Download IJEMS 3(6) 243-247

IndianJournalof En~neering& MaterialsSciences
Vol. 3, December1996,pp. 243-247
,
'"
-,
Reductive dissolution of hematite in hydrochloric acid medium by some
inorganic and organic reductants: A comparative study
KMParida&NNDas
RegionalResearchLaboratory(CSIR),Bhubaneswar751013,India
Received18April 1996;accepted6 September1996
Dissolution behaviourof naturally occurring hematite(> 97% Fe203)in hydrochloric acid medium with or without the presenceof reductantshas beeninvestigatedas a function of acid concentration, temperature,liquid/solid ratio, reductant concentrationand particle size. FeCl2 and SnCl2 are
found as the best reductants among all the organic and inorganic reductants.Addition of FeCl2 or
SnCl2to the extentof 2 wt % enhancesthe formation of Fe2+ in the solution which in turn increases
the percentageof dissolutionand alsolowersthe time of dissolution(> 90%)from 6 to 3h.
The dissolution of iron oxides is an important
chemical reaction that takes place in several fields
of chemistry, viz., leaching of oxide ores in hydrometallurgy,1 removal of iron from low iron
containing minerals,2 the development of suitable
processes for the chemical cleaning and decontamination of steel surfaces,3.4 availability and
transformation of iron in soils, water and sediments.5 Among several mineral acids used for the
dissolution of hematite,6-S HCI is found to be
most effective due to relatively higher romplexing
Experimental Procedure
Naturally occurring iron oxide powder collected
from Barsua Iron Ore Mines, Sundargarh, Orissa,
were sieved to different size fractions: -150
+ 751A.,-75 + 451A.and -45 + 371A..
Mineralogical studies employing X-ray diffraction revealed
that the samples contained mainly a-Fe203 with
traces of magnetite (Fe304) as the iron bearing
phases besides quartz (a-SiO2). The weight per
cent of major elements in -150 + 751A.,-75
+ 451A.and -45 + 371A.samples are: Fe(II), 66.5,
ability of CI- with Fe3+ and solubility of FeCl3
compared to other ferric salts. The dissolution
process is, however, greatly enhanced by the presence of reductant and/or complexing agents4.9-14.
For example, the dissolution of hematite in HCI
or H2SO4 in presence of various redox couples
(e.g. V(III)-V(IV), 1-12, Br-Br2' Ce(III)-Ce(IV),
Fe(II)-Fe(III) etc.) and complexing agents (e.g. ED
TA, crphenanthroline, 2,2'-Bipyridine, NTA etc.)
has been found to increase than that in absence of
such redox couples or complexing agents. Although a considerable amount of studies have
been carried out on the dissolution of hematite in
the presence and absence of reductants, no report
on dissolution using both organic and inorganic
reductants and their comparison is available in literature. In the present work, the dissolution of
naturally occurring hematite in HCI medium in
presence of several organic and inorganic reductants in order to find the most effective reductant
for the dissolution process has been reported.
The promoting effect of various reductants and its
probable reasons are also discussed.
68.0 and 69.5; Fe(III), 0.18, 0.35 and 0.92; SiO2,
4.43, 2.80 and 0.66 and Al2O3' 0.09, 0.07 and
0.01, respectively. Most of the experiments were
performed with samples having size fraction -75
+ 451A..
All the reagentsused were of AR grade.
Dissolution experiments were carried out in a
three necked 500 mL capacity flat bottom reaction vessel containing 200 mL of hydrochloric acid of desired concentration. A reflux condenser
and a thermometer port were fitted in its two
openings while the third opening was used for
charging samples and withdrawing leach liquor.
The reactor was magnetically. stirred and heated
with a heating mantle. A critical speed (500 rpm)
was maintained to prevent diffusion of reactants
in the bulk solution from being the rate limiting.
When the desired temperature is reached, 20g of
sieved hematite with or without reductants were
introduced into the reactor. At pre-determined
time intervals, 2 mL of aliquot was withdrawn for
estimation of Fe2+, F~olal and free acid content
using standard methodsl5. At the end of the dissolution experiment, the undissolved mass was fil-
r
I
244
INDIANJ. ENG.MATERSCI.,DECEMBER1996
100
100
80
:'"
.
