Fluoride recovery from spent fluoride etching solution through
crystallization of Na3AlF6 (synthetic cryolite)
Chih-Wei Lin1 and Chi-Wang Li2*
1
Master student and 2 Professor
Department of Water Resources and Environmental Engineering, Tamkang University 151 Yingzhuan Road,
Tamsui district, New Taipei City 25137, Taiwan
Corresponding author: Email: chiwang@mail.tku.edu.tw, (O) +886-2-26239343 (FAX) +886-2-26209651
ABSTRACT: CaF2 precipitation through addition of calcium chloride or lime is the most frequent
applied method in Taiwan to remove fluoride from fluoride-containing wastewater of semiconductor or
optoelectronic industries. Due to very fine CaF2 precipitates (~0.1m), coagulants/flocculants are needed to
facilitate sedimentation of CaF2. In turn, large amount of sludge is produced by CaF2
precipitation/sedimentation process. In this study, removal of fluoride from spent fluoride etching solution by
cryolite synthesis was investigated. Experimental results showed that good control of reaction pH and Al:F
molar ratio is the key to form cryolite successfully. The cryolite precipitates have particle size in the range of
3µm ~15µm and are much larger than CaF2 precipitates of 0.1 µm, resulting in rapid sedimentation.
Meanwhile, cryolite crystallization process produces much less sludge volume than does by CaF2
precipitation/sedimentation process. The proposed process generates useful resource and produces less
wasted sludge.
Key Words: Cryolite, Crystallization; etching solution
1.
INTRODUCTION
Removal of fluoride from wastewater generated from semiconductor or optoelectronic industries through
CaF2 precipitation is the most frequently applied method in Taiwan [1]. Both calcium chloride and lime have
been used as the calcium sources to promote precipitation of CaF2. The former is preferred due to relatively
less sludge generated [2]. However, due to the very fine CaF2 precipitates (~0.1m) [2] coagulants or
flocculants are usually needed to facilitate sedimentation of CaF2 [1, 2]. In turn, treatment of
fluoride-containing wastewater by CaF2 precipitation/sedimentation processes still produces large amount of
sludge.
Cryolite is an important ingredient for optical application and is used in aluminum production through
electrolysis process [3]. The price of synthetic cryolite can be as high as US $1000 per metric ton depending
on the quality of cryolite [4]. Removal and recovery of fluoride by cryolite synthesis was studied by Wang et
al. [5]. In the solution containing fluoride, aluminum salt (aluminum sulfate or aluminum nitrate) was added
to form aluminum/fluoride complexes, and then caustic conversion solution, i.e., NaOH, was added to bring
pH up to 4.5 to 5.5 to precipitate cryolite under temperature of 95 oC [5, 6]. After precipitates was filtered and
dried (at temperature of 383 K), the crystal phases of precipitates were examined using XRD analysis, and the
result matches very well with commercial cryolite under optimum conditions. Using sodium carbonate as the
Na source, Kumar et al. [7] studied recovery of fluoride from acid leach liquor for refining low-grade
molybdenite concentrates. It was shown that recovery of cryolite was maximized at temperature of 50 oC, and
decreased with further increases in temperature. Although data not shown, the authors indicated that addition
of ‘seed’, i.e., cryolite, is beneficial for recovery of fluoride.
No recovery of fluoride by cryolite crystallization from spent fluoride etching solution has been reported so
far. In this study, effects of Al/F ratio, reaction pH and temperature on recovery of fluoride by cryolite
crystallization from spent fluoride etching solution were investigated.
2.
