RU2533510C1 - Method for manufacturing high-porous ceramic blocks - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: reaction mixture is prepared containing oxidiser, deoxidiser, high melting component, gasifier and liquid vehicle gasifiable by thermal decomposition. Workpiece is obtained by reaction mixture prepressing in shape-generating mandrel. The workpiece is dried in shape-generating mandrel followed by the workpiece removal and curing. Self-spreading high-temperature synthesis of unit from reaction mixture of the workpiece is performed outdoors in mandrels from fire-resistant, gas-permeable, carbon-containing material.

EFFECT: obtaining high-porous long ceramic units with relation of altitude to diameter up to 5 under increase of open porousness, thermal resistance, compression strength, as well as improving items quality and simplifying tooling structure.

4 cl, 1 dwg, 2 tbl, 2 ex

Description

The invention relates to powder metallurgy and thermal energy, in particular to the production of highly porous ceramic blocks, which can be used both as a carrier of catalytic monoblocks for processing hydrocarbon raw materials and nozzles for organizing combustion, for example gas-air mixtures.

A known method of manufacturing porous pipes from refractory materials, including the preparation of the reaction mixture of powders, coaxial placement in the form of a hollow mandrel, molding the workpiece by filling the reaction mixture of powders in the gap between the mold and the mandrel, local ignition of the reaction mixture of the workpiece, and before local ignition, the mandrel cavity is filled with powder a forming material selected from the group: aluminum oxide, zirconium oxide, magnesium oxide, after which the mandrel is removed (copyright certificate in the USSR No. 1499813, publ. 27.02.2000, IPC B22F 3/10). The invention is aimed at improving the manufacturability and economy of the method due to the repeated use of the forming material.

The disadvantages of this method include:

- low mechanical strength of large-sized porous pipes arising due to a sharp temperature gradient along the radius of the pipe due to the contact of the outer wall of the workpiece with the inner wall of the metal mold, which leads to transverse cracks due to the presence of impermeable metal walls leading to high hydraulic resistance to the flow out gases

- the presence of pore heterogeneity in the finished product.

Also known is a method of manufacturing porous products from powder materials (RF patent No. 1790806, publ. 12/27/1996, IPC B22F 3/10), which consists in the fact that self-propagating high-temperature synthesis (SHS) is carried out in a non-stationary mode, locally initiating combustion so that the combustion front passes perpendicular to the selected direction of the pore channels, cooling the opposite sides of the product at different speeds.

The disadvantages of this method include:

- variable through porosity in the finished product;

- anisotropy of operational characteristics, such as thermal conductivity, permeability, mechanical strength;

- porosity range (45-70)%;

- obtaining products with a pore size of less than 100 microns, which excludes their use as nozzles for gas burners.

A known method of manufacturing porous pipes (mainly large) by the method of self-propagating high-temperature synthesis (USSR author's certificate No. 1818880, publ. 07.20.1996, IPC C22F 1/04). The essence of the invention lies in the fact that between the form and the reaction mixture is additionally placed mandrel. Refractory material with a thickness of at least 10 mm and a grain size (0.5-3) mm is poured into the mandrel. The material to be filled has a melting point above the reaction temperature. The porosity of the obtained product is equal to 75%. The tensile strength of the obtained products reaches (0.46-0.5) MPa due to the creation of close heat and mass transfer conditions for the inner and outer walls of the pipe.

The disadvantages of this method include:

- low (experimentally confirmed) heat resistance of the product (1000 ° C);

- the need to maintain an inert atmosphere during the combustion process;

- the impossibility of using these products as homogeneous catalytic blocks due to the loss of stability of the catalyst due to carburization of pores during its operation in the processing of gas-air mixtures.

The closest in technical essence to the claimed technical solution is a method of manufacturing large-pore refractory pipes (RF patent No. 2318633, publ. 10.03.2008, B22F 5/12).

The essence of the invention lies in the fact that a reaction mixture is prepared on the basis of aluminum and iron oxide, into which a refractory component and a gasifier are introduced. Two mandrels of various diameters are coaxially placed in the form, and in the center is a heat exchanger. The cavities between the heat exchanger and the mandrel of a smaller diameter and between the outer shell of the mold and the mandrel of a larger diameter are filled with a forming material and prepressed. Mandrels are deleted. By prepressing, the cavities formed after removal of the mandrels are filled with the reaction mixture and the synthesis is carried out in air in the mode of self-sustaining combustion. A large-sized metal-ceramic product is obtained with a porosity of up to 75%, an average pore size of up to 1000 μm, a heat resistance of up to 1500 ° C, and a compressive strength of up to 26 MPa.

The disadvantages of this method include:

- the structural complexity of the technological equipment in which the SHS process is implemented;

- the possibility of the appearance on long products of delaminations and heterogeneity of the structure associated with the presence of a pressure drop in the gas discharge forming material in the central axial direction.

