Recovery of Calcium Fluoride from Highly Contaminated Fluoric/Hexafluorosilicic Acid Wastewater
2018 Volume 59 Issue 2 Pages 290-296
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2018 Volume 59 Issue 2 Pages 290-296
We investigated the recovery of calcium fluoride (CaF2) from highly concentrated hexafluorosilicic acid wastewater by neutralization–purification. Neutralization was achieved by dosing calcium hydroxide, whereas purification was carried out by alkaline leaching with sodium hydroxide. X-ray diffraction, X-ray absorption fine structure, mineral liberation analyzer, Fourier transform infrared spectroscopy and Inductively-coupled plasma atomic emission spectroscopy were used to quantify the neutralization and leaching performance and to elucidate their mechanisms. The precipitation behavior was strongly dependent on the calcium (Ca)/silicon (Si) molar ratio. For a Ca/Si ratio of 1.12, approximately 25% of the total fluorine was precipitated selectively as CaF2. By increasing the Ca/Si ratio to 3.91, the recovery yield increased to 100% because of the precipitation of CaSiF6 and the hydrolytic decomposition of hexafluorosilicate ion (SiF62−) to CaF2. The hydrolytic decomposition of SiF62− resulted in the precipitation of silicon dioxide on the surface of the previously formed CaF2. Alkaline leaching by sodium hydroxide at 70℃ resulted in an efficient removal of the silicon phase from the neutralized sludge with the formation of a CaF2 product with a grade above 90%. Leaching parameters, such as the kinetic constant and the activation energy, were determined by assuming first-order kinetics. The residual silicon phase in the final product could not be dissolved because of the formation of non-leachable SiO32−.
Fluorine (F) is used widely in many industrial fields, such as in the manufacture of hydrofluoric acid (HF), fluorocarbons and glass. A natural source of fluorine is mineral ore that contains fluorite (CaF2), which is processed mostly in China and Mexico1). Because of restrictions in the export of rare-earth metals from China between 2005 and 20102), it is believed that a short supply of fluorine could result because of the international geopolitical situation3). Therefore, F recycling in addition to mineral-ore availability is desirable to ensure a stable F supply4).
Fluoride can be recovered from wastewater that is generated in the manufacture of glass5), semiconductors6), hydrofluoric acid7), organic fluorine chemical products8) and fertilizers9). An ideal source of recoverable fluorine is wastewater that is produced in the chemical mechanical polishing (CMP) of silicon wafers10); this source is even more promising considering the constant expansion of the electronics industry11).
A successful selective recovery of fluorine as CaF2 from CMP wastewater would be beneficial from an economic and environmental perspective. CaF2 that is used in the production of HF requires a grade (acid grade) higher than 97 mass%12), whereas that to be used in steel manufacture must have a CaF2 content of 60–72.5% (metallurgical grade)13). Because the achievement of these grades requires the removal of impurities, such as silicon dioxide (SiO2), calcium carbonate and phosphorus14), the recovery of fluorine would reduce the costs related to the purification operations significantly. Because an excessive absorption of fluorine is hazardous for human health, the recovery of fluorine from wastewater would lower the remediation costs related to the immobilization operations that are used most frequently, such as coagulation, ion-exchange and adsorption15). Fluorine recovery from high concentrations of fluorine wastewater mitigates the depletion of natural raw material. For these reasons, the selective recovery of fluorine from CMP wastewater is highly anticipated.
Previous researchers have focused on the recovery of fluorine as CaF2 from wastewater that is generated in the silicon industry16) or in fertilizer plants17) by neutralization/precipitation. However, in their research, the only source of fluorine was HF7,16) or, when hydrogen hexafluorosilicate (H2SiF6) was also present, the total fluorine concentration was lower than ～1 g/L18). This implies a direct recovery of CaF2 without any purification from the silicon impurities.
The goal of this research was to recover fluorine as CaF2 from a highly concentrated H2SiF6 CMP wastewater by precipitation/neutralization with calcium hydroxide (Ca(OH)2). Ca(OH)2 results from the lower solubility of calcium fluoride (KsCaF2 (25℃) = 3.46 × 10−11 mol3 × L−3) compared with other fluorides (e.g., the solubility of sodium fluoride exceeds 40 g/L at 20℃)19). Unlike in previous research, we focused on wastewater that exceeded 100 g/L H2SiF6, which can contaminate CaF2 with silicon impurities because of the possible precipitation of SiO2 and CaSiF6. Therefore, to recover CaF2, we investigated the precipitation behavior and the formation of silicon compounds in the neutralization process.
