Surface tension of pseudo binary and ternary ...

Surface tension of pseudo binary and ternary sulphide matte phase systems Ni3S2, Cu2S and FeS by sessile drop technique J. Hamuyuni*1,2, P. Taskinen1, G. Akdogan2 and S. Bradshaw2 The surface tensions of pseudo binary systems Ni3S2–FeS, Cu2S–FeS and Cu2S–Ni3S2 and pseudo ternary system FeS–Cu2S–Ni3S2 as a function of composition, at a constant temperature of 1200uC, have been determined. For the Ni3S2–FeS and Cu2S–Ni3S2 pseudo binary systems, the surface tensions increased with an increase in Ni3S2 content. The amount of Cu2S has a small effect on the surface tension of the Cu2S– containing pseudo systems investigated in this study. On a de´termine´ la tension superficielle de syste`mes pseudo binaires Ni3S2–FeS, Cu2S–FeS, Cu2S–Ni3S2 et du syste`me pseudo ternaire FeS–Cu2S–Ni3S2 en fonction de la composition, a` une tempe´rature constante de 1200uC. Pour les syste`mes pseudo binaires Ni3S2–FeS et Cu2S–Ni3S2, la tension superficielle augmentait avec une augmentation de la teneur en Ni3S2. La quantite´ de Cu2S avait un petit effet sur la tension superficielle des pseudos syste`mes contenant du Cu2S, examine´s dans cette e´tude. Keywords: Surface tension, Sulphide mattes, Cu2S, Ni3S2, FeS

Introduction Smelting of sulphide concentrates is known to be driven by a number of physicochemical properties. The most influential physical properties are density, viscosity and surface tension of molten mattes.1–4 For nickel and copper mattes–slags, surface tension data are the scarcest.1 Most recently, Andersson et al.3 in their study reported key properties for optimising the smelting process. Notably they reported the importance of surface tension data. At various conditions of matte grade, temperature2 and oxygen partial pressure, surface tension in relation to other mentioned properties have been found to result in a specific outcome in phase interactions. Therefore, in order to predict the outcome in phase interaction, surface tension data are essential. In copper smelting for example, copper losses to slag translate into significant revenue loss. Biswas and Davenport1 reported a 1–3% loss of copper metal from that contained in the feed for smelters with no copper recovery from slag stage. They further reported that of

1 Metallurgical Thermodynamics and Modelling Research Group, Aalto University School of Science and Technology, PO Box 16200, FI-00076 Aalto, Finland 2 Department of Process Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, Stellenbosch, South Africa

*Corresponding author, email [email protected]

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ß 2013 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute Received 29 June 2012; accepted 3 September 2012 DOI 10.1179/1879139512Y.0000000044

this loss, y70% is mechanically entrained in drops of matte. Investigation of these properties that influence matte–slag separation therefore becomes important not only for smelting efficiency and improvement of operations, but also for theoretical study of the smelting process.4–10 Surface tensions of mattes–slags vary with temperature, composition and oxygen partial pressures, among other significant factors.11–18,20 This is observed at high temperatures where these sulphides are molten.14,15 Surface tensions of mattes–slags have been reported to be composition sensitive to some species in Kucharski et al.,2 where they used the maximum bubble pressure method.19,21 Therefore, in this study, surface tension of Cu2S, Ni3S2 and FeS binary and ternary systems has been investigated as a function of composition. An improved sessile drop method suitable for such measurements was used.22–25 The method which fits the entire shape of the edge of the drop to the Laplace equation has been employed.23,24 The automated procedure was done in ˚. software as provided by FTA

Experimental Principle Figure 1 is a schematic of the sessile drop resting on a substrate.

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1 Definition of characteristic dimensions R and h for sessile drop

The shape of an axisymmetric sessile drop depends on a single parameter referred to as the bond number.25 This bond number is a measure of the relative importance of gravity to surface tension in determining the shape of the liquid drop (see equation (1)) c~

Drgb2 b

(1)

where r is the density, g is the gravitational constant, b is the radius of curvature and b is a bond number. When bond numbers approach zero, the surface tension dominates, causing the drop to be nearly spherical. For large bond numbers, the drop is significantly deformed.25 In principle, the method involves comparison of drop image with theoretical profiles.

