CA1145542A - Direct reduction rotary kiln with improved air injection - Google Patents

Abstract

Abstract:

In the direct reduction process for reducing iron ore to sponge iron of the type disclosed in U.S. Pat. No.
3,890,138 to Hockin wherein all of the air supply tubes along the rotary kiln inject air toward the discharge end of the kiln it has been round that by reversing the direction of injection of the air from one or more of the tubes in the preheat zone of the kiln the condition of the off-gases can be considerably improved without affecting the process in the reduction zone and ulti-mately the quality of the resulting product and im-proved preheating of the kiln charge can be effected without the formation of undesirable kiln accretions.

Description

~i~5542 DIR~CT REDUCTION ROTARY KILN
WITH IMPROVED AIR INJ~CTION

Background of the Invention The present invention relates to a direct reduction process for the reduction of iron ores in a rotary kiln provided with air injection pipes along its length and using countercurrent f low of gas and charge, and more particularly, to an improved method and means f or in-troducing the air flow into the kiln Various methods have been suggested and used for carry-ing out the direct reduction process using high vola-tile coal as heating agent and reductant in a rotary kiln. For example, in some of these processes the coal is fed into the kiln through the discharge end by mechan-ical or pneumatic means, such as disclosed in U.S. Pat.
No. 3,505,060 to Heitmann, and in some it is fed at the center of or along the kiln, such as disclosed in U.S.
Pat. No. 3,206,299 to Senior et al. However, consider-able disadvantages have arisen in blowing all of the high volatile coal into the kiln from the discharge end, and in feeding such coal at the center of the kiln.
Because air is supplied to the kiln at a constant rate, unless altered by the intervention of the operator, and the composition of the chamber gas is subject to fluc-tuation, the reducing and combustion processes are not uniform, and the control of the process is adversely 11455'~

affected. When the reducing agent used has a high con-tent of volatile matter or moisture, such as is the case with many low grade coals, the pressure in the rotary kiln is also subject to general and local fluc-tuations which further affect control of the process and which lead to a nonuniform discharge of solids from the kiln. As the distribution of the coal throughout the kiln is so highly critical, addition of all the coal from the discharge end makes the process difficult to control for simple mechanical and metallurgical rea-sons.

Further, although benefits are derived in regard to fixed carbon consumption when feeding all the fuel and reductant requirements in the form of a high volatile coal from the discharge end of the kiln, the control of the operation can be very difficult due to the large amount of fuel and reductant that has to be fed and the need to have highly precise distribution of the fuel if a high degree of reduction is to be achieved. It has been found in practice that it is not possible to main- -tain this fuel distribution, and conse~uently variable reduction results. It has also been found that the in-corporation of high volatile coal into the kiln bed at the discharge end of the kiln results in impaired reduc-tion capability in the kiln due to variations in the CO/C02 ratio in the bed and in the chamber gas, and that this situation tends to limit the degree of metal-lization to a level below that required for commercial practicality.

On the other hand, feeding of a high volatile coal from the feed end of a countercurrent flow system leads to a loss of volatile material in the first section or pre-heat zone of the kiln. These volatiles are removed by the combustion gas flow and are thus lost to the process and increase the heat value of the kiln off gas, and only a portion of the gases from the low temperature distillation of the coal can be used for the process.
The increased heat value of the off gas can further cause operating difficulties in the off-gas exhaust or processing system.

The disadvantages of these various approaches have been overcome by feeding a portion of the coal from the dis-charge end of the kiln sufficient to control the tem-perature profile throughout the kiln and feeding the remaining portions of the coal at the feed end while ensuring that the coal from the discharge end is dis-tributed in the kiln in such a manner that substan-tially no coal lands in the reducing zone within the last 15~ of the kiln length and is distributed to with-in the feed end zone of the kiln. The rotary kiln is fitted along its length with air injection devices which blow air countercurrent to the general flow of reducing gases within the kiln to produce mixture there-between. A system of this type is disclosed in ~.S.

~55 Pat. No. 3,890,138 to Hockin, particularly for use in reducing ilmenite. However, while this latter tech-nique improves upon the other coal feeding methods in the direct reduction process, when it is used for re-ducing iron ore to sponge iron, certain problems still remain in the content of the exhaust or off gases re-quiring special attention in the off-gas processing or cleaning system.

