Method for manufacturing unburned magnesia-chrome brick

Abstract

The present invention addresses the problem of suppressing structural spalling in an unburned magnesia-chrome brick. In the present invention, in order to solve this problem, a refractory raw material blend is kneaded, is then subjected to press molding, and is then heated to 60-1000°C, wherein the refractory raw material blend contains magnesia-chrome-based raw materials in a total amount of 73-98 mass% and alumina-based raw materials in a total amount of 1-25 mass% and further contains a binder.

Description

Manufacturing method of unfired magnesia-chrome bricks

[0001] The present invention relates to a method for producing unburned magnesia-chrome bricks used in steelmaking, particularly secondary refining.

[0002] In the secondary refining process, basic bricks such as magnesia-chrome bricks are generally used. Magnesia-chrome bricks are fired at 1700°C or higher, which develops a spinel containing chromium, forming a strong bond and providing high corrosion resistance. On the other hand, the production of magnesia-chrome bricks requires ultra-high temperature firing at 1700°C or higher, and CO ２ However, there are also disadvantages such as emissions and increased costs. Therefore, methods for producing unfired magnesia-chrome bricks that do not require high-temperature firing have been investigated.

[0003] For example, Patent Document 1 discloses a method for producing a composite material containing Li as a binder, for 100 parts by weight of aggregate adjusted to a range in which the MgO component exceeds 50% by mass. ２ O / P ２ O ５ Patent Document 2 discloses a method for producing an unfired refractory material, characterized by using 0.5 to 5 parts by weight of condensed lithium phosphate having a weight ratio of 2.5 / 7.5 to 1.5 / 8.5. Patent Document 2 also discloses a method for producing an unfired magnesia-chrome brick, which comprises blending 0.5 to 3.5 parts by weight of sodium aluminate with 100 parts by weight of a refractory composition containing a magnesia raw material and a chromia raw material, kneading the mixture, and then pressure-molding and drying at 100 to 500°C.

[0004] Japanese Patent Publication No. 63-24944 Publication No. 11-157917 Japanese Patent Publication No. 4-6150

[0005] The inventors conducted tests on unfired magnesia-chrome bricks and found that they are much more susceptible to structural spalling than fired magnesia-chrome bricks. Since magnesia-chrome bricks are inherently more susceptible to slag penetration than graphite-containing bricks such as magnesia-carbon bricks, structural spalling is often a cause of wear during use. However, it was found that the most significant cause of wear is the extremely high degree of structural spalling, particularly in the case of unfired magnesia-chrome bricks. "Structural spalling" refers to the phenomenon in which slag penetration creates an altered zone with a significantly different structure from the original zone, and when this altered zone experiences a sudden temperature rise or fall during use in a molten metal vessel, cracks form at the boundary between the original zone and the altered zone, causing separation.

[0006] On the other hand, in the examples of Patent Document 3, an aggregate containing sintered magnesia clinker (particle size 5 mm or less), natural chromium ore (particle size 5 mm or less), and chromium oxide (fine powder) is mixed with an alumina-based raw material having a particle size of 0.35 to 1.0 mm in an outer percentage of 1 to 5 wt %, and then the mixture is subjected to a pressure of 760 kgf / cm ２ Patent Document 3 discloses a magnesia-chrome refractory obtained by molding a refractory under a pressure of 1000 kJ / cm2 and then firing the molded body at 1800°C or higher. That is, as described in the "Action" section in the upper right column of page 2 of Patent Document 3, during firing, the alumina-based raw material reacts with magnesia in the aggregate to form a spinel, causing volume expansion and generating microcracks in the fibers of the refractory. The microcracks thus introduced into the refractory in advance prevent or suppress the growth of cracks that occur in the refractory during use, thereby improving spalling resistance.

[0007] However, Patent Document 3 is merely a technology for improving the spalling resistance of fired magnesia-chrome bricks, and is not a technology for suppressing structural spalling, which is a particular problem in unfired magnesia-chrome bricks.

[0008] The problem to be solved by the present invention is to suppress structural spalling in unfired magnesia-chrome bricks.

[0009] The gist of the present invention is as follows: 1. A method for producing unfired magnesia-chrome bricks, comprising kneading a refractory raw material mixture containing a total of 73 to 98% by mass of magnesia-chrome raw materials, a total of 1 to 25% by mass of alumina-chrome raw materials, and a binder, and then press-molding the mixture and heat-treating it at 60 to 1000°C. 2. A method for producing unfired magnesia-chrome bricks as set forth in 1, wherein the particle size of the alumina-chrome raw materials is less than 0.1 mm. 3. A method for producing unfired magnesia-chrome bricks as set forth in 1 or 2, wherein the binder is silica having a particle size of less than 75 μm. 4. A method for producing magnesia-chrome bricks as set forth in 3, wherein the refractory raw material mixture contains a total of 0.2 to 30% by mass of at least one of magnesia and magnesia-chrome having a particle size of less than 75 μm as magnesia-chrome raw materials, and 0.3 to 2.5% by mass of silica having a particle size of less than 75 μm as a binder.

