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Fatty acid collectors for phosphate flotation and their adsorption behavior using QCM-D
International Journal of Mineral Processing 95 (2010) 1–9
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International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Fatty acid collectors for phosphate flotation and their adsorption behavior using QCM-D J. Kou a,b, D. Tao b,⁎, G. Xu b a b
School of Civil and Environment Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083, PR China Department of Mining Engineering, University of Kentucky, Lexington, KY 40506, USA
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 5 March 2010 Accepted 11 March 2010 Available online 25 March 2010 Keywords: Fatty acid FTIR Hydroxyapatite Phosphate flotation QCM-D Zeta potential
a b s t r a c t In this paper the relationship between the flotation performance of phosphate collectors and their adsorption behavior was evaluated using a variety of techniques including the Crystal Microbalance with Dissipation technique (QCM-D). The adsorption of the collectors on the surface of hydroxyapatite was primarily characterized using QCM-D, which is a high sensitivity in-situ surface characterization technique. Additionally, the collectors were evaluated via zeta potential and FTIR analyses. The flotation performance of the collectors was evaluated using a laboratory mechanical flotation cell at different process parameters such as pH, collector dosage, diesel dosage and flotation time. The two collectors evaluated were a commercial plant collector and a refined tall oil fatty acid. The QCM-D data showed that the refined tall oil fatty acid adsorbed on phosphate more readily and produced stronger hydrophobicity and better flotation performance than the plant collector. The chemisorption and surface precipitation mechanisms of the refined tall oil fatty acid on the surface of hydroxyapatite were demonstrated by means of zeta potential measurements and FTIR analysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the conventional phosphate flotation (Crago) process, a significant amount of the silica present in the feed is floated twice, first by fatty acid, and then by amine (Zhang et al., 1997). The Crago process is therefore inefficient in terms of collector efficiency. The phosphate mining industry is faced with higher fatty acid prices, lower feed grade, and stricter environmental regulations (Sis and Chander, 2003). To meet the market demand for higher effectivity, lower cost and better selectivity of phosphate flotation collectors, there is a need to evaluate surface adsorption techniques that may help researchers develop better collectors by understanding how the adsorption behavior of materials affects their performance as flotation collectors. In order to evaluate this relationship, a plant collector of proprietary composition and a refined tall oil fatty acid were compared. The refined tall oil fatty acid, referred to as GP193G75, was comprised of 47% oleic and 33% linoleic acids.1 Flotation tests were performed at varying process parameters such as pH, collector dosage and flotation time with phosphate ore from CF Industries' phosphate rock mine in Hardee County, Florida. To better understand the behavior of the collectors on an apatite surface, their adsorption on the surface of a hydroxyapatite-coated sensor was studied using the QCM-D tech-
⁎ Corresponding author. Tel.: + 1 859 257 2953; fax: +1 859 323 1962. E-mail address: [email protected] (D. Tao). 1 The refined tall oil fatty acid was supplied by Georgia-Pacific Chemicals, LLC under the name GP 193G75.
0301-7516/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2010.03.001
nique. The adsorption and flotation characteristics of the two collectors were then compared. Most of the studies about the adsorption mechanism of collectors on mineral surface were conducted based on ex-situ measurements such as contact angle, adsorption isotherm, FTIR spectroscopy, and zeta potential, which unfortunately cannot monitor the real-time formation process and characteristics of adsorbed layer. QCM-D is the second generation of QCM, which has been shown by many investigators to be a sensitive tool for studying the behavior of protein and surfactant adsorption in aqueous solutions, with sensitivity in the ng/cm2 (submonolayer) region (Hook et al., 1998). It can simultaneously determine changes in frequency and energy dissipation of a quartz crystal at nanoscale in real-time and derives valuable in-situ information on adsorbed mass as well as the mechanical (viscoelastic)/structural properties of the adsorbed layer from experimentally obtained data of energy dissipation in relation to frequency shift (Paul et al., 2008). The purpose of this study was to investigate in-situ the adsorption behavior of two collectors on the hydroxyapatite surface by means of QCM-D technique and to determine whether the differences observed may lead to differences in flotation performance. 2. Experimental 2.1. Materials The phosphate from the CF Industries' phosphate rock mine was a mixture of apatite with gangue minerals such as quartz and clay
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minerals. The moisture of the sample was 20.88%. Wet sieving was conducted with the as-received ore and the particle size distribution for this particular phosphate sample is shown in Table 1. As shown, the sample contained only 5.47% +30 mesh materials and 0.49% smaller than 200 mesh particles. The dominant size fractions were −30 + 50 mesh and −50 + 70 mesh fractions which accounted for 29.06% and 51.36% of the total sample, respectively. Each size fraction of the feed sample was assayed for its grade (P2O5%) and acid insoluble (A.I.) content (see Table 1 results). The overall feed grade was 7.54% P2O5% and 65.05% A.I. The flotation collector blend was prepared using the collectors as supplied and Chevron #2 diesel fuel. Soda ash in the form of 15% water solution was used for pH adjustment of ore slurry. The sensor used in QCM-D analysis was an AT-cut quartz disc with 5 nm Cr, 100 nm Au, 50 nm Ti and 10 nm hydroxyapatite that were sputter-coated onto the crystal surface successively. The sensors and Q-sense E4 system were supplied by Q-sense Co. Zeta potential and FTIR tests were conducted with hydroxyapatite powder (50% less than 16.93 µm, 90% less than 85.90 µm) made of pulverized hydroxyapatite crystals purchased from Ward's Natural Science, and the XRD results did not show any other impurity in the samples. 2.2. Flotation tests Flotation tests were conducted using a Denver D-12 lab flotation machine equipped with a 2-liter tank and a 2–7/8 in. diameter impeller. The slurry was first conditioned with soda ash solution for pH adjustment. It was then conditioned in a bucket at about 70% solids concentration after the addition of collector and fuel oil at certain dosages. After that, the conditioned slurry was transferred to the 2liter flotation cell and water was added to dilute it to 25% solids by weight. Unless otherwise specified, flotation lasted for 1–2 min. Tap water was used in all flotation tests. To evaluate the collector's performance and optimize the process parameters, flotation tests were carried out at different pHs, collector dosages, diesel percentage and flotation times (kinetic tests). The flotation products were assayed for grade (P2O5%) and acid insolubles (A.I.). Flotation recoveries were calculated based on the analyses of the separation products (concentrates and tailings). Flotation efficiency is a composite parameter used for evaluating the flotation performance. It was calculated from recovery and A.I. rejection in the equation
The QCM-D experiments were conducted at pH 10.0 and 25 °C (± 0.02 °C). The stock solution was prepared by dissolving the appropriate amount of collector with sodium hydroxide in deionized water. To ensure dissolution and degassing, the solutions were left in an ultrasonic bath for 5–10 min. For each experiment, the data generated with the solvent only (sodium hydroxide solution in this study) were accepted as baseline when the process became stable. The fatty acid solutions were injected into the measurement system by a chemical feeding pump capable of precise flow rate control. The flow rate in the experiment was kept at 0.5 mL/min. Software QTools 3.0 was used for data modeling and analysis. For a rigid, thin, uniform film, the change in dissipation factor ΔD is smaller than 10− 6 for a 10 Hz frequency change (Paul et al., 2008). The Sauerbrey equation (Eq. (3)) can be used for mass determination. Δm = −
ρq tq Δf ρq vq Δf CΔf =− =− f0 n n 2f02 n
D=
Edissipated 2πEstored
ð4Þ
where Edissipated is the energy dissipated during one oscillation, and Estored is the energy stored in the oscillating system (Ekholm et al., 2002). When adsorption causes a great shift in the D value (ΔD N 1 × 10−6 ) as a result of the adsorption of a viscous and soft layer, Voigt modeling (Eqs. (5) and (6)) (Voinova et al., 1999) can be used: 1 Δf ≈ − 2πρ0 h0
ΔD ≈ −
A.I. rejection can be calculated from Eq. (2) Tt ′ 100 Cc′ + Tt ′
ð2Þ
in which t′ and c′ are A.I. of tailing and concentrate, respectively.
Table 1 Particle size distribution of phosphate sample.
