Insulating Compositions And Structures Formed Therefrom For Use In Hot Topping Comprising Fibrous Wollastonite

Abstract

Insulating compositions and hot topping structures formed therefrom comprising from about 5 to about 95 wt. % fibrous Wollastonite; at least about 3 wt. % of a second fibrous material which may be inorganic, organic, or a mixture of both; from 0 to about 90 wt. % of an inorganic particulate filler; and from about 2 to about 8 wt. % of an inorganic or organic binder.

Description

BACKGROUND OF THE INVENTION

This invention relates to the hot topping art and more particularly to insulating compositions and structures formed therefrom for use in hot topping.

As is well known in the art, the principal use of hot tops is in the manufacture of steel ingots. Conventionally, hot tops are either superimposed upon, or are inserted into the top of the ingot mold and act as reservoirs to feed molten metal to the ingot mold during the process of ingot formation.

As molten steel cools and solidifies, it shrinks. The cooling occurs more rapidly near the peripheral walls of the ingot mold than in the center of the ingot. Accordingly, a shrinkage cavity tends to form vertically through the center of the body of metal in the ingot mold, as the molten metal in the center feeds itself downwardly and outwardly in response to the lessening volume of cooler metal near the periphery.

Impurities as well as certain formulated ingredients in the steel composition tend to migrate toward the ever-decreasing pool of molten metal, resulting in a segregation of these impurities and formulated ingredients in the last metal to solidify. This portion is unusable and must be discarded, reducing the volume of sound metal which can be obtained from the ingot.

By superimposing or inserting a hot top in the ingot mold or by insulating the top portion of the mold to form an in-the-mold hot top, the effect is one of minimizing or eliminating the shrinkage cavity in the body of the ingot by maintaining a reservoir of molten steel in the hot top which feeds the ingot. This permits recovery of a higher percentage of sound metal from the metal forming the ingot.

To insure that the hot top serves its function of feeding molten metal to the ingot mold as the metal in the latter solidifies, the interior surfaces of the hot top are provided with a lining of highly insulative material which slows the transfer of heat out of the molten metal. Since the ingot mold is not insulated, its contents will cool before the contents of the hot top do. The present invention is addressed to providing improved insulating compositions and structures formed therefrom for use in hot topping.

The insulating compositions and structures of the present invention are particularly useful in connection with so-called "reusable" hot tops. These comprise outer shells of cast iron or the like formed integrally with or separately from the ingot mold, and an insulating lining which is replaceable. The replaceable lining is generally formed from at least one, but frequently a plurality of self-supporting panels or boards which will generally range from about one-half inch to not more than about 11/4 inch in thickness. The shape of the panels or boards will conform generally to the surface configuration of the hot top, and accordingly, may be flat or curved. In some instances, the board is formed of a plurality of connected sections of flat or curved elements which may be folded to form two or more sides of the lining.

In addition to the replaceable panels or boards, many hot tops in use today are provided with a replaceable bottom ring or gasket. By convention the structure is referred to as a "bottom ring" when associated with in-the-mold hot tops, and a "gasket" when associated with superimposed hot tops. The bottom ring or gasket, which may be from one-half to 13/4 inches thick, is disposed at the bottom of a hot top and forms a seal between the hot top casting and the ingot wall to prevent molten metal leakage between the juncture; protects the hot top casting from molten metal; seals off molten metal leakage behind the insulating liner; and supports the lower edges of the replaceable panels or boards.

While the insulating composition forming a bottom ring may be the same as that forming the panels or boards, it is generally preferred that a gasket be formed of a composition which is softer, less dense, and more resilient than that forming the boards or panels. A softer and more resilient gasket forms a better seal with the bottom edges of the panels or boards, and between the hot top and ingot mold.

In addition to possessing a low coefficient of thermal conductivity, compositions useful in forming panels, boards and bottom rings for hot tops preferably possess no more than nominal quantities of water and decomposable organic constituents, and should be relatively free of materials which will adversely affect the quality of the steel being manufactured. The panels should be light weight to facilitate handling.

The compositions of the present invention fall within the foregoing criteria and in addition, permit the formation of structures which have a wide variety of densities, ranging from about 30 lbs. per cubic foot to about 60 lbs. per cubic foot. Moreover, the compositions have proven to be suitable replacements for insulating compositions employing silica and asbestos.

It is therefore an object of the present invention to provide improved insulating compositions for use in hot topping.

A further object of the invention is to provide improved hot topping structures, such as panels, boards, and bottom rings and gaskets formed of improved insulating compositions.

Another object of the invention is to provide insulating compositions for use in hot topping which have very good thermal insulating properties, are relatively free of water, and contain only nominal quantities of decomposable organic materials.

Yet another object of the invention is to provide insulating compositions which can be formed without the use of asbestos or silica.

