CS210217B1 - High temperature lime-silica elements - Google Patents

Abstract

The invention relates to high-temperature calcium-silicate elements based on microcrystalline xonotlite as the main binding component, prepared from lime and a silica component, intended for thermal insulation and structural insulation materials up to temperatures of 00 and 1,000 °C. The silica component consists of a mixture of fly ash from high-temperature metallurgical processes for the production of silicon and its alloys with diatomite.

Description

The invention relates to high-temperature calcium silicate elements based on hydrous calcium silicates, the structure of which is formed predominantly by microcrystalline ixonolite with a composition of 5 CaO. 5 Si0 ₂ . HgO and which are intended for use temperatures of approximately up to 1,000 °C.

Thermal insulation and structural insulation materials prepared by hydrothermal processing of a mixture of calcium and silica components and consisting essentially of hydrous calcium silicates are known. These materials usually contain a certain proportion of reinforcing fibers, especially inorganic ones, such as asbestos, and may contain other additives, such as bentonite, kaolin, alkalis, etc. Quality products of a certain chemical composition and with stable physical properties are characterized by a structure consisting essentially of microcrystalline hydrosilicates tobermorite 5 CaO . 6 SiO . 5 HgO, xonotlite 5 CaO . 5 SiO . HgO or mixtures thereof. They are produced in various volumetric weights in relation to the required function they are to perform. They are used in the construction of thermal aggregates, such as furnaces, dryers, reactors, for insulation of industrial equipment, in the smelting of non-ferrous metals, in shipbuilding, in construction and many other fields. They are applicable for working temperatures of 600 °C to 1,050 °C. The thermal resistance and thus the limiting temperatures at which the products can be used are determined primarily by the type of potassium hydrosilicate, which forms the bulk of the mass of the resulting product. If the products contain predominantly tobermorite with a composition of 5 CaO. 6 SiOg. . 5 HgO, which is characterized by a relatively significant content of bound water, they are subject to significant changes when exposed to temperatures above 650 °C, exhibiting excessive shrinkage and the formation of cracks. In contrast, xonotlite is a potassium hydrosilicate with a composition of 5 CaO. 5 SiO ₂ * HgO, which contains approximately five times less water in the molecule compared to tobermorite. Due to the significantly smaller amount of water bound in the xonotlite lattice, as well as the fibrous structure, xonotlite-based products exhibit high volume stability, low shrinkage and withstand temperatures relatively well up to approximately 1,000 °C. For this reason, for temperatures above oo and 650 °C, it is expedient to use calcium-silicate elements, the structure of which consists mainly of microcrystalline xonotlite.

Selective preparation of xonotlite places higher demands primarily on the silica component used for the reaction with lime. Meanwhile, for the preparation of calcium-silica elements of the tobermorite type, it is possible to use silica raw materials that are not particularly pure and contain, in addition to crystalline or amorphous Si0 _{2 ,} a higher proportion of other oxides, such as alumina Al ₂ 0^, ferric oxide Pβ ₂ 0^, etc., as is the case, for example, with natural diatomites. When preparing elements based on xonotlite, it can be observed that some silica raw materials and mainly accompanying impurities prevent the formation of xonotlite as the final product of the hydrothermal reaction between the calcium and silica components. In addition, to avoid difficulties in preparing elements based on xonotlite and to achieve a smooth technological process, it is necessary to use a highly pure silica material, preferably a material having an amorphous structure and containing over 90%, preferably over 95% SiO_ and only a small proportion of multivalent metal oxides, in particular a trace or very small amount of Al ₂ O^. It is further expedient to use a highly dispersed material with a particle size preferably below 50 μm.

It was proposed to use as a siliceous component for the preparation of high-temperature calcium-silicate thermal-insulating and structural-insulating elements based on microcrystalline xonotlite as the main binding component fly ash from high-temperature metallurgical processes of silicon and its alloys production, especially from the production of crystalline silicon. Fly ash from the production of crystalline silicon is characterized by high purity (Si0 ₂ content 95 to 97%), non-crystalline structure of the present Si0 ₂ , favorable for the formation of xonotlite, high fineness (primary particle size below lytun), low content of aígO^ and Fe ₂ 0j (below 1 ¢) and other favorable properties for the above-mentioned use. Calcium-silicate elements prepared using fly ash from the production of crystalline silicon as a silicon component are characterized by very good thermal-technical and physical-mechanical properties and their heat resistance reaches 1,050 °C. The filterability of reaction mixtures with fly ash in a technological process using pre-reaction at temperatures up to 100 °C and subsequent processing by filter pressing and autoclaving is, however, somewhat lower than when using diatomaceous earth or silica sand. Impaired filterability reduces the capacity of the production line and can cause technological difficulties.

