Fluoride recovery from spent fluoride etching solution through crystallization of NaAlF (synthetic cryolite)

trextemperMécanique

22 févr. 2014 (il y a 3 années et 3 mois)

96 vue(s)

Fluoride recovery from spent fluoride etching solution through
crystallization of Na
3
AlF
6

(synthetic cryolite)

Chih
-
Wei Lin
1

and Chi
-
Wang Li
2*

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:

CaF
2

precipitation
through addition of
calcium chloride or lime is the most frequent
applied method in Taiwan to remove fluoride from fluoride
-
conta
ining wastewater of semiconductor or
optoelectronic industries. Due to very fine CaF
2

precipitates (~0.1

m), coagulants/flocculants are needed to
facilitate sedimentation of CaF
2
. In turn, large amount of sludge is produced by CaF
2

precipitation/sedimentat
ion 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 cry
olite precipitates have particle size in the range of
3µm ~15µm and are much larger than CaF
2

precipitates of 0.1 µm, resulting in rapid sedimentation.
Meanwhile, cryolite crystallization

process

produces much less sludge volume than does by CaF
2

precipita
tion
/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
optoelec
tronic

industries
through
CaF
2

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 CaF
2
. The former is
preferred

due to relatively
less sludge g
enerated
[
2
]
. However, due to
the

very fine
CaF
2

precipitates
(~0.1

m)

[
2
]

coagulants

or
floccul
a
nts are usually needed to facilitate
sedimentation

of
CaF
2

[
1
,
2
]
. In turn, treatment of
fluoride
-
containing wastewater by
CaF
2

precipitation
/
sedimentation

pro
cesses 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 dependi
ng
on the quality of cryolite
[
4
]
. Removal and recovery of fluoride by cryolite synthesis was studied by Wang et
al.
[
5
]
. In the
solut
ion containing fluoride, aluminum salt (aluminum sulfate or aluminum nitrate) was added
to form aluminum/fluoride complexe
s, 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
o
C

[
5
,
6
]
. A
f
ter precipitates was filtered and
dried (at temperature of 383 K), the crystal phases of precipitates w
ere 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

molybd
enite concentrates.
It was shown
that recovery of
cryolite
was maximized
at temperature

of
50
o
C
, and
decreased with further increases in temperature
.

Although data not shown, t
he 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 for
mation 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

(Na
3
AlF
6
). 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
AlF
3

and
4
Al F

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
.

0
20
40
60
80
100
Al/F = 6

6
% Al
3+
in solids
0
20
40
60
80
100
Al/F = 2

6
% Al
3+
in solids
Al(OH)3
cryolite
0
20
40
60
80
100
Al/F = 1

6
%Al
3+
in solids
0
20
40
60
80
100
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Al/F = 0.8

6
% Al
3+
in solids
pH





Previous

studies
[
5
-
7
]

on removal and recovery of fluoride by cryolite synthesis were conducted at elevated
temperature (95 and 50
o
C
). It will make
economic

sense to operate cryolite formation process under ambient
temperature. According to
th
ermodynamic 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.

3 33.84
3 6 Cr yol i t e K 10,9.802
sp
KJ
Na Al F H
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
0
20
40
60
80
100
1
2
3
4
5
6
7
8
9
10
11
12
13
14
% F
-
in cryolite
pH
Al/F = 0.8

6
Al/F = 1

6
Al/F = 2

6
Al/F = 6

6
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

Ⱐ 慮搠 fi硥x
捯湣敮cr慴i潮猠潦 F
-

and Na
+
at 0.06 and 0.03 M,
respectivel
y.