A further object of the invention is to provide improved hot topping structures having densities within the range of about 30 to about 60 lbs./ft..sup.3.

These and other objects and advantages of the invention will become apparent from the following detailed discussion, including the best mode presently contemplated for practicing the invention.

In accordance with one aspect of the present invention, there is provided a composition comprising, on a dry basis, from about 5 to about 95 wt. % fibrous Wollastonite; at least about 3 wt. % of a second fibrous material selected from the group consisting of inorganic fibrous materials (i.e., other than fibrous Wollastonite), organic fibrous materials and mixtures thereof, the inorganic fibrous material being present in an amount within the range of 0 to about 20 wt. % and the organic fibrous material being present in an amount within the range of 0 to about 8 wt. %; an inorganic particulate filler present in an amount within the range of 0 to about 90 wt. %; and from about 2 to about 8 wt. % of a binder selected from the group consisting of inorganic or organic binders.

Wollastonite is a naturally occurring calcium metasilicate. It is the only pure white mineral available commercially which is wholly fibrous. Fiber lengths average thirteen to fifteen times the diameter. The properties of Wollastonite are as follows:

Chemical Formula CaSiO.sub.3 Molecular Weight 116 Crystaline Structure acicular Specific Gravity 2.9 Coefficient of Expansion (mm/.degree.C) 6.5 .times. 10.sup.6 Refractive Index 1.63 Transition Point 1200 (to pseudo-Wollastonite, .degree.C) Melting point (.degree.C) 1540 Solubility in H.sub.2 O (gm/100 cc) 0.0095 Hardness (Mohs' scale) 4.5 Color Brilliant White Weight (lbs./solid gallon) 24.2 Bulking Value (Gal./lb.) 0.0413 pH (10% slurry) 9.9 Moisture Content (per cent) 0.5

Wollastonite is also available in powder form, and as such can be used as the inorganic particulate filler ingredient of the composition as described more fully hereinafter.

Consistent with the function of fibrous materials, fibrous Wollastonite serves to provide high green strength for hot topping structures formed from compositions containing the same.

Compositions of the present invention should also contain, in addition to the fibrous Wollastonite, at least about 3 wt. % of a second fibrous material selected from the group consisting of inorganic fibrous materials, organic fibrous materials and mixtures thereof. This second fibrous material tends to enhance the green strength of the compositions since these fibers are generally longer than Wollastonite fibers. Suitable inorganic materials serving this function include rock wool, slag wool, glass fiber and the like. Cellulosic fiber, e.g. finely divided paper or paper fiber, is the least expensive and most readily available organic fiber, although other synthetic organic fibrous materials may also be used.

While it is not necessary to use more than about 3 wt. % of a second fibrous material, under certain circumstances it may be desirable to use higher amounts. However, it has been found in general that where an inorganic fibrous material is employed, its concentration should not exceed about 20 wt. %, and where an organic fibrous material is employed, its concentration should not exceed about 8 wt. %.

As the inorganic particulate filler material which may be present in an amount within the range of 0 to about 90 wt. %, use may be made of a wide variety of conventional materials such as the diatomaceous earths, perlite, burnt rice hulls, olivine, kyanite, mullite, fly ash and the like. A preferred inorganic particulate filler material is powdered Wollastonite, due to its relative purity, low water content and essential freedom from uncombined silica.

Both fibrous and powdered Wollastonite are available from Interpace Corporation. The fibrous grade Wollastonite is sold under the designation F-1, and powdered Wollastonite under a variety of designations, the most preferred of which is C-6. This powdered Wollastonite comprises almost 90% minus 200 mesh (74 micron) particles by wet sieve analysis.

The preferred binder is a phenol-formaldehyde resin, although other organic, e.g. urea-formaldehyde resins, and inorganic, e.g. sodium silicate, binders may also be used.

Compositions of the present invention may be prepared by adding the fibrous ingredients, e.g. the fibrous Wollastonite and second fibrous material to water with adequate stirring to uniformly disperse the fibrous materials in the water. Thereafter, the inorganic particulate material and the binder are added with stirring until uniformly dispersed.

The slurry is then placed in a suitable mold dimensioned to the shape of the desired hot topping structure, e.g. a panel, board, wrap-up liner, or gasket.

The mold preferably is a filtering-type mold having foraminous surfaces. If desired, the rate of dewatering can be increased by applying vacuum, pressure, or both to the mold and its contents. The compositions of the present invention have proved particularly suitable for forming with a pressure differential dewatering technique in a filtering-type mold.

Where dimensional stability is important, the molded structure should be at least partially dried within the mold.

It has been found that for most purposes, adequate dimensional stability is achieved by partially drying the structure in the mold and then removing the structure from the mold and free-drying it. Where a very high level of dimensional stability is required, the structure may be completely dried while in the mold.

