JP2011074438A - Method for producing reduced iron with moving type hearth furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing reduced iron with which carbon content in the reduced iron can be controlled and also, the melting in a refining furnace is easy. <P>SOLUTION: The production of reduced iron with a moving type hearth furnace, is performed as the followings, that is, the iron-containing oxide is reduced by loading mixed raw material containing the iron-containing oxide, carbonaceous solid reducing material and slag-making material on the moving furnace hearth and heating, to reduce the iron-containing oxide, and also, when the reduced iron is produced by melting and introducing the separation of iron and slag, the reduced iron is produced by using the blended material so that the relation of the average grain diameter of the iron-containing oxide and the carbonaceous material ratio shown as the ratio between the oxygen concentration of the iron to be reduced and the carbon concentration in the mixed raw material, becomes 0.5 to 1.6. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  In the present invention, the raw material on the hearth moving in the heating furnace is heated, and the raw material is heated and reduced in the process of moving the hearth in the furnace. The present invention relates to a method for producing reduced iron from an oxide.

  Typical methods for producing crude steel include a method using a blast furnace-converter and a method using an electric furnace method. Among these methods, the electric furnace method is a method in which scrap or reduced iron is used as an iron raw material, and these are heated and melted by electric energy and, depending on the case, further refined into steel. At present, scrap is the main raw material. However, in recent years, demand for scrap is becoming tighter and products are expected to be upgraded by the electric furnace method, so the use of reduced iron instead of scrap is being considered.

  As a technique for producing reduced iron typified by a reduced metal, a method as disclosed in Patent Document 1 is known. As this known method, iron ore and a solid reducing material are loaded on a moving hearth moving in the horizontal direction in the heating furnace, and iron ore is reduced by heating by radiant heat from above, and then the reduction generation is performed. There is a well-known mobile hearth furnace method in which an object is melted on the hearth to separate slag and metal into reduced iron.

  In general, a mobile hearth furnace usually takes the form of a rotational movement that moves on an annular track, and therefore this type of mobile hearth furnace is called a rotary hearth furnace. FIG. 1 shows an example of a typical rotary hearth furnace, which has a heating furnace body 10 partitioned into a pre-tropical zone 10a, a reduction zone 10b, a melting zone 10c, and a cooling zone 10d. The moving hearth 11 has a structure arranged so as to be rotatable. On the moving hearth 11, for example, a mixed raw material 12 made of iron ore and a solid reducing material is loaded. As the mixed raw material 12, in addition to powder, a lump of carbonaceous material interior pellets or the like is used. Here, the mobile hearth 11 is covered with a furnace body 10 lined with a refractory, but as disclosed in Patent Document 1, in order to protect the hearth refractory, the raw material layer is further protected. In some cases, a carbon material, that is, a solid reducing material is spread as a floor material. A burner 13 is installed on the upper side wall of the furnace body 10, and the iron ore on the moving hearth 11 is reduced using the burner 13 as a heat source. In FIG. 1, 14 is a charging device for charging and loading the mixed raw material on the moving hearth 11, and 15 is a discharging device for discharging the reduced product. The temperature of the heating atmosphere in the heating furnace is usually maintained at around 1300 ° C., but in particular, the melting zone is usually at a high temperature around 1500 ° C.

  Iron-containing oxides, such as iron ore, as a mixed raw material usually contain many gangue components, although the amount varies depending on the place of production. Also, ash is contained in coal, coal char, and coke, which are typical carbon-based solid reducing materials. For this reason, in the operation of the mobile hearth furnace method in which only heat reduction proceeds, it is not possible to avoid mixing gangue into the reduced iron that is the product, and further, ash from the reducing material is mixed into the product. There is also a fear. However, in the operation of the mobile hearth furnace, the reduced raw material is melted on the mobile hearth, so that the metal generated by the reduction and the slag as the residue can be easily separated.

  As a technique for promoting the separation of metal and slag, for example, Patent Document 2 proposes a method of supplying the mixed raw material in a lump form.

Japanese Patent Laid-Open No. 11-172121 JP 2002-339909 A

When using reduced iron in an electric furnace, it is advantageous to use reduced iron with a high carbon concentration. The reason is as follows.
(1) Reduced iron containing a large amount of carbon has a low melting point and is easy to dissolve.
(2) The carbon contained in the reduced iron is used as a heat source for heating the molten steel by combustion when oxygen is blown in the steel making process. This helps to reduce the basic unit of expensive power and can contribute to cost reduction.

  By the way, the conventional techniques including the disclosure of the above-mentioned patent documents are techniques for producing reduced iron containing a carbon content of less than 2 to 4 mass%, and control the carbon concentration of the reduced iron. This is a technology that is not supposed at all.

