JP2012126947A - Cavity-maldistributed baking pellet and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a baking pellet whose strength does not drop but whose reducibility remains excellent while stored.SOLUTION: The pellet is made by baking iron ore powder and granulated substances mixed with auxiliary material ore powder including CaCOand/or MgCO. The cavity-maldistributed baking pellet features that the (x1) surface part is made of a cavity communicating structure with porosity of 20% or more and the (x2) center part is made of the cavity block structure with porosity of 20% or below.

Description

  The present invention relates to a fired pellet for charging a blast furnace in which pores are unevenly distributed in a surface layer portion.

  Generally, calcined pellets that are conventionally used as part of raw materials for blast furnace charging are inferior in reducing properties at high temperatures compared to sintered ore, so when using ore in a blast furnace, In comparison with this, the cohesive zone formed in the blast furnace is enlarged, and the air permeability in the furnace is deteriorated or the ratio of the reducing material is increased.

  The above disadvantages of the fired pellet are considered to be due to the following reason. The calcined pellets are heated by the reducing gas that rises in the furnace as they descend in the blast furnace, reducing the iron oxide, but the reduced iron produced in the surface layer at the beginning of the reduction forms a dense outer shell. In addition, since the diffusion of the reducing gas into the fired pellet is hindered, the reduction rate of the iron oxide is remarkably reduced in the latter stage of reduction.

  For this reason, the calcined pellets are inferior to the sintered ore in overall reducibility. Furthermore, since the fired pellets have a form in which slag concentrates and remains in the central portion even when reaching a region of 1200 ° C. or higher where the cohesive zone is formed, the shrinkage at the cohesive zone is large.

  As a measure for improving the above-described drawbacks of the fired pellets, attempts have been made to add CaO and / or MgO to self-solubilize the fired pellets to improve the reducibility. For example, Patent Document 1 discloses that MgO has a higher reducing effect than CaO.

In Patent Document 2, the concentration of CaO or MgO and the particle size of the CaO or MgO source are optimized, CaO / SiO ₂ is 0.8 or more, MgO / SiO ₂ is 0.4 or more, and the particle size is It is disclosed that the reduction rate at a high temperature is further improved if the thickness is 44 μm to 1 mm.

  However, when self-solubilized calcined pellets (self-soluble calcined pellets) absorb rain and moisture in the air, the hydration reaction of CaO, MgO, etc. in the slag in the pellets progresses over time. Due to the accompanying volume expansion, the tissue collapses and the crushing strength is reduced (hereinafter referred to as “aging over time”).

  In Patent Document 3, as a measure for solving the deterioration with time of the self-solvent fired pellets, contrary to the measure disclosed in Patent Document 2, the particle size of the CaO or MgO source is reduced, specifically, 44 μm or less. It is disclosed that the content is 20% or more.

  As described above, self-solubilization of the calcined pellets improves the reducibility of the calcined pellets, but on the other hand, it has a problem of deterioration with time, so far, both reducibility and deterioration with time have been improved at the same time. No strategy has been proposed.

Japanese Patent Laid-Open No. 50-021917 Japanese Patent Laid-Open No. 01-136936 JP 07-197137 A

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide fired pellets that are not deteriorated with time during transportation and storage of the pellets and have excellent reducibility, and a method for producing the same.

  According to the study of the present inventors, if the porosity is simply increased or the pore diameter is increased, the reducibility of the fired pellets is improved, but the pellet crushing strength remains at a low level, The amount of decrease in crushing strength due to deterioration over time of the raw material storage period increases.

Simply increasing the porosity or expanding the pore diameter increases the amount of decrease in the crushing strength of the calcined pellets during the raw material storage period because the amount of moisture absorbed increases through the pores of the calcined pellets. Hydration reaction of CaO, MgO, etc. in slag progresses, and tissue collapse due to hydration reaction (tissue destruction due to volume expansion when Ca (OH) ₂ and Mg (OH) ₂ are produced from CaO and MgO) It is thought that this is because it is promoted.