The above-mentioned disadvantages are eliminated by high-temperature calcium-silicate elements based on microcrystalline xonotlite as the main binder component, prepared from lime and a silica component in a mutual ratio corresponding to a molar ratio of CaO:SiO2 of 0.8:1 to 1.3:1, preferably 0.95:1 to 1.1:1, optionally containing fibrous reinforcement and other additives, according to the invention, the essence of which is that the silica component consists of a mixture of fly ash from high-temperature metallurgical processes for the production of silicon and its alloys with diatomaceous earth, with the total _Al2O3 content in the mixture of fly ash and diatomaceous earth being below 3.0, preferably below 2.0%.

It was found that the addition of pure diatomaceous earth, only slightly contaminated with clay minerals, to the fly ash from high-temperature metallurgical processes for the production of silicon and its alloys, especially from the production of crystalline silicon, in an amount of up to 50 wt. %, based on the entire siliceous component entering into the reaction with lime, does not have a significant effect on the formation of xonotlite as the main binding component during the subsequent hydrothermal processing. Although diatomaceous earth induces the formation of 11 & - tobermorite from approximately 10 % of the additive, in very small quantities and up to approximately 50 % of the additive of pure diatomaceous earth, the main product of the hydrothermal reaction remains xonotlite, which determines the high thermal resistance of the product. Only when the addition of diatomaceous earth brings the total AlgO^ content in the siliceous component to 2 to 3 wt. %, does the formation of xonotlite decrease significantly. In the case of using diatomaceous earth heavily contaminated with clay fractions, the amount of its additive must be reduced accordingly so that the total content of Al ₂ O^ introduced into the reaction system by the mixed silica component does not exceed the stated limit of 3 wt. %, preferably 2 wt. %.

It is advantageous to use a small amount of reinforcing inorganic fibers, such as asbestos, alkali-resistant glass fibers, or even suitable organic fibers, in the preparation of high-temperature elements according to the invention in order to improve the processability of the mixture and the physical-mechanical properties of the final products. It is expedient to use the Fibers V ΓΟΖΠΙΘΖί Z ais 40, preferably 5 to 20 wt. based on the total weight of solid components. For fibers that can negatively affect the formation of xonotlite, such as asbestos, it is expedient to significantly limit the amount of the additive. It is also advantageous to use an additive of finely dispersed xonotlite or wollastonite to the starting suspension in order to accelerate the hydrothermal reaction.

The procedure is as follows: first, high-quality quicklime is quenched with intensive stirring in 3 to 4 times the amount of hot water, after which, after about 30 minutes, it is added with stirring to an aqueous suspension of fly ash from the production of crystalline silicon, diatomaceous earth and, if necessary, asbestos fibers. The asbestos is first broken up in some suitable device ensuring a high degree of separation of individual fibers from bundles. The amount of lime, fly ash and the corresponding diatomaceous earth additive is selected so as to achieve a molar ratio of CaO/SiO2 in the initial suspension of approximately one. The resulting mixture is diluted with water to the required concentration with respect to the further processing method.

For the production of elements according to the invention by the filter molding method, this starting suspension is first heated with occasional stirring for 1 to 3 hours at a temperature of 95 °C. After this pre-reaction, the gel-like product is drained in a molding device and pressed into the final shape. The molded beads are then subjected to autoclaving. The autoclaving process must be carried out in accordance with known laws and rules concerning the course of increasing and decreasing pressure; the temperature and pressure of the isothermal hold, as well as the holding time at this temperature, must be adapted to the optimal conditions for the formation of xonotlite. In general, it is appropriate to use a higher pressure than in the preparation of elements based on tobermorite, for example 10 to 10 atm for a period of 6 to 10 hours. After autoclaving, the products are dried and, if necessary, surface-treated.

The use of a combined siliceous component, containing, in addition to fly ash from high-temperature metallurgical processes for the production of silicon and its alloys, also chalcopyrite in the specified range, allows improving the processability of the reaction mixture in the filter pressing technology with pre-reaction, in particular improving dewatering and increasing the productivity of the production equipment.

Examples

Example 1

Mixtures of diatomaceous earth and fly ash from the production of crystalline silicon were prepared in graduated proportions. These mixtures were mixed with lime so that the molar ratio CaO/SiO2 was always equal to 1.0. Aqueous suspensions of this mixture were heated for 2.5 hours at 95 °C and then autoclaved for 5 hours at 203 °C. The content of xonotlite in the product was evaluated by X-ray diffraction. A sample of pure fly ash from the production of crystalline silicon and lime was prepared as a control. Pure diatomaceous earth treated by sorting, with a low AlO2 content (i), untreated diatomaceous earth with a low clay content (li), heat-treated filter diatomaceous earth with a high _Al2O3 content (li) and untreated diatomaceous earth with a high clay mineral content were used to prepare the mixture. The composition of the fly ash and the used diatomaceous earths in mass. % is shown in Table 1.

