KR100778683B1 - Method for manufacturing direct reduced iron with high strength for blast furnace - Google Patents

Abstract

Pellets for blast furnace and a method for manufacturing the same are provided, wherein the pellets for blast furnace have high strength and reduction ratio by varying the weight ratio of carbon and iron in a central part and a surface layer part thereof. A method for manufacturing direct reduced iron with high strength for blast furnace comprises: a central part forming step of manufacturing raw pellets having a central part of which a carbon to iron ratio is 0.2 to 0.25% by weight; a surface layer part growing step of growing a surface layer part from an outer surface of the central part of the raw pellets using a raw material of which a carbon to iron ratio is 0.16 to 0.2% by weight; and a reduction step of reducing the raw pellets in a rotary hearth furnace. A raw pellet comprises a central part which has a diameter of 8 to 10 mm, and of which a carbon to iron ratio is 0.2 to 0.25% by weight; and a surface layer part which is formed on an outer surface of the central part, and of which a carbon to iron ratio is 0.16 to 0.2% by weight, wherein the raw pellet has a diameter of 12.5 to 13.2 mm.

Description

Method for Manufacturing Direct Reduced Iron with High Strength for Blast Furnace}

1 is a flow chart showing a method for producing high strength reduced iron for blast furnace according to an embodiment of the present invention.

Figure 2 is a schematic diagram showing the process flow from the mixing of raw materials to the production of reduced iron in the production of high-strength reduced iron for blast furnaces using a rotary hearth type reduction furnace according to an embodiment of the present invention.

3 is a cross-sectional view showing an experimental furnace that simulates the conditions of a rotary hearth type reduction furnace.

Figure 4 is a graph showing the change in the reduction rate according to the ratio of the weight of carbon and iron in the raw pellet for the temperature of 1250 ℃ and 1300 ℃.

5 is a graph showing the change in strength of the reduced iron according to the ratio of iron weight and carbon weight in the raw pellets at temperatures of 1250 ° C and 1300 ° C.

Figure 6 is a graph showing the correlation between the porosity in the reduced iron according to the ratio of the weight of the carbon component and the iron component in the raw pellet at a reduction temperature of 1250 ℃.

Figure 7 is a schematic diagram of the raw pellet prepared according to the embodiment of the present invention.

<Description of reference numerals for the main parts of the drawings>

201: ferrous iron raw materials and carbon materials 203: mixer

205: first pelletizer 207: ferrous iron raw material and carbonaceous material

209: second pelletizer 211: the first screen

213: second screen 215: pellet dryer

217: rotary hearth type reduction furnace 219: cooler

221: storage

The present invention relates to a method for producing high strength reduced iron for blast furnaces. More specifically, the present invention relates to a blast furnace pellet having a high reduction rate and high strength using a rotary hearth furnace, and a method of manufacturing the same.

In the related art, reduced iron produced from a rotary hearth type reduction furnace was used as a substitute for scrap in steelmaking or electric furnace processes. Therefore, the main purpose was to manufacture reduced iron having high purity and high reduction rate and high metallization rate. Was not highlighted. Therefore, since the reduced iron produced by the conventional rotary hearth type reduction furnace method has a high reduction rate and low strength, its use has been limited to an electric furnace, a converter and the like.

In addition, in order to produce reduced iron having a high reduction rate by a rotary hearth type reduction furnace method, a high temperature and a large amount of coal of about 1400 to 1450 ° C. are required. In this case, the energy efficiency decreases, and the inflow of ash from coal is increased to reduce the purity of the reduced iron pellets, and the melting point of the reduced iron is reduced by melting the added iron by carburizing the inside of the reduced iron, thereby reducing the reduced iron; Problems such as fusion on the furnace occur.

On the other hand, it is important to manufacture a high-strength reduced iron in order to use directly in the blast furnace (steel) -type steelmaking process. The reason for this is that in the blast furnace, about 2,000 to 8000 tonnes of ore and coke are stacked, so the solid charges that are introduced into the furnace are considerably loaded because the solid charges must not support and differentiate into the load.

It is known that the required compressive strength is at least 50kg / cm ² or more, but in general, the fired pellets used in the blast furnace process have a high compressive strength of about 200 kg / cm 2, so that the reduced iron is also close to the compressive strength of the fired pellets. It can be said to be advantageous for the operation of the steelmaking process.

