US2829042A - Furnacing process - Google Patents

Description

April 1, 1958 O. MOKLEBUST FURNACING PROCESS Filed Jan. 9, 1956 I94OF [lsoow I Fig. 2'.

INVENTOR.

Olov Moklebusi United States FURNACING PROCESS Olav Moklebust, Birmingham, Ala., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey Application January 9, 1956, Serial No. 557,976

3 Claims. (Cl. 75-36) This invention relates to a process for selectively establishing and maintaining specific temperature areas during the furnacingof materials such as ores and ore concentrates. More specifically, it relates to an improved and commercially attractive method for reducing ores such as iron ores and the like.

Rotary funaces or kilns have long been used in the chemical and metallurgical fields. For example, they are extensively employed in the production of cement and also in the calcination of pigments, such as titanium dioxide. Much effort has been made to adapt the rotary kiln technique to the reduction of various ores so as to recover the metallic values therefrom, and the art is rather replete with reference to such operations. Some of these references extend back nearly a hundred years; however, until applicants invention and recognition of the instant process as hereinafter described and claimed, there was essentially no process for the reduction of iron ores employing rotary kiln type furnacing which could be considered as an economical and commercial reality. Many individuals and agencies tried to develop the technique of rotary kiln reduction with little or no commercial success. Much difiiculty was encountered with ringing/f wherein the iron ore and various reduction products sintered and adhered in massive form to the walls of the rotary kiln. This phenomenon which, for the most part, is attributable to the previous lack of adequate temperature control throughout the system caused much trouble. The formation of sintered adherent masses impeded the flow of materials. Further, such ringing damaged the interior walls of the kiln and necessitated long interruptionsin operation of the reduction equipment. Operation below temperatures causing sintering resulted in reaction rates too low to be practical.

An object of the instant invention, therefore, is to provide a method for selectively establishing desired heat input and temperature conditions within rotary kiln type apparatus. Another object lSLtO selectively establish a number of varied temperature zones in such apparatus.

Still another object is to provide a method for accurately establishing and controlling the temperature within any part of a rotary kiln. Another object is to provide a method for selectively liberating heat values in rotary kiln type apparatus, which is particularly useful in the reduction of iron ores, concentrates, and the like. Another object is to provide a process for accurately controlling temperature conditions within a rotary kiln whereby ringing is prevented. Still an additional object is to provide a novel method for heat liberation which will eifectively permit the use of much larger equipment than heretofore considered feasible.

Another important object is to provide a method for accurately. establishing and controlling the temperature within any part of the kiln so as to permit the controlled release of energy to effect endothermic chemical changes in a reaction mass at a maximum rate consistent with safe operating temperatures.

Another important object isto provide a method for "ice accurately controlling the release of thermal energy to effect endothermic chemical changes in a reaction mass at maximum rate consistent with safe operating temperatures.

These and other objects will become clear from the following more complete description including the examples presented herein.

Broadly, the instant invention is related to a process wherein a kiln, as for example a rotary kiln, is employed to furnace materials such as ores and ore concentrates which comprises obtaining a selective liberation of heat values at any desired area in the kiln by introducing at one end of the kiln a gaseous composition comprising partially combusted gases, progressively transferring the combustible gases through the kiln to a forward area wherein a specific heat condition is desired, at that area introducing an amount of oxygen containing gas calculated as sufiicient to react with part of the combustible gases and to liberate the heat values (thermal units) necessary for said heat condition, progressively transerring the remainder of the combustible gases to yet another forward position and repeating the desired liberation of heat values in the same manner, and finally toward the other end of the kiln, burning the remaining combustible gases to obtain substantially complete combustion thereof.

The instant process is adaptable to the furnacing of many materials and has been found to be particularly useful in the reduction of iron ores and ore concentrates, such as, for example low grade iron ores, brown ores, hematite, magnetite, non-magnetic taconite, Alabama ferruginous sandstones, Alabama Big Seam iron ore, hematite wash ore, Rana (Norwegian taconite) and the like.

The term iron ore is, for the purposes of this specification, intended also to include ores containing the other valuable mineral constituents, such as, for example, titanium. Examples of these'are the various ilmenite ores, titaniferous magnetite, etc.

construed as coming under this term.

In the reduction of iron ores, it is desirable to introduce a mixture of the iron ore and a carbonaceous reducing material such as coke, disco coke, coke breeze, coal chars, anthracite culm or the like, into the rotary kiln. The cokeor'other reducing material is generally employed in quantity in excess of that required to react with the oxygen values of the ore and thus permit the formation of metallic iron values. According to a preferred embodiment of this invention, the combustible gas, i. ei, carbon monoxide, which is produced during the reduction phase, may be used to supplement and add to the combustible gases which are initially introduced as previously described. For the purpose of this invention, the partially combusted gases which are initially introduced and that carbon monoxide which may form by reaction between the carbon and the oxygen values of theme will be considered as the combustible gas mixture.