ð3Þ
where ρq and tq are the density and thickness of quartz crystal, respectively, and vq is the transverse wave velocity in quartz. The constant C has a value of 17.8 ng cm− 2 Hz− 1, and n is the harmonic number (when n = 1, f0 = 5 MHz). If the adsorbed film is “soft” (viscoelastic), it will not fully couple to the oscillation of the crystal (Ekholm et al., 2002), and this will cause energy dissipation of system. The dissipation factor D is proportional to the power dissipation in the oscillatory system (Eq. (4)) and can give valuable information about the rigidity of the adsorbed film (Ekholm et al., 2002):
(
ð1Þ
Flotation efficiency = P2 O5 recovery + A:I: rejection−100
A:I: rejection =
2.3. QCM-D analyses
1 2πf ρ0 h0
" #) 2 ηj ω2 η3 η3 + ∑ hj ρj ω−2hj δ3 δ3 μj2 + η2j ω2 j = 1;2
(
" #) μj ω2 η3 η 2 : + ∑ 2hj 3 δ3 δ3 μj2 + η2j ω2 j = 1;2
ð5Þ
ð6Þ
According to the Voigt model for viscous adsorption layer, Δf and ΔD depend on the density (ρ), thickness (h), elastic shear modulus (µ) and shear viscosity (η) of the adsorption layer, and j is the number of adsorbed layers. The Sauerbrey equation and the Voigt model are the theoretical basis for data modeling in the QCM-D analysis. 2.4. Zeta potential measurements
Size (mesh)
Wt (%)
Grade (P2O5%)
A.I. (%)
Cumulative overscreen (%)
Cumulative overscreen grade (P2O5%)
Cumulative overscreen A.I. (%)
N 30 30–50 50–70 70–100 100–200 b 200
5.47 29.06 51.36 6.34 7.28 0.49
4.13 11.95 6.56 5.07 1.78 4.76
91.44 32.86 75.44 91.41 75.53 92.89
5.47 34.53 85.89 92.23 99.51 100.00
4.13 10.71 8.23 8.01 7.56 7.54
91.44 42.14 62.05 64.07 64.91 65.05
The zeta potential measurements were made with Zeta-plus analyzer of Brook Haven Instruments Corporation. All experiments were conducted with 1 mM KCl solution at laboratory atmosphere and temperature. 1.0 g hydroxyapatite powder was first conditioned in 50 mL of 1 mM KCl solution with magnetic stirrer for 1 h, during which 1.25 mg GP139G75 was added into solution to make the concentration of 25 ppm, and pH was adjusted by NaOH or HCl solutions. The mineral suspension was filtered using Whatman filter
J. Kou et al. / International Journal of Mineral Processing 95 (2010) 1–9
paper (pore size 25 µm) and then poured into the rectangular cell for zeta potential measurements. The pH was measured again at the end of test as the final pH.
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The infrared transmission spectra were recorded on a Thermo Nicolet Nexus 470 FTIR spectrometer. The hydroxyapatite powders were conditioned with 50 mL collector solution at different pHs and different concentrations, while being agitated with magnetic stirrer for 0.5 h to make 1% suspension. The suspension was then filtered with Whatman filter paper (pore size 25 µm), and the solids were airdried overnight at the room temperature. The samples were prepared by dispersing 0.025 g air-dried powder in 5 g KBr followed by pressing into a transparent tablet for scanning. The untreated (initial) hydroxyapatite powder was used as reference. Each spectrum is an average of 250 scans.
dosage and the A.I. with the plant collector remained higher (i.e. worse) than with the refined tall oil fatty acid. When the refined tall oil fatty acid dosage increased from 0.3 kg/t to 0.45 kg/t, the increase in A.I. was less significant than with the plant collector, which indicates that the refined tall oil fatty acid showed better selectivity and higher acid insolubles rejection than the plant collector. Fig. 2 demonstrates the relationship between P2O5 recovery and grade with the refined tall oil fatty acid and the plant collector to compare the separation sharpness of these two reagents. The results indicate that the flotation P2O5 recovery with the refined tall oil fatty acid was higher than with the plant collector at a given P2O5 grade ranging approximately from 20% to 30%. When the grade decreased from 29.0% to 25.9%, the recovery with the refined tall oil fatty acid increased from 72.1% to 82.5% while the recovery with the plant collector showed no significant increase. All of the above data indicates that the refined tall oil fatty acid performed better than the plant collector.
3. Results and discussion
3.2. Effect of diesel dosage
3.1. Effect of collector dosage
Diesel plays an important role in phosphate flotation. It acts as solvent or booster of tall oil fatty acid collectors and has a significant effect on foam controlling (Snow et al., 2004). To understand the effect of diesel on the performance of the collectors, flotation tests were conducted by adding the mixture of the refined tall oil fatty acid and diesel as collector at the same dosage but in different ratios (by weight) at pH 9.5 and the results of flotation recovery and P2O5 grade are presented in Fig. 3. Fig. 3 shows that the P2O5 recovery increased with increasing the diesel percentage from 10% to 50% at both dosages and the highest recovery was achieved when the ratio of the refined tall oil fatty acid to diesel was 1:1. However, a further increase in the diesel percentage from 50% to 80% decreased the recovery from 88.4% to 32.9% at the dosage of 0.9 kg/t and from 63.9% to 15.7% at the dosage of 0.6 kg/t. The highest grade was achieved at 30% diesel percentage with 0.6 kg/t collector and 60% diesel percentage with 0.9 kg/t collector. It is interesting to see that 60.1% recovery and 17.5% P2O5 grade were achieved at 30% diesel percentage with 0.27 kg/t diesel and 0.63 kg/t GP193G75 but 63.9% recovery and 15.8% P2O5 grade were also obtained at the dosage of only 0.3 kg/t GP 193G75 with 0.3 kg/t diesel. This indicates that increasing diesel ratio may decrease the consumption of the refined tall oil fatty acid but with insufficient amount of collector, increasing diesel ratio cannot achieve optimum performance. When both recovery and grade are considered at the same time, a collector dosage of 0.9 kg/t was significantly better than that of 0.6 kg/t.
2.5. FTIR analysis
The effects of dosage on rougher phosphate flotation performance were studied by a series of flotation tests with the refined tall oil fatty acid in conjunction with the plant collector. The pH, conditioning time and impeller rotating speed were fixed constant at pH 10, 6 min and 1500 rpm, respectively and the ratio of collector to diesel was 3:2. The dependence of flotation efficiency and A.I. on collector dosage is shown in Fig. 1. It was quite obvious that the refined tall oil fatty acid generated higher flotation efficiency than the plant collector at all three dosages tested. The flotation efficiency increased significantly with increasing dosage of the refined tall oil fatty acid from 0.15 kg/t to 0.3 kg/t, but decreased slightly when the refined tall oil fatty acid dosage increased further from 0.3 kg/t to 0.45 kg/t. When the plant collector dosage increased from 0.15 kg/t to 0.3 kg/t, the increase in flotation efficiency was less significant than for the refined tall oil fatty acid. The highest flotation efficiency of 79.5% was achieved with 0.3 kg/t of the refined tall oil fatty acid, which was about 10% higher than the flotation efficiency with the plant collector at the same dosage or more than 6% higher than the maximum flotation efficiency achieved with the plant collector at 0.45 kg/t. However, a further increase in the dosage to 0.45 kg/t decreased the flotation efficiency to 77.8%, which was still about 5% higher than the plant collector at the same dosage. The concentrate A.I. increased with increasing the
Fig. 1. Effect of collector dosage on flotation efficiency and concentrate A.I.
Fig. 2. Relationship between the P2O5 recovery and grade with GP193G75 and plant collector at different dosage.
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Fig. 3. Effect of diesel percentage on flotation recovery and grade (pH 9.5, dosage of GP193G75 and diesel mixture at 0.6 kg/t and 0.9 kg/t).
3.3. Effect of pH Flotation tests were conducted to investigate the effect of slurry pH on the collector performance in apatite flotation by varying the pH from 9.5, 10, to 10.5. Fig. 4 demonstrates the flotation efficiency/A.I. as a function of slurry pH in the presence of 0.3 kg/t of the refined tall oil fatty acid or the plant collector conditioned for 6 min. It can be seen that the flotation efficiency increased with increasing pH values and the refined tall oil fatty acid showed significantly higher flotation efficiency than the plant collector at all three pHs. The flotation efficiency obtained with the refined tall oil fatty acid increased from 72.8% to 82.6% as the pH increased from 9.5 to 10.5. Meanwhile, the A. I. with the refined tall oil fatty acid and the plant collector increased with increasing slurry pH but was always lower with the refined tall oil fatty acid at all three pHs, which indicates that the refined tall oil fatty acid results in better selectivity than the plant collector. The relationship between P2O5 recovery and grade with the refined tall oil fatty acid and the plant collector is shown in Fig. 5. The curves closer to the upper right corner represent more efficient separation. Compared with the plant collector, the refined tall oil fatty acid collector increased P2O5 recovery by 6% at the same concentrate grade. Obviously, the refined tall oil fatty acid has a better flotation performance than the plant collector. It can also be concluded from the results that higher pulp pH had positive effects on phosphate
Fig. 4. Effect of slurry pH on flotation efficiency and concentrate A.I.
Fig. 5. Relationship between the P2O5 recovery and grade with GP193G75 and plant collector at different pH.
flotation. This observation is in agreement with the previous studies. Feng and Aldrich (2004) investigated the effect of some operating parameters such as pulp pH and collector dosage on the kinetics of apatite flotation and obtained the best flotation performance at a pH level of 12.3 with fatty acid and sulphonate as collector. They reported that an elevated pulp pH increased the flotation recovery and kinetics, probably by softening the process water and speeding up the electrolysis of the fatty acid. According to the work by Robert Pugh and Per Stenius (1984), the minimum surface tension and the formation of pre-micella associated species occurred at higher pH in lower concentrations of sodium oleate solution, which attributed to the better flotation recovery of apatite. 3.4. Kinetic flotation tests The kinetics of flotation studies the variation in floated mineral mass as a function of flotation time. To characterize the effect of different collectors on flotation rate, kinetic flotation tests were performed with both the refined tall oil fatty acid and plant collector. During the tests concentrate samples were collected at a time interval of 10 s in the first 30 s and also finally at 90 s. Both reagents were tested under the same conditions. The dosage of collector was 0.45 kg/t mixed with 0.4 kg/t fuel oil and the pH of ore pulp was 10. The results of recovery and concentrate grade versus flotation time are shown in Fig. 6. It can be seen that the recovery increased about 30% in the first
Fig. 6. P2O5 grade and recovery versus flotation time.
