Understanding the Self-Sintering Process of BOS Filter Cake for Improving Its Recyclability

Abstract

Basic oxygen steelmaking (BOS) filter cake has been found to undergo a self-sintering process, improving its mechanical properties to allow easier recycling and utilization on plant. The aim of this study was to gain an understanding of the self-sintering of the BOS filter cake in terms of what reactions occurred, and how strength developed in the filter cake during self-sintering. The approach used was to characterize samples before reaction, and to measure the reactivity of the BOS filter cake during heating in air. Reacted samples were characterized and compared to self-sintered samples from the plant. The BOS filter cake consisted of very fine particles (200–500 nm) of metallic iron and wüstite. Upon heating in air from 100 to 1000 °C, the BOS filter cake underwent a sequence of drying, oxidation, and calcination events. The primary reactions in self-sintering were found to be the oxidation of metallic iron and wüstite to hematite and zinc ferrite, beginning at approximately 120 °C and were largely completed by 500–600 °C. These exothermic oxidation reactions at low temperatures were likely driven by the very fine particle size, and provided the energy required to heat the stockpiles and drive self-sintering. The strength required for recycling the BOS filter cake appeared to result from a network of particle–particle bonds that formed between the very fine iron oxide particles in the matrix during oxidation at elevated temperatures. Temperatures between 600 and 800 °C under oxidizing conditions are likely sufficient to form adequately strong material for transport and recycling in the BOS.

Appendix

Heat and Mass Balance for Enthalpy calculations

The enthalpies as measured by DSC were compared against the theoretical several reaction schemes, as given in Table 2. The enthalpies were calculated using the measured mass change during the reaction and the initial composition of the BOS filter cake. The overall mass balance used is given in Eq. 2.

$$ \Delta m \, = \, n_{{{\text{Fe}}}} \cdot M_{{({\text{Fe reaction}})}} + \, n_{{{\text{FeO}}}} \cdot M_{{({\text{FeO reaction}})}} + \, n_{{{\text{ZnO}}}} \cdot M_{{\left( {{\text{ZnFe}}_{{ 2 {}}} {\text{O}}_{{ 4 {}}} } \right)}}, $$

(16)

where Δm is the measured mass change (from the TGA curve) during the Stage 2, n_i is the number of moles of species i that have reacted, as defined in Eqs. 3–5, M_{(Fe reaction)} is the mass change per mole associated with the reaction of iron, M_{(FeO reaction)} is the mass change per mole associated with the reaction of wüstite, and ${\mathrm{M}}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is the mass change per mole associated with the formation of ZnFe₂O₄. For reaction scheme 1, M_{(Fe reaction)} is associated with reaction 3, M_{(FeO reaction)} is associated with reaction 4, while ${M}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is zero. For reaction scheme 2, M_{(Fe reaction)} is associated with reaction 5, M_{(FeO reaction)} is associated with reaction 5, while ${M}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is associated with reaction 8. For reaction scheme 3, M_{(Fe reaction)} is associated with reaction 9, M_{(FeO reaction)} is zero, while ${M}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is associated with reaction 10.

$$ n_{{{\text{Fe}}}} = \frac{{ ( {\ 1 } - f_{{{\text{H}}_{{ 2 {}}} {\text{O}}}} ) {} \cdot f_{{{\text{Fe}}}} \cdot m_{{{\text{reacted}}}} }}{{M_{{{\text{Fe}}}} }} $$

(17)

$$ n_{{{\text{FeO}}}} = \frac{{ ( {\ 1 } - f_{{{\text{H}}_{{ 2 {}}} {\text{O}}}} ) {} \cdot f_{{{\text{FeO}}}} \cdot m_{{{\text{reacted}}}} }}{{M_{{{\text{FeO}}}} }} $$

(18)

$$ n_{{{\text{ZnFe}}_{{ 2 {}}} {\text{O}}_{{ 4 {}}} }} = \frac{{ ( {\ 1 } - f_{{{\text{H}}_{{ 2 {}}} {\text{O}}}} ) {} \cdot f_{{{\text{ZnO}}}} \cdot m_{{\text{o}}} }}{{M_{{{\text{ZnO}}}} }}, $$

(19)

where f_H2O is the mass fraction of water in the unreacted BOS filter cake; and f_Fe, f_FeO, and f_ZnO are the mass fractions of metallic iron, wüstite, and zinc oxides, respectively, in the BOS filter cake on a dry basis, m_reacted is the mass of the BOS filter cake sample that reacted, m_o is the original sample mass, and M_i is the molar mass of species i. f_Fe and f_FeO were estimated by semiquantitative XRD to be 0.27 and 0.45, respectively, while f_H2O and f_ZnO values are listed in Table 1. Substituting Eqs. 17–19 into Eq. 16 allowed m_reacted to be calculated iteratively, and the values for nFe, nFeO, and ${n}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ to be determined.

These values for nFe, nFeO, and ${n}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ were than used to calculate the enthalpy based on the enthalpies of reaction for each reaction in the different schemes, as given in Eq. 2. In Eq. 2, the values of each ΔH term vary, depending on which reaction scheme was considered. For reaction scheme 1, ΔH_{(Fe reaction)} is associated with reaction 3, ΔH_{(FeO reaction)} is associated with reaction 4, while ${\Delta \mathrm{H}}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is associated with reaction 7. For reaction scheme 2, ΔH_{(Fe reaction)} is associated with reaction 3, ΔH_{(FeO reaction)} is associated with reaction 4, while ${\Delta \mathrm{H}}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is associated with reaction 8. For reaction scheme 3, ΔH_{(Fe reaction)} is associated with reaction 9, ΔH_{(FeO reaction)} is zero, while ${\Delta \mathrm{H}}_{({\mathrm{Z}\mathrm{n}\mathrm{F}\mathrm{e}}_{2}{\mathrm{O}}_{4})}$ is associated with reaction 10.