METHOD OF OPERATING MOVABLE HEARTH FURNACE

Abstract: With a solid fine reducing material containing hydrogen placed as a lower layer on a movable hearth, a powdered mixture of an oxide powder dominantly of a fine iron ore and a solid fine reducing material is stacked in layered arrangement, and subsequent reduction operation is performed.

Description: The present invention relates to a method of operating a movable hearth furnace, comprising the steps of introducing a powdered mixture of oxide powder dominantly of a fine iron ore and a solid fine reducing material on a horizontally movable hearth and subjecting said oxide powder to reduction by means of radiant heat transfer resulting from an upper portion within said furnace. Such a method is known from US-A-1 452 627. The production of crude steel is carried out by those methods broadly classified into a blast furnace-converter process and an electric furnace process. Of such methods, the electric furnace process produces steel by melting scrap or reduced iron used as a feed stock through the action of electric energy and, where needed, by further refining the melt. The scrap is now dominant as the feed stock, but the reduced iron has of late been in growing demand to compensate for shortage of the scrap and also to meet the trend of steel product of high quality obtainable by the electric furnace method. A certain process for the production of reduced iron is known as disclosed in Japanese Unexamined Patent Publication No. 63-108188. This process is contrived to stack an iron ore and a solid reducing material in layered formation on a horizontally rotary hearth, followed by reduction of the iron ore with heat applied from above by means of radiant heat transfer, whereby a reduced iron is provided. Such known process is advantageous in respect of possible construction of equipment at rather a low cost, less frequent trouble during operation and the like. In many instances, a horizontally movable hearth takes the form of a rotary hearth as seen in FIG. 1A and FIG.1B of the accompanying drawings, the two figures being taken to explain the structural details of such rotary hearth. On a movable (rotary) hearth 3 is stacked a layer 1 as a powdered mixture of an iron ore (oxide powder mainly of a particulate iron ore) and a solid reducing material. The movable hearth 3 is covered by a furnace body 4 lined with a refractory material, and a burner 5 is located upstream of the furnace body. By use of the burner as a source of heat, the iron ore put in place on the movable hearth is subjected to reduction. In FIG. 1A, reference numeral 6 refers to a charging unit of feed stocks, and 7 to a discharge unit. In the operation of the foregoing type of furnace, the internal furnace temperature is usually set at around 1300°C. Furthermore, after the reduction is completed, the finished reduced iron is generally cooled on the rotary hearth with the use of a cooling unit and thereafter taken out of the furnace. This prevents such reduced iron from becoming oxidized outside the furnace and also renders the same easy to handle. The reduction taking place in such method of the production of reduced iron proceeds as a direct reduction between an iron ore and a solid reducing material. The direct reduction is an endothermic reaction, and the supply of heat and the rate of reduction are the determinants of ultimate productivity. The heat supply is conducted, the layer composed of an iron ore and a solid reducing material, by means of radiant heat transfer derived from a burner flame or from an inner wall of the furnace by the use of a burner as a source of heat. With the heat thus supplied, the reduction rate is decided by the thermal conductivity within the layer composed of an iron ore and a solid reducing material and further by the difference in temperature between a reaction temperature and a furnace temperature. Document US-A-3 383 199 relates to a treatment of iron oxide values such as iron ores, and more particularly to production from such ores of hard, porous, high strength, shock-resistant pre-shaped materials. The method disclosed in US-A-3 383 199 features the continuous production at high rates of a highly reactive combined feed material containing ore values and coke for reduction furnace operation, such as in blast furnace for production of iron. It has been found accordind to the teaching of US-A-3 383 199 that a combined feed material containing oxidic ore values and coke and suitable for reduction furnace operations such as blast furnace may be produced and in a highly efficient manner by subjecting a moving mass of shaped bodies containing oxidic ore and coking-coal to hot combustion gases under coking conditions of a hydrocarbon liquid capable of slow and continuous gasification at the surface of the shaped bodies during coking. The method is realized in that a lower layer of solid fuel is deposited on a moving bed. The solid fuel that is been used may be any solid carbonaceous fuel, such as bituminous coal, steam coal, anthracite, subbituminous coal, lignite coal and coke breeze, but it is also stated that in the more preferred forms of practice steam coal is employed as the solid fuel. Then after the solid fuel has been deposited shapes of ore and coking coal are deposited uniformly in controlled amounts on the underlayer of solid fuel. Before the deposition of the coking material the underlying solid fuel is ignited by direct flame impingement by an ignitor closely positioned over the moving bed of solid fuel. The