Manufacturing method of unfired bricks

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

Technical Field

[0001] The present invention relates to a method for manufacturing unburned bricks used in coke ovens, blast furnaces, hot blast stoves, CDQs, steel melting pots, hot metal ladles, secondary refining furnaces, etc.

Background Art

[0002] In coke ovens, blast furnaces, hot blast stoves, CDQs, steel melting pots, hot metal ladles, secondary refining furnaces, etc., fired bricks having Al2O3 component and / or SiO2 component as the main chemical components (hereinafter referred to as "Al2O3 / SiO2 based fired bricks") are used. For example, alumina bricks, alumina-silica bricks, clay bricks, silica bricks, alumina-magnesia bricks, alumina-spinel bricks, spinel bricks, etc. These bricks are generally manufactured by kneading a refractory raw material mixture containing one or more of alumina raw materials, alumina-silica raw materials, silica raw materials, and spinel raw materials, then pressure-molding with a molding machine and firing at a high temperature (for example, Patent Documents 1 to 3). However, in the manufacturing method of Al2O3 / SiO2 based fired bricks, there are problems of high CO2 gas emission and high energy cost due to firing at a high temperature for a long time.

[0003] Therefore, the non-firing of Al2O3 / SiO2 based fired bricks has been studied for a long time. For example, in Patent Document 4, an amorphous refractory mainly composed of fused silica is poured and molded, and after demolding, it is dried to produce a silica-based precast block. However, in such a manufacturing method of precast blocks by pouring and molding, there is a problem of poor molding efficiency because it takes 12 to 24 hours from pouring the material into the molding frame to demolding.

[0004] On the other hand, unfired bricks using alkali metal silicates or alkali metal phosphates as binders are known for alumina and alumina-silica bricks. For example, Patent Document 5 discloses a method for producing unfired bricks using alumina raw material and magnesia raw material containing 90% or more by mass 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, alkali metal oxides in the binder tend to react with other refractory raw materials to form low-melting-point substances, which leads to problems such as reduced corrosion resistance and creep resistance. Furthermore, when alkali metal phosphates are used as binders, there are concerns about the leaching of phosphorus components into the molten steel, so the use of phosphates is undesirable.

[0005] On the other hand, Patent Document 6 discloses an alumina-based unfired brick to which 0.3% by mass of phosphate has been 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 Publication No. 2017-65956 [Patent Document 3] Japanese Patent Publication No. 2016-64956 [Patent Document 4] Patent No. 6823042 [Patent Document 5] Japanese Patent Publication No. 2007-145684 [Patent Document 6] Japanese Patent Application Publication No. 8-109062 [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide a method for producing unfired bricks that can suppress the decrease in corrosion resistance and creep resistance caused by the formation of low-melting-point materials, while also exhibiting sufficient strength. [Means for solving the problem]

[0008] According to one aspect of the present invention, a method for producing the following unfired brick is provided. Magnesia with a particle size of less than 75 μm As lightly baked magnesia 0.2% by mass or more 5 A method for producing unfired bricks, comprising: adding water to a refractory raw material mixture containing 0.3% to 10% by mass of silica with a particle size of less than 44 μm and the remainder mainly consisting of one or more of alumina raw materials, alumina silica raw materials, silica raw materials, and spinel raw materials, and having a total content of alkali metal phosphates and alkali metal silicates of 1% by mass or less (including 0); 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. Law. Furthermore, according to another aspect of the present invention, the following method for producing unfired bricks is provided. Magnesia with a particle size of less than 75 μm As lightly baked magnesia A method for producing unfired bricks involves adding water to a refractory raw material mixture containing 0.2% to 5% by mass of , 10% or less by mass (excluding 0) of magnesia with a particle size of 75 μm or larger, and 0.3% to 10% by mass of silica with a particle size of less than 44 μm, with the remainder mainly consisting of alumina raw materials, and the total content of alkali metal phosphates and alkali metal silicates being 1% or less by mass (including 0), kneading the mixture, press-forming it, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or lower. Law. [Effects of the Invention]

[0009] In the manufacturing method of the present invention, although the details will be described later, magnesium silicate hydrate is formed as a binder, so that the decrease in corrosion resistance and creep resistance due to the formation of low-melting-point materials can be suppressed, and an unfired brick that can exhibit sufficient strength can be obtained. [Modes for carrying out the invention]

