Novel High-Temperature Experimental Setup to Study Dynamic

Article pubs.acs.org/IECR

Novel High-Temperature Experimental Setup to Study Dynamic Surface Tension Phenomena in Oxide Melts Mirco Wegener,* Luckman Muhmood, Shouyi Sun, and Alex V. Deev CSIRO Process Science and Engineering, Clayton, VIC 3169, Australia S Supporting Information *

ABSTRACT: In many pyrometallurgical applications, subprocesses such as emulsification, droplet, bubble or jet formation, coalescence, and surfactant adsorption occur at small time scales (typically milliseconds to fractions of seconds), both at slag/ metal and slag/gas interfaces. These phenomena are surface tension driven anddue to the high-temperature environment very difficult to investigate in a quantitative manner. Under these dynamic conditions, the instantaneous surface tension of slags may vary in time as well as along its surface and may change dramatically the rate of the involved processes. This paper presents a new high-temperature experimental setup to study and measure the dynamic surface tension of slags, the mechanisms of slag jet and droplet formation, and the capillary breakup of molten slag jets. It features a three zone furnace with optical access, and a droplet generation device incorporating a back-pressure system in combination with a stopper for precise slag flow control. The first successful results of controlled formation of calcia/alumina droplets and coherent jets in an argon environment are discussed. Various time-dependent phenomena such as droplet formation and elongation, necking, breakup, oscillation, satellite formation, and jet disintegration were investigated and quantified using a high-speed camera system. A dynamic pendant drop method was applied to determine the surface tension. The obtained values are in excellent agreement with literature data.

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INTRODUCTION Interfacial phenomena are of fundamental importance for iron and steelmaking applications, since they have significant effect on the mass transfer and fluid dynamics and hence on the diverse reactions occurring across the interfaces.1 For decades, the importance of interfacial phenomena has been demonstrated by numerous researchers in this field.2−8 Detailed knowledge of those interfacial phenomena and the involved physical properties of slags is constantly accumulating, and there is a demand for reliable experimental data, including the interfacial and surface tension of the melt phases in pyrometallurgical processes.9 In iron and steelmaking, molten slag is used for different purposes such as protection of the metal phase against atmosphere (sealing the metal from oxygen, nitrogen); removal of undesired impurities (e.g., sulfur, phosphorus), nonmetallic inclusions; reduction of heat losses; melting point reduction through optimum slag composition; low refractory interaction (e.g., protection of the lining from the arc in an Electric Arc Furnace).10−13 Slag has a lesser density than metal and floats above the molten metal, forming an interface with the metal underneath it and an interface with the gas above it. Thus, slag is involved in most mass transfer and fluid dynamic related operations. Slags exhibit a unique range of physical properties. For example, the surface tension is usually lower than that of most metals, but far higher compared to aqueous systems, typically in the range 300−700 mN/m.14 By contrast, the viscosity is usually far higher than that of metals, increasing drastically with decreasing temperature and increasing polymerization, or structural networking. Much attention has been paid to the metal/slag interface where the most important reactions and mass transfer © 2013 American Chemical Society

processes take place, e.g. steel refining. A prominent example is the basic oxygen furnace where a jet of oxygen is blown into the steel melt to remove carbon. In vacuum degassing or refining, an inert gas is used as a stirrer to increase surface area and promote mass transfer between steel and slag during desulphurisation for instance. In both cases an emulsion of metal, slag and gas is formed. Hence, the slag/gas interface is also of particular interest. Either by the formation of carbon monoxide or by the presence of oxygen in the melt, gas bubbles form and eventually burst at the melt surface leading to the ejection of droplets or jets, as schematically shown in Figure 1, left. If the kinetic energy is high enough, a penetration crater forms at whose upper end a ligament may form with consecutive droplet detachment. The processes are quite dynamic and are controlled to a large extent by the interfacial or surface tension σ of the system.15 Thus, its knowledge is essential for all subsequent calculations, developments of analytical models, and computational fluid dynamic (CFD) calculations. For example in emulsions, to calculate the specific surface area available for chemical reactions and heat and mass transfer, the Sauter mean diameter is required which results from the population balance equation in which surface tension plays a significant role in the breakup and coalescence kernels.16,17 Another example of an application with strong involvement of surface tension driven phenomena at slag/gas interfaces is the granulation of blast furnace slag droplets into then amorphous (glassy) particles by a rotating disc atomizer. Received: Revised: Accepted: Published: 16444