However, since the strength of the reduced iron produced by the operation method (manufacture of reduced iron having a reduction rate of 90% or more) in the conventional rotary hearth type reduction furnace method shows about 20 to 30 kg / cm ² , such reduced iron is used in the blast furnace steelmaking process. If so, it is easily compressed in the furnace and discharged as dust, so that not only the recovery rate as iron raw material is deteriorated but also the air permeability in the furnace is deteriorated, which makes operation management difficult. Therefore, the reduced iron produced by the conventional rotary hearth type reduction furnace method cannot be used directly in the blast furnace steelmaking process.

There is a method of manufacturing high strength briquettes by compressing low-strength high reduction rate reduced iron again with a hot briquette device and using it in a blast furnace steelmaking process, but in this case, a separate compression molding process is added. There is this.

On the other hand, in Japanese Patent Laid-Open No. 2002-194410, the carbonaceous material-containing molded body (pellets or briquettes) is formed at a temperature of 1200 ° C. or more at the same time under which the reduction reaction and the sintering are simultaneously performed. It has been found that the residence time is a very important indicator, and that reduced iron with high strength can be produced especially when the weight ratio of metal iron in the reduced iron is 40% or more.

In addition, Japanese Patent Application Laid-Open No. 2004-218019 specifies the particle size of the raw material constituents and the chemical composition of the raw material powder, and manufactures high-strength reduced iron having a weight of 50 kg / cm ² or more by discharging the reduced iron when the weight ratio of the metal iron is about 45%. It proposed a method.

However, the above methods not only overlooked the fact that the strength of the reduced iron was determined not only by the proportion of the iron but also by the porosity inside the reduced iron pellets.

In addition, Japanese Patent Laid-Open No. 2000-119722 sets the weight ratio of the iron component in the raw material to 45-65%, and sets the ratio (% C /% O) of the weight of the carbon component to the weight of oxygen (present as iron oxide) in the range of 0.45 to 0.80. A method for producing high-strength reduced iron was proposed by defining the chemical composition of carbonaceous pellets within the range of. In this method, the strength of the reduced iron pellets required in the blast furnace steelmaking process is 50 kg / cm ² , but the strength of the reduced iron is suitable for small furnaces, but it is somewhat insufficient in large blast furnaces with high loadings.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a blast furnace pellet and a method for producing the same having high strength and reduction rate because the weight ratio of carbon and iron varies depending on the center portion and the surface layer portion.

The present invention, the core forming step of producing a green pellet (Green Pellet) having a central portion of the carbon and iron ratio of 0.2 to 0.25 by weight, a raw material having a carbon and iron ratio of 0.16 to 0.2 by weight It provides a surface layer growth step of growing a surface layer portion from the outer surface of the center of the pellet using, and a reduction step of reducing the pellet in a rotary hearth type reduction furnace.

The diameter of the central portion of the fresh pellets may be 8 to 10 mm, the diameter of the fresh pellets after the surface layer is grown may be 12.5 to 13.2 mm.

The raw pellets may be reduced to a temperature of 1240 to 1260 ° C. in a rotary hearth type reduction furnace.

In addition, the present invention, the weight ratio of carbon and iron in the center of 0.2 to 0.25 and the diameter of 8 to 10 mm on the basis of weight percent, and the surface layer portion of 0.16 to 0.2 ratio of carbon and iron on the outer surface of the center by weight percent It provides a method for producing a raw pellet having a diameter of 12.5 to 13.2 mm.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First of all, in adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are used as much as possible even if displayed on different drawings. In describing the present invention, when it is determined that the detailed description of the related well-known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

1 is a flow chart showing a method for producing high strength reduced iron for blast furnace according to an embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing high-strength reduced iron for blast furnace according to an embodiment of the present invention includes forming a central portion (S101), a surface layer growth step (S103), and a reducing step (S105) of raw pellets.

The center forming step (S101) is a step of forming a center having a ratio of carbon to iron in a weight percent of 0.2 to 0.25 and a diameter of 8 to 10 mm.

Surface layer growth step (S103) After the center portion is formed is a step of forming a surface layer portion on the outer surface of the center, the surface layer portion of the carbon and iron ratio of 0.16 to 0.2 based on the weight percent, the diameter of the raw pellet after the surface layer portion is formed 12.5 to 13.2 mm.