[0010] The inventors have discovered that by adding water to a refractory raw material mixture containing magnesia fine powder and silica fine powder, kneading it, and then press-molding it, leaving it at room temperature or heat-treating it at a temperature of 1000°C or lower, an unfired brick with sufficient corrosion resistance, creep resistance, and strength can be obtained. The mechanism is thought to be as follows: Magnesia and silica fine powders form a gel-like magnesium silicate hydrate in the presence of water, so even when left at room temperature, the moisture content in the brick decreases, and they function as a binder in the brick matrix. Subsequently, even if the temperature of the brick rises due to heat treatment and the magnesium silicate hydrate dehydrates, an amorphous magnesium silicate bond is formed, maintaining strength. Furthermore, when heated, the amorphous magnesium silicate becomes forsterite, suppressing the decrease in strength until the usage temperature. Moreover, forsterite has a high melting point and is stable during use without forming low-melting-point substances with other raw materials, so it hardly adversely affects creep resistance or corrosion resistance.

[0011] In one aspect of the present invention, magnesia fine powder is used as a magnesia component for producing magnesium silicate hydrate in a content of 0.2% to 15% by mass in 100% by mass of the refractory raw material formulation. Since the smaller the particle size of the magnesia fine powder, the easier it is to form hydrate in the presence of water, specifically, a particle size of less than 75 μm is used. That is, in one aspect of the present invention, the refractory raw material formulation contains 0.2% to 15% by mass of magnesia fine powder, which is magnesia with a particle size of less than 75 μm. If the magnesia fine powder content is less than 0.2% by mass, the connective tissue in the brick structure is insufficient, resulting in low strength. If it exceeds 15% by mass, the amount of reaction products (secondary spinel, enstatite, etc.) and low-melting-point compounds (cordierite, etc.) with different volumes from the surrounding raw materials increases. In alumina-silica and silica-based materials, a decrease in creep resistance becomes a problem, and in alumina and spinel-based materials, residual expansion increases, making cracking more likely during use.

[0012] As the content of magnesia fine powder increases, residual expansion increases, for example, in alumina-based materials. On the other hand, magnesia raw materials are effective refractory materials for ensuring corrosion resistance. Therefore, in another aspect of the present invention, the upper limit of the content of magnesia fine powder is reduced to 5% by mass from the viewpoint of reducing residual expansion and further improving volume stability, and magnesia with a particle size of 75 μm or larger is included at a content of 10% by mass or less (excluding 0) from the viewpoint of ensuring corrosion resistance.

[0013] In this invention, calcined magnesia can also be used as magnesia powder. Calcined magnesia is obtained by calcining magnesite, magnesium hydroxide, etc., at a relatively low temperature, for example, 1400°C or lower, usually 1000 to 1400°C, and is generally called activated magnesia or calcined magnesia, with an average particle size of 1 μm or less. Because this calcined magnesia has a large specific surface area and high activity, it dissolves more easily in water compared to sintered magnesia and electrofused magnesia, so that magnesium silicate bonds develop well in the brick structure and the strength of the brick can be increased. This calcined magnesia can be used in a content of 0.2% to 5% by mass in 100% by mass of the refractory raw material mixture.

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

[0015] As silica fine powder, silica flower, fused silica, silica sol, silica brick waste, or silica fine powder can be used. Since silica sol is in liquid form, its content should be calculated on an SiO2 basis. In the case of silica flower, due to its high reactivity, its content can be between 0.3% and 2.5% by mass. In the present invention, silica flower refers to fine particle silica with an average particle size of 10 μm or less, and generally refers to silica fume, fumed silica, and white carbon. Silica fume is a by-product obtained by collecting exhaust gas generated during the refining process of ferrosilicon, metallic silicon, fused zirconia, etc., and can be obtained relatively inexpensively. Fumed silica is produced by hydrolyzing silicon compounds such as silicon tetrachloride in a flame of oxygen and hydrogen as raw materials. Also, white carbon is obtained by decomposition using an acid or an alkali of sodium silicate. Since these silica flowers are easily soluble in alkaline water in which part of the magnesia fine powder is dissolved, the bond of magnesium silicate can be well developed in the brick structure to enhance the strength of the brick.

[0016] In one aspect of the present invention, the raw material composition other than the magnesia fine powder and silica fine powder contained in the refractory raw material formulation, that is, the raw material composition of the remainder of the refractory raw material formulation, is mainly one or more of alumina raw materials, alumina-silica raw materials, silica raw materials, and spinel raw materials. In another aspect of the present invention, the raw material composition other than the magnesia fine powder, magnesia with a particle size of 75 μm or more, and silica fine powder contained in the refractory raw material formulation, that is, the raw material composition of the remainder of the refractory raw material formulation, is mainly an alumina raw material. As the alumina raw material, one or more of fused alumina, sintered alumina, calcined alumina, bauxite, and banket can be used. As the alumina-silica raw material, one or more of mullite, sillimanite, andalusite, kyanite, wax stone, 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, those with an Al2O3 content of 60 to 90% by mass, a MgO content of 10 to 40% by mass, and a total content of Al2O3 and MgO of 95% by mass or more can be used.