X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), mineral liberation analysis (MLA), ion-chromatography (IC) and inductively-coupled plasma spectrometry (ICP-AES) were used to investigate the influence of the Ca/Si molar ratio on the crystallization behavior and the recovery performances.
We also investigated the removal of silicon impurities from the precipitation sludge by alkaline leaching with sodium hydroxide (NaOH). NaOH was preferred over other possible strong bases, such as potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2) and Ca(OH)2 because of its lower cost and because of its ability to dissolve silicon without forming insoluble silicate salts (e.g., the solubility of calcium silicate is 0.095 g/L20)). The influence of temperature on leaching products and performance was elucidated by Fourier-transform infrared spectroscopy (FT-IR) and a kinetic analysis of the reaction was performed by coupling experimental data with thermodynamic calculations by using PHREEQC software (User's Guide to PHREEQC).
Neutralization tests were performed at room temperature on 1-L samples of synthetic wastewater solutions that contained 1.28 M HF and 0.90 M H2SiF6 that were prepared by mixing 46.5 mL of 48% HF, 232.5 mL of 40% H2SiF6 and pure water (Aquarius RFD 240NA，ADVANTEC, Japan). Synthetic wastewater was prepared based on the concentrations of a real wastewater that was produced in Japan from the cleaning of glass substrates. In the neutralizations tests, 15 mass% lime hydrate (Ca(OH)2) slurry was dripped at 100 mL/h (0.20 mol-Ca/30 mins) into the wastewater solution with stirring. After 210 min, the suspension was centrifuged (Himac CR-21, Hitachi, Japan) for 10 min at 5000 rpm. Suction filtration was conducted with 0.10-µm membrane filters (A010A090C, ADVANTEC). The concentrations of Ca and Si in the filtrate were measured by ICP-AES (SPS-4000, Seiko Instruments, Chiba, Japan), whereas the F concentration was determined by IC (IC850, Metrohm, Tokyo, Japan).
All chemicals used were analytical-grade reagents from Wako-Chemical, Japan.
Sludge that was generated in the neutralization process was dried at 25℃ and analyzed by XRD (RINT Ultima III, Rigaku, Japan) to determine the phase composition. To identify the chemical form of silicon in the sludge, XAFS analysis (BL6N1 in Aichi Synchrotron Radiation Center) was conducted by using a fluorescence method. Specifically prepared CaSiF621) and amorphous SiO2 (Kanto Chemical, highest quality of JIS, Japan) were measured as reference materials.
MLA was conducted to elucidate the evolution of mineral species in the sludge during the precipitation of CaF2 and silicate. The MLA equipment was composed of a scanning electron microscope (SEM) (Quanta 250G, FEI) and an energy dispersive X-ray spectrometer (EDS) (XFlash Detector 5030, Bruker). To perform the MLA analysis, solid samples were molded into resin blocks whose surface had been polished by using sandpaper. Diamond paste and DP-Lubricant Blue (Struers, Japan) were used for the final polishing. After polishing, the sample was coated (SC-701, SANYU ELECTRON, Japan) to prevent charging. MLA mapping, was performed in the grain X-ray mapping measurement mode (GXMAP) mode because of the close back scattered electron (BSE) value of the CaF2 and the silicate.
2.2 Alkali leaching experimentThe neutralized sludge was dried at 45℃ and crushed by using a mortar to obtain 850–1700-µm samples for leaching by NaOH. The 1 M NaOH solution that was to be used as a leaching agent was heated on a hot stirring plate to the desired temperature. The leaching pulp density was fixed at 50 g sample/L NaOH. After leaching, solid–liquid filtration was performed with 0.10-µm membrane filters (H010A090C, ADVANTEC). The Ca and Si concentrations were measured by ICP-AES, whereas the F concentration was determined by IC. The leaching residues were washed and dried at 45℃ for 24 h. After drying, Fourier-transform infrared spectroscopy (FT-IR4200, JASCO, Japan) was used to investigate the silica transformation after leaching. FT-IR analyses were carried out by using the transmission method, by diluting the sample with potassium bromide (KBr) to 0.3 mass%. The sample and KBr were mixed in an agate mortar and pressed into a 10-mm-diameter pellet. The measurement was conducted with a scanning speed and a resolution of 2 mm·s−1 and 4 cm−1, respectively. Amorphous SiO2 and Na2SiO3 were measured as references.
