INITIAL WETTING AND SPREADING PHENOMENA OF SLAGS ON REFRACTORY CERAMICS
1Yongsug Chung, 2Tae Hee Yoon and 3Kyuyong Lee
1Professor and 2Graduate Student, Department of Advanced Materials Engineering
3Professor, Department of Consilience
Korea Polytechnic University, Siheung-si, Gyunggi-do, 15073, Korea (ROK) Keywords: Wettability, Spreading rate, Refractory ceramics, Slag, Reactive wetting
Abstract
Wetting angle and spreading rate between slags and refractory ceramics such as Al2O3, MgO, MgO-C or SiC have been recently determined by using dispensed drop technic with a high speed camera. Intrinsic value of wetting angle and the effect of reactions on wetting and spreading are reviewed and discussed. Role of carbide or graphite in refractories are also reviewed when these were in contact with reducible slags. Driving force for spreading and a spreading model (non-reactive viscous model) are discussed.
Introduction
It is important to know wetting and spreading phenomena between slags and refractories.
This knowledge provides a better understanding of inclusion removal of refractory ceramics where they are indigenously or exogenously formed during steelmaking, and of penetration behavior of slags into refractories which causes degradation [1, 2].
The penetration behavior is considerably affected by wetting and spreading of liquid slag on refractory material as shown in Eq. (1) [2].
h = ( r σ cosθ t/2η)1/2 , (1)
where h is depth of penetration, r is radius of pore of refractory, σ is surface tension of slag, θ is wetting angle between slag and refractory, η is viscosity of slag, and t is time.
Although the values of surface tension and viscosity of slag can be easily found, the contact angles cannot be obtained in open literatures due to difficulty of the measurement. The conventional sessile drop technic has some limitation to obtain an intrinsic angle since a reaction can be occurred while temperature elevates.
Several successful measurements have been carried out by using the dispensed drop technic.
[3, 4] Recently, the dispensed drop technic with a high speed camera provided intrinsic values both of wetting angle and spreading rate for the system between slags and Al2O3, MgO, MgO-C or SiC [5-8] In those studies, reactive wetting was quantitatively considered [5, 6, 8].
Wettability is divided into two categories: non-reactive wetting and reactive wetting. In non- reactive wetting, mass transfer through the solid/liquid interface is very limited and the wetting is mostly driven by the physical forces such as inertial, gravitational, and viscous
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
force. In reactive wetting, wettability is strongly influenced by the reaction at the solid/liquid interface such as diffusion and the formation of intermediate compounds or bubbles at the interface.
In most non-reactive wetting, spreading of droplet is either inertial force dominant spreading or viscous force dominant spreading. In inertial spreading for low viscosity liquid, local equilibrium is rapidly established at the triple line, thus the contact angle approaches the equilibrium angle immediately. Then, spreading is controlled by inertial force due to the Laplace pressure difference caused by the curvature difference between the bulk and the region near the triple line. On the other hand, viscous spreading is controlled by viscous friction inside the droplet and the dissipation of energy at the triple line of the droplet. The driving force in this case is the change in the surface and the interfacial energy of the system caused by the displacement of the triple line [9, 10].
In this paper, wetting phenomena between slags and refractory ceramics such as Al2O3, MgO, MgO-C or SiC are reviewed and the intrinsic contact angles are discussed based on a comparison of the similar systems; oxide ceramics and C containing ceramics. Spreading phenomena by De Gennes’s non-reactive model are also reviewed for systems between oxide ceramics and slags [11].
Experimental Methods
The slag and substrate were separately heated and the molten liquid droplet of slag was dispensed onto the substrate by the push bar, so droplet was contacted with substrate in isothermal condition (Fig.1). The distance between crucible and solid substrate was carefully set to 1cm for each run to maintain consistent droplet shape. A high speed camera was used to capture the images of the initial wetting and spreading behaviors.
The chamber was sealed and evacuated to 2.0 x 10-2 tor using a rotary vacuum pump and filled with 99.999wt% Ar gas. This process was repeated more than three times. Then the chamber was heated up to 1600℃ at the rate of 10℃/min. When the droplet contacts the substrate, the high speed camera was used to capture the initial contact angle at 1000~1500 frame/s.
Fig. 1 Schematics of experimental apparatus of dispensed drop technic
Results and Discussion 1. Intrinsic wetting angle between slags and oxide ceramics
Most popular oxide refractory ceramics used for steelmaking are mainly composed with Al2O3 or MgO. An intrinsic value of wetting angle is not easy to obtain due to a reaction between two solid-liquid oxides. Dissolution of refractory component into slag occurs in most cases. Therefore, in order to remove the dissolution effect, a saturation condition is required. Kim et al. [5] measured wetting angle between single crystal Al2O3 and CaO-Al2O3
binary slag at saturated and non-saturated condition at 1550oC as can be seen in Fig. 2. The compositions of Al2O3 were 57.9 wt% at saturated condition and 42.8 wt% at non-saturated condition, respectively. The apparent wetting angle rapidly decreases to almost 20 ° within 1 second. From 0 to 3 seconds, the apparent contact angle of the two kinds of slag is similar.
