US3005422A - Refractory roof - Google Patents

Description

Oct. 24, 1961 R. P. HEUER 3,005,422

REFRACTORY ROOF Filed Nov. 2a. 1958 INVENTOR flzpssefc Rearce Hews? ATTORNEY S i ate vania Filed Nov. 28, 1958, Ser. No. 776,885 4 Claims. (Cl. 110-99) The present invention relates to refractory roofs of the character employed in metallurgical furnaces such as open hearth steel furnaces, copper refining furnaces, and the like.

The present application is a continuation-in-part of my copending application Serial No. 408,897, filed February 8, 1954, for Refractory Roof which is now abandoned.

A purpose of the invention is to reduce the pronounced tendency of basic refractory roof brick to spall parallel to the hot face.

A further purpose is to provide roof brick having internal oxidizable metallic plates which are comolded into the brick and extend at least over a major portion of the longitudinal dimension of the brick, and to place such bricks in a curved arch of the brick.

A further purpose is to dispose the internal plates substantially radially in a curved arch.

A further purpose is to construct a curved arch having means of support for the brick and internal oxidizable metallic plates which are comolded into the brick and extend at least over a major portion of the longitudinal dimension of the brick.

A further purpose is to dispose the internal plates substantially radially in a curved arch together with external plates in the transverse joints of the roof placing the external plates on the bricks by comolding or by introducing the plates at the time of assembly.

Further purposes appear in the specification and in the claims.

In the drawings I have chosen to illustrate a few only of the numerous embodiments in which my invention may appear, selecting the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

FIGURE 1 is a fragmentary diagrammatic section transversely of a suspended roof arch according to the invention.

FIGURE 2 is a fragmentary section in the position indicated by the line 22 of FIGURE 1, substantially in plan.

FIGURE 3 is a fragmentary diagrammatic vertical section in the transverse direction through a modified arch of the invention.

FIGURE 4 diagrammatically represents a radial joint in a refractory roof, in which joint are an oxidizable metallic plate and an asbestos separator, the latter occurring near the hot end of the joint.

Describing in illustration but not in limitation and referring to the drawings;

Basic refractory brick have been employed in roof construction with oxidizable metallic spacer plates applied on all faces. As the furnace continues in service, the metal of the spacer plate, which is usually iron or steel, oxidizes, and the iron oxide reacts with the basic refractory of the brick at either side, which is usually of chromite, magnesia or a mixture of the two predominating in either of the components. This tends to integrate the roof together, preventing leakage of gases at the joints, preventing infiltration of air, and reducing the tendency of the brick to crack and spall.

The metallic pacer plates in the joints may have one objection in that the oxidation which occurs in service increases the thickness of the plates by percent or more. Furthermore, the oxidation is not uniform throughout the length of the brick since itis practically negligible at the cold face and increases with increasing distance toward the hot face. This non-uniform degree of oxidation causes non-uniform dimensional changes in the brick which may localize the strains in a curved arch and concentrate them at or near points where oxidation has been the greatest.

Difiiculty has been encountered in sprung arches, in suspended arches and in combined sprung arches and suspended arches if the tendency of the arch to elongate in the direction of the are when the plates oxidim at the radial joints is excessive. This develops a com paratively high and localized pressure in the direction of the arc, subjecting the brick to high compression forces which may cause failure or cracking.

The basis refractory brick of the character which have previously been used with oxidizable metallic plates on the lateral faces have shown a tendency to spall along a line parallel to the hot face and at a distance of one to three inches from it.

I have discovered that if an oxidizable metallic plate is comolded in the interior of the basic brick containing at least 10 percent of magnesia, and extends longitudinally of the brick throughout the major portion of the length of the brick, a wholly different and more advantageous behavior occurs. As the plate which is buried in the brick by comolding oxidizes, it does not cause any overall dimensional change. It appears that the iron oxide formed by the oxidation of the plate reacts with the basic ingredients of the refractory, for example with the magnesia present in the refractory, to form magnesioferrite, which diffuses into the refractory without causing an overall volume change. The same thing occurs in a brick of chromite where the magnesia content exceeds 10 percent by weight.

