CA1091022A - Method of reducing deterioration of electric furnace refractory metal components - Google Patents

Abstract

Abstract A process is described for the reduction or prevention of deterioration of refractory metal components of an electric furnace by chromia-containing oxide melts.
The process is based on the discovery of an unexpected series of oxidation/reduction reactions leading to formation of chromium trioxide and subsequent oxidation of the refrac-tory metal (e.g., molybdenum or tungsten) by the chromium trioxide. The process comprises adding to the mixture of oxides in the furnace a component which is oxidized to an oxide with a lower free energy of formation per mole of oxygen than the refractory metal oxides (such as Mo02, Mo03, W02 or W03) and chromia, and which thus reduces Cr03 to a compound which is inert to the refractory metal. In a preferred embodiment the additive is a carbonaceous material and may be in the form of powdered coal.

Description

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METHOD OF REDUCING DETERIORATION OF
1 ELECTRIC FURNACE ~EFRACTORY METAL COM~ONNTS

Backgrcund of the Invention The invention herein relates to processes for melting refractory oxides in electric furnaces. More partic~larly it relates to a method for reducing or pre-venting the oxidative deterioration of refractory metal electrodes and orifices in such furnaces.
For a number years electric melt resistance furnaces have been used to melt refractory inorganic oxides, 10 commonly silica and alumina. These molten oxides can then be spun or blcwn into attenuated fibers which are used as thermal insulations. Those fibers which have relatively lower melting points are commonly referred to as "glass ' fibers" while those with relatively higher melting points are referred to as "refractory fibers". The glass fibers are used as thermal insulation for homes, office buildings and the like, while the refractory fibers are used as thermal insulation for furnaces, kilns and similar devices.
These various types of fibers have melting points ranging from about 1000F to about 3300F or higher (about 540C to about 1815C or higher). There is no absolute line of demarcation between the fibers commonly termed "glass" and those commonly termed "refractory", but normally the latter are considered to be those usable at temperatures above about 1200F (65~C).
Electric furnaces for melting such oxides commonly have the general shape of a bowl witn a drain orifice in the center. Norm211y three or more electrodes are positioned around the orifice. The refractory oxides to be ~elted are charged, usually in powdered or granulated fo~m~ into the bowl shaped furnace such that they cover the oriiice and the -?- , J ~

electrodes. After start-up using a high temperature flame to form an initial molten pool of oxides, electrical current passing through the electrodes and the molten oxides causes resistance heating of the oxides and in turn continued melting of the oxide materials. As the oxides melt they drain through the refractory metal orifice into the fiber forming apparatus. In the meantime raw materials in the form of oxide powders are continually added to the furnace and in turn melted for fiberizing, thus resulting in con-tinuous formation of an oxide melt and subsequent fibers.
Freguently a melting and fiberizing operation, once started, will be run continuously for periods of many days, weeks or months. This means that the electrodes must operate satis-factorily for such periods, for once the melting operation has started there is often no opportunity to replacq or repair the electrodes without shuting down the furnace.
Even with those furnaces where electrodes can be replaced while the furnace remains in service, such replacement is both difficult and costly.
Similarly, the orifice must remain substantially unchanged in shape or dimensions during the entire run, for the flow of molten oxide into the fiberizing unit i, con-trolled by this orifice. The orifice is formed with a specific and carefully calculated diameter, designed such that a closely controlled predetermined quantity of molten oxide drains down to the fiberizing unit located below the furnace. The quantity and especially the quality of the fiber formed is directly related to the flow rate of the oxide melt from the furnace through the orifice to the fiberizing unit. Control o~ such flow rate is main~ained by a needle-like flow controller ~;hich operates in cooperation with the orifice to control the guantity of oxide flowing t U~

