Casting materials for sintering furnaces

Abstract

This invention provides a sintering furnace casting material that eliminates the need for a drying step after casting, suppresses physical impact from furnace contents, and inhibits alkali attack. The sintering furnace casting material of this invention is made by adding colloidal silica to a refractory material mixture containing at least 75% by mass of one or more of alumina, alumina-silica, and silicon carbide raw materials, and containing 1% to 15% by mass of silica powder, and with a CaO content of 0.5% by mass or less (including zero) derived from alumina cement.

Description

Technical Field

[0001] This invention relates to casting materials for sintering furnaces used in incinerators, moving bed furnaces, industrial waste kiln treatment furnaces, circulating flow bed (CFB) boilers, cement manufacturing equipment furnaces, gasification melting furnaces, coal supply furnaces, etc. Prior Technology

[0002] Castable materials are a general term for monolithic refractories that can be cast, also known as castable refractories or castable materials. While cement (alumina cement, Portland cement, magnesia cement, etc.) is widely used as a binder in casting materials, its use causes a hydration reaction at room temperature, increasing the strength of the cast body and making dehydration difficult. If the furnace temperature rises, there is a risk of cracking. Therefore, a drying process after casting is necessary, but this process is costly and requires extended downtime.

[0003] On the other hand, silica sol, or colloidal silica, is also known as a casting material with a binder (e.g., Patent Documents 1-4). However, in sintering furnaces, such as incinerators, where materials are heat-treated (sintered, incinerated), the casting material that forms the furnace wall is susceptible to physical impact and erosion from the materials flowing inside the furnace. [Previous Technical Documents] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2015-168596 [Patent Document 2] Japanese Patent Application Publication No. 2018-111612 [Patent Document 3] Japanese Patent Application Publication No. 2019-119653 [Patent Document 4] Japanese Patent Application Publication No. 2019-142727 Summary of the Invention

[0005] [The problem the invention aims to solve] After a detailed investigation of the erosion reactions occurring in the sintering furnace, the inventors determined that alkali erosion, originating from alkaline components (Na₂O, K₂O, CaO, etc.) within the furnace contents, is the primary factor in the erosion reaction. Specifically, in the sintering furnace, alkali erosion lowers the viscosity of the operating surface (the surface of the workpiece), allowing alkali to easily penetrate into the workpiece. As a result, peeling occurs due to the expansion difference between the alkali-impregnated layer and the non-impregnated layer, thus initiating erosion.

[0006] In view of the above, the problem to be solved by the present invention is to provide a casting material for sintering furnaces that does not require a drying step after casting construction, can suppress physical impact from furnace contents and suppress alkali attack. [Methods used to solve problems]

[0007] In order to obtain a means to suppress alkali penetration caused by alkali corrosion in casting materials using colloidal silica as a binder, the inventors have repeatedly conducted experiments and research. The results show that generating a high-viscosity glass on the surface (operating surface) of the casting material is effective. Therefore, controlling the amount of silicate bonds in the matrix of the casting material is important. Specifically, it has been found that it is crucial to include silica powder in the refractory raw material formulation of the casting material at a specific content range, while simultaneously setting the addition rate of colloidal silica as a SiO2 component within a specific range.

[0008] The present invention is conceived based on these insights and technical ideas, and according to one viewpoint, provides the following casting material for sintering furnaces. A casting material for a sintering furnace is made by adding colloidal silica to a refractory material mixture containing at least 75% by mass of one or more of alumina raw materials (excluding calcined alumina) and alumina-silica raw materials, and containing at least 1% by mass and at least 15% by mass of silica powder, at least 4% by mass and at least 10% by mass of calcined alumina, and at least 0.5% by mass (inclusive) of CaO content derived from alumina cement, at an addition rate of 0.5% by mass and at least 5% by mass based on SiO2 content. Furthermore, according to other aspects of the present invention, the following casting materials for sintering furnaces are provided. A casting material for sintering furnace is made by adding colloidal silicon oxide to a refractory material mixture containing more than 75% by mass of silicon carbide raw material and more than 1% by mass and less than 15% by mass of silicon oxide powder, and the content of CaO derived from alumina cement being less than 0.5% by mass (inclusive). [Invention Effects]

[0009] According to the present invention, no drying step is required after the casting process, which can suppress physical impact from the furnace contents and suppress alkali corrosion. Implementation

[0010] The casting material for sintering furnaces of the present invention (hereinafter also referred to as "the casting material of the present invention") is made by adding colloidal silica to a refractory material mixture containing at least 75% by mass of one or more of alumina raw materials, alumina silica raw materials and silica carbide raw materials, and containing at least 1% by mass and less than 15% by mass of silica powder, and the content of CaO derived from alumina cement is at least 0.5% by mass (including zero), based on an addition rate of 0.5% by mass and less than 5% by mass of SiO2.