I have also discovered that the oxidizable metallic spacer plate which extends throughout the interior of the brick exerts a pronounced tendency to prevent spal-ling oif of the hot ends of the brick. This would appear to be due to the fact that the magnesioferrite is highly refractory and tends to create a strengthening rib running lengthwise through the brick, and also the magnesioferrite tends to break up the force distribution and pre vent crack propagation across from one side to the other of the magnesioferrite band.

To be most effective the buried plate should extend almost to the hot face or preferably to a distance not in excess of one inch from it. It is less important that the plate extend to the cold end, although it is desirable to extend to a position close to the cold end.

The number of plates which are buried in the interior of the brick by comolding may vary. In some cases a single plate is suflicient, although in other cases two or more plates extending in the longitudinal direction will be used.

External plates are used on the joints in the transverse direction. The plates on the transverse faces are conveniently comolded with the brick, although they will in some cases be assembled to the brick when the roof is made. The plates which are comolded on the transverse faces perform the desirable function of strengthening the brick during shipment.

A considerable amount of testing experience has now been had with the improved basic refractory roof of the present invention applied to open hearth furnaces. For example, one open hearth furnace is oil fired and has a regular capacity of 3 50 tons. Previously the main roof of this furnace was equipped with so-ca-lled Zebra construction of alternate rows of basic and acid refractories, and the balance of this roof was silica. The

Zebra roof would last approximately 140 heats with extensive patching and repairs, after which the entire roof required replacement. The main roof of this furnace was constructed to eliminate silica bricks by using basic bricks comprising a mixture of chrome ore and magnesite, each brick being of key shape and molded to have steel plates approximately inch thick on four major external surfaces and internal plates disposed along the major axis of the brick in two planes radial of the arch which were spaced approximately equidistant from each other and from adjoining surfaces of the brick.

These bricks were laid in an arch formed between fixed skewbacks. The roof was 12 inches thick with 15 inch ribs. Allowance for expansion was made in the radial joints at the hot face of the brick by insertion of asbestos strips two inches wide and inch thick placed between alternate courses of the brick for a distance approximately six feet out from each skewback, and in every fourth course of brick for the remaining fourteen feet of the arch. The percentage of expansion allowance represented by the asbestos was approximately 0.6 percent measured along the curved inner surface of the arch between the skewbacks.

Prior to the installation of this roof I learned that because of the increased cost of the basic refractory it would be necessary for the roof to last slightly more than 200 heats in order for it to be economical as compared to the best prior art roofs.

The roof of this furnace has the following features:

(1) The arch is sprung from rigid skewbacks, the arch length being 24 feet between skewbacks.

(2) There are steel plates on the radial faces of the basic refractory brick.

3) There are internal plates extending radially in the basic refractory brick.

(4) Between the hot ends of the basic refractory brick, asbestos sheet is introduced.

The roof of this new open hearth furnace stood up for approximately 180 percent of the minimum requirement of heats.

Since the success of this first basic open hearth furnace, there has been a major change to basic open hearth roofs in accordance with the present invention.

More than 60 open hearth furnaces have been converted to the basic arch roof of the present invention since the beginning of the year 1958. Some of these furnaces are located in typical steel plants in Wierton, West Virginia; Detroit, Michigan; Buffalo, New York; Cleveland, Ohio; Chicago, Illinois; Peoria, Illinois; Homestead, Pennsylvania and Geneva, Utah.

There are several opposing forces which lead to the surprising behavior of the open hearth roof of the present invention. In order to understand this behavior, I will compare with the practice using wire screen, mesh or gauze at the radial joints according to my US. Patent 2,779,233. I have had considerable experience with the type of roof which uses the wire screen, mesh or gauze at the radial joints in Europe.

(1) Continuous steel plates in the radial joints of a sprung arch basic refractory roof in accordance with the present invention unite the bricks in the line of the arc of the roof to form a monolithic or integrated structure, which greatly strengthens the arch and guards against portions of individual bricks dropping out prematurely.

(2) Where the continuous steel plates are used in the radial joints of the sprung arch, the steel plates expand due to formation of iron oxide, which has a lower density than iron. If nothing else is done, therefore, the presence of continuous steel plates in the radial joints causes a plus dimensional change or a dimensional increase.