1 through the orifice. If the orifice deteriorates, it becomes substantially enlarged and it can no longer cooperate with the "needle valve" control apparatus to resulate the flow of oxide melt through the orifice. The fiberizing unit thus becomes essentially "flooded" with excess oxide melt and can no longer function to produce satisfactory fiber. This deterioration of the ori~ice is more critical to the operation than the deterioration of the electrodes, for to some extent the electrodes can be moved and/or the electric current flow can be ad~usted so as to compensate to some extent for the deterioration of the electrodes. However, the fixed orifice cannot read;ly be adjusted to compens~te for the removal oF
the refractory orifice metal and resulting increase in the orifice diameter.
As noted, electric furnaces have been used success-~ fully in the past to form "glass" melts which are primarily s composed of mixtures of silica and soda, and "refractory"
melts which are primarily composed of mixtures of silica and alumina. Frequently small amounts of other oxides, such as titania, zirconia, iron oxides or the like may be present, particularly with the lower ternperature "glass fibers".
Herefore it has not been observed that any of these materials has had any unduly detrimental effects on the operation of the electric furnaces.
In recent years, however, refractory oxide fibers containing silica and alumina and up to aboui 10% chromia have found widespread use. Such fibers have been fo!:nd to be very effective thermal insulations at temperatures up to about 2700F (1480C). Typical of such chromia containing 3~ fibers are those sold under the trademark FIBERCHROME by the Johns-Manville Corporation and described in U.S. Patent No.
3,449,137 to Ekdahl.

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1 Initially the chromia containing oxide melts were made in car~on arc furnaces for fiberization. Success of refractory metal electrode electric furnaces with other oxide melts led to attempts to form chromia containing melts in such furnaces. However, it was discovered that when melts containing approximately 0.5% to 10% or more by weight , of chromia were used in electric furnaces, the refractory electrodes and orifices rapidly deteriorated and wore away.
In some instances electrodes and orifices which would have served for a number of months with other types of oxide melts deteriorated in the presence of chromia so rapidly that the furnaces had to be shut down in a matter of a few hours or days. Prior to the invention herein, all such , attempts to melt chormia containing oxides in electric furnaces with refractory metal electrodes and orifices have resulted in rapid deterioration of the electrodes and orifices and have been considered totally unsuccessful.
It is therefore an object of this invention to provide a method for permitting the melting of chromia containing oxide mixtures in electric furnaces in the presence of refractory metal electrodes and orifices (such as molybdenum) while still providing a satisfactory service life of the orifice and electrode components.
Brief Description of the Invention The invention herein is based upon the surprisins discovery that, unlike other oxides normally found in refractory oxide melts, chromia (Cr203, with a chromium valence of 3) oxidizes in the presence of the air which is - normally in contact with the surface of the oxide melt to form chromium dioxide (CrO2, with a chromium valence of 4) and chromium trioxide (CrG3, with a chromium valence of 6).
The chromium dioxide and trioxide compounds subsequently 1(J'31(1~
come into cont~t ~i th th~ re~ract~-ry metal electrodes and orifices where they are reduced by the refractory metal back to chromla causing oxidation of the refractory metal. The refractory metal oxides are then fluxed into the oxide melt resulting in deterioration and wear of the refractory metal furnace components.
This discovery of the oxidation/reduction mechanism of the chromium oxides and the resulting oxidation of the refractory metal orifice and electrodes in the presence of the chromia containing melts thus leads directly to the method of the present invention for preventing such deterioration, which method comprises supplying to the furnace, in addition to the oxide mixture, an additive material which is in less than its maximum valence or oxidation state and which will react with the higher valence chromium oxides to form one or more oxides which have lower free energies of formation per mole of oxygen than the oxides of the refractory metal such that the reduced compounds will not be reduced by refractory metal (e.g., molybednum) electrodes or orifices. The oxides of the additive 80 formed must also have a lower free energy of formation than the higher chromium oxides, such that the additive material will also act to reduce any chromium dioxide or trioxide which is formed before it can attac~ the molybdenum components. The additive material may be in the elemental state or may be an oxide wherein the non-oxygen element in the oxide is in less than its maximum valence state. In one specific embodiment of the invention the additive is carbonaceous, and may be in the form of powdered coal~
In one particular aspect the present invention provides a process for the reduction or prevention of chromia-induced oxidative deterioration of components made of refractory metal jl/- -5-selected from the group cons~sting of molybdenum, tungsten, rhenium, tantalum, osmium and iridium and alloys thereof in an electric furn~ce in which a chromia-containing oxide mixture is melted, which process comprises:
supplying to the furnace in addition to the oxide mixture an additive material which is in less than its maximum valence state and which can be oxidized by chromium dioxide or chromium trioxide to an oxide which has a lower free energy of formation per mole of oxygen than the oxides of the refractory metal and chromia, such that in operation the additive material is oxidized in preference to the oxidation of the refractory metal, acts to reduce the higher chromium oxides to chromia, and forms an oxide which is inert to the refractory metal, the reactive quantity of said additive being present as 25 to 50 percent by weight based on weight of chromia in the mixture.
In another particular aspect the present invention ;I provides a process for the reduction or prevention of chromia-induced oxidative deterioration of compone~ts made ~0 of refractory metal selected from the group consisting of molybdenum, tungsten and tantalum and alloys thereof in an . electric furnace in which a chromia-containing oxide mixture ls melted, which process comprises:
supplying to the furnace in addition to the oxide mixture an additive material which is in less than its maximum valence state and which can be oxidized by chromium dioxide or chromium trioxide to an oxide which has a lower free energy of formation per mole of oxygen than the oxides of the refractory metal and chromia, such that in operation the additive material is oxidized in preference to the oxidation of the refractory metal, acts to reduce the higher chromium oxides to chromia, and forms an oxide which is inert to the refractory metal, the reactive quantity of said additive being present as ~5 to 50 percent by weight based on weight of chromia in the mixture.

