Research on the ring formation mechanism of magnesian flux pellets in rotary kiln

doi:10.21203/rs.3.rs-2108995/v1

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Research on the ring formation mechanism of magnesian flux pellets in rotary kiln

https://doi.org/10.21203/rs.3.rs-2108995/v1

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published 10 Feb, 2023

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The forming of rings was a key problem of magnesian flux pellets in rotary kiln, which seriously limited production efficiency. Pellet powder and flux were the raw materials of the ring. Based on this, the bonding strength, melting behavior and microstructure of pellet powder and its mixed powder with flux were investigated. The influence of basicity on ring behavior of pellet powder was analyzed, and ring formation mechanism of magnesian flux pellets was clarified. The results showed that acid pellet powder was not easy to form rings due to lower bonding strength of briquettes. Due to changes in bonding process after mixing of flux, magnesian flux pellet powder produced ferrite and silicate liquid phase with lower melting point, which promoted diffusion and recrystallization of hematite and enhanced compressive strength of briquettes, then finally caused rings to form. Moreover, it is required to control roasting temperature below 1200℃, which is a necessary condition for magnesian flux pellet powder to form an initial ring which was easy to be destroyed.

Basicity

Magnesian flux pellet powder

Rotary kiln

Ring formation

Formation mechanism

With the dual measures of vigorously cutting excessive industrial capacity and environmental renovation, China's steel industry was under the pressure of structural adjustment and upgrading^[1], which forced iron and steel enterprises to embark on the clean, efficient and high-quality development route. Magnesian flux pellet has become a high-quality and efficiency blast furnace raw material with high grade, low energy consumption and environmental protection^[2–4]. According to statistics, compared with sintering process, pollutants CO₂, SO₂ and NO_x produced by tons of products in pelletizing process were reduced respectively by 75%, 53% and 16%, and energy consumption in pelletizing process was reduced by 11.9%^[5–6]. Therefore, pelletizing process was more environmental friendly than sintering process.

Production processes of pellets mainly included shaft furnace, belt roaster and grate rotary kiln^[7–9]. Grate rotary kiln was compatible with a variety of fuels for heating^[10]. Moreover, China is rich in coal resources, with an output accounting for about 60% of total pellet production^[11]. Grate rotary kiln process occupied a mainstream position in China's pellet production. However, rotary kiln process could easily form rings in production of pellets. Especially in production process of magnesian flux pellets, rings were formed frequently in a short cycle, which has seriously limited the process of industrial production of magnesian flux pellets.

Few reports today focus on growth behavior and formation mechanism of rings of magnesian flux pellets in rotary kiln, while most of them primarily talked about reduction process of acid pellet rings, low-grade iron ore and the interface reaction between rings and refractory of rotary kiln. Carrie et al. ^[12–13] studied characteristics of particles in flue gas during coal combustion and formation mechanism of particles in rotary kiln, then explored the mechanism of ring formation in rotary kiln. Guo et al. ^[14] studied the influence of Na₂O content in ring on tendency of slagging/forming rings, and found that adhesion of rings increased with increase in Na₂O content. Researches of Zhong et al. ^[15] showed that due to insufficient recrystallization of Fe₂O₃, it was difficult for pure pellet powder to form rings, but extra coal ash could strengthen bonding strength and make initial rings difficult to be damaged, which resulted in formation of rings in rotary kiln. Wang et al. ^[16] focused on ring formation mechanism in rotary kiln, during process of producing hematite pellets, he found that rings mainly came from preheated pellet powder and coal ash. Then he described formation process of rings. According to him, formation mechanism of rings in low temperature was that unburned pulverized coal reduced hematite to FeO, which reacted with coal ash to form silicate phase with low melting point that produced liquid phase in low temperature area. Silicate phases, as bonding phases, adhered separated and independent hematite grains together and promoted bonding of hematite particles. In high temperature area, formation of ring was mainly due to crystallization and diffusion of hematite, in which, liquid phase played a secondary role. After studies on the effect of adding biomass to coal on ring formation in rotary kiln, Sefidari et al. ^[18] established the relationship between ring formation tendency and ash melting viscosity. Jesper et al. ^[19] studied the reaction mechanism of interface between refractory brick and rings during roasting in rotary kiln, and analyzed evolution law of mineral phase structure and element migration law of interface between refractory brick and rings. Liu et al. ^[20] studied growth behavior of rings in pre-reduction rotary kiln and put forward formation mechanism of layered structure of rings. However, ring formation behavior of magnesian flux pellet in rotary kiln still remained unclear due to complexity of reactions in rotary kiln and variability of raw materials. Therefore, studies on ring formation mechanism of magnesian flux pellet in rotary kiln are crucial for this study.

