Kinetics of Reactive Droplets in BOF Steelmaking

By: Glendon BrownSupervisor: Dr. K. Coley

Jan 13th, 2012

1

Outline

• Purpose– Comprehensive model of BOF refining

• Introduction– BOF Steelmaking– Importance of Droplets

• Elements of the model– Droplet generation– Droplet residence time– Droplet swelling

• Results– Cr Reduction Work

• Future Work2

Purpose

• Long Term: – To create a model that can accurately predict the

complex process of BOF refining• The high degree of complexity

• Current work:– Focus on specific reactions of interest

3

BOF Steelmaking

• Blast furnace hot metal is charged into Oxygen converters to be made into Steel

• Oxygen jet blows onto the hot metal, ejecting metal droplets into the slag

Image: T. Letcher, Chemical Thermodynamics for Industry, 20044

Importance of Droplets

• Droplets offer a greatly increased SA/V ratio accelerating refining reactions– Desulphurization– Dephosphorization– Desiliconization– Decarburization

• Wide range of opinions on importance of droplets. Some workers1 suggest up to 50% of metal in the slag at a given time.

1. Urquhart and Davenport, Canadian Metallurgical Quarterly, 1973, Vol.12, No.45

Phosphorous

• Phosphorous is refined to very low levels• Competition between decarb, and dephos

(Limited Oxygen Supply)• C has stronger affinity for oxygen

– P needs to be oxidized to PO2.5 no gas nucleation required

– C oxidation requires CO nucleation

6

Droplet Generation

1. Subagyo, Brooks, Coley, Irons, ISIJ International, Vol 43, 20032. Koria and Lange: Met Trans. B, vol 15B, 19843. He and Standish, ISIJ International, Vol 30, 19904. Standish and He, ISIJ International, Vol 29, 1989

• Subagyo et al1

– Created the blowing number (NB) which correlated to generation rate

– Results were coincident with previous works2,3

7

Subagyo, et al1

Standish and He3,4

Droplet Residence Time• Subagyo et al1

– Developed a model based of ballistic motion of metal droplets in slag, predicted residence times up to 60 times too small (order of 1 Second)

• Brooks et al2

– Recognized that swelling of droplet observed by Molloseau and Fruehan3 would affect predictions of Subagyo et al.

1. Subagyo, Brooks, Coley, Canadian Met. Quarterly , Vol 44, 2005

82. Brooks, Pan, Subagyo, Coley, Met Trans B, Aug 2005

3. Molloseau and Fruehan, Met Trans B, June 2002

Droplet Residence Time• Subagyo et al1

– Developed a model based of ballistic motion of metal droplets in slag, predicted residence times up to 60 times too small (order of 1 Second)

• Brooks et al2

– Recognized that swelling of droplet observed by Molloseau and Fruehan3 would affect predictions of Subagyo et al.

1. Subagyo, Brooks, Coley, Canadian Met. Quarterly , Vol 44, 2005

92. Brooks, Pan, Subagyo, Coley, Met Trans B, Aug 2005

3. Molloseau and Fruehan, Met Trans B, June 2002

Droplet Swelling

• Molloseau and Fruehan data inadequate for general prediction

• Chen1 proposed a model for swelling by balancing internal gas generation with escape rate– Rate of gas generation controlled by internal nucleation

• Pomeroy2 modelled the incubation time before the onset of swelling

1.Chen and Coley, Iron. and Steelamking, Vol 37, 2010 2. Pomeroy, MSc Thesis, McMaster university, 2011 10

111. E. Chen, Ph.D. Thesis, McMaster University, 2011.

12

8 10 12 14 16 18 20 22 24 26 28 301

1.5

2

2.5

3

3.5

4

Bulk Slag FeO (%)

Init

ial C

(%

)

