JPH031262B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a refractory used as a lining for melting furnaces, holding furnaces, ladles, gutters, etc. that come into contact with molten aluminum or aluminum alloys. [Prior Art] Refractories used as linings for parts that come into contact with molten aluminum or aluminum alloys (hereinafter abbreviated as molten aluminum) are used as linings for other molten metals whose molten metal temperature is high, such as hot metal or molten steel. Melting loss, which is a typical damage, is not observed, and the damage progresses by another mechanism. In other words, unlike other molten metals, molten aluminum has the property of easily penetrating into the structure of refractories and has an extremely strong reducing power, so the molten metal that has penetrated into the refractory from the contact area gradually While the infiltration area expands into the interior of the refractory, components such as silicic acid (SiO ₂ ) contained in the refractory are reduced (metalized) and damage progresses. In other words, the damage to the refractory caused by molten aluminum is that an altered layer of the refractory, which has lost its function as a refractory, is formed on the surface of the refractory and gradually develops inside. In addition, in the altered layer that the molten aluminum has come into contact with and penetrated, deposits containing molten metal oxides (Al ₂ O ₃ ), etc. produced as a result of the reduction of silicic acid are formed on the refractory surface.
So-called "ghosts" may occur that gradually grow and block the space inside the furnace, causing problems in melting operations. For this reason, it is important to prevent molten aluminum from penetrating into refractories, and efforts have been made to prevent molten metal from penetrating. Conventional methods for suppressing molten metal intrusion can be roughly divided into two methods: making the refractory structure dense so that molten metal does not easily penetrate, and creating a refractory composition using materials that are difficult to wet with molten metal. There are the following: (a) To make the refractory structure dense, a molding machine such as a high-pressure press or a rubber press is used, and high-temperature firing is performed using a firing furnace at a temperature of 1,600°C or higher, for example. (b) Other methods include using binders and sintering aids. (c) If a refractory is to be used that is difficult to wet with molten metal, it is common to use a refractory whose composition is silicon carbide (SiC) or silicon nitride (Si ₃ N ₄ ). for example,
Silicon carbide bricks bonded with silicon nitride are examples of refractories that are difficult to wet with molten aluminum. This involves mixing silicon carbide particles with silicon, molding them under high pressure, and then bonding the silicon carbide while generating silicon carbide in a nitriding gas flow. (d) Also, since it is difficult to get wet with molten aluminum, there are nitride compositions such as aluminum nitride (AlN) and boron nitride (BN) as refractories that suppress penetration. These are produced by sintering a reactant produced by reacting with nitrogen, such as AlN, or if this is difficult, by sintering under pressure at high temperature like BN. (e) There is also the idea of adding a compound containing boron to the refractory composition. For example, U.S. Patent No. 2997402
No. _B2O3 15 _~ 80%, CaO5~50%, _{Al2O3 2} ~
This method involves synthesizing a 60% glassy frit and adding its powder. (f) U.S. Pat. No. 4,158,568 also states that B ₂ O ₃ 20~
40%, ZnO 50~60%, _SiO2 8~12%, _{Al2O3 0} ~
A method has been proposed in which a zinc borosilicate glass frit having a composition of 10% is added to the refractory composition to suppress the penetration of molten aluminum. [Problems to be solved by the invention] Each of the above-mentioned (a) to (f) has the following problems. (a) has the problem of requiring expensive special equipment. In (b), if a binder or sintering aid is used, there is a problem in that thermal spalls occur when refractories are used, resulting in brittleness. In (c), there are problems in that silicon carbide and silicon nitride are artificially synthesized and expensive raw materials, and the sintering manufacturing process is special and cannot be carried out easily. In (d), the materials are expensive and require a special manufacturing process, so the manufacturing cost is extremely high, and it is technically difficult to manufacture large quantities of refractories with dimensions and shapes larger than ordinary bricks. There are problems in that it is difficult to use and is a refractory that lacks practicality. In (e), a glass phase is formed in the refractory structure due to its composition, and there is a problem that the resistance to thermal spalling decreases due to the development of the well-known glass phase. In other words, this promotes destruction due to thermal stress during repeated heating and cooling during operation, eventually weakening the refractory structure and allowing molten aluminum to penetrate. The other major problem with this composition is that, as is well known in glass engineering, B ₂ O ₃ is high in SiO ₂