2.1. Materials

HBIS Group provided iron concentrate powder, flux and bentonites which were required for the experiment. Their chemical compositions was shown in Table 1. The basicity, MgO content and SiO₂ content of the powder in rotary kiln were adjusted by adding dolomite and limestone. After the adjustment, the iron concentrate powder, flux and bentonite were mixed and prepared into a cylinder with a diameter of 10mm and a height of 10mm. The cylinder was preheated at 950℃ for 10min and roasted at 1250℃ for 10min in a tubular furnace afterwards. When temperature dropped to room temperature, the cylinders were prepared into powder grain by the prototype for the simulation and replacement of pellet powder produced in rotary kiln for raw briquette materials. Based on screening by a standard sieve, the proportion of grain composition below 200 mesh was 80%, and the proportion of grain composition above 200 mesh was 20%. The obtained powder was prepared again into a cylinder with a diameter of 10mm and a height of 10mm under a press at a pressure of 15 MPa, then the briquette was obtained.

Table 1

Chemical compositions of experimental raw materials (wt%).
Raw materials	TFe	FeO	SiO₂	CaO	MgO	Al₂O₃	burnt loss
PMC mine	66.70	28.57	0.76	0.63	2.56	0.094	-1.60
Yanshan mine	65.40	24.28	5.91	0.88	0.59	0.45	-2.50
Miaogou mine	65.64	26.86	6.44	0.65	0.65	1.76	-3.00
Dolomite	-	-	0.86	30.23	22.24	0.24	38.52
Limestone	-	-	4.02	51.36	0.46	1.22	42.58
Bentonite	-	-	55.96	4.16	2.01	11.73	-

2.2. Experimental method

Prepared raw material powders with different basicities, MgO contents and SiO₂ contents were pressed into briquettes (cylinders with a diameter of 10 mm and a height of 10 mm) which were preheated again at 950℃ for 10 min and roasted at 1250℃ for 10 min. Compressive strength of briquettes were measured when briquettes were cooled to room temperature. On this basis, compressive strength of briquettes with R_1.0Mg_2.0Si_5.0 was measured at different roasting temperatures. The basicity is binary basicity and R is CaO/SiO₂. Experimental plan was shown in Table 2.

Table 2

Experimental plan, including water (wt%).
Raw materials	R_0.6Mg_2.0Si_5.0	R_0.8Mg_2.0Si_5.0	R_1.0Mg_2.0Si_5.0	R_1.2Mg_2.0Si_5.0	R_1.4Mg_2.0Si_5.0
PMC mine	16.93	16.92	16.91	16.90	16.89
Yanshan mine	35.35	35.35	35.35	35.35	35.35
Miaogou mine	42.16	40.27	38.42	36.60	34.81
Dolomite	4.87	4.80	4.74	4.68	4.63
Limestone	1.63	3.57	5.47	7.35	9.19

2.3. Microscopic analysis

First, briquettes without cracks were used for polishing, followed by an analyzation of microstructure and mineral phase structure of briquettes with Quanta 650 field emission scanning electron microscope and DM4500P research polarizing microscope. Besides, XRD analysis of briquette powder was carried out by D/max2500pc X-ray diffraction.

