US2508004A - Electric salt bath furnace - Google Patents

Description

wn ADAM, JR 2,508,004

ELECTRIC SALT BATH FURNACE:

7 Shee's-Sheet 1 l l l May 16, 1950 Filed March 13, 1948 IN2-ron ATTORNEYS l I l l E l ai I Ll May 16, 1950 w. ADAM, .JR 2,508,004

ELECTRIC SALT BATH FURNACE AFiled March 15, 1948 vi sheets-sheet 2 INVENTOR ATTORNEY May 16, 1950 w. ADAM, JR

ELECTRIC SALT BATH FURNACE 7 Sheets-Sheet 5 Filed March 13. 1948 INVENTOR ATTORNEYS May 1'6, 1950 w. ADAM, JR 2,503,004

ELEc'rRic SALT BATH FURNACE Filed March 15, 1948 '7 Sheets-Sheet 4 INVENTOR May 16, 1950 w. ADAM, JR 2,508,004

ELECTRIC SALT BATH FURNACE Filed Maron 1s, 194e 7 sheets-sheet 5 INVENTOR ATTORNEYS 7 Sheets-Sheet 6 W. ADAM, JR

ELECTRIC SALT BATH F'URNACE May 16, 1950 Filed Maron 13, 194e y f6. JZJ

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May 16 1950 w. ADAM, JR 2,508,004

ELECTRIC SALT BATH FURNACE INVENTOR Patented May 16, 1950 2,508,004 ELECTRIC SALT BATH FUBNACE William Adam, Jr., Camden, N. J., assigner to Ajax Electric Company, Inc., Philadelphia, Pa., a corporation o! Pennsylvania Application March 13, 1948, Serial No. 14,733

(Cl. 13-23i 4 Claims.

The present invention relates to submerged electrode salt bath furnaces.

A purpose of the invention is to facilitate the construction and operation of submerged electrode salt bath furnaces, particularly very large or very deep furnaces.

A further purpose is to take advantage ci the inherent shrinkage of the salt to permit the top of the electrodes to be covered by the salt bath during normal operation but to expose a closespaced portion of the electrodes when the salt bath cools down for the purpose of starting.

A further purpose is to minimize the necessity of chiseling out salt from a salt bath furnace when the bath solidies either because of the intention of the user or through an unexpected power failure.

A further purpose is to permit arrangement of electrodes in cascade whereby one electrode set can deliver molten salt which will be used to start another electrode set.

A further purpose is to arrange sets of electrodes at different vertical levels and in position so that upper electrode sets can start lower electrode sets which are below and adjacent thereto, preferably not more distant than six inches to two feet for most installations, depending upon the conditions.

A further purpose is to increase the life of electrodes and reduce the loss incident to electrode replacement.

A further purpose is to secure adequate stirring and proper uniformity of temperature in very large and very deep salt baths.

A further purpose is to simplify and improve salt bath electrode construction.

A further purpose is to further reduce the likelihood of having current flow through the work.

A further purpose is to assure adequate wide spacingl of electrodes outside the bath without necessitating expensive electrode designs.

A further purpose is to permit electrodes to be carried out through the furnace wall when a metallic pot is employed.

Furtherl 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, choosing the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear dem onstration of the principles involved.

Figure 1 is a fragmentary top plan view of a large salt bath furnace to which my invention has been applied.

Figure 2 is a longitudinal vertical section of Figure l on the line 2 2.

Figure 3 is a transverse vertical section of Figure 1 on the line 3 3.

Figure 4 is a view corresponding to Figure 2 showing a modification.

Figure 5 is a sectional perspective showing an electrode passing through the furnace wall in chlorides and the like.

2 the form of Figures 1 to 3 inclusive, or Figure 4.

Figures 6 and 'I are enlarged vertical transverse sections intended to assist in explaining the exposure of the electrodes by shrinkage of the sait.

Figures 8, 9 and l0 are fragmentary diagrammatic horizontal sections showing different electrode positions with respect to a refractory furnace wall.

Figure il is a fragmentary transverse section corresponding to Figure 3 showing a window near the top of a metallic pot.

Figure l2 is a fragmentary interior elevation of the window and pot of Figure 11.