A system with a small amount of glass cannot produce a stable glass, and this frit also corresponds to such a composition. Furthermore, in this type of glassy frit, for example, there is a risk that B ₂ O ₃ absorbs moisture and deteriorates even during storage, and the description of the invention also mentions that the additive dissolves in water. being done. Additives that change in quality during storage are not suitable as raw materials for refractories, but even if this is not the case, if the additives change in quality or dissolve during kneading with water during manufacturing, and the pH changes, the kneading The viscosity of the material changes, the binder reacts and hardens, and it becomes difficult to form a desired refractory structure. These problems go beyond just brick manufacturing;
Problems also arise with monolithic refractories. For example, castables using alumina cement as a binder are cast at the construction site after being mixed with water, but if B ₂ O ₃ is leached into the additive, the alumina cement will not harden sufficiently due to changes in pH. There is a possibility that it may become impossible to form a satisfactory construction body. In order to solve these problems and increase the stability of boron-containing compounds, namely, B ₂ O ₃
The idea of vitrifying glass by adding components such as SiO ₂ to make it a stable glass is the invention of the US patent shown in (f) above. In (f), although the stability of frits has certainly been improved, the problem of reduced thermal spalling properties of refractories due to their glassy nature has not been resolved, and new problems have arisen. Problems also need to be resolved. In other words, when SiO ₂ is included, especially when amorphous silica is present, molten aluminum causes the reaction shown in the following reaction formula.
The reduction reaction of SiO ₂ proceeds. 4Al＋3SiO ₂ →3Si＋2Al ₂ O _3This reaction does not proceed rapidly, but it has the problem of accelerating the penetration of molten aluminum and deteriorating the refractory structure. It is necessary to consider a refractory structure that does not contain SiO ₂ because it causes adhesion and growth on the surface, so-called "ghosts". [Means for solving the problem] The means described in this manual uses BH, which is a boron compound,
B ₄ C, ZrB ₂ , SrB ₂ , LaB ₆ , 3MgO・B ₂ O ₃ ,
2MgO・B ₂ O ₃ , 3CaO・B ₂ O ₃ , 3SrO・B ₂ O ₃ ,
3BaO・B ₂ O ₃ , 3CoO・B ₂ O ₃ , Sc ₂ BO ₄ , 3LaO ₃・
One or more of B ₂ O ₃ , ThO ₂ and B ₂ O ₃ is added to a refractory powder made of alumina powder and a refractory binder so that the total boron content is 0.1 to 8.0 parts by weight. It is characterized by being dispersed and contained in a refractory. Each of the above boron compounds satisfies the common conditions of not dissolving or eluting in water, being crystalline, not containing silicic acid or silicon, and not melting or decomposing at 1200° C. or lower. As the alumina powder, sintered alumina powder or calcined alumina powder can be used, and as the refractory binder, fireclay or alumina cement can be used. The alumina powder preferably contains less silicon and silicic acid. [Operation] The presence of the boron compound in this inventive means is
It has the effect of suppressing the penetration of molten aluminum into the refractory structure. Although the suppressing mechanism is not necessarily clear, it can be roughly explained as follows. For example, when BN is contained in a refractory, at the first stage of contact with the molten metal, it is difficult to wet with the molten metal, which is a common characteristic with nitrides such as Si ₃ N ₄ and carbides such as SiO. As a result, contact with molten aluminum is limited, but oxidation by the atmosphere at the contact interface above and below the meral line gradually progresses, producing B ₂ O ₃ . This B ₂ O ₃ is reduced at the contact interface with the molten metal and reacts with aluminum, forming a viscous thin film at the refractory interface to suppress penetration of the molten metal. on the other hand,
On the surface of the refractory, B ₂ O ₃ is an oxide of the molten metal, Al ₂ O ₃
Alternatively, it reacts with Al ₂ O ₃ in the refractory composition to produce 9Al ₂ O ₃ .3B ₂ O ₃ , which is a strong bond, forming a structure that is difficult for molten metal to penetrate. When the film at the molten metal interface washes away, the 9Al ₂ O ₃ 3B ₂ O ₃ compound is exposed at the refractory interface, but B ₂ O ₃ is gradually reduced by the molten aluminum and reacts with aluminum again. Forms a thin, viscous film on the interface of refractories to prevent molten metal from penetrating. The action of such a film suppresses the penetration of molten aluminum into compounds containing boron, whether they are carbides, nitrides, or oxides. The melting and decomposition temperature of the boron compound is required to be 1200°C or higher because the boron compound, which is the boron source for the film, must be at about 1200°C or lower, which is the maximum temperature range of the furnace in which the refractory is used. or melted with
If it is something that can be decomposed, for example, 4P _b O.
