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Flotation of monazite in the presence of calcite part I: Calcium ion effects on the adsorption of hydroxamic acid
Minerals Engineering 100 (2017) 40–48
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Flotation of monazite in the presence of calcite part I: Calcium ion effects on the adsorption of hydroxamic acid W. Zhang, R.Q. Honaker ⇑, J.G. Groppo Department of Mining Engineering, University of Kentucky, Lexington, KY, USA
a r t i c l e
i n f o
Article history: Received 19 July 2016 Revised 6 September 2016 Accepted 24 September 2016
Keywords: Flotation Monazite Calcite Calcium ions Hydroxamic acid
a b s t r a c t The dissolution of minerals provides elevated concentrations of alkaline earth metal ions such as calcium (Ca2+) in water which may have a significant influence on flotation performance. The effects of Ca2+ ions dissolved from calcite surfaces on the flotation performance of a monazite-calcite system have been investigated using micro-flotation tests followed by fundamental collector adsorption studies. The results indicate that specially adsorbed CaOH+ via hydrogen bonding competed with hydroxamic acid for the phosphate-oxygen active sites on the monazite surface and decreased its hydrophobicity due to hydration. Electrostatically adsorbed CaOH+ on monazite increased the adsorption density of hydroxamic acid through a chelating interaction. At higher dosages of calcium and hydroxamic acid, bulk and surface precipitation of calcium-hydroxamate occurs in the monazite-calcium-hydroxamic acid system. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth elements (REEs) are a group of 15 lanthanide elements (atomic numbers 57–71) and two transition metal elements (scandium and yttrium). There is a high demand for REEs due to their applications in catalysts, alloys, magnets, and glass polishing (Chelgani et al., 2015; Zhang et al., 2015). Monazite ((Ce, La, Y, Th)PO4) is a commercial rare earth mineral and usually associated with heavy minerals such as zircon, rutile, and ilmenite (Abeidu, 1972; Cheng et al., 1993; Pavez and Peres, 1994). Besides gravity, magnetic, and electrostatic separation methods, flotation has been widely used in monazite concentration due to its advantages in fine particle processing (Cheng et al., 1993). Monazite flotation has been investigated by many researchers (Abeidu, 1972; Cheng, 2000; Cheng et al., 1993; Cross and Miller, 1989; Dixit and Biswas, 1969; Pavez and Peres, 1994, 1993; Qi, 1993). Satisfactory flotation performance is achieved at pH 8–9 by using sodium oleate as a collector for monazite and sodium silicate as a depressant for zircon, rutile, and ilmenite (Cheng et al., 1993; Pavez and Peres, 1994, 1993; Qi, 1993). However, studies on the flotation separation of monazite from carbonate minerals such as calcite and dolomite or phosphate minerals such as apatite are limited. As reported by Chan (1991), small amounts of dolomite and apatite exists in a monazite ore located in Mt Weld, Australia. Moreover, for some promising rare earth
⇑ Corresponding author. http://dx.doi.org/10.1016/j.mineng.2016.09.020 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.
secondary resources, such as coal fine refuse and phosphates, monazite is usually associated with carbonate and phosphate minerals (Kandil et al., 2010; Zhang et al., 2015). Minerals such as calcite, dolomite, and apatite belong to sparingly soluble minerals. Lattice ions of these minerals such as calcium ions in calcite are dissolved from the mineral surfaces into solution, which influences collector adsorption and thus flotation performance. A number of studies have been previously conducted to evaluate calcium and multivalent cation adsorption on mineral surfaces (Ananthapadmanabhan and Somasundaran, 1985; Atesok et al., 1988; Clark and Cooke, 1968; James and Healy, 1972). Adsorption of calcium on minerals such as silica and Na-kaolinite usually increased sharply at pH values above 10 where significant amounts of calcium ions hydrolyze to CaOH+ ions and Ca(OH)2 precipitates (Atesok et al., 1988; Clark and Cooke, 1968). The higher affinity of CaOH+ compared with Ca2+ is mainly due to the decrease in the unfavorable solvation energy and the increase in favorable chemical energy changes (Ananthapadmanabhan and Somasundaran, 1985; James and Healy, 1972). Specific adsorption of calcium hydrolyzed species is well documented for the apatite/calcite system (Amankonah et al., 1985; Ananthapadmanabhan and Somasundaran, 1984; Mishra, 1978; Somasundaran, 1968; Somasundaran and Agar, 1972). In addition, the free Ca2+ ions attach to apatite surfaces in the form of Ca5(PO4)3(OH) via the following equation (Elgillani and Abouzeid, 1993):
Ca5 ðPO4 Þ3 ðOHÞðsÞ þ 4Hþ ¼ 3CaHPO4ðaqÞ þ 2Ca2þ þ H2 O:
W. Zhang et al. / Minerals Engineering 100 (2017) 40–48
Despite similarities in the calcium adsorption mechanisms on mineral surfaces, the effects of calcium ions on flotation are more complicated and have been found to be a function of mineral characteristics, calcium ion concentration, collector type, solution pH values, etc. (Clark and Cooke, 1968; Demir et al., 2003; Liu et al., 2015; El-Salmawy et al., 1993; Santos et al., 2010; Scott and Smith, 1993). Calcium ions serve as an activator for quartz and spodumene when using anionic collectors such as sodium oleate and sodium alkysulfonates (Clark and Cooke, 1968; El-Salmawy et al., 1993; Liu et al., 2015). However, depression of quartz and Na-feldspar occurred in the presence of calcium ions when using a cationic amine collector (Demir et al., 2003; Scott and Smith, 1993). In the current study, the separation of monazite from calcite using froth flotation and hydroxamic acid as the collector was studied systematically using a micro-flotation apparatus. The effects of calcium ions on the adsorption of hydroxamic acid onto monazite surfaces were fundamentally investigated through electrophoresis and adsorption studies and supported by Fourier transform infrared spectroscopy (FTIR) analysis. Finally, an interaction model for the adsorption of hydroxamic acid onto monazite in the presence of calcium ions was proposed. 2. Material and methods 2.1. Materials Monazite pre-concentrate samples collected from a rare earth mine in China were purified using a shaking table and highgradient magnet. A monazite concentrate with a purity level of nearly 98% was obtained as determined by inductively coupled plasma mass spectrometry (ICP-MS). The X-ray diffraction analysis indicated that a minor amount of cerium oxide existed in the sample. Chunk samples of calcite collected from a limestone mine located in Kentucky, USA, were crushed and ground to minus 300 lm. X-ray diffraction analysis showed that calcite content in the sample was more than 95% and thus directly used as pure minerals. For the micro-flotation tests, the monazite concentrate and calcite material were screened to obtain material having a particle size smaller than 300 lm and larger than 150 lm. The remaining samples were ground using a corundum pestle and mortar for zeta potential, adsorption, Fourier transform infrared spectroscopy (FTIR) tests. Octanohydroxamic acid of minimal 95% purity was purchased from TCI America. Methyl isobutyl carbinol (MIBC) of more than 99% purity was used as frother and purchased from Alfa Aesar. Other chemicals were of reagent grade and purchased from We Enable Science. Type I deionized water with a resistivity value of 18.2 MXcm at 25 °C was used for all the experiments. 2.2. Methods 2.2.1. Micro-flotation tests A micro-flotation cell with a volume of 50 ml was used for the flotation tests (Fig. 1). During the tests, a suspension of the particles was achieved using a magnetic stirrer. To reduce interference of carbon dioxide, air bubbles were produced by passing pure nitrogen through a porous frit. Conditioning was conducted in a glass beaker with one gram of pure minerals in 50 ml solution. For the mixed mineral flotation tests, the ratio of monazite-tocalcite was 1:1 by weight. Solution pH values were first adjusted to predetermined values followed by the addition of metal salts and/or collectors with each conditioned for 5 min. Frother was
41
Fig. 1. Schematic diagram of the micro-flotation apparatus.
