RU2479552C1 - Method of producing heat insulating material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to production of heat insulating materials and can be used to increase energy efficiency of thermal equipment, to make a heat insulating layer for industrial installations operating at high temperatures, as well as provide fire safety for installations, buildings and structures. The method of producing heat insulating material involves preparation of a crude mixture consisting of potassium titanate and amorphous silica, forming an article using said mixture and heat treatment thereof. The crude mixture contains amorphous potassium titanate which is characterised by molar ratio TiO₂:K₂O which varies from 4.2 to 5.3; the amorphous silica is added in an amount required to obtain molar ratio of oxides (TiO₂+SiO₂):K₂O of 6:1, wherein content of SiO₂ in the mixture, in terms of dry residue, is not less than 7 wt % or is not more than 18.5 wt %. The article is fired at temperature not lower than 940°C. When using moist powder of amorphous potassium titanate and silica or aqueous dispersions thereof which contain 15-40% water, the articles are fired after drying.

EFFECT: high mechanical strength of lightweight material and improved heat insulating and heat-reflecting properties in a wide temperature range.

2 cl, 1 tbl, 5 dwg

Description

The invention relates to the field of production of heat-insulating materials for engineering, the building and refractory materials industry and construction, and can be used to increase the energy efficiency of thermal equipment, to perform a heat-insulating layer of industrial plants operating at high temperatures (furnaces, thermal and refrigeration units; liquefied gas pipelines , thermal insulation of the foundations of buildings and structures operating in permafrost (low temperatures), as well as for fire safety cookies for installations, buildings and structures.

Fibrous crystalline potassium titanates are widely used as a component of lightweight refractory materials and granular fillers. This is due to their high mechanical strength (tensile strength up to 2 GPa), low thermal conductivity (0.02-0.03 W / (cm · K)) and high reflectivity for radiation in the near infrared region (thermal radiation) , which constitutes 93-97% for its various modifications with a thickness of 0.3 mm (see, for example, the article: Mitsuhashi T., Tanaka H., Fujiki Y., Thermal properties of sintered potassium hexatitanate. J. Ceram. Soc. Jap . - 1982. - V.90. - No1047. - P.676-678).

The simplest version of heat-insulating materials and lightweight fillers based on fibrous crystalline potassium titanates are materials obtained by drying aqueous dispersions consisting of potassium titanate or its mixtures with other mineral fibers and heat-insulating materials.

For example, the known method according to Japanese patent No. 60065199 from 1985.04.13, IPC: B01D 39/20; D21H 13/46, in which sheet thermal insulation material is obtained on the basis of fibrous potassium titanates (90-10%) and aqueous dispersions of ultrathin mineral aluminosilicate fiber (10-90%), and expanded mica (20% over the total fiber content); the material is suitable for operation without deformation at temperatures up to 800 ° C. Such materials have excellent thermal insulation characteristics; however, the disadvantage of this method is the very low mechanical strength of the materials obtained, they easily crumble with minimal mechanical stress. In this regard, thermal insulation materials based on mixtures of potassium titanates and a liquid glass binder are more attractive.

So the known method of producing granular refractory material according to Japanese patent No. 63050373 from 1988.03.03, IPC SB 38/00; the resulting material is suitable for use in the manufacture of heat-insulating materials, filters and catalyst supports, which are made on the basis of a mixture of fibrous micro- or nanocrystals K ₂ Ti ₆ O ₁₃ having a length / diameter ratio in the range of 50-500 (100 weight parts), and 23% aqueous solution of lithium silicate (25 parts by weight) and water (70 parts by weight). The material obtained on the basis of spherical granules of this mixture and a binder after firing at 800 ° C has a thermal conductivity of 0.0021 W / (m · K) at 350 ° C, but its mechanical strength does not exceed 0.2 MPa.

Another variant of the simplest thermal insulation materials based on potassium titanates is made on the basis of compositions containing, in addition to fibrous crystalline potassium titanates, clay minerals used as a temporary technological binder. So, for example, a known method of obtaining a porous heat-insulating material having high mechanical strength, according to Japanese patent No. 63236777 from 1988.10.03, IPC SB 38/00. The material is made on the basis of a mixture of layered clay minerals (for example, montmorillonite) and fibrous crystals (for example, potassium titanate, taken in a 5: 1 weight ratio), which are dispersed in a solvent and then dried at room temperature to form a porous sheet material with a density of 0 , 23-0.30 g / cm ³ , thermal conductivity 0.057 kJ / (m · K). However, the bending strength of the resulting material is only 0.12 MPa.