Figure i3 is a view corresponding to Figure 1i showing a window located at a point below the top of the bath.

Figure 14 is a fragmentary interior elevation of the window of Figure 13.

Figure 15 is a fragmentary vertical section of a salt bath furnace having an echelon arrangement of electrodes.

Figure i6 is a vertical section of a metal melt ing furnace.

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

In the prior art the problem of starting submerged electrode salt bath furnaces which have cooled below operating temperature, usually to the point of solidication, has in some cases presented serious diiculties. The various salt bath salts are fairly good conductors when molten, but are good insulators when solid. 'In prior art designs where electrodes are carried straight out of the bath through the top, the practice has been to fuse a small amount of salt at the top between the electrodes in order to establish a current path for starting purposes. Fusing is sometimes accomplished by fuel heat applied from a torch or the like and sometimes by creating an electrical resistance path, as by applying powdered carbon between the electrodes.

The furnace designs in which the electrodes are carried out through the top of the bath present the difficulty that the metallic electrodes are exposed to the air at the surface of the bath, and therefore are subject to serious deterioration, especially from attack by oxygen, oxides, oxy- This has frequently resulted in replacement of electrodes which have failed entirely at a point corresponding to the surface of the bath, but which are otherwise suitable for use.

Electrode designs have been produced in which the electrodes are carried out of the bath fat the side below the top, and these have the advantage of protecting against electrode corrosion, erosion or other attack at the top of the bath. In such designs, however, it has been necessary to keep the power on at all times, as solidification of the bath due to power failure or the like creates almost an insuperable problem in restarting the bath under such conditions. It is then neces- 3 sary to attempt to chip or drill salt out of the bath to considerable depths in order to restart operation.

Furthermore, in prior art attempts to protect against electrode attack at the surface, it has been necessary to employ refractory pots or furnace walls exclusively, as metallic pots could not be employed without short circuiting the device.

By the present invention it is possible to greatly facilitate the starting not only at the top of the bath, but also in very large or deep furnaces whereit is necessary to carry the furnace depth greater than that of a single electrode set.v

Considering first the form of Figures 1 to 3 inclusive, a salt bath furnace 20 is illustrated having an outer structural support, not shown, a heat insulating layer 2l and a refractory furnace wall or pot 22 inside the heat insulating layer (in other embodiments of the inventionthe wall or pot may be of metal). The refractory pot 22 has side or end walls 23 and a bottom 24. The top 25 is shown as being open, although it will be evident of course that any suitable cover not illustrated may be employed.

The furnace has an interior salt bath space 25, the center portion 2l of which is entirely free from electrodes and remote from the electric heating current path, so that work in the space 21 is remote from the current and not likely to be subject to any non-uniformity of heating on that account. Electric heating and electrodynamic circulation are accomplished by electrodes 28 which are preferably adjacent tov a furnace side wall 30.

While for many aspects of the present invention the electrodes may vary widely in structure, the preferred embodiment employs L-shaped electrodes having lateral support and terminal portions 3|, comprising preferably an opposite branch of the L, and vertical heating and stirring portions 32. The branches of the L meet at the bend 33, at which point one branch preferably overlaps the other and is suitably joined thereto as by a weld 34. The axes of the two branches of the L are not in the same plane but displaced, and the electrodes are arranged in sets, suitably pairs, with the electrodes whose lateral or terminal portions are more widely spaced co operating. Thus where the vertical portions have close spacing substantially throughout the length at 35, the terminal portions of the corresponding electrodes are more widely spaced at 36. This is important as the temperature near the inside of the furnace wall is likely to be above the melting point of the salt, and intrusion of molten salt around the electrode terminals is likely to occur, so that the Wider spacing of the terminals is helpful to prevent short circuiting.

The electrode construction thus employs electrodes of opposite counterpart character, for example right-hand electrodes 31 and left-hand electrodes 38, arranged in pairs, with the vertical branch of the L on the side of the horizontal terminal toward the opposite electrode of the set in each case. The vertical branches 32 preferably form uniform close spaced portions 40, extending fully up to the top of the electrodes, but in any case it will be evident that a position of close spacing of the electrodes exists very close to the top, as shown. The electrodes may of course have other relative relationships, with or without the uniformity of close spacing which is here shown.

