The Leaching of Hematite in Acid Solutions

HIROSHI MAJIMA, YASUHIRO AWAKURA, and TAKUMI MISHIMA

The reactions of hematite in aqueous hydrochloric acid, perchloric acid, and sulfuric acid solutions
with or without the addition of common or uncommon salts were studied using monosized particulates
in a well-stirred reactor and dilute solid concentration to obtain fundamental details of the reaction
kinetics. The experimental rate data suggest that the entire leaching reaction is controlled by a chemical
process. The leaching rate of hematite was seen to be first order with respect to hydrogen ion activity,
a(H+), in hydrochloric acid or perchloric acid solutions, with or without the addition of common salts,
while the rate was of a half order in sulfuric acid solutions with or without the addition of sodium
sulfate. A theoretical analysis showed that the anions next to the surface in the double layer were
chloride ion and perchlorate ion in hydrochloric acid and perchloric acid solutions, respecuvely, and
sulfate ion in sulfuric acid solutions, with or without the addition of sodium sulfate. The fact that the
leaching rates of hematite were quite different in various acids having identical a (H +) values indicates
the importance of anion adsorption. The dependency of the leaching rate upon a (H § appeared to be
controlled by adsorbed anions next to the surface in the double layer.

I. INTRODUCTION from the Bailadila Mine, India. Chemical analysis of this

material yielded an iron content of 66.8 pct, corresponding
ACIDleaching reactions of metal oxide ores of Zr, U, Be, to a Fe,O3 content of 95.4 pct. X-ray diffraction showed the
Ti, Cu, and others are widely utilized in hydrometallurgy.
existence of no particular contaminating minerals. Spec-
In a paper by Warren and Devuyst, 1 a general mechanism
trographic analysis indicated the presence of only traces of
for the leaching of metal oxides was proposed. However,
A1, Si, Cu, and Zn. The mineral was crushed and sized by
some features of the leaching mechanism still remain to
be investigated. wet screening to provide a clean - 1 5 0 +200 mesh (Tyler
standard) fraction for these leaching studies. All chemicals
Recently Majima et al. 2 studied the dissolution reaction
used were of reagent grade. Deionized water was used in the
of cupric oxide in various acid solutions and found that the
preparation of all the solutions.
dissolution rate was first order with respect to the activity of
hydrogen ions for perchloric, nitric, and hydrochloric acids,
while it was a half order for sulfuric acid. They also ob- B. Experimental Procedures
served that the addition of electrolytes having anions com-
mon to the acid solutions resulted in an acceleration of the Particulate leaching experiments were carried out in a
dissolution rates due to increases m the activity of the hydro- covered one liter Pyrex separable flask immersed in a
gen ions. The addition of electrolytes having uncommon thermostatically-controlled water bath. The temperature
anions produced a markedly different effect on dissolution within the flask was maintained -+ 0.1 K of the noted val-
rate. From these observations the importance of the role of ues. The leaching solution containing monosized particu-
anions in dissolution reactions was clearly established. At lates of hematite was agitated with a Teflon stirrer. Solution
that time the role of anions in affecting the dissolution rates samples were withdrawn through a fritted filter at certain
in acid solutions was not determined.-" time intervals.
Hematite is one of the interesting oxide minerals from a
leaching viewpoint. Many fundamental studies have been C. Analytical
made on the leaching of iron oxide, 13-" but some features The concentrations of iron in the solutions were analyzed
of the leaching mechanism still remain to be elucidated. by means of an atomic absorption spectrophotometer.
In the present study the authors examined the dependence
of leaching rate on hydrogen ion activity and the role of
anions in the leaching of hematite in various acids, provid- D. Measurements of Hydrogen Ion Activity in Various
ing some detail toward a fuller understanding of the dis- Acid Solutions
solution mechanism. The approximate values of the activities of hydrogen ions
in solutions containing hydrochloric, perchloric, or sulfuric
acid were determined by measuring the electromotive force
II. EXPERIMENTAL of a cell at 298 K with transport as shown:
A. Materials AgC1-Ag (reference electrode)]3.3 mol 9 3 KC1H
The hematite used for these experiments was in the form test solution, H, (1 atm)lPt-Pt black
of high grade lumps about 5 cm in diameter each, obtained The Pt-Pt black electrode was prepared according to the
HIROSHI MAJIMA, Professor, and YASUHIRO AWAKURA, In- procedure discussed by Ives and Janz.~2 An AgC1-Ag elec-
structor, are with the Department of Metallurgy, Kyoto University, Kyoto, trode supplied commercially was used as the reference
Japan 606. TAKUMI MISHIMA, formerly Graduate Student, Kyoto
Umverslty. is now Research Engineer, Mitsub~shi Metal Corporanon,
electrode. Agar media containing 3.3 mol 9 dm -5 KC1 or
Osaka Refinery. Kita-ku, Osaka, Japan 530. 5.4 tool 9 dm -~ NaC1 were used as salt bridges for the solu-
Manuscnpt submitted May 16, 1984 tions of hydrochloric and sulfuric acids or perchloric acid,
METALLURGICALTRANSACTIONS B VOLUME 16B, MARCH 1985-- 23
respectively. To avoid the formation of insoluble potassium 11, i i

