3.1. The Effect of Aluminium Hydroxide on the Mobility of F− Ions in the Silica Gel Waste Samples
It is known that aluminum hydroxide has amphoteric properties, i.e., it is able to donate or accept protons and react with both acids and bases. Furthermore, Al(OH)
3 dissolves in water, and—under certain conditions—produces H
+ or OH
− ions:
As the pH of the silica gel waste is strongly acidic, different amounts of aluminum hydroxide, which were equivalent to 2.5, 5, 7.5 and 10% Al3+ ions from the mass of the dry materials, were added to the SGW samples and mixed in a homogenizer (Turbula type T2F) for 1 h at 30 rpm. Later, the samples were treated with water under static and dynamic conditions.
It was determined that, after 24 h of the treatment of the SGW with Al(OH)
3 additive (2.5–10%) under static conditions, the intensity of the diffraction peaks characteristic to AlF
3·3H
2O only slightly decreased, when the water-to-solid ratio was equal to 6 (
Figure 2). Meanwhile, the intensity of the main diffraction maximum of Al(OH)
3 (d-spacing: 0.547 nm) increased from 0 (without additive) to 185 cps (when 10% additive was used). Meanwhile, the increment of the water-to-solid ratio to 100 had a positive effect on the decomposition of the AlF
3∙3H
2O in the mixtures with 2.5% Al
3+ ions, because the intensities of the diffraction maximums typical to the latter compound decreased more than five times (
Figure 2). It should be noted that, when the Al(OH)
3 additive is not used and the SGW is treated with water (w/s = 100, 24 h), the intensity of the diffraction peaks characteristic of AlF
3∙3H
2O decreased only three times [
19].
Moreover, it was found that the pH values of the liquid medium separated from the SGW samples after the treatment with water (w/s = 6) depended on the amount of Al
3+ ions used (
Table 1). When 2.5% of the mentioned ions were added to the system, the pH of the liquid medium reached 2.54; meanwhile, at higher concentrations of Al
3+ ions (≥5%), this parameter was increased to 4.41–4.73 (
Table 1). It should be noted that, in the same treatment conditions, the pH value of the liquid medium was ~2.4 times higher in comparison with the samples without aluminum ions [
19]. However, the amount of F
− ions released to the liquid medium was insignificant (
Table 1).
Furthermore, when the w/s ratio was increased to 100 and 2.5% Al
3+ ions were used, the pH value of the liquid medium reached 3.06 (
Table 1). In this case, more than 55% of the initial amount of F
− ions were released from the structure of the sample into the liquid medium (
Table 1). It should be noted that this is ~6% more than in the samples without an additive [
19].
Since the amount of Al(OH)
3 additive does not affect the stability of AlF
3∙3H
2O, the leaching in cycles under dynamic conditions (10 g SGW was treated with 50 mL liquid medium) was achieved by the use of Al(OH)
3, the quantity of which corresponded to 2.5% Al
3+ ions. It was determined that the intensities of the diffraction peaks characteristic to AlF
3∙3H
2O only slightly decreased, and the concentration of F
− ions decreased only to ~9% when the w/s ratio was equal to 20 (
Figure 3,
Table 2). By increasing the w/s ratio to 100, it was observed that a small amount of AlF
3∙3H
2O was still present in the sample, because only low intensity diffraction peaks were identified in the XRD patterns (
Figure 3). Moreover, the quantity of the fluorine ions released into the liquid medium was estimated to be ~2.5% higher (
Table 2) than in the samples treated under static conditions (
Table 1).
The results of the liquid medium analysis showed that the duration of one cycle (filtration/interaction) was equal to ~40 s; thus, the total interaction time was 13 min, which is significantly shorter in comparison to the static conditions (24 h). Moreover, the pH value of the liquid medium reached ~3 when the w/s ratio was equal to 10 (
Figure 4). The further increment in the w/s ratio (30–100) resulted in a pH value of 4.2.
In a previous work, it was determined that, by the use of treatment with water at 25 °C, the F
− ion concentration in silica gel waste can be reduced by up to 49% [
19]. Meanwhile, by adding 2.5% Al
3+ ions to SGW, the F
− ion concentration can be reduced by up to 58% (
Table 1 and
Table 2). Thus, it can be stated that the Al(OH)
3 additive positively affected the removal of F
− ions from the structure of the silica gel in the liquid medium.
In the next part of this work, it was decided to investigate the influence of another hydroxide (calcium hydroxide) on the mentioned process.