  Therefore, the present inventors have focused on the pore structure and structure of the fired pellets, which has not been studied in the past, and adjusted the pore structure and structure of the surface layer portion and the central portion while maintaining the overall porosity high. By doing this, it was aimed to produce calcined pellets that are excellent in reducibility and have a small amount of deterioration over time in the crushing strength during the raw material storage period.

  The inventors of the present invention newly manufactured fired pellets using various brands of powdered iron ore and auxiliary materials, focusing on the characteristics of the iron ore itself, and investigated the properties and morphology of the structure. As a result, it has been found that the pore structure after firing of the self-solvent fired pellets varies greatly depending on the crystal grain size of iron ore (hereinafter, “particle size” means the diameter).

  That is, the following knowledge was obtained.

(I) When a mixed granular material of iron ore powder such as Carajas iron ore powder composed of hematite crystal grains having an average particle diameter of 20 μm or less and auxiliary raw material ore powder containing CaCO ₃ and / or MgCO ₃ such as dolomite is calcined. The surface layer part is a pore-communication structure that does not contain slag, whereas the center part is a calcined pellet that is a pore-closed structure containing calcium-silicate slag, that is, the pores are unevenly distributed in the surface part compared to the center part. It becomes “a pore uneven distribution type fired pellet”.

  (Ii) Pore-distributed calcined pellets (hereinafter sometimes referred to as “pore-distributed calcined pellets”) have little deterioration over time (the amount of deterioration over time of the crushing strength during the raw material storage period is small) and are highly reducible. Are better.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) A pellet obtained by firing a mixed granule of iron ore powder and a secondary raw material ore powder containing CaCO ₃ and / or MgCO ₃ ,
(X1) The surface layer portion is composed of a pore-communication structure having a porosity of 20% or more,
(X2) A pore-localized sintered pellet characterized in that the central portion is composed of a pore-occluding tissue having a porosity of less than 20%.

  (2) The slag content in the pore-connected tissue is 5% by mass or less, and the slag content in the pore-occluded tissue is 10% by mass or more. .

(3) In the method for producing the pore-localized fired pellets according to (1) or (2),
(Y1) iron ore containing hematite crystal grains having an average particle size of 20 μm or less; and (y2) an auxiliary material containing CaCO ₃ and / or MgCO ₃ , SiO ₂ is 1.8 to 2.2, composite basicity (CaO + MgO ) / SiO ₂ is mixed to be 2.0 to 1.5,
(Y3) A method for producing pore-localized fired pellets, characterized in that a granulated material obtained by granulating the mixture is fired.

  (4) The method according to (3), wherein the firing is performed at 1200 to 1350 ° C. for 25 to 50 minutes.

  ADVANTAGE OF THE INVENTION According to this invention, the calcined pellet with little deterioration over time of the crushing intensity | strength of the pellet storage period and excellent in reducibility can be provided as a blast furnace charging raw material.

It is a figure which shows the cross-sectional structure | tissue of this invention pellet imaged with the optical microscope. It is a figure which shows the structure | tissue (magnification: 500 times) of the center part and surface layer part of this invention pellet imaged with the optical microscope. (A) shows the structure of the central part, and (b) shows the structure of the surface layer part. It is a figure which shows the structure | tissue (magnification: 500 times) of the center part and surface layer part of the conventional pellet imaged with the optical microscope. (A) shows the structure of the central part, and (b) shows the structure of the surface layer part. It is a figure which compares and shows the structure | tissue structure of the pellet of the conventional pellet and this invention typically.

  The present invention will be described in detail.

  As described above, the present inventors pay attention to the pore structure and structure of the baked pellets, which has hardly been studied in the past, and maintain the overall porosity high, while maintaining the pore structure and structure of the surface layer portion and the central portion. Aiming to produce calcined pellets that are excellent in reducibility and have little crushing strength degradation over time during the raw material storage period by adjusting the amount of calcined pellets using various brands of powdered iron ore and auxiliary materials And the properties and aspects of the tissue were investigated.

(Results of basic survey)
The following describes the results of the survey.

