FI71719C - Refractory coating with high emissivity, process for making it and coating composition for use therein. - Google Patents

Description

1 71719

The present invention relates to a high emitting refractory coating which enables a very efficient use of thermal energy in industrial furnaces or other heating devices, to a method for producing a highly emitting refractory coating. composition when appropriate.

A one-component coating composition is known which is prepared using a mixture of silicon carbide and phosphate as a binder. This composition can be applied, for example, to the wall of an industrial furnace to improve its thermal efficiency. However, the refractory coating formed with this composition has a few disadvantages. At high temperatures, e.g., 1000 ° C or above, in addition to being likely to peel off the furnace wall, it also tends to cause the surface layers of the bricks to peel off the wall. Even at moderate temperatures, chemical reactions and thermal stress develop both within the contact surface and the binder-impregnated refractory material, and depend on the refractory material. They cause the structure to break and become brittle, leading to cracking and peeling of the coating. In particular, in the case of highly corrosive refractory materials having a low coefficient of elasticity, their low compressive strength tends to cause the refractory materials to decompose because the stress is concentrated in the part whose thermal and physical properties have changed due to foreign material contact or impregnation and the rest. area.

2,71719 GB patent 1,095,191 describes high temperature resistant inorganic coating compositions using lithium sodium silicate as a carrier. GB patent application 2,066,802 relates to a refractory material for casting, in which the refractory component consists of compounds A 2 O 2, ZrO 2 and SiO 2. U.S. Patent No. 3,810,768 discloses a refractory composition comprising coarse clay or bauxite and carbon particles, and U.S. Patent No. 3,892,584 relates to monolithic refractory materials comprising alumina, silicon carbide and carbon, as well as a chemical binder. Abstr. Vol. 93 (1980), 154 863r describes a refractory mortar.

The present invention relates to a refractory coating having a high black body emissivity which adheres to the refractory wall of the tii-15 bevel furnace, forming a refractory and mechanically strong black base thereon and which lasts for a long time without peeling.

Second, the present invention relates to a method of making a high emitting refractory coating having the properties described above.

The present invention further relates to a coating composition suitable for the manufacture of the highly emitting refractory coating described above.

This invention relates to a highly emitting refractory coating consisting of a cured coating composition consisting of a powder mixture containing 17-22% by weight of silicon carbide, 6-11% by weight of graphite, 17-22% by weight of alumina, 2-5% by weight of silicon nitride, 8-12% by weight of 12% by weight of tricobalt tetraoxide, 18-22% by weight of cobalt alumina and 18-23% by weight of iron, based on the total weight of the powder mixture; and an aqueous solution of potassium silicate, and a method of preparing a high emitting refractory coating, the method comprising mixing a powder mixture and an aqueous solution of potassium silicate to form a suspension, applying this suspension to the surface of furnace refractories or ceramic fibers and drying, curing, for the manufacture of refractory coatings.

3,71719

Brief explanation of the drawings

Figure 1 is a graphical representation of changes in emissivity (£) of refractory coatings as a function of time.

5 Figure 2 illustrates the arrangement used to study pavement deformations in Example 3.

Figures 3- (A) and 3-— (B) are photographs of the deformations of the coatings in Example 4 and Comparative Example 1.

The present invention preferably uses silicon carbide having a purity of at least 85%. The silicon carbide preferably has a particle size of 280 mesh (about 0.051 mm) or more, even more preferably 300-340 mesh (about 0.048-0.042 mm) and most preferably 320 mesh (about 0.044 mm). Silicon carbide is generally present in 17-22% of the total weight of the powder mixture. It forms a hard-15-bond bond with graphite and results in a coating that is able to withstand sudden heat fluctuations, aging caused by deformations and corrosion, and has a high thermal emission capacity. The graphite preferably has a solid carbon content of at least 94%, more preferably 97% 20 or greater, and preferably has a particle size of 280 mesh (about 0.051 mm) or greater, even more preferably 280-350 mesh (about 0.051-0.041 mm). ) and most preferably 300 mesh (about 0.048 mm), and forms a covalent bond with silicon carbide, giving the coating a high thermal emissivity, a high melting point and a high degree of hardness. Graphite is usually present in an amount of 6-100% based on the total weight of the powder mixture. The alumina preferably has a purity of at least 79% and a particle size of 280 mesh (about 0.051 mm) or greater, more preferably 280-350 mesh (about 0.041 mm) and most preferably 300 mesh (about 0.048 mm), 30 and may contain Fe 2 C > 2, SiO 2 and MgO. It gives the coating a high degree of hardness and good heat resistance and enhances the reaction of the coating ingredients. Alumina is generally present in 17-22% of the total weight of the powder mixture. Silicon nitride helps the applied film retain its strength when sintered and forms an oxide barrier layer that protects the inside of the coating. The silicon nitride preferably has a silicon content of 50-70%, more preferably 55-60%, and a particle size of 280 mesh (approx.

