Colloidal silica-bonded MGO-containing refractory castable

Abstract

Provided is a composition for producing a magnesia-containing refractory material, comprising (i) magnesia and (ii) modified colloidal silica. The colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group. The composition may optionally further contain alumina and / or water.

Description

Technical Field

[0001] The present invention relates to a composition for producing magnesia refractories, and more particularly to an aqueous composition comprising magnesia, modified colloidal silica, and optionally alumina. The present invention also relates to a method for producing magnesia refractories, and to the use of modified colloidal silica particles for preparing refractories, and for reducing cracks in refractories, and to a composition as a press aid for magnesia sinters for preparing refractories.

Background Art

[0002] Magnesia (MgO) is widely used in the steel industry for producing refractories due to its refractoriness (MgO has a melting point above 2800 °C), resistance to alkaline slag, and competitive cost due to readily available raw materials worldwide. Basically, there are two methods for preparing magnesia refractories that are currently mainly used. In the first method, MgO bricks are usually prepared at a central factory by pressing magnesia in the form of magnesia particles and shaping the binder into the desired shape. The resulting pressed and compacted material (so-called green material or green state) is usually pre-sintered at about 1300 °C to 1700 °C for about 1 to 5 hours to obtain a pre-sintered material (also called the pressing method). The pre-sintered material thus obtained is then shipped to the installation site, and the pre-sintered bricks are used to construct the desired shape of the intended structure, such as a furnace, and then sintered again in a second sintering process to produce the final refractory magnesia material. For example, during such a second sintering process, the entire structure is heated to the operating temperature using gas burners. Usually, this is a process that takes one or two days to complete because of the large quantity of the structure and to avoid thermal shock during the initial heating. After reaching the operating temperature, the material is used, i.e., it comes into contact with molten metal. This method has the advantage that the pre-sintered bricks are not so strict about the installation conditions and do not require a specific heating schedule, and a desirable compact structure is generated after initial heating to the operating temperature. However, the disadvantage of this method is that the pre-sintering process required to provide a green material with sufficient strength for shipping requires a high amount of thermal energy that is completely lost due to the necessary cooling of the green material to ambient temperature before shipping. Also, some typical binders used in this method, such as polyvinyl alcohol, require the presence of a certain amount of water to provide optimal strengthening of the strength of the compressed material in the green state. However, MgO is sensitive to hydroxylation and forms magnesium hydroxide, especially in the presence of water such as, for example, the moisture in the air or added water. Since hydroxylated magnesia has a different density from MgO, the excessive formation of hydroxylated magnesia causes internal tension in the magnesia refractory, causes undesirable cracks, and significantly impairs the properties of the refractory.

[0003] In a second method of preparing magnesia refractories, a suspension is produced in water with magnesia and optionally additives suspended therein, formed at the desired installation site and then further processed as a castable (also referred to below as the castable process). The suspension is then first gelled, then dried, and then heated at a temperature of about 1300 °C to 1700 °C before use. Such a method avoids a pre-sintering step and is thus, in principle, more desirable from an environmental point of view. However, due to the use of the suspension, a significant amount of water is required, and the problem of undesirable magnesium hydroxide formation becomes even more severe. In addition, the suspension requires a delicate balance of additives so that gelation of the suspension is achieved within an acceptable time frame, which is usually from a few minutes to a few hours depending on the particular application. It is further desirable that a very homogeneous suspension is formed and homogeneity is maintained during gelation, i.e., for example, no supernatant is formed during gelation. Furthermore, due to the presence of a large amount of water, the preparation of such refractories requires a very specific heating schedule to prevent explosive spalling by pressurized steam. Furthermore, the structure produced by this second method is not as compact as the structure obtained by the first method described above.

[0004] Another base material widely used for refractories is alumina (Al2O3), especially calcined alumina. Alumina may be used as the sole base material or in combination with magnesia. To improve the properties of sintered alumina-containing refractories, especially their rigidity, it is known to add unmodified "normal" colloidal silica as a binder. In particular, the addition of colloidal silica to alumina reduces the maximum sintering temperature required, i.e., sufficient strength can be achieved at a lower temperature, further increases the relative density, i.e., reduces the total porosity, and thus increases the compressive and flexural strength of the final refractory material. However, all attempts to use such unmodified "normal" colloidal silica in magnesia-containing refractories have completely failed, and it is generally considered in the refractory art that magnesia and colloidal silica are incompatible.

[0005] In order to provide high rigidity by means of a sintered refractory, there are hydrated alumina (HA) and calcium aluminate cement (CAC) as alternative binders generally added to the refractory composition. However, these binders as well as unmodified “normal” colloidal silica have the drawback that they do not sufficiently suppress the formation of magnesium hydroxide from magnesia in a high humidity state. Further, hydrated alumina tends to form a very low permeability structure that presents a high risk of explosive emission during drying. Calcium aluminate cement may reduce the refractoriness in compositions containing Al2O3, MgO and SiO2 by the formation of low melting point compounds. CAC also increases the pH of the castable, thereby increasing the driving force required for the hydroxylation of MgO.

[0006] Accordingly, there is a need for improved magnesia refractories that have a particularly low tendency to form magnesium hydroxide compared to generally used magnesia refractories. Also, it is desirable that such magnesia refractories have a lower tendency to form cracks than generally used magnesia refractories. Even further, it is desirable that such improved magnesia refractories be suitable for both the preparation by pressing the unfired material and the preparation as a castable using a suspension. It is even more desirable that such castables allow the use of lower pre-sintering and / or sintering temperatures than generally used magnesia refractories. Even further, it is desirable that such castables be homogeneous and do not form a supernatant during gelation. Even further, it is desirable that the sintered refractory obtained from such an improved magnesia refractory composition have a rigidity comparable to that of the sintered refractory obtained from a refractory magnesia composition containing hydrated alumina (HA) and calcium aluminate cement (CAC), which are the binders generally applied today. Further, it is desirable that fused magnesia can be used, and its production requires less energy than, for example, the production of fused magnesia such as electromelted magnesia. SUMMARY OF THE INVENTION

[0007] The present invention provides a composition for producing a magnesia-containing refractory material containing magnesia and modified colloidal silica, wherein the colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group, optionally alumina, and optionally water.

[0008] The present invention further relates to a method for preparing a refractory material, comprising the steps of: (i) preparing a composition containing magnesia, modified colloidal silica, optionally alumina, and optionally water, or alternatively, magnesia, modified colloidal silica, optionally alumina, and optionally water as individual components; (ii) mixing the composition or the individual components of magnesia, modified colloidal silica, optionally alumina, and optionally water; (iii) pressing the resulting mixture into a desired shape; (iv) optionally curing the resulting mixture, preferably at 1 to 80 °C; (v) optionally drying the resulting mixture, preferably at 80 to 400 °C; and (vi) sintering the dried mixture, preferably at 800 to 2000 °C.

[0009] The present invention further relates to the use of modified colloidal silica particles for preparing a refractory material and the use of a composition for preparing a refractory material, wherein the modified colloidal silica particles contain at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

[0010] The present invention further relates to the use of modified colloidal silica particles for reducing cracks in a refractory material, preferably for reducing cracks formed by the formation of magnesium hydroxide, wherein the modified colloidal silica particles contain at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

[0011] The present invention further relates to the use of modified colloidal silica particles as a pressing aid for magnesia sintering, wherein the modified colloidal silica particles comprise at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

[0012] The present invention can be suitably applied to the preparation of magnesia-containing refractories, and is particularly suitably applied to the preparation of magnesia-containing refractories under conditions where water is present and water comes into contact with magnesia.

[0013] In the following discussion, "organosilane-modified colloidal silica" can be referred to as "organosilane-functionalized colloidal silica".

Brief Description of the Drawings

[0014]

Figure 1

Figure 2a

Figure 2b

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0015] The composition of the present invention has been found to provide improved performance in refractory applications. In particular, the composition of the present invention containing magnesia has been found to have a lower tendency to form magnesium hydroxide than commonly used magnesia refractory compositions. Further, the composition of the present invention containing magnesia has been found to have a lower tendency to form cracks than commonly used magnesia refractory compositions. Further, the composition of the present invention containing magnesia has been found to be suitable for both preparation by pressing a green material and preparation as a castable using a suspension. Further, the composition of the present invention containing magnesia has also been found to enable the use of a lower pre-sintering temperature and / or sintering temperature than commonly used magnesia refractories. Further, the composition of the present invention has been found to enable the use of an environmentally preferable magnesia sintered product because its production requires less energy, for example, as the preparation of fused magnesia such as electrofused magnesia.