perchlorate, sodium chloride was used as the salt bridge for

perchloric acid solutions. Temp 328 K
to 600 rpm C (H2504)
The electromotive forces of these cells are expressed as 10 I m ol.drn-3~
EMF= - 0 . 2 0 6 + 0.05916 9 l o g a ( H ~) + V(jp) [1]
where V(jp) is a liquid junction potential occurring in the
boundary region between the test solution and the salt 2.0
bridge. The values of the liquid junction potential were
evaluated with a first order approximation by applying m8
Henderson's equation. ~3The concentrations of ionic species 0
required for the calculation of V(jp) in sulfuric acid solu- E
7
tions were calculated based on the relationship, as shown 'O
later in Eq. [5], described by Baes for the dissociation of
bisulfate ions.l~ The approximate values of hydrogen ion 6 1.5
activity were then calculated from Eq. [1]. 1516 0
U_

o5
2
III. E X P E R I M E N T A L RESULTS O
ffl 1.0
A. Dissolution Rate Curves i:5 4
The effects of acid concentration on the quantities of iron
leached by hydrochloric and sulfuric acids at various time 3
intervals are shown in Figures 1 and 2, respectively. The 0.5
general curvature of the plots of iron leached against time,
shown in these figures, varies markedly for different acids.
In hydrochloric acid solutions, particulate hematite dis-
solved at a constant rate except in the initial stage of dis- 0.1
solution. Similar experiments with sulfuric acid solutions

101 [ I I I
o 60 120 180 240
t Temp : 328 K Time / rain
9 to : 600 rpm C(HC[) Fig. 2--Typical example of hemaUte leaching curves in sulfuric acid
/ mol.d m-: soluUons at 328 K.

are depicted in Figure 2. Again, linear kinetics were ob-

served over the almost whole reaction period, although a
7 slight loss in linearity was observed when the leaching ex-
0 periment was done in 2.0 mol 9 dm -3 H2SO4. The shape of
E the leaching rate curves obtained in perchloric acid solutions
~, 6 was very close to that of hydrochloric acid solutions. Based
o 3,5 on these observations, a leaching rate could be determined
from a slope of the linear portion of the curve in hydro-
chloric acid and perchloric acid solutions. In sulfuric acid
solutions, however, an average rate of leaching was calcu-
| lated from the amounts of Fe(III) dissolved at 30 minutes
> and 240 minutes because the less linear relationship be-
"6 tween Fe(III) dissolved and time can be observed as shown
m 3.0
a / /'~( in Figure 2.
Rate data obtained in this manner were used to examine
the effects of various factors on the leaching of hematite.

. _ ~ ~ / 1.5
B. Kinetic Limitations
The effect of temperature on the leaching rate of hematite
in both hydrochloric acid and sulfuric acid solutions was
0, o_
examined. Figure 3 is an Arrhenius plot showing a greater
0 ,
0 60 120 180 240 temperature dependence of reaction rate for leaching in hy-
Time/rain drochloric acid than in sulfuric acid. It also shows that at
Fig. 1 --Typical example of hematite leaching curves m hydrochloric acLd higher temperatures the rate is greater in 1 mol" dm -3
solutions at 328 K H2SO4 than in 1 mol 9 dm -3 HCI. In a 1 mol 9 -3 HCI

24-- VOLUME 16B, MARCH 1985 METALLURGICAL TRANSACTIONS B

 Temp / K , l ,
328 323 318 313 308 T e m p 328 K
1 I I l 1
J
r-
9.5
E

.tool -1 'E 1 mot.dr'n3HCt

U O O
9
O

'E 10.0 E 2
~h 1 mol. d r'n-3H2SO~
'C~
77.7 kJ.mol ' O
O
E
O
rY
t"t"