July 15, 2013 October 23, 2013 October 24, 2013 November 8, 2013 dx.doi.org/10.1021/ie4022623 | Ind. Eng. Chem. Res. 2013, 52, 16444−16456

Industrial & Engineering Chemistry Research

Article

If we apply these considerations to the above examples of droplet breakup in slag splashing or the breakup of slag droplets from ligaments in the granulation process, the time scale has to be taken into consideration. If the time scale of the equilibrating transport processes between bulk and surface of the considered slag system are of the same order of magnitude (or larger) as the time scale of droplet breakup, which is likely to be in the order of milliseconds,36,37 the equilibrium surface tension values will not be applicable and would consequently lead to erroneous calculations. After reviewing a considerable amount of literature data, to the authors’ best knowledge, no time-resolved surface tension measurements for molten oxides with additions of surfactants have been published so far. However, the breakup of liquid metal jets at moderate temperatures (0.25 bar, hence we chose a pressure difference of p = 0.35 bar for our experiments. There was no imposed external excitation, hence “natural” breakup was observed triggered by the inherently present perturbations from the surroundings (e.g., heating elements, fans in outer shell, pressure fluctuations of purging gas, vibrations, etc.) in all sequences. Consequently, the jet length is not constant as it would be using an external perturbation at a specified frequency corresponding to the maximum growth rate of the system. Hence, in our case, the unbroken jet length varies considerably depending on the actual degree of external “arbitrary” perturbations. The shortest jet length found from the recorded sequence was 9.5 cm which is slightly above but close the predicted value from eq 6. Figure 12 shows a sequence of images of a disintegrating calcia/alumina jet. The time difference between each image is 1.55 ms, the camera position is constant. Necks and swells have developed as a consequence of the growth of instabilities. The

dP = 1.89d jet

(8)

which gives a theoretical droplet diameter of 3.78 mm for a 2 mm jet. Figure 13 shows the frequency distribution of droplets formed from a disintegrating calcia/alumina jet, again with djet = 2 mm at 1570 °C.

Figure 13. Number frequency distribution of a disintegrating calcia/ alumina jet emerging from a 2 mm graphite capillary plotted over the droplet mean diameter. The theoretical diameter of the main droplet as predicted by the linear stability analysis can be obtained from eq 8 and gives 3.78 mm. The peak representing the main droplet is around 3.4−3.5 mm. The distribution is bimodal, the peak at around 1.8−1.9 mm reflects the size of the satellite droplets.

Every 150th frame out of 27 000 frames (corresponding to a 3 s sequence at 9000 fps) has been analyzed. Only objects with an aspect ratio between 0.5 and 1.5 have been measured. 312 objects meeting these criteria have been found, the maximum frequency being in the interval 3.4−3.5 mm which is only slightly smaller than the theoretical value. 67% of all droplets were between 3.2 and 3.8 mm.

Figure 12. Sequence of a liquid calcia/alumina slag jet exhibiting necks and swells due to instability growth. The wavelength λ is given as an example in the first image with λ = 13.25 mm (k* = kR = 0.47). The time difference between each image is 1.55 ms, the temperature was 1570 °C, the pressure difference was set to 0.35 bar. 16452

dx.doi.org/10.1021/ie4022623 | Ind. Eng. Chem. Res. 2013, 52, 16444−16456


Article

shaped liquid thread with two loose ends is formed. With time, the thread retracts and forms an oblate droplet which oscillates shortly around its spheroidal shape. Below the dumbbell-shaped droplet, two more droplets have already formed. The three droplets are quite different in size, the mean diameter are 2.16, 3.74, and 1.41 mm, from top to bottom, respectively. Another example of a breakup mechanism is shown in Figure 15. Here, the middle droplet detaches from the droplet above it