The cross section of the stock making up the electrodes is preferably rectangular to permit flat cooperating faces between the electrodes and flat faces against the furnace walls, and for most practical purposes the electrode cross section may be approximately square. Other cross sections may however be used.

The electrodes may of course be of any suitable alloy, preferably of a high temperature alloy such as iron-chromium alloy, iron-nickel alloy, ironchromium-nickel alloy, chromium-nickel alloy or the like.

The terminal branches 3| of the electrodes are conveniently carried through the refractory wall at 4l and through the adjoining heat insulation as shown in Figure 5. The outer ends of the electrodes may to advantage be water cooled as by providing water cooling passages 42 which receive water or other cooling medium through a pipe 43 extending into the inner end of the passage and discharging the water at 44. The cooling medium then flows around the pipe 4I and is withdrawn by a branch pipe 45, which may also provide electrical connection to the electrode.

The depending branch 32 of the electrode will preferably be in close contact with the furnace wall where a refractory pot is used. This has the advantage of increasing the safe available work space, and surprisingly I find that the electrodynamic stirring as described in Adam U. S. Patent 2,145,677 is not substantially influenced by backing the electrode up against the furnace wall or even by submerging the electrode in the furnace wall, providing there ls adequate electrode exposure. In the form of Figures 1 to 3, 5 and 8, the electrodes are shown with the backs of the depending portions engaging the furnace wall at 46, but not submerged in the furnace wall. As shown in Figure 9, the sides of the electrodes may be partially submerged in the furnace wall as shown at 4l in Figure 9, or fully submerged as shown at 48 in Figure 10, where only the outer faces 50 are exposed to the bath. The electrode arrangements of Figures 8, 9 and 10 are very effective indeed to keep the current out of the Work, where a refractory pot is used, that is, a furnace lining material such as magnesite, alumina, chromite, silica or the like, depending upon the salt bath.

In connecting the electrodes it will be noted that a number of cooperating sets are shown in the fragmentary showing of Figures 1 and 2. It will be evident of course that these individual sets may be connected in a single phase or polyphase circuit as well known in the art.

The carrying of the close spaced portion of the electrodes up to the top thereof is of great advantage in starting, because for the first time it is possible to have the advantage of full submergence of the electrodes during bath operation without any diiliculty due to corrosion or erosion from carrying the electrodes out through the top of the bath, but without the accompanying difilculty in starting.

This feature is accomplished in connection with the structure of the present invention by proper co-relation between the depth of the bath when y molten, the volume of the bath, and the contracfles the shrinkage will bring the top of the bath down to a level 52 below the tops of the electrode close spaced portions as shown in Figures 2, 3 and Thus when the bath is molten and the electrodes do not pass up through the top of the bath and the very hot portion of the electrodes in contact with the bath `is not also directly exposed to the air at an interface, since the portion of the bath 53 above the tops of the electrodes acts as a protection. On the other hand the bath when it cools automatically exposes the tops of the electrodes at a close spaced portion 54 (Figure 2) to permit fusing of a small current flow path through the salt in order to start the bath by applying an external source of heat at this point or by building aresistance channel at the top of the salt, for example through powdered graphite or the like applied across the electrodes at 54. It will thus be evident that the close spaced portions of the electrodes are within the shrinkage distance of the salt from the top of the bath. This protection feature greatly increases the life of the electrodes and avoids the cutting action which otherwise takes place at the top.

The invention is applicable to various salts, one example being barium chloride which might lbe used over a temperature range between 1850* and 2400u F. This material exhibits a shrinkage of about 23% between operating temperature and room temperature, most of which occurs at solidication.

The fact that the present invention involves conducting the terminal portions of the electrodes laterally through the furnace wall at apoint below the bath does not constitute a substantial disadvantage since the cooling of the electrodes tends to solidify liquid salt which would otherwise leak out, thus preventing any fire hazard to persons or damage to transformers which might otherwise result.

It will be evident that while the L-shaped arrangement with electrodes having flat faces directed toward one another and parallel to one another is very desirable, it is not essential and the invention may be used without employing these features.

