Method for manufacturing non-fired brick

Abstract

The present invention provides a method for manufacturing a non-fired brick whereby reductions in corrosion resistance and creep resistance due to the generation of low melting point substances can be suppressed, and sufficient strength can be exhibited. In the present invention, water is added to and mixed with a refractory raw material formulation containing 0.2-15 mass% of magnesia having a particle diameter of less than 75 μm and 0.3-10 mass% of silica having a particle diameter of less than 44 μm, with the remainder being 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 ratio of alkali metal phosphates and alkali metal silicates being less than 1 mass% (including 0). Thereafter, the mixture is press molded and then left standing at room temperature or heat treated at a temperature of 1000°C or less.

Description

Unburned brick manufacturing method

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

[0002] Coke ovens, blast furnaces, hot stoves, CDQs, molten steel ladles, molten iron ladles, secondary refining furnaces, etc. contain Al as a chemical component. ２ O ３ component and / or SiO ２ Fired bricks whose main component is Al ２ O ３ / SiO ２ These bricks are generally manufactured by kneading a refractory raw material mixture containing one or more of alumina raw material, alumina silica raw material, silica raw material and spinel raw material, and then pressurizing and molding the mixture in a molding machine and firing it at a high temperature (see, for example, Patent Documents 1 to 3). However, Al bricks are often used. ２ O ３ / SiO ２ In the manufacturing method of fired bricks of this type, firing is carried out at high temperatures for a long time, so CO ２ There are problems with gas emissions and high energy costs.

[0003] For this reason, Al ２ O ３ / SiO ２ Studies have been conducted on non-firing fired bricks of this type. 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 it. However, this method of producing a precast block by casting requires 12 to 24 hours after pouring the material into the form until the form is removed, which results in poor molding efficiency.

[0004] On the other hand, unfired alumina and alumina-silica bricks using alkali metal silicate or alkali metal phosphate as a binder 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 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 oxide in the binder is likely to react with other refractory raw materials to produce 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 phosphorus components may leach into molten steel, so the use of phosphate is not preferred.

[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.

[0006] Japanese Patent Publication No. 53-13642 Japanese Patent Application Laid-Open No. 2017-65956 Japanese Patent Application Laid-Open No. 2016-64956 Japanese Patent No. 6823042 Japanese Patent Application Laid-Open No. 2007-145684 Japanese Patent Application Laid-Open No. 8-109062

[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.

[0008] According to one aspect of the present invention, there is provided the following method for producing an unburned brick: A method for producing an unburned brick, comprising adding water to a refractory raw material blend containing from 0.2 to 15% by mass of magnesia having a particle size of less than 75 μm and from 0.3 to 10% by mass of silica having 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, and having an alkali metal phosphate and alkali metal silicate content of less than 1% by mass (including 0), followed by kneading the blend and press-molding, followed by leaving the blend at room temperature or heat-treating at a temperature of 1,000° C. or less. According to another aspect of the present invention, there is provided the following method for producing an unburned brick: A method for producing an unfired brick, comprising the steps of adding water to and kneading a refractory raw material blend containing 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 (inclusive of 0 mass) 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 being less than 1% by mass (inclusive of 0 mass), and then press-molding the blend, followed by leaving the blend at room temperature or heat-treating the blend at a temperature of 1000°C or less.

[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.

[0010] The present inventors have discovered that unfired bricks with sufficient corrosion resistance, creep resistance, and strength can be obtained by adding water to a refractory raw material blend containing magnesia fine powder and silica fine powder, kneading the blend, press-molding the blend, and then leaving the blend at room temperature or heat-treating the blend at a temperature of 1000°C or less. The mechanism behind this is believed to be as follows: Magnesia fine powder and silica fine powder form gel-like magnesium silicate hydrate in the presence of water. Therefore, leaving the brick at room temperature reduces the moisture content, allowing the brick to function as a binder in the brick matrix. Subsequent heat treatment increases the brick temperature, dehydrating the magnesium silicate hydrate, but forming an amorphous magnesium silicate bond maintains 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, preventing the formation of low-melting-point compounds with other raw materials, and therefore poses little 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, a specific 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. If the magnesia fine powder content is less than 0.2% by mass, the brick structure will lack a sufficient bonding structure, resulting in low strength. If the magnesia fine powder content exceeds 15% by mass, the amount of reaction products (e.g., secondary spinel, enstatite) with different volumes from the surrounding raw materials and low-melting-point compounds (e.g., cordierite) will increase, resulting in a decrease in creep resistance in alumina-silica and silica-based bricks, and increased residual expansion in alumina- and spinel-based bricks, making them more susceptible to cracking during use.

[0012] As described above, when 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 easily 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 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, while if it exceeds 10 mass%, the corrosion resistance and creep resistance decrease.

