3.1. Micro-Flotation Experiments
Figure 2 shows the effect of pulp pH and dosage of AWG on the recovery of fluorite and calcite with NaOL as a collector and AWG as a depressant. Previous studies have shown that the optimal separation pH of calcite and fluorite appears in the weak alkaline environment close to 9, which is due to the problems of increased mineral solubility, poor reagent selectivity, and difficult adjustment in the acidic and weak alkaline environment. In this study, the pH range is 7–11, including the optimal pH for the separation of fluorite and calcite [
15,
23,
28]
Figure 2a shows that, in the pH range of 7–11, the flotation recovery of fluorite increases first and then decreases with the increase of solution pH, and the flotation recovery of calcite decreases first and then increases with the increase of solution pH. For pH > 9, the recovery of fluorite was greater than that of calcite. For pH = 9, the flotation recovery of the two minerals was similar, and the flotation recovery of calcite was the lowest. For pH < 9, the flotation recovery of fluorite was lower than that of calcite.
Figure 2b shows that, when pH = 9, the flotation recovery of fluorite and calcite decreases with the increase of AWG dosage, and the flotation recovery of fluorite is always lower than that of calcite. When the dosage of AWG reached 30 mg/L, the recovery of calcite is significantly reduced to 22.11%, and the recovery of fluorite remained unchanged, which is most conducive to the separation of the two. These results show that the pulp pH has a great influence on the inhibition of AWG. When AWG is used as a depressant alone, fluorite and calcite will be strongly inhibited. The difference between their floatability is small, so it is difficult to separate fluorite from calcite.
Figure 3 shows the effects of NaF dosage and pulp pH on the flotation recovery of fluorite and calcite with NaOL as collector and AWG as a depressant. The results show that, with the increase in the amount of sodium fluoride, the flotation recovery of the two minerals gradually increases, and the flotation recovery of fluorite increases greatly. When the recovery of fluorite reached 88.72%, it was much higher than that of calcite at 34.85%. The difference between the flotation recovery of fluorite and calcite is the largest under neutral or weakly alkaline conditions, which is most conducive to the separation of fluorite and calcite.
To verify the effect of the fluoride ion on the flotation separation of fluorite and calcite with AWG as a depressant, the flotation tests of inhibiting calcite and collecting fluorite were designed for the artificial mixed ore. The experimental conditions were as follows: pH = 9, AWG (30 mg/L) as depressant, NaOL (250 mg/L) as collector, and NaF (80 mg/L) as auxiliary reagent. The mixture was passed through a roughing stage to obtain fluorite concentrate. The experimental results are shown in
Table 1 and
Table 2. After adding NaF, the grade of fluorite in the concentrate increased from 35.26% to 61.08%, and the grade of calcite decreased from 64.74% to 38.92%, indicating that the addition of a fluoride ion can significantly improve the flotation separation effect of fluorite and calcite.
3.2. Solution Chemistry
The Visual MINTEQ model is widely used to calculate the morphology, solubility equilibria, and adsorption of ions in solution [
29,
30,
31,
32]. It can reflect the ion concentration and potential products in flotation pulp. The effect of pH on the composition and content of the water glass solution system is shown in
Figure 4.
According to
Figure 4, it is inferred that the main reactions in the water glass solution system are as follows:
Figure 4 shows that in the water glass solution, the dissociated SiO
2 exists in the form of Si(OH)
4, SiO(OH)
3−, and SiO
2(OH)
22−. A large number of studies showed that SiO(OH)
3− and Si(OH)
4 are the main components of water glass to inhibit calcite and fluorite [
13,
33]. Comparing
Figure 2a, when pH < 9.4, Si(OH)
4 is the dominant component, and the recovery rate of fluorite is higher than that of calcite. When 9.4 ≤ pH ≤ 12.6, SiO(OH)
3− is the dominant component, and the recovery of calcite is higher than that of fluorite. The results show that Si(OH)
4 is the main component to inhibit calcite, while SiO(OH)
3− is the main component to inhibit fluorite. In AWG, the H
+ from acid will combine with SiO
2(OH)
22− and SiO(OH)
3− ions in water glass solution, promoting the forward progress of reactions (4) and (5), generating more Si(OH)
4, and enhancing the inhibitory effect of sodium silicate on calcite.
Solution chemical calculations are widely used to study dissolution/precipitation characteristics and surface species transformation [
34,
35].
Ksp is called the solubility product of minerals, which depends on ionic strength and temperature. During the dissolution of CaF
2, CaCO
3, and CaSiO
3, with the hydrolysis of Ca
2+ and the proton addition reaction of F
−, CO
32−, and SiO
32−, the solubility of minerals will be affected accordingly. The conditional solubility product
K′
sp of CaF
2, CaCO
3, and CaSiO
3 is calculated using the equilibrium formula and thermodynamic data in
Table 3 [
34,
36,
37].
Figure 5 shows the pH–log
K′
sp relationships for the three minerals. The smaller the value of log
K′
sp, the more difficult it is to dissolve the precipitate.
Figure 5 shows that when 6 < pH < 9, the order of conditional solubility products is CaF
2 < CaCO
3 < CaSiO
3. When 9 < pH < 12, the order of conditional solubility products is CaF
2 < CaSiO
3 < CaCO
3. When the pulp pH is in the range of 6–12, the conditional solubility product of CaF
2 is smaller than that of CaCO
3 and CaSiO
3, and CaF
2 is easier to form than CaCO
3 and CaSiO
3. Therefore, adding F
− to the pulp will preferentially generate CaF
2 precipitate and transform the original CaCO
3 and CaSiO
3 precipitates. This indicates that fluoride ions may restore the floatability of fluorite by converting CaSiO
3 on the surface of fluorite into CaF
2, but the selectivity of fluoride ions is not clear and needs to be further studied.
