US2199384A - Calcining apparatus - Google Patents

Description

May 7 1940- v. J. AzBE 2,199,384

GALCINING APPARATUS Filed Feb. 26, 1938 v'.5 Sheets-Sheet l May 7, 1940. v, J, AZBE 2,199,384 l CALCINING' APPARATUS Filed Feb. 2G, 1958 3 Sheets-Sheet 2 FBGEO. HG1 i.

May 7, 1940. v. J. AzBE 2,199,384

CALCINING APPARATUS Filed Feb. 26, 1938 3 Sheec.s-She.=.'rl 3 Patented May 7, 1940 'UNirEo STATES cammino lirrlimrrus- VVictor J. Azbc, Webster Groves, Mo. y u Application February 28, 1938, Serial lNo. 192,674

2z claims.

This invention relates to calcining apparatus, and with regard to certain more specific features to apparatus for burning limestone to produce lime and CO2 gas.

l The present application is av continuation-inpart of my patent application Serial No. 148,190,y

dated June 14,1937, for Lime kiln.

Among the several objectsof the invention may y be noted the provision of apparatus for producing 10 a high-grade, soft-burned, high-availability lime,

with minimum of core, from low-grade or highgrade limestone, the production being at a high rate and at high efficiency; the provision of means for obtaining high capacities vfrom small kilns ll without excessive deterioration of kiln linings and with long kiln life; the provision of means for Iburning spalls heretofore wasted or burned in rotary kilns; the provision for central injection means for gaseous fuels and control of (a) com- 20 bustion, (b) hydrocarbon cracking and (c) temperature, in order to obtain high fuel efllciency and high quality of lime; and theprovision of .means for obtaining CO2 gas of very high strength. Other objects will be in part obvious g and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which will be exemplified in the structures hereinafter 3o described. and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings, in which is illustrated one of various possible embodiments of the invention,

35 Fig. 1 is an isometric view of the kiln, parts being broken away:

Fig. 2 is a side elevation of a charging apparaturS:

Fig. 3 is a cross section taken on line 3 3 of 40 Fig. 2, parts being broken awayfor clarity;

Fig. 4 is a horizontal section taken on line 4 4 of Fig. l; v

Fig. 5 is a horizontal section taken on line 5 5 of Fig.l l; l

Fig. 6 is a horizontal section taken on line 6 6' 50 of Fig. 1;

Fig. 9 is a vertical section of a center burner' bridge wall; f l

Fig. 10 is a vertical section taken on line N IU n of Fig. 9; V

Fig. l1 is a view similar to Fig. 10 showing an alternative form of center burner: y

Fig. 12 is a detail section of a flue control;

Fig. 13 is a detail section of certain draw gates;`

Fig. 14 is a vertical section of the invention 5 with an alternative feature; and, c

Fig. 15 is a fragmentary and enlarged section taken on line I 5 1 5 oi' Fig. l.

Similar reference characters indicate correspending parts throughout the several views of 1o the'drawings.

The following imperfections in prior vertical, shaft-type limekilns are generally known:

1. They have a low capacity for. a given investment;

2. They require considerable fuel for a given amount of lime;

3. They need to be repaired frequently;

4. They need to have large-sized limestone rock;

5. The rock needs to be of best quality to insure fair lime;

6. 'I'hey are usually incapable of producing good, readily slaking lime out of poor rock:

7. 'Ihey are incapable of giving uniformly burned lime;

8. 'I'hey cannot be regulated so as consistently to produce lime of different characteristics when desired from the same stone; and,

9. 'I'he kiln gases have a lowCOn are very hot. g

Many lime'plants have large piles of wasted spalls that have been accumulating through Ayears and for which there has been no use. This vis because only rock of large size could be'A burned in vertical kilns, since gas could not Well be equally distributed when the shaft was fllled'with small rock or spalls. By means oi' this invention, the burning of spalls in vertical kilns not only becomes practical. but preferable to burning of large shapes that tend to' presenta relatively small surface for heat absorption.

All limestones contain a certain ,amount of silica, alumina and iron compoundsthatare ordinarily considered to be impurities. vIn some limestones thereis a considerable amountyof these content and undesirables. IIn the limekiinv1 they cciilBiiiSimt-mw the calcium to form-silicates, aluminates and ferrates, which -reduce the amount i `of. free lime'. Furthermore,these compounds tend to destroyiihe sponginess of the lime and formliquid slags which upon cooling form concretions'which'enclosefree calcium oxide, making it unavailable.l Because of.

this, the lime becomesmuch less ydesirable Afor 5'5- some chemical processes, and sometimes actually useless.