Fig. 1 in the accompanying drawing is a schematic representation of a substantially horizontal rotary kiln apparatus which may be suitably employed in the practice of this invention. The rotating kiln body is represented generally by A. At the feed end housing. B is located through which appropriate feeding means such as, for example, screw conveyor 19 passes. Housing B is also equipped with flue or stack 12 having damper 14 located therein. According to a preferred embodiment, the stack 12 is furtherprovided with or connected to fan 13 Which assists in controlling the draft within the kiln.;

The other end, i. e. the discharge end of the kiln, isfin close proximity to combustion chamber C which is" equipped with burner 16 forthe' introduction and partial Patented Apr. 1, 1958 Also iron ores containing valuable quantities of manganese, nickel, etc., may be' combustion of fuel, such as oil, gas, and the like. There is also provided appropriate discharge means 18 for the removal of the furnace products.

Considering now the rotary kiln body A, it may be seen that a number of tubes 20 are positioned along the length of the kiln body and extend generally to the horizontal axis thereof. Each tube is provided with valve 22 which can be opened or closed independently of the others, so as to permit entrance of the calculated amount of air or oxygen containing gas through the tube.

The novel technique which is the basis of this invention can be and is employed both to establish a desired heat condition in the rotary kiln before the furnacing operation actually commences and also during the furnacing of-the materials themselves. In order to more clearly il lustrate the instant invention, the following detailed example is presented wherein apparatus, as schematically represented in the drawing, is employed to reduce a low grade iron ore.

Example A mixture of gas and air was introduced into the burner and ignited under conditions of complete oxidation. The heat values so produced were employed to heat the combustion chamber and part of the rotary kiln initially. When the area of this kiln immediately adjacent to the combustion chamber reached the desired temperature in the use about 1940 F., the amount of gas which was introduced into the burner was increased thereby yielding a partially combusted mixture. ous mixture was then transferred to a more forward position in the rotary kiln generally represented at point I. The valve 22 of air tube 20 which is positioned at this point was then adjusted so as to permit the introduction of air and thus effect. the combustion of enough of the combustible mixture so as to result in liberation of heat values sufiicient to etfect the desired reaction and maintain the temperature of the kiln at that point as was desired. The process of progressively transferring heated combustible gaseous mixture to a more forward point in the kiln and selectively liberating the required heat values (thermal units) so as to effect the desired chemical reaction and at the same time establish the desired temperature condition was continued by employing the air tubes along the length of the kiln as described and by alsoregulatingthe draftby means of the valve 14 and fan 13. At the point generally identified as II, which point is near the feed end of the kiln, the combustible gases are completely burned and the exhaust. gases passing out of the line show a compositionof substantially complete combustion. By

this ismeant that only a fraction of a per cent of carbon monoxide can. be detected in the flue gas.

According to a typical operation wherein the kiln which was employed was heated as above and was ap proximately 150 feet long and 9 feet in diameter, a mixture of 7 tons of iron are (containing about 34-35% total iron as iron oxides) and about 5 tons of coke (about 80 percent carbon) were introduced per hour into the kiln. This feed rate was continued on a 24 hour basis. The materials so entering the kiln built up a loose bed occupying 50% of the total kiln volume. This mixture of iron ore and coke in bed form traveled progressively through the kiln generally countercurrent to the gas flow and was progressively and controllably heated to and maintained at the desired reduction temperature, in this case about 1940 F. At this temperature, the carbon in the coke reacted with the oxygen values of the iron ore according to the following equation:

The carbon monoxide which is evolved mixes with the combustible gases being fed into the kiln from the burner and combustion chamber and in fact supplements the combustion gases. This combustible mixture is then progressively transferred through the kiln according to the operation as described and combusted in that area where This combustible gaseit is required to liberate heat values in order to effect the reduction of the iron oxide according to the above equation and maintain the maximum safe temperature condition. It is well known that the reduction of iron ore by carbon does not proceed at a constant rate; therefore, it is necessary when operating as near as possible to a safe maximum operating temperature, as is the case in the instant practice, to liberate varying controlled .amounts of thermal energy in order to effect the most ethcient chemical change or reduction in any and all parts of the reaction mass.