The reduction step (S105) is a step in which the raw pellets formed to the surface layer portion are finished by reducing iron at a temperature of 1240 to 1260 ° C in a rotary hearth reduction furnace.

Figure 2 is a schematic diagram showing the process flow from the mixing of raw materials to the production of reduced iron in the production of high-strength reduced iron for blast furnaces using a rotary hearth type reduction furnace according to an embodiment of the present invention.

 Referring to FIG. 2, the iron-containing raw material 201 and the reducing agent are mixed in the mixer 203 so that the ratio of carbon and iron in the first pelletizer 205 is 0.2 to 0.25 and the diameter is 8 to 10 mm in weight%. It is formed of raw pellets having a phosphorus center, and when the iron-containing raw material 207 and the reducing agent are added to the raw pellets thus formed, the ratio of carbon and iron in the second pelletizer 209 is 0.16 to 0.2 based on the weight%. The surface layer portion is formed. In this case, the diameter of the fresh pellet in which the surface layer part was formed is 12.5-13.2 mm.

Meanwhile, between the first pelletizer 205 and the second pelletizer 209, the first screen 211 is provided to select only pellets having a particle diameter of 8 to 20 mm to separate the second pelletizer 209. ) And the remaining pellets are returned to the mixer 203 for crushing and mixing. In addition, only the raw pellets having a particle diameter of 10 to 20 mm among the raw pellets manufactured by the second pelletizer 209 are screened by the second screen 213 and moved to the pellet dryer 215. The reason for this sorting is that pellets having a particle size of less than 10 mm are melted or fused to the furnace when supplied to the rotary hearth type reduction furnace 217. In the case of pellets having a particle size of more than 20 mm, uniform quality (reduction rate or Strength) can not be expected.

Since the pellets from the second pelletizer 209 contain about 10 to 15% of moisture, the pellets are removed from the pellet dryer 215 and then supplied to the rotary hearth type reduction furnace 217.

In the rotary hearth type reduction furnace 217, the pellets are reduced to a temperature of 1240 to 1260 ° C. The reduced pellet is also cooled in the cooler 219 and sent to the reservoir 221.

Example

In the present invention, as shown in Fig. 3, various experiments were conducted using an experimental furnace that simulates the conditions of a rotary hearth type reduction furnace. The raw pellets 301 were heated to a high temperature by the heating element 305 in a nitrogen gas atmosphere contained in a stainless steel board 303 provided in the quartz reaction tube 302, and the temperature change of the pellets 301 was a thermocouple for temperature measurement. Measured by 307 and recorded in recorder 309.

The raw pellets are used to add carbonaceous material to iron ore with 60% by weight of total iron (% T.Fe) so that the carbon content is 4 to 10% by weight and 12.5 to 13.2 mm in diameter. Raw pellets were prepared to change the weight of the total iron in the range of 47-57%. The reduction rate, strength, and porosity of the reduced iron obtained by reacting the prepared pellets at furnace temperatures of 1250 ° C. and 1300 ° C. were investigated, and the conditions under which high-strength reduced iron of high reduction rate could be obtained were derived.

4 is a graph showing the change in the reduction rate according to the ratio ((% C) / (% Fe)) of the carbon component weight and iron component weight in the pellets at temperatures of 1250 ° C and 1300 ° C.

The reason why the carbon material mixture ratio is expressed as the ratio of the carbon component weight to the iron weight ratio ((% C) / (% Fe)) is that the oxygen to be removed by the carbon shows the form of an oxide combined with the metal component. This is because the oxide that can be reduced by carbon in the operating temperature range (1200 to 1400 ° C.) of the reduction furnace is iron oxide.

If the total oxygen content must be analyzed to determine the carbon content required for reduction, the hassle of analyzing the content of the metal constituting all the oxides is inevitable. Therefore, in order to easily determine the carbonaceous compounding ratio can be easily determined by analyzing the content of the iron component of the raw material components.

As shown in FIG. 4, the reduction rate increases linearly with increasing carbon content, and the reduction rate at 1250 ° C. is about 1 to 2% lower than the reduction rate at 1300 ° C., but there is no significant difference.

On the other hand, the change in the reduction rate according to the carbonaceous material mixing conditions at each temperature could be expressed by the following equation.