added last followed by 1 min of conditioning. The solution pH values were measured and adjusted during conditioning every 2.5 min and just prior to the start of the flotation test. Given that the majority of the material floated within the first 30 s of the test, the initial pH values were considered to be more representative of the flotation conditions. The difference in the pH values measured before and after each flotation test was typically 0.2 pH units. During the flotation tests, nitrogen flow rates were maintained at 55 ml/min while froth products were continuously removed over a period of 4 min. The concentrate and tailing samples were dried in an oven for 12 h and then cooled in a desiccator. Finally, flotation recovery values were calculated based on the sample weights. For the mixed mineral flotation tests, monazite and calcite recovery values were calculated by using the assay values obtained from acid digestion. Three repeat tests were conducted under the same condition and the experiment error measured from the recovery data indicated a standard deviation value of 1.10%. 2.2.2. Adsorption tests Adsorption tests were conducted in a 20 ml glass beaker. Solid concentration values of 0.5% and 1% were used for monazite and calcite, respectively. Conditioning procedures were similar to that used for micro-flotation tests. After conditioning, 10 ml of the suspension was removed to a centrifuge tube and centrifuged in an IEC Clinical centrifuge at 3175 rpm. Hydroxamic acid concentration was determined using a Shimadzu 1280 UV–vis spectrophotometer based on the ferric hydroxamate method. Ferric chloride (5 mM) of 10 ml was added into 5 ml of centrifuged supernatant in a glass tube. Full wave scanning of the obtained solution showed that peak absorption occurred at 503 nm which was used for hydroxamic acid determination. For the adsorption tests at pH 11.0, pH values of the supernatant were readjusted to below 9.0 before ferric chloride addition to avoid the precipitation of ferric ions. The calcium concentration in solution was determined using induced coupled plasma (ICP) analysis. Three tests were conducted under the same condition and a standard deviation value of 0.30 lM/m2 was obtained. 2.2.3. Zeta-potential tests Zeta potential tests were conducted to study the effects of calcium ions on the electro-kinetic properties of monazite particles. A solids concentration of 0.05% by weight was used and conditioning procedures were similar to that used for the micro-flotation tests. Before conditioning, the solution was subjected to sonification for 1 min to thoroughly disperse the solid particles. A potassium chloride solution of 1 mM was used as a supporting electrolyte for all measurements. A standard deviation value of
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2.71 mV was determined from four repeat tests when conducted under the same conditions. 2.2.4. Fourier transform infrared spectroscopy (FTIR) analysis Pure monazite and calcite were ground to minus 2 lm for FTIR tests to avoid light scattering. Conditioning procedures were similar to that used for the flotation tests. After conditioning, the sample was centrifuged and the supernatant decanted followed by three repeated rinses to remove any residual or physically adsorbed collectors. FTIR tests based on the attenuated total reflection (ATR) method were conducted using a Varian 7000e spectrometer. A few milligrams of mineral powders was placed directly on the optic window. The analysis was conducted from 4000 to 700 cm1 using 32 scans with a resolution of 4 cm1. The FTIR results were analyzed using the drawing and peak fitting tools of the Original software. 3. Results 3.1. Micro-flotation tests Mineral floatability is closely related to solution pH values, which influences the solution species of collectors and ions present in solution. Fig. 2 shows the flotation recovery results obtained when monazite and calcite are floated separately in singlemineral flotation tests using a collector dosage of 1 104 M. A maximum recovery of around 90% was obtained for monazite at pH 9.0 while calcite recovery at the same pH value was only around 22%%. Maximum calcite recovery was realized at pH 10.0. The results imply that an effective separation could be realized using hydroxamate at a solution pH of 9.0. However, tests results obtained when mixing the two minerals at a 1:1 ratio provided a different outcome. While maintaining the solution pH at a value of 9.0, hydroxamate dosage was varied in tests involving both single-mineral and mixed-mineral flotation systems. As shown in Fig. 3, the presence of calcite suppressed the flotation of monazite when using hydroxamate dosages less than 2.5 104 M. Furthermore, the recovery of calcite reached values approaching 100% at high collector dosage levels. As such, selectivity was significantly lower in the mixed mineral system and declined substantially with an increase in hydroxamate concentration. At the dosage of 1 104 M as used in the pH experiments (Fig. 2), the monazite recovery in the
Fig. 3. Effects of hydroxamic acid concentration on monazite and calcite flotation at pH 9.0.
Fig. 4. Effects of calcium and carbonate ions on the flotation of monazite using 1 104 M hydroxamic acid.
mixed-mineral system was only around 40% compared to the 90% value obtained from the single mineral tests while the calcite recovery remained relatively unchanged. Calcite is a sparingly soluble mineral and crystal ions will dissolve from its surfaces in water. As previously discussed, the influence of calcium and carbonate ions on flotation have been reported in literature. Fig. 4 shows the effects of calcium and carbonate ions on monazite flotation. Monazite recovery values decreased exponentially with an increase in calcium ion concentration while carbonate ions showed negligible effects. Therefore, the depression in monazite recovery in the presence of calcite shown in Fig. 3 may be attributed to the dissolution of calcium ions from the calcite surfaces. 3.2. Adsorption tests
Fig. 2. Effects of pH values on monazite and calcite flotation when using 1 104 M hydroxamic acid.
In an effort to develop a fundamental understanding of the flotation results, adsorption density was measured over a range of solution pH values and calcium ion concentrations. As shown in Fig. 5, maximum adsorption density values for the monazite and calcite were obtained at pH 9.0 and 10.0, respectively, which
W. Zhang et al. / Minerals Engineering 100 (2017) 40–48
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2.1 lM/m2 with an elevation in the calcium concentrations from 5 105 to 5 103 M (2–200 ppm). For concentrations below 5 105 M, adsorption tests were not conducted due to the lower limits of the ICP-OES analyzer. However, it is reasonable to assume that the adsorption density was below 1.5 lM/m2 for the lower calcium ion concentrations. Monolayer coverage of bare dehydrated Ca2+ ions corresponds to an adsorption density of 52.9 lM/m2. However, James and Healy (1972) found that inner hydration sheaths of metallic cations remain adsorbed on a silica surface. The radius used for monolayer coverage calculations was the radius of bare cations plus the diameter of a water molecule. In this case, the monolayer coverage of calcium corresponds to 3.76 lM/m2. Using this value, only half of the monazite surface was covered by calcium ions at pH 9.0 within the studied calcium concentrations. 3.3. Electro-kinetic tests Fig. 5. Effect of pH on the adsorption of hydroxamic acid on monazite and calcite.
agreed well with the optimum recovery values obtained from the micro-flotation tests (Fig. 2). The vertically oriented monolayer adsorption density of the hydroxamic acid was estimated to be 8.1 lM/m2 assuming that the cross-sectional area of the hydroxamic acid molecule to be 20.5 Å2 (the number of adsorbed monolayers = molecular size (m2) adsorption density (M/m2) Avogadro number (6.02 1023)) (Ni and Liu, 2012). As such, 75% of the monazite surface was covered by hydroxamic acid at pH 9.0 when the bulk concentration was 1 103 M. Micro-flotation tests indicated that calcium ions decreased monazite recovery most likely due to a negative effect on the surface hydrophobicity. To better understand the effect, the adsorption of calcium ions onto the monazite surfaces and the corresponding impact on hydroxamic acid adsorption were also evaluated. The adsorption density of hydroxamic acid decreased initially with an increase in calcium concentration before reaching a minimum at a concentration of 5 106 M (Fig. 6). The adsorption density of the calcium ions increased slightly from 1.5 to
The adsorption mechanism was investigated using electrokinetic tests based on the fact that the isoelectric point (IEP) of mineral particles typically shift when specific adsorption occurs in the Stern layer. As shown in Fig. 7, the IEP of pure monazite was located at a solution pH value of 6.5 which agreed with values reported in literature (Cheng et al., 1993; Cheng, 2000). The zeta potential of monazite is pH sensitive which indicates that hydrogen and hydroxyl ions are potential determining ions for monazite. The influence of calcium ions on the electro-kinetic properties of monazite was more evident in basic solutions. For a calcium concentration of 1 106 M, the effect was negligible. However, with the addition of 1 105 M and 1 104 M calcium ions, the absolute charges of monazite decreased when pH values were larger than 8.2 and 7.0, respectively. Since the concentrations of calcium ions were much smaller than that of the supporting electrolyte (1 mM KCl), compression of the electrical double layer was minor and specific adsorption of the calcium ions on monazite was expected to occur. When the calcium ion concentration was increased to 1 103 M and 5 103 M, the IEP of the monazite was shifted to the right significantly which provided evidence of strong specific adsorption.
Fig. 6. Effects of initial concentration of calcium ions on the adsorption of 1 103 M hydroxamic acid and calcium ions on monazite surface at pH 9.0.
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3.4. FTIR analysis
3248
2747
1389 1420 1466 1562 1620 1659
2847 2916 2943 2954 3063
1292 1331
(a)
(b) 945 982
1604
(c)
(d)
1604
Transmittance
895 968 1030 1076 1119
Fig. 7. Effects of calcium ions on the electro-kinetic properties of monazite at different pH values.
