Abstract:
A method of pyroprocessing mineral ores, such as iron ore. The method includes receiving a preheated product stream of iron-containing pellets at an infeed end of a rotary kiln and introducing an oxidizing gas into the tumbling bed toward the infeed end of the rotary kiln. Additionally, a combustible fuel is introduced through ports above the tumbling bed such that combustion of the introduced fuel increases the temperature of the product stream toward the infeed end. The increase in the temperature of the product stream allows the intensity of the flame from the centerline burner to be decreased, resulting in a reduction in the production of NO x . The apparatus for introducing both the oxidizing gas and fuel into the rotary kiln are common with each other.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to the conversion of fine grained iron ore pellets into hardened iron oxide pellets suitable as a feed material for making steel. More specifically, the present invention relates to a rotary kiln useful in heat hardening the iron ore pellets while reducing the overall fuel consumption and NO x  production. More generally, the present invention relates to a high temperature rotary kiln process where a longitudinal distribution of air and fuel along the length of the kiln enhances the processing condition of the solids being treated. 
     It is well known that most mineral ores in their natural state are low in concentration relative to a specific element and therefore require beneficiation which renders this material ore to a fine, powdery consistency. In the case of magnetite and hematite iron ores, this fine powdery state is unsuitable for transportation and many times unsuitable for further processing such as a feed to blast furnaces, open hearth furnaces or electric arc furnaces. A number of methods have been tried, with varying degrees of success, for agglomerating such fine ore particles to make the particles more suitable for handling, shipment and use in downstream processing equipment. In the following discussion, magnetite ore will be used as the feed material in the illustrations provided. 
     A well known agglomerating method involves converting the fine, powdery ore into balls or pellets that are transferred onto a traveling grate where the pellets are subjected to a cross-flow of hot gases to dry and pre-harden the pellets. While being heated, the iron ore pellets begin to oxidize, at which time the magnetite (Fe 3 O 4 ) phase of iron begins to transform into hematite (Fe 2 O 3 ). The pellets are then discharged from the traveling grate into a directly fired rotary kiln where the pellets are tumbled and subjected to further heating by a centerline burner in a counter-flowing oxidizing atmosphere with temperatures ranging between 2200-2450° F. The pellets are subsequently discharged into an annular cooler where they are further oxidized to the point of almost 100% oxidation. Since the oxidation of the ore is an exothermic reaction, the further oxidation of the product stream in the annular cooler raises the temperature within the cooler, which decreases its cooling efficiency. 
     The Rierson U.S. Pat. No. 5,248,330, incorporated herein by reference, improves upon the above-identified, generally known process by introducing an oxidizing gas, such as air, beneath the bed of pellets within the rotary kiln such that the oxidizing gas flows radially upward through the tumbling bed of pellets. The radially injected under-bed oxidizing gas intimately contacts the surface of the tumbling pellets and oxidation occurs. The process disclosed in the Rierson &#39;330 patent causes nearly the entire oxidation to occur before the annular cooler, which relieves the burden on the downstream cooler, in which 30-40% of the oxidation previously occurred. The introduction of the oxidizing gas into the rotary kiln causes the oxidation to occur within the rotary kiln rather than in the annular cooler, which allows the annular cooler to operate more efficiently in cooling the iron oxide product. 
     Although the disclosure of the Rierson &#39;330 patent has proved to be successful, a problem exists in the amount of NO x  produced during the transformation of the magnetite into hematite. Specifically, the high temperature firing of the centerline burner required to maintain the temperature within the rotary kiln generates a significant amount of NO x , which is considered to be an environmental pollutant. The production of NO x  occurs due to the high intensity firing of the centerline burner contained within the rotary kiln. 
     Therefore, it is an object of the present invention to decrease the amount of NO x  produced during the oxidation of mineral ores such as magnetite. An additional object of the invention is to provide a system for oxidizing ores using a traveling grate and rotary kiln in which the ore is substantially oxidized prior to being discharged from the rotary kiln. Further, it is an object of the present invention to maintain the level of oxidation of iron ore prior to its introduction into the cooler while at the same time reducing the external fuel consumption and decreasing the NO x  production. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for use in oxidizing an iron ore product stream. The rotary kiln of the invention includes a plurality of fuel distribution pipes that are positioned along a portion of the outer surface of the rotary kiln. The fuel distribution pipes are each associated with a supply pipe having a plurality of individual ports that provide a flow passageway from the fuel distribution pipe to the open interior of the rotary kiln. 