[0017] In addition, in one aspect of the present invention, as refractory raw materials other than the above-described alumina raw material, alumina-silica raw material, silica raw material, and spinel raw material in the remainder of the refractory raw material formulation, one or more of metal powder, silicon carbide, graphite, pitch, carbon black, zirconia, glass powder, zirconia, zircon, and boron carbide, etc. can be contained at a content rate of 5% by mass or less in the proportion occupied in 100% by mass of the refractory raw material formulation. Thus, in one aspect of the present invention, the raw material composition other than the magnesia fine powder and silica fine powder contained in the refractory raw material formulation, that is, the raw material composition of the remainder of the refractory raw material formulation, is, at most, mainly 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 becomes an unfired brick mainly composed of the Al2O3 component and / or SiO2 component as chemical components. Also, in another aspect of the present invention, in the remainder of the refractory raw material formulation, as refractory raw materials other than the above-described alumina raw material, one or more of metal powder, silicon carbide, graphite, pitch, carbon black, zirconia, glass powder, zirconia, zircon, and boron carbide, etc. can be contained at a content rate of 5% by mass or less in the proportion occupied in 100% by mass of the refractory raw material formulation. Thus, in another aspect 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 formulation, that is, the raw material composition of the remainder of the refractory raw material formulation, is, at most, mainly composed of the alumina raw material. Therefore, the unfired brick according to the present invention becomes an unfired brick mainly composed of the Al2O3 component as chemical components.

[0018] In this 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 unfired bricks, and therefore they do not need to be used, although they can be used as auxiliary binders. However, in this case, the total content should be less than 1% by mass, preferably less than 0.2% by mass. If the total content of these is 1% by mass or more, corrosion resistance and creep resistance will decrease. In the case of alkali metal phosphates, there is a concern that the phosphoric acid component may dissolve into the molten steel. Here, alkali metal silicates are sodium silicate and potassium silicate, and alkali metal phosphates are sodium phosphate and potassium phosphate. Furthermore, in this invention, sufficient strength can be obtained without using other binders, but it is also possible to use other binders in combination. In other words, in this invention, binders commonly used for refractory bricks can also be used. However, it is preferable that the total content of such binders be 1.5% by mass or less, and more preferably less than 1% by mass.

[0019] In the method for producing unfired bricks of the present invention, water is added to the above-mentioned refractory raw material mixture and kneaded, and then pressed and molded. Water should be added in an appropriate amount depending on the raw material composition to produce magnesium silicate hydrate as a binder and to ensure the moldability of the clay. Specifically, it can be added at an addition rate of 0.5% to 8% by mass per 100% by mass of the refractory raw material mixture. In this case, organic solvents such as ethylene glycol or methanol, which are compatible with water, may be used in combination with water. After press molding, magnesium silicate hydrate is formed, and removing some of the moisture ensures sufficient handling strength. Therefore, even leaving the bricks at room temperature provides sufficient strength for handling. Here, room temperature refers to the temperature inside the brick manufacturing plant, specifically between 25°C and 60°C. The leaving time should be adjusted according to the size and temperature of the bricks; for example, 12 to 120 hours is sufficient. Furthermore, depending on the intended use and conditions of the bricks (e.g., preheating temperature and time), if the factory temperature is below 25°C, or if it is desired to complete the product 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 as used in this invention refers to the size of the sieve opening when refractory raw material particles are separated by sieving. For example, magnesia with a particle size of less than 75 μm is magnesia that can pass through a sieve with an opening of 75 μm, and magnesia with a particle size of 75 μm or more is magnesia that cannot pass through a sieve with an opening of 75 μm. Furthermore, in this invention, the average particle size refers to the volume-average particle size corresponding to the central cumulative value (D50) of the cumulative curve measured by a laser diffraction scattering particle size analyzer. [Examples]

[0021] Tables 1 to 4 show the composition of the refractory raw material formulations in the examples and comparative examples of the present invention, and the evaluation results of the obtained bricks. Note that the brick materials differ in Tables 1 to 4: Table 1 shows alumina-silica unfired bricks, Table 2 shows silica-based unfired bricks, Table 3 shows spinel-based unfired bricks, and Table 4 shows alumina-based unfired bricks.

