JPH0225877B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to carbonaceous products such as carbon electrodes or graphite electrodes, especially those coated with an antioxidant coating,
It relates to a method for producing carbonaceous products that are protected against oxidation and erosion at high temperatures. In a more specific aspect, the invention is directed to improved oxidation protection of carbon or graphite electrodes at high temperatures in steelmaking electric arc furnaces. Carbonaceous products are widely used today in the chemical and metallurgical industries because they exhibit special properties at high temperatures. For example, graphite electrodes are used in electric arc furnaces. This is because graphite electrodes are excellent conductors in the high temperatures of furnaces, and also exhibit excellent mechanical strength and excellent heat resistance. However, a major drawback of carbonaceous products is that they rapidly carbonize and erode at high temperatures, above about 500°C. This problem is particularly acute in the case of graphite electrodes for electric arc furnaces. As such electrodes oxidize and erode, they must be replaced periodically, which has a significant impact on the final cost of the steel. It has been proposed to use antioxidant coatings on the surface of carbonaceous products, especially carbon or graphite electrodes. These protective coatings are composed of carbides, silicides and metals, for example. These coatings are heated by electric arc heating,
It has been applied onto the surface of the product using various known methods such as flame spray heating or vacuum heating.
However, a problem with these protective coatings has been their inability to withstand severe thermal shock. For example, a coated graphite electrode has approximately 1200
This condition occurs when the furnace is heated to a high furnace temperature of 0.degree. C. or higher and then cooled rapidly. Protective coatings can crack and even spall under these harsh conditions, thus exposing the graphite matrix to oxidative attack. The main cause of such failure of the protective coating is that the coefficient of thermal expansion of the coating does not match that of the graphite matrix and that the coating does not adhere well to the graphite. In this way, the protective coating and the graphite matrix expand and contract at different rates, thereby creating high internal stresses within the coating and at the coating/graphite interface. GB 1 166 429 discloses shaped carbon or graphite bodies with an antioxidant coating. The coating includes a primer layer applied to the coating and a metal surface layer applied to the primer layer. Primer layer is 90~
It consists of 100% by weight of silicon and 0 to 10% by weight of one or more of sodium, nitrogen, magnesium, calcium, boron and aluminum. The metal surface layer consists of 85-100% by weight aluminum and 0-15% by weight of one or more metals or compounds including, for example, sodium, magnesium, boron, silicon and phosphorous. The primer layer and the metal surface layer are applied to the carbon or graphite body by flame spraying techniques. The metal surface layer melts or softens around the operating temperature of the electrode, sealing pores created during its application within the coating. However, the disadvantage of this double layer coating is that the silicon metal-containing primer layer is difficult to apply, expensive, and does not easily adhere to graphite substrates. moreover,
Low temperature oxidation products in the coating are subject to spalling and erosion. US Pat. No. 3,140,193 (Kane) discloses a graphite electrode with a double layer protective coating.
The inner layer of the coating consists of porous silicon carbide and the outer layer consists of metallic silicon. This protective coating has the same limitations because the inner layer does not adhere to the graphite. It thus avoids the problems of the prior art, provides very effective protection against oxidation even in the most severe or abusive conditions, is inexpensive, and is compatible with large-scale production. There is a need in the industry for improved antioxidant coatings for use in carbonaceous products, particularly carbon or graphite electrodes. According to the invention, a method for producing a carbonaceous product protected against oxidation and erosion at elevated temperatures comprises: (a) 30% by weight relative to the total weight of dry ingredients;
75% to 75% by weight mullite, up to 12% by weight silica, 15 to 60% by weight aluminum, 5 to 11
applying a particulate mixture comprising % by weight of a silicon-containing binder to the surface of a carbonaceous substrate; (b) heating the particulate mixture to a temperature at which the silica is reduced by aluminum to form aluminum oxide and silicon; A method is provided that includes the steps of: The antioxidant coating of ceramic refractory material thus applied to the surface of the carbonaceous substrate is compliant and porous, so that the coating has a coefficient of thermal expansion between the coating layer and the carbonaceous substrate. Absorbs most of the high internal stress caused by the difference. Furthermore, since ceramic refractory materials have low strength,
Since its tensile strength is less than the shear force at the bond between the coating layer and the substrate, the coating relieves the very high internal stress by creating microcracks within its structure, and the coating layer/substrate interface It is possible to prevent definitive destruction in Preferably as a primer layer or underlayer,
Using the same flexible porous low strength coating as described above, a dual layer antioxidant coating is applied to the surface of the carbonaceous product. This lower layer (base
A cover layer is placed over and bonded to the top layer, and the top layer is made of a rigid glassy material that becomes plastic and flows at temperatures above about 750°C. Optionally, the method of the invention can include the additional step of forming a rigid vitreous cover layer. (c) applying to the granular material of step (a), before or after step (b), a second granular material containing a glassy material having a melting point of about 750°C or higher; (d) heating the second particulate material to a high enough temperature to melt or soften the glassy material; and (e) cooling the glassy material to form a rigid glassy top layer. . Preferably, the first particulate material is applied to the carbonaceous product in the form of an aqueous slurry and then dried under conditions that result in a continuous layer that adheres to the surface of the product. A second particulate material, also in the form of an aqueous slurry, is then applied to this continuous layer and dried under conditions that result in a second continuous layer that is adhered to the first continuous layer. The coated article is then heated to a high temperature to simultaneously cure the coating composition in the first and second layers. Mullite (3Al ₂ O ₃ .2SiO ₂ ) is a very well known ceramic material. It is widely used in the ceramics industry for various applications. Mullite consists of about 1300 clay, feldspar, kyanite, etc.
Produced by heating to temperatures above ℃.
Methods for making mullite are disclosed in US Pat. Nos. 3,857,923 and 3,922,333. The invention applies to the surface of a carbonaceous product a granular material containing a mixture of mullite, silica, aluminum or aluminum alloy and a silicon-containing binder, such as sodium silicate;
It is based on the discovery that a flexible, porous, low strength ceramic refractory coating can be bonded to the surface of a carbonaceous product by subsequently heating this particulate material to high temperatures of about 1000°C or higher. At this heating temperature, the aluminum reacts with the silica to form particles of aluminum oxide and silicon. Aluminum also decomposes the silicon-containing binder, creating a large number of empty spaces between the particles. This greatly aids the porosity of the coating. Furthermore, during the reaction, the particles of mullite, aluminum oxide and metallic silicon are weakly sintered with each other, reducing the strength and stiffness of the coating. It has also been unexpectedly discovered that mullite, silica, aluminum or aluminum alloys interact at high temperatures to produce unique fibrous ceramic materials. These fibers are formed as agglomerates located within the interparticle void spaces and greatly contribute to the low stiffness and flexibility properties of the coating. It has also been discovered that these fibers form within the pores of the graphite matrix and form strong bonds at the coating/substrate interface. In carrying out this invention, mullite, silica,
The coating composition is created by mixing together particles of aluminum and a silicon-containing binder, such as sodium silicate. The coating composition can include about 30-75% by weight mullite, up to 12% by weight silica, about 15-60% by weight aluminum, and about 5-11% by weight silicon-containing binder. Sufficient water is added to the mixture to form a slurry having a consistency that allows the coating composition to be evenly applied to the surface of the carbonaceous article by brushing, spraying, or similar means. The amount of aluminum in the coating composition used to form the lower layer of the dual layer coating is preferably a dry mixture (e.g. mullite,
silica, aluminum and solid sodium silicate). If the amount of aluminum is below this required range, the coating will not adhere to the graphite substrate during high temperature heating cycles, resulting in spallations.
On the other hand, if the coating composition contains an amount of aluminum above this range, in the case of a dual layer coating, the silicon carbide containing upper layer will tend to coat the lower layer non-uniformly. The particle sizes of the various mixture components used in the coating composition are not strictly critical. Generally, the mullite, silica and aluminum particles are about 5
kept within the range of ~350 microns. For example, primarily -74 to +44 micron commercial grade aluminum powder can be used. The silicon-containing binder is preferably sodium silicate, although other materials such as potassium silicate, ethyl silicate and colloidal silica can also be used. Sodium silicate is used as a dry powder,
Or it can be used as a liquid, for example water glass. The slurry is applied to the carbonaceous product using conventional methods such as brushing or spraying.