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30 s with both the refined tall oil fatty acid and the plant collector and additional 7% and 11.7% in the last 60 s, respectively. The grade of concentrate generated by the plant collector decreased 5% with the increase in recovery in the first 30 s. On the other hand no grade decrease was observed with the refined tall oil fatty acid during the same period of time. At 90 s the refined tall oil fatty acid showed a higher recovery of 91.7% and a higher grade of 26.5% compared to the plant collector (88.6% recovery and 20.6% grade). The above data shows that the refined tall oil fatty acid had considerably better selectivity and flotation efficiency than the plant collector. 3.5. Adsorption behavior of fatty acid onto hydroxyapatite surface To characterize the adsorption behavior of the refined tall oil fatty acid and plant collector on the hydroxyapatite surface, a highly sensitive in-situ surface characterization technique QCM-D was em-
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ployed in conjunction with zeta potential measurement and FTIR spectra analysis. 3.5.1. QCM-D measurements Fig. 7 shows the real-time response curves of frequency shift (Δf) and dissipation shift (ΔD) from the third overtone (15 MHz) associated with the refined tall oil fatty acid and the plant collector adsorption onto a hydroxyapatite surface at a concentration of 500 ppm. Fig. 7A displays the frequency shift (Δf) for the refined tall oil fatty acid and the plant collector adsorption on the hydroxyapatite surface. Arrow a indicates the beginning of injection of collector solution into the system. It can be observed that after the injection of the refined tall oil fatty acid, Δf had an immediate sharp decrease simultaneous with a sharp increase in ΔD (Fig. 7B, arrow a). These sharp changes in Δf and ΔD indicate the quick adsorption of the refined tall oil fatty acid on the apatite surface and the high ΔD value (ΔD N 1E
Fig. 7. QCM-D experimental data of frequency shift (A) and dissipation shift (B) (measured at 15 MHz) of GP193G75 and plant collector adsorption onto a hydroxyapatite surface at concentration of 500 ppm.
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−6) suggests the formation of a dissipated layer. The highest adsorption thickness obtained at arrow b was 70 nm which was calculated using the Voigt model. It should be noted that Δf continued to increase after the adsorption thickness reached the maximum and a steady state was not reached until after 90 min at arrow c. The adsorption thickness at the stable frequency shift was 11 nm. This phenomenon has rarely been reported in the previous studies of adsorption of proteins and surfactants. According to the work by Mielczarski and Mielczarske (1995), who applied the FTIR reflection spectroscopy in the analysis of the molecular orientation and thickness of oleate adsorption on the apatite surface, the oleate monolayer on the apatite was well-organized, but the second layer adsorbed on top of the first well-ordered layer was randomly spread and oriented almost parallel to the interface. Since the FTIR is an ex-situ analysis method and QCM-D is an in-situ analysis technique, this phenomenon of increase in Δf (Fig. 7A, from arrow b to arrow c) probably reflected the orientation process of the second and subsequent layers of molecules, which led to a low free energy state in the solution system. As reported by Mielczarski and Mielczarske (1995), water molecules and calcium ions on the surface of apatite formed an intramolecular layer, which was not uniform for the carboxylate group. The higher ΔD at arrow b probably indicated the formation of soft and water-rich multi-layers of fatty acid on the apatite surface at the very beginning. ΔD decreased with the increase of Δf (Fig. 7B from arrow b to arrow c), which indicated the formation of more rigid multi-layers due to the
change in orientation of top layers of molecules and the loss of water caused by layer compression. Compared with the refined tall oil fatty acid, the plant collector generated a gradual decrease of Δf and increase of ΔD. However, Δf reached a stable value after 30 min when the ΔD value was still increasing. This indicates a slow adsorption process as well as the formation of much softer and more porous multi-layers. Rodahl et al. (1997) postulated that if the porous film is deformed by shear oscillation, liquid can be “pumped” in and out of the film and also be subjected to oscillatory “flow” within the film as the pores in the film change in shape and size. This may cause the continuing increase of ΔD even when the Δf was stable. The final thickness of the plant collector adsorption layer on the surface of hydroxyapatite calculated using the Voigt model was 4 nm, which was not only thinner, but also more dissipated and less rigid (plant collector ΔD N the refined tall oil fatty acid ΔD) than the adsorption layer for the refined tall oil fatty acid. The frequency shift and dissipation shift caused by adsorption of the refined tall oil fatty acid and the plant collector at a concentration of 1000 ppm showed almost the same trend as at 500 ppm. It should be noted that the stable Δf at 1000 ppm appeared 1.3 h earlier than at 500 ppm for the refined tall oil fatty acid. This indicates that the refined tall oil fatty acid adsorbed on the hydroxyapatite surface more rapidly at higher concentrations. The measured frequency and dissipation shifts at 500 ppm and 1000 ppm were summarized as ΔD–Δf plots in Fig. 8A–D. ΔD–Δf plots
Fig. 8. ΔD–Δf plots for (A) plant collector at 500 ppm, (B) plant collector at 1000 ppm, (C) GP193G75 at 500 ppm, and (D) GP193G75 at 1000 ppm, adsorbed on the hydroxyapatite surface.
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can provide information on the energy dissipation per unit mass added to the crystal. The slope of the plots was defined as K (K = ΔD/Δf, absolute value) which is indicative of kinetic and structure alternation during adsorption process (Paul et al., 2008). A more rigid and compact adsorption mass is expected to yield a small K value, and a soft and dissipated layer is expected to yield a higher K value. Only one slope is associated with the adsorption process without kinetic or conformational change. More than one slope suggests the adsorption associated with orientation change or hydrodynamically-coupled water in the adsorbed layer (Paul et al., 2008; Rodahl et al., 1997). Fig. 8A and B shows the ΔD–Δf plots for the adsorption of the plant collector on the hydroxyapatite surface at 500 ppm and 1000 ppm, respectively. Two slopes were observed based on the gross trend of curves in Fig. 8A and B. It can be observed that the 500 ppm and the 1000 ppm tests have similar K1 values of 0.106 and 0.092, respectively, while K2 was higher than K1 at both concentrations, and the trendlines were almost vertical to the X axis. This indicates that the initially-formed layer was less dissipated, but with the increase of adsorbed mass and trapped water molecules, it was getting softer and more dissipated. At the second stage with a slope of K2, the ΔD increased rapidly while the Δf was stable at about − 10 Hz. As mentioned before, this elevated ΔD was probably due to the structural change of the adsorbed layer caused by trapped water. Fig. 8C and D shows the ΔD–Δf plots for the adsorption of 500 ppm and 1000 ppm of the refined tall oil fatty acid on the hydroxyapatite surface. The trend of the curve shows three distinguishable stages (three K values) in the adsorption process. Comparing the K values in Fig. 8C and D reveals that K2 (0.159 at 500 ppm and 0.029 at 1000 ppm) was smaller than K1 (0.305 at 500 ppm and 0.220 at 1000 ppm). This indicates that although the initial adsorption of the refined tall oil fatty acid on hydroxyapatite was rapid, the adsorbed layer was soft and dissipated. With decreasing Δf, ΔD increased slowly at the second stage, which yielded the lower K2. It must be noted that, in comparison with the plant collector, the refined tall oil fatty acid has a third adsorption stage at which the Δf increased as the ΔD decreased to smaller than 1E−6. This indicates the loss of water caused by layer compression or the change in orientation of molecules. The dissipation shift per unit mass at this stage was the same as the first stage. 3.5.2. Zeta potential measurements Fig. 9 shows the zeta potentials of pure hydroxyapatite before and after conditioning with the refined tall oil fatty acid or the plant collector at the constant ionic strength with respect to pH. It can be seen that the zeta potential of pure hydroxyapatite was zero at pH 4 and the shapes of curves agreed well with the results reported by Rao
Fig. 9. Zeta potential of hydroxyapatite and hydroxyapatite conditioned with 25 ppm GP193G75 and plant collector at constant ionic strength as a function of pH.
7
et al. (1990). The results indicate that after conditioning with collectors, the zeta potential of hydroxyapatite became more negatively charged and shifted the i.e.p. towards lower than pH 4. In addition, the zeta potential with the refined tall oil fatty acid was lower than that with the plant collector at pHs ranging from 5 to 11, which is in good agreement with the QCM-D results that showed the refined tall oil fatty acid is associated with higher affinity and adsorption density than the plant collector. According to Rao et al. (1990), the more negative charge of apatite as a result of conditioning with sodium oleate indicates the high affinity of oleate with surface Ca-sites. Either the chemisorption of the refined tall oil fatty acid, which forms the monocoordinated complex (i.e. 1:1 oleate-lattice calcium complex) or the precipitation of its calcium salt may take place, especially at high concentrations of the collector. As shown in QCM-D results, the sharply decreased frequency shift from the beginning of adsorption demonstrates that the consistent mass increase happened on the hydroxyapatite surface due to the simultaneous chemisorption and surface precipitation on hydroxyapatite, which formed the 70 nm adsorption layer at 500 ppm solution with the refined tall oil fatty acid. The QCM-D results show not only the adsorption process, but also the process of layer compression and molecular orientation (K2 b K1). According to Mielczarski et al. (1993), the hydrophobicity of apatite is closely related to the structure of the adsorbed layer on the surface, wherein the higher packing density involves hydrophobic character of sample, a poorly organized structure does not produce a high hydrophobicity of apatite. When the adsorption became stable, the adsorbed layer of the refined tall oil fatty acid had a lower ΔD than the plant collector, which indicated better organized structure of the refined tall oil fatty acid on hydroxyapatite than the plant collector, which caused higher hydrophobicity of hydroxyapatite conditioned with the refined tall oil fatty acid. 3.5.3. FTIR spectra Infrared transmission spectra of hydroxyapatite after adsorption in the refined tall oil fatty acid solution at different pHs and concentrations are shown in Fig. 10 and Fig. 11. According to many researchers (Antti and Forssberg, 1989; Mielczarski et al., 1993; Rao et al., 1991), the absorbance bands which can provide useful information about the nature of the adsorbed fatty acid such as oleic acid can be observed in two regions of frequency, i.e., (1) most peaks at wavenumbers from 3100 cm− 1 to 2900 cm− 1 are related to the hydrocarbon chain. (2) Most peaks at wavenumbers from 1700 cm− 1 to 1400 cm− 1 are
Fig. 10. Infrared transmission spectra of hydroxyapatite after adsorption of GP193G75 at different concentrations and pHs (concentration ppm/pH) for the alkyl chain region 2750–3100 cm− 1.