A smaller peak representing the satellite droplets is located around the class 1.8−1.9 mm, thus very close to the jet or capillary diameter, respectively. One finds dmain/dsat ≈ 1.9. The number of satellite droplets is considerably smaller, roughly 7% of the total number of droplets belong to this group, 21% of all droplets are smaller than 3 mm. The linear stability analysis fails at the point of jet breakup and cannot predict the occurrence and size of satellite droplets,67 and nonlinear terms have to be introduced to predict satellite droplet formation from disintegrating liquid jets.70 Theoretical considerations were supported by water jet experiments and showed that the ratio of the size of main droplets to the size of satellite droplets varies in a characteristic manner with the dimensionless wavenumber of the perturbation. The larger the wavenumber (the shorter the wavelength), the smaller the satellite droplets become. For very low wavenumbers, the satellites were found to be even bigger than the main droplets.70−72 Viscosity has stabilizing effects on the liquid thread which leads to fewer and smaller satellite droplets than compared to less viscous liquids.72 If the theoretical curves in Figure 4 in the paper presented by Lafrance70 were applied with the therein defined ordinate r* = rP/rjet, one would obtain r* ≈ 1.7 for the main droplet, and hence k* > 1 which suggests that no satellites could be formed. It seems that the figure cannot be applied for highly viscous slags, but interestingly, if k* = kR = 0.47 from our experiments is used and if the ratio of the sizes of main to satellite droplets from the theoretical curves from Figure 4 in the paper by Lafrance70 at that wavenumber is calculated, one obtains approximately r* = rmain/rsat = dmain/dsat ≈ 2.05/1.075 = 1.9, which is in astonishing agreement with the ratio between main and satellite droplets obtained from the disintegrating slag jet (see Figure 13). Unfortunately, the images obtained from the sequences did not permit the determination of the growth rate. As can be seen from Figure 12, the sequence is a nonlinear example for a “negative” growth rate since the diameter of the lower swells is smaller than the diameter of the upper swells. Or, in other words, the swell further upstream grows faster in time than the one further downstream leading finally to intermediate droplet breakup as described above. Hence, a surface tension calculation based on the evolution of neck and swell diameter along the jet axis as described in literature (see, e.g., in Donnelly and Glaberson73 or Ronay74) was not possible. Figure 14 shows the intermediate breakup in more detail. Again, breakup between two swells occurred, and a dumbbell-

Figure 15. Another example of multiple droplet breakup from a slag jet. In this example, three droplets are formed with mean diameter ranging from 2.66 to 3.76 mm. The time difference between each image is here 0.77 ms.

which is still connected via a liquid ligament with the main jet. The middle droplet exhibits a bulbous or an upside down “microphone”-like shape, a more or less cone-shaped droplet which relaxes quickly to form an oblate shaped droplet. The last example, Figure 16, shows coalescence between a smaller and a

Figure 16. Coalescence of two slag droplets, 2.2 and 3.45 mm. After successful merging, the resulting droplet oscillates from prolate to oblate shape. The time difference between each image is 1.55 ms.

larger droplet. After reviewing a large number of different coalescence phenomena, it never occurred that coalescence was not successful. In other words, a rebound was never observed in all screened cases.

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SUMMARY AND OUTLOOK We presented in this paper a new high-temperature measurement device which has been designed to investigate the dynamics of surface tension driven phenomena of droplet formation and jet disintegration of liquid slags. These phenomena comprise droplet elongation, liquid bridge necking, satellite droplet formation, detachment, formation of instabilities, oscillation, and coalescence. The test liquid system in this study was a 49/51 wt % calcia/alumina slag kept in a graphite crucible emerging from a nonwetting graphite capillary into the argon purged heating chamber. Slag flow and droplet formation could be controlled precisely and in a highly repetitive manner using a back-pressure system in combination with an accurate stopper positioning device. A high-speed

Figure 14. Retracting dumbbell-shaped liquid thread occurring from multiple jet breakup. The mean diameter of the small droplet on the bottom of the first image is 1.41 mm, the bigger one above it is 3.74 mm in size, and the one formed from the dumbbell-shaped body 2.16 mm. The time difference between each image is 1.55 ms. 16453