Even when limestone is very pure and impure compounds cannot form to any great degree, exposure to excessive temperatures for variable lengths of time impairs the quality. This is because of shrinkage and reduction of free space between unit cells of calcium oxide. Reduction need not be great for the free space to become less than-the dimensionsjof water molecules, thus interfering with slaking. Hence settling rates, availability and plasticity, and all other important factors, become impaired.

The kiln incorporates features to be explained which effectively avoid the occurrence of this undesirable action through provision for control of zonal temperature, distribution of heat, rate of draw, and location of draw.

The invention also includes a novel method of introducing 'gas for combustion and a novel method of delayed combustion that tends to produce highest temperature when deleterious action on the stone is least likely to take place, and lowest temperature when such action is most likely to occur.

Prior limekilns also present difficult refractory problems. Highest temperatures are developed along the walls, and the calcium base of the lime combines with the comparatively acid silica of the refractory. This is aided by the alumina and fluxes of the refractory and by the impurities in the lime. As a result, the surface of the highmelting-point brick changes to a low-meltingpoint slag which, at ordinary kiln temperatures, tends to retain its shape, but which is semi-iiuid. The action penetrates deeper with progress of time. This converted mass finally melts and runs when temperatures become higher. Thus some kilns needed to be repaired every few months, and all have had comparatively short lives. This invention overcomes this slagging difliculty very effectively by removing the high temperature zone away from the kiln lining, and by other means to be described.

Many conventional vertical kilns have three operating zones, namely, pre-heating, calcining and cooling. Some operate with only two zones, namely, pre-heating and calcining, these two being very indefinitely denedl.

The limekiln of the present invention is divided into six zones, namely, (l) storage and drying zone, (2) pre-heating zone, (3) upper calcining zone, (4) lower calcining zone, (5) finishing zone, and (6) cooling zone. By this means. control is obtained that greatly enhances quality, quantity, and eiciency.

I have discovered that the color of lime is often impaired by reason of oxidation while it is in the cooling zone. In the present kiln there are means to avoid this, with consequent production of a much whiter product. This is accomplished by retaining of lime in a reducing atmosphere and subsequent subjection to sudden cooling.

The present kiln also includes features permitting of partial hydration of lime by admitting low pressure steam with air. I'his retains some of the heat of hydration in the kiln, and increases by a substantial amount the heat available for calcining. This partial hydration conditions the lime for special processes.

By means of the present invention, combustion, even with natural gas, is caused to be very luminous, and is attained by pre-heating'and cracking of hydrocarbons and by special methods of introducing air for combustion. A luminous,

long name is very much desired for lime burning, as it produces better lime and greatly increases heat transfer. In this kiln, ame can be made as long or as short as desired, and either luminous or non-luminous.

In prior limekilns, there is very little or no vcontrol over the amount of lime that can be drawn from different sections of thekiln. In the present kiln, there are four outlet sections in which the amount of lime drawn from each can be increased or reduced independently of the other. At the same time, the finishing heat to each of these sections can also be controlled independently, with complete exclusion from some sections, if desired.

Another novel feature is the production of high-strength CO2 gas that may be cooled and compressed immediately without the requirement of the usual purification and absorbing system.

Ordinary limekilns give off gases containing only 20% to 40% of CO2. This kiln is arranged for intermittent operation as a retort, self-heated from within, to provide CO2 of substantially 100% strength.

There is also an arrangement for exact supplemental electrical heating during the retorting period in which CO2 is driven off. This optional supplementary use of electricity lengthens the period during which CO2 is given off. About one kilowatt-hour of energy produces 2% pounds of lime and 2 pounds of pure CO2 gas. By this method of CO2 generation, gas can be obtained uncontaminated with products of combustion, and suitable for dry ice manufacture. A CO2 plant construction can thus be simplified in operation and reduced in cost.

The kiln includes means to aid uniform gas introduction and distribution into the limestone charge; means to regulate the flow of stone in different sections; means to assure hanging of charge when drawing, to enable trimming, and of clearing of the walls; and means for aiding practical operation and avoiding possible difficulties caused by excessive temperatures or expansion and contraction due to heating of structural members.

Referring now more particularly to Fig. 1, there is shown at numeral I the cylindric outside metallic body of the kiln in which is arranged a refractory brick lining 3. It is shown circular in cross section but an ovate form is sometimes preferable The kiln is divided into six zones, as above noted, these being identified as the storage and drying zone S, the pre-heating zone P, the upper, calcining zone HC, the lower, calcininf,r zone LC, the finishing zone F, and the cooling zone K. :All zones except the last two are single pass; that is, all ow through the zones S, P, HC and LC is bounded by a single cylinder.

