CA1208902A

CA1208902A - Method of manufacturing refractory products having continuous channels therein and products made by such a method

Info

Publication number: CA1208902A
Application number: CA000419088A
Authority: CA
Inventors: Mario Heinricher; Bernd Grabner; Gunther L. Mortl
Original assignee: Osterreichisch Amerikanische Magnesit AG
Current assignee: Osterreichisch Amerikanische Magnesit AG
Priority date: 1982-01-12
Filing date: 1983-01-07
Publication date: 1986-08-05
Also published as: ES8403361A1; EP0083919B1; EP0083919A2; ES518889A0; ATA8182A; EP0083919A3; DE3277015D1; AT384769B

Abstract

Abstract of the Disclosure Fired refractory products are formed having continuous channels therethrough. A particulate refractory material is formed into a compacted product with metal grids therein. Upon firing the compacted product the metal in the rods forms species that diffuse into the refractory material leaving openings in the fired product in the form of the grids originally placed in the refractory material.

Description

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The invention relates to a method for manufacturing fired, refractory bricks having continuous channels thereinJ consisting of a basic, refractory material from the group of magnesia, mixtures of magnesia and chromite, as well as spinels hased on MgO.A12O3 and MgO.Cr2O3 having a MgO content of 20 to 37 weight percent. The refractory material used in the manufacture of the bricks is placed in particulate form in a mold, and compacted therein with filaments of metal for the purpose of forming the channels in the bricks. The basic, refractory materials may be sintered or fused materials, and as mixtures of magnesia and chrome ore and further as MgO.Cr2O3 spinels there may be used a]so simultaneous sinters, i.e. sintered materials obtained by simultaneously sinter-ing raw materials consisting of MgO and Cr2O3, or yielding these compounds on firing.
~efractory products having continuous channels which are often called continuous pores or directed pores, are frequently used in various areas of the industry, especially in metallurgical furnaces and vessels. They are primarily used for feeding gases or finely dispersed solid materials through their channels, for instance into molten metal. Such products as tuyere bricks for the bottom of converters, specifically Thomas converters, are for instance manu-factured in a manner wherein a particulate refractory material, such as sintered magnesia, is placed in a mold having therein a so-called needle~plate or core-pla~e, which has needles or cores thereon for purposes of forming the channels in the bricks. The cores are arranged vertically and firmly in the plate in the mold. The material is co~pacted by tamping, shaking, vibrating or pressing.
When the products are taken from the mold, they exhibit channels in those places where the cores in the mold penetrated into the refractory material. The chan-nels in the tuyere bricks obtained in such a manner have generally a diameter of 12 to 16 mm and a length of more than 500 mm.

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~, ~2~¢1\2 This method of producing molded bricks having continuous channels therein becomes more difficult, primarily when the diameter is decreased, and also when the length of the channels is increased, because then ~he needles or cores used to form the channels easily become deformed in the process of molding the articles during compaction of the particulate refractory material in the mold, and especially if this material is compressed~
Large refractory products having channel diameters of less than 10 mm, for instance approximately 5 to 6 mm, and channel lengths which are approxi-mately 70 to 200 times and more that of the channel diameter, can be obtained by pouring a castable refractory mass into a mold in which there are needles which are secured into their positions designated for the formation of the channels. After the mass has solidifiad, the end product with the needles therein is removed from the mold. rhe castable refractory mass may have a MgO
content of at least 85 weight percent, and the needles may consist either of meltable material, for instance such as a synthetic, which melts out or evaporates during the drying process; or the needles may be a wire-like construc-tion which can be removed from the molded brick leaving smooth inner walls therein ~AT-PS 248 936). Refractory products obtained from a castable mass, however, can only be obtained by shaking or vibrating the mass, and are inferior in some respects to bricks obtained by compacting pressure.
A urther method for producing refractory articles having continuous pores or channels therein is known in which a mesh of at least a partially burnable warp and/or weft filament is compacted in layers with the refractory material. The layered bric~ is dried and fired, pores or channels being formed as a result of burning out the mesh (DE-OS 25 11 979). But meshes and other easily deformable materials, such as synthetics, because of their pliability ~ .