combustion of the solid fuel rise upwardly through the ore-coal shapes to effect coking. Thus as stated in US-A-3 383 199 an important feature of the method is that the provision of preformed bodies or shapes in which the oxide ore and coking coal are intimately admixed with a hydrocarbon liquid capable of gasification at the surface. It is also stated that the direct and uniform ignition of the upper portion of the bed of solid fuel is important in order to obtain good results. In US-A-1 452 627 iron ore is mixed with a solid fine reducing material such as carbonaceous material. In order to obtain finely divided carbonaceous material one could either crush or powder coal Furthermore, it is desirable that the iron ore shall be in finely divided condition of e.g. 30 mesh or lower. Once the iron ore and the solid fine reducing material are in a desirable condition, they are mixed and thereafter deposited on the rotary hearth. Eventually the ore is brought to reduction by means of radiant heat transfer from an upper portion within said furnace. The application of heat by radiation from a surface which is not in contact with the ore is an important feature. Since the heat is radiated from a source of heat from above the hearth reduction is carried out in a relatively quiescent reducing atmosphere and without direct contact with the surface applying the heat. It is an objective of the invention to improve the method known from-US-A-1 452 627 by shortening the reduction time thereby improving productivity. A further objective is to lower the temperature at which the reduction reaction is initiated of the known process. As a solution to the objective the present invention provides a method as defined in claim 1. Preferred embodiments of the invention are defined in claims 2 and 3. By the auxiliary feed stock used herein is meant a fluxing agent such as limestone, fluorite, serpentine and dolomite. Brief Description of the Drawings FIG. 1A and 1B are views explanatory of a rotary hearth furnace, FIG. 1A showing the whole furnace, and FIG.1B showing the furnace in cross section as taken along a line 1B-1B. FIG. 2 is a view explanatory of an apparatus used for experimentation. FIG. 3 graphically represents the relationship between the content of H 2 in a carbon material and the temperature at which to initiate reduction. FIG. 4 is a view explanatory of the manner in which feed stocks are stacked in layered arrangement for acceptance by the present invention. FIG. 5 are graphs showing the relationship between the metallizing ratio and the lapse of time as regards acceptable and comparative examples and also the relationship between the concentration of hydrogen in a layer of a mixture of an oxide and a carbon material and the lapse of time as regards acceptable and comparative examples. [Explanation of Reference Numerals] 1 layer of powdered mixture of iron ore (oxide) and reducing material 2 solid fine reducing material containing hydrogen 3 movable (rotary) hearth 4 furnace body 5 burner 6 layup unit of feed stocks 7 discharge unit 8 annular furnace 9 furnace core tube for gas seal 10 grating for hydrogen introduction 11 heating medium 12 introduction of hydrogen 13 discharging of gas Mode of Carrying Out the Invention When a powdered mixture of an oxide powder composed dominantly of a fine iron ore with a solid fine reducing material is heated from outside, a reduction reaction proceeds with those reactions continuously initiated as represented by the formulae (1) and (2) below: CO + FeO -- Fe + CO 2 (adjacent to oxide) CO 2 + C -- 2CO (adjacent to reducing material) The reduction reaction of the formula (1) proceeds generally at a temperature of 600°C or above, whereas the reaction of the formula (2) fails to takes place unless the temperature elevates at a higher level. Additionally, it is required that, for both of the reactions to continuously proceed, CO 2 formed in the formula (1) be smoothly diffused in the solid fine reducing material, and CO formed in the formula (2) be smoothly diffused in the oxide powder. In the case of use of an iron ore and a solid reducing material which are commonly employed, a temperature of about 1100°C is a reaction-initiating temperature expected to show a rate of reaction suited for practical application. In the movable hearth furnace according to the present invention, the heat supply is conducted, with respect to a layer composed of an iron ore and a solid reducing material, by means of radiant heat transfer derived from a burner flame or from an inner wall of the furnace with use of a burner as a source of heat. Along with the heat supplied in that way, the reduction rate is determinable by the thermal conductivity within the layer composed of an iron ore and a solid reducing material and further by the difference in temperature between a reaction temperature and a furnace temperature. Thus, the reduction reaction proceeds by taking advantage, as a driving force of heat transfer, of the difference in temperature between a reaction temperature of 1100°C and a furnace temperature of 1300°C. On the other hand, hydrogen takes part in the rate of reaction. With particular attention paid to the fact that the reactions represented by the following formulae (3) and (4) are allowed to proceed at a higher rate than at that of the formulae (1) and (2), experiments were conducted with use of an experimental apparatus shown in FIG. 2. H 2 + FeO -- Fe + H 2 O (adjacent to oxide) H 2 O + C -- CO + H 2 (adjacent to reducing material) In FIG. 