'I'he body is capped by an enclosure 5 having an opening 2 for which is provided a warp-proof, charging and explosion door A having wheels I3 travelling on rails 9. The rails 9 have cam depressions II into which the wheels I3 of the door A sink when the door is in position over opening 2. Thus the door, in its closed position, lowers to a seat I5 that is triangular in vertical section, upon the upper edge of which dirt cannot collect. The seat I5 surrounds the charging opening 2 and the rails 9 are outside of this seat. Thus, the closing of opening 2 is always tight. During natural draft operation, this door serves as draft regulator as well, enabling precise adjustment of pressure under the enclosure 5. Charging apparatus is diagrammatically illustrated l ing, is discharged to waste at outlet valve damper at I1. and a hopper at I4. As shown in Figures 2 and 3, the door A moves under the hopper I4. Suitable manual or automatic means (not shown) are adapted to draw the door by member I5.

Gases may be withdrawn at all or any of three points, each with a special purpose. The purpose of the uppermost exhaust at i 2 is to dispose of leakage air which enters the charging opening 2, and to dispose of steam created while drying the stone in the storage chamber S.

The purpose of the middle and main exhaust through pipe V is to discharge the cooled gases that have served their purpose in pre-heating the stone. Due to the upper exhaust at |2, these gases are, if desired, undiluted with air or with steam from the drying of the rock in zone S or from melting of the ice and snow from this rock during winter periods.

The third withdrawal point is from the hollow oitake bridge 51 at the top of the calcining zone HC where there is a temperature of around 1500" F. Inasmuch as the gases coming through the calcining zone HC have a surplus of heat necessary for pre-heating in zone P, a portion may be withdrawn at this point through openings 53 and utilized for pre-heating of natural gas (if this be used), or for pre-heating air for a gas producer, or for the generation of steam for injection with air into the cooling zone K, or for other use.

This pre-heating not only has the effect of saving the heat for use in the pre-heating process, but permits the combustion process to be started at a temperature higher than atmospheric, with the result that the calcining temperature is more easily and economically reached than would otherwise be possible.

The kiln also has arrangements for making useful the surplus relatively cooler gas from the top of the calclning zone HC, LC. This relatively cooler gas is recirculated for the purposes of lowering temperatures in the lower calcining zone and making them more uniform; hence eliminating hot spots which ordinarily would. occur in zone LC. There is also effected increased velocity of gas flow and better heat transfer. This is accomplished, without the gasesleaving the kiln, through special passages 35 built in the kiln wall, and the flow is created by injector effect at burner nozzles (Fig. 6) and 22 (Fig.. 15) and without using a fan for propelling these gases. Due to the great heat of these gases, a fan would be impractical. A further value of recirculation of these hot gases is that, the kiln shaft is insulated with hot gases which tends to reduce heat loss, as well as to cool the walls, thus lengthening their life. The cooling is by reason of the fact that the gases, although hot enough to reduce the temperature gradient through the wall, are relatively cool enough to effect the cooling desired.

The kiln is ona y,foundation I3 and has four lower tapered outlets 2 I. Each outlet has chargereleasing doors 23 movable to shut (solid-line positions) or to open (dotted-line positions) Each outlet 2| is also provided with draft inlets 25 leading from an air line 21 (Fig. 8). In the air line 21 are a forced-draft fan O and a control damper 24.

A second set of air inlets is at 23 supplied from a branch aii pipe 30 in which is a damper 32.

A fan B draws suilicient hot gas through the storage zone S to dry the rock. This gas, with its moisture and with all such air as may have entered through the charging door while charg- 29. By this means, the storage zone S is utilized for useful purposes other than mere storage and the gases passing through the main exhaust 3| from pipe V are undiluted withleakage air or moisture from stone.

When fan B carries of! gas from zone S, a damper 33 is open and a damper 35 in a by-pass pipe D is closed. When damper 33 is closed and damper 35 ir. open, the gas from zone S flows to lthe exhaust pipe 3| by way of both pipes V and D. An exhaust ian 31 draws gases from the exhaust 3|. Thus when the presence 0f air or steam in the main exhaust stream at 3| is not harmful, the fan B is out off and damper 3 3 is closed. The storage zone S is nevertheless utilized as a mild pre-heater and dryer by t e passage of small controllable amounts of gas rough the by-pass line D. A damper 43 in line 3| permits of further control. Damper 34 permits control of all ow through pipev3l. It is clear that Whatever gas does not go through pipeD, goes through pipe V, and hence the flow of gas from the remainder of the kiln into pipe 3| remains substantially constant, regardless of the setting of damper 35. All dampers are of the specially devised, thin, flexible edge type that accommodate themselves to the contour of the opening, and which seal tightly.

