Xuefeng She

Effect of carbon species on the reduction and melting behavior of boron-bearing iron concentrate/carbon composite pellets

Iron nugget and boron-rich slag can be obtained in a short time through high-temperature reduction of boronbearing iron concentrate by carbonaceous material, both of which are agglomerated together as a carbon composite pellet. This is a novel flow sheet for the comprehensive utilization of boron-bearing iron concentrate to produce a new kind of man-made boron ore. The effect of reducing agent species (i.e., carbon species) on the reduction and melting process of the composite pellet was investigated at a laboratory scale in the present work. The results show that, the reduction rate of the composite pellet increases from bituminite, anthracite, to coke at temperatures ranging from 950 to 1300°C. Reduction temperature has an important effect on the microstructure of reduced pellets. Carbon species also affects the behavior of reduced metallic iron particles. The anthracite-bearing composite pellet melts faster than the bituminitebearing composite pellet, and the coke-bearing composite pellet cannot melt due to the high fusion point of coke ash. With anthracite as the reducing agent, the recovery rates of iron and boron are 96.5% and 95.7%, respectively. This work can help us get a further understanding of the new process mechanism.

Reduction Characteristics and Kinetics of Bayanobo Complex Iron Ore Carbon Bearing Pellets

The kinetics of isothermal reduction of the carbon bearing pellets, which were mainly composed of Bayanobo complex iron ore and pulverized coal, was investigated by thermogravimetry at the temperature of 1 273–1 673 K. The effects of xc/xo and the atmospheres on the extent of reduction also were investigated. The results indicate that the fractional reaction increased proportionally with temperature increasing and heating temperature is the significant influence factor to the reaction of carbon bearing pellets. The optimum xc/xo is 1. 2 and the effect of atmosphere on the reduction of iron oxides is almost negligible. The results can be interpreted that the reaction was initially controlled by a mixed controlled mechanism of carbon gasification and interface chemical reaction, and in the later stage, interface chemical reaction became the rate-controlling step. Apparent activation energy values of reduction at different levels of fractional reaction were calculated. Before F (fraction of reaction)=0. 5, the apparent activation energy ranges from 66.39 to 75.64 kJ/mol, while after F=0.5, the apparent activation energy is 80. 98 to 85. 37 kJ/mol.

Reduction Behavior of Sinter Based on Top Gas Recycling-Oxygen Blast Furnace

In order to explore the behavior laws of sinter reduction in TGR-OBF (top gas recycling-oxygen blast furnace), reduction experiments of sinter have been conducted by thermal balance mass loss method with different atmospheres, temperatures and volume flows. The changes of RI (reduction degree of Fe2O3), RI (reduction rate of Fez O3 ) and r ( reduction degree of FeO) have been examined. The results show that the reduction of sinter was significantly improved under TGR-OBF atmosphere, and the Rl and r were measured up to 98.2% and 97.8% at 900 °C respectively. With increasing of the reduction temperature, the reduction of sinter speeded up greatly, and the reduction time-duration shortened from 117 min at 900 “C to 63 min at 1100 °C. Moreover, the reduction of sinter enhanced with increasing of the reductive gas flow. When the flow increased from 10 to 15 L/min, the initial reduction rate of sinter increased from 2. 47%/min to 3. 73%/min. While increasing H2 and CO contents in the reductive atmosphere, the reduction of sinter was promoted. Besides, Hz influenced more evidently than CO to the reduction of sinter, especially in the later stage of the reduction process, for instance, the reduction of wustite will be improved enormously when increasing the H2 content in the reductive atmosphere.

Removal Mechanism of Zn, Pb and Alkalis from Metallurgical Dusts in Direct Reduction Process

The high-temperature tube furnace was applied to simulate the rotary hearth furnace (RHF) for the direct reduction of zinc-bearing dusts from steel plants. The removal mechanism of Zn, Pb and alkalis from cold bonded briquettes made by mixing metallurgical wastes, such as dust from bag house filter, OG sludge, fine converter ash and dust from the third electric field precipitator of the sinter strand, in various proportions was investigated. More than 70% of metallization rate, more than 95% of zinc removal rate, 80% of lead removal as well as more than 80% of K and Na removal rates were achieved for the briquettes kept at 1473–1603 K for 15 min during the direct reduction process respectively. The soot generated in the direct reduction process was studied by chromatography, X-ray difraction (XRD) and scanning electron microscopy (SEM). The results suggested that the main phases of the soot were ZnO, KCl, NaCl and 4ZnO • ZnCl2 • 5H2O. Furthermore, the content of Zn reached 64.2%, which could be used as secondary resources for zinc making. It was concluded that KCl and NaCl in secondary dust resulted from the volatilization from the briquettes, whilst ZnO and PbO were produced by the oxidation of Zn or lead vapour from briquettes by direct reduction.

Numerical simulation of the direct reduction of pellets in a rotary hearth furnace for zinc-containing metallurgical dust treatment

A mathematical model was established to describe the direct reduction of pellets in a rotary hearth furnace (RHF). In the model, heat transfer, mass transfer, and gas-solid chemical reactions were taken into account. The behaviors of iron metallization and dezincification were analyzed by the numerical method, which was validated by experimental data of the direct reduction of pellets in a Si-Mo furnace. The simulation results show that if the production targets of iron metallization and dezincification are up to 80% and 90%, respectively, the furnace temperature for high-temperature sections must be set higher than 1300°C. Moreover, an undersupply of secondary air by 20% will lead to a decline in iron metallization rate of discharged pellets by 10% and a decrease in dezincing rate by 13%. In addition, if the residence time of pellets in the furnace is over 20 min, its further extension will hardly lead to an obvious increase in production indexes under the same furnace temperature curve.