[0016] The composition of the present invention for producing a magnesia-containing refractory contains magnesia, preferably a magnesia sintered product, and a modified colloidal silica, and the colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

[0017] Magnesia is not limited, and basically any type of magnesia such as commercially available magnesia sinter (also called dead-burned magnesia), or fused magnesia, for example, electrofused magnesia, can be used in the present invention. Typical suppliers of magnesia are, for example, RHI-Magnesita in Brazil or IBAR in Brazil. In principle, magnesia can be used in any form such as powder or particles. However, the use of magnesia particles is preferred. The magnesia particles typically used may have a size in the range of 1 to 600 μm. In a preferred embodiment, magnesia contains, for example, at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 50 wt%, at least 70 wt%, at least 80 wt%, or even at least 90 wt% of magnesia sinter based on the total amount of magnesia, and preferably consists of magnesia sinter. Magnesia sinter (also called sintered magnesia here) is a magnesia material obtained by a sintering process, that is, a process of compressing and forming the solid mass of magnesia by pressure or heat without melting to the liquefaction point. In contrast, fused magnesia is obtained, for example, by a process in which magnesia is heated above its melting temperature by electrical energy. After melting and cooling, the magnesia is crushed. Since the initial molten block has large magnesia crystals in at least the millimeter range, the obtained fused magnesia particles are highly stable and do not contain any grain boundaries. Also, it is possible to use a mixture of various types of magnesia, for example, a mixture preferably containing at least a part of magnesia sinter. In an embodiment, the amount of magnesia sinter may be 10 to 99 wt%, for example, 20 to 99 wt%, 30 to 99 wt%, 50 to 99 wt%, 70 to 95 wt%, or even 80 to 90 wt% etc. based on the total amount of magnesia respectively. Since its manufacturing process has low energy requirements, for example, as the preparation of fused magnesia such as electrofused magnesia, it is preferable to keep the amount of magnesia sinter in the total magnesia high.In some embodiments, the amount of all other types of magnesia (i.e., all magnesias other than magnesia sintered products, such as fused magnesia like electrofused magnesia) may be from 1 to 90% by weight, for example, based on the total amount of magnesia respectively, it may be 1 to 80% by weight, 1 to 70% by weight, 1 to 50% by weight, 5 to 30% by weight, or 10 to 20% by weight, etc.

[0018] The modified colloidal silica contains colloidal silica particles, and the colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group particle. In other words, in the modified colloidal silica used in the present invention, at least a part of the surface silanol groups is substituted with one or more chemically bonded organosilane groups. The modified colloidal silica generally acts as a binder in the composition of the present invention. Without wishing to be bound by theory, it is believed that the modified silica fills the gaps in the magnesia structure and thus contributes to providing a denser structure. In particular, the modified silica in magnesia is thought to induce spinel-like grain growth resulting in a harder refractory material.

[0019] The chemically bonded organosilane group has a silicon atom bonded to the group -R m There may be a configuration in which 1 to 3 -R m groups are present on the silicon atom of the organosilane moiety. Typically, at most 2 -R m groups are present, and in some embodiments only 1 is present. If there are two or more -R m groups, they may be the same as each other or different from each other.

[0020] The organosilane-functionalized colloidal silica can be produced by conventional processes, for example, as described in WO2004 / 035473 and WO 2004 / 035474. Typically, the organosilane-functional colloidal silica is the organosilane reactant T 4-y Si-[R m ​y and is formed from the reaction between one or more silanol groups on the silica surface, i.e., the [SiO2]-OH groups. In the organosilane reactant, each T is typically independently selected from C 1-6 alkoxy, C 1-6 haloalkoxy, hydroxy, and halide. Another option is the use of siloxanes, for example, of the formula [R m b T 3-b Si{-O-SiT 2-c [R m c} a -O-SiT3 -b [R m b where a is an integer of 0 or 1 or more, typically 0 to 5, b is 1 to 3, and c is 1 to 2. Another example is the use of disilazanes of the formula {[R m b T 3-b Si}2-NH (where b is 1 to 3). Among the haloalkoxy groups, fluoro and chloro are preferred halo substituents. Alkoxy groups and halides are often preferred as T species. Among the halides, chloride is a suitable choice. Among the alkoxy groups, C 1~4 alkoxy groups such as methoxy, ethoxy, propoxy, or isopropoxy are preferred choices. In some embodiments, the organosilane reactant may be pre-hydrolyzed to convert one or more T groups to -OH groups, as described, for example, in Greenwood and Gevert, Pigment and Resin Technology, 2011, 40(5), pp 275-284.

[0021] The organosilane reactant reacts with the surface silanol groups to form one to three Si-O-Si linkages between the silica surface and the organosilane silicon atoms, i.e., {[SiO2]-O-} 4-y-z [T] z Si-[R m y ​​​​​(wherein 4 - y - z is from 1 to 3, usually in the range of 1 to 2) can be formed. As a result, the corresponding number of T groups are removed from the organosilane. For example, when T is an alkoxy unit, alcohol is produced.

[0022] Also, it is possible that at least a portion of the organosilane is in dimeric or oligomeric form before binding to the colloidal silica, i.e., two or more organosilane moieties are bonded to each other via Si - O - Si bonds.

[0023] The chemically - bonded organosilane group has the formula [{SiO2}-O-] 4-y-z [Z] z Si-[R m y and can be represented by. The {SiO2}-O- group represents an oxygen atom on the silica surface. The organosilane silicon atom has at least one, and optionally up to three, such bonds to the silica surface, i.e., 4 - y - z is at least 1 and 3 or less. The group Z is optionally present and z is in the range of 0 to 2. The organosilane silicon atom has 1 to 3 [R m groups, i.e., y is 1 to 3, typically 1 to 2. When there are two or more R m groups, they may be the same or different.

[0024] When z is not zero, the organosilane silicon has unreacted T groups and / or hydroxyl groups from which the T groups have been removed, for example, by a hydrolysis reaction. Alternatively or in addition, the Si - O - Si bond can be formed by the silicon atoms of adjacent organosilane groups. Thus, in the formula {[SiO2]-O-} 4-y-z [Z] z Si-[R m y the group Z can be selected (individually) from the groups defined under T above and the hydroxyl group and - O - [SiR m ’ group where the [SiR m ’ group is an adjacent organosilane group. ​​

[0025] R m is an organic moiety and typically has from 1 to 16 carbon atoms, such as from 1 to 12 carbon atoms, or from 1 to 8 carbon atoms. It is bonded to the organosilane silicon by a direct C—Si bond.

[0026] Plural R m groups, when present (i.e., when y is greater than 1), each R m may be the same or different.

[0027] R m is selected from alkyl, alkenyl, epoxyalkyl, aryl, heteroaryl, C 1-6 alkylaryl, and C 1-6 alkylheteroaryl groups and may optionally be substituted with one or more groups selected from ER n , isocyanates, and isocyanurates.

[0028] ER n in which E is absent or is selected from —O—, —S—, —OC(O)—, —C(O)—, —C(O)O—, —C(O)OC(O)—, —N(R p )—, —N(R p )C(O)—, —N(R p )C(O)N(R p )— and —C(O)N(R p )(wherein R p is H or C 1-6 alkyl). Preferably, E is —O—, —N(Rp)— or —N(R p )C(O)N(R p )—.

[0029] R n is linked to E or, when E is absent, is directly linked to R m and is selected from halogen (typically F, Cl, or Br), alkyl, alkenyl, aryl, heteroaryl, C 1-3 alkylaryl, and C 1-3 alkylheteroaryl. Rn is optionally substituted with one or more groups selected from hydroxyl, halogen (typically F, Cl, or Br), epoxy, -OR p , or -N(R p )2, wherein each Rp is as defined above. When E is present, R n may be hydrogen.

[0030] In the above definitions, the alkyl and alkenyl groups may be aliphatic, or cyclic, or may have both an aliphatic portion and a cyclic portion. The aliphatic group or aliphatic portion may be straight-chain or branched-chain. When any group or substituent contains halogen, the halogen is preferably selected from F, Cl, and Br.

[0031] Some groups can undergo endothermic reactions under the conditions experienced in a colloidal silica medium. Thus, groups containing moieties such as halides, acyloxy, (meth)acryloxy, and epoxy groups can hydrolyze to form the corresponding carboxyl, hydroxyl, or glycol moieties.

[0032] In some embodiments, one or more R m groups are optionally C 1-8 alkyl, C 1-8 haloalkyl, C 1-8 alkenyl, or C 1-8 haloalkenyl having a halide (e.g., chloride) substituent, typically C 1-8 alkyl, or C 1-8 alkenyl. Examples include methyl, ethyl, chloropropyl, isobutyl, cyclohexyl, octyl, and phenyl. These C 1~8 groups may be C 1~6 groups in some embodiments, or C 1~4 groups in other embodiments. The longer the carbon chain, the lower the solubility tends to be in aqueous systems, and the more complex the synthesis of organosilane-modified colloidal silica becomes.