FTIR analysis was conducted to evaluate the surface chemical changes of monazite after the adsorption of hydroxamic acid. Fig. 8 shows the spectrum of octanohydroxamic acid, monazite, and monazite conditioned in hydroxamic acid. Assignments of selected FTIR bands of solid hydroxamic acid are shown in Table 1. The bands of monazite at 945 cm1 and 982 cm1 were assigned to rare earth oxygen and phosphate oxygen bonds. After adsorption of hydroxamic acid, new bands at 1604 cm1 appeared indicating the occurrence of chemical adsorption. Hydroxamic acid showed two peaks around 1609 cm1, i.e., 1562 cm1 and 1620 cm1, representing the CAN stretch/NAH bend and C@O stretch, respectively. Therefore, it is reasonable to suggest that the chemical reaction which occurred between hydroxamic acid and monazite involved the CAN, NAH, and C@O groups. For the calcite system, no new peaks appeared for calcite after conditioning with 1 103 M hydroxamic acid as shown in Fig. 9 which indicates weak adsorption at the lower collector dosage level. When the dosage was increased to 5 103 M hydroxamic acid, adsorption peaks occurred at 1608, 2855, 2924, and
500
1000
1500
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 8. FTIR spectrum of monazite and octanohydroxamic acid: (a) octanohydroxamic acid; (b) monazite conditioned in water at pH 9.0; (c) monazite conditioned in 1 103 M hydroxamic acid at pH 9.0; (d) monazite conditioned in 3 103 M hydorxamic acid at pH 9.0.
Table 1 Assignments of selected FTIR bands from solid octanohydroxamic acid. FTIR (cm1)
Assignment
FTIR (cm1)
Assignment
895 968 1030 1076 1119 1292 1331 1389 1420 1466
CACAC skeletal stretch NAO stretch CAO stretch CACAC stretch CAH symmetric deformation (CH2)n wag CH3 deformation AOH bend of NAOAH NAOAH in place bend CAN stretch
1562 1620 1659 2747 2847 2916 2943 2954 3063 3248
CAN stretch, NAH bend C@O stretch C@O stretch OAH stretch CAH symmetric stretch for CH2/CH3 CAH symmetric stretch for CH2/CH3 CAH antisymmetric stretch for CH2/CH3 CAH antisymmetric stretch for CH2/CH3 Fermi resonance of CAN stretch and NAH bend NAH stretch
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2954 cm1 which existed even after rinsing. As such, the evidence indicated the existence of strong chemical adsorption of the collector onto the calcite surfaces at the higher collector concentration. The similar peaks (1604 cm1 for monazite and 1608 cm1 for calcite) indicate a nearly identical chemical adsorption mechanism given that the 4 cm1 shift may be due to the difference in the active sites. As shown in Figs. 3 and 4, the presence of calcium ions has a negative effect on monazite flotation recovery which is likely due to the impact on collector adsorption. When Ca2+ was added to the monazite system, the FTIR adsorption peak for monazite conditioned in hydroxamic acid solution shifted from 1604 cm1
to 1607 cm1, and to 1608 cm1 (Fig. 10). Therefore, in terms of hydroxamic acid adsorption, the surface characteristics of monazite gradually transformed to that of calcite when calcium concentration was increased in solution. Monazite conditioned in 5 103 M Ca2+ and 3 103 M hydroxamic acid showed very strong adsorption peaks at 1608, 2855, 2924, 2954 cm1, which did not occur for monazite and calcite conditioned in the same dosage of collector without the addition of Ca2+. It is reasonable to suggest that precipitation occurred in the monazite-calciumhydroxamic acid system. A titration test of a solution containing 5 103 M Ca2+ and 3 103 M hydroxamic acid showed that precipitates of white color started forming at pH 8.1.
(a)
1797 713 873 1403
Transmittance
(b)
2855 2954 2924
1608
(c)
500
1000
1500
(d)
2855 2954 2924
1608
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 9. FTIR spectrum of calcite (a) conditioned in water at pH 9.0; (b) conditioned in 1 103 M hydroxamic acid at pH 9.0; (c) conditioned in 5 103 M hydroxamic acid at pH 9.0 without rinsing; (d) conditioned in 5 103 M hydroxamic acid at pH 9.0 with rinsing.
(a)
Transmittance
1604
(b)
1607
(c) 2855 2954 2924
1458 723 1608 1512 868
500
1000
1500
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 10. Effect of Ca2+ on the adsorption of octanohydroxamic acid on monazite: (a) monazite conditioned in 1 103 M hydroxamic acid at pH 9.0; (b) monazite conditioned in 1 103 M Ca2+ and 1 103 M hydroxamic acid; (c) monazite conditioned in 5 103 M Ca2+ and 3 103 M hydroxamic acid.
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4. Discussion 4.1. Crystal characteristics of monazite Monazite crystal occurs in monoclinic space group with P21/n settings. Cleavages usually occur along (1 0 0) planes (Fig. 11). The rare earth and oxygen atoms exposed on the fresh surface are unsaturated. When placed in water, hydrolysis occurs due to electrical compensation leaving the surface sites divided into two different types, i.e., rare earth oxygen (REE-OH) and phosphate oxygen (P-OH) sites. The dissolution rate of monazite at 25 °C and pH 9.0 is less than 1017 mol/cm2/s (Oelkers and Poitrasson, 2002). Therefore, the dissolution of rare earth and phosphate atoms is minimal thereby eliminating the possibility of specific adsorption for the pure mineral-water systems used in this study. As such, the isoelectric point (IEP) and the point-of-zero charge (PZC) are equal with a value of 6.5 as shown in Fig.7. The surface charge of monazite can be represented by the single site surface protonation reactions i.e.,
SO þ Hþ SOH; SOHþ Hþ SOHþ2 ; where SO , SOH, and SOHþ 2 represent surface species, i.e., rare earth – oxygen and phosphate – oxygen sites, and Hþ represents the hydrogen ion near the charged surface (Parks, 1965; Somasundaran and Agar, 1967; Yoon et al., 1979). The surface charge was controlled by the equilibrium of the above equations. At the pH values beyond 6.5, more surface sites were deprotonated and SO was dominant. 4.2. Adsorption of calcium ions on monazite Concentrations of different calcium species in solution are controlled by the following reversible reactions (Somasundaran and Agar, 1967):
Ca2þ þ OH þ CaOHþ ;
K 1 ¼ 101:40 ;
CaðOHÞþ þ OH þ CaðOHÞ2ðaqÞ ; CaðOHÞ2ðaqÞ þ CaðOHÞ2ðsÞ ;
K 2 ¼ 101:37 ;
K 2 ¼ 102:45 :
Fig. 12. Specification diagram of calcium ions in solution.
The equilibrium concentrations of Ca2+, CaOH+, and Ca(OH)2(aq) in a solution containing 5 103 M of total calcium ions were calculated and shown in Fig. 12. Ca2+ ion is the dominant species in the pH range of 3–12 while concentrations of hydrolyzed species of calcium, i.e., CaOH+, increase with a rise in the solution pH value. Electro-kinetic tests in Fig. 7 indicate that adsorption of calcium onto the monazite surface occurred more readily in the basic pH environment, which coincided with the pH range where hydrolysis of calcium ions occurred. Based on these findings, the solution species resulting in the specific adsorption was mainly Ca(OH)+. The precipitation of calcium hydroxide occurs at pH values larger than 13.1. As such, at pH 9.0, the adsorption of hydroxamic acid by calcium hydroxide forms in solution does not need to be considered. As previously presented in this paper, the adsorption study indicated that calcium adsorption on monazite was less than a monolayer. The adsorption occurred within the Stern layer and thickness of the adsorbed layer was the radius of the calcium ions plus the diameter of a hydroxyl group, i.e., 1.00 Å plus 2.66 Å. With the addition of 1 104 M Ca2+, the adsorption density of the calcium ions onto monazite surfaces is around 1.5 lM/m2. Based on the adsorption layer thickness, adsorption density, and dissociation reactions of calcium in solution, the Ca(OH)+ concentration and pH value corresponding to calcium precipitation near the monazite surface were calculated to be 1.5 M and 11.30,
Fig. 11. Ball and stick model of the cleavage of monazite (CePO4) occurring on (1 0 0) plane.