Care must be taken to cover the entire area to be protected with a relatively thick, moist, even granular layer. This layer is suitably dried at room temperature for about 1-2 hours. The layer can also be dried under accelerated conditions at moderately high temperatures, for example in a drying oven. If necessary, the slurry can be reapplied in a similar manner to the dried first layer to increase the thickness of the underlying layer. Preferably, the dry layer thickness will be in the range of about 0.020 to 0.100 inches. After drying is complete, the product is heated to an elevated temperature of about 1000° C. or higher, if desired, to cure the composition into a single layer coating according to the invention. This single layer coating typically provides excellent protection for carbonaceous products in moderately oxidizing conditions. However, for optimal oxidation protection, it is preferred to use a double layer coating on the surface of the carbonaceous product. This double layer coating is preferably applied to the lower layer after drying.
It is obtained by applying a top layer consisting of a rigid vitreous material with a low melting point of 750°C. At such temperatures, the top glass layer becomes plastic and fluid, sealing microcracks that form in the bottom layer during heating cycles. At the same time, the lower layer is flexible with a low elastic modulus due to the porosity and weak sintering between the particles. As a result, the underlayer is able to absorb high internal stresses caused by the difference in thermal expansion coefficients between the coating and the graphite matrix. Furthermore, the tensile strength of the bottom layer is much smaller than the shear force at the interface between the bottom layer and graphite. Therefore, the underlying layer is less strong than the adhesion, and if exceptionally high stresses occur, the underlying layer will lose tension by microcracking before the coating detaches from the interface or fractures. . Microcracks that occur in the lower layer are sealed by the glassy fine material, if necessary by heating again to a high temperature. Various vitreous materials can be used to produce the top layer according to the invention. The top layer can be made from other types of coating compositions, such as those made by mixing particles of mullite, silica, silicon carbide or boron carbide, or both, and a silicon-containing binder such as sodium silicate. This coating composition contains up to 70% by weight mullite, up to 14% by weight silica,
about 5-95% by weight metal carbide and about 5-14% by weight
and a silicon-containing binder. Preferably, silicon carbide is used as the metal carbide in the coating composition. As before, sufficient water is added to the mixture to form a slurry having a consistency that allows uniform application of the coating composition to the underlying dry surface by brushing, spraying, or similar methods. Added. The silicon carbide or boron carbide used in the coating composition is used in an amount of at least about 5% of the amount of the mixture. If less than this amount is used, the glassy upper layer will become flaky and will not stick to the lower layer. The particle sizes of the mixture components used in this coating composition vary, but are preferably kept within the range of, for example, about 5 to 350 microns.
The silicon carbide particles can be commercial grade SiC powder of 74 microns or less. Again, the silicon-containing binder is preferably sodium silicate, although other materials such as potassium silicate, ethyl silicate or colloidal silica can also be used. Sodium silicate can be used as a dry powder or as a liquid, for example as water glass. The slurry is applied to the dried but uncured subsurface using conventional methods such as brushing or spraying. Care is taken to cover all lower surfaces with a relatively thick, moist, uniform layer of berries. This layer is then dried to remove moisture, resulting in a hard continuous layer that is adhered to the underlying layer. This layer can be dried at ambient temperature or in a drying oven at moderately high temperatures. Again, if necessary, a second application of slurry can be carried out on the dried first layer to increase its thickness. Preferably the thickness of the dry top layer is maintained within the range of about 0.25 to 1.27 mm (0.010 to 0.050 inch). The double coated product is then heated to a high temperature of about 1000° C. or higher to simultaneously fire the bottom layer and the glassy top layer. Optionally, the bottom layer can be fired prior to application and curing of the top layer. The upper and lower layers are chemically bonded to each other and adhere very well. When sodium silicate is used as a binder, it is decomposed by the aluminum in the lower layer and most of the sodium migrates to the upper layer,
There it acts as a glass forming agent. The rigid vitreous top layer made as described above has a melting point of about 1100° C. or higher, which is satisfactory for many applications. However, in a few applications, such as melting furnace electrodes, it is desirable to have a vitreous top layer with a much lower melting point, for example around 750°C. For example, in the case of melting furnace electrodes, when the electrode is cycled and rapidly cooled, the carbonaceous matrix is exposed to oxidative action through the many microcracks that are generated in the coating by thermal stresses. Such microcracks are sealed to prevent oxidation only when the glassy top layer reaches its melting point and becomes plastic/flowable. If the melting point of the top layer is too high, oxidation protection will not be supported at the beginning of the next cycle, when the electrode is heated again to steelmaking temperature. A rigid vitreous top layer with a significantly lower melting point
For example, it can be made by using boron-containing compounds such as boron oxide B ₂ O ₃ in the coating. Glass frits, such as borosilicate frits, are an excellent choice for this purpose. The main components of borosilicate frit are Na ₂ O,
Contains B ₂ O ₃ and SiO ₂ (G. W. Moray in Glass Technology Association Journal 35, 270 (1951), Na ₂ O−
_{Other oxides} found in this _borosilicate frit are
Contains CaO, _MgO and _Al2O3 . A variety of borosilicate frits are commercially available. Frits manufactured by Ferro Corp in the practice of this invention
No. 3227 was used and was effective. It has been discovered that in order to obtain a glassy top layer with a sufficiently low melting point for use in coating melting furnace electrodes, the borosilicate frit must have the following composition. namely, about 7 _to about 52% by weight _Na2O , about 8 to about 64% by weight _B2O3 , and about 6 to about 68% by weight _SiO2 . The melting point of such borosilicate frits is typically about 700-900°C. The top layer can be made from a particulate mixture containing borosilicate frit material and a silicon-containing binder, such as sodium silicate, as before. A slurry containing fine borosilicate (eg, about 200 microns) and binder is applied in a relatively thick layer. This layer is dried and fired at a high temperature of approximately 700°C or higher. Fillers can be used in the particulate mixture if desired. This filler thickens the mixture and gives it the proper viscosity. Most refractory fillers can be used as long as they are compatible with the other components of the coating. Examples of suitable fillers are:
Contains Al ₂ O ₃ , SiC, Mullite, SiO ₂ and B ₄ C. The amounts of borosilicate frit, binder and filler used in the granular mixture are not strictly critical. However, it has been discovered that good results are obtained when the mixture components are kept within the following ranges. i.e. about 25 to about 85% by weight borosilicate;
from about 5 to about 15% by weight silicon-containing binder, and from about 0 to about 65% filler. A granular mixture containing borosilicate frits and a binder can be applied to the bottom layer after drying as described above, or this granular mixture can be added on top of the double layer coating made as described above. It can be applied as an outer layer. In this case, a layer containing metal carbide (for example silicon carbide) particles is used as an intermediate layer to obtain a multilayer coating. FIG. 1 schematically depicts a carbonaceous product with a double layer antioxidant coating according to the invention. As shown, the coating includes a lower layer 10 of refractory material adhered to the top side of a graphite substrate 12 and a rigid vitreous upper layer 14. The lower layer 10 consists of a homogeneous mixture of loosely bound mullite particles 16, aluminum oxide particles 18 and metal silicon particles 20. Each silicon particle 20 is surrounded by an aluminum oxide body such as 22. The lower layer also includes spherical hollow shell particles 24 made of aluminum oxide. The particles of mullite, aluminum oxide and silicon are loosely sintered together with many voids between them as shown at 26.
There is a space in between. Ceramics refractory material 2
8 fibers inside the void space and 3
As shown by 0, the aluminum oxide shell is arranged as an agglomerate inside the aluminum oxide shell. The bottom layer 10 is interfacially bonded 32 to the graphite matrix 12.
strongly held by This bond consists primarily of fibers 28 of refractory ceramic material. As will be explained in more detail below, the fibers 28 are widely distributed in a random direction within the graphite pores located within the interfacial bonding area. The upper layer 14 of the coating consisted of a rigid glass, which in this case was uniformly distributed mullite particles 34 and silicon carbide particles 36. Top layer 14 is chemically bonded to bottom layer 10 and is plastic/flowable at temperatures above about 1100°C. The double layer coating described above can withstand severe thermal shock. The graphite electrode protected by the double layer coating could be used at temperatures above 1400° C. and under repeated heating cycle conditions without losing its protective ability due to melting or spalling. The rigid glass upper layer 14 and the lower layer 10 are made of a graphite matrix 1
It has a coefficient of thermal expansion (CTE) greater than 2.
