Subscriber access provided by UNIV OF NEBRASKA - LINCOLN
Article
Surface charge anisotropy of scheelite crystals Zhiyong Gao, Yuehua Hu, Wei Sun, and Jaroslaw W. Drelich Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01252 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Surface charge anisotropy of scheelite crystals Zhiyong Gao,a Yuehua Hu,a,* Wei Sun,a and Jaroslaw W. Drelichb,* a School
of Minerals Processing and Bioengineering, Central South University, Changsha 440083, China
b Department
of Materials Science and Engineering, Michigan Technological University, Houghton 49931, MI, USA
Abstract: Atomic force microscopy (AFM) was employed to measure the colloidal interactions between silicon nitride cantilever tips and scheelite crystal surfaces in 1mMKCl solutions of varying pH. By fitting the DLVO theoretical model to the recorded force-distance curves, the surface charge density and surface potential values were calculated for three crystallographic surfaces including {112}, {101} and {001}.The calculated surface potential values were negative in both acidic and basic solutions and varied among crystallographic surfaces. The surface potential values determined were within zeta potential values reported in the literature for powdered scheelite minerals. The surface {101} was the most negatively charged surface, followed by {112} and {001}. The surface potential for {001} was only slightly affected by pH whereas surface potential for both {112} and {101} increased with increasing pH. Anisotropy in surface charge density was analyzed in relation to surface density of active oxygen atoms; i.e., the density of oxygen atoms with one or two broken bond(s) within tungstate ions located in the topmost surface layer. On a surface with a higher surface density of active oxygen atoms, a larger number of OH- is expected to adsorb through hydrogen bonding, leading to a more negative charging. 1
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Keywords: Keywords scheelite; surface charge; zeta potential; anisotropy; AFM
1. INTRODUCTION The surface characteristics of minerals are of both fundamental and practical importance in developing a suitable reagent regime for mineral flotation. The information about surface charge of minerals is often crucial in selecting the type and dosage of the collector, particularly when adsorption of the collector is controlled by electrostatic interactions. It is less relevant to surface property however, in selecting chemisorbing collectors1. Scheelite (CaWO4) is the most important mineral source of tungsten, an element used in lamp filaments2, cutting steels3, super-hard moulds4, and catalysts5, to name a few. This mineral often coexists with other calcic minerals in ore deposits such as calcite (CaCO3) and fluorite (CaF2). To separate scheelite selectively from calcite and fluorite by flotation requires the use of chemisorbing collectors, selection of which is a challenge because the same active Ca sites are available on the surfaces of all three minerals6. Recent work showed that cationic collectors of quaternary ammonium salts improve selectivity of flotation in separation of scheelite from calcite, and this effect is likely the result of differences in the surface charge of the two minerals7, 8. Negative surface charges of scheelite particles, typically somewhat influenced by pH, are well reported in the literature6,
8-14.
Electrophoretic mobility measurements are the
most widely used in studying (global) surface potential of scheelite particles15. The downsides of this method include unrealistic assumptions on mineral particle spherical
2
ACS Paragon Plus Environment
Page 2 of 31
Page 3 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
geometry and uniformity of surface charge distribution over particle surface16. Inability of electrophoresis to quantify the surface charges locally – over either different locations of charge on heterogeneous surfaces or for different crystallographic and cleavage planes – is a significant drawback of this method that slows down understanding and quantification of stochastic-nature colloidal interactions in multiphase systems involving mineral particles17, 18. Scheelite particles are usually comprised of platy and tetragonal dipyramid forms that have different crystal surfaces exposed19-21. The anisotropic nature and lateral variation of charges for such scheelite particles and their surfaces in electrolyte solutions have not been studied in the past. Developments in atomic force microscopy (AFM) made in the last two decades have opened new opportunities in studying various surface characteristics including surface charge anisotropy of mineral crystals. Surface charge anisotropy was studied for several oxide and silicate minerals in recent years including geothite22, alumina sapphire23, 24, rutile25, muscovite26, talc26,
27,
chlorite16 and chrysotile28. The surface potential that is
orientation dependent was also reported for fluorite29. The observed and recorded differences in surface charge density and surface potential of the anisotropic minerals are closely related to their crystal structure and surface ionization characteristics. Recent X-ray diffraction (XRD) analysis demonstrated that the three most commonly exposed surfaces in scheelite particles are {112}, {101} and {001}19, 30-34. These three surfaces exhibit distinct differences in arrangements of atoms, affecting their surface (free) energy values and sensitivity to adsorption of water molecules and flotation reagents. It is 3
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
also tenable to hypothesize that all three scheelite surfaces have different charge distribution characteristics. In this work, AFM was used to quantify colloidal interactions between nano-sized silicon nitride probes and scheelite {112}, {101} and {001} surfaces in 1mMKCl solutions of varying pH. The force-distance curves recorded were analyzed using the DLVO theoretical model and both surface charge density and surface potential were determined for all three surfaces.
