Unburned brick manufacturing method

Abstract

A method for producing unburned bricks is provided that can suppress the deterioration of corrosion resistance and creep resistance due to the formation of low-melting-point substances and can also exhibit sufficient strength. [Solution] A refractory raw material blend containing 0.2 to 15% by mass of magnesia with a particle size of less than 75 μm and 0.3 to 10% by mass of silica with a particle size of less than 44 μm, with the remainder mainly being one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material, in which the total content of alkali metal phosphate and alkali metal silicate is less than 1% by mass (including 0), the total content of Na2O and K2O components is 0.5% by mass or less (including 0), and the content of P2O5 components is 0.7% by mass or less (including 0), is added with water and kneaded, press-molded, and then left at room temperature or heat-treated at a temperature of 1000°C or less.

Description

[Technical Field]

[0001] The present invention relates to a method for producing unburned bricks for use in coke ovens, blast furnaces, hot blast stoves, CDQs, molten steel ladles, molten iron ladles, secondary refining furnaces, and the like. [Background technology]

[0002] Sintered bricks whose chemical components are primarily Al2O3 and / or SiO2 (hereinafter referred to as "Al2O3 / SiO2-based sintered bricks") are used in coke ovens, blast furnaces, hot stoves, CDQs, molten steel ladles, molten iron ladles, secondary refining furnaces, etc. Examples include alumina bricks, alumina-silica bricks, clay bricks, silica bricks, alumina-magnesia bricks, alumina-spinel bricks, and spinel bricks. These bricks are generally produced by kneading a refractory raw material blend containing one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material, then pressurizing and molding the blend in a molding machine and firing the blend at high temperatures (see, for example, Patent Documents 1 to 3). However, the method for producing Al2O3 / SiO2-based fired bricks involves firing at high temperatures for long periods of time, which poses problems of CO2 gas emissions and high energy costs.

[0003] For this reason, studies have been conducted on non-fired Al2O3 / SiO2-based fired bricks. For example, in Patent Document 4, a siliceous precast block is produced by casting a monolithic refractory material mainly composed of fused silica, removing the form, and then drying. However, this method of producing precast blocks by casting requires 12 to 24 hours from the time the material is poured into the form until it is removed from the form, which results in poor molding efficiency.

[0004] On the other hand, unfired alumina and alumina-silica bricks using alkali metal silicates or alkali metal phosphates as binders are known. For example, Patent Document 5 discloses a method for producing unfired bricks using an alumina raw material and a magnesia raw material containing 90% by mass or more of fine powder of 0.5 mm or less, and using sodium silicate and potassium silicate as binders. However, when sodium silicate or potassium silicate is used as a binder as in Patent Document 5, the alkali metal oxides (NaO and KO) in the binder tend to react with other refractory raw materials to form low-melting-point substances, resulting in reduced corrosion resistance and creep resistance. Furthermore, when an alkali metal phosphate is used as a binder, there is a concern that the phosphorus component (PO) may leach into molten steel, making the use of phosphates undesirable.

[0005] On the other hand, Patent Document 6 discloses an alumina unburned brick to which 0.3 mass % of phosphate is added as a binder, but there is a problem that the amount of phosphate is too small to obtain sufficient strength. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 53-13642 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-65956 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-64956 [Patent Document 4] Patent No. 6823042 [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-145684 [Patent Document 6] Japanese Patent Application Publication No. 8-109062 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide a method for producing unburned bricks which can suppress the deterioration of corrosion resistance and creep resistance due to the formation of low-melting-point substances and which can also exhibit sufficient strength. [Means for solving the problem]

