Fundamental Study On Separating Nonmetallic Inclusion From Molten Metal By Super Gravity

With increasing strict demand for high-quality metal materials from the automobile manufacturing,aerospace and construction industry,further reducing the level of inclusion from melt and improving the cleanliness of metal is a major developing direction for metal smelting.The conventional purification methods can effectively remove large-size inclusion,but these methods have the disadvantages of low efficiency for removing fine-size inclusion and tending to pollute the melt or environment.Therefore,it is very important to explore a high-efficiency and innovative method for purifying the melt.In this work,super-gravity separation technology is applied into the melt purification.The flow field in the melt and the moving behavior of inclusion has been theoretically studied with the FLUENT simulation software.The aluminum melt with MgAl₂O₄ particles has been isothermally purified and filtered by super gravity.Besides,the separating behavior of Al₂O₃ particle in the Fe-Al-O melt has been investigated under super-gravity field.The purpose of this work is supplying some scientific basis and theoretical guidance for purifying the melt by super gravity.The simulating results about the flow field in the aluminum melt under super-gravity field using FLUENT software showed that only rotating velocity field existed in the melt,no obvious convection appeared in the melt along the direction of super gravity.The pressure in the melt gradually increased along the direction of super gravity and the pressure became large with increasing the gravity coefficient,for example,the pressure was up to 5.3×105 Pa when the gravity coefficient G=500.Moreover,the rotating tangential velocity increased gradually with the increase of the gravity coefficient.The results obtained by force analysis and simulation for the movement of inclusion in the aluminum melt showed that compared with normal gravity,super gravity could effectively intensify the directed separating process of inclusion in the melt.Only taking Mass force?FG?and Viscous drag?FD?into consideration,the moving behavior of inclusion particle in the viscous liquid was determined by Stokes' law.Namely,the particle could reach the terminal velocity instantaneously,and the velocity of particle was proportional to the gravity coefficient,density difference between inclusion and melt,and the square of particle diameter.The moving behavior of particle was studied using Discrete Phase Mode?DPM?,along the direction of normal gravity,fine-size inclusion particle was imposed by the main forces including Mass force?FG?,Viscous drag force?FD?,Brown force?FB?and Virtual mass force?Fv?.However,along the direction of super gravity,the main forces was FG and FD,due to the fact that the large super gravity could weak the effect of FB and Fv on the movement of inclusion particle,and the moving behavior of particle approximately obeyed the Stokes' law.The experimental study on the separation of MgAl₂O₄ particles in the aluminum melt by super gravity has been investigated.The results showed that super gravity could effectively separate the nonmetallic inclusion in the melt.After treatment of super gravity,the significant region with inclusions accumulated appeared in the bottom area of the sample,and the purifyting efficiency of the sample increased with the increase of gravity coefficient and separating time,for example,the purifying efficiency was up to 90%at the gravitycoefficient of G=500,separating time of t=2min.The moving velocities obtained experimentally agreed well with the theoretical ones calculated by Stokes' law at G?20,t?5 min.However,there were obvious differences between the experimental and theoretical moving velocities under conditions of G?100 and t=2min,due to the fact that the higher inclusion particle concentration caused the greater viscous drag,hindering the migration of inclusion particle.The experimental study on purifying the molten aluminum by super-gravity filtration showed that the inclusion could be thoroughly removed from aluminum melt by filtration under super-gravity field.All inclusions?MgAl₂O₄ and Al₃Ti particles?were nearly intercepted in the upper residue by filter felt and the melt flowed through the filter medium to the lower container.The gravity coefficient and filtering temperature were the major influential factors for the purifying effect of the melt,and the influences of filtering time and the thickness of filter medium on purification of the melt were not obvious.When the gravity coefficient G<50,the melt could not flow through the filter felt,while the purified melt was obtained by flowing through the filter medium to the lower container at the gravity coefficient of G?50.When the gravity coefficient G=500,the removal eff-iciencies of MgAl₂O₄ and Al3Ti particles were 99.22%and 92.09%,respectively,the yield of purified aluminum was up to 92.24%.Although the mechanisms of cake-mode and deep-mode filtrations were observed in the super-gravity filtration experiments,cake-mode filtration was the major one.Al₂O₃ particles in the high-temperature melt of Fe-Al-O system were separated by super gravity and the experimental results showed that the oxide inclusion was separated directionally from the melt.After treated by super gravity,the number of Al₂O₃ particles in the bottom and middle areas was reduced obviously,and most of oxide inclusions were gathered in the upper area.The average diameter of oxide inclusion gradually increased along the direction from the bottom to the top of sample.The contents of oxygen in the bottom and middle areas were reduced markedly;the content of oxygen in the middle area of sample decreased from 5.7×10-3%to 2.2×10-3%,and the maximum removal efficiency of oxygen was up to 61.4%.Therefore,it is feasible to apply the super gravity technology into the purification of molten steel.