US1352387A - saunders - Google Patents

Description

L. E. SAUNDERS.

METHOD OF CONVERTING REFRACTORY OXID. APPLIQATION FILED APR. 15. 1919.

1, 352,887. PatentedSept. 7, 1920.

2 SYHEETSSHEET l.

I /gz L. E. SAUNDERS.

METHOD OF CONVERTING REFRACTORY OXID. APPLICATION FILED APR. 15. 1919.

1,352,387. PawntedSept. 7, 1920.

2 SHEETS-SHEET 2.

amen to; I

55*; QKVMQWMQ Q W UNITED, STATES PATENT OFFICE.

LEWIS SAUNDERS, OE NIAGARA FALLS, NEW YORK, ASSIGNOR TO NORTON COM- IPANY, OE WORCESTER, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS.

METHOD OF CONVERTING REFRACTORY OXID.

of Converting Refractory Oxids, of which the following is a' specification.

This invention is an electric furnace method whereby various so-called refractory oxids' or mixtures thereof may be subjected I their fusing points.

to any desired temperature up to or above The method is applicable to the conversion (including shrinking, agglomeration, sintering, or fusion) of loose charges of oxids or oxid mixtures, and

may be applied at will and with high electrical and thermic efficiency, to very' large charges, amounting if desired to several tons. The method is applicable to all such oxids or oxid mixtures as are stable at or somewhat below their melting points and acquire at or below the conversion temperature suflicient electrical conductivity to carry the current necessary for the conversion of the surrounding portions of the charge. For example, the method is applicable to the conversion of such oxids as those of magnesium, aluminum, (including bauxite), zirconium, titanium, chromium, (including chromite), certain iron oxids and oxid mixtures, and the like.

' The principle involved in the novel operation is as follows: A conductive path of relatively very restricted cross section is established within the body of an oxid charge,

7 preferably above the median horizontal line of the charge. For example, a thin carbon rod, extending between heavy carbon terminals, may be completely embedded in the upper portion of the oxid or oxid mixture to be converted. Current is then caused to traverse the rod in sufiicient volume to agglomerate or fuse the immediately adjacent portion of the charge, and to raise the same to the temperature at which it acquires enough conductivity to carry the electrical load. The carbon resistor is essentially fugi tive, and before its disruption, or at about the moment of its disruption the impressed E. M. F. isincreased sufliciently to maintain,

through that portion of the charge which has become conductive in the initial phase of theoperation, current adequate to maintain the "required temperature in the initially- Specification of Letters Patent.

takes place is 2600 C.

Patented Sept. 7, 1920.

Application filed A ruis, 1919. Serial No. 290,223.

produced core, and progressively to convert the adjacent portions of the charge. Under these conditions the conductive cross sectlonal area progressively increases, and the voltage may be gradually reduced, the electrical conditions being preferably so controlled as to maintain a substantially con stant load on the furnace until the close of the heating stage of the operation. In cases where full fusion is reached, as for example in the melting of alumina and other fusible oxids, there is a marked tendency to lateral spreading of the melt, and it is necessary to provide some adequate means for limiting this, as for example by the provision of water-cooled side walls.

At the close of the heating stage the current may be cut off and the converted product permitted to cool at the normal rate; or

in cases where slow cooling, or even re-heatmg are deemed desirable, the load may be gradually reduced or varied, being held for any required time at any desired temperature. In this way a special heat treatment of oxids becomes possible, analogous in some respects to the heat treatment of steels.

A typical illustrative example of the practice of the invention is the shrinking of magnesia, preferably at the temperature at which sintering, agglomeration, or even fusion occurs. he approximate temperature at which the sinterin of pure magnesia ommercial grades of magnesia sinter at a somewhat lower temperature, depending upon the nature and proportion of the impurities present.

The method may be carried into effect in a furnace of the character illustrated in the accompanying drawings, wherein- Figure 1 is a horizontal section of the furnace on line I-I of Fig. 2;

Fi 2 is a central longitudinal section on line 1-41 of Fig. 1;

Fig. 3 is a resistance curve illustrating the sintering of magnesia under specific conditions as described below; and

Fig. 4 is a vertical central sectional view showing one end of a modified furnace construction.

The furnace illustrated inFigs. 1 and 2 has side walls 1, 1, end walls 2, 2, and a base 3, all of which may be of fire-brick or other suitable refractory material, basic in character. Magnesia brick may be used. The terminal electrodes 4, 4 are of preferably graphite or other form of carbon and may extend from side to side of the furnace, as

shown.- Below the electrodes 4, '4, the end may if desired consist oftwo or more sections which are initially in-electrical contact with each other; or in general'l may employ any arrangementwhich will afford a continuous but restricted pathfor the current between the terminals 4, .l. .1

The method as carried out in a simple fun nace of this kind is as followsz-The magnesia, preferably calcined, is charged into the furnace completely filling "the same and being packed over and around rod 0 and in sleeve midway between the electrodes. The

interior length ofthe furnace was 94: inches,

its width 30 inches, and its depth 371}; inches.