It has generally been the practice in the direct reduc-tion art to direct the air supply within the kiln along its length toward the feed end in keeping with the ear-ly teachings of Moklebust, for example, in U.S. Pat. No~

2,829,042 and subsequently in U.S. Pat. No. 3,170,786, as well as the teaching in the previously cited U.S.
Pat. No. 3,206,299 to Senior et al. However, Meyer et al in U.S. Pat. No. 3,235,375 teaches the directing of the air flow preferably toward the discharge end or in either or both directions to achieve improved heat dis-tribution and more effective combustion of carbon mon-oxide gas while obviating localized overheating of the charge mixture and formation of wall accretions.

The practice in the process of Hockin, U.S. Pat. No.

3,890,138, when reducing iron ore to sponge iron has been to direct the air toward the discharge end of the kiln countercurrent to the reducing gas flow to enhance mixing. Such is the arrangement shcwn in the prior art diagram of Fig. 1 which illustrates the components of a ~l~S5~2 direct reduction plant for producing sponge iron in ac-cordance with the HOCKIN process. Although the HOCKIN
process overcomes many of the problems af the prior art, still as noted above, difficulties have been en-countered in handling the off-gases. The present in-vention involves certain i~provements which have been discovered in the operation of the illustrated plant for the HOCKIN process.

~4554;~

Summary of the Invention In accordance with the present invention, it has been found that in a direct reduction process of the HOCXIN
type when the introduction of air into the kiln is di-rected oppositely in the preheat zone from the reducing zone, that is, toward the feed end rather than the dis-charge end, substantially complete reaction of the air with the combustible components in the preheat zone is achieved, and thus the composition of the off-~as is considerably improved, and improved preheating of the kiln charge can be effected without the formation of undesirable kiln accretions. More particularly, rather than blowing all of the air countercurrent to the com-bustion gases as in the past, with the present inven-tion, while the air injection tubes in the reducing or working zone are directed to blow the air toward the discharge end of the kiln, one or more of the air injec-tion tubes in the preheat zone are oriented to blow the air toward the feed end. The improved condition of the waste or off-gas resulting ~rom this improved air blow-ing arrangement obviates the need for using an after-burner in the off-gas system, as has been used in some of the other direct reduction kilns wherein all of the air has been directed toward the feed end of the kiln.
The approach of the present invention differs from that of Meyer et al in previously-noted U.S. Pat. No.

3,235,3Z5 which, while suggesting the opposite directing 11~55~2 ,7-of the introduced air flow, does not limit the opposite directing of the air flow to and within either of the different zones of the kiln. Unlike Meyer et al, no supplementary fuel is used in the process of the present invention, and by virtue of reversing the tubes only in the feed end or preheating zone, relatively large quan-tities of air are introduced to combust volatiles with-out affecting the desired temperature profile of the charge material in the entire length of the kiln and without causing sticking of gangue material on the pel-lets and/or sticking of particles of the charge on the wall of the kiln, such as may occur in the Meyer et al process.

~14554Z

Brief Description of the Drawings Fig. 1 is a diagrammatic view of a prior art direct re-duction plant for the production of sponge iron, which plant is of the type on which the present invention improves.

Fig. 2 is a diagrammatic view of a reduction kiln of the type used in the plant of Fig. 1, but which incor-porates the present invention.

5~
g Detailed Description of the Preferred Embodiment A direct reduction plant for the production of sponge iron of the type utilizing a rotary kiln operated in accordance with the HOCKIN process is shown in Fig. 1.
The plant comprises an array of feed bins respectively including: a bin 1 for supplying ore in the form of iron oxide pellets; a bin 2 for providing limestone or dolomite; a bin 3 for supplying recycled char; and a bin 4 for providing a carbonaceous reducing agent in the form of coal of less than 1 inch nominal diameter particles. The iron ore pellets, coal r return char and dolomite or limestone are accurately proportioned and fed continuously as a charge to the feed end 6a of the reduction kiln 6. A remaining bin 5 supplies coal of less than 3/8 inch nominal diameter particles to the feed end 6b of the rotary kiln 6, where carefully con-trolled quantities are blown in together with carrier air from a suitable source 7 through a coal injection pipe 8 which can be adjusted to achieve the optimum trajectory for this coal.

The reduction kiln 6 may be typically 11.5 feet (3.5 meters) in outside shell diameter and 148 feet (45 meters) long, sloped at 3%. It may be supported on two tires driven by a 200 horsepower variable speed D.C.
motor and lined with 8 inches of castable refractory.