[0008] According to one aspect of the present invention, there is provided the following method for producing unburned bricks. A method for producing unfired bricks, comprising adding water to a refractory raw material blend containing 0.2 to 15% by mass of magnesia having a particle size of less than 75 μm and 0.3 to 10% by mass of silica having a particle size of less than 44 μm, with the remainder primarily being one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material, the total content of alkali metal phosphate and alkali metal silicate being 1% by mass or less (inclusive), the total content of NaO and KO being 0.5% by mass or less (inclusive), and the content of PO5 being 0.7% by mass or less (inclusive), the blend being kneaded, press-molded, and then left at room temperature or heat-treated at a temperature of 1000°C or less. According to another aspect of the present invention, there is provided the following method for producing unburned bricks. A method for producing unfired bricks, comprising the steps of adding water to and kneading a refractory raw material blend containing 0.2% by mass to 5% by mass of magnesia having a particle size of less than 75 μm, 10% by mass or less (inclusive) of magnesia having a particle size of 75 μm or more, and 0.3% by mass to 10% by mass of silica having a particle size of less than 44 μm, with the remainder being primarily an alumina raw material, and having an alkali metal phosphate and alkali metal silicate total content of 1% by mass or less (inclusive), a NaO and KO total content of 0.5% by mass or less (inclusive), and a PO5 content of 0.7% by mass or less (inclusive), and then press-molding the blend, followed by leaving it at room temperature or heat-treating it at a temperature of 1000°C or less. [Effects of the Invention]

[0009] In the production method of the present invention, as will be described in detail later, magnesium silicate hydrate is formed as a binder, which makes it possible to suppress the deterioration of corrosion resistance and creep resistance due to the formation of low-melting-point substances, and also to obtain unburned bricks that can exhibit sufficient strength. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present inventors have found that unfired bricks having sufficient corrosion resistance, creep resistance and strength can be obtained by adding water to a refractory raw material compound containing magnesia fine powder and silica fine powder, kneading the mixture, press-molding the mixture, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or less. The mechanism is thought to be as follows: Magnesia fine powder and silica fine powder form gel-like magnesium silicate hydrate in the presence of water. Therefore, by leaving the brick at room temperature, the moisture in the brick decreases, and the magnesium silicate hydrate functions as a binder in the brick matrix. Subsequent heat treatment raises the brick temperature, and even when the magnesium silicate hydrate dehydrates, it forms an amorphous magnesium silicate bond, maintaining strength. Further heating converts the amorphous magnesium silicate to forsterite, preventing a decrease in strength up to the operating temperature. Furthermore, forsterite has a high melting point and is stable during use, without forming low-melting-point compounds with other raw materials, and it has almost no adverse effect on creep resistance or corrosion resistance.

[0011] In one embodiment of the present invention, magnesia fine powder is used as the magnesia component for producing magnesium silicate hydrate in a content of 0.2% by mass to 15% by mass based on 100% by mass of the refractory raw material blend. Since the smaller the particle size of the magnesia fine powder, the easier it is to form a hydrate in the presence of water. Specifically, a magnesia fine powder with a particle size of less than 75 μm is used. That is, in one embodiment of the present invention, the refractory raw material blend contains 0.2% by mass to 15% by mass of magnesia fine powder, i.e., magnesia with a particle size of less than 75 μm. A content of magnesia fine powder less than 0.2% by mass results in insufficient bonding structure in the brick structure, resulting in low strength. A content of magnesia fine powder greater than 15% by mass results in increased amounts of reaction products with different volumes from the surrounding raw materials (e.g., secondary spinel, enstatite) and low-melting-point compounds (e.g., cordierite). This leads to a decrease in creep resistance in alumina-silica and silica-based materials, and increased residual expansion in alumina- and spinel-based materials, making them more susceptible to cracking during use.

[0012] As the content of magnesia fine powder increases, residual expansion increases, for example, in alumina. On the other hand, magnesia raw material is a refractory raw material that is effective in ensuring corrosion resistance. Therefore, in another embodiment of the present invention, the upper limit of the content of magnesia fine powder is reduced to 5 mass% from the viewpoint of reducing residual expansion and further improving volume stability, and magnesia with a particle size of 75 μm or more is contained at a content of 10 mass% or less (not including 0) from the viewpoint of ensuring corrosion resistance.

[0013] In the present invention, light-burned magnesia can also be used as the magnesia fine powder. Light-burned magnesia is obtained by firing magnesite, magnesium hydroxide, or the like at a relatively low temperature, for example, below 1400°C, usually between 1000 and 1400°C. It is generally called activated magnesia or calcined magnesia, and has an average particle size of 1 μm or less. This light-burned magnesia has a large specific surface area and is highly active, making it more soluble in water than sintered magnesia or electrofused magnesia. This allows for better development of magnesium silicate bonds in the brick structure, thereby increasing the strength of the brick. This light-burned magnesia can be used in a content of 0.2% by mass or more and 5% by mass or less based on 100% by mass of the refractory raw material composition.

