Showing papers by "Lifeng Zhang published in 2010"

Removal of Inclusions from Aluminum Through Filtration

Abstract: Filtration experiments were carried out using both an AlF3 slurry-coated and an uncoated Al2O3 ceramic foam filter to study the removal of nonmetallic inclusions and impurity elements. The results showed that the 30-ppi ceramic foam filter removed up to 85 pct inclusions from aluminum. Several pictures of two- and three-dimensional morphologies of both nonmetallic and intermetallics inclusions also have been presented. The following contributing mechanisms for the removal of nonmetallic inclusions in the deep-bed filtration mode are proposed: (1) collision with walls and interception effect and (2) the formation of both intermetallic and nonmetallic inclusion bridges during filtration. Fluid dynamics modeling of inclusion attachment to the filter walls showed that most inclusions, especially those with larger sizes, are entrapped at the upper part of the filter, whereas smaller inclusions are dispersed well throughout the filter. The calculated inclusions removal fractions for the 30-ppi filter showed that almost all inclusions >125 μm are removed, and inclusions ~5 μm in size are removed up to 85 pct. The interfacial energy between two collided same-size inclusions was calculated, indicating that a strong clustering of inclusions may result within the filter window. Magnesium impurities were removed up to 86 pct by the AlF3 slurry-coated filter. The filter acted in active filtration mode in addition to the contribution of the air oxidation of dissolved [Mg], which was calculated to be 13 pct. The total mass transfer coefficient of dissolved [Mg] to the reaction interface was calculated to be 1.15 × 10−6 m/s.

Kinetic Modeling on Nozzle Clogging During Steel Billet Continuous Casting

Abstract: In the current paper, a kinetic model was developed to study the entrapment of inclusions in the molten steel flowing through a Submerged Entry Nozzle (SEN) during billet continuous casting process. The trajectory of inclusions was calculated by considering the drag force, lift force and gravitational force. The entrapment locations of inclusions on SEN wall were predicted. The effects of nozzle diameter, casting speed, billet dimension, and inclusion diameter on SEN clogging were quantitatively discussed. The results indicate the inclusions with diameter larger than 100 μm are not able to be entrapped by the nozzle wall; and the entrapment probability will increase quickly with decreasing size of inclusions. The distribution of the entrapped inclusions along the nozzle length is non-uniform and the volume fraction of inclusions in the clogging materials should be considered in order to more precisely predict the accumulated weight of molten steel that can be poured before the nozzle is fully blocked by clogging. Under the conditions assumed: 150 mm×150 mm billet, 2.0 m/min casting speed, approximately 25°C superheat, 1 m length of the SEN (Al2O3–C materials), 20 μm inclusions diameter in a single size, 30 ppm T.O and 40 mm nozzle diameters, the prediction shows that ~351 ton steel can be poured for the current billet continuous caster.

Transient Fluid Flow Phenomena during Continuous Casting: Part II—Cast Speed Change, Temperature Fluctuation, and Steel Grade Mixing

Abstract: Many complicated phenomena are associated with transient stages during the continuous casting process, especially inside the casting mold cavity. A deep understanding of the phenomena and optimized control over the process ensure the high quality product and low costs. This work presents a three-dimensional numerical simulation of transient fluid flow phenomena during steel continuous casting. In the Part II, the process of casting speed change (increase or decrease), temperature fluctuation, and steel grade mixing in a slab mold were investigated. The phenomena, such as flow pattern, inclusion removal fraction, temperature variation, top surface profile, and steel transportation, were addressed to evaluate the process and enhance product quality.

Behavior of MgO • Al2O3 Based Inclusions in Alloy Steel During Refining Process

Abstract: The transformation of MgO • Al2O3 based inclusions in alloy steel during refining has been studied by industrial trials Besides Factsage software is used to study the formation and modification of spinel inclusions in alloy steel using calcium treatment during refining process The results show that the transformation sequence of inclusions is: MgO • Al2O3→CaO-Al2O3-MgO complex inclusions→MgO • Al2O3, and under present experimental condition, in order to avoid forming MgO • Al2O3 inclusions the content of dissolved Ca in the molten steel has to reach 1×10−6 Also the results show that when more calcium was added to molten steel, the content of Al2O3 and MgO will be lower Besides, increasing the content of CaO in the inclusions will increase even if the content of SiO2 changes little

Transient Fluid Flow Phenomena during Continuous Casting : Part I-Cast Start

Abstract: This work presents a three-dimensional numerical simulation of transient fluid flow phenomena during continuous casting of steel. In the Part I, the process of cast start was investigated. Cast start involved the filling process and the caster start process. In the filling process, molten steel was continuously filled into the mold cavity with surrounding mold walls and a dummy bar head at the bottom. To simulate the interface between the molten steel and the air, an algorithm of volume of fluid has been employed. In the cast start process, the dummy bar was gradually dragged out with a low casting speed. The movement of dummy bar was represented using dynamic mesh, and the moving speed of the boundary was controlled by a subroutine. The shape of the air/steel surface was tracked over the time, and the speed variation at a series of points close to the surface was monitored to reflect the fluctuation at top surface during cast start.