In addition to the introduction of carrier air through 1~554Z

pipe 8, the kiln shell is equipped with a series of air injection tubes 9 which are spaced along its length and extend into the interior of the kiln for drawing air from the outside and injecting it along the kiln axis.
Each of the tubes 9 is equipped with its own fan and motor combination 10 so that the rate of injection may be properly regulated along the kiln. ~n existing plants of this type the air injecting ends 9a of the tubes 9 are all directed toward the discharge end 6b of the reduction kiln to produce air flow in that di-rection for better mixing of the air with the counter-current combustion and exhaust gases.

The hot waste or off-gases exhaust from Lhe feed end 6b of the kiln and pass into an off-gas processing or cleaning system. In a suitable cleaning system the gases are passed first to twin refractory-lined scalp-ing cyclones 11 and then to a 57 feet high by 11.5 feet inside diameter spray cooling tower 12 where they are cooled to 500 F before passing to an 8-cell bag house 13 equipped with glass fiber bags. The cleaned gases exit via an induced draft fan and a 100 feet high stack 14.

The material discharged from the discharge end 6b of reduction kiln 6 consists of a mixture of sponge iron, coal char, coal ash and desulfurizing agent. This ma-terial is cooled in a rotary cooler 15 fitted with lifters and cooled externally with water. The cooled ~ ~554~

mixture is then passed from the cooler 15 to a screening system 16 and screened. The two oversized fractions are subject to magnetic separation in respective magnetic separators 17 and 18. Separator 17 removes the sponge iron from waste material and delivers the sponge iron to the product loadout area. The nonmagnetic fraction is conveyed to the return char hin 3.

While a plant such as shown and described in connection with ~ig. 1 has, with proper control of combustion con-ditions over the kiln bed and reduction conditions in the bed, yielded high rates of heat transfer and opti-mum utilization of kiln volume with metallization of iron consistently in a 9~%-95% range, still when high volatile coal is used at the feed and discharge ends, problems may arise with regard to the condition of the off-gases.

As the high-volatile coal introduced at the feed end is heated in the preheating zone of the kiln from ambient to the temperature of operation in the reduction zone, volatile hydrocarbons are distilled as gaseous mixtures from the coal. These hydrocarbon gases are carried by the process combustion gases out of the feed end of the kiln and into the gas handling and cleaning equipment.
In this equipment their concentration by volume may be sufficient to form combustible mixtures if air be acci-dentally admitted therein creating an explosion risk.

In the absence of air they may condense inside the dust ~1~55~2 cleaning equipment, e.g., on the baghouse bags, causing impaired performance thereof, and/or they may condense to a relatively stable aerosol suspended in the ambient air adjacent to and mixed with the total gas flow from the stack exit causing the opacity of the stack gas plume to exceed the opacity permitted by environmental regulations. In some prior art direct reduction sys-tems it has become necessary to install afterburning equipment in the off-gas system to deal with problems of this type. However, it has been discovered that these problems can be solved without the need for afterburning equipment by modifying the direction of air injection in accordance with the present invention.

More particularly, the improved modification of the kiln 6 to conform to the present invention is shown in greater detail in Fig. 2. The rotary kiln 6 fitted with air injection devices 9, ten in number, is oper-ated in accordance with the HOCKIN process, that is, using a high volatile, non-caking coal as the reductant and fuel and characterized in that part of the coal is added from the discharge end 6b of the kiln in such manner that substantially no coal is incorporated in the kiln bed within at least the last 15%, and prefer-ably the last 20%, of the kiln length and so that some of the coal added from the discharge end of the kiln is distributed to within the feed-end region or zone of the kiln, and in that the remainder of the coal is 11~5~2 added at the feed end 6a of the kiln. The amount of coal fed into the kiln from the discharge end 6b to maintain a satisfactory and controllable temperature profile is preferably within the range of 15%-30% by weight of the total coal feed. The rate of feed and the particle size of the coal are suitably controlled to make sure that the desired operating conditions are obtained. As indicated in Fig. 2, operationally the kiln is divided into two zones, that is, a preheat zone toward the feed end of the kiln, which extends for ap-proximately the first one-third, but perhaps as far as the first one-half, of the kiln length, and a working or reduction zone which extends through the remainder of the kiln to the discharge end. In the preheat zone the ore, limestone or dolomite, coal, and recycled char are preheated gradually to the reduction temperature of aLproximately 1800 to 1950 F. The volatiles from the coal and the carbon monoxide formed by reduction in the kiln bed are combusted progressively by air admitted to the kiln through the spaced air tubes 9 mounted in the wall of the kiln. The temperature profile within the kiln is dependent on a number of factors and will differ with the type of coal used, its fixed carbon content, the volatile matter and its charring tempera-ture and its ash softening temperature. The kiln tem-peratures are measured with twelve thermocouples 19 along the kiln 6 which are designed to separately mea-l~S5~2 sure the temperature of the charge in the kiln and the gas temperature.