[0010] According to the present invention, structural spalling can be suppressed in unfired magnesia-chrome bricks, thereby significantly improving the life of the bricks.

[0011] Fig. 1 is an explanatory diagram showing the shape of the test bricks used in the rotational erosion test. Photographs showing examples of evaluation of structural spalls (examples rated as ⊚). Photographs showing examples of evaluation of structural spalls (examples rated as ×).

[0012] The inventors have discovered that when a specific amount of alumina-based raw material is added to the refractory raw material mixture in the manufacturing method of unfired magnesia-chrome bricks, a Cr-containing spinel with a high Al content is produced in addition to the usual secondary spinel (magnesia-alumina spinel) when used as a lining material for a molten metal vessel. In other words, while only one type of secondary spinel (magnesia-alumina spinel) is produced in the matrix of ordinary magnesia-chrome bricks, the production of two or more types of spinel with different thermal expansion coefficients in the matrix is ​​thought to alleviate stress at the boundary between the slag-infiltrated layer and the sound layer, thereby suppressing structural spalling.

[0013] In the present invention, the content of the alumina-based raw material is set to 1% by mass or more and 25% by mass or less, based on 100% by mass of the refractory raw material blend. If the content of the alumina-based raw material is less than 1% by mass, the effect of suppressing structural spalling becomes insufficient, and if it exceeds 25% by mass, it causes a decrease in corrosion resistance. The content of the alumina-based raw material is preferably 5% by mass or more and 25% by mass or less.

[0014] Here, in the present invention, the term "alumina-based raw material" refers to Al ２ O ３ Such alumina-based raw materials include alumina, mullite, spinel, alumina zirconia, calcium aluminate, etc. In the present invention, the alumina-based raw materials are selected from those having a high melting point and a low Cr content. ２ O ３ It is preferable to use one or more materials selected from alumina, mullite, spinel, and alumina zirconia, because they are unlikely to react with the alumina to produce hexavalent chromium. The particle size of the alumina-based raw material used in the present invention is preferably less than 0.1 mm, because this promotes the production of secondary spinel in the matrix.

[0015] Among these alumina-based raw materials, electrofused alumina, sintered alumina, and calcined alumina can be suitably used as alumina, or bauxite, banquet, etc. can also be used. In particular, calcined alumina with an average particle size of 10 μm or less has high reactivity and therefore provides a greater effect. In addition, when corrosion resistance is more important, Al ２ O ３ Alumina having a content of 90 mass % or more can also be used. ２ O ３ and SiO ２ The spinel preferably contains Al in an amount of 90% by mass or more, and for example, sintered mullite or electrofused mullite can be used. ２ O ３ The content of Al is 60 to 90 mass %, the content of MgO is 10 to 40 mass %, ２ O ３A spinel having a total content of 95% by mass or more of Al and MgO can be suitably used. Spinel having such a composition is also called magnesia-alumina spinel, alumina spinel, etc., but in the present invention, it is simply called "spinel." Alumina zirconia is a material containing Al ２ O ３ The content of is 50 to 70 mass%, ZrO ２ The content of Al is 30 to 50 mass %. ２ O ３ and ZrO ２ A composition having a total content of the components of 95% by mass or more can be suitably used.

[0016] In the present invention, magnesia-chrome-based raw materials are used as the main raw materials to produce unfired magnesia-chrome bricks. In this context, "magnesia-chrome-based raw materials" refers to refractory raw materials that are the main raw materials for magnesia-chrome bricks, i.e., the magnesia source and chromium source. Magnesia and magnesia-chrome can be used as the magnesia source, and chromium ore, chromium oxide, and magnesia-chrome can be used as the chromium source. The above-mentioned magnesia-chrome-based raw materials are appropriately selected and used depending on the type of magnesia-chrome brick to produce that type of magnesia-chrome brick. For example, in the case of a rebonded type, magnesia-chrome is primarily used; in the case of a semi-rebonded type, magnesia and chromium ore or chromium oxide are used together with magnesia-chrome; and in the case of a direct-bonded type, magnesia and chromium ore or chromium oxide are primarily used. In any case, in the present invention, the magnesia-chrome-based raw materials are used in a total content of 73% by mass to 98% by mass based on 100% by mass of the refractory raw material blend.