In some cases it is desirable to apply the invention to furnaces having metallic pots. In this case of course the electrodes may be spaced well away from the pot so that they will not be short circuited, such spacing being shown at 55 in Figures l1 and 13, although of course this feature need not be employed if it is desired to conduct current in the wall of a metal pot. In order to carry the lateral branches of the L out below the level of the molten salt bath, windows 56 are provided in the metallic pot wall 51, at positions corresponding to individual electrodes, or preferably sets of electrodes, as shown in Figures l1 to 14.

Each window preferably takes the form of a receptacle or box, having a rim 58 around its inner edge, a bottom wall 60 and a rim 6i around its outer edge. Where the window is provided near the top of the pot as in Figures 1l and 12, the top wall 62 of the window may be limited to the flange of the pot, but where the window is employed at a point deep down in the pot (referring to Figures 13 and 14) later to be described, the top wall 62' will preferably extend as far out as the bottom wall. Adequate clearance is provided all around the electrodes for insertion of electrical insulating refractory 63 to support, insulate and seal the electrodes. The refractory may be magnesia, chromite, alumina or other well 8 known type, preferably set with a binder such as sodium silicate or other well known binder.

For a very deep bath construction there are serious electrical, mechanical and practical dimculties in the way of carrying a single set of electrodes to the bottom of the bath. The electrical resistance is likely to be so high that very little voltage is available and very little power obtainable at the lower portion, especially as the electrodes are usually made of high resistance alloys. From an electrical standpoint also the problem will be appreciated when it is understood that the current density on the cooperating faces of the electrodes may be of the order of amperes per square inch, so that in the case of a very deep bath the requirements for conductors to a given electrode and for electrode cross section to carry the current to the point of conduction into the bath would be very considerable. When it isv appreciated also that the electrode spacing is preferably of the order of 1/2 to 2 inches and that it is likely to approximate the cross sectional dimensions of a square electrode, it will be understood that any great increase in electrode cross section is likely to interfere with structural design. From the standpoint of support, a single electrode extending to the bottom of a very deep furnace presents a, serious problem, both with a refractory and with a metallic pot. It will further be appreciated that in case of replacement of an electrode due to localized failure, the expense would be considerable.

In accordance with the invention as illustrated in Figure 4 this problem is solved by arranging the electrodes in a plurality of cascades, each of which at a given level may include any desired number of sets oi electrodes arranged side by side, and preferably with one electrode set in an upper cascade immediately above an electrode set in the next lower cascade.

Thus Figure i shows four cascades 6I, 65, 58 and 6l one below another on the same side wall of the furnace, and each consisting of sets of L-shaped electrodes as previously described. In each cascade the electrodes are vertically aligned above the cascades below, and below the cascades above, so as to produce columns of vertically aligned electrode sets. For example there are four such columns shown, 68, 10, 1I and 12. Accordingly the spacing at the bottom between a pair of vertically aligned electrodes in the preferred form is immediately above the spacing at the top between the next lower set. The individual cascades are preferably placed close enough to the next lower cascade to permit effective starting of a cold bath by starting at the top cascade in the manner already described (preferably employing the feature of maintaining the bath when molten above the top of the top cascade but within the shrinkage distance so that it will expose close spaced portions of the top cascade when at solid size) and then as soon as salt in the top of the bath is melted taking advantage of the end effect of each set of electrodes as discovered by me to melt salt below such set suiciently to start the next lower set of electrodes. Thus the starting action will continue successively down from cascade to cascade.

Accordingly it will be seen that the cascade arrangement in Figure 4, whether it employs a single column or multicolumns, permits the constr uction of a very deep bath with only moderate sized inlet leads to the individual electrodes and moderate electrode cross sections, with a moderate supporting lead on each electrode and reasonable cost in replacing an individual electrode, but without loss in starting advantage since the cascadesstart one another as already explained. Even if the lower cascade is not lined up below, there is an advantage from the invention. For very good results the distance between the bottom of an upper set of electrodes and the close spaced top portion of the next lower set should not exceed two feet, and for best results not exceed six inches for most installations, although of course some advantage of the invention can be obtained without this feature or even by staggering the electrodes in a given column. Staggering is not preferred because it minimizes the advantage of end effect.

The end spacing will of course vary with the current flowing between the electrodes, the heat conductivity of the particular salt and the temperature potential or degrees of superheat under which the salt operates.

