Table of Contents
Copper Activation Sphalerite & Xanthate: Unlike most other sulfide minerals, sphalerite responds poorly to short-chain thiol collectors. The problem is attributed to the high solubility of zinc-thiol compounds in water. Also, due to its large band gap (3.7 eV), sphalerite is a poor catalyst for mixed potential reactions between xanthate and oxygen.
The reactivity of sphalerite with xanthate improves when the mineral is activated by heavy metal ions, such as copper, to form a copper sulfide, as follows:
ZnS + Cu²+ → CuS + Zn²+……………………………………………………….(1)
which in turn reacts with the collector to form stable copper xanthate to render the mineral surface hydrophobic.
It is well known that sphalerite flotation is sensitive to pH. Kartio, conducted XPS studies on copper-activated sphalerite in alkaline pH. The results showed that a CuS-like activation product is formed in deoxygenated solutions, while copper polysulfide is formed in air-saturated solutions. This finding provides an explanation for the collectorless flotation of copper-activated sphalerite. Other investigators identified elemental sulfur, rather than the polysulfide, as the hydrophobic species when sphalerite was copper-activated in acidic solutions.
It is well known that sphalerite flotation is difficult in the neutral pH range. Sutherland attributed the difficulty to the precipitation of copper xantahate, which in turn depletes the solution of xanthate ions. The Hallimond tube flotation experiments conducted by Steininger (1968) showed that sphalerite flotation is suppressed in the pH range of 6 to 9. The higher the copper sulfate addition, the wider the pH range of flotation suppression became. Steininger attributed the suppression to the formation of a basic copper xanthate (Cu2(OH)2X2), rather than the copper xanthate, at the near neutral pH. Laskowski also showed the difficulty in floating sphalerite at near neutral pH, and attributed it to the presence of CuOH+ ions which, according to the authors, do not form a ‘flotation active’ activation product such as CuS on the surface of sphalerite. These authors showed, however, that the sphalerite flotation at neutral pH improves after a long conditioning time.
It is the objective of the present work to study the mechanism of sphalerite flotation by measuring the surface forces relevant to bubble-particle interactions. Many investigators conducted direct force measurements between two solid surfaces using surface force apparatus (SFA) and atomic force microscope (AFM). Some investigators extended these techniques to directly measure the surface forces between air bubbles and solid surfaces. However, the measurement proved to be more difficult than anticipated. In a given force measurement, a bubble surface deforms as it approaches a solid surface, which makes it difficult to accurately determine the distance separating the air bubble and solid surface. Consequently, there are considerable discrepancies among the results reported by different investigators. Furthermore, some of the results are not consistent with the previous work reported in flotation literature. To alleviate the problem associated with the deformation of bubble surface, the authors of the present communication conducted force measurements using a hydrophobized glass sphere as a ‘simulated’ air bubble. This technique is based on the finding that air bubble is hydrophobic at low surfactant concentrations. The measurements conducted for the quartz/amine and covellite/xanthate systems are consistent with the respective flotation data.
Surface forces (F) measured using either SFA or AFM are routinely analyzed using the DLVO theory:
F = Fd + Fe………………………………………………………..(2)
in which Fd is the London-van der Waals dispersion force and Fe is the ion-electrostatic force. The magnitude of the former is represented by the Hamaker constant (A), while that of the latter is represented by the double-layer potential (Ψ). It is well known, however, that the DLVO theory is applicable only to those colloidal particles which are weakly hydrophobic. For strongly hydrophobic surfaces, it is necessary to extend the theory by including the contributions from the hydrophobic forces (Fh), as follows:
F = Fe + Fd + Fh……………………………………………………..(3)
which is frequently referred to as extended DLVO theory. It has been shown that the hydrophobic forces can be conveniently represented in the following form:
Fh = -KR/6H²………………………………………………(4)
in which R is the radius of curvature of the spherical (or cylindrical) object used for direct force measurement, H is the closest distance separating the two surfaces, and K is a constant.
Eq. [4] is of the same form as the one commonly used for the dispersion force. Therefore, K can be directly compared with A.
Experiment
Specimen-grade sphalerite from Santander, Spain, was obtained from the Ward’s Natural Science Est. The sphalerite surface was wet-polished, first, using 600 grit silicon carbide paper and, then, using 3-micron Buehler silicon carbide paper. The final polishing was done on Buehler polishing cloth using an aluminum oxide (30 nm) suspension. The polished mineral surface was rinsed with ethanol and then dried by blowing nitrogen. The polished sphalerite surface was used as a plate in AFM force measurements. The AFM images of the sphalerite samples prepared in a manner described above showed roughnesses in the range of 12-15 nm. Thus, the surfaces would not be smooth enough to measure the forces in short separation distances; however, the forces measured at longer separation distances may be reliable.
Force measurements were conducted in solutions of potassium ethyl xanthate (KEX). All of the solutions were prepared using the conductivity water from a Barnstead Nanopure II water purification system. The pH of the KEX solutions were adjusted by adding aliquots of reagent-grade NaOH and/or HCl solutions. The KEX used in the present work was obtained by dissolving an industrial-grade reagent in acetone and recrystallyzing it in ether.
Apparatus and Procedure
Figure 1 is a schematic representation of the atomic force microscope (AFM), Digital Instruments Nanoscope III scanning probe microscope, that was used for direct force measurements. A hydrophobic glass sphere (a), which represents an air bubble in flotation, is attached at the end of a cantilever spring (b). A polished sphalerite plate (c) is placed on a piezo-electric crystal (d), which is designed to move the mineral relative to the glass sphere. If a repulsive force exists between the glass sphere and the sphalerite plate, the cantilever will be deflected upward. On the contrary, if there exists an attractive force between the sphere and plate, the cantilever will be deflected downward. The cantilever deflection is monitored by the laser beam, which is emitted by its source (e), deflected off the cantilever, and enters a detector (f). From the cantilever deflection, one can determine i) the closest distance (H) between the glass sphere and sphalerite plate, and ii) the force (F) between the two surfaces if the spring constant of the cantilever is known. For the measurement of repulsive forces, the standard triangular silicon nitride (Si3N4) cantilevers were used. To measure strong attractive forces, rectangular TESP and FESP cantilevers were used. All experiments were conducted in aqueous media using a liquid cell provided by Digital Instrument.
Contact Angle Measurements
Equilibrium contact angles of a polished sphalerite electrode were measured using the captive bubble technique by means of a Rame-Hart Model 100 goniometer. The electrode was immersed in a solution of interest, with its polished surface facing down and an air bubble approaching the surface from below. The angles were measured through the aqueous phase.
The contact angle of the hydrophobized glass sphere was determined by conducting the measurement on a glass plate that had been hydrophobized in the same OTS solution as that used for the glass sphere. The sessile drop technique was used for the measurement. The glass spheres, whose equilibrium contact angle (θ) is 109°, were used as simulated air bubbles in the surface force measurements.
The Hamaker constant of the sphalerite sample was calculated from its methylene iodide contact angle, as described previously. The contact angle was determined using the sessile drop technique.