80
"'
~
c
.0
c'
0
:;-
:= 60
60
0
~
III
-III
0
.-
=
"0
.
.N.
:0
4
40
~
""
14
20
14
14
14
2
0
,
0
0
0
1
2
3.'
5
6
1
2
[Free
]
Qcid),M
4
Fig. 2-Correlation of free acid concentration with dissolution
of hematite at different time intervals (Liquid/solid = 10:1,
7
Tlm~,h
T= 106°C)
Fig. I-Dissolution of hematitein HCI asa functionof time.
(Liquid/solid= 10:1,T = 106°C)
g HCI (~ 200 mL 1.97 M HCI) only 5.51:>g hem-
tered, washed thoroughly, dried at 120°C and finally subjected to chemical analyses for mass balance and XRD study for identification of mineral
phases. XRD patterns of the residue showed
characteristic Peaks of Fe o and Fe o of re2 3
3 4
duced intensities and prominent peaks for a-SiO2.
The effect of different variables, viz., acid concentration, temperature, particle size and liquid/solid
ratio, reductant concentration and time of reaction were also studied.
Results and Discussion
Effect of acid concentration-The
effect of
[HCI] (1.97-7.88 M) on dissolution of hematite
was studied keeping the temperature, liquid/solid
ratio and particle size constant (Fig. 1). As expected, the percentage of dissolution increases
.., [HCI]. Approximately parabolic-type increase
.
f
with
m the % of dissolution as well as formation 0
~~/
r~
n)
'
m
th
I
e
h
eac
li
quor
. th
Wi
t
respec
t
o.
[HCI]
atite comes into solution after 8 h against the expect~d 1?7~ hematite. This indicates .~at the
reactio? is Virtually stopped when the acid ~o~centration .becomes 0.9M (= 6.56 goHC!). Thi~ is
further evident. from. the. plots of Yo dissolution
versus free acid which mtersect at a common
.
pomt (F.ig. 2 ).
Effect of temperature-To study the effect of
temperature, experiments were carried out in the
temperature range 70-106°C keeping all other
conditions constant. Fig. 3 shows that the dissolution of hematite increased almost linearly at low
temperatures and becomes assymptotic at higher
temperatures. This is due to higher initial rate of
dissolution at higher temperatures. Under identical conditions, the formation Fe(n) increases with
increase in temperature, which may also attribute
to the higher rate of dissolution.
E~ect 0if part lcI e Slze- The fracti on 0f iron
'
ex,
..,
.
tracted
versus
time
.
plots
i
,., ,
.
for
different
.,
pamcle
,
SiZe
.
were observed from the plots. This increase of fractions at ~ed tempe,ratu~e, liquid/solid ratio
d
I ti'
. th .'
[HCI]
..
b d t
and
[HCI]
are illustrated
m Fig.
4. It shows
that
as
iSSO u on
Wi
mcrease
m
is ascn
eo.
...
the increase in [CI-] and also [H+P6. Besides this ~e pamcl~ s~e ~ecrease~ the ~ate of dissol~ti~n
an increase in the formation of Fe2+ in leach li- mcreas~s, mdicating ~e dissolution of hema~te m
quor with increase in acid concentration is an HCI ffilght be. cheffilcally. contro~ed. Th~ higher
added factor for higher dissolutionl.
~e(n) content m sm~er SiZe fraction and m leach
The main reaction taking place during hematite li9uor ~y also attnbute to the enhanced rate of
dissolution in HCI is given by Eq. (1):
dissolution.
Fe2O3 + 6HCI = 2FeCl3 + 3H20
...(1)
Effect of liquid/solid ratio-The effect of liquid/
A simple calculation using Eq. (1) shows that 20 g solid ratio (mUg) on dissolution of hematite was
iron ore (= 19.44 g hematit~, Fe203) requires studied keeping other parameters constant (Fig.