[0014] In the present invention, fine silica powder, which is silica having a particle size of less than 44 μm, is used as the silica component for producing magnesium silicate hydrate in a content of 0.3 to 10 mass% based on 100 mass% of the refractory raw material blend. If the content of fine silica powder is less than 0.3 mass%, the bonding strength of the brick structure becomes insufficient, and if it exceeds 10 mass%, corrosion resistance and creep resistance decrease.

[0015] Examples of silica fine powder that can be used include silica flour, fused silica, silica sol, silica brick chips, and silica fine powder. Note that since silica sol is in liquid form, its content is calculated as SiO2. In addition, since silica flour has high reactivity, its content can be 0.3% by mass or more and 2.5% by mass or less. In the present invention, silica flour refers to finely divided silica particles with an average particle size of 10 μm or less, and collectively refers to silica fume, fumed silica, and white carbon. Silica fume is a by-product obtained by collecting exhaust gases generated during the refining process of ferrosilicon, metallic silicon, electrofused zirconia, etc., and is relatively inexpensive. Fumed silica is produced by hydrolysis of silicon compounds, such as silicon tetrachloride, in an oxygen and hydrogen flame. White carbon is obtained by decomposing sodium silicate using an acid or alkali. These silica flours are easily soluble in alkaline water in which magnesia fine powder is partially dissolved, allowing magnesium silicate bonds to develop well in the brick structure, thereby increasing the strength of the brick.

[0016] In one embodiment of the present invention, the raw material composition other than the magnesia fine powder and silica fine powder contained in the refractory raw material blend, i.e., the raw material composition of the remainder of the refractory raw material blend, is mainly one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material. In another embodiment of the present invention, the raw material composition other than the magnesia fine powder, magnesia and silica fine powder with a particle size of 75 μm or more contained in the refractory raw material blend, i.e., the raw material composition of the remainder of the refractory raw material blend, is mainly an alumina raw material. As the alumina raw material, one or more of electrofused alumina, sintered alumina, calcined alumina, bauxite, and bauxite can be used. As the alumina silica raw material, one or more of mullite, sillimanite, andalusite, kyanite, pyrophyllite, chamotte, and clay can be used. As the silica raw material, one or more of fused silica and silica stone can be used. The spinel raw material may contain Al2O3 at 60 to 90 mass %, MgO at 10 to 40 mass %, and the total content of Al2O3 and MgO at 95 mass % or more.

[0017] In one embodiment of the present invention, the remainder of the refractory raw material blend may contain, as refractory raw materials other than the above-mentioned alumina raw material, alumina-silica raw material, silica raw material, and spinel raw material, one or more of metal powder, silicon carbide, graphite, carbon black, zirconia, glass powder, zirconia, zircon, boron carbide, etc., in an amount of 5 mass% or less relative to 100 mass% of the refractory raw material blend. Thus, in one embodiment of the present invention, the raw material composition other than the magnesia fine powder and silica fine powder contained in the refractory raw material blend, i.e., the raw material composition of the remainder of the refractory raw material blend, is primarily composed of one or more of the alumina raw material, alumina-silica raw material, silica raw material, and spinel raw material. Therefore, the unfired brick according to the present invention is an unfired brick whose chemical components are primarily composed of an Al2O3 component and / or a SiO2 component. In another embodiment of the present invention, the remainder of the refractory raw material blend may also contain, as refractory raw materials other than the above-mentioned alumina raw material, one or more of metal powder, silicon carbide, graphite, carbon black, zirconia, glass powder, zirconia, zircon, boron carbide, etc., in an amount of 5 mass% or less relative to 100 mass% of the refractory raw material blend. Thus, in another embodiment of the present invention, the raw material composition other than the magnesia fine powder, magnesia and silica fine powder having a particle size of 75 μm or more contained in the refractory raw material blend, i.e., the raw material composition of the remainder of the refractory raw material blend, is strictly composed mainly of an alumina raw material, and therefore the unfired brick according to the present invention is an unfired brick containing Al2O3 as its main chemical component.