[0033] In some embodiments, R m may be an alkyl isocyanate, such as propyl isocyanate. Also, R m may have an isocyanurate moiety, for example, Rm may be a propyl isocyanurate moiety, or may include a propyl isocyanurate moiety.

[0034] In a preferred embodiment, R m is a hydrophilic moiety. In an embodiment, R m is a hydrophilic moiety containing at least one group selected from hydroxyl, thiol, carboxyl, ester, epoxy, acyloxy, ketone, aldehyde, glycol ether (preferably polyglycol ether (PEG), etc.), (meth)acryloxy, amino, amide, ureido, isocyanate, or isocyanurate, preferably hydroxyl, amino, glycol ether (preferably polyglycol ether (PEG), etc.), ureido or ethylene glycol silane, even more preferably a hydrophilic moiety containing at least one group selected from hydroxyl, amino, ureido or ethylene glycol silane. In a further embodiment, the hydrophilic moiety contains at least one heteroatom selected from O and N and contains three or fewer consecutive alkylene (-CH2-) groups linked together.

[0035] In some embodiments, R m is a group containing 1 to 8 carbon atoms, for example, a C 1-8 alkyl group, and further, E is oxygen, and R n is an optionally substituted C 1-8 -epoxyalkyl and a C 1-8 hydroxyalkyl selected ER n substituent. Alternatively, R n may be an optionally substituted alkyl isocyanurate. Examples of such ER n substituents include 3-glycidoxypropyl and 2,3-dihydroxypropoxypropyl.

[0036] In some embodiments, R m is a group containing 1 to 8 carbon atoms, such as C 1~8 alkyl group, and further, in the absence of E, ER n contains a substituent, and R n is epoxyalkyl, such as epoxycycloalkyl. An example of such R m group is beta-(3,4-epoxycyclohexyl)ethyl. The epoxy group may alternatively be two adjacent hydroxyl groups, for example, R n may be dihydroxyalkyl such as dihydroxycycloalkyl, and R m is (3,4-dihydroxycyclohexyl)ethyl.

[0037] In embodiments where there are multiple R m groups on the Si atom of the organosilane, at least one is C 1-8 alkyl group or alkenyl group.

[0038] Examples of organosilane reactants that can be used to produce such functional colloidal silica include octyltriethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, tris-[3-(trimethoxysilyl)propyl] isocyanurate, 3-mercaptopropyltrimethoxysilane, beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, silanes containing epoxy groups (epoxysilanes), glycidoxy groups, and / or glycidoxypropyl groups, such as 3-(glycidoxypropyl)trimethoxysilane (also referred to as trimethoxy[3-oxiranylmethoxy)propyl]silane), (3-glycidoxypropyl)triethoxysilane, (3-glycidoxypropyl)hexyltrimethoxysilane)3-(glycidoxypropyl)methyldiethoxysilane, beta-(3,4-epoxycyclohexyl)-ethyltriethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriisopropoxysilane, 3-methacryloxypropyltriethoxysilane, octyltrimethoxysilane, ethyltrimethoxysilane, propyltriethoxysilane, phenyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, cyclohexyltrimethoxysilane, cyclohexyltriethoxysilane, dimethyldimethoxysilane, 3-chloropropyltriethoxysilane, 3-methacryloxypropyltrimethoxysilane, i-butyltriethoxysilane, trimethylethoxysilane, phenyldimethylethoxysilane, hexamethyldisiloxane, trimethylsilyl chloride, ureidomethyltriethoxysilane, ureidoethyltriethoxysilane, ureidopropyltriethoxysilane, hexamethyldisilazane, and mixtures thereof. U.S. Patent Application Publication No. 4,927,749 discloses additional suitable silanes that can be used in the present invention.

[0039] The most preferred organosilanes have epoxy groups, such as epoxyalkylsilanes or epoxyalkyloxyalkylsilanes. Hydroxyl substituents, such as hydroxyalkyl and hydroxyalkyloxyalkyl groups containing one or more hydroxyl groups, such as one or two hydroxyl groups, are also preferred. Examples include organosilanes containing glycidoxy groups, glycidoxypropyl groups, dihydropropoxy groups, or dihydropropoxypropyl groups. These may be derived from organosilane reactants such as (3-glycidoxypropyl)trimethoxysilane, (3-glycidoxypropyl)triethoxysilane, and (3-glycidoxypropyl)methyldiethoxysilane. In the compositions of the present invention, the epoxy groups can hydrolyze to form the corresponding vicinal diol groups. Thus, the present invention also encompasses the diol equivalents of the above epoxy group-containing compounds.

[0040] For example, when organosilane-modified silica is produced by reacting a mixture of two or more organosilanes with colloidal silica or by mixing two or more separately prepared organosilane-modified colloidal silicas, two or more different organosilanes may be present in the modified (or functionalized) colloidal silica.

[0041] Silane compounds can form stable covalent siloxane bonds (Si-O-Si) with silanol groups. In addition, they can be linked to silanol groups on the surface of colloidal silica particles, for example, by hydrogen bonding. It is possible that not all silica particles are modified by organosilanes. The proportion of colloidal silica particles functionalized with organosilanes depends on various factors, such as the size and available surface area of the silica particles, the relative amount of organosilane reactant to colloidal silica used to functionalize the colloidal silica, the type of organosilane reactant used, and the reaction conditions.

[0042] The degree of modification (DM) of the silica surface with organosilane can be expressed according to the following calculation (Equation 1) in terms of the number of silane molecules per square nanometer of the silica surface.

Number

[0043] DM may be at least 0.8 silane molecules per 1 nm 2 and preferably ranges from 0.8 to 4 molecules per 1 nm 2 . In a preferred embodiment, it has a DM in the range of 1 - 3, for example 1 - 2.

[0044] In the above Equation 1, the surface area of the silica can be suitably measured by shear titration.

[0045] The colloidal silica used in the composition of the present invention is a stable colloid. "Stable" means that the organosilane-functionalized colloidal silica particles dispersed in a (usually aqueous) medium do not substantially gel or precipitate within at least 2 months, preferably at least 4 months, more preferably at least 5 months during normal storage at room temperature (20 °C).

[0046] Preferably, the relative increase in the viscosity of the silane-functionalized colloidal silica dispersion between its preparation and 2 months after preparation is less than 100%, more preferably less than 50%, and most preferably less than 20%.

[0047] The relative increase in the viscosity of the silane-functionalized colloidal silica is preferably less than 200%, more preferably less than 100%, and most preferably less than 40% within up to 4 months from its preparation.

[0048] The silica particles in the silica sol can be modified with one or more additional oxides, such as aluminum oxide or boron oxide. Boron-modified silica sols are further described, for example, in US2,630,410. Alumina-modified silica particles suitably have an Al2O3 content of about 0.05 to about 3 wt%, such as about 0.1 to about 2 wt%. Procedures for preparing alumina-modified silica sols are also described, for example, in "The Chemistry of Silica" (Iler, K. Ralph, pages 407-409, John Wiley & Sons (1979)) and US 5 368 833.

[0049] Typically, the silica contained in the colloidal silica does not contain any additional oxides and in each case contains an amount below trace or impurity levels, for example, each additional oxide is contained at less than 1000 weight ppm. Typically, the total amount of non-silica oxides present in the sol is less than 5000 ppm, preferably less than 1000 ppm.

[0050] The colloidal silica particles preferably have an average particle diameter in the range of 2 to 150 nm, preferably about 3 to about 50 nm, and most preferably 5 to 25 nm. In a preferred embodiment, the average particle diameter is in the range of 6 to 20 nm. Suitably, the colloidal silica particles have a surface area of 20 to 1500 m 2 g -1 , preferably 50 to 900 m 2 g -1 , more preferably 70 to 600 m 2 g -1 , such as 70 to 400 m 2 g -1has a specific surface area. The surface area is often expressed as that of the "naked" or "non-functionalized" colloidal silica used in the synthesis. This is because the functionalization of the silica surface can complicate the surface area measurement. The surface area can be measured using the Sears titration (G.W. Sears; Anal. Chem., 1956, 28(12) pp1981 - 1983). The particle size can be calculated from the titration surface area using the method described by Iler, K. Ralph in “The Chemistry of Silica”, page 465, John Wiley & Sons (1979). Assuming that the silica particles have a density of 2.2 g cm -3 and all the particles are of the same size, have a smooth surface area, and are spherical, the particle size can be calculated from Equation 2. [Number]

[0051] Colloidal silica particles are preferably dispersed in water in the presence of a stabilizing cation, which is typically selected from K + , Na + , Li + , NH4 + , organic cations, quaternary amines, tertiary amines, secondary amines, and primary amines, or mixtures thereof, to form an aqueous silica sol. The dispersion can also contain an organic solvent, typically an organic solvent that is miscible with water, such as lower alcohols, acetone, or mixtures thereof, preferably those with a volume ratio to water of 20% or less. Preferably, no solvent is added to the colloidal silica or functionalized colloidal silica. The organic solvent in the composition can arise during the synthesis of organosilane-functionalized colloidal silica due to the reaction of the organosilane reactant with the silica. For example, if the organosilane reactant is an alkoxide, the corresponding alcohol is produced. The amount of any organic solvent is preferably kept below 20% by weight, preferably below 10% by weight.

