METHOD FOR MELT-REDUCING IRON ORE

Abstract: Improvements in a process of so-called direct iron making are suggested. Particularly a method for preventing clustering in a preliminary reducing furnace and attaining an improvement of the reduction efficiency in the preliminary reducing furnace are described.

Description: The present invention relates to an improvement in process of a so-called direct iron making process. More particularly, it is concerned with a method for preventing clustering in a preliminary reducing furnace and attaining an improvement of the reduction efficiency in the preliminary reducing furnace. As the technique for manufacturing pig iron without using a blast furnace, a so-called direct iron making process is being spotlighted, and there have been developed KR method, COIN method, Kawa-Tetsu method and Sumi-Kin method as small-scale iron manufacturing techniques. These methods require both a preliminary reducing furnace and a melt-reducing furnace. As examples of the preliminary reducing furnace, a shaft furnace and a fluidized bed furnace have been considered. The principle of the direct iron making process is as illustrated in Fig. 2, in which a high-­ temperature gas G₁ evolved in a melt-reducing furnace 2 and containing CO as a main component (also containing CO₂, H₂ and H₂O) is fed as both a heat source and a reducing agent to a preliminary reducing furnace 1, while pig iron is placed beforehand in the melt-reducing furnace 2 and held in a molten state, then carbon material and oxygen gas (or air) are introduced therein to effect the supply of heat source and reducing gas, and iron ore (hereinafter referred to as "preliminary reduced iron C") which has been rduced partially in the preliminary reducing furance 1 is charged into the melt-­ reducing furnace, allowing reduction to be finally completed therein, then the product obtained is taken out as pig iron F. In the method of the present invention, carbon material and oxygen gas are introduced into molten iron heated temperature exceeding 1,500°C, so if it is introduced directly into the preliminary reducing furnace 1, there will occur fusing (clustering phenomenon) of iron ore pieces with each other irrespective of whether the preliminary reducing furnace is a shaft furnace type or a fluidized bed type, resulting in that the discharge from the preliminary reducing furnace becomes difficult. In an effort to remedy this drawback, the present inventors have complete a technique of charging hydrocarbons (from any source, e.g. petroleum or coal source) into the preliminary reducing furnace 1 and coating iron ore (Or in Fig.1) in the same furance with carbon as indicated by broken line in Fig. 1, and applied for patent (Japanese Patent Application Nos. 163950/84 and 191330/84). As to the carbon coating method, its details are explained in the specifications of the above prior applications, but typical means are as exemplified below: (1) Method in which a hydrocarbon material such as heavy oil, pitch or asphalt obtained in the course of petroleum refining, or a hydrocarbon material such as coal tar or pitch obtained from coal, is sprayed to iron ore and decomposed thermally at 400-600°C to deposit coke on the surface of the iron ore. (2) Method in which the above hydrocarbon material and iron ore are mixed and heated, allowing the iron ore surface to be coated with coke formed by thermal decomposition. However, the carbon coating method is not limited to the above means. Various means are adoptable. On the other hand, in the melt-reducing furnace, as means which meets the desire that the amount of coal used be decreased, there is known a means (so-­ called post combustion) in which combustion is allowed to take place just after the evolution of CO and H₂ in the foregoing high-temperature gas G₁, that is, combustion is allowed to proceed on the surface of pig iron bath, and the recovery of heat is effected. This means is significant in that its own purpose is attained. But as a result of the above combustion, the concentration of CO₂ and H₂O is increased and that of CO and H₂ decreased by the following reactions: 2CO + O₂ 2CO₂ .... (1) 2H₂ + O₂ 2H₂O .... (2) Consequently, the reducibility as the gas G₁ is deteriorated and it is inevitably required to increase the feed into the preliminary reducing furnace. However, the temperature of the gas G₁ is at a further raised level because the above combustion is an exothermic reaction, and so the foregoing clustering phenomenon is more likely to occur. Consequently, the operating temperature in the preliminary reducing furnace is limited from the standpoint of prevention of clustering, and in a shaft furnace using pellets or lump ore it is generally considered that the upper limit of the operating temperature is 830°C or so. In a fluidized bed furnace, a temperature of 780°C or so is considered to be an upper lilmit. Therefore, reducing the temperature of the gas G₁ to be fed to the preliminary reducing furnace, by the use of a