EXAMPLE I

A 52% uniform slurry of, by weight, water, 1.22% of rock wool, 18.3% of a 5% aqueous dispersion of pulp stock, 2.52% of F-1 fibrous Wollastonite, 24.43% of C-6 Wollastonite powder and 1.53% of phenol-formaldehyde resin was made by stirring to produce uniform dispersment.

The composition was then poured into molds, dewatered and dried. The resulting composition had a density of about 55 lbs. per cubic foot. On a dried basis, this produced a composition in the following proportions:

Fibrous Wollastonite 8 wt. % Rock Wool 4 wt. % Paper Fiber 3 wt. % Powdered Wollastonite 80 wt. % Resin Binder 5 wt. %

EXAMPLE II

A lower density product was prepared in a manner similar to that described in Example I, the ingredients being present in the proportions indicated below (dry basis):

Fibrous Wollastonite 94 wt. % Paper Fiber 2.5 wt. % Glass Fiber 1 wt. % Resin Binder 2.5 wt. %

This composition produced a structure which was much softer than that produced from the composition of Example I, and which had a density of about 31 lbs. per cubic foot. This composition was deemed to be more useful in the manufacture of gaskets than panels or boards, while the denser composition of Example I was deemed to be more useful in the manufacture of panels or boards.

EXAMPLE III

An intermediate density product was prepared in a manner similar to that described in Example I, the ingredients being present in the proportions indicated below (dry basis):

Fibrous Wollastonite 20 wt. % Glass Wool 8 wt. % Perlite 3.5 wt. % Powdered Wollastonite 65 wt. % Resin Binder 3.5 wt. %

This composition provided a structure having a density of about 42 lbs./ft..sup.3, using inorganic fibrous materials exclusively.

EXAMPLE IV

Another intermediate density product was prepared in a manner similar to that described in Example I, the ingredients being present in the proportions indicated below (dry basis):

Fibrous Wollastonite 30 wt. % Paper Fiber 6 wt. % Powdered Wollastonite 60 wt. % Resin Binder 4 wt. %

This composition provided a structure having a density of about 43 lbs./ft..sup.3, using a second fibrous material consisting of organic fibers.

The compositions of Examples III and IV produced insulating panels exhibiting a high degree of impact resistance and an ability to undergo moderate bending without breaking.

Although the compositions of the present invention permit the exclusion of asbestos and silica, it will be appreciated that the invention may be practiced with the inclusion of these ingredients.

Claims

1. An insulating composition for use in hot topping comprising, on a dry basis, from about 5 to about 95 wt. % fibrous Wollastonite; at least about 3 wt. % of a second fibrous material selected from the group consisting of inorganic fibrous materials, organic fibrous materials and mixtures thereof, said inorganic fibrous material being present in an amount within the range of 0 to about 20 wt. % and said organic fibrous material being present in an amount within the range of 0 to about 8 wt. %; an inorganic particulate filler present in an amount within the range of 0 to about 90 wt. %; and from about 2 to about 8 wt. % of a binder selected from the

2. The composition defined in claim 1 wherein said second fibrous material

3. The composition defined in claim 1 wherein said second fibrous material

4. The composition defined in claim 1 wherein said inorganic particulate

5. The composition defined in claim 1 wherein said binder comprises a

6. An insulating composition for use in hot topping comprising, on a dry basis, about 8 wt. % fibrous Wollastonite, about 4 wt. % second inorganic fibrous material, about 3 wt. % cellulosic pulp, about 80 wt. % powdered

7. An insulating composition for use in hot topping comprising, on a dry basis, about 94 wt. % fibrous Wollastonite, about 1 wt. % second inorganic fibrous material, about 2.5 wt. % cellulosic fiber, and about 2.5 wt. %

8. An insulating structure for use in hot topping defined by a body having a thickness within the range of about one-half inch to about 13/4 inch, at least in part formed of a composition comprising, on a dry basis, from about 5 to about 95 wt. % fibrous Wollastonite; at least about 3 wt. % of a second fibrous material selected from the group consisting of inorganic fibrous materials, organic fibrous materials and mixtures thereof, said inorganic fibrous material being present in an amount within the range of 0 to about 20 wt. % and said organic fibrous material being present in an amount within the range of 0 to about 8 wt. %; an inorganic particulate filler present in an amount within the range of 0 to about 90 wt. %; and from about 2 to about 8 wt. % of a binder selected from the group

9. The insulating structure defined in claim 8 wherein said body is formed entirely from said composition, said structure having a thickness within

10. The insulating structure defined in claim 9 in the shape of a board.

11. The insulating structure of claim 9 in the shape of a curved board.

14. The insulating structure defined in claim 9 wherein said composition comprises, on a dry basis, about 8 wt. % fibrous Wollastonite, about 4 wt. % second inorganic fibrous material, about 3 wt. % cellulosic fiber, about 80 wt. % powdered Wollastonite and about 5 wt. % phenol-formaldehyde resin binder.