SURFACE PROPERTIES OF MOLTEN FLUORIDE-BASED SALTS Thomas Villalón Jr.1, Shizhao Su1, Uday Pal*1, 2
1Division of Materials Science and Engineering, Boston University, Brookline, MA 02446
2 Department of Mechanical Engineering, Boston University, Boston, MA 02215 Keywords: Surface Tension, Molten Fluoride Salts, Sessile Drop, Maximum Bubble Pressure
Abstract
Two techniques were used to determine the surface characteristics of a molten fluoride-based salt.
The maximum bubble pressure method was used to measure the surface tension of the molten salt.
Regulated Ar-2%H2(g) was passed through a steel tube into a molten salt at high temperature. The surface tension of the gas-liquid interface was calculated by measuring the maximum pressure within the tube using a pressure transducer. Contact angles between the flux and solid interfaces were measured by imaging droplets of the molten flux on various surfaces using a camera with a high shutter speed. The camera took images of the high temperature sessile droplet on a solid surface through an optically clear pane. This image was processed through multiple software packages to determine the contact angle and surface tension between the solid-liquid interfaces.
The surface properties measured were used to optimize the salt for electrolytic metals production processes.
Introduction
In order to design an efficient electrolysis process for metals production, an electrolytic cell must use materials that have optimal surface tension characteristics. This is a key requirement at electrodes where wetting between liquids and solids provides low contact and charge transfer resistances. In addition, the surface tension of the flux is a key physical property that also influences the fluid behavior and its interactions with the gas phase when bubbling is used to stir the flux. To characterize the surface tension behavior of molten flux, two different types of experiments were conducted: a). maximum bubble pressure experiment and b). sessile drop experiment. The former helps determine gas-liquid surface tension by blowing gas through a liquid [1]. The latter finds an equilibrium condition between gas-solid-liquid interfaces by observing the characteristics of a molten drop with respect to the surface it lies on and the gas surrounding it [1- 2]. However, these experiments, although simple in theory, can become very complex when attempted under high temperature conditions. In order to properly acquire the data, materials must be selected that do not interact and affect the experiment while being cost effective and safe. The nature of high temperature experiments quickly limits the range of materials that can be used over 1000°C. To cope with these high temperatures, many experimental components were replaced with either steel or ceramic counterparts. Additionally, all experiments were designed to work in a reducing environment to prevent oxidation from altering the surfaces from their original condition. Running these experiments several times, the goal was to validate the effectiveness of these techniques in high temperature conditions in addition to acquiring data that would help characterize the materials for use in an electrolytic cell.
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
Experimental
A. Maximum Bubble Pressure Setup
Figure 1 shows the schematic of a maximum bubble pressure setup. The setup consists of mild steel crucible holding the molten salt that was heated to 1200°C. The steel crucible was selected because it is inert when exposed to a eutectic magnesium fluoride-calcium fluoride based molten salt while not introducing any impurities to the salt. This crucible was held in forming gas environment (98% argon-2% hydrogen gas by volume) to create a slightly reducing environment.
The forming gas flowrate was 100 mL/min. 400 grams of powdered flux (45 wt% MgF2 – 55 wt%
CaF2 containing 6 wt% CaO, 3wt% Al2O3 and 4 wt% YF3) was placed inside the crucible which was heated to the desired temperature for the measurement. To find the maximum bubble pressure, a 1/8” inner diameter, ¼” outer diameter stainless steel tube was immersed into the flux at differing heights below the surface of the flux. In line with the steel tube there was a pressure transducer and a source of the forming gas. The pressure transducer measured the bubble pressure when the gas line was opened and the gas was allowed to flow into the molten salt.
Figure 1: Maximum Bubble Pressure Setup
Procedure: Prior to the experiment, the powders were dried, massed into stoichiometric quantities, and dry mixed in a ball mill overnight. Then, the powders were put into the steel crucible and the unit was heated to the desired temperature. Once at temperature, the stainless steel tube was placed near the surface of the flux. A port was then opened, and the stainless steel tube was used to find the surface of the flux. This was accomplished by finding a location where the pressure was zero and just below the pressure spiked due to the liquid creating a back pressure. The stainless steel tube was immersed 1/8” below the surface of the flux and allowed to equilibrate. Once at equilibrium, the gas line was cracked open as little as possible to allow gas through at a very slow rate. The intent was to permit the least amount of gas through and observe the pressure changes
and record the peak pressure. To aid with this procedure, the pressure transducer signal was recorded in a laptop and the pressure was actively monitored. Once this was done, the stainless steel tube was lowered another 1/8” and the process was repeated.
