The importance of materials properties in high-temperature ... › 2013 › images › papers ›...
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The importance of materials properties in high-temperature processes
Ken MILLS1,2, Muxing GUO3
1 Dept. Materials, Imperial College, London SW& 2AZ,
2 University of Manchester 3 Dept. Metallurgy & Materials Eng. KU Leuven,
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Physical properties &high-temperature processes
• Phys. Chem. Properties- useful in solving problems with process control and product quality.
• First law of high temperatures • “At high temperatures everything reacts with
everything else” • Second Law: • “They react bloody quickly and situation worsens
rapidly as the temperature increases”. • Many problems in high temp. processes- disaster
never far away
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Example- variable weld penetration
TIG/GTA welding-robotic process-many welds
-set up optimum welding- Deep weld penetration - Change steel- meets
spec. -→shallow welds - Forces acting in pool
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Effect of tiny differences in S
1400
1600
1800
2000
1500 1600 1700 1800 1900
Temperature, T/K
Surf
ace
tens
ion,
mN
m-1
• Hot liquid → melt back <50 ppm S –flow outward-
melt back-shallow weld >50 ppm S –flow inward &
down → deep weld
Surface tension >50steels 50ppm S →25% ↓ in γ d γ/ dT → ─ve to + ve
30 ppm S
80 ppm S
S< 50 ppm S> 50ppm
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Slag properties and structure
Problem caused by ppm differences in S
Math models improved → insights to mechanisms
-Slags- are ionic 4Al steel+3SiO2 slag=3Si steel+ 2Al2O3 slag
4Al + 3Si4+ = 3Si + 4Al3+. Slag props –depend on slag
structure
Silicates- building block Si- 4O tetrahedron with O
joining 2 terahedra Cations,Na+ - break bonds
BOs, NBOs and free O’s
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Alumino-silicates
Add Al2O3-network breakers↓
→ polymerisation ↑. (4)Al High Al2O3→some (5)Al, (6)Al NBO/T=2 (ΣXMO + ΣXM2O-XAl2O3 )/
(XSiO2+ 2 XAl2O3) Q=( 4- NBO/T) =measure of
polymerisation TiO2 behaves like SiO2? Fe2O3,Cr2O3-behave as Al2O3 ?
Al Na+
Charge-balancing
Si4+; Al3+; →(NaAl)4+; Na+ cannot act as network-breaker- polymerisation ↑
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Effect of structure on slag properties
Polymerisation (Q) has great effect on props
Q↑ as %SiO2 ↑ as CaO%↓ Alumino –silicates May be Al-O bond weaker
than Si-O bond
-2
2
6
10
14
18
-1 0 1 2 3 4Q
ln η
1900
K, (
dPas
)
-2
0
2
4
6
8
-1 0 1 2 3 4Q
ln η
1900
K, (
dPas
)
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Factors affecting properties
Slags- crystals or glasses Crystals-atoms fixed pos’n Glasses-disorder- forms
scl- high Sconfig; Nature cations→structure Bond strength- (z/ r2) (z/ r2)↑→ ( Qn→Qn-1+Qn+1) ↑ → Coord no ((6) N) ↑ Size: elect cond. Diffusion No. (n):Na2O n=2;CaO,n=1
Effect of structure- props 1. Viscosity(η) El cond
(κ)/ Res(R)Diff coeff(D) Liq: η = AA exp(BA/T) Scl: η = AV exp(BV/(T-To)) 1.Resistance cation movement 2. Mobility of cations size Equivalent: η; R; (1/D)
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ln η1900; ln R1900;Bη;BR as f (Q)
• )
-2
2
6
10
14
18
-1 0 1 2 3 4Q
ln η
1900
K, (
dPas
)
0
20
40
60
0 1 2 3 4Q
B/1
000
-3-113
579
11
0 1 2 3 4Q
ln R
1900
K
0
10
20
30
0 1 2 3 4Q
BR
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Density (ρ),Thermal expansion coeff (α,β) Surface and Interfacial tension (γsl, γmsl)
• α ↑ as Q↓ and (z/r2) ↓ • αglass ↑sharply at Tg-
• glass→ scl: • Tg –Tliq: αscl ≈ 3 αcrys: ρ slightly affected by structure ρcrys>ρglass- better packing
γsl, γmsl surface not bulk property
Surfactants- in surface layer eg. S,O in steel
slags, B2O3, CaF2,K2O, Na2O γmsl = γm+ γsl- 2φ( γm γsl)0.5 γm ≈4 γsl : γm most important - S, O contents in metal
0
2000
4000
6000
8000
200 400 600 800 1000Temperature, T/K