With the exit nozzles 9a of the air injection tubes 9 all directed to introduce air flow countercurrent to the reducing gas flow in the kiln, that is, directed toward the discharge end 6b of the kiln as in Fig. 1, the composition of the off-gas from the feed end of the kiln may be such as to cause problems in the off-gas cleaning system as indicated above. It has been dis-covered, however, that by reversing the orientation of the nozzles 9a of one or more of the air injection tubes in the preheat zone of the kiln, preferably the three tubes 9', as shown in Fig. 2, the off-gas compo-sition can be sufficiently improved to obviate any mod-ifications to the off-gas cleaning system, such as the use of an afterburner, while the optimum temperature profile within the kiln can be maintained.

More particularly, it should be appreciated that with the exit nozzles 9a of the three air injection tubes 9' all directed in accordance with the prior practice to introduce air flow countercurrent to the combustion gas flow in the kiln, i.e., directed toward the discharge end 6b of the kiln as in Fig. 1, the air volume flow in the three tubes is required to be limited to the range from ten to thirty percent of the total air volume flow through all ten tubes of the kiln. This is necessary to prevent excessive heating of the kiln internal sur-~5S~2 faces, by the otherwise complete oxidation and combus-tion of the volatile hydrocarbon gases to carbon dioxide and water vapor, which would cause sticking of the charge particles to each other and to the kiln internal surfaces, and to prevent overheating and consequent damage to the metal equipment feeding the charge ma-terials into the kiln. It also prevents loss of con-trol of the optimum temperature profile of the charge axially along the kiln in the preheating zone and t.~e first one-third of the reduction zone.

However, with the exit nozzles 9a of the three air in-jection tubes 9' all directed to introduce air flow concurrent to the combustion gas flow in the kiln, i.e., directed toward the feed end 6a of the kiln in accord-ance with the present invention as shown in Fig. 2, the air volume flow in the three tubes 9' may advantageously be increased to the range from 65~ to 85% of the total air volume flow through all ten air tubes of the kiln, thereby partially or substantially oxidizing and com-busting the volatile hydrocarbon gases to carbon diox-ide and ~ater vapor in such a manner that the tempera-ture of the total gas and air stream at their exit (6a) from the kiln does not cause excessive heating of the kiln internal surfaces in the preheating zone. In ad-dition, it does permit maintaining the optimum tempera-ture profile of the charge axially along the kiln in the preheating zone and the first one-third of the ~1~5542 reduction zone. Substantially complete oxidation and combustion of the volatile hydrocarbon gases by the air from the three tubes 9' is effected witllin the pre-heating zone of the kiln and/or outside the feed end 6a of the ~iln so that the aforementioned hydrocarban gases do not cause the problems and difficulties in the gas cleaning system described above.

It will therefore be seen that by reversing the direc-tion of injection of the air from one or more of the air supply tubes in the preheat zone of a kiln operat-ing in accordance with the described process, the con-dition of the off-gases can be considerably improved without affecting the process in the reduction zone and ultimately the quality of the resulting product.

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a rotary kiln for the direct reduction of materials containing iron oxides using a solid carbonaceous reducing agent as the reductant and fuel and having a pre-heat zone toward the charge feed end, a reduction zone toward the discharge end and of the type comprising:
means for adding part of the solid carbonaceous reducing agent through the discharge end of the kiln con-currently with the flow of the combustion gases within the kiln;
means for adding the remainder of the solid carbonaceous reducing agent through the feed end of the kiln out of which end the combustion gases exhaust; and means spaced along the length of the kiln for injecting air under pressure axially therein; the improve-ment wherein said air injecting means comprises:
means at the feed end of the kiln for injecting air toward said feed end concurrently with the flow of the combustion gases; and means along the remainder of the kiln for inject-ing air toward the discharge end of the kiln against the combustion gases flowing toward the feed end.

2. A kiln as in claim 1, wherein said air injecting means comprises ten air injection tubes, each having an injection nozzle means for directing the flow of air coming through the tube along the kiln axis and wherein said means at the feed end comprises at least the nozzle means nearest the feed end which directs the flow of air toward the feed end.