Reduction (%) = 5.60 + 405.4a / b (1250 ℃)

Reduction (%) = 5.57 + 414.5a / b (1300 ℃)

Where a is the weight of the carbon component and b is the weight of the iron component.

5 is a graph showing the change in the strength of the reduced iron according to the ratio of the weight of iron and carbon in the pellet with respect to the temperature of 1250 ℃ and 1300 ℃, the strength of the reduced iron pellet prepared at 1250 ℃ 20kg / cm ² is higher than the average strength. In addition, regardless of temperature, the strength of the reduced iron is the highest (about 170 ~ 220kg / cm ² ) in the ratio ((% C) / (% Fe)) of the weight of the carbon component and iron weight of about 0.16 ~ 0.2, which is It is similar to the strength of calcined pellets. This corresponds to the addition of about 8 to 10% by weight of the carbon component to the iron ore, the reduction rate of reduced iron is about 70 ~ 85%, the reduction rate is higher than 85% reduced the strength of the reduced iron.

On the other hand, the strength of the reduced iron according to the carbonaceous material mixing conditions at each temperature could be expressed by the following equation.

Compressive Strength (kg / cm2) = -126 + 3770a / b-11196 (a / b) ² (1250 ℃)

Compressive Strength (kg / cm2) = -123 + 3508a / b-10566 (a / b) ² (1300 ℃)

The reduction rate and strength of the reduced iron are greatly affected by the coal ashing conditions, and in particular, it is confirmed through the experimental results that the strength of the reduced iron is affected by the coal ashing conditions.

Reduction of the carbonaceous pellet is made by the reaction of carbon in the pellets and oxygen combined with iron, and since the CO gas, which is a result of the reduction reaction, is discharged out of the reduced iron after generation, pores are inevitably formed inside the reduced iron.

6 is a graph showing the correlation between the porosity in the reduced iron according to the ratio ((% C) / (% Fe)) of the carbon component weight and the iron component weight in the pellet at a reduction temperature of 1250 ° C. The ratio of the weight of the iron component ((% C) / (% Fe)) is about 0.16 to 0.2, indicating a minimum value of less than 25% of the porosity. In addition, when compared with FIG. 5, it can be seen that the porosity and the strength of the reduced iron has a reverse correlation, and in order to manufacture the reduced iron of the maximum strength, it can be seen that the porosity should be controlled to less than 25%.

Under these conditions, the strength of the reduced iron pellets having a reduction rate of 80% or more indicates the strength of 175 kg / cm ² or less. In order to stably use the reduced iron as the fired pellets in the blast furnace steelmaking process, the strength level of the fired pellets (about 200 kg / It can be seen that the strength needs to be further increased to maintain cm ² ).

In the present invention, the above problems can be solved by forming the pellets by reducing the center portion and the surface layer portion according to the charcoal content and reducing the pellets.

Figure 7 is a schematic diagram of the raw pellet prepared according to the embodiment of the present invention.

Referring to FIG. 7, the center portion 701 of the pellet 700 has a ratio of (0.2% to 0.2%) of the carbon content and the iron content ((% C) / (% Fe)) so as to have a diameter of 8 to 10 mm. The surface layer portion 703 is formed on the outer surface of the central portion 701 by using a raw material obtained by mixing the ratio of the carbon component weight to the iron weight ratio ((% C) / (% Fe)) at 0.16 to 0.2. The overall structure of the 700 is a dual structure of the central portion 701 and the surface layer portion 703, the diameter of the pellet 700 is 12.5 ~ 13.2mm.

As described above, pellets having different carbon content in the central part and the surface layer were manufactured, and a heat treatment (reduction and sintering) experiment was conducted in an experimental furnace simulating a rotary hearth type reduction furnace. The experimental conditions are as follows, and the results are shown in Table 1.

Experimental conditions

(1) fresh pellets produced by the first pelletizer

Prepare raw pellets with a diameter of 8-10 mm by supplying a mixed raw material having a ratio ((% C) / (% Fe)) of carbon component weight to iron component weight of 0.21.

(2) fresh pellets produced in the second pelletizer

The raw pellets (diameter 8-10 mm) manufactured in the first pelletizer were supplied by supplying a mixed raw material having a ratio ((% C) / (% Fe)) of carbon component weight to iron component weight (0.1%). The diameter was 12.5-13.2mm. Secondary growth.