26.7 g HCI for complete dissolution as FeCI3. 5). As expected, the rate of dissolution increases
However, experimentally it is found that in ~4.36 as the liquid/solid ratio changes from 5:1 to 20:1.
.
,
..
I
PARmA& DAS:REDUcnVEDISSOLUTION
OF HEMATITE
245
100
100
8
~
..~
~
c
0
80
c
0
-"
6
-
.-0
-WI
"
60
WI
"0
:0
WI
WI
:0 '0
.0
~
.
5.1
661
A
.40.
~
..20:
10: 1
1
2
20
0
5
Liquid/solid
8
1m_,
90
3
R_ductants
1 So4ium
lulp'ito
Z Zi..
J O.ali. a.id
~
80
8
.
.~
C
0
9 Sta..ou.
a.id
.h'orido
10 Forroul .hlorido
, Hydroqui.ono
~
C
.-0
..60
:.
Particl.
0
.";
size
-
WI
0
.~ 40
III
.~
10
1 -150,41
..
'0
.L&o
7 Hydro'7Iami..
hydro.hlarldo
8 Hydra.i.o
, Glu.o.o
S A..orbi.
~
mL,9
Fig. 5-Effect of.liquid/solidratio on dissolutionof hematite.
([HCl]= 5.90M,T = 106 .C)
Fig. 3-Dissolutionof hematiteat differenttemperature
as a
functionof time(Liquid/solid= 10:1,[HCl] = 5.90M)
100
fatlo,
2
-15,41
'a
3
-451J
L..
20
0
60
0
3
Time.h
'
Fig. 4-Effect of particlesizeon dissolutionof hematiteas a
functionof time(Liquid/solid= 10:1,[HCl] = 5.90M,
T=106.C)
Effect
."
of reducing
agents-The
dissolutIon
of
hematite in presence of varying amount of reductants (0,.5 wt.%) studied at fixed temperature, liquid/solid ratio and [HCl] are illustrated in Fig. 6.
It is interesting to note that, with increase in reducing agents from 0 to 2%, the dissolution increases sharply except in the case of sodium sulphite where a marginal increase is found even at
5%. However, further increase in reductant con.centrations (upto 5%) do not show an appreciable
increase in dissolution except in cases of SnCI2,
FeCI2,oxalic acid and glucose.
The representative time plots for dissolution of
hematite in presence of reductants are given in
500
2
3
[R.ductantJ,%
5
Fig. 6-Dissolution 01 hematite as function of reductant concentration ([HCl] = 5.90M, T'= 106 .C)
Fig. 7. Similar plots were also obtained in the
cases of other reductants. For better comparison,
the reductants are divided. into three categories
based on their electrochemical reactions: (i) reductants which consume H+ with release of electrons, viz., Zn, Na2S03, (ii) reductants which release H+ along with electrons, viz., ascorbic acid,
glucose, hydro-quinone, oxalic acid, hydroxylamine hydrochloride and hydrazine and (ill) reductants which releas~ H + ions and electrons
consuming Cl- ion.
246
INDIANJ. ENG.MATERSCI.,DECEMBER1996
Table 1-Fe(II) formationafterfour hoursof dissolution
andits comparison
withexpected
values[Hematite(-75 + 45 14)=
20g,[reductant]= 2%{w/w),[HCl] = 5.9 M (200 mL)andtemperature= 106.C]
Sl. Reductants
No.
Electron
released/
[Reductants],M Fe(II)expected',g MaximumFe(II)
expected",
g
Fe(II)found,g
.
molecule
1
2
3
4
5
6
7
8
Nil
SodiumSulphite
2,
0.158
Zinc
2
0.0305
Oxalicacid
2
0.0159
Glucose
2
0.0111
Ascorbicacid
2
0.0113
Hydroquinone
2,
0.0180
Hydroxylamine
2
0.0287
hydrochloride
9 Hydrazine
4
0.0154
10 Tinchloride
2
0.0169
11 Ferrouschloride
1
0.0158
*Fe(II)calculated
basedonelectrochemical
reactionof reductants
**Sumof Fe(II)expected
1and Fe(II)formedwithoutreductant
(0.042g)
10
80
.SnCl2
;c
~ 60
~
"'
:0
.."
