US2421542A - Method and furnace apparatus for calcining carbonate material and for other purposes - Google Patents

Description

June 3, 1947.

GE. CONNOLLY METHOD AND FURNACE APPARATUS FOR CALCINING CARBONATE MATERIAL AND FOR OTHER PURPOSES Filed April 21. 1943 2 Sheets-Sheet 1 IN VE N TOR GEOEG'EE. CONNOLLK y. ATTORNEYS June 3, 1947. E. CONNOLLY 2.421.542 v uz'r AND I NA APPARATUS FOR cnmm on ATE MATERI AND FOR o-runn nrosss Filed April 21, 1943 2 Sheets-Sheet 2 nvmvrox GEORGEE CONNOLLL ATTORNEYS Patented June 3, 1 947 ME' I'HOD AND FURNACE APPARATUS FOR CALCINING CARBONATE MATERIAL AND FOR OTHER PURPOSES George E. Connolly, San Francisco, Calif., assignor to Nichols Engineering & Research Corporation, a. corporation of Delaware I Application April 21, 1943, Serial No. 483,920

9 Claims. (Cl. 263-26) This invention relates to furnace apparatus and methods adapted, among other uses, for calcining carbonate materials and for the simultaneous recovery of oxide products and gases having a high carbon dioxide content.

In accordance .with the present invention, materials such as limestone fines and magnesite fines may be calcined in a multiple hearth furnace of the well-known so-called Herreshoif class. While it has heretofore been proposed to calcine limestone and the like in furnaces of this class, it has generally been found difficult or impossible to do so economically while at the same time obtaining a carbon dioxide gas product of high concentration and free to a desired extent from gaseous products of fuel combustion. 'That is, in order to economically heatthe furnace to the necessary high temperatures for such a process, it has been considered neces'sary to burn fuel in the presence of the material being calcined, and this in turn inherently involves, dilution of the carbon dioxide product with gaseous products of fuel combustionand air, in excessive amounts difficult to control or regulate. It is possible to apply muffle heating by known methods to furnaces of this class, to thereby avoid burning the fuel in the presence of the material under treatment, and it is contemplated that various fea-' tures of the present invention may be utilized with muille heating of the furnace. However, mufile heating arrangements are generally relatively inefiicient andthe passages and cavities of the mutlle heaters are. diflicult to keep clean for proper heat transfer, and free from being blocked by dust and soot. Consideration has also been given to heating furnaces of this class electrically in cases where burning of fuel in the presence of the material being calcined is undesirable. But so far as is known, no method or apparatus has heretofore been available for electrically heating furnaces of this class in a dependable and economical way and with which the resistors will withstand the relatively high temperatures necessary for calcining carbonates, or comparable uses.

In accordance with the preferred form of the present invention, provision is made for dividing the multiple hearth furnace into three zones which are preferably separated against free passage of gases between adjacent zones, the first zone being adapted for drying and preheating, the second zone being heated by a novel arrangement of electrical resistance units so that the burning of fuel in this zone is avoided, and whereby the greater part of the carbon dioxide gas available ered from this zone, largely or wholly free of air or combustion gases. The third zone provides a chamber or chambers within which the material under treatment may be further dissociated to the desired extent and preferably by higher temperatures. This latter zone may be economically heated by the combustion of fuel therein and the resulting gaseous products may be conveyed into the first zone to provide heat for the drying and preheating purposes. With this arrangement the relatively expensive electrical heating may be economically confined to the second zone which is preferably heated only to a moderately high temperature which the resistors will safely withstand, and from which the gases of high carbon dioxide content are recovered, whereas the heat for drying, preheating and also for the final thorough high temperature calcining, may be supplied at less expense by the burning of fuel.

The invention further embodies novel features of construction and arrangement of the electrical heating means and novel features of the furnace construction, particularly adapted among other uses, for carrying out the above indicated method.

Various further and more specific objects, features and advantages will more clearly appear from the detailed description given below taken in connection with the accompanying drawings which form a part of this specification and illustrate merely by way of example, a preferred form of the invention. The invention consists in such novel features, arrangements and combinations of parts as may be shown and described in connection with the apparatus herein disclosed and also such novel methods as are disclosed and described hereinafter.