There is also known a method of obtaining a lightweight thermal insulation material, which, according to Japanese patent No. 6316467 dated 1994.11.15, IPC B28B 1/52; C04B 32/00; C04B 35/00; C04B 35/63; C04B 35/80; С04В 35/81, is obtained by drying mixtures based on K ₂ TiO ₃ nanofibers (5-80%) and mineral aluminosilicate fiber (20-95%) dispersed in an aqueous suspension of a clay mineral. However, the mechanical strength of such materials is also low.

On the other hand, the introduction into the composition of raw mixes based on potassium titanates a significant amount of mineral binders that increase the mechanical strength of the final product (clay minerals, colloidal silica, calcium-aluminate cement), especially during subsequent high-temperature firing, dramatically reduces the heat-insulating and heat-reflecting properties of the final material . For example, in the method for producing a lightweight thermal insulation material according to Japan Patent No. 2007161561 dated 2007.06.28, IPC C03B 32/00, C03B 23/02, C03B 38/00, a lightweight thermal insulation material is obtained on the basis of mixtures containing (in parts by weight): fibrous potassium titanate (25-35), colloidal silica (100), ZrO ₂ and / or SiC (30-40) and an additional fibrous reinforcing filler (10-20); the mixture is prepared in the form of an aqueous suspension (200-300 parts by weight of H ₂ O per 100 parts by weight of the solid components of the mixture), and then used to form articles under pressure and solidify. The dense material thus obtained is no longer heat insulating, but a refractory structural material.

The same group of materials includes refractory material obtained according to the method according to US patent No. 3253936 dated 1966.31.05, IPC С04В 28/24, made on the basis of plastic mixtures of K ₂ TiO ₃ fiber crystals (40-80 weight parts) of a hydrated binder, for example calcium-aluminate cement (20-40 parts by weight) with the addition of a non-hydratable binder, for example a colloidal dispersion of kremenesem, silicate or aluminate (45-85%) in water (5-40%), hardening for no more than 3 hours with low shrinkage (not more than 3%). Thus, it is possible to obtain products of complex shape without the use of pressing and additional compaction of mixtures. The disadvantage of this method is the high density of the materials obtained (about 1.75 g / cm ³ ) and they cannot be used as thermal insulation.

Closest to the proposed invention is a method of teaching a structured heat-insulating material, US patent No. 3629116 dated 1971.12.21, IPC С04В 43/04, in which the material is obtained on the basis of raw mixes, including mineral or ceramic fiber (15-89%), potassium titanate ( 10-70%) and a binder based on amorphous silica (1-35%), prepared in the form of 15-40% aqueous dispersion, followed by molding in dehydrogenating form. When vacuum molding and subsequent drying at a temperature of about 150 ° C for 1-1.5 hours, the resulting material has a tensile strength of 0.2 MPa, at a density of 0.3 g / cm ³ and a thermal conductivity of 0.013 W / (cm · K) at 540 ° C and a reflection coefficient of thermal radiation of 75-80% (when using MgO as a reference). In the description of the invention it is noted that the material obtained during drying can be subjected to subsequent high-temperature firing and retains its properties to temperatures of the order of 920-975 ° C.

The main disadvantages of this method are low mechanical strength, insufficiently high reflection coefficient of thermal radiation, resulting from the introduction of a large amount of mineral fiber and colloidal silica into the composition of the raw material mixture, which reduces the thermal insulation properties and increases the apparent density of the material, as well as the inability to use the material at elevated temperatures .

The objective of the present invention is to increase the mechanical strength of a lightweight material consisting of fibrous crystals of potassium titanate and to improve the heat-insulating and heat-reflecting properties in an extended temperature range.

The technical result achieved in solving the problem is to obtain a material having high mechanical strength and low thermal conductivity with an increased value of the reflection coefficient of thermal radiation.

This object is achieved in that in a method for producing a heat-insulating material, including preparing a raw material mixture consisting of potassium titanate and amorphous silica, molding the product on its basis and its subsequent heat treatment, according to the proposed technical solution, amorphous potassium titanate, characterized by the molar ratio of TiO ₂ : K ₂ O, varying from 4.2 to 5.3; and amorphous silica is introduced in an amount necessary to obtain a molar ratio of oxides (TiO ₂ + SiO ₂ ): K ₂ O equal to 6: 1; the content of SiO ₂ in the mixture in terms of dry residue should not be less than 7 wt.% or exceed 18.5 wt.%, then subject the product to firing at a temperature of at least 940 ° C.

