Elsevier

Chemical Engineering Journal

Volume 374, 15 October 2019, Pages 648-657
Chemical Engineering Journal

A kinetic comparison between microwave heating and conventional heating of FeS-CaO mixture during hydrogen-reduction

https://doi.org/10.1016/j.cej.2019.05.226Get rights and content

Highlights

  • Microwave irradiation enhances the rate of chemical reactions.

  • Gas diffusion is rate controlling step during microwave irradiation.

  • Fe shell changes the rate controlling mechanism during conventional heating.

Abstract

Kinetics of iron production via hydrogen-reduction of FeS-CaO mixture during both microwave heating and conventional heating was investigated at 750, 570, and 460 °C to clarify the effect of heating method on dominant rate-controlling mechanism. Results of conventional heating revealed that the rate controlling step during treatment in the first 500 s at 750 °C, 800 s at 570 °C, and 1000 s at 460 °C is interfacial chemical reaction; then it is changed to gas diffusion in micro pores of reduced Fe owing to the formation of Fe shell on the surface of FeS particles. In microwave treatment, the interfacial chemical reaction is rapidly progressed and gas diffusion is the dominant rate-controlling mechanism with an activation energy of 22.3 kJ.mol−1. This study demonstrated that the microwave irradiation enhances the rate of chemical reaction during iron production from FeS-CaO mixture.

Introduction

Reduction of sulfide minerals in the presence of lime (CaO) has been studied in previous literature [1], [2] to prevent SO2 emission during production of metal from metal sulfides owing to a high tendency of lime for reaction with sulfur to form CaS in a reducing atmosphere [1], [3]. For instance, Hara et al. [1] produced metallic Cu, Co and Fe from a sulfide concentrate via a carbothermic reduction in the presence of CaO at 1000 °C. Jha et al. [4] showed that the reduction of FeS:CaO:C = 1:1:2 mixture initiates after a heat treatment at 850 °C for > 90 min. Kutsovskaya et al. [5] attained a reduction degree of 95% after treating a FeS:CaO:C = 1:1.1:1.1 mixture at 1050 °C in 30 min.

On the other hand, reduction rate of iron oxide in H2 is much higher than that in CO [6], [7], [8], [9] and also the interdiffusion coefficient of H2/H2O gas in iron ore sintered particles is three times higher than that of CO/CO2 at 500 °C [10]. Such advantages of hydrogen-reduction, in addition to a large demand for eco-friendly ironmaking have resulted in employing H2 as a reducing agent, instead of carbonaceous materials [6], [7], [8], [9], [10], [11] to mitigate CO2 emission. Moreover, Habashi et al. [12] indicated that the CaO favors the hydrogen-reduction of metal sulfides (cupper sulfides and cupper-iron sulfides) at 800 °C. Therefore, hydrogen-reduction of FeS in the presence of lime is likely an alternative method for iron production from iron sulfide minerals that can be considered as a two-stage reaction [3], [13]; ion exchange reaction, Eq. (1), and reduction reaction, Eq. (2) [14]. The total reaction is represented by Eq. (3).FeS(s)+CaO(s)=FeO(s)+CaS(s)ΔG1°=-4615-0.205TJ.mol-1FeO(s)+H2(g)=Fe(s)+H2O(g)ΔG2°=16240-8.45TJ.mol-1FeS(s)+CaO(s)+H2(g)=Fe(s)+CaS(s)+H2OgΔG3°=11625-8.655TJ.mol-1

A higher stability of the metal oxide than metal sulfide demonstrates a greater reactivity of mineral sulfide in the ion exchange reaction [3]. For example, the Gibbs free energy of the ion exchange reaction between FeS and CaO is negative indicating higher stability of the FeO than FeS. Furthermore, metallization of Fe is restricted by the reduction reaction owing to a higher stability of FeO than metallic iron [3].

In addition, microwave irradiation is an energy-efficient and a rapid-heating method to decrease activation energy of chemical reactions owing to both thermal and non-thermal effects of microwave photons. The non-thermal effect of microwave irradiation on the rate of chemical reactions, has attracted the attention of researchers on microwave irradiation energy in terms of speeding up chemical reactions [15], [16], [17], [18]. The thermal effect of microwave irradiation has the potential to mitigate CO2 emission and decrease the amount of carbonaceous materials required for carbothermic reduction of iron resources due to its specific characteristics, such as rapid and selective heating, volumetric heating, and high-efficiency heating. For example, the authors have investigated carbothermic reduction of FeS in the presence of lime via microwave heating wherein the results revealed that the average temperature required for attaining a certain reduction degree is lower during microwave heating than in the conventional heating [19].

As a novel idea for a further mitigation of CO2 emission, hydrogen-reduction of FeS-CaO mixture during microwave irradiation is investigated in the present study to combine the advantages of microwave irradiation and using H2 as reducing agent during iron production. Such hydrogen-reduction during microwave processing has been also applied for magnetite reduction in our previous work [20]. Furthermore, hydrogen-reduction of FeS-CaO mixture during conventional heating is also conducted in an electric resistance furnace to make a better understanding of the effect of heating method (conventional and microwave) on the kinetics of iron production during reduction reaction.

Section snippets

Materials

Reagent CaCO3 powder (purity, 99.5%) was calcined at 1300 °C for 600 min followed by crushing and grinding to obtain a CaO with grain size of less than 45 μm. Formation of the CaO after calcination process was confirmed by X-ray diffraction analysis; however, small amount of CaCO3 possibly remain which cannot be detected in the XRD pattern. Reagent FeS (purity, 99%) was crushed and ground to the grain size of less than 45 μm. Then, the crushed CaO and FeS were mixed well by a stainless steel

Heating profile during microwave treatment

In the present study, the microwave treatment was conducted at least twice at each output power and the average temperature was considered as the temperature corresponding to the applied power. Fig. 2 shows average temperature profiles of samples reduced in H2 during microwave irradiation at powers of 1275, 1125, and 975 W. Error bars represent standard deviations. The variation of temperature deviation with treatment time is explained in Section 3.2. The average temperature of samples

Kinetic model

Un-reacted core model has been largely applied in previous investigations [25], [31], [32], [33], [34] to study the kinetics of chemical reactions which progress topochemically. Fu et al. [33] reported that the un-reacted core model is the most frequently used ore reduction model. Bai et al. [31] also reported that “the un-reacted core model has been widely applied and confirmed in the area of gas-solid reactions”. In this model, four mechanisms can contribute to reaction progress: gas

Kinetic analysis

With regards to the microscopic observations that confirm the topochemical progress of reaction in the present study, un-reacted core model was employed to clarify the dominant rate-controlling mechanism in reduction reaction (Eq. (2)) during treatment under H2 atmosphere. To simplify calculations, Usui et al. [37] suggested that the sample shape (cylinder) can be assumed as a sphere with a volume same as cylindrical sample. Therefore, the radius of the assumed spherical sample (ri, cm) was

Conclusions

Kinetics of iron production during hydrogen reduction of FeS-CaO mixture was investigated under microwave irradiation and conventional heating to clarify the effect of microwave irradiation on the dominant rate-controlling mechanisms. The results are summarized as follows:

  • 1.

    Reduction of FeS-CaO mixture in H2 can be considered as a two-step reaction: An ion exchange reaction between FeS and CaO that forms iron oxide on the surface of FeS particles. Then, H2 reduces the iron oxide (product of the

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