In the drawings:

Fig. 1 is a vertical sectional view of a. multiple hearth furnace constructed in accordance with the invention;

Fig. 2 is a horizontal sectional view taken substantially along line 2-2 of Fig. 1;

Fig. 3 is an enlarged vertical sectional view taken substantially along line 3-3 of Fig. 2;

Fig. 4 is an enlarged view showing one of the electrical heating units and its mounting;

Fig. 5 is a horizontal sectional view taken substantially along line 5-5 of Fig. 1 and showing novel features of internal construction of a shaft section of the rabbling structure; and

Fig. 6 is a vertical sectional view of the structure of Fig. 5.

As shown in Fig. 1, the furnace may comprise 3 a cylindrical wall In embraced by an outer metal shell as at H. The furnace may be provided with a plurality of superposed hearths as at 12-22 inclusive, each accompanied by two or more rabble arms as at 23 carried by a central vertical shaft 24 adapted to be rotated through gearing as at 25 connected to a suitable source of power. The even numbered hearths may be formed with peripheral ports or openingsflas at 26, the intervening odd numbered hearths being formed with central ports or drop openings as at 21, whereby as the rabbling structure is rotated, rabble teeth as at 28 act to periodically agitate and gradually advance the material being treated over each hearth and from hearth to hearth down through the furnace, the material being passed outwardly and inwardly respectively on alternate hearths. The upper hearth l2 or furnace top which may serve as a preliminary dryin hearth may have its port or ports fitted with some well-known type of feeding device as schematically indicated at 29. Gases may be withdrawn from within the upper part of the furnace through an outlet flue 30, and the treated calcines may be discharged from the bottom of the furnace through any suitable known type of outlet construction as at 3|. The features of construction as thus far described in this paragraph, are in accordance with wellknown types of Herreshoff furnaces, one example of which is disclosed in further detail in U. S.

patent to Baird 1,669,925, granted May 15, 1928, reference to which may be had as to further details of construction.

While in the most common furnaces of this type the gases are free to travel countercurrent to the material being treated, from the lower hearths up through the various hearth ports and out from the top of the furnace, on the other hand, with the preferred embodiment of the present invention, the hearths are divided into groups to form three zones, that is, an upper zone located within the furnace between hearths l2 and I6 for example, an intermediate or middle zone located between hearths l 6 and for example, and a lower zone between hearths 20 and 22. The upper and middle zones, and similarly the middle and lower zones, may be isolated against the direct or free passage of gases from one zone to the next, by so-called ore luted feed devices as at 32, 33 of a suitable known type. These devices may'comprise for example a shelf or plate 34 mounted beneath the hearth port so that suificient material will fall and accumulate on the plate to normally keep the port closed against free passage of gas. The rabble arms at this region may be provided with feeding arms or scoop members as at 35 so that each time the rabble arm passes the plate 34, a predetermined amount of the material will be dislodged from the plate and fed to the hearth below. The furnace is thus effectively divided into an upper drying and preheating zone, an intermediate calcining zone, and a lower zone for completion of the calcining.

The lower zone may be heated by the combustion of fuel therein, for example by the use of oil burners as at 36. The resulting gases in this zone may be conducted from the upper part of this zone, for example from a point just beneath hearth 20, by a conduit 31 extending up into the upper zone preferably to the lower part of the upper zone, or at a point just under hearth No. 15. Thus the hot gaseous products of combustion of the fuel in the lower zone, together with some air and gases dissociated from the material under final treatment, are carried up 4 to the drying and preheating zone to normally supply sufllcient heat to the upper zone. If desired, additional heat may be obtained by burning fuel in the upper zone, as by the use of one or more oil burners such as indicated at 38.

, As shown, the hearth spaces in the middle zone may be heated without combustion of fuel in this zone by the use of electrical resistance heating units indicated at and hereinafter more fully described in connection with Figs. 2-4.

It is intended to carry on the calcining process in the furnace continuously and since the intermediate zoneas thus far described may be substantially sealed against the entrance of air or gases of combustion, once the process is well under way, the atmosphere in the middle zone will comprise solely the gases evolved from the material under treatment. These may be withdrawn through an outlet conduit as at 4|.