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Fig. 13. Model of interaction among monazite, calcium ions, and hydroxamic acid.
respectively. Hydroxyl ions were likely repelled from the surface due to electrostatic repulsion, which means pH values beyond 11.30 in bulk solution is required for the occurrence of surface precipitation. Therefore, at pH 9.0, calcium ions were adsorbed as hydrolyzed species (CaOH+) on the monazite surfaces, which is also true for the higher concentrations investigated in the current study. The adsorption of CaOH+ on monazite is more likely to occur on the neutralized and deprotonated rare earth oxygen and phosphate oxygen sites via hydrogen bonding and electrostatic interactions, respectively, which could be represented by the following equations:
P
OH
OH
O
REE
OH
NH
C
formation of chelates on monazite surface. Furthermore, adsorption of hydroxamic acid molecules onto phosphate oxygen sites was also likely due to the existence of hydrogen bonding. The phosphate oxygen-to-rare earth oxygen sites ratio is 1:1 based on both the crystal data and stoichiometry of monazite. The adsorption of hydroxamic acid on phosphate oxygen sites is further proved by the adsorption test, i.e., near 75% of monazite surface was occupied by the hydroxamic at the predetermined conditions. Overall, the adsorption of hydroxamic acid on monazite can be represented by the following expression:
O P R
REE
OH O
OH NH R C
O
NH
REEOH þ CaOHþ REEOH OHCaþ ; REEO þ CaOHþ ¢ REEOCaOH; POH þ CaOHþ ¢ POH OHCaþ ; PO þ CaOHþ ¢ POCaOH: 4.3. Adsorption mechanism of hydroxamic acid on monazite As a typical chelating reagent, adsorption of hydroxamic acid has been well studied for many minerals such as chryscolla, iron oxide, manganese dioxide, and bastnaesite (Fuerstenau et al., 1970, 1967; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1985, 1983; Raghavan and Fuerstenau, 1975). In spite of some controversial opinions such as questions regarding the active species of hydroxamic acid in solution, chelates of metal complexes are generally considered to be formed by the hydroxamic acid and characterized by a ring structure with the metal as the central atom surrounded by the N and O atoms (Nagaraj, 1988). FTIR tests in this study indicated the involvement of the C@O, NAH, and CAN in the chemical adsorption of hydroxamic acid on monazite, which proved the
C
R
4.4. Effects of calcium on the adsorption of hydroxamic acid on monazite Surface hydrolysis has been proved necessary for hydroxamic adsorption on minerals (Fuerstenau et al., 1970, 1967; Nagaraj, 1988; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1983, 1985). In the current study, the importance of surface hydrolysis was also observed, i.e., optimum flotation and adsorption occurred at pH 9.0 where concentrations of rare earth hydrolyzed species reached maximum. Based on the fact that hydroxamic acid is strongly complexed with the rare earth atoms, the effects of CaOH+ on the adsorption of hydroxamic acid on rare earth oxygen sites may be ignored. However, for the phosphate oxygen sites, both CaOH+ and hydroxamic acid adsorbed via hydrogen bonding and competitive adsorption was expected to occur, which explains the decreases of hydroxamate adsorption when small dosage of calcium ions was added. Adsorption density of hydroxamic acid was increased with the addition of higher dosage of calcium ions (see Fig. 6). It was reasonable to suggest that the phosphate oxygen sites were covered with a layer of Ca(OH)+ attached via hydrogen bonding or electrostatic attraction. The electrostatically adsorbed CaOH+ with the hydroxyl group oriented away from the solid surface provides alternative
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active sites for hydroxamic acid adsorption, which agrees with the fact that the hydrolysis of metal atoms on minerals is required for formation of chelates with hydroxamic acid (Fuerstenau et al., 1970, 1967; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1983, 1985). The shifts of the adsorption peaks (1604, 1607, 1608 cm1) of monazite which interacted with both calcium and hydroxamic acid also suggested the formation of calcium-hydroxamate chelates on monazite surfaces. The primary hydration sheath of CaOH+ adsorbed via hydrogen bonding was difficult to remove, which increased the surface hydrophilicity. The combined effects were that stronger surface hydrophobicity was not observed with the addition of calcium ions despite the adsorption of additional hydroxamic acid. The interaction model between monazite, hydroxamic acid, and calcium ions is shown in Fig. 13. However, surface and bulk precipitation of calcium hydroxamate occurred with the addition of higher dosages of calcium and hydroxamic acid, i.e., 5 103 M of Ca2+ and 3 103 M hydroxamic acid in the current study.
5. Conclusions The flotation of monazite in the presence of a calcium-based mineral (i.e., calcite) has been systematically studied using hydroxamic acid as the collector. Test results indicate that the presence of calcium ions, which exist thorough dissolution of the calciumbased mineral, suppress the floatability of the monazite at collector concentrations below 3.0 104 M. Furthermore, calcite readily floats at or above this concentration thereby eliminating any selectivity between the two minerals. Both rare earth oxygen and phosphate oxygen sites exist on monazite surfaces. The adsorption of CaOH+ is a result of both hydrogen bonding and electrostatic attraction for which the orientation of the ion is determined by the adsorption mechanism. At low collector concentrations and in the presence of calcium ions, competitive adsorption of CaOH+ and hydroxamic acid occurs on the phosphate oxygen sites. The competitive adsorption leads to a decrease in the hydroxamic acid adsorption density on the monazite surfaces thereby resulting in low flotation recovery values. As the calcium concentration is increased in solution, the electrostatically adsorbed CaOH+ provides an additional site for the adsorption of the hydroxamic acid collector. Although this action results in an increase in the adsorption density of the hydroxamic acid, hydrophobicity and flotation recovery of the monazite decreases due to hydration of the hydrogen bonded CaOH+ sites on the monazite surface. Acknowledgements The research results presented in this publication was funded in part by the Kentucky Energy and Environment Cabinet (Project ID No. PON2 127 150002394 1). References Abeidu, A.M., 1972. The separation of monazite from zircon by flotation. J. Less Common Met. 29 (2), 113–119. Amankonah, J.O., Somasundaran, P., Ananthapadmabhan, K.P., 1985. Effects of dissolved mineral species on the dissolution/precipitation characteristics of calcite and apatite. Colloid Surf. 15, 295–307. Ananthapadmanabhan, K.P., Somasundaran, P., 1985. Surface precipitation of inorganics and surfactants and its role in adsorption and flotation. Colloid Surf. 13, 151–167. Ananthapadmanabhan, K.P., Somasundaran, P., 1984. The role of dissolved mineral species in calcite-apatite flotation. Miner. Metall. Process 1 (1), 36–42. Atesok, G., Somasundaran, P., Morgan, L.J., 1988. Adsorption properties of Ca2+ on Na-kaolinite and its effect on flocculation using polyacrylamides. Colloid Surf. 32, 127–138.
Chan, T.N., 1991. A new beneficiation process for the treatment of supergene monazite ore. In: Proc. Rare Earths—Resources, Science, Technology and Applications. TMS, Warrendale, pp. 77–94. Chelgani, S.C., Rudolph, M., Leistner, T., Gutzmer, J., Peuker, U.A., 2015. A review of rare earth minerals flotation: monazite and xenotime. Int. J. Min. Sci. Technol. 25 (6), 877–883. Cheng, T.W., 2000. The point of zero charge of monazite and xenotime. Miner. Eng. 13 (1), 105–109. Cheng, T.W., Holtham, P.N., Tran, T., 1993. Froth flotation of monazite and xenotime. Miner. Eng. 6 (4), 341–351. Clark, S.W., Cooke, S.R.B., 1968. Adsorption of calcium, magnesium, and sodium ion by quartz. Trans. AIME 241, 334. Cross, W.M., Miller, J.D., 1989. Bubble attachment time measurements for selected rare-earth phosphate minerals in oleate solutions. Rare Earths: Extract. Prep. Appl., 45–55 Demir, C., Bentli, I., Gülgönül, I., Celik, M.S., 2003. Effects of bivalent salts on the flotation separation of Na-feldspar from K-feldspar. Miner. Eng. 16 (6), 551–554. Dixit, S.G., Biswas, A.K., 1969. PH-dependence of the flotation and adsorption properties of some beach sand minerals. Trans. AIME 244 (2), 173–178. El-Salmawy, M.S., Nakahiro, Y., Wakamatsu, T., 1993. The role of alkaline earth cations in flotation separation of quartz from feldspar. Miner. Eng. 6 (12), 1231– 1243. Elgillani, D.A., Abouzeid, A.Z., 1993. Flotation of carbonates from phosphate ores in acidic media. Int. J. Miner. Process. 38 (3), 235–256. Fuerstenau, M.C., Harper, R.W., Miller, J.D., 1970. Hydroxamate vs fatty-acid flotation of iron oxide. Trans. AIME 247 (1), 69–73. Fuerstenau, M.C., Miller, J.D., Gutierrez, G., 1967. Selective flotation of iron oxide. Trans. AIME 238, 200–203. James, R.O., Healy, T.W., 1972. Adsorption of hydrolyzable metal ions at the oxide— water interface. III. A thermodynamic model of adsorption. J. Colloid Interface Sci. 40 (1), 65–81. Kandil, A.T., Aly, M.M., Moussa, E.M., Kamel, A.M., Gouda, M.M., Kouraim, M.N., 2010. Column leaching of lanthanides from Abu Tartur phosphate ore with kinetic study. J. Rare Earth 28 (4), 576–580. Liu, W., Zhang, S., Wang, W., Zhang, J., Yan, W., Deng, J., Feng, Q., Huang, Y., 2015. The effects of Ca (II) and Mg (II) ions on the flotation of spodumene using NaOL. Miner. Eng. 79, 40–46. Mishra, S.K., 1978. The electrokinetics of apatite and calcite in inorganic electrolyte environment. Int. J. Miner. Process. 5 (1), 69–83. Nagaraj, D.R., 1988. The Chemistry and Application of Chelating or Complexing Agents in Minerals Separations. Marcel Dekker Inc., Reagents in Mineral Technology, pp. 257–334. Natarajan, R., Fuerstenau, D.W., 1983. Adsorption and flotation behavior of manganese dioxide in the presence of octyl hydroxamate. Int. J. Miner. Process. 