  Accordingly, an object of the present invention is to make it possible to control the carbon concentration in the reduced iron to some extent, and to produce reduced iron that can be easily dissolved in a smelting furnace. The object is to propose a technique for producing reduced iron containing carbon at a carbon concentration of about 4% by mass.

The inventors have arrived at the present invention according to the following summary configuration during research aimed at realizing the above object. That is, the present invention moves within a mobile hearth furnace in a state where a mixed raw material containing an iron-containing oxide, a carbon-based solid reducing material, and a slagging material is loaded on the mobile hearth of the mobile hearth furnace. In the method for producing reduced iron by reducing the iron-containing oxide by heating and reducing the iron-containing oxide and leading to soot separation, the average particle size of the iron-containing oxide and Reduced iron by a mobile hearth furnace characterized by using a carbonaceous material ratio expressed as a ratio of the oxygen concentration to be reduced and the carbon concentration in the mixed raw material so as to satisfy the following formula: It is a manufacturing method.
0.5 <(carbon material ratio) <1.6 and log (1 / r) <(− 2.0 × (carbon material ratio) +2.5)
r: Radius of iron-containing oxide Carbon material ratio: (carbon concentration in mixed raw material) / (reduced oxygen concentration in mixed raw material) / 12 × 16

In the present invention,
(1) When the mobile hearth furnace is heated, CO or H ₂ generated during the reduction reaction is subjected to secondary combustion by blowing air or oxygen-added air into the furnace.
(2) First loading a carbon-containing substance on the moving hearth, loading the powdered or lump mixed raw material thereon, and performing heat reduction.
(3) The iron-containing oxide is a powder or lump made of one or more of iron ore, dust or sludge,
(4) The slag material is one or more of limestone, dolomite and serpentine,
Is a more preferable solution.

  According to the present invention, by making the carbonaceous material ratio represented by the carbon concentration and the reducible oxygen concentration in the mixed raw material and the average particle diameter of the iron-containing oxide in the mixed raw material have a certain relationship, It becomes possible to reliably and easily produce reduced carbon-containing reduced iron having a reduced iron concentration of 4 mass% or more. As a result, it is easy to dissolve reduced iron in an electric furnace or the like, which can contribute to a reduction in product cost.

In addition, according to the present invention, a carbon-containing substance is loaded under the mixed raw material layer on the moving hearth, and heat treatment is performed by blowing air, oxygen-enriched air, or the like into the furnace, so that CO generated by the reduction reaction is generated. And H ₂ can be efficiently subjected to secondary combustion, which contributes to a reduction in energy costs.

  In addition, the effect of this invention exhibits the same effect not only when using a powder raw material directly, but also when using agglomerated raw materials, such as a pellet and a briquette.

It is a schematic diagram of a rotary hearth furnace. It is a schematic diagram of the electric furnace used for simulation. It is a graph which shows transition of the furnace temperature at the time of electric furnace heating, and sample temperature. It is the external view and cross-sectional photograph of the sample used for experiment. It is a schematic diagram of the sample cross section at the time of a smelting reduction reaction. It is a graph which shows the relationship between carbon concentration and carbonaceous material ratio. It is a graph which shows the relationship between a carbon concentration and the reduction rate in 1350 degreeC. It is a figure which shows the relationship between the carbon material ratio given to a reduction rate, and a particle size (specific surface area). It is a graph which shows carbon concentration transition in an Example.

First, the inventors describe the contents of an experiment that led to the idea of the present invention. In this experiment, heat reduction is performed using a small electric furnace that can simulate the production process of reduced iron in a mobile hearth furnace. A schematic diagram of the 20 KW graphite electric furnace used in this experiment is shown in FIG. This electric furnace is provided with a graphite crucible 2 having an inner diameter of 60 mm and a lifting platform 3 in a cylindrical furnace body 1, and a heater 4 is disposed outside the furnace body 1. In this furnace, the crucible 2 using the lifting platform 3 moves up and down (moves). For example, when the crucible 2 is raised, the graphite crucible 2 enters the electric furnace body 1 heated to 1500 ° C. In this crucible 2, nitrogen was circulated at 1.8 Nm ³ / H in order to prevent oxygen from entering the crucible 2.