Reduction Behaviors of Pellets Under Different Reducing Potentials

Owing to the change of gas composition in top gas recycling-oxygen blast furnaces compared with traditional blast furnace, many attentions are attracted to the research on iron oxide reduction again. In order to study the influence of H2 and CO on the reduction behavior of pellets, experiments were conducted with H2-N2, CO-N2 or H2-CO gas mixtures at 1173 K by measuring the mass loss, respectively. It was found that the reduction degree increased with increasing the ratio of H2 or CO in the gas mixture, but the reduction with hydrogen was faster than that with carbon monoxide The reduction degree could reach 96.72% after 65 min for the reduction with 50% H2+50% N2, while it is only 53.37% for the reduction with 50% CO+50% N2. The addition of hydrogen to carbon monoxide will accelerate the reduction because the hydrogen molecules are more easily chemisorbed and reacted with iron oxide than carbon monoxide. A scanning electron microscope was used to characterize the structures of reduced samples. Dense structure of iron was obtained in the reduction with hydrogen while the structure of iron showed many small fragments for the reduction with carbon monoxide. At the later stage of reduction with the gas mixtures containing carbon monoxide, the reduction curves showed a descending trend because the rate of carbon deposition caused by the thermal decomposition of carbon monoxide was faster than the rate of oxygen loss. Compared with the reduction with CO-N2 and H2-CO gas mixtures, H2 gas could enhance the carbon deposition while N2 gas would reduce this phenomenon. The results of X-ray diffraction and chemical analysis demonstrated that the carbons ar — mainly in the form of cementite (Fe3C) and graphite in reduced sample.

REDUCTION MECHANISM OF TITANOMAGNETITE CONCENTRATE BY CARBON MONOXIDE

Titanomagnetite concentrate was reduced by CO-Ar gas mixtures in a laboratory fixed bed reactor in the temperature range from 1123 to 1323 K. The influences of reductive conditions on the reduction rate and metallization degree including reduction temperature, reduction time and carbon monoxide content were studied. And the characteristics of reduced samples were analyzed by XRD, BES and EDS. Results shown that both the reduction and metallization degree increased with the increasing of temperature and monoxide content. The low reduction degree was owing to the low iron oxides content and high impurities content such as magnesium oxide in titanomagnetite concentrate. Above 1123 K, the reduction is controlled by interfacial chemical reaction at early stage of the reaction and then turns to the internal diffusion controlling with reaction processing. The reduction path at temperatures above 1123 K is suggested as follow: Fe3-xTixO4 -> (x+y-1)FeO + Fe4-2x-yTixO4-y -> (x+y-1)Fe+Fe4-2x-yTixO4-y -> (3-x)Fe+xTiO(2) (0

Microstructure evolution during softening and melting process in different reduction degrees

This paper concerns the degree of indirect reduction in a burden rising substantially in an oxygen blast furnace. It studies the pellet, sinter and a mixture of both in different cases. The paper concerns experiments on single particle load softening to investigate the microstructural evolution of different burdens during the softening and melting process. The results of the experiments show that the degree of reduction impacted the softening and melting behaviour. In the case of a low degree of reduction, a slag phase substrate and a myrmekitic iron structure were formed on the periphery area of the molten burden, whereas slag phase substrate and disperse island wüstite structure were formed in the centre area. Both peripheral and central areas had a slag phase substrate and myrmekitic iron texture. The slag–iron distribution had a structure in which the slag phase was cut in the metal iron phase. The content of 2FeO.SiO2 as a low melting point phase in the slag decreased sharply, and this resulted in the increase in slag–iron separation temperature. The variation of the Ca/Si ratio in the interface between the pellet and the sinter indicated that enhancement of the reduction degree caused the initial temperature of the interaction in the mixed burden to rise and the interaction distance to decrease.

Softening and Melting Behavior of Ferrous Burden under Simulated Oxygen Blast Furnace Condition

The softening and melting behavior of sinter, pellet and mixed burden was researched through high temperature reaction tests under load simulating traditional blast furnace (T-BF) and oxygen blast furnace (OBF) conditions. The results indicated that compared with T-BF, the softening zone of sinter and pellet became wide, but the melting zone became narrow in OBF. The permeabilities of both sinter and pellet were improved in OBF. Under the condition of OBF, the temperature of softening zone of mixed burden was increased by 63 K, but the temperature of melting zone was decreased by 76 K. Therefore, the permeability of material layer was significantly improved. This was mainly caused by the change of the melting behavior of pellet. In addition, the quality of dripping iron in OBF was much better than that of T-BF.

Numerical analysis of carbon saving potential in a top gas recycling oxygen blast furnace

Aiming at the current characteristics of blast furnace (BF) process, carbon saving potential of blast furnace was investigated from the perspective of the relationship between degree of direct reduction and carbon consumption. A new relationship chart between carbon consumption and degree of direct reduction, which can reflect more real situation of blast furnace operation, was established. Furthermore, the carbon saving potential of hydrogen-rich oxygen blast furnace (OBF) process was analyzed. Then, the policy implications based on this relationship chart established were suggested. On this basis, the method of improving the carbon saving potential of blast furnace was recycling the top gas with removal of CO2 and H2O or increasing hydrogen in BF gas and full oxygen blast. The results show that the carbon saving potential in traditional blast furnace (TBF) is only 38–56 kg · t−1 while that in OBF is 138 kg · t−1. Theoretically, the lowest carbon consumption of OBF is 261 kg · t−1 and the corresponding degree of direct reduction is 0.04. In addition, the theoretical lowest carbon consumption of hydrogen-rich OBF is 257 kg · t−1. The modeling analysis can be used to estimate the carbon savings potential in new ironmaking process and its related CO2 emissions.