The objective of the invention is to simplify the method of manufacturing highly porous ceramic blocks by simplifying the design of tooling while increasing the yield of high-quality long products.

Using the present invention provides the following technical result with a ratio of height to block diameter of up to 5:

- open porosity (65-85)%;

- heat resistance from 1500 to 1700 ° C;

- lack of delamination in the final product;

- compressive strength up to 50 MPa;

- structural simplification of technological equipment while reducing the totality of technological operations during the SHS of the catalytic unit;

- reduction of the cost of the final product by 30%.

This problem is solved, and the technical result is achieved by the fact that in the method of manufacturing highly porous ceramic blocks from a reaction mixture containing an oxidizing agent and a reducing agent, a refractory component and a gasifier, including preparing the reaction mixture, preparing a workpiece by pressing the reaction mixture in a forming mandrel, and self-propagating high-temperature synthesis of blocks in air, according to the invention, a liquid binder is introduced into the composition of the reaction mixture, gasified by thermal decomposition After drying, the billet is dried in a forming mandrel with the subsequent extraction of the billet from it and the curing of the billet, and self-propagating high-temperature synthesis is carried out in mandrels of a refractory gas-permeable carbon-containing material.

Wherein:

- as a liquid binder can be used (3-5)% aqueous solution of polyvinyl alcohol;

- alcohol is introduced into the reaction mass in an amount of (19-25)% by weight of the reaction mixture;

- the preform is dried in air at a temperature of (50-70) ° C, and curing at a temperature of (120-150) ° C for (2-3) hours.

The manufacture of highly porous ceramic blocks from a reaction mixture containing an oxidizing agent and a reducing agent, a refractory component and a gasifier by preparing the reaction mixture, obtaining a workpiece by prepressing the reaction mixture in a forming mandrel provide such a well-known technical effect as the fundamental possibility of synthesizing a ceramic block by SHS. At the same time, it is possible to control both the heat resistance of the product and the average pore size by selecting the particle size distribution of the components of the reaction mixture.

The introduction of a liquid binder, gasified during thermal decomposition, provides such a technical effect as the principal technological possibility of prepressing the reaction mixture when forming a homogeneous low-porous preform by increasing the wettability and adhesion of the particles of the reaction mixture to each other, on the one hand, and on the other hand, provides such an effect as an increase in the compressive strength of samples of homogeneous porosity of the nominal size compared with the figures in the prototype up to 2.7 times.

Drying the workpiece in a forming mandrel at a temperature of (50-70) ° C provides such a technical effect as the fundamental possibility of extracting a large workpiece from the mandrel due to the radial shrinkage of the workpiece, and also provides such an effect as its resistance to mechanical stress arising from further technological manipulations with the workpiece while maintaining its shape and size, due to the starting process of curing polyvinyl alcohol (self-assembly of alcohol macromolecules into branched agl omerates), which, in turn, eliminates the formation of cracks and delamination in the workpiece.

The curing of the workpiece in an oven provides such a technical effect as the fundamental possibility of making long workpieces of highly porous ceramic blocks, and also provides such a technical effect as a fundamental increase in their stiffness, which makes it possible to form workpieces with an L / d ratio of less than or equal to 5, while simplification of the technological process of their further redistribution into highly porous ceramic blocks.

The burning of the workpiece in the SHS mode is realized in mandrels made of refractory gas-permeable carbon-containing material of the corresponding thermal conductivity and porosity, which allows simplifying the design of technological equipment, on the one hand, and on the other hand, provides radial filtering of gases during combustion of the workpiece, which, in turn, , provides such a technical effect as relaxation of thermal stresses of the system and uniformity of the size of the pore channels, their radial orientation, the absence of separation Nij adjustment of the final product.

Figure 1 shows a schematic diagram of a tool for burning a tubular billet in the SHS mode, where:

1 - inner cylindrical mandrel;

2 - an external cylindrical mandrel;

3 - a pallet of insulating material;

4 - a cover with a central hole "A";

5 - resistive elektrozapal;

6 - initiating charge (quick-burning, gas-free);

7 - tubular billet;

8 - plate low porosity.

The invention is illustrated by the following examples.