Despite this difference in CTE, dual layer coatings can withstand high shear stresses occurring in both layers and at the coating/graphite interface without failure. If exceptionally high stress occurs, this stress is relieved by the formation of microcracks such as 38. These microcracks form in the glass layer 14 and propagate within the coating towards the graphite matrix 12. This phenomenon occurs because both layers have a tensile strength that is much less than the shear force of the interfacial bond 32. FIG. 2 is a photomicrograph of a graphite product with an actual double layer coating according to the invention. This photo was taken at 50X magnification and shows particles of mullite, aluminum oxide, silicon carbide, and silicon metal in the lower and upper layers of the coating. The coated product shown in Figure 2 was mounted in an epoxy potting resin using standard metallographic techniques. The dark areas seen in this photo are the impenetrable void spaces of the embedding resin, while the bright areas near the exterior surface are resin-filled pores. This applies to all other micrographs unless otherwise specified. FIG. 3 is a photomicrograph of a graphite article with another double layer coating according to the invention taken at 200X magnification. The top layer of this coating includes silicon carbide. This coating includes microcracks 40 that begin at the surface of the upper layer and propagate toward the graphite matrix and through both the upper and lower layers to the lower layer. It can be seen that this crack passes through large bubbles or voids 42 and mullite grains 44 in the upper layer. The cracks also run around large metal silicon particles 46 in the lower layer. The crack terminates in a large void 48 directly above the graphite matrix. Also shown in this photograph are several aluminum oxide shells 50. The absence of metal voids at the interface of this specimen indicates a good bond between the coating and graphite. FIG. 4 is a photomicrograph taken at 200X magnification of a graphite article with another coating according to the invention, but in this case the coating comprises a single layer of refractory material. As can be seen in this photograph, the coating contains microcracks 52 that pass through the monolayer and virtually reach the interface between the coating and the graphite matrix. It can also be seen that in this sample piece, the microcracks pass through several voids and the metal silicon particles 54. 5 and 6 illustrate the self-sealing properties of the double layer coating according to the invention as described in Example 1. FIG. These two photographs show glass layer samples cut from a coated product heated to a temperature of 1350°C. The coated product was the same sample made in Example 1 below. The coating includes a top layer incorporating silicon carbide particles. The sample was mechanically fractured to form large tensile cracks and then reheated to 1350°C. Graphite sample with coating, 12.7×12.7×38.1mm
(1/2″ x 1/2″ x 1 1/2″) was subjected to tensile cracking by loading in a three-point bending mode with its coating side down. This arrangement placed the coating in tension. The load was increased until cracks developed in the coating and partially propagated into the graphite.The fractured specimens were inspected to confirm that the coating retained good adhesion to the graphite. The tensile cracks were considerably larger than the typical microcracks that occur in a glass layer subjected to thermal shock. Figure 6 shows this same sample heated to a temperature of 1350°C. In this second photograph it can be seen that the cracks have been sealed and a substantially smooth layer of glass remains. Some small spherical voids are shown as black spots in the glass layer. Figure 7 is a photomicrograph of a fractured sample taken from a double layer coating according to the invention. The substrate was taken at 1000X magnification. As seen in this photo, the substratum contains many relatively large irregularly shaped pores shown in dark areas. Also, inside the pores are A large number of elongated refractory fibers are formed, and these fibers appear to grow in clumps around and between the particles.
The fiber agglomerates are more obvious in the photograph of FIG. This is a 4000X magnification of the same photo. Figures 9 and 10 are photomicrographs of similar fracture samples taken from dual layer coatings according to the invention. The photographs in this case show part of the substratum area close to the interface and were taken at magnifications of 1000X and 4000X, respectively. As can be seen in these photographs, this lower region close to the interface with graphite contains fewer pores and the diameter of these pores is much smaller. It can also be seen that a large number of elongated refractory fibers are formed in this area, which have an identical appearance except that some of the fibers are straight, eg needle-like. The coatings of Figures 7-10 were also made from a top layer containing hydrocarbon particles. An analysis was performed to assess the exact nature of the interfacial bond between the graphite matrix and the bilayer coating.
In this study, it was surprisingly discovered that this bond is primarily mechanical rather than chemical, although the possibility of chemical bonding cannot be definitively excluded. It is believed that a large number of microscopic refractory fibers are formed in the pores of the interfacial structure just below the interface, resulting in the mechanical bonding described above. These fibers appear to form in situ during the coating reaction at high temperatures. Electron microscopy also discovered that each fiber contained the elements aluminum, silicon, nitrogen and oxygen. Indeed, recent X-ray diffraction analyzes have confirmed these studies and hinted at the presence of an aluminosilicate nitrogen compound, Al ₈ Si ₉ N ₁₂ O ₁₂ . Analytical studies were performed using standard electronic techniques: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and electron impact spectroscopy (AES). These studies were conducted using samples made according to Example 1 below. For this study, a double layer coating was applied to each area.
, etc., and each area was analyzed separately. The zone is a rigid glass top layer made of silicon carbide particles, the zone is an intermediate layer or bottom layer,
The area also had a graphite matrix. Three additional areas were analyzed. These are the area between the rigid glass layer and the bottom layer/(A), the area/(A) inside the bottom layer close to the bottom layer/graphite interface, and finally the area/(B) inside the graphite base layer close to the interface. The results of these analyzes are shown in the table.

【table】
It was done.
50V

The analytical results presented in the table confirm that the rigid glassy top layer consists primarily of oxides of aluminum, sodium and silicon, and silicon carbide. It is interesting to note from this table that the zone, the glass layer, is rich in sodium, while the zone is completely devoid of sodium. Most of the sodium from the preferred silicate binder in the lower layer migrates to the zone where it acts as a glass former. Area close to the interface, i.e. area/
(A) was rich in nitrogen, aluminum and silicon. X-ray diffraction data was also obtained for each area of the coating. In the first zone, close to the outer surface of the vitreous top layer, the structure was found to be mostly amorphous. Crystalline phases were found in this area, which were identified as silicon carbide and mullite. In the second zone inside the glass layer, the structure was also amorphous and the crystalline phases were identified as silicon carbide and mullite. One or more additional phases were also discovered. For example, sodium hydroxide. The third area analyzed was within the lower layer. This area is also mostly amorphous and only weakly crystalline. i.e. mullite, metallic silicon, etc. The fourth area analyzed was within the underlying layer, but adjacent to the interface with the graphite matrix. This area was also mostly amorphous, but contained strongly crystalline parts, which were alpha Al ₂ O ₃ , graphite and mullite. A pattern that could be identified as Al ₈ Si ₉ N ₁₂ O ₁₂ was also discovered in this area. The physical properties of the double layer coating according to the invention were also examined. For example, the elastic modulus of the bottom layer is
41.5×10 ⁴ when measured by ASTM C-747 method
Kg/cm ² (5.9×10 ⁶ lbs/in 2 ), while the elastic modulus of the upper layer containing silicon carbide particles was approximately 56.2×10 ⁴ Kg/cm ² (8×10 ⁶ lbs/in 2 ). It was hot. This estimate is based on cited literature showing that this type of glass has a modulus of elasticity of about 70.3 x 10 ⁴ Kg/cm ² (10 x 10 ⁶ pounds per square inch) when fully dense. Since this upper layer is porous, its elastic modulus will be slightly lower.
