Interaction between iron ore and magnesium additives during induration process of pellets

Journal Pre-proof Interaction between iron ore and magnesium additives during induration process of pellets

Rongrong Wang, Jianliang Zhang, Zhengjian Liu, Xingle Liu, Chenyang Xu, Yang Li PII:

S0032-5910(19)30953-2

DOI:

https://doi.org/10.1016/j.powtec.2019.11.006

Reference:

PTEC 14888

To appear in:

Powder Technology

Received date:

20 March 2019

Revised date:

14 October 2019

Accepted date:

4 November 2019

Please cite this article as: R. Wang, J. Zhang, Z. Liu, et al., Interaction between iron ore and magnesium additives during induration process of pellets, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2019.11.006

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© 2019 Published by Elsevier.

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Interaction between iron ore and magnesium additives during induration process of pellets Rongrong WANG 1, Jianliang Zhang1,2, Zhengjian Liu1*, Xingle Liu1, Chenyang Xu1, Yang Li1 (1. University of Science and Technology Beijing, Beijing, 100083, China; 2. School of Chemical Engineering, The

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University of Queensland, St Lucia,QLD 4072, Australia)

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Abstract: To eliminate the adverse effect of incompletely mineralized magnesium

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additive on pellets strength, clarify the interaction between additives and iron ore

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during the induration process, two types of interaction couples were prepared and

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scanning electron microscopy-Energy dispersive spectrometer was used to study the diffusion of elements and phase transformation during the induration. Results show

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that the diffusion of Mg dominates both interactions. The diffusion of Fe from ore to

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magnesia fines and magnesium olivine are accompanied by the migration of Si, the

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former generates Magnesioferrite and Forsterite while the latter also generates Fayalite. For the diffusion of Mg from magnesia fines to ore, Mg only reacts with iron oxides and forms Magnesioferrite, but in the ore-olivine couple, Mg both diffuses into iron oxides and silicate, which indicates that the reaction mechanism of two couples is different. The interaction schematic diagrams of each couple are given in this paper. Keywords: iron ore; magnesium olivine; magnesia fines; induration process; interaction mechanism

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* Corresponding author. Tel.: +86-10-62332550.E-mail address: [email protected] (Z. Liu)

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1. Introduction It is predictably that with the improvement of environmental protection standard and development of large blast furnaces in steel enterprises in China, pellets, compared with sinter, application fraction of which in blast furnace will gradually increase due to their advantages of higher grade, lower energy consumption, greater

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strength and better high temperature performance.

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Magnesium containing pellets were first produced and used in European and

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North American countries in 1980s, and now they have been added in BF for almost

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100% in LKAB and so on European as well as North American enterprises. The

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research and utilization of magnesium containing pellets have also been conducted in Chinese enterprises in recent years, such as Shougang Jingtang, Baotou Steel and

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Ansteel, which has gained good application results[1-3]. It can be seen that magnesium

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containing pellets have good development prospect.

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However, although magnesium containing pellets have the advantages of good reducibility, low reduction differentiation rate, low reduction swelling index and excellent softening and melting performance[4-13], it cannot be ignored that adding magnesium additives will have certainly influence on the compressive strength of pellets, which have been proved by many researchers[14-19]. Our previous study[20] has also validated this phenomenon, and founded that there was incompletely mineralized additive (which had not totally reacted with iron ore) existed in pellets, which increased the porosity of pellets and hindered the recrystallization of hematite, further reduced the strength of pellets.

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In order to clarify the interaction between additives and iron ore during the induration process of pellets, interaction couple experiments[21-22] were conducted and diffusion performance of elements was studied. It is aimed to lay a theoretical foundation for the improvement of additive mineralization and promotion of pellets.

2. Experimental

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2.1 Raw materials

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Two types of iron ore powder provided by an iron and steel enterprise, magnesia

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fines and magnesium olivine were applied to prepare interaction couples, the

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composition and size properties of raw materials are shown in Table 1 and Table 2,

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respectively. The magnesia fines have a large MgO content of 83.30%, while magnesium olivine is a naturally occurring mineral composed of SiO 2 and MgO, with

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2.2 Sample Preparation

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an effective MgO content of 48.73%.