B. Maximum Bubble Pressure Chemicals, Materials, and Electronics
Calcium fluoride (99.5%), magnesium fluoride (99.5%), calcium oxide (reagent grade), yttrium (III) fluoride (99.9%), and aluminum oxide (99.5%) were supplied by Alfa Aesar (Ward Hill, MA).
The stainless steel tube and mild steel crucible were made from steel tubes and round supplied by McMaster-Carr (Robbinsville, NJ). The pressure transducer was a High Speed USB Output Pressure Transducer, rated for 0 to 5 PSIG (gauge PSI). It was acquired from OMEGA Engineering, Inc. (Stamford, Connecticut).
C. Surface Tension Characteristics of a Gas-Liquid Interface
The gas-liquid surface tension was characterized between the flux and the forming gas by passing the forming gas through the flux at varying heights. By viewing the system as having hydrostatic and surface tension components, γg/l can be calculated based on the following two equations:
𝑃𝑚𝑎𝑥= 𝑃𝑠𝑡+ 𝑃ℎ𝑦𝑑𝑟𝑜=2𝛾𝑔/𝑙
𝑟𝑒𝑓𝑓+ 𝜌𝑔ℎ (1) 𝑟𝑒𝑓𝑓= √𝑟2(1 + 2𝛼𝑠𝑡𝑎𝑖𝑛𝑙𝑒𝑠𝑠∆𝑇) (2)
Here, Pmax is the total pressure recorded by the pressure transducer. It is made of two separate components. The first component is the differential pressure created by the surface tension interaction between the liquid salt and the forming gas inside the bubble, Pst. Pst has two variables:
γg/l, the gas-liquid surface tension, and reff, the effective radius of the tube that is used for gas flow.
The second component of the first equation, Phydro, is the hydrostatic pressure in the system. This is composed of three values: ρ, the density of the flux, g, the acceleration due to gravity, and h, the height of the flux column. The second equation is used to compensate for the thermal expansion that occurs in the stainless steel tube when immersed at high temperatures for prolonged periods.
The thermally expanded radius, reff, has three associated variables: r, the initial radius of the tube, αstainless, the coefficient of thermal expansion, and ΔT, the change in temperature from the start of the experiment.
D. Post Experiment Data Analysis
The data from the pressure transducer was recorded and exported by a software package supplied by OMEGA. This data was then analyzed in Microsoft Excel and sampled to find where bubble peaks occurred. Selecting an area of pressure peaks, an average value for peak pressure is acquired for every height. These values are then plotted to see the trend of pressure as a function of submersion depth.
E. High Temperature Sessile Drop Setup
Figure 2 shows the schematic for a high temperature sessile drop setup. The setup revolves around using a horizontal tube furnace heated to 1200°C. The sample is held in a horizontal alumina tube (3/4” inner diameter, 1” outer diameter) that is capped on both ends to create a slightly reducing environment. This is accomplished by purging forming gas (98% argon-2% hydrogen gas) into the
sealed tube. To hold the sample, an open top stainless steel crucible was made that holds 40 grams of silver. On top of the silver, a sample of the cathode substrate material was placed. For this particular experiment, the cathode material selected was titanium diboride. The design of the crucible serves two purposes. The first is to use the silver as a self-leveling mechanism in the event that the cathode material and salt sample are not perfectly vertical. The second is to catch any molten salt that might overflow the surface of the cathode material within the silver bath. On top of the cathode material, a sample of homogenized molten salt (45 wt% MgF2 – 55 wt% CaF2
containing 6 wt% CaO, 3 wt% Al2O3 and 4 wt% YF3) was equilibrated, approximately weighing 0.1 grams. This sample was made by solvent mixing the powders overnight, melting the powders, and then crushing the resulting salt block. The melting and powder crushing procedure was repeated up to three times to ensure homogeneity. At one end of the alumina tube, the end cap must be transparent to see the sample inside the furnace. For this particular experiment, an assembly was made that used two aluminum disks, a quartz viewing port, sealing gaskets, and tensioning screws.
