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.sub.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.sub.3 O.sub.4) phase of iron begins to transform into hematite (Fe.sub.2 O.sub.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.degree. 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 '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 '330 patent has proved to be successful, a problem exists in the amount of NO.sub.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.sub.x, which is considered to be an environmental pollutant. The production of NO.sub.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.sub.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.sub.x production.