3.2. The Effect of Calcium Hydroxide on the Mobility of F− Ions in Silica Gel Waste Samples
According to the literature [
33,
34], the application of Ca(OH)
2 can not only induce the release of F
− ions, but can also bind these ions into a stable calcium fluoride compound (CaF
2).
The treatment of contaminated silica gel with a Ca(OH)
2 additive was first performed under static conditions. In order to decrease the amount of F
− ions, 6.5 and 20% Ca(OH)
2 were added to the SGW sample, along with a w/s ratio equal to 10 and a treatment temperature of 25 °C. It is worth noting that the maximum amount of the additive (20%) was sufficient enough to combine all of the fluorine ions into CaF
2. It was determined that, after the treatment of the SGW samples with 6.5% Ca(OH)
2, the AlF
3∙3H
2O was fully decomposed, and the F
− ions were bound into CaF
2 (
Figure 5). Furthermore, aluminum fluoride hydroxide hydrate (AlF
1.5(OH)
1.5∙0.375H
2O;
d-spacing: 0.568, 0.296, 0.284 nm), a product of AlF
3∙3H
2O decomposition, was identified in the XRD patterns. Moreover, the increment in the quantity of the additive to 20% did not affect the mineral composition of the formed compounds (
Figure 5). The results of chemical analysis showed that, in both cases, F
− ions were not released into the liquid medium. Furthermore, it was found that the amount of the additive determined the pH of the liquid medium, the values of which were equal to 7.1 (by the use of 6.5% Ca(OH)
2) and 11.7 (by the use of 20% Ca(OH)
2).
It can be stated that the Ca(OH)
2 additive has a positive effect on AlF
3∙3H
2O decomposition; however, the F
− ions were bound into CaF
2, or were absorbed into the silica gel sample and remained in the solid material. Similar results were obtained by Iljina et al. [
22]. These authors showed that, by treating SGW under static conditions at different temperatures and w/s ratios, the F
− ion concentration can be reduced to only 6–7 wt%. For this reason, in the next stage of this work, the silica gel samples were treated with a saturated Ca(OH)
2 solution under dynamic conditions.
It was determined that the treatment conditions affect the stability of AlF
3∙3H
2O and the removal of F
− ions. After the leaching of the F
− ions by the use of continuous liquid medium flow (w/s = 25, 50, 100), low diffraction peaks characteristic of AlF
3∙3H
2O and its decomposition product, AlF
1.5(OH)
1.5∙0.375H
2O, were still observed in the XRD patterns (
Figure 6, curves 1–4). As expected, together with the mentioned compounds, CaF
2 and CaCO
3 (formed due to the interaction between Ca(OH)
2 and atmospheric CO
2) were detected. Meanwhile, when the silica gel waste was treated in cycles (w/s = 100), AlF
3∙3H
2O was completely decomposed. However, alongside CaF
2 and CaCO
3, two diffraction peaks (
d-spacing: 0.762, 0.385 nm)—which did not correspond to any compound indexed in the PDF−4 database—were also noted (
Figure 6). Calcium aluminates with unknown structures and compositions were probably formed during the leaching.
Regardless of the chosen treatment method (in cycles or not), fine solid particles were formed after the filtration in the liquid medium (
Figure 7). For this reason, in order to induce the sedimentation and chemical reaction, the liquid medium was additionally maintained for 24 h under static conditions. Later on, the obtained precipitates were filtered and dried at 50 ± 5 °C for 24 h. The XRD analysis of the precipitates showed that, during the leaching experiments, Ca(OH)
2 reacted with both fluoride and aluminum ions (formed by the decomposition of AlF
3∙3H
2O) ions, which lead to the formation of CaF
2 and katoite (Ca
3Al
2.85O
2.55(OH)
9.45,
d-spacing: 0.509, 0.333, 0.279 nm) in the products (
Figure 6, curve 5). It is worth mentioning that CaCO
3 and two diffraction maximums (d-spacing: 1.248, 0.627 nm), which did not correspond to any compound indexed in the PDF−4 database, were observed in the XRD patterns of the obtained precipitates as well (
Figure 6, curve 5). Based on the obtained results, it can be stated that the rate of CaF
2 crystallization was great; therefore, this compound was detected in both the solid material and the precipitates. The obtained results were in good agreement with the literature data [
23,
33,
34].
