Thermodynamic studies on behavior of newly formed metallic iron on surface of particles

doi:10.11949/0438-1157.20190931

Abstract

Abstract:

The influence of reduction conditions on the morphology of newly formed metallic iron during high temperature gas-based reduction of iron ore are studied from the standpoint of thermodynamic analysis, and the solutions for controlling iron morphology through regulation of reduction conditions are proposed. Experimental results showed that addition of H₂ in CO could not only accelerate the growth rate of iron grains, but also increase the amounts of iron nuclei, resulting in the transformation of fibrous iron into dense one, addition of CO₂ in CO could transform the“sharp-pointed”iron whiskers into“cactus-like”ones, decrease of reduction temperature could lower the strength of fibrous iron.

Key words: precipitation behavior of iron, thermodynamics, regulation of iron morphology, reduction, granular materials

摘要：

从热力学角度出发，揭示了还原条件对高温气基还原过程中颗粒表面铁析出行为的影响规律，提出了通过改变还原气氛调控铁析出形貌的操控方法。研究表明在CO中混入H₂可以加快铁晶粒的生长速率，同时增加还原初期矿粉表面的铁形核数量，导致矿粉表面新生成的金属铁由晶须状转变为致密状。随着CO-CO₂中CO₂含量的升高，矿粉表面新生成的金属铁会由“锋利”的晶须状转变为“仙人掌状”，并且表面铁的分布密度会变小。随着还原温度的降低，矿粉表面铁晶须的强度会变弱。

关键词: 铁析出规律, 热力学, 形貌调控, 还原, 颗粒物料

CLC Number:

TQ 021.2

Zhan DU. Thermodynamic studies on behavior of newly formed metallic iron on surface of particles[J]. CIESC Journal, 2019, 70(11): 4143-4152.

杜占. 颗粒表面金属铁析出规律的热力学研究[J]. 化工学报, 2019, 70(11): 4143-4152.

Figures/Tables 14

Table 1 Chemical composition of Chilean iron ore

Composition	Content /%(mass)
TFe	61.13
FeO	25.43
CaO	0.83
MgO	4.32
Al₂O₃	0.84
SiO₂	6.02
TiO₂	0.11

Fig.1 Surface morphology of Chilean iron ore

Fig.2 Experimental apparatus used for fluidized bed reduction1—gas cylinder; 2—shutoff valve; 3—mass flowmeter; 4—fluidized bed; 5—electric resistance furnace; 6—thermocouple; 7—temperature controller; 8—pressure sensor; 9—computer

Fig.3 Metallization curves of iron ore fines as a function of time in CO-H2 mixtures at 800℃

Fig.4 Surface morphology of as-reduced samples in CO-H2 mixtures at 800℃(metallization degree is about 22%)

Fig.5 Cross sectional views of as-reduced samples in H2 and CO during initial reduction period(metallization degree is about 5%)

Fig.6 Conceptual diagram for precipitation mechanism of iron in H2 and CO

Fig.7 Surface morphology of as-reduced sample by pre-reduction in H2/(H2+CO)=30%(vol) for 5 min, metallization degree is about 22%

Fig.8 Metallization curves of iron ore fines as a function of time in CO-CO2 mixtures at 800℃

Table 2 Thermodynamic driving forces in CO-CO2 mixtures at 800℃

CO₂ /(CO+CO₂)/%(vol)	ΔG/ (kJ/mol)
10	-33.8
20	-18.3
30	-8.7
CO_{2 Fe/oxide eq}=41.1%(vol)	0

Fig.9 Surface morphology of as-reduced samples in CO-CO2 mixtures at 800℃(metallization degree is about 15%)

Fig.10 Metallization curves of iron ore fines as a function of time in CO at different temperatures

Fig.11 Surface morphology of as-reduced samples in CO at different temperatures(metallization degree is about 20%)

Fig.12 Thermodynamic analyses of reduction of iron oxides in CO

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