Recovery of iron and calcium aluminate slag from high-ferrous bauxite by high-temperature reduction and smelting process

A high-temperature reduction and smelting process was used to recover iron and calcium aluminate slag from high-ferrous bauxite. The effects of w（CaO）/w（SiO_2） ratio, anthracite ratio, and reduction temperature and time on the recovery and size of iron nuggets and on the Al_2O_3 grade of the calcium aluminate slag were investigated through thermodynamic calculations and experiments. The optimized process conditions were the bauxite/anthracite/slaked lime weight ratio of 100：16.17：59.37, reduction temperature of 1450°C and reduction time of 20 min. Under these conditions, high-quality iron nuggets and calcium aluminate slag were obtained. The largest size and the highest recovery rate of iron nuggets were 11.42 mm and 92.79wt%, respectively. The calcium aluminate slag mainly comprised Ca_2 SiO_4 and Ca_（12）Al_（14）O_（33）, with small amounts of Fe Al_2O_4, CaAl_2O_4, and Ca_2Al_2SiO_7.

Reduction and melting behavior of carbon composite lateritic bauxite pellets

Direct reduction of low-grade lateritic bauxite was studied at high temperature to recover Fe and beneficiate AlzO3 slag. The re- sults show that a metallization rate of 97.9% and a nugget recovery rate of 85.1% can be achieved when the reducing and melting tempera- tures are 1350 and 1480℃, respectively. Moreover, a higher-grade calcium aluminate slag （A1203 = 50.52wt%） can also be obtained, which is mainly composed of ct-A1203, hercynite （FeAI：O4）, and gehlenite （Ca2A12SiO7）. In addition, high-quality iron nuggets have been produced from low-grade lateritic bauxite. The nugget is mainly composed of iron （93.82wt%） and carbon （3.86wt%）, with almost no gangue （slag）.

Numerical simulation of Marangoni effect induced by species transfer across iron droplet–molten slag interface

A mathematical model accounting for unsteady mass transfer across interface of a stationary iron droplet immersed into molten slag was established through the volume of fluid coupled with level set method.The Marangoni effect induced by mass transfer was reproduced successfully,and the hydrodynamic instability phenomena at the interface,such as the Marangoni convection flow,the evolution of the interfacial tension during the mass transfer,and the influence of Marangoni effect on the mass transfer rate,were revealed.The results show that the Marangoni convection flow develops quickly and behaves as an ordered structure in the forms of four pairs of the convection cell at the edge of the droplet once the oxygen transfer across the interface starts.The average convection flow velocity along the interface is high,even more than 0.025 m/s,depending on the droplet diameter,which facilitates the mass transfer.The Marangoni convection flow of the large droplet develops more easily than that of the small droplet,and the larger the droplet diameter is,the higher the convection flow velocity and the mass transfer rate are.Moreover,it is shown that the droplet diameter influences the impacting region of the Marangoni convection flow and its duration period.

Influence of Cohesive Zone Shape on Solid Flow in COREX Melter Gasifier by Discrete Element Method

Based on the principle of discrete element method （DEM）, a 2D slot model of a COREX melter gasifier was established to analyze the influence of cohesive zone shape on solid flow, including mass distribution, velocity distribution, normal force distribution and porosity distribution at a microscopic level. The results show that the co- hesive zone shape almost does not affect the particle movement in the upper shaft and deadman shape. The particles in the lower central bottom experience large normal force to support the particles above them, while particles around the raceway and in the fast flow zone exhibit weak force network. The porosity distribution was also examined under three kinds of cohesive zones. Like the velocity distribution, the whole packed bed can be divided into four main re- gions. With the increase of cohesive zone position, the low porosity region located in the root of cohesive zone increa- ses. And the porosity distribution becomes asymmetric in the case of biased cohesive zone.

Numerical simulation on characteristics of COREX shaft furnace with central gas distribution

A three-dimensional steady-state mathematical model, considering the chemical reactions and transfers of momentum, heat and mass between the gas and solid phases, was developed to investigate the characteristics of the shaft furnace with the central gas distribution (CGD) device. The model was verified by the practical production data of a COREX-3000 shaft furnace, and then, it was used to study the inner characteristics of the shaft furnace with CGD. The results show that, compared with the COREX shaft furnace without CGD, the gas utilization rate (UR) and solid metallization rate (MR) increase from 33.66% to 34.18% and 60.4% to 61.8%, respectively. Especially, the standard deviation of solid MR decreases from 6.9% to 0.8%, which means that the MR of direct reduced iron from the furnace with CGD is more uniform than that without CGD. Additionally, the effects of operational conditions and CGD design on gas UR, solid MR and direct reduced iron uniformity were further discussed and the optimized conditions were suggested.