The total bharge, including additions dur ing the operation, consisted of about 3,500

volts, and was increased by short stages tor a 2,330 amperes at 109 volts at the endof fourminutes.

creased over a period of about lfiininutes,

The resistance then, rapidly induring which the voltage was progressively increased so as approximately to maintain 190 and the amperes 1,460. The resistance then gradually declined, subject-to fluctuations, the conditions ultimately becoming approximately constant at 131 volts and 2,000 to 2,100 amperes.

lit will be observed that this particular curve indicates three well-defined stages of the operation. it maybe assumed'that the rapid fall in resistance from A to B correclose contact with the terminal electrodes a, sponds to the increase of conductivityof the t. A heavy current is then applied, which in the first instance, traverses the carbon rod 0. -At the beginning of the operation, the

amperage and the voltage of the current are so adjusted with reference to the cross sec. tion of the rod 0, that the portion ofthe charge immediately surrounding the ro d is caused to agglomerate and to become elec-' trically conductive. At the high temperatures thus developed in the rod, the carbon disappears completely in the course of a very few minutes, being presumably oXidized by the magnesia, the reduced magnesium being reoxidized to magnesia in the course of its passage through the charge. It

is essential to the success of the' operation I that in the very short interval of time before the rod is destroyed a sufilci'ent cross-section of the surrounding charge should be brought to a sufliciently conductive state, so

that the magnesia thus converted may, usu-v ally under a somewhat higher impressed fvoltage, assume the role of a resistance body,

as the carbon rod fails. its a rule, it is necessary to increase the impressed voltage at a rapid rate as the carbon rod disappears in order properly to maintain the load.

1 The resistance curve, Fig. 3, will serve to illustratethe changes occurring in a furnace durin thefcourse of a particular run,

it being understood, however, that such fluctuations as are here-indicated are not neces-' sarily characteristic of the practice of the method. lln the curve, the'solid line represents the actual fluctuations of the" resistance, and the dotted line represents a simplified expression of the changes in resistance.

' lnthe run referred to, two parallel carbon It is of course to carbon rod under the rapidly rising temperature: The following rise in resistance from B to C probably corresponds to the destruction of the rod, by oxidation: From the point C the current was carried chiefly or entirely by the magnesia, the gradual decline in resistance corresponding to the increase in cross-section of the magnesia core, by conversion of the surrounding charge, subject to fluctuations which in part at least aredue to temperature changes, and in part to the presence of molten magnesia.

, At theconclusion of the run, after cooling the furnace, the magnesia was found in the form of a sintered mass or pig weighing about 1,900 pounds and bedded in the unconverted portions ofv the charge. The latter portions being unsintered, were easily brushed 0d, and were immediately available, with the necessary additions, for a.

repetition of theoperation. The pigeonsisted of fullyv shrunk material having a the load, the voltage after 20'minutes being,

carbon rod remained.

During the course fthe fchargii decreases gradually in volnnie,-ai1d is'preferably maintained by adding -further porbiQnS Of 3:3 requi d i" tions under' rdin'ary conditions ma am approximatel t the weight of charge originally taken-.

a specificoperation, as are abovei givenifor the Purpose of a clear and full-explanation 125 of thefnature'of. the invention." The spa eific operating conditions will of course vary y understoodthat invention is not restricted to such details-of not only according to the particular oxid. I or oxid mixture tov be convertedfland the:

temperature to which it is to' be heated, but

also according to the scale of operations.

ductive portions of the charge: As already explained this requires a suflicient increase in the applied electromotive force to effect the transfer before the complete failure of the connection initially established through the starting rod.

It will be observed by referring to Fig. 2 that the stationary rod is located above the horizontal axis or median line of the charge considered as a whole. This is highly desirable on account of the tendency of the conductive portions of the charge to subside;

and the arrangement is such that any subsidence of the conductive portions of the mass will tend to bring such portions into' the direct current terminals 4, 5.

A modified arrangement having the same general purpose in view is illustrated in Fig. 4. In this-modification, 6 represents the fire-brick base of the furnace, and 7 a bottom lining of magnesia. The sides may be water-cooled metal walls. 8 is a carbon end wall faced with carbon blocks 9, and carrying the main terminal electrode 10, which may be of graphite, and is preferably capable of limited horizontal movement, sliding freely in the aperture of the 'end wall. 11 represents the graphite leading-in rod, to whichthe bus-bars 12 may be secured by means of a copper band 13. Loosely mounted upon the inner or working end of the electrode 10 is an auxiliary elecpath between the main trode 14, which may also be of graphite and is electrically connected to the band 13 by means of a flexible metal cable 15. The starting rod 16 is attached to or contacts with the auxiliary electrode 14 and extends to a like terminal construction at the opposite end of the furnace structure.