In the neutralization tests, for a Ca/Si molar ratio below 1.12, only a small amount of Si (2.42%) precipitated (Fig. 1). By increasing the Ca/Si molar ratio, the amount of precipitated Si also increased. However, for a Ca/Si molar ratio of 3.91, wastewater neutralization was achieved (pH 7.43) along with a quantitative removal of all components as precipitates. Up to a Ca/Si ratio of 1.12, whereas ～25% of the fluorine was precipitated, almost all silicon remained dissolved in the wastewater. This result suggests that the removed fluorine was fluoride ion from HF, which precipitated as CaF2. In our wastewater solutions, fluoride comprised ～25% of the total fluorine, whereas the remaining 75% was hexafluorosilicate ion (SiF62−), which is well-known for its difficult decomposition10) and higher solubility (KsCaSiF6 (25℃) = 0.303 mol2 × L−2)22).
Distribution ratio of each elements to solid against total amount of input by neutralization at different Ca/Si molar ratios.
The XRD spectra of the neutralized sludge confirmed that the precipitated solid was crystalline CaF2 (ICSD:00-035-0816). By increasing the Ca/Si ratio to 3.36, the XRD peak of CaF2 sharpened as a result of the increased crystallinity. At a Ca/Si ratio of 3.91, the CaF2 peak broadened again, which suggests that the CaF2 precipitate could be contaminated by other compounds. This evidence implies that, up to a Ca/Si ratio of 1.12, neutralization can ensure a partial selectivity for fluorine removal.
The pH decreased slightly up to a Ca/Si ratio of 2.24 and then increased again (Fig. 1). This evidence suggests that SiF62− decomposed to HF and Si(OH)4 because of the hydration of SiF62− as shown in eq. (1)18):
| \[ {{\rm SiF}_{6}}^{2-} + 4{\rm H_{2} O} \to 4{\rm H}^{+} + 6{\rm F}^{-} + {\rm Si(OH)}_{4} \] | (1) |
Because silicon compounds were not detected by XRD (Fig. 2), despite the hydrolytic decomposition of SiF62−, we speculate that they precipitated as amorphous materials. To elucidate this aspect, the edge region of the XAFS spectra (XANES) of the neutralized precipitate was studied at the Si K-edge by measuring CaSiF6 and amorphous SiO2 as reference materials.
XRD patterns of the neutralized sledges.
For the precipitate that was obtained at a Ca/Si ratio of 1.12, a double peak in the XANES region at ～1853 eV and 1855 eV was observed (Fig. 3). In comparison with the reference CaSiF6, we see that the double peak corresponded to CaSiF6. By increasing the Ca/Si molar ratio to 2.24, the CaSiF6 peak weakened, whereas another peak at ～1849 eV, which corresponded to amorphous SiO2 appeared. With a further increase of Ca/Si ratio to 3.91, the only peak that was observed in the XANES spectrum resulted from amorphous SiO2.
Si K-edge XANES spectrum of the precipitate from neutralization; (dashed lines are the result of fitting with CaSiF6 and amorphous SiO2 reference materials).
These results confirm the previous hypothesis on the partial precipitation of SiF62− as CaSiF6 in addition to CaF2 for low amounts of Ca(OH)2, and the selective precipitation of CaF2 and amorphous SiO2 upon decomposition of SiF62− for larger amounts of Ca(OH)2.
To quantify the CaSiF6 and amorphous SiO2 in the precipitate, a fitting of the XANES spectra was conducted against reference CaSiF6 and amorphous SiO2. As shown in Table 1, at a Ca/Si ratio of 1.12, most silicon in the precipitate consisted of CaSiF6, whereas at a Ca/Si ratio of 3.91, ～95% of silicate was present as amorphous SiO2. These results support the abovementioned discussion on the enhancement of hydration and decomposition of SiF62− by CaF2 precipitation for large additions of Ca(OH)222).
