ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
High Temperature Mineralization Behavior of Mixtures during Iron Ore Sintering and Optimizing Methods
Min GanXiaohui Fan Zhiyun JiXuling ChenLiang YinTao JiangGuanghui LiZhiyuan Yu
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2015 Volume 55 Issue 4 Pages 742-750

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Abstract

The structure of sinter composes of a melt zone and unfused ores. Sinter strength is mainly subjected to the properties of melt zone since unfused ores are wrapped by melt zone is proposed. It facilitates obtaining sinter of high strength with the increase of liquid phase generated in melt zone and the formation of columnar & acicular SFCA (calcium ferrite containing silicate and alumina) during melt condensation. The mineralization behaviors show that, when Ca/Fe (molar ratio) in melt zone is 0.3–0.4, the content of SiO2 (mass ratio) is about 5%, Al2O3 is less than 1.8% and MgO is as low as possible, it would benefit the generation of liquid phase and columnar & acicular SFCA. Research of optimizing ore blending indicates that, as the chemical components in melt zone satisfies the conditions of mineralization, the yield and tumbler strength of sinter can be improved, and the solid fuel consumption can be reduced.

1. Introduction

Iron ore sintering is a heat treatment process for agglomerating fine particles into larger lumps, which then serve as the major burden for blast furnace. The production of high-quality sinter is critical for efficient blast furnace operation. The quality and yield of sinter are determined to great extent by the properties of iron ore. Especially, the high-temperature mineralization properties of sintering materials, such as the physical behaviors and chemical reactions during iron ore sintering process, affect the structure and mineral compositions of sinter and have a remarkable influence on sintering yield-quality indexes and fuel consumption.1,2,3)

Mineralization reaction mainly occurs in preheating layer, combustion layer and initial cooling layer during the sintering process, which is reflected by the ability of solid-phase reactions, the capacity for the generation of liquid phase and the behavior of condensation and crystallization.4,5) In recent years, a vast of research has been carried out on the mineralization behavior of iron ores.6,7,8,9) Current research focuses on mineralization of single iron ore, and tries to evaluate mineralization abilities by testing the assimilation performance, the fluidity of liquid phase, penetration behaviour of melt etc.10,11,12,13) However, it is difficult to predict the sintering indexes according to the mineralization properties of single iron ore since the generation ability and crystallisation behavior of liquid phase are attributed to the interaction between diverse iron ores and fluxes. Consequently, no effective method of optimizing ore blending is available to regulate the high-temperature mineralization behaviour of sintering mixtures. This investigation analyses the structural characteristics of sinter and studies the major factors influencing the mineralization properties. Suitable conditions for mineralization are then revealed, and methods to optimize the mineralization process are also found out.

2. Materials and Methods

2.1. Properties of Raw Materials

The raw feed for sinter production comprises mainly iron ores with lesser amounts of coke breeze, return fines and fluxes. The chemical compositions and size characteristics (−0.5 mm fraction) of raw materials are given in Table 1.

Table 1. Physicochemical properties of sintering materials.
Material typesFraction of sizeChemical component/%Fraction of −0.5 mm/%
TFeFeOCaOMgOSiO2Al2O3LOI
Brazilall63.865.020.100.114.591.142.2340.00
Ore A−0.5 mm62.632.510.060.225.831.312.56
Brazilall67.220.140.070.271.041.001.7634.20
Ore B−0.5 mm66.520.220.030.211.161.112.00
Australiaall62.132.590.330.263.801.934.5245.20
Ore A−0.5 mm62.810.530.680.084.543.376.19
Australiaall61.930.860.040.182.931.776.2246.81
Ore B−0.5 mm62.410.570.070.263.312.105.36
Australiaall58.621.220.100.204.661.988.2221.23
Ore C−0.5 mm57.570.100.770.285.341.998.71
Australiaall57.211.290.010.105.621.3711.0720.45
Ore D−0.5 mm56.340.720.060.286.342.4011.35
Indiaall63.321.720.050.162.842.323.5247.75
Ore A−0.5 mm63.840.500.070.183.332.252.74
Indiaall62.801.540.290.344.792.642.0246.31
Ore B−0.5 mm61.101.980.260.356.033.52.22
Southall65.040.570.150.174.011.440.9816.30
Africa Ore−0.5 mm63.000.860.260.225.381.991.59
Chineseall63.4622.421.190.825.970.891.1194.62
Ore A−0.5 mm63.5523.101.020.775.850.831.10
Chineseall60.0718.111.330.937.002.163.0298.90
Ore B−0.5 mm60.1318.351.220.906.892.122.95
Quicklimeall0.400.2380.661.182.861.2012.3657.13
Limestoneall0.210.1350.662.281.490.7240.7234.57
Dolomiteall0.140.1032.6419.830.710.4346.4738.68
Coke breezeall0.800.115.720.600.124.8486.5958.27
Return finesall56.816.259.021.865.112.000.0037.51