Next, Table 1 shows the composition of coal and iron ore used in this experiment. In a graphite crucible 2 having a diameter of 70 mm and a height of 60 mm, first, 10 g of coal 5 was laid on the lower layer, and then mixed raw material powder 6 was charged thereon. In this experiment, coal was blended with iron ores A and B, and limestone was blended in the mixed raw material powder 6 so that the basicity, which is the ratio of the SiO ₂ concentration and the CaO concentration, was 1.2. Here, in the present invention, the carbon material ratio was defined as follows, and the raw material was blended so that the carbon material ratio was 0.5 or more and less than 1.6, and an experiment was attempted.
Carbon material ratio = (carbon concentration in mixed raw material) / (reduced oxygen concentration in mixed raw material) / 12 × 16

Table 2 shows the particle size of iron ore. Here, the area-based average particle diameter was determined by the following method and used for evaluation.

The reduction rate was determined from the weight loss of the sample as follows.
(Reduction rate) = 1− (W−W′−W _min ) / (W ₀ −W′−W _min )
W ′: moisture contained in mixed powder, coal volatile matter, CO ₂ amount in limestone (CaCO ₃ ) W ₀ : mass of sample before heating W _min : mass of sample after heating for 900 sec

  FIG. 3 is a diagram showing the change over time of the sample temperature measured during this experiment. Moreover, FIG. 4 is a photograph of a sample overview and a cross section at each time. As can be seen from the photograph shown in FIG. 4, it can be seen that the sample exists as a mixture as it is for 5 minutes after the start of the experiment, and a solid reduction reaction (gas reduction reaction) occurs. On the other hand, after 7 minutes, the melting and separation start at a temperature exceeding 1350 ° C. at which FeO melts. The melted and separated slag and metal are reduced and melted as shown in the photograph after 9 minutes, and it can be seen that a large number of CO gas bubbles are generated. FIG. 5 is a diagram showing a schematic diagram at the time of bubble generation observed in the sample during the smelting reduction reaction.

  Of the reduced iron thus obtained, a portion of the metal sample after 15 minutes was subjected to chemical analysis, and the carbon concentration was measured. FIG. 6 shows the relationship between the carbon ratio and the carbon concentration obtained by the experiment for each of the particle sizes shown in Table 2 and the ore brands (A, B) shown in Table 1.

  As can be seen from FIG. 6, the carbon concentration was lower as the carbon material ratio was higher and the particle size was smaller (distribution 1, 2). The condition that the carbon material ratio is high and the particle size is small is a condition that the so-called solid reduction reaction is favorably performed. Therefore, FIG. 7 shows the relationship between the reduction rate at 1350 ° C. and the carbon concentration after 7 minutes from the melt reduction reaction to the change of the reduction reaction by melt separation.

  Through the above experiments, the inventors have found that the carbon concentration decreases as the reduction rate at 1350 ° C. increases even under different conditions of ore brand, particle size, and carbonaceous material ratio. From this, it can be inferred that the carbon concentration of the reduced iron can be controlled if the reduction rate at the time of reaching 1350 ° C. can be controlled. However, a low reduction rate means that the FeO concentration in the slag at the start of melting is high. This is because FeO in slag is considered to be a causative substance that lowers the carbon concentration in the metal because it consumes the carbon carburized in the metal.

  Therefore, the inventors made a more detailed observation regarding this. As a result, as shown in FIG. 5, the molten slag foams vigorously and flows vigorously due to the bubbles 7 generated by the melting reduction reaction. And it was observed that the molten metal 9 which exists under the slag 8 flows accompanying flow.

From this, the inventors reached the following conclusion regarding the relationship between the reduction rate and the carbon concentration. The carburization reaction in the molten metal
(1) Dissolution process at the metal-carbon material interface,
(2) Diffusion process in which dissolved carbon atoms diffuse throughout
Because it is thought that it will progress through
Compared to the stationary state with metal flow, diffusion is promoted in the convection or flow state, and the process (2) is dramatically accelerated. As a result, carbon atoms exceeding the consumption by the smelting reduction reaction are supplied into the molten metal, and the carbon concentration in the metal is considered to increase.

  By the way, when the relationship in FIG. 7 is seen in detail, it can be seen that the reduction rate of the carbon concentration with respect to the reduction rate clearly changes between the reduction rate of 90% or more and less than 90%. This is presumably because the carbon concentration became 4 mass% or more when the reduction rate was less than 90%, and reached a carbon saturation concentration close to 5 mass%, so that the carburization reaction rate decreased.

Next, the inventors examined the operating conditions for achieving a reduction rate of 90% or less at 1350 ° C. As a result, the reduction proceeds by a so-called solid reduction reaction up to 1350 ° C. as described above. The solid reduction reaction occurs by a CO gas or N ₂ gas generation process by gas decomposition of a flooring material (carbon-containing substance) that is a solid reducing material, and a gas reduction reaction process with iron ore. In general, the gas reduction process is said to be a rate-determining process.