[0052] The silica content of the functionalized silica sol before mixing with the accelerator is preferably in the range of 5 to 60% by weight, more preferably 10 to 50% by weight, and most preferably 15 to 45% by weight. This is expressed as the weight percentage of non-functional silica and is calculated from the weight percentage of silica in the colloidal silica source before modification with organosilane. In the presence of the accelerator, the silica content in the final composition is typically in the range of 3 to 58% by weight, for example, 10 to 55% by weight, for example, 15 to 50% by weight, expressed as the weight percentage of non-functional silica (i.e., as SiO2).

[0053] The pH of the functionalized silica sol is preferably in the range of 1 to 13, more preferably 2 to 12, for example 4 to 12, or 6 to 12, and most preferably 7.5 to 11. When the silica is aluminum-modified, the pH is preferably in the range of 3.5 to 11.

[0054] The functional colloidal silica preferably has an S value of 20 to 100, more preferably 30 to 90, and most preferably 60 to 90 before being mixed with the accelerator.

[0055] The S value indicates the degree of aggregation of colloidal silica particles, i.e., the degree of aggregate or microgel formation. The S value can be measured and calculated by the formula of Iler, R. K. & Dalton, R. L. described in J. Phys. Chem., 60 (1956), 955 - 957.

[0056] The S value depends on the silica content, viscosity, and density of the colloidal silica particles. A high S value indicates a low microgel content. The S value represents the amount of SiO2 in weight percent present in the dispersion phase of the silica sol. The degree of microgel can be controlled during the production process as further described in US 5,368,833.

[0057] In some embodiments, the weight ratio of the organosilane to the silica in the silane-functionalized silica sol is from 0.003 to 1.5, such as from 0.013 to 1.5, from 0.015 to 1.5, from 0.015 to 1, from 0.015 to 0.5, or from 0.015 to 0.33, preferably from 0.006 to 0.5, such as from 0.013 to 0.33, or from 0.015 to 0.33, and most preferably from 0.015 to 0.25, such as from 0.02 to 0.2, from 0.03 to 0.15, or from 0.05 to 0.1.

[0058] In some embodiments, the modified colloidal silica has a weight ratio of the organosilane to the silica from 0.013 to 1.5, such as from 0.015 to 1.5, from 0.025 to 1.5, or from 0.025 to 0.5, from 0.015 to 1, from 0.015 to 0.5, or from 0.015 to 0.33, preferably from 0.006 to 0.5, such as from 0.013 to 0.33, or from 0.015 to 0.33, and most preferably from 0.015 to 0.25, such as from 0.02 to 0.2, from 0.03 to 0.15, from 0.04 to 0.1, or from 0.04 to 0.06.

[0059] Here, the weight of the organosilane in the dispersion is calculated as the total amount of free organosilane compounds and organosilane derivatives or groups that may be bonded or linked to the silica particles, i.e., based on the total amount of one or more organosilane reactants initially added to the colloidal silica to produce the organosilane-modified silica, and is not necessarily calculated based on directly measuring the amount of organosilane actually chemically bonded to the silica.

[0060] Such a composition comprising magnesia and modified colloidal silica, wherein the colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group, has been found to be particularly suitable for the production of magnesia-containing refractories via a pressing method, in order to enable a reduction in the temperature of pre-sintering required for such a composition. For example, the rigidity of a green body containing modified colloidal silica modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group, after pre-sintering at, for example, about 1000 - 1300 °C, has been found to be similar to that of a green body not containing such silica after pre-sintering at the currently used standard temperature of 1500 °C. In other words, the addition of the specific modified silica described in the present disclosure enables a reduction of at least 200 °C to 500 °C in the pre-sintering step, and can be reduced without impairing the quality of the pre-sintered material. Such a temperature reduction reduces the carbon dioxide footprint of the entire process and also significantly saves costs. Furthermore, when adding the specific modified silica described in the present disclosure to magnesia-containing refractories, no signs of damage due to hydroxylation of magnesia were found. In other words, it has been unexpectedly found that the specific modified silica described in the present disclosure helps to suppress the hydroxylation of magnesia.

[0061] Since the contents of the magnesia and the modified colloidal silica described in the present disclosure in the composition of the present invention are not particularly limited, in principle, any amount of these components of the composition can be used. For example, in some embodiments, the content of the magnesia is 1 to 99.9% by weight, such as 1 to 99% by weight, 11 to 99% by weight, 12 to 99% by weight, 15 to 99% by weight, 1 to 95% by weight, 1 to 90% by weight, 1 to 85% by weight, or 1 to 80% by weight, preferably 1 to 70% by weight, such as 11 to 70% by weight, 12 to 70% by weight, or 15 to 70% by weight, more preferably 5 to 60% by weight, still more preferably 10 to 50% by weight, such as 11 to 50% by weight, 12 to 45% by weight, 15 to 40% by weight, or 17 to 35% by weight, and most preferably 20 to 30% by weight. The % by weight is based on the total weight of the composition for producing the magnesia-containing refractory.

[0062] In a further embodiment, the content of the magnesia is 5 to 99.9% by weight, such as 10 to 99.9% by weight, 11 to 99.9% by weight, 12 to 99.9% by weight, 15 to 99.9% by weight, 20 to 99.9% by weight, or 30 to 99.9% by weight. The % by weight is based on the total weight of the composition for producing the magnesia-containing refractory. These content ranges are particularly suitable when the composition of the present invention contains magnesia and the modified colloidal silica described in the present disclosure as the main components. However, when the composition of the present invention additionally contains alumina and / or water, these ranges can of course also be used.

[0063] The content of the modified colloidal silica is 0.1 to 99% by weight, such as 0.1 to 95% by weight, 0.1 to 90% by weight, 0.1 to 80% by weight, or 0.1 to 70% by weight, preferably 0.1 to 50% by weight, more preferably 0.5 to 40% by weight, still more preferably 1 to 30% by weight, such as 5 to 20% by weight, and most preferably 7 to 15% by weight, and the % by weight is based on the total weight of the composition for generating the magnesia-containing refractory.

[0064] For example, the composition of the present invention may contain 1 to 99.9% by weight of magnesia and 0.1 to 99% by weight of the modified colloidal silica described in the present disclosure. In some embodiments, the composition of the present invention contains 20 to 99.9% by weight of magnesia and 0.1 to 80% by weight of the modified colloidal silica described in the present disclosure. In some other embodiments, the composition of the present invention contains 30 to 99.9% by weight of magnesia and 0.1 to 70% by weight of the modified colloidal silica described in the present disclosure. In some other embodiments, the composition of the present invention contains 50 to 99.9% by weight of magnesia and 0.1 to 50% by weight of the modified colloidal silica described in the present disclosure. In some other embodiments, the composition of the present invention contains 70 to 99% by weight of magnesia and 1 to 30% by weight of the modified colloidal silica described in the present disclosure. In a preferred embodiment, the composition of the present invention contains 75 to 98% by weight of magnesia and 2 to 25% by weight of the modified colloidal silica described in the present disclosure. In a particularly preferred embodiment, the composition of the present invention contains 80 to 95% by weight of magnesia and 5 to 20% by weight of the modified colloidal silica described in the present disclosure. All % by weight are based on the total weight of the composition for producing the magnesia-containing refractory.

[0065] The composition of the present invention may further contain alumina (aluminum oxide, Al2O3). The alumina is not limited, and basically any type of alumina, such as commercially available alpha alumina (calcined alumina), or rho alumina (hydrated alumina) can be used in the present invention. These names are based on the identification of the crystallographic phases of the respective aluminas. Typical suppliers of alumina include, for example, Almatis, USA, Alcoa Aluminio, Brina, or Rio Tinto, USA. The most common source of alumina used in refractories is alpha alumina, and alpha alumina is also the preferred alumina to be used in the present invention. There are also grades of alpha alumina, and its microstructure is different, such as calcined alumina, fused alumina, and tabular alumina (typically in the form of coarse particles or aggregates). All of these various grades of alumina can be equally used in the present invention.