cooler, and the resulting lowering of the preliminary reduction efficiency, are unavoidable. Under the circumstances, the carbon coating for the iron ore in the preliminary reducing furnace is becoming more and more important, and now it is considered to be an essential means in the direct iron making process, aside from whether the coating step is to be applied before charging into the preliminary reducing furnace or within the same furnace. In the inventions of the foregoing prior applications, however, no reference is made to other techniques than carbon coating, nor is given any consideration about the utilization of gas evolved in the coating step. According to the present invention, which has been accomplished in view of the above circumstances, there is provided a method capable of not only preventing the clustering phenomenon in the preliminary reducing furnace but also effectively utilizing the gas evolved by the carbon coating and capable of attaining the cooling of the high-temperature gas G₁ and improvement of the reducibility. The gist of the present invention resides in that a preliminary reduction is performed in a coated state of the iron ore surface with carbon, and H₂ and hydrocarbon gas evolved in the coating treatment are fed to the preliminary reducing furnace to accelerate the reduction rate. According to our finding, the carbon coating on the starting iron ore not only prevents the fusing of iron ore pieces with each other, but also reacts partially with CO₂ and H₂O incorporated in the high-­ temperature gas G₁ in the following manner; thereby enhancing the reproducibility of the gas G₁: C + CO₂ → 2CO .... (3) C + H₂O → CO + H₂ .... (4) On the other hand, in the step of coating the iron ore surface with carbon, there is adopted a means of heating petroleum or coat hydrocarbons and coating the iron ore surface therewith, so the evolution of cracked gas G₂ has been presumed. As a result of experiment on one example thereof, there were obtained such results as shown in Table 1. The gas temperature was about 400°C. The following became clear. The composition and proportion as well as the total amount of gas evolved differ depending on the hydrocarbon material used for the coating, and particularly the total amount of gas evolved differs depending on the coating temperature. Generally, the proportion of the gas evolved was found to be about 10 to 40% by weight based on the amount of the hydrocarbon material used. On the other hand, mention will now be made of the CO-predominating high-temperature gas G₁. As the carbon material to be introduced into the melt-reducing furnace there generally employed coal or coke, and its amount to be introduced in almost determined by the amount of unreduced iron ore contained in the preliminary reduced iron C which is brought into the melt-reducing furnace; usually it is 350 to 750 kg per ton of molten iron. The carbon material is gasified in the melt-reducing furnace, whereby the CO-predominating high-temperature gas G₁ is evolved. The composition, proportion and total amount of the gas G₁ differ depending on the composition and amount of the carbon material and various other conditions, but a typical composition is as set out in Table 2. The evolved gas G₁ is discharged at a high temperature above 1,500°C, so by incorporating the cracked gas G₂ (about 400°C) therein, a cooling is effected to a gas temperature suitable for the preliminary reduction and not causing clustering. Thus, the heat loss as in the prior art in which heat is released to the exterior by taking the trouble to use a cooler. In the composition of the cracked gas G₂, the other components than H₂ shown in Table 1, as they are, will not exhibit any function as a reducing agent. But is there is adopted a means for mixing the evolved gas G₁ and the cracked gas G₂ as mentioned above, decompositions will take place as represented by the following chemical reactions, whereby the formation of CO and H₂ having a strong reducing function is expected: CH₄ + CO₂ →2CO + 2H₂ .... (5) CH₄ + H₂O →CO + 3H₂ .... (6) The oxygen fed from the iron ore side also contributes to these decomposition reactions. Moreover, it is generally known that the above decomposition reactions efficiently in the presence of metal iron as catalyst. In this connection, metal iron comes to be present in the preliminary reducing furnace as the reduction proceeds partially, and so also in this point it is extremely effective to perform the above reactions in the preliminary reducing furnace. If there is adopted such means as illustrated in Fig. 1 in which a carbon coating apparatus 3 for iron ore is disposed before the preliminary reducing furnace 1 and the cracked gas G₂ from the coating apparatus 3 is mixed with the evolved gas G₁, followed by introducing into a lower portion of the prelimiary reducing furnace 1, then the above decomposition reactions (5) and (6) will be completed beforehand because the foregoing reduced iron is present at the bottom of the preliminary