Eleven kinds of typical iron ores are used in this investigation, which are of great representativeness. They are Brazilian hematite with high total iron content, Australian limonite, Indian iron ore with high alumina content, South African dense hematite and Chinese fine-grained magnetite respectively. The chemical compositions of iron ores and their proportion of −0.5 mm fraction are used to design the compositions of sinter and melt zone in sinter respectively. A range of iron ores are used in varying proportions to prepare ore blends with different compositions, so that Fe, SiO2 and Al2O3 contents of sinter can be adjusted at a relatively wide range by blending ores. And fluxes with conventional compositions and grain sizes from the ordinary sintering plants, including limestone, dolomite and quicklime, are used to provide needed CaO and MgO components for sinter. Also, coke breeze and return fines are added into mixtures as producing sinter.

2.2. Experimental Methods

2.2.1. Mineralization Test

Mini-sintering test was used to research the mineralization reactions, such as liquid generation, crystallisation behaviour, ect., under high temperature. Horizontal heating furnace whose temperature and atmosphere could be controlled by program was adopted to conduct mini-sintering experiment. The device was shown in Fig. 1.

Fig. 1.

Device of mini-sintering test.

In order to simulate sintering process exactly, sintering process was divided into preheating layer, reaction layer, melt layer, solidification layer and sinter layer. Based on the physicochemical characteristics of each layer and the actual temperature curve of sintering bed, the heating program and atmosphere simulated during mini-sintering test were shown in Table 2 and Fig. 2. The actual temperature curve was tested on the conditions of suitable content of coke breeze and mixture moisture, as the sintering indexes could be obtained optimally. As shown in Fig. 2, the maximum temperature of sintering bed is about 1300°C, which was used to study the mineralization behaviors.

Table 2. Heating program and atmosphere for min-sintering test.
Temperature/°CHeating up time/minAtmosphere
Preheating layer60→7001N2
Reaction layer700→12001CO:O2:CO2=1:1:5
Melt layer1200→1300With the velocity of 10°C/minCO:O2:CO2=1:1:5
Solidification layer1300→10002Air
Sintered layer1000→7001Air
Fig. 2.

The change of temperature in sintering bed.

Liquid generation was determined by mini-sintering method, which included ore blending, briquetting, sintering and cooling. The principle of ore blending was that adhesive fines (−0.5 mm) of iron ore were mixed with all fluxes. The mixture then was ground to −0.074 mm and compressed into a standard pyrometric cone with 20 mm height and a regular triangle bottom whose side lengths were 5 mm. Samples of pyrometric cone were settled on a pallet, and then the pallet was sent into the heating furnace, which was roasted under the conditions outlined in Table 2.

There is relationship between the generation of liquid phase and the shrinkage degree of pyrometric cone. The easier the mixture generate liquid phase, the more susceptible to shrinkage of pyrometric cone during sintering. Therefore, the apparent amount of liquid phase(η) could be reflected by the deformation of pyrometric cone, which was defined as the variance of projection area between 1000°C and 1300°C. Liquid phase content was calculated by Eq. (1). While roasted, a group of triangular pyramid shape pictures were taken by camera per 10 seconds, and the areas of pyrometric cone at 1000°C and 1300°C could be obtained through image processing software.   