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In summary, infrared studies showed that the adsorption of the refined tall oil fatty acid on apatite involves both chemical reaction between the carboxylate species and surface calcium ions and surface precipitation, which agrees well with the zeta potential and QCM-D measurements. 4. Conclusions
Fig. 11. Infrared transmission spectra of hydroxyapatite after adsorption of GP193G75 at different concentrations and pHs (concentration ppm/pH) for the carboxylate group region 1600–1400 cm− 1.
related to the carboxylate radical. It must be noted that the infrared spectra in the case of calcite are found to be hard to interpret due to the highly interfering carbonate absorption in the same frequency region as that of the carboxylate radical (Rao and Forssberg, 1991). Fig. 10 shows the typical absorption bands of alkyl chain in the region from 2750 cm− 1 to 3100 cm− 1, wherein the 2923 cm− 1 and 2852 cm− 1 bands were the asymmetric and symmetric stretching vibrations in the CH2 radical, respectively and 2956 cm− 1 was the asymmetric stretching vibration in the CH3 radical. Even though the pH did not show significant effect on the intensities of adsorption peaks, the spectra in alkyl region showed absorption bands in the range of 2750–3100 cm− 1 and the intensities of these bands were found to increase with increasing concentration of the refined tall oil fatty acid, as shown in Fig. 10. Fig. 11 shows the infrared transmission spectra of hydroxyapatite after adsorption of the refined tall oil fatty acid at different concentrations and pHs for the carboxylate group region from 1400 cm− 1 to 1600 cm− 1. Since the hydroxyapatite sample itself shows characteristic absorption bands between 1400 cm− 1 and 1500 cm− 1 and the carboxyl absorbance bands appear in the narrow range between 1540 cm− 1 and 1580 cm− 1, it is difficult to identify the typical absorption bands of carboxyl. It was observed that the carboxylate group in the refined tall oil fatty acid gave rise to characteristic absorption bands in the region of 1600–1400 cm− 1 at the higher concentration of 1000 ppm, wherein the 1450 cm− 1 band corresponds to the asymmetric deformation of the CH3 radical. In ionized fatty acids, the band at 1710 cm− 1 is replaced by two new bands due to vibrations in the COO– radical including 1610– 1550 cm− 1 and 1420–1300 cm− 1 (Antti and Forssberg, 1989). The adsorption bands that occur at 1576 cm− 1 and 1537 cm− 1 are assigned to the asymmetric stretching vibration of COO– radical. This corresponds to the monocoordinated complex or the surface precipitation of calcium salt fatty acid at the concentration of 1000 ppm, which is also demonstrated by QCM-D measurements that showed the frequency shift decreased significantly at the beginning of adsorption as a result of the formation of a thick and soft adsorption layer. According to Mkhonto et al. (2006), all surfactants containing carbonyl and hydroxy groups interact strongly with the apatite surfaces and bridging between two or more surface calcium ions of apatite is the preferred mode of surfactant adsorption.
The performance of a commercial phosphate collector and a refined tall oil fatty acid was investigated in this study. Flotation tests were performed under different process parameters. It was found that under the same process conditions, the refined tall oil fatty acid achieved a considerably better flotation performance than the plant collector. The recovery and concentrate grade were 91.7% and 26.5%, respectively when the refined tall oil fatty acid was used at a dosage of 0.45 kg/t at pH 10.0 with a 9:8 (by mass) concentration ratio of fatty acid to diesel. High pulp pH was found to have positive effects on phosphate flotation. Use of diesel as synergist can significantly decrease the dosage of the refined tall oil fatty acid, but an increase in diesel dosage cannot achieve good performance without a sufficient amount of the refined tall oil fatty acid. The QCM-D results indicated that the refined tall oil fatty acid had different adsorption behavior than the plant collector. The plant collector had two adsorption stages and K2 was greater than K1, which means the adsorbed layer become more dissipated with increasing adsorption time. On the contrary, the adsorption layer of the refined tall oil fatty acid became more rigid when K2 was smaller than K1. It also showed a compression stage represented by K3. Therefore, the adsorption layer of the refined tall oil fatty acid formed on a hydroxyapatite surface was more organized and rigid than the plant collector at the concentrations of 500 and 1000 ppm. Since the higher packing density of adsorbed layer results in hydrophobic character and a poorly organized structure does not produce a high hydrophobicity, the hydroxyapatite surface adsorbed by the refined tall oil fatty acid had higher hydrophobicity than the plant reagent after the adsorption reached the steady state. The results of zeta potential measurements and FTIR analyses indicated strong electrostatic interaction and high affinity of the refined tall oil fatty acid on the hydroxyapatite surface. Both chemisorption and surface precipitation mechanisms were demonstrated by FTIR analysis and QCM-D measurement. References Antti, B.M., Forssberg, K.S.E., 1989. Pulp chemistry in industrial mineral flotation, studies of surface complex on calcite and apatite surfaces using FTIR spectroscopy. Miner. Eng. 2, 217–227. Ekholm, P., Blomberg, E., et al., 2002. A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface. J. Colloid Interface Sci 247, 342–350. Feng, D., Aldrich, C., 2004. Influence of operating parameters on the flotation of apatite. Miner. Eng. 17, 453–455. Hook, F., Rodahl, M., Brzezinski, P., Kasemo, B., 1998. Energy dissipation kinetics for protein and antibody–antigen adsorption under shear oscillation on a quartz crystal microbalance. Langmuir. 14, 729–734. Mielczarski, J.A., Mielczarske, E., 1995. Determination of molecular orientation and thickness of self-assembled monolayers of oleate on apatite by FTIR reflection spectroscopy. J. Phys. Chem 99, 3206–3217. Mielczarski, J.A., Cases, J.M., Bouquet, E., Barres, O., Delon, J.F., 1993. Nature and structure of adsorption layer on apatite contacted with oleate solution. 1. Adsorption and fourier transform infrared reflection studies. Langmuir. 9, 2370–2382. Mkhonto, D., Ngoepe, P.E., Cooper, T.G., Leeuw, N.H., 2006. A computer modeling study of the interaction of organic adsorbates with fluorapatite surfaces. Phy. Chem. Minerals 33, 314–331. Paul, S., Paul, D., Tamara, B., Ray, A.K., 2008. Studies of adsorption and viscoelastic properties of proteins onto liquid crystal phthalocyanine surface using quartz crystal microbalance with dissipation technique. J. Phys. Chem. C. 112, 11822–11830. Pugh, R., Stenius, P., 1984. Solution chemistry studies and flotation behavior of apatite, calcite and fluorite minerals with sodium oleate collector. Int. J. Miner. Process. 15, 193–218. Rao, K.H., Forssberg, K.S.E., 1991. Mechanism of fatty acid adsorption in salt-type mineral flotation. Miner. Eng. 4, 879–890. Rao, K.H., Antti, B.M., Forssberg, E., 1990. Mechanism of oleate interaction on salt-type minerals, part II. Adsorption and electrokinetic studies of apatite in the presence of sodium oleate and sodium metasilicate. Int. J. Miner. Process. 28, 59–79.
J. Kou et al. / International Journal of Mineral Processing 95 (2010) 1–9 Rao, K.H., Cases, J.M., Forssberg, K.S.E., 1991. Mechanism of oleate interaction on salttype minerals V. Adsorption and precipitation reactions in relation to the solid/ liquid ratio in the synthetic fluorite-sodium oleate system. J. Colloid Interface Sci 145, 330–348. Rodahl, M., Hook, F., Fredrikson, C., et al., 1997. Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion. Faraday Discuss. 107, 229–246. Sis, H., Chander, S., 2003. Reagents used in the flotation of phosphate ores: a critical review. Miner. Eng. 16, 577–585.
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Snow, R., Zhang, P., Miller, J.D., 2004. Froth modification for reduced fuel oil usage in phosphate flotation. Miner. Eng. 74, 91–99. Voinova, M.V., Rodahl, M., et al., 1999. Viscoelastic acoustic response of layered polymer films at fluid–solid interfaces: continuum mechanics approach. Phys. Scr. 59, 391–396. Zhang, P., Yu, Y., Bogan, M., 1997. Challenging the “Crago” double floate process II. Amine-type-fatty acid flotation of silicious phosphates. Miner. Eng. 10, 983–994.