[0018] In the present invention, since magnesium silicate hydrate functions as a binder, alkali metal phosphates and alkali metal silicates do not need to be contained as binders as in conventional unburned bricks, and therefore can be omitted, but can also be used as supplementary additives. However, in this case, the total content should be 1% by mass or less, preferably 0.2% by mass or less. If the total content exceeds 1% by mass, corrosion resistance and creep resistance will decrease. Furthermore, in the case of alkali metal phosphates, there is a concern that phosphoric acid components will leach into molten steel. Here, alkali metal silicates include sodium silicate and potassium silicate, and alkali metal phosphates include sodium phosphate and potassium phosphate. As described above, in the present invention, since it is basically not necessary to use conventional alkali metal silicates or alkali metal phosphates as binders, the total content of the NaO and KO components in the refractory raw material blend can be set to 0.5 mass% or less (including 0), and the content of the P2O5 component can be set to 0.7 mass% or less (including 0). This makes it possible to suppress a decrease in corrosion resistance and creep resistance, and to suppress the elution of phosphorus into molten steel. Note that the NaO, KO, and P2O5 components are contained in the alkali metal silicates and alkali metal phosphates that serve as binders, as well as in refractory raw materials such as the alumina raw material. Therefore, the content in the refractory raw material blend is the sum of the content derived from the binder and the content derived from the refractory raw materials. Furthermore, in the present invention, it is basically not necessary to use organic binders such as pitch, phenolic resin, molasses, pulp waste liquor, dextrin, methylcellulose, lignin sulfonate, polyvinyl alcohol, etc., which are used in conventional unburned bricks. However, they can be used supplementarily in an amount of 1.5% by mass or less, more preferably 1% by mass or less, to supplementarily increase the strength after molding. Note that the content of organic binders containing solvents is the content as solids.

[0019] In the method for producing unburned bricks of the present invention, water is added to the above-mentioned refractory raw material mixture, and the mixture is kneaded and press-molded. Water may be added in an appropriate amount depending on the raw material composition, etc., to produce magnesium silicate hydrate as a binder and to ensure the moldability of the clay, and specifically, the amount may be 0.5 to 8% by mass based on 100% by mass of the refractory raw material blend. In this case, organic solvents compatible with water, such as ethylene glycol and methanol, may be used in combination with water. After press molding, magnesium silicate hydrate is produced, and removing some of the moisture provides sufficient handling strength. Therefore, sufficient strength for handling bricks can be obtained by leaving them at room temperature. Here, room temperature refers to the temperature inside a brick manufacturing plant, specifically, between 25°C and 60°C. The leaving time can be adjusted depending on the size and temperature of the brick; for example, 12 hours to 120 hours is sufficient. In addition, depending on the brick's intended use and conditions of use (for example, preheating temperature and time), if the temperature in the factory is below 25°C, or if the product needs to be completed quickly, heat treatment can be performed by selecting appropriate temperature conditions within the temperature range of 60°C to 1000°C.

[0020] Here, the particle size referred to in the present invention refers to the size of the sieve openings when the refractory raw material particles are sieved and separated. For example, magnesia with a particle size of less than 75 μm means magnesia that passes through a sieve with 75 μm openings, and magnesia with a particle size of 75 μm or more means magnesia that does not pass through a sieve with 75 μm openings. The average particle size in the present invention refers to the volume average particle size corresponding to the median cumulative value (D50) of the cumulative curve measured with a laser diffraction / scattering particle size distribution analyzer. [Example]

[0021] Tables 1 to 4 show the compositions of the refractory raw material blends in the examples of the present invention and comparative examples, and the evaluation results of the resulting bricks. The brick materials in Tables 1 to 4 are different: Table 1 shows alumina-silica unburned bricks, Table 2 shows silica unburned bricks, Table 3 shows spinel unburned bricks, and Table 4 shows alumina unburned bricks.

