Elsevier

Powder Technology

Volume 366, 15 April 2020, Pages 709-721
Powder Technology

Comparative investigation on the reduction behavior of blast furnace dust particles during in-flight process in hydrogen-rich and carbon monoxide atmospheres

https://doi.org/10.1016/j.powtec.2020.03.025Get rights and content

Highlights

  • H2-rich in-flight reduction is an efficient iron-bearing dust recycling technology.

  • A maximum reduction degree of 93.4% of BF dust is achieved during in-flight process.

  • Morphology and microstructure evolutions of reduced BF dust particle are revealed.

  • Heat and mass transfer characteristic between particle and reducing gas is analyzed.

Abstract

Blast furnace (BF) dust is a typical fine-grained solid waste generated as by-product during the ironmaking process. A novel in-flight reduction technology has been developed to recovery the fine iron-bearing dusts efficiently and cleanly to bypass the sintering/pelletization steps. In this work, the reduction behavior of BF dust particle during in-flight process in H2-rich and CO atmosphere were investigated comparatively. A maximum reduction degree of 93.4% is obtained in H2-rich atmosphere at temperature of 1573 K within 1.6 s of reduction time. Morphological observations reveal that dust particles reduced in H2-rich atmosphere is more coarse and porous than in CO. The carbon contained in dust is confirmed to participate in the reduction process in the form of gasification. The heat and mass transfer characteristics between dust particle and reducing gas during in-flight process are also analyzed, which provide fundamental knowledge for effective utilization of BF dust with in-flight reduction technology.

Introduction

Blast furnace (BF) dust is a fine-grained solid waste generated as by-product [1,2] during the ironmaking process. According to statistics, the average amount of BF dust separated from off-gas of blast furnace is estimated about 18 kg per ton of hot metal produced [3], and the annual emissions of BF dust in China alone can reach up to 10 million tons [4]. In general, the main components of BF dust are valuable contents of iron oxides (25%–40% TFe), unburned carbon (~20% C) and small amounts of calcium, aluminum, silicon oxides [[5], [6], [7]]. In view of the contained recyclable elements, BF dust can be recognized as an excellent secondary resource and possibly reused as raw materials. However, up to now, the comprehensive utilization rate of BF dust is reported to be less than 20% [8,9]. Most of the dust is stored or dumped directly as solid waste at landfill sites in the iron & steel plants. Not only does the dust take up much land, it also causes serious environmental pollution and resource waste. Therefore, it is of significant interest to explore an efficient and clean treatment process to utilize this dust for recycling the valuable components.

Several different treatment processes have been proposed to extract and utilize the valuable metal elements from metallurgical dust, including pyrometallurgical, hydrometallurgical and physical methods. Among them, the sintering [[10], [11], [12]] and rotary hearth furnace (RHF) process [[13], [14], [15], [16]] are used most extensively. The high contents of iron and carbon materials contained make BF dust a favorable material for recycling as an alternative to raw materials and solid fuels for sintering process and consequently fed back to blast furnace. However, limitations for recycling the dust through sintering usually arise from its fine particle size and moisture contents, which can cause poor gas permeability and reduce sintering quality. Meanwhile, the accumulation of problematic elements (zinc, lead or alkaline metal) blocks the upward pipes and shortens BF campaign life. Rotary hearth furnace (RHF) process is considered as a reliable way to treat the high zinc content metallurgical dusts. In a typical RHF process, the fine metallurgical dust, scale and pulverized coal are agglomerated into pellets or briquettes, and then charged into furnace with counter current high temperature (1423–1673 K) reducing gas flow. Zinc oxides contained in the dust are reduced to zinc in the furnace and exhausted into the off-gas, and then the accumulated zinc can be separated and recovered through a further purification process. Nevertheless, the product DRI pellets have comparatively higher sulfur content which is emitted from the reducing agent of pulverized coal. On the other hand, it should be noted that the heat transfer efficiency between reducing gas and pellets layers in RHF is relatively low, and high operating temperature with high energy consumption reduce the commercial profits. In recent years, a novel ironmaking technology called in-flight [[17], [18], [19], [20]] or flash reduction technology [[21], [22], [23], [24]] has been developed, which is treated as a flexible technology that can utilize the large quantities of iron ore fines directly to bypass the sintering/pelletization and conventional coke-making steps. During the in-flight reduction process, the micro-size iron ore particles are heated and pre-reduced rapidly by the high temperature reducing gas in a few seconds flight time, while the final reduction is completed in molten bath. The main product is premium grade hot metal, and the gangue materials can be separated into slag easily by density. In addition, the high reactivity and clean reducing gas such as hydrogen, natural gas or a gas generated by the partial combustion of coal are utilized as the reducing agent, which make this process more energy efficient and significantly lowering the CO2 emission. Considering the fewer restrictions of the raw material used, the in-flight reduction technology can be applied to treat the fine iron-bearing metallurgical dust with high level of impurities, in which the iron oxides contained in the dust is expected to be pre-reduced during the in-flight process and extracted to hot metal finally, while the gangue materials are removed to the molten slag. Besides, the ZnO is reduced to metallic zinc by reducing gas and recovered in the off-gas treating system simultaneously.