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Brief Desc~pt:ton o~ the Drawtn~s Figs. 1 and 2 are graph~ of variation wlth temperature of the free energy of formation per mole of ,, ~!

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1 oxysen of v~rious oxides of chromium, molybdenum, tungsten, silicon, aluminum and carbon. Fig. 1 indicates the typical operating temperature range for refractory fiber furnaces and Fis. 2 indicates the typical operating temperature range for fiber glass furnaces Detailed Description and Preferred Embodiments The invention herein has as its basis the discovery of the mechanism of deterioration of refractory metal (e.g., molybdenum) electrodes and the orifice in an electric ~urnace in which chromia containing oxides are melted; i.e., the discovery that chromia initiates a series of unexpected oxidation/reduction reactions ultimately leading to the 'formation of refractory metal oxides which flux into the melt. This discovery led to the process of the present invention, which is a method for eliminating or substan-' tially reducing the tendency for this series of reactions to occur. Therefore the process of t.his invention is best understood by first considering the series of oxidation/
reduction reactions which it has been discovered are occuring.
To illustrate this series of reactions, they will be discussed in terms of compositions of the type described in the aforesaid U.S. Patent No. 3,449,137 and with respect to an electric furnace with molybdenum components.
In this type of refractor~ fiber melt a typical composition is composed of 40 to 60 weight percent silica, 35 to 55 weight percent alumina and 1 to 8 weight percent chromiag the last being the sesquioxide or "chromic o.Yide,"
Cr203. As noted in that patent the prior art comprised aluminosilicate fibers formed of roughly equal mixtures of alumina and silica. These are still very widely used and are readily melted in electric furnaces with no significant problems Gf undue deterioration af the molybdenum e1ectrodes lV'~