[0022] First, the refractory raw materials used in Tables 1 to 4 will be described. As the alumina raw materials, fused alumina, sintered alumina, and calcined alumina each having an Al2O3 content of 98 mass % or more were used. The alumina-silica raw materials used were andalusite with an Al2O3 content of 60 mass% and an SiO2 content of 37 mass%, sintered mullite with an Al2O3 content of 71 mass% and an SiO2 content of 27 mass%, chamotte with an Al2O3 content of 33 mass% and an SiO2 content of 64 mass%, and clay with an Al2O3 content of 40 mass% and an SiO2 content of 45 mass%. As the silica raw materials, fused silica with an SiO2 content of 99 mass% and silica stone with an SiO2 content of 98 mass% were used. The spinel raw material used was an electrofused spinel containing 22 mass % of MgO and 73 mass % of Al2O3. For the magnesia fine powder, sintered magnesia with an MgO content of 98% by mass or more was used for magnesia with a particle size of less than 75 μm, and light-burned magnesia with an MgO content of 98% by mass or more was used.Furthermore, for magnesia with a particle size of 75 μm or more, electrofused magnesia with an MgO content of 98% by mass or more was used. As the silica fine powder, silica stone with a particle size of less than 44 μm and silica flour with an SiO2 content of 98 mass % were used. As the alkali metal phosphate, sodium metaphosphate with a P2O5 content of 65 mass% was used. As the alkali metal silicate, sodium silicate used was sodium metasilicate with an Na2O content of 30 mass %.

[0023] Water was added at a rate of 3% by mass to the refractory raw material composition of each example shown in Tables 1 to 4, and the mixture was kneaded. The mixture was then formed into a shape of 230 × 114 × 100 mm using an oil press, and heat-treated at 250°C for 5 hours to produce unfired bricks. For some examples, bricks were left at room temperature (left at 25°C for 120 hours), and some were heat-treated at different temperatures. Samples for measuring physical properties were cut out from these bricks, and their bending strength was measured at room temperature. For samples heat-treated at 250°C, their corrosion resistance, residual expansion resistance, and creep resistance were evaluated.

[0024] The bending strength at room temperature was measured by three-point bending. Specifically, strip samples of 20 × 20 × 80 mm were prepared, and the bending strength was measured with a span of 50 mm and a compression rate of 1.65 kN / s. Corrosion resistance was evaluated for spinel and alumina materials, assuming their use as lining materials for molten metal vessels. Residual expansion was also evaluated for spinel and alumina materials. On the other hand, alumina-silica and silica materials are often used in applications that do not come into contact with molten iron or molten steel, so load-softening tests were conducted to evaluate creep resistance.

[0025] Corrosion resistance was evaluated by a rotational corrosion test. In the rotational corrosion test, the inner surface of a drum with a horizontal rotation axis was lined with test bricks, and slag was added and heated to erode the brick surface. The heat source was an oxygen-propane burner, the test temperature was 1700°C, and the slag composition was 30% by mass of CaO, 30% by mass of SiO2, 20% by mass of Al2O3, and 20% by mass of FeO + Fe2O3. The slag was discharged and added 10 times every 30 minutes. After the test, the erosion depth (mm) was calculated from the difference in thickness (mm) before and after the test at the most eroded part of each brick. A corrosion index was then calculated, with the erosion depth (mm) of the brick from "Comparative Example 11" listed in Table 3 being set to 100. A smaller corrosion index indicates better corrosion resistance.

[0026] Residual expansion was evaluated by using 20 x 20 x 80 mm strip samples, heat treating them in an air atmosphere at 1400°C for 5 hours, and then comparing the long dimensions before and after heat treatment. A dimensional change rate of less than 0.5% after heat treatment compared to before heat treatment was rated as ◯ (good), 0.5% to less than 1.0% as △ (passable), and 1.0% or more as × (poor).