3.1. Effect of basicity and roasting temperature on compressive strength of briquettes

Figure 1(a) showed that there was a positive correlation between compressive strength of briquettes and basicity. The compressive strength of acid briquettes was 165.4KgF/cm², which was lower than basicity of briquettes. When basicity was 0.6, compressive strength of briquettes was 726.8KgF/cm². When basicity ranged from 0.6 to 1.0, compressive strength of briquettes changed little. When basicity exceeded 1.0, compressive strength of briquettes increased. When basicity was 1.4, compressive strength of briquettes was 861.8KgF/cm².

There was a positive correlation between the compressive briquette strength and the basicity. On the one hand, the flux changed the composition of pellet powder and improved the bonding process of pellet powder. On the other hand, the increase of basicity also leaded to the promotion of CaO content, so that the excess CaO entered the slag phase and formed a liquid phase with low melting point. These formed liquid phases would benefit the promotion of the hematite recrystallization and greatly enhanced the compressive strength of briquettes, which made the compressive strength changed greatly.

Acid briquettes have a low compressive strength, so did rings formed in rotary kiln. Under the condition of original basicity. It was very easy to be destroyed and it was difficult to form initial rings, which made the ring formation cycle of acid pellets longer in the production process. However, upon increase in basicity, pellet powder in rotary kiln was highly improved in compressive strength, and form unbreakable initial rings very easily. With an increase in pellet powder in rotary kiln, initial rings gradually aggravated, which hindered the movement and circulation of materials and hot gas flow, causing a reduction in magnesian flux pellets’ quality and production.

Furthermore, Fig. 1(b) showed that with a decrease in roasting temperature, the compressive strength of briquettes gradually decreased to 190.1KgF/m² at 1200℃, which was close to that of acid briquettes. Therefore, under the condition of ensuring pellet strength, roasting temperature should be controlled below 1200℃, which is a necessary condition for production of magnesian flux pellets.

3.2. The effect of basicity on melting temperature of briquettes

As shown in Fig. 2, there was a negative correlation between the deformation and softening temperature of briquettes and the basicity. According to the Fe₂O₃-CaO phase diagram^[9], when w(CaO) was less than 20%, liquidus temperature decreased gradually with an increase in CaO content. Therefore, the CaO content in briquettes would increase with the increase of basicity, which promoted the formation of low-melting-point mineral phase and the amount of liquid phase, leading to a lower deformation temperature and softening temperature of briquettes.

3.3. Mineral phase structure analysis

Figure 3 showed that metal phase of briquettes was mainly composed of a large amount of hematite and a small amount of magnetite, while the bonding phase was mainly composed of calcium ferrite, magnesian ferrite and olivine. With the increase of basicity, the ferrite phase and olivine phase gradually increased (as shown in formulas 1, 2 and 3), and the iron olivine phase gradually transmuted into the calcium iron olivine phase with a lower melting point, which increased the amount of liquid phase, accelerated the diffusion of hematites and filled the pores of the briquette, leading to a larger compressive strength of briquettes.

Fe₂O₃(s) + CaO(s)→CaO·Fe₂O₃(l)	(1)
FeO(s) + SiO₂(s)→FeO·SiO₂(l)	(2)
CaO(s) + FeO(s) + SiO₂(s)→CaO·FeO·SiO₂(l)	(3)

Figure 4 showed a dense and uniform microstructure distribution of briquettes with different basicities and also a uniform distribution of pores which are of different sizes and irregular shapes. The ore phase was mainly composed of hematite and a small amount of magnetite, calcium ferrite and calcium iron olivine, which revealed the same results of XRD analysis.