0.051 mm) or more, more preferably 280-350 mesh 471719 (about 0.051-0.041 mm), and contains other elements such as N, Fe and Al. It gives the coating abrasion resistance, breakage resistance and corrosion resistance. Silicon nitride is generally present in 2-5% of the total weight of the powder mixture.

Tricobalt tetroxide, Co 2 O 2, preferably has a molecular weight of 240, preferably has the analytical purity shown below, and preferably has a particle size of 200 mesh (about 0.074 mm) or greater, more preferably 200-300 mesh (about 0.074-0.048). mm) and most preferably 200 mesh (about 0.074 mm). Co 2 O 2, tri-10 cobalt tetroxide, having a spinel structure Co ** Co 11 o 4 (3K crystal, lattice constant α = 8 A and a melting point of 1950 ° C) and a purity of at least 95%, is a catalyst and improves together with carbide adhesion of the coating to refractories. Tricobalt tetroxide is generally present in 8-12% of the total weight of the powder-15 mixture.

The aluminum content of cobalt alumina (Al-Co) is preferably about 40-60%, more preferably 50-60%, and the particle size is 200 mesh (about 0.074 mm) or greater, even more preferably 200-300 mesh (about 0.074-0.048 mm). , and acts as a catalyst 20 such as tricobalt tetroxide. To form a stable and strong bond between the coating and the refractories, it is important to use a suitable combination of cobalt alumina and tricobalt tetroxide. Cobalt alumina is generally present in an amount of 18-22% of the total weight of the powder mixture. If only one of them is used or their ratio is not suitable, it is impossible to achieve the advantages of the present invention.

The silicon content of the iron is preferably at least 75%, even more preferably 89% or more, and the particle size is 200 mesh (about 0.074 mm) or more, even more preferably 200-30,300 mesh (about 0.074-0.048 mm), and forms a high in the heat state of a SiC ^ sn or Si ^ N ^ rn-like compound. It is inert and excellent in providing heat resistance and creep strength. In addition, the iron is polymerized by reaction with alumina and cobalt oxide to improve the stability and activity of the catalyst and the strength of the coating at high temperature. Iron is usually present in 18-23% of the total weight of the powder mixture.

Each of these components is finely ground with an ultra-fine particle size to the above-mentioned particle size, and mixed in a predetermined ratio into a powder mixture.

An aqueous solution of potassium silicate 1 → 0 S1O2 », preferably n = 3 or 4, is prepared by adding 55-65 5 parts by weight of water to 50-60 parts by weight of potassium silicate.

The mixture of it and the powder mixture forms half-melted, transition-state crystals on heating, and the solid solution forms a strong bond with the refractories.

The powder mixture and the aqueous potassium silicate solution in a weight ratio of 1: 1 to 1: 1.5 are mixed together, and the mixture is mixed in a suitable container to form a homogeneous suspension. The suspension is applied to the surface of the refractories evenly by spraying or by any other suitable method. The amount of suspension to be applied depends on the conditions used in the oven.

In general, the preferred amount of suspension to be applied is about 2,400-3000 g / m 2. The coating thus applied hardens if it is allowed to dry for about 3 hours, but it is better to sinter it by heating at 200-600 ° C for about 6 hours.

The highly emitting refractory coating obtained by the method of the present invention is inert and has a dense structure. It is thermally and chemically stable. It does not undergo any changes at high temperatures and is highly resistant to any corrosive substances. Its adhesion strength to furnace refractories is much better than any similar coating known in the art.