Pipe V constitutes a submerged gas oitake pipe preferably of heat resisting metal which is located crosswise of the kiln at a position substantially between the storage zone S and preheating zone P. It is slotted on its bottom at 39 so that gases are drawn uniformly from the entire cross section of the shaft. The slots are on the underside of the pipe F and thus cannot become clogged. Furthermore, an open pocket forms in the material under the pipe V. The gas oi'ftake pipe V divides the storage from the preheating zones. and, in fact, creates these two zones. Whenever the kiln is drawn, it receives fresh rock from the storage zone S, although kiln charging may take place less often than kiln drawing. This prevents wide fluctuations in gas temperatures and draft. The presence of the cross pipe V in connection with the constriction 5 between the storage zone S and main kiln shaft effectively breaks the fall of rock and reduces rock breakage and excessive packing.

Within the gas oitake pipe V there is a thermostat bar 4I having one end anchored at 34 while the other is articulated with thev door H.

When temperatures become excessive, the lengthening of the bar opens the door, admitting cold air, and by this automatic means, damage to the fan 31 is prevented.

'Ihe pipe V is subject to considerable expansion by heat, and hence is not directly connected to the external pipe ai. it lies independently within the kiln left-hand wall as shown at 45. The external pipe 3| is welded to the shell at flange 36, and thus a tightconnectionfis established that cannot possibly develop a leak. The door H is hinged to the pipe V and pipe V is welded to the kiln body at 200. The bar 4| expands faster than pipe V and therefore can open the door H'despite the fact that with heating the anchor 34 moves to the left.

way'. If the capacity desired is less than half normal, forced draft is not necessary, and nat'- kiln shaft are independent of one another.

ural draft is sufficient, asby opening the top door A. If a still smaller output is desired, it can be efficiently obtained by regulating this top door A so that a back-pressure is created, counteracting the normal draft of the hot kiln, and thus reducing the air admitted at the bottom to correspond with the fuel to be burned. ILno. lime isvwanted at all, the door A is closed tightly and then as no gases can escape (all dampers being closed), no air can enter, and hence no heat is lost, except through normal radiation through the kiln walls. In this way, the kiln can be maintained on the line red hot for a week without any burning of fuel, and be ready at the end of that time to get under production immediately.

At J is shown a bridge arch with a wall 42 beneath. This arch serves to retard the ow of lime and aids its hanging, and thus permits proper trimming of the charge. It is hollow, thus enabling admission of combustible gases to the kiln center. It is arranged so that the rate of gas ow to either of several sections of the This is accomplished by arranging eight valves 83 respectively on the eight nozzles 20, each of which controls the flow of gas into a separate duct 84 in the center burner bridge J. Each duct 84 has an outlet 86 (Fig. 10). A secondary center air inlet is shown at 46 under control of dampers 48 and having ports in the bridge J above the fuel ports 86.

In an alternative form of center burner shown in Fig. l1, a single passage 88 is used for gas with a multiplicity of outlets 90. This type of bridge is used for producer gas. Both bridges) of Figures l0 and l1 are arranged so that the ports cannot clog. This is done by providing overhanging portions 82.

By means of this bridge, there is obtained the effect of a center burner. No outside burners are necessary, although they may be used for special purposes, as in the cases where espcially small-sized limestone is to be burned, to be referred to later in connection with Fig. 14. Thus greatest heat is removed from the kiln walls, and placed closer to the center, and air coming from the cooler mixes better with the combustible gas.

The kiln from this point down in zones F and K is partitioned by walls 42 and 58 into four equal sized sections or quadrants 51, each of which can be independently drawn at its bottom doors 23, and each of which can receive independently any desirable amount of combustible gas and air. Thus, the kiln becomes, to all practical purposes, one with four independent production units, if the bridge J of Figures 9 and 10 be used. Each of the four units is, for example, readily capable of producing the equal to one or two ordinary kilns. One-unit limekilns of large size are ordinarily poor performers, but this invention increases capacity and improves performance.

In addition to the upper firing level at bridge J, there is a lower firing level shown at ports L. Here to the air coming up from the cooling zone K there is added a suiiicient vamount of gas to heat a limited amount, for example, not more than l800 F. Expressing it differently, the gas is burned with such a large amount of excess air that temperatures cannot be high. By this means, a new, heretofore never used, limekiln zcne is created, that is, the controllably heated finishing zone F; which extends from the level of burners L to the level of the burner outlets 86 l in the bridge J. In this finishing zone F the core areassa of the lime is calcined and the last amounts of CO2 are driven off.

