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2023

Modeling and Experimental Study of Ore-Carbon Briquette Reduction under CO–CO2 Atmosphere (1)

Keywords:

ore-carbon briquette,CO–CO2 atmosphere,reduction, simulation, re-oxidation

Source: internal company

Abstract

Iron ore-carbon briquette is often used as the feed material in the production of sponge iron via coal-based direct reduction processes. In this article, an experimental and simulation study on the reduction behavior of a briquette that is made by hematite and devolatilized biochar fines under CO–CO₂ atmosphere was carried out. The reaction model was validated against the corresponding experimental measurements and observations. Modeling predictions and experimental results indicated that the CO–CO₂ atmosphere significantly influences the final reduction degree of the briquette. Increasing the reduction temperature did not increase the final reduction degree but was shown to increase the carbon that was consumed by the oxidative atmosphere. The influence of the CO–CO₂ atmosphere on the briquette reduction behavior was found to be insignificant in the early stage but became considerable in the later stage; near the time of the briquette reaching its maximum reduction degree, both iron oxide reduction and metallic iron re-oxidation were able to occur.

1. Introduction

An ore-carbon briquette is a composite briquette consisting of iron-bearing oxide and carbonaceous materials that were used as feed material in some coal-based direct reduction processes, such as FASTMET® (FASTMET is a trade mark of MIDREX Co., USA.) and ITMK3® (ITMK3 is a trademark of KOBE Steel Co., Japan.) . The use of these briquettes offers advantages, such as a high reduction rate, utilization of non-coking coal, and biochar for producing sponge iron economically. The ore-carbon briquette reduction technology is often used in treating various metallurgical dust and sludge recovering valuable metals, such as nickel and titanium, from complicated minerals, and upgrading refractory iron ores by removing detrimental minerals, such as quartz and alumina. The reduction of ore-carbon briquettes is usually carried out at the industrial level using rotary hearth furnace (RHF) reduction technology. In an RHF process, the briquette is fed onto the rotating hearth of the RHF and is reduced into sponge iron in a high-temperature environment. The flue gas from the combustion of coal gas, composed of CO, CO₂, H₂, and H₂O, forms a weakly oxidative atmosphere on the surface of the briquette. The CO₂ or H₂O levels may allow for the metallic iron oxidation of the ore-carbon briquette during the RHF process, which would have a negative effect on the quality of the products. Therefore, studies are required to address the reduction behavior of ore-carbon briquettes under the RHF reactive atmosphere.

The reaction kinetics of iron ore-carbon briquettes under inert atmosphere (nitrogen or argon) have been extensively studied, and their major features are well established. The reduction of the briquette proceeds rapidly under high temperatures and generates a large amount of gas inside the briquette, making it more complicated than the reaction behavior of an iron ore or coal briquette. The reaction kinetics depend significantly on the chemical composition and physical properties of the briquette. Some studies on ore-carbon reduction have been conducted under the oxidative atmosphere by Singh et al. and by Ghosh et al., but such studies are scarce. There are several mathematical descriptions of the reduction phenomenon of the iron ore-carbon briquette. For example, Moon et al. developed a model with an assumption of uniform conversion of iron oxide and carbon particles in the briquette, Sun and Lu and Shi et al. developed models including the expressions of chemical kinetics, equations of mass transfer, and equations of heat transfer, and Donskoi et al. developed a model when considering the swelling/shrinkage of the briquette. Although these models attempt to give a comprehensive understanding of the reduction behavior of the briquette, the interaction between the briquette and the oxidative atmosphere has not been included. Therefore, the effect of the oxidative atmosphere on the reduction behavior of ore-carbon briquette has been overlooked in the existing studies.