| Ca/Si Molar ratio |
CaSiF6 (%) | Amorphous SiO2 (%) |
R value |
|---|---|---|---|
| 1.12 | 100 | 0 | 0.033 |
| 2.24 | 27.4 | 72.6 | 0.023 |
| 3.35 | 17.5 | 82.5 | 0.022 |
| 3.91 | 4.93 | 95.1 | 0.016 |
From XRD and XANES analyses, we see that neutralization by Ca(OH)2 at a lower Ca/Si molar ratio was controlled by the direct precipitation of CaF2 and CaSiF6 as in (2) and (3).
| \[ {\rm 2HF} + {\rm Ca(OH)}_{2} \to {\rm CaF}_{2} + 2{\rm H_{2}O} \] | (2) |
| \[ {\rm H_{2}SiF}_{6} + {\rm Ca(OH)}_{2} \to {\rm CaSiF}_{6} + 2{\rm H_{2}O} \] | (3) |
By adding more Ca(OH)2, the neutralization was controlled by the precipitation of CaF2 and amorphous SiO2 as result of the hydrolytic decomposition of SiF62−, as in (4).
| \[ {\rm H_{2}SiF}_{6} + {\rm 3Ca(OH)}_{2} \to {\rm 3CaF}_{2} + {\rm SiO}_{2} + 4{\rm H_{2}O} \] | (4) |
For a proper choice of purification operation, it is convenient to know where the impurities are concentrated. For this reason, to elucidate whether the silicon products that form for a high Ca/Si molar ratio coat the surface of the precipitated CaF2, we conducted MLA analysis and performed image analysis of the SEM-EDS mapping (Fig. 4) to determine the free-surface ratio of each precipitated particle.
SEM-EDS mapping of the precipitate products.
The free-surface ratio of the precipitated CaF2 particles and their relationship with the Ca/Si molar ratio is shown in Fig. 5.
Free surface ratio of precipitated CaF2 particles.
At a Ca/Si ratio of 1.12, CaF2 exhibited a mostly silicon-free surface as a result of selective precipitation. At a Ca/Si ratio of 2.24, the free surface of the CaF2 phase decreased partially. By increasing the Ca/Si ratio, a large part of the CaF2 phase was coated by silicate, which confirmed that the precipitation of SiO2 followed the precipitation of CaF2 and CaSiF6.
Based on results of the XANES fitting and stoichiometric considerations (fluorine that was present as CaF2 and CaSiF6; silicon that was present as amorphous SiO2), we calculated the chemical composition of the precipitates that were obtained for different Ca/Si ratios. The results are shown in Table 2.
| Ca/Si molar ratio |
CaSiF6 (%-g/g) |
SiO2 (%-g/g) |
CaF2 (%-g/g) |
F recovery rate (%) |
|---|---|---|---|---|
| 1.12 | 6.42 | 0 | 93.58 | 23.45 |
| 2.24 | 8.48 | 7.41 | 84.11 | 56.72 |
| 3.35 | 9.00 | 14.85 | 76.15 | 95.22 |
| 3.91 | 2.53 | 16.11 | 81.36 | 99.97 |
At a low Ca/Si molar ratio, a high grade CaF2 can be recovered, but the removal efficiency towards the fluorine is probably very low for practical applications. In contrast, for a higher Ca/Si molar ratio, fluorine can be removed quantitatively, but the recovered CaF2 exhibits a lower grade. Therefore, a process that involves the quantitative precipitation of fluorine at a high Ca/Si molar ratio followed by purification seems to be the most desirable process option.
Based on the abovementioned results that show the presence of mixed phases of CaF2–CaSiF6 coated by amorphous SiO2, we chose to purify CaF2 by alkaline leaching.
3.2 Neutralization mass balanceThe XRD and XANES analyses indicate that the neutralization by Ca(OH)2 at a lower Ca/Si molar ratio was controlled by the direct precipitation of CaF2 and CaSiF6 as shown in eqs. (1) and (2). In particular, at a Ca/Si ratio of 1.12, the silicon removal was ～2.5%, which corresponded to 0.59 mol/L of residual silicon in solution. In this case, the precipitates consisted mostly of CaF2 as shown in Table 2, whereas the dissolved concentrations of fluorine and calcium were 3.5 mol/L and 0.28 mol/L, respectively. Because the added Ca(OH)2 was consumed by the formation of insoluble CaF2 and neutral CaSiF6, whereas the hydrolysis of SiF62− released H+ in solution, the pH decreased slightly.