[0011] Thus, in the casting material of the present invention, the refractory raw material formulation is characterized by using one or more of alumina raw material, alumina silica raw material and silica carbide raw material as the main material, the silica powder contains more than 1% by mass and less than 15% by mass, and the content of CaO component derived from alumina cement is less than 0.5% by mass (including zero).

[0012] When the silica powder content is less than 1% by mass, sufficient silicate bonds cannot be formed in the matrix of the refractory material. Therefore, the effects of inhibiting alkali penetration and suppressing physical impact from furnace materials cannot be achieved. On the other hand, when the silica powder content exceeds 15% by mass, the refractory material becomes overly dense due to its fine powder composition. Therefore, if the furnace temperature rises, there is a risk of cracking. Furthermore, an excessive amount of fine powder in the refractory material formulation results in a relatively lower aggregate content, leading to a decrease in the compressive strength of the refractory material and a reduction in the effect of suppressing physical impact from furnace materials. The preferred silica powder content is 3% to 10% by mass.

[0013] Furthermore, in this invention, the content of CaO derived from alumina cement in the refractory raw material formulation is limited to 0.5% by mass or less (including zero). This is because CaO derived from alumina cement promotes alkali penetration caused by alkali corrosion. Moreover, CaO derived from alumina cement is a cause of explosions as described above, therefore a drying step must be performed after casting. Based on these considerations, the lower the content of CaO derived from alumina cement, the better, and ideally, it should be zero.

[0014] In the casting material of this invention, the refractory raw material formulation preferably contains 4% to 10% by mass of calcined alumina. By including calcined alumina within this range, the castable body can be appropriately densified, improving the effect of inhibiting alkali penetration and suppressing physical impact from materials inside the furnace. Furthermore, in this invention, calcined alumina is not equivalent to alumina raw material, which is one of the main materials of the refractory raw material formulation. That is, in this invention, the content of alumina raw material does not include the content of calcined alumina.

[0015] As described above, in this invention, colloidal silica is used as a binder, and its addition rate, based on the SiO2 content, is set to be 0.5% by mass or more and 5% by mass or less. When the addition rate of colloidal silica (referring to the addition rate based on the SiO2 content, hereinafter the same) is less than 0.5% by mass, sufficient silicate bonds cannot be formed in the matrix of the workpiece. Therefore, the effect of suppressing alkali penetration and the effect of suppressing physical impact from furnace contents are insufficient. On the other hand, when the addition rate of colloidal silica exceeds 5% by mass, the compactness of the workpiece is excessively reduced due to reasons such as increased porosity, and the compressive strength of the workpiece is also reduced. As a result, the effect of suppressing alkali penetration and the effect of suppressing physical impact from furnace contents are reduced. The addition rate of colloidal silica is preferably 1.5% by mass or more and 5% by mass or less.

[0016] In this invention, although colloidal silicon oxide is added as a binder to the above-mentioned refractory raw material formulation, various additives such as dispersants, hardening modifiers, and cracking inhibitors may also be added as appropriate. Dispersants are substances that impart fluidity during application. Specific examples include inorganic salts such as sodium tripolyphosphate, sodium hexametaphosphate, sodium hyperpolyphosphate, acidic sodium hexametaphosphate, sodium borate, sodium carbonate, and polymetaphosphate, as well as sodium citrate, sodium tartrate, sodium polyacrylate, sodium sulfonate, polycarboxylate, β-naphthalenesulfonate, and naphthalenesulfonic acid. Hardening modifiers include hardening accelerators and hardening retarders. Specific examples of hardening accelerators include quicklime, calcium chloride, gypsum, magnesium oxide, sodium aluminate, and lithium carbonate. Specific examples of hardening retarders include boric acid, oxalic acid, citric acid, gluconic acid, sodium carbonate, and sugar. Specific examples of anti-burst agents include organic fibers, organic foaming agents, alkaline aluminum lactate, and metallic aluminum. Specific examples of organic fibers include high molecular weight organic fibers such as vinylon (including polyvinyl alcohol), rayon, polyester, nylon, polypropylene, and polyethylene. These additives, like colloidal silicon dioxide used as a binder, are added at a rate of 100% by mass relative to the refractory material formulation. The addition rate can be the same as for general casting materials.

