Study On The Mechanism Of Impurities Removal From Fe,Ni,and Mo By Hydrogen Plasma Arc Melting

High purity metal has been widely used in high technology industries such as electronic information,for its excellent physical and chemical properties.Impurities in high purity metals have negative effects on the above properties and the application.As the increasing quality requirements of targets in the electronic information industry,it is difficult to achieve the demands of high-quality targets by conventional purification methods.In recent years,the hydrogen plasma arc melting?HPAM?technique has been approved in producing high-purity metals.However,few investigations about nonmetallic and metallic impurities removal by HPAM have been done throughly.And the mechanism of impurities removal by HPAM is needed to be discussed.In this dissertation,the purification mechanism of HPAM from Fe,Ni,and Mo was studied.The effects of hydrogen proportion in plasma gas and atmospheric pressure on the removal of nonmetallic impurities from Fe,metallic impurities from Ni and Mo were investigated respectively.According to the kinetics and thermodynamics analysis on nonmetallic and metallic impurities removal by HPAM,the mechanism of HPAM was studied by experiments and numerical simulations.Based on the influence of magnetic field on melt flow,the effect of magnetic field on HPAM was studied.The main conclusions are as follows:?1?The plasma gun was optimizied.The hydrogen proportion in plasma gas is increased from 20%to 40%by adjusting the cooling structure in the plasma gun.The plasma gun can be used under negative pressure by means of optimizing the sealing structure between cooling water and plasma gas.?2?The effect of hydrogen content on the non-metallic impurities removal by HPAM was investigated.The deep deoxidation experimental results show that the deoxidation process by HPAM obeys the first-order rate law.The reduction rate of oxygen increases proportionally to the approximately 0.44th power of the hydrogen content in plasma gas.The rate of deoxidization increases with the hydrogen content.The oxygen content can be decreased from 100 ppm to 1 ppm after 120 min melting by?Ar+40%H₂?HPAM.The desulfurization experimental results show that the desulfurization process by HPAM obeys the first-order rate law.The reduction rate of sulfur increases proportionally to the approximately 0.8th?1.2th power of the hydrogen content in plasma gas.The rate of desulfurization increases with the hydrogen content.The sulfur content was reduced from the 840 ppm to 10 ppm after120 min melting by?Ar+40%H₂?HPAM.The products of decarburization and desulfurization by HPAM are CS and H₂S gas,due to[C]reacting with[S]to CS and the active hydrogen atoms reacting with[S]to H₂S.The thermodynamic analysis indicates that nonmetallic impurities removal power of hydrogen is H⁺>H>H₂,because of the reaction Gibbs free energy of H⁺,H,H₂ with non-metallic impurities increases sequentially.?3?The effect of hydrogen content on the metallic impurities removal by HPAM was investigated.The experimental results of manganese removal from nickel show that the manganese removal process by HPAM obeys the first-order rate law.The reduction rate of manganese removal increases proportionally to the approximately0.73th?0.75th power of the hydrogen content in plasma gas.The rate of manganese removal increases with the increase in hydrogen content and drop in melting pressure.Based on the principle of manganese removal from nickel by HPAM,the refining experiment of Mo by HPAM was carried out.The results show that the purity of Mo?except W,O,N,C,and S?is improved from 3N8 to 5N7 after 30 min melting by under the pressure of 0.05 MPa or 60 min melting under atmosphere pressure.The removal rate of metallic impurities increases with the reduction of atmosphere pressure.The thermodynamic analysis indicates that metallic impurities could be removed by active atom H in plasma.?4?The process of impurities removal by HPAM was simulated.The physical and mathematical model of proceeding in HPAM has been build.The results showed that the current density at the cathode spot increased from 4×10⁷ A/m² to 7×10⁷ A/m²,as the hydrogen proporition in the plasma gas increased from 0 to 40%.The temperature of the plasma increased from 3.2×10⁴ K to 5.7×10⁴ K and the flow rate of plasma increased from 1×10³ m/s to 3×10³ m/s with the addition of 40%H₂.In addition,the temperature of the melt increased from 2.5×10³ K to 1.2×10⁴ K and the flow rate of melt increased from 4×10^-2 m/s to 7×10^-2 m/s by adding 40%H₂ in plasma gas.The simulation results indicated that the elevating of melt temperature accelerated the reaction rate between hydrogen atoms and impurities and the increase of melt flow rate enhanced the mass transfer in the melt,which improved the rate of impurity removal by HPAM.?5?The mechanism of impurities removal by HPAM was investigated.The results of the calculation in COMSOL are compared with the experimental data,which includes nonmetallic and metallic impurities removal by HAPM.The mechanism is that the counter-diffusion of hydrogen atoms will react with impurities in the melt or on the melt-gas boundary layer,and the hydrides will transfer to the gas phase.In other words,the activated hydrogen atoms are considered to act as a carrier of impurities.The rate of impurity removal is limited by the mass transfer from melt to the melt-gas boundry.Based on the experimental data,a formula can be derived for estimating the impurities removal degree by HPAM.The formula is?%?=RD₀?[1+?%H₂?^0.08].?6?The effect of magnetic field on the sulfur removal by HPAM was investigated.The effect of static magnetic field on sulfur removal by HPAM was studied.Results indicated that the application of the static magnetic field enhanced the sulfur removal efficiency.Based on simulation by COMSOL,the dumbbell-shaped melt flow in the vertical plane of the melt is converted into a spiral flow.Under the static magnetic field,the impurities transfer is enhanced because the melt flow rate is improved from the level of 10^-2 m/s to 10^-1 m/s.During solidification,the static magnetic field caused the decrease of impurities concentration at the liquid-solid front,which improved the purity of the solid phase.