11 (2), 139–153. Ni, X., Liu, Q., 2012. The adsorption and configuration of octyl hydroxamic acid on pyrochlore and calcite. Colloid Surf. A 411, 80–86. Oelkers, E.H., Poitrasson, F., 2002. An experimental study of the dissolution stoichiometry and rates of a natural monazite as a function of temperature from 50 to 230 °C and pH from 1.5 to 10. Chem. Geol. 191 (1), 73–87. Parks, G.A., 1965. The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chem. Rev. 65 (2), 177–198. Peterson, H.D., Fuerstenau, M.C., Rickard, R.S., Miller, J.D., 1965. Chrysocolla flotation by the formation of insoluble surface chelates. Trans. AIME 232, 388–392. Pavez, O., Peres, A.E.C., 1994. Technical note bench scale flotation of a Brazilian monazite ore. Miner. Eng. 7 (12), 1561–1564. Pavez, O., Peres, A.E.C., 1993. Effect of sodium metasilicate and sodium sulphide on the floatability of monazite-zircon-rutile with oleate and hydroxamates. Miner. Eng. 6 (1), 69–78. Pradip, Fuerstenau, D.W., 1985. Adsorption of hydroxamate collectors on semisoluble minerals Part II: effect of temperature on adsorption. Colloid Surf. 15, 137–146. Pradip, Fuerstenau, D.W., 1983. The adsorption of hydroxamate on semi-soluble minerals. Part I: Adsorption on barite, calcite and bastnaesite. Colloid Surf. 8 (2), 103–119. Qi, G.W., 1993. Use of the QEM⁄ SEM analysis in flotation testwork on a phosphate ore containing monazite. Int. J. Miner. Process. 37 (1), 89–108. Raghavan, S., Fuerstenau, D.W., 1975. The adsorption of aqueous octylhydroxamate on ferric oxide. J. Colloid Interface Sci. 50 (2), 319–330. Santos, M.A., Santana, R.C., Capponi, F., Ataíde, C.H., Barrozo, M.A., 2010. Effect of ionic species on the performance of apatite flotation. Sep. Purif. Technol. 76 (1), 15–20. Scott, J.L., Smith, R.W., 1993. Calcium ion effects in amine flotation of quartz and magnetite. Miner. Eng. 6 (12), 1245–1255. Somasundaran, P., Agar, G.E., 1972. Further streaming potential studies on apatite in inorganic electrolytes. Trans. AIME 252, 348–352. Somasundaran, P., 1968. Zeta potential of apatite in aqueous solutions and its change during equilibration. J. Colloid Interface Sci. 27 (4), 659–666. Somasundaran, P., Agar, G.E., 1967. The zero point of charge of calcite. J. Colloid Interface Sci. 24 (4), 433–440. Yoon, R.H., Salman, T., Donnay, G., 1979. Predicting points of zero charge of oxides and hydroxides. J. Colloid Interface Sci. 70 (3), 483–493. Zhang, W., Rezaee, M., Bhagavatula, A., Li, Y., Groppo, J., Honaker, R., 2015. A review of the occurrence and promising recovery methods of rare earth elements from coal and coal by-products. Int. J. Coal Prep. Util. 35 (6), 295–330.
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Flotation of monazite in the presence of calcite part I: Calcium ion effects on the adsorption of hydroxamic acid W. Zhang, R.Q. Honaker ⇑, J.G. Groppo Department of Mining Engineering, University of Kentucky, Lexington, KY, USA
a r t i c l e
i n f o
Article history: Received 19 July 2016 Revised 6 September 2016 Accepted 24 September 2016
Keywords: Flotation Monazite Calcite Calcium ions Hydroxamic acid
a b s t r a c t The dissolution of minerals provides elevated concentrations of alkaline earth metal ions such as calcium (Ca2+) in water which may have a significant influence on flotation performance. The effects of Ca2+ ions dissolved from calcite surfaces on the flotation performance of a monazite-calcite system have been investigated using micro-flotation tests followed by fundamental collector adsorption studies. The results indicate that specially adsorbed CaOH+ via hydrogen bonding competed with hydroxamic acid for the phosphate-oxygen active sites on the monazite surface and decreased its hydrophobicity due to hydration. Electrostatically adsorbed CaOH+ on monazite increased the adsorption density of hydroxamic acid through a chelating interaction. At higher dosages of calcium and hydroxamic acid, bulk and surface precipitation of calcium-hydroxamate occurs in the monazite-calcium-hydroxamic acid system. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth elements (REEs) are a group of 15 lanthanide elements (atomic numbers 57–71) and two transition metal elements (scandium and yttrium). There is a high demand for REEs due to their applications in catalysts, alloys, magnets, and glass polishing (Chelgani et al., 2015; Zhang et al., 2015). Monazite ((Ce, La, Y, Th)PO4) is a commercial rare earth mineral and usually associated with heavy minerals such as zircon, rutile, and ilmenite (Abeidu, 1972; Cheng et al., 1993; Pavez and Peres, 1994). Besides gravity, magnetic, and electrostatic separation methods, flotation has been widely used in monazite concentration due to its advantages in fine particle processing (Cheng et al., 1993). Monazite flotation has been investigated by many researchers (Abeidu, 1972; Cheng, 2000; Cheng et al., 1993; Cross and Miller, 1989; Dixit and Biswas, 1969; Pavez and Peres, 1994, 1993; Qi, 1993). Satisfactory flotation performance is achieved at pH 8–9 by using sodium oleate as a collector for monazite and sodium silicate as a depressant for zircon, rutile, and ilmenite (Cheng et al., 1993; Pavez and Peres, 1994, 1993; Qi, 1993). However, studies on the flotation separation of monazite from carbonate minerals such as calcite and dolomite or phosphate minerals such as apatite are limited. As reported by Chan (1991), small amounts of dolomite and apatite exists in a monazite ore located in Mt Weld, Australia. Moreover, for some promising rare earth
⇑ Corresponding author. http://dx.doi.org/10.1016/j.mineng.2016.09.020 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.
secondary resources, such as coal fine refuse and phosphates, monazite is usually associated with carbonate and phosphate minerals (Kandil et al., 2010; Zhang et al., 2015). Minerals such as calcite, dolomite, and apatite belong to sparingly soluble minerals. Lattice ions of these minerals such as calcium ions in calcite are dissolved from the mineral surfaces into solution, which influences collector adsorption and thus flotation performance. A number of studies have been previously conducted to evaluate calcium and multivalent cation adsorption on mineral surfaces (Ananthapadmanabhan and Somasundaran, 1985; Atesok et al., 1988; Clark and Cooke, 1968; James and Healy, 1972). Adsorption of calcium on minerals such as silica and Na-kaolinite usually increased sharply at pH values above 10 where significant amounts of calcium ions hydrolyze to CaOH+ ions and Ca(OH)2 precipitates (Atesok et al., 1988; Clark and Cooke, 1968). The higher affinity of CaOH+ compared with Ca2+ is mainly due to the decrease in the unfavorable solvation energy and the increase in favorable chemical energy changes (Ananthapadmanabhan and Somasundaran, 1985; James and Healy, 1972). Specific adsorption of calcium hydrolyzed species is well documented for the apatite/calcite system (Amankonah et al., 1985; Ananthapadmanabhan and Somasundaran, 1984; Mishra, 1978; Somasundaran, 1968; Somasundaran and Agar, 1972). In addition, the free Ca2+ ions attach to apatite surfaces in the form of Ca5(PO4)3(OH) via the following equation (Elgillani and Abouzeid, 1993):
Ca5 ðPO4 Þ3 ðOHÞðsÞ þ 4Hþ ¼ 3CaHPO4ðaqÞ þ 2Ca2þ þ H2 O:
W. Zhang et al. / Minerals Engineering 100 (2017) 40–48
Despite similarities in the calcium adsorption mechanisms on mineral surfaces, the effects of calcium ions on flotation are more complicated and have been found to be a function of mineral characteristics, calcium ion concentration, collector type, solution pH values, etc. (Clark and Cooke, 1968; Demir et al., 2003; Liu et al., 2015; El-Salmawy et al., 1993; Santos et al., 2010; Scott and Smith, 1993). Calcium ions serve as an activator for quartz and spodumene when using anionic collectors such as sodium oleate and sodium alkysulfonates (Clark and Cooke, 1968; El-Salmawy et al., 1993; Liu et al., 2015). However, depression of quartz and Na-feldspar occurred in the presence of calcium ions when using a cationic amine collector (Demir et al., 2003; Scott and Smith, 1993). In the current study, the separation of monazite from calcite using froth flotation and hydroxamic acid as the collector was studied systematically using a micro-flotation apparatus. The effects of calcium ions on the adsorption of hydroxamic acid onto monazite surfaces were fundamentally investigated through electrophoresis and adsorption studies and supported by Fourier transform infrared spectroscopy (FTIR) analysis. Finally, an interaction model for the adsorption of hydroxamic acid onto monazite in the presence of calcium ions was proposed. 2. Material and methods 2.1. Materials Monazite pre-concentrate samples collected from a rare earth mine in China were purified using a shaking table and highgradient magnet. A monazite concentrate with a purity level of nearly 98% was obtained as determined by inductively coupled plasma mass spectrometry (ICP-MS). The X-ray diffraction analysis indicated that a minor amount of cerium oxide existed in the sample. Chunk samples of calcite collected from a limestone mine located in Kentucky, USA, were crushed and ground to minus 300 lm. X-ray diffraction analysis showed that calcite content in the sample was more than 95% and thus directly used as pure minerals. For the micro-flotation tests, the monazite concentrate and calcite material were screened to obtain material having a particle size smaller than 300 lm and larger than 150 lm. The remaining samples were ground using a corundum pestle and mortar for zeta potential, adsorption, Fourier transform infrared spectroscopy (FTIR) tests. Octanohydroxamic acid of minimal 95% purity was purchased from TCI America. Methyl isobutyl carbinol (MIBC) of more than 99% purity was used as frother and purchased from Alfa Aesar. Other chemicals were of reagent grade and purchased from We Enable Science. Type I deionized water with a resistivity value of 18.2 MXcm at 25 °C was used for all the experiments. 2.2. Methods 2.2.1. Micro-flotation tests A micro-flotation cell with a volume of 50 ml was used for the flotation tests (Fig. 1). During the tests, a suspension of the particles was achieved using a magnetic stirrer. To reduce interference of carbon dioxide, air bubbles were produced by passing pure nitrogen through a porous frit. Conditioning was conducted in a glass beaker with one gram of pure minerals in 50 ml solution. For the mixed mineral flotation tests, the ratio of monazite-tocalcite was 1:1 by weight. Solution pH values were first adjusted to predetermined values followed by the addition of metal salts and/or collectors with each conditioned for 5 min. Frother was
41
Fig. 1. Schematic diagram of the micro-flotation apparatus.