2. EXPERIMENTAL 2.1.Scheelite crystals and their characterization Single crystals of scheelite with dimensions of 5 to 10 mm were obtained from an ore in the Huili Mine located in Sichuan, China. The crystallographic orientations of scheelite crystal specimens were determined by single-crystal X-ray diffraction. The X-ray diffractometer (D8-ADVANCEBruker-AXS) was run in the reflection mode with Cu-Kα radiation (λ= 0.15406 nm, tube potential 40 mV, and tube current 40 mA), and a goniometer speed of 4°/min (2θ). The single-crystal diffraction patterns with a 0.01° precision of interlayer spacing d measurements were conducted from 5° to 80° (2θ). The XRD analysis was conducted on 20 scheelite crystal samples and three crystals were selected for the AFM study. The crystals were too small to mount them in the AFM fluid cell. Therefore, the crystals were mounted inside ~15 mm diameter discs made of a two-part epoxy resin. Orientation of the crystals along either {001}, {112} or {101} surface was preserved during 4
ACS Paragon Plus Environment
Page 4 of 31
Page 5 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
mounting. After curing, the mounted crystals were polished with 0.06 µm alumina powder, then sonicated in and washed with deionized water (having a resistivity of >18MΩ×cm), and finally washed with absolute ethanol. The samples were additionally cleaned from organic contaminants before AFM force measurements by placing them for approximately 30 min in a UV cleaner (Bioforce Labs, Ames, IA, USA). This type of cleaning is
acceptable in surface treatment of salt-type minerals, which are less susceptible to oxidization than, for example, sulfide minerals. A Nanoscope III Dimension 3000 atomic force microscope (Digital Instruments, Santa Barbara, CA, USA) was used in a tapping mode operation for topographical imaging of specimens
and
determination
of
their
surface
roughness.Budget
Sensors
Tap300Alcantilevers made of silicon with an aluminum reflex coating, and an estimated tip radius of 10 nm were used in this study.
2.2.Force measurements Contact-mode silicon nitride (Si3N4) AFM cantilevers of a V-frame shape (Bruker AFM Probes, Camarillo, CA), having pyramidal-shaped tips and a spring constant of either approximately0.12 N/m and 0.58 N/m were used in this study. An accurate value of the spring constant was determined using the thermal tune method available in the AFM software package. The apex curvature radius for each tip was measured from images of the tip captured under the field emission scanning electron microscope (S-4700 FE-SEM, Hitachi High Technologies America, Schaumburg, IL). Previous research showed that at a certain pH, the ionic strength (the 5
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
concentration of simple electrolytes from 10-6 to10-3mol/L) has little influence on the zeta potential of scheelite particles10,
11.
Therefore, the colloidal force measurements were
carried out in 1mM KCl solutions of varying pH (adjusted with KOH or HCl stock solutions) using a NanoscopeII AFM (Digital Instruments Inc. - currently Bruker Corp.) equipped with a fluid cell. Several random locations on the scheelite crystals were selected during the force measurements. All the measurements were performed at a scan rate of 4 Hz, and captured at a resolution of 512 points/measurement. The force curves were then analyzed with SPIP software (Image Metrology, Lyngby, Denmark), which converts the deflection-distance data to force-separation curves. The additional processing of the force-distance curve included baseline correction and hysteresis correction. From 12 to 20 force curves were recorded, and 5 to 7 force curves were selected from each set for detailed analysis for each specimen and at each pH. Surface charge density and surface potential values reported are values calculated from experimental force curves after fitting the theoretical equations presented in Section 3.2.