It was determined that, after the treatment of the SGW with the saturated Ca(OH)
2 solution under dynamic conditions, the pH values of the liquid medium were greater than 8, i.e., the liquid medium had alkaline properties (
Table 3). Meanwhile, in a case of the samples with Al(OH)
3, the pH of the liquid medium was acidic (
Table 1). It was measured that the duration of the interaction (filtration) depends on the amount of water, and it increased from 55 s (w/s = 25) to 283 s (w/s = 100) (
Table 3). The chemical analysis data of the treated SGW (w/s = 100) showed that the amount of F
− ions released into the liquid medium was equal to 35.19% (
Table 3). It is worth mentioning that the mentioned value was two times higher in comparison to the value obtained after the treatment of SGW without the additives (16.9%) [
19].
It was determined that, after the treatment of the samples in cycles—already at the beginning of the process (w/s = 10)—the pH of the liquid medium was alkaline, i.e., pH > 7 (
Figure 8, curve 2). Meanwhile, after 4 cycles (w/s = 20), the values of the pH of the liquid medium became equal to the ones of the initial solution, and the further increment in the w/s ratio had no effect on the latter values. It was observed that, under these experimental conditions, the duration of the interaction (filtration) depended on the number of cycles, i.e., the duration increased with each cycle, and was more than five times longer at the end of the process (112 s) than at the beginning (21 s) (
Figure 8, curve 1). Presumably, this was caused by the crystallization of CaF
2 and/or Ca
3Al
2.85O
2.55(OH)
9.45 in the filter pores. Although the duration of the interaction (filtration) was longer than it was in the samples with Al(OH)
3 (40 s), the amount of F
− ions released into the liquid medium (39.98%) during the treatment in cycles was ~18% lower than the amount formed by the use of the Al(OH)
3 additive (
Table 2 and
Table 3). The greater amount of residual F
− ions can be explained by the fact that CaF
2 remains in the sample (
Figure 6). The chemical analysis data of the liquid medium showed that all of the F
− ions were combined to CaF
2, because the mentioned ions’ concentration did not exceed 0.001%. Thus, the calcium ions influenced both the composition (mineral and chemical) and the properties of the silica gel waste and the liquid medium.
As can be seen from the results obtained in parts 3.1 and 3.2, the hydroxide additives not only promote the release of fluorine ions into the liquid medium but also alter the mineral composition of the silica gel waste. Therefore, it was decided to treat the investigated samples with soluble alkaline solutions.
3.3. The Effect of Alkaline Solutions on the Mobility of F− Ions in Silica Gel Waste Samples
In this part of work, the NaOH and NH
4OH solutions were used for the leaching of the F
− ions in the experiments. At the beginning, the treatment of the SGW with 0.01% and 0.05% NaOH solutions under dynamic conditions was performed. It was determined that the NaOH solutions (w/s = 20) do not have a significant influence on the stability of AlF
3∙3H
2O, because the intensities of the diffraction maximums typical to the latter only slightly decreased (
Figure 9). Furthermore, low intensity diffraction peaks characteristic of AlF
1.5(OH)
1.5·0.375H
2O were detected.
Moreover, it was found that the duration of the interaction (
Table 4) was very close to the one obtained by the treatment of the SGW sample with distillated water (20–30 s). Meanwhile, the pH value of the liquid medium had already reached more than 3 at the beginning of the process (w/s = 10). It should be indicated that the pH values of the initial NaOH solutions were equal to 11.17 (0.01%) and 11.73 (0.05%).
The results of the chemical analysis showed that, after treatment with a 0.05% NaOH solution (w/s = 20), the concentration of fluorine ions in the SGW sample decreased by ~1.6 times, i.e., to 6.23%. Although the diffraction peaks characteristic of AlF3∙3H2O remained intensive, the intense release of fluorine ions into the liquid medium proceeded.
Due to the intense release of fluorine ions into the liquid medium, during the leaching experiments with the NaOH solution, it was decided to use 0.01% and 0.5% ammonia water solutions—which are currently applied in JSC ‘Lifosa’—for further research.
The treatment of the SGW samples was performed in cycles when the maximum w/s ratio 20 was used (four cycles in total). It was determined that a slight decrease in the intensities of the diffraction peaks typical to AlF
3∙3H
2O was observed in the XRD patterns when the sample was treated with a 0.01% ammonia water solution (w/s = 20) (
Figure 10). Meanwhile, after the treatment with a more highly-concentrated solution (0.5%, w/s = 10), it was noted that AlF
3∙3H
2O fully recrystallized to Al(OH,F)
3 (
d-spacing: 0.570, 0.289, 0.284, 0.189 nm).