The component composition shown in Table 1 is composed of Brazilian Carajas ore or iron ore powder obtained by pulverizing Brazilian MBR ore to a particle size of 125 μm or less, and auxiliary raw material ore powder containing MgCO ₃ and CaCO ₃ as main components. The mixed raw materials were granulated to produce granules having a particle size of 8 to 15 mm.

  The above granulate is fired in a firing furnace at a firing temperature of 1280 ° C. and a firing time of 36 minutes, and then fired pellets (hereinafter referred to as “the present invention pellet” using the Carajas ore, Conventional pellets ”) were manufactured and their structure and properties were investigated.

  Here, the crystal particle diameter of the carajas ore is approximately 1 to 20 μm, whereas the crystal particle diameter of the MBR ore is approximately 50 to 100 μm.

  Table 2 shows representative numerical values of the results of a comparative investigation on the quality of the pellets of the present invention and the conventional pellets. It has been confirmed that the pellets of the present invention have an advantageous effect that they are excellent in reducing property at the time of weight-softening and are less deteriorated with time in crushing strength than conventional pellets.

  Next, the difference in structure between the pellets of the present invention and the conventional pellets will be examined.

  In FIG. 1, the cross-sectional structure | tissue of this invention pellet imaged with the optical microscope is shown. Although a structure mainly composed of hematite and pores is formed as a whole by a normal melt reaction, it is understood that the structure of the structure other than the pore structure and the hematite is different between the central portion and the surface layer portion.

  In FIG. 2, the structure | tissue of the center part and surface layer part of this invention pellet imaged with the optical microscope is shown. 2A shows the structure of the central part, and FIG. 2B shows the structure of the surface layer part. It can be seen that the pore structure and the porosity are clearly different in the structure of the center portion and the surface layer portion of the pellet.

  As a result of detailed analysis by the inventors, the following has been found.

(I) In the central part of the pellet of the present invention, a structure in which a slag part obtained by solidification of a calcium-silicate (chemical formula: CaO.SiO ₂ ) melt generated by a firing reaction closes pores (hereinafter referred to as “pore closed structure”). ").

  (Ii) In the surface layer portion, the pores generated by the firing reaction are gathered in a shape almost as they are formed, and the slag portion in which the calcium-silicate melt is solidified (hereinafter referred to as “pore-connected structure”). .)

  On the other hand, FIG. 3 shows a comparison between the pore structure and the structure of the center portion and the surface layer portion of the conventional pellet. FIG. 3 (a) shows the pore structure and tissue aspect of the central part, and FIG. 3 (b) shows the pore structure and tissue aspect of the surface layer part. In the surface layer portion of the conventional pellet shown in FIG. 3 (b), there is a slag portion in which the calcium-silicate melt is solidified, and compared with the pore structure in the center portion of the conventional pellet shown in FIG. 3 (a). It can be seen that there are few organizations.

  Thus, the pore structure and structure of the pellet of the present invention are significantly different from those of conventional pellets.

  Table 3 shows a comparison of measurement examples of the area ratio of pores and slag, together with the qualitative differences in the above-described structure structure in the surface layer portion and the central portion of the pellets of the present invention and the conventional pellets.

  In the surface layer portion of the pellet of the present invention, the porosity is remarkably higher than the surface layer portion of the conventional pellet, and the ratio (slag ratio) of the slag portion where the calcium-silicate melt is solidified is extremely small as 1.7%. . Further, it is quantitatively shown that the porosity of the pellet of the present invention is lower than that of the surface layer of the conventional pellet and the ratio (slag ratio) of the slag portion in which the calcium-silicate melt is solidified is large. .

  According to observation, it seems that the structure of the pores changes with the porosity of 20%. If it was 20% or less, it was estimated that the pores were closed because the cross-sectional shape was circular. Moreover, when it exceeded 20%, the cross-sectional shape increased unevenness, and it was reminded that the pores were continuous.

  As described above, the pellet of the present invention has a pore communicating structure in which pores contributing to reducibility in the blast furnace are collected and remain in the surface layer portion, and a slag in which the calcium-silicate melt is solidified in the central portion. It can be said that it is a calcined pellet having a pore-occluded tissue in which the portion clogs pores.