under pressure, are hardly capable of retaining their shape during compaction of the refractory material, and they result in a product naving irregular and deformed channels (AT-PS 277 036) which do not meet the requirements of the practice.
This is also applicable for instance to refractory bricks having high gas permeability, consisting of 1 to 75% alumina, the rest silicon dioxide and binding materials, having con~inuous channels on all sides. The channels being helically twisted and having a diameter from 0.001 to 0.1 mm~ are formed by filling a mold wi~h a ~oist mixture of the refractory starting material, into which are simultaneously placed helically crimped synthetic filaments having a diameter from 0.001 to 0.1 mm. The ends of the wires touch the inner walls of the mold. The contents of the mold are co~pacted. The brick thus obtained is dried and then heated with a hot gas and fired, whereby the synthetic filaments are melted and expelled gaseously (AT PS 286 860).
Thus it is difficult with all these known methods to produce fired, refractory bricks having continuous channels therein, and having directed porosity when using a non-castable, particulate, refractory mass, and to produce channels which would be in a desired position in the finished brick, having across their entire length the provided desired dimensions. To overcome these difficulties, it has also been attempted to provide molds for purposes of obtaining the clesired pores, in which molds two of the walls opposite one another would have holes bored therein through which holes filaments, metal wires, or refractory tubes would be placed across the mold before adding the refractory material. The wires or tubes would be embedded into the refractory material during its shaping, and they would remain in the brick when removing the brick from the mold. When using burnable filaments, they could either be 9~

burned out, or in the case of metal wlres, they could be pulled out before firing the brick (US PS 35 39 667). As refractory materials such as corundum, mullite, bauxite, sillimanite, magnesia, a material containing silica, and zircon can be used. But this method, too, is not satisfactory, because it is time-consuming to arrange the fila~nents or other elements for the formation of the pores, and because, during the removal of the metal wires from the fired bricks when these are not formed of a castable mass, but rather of a particulate, refractory material, the channels can be damaged. For this reason, it has lately been proposed to build up the refractory bricks having continuous channels therein, with several, juxtaposed plate-shaped brick parts, which have at theix abutting lateral surfaces corresponding recesses, which together present a continuous channel (European patent application 21 861 published January 7, 1981~. Suitably these brick parts are held together by a sheet metal mantel, which has a gas connection pipe.
The manufacture of such bricks is, however, complex, because the individual brick parts must be carefully constructed.
The present invention is directed to providing a method ~0 for manufacturing fired, refractcry bricks of the type mentioned above, having continuous channels therein., and which can be produced by simple means.
It is desirable to use a method which pos.itively facilitates the provision of continuous channels with equal dimen-sions in the brick, and to obtain bricks having satisfactory characteristics in every other respect, including acceptable costs.
It has been found that this can be realized when using a metal grid for purposes of forming the channels in the bricks, and when the ~lZ~8~

grid is left in the brick during firing.
According to the present invention, there is provided a method for manufac~uring a fired refractory product having continuous channels therein, said method comprising the steps of:

- 4a -a) placing a particulate refractory material in a mold, said refractory material being selected from the group consisting o~
magnesia, mixturas of magnesia and chromite, and spinels based on MgO.A12O3 and MgO.Cr2O3 having an MgO content of 20 to 37 weight percent; b) placing at least one metal grid within said par~iculate refractory material in said mold, said grid being ~ormed of a plurality of parallel me~al rods which are connected at each extrem-ity by metal crossbars, said grid being disposed to form continuous channels in said refractory material, said metal having a composi-tion such that species soluble in said refractory materia' areformed during firing such that channels in said refra~tory material are formed at the location of said grid; c) compacting said refractory material wi-th said grid therein to ~orm a compacted product; d) removing said compacted product from said mold; and e) firing said compacted product to ~orm said fired refractory product and to induce diffusion of said soluble species from said metal grid into said fired refractory product thereby forming continuous channels in said fired refractory product.
The present inVention may also be defined as a method o~
making a refractory brick having continuous channels therein, said method comprising the steps of: a) placing a particulate refrac-tory material in a mo~d, said particulate refractory material comprising a material selected from the group consisting of magnesia, mixtures of magnesia and chromite, and spinels based on Mg~.A12O3 and MgO.Cr2~3 having an MgO content of 20 to 37 weight percent; bl placing a plurality of mekal grids within said particulate refractory material in said mold, said grids being ~8~Z

