CFD simulation of fluidized magnetic roasting coupled with random nucleation model

doi:10.1016/j.ces.2020.116148

Chemical Engineering Science

Volume 229, 16 January 2021, 116148

https://doi.org/10.1016/j.ces.2020.116148 Get rights and content

Highlights

•
Fluidized magnetic roasting is simulated coupled with random nucleation model.
•
Mass transfer within bubble phase is taken account for bubbling bed approach.
•
Fluid dynamics, chemical kinetics and flow structure determine fluidized roasting.

Abstract

The fluidized magnetic roasting is recognized as the most effective treatment of low-grade/complex iron ores for the sustainable development of steel industry. For a deeper investigation of the technology, the computational fluid dynamics (CFD) simulation of this gas-solid reaction process is conducted through combining the intrinsic chemical kinetics with random nucleation model, which shows a good agreement with the experimental data for considering the mixed-effect of mass transfer resistance and nucleation mechanism. Furthermore, in order to predicate the gas-solid noncatalytic reaction rate more accurately, the contribution of mass transfer within the bubble phase has been also taken account for the bubbling bed approach. It is demonstrated that the fluid dynamics, chemical kinetics and heterogeneous flow structure determine the fluidized roasting behavior simultaneously.

Introduction

Compared with the metallurgical apparatus of rotary kiln, shaft furnace and sintering machine for the recovery of zinc, copper, nickel and other metals, the fluidized-bed roaster has been dominating the pyrogenic extraction industry for its lower energy consumption and higher production efficiency (Battle et al., 2015, Kunii and Levenspiel, 1991, Sohn and Wadsworth, 2013, Yang, 2003). Among the oxidizing, sulfatizing, chloridizing, reducing and other roasting processes, the fluidized magnetic roasting followed by magnetic separation, which has been recognized as the most effective treatment of low-grade/complex iron ores and attracted much more attentions from the economic and environmental aspects (Guo, 1979, Priestley, 1957, Uwadiale, 1992, Yu et al., 2019). A comprehensive investigation of flow hydrodynamics and reaction mechanism for the magnetic roasting process will be of great help to the design and integration of fluidized metallurgical engineering.

Until now, extensive researches have been carried out on the fluidized magnetic roasting for transforming nonmagnetic minerals of hematite (Fe₂O₃), limonite (FeO(OH)·nH₂O) and siderite (FeCO₃) into magnetite (Fe₃O₄), which involves gas-solid reaction at elevated temperature in a weakly reducing atmosphere (Fine and Prasky, 1966, Jacobs, 1970, Nabi and Lu, 1968, Yu and Qi, 2011). These studies either publish on the experimental analysis of reduction gas composition, detention time, particle size and roasting temperature for the optimum reaction conditions, or focus on the mathematical modeling of reduction kinetics with the rate-controlling mechanism interpreted in terms of nucleation, non-topochemical, phase boundary or contracting sphere models, all of which have provided the fundament for technology development and process control of the industrial fluidized magnetic roasting applications. Whereas, it should be admitted that the CFD is an efficient tool to study the complex gas-solid flow behavior and to predict the product quality under different operating conditions, which is able to obtain the constructive information about fluid dynamics, reaction behavior and reactant distribution throughout the bed that the traditional experimental study or mathematical modeling can hardly provide directly (Chung, 2010, Pannala, 2010). Until now, numerous benefits from the implementation of CFD have been reported in the fluidized applications of fluid catalytic cracking, biomass gasification, char combustion, food drying and so on (Davidson, 2001), but little information is available in the literature concerning the CFD simulation of fluidized mineral roasting, which hampers the equipment improvement and technology optimization in some degree.

For the simulation of fluidized reaction, the complex chemical kinetics is coupled with CFD model and determines the computed results directly. Generally speaking, the gas-solid reaction kinetics is hypothetically classified into the shrinking spherical/core model with the internal/external diffusion or chemical reaction control mechanism (Levenspiel, 1999). Whereas, numerous other factors of non-uniform temperature distribution, varied particle density/porosity, sintering phenomenon and nucleation behavior may also affect the overall reaction process and lead to different kinetic mechanisms in reality (Szekely, 2012). Based on the previous kinetics modeling study (Hurst et al., 1982), the reduction of some metal oxides (Fe₂O₃, NiO and CuO) is a typical nucleation-controlled process with the sigmoid (S) kinetic curve being characterized by an initial induction period of producing nuclei, followed by an acceleratory period of crystal growth before falling off in the final stage. This behavior is commonly observed for the topochemical reaction and mainly analyzed by the autocatalytic mechanism (Avrami, 1939, Bhatia and Perlmutter, 1979, Hurst et al., 1982, Tiernan et al., 2001), but it often overestimates the reaction rate for not considering the influence of mass transfer resistance during the actual reaction process (Piotrowski et al., 2007, Yu et al., 2017). Therefore, it is more reasonable to conduct the numerical investigation of fluidized magnetic roasting with taking account of random nucleation mechanism and mass transfer behavior, comprehensively.

