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

rapidcrimsonMécanique

22 févr. 2014 (il y a 3 années et 4 mois)

60 vue(s)



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

(
synthetic
cryolite)

Chih
-
Wei
Lin

and

Chi
-
Wang Li


Department of Water Resources and Environmental Engineering, Tamkang
University 151 Yingzhuan Road, Tamsui district, New Taipei City 25137, Taiwan


Corresponding

author
:
Email:
c
hiwang
@mail.tku.edu.tw
, (O)
+886
-
2
-
26239343
(FAX) +886
-
2
-
26209651

Abstract

CaF
2

precipitation
is
the
most
frequent applied method in Taiwan

to r
emov
e

fluoride
from

wastewater
of
semiconductor

industries
.

C
alcium chloride and lime

are

used to promote precipitation of CaF
2
.
D
ue to
the

very fine

CaF
2

precipitates

(~0.1

m)
,

coagulants
/
f
loccul
a
nts

are needed to facilitate
sedimentation

of
CaF
2
. In
turn,
large amount of sludge is produced by

CaF
2

precipitation
/
sedimentation

process.

In this study, removal
of
fluoride
from spent fluoride etching solution
by cryolite
synthesis was
investigated
.
While
cryolite
crystallization happened
instantaneously
,
e
xperimental results show
ed

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
CaF
2

precipitates
of
0.1

µm
,
resulting
in
rapid

s
edimentation
. Meanwhile,
cryolite
crystallization
produces much less
sludge
volume than
does
by
CaF
2

precipit
ation.

Depending on the type of aluminum salts used, r
esidual fluoride is in the ranges
of from 140 to
380
mg/L

with
a
lumin
um

chloride
produc
ing

the lowest residual
concentration
.

Recovery of F by
cryolite

in fluidized bed
crystallization

reactor
followed by removal of F by conventional CaF
2

precipitation is
investigated
.

The
proposed process

generates

useful
resource and produces less
wasted
sludge.