comprised of a plurality of parallel and juxtaposed metal rods, said metal comprising iron and said rods having a diameter in the range of from 0.5 mm to 3 mm, said grids and said rods being placed in said mold in alternating layers; c) compacting said particulate refractory material with said grids therein by pressing to form a compacted product; d) removing said compacted product from said mold; and e) firing said compacted product at a temperature in the range of from 1550C to 1880C.
In another aspect, the invention provides a fired refractory product having continuous channels therein and formed of a basic magnesia refractory material with oxidised metal material in combination with and/or absorbed within said refractory material.
Preferably, fired, refractory bricks are manufactured having continuous channels therein. The bricks consist of a basic refractory material preferably selected from the group consisting of magnesia, mixtures of magnesia and chromite, and splnels based on MgO.A12O3 and MgO.cr2o3 having a MgO content of 20 to ~7 weight percent. The refractory material used in the production of the bricks is placed in a mold, and is compacted therein with filaments of metal intended to form the continuous channels. Thus, a layer of refractory, particulate material is placed in the mold and a metal grid is placed onto the material.

- 5a -~ ~z~

The grid preferably consists of a plurality o~ parallel and juxtaposed rods, each of the rods being connected at both its ends to a crossbar. The grid is loosely placed and further covered with a layer of the refractory material. The contents of the mold are then compacted, preferahly by pressing. The formed bricks are fircd, whereafter the end pieces of the bricks with the crossbars are rcmoved. If it is desired, the bricks are covered with a sheet metal mantel, which may have a gas connection pipe. The bricks obtained according to the in-vention may contain several metal grids, and to produce the bricks, alternate layers of refractory materials and the metal grids are placed into the mold.
During firing o the brick the me~al or wire grid oxidizes and the oxide built up during the oxidation process, ~or instance FeO, NiO and CuO, diffuses in the surrounding brick material and is absorbed, as for instance by the formation of magnesium ferrite. As a result, in the area in which the metal grid originally was, a hollow space is formed which constitutes the desired con-tinuous channel. The refractory material should contain at least 10 weight percent of free MgO to ensure good absorption, or respectively, conversion, of the metal oxide built up from the grid during the firing process of the brick.
The term "free MgO" is to be understood to mean MgO which is not already bonded, as is for instance the case with spinels, such as MgO.A12O3, MgO.Cr2O3, or MgO~Fe2O3, or magnesium silicates, such as CaO.MgO.SiO2 (monticellite) or 2 MgO~SiO2 (forsterite).
In practice, the manufacture of the bricks according to the invention is best carried out in a manner in which the refractory material and the metal grid are placed in alternate layers into the brick mold, and in which the con-tents of the mold are compacted, preferably under pressing pressure. Thereafter the bricks are removed from the mold and are rapidly dried and fired. The metal ~ ~a~f~q~