30 Adhesion strength refers to the ultimate strength of the bottom of a pavement when the bottom is loaded with a certain amount at different temperatures. It has a high degree of volume stability at elevated temperature (e.g., about 1800 ° C) and high thermal emission capacity. It greatly improves the heat transfer in the furnace and increases the effective heat of the furnace, thus enabling the furnace to operate with a high energy efficiency. Thermal emissivity is measured with an optical pyrometer at a wavelength of transmitted light of about 0.65. The method of the present invention is suitable for regular shaped bricks, cast refractories and ceramic fibers which form e.g. furnace walls, furnace bottoms, combustion chamber columns and inner walls of various types of industrial furnaces used in the petroleum, petrochemical and steel industries.

The coating made in accordance with the present invention retains a high emissivity (£) for a longer time than any conventionally used coating. For example, it maintains an emissivity value (E,) of 0.9-0.95 at 1000 ° C for 15 months, in contrast to conventional materials such as chamotte materials, as shown in Figure 1. The refractory coating of this invention forms an ideal a black body that is western deep black in color.

The invention will now be described in more detail by means of the following examples. However, these examples do not limit the scope of the invention.

Example 1

The following substances were thoroughly mixed together in a mixer equipped with a mixer to obtain 57.5 kg of powder mixture:

Substance Amount (% by weight) Size _ _ __________ (mesh) (mm.)

Silicon Carbide 20 320 n.0,044

Graphite (carbon content 97%) 7 300 n.0,048 30 Alumina (purity 79%) 20 300 n.0.048

Silicon nitride (silicon content 55%) 3,325 n.0,044

Tricobalt tetroxide (95% purity) 10,200 n.0,074

Al-Co (Al content 50%) 20,200 n.0,074 35 Draw (silicon content 89%) 20,200 n.0,074 7,71719

75 kg of potassium silicate solution was prepared by dissolving 30 kg of potassium silicate in 45 kg of water. The powder mixture was added to the solution and mixed thoroughly to form a uniform suspension. The Levi-5 suspension was applied to the ceiling and side walls of a 500 g / m 2 spray gun with a push beam type continuous annealing furnace having a capacity of 30 t / h and constructed of refractory bricks (SK38) containing ceramic fibers. The furnace was heated with heavy oil (C), 10 and operated at 1100-1350 ° C for 8 months continuously. The deformation of the surface of the coating beaten with a soft iron ball weighing 1.5 kg at a distance of 100 cm was examined, and the blackness of the surface was measured with an optical pyrometer (^ - 0.65 μm). The coating 15 remained firmly attached to the brick, and the surface remained black (£ = 0.94). Its brightness and density were excellent. No oxide layer and gas binding were observed. The results can be summarized as follows: 1. Oven: Beam-type continuous annealing furnace.

2. Capacity and fuel: 30 t / h; heavy oil C.

3. Test object: Fuel consumption (1 / t) during 6 months.

4. Oven temperature: 1350 ° C; preheating zone 1100 ° C, 12 30 ° C.

25 5. Bricks: Refractory SK38 bricks and ceramic fibers.

6. Material used: Metal bars.

7. Covered area: 235 m2.

8. Fuel consumption: 30 (a) Before coating: 51 1 / t (b) After coating: 44 1 / t, which means a reduction of 13% 9. Operating time: March 1981-31 October 1981 10. Average monthly fuel consumption: 35 51 1 / tx 14 000 t = 714 000 1 s 71719 11. Average monthly operating costs: 14 000 t (production) x 7 1 (fuel savings per tonne; $ 1 1 - 44 1) x 67 yen (oil price) = 6 566 000 yen.

5 12. Average energy (fuel) savings 13%.

Example 2_

The following substances were thoroughly mixed in a ball mill equipped with a stirrer to give 12 kg of powder mixture.