Previous studies on the in-flight or flash reduction process were mainly focused on the fine iron ores, and laboratory scale drop tube furnace which could measure the rapid chemical reaction rate of iron ore fines at high temperature was usually employed as the reactor. Sohn and co-workers [21,22] fabricated an experimental drop tube furnace at the University of Utah, USA to study the flash reduction kinetics of fine iron ore concentrates (hematite, magnetite) under various gas compositions, temperatures and reduction times. Most importantly, they have reported that both the magnetite and hematite concentrate can be reduced to more than 90% within several seconds by hydrogen. The goal of their research was to perform a measurement of the reduction kinetics and derive the rate expression that will be used for the design of flash ironmaking reactor. Abolpour et al. [17,18] made up a vertical ceramic tube furnace with 3 kW electrical maximum power supply at the University of Kerman, Iran and investigated the in-flight reduction kinetics of magnetite concentrate particles with CO and H2. The activation energy was determined to be 219.6 and 74.2 kJ/mol·K, respectively based on the nucleation and growth kinetic model. Qu et al. [19,20] also conducted a series of experiments and studied the in-flight reduction kinetics of hematite concentrate with 50 μm median size by a gaseous mixture of 42.2% CO + 57.8% CO2 at the temperature range from 1550 to 1750 K in a laboratory high temperature drop tube furnace. The final conversion less than 30% was obtained without metallic iron production. Shimizu et al. [25]. investigated the mechanism and the kinetic of the rapid in-flight reduction of spherical wustite by CH4 at the temperature varying from 1373 to 1573 K. As the result, the reduction fraction of spherical wustite by CH4 reached over 80% at 1573 K within 1 s, and the reduction rate determined step was chemical reaction on the reaction interface. Tsukihashi et al. [26] studied the reduction behavior of molten iron oxide in CO conveyed system at high temperature of 1723 K and analyzed the reduction kinetics by considering the mass transfer in gas film and chemical reaction at the gas-liquid interface. As shown above, almost all of the previous work focused on the high grade iron ore concentrate, but no specific studies have been reported on the reduction behavior of iron-bearing dust during in-flight process, which is essential for evaluating the possibility and efficiency to apply the in-flight reduction technology to recycle iron-bearing dusts.

Considering that the iron, carbon and various complex impurities compositions coexist in BF dust, the reduction behavior of which is presumed to be different from ordinary fine iron concentrates. Meanwhile, taking account of the extremely short reduction time of BF dust particle during in-flight process, a reducing gas agent with strong reducing ability is required to achieve high reduction degree of iron oxides contained in BF dust. Therefore, in the present study, the reduction behavior of BF dust during in-flight process by using H2 and CO as reducing agent is investigated comparatively. The experiments were carried out in a lab-scale high temperature drop tube furnace. The effects of reduction temperature and reducing gas mixture composition on the reduction degree are examined. The morphology and microstructure changes of individual particle during the in-flight process for different reducing agents of H2-rich and CO atmospheres are also observed respectively. Furthermore, the influences of diffusion coefficient and thermal conductivity of the H2-rich and CO based gas on the in-flight reduction process are analyzed. Additionally, the detailed temperature profiles and reduction degree variations of the dust particles during the in-flight process are obtained based on a mathematical model. The aim of this study is to clarify the reduction behavior of BF dust under H2-rich atmosphere during in-flight process, and provide fundamental information for recycling the fine iron-bearing dusts with in-fight reduction technology in H2-rich atmosphere.

Section snippets

Experimental material and apparatus

BF dust samples used in this study were separated from dust cyclone, which was supplied by Baosteel, China. In order to eliminate the fluctuation of chemical composition, 56 samples during an operation period of 7 days were selected and fully mixed for the detailed investigations in this study. The main chemical compositions of BF dust samples are presented in Table 1, showing that the mass fraction of total iron and carbon in the dust are as high as ~43.6% and 20.5%, respectively. Due to the

Reduction degree under different reducing gas agents

For the proposed in-flight reduction technology, an important concerning point is obtaining high reduction degree within a few seconds of the residence time. One of the feasible methods is to use a gas with strong reducing ability as reducing agent. According to the definition of reduction degree (R) in Eq. (4), the experimental results of reduction degree under different reduction temperatures and reducing gas compositions are calculated and shown in Fig. 4. Fig. 4(a) and (b) present the

Conclusions

In the present work, the reduction behavior of BF dust particles in hydrogen-rich and carbon monoxide atmosphere during in-flight process was investigated comparatively. The following conclusions can be drawn:

  • (1)

    Effects of different reduction temperatures and reducing gas agents on the reduction degree of BF dust particles during in-flight process are significant. A maximum reduction degree of 93.4% is obtained by using H2 as reducing gas agent at a temperature of 1573 K within 1.6 s of reduction

Declaration of Competing Interest

The authors declare no competing of interest.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 51574066, 51774072, 51574065, 51774073). The first author acknowledges the China Scholarship Council (CSC No. 201806080122) for providing the scholarship to support his research at Monash University.

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