LL
1
2
3
4
5
Nil
Oxalicacid
Hydroxylamine
hydrochlorid.
SnCI2
F.CI2
2
0.3529
0.6814
0.3545
0.2479
0.2524
0.4021
0.6411
0.3949
0.7233
0.3965
0.2899
0.2944
0.4441
0.6831
0.042
0.0479
0.0804
0.2404
0.2665
0.2856
0.3613
0.3782
0.6862
0.3764
-0.4420
0.7282
0.4184
0.4310
0.4280
0.4550
RCHO -+ RCOOH + 2e
...(6)
C6H6O2 -+ C6H4O2 + 2H+ + 2e
...(7)
N2Hs+ -+ N2 + 5H+ + 4e
...(8)
2NH3OH+ -+ H2N2O2 + 6H+ + 4e
...(9)
+ 2HCl -+ SnCl4 + 2H+ + 2e
...(10)
(whereR = CSHIIOS)
In case of glucose, oxidation of aldehyoe group
to the acidic group has been considered whereas
th lib
d SO f
d
1 hi ..
e
erate
2 rom so IUm SUp te IS OXIdisedtoSO~- in presence of Fe3+(ref. 17).
..
Table 1 presents the concentration of vanous
reductants used; expected Fe2+ comes into solution based on the electrochemical reactions and
Fe2+ actually found after 4 h of dissolution. A
1
.
2
Time,h
Fig. 7-Dissolutionof hematitein presence
of reductants
asa
functio~Qftime ([HCl] = 5.90~. [reductant!= 2 wt. %
T= 106 C)
much lower value of Fe2+ formation in the case
of Na2S03 is probably due to the incomplete utilisation of evolved S02' In case of zinc, because of
its high electropositive nature, conversion to Zn2+
is completed before any appreciable amount of
Fe203 dissolution takes place. In the second category (SI Nos 4-9) the formation of Fe2+ is also
The presence of reducing agents during dissolution reduce a part of FeCI3 to FeCI2. The extent
of reduction is a complicated process w-rnch is a
function of several factors such as concentration
and nature of reductants, reduction potentials and
reaction medium. So it is worthwhile to present
~he °.xidation reactions ~f variou~ reductan~s us~d
III this study. The reactions taking place III acId
medium are as follows:
Na2S03 + HCl -+ S02 + H2O + 2NaCI
...(2)
S02 + 2H2O -+ SO~- + 4H+ + 2e
...(3)
Zn + 2HCl -+ ZnCI2 + H2 + 2e
...(4)
H2C2O4 -+ 2H+ + 2C02 + 2e
...(5)
lower, but comparable with expected values indicating incomplete reaction of the reductants under the expelimental conditions. The high values
of Fe2+ in case of hydroxylamine hydrochloride
is, perhaps, due to its high concentration whereas
in case of hydrazine sulphate, the same is due to
more" number of electrons released per molecule.
In case of t?e third category, (Sl Nos :0 & 11),
Fe2t formation are remakably same WIth that of
expected values. In order to understand the importance of Fe2+ formation during dissolution,
Fe3+IFe2+ ratios at different time intervals are
calculated and compared with that of Fe203 dis'solution.
0
0
"-
'"
J
.
,
PARIDA & DAS: REDUCI1VE DISSOumON OF HEMATI'I'E
247
Table 2-Relationshlp betweenFe3+IFe2+ratio with hematitedissolutionat different time intervals
[Hematite( -75 + 4.5 It) = 20g,[Reductant]= 2% (w/w), [HCl] = 5.9 M (200 nIL) and temperature= 106 .C]
S
No.