By this technique, it was possible to establish and maintain a definite temperature condition in every part of the kiln with a controllable accuracy of about :5 F. In the instant example, the iron ore was most etficiently reduccd at a temperature of about 1940 F. and this temperature was maintained with the indicated accuracy for more than half of the kiln length. This condition of operation resulted in excellent reduction of the iron oxides (-90% total metallization of the iron values). Further, although the temperature of operation was just below the probable sintering temperature, the degree of temperature control in fact precluded the formation of any significant sinter. Therefore, no ringing took place and there was no blockage of the free passage of materials. Neither was there any damage to the kiln walls which might have necessitated early shutdown of the apparatus.

In Fig. 2 there is shown the typical temperature condition as was maintained in the kiln during the reduction operation. The reason for maintaining the feed end of the kiln at between 800 F. and 1600 F. as indicated was because of the limitations of the materials of construction used in the feed mechanism, etc. It is possible, however, if desired, to maintain for example a high temperature of about 1900 F. in the area adjacent the combustion chamber, a temperature of 800 F. in the center of the kiln, and a temperature of 2200 F. at the feed end. Likewise it is just as possible to produce areas of high temperature in the center and low temperature at the ends or alternating high and low temperature areas. As has been explained, it is only necessary to liberate the required heat values in the proper area by introducing the oxygen or air for combustion of the gaseous mixture.

It should be emphasized that excepting for the carbon monoxide developed in the bed by the reduction of the iron oxide values that essentially none of the carbon values of the coke were combusted. The kiln products which were discharged comprised the excess coke, the reduced iron values, and some ash and gangue materials. The last of the combustible gases were essentially burned in the area identified by H in Fig. l and the exhaust gases which were examined were found to have the following analyses:

0.2% CO 0.8% CH Remainder N In a continued operation over an extended period of time, it is possible to obtain excellent results by adjusting the flue damper and by regulating the introduction. of gas at the burner. If necessary, the air valves on the tubes may also be adjusted to permit more or less air to enter as required but this latter adjustment is not often necessary once conditions have been reasonably stabilized.

The rotary kilns as presently employed for most applications rarely exceed -175 feet in length. One of the reasons for the restriction in length is that theliberation of all of the heat values at one end of the kiln as normally accomplished, obviously yields a normal tendency for development of a temperature gradient throughout some or possibly the entire kiln length. By providing suitable means for introducing air as disclosed herein and by practicing the instant technique of progressively moving a combustible gas mixture to a desired area, it is possible to extend the length of rotary kilns to 300 feet or more. Such extension would be an 0bvious improvement in many instances.

Although, as has been emphasized, this operation'is particularly adaptable to employment in connection with a rotary type kiln, it is not necessarily restricted by apparatus design. It would be possible to adapt the principle of operation for use with other kilns such as tunnel kilns, rocking kilns, etc. It is therefore intended to include such apparatus when speaking of kilns of the rotary type.

While the method of the instant invention has been more completely described by reference to an operation wherein iron ore is reduced to metallic state, it should be appreciated that the invention is broader and is applicable to many other furnacing operations involving the use of rotary kilns wherein it is important to establish and control heat input and temperature conditions with accuracy. As was indicated, depending upon the particular materials being furnaced, the combustible gases may all be introduced through the burner or some may be evolved as described. Modifications and variations may be employed within the scope of the following claims.

I claim:

1. The steps in a process of reducing iron oxide containing materials in a kiln of the rotary type comprising establishing a moving materials bed Within the kiln, said bed occupying between about 30 percent and about 50 percent of the kiln volume, by introducing a mixture of the iron oxide containing material and of carbonaceous material through the feed end of the kiln the carbonaceous material being present in amount in excess of that calculated as theoretically necessary to metallize the total iron values in the iron oxide containing material and being sufiicient to shield the iron values in the bed from a combustible atmosphere above the bed, selectively liberating heat values at various areas in the kiln and reducing the iron values by employing the carbon in the carbonaceous reducing material in the bed, in a controlled manner and at a maximum safe rate Without any appreciable sintering by introducing heated combustible gas at the opposite end of the kiln, transferring the combustible gas to a forward area closer to the feed end, at which forward area a specific heat condition is desired, introducing at a point within said area the amount of oxygen containing gas required to liberate the heat values necessary to establish said heat condition in said area, progressively transferring the combustible gas to a more forward area and repeating the step as described to liberate a controlled amount of heat values, and finally near the feed end of the kiln burning the remaining gas to obtain essentially complete combustion thereof.