In cases where it is unnecessary that the gases from outlet 4| entirely comprise gas evolved from the material under treatment, it may be desirable to introduce into the middle zone some of the gas drawn oil. from the lower zone. This may be done by using a conduit 42 branching off from conduit 31 and entering the middle zone preferably at its lower part, for example just below hearth I9. The amount of this gas may be varied or regulated as by a damper 43 in conduit 42. In cases where a certain amount of combustion gases and/or air is not objectionable in the middle zone, the middle zone ma generally be heated with less expense by thus supplementing the electrical heaters with a predetermined controlled or regulated amount of the gases from the lower Z0116.

A present preferred construction and arrangement of the electrical heating units for the middie zone is shown in Figs. 2-4. These units may comprise for example non-metallic ceramic rods as at 40, of silicon carbide material, one form of which is now commercially available under the name Globar. As best shown in Fig. 2, these rods may be placed in a polygonal arrangement, for example a hexagonal pattern as shown, around the inside walls of each hearth space. As shown 3, several of the rods, for example four, may be mounted in superposed positions at each side of the polygon, The ends of each resistance rod or unit may be slidably mounted in insulation bushing members as at 46. The interior wall surfaces of the furnace may be suitably formed or cut away as at 41 to afford space between the rods and the wall surfaces. The heating units may thus be largely inset with respect to the normal interior contour line of the furnace wall so that the operation of the rabble arms is not interfered with and also the material under treatment is free to fall through the hearth ports 26 without falling on the heating elements.

Heating units of the above mentioned type are ordinarily formed with a high resistance central portion and low resistance ends so that the end portions will be at a relatively lower temperature and thus not injure the mounting means. The exterior wall surface of the furnace may be formed at circumferentially spaced points with cavities as at 48 lined if desired with suitable heat-resistant metal pieces as at 49 for strengthening and protecting these portions of the walls and providing spaces for mounting and for ready access to the electrical terminals 50 of the heating units. It' will be further noted that the cavities 48 and the lining members 49 therefor are al- I so so shaped that any of the resistance units may be withdrawn or reinserted by sliding the same longitudinally through their insulation bushings l8. 1

The terminals 50 may be of a suitable known type readily removable to permit the insertion or replacement of the resistance units to which they are connected. As will readily be understood, the variouselectrical resistance units shown may all be connected in parallel to a source of power or in various known series-parallel arrangements if desired, depending upon the voltages available *by heating the latter zone to a somewhat higher and the resistance values of the particular units 1 1, .1mm. Thefpolyg' onal arrangement -oi! the heating units has a number of important advantages. In

case the material being treated, as is usual, has a tendency to cause considerable dust to arise within the furnace, quantities of such dust will tend to accumulate on the resistance rods, and thereby seriously impair the heating efficiency. It is therefore desirable to provide an arrangement whereby such dust may be frequently and quicklyremoved from the heating units without interrupting the operation of the furnace. This may be easily accomplished with the above de-' scribed construction merely by electrically disconnecting and sliding out each unit so that the dust is scraped off as the unit slides through one of its insulation supporting bushings 46. The polygonal pattern of the units extending horizontally along the inside walls of the furnace further provides a convenient arrangement whereby the electrical terminals and connections may all be located entirely outside the furnace and thus free from overheating and in a position where they can be quickly and easily disconnected whenever it is desired to inspect, clean,

repair or replace any unit while the furnace is in direct firing. In locations where electric power is inexpensive, the same arrangement of heating units may of course be extended to other hearths in addition to the middle zone of the furnace.