A Simple Model to Calculate Dendrite Growth Rate during Steel Continuous Casting Process

Abstract: In the current paper, the relationship between the casting speed and the dendrite growth during steel continuous casting process was studied; and a simple mathematical model to calculate the primary dendrite growth rate according to the casting speed was developed. The model was used to predict the distribution of the dendrite growth rate along width direction and casting direction during a continuous casting process, indicating a dendrite growth rate of 0.1–3.0 mm/s, which is much higher than that in regular ingot casting. The dendrite growth rate fluctuates along casting direction because of the different cooling intensity in each zone of the continuous caster; and the distribution of the dendrite growth rate along width direction was non-uniform due to the uneven transverse distribution of cooling intensity. The predicted results agree well with the measurements.

Modeling on Dendrite Growth of Medium Carbon Steel during Continuous Casting

Abstract: Dendrite growth is an important phenomenon during steel solidification. In the current paper, a numerical method was used to analyse and calculate the dendrite tip radius, dendrite growth velocity, liquid concentration, temperature gradient, cooling rate, secondary dendrite arm spacing, and the dendrite tip temperature in front of the solid/liquid (S/L) interface for the solidification process of medium carbon steels during continuous casting. The current model was well validated by published models and measurement data. The results show that in the continuous casting process, the dendrite growth rate is dominated by the casting speed. Dendrite growth rate, liquid concentration at the S/L interface, temperature gradient and cooling rate decrease with proceeding solidification and solid shell thickness growth, while other parameters such as dendrite tip radius, secondary dendrite arm spacing, and dendrite tip temperature in front of the S/L interface become larger with solidification progress and solid shell thickness growth. Parametric investigations were carried out. The effects of the stability coefficient, temperature gradient and casting speed on the micro-structural parameters were discussed. Under the same conditions, higher casting speed promotes coarser secondary dendrite arm spacing and enlarges the dendrite tip radius, while decreasing temperature gradient, reducing the dendrite growth rate and making the solute distribute more uniform.

Gas−Liquid Two-Phase Flow in Minichannels with Liquid Side Introduction

Abstract: In the present work, computational fluid dynamics (CFD) modeling with a volume of fluid (VOF) method was used to understand gas-liquid two-phase flow in PEM fuel cell flow channels under typical fuel cell flow conditions. Simulations with different liquid water introduction manners to flow channels were discussed to resemble liquid water transport in flow channels in active fuel cells. Those introduction methods included side liquid introduction through a number of ports, continuous liquid introduction along the channel with uniform and nonuniform flow rates, and liquid water introduction from the inlet together with the gas phase. The simulated results showed that the side introduction of water increases the water accumulation near the end of the channel and the gas slugs are significantly elongated, compared to the cases with water introduction from the inlet. When water was introduced nonuniformly along the channel with more near the outlet of the channel, there appears to be less probability of slug formation in the channel. With low liquid to gas volumetric flow ratios as encountered in fuel cells, the flow pattern changed between slug flow and stratified flow, depending on different liquid introduction manners. When water was introduced into the channel from ports on the bottom wall, it emerged and accumulated near the port region before slugs formed, detached, and purged out of the channel, causing high pressure drop and pressure drop fluctuations. When the wettability, or contact angle, of the channel walls varied, liquid water tended to attach to the wall surface with low contact angles, namely, more hydrophilic surface. In addition, the change in surface contact angle can also lead to the change in flow patterns. The slug frequency decreased with an increase in the port size. With increasing of the port number, more liquid tended to accumulate in the channel.

Evolution of Mgo·Al 2 O 3 Based Inclusions in Alloy Steel During the Refining Process

Abstract: The transformation of MgO·Al2O3 based inclusions in alloy steel during refining has been studied by industrial trials. Besides Factsage software is applied to study the formation and modification of spinel inclusions in alloy steel using calcium treatment during refining process. The results show the transformation sequence of inclusions is: MgO·Al2O3 → CaO– Al2O3–MgO complex inclusions → MgO·Al2O3, and under present experimental condition, in order to avoid forming MgOAl2O3 inclusions the content of dissolved Ca in the molten steel has to reach 1ppm. Also the results show that when more calcium was added into molten steel, the content of Al2O3 and MgO will be lower. Besides, increases the content of CaO in the inclusions will increase even if the content of SiO2 changes little.

Transient Fluid Flow Phenomena in Continuous Casting Tundishes

Abstract: Transient fluid flow phenomena in a 60-ton continuous casting tundish were investigated using mathematical models and industrial trials. The study provides useful information to help improve steel cleanliness during continuous casting.