added last followed by 1 min of conditioning. The solution pH values were measured and adjusted during conditioning every 2.5 min and just prior to the start of the flotation test. Given that the majority of the material floated within the first 30 s of the test, the initial pH values were considered to be more representative of the flotation conditions. The difference in the pH values measured before and after each flotation test was typically 0.2 pH units. During the flotation tests, nitrogen flow rates were maintained at 55 ml/min while froth products were continuously removed over a period of 4 min. The concentrate and tailing samples were dried in an oven for 12 h and then cooled in a desiccator. Finally, flotation recovery values were calculated based on the sample weights. For the mixed mineral flotation tests, monazite and calcite recovery values were calculated by using the assay values obtained from acid digestion. Three repeat tests were conducted under the same condition and the experiment error measured from the recovery data indicated a standard deviation value of 1.10%. 2.2.2. Adsorption tests Adsorption tests were conducted in a 20 ml glass beaker. Solid concentration values of 0.5% and 1% were used for monazite and calcite, respectively. Conditioning procedures were similar to that used for micro-flotation tests. After conditioning, 10 ml of the suspension was removed to a centrifuge tube and centrifuged in an IEC Clinical centrifuge at 3175 rpm. Hydroxamic acid concentration was determined using a Shimadzu 1280 UV–vis spectrophotometer based on the ferric hydroxamate method. Ferric chloride (5 mM) of 10 ml was added into 5 ml of centrifuged supernatant in a glass tube. Full wave scanning of the obtained solution showed that peak absorption occurred at 503 nm which was used for hydroxamic acid determination. For the adsorption tests at pH 11.0, pH values of the supernatant were readjusted to below 9.0 before ferric chloride addition to avoid the precipitation of ferric ions. The calcium concentration in solution was determined using induced coupled plasma (ICP) analysis. Three tests were conducted under the same condition and a standard deviation value of 0.30 lM/m2 was obtained. 2.2.3. Zeta-potential tests Zeta potential tests were conducted to study the effects of calcium ions on the electro-kinetic properties of monazite particles. A solids concentration of 0.05% by weight was used and conditioning procedures were similar to that used for the micro-flotation tests. Before conditioning, the solution was subjected to sonification for 1 min to thoroughly disperse the solid particles. A potassium chloride solution of 1 mM was used as a supporting electrolyte for all measurements. A standard deviation value of
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2.71 mV was determined from four repeat tests when conducted under the same conditions. 2.2.4. Fourier transform infrared spectroscopy (FTIR) analysis Pure monazite and calcite were ground to minus 2 lm for FTIR tests to avoid light scattering. Conditioning procedures were similar to that used for the flotation tests. After conditioning, the sample was centrifuged and the supernatant decanted followed by three repeated rinses to remove any residual or physically adsorbed collectors. FTIR tests based on the attenuated total reflection (ATR) method were conducted using a Varian 7000e spectrometer. A few milligrams of mineral powders was placed directly on the optic window. The analysis was conducted from 4000 to 700 cm1 using 32 scans with a resolution of 4 cm1. The FTIR results were analyzed using the drawing and peak fitting tools of the Original software. 3. Results 3.1. Micro-flotation tests Mineral floatability is closely related to solution pH values, which influences the solution species of collectors and ions present in solution. Fig. 2 shows the flotation recovery results obtained when monazite and calcite are floated separately in singlemineral flotation tests using a collector dosage of 1 104 M. A maximum recovery of around 90% was obtained for monazite at pH 9.0 while calcite recovery at the same pH value was only around 22%%. Maximum calcite recovery was realized at pH 10.0. The results imply that an effective separation could be realized using hydroxamate at a solution pH of 9.0. However, tests results obtained when mixing the two minerals at a 1:1 ratio provided a different outcome. While maintaining the solution pH at a value of 9.0, hydroxamate dosage was varied in tests involving both single-mineral and mixed-mineral flotation systems. As shown in Fig. 3, the presence of calcite suppressed the flotation of monazite when using hydroxamate dosages less than 2.5 104 M. Furthermore, the recovery of calcite reached values approaching 100% at high collector dosage levels. As such, selectivity was significantly lower in the mixed mineral system and declined substantially with an increase in hydroxamate concentration. At the dosage of 1 104 M as used in the pH experiments (Fig. 2), the monazite recovery in the
Fig. 3. Effects of hydroxamic acid concentration on monazite and calcite flotation at pH 9.0.
Fig. 4. Effects of calcium and carbonate ions on the flotation of monazite using 1 104 M hydroxamic acid.
mixed-mineral system was only around 40% compared to the 90% value obtained from the single mineral tests while the calcite recovery remained relatively unchanged. Calcite is a sparingly soluble mineral and crystal ions will dissolve from its surfaces in water. As previously discussed, the influence of calcium and carbonate ions on flotation have been reported in literature. Fig. 4 shows the effects of calcium and carbonate ions on monazite flotation. Monazite recovery values decreased exponentially with an increase in calcium ion concentration while carbonate ions showed negligible effects. Therefore, the depression in monazite recovery in the presence of calcite shown in Fig. 3 may be attributed to the dissolution of calcium ions from the calcite surfaces. 3.2. Adsorption tests
Fig. 2. Effects of pH values on monazite and calcite flotation when using 1 104 M hydroxamic acid.
In an effort to develop a fundamental understanding of the flotation results, adsorption density was measured over a range of solution pH values and calcium ion concentrations. As shown in Fig. 5, maximum adsorption density values for the monazite and calcite were obtained at pH 9.0 and 10.0, respectively, which
W. Zhang et al. / Minerals Engineering 100 (2017) 40–48
43
2.1 lM/m2 with an elevation in the calcium concentrations from 5 105 to 5 103 M (2–200 ppm). For concentrations below 5 105 M, adsorption tests were not conducted due to the lower limits of the ICP-OES analyzer. However, it is reasonable to assume that the adsorption density was below 1.5 lM/m2 for the lower calcium ion concentrations. Monolayer coverage of bare dehydrated Ca2+ ions corresponds to an adsorption density of 52.9 lM/m2. However, James and Healy (1972) found that inner hydration sheaths of metallic cations remain adsorbed on a silica surface. The radius used for monolayer coverage calculations was the radius of bare cations plus the diameter of a water molecule. In this case, the monolayer coverage of calcium corresponds to 3.76 lM/m2. Using this value, only half of the monazite surface was covered by calcium ions at pH 9.0 within the studied calcium concentrations. 3.3. Electro-kinetic tests Fig. 5. Effect of pH on the adsorption of hydroxamic acid on monazite and calcite.