Effect of screw casing structure on descending of burdens in COREX shaft furnace

COREX shaft furnace（SF）is a typical screw feeder with a storage container coupled with eight screw casings and screws.The structure of screw casing plays an important role in the moving behavior of burdens,stress distribution,abrasive wear of screws,and energy consumption during the operation of SF.Therefore,a three-dimensional semi-cylindrical model of actual size of COREX-3000 SF was established based on discrete element method to investigate the influences of screw casing structure.The results show that the increase in the gap between the outside of screw flight and screw casing is beneficial for the smooth operation of SF,resulting in uniform descending velocity along the radius of SF in the lower part,decreasing the size of recirculation region,and alleviating stress concentration in the screw casing.Moreover,raising the gap appropriately is also beneficial to weaken screw abrasive wear,decrease energy consumption,and then prolong the service life of the screws.However,enlarging the gap also leads to more undesired high temperature reduction gas into the SF from melter gasifier,thereby deteriorating the operation of SF.Thus,an ideal distance exists between the outside of the screw flight and the screw casing,which is suggested to be equal to the average of particle diameter.

Mathematical Modeling of Multi-sized Argon Gas Bubbles Motion and Its Impact on Melt Flow in Continuous Casting Mold of Steel

The 3D turbulence k-ε model flow of the steel melt （continuous phase） and the trajectories of individual gas bubbles （dispersed phase） in a continuous casting mold were simulated using an Eulerian-Lagrangian approach. In order to investigate the effect of bubble size distribution, the radii of bubbles are set with an initial value of 0. 1- 2.5 mm which follows the normal distribution. The presented results indicate that, in the submerged entry nozzle （SEN）, the distribution of void fraction is only near the wall. Due to the fact that the bubbles motion is only limited to the wall, the deoxidization products have no access to contacting the wall, which prevents clogging. In the mold, the bubbles with a radius of 0. 25--2.5 mm will move to the top surface. Larger bubbles issuing out of the ports will attack the menis- cus and induce the fluid flows upwards in the top surface near the nozzle. It may induce mold powder entrapment into the mold. The bubbles with a radius of 0.1--0.25 mm will move to the zone near the narrow surface and the wide surface. These small bubbles will probably be trapped by the solidification front. Most of the bubbles moving to the narrow surface will flow with the ascending flow, while others will flow with the descending flow.

DEM Simulation of Solid Flow Including Asymmetric Phenomena in COREX Shaft Furnace

Based on the principles of the discrete element method （DEM）, a scaled-down model was established to analyze burden descending behavior, including asymmetric phenomena, throughout an entire COREX shaft furnace （SF）. The applicability of the DEM model was validated by determining its accordance with a previous experiment. The effects of discharge rate and abnormal conditions on solid flow were described in terms of solid flow pattern and microscopic analysis. Results confirmed that the solid flow of the COREX SF can be divided into four different flow regions; the largest normal force exists at the top of the man-made dead zone, and the weak force network exists in the funnel flow region. The basic solid flow profile was identified as a clear Flat→U→W type. Increasing the dis- charge rate decreased the quasi-stagnant zone size, but did not affect the macroscopic motion of particles or the shape of patterns above the bustle. For asymmetric conditions, in which particles were discharged at different rates, the solid flow patterns were asymmetric. Under an abnormal condition where no particles were discharged from the left outlet, a sizeable stagnant zone was formed opposite to the working outlet, and ＂motionless＂ particles located in the left stagnant zone showed potential to increase the period of static contacts and sticking effect.

Medium oxygen enriched blast furnace with top gas recycling strategy

Top gas recycling oxygen blast furnace（TGR-OBF）process is a promising ironmaking process.The biggest challenge of the TGR-OBF in operation is the dramatic decrease of top gas volume（per ton hot metal）,which once led to hanging-up and shutdowns in practice of the Toulachermet.In order to avoid this weakness,the strategy of medium oxygen blast furnace was presented.The maneuverable zone of the TGR-OBF was determined by the top gas volume,which should not be far from the data of the traditional blast furnace.The deviation of ±12.5% was used,and then the maneuverable blast oxygen content is from 0.30 to 0.47 according to the calculation.The flame temperature and the top gas volume have no much difference compared to those of the traditional blast furnace.The minimum carbon consumption of 357 kg per ton hot metal in the maneuverable zone occurs at the oxygen content of 0.30（fuel saving of 14%）.In the unsteady evolution,the N2 accumulation could approach nearly zero after the recycling reached 6 times.Thus far,some TGR-OBF industrial trials have been carried out in different countries,but the method of medium oxygen enriched TGR-OBF has not been implemented,because the accumulation of N2 was worried about.The presented strategy of medium oxygen enriched TGR-OBF is applicable and the strategy with good operational performance is strongly suggested as a forerunner of the full oxygen blast furnace.