CHEMICAL EQUILIBRIUM MODELING
Commercial chemical equilibrium software, Mineql+ [8], was used to model chemical equilibrium of
cryolite formation. Effects of pH and Al/F molar ratio on cryolite formation were modeled at fixed
concentrations of F- and Na+ at 0.06 and 0.03 M, respectively. As indicated in Figure 1, pHs of between 3 and
7 are the optimal range for the formation of cryolite under molar ratio of Al/F being less than theoretic Al/F
molar ratio of 1/6 for cryolite (Na3AlF6). Meanwhile, formation of Al(OH)3 is dominated with Al/F molar
ratio of higher than 1/6,
Figure 2 shows the percentage of fluoride in the form of cryolite as function of pH and Al/F molar ratio,
indicating that Al/F molar ratio of higher than 1/6 decrease the formation of cryolite due to domination of

AlF3 and Al F4 species with increasing Al concentration. In this study, experiments will be conducted with
Al/F molar ratio of 1/6 to avoid the formation of Al(OH)3 over cryolite.
100
100
Al/F = 0.8：6
Al/F = 1：6
80
60
% F- in cryolite
Al/F = 6：6
% Al3+ in solids
80
40
20
Al/F = 2：6
Al/F = 6：6
60
40
0
100
20
Al(OH)3
cryolite
Al/F = 2：6
% Al3+ in solids
80
0
60
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pH
40
Figure 2. Effect of pH and Al/F molar ratio on
cryolite formation. Chemical equilibrium
analysis
using
Mineql+.
Conditions:
Temperature of 25℃, and fixed concentrations of
F- and Na+ at 0.06 and 0.03 M, respectively.
20
0
100
Al/F = 1：6
%Al3+ in solids
80
60
40
20
0
100
Al/F = 0.8：6
% Al3+ in solids
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pH
Figure 1. Effects of pH and Al/F molar ratio on
formation of aluminum-containing solid through
chemical equilibrium analysis using Mineql+.
Conditions: Temperature of 25 ℃ , and fixed
concentrations of F- and Na+ at 0.06 and 0.03 M,
respectively.
Figure 3. Effect of temperature on cryolite
formation. Chemical equilibrium analysis using
Mineql+. Initial conditions: Concentration of
Na+, Al3+, F- are all 0.03, 0.01, and 0.06 M,
respectively.
Previous studies [5-7] on removal and recovery of fluoride by cryolite synthesis were conducted at elevated
temperature (95 and 50 oC). It will make economic sense to operate cryolite formation process under ambient
temperature. According to thermodynamic data obtained from Mineql+, as shown in Eq (1), formation of
cryolite is exothermic reaction and equilibrium toward formation of cryolite is less favor under elevated
temperature.
3Na  Al
3
 6F   Cr yol i t e Ksp  1033. 84, H  9. 802 KJ
mol
(1)
Figure 3 shows the effect of temperature on the formation of cryolite under different pHs. The percentage of
fluoride in the form of cryolite increases rapidly with increasing pH from 2 to 4, and then increases gradually
for pH from 4 to 7 to 8 depending on temperature which is followed by rapid decreases with further
increasing pHs. Meanwhile, the maximum amount of cryolite formed increases with decreasing temperature.
The simulation result is different from those reported by others [5-7]. In this study, effect of temperature (20,
55, and 90℃) on the formation of cryolite and fluoride removal will be investigated.
3.
MATERIALS AND METHODS
Synthetic fluoride solution was prepared by adding 1.26 g of reagent grade sodium fluoride to 0.25 L of DI
water, corresponding to 120 mM of fluoride, and wasted hydrofluoric acid etching solution containing 22%
of fluoride was obtained from an optoelectronic manufacturer.
Different aluminum salts could be used for cryolite precipitation, including aluminum sulfate, aluminum
chloride, and aluminum nitrate. The price obtained from website is listed in Table 1, and it should be noted
that the price will be different with different quality of chemicals and the amount of orders. The prices in
terms of kg of Al ions for aluminum sulfate and aluminum chloride are more of less the same and are much
cheaper than aluminum nitrate. The price for removing one kg of fluoride based on theoretic Al/F and Ca/F
molar ratios for formation cryolite and calcium fluoride, respectively, is also listed in the table, indicating that
the cost of aluminum using aluminum sulfate or aluminum chloride for cryolite formation is comparable to
the chemical cost of calcium for formation of calcium fluoride. In this study, aluminum sulfate is used to
provide aluminum ions needed for cryolite formation. Due to quite acidic nature of hydrofluoric acid etching
solution (pH of less than 1.0), sodium hydroxide is added to bring reaction pH to the optimum range of 3-7
for formation of cryolite. Concentration of sodium ions from caustic soda addition is more than
stoichiometric ratio needed for formation of cryolite and therefore, no additional Na+ is required.
