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hydrcmetallulgy
Hydrometallurgy 47 (1997) 113-125
Zinc recovery from blast furnace flue dust
B. Asadi Zeydabadi u, D. Mowla u'*, M.H. Shariat b,
J. Fathi Kalajahi "
" Chemical Engineering Department, School of Engineering, Shiraz Uniuersity, Shiraz lran
b Materials Science and Engineering Department, School of Engineering, Shiraz IJniuersity, Shiraz, Iran
Received 2 October 1996: accepted 17 June 1997
ELSEVIER Hydrometallurgy 4t (1991) lI3-125
hydrcmetallurgy
Zinc recovery from blast furnace flue dust
B. Asadi Zeydabadi u, D. Mowla u'*, M.H. Shariat b,
J. Fathi Kalajahi "
, " Clrcmical Engineering Department, School of Engineering, ShiraT Uniuersity, Shirctz., Iran
" Materials Science and Engineering Department, Scltool of Engineering, Shtral Uniuersity, Shiraz, Iran
Received 2 October 1996 accepted I.7 hne 1997
Abstract
that it is not possible to lecycle this dust dilectly or to reject it as landfill, makes it necessary to
consider the lecovery of the valuable elements contained in it and to obtain a non-hazardous
n agglomeration units in iron-making
erations which consists of leaching,
rocesses is required. In this research a
with some zinc and other elements
oxides show that it
valuab low acid co
giving he pregnant
purific aminants, an
before electlowinning the zinc. @ 1997 Elseviel Science B.V.
1. Introduction
Many metallurgical processes produce waste materials that until recently have been
uneconomical and/or legally unnecessary to be treated. In the case of iron-making
industry, there is a production of waste flue dust from blast furnaces, which in Iran
could be as high as 42,000 ton/year. These materials are considered as hazardous, from
* Cor'esponding author. Fax: +98-71-52'.,25: E-mail: dmowla@succ.shirazu.ac.ir.
0304-386X/91 /$17.00 O 1997 Elsevier Science B.V. Al1 rishts reserved.
P/1 S0304-3 86X(97)00039-X
B. Asadi Zeydabadi et al. / Hydrometallurgy 47 (1997) 1t3-125
RAW
MATM.L{.L - Puifrcation Colm CATIIODIC
aNc
DIRECT
CIJRRETI
Fig. 1. Simplified diagram of sequential hydrometallurgical processes used in this research work.
the environmental point of view, but also due to the content of valuable metals,
principally zinc. They could be considered as a source for recovery of valuable metals
thus reducing environmental impact and several irregularities usually produced in blast
fumaces [t]. One of these problems is the formation of scaffolds in the blast furnace.
Scaffolds are accretion of materials which build up on the furnace wall and project
towards the furnace center. A wedging or bridging of the charge materials occurs across
the horizontal cross section as well as vertically along a part of the lining which
intemrpts smooth descent of the stock [2]. The scaffold can be kept under control by
controlling the composition of input materials that cause it or by facilitating their exit
from the fumace and also by taking actions to minimize its formation and promote its
removal, as early as possible, when formed.
In this work, the main aim is concentrated on the first method. By separation of zinc
and alkali metals from blast furnace flue dust, zinc as a valuable metal is produced and
secondly, scaffolds and the other similar irregularities in the blast furnaces are con-
trolled. For this purpose a hydrometallurgical process is used with the following
conceptual steps [3,4] (see Fig. 1):
. Direct atmospheric leaching.
. Removal of some impurities of the leached liquor.
. Selective zinc extraction from the pregnant liquor.
. Zinc stripping from the organic extractant.
. Electro-deposition of zinc metal.