η=( S 1   000 - S 1   300 )/ S 1   000 ×100% (1)
where η is the amount of liquid phase, S1300 is the area of pyrometric cone in 1300°C, S1000 is the area of pyrometric cone in 1000°C.

The research regarding the formation behavior of SFCA (Quaternary compound of calcium ferrite containing silicate and alumina) also adopted mini-sintering method. After ore blending, the mixture was compacted into a cylinder with the size of Φ30×25 mm under the pressure of 300 kg/cm2 for 1 min. Then the cylinders were sintered according to the heating programs in Table 2. The sintered samples were used to observe the mineralization of sintering mixture. The agglomerates were mounted with epoxy resin, and then polished to form a section, in which the microstructures were observed by optical microscope and SEM, and mineral components were detected by image analysis software. Columnar & acicular SFCA was defined as the ratio of length-diameter was bigger than 2.5. The graphic processing software could recognize the SFCA and figure out its content. The process consisted of image reading, image filtering, identification and segmentation, length-diameter ratio detection, and statistics of selected area.

Fracture toughness can objectively measure the crack extension of the mineral, which reflects the resistance of minerals fracture. Vickers indentation test method was used to measure the fracture toughness of minerals. This method adopted Japanese Micro-hardness Tester FM-700 durometer. SFCA in different morphologies were suppressed on their surface by the indenter of durometer. The generation and development of cracks were studied under different pressures to measure the mineral microstrength.14)

2.2.2 Sinter Pot Test

Sintering process was simulated in a sinter pot of Φ180×700 mm. Raw materials having been blended and granulated were charged into the sinter pot. Under the mixtures, a hearth layer of approximate 20 mm thick was previously prepared to protect the grate from thermal erosion. After charging, the fuel in the surface layer was ignited at 1150±50°C by an ignition hood initially, and then the combustion front moved downwards with the support of downdraught system, which was mainly a draught fan used to enable sufficient air to be sucked into sinter pot from top at a negative pressure of 10 kpa. As sintering end, the total sintering time was record which started from ignition to the point where the sinter waste gas had reached a maximum temperature. Before unloading, 3 min for cooling was necessary. Then dropping test (2 m×3 times), screening and drum strength determination were carried out to value the sintering indexes, which were sintering vertical speed, yield, productivity and tumbler index.

3. Results and Discussion

3.1 Structure Model of Sinter

The mineralization process of mixtures during sintering was researched,15) and the conclusions are shown in Fig. 3. After granulation, the granulated mixtures are used to sinter. Original granules are composed of adhesive layers and nuclear particles. Solid-phase reactions occur between fine particles of iron ores and fluxes in the adhesive layer to form low-melting compounds. When the temperature is raised to melting point of compounds, the primary liquid phase begins to generate in the adhesive layer. Under the action of primary liquid phase, nuclear particles of fluxes, such as limestone, dolomite and quicklime, are melted in the liquid phase to develop the amount of liquid, but iron ore nuclei almost would not participate in the reaction for its low reacting speed. As the temperature rises continually, liquid phase covers together due to the improvement of liquid fluidity, and pores emerge by the shrinkage of liquid phase at the maximum temperature. During temperature-fall process, crystals start to forming with the condensation of liquid phase. Therefore, the liquid bonds unmelted ores to form the final sinter.

Fig. 3.

Schematic diagram of sintering mixture mineralization processes.

The structures of sinter are shown in Fig. 4. The macrostructure of sinter can be divided into two parts, the melt zone and unfused ores, which is composed of the melt bonding the unfused ores together. The author has discovered that almost all fluxes participated in mineralization during sintering, and the particle size of iron ores involved in mineralization was proved to be less than 0.5 mm,16) which indicated that fluxes reacted with the fine iron ores(−0.5 mm) to form the melt zone, while the coarse iron ores(+0.5 mm) remained as the unfused zone. Since unfused ores are wrapped in melt zone, the strength of sinter is subjected to the properties of melt zone.