O1
o 10.5 ~ : 600rprn ~'x~ -
0 : 1 rnob dm~H2SO4 ~ ' ~%x, 10 200
i 400 600
[] : 1 mobdm3HCI xOxx to / r p m
Fig. 4 - - E f f e c t of agitation speed on leaching of hematite.
I I J I I
3.00 3.05 3,10 3.15 3.20 3.25
I"-1 / 10-3K-1 ! 1
/ |

Fig. 3--Arrhemus plot for hematite leaching m hydrochloric acid and 2.12 Temp 328 K O//O
sulfuric acid solutions.

solution an apparent activation energy of 77.7 kJ 9 mol -l

was calculated by regression analysis, while an activation -c1.5
energy in a 1 mol" dm -3 H2SO4 solution was calculated E
to be 104 kJ 9 mo1-1. The activation energies for the leach-
ing of hematite by hydrochloric acid, sulfuric acid, and t)
perchloric acid are compared in Table I. The magnitude of '~ 1.12
these activation energies is high enough to support the
chemical reaction mechanism for the leaching of hematite, l0
Several tests were performed to determine the effect of
agitation speed on the reaction kinetics in hydrochloric acid "0.5
and sulfuric acid solutions. No significant change in reaction
rates was observed as the agitation speed was increased as
shown in Figure 4. Based on the facts that the reaction
shows the higher activation energies and that almost con- t I l I
stant reaction rates are obtained at the various agitation 0 1 2 3 4
speeds under the experimental conditions studied, it is rea- C ( H C I ) / tool-din -3
sonable to expect the chemical limitations are the major
Fig 5 - - E f f e c t o f h y d r o c h l o r i c acid c o n c e n t r a t i o n on rate o f l e a c h i n g o f
leaching determinants. h e m a u t e in h y d r o c h l o r i c acid s o l u t i o n s at 3 2 8 K

C. Effect of Acid Concentration

centration, as shown in Figure 5. A similar response of
The hematite leaching rates were measured at different hematite leaching rate on hydrochloric acid concentration
concentrations of hydrochloric acid and sulfuric acid. was reported by Surana and Warren.~~ The dependence of the
Figures 5 and 6 depict the experimental results obtained leaching rate on acid concentration in perchloric acid solu-
with hydrochloric acid and sulfuric acid, respectively, The tions is quite similar to that in hydrochloric acid solutions.
leaching rate of hematite in hydrochloric acid solutions in- The effect of sulfuric acid concentration on hematite
creased sharply with an increase in hydrochloric acid con- leaching was significantly different from the effect of the
acid concentrations of hydrochloric acid and perchloric
Table I. Activation Energies in kJ 9 mo1-1 for the acid. In this case, the leaching rate exhibited a parabolic
Leaching of Hematite by HCI, H2S04, and HClO4 relation to sulfuric acid concentration, as shown in Figure 6.
The authors pointed out in a previous paper that hydrogen
Acid Ref. 9 Ref. 11 Ref. 6 This Work
ion activity rather than acid concentration is the appropriate
HC1 90.3 95.7 91.5 77.5 rate-law parameter. 2 In order to examine the relationship
H2SO4 -- -- 76.1 104 between leaching rate and hydrogen ion activity, the activ-
HC104 86.9 80.3 -- ities of hydrogen ions in aqueous solutions of hydrochloric

METALLURGICAL TRANSACTIONS B VOLUME 16B. MARCH 1 9 8 5 - - 2 5

 I I i I I I t

Temp 328 K 1.0 Temp : 298 K

to 600 r p m
3.0
0.8

O
u ~: 0.6
"6
~, 2.0
O C~
E oOZ,,
o O
'O 9 : HCI04 § NoCIO 4
0.2 ~ I) : HCIO4 § NQC[
n- 1.0 13~'~ (]l: HCIOj,~Na2S04
\ ~ z~ : H2S04. Na2S04
0 ,n : H2SO4. NaCI
I I I I