1 or orifices. It is only when chromia is added as a component that the difficulties arise. It has now been discovered, as part of the discovery of the mechanism of the chromia induced deterioration, that the alumina and silica are essentially inert to the molybdenum metal because the si,icon and aluminum in these respective oxides are each in their highest valence state and thus cannot be oxidized to a higher state. The chromia, however, represents a critically different situation.
The problem with chromia can be best understood by reference to Figs. 1 and 2. These are plots of the variation with temperature of the free energy of formation per mole of oxygen of several of the oxides of interest herein. (The oxide curves are identical in each drawing; the drawings differ only in the indication of the operating ranges of the two groups of electric furnace operations.) If at a given temperature the free energy of formation of an oxide of a metal M is less (i.e. more negative or lower on the graph) than the oxide of a metal X, contact of metallic M with the X metal oxide will cause the M metal to be oxidized and the X metal oxide to be reduced. Thus, in FIGS. 1 and 2 the free energy of formation of either of the molybdenum oxides is less (or lower) than the free energy of formation of chromium trioxide. Consequently the contact of metallic molybdenum with chromium trioxide will result in oxidation of molybdenum and reduction of the chromium trioxide to chromia.
Conversly, if the free energy of formation of the oxide of metal M is greater (i.e. less negative or higher on the graph) than the free energy of formation of an oxide of metal X, the X metal oxide will be inert to the metal M and will not cause oxidation of the latter. Thus, in FIGS. 1 and 2 the free ener~ies of formation of the molybdenum 1 oxides are greater (or higher) than the free enersy of formation of chromia. Therefore, chromia in contact with the molybdenum metal will not oxidize the latter.
The graphs of FIGS. 1 and 2 emphasize the sur-prisins nature of the discovery of this invention. It has been known from such published graphs that alumina and silica have curves lying below the molybdenum oxide curves, thus indicating their inertness to metallic molybdenum. As noted above, these oxides have been found by experience indeed to be inert to the molybdenum electrodes and orifices of furnaces. Similarly, the same published graphs show chromia also to lie below the molybdenum oxide curves, thus clearly indicating that it too should be inert to the molybdenum. The addition of chromia to a melt, however, was as noted found to lead to rapid destruction of the molybdenum components. It was the surprising discovery of this invention that the chromia was being oxidized by contact with air to a higher valence form such as chromium trioxide, whose curve lies above the curves for the molybdenum oxides, that led to the process of this invention.
The process of this invention therefore comprises incorporating into a refractory oxide melt containing chromia a component (or "additive") which will minimize the opportunity for the chromia to be oxidized to chromium dioxide or trioxide and will act to reduce any such higher chromium oxides which are formed back to chromia before the higher oxides can come into contact with the re~ractory metal furnace electrodes and/or orifice.
For simplicity herein, the invention has been discussed primarily with respect to molybdenum metal com-ponents. However, the invention is not limited solely to furnaces utilizing molybdenum components. Other materials, including tungsten an~ tancalum metdls and tungsten and molybdenum alloys are used or have been suggested for use in electric furnaces, and are thus "refractory metals" as defined for the purposes of this invention, for they will all be susceptible to chromia-induced oxidative deterioration. Other like materials and alloys thereof, including rhenium, osmium and iridium, will be readily apparent to those skilled in the art, and are to be included within the scope of this invention.
A number of materials can be used as the additive component. These can be readily determined from published tables or graphs similar to those of Figs. 1 and 2. A typical set of such tables and graphs will be found in Elliott et al, Thermochemistry For Steelmaking, Volume I (1960), especially pages 214-215. From these data it can be readily determined that materials such as calcium, aluminum, titanium, zirconium, magnesium, cerium, vanadium, silicon, and even chromium metal itself, may be used. Other materials such as manganese, iron, niobium, and sodium may also be used with lower temperature oxide melts. The additive material may also be an oxide in which the non-oxygen element is in less than its maximum oxidation state. Various mixtures and/or alloys of these materials are also suitable.
In this invention, however, it is preferred to use carbon or a carbonaceous material as the additive. One reason for this preference is that the metallic elements mentioned above form oxides which themselves become incorporated into the oxide melt and thus into the fibers thereafter formed. These oxides can serve as fluxes and can also adversely affect the temperature limitations of the fibers when used as thermal insulations. In many cases, particularly with the "glass fibers", such additions are not critical and the effects on the temperature limitations are of no consequence jl/- -9-1 since the the fibrous i~sulations are used at relatively 1QW
service temperatures. However, for the higher temperature refractory fibers such as those descr,bed in the aforesaid U.S. Patent No. 3,449,137, maximum service temperatures of the insulation fibers are very important and thP adverse ! effects of significant amount~ of other metallic oxides in the fiber composition often cannot be tolerated. Unl-ke the metals, however, carbon forms gaseous oxides which are not incorporated to any significant degree into the oxide melt which flows to the fiberizinc unit. Conse~uently the melt for fiberizing is not contaminated or diluted by oxides other than thGse which are intended to be incorpo.ated.
Another reason .or the preference for carbon or carbonaceous materials is economic. Normally such materials are cheaper and more readily available than many of the metals listed above.
A third reason for the preference will be seen from an examination of FIGS. 1 and 2. Unlike the metall;c oxides whose free energies of formation increase with increasing temperature, the free energy of formation of carbon monoxide decreases rapidly with increasing tempera-ture. Consequently the higher the temperature tl)e greater the tendency for the carbon to be oxidized in preference tG
the chromia.
The amount of additive required to substantially eliminate the oxidative erosion of the refractory metal electrcdes and orifices in electric furnaces ,s depend~nt upon the amount of chromia present in the oxide melt.
Normal amounts are from 25 to 50 weight percent, preferably 30 to 40 weight percent, of additive based on weight of chromia present in the oxide ~ixture. If desired there may be some excess of additive to ensure suppression of the -~0-lV'~