[0066] Surprisingly, the compositions of the present invention containing magnesia, alumina, and in particular modified colloidal silica (colloidal silica modified with at least one organosilane moiety containing silicon atoms bonded to carbon atoms of organic groups) have been found to be particularly suitable for the preparation of magnesia-containing refractories via a castable process because in such compositions, desirably low magnesia shows a tendency to hydroxylate. Thus, the formation of cracks induced by magnesium hydroxide is reduced by the presence of the modified colloidal silica described in the present disclosure.

[0067] Since the content of alumina in the compositions of the present invention is not particularly limited, any amount thereof can in principle be used. For example, the content of alumina may be 10 to 98.9% by weight, such as 10 to 79% by weight or 10 to 75% by weight, preferably 20 to 95% by weight, more preferably 30 to 90% by weight, even more preferably 40 to 80% by weight, such as 40 to 79% by weight, or 40 to 75% by weight, and most preferably 50 to 70% by weight, and the % by weight is based on the total weight of the composition for producing the magnesia-containing refractory.

[0068] Accordingly, when alumina is present in the composition of the present invention, the composition of the present invention may comprise 1 to 70 wt% magnesia, 10 to 98.9 wt% alumina, and 0.1 to 50 wt% of the modified colloidal silica described in the present disclosure. In some embodiments, the composition of the present invention may comprise 5 to 60 wt% magnesia, 20 to 94.5 wt% alumina, and 0.5 to 40 wt% of the modified colloidal silica described in the present disclosure. In still other embodiments, the composition of the present invention may comprise 5 to 60 wt% magnesia, 20 to 94.5 wt% alumina, and 0.5 to 40 wt% of the modified colloidal silica described in the present disclosure. In still other embodiments, the composition of the present invention may have a configuration comprising 10 to 50 wt%, preferably 15 to 40 wt% magnesia, 30 to 89 wt%, preferably 40 to 78 wt% alumina, and 1 to 30 wt%, preferably 2 to 20 wt% of the modified colloidal silica described in the present disclosure. In a preferred embodiment, the composition of the present invention may have a configuration comprising 20 to 30 wt% magnesia, 50 to 75 wt% alumina, and 5 to 20 wt%, for example, 5 to 15 wt% of the modified colloidal silica described in the present disclosure. All weight percentages are based on the total weight of the composition for producing the magnesia-containing refractory.

[0069] The composition of the present invention may further preferably contain water in an amount of 1 to 50 wt%, such as 2 to 45 wt%, more preferably 5 to 40 wt%, even more preferably 8 to 30 wt%, and most preferably 10 to 20 wt%, and the weight percentage is based on the total weight of the composition for producing the magnesia-containing refractory.

[0070] Accordingly, when water is present in the composition of the present invention, the composition of the present invention may be configured to include 1 to 98.9% by weight of magnesia, 0.1 to 98% by weight of the modified colloidal silica described in the present disclosure, and 1 to 50% by weight of water. In some embodiments, the composition of the present invention may be configured to include 20 to 97.9% by weight of magnesia, 0.1 to 78% by weight of the modified colloidal silica described in the present disclosure, and 2 to 45% by weight of water. In still other embodiments, the composition of the present invention may be configured to include 30 to 94.9% by weight of magnesia, 0.1 to 65% by weight of the modified colloidal silica described in the present disclosure, and 5 to 40% by weight of water. In still other embodiments, the composition of the present invention may be configured to include 50 to 94.9%, preferably 70 to 91% by weight of magnesia, 0.1 to 50%, preferably 1 to 30% by weight of the modified colloidal silica described in the present disclosure, and 5 to 40%, preferably 8 to 30% by weight of water. In a preferred embodiment, the composition of the present invention may include 75 to 93% by weight, such as 80 to 87% by weight or 80 to 85% by weight of magnesia, 2 to 25% by weight of the modified colloidal silica described in the present disclosure, such as 5 to 20% by weight, and 5 to 40% by weight of water, such as 8 to 30% by weight or 10 to 20% by weight. All percentages by weight are based on the total weight of the composition for producing the magnesia-containing refractory.

[0071] When alumina and water are present in the composition of the present invention, the composition of the present invention may contain 1 to 70% by weight of magnesia, 10 to 97.9% by weight of alumina, 1 to 50% by weight of water, and 0.1 to 50% by weight of the modified colloidal silica described in the present disclosure. In an embodiment, the composition of the present invention may contain 5 to 60% by weight of magnesia, 20 to 92.5% by weight of alumina, 2 to 45% by weight of water, and 0.5 to 40% by weight of the modified colloidal silica described in the present disclosure. In still other embodiments, the composition of the present invention may have a configuration that includes 5 to 60% by weight of magnesia, 20 to 89.5% by weight of alumina, 5 to 40% by weight of water, and 0.5 to 40% by weight of the modified colloidal silica described in the present disclosure. In still other embodiments, the composition of the present invention may have a configuration that includes 10 to 50% by weight, preferably 15 to 40% by weight of magnesia, 30 to 81% by weight, preferably 40 to 78% by weight of alumina, 8 to 30% by weight, preferably 8 to 25% by weight of water, and 1 to 30% by weight, preferably 2 to 20% by weight of the modified colloidal silica described in the present disclosure. In a preferred embodiment, the composition of the present invention may have a configuration that includes 20 to 30% by weight of magnesia, 50 to 65% by weight of alumina, 10 to 20% by weight of water, and 5 to 20% by weight, for example, 5 to 15% by weight of the modified colloidal silica described in the present disclosure. All percentages by weight are based on the total weight of the composition for producing the magnesia-containing refractory.

[0072] The composition of the present invention may, in principle, contain any further additives commonly used in compositions for producing magnesia-containing refractories. For example, such compositions may contain further binders such as, for example, calcium aluminate cement, hydrated alumina, colloidal alumina, phosphate binders, phenolic resins, bentonite, and gypsum. Furthermore, such compositions may contain a dispersant, examples of which include poly(ammonium acrylate) lignosulfonate, citric acid, poly(ethylene glycol), etc. These further additives are usually included in the composition of the present invention in a total amount of 20% by weight or less, for example 15% by weight or less, or 10% by weight or less. It is preferred that these further additives are included in the composition of the present invention in a total amount of 8% by weight or less, such as 5% by weight or less, or 3% by weight or less, or even 1% by weight or less. In certain embodiments, the composition of the present invention does not contain further additives and, as a result, consists only of magnesia, the modified colloidal silica described in this disclosure, optionally alumina, and optionally water. When the composition of the present invention consists of magnesia, the modified colloidal silica described in this disclosure, optionally alumina, and optionally water, these components may be present in the amounts defined above.

[0073] In some embodiments, the composition of the present invention contains less than 10% by weight of calcium aluminate cement, for example less than 5% by weight, less than 3% by weight, or even less than 2% by weight of calcium aluminate cement. In some embodiments, the composition of the present invention is substantially free of calcium aluminate cement. The term "substantially free" means that the amount of calcium aluminate cement is less than 1% by weight or even less than 0.5% by weight. In other embodiments, the composition of the present invention does not contain calcium aluminate cement at all, i.e., the composition of the present invention contains no calcium aluminate cement whatsoever.

[0074] In some embodiments, the composition of the present invention contains less than 8% by weight of silicon, for example, less than 5% by weight, less than 3% by weight, or even less than 2% by weight of silicon. In some embodiments, the composition of the present invention is substantially free of silicon. The term "silicon" means the metallic form of the element Si. The term "substantially free of" means that the amount of silicon is less than 1% by weight, or even less than 0.5% by weight. In other embodiments, the composition of the present invention does not contain silicon at all, that is, the composition of the present invention contains no silicon.

[0075] The present invention further relates to a magnesia-containing refractory obtained from a composition for producing a magnesia-containing refractory as described in the present disclosure.

[0076] For example, according to the present invention, the magnesia-containing refractory may be prepared using the following process for preparing a refractory containing the following: (i) preparing the composition described in the present disclosure, or alternatively, the individual components magnesia, modified colloidal silica, optionally alumina, and optionally water; (ii) mixing the composition or the individual components magnesia, modified colloidal silica, optionally alumina, and optionally water; (iii-a) optionally, pressing the resulting mixture into a desired shape, or (iii-b) optionally, curing the resulting mixture, preferably at 1 to 80 °C, and (iv) optionally, drying the resulting mixture, preferably at 80 to 400 °C, and (v) sintering the mixture, preferably at 800 to 2000 °C.

[0077] In step (i), a previously prepared composition for producing a magnesia-containing refractory as described in the present disclosure is prepared. Alternatively, of course, it is also possible to prepare the individual components magnesia, modified colloidal silica, optionally alumina, and optionally water separately.