The degree of burn of lime can thus be very readily controlled, as it depends upon the temperature to, which it is exposed and the time it requires to pass the finishing zone F. The temperature depends upon the simple matter of adjusting the amount of gas admitted through the valves |02 of the lower burners 22. Lime thus can'be obtained hard or soft burned. As the lower divided section F of the kiln is utilized for controlled finishing, it is possible consistently to obtain one kind of lime from one section and another kind from another section; and this very simply and quite independently.

There is the further advantage that the CO2 content of the finishing zone F is very low, thus reducing C02 tension, and so reducing the temperature required for calcination and eliminating the possibility of recarbonization, which together bring on the advantage that the lime drawn has a higher calcium oxide content, due to lower loss on ignition. Further, the lime, before it is fully calcined and so before it has acquired the temperature of the combustion zone HC, LC, passes into the low temperature zone F. Hence, the impurities do not tend `to combine. This is another reason for the higher amount of calcium oxide obtained.

If desired, it is possible to use the finishing zone F according to the principle of the flreless cooker, calcining the core with stored heat remaining after the lime passes down below the upper burners in the bridge J. In this event, the finishing zone F is by-passed by the air which comes up through the cooling zone K. At this time, the injecting nozzles 22 are turned oif, but the nozzles 20 remain on. Thus, the upper gas injecting nozzles 20 function to draw air through the ports L and through the lower portions of passages 85 of the kiln wall. This air is then forced into the center burner J, thus by-passing the finishing zone F, and acting as primary air in the center burner J. The flnishing zone F would, in this case, have no flow excepting for the normal convection currents.

The ability to by-pass the air from the cooling zone K through the ports L through passage 85 to burners 20 and out through the burner arch J permits of obtaining a whiter lime, because a reducing atmosphere is maintained in the finishing zone F. 'I'his is becausesome gas may be drawn down from zone LC. This also permits of drawing in a large amount of air through the cooling zone K without affecting the finishing zone F, so that the lime passes suddenly from the finishing zone F into the cooling zone K. This prevents deterioration in color.

The cooling zone K receives air from passages 25, and/or 28, either through natural draft, or forced draft, from the blower O, and is conditioned with steam from jets T, if partial kiln hydration is desired. Natural draft is obtained by a slightly open condition of the gates 2|, 23 to a degree that will not permit falling out of ivo the charge, but will permit entry of the air. The

There is no intention ordinarilyto hydrate in u the kiln, except' for special reason, nor completely to hydrate lime therein. Only partial hydration is intended, when used at all, as limited by the diiliculties due to lime expansion and disintegration. But absorption of even small quantities of steam is sometimes advantagedus. -The sensible heat and the latent heat of the steam, and the heat of hydration of lime, are thus liberated and in turn passed on to the air. For each pound of steam, 1600 B. t. u. is given off, which in the ldln all serves as heat of high elevation capable of making lime. One pound of steam absorbed gives 1.15 pounds of additional lime, without any more fuel. A boiler is shown at 'l5 which may be out in by means of damper 11. The steam line 'I9 fromthe boiler 15 leads to the jets T. Thus, air entering the cooler K may be humidified by steam from the jets T. In the cooler, lime is cooled and water vapor absorbed, which in turn releases heat for re-heating of air.

Above the cooling zone K, the air receives a certain amount of combustible at the lower firing ports L, suilicient to create a desirable temperature in the finishing zone F. At the top of the finishing zone F, at bridge J, the full amount of combustible is admitted in excess of that required to'satisfy the air coming up from the cooling zone K This combustible is pre-heated at preheaters 'Il and 13 and hydrocarbons partially cracked. Burning this gas with insuiiicient air gives a very luminous, long, and mild ame. At openings 50, the remaining amount of air is admitted and combustion is completed. More air may be admitted at Q, or further firing may be effected through these op pings Q Where the air otherwise admitted is inxcess for a given limestone.. Either one or both heat transfer apparatuses 1| and 'l5 may be used. If both are used, then not only Would all heat be recovered for vcarbonate dissociation, but the heat of hydration Aary gas at J is with turbulent, high-Velocity flow,

thus aiding mixing and preventing hot spots or heat channels, and thus greatly aiding heat transfer.

The storage zone S is iiared out, as shown. The pre-heating zone P is narrower and constricted. In addition, and this is novel, the finishing zone F and cooling zone K are made of large cross-sectional area, as indicated at 89. In addition, a choke is used at 9| for the purpose of breaking the fall of rock and loosening the lime in the hot zone so that the material in it is not so densely packed, and so that the gas can circulate more readily through it. Trimming to permit the fall of the charge is effected through poke holes 93.