     Each of the fuel distribution pipes extending along the exterior of the rotary kiln is coupled to a common fuel distribution manifold through a fuel valve. The fuel valve can be selectively operated between a closed position to prevent the flow of fuel into the associated supply pipe and thus the open interior of the rotary kiln or an open position to allow the fuel to flow through the associated supply pipe and into the kiln interior. 
     In addition to the plurality of fuel distribution pipes, the rotary kiln of the present invention includes a plurality of air distribution pipes positioned adjacent to the fuel distribution pipes along the outer surface of the rotary kiln. The air distribution pipes are also each associated with one of the supply pipes having a plurality of individual ports that provide the flow passageway into the open interior of the rotary kiln. Thus, the ports serve the dual function of providing the flow passageway into the kiln for both fuel and air at different times during the rotation of the kiln. 
     Each of the air distribution pipes extending along the exterior of the rotary kiln is coupled to a second and independent air distribution manifold through an air valve. Each of the air valves can be selectively operated between a closed position to prevent the flow of air into the associated supply pipe and an open position to allow air to flow through the associated supply pipe and into the kiln interior. The fuel and air distribution systems discussed above are completely independent from each other and are designed to permit flow into the kiln&#39;s interior at different angular orientations as the rotary kiln rotates. 
     The air valves spaced around the circumference of the rotary kiln and positioned in the air distribution pipes are each operated in accordance with the invention to allow the oxidizing gas (air) to enter into the open interior of the rotary kiln when the rows of ports associated with the air distribution pipe are in the appropriate angular position(s). In the case where magnetite is being processed, air can be introduced when the rows of ports are oriented both under the bed and over the bed. When air is introduced under the bed, the oxidizing gas flows radially upward through the tumbling product stream to aid in the oxidation process of the product stream. When the oxidizing gas (air) is introduced above the bed, the air aids in the combustion of fuel added to the over bed space. 
     As the kiln continues to rotate, each of the fuel valves positioned in the fuel distribution pipes are opened to allow fuel to flow through the associated supply pipe and associated rows of ports when the fuel distribution pipe and rows of ports are positioned above the product stream bed. The fuel valve remains open to allow fuel to flow into the open interior during a predetermined fuel flow arc of travel. The introduction and distribution of fuel above the product stream causes the fuel to combust over the length of the ported zone and within the heated, oxidizing environment contained in the open interior of the rotary kiln. 
     Combustion of the fuel introduced, with air, through the rows of ports above the product stream bed lengthens the hot zone temperature of the product stream. This is not an intense combustion zone and is therefore without the production of NO x . The increase in temperature of the product stream near the infeed end of the rotary kiln allows the amount of fuel consumed by the centerline burner to decrease while still providing the required temperature within the rotary kiln. The amount of fuel no longer required by the centerline burner can be diverted to the ports such that the introduction of fuel near the infeed end does not increase the total amount of fuel consumed by the rotary kiln process. The decrease in fuel consumption by the centerline burner results in a decrease in the intensity of the flame produced by the centerline burner, thereby decreasing the amount of NO x  produced in this process. 
     Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
     FIG. 1 is a schematic illustration of an iron ore processing system including a rotary kiln of the present invention for reducing the production of NO x ; 
     FIG. 2 is a perspective view, including a partial section, illustrating a prior art rotary kiln; 
     FIG. 3 is a graphic depiction of the temperature profile as the product stream travels along the length of the kiln for both the prior art system of FIG.  2  and the rotary kiln illustrated in FIG. 1; 
     FIG. 4 is a schematic illustration of the rotary kiln of the present invention illustrating the fuel distribution pipes and air distribution pipes for introducing air and fuel into the interior of the rotary kiln; 
     FIG. 5 is a partial section view of the rotary kiln of the present invention illustrating the introduction of an oxidizing gas into the product stream and fuel into the open interior of the rotary kiln above the product stream; and 
     FIG. 6 is a section view taken along line  6 — 6  of FIG. 4 illustrating the introduction of air and fuel into the rotary kiln through the common induction ports. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a processing system  10  used to oxidize a wide variety of lump ores or pellets to produce an oxidized product stream. In the preferred embodiment of the invention, the processing system  10  will be discussed in connection with an iron ore (magnetite) pelletizing product stream. The processing system  10  includes a supply of a binder from bin  12  and a supply of ore fines from a bin  14  that are combined to form green balls in a balling drum  16 . The green balls of metal-bearing material are deposited on a traveling grate  18  that travels through a furnace  20 . The furnace  20  is divided into drying and preheating zones such that the pellets on the traveling grate  18  are exposed to a cross-flow of hot oxidizing gases that dry and harden the pellets in preparation for transfer to a rotary kiln  22 . The pellets must be sufficiently strengthened through heat hardening before reaching the rotary kiln  22  to avoid breaking and crumbling of the pellets. 
     When the pellets are on the traveling grate  18 , hot oxidizing gas from the rotary kiln  22  is drawn through the bed of pellets to harden the pellets and to begin the oxidizing process. In the preheating zone of the furnace  20 , the pellets are heated to between 1600-2000° F. Typically, approximately 60-70% of the total oxidation of the iron ore pellets occurs while the pellets are on the traveling grate  18  within the furnace  20 . 
     After the pellets have passed completely through the furnace  20 , the pellets are delivered to an infeed end  24  of the rotary kiln  22 . The rotary kiln  22  is inclined such that as the rotary kiln rotates, the pellets form a continuous product stream that is urged to travel from the infeed end  24  to a discharge end  26  of the rotary kiln  22 . The amount of time each pellet remains in the rotary kiln  22  is determined by the slope of the kiln and the rate of rotation. As an illustrative example, each pellet may remain in the kiln  22  for 20-40 minutes. 
     While the product stream of iron ore pellets is contained within the rotary kiln  22 , the iron ore is further oxidized by the introduction of an oxidizing gas, such as air, beneath the bed of pellets such that approximately 99% of the total oxidation of the pellets from magnetite (Fe 3 O 4 ) to hematite (Fe 2 O 3 ) occurs prior to the product stream reaching the discharge chute  28 . The discharge chute  28  transfers the hot oxidized product stream to an annular cooler  30  that cools the oxidized product stream before its discharge onto a conventional product handling system  32  consisting of conveyors, screens and bins. 
     Referring now to FIG. 2, in a conventional, prior art rotary kiln  22   a , the product stream enters into the infeed end of the rotary kiln  22   a  and proceeds through the rotary kiln  22   a  toward a centerline burner  34 . The centerline burner  34  generates a flame  36  that provides the required heat within the rotary kiln  22   a  to indurate the ore pellets contained in the product stream. As can be seen by the temperature profile line  37  in the graph of FIG. 3, the temperature of the solids within the product stream increases as the product stream nears the centerline burner  34  and peaks at point  38  which is generally aligned with the flame  36  of the centerline burner  34 . In the prior art processing systems including the rotary kiln  22   a , the remaining 30-40% of the oxidation of the pellets occurred in a downstream cooler. Since the oxidation process is exothermic, the oxidation of the pellets in the cooler obviously decreases the efficiency of the cooler. 
     Although the rotary kiln  22   a  shown in FIG. 2 has proven effective in an overall system for indurating the ore contained in the product stream, the high intensity of flame  36  generates NO x , such as NO or NO 2 , which is considered an environmental pollutant. As an example, the flame  36  may burn at 3000° F. to create an ambient temperature of 2400° F. in the rotary kiln  22   a . It is well known that the intensity of the flame  36 , not the amount of fuel consumed by the centerline burner  34 , is critical in determining the amount of NO x  formed in the rotary kiln  22 . 