(3) drying

Dried for 3 hours at 105 ℃ oven to produce less than 2% moisture content.

(4) heat treatment conditions

Reaction temperature 1200, 1250 and 1300 ° C., temperature increase time 5 minutes, reaction time 10 minutes.

[Table 1] Reduction rate and strength measurement results of reduced iron according to the reaction temperature for the raw pellet of FIG.

division Temperature (℃) 1200 1250 1300 Reduction rate (%) 84.3 87.1 88.7 Compressive strength (kg / cm ² ) 122.8 214.2 188.3

Referring to Table 1, as the temperature is increased, the reduction rate tends to increase, but the strength of the reduced iron shows the maximum strength at 1250 ° C.

Since the reduction reaction by carbon as a reducing agent is a strong endothermic reaction, the degree of reaction increases as the temperature increases, and the reduction rate increases. As the temperature increases from 1250 ° C to 1300 ° C, the increase is slowed.

In addition, the strength of reduced iron showed the maximum value at 1250 ° C. As a result of the structure observation, the inside of the reduced iron had large and many pores, but the outside of about 2 to 2.5 mm had small pores and a very dense metal iron network was formed. As a result, the compressive strength of the reduced iron was confirmed to represent 200kg / cm ² or more, which is comparable to that of the calcined pellets.

On the other hand, in the case of 1200 ° C, a local metal iron network was formed, in the case of 1300 ° C, there was local melting, and a molten slag layer was found between some of the metal iron networks. It was confirmed that the weaker than the reduced iron reduced.

Therefore, in the production of reduced iron having a reduction rate of 85% or more and a compressive strength of 200 kg / cm ² or more, it is preferable to reduce the carbonaceous material-containing pellets at a temperature range of 1240 to 1260 ° C., which is lower than that of the conventional method. In addition, when the pellet is manufactured, pellets are prepared by supplying a raw material in which the ratio of the carbon component weight and iron component weight ((% C) / (% Fe)) is 0.2 to 0.25 at a diameter of 8 to 10 mm. A pellet prepared by growing a pellet to 12.5 to 13.2 mm by supplying a raw material in which the ratio of weight and iron content ((% C) / (% Fe)) is 0.16 to 0.2 is grown in a temperature range of 1240 to 1260 ° C. By reducing, it was possible to derive a method for producing reduced iron having a high reduction rate and high strength that can be directly used in a blast furnace steelmaking process.

The above description is merely illustrative of the present invention, and those skilled in the art may make various modifications and changes without departing from the essential characteristics of the present invention. In addition, the protection scope of the present invention should be interpreted by the claims, and all technical ideas within the equivalent scope should be construed as being included in the scope of the present invention.

According to the present invention, by varying the weight ratio of carbon and iron according to the central portion and the surface layer portion, there is an effect of providing a method for producing reduced-iron for blast furnace having high strength and reduction rate.

Claims (5)

A core forming step of preparing a raw pellet having a central portion in which a ratio of carbon and iron is 0.2 to 0.25 based on the weight%; A surface layer growth step of growing a surface layer portion from an outer surface of the central portion of the raw pellet by using a raw material having a ratio of carbon and iron in a weight percent of 0.16 to 0.2; And Reduction step of reducing the raw pellets in a rotary hearth type reduction furnace Method for producing high strength reduced iron for blast furnace comprising a. The method of claim 1, The diameter of the center portion of the pellet is a method for producing high strength reduced iron for blast furnace, characterized in that 8 to 10 mm. The method of claim 2, The diameter of the pellet after the surface layer is grown is a method for producing high strength reduced iron for blast furnace, characterized in that 12.5 to 13.2 mm. The method of claim 3. The pellet is a method for producing high-strength reduced iron for blast furnace, characterized in that the reduction in the rotary hearth type reduction furnace at a temperature of 1240 to 1260 ℃. A central portion of carbon to iron in a weight percent of 0.2 to 0.25 and a diameter of 8 to 10 mm; And It is formed on the outer surface of the center portion, the surface layer portion of the carbon and iron ratio of 0.16 to 0.2 by weight% Including but not limited to: Fresh pellets, characterized in that the diameter of 12.5 to 13.2 mm.

Images