This is because when B ₂ O ₃ is melted, B ₂ O ₃ is released all at once and flows out into the molten metal, and boron cannot be gradually supplied, so it is not possible to continuously suppress the penetration of the molten metal. The reason for setting the boron content in the refractory composition to 0.1 to 0.8 parts by weight is as follows. In a composition containing 0.03% boron, the effect of suppressing the penetration of molten aluminum is only partially observed, but in a composition containing 0.1% or more, the effect of suppressing penetration is observed throughout the entire surface of the refractory in contact with the molten metal. For example, by adding 0.23% BN, the required boron can be supplied at 0.1%, and excellent corrosion resistance can be imparted to the entire surface of the refractory. In addition, when the boron content exceeds 6%, the degree of improvement in the penetration suppression effect begins to decrease, and when the boron content exceeds 8%, no improvement in the effect is observed despite an increase in the amount added, and even There is a possibility that it may react with other components and cause a decrease in heat resistance. Therefore, the upper limit of the boron content is preferably 8%.
By the way, if 18.45% of BN is added, 8% of boron will be included in the refractory composition. The refractory according to the present invention can be easily obtained by adding one or more of the boron-containing compounds in combination to a conventional refractory composition. At that time, the boron-containing compound is dispersed and present in the refractory composition, but the smaller the particle size of the boron compound, the more effective it is, and it is desirable that it be a fine powder of at least 0.3 mm or less. In conventional refractories, usually
The skeletal part, called the aggregate, which makes up the majority of the composition, is difficult for molten aluminum to penetrate, and is composed of thermally stable coarse particles of 0.3 mm or more, so molten aluminum mainly penetrates into the skeletal part. This is done in the fine powder part that binds. Therefore, it is important to suppress the penetration of this fine powder portion, and for this reason, in order to effectively disperse the boron compound in the fine powder portion, it is desirable that the particle size of these particles be 0.3 mm or less. [Example] First, a castable refractory will be described below as an example of a monolithic refractory. Table 1 shows the formulations, arrangement ratios, etc. of Examples 1 to 4 and Comparative Examples 1 and 2.

[Table] Comparative Example 1 is a high alumina castable that has been known to have high corrosion resistance against molten aluminum, and Comparative Example 2 is a castable with a high alumina content that is known to have high corrosion resistance against molten aluminum. In Comparative Example 3, the amount of boron was increased. The main qualities of the raw materials used in Examples 1 to 4 and Comparative Examples 1 to 3 are as follows. Sintered alumina is composed of αAl ₂ O ₃ (corundum) crystals with an Al ₂ O ₃ content of 99.5%, and has a diameter of 4 to 1 mm.
It is a commercially available product pulverized to particle sizes of 1 mm or less and 0.3 mm or less. Calcined alumina has _Al2O3 content _of 99.8%
Although it is composed of aAl ₂ O ₃ crystals, it can be used in a relatively low temperature range (1100℃
This is a commercial product of fine powder alumina with a particle size of 0.1 mm or less manufactured by High alumina cement contains _{Al2O3 79} %, CaO18
_This is a commercially available hydraulic _cement for refractories _, which has _a composition of Boron nitride is a commercially available hexagonal crystal type product with 99.5% BN and a fine particle morphology of 0.1 mm or less. Boric anhydride is a commercially available reagent with a B ₂ O ₃ content of 99.5% and a particle morphology of 0.3 mm or less. Magnesium borate is produced by molding a mixture of the reagents boric acid and magnesium hydroxide as starting materials based on the theoretical ratio at a pressure of 200 kg/ ^cm2 .
