Kexin Jiao

Formation mechanism of the protective layer in a blast furnace hearth

A variety of techniques, such as chemical analysis, scanning electron microscopy−energy dispersive spectroscopy, and X-ray diffraction, were applied to characterize the adhesion protective layer formed below the blast furnace taphole level when a certain amount of titanium- bearing burden was used. Samples of the protective layer were extracted to identify the chemical composition, phase assemblage, and distribution. Furthermore, the formation mechanism of the protective layer was determined after clarifying the source of each component. Finally, a technical strategy was proposed for achieving a stable protective layer in the hearth. The results show that the protective layer mainly exists in a bilayer form in the sidewall, namely, a titanium-bearing layer and a graphite layer. Both the layers contain the slag phase whose major crystalline phase is magnesium melilite (Ca2MgSi2O7) and the main source of the slag phase is coke ash. It is clearly determined that solid particles such as graphite, Ti(C,N) and MgAl2O4 play an important role in the formation of the protective layer, and the key factor for promoting the formation of a stable protective layer is reasonable control of the evolution behavior of coke.

Reduction Kinetics of Vanadium Titano-Magnetite Carbon Composite Pellets Adding Catalysts Under High Temperature

Experiments were carried out by adding CaF2 and NaF as catalysts in an Ar atmosphere to study the isothermal reduction kinetics of vanadium titano-magnetite carbon composite pellets under high temperature in the range from 1473 to 1673 K. The scanning electron microscope (SEM) was used to characterize the microstructure of product. By analyzing reduction mechanism, it was foundthat the rate controlling step was gas diffusion, and the activation energy was 178.39 kJ/mol without adding any catalysts. Adding CaF2 or NaF of 3% to vanadium titano-magnetite carbon composite pellets can decrease the apparent activation energy of reduction, and the decrease extent was 14.95 and 15.79 kJ/mol, respectively. In addition, temperature was an important factor influencing on reaction rate.

Reduction mechanisms of pyrite cinder-carbon composite pellets

The non-isothermal reduction mechanisms of pyrite cinder-carbon composite pellets were studied at laboratory scale under argon (Ar) atmosphere. The composite pellets as well as the specimens of separate layers containing pyrite cinder and coal were tested. The degree of reduction was measured by mass loss. The microstructures of the reduced composite pellets were characterized by scanning electron microscopy (SEM). It is found that the reduction processes of the composite pellets may be divided into four stages: reduction via CO and H2 from volatiles in coal at 673–973 K, reduction via H2 and C produced by cracking of hydrocarbon at 973–1123 K, direct reduction by carbon via gaseous intermediates at 1123–1323 K, and direct reduction by carbon at above 1323 K. Corresponding to the four stages, the apparent activation energies (E) for the reduction of the composite pellets are 86.26, 78.54, 72.01, and 203.65 kJ·mol−1, respectively.

Interfaces Between Coke, Slag, and Metal in the Tuyere Level of a Blast Furnace

An in-depth understanding about the reactions in the high-temperature zone of a blast furnace is significant to optimize both the current and future blast furnace process. The interfaces between coke, slag, and metal were observed using scanning electronic microscope with samples obtained from the tuyere level of a blast furnace. Two types of slag phases were identified, one originating from coke ash and the other from the bosh slag. Slag formed by coke ash was seen to cover the coke surface, which may hinder the reaction of coke with both gas and liquid iron. The reduction of FeO from the bosh slag (originated from the primary slag) occurs in the coke/slag interface with the reduced iron forming a metal layer surrounding the coke surface. The reduction of SiO2 occurs both in and outside the coke, and the reduced silicon reacts with iron to form iron silicide if the two species come into contact. Further study is proposed based on the results of this study.

Devolatilization Characteristics and Kinetic Analysis of Lump Coal from China COREX3000 Under High Temperature

A devolatilization study of two lump coals used in China COREX3000 was carried out in a self-developed thermo-gravimetry at four temperature conditions [1173 K, 1273 K, 1373 K, and 1473 K (900 °C, 1000 °C, 1100 °C, and 1200 °C)] under N2. This study reveals that the working temperature has a strong impact on the devolatilization rate of the lump coal: the reaction rate increases with the increasing temperature. However, the temperature has little influence on the maximum mass loss. The conversion rate curve shows that the reaction rate of HY lump coal is higher than KG lump coal. The lump coals were analyzed by XRD, FTIR, and optical microscopy to explore the correlation between devolatilization rate and properties of lump coal. The results show that the higher reaction rate of HY lump coal attributes to its more active maceral components, less aromaticity and orientation degree of the crystallite, and more oxygenated functional groups. The random nucleation and nuclei growth model (RNGM), volume model (VM), and unreacted shrinking core model (URCM) were employed to describe the reaction behavior of lump coal. It was concluded from kinetics analysis that RNGM model was the best model for describing the devolatilization of lump coals. The apparent activation energies of isothermal devolatilization of HY lump coal and KG lump coal are 42.35 and 45.83 kJ/mol, respectively. This study has implications for the characteristics and mechanism modeling of devolatilization of lump coal in COREX gasifier.