The bending strength of the lower layer was also found to be approximately 176 kg/cm ² (2500 lbs/in 2 ). The thermal expansion coefficient of the lower layer is approximately 5.5×10 ^-6 cm/cm/℃,
The thermal expansion coefficient of the glassy upper layer is approximately 40×10 ^-6 cm/cm/
It is ℃. In comparison, electrode grade graphite typically has a coefficient of thermal expansion of less than 1.5 x 10 ^-6 cm/cm/°C. The large difference in CTE between graphite and glassy overlayers usually results in interfacial bond failure in the absence of a flexible base layer. In a more specific aspect, the invention relates to coated graphite electrodes used at high temperatures in steelmaking electric arc furnaces. During operation of this electric arc furnace, a current is passed through the graphite electrode, creating an electric arc between the tip of the electrode and the metal bath. The electrode column is heated by an electric arc to a high temperature of approximately 1000° C. or higher, and is periodically rapidly cooled, for example, when pouring out molten metal. A graphite electrode protected by a double layer coating according to the invention can operate effectively under these conditions for a substantially longer period of time than a graphite electrode without a coating. This significantly reduces the operating costs of the furnace and thus the cost of the final product. The invention also provides an electric arc furnace electrode with an unfired or "green" anti-oxidant coating on the surface. This green or green coating can be applied to the graphite electrode in the same manner as described above using a coating composition containing the same components in the same proportions as described above. However, in this case the electrode manufacturer does not roast or bake the coating at high temperatures before storage or shipping. The double layer coating of this invention is particularly suitable for such storage or shipping. This is because the green, dry coating adheres well to the graphite electrode, is hard, and is very durable. However, it should be noted that since the pre-fired coating is water soluble, the green coating should not be directly exposed to the environment for extended periods of time. The invention also includes new and improved coating compositions used to protect surfaces of carbonaceous products, such as carbon or graphite electrodes, against oxidation. This coating composition is
It is preferably applied to the surface of the carbonaceous article by brushing, troweling or similar techniques in the form of a wet paste containing the same components in the same proportions as described above. Alternatively, the coating composition can be applied to the surface of the carbonaceous product by spraying or dipping. In this case additional amounts of water are added to adjust the consistency of the coating to the desired level. In the bilayer embodiment of this invention, the preferred composition of the bottom layer calculated on a dry basis is as follows. 48%-58% mullite, 9%-18% silica, 20-34% aluminum and 8-11% solid sodium silicate. Sufficient water is added to produce the desired slurry concentration required for application methods such as brushing, spraying, etc. The preferred composition of the silicon carbide-containing upper layer, also calculated on a dry basis, is as follows: 52-61% mullite, 10-12.0% silica, 16.0-27% silicon carbide, 9-12% solid sodium silicate. . Next, as in the case of the lower layer, an amount of water is added to produce the desired slurry consistency. In the following, the invention will be further explained with reference to a number of embodiments. It should be noted that some of these examples also include comparative examples that are not based on the present invention. Example 1 Mullite (3Al ₂ O ₃・2SiO ₂ ) and silica (SiO ₂ )
A slurry was prepared using a commercially available refractory cement containing and sold under the trademark "Super 3000". This is a product of Barry Fogy, PA. This cement had the following composition: Approximately 57.0
wt% mullite, 11.4wt% silica and
31.6% water glass by weight. The composition announced by the manufacturer was calculated based on chemical analysis of the cement according to ASTM C-753, which contained 52.2% by weight Al ₂ O ₃ and 44% by weight SiO _{2 .} , containing 0.2 wt. Fe ₂ O ₃ and 3.6 wt. % alkali (Na ₂ O). Water glass has an approximate composition
33.4% by weight of sodium silicate in _Na2O・_3.8SiO2
It is a solution. Water glass can also be expressed as 10.6% by weight of Na ₂ O.3.8SiO ₂ and 21% by weight of water based on the total weight of cement. Aluminum powder is added to the cement to form a slurry, approximately 30% to the total weight of the cement.
It was added at a ratio of The final composition of the slurry is 43.8% mullite, 8.7% silica, 24.2% by weight
water glass, 23.1% by weight aluminum. The water glass can be expressed as 8.1% by weight of Na ₂ O.3.8SiO ₂ and 16.1% by weight of water relative to the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 52.3% by weight mullite,
10.4 wt% silica, 26.7 wt% aluminum, and _9.6 wt% solid sodium silicate as _Na2O.3.8SiO2 . The aluminum used in the slurry was commercially available aluminum powder, i.e.
Metco, Westbury, NY, grade 54 aluminum powder. Most of the aluminum powder was -200/+325 mesh. Diameter 12.7cm (5 inches), height 15.2cm (6 inches)
The slurry was applied with a brush to areas other than the bottom surface of the graphite electrode sample. The slurry was then dried in air at room temperature until the moisture was removed. Next, this drying step
A second slurry layer was brushed over the layer. The two coats of slurry bonded very well to the graphite surface and had a total dry thickness of about 0.05 inch. In this case, a second slurry was made using the above refractory cement to which silicon carbide particles were added. Silicon carbide was added at a rate of about 20% by weight based on the total weight of the cement. The second slurry had the following components: 47.4% by weight mullite, 9.4% by weight silica, 26.1% by weight water glass and 16.7% by weight
of silicon carbide. The water glass could be expressed as 8.7% Na ₂ O.3.8SiO ₂ plus 17.4% water based on the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 57.7 wt% mullite, 11 wt% silica, 20.3 wt% silicon carbide, and 10.6 wt% _solid sodium silicate as _Na2O.3.8SiO2 , silicon carbide is commercially available SiC
The powder was Niagara Fol, NY, Carborundum, Grade 280RA, silicon carbide. Most of the silicon carbide particles were -74 microns. This second slurry was then brushed onto the surface of the aluminum-containing lower layer of the graphite electrode and dried in air at room temperature until all moisture was removed. The graphite and bilayer coating had a dry weight of 3720 g. Globar coated electrode
It was placed in a furnace and an oxidation test was performed. The oxidation test was performed by heating to approximately 500°C and holding at this temperature for approximately 1 hour. Next, the temperature of the furnace was increased to approximately 1350℃ after 3 hours, and during the oxidation test, approximately 7.5SCFH was generated inside the furnace.
to scavenge the air. After 2 1/2 hours, the coated electrode was removed from the oven and allowed to cool by allowing it to stand at room temperature. This electrode is about 10
Remained red hot for ~15 minutes. There was no evidence of puffing or pinhole formation. This double layer coating was examined under a microscope at 20X magnification and no cracks were found on its surface. However, numerous pores were visible throughout the coating. The coating bonded very well to the graphite with no evidence of bumps. Figures 2 and 3 are micrographs of this coating. The final weight of the coated electrode after cooling was 3700 g. This corresponds to a loss of 20g or about 0.54%. A billet without the coating subjected to the same oxidation test lost approximately 11.4%. Therefore,
The double layer coating was effective in reducing typical oxidation losses by about 90%. Example 2 Two applications of the same slurry as in Example 1 were applied to form the bottom layer of a double layer coating of a 5 inch (12.7 cm) diameter, approximately 15.2 cm (6 inch) long graphite electrode. However, no aluminum was added to the slurry. A top layer was then applied using the same slurry as in Example 1 containing approximately 20% by weight SiC particles. The coated electrode was placed in a Globa furnace and subjected to the same oxidation test at 1350°C. Thereafter, the electrode was taken out of the furnace and cooled at room temperature in the same manner. The electrode remained red-hot for approximately 10 minutes, and a loud cracking sound was heard as it cooled. Upon inspection the next day, large cracks were discovered in the coating. Although the coating was easily removed from the electrode, the surface beneath the coating appeared to be well protected. This indicates that the cracks formed during cooling. The initial weight of the coated electrode is 3541g
It was hot. After oxidation test, electrode weight is 3500g
It was hot. This corresponds to a loss of 41g or 1.16%. It can be seen from this example that a double layer coating that does not contain aluminum in the underlying layer is not an effective coating. Example 3 A slurry was made in the same manner as in Example 1. However, in this case, approximately 10
Aluminum powder in an amount of % by weight was added to the slurry. The final composition of the slurry is 51.7% by weight mullite;
It was 10.3% by weight silica, 28.5% by weight water glass, and 9.1% by weight aluminum. The water glass contains 9.5% by weight of _Na2O・ based on the total weight of the slurry.
It can be expressed as 3.8Si ₂ O and 19.0% by weight water. On an anhydrous basis, the dry ingredients had the following composition: 64.1 wt% mullite, 12.8 wt% silica, 11.3 wt% aluminum and _Na2O .
11.8% by weight solid sodium silicate as _3.8SiO2 . 12.7cm of the same grade as that used in Example 1
(5 inch) diameter graphite electrodes were coated twice with the slurry and dried. Then, using a similar second slurry containing about 20% by weight silicon carbide particles,
The top layer was applied. The coated electrode was placed in a Globa furnace and held at 500°C for 1 hour, after which the furnace temperature was increased to 1350°C. After being held at this temperature for 2 1/2 hours, the electrode was removed from the furnace and allowed to cool to room temperature. The electrode remained red hot for about 10 minutes. When inspected the next day, the coating on half of the electrodes was severely cracked and some of the coating had flaked off the graphite surface. However, the exposed surface of graphite was not oxidized. This indicates that the glass was formed upon cooling or thermal shock. The other side of the electrode showed a slightly harder glass layer and appeared to bond well to the graphite. No cracks were observed. The coated electrode weighed 3658 g before testing. 3628 with 30g or 0.28% loss after testing
It was hot at g. This example shows that a double layer coating containing only 10% aluminum in the bottom layer is still not effective as it suffers from some spallation. Example 4 Another slurry was made as in Example 3 except that the amount of aluminum added was increased to about 15% by weight based on the weight of the refractory cement. The final composition of this slurry is 49.5% by weight mullite, 9.8% by weight silica, 27.3% by weight water glass and 13.0% by weight.
% aluminum by weight. The water glass contains 9.1% by weight of _Na2O・_3.8SiO2 based on the total weight of the slurry.
and 18.2% water by weight.