0 1 2 3 4
Salt Concentration / tool.din -3
0.5 1.0 1.5 2.0 Fig. 8 - - A c t i v i t i e s o f hydrogen ions in ] tool 9 dm 3 acid solutions con-
C(H2SO 4) I t o o l . d i n -3 taining c o m m o n salts
Fig. 6-- Effectof sulfuricacid concentrationon rate of leachingof hema-
tite in sulfuric acid solutionsat 328 K.
acids containing their common sodium salts are depicted
in Figure 8.
acid, sulfuric acid, and perchloric acid were determined at
The activities of hydrogen ions in hydrochloric acid, sul-
298 K as a function of acid concentration.
furic acid, and perchloric acid solutions sharply increase
The measured values of hydrogen ion activity are shown
with an increase in molarities of these acids. The activities
in Figures 7 and 8. In Figure 7 the activities of hydrogen
of hydrogen ions in l tool 9 dm -3 of hydrochloric acid and
ions in various acids are plotted against acid concentrations.
perchloric acid solutions containing common salts and
The activities of hydrogen ions in 1 mol- dm -3 of three
1 mol" dm -3 of sulfuric acid solution containing sodium
chloride also increase with an increase in molarities of salts
I [ I I
added. The activity of hydrogen ions in 1 tool 9 dm -3 of
sulfuric acid and perchloric acid solutions containing so-
Temp 298 K dium sulfate, however, decreases with an increase in mo-
larity of the salt.
The leaching rates of hematite in hydrochloric acid and
sulfuric acid solutions at 328 K as shown in Figures 5 and
6 are replotted against corresponding values of hydrogen ion
activity at 298 K in Figures 9 and 10. The clear relationship
between hydrogen ion activity and the leaching rate of he-
matite in perchloric acid solutions is also demonstrated in
Figure 9. The first order dependence of the rate of leaching
"l- of hematite in hydrochloric acid and perchloric acid solu-
v
O tions on hydrogen ion activity was deduced from this study,
O~ while the half order dependence of leaching rate on hydro-
O
gen ion activity was observed for the leaching of hematite in
sulfuric acid solutions.
Compared to the leaching rate of hematite in sulfuric acid
and hydrochloric acid solutions, the rate of hematite leach-
O 9 HCI ing m perchloric acid is low as can be expected from the
results shown in Figures 9 and 10. In other words, per-
9 " HClO~, chloric acid leaches hematite very slowly. If the dissolution
reaction of hematite in an acid solution is controlled simply
H2SO4
by the action of hydrogen ions, the same leaching rate
-1 should be obtained if the leaching rates in different acid
solutions having the same level of a(H +) are compared.
I I I
However, the leaching rates of hematite differ greatly ac-
0 1 2 3
cording to the acids used. The dependence of leaching rate
Acid Concentration tool.din -3 on a(H § in sulfuric acid solution is different from that
Fig. ? - - A c t x v i u e s of h y d r o g e n ions m aqueous acid soluuons in hydrochloric acid and perchloric acid. Furthermore,

26 VOLUME 16B, MARCH 1985 METALLURGICAL TRANSACTIONS B

 I III I I I I I I III I I I I I I Ill I 10-9 I I 1 l I [111 [ I [ I I I l I

Temp: 328 K Temp 328 K

~o : 600 r p m ,45/" uJ 600 rpm

10-9 9 : NaCI
O : HCI
HCI. //0 OAS~

: HcIo
9 HC~- :
IE 9 H;SOi 9 N c l ~

'E
E u 10-i0
0
o. i0_i0
~'~: E
I11
0 c'r"
E 2
2-

1041 10-1 t I 1 i i iltl i i i i i i i

10-1 10 o 101
a(H §
1 Fig 1 0 - - E f f e c t o f h y d r o g e n ion activity on rate o f l e a c h i n g o f h e m a t i t e
m sulfuric acid solution at 3 2 8 K

