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

rapidcrimsonMechanics

Feb 22, 2014 (4 years and 10 months ago)

218 views



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.