Contents lists available at ScienceDirect
International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Fatty acid collectors for phosphate flotation and their adsorption behavior using QCM-D J. Kou a,b, D. Tao b,⁎, G. Xu b a b
School of Civil and Environment Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083, PR China Department of Mining Engineering, University of Kentucky, Lexington, KY 40506, USA
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 5 March 2010 Accepted 11 March 2010 Available online 25 March 2010 Keywords: Fatty acid FTIR Hydroxyapatite Phosphate flotation QCM-D Zeta potential
a b s t r a c t In this paper the relationship between the flotation performance of phosphate collectors and their adsorption behavior was evaluated using a variety of techniques including the Crystal Microbalance with Dissipation technique (QCM-D). The adsorption of the collectors on the surface of hydroxyapatite was primarily characterized using QCM-D, which is a high sensitivity in-situ surface characterization technique. Additionally, the collectors were evaluated via zeta potential and FTIR analyses. The flotation performance of the collectors was evaluated using a laboratory mechanical flotation cell at different process parameters such as pH, collector dosage, diesel dosage and flotation time. The two collectors evaluated were a commercial plant collector and a refined tall oil fatty acid. The QCM-D data showed that the refined tall oil fatty acid adsorbed on phosphate more readily and produced stronger hydrophobicity and better flotation performance than the plant collector. The chemisorption and surface precipitation mechanisms of the refined tall oil fatty acid on the surface of hydroxyapatite were demonstrated by means of zeta potential measurements and FTIR analysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the conventional phosphate flotation (Crago) process, a significant amount of the silica present in the feed is floated twice, first by fatty acid, and then by amine (Zhang et al., 1997). The Crago process is therefore inefficient in terms of collector efficiency. The phosphate mining industry is faced with higher fatty acid prices, lower feed grade, and stricter environmental regulations (Sis and Chander, 2003). To meet the market demand for higher effectivity, lower cost and better selectivity of phosphate flotation collectors, there is a need to evaluate surface adsorption techniques that may help researchers develop better collectors by understanding how the adsorption behavior of materials affects their performance as flotation collectors. In order to evaluate this relationship, a plant collector of proprietary composition and a refined tall oil fatty acid were compared. The refined tall oil fatty acid, referred to as GP193G75, was comprised of 47% oleic and 33% linoleic acids.1 Flotation tests were performed at varying process parameters such as pH, collector dosage and flotation time with phosphate ore from CF Industries' phosphate rock mine in Hardee County, Florida. To better understand the behavior of the collectors on an apatite surface, their adsorption on the surface of a hydroxyapatite-coated sensor was studied using the QCM-D tech-
⁎ Corresponding author. Tel.: + 1 859 257 2953; fax: +1 859 323 1962. E-mail address: [email protected] (D. Tao). 1 The refined tall oil fatty acid was supplied by Georgia-Pacific Chemicals, LLC under the name GP 193G75.
0301-7516/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2010.03.001
nique. The adsorption and flotation characteristics of the two collectors were then compared. Most of the studies about the adsorption mechanism of collectors on mineral surface were conducted based on ex-situ measurements such as contact angle, adsorption isotherm, FTIR spectroscopy, and zeta potential, which unfortunately cannot monitor the real-time formation process and characteristics of adsorbed layer. QCM-D is the second generation of QCM, which has been shown by many investigators to be a sensitive tool for studying the behavior of protein and surfactant adsorption in aqueous solutions, with sensitivity in the ng/cm2 (submonolayer) region (Hook et al., 1998). It can simultaneously determine changes in frequency and energy dissipation of a quartz crystal at nanoscale in real-time and derives valuable in-situ information on adsorbed mass as well as the mechanical (viscoelastic)/structural properties of the adsorbed layer from experimentally obtained data of energy dissipation in relation to frequency shift (Paul et al., 2008). The purpose of this study was to investigate in-situ the adsorption behavior of two collectors on the hydroxyapatite surface by means of QCM-D technique and to determine whether the differences observed may lead to differences in flotation performance. 2. Experimental 2.1. Materials The phosphate from the CF Industries' phosphate rock mine was a mixture of apatite with gangue minerals such as quartz and clay
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minerals. The moisture of the sample was 20.88%. Wet sieving was conducted with the as-received ore and the particle size distribution for this particular phosphate sample is shown in Table 1. As shown, the sample contained only 5.47% +30 mesh materials and 0.49% smaller than 200 mesh particles. The dominant size fractions were −30 + 50 mesh and −50 + 70 mesh fractions which accounted for 29.06% and 51.36% of the total sample, respectively. Each size fraction of the feed sample was assayed for its grade (P2O5%) and acid insoluble (A.I.) content (see Table 1 results). The overall feed grade was 7.54% P2O5% and 65.05% A.I. The flotation collector blend was prepared using the collectors as supplied and Chevron #2 diesel fuel. Soda ash in the form of 15% water solution was used for pH adjustment of ore slurry. The sensor used in QCM-D analysis was an AT-cut quartz disc with 5 nm Cr, 100 nm Au, 50 nm Ti and 10 nm hydroxyapatite that were sputter-coated onto the crystal surface successively. The sensors and Q-sense E4 system were supplied by Q-sense Co. Zeta potential and FTIR tests were conducted with hydroxyapatite powder (50% less than 16.93 µm, 90% less than 85.90 µm) made of pulverized hydroxyapatite crystals purchased from Ward's Natural Science, and the XRD results did not show any other impurity in the samples. 2.2. Flotation tests Flotation tests were conducted using a Denver D-12 lab flotation machine equipped with a 2-liter tank and a 2–7/8 in. diameter impeller. The slurry was first conditioned with soda ash solution for pH adjustment. It was then conditioned in a bucket at about 70% solids concentration after the addition of collector and fuel oil at certain dosages. After that, the conditioned slurry was transferred to the 2liter flotation cell and water was added to dilute it to 25% solids by weight. Unless otherwise specified, flotation lasted for 1–2 min. Tap water was used in all flotation tests. To evaluate the collector's performance and optimize the process parameters, flotation tests were carried out at different pHs, collector dosages, diesel percentage and flotation times (kinetic tests). The flotation products were assayed for grade (P2O5%) and acid insolubles (A.I.). Flotation recoveries were calculated based on the analyses of the separation products (concentrates and tailings). Flotation efficiency is a composite parameter used for evaluating the flotation performance. It was calculated from recovery and A.I. rejection in the equation
The QCM-D experiments were conducted at pH 10.0 and 25 °C (± 0.02 °C). The stock solution was prepared by dissolving the appropriate amount of collector with sodium hydroxide in deionized water. To ensure dissolution and degassing, the solutions were left in an ultrasonic bath for 5–10 min. For each experiment, the data generated with the solvent only (sodium hydroxide solution in this study) were accepted as baseline when the process became stable. The fatty acid solutions were injected into the measurement system by a chemical feeding pump capable of precise flow rate control. The flow rate in the experiment was kept at 0.5 mL/min. Software QTools 3.0 was used for data modeling and analysis. For a rigid, thin, uniform film, the change in dissipation factor ΔD is smaller than 10− 6 for a 10 Hz frequency change (Paul et al., 2008). The Sauerbrey equation (Eq. (3)) can be used for mass determination. Δm = −
ρq tq Δf ρq vq Δf CΔf =− =− f0 n n 2f02 n
D=
Edissipated 2πEstored
ð4Þ
where Edissipated is the energy dissipated during one oscillation, and Estored is the energy stored in the oscillating system (Ekholm et al., 2002). When adsorption causes a great shift in the D value (ΔD N 1 × 10−6 ) as a result of the adsorption of a viscous and soft layer, Voigt modeling (Eqs. (5) and (6)) (Voinova et al., 1999) can be used: 1 Δf ≈ − 2πρ0 h0
ΔD ≈ −
A.I. rejection can be calculated from Eq. (2) Tt ′ 100 Cc′ + Tt ′
ð2Þ
in which t′ and c′ are A.I. of tailing and concentrate, respectively.
Table 1 Particle size distribution of phosphate sample.