Where the method and apparatus are used for site, sufllcient fuel may be burned in the lower calcining materials such as limestone or magnezone, supplemented by such fuel, if any, as is necessary in the upp r zone, so that the temperature of the calcines upon passing into the middle zone may, for example, be in the neighborhood of 1000 F. At this temperature little or no carbon dioxide will be given off. Then in the middle zone, sufficient electrical heating may preferably be provided to heat the calcines to a temperature for example of 1600 F. before same pass to the lower zone. This temperature and the normaltime of treatment in the middle zone (from twenty to sixty minutes) will be-suiiicient 70 to cause the greater part of the carbon dioxide gas to be evolved from the material, but normally insufficient to completely dissociate the carbonate in this zone. The dissociation may be temperature, for example several hundreds of degrees higher. 7

The most desirable temperatures in the middle and lower zones will depend upon the relative Eases, the damper 43 may be opened somewhat or regulated so that some hot gases from the lower zone will pass through conduit 42 and contribute to the heating of the middle zone and thus supplement the generally more expensive electrical heating, within limits permissible by the character of gas desired from the middle zone. When the damper 43 is kept closed, the gases passing out through conduit II from the middle zone will consist almost entirely of carbon dioxide where the material under treatment comprises limestone or magnesite. The gases passing out through flue 30 will be relatively low in carbon dioxide.

The division of the furnace into three 'zones arranged as above described, makes possible the use of fuel for the preliminary drying and'preheating and final calcining operations, thus conflning the generally more expensive electrical heating to the middle zone. Also the performance of the calcining steps in two successive zones, permits the middle or electrically heated zone to be operated at a temperature somewhat lower than that required for thorough calcining in the lower zone. This permits the electrical heaters to be operated at temperatures which-will not be unduly destructive of the unit, yet high enough to permit recovery from the middle zone of the greater part of the available carbon dioxide, while confining the higher temperatures necessary for thorough dissociation to the lower zones having no electrical heaters subject to being burned out.

The exclusion of gases of fuel combustion from the middle or main calcining zone also has the advantage of greatly reducing the gas volume carried out of this zone. This in turn reduces the amount of sensible heat loss from the furnace in this gas, because of the less weight of such gas. Also the reduced gas volume exited from the middle zone reduces the dust loss from this zone, because of the lowervelocity of the small quantity of gas.

While in the example shown, the furnace is provied with 10 hearths in addition to the top drying hearths, it will be understood that the principles of the invention are applicable to furnaces having a greater or lesser number of hearths in each zone.

In view of the high temperatures at which the furnace is operated, it is practically necessary to provide internal cooling air conduits within the rabbllng structure. As in the types of Herreshoif furnaces heretofore available, cooling air may be introduced through a conduit 5i connected by suitable known means to the base of the central shaft. As shown in Fig. 5, the shaft 24 may be double walled, i. e., with an inner shaft 52 comprising a, cooling fluid conduit and with a space 53 between the inner and outer walls providing a return path to the top of the shaft, for the cooling fluid after it has been conducted into and out of the various rabble arms. The above completed to the extent desired in the lower zone mentioned Baird patent discloses one suitable either extended radially, or tangentiallyto both sides of each rabble arm socket in the inner shaft as shown in-Fig. 11 of the above mentioned Baird patent. However, if the furnace temperatures are subject to wide variations or are quite high, as necessary in the lower zone as operated in accordance with this invention, there will be substantial danger that the expansion or contraction of the shaft sections due to temperature changes or diiferences, will cause such ribs or webs as heretofore used, to crack. That is, the inner and outer walls of each shaft section being rigidly held in place by the integralribs or webs,

will not be sufficiently free to move in any direc-.

tion enough to accommodate the expansion or contraction of the parts, particularly if blasts of cool air should be introduced at any time into the central cavity, or where the incoming cooling air is of a necessary low temperature to adequately cool the rabble structure at high temperature hearths such as involved with the present invention. And if these ribs or webs become cracked, the rabble arm sockets in the inner conduit 52 will be ineffective for securely retaining the rabble arms in position when the rabbling force is applied thereto, as the inner conduit 52 may become more or less free to move out of place or also become cracked, or open up at the joints between adjacent sections.

To eliminate these difficulties, I have devised an improved arrangement of these webs or ribs as shown in Figs. 5 and 6. That is, a plurality of the ribs are provided as at 5 extending substantially along successive tangents to the inner wall, and all in the same direction, to the right or left, from the points of tangency adjacent each arm socket, whereby upon expansion or contraction of the webs, or upon relative changes of diameter of the inner and outer walls, all of the webs will cooperate in tending to rotate the inner wall slightly with respect to the outer wall,

in the same direction, clockwise or counterclockwise. This slight relative'rotation of the inner wall with respect to the outer wall, thus relieves the stresses which occur due to expansion or contraction of the various parts of the structure upon temperature changes. As a result, the tendency of the ribs to crack is eliminated. Yet it is still possible to cast the inner and outer walls and ribs of each shaft section integrally, and rigidly to the extent necessary for proper support of the rabble arm sockets.