2. Experimental procedure
A sample of blast fumace flue dust 'A' from Isfahan Ironmaking complex (IIC)
furnaces in Iran was used in this investigation. This sample was magnetically concen-
trated with a permanent laboratory magnet of low intensity to obtain two different
fractions: magnetic fraction 'AM' and non-magnetic fraction 'ANM'. According to the
peaks detected and their intensities, the results of X-ray fluorescence were:
. majority elements: Si, Fe, C, Caf
. minority, first order: A7, Mg, Zn;
. minority, second order: P, S, Mn, Ti, K, Na, Pb, Mo;
. trace: Cd, As, Sb, Co.
RESIDI]E AQUEOUS ORGANIC E,ECTROIJ]TE
CYCLE CTCIIE CYCIJE
Table 1
Chemical
B. Asadi Zeydabadi et al. / Hydrometallurgy 47 ( j99n I l3_125
is of the blast furnace flue dust
Compounds Sample "AM" Sample'ANM
sio2
D; A
TFe
FeO
Fero.'
Al2o3
CaO
Mgo
P
S
MnO
ZnO
Tlo2
Kzo
NarO
Pbo
Mo
cdb
Asb
sbb
Cob
9.42
27.3
24.0
4.O3
30.0
2.21
1'7.2
5.35
0.19
0.91
0.67
2.52
0.83
0.28
0.12
o.04
0.02
5.62
4.51
6.01
4.31
9.30
25.6
25.5
4.30
31.6
2.28
16.9
5.28
0.19
0.90
0.78
2.48
0.81
0.29
0.13
0.03
0.10
3.50
3.62
2.03
3.20
9.62
29.3
21.3
J. J.1
26.7
2.62
t7.70
s.45
0.19
0.90
0.11
2.4'7
0.87
0.31
0.13
0.03
1.01
7.12
8.20
7.62
6.01
A11 values in percentages unless otherwise stated.
" Residual insoluable after leaching with aqua regia, including carbon.
" ln g/t.
A quantitative chemical analysis of all the samples was carried out based on the
results obtained by X-ray fluorescence examination. The samples were leached with
aqua rcgia and the elements determined by atomic absorption spectrometry (AAS).
100
L
O
.. o
:N
E@
ox
+6
n6
0
0.00
Particle size , mm
Fig. 2. Cumulative amount smaller than stated size of particles in the blast furnace flue dust as a function of
parlicle size.
020
010 040
B. Asadi Zeydctbadi et ol. / Hydrometallurgy 47 (1997) Ii3 125
These analyses are shown in Table 1. By looking at the analysis, the need for magnetic
separationlwas superfluous, and all the experiments were carried out on the original flue
dust 'A'. Fig. 2 is a particle size analysis of sample 'A' obtained by dry sieving. In this
figure, the weight percent accumulation data is plotted as a function of size of particles.
2.L Leaching process
In the first step, leaching is used to transfer zinc to the solution phase. Selective
solubility of zinc relative to iron compounds in this step is very critical. For this reason
not every acid or base is suitable for leaching the blast furnace flue dust. Although
hydrochloric acid, sulphuric acid, nitric acid and sodium hydroxide are employed for
this purpose, sulphuric acid is chosen due to its high selectivity toward zinc extraction.
Leaching studies to determine optimum operational conditions, such as residence
time, liquid-to-solid ratio, concentration of solution and temperature have been carried
out in a magnetically stirred reactor. In these experiments, the reaction conditions were:
stired rate 800 min-l, variable solid-to-liquid ratio, leaching reagent concentration
varying between 0.125 and 1,0 M, maximum reaction time 1.0 h, and, temperature
varying between 18 and 65"C in a reactor of 60 ml. In these experiments, the zinc and
iron concentration and dissolution percent was determined by AAS. Minimum residence
time was determined at different operating conditions. Then, considering this residence
time, the effects of other parameters on zinc and iron dissolution were studied. Results
of these experiments are described in the following sections.