ð3Þ
where ρq and tq are the density and thickness of quartz crystal, respectively, and vq is the transverse wave velocity in quartz. The constant C has a value of 17.8 ng cm− 2 Hz− 1, and n is the harmonic number (when n = 1, f0 = 5 MHz). If the adsorbed film is “soft” (viscoelastic), it will not fully couple to the oscillation of the crystal (Ekholm et al., 2002), and this will cause energy dissipation of system. The dissipation factor D is proportional to the power dissipation in the oscillatory system (Eq. (4)) and can give valuable information about the rigidity of the adsorbed film (Ekholm et al., 2002):
(
ð1Þ
Flotation efficiency = P2 O5 recovery + A:I: rejection−100
A:I: rejection =
2.3. QCM-D analyses
1 2πf ρ0 h0
" #) 2 ηj ω2 η3 η3 + ∑ hj ρj ω−2hj δ3 δ3 μj2 + η2j ω2 j = 1;2
(
" #) μj ω2 η3 η 2 : + ∑ 2hj 3 δ3 δ3 μj2 + η2j ω2 j = 1;2
ð5Þ
ð6Þ
According to the Voigt model for viscous adsorption layer, Δf and ΔD depend on the density (ρ), thickness (h), elastic shear modulus (µ) and shear viscosity (η) of the adsorption layer, and j is the number of adsorbed layers. The Sauerbrey equation and the Voigt model are the theoretical basis for data modeling in the QCM-D analysis. 2.4. Zeta potential measurements
Size (mesh)
Wt (%)
Grade (P2O5%)
A.I. (%)
Cumulative overscreen (%)
Cumulative overscreen grade (P2O5%)
Cumulative overscreen A.I. (%)
N 30 30–50 50–70 70–100 100–200 b 200
5.47 29.06 51.36 6.34 7.28 0.49
4.13 11.95 6.56 5.07 1.78 4.76
91.44 32.86 75.44 91.41 75.53 92.89
5.47 34.53 85.89 92.23 99.51 100.00
4.13 10.71 8.23 8.01 7.56 7.54
91.44 42.14 62.05 64.07 64.91 65.05
The zeta potential measurements were made with Zeta-plus analyzer of Brook Haven Instruments Corporation. All experiments were conducted with 1 mM KCl solution at laboratory atmosphere and temperature. 1.0 g hydroxyapatite powder was first conditioned in 50 mL of 1 mM KCl solution with magnetic stirrer for 1 h, during which 1.25 mg GP139G75 was added into solution to make the concentration of 25 ppm, and pH was adjusted by NaOH or HCl solutions. The mineral suspension was filtered using Whatman filter
J. Kou et al. / International Journal of Mineral Processing 95 (2010) 1–9
paper (pore size 25 µm) and then poured into the rectangular cell for zeta potential measurements. The pH was measured again at the end of test as the final pH.
3
The infrared transmission spectra were recorded on a Thermo Nicolet Nexus 470 FTIR spectrometer. The hydroxyapatite powders were conditioned with 50 mL collector solution at different pHs and different concentrations, while being agitated with magnetic stirrer for 0.5 h to make 1% suspension. The suspension was then filtered with Whatman filter paper (pore size 25 µm), and the solids were airdried overnight at the room temperature. The samples were prepared by dispersing 0.025 g air-dried powder in 5 g KBr followed by pressing into a transparent tablet for scanning. The untreated (initial) hydroxyapatite powder was used as reference. Each spectrum is an average of 250 scans.
dosage and the A.I. with the plant collector remained higher (i.e. worse) than with the refined tall oil fatty acid. When the refined tall oil fatty acid dosage increased from 0.3 kg/t to 0.45 kg/t, the increase in A.I. was less significant than with the plant collector, which indicates that the refined tall oil fatty acid showed better selectivity and higher acid insolubles rejection than the plant collector. Fig. 2 demonstrates the relationship between P2O5 recovery and grade with the refined tall oil fatty acid and the plant collector to compare the separation sharpness of these two reagents. The results indicate that the flotation P2O5 recovery with the refined tall oil fatty acid was higher than with the plant collector at a given P2O5 grade ranging approximately from 20% to 30%. When the grade decreased from 29.0% to 25.9%, the recovery with the refined tall oil fatty acid increased from 72.1% to 82.5% while the recovery with the plant collector showed no significant increase. All of the above data indicates that the refined tall oil fatty acid performed better than the plant collector.
3. Results and discussion
3.2. Effect of diesel dosage
3.1. Effect of collector dosage
Diesel plays an important role in phosphate flotation. It acts as solvent or booster of tall oil fatty acid collectors and has a significant effect on foam controlling (Snow et al., 2004). To understand the effect of diesel on the performance of the collectors, flotation tests were conducted by adding the mixture of the refined tall oil fatty acid and diesel as collector at the same dosage but in different ratios (by weight) at pH 9.5 and the results of flotation recovery and P2O5 grade are presented in Fig. 3. Fig. 3 shows that the P2O5 recovery increased with increasing the diesel percentage from 10% to 50% at both dosages and the highest recovery was achieved when the ratio of the refined tall oil fatty acid to diesel was 1:1. However, a further increase in the diesel percentage from 50% to 80% decreased the recovery from 88.4% to 32.9% at the dosage of 0.9 kg/t and from 63.9% to 15.7% at the dosage of 0.6 kg/t. The highest grade was achieved at 30% diesel percentage with 0.6 kg/t collector and 60% diesel percentage with 0.9 kg/t collector. It is interesting to see that 60.1% recovery and 17.5% P2O5 grade were achieved at 30% diesel percentage with 0.27 kg/t diesel and 0.63 kg/t GP193G75 but 63.9% recovery and 15.8% P2O5 grade were also obtained at the dosage of only 0.3 kg/t GP 193G75 with 0.3 kg/t diesel. This indicates that increasing diesel ratio may decrease the consumption of the refined tall oil fatty acid but with insufficient amount of collector, increasing diesel ratio cannot achieve optimum performance. When both recovery and grade are considered at the same time, a collector dosage of 0.9 kg/t was significantly better than that of 0.6 kg/t.
2.5. FTIR analysis
The effects of dosage on rougher phosphate flotation performance were studied by a series of flotation tests with the refined tall oil fatty acid in conjunction with the plant collector. The pH, conditioning time and impeller rotating speed were fixed constant at pH 10, 6 min and 1500 rpm, respectively and the ratio of collector to diesel was 3:2. The dependence of flotation efficiency and A.I. on collector dosage is shown in Fig. 1. It was quite obvious that the refined tall oil fatty acid generated higher flotation efficiency than the plant collector at all three dosages tested. The flotation efficiency increased significantly with increasing dosage of the refined tall oil fatty acid from 0.15 kg/t to 0.3 kg/t, but decreased slightly when the refined tall oil fatty acid dosage increased further from 0.3 kg/t to 0.45 kg/t. When the plant collector dosage increased from 0.15 kg/t to 0.3 kg/t, the increase in flotation efficiency was less significant than for the refined tall oil fatty acid. The highest flotation efficiency of 79.5% was achieved with 0.3 kg/t of the refined tall oil fatty acid, which was about 10% higher than the flotation efficiency with the plant collector at the same dosage or more than 6% higher than the maximum flotation efficiency achieved with the plant collector at 0.45 kg/t. However, a further increase in the dosage to 0.45 kg/t decreased the flotation efficiency to 77.8%, which was still about 5% higher than the plant collector at the same dosage. The concentrate A.I. increased with increasing the
Fig. 1. Effect of collector dosage on flotation efficiency and concentrate A.I.
Fig. 2. Relationship between the P2O5 recovery and grade with GP193G75 and plant collector at different dosage.
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Fig. 3. Effect of diesel percentage on flotation recovery and grade (pH 9.5, dosage of GP193G75 and diesel mixture at 0.6 kg/t and 0.9 kg/t).
3.3. Effect of pH Flotation tests were conducted to investigate the effect of slurry pH on the collector performance in apatite flotation by varying the pH from 9.5, 10, to 10.5. Fig. 4 demonstrates the flotation efficiency/A.I. as a function of slurry pH in the presence of 0.3 kg/t of the refined tall oil fatty acid or the plant collector conditioned for 6 min. It can be seen that the flotation efficiency increased with increasing pH values and the refined tall oil fatty acid showed significantly higher flotation efficiency than the plant collector at all three pHs. The flotation efficiency obtained with the refined tall oil fatty acid increased from 72.8% to 82.6% as the pH increased from 9.5 to 10.5. Meanwhile, the A. I. with the refined tall oil fatty acid and the plant collector increased with increasing slurry pH but was always lower with the refined tall oil fatty acid at all three pHs, which indicates that the refined tall oil fatty acid results in better selectivity than the plant collector. The relationship between P2O5 recovery and grade with the refined tall oil fatty acid and the plant collector is shown in Fig. 5. The curves closer to the upper right corner represent more efficient separation. Compared with the plant collector, the refined tall oil fatty acid collector increased P2O5 recovery by 6% at the same concentrate grade. Obviously, the refined tall oil fatty acid has a better flotation performance than the plant collector. It can also be concluded from the results that higher pulp pH had positive effects on phosphate
Fig. 4. Effect of slurry pH on flotation efficiency and concentrate A.I.
Fig. 5. Relationship between the P2O5 recovery and grade with GP193G75 and plant collector at different pH.
flotation. This observation is in agreement with the previous studies. Feng and Aldrich (2004) investigated the effect of some operating parameters such as pulp pH and collector dosage on the kinetics of apatite flotation and obtained the best flotation performance at a pH level of 12.3 with fatty acid and sulphonate as collector. They reported that an elevated pulp pH increased the flotation recovery and kinetics, probably by softening the process water and speeding up the electrolysis of the fatty acid. According to the work by Robert Pugh and Per Stenius (1984), the minimum surface tension and the formation of pre-micella associated species occurred at higher pH in lower concentrations of sodium oleate solution, which attributed to the better flotation recovery of apatite. 3.4. Kinetic flotation tests The kinetics of flotation studies the variation in floated mineral mass as a function of flotation time. To characterize the effect of different collectors on flotation rate, kinetic flotation tests were performed with both the refined tall oil fatty acid and plant collector. During the tests concentrate samples were collected at a time interval of 10 s in the first 30 s and also finally at 90 s. Both reagents were tested under the same conditions. The dosage of collector was 0.45 kg/t mixed with 0.4 kg/t fuel oil and the pH of ore pulp was 10. The results of recovery and concentrate grade versus flotation time are shown in Fig. 6. It can be seen that the recovery increased about 30% in the first
Fig. 6. P2O5 grade and recovery versus flotation time.