(3) The steel screen, mesh or gauze in the radial joints, when it oxidizes, does not form a monolithic structure. The reason for this is that the basic brick surface and the intervening gauze only make contact along the surfaces of the wires and tlr's surface contact is not sufficient to bond the structure monolithically.

(4) When the steel mesh, screen or gauze in the radial joints oxidizes, the oxide formed does not cause a dimensional increase but diffuses into and reacts with the basic refractory and the net result is a minus dimensional change or a dimensional decrease in the direction of the arc of the arch.

(5) Asbestos layers between the radial joints at the hot end cause a net dimensional decrease or a minus dimensional change.

Summarizing the above, the steel wire screen, mesh or gauze is unsatisfactory because it does not produce a monolithic joint at the radial joints. The continuous plate at the radial joints produces a monolithic joint but when used alone it has the undesirable feature of cansing a dimensional increase. By using the internal plates which also extend radially it is possible to avoid a plus dimensional change when both the external and internal plates oxidize, because as mentioned above the internal plate forms iron oxide which reacts with the magnesia to form magnesioferrite, and the magnesioferrite diffuses into the refractory which produces a minus dimensional change.

Thus it is possible to use rigid skewbacks, and even if spring loaded skewbacks are used, the need for dimensional adjustment would not be substantial.

On the other hand, if steel screen wire, mesh or gauze were used at the radial joints, there is a minus dimen sional change and the radial joints are not monolithic.

Considering now the curved arch as shown in'FIG- URES 1 and 2, I there illustrate suitably shaped overhead steel supporting members 25 running in the arc direction, and receiving and supporting at intervals T-shaped hangers 26 having hooked upper ends 27 engaged over the supporting element, and having opposed projections 28 at the lower ends which are received within hanger openings 34) of the respective brick. It is immaterial from the standpoint of the present invention whether the hanger recesses 39 are of the character which are entirely formed of refractory, entirely formed of metal imbedded in the refractory, or formed of a combination of the two.

Basic refractory roof brick 31 suspended from the hangers are also partially supported by skewbacks 32 as well known (only one skewback is shown). The skewbacks may be backed up by springs as is well known (not shown).

Each of the brick 31 has radial faces 33, transverse faces 34-, hot ends 35, and cold ends 36. The brick are wedge shaped as shown in 'FIGURE 1.

Extending in the radial direction through the basic refractory brick and imbedded or buried therein by co molding with the refractory is an oxidizable metallic plate 37. When a single plate is used it is preferably located in the middle. The plate 37 (as where formed of aligned plate portions 37a and 37b as indicated in FIGURE 2) preferably extends over the major portion of the length and width of the brick and is in no respect comparable with separate wires or a mere screen mesh. The plate extends desirably to about one inch from the hot end, and this distance should not exceed 5 inches.

The plate 37 will normally extend to the hanger socket which is adjacent the cold end and will if required be cut out to avoid the hanger.

The plates 37 are of any suitable oxidizable metal, preferably low alloy or plain carbon steel, but permissibly also stainless steel. The plate thickness will normally be less than inch and preferably between and inches.

In the preferred embodiment the internal plates are comolded with the refractory. External plates 38 are provided on those faces of the brick which are placed in the transverse joints. The external plates cover approximately the full width of the brick and the pre.

ponderance of the distance between the hot face and the cold face. The internal plates are preferably in two approximately equal pieces 37a and 37b, which pieces are supported by external plates on opposite faces of the brick and the pieces extend into the refractory toward each other from these external plates as by welded afl'lxation to the external plates. The external lates are preferably the same material as the internal plates, their thickness being less than 4 inch and preferably between and 7 inch. Instead of a welded assembly of the internal and external plates, it is also desirable in some cases to use a single plate formed to provide an external portion and an internal portion disposed at a right angle to each other.

When the molding operation is effected, one external plate with preferably attached internal plate or plates protruding upward, is placed in the bottom of the mold and the refractory mixture is placed on top of it in the mold. The other external plate with its internal plate or plates protruding downward is placed in the top of the mold or alfixed to the top die of the molding press.

The pressing operation is completed by pressing upon the external plates to comold the refractory to the plates. The internally molded plates can if desired be placed approximately radial and so located in the back that in the assembled roof the distance between the internal plates in the direction across the arch of the furnace is approximately uniform.