In Fig. I4 is shown the value of the enlarged portion 89. adjacent the center burner J. This enlarged portion permits the rock, particularly ne rock such as spalls, to assume a position, ac-

cording to its angle of repose, which is peripherally spaced from the enlargement 89 (see dotted lines H2). At the enlargement l89, auxiliary external burners 95 may be used at an angle, as shown or at any other convenient position, to inject a combustible into the space thus left. It will be understood that with spalls or other tine rock the center burner J may not distribute the combustible as satisfactorily as might be desired, and ,that the auxiliary distribution into the nozzles 95 assures that all spall areas are exposed to heating.

The Wide lower section also increases the A pressures.

length of stay of lime in the nishlng and cooling zones and reduces the pressure necessary to deliver the air.

By the time that the calcining zone HC has been traversed by the gases, they have cooled to below the temperature of dissociation of the carbonate and may then only be used forpre-heating of stone in the pre-heating zone P. The division between zones HC and P depends upon the kind oflimestone calcined, but may be around l500 F.

Since there is a surplus of heat available for pre-heating of stone, some of the gas can be withdrawn at the junction joint between zones HC and P and utilized in a way that makes this heat of low elevation (not any more available for calcinationl again very useful for this purpose. This can be accomplished in several ways. It is possible to draw olf through openings in the wall, but this is unsatisfactory. A better means is by use of the hollow, refractory bridge 6l sprung across at the transition point, as

shown, having'openings 69 in its lowei` portion,

through which the gases enter and pass either to the fan 3l by way of pre-heater 'll and damper H4, or to other heat-exchange gas apparatus, such as the boiler l5. Or it may in part be recirculated by means of the injector effect at gas entrances at nozzles 20 and 22.

In the case of natural-gas fuel, it is first preheated in the first stage pre-heater located at 13 in the main exhaust line 3l. It is heated to almost the cracking point in the secondary hightemperature pre-heater ll. Delivered in lower bridge J, it heats and cracks upon entering the burning zone of the kiln. Thus is recovered heat which might otherwise be wasted, and improvement of heat transfer by effecting higher luminosity.

The gases withdrawn at ports 69 can also readily serve to make steam for such purposes as steam may be desired, but from kiln efficiency standpoint, preferably for injection into the cooler as at jets T.

In respect to the re-circulation of gas through passage for conditioning combustion, it should be noted that the gases never come out ofthe kiln shell, but pass through the channel 85 in the Wall, propelled by the suction effect of the injection at 20 and 22. It is recirculation with no substantial loss of heat, altogether different from re-circulation practiced heretofore, when gases were brought out of the kiln to be inevitably cooled, and then reinjected by fans, and often causing more harm than good.

If all air inlets and all poke-hole doors are closed, a pressure is created in the kiln which effectively prevents any air entering through the draw gates 23. If all combustible is shut off simultaneously, the kiln becomes a retort in which the heat stored dissociates the carbonates and generates pure CO2 gas. This gas in substantially pure undiluted form is discharged at a rate adapted to retain gas pressure within the kiln, and thus avoid any chance leaks and possible dilution.

The flow is controlled by means of a damper 91 shown in Fig. l2, which is regulated automatically by the pressure of the gas et the base of the kiln. The pressure pipe 99 in this event is connected to the usual pneumatic relay 'apparatus (not shown) which is responsive to-kiln When the pressure builds up, the damper valve 91 opens so that the CO2 under pressure may be relieved and vice-versa. This control apparatus hask been shown merely diagrammatically, because it is known.

Exclusive heating of limekilns by means of electric current is uneconomical. In this case,it is made practical by partial use at opportune times. In the wing walls 42 and 58 of the nishing zone F, readily removable and replaceable electrical heating elements 82 are inserted. When the kiln is sealed for operation as a retort, the electric current may also be turned on and through its heating of the structure and dissemination of this heat by convection to the lime and limestone, the period of exclusive CO2 generation is greatly prolonged.

Kiln design is in conformity with definite controlling factors, as surface presented for heat absorption, capacity desired, temperature permitted, resistance to gas flow created, time element required, kind of lime wanted, kind and cost of fuel, labor obtainable, etc.

The lime from each of the four units of zone K may be drawn intermittently. During the draw, the gas is shut oif from the respective unit, with other units remaining under normal operation. In this way, interruption is reduced to a minimum, and is only suiiicient properly to trim the particular kiln section. If desirable, the drawing may be continuous.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in carrying out the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In a kiln having a charging inlet, means providing a storage zone within said charging inlet, conduit means for drawing off gases at the bottom of said storage zone, a second conduit means adjacent said inlet for drawing off gases at the top of the storage zone, both of said conduit means being joined, suction means for drawing gases through both of the conduits, and auxiliary suction means adapted to draw gas from the conduit connecting with the upper portion of said storage zone.