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

[0023] Unfired bricks were produced by adding water at a rate of 3% by mass to the refractory raw material formulations of each example listed in Tables 1-4, kneading the mixture, and then forming it into a shape of 230 × 114 × 100 mm using an oil press. The bricks were then heat-treated at 250°C for 5 hours. In addition, for some examples, bricks were prepared by leaving them at room temperature (25°C for 120 hours) and by varying the heat treatment temperature. Samples for physical property measurement were cut from these bricks, and their bending strength at room temperature was measured. For those heat-treated at 250°C, the corrosion resistance, residual expansion resistance, and creep resistance were evaluated.

[0024] The bending strength at room temperature was measured using a three-point bending method. Specifically, a 20 x 20 x 80 mm strip sample was prepared, and its bending strength was measured with a span of 50 mm and a pressure rate of 1.65 kN / s. Corrosion resistance was evaluated for spinel and alumina materials, assuming their use as lining material for molten metal containers. Residual expansion properties were also evaluated for spinel and alumina materials. On the other hand, since alumina-silica and silica materials are often used in applications that do not come into contact with molten iron or molten steel, a load softening test was performed to evaluate their creep resistance.

[0025] Corrosion resistance was evaluated by a rotary erosion test. In the rotary erosion test, the inner surface of a drum with a horizontal rotation axis was lined with the test brick, 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% by mass, SiO2: 30% by mass, Al2O3: 20% by mass, FeO + Fe2O3: 20% by mass. The discharge and addition of slag was repeated 10 times every 30 minutes. After the test, the amount of erosion (mm) was determined from the difference in thickness (mm) before and after the test at the most eroded part of each brick. Then, the erosion index was calculated, with the amount of erosion (mm) of the brick in "Comparative Example 11" listed in Table 3 set to 100. A smaller value of this erosion index indicates better corrosion resistance.

[0026] Residual expansion properties were evaluated using 20 x 20 x 80 mm strip samples. These were heat-treated at 1400°C for 5 hours in an air atmosphere, and the dimensions of the longer strips were compared before and after heat treatment. A dimensional change rate of less than 0.5% after heat treatment was rated as ○ (good), 0.5% to less than 1.0% as △ (acceptable), and 1.0% or more as × (poor).

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

[0028] [Table 1]

[0029] [Table 2]

[0030] [Table 3]

[0031] [Table 4]

[0032] In Table 1, Examples 1 to 7 show that although the content of magnesia fine powder in the refractory raw material formulation differs in alumina silicate unfired bricks, they are within the scope of the present invention, exhibiting sufficient strength and excellent creep resistance. On the other hand, Comparative Example 1 did not contain magnesia fine powder, resulting in insufficient strength after heat treatment. 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, where the sodium silicate content was within the scope of the present invention.

[0033] In Table 1, Examples 8-12 show that while the silica fine powder content in the refractory raw material formulation differs for alumina-silica unfired bricks, it remains within the scope of the present invention, exhibiting sufficient strength and excellent creep resistance. On the other hand, Comparative Example 4 did not contain silica, resulting in insufficient strength after heat treatment. Comparative Example 5 exceeded the upper limit of the present invention in silica fine powder content, resulting in poor creep resistance. In Examples 8-10, heat treatment was performed at different temperatures, but sufficient strength was obtained in all cases.

[0034] In Table 2, Examples 13 to 20 show that while the content of magnesia fine powder in the refractory raw material formulation differs in siliceous unfired bricks, it remains within the scope of the present invention, exhibiting sufficient strength and excellent creep resistance. On the other hand, Comparative Example 6 did not contain magnesia fine powder, resulting in insufficient strength after heat treatment. Comparative Example 8 exceeded the upper limit of the present invention in terms of magnesia fine powder content, resulting in poor creep resistance. Furthermore, Comparative Example 7 exceeded the upper limit of the present invention in terms of sodium silicate content, resulting in poor creep resistance compared to Examples 15 and 16, where the sodium silicate content was within the scope of the present invention.

[0035] In Table 2, Examples 21-25 show that while the silica fine powder content in the refractory raw material formulation differs for siliceous unfired bricks, they remain within the scope of the present invention, exhibiting sufficient strength and excellent creep resistance. On the other hand, Comparative Example 9 did not contain silica fine powder, resulting in insufficient strength after heat treatment. Comparative Example 10 exceeded the upper limit of the present invention in silica fine powder content, resulting in poor creep resistance. Furthermore, although heat treatment was performed at different temperatures in Examples 21-23, sufficient strength was obtained in all cases.