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The preparation of interaction couple was conducted as follows. Iron ore powder mixtures, magnesia fines and magnesium olivine were placed in a die separately and pressed into briquettes in a uniaxial hydraulic press (as shown in Fig 1) by applying a pressure of 125MPa for 2min, then the iron ore powder briquettes (short as ore briquette), magnesia fines briquettes (short as fines briquette) and magnesium olivine briquettes (short as olivine briquette) were obtained. After that, all types of briquettes were sintered in a muffle furnace for 10h under 1300°C. After cooled to room temperature, the briquettes were cut and polished to obtain mirror surface, then the mirror surface of different briquettes were fit closely to make ore-fines couple and

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ore-olivine couple. The prepared couples were roasted in the muffle furnace under 1300°C subsequently to simulate the roasting process of pellets. Fig.2 shows the schematic diagram of interaction couple before and after roasting. 2.3 Measurement of Product Layer Thickness The thickness of product layer in additive briquette (product layer 2 in Fig.2) was

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determined by the SEM images observed before. The SEM image was first divided

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into 14 parts uniformly, as shown in Fig.3. Thickness of each product layer was

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named as L1, L2, L3, ...... L15, respectively. Then the average thickness (L) could be in ore briquette (product layer

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obtained by Eq.(1). The thickness of product layer

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1 in Fig.2) was determined by the mapping results of SEM images observed before and measured by the same method.

L1  L2  ...  L15 15

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L

(1)

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3. Results and discussion

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3.1 Initial microscopic morphology and phase composition of briquettes In order to clarify the material migration and phase transition in the interaction couple during induration process, the microstructure and phase composition of three types of briquettes (iron ore powder, magnesia fines and magnesium olivine) were analyzed in this section. Fig. 4~6 presents the SEM images of briquettes which have been roasted under the temperature of 1300°C for 10 h. The corresponding phases of points marked in each Figure were identified by EDS, as shown in Table 3 ~ Table 5. It should be noted that when an elemental content is less than 1 wt%, the number is only indicative due to the accuracy limits of EDS analysis.

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It can be seen from Fig. 4 and Table 3 that in ore briquette, the bright areas (P1 and P2) represent the particles of Hematite with a high purity and almost no impurities exists. The darker areas (P3 and P4) represent the Silicate phase, which is distributed among Hematite particles. The mass ratio of Mg is about 2 wt% and the initial content of Si is 33 wt%. The black areas (P5 and P6) represent the pores

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between Hematite particles with irregular shapes.

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Fig. 5 and Table 4 indicate that in fines briquette, most of areas (P1 to P4) have

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the Mg content of about 70%, as well as a small amount of Si and Ca, that means the

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phase is Magnesia. Brighter phase in SEM image, as marked in P5 has relative higher

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Si and Ca content, indicating that the phase here is Silicate phase. As can be seen from the whole SEM image, the content of Silicate in the briquette is very small, and

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Magnesia is the main phase.

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As can be seen from Fig.6 and Table 5, dark areas dominate the SEM image,

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EDS analysis results of P1 and P2 show that the corresponding phase is Forsterite, which has a high degree of crystallization, the mass ratio of Fe in this phase is about 6.8 wt%. According to the EDS results of P3 and P4, the white area is mainly Magnesioferrite with a high Fe content, and the content of Mg is about 9 wt%. The ratio of Magnesioferrite in olivine briquette is relatively low. In addition, there are a few pores in the olivine briquette. 3.2 Interaction between iron ore and magnesia fines The ore-fines couple was roasted in the muffle furnace under 1300°C to simulate the induration process of pellets. The elemental distribution of product layer in fines

Journal Pre-proof briquette is shown in Fig.7. The diffusion thickness of Fe was measured at 204μm. Fig.8 presents the SEM image of product layer marked in Fig.7 at a higher magnification. As can be seen from the elemental distribution results, Si also exists in the enrichment area of Fe in the briquette, indicating that both elements diffused into

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magnesia fines and reacted during the reaction process. EDS analysis results of

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reaction interface outer layer show that the main product is Magnesioferrite, Mg

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content of which is 7.8 wt%; As Fe further spreads into inner magnesia fines, the

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reaction product changed into Forsterite, among which the content of Fe is much less

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than the outer layer, and content of Si is about 25~30 wt %, it is much higher than the initial content in magnesia fines before induration. Based on above analysis, a

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conclusion could be drawn that during the reaction process, most of the Fe reacts with

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magnesia fines in the outer layer and generates Magnesioferrite, a small number of Fe