ΔL/L
293 p
pm
02468
1012141618
0 1 2 3 4Q
105 α
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Thermal conductivity (k) thermal diffusivity (a)
00.20.40.60.8
11.21.41.61.8
2
0 200 400 600 800 1000 1200 1400
Temperature, T/oC
The
rmal
con
duct
ivity
,Wm-1
K-1
00.20.40.60.8
11.21.41.61.8
2
0 200 400 600 800 1000 1200 1400 1600
Temperature, T/K
The
rmal
con
duct
ivity
,Wm
-1K
-1
Heat transfer over slag film -complex-cond’n,rad’n, conv Solid: keff= klat+ kR. kR=16σn2T3/ 3α* liq glass: kR=10klat. kR↓ by (i) FeO add’n (ii) cryst kcrys≈ 2kglass
2 methods: LP and THW kLP ≈ 5-10 kTHW. Problem un-resolved
LP
THW
THW
LP
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Manipulation of properties for process optimisation-liquidus temp (Tliq)
1. Liquidus temp (Tliq) Freeze linings- slag attack
of refractory – Lurgi -Slag coating of refractory Slag splashing- BOS-LD Pig Fe+O2→FeO; Si→SiO2 CaO added → slag Great turbulence → Extend refr life and
minimise “downtime” Optim. slag:13%FeO:8%MgO
Forms MgO.Fe2O3- high mp- >50,000 heats without re-lining
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Manipulation of slag Tliq, viscosity(η)
Drainage rate- (1/ η) Coal gasification ash →↑ η slag CaO added →↓ η Low melting regions CaO- SiO2 : XCaO = 0.4-0.6 CaO- Al2O3 :XCaO=(0.5-0.7) Fixed amount CaO-
disasters Ladle glazes→inclusions
-2
2
6
10
14
18
-1 0 1 2 3 4Q
ln η
1900
K, (
dPas
)
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Manipulation of surface (γsl) and interfacial tension (γmsl)
Slag & metal entrainment -Loss of precious metals -Inclusions in CC steel
Velocitymetal > Velocityslag . mentrap=1.06x10-7 (ηsl) -0.255. (γmsl) -2.18
Min by incr ηsl and γmsl.
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Manipulation of surface (γsl) and interfacial tension (γmsl)
Math Model of CC process Predicts standing wave
,vortices, “necking and detachment”
γmsl = γm+ γsl- 2φ( γm γsl)0.5 γmsl ↑ as S, O in metal ↓
B2O3, K2O, Na2O ↓
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Other problems involving surface and interfacial tension
1.Removal of inclusions By Ar bubbling 2. Scale formation Na2O, K2O cause slag to
stick to metal-reacts with FeO→FeSiO3 (scale)
3. Separation of metal droplets
Fe3O4
FeO
steel
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Manipulation of heat transfer via freeze lining
Heat transfer-very complex Cracking and stickers Heat flux, q – ds, kR, kcryst ≈2kglass ; qcryst< qglass. Cryst ↑→ kR ↓ Heat flux manipulated by 1. ds →Tsol ; ds ↑ as Tsol ↑ 2. % crystals in slag film
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Harvesting enthalpy in slag
Steelmaking Energy ca. 30% total costs BOS slag –granulated –
water spray on molten slag→ flame
H2- decomposition of H2O Harvest H2. Problem – O2 + H2 mixture Tata- reseach programme-
estimated saving- 30 million $ p.a for average steelworks
Produced >70%H2+N2+CO2
H2 could be used for energy or as reductant
Pilot –ferrochrome plant
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Discussion
• Vast range slag compositions high temp processes • Compositions may change on daily basis • Need for models to calculate props from chem comp. • Models should include structural characteristics • Can represent structure in terms of BO, NBO or Q or
thermodynamic functions • Challenge will be in representing certain structural
features eg. Ti4+ prefers company of otherTi4+ to Si4+
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Conclusions
1. Slag properties proved very useful for solving process problems
2. Mathematical models can give insights into mechanisms and help solve problems.
3. Knowledge of factors affecting physical properties allows one to manipulate processes- including valorisation
4. Vast range of slag compositions in pyro-metallurgy - will need good models to estimate properties from chemical comp’n- where structure comes in.