocks 3 and 4 had lost 3.5 and 8% of their weight, respectively. A close examination of these last two blocks revealed that, for block 3, weight loss was associated at two points of approximately 1 mm in diameter where the surface was not coated, and that, for block no. 4, the weight loss was associated with an absence of deposit in one of the corners of the block. The coatings of blocks 3 and 4 therefore provide excellent protection against oxidation which the Applicant attributes to the formation of a protective refractory layer during the calcination operation.

Trial 2

This test involved stainless steel blades of approximately 1 x 12 x 20 mm.

A slip was prepared, following the same procedure as for test 1, with the following composition:

- metal compound charge: 44.9% by weight of a calcined alpha alumina powder (technical alumina reference P172SB from the company Aluminum Pechiney) having a D50 of 0.5 μm and a BET specific surface of 6 to 8 m ² / g. The alumina was finely ground (particle size typically between 0.2 μm and 1.5 μm);

- silicone resin: 14% by weight of a polymethylsiloxane MK from the company Wacker, which is a tri-functional resin with approximately 1% of OH groups. This resin was composed of approximately 80% of silica equivalent and 20% of methyl groups, which decompose at a temperature of the order of 450 ° C; - organic solvent: 39.8% by weight of xylene;

- wetting agent: 1.35% by weight of polysilane Dynasylan ^® 4140 from Degussa-Huls (about 3% by weight relative to the quantity of TiB in all cases).

A blade was covered with four successive layers of the slip thus obtained. The slide underwent a calcination operation at 900 ° C. after each deposition.

The coated slide and an uncoated control slide were tested by immersion for 8 hours in a flow of liquid aluminum at about 750 ° C. The coated blade was hardly attacked by the liquid metal while the uncoated blade largely dissolved in the liquid metal.

Claims

1. Coating precursor comprising a silicone resin, an inorganic filler and an organic solvent capable of dissolving said resin and of suspending the inorganic filler, the silicone resin and the inorganic filler being capable of reacting chemically so as to produce a solid layer on a substrate after evaporation of the organic solvent and a cohesive refractory layer after a calcination operation.

2. Coating precursor according to claim 1, characterized in that the siloxane units of the silicone resin include tri- or quadri-functional units.

3. coating precursor according to claim 1 or 2, characterized in that the silicone resin is a polysiloxane comprising a proportion of groups

OH.

4. coating precursor according to claim 3, characterized in that said polysiloxane is a polymethylsiloxane, a polydimethylsiloxane, a polymethylsilsesquioxane, or a mixture thereof, comprising a proportion of OH groups substituted for methyl groups.

5. Coating precursor according to claim 3 or 4, characterized in that the proportion of OH groups is between approximately 0.5% and approximately 2%.

6. Coating precursor according to any one of claims 1 to 5, characterized in that said organic solvent is apolar.

7. Precursor according to claim 6, characterized in that the apolar organic solvent is a xylene or a toluene.

8. Precursor according to any one of claims 1 to 7, characterized in that the proportion of solvent in the precursor is between 20% and 60% by weight.

9. Precursor according to any one of claims 1 to 8, characterized in that the proportion of mineral filler in the precursor is between 30% and 55% by weight.

10. coating precursor according to any one of claims 1 to 9, characterized in that the mineral filler is chosen from metal oxides, metal and non-metal carbides, metal and non-metal borides and metal and non-metal nitrides, or a combination or mixture thereof.

11. Coating precursor according to claim 10, characterized in that the mineral filler comprises a calcined alpha alumina.

12. coating precursor according to claim 10 or 11, characterized in that the mineral filler is chosen from the group comprising ZrO ₂ , ZrB ₂ , TiB, Ti0, boron nitride, boron carbide, and a mixture or a combination of them.

13. Coating precursor according to any one of claims 1 to 12, characterized in that the mineral filler is in the form of a fine powder whose grain size is between 0.05 μm and 5 μm.

