3.1. Microflotation tests
Micro-flotation was carried out under the natural suspension pH which was set to approximately 5.0. To observe the floatability of both minerals by CTAC, flotation recovery was examined as a function of CTAC concentration as shown in
Figure 3.
As seen in
Figure 3, a similar increasing floatability trend was observed for quartz and hematite with the increase of CTAC concentration. With increasing CTAC dosages from 0.00 to 43.75 ×10
-4 mmol/L, the hematite and quartz recovery increased from 0.21 to 37.29% and 0.13 to 90.04%, respectively. Moreover, the recoveries of the two minerals indicated that CTAC exhibited much better collection ability towards quartz than hematite, and a significant recovery difference (about 60.47%) between them was obtained when the concentration of CTAC was 26.25×10
-4 mmol/L. This observation may be attributed to two main reasons. First, CTAC as a quaternary ammonium salt could completely dissolve in water and generated positively charged C
19H
42N
+ and negatively charged Cl
— with the following equation [
13].
Second, isoelectric points of hematite and quartz were approximately pH 4.8 and pH 2.5, respectively [
17,
18], and the surface potential of quartz was more negative than that of hematite at pH 5.0. Therefore, C
19H
42N
+ with cationic functional groups exhibited stronger affinity towards quartz than hematite due to the same charge will repel, different charges will attract.
To further verify the selectivity of CTAC between hematite and quartz, the mixed minerals separation tests were conducted at a mass ratio of 1:1, and the corresponding assay results in the froth products are shown in
Table 2.
From
Table 2, the yield and hematite recovery of froth product increased from 51.27 to 71.12% and from 26.43 to 52.82%, respectively, with CTAC concentration increasing from 17.50 to 35.00 ×10
-4 mmol/L. In addition, the recovery of quartz increased from 76.11 to 92.72% with CTAC dosages from 17.50 to 26.25 ×10
-4 mmol/L, and a stable trend was observed as the concentration increased further. Compared with single mineral flotation results, the recovery of quartz was rarely changed. However, the recovery of hematite in artificial mixed minerals was about 1.8 times higher than that in the single hematite flotation under the same concentration of CTAC. The reason for the increase of hematite recovery was likely due to the partial hetero-aggregation and entrapment between positively charged hematite and quartz that had negative surface charge in suspension, and they floated together into the froth phase especially in the presence of higher dosages of CTAC. Overall, hematite and quartz still had a recovery difference of 45.46% with a CTAC concentration of 26.25 ×10
-4 mmol/L, indicating that the CTAC remained a high selective collection capacity for quartz in mixed minerals.
3.2. Zeta-potential measurements
CTAC dissociates in water and exists in the form of positively charged C19H42N+, the adsorption of which on mineral surface can change the electrical property of mineral/liquid interface. Here, the adsorption capacity of CTAC on the surface of hematite and quartz was analyzed by measuring surface potential changes of minerals before and after the interaction with CTAC. The zeta-potential measurements were conducted under different CTAC concentrations at pH 5.0, and the results are presented in
Figure 4.
As shown in
Figure 4, the zeta potentials of hematite and quartz were -16.37 and -41.87 mV, respectively, without any reagents. It can be observed that as the CTAC concentration increased from 0 to 26.25 ×10
-4 mmol/L, the zeta potential of hematite and quartz positively increased by 15.58 and 55.97 mV, respectively. These results indicated that the positively charged C
19H
42N
+ cation was adsorbed on the surface of hematite and quartz, increasing the zeta potential value of the two mineral surfaces. In addition, the increase of surface zeta potential of quartz was significantly higher (about 3.59 times) than that of hematite, indicating that the adsorption amount of C
19H
42N
+ on quartz surface was far greater compared with hematite, which was in agreement with the micro-flotation results.
3.3. XPS measurements
In order to further reveal the adsorption difference and mechanism of CTAC on the mineral surface, the XPS analysis was detected with a CTAC concentration of 26.25 ×10-4 mmol/L at pH 5. The XPS spectra of hematite and quartz before and after treatment with CTAC are shown in
Figure 5 and
Figure 6, respectively.
From
Figure 5, the Fe2p1, Fe2p2 and O1s peaks were detected at approximately 724, 711 and 531eV, which was attributable to the iron and oxygen elements on the surface of hematite [
19].
Figure 6 shows that O1s and Si2p peaks were detected at binding energy 102 and 531 eV, respectively, which indicated SiO
2 as the main component of quartz and no significant contaminant was detected [
12,
13]. The C1s peaks in spectra were positioned at around 284.8 eV, mainly attributable to carbon dioxide in the air and the CTAC molecules [
20,
21,
22]. The peak intensities of C1s in
Figure 5a,b were 16516.7 and 18096.7, respectively, and the offset value was 1580 after treatment with CTAC. In contrast, the peak intensities of C1s in Figure 6a,b were 14933.3 and 20283.3, respectively, with a much higher offset value of 5350 by CTAC. Above results indicated that the adsorption of CTAC could enhance peak intensities of C1s for both hematite, however, the increment of quartz was 3.39 times higher than that of hematite, suggesting a stronger affinity of CTAC with quartz surface. In addition, N1s peaks were detected in 400.23 eV (
Figure 6b) of quartz after CTAC treatment, and there was no obvious change in the spectrum of hematite before and after treatment with CTAC. These results indicate that in addition to carbon dioxide in the air, CTAC was also adsorbed on the surface of minerals, and the adsorption density of CTAC on the quartz surface was far greater than that of hematite.
Table 3 quantifies the effect of CTAC on the content of mineral surface elements, showing the content of surface elements of hematite and quartz before and after treatment with CTAC.
As can be seen, there was a slight change in the atomic concentration of the main element on hematite surface after interaction with CTAC, indicating weak adsorption, which was supported by the low offsets of C1s (2.40%) N1s (0.43%). However, after treating quartz with CTAC, the atomic concentrations of C1s and N1s were 23.74% and 1.22%, respectively. In contrast with hematite sample, the atomic concentration offsets of C1s and N1s of quartz after CTAC treatment were 4.25 and 2.84 times higher, respectively. This proved that CTAC strongly adsorbed onto quartz rather than hematite. In comparison with hematite, the stronger adsorption of CTAC onto quartz enhances its good flotation performance, thereby achieving efficient removal of quartz from hematite via reverse flotation.
As an amine cationic collector, CTAC was mainly adsorbed on the mineral surface by electrostatic adsorption and hydrogen bonding [
12,
13]. Therefore, in order to obtain the difference in the adsorption strength of CTAC on the surface of hematite and quartz, the anions (i.e., O1s) on the surface of the two minerals before and after treatment with CTAC were fitted and analyzed, and the results are shown in
Figure 7 and
Figure 8.
Figure 7 shows that the O1s XPS spectra of hematite before and after treatment with CTAC shifted from 529.50 (
Figure 7a) to 529.58 eV (
Figure 7b), which was an offset of 0.08 eV, indicate that the interaction between CTAC and oxygen sites on the surface of hematite was weak. However, after treating quartz with CTAC, the binding energy of O1s shifted from 530.11 (
Figure 8a) to 529.79 eV (
Figure 8b), which was an offset of 0.32 eV. After treatment with CTAC, the deviation of the binding energy of O1s on the surface of quartz was four times of that of hematite, which indicated that the adsorption capacity of CTAC on the surface of quartz was much greater than that of hematite. In summary, the XPS analysis results was consistent with those of the flotation tests and zeta-potential measurements, thus providing further support for the efficient separation of hematite from quartz using CTAC as a collector via reverse flotation.