Specimen, apparatus and procedure Pure sulphide powders of Ni3S2 (99?99%), Cu2S (99?99%) and FeS (99?95%) (metal basis), at 2200, 2200 and 2100 mesh respectively, employed in measurements were purchased from Alfa Aesar (Germany). For compositions, see Table 1. The sulphide droplets, of various compositions, were prepared by using a hydraulic press. Substrates of dense alumina (99?95) were used in the experiment. Contact angles in the range 90–110u were observed for all experiments, representing a non-wetting environment. Measurements were performed in an inert atmosphere of purified argon (99?999%) at 20 mL min21. The experimental set-up used in this study is depicted in Fig. 2. It consists of an ENTECH, ETF 50 – 70/15 – S horizontal tube furnace with an alumina tube of 100 mm length and 40 mm inside diameter. It is built with SiC heating elements placed parallel and around the tube. The furnace is designed for performing measurement in high vacuum or inert gas atmosphere at temperatures up to 1500uC. The argon gas used in the experiment was purified using two types of cartridges (Messer, Krefeld, Germany: Hydrosorb for moisture removal and Oxisorb for oxygen Table 1 Chemical analysis of sulphides Composition/mass-% Sulphides

Cu

Ni

Fe

S

Cu2S Ni3S2 Fes

79.58 … …

… 69.53 …

… … 63.87

20.41 30.46 36.08

Surface tension of Ni 3 S 2 , Cu 2 S and FeS by sessile drop technique

2 Schematic set-up of experimental apparatus

removal). This ensured that oxygen partial pressures were kept low (,1028 atm). This was compensated by the fact that the composition of pure substances did not change over the considered period. The temperature is measured by an S type (Pt–Pt, 10% RH) thermocouple placed near the sample. The thermocouple was calibrated using pure copper. The ambient room temperature measurement was done with a Pt100 sensor (platinum resistance thermometer) instead of a mercury thermometer. The sensor was calibrated against ice water at 273?15 K and the obtained resistance value was R05100?013 V. This value was entered into NI LabVIEW temperature logging programme. The manufacturer reported that the tolerance of the sensor is in accordance with the DIN IEC 751 standard B1/10 and obtains an accuracy of ¡0?03 K. The system of sample holder, substrate and sulphide specimen was placed in the hot zone inside the alumina tube using a pulling rod. Thermo profiles were obtained to ensure that the droplet formed was within ¡1uC (5 cm) identified zone. The sample holder allows movements for horizontal positioning of substrate. For visibility of the droplet and illumination, the furnace tube was fitted with borosilicate glass at both ends. A system’s rectangular shield prevents the excessive heat loss from the furnace zone. When an experiment was conducted, the droplet resting on the substrate was carefully placed in the uniform temperature zone with the help of a pulling rod. At this point, the furnace tube was gas tight sealed. The experiments were conducted under controlled partial pressure of oxygen (222(log Po2(214). A high flow of purified (80 mL min21) argon gas was flushed in the furnace to remove oxygen and any other possible reactive gas, for 30 min. This flowrate was reduced to 20 mL min21 during the experiment. The furnace was heated to the set point at a rate of 4uC min21. The temperature was held for another 30 min at set point to ensure that the system had enough time to reach equilibrium. The special optical system used was acquired from SKS Vision Systems Oy of Finland. VISI50 SMART Integrated machine vision sensor was used to record the image of the drop’s contour. This is an integrated machine vision sensor for measurement and quality control. It has been mainly used commercially for edge position measurement, centre position measurement, width measurement, area measurement and object counting. Unlike the commonly used charge coupled device cameras, Visi50

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3 Isotherm of surface tension of Cu2S–Ni3S2 system at 1200uC