2.2. Purification process
Precipitation and cementation techniques are carried out for removal of the impurities
from the leach liquor, principally iron, aluminum and manganese [5]. By controlling
temperature and pH of solution, iron and aluminum ions existing in the pregnant
solution were precipitated. In this step, the precipitation of ammonium jarosite
(NH4Fe3(SO,r)r(OU)u) is applied in hydrometallurgical zinc refining to remove iron as
a solid substance from acidic zincleach solution [9,10]. larosite is also used as an outlet
for impurities which would otherwise accumulate in the leaching circuit and which
originate from the blast furnace flue dust. Examples of these are arsenic, aluminum and
other minor and trace elements. The precipitation of jarosite is given by the following
reaction:
NHi + 3Fe+3 + 2SO;2 + 6H2O -- NH+Fe:(SO4)r(OH)6 + 6H+ ( 1)
For separation of ions such as cadmium and manganese, a fluidized bed column
containing zinc powder or granules was used. In this reactor, cementation of cadmium
and similar ions from solution occurs on zinc metal particles. The general reaction can
be described as follows.
x*2 I znj __> zn+2 + X0
(in solution) (metal) (in solution) (cemented metallic form on zinc oarticles) (2)
The pregnant solution produced in the purification process contains the above impurities
at concentrations near to allowable concentrations. siven in Table 2.
B. Asadi Zeydabadi et al. / Hydrometallurgy 47 ( 1997 I I 3_125
Table 2
Allowable concentlation of some elements in zinc electrowinning process 17]
rtl
Concentration (g/l)
Cadmium
Iron
Arsenic
Antimuan
Cobalt
0.002-0.004
0.02-0.03
0.0001
0.0001
0.003-0,007
2.3. Extraction process
Selective zinc extraction from the pregnant liquor formed in the previous step, using
liquid cationic exchangers such as LIX 622 atd LIX 984, resulted in formation of an
organic extract containing nearly all the leached zinc and an acid aqueous raffinate
depleted of zinc. The general reaction for this step is as follows.
2RHo,s +Zn{l :RrZno,r+ 2H:q (3)
The equilibrium extraction isotherms for LIX 622 and 984 under low acid (pH: l, 2, 3
and 4) conditions ar 23 and,55"C are obtained experimentally and will be reported
below.
2.4. Stripping process
The next step is the zinc stripping from the organic extract by means of a high acidity
electrolyte.
R rZn o,, + 2H:q : 2RH o,, + Zn (4)
In this step, both loaded electrolyte of high purity, and organic phase are produced, the
latter being recycled back to the previous extraction step. Similar to extraction process,
the equilibrium stripping isotherms for LIX 622 and 984 under high acidity (200, 350,
450 and 500 g/I) conditions at23 and 55oC were obtained experimentally and reported
in the following sections.
2. 5. Electrodeposition proces s
The last step of the process is the electrodeposition of zinc on aluminum cathodes
from the loaded electrolyte according to Eq. (5). In this step, an acidic electrolyte which
is stripped of zinc is produced and recycled back to the previous stripping step [7].
Zn + H2O :Zno + ZH:q+ +O2
Thus, a closed electrolyte circuit is established,
transferring the zinc from the organic extract to
(s)
fulfilling the following two tasks: (a)
the electrowinning operation; and (b)
118 B. Asadi Zeydabad.i et al. / Hydrometallurgy 47 (j997) j13-125
transferrilg the corresponding acidic solution generated by the electrolysis of zinc to the
organic r'qffinate.
3. Results and tliscussion
In the leaching step, a comparison between different mineral acids and bases showed
that sulphuric acid is an ideal leachant for separation of zinc and alkali metals from the
blast furnace flue dust [8]. Selective leaching of zinc with respect to iron is the main
characteristic of the sulphuric acid leaching process. Application of a fluidized bed
reactor in the purification process is suitable for removal of some impurities from the
pregnant solution produced by leaching. Because oflow concentration of zinc ions in the
pregnant solution produced from the leaching process, extraction and stripping steps are
used to concentrate this solution. In these steps, employing two cationic exchangers such
as LIX 622 and LIX 984 gives an electrolyte with at least 60 g of zitc ions per liter of
solution. Use of standard conditions in the electrodeposition process can produce
extra-pure zinc metal and a high acid solution which can be employed in the leaching
step. Results of these sequential steps are given as follows.