It is possible to prepare a raw mixture based on dry or moistened powders of amorphous potassium titanate and silica or their aqueous dispersions. When using moistened powders or their aqueous dispersions containing 15-40% water, the products are fired after drying.

The invention is illustrated by drawings, in which Fig. 1 is a photomicrograph of a starting potassium titanate powder illustrating its macrostructure; figure 2 - x-ray diffraction pattern of the original amorphous potassium titanate; figure 3 is a thermogram of a raw material mixture containing 90% amorphous potassium titanate and 10% amorphous silica; figure 4 - x-ray diffraction patterns of the insulating material obtained by firing this mixture at 940 ° C (A), as well as used in the preparation of the mixture of potassium titanate without the addition of amorphous silica (B), 1 - diffraction maxima corresponding to the structure of K ₂ Ti ₄ O ₉ (potassium tetratitanate), 2 - to the structure of K ₂ Ti ₆ O ₁₃ (potassium hexatitanate); and figure 5 is an electron micrograph of this material.

A method of obtaining a thermal insulation material is as follows. By thorough mixing, a raw mixture is prepared consisting of powders of potassium titanate and amorphous silica, while potassium titanate in the composition of the raw mix is used in an amorphous form characterized by a molar ratio of TiO ₂ : K ₂ O varying from 4.2 to 5.3 ; amorphous silica is introduced in an amount necessary to obtain in the composition of the raw material mixture the molar ratio of components (TiO ₂ + SiO ₂ ): K ₂ O equal to 6: 1; the content of SiO ₂ in the mixture, calculated on the dry residue should not be less than 7 wt.% in the case of potassium titanate, characterized by the maximum allowable value of TiO ₂ : K ₂ O = 5.3, and should not exceed 18.5 wt .% in the case of using potassium titanate, characterized by the minimum acceptable value of TiO ₂ : K ₂ O = 4,2. Next, molding products based on the obtained raw material mixture and its subsequent heat treatment by firing at a temperature of at least 940 ° C are carried out. It is allowed to use moistened raw mixes or to prepare them based on colloidal solutions (aqueous dispersions) of amorphous silica; in this case, before drying, the molded products are dried.

Examples

A raw mixture is prepared comprising an amorphous potassium titanate powder characterized by a different molar ratio of TiO ₂ : K ₂ O; and amorphous silica powder (microsilica) taken in various weight ratios. The mixture is prepared in dry form or using an aqueous dispersion of amorphous silica containing 30% water. The resulting raw material mixture is thoroughly mixed in a homogenizer (mixer, ball mill) and products based on it are formed in the form of rectangular plates 10 × 10 × 2 cm in size by filling in stainless steel molds. The resulting products are subjected to heat treatment by firing (using dry mixtures) or, sequentially, drying at 150 ° C for 1.5 hours and firing.

Table 1 presents data on the effect of the content of powders of amorphous potassium titanate and silica in the raw material mixture when using potassium titanate of different chemical composition, as well as the firing temperature on the properties of the insulating material when using dry and moistened raw mixes.

From the examples shown in table 1, it follows that the use of amorphous potassium titanate in the composition of the raw material mixture, characterized by a molar ratio of TiO ₂ : K ₂ O of less than 4.2 and more than 5.3, significantly reduces the mechanical strength of the obtained products; and in the case of TiO ₂ : K ₂ O less than 4.2, the reflection coefficient of thermal radiation also drops sharply. The minimum possible content of SiO ₂ in the composition of the material, calculated on the dry residue, is 7.0 wt.% (When using potassium titanate with a maximum molar ratio of TiO ₂ : K ₂ O = 5.3), and its maximum possible content is 18.5 wt.% (When using potassium titanate with a minimum allowable molar ratio of TiO ₂ : K ₂ O = 4,2).

A firing temperature of less than 940 ° C leads to a sharp decrease in the mechanical properties of the obtained products, and the method of forming the product (plastic or semi-dry molding) does not significantly affect the mechanical and thermal insulation properties of the obtained material, provided that the formed blanks are dried effectively.

Thus, from the above examples it follows that the claimed technical solution allows us to synthesize a porous heat-insulating material with high mechanical strength, low thermal conductivity and high heat-reflecting ability.