[0017] The magnesia-chrome used in the present invention is a synthetic raw material obtained by melting magnesia and chromium ore in an arc furnace or by firing in a kiln. It is also called magnesia-chrome clinker, electrofused magnesia-chrome clinker, electrofused magnesia-chrome, or fused magnesia-chrome. Materials commonly used as raw materials for refractories can be used. Recycled materials obtained by crushing and sizing broken or used magnesia-chrome bricks can also be used. The magnesia used in the present invention can also be materials commonly used as raw materials for refractories, such as electrofused magnesia and sintered magnesia. As described above, at least one of chromium ore and chromium oxide can be used as the chromium source for the magnesia-chrome brick in the present invention. In this case, the chromium ore content can be 40% by mass or less, and the chromium oxide content can be 10% by mass or less. Naturally occurring chromium ore can be used as the chromium ore. In the present invention, the refractory raw material blend may contain one or more of metal powder, silicon carbide, graphite, pitch, carbon black, boron carbide, and titania as refractory raw materials other than the above-mentioned alumina-based raw material and magnesia-based raw material in an amount of 5 mass% or less.

[0018] Furthermore, in the present invention, a binder is used to ensure that the unfired magnesia-chrome bricks develop strength when heat-treated at 60° C. to 1,000° C. Known binders such as lithium phosphate, sodium aluminate, magnesium chloride, fine silica powder, magnesium sulfate, gypsum, and phenolic resin can be used. Of these, fine silica powder is easy to use because it generates almost no toxic gases, odors, or smoke when heated or when hexavalent chromium is produced, has excellent corrosion resistance, and has sufficient hot strength at intermediate temperatures.

[0019] Specifically, silica fine powder forms a gel-like magnesium silicate hydrate in the presence of water with at least one of magnesia fine powder and magnesia-chrome fine powder, which are types of magnesia-chrome-based raw materials in the refractory raw material blend. Therefore, even when heat-treated at 60°C or higher, the moisture in the brick is reduced, allowing it to function as a binder in the brick matrix. Even when the brick temperature subsequently rises and the magnesium silicate hydrate dehydrates, the formation of 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, this magnesium silicate bond is characterized by high hot strength, particularly at 600-1000°C. When used as a lining material for a molten metal vessel, it effectively prevents peeling of the working surface due to weakening of the brick structure.

[0020] Specifically, in the present invention, it is preferable to use silica fine powder as the silica component for producing magnesium silicate hydrate in a content of 0.3 to 2.5 mass% based on 100 mass% of the refractory raw material blend. In this case, it is also preferable to use at least one of magnesia fine powder and magnesia-chrome fine powder as the magnesium component for producing magnesium silicate hydrate in a total content of 0.2 to 30 mass% based on 100 mass% of the refractory raw material blend.

[0021] In the present invention, the smaller the particle size of the silica fine powder, magnesia fine powder, and magnesia-chrome fine powder, the easier it is to form hydrates in the presence of water, so specifically, particles of less than 75 μm in particle size are used. That is, in this specification, the silica fine powder, magnesia fine powder, and magnesia-chrome fine powder refer to silica, magnesia, and magnesia-chrome having a particle size of less than 75 μm, respectively.

[0022] As the silica fine powder, silica flour, fused silica, silica sol, or silica stone fine powder can be used. Note that since silica sol is in a liquid state, its content is SiO ２The silica flour used in this invention is calculated as a percentage of the total mass of silica particles. Furthermore, in the present invention, silica flour refers to finely divided silica particles having 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 available relatively inexpensively. 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.

[0023] 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 magnesium silicate bonds to develop well 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.

[0024] 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.

[0025] In the method for producing the unfired magnesia-chrome brick of the present invention, the above-mentioned refractory raw material mixture is kneaded with water, an organic solvent, etc., if necessary, and then press-molded and heat-treated at a temperature of 60° C. to 1000° C.

[0026] Table 1 shows the compositions of the refractory raw material blends in the examples of the present invention, comparative examples and reference examples, and the evaluation results of the resulting bricks.

[0027]

[0028] Among the refractory materials, the magnesium chrome contains 74% by mass of MgO and 10% by mass of Cr. ２ O ３ The magnesia used was fused magnesia and light-burned magnesia with an MgO purity of 98% by mass. The chromium ore used was Cr ２ O ３ 55% by mass, Fe ２ O ３ 14% by mass, MgO 16% by mass, Al ２ O ３ The sintered alumina and the calcined alumina contained 11% by mass of Al. ２ O ３ The mullite is 98% by mass of Al. ２ O ３ 77% by mass, SiO ２ 23% of spinel, and Al ２ O ３ 75% by mass and 25% by mass of MgO, and alumina zirconia is Al ２ O ３ 70% by mass, ZrO ２ Furthermore, as the silica fine powder having a particle size of less than 75 μm, SiO ２ 98% by mass of silica flour, and SiO ２ The silica fine powder used was 98% by mass.