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30 s with both the refined tall oil fatty acid and the plant collector and additional 7% and 11.7% in the last 60 s, respectively. The grade of concentrate generated by the plant collector decreased 5% with the increase in recovery in the first 30 s. On the other hand no grade decrease was observed with the refined tall oil fatty acid during the same period of time. At 90 s the refined tall oil fatty acid showed a higher recovery of 91.7% and a higher grade of 26.5% compared to the plant collector (88.6% recovery and 20.6% grade). The above data shows that the refined tall oil fatty acid had considerably better selectivity and flotation efficiency than the plant collector. 3.5. Adsorption behavior of fatty acid onto hydroxyapatite surface To characterize the adsorption behavior of the refined tall oil fatty acid and plant collector on the hydroxyapatite surface, a highly sensitive in-situ surface characterization technique QCM-D was em-
5
ployed in conjunction with zeta potential measurement and FTIR spectra analysis. 3.5.1. QCM-D measurements Fig. 7 shows the real-time response curves of frequency shift (Δf) and dissipation shift (ΔD) from the third overtone (15 MHz) associated with the refined tall oil fatty acid and the plant collector adsorption onto a hydroxyapatite surface at a concentration of 500 ppm. Fig. 7A displays the frequency shift (Δf) for the refined tall oil fatty acid and the plant collector adsorption on the hydroxyapatite surface. Arrow a indicates the beginning of injection of collector solution into the system. It can be observed that after the injection of the refined tall oil fatty acid, Δf had an immediate sharp decrease simultaneous with a sharp increase in ΔD (Fig. 7B, arrow a). These sharp changes in Δf and ΔD indicate the quick adsorption of the refined tall oil fatty acid on the apatite surface and the high ΔD value (ΔD N 1E
Fig. 7. QCM-D experimental data of frequency shift (A) and dissipation shift (B) (measured at 15 MHz) of GP193G75 and plant collector adsorption onto a hydroxyapatite surface at concentration of 500 ppm.
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−6) suggests the formation of a dissipated layer. The highest adsorption thickness obtained at arrow b was 70 nm which was calculated using the Voigt model. It should be noted that Δf continued to increase after the adsorption thickness reached the maximum and a steady state was not reached until after 90 min at arrow c. The adsorption thickness at the stable frequency shift was 11 nm. This phenomenon has rarely been reported in the previous studies of adsorption of proteins and surfactants. According to the work by Mielczarski and Mielczarske (1995), who applied the FTIR reflection spectroscopy in the analysis of the molecular orientation and thickness of oleate adsorption on the apatite surface, the oleate monolayer on the apatite was well-organized, but the second layer adsorbed on top of the first well-ordered layer was randomly spread and oriented almost parallel to the interface. Since the FTIR is an ex-situ analysis method and QCM-D is an in-situ analysis technique, this phenomenon of increase in Δf (Fig. 7A, from arrow b to arrow c) probably reflected the orientation process of the second and subsequent layers of molecules, which led to a low free energy state in the solution system. As reported by Mielczarski and Mielczarske (1995), water molecules and calcium ions on the surface of apatite formed an intramolecular layer, which was not uniform for the carboxylate group. The higher ΔD at arrow b probably indicated the formation of soft and water-rich multi-layers of fatty acid on the apatite surface at the very beginning. ΔD decreased with the increase of Δf (Fig. 7B from arrow b to arrow c), which indicated the formation of more rigid multi-layers due to the
change in orientation of top layers of molecules and the loss of water caused by layer compression. Compared with the refined tall oil fatty acid, the plant collector generated a gradual decrease of Δf and increase of ΔD. However, Δf reached a stable value after 30 min when the ΔD value was still increasing. This indicates a slow adsorption process as well as the formation of much softer and more porous multi-layers. Rodahl et al. (1997) postulated that if the porous film is deformed by shear oscillation, liquid can be “pumped” in and out of the film and also be subjected to oscillatory “flow” within the film as the pores in the film change in shape and size. This may cause the continuing increase of ΔD even when the Δf was stable. The final thickness of the plant collector adsorption layer on the surface of hydroxyapatite calculated using the Voigt model was 4 nm, which was not only thinner, but also more dissipated and less rigid (plant collector ΔD N the refined tall oil fatty acid ΔD) than the adsorption layer for the refined tall oil fatty acid. The frequency shift and dissipation shift caused by adsorption of the refined tall oil fatty acid and the plant collector at a concentration of 1000 ppm showed almost the same trend as at 500 ppm. It should be noted that the stable Δf at 1000 ppm appeared 1.3 h earlier than at 500 ppm for the refined tall oil fatty acid. This indicates that the refined tall oil fatty acid adsorbed on the hydroxyapatite surface more rapidly at higher concentrations. The measured frequency and dissipation shifts at 500 ppm and 1000 ppm were summarized as ΔD–Δf plots in Fig. 8A–D. ΔD–Δf plots
Fig. 8. ΔD–Δf plots for (A) plant collector at 500 ppm, (B) plant collector at 1000 ppm, (C) GP193G75 at 500 ppm, and (D) GP193G75 at 1000 ppm, adsorbed on the hydroxyapatite surface.
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can provide information on the energy dissipation per unit mass added to the crystal. The slope of the plots was defined as K (K = ΔD/Δf, absolute value) which is indicative of kinetic and structure alternation during adsorption process (Paul et al., 2008). A more rigid and compact adsorption mass is expected to yield a small K value, and a soft and dissipated layer is expected to yield a higher K value. Only one slope is associated with the adsorption process without kinetic or conformational change. More than one slope suggests the adsorption associated with orientation change or hydrodynamically-coupled water in the adsorbed layer (Paul et al., 2008; Rodahl et al., 1997). Fig. 8A and B shows the ΔD–Δf plots for the adsorption of the plant collector on the hydroxyapatite surface at 500 ppm and 1000 ppm, respectively. Two slopes were observed based on the gross trend of curves in Fig. 8A and B. It can be observed that the 500 ppm and the 1000 ppm tests have similar K1 values of 0.106 and 0.092, respectively, while K2 was higher than K1 at both concentrations, and the trendlines were almost vertical to the X axis. This indicates that the initially-formed layer was less dissipated, but with the increase of adsorbed mass and trapped water molecules, it was getting softer and more dissipated. At the second stage with a slope of K2, the ΔD increased rapidly while the Δf was stable at about − 10 Hz. As mentioned before, this elevated ΔD was probably due to the structural change of the adsorbed layer caused by trapped water. Fig. 8C and D shows the ΔD–Δf plots for the adsorption of 500 ppm and 1000 ppm of the refined tall oil fatty acid on the hydroxyapatite surface. The trend of the curve shows three distinguishable stages (three K values) in the adsorption process. Comparing the K values in Fig. 8C and D reveals that K2 (0.159 at 500 ppm and 0.029 at 1000 ppm) was smaller than K1 (0.305 at 500 ppm and 0.220 at 1000 ppm). This indicates that although the initial adsorption of the refined tall oil fatty acid on hydroxyapatite was rapid, the adsorbed layer was soft and dissipated. With decreasing Δf, ΔD increased slowly at the second stage, which yielded the lower K2. It must be noted that, in comparison with the plant collector, the refined tall oil fatty acid has a third adsorption stage at which the Δf increased as the ΔD decreased to smaller than 1E−6. This indicates the loss of water caused by layer compression or the change in orientation of molecules. The dissipation shift per unit mass at this stage was the same as the first stage. 3.5.2. Zeta potential measurements Fig. 9 shows the zeta potentials of pure hydroxyapatite before and after conditioning with the refined tall oil fatty acid or the plant collector at the constant ionic strength with respect to pH. It can be seen that the zeta potential of pure hydroxyapatite was zero at pH 4 and the shapes of curves agreed well with the results reported by Rao
Fig. 9. Zeta potential of hydroxyapatite and hydroxyapatite conditioned with 25 ppm GP193G75 and plant collector at constant ionic strength as a function of pH.
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et al. (1990). The results indicate that after conditioning with collectors, the zeta potential of hydroxyapatite became more negatively charged and shifted the i.e.p. towards lower than pH 4. In addition, the zeta potential with the refined tall oil fatty acid was lower than that with the plant collector at pHs ranging from 5 to 11, which is in good agreement with the QCM-D results that showed the refined tall oil fatty acid is associated with higher affinity and adsorption density than the plant collector. According to Rao et al. (1990), the more negative charge of apatite as a result of conditioning with sodium oleate indicates the high affinity of oleate with surface Ca-sites. Either the chemisorption of the refined tall oil fatty acid, which forms the monocoordinated complex (i.e. 1:1 oleate-lattice calcium complex) or the precipitation of its calcium salt may take place, especially at high concentrations of the collector. As shown in QCM-D results, the sharply decreased frequency shift from the beginning of adsorption demonstrates that the consistent mass increase happened on the hydroxyapatite surface due to the simultaneous chemisorption and surface precipitation on hydroxyapatite, which formed the 70 nm adsorption layer at 500 ppm solution with the refined tall oil fatty acid. The QCM-D results show not only the adsorption process, but also the process of layer compression and molecular orientation (K2 b K1). According to Mielczarski et al. (1993), the hydrophobicity of apatite is closely related to the structure of the adsorbed layer on the surface, wherein the higher packing density involves hydrophobic character of sample, a poorly organized structure does not produce a high hydrophobicity of apatite. When the adsorption became stable, the adsorbed layer of the refined tall oil fatty acid had a lower ΔD than the plant collector, which indicated better organized structure of the refined tall oil fatty acid on hydroxyapatite than the plant collector, which caused higher hydrophobicity of hydroxyapatite conditioned with the refined tall oil fatty acid. 3.5.3. FTIR spectra Infrared transmission spectra of hydroxyapatite after adsorption in the refined tall oil fatty acid solution at different pHs and concentrations are shown in Fig. 10 and Fig. 11. According to many researchers (Antti and Forssberg, 1989; Mielczarski et al., 1993; Rao et al., 1991), the absorbance bands which can provide useful information about the nature of the adsorbed fatty acid such as oleic acid can be observed in two regions of frequency, i.e., (1) most peaks at wavenumbers from 3100 cm− 1 to 2900 cm− 1 are related to the hydrocarbon chain. (2) Most peaks at wavenumbers from 1700 cm− 1 to 1400 cm− 1 are
Fig. 10. Infrared transmission spectra of hydroxyapatite after adsorption of GP193G75 at different concentrations and pHs (concentration ppm/pH) for the alkyl chain region 2750–3100 cm− 1.