2. In a kiln, a storage zone, a pre-heating zone therebelow, a gas oiftake member extending across the kiln between said storage and preheating zones, a calcining zone below said preheating zone, gas offtake means extending across the kiln between the pre-heating andl calcining zones, an inlet member for combustibles extending across the kiln at the base of the calcining zone, a finishing zone below said calcining zone, said finishing zone being divided into a plurality of compartments, each having a separate outlet, and a, cooling zone including a part of each compartment below each respective finishing zone compartment.

3. In a kiln, a. storage zone, a pre-heating zone therebelow, a gas offtake member extending across the kiln between said storage and preheating zones, a calcining zone below said preheating zone, gas offtake means extending across the kiln between the pre-heating and calcining zones, an inlet member for combustibles extending across the kiln at the base of the calcining zone, and a finishing zone below said calcining zone.

4. In a kiln, a storage zone, a pre-heating zone therebelow, a gas oiitake member extending across the kiln between said storage and preheating zones, a calcining zone below said preheating zone, gas offtake means extending across the kiln between the pre-heating and calcining zones, an inlet member for combustibles extending across the kiln at the base of the calcining zone, a finishing zone below said calcining zone, a cooling zone below the finishing zone, and means for admitting air to the cooling zone for cooling, whereby the air becomes pre-heated and provides a supply of air for primary combustion.

5. In a limekiln, a shaft having a calcining zone, means for introducing a substantial quantity of combustible gas into said calcining zone, said shaft having a finishing zone below said calcining zone and below the point of entry of said combustible gases, and means for introducing combustible gas with an excess of air for combustion below the level of said finishing zone.

6. InV a limekiln, a shaft having a calcining zone, means for introducing a substantial quantity of combustible gas into said calcining zone, said shaft having a finishing zone below said calcining zone and below the point of entry of said combustible gases, means for introducing combustible gas with an excess of air for combustion below the level of said finishing zone, said excess of air combining with the gas in the calcining zone, and means for introducing additional air at the calcining zone.

7. In a kiln, a shaft having a calcining zone and a finishing zone therebelow, an injection burner at one level between the calcining and the finishing zones, a second injection burner below the finishing zone, air inlet means below the finishing zone, ue means between said burners, and means for turning off the burner below the finishing zone whereby under continuous action of the burner above the finishing zone air is drawn in from the air inlet through the lower injection burner connections and into the upper burner connections, thereby by-passing said iinishing zone to permit the latter to drive off CO2 without auxiliary heating of the lime core that remains in the finishing zone.

8. A limekiln comprising a shaft having therein a single calcining zone in which limestone is heated by combustion, said shaft being downwardly divided into a plurality of finishing compartments in communication with said calcining zone into which finishing compartments products from the calcining zone flow respectively, means in the finishing compartments for heating the products therein, the heating in each compartment being independent of the heating in others, and means for independently drawing the respective finishing compartments, whereby a different type of lime may be produced in each compartment.

9. A limekiln comprising a hollow shaft having openings therein, means for introducing limestone into said shaft, means for introducing burning gases and passing them therethrough to heat the limestone, means adapted to stop introduction and passage of burning gases from time to time and at the same time to close the openings ln the shaft, whereby self-contained heat produces CO2 from the limestone without substantial dilution, outlet means for the CO2 and electric heating means adapted to supply heat to the shaft during the time that the kiln is closed.

10. In a lime kiln, a shaft forming a calcining zone, a hollow burner bridge extending across thershaft at the lower region of said calcining 'amaca-i zone and having at least one burner outlet, a hollow oitake bridge at the upper portion of the calcining zone having at least one opening to receive gases from the calcining zone, means procirculation of gases through the calcining zone and at least one burner arranged to fire into the burner bridge and to inject gases received from said oiftake bridge and to force them from the burner bridge during the process of combustion.

1l. In a lime kiln, a shaft forming a calcining zone, a hollow burner -bridge extending 'across the shaft at the lower region of said zone and having at least one burner outlet, a hollow offtake bridge `at the upper portion of the vupper section of the calcining zone having at least one opening to receive gases from the calcining zone, said shaft having hollow wall means providing a connection between said hollow offtake bridge and the burner bridge to provide for recirculation of gases through the calcining zone, said gases serving also as heat insulators within the shaft wall adjacent the calcining zone.v

12. In a lime kiln, a shaft forming a calcining zone, a hollow burner bridge extending across the shaft at the lower region of said calcining zone and having at least one burner outlet, walls below and supporting the burner bridge and separating the shaft into a plurality of compartments, al hollow offtake bridge at the upper portion of the upper section of the calcining zone having at least one opening to receive gases from the calcining zone, hollow means providing a connection between said hollow oiftake bridge and the burner bridge to provide for recirculation of gases through the calcining zone, and at least one gas nozzle arranged to fire into the burner bridge and to inject gases received from said offtake bridge and to force them from the burner bridge.