  FIG. 4 schematically shows the entire pore structure and structure of the pellet of the present invention in comparison with the conventional pellet. Conventional pellets have a generally homogeneous pore structure and structure as a whole, but the pellet of the present invention has a slag in which the surface layer portion is composed of a pore-communication structure and the calcium silicate melt is solidified at the center portion. The part consists of a pore-occluding tissue in which the pores are obstructed.

  It is a novel finding that forms the basis of the present invention to have a two-layer structure in which the surface layer portion is a pore communicating tissue and the central portion is a pore-occluding tissue.

  The reason why the novel pore structure and structure shown in FIG. 2 are realized in the fired pellet of the present invention is presumed as follows.

  Original ore particles (ore particles) that are raw materials of the present invention pellets are composed of a texture of fine hematite crystal grains of 20 μm or less, and when the heated ore structure is softened and fused, Individually divided at grain boundaries up to fine hematite crystal grain units of 20 μm or less.

The voids generated when the individually divided crystal grains recombine and the voids generated by the dolomite decarboxylation reaction (CO ₂ gas generation) become numerous fine pores and remain inside the pellet.

  On the other hand, since the ore particles that are individually divided are bonded while including a large number of fine pores, a sponge-like skeleton having a fine and complicated pore shape is formed inside the pellet. As the temperature rises further, a melt starts to be generated from the surface layer portion having a higher temperature.

  Although the melt is gradually formed in the center, the pellet has a porous and spongy pore structure, unlike a normal pellet. Get inside the pellet. The melt that has entered the inside of the pellet forms a stable spherical shape at the center of the pellet.

  For this reason, while the surface layer part of a pellet becomes a spongy structure from which the melt part has been removed, the center part has a dense structure.

  In the pellet having a novel structure according to the present invention, the surface layer portion becomes a spongy structure, so that the reducibility is extremely high. In the center, the reducing gas easily penetrates to the center of the pellet through the pores of the spongy structure, so that the reduction in the center of the pellet is likely to proceed.

  The reason why the temporal deterioration of the crushing strength is small in the pellets having a new structure is presumed as follows.

  (I) Since the melt produced by the firing reaction concentrates and solidifies in the center of the pellet, there is little opportunity for the solidified slag (slag unstable to water) to come into contact with moisture in water or air Thus, the generation of slaked lime with volume expansion is eliminated.

  (Ii) Even if slaked lime is generated and there is volume expansion at the center, the sponge-like pore communicating structure formed in the surface layer absorbs internal stress generated by volume expansion.

  In other words, if the porosity of the pellet surface layer is high and the proportion (%) of fine pores of 1 μm or less is high, even if a large internal stress is generated in the center, it can be absorbed. No cracking occurs.

(Detailed Description of the Invention of Claim 1)
The pore-localized fired pellet of the present invention (the present pellet) is based on the above findings,
(X1) The pore layer communicating structure in which the surface layer does not contain the slag structure, and
(X2) a pore-occluded tissue in which the slag portion occludes the pores at the center,
It consists of the basic features.

  Here, the center of the pellet is in the range from the center of the pellet to 1/3 of the pellet equivalent diameter, and the surface layer is in the range from the surface of the pellet to 1/3 of the pellet equivalent diameter. To do. In general, the intermediate portion exhibits a mixed organization.

  However, the range of the center part and surface part of a pellet is not limited to the said range. The ranges of the central part and the surface layer part of the fired pellets may be appropriately set in consideration of the overall structure and / or partial aspect of the pore structure and structure.

  As described above, since the pore communicating structure of the pellet surface layer part is a structure in which the crushing strength is small in deterioration with time and excellent in reducibility, the porosity of the pellet surface layer part is not particularly limited. It has already been mentioned that communication with a porosity of 20% is the boundary.

  For this reason, the porosity of the surface layer portion is 20% or more, and the reducibility of the pellets in the blast furnace is remarkably improved. From the viewpoint of further improving the reduction rate, the porosity is more preferably 30% or more. Moreover, although the upper limit of the porosity of a surface layer part is not defined in particular, what exceeds 40% is not implement | achieved.