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In summary, infrared studies showed that the adsorption of the refined tall oil fatty acid on apatite involves both chemical reaction between the carboxylate species and surface calcium ions and surface precipitation, which agrees well with the zeta potential and QCM-D measurements. 4. Conclusions
Fig. 11. Infrared transmission spectra of hydroxyapatite after adsorption of GP193G75 at different concentrations and pHs (concentration ppm/pH) for the carboxylate group region 1600–1400 cm− 1.
related to the carboxylate radical. It must be noted that the infrared spectra in the case of calcite are found to be hard to interpret due to the highly interfering carbonate absorption in the same frequency region as that of the carboxylate radical (Rao and Forssberg, 1991). Fig. 10 shows the typical absorption bands of alkyl chain in the region from 2750 cm− 1 to 3100 cm− 1, wherein the 2923 cm− 1 and 2852 cm− 1 bands were the asymmetric and symmetric stretching vibrations in the CH2 radical, respectively and 2956 cm− 1 was the asymmetric stretching vibration in the CH3 radical. Even though the pH did not show significant effect on the intensities of adsorption peaks, the spectra in alkyl region showed absorption bands in the range of 2750–3100 cm− 1 and the intensities of these bands were found to increase with increasing concentration of the refined tall oil fatty acid, as shown in Fig. 10. Fig. 11 shows the infrared transmission spectra of hydroxyapatite after adsorption of the refined tall oil fatty acid at different concentrations and pHs for the carboxylate group region from 1400 cm− 1 to 1600 cm− 1. Since the hydroxyapatite sample itself shows characteristic absorption bands between 1400 cm− 1 and 1500 cm− 1 and the carboxyl absorbance bands appear in the narrow range between 1540 cm− 1 and 1580 cm− 1, it is difficult to identify the typical absorption bands of carboxyl. It was observed that the carboxylate group in the refined tall oil fatty acid gave rise to characteristic absorption bands in the region of 1600–1400 cm− 1 at the higher concentration of 1000 ppm, wherein the 1450 cm− 1 band corresponds to the asymmetric deformation of the CH3 radical. In ionized fatty acids, the band at 1710 cm− 1 is replaced by two new bands due to vibrations in the COO– radical including 1610– 1550 cm− 1 and 1420–1300 cm− 1 (Antti and Forssberg, 1989). The adsorption bands that occur at 1576 cm− 1 and 1537 cm− 1 are assigned to the asymmetric stretching vibration of COO– radical. This corresponds to the monocoordinated complex or the surface precipitation of calcium salt fatty acid at the concentration of 1000 ppm, which is also demonstrated by QCM-D measurements that showed the frequency shift decreased significantly at the beginning of adsorption as a result of the formation of a thick and soft adsorption layer. According to Mkhonto et al. (2006), all surfactants containing carbonyl and hydroxy groups interact strongly with the apatite surfaces and bridging between two or more surface calcium ions of apatite is the preferred mode of surfactant adsorption.
The performance of a commercial phosphate collector and a refined tall oil fatty acid was investigated in this study. Flotation tests were performed under different process parameters. It was found that under the same process conditions, the refined tall oil fatty acid achieved a considerably better flotation performance than the plant collector. The recovery and concentrate grade were 91.7% and 26.5%, respectively when the refined tall oil fatty acid was used at a dosage of 0.45 kg/t at pH 10.0 with a 9:8 (by mass) concentration ratio of fatty acid to diesel. High pulp pH was found to have positive effects on phosphate flotation. Use of diesel as synergist can significantly decrease the dosage of the refined tall oil fatty acid, but an increase in diesel dosage cannot achieve good performance without a sufficient amount of the refined tall oil fatty acid. The QCM-D results indicated that the refined tall oil fatty acid had different adsorption behavior than the plant collector. The plant collector had two adsorption stages and K2 was greater than K1, which means the adsorbed layer become more dissipated with increasing adsorption time. On the contrary, the adsorption layer of the refined tall oil fatty acid became more rigid when K2 was smaller than K1. It also showed a compression stage represented by K3. Therefore, the adsorption layer of the refined tall oil fatty acid formed on a hydroxyapatite surface was more organized and rigid than the plant collector at the concentrations of 500 and 1000 ppm. Since the higher packing density of adsorbed layer results in hydrophobic character and a poorly organized structure does not produce a high hydrophobicity, the hydroxyapatite surface adsorbed by the refined tall oil fatty acid had higher hydrophobicity than the plant reagent after the adsorption reached the steady state. The results of zeta potential measurements and FTIR analyses indicated strong electrostatic interaction and high affinity of the refined tall oil fatty acid on the hydroxyapatite surface. Both chemisorption and surface precipitation mechanisms were demonstrated by FTIR analysis and QCM-D measurement. References Antti, B.M., Forssberg, K.S.E., 1989. Pulp chemistry in industrial mineral flotation, studies of surface complex on calcite and apatite surfaces using FTIR spectroscopy. Miner. Eng. 2, 217–227. Ekholm, P., Blomberg, E., et al., 2002. A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface. J. Colloid Interface Sci 247, 342–350. Feng, D., Aldrich, C., 2004. Influence of operating parameters on the flotation of apatite. Miner. Eng. 17, 453–455. Hook, F., Rodahl, M., Brzezinski, P., Kasemo, B., 1998. Energy dissipation kinetics for protein and antibody–antigen adsorption under shear oscillation on a quartz crystal microbalance. Langmuir. 14, 729–734. Mielczarski, J.A., Mielczarske, E., 1995. Determination of molecular orientation and thickness of self-assembled monolayers of oleate on apatite by FTIR reflection spectroscopy. J. Phys. Chem 99, 3206–3217. Mielczarski, J.A., Cases, J.M., Bouquet, E., Barres, O., Delon, J.F., 1993. Nature and structure of adsorption layer on apatite contacted with oleate solution. 1. Adsorption and fourier transform infrared reflection studies. Langmuir. 9, 2370–2382. Mkhonto, D., Ngoepe, P.E., Cooper, T.G., Leeuw, N.H., 2006. A computer modeling study of the interaction of organic adsorbates with fluorapatite surfaces. Phy. Chem. Minerals 33, 314–331. Paul, S., Paul, D., Tamara, B., Ray, A.K., 2008. Studies of adsorption and viscoelastic properties of proteins onto liquid crystal phthalocyanine surface using quartz crystal microbalance with dissipation technique. J. Phys. Chem. C. 112, 11822–11830. Pugh, R., Stenius, P., 1984. Solution chemistry studies and flotation behavior of apatite, calcite and fluorite minerals with sodium oleate collector. Int. J. Miner. Process. 15, 193–218. Rao, K.H., Forssberg, K.S.E., 1991. Mechanism of fatty acid adsorption in salt-type mineral flotation. Miner. Eng. 4, 879–890. Rao, K.H., Antti, B.M., Forssberg, E., 1990. Mechanism of oleate interaction on salt-type minerals, part II. Adsorption and electrokinetic studies of apatite in the presence of sodium oleate and sodium metasilicate. Int. J. Miner. Process. 28, 59–79.
J. Kou et al. / International Journal of Mineral Processing 95 (2010) 1–9 Rao, K.H., Cases, J.M., Forssberg, K.S.E., 1991. Mechanism of oleate interaction on salttype minerals V. Adsorption and precipitation reactions in relation to the solid/ liquid ratio in the synthetic fluorite-sodium oleate system. J. Colloid Interface Sci 145, 330–348. Rodahl, M., Hook, F., Fredrikson, C., et al., 1997. Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion. Faraday Discuss. 107, 229–246. Sis, H., Chander, S., 2003. Reagents used in the flotation of phosphate ores: a critical review. Miner. Eng. 16, 577–585.
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Snow, R., Zhang, P., Miller, J.D., 2004. Froth modification for reduced fuel oil usage in phosphate flotation. Miner. Eng. 74, 91–99. Voinova, M.V., Rodahl, M., et al., 1999. Viscoelastic acoustic response of layered polymer films at fluid–solid interfaces: continuum mechanics approach. Phys. Scr. 59, 391–396. Zhang, P., Yu, Y., Bogan, M., 1997. Challenging the “Crago” double floate process II. Amine-type-fatty acid flotation of silicious phosphates. Miner. Eng. 10, 983–994.