The first aim of this study was to conduct kinetic experiments of the isothermal reduction behavior of the ore-carbon briquette under a simulated RHF atmosphere (CO–CO₂ atmosphere with P_CO/P_CO2 = 1.0). The second aim was to establish and develop a reaction model of ore-carbon briquette reduction that includes the reaction of metallic iron with CO₂. The simulation results were compared to the experimental results in respect to briquette mass change, briquette carbon conversion, briquette reduction degree, and briquette reduction progress. The reduction behavior of the briquette under the CO–CO₂ atmosphere was also analyzed.

2. Experiments

2.1. Materials and Briquette Preparation

The iron ore sample was from Tangshan Iron and Steel Company (Tangshan), China. The carbonaceous reductant sample was prepared by carbonizing the biochar under 1273 K for 1 h. The chemical composition of the employed biochar is given in. Chemical composition of the ore sample is listed in Table 1. Fe₂O₃ content in Table 1 was analyzed by chemical analysis (iron chloride method) and contents of other components in Table 1 were examined by energy-dispersive X-ray fluorescence spectrometry (XRF) using an XRF 1800 spectrometer (Shimadzu Co., Kyoto, Japan), and the proximate analysis of the carbonaceous reductant sample is listed in Table 2. Both of the samples were ground using a F-P400 ball mill (Focucy Co., Changsha, China), and the average sizes of the ore fines and reductant fines were 100 and 80 µm, respectively. The mixture was thoroughly mixed with an addition of 2% cellulose binder (Dingshengxin Co., Tianjin, China), 5% reagent-grade CaO powder (Xilong Co., Shantou, China), and 10% distilled water. Molar ratio of fixed carbon in the reductant fines to oxygen in the iron oxide of the ore fines was 1.0. The moistened fines were pressed with a die under a pressure of 40 MPa to make the briquettes. The briquettes were then air-dried for 24 h, followed by drying at 473 K for 2 h. The prepared briquettes had a cylindrical shape with a diameter (D) of 20 mm, a height (H) of 10 mm, and a mass of approximately 6.0 g.

Table 1. Chemical composition of the iron ore sample (wt %).

Fe2O3

SiO2

CaO

Al2O3

MgO

MnO

LOI

91.77

2.9

0.1

4.05

0.56

0.14

0.48

LOI: loss on ignition.

Table 2. Proximate analysis of the carbonaceous reductant sample (wt %).

Volatile	Fixed Carbon	Ash
0.41	96.3	3.29

2.2. Experimental Setup and Procedures

The experimental device, schematically presented in Figure 1, includes a gas supply system, an electronic scale with an accuracy of ±0.001 g, and a temperature-controlled furnace with an accuracy of ±2 K. The furnace was heated by MoSi₂ elements, producing a 50 mm hot zone in the reaction tube, with an inner diameter of 60 mm. The sample holder was made of a heat-resistant alloy wire (Fe–Cr–Al).

Figure 1. Schematic diagram of the experimental setup.

The furnace was preheated to the desired temperature under N₂. One briquette was then loaded into the sample holder, preheated at 773 K for 5 min in the upper part of the reaction tube, and then introduced into the hot zone. At this time, the N₂ feed was replaced with a CO–CO₂ gas mixture, P_CO/P_CO2 = 1.0, and the mass loss/gain of the briquette was measured by an electronic scale and was recorded by a computer at an interval of 2 s. After the predetermined time, the briquette was withdrawn from the reaction tube and quenched by a N₂ stream. In all of the individual tests, a constant gas flow rate of 1600 cm³/min (Standard Temperature and Pressure) at the gas inlet was maintained. In addition, some reduced briquettes were subjected to carbon content analysis, scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) examinations. The carbon analysis was conducted using a CS-2800 infrared carbon sulfur analyzer (NCS, Beijing, China); SEM and EDS were performed using a Quanta-250 scanning electron microscope (FEI, Hillsboro, OR, USA). The defifinitions and calculation methods of mass-loss fraction (f_m), reduction degree (fo) and carbon conversion (fc) of the tested briquette are f_m = ∆m_t/(m_C + m_O), fc = ∆m_C/m_C = 1.0 − (m_b − ∆m_t)[C]_t/m_C, and fo = ∆m_O/m_O = fm + m_C/m_O(fm − fc), respectively [30], where, ∆mt , ∆mC, and ∆mO are the mass loss, carbon mass loss, and oxygen mass loss of the briquette at time t, respectively; mb , mC,and mO are the initial mass, initial carbon mass, and initial iron-oxide oxygen mass of the preheatedbriquette, respectively; and [C]_t is the carbon content of the sample at time t. m_b , m_C, and m_O areavailable, according to the preparation procedure of the briquette.