utilises a complementary metal oxide semiconductor sensor of 5 megapixels (256061920) and high performance digital signal processor for image processing tasks. The complementary metal oxide semiconductor image sensor technology enables freely programmable measurement and detection of areas (regions of interest). The device can be used as an area or line scan camera. High line resolution enables outstanding measurement accuracy even in line scan applications. Visi50 can work as a fully independent stand-alone measurement, control or guiding device or it can be connected to automation system with versatile input/output interface. Object illumination as an option is done especially in cases where natural light is insufficient. Object illumination may be done with integrated light emitting diode or with other light sources. In this experimental study, the device was connected to an automation system and light emitting diode light was used for illumination. VISI50 SMART integrated machine vision sensor comes with a software compatible program which is capable through a specialised video acquisition board to reproduce the images captured on the computer’s display, to convert and save them into a standard graphic format in real time, without introducing geometrical distortions. The images can be saved as a movie or separate images. At the same time, the computer through the RS-232 serial interface, using a Keithley 2000 digital multimeter, records the value of the measured temperature. The image data obtained are stored to files and used to obtain the values of the surface tension using the geometrical parameters of the drop. For surface tension calculations, FTA32 software program (supplied by Crelab Instruments, Sweden) was employed using the dimensions of the droplet at a set magnification. One important parameter, which must be known in calculations of surface tension, is density of matte droplet. Density values used in this study were adapted from the work by Kucharski et al.2 The first step involved calibration of the optical system. This involves comparison of drop image with

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4 Isotherm of surface tension of FeS–Ni3S2 system at 1200uC

theoretical profiles. Images of the order 6406480 pixels suitable for calculations in software were obtained. The drop in the images that we used for the calculations occupied 75% (which is required for accurate results). By using the two dimensions (actual and measured), the magnification was obtained. Once all the parameters were obtained, surface tension was calculated in this ˚. software program provided by FTA

Results and discussion The surface tensions of Ni3S2–FeS, Cu2S–FeS and Cu2S–Ni3S2 pseudo binary systems and FeS–Cu2S– Ni3S2 pseudo ternary system were determined. These measurements at various compositions were done at 1200uC when the system had reached equilibrium. This temperature was sufficient to bring sulphide mattes to molten state, which is necessary for these experiments. Figures 3–5 illustrate the results of binary systems obtained in these experiments. Each point on the

5 Isotherm of surface tension of FeS–Cu2S system at 1200uC

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Surface tension of Ni 3 S 2 , Cu 2 S and FeS by sessile drop technique

increase in Ni3S2 content. Cu2S has a small effect on the Cu2S–FeS system as well as the pseudo ternary system.

Acknowledgements The authors are grateful to Stellenbosch University OSP Fund and Improved Sulfide Smelting (ISS) project of the ELEMET program and Tekes, the Finnish Funding Agency for Technology and Innovation, for financial support.

References

6 Pseudo ternary surface tension diagram for FeS–Cu2S– Ni3S2 system at 1200uC: surface tension and iso tension values are in mN m21

experimental results of this study represents 10 independent experimental values. From Fig. 3, the sensitivity of surface tension to amounts of Ni3S2 in Ni3S2–Cu2S pseudo binary system is observed. The surface tension of Ni3S2–FeS pseudo binary system increases with a corresponding increase in the amount of Ni3S2. In Fig. 4, the surface tension of the Ni3S2–FeS also increases with increasing Ni3S2 content. The effect of Cu2S in the Cu2S–FeS pseudo binary system is small as seen in Fig. 5. From these results, an R2 value of 0?98, for systems in Fig. 4, is observed, showing that a linear model which explains 98% of the variability of the data. Figures 3 and 5 however show a non-linear relationship. The 10 independent experimental values for these points in the binary system present very small standard deviations from the mean, with the highest standard deviations for these results are 10?85, 6?77 and 14?61 mN m21 for results in Figs. 3–5 respectively. The results of these binary systems compare well with those obtained by Kucharski et al.2 The nonlinearity behaviour was observed in the results. This could be to a lesser extent due to the non-ideality of the mixtures used in these experiments. Figure 6 illustrates the surface tension results obtained from the FeS–Cu2S–Ni3S2 pseudo ternary system. The numbers on each of the closed circles represent the surface tension at a point in mN m21. It can be observed that the surface tension of mattes is sensitive to composition changes, particularly, the amount of Ni3S2. The surface tension increases with an increase in Ni3S2 content. This can be seen by the high surface tension values in the Ni3S2 rich corner of the pseudo ternary diagram.

Conclusion The surface tensions of Ni3S2–FeS, Cu2S–FeS and Cu2S–Ni3S2 pseudo binary systems and FeS–Cu2S– Ni3S2 pseudo ternary system as a function of composition at a fixed temperature of 1200uC have been determined. For the Ni3S2–FeS and Cu2S–Ni3S2 pseudo binary systems, the surface tensions increased with an

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