In some cases further difficulties will 'occur due to uneven expansion and contraction of the inner and outer walls of the shaft sections. For example, if the socket regions at the upper part of the shaft section of Fig. 6 should become heated much more than the lower part of this section, then there will be a tendency for the inner wall of the section to be twisted and become cracked, or the excessive axial expansion of the upper portion of the inner wall of the section may-set up axial stresses, tending to crack the integral ribs 54. To eliminate these difficulties, the inner wall of the section may be severed 8 as at 55 at a region intermediate the elevations of the two sets of sockets. The severed ends of the inner conduit may be formed with flanges as at 56 and so made as to be adapted to receive a ring member 51, which provides a slidable and rotatable joint. This joint thus permits the upper and lower portions of the inner conduit to relatively rotate as well as to also move axially with respect to each other, to the extent necessary to relieve the stresses. The ring 56 may for example comprise a split sheet metal ring adapted to be inserted to spring into place after the remainder of the casting has been completed.

While the invention has been described in detail with respect to a particular preferred example, it will be understood by those skilled in the art after understanding the invention that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent is:

1. Method for calcining carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising passing the material successively over a plurality of superposed hearths, including an upper hearth or hearths forming a drying and preheating zone, an enclosed plurality of hearths forming an intermediate zone and another enclosed hearth or hearths forming a lower zone, subjecting the material in said lower zone to a time and temperature treatment in the presence of burning fuel, suflicient to largely complete dissociation of the carbonate, conducting resulting hot gases from said lower zone into the upper zone to heat the latter, subjecting the material in the intermediate zone to a time and temperature treatment suificient to dissociate the carbonate therein to a substantial extent while largely or wholly excluding gases of combustion therefrom, and withdrawing the resulting gas of high carbon dioxide content from said intermediate zone independently of the gases passed from the lower zone into the upper zone.

2. Method for calcining carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising passin the material successively over a plurality of superposed hearths, including an upper hearth or hearths forming a drying and preheating zone, an enclosed plurality of hearths forming an intermediate zone and another enclosed hearth or hearths forming a, lower zone, maintaining separation between said zones against free passage of gases between adjacent zones, subjecting the material in the intermediate zone to a time and temperature treatment suflicient to dissociate the carbonate therein to a substantial extent, by introducing heat into such zone electrically, subjecting the material in said lower zone to a time and temperature treatment in the presence of -'b fuel, to further dissociate the material, conducting resulting hot gases from said lower zone into the upper zone to heat the latter, and withdrawing the resulting gas of high carbon dioxide content from said intermediate zone independently of the gases passed from the lower zone into the upper zone.

3. Method for calcining carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising passing the material successively over a plurality of superposed hearths, including an upper hearth or hearths forming a drying and preheating zone, an enclosed plurality of hearths forming an intermediate zone ment in the presence of burning fuel, to furtherdissociate the material, conducting resulting hot gases from said lower zone into the upper zone to heat the latter, also conducting a controlled portion of said hot gases from said lower zone into the intermediate zone, and withdrawing resulting gas of high carbon dioxide content from said intermediate zone independently of the part, of

the gases passed from the lower zone into the upper zone.

4. A furnace for calcining carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising a plurality of superposed hearths, rabbling structure for advancing the material over each hearth and from hearth to hearth down through the furnace, said hearths including an upper hearth or hearths forming a drying and preheating zone, another plurality of hearths forming an intermediate zone, another hearth or hearths formin a lower zone, means separating said zones against free passage of gases between adjacent zones, means for heating the intermediate zone without combustion of fuel therein to a temperature sumcient to dissociate the carbonate therein to a substantial extent, means for burning fuel in said lower zone t heat same to a temperature sufficient to cause further dissociation of the carbonate therein, means for conducting the gases resulting in said lower zone into the upper zone for heating the latter, and outlet means for conducting away the gas of high carbon dioxide content formed in said intermediate zone, independently of'the gases conducted into the upper zone from the lower zone.