It was measured that, when the silica gel waste was treated with 0.01% ammonia water, the filtration time and the pH values increased with the increasing w/s ratio, and at the end of the experiment (w/s = 20) were equal to 55 and 6.54 s, respectively (
Table 5). Meanwhile, after the treatment with a higher concentration of the solution, the structure of the silica gel was destroyed, and fine particles clogged the pores of the filter, because even by applying vacuum 0.6–0.7 bar, ~900 s (cycle 1) and ~6000 s (cycle 2) filtration durations were reached (
Table 5). Thus, the w/s ratio was not increased to 20. It is worth noting that the pH values obtained in these conditions were >8.
It was determined that the amount of F
− ions released from the structure of the SGW sample into the liquid medium depends on the concentration of the NH
4OH solution (
Table 6). It was found that, after treatment with 0.01% ammonia water (w/s = 20), the amount of F
− ions released was almost three times lower that than in the sample which was treated with 0.5% ammonia water (w/s = 10) (55.04%). It should be noted that a similar amount of fluorine ions were released into the liquid medium (>50%) obtained by treating the SGW only with a high volume of water (w/s = 100) (
Table 1 and
Table 2). Thus, it can be stated that the structure of the SGW decomposes in NH
4OH solution, and that the SGW lost its adsorption properties.
3.4. The Effect of a Combined Treatment on the Mobility of F− Ions in Silica Gel Waste Samples
As can be seen from the results discussed above that the pH value of the liquid medium obtained after the treatment increases significantly with the addition of different hydroxide additives (NH
4OH, Ca(OH)
2, NaOH) (
Table 3,
Table 4,
Table 5 and
Table 6 and
Figure 8). This is likely due to the peculiarities of the AlF
3 production process; a significant amount of one of the starting materials, H
2SiF
6 acid, remained in the investigated sample, and it can theoretically be converted into soluble compounds only by the use of KCl and NaOH solutions:
Therefore, in the last stage of this research, the SGW was treated with 5% KCl and 5% NaOH solutions: the KCl solution (w/s = 0.5) was poured onto the sample, stirred vigorously for 5 min (60 rpm), and filtered off; later, the same procedure was repeated with an NaOH solution (w/s = 0.5), and, in the final step, the sample was treated with water (w/s = 0.5).
The results of the XRD analysis show that a slight decrease in the intensities of AlF
3∙3H
2O diffraction peaks was observed, when the SGW sample was treated with KCl solution (
Figure 11). Meanwhile, AlF
3∙3H
2O recrystallized to a related compound, Al
2(OH)
2.76F
3.24(H
2O), after treatment with an NaOH solution (
Figure 11). It was found that, due to the presence of Al
3+ ions in the silica gel samples, the F
− ions were bound to low solubility compounds—K
2NaAlF
6 (
d-spacing: 0.287, 0.234, 0.203 nm) and Na
3AlF
6 (
d-spacing: 0.194, 0.275, 0.157 nm)—therefore, they were identified even after the treatment with water (
Figure 11).
As JSC ‘Lifosa’ preferentially uses NH
4OH in the production process, the NaOH solution was replaced with 5% ammonia water. It was found that the AlF
3∙3H
2O remained stable, as a slight decrease in the intensities of the diffraction peaks typical to the mentioned compound was observed in the XRD patterns (
Figure 12). Besides this, K
2NaAlF
6 or Na
3AlF
6 did not form under these treatment conditions.
It was also found that the SGW sample was neutralized when KCl and NaOH solutions were used together, as the pH value of the liquid medium was equal to 6.69 after the treatment with water (
Table 7). Meanwhile, when the NaOH solution was replaced with 5% ammonia, the pH values were equal to 2.36 and 3.01. It should be noted that, after the leaching of the silica gel samples with ammonia water (without KCl), the pH value of the liquid medium had already reached >6 at the beginning of the process (
Table 5).
Thus, a combined treatment method was found to have no significant effect on the fluorine ion content in the investigated sample, as the concentration of these ions in the sample was reduced only by 13–16% (
Table 7), which was four times lower than that in the sample which was treated with NH
4OH (
Table 6).
To summarise the results, it can be stated that, by the use of an Al(OH)
3 additive, it is possible to reduce the concentration of F
− ions in the sample from 10.0% to ~4.5%; however, a large amount of water is required for this process (w/s > 100) (
Table 8). The amount of liquid medium required can be significantly reduced by the use of an NH
4OH solution (
Table 8). To our knowledge, the more successful results were obtained only by Krysztafkiewicz et al. [
23]; the F¯ ions’ concentration was reduced by eight times; however, the initial concentration of the mentioned ions was equal only to 1.6%, and there is no data about the water-to-solid ratio.
In the future research, the treated silica gel waste will be used as an SiO2 source for the production of environmentally-friendly cementitious materials.