Refining of Liquid Metal by Refining of Liquid Metal by Hydrogen Cold PlasmaHydrogen 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 Te and ion temperature Ti

When Te≈Ti， 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 Te>>Ti， 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 power2-Pulse generator3-Reaction chamber4-Electrode5-Support rods of electrode6-Vacuum pump7-Sealing plug8-Thermocouple9-Pressure gauge10-Vacuum meter11-Flow meter12-H2 container13-N2 container14-Furnace

plasma

crucible

melt

14

Experimental apparatus

0 20 40 60 80 100 1200

5

10

15

20

25

30

35

40

45

50hydr ogen col d pl asma( P=4000Pa)

Ar cold plasma (P=4000Pa)

[S], ppm

Ti me t , mi n

Experimental results

System: Sn-S

Temp. : 650°C

Distance between two electrodes: 3 cm

Voltage: 350-600V

Plus frequency : 1600Hz

Crucible size : 25x30 cm

0 20 40 60 80 100 120 140 1600

5

10

15

20

25

30

35

40

45

50

H2*( - ) : mel t as cat hode

H2*( +) : mel t as anode

H2*(-)

(P=4000Pa)

H2*(+)

(P=4000Pa)

H2

(P=4000Pa)

[S] ppm

Ti me t , mi n

Experimental results

Influence of gas type and polarity on refining

0 50 100 150 200 2500

100

200

300

400

500

600

700

800

900

H2*

(P=4000Pa) Ar*(P=4000Pa)

H2

(P=4000Pa)

Ar

( P=105Pa)

[S]

ppm

，

Ti me t mi n，

Experimental results

0 20 40 60 80 100 1200

200

400

600

800

1000

1200[S], ppm

Ti me t , mi n

hydr ogen col d pl asma ( 400Pa） hydr ogen col d pl asma ( 4000Pa） ar gen col d pl asma ( 4000Pa） ar gen/ hydr ogen col d pl asma ( 4000Pa）

Experimental results

0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

1550℃ 4000Pa、

hydr ogen gas

[N],

ppm

Ti me t , mi n

hydr ogen col d pl asma

Experimental results

System: low carbon steel

Temp. : 1550°C

Distance between two electrodes: 3 cm

Voltage: 250 - 350V

Plus frequency : 1250Hz

Crucible size : 2.5x30 cm

Nitrogen removal from liquid steel

0 10 20 30 40 50 600

10

20

30

40

50

60

70

hydr ogen col d pl asma

ar gen/ hydr ogen col d pl asma

ar gen col d pl asma

1550℃ 4000Pa

[N], ppm

Ti me, mi n

Experimental results

Mechanism of refining with hydrogen cold plasma

Melt as a cathode

2H++[S]=H2S↑

2H*+[S]=H2S↑

H2SH2SH+H*

Melt as an anode

H2SH2SH*

H+ : hydrogen ionsH* : 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

Sx, N2Sx, N2

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 NH3, H2S and

PH3. Because the compounds are insoluble into the

melts, they will be carried off by fluent gas. Finally the impurity elements are removed from metal.

Thanks