With changes in basicity of briquettes, content of hematite changed little, which increased the content of calcium ferrite and calcium iron olivine. When basicity ranged from 0.8 to 1.0, hematite crystals were recrystallized and grown, which gradually formed slag phase consolidation with an increase of calcium ferrite, and the pores decreased due to the filling of iron calcium olivine. When the basicity ranged from 1.0 to 1.2, there would be little changes in pores and a large area of continuous crystallization of hematites. Moreover, a large amount of iron calcium olivine and some calcium ferrite were intertwined between hematite crystals, which enhanced recrystallization of hematites, thus, increased the density of briquettes.

To sum up, with the increase of basicity, the hematite recrystallized and grew while a large amount of iron calcium olivine and some calcium ferrite were intertwined between hematite crystals to enhanced recrystallization of hematite. This caused briquettes to have a smaller porosity, a higher density and a larger compressive strength, which made the rings in rotary kiln more dense and unbreakable.

3.4. SEM analysis

As shown in Fig. 5, the element composition at 3# showed that this phase was hematite; the element composition at 2# showed that the phase was magnetite with a small amount of Mg, and Mg²⁺ limited the oxidation of magnetite in the magnetite lattice, which made magnetite amorphous and almost not distributed around hematite; the element composition at 1# showed that this phase was liquid phase which distributed around hematite, mainly due to the reaction of hematite with silicate phase to form a large number of ferrite liquid phase with low melting point, which accelerated the mass transfer of Fe³⁺, improved the recrystallization ability of hematite and enhanced the compressive strength of briquettes. This also showed that higher basicity would make briquettes have a larger compressive strength and exacerbate the ring formation in the rotary kiln, which impeded the production of magnesian flux pellets.

3.5. Mechanism of ring formation

As shown in Fig. 6, during the ring formation process of magnesian flux pellet powder in the rotary kiln, magnetite was oxidized into hematite in the grate, with some hematite powder produced due to lower compressive strength of magnesian flux pellets. Pure hematite powder was difficult to form initial ring with high strength at 1250℃. However, existence of CaO and MgO fluxes made hematite powder and fluxes able to produce liquid phases in the high-temperature rotary kiln, which promoted the diffusion and recrystallization of hematite in the rotary kiln, thus, improved the strength of initial ring and aggravated the ring formation in rotary kiln.

(1) With the increase of basicity, the briquette compressive strength of magnesian flux pellet powder increased gradually, due to which, rings were formed easily and it is difficult for briquettes of acid pellet powder to form rings for its lower compressive strength. Furthermore, under the condition of ensuring the pellet strength, the roasting temperature should be controlled below 1200℃, which is a necessary condition for magnesian flux pellet powder to form the initial ring which was easy to be destroyed.

(2) The briquette of magnesian flux pellet powder was mainly composed of hematite, magnetite, calcium ferrite, magnesium ferrite and olivine phases. With the increase of basicity, ferrite phase and olivine phase gradually increased, and iron olivine phase gradually became calcium iron olivine phase with lower melting point.

(3)The aggravation of rings in rotary kiln was mainly due to the formation of liquid phase of magnesian flux pellet powder. In the rotary kiln, there were Cao and MgO fluxes produced by magnesian flux pellets, causing hematite powder and fluxes to produce liquid phases in the high-temperature rotary kiln, which promoted the diffusion and continuous crystallization of hematite, thus, improved the strength of the initial ring, and then intensified the ring formation in the rotary kiln.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Author Contributions

Formal analysis, T.L. Tian, Y.Z. Zhang; Funding acquisition, T.L. Tian; Investigation, writing, Z.H. Guo.

Competing Interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to T.L.Tian.

Reprints and permissions information is available at www.nature.com/reprints.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afliations.

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You are reading this latest preprint version

Research on the ring formation mechanism of magnesian flux pellets in rotary kiln

Status:

Journal Publication

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Materials

2.2. Experimental method

2.3. Microscopic analysis

3. Results And Discussions

3.1. Effect of basicity and roasting temperature on compressive strength of briquettes

3.2. The effect of basicity on melting temperature of briquettes

3.3. Mineral phase structure analysis

3.4. SEM analysis

3.5. Mechanism of ring formation

4. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

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