Figure
2
. Effect of pH and Al/F molar ratio on
cryoli
te formation. Chemical equilibrium
analysis using Mineql+. Conditions:
Temperature of 25

Ⱐa湤nfix敤e捯湣c湴r慴i潮猠潦
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
+
, Al
3+
, F
-

are all 0.03, 0.01, and 0.06 M,
respectively.


for pH from 4 to 7 to 8 depending on temperature which is followed by rapid decreases
with

f
urther
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 w
asted 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. Th
e 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 mo
re 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 fo
rmation. 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 i
s 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

Aluminum
sulfate

(Al
2
(SO
4
)
3


13~14H
2
O)

Aluminum
chloride

(AlCl
3

6H
2
O)

Aluminum
nitra
te

(Al(NO
3
)
3

9H
2
O)

Calcium
chloride
(CaCl
2
)

Price
($/kg)


0.2~0.4

[
9
]

0.3~0.46

[
10
]

0.3~0.5

[
11
]

0.13~0.2

[
12
]

Price
($/kg
-
Al or
Ca)

1.27~2.54

1.48~2.27

2.37~3.95

0.36~0.55

Price
($/kg
-
F)

0.3~0.6

0.
35
~0.54

0.56~0.93

0.38~0.58


Experiments to study the
effect
s

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
an
d stored in
glass v
ial. 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 f
or 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
chromatograph
y (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
emp
loyed for solid analysis. Particle size is analyzed with a l
aser 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 wi
th 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 peak
s 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 mig
ht qualitatively reflect the
purity of sample.












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 cr
yolite. On the other
hand, no
distinguish
able patter of peaks could be found in those particles produced at Al/F molar ratios

of
high
er 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
.


4.
2

Effect of temperature

Formation of cryolite has been shown to be more efficient at higher temperature

[5
-
7].

However, these
result
s
are inconsistent with that predicted by chemical
equilibrium

modeling

which shows
the maximum amount of
cryolite formed increa
ses with decreasing temperature
. In this
study
, e
ffect of temperature (20, 55, and 90

)
on the formation of cryo
lite 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 format
ion of cryolite.

Figure
4
. Effects of pH on fluoride removal.
Experimental c
onditions: 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.


0
10
20
30
40
50
60
70
80
90
100
Intensity
2
θ
pH=3
pH=4
pH=5







Na
5
Al
3
F
14


pH=5.5
pH=7
pH=6






pH=9

cryolite
Figure
5
. XRD analysis of solid produced at
various pHs. Experimental condition is the same
as that shown in
Figure
4
.

Figure
6
. XRD analysis of solid
produced at
various Al/F ratios.
(a) Al

F =
〮㈵


㘬6
(戩 Al

F =
〮0


(挩 Al

F = 1

6
, (搩


F = 1


6

(攩


F =
2

6
Ⱐ慮搠
(
f

捯cm敲捩慬
慶慩l慢a攠 synt桥hi挠 捲yolite
⸠ 䕸灥物m敮e慬
捯湤cti潮o 灈 = 㔮㔮5Fl畯uid攠捯湣敮cr慴i潮o=

ㄶ〠

.


80
90
100
110
120
130
140
150
160
170
180
20
55
90
F
-
concentration (mg/L)
Temperature (

)
F- concentration
F- removal
F
-
removal (%)

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/F
int
)
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 a
bove 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 rem
oved 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/F
residual
) is much higher during the second and third Al addition. As indicated in
Table
2
, t
h
e
actu
al
Al/F
residual

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 con
centration and removal efficiecny with s
equential addition of aluminum on formation of
cryolite. Experimental condition: Initial fluoride concentration (F
int
) = 2.28 M (
4
3332

mg/L
), Al
3+

added each
step equals to
Al
/F
int

molar ratio of
0.25

6
,
pH5.5
.

Fluoride concn.

(mg/L)

43332

6714

4937

231

Fluoride removal rate

(%)

-

84.5

88.6

99.5

C
umulative
Al/F
int

mol
ar

ratio

0.25

6

〮0

6

〮㜵

6

-

Al/F
residual

mol
ar

ratio

0.25

6

ㄮ㐹

6



6

-


周攠d潳o湧n
獴r慴敧
y w慳 t桥h 捨慮来搠批 慤摩湧n慬畭i湵m

i潮猠i渠t桥h捯湣敮cr慴i潮oe煵慬 t漠Al t漠r敳i摵慬 F
(
Al/F
residual
)
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 10
th

Al addition (corresponding to Al/F
int

molar rati
o of 0.59:6).

The
overall fluoride removal efficiency is 93.6% and the residual
fluoride

concentration is 2570 mg/L.