After 3 s, the apparent contact angle of the saturated slag slowly reaches to 17 °. In contrast, that of the non-saturated slag slowly decreases to the angle less than 17 °. The lower value for the non-saturated slag was induced by a crater formation at the substrate due to dissolution during the spreading as can be seen in Fig. 3 (b).
0 5 10 15 20
0 20 40 60 80 100 120 140 160 180
Apparent Contact angle ,)
Time (s)
Non-saturated Slag Saturated Slag
Fig. 2 Apparent wetting angles between two kinds of slag droplet and Al2O3 single crystal at 1550oC [5]
Fig. 3 Stereoscopic microscope image of cross section. (a) Saturated slag and Al2O3
substrate. (b) Non-saturated slag and Al2O3 substrate. [5]
Similar study has been carried out for the system MgO and CaO-SiO2 (50: 50 in weight) slag, of which condition was non-saturated. Park et al. [6] determined wetting angle between MgO and CaO-SiO2 slag by using single crystal and poly crystal of MgO at 1600oC. As can be seen in Fig. 4(a), the wetting angle of single crystal rapidly decreased to the equilibrium angle (about 5°) within 0.5 seconds which is much lower and faster than the one of Al2O3. They found 7% increase of MgO content in the slag after quenching but did not find any formation of the crater so that the interface remained to be flat after spreading.
0.0 0.2 0.4 0.6 0.8 1.0
0 20 40 60 80 100 120 140 160 180 200
MgO single MgO poly Contact Angle ( 0 )
Time(s)
(a)
Fig. 4 Initial wetting angles of slag droplet on MgO single and poly crystal substrates at 1600oC.[6]
2. Reactive wetting between slags and C containing refractory ceramics.
Carbon is used for refractory as carbide or graphite. Silicon carbide is of great technological interest because of its good mechanical properties, high thermal conductivity, and good thermal shock resistance. MgO-C is also widely used due to its excellent chemical corrosion resistance, thermal resistance and mechanical properties. However, refractories containing C can be reacted with reducible slag components such as FeO, MnO and possibly SiO2 at steelmaking temperature.
Recently, wetting angle has been measured with various CaO-SiO2-MnO slags [8]. Figure 5 shows the changes of contact angle at C/S = 0.8, in which the contact angle rapidly decreases with time, reaches 30 degrees within 1 second, and slowly decreases to 20 degrees in 20 seconds. While this overall tendency is observed in both cases of two different MnO contents (about 12.5% vs 37.9%), a significant variation of contact angle occurs at 8 seconds in the case of 37.9% MnO. This abrupt change in contact angle reflects the bubble formation at the interface. Based on thermodynamic consideration, the bubble formation reactions can be suggested as follows [12]:
SiC(s) + 3 MnO (in slag) = SiO2(in slag) + 3Mn(g) + CO(g) △G◦= 963120-474T(J/mol) (2) SiC + MnO (in slag) = SiO(g) + Mn(g) + C △G◦= 605630 - 276T(J/mol) (3)
SiC(s) + 2SiO2 (in slag) = 3SiO(g) + CO(g) △G◦= 1457790 - 692T(J/mol) (4)
Fig. 5 Apparent wetting angles of slags on SiC substrates at 1550oC at different MnO content[8]
Graphite plays an important role in chemical corrosion where it provides poor wettability to slags. Figure 6 clearly shows the changes of wetting angles of CaO-55%SiO2 slags in contact with different content of carbon in MgO-C. The initial contact angles were 115° for the pure graphite, 92° for MgO-17%C and 40° for MgO-4%C, showing that the contact angle increased with increasing carbon content in MgO-C. A slight decrease in the wetting angle of MgO-17%C as time passes can be understood from the following reactions [7]:
MgO(s) + C(s) = Mg(g) + CO(g) △G◦= 619,800 – 292.5T(J/mol) (5) SiO2(l) + C(s) = SiO(g) + CO(g) △G◦= 198,899 – 199.4T(J/mol) (6)
In this case, we did not find bubble formation which was found in SiC case. The reason could be either because the slag was in poor contact with MgO-17%C, or because the reactions proceeded very slowly.
Fig. 6 Apparent wetting angles between CaO-SiO2 slag and MgO-C substrates at 1600oC [7]
Values of the initial contact angle and the observed reactions are summarized in Table 1.