[0015] As the silica fine powder, silica flour, fused silica, silica sol, silica brick chips, silica fine powder, etc. can be used. Note that since silica sol is in a liquid state, its content is SiO ２The total mass of silica flour is calculated as a percentage of the total mass of the brick. Because silica flour is highly reactive, its content can be set to 0.3% by mass or more and 2.5% by mass or less. In the present invention, silica flour refers to fine 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, silicon metal, electrofused zirconia, etc., and is relatively inexpensive. Fumed silica is produced by hydrolysis of silicon compounds, such as silicon tetrachloride, in an oxygen-hydrogen flame. White carbon is obtained by decomposing sodium silicate with 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 remaining portion of the refractory raw material blend, is primarily 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 having a particle size of 75 μm or more, and silica fine powder contained in the refractory raw material blend, i.e., the raw material composition of the remaining portion of the refractory raw material blend, is primarily 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, pyrolite, chamotte, and clay can be used. As the silica raw material, one or more of fused silica and silica stone can be used. As the spinel raw material, Al ２ O ３ The content of Al is 60 to 90 mass %, the content of MgO is 10 to 40 mass %, ２ O ３ and MgO in a total content of 95% by 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, pitch, carbon black, zirconia, glass powder, zirconia, zircon, boron carbide, etc., in a content 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 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 does not contain Al as a chemical component. ２ O ３ component and / or SiO ２ The result is an unfired brick containing the alumina raw material as the main component. Also, in another 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, one or more of metal powder, silicon carbide, graphite, pitch, 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 solely composed of the alumina raw material, and therefore the unfired brick according to the present invention does not contain Al as a chemical component. ２ O ３ The resulting bricks are unfired and contain the main 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 included as binders as in conventional unburned bricks. Although these can be used as supplementary binders, they can be omitted. However, in this case, the total content is less than 1 mass%, preferably less than 0.2 mass%. If the total content exceeds 1 mass%, corrosion resistance and creep resistance decrease. Furthermore, in the case of alkali metal phosphates, there is a concern about the leaching of phosphoric acid components into molten steel. Here, alkali metal silicates include sodium silicate and potassium silicate, and alkali metal phosphates include sodium phosphate and potassium phosphate. Furthermore, in the present invention, sufficient strength can basically be obtained without the use of other binders, but other binders can also be used in combination. That is, binders commonly used in refractory bricks can also be used in combination. However, the total content is preferably 1.5 mass% or less, more preferably less than 1 mass%.

[0019] In the method for producing unburned bricks of the present invention, water is added to the above-described refractory raw material mixture, kneaded, and then press-molded. Water may be added in an appropriate amount depending on the raw material composition, etc., to generate magnesium silicate hydrate as a binder and ensure moldability of the clay. Specifically, water may be added at a rate of 0.5 to 8 mass% based on 100 mass% of the refractory raw material mixture. In this case, a water-compatible organic solvent, such as ethylene glycol or methanol, may be used in combination with water. After press-molding, magnesium silicate hydrate is generated, and removing some of the water ensures sufficient handling strength. Therefore, sufficient strength for handling can be obtained by leaving the mixture at room temperature. Here, room temperature refers to the air temperature in 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 to 120 hours is sufficient. Furthermore, depending on the brick's intended use and conditions of use (e.g., preheating temperature and time), if the temperature in the factory is below 25°C, or if it is desired to complete the product quickly, appropriate temperature conditions can be selected within the temperature range of 60°C to 1000°C for heat treatment.

[0020] The particle size referred to in the present invention is the size of the sieve openings when the refractory raw material particles are sieved and separated. For example, magnesia having a particle size of less than 75 μm means magnesia that passes through a sieve with 75 μm openings, and magnesia having a particle size of 75 μm or more means magnesia that does not pass through a sieve with 75 μm openings. Furthermore, the average particle size referred to in the present invention is 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.

[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 were each Al ２ O ３ The alumina silica raw material used was one with a content of 98% by mass or more. ２ O ３ Content is 60% by mass, SiO ２ The sintered mullite has an Al content of 37% by mass. ２ O ３ Content is 71% by mass, SiO ２ The chamotte contains 27% by mass of Al. ２ O ３ Content is 33% by mass, SiO ２ The clay has an Al content of 64% by mass. ２ O ３ Content is 40% by mass, SiO ２ The silica raw material used was fused silica with a content of 45% by mass. ２ The silica content is 99% by mass, and the silica content is SiO ２ The spinel raw material used was a material with an MgO content of 22% by mass and Al content of 98% by mass. ２ O ３ The magnesia fine powder used was sintered magnesia with an MgO content of 98% by mass or more 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 for magnesia with a particle size of 75 μm or more. The magnesia with an MgO content of 98% by mass or more for magnesia with a particle size of 75 μm or more was fused magnesia. The silica fine powder used was silica stone with a particle size of less than 44 μm and SiO ２ Silica flour with a content of 98% by mass was used. As the alkali metal phosphate, sodium phosphate was used. ２ O ５ Sodium metaphosphate with a content of 65% by mass was used. As the alkali metal silicate, sodium silicate was used. ２ Sodium metasilicate with an O content of 30% by mass was used.