Fig. 4.

Structure of sinter.

Liquid phase is the basis of sinter solidification, the quantity of liquid phase generated in melt zone therefore is one of the most important factors which determine the sinter strength. The influences of the amount of liquid phase on tumbler strength of sinter are researched in the paper. According to 60 groups of sintering test data, a good linear relationship between tumbler strength and the amount of liquid phase was observed (Fig. 5). With the improvement of liquid production in melt zone, tumbler strength achieves a gradual increase. As the amount of liquid phase increases by 1%, tumbler strength increases by 0.14%.

Fig. 5.

Influence of liquid phase amount on tumbler strength of sinter.

The sinter strength is not only related to the amount of liquid phase, but associated with the composition and morphology of minerals formed during liquid condensation. As shown in Fig. 6, the microstructure of melt zone mainly presents as the “corrosion structure” (the texture of precipitated crystals forms from molten phase during condensation process, which presents closely intertwined status in sinter) of magnetite and SFCA, which accounts for 80%–90% in melt zone. And a few partial areas rich in porosity present as mixed structure of hematite and SFCA, or the eutectic structure of SFCA and silicate. Among the three structures, SFCA is the most important bonding phase in melt zone. Following from the difference in the morphology characteristics of calcium ferrite, SFCA can be divided into four types of structure, including plate-type, sheet-type, columnar-type, and acicular-type. Consider the differences from the ratio of length-diameter, the sheet-type and plate-type SFCA is lower than that of columnar-type and acicular-type SFCA. In addition, the columnar and sheet structures can be differentiated from the shapes. Columnar-type SFCA is the regular long strip relatively, but sheet-type SFCA is no uniform shape.

Fig. 6.

Structural morphology of SFCA.

The results of energy spectrum analyses and fracture toughness tests for different types of SFCA are shown in Table 3. It can be seen that SFCA of acicular-type and columnar-type have lower Fe2O3 content than plate-type and sheet-type, but higher contents of Ca and Si. There is no obvious difference in the content of Al2O3 among four structures, while SFCA of columnar & acicular-type has lower content of MgO than plate & sheet-type. The components of columnar-SFCA is closed to calcium diferrite (ω (CaO) = 14.9), and acicular-SFCA has a relative component between calcium diferrite and the eutectic chemicals of CaO·2Fe2O3–CaO·Fe2O3.

Table 3. Component and fracture toughness of various SFCA.
Structure of SFCAFracture toughness/
MPa·m−2
Chemical component/%
CaOFe2O3SiO2Al2O3MgO
Platy-type0.859.07–10.6582.50–89.002.59–4.052.83–4.160.55–1.92
Sheet-type0.9110.75–12.4971.79–85.663.73–6.663.03–4.110.92–2.28
Columnar-type1.3313.27–15.4368.07–78.867.09–9.263.39–4.440.40–1.43
Acicular-type1.3913.99–17.0470.57–75.376.60–9.993.20–4.250.57–0.80

Fracture toughness was used to measure the microstrength of various types of SFCA. As shown in Table 3, the order of the strength of four kinds of SFCA is acicular-type>columnar-type>sheet-type>platy-type, while the strength of the sheet-type and plate-type are close, as well as the acicular-type and columnar-type. Due to their similar strength, acicular-type and columnar-type are named as columnar & acicular-type, as well as the plate & sheet-type. The corrosion structure of magnetite and the columnar & acicular-type is the best microstructure with the highest strength.

Consequently, increasing the content of liquid phase in melt zone and developing the bonding phase that is mainly composed by columnar & acicular-type SFCA seem to be effective measures to improve sinter strength.

