I4Future Plasma diagnostics by imaging and spectral data · 2019-05-20 · Plasma diagnostics by imaging and spectral data Doctoral researcher Henri Pauna Process metallurgy research

I4FuturePlasma diagnostics by

imaging and spectral data

Doctoral researcher Henri Pauna

Process metallurgy research unit, University of Oulu

University of Oulu

About myself

Laser ablation study in progress at Nano and Molecular Systems Research Unit (NANOMO)

‒ Physics studies at the University of Oulu

(2012 – 2017)

- After Bachelor’s degree in 2015 at NANOMO

‒ M. Sc. from physics in 2017

- Thesis title: Time-of-flight Mass Spectroscopic

Study of Laser Ablated Oxidized Solid Lithium

‒ Joined the OSCANEAF project 1 day after

the M. Sc. degree

- Objective: process control system for

electricity based melting in the steel industry

2 15.05.2019 I4Future, Methods for Image and Spectral Data Analysis and Interpretation

University of Oulu

Wait, OSCANEAF?What?How?Why?

What about the plasma and the spectra?

3

Before we go deeper into that acronym, lets make a backgroud check.

15.05.2019 I4Future, Methods for Image and Spectral Data Analysis and Interpretation

University of Oulu4

Background: The steel industry

Outokumpu's stainless steel EAF in Calvert, Alabama; https://www.youtube.com/watch?v=2UiQERnDips

1808.6 million tons of steel in 2018https://www.worldsteel.org/media-centre/press-releases/2019/Global-crude-steel-output-increases-by-4.6--in-2018.html

Two main routes for steel production:

‒ Blast oxygen furnace (approx. 2/3)

- Ore-based steelmaking

- Oxidized components (e.g. 𝐹𝑒2𝑂3)

- Reduction process; oxygen is removed with

carbon injection

- Induces carbon dioxide emissions

‒ Electric arc furnace (EAF, approx. 1/3)

- Scrap-based steelmaking using electricity

- Usually recycled scrap metal

- No need for reduction process

- Significantly lower carbon dioxide emissions

- If the electricity is produced sustainably


https://www.youtube.com/watch?v=2UiQERnDips

https://www.worldsteel.org/media-centre/press-releases/2019/Global-crude-steel-output-increases-by-4.6--in-2018.html

University of Oulu

Charge material in EAFs; the scrap


University of Oulu6

Background: Slag

Slag runoff after melting

‒ The impurities from the molten metal

accumulate on top of the metal bath

- This impurity layer is called slag

- Separates from the steel due to difference

between the densities

‒ Most common components are lime (𝐶𝑎𝑂),

silica (𝑆𝑖𝑂2), magnesia (𝑀𝑔𝑂), and 𝐴𝑙2𝑂3

- Other components; 𝑀𝑛𝑂, 𝐶𝑎𝐹2, 𝑆, 𝑃

‒ Some steel constituents (𝐶𝑟, 𝐹𝑒) might

(and will) dissolve into the slag

- This can be reduced with material additions

during melting

- E.g. 𝐶𝑎𝑂, 𝑆𝑖𝑂2, 𝐶𝑎𝐹2- Changes e.g. basicity of the slag

- Slag composition must be monitored


University of Oulu

What to do with slag?


University of Oulu

OSCANEAF

A schematic figure of the measurement set-up [1]

‒ On-line slag composition analysis for

electric arc furnaces (RFCS project)

- Steel scrap (recycled) is used as raw

material in electric arc furnaces

- Composition of scrap varies between

different melts

- Results into varying quality and

properties of steel

- Usually slag composition is analysed off-line

- X-ray fluorescence (XRF) analysis for a

slag sample (cool down, grind)

- Takes a lot of time!

- On-line analysis would allow modifying steel

properties during the melting process

- Our solution: on-line in situ optical

emission spectroscopy (OES)

- Spectrometers from Luxmet Oy (Oulu)


University of Oulu

Background: Emission of light

Allowed electronic transitions (above) and three low wavelength transitions of lithium [2] (to the right)

‒ By studying the emission spectrum of a

substance, its composition can be

determined

- Each emission line in a spectrum

corresponds to a specific electronic transition

- Emission of light arises from relaxation of an

excited state of a particle

- Requires sufficient amount of energy for

excitation


University of Oulu

Background: EAF OES

A common slag composition of an EAF slag in % (above) and a laboratory EAF UV emission spectrum (right)

‒ In case of EAF, the compositions of the

metal and the slag are complicated

- The spectrum will consist of emission lines

from individual elements (and/or molecules)

‒ Extreme environment

- Temperature of the melt ~1600 ⁰C

- Arc temperature 5000 – 20000 K!