3.3. XPS Analysis
The relative contents of the main elements on the surface of fluorite and calcite treated with AWG or AWG + NaF were analyzed using XPS. The results are shown in
Table 4 and
Table 5. After F
− treatment, the F1s content on the surface of fluorite and calcite increased by 1.42% and 0.99%, respectively, and the Si2p content decreased by 0.83% and 1.17%, respectively. This shows that fluoride ions are adsorbed on the surfaces of fluorite and calcite, and desorption of silicon-containing substances happens on the surfaces of fluorite and calcite.
To explore further the effect of fluoride ions on the adsorption of AWG on the surface of fluorite and calcite, the high-resolution XPS spectrum of the Si2p peak was analyzed, and the results are shown in
Figure 6. The Si2p XPS spectra of the fluorite surface treated with AWG and AWG + NaF are shown in
Figure 6a,b. The peaks at 101.87 and 102.60 eV in
Figure 6a may be attributed to Na
2SiO
3 and CaSiO
3, respectively [
38]. After F
− treatment, it is observed in
Figure 6b that the peak area corresponding to CaSiO
3 decreases from 62.02% to 34.31%, indicating that part of CaSiO
3 on the surface of fluorite is converted into CaF
2 by fluoride ions, so the inhibition of fluorite is weakened.
The Si2p spectra of the calcite surface treated with AWG and AWG + NaF are shown in
Figure 6c,d. The peaks at 101.64 and 102.41 eV in
Figure 6c can also be attributed to Na
2SiO
3 and CaSiO
3 [
39,
40]. It can be clearly observed that a peak different from fluorite appears at 103.21 eV on the calcite surface, which can be attributed to the physical adsorption of Si(OH)
4 on the calcite surface [
13,
33,
41]. After F
− treatment, it is observed in
Figure 6d that the peak area corresponding to CaSiO
3 decreases from 47.65% to 43.18%, and the peak area corresponding to Si(OH)
4 decreases from 23.21% to 20.89%, indicating that although CaSiO
3 on the calcite surface will also be desorbed by fluoride ions, the desorption ability of fluoride ions to Si(OH)
4 is very weak, and Si(OH)
4 is the main component to inhibit the floatability of calcite. As a result, calcite is still strongly inhibited. This shows that, although CaSiO
3 on the surface of calcite will also be converted by fluoride ions, the conversion ability of fluoride ions with Si(OH)
4 is very weak, which is the mechanism of fluoride ions improving the flotation separation efficiency of fluorite and calcite.
3.4. ICP–MS Tests
Fluoride ions improve floatability by desorbing silicon adsorbed on the surface of fluorite and calcite. Under the condition of 30 mg/L AWG dosage, NaF of different concentrations was added and filtered after reaction, and the silicon concentration in each filtrate was measured using inductively coupled plasma–mass spectrometry (ICP–MS). The change of silicon concentration in the filtrate with the amount of NaF is shown in
Figure 7. With the increase in the number of fluoride ions, the desorption of fluoride ions is enhanced, the silicon on the mineral surface is gradually reduced, and the silicon concentration in the filtrate is increased. Under different NaF dosages, the silicon concentration in fluorite filtrate is higher than that of calcite, which proves that the effect of F
− on the fluorite surface is stronger than that of the calcite surface, which is consistent with the conclusions of the microflotation experiment and XPS analysis.
3.5. Image Analysis of Flotation Froth
Flotation involves increasing the floatability difference between useful minerals and gangue minerals using additives so that hydrophobic minerals can be attached to flotation froth for separation purposes. Therefore, the flotation effect can be directly judged by observing the mineral attachment to the floating foam.
Figure 8a,b shows the fluorite flotation froth image processed with AWG or AWG + NaF and 80 mg/L NaOL. The floating foam in
Figure 8a shows a large area of the transparent state, which means that fluorite is inhibited by AWG and cannot adsorb on the foam. After F
− treatment, it was observed that the surface of the
Figure 8b foam was evenly covered by fluorite and showed good adsorption properties. Fluorite ore activated by fluoride ions shows strong adsorption capacity to foam, which greatly improves the floatability of fluorite, which is inhibited by AWG.
Figure 8c,d shows the image of calcite flotation froth treated with AWG or AWG + NaF and 80 mg/L NaOL. Comparing
Figure 8c with
Figure 8d, it is found that, before and after NaF addition, the center of the flotation cell shows a transparent part, and the area changes little. This indicates that the inhibition effect of AWG on calcite did not change with the addition of NaF, and the calcite inhibited by the reaction could not be adsorbed onto the foam to be floated. It can be seen from the flotation froth image that the fluoride ion selectively improves the flotation effect of fluorite.
3.6. Contact Angle Measurement Results
The contact angle measurement results of fluorite and calcite under different conditions are shown in the
Table 6. The contact angle is directed related to the floatability of mineral, and a larger contact angle generally indicates a more hydrophobic mineral surface [
42].
As shown in the
Table 6, under the condition of pH = 9, the contact angle between the untreated fluorite sample and calcite sample is close, indicating that their floatability is similar in natural state. After AWG treatment, the contact angle of fluorite samples decreased by 20.46°, and that of calcite samples decreased by 18.67°, indicating that AWG had a strong inhibitory effect on calcite and fluorite. After AWG and NaF treatment, compared with AWG treatment alone, the contact angle of fluorite increased by 24.40°, and the floatability was greatly improved; The contact angle of calcite only increased by 0.76°, and the change of floatability was small. The change of contact angle between fluorite and calcite shows that the fluoride ion selectively improves the floatability of fluorite inhibited by AWG, which is consistent with the above experimental conclusions.