3.2. The Influencing Factors of Mineralization in Melt Zone

The chemical composition of melt zone plays a considerably important role in liquid phase. The chemical composition of melt zone can be calculated by Eq. (2). On the basis of that, all of the fluxes react with the fine iron ores less than 0.5 mm to form the melt zone. According to the equation, as knowing the composition of raw materials, adhesive fines (−0.5 mm) content and the proportions of raw materials, the chemical compositions in melt zone can be figured out.   

w(Q)= x i x i -0.5 w i Q + x j w j Q ( x i x i -0.5 + x j ) ( 1- x i x i -0.5 w i LOI - x j w j LOI ) (2)
Where w(Q) is the content of chemical composition Q in melt zone, %;

xi is the ratio of iron ore i in the mixture, %;

xi−0.5 is the content of fine grains (−0.5 mm) in ore i, %;

wiQ is the content of chemical composition Q in adhesive fines (−0.5 mm) of ore i, %;

wiLOI is the loss on ignition of fraction −0.5 mm in ore i, %;

xj is the ratio of flux j in the mixture, %;

wjQ is the content of chemical composition Q in flux j, %;

wjLOI is the loss on ignition of flux j,%.

The influences of the molar ratio of Ca/Fe, the contents of SiO2, Al2O3 and MgO on the generation features of liquid phase in melt zone were studied. The results are shown in Fig. 7. The molar ratio of Ca/Fe and the content of MgO were changed by adding calcium or magnesium fluxes, and the contents of SiO2 and Al2O3 were changed by regulating the types of iron ores used. With the increase of the molar ratio of Ca/Fe, the content of liquid phase in melt zone increases. When the ratio exceeds 0.3, more than 80% of the materials can be translated into liquid phase. As Ca/Fe ranges from 0.35 to 0.5, all of the melt zone can be formed into liquid phase. For the content of SiO2, the production of liquid phase varies slightly as SiO2 ranges between 4.4%–5.4%. While the content of SiO2 exceeds 5.4%, the liquid phase decreases instead. As increasing the content of Al2O3 in melt zone, the amount of generated liquid phase decreases slowly when Al2O3 is below 1.8%. However, the content of liquid phase would reduce sharply as the proportion of Al2O3 exceeds 1.8%. With the increase of MgO content, the production of liquid phase declines gradually. The reason is that minerals containing magnesium with high melting temperature are formed in the process of mineralization, including forsterite (melting point 1890°C), monticellite (melting point 1454°C), manganolite(melting point 1570°C), akermanite (melting point 1454°C) and so on. While the content of MgO increases from 1.38% to 6.48%, the liquid phase decreases from 100% to 67.24%.

Fig. 7.

Influence of chemical components on the generation of liquid phase in melt zone.

The influences of Ca/Fe, SiO2, Al2O3 and MgO on the generation of SFCA in melt zone were studied. The results are shown in Figs. 8 and 9. The total content of SFCA increases with the improvement of the molar ratio of Ca/Fe, but columnar & acicular-SFCA increases first and then decreases. When Ca/Fe is 0.23, the morphology of SFCA is mainly platy-type (Fig. 9(a)). As Ca/Fe reaches to 0.3–0.4, the main form of SFCA exists as columnar & acicular-type, which reaches the maximum amount (Figs. 9(b) and 9(c)). When Ca/Fe increases to 0.5, the content of columnar & acicular-SFCA decreases instead, and sheet-type SFCA of interconnection mode forms remarkably (Fig. 9(d)).

Fig. 8.

Influence of the chemical component on the generation of SFCA in melt zone.

Fig. 9.

Influence of chemical component to the microstructure of fusion zone.

For the influence of SiO2, when the content of SiO2 increases from 4.3% to 5.0%, the content of columnar & acicular-SFCA increases to some extent (Figs. 9(e) and 9(b)). SiO2 content of 5.0% would benefit the generation of columnar & acicular-SFCA because SiO2 is an important ingredient for SFCA formation. While SiO2 exceeds 5.0%, the reaction between CaO and SiO2 occurs more easily than that between CaO and Fe2O3.17) As a consequence, the amount of SFCA coming from the reaction between CaO and Fe2O3 is reduced. And it facilitates producing platy SFCA other than columnar & acicular-type since more silicate is generated (Fig. 9(f)).