Accordingly, the inventors mapped ore A and ore B with the carbonaceous material ratio (−) and the specific surface area log (1 / r) as shown in FIG. 8. As is apparent from FIG. 8, the regions where the reduction rate is less than 90% and 90% or more are clearly different. From the experimental results, the following areas were defined as areas with a reduction rate of 90% or less.
log (1 / r) <(− 2.0 × (carbon material ratio) +2.5)
r: radius of the iron-containing oxide
(However, 0.5 <carbon material ratio <1.6)

  By satisfying the above relationship as described above, the reduction rate can be suppressed to 90% or less, and thereby the carbon concentration of the produced reduced iron can be set to 4.0 mass% or more.

  In summary, the carbon concentration of reduced iron can be controlled by appropriately controlling the particle size and the carbonaceous material ratio as in the method of the present invention, and the reduced iron having a carbon concentration of 4 mass% or more can be controlled. Can be stably produced.

In order to confirm the usefulness of the method of the present invention, an experimental operation was conducted in a rotary transfer furnace.
A rotary furnace with the specifications shown in Table 3 was used for this operation. The ores and carbon materials shown in Table 1 were used, and each iron ore was blended for each particle size shown in Table 2 after sizing. The rotary moving furnace was provided with a sampling port for taking a sample during heating, and a sample was collected at a furnace temperature of 1350 ° C. that was moved (rotated) by 50% of the whole process. This sample was subjected to chemical analysis to calculate a reduction rate, and expressed as “50% reduction rate of the process”.

  The reduced iron after the heating step was subjected to chemical analysis to measure the carbon concentration. Some raw materials were used after agglomeration with a briquette machine after adding an organic binder. In addition, an experiment was carried out in which 50 mm of coal was loaded as a flooring material on the moving hearth and mixed raw materials were charged thereon.

  Table 4 and FIG. 9 show the operation results. It has been found that by using a method suitable for the present invention, the carbon concentration in the reduced iron can always be stably achieved at 4 mass% or more, and reduced iron that is easy to use in electric furnace operation can be provided. In addition, from the results of the above-mentioned experimental operation, the same effect is achieved even when coal (floor material), which is a carbon-containing material, is loaded on the moving hearth, or even if it is a bulk mixed raw material obtained by agglomerating powder raw materials It was confirmed that

  The present invention is useful as a technique for producing reduced iron using a mobile hearth furnace, particularly a rotary hearth furnace, but is also an effective method as a technique for reducing other metals.

DESCRIPTION OF SYMBOLS 1 Furnace body 2 Graphite crucible 3 Lifting stand 4 Heater 5 Coal 6 Mixed raw material powder 7 Bubble 8 Slag 9 Molten metal 10a Pre-tropical 10b Reduction zone 10c Melting zone 10d Cooling zone 10 Furnace body 11 Moving hearth 12 Mixed raw material 13 Burner 14 Installation Input device 15 Discharge device

Claims (5)

By heating while moving in the mobile hearth furnace with a mixed raw material containing iron-containing oxide, carbon-based solid reducing material and ironmaking material loaded on the mobile hearth of the mobile hearth furnace In the method for producing reduced iron by reducing the iron-containing oxide and melting and leading to soot separation, the mixed raw material includes an average particle size of the iron-containing oxide and a coating in the mixed raw material. A method for producing reduced iron using a mobile hearth furnace, characterized in that a carbonaceous material ratio expressed as a ratio of reduced oxygen concentration and carbon concentration is blended so as to satisfy the following formula.
0.5 <(carbon material ratio) <1.6 and log (1 / r) <(− 2.0 × (carbon material ratio) +2.5)
r: Radius of iron-containing oxide Carbon material ratio: (carbon concentration in mixed raw material) / (reduced oxygen concentration in mixed raw material) / 12 × 16 2. The mobile type according to claim 1, wherein when the mobile hearth furnace is heated, air or oxygen-added air is blown into the furnace to cause secondary combustion of CO or H ₂ generated during the reduction reaction. A method for producing reduced iron using a hearth furnace. 3. The mobile hearth furnace according to claim 1, wherein a carbon-containing substance is first loaded on a moving hearth, and the mixed raw material in powder or lump is loaded thereon to perform heat reduction. A method for producing reduced iron. The mobile furnace according to any one of claims 1 to 3, wherein the iron-containing oxide is a powder or lump of iron ore, dust or sludge. A method for producing reduced iron using a floor furnace. The method for producing reduced iron by a mobile hearth furnace according to any one of claims 1 to 4, wherein the slagging material is one or more of limestone, dolomite, and serpentine. .

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