[0036] In Table 3, Examples 26-32 show different magnesia fine powder content in the refractory raw material formulation of spinel-based unfired bricks, but all fall within the scope of the present invention, exhibiting sufficient strength, excellent corrosion resistance, and good residual expansion. On the other hand, Comparative Example 11 did not contain magnesia, resulting in insufficient strength after heat treatment. Comparative Example 13 exceeded the upper limit of the present invention in terms of magnesia fine powder content, resulting in increased residual expansion. Furthermore, Comparative Example 12 exceeded the upper limit of the present invention in terms of sodium phosphate content, resulting in inferior corrosion resistance compared to Examples 27 and 28, where the sodium phosphate content was within the scope of the present invention.

[0037] In Table 3, Examples 33-37 show different silica fine powder content in the refractory raw material formulation of spinel-based unfired bricks, but all fall within the scope of the present invention, exhibiting sufficient strength, excellent corrosion resistance, and good residual expansion. On the other hand, Comparative Example 14 did not contain silica fine powder, resulting in insufficient strength after heat treatment. Comparative Example 15 exceeded the upper limit of the present invention in silica fine powder content, resulting in poor corrosion resistance. Furthermore, although heat treatment was performed at different temperatures in Examples 33-35, sufficient strength was obtained in all cases.

[0038] In Table 4, Examples 38 to 43 show that while the content of magnesia fine powder in the refractory raw material formulation differs in alumina-based unfired bricks, it remains within the scope of the present invention, exhibiting sufficient strength, excellent corrosion resistance, and good residual expansion. On the other hand, Comparative Example 16 did not contain magnesia fine powder, resulting in insufficient strength after heat treatment. Comparative Example 19 exceeded the upper limit of the present invention in terms of magnesia fine powder content, resulting in increased residual expansion. Furthermore, Comparative Example 18 exceeded the upper limit of the present invention in terms of sodium phosphate content, resulting in inferior corrosion resistance compared to Examples 40 and 41, where the sodium phosphate content was within the scope of the present invention. In addition, Comparative Example 17 did not contain silica fine powder but contained 0.3% by mass of sodium phosphate, resulting in insufficient strength.

[0039] In Table 4, Examples 44-47 show that while the silica fine powder content in the refractory raw material formulation differs for alumina-based unfired bricks, it remains within the scope of the present invention, exhibiting sufficient strength, excellent corrosion resistance, and good residual expansion. On the other hand, Comparative Example 20 did not contain silica fine powder, resulting in insufficient strength after heat treatment. Comparative Example 21 exceeded the upper limit of the present invention in silica fine powder content, resulting in poor corrosion resistance. Furthermore, although heat treatment was performed at different temperatures in Examples 44-46, sufficient strength was obtained in all cases.

[0040] In Table 4, Examples 48 and 49 are alumina-based unfired bricks in which magnesia with a particle size of 75 μm or more is included in the refractory raw material formulation. These examples are within the scope of the present invention and exhibit superior residual expansion compared to Examples 39 and 42, which have a similar level of magnesia raw material content, as well as sufficient strength and corrosion resistance.

Claims

1. A method for producing unfired bricks, comprising adding water to a refractory raw material mixture containing 0.2% to 5% by mass of light-fired magnesia as 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 containing one or more of alumina raw materials, alumina silica raw materials, silica raw materials, and spinel raw materials, and wherein the total content of alkali metal phosphates and alkali metal silicates is 1% by mass or less (including 0), kneading the mixture, press-forming it, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or less.

2. A method for producing unfired bricks according to claim 1, wherein silica flower is used as silica with a particle size of less than 44 μm.

3. A method for producing unfired bricks, comprising adding water to a refractory raw material mixture containing 0.2% to 5% by mass of light-fired magnesia as magnesia with a particle size of less than 75 μm, 10% or less by mass of magnesia with a particle size of 75 μm or more (excluding 0), and 0.3% to 10% by mass of silica with a particle size of less than 44 μm, with the remainder mainly consisting of alumina raw materials, and the total content of alkali metal phosphates and alkali metal silicates being 1% or less by mass (including 0), kneading the mixture, press-forming it, and then leaving it at room temperature or heat-treating it at a temperature of 1000°C or lower.

4. A method for producing unfired bricks according to claim 3, wherein silica flower is used as silica with a particle size of less than 44 μm.

Images