For a Ca/Si ratio of 2.24, the silicon removal increased to ～30%, which corresponds to 0.31 mol/L of silicon in solution. According to the XANES fitting, 84.11% of CaF2 and several percent of CaSiF6 and SiO2 were produced and 1.5 mol/L of fluorine and 0.20 mol/L of calcium were obtained in solution. Therefore, in this case as well, the formation of CaSiF6 along with the increased hydrolysis of SiF62− prevented the pH from increasing.
By adding more Ca(OH)2 (Ca/Si ratio of 3.36), the experimental silicon removal increased to ～90%, which corresponds to 0.03 mol/L of silicon in solution. In this case, most fluorine and calcium was precipitated (Fig. 1 and Table 2), which leaves 0.13 mol/L of fluorine and 0.01 mol/L of calcium in an acidic solution.
The further addition of Ca(OH)2 to a Ca/Si ratio of 3.96 yielded the complete removal of silicon and fluorine with a consequent increase in pH because of the excess of Ca(OH)2.
3.3 Purification by alkali leaching: leaching performancesTo purify the fluorine product that was obtained at a Ca/Si molar ratio of 3.91, we performed an alkali leaching of the precipitated CaF2 by NaOH and investigated the effect of three different temperatures on the leaching performances. The results are shown in Fig. 6.
Leaching rate of Si, F and Ca.
According to the obtained results, regardless of the temperature, Si was leached from the precipitate preferentially, whereas F and Ca were dissolved only slightly. This evidence suggests that CaSiF6 and amorphous SiO2 were extracted selectively from the precipitate along with smaller amounts that released the fluoride ion into solution. All extracted calcium precipitated as Ca(OH)2 (KsCa(OH)2 (25℃) = 5.5 × 10−6 mol3 × L−3)19).
A higher temperature yielded an increased and faster silicon dissolution, as reported in previous research23). To evaluate important leaching parameters, such as kinetic constants and activation energy, we assumed that the dissolution of the solid products follows independent first-order kinetics as given by eq. (5):
| \[ \frac{{\rm d}C}{{\rm d}t} = - kC \] | (5) |
| Temperature (K) |
CaSiF6 (s−1) |
SiO2 (s−1) |
CaF2 (s−1) |
|---|---|---|---|
| 298.15 | 2.0 × 10−3 | 8.5 × 10−5 | − |
| 318.15 | 5.0 × 10−3 | 1.5 × 10−3 | − |
| 343.15 | 1.0 × 10−2 | 1.0 × 10−2 | 3.0 × 10−6 |
To determine the activation energy for the alkali leaching reactions that were involved in the purification operations, we plotted the kinetic constant of Table 3 with the Arrhenius eq. (6) in its linearized form (7):
| \[ k = {\rm A}\ \exp \left(-\frac{E_{a}}{{\rm R}T}\right) \] | (6) |
| \[ \ln k = - \frac{E_{a}}{{\rm R}T} + \ln A \] | (7) |
By a linear regression of the Arrhenius plot in Fig. 7, we determined the estimated activation energies for the leaching of CaSiF6 and SiO2 as 30.37 and 89.75 kJ/mol, respectively.
Arrhenius plot fitting for CaSiF6 and SiO2.