[0017] In addition to the aforementioned refractory raw material formulation, binder, and additives, the casting material of this invention may also use coarse particles with a particle size of 8 mm or more. These coarse particles have the function of preventing crack propagation within the refractory structure. As a material, similar to the main material of the refractory raw material formulation, one or more of alumina, aluminosilicate, and silicon carbide can be used. However, in the casting material of this invention, the coarse particles are not included in the refractory raw material formulation. That is, in the casting material of this invention, the coarse particles are added externally at 100% by mass relative to the refractory raw material formulation. In other words, in the casting material of this invention, the so-called refractory raw material formulation is a formulation of refractory raw materials with a particle size of less than 8 mm. [Example]

[0018] Table 1 shows the raw material preparation and evaluation results of the embodiments and comparative examples of the present invention. The evaluation items and evaluation methods in the embodiments and comparative examples are as follows.

[0019] <Alkali penetration resistance> A specific amount of water was added to the raw material formulations in each example and mixed to obtain crucible test pieces with φ30×35mm holes in a 70×70×65mm prismatic shape. 20g of commercially available sodium carbonate was placed into the hole of the crucible test piece, and it was heated at 1200℃ for 12 hours. After heating, it was cooled to room temperature (natural cooling), and then cut at the center of the crucible test piece (axial section of the hole), and the cut surface was observed. In the observation of the cut surface, the residual thickness at the site of maximum erosion was measured, and the retention rate relative to the thickness before the test was calculated. Alkali penetration resistance was evaluated using the erosion index when the retention rate of Comparative Example 1 was set to 100. The lower the erosion index, the better the resistance to alkali penetration, that is, the higher the effect of inhibiting alkali penetration caused by alkali attack. When the erosion index is less than 80, it is rated as ◎ (good); when it is 80 or above but less than 90, it is rated as 〇 (acceptable); when it is 90 or above but less than 100, it is rated as △ (unacceptable); and when it is 100 or above, it is rated as × (poor).

[0020] <Blast Resistance> A specific amount of water was added to the raw material formulation in each example and mixed. The mixture was then cast into a cylindrical mold frame (100mmφ×100mmh) and cured for 24 hours. Two cylindrical test pieces were obtained after removing the mold frame and placed in an electric furnace at 900°C for heating. After maintaining the temperature for 30 minutes, the test pieces were removed and evaluated for any cracking or bursting. The evaluation was as follows: no cracking was rated ◎ (Good); minor cracking was rated 〇 (Acceptable); partial surface fracture was rated △ (Unacceptable); and fracture or more than 50% damage was rated × (Poor). Each test piece was evaluated separately. The inventors confirmed that if the crack resistance evaluation is ◎ (Good) or 〇 (Acceptable), a drying step is not required after actual furnace casting.

[0021] <Abrasion Resistance> A specific amount of water was added to the raw material formulations in each example and mixed. The mixture was then cast into a 115×115×65mm mold and cured for 24 hours. After removal from the mold and drying, the resulting test pieces were preheated at 800°C for 5 hours. The test pieces were then tested according to ASTM C704 (Abrasion Resistance of Refractory Materials at Room Temperature). Specifically, the 115×115mm face was tilted at 45° relative to the direction of abrasive ejection and fixed. The test piece was then rotated at 15 rpm around the vertical centerline of the 115×115mm face while 1 kg of abrasive was continuously blown onto the 115×115mm face. Silicon carbide particles with a particle size adjusted to 1~0.3mm were used as the abrasive. The blowing air pressure was 0.4 MPa. The mass of the test piece before and after the test was measured, and the abrasion volume was calculated based on the mass change and the loose density of the test piece. Abrasion resistance was evaluated using the abrasion index, where the abrasion volume of Comparative Example 1 was set to 100. The smaller the abrasion index, the better the abrasion resistance, meaning a higher effect in suppressing physical impacts from materials inside the furnace. An abrasion index below 80 was rated ◎ (good), 80 or above but below 90 was rated 〇 (acceptable), 90 or above but below 100 was rated △ (unacceptable), and 100 or above was rated × (poor).