  The porosity of the pore-occluded tissue at the center of the pellet of the present invention is 20% or less, more preferably 10% or less. When the porosity is 20% or less, deterioration with time of the crushing strength of the fired pellet is small, and the crushing strength itself can be increased.

  Note that the pore-closed structure at the center of the pellet is a structure with a significantly smaller porosity than the structure of the surface layer, but the porosity of the entire fired pellet including the porosity of the surface layer is the porosity of the conventional pellet. Therefore, there is no problem of a decrease in crushing strength due to an increase in the overall porosity.

(Detailed Description of the Invention of Claim 2)
Many of the effects of the present invention can be achieved by realizing the above-described pore structure, and more preferable results can be obtained by proper arrangement of the slag. That is, the slag content of the pore-penetrating tissue in the surface layer portion is set to 5% or less, and that of the pore-occluding tissue in the central portion is distributed to 10% or more.

  As a result, as a main structure, the slag structure in which the calcium-silicate melt causing solidification of the crushing strength with the progress of the hydration reaction of CaO and MgO in the slag is solidified more than the surface layer part. Contain.

  The pore-occluded tissue in the center of the pellet is a tissue whose porosity is significantly smaller than that of the surface layer, so that the progress of the hydration reaction of CaO, MgO, etc. is suppressed. As a result, the volume expansion due to the hydration reaction Degradation with time of the crushing strength due to this is reduced.

(Detailed Description of the Invention of Claim 3)
The basic requirements of the method for producing the pellets of the present invention will be described. The iron ore as the raw material needs to have a hematite crystal particle size of 20 μm or less. Thereby, when the heated ore structure is softened and fused, the ore particles are individually divided at the grain boundaries to promote densification of the central part.

  Currently, the only hematite iron ore that satisfies this condition is the carajas ore, but the present invention is not limited to the carajas ore. In addition, it is considered that the crystal particle diameter of goethite ore (maramanba ore or pisolite ore) is generally 20 μm or less.

However, since these ores currently have high SiO ₂ and Al ₂ O ₃ contents, the chemical composition of the raw material exceeds the range specified in Table 1 and the amount of slag produced becomes excessive, and the calcined pellets having the structure of the present invention It will not be.

Further, in the slag forming component, it is necessary to adjust the complex basicity (CaO + MgO) / SiO ₂ to 1.5 to 2.0 in the range of SiO ₂ to 1.8 to 2.2% by mass. The regulation of the amount of SiO ₂ determines the total slag amount in combination with the regulation of complex basicity, and if it is outside this range, the desired pellet structure cannot be obtained.

  In the definition of the complex basicity, when it is less than 1.5, a melt starts to be generated at a lower temperature, so that the amount of the melt is excessively increased, and the formed pores are blocked and the porosity is lowered. On the other hand, if it exceeds 2.0, the adverse effect of the excessive amount of melt in the pellet structure is reduced, but the slag volume is increased when used in the blast furnace.

  The basicity of the pellet is preferably adjusted to a composite basicity that maximizes the reducing properties and does not increase the slag volume as much as possible.

  In the pellets of the present invention, the presence form of the pores greatly changed depending on the total amount of CaO and MgO, but no significant difference was observed between CaO and MgO in the influence on the existence form of the pores.

  Therefore, in order to obtain the required pore structure in the pellets of the present invention, as described above, the composite basicity is adopted as an index, and the chemical components of the granular material before firing produced by mixing iron ore powder and auxiliary materials are defined. ing.

  The pellet of the present invention is obtained by agglomerating the above-described raw materials. At this time, the particle size of the agglomerated material may be the particle size of normal pellets, but is preferably 10 mm or more and 15.0 mm or less.

(Detailed Description of the Invention of Claim 4)
With respect to the granulation and firing steps, a method according to an ordinary fired pellet manufacturing method may be used. That is, the above-mentioned raw material is granulated with a granulator and fired in a firing furnace. The firing temperature and firing time may be normal firing temperature and firing time. The firing temperature is 1200 to 1350 ° C., preferably 1250 to 1300 ° C., and the firing time is 25 to 50 minutes, preferably 35 to 40 minutes.