10 Substance Weight Particle size ___________ __ (%! __ (mesh) (mm.)

Silicon carbide (purity 85%) 18,324 n.0,044

Graphite (carbon content 97% 10,300 n.0,048

Alumina (purity 79%) 20 300 n.0,048 15 Silicon nitride (Si content 55%) 4 325 n.0,044

Tricobalt tetroxide (95% purity) 8,200 n.0,074

Al-Co (Al content 50%) 22 220 n.0.070

Draw (Si content 89%) 18,200 n.0,074 20 An aqueous solution of 14.5 kg of potassium silicate was prepared by dissolving 7 kg of potassium silicate in 7.5 kg of water. The finely divided powder mixture was dispersed in the solution to prepare a uniform suspension. The Levi-2 suspension was applied with a 500 g / m spray gun to all the inner walls of the above-mentioned type 25 electric furnace.

The coating was allowed to dry for about 3 h and then dried, gradually raising the temperature from 150 ° C and maintaining it at 520 ° C for about 4 h. Summary of results: 1. Oven: Batch type electric oven No. 13 30 2. Capacity and heat source: 3290 kg / month; electricity 3. Test object: Power consumption (kg / kWh) during 6 months 4. Oven temperature: 520 ° C.

5. Material used: Aluminum sheets 2 6. Coated area: 50 m 35 7. Energy consumption: (a) Before coating: 3 789 511 Kcal (average per charge) 9 71719 8. Frequency of use: Average 10 charges per month 9. Power consumption: 13 140 kW / month on average.

10. Power savings: 34,078 kWh - 26,240 kWh = 7,838 kWh.

Example 3 The process of the invention was used to coat a ceramic fiber substrate.

1. Test method (1) Coating material: Two KAOWOOL 1400 sheets (Isolite Industrial Co., Ltd., Japan), each measuring 600 mm x 400 mm x 20 mm (2) Coating material: Highly emitting refractory according to the invention , prepared 3 by dissolving 300 g of powder mixture in 450 cm of solvent. The powder mixture contained 22% silicon carbide, 7% graphite, 20% alumina, 8% tricobalt tetroxide, 18% cobalt alumina and 23% drawing. The solvent was 55% aqueous potassium silicate.

2. Application Method (1) Application Device: A spray gun of the general 20 types used for coating purposes.

(2) Amount applied:

Sample A B C

_______ (bottom) Coated 2i range 550 nm x 350 nm 550 nm x 350 nm - amount of powder (g) 90 200 0

Solvent (s) 150 3300 0 Coating 30 thickness (nm) 0.3 0.7 0 3. Heating conditions Temperature 950 ° C; time 150 h.

The temperature was raised and lowered 4 times and ranged from about 500 ° C to 950 ° C for 24 h. Samples A, B 35 and C were placed horizontally in an electric oven and maintained at the above conditions.

4. Examination of deformations

Each sample 2 was placed on a table and the distance between the ruler 1 and sample 2 was measured at each end b of sample 5 and at its center a. This was done both longitudinally (600 mm) and widthwise (400 mm) before and after heating the sample to see if the shape of the sample changed under the influence of heat.

10 5_. Test results Sample Appearance Deformation

Width Length A Unchanged 1 mm 0.5 mm B No coating peeling. _ where from. 1 mm 1 mm

Constant

C

(bottom) Unchanged 0.5 mm 0.5 mm

As shown in the table, the shape of the samples did not change under the influence of heat.

The refractory coating according to the invention did not show any softening or creep, although they were heated rapidly and a certain load was applied to the substrate. Many experiments demonstrated the effectiveness of the coating prepared according to the invention.

The refractory coating according to the invention withstands the highest tested temperature of 1700 ° C.

400 g of a powder mixture was prepared by mixing the following materials: 11,71719

Material Weight Particle size _ (%) (mesh) (mm.)

Silicon carbide (purity 85%) 19,325 n.0,044

Graphite (carbon content 97%) 8,280 n.0,051

Alumina (purity 79%) 20 300 n.0,048 5 Silicon nitride (Si content 55%) 3 220 n.0,070

Tricobalt tetroxide (purity 95%) 10,230 n.0,068

Al-Co (Al content 50%) 22 220 n.0.070

Draw (Si content 89%) 18,200 n.0,074

A 590 g aqueous solution of potassium silicate was prepared by dissolving 270 g of potassium silicate in 320 g of water. The powder mixture was added to the solution and mixed thoroughly to prepare a uniform suspension. Suspension 2 was applied with a brush to the inner walls (burner-flame parts) of an 800 g / m continuous furnace 15 (Petroleum Ethylene Cracker) constructed of refractory bricks (LBK28, large porosity). The furnace was heated with natural gas and maintained at 1100-1200 ° C for 12 months, after which it was cooled to room temperature (Sample A in Example 4).