Table * diatomaceous earth

Flight i. II. III. IV. Si0 ₂ 97.27 85.25 82.17 83.11 68.62 A1 ₂ O ₃ 0.51 3.37 5.85 12.63 18.33 ^Fe 2°3 0.10 3.89 1.78 1.34 2.07 ₂ people 0.01 0.21 0.17 0.57 0.63 CaO 0.54 1.01 1.70 0.59 0.42 MgO 0.37 0.82 1.88 0.28 0.28 to ₂ o 0.40 0.17 0.32 0.03 0.83 At ₂ 0 0.06 0.21 0.29 0.85 0.14 loss on annealing 0.68 5.20 5.46 0.35 8.64

The percentage content of AlgO^ in mixtures with different ratios of diatomaceous earth and fly ash is given in Table 2.

Table 2

Ratio of beehive/diatomaceous earth Percentage of diatomaceous earth A1 ₂ O ₃ content when used I. II. III. IV. 90/10 0.79 1.03 1.71 2.28 80/20 1.07 1.57 2.93 4.07 70/30 1.36 2.11 4.14 6o/4o 1.65 2.64 50/50 1.9* 3.18 40/60 2.22 3.71

The control sample of pure fly ash and lime yielded very pure and well-developed xonotlite after hydrothermal treatment. In a mixture prepared by gradual replacement of fly ash with diatomaceous earth

I. and in mixtures of fly ash with diatomaceous earth II., it was possible to observe the formation of small amounts of 11 & tobermorite, with the intensities of the diffraction lines of xonotlite remaining at the level of the control sample, starting from approximately 10% diatomaceous earth (based on the total silica component).

In mixtures containing 20 to 40% of diatomaceous earth I., as well as in mixtures containing the same content of diatomaceous earth II., there was a slight increase in the formation of 11 8. tobermorite, accompanied only by a slight decrease in the intensities of xonotlite reflections. In products prepared using 50% of diatomaceous earth II., however, there was a significant decrease in the content of xonotlite, and in mixtures containing 60% of diatomaceous earth II., 11 8 - tobermorite, or its imperfectly crystalline form, predominated in the product. A similar decrease in the intensities of xonotlite diffraction lines was observed in mixtures containing 60% or more of diatomaceous earth I.

In mixtures of fly ash with diatomaceous earth III. and diatomaceous earth IV., a small amount of 11 8 - tobermorite was formed at a content of 10% diatomaceous earth, while the diffraction lines of xonotlite showed a degree of integral intensities approximately at the same level as in the control sample from pure fly ash. However, in samples with 20% diatomaceous earth III. and especially IV., there was already a significant suppression of the formation of xonotlite) the tobermorite and xonotlite phases were formed in approximately equivalent proportions. Increasing the content of diatomaceous earth III. and IV. in the resulting mixtures to 30% already led to a clear suppression of the formation of xonotlite, which was demonstrated in the products in a very low content; the predominant component was 11 & tobermorite, in addition to small amounts of imperfectly crystalline tobermorite and the ^c SH _n phase

Example 2

A mixture of calcium and silica components was prepared with a composition corresponding to a molar ratio of CaO/SiO2 of 1.0 and containing, in addition to fly ash from the production of crystalline silica and lime, 5% diatomaceous earth with a SiO2 content of 72.5, 10% ochresite asbestos and 5% amosite asbestos, based on the total weight of the solid components. The aqueous suspension of this mixture was heated for 2.5 hours at 95 °C; then the suspension was drained and pressed under suction into plates measuring 0.5 x 1.0 m and 2.5 mm thick. The plates were autoclaved for 8 hours at 203 °C. The average bulk density of the resulting plates after drying was 314 kg/m3, and the tensile strength during bending was 2.13 MPa. X-ray diffraction analysis showed that the plates are composed of very pure xonotlite in terms of phase composition. The shrinkage values based on dilatometric curves were 1.8% and 2.0% for samples taken in two mutually perpendicular directions after heating to 1,000 °C.

Claims (3)

High-temperature calcium-siliceous elements based on microcrystalline xonotlite as the main binder component consisting of lime and siliceous component in a ratio to each other corresponding to a molar ratio of CaO: SiOg of 0.8: 1 to 1.3: 1, preferably of 0.95: 1 to 1.1: 1, optionally comprising fibrous reinforcement and other additives, characterized in that they contain as a silica component a particulate blend of high temperature metallurgical processes for the production of silicon and its alloys and diatomaceous earth, the total AlgOO content of the particulate blend and diatomaceous earth being below 3; , 0 # · 2. The high-temperature calcium-siliceous elements of claim 1, wherein the siliceous component consists of 99 to 85 wt. % drift and 1 to 15 wt. kieselguhr containing up to 20 wt. % AlgO4. 3. The high-temperature calcium-siliceous elements of claim 1, wherein the siliceous component consists of 99 to 50 wt. % drift and 1 to 50 wt. diatomaceous earth containing up to 6 wt. % AlgO4.