1 molybdenum deterioration. However, the tolerable amount of excess additive will depend on the effect of the excess additive material on other oxides in the oxide melt and to the extent that the excess remains in the melt. As dis-cussed above~ ir some instances large amounts of additive materials can be tolerated in the fin~l fiber, while in other instances they cannot.
When the additive is powdered or granular, as is usual, it preferably should be in a relatively finely divided state rather than being coarse particles. The exact dimensions of "fine" as compared to "coarse" particles will vary with the different types of additives, but can be readily determined by those skilled in the art. For the preferred carbonaceous additive, powdered coal, typical dimensions are, for "fine coal", 14% greater than 3p mesh (Tyler Seive Series) and 86% less than 30 mesh, and for "coarse coal", 63% great.er than 30 mesh and 37% less than 30 mesh.
The additive component may be added to the oxide melt in the same manner that the oxide raw materials are added to the furnace. In one commonly used method the oxides are granulated to fine powders, mixed, and contin-uously spread in powdered form over the surface of the melt.
The additive material, such as a carbonaceous material, may be similarly granulated and mixed with the oxide powders prior to spreading over the surface of the melt. Fresh additive material must be continued to be added to the oxide melt along with fresh oxide raw materials, for the addjtive material is continually depleted by its oxidation.
The process of this invention was tested in a refractory fiber electric furnace having three molybdenum electrodes and a molybdenum orifice. The oxide melt used 1 contained approximately 55 weight percent silica, 40 weight percent alumina and 4 weight percent chromia, with less than about one half weight percent of other oxides. This material was melted at a temperature of 3600F (1980C). In previous e~periments using the same typ~ of furnace and essent;ally the same composition, electrode and orifice deterioration had been so rapid that s~tisfactory fiber formation runs lasted no more than about forty hours. Since only a small , quantity of fiber could be produced in that short time, and ' 10 since the entire electrode and orifice system therafter required complete rebuilding, such runs were considered uneconomical and totally unsatisfactory.
Thereafter, in order to test the process of this invent;on, carbon ;n the form of powdereu bituminous coal ;n an amount of 36% by weight based on the weight of the chromia was added as a part of the oxide continuously over the length of the run. This run lasted for 21 days and was terminated only when the des;red amount of f;ber had been formed. Thereafter the electrodes and orifice were in-spected and were found to have suffered substantially less deterioration from the chromia than the electrodes in the control run described above.
The ;nvention has been here described with reference to deterioration of refractory metal electrodes and/or orifices in electric furnaces. It will of course be under-stood that the invention also applies to any refractory metal component of the furnace which may come into contact with the chromia containing oxide melt and which is suscep-tible to chromia-induced oxidative deterioration.