5. A furnace for calcining carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising a plurality of super-- posed hearths, rabbling structure for advancing the material over each hearth and from hearth to hearth down through the furnace, said hearths including an upper hearth or hearths forming a drying and preheating zone, another plurality of hearths forming an intermediate zone, another hearth or hearths forming a, lower zone, means separating said zones against free passage of gases between adjacent zones, means for burning fuel in said lower zone to heat same for final dissociation treatment of the material, means for conducting the resulting gases from said lower zone into the upper zone for heating the latter, electrical resistance means within the intermediate zone for heating same without combustion of fuel therein, to a temperature sufficient to dissociate the carbonate therein to a substantial extent, and outlet means for conducting away the gas of high carbon dioxide content formed in said intermediate zone.

6. A furnace for calcining material subject to dissociation by heat to form a gaseous product and a solid product, comprising a plurality of superposed hearths, rabbling structure for advancing the material over each hearth and from hearth to hearth down through the furnace, said hearths including an upper hearth or hearths forming a drying and preheating zone, another 10 plurality of hearths zone, another hearth or hearths forming a lower zone, means separating said zones against free passage of gases between adjacent zones, means for heating the intermediate zone without combustion of fuel therein to a temperature sufilcient to dissociate the material therein to a substantial extent, means for burning fuel in said lower zone to heat same to a temperature sufficient to cause further dissociation of the material therein, means for conducting the resulting gases from said lower zone into the upper zone for heating the latter, and outlet means for conducting away the said gaseous product formed in said intermediate zone, independently of the gases conducted into the upper zone from the lower zone.

'7. Method for continuously calcining supplies of carbonate material for the recovery of oxide and gas of high carbon dioxide content, comprising, drying and preheating the material by applying thereto gaseous products of fuel combus tion, introducing the resulting hot material into a chamber heated electrically to a temperature sufficient to cause dissociation of the material to a substantial extent, withdrawing from said chamher the carbon dioxide gas as formed therein, transferring the material from said chamber while hot to another chamber, applying heat by burning fuel in the latter chamber, to cause further dissociation, and utilizing gases resulting in the latter chamber as a supply of said gaseous products of fuel combustion for said drying and. preheating step. t

8. In a furnace construction comprising a plurality of superposed hearths and rabbling arms carrying teeth for advancing material over said hearths and from hearth to hearth down through the furnace, a central rotatable vertical shaft for carrying said arms, said shaft beingformed of double walled sections providing an inner cooling fluid conduit surrounded by another fluid cavity between the inner and outer walls, said walls respectively being formed with aligned sockets for receiving each of said arms, a plurality of said sockets being provided at each of two elevations in each of said shaft sections, the inner wall of each section being formed with a slidable and rotatable joint at a region intermediate said two elevations, and a plurality of rib means at both the upper and lower regions in each section integrally formed with both the inner and outer walls and extending from the inner wall to the outer wall substantially along successive tangents to the inner wall, and all in the same direction, to the left or right, from the points of tangency, whereby upon expansion or contraction of the webs, all will cooperate in tending to rotate the inner wall slightly with respect to the outer wall, in the same direction, clockwise or counterclockwise.

9. In a furnace construction, rotary rabble arm supporting means comprising a central rotatable vertical shaft, said shaft being formed of double walled sections providing an inner fluid conduit surrounded by another fluid cavity between the forming an intermediate 11 limited relative vertical and rotational move- Nu b ment. 1,549,379 GEORGE E. CONNOLLY. 1,861,213 1,678,875 REFERENCES CITED 2,225,199 The following references are of record in the $263,464 file of this patent: UNITED STATES PATENTS 1:088:496 Number Name Date 10 4 1,912,811 Wechter June 6, 1933 Name Date Pike Aug. 11, 1925 Greene May 31, 1932 Rohn July 31, 1928 Abbott Dec. 17, 1940 Seipel Dec. 30, 1941 Paitrowitz Apr. 8, 1930 Baird May 15, 1928 Wedge Feb. 24, 1914 Wedge Feb. 10, 1914

Images