Flotation of monazite in the presence of calcite part II: Enhanced separation performance using sodium silicate and EDTA

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Flotation of monazite in the presence of calcite part II: Enhanced separation performance using sodium silicate and EDTA Wencai Zhang, Rick Q. Honaker

⁎

University of Kentucky, Lexington, KY 40506-0107, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Flotation Monazite Calcite Sodium silicate Sodium hexametaphosphate Citric acid EDTA

Selective depression of calcite in a monazite-calcite flotation system was systematically studied using microflotation and electrokinetic tests as well as X-ray photoelectron spectroscopy (XPS) characterization and solution chemistry calculations. Monazite was found to be depressed by both sodium silicate and sodium hexametaphosphate when calcium ions appeared in solution. It was hypothesized that co-adsorption of calcium species (CaHCO3+ and CaCO3(aq)) and Si (OH )4 as well as SiO (OH )−3 via hydrogen bonding formed a compact layer over the monazite surface, thereby reducing octanohydroxamic acid adsorption by steric hindrance. The use of citric acid or EDTA formed soluble chelates (e.g. CaCit− and CaEdta2−) with the calcium ions which negated the flotation suppression effect. An XPS study indicated that both EDTA and citric acid chemically adsorbed on the monazite surface, while the adsorption on the calcite surface was achieved mainly through hydrogen bonding. EDTA was more effective than citric acid due to two reasons: (i) citric acid itself presented stronger depression effects and (ii) citric acid has weaker chelating properties for calcium. Using 2.5 × 10−4 M octanohydroxamic acid, 0.05 g/L sodium silicate and 6 × 10−5 M EDTA, monazite recovery of more than 90% was achieved while recovering only 20% of the calcite.

1. Introduction Monazite is a rare earth mineral normally associated with heavy mineral sands which are a class of ore deposits and an important source of zircon, rutile and ilmenite (Cheng, 2000; Pavez and Peres, 1994). Concentration of monazite from the heavy minerals using froth flotation has been well studied (Abeidu, 1972; Cheng et al., 1993; Cheng, 2000; Dixit and Biswas, 1969; Nduwa-Mushidi and Anderson, 2017; Pavez and Peres, 1993, 1994; Qi, 1993; Zhang et al., 2016). The selective recovery of the monazite required the use of appropriate collectors (e.g., fatty acid) under mild alkaline conditions and the addition of depressants (e.g., sodium silicate) (Nduwa-Mushidi and Anderson, 2017; Pavez and Peres, 1994; Qi, 1993). Due to an increasing demand for rare earth elements, alternative rare earth resources have been explored including materials such as coal, fly ash and red mud (Franus et al., 2015; Fulford et al., 1991; Honaker et al., 2014, 2016b, 2017; Zhang et al., 2015, 2017a, 2018b). The recovery of monazite from coal and coal byproducts using flotation was somewhat successful as reported by Honaker et al. (2014, 2017) and Zhang et al. (2015, 2017a). However, other minerals associated with the sources such as calcite, dolomite, rutile and pyrite were found to interfere with the concentration process. Espiritu and Waters (2018)

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also reported on the presence of dolomite in a monazite deposit as the main host mineral which results in a difficult flotation system due to its solubility published reports on the separation of monazite from minerals with higher solubility (e.g., calcite and dolomite) using a froth flotation process are few. Research have been performed regarding the flotation separation of the other types of rare earth minerals from the more soluble minerals, e.g., calcite, dolomite, and barite (Cui and Anderson, 2017a, b; Fuerstenau et al., 1992; Pradip and Fuerstenau, 1991, 1983). Pradip and Fuerstenau (1991) did an adsorption study of octanohydroxamic acid on bastnaesite and calcite surfaces. It was found that the hydroxamate adsorption on calcite increased significantly at higher collector dosages at pH 9.3 due to surface and/or bulk precipitation of calcium hydroxamates. Cui and Anderson (2017a, b) reported that surface precipitation of CaCO3 and SrCO3 could occur on ancylite [Sr(Ce,La) (CO3)2(OH)%H2O] surface in basic conditions, which reduced the hydroxamate adsorption on the rare earth mineral surface. In the current study, the separation of monazite from calcite in a combined system using froth flotation with appropriate depressants (e.g. sodium silicate) and regulators (e.g., citric acid and EDTA) was systematically studied by micro-flotation and electrokinetic tests as well as X-ray photoelectron spectroscopy (XPS) characterization and

Corresponding author. E-mail address: [email protected] (R.Q. Honaker).

https://doi.org/10.1016/j.mineng.2018.01.042 Received 11 August 2017; Received in revised form 30 January 2018; Accepted 31 January 2018 0892-6875/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Zhang, W., Minerals Engineering (2018), https://doi.org/10.1016/j.mineng.2018.01.042

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Fig. 1. XRD patterns of (a) pure monazite (M) and (b) calcite (C) minerals.

mixed with 50 mL of deionized water in a glass beaker and conditioned using a magnetic stirrer. Next, the solution pH value was adjusted via the addition of sodium hydroxide or hydrochloric acid solutions. Subject to the purposes of tests, calcium ions, regulators, depressants and collector were added sequentially with 5 min of conditioning after each addition. Finally, frother was added and the solution was conditioned for another 30 s. The solution pH values were adjusted every 2.5 min during the conditioning process and the values measured directly before the flotation tests were recorded. After conditioning, the solution was transferred into the cell and flotation was conducted continuously for a period of 3 min using nitrogen at a flow rate of 55 mL/min. The flotation concentrates and tailings were dried in a desiccator and the dry weights were measured and recorded for recovery calculations. For the combined mineral flotation tests, monazite was mixed with calcite at a mass ratio of 1:1 and recovery values were calculated 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%.

solution chemistry calculations. An in-depth understanding was obtained for the important role calcium ions play in the monazite-calcite flotation system in terms of selective depression. An efficient reagent scheme was formulated based on the test results and evaluated for its performance in achieving an efficient monazite-calcite separation. 2. Materials and methods 2.1. Materials Monazite pre-concentrate was obtained from a rare earth mine in China and calcite samples were collected from a limestone mine in Kentucky, USA. The monazite pre-concentrate was upgraded using a shaking table and a permanent neodymium magnet bar. The permanent magnet primarily removed hematite from the pre-concentrate. As a result of the upgrading, monazite and calcite concentrates with a purity level of nearly 95% were obtained for the study as determined by inductively coupled plasma mass spectrometer (ICP-MS) and X-ray diffraction (XRD) analysis (Fig. 1). The material used for micro-flotation tests was from a narrow particle size fraction of 300 × 150 μm which was attained by screening the monazite material. Crushing and screening was needed for the calcite sample. For the zeta potential and adsorption studies, representative portions of the two materials were ground using a corundum pestle and mortar to achieve a particle size below 5 μm. Octanohydroxamic acid of minimal 95% purity was purchased from TCI America for use as the collector. Methyl isobutyl carbinol (MIBC) of more than 99% purity was used as the frother and purchased from Alfa Aesar. Technical grade sodium silicate and ACS grade sodium hexametaphosphate were purchased from Fisher Scientific for use as calcite depressants. Citric acid and ethylenediaminetetraacetic acid (EDTA) of ACS grade were used as regulators and purchased from VWR. Trace metal grade hydrochloric acid and ACS grade sodium hydroxide that purchased from Fisher Scientific were used to regulate slurry pH values. All other chemicals used in the study such as the calcium chloride and potassium chloride were of ACS grade and purchased from Fisher Scientific.

2.2.2. Electrokinetic tests Zeta potential measurements were conducted to study the effects of calcium, regulators and depressants on electrokinetic properties of monazite and calcite. Particles of monazite and calcite finer than 5 μm were used in the study. Approximately 0.04 g solids were dispersed in 40 mL of 1 × 10−3 M potassium chloride solution by 30 s of sonification. Conditioning procedures were similar to that used for the microflotation tests. After conditioning, the suspension was settled for 5 min and the supernatant was collected for measurement. Electro-kinetic tests were conducted using a ZetaPlus zeta potential analyzer manufactured by Brookhaven Instrument Corporation. Three repeated tests were conducted under the same condition and the experimental error measurement indicated a standard deviation value of 2.23%. 2.2.3. X-ray photoelectron spectroscopy (XPS) characterization XPS characterization was performed to study the surface chemistry changes of the minerals after interacting with the different reagents. Monazite and calcite particles finer than 5 μm were used in the study. The conditioning procedures were similar to that used for the microflotation and electrokinetic tests. The samples were dried at room temperature in an oven. The characterization was conducted using a PHI VersaProbe III XPS equipment. Spectra were acquired using a monochromatic Al K-alpha X-ray source (1486.6 eV) at 100 W over 1400 × 100 μm area at 45° angle. The samples were prepared in the

2.2. Methods 2.2.1. Micro-flotation tests A micro-flotation cell with a volume of 50 mL was used in the study. Air bubbles were generated by sparging nitrogen through a porous material fixed at the bottom of the cell. Pure minerals of 1 g were first 2

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Table 1 Reactions and corresponding thermodynamic reaction constants used for solution chemistry calculations. Reactions

Reaction Constants

H2 O ⇆ H + + OH−

10−14

Ca2 + + OH− ⇆ Ca (OH )+

101.4

Ca2 + + 2OH− ⇆ Ca (OH )2(aq)

102.77

Ca2 + + 2OH− ⇆ Ca (OH )2(s ) CO2(g ) ⇆ CO2(aq)

105.22

CO2(aq) + H2 O ⇆ CO32 − + 2H +

10−16.68

CO32 − + H + ⇆ HCO3−

1010.33

Ca2 + + CO32 − ⇆ CaCO3(aq)

103.22

CaCO3(s ) ⇆

Ca2 +

+

10−1.47

10−8.34

CO32 −

Ca2 + + CO32 − + H + ⇆ CaHCO3+ SiO2(s ) + 2H2 O ⇆ Si (OH )4(aq)

1011.44

Si (OH )4 + OH− ⇆ SiO (OH )− 3 + H2 O

10 4.29

2− − SiO (OH )− 3 + OH ⇆ SiO2 (OH )2 + H2 O

100.99

4Si (OH )4 +

⇆ Si4 O6 (OH )62 − CitH2 −

2OH−

H + + Cit 3 − ⇆

10−2.64

+ 6H2 O

106.33

2H + + Cit 3 − ⇆ CitH2−

1011.05

3H + + Cit 3 − ⇆ CitH3

1014.18

Ca2 +

1011.35

+

Cit 3 −

⇆

CaCit−

Ca2 + + Cit 3 − + H + ⇆ CaCitH

1018.48

H + + Edta4 − ⇆ EdtaH3 −

109.96

2H + + Edta4 − ⇆ EdtaH22 −

1016.61

EdtaH3−

1018.86

4H + + Edta4 − ⇆ EdtaH4

1020.93

5H + + Edta4 − ⇆ EdtaH5+

1023.464

Ca2 + + Edta4 − ⇆ CaEdta2 −

1012.4

Ca2 + + Edta4 − + H + ⇆ CaEdtaH−

1016

3H +

+

Edta4 −

⇆

Fig. 2. Effects of sodium silicate on the single mineral and mixed monazite and calcite flotation systems when using 2.5 × 10−4 M octanohydroxamic acid at pH 9.0.

1015.03

calcite floatability in combined-mineral flotation systems (Feng et al., 2015; Gao et al., 2018). Effects of depressant type and dosage were measured when floating monazite and calcite in separate tests and when the two minerals were mixed. All micro-flotation tests were conducted at a solution pH value of 9.0. Based on a previous study, maximum flotation recovery of monazite using octanohydroxamic acid occurred at pH 9.0 (Zhang et al., 2017b). As shown in Figs. 2 and 3, both sodium silicate and SHMP had minor effects on the floatability of monazite in the single mineral tests. However, calcite recovery values decreased significantly with an increase in depressant dosage. Complete depression occurred using either 5 × 10−6 M SHMP or 0.025 g/L sodium silicate. The individual mineral flotation test findings indicate that both SHMP and sodium silicate are promising depressants for the monazite-calcite flotation system. However, when flotation tests were conducted in a mixed system of monazite and calcite, monazite was depressed using either SHMP or sodium silicate (Figs. 2 and 3). Given that both monazite and calcite were depressed, the data suggests that an efficient separation of monazite and calcite at pH 9.0 when using SHMP and 2.5 × 10−4 M octanohydroxamic acid was not possible. However, using a small sodium silicate dosage (i.e., 0.01 g/L), monazite recovery of nearly 90% was obtained while only recovering 30% of the calcite (Fig. 2). Dissolution of calcium and carbonate from calcite surfaces occurs due to the relatively high solubility of calcite. In a mixed suspension

(Data sources: Marinakis and Shergold, 1985; Westin and Rasmuson, 2005; Zhang et al., 2017b. Note: CitH and Edta represent citric acid and EDTA, respectively).

natural atmosphere and hydrocarbon contaminants occurred on their surfaces. As such, all binding energies were referenced to adventitious CeC at 284.8 eV, which is commonly utilized to characterize air-exposed samples. Chemical states of the elements were assigned based on the PHI and NIST XPS Databases. 2.2.4. Solution chemistry calculation An aqueous chemistry study was performed to determine the active species in solution and obtain a fundamental understanding of the system. It is well known that, when metallic cations (e.g., Ca2+) and/or organic acids (e.g., citric acid and EDTA) are added to water, a series of reactions occur and each reaction is controlled by a thermodynamic reaction constant. The overall system obeys mass conservation and charge balance laws. The species concentration at certain pH values can be calculated using the reaction constants, mass conservation, etc. In the current study, the distribution of solution species as a function of pH values was calculated for four different systems, i.e., silica in water, calcium in water, calcium plus citric acid in water, and calcium plus EDTA in water. The reactions and constants used for calculation are listed in Table 1. The partial pressure of carbon dioxide utilized for the calculations was 10−3.5. 3. Results and discussion 3.1. Micro-flotation tests The selectivity achieved by the froth flotation process can sometimes be enhanced by the use of an appropriate depressant and dosage level. In the current study, sodium silicate and sodium hexametaphosphate (SHMP) were used as depressants in an attempt to improve selectivity when concentrating monazite in the presence of a carbonate mineral, i.e., calcite. Both reagents have been widely utilized to depress

Fig. 3. Effects of sodium hexametaphosphate (SHMP) on the single mineral and combined flotation of monazite and calcite systems when using 2.5 × 10−4 M octanohydroxamic acid at pH 9.0.

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Fig. 6. Effects of citric acid and EDTA on single mineral flotation of the monazite and calcite.

Fig. 4. Effects of calcium ions and sodium silicate on the monazite single mineral flotation (the solid line with open symbols represents sodium silicate was added before calcium ions).

monazite surfaces. Both the single and combined mineral flotation tests indicated that sodium silicate performed better than SHMP in the presence of calcium ions (Figs. 2–5). As such, in subsequent tests, sodium silicate was used as the depressant for calcite. To achieve an efficient separation of monazite from calcite, the negative effects of calcium ions was remedied using appropriate regulators. In the current study, two chelating reagents, i.e., citric acid and EDTA, were used to provide a cleaning action for the monazite surfaces by chelating with and removing the calcium ions. The effects of the two regulators were evaluated for the flotation of both the pure monazite and calcite minerals. As shown in Fig. 6, both monazite and calcite recovery values were decreased with an increase in regulator dosage. Citric acid had stronger depression effects compared to EDTA and had the largest effect on reducing the floatability of calcite. Depression was not expected based on the fact that the purpose was to remove calcium ions from the monazite surfaces. However, when using a regulator dosage less than 5 × 10−4 M, the depression effect was negligible. To evaluate the efficiency of citric acid and EDTA to remove calcium from the monazite surfaces, flotation tests were conducted on a system containing monazite and calcium ions that was treated with sodium silicate at a given concentration and a chelating agent over a range of dosages. Calcium, chelating reagents (i.e., citric acid and EDTA), and sodium silicate were added sequentially into the flotation system. As shown in Fig. 7, flotation recovery of the monazite increased from 10%

Fig. 5. Effects of calcium ions and SHMP on the monazite single mineral flotation (the dashed line with open symbols represents SHMP was added before calcium ions).

containing both monazite and calcite, the dissolved calcium ions adsorb onto the monazite surface, which hinders collector adsorption and thus suppressing the floatability of monazite (Zhang et al., 2017b). Figs. 4 and 5 show the effect of adding calcium ions and sodium silicate and SHMP over a range in concentrations on the flotation behavior of monazite in a single mineral system. The flotation recovery values were sensitive to calcium concentration when calcium ions and sodium silicate or SHMP co-existed in the system. For example, with the addition of 0.05 g/L sodium silicate, the recovery values decreased from nearly 85% to less than 10% with an elevation in calcium concentration from 1 × 10−4 to 2.5 × 10−4 M. Previous studies showed that SHMP can interact with mineral surfaces via chelation with cationic ions such as Mg2+ and Ca2+ (Lu et al., 2011; Hong et al., 2012; Castellini et al., 2013). As such, the depression effect of SHMP on monazite flotation in presence of calcite is due to the complexation of SHMP with calcium species on the monazite surface, which prevents collector adsorption. Additional flotation tests were conducted to determine if adding the dispersants prior to the addition of the calcium ions has an impact on the monazite flotation response. As shown in Figs. 4 and 5, the sequence of additions did not provide a significant impact, indicating that, instead of an ordered structure with calcium immediately above monazite surfaces and depressant next to the calcium layers, the co-adsorbed species more likely formed a disordered compact layer around the

Fig. 7. Effects of citric acid and EDTA dosages on the pure monazite flotation in a system containing 2.5 × 10−4 M Ca2+ and 0.05 g/L sodium silicate.

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Fig. 8. Effects of citric acid and EDTA dosages on the monazite and calcite combined mineral flotation separation system using 0.05 g/L sodium silicate and 2.5 × 10−4 M octanohydroxamic acid.

Fig. 11. Effects of calcium, citric acid and EDTA on the zeta potential of monazite as a function of solution pH.

EDTA on the monazite surfaces. Combined mineral flotation tests were conducted using both monazite and calcite to evaluate the effects of citric acid and EDTA on monazite-calcite separation performance. As shown in Fig. 8, both citric acid and EDTA improved monazite recovery from 60% to about 90% while having no impact on calcite recovery which was relatively constant at around 20% over the entire dosage range. As such, the test results indicate that the negative effects caused by calcium ion dissolution from the calcite surfaces can be eliminated using appropriate regulators such as citric acid and EDTA. Furthermore, EDTA performed better than citric acid, which agreed with the test results presented in Figs. 6 and 7. The inferior performance of citric acid was related to its stronger depression effects on the monazite floatability (Fig. 6). 3.2. Electrokinetic tests Adsorption of chemicals such as the depressants on mineral surfaces potentially changes the electrokinetic properties such as the isoelectric point (IEP) and potential distribution. As such, electrokinetic tests were conducted to evaluate the adsorption of calcium, sodium silicate, citric acid and EDTA on monazite surfaces. Fig. 9 shows the effects of 1 × 10−4 M Ca2+ and/or 0.05 g/L sodium silicate on the monazite surface potential. The IEP occurred at pH 5.3, which agreed with data reported by Cheng (2000) and Zhang et al. (2017b). Zeta potential changed more significantly in basic environments with the addition of 1 × 10−4 M Ca2+, which was due to the specific adsorption of the calcium species as reported by Zhang et al. (2017b). Strong specific adsorption of sodium silicate on monazite surfaces occurred as indicated by a significant decrease in the IEP value. Based on the fact that sodium silicate has no effect on monazite flotation when using octanohydroxamic acid as the collector (Fig. 2), hydroxamic acid has a higher affinity for monazite than sodium silicate. For pH values less than 5.5, the addition of sodium silicate together with calcium ions generated a nearly equal change in the electrokinetic properties as compared to the addition of sodium silicate only. However, for pH values above 5.5, the absolute zeta potential value of monazite conditioned with both sodium silicate and calcium ions remained nearly constant with an increase in the solution pH value. As such, co-adsorption of calcium and sodium silicate likely occurred on the monazite surfaces when pH values exceeded 5.5. The IEP value of the calcite used in the study occurred at about pH 10.5 (Fig. 10). Decreases in zeta potential was observed when adding sodium silicate, citric acid or EDTA and the isoelectric points did not occur within the studied pH range, which indicated the occurrence of specific adsorption. Furthermore, hydroxamic acid adsorption on

Fig. 9. Effects of calcium and/or sodium silicate on the monazite zeta potential.

Fig. 10. Effects of sodium silicate, citric acid and EDTA on the calcite zeta potential.

to about 85% and 90% at optimum dosage levels for both citric acid and EDTA, respectively. As such, both chelating agents eliminated the suppression of monazite flotation by preventing the co-adsorption of sodium silicate and calcium species on monazite surfaces. However, flotation recovery decreased when using dosages higher than the optimum level, which is due to the adsorption of excessive citric acid and 5

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Fig. 12. Ce 3d photoelectron spectra of (a) monazite conditioned in deionized water at pH 9.0, (b) monazite conditioned in 0.001 M EDTA solution at pH 9.0, and (c) monazite conditioned in 0.001 M citric acid solution at pH 9.0.

the zeta potential values with and without the presence of calcium ions were nearly equal, which was due to the lack of hydrolyzed calcium species. However, in high pH solutions, hydrolyzed calcium species specifically adsorbed onto the monazite surface. In addition, both citric acid and EDTA form soluble complexes and remove the calcium ions from the monazite surface. As such, when calcium ions were added together with citric acid and/or EDTA, the zeta potentials of monazite were expected to remain unchanged. Fig. 11 shows that, for high pH values, the zeta potential of the monazite conditioned with both calcium and regulators, especially EDTA, was close to the untreated monazite which provided evidence of calcium ion removal from the monazite surfaces.

calcite surfaces is fairly weak due to the lower stability of the hydroxamate-calcium complex (Pradip and Fuerstenau, 1983; Ni and Liu, 2012; Zhang et al., 2017b). As such, sodium silicate, citric acid or EDTA competes well with hydroxamic acid for the calcite surface sites, which results in reduced floatability. The IEP of monazite decreased significantly when conditioned with 1 × 10−4 M citric acid or EDTA, indicating the occurrence of specific adsorption (Fig. 11). However, both citric acid and EDTA at the same concentration had minimal effects on monazite floatability when using 2.5 × 10−4 M octanohydroxamic acid as the collector, which indicated a stronger interaction between the collector and monazite surfaces. Furthermore, as shown in Fig. 11, citric acid generated a more negative charge on the monazite surfaces as compared to EDTA at the same dosage, which agreed with the findings that citric acid reduced the floatability of monazite to a greater degree. When calcium ions were added together with the regulators, the zeta potentials were between the values observed for monazite conditioned with regulators and untreated monazite. In the low pH solutions,

3.3. X-ray photoelectron spectroscopy characterization To investigate the surface chemistry changes of monazite and calcite after interaction with the different reagents, X-ray photoelectron spectroscopy (XPS) characterization was performed. The effects of citric 6

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Fig. 13. Ca 2p photoelectron spectra of (a) calcite conditioned in deionized water at pH 9.0, (b) calcite conditioned in 0.001 M EDTA at pH 9.0, and (c) calcite conditioned in 0.001 M citric acid at pH 9.0.

acid and EDTA on the monazite and calcite surface chemistry were first studied. Two peaks (882.23 eV and 885.91 eV) were observed from Fig. 12, which belongs to the Ce 3d5/2 spin-orbit components (Cui et al., 2012; Mullins et al., 1998; Praline et al., 1980). The dominant peak (882.23 eV) shifted to 881.99 eV and 882.02 eV, respectively, after conditioning with 0.001 M EDTA and 0.001 M citric acid. As such, EDTA has a stronger interaction with the cerium species on the monazite surface compared with citric acid (0.24 eV versus 0.21 eV binding energy shifts). In addition, the large binding energy changes indicated that citric acid and EDTA were chemically adsorbed onto the monazite surface, which agrees with the isoelectric point (IEP) changes of monazite after interaction with the two chemicals (Fig. 11). It has to be notified that, despite EDTA having a stronger interaction with monazite, its depression effect on monazite flotation is weaker than citric acid (Fig. 8).

Two bands were observed for the Ca 2p spectra of the calcite surface, which were assigned to the Ca 2p1/2 and Ca 2p3/2 spin-orbit doublet (Gao et al., 2018; Zhang et al., 2018a). The Ca 2p3/2 peak at 346.9 eV of the calcite shifted slightly (0.1 eV) after being conditioned with 0.001 M EDTA and no shift was observed with the addition of 0.001 M citric acid (Fig. 13). Zeta potential tests indicated that both EDTA and citric acid significantly shifted the IEP of the calcite. As such, instead of chemical adsorption, the two chemicals were adsorbed via hydrogen bonding. This finding also indirectly proved the weak adsorption of hydroxamate collectors on the calcite surface. Both the micro-flotation and electrokinetic test results indicated that calcium species co-adsorbed with silicate species on the monazite surface, which reduced the collector adsorption. XPS characterization was performed to investigate the adsorption of calcium species on the monazite surfaces. As shown in Fig. 14, with an increase in the calcium 7

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Fig. 14. C 1s photoelectron spectra of (a) monazite conditioned in 0.0001 M Ca2+ solution at pH 9.0, (b) monazite conditioned in 0.001 M Ca2+ solution at pH 9.0, and (c) calcite conditioned in deionized water at pH 9.0.

based on a 1 × 10−3 M silica concentration was developed as shown in Fig. 16. The findings indicated that Si(OH)4 and SiO(OH)−3 species are dominant in solutions having pH values below and above 9.8, respectively. Both species have hydroxyl groups, which explains the specific adsorption of sodium silicate on the monazite and calcite surfaces via hydrogen bonding. Fig. 17 (a) shows the speciation diagram of 2.5 × 10−4 M Ca2+ in water. CaHCO3+ and CaCO3(aq) are the dominant species at pH 9.0 in the system, which agrees with the XPS study. The concentration of the two species increases with the increase in solution pH values, which also agrees with the conclusion of electrokinetic study that stronger specific adsorption of calcium species on the monazite surface occurred in basic conditions. Aqueous carbonate species such as CO32− and HCO3− can form hydrogen bonds with water molecules, which indirectly supports the formation of hydrogen bonding between the calcium carbonate species (CaHCO3+ and CaCO3(aq)) and silicate species on the monazite surface (Kumar et al., 2008). The aqueous calcium carbonate species and silicate species form a compact hydrophilic layer on the monazite surface, which reduced the collector adsorption by

concentration, the C 1 s peak corresponding to the C]O group shifted from 288.85 eV to 289.02 eV which was close to that of pure calcite (289.43 eV). As such, calcium carbonate species occurred on the monazite surface and the surface characteristic of the monazite was similar to that of the calcite at high dosages of calcium. Ca photoelectron spectra on the monazite surface were characterized to understand the co-adsorption of calcium and silicate species on the surface. Ca 2s spectra were selected due to the overlap of binding energies between Ca 2p and Th 4f. As shown in Fig. 15, no binding energy shift was observed when sodium silicate was added into the monazite plus calcium system. As such, instead of chemisorption, the co-adsorption of calcium and silicate species on the monazite surface was more likely through physical interactions.

3.4. Solution chemistry study Sodium silicate at a concentration of 0.05 g/L as used for the flotation and electrokinetic tests contains 8.33 × 10−4 M of silica assuming the SiO2 content was 100%. As such, a speciation diagram 8

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Fig. 15. Ca 2s photoelectron spectra of (a) monazite conditioned in 0.001 M Ca2+ solution at pH 9.0 and (b) monazite conditioned in 0.001 M Ca2+ and 0.4 g/L sodium silicate solution at pH 9.0.

on the monazite surface did not occur. The speciation diagrams of two different systems, i.e., calcium-citric acid-water and calcium-EDTA-water, are shown in Fig. 17(b) and (c), respectively. Instead of the Ca2+ or CaCO3(aq) species, CaCit− and CaEdta2− were the dominant species when calcium was mixed with citric acid and EDTA, respectively. The CaCit− and CaEdta2− chelates are more soluble in water which favors calcium staying in solution instead of being adsorbed on the monazite surfaces. As such, the negative effects of calcium species were removed using the chelating agents. As shown in Table 1, the stability constants of the complexes formed between EDTA and calcium are larger than that of the complexes formed between citric acid and calcium ions, which also explains the better performance of EDTA (Fig. 8). In addition, CaCO3(s) did exist in all the three systems shown in Fig. 17(a)–(c), which excludes the possibility that CaCO3(s) formed on the monazite surface and reduced collector adsorption. 4. Conclusions Fig. 16. Speciation diagram of silicate in water with a total concentration of 1 × 10

−3

M.

In a mixed flotation system of monazite and calcite, the effects of calcium ions on depressing the floatability of the two minerals were systematically evaluated with and without the use of sodium silicate and sodium hexametaphosphate as depressants. Micro-flotation and electrokinetic tests as well as X-ray photoelectron spectroscopy (XPS) characterization and solution chemistry calculations were utilized for the study. Monazite was found to be depressed by both sodium silicate and sodium hexametaphosphate when small amounts of calcium ions (e.g., 1 × 10−4 M) were present in solution. Based on the electrokinetic tests, XPS study and solution chemistry calculations, the depression was attributed to the co-adsorption of calcium species (CaHCO3+ and CaCO3(aq)) and Si(OH)4 as well as SiO(OH)−3 , which formed a compact layer on the monazite surfaces and thereby reducing the collector

steric hindrance. In addition to hydrogen bonding, surface reaction has also been suggested to occur between calcium and silicate ions on calcite surfaces (Mishra, 1982; Marinakis and Shergold, 1985; Rao et al., 1988, 1990, 1989; Feng et al., 2015). The surface reactions were represented as follows: + − 2+ Ca(2surf ) + SiO (OH )3 ⇄ Ca −SiO (OH )3(surf )

Ca2 +−OH(surf ) + SiO (OH )4 ⇄ Ca2 +−OSi (OH )3(surf ) + H2 O However, the XPS characterization of the current study showed that the chemical reaction between calcium carbonate and silicate species 9

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Analysis Laboratory at Virginia Tech for the XPS characterization and the XPS data analysis. References Abeidu, A.M., 1972. The separation of monazite from zircon by flotation. J. Less Common Met. 29 (2), 113–119. Castellini, E., Berthold, C., Malferrari, D., Bernini, F., 2013. Sodium hexametaphosphate interaction with 2: 1 clay minerals illite and montmorillonite. Appl. Clay Sci. 83, 162–170. Cheng, T.W., 2000. The point of zero charge of monazite and xenotime. Miner. Eng. 13, 105–110. Cheng, T.W., Holtham, P.N., Tran, T., 1993. Froth flotation of monazite and xenotime. Miner. Eng. 6 (4), 341–351. Cui, H., Anderson, C.G., 2017a. Alternative flowsheet for rare earth beneficiation of Bear Lodge ore. Miner. Eng. 110, 166–178. Cui, H., Anderson, C.G., 2017b. Fundamental studies on the surface chemistry of ancylite, calcite, and strontianite. J. Sustain. Metall. 3 (1), 48–61. Cui, J., Hope, G.A., Buckley, A.N., 2012. 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Fig. 17. Speciation diagrams: (a) 2.5 × 10−4 M Ca2+ in deionized water; (b) 2.5 × 10−4 M Ca2+ and 1 × 10−4 M citric acid in deionized water; and (c) 2.5 × 10−4 M Ca2+ and 1 × 10−4 M EDTA in deionized water. (Note: A CO2 partial pressure of 10−3.5 was utilized for all the calculations).

adsorption by steric hindrance. Both citric acid and EDTA form soluble chelates (e.g. CaCit− and CaEdta2−) with calcium ions. By the addition of the chelating agents, the calcium species were removed from the monazite surface which eliminated negative floatability effects resulting from the presence of calcium. The chelating agent EDTA was found to perform better than citric acid due to two reasons: (i) citric acid provided a stronger depression effect and (ii) citric acid was a weaker chelator for the calcium ion. The XPS study indicated that EDTA and citric acid were chemically adsorbed on the monazite surface, while the adsorption on calcite was achieved mainly through physical adsorption (hydrogen bonding). Finally, monazite recovery of more than 90% was achieved using 2.5 × 10−4 M octanohydroxamic acid, 0.05 g/L sodium silicate and 6 × 10−5 M EDTA while only recovering 20% of calcite. Acknowledgements We appreciate the help provided by Dr. Xu Feng and the Surface 10

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middlings. Fuel 215, 551–560. Zhang, W., Honaker, R.Q., Groppo, J., 2017a. Concentration of rare earth minerals from coal by froth flotation. Miner. Metall. Proc. 34, 132–137. Zhang, W., Honaker, R.Q., Groppo, J., 2017b. Flotation of monazite in the presence of calcite part I: calcium ion effects on the adsorption of hydroxamic acid. Miner. Eng. 100, 40–48. Zhang, W., Honaker, R., Groppo, J., 2016. Fundamental Study of the Monazite-Calcite Flotation Separation. XXVIII International Mineral Processing Congress, Sep. 11–15, Québec, Canada. 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.

carbonate gangue using sodium oleate as collector and sodium silicate as modifier. Int. J. Miner. Process. 26 (1–2), 123–140. Rao, K.H., Antti, B.M., Forssberg, E., 1988. Mechanism of oleate interaction on salt-type minerals part I. Adsorption and electrokinetic studies of calcite in the presence of sodium oleate and sodium metasilicate. Colloid Surf. 34 (3), 227–239. Westin, K.J., Rasmuson, Å.C., 2005. Nucleation of calcium carbonate in presence of citric acid, DTPA, EDTA and pyromellitic acid. J. Colloid Interf. Sci. 282 (2), 370–379. Zhang, C., Wei, S., Hu, Y., Tang, H., Gao, J., Yin, Z., Guan, Q., 2018a. Selective adsorption of tannic acid on calcite and implications for separation of fluorite minerals. J. Colloid Interf. Sci. 512, 55–63. Zhang, W., Yang, X., Honaker, R.Q., 2018b. Association characteristic study and preliminary recovery investigation of rare earth elements from Fire Clay seam coal

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