2. In a process for reducing iron oxide containing materials wherein reduction is efiected at temperatures below that which produces molten iron the steps for permitting the controlled release of energy to effect endothermic changes in the iron oxide material at a maximum rate and in the absence of any significant sintering comprising establishing a moving materials bed in a kiln of the rotary type, said bed occupying between about'30 percent and about 50 percent of the total kiln volume, by introducing a mixture of solid carbonaceous reducing matter and the iron oxide containing material through 6 the feed end of the kiln the carbonaceous matter being present in excess of that amount calculated as theoret1cally necessary to metallize the iron values and being sufficient to shield the iron values in the bed from a combustible atmosphere above the bed, introducing heated combustion gas at the opposite end of the kiln, transferring the combustible gas to a forward area closer to the feed. end at which area a specific heat condition is desired to effect reduction of the iron values in said forward area, introducing at a point within said area the amount of oxygen containing gas required to react with the combustible gas to liberate the heat values necessary the carbonaceous matter on the iron oxide containing material being used to supplement and add to the combustible gas originally introduced and being burned therewith, and finally near the feed end of the kiln burning the remaining combustible gas to obtain essentially complete combustion thereof.

3. In a process wherein a kiln of the rotary type is employed to reduce iron oxide containing materials the steps comprising establishing a moving materials bed within said kiln by introducing a mixture of solid carbonaceous reducing matter and the iron oxide containing material through the feed end of the kiln, said bed occupying between about 30 percent and about 50 percent of the total kiln volume, the reducing matter being present in amount about five times that calculated as theoretically necessary to metallize the total iron values in the iron oxide containing material and being sufiicient to shield the iron values in the bed from a combustible atmosphere above the bed, reducing the iron values in the bed by the carbon in the carbonaceous reducing matter, maintaining a combustible atmosphere over the material in the kiln by introducing heated combustion gas at the opposite end of the kiln, transferring the combustible gas to a forward area closer to the feed end, at which forward area a specific heat condition is desired, introducing at a point within said forward area the amount of oxygen containing gas required to react with the combustible gas to liberate the heat values necessary to establish said heat condition in said area, progressively transferring combustible gas to a more forward area and repeating the step as described to liberate controlled amount of heat values, the carbon monoxide which is produced in the kiln by the reducing action of the carbonaceous matter on the iron oxide containing material being used to supplement and add to the combustible gas originally introduced and being burned therewith, and finally near the feed end of the kiln burning the remaining combustible gas to obtain essentially complete combustion thereof.

Landers May 15, 1928 Newkirk et al May 27, 1930

Claims (1)

1. THE STEPS IN A PROCESS OF REDUCING IRON OXIDE CONTAINING MATERIALS IN A KILN OF THE ROTARY TYPE COMPRISING ESTABLISHING A MOVING MATERIALS BED WITHIN THE KILN, SAID BED OCCUPYING BETWEEN ABOUT 30 PERCENT AND ABOUT 50 PERCENT OF THE KILN VOLUME, BY INTRODUCING A MIXTURE OF THE IRON OXIDE CONTAILNING MATERIAL AND OF CARBONACEOUS MATERIAL THROUGH THE FEED END OF THE KILN THE CARBONACEOUS MATERIAL BEING PRESENT IN AMOUNT IN EXCESS OF THAT CALCULATED AS THEORETICALLY NECESSARY TO METALLIZE THE TOTAL IRON VALUES IN THE IRON OXIDE CONTAINING MATERIAL AND BEING SUFFICIENT TO SHIELD THE IRON VALUES IN THE BED FROM A COMBUSTIBLE ATMOSPHERE ABOVE THE BED, SELECTIVELY LIBERATING HEAT VALUES AT VAROUS AREAS IN THE KILN AND REDUCING THE IRON VALUES BY EMPLOYING THE CARBON IN THE CARBONACEOUS REDUCING MATERIAL IN THE BED, IN A CONTROLLED MANNER AND AT A MAXIMUM SAFE RATE WITHOUT ANY APPRECIABLE SINTERING BY INTRODUCING HEATED COMBUSTIBLE GAS AT THE OPPOSITE END OF THE KILN, TRANSFERRING THE COMBUSTIBLE GAS TO A FORWARD AREA CLOSER TO THE FEED END, AT WHICH FORWARD AREA A SPECIFIC HEAT CONDICTION IS DESIRED, INTRODUCING AT A POINT WITHIN SAID AREA THE AMOUNT OF OXYGEN CONTAINING GAS REQUIRED TO LIBERATE THE HEAT VALUES NECESSARY TO ESTABLISH SAID HEAT CONDITION IN SAID AREA, PROGRESSIVELY TRANSFERRING THE COMBUSTIBLE GAS TO A MORE FORWARD AREA AND REPEATING THE STEP AS DESCRIBED TO LIBERATE A CONTROLLED AMOUNT OF HEAT VALUES, AND FINALLY NEAR THE FEED END OF THE KILN BURNING THE REMAINING GAS TO OBTAIN ESSENTIALLY COMPLETE COMBUSTION THEREOF.

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