Ih
Reductant
1
2
3
4
5
6
7
8
4h
Fe3+/Fe2+ Fe dissolution,% Fe3+/Fe2+ Fe dissolution,% Fe3+/Fe2+ Fe dissolution,%
Nil
SodiumSulphite
Zinc
Oxalic acid
Glucose
Ascorbic AcidHydroquinone
Hydroxylamine
hydrochloride
Hydrazine
Tin chloride
Ferrouschloride
9
10
11
2h
223
191
122
59
49
40
32
27
57.5
58.5
60.4
63.5
64.6
71.8
71.5
72.2
228
209
142
88
51
.42
34
30
70.7
74.0
76.1
79.5
81.7
83.4
84.5
85.3
.278
251
151
51
46
43
33
33
86.1
89.0
90.1
91.3
91.7
91.9
94.0
94.3
24
23
22
72.7
72.9
73.3
26
27
25
85.3
85.6
86.3
29
29
27
95.0
95.5
96.5
Table 2 presents Fe3+IFe2 + ra~,o,s in the leach
ence to the extent <;>f2 w.t, 9/0 reduce the dissolu-
liquor found after 1,2 and 4 h"of
dissolution
tion (> 90°/9 of Fe203) time from 6 to 3h,
along with the percentage Fe2O3 dissolution, It is
f
thi blth
th d ' I '.
Ac know Iedgemen ts
seen rom
s ta e
at e 1SS0ut10n Increases
Th eau thors are thankfu I t 0 Pr0f , H S Ray, D1'Simil
'
b
th d
' r 3+ /r= 2+
'
W1
ecrease In re
/ J.'e rattos,
ar 0 serv.
h
al
b
d
b
Chi
,.
d
H
rector,
Regional Research Laboratory, Bhubanes,
.4
d D S BR
H din
' Ch
' als
ations
ave so een ma e y
anzm an
orTh
0/ fth
d
d .war,
an
r
ao, ea,
organic
effilc
D '"
1V1S10n to r encouragemen
t an d In
' t eres t In
'
thi s
Wltz ,
e wt /0 0 .. e re uctants are expresse ( In
.
terms
Table
of
co?centration
for
better
coml?anson
1). Since the molar concentrations
see
of the
reductants are not the same in all cases, comparison of their effec~veness cannot be done t~ing
all of them at a time, However, among sodiUm
sulphite, oxalic acid, tin chloride and ferrous chloride, where the concentration of promoter, are almost same (- 0,015 M), their effectiveness follow
d
1:- CI
S CI >
ali
.d > sod rum
.lear
an or er re
2
n 2
ox c aC1
sulphite. Similarly, among zinc, hydroxylamine hy-
-
drochloride"' and hydrazine sulphate where the
concentration is almost equal to 0,03 M, the order of effectiveness is: hydrazine sulphate -hyd
I . h d
hl 'd > '
roxy amme y roc on e
ZinC,
Conclusions
Th d .
I .
f h
"
HC I .I'
e 1SS0ution. 0 .ematite 1?
1Sa comp 1cated process which 1S a function of several factors such as acid concentration, temperature, particle size and liquid/solid
ratio, It increases wi$
increase in free acid and Fe2 + concentration in
.
f
th
~eleac,h liquor, In ~e I?resence 0 reductant
e
d1ssolution of hematite 1S greatly enhanced, The
priniary function of the reducing agents is to reduce a part of Fe3+ to Fe2+ and thereby lowers
Fe3+ IFe2+ ratio, Reductants to the extent of 1 to
d
th
d '
I ti.
t '
5 to/ f h
t 't
W
,/0
0
ema
1 e
re
uce,
e
1SS0
u
on
lffie
ap-
prec1ably from that of WlthOUt reductants, FeCI2
and SnCI2 are found to be the best and their pres-
work'
'
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"
,
3 Blesa M,~. & Marot.oA J G, ill Dec,ontamlnatlon
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'
,
5 Zinder B, Furrer G & Stumm W, Geochim Cosmochim
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(1985)23,
15 Vogel A I, A text book of quantitative analysis,3rd ed,
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,
16
Warren
(1987)
G
57.
M,
Kill
S
-H
&
Hemen
H,
Met
Trans,
18B
17 Tiwari B L, Kolbe J & Hayden H W (Jr), Met Trans,lOB
(1979)607.