(One form of manufacturing method)
Next, one form of the manufacturing method of this invention pellet is demonstrated. Fine powder obtained by pulverizing carajas iron ore and dolomite powder are mixed to obtain a raw material having a component composition shown in Table 1, and the raw material is granulated by a dish granulator to form a granular material. The fine iron ore powder is preferably as the particle size is small, and more preferably 50 μm or less. Dolomite powder is also preferred as the particle size is smaller, and more preferably 50 μm or less.

  Here, the reason why dolomite was used for adjusting the complex basicity is that it contains CaO and MgO to the same extent and both components can be adjusted at the same time, but this is not restrictive. The firing method is as described in (Detailed description of the invention according to claim 4).

  The porosity of the pellet can be controlled by appropriately setting the amount of dolomite added according to the desired porosity. However, when the amount of dolomite added is restricted due to use conditions in the blast furnace, it can also be adjusted by adding a carbonaceous material. Since the carbonaceous material is gasified during firing, the amount of pores increases due to an increase in the amount of the carbonaceous material added.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

As an actual scale production test of the pellets of the present invention, a 10-day test production was performed using a great pellet baking machine having an effective area of 500 m ² . Carajas iron ore and dolomite were used as raw materials, and coke breeze was added to control porosity and reduce fuel for heating. The used calajas ore was sampled as appropriate, filled and polished with resin, and the cross section was observed with a microscope and the crystal particle diameter was measured, and it was in the range of 1 to 10 μm.

  After pulverizing these raw materials with a ball mill and a roller press until the ratio of the particle size of 50 μm or less is 90% by mass or more, granulating with a dish type granulator while adjusting the water content to 9 to 11% by mass, 10-15 mm raw pellets were produced. The temperature in the last stage of baking was adjusted at 1250-1300 degreeC.

  Table 4 shows the results of the test operation as average data for one day.

During the 10 days, SiO ₂ is 1.8 to 2.2 mass%, and the composite basicity was adjusted in the range of 1.5 to 2.0. As a result, CaO changed in the range of 2.1 to 2.7% by mass and MgO in the range of 1.0 to 1.4% by mass. Moreover, the powder coke was adjusted in the range of 62.0-98.7 kg / t, confirming the transition of the reduction rate.

  As a result, a self-fluxing fired pellet having excellent crushability and low strength deterioration with time, with a crushing strength of 247 to 291 daN, a JIS reduction rate of 86.3 or more and a strength deterioration of 9.8 daN after 60 days over 10 days. Could be manufactured.

  As described above, according to the present invention, since pellet strength does not deteriorate with time during pellet storage, a fired pellet having high crushing strength and excellent reducibility is provided as a blast furnace charge. be able to. Therefore, the present invention has high applicability in the steel industry.

Claims (4)

A pellet obtained by firing a mixed granular material of iron ore powder and auxiliary raw material ore powder containing CaCO ₃ and / or MgCO ₃ ,
(X1) The surface layer portion is composed of a pore-communication structure having a porosity of 20% or more,
(X2) A pore-localized sintered pellet characterized in that the central portion is composed of a pore-occluding tissue having a porosity of less than 20%.   The slag content rate of the said pore continuous structure | tissue is 5 mass% or less, and the slag content rate of the said pore obstruction | occlusion structure | tissue is 10 mass% or more, The pore uneven distribution calcination pellet of Claim 1 characterized by the above-mentioned. In the method for producing pore-localized fired pellets according to claim 1 or 2,
(Y1) iron ore containing hematite crystal grains having an average particle size of 20 μm or less; and (y2) an auxiliary material containing CaCO ₃ and / or MgCO ₃ , SiO ₂ is 1.8 to 2.2, composite basicity (CaO + MgO ) / SiO ₂ is mixed to be 2.0 to 1.5,
(Y3) A method for producing pore-localized fired pellets, characterized in that a granulated material obtained by granulating the mixture is fired.   The said baking is performed at 1200-1350 degreeC and 25-50 minutes, The manufacturing method of the pore uneven distribution baking pellet of Claim 3 characterized by the above-mentioned.