On an anhydrous basis, the dry ingredients had the following composition: 60.8 wt% mullite, 12.0 wt% silica, 16.0 wt _% aluminum, _Na2O.3.8SiO2
as 11.1% by weight solid sodium silicate. 12.7cm of the same grade as that used in Example 1
A (5 inch) diameter graphite electrode was coated twice with this slurry to form the underlayer. The top layer of the coating was applied using the same silicon carbide-containing slurry made according to Example 1. The coated electrode was placed in a globe furnace and a similar oxidation test was conducted at 1350°C. After 2 1/2 hours, the electrode was taken out of the furnace and allowed to cool to room temperature. The coating bonded extremely well to the graphite with no evidence of pinholes, puffing or cracking. Before the oxidation test, the graphite electrode and coating were
The weight was 3575g, but after the test it was 3548g with a loss of 27g or 0.76%. This example shows that a double layer coating containing 15% aluminum in the bottom layer is effective because it does not cause spalling. Example 5 The amount of aluminum added is approximately
Another slurry was made in the same manner as in Example 1 except that the slurry was increased to 40% by weight. The final composition of the slurry is as follows. 40.6% by weight mullite, 8.1% by weight
of silica, 22.4 wt% water glass, and 28.6 wt% aluminum. The water glass could be expressed as 7.5% by weight of Na ₂ O.3.8SiO ₂ plus 14.9% by weight of water based on the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 47.9% by weight
solid sodium silicate as mullite, 9.6 wt% silica, 33.7 wt% aluminum, 8.9 _wt % _Na2O.3.8SiO2 . Using this slurry, 12.7 cm of the same grade
(5 inch) diameter graphite electrode was applied twice. next,
A silicon carbide-containing slurry made as in Example 1 was used to coat the top layer. The coated electrode was placed in a glober furnace and the same oxidation test was performed at 1350°C. After 2 1/2 hours, the electrode was removed from the furnace and allowed to cool to room temperature. The coating bonded well to the graphite with no puffing or cracking. The electrode weight before the oxidation test was 3677g, and after the test it was 3652g with a loss of 35g or 0.68%.
It was hot. This example demonstrates the effectiveness of a dual layer coating containing 90% aluminum in the bottom layer. Example 6 Example 1 except that the amount of aluminum added was increased to about 50% by volume based on the weight of the refractory cement.
I made other slurries in the same way. The final composition of the slurry was 38.0% mullite, 7.5% silica, 20.9% waterglass and 33.3% aluminum by weight. The water glass could be expressed as 7.0 wt% _{Na2O.3.8SiO2 plus} 13.9 wt% water based on the total slurry weight. On an anhydrous basis, the dry ingredients had the following composition: 44.2 wt% mullite, 8.7 _wt % silica, 33.8 wt% aluminum, and 8.1 wt% _Na2O3.8SiO2
as solid sodium silicate. Using this slurry, 12.7 cm (5 cm) of the same grade
The bottom layer was made by applying two coats of (inch) diameter graphite electrode. The top layer of the coating was applied using a silicon carbide-containing slurry similar to that of Example 1. Place the coated electrode in a Grover furnace;
The same oxidation test was performed at 1350°C. After 2 1/2 hours, the electrode was removed from the furnace and allowed to cool to room temperature. When this was examined, large foamy islands of silicon carbide were formed with a hard glass layer with a network of cracks running between these islands. However, the coating appeared to bond well to the graphite.
The electrode weight before the oxidation test is 3576g, after the test it is 26g.
In terms of g, it was 3350g with a loss of 0.73%. It can be seen from this example that using 50% aluminum in the bottom layer forms an effective double layer coating because it bonds well with the graphite. Example 7 Another slurry was made as in Example 3 except that the amount of aluminum added was increased to about 100% by weight based on the weight of the refractory cement. The final composition of the slurry was as follows. 28.4% by weight mullite,
6.6 wt% silica, 15.6 wt% water glass, and 50.0 wt% aluminum. The water glass was expressed as 5.2 wt% _{Na2O.3.8SiO2 plus} 10.4 wt% water based on the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 31.8% by weight mullite, 6.3% by weight silica,
56.0 wt% aluminum, and 5.9 wt%
Solid sodium silicate _{as Na2O.2.8SiO2} . This slurry was applied twice to a 12.7 cm (5 inch) diameter graphite electrode of the same grade as in Example 1 to form a lower layer. The same silicon carbide-containing slurry as in Example 1 was used to apply the top layer of the coating. The coated electrode was placed in a globe furnace and the same oxidation test was performed at 1350°C. 2
After 1/2 hour, the electrode was removed from the furnace and allowed to cool to room temperature. When examined, a network of microcracks formed between the silicon carbide islands in the aluminum-containing sublayer was observed. However, the base layer appeared to be well bonded to the graphite. The coated electrode weight before the oxidation test was 3586 g and after the test it was 3550 g for a loss of 33 g or 0.92%. From this example, the amount of aluminum in the lower layer is
It can be seen that although it can be increased to 100%, the effectiveness of the vitreous top layer is reduced due to its non-uniformity. Example 8 Using the first slurry of Example 1 containing about 30% aluminum by weight, a 6 inch long
Two coats of 12.7 cm (5 inch) diameter graphite electrode were applied to form the bottom layer of the dual layer coating. The top layer of this coating was made using a second slurry similar to Example 1 but without the addition of silicon carbide. Further, the graphite electrode was of the same grade as in Example 1. After the coating was dry, the electrode was placed in a globe furnace and heated to approximately 500°C. The oven was held at this temperature for approximately 1 hour. The temperature of the furnace was then raised to approximately 1350°C after 3 hours. During the oxidation test, air was purged into the furnace at approximately 7.5 SCFH. After 2 1/2 hours, remove the coated electrode from the furnace and
It was kept at room temperature and cooled. The electrode remained red-hot for about 10 minutes, and a cracking sound could be heard while it cooled. Upon subsequent inspection, a network of cracks was seen forming on the surface of the coating. The coating was bonded to the graphite in some areas, but in other areas there was no evidence of bonding. This loss of bond in some areas is likely due to excessive expansion of the top layer during curing. The coated electrode weighed 3535 g before the oxidation test and 3500 g after the test for a loss of 35 g or 0.99%. Example 9 Example 1 containing about 30% aluminum by weight
The first slurry was used to apply two coats of approximately 5 inch long 5 inch diameter graphite electrodes of the same grade to form the lower layer of the dual layer coating. The upper layer of this coating is approximately 5% by weight.
Example 1 was carried out using the second slurry of Example 1 containing silicon carbide. The final composition of the second slurry was 54.2% mullite, 10.8% silica, 29.9% waterglass and 4.8% silicon carbide by weight. The water glass could be expressed as 10.0% by weight of Na ₂ O.3.8SiO ₂ plus 19.9% by weight of water based on the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 67.9% by weight mullite,
Solid sodium silicate as 13.5% by weight silica, 6.0% by weight silicon carbide and 12.5 _% by weight _Na2O.3.8SiO2 . After the coating has dried,
The electrode was placed in a globe furnace heated to approximately 500°C. The oven was held at this temperature for approximately 1 hour. The furnace was then increased to 1350°C after 3 hours. During the oxidation test, air was purged into the furnace at approximately 7.5 SCFH. 2
After 1/2 hour, the coated electrode was removed from the oven and rapidly cooled to room temperature. Upon inspection,
The coating appeared to bond well to the graphite, with some microcracks visible under the microscope. This coating seemed a little polished. The coated sample weighed 3506 g before the oxidation test and 3476 g after the test for a loss of 30 g or 0.86%. It has been found that effective double layer coatings are obtained when the amount of silicon carbide in the vitreous top layer is increased to about 5%. Example 10 The first slurry of Example 1 containing about 30% aluminum by weight was used to coat two 5-inch diameter graphite electrodes of the same grade about 6 inches long. , formed the lower layer of the double layer coating. A second slurry similar to Example 1 but with the addition of 40% by weight silicon carbide was used to apply the top layer of the coating. The final composition of the second slurry is 40.6
It was weight percent mullite, 8.1 weight percent silica, 22.4 weight percent water glass, and 28.6 weight percent silicon carbide. water glass relative to the total weight of the slurry
It could be expressed as 7.5% by weight of Na ₂ O and 3.8SiO ₂ plus 14.9% by weight of water. On an anhydrous basis, the dry ingredients had the following composition: Solid sodium silicate as 47.9 wt% mullite, 9.6 wt% silica, 33.7 wt% _silicon carbide and 8.9 wt% _Na2O.3.8SiO2 . After the coating was dry, the electrode was placed in a globe furnace and a similar oxidation test was performed at 1350°C. After 2 1/2 hours, the electrode was removed from the furnace and allowed to cool to room temperature.