The operation of this furnace is essen tially as described above. The length of the furnace will of course depend upon the conductivity of the oxid or oxid mixture to be converted, the furnace being somewhat shortened for the conversion of oxids of relatively low conductivity at the conversion temperature. The loose oxid or homogeneous mixture is bedded to and above the starting rod, which is kept covered throughout the initial operating stage, and until its disruption or disappearance. As soon as the starting rod has ceased to function the The essential auxiliary electrodes may be swung out of position or removed, and the load will be assumed by the conductive portions of the charge, now forming a continuous circuit between the principal terminals, which'may be advanced if necessary to' establish or maintain sufficient electrical contact with the conductive core.

This operation may as stated above be applied to a wide variety of'oxids and oxid mixtures, including so-called composite oxids (chromite, etc.), as well as natural oxid mixtures such as calcined bauxite.

The method presents many advantages as compared with the commonly practised method of fusing such oxids in furnaces of the arc type, having a strongly localized heating zone. Among such advantages may be mentioned the following:

1. The operation is electrically far more efficient, its power requirement being only about that of an ordinary arc fusion of the same oxid.

2. The great bulk of the charge is kept free from contact with or proximity to carbon surfaces, and is hence practically free from objectionable reduction products.

3. The conversion may be carried out under strictly neutral conditions, or under oxidizing or reducing conditions as may be desired; whereas in the arc fusion as commonly practised reducing conditions are un-. avoidable. This renders it possible in the present operation to incorporate with the charge, consisting for example of alumina, small proportions of reducible oxids, such as oxids of chromium, cobalt, manganese or the like, whether for obtaining special color effects in the product, or for modifying the character of the grain for abrasive purposes, or for other special purposes.

. 4; It is possible to hold the charge for an indefinite period at the maximum or any other ,desired temperature; and to control in an absolute manner the rate of cooling. In this way the size and other characteristics of the crystals may be controlled at will, which is often" a matter of the utmost importance, particularly in the art of preparing aluminous abrasives.

5. The method is readily and economically applicable both to small and large scale operations, and the operation in either case proceeds smoothly, quietly, and with practical freedom from dust or loss by vaporization.

The present applicatiomis a continuation in part of my prior application Serial No. 183,367, filed July 28, 1917.

I claim:

1. An electrical method of converting refractory oxids or oxid mixtures, comprising locally heating the oxid charge under conditions to establish a current path therethrough, and progressively converting fura lame line, and progressively fusingfurther portions of said charge by means of an electric current traversing the conductive portions thereof. I i

'ther 'portionsof saiol charge by means of an electric current traversing the conductive fportions thereof. c

I 2 Anelectrical method of fusing-alumina rent path ithelrethrough above media a; An ectrical method of fusing alumina, I -i 'anol similar gelractory .oxids or oxid .i'iiixkf ,tures, comprising locally heating an qoxiol charge under conditions to establish aptitvelopedfb'y an electric current traversing the conductive portions thereof.

In testimony whereof I ailix my signature.

" LEWIS a. satinnnns.

5 and Similar refractory oxids. or .oxid 1nu'ix+ 51, An -electrical method of convertiiig re- 30 tures, comprising locally heatinganfoxid fractoryoxidsor oxidvmixtures, which concharge; under conditions to .establishjja Ecursi'stsinestablishing a-current path of elecrent path therethrough,'and progres"s iv elyftrica-lly-c'onductive. oxid "in the body of a. gfu'sing furtherportions of said'chargehy substantially non-conductive chargeof-the w means of an electric current traversing the same *oxid,f and progressively converting s5 conductive'portions thereof; f I {said chargeby nieansof heat developed by 3. 'An electrical method of converting r'ean electric currenttrave'rsing the conductive v fractory oxids or oxid mixtures, comp'ri's- .portionsllthereofijj ing' locally heating; the. oxiol chargefuncler .6, electrical method of fusing alumina '15 conditions to establish a current path there- ,1;iandsimilar;refractory oxids and oxid mix- 40 through above the median" linefl'of the "til-res;fwhichfconsists in establishing a our charge, andffpro'gressively converting fur .rentpath"of"electrically-conductive alumina ther portions'ot said charge by means of an in, the hO JYQOf substantially non-conducelectric current traversing the conductive tive: cliargefot alumina, and progressively 226D portions hereof. fusing said charge by means of heat de- 45

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