- A lot of gases in the atmosphere

- Affects the spectra!

10

𝐶𝑎𝑂 𝑆𝑖𝑂2 𝑀𝑔𝑂 𝐴𝑙2𝑂3 𝐶𝑎𝐹2 𝑆 𝐹𝑒2𝑂3 𝑀𝑛𝑂 𝐶𝑟2𝑂3

53.0 6.0 6.0 26.0 6.0 1.5 0.1 0.1 0.1


University of Oulu

Background: Plasma

Saha-Boltzmann equation and its plot for Fe I and Fe II lines [3] (above) and arc in a laboratory EAF (right)

‒ Plasma is a state of matter that consists

of “free” particles (electrons, atoms,

molecules, etc.)

- Typical case in high temperatures (as in EAF)

- Plasma properties with- Boltzmann plot Plasma temperature

- Saha-Boltzmann equation Electron density

- Stark broadening Electron density


University of Oulu

Plasma analyticsConstants from NIST atomic spectra database [4]

Boltzmann-plot equation [3]

12

Modified Saha-Boltzmann equation and

McWhirter local thermodynamic

equilibrium (LTE) criterion [3]


University of Oulu

Industrialrelevance

‒ On-line slag composition

- Monitoring of the process conditions

- Changes in emission spectra relate

to the changes in atmosphere and arc

- Instant information on slag composition

- Helps the furnace operator to determine

the amount of additive materials

- Timing of e.g. carbon injection and

oxygen lancing

‒ Plasma analysis

- Validation tool for spectrum analysis

- Information on properties of the

electric arc itself

- Correlations between electrode/arc

parameters with plasma properties


University of Oulu

Laboratory EAF (KTH, Sweden)

‒ 10 kg EAF

- Two graphite electrodes

‒ Two cameras, three spectrometers

- Three spectrometers to cover ultraviolet,

visible and near-infrared

- Cameras to capture the movements and

brightness of the electric arc

- The cameras were filtered with 500 nm and

1000 nm band-pass filters

- 500 nm: the electric arc

- 1000 nm: the heat radiation from slag

14

Laboratory EAF and measurement set-up at KTH Royal Institute of Technology


University of Oulu

Laboratory EAF (KTH, Sweden)

15

Foaming test results [5]; plasma temperatures with Cr I emission lines

‒ Foaming test

- Oxygen is lanced into the melt to form

𝐶𝑂 bubbles

- Creates ”foam” that

- Protects the furnace lining

- Captures the heat from the

arc into the slag

- Reduces noice pollution


University of Oulu

Pilot-scale EAF(IOB, Germany)

Pilot-scale testing at RWTH Aachen University, Germany (above) and illustration of the pilot-scale EAF and view cone (right, view cone marked with green circle)

‒ Capacity: 200 kg of liquid steel

- Equipped with two graphite electrodes

‒ One camera, three spectrometers

- Same spectrometer configuration as in KTH

- Camera to capture the movement and

dimensions of the electric arc

https://www.iob.rwth-aachen.de/en/research/equipment/electric-arc-furnace/


https://www.iob.rwth-aachen.de/en/research/equipment/electric-arc-furnace/

University of Oulu

Visual presentation of IOB data [6]

17

‒ Length correlates with voltage

‒ Area visibility correlates with plasma temperature

‒ Electrical data of the furnace

‒ Electron densities with Ca I and Ca II lines

‒ Temperatures with Cr I lines


University of Oulu

Industrial EAF

18

‒ Capacity: 50 – 150 t

‒ Power: 40 – 120 MW

‒ Melt temperature around 1650 ⁰C

‒ Plasma temperature 5000 – 20000 K

‒ Slag splashes, gas bursts, dusts, etc.