3.1. Leaching step
In this process various parameters which affect the rate of dissolution of zinc with
respect to iron are investigated. Optimum values for these variables are determined
experimentally and shown in the following subsections.
3.1.1. Effect of sulphuric acid concentration
The effect of sulphuric acid concentration is investigated in the concentration range
of 0'125-1.0 M. The results obtained are shown in Figs. 3 and 4. From these tests it
could be deduced that the rates of zinc and iron dissolution are functions of the acid
S/L=L/r5 E/cm
T=18.0'c -'
o_!qD Cr=o,125
r-!!rr C2=0,250
^44AA Ca=0.500
^_4a! Ca=1.000
M
M
M
M
010203040506070
Time , mrn
3. Zinc dissolution percent as a function of time in H2Soa leaching process for sample .A'.
100
!
O
860
o
o40
@
@
a20
N
0
Fig
B. Asadi Zeydabadi et al. / Hydrometalturgy 47 (1992) I13_125
oj].qo C,=0.125 M
u!!! cr=o.250 M
a_Aqa Q.=g.gee 11.{
^-A-a-! C4=1.000 M
Time , mrn
Fig. 4. hon dissolution percent as a function of time in HrSOa leaching process for sample ,A'.
concentration in the investigated range. Fig. 3 shows that the optimum value of acid
concentration of about 1.0 M is suitable for the leaching process.
3.1.2. Effect of temperature
The effect of temperature on extent of dissolution was studied at 18, 35, 50 and 65"C.
The results are given in Figs. 5 and 6. Although the rate of dissolution of the zinc is not
strongly dependent on the temperature, the dissolution rate of the iron species is
sensitive to the temperature, which means that it is necessary to work at room
temperature to obtain simultaneously high extraction rate of zinc and low dissolution
rate for iron.
3.1.3. Effect of solid-to-liquid ratio
According to the results obtained above, the best sulphuric acid concentration may be
1.0 M' when the usual solid-to-liquid ratio is selected. In this state, sulphuric acid
concentrations less than 0.5 M are not sufficient to leach these species completely.
s1t=t/rs g/cm"
Uoncentration=0-5O
oEoEo tI.=1A o'a
r_!!r. Te=35.0'C
^_34AA T3=50.0,C
^-41a4 T4=65.0'C
0102o3040506070
Time , mrn
Fi'g' 5 ' zinc dissolution percent as a function of time in H, soo leaching process for sample ,A'
119
10
!o
\o-
\ob r.
5^
j"
aL
b'
a
.iq
d9
o
!.
010
100
o
F60
o
o40
@
@
d
o20
N
0
B. Asadi Zeydabadi et aI. / Hydrometallurgy 47 ( I 997) I I 3 I 25
10
S/L=I/I5 E/cmg
Concentration=0.50
o_!!qo Tr=18.0'C
u!Lr Te=35.0,C
^]:aa T3=50.0'C
^4^ Ta=65.0,C
0102030405060?o
Time , mrn
Fig. 6. hon dissolution percent as a function of time in H, soo leaching process for sample .A,.
100
o^
OE
o
oL
b
o
o
o
160
o40
a
!oZQ
N
tr_llqo L/S=s
r-!!.Lr L/S=10
Concentmtion=0.b0 -Ml ^_aaA^ L/S=lb
f-ure=18.0 c | Lrtrrt'/S=2O
10 20 30 40 50 60 70
Time , rrrin
Fig. 7 . Zinc dissolution percent as a function of time in H, Soo leaching process for sample ,A'.
o
ts
o
a
@
o
L2
10
I
6
4
0
EtrtrEo L/s=5
rrrrr T.,/S=ln
AAMA L/S=15
^t!L L:/S=ZO
Concentration=o.s0 M
Tenperature=18.0,C
20 30 40 50 60 70
Time , rnin
Fig. 8. kon dissolution percent as a function of time in HrSOo leaching process for sample ,A,.