3. THEORETICAL ANALYSIS 3.1. Unit cell analysis and computer simulation Scheelite has a tetragonal crystal structure with a=b=5.243 Å, c=11.376 Å, and α=β=γ=90°35. All calculations and simulations were performed in the Accelrys Material Studio 6.0 (MS) modeling package. The crystal structure of scheelite was built in the Crystal Builder module using the structure data reported elsewhere36. A range of surface slabs, namely {001}, {101} and {112}, were then created from the bulk unit cell of scheelite 6
ACS Paragon Plus Environment
Page 6 of 31
Page 7 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
at its Miller indices by the Surface Builder module in MS. Tungstate ions (WO42-) were kept intact and Ca-O ionic bonds were assumed to broken when each surface was simulated. During the surface simulation, oxygen loses certain number of coordinated bonds to Ca ions and becomes active. The density of active oxygen on each surface was calculated according to the followed equation: DO = NO /S
(1)
where Do and No represent active oxygen density and active oxygen number per unit cell area on a certain surface, respectively. S is the area of the surface unit cell.
3.2. DLVO model for force – distance curves The cantilevers used in the experiments had pyramidal shaped tips. The shape of the tip was approximated as conical with a spherical cap at its apex. The equations on DLVO forces (van der Waals plus electrostatic forces) for such geometry were derived previously37 and only final equations are shown here. The van der Waals forces (FvdW) were modeled according to the following equation: F vdW =
A ( R + D) − 2 L1 R − D A 1.0 R sin α tan α − D − R(1 − cos α ) − − + 2 2 6 L1 D 3 tan 2 α L1 L12
(2)
The equation describing the electrostatic force (Fedl, constant surface charge density case) is as follows:
F edl =
4π
ε 0εκ
σ T σ S ( a0e−κ D − a1e−κ L ) + 1
2
2π
ε 0εκ
2
(σ
2 T
+ σ S2 ) ( a2e−2κ D − a3e−2κ L1 ) +
(σ T2 + σ S2 ) e−2κ L1 4π b1σ T σ S e −κ L1 + b2 + ε 0εκ tan α 2 where:L1=D+R(1-cosα), a0=κR-1, a1=κRcosα-1, a2=a0+0.5, and a3=a1+0.5 7
ACS Paragon Plus Environment
(3)
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
D + R(1 − cos α ) 1 b1 = R sin α − + tan α tan α
1 L1 + κ
D + R(1 − cos α ) 1 b2 = R sin α − + tan α tan α
1 L1 + 2κ
A is the Hamaker constant, α and β are the geometrical angles for the spherical cap at the tip end and conical tip with α+β=90°, D is the distance from the end of tip to the substrate, L1 is the distance between a differential surface section of the tip and the substrate, r is the radius of the circle of the tip at a given vertical position, R is the radius of the spherical cap at the tip end, ε is the dielectric constant of the solution in this system, ε0 is the permittivity of vacuum, 1/κ is the Debye length, σ is the surface charge density, and subscripts S and T refer to substrate and tip, respectively. More details are provided in the previous publication.37
4. RESULTS 4.1. Scheelite crystal characterization In total, 23 pieces of scheelite crystals were randomly picked up from the crushed ore samples and characterized with XRD. The XRD results revealed 13 specimens with {112} surface, 7 specimens with {101} and only 3 specimens with {001}, indicating that the {112} plane is the most commonly exposed cleavage surface for scheelite crystal, followed by a moderate {101} and a weak {001}, in agreement with the previous reports32,33. Only selected XRD patterns for the scheelite crystals are shown in Figure 1. Three scheelite crystals of larger dimensions were selected and mounted inside epoxy resin as shown in Figure 2. Orientation of the crystals along each surface was 8
ACS Paragon Plus Environment
Page 8 of 31
Page 9 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
preserved and exposed to further polishing during mounting. Each surface of the mounted sample was then polished with alumina powder, rinsed with deionized water and imaged under AFM before force measurement (Topographic AFM images of the sample surfaces tested are shown in Supporting Information for Publication).The surface roughness
characteristics of the three specimens studied are summarized in Table 1. Roughness characterization included root-mean-square (RMS; also often called geometrical roughness (Rq)) that represents a measure of the standard height deviation for the analyzed image area, and the arithmetic average of the absolute values of surface height deviations from the mean (Ra). Table 1 shows the average values and standard deviation for roughness parameters calculated based on analysis for five 5x5 µm (or 2x2µm)and five 1x1 µm images. The samples of three orientations demonstrated smooth surfaces with RMS and Ra roughness parameters at a level of 1-2 nm and