In the present work, a CFD simulation integrated with random nucleation model and chemical reaction kinetics has been conducted to investigate the fluidized reduction of Fe₂O₃ to Fe₃O₄. Based on the validation against experimental data and comparison with other computational model results, the characteristics of fluidized magnetic roasting are accurately predicted and extensively analyzed on account of the CFD coupling simulation, which is expected to give a thorough analyzing and further exploration of the overall gas-solid reaction behavior for the fluidized roaster design and optimization.

Section snippets

Simulated system

The fluidized reduction kinetics of natural hematite ore (containing 93.5 wt%-Fe₂O₃ and 6.5 wt%-SiO₂) to Fe₃O₄ obtained from the doctoral dissertation of Yu (2017) is adopted to verify the CFD simulation of magnetic roasting based on random nucleation model and chemical reaction kinetics. The reduction process of Fe₂O₃ to Fe₃O₄ can be represented by the following equations: $3 F e_{2} O_{3 (s)} + C O_{(g)} = 2 F e_{3} O_{4 (s)} + C O_{2 (g)}$

The experiments were conducted in a cylindrical quartz fluidized bed with an inner diameter of

Modeling approach

In this work, the multiphase flow model is used to perform CFD simulation with several new features. The random nucleation model integrates with intrinsic reaction kinetics, which also takes account of the mass transfers within both emulsion and bubble phases, is incorporated to calculate the fluidized roasting behavior.

Simulation setup

The simulation of fluidized magnetic roasting process was conducted in a 2D condition (0.025 × 0.10 m², displayed in Fig. 4) for the large computational complexity of CFD coupled with the user-defined scalar (UDS) solving. The mass fractions of CO component in the emulsion and bubble phases ( $X_{e} = \frac{C_{e}}{ρ_{g}}$ , $X_{b} = \frac{C_{b}}{ρ_{g}}$ ) were defined as UDS-0 and UDS-1 with the same initial value of 0.135. The mixed gaseous reactant was fed from the bed bottom distributor defined as a velocity inlet and escaped through the

Results and discussion

In order to validate the CFD simulation of fluidized magnetic roasting process conducted through combining the intrinsic chemical kinetics with random nucleation model, three typical reaction conditions for the reduction of Fe₂O₃ to Fe₃O₄ are selected and tabulated in Table 5. Furthermore, the characteristics of magnetic roasting reaction rate, effective reaction area, gas reaction rate within different phases, solid holdup distribution and relative gaseous reactant concentration within the bed

Conclusions

Combining the chemical reaction kinetics with random nucleation model, a CFD simulation based on the bubbling bed approach has been conducted to investigate the fluidized magnetic roasting process. It is proved that the numeration integrated with random nucleation model taking account of the mass transfers within both emulsion and bubble phases predicts more accurately than the other methods, which are lack of the consideration of mass transfer resistance, nucleation mechanism or reaction rate

CRediT authorship contribution statement

Zheng Zou: Investigation, Writing - original draft, Project administration. Jingyi Zhu: Data curation, Investigation. Dong Yan: Software, Investigation. Yitong Wang: Software, Validation. Qingshan Zhu: Supervision, Writing - review & editing. Hongzhong Li: Conceptualization, Methodology, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China under Grant No. 21878304, 21736010 and 21908227, the Science Fund for Creative Research Groups of the National Natural Science Foundation of China under Grant No. 21921005, and the Fund of State Key Laboratory of Multiphase complex systems under Grant No. MPCS-2019-A-07.

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CFD simulation of fluidized magnetic roasting coupled with random nucleation model

Highlights

Abstract

Introduction

Section snippets

Simulated system

Modeling approach

Simulation setup

Results and discussion

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Chem. Eng. Sci.

Powder Technol.

Chem. Eng. J.

Chinese. J. Chem. Eng.

Powder Technol.

Chem. Eng. J.

Chem. Eng. J.

Chem. Eng. Sci.

Chem. Eng. Sci.

Chem. Eng. Sci.

Chem. Eng. J.

Kinetics of phase change. I General theory

J. Chem. Phys.

Kinetics of phase change. II transformation-time relations for random distribution of nuclei

J. Chem. Phys.

Drying, roasting, and calcining of minerals

A population balance approach to the modeling of solid phase reactions

Aiche J.

Computational fluid dynamics

The enterprise-wide application of CFD in the chemicals industry, the 6th World Congress of Chemical Engineering

Fluidised Particles

Magnetic roasting of iron ores with ferrous scrap

Multiphase flow and fluidization: continuum and kinetic theory descriptions

Fluidized roasting of oxidic Chinese iron ores

Sci. Sinica

Temperature programmed reduction

Catal. Rev. Sci. Eng.

Magnetic roasting and leaching for upgrading minnesota manganiferous iron ores