Under these conditions an 7/I0, and, a residence time of
1.0 h, 82Vo of Zn and 5Vo show that although the extent
dissolution of zinc is not , this is not the case for iron.
B. Asadi Zeydabadi et al. / Hydrometallurgy 47 ( 1997) I t3_j25 121
Therefore, it could be deduced that the optimum operational conditions for leaching is
the use oI a 1.0 M solution of sulphuric acid with a residence time of l0 min and a
solid-toJiquid ratio of I / l0 at room temperature.
3.2. Extraction and stripping steps
In solvent extraction and stripping steps isotherms at23"C for organic extractant, LIX
622 and 984 are obtained experimentally and are shown in Figs. 9-12. In these
extraction experiments, the liquid phases nthetic solutions containing
similar amount of zinc ions which are ob t furlace flue dust leaching
process. In Fig. 9a and b the extraction is Lrx 622 (15 volvo in Shell
r40) at o/A ratio of r/7 are shown, and the effect of aqueous solution pH on
extraction isotherm is indicated in these figures. Fig. 10 shows stripping isotherms at
various acid phase concentrations, with an O / A ratio of 5 /l and at room temperature.
Fig. l1a and b and Fig. 72 are the same isotherm but for LIX 984 as the other orsanic
(a) o'o
EoDEo hH=1 O
rrrr pH=z.0
a-444a p]l=3.0
1.0 2.0 3.0 4.O 5.0
Zinc concentration in
aqueous phase, (g/L)
(b) to.o
ro_s_Lo pH=4.0
Zinc concentration in
aqueous phase, (g/L)
Fig. 9. (a) Extraction isotherms forL\X622, 15 volTo in kerosene (Shell
isotherm for LIX 622, 15 vol%o in kerosene (Shell 140) for sample .A,. 140) for sample 'A'. (b) Extraction
E,-
trbo
'J Lo
!o
ri@
ad
oo cn
Od
s:r
NO
0.0
''ar o.o
si
obn
.iv
{o; uo
id
9,!
EA+.0
OO
o.i
O6
I F! z.o
NO
0.00
60
060
040 100
122 B. Asadi Zeydabadi et al. / Hydrometallurgy 47 ( 1997 ) I I 3 - 125
1.0
\-
\l .1 0.8
H\
oEt
'nv
i oi 0.6
ri@
9'q
F 0.4
o.i
O6
HP ^O
NO
o/A=5/r
Temperature=23'C
tr__S_S_S_o CEso.=200
r! Lr CE So.=350
a A-eAA Cn"so.=450
^1f-L CH"so.=500
s./L
s./ L
e./ t
g/L
00
Fig. 10
0.0 10.0 20.0 30,0 40,o 50.0
Zinc concentration in
aqueous phase, (g/L)
Stripping isotherms for LIX 622, 15 vol%o in kerosene (Shell 140) for samDle .A,
oo m o p!l=1.0
rrilr pH=e.0
a344a pH=3.0
1.0 2.0 3,0 4,o 5.0
Zinc concentration in
aqueous phase, (g/L)
E-ase! pH=4.0
0.00.00 0.20 0.40 0.60 0.80 1.00
Zinc concentration in
aqueous phase, (g/L)
Fig. 11. (a) Extraction isotherms for LIX 984, 15 volVo in kerosene (Shell 140) for sample 'A'. (b) Extraction
isotherm for LIX 984, 15 vol%o in kerosene (Shell 140) for sample 'A',
(a; e'o
Fl
c\
oEn
'alv
lio
ldi
9r
90
Oo
-d2.o
0d
F9D
NO
/h\ 1o.o
F] 8.0
tr\
obt
'iv
d-
ho 6.0
Ph
trd
;a:
o.n
Od
.E il z.o
NO
00
B. Asadi Zeydabadi et aL / Hydrometallurgy 47 ( IggT) I I 3_125 123
0.0 10.0 20.0 30.0 40.0 50.0
1Lt"""n";:l*""l.tru?ir
Fig. 12. Stripping isorherms for LIX 9g4, 75 volTo in kerosene (Shell 140) for sample ,A,.