agreed well with the optimum recovery values obtained from the micro-flotation tests (Fig. 2). The vertically oriented monolayer adsorption density of the hydroxamic acid was estimated to be 8.1 lM/m2 assuming that the cross-sectional area of the hydroxamic acid molecule to be 20.5 Å2 (the number of adsorbed monolayers = molecular size (m2) adsorption density (M/m2) Avogadro number (6.02 1023)) (Ni and Liu, 2012). As such, 75% of the monazite surface was covered by hydroxamic acid at pH 9.0 when the bulk concentration was 1 103 M. Micro-flotation tests indicated that calcium ions decreased monazite recovery most likely due to a negative effect on the surface hydrophobicity. To better understand the effect, the adsorption of calcium ions onto the monazite surfaces and the corresponding impact on hydroxamic acid adsorption were also evaluated. The adsorption density of hydroxamic acid decreased initially with an increase in calcium concentration before reaching a minimum at a concentration of 5 106 M (Fig. 6). The adsorption density of the calcium ions increased slightly from 1.5 to
The adsorption mechanism was investigated using electrokinetic tests based on the fact that the isoelectric point (IEP) of mineral particles typically shift when specific adsorption occurs in the Stern layer. As shown in Fig. 7, the IEP of pure monazite was located at a solution pH value of 6.5 which agreed with values reported in literature (Cheng et al., 1993; Cheng, 2000). The zeta potential of monazite is pH sensitive which indicates that hydrogen and hydroxyl ions are potential determining ions for monazite. The influence of calcium ions on the electro-kinetic properties of monazite was more evident in basic solutions. For a calcium concentration of 1 106 M, the effect was negligible. However, with the addition of 1 105 M and 1 104 M calcium ions, the absolute charges of monazite decreased when pH values were larger than 8.2 and 7.0, respectively. Since the concentrations of calcium ions were much smaller than that of the supporting electrolyte (1 mM KCl), compression of the electrical double layer was minor and specific adsorption of the calcium ions on monazite was expected to occur. When the calcium ion concentration was increased to 1 103 M and 5 103 M, the IEP of the monazite was shifted to the right significantly which provided evidence of strong specific adsorption.
Fig. 6. Effects of initial concentration of calcium ions on the adsorption of 1 103 M hydroxamic acid and calcium ions on monazite surface at pH 9.0.
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3.4. FTIR analysis
3248
2747
1389 1420 1466 1562 1620 1659
2847 2916 2943 2954 3063
1292 1331
(a)
(b) 945 982
1604
(c)
(d)
1604
Transmittance
895 968 1030 1076 1119
Fig. 7. Effects of calcium ions on the electro-kinetic properties of monazite at different pH values.
FTIR analysis was conducted to evaluate the surface chemical changes of monazite after the adsorption of hydroxamic acid. Fig. 8 shows the spectrum of octanohydroxamic acid, monazite, and monazite conditioned in hydroxamic acid. Assignments of selected FTIR bands of solid hydroxamic acid are shown in Table 1. The bands of monazite at 945 cm1 and 982 cm1 were assigned to rare earth oxygen and phosphate oxygen bonds. After adsorption of hydroxamic acid, new bands at 1604 cm1 appeared indicating the occurrence of chemical adsorption. Hydroxamic acid showed two peaks around 1609 cm1, i.e., 1562 cm1 and 1620 cm1, representing the CAN stretch/NAH bend and C@O stretch, respectively. Therefore, it is reasonable to suggest that the chemical reaction which occurred between hydroxamic acid and monazite involved the CAN, NAH, and C@O groups. For the calcite system, no new peaks appeared for calcite after conditioning with 1 103 M hydroxamic acid as shown in Fig. 9 which indicates weak adsorption at the lower collector dosage level. When the dosage was increased to 5 103 M hydroxamic acid, adsorption peaks occurred at 1608, 2855, 2924, and
500
1000
1500
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 8. FTIR spectrum of monazite and octanohydroxamic acid: (a) octanohydroxamic acid; (b) monazite conditioned in water at pH 9.0; (c) monazite conditioned in 1 103 M hydroxamic acid at pH 9.0; (d) monazite conditioned in 3 103 M hydorxamic acid at pH 9.0.
Table 1 Assignments of selected FTIR bands from solid octanohydroxamic acid. FTIR (cm1)
Assignment
FTIR (cm1)
Assignment
895 968 1030 1076 1119 1292 1331 1389 1420 1466
CACAC skeletal stretch NAO stretch CAO stretch CACAC stretch CAH symmetric deformation (CH2)n wag CH3 deformation AOH bend of NAOAH NAOAH in place bend CAN stretch
1562 1620 1659 2747 2847 2916 2943 2954 3063 3248
CAN stretch, NAH bend C@O stretch C@O stretch OAH stretch CAH symmetric stretch for CH2/CH3 CAH symmetric stretch for CH2/CH3 CAH antisymmetric stretch for CH2/CH3 CAH antisymmetric stretch for CH2/CH3 Fermi resonance of CAN stretch and NAH bend NAH stretch
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2954 cm1 which existed even after rinsing. As such, the evidence indicated the existence of strong chemical adsorption of the collector onto the calcite surfaces at the higher collector concentration. The similar peaks (1604 cm1 for monazite and 1608 cm1 for calcite) indicate a nearly identical chemical adsorption mechanism given that the 4 cm1 shift may be due to the difference in the active sites. As shown in Figs. 3 and 4, the presence of calcium ions has a negative effect on monazite flotation recovery which is likely due to the impact on collector adsorption. When Ca2+ was added to the monazite system, the FTIR adsorption peak for monazite conditioned in hydroxamic acid solution shifted from 1604 cm1
to 1607 cm1, and to 1608 cm1 (Fig. 10). Therefore, in terms of hydroxamic acid adsorption, the surface characteristics of monazite gradually transformed to that of calcite when calcium concentration was increased in solution. Monazite conditioned in 5 103 M Ca2+ and 3 103 M hydroxamic acid showed very strong adsorption peaks at 1608, 2855, 2924, 2954 cm1, which did not occur for monazite and calcite conditioned in the same dosage of collector without the addition of Ca2+. It is reasonable to suggest that precipitation occurred in the monazite-calciumhydroxamic acid system. A titration test of a solution containing 5 103 M Ca2+ and 3 103 M hydroxamic acid showed that precipitates of white color started forming at pH 8.1.
(a)
1797 713 873 1403
Transmittance
(b)
2855 2954 2924
1608
(c)
500
1000
1500
(d)
2855 2954 2924
1608
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 9. FTIR spectrum of calcite (a) conditioned in water at pH 9.0; (b) conditioned in 1 103 M hydroxamic acid at pH 9.0; (c) conditioned in 5 103 M hydroxamic acid at pH 9.0 without rinsing; (d) conditioned in 5 103 M hydroxamic acid at pH 9.0 with rinsing.
(a)
Transmittance
1604
(b)
1607
(c) 2855 2954 2924
1458 723 1608 1512 868
500
1000
1500
2000
2500
3000
3500
4000
Wave number (cm-1) Fig. 10. Effect of Ca2+ on the adsorption of octanohydroxamic acid on monazite: (a) monazite conditioned in 1 103 M hydroxamic acid at pH 9.0; (b) monazite conditioned in 1 103 M Ca2+ and 1 103 M hydroxamic acid; (c) monazite conditioned in 5 103 M Ca2+ and 3 103 M hydroxamic acid.
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4. Discussion 4.1. Crystal characteristics of monazite Monazite crystal occurs in monoclinic space group with P21/n settings. Cleavages usually occur along (1 0 0) planes (Fig. 11). The rare earth and oxygen atoms exposed on the fresh surface are unsaturated. When placed in water, hydrolysis occurs due to electrical compensation leaving the surface sites divided into two different types, i.e., rare earth oxygen (REE-OH) and phosphate oxygen (P-OH) sites. The dissolution rate of monazite at 25 °C and pH 9.0 is less than 1017 mol/cm2/s (Oelkers and Poitrasson, 2002). Therefore, the dissolution of rare earth and phosphate atoms is minimal thereby eliminating the possibility of specific adsorption for the pure mineral-water systems used in this study. As such, the isoelectric point (IEP) and the point-of-zero charge (PZC) are equal with a value of 6.5 as shown in Fig.7. The surface charge of monazite can be represented by the single site surface protonation reactions i.e.,
SO þ Hþ SOH; SOHþ Hþ SOHþ2 ; where SO , SOH, and SOHþ 2 represent surface species, i.e., rare earth – oxygen and phosphate – oxygen sites, and Hþ represents the hydrogen ion near the charged surface (Parks, 1965; Somasundaran and Agar, 1967; Yoon et al., 1979). The surface charge was controlled by the equilibrium of the above equations. At the pH values beyond 6.5, more surface sites were deprotonated and SO was dominant. 4.2. Adsorption of calcium ions on monazite Concentrations of different calcium species in solution are controlled by the following reversible reactions (Somasundaran and Agar, 1967):
Ca2þ þ OH þ CaOHþ ;
K 1 ¼ 101:40 ;
CaðOHÞþ þ OH þ CaðOHÞ2ðaqÞ ; CaðOHÞ2ðaqÞ þ CaðOHÞ2ðsÞ ;
K 2 ¼ 101:37 ;
K 2 ¼ 102:45 :
Fig. 12. Specification diagram of calcium ions in solution.