[0022] <Overall Evaluation> When all three evaluations for alkali penetration resistance, crack resistance, and abrasion resistance are ◎, the evaluation is ◎ (Good); when at least one of these evaluations is ○, the evaluation is ○ (Acceptable); when at least one of these evaluations is △, the evaluation is △ (Unacceptable); and when at least one of these evaluations is ×, the evaluation is × (Poor). In other words, the overall evaluation is displayed in the order of ◎, ○, △, ×, indicating a poor rating. An overall evaluation of ◎ or ○ is considered acceptable, while △ and × are unacceptable.

[0023]

[0024] Examples 1-6 are examples in which the main material of the refractory raw material formulation is alumina, and the content of silicon oxide powder varies within the range of this invention. The overall evaluation is ◎ or ○, indicating good results. Among them, Examples 2-4 and 6, in which the content of silicon oxide powder is within the preferred range of 3% by mass to 10% by mass, have an overall evaluation of ◎, indicating particularly good results. In contrast, Comparative Example 1, in which the chlorine content of the silicon oxide powder is below the lower limit of the present invention, shows a significant reduction in alkali penetration resistance and abrasion resistance. Furthermore, Comparative Example 2, in which the chlorine content of the silicon oxide powder exceeds the upper limit of the present invention, shows a significant reduction in crack resistance and insufficient abrasion resistance.

[0025] Example 7 is an example where the CaO content derived from alumina cement contains 0.5% by mass. Compared to Example 6, which does not contain CaO derived from alumina cement, the alkali penetration resistance and crack resistance are slightly reduced, but still within acceptable limits. In contrast, Comparative Example 3 is an example where the CaO content derived from alumina cement is 1% by mass, exceeding the upper limit of the present invention, resulting in a significant reduction in alkali penetration resistance and crack resistance.

[0026] Reference Example 8 is an example without calcined alumina. Compared with Example 6, which contains calcined alumina, the alkali penetration resistance and wear resistance are slightly reduced, but still within acceptable limits. Examples 9 and 10 are examples of varying the content of calcined alumina, but similar to Example 6, the overall evaluation is ◎, indicating good results.

[0027] Examples 11-13 involve changes to the main materials of the refractory raw material formulation, but all remain within the scope of this invention and yield good results. By comparing Examples 12 and 13, it can be seen that when the main material of the refractory raw material formulation is silicon carbide, good results can be obtained even without calcined alumina.

[0028] Examples 14-16 are examples in which alumina is used as the main raw material in the refractory material formulation, and the addition rate of colloidal silica is varied within the range of this invention. The overall evaluation is ◎ or ○, indicating good results. Among them, Examples 15 and 16, where the addition rate of colloidal silica is preferably within the range of 1.5% by mass to 5% by mass, receive an overall evaluation of ◎, indicating particularly good results. In contrast, Comparative Example 4, in which the addition rate of colloidal silicon oxide was lower than the lower limit of the present invention, showed a significant decrease in abrasion resistance and insufficient resistance to alkali penetration. Comparative Example 5, in which the addition rate of colloidal silicon oxide exceeded the upper limit of the present invention, showed a significant decrease in both alkali penetration resistance and abrasion resistance.

Claims

1. A casting material for a sintering furnace, comprising adding colloidal silica to a refractory material mixture containing at least 75% by mass of one or more of alumina raw materials other than calcined alumina and alumina silica raw materials, and containing at least 1% by mass and at least 15% by mass of silica powder, at least 4% by mass and at least 10% by mass of calcined alumina, and at least 0.5% by mass (inclusive) of CaO content derived from alumina cement, at an addition rate of at least 0.5% by mass and at least 5% by mass based on SiO2 content.

2. A casting material for a sintering furnace, which is made by adding colloidal silica to a refractory material mixture containing 75% or more silicon carbide raw material and 1% or more to 15% or less silica powder, and whose CaO content derived from alumina cement is 0.5% or less (inclusive) or less (inclusive) based on SiO2 content.

3. The casting material for the sintering furnace as requested in item 1 or 2, wherein the content of silicon oxide powder is more than 3% by mass and less than 10% by mass.

4. For the casting materials used in the sintering furnaces as requested in item 1 or 2, the addition rate of colloidal silicon oxide is 1.5% by mass or more and 5% by mass or less, based on the SiO2 content.