0.15s 0.25s 0.35s

0.5s

0.75s

1s

1.5s

1.3s

1.0 g, 0.005 % S, 1773 K

1.03 s ± 0.2 1..47 s ± 0.2

1.73 s ± 0.2

1.53s ± 0.2

1. Pomeroy, MSc Thesis, McMaster university, 2011

Cr Reduction

• Motivation– Low oxygen potential– Explain previous work by

Simukanga1; sulphur increased the rate of Chromium reduction from slag by Fe-C-S

– Hypothesis: slag metal emulsification

1. S. Simukanga, Ph.D. Thesis, University of Strathclyde, 1990. 13

Cr Reduction

• Simukanga: slag metal emulsification

• Pomeroy: x-ray showed no emulsification

• Hypothesis: CO nucleation rate determining step

1. S. Simukanga, Ph.D. Thesis, University of Strathclyde, 1990.14

Experimental Apparatus

15

Quench zone (optional)

Gas out/pressure transducer

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.160.0E+00

1.0E-08

2.0E-08

3.0E-08

4.0E-08

5.0E-08

6.0E-08

7.0E-08

8.0E-08

9.0E-08

1.0E-07

CO Evolution with Sulphur Content

Metal Sulphur Content (wt %)

CO

Evo

lutio

n R

ate

(mol

e /

s)

1. M. Pomeroy, G. Brown, Dr. K. Coley, AISTech 2011, Conference Proceeding, May 2010. 16

Results

• Increase as seen with Simukanga• Subsequent decrease in reaction rate as surface

poisoning takes effect• The increased reaction rate is a result of lowered

surface tension, as proposed by Chen, which causes enhanced CO evolution

• Gas evolution peak was consistent with previous work by Chen, Pomeroy, and Fruehan

• Droplet swelling was not observed (consistent with predictions of Pomeroy)

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• A gas halo was instead

18

0.05 0.10 0.15 0.20 0.25 0.300.00E+00

5.00E-08

1.00E-07

1.50E-07

2.00E-07

2.50E-07

3.00E-07

CO Evolution Volume Dependency

Droplet Volume (cm -3)

CO

Evo

lutio

n R

ate

(mol

e /

s)

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Results

• The linear correlation clearly indicates a volumetric dependency of gas evolution

• Conclusion: a thin layer of internal nucleation exists, labelled δ, where sufficient oxygen has diffused to nucleate CO

Thickness δ

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δ Analyzed

• Volume of this layer is:

• Expected rate of gas generation:

• Numerical solutions (Pomeroy) of in relevant droplet size range show: δ ≈ 0.07R

Rate

Next Steps

• Dephosphorization– High residence time desirable

• High CO nucleation rate desirable

– However, C in competition with P• High CO nucleation rate undesirable

– Looking for a “sweet spot”

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Next Steps

• Fe-C-P-S droplets will be studied with C - ~4 wt%, P – 0.02 wt%, S – 0 to 0.02 wt%

• 2 different methods– CPVI and XRF to measure gas evolution and

swelling– Quenching experiments to investigate the

progress of dephos via ICP

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Tying things together

• Incorporation of the Cr reduction data and dephosphorization data into the model developed by Chen and Pomeroy

• Expansion of the model to include more of the significant reactions of steelmaking will make this model useful and accurate in predicting BOF refining to increase yield and improve economics of the processes

24

Thanks

• NSERC• McMaster SRC• Dr. K. Coley• Members of my research group

25

Questions?

• Thank you for your attention

26

Appendix

• NB

– When the Kelvin Helmholtz instability criterion is exceeded and the interface breaks down

– Li and Harris1 found that splashing occurred at:

• Surface Tension modification parameter of Chen:

271. Li and Harris, Pyrometallurgy 95 Conf. Proc., IMM, London, 1995

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

2

4

6

8

10

12

0

5

10

15

20

25

Gas Evolution Comparison Data

Cr Reduction DataPomeroy workLogarithmic (Pomeroy work)Chen's work

S (wt %)

Gas E

volu

tion

x108

(mol

e/s)

Gas E

volu

tion

x105

(mol

e/s)

• Molloseau also reported a peak at 0.011 wt% S with subsequent decrease