The coating was polished and bonded well to the graphite. When viewed under a microscope, a small number of microcracks were seen. coated electrodes before oxidation test
It was 3588g and after the test it was 3559g for a loss of 29g or 0.80%. This example shows that an effective double layer coating can be obtained with 40% silicon carbide in the vitreous top layer. Example 11 Example 1 containing about 30% aluminum by weight
The first slurry was used to apply two approximately 6 inch lengths of the same grade 5 inch diameter graphite electrode to form the lower layer of the dual layer coating. The top layer of the coating was applied using a second slurry identical to Example 1, but with the addition of 60% by weight silicon carbide. The final composition of the slurry is
35.6% by weight mullite, 7.1% by weight silica, 19.7% by weight
% water glass and 37.5% silicon carbide by weight. The water glass is based on the total weight of the slurry.
It was 6.6% _{Na2O.3.8SiO2 plus} 13.1% water. On an anhydrous basis, the dry ingredients had the following composition: It was 41.0% by weight mullite, 8.2% by weight silica, 43.2% by _weight silicon carbide, and 7.6% by weight solid sodium silicate as _Na2O.3.8SiO2 . After the coating was dry, the electrode was placed in a globe furnace and a similar oxidation test was performed at 1350°C. After 2 1/2 hours, the electrode was removed from the furnace and cooled to room temperature. The coating bonded well to the graphite and there was no evidence of puffing or cracking during the coating. The coated sample weighed 3516 g before the oxidation test and 3489 g after the test for a loss of 27 g or 0.77%. This example shows that an effective double layer coating can be obtained with 60% silicon carbide content in the vitreous top layer. Example 12 The first slurry of Example 1, again containing about 30% aluminum by weight, was applied twice to a 5 inch diameter graphite electrode of the same grade, about 6 inches long. to form the lower layer of the double layer coating. 30% by weight sodium silicate and
A second slurry containing 70% by weight silicon carbide particles was used to apply the top layer of the coating. In producing this slurry, the refractory cement used in each of the above examples was not used.
After the coating was dry, the electrode was placed in a globe furnace and heated to approximately 500°C. The oven was held at this temperature for approximately 1 hour. The furnace temperature is about 1350 after 3 hours.
It increased to ℃. After 2 1/2 hours, the electrode was taken out of the furnace and cooled to room temperature. The electrode remained red-hot for some time and had no evidence of cracking or puffing when cooled. The upper layer was slightly flaky and bonded to the graphite. The coated electrode weighed 3518 g before the oxidation test and 3502 g after the test for a loss of 16 g or 0.45%. This example shows that an effective double layer coating is obtained when the vitreous top layer consists primarily of silicon carbide. Example 13 Using the slurry of Example 1 containing 30% aluminum, one 5 inch diameter graphite electrode of the same grade approximately 6 inches long was prepared.
It was coated twice to form the lower layer of the double layer coating. After the first layer was dry, the top layer was applied using the same slurry as in Example 1 containing about 20% by weight SiC particles. The coated electrodes were dried and then placed in a globe furnace and subjected to similar oxidation tests at 1350°C. Next, the electrode was taken out of the furnace and cooled to room temperature. The sample remained red-hot for over 10 minutes with no puffing or pinholes. When inspected, the coating was polished with no cracks and was well bonded to the graphite. However, many pores were observed on the surface of the coating. The coated electrode weighs 3615g before the oxidation test and after the test
It was 3596g for a loss of 19g or 0.53%. It can be seen from this example that an efficient double layer coating is created using only one application of the first slurry. EXAMPLE 14 A 6-inch long tube of the same grade was made as in Example 1, but using a slurry containing up to 40% aluminum based on the total weight of the cement. inch) diameter graphite electrode 2
Applied twice. The aluminum was Metco No. 54 high purity spherical powder. The additional top layer containing SiC was omitted. Figure 4 is a photograph of this coating. The coated electrode was placed in a globe furnace and maintained at 500°C for 1 hour. The furnace temperature was then increased to 1350° C. after 3 hours and held at this temperature for 2 1/2 hours. The electrode was then removed from the furnace and cooled to room temperature. While it was still hot (approximately 900 degrees Celsius) during cooling, a cracking sound was heard after 5 minutes. Upon inspection, the coating appeared to bond well to the graphite, with numerous microcracks found but no broaching. A slight oxidation was observed at the bottom of the electrode. This example shows that an aluminum-containing single layer coating according to the present invention can be used to effectively preserve the graphite surface. Example 15 Example 1 containing about 30% aluminum by weight
The first slurry was used to apply two coats of a 5 inch diameter graphite electrode approximately 6 inches long. After the coating is dry, place the coated electrode inside a globe furnace and
heated to ℃. The oven was held at this temperature for approximately 1 hour. The furnace temperature was then increased to approximately 1350°C after 3 hours. During the oxidation test, air was purged into the furnace at approximately 7.5 SCFH. After 2 1/2 hours, the electrode was removed from the oven and allowed to cool by leaving the electrode at room temperature. Upon subsequent inspection, a network of cracks was observed to form on the surface of the coating. The coating appeared to bond well to the graphite. The coated electrode before oxidation test weighs 3666g and after test it weighs 17g or 0.46
% loss was 3649g. Example 16 Using a method similar to Example 1, the slurry of Example 1 containing 30% by weight aluminum was added to approximately 25.4 cm
Two coats were applied to a (10 inch) long 50.8 cm (20 inch) diameter graphite electrode to form the lower layer of the dual layer coating. This graphite electrode sample was taken from a 20 inch by 96 inch electrode with the top layer of coating applied using the second slurry of Example 1 containing about 20% by weight SiC. This electrode was placed in an induction furnace and two graphite spacers were placed at the top of the electrode. The furnace is heated under non-oxidizing conditions to approx.
It was held at 500°C for an hour and then increased to a temperature of 1350°C for about 3 hours. The oven was held at this temperature for 1 1/2 hours and then cooled to ambient temperature. Upon inspection, the coating was well bonded to the graphite with some cracking visible on the sides of the coating. However, these cracks resulted from the pinching action of the gripping device used to remove the electrode from the furnace. This example shows that the double layer coating according to the invention can be effectively used on large diameter furnace electrodes. Example 17 A slurry was prepared in the same manner as in Example 1, but in this case aluminum and silicon carbide were added at the same time, each in a proportion of about 20% by weight based on the total weight of the refractory cement. The final composition of the slurry was 40.6% mullite, 8.1% silica, 22.4% waterglass, 14.3% aluminum, and 14.3% SiC. The water glass could be expressed as 7.5% Na ₂ O 3.8SiO ₂ plus 14.9% water based on the total weight of the slurry. On an anhydrous basis, the dry ingredients include the following composition: 47.9 wt% mullite, 9.6 wt% silica, 16.8 wt% aluminum, 16.8 wt% SiC, and 8.9 wt%
Solid sodium silicate _{as Na2O.3.8SiO2} . 5 of the same grade as Example 1 with a length of approximately 6 inches.
The slurry was applied twice to inch diameter electrodes and allowed to dry. Next, place the electrode in a globe furnace and heat it to 500℃.
and held at this temperature for 1 hour. The furnace temperature was then increased to 1350° C. after 3 hours and held at this temperature for an additional 1 1/2 hours. The electrodes were inspected periodically and there were no signs of pinholes or puffing. The electrode was cooled to room temperature and inspected. Mild puffing was detected, but numerous pinholes were scattered throughout the coating. The coating rapidly lost its glow, probably due to a lack of bonding. When cooled, the coating was easily removed and showed evidence of oxidation on the graphite surface. Before the oxidation test, the coated electrode weighed 3658g, but after the test it weighed 38g or
It was 3520g with a loss of 3.77%. As is clear from this example, coatings made with slurries containing aluminum and silicon carbide are not effective. Example 18 A slurry was prepared using the same refractory cement as in Example 1 except that 20% by weight of metallic silicon powder was used instead of aluminum. The final composition of the slurry was 47.4% mullite, 9.4% silica, 26.1% waterglass, and 16.7% silicon by weight. Water glass is 8.7% of the total slurry weight.