extractlng agent. A simple comparison between these isotherms and those obtained for
3.3. Electrodeposition step
It is well-documented that zirrc deposition in the electrowinning process is very
sensitive to small quantities of ceftain impurities [7]. Previous studies have indicated that
10
o.B
o6
j
d\
ob0
'f1v
d-
!O
+@
9.q
OO
Od
NO
00
Fig' 13' SEM photomicrograph (x 100) showing the zinc deposit morphology, under rhe following electrolysis
conditions: 60 g/1 zn2+,250 g/l H2so4, 35"c temperarure, 3.0 h deposition time, 50 mA/cm2 crttent
density, animal glue-free electrolyte, commercial Al cathode and pure pb anode.
o/^=5/r
o--s!-Lo cg,so,=200 g/L
.-!!-Lr UEtso.=350 g/L
anqa Cuso.=450 g,/L
^4rr^ Cg,so,=500 E/L
B. Asadi Zeydabadi et aL / Hydrometallurgy 47 ( 1997) I t3_ 125
Fig. 14. SEM photomicrograph ( x 500) showing the zinc deposit morphology, under the following electrolysis
conditions: 6o g/l zn2+,250 g/1H2so4, 35"c temperature, 3.0 h deposition time, 50 mA/cm2 crtient
density, animal glue-free electrolyte, commercial Al cathode and pure pb anode.
very small amounts of cadmium and arsenic in the ranges of parts per billion can greatly
reduce the current efficiency [Z]. In ttre purification process used in this work, we have
tried to reach low levels in actual practice. Chemical analysis of the pregnant aqueous
solution produced from the stripping cycle indicates that this is a suitable electrolyte for
Fig. 15' SEM photomicrograph^(X1000) showing the zinc deposit morphology, under the following
electrolysis conditions: 60 g/lznz+,250 g/1H2so4, 35'c temperature, 3.0 h deposition time, 50 mA/cri
current density, animal glue-free elecfolyte, commercial A1 cathode and pure pb anode.
B. Asctdi Zeydabadi et al. / Hydrometallurgy 47 ( 1997) I j3-125
zinc deposition on aluminum cathodes. By application of standard conditions used in
zinc electrodeposition cells, extra-pure zinc metal is obtained.
The SEM photomicrograph in Figs. 13-15 show the morphology of zinc deposits
obtained under experimental conditions described below each figure. As shown in these
figures, because of high-aqidity of electrolyte and some additional impurities, hydrogen
evolution takes place and therefore current efficiency greater than'75Vo is not obtained.
4. Conclusions
Recovery and separation of metals, especially zinc, from blast furnace flue dust is a
practical idea in iron-making industries. The fact that it is not possible to recycle this
dust directly or to reject it as landfill, makes it necessary to consider the proposed
process used in this work, to obtain a non-hazardous residue that can be stored without
problem or can be used in agglomeration units. By application of sequential hydrometal-
lurgical processes as described, high purity zinc is produced while at the same time, by
avoiding the possibility of production of scaffolds in blast furrraces, the smooth
operation of blast fumaces will increase productivity and reduce the cost of pig iron.
t25
Acknowledgements
The authors wish to express their gratitude to
Complex (IIC) for their financial support and
Chemical, Materials Science and Engineering
Shiraz University and Central Laboratory of
chemical analysis of the sample is appreciated.
the management of Isfahan Ironmaking
assistance in this study. The help of
and Civil Engineering Departments of
Isfahan Ironmaking Complex for the
t5l
t6l
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