grid is constructed in a simple manner. For instance welding wires having the prescribed diameter are placed next to one another at distances which correspond to the desired distances of the channels in the finished bricks, ~he ends of the wires being spot welded to a crossbar. Thereafter ~he brick parts in which the crossbars are disposed are removed by cutting, and the remaining brick may be ground, if necessary. The grinding or polishing is especially then appropriate when the bricks thereafter are to be covered with a sheet metal mantel and are to be used as gas scavenging bricks in ducts feeding gases into me~allurgical furnaces or vessels. The application of the sheet metal mantel or metal cover respectively can be carried out in any one of the several known methods, for instance: by simple coating of the bricks with a preformad sheet metal casing;
by stretching of a U-shaped sheet metal casing and subse~uent fastening of one or two further sheet me~al plates onto the rem~ining free lateral side surfaces of the bricks; by glueing the sheet metal plates onto the bricks, etc. In the case of gas scavenging bricks the sheet metal mantels are also provided with a connection pipe for purposes of feeding gas.
Preferably steel or iron is used as material for the metal grid, and the diameters of the rods of ~he metal grid are suitably 0.5 to 3 mm, preferably 0~8 to 1.8 mm.
The invention is explained further in the following examples, wherein all percentages with the exception of porosity, are weight percents.
Example 1:
For manufacturing of magnesia bricks having directed porosity, highly pure sintered magnesia was used in the following composition and particle size:
SiO2 O. 1~%
Fe203 0.11%
A123 0.16%

~2~ 2 Mn304 0.03%
Cr203 0.36%
CaO 0.80%
MgO 98.40%
15% singered magnesia 3 - 5 mm 52% sintered magnesia 0.3 - 3 mm 33% sintered magnesia O - 0.12 mm.
1 weight percent dry sulfite cellulose waste liquor served as a binder~ and the mixture of the refractory material and binder was pressed into bricks with a metal wire grid therein after the additi~n of 2.1 1 water/100 kg brick mixture, the individual rods of the grid having a diame~er of 1.6 mm.
The bricks were fired at a temperature of 1600C, and exhibited channels in places in which the rods of the wire grid had been. The channels had a diameter of 1.6 mm.
The characteristics of the bricks after cutting off the brick parts containing the crossbars were as follows:

In Channel ~ertical to Direction Channel DirectiGn Bulk Density: 3.10 g/cm 3.05 g/cm Open porosity: 12.9% 13.8%
Cold crushing strength: 63.4 N/mm2 39.2 N~mm2 Gas permeability: >150 nPm 2.1 nPm Example 2:
As starting material a prereacted magnesia-chrome ore material ~simultaneously sintered material "SSM") of the following composition was used:
SiO2 1.23%
Fe203 11.14%

A123 ~.76%
Mn304 0.25%
Cr203 18.94%
CaO 0.96%
MgO 62.72%
This starting material was mixed with 5% sintered magnesia of the composition of Example 1 in the particle size shown hereafter, with 1% dry sulfite cellulose waste liquor and 1.8 1 kieserite solution (29Bé), to 100 kg of the brick mix~ure.
67% SSM 0.3 - 3 ~n 28% SSM under 0.12 mm 5% SS~ under 0.12 mm.
The mixture was pressed in a mold into bricks with a wire grid having a gauge of 1 mm, and the bricks were fired at 1550C. After firing, the bricks had channels with diameters of 1 mm. The characteristics of these bricks after removal of their external parts containing the crossbars were as follows:

In Channel Vertical to Direction Channel Direc~ion Bulk Density: 3.23 g/cm 3.25 g/cm ~o Open porosity: 17.4% 16.6%
Cold c~ushing strength:91.4 N/mm2 90.2 N/~m2 Gas permeability: 150 nPm 1.2 nPm Example 3:
To manufacture MgO.Cr2O3 spinel bricks, 10% of the sin-tered magnesia of the composition of example 1 having a particle size of from O to 0.12 mm, and 90% of a sintered material of the following analysis (70% 0.2 to 2 mm, 20% 2 to 4 mm) was mixed with 3 kg magnesium sulfate solution/100 kg:

_ g _ 3LZ~1~9~

Si02 0.~6%
Fe203 1.78%
A1203 0.85%
Cr203 79.00%
CaO 0.60%
MgO 17.31%
The mixture was pressed into bricks with wire grids having a gauge of 1.8 mm, and the bricks were fired at 1800C. After firing the bricks had smooth~ continuous channels and the following characteristics:
In Channel Vertical to Direction Channel Direction Bulk Density: 3.85 g/cm3 3.97 g/cm Open porosity: 14.5% 15.3%
Cold crushing strength:41.0 N/mm2 38.5 N/mm2 Gas permeability: >150 nPm 2.8 nPm Example 4:
The following starting materials were used to form spinel bricks of MgO.A1203:
Si02 0.16%
2n Fe203 0.10%
A1203 69.~0%
Cr2O3 0.10%
CaO 0.41%
MgO 29.83%
The material was pressed into bricks in par-ticulate form (70% 0.3 to 4 mm, 30% below 0.2 mm), with 5% sintered magnesia having the composition of Example 1 and a particle size less than 0.12 mm, under addition of a magnesium sulfate solution as a binder, together with a metal grid having a gauge of 0.8 mm, and the bricks were then fired at 1700C.

~z~ z The characteristics of the bricks obtained having directed porosity are shown in the following Table:

In Channel Vertical to Direction Chalmel Direction Bulk Density: 3.05 g/cm3 3-09 g/cm Open porosity: 18.8% 17.9%
Cold crushing strength: 38.5 N/mm2 36.0 N/mm2 Gas permeability: >150 nPm 4.5 nPm This test data does not change essentially when the amount of the sintered magnesia in the brick mixture is increased from 5% to 10%.
By repeating examples 1 to 4 and by using a wire grid having a diameter of 0.5 and 3 mm respectively, equally satisfactory results were obtained.
The bricks obtained by compression exhibit better characteristics than those obtained by tamping, shaking or vibrating. The firing is carried out suitably in all cases at a temperature of at least 1550C and not above 1800C9 prefer-ably at a temperature of 1560 to 1600C.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for manufacturing a fired refractory product having continuous channels therein, said method comprising the steps of:
a) placing a particulate refractory material in a mold, said refractory material being selected from the group consisting of magnesia, mixtures of magnesia and chromite, and spinels based on MgO.Al2O3 and MgO.Cr2O3 having an MgO content of 20 to 37 weight percent;
b) placing at least one metal grid within said particulate refractory material in said mold, said grid being formed of a plurality of parallel metal rods which are connected at each extrem-ity by metal crossbars, said grid being disposed to form continuous channels in said refractory material, said metal having a composi-tion such that species soluble in said refractory material are formed during firing such that channels in said refractory material are formed at the location of said grid;
c) compacting said refractory material with said grid therein to form a compacted product;
d) removing said compacted product from said mold; and e) firing said compacted product to form said fired refrac-tory product and to induce diffusion of said soluble species from said metal grid into said fired refractory product thereby forming continuous channels in said fired refractory product.

2. The method of claim 1 wherein said particulate refractory material is: compacted by pressing.

3. The method of claim 1 wherein the ends of said compacted product containing said crossbars are removed.

4. The method of claim 1 wherein said rods have a diameter of from 0.5 mm to 3 mm.

5. The method of claim 4 wherein said rods have a diameter of from 0.8 to 1.7 mm.

6. The method of claim 1 including the step of adding particulate refractory material to said mold alternately with respect to the adding of grids to said mold.

7. The method of claim 1 wherein said refractory material contains at least 10 weight percent free MgO.

8. The method of claim 1 wherein said metal grid contains iron.

9. The method of claim 1 wherein said product is a brick.

10. The method of claim 1 including the step of placing a metal cover on said product.

11. A method of making a refractory brick having continuous channels therein, said method comprising the steps of:
a) placing a particulate refractory material in a mold, said particulate refractory material comprising a material selected from the group consisting of magnesia, mixtures of magnesia and chromite, and spinels based on MgO.Al2O3 and MgO.Cr2O3 having an MgO content of 20 to 37 weight percent;

b) placing a plurality of metal grids within said particulate refractory material in said mold, said grids being comprised of a plurality of parallel and juxtaposed metal rods, said metal comprising iron and said rods having a diameter in the range of from 0.5 mm to 3 mm, said grids and said rods being placed in said mold in alternating layers;
c) compacting said particulate refractory material with said grids therein by pressing to form a compacted product;
d) removing said compacted product from said mold; and e) firing said compacted product at a temperature in the range of from 1550°C to 1880°C.