Procedure: A camera is then placed collinearly with the sample, inside the alumina tube, by using a scissor lift. The distance of the camera from the sample is adjusted along with focusing the lens prior to starting the experiment. To ensure that no distortion effects occur during the experiment, both the tube and the camera are made level and test images are taken to create a 2D image of the cathode material and salt sample. Prior to the experiment, the end cap was taken off and cleaned with ethanol. A picture was then taken of the sample to ensure optical clarity and final assembly collinearity. As the experiment progressed, the camera’s shutter speed was progressively decreased as the luminosity of the furnace kept increasing. At 1200°C, the camera was using a shutter speed of 1/4000th of a second. Five images were taken over the span of 15 minutes after permitting the system to equilibrate at 1200°C for over an hour.
Figure 2: High Temperature Sessile Drop Setup
F. High Temperature Sessile Drop Chemicals, Materials, and Electronics
Calcium fluoride (99.5%), magnesium fluoride (99.5%), calcium oxide (reagent grade), yttrium (III) fluoride (99.9%), aluminum oxide (99.5%), and silver castings were supplied by Alfa Aesar (Ward Hill, MA). The stainless steel tube, stainless steel round, aluminum round, gasket seals,
socket cap hex screws, and quartz endcap were supplied by McMaster-Carr (Robbinsville, NJ).
The camera used in the experiment was a Nikon D610 with a 24-85 mm VR lens acquired from Adorama (New York City, NY). The grease used to help make the seals airtight was DOW Corning high vacuum grease, supplied by DOW Chemicals. To support the camera, an 8” x 8” scissor lift was procured from Amazon. The furnace used to run this operation was a Barnstead Thermolyne 21110 Horizontal Tube Furnace. The titanium diboride was provided by Infinium, Inc. (Natick, MA) and machined via wire-cut electrical discharge machining into a square shape.
G. Sessile Drop Characteristics for a Molten Salt and Solid Substrate
Given the three phase equilibrated system of cathode material, molten salt, and the gas, the contact angle (θ) can be used to relate all three surface tensions. This is shown in Equation 3.
𝑐𝑜𝑠𝜃 =𝛾𝑔/𝑠 − 𝛾𝑙/𝑠
𝛾𝑔/𝑙 (3)
Using the contact angle acquired and the previously calculated 𝛾𝑔/𝑙, (gas-liquid surface tension) the 𝛾𝑙/𝑠 (liquid-solid surface tension) can be found if a literature value for 𝛾𝑔/𝑠 (gas-solid surface tension) exists. By analyzing the contact angle, an assessment of the wettability between the liquid and the solid can be determined.
H. Image Post Processing and Data Analysis
After the pictures were taken, they were imported into Adobe Lightroom and modified to create a high level of contrast between the sessile drop and the background. The images were then converted into black and white, maintaining a high level of contrast. The images were subsequently imported into ImageJ. Contact angles for each image were then found by running the images through the DropSnake plugin.
Results
A. Maximum Bubble Pressure Results
Figure 3 shows an example of the data produced when the tube was submerged half an inch into the molten salt. Note the repeating increasing pressure followed by a sudden drop. This cyclical behavior is indicative of bubbles forming and subsequently escaping.
Figure 3: Maximum Bubble Pressure Raw Data - 0.50" Depth
Figure 4 shows a plot of peak pressure values per bubble event within the region highlighted in green.
Figure 4: Peak Pressure Values - 0.50" Depth
Using the values from the 0.50”, 0.75”, and 1.00” submersion depths, a plot can be made comparing immersion depth and peak pressure. This plot then provides a slope and a y-intercept as seen in Figure 5. Data is provided in Table I.