Table 1. Price of aluminum salts and calcium chloride
Types
Price ($/kg)
Price ($/kg-Al or
Ca)
Price ($/kg-F)
Aluminum
sulfate
(Al2(SO4)3
·13~14H2O)
0.2~0.4 [9]
Aluminum
chloride
(AlCl3·6H2O)
Aluminum
nitrate
(Al(NO3)3·9H2O)
Calcium
chloride
(CaCl2)
0.3~0.46 [10]
0.3~0.5 [11]
0.13~0.2 [12]
1.27~2.54
1.48~2.27
2.37~3.95
0.36~0.55
0.3~0.6
0.35~0.54
0.56~0.93
0.38~0.58
Experiments to study the effects of pH and Al/F ratio were carried by mixing 250 mL of fluoride solution
with 250 mL of pre-determined concentration of aluminum sulfate to make up various Al/F molar ratios.
During rapid mixing at 90 rpm for 3 min, desired solution pH was adjusted using NaOH or HCl. Solution was
then slow mixing (30 rpm) for 20 min, followed by settling under quiescent condition for 10 min. Sample for
fluoride analysis was taken and filtered (0.45 m). Solid retained on the filter was scrapped and stored in
glass vial. After being dried at oven (103~105℃) for 2 hrs, solid was grinded and sieved through ASTM#200
sieve. The resulted powder was fixed at a glass slide using Vaseline for XRD analysis, and was gold plated
and fixed onto SEM holder using copper foil tape for SEM analysis.
Fluoride concentration analyzed by EPA method 340.1 with addition of SPADNS reagent to generate color
for absorbance at 580 nm is not suitable due to the interference of aluminum ions. Therefore, ion
chromatography (ICS-1000, Dionex, USA) is employed for fluoride concentration analysis. Scanning
Electron Microscope (SEM, HITACHI S-3000N) with Energy Dispersive X-ray Spectrometer (EDX, Horiba
EMAX550) and X-ray diffraction (XRD) analysis (Bruker AXSD8 ADVANCEX-ray diffraction system) are
employed for solid analysis. Particle size is analyzed with a laser particle size analyzer (LA-300, Horiba).
4.
RESULTS AND DISCUSSION
4.1 Effect of reaction pH and Al/F molar ratio
Effects of reaction pH on the fluoride removal are shown in Figure 4 along with the equilibrium
concentration of fluoride modeled by Mineql+. The experimental data follows the modeling result quite well.
The optimum pH region for fluoride removal is around 3-7, corresponding to the formation of cryolite as
indicated in Figure 1.
XRD analysis of solid produced at various pHs along with commercial available synthetic cryolite is shown
in Figure 5. Apparently, cryolite was formed with addition of aluminum sulfate at pH ranging from 3 to 7. At
reaction pH of 9, some of refraction peaks match those of commercial available synthetic cryolite. However,
the intensity of refraction peaks is much less. As indicated in Figure 1, at pH 9.0 around 60% of aluminum
added is precipitated in the form of Al(OH)3. Consequently, the peak intensity might qualitatively reflect the
purity of sample.
pH=9
★★
Intensity
pH=6
★★
pH=5.5
★★
pH=5
★★
pH=4
★★
pH=3
★★
cryolite
0
Figure 4. Effects of pH on fluoride removal.
Experimental conditions: F- concentration = 0.06 M.