Claims (18)

WHAT IS CLAIMED IS:

1. A process for the reduction or prevention of chromia-induced oxidative deterioration of components made of refractory metal selected from the group consisting of molybdenum, tungsten, rhenium, tantalum, osmium and iridium and alloys thereof in an electric furnace in which a chromia-containing oxide mixture is melted, which process comprises:
supplying to the furnace in addition to the oxide mixture an additive material which is in less than its maximum valence state and which can be oxidized by chromium dioxide or chromium trioxide to an oxide which has a lower free energy of formation per mole of oxygen than the oxides of the refractory metal and chromia, such that in operation the additive material is oxi-dized in preference to the oxidation of the refractory metal, acts to reduce the higher chromium oxides to chromia, and forms an oxide which is inert to the refractory metal, the reactive quantity of said additive being present as 25 to 50 percent by weight based on weight of chromia in the mixture.

2. A process as in Claim 1 wherein the reactive quantity of said additive is present as 30 to 40 percent by weight based on weight of chromia in the mixture.

3. A process as in Claim 1 wherein said additive is in the elemental state or is an oxide wherein the non-oxygen element is in less than its maximum valence state.

4. A process as in Claim 3 wherein said additive is carbon in the form of carbonaceous material containing carbon in the elemental state.

5. A process as in Claim 4 wherein said carbo-naceous material is coal.

6. A process as in Claim 5 wherein the reactive quantity of said coal is present as from 30 to 40 percent by weight of the chromia in the mixture.

7. A process as in Claim 3 wherein said additive is elemental calcium, aluminum, titanium, zirconium, magnesium, cerium, silicon, vanadium, chromium, manganese, iron, niobium, or sodium, or combinations thereof.

8. A process for the reduction or prevention of chromia-induced oxidative deterioration of components made of refractory metal selected from the group consisting of molybdenum, tungsten and tantalum and alloys thereof in an electric furnace in which a chromia-containing oxide mixture is melted, which process comprises:
supplying to the furnace in addition to the oxide mixture an additive material which is in less than its maximum valence state and which can be oxidized by chromium dioxide or chromium trioxide to an oxide which has a lower free energy of formation per mole of oxygen than the oxides of the refractory metal and chromia, such that in operation the additive material is oxidized in preference to the oxidation of the refractory metal, acts to reduce the highter chromium oxides to chromia, and forms an oxide which is inert to the refractory metal, the reactive quantity of said additive being present as 25 to 50 percent by weight based on weight of chromia in the mixture.

9. A process as in Claim 8 wherein the reaction quantity of said additive is present as 30 to 40 percent by weight based on weight of chromia in the mixture.

10. A process as in Claim 8 wherein said additive is in the elemental state or is an oxide wherein the nonoxygen element is in less than its maximum valence state.

11. A process as in Claim 10 wherein said additive is carbon in the form of carbonaceous material containing carbon in the elemental state.

12. A process as in Claim 11 wherein said carbo-naceous material is coal.

13. A process as in Claim 12 wherein said coal is present as from 30 to 40 percent by weight of the chromia in the mixture.

14. A process as in Claim 10 wherein said additive is elemental calcium, aluminum, titanium, zirconium, magnesium, cerium, silicon, vanadium, chromium, manganese, iron, niobium, or sodium, or combinations thereof.

15. A process as in Claim 8 wherein said refractory metal is molybdenum.

16. A process as in Claim 8 wherein said refractory metal is tungsten.

17. A process as in Claim 11 wherein said refractory metal is molybdenum.

18. A process as in Claim 12 wherein said refractory metal is molybdenum.