[0078] In step (ii), the compositions described in the present disclosure are mixed to uniformly disperse the components. Alternatively, the individual components, magnesia, modified colloidal silica, optionally alumina, and optionally water, are mixed together to form a homogeneous mixture. It is also possible to first mix two or three of the components separately and then add additional components. The mixing can be carried out using any commonly known mixing device, such as a ball mill, paddle mixer, concrete mixer, or Ehrlich mixer.

[0079] In step (iii-a), the mixture obtained in step (ii) is optionally pressed into the desired shape. This step is typically carried out when refractories are prepared using a pressing process. Pressing or compressing may be carried out using standard methods and apparatus well known to those skilled in the art, for example, by using a set comprising a lower actuator, a cavity of the desired shape, and an upper actuator. The pressing conditions may vary depending on the particular composition used, but the pressure applied should be sufficient to form a stable green body that can be subjected to a sintering process. Typically, a compression pressure of 1 to 100 MPa is used.

[0080] In step (iii-b), which is an alternative to step (iii-a), the mixture obtained in step (ii) is optionally cured. Curing the mixture basically means gelling the mixture. This step is usually carried out when the refractory is prepared using a castable process to ensure that the castable has a defined shape and is stable enough to be subjected to a sintering process. The curing of the mixture is preferably carried out at 1 to 80 °C, more preferably 10 to 80 °C, even more preferably 40 to 70 °C, and most preferably at about 60 °C. The curing should be slow enough to allow proper mixing of the components and to shape the composition into the desired form. As will be understood by those skilled in the art, the minimum curing time required is temperature-dependent, i.e., curing at a higher temperature requires less time than at a lower temperature. Those skilled in the art can readily determine the curing time suitable for a given composition. Typically, the curing should require at least 2 minutes, preferably at least 5 minutes, even more preferably at least 10 minutes, or even at least 30 minutes. Curing can require longer times, such as from several hours to several days, especially when lower curing temperatures, such as ambient temperatures in the range of 20 to 25 °C, are used. To ensure sufficient curing, the curing should be carried out for at least 1 hour, or at least 5 hours, or at least 10 hours. For example, the curing may be carried out for 12 to 200 hours, preferably 16 to 168 hours, more preferably 20 to 120 hours, for example, 24 to 72 hours or 24 to 36 hours, etc. Most preferably, the curing is carried out for about 24 hours.

[0081] In step (iv), optionally, the mixture is dried. Since water in the composition evaporates during sintering in the sintering material and causes cracking, it is desirable to dry to reduce the amount of water in the composition and decrease the water in the composition before sintering. Depending on the amount of water and rigidity of the sintering material, the sintering material may also explode. Drying the mixture obtained in step (ii) or after an additional step (iii-a) or (iii-b) may be carried out at 80 - 400 °C, i.e., at a temperature close to or higher than the boiling point of water but well below the sintering temperature. Drying may be carried out for 1 minute to a maximum of several days. The duration of drying may depend on the temperature used for drying. Typically, the compositions of the present invention are dried for 1 minute to a maximum of 24 hours, such as 5 minutes to 12 hours.

[0082] In step (v), the mixture obtained in step (ii), or after an additional step (iii-a) or (iii-b), or after an optional drying step (iv) is sintered. Typical sintering temperatures are in the range of 800 - 2000 °C, such as in the range of 1200 - 1800 °C or in the range of 1300 - 1500 °C. Sintering is usually carried out for at least 10 minutes to a maximum of 24 hours, such as a maximum of 12 hours, a maximum of 10 hours, or a maximum of 8 hours. However, it is also conceivable that refractory materials are used directly after the sintering step without cooling for weeks or months. By sintering, the refractories are strengthened. Without being sintered, the composition in the pressed or compressed state is weak and can be easily decomposed even by hand.

[0083] In a further aspect, the present invention relates to the use of modified colloidal silica particles for preparing refractories, preferably magnesia-containing refractories, and compositions for preparing refractories, preferably magnesia-containing refractories, wherein the modified silanized colloidal silica particles contain at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group. The modified colloidal silica particles, as well as the compositions for preparing refractories, preferably magnesia-containing refractories, and their components are the same as those already described in the present disclosure.

[0084] In yet a further aspect, the present invention relates to the use of modified colloidal silica particles for reducing cracks in refractories, preferably in magnesia-containing refractories formed by the formation of magnesium hydroxide, the modified colloidal silica particles comprising at least one surface-bonded organosilane moiety comprising a silicon atom bonded to a carbon atom of an organic group. The modified colloidal silica particles as well as the refractories and their components are the same as those already described in the present disclosure.

[0085] In yet a further aspect, the present invention relates to the use of modified colloidal silica as a pressing aid for magnesia sinters, the modified colloidal silica particles comprising at least one surface-bonded organosilane moiety comprising a silicon atom bonded to a carbon atom of an organic group. The modified colloidal silica particles as well as the magnesia sinters are the same as those already described in the present disclosure.

Examples

[0086] The following non-limiting examples illustrate how the present invention may be practiced.

[0087] Colloidal silica 1 (CS1) 220 m 2 / g surface area and 12 nm particle size of organosilane-modified colloidal silica. The modified colloidal silica contained 39.81 wt% silica and had a pH of 6 - 8. The degree of modification was 1.7 molecules / nm of the silica surface 2 as measured by shear titration. The organosilane compound used to modify the silica was (3-glycidyloxypropyl)triethoxysilane.

[0088] Colloidal silica 2 (CS2) 360 m 2 / g surface area and 8 nm particle size of organosilane-modified colloidal silica. The modified colloidal silica contained 32.12 wt% silica and had a pH of 6 - 8. The degree of modification was 1.4 molecules / nm of the silica surface as measured by shear titration 2It was. The organosilane compound used to modify silica was (3-glycidyloxypropyl)triethoxysilane.

[0089] Colloidal silica 3 (CS3) 300 m 2 / g surface area, 9 nm particle size, containing 31.71 wt% silica, and having a pH of 9.0 - 10.5, non-functional colloidal silica.

[0090] Colloidal silica 4 (CS4) 250 m2 / g surface area, 11 nm particle size, containing 41.48 wt% silica, and having a pH of 9.0 - 10.5, non-functional colloidal silica.

[0091] Colloidal silica 5 (CS5) 200 m2 / g surface area, 14 nm particle size, containing 39.95 wt% silica, and having a pH of 9.0 - 10.5, non-functional colloidal silica.

[0092] Colloidal silica 6 (CS6) 160 m 2 / g surface area, 17 nm particle size, containing 31.31 wt% silica, and having a pH of 2 - 4, non-functional colloidal silica.

[0093] Colloidal silica 7 (CS7) 160 m 2 / g surface area, 17 nm particle size, containing 30.72 wt% silica, and having a pH of 2 - 4, cation-modified colloidal silica.

[0094] Colloidal silica 8 (CS8) Surface area 500 m 2 / g, particle size 5 nm, containing 16.61 wt% silica, and having a pH of 9.0 - 10.5, Al-modified colloidal silica.

[0095] Colloidal silica 9 (CS9) 750 m 2Non-functional colloidal silica having a surface area of

[0096] Colloidal silica 10 (CS10) 170 m 2 / g, a particle size of 4 nm, containing 16.01 wt% silica, and having a pH of 9.0 - 10.5

[0097] Colloidal silica 11 (CS11) 110 m 2 / g, a particle size of 16 nm, containing 39.95 wt% silica, and having a pH of 9.0 - 10.5

[0098] Colloidal silica 12 (CS12) 1100 m 2 / g, a particle size of 2 nm, containing 7.78 wt% silica, and having a pH of 9.0 - 10.5

[0099] Colloidal silica 13 (CS13) 80 m 2 / g, a particle size of 34 nm, containing 51.20 wt% silica, and having a pH of 9.0 - 10.5

[0100] The core properties of colloidal silicas 1 - 13 used in the following examples are summarized in Table 1 below.

[0101]

Table 1

[0102] Description of types. Na stab is a standard grade of colloidal silica having Na as a counter ion. The Di type is based on the Na stab product but involves an additional step of ion exchange. The Cat product is a Na stab product modified to have a cationic surface charge. The Al-modified is a Na stab product surface-modified with sodium aluminate. The silane-modified is a Na stab product modified with silane (gamma-glycidoxypropyltriethoxysilane) followed by an additional step of partial ion exchange. Colloidal silica 1 has a surface coverage (degree of modification) of 1.7 silanes / nm 2 Colloidal silica has a surface coverage (degree of modification) of the surface area, and colloidal silica 2 has a surface coverage of 1.4 silanes / nm 2 Colloidal silica has a surface coverage (degree of modification) of the surface area.