Al/F molar ratio =1/6. Chemical equilibrium analysis
using Mineql+. Initial conditions: F- concentration =
0.06 M. Al/F molar ratio =1/6. Na+=0.03 M. Sulfate
concentrations = 0.015 M.
★ Na5Al3F14
★
pH=7
10
★
20
30
★
40
50
2θ
60
70
80
90
100
Figure 5. XRD analysis of solid produced at
various pHs. Experimental condition is the same
as that shown in Figure 4.
Figure 6 shows the XRD analysis of solid produced at various Al/F ratios. XRD of particles produced at Al/F
molar ratios of less than 1/6 match quite well with those of the commercial synthetic cryolite. On the other
hand, no distinguishable patter of peaks could be found in those particles produced at Al/F molar ratios of
higher than 1/6 (d and e). The result is consistent with that shown in Figure 1, indicating that the domination
of amorphous Al(OH)3 precipitates forming at Al/F molar ratios of higher than 1/6.
Figure 6. XRD analysis of solid produced at
various Al/F ratios. (a) Al：F = 0.25 : 6, (b) Al：
F = 0.5：6, (c) Al：F = 1：6, (d) Al：F = 1.5：
6, (e) Al ： F = 2 ： 6, and (f) commercial
available synthetic cryolite. Experimental
condition: pH = 5.5. Fluoride concentration =
160 mM.
4.2 Effect of temperature
Formation of cryolite has been shown to be more efficient at higher temperature [5-7]. However, these results
are inconsistent with that predicted by chemical equilibrium modeling which shows the maximum amount of
cryolite formed increases with decreasing temperature. In this study, effect of temperature (20, 55, and 90 ℃)
on the formation of cryolite and fluoride removal was investigated. As indicated in Figure 7, the highest
residual fluoride concentration is found at reaction temperature of 90 ℃. The residual fluoride concentration
at 20 and 55℃are similar. Meanwhile, XRD analysis of particles produced in three temperature conditions all
matches the XRD pattern of commercial synthetic cryolite (data not shown). The result indicates that no extra
heat energy is required for obtaining higher removal of fluoride and formation of cryolite.
180
F- removal (%)
F- concentration (mg/L)
170
F- concentration
F- removal
160
Figure 7. Effect of temperature on the
removal of fluoride. Experimental
conditions: Initial fluoride concentration
= 60 mM, Al/F molar ratio =1/6, pH5.5
150
140
130
120
110
100
90
80
20
55
Temperature (℃)
90
4.3 Comparison of fluoride removal by cryolite and calcium fluoride precipitation
Removal of fluoride by cryolite and calcium fluoride precipitation were compared using spent hydrofluoric
acid etching solution containing 40 to 43 g/L of fluoride. The formation of cryolite was done by adding
aluminum ions three times at Al to initial F (Al/Fint) molar ratio of 0.25：6 for each addition and fixed pH of
5.5.
Figure 8 shows the pictures taken during fluoride removal by cryolite precipitation. After solution being
mixed for 10 min, the solution was allowed to settle at quiescent conditions. A clear sludge interface could be
seen within 2 min, and after 10 min all particles are settled down. For the second addition of aluminum ions,
the liquid above sludge interface is a bit cloudy after 10 min, and the sludge volume is large than those of first
Al addition. After the third Al addition, the sludge volume almost occupied the whole solution. As indicated
in Table 2, 84.5% of initial fluoride is removed after the first Al addition, and fluoride concentration in the
solution is 6714 mg/L. The fluoride concentrations are 4937 and 231 mg/L, respectively, after the second and
third Al addition. The dosage of Al for each addition is calculated based on Al to initial F (43332 mg/L)
molar ratio of 0.25:6. Howevre, after fluoride was removed by the first Al addition, the actual Al to residual F
molar ratio (Al/Fresidual) is much higher during the second and third Al addition. As indicated in Table 2, the
actual Al/Fresidual molar ratios are 1.49:6 and 34:6 for the second and third Al additions. As the consequence,
Al(OH)3 solids are the dominant solids in the solution in the second and third Al addition, resulting in slow
settling solids.