Example 1. Prepare a reaction mixture composition, wt.% (See table 1): iron oxide - 50; aluminum - 14.2; magnesium oxide - 25; calcium carbonate - 6; ammonium carbonate - 4.8%; an aqueous solution of polyvinyl alcohol (5%) - 25 (in excess of 100%). The components are mixed and the resulting mixture is loaded into the forming mandrel in stages - in separate portions (for the manufacture of a hollow cylinder with an external diameter of 400 mm, a wall thickness of 40 mm and a height of up to 2 m), a step-by-step prepressing of each portion with an effort of not more than 5 kg / cm ^{2 is} performed (at This process of forming lengthy workpieces with individual webs is the know-how of the authors, which with high reliability ensures the absence of stratification of long blocks along the borders of technological hangings after the CB process ). The resulting preform is dried directly in the forming mandrel in air at a temperature of (50-70) ° C. After removing the preform, the curing process is carried out in air at a temperature of 150 ° C for 2 hours. To conduct SHS, the preform is placed (see Fig. 1) in a cavity formed by inner 1 and outer 2 cylindrical mandrels of refractory gas-permeable carbon-containing material, for example, silicon carbide of the required porosity, coaxially arranged with respect to each other, which are placed on a non-porous tray 3 of heat insulating material. The workpiece 7 is installed between mandrels 1 and 2 with a diametrical clearance of (2-4) mm. The SHS is launched through the central hole A of the cover 4 by a resistive electric valve 5 by igniting a quick-burning gas-free initiating charge 6, which, in contact with the surface of the tubular billet 7, is clamped between the non-porous loaded external force cover 4 and the supporting low-porous plate 8.

Table 1 The composition of the reaction mixture Component Name The content of the component, wt.% Iron Oxide (Fe ₂ O ₃ ) fifty Aluminum (Al) 14.2 Magnesium Oxide (MgO) 25 Calcium Carbonate (CaCO ₃ ) 6 Ammonium carbonate (NH ₄ HCO ₃ ) 4.8 5% aqueous solution of polyvinyl alcohol (in excess of 100%) 25

A large-sized refractory cermet product is obtained in the form of a pipe with a porosity of 70%, an average pore size of 1000 μm, heat resistance up to 1500 ° C, and compressive strength up to 26 MPa.

Refractory foamed gas-permeable carbon-containing material used as mandrels in the implementation of SHS must meet the requirements for effective thermal protection and low hydraulic resistance to gas flow in the radial direction during the SHS process. For example, instead of silicon carbide, you can use a material containing (12-20) wt.% Silicon carbide, (25-51) wt.% Kyanite and mineral fillers - the rest. Such materials have the following characteristics: porosity - (60-81)%, thermal conductivity at 20 ° C (0.08-0.18) W / m × K, heat resistance - (1400-1700) ° C.

Example 2. The method is carried out in the tooling of example 1 when using the reaction mixture composition, wt.% (See table 2): nickel oxide - 48.62; aluminum - 6.25; magnesium - 8.05; aluminum oxide - 18.75; calcium carbonate - 2.08; sugar - 16.25; an aqueous solution of polyvinyl alcohol (5%) - 25 (in excess of 100%).

table 2 The composition of the reaction mixture Component Name The content of components, wt.% Nickel Oxide (NiO) 48.62 Aluminum (Al) 6.25 Magnesium (Mg) 8.05 Alumina (Al ₂ O ₃ ) 18.75 Calcium Carbonate (CaCO ₃ ) 2.08 Sugar (C ₁₂ H ₂₂ O ₁₁ ) 16.25 5% aqueous solution of polyvinyl alcohol (in excess of 100%) 25

A large refractory ceramic-metal product is obtained in the form of a pipe with a porosity of 80%, an average pore size of 1500 μm, heat resistance up to 1700 ° C, and compressive strength up to 50 MPa.

Using all the essential features of the claimed technical solution allows to obtain long products in the form of a pipe with a ratio of length to diameter less than or equal to 5, with open porosity (65-85)%, having an experimentally determined average value of the pore cross section from 0.25 mm to 2 mm, providing heat resistance up to 1700 ° C, compressive strength from 0.5 to 50 MPa.

Claims (4)

1. A method of manufacturing a highly porous ceramic block from a reaction mixture containing an oxidizing agent, a reducing agent, a refractory component and a gasifier, including preparing the reaction mixture, preparing a workpiece by prepressing the reaction mixture in a forming mandrel and self-propagating high-temperature synthesis in air, characterized in that the composition of the reaction mixture additionally, a liquid binder is introduced, gasified during thermal decomposition, and the workpiece is dried in a forming mandrel with the next extraction and curing of the workpiece, and self-propagating high-temperature synthesis is carried out in mandrels of refractory gas-permeable carbon-containing material. 2. The method according to claim 1, characterized in that as a liquid binder use 3-5% aqueous solution of polyvinyl alcohol. 3. The method according to claim 2, characterized in that the solution of polyvinyl alcohol is introduced into the reaction mass in an amount of 19-25 wt.% By weight of the reaction mixture. 4. The method according to claim 1, characterized in that the billet is dried in air at a temperature of 50-70 ° C, and curing is carried out at a temperature of 120-150 ° C for 2-3 hours.

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