10-'~2 IIII I I I I IIIi I 1 I I l 11111 I the hydrogen ion activity of the sulfuric acid solution. The
i0 -I 100 101 effect of the addition of sodium chloride to 1 mol 9 dm -3
a(H ~ ) H2504 solutions was also examined. The concentrations of
Fig. 9 - - Effect o f h y d r o g e n ion activity on rate o f l e a c h i n g o f h e m a m e in sodium chloride in the solutions were adjusted to be 0.5,
h y d r o c h l o r i c or p e r c h l o r i c acid s o l u t i o n s at 3 2 8 K. 1.0, and 2.0 mol 9 dm -3. Hydrogen ion activity of the sul-
furic acid solution was increased by the addition of sodium
we must note that strong solutions of hydrochloric acid chloride, and this resulted in an increase in leaching rate. It
leach hematite more rapidly than equivalent solutions of should be noted that the characteristic leaching behavior of
sulfuric acid; at low concentrations sulfuric acid is the more hematite in these solutions deviates from that in sulfuric acid
active reagent. solutions with the addition of sodium chloride; the leaching
rate of hematite tended to approach that of the hydrochloric
D. Effect of the Addition of Soluble Salts acid solutions. In order to obtain a fuller understanding of
the role of anions in hematite leaching, the effects of the
Hematite leaching was found to be enhanced by the ad- addition of sodium chloride and sodium sulfate on the rate
dition of sodium chloride to aqueous hydrochloric acid of leaching of hematite in 1 tool 9 dm -~ HC104 were inves-
solutions or sodium perchlorate to aqueous perchloric acid tigated. Figure 11 shows these experimental results. The
solutions. The activities of hydrogen ions of hydrochloric three solid lines in this figure indicate log R - a(H +) re-
acid and perchloric acid solutions increased with the addi- lationships of hematite leaching in hydrochloric, perchloric,
tion of the common salts of each acid. Leaching experiments and sulfuric acid solutions. The circles on each line indicate
were made in 1 tool. dm -3 HC1 solutions containing so- the leaching rate in each acid solution whose concentration
dium chloride at concentrations of 0.1, 0.5, and 1.0 tool 9 was 1 tool" dm -3. The addition of sodium chloride to
dm -3. This relationship between the rate of leaching and 1 mol 9 -3 HCIO4 solution resulted in a sharp increase in
a(H § in the presence of sodium chloride is graphically the rate of leaching despite the small increase in hydrogen
presented in Figure 9. It was found that the accelerated ion activity as shown in Figure 11. For example, the leach-
effect of leaching due to the addition of sodium chloride ing rate of hematite in perchloric acid containing 1 mol"
could be attributed to an increase in a ( H +) as is obvious dm 3 NaC1 increased by a factor of 15 compared to that of
in this figure. Similar results were obtained for the leaching the same solution without sodium chloride. By adding so-
of hematite in 1 tool 9 dm -3 HC104 with the addition of dium chloride to the perchloric acid solution, the leaching
sodium perchlorate. rate of hematite tended to approach that of the hydrochloric
The effect of the addition of sodium sulfate to sulfuric acid solution. Again, the leaching rate of hematite in per-
acid solutions was also examined. Leaching experiments chloric acid solution containing sodium sulfate approached
were made in 1 tool 9 dm -~ HzSO4 solutions whose sodium that of sulfuric acid despite the decreased hydrogen ion
sulfate concentrations were 0.1, 0.5, and 1.0 tool 9 dm 3. activity of the solution.
As is obvious in Figure 8, the addition of sodium sulfate
to 1 tool 9 dm -3 H2SO4 resulted in a lowering at hydrogen IV. DISCUSSION
ion activity. Experimental results are plotted on a log R -
a ( H +) plot, shown in Figure 10. As can be seen in this It is important to elucidate the role of anions in the
figure, the function of sodium sulfate is simply to decrease leaching of hematite in order to obtain a fuller understanding

METALLURGICAL TRANSACTIONS B VOLUME 16B. MARCH 1 9 8 5 - - 2 7

 10-9 iiii I I I I I IIll I I I T I IIII Since the surface is positively charged, the ions adsorbed
O : HCtO4 next to the surface, or in the Stern plane, would be anions
9 : HCIO4 § from the solution. The anions present in the solutions in this
HCl
study were hydroxyl ions, anions dissociated from the acids
9 : HClOz..Na2S Q
and salts added, and complex anions of Fe(III).
: HCIQ 9 NctCl
Solutions used for the leaching experiments were strongly
3.5 H2$O~' acidic. Thus the concentration of hydroxyl ions was negli-
ldlO gibly small compared to that of dissociated anions from the