Table 1. Initial contact angles between slags and refractory ceramics
Al2O3
(1550oC) MgO (1600oC)
SiC (1550oC)
C (1600oC)
MgO- 17%C (1600oC)
MgO- 4%C (1600oC)
Reaction remarks
CaO- 57.9%Al2O3
20 - - - - - -
CaO-42.8%
Al2O3
17 - - - - - Dissolution
[Crater formation]
CaO-50%
SiO2
- <5 - - - - Dissolution
[No Crater formation]
CaO-SiO2- 12.5%MnO
(C/S=0.8)
- - 20 - - - -
CaO-SiO2- 37.9%MnO
(C/S=0.8)
- - 20 ~ 40 - - - Bubble Formation
CaO-55%
SiO2
- - - 115 92 40 -
3. Driving force for spreading between slags and oxide ceramics
Slags and oxide refractory ceramics promptly wet each other. Spreading nature can be discussed in this case. When θ ≪90 ˚, the ratio of inertial force to viscous force is given by
𝑓𝑖𝑛
𝑓𝑣 = 0.0024 𝜌𝜎𝜂𝐿𝑉2 𝜃5𝑅 , (7)
where 𝑓𝑖𝑛 is the inertial force, 𝑓𝑣 is the viscous force, 𝜌 is the density, 𝜎LV is the liquid/vapor surface tension, 𝜂 is the viscosity, 𝜃 is the contact angle, and 𝑅 is the droplet radius[13]. The ratio can be used as a measure to determine which effect is dominant in the spreading. As shown in Fig. 7, the viscous force is dominant over the inertial force thro ughout the spreading for both systems. [5, 6]
-2.0 -1.5 -1.0 -0.5 0.0 0.5
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
log Fin/Fv
log Time (s)
Non-saturated slag Saturated slag
(a)
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
-6 -5 -4 -3 -2 -1 0 1
dominance of viscous force dominance of inertial force
MgO single MgO poly
log Fin / Fv ratio
log Time (s)
(b)
Fig. 7 The ratio of inertial to viscous force for AlO/CaO- AlO(a) and for MgO/CaO-SiO(b).
4. Non-reactive viscous model for spreading rate.
As previously mentioned, viscous force was dominant during the early stage of the spreading.
And, variation of wetting angle and spreading radius with time for the saturated slag with alumina (there was no reaction) was not significantly different from the ones for the non- saturated slag (there was a dissolution reaction). De Gennes’s non-reactive model can be applied to the spreading of the slag droplets [9]. The non-reactive viscous equation is given by
U =3𝐾𝜂𝜎𝐿𝑉𝑡𝑎𝑛𝜃(𝑡)(𝑐𝑜𝑠𝜃𝐹− 𝑐𝑜𝑠𝜃(𝑡)) (8) U =𝜎6𝜂𝐾𝐿𝑉𝜃(𝜃2− 𝜃𝐹2) . (9)
This equation is valid for θ<90°. For θ <45 °, the tan 𝜃 is replaced by 𝜃 and the cos 𝜃 by (1- 𝜃 2/2) as a good approximation so the Eq. (8) becomes Eq. (9). U is spreading rate, σLV is the liquid/vapor surface tension (J/m2), η is the viscosity (Pa∙s), 𝜃 is the contact angle, 𝜃 F
is the final contact angle at 20 s (𝜃𝐹𝑆= 17 °, 𝜃𝐹𝑁𝑆= 13°), and K(ln ∣𝑋𝑋𝑚𝑎𝑥
𝑚𝑖𝑛∣, x ≈ radius of droplet) is 10.
The experimental spreading rate evaluated by droplet radius and the theoretical value calculated by De Gennes’s equation are compared. Fig. 8 shows the experimentally obtained spreading rate and theoretical spreading rate against contact angle for Al2O3/CaO- Al2O3 s ystem (a) and for MgO/CaO-SiO2(b). The experimental values are in good agreement with the theoretical values. It means that although dissolution occurs during spreading, wettability between oxide slags and solid oxides are strong so that spreading behavior appears to be a non-reactive system.
0 20 40 60 80
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
Spreading rate (m/s)
Angle () NS Theoretical S Theoretical S Slag NS Slag
(a)
0 20 40 60 80
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
MgO poly MgO single theoretical
Spreading rate (m/s)
Angle ( 0 )
(b)
Fig. 8 Measured and theoretical spreading rate for Al2O3/CaO- Al2O3(a) and for MgO/CaO- SiO2(b) based on non-reactive viscous spreading model[5,6]
Conclusions
1) Initial contact angle between CaO-Al2O3 slag and Al2O3 at dissolutive condition resulted in 3o lower than the one (20o) at saturated condition due to a crater formation during spreading of the slag. Initial contact angle between CaO-SiO2 slag and MgO rapidly decreased to the equilibrium angle (about 5°) within 0.5 seconds which is much lower and faster than the one of Al2O3.
2) SiC in refractory reacted with reducible slags during spreading and created a bubble and C in MgO provided poor wettability against slags as its content increased.
3) Viscous spreading was dominant for wettings between CaO-SiO2/MgO and CaO-Al2O3
/Al2O3. In both systems, experimentally obtained spreading rate agrees with the calculated value using the viscous model, which suggests that the initial spreading can be regarded as non-reactive wetting controlled by viscous friction.
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