     Referring now to FIGS. 1 and 4, the rotary kiln  22  of the present invention includes an air distribution manifold  40  and a fuel distribution manifold  41  that each surround the outer circumference of the shell  42  of the rotary kiln  22 . The air distribution manifold  40  is coupled to a supply of oxidizing gas and the fuel distribution manifold  41  is coupled to a supply of fuel. In the preferred embodiment of the invention, the oxidizing gas is air that is supplied through a conventional blower  43 . 
     Referring now to FIG. 4, in the preferred embodiment of the invention, the supply of fuel is a supply of fluid, which could be natural gas (CH 4 ), supplied to the fuel distribution manifold  41  by a fuel supply pipe  44 . The fuel supply pipe  44  is connected to a fuel-on system  45  by a rotary union connection  46 . 
     As can be seen in FIGS. 1 and 4, a plurality of supply pipes  47  extend along the exterior of the rotary kiln. Each of the supply pipes  47  includes a row of spaced ports  49  that extend from the respective supply pipe  47  through an opening formed in the shell  42  of the rotary kiln. The ports  49  are equally spaced along the supply pipe  47  and provide a pathway for either fuel or oxidizing gas to pass from the supply pipe  47  into an open interior  50  of the rotary kiln  22 , as shown in FIG.  5 . In the preferred embodiment of the invention, 8-12 individual supply pipes  47  are equally spaced around the circumference of the shell  42 . 
     Each of the supply pipes  47  is connected to the air distribution manifold  40  by an air distribution pipe  48 . Since the rotary kiln  22  includes 8-12 supply pipes  47 , the same number of air distribution pipes  48  are also spaced along the outer circumference of the rotary kiln  22  and connected to a corresponding supply pipe  47 . 
     Each air distribution pipe  48  includes its own air valve  51  that is selectively operable to control the flow of air from the air distribution manifold  40  to the associated supply pipe  47 . Each air valve  51  can either be closed to prevent air flow from the air distribution manifold  40  to the supply pipe  47  or opened to permit air to flow from the air distribution manifold  40 , through the air distribution pipe  48 , and into the associated supply pipe  47 . 
     Referring back to FIGS. 1 and 4, the fuel distribution manifold  41  is connected to the same plurality of supply pipes  47  by a corresponding number of fuel distribution pipes  52 . Each fuel distribution pipe  52  extends between the fuel distribution manifold  41  and one of the supply pipes  47  spaced around the outer circumference of the outer shell  42  of the rotary kiln  22 . Each fuel distribution pipe  52  includes its own fuel valve  53  that can be selectively opened or closed. When the fuel valve  53  is closed, the fuel valve  53  prevents the flow of fuel from the fuel distribution manifold  41  into the open interior of the rotary kiln  22  through the associated row of ports  49 . When the fuel valve  53  for each fuel distribution pipe  52  is opened, fuel can flow from the fuel distribution manifold  41 , through the fuel distribution pipe  52 , and into the open interior of the rotary kiln through the associated supply pipe  47  and ports  49 . As can be understood in FIGS. 1 and 4, the fuel distribution pipes  52  and air distribution pipes  48  are positioned adjacent to each other and are connected to the same supply pipes  47 . In this manner, the plurality of supply pipes  47  and corresponding rows of ports  49  provide a passageway for both fuel and air into the open interior  50  of the rotary kiln  22 . 
     In the preferred embodiment of the invention and for this specific oxidation process, the combined length of the port system is approximately ½ of the overall length of the rotary kiln  22 . As shown in FIG. 1, each of the supply pipes  47  terminates toward the infeed end  24  of the rotary kiln  22 . 
     Referring now to FIG. 6, as the rotary kiln  22  rotates in the direction shown by arrow  55 , the air valve  51  for each air distribution pipe  48  is opened to allow oxidizing gas to flow through the air distribution pipes  48  and associated supply pipes  47  including the ports  49  when the associated supply pipe  47  is beneath the product stream  56  contained within the rotary kiln, as indicated by the air flow arc  57 . In the preferred embodiment of the invention, each air valve  51  is opened to allow the passage of oxidizing gas when the supply pipe  47  nears the six o&#39;clock position and the air valve  51  is closed when the supply pipe  47  is between the three o&#39;clock and the six o&#39;clock position during rotation of the rotary kiln  22 . The resultant air flow arc  57  illustrated in FIG. 6 ensures that the oxidizing gas is introduced beneath the product stream such that the oxidizing gas flows radially upward through the tumbling bed to improve the contact between the oxidizing gas and the tumbling pellets in the product stream and to cause oxidation to occur. 