{ 3.1
leachant. On the other hand, the concentration of Fe(III)
after the leaching experiments was usually less than 1 x
10-~ tool 9 dm -3. The formation constants of Fe(III) chloro-
2.0 and Fe(IIl) sulfato-complexes at the ionic strength of zero
and 1.2 are reported, respectively, as shown in Table II. 2~
E By using these values, the concentrations of Fe(III) sulfato-
~.10-1 and Fe(III) chloro-complexes in the solutions after leaching
were calculated as listed in Table III. In this table, the
figures on the upper line indicate the results obtained using
complex formation constants at zero ionic strength, and the
figures on the lower line indicate the results at the ionic
Temp : 328 K 1
strength of 1.2. As can be seen in this table, Fe(SO4)~- is
to : 600 r p m a principal complex ion in 1 tool. dm -3 H 2 8 0 4 solution
10 -II tIll [ I I I Illll I I I I IIIII I and l tool 9 dm -3 H2SO4 solution containing 1 mol 9 dm -3
10-1 100 101 Na2SQ. The concentrations of Fe(SO4);- in both solutions,
a(H') however, are far less than those of bisulfate and sulfate ions.
Fig. l 1 - - E f f e c t of the addition of soluble salts on rate of leaching of No significant amount of other complex anions exists in
hemame in 1 tool 9 dm 3 HCIO4 solutions at 328 K. Numerals shown those solutions. In 1 mol" dm -3 HC1 solutions with or
beside e, u and 9 indicate concentrations of NaCIO4, NaC1, and Na.,SO~ without the addition of sodium chloride, although FeC1] is
added in tool - dm the most predominant complex at an ionic strength of 1.2,
FeC1; is the most predominant among complex ion species,
showing no indication of the formation of complex anions.
of the overall leaching mechanism. This study showed that
Therefore, it seems reasonable to assume that the important
the leaching rates of hematite differed greatly with different
anions are bisulfate and sulfate ions in sulfate systems,
acids as leachants although their hydrogen ion activities were
chloride ions in chloride systems, and perchlorate ions in
adjusted to the same value. Also, the addition of sodium
perchlorate systems.
chloride or sodium sulfate to a perchloric acid solution re-
The concentration of ions at any point in the double layer
suited in a sharp increase in the leaching rates as shown in
can be calculated according to Boltzmann's distribution re-
Figure 11. From these observations, it is hypothesized that
adsorption of anions onto the mineral surface may directly lation, Eq. [3]
determine leaching rate. It has been impossible to apply a n = n ~ exp(-zeto,/kT) [3]
rigorous equilibrium concept to the mineral-solution inter-
face where the dissolution is actually occurring. However, where n is the concentration of ions in the double layer at
theoretical considerations of an equilibrium concept would any given point, n ~ is the concentration of ions of the same
provide useful information on the driving force of the reac- species in the bulk solution, and tO, is the potential at the
tion which appears at the interface. point under consideration in the double layer.
The potential on the surface of hematite is determined by A general way to estimate the magnitude of tO, at the
both the hydrogen and hydroxyl ions. B2r definition, the plane where the adsorbed anions lie has not been estab-
magnitude of the surface potential tOois controlled by the pH lished: however, the values of tO, must lie between the sur-
of the bulk solution. The total double layer potential tOo in face potential tOo and the zeta potential, ~', the potential
the mineral surface is generally given by ]7 located at the slipping plane. The distance between the inner

kT C+ kT C- Table II. Chloro- and Sulfato-Complex

~0=-- In - In-- [2]
ze C~ ze C~ Formation Constants of Ferric Ion at the
Ionic Strength of Zero and 1.2 at 298 K
where C + and C are the concentrations of the potential-
determining ions in the solution, and C~ and C~ are the Log k,
concentrations of these ions at the point of zero charge. Complex Formation Reaction I = 0 I = 1.2
Also, k is Boltzmann's universal constant, z is the valence Fe 3~ + CI = FeCI2t 1.48 0.66
of the ions, and e is the charge of the electron. The reported FeC12- + CI- = FeCI~ 0.65 0.15
values of the point of zero charge of hematite were scattered FeC17 + C1 = FeCI~ -1 O0 0.02
ranging from 5.2 to 9.3. TM However, it is assumed that the Fe3+ + SO] = FeSO2 4.04 2.23
point of zero charge of natural hematite occurs at pH 6.7, ~9 FeSO2 + SO]- = Fe(SO~)~- -- 2.00
Co~ = 1 x 10 6 7 and the total double layer potential, tOo, Fe 3+ + HSOg = FeHSO] + 1.78" 0.78
of hematite at pH 0 should be + 396 mV as a result of the
*complex formauon constant at the ~omcstrength of 0.15
adsorption of pontential-determining hydrogen ions.