3. Mathematical Model

A mathematical model was established for the reduction process on a single cylindrical briquette. According to the symmetry of the geometry and the experimental conditions, a simplified geometrical model is shown in Figure 2a. The computational domain was chosen as 0.5 radian, and three types of boundary conditions, including wall, symmetry, and axis, were used in the simulations. Figure 2b schematically shows the structure’s grid system, which used a grid of 40 × 20.

Figure 2. Geometrical and mathematical model for the modeling of a single briquette: (a) 0.5 radian computational domain; and, (b) grid system.

Several assumptions were made during model development to express the behavior of a briquette. Briquette reduction was considered under isothermal conditions, swelling/shrinking of the briquette was not considered, and the porosity of the briquette was assumed to be constant. Similar assumptions can be found in many studies involving the modeling of the reduction of the ore-carbon briquette, and therefore it is considered that these assumptions would not cause considerable errors.

The overall reduction process is believed to occur through an intermediate gas phase. The reactions that take place in the briquette are the stage-wise reductions of iron oxide by CO (Equations (1)–(3)), the Boudouard reaction of carbon particles (Equation (4)), and metallic iron oxidation (Equation (5)). The reduction of iron oxide by solid carbon may also occur; however, it is difficult to estimate the extent of this reaction, and the share of the solid-state reduction is minimal when considering the much larger gas–solid contact area. Therefore, no solid–solid reaction was taken into account.

3 Fe2O3(s) + CO(g) = 2 Fe3O4(s) + CO2(g) (1)

Fe3O4(s) + CO(g) = 3 FeO(s) + CO2(g) (2)

FeO(s) + CO(g) = Fe(s) + CO2(g) (3)

C(s) + CO2(g) = 2CO(g) (4)

Fe(s) + CO2(g) = FeO(s) + CO(g) (5)

The gas phase in the briquette was considered to be ideal and to consist of CO and CO2.

Dependencies of the gas properties on local temperature and composition were calculated according to the ideal gas law in the FLUNET material database.

Gas transfer in the reduction of an ore-carbon briquette is driven by two factors: the concentration gradient and the pressure gradient. Gas transfer by pressure gradient is negligible in most cases; therefore, the total gas pressure on the computational domain was fixed at atmospheric pressure, and it is shown below in Equation (6).

The equation of species CO₂ on the computational domain was then given by Equation (7) using the effective coefficient with Fick’s law:

where SCO2 = MCO2 /MO(R1 + R2 + R3) − MCO2 /MCR4 − MCO2 /MOR5 . The equation of species CO is shown below in Equation (8).

D_eff in Equation (7) depends on the porous structure of the briquette, and it was determined using the Weisz–Schwartz relationship, given by Equation (9).

Equations (6)–(8) formed the governing equations of the gas phase. The boundary and initial conditions for Equation (7) were given by Equations (10)–(13). Under experimental conditions, the velocity and other gas properties of the furnace atmosphere in Equations (10) and (11) were considered as equal to their respective values at the gas inlet of the furnace. Additionally, the equivalent diameter of the cylindrical briquette was assumed as the overall diameter.

The governing equations used for the solid phase are given below as Equations (14)–(18).

P_{CO} + P_{{CO}_{2}} = 1.01 \times 10^{5}