reducing furnace 1, and consequently the gaseous mixture is fed to the furnace 1 after control of reducing agent component and temperature. This is more convenient. Thus, by mixing the cracked gas G₂ with the evolved gas G₁, there is fed to the preliminary reducing furnace a reducing gas having a high reducing component concentration and a temperature of a level not causing clustering (and suitable for the reducing reaction). Consequently, a great contribution can be made to the improvement in efficiency of the preliminary reduction and the prevention of clustering. Further, the improvement in efficiency of the preliminary reduction means contributing to the reduciton in load of the melt-­ reducing furnace, and thus it becomes possible to decrease the amount of carbon material, etc. to be supplied to the melt-reducing furnace. This is convenient also from the economic point of view. Besides, as shown in the decomposition reaction formulae (5) and (6), CO₂ and H₂O in the evolved gas G₁ are converted to reducing components and thereby utilized effectively. Even in the event the reaction of the formula (5) does not proceed sufficiently and a portion of CO₂ remains unreacted and in this state enters the preliminary reducing furnace, the carbon as coating over the iron ore is converted to CO as shown below and contributes to the improvement of reducibility: CO₂ + C → 2CO .... (7) [Example] Fig. 1 is a diagram showing a typical example of the present invention, in which the same functional portions as in Fig. 2 are indicated by the same reference numerals. Iron ore Or and hydrocarbon are introduced into the carbon coating apparatus 3 and the iron ore coated with carbon is fed to the preliminary reducing furnace 1. On the other hand, the cracked gas G₂ as a discharged gas from the carbon coating apparatus 3 is joined with the evolved gas G₁ flowing toward the preliminary reducing furnace 1, whereby the decomposition reactions are allowed to take place to enhance reducibility and lower the temperature to an extent not causing fusing, before introducing into the preliminary reducing furnace 1. The following are charge and production on the gas side in the above process. Charge of iron ore: 1.43 ton Charge of coal tar (carbon coating apparatus): 100 kg/t pig iron Charge of carbon material (melt-reducing furnace): 640 kg/t pig iron Feed of oxygen (- ditto -): 430 Nm³/t pig iron Production of pig iron: 1 ton Gas G₁ (the composition of Table 2): 1,200 kg/t pig iron Gas composition at the lower portion of the preliminary reducing furnace: Table 3 [Effect of the Invention] According to the present invention constructed as above, since the iron ore surface is coated with carbon, the clustering based on melting of the iron ore surface in the preliminary reducing furnace is prevented, and since there is adopted means which effectively utilizes the cracked gas resulting from carbon deposition on the iron ore, it is possible to effectively lower the temperature of the evolved gas from the melt-reducing furnace and contribute to the improvement of reducibility. Consequently, the preliminary reduction efficiency in the preliminary reducing furnace is improved. Further, as a result thereof, it became possible to decrease the amount of carbon material, etc. to be fed to the melt-reducing furnace and make a great contribution to the reduction of cost required in the direct iron making process. 4. Brief Description of the Drawings Fig. 1 is a conceptual diagram showing an example of equipment for practising the method of the present invention, and Fig. 2 is a conceptual diagram of a conventional equipment. 1 ... preliminary reducing furnace 2 ... melt-reducing furnace 3 ... carbon coating apparatus

Claims: 1. A method for melt-reducing iron ore by subjecting the iron ore in a solid state to a preliminary reduction in a preliminary reducing furnace and then subjecting it to a melt reduction in a melt-­ reducing furnace, characterized in that the surface of the iron ore is coated with carbon in a coating apparatus and is fed to preliminary reduction, and H₂ gas and hydrocarbon gas evolved during said coating treatment are fed to said preliminary reducing furnace to accelerate the reduction rate. 2. A method according to claim 1, wherein a hydrocarbon material is sprayed to or mixed with the iron ore and then decomposed thermally to coat the surface of the iron ore with carbon and in this state the preliminary reduction is performed. 3. A method according to claim 1, wherein H₂ gas and hydrocarbon gas evolved during the coating of the surface of the iron ore with carbon are mixed with a high-temperature gas evolved from the melt-reducing furnace and the resulting gaseous mixture is introduced into the preliminary reducing furnace from a lower part of the same furnace.

Inventors: IMANISHI, NOBUYUKI, NISHIDA, REIJIRO, SEKI, YOSHIKAZU, WATANABE, RYO
Applicants: KABUSHIKI KAISHA KOBE SEIKO SHO
Application year: 1986
Main classification: C21B13
IPC codes: C21B11, C21B13