Formation mechanism of the graphite-rich protective layer in blast furnace hearths

A long campaign life of blast furnaces is heavily linked to the existence of a protective layer in their hearths. In this work, we conducted dissection studies and investigated damage in blast furnace hearths to estimate the formation mechanism of the protective layer. The results illustrate that a significant amount of graphite phase was trapped within the hearth protective layer. Furthermore, on the basis of the thermodynamic and kinetic calculations of the graphite precipitation process, a precipitation potential index related to the formation of the graphite-rich protective layer was proposed to characterize the formation ability of this layer. We determined that, under normal operating conditions, the precipitation of graphite phase from hot metal was thermodynamically possible. Among elements that exist in hot metal, C, Si, and P favor graphite precipitation, whereas Mn and Cr inhibit this process. Moreover, at the same hot-face temperature, an increase of carbon concentration in hot metal can shorten the precipitation time. Finally, the results suggest that measures such as reducing the hot-face temperature and increasing the degree of carbon saturation in hot metal are critically important to improve the precipitation potential index.

Mechanisms of Swelling of Iron Ore Oxidized Pellets in High Reduction Potential Atmosphere

The influences of the time, temperature and atmosphere on the reduction swelling of oxidized pellets were investigated by single factor experiments. The mechanisms of reduction swelling of oxidized pellets were analyzed and investigated by SEM (scanning electron microscopy) and XRD (X-ray diffractometer) analysis. The results show that the change rules of reduction swelling index of oxidized pellets in different reduction atmospheres are very similar. With the increase of reduction time, the reduction swelling index moves up firstly and then down. When the reduction temperature is above 900 °C, α-quartz turns into α-tridymite, and the transition generates additional volume expansion effect. The reduction swelling index changes faster in H2 atmosphere than in CO atmosphere. Increasing H2 content in the reduction atmosphere is useful to decrease the reduction swelling index, but it is also easy to cause oxidized pellets cracking.

Dripping and evolution behavior of primary slag bearing TiO2 through the coke packed bed in a blast-furnace hearth

To investigate the flow of primary slag bearing TiO2 in the cohesive zone of blast furnaces, experiments were carried out based on the laboratory-scale packed bed systems. It is concluded that the initial temperature of slag dripping increases with decreasing FeO content and increasing TiO2 content. The slag holdup decreases when the FeO content is in the range of 5wt%–10wt%, whereas it increases when the FeO content exceeds 10wt%. Meanwhile, the slag holdup decreases when the TiO2 content increases from 5wt% to 10wt% but increases when the TiO2 content exceeds 10wt%. Moreover, slag/coke interface analysis shows that the reaction between FeO and TiO2 occurs between the slag and the coke. The slag/coke interface is divided into three layers: slag layer, iron-rich layer, and coke layer. TiO2 in the slag is reduced by carbon, and the generated Ti diffuses into iron.

Combustion characteristics and kinetics of anthracite with added chlorine

The combustion process of Yangquan anthracite (YQ) with the addition of 0.045wt%, 0.211wt%, 1.026wt%, and 2.982wt% chlorine was investigated using a thermogravimetric method from an ambient temperature to 1173 K in an air atmosphere. Results show that the YQ combustion characteristics are not significantly affected by an increase in chlorine content. Data acquired for combustion conversion are then further processed for kinetic analysis. Average apparent activation energies determined using the model-free method (specifically the KAS method) are 103.025, 110.250, 99.906, and 110.641 kJ/mol, respectively, and the optimal kinetic model for describing the combustion process of chlorine-containing YQ is the nucleation kinetic model, as determined by the z(α) master plot method. The mechanism function of the nucleation kinetic model is then employed to estimate the pre-exponential factor, by making use of the compensation effect. The kinetic models to describe chlorine-containing YQ combustion are thus obtained through advanced determination of the optimal mechanism function, average apparent activation energy, and the pre-exponential factor.

Effect of chlorine on the viscosities and structures of blast furnace slags

The viscosity of CaO–SiO2–Al2O3–MgO–CaCl2 slags (C/S = 1.12) were investigated to elucidate the effects of chlorine ranging from 0.02 to 0.53 mass% on the blast furnace slags at high temperatures. Moreover, the Raman spectra of the quenched slags and the X-ray diffraction patterns of the slags cooled in air after viscosity measurement were thoroughly analysed to interpret the transformation of the structures of the slags with increasing the content of chlorine. The viscosity was found to decrease slightly with the increase of chlorine at a given temperature higher than 1673 K, and the critical temperature (TCR) decreased from about 1660 to 1590 K simultaneously which was possibly deriving from the precipitation of Ca2Al2SiO7, Ca3Al2(SiO4)3–xCl4 x and SiO2 in higher chlorine content. The degree of polymerisation for silicon–oxygen tetrahedra was found to decrease estimating from the decrease of the average amount of bridging oxygen calculated from the deconvolution results of the Raman spectra of the quenched slags, which provided the explanation for the decrease in viscosity. And that the apparent activation energy of the slags was commonly reduced by chlorine increasing demonstrated the decrease in the degree of polymerisation of molten slags simultaneously.