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further spreads into inside and accompanied by Si, then forms Forsterite. The elemental distribution of product layer in ore briquette is shown in Fig.9. The diffusion thickness of Mg was measured at 316μm. Fig.10 presents the SEM image of product layer at a higher magnification. According to the element distribution results, there is no enrichment phenomenon of other elements in the area enriched by Mg in the ore briquette, indicating that only Mg diffused into the iron ore and reacted during the induration process. Fig. 10 shows that the reaction product is mainly Magnesioferrite which exists in the area of initial Hematite. Mg content in the generated Magnesioferrite is

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about 6 wt% and as Mg further diffused into inner area of iron ore, the content decreases slightly. EDS results of silicate phase show that Mg content was the same as that in the initial iron ore briquette, indicating that there is no Mg diffused into silicate phase of iron ore. By analyzing the diffusion of Mg and Fe in ore-fines couple, the mechanism

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diagram of the reaction between iron ore and magnesia fines can be obtained, as

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shown blow:

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As can be seen from Fig. 11, Fe, Si and Mg diffuse during the reaction and the

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products are Magnesioferrite and Forsterite. Most Fe diffuses into magnesia fines and

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generates Magnesioferrite which distributed evenly in the outer layer, the rest of Fe and Si from the silicate phase further diffuse and generate Forsterite. Mg only diffuses

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into the Hematite phase of iron ore and generates Magnesioferrite.

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3.3 Interaction between iron ore and magnesium olivine

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The ore-olivine couple was roasted in the muffle furnace under 1300°C to simulate the induration process of pellets. The SEM image of product layer in olivine briquette is shown in Fig.12(a). The diffusion thickness of Fe was measured at 58μm. Fig. 12(b) presents the SEM image of product layer marked in Fig.12(a) at a higher magnification. As can be seen from Fig.12, no obvious diffusion layer forms during the diffusion of Fe in olivine briquette, the products are mainly Magnesioferrite, Forsterite(higher Fe content) and Fayalite. Magnesioferrite particles distributes evenly in the reaction interface, size of these particles is much greater than the initial particle

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size in olivine briquette before roasting. Mg content of these Magnesioferrite particles is 7.18 wt%, which is lower than that of particles in the initinal olivine briquette, thus it can be deduced that most of Fe diffuses into the initial Magnesioferrite particles in briquette and lead to the increase of particle size. The generated Forsterite(higher Fe content) mainly exists in the area of original forsterite area, compared with the initial

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composition of forsterite, the content of Fe and Si increases while content of Mg

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decreases, indicating that both Fe and Si diffuses to the forsterite. Moreover, Fayalite

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exists mainly nearby the Magnesioferrite particles with a Fe content of 22.46 wt% and

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Mg content of merely 4.09 wt%, which indicates that Fe diffuses into the forsterite

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phase and replaces Mg in the original phase then forms Fayalite. Meanwhile, the content of Si in Fayalite is 38.60 wt%, further demonstrated that the diffusion of Fe is

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accompanied by the diffusion of Si.

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The elemental distribution of product layer in ore briquette is shown in Fig.13.

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The diffusion thickness of Mg was measured at 271μm. Fig.14 presents the SEM image of product layer at a higher magnification. According to the element distribution results, there is no enrichment phenomenon of other elements in the area enriched by Mg in the ore briquette, indicating that only Mg diffuses into the iron ore and reacts during the induration process. Fig. 14 shows that the reaction product is mainly Magnesioferrite, Mg content of which is about 6 wt%. EDS result of silicate phase shows that compared with the initial iron ore briquette, Mg content increases into 3.9 wt%, indicating that Mg diffuses into both silicate phase and hematite.

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By analyzing the diffusion of Mg and Fe in ore-olivine couple, the mechanism diagram of the reaction between iron ore and magnesium olivine can be obtained, as shown blow: As can be seen from Fig. 15, Fe, Si and Mg diffuse during the reaction and the products are Magnesioferrite, Fayalite and Forsterite. Most of Fe diffuses into the

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initial Magnesioferrite particles, which leads to the decrease of Mg content and the

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increase of particle size. The accompanied diffusion of Si leads to the formation of

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Fayalite near the Magnesioferrite particles. The rest of Fe diffuses uniformly in the

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and silicate phases of iron ore.