[0103] Example 1: a) A mixture of magnesia sinter and modified colloidal silica Several mixtures of magnesia sinter and modified colloidal silica with various amounts of components were prepared by mixing various amounts of colloidal silica 1 (2.7 to a maximum of 15.1 wt%) and 97.3 to 84.9 wt% of magnesia sinter powder (micronized magnesia sinter, available as MS30 from RHI-Magnesita, Brazil). A blank test containing only magnesia was also prepared.

[0104] Since colloidal silica 1 has a solid content of 39.81 wt% in water, the above amounts are in the range of 2.7 to 15.1. The wt% of colloidal silica 1 corresponds to 1.08 to 6.01 wt% of solid colloidal silica in the final sinter mixture.

[0105] First, a portion of the magnesia sintered product was weighed and spread on a 30×30 cm tray placed on a mass recorder. Subsequently, colloidal silica 1 was sprayed onto the particles in various amounts using a household spray device. The amount of colloidal silica 1 was recorded for each sample, and the composition was homogenized with a spatula. After homogenization, the mixture was pressed as a 70×20×10 mm prism for 30 seconds using a compression pressure of 40 MPa. All samples were dried overnight at 80°C before the tests were carried out.

[0106] For all unsintered dry samples, the flexural elastic Young's modulus was measured by excitation using impulse non-destructive technology (see Example 2 below for a detailed description of this technology). Since the compositions of the samples presented slightly different amounts of colloidal silica 1, these results are shown in Figure 1 as a function of their total solid silica content.

[0107] For the unsintered dry samples, together with the amount of silica added to the composition, the flexural Young's modulus increased linearly (from 10 GPa for the silica-free reference sample to 29.9 GPa for the sample containing 6 wt% silica). This three-fold increase in stiffness is remarkable and indicates that the addition of colloidal silica 1 significantly improves the strength of the unsintered bricks. Furthermore, no signs of damage due to potential hydroxylation (i.e., the formation of magnesium hydroxide) were observed in these samples.

[0108] After the characterization in the unsintered state, the samples were sorted in ascending order of stiffness, numbered, and divided into two groups. The even-numbered samples were sintered at 1100°C, and the odd-numbered samples were sintered according to the following sintering schedule: from 1°C / min to a maximum of 400°C, from 2°C / min to a maximum of 800°C, from 5°C / min to a maximum of 1100°C, and at 1500°C for 3 hours, cooled at 5°C / min to 800°C, and then naturally cooled to room temperature.

[0109] In both cases, the addition of colloidal silica 1 increased the rigidity of the structure after sintering (up to twofold for those sintered at 1500 °C). Furthermore, those samples sintered at 1100 °C and containing up to 6 wt% silica exhibited a rigidity close to that of the silica-free samples sintered at 1500 °C. This indicates that the addition of colloidal silica 1 reduces the necessary pre-sintering temperature of the bulk by about 400 °C.

[0110] b) Comparative mixture of magnesia sinter and dissolved polyvinyl alcohol (PVAl) A mixture of magnesia sinter and dissolved polyvinyl alcohol (PVAl) was prepared by mixing 90 wt% magnesia sinter powder (fine magnesia sinter, Brazil, available as MS30 from RHI-Magnesia) with a 10 wt% mixture containing 80 wt% water and 20 wt% PVAl.

[0111] First, a portion of the magnesia sinter was weighed out and spread on a 30×30 cm tray placed on a mass recorder. Subsequently, a mixture containing 80 wt% water and 20 wt% PVAl was sprayed onto the particles using a household spray device. The composition was homogenized with a spatula. After homogenization, the mixture was pressed as a 70×20×10 mm prism for 30 s using a compression pressure of 40 MPa. The samples were dried overnight at 80 °C and then sintered at 1500 °C using the sintering schedule described above for the mixture of magnesia sinter and modified colloidal silica.

[0112] Electron micrographs of the surface of the comparative mixture of magnesia sinter and sintered dissolved PVAl, and of the sintered inventive mixture of magnesia sinter and modified colloidal silica, prepared using a field emission scanning electron microscope (FEG-SEM, FEI 7500F, Holland), are shown in Fig. 2a (comparative mixture of magnesia sinter and dissolved PVAl) and Fig. 2b (inventive mixture of magnesia sinter and modified colloidal silica).

[0113] As can be seen from Figure 2a, the sample without silica presented a standard granular microstructure for magnesia sintering. Some pores can be seen as a result of the lower time-temperature conditions used. However, the pores impair the rigidity of the sintered material.

[0114] On the other hand, as can be seen from Figure 2b, the addition of colloidal silica 1 results in a configuration where the silica particles fill the space between the magnesia particles and act as a high-temperature binder, thus increasing its rigidity. Figure 2b shows that the material of the present invention containing colloidal silica 1 has far fewer pores than the comparative material shown in Figure 2b.

[0115] Example 2: In addition to the above-described colloidal silicas 1 to 14, in Example 2, the following raw materials described in Table 2 below were used.

[0116] [Table 2]

[0117] All products were mixed with the same amount of magnesia sinter. All formulation amounts are shown in Table 3. The water in the addition of these colloidal silicas represents 1.5 times the stoichiometric amount of water for completely hydroxylation of magnesia. All of these contain the same amount of magnesia sinter, MS, and water, but contain various amounts of solid silica particles.

[0118] [Table 3]

[0119] The suspension was held at 60 °C for up to 7 days to simulate the typical conditions seen during the processing of MS in many industrial processes and compared with other studies using similar conditions. For each condition, at various intervals, three equivalent samples were taken out of the oven, dried at 120 °C for 24 hours, weighed (M H , g), fired at 900 °C for 5 hours at 5 °C / min, and weighed again (MC , g) was measured. The experimental degree of hydroxylation (W H_EXP , %, in the range of 0 to 100%) was calculated according to Equation 3.

Equation

[0120] In the formula, 0.45 is M H and M C are, respectively, Mg(OH)2 (58.3 g·mol -1 ) and MgO (40.3 g·mol -1 ) and refers to the theoretically obtained degree of hydroxylation when they are equal to the molecular weights of [Ceramics International (2007), 33(5), pp. 803 - 810 and Ceramics International (2015), 41(10), pp. 13998 - 14007]. The results of this evaluation are shown in Table 4 together with the observations regarding gel formation and general stability.

[0121]

Table 4

[0122] The results presented in Table 4 show that only Examples 12 and 13 according to the present invention meet all requirements, such as having a long gel time, a homogeneous phase (i.e., no separation / formation of supernatant), and a low degree of hydroxylation required for magnesia castables.

[0123] b) Evaluation of colloidal silica 1 (CS1; silane-modified colloidal silica according to the present invention) as a binder in magnesia sintered refractories (compared with commercially available additives): The components used in these experiments and their respective amounts (in weight units) are shown in Table 5 below.

[0124]

Table 5

[0125] Sample preparation: Calcined alumina, magnesia sinter, binder (calcium aluminate cement and hydrated alumina (ρ-Al2O3)), and dispersant were dry-mixed and added to an appropriate content of distilled water or colloidal silica. For the sample containing colloidal silica, only calcined alumina, magnesia sinter, and dispersant were dried and wet-mixed. The wet mixing was carried out at 1000 rpm for 3 minutes in a paddle mixer. After mixing, the sample was cast in a cylindrical non-adhesive mold (length 70 mm × diameter 16 mm).

[0126] All samples were held at 60 °C for 72 hours in an environment with a humidity close to 100%. After demolding, the samples were maintained at 60 °C for 24 hours in a ventilated environment, dried at 120 °C for 24 hours, and then sintered at 1500 °C for 3 hours (heating rate of 2 °C / min up to 800 °C, heating rate of 5 °C / min up to 1500 °C, cooling rate of 10 °C / min to room temperature). -1 to room temperature).

[0127] The samples were measured (length = L i , diameter = D i , cm) and the weights (M i , g) were measured before and after sintering. Their permanent thermal linear variation (PTLV, %) after sintering was calculated by Equation 4 (the i index is related to the unsintered sample 0 or the sintered sample F).

Equation

[0128] The elastic modulus (E, GPa) of the sintered sample was measured by impulse excitation (sonelastic, Brazil) of the vibration technique in accordance with ASTM E 1876-01 standard (Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ration by Impulse Excitation of Vibration). The impulse excitation technique characterizes the elastic modulus and attenuation of the material based on the acoustic response emitted by the sample after receiving an optical impulse excitation. This acoustic response includes the specimen natural frequency of vibration proportional to the elastic modulus and shows an amplitude attenuation proportional to the attenuation. For regular geometric shapes such as bars, cylinders, disks, and rings, there are analytical expressions related to the elastic modulus and the natural frequency of the sample of vibration, dimensions, and mass. In this technique, a prismatic sample is placed on a polymer foam holder and a gentle impact is applied to one of its edges. Under these conditions, the sample begins to vibrate at its natural frequency, generating a characteristic sound (having a specific wavelength and frequency) recorded by an omnidirectional microphone. Subsequently, using the natural frequency and the length, diameter, and mass of the sample, its flexural modulus (or flexural Young's modulus) can be calculated in a non-destructive manner.