Article

Notes

camera with fast data transfer was used to capture the phenomena through the window in the horizontal alumina protection tube forming a cross with the vertical one. The dynamics of droplet formation, liquid bridge necking and oscillation during free fall were captured with the highspeed camera and analyzed with an image processing tool. The droplet length, the neck diameter and the droplet projection area were quantified as a function of time for different temperatures. The effect of viscosity was demonstrated in all cases. The empirical correlation of Scheele and Meister51 was found to predict the volume of the formed droplets in a satisfactory manner. At high frame rates, in some cases with relatively high slag viscosity, satellite droplets were found. Surface instabilities on the retracting liquid thread were clearly shown, resulting in nearly simultaneous pinch-off at both ends to form free satellite droplets of small size (order of 102 μm). The coalescence of the satellites both with the remaining liquid at the nozzle and with the free falling main droplet was found. At higher pressure differences, a coherent slag jet was formed. Jet disintegration was triggered by random disturbances present in the immediate environment, thus “natural” breakup occurred at wavenumbers lower than the wavenumber corresponding to the maximum growth rate, also resulting in nonlinear behavior, such as “negative” growth rate and intermediate satellite droplet formation. Various forms and shapes of the detaching droplets were filmed, some of them were presented in this paper. The frequency distribution of the satellite droplets showed a bimodal distribution in very good agreement with theoretical considerations. The jet length was not constant due to random nonlinear disintegration, although in relatively close agreement with available jet length predictions. Future work will include the use of controlled perturbances, as for instance used by Benda75 in a hightemperature setup, to improve the uniformness of droplet formation. In the pendant drop method setup, the surface tension of calcia/alumina slags at different temperatures in a controlled argon atmosphere was obtained as a function of time. The measurements were shown to be highly reproducible. The results show close agreement with published literature data and with the additivity rule (deviation was less than 3%). These first encouraging results obtained with the new hightemperature device are the foundation to carry out measurements in slags with additions of surfactants to elucidate the impact of surface active materials on the interfacial phenomena in slags. The aim is to use optical high-speed measurements in combination with the pendant drop and the oscillating jet technique to capture the time-dependent behavior of surface tension.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Tetlow Kilns & Furnaces for their invaluable and exhaustive support in designing and engineering the furnace. A special thanks goes to Roger Brockway and Stefan Teran for all their technical support and the technical drawings. The extraordinary help of Shane Mullenger from Mersen Oceania (formerly Carbon Lorraine) to build the gigantic graphite crucible is also highly appreciated. Finally, the authors thank the CSIRO Division for Process Science and Engineering, more precisely the Capability Development Theme, for funding this project.

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Latin Letters

a = thinnest neck diameter, m A, B, C = parameter de, ds = diameter used in selected plane method, m dP = droplet diameter, m D = diameter, m g = gravity, m s−2 H−1 = parameter in selected plane method, − ID = inner diameter, m k = wavenumber, m−1 k* = dimensionless wavenumber, − L = length, m Lb = breakup length of jet, m Lchar = characteristic length scale in dimensionless numbers, m OD = outer diameter, m p = pressure, Pa pO2 = oxygen partial pressure, Pa Q = volume flow rate, m3 s−1 rP = droplet radius, m rjet = jet radius, m r* = radius ratio rP/rjet, − R = capillary radius, m S = ds/de = parameter in selected plane method, − SD = standard deviation t = time, s td = droplet detachment time, s T = temperature, K v = velocity, m s−1 VP = droplet volume, m3 Xi = molar fraction, mol mol−1 Greek Letters

ε0 = initial disturbance, m ϑ = temperature, °C λ = wavelength, m μ = dynamic viscosity, Pas ρ = density, kg m−3 σ = surface tension, N m−1 ΦHB = Harkins and Brown correction factor, −

ASSOCIATED CONTENT

S Supporting Information *

Detailed information about the experimental setup, especially the design of the alumina cross tube, and about the slag preparation procedure. This material is available free of charge via the Internet at http://pubs.acs.org.

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NOTATION

Subscripts

0 = initial, at t = 0 AR = aspect ratio cap = capillary d = detachment f = formation g = gas

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 16454



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i = inner jet = jet main = main droplet min = minimum pred = predicted P = droplet sat = satellite droplet w = water Dimensionless Numbers

Oh = Ohnesorge number, Oh = (We) 1/2 Re −1 = μ(ρσLchar)−1/2 Re = Reynolds number, Re = ρvLcharμ−1 We = Weber number, We = ρv2Lcharσ−1

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Novel High-Temperature Experimental Setup to Study Dynamic

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