3.2. Zeta Potential Analysis
In the flotation process, zeta potential analysis is an important reference method used for interpreting mineral surface properties. Flotation agents exist in different forms at different pH conditions, and they are adsorbed on the surface of minerals thus affecting the zeta potential of minerals [
31,
32,
33].
Figure 5 shows the variation in zeta potential of bastnaesite and calcite samples with/without 4 × 10
−4 mol/L collector (NHA, SHA, NaOL) by pH value. The isoelectric points (IEP) of pure bastnaesite and calcite are about 7.2 and 9.3, respectively, which are consistent with previous studies [
19,
32]. In the presence of collectors (SHA, NaOL), the zeta potentials of the bastnaesite and calcite samples are more negative than those of the pure bastnaesite and calcite samples. The change in IEP can be explained by the collector being adsorbed onto the surface of bastnaesite or calcite, thereby reducing the IEP. The three kinds of collectors are anionic collectors so when the pulp pH value is greater than the IEP, and if there is only electrostatic force, the collector should not be adsorbed onto the surface of bastnaesite or calcite. However, when the surface is negatively charged, the zeta potential of bastnaesite or calcite still decreases. The results show that there is a new effect between NaOL or SHA and bastnaesite or calcite, which is much stronger than the electrostatic effect. In addition, compared with calcite, SHA can significantly reduce the zeta potential of bastnaesite, which could explain why SHA is more easily adsorbed onto the surface of bastnaesite. When pH > 7, there were 20 potential point differences between NHA-treated bastnaesite and pure bastnaesite, indicating that NHA adsorption onto the surface of bastnaesite involved not only physical adsorption but also chemical adsorption. When pH > 10, the calcite site of NHA was almost unchanged, which may indicate that there is no chemical adsorption between NHA and calcite. NHA is an anionic collector. When calcite has a positive potential, its adsorption behavior involves electrostatic adsorption or van der Waals forces and other physical adsorption, resulting in a change in calcite potential.
3.3. Fourier Transform Infrared (FTIR) Spectroscopic Test Results
Figure 6 shows the FTIR spectra before and after the interaction of minerals and reagents.
Table 5 shows the main bands corresponding to the relevant chemical bonds. In the FTIR spectra of NHA, 3752 cm
−1 and 3625 cm
−1 are the stretching vibrations of -OH with CH
3 on the aromatic ring; the characteristic peak near 1535 cm
−1 in the spectrum corresponds to the stretching vibration of naphthalene rings C=H and C=C. The spectra at 1101 cm
−1 and 875 cm
−1 are the symmetric and asymmetric stretching vibrations of =N-O and -C-N, respectively, which are the main functional groups of NHA [
28,
33].
Figure 6a shows the FTIR spectra of bastnaesite samples with and without NHA treatment. The characteristic peaks of
group in pure bastnaesite spectrum are 729 cm
−1, 868 cm
−1, 1087 cm
−1 and 1448.56 cm
−1 [
34]. The new characteristic peaks at 3732 cm
−1, 3672 cm
−1 and 3648 cm
−1 of bastnaesite treated with NHA are caused by the tensile vibration of N-H and C-O [
35,
36], and the characteristic peaks at 665 cm
−1 are caused by the bending of naphthyl ring C-H out of plane [
28,
36,
37].
Figure 6b shows the infrared spectra of calcite treated with/without NHA. For pure calcite, the characteristic peaks at 872 cm
−1 and 710 cm
−1 are attributed to the deformation vibration of C-O, and the tensile vibration of C-O corresponds to the characteristic peak at 1421 cm
−1 [
38,
39]. After the calcite was treated with NHA, there was no new band in NHA, indicating that NHA had little or no chemical adsorption on the calcite surface.
The FTIR test showed that NHA formed a new characteristic peak after interaction with bastnaesite, while calcite had no significant change after NHA treatment, indicating that NHA did not undergo chemical adsorption on the surface of calcite, which further confirmed the selective adsorption of NHA on the surface of bastnaesite.
3.4. XPS Measurement Results
XPS is an analytical method used to study the adsorption behavior of reagents on the surface of minerals. The adsorption behavior of NHA on two mineral surfaces was studied by XPS method. The concentration of NHA was 4 × 10
−4 mol/L [
40,
41].
The relative atomic concentration changes before and after the interactions between NHA and the minerals are shown in
Table 6. Before NHA treatment, the XPS spectra of pure bastnaesite and calcite did not contain the peak of impurity N1
S, indicating the high purity of the tested bastnaesite and calcite samples. After NHA treatment, the N atom concentrations on the surface of ceria and calcite were 2.91% and 1.22%, respectively, and the C atom concentration was higher than that of pure ceria and calcite. The surface relative atomic concentration of bastnaesite was more obvious than that of calcite. In particular, the significant increase in N content indicates that a large amount of NHA was adsorbed on the surface of bastnaesite. In contrast, only a small amount of NHA was adsorbed on the calcite surface. To further investigate the adsorption of NHA on the surface of bastnaesite and calcite, the spectra of XPS with/without flotation reagents were investigated and are shown in
Figure 7. After NHA treatment, the spectrum of calcite did not change significantly from that before NHA treatment, which indicates that the adsorption of NHA on the surface of calcite is very weak. However, a new N1
S peak appeared on the surface of the bastnaesite after NHA treatment, indicating that a large amount of NHA covered the bastnaesite surface. Thus, the strong adsorption of NHA on bastnaesite contributes to the selective recovery of bastnaesite, which corresponds to the flotation results.
