Refining of Liquid Metal by Hydrogen Cold Plasma Shanghai University Weizhong Ding School of Material Science and Engineering Shanghai University

Plasmas play important roles in several scientific and industrial applications, which include: ¨ Manufacture of semiconductor devices ¨ Modification of the surface of polymers ¨ Chemical processes ¨ Metallurgical refining ¨ Environmental pollution Plasma applications

Two types of plasma Electron temperature T e and ion temperature T i When T e ≈T i ， it is called as thermal plasma or equilibrium plasma, such as plasma torch 、 arc. Thermal plasma usually serves as a high- enthalpy source in metallurgical process. When T e >>T i ， it is defined as cold plasma or non-equilibrium plasma, like glow discharge 、 corona, etc.

DC pulsed glow discharge plasma The advantage of DC pulsed glow discharge plasma are: simple equipment easy to control less thermal energy input suitable for industrial applications in large scale

The refining effect of cold plasma on liquid metal. The purification of metal can be enhanced by cold plasma. Under hydrogen cold plasma atmosphere, nitrogen, sulfur and phosphorus in the melts are converted into volatile compounds and then distilled off. An interesting phenomenon

Experimental apparatus 14 1-DC power 2-Pulse generator 3-Reaction chamber 4-Electrode 5-Support rods of electrode 6-Vacuum pump 7-Sealing plug 8-Thermocouple 9-Pressure gauge 10-Vacuum meter 11-Flow meter 12-H 2 container 13-N 2 container 14-Furnace plasma crucible melt 14 Experimental apparatus

Experimental results System: Sn-S Temp. : 650°C Distance between two electrodes: 3 cm Voltage: V Plus frequency : 1600Hz Crucible size :  25x30 cm

Experimental results Influence of gas type and polarity on refining

Experimental results

System: low carbon steel Temp. : 1550°C Distance between two electrodes: 3 cm Voltage: V Plus frequency : 1250Hz Crucible size :  2.5x30 cm Nitrogen removal from liquid steel

Experimental results

Mechanism of refining with hydrogen cold plasma Melt as a cathode 2H + +[S]=H 2 S↑ 2H*+[S]=H 2 S↑ H2SH2SH2SH2S H+H+ H* Melt as an anode H2SH2SH2SH2S H* H + : hydrogen ions H* : hydrogen atoms If melt serves as a cathode, both hydrogen ions and atoms impact on the surface of melt and react with dissolved sulfur to form volatile compounds.

Mechanism of refining with argon cold plasma S x, N 2 Ar + Melt as a cathode Argon ions will be accelerated by electrical field and impact on the surface of the melt.,Thus sulfur or nitrogen escape from the melts. Inert gas as a refining agent

Conclusion The molecules of hydrogen in the gas are ionized when applied a DC pulsed electric power. Thus the different radicals are produced. The refining ability of hydrogen is enhanced in such process. While the radicals reach the surface of the melt, they react with the impurities to form some volatile compounds, such as NH 3, H 2 S and PH 3. Because the compounds are insoluble into the melts, they will be carried off by fluent gas. Finally the impurity elements are removed from metal.

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