It is not necessary that the external plates be placed perpendicular to the internal plates. Internal plates can be placed in a position at right angles to movement of the molding die and parallel to the external plates if any are present. In such case the external plates would be placed in the mold first, then part of the refractory mixture, then an internal plate, then more refractory mixture, etc. until the desired number of plates have been included in the mold with the final external plate on the top. This molding procedure is not so easily affected as the molding process with vertical internal plates, nevertheless, the brick is quite suitable for the purpose.

The number of internal plates may be varied and the number should be increased as the size of the brick increases. For general uses I prefer brick of cross-section 6 x 3 inches or smaller, as for example 4 x 2 /2 inches. In cases where the pair of opposed internal plates extends through the minor dimension of the brick, such as the 2 /2 inch dimension of 4 /2 x 2 /2 inch brick, I have found that two pairs of opposed plates spaced 1 /2 inches apart and 1% inches from each lateral face are satisfactory. For best results the distance from the internal plate or plates to the nearest lateral face should not exceed 2 /2 inches.

By using internal plates within the brick which are disposed in a radial position in a curved arch, I find that I have many choices of the method of treating the radial joints themselves. A single loose plate may be inserted between the brick in the radial joint or in way of further illustration plates may be pressed on every brick or on alternate brick, thereby providing a single or double thickness of metal in the radial joint.

As shown in FIGURE 4, the brick 21 has on the radial face a loose plate 54 which hooks at 55 over the cold end 36, and extends to a point 56 which is suitably an inch or more removed from the cold end 35. A strip of asbestos paper 58' is disposed over the radial face be tween the brick at the hot end. Loose oxidizable metallic plates, or plates which are attached to the brick at the time of pressing, single plates, or double plates may be present in the joints together with the asbestos inserts.

It will be understood that the refractory used is preferably of the type suited for use without kiln firing. The molded brick are cured and/or dried and are then suitable for use. The external plates 38 on the unfired refractory brick tend to protect the brick during shipment.

In some cases separate oxidizable metallic spacer plates are inserted on the transverse faces.

In some cases the invention is applied to a sprung arch having no hangers as in FIGURE 3. Except for the absence of hangers and hanger sockets, and the use of two internal lines of plates 37 in each brick, the construction of FIGURE 3 conforms with that of FIGURES l and 2.

When reference is made herein to oxidizable metallic plates, it is intended to mean continuous plates as opposed to mesh or screen.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the method and structure shown, and I therefore claim all such insofar as they fall within the reasonable spirit of and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

l. A refractory roof of curved arch form, comprising skewbacks in spaced relation, and between the skewbacks a plurality of tapered basic refractory roof brick each having two non-parallel radial sides, said brick being of the class consisting of magnesia and mixtures of chromite and magnesia containing at least 10 percent magnesia by weight, there being in each of the brick an oxidizable metallic internal plate in intimate contact with the refractory of the brick extending through the interior of the brick at a position remote from the outside of the brick in a radial plane over a major portion of the longitudinal and lateral dimension of the brick from a position adjacent the hot end, and there being in each radial joint between bricks in the direction of the load in the arch an oxidizable metallic continuous plate extending over a major portion of the longitudinal and lateral dimension of the brick, whereby on oxidation of the internal plates their resulting oxide diffuses into the brick leaving open spaces for thermal expansion where the internal plates were initially present and reacts with amounts of the magnesia present to form a magnesioferrite structure resistant to spalling, and the plates at the radial joints on oxidation produce oxide with positive dimensional increase.

2. A refractory roof of claim 1, in which there are a plurality of plates extending in radial planes through the interior of each brick in the arch.

3. A refractory roof of claim 1, in which there are external oxidizable continuous metallic plates on all four lateral joints between adjoining bricks.

4. A refractory roof of claim 1, in combination with separators of asbestos interposed between brick at radial joints near the hot end.

References Cited in the file of this patent UNITED STATES PATENTS 2,105,804 Beall Jan. 18, 1938 2,236,920 Robertson Apr. 1, 1941 2,547,322 Heuer Apr. 3, 1951 2,641,207 Pollen June 9, 1953 ,799,233 Heuer July 16, 1957 2,903,254 Heuer Sept. 8, 1959

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