It could be expressed as % _Na2O・_3.8SiO2 plus 17.4% water. On an anhydrous basis, the dry ingredients had the following composition: 57.7% water, 11.4% silica, 20.3% silicon, 10.6% by weight
of solid sodium silicate as Na ₂ O.3.8SiO ₂ . The metallic silicon used in this example was 10%
-100 meshes or 10% -200 meshes. This slurry was used to coat two approximately 6 inch long 5 inch diameter electrodes of the same grade to form the lower layer of the dual layer coating. A second slurry containing 20% by weight SiC as described in Example 1 was used to apply the top coating layer. The coated electrode was placed in a global furnace and heated to a temperature of 500° C. for 1 hour. The furnace temperature was then increased to about 1350° C. for 3 hours and held at this temperature for 2 1/2 hours. The electrode was taken out of the furnace and cooled to room temperature. The coating surface was covered with cracks and pinholes, and there was no evidence of bonding between the coating and the graphite. The coated electrode weighed 3575 g before oxidation testing. After testing it was 3462g for a loss of 113g or 3.16%. This example shows that a double layer coating containing silicon instead of aluminum in the lower layer does not provide effective protection for the graphite electrode. Example 19 Example 1 containing about 30% aluminum by weight
The slurry was used to apply two approximately 6 inch lengths of the same grade 5 inch diameter graphite electrode to form the lower layer of the dual layer coating. A top layer was then applied using a second slurry containing about 20% by weight of boron carbide particles based on the weight of the cement. The second slurry had the following composition. 47.4% by weight mullite, 9.4% by weight silica,
26.1% by weight water glass, and 16.7% by weight boron carbide. Water glass is 8.7% of total slurry weight.
Expressed as wt% _{Na2O.3.8SiO2 plus} 17.4wt% water. On an anhydrous basis, the dry ingredients have the following composition: 57.7% by weight mullite, 11.4% by weight
silica, 20.3% boron carbide, and 10.6% by weight
Solid sodium silicate _as wt% _Na2O.3.8SiO2 . Electrodes with coatings at approximately 500℃
The sample was placed in a glober furnace heated to 100 ml and held at this temperature for about 1 hour. Next, increase the furnace temperature to approx.
The temperature was increased to 1350℃. After 2 1/2 hours, the electrode was taken out of the furnace and allowed to cool to room temperature. When examined under a microscope, numerous microcracks were observed in the coating and one pinhole was observed. The coating was relatively hard and appeared to bond well to the graphite. The coated electrode weighed 3596 g before the oxidation test and 3570 g after the test for a loss of 36 g or 1.00%. This example shows that an effective double layer coating is created using boron carbide instead of silicon carbide in the vitreous top layer. Example 20 Example 1 containing about 20% aluminum by volume
The slurry was used to form the bottom layer of the multilayer coating by applying three approximately 6 inch lengths of the same grade 5 inch diameter graphite electrode. about
The second slurry of Example 1 containing 20% by weight SiC was used to apply the interlayer. The top layer was applied using a third slurry containing about 76.1% by weight borosilicate frit and water glass binder. The borosilicate frit was Frit No. 3227 manufactured by Ferro and had the following weight composition. 4.1% _K2O , 13.9%
_Na2O , 15.6% _Al2O , 28.5% _B2O , and
37.9% _SiO2 . The third slurry had the following composition. 76.1% by weight borosilicate frit and
23.9% water glass by weight. The water glass contains 9.1% by weight of _Na2O and _3.2SiO2 plus with respect to the total weight of the slurry.
It can be expressed as 14.7% water by weight. On an anhydrous basis, the dry ingredients had the following composition: 89.2% by weight borosilicate frit and 10.8% by weight sodium silicate. The coated electrode is placed in a glober furnace heated to approximately 500℃ and scavenged.
This temperature was maintained for approximately 1 hour. The furnace temperature was then increased to approximately 1350°C for 4 hours. After 2 1/2 hours, the electrode was taken out of the furnace and cooled to room temperature. The appearance of the coating was good, although the top layer was a little uneven. Small microcracks were visible under the microscope and two pinholes were visible near the top edge of the coating. The bond between the coating and the substrate was good with no evidence of sporulation. The coated electrode weighed 3539g before oxidation testing.
and after the test 3570g, i.e. 43g
Or a loss of 1.22%. This example shows that an effective multilayer coating can be made by using a top layer of borosilicate frit. This particular frit had a melting temperature of about 750°C. Example 21 The same first slurry and second slurry were used to make the bottom and middle layers, respectively, as described in Example 20.
The slurry was used to coat a 5 inch diameter graphite electrode of the same grade approximately 6 inches long with the bottom and middle layers. A top layer was then applied using a third slurry containing borosilicate frit, water glass and filler. In this example, the same borosilicate frit, Frit No. 3227, was used. The filler used _in the third slurry was _Al2O3 . This third slurry had the following composition. 59.5% by weight borosilicate frit, 23.9% by weight water glass, and 16.6% by weight
filling material. water glass relative to the total weight of the slurry
It can be expressed as 9.1% by weight of _Na2O.3.2SiO2 plus 14.7% by _weight of water. On an anhydrous basis, the dry ingredients had the following composition: 69.8 wt% borosilicate frit, 19.5 wt% _Al2O3 , _and
10.8% by weight of solid sodium silicate _{as Na2O.3.2SiO2} . The coated electrode was placed inside a scavenged globe furnace heated to about 500° C. and held at this temperature for about 1 hour. After 4 hours, the furnace temperature was increased to approximately 1350°C. Approximately 2 to this temperature
It was held for 1/2 hour, then removed from the oven and cooled to room temperature. The coating had a good appearance and bonded well to the carbon electrode. A large number of microcracks were observed under the microscope. The outer layer of hard glass contained bubbles and a portion of the glass layer had melted off the top edge, exposing a portion of the base layer. However, no oxidation of the substrate was observed. The coated electrode weighed 3415 g before the oxidation test and 378 g after the test for a loss of 1.08%. This example shows that an effective multilayer coating was obtained using a top layer made of borosilicate frit, filler and water glass. Example 22 Example 1 containing about 30% aluminum by weight
Approximately 50.2 mm (2 inches) using the slurry described in
Three coats of 31.7 mm (1 1/4 inch) diameter graphite electrode samples of the same length were applied to create the bottom layer of the dual layer coating. A top layer was then applied using a second slurry containing borosilicate frit and water glass. The borosilicate frit is the same Ferro Frit No.
It was 3227. The second slurry had the following composition. 76.1% by weight borosilicate fritted plus
23.9% water glass by weight. The water glass could be expressed as 9.1% by weight of Na ₂ O.3.2SiO ₂ and 14.7% by weight of water relative to the total weight of the slurry. On an anhydrous basis, the dry ingredients had the following composition: 89.2
% by weight borosilicate frit, and 10.8% by weight
of solid sodium silicate as Na ₂ O.3.2SiO ₂ . The coated electrodes were placed inside a CM furnace heated to various temperatures for approximately 3 hours without scavenging. Next, the electrodes were taken out and tested. Samples were fixed as described below. (1) 700°C/3 hours - The coating of the sample was well bonded and had a hard glass layer with some microcracks forming inside the coating. Numerous small bubbles were observed in the hard glass layer at locations where no reaction occurred. (2) 800°C/3 hours - The coated sample was also well bonded and had a hard glass layer with microcracks forming in the coating. A small amount of foam was generated in a small area where no reaction occurred. (3) 900°C/3 hours - The coated sample was well bonded with a hard glass layer and showed some microcracks but no bubble formation. (4) 1000°C/3 hours - Coating samples were well bonded with a hard glass layer, some microcracks, but no bubble formation. From this example, an effective double layer coating was created using a glassy outer layer made from borosilicate frits, and foaming was eliminated by firing the glass layer at a high enough temperature to ensure a safe reaction. I can see what will happen. Example 23 Example 11 containing about 30% aluminum by weight
The slurry was used to coat approximately 2 inches long 1 1/4 inch diameter graphite electrode samples of the same grade to form the bottom layer of the dual layer coating. A top layer was then applied using a second slurry containing borosilicate frit, water glass and filler. Also in this example, Ferro frit No. 3227 was used and the filler was silicon carbide. The second slurry had the following composition. 59.5% by weight borosilicate frit, 16.7% by weight silicon carbide and 23.8% by weight water glass. Water glass is 9.1% of total slurry weight
It could be expressed as wt% _{Na2O.3.2SiO2 plus} 14.7wt% water. On an anhydrous basis, the dry ingredients have the following composition: 69.8% by weight borosilicate frit, 19.6% by weight silicon carbide, and 10.7% by weight
% Na ₂ O.3.2SiO ₂ by weight. The coated electrodes were then placed in a CM furnace and heated to various temperatures for approximately 3 hours without scavenging. The electrode was then removed from the furnace and inspected. Coating samples were fixed as follows. (1) 700°C/3 hours - Coating sample bonds to substrate and shows some microcracks. The hard glass layer was slightly foamy and contained some bubbles. (2) 800°C/3 hours - The coating was bonded to the substrate in the area near the top and contained some microcracks. The upper layer is a clear hard glass layer with some bubbles. (3) 900°C/3 hours - Coating is well bonded to the substrate and shows some microcracks. The outer hard glass layer is smooth, transparent and bubble-free. (4) 1000°C/3 hours - Coating bonds well to substrate with some microcracks. The outer hard glass layer is smooth and transparent and free of bubbles. This invention is not limited to the above description, and can be modified and implemented as desired within the scope of the spirit thereof.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of the double layer coating according to the present invention, and FIGS. 2 to 10 are micrographs showing the particle structure or fiber shape present on each surface. is an optical micrograph at a magnification of 50X showing a cross section of an actual double layer coating according to the present invention, FIG. 3 is an optical micrograph at a magnification of 200X showing a cross section of the same double layer coating as in FIG. Figure 3 and similar figure at 200X magnification showing a cross-section of a single layer coating by
Figure 5 is a photograph of the top surface of an actual vitreous upper layer containing cracks at room temperature, Figure 6 is a photograph of the upper surface of the same vitreous upper layer after it has been heated to 1350°C, and Figure 7 is a photograph of the top surface of the same vitreous upper layer after it has been heated to 1350°C. Optical micrograph at 1000X magnification showing the central cross section of the lower layer; Figure 8 is a magnification showing the same cross section of the lower layer in Figure 7 in more detail.