13. In a lime kiln, a shaft forming a calcining zone, a hollow burner bridge extending across the shaft at the lower region of said zone and having burner outlets, gas burners arranged to re into the burner bridge, and means built into the burner bridge for introducing secondary air into the calcining zone comprising outlets above and separate from the burner outlets but forming part of the bridge.

14. A lime kiln comprising a hollow shaft for containing limestone and determining draft,

comprising a section providing a calcining zone,

means for forcing combustible into the shaft at a lpoint below the calcining zone, and means for drawing off gases at a point-above'the calcining zone at a rate to produce a draft-balancing effect to improve distribution of gaseous products and heat transfer within the limestone.

15. A lime kiln 'comprising a hollow shaft for containing limestone. and determining draft, comprising a section providing a calcining zone, a pre-heating zone thereabove and a finishing zone therebelow, means for introducing air into the shaft at a point below the finishing zone, means for drawing off gases at a point above the preheating zone, wherebya draft-balancing effect may be obtained to improve distribution of gaseous products andheat transfer within the limestone, means comprising a firing opening for introducing combustible fuel below the calcining zone, means comprising a withdrawal opening above the calcining zone through which gases of combustion may be withdrawn, and means concining zone through which recirculation of gases is effected through the calcining zone.

16. A lime kiln comprising a hollow shaft for containing limestone and determining draft, comprising a section providing a calcining zone,

a finishing zone therebelow, means for introducing primary air vinto the shaft at a point below the finishing zone, means comprising an opening-for introducing combustible gases below the calcining zone, means'comprisingv an opening above the calcining zone through which gases may be withdrawn, means connecting the lastnamed opening above the calcining zone with said gas opening below the calcining zone through which recirculation of gases may be effected through the calcirflng zone, and means for introducing secondary air adjacent and above the point of gas introduction.

1'7. A kiln comprising a shaft for receiving and passing a charge, said shaft including a calcining zone, said charge moving downwardly through the kiln, a firing bridge for combustible gases located crosswise of the shaft and below the calcining zone and around which the charge passes, a gas offtake bridge member arranged across the kiln above the calcining zP-ne and around which the chargev also passes before reaching the firing bridge, said firing bridge and said gas offtake bridge each having gas openings spaced from the sides of the shaft.

18. A kiln comprising a shaft for receiving and passing a charge, said shaft including a calcining zone, said charge moving downwardly through the kiln, a firing bridge for combustible gases located crosswise of the shaft and below the calcining zone and around which the charge passes, a gas offtake bridge member arranged across the kiln above the calcining zoneand around which the charge also passes before reaching the firing bridge, said ringbridge having inlet openings spaced fromthe sides of the shaftand laterally directed, and said gas oftake bridge member having outlet openings spaced from said shaft and directed downwardly.

19. In a kiln, a shaft providing a calcining zone, a firing bridge across said shaft near the base of the calcining kiln for introducing combustible gases, said ring'bridge having a plurality of openings leading into the shaft, separate passages within the bridge connecting with the respective openings, separate ring means for said separate passages, and independent control means for the respective firing means.

20. In a kiln, a shaft providing a calcining zone, a firing bridge built across said shaft near the base of said zone, a plurality of independent passages in said bridge, each passage having a separate outlet at a point adjacent the base of said calcining zone, independently controllable gas burner members in the respective passages and separate finishing compartments joined to the calcining zone at said bridge to receive in- `base of said calcining zone and adjacent the inlet of one finishing zone,'and independently controllable gas burner members in the respective passages, said burners being arranged to project combustible gas into the calcining zone at points respectively above the respective nishlng zones, whereby the materials passing to the different nishing zones may be controllably and differently treated.

22. In a kiln, a shaft providing a calclning zone. a tiring bridge built across said shaft near the base of said zone, a plurality of independent passages in said bridge, each passage having a separate outlet at a point adjacent the base of said calcining zone, independently controllable gas burner members in the respective passages, said burners being arranged with respect to the passages to project combustible gas and to inject therewith auxiliary gases, and at least one other independent passage in the bridge and having at least one opening into the shaft for the independent introduction of air for combustion.

VICTORI J. AZBE.

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