[0029] Except for the reference example, which was a fired magnesia-chrome brick fired at 1750°C, unfired magnesia-chrome bricks were produced by adding an appropriate amount of water to the refractory raw material mixture, kneading it, and molding it into a shape of 230 mm x 114 mm x 100 mm using an oil press, followed by heat treatment at 250°C for 5 hours. Samples for measuring physical properties were cut out from these bricks, and bending strength at room temperature and hot bending strength at 600°C were measured, and structural spalling and corrosion resistance were evaluated.

[0030] The bending strength at room temperature was measured by three-point bending. Specifically, a rectangular specimen of 20×20×80 mm was prepared, and the bending strength was measured at a pressure rate of 1.65 kN / s.

[0031] The structural spalling and corrosion resistance were 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 was discharged and charged 10 times every 30 minutes. After the test, each brick was cut in the center parallel to the rotation axis and observed for cracks and corrosion. The dimensions of the test bricks were A = 100 m, B = 110 mm, and C = 40 m, as shown in Figure 1. The brick surface that came into contact with the slag during the test, i.e., the working surface, was as shown in Figure 1, and the cut position of the brick after the test was the position indicated by the dotted line in Figure 1.

[0032] Regarding structural spalling, the size of cracks parallel to the working surface (see Figure 1) that occurred at the boundary between the slag-infiltrated layer and the sound layer on the cut surface was evaluated using three levels: large (×), small (◯), and absent (◎). ×: The total length of the cracks was 40% or more of the height B of the test piece. ○: The total length of the cracks was less than 40% of the height B of the test piece. ◎: No cracks. Note that Figure 2 shows an example of ◎ (Example 2), and Figure 3 shows an example of × (Comparative Example 1).

[0033] The corrosion resistance was evaluated by determining the amount of corrosion (mm) from the difference in thickness (mm) between before and after the test at the most corroded portion of the cut surface. A corrosion index was calculated, with the amount of corrosion (mm) of the brick of "Comparative Example 1" shown in Table 1 taken as 100. A smaller corrosion index indicates better corrosion resistance.

[0034] Although the contents of the alumina-based raw materials in Examples 1 to 4 were different, they were within the range of the present invention, and both corrosion resistance and structural spalling were good. In contrast, in Comparative Example 1, the content of the alumina-based raw material was below the lower limit of the present invention, and structural spalling occurred significantly. In Comparative Example 2, the content of the alumina-based raw material exceeded the upper limit of the present invention, and corrosion resistance was significantly reduced.

[0035] Examples 5 to 8 use different types of alumina-based raw materials, but are within the scope of the present invention and provide good results in both corrosion resistance and structural spalling. Examples 9 to 11 use different types of binders, but are within the scope of the present invention and provide good results in both corrosion resistance and structural spalling. Examples 12 and 13 are semi-rebonded types, and Example 14 is a direct-bonded type, but both are within the scope of the present invention and provide good results. Examples 15 and 16 use different silica fine powder contents, and Examples 17 and 18 use different light-burned magnesia contents, but all are within the scope of the present invention and provide good results.

[0036] The reference example was fired at 1750°C, and exhibited good corrosion resistance and structural spalling without any problems.

Claims

1. A method for producing unfired magnesia-chrome bricks, comprising kneading a refractory raw material mixture containing a total of 73% by mass or more and 98% by mass or less of magnesia-chrome raw materials, 1% by mass or more and 25% by mass or less of alumina-based raw materials, and a binder, and then press-molding the mixture and subjecting it to a heat treatment at 60°C or more and 1000°C or less.

2. The method for producing unfired magnesia-chrome bricks according to claim 1, wherein the particle size of the alumina-based raw material is less than 0.1 mm.

3. A method for producing unfired magnesia-chrome bricks according to claim 1 or 2, wherein the binder is silica having a particle size of less than 75 μm.

4. A method for producing unfired magnesia-chrome bricks as described in claim 3, wherein the refractory raw material mixture contains at least one of magnesia and magnesia-chrome having a particle size of less than 75 μm as magnesia-chrome raw materials in a total amount of 0.2% by mass to 30% by mass, and silica having a particle size of less than 75 μm as a binder in an amount of 0.3% by mass to 2.5% by mass.

Images