It is produced by reaction in an oxygen-propane gas combustion furnace at a temperature of 1900℃, and 3MgO・B ₂ O ₃
It is expressed by the molecular formula of The resulting sintered body was roughly crushed with a hammer, further crushed with a machine crusher, and finally finely crushed with a ball mill to a size of 0.3 mm or less and classified into each particle size. In Examples 1 to 4 and Comparative Examples 1 to 3, 5 kg of each of the formulations shown in Table 1 was prepared, mixed in a 10-capacity experimental V-type mixer, water was added, and a 10-volume experimental blade-equipped mixer was prepared. Kneaded with a mixer. The amount of water added was adjusted so that the flow value was approximately 190 using a flow test device specified in JIS R2254 so that the castable had sufficient fluidity to be poured into the mold. Next, using a part of the above kneaded material, in order to determine the pot life of the kneaded material necessary for actual pouring work, in other words, whether the flow value at which pouring becomes difficult reaches 120 after 30 minutes of kneading. was measured. The kneaded material determined to be pourable based on this measurement was then poured into a mold with an inner diameter of 40 mm and a height of 40 mm.The mold was then placed in a constant temperature and humidity chamber at a temperature of 20°C and a humidity of 80%. After holding for 12 hours, the molded cured product was taken out from the mold, and its compressive strength was measured using an electric pressurized uniaxial compression tester to examine its curing properties. On the other hand, immediately after adding water, the remainder of each kneaded product was immediately placed inside a cylindrical molded body (outer diameter 80 mm, height 65 mm) so that the wall thickness of each part was 25 mm.
It was poured into a mold capable of forming a crucible with a hole of mm height, left at room temperature for 24 hours, removed from the mold, and further heated in an electric hot air dryer for 110 min.
℃ for 12 hours, and then fired at a temperature of 800℃ in a resistance electric heating furnace using silicon carbide elements. A molten aluminum alloy containing 5.5% magnesium, known as the toughest aluminum alloy for refractories, is injected into each crucible and electrically heated. The crucibles were inserted into a furnace and tested under harsh conditions by raising the temperature to 950°C, which is quite high for molten aluminum, and holding it for 20 hours to try to erode each crucible with the molten metal. After this erosion test was completed, each crucible was cooled, taken out from the furnace, cut with a diamond cutter, and the state of erosion of the molten metal into the crucible was observed. The results of each observation are shown in Table 2, and the main points are summarized as follows.

[Table] (1) Comparative example 2, in which water-soluble B ₂ O ₃ was added,
It maintains sufficient fluidity, but does not harden.
It was not possible to obtain a molded body. (2) Examples 1 to 4 in which BN and 3MgO・B ₂ O ₃ , which are poorly soluble in water, were added maintained sufficient fluidity and had a longer pot life than the conventional product (Comparative Example 1). No difference was observed in either case. (3) Examples 1 to 4 suppress penetration or erosion of molten aluminum and improve corrosion resistance; Comparative example 3, in which 10% was added, does not have sufficient corrosion resistance compared to samples 3 and 4. Next, high alumina refractory bricks will be shown as an example of a shaped refractory. The formulations and proportions of Example 5 and Comparative Examples 4 and 5 are shown in Table 3. This is a conventional high-alumina brick known for its high corrosion resistance against molten aluminum. Comparative Example 5 is a mixture of Comparative Example 4 in which a portion of the calcined alumina is replaced with borosilicate frit. The raw materials used in these Examples and Comparative Examples are:
Most of the raw materials used are the same as those used for the monolithic refractories mentioned above, but the main qualities of the newly used raw materials are as follows.

〔Effect of the invention〕

According to the present invention, by simply adding one or more of the specified boron compounds, excellent aluminum and aluminum alloy refractories can be obtained through a process that is almost the same as conventional methods. That is, since no particularly expensive equipment is required, manufacturing costs do not increase. The present invention can be applied to both monolithic refractories and fixed refractories, and the same equipment as conventional ones can be used for both, and refractories superior to conventional ones can be obtained. Furthermore, this invention can be applied to the field of sprayed refractories and coating materials for molten aluminum, although they are not used much at present, and also to the field of ceramic sprayed refractories for molten aluminum, which will be developed in the future.

Claims (1)

[Claims] 1 Boron compounds BN, B ₄ C, ZrB ₂ , SrB ₂ ,
LaB ₆ , 3MgO・B ₂ O ₃ , 2MgO・B ₂ O ₃ , 3CaO・
B ₂ O ₃ , 3SrO・B ₂ O ₃ , 3BaO・B ₂ O ₃ , 3CoO・
One or more of B ₂ O ₃ , Sc ₂ BO ₄ , 3LaO ₃・B ₂ O ₃ , ThO ₂・B ₂ O ₃ is added to a refractory powder consisting of alumina powder and a refractory binder to reduce the overall Boron content
A refractory for aluminum and aluminum alloys, which is dispersed in a refractory in an amount of 0.1 to 8.0 parts by weight.