2, reference numeral 1 refers to a powdered mixture of an oxide powder dominantly of a fine iron ore and a solid fine reducing material, 8 to an annular furnace, 9 to a furnace core tube for gas seal, 10 to a grating for introducing hydrogen therethrough, 11 to a heating medium, 12 to introduction of hydrogen and 13 to discharging of gas. Into the annular furnace maintained at a given temperature was charged a powdered mixture of an oxide powder and a solid reducing material, and hydrogen was caused to flow in a small amount into the furnace from a lower portion of the latter. On the assumption that the solid reducing material is mixed in an amount of 600 kg based on 1000 kg of iron, the hydrogen was introduced into the furnace at from its lower portion in such an amount as being regarded as equivalent to that generated from the solid reducing material, namely the carbon material. Examination was made of reduction-initiating temperatures in the above manner. The experimental results are given in FIG. 3 as a graphical representation of the relationship between the content of H 2 in a carbon material and the temperature at which to initiate reduction. As is evident from FIG. 3, increased content of hydrogen in a carbon material contributes greatly to lowered reduction-initiating temperature at which a practically applicable rate of reduction is attainable. A drop in reduction-initiating temperature, say about 100°C, can be expected to be gained with an amount of about 0.5 % by weight of hydrogen. Hence, the difference between a reaction temperature (reduction-initiating temperature) and a furnace temperature can be increased from a conventionally known level of 200°C to as high as 300°C so that both the rate of reaction and the rate of heat transfer are raised, respectively, to 1.5 times on the average. Those beneficial effects are thought to accrue from two aspects set forth below. Firstiy, hydrogen itself leads to improved rate of reaction in the formulae (3) and (4), particularly in the formula (4). Next, the diffusion rates of H 2 and H 2 O are as high as 3.7 times and 1.6 times, respectively, as compared to those of CO and CO 2 so that the concentration of H 2 O on a surface of the carbon material becomes greater with a higher rate of reaction in the formula (4). In the layer of a powdered mixture of an oxide powder and a solid reducing material, a reaction of gas shift according to the formula (5) below occurs in addition to the reactions of the formulae (1) to (4). The presence of hydrogen in a small amount, therefore, produces improved rate of reaction as a whole. H 2 O + CO -- H 2 + CO 2 The rate of reaction is improved in the presence of hydrogen as mentioned above. However, since the resultant effect gets saturated at a concentration of hydrogen of more than 0.5% by weight in a carbon material as shown in FIG. 3 above, the concentration of hydrogen in a carbon material should be set to be not less than 0.5 % by weight. With regard to those methods used to stack feed stock, the present inventors have done extensive research in light of the results obtained from the above fundamental experiments. As a consequence, the inventors have now found a noticeably effective stack method. That is, as viewed in FIG. 4 wherein a stack method of feed stock suited for the present invention is explained, a solid fine reducing material 2 containing hydrogen is firstly placed in a given thickness on a refractory movable hearth furnace, and a powdered mixture 1 of an oxide powder and a solid fine reducing material is then stacked in a layered posture on the first-laid reducing material. Reduction is then caused to proceed by means of radiant heat transfer applied from above. In the reduction operation of the feed stock thus stacked in layered arrangement, heat-up proceeds in a relatively short length of time up to a temperature at which an endothermic reaction initiates, and upon initiation of such reaction, the heat-up temporarily stop. A H atom contained in the reducing material is in most cases held in covalently bonded relation to a C atom, and an elimination reaction between the two atoms occurs at a temperature of 800°C to 900°C. In the case where a hydrogen-containing reducing material is stacked beneath a powdered mixture of an oxide powder and a solid reducing material, the temperature of a lower layer reaches 800°C to 900°C at the time the temperature of an upper layer elevates at 1000°C to 1100°C, whereupon the elimination reaction starts. Hydrogen generating in a lower layer moves toward an upper layer with eventual improvement in reaction rate in line with the reactions of the formulae (3) and (4). As contrasted, according to a method in which a reducing material containing hydrogen is mixed with an oxide, the hydrogen becomes generated at the beginning of a heat-up stage. Because most of the hydrogen has already been discharged outside the mixture when the heat-up arrives at a temperature at which to initiate reduction, no advantage can be expected of gaining promoted reduction. Thus, as means for enhancing the rate of reaction with use of a hydrogen-containing reducing material, a specific method of the operation of a movable hearth furnace is conspicuously effective in reducing an oxide. In this specific method, when a layer composed of an oxide powder and a solid fine reducing material is stacked on a horizontally movable hearth, a layer of a solid fine reducing material containing hydrogen is placed as a lower layer on the hearth, and a powdered mixture of a