The following materials were thoroughly mixed as a control:

Material Weight Particle size (%) (mesh) (mm.) 25 Silicon carbide (SiC) 40,285 (approx. 0.051)

Zirconia (ZrO 2) 2 260 (n.0,058)

Aluminum phosphate / A1 (H „P0.), PH 37 4 50 ---

Water 8 --- 30 The obtained mixture solution was applied to the inner walls of the oven, and the procedure of Example 4 (Sample B in Comparative Example 1) was repeated. Samples A and B were compared for coating deformation and blackness. The results are shown in the photographs (Figures 3- (A) and 3- (B)).

As shown in Figure 3- (A), sample A according to the invention formed a black surface with a solid structure and good adhesion. The emissivity (i) of Sample A was measured with an optical pyrometer. As a result, the emissivity value (£) 0.95 was maintained.

5 This value (L) remained almost unchanged compared to the value measured at the time of coating.

On the other hand, it can be seen from Fig. 3- (A) that in Sample B, the oxidation reaction, rapid thermal expansion changes, and rupture of the phosphate-moistened refractory base-10 material cause both internal brick damage and pavement fracture and peeling at high temperature. In the light of the high temperature physical properties of SiC, the cracking of the coatings can be considered as deformations caused by the difference between the high temperature thermal expansion and the Ald 2 PO 2 coefficient of longitudinal expansion and the chemical reaction between the refractory bricks and the molten solution.

In addition, the emissivity (s) of the remaining sample B coatings were measured with an optical pyrometer.

It was as low as 0.71 due to the softening damage of SiC caused by oxidation as well as the conversion of SiC to SiO 2.

In addition, the coatings formed on the bricks 25 (Samples A and B) were partially removed and the surfaces of the bricks from which the coating had been removed were examined, photograph of Figure 3- (B). As shown in Fig. 3- (B), no damage to the brick surface was observed in the case of Sample A, while corrosion and damage to the brick surfaces was observed in the case of Sample B.

From the above results, it can be seen that the coating according to the invention retained good adhesion to the brick as well as high surface blackness.

13 71719

Although the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications may be made to the invention without departing from the spirit and scope thereof.

Claims (9)

1. High emitting, refractory surface coating composition, characterized in that it consists of a powder mixture containing 17-22 wt.% Silicon carbide, 6-11 wt.% Graphite, 17-22 wt.% Alumina, 2-5 wt. % silica nitride, 8-12 wt% tricobalt tetroxide, 18-22 wt% cobalt alumina ox 18-18 wt% ferro silicon; and an aqueous solution of potassium silicate. Refractory coating composition according to claim 1, characterized in that the weight ratio of powder mixture to aqueous potassium silicate is in the range of 1: 1 to 1: 1.5. Refractory coating composition according to claim 1, characterized in that the aqueous potassium silicate solution has a potassium silicate content of 50-60% by weight. 4. High emitting, refractory coating, characterized in that it comprises a cured coating composition consisting of a powder mixture containing 17-22 wt% silicon carbide, 6-11 wt% graphite, 17-22 wt% alumina, 2 -5 wt% silicon nitride, 8-12 wt% tricobalt tetroxide, 18-22 wt% cobalt alumina and 18-23 wt% ferro silicon; and an aqueous solution of potassium silicate. 25 Process for the preparation of a high emitting, refractory coating, characterized in that a powder mixture containing 17-22 wt% silicon carbide, 6-11 wt% graphite, 17-22 wt% alumina, 2-5 wt% silicon nitride, 8-12 wt% tricobolt tetroxide, 18-22 wt% cobalt aluminum oxide and 18-23 wt% ferro silicon, and an aqueous solution of potassium silicate are mixed together to give a suspension; the suspension is applied to the surface of a material to form a refractory coating thereon; and the coating is dried to cure the same. 35 6. A process for producing a high emitting, refractory coating according to claim 5, characterized in that the drying is carried out by heating the coating to a temperature in the range 200-600 ° C.