Two-Dimensional Simulations of Gas-Liquid Two-Phase Flow in Mini channels of PEM Fuel Cell Flow Field

Abstract: Effective water management is one of the key strategies for improving the performance and durability of PEM fuel cells. Phenomena such as membrane dehydration, catalyst layer flooding and two-phase flow in flow-channels are all determined by the distribution and movement of water during cell operation. In this study, gas-liquid flow in mini-channels relevant to fuel cells was numerically studied using a CFD two-phase flow model in combination with a volume of fluid (VOF) method. The results show that the surface wettability of the channel wall can greatly affect the flow pattern, especially when the channel walls and the gas diffusion layer (GDL) surface possess different contact angles. When the channel walls are more hydrophobic, more water is accumulated on the GDL. An increase in the surface tension results in a slight increase of slug frequency and a slight decrease in slug length. The onset of slugging along the channel is determined by the gas-liquid mixture velocity, gas-to-liquid flow ratio and the way water is introduced into the gas flow channel. Furthermore, the calculated pressure drop fluctuations show a strong dependence on the channel liquid content and the slug length.

Purification of solar grade silicon using electromagnetic field

Abstract: Non-metallic particles and the metallic impurity elements in the solar cell silicon have a strong detrimental effect on the conversion efficiency of the solar cell. Removing these impurities is one of the important tasks for silicon refining. The current paper proposed a new approach to purify silicon - electromagnetic (EM) separation. Since the non-metallic particles and the metallic impurity elements are non- or less-conductive while the molten silicon is well conductive, under EM field, the Lorenz force will push the particles to the boundary layer, thus separate these inclusions. In the current study, a high frequency EM field was imposed on the silicon melt in laboratory scale experiments with a frequency of 60 kHz and 15.0 A current, the non-conductive SiC particles were successfully pushed to the boundary layer close to the crucible wall.

Characterization of Disintegrator Milled Electronic Waste Powders for Materials Recovery

Abstract: In this paper, the mechanical milling of the Printed Circuit Boards (PCB) was carried out. The new air classification stand was developed for testing the separation of lightweight particles like tinfoil stripes and plastics. The test results for separation of heavier fractions like non-ferrous metals (Al, Cu) are presented. For milled materials characterization the sieve analysis, laser diffraction analysis and a scanning electron microscope were used. The chemical composition of the PCB powders was studied by means of the energy dispersive X-ray microanalysis. Then, the pyrometallurgical recycling PCB was executed. The mechanisms of thermal degradation and combustion are investigated using TG/DTA and MS machines with the aim of separating and recovering the organic and metallic materials. The mass loss, conversion fraction with the temperature, activation energy etc. are investigated. Some alkalis, such as Na2CO3, NaHCO3, NaOH and CaCO3 were used to control the exhausted toxic gas such as HBr and benzene.

Inclusions and Nozzle Clogging During the Billet Continuous Casting Process

Abstract: A simple kinetic clogging model was developed to predict the entrapment probability of inclusions to the nozzle lining wall and the number of heats that can be poured before the nozzle is entirely clogged. Factors affecting SEN clogging are discussed.

Non-metallic particles in Solar Grade Silicon (SoG-Si)

Abstract: This study investigated the non-metallic inclusions in Solar Grade Silicon (SoG-Si), especially the distribution of inclusions in the top 15mm layer of multicrystalline silicon ingot. The SoG-Si ingot produced from directional solidification process usually pushes the impurities to the top and finally cut off and discarded, which leads to material loss. The hard inclusions lead to wire breakages during the cutting of the ingot into wafers. The main kinds of inclusions found in top-cut silicon scraps from two manufacturers have been investigated using acid extraction, automated feature analysis techniques and SEM-EDS and optical Microscope: they are needle-like Si 3 N 4 and lumpy SiC inclusions. Surface observations of the scraps before polishing revealed that, Si3N4 inclusions are usually bigger and in some cases can be about a few millimeters. SiC inclusions are usually smaller, ∼200µm but can be ∼500µm in some cases. For the directional solidified silicon ingot, it was determined that an approximate distance of ∼10mm is a good enough cutoff thickness.

PURIFICATION OF ALUMINUM THROUGH Al2O3 - AlF3 ACTIVE FILTRATION

Abstract: Filtration is the most cost effective method to purify aluminum. In this paper, filtration experiments have been carried out using both AlF3 slurry coated, and uncoated Al2O3 ceramic foam filter (CFF) to study the removal of both nonmetallic inclusions and impurity elements. The results showed that the 30 ppi CFF removed up to 85% inclusions from aluminum. The contributing mechanisms for the removal of nonmetallic inclusions in the deep bed filtration mode are proposed: (1) collision with walls and interception effect; and (2) the formation of both intermetallic and nonmetallic inclusion bridges during filtration. Fluid dynamics modeling of inclusions attachment to the filter walls showed that most inclusions, especially those with larger sizes, are entrapped at the upper part of the filter while smaller inclusions dispersed well throughout the filter. The active filter could [Mg] remove impurity elements up to 86 %.