[0129] The flexural strength at break (σ R ) was measured by a three-point bending test (MTS 810 TestStar II, 2 N.s -1 load-inducing acceleration, see Equation 5).

Number

[0130] F is the maximum applied load (N), S is the distance between the two support points (m), and D (m) is the average diameter of the test sample. In all cases, three samples were tested for each condition. The results are summarized in Table 6.

[0131]

Table 6

[0132] These results indicate that only the composition of Example 14 according to the present invention containing colloidal silica 1 (CS1) provides a material with good strength and high elastic modulus, combined with good dimensional stability manifested as low dimensional change (PTLV (%)).

[0133] c) Evaluation of the effects of various amounts of modified silica in magnesia-containing refractory compositions. Refractory compositions containing colloidal silica 1 (CS1) were prepared in various amounts as shown in Table 7 below.

[0134] [Table 7]

[0135] The flexural Young's modulus and flexural strength of the dried green bodies (at 120 °C for 24 hours) and sintered compositions (using sintering scheduled as described above at 1500 °C for 3 hours) were measured. The results are shown in Figure 3.

[0136] Figure 3 shows that the flexural Young's modulus and flexural strength of the compositions of Experiments 15 - 18 of the present invention do not change significantly with various amounts of modified silica. Furthermore, the Young's modulus and flexural strength of the compositions of Experiments 15 - 18 of the present invention are similar to the values obtained for the compositions of Reference Examples 4 and 5, which show refractory compositions containing commonly used binders such as hydrated alumina (Reference Example 4) and calcium aluminate cement (Reference Example 5).

[0137] Furthermore, the drying behavior of the compositions of Experiments 15 and 18 and Reference Experiments 4 and 5 (Reference Examples 4 and 5) was examined. The results are shown in Figure 4.

[0138] Figure 4 shows the mass loss rate in the temperature rise of the investigated compositions. A mass loss rate > 0 indicates that water evaporates from the composition. It can be seen that the lower the amount of modified silica, the lower the temperature at which water evaporates. A lower temperature is desirable for water evaporation. It is further found that the temperature at the peak water evaporation of the modified silica containing the composition of the present invention is in a region similar to that of the composition of Reference Example 4 containing hydrous alumina. The graph of Reference Example 5 containing CAC shows that there is a significant amount of magnesium hydroxide formed as a significant amount of water evaporates at about 200 to 300 °C. This water is derived from magnesium hydroxide.

Claims

1. A composition for producing a magnesia-containing refractory, comprising: (i) magnesia; and (ii) a modified colloidal silica, wherein the colloidal silica is modified with at least one organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

2. The composition according to claim 1, further comprising: (iii) alumina.

3. The composition according to claim 1 or 2, wherein the magnesia comprises sintered magnesia in an amount of preferably at least 10% by weight based on the total amount of the magnesia.

4. wherein the organosilane moiety includes one or more groups R bonded to the silicon atom of the organosilane ｍ and Each R ｍ is independently optionally substituted with one or more groups selected from ERn, isocyanate, and isocyanurate, and is an alkyl, alkenyl, epoxyalkyl, aryl, heteroaryl, C １－６ alkylaryl, and C １－６ alkylheteroaryl group, E is absent or is a linking group selected from -O-, -S-, OC(O)-, -C(O)-, -C(O)O-, -C(O)OC(O)-, -N(R ｐ ), -N(R ｐ ), -N(R ｐ ), -N(R ｐ ), and -C(O)N(R ｐ ), where R ｐ is H or C １～６ alkyl, R ｎ is connected to E or, if E does not exist, is directly connected to ｍ R R ｎ is selected from halogen (typically F, Cl, or Br), alkyl, alkenyl, aryl, heteroaryl, C １－３ alkylaryl, and C １－３ alkylheteroaryl, and each R ｎ is optionally substituted with one or more groups selected from hydroxyl, halogen (typically F, Cl, or Br), epoxy, -OR ｐ , or -N(R ｐ ) ２ and is optionally substituted with one or more groups selected from the group consisting of When E is present, R ｎ may also be hydrogen, The composition according to any one of claims 1 to 3, wherein the alkyl group may be aliphatic, cyclic, or may contain both an aliphatic moiety and a cyclic moiety, and any aliphatic group may be straight-chain or branched-chain.

5. The composition according to any one of claims 1 to 4, wherein the modified colloidal silica is purified using a weight ratio of organosilane to silica sol in the range of 0.013 to 1.5, preferably 0.015 to 1.0, more preferably 0.025 to 0.5, even more preferably 0.03 to 0.15, and most preferably 0.04 to 0.

06.

6. The composition according to any one of claims 1 to 5, wherein the organosilane moiety contains at least one group selected from hydroxyl, thiol, carboxyl, ester, epoxy, acyloxy, ketone, aldehyde, glycol ether, (meth)acryloxy, amino, amide, ureido, isocyanate, or isocyanurate.

7. The composition according to any one of claims 1 to 6, wherein the organosilane moiety contains an epoxy group or at least one hydroxyl group, preferably an epoxy group.

8. The composition according to any one of claims 1 to 7, further containing water, preferably in an amount of 1 to 50% by weight, more preferably 5 to 40% by weight, even more preferably 8 to 30% by weight, and most preferably 10 to 20% by weight, based on the total weight of the composition for producing the magnesia-containing refractory.

9. The composition according to any one of claims 1 to 7, substantially free of calcium aluminate cement.

10. The composition according to any one of claims 1 to 9, substantially free of silicon.

11. (i) The content of magnesia is 1 to 99.9% by weight, for example 1 to 99% by weight, 1 to 95% by weight, or 1 to 90% by weight, preferably 1 to 70% by weight, more preferably 5 to 60% by weight, even more preferably 10 to 50% by weight, for example 11 to 50% by weight, 12 to 45% by weight, or 15 to 40% by weight, and most preferably 20 to 30% by weight, where the % by weight is based on the total weight of the composition for producing the magnesia-containing refractory, and the composition according to any one of claims 1 to 10.

12. (ii) The content of the modified colloidal silica is 0.1 to 99% by weight, for example 0.1 to 95% by weight, 0.1 to 90% by weight, or 0.1 to 80% by weight, preferably 0.1 to 50% by weight, more preferably 0.5 to 40% by weight, even more preferably 1 to 30% by weight, for example 5 to 20% by weight, and most preferably 7 to 15% by weight, where the % by weight is based on the total weight of the composition for producing the magnesia-containing refractory, and the composition according to any one of claims 1 to 11.

13. (iii) The content of alumina is 10 to 98.9% by weight, for example 10 to 79% by weight or 10 to 75% by weight, preferably 20 to 95% by weight, more preferably 30 to 90% by weight, even more preferably 40 to 80% by weight, for example 40 to 79% by weight, or 40 to 75% by weight, and most preferably 50 to 70% by weight, where the % by weight is based on the total weight of the composition for producing the magnesia-containing refractory, and the composition according to any one of claims 2 to 12.

14. A magnesia-containing refractory obtained from the composition according to any one of claims 1 to 13.

15. A process for preparing a refractory, comprising: (i) preparing the composition according to any one of claims 1 to 13, or alternatively, the individual components magnesia, modified colloidal silica, optionally alumina, and optionally water; (ii) mixing the composition or the individual components magnesia, modified colloidal silica, optionally alumina, and optionally water; (iii-a) optionally, pressing the obtained mixture into a desired shape; or (iii-b) optionally, curing the obtained mixture, preferably at 1 to 80 °C; and (iv) optionally, drying the obtained mixture, preferably at 80 to 400 °C. A process comprising the step of sintering the mixture preferably at 800 to 2000 °C. **Claim 16** Use of modified colloidal silica particles for preparing a refractory, preferably a magnesia-containing refractory, and a composition for preparing a refractory, preferably a magnesia-containing refractory, wherein the modified colloidal silica particles comprise at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group. **Claim 17** Use of modified colloidal silica particles for reducing cracks in a refractory, preferably a magnesia-containing refractory, in particular for reducing cracks formed by the formation of magnesium hydroxide, wherein the modified colloidal silica particles comprise at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group. **Claim 18** Use of modified colloidal silica particles as a press aid for magnesia sinters, wherein the modified colloidal silica particles comprise at least one surface-bonded organosilane moiety containing a silicon atom bonded to a carbon atom of an organic group.

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