4000X optical micrograph, Figure 9 is a 1000X optical microscope photograph showing a cross section of the lower layer near the interface between the lower layer and the graphite matrix, and Figure 10 is a more detailed 4000X magnification of the same cross section of the lower layer in Figure 9. This is an optical microscope photograph. 10...lower layer, 12...substrate, 14...upper layer,
16...Mullite particles, 18...Aluminum oxide particles, 20...Metal silicon particles.

Claims (1)

Claims: 1. A method for producing a carbonaceous product protected against oxidation and erosion at elevated temperatures, comprising: (a) 30% by weight relative to the total weight of dry ingredients;
75% to 75% by weight mullite, up to 12% by weight silica, 15 to 60% by weight aluminum, 5 to 11
applying a particulate mixture comprising % by weight of a silicon-containing binder to the surface of a carbonaceous substrate; (b) heating the particulate mixture to a temperature at which the silica is reduced by aluminum to form aluminum oxide and silicon; A method comprising steps. 2. A method according to claim 1, wherein said silicon-containing binder is selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 3. The granular mixture contains 52.3% by weight of mullite;
A process according to claim 1, comprising 10.4% by weight silica, 26.7% by weight aluminum and 9.6% by weight sodium silicate. 4. A method according to claim 1, wherein the particulate mixture is applied to the surface of the carbonaceous substrate in the form of an aqueous slurry. 5. A method according to claim 4, wherein the slurry is dried under conditions that produce a hard continuous layer fixed to the carbonaceous substrate. 6. A method according to claim 5, wherein the slurry is dried in air at room temperature. 7. A method according to claim 5, wherein the dried continuous layer is heated to a temperature of at least 1000°C. 8. In a process for producing carbonaceous products protected against oxidation and erosion at high temperatures, (a) from 30 to 75% by weight relative to the total weight of dry ingredients;
12% mullite, up to 12% silica, 15% by weight
applying to the surface of the carbonaceous substrate a first particulate mixture comprising from 60% to 60% aluminum and from 5 to 11% silicon-containing binder; (b) 70% by weight relative to the total weight of dry ingredients; A second particulate mixture comprising up to 14% silica, up to 14% silica, a metal carbide selected from the group consisting of 5 to 95% silicon carbide and boron carbide, and 5 to 14% silicon metal binder to said first (c) heating said first particulate mixture, either before or after step (b), to a temperature at which silica is reduced by aluminum to form aluminum oxide and silicon; (d) heating the second particulate mixture to a temperature at which a rigid glass top layer is formed, either simultaneously with or after step (c). 9. A method according to claim 8, wherein the silicon-containing binder in the first particulate mixture is selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 10. A method according to claim 9, wherein the first particulate mixture comprises 52.3% by weight mullite, 10.4% by weight silica, 26.7% by weight aluminum and 9.6% by weight sodium silicate. 11. The method according to claim 8, wherein the silicon-containing binder in the second particulate mixture is selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 12. A method according to claim 11, wherein the second particulate mixture comprises 57.7% by weight mullite, 11.0% by weight silica, 20.3% by weight silicon carbide, and 10.6% by weight sodium silicate. 13. A method according to claim 12, wherein the first particulate mixture is applied to the surface of the carbonaceous substrate in the form of an aqueous slurry. 14. A method according to claim 13, wherein the slurry is dried under conditions that produce a rigid continuous layer adhered to the carbonaceous substrate. 15. A method according to claim 14, wherein the slurry is dried in air at room temperature. 16 said dried continuous layer has at least 1000
15. A method according to claim 14, wherein the method is dried to a temperature of °C. 17. A method according to claim 14, wherein said second particulate mixture is applied to said first particulate mixture in the form of an aqueous slurry. 18. A method according to claim 17, wherein the slurry is dried under conditions that result in a hard continuous layer fixed to the first particulate mixture. 19. A method according to claim 18, wherein the slurry is dried in air at ambient temperature. 20 The dried continuous layer is heated to at least 1000°C.
19. A method according to claim 18, wherein the method is heated to a temperature of . 21 A third particulate mixture is applied to the second particulate mixture, the third particulate mixture having a particle size of 25 to 85
% by weight of borosilicate frits and 5 to 15% by weight.
a silicon-containing binder, and wherein said third particulate mixture is heated to a temperature at which a rigid vitreous top layer is formed. 22. A method according to claim 21, wherein said third particulate mixture further comprises up to 65% by weight of filler. 23 The borosilicate frit contains 7 to 52% by weight of Na ₂ O, 8 to 64% by weight of B 2 O 3 , and 6 to 52% by weight of B ₂ O ₃ .
68% by weight of SiO ₂ . 24. A method according to claim 21, wherein said silicon-containing binder is selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 25 The filler is Al ₂ O ₃ , SiC, mullite,
A method according to claim 22, wherein the method is selected from the group consisting of SiO ₂ and B ₄ C. 26. A method for producing a carbonaceous product protected against oxidation and erosion at elevated temperatures, comprising: (a) 30 to 75% mullite and up to 12% silica, as % by weight relative to the total weight of the dry ingredients; 60% aluminum and 5% to 11% aluminum
(b) 25 to 85% by weight of borosilicate frit, based on the total weight of the dry ingredients; applying to said first particulate mixture a second particulate mixture comprising 15% silicon-containing binder; heating one particulate mixture before or after step (b); and (d) heating the second particulate mixture to a temperature sufficient to form a top layer simultaneously with or after step (c). and heating the method. 27. A method according to claim 26, wherein said second particulate mixture further comprises up to 65% by weight filler. 28. A method according to claim 26, wherein the silicon-containing binder in the first particulate mixture is selected from sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 29. A method according to claim 28, wherein the first particulate mixture contains 52.3% by weight mullite, 10.4% by weight silica, 26.7% by weight aluminum and 9.6% by weight sodium silicate. 30 The borosilicate frit comprises 7 to 52% by weight of _Na2O , 8 to 64% by weight of _B2O3 , and 6 to 68% by _weight of _SiO2 .
Method according to Section 6. 31. A method according to claim 26, wherein said silicon-containing binder is selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica. 32 The above filler is Al ₂ O ₃ , SiC, mullite,
28. A method according to claim 27, selected from the group consisting of SiO ₂ and B ₄ C. 33. A method according to claim 26, wherein said first particulate mixture is applied to the surface of said carbonaceous substrate in the form of an aqueous slurry. 34. A method according to claim 26, wherein the slurry is dried under conditions that result in a hard continuous layer adhered to the carbonaceous substrate. 35. A method according to claim 26, wherein the slurry is dried in air at ambient temperature. 36 said dried continuous layer has at least 1000
27. A method according to claim 26, wherein the method is heated to a temperature of °C. 37. Claim 2, wherein said second particulate mixture is applied to the first particulate mixture in the form of an aqueous slurry.
Method according to Section 6. 38. A method according to claim 37, wherein the slurry is dried under conditions that result in a hard continuous layer that adheres to the first particulate layer. 39. A method according to claim 38, wherein the slurry is dried in air at ambient temperature. 40. The method of claim 39, wherein the dry continuous layer is heated to a temperature of at least 1000°C.