14. Precursor according to any one of claims 1 to 13, characterized in that the proportion of silicone resin in the precursor is between 5% and 30% by weight, and preferably between 7.5 and 20% by weight.

15. Coating precursor according to any one of claims 1 to 14, characterized in that said coating precursor further contains a wetting agent capable of promoting the formation of a thin layer.

16. Coating precursor according to claim 15, characterized in that said wetting agent is a polyether silane.

17. coating precursor according to one of claims 15 or 16, characterized in that the proportion of wetting agent in the precursor is between approximately 0.5 and 5%, and preferably between 1 and 3%, by weight .

18. Precursor according to any one of claims 1 to 17, characterized in that it is in the form of a suspension or slip.

19. Method for coating a determined surface of a substrate with at least one refractory layer containing silicon in which:

- Coating said surface with a coating precursor according to any one of claims 1 to 18, so as to form a green layer;

20. The method of claim 19, wherein there is further carried out a preparation of the substrate surface before coating, such as cleaning and / or degreasing.

21. The method of claim 19 or 20, wherein the coating is carried out by brushing, dipping, spraying or spraying.

22. Method according to any one of claims 19 to 21, in which the substrate is brought to a temperature above ambient before coating.

23. Method according to any one of claims 19 or 22, in which said green layer is dried before said calcination treatment.

24. Method according to any one of claims 19 to 23, wherein said calcination treatment comprises at least one step at a temperature between 800 and 1300 ° C capable of transforming the raw layer into a refractory ceramic.

25. A method according to any one of claims 19 to 24, wherein said calcination treatment is carried out in a non-oxidizing atmosphere.

26. Method according to any one of claims 19 to 25, in which said refractory layer is formed by several successive layers.

27. A method according to any one of claims 19 to 26, characterized in that said substrate is made of metal, of refractory material or of carbonaceous material, or a mixture or a combination of these.

28. The method as claimed in any one of claims 19 to 27, in which said substrate is a molten salt electrolysis cell element for the production of aluminum.

29. The method of claim 28, wherein said element is an anode of carbonaceous material, a support element of an anode, a coating element of an electrolytic cell and / or a cathode block of carbonaceous material.

30. Use of the precursor according to any one of claims 1 to 18 or of the method according to any one of claims 19 to 26 for the protection of a material and / or an element of electrolysis cell in molten salt for the production of aluminum.

31. Use according to claim 30, characterized in that said material is a metal, a refractory or a carbonaceous material, or a mixture or a combination of these.

32. Use according to claim 30, characterized in that said element is an anode of carbonaceous material, a support element of an anode, an element or part of an electrolytic cell, a coating element of a cell of electrolysis and / or a cathode block of carbonaceous material.

33. Element of electrolysis cell in molten salt for the production of aluminum, characterized in that at least a part of the surface comprises at least one refractory layer obtained by using a precursor according to any one of claims 1 to 18 or by using the method according to any one of claims 19 to 26.

34. Element according to claim 33, characterized in that it is made of metal, of refractory material or of carbonaceous material, or a mixture or a combination of these.

35. Element according to claim 33, characterized in that it is chosen from the group comprising anodes made of carbonaceous material, the support elements of an anode, the elements or parts of an electrolysis cell, the coating elements of '' an electrolysis tank and cathode blocks made of carbonaceous material or a mixture of carbonaceous materials.

36. Element according to claim 35, characterized in that the said anode support elements are chosen from the group comprising the anode rods and the anode logs.

37. Element according to claim 35, characterized in that the said elements or parts of electrolytic cell are chosen from the group comprising the caissons and the box gunwales.

38. Element according to claim 35, characterized in that the said coating elements are chosen from the group comprising the refractory bricks and the brazing elements.

39. Element according to claim 35, characterized in that said cathode blocks contain graphite.

40. Molten salt electrolysis cell for the production of aluminum comprising at least one element according to any one of claims 33 to 39.