[0027] Creep resistance was evaluated by measuring the softening point under load. The softening point under load was measured in accordance with JIS R2209 under a load of 0.2 MPa. When the difference in the 2% softening point temperature (T2) of the brick of each Example and Comparative Example relative to the 2% softening point temperature (T2) of the brick of the reference Example was less than 100°C, it was evaluated as ◯ (good); when it was 100°C or higher but less than 200°C, it was evaluated as △ (passable); and when it was 200°C or higher, it was evaluated as × (poor). Here, for Examples 1 to 7 and Comparative Examples 1 to 3, Example 1 was used as the reference; for Examples 8 to 12 and Comparative Examples 4 to 5, Example 8 was used as the reference; for Examples 13 to 20 and Comparative Examples 6 to 8, Example 13 was used as the reference; and for Examples 21 to 25 and Comparative Examples 9 and 10, Example 21 was used as the reference.

[0028] [Table 1]

[0029] [Table 2]

[0030] [Table 3]

[0031] [Table 4]

[0032] In Table 1, Examples 1 to 7 show alumina-silica unfired bricks in which the content of magnesia fine powder in the refractory raw material blend was different, but within the range of the present invention, and the bricks had sufficient strength and excellent creep resistance. On the other hand, Comparative Example 1 did not contain magnesia fine powder, and the strength after heat treatment was insufficient. In Comparative Example 3, the content of magnesia fine powder exceeded the upper limit specified in the present invention, resulting in poor creep resistance. In Comparative Example 2, the content of sodium silicate and the total content of Na2O and K2O components (Na2O + K2O) in the refractory raw material blend exceeded the upper limit specified in the present invention, resulting in poor creep resistance compared to Examples 3 and 4, in which these contents were within the range of the present invention.

[0033] In Table 1, Examples 8 to 12 are alumina-silica unfired bricks in which the content of silica fine powder in the refractory raw material blend was different, but within the range of the present invention, and they had sufficient strength and excellent creep resistance. On the other hand, Comparative Example 4 was an example in which no silica was contained, and the strength after heat treatment was insufficient. Comparative Example 5 had a content of silica fine powder that exceeded the upper limit specified in the present invention, resulting in poor creep resistance. Note that Examples 8 to 10 were heat-treated at different temperatures, but all of them obtained sufficient strength.

[0034] In Table 2, Examples 13 to 20 show unfired siliceous bricks in which the content of magnesia fine powder in the refractory raw material blend was different, but within the range of the present invention, and exhibited sufficient strength and excellent creep resistance. On the other hand, Comparative Example 6 did not contain magnesia fine powder, and the strength after heat treatment was insufficient. Comparative Example 8 had a magnesia fine powder content exceeding the upper limit of the present invention, resulting in poor creep resistance. Furthermore, Comparative Example 7 had a sodium silicate content and a total content of Na2O and K2O components (Na2O + K2O) in the refractory raw material blend exceeding the upper limit of the present invention, resulting in poor creep resistance compared to Examples 15 and 16, in which these contents were within the range of the present invention.

[0035] In Table 2, Examples 21 to 25 show unfired siliceous bricks in which the content of silica fine powder in the refractory raw material blend was different, but within the range of the present invention, and the bricks had sufficient strength and excellent creep resistance. On the other hand, Comparative Example 9 did not contain silica fine powder, and the strength after heat treatment was insufficient. Comparative Example 10 had a silica fine powder content exceeding the upper limit of the present invention, resulting in poor creep resistance. Furthermore, Examples 21 to 23 were heat treated at different temperatures, but all of them achieved sufficient strength.

[0036] In Table 3, Examples 26 to 32 show spinel unfired bricks in which the magnesia fine powder content in the refractory raw material composition was different, but within the range of the present invention, and the bricks had sufficient strength and excellent corrosion resistance and residual expansion. On the other hand, Comparative Example 11 did not contain magnesia, and the strength after heat treatment was insufficient. In Comparative Example 13, the magnesia fine powder content exceeded the upper limit of the present invention, resulting in large residual expansion. In Comparative Example 12, the sodium phosphate content and the P2O5 component content in the refractory raw material composition exceeded the upper limit of the present invention, resulting in inferior corrosion resistance compared to Examples 27 and 28, in which these contents were within the range of the present invention.