Al2O3 is also an influencing factor for SFCA. When the content of Al2O3 is under 1.8%, melt zone mainly consists of the corrosion structure formed by SFCA and magnetite. But the microstructure would change markedly when the content of Al2O3 is more than 1.8%. The columnar & acicular-SFCA is suppressed and the platy SFCA gets developed (Fig. 9(g)). When the content of Al2O3 is excessively high, it would not only increase the melting point of sintering mix, but also increase the viscosity of liquid phase.18,19) Thus the fluidity of liquid phase is worsened, which makes it difficult for the precipitation of columnar & acicular-SFCA that crystallises along one-way extension, and makes the size and quantity of the pores in the melt zone increase.

With the increase of the content of MgO, the content of SFCA decreases. And the main reason is that Mg2+ entered into the crystal lattice of magnetite, forming magnesiaspinel [(Fe,Mg)O·Fe2O3].17) The crystal lattice of magnetite is stabilised by solid solution of Mg2+. As a result, it would suppress the formation of SFCA by preventing the oxidising reaction from magnetite to hematite. With the increase of MgO from 1.4% to 6.4%, the content of columnar & acicular-SFCA decreases from 34.67% to 18.17%. When the content of MgO is excessively high, lumpy pieces of recrystallisation magnetite are generated in melt zone.

Synthesising the impact of CaO/Fe2O3, SiO2, Al2O3, and MgO on the amount of generated liquid phase and columnar & acicular-SFCA, the suitable chemical components of melt zone which are advantage to mineralization have been proved to be that, the molar ratio of Ca/Fe is 0.3–0.4, the content of SiO2 is about 5%, the content of Al2O3 is less than 1.8%, the content of MgO should be controlled as low as possible under the condition guaranteeing the slag-making of blast furnace. In accordance with the principles introduced above (Table 4), the performance of mixtures on the mineralization can be optimized.

Table 4. Suitable ranges of chemical component in melt zone for mineralization.
Suitable rangesCa/Fe (molar ratio)SiO2 content/%Al2O3 content/%MgO content/%
For liquid phase≥0.3≤5.4≤1.8≤3.4
For SFCA0.3–0.4about 5.0%≤1.8Low as possible
Comprehensive the mineralization0.3–0.4about 5.0%≤1.8Low as possible

3.3 Methods of Optimizing Mineralization

There are 11 kinds of iron ores used for blending in this test, whose contents of −0.5 mm fraction and the corresponding chemical component are shown in Table 1. The experimental conditions are that ore blends, coke breeze and fluxes, including dolomite, limestone, quicklime are mixed together, which produces sinter with SiO24.8%, basicity (CaO/SiO2) 2.0 and MgO 2.0%. The chemical components in melt zone can be regulated by adjusting the blending schemes of iron ores to improve the generation of liquid phase and columnar & acicular-SFCA.

The basic ore blending scheme is shown in Table 5 and the chemical composition of melt zone is shown in Table 6. According the suitable ranges of relative components, the molar ratio of Ca/Fe and the contents of SiO2, Al2O3 are all beyond their appropriate values in basic scheme. Both the liquid phase and columnar & acicular-SFCA are low because the chemical component of melt zone fails to meet the expected match.

Table 5. Ore blending schemes/%.
SchemeBO ABO BAO AAO BAO CAO DIO AIO BSAOCO ACO B
Basic5.57.010.08.55.55.07.020.04.016.011.5
Optimization A18.00.010.08.55.55.05.05.020.021.02.0
Optimization B24.03.05.010.05.55.00.010.018.014.55.0

BO—Brazil Ore, AO—Australia Ore, IO—India Ore, SAO—African Ore, DO—Chinese Ore

Table 6. Chemical component of melt zone and the property of mineralization.
SchemeChemical component of melt zoneContent of liquid phase/%Content of C.&A.-SFCA/%
Molar ratio of Ca/FeSiO2/
%
Al2O3/
%
MgO/
%
Basic0.285.432.032.9875.8523.86
Optimization A0.325.081.783.0896.5632.66
Optimization B0.335.161.823.1492.3334.27