The equilibrium concentrations of Ca2+, CaOH+, and Ca(OH)2(aq) in a solution containing 5 103 M of total calcium ions were calculated and shown in Fig. 12. Ca2+ ion is the dominant species in the pH range of 3–12 while concentrations of hydrolyzed species of calcium, i.e., CaOH+, increase with a rise in the solution pH value. Electro-kinetic tests in Fig. 7 indicate that adsorption of calcium onto the monazite surface occurred more readily in the basic pH environment, which coincided with the pH range where hydrolysis of calcium ions occurred. Based on these findings, the solution species resulting in the specific adsorption was mainly Ca(OH)+. The precipitation of calcium hydroxide occurs at pH values larger than 13.1. As such, at pH 9.0, the adsorption of hydroxamic acid by calcium hydroxide forms in solution does not need to be considered. As previously presented in this paper, the adsorption study indicated that calcium adsorption on monazite was less than a monolayer. The adsorption occurred within the Stern layer and thickness of the adsorbed layer was the radius of the calcium ions plus the diameter of a hydroxyl group, i.e., 1.00 Å plus 2.66 Å. With the addition of 1 104 M Ca2+, the adsorption density of the calcium ions onto monazite surfaces is around 1.5 lM/m2. Based on the adsorption layer thickness, adsorption density, and dissociation reactions of calcium in solution, the Ca(OH)+ concentration and pH value corresponding to calcium precipitation near the monazite surface were calculated to be 1.5 M and 11.30,
Fig. 11. Ball and stick model of the cleavage of monazite (CePO4) occurring on (1 0 0) plane.
W. Zhang et al. / Minerals Engineering 100 (2017) 40–48
47
Fig. 13. Model of interaction among monazite, calcium ions, and hydroxamic acid.
respectively. Hydroxyl ions were likely repelled from the surface due to electrostatic repulsion, which means pH values beyond 11.30 in bulk solution is required for the occurrence of surface precipitation. Therefore, at pH 9.0, calcium ions were adsorbed as hydrolyzed species (CaOH+) on the monazite surfaces, which is also true for the higher concentrations investigated in the current study. The adsorption of CaOH+ on monazite is more likely to occur on the neutralized and deprotonated rare earth oxygen and phosphate oxygen sites via hydrogen bonding and electrostatic interactions, respectively, which could be represented by the following equations:
P
OH
OH
O
REE
OH
NH
C
formation of chelates on monazite surface. Furthermore, adsorption of hydroxamic acid molecules onto phosphate oxygen sites was also likely due to the existence of hydrogen bonding. The phosphate oxygen-to-rare earth oxygen sites ratio is 1:1 based on both the crystal data and stoichiometry of monazite. The adsorption of hydroxamic acid on phosphate oxygen sites is further proved by the adsorption test, i.e., near 75% of monazite surface was occupied by the hydroxamic at the predetermined conditions. Overall, the adsorption of hydroxamic acid on monazite can be represented by the following expression:
O P R
REE
OH O
OH NH R C
O
NH
REEOH þ CaOHþ REEOH OHCaþ ; REEO þ CaOHþ ¢ REEOCaOH; POH þ CaOHþ ¢ POH OHCaþ ; PO þ CaOHþ ¢ POCaOH: 4.3. Adsorption mechanism of hydroxamic acid on monazite As a typical chelating reagent, adsorption of hydroxamic acid has been well studied for many minerals such as chryscolla, iron oxide, manganese dioxide, and bastnaesite (Fuerstenau et al., 1970, 1967; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1985, 1983; Raghavan and Fuerstenau, 1975). In spite of some controversial opinions such as questions regarding the active species of hydroxamic acid in solution, chelates of metal complexes are generally considered to be formed by the hydroxamic acid and characterized by a ring structure with the metal as the central atom surrounded by the N and O atoms (Nagaraj, 1988). FTIR tests in this study indicated the involvement of the C@O, NAH, and CAN in the chemical adsorption of hydroxamic acid on monazite, which proved the
C
R
4.4. Effects of calcium on the adsorption of hydroxamic acid on monazite Surface hydrolysis has been proved necessary for hydroxamic adsorption on minerals (Fuerstenau et al., 1970, 1967; Nagaraj, 1988; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1983, 1985). In the current study, the importance of surface hydrolysis was also observed, i.e., optimum flotation and adsorption occurred at pH 9.0 where concentrations of rare earth hydrolyzed species reached maximum. Based on the fact that hydroxamic acid is strongly complexed with the rare earth atoms, the effects of CaOH+ on the adsorption of hydroxamic acid on rare earth oxygen sites may be ignored. However, for the phosphate oxygen sites, both CaOH+ and hydroxamic acid adsorbed via hydrogen bonding and competitive adsorption was expected to occur, which explains the decreases of hydroxamate adsorption when small dosage of calcium ions was added. Adsorption density of hydroxamic acid was increased with the addition of higher dosage of calcium ions (see Fig. 6). It was reasonable to suggest that the phosphate oxygen sites were covered with a layer of Ca(OH)+ attached via hydrogen bonding or electrostatic attraction. The electrostatically adsorbed CaOH+ with the hydroxyl group oriented away from the solid surface provides alternative
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active sites for hydroxamic acid adsorption, which agrees with the fact that the hydrolysis of metal atoms on minerals is required for formation of chelates with hydroxamic acid (Fuerstenau et al., 1970, 1967; Natarajan and Fuerstenau, 1983; Peterson et al., 1965; Pradip and Fuerstenau, 1983, 1985). The shifts of the adsorption peaks (1604, 1607, 1608 cm1) of monazite which interacted with both calcium and hydroxamic acid also suggested the formation of calcium-hydroxamate chelates on monazite surfaces. The primary hydration sheath of CaOH+ adsorbed via hydrogen bonding was difficult to remove, which increased the surface hydrophilicity. The combined effects were that stronger surface hydrophobicity was not observed with the addition of calcium ions despite the adsorption of additional hydroxamic acid. The interaction model between monazite, hydroxamic acid, and calcium ions is shown in Fig. 13. However, surface and bulk precipitation of calcium hydroxamate occurred with the addition of higher dosages of calcium and hydroxamic acid, i.e., 5 103 M of Ca2+ and 3 103 M hydroxamic acid in the current study.
5. Conclusions The flotation of monazite in the presence of a calcium-based mineral (i.e., calcite) has been systematically studied using hydroxamic acid as the collector. Test results indicate that the presence of calcium ions, which exist thorough dissolution of the calciumbased mineral, suppress the floatability of the monazite at collector concentrations below 3.0 104 M. Furthermore, calcite readily floats at or above this concentration thereby eliminating any selectivity between the two minerals. Both rare earth oxygen and phosphate oxygen sites exist on monazite surfaces. The adsorption of CaOH+ is a result of both hydrogen bonding and electrostatic attraction for which the orientation of the ion is determined by the adsorption mechanism. At low collector concentrations and in the presence of calcium ions, competitive adsorption of CaOH+ and hydroxamic acid occurs on the phosphate oxygen sites. The competitive adsorption leads to a decrease in the hydroxamic acid adsorption density on the monazite surfaces thereby resulting in low flotation recovery values. As the calcium concentration is increased in solution, the electrostatically adsorbed CaOH+ provides an additional site for the adsorption of the hydroxamic acid collector. Although this action results in an increase in the adsorption density of the hydroxamic acid, hydrophobicity and flotation recovery of the monazite decreases due to hydration of the hydrogen bonded CaOH+ sites on the monazite surface. Acknowledgements The research results presented in this publication was funded in part by the Kentucky Energy and Environment Cabinet (Project ID No. PON2 127 150002394 1). References Abeidu, A.M., 1972. The separation of monazite from zircon by flotation. J. Less Common Met. 29 (2), 113–119. Amankonah, J.O., Somasundaran, P., Ananthapadmabhan, K.P., 1985. Effects of dissolved mineral species on the dissolution/precipitation characteristics of calcite and apatite. Colloid Surf. 15, 295–307. Ananthapadmanabhan, K.P., Somasundaran, P., 1985. Surface precipitation of inorganics and surfactants and its role in adsorption and flotation. Colloid Surf. 13, 151–167. Ananthapadmanabhan, K.P., Somasundaran, P., 1984. The role of dissolved mineral species in calcite-apatite flotation. Miner. Metall. Process 1 (1), 36–42. Atesok, G., Somasundaran, P., Morgan, L.J., 1988. Adsorption properties of Ca2+ on Na-kaolinite and its effect on flocculation using polyacrylamides. Colloid Surf. 32, 127–138.
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