Figure
7
. Effect of temperature on the
removal of fluor
ide.
Experimental
conditions: Initial fluoride
concentration

= 60 mM, Al/F molar ratio =1/6, 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, sol
ution is allowed to settle for 3 hrs
and result
solution

is shown in
Figure
10
. The reason that CaF
2

is very difficult to settle could be explained by
the fine particle
size

of CaF
2

precipitates. As indicated in
Figure
11
, the size of CaF
2

is around 0.1

m while
the size of cryolite is about 100 times bigger than that of CaF
2
.


Figure
9
. Sequential addition of aluminum on formation of cryolite. Experimental condition: Initial fluoride
concentration = 2.11 M (
4
0000

mg/L
), Al
3+

added each step equals to
A
l
/F
residual

molar ratio of
0.25
:
6
,
pH5.5
.

Figure
8
. Sequential addition of
aluminum on formation of cryolite.
Experimental condition: Initial fluoride
concentration (F
int
) = 2.28 M (
4
3332

mg/L
), Al
3
+

added each step equals to
Al
/F
int

molar ratio of
0.25

6

pH㔮5
.







4.

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
successful
ly 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
f
ormation.

T
he highest residual fluoride concentration is found at
reaction t
emperature 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 fo
r 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 CaF
2

is very difficult to settl
e could be explained
by the fine particle size of CaF
2

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.

REFERENCES

[1] T.C. Chuang, C.J. Huang
, J.C. Liu, Treatment of semiconductor wastewater by dissolved air flotation, J.
Environ. Eng., 128 (2002) 974
-
980.

[2] M.F. Chang, J.C. Liu, Precipitation removal of fluoride from semiconductor wastewater, J. Environ. Eng.,
133 (2007) 419
-
425.

[3] G. Scho
lz, O. Korup, High
-
energy ball milling
-
a possible synthesis route for cryolite and chiolite, Solid
State Sciences, 8 (2006) 678
-
684.

[4]
http://www.alibaba.com/showroom/cryolite.html
, Access date: 2012/01/03

[5] L. Wang, C. Wang, Y. Yu, X. Huang, Z. Long,
Y. Hou, D. Cui, Recovery of fluorine from bastnasite as
synthetic cryolite by
-
product, J. Hazard. Mater., 209
-
210 (2012) 77
-
83.

[6] C. Wang, X. Huang, D. Cui, Z. Long, X. Luo, Synthesis of cryolite by fluoride aluminum complex
Figure
10
. Fluoride removal by addition of
calcium
chloride

to form calcium
fluorid
e
precipitates. Initial conditions: Initial fluoride
concentrati
on = 2.11 M (
4
000

mg/L
), Ca
/F

molar ratio of 1




6

⸠(
灯py慬umi湵m
捨c潲i摥d (PAC) 捯慧畬a湴

慮搠 潲条湩c
灯pym敲
fl潣捵o慮a

w敲攠a摤敤d t漠fa捩lit慴e
灡pti捬敳⁳ ttli湧n


Fi杵g攠

⸠偡Pti捬攠獩穥z 摩獴ri扵bi潮o 潦 捲y潬it攠慮搠
捡c捩畭⁦l畯ui摥⁰d潤畣敤⁩n⁴桩猠st畤y.

solution, Xiyou Jinshu / Chin
ese Journal of Rare Metals, 33 (2009) 737
-
741.

[7] M. Kumar, M.N. Babu, T.R. Mankhand, B.D. Pandey, Precipitation of sodium silicofluoride (Na2SiF6)
and cryolite (Na3AlF6) from HF/HCl leach liquors of alumino
-
silicates, Hydrometallurgy, 104 (2010)
304
-
307.

[8] MINEQL+ Version 4.6, Environmental Research Software
.

[9]
http://www.alibaba.com/product
-
gs/605936375/aluminium_sulphate_price.html
, Access date: 2013/07/08

[10]
http://www.alibaba.com/product
-
gs/497674365/competitive_price_aluminium_chloride_hexahydr
ate.html
,
Access date: 2013/07/08

[11]
http://www.alibaba.com/product
-
gs/436671735/Techn_grade_98_Aluminium_Nitrate_Nonahydrate.html
,
Access date: 2013/07/08

[12]
http://www.alibaba.com/product
-
gs/673074073/Calcium_chloride_best_price_2013.html
, Access dat
e:
2013/07/08