[0023] Water was added to the refractory raw material compositions of each example shown in Tables 1 to 4 at a rate of 3% by mass, 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 strengths were 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 measuring 20 × 20 × 80 mm were prepared, and bending strength was measured at a span of 50 mm and a pressure rate of 1.65 kN / s. Corrosion resistance was evaluated for the spinel and alumina materials, assuming their use as lining materials for molten metal vessels. Residual expansion was also evaluated for the spinel and alumina materials. On the other hand, a load-softening test was performed on the alumina-silica and silica materials to evaluate their creep resistance, since they are often used in applications that do not come into contact with molten iron or molten steel.

[0025] The corrosion resistance was evaluated by a rotary erosion test. In the rotary erosion test, the inner surface of a drum with a horizontal rotating shaft 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 CaO: 30 mass%, SiO ２ :30% by mass, Al ２ O ３ :20% by mass, FeO+Fe ２ O ３ The slag content was 20% by mass, and the slag discharge and charging were repeated 10 times every 30 minutes. After the test, the corrosion amount (mm) was calculated from the difference in thickness (mm) between before and after the test at the most corroded part of each brick. A corrosion index was calculated, with the corrosion amount (mm) of the brick of "Comparative Example 11" listed in Table 3 taken as 100. A smaller corrosion index value indicates better corrosion resistance.

[0026] Residual expansion was evaluated by comparing the longitudinal dimensions of a 20 x 20 x 80 mm strip sample before and after heat treatment at 1400°C for 5 hours in an air atmosphere. A dimensional change rate of less than 0.5% after heat treatment compared to before heat treatment was evaluated 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 and 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]

[0029]

[0030]

[0031]

[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. Comparative Example 3 had a magnesia fine powder content exceeding the upper limit of the present invention, resulting in poor creep resistance. Furthermore, Comparative Example 2 had a sodium silicate content exceeding the upper limit of the present invention, resulting in poor creep resistance compared to Examples 3 and 4, which had sodium silicate contents within the range of the present invention.

[0033] In Table 1, Examples 8 to 12 show 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 the bricks had sufficient strength and excellent creep resistance. On the other hand, Comparative Example 4 did not contain silica, and the strength after heat treatment was insufficient. Comparative Example 5 had a silica fine powder content exceeding 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 achieved 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 the bricks had 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 exceeding the upper limit of the present invention, resulting in poor creep resistance compared to Examples 15 and 16, which had sodium silicate contents within the range of the present invention.

[0035] In Table 2, Examples 21 to 25 show unburned 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 specified in 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 are spinel 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 corrosion resistance and residual expansion. On the other hand, Comparative Example 11 was a case in which no magnesia was contained, and the strength after heat treatment was insufficient. In Comparative Example 13, the content of magnesia fine powder exceeded the upper limit of the present invention, resulting in large residual expansion. In Comparative Example 12, the content of sodium phosphate exceeded the upper limit of the present invention, resulting in inferior corrosion resistance compared to Examples 27 and 28, which had sodium phosphate contents 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 magnesia fine powder content 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 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 exceeding the upper limit of the present invention, resulting in inferior corrosion resistance compared to Examples 40 and 41, which had sodium phosphate contents 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 brick 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 of 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 are superior in residual expansion, and also have sufficient strength and corrosion resistance.

Claims

1. A method for producing unfired bricks, comprising the steps of adding water to a refractory raw material mixture containing 0.2% by mass to 15% by mass of magnesia having a particle size of less than 75 μm, 0.3% by mass to 10% by mass of silica having a particle size of less than 44 μm, and the remainder being primarily one or more of an alumina raw material, an alumina-silica raw material, a silica raw material, and a spinel raw material, with the total content of alkali metal phosphate and alkali metal silicate being less than 1% by mass (including 0), and then kneading the mixture with water, press-molding the mixture, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or less.

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

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

4. A method for producing unfired bricks, comprising the steps of adding water to a refractory raw material mixture containing 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 (not 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 mainly being alumina raw material, and the total content of alkali metal phosphate and alkali metal silicate being less than 1% by mass (including 0), and then kneading the mixture with water, press-molding the mixture, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or less.

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. A method for producing an unfired brick 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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