Table 2. Fluoride concentration and removal efficiecny with sequential addition of aluminum on formation of
cryolite. Experimental condition: Initial fluoride concentration (Fint) = 2.28 M (43332 mg/L), Al3+ added each
step equals to Al/Fint molar ratio of 0.25：6, pH5.5.
Fluoride concn. (mg/L)
Fluoride removal rate (%)
Cumulative Al/Fint molar
43332
0.25：6
6714
84.5
0.5：6
4937
88.6
0.75：6
231
99.5
-
Al/Fresidual molar ratio
0.25：6
1.49：6
34：6
-
ratio
The dosing strategy was then changed by adding aluminum ions in the concentration equal to Al to residual F
(Al/Fresidual) molar ratio of 0.25:6 for each Al addition. As indicated in Figure 9, clear liquid above sludge
interface could be seen even after the 10th Al addition (corresponding to Al/Fint molar ratio of 0.59:6). The
overall fluoride removal efficiency is 93.6% and the residual fluoride concentration is 2570 mg/L.
Figure 8. Sequential addition of
aluminum on formation of cryolite.
Experimental condition: Initial fluoride
concentration (Fint) = 2.28 M (43332
mg/L), Al3+ added each step equals to
Al/Fint molar ratio of 0.25：6, pH5.5.
Removal of fluoride by calcium fluoride precipitation was conducted by mixing calcium chloride at Ca/F
molar ratio of 1:1 and fixed pH of 6.5. However, no clear sludge interface could be seen even after more than
5 hrs settling. Therefore, polyaluminum chloride (PAC) coagulant and organic polymer flocculant were
added to facilitate particles settling. After coagulant/flocculant added, solution is allowed to settle for 3 hrs
and result solution is shown in Figure 10. The reason that CaF2 is very difficult to settle could be explained by
the fine particle size of CaF2 precipitates. As indicated in Figure 11, the size of CaF2 is around 0.1 m while
the size of cryolite is about 100 times bigger than that of CaF2.
Figure 9. Sequential addition of aluminum on formation of cryolite. Experimental condition: Initial fluoride
concentration = 2.11 M (40000 mg/L), Al3+ added each step equals to Al/Fresidual molar ratio of 0.25:6, pH5.5.
Figure 10. Fluoride removal by addition of
calcium chloride to form calcium fluoride
precipitates. Initial conditions: Initial fluoride
concentration = 2.11 M (4000 mg/L), Ca/F
molar ratio of 1：1, pH 6.5. (polyaluminum
chloride (PAC) coagulant and
organic
polymer flocculant were added to facilitate
particles settling)
4.
Figure 11. Particle size distribution of cryolite and
calcium fluoride produced in this study.
SUMMARY
Control of pHs in the optimal range of 3 to 7 while keeping Al/F molar ratio of less than theoretic Al/F molar
ratio of 1/6 is for cryolite (Na3AlF6) is the key to successfully produce cryolite. In contradiction to others’
study, equilibrium toward formation of cryolite is less favor under elevated temperature which is consistent
with the exothermic reaction of cryolite formation. The highest residual fluoride concentration is found at
reaction temperature of 90℃ while those at 20 and 55℃are similar. Although, XRD analysis of particles
produced in three temperature conditions all matches the XRD pattern of commercial synthetic cryolite. The
result indicates that no extra heat energy is required for obtaining higher removal of fluoride and formation of
cryolite. With good control of Al/F molar ratio, less sludge is produced in the cryolite formation than that
produced in calcium fluoride precipitation. The reason that CaF2 is very difficult to settle could be explained
by the fine particle size of CaF2 precipitates which is around 0.1 m and is about 100 times smaller than that
of cryolite.
5.
ACKNOWLEDGEMENTS
The study has been supported by the National Science Council of Taiwan under Grant Numbers
100-2628-E-032-002-MY3 and 99-2622-E-032-002-CC3.
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