fine iron ore and a solid fine reducing material, or a powdered mixture of a fine iron ore, additives and a solid fine reducing material is then superposed on the lower layer. The reducing material as the lower layer contains hydrogen in an amount of not less than 0.5% by weight. Here, the additives denotes a fluxing agent such as limestone, fluorite, serpentine and dolomite. EXAMPLES By the use of a rotary hearth furnace shown in FIG. 1 above and constructed with a roraty hearth of 2.2 m in diameter, a burner located upstream of the hearth and a furnace body disposed to wholly cover the associated parts, the following experimental operation was performed. The movable hearth 3 of this rotary hearth furnace is lined on its outer surface with a refractory material of an alumina type. A cooling unit is placed before discharging the products so as to cool the products, and the products are discharged using a screw type discharge unit 7. A fine iron ore and a solid fine reducing material are arranged at a supply port as seen in FIG. 4 above such that a layer 2 of a solid fine reducing material and a layer 1 of a mixture of a fine iron ore and a solid fine reducing material are stacked in two-layered formation, and subsequent reduction is effected in the furnace. Here, both the fine iron ore and the solid fine reducing material were adjusted, prior to use, to pass through a screen opening of less than 3 mm. The furnace temperature in a reduction region was maintained at 1300°C by means of controlled combustion of the burner. In Table 1, the results related to reduction times and metallizing ratios are tabulated which were obtained when the reduction operation was carried out with varying kinds and amounts of the fine reducing materials, i.e., carbon materials, and with varying stack of feed stock. As shown in Table 1, use was made of three kinds of carbon materials of different hydrogen contents such as coal (hydrogen 4%), mild char (hydrogen 2%) and coke (hydrogen 0.3%), and the carbon material was mixed in an amount of 500 to 600 kg/t with an oxide, which amount was required for reduction to proceed. The reduction operation was conducted with varied amount of the carbon material for use as the lower layer. As is clear from Table 1, Comparative Example 1 in which coal was uniformly mixed and Comparative Example 2 in which hydrogen-free coke was employed as a lower layer were found to be beyond 92% in reduction ratio with 25 minutes and 28 minutes in reduction time, respectively. In contrast, Acceptable Examples 1 to 3 using coal as a lower layer and falling within the scope of the present invention can exhibit a metallizing ratio of more than 92% with a reduction time of 18 minutes to 16 minutes. Moreover, an increased amount of the carbon material in the lower layer is capable, though slightly, of giving shortened reduction time. Mild char needs no quantity of heat as in the decomposition of coal, and hence, hydrogen has proved to exert more noticeable effects. As regards two examples using typical operation conditions, FIG. 5 represents graphs as to the relationship between the metallizing ratio and the lapse of time and as to the relationship between the concentration of hydrogen gas in a mixed layer of an oxide and a carbon material and the lapse of time. In Comparative Example 1, hydrogen starts discharging at the very beginning of the reaction and becomes completely discharged even with a maximum hydrogen concentration up to 18% before a reduction reaction initiates. A reduction time of about 25 minutes is thus needed in achieving a metallizing ratio of higher than 92%. On the other hand, Acceptable Example 2 reveals a period of time for hydrogen discharging in the shape of a rearwardly gentle curve. This means that a metallizing ratio of not less than 92% is attainable with a reduction time of about 17 minutes for the reasons of promoted reduction with hydrogen, lowered temperature at which to initiate reduction, substantially increased amount of heat transfer and the like. Industrial Applicability According to the present invention, a layer composed of a mixture of a fine iron-ore and a reducing material and a layer of a reducing material containing at least 0.5 wt% hydrogen placed therebeneath are stacked on a movable hearth, whereby the fine iron ore is reduced in a movable hearth furnace. The invention leads to a sharp shortening in reduction time, thus contributing greatly to improved productivity or saved equipment cost of the reduction furnace.

Claims: A method of operating a movable hearth furnace, comprising the steps of: Supplying a powdered mixture (1) of an oxide powder dominantly of a fine iron ore and a solid fine reducing material on a horizontally movable hearth (3); and subjecting said oxide powder to reduction by means of radiant heat transfer resulting from an upper portion within said furnace, characterized in that said powdered mixture (1) is stacked in layered arrangement on a layer of a solid fine reducing material (2) containing at least 0.5 wt% of hydrogen. The method according to claim 1, characterized in that the powdered mixture (1) contains fine additives. The method according to claim 1 or 2, wherein said movable hearth is a rotary hearth (3).

Inventors: SAWA YOSHITAKA, TAKEDA KANJI, SAWA, YOSHITAKA, TAKEDA, KANJI, SAWA, YOSHITAKA-TECHNICAL RESEARCH LABORATORIES, TAKEDA, KANJI-TECHNICAL RESEARCH LABORATORIES
Applicants: JFE STEEL CORP, JFE STEEL CORPORATION
Application year: 1998
Main classification: C21B13
IPC codes: F27B9, C21B13