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outer layer and increases the Fe content of Forsterite. Mg diffuses into both Hematite

4. Conclusion

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The interaction between iron ore and two types of magnesium additives as well

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as the mechanism during induration process of pellets are investigated in the present

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work. The main findings are summarized below: (1). The product layer thickness of Fe and Mg in iron ore powder-magnesia fines couple is 204μm and 306μm respectively, while in iron ore powder -magnesium olivine couple, the thickness is 58μm and 271μm respectively. (2). The migration of Mg dominates the reaction between iron ore powder and both additives. Iron ore powder is more likely to react with magnesia fines than magnesium olivine. (3). The diffusion of Fe from ore to magnesia fines is accompanied by the migration of Si, which reacts with Magnesia and generates Magnesioferrite as well as

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Forsterite with a small content of Fe; For the diffusion of Mg from magnesia fines to ore, Mg only reacts with Hematite and forms Magnesioferrite, without the diffusion process to Silicate. (4). The diffusion of Fe from ore to magnesium olivine mainly generates Magnesioferrite, Forsterite with a higher Fe content and Fayalite; the diffusion of Mg

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contains both the migration from olivine to Hematite and the migration to silicate ,

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most of which diffuses to the Hematite and generates Magnesioferrite.

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Acknowledgments

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The authors acknowledge the support of the National Natural Science

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Foundation of China (51874025) and National Key Research and Development Program of China (2017YFB0304300 & 2017YFB0304302).

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Reference:

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Journal Pre-proof Table 1

Chemical composition of raw materials, wt%

FeO

SiO2

CaO

MgO

Al2O3

TiO2

S

P

Ore A

69.30

29.70

1.84

0.31

0.67

<0.05

0.074

0.20

0.004

Ore B

59.52

27.0

7.58

1.90

2.38

1.37

0.22

0.65

0.054

Magnesium fines

0.42

0.033

9.06

1.51

83.30

0.62

-

0.051

-

Magnesium olivine

5.18

6.42

41.63

0.116

48.73

0.642

-

-

-

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Pr

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TFe

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Size properties of raw materials -0.074mm, %

Ore A

1404.3

99.95

Ore B

2344.3

99.90

Magnesium fines

6366

100.00

Magnesium olivine

4344.3

96.49

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Pr

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Specific surface area, cm2/g

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Table 3

Corresponding phases of points marked in Fig. 4 Phase identified

Point NO.

Phase identified

1

Hematite

4

Silicate

2

Hematite

5

Pore

3

Silicate

6

Pore

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Pr

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Point NO.

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Table 4

Corresponding phases of points marked in Fig. 5 Phase identified

Point NO.

Phase identified

1

Magnesia

4

Magnesia

2

Magnesia

5

Silicate

3

Magnesia

-

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Pr

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Point NO.

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Table 5

Corresponding phases of points marked in Fig. 6 Phase identified

Point NO.

Phase identified

1

Forsterite

4

Magnesioferrite

2

Forsterite

5

Pore

3

Magnesioferrite

6

pore

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Pr

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Point NO.

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Schematic diagram of uniaxial hydraulic press

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Fig.1

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Schematic diagram of interaction couple

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Fig.2

Fig.3

Schematic diagram of product layer thickness measurement

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SEM image and EDS results of ore briquette

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Fig.4

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Fig.5

SEM image and EDS results of fines briquette

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SEM image and EDS results of olivine briquette

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Pr

Fig.6

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Fig.7

SEM image and mapping results of product layer in fines briquette

SEM image and EDS results of product layer in fines

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Fig.8

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briquette

Fig.9

SEM image and mapping results of product layer in ore

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SEM image and EDS results of product layer in ore

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briquette

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Fig.10

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briquette

Fig.11

Mechanism diagram of the reaction

SEM image and EDS results of product layer in olivine

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Fig.12

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Pr

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briquette

Fig.13

SEM image and mapping results of product layer in ore briquette

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SEM image and EDS results of product layer in ore

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briquette

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Mechanism diagram of the reaction

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Fig.15

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Fig.14

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1. Diffusion property of elements between magnesium additives and iron ore is discussed. 2. The diffusion of Mg dominates the reaction in both interaction couples. 3. Iron ore is more likely to react with magnesia fines than magnesium olivine.

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4. The interaction schematic diagrams of each interaction couple

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are given.

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☒ The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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