In order to optimize the chemical components of melt zone, it is necessary to increase the molar ratio of Ca/Fe and reduce the contents of Al2O3 and SiO2 in the fraction of −0.5 mm on the basis of basic scheme. The methods of optimization are decreasing the content of −0.5 mm in blending ores to raise the Ca/Fe, and decreasing the ores of high Al2O3 and SiO2. So the optimization schemes are reducing the ratio of Chinese ore B, Australian ore A, Indian ore A, and Indian ore B, at the same time increasing the ratio of Brazilian ore A, Chinese ore A and South African ore. Two ore blending schemes of optimization are shown in Table 5.

The chemical components of melt zone are shown in Table 6 after optimizing the ore blending. It can be seen that the molar ratio of Ca/Fe of Optimization A and Optimization B increased to 0.32 and 0.33 respectively, the content of SiO2 decreased to 5.08% and 5.16%, and Al2O3 decreased to 1.78% and 1.82%, all reaching or approaching the suitable range for mineralization. Compared with the basic scheme, the liquid phases of two optimized schemes increase from 75.85% to 96.56%, 92.33% respectively, and the contents of columnar & acicular-SFCA increase from 23.86% to 32.66%, 34.27% respectively.

The influences of mineralization improvement by optimizing the ore blending on sintering are shown in Table 7. When the proportion of coke breeze remained at 5%, sintering speed of two optimizing schemes increases from 21.94 mm/min to 23.55 mm/min, 23.78 mm/min, the yield increases from 72.66 to 74.05%, 73.69%, the productivity increases from 1.48 t/(m2·h) to 1.60 t/(m2·h), 1.59 t/(m2·h), and the tumbler strength increases from 65.00% to 66.40%, 66.45%. When the production and quality indexes are comparative, the ratio of coke breeze can be decreased from 5.0% to 4.7%, and the solid fuel consumption reduces by 5.6%.

Table 7. Iinfluence of the optimization of mineralization to the sintering.
SchemeRatio of coke breeze/%Sintering speed/
mm·min−1
Yield/%Tumbler index/%Productivity/
t·m−2·h−1
Standard5.021.9472.6665.001.48
Optimization A5.023.5574.0566.741.60
Optimization A4.722.2271.8864.561.48
Optimization B5.023.7873.6966.451.59

4. Conclusions

(1) There is a good linear relationship between the amount of generated liquid phase of fusion zone and the tumbler index of sinter. With the amount of generated liquid phase rising, the tumbler index increases.

(2) SFCA can be divided into four structural types, including plate-type, sheet-type, columnar-type, and acicular-type. The strength of columnar & acicular-type SFCA is better than that of plate & sheet-type.

(3) The suitable chemical components for mineralization of the melt zone are that, the molar ratio of Ca/Fe is 0.3–0.4, the content of SiO2 is about 5%, the content of Al2O3 is less than 1.8%, and the content of MgO should be controlled as low as possible for guaranteeing the slag-making of blast furnace, which synthesis the impact on liquid phase and columnar & acicular-SFCA generation.

(4) After optimizing ore blending, the molar ratio of Ca/Fe in melt zone increases from 0.28 to 0.32, the content of SiO2 decreases from 5.43% to 5.08%, and the content of Al2O3 decreases from 2.03% to 1.78%. Relatively, the amount of generated liquid phase increases from 75.85% to 96.56%, the content of columnar & acicular-SFCA increases from 23.86%,to 32.66%, which result in that the yield increases to 74.05% from 72.66%, the tumbler strength increases to 66.40% from 65.00%. And the solid fuel consumption is reduced by 5.6%.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (no. 51304245) and the Postdoctoral Science Foundation (no. 2013M540639 and no. 2014T70691) for supporting this research.

References
 
© 2015 by The Iron and Steel Institute of Japan
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