Crystallization behavior of blast furnace slag modified by adding iron ore tailing

Blast furnace （BF） slag is a by-product of the ironmaking process and could be utilized to manufacture slag fiber by adding iron ore tailing. The crystallization behavior of the modified BF slag is significant to the fi- brosis process. To investigate the influence of basieity on the crystallization behavior, BF slag was modi- fied by adding iron ore tailing at room temperature and melted at 1 500 ℃. FactSage simulation, X-ray dif- fraction, scanning electron microscopy backscattered electron imaging coupled to an energy dispersive spectrometer, and hot thermocouple technique analysis were performed to explore the crystallization be- havior of the modified BF slag during the cooling process. It was found that the initial crystallization tem- perature increased with the increase in basicity. Melilite, anorthite, clinopyroxene, and wollastonite could be generated during the cooling process as basicity ranged from 0.7 to 0.9. Spinel could be found as one of the phases; however, wollastonite disappeared under a basicity of 1.0. The initial crystallization tempera- ture was controlled by the crystallization of melilite during the cooling process when the basicity of the modified BF slag ranged from 0. 7 to 1.0. Moreover, the cooling rate could also influence the crystalliza- tion of the modified BF slag.

Influence of Screw Design on Burden Descending Velocity and Particle Segregation in COREX Shaft Furnace

COREX shaft furnace（SF）is an industrial system that employs screw feeders;thus,the burden descending velocity and particle segregation in the SF can be directly affected by the design of screw.A three-dimensional actual size model of COREX-3000 SF was established using the discrete element method.Four types of burdens,including pellet,ore,flux and coke,were considered in this model.With this consideration,the effect of screw design on solid flow was investigated.Results showed that,in the base case,burdens fell primarily down from the first flight of the screw.The burden descending velocities were nearly uniform in the peripheral direction and decreased along the radial direction.In addition,the normalized particle size increased in the center area and decreased in the wall area.Reducing the flight diameter of screw benefited an even flow pattern and restrained the rolling tendency of burden from the edge to center areas.An optimized case was also proposed,in which a uniform solid flow profile could be obtained and the evenness of descending velocity along the radius could be greatly improved.

Recycling of High Ferrous Bauxite Reducing Slag for Synthesis of CaAl_2Si_2O_8-Al_2O_3-CaAl_(12)O_(19) Composite

CaAl2 Si2 O8- Al2 O3- CaAl12 O19） （CAS2- Al2 O3 -CA6 ） composite was synthesized through reaction sintering alumina and bauxite reducing slag. The CAS2-Al2 O3-CA5 composite was mainly composed of α-Al2O3 , CAS2 , and CA6. Gehlenite （Ca2 Al2 SiO7 , C2 AS） phase was effectively transformed to CASe and CA6 through high-temperature reaction sintering under weak oxidizing atmosphere at 1400 ℃for 4 h. SEM （scanning electron microscopy） and EDS （energy dispersive spectroscopy） analysis indicated that black and needle-shaped Al2O3, rhombic or irregular polygonal shaped FeAl2O4 , and glassy phase Ca2 Al2 SiO7 disappeared after the reaction sintering. The light gray and flaky hexagon crystals of CaAl12 O19 （10 μm） and the grainy particles of Al2O3 （2-7 μm） were observed in the CASe-Al2 O3-CA6 composite. The gray crystals of CASe act as the binding phase and are distributed around CA6 and Al2O3. CAS2-Al2O3-CA6 composite exhibits high refractoriness and service temperature, which are 1 650 ℃ and 1 450 ℃, respectively. Reaction sintering of alumina and bauxite reducing slag is a feasible method for the synthesis of CAS2-Al2 O3-CA6 composite.

Definition of Raceway Boundary Using Fractal Theory

The particle velocity contours were obtained by tracking the tracer particles in the raceway region of the COREX melter gasifier model and the contours were irregular. According to the fractal theory, the fractal dimen sions of different particle velocity contours were determined. Through the analysis of the fractal dimensions, a new method for precise determination of the raceway boundary was proposed. The results show that, when the velocity is less than 0.18 m/s, the particles are located in the stagnant zone and the fractal dimensions of particle velocity con- tours are almost constant as 1.41; when the velocity increases from 0.18 to 0.83 m/s, the particles are located in the rapid movement zone and the fractal dimensions decrease gradually from 1.41 to 1.05 ＇when the velocity is grea- ter than 0.83 m/s, the particles are located in the cavity zone and the fractal dimensions are again almost constant as approaching to 1.00. Therefore, the velocity contour of 0.18 m/s, which is critical to distinguish the rapid move- ment zone and stagnant zone, can be used to define the raceway boundary. Based on this method, the effect of blo wing rate on raceway size was calculated and the results show that the penetration depth and height of the raceway increase with the increase of blowing rate.