     Since oxidation of the iron ore pellets is an exothermic reaction, the simple introduction of the oxidizing gas into the tumbling product stream toward the infeed end  24  of the rotary kiln  22  results in an increase in the temperature within the rotary kiln  22 . This elevation in the temperature within the rotary kiln decreases the amount of heat required from the centerline burner  34  to drive the oxidizing reaction and complete pellet induration. This decrease in fuel causes a decrease in the production of NO x . 
     As the rotary kiln  22  continues to rotate in the direction shown by arrow  55 , the fuel valve  53  for each fuel distribution pipe  52  is opened to allow fuel to flow through the fuel distribution pipe  52  and associated supply pipe  47  including the ports  49  when the supply pipe  47  passes through a fuel flow arc of rotation  60 . As the ports  49  pass through the fuel flow arc  60 , the fuel is introduced into the kiln&#39;s overbed space. 
     In addition to the introduction the oxidizing gas directly into the product stream  56  during the air flow arc  57 , each of the air valves  51  is opened a second time to allow air from the air distribution manifold  40  to be introduced into the supply pipes  47  and pass through the ports  49  during rotation of the kiln  22  through a flow arc  62 . The air introduced through the ports  49  during rotation through the second flow arc  62  is introduced above the bed of pellets such that the air mixes with the fuel that was introduced during the fuel flow arc  60 . The fuel introduced through the ports  49  mixes with the oxygen contained in the open interior  50  and combusts within heated open interior  50 . The combustion of the fuel within the open interior  50  further increases the temperature toward the infeed end of the rotary kiln  22 , as illustrated by the temperature profile line  61  in the graph of FIG.  3 . 
     As the temperature profile line  61  in FIG. 3 illustrates, the temperature of the solids within the product stream rapidly increases toward the infeed end of the rotary kiln  22  as compared to the prior art system including only the centerline burner  34 . Because of the increase in temperature near the infeed end  24  due to both the introduction of the oxidizing gas beneath the product bed and the introduction of fuel and air above the product bed, the intensity of the flame  36  of the centerline burner  34  can be reduced while the temperature within the kiln remains at approximately 2400° F. The reduction in the flame intensity of the centerline burner  34  decreases the amount of NO x  produced within the rotary kiln  20  and also reduces the amount of fuel consumed by the centerline burner  34 . The fuel savings from the centerline burner  34  can be diverted to the ports  52  and introduced into the rotary kiln along the fuel flow arc  60 . In this manner, the total fuel consumption for the entire system remains generally the same or can be slightly reduced. 
     Although not shown in the Figures or described in the above description, appropriate control mechanisms are provided around the outer circumference of the rotary kiln  22  to open and close the air valves  51  and the fuel valves  53  so that the oxidizing gas and fuel are injected when the ports  49  are in the desired positions as indicated by the air flow and fuel flow arcs of FIG.  6 . In a proposed embodiment of the invention, each of the air valves  51  and fuel valves  53  can be a conventional tripper valve whose opening and closing is controlled by a conventional tripper mechanism. Such a system, including the valves and nozzles, is disclosed in U.S. Pat. No. 3,946,949. Conventional tripper valves and tripper mechanisms are well known in the industry and their details will not be discussed in the present disclosure. 
     As can be understood by the foregoing description, the introduction of air and fuel above the product stream toward the infeed end of the rotary kiln increases the temperature within the kiln. The increase in temperature toward the infeed end of the rotary kiln allows the centerline burner  34  to maintain the product stream at a desired temperature by using a centerline flame  36  having a decreased intensity. Although the centerline flame  36  still produces some NO x , the amount of NO x  produced is reduced without any adverse affects in the process of iron oxide pellet induration. 
     Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.