28--VOLUME 16B. M A R C H 1985 MET,~LLURGICAL T R A N S A C T I O N S B

 Table III. Concentrations of Ionic Species in Solutions after Leaching of Hematite
[Fe(III)] [Fe3§ [FeC12§ [FeCI~] [FeCI~] [FeS02] [Fe(S04)s [FeHSO2§
Leachant • 104 • 104 • 104 • 104 x 104 • 10-~ x 105 • l0 s I*
1 mol 9 -3 H C I 1 0.006 0.168 0.751 0.075 -- -- -- 0.0
0.046 0.205 0.366 0.383 -- -- -- 1.2
1 mol 9 dm s HC1 + 1 0.002 0.085 0.761 0. 152 -- -- -- 0.0
1 mol 9 dm 3 NaC1 0.005 0.086 0.294 0.615 -- -- -- 1.2
1 mol 9 d m -3 I-I2SO4 0.1 0.0001 -- -- -- 0.986 -- 0,013 0.0
0.0002 -- -- -- 0.033 0.962 0.003 1.2
1 tool 9 dm -3 H~_SO4 + 0.0000 -- -- -- 0.997 -- 0.003 0.0
1 mol 9 dm 3 Na2SO4 0.1 0.0000 -- -- -- 0,007 0.992 0.001 1.2
concentration unit: mo] 9 d m -3
9 Ionic strength to which complex formation constants refer

Helmholtz plane and the hematite surface in hydrochloric sulfate ions act as counter ions when the solution contains
acid solutions must be different from that in sulfuric acid sulfuric acid or sodium sulfate.
solutions, since the ionic radii of dehydrated anions are quite As shown in Figure 11, the rate of hematite leaching in
different from each other. In regard to defining 0,, Gaudin dilute sulfuric acid solutions was greater than that in hydro-
and Fuerstenau 23 took the mean value of 00 and ~r as 0~ in chloric acid solution of equivalent concentration. If we
their study of quartz flotation with a cationic collector. In the consider 0.0l m o l . dm 3 acid solutions, the total double
current study, we have assumed that 0, is the average of tO0 layer potential should be about +278 mV, and thus 0, =
and ~', according to Gaudin and Fuerstenau, although the 139 mV. The concentration of sulfate ions in the double
actual 00 values for chloride and perchlorate ions or bi- layer next to the hematite surface is about two hundred times
sulfate and sulfate ions might be different. The zeta potential larger than that of chloride ions according to Eq. [4]. This
of hematite in strong acid solutions is considered to be fact may account for the faster leaching of hematite in sul-
nearly zero; thus the value of 0~ is assumed to be about furic acid than hydrochloric acid in dilute solutions.
00/2. Thus we obtained ~p, = 198 mV at pH 0.
Sulfuric acid is diprotic and ionized to bisulfate and sul-
fate ions. While the first ionization is almost complete, the V. CONCLUSIONS
second ionization is incomplete. Therefore we should de- The leaching of monosized particulate hematite in aque-
termine whether the bisulfate or sulfate ion is the actual ous hydrochloric acid, perchloric acid, and sulfuric acid
absorbed species on the hematite surface. Applying Eq. [3] solutions was studied kinetically. The experimental rate data
to both the concentrations of monovalent and divalent an- show that the over-all leaching reaction is controlled by a
ions in strong acid solutions, the following expression can chemical reaction process. The leaching rate of hematite is
be obtained: first order with respect to a ( H - ) in hydrochloric acid or
perchloric acid solutions with or without their common salts
log(nL/n2) = log (n~/n~) - (zl - z2) "2'
, ,
added, while it is of a half order in sulfuric acid solutions,
2.303RT two/ ~ [4]
with or without the addition of sodium sulfate,
where nl/n2 is the ratio of concentration of monovalent The potential at the hematite-solution interface was deter-
anion to that of divalent anion next to the surface at 298 K, mined by hydrogen ions in acid solutions. A theoretical
and nT/n~ is the value of n~/n2 in the bulk solution. The analysis shows that the anions next to the surface in the
second ionization constant, K2, of concentrated sulfuric acid double layer are chloride ions and perchlorate ions in hydro-
at 298 K is reported to be 14 chloric acid and perchloric acid, respectively, while they are
sulfate ions in sulfuric acid solutions, with or without the
tog/(2 = log 0.0102 + 2 . 0 3 6 V ~ / ( 1 + 0.4X/I) [5]
addition of sodium sulfate. The fact that the leaching rates
where I is the ionic strength. By using this ionization con- of hematite were quite different in various acids having the
stant, we calculate the concentration of sulfate