University of Oulu

EAF process steps (1)

19

Ignition

‒ Start of the melting process

- No spectra are observed

- Charge material (scrap metal) level

is very high

[8]


University of Oulu


20

Boring (=“drilling”)

‒ The electrodes are lowered

- Heat radiation from the

melting scrap

[8]


University of Oulu


21

Formation of the melt

‒ Electrodes are at their lowest position

- Scrap still blocks the arcs

- Emission lines from alkali metals

(Na, K, Li, Rb)

[8]


University of Oulu


22

Main heating period

‒ Increase in light intensity

- Heat radiation rises a lot

- Solid scrap still blocks the view to

the arc

[8]


University of Oulu


23

Down melting period

‒ Most of the scrap is molten

- Light emissions from the arc

plasma

Strong self-absorption of the alkali

[8]


University of Oulu


24

Heating

‒ Near the end of the melting

process

- The charge material is liquid

- Arc emission spectra are observed

[8]

University of Oulu25

Industrial EAF VIS and NIR spectra from EAF

with light emissions from the arc.

Fe I, Cr I, Ca I, Mn I, Mg I, Ca II, (Na I, third spectrum)



‒ Case studies

- Spectra are observed 5 – 20 min

before the end of melts

- Sufficient time scale for on-line slag

composition analysis

- Cr and Fe are more uniform than Ca

- Towards the end of the melt,

the temperatures converge

- Promising results already!

- Correlations between OES and

XRF slag analyses

Industrial EAF

Ca I/Mg I line ratio time evolution.



Ladle furnace (LF)

Pouring molten charge from the furnace to the ladle.

‒ After EAF processing, the melt is poured into a ladle furnace

‒ Purpose of ladle furnace is to

- Fine-tune the temperature of the

melt for casting

- Adjust the chemical composition to

the product standards

- Additive materials

- Same principles as in EAF,

but smaller

‒ New measurement campaign started on 7.5.2019

- Slag composition analysis for

ladle furnaces


University of Oulu

Summary

Figure 12: Fe I, Mg I, Ca I, and Cr I emission lines around 520 nm.

OES and plasma analysis for EAFs

‒ Process monitoring

- State of the electric arc and atmosphere

‒ Slag composition

- Possibility for on-line analysis- Fine-tuning of slag/steel composition

accordingly during the melting- Very promising results for correlation

between the OES and XRF slag analyses

‒ Plasma analysis

- Data evaluation tool- Tells when the spectra are most reliable

‒ Image analysis

- Characteristics for different arc types- With respect to e.g. the arc length


University of Oulu

References[1] Anonymous: Teräskirja, Metallinjalostajat ry, Helsinki, Tampere (2009), 104

[2] G. Schriever, K. Bergmann, and R. Lebert,

J. Appl. Phys., Vol. 83, No. 9, 1 May 1998,

”Narrowband laser produced extreme ultraviolet sources adapted to silicon/molybdenum multilayer optics”

[3] C. Aragón, and J.A. Aguilera,

Spectrochimica Acta Part B 63 (2008) 893–916,

”Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods”

[4] NIST Atomic Spectra Database, https://www.nist.gov/pml/atomic-spectra-database

[5] M. Aula et al., ”Arc plasma emerging from foaming slag – Plasma diagnostics and visual observations”, 7th International

Congress on Science and Technology of Steelmaking, Venice, Italy (2018)

[6] H. Pauna et al., ”Prospects of plasma analysis for electric arc furnaces”, 7th International Congress on Science and

Technology of Steelmaking, Venice, Italy (2018)

[7] B. Bowman and K. Krüger, Arc furnace physics, Düsseldorf : Stahleisen, 2009.

[8] M. Aula, POHTO 2017 presentation (Oulu, Finland)


https://www.nist.gov/pml/atomic-spectra-database

University of Oulu

Some things for the future

‒ Molecular emission/absorption

of the slag components

- Especially 𝑆𝑖𝑂2 and 𝐶𝑎𝐹2

‒ Emission/absorption of light

by gases

- 𝐶𝑂, 𝐶𝑂2, 𝑁2, air in general

- Even 𝑯𝟐𝑶

‒ AI implementation

- Big question mark here!


University of Oulu

Thank you

Documents

I4Future Plasma diagnostics by imaging and spectral data · 2019-05-20 · Plasma diagnostics by imaging and spectral data Doctoral researcher Henri Pauna Process metallurgy research