The achieved effect is due to the fact that the raw materials in amorphous form (Fig. 1) in the form of agglomerated particles (Fig. 2) have increased reactivity, as a result of which chemical interaction proceeds between them, accompanied by the formation of a combined amorphous phase , which, according to the differential thermal analysis (Fig. 3), crystallizes at a temperature of 960 ° C with the formation of a crystalline phase, which, in its structure and properties, is similar to potassium hexatitanate (phi g.4). In this case, silicon atoms occupy the positions of titanium atoms, forming fibrous crystals of a solid solution of the composition K ₂ Ti _(6-x) Si _x O ₁₃ , while the initial amorphous potassium polytitanate of the composition K ₂ O · nTiO ₂ (n = 4,2- 5.3) when heated under similar conditions, it crystallizes with the formation of a crystal lattice of potassium tetratitanate and a low-strength amorphous phase (see article Sanchez-Monjaras T., Gorokhovsky AV, Escalante-Garcia JI Molten salt synthesis and characterization of polytitanate ceramic precursors with varied TiO ₂ / K ₂ O molar ratio // J. Am. Ceram. Soc. - 2008. - V.91. - No. 9. - P.3058-3065).

Table 1. Properties of thermal insulation materials. The composition of the original potassium titanate The content of components (in terms of dry residue), wt.% Firing temperature, ° С Compressive strength, MPa Reflection coefficient of thermal radiation,% Potassium titanate Amorphous silica K ₂ O · 3,7TiO ₂ 75.9 24.1 900 0.11 - 940 0.35 61 980 0.48 - K ₂ O · 4,2TiO ₂ 81.5 18.5 900 0.9 - 940 6.5 81 980 6.9 - 980 ^∗ 6.8 83 K ₂ O · 4,7TiO ₂ 86.8 13,2 900 1,2 - 940 10,2 86 980 10.3 - 980 ^∗ 9.9 84 980 ^∗∗ 2.1 71 K ₂ O · 5,3TiO ₂ 93.0 7.0 900 1,1 - 940 8.0 91 980 9.2 - K ₂ O · 5,7TiO ₂ 96.9 3,1 900 0.8 - 940 1.9 93 K ₂ O 6 TiO ₂ one hundred - 900 0.7 95 940 1,2 97 ^∗ Plastic molding with the introduction of amorphous silica in the form of an aqueous dispersion containing 30% water with preliminary drying before firing. ^∗∗ Plastic molding with the introduction of amorphous silica in the form of an aqueous dispersion containing 30% water without drying before firing.

The effect of incorporation of silicon into the structure of potassium hexatitanate is achieved with a general ratio of raw materials that provides a molar ratio of oxides in the system corresponding to K ₂ O: (TiO ₂ + SiO ₂ ) ≈1: 6.

At the same time, high-strength needle crystals of K ₂ Ti _(6-x) Si _x O ₁₃ can be formed only if the content of SiO ₂ in the system does not exceed 18.5 wt.%; otherwise, the excess silicon oxide crystallizes in the form of quartz, which dramatically reduces the mechanical strength of the obtained ceramic material and its heat-reflecting ability. When this forms a porous structure formed by intertwining high-strength needle crystals of potassium titanate-silicate (figure 5).

On the other hand, a decrease in the content of SiO ₂ in the reaction system below 3 wt.% Also reduces the mechanical strength of the obtained material due to the fact that the decrease in the concentration of K ₂ O does not allow firing to form, in addition to a solid solution of the composition K ₂ Ti _{(6- x)} Si _x O ₁₃ , residual glass phase, which provides adhesion of fibrous crystals and the formation of a durable sintered ceramic shard.

An increase in the temperature of firing and / or operation of heat-insulating materials, according to the considered technical solution, above 1350 ° C is impractical, since at this temperature a solid solution of the composition K ₂ Ti _(6-x) Si _x O ₁₃ melts.

Claims (2)

1. A method of obtaining a heat-insulating material, including the preparation of a raw material mixture consisting of potassium titanate and amorphous silica powders, molding products based on it and subsequent heat treatment, characterized in that the potassium titanate in the composition of the raw material mixture is used in an amorphous form characterized by a molar ratio of TiO ₂ : K ₂ O, varying from 4.2 to 5.3; amorphous silica is introduced in an amount necessary to obtain a molar ratio of components (TiO ₂ + SiO ₂ ): K ₂ O equal to 6: 1 in the composition of the raw material mixture; the content of SiO ₂ in the mixture in terms of dry residue should not be less than 7 wt.% or exceed 18.5 wt.%, and heat treatment is carried out by firing at a temperature of at least 940 ° C. 2. A method of obtaining a heat-insulating material according to claim 1, characterized in that the raw mixture is prepared using moistened powders of potassium titanate and silica or their aqueous dispersions containing 15-40% water, and the fired products are fired after drying.

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