Figure 5: Tube Submersion Depth vs. Average Peak Pressure
Table I: Tube Depth vs. Average Peak Pressure Data Values
Tube Depth [in] Tube Depth [m] Peak Gauge Pressure [PSI] Peak Gauge Pressure [Pa]
0.50 0.0127 0.0638 440.23
0.75 0.01905 0.0909 626.42
1.00 0.0254 0.1170 806.22
Extrapolating the plot to the y-intercept gives a value of 75.305 pascals. This indicates that at zero hydrostatic pressure the total pressure, exerted only by the surface tension, is 73.305 pascals.
Applying the condition of zero hydrostatic pressure to Equation 1, Equation 4 is created.
𝛾𝑔/𝑙=𝑃𝑚𝑎𝑥𝑟𝑒𝑓𝑓
2 (4) Using equation 4, the y-intercept pressure, and the effective tube outer radius, the surface tension of the gas-liquid interface is calculated to be 121.95 mN/m. Applying statistical analysis, the standard deviation for this system is 9.29 mN/m. Thus, the surface tension of the gas-molten salt interface is 121.95 ± 9.29 mN/m. The relatively low surface tension value of our fluoride salt is similar to that of cryolite-based melts [3] and could be attributed to the lower melting point (~970°C) as measured by differential thermal analysis as seen in Figure 6.
Figure 6: Differential Scanning Calorimetry Data of the BU Salt (45 wt% MgF2 – 55 wt% CaF2
containing 6 wt% CaO, 3wt% Al2O3 and 4 wt% YF3)
Further analysis of the salt also shows that the flux has very low volatility 0.171 µg/cm^2*s indicating that the composition will not change during electrolysis. It is to be also noted that the low surface tension value of the flux would allow efficient gas-stirring of the melt.
In addition to the surface tension, analyzing the slope of the submersion depth vs. maximum bubble pressure plot in Figure 5 provides data that can be used to calculate the density of the flux. Equating the slope to the second term of Equation 1, specifically ρ*g, a density of 2.94 g/cm^3 is found for the salt.
B. High Temperature Sessile Drop Results
Taking the images and processing them, contact angles were found over the course of 15 minutes.
Each image was taken over a three minute interval. These images can be seen with contact angles in Figure 7 with data values supplied in Table II.
Figure 7: Contact Angle of Flux Samples
Table II: Contact Angle Data for Flux Samples
Sample Contact Angle [Degrees]
1 8.473
2 11.697
3 10.739
4 7.022
5 8.839
Average 9.354
Standard Deviation 1.357
The images analysis show that the contact angle is 9.354°±1.357°. From the contact angle measurement, it can be concluded that the flux does wet the titanium diboride cathode substrate quite well. This is beneficial in two regards. It would lower both the charge transfer and contact resistances at the electrodes.
Conclusions
Two types of experiments were conducted, showing the ability to characterize surface behavior via a maximum bubble pressure measurement and sessile drop observation. The former experiment found that a molten fluoride salt, under an environment of Ar-2%H2(g) at 1200°C, has a gas-liquid surface tension of 121.95 ± 9.29 mN/m which is similar to that of cryolite-based salts.
Additionally, data analysis of the plot indicates that the salt’s density is 2.94 g/cm^3. The second experiment noted that a molten fluoride salt, placed on a titanium diboride substrate and exposed to the same gas environment, produces a contact angle value of 9.354°±1.357° which indicates good wetting between the cathode substrate and the molten salt. Both values show promise for the salt and titanium diboride to be used in an electrolytic cell.
Acknowledgements
This project was financially supported by the Department of Energy (DOE) through ARPA-E REMOTE program. The authors would like to thank Matthew Mirek for assisting with the preparation and execution of these experiments.
References
[1] Adamson, Arthur W., and Alice P. Gast. Physical Chemistry of Surfaces. New York: Wiley, 1997. Print.
[2] Zisman, W. A. "Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution." Advances in Chemistry Contact Angle, Wettability, and Adhesion (1964): 1-51.
Web.
[3] Fernandez, R., and T. Østvold. "Surface Tension and Density of Molten Fluorides and Fluoride Mixtures Containing Cryolite." Acta Chem. Scand. Acta Chemica Scandinavica 43 (1989):
151-59. Web.