Based on the initial composition of the neutralized sludge (Table 2), by considering the ICP results and the stoichiometry of leaching reactions (8) and (9),
| \[ {\rm CaSiF}_{6} + {\rm 6NaOH} \to {\rm 6NaF} + {\rm Na_{2}SiO}_{3} + {\rm Ca(OH)}_{2} \] | (8) |
| \[ {\rm SiO}_{2} + {\rm 2NaOH} \to {\rm Na_{2}SiO}_{3} + 2{\rm H_{2}O} \] | (9) |
We performed a mass balance of solid species (Table 4) and calculated the grade of the final CaF2 product as in (10):
| \[ {\rm CaF}_{2}(\%) = \frac{{\rm m}_{\rm CaF2}}{{\rm m}_{\rm CaF2} + {\rm m}_{\rm SiO2} + {\rm m}_{\rm Ca(OH)2}} \times 100 \] | (10) |
| Species | Temperature (℃) |
Initial amount (mmol/g) |
Dissolution (%) |
Amount in the solid phase (mmol/g) |
Fluorine recovery (%) |
CaF2 grade (%) |
|---|---|---|---|---|---|---|
| CaF2 | 25 | 10.42 | 0.6 | 10.35 | 95.5 | 91.1 |
| CaSiF6 | 0.14 | 100 | - | |||
| SiO2 | 2.68 | 57.4 | 1.14 | |||
| Ca(OH)2 | - | - | 0.14 | |||
| CaF2 | 45 | 10.42 | 0.5 | 10.37 | 95.7 | 91.4 |
| CaSiF6 | 0.14 | 100 | - | |||
| SiO2 | 2.68 | 58.9 | 1.10 | |||
| Ca(OH)2 | - | - | 0.14 | |||
| CaF2 | 70 | 10.42 | 3.3 | 10.07 | 92.9 | 91.8 |
| CaSiF6 | 0.14 | 100 | - | |||
| SiO2 | 2.68 | 62.7 | 1.00 | |||
| Ca(OH)2 | - | - | 0.14 |
To elucidate the mechanism of silicate dissolution, FT-IR analyses of solid residues before, during and after alkaline leaching were performed. The results are shown in Fig. 8 along with the IR patterns of NaSiO3･9H2O and amorphous SiO2, which were analyzed as references.
FT-IR spectra of the sludge before and after leaching at 25℃, 45℃ and 70℃.
In general, the Si atom in silica has a tetrahedral coordination, with four oxygen atoms that surround a central Si in a shared SiO4 tetrahedra (net chemical formula SiO2) with four bonds, namely (Qn), where n = 0,1,2,3,426). As shown in Fig. 8, Si-O bond (Q4) stretching around 1100 cm−1, which was identified in the initial precipitate and amorphous SiO2 reference, shifted gradually to lower wavelengths (1000 cm−1) as the leaching proceeded for 30 min at 25℃. This shift corresponded to an increase in silicon portions that are bound with non-bonding oxygens (NBO)27,28). As a result, the Si-O stretching (Q3) peak appeared. After 2 h of leaching at 25℃, a new peak appeared at ～900 cm−1. This peak was assigned to the Si-O stretching (Q2)29). This Si-O (Q2) bonding was found also in the solid residues from high-temperature leaching and it was similar to the Si-O bonding in Na2SiO3. A further shift to (Q1) (one more replacement by OH−) would be required to transform the silicon compound to a soluble product30,31). Therefore, it is suggested that the final chemical form of silicate in the leaching residue was not amorphous SiO2 but H2SiO3, NaHSiO3 or Na2SiO3. These results also highlight that leaching stopped as a result of the transformation of the Si-O bounding (Q4) to the Si-O bounding (Q2), which has a silicon site with non-bonding oxygens (Fig. 9).
Stagewise breaking of silicon–oxygen bonds from SiO227).
The recovery of CaF2 from highly-concentrated hexafluorosilicic acid wastewater that consisted of 1.28 M HF and 0.9 M H2SiF6 by neutralization–purification was investigated. A first operation of neutralization by Ca(OH)2 allowed for the removal of fluorine from the liquid media. The precipitation behavior was found to be strongly dependent on the Ca/Si molar ratio. Although the best removal performance could be obtained for high Ca/Si molar ratios, the obtained sludge contained impurities, such as SiO2 and CaSiF6, and it required purification to achieve a higher CaF2 grade.
The Si phase was removed from the precipitated CaF2 by alkaline leaching with NaOH for 10 min between 45 and 70℃. Leaching at lower temperatures allowed for a larger recovery of fluorine as CaF2 but with a lower grade. In contrast, at higher temperatures, the fluorine recovery was lower, but the CaF2 grade was slightly higher.
We thank the staff of the Aichi Synchrotron Radiation Center (Aichi, Japan) for their support in the XAFS analyses. We thank Laura Kuhar, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
CMP: Chemical mechanical polishing
EDS: Energy dispersive X-ray spectroscopy
FT-IR: Fourier transform infrared spectroscopy
ICP-AES: Inductively-coupled plasma atomic emission spectroscopy
IC: Ion chromatography
MLA: Mineral liberation analyzer
SEM: Scanning electron microscope
XRF: X-ray fluorescence
XRD: X-ray diffraction
XAFS: X-ray absorption fine structure
XANES: X-ray absorption near edge structure