3. A kiln as in claim 2, wherein the three nozzle means nearest the feed end direct the flow of air toward the feed end.

4. Apparatus for the direct reduction of iron oxides using a solid carbonaceous reducing agent, such as coal, as the reductant and fuel comprising:
a rotary kiln with a charge feed end and a discharge end and having a preheat zone toward the charge feed end and a reduction zone toward the discharge end and wherein the combustion gases flow countercurrent to the charge and exhaust through the charge feed end;
means for feeding said reducing agent through both the charge feed end and the discharge end; and means for injecting air under pressure axially along the length of the kiln comprising:
means for directing the air injected into at least the first part of the preheat zone of the kiln con-currently with the combustion gases toward the charge feed end; and means for directing the remainder of said air injected into the kiln in the direction of the discharge end against the flow of the combustion gases.

5. Apparatus as in claim 4, wherein said air inject-ing means comprises ten air injectors spaced along the length of the kiln and wherein at least the air injector at the position nearest the charge feed end is directed toward the feed end.

6. Apparatus as in claim 5, wherein said air injec-tors disposed along about the first third of the kiln length at the charge feed end direct the injected air toward the feed end.

7. Apparatus as in claim 4, wherein said means for directing air toward the charge feed end injects from 65%
to 85% of the total volume of air injected into the kiln.

8. Apparatus for the direct reduction of ore or other materials containing iron oxides and using a solid carbonaceous reducing agent, such as high volatile coal, as the reductant and fuel comprising:
an inclined rotary kiln having a preheat zone toward the upper charge feed end and a reduction zone toward the lower discharge end and wherein the combustion gases flow countercurrent to the charge and exhaust through the charge feed end;
means for feeding part of the coal through the discharge end of the kiln;
means for feeding the remainder of the coal through the charge feed end of the kiln; and air injecting means disposed along the length of the kiln for injecting air under pressure axially along the length of the kiln, said injecting means directing the injected air in at least a part of the preheat zone concurrently with the combustion gases toward the charge feed end and injecting the remainder of the air toward the discharge end of the kiln against the combustion gas flowing toward the charge feed end.

9. Apparatus as in claim 8, wherein said air injec-ting means comprises a plurality of air injection tubes, each having an injection nozzle means for directing the flow of air coming through the tube along the kiln axis and wherein at least the air injection tube nearest the feed end has its nozzle means directing the flow of air toward the feed end.

10. A method for introducing an oxygen-containing gas into a rotary kiln for the direct reduction of materials containing iron oxides using a solid carbonaceous reducing agent as the source of reductant and fuel, the kiln having a preheat zone toward the charge feed end through which some of the reducing agent is added and a reduction zone toward the discharge end through which the remainder of the reducing agent is added and wherein the combustion gases within the kiln flow toward and exhaust through the charge feed end, comprising the steps of injecting the oxygen-containing gas under pressure axially along the length of the kiln and directing said gas injected into at least the first part of the preheat zone toward the charge feed end concurrent with the combustion gases while directing the remainder of said gas injected into the kiln in the direction of the discharge end countercurrent to the combustion gases.

11. A method as in claim 10, wherein the volume flow of said oxygen-containing gas directed toward the feed end is in the range from 65% to 85% of the total volume of said gas injected into the kiln.

12. A method as in claim 10, wherein said oxygen-containing gas injected toward the feed end is injected along about the first third of the kiln length at the feed end.

13. A method as in claim 10, wherein said oxygen-containing gas is injected at ten positions along the length of the kiln and at least the gas injected at the position nearest the charge feed end is directed toward the feed end.

14. In the process for reducing materials containing iron oxides using a solid carbonaceous reducing agent and an oxygen-containing gas in a rotary kiln fitted with gas injection devices along its length and having a preheat zone toward the charge feed end and a reduction zone toward the discharge end and wherein part of the reducing agent is added through the discharge end of the kiln and the remainder is added with the charge at the feed end of the kiln, and wherein the oxygen-containing gas from the injection devices is injected under pressure in the direction toward the discharge end of the kiln counter-current to the combustion gases flowing toward and exhausting through the charge feed end, the improvement comprising reversing the direction of injection of the oxygen-containing gas in at least a part of the preheat zone to direct it toward the feed end concurrent with the combustion gases.

15. A process as in claim 14, wherein the volume flow of the gas redirected toward the feed end is in the range from 65% to 85% of the total volume of oxygen-containing gas injected into the kiln.

16. A process as in claim 14, wherein about one-third of the gas injection devices are redirected to produce oxygen-containing gas flow in the direction of the kiln feed end.

17. A method as in claims 10 or 15, wherein said solid carbonaceous reducing agent comprises coal and said oxygen-containing gas comprises air.