In Figure an echelon cascade arrangement of electrodes is shown, in which one upper set of electrodes 73 is located close enough to the top (but below the top) so as to be submerged by the molten salt bath but exposed by the solidified bath as earlier explained, while successive electrode sets 14, 15, 16, etc. beside the set 13 are progressively placed lower so as to overlap laterally, and stagger the locations at which end effect r must operate to start electrodes of the next lower cascade 11, 18, etc. This arrangement may be used on as many of the side walls of the furnace as desired.

It will be evident that the invention finds its widest application in heat treating salt bath furnaces employed for the heating, cooling. welding, brazing or other like operations on metals, but it will be understood that the invention can also be applied toA submerged electrode furnaces employing such media for other purposes or ernploying similar heating media. One example is for melting metals, especially aluminum and magnesium.

In some cases the principles of the invention may be applied in melting metals such as aluminum or magnesium in a submerged electrode salt bath furnace as shown in Figure 16, having a floating molten metal bath 80, on top of the salt bath (which is constructed as in Figure 3). A slag 8| may float on the metal. In case it is necessary to allow the salt bath to solidify, the molten metal bath may first be poured off, or the voltage can be lowered in starting until the electrodes are completely immersed and the metal no longer contacts the electrodes.

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 process and structure shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scopeof my claims.

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

1. In a subr `:rged electrode salt bath furnace, a salt bath, l. set of electrodes having a close spaced portici .ear the top of the bath, in combination with a furnace side wall and bottom, the electrodes extending into the side wall below the upper surface of the salt when the salt is fully molten, said salt bath having a shrinkage which is greater than the volume of the bath which is above the electrodes when the bath is fully molten which exposes the electrodes due to shrinkage for starting purposes when the bath has been allowed to cool down.

2. In a submerged electrode salt bath furnace, a set of electrodes extending vertically and having a close spaced portion near their top and then extending laterally, a furnace wall through which the laterally extending portion of the electrodes pass adapted to contain a salt bath, and a salt bath contained within the wall and between the electrodes having a level when molten which is high enough to submerge the portion of the electrodes inside the bath space completely and having a shrinkage which is greater than the volume of the bath above the electrodes when the bath is fully molten so that when the bath il cooled down it exposes the electrodes close spaced near the top.

3. In a submerged electrode salt bath furnace. a furnace .wall, a plurality of sets of submerged electrodes exposed inside the furnace wall, one set being above another set, and the top set extending laterally through the f-urnace wall, and a salt bath which when molten is above the top of the top set so as to completely submerge the portion thereof inside the furnace, and having a shrinkage which is greater than the volume of the bath above the top set when the bath is fully molten, whereby the top of the top set of electrodes is exposed for starting.

4. In a submerged electrode salt bath furnace. av furnace wall, a salt bath within the wall, a metal bath on top of the salt bath, and a set of submerged electrodes extending through the furnace wall and having a close spaced portion near the top of the bath, said salt bath extending over the top of the electrodes when the bath is molten and having a shrinkage which is greater than the volume of the bath which is above-the top of the electrodes when the bath is fully molten. so as to expose the electrodes due to shrinkage for starting purposes when the bath has been allowed to cool down.

WILLIAM ADAM, Jn.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 696,004 Burton Mar. 25, i902 771,249 Horry Oct. 4, 1904 771,250 Horry Oct. 4, 1904 896,429 Becket Aug. 18, 1908 1,069,255 Heroult Aug. 5, 1913 1,267,317 Erskine May 21, 1918 1,373,615 Jacobs Apr. 5, 1921 1,637,167 Weckerle July 26, 1927 2,234,476 Jessop Mar. 1i, 1941 2,336,412 Messinger Dec. 7, 1943 2,349,678 Rolnick May 23, 1944 2,415,494 Holden Feb. 11, 1947 2,421,224 Solakian et al May 27, 1947 FOREIGN PATENTS Number Country Date 28,536 Great Britain 1910 376,363 Germany May 28, 1923 411,278 Germany Mar. 21, 1925 586,254 Germany Oct. 19, 1933 629,708 Germany May 9, 1936 682,611 Germany Oct. 18, 1939 74,342 Sweden Sept. 16, 1926

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