[0037] In Table 3, Examples 33 to 37 show spinel unfired bricks in which the content of silica fine powder in the refractory raw material blend was different, but within the range of the present invention, and the bricks had sufficient strength and excellent corrosion resistance and residual expansion. On the other hand, Comparative Example 14 did not contain silica fine powder, and the strength after heat treatment was insufficient. Comparative Example 15 had a silica fine powder content exceeding the upper limit specified in the present invention, resulting in poor corrosion resistance. Furthermore, Examples 33 to 35 were heat treated at different temperatures, but all of them achieved sufficient strength.

[0038] In Table 4, Examples 38 to 43 show alumina unfired bricks in which the content of magnesia fine powder in the refractory raw material composition was different, but within the range of the present invention, and the bricks had sufficient strength and excellent corrosion resistance and residual expansion. On the other hand, Comparative Example 16 did not contain magnesia fine powder, and the strength after heat treatment was insufficient. Comparative Example 19 had a magnesia fine powder content exceeding the upper limit of the present invention, resulting in large residual expansion. Furthermore, Comparative Example 18 had a sodium phosphate content and a P2O5 content exceeding the upper limit of the present invention in the refractory raw material composition, resulting in inferior corrosion resistance compared to Examples 40 and 41, whose contents were within the range of the present invention. Furthermore, Comparative Example 17 did not contain silica fine powder and contained 0.3 mass% sodium phosphate, but the strength was insufficient.

[0039] In Table 4, Examples 44 to 47 are alumina unfired bricks in which the content of silica fine powder in the refractory raw material blend was different, but within the range of the present invention, and the bricks had sufficient strength and excellent corrosion resistance and residual expansion. On the other hand, Comparative Example 20 was a case in which no silica fine powder was contained, and the strength after heat treatment was insufficient. Comparative Example 21 had a silica fine powder content exceeding the upper limit specified in the present invention, resulting in poor corrosion resistance. Furthermore, Examples 44 to 46 were heat treated at different temperatures, but all of them obtained sufficient strength.

[0040] In Table 4, Examples 48 and 49 are alumina unfired bricks containing magnesia having a particle size of 75 μm or more in the refractory raw material composition. These examples are within the scope of the present invention, and compared with Examples 39 and 42, which contain the same level of magnesia raw material content, they have excellent residual expansion properties and sufficient strength and corrosion resistance.

Claims

1. The composition contains 0.2% by mass or more and 15% by mass or less of magnesia having a particle size of less than 75 μm, 0.3% by mass or more and 10% by mass or less of silica having a particle size of less than 44 μm, and the balance is mainly one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material, and the total content of an alkali metal phosphate and an alkali metal silicate is 1% by mass or less (including 0), and Na ２ O component and K ２ The total content of O components is 0.5 mass% or less (including 0), and P ２ O ５ A method for producing unfired bricks, comprising adding water to a refractory raw material compound having a component content of 0.7% by mass or less (including 0), kneading the mixture, press-molding the mixture, and then leaving the mixture at room temperature or heat-treating the mixture at a temperature of 1000°C or less.

2. 2. The method for producing unfired bricks according to claim 1, wherein light-burned magnesia is used as the magnesia having a particle size of less than 75 μm.

3. 3. The method for producing unburned bricks according to claim 1 or 2, wherein silica flour is used as the silica having a particle size of less than 44 μm.

4. The composition contains 0.2% by mass or more and 5% by mass or less of magnesia having a particle size of less than 75 μm, 10% by mass or less (including 0) of magnesia having a particle size of 75 μm or more, and 0.3% by mass or more and 10% by mass or less of silica having a particle size of less than 44 μm, with the remainder being mainly an alumina raw material, and the total content of alkali metal phosphate and alkali metal silicate is 1% by mass or less (including 0), and Na ２ O component and K ２ The total content of O components is 0.5 mass% or less (including 0), and P ２ O ５ A method for producing unfired bricks, comprising adding water to a refractory raw material compound having a component content of 0.7% by mass or less (including 0), kneading the mixture, press-molding the mixture, and then leaving the mixture at room temperature or heat-treating the mixture at a temperature of 1000°C or less.

5. 5. The method for producing unfired bricks according to claim 4, wherein light-burned magnesia is used as the magnesia having a particle size of less than 75 μm.

6. 6. The method for producing unburned bricks according to claim 4 or 5, wherein silica flour is used as the silica having a particle size of less than 44 μm.

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