Figure 8 shows the high-resolution XPS spectra of Ce 3d
5/2, C 1s, O 1s and N 1s treated with/without NHA. In the pure bastnaesite spectrum, the three-dimensional Ce spectrum is composed of spin-orbit splitting 3d
5/2 and 3d
3/2 nuclear pores, and the 3d
5/2 and 3d
3/2 spectra are closely linked. Usually, the binding energy of 3d
3/2 is 18–19 eV higher than that of 3d
5/2 and the strength ratio of 3d
5/2 to 3d
3/2 is 1.5 [
16,
19].
Figure 8a shows the high-resolution Ce 3d
5/2 XPS spectra of pure and NHA-treated bastnaesite samples, and
Table 7 shows the detailed analysis results of Ce 3d
5/2 XPS spectra. The peaks near 885.66 eV and 882.84 eV are characteristic spectral characteristics of Ce(III) [
19]. The peaks at 888.18 eV and 888.18 eV are more likely to be related to Ce(IV), which may be due to the complex electronic configuration of the Ce atom and the influence of the F atom on the crystal structure of bastnaesite [
16,
19]. The Ce 3d
5/2 peak has been confirmed to decrease by 0.18 ± 0.02 eV after NHA treatment. The lower binding energy means that NHA adsorbs Ce onto the surface of bastnaesite through chemical adsorption.
Figure 8b shows the high-resolution C 1s XPS spectra of pure and NHA-treated bastnaesite samples, and
Table 8 shows the detailed analysis results of C 1s XPS spectra. The results show that the C 1s of pure bastnaesite can be fitted to two component peaks at binding energies of 284.77 eV and 289.31 eV from C-C and
, respectively. In the C 1s XPS spectra of NHA-treated bastnaesite, the peaks at 284.77 eV and 289.31 eV moved forward 0.22 ± 0.02. This may be due to the superposition of
on the surface of bastnaesite and the C=O in NHA [
19,
34,
42].
Figure 8c shows the high-resolution O 1s XPS spectra of pure and NHA-treated bastnaesite samples, and
Table 9 shows the detailed analysis results of O 1s XPS spectra. The results show that the O 1s spectrum of pure bastnaesite can be fitted to two component peaks at binding energies of 531.55 eV and 533.11 eV from
and Ce-OH, respectively [
19,
42]. In the O 1s XPS spectra of NHA-treated bastnaesite, compared with pure bastnaesite, the O 1s XPS peak of
shifts 0.23 eV in the positive direction because the naphthalene ring in NHA binds to the -OH group [
34,
43]. The positive shift of the O 1s XPS peak in Ce-OH at 0.21 eV is due to the C=O in NHA [
21,
43].
In addition, the detailed analysis results of N 1s XPS spectra from
Figure 8d and
Table 10 show that N 1s spectra were also detected for the NHA-treated bastnaesite surface, which is due to the N atom in -C(OH)=NO- and -C(=O)-NHO- It was further confirmed that NHA chelates with bastnaesite to form new bonds and chemical adsorption occurs [
28,
44].
Figure 9a shows the high-resolution Ca 2p XPS spectra of pure and NHA-treated calcite samples. In the XPS spectra of pure calcite samples, the peaks of Ca 2p
3/2 and Ca 2p
1/2 are located at binding energies of 347.78 eV and 351.30 eV, respectively [
45]. The spectra of calcite samples treated with NHA show that the binding energies of the Ca 2p
3/2 and Ca 2p
1/2 peaks shift slightly compared with those of pure calcite samples, and the deviations are 0.10 eV and 0.02 eV (<0.20 eV), respectively, which are less than the instrument error.
Figure 9b shows the high-resolution O 1s XPS spectra of pure and NHA-treated calcite samples. In the XPS spectra of pure calcite samples, the O 1s peak is attributed to
[
40,
45]. In the XPS spectra of calcite samples treated with NHA, no characteristic peaks of the naphthalene ring and C=O of NHA appeared, and the binding energy deviation of the O 1s peak was also within the instrument error range.
Figure 9c shows that the N 1s spectra of calcite treated with NHA are chaotic, and no specific N 1s peaks are found. This result better indicates that NHA is not adsorbed onto the surface of calcite.
As discussed, the different active adsorption sites (Ce3+ or Ca2+ ions) exposed to bastnaesite and calcite surfaces led to the different adsorption behaviors of NHA. The XPS test results are consistent with the above experimental results; that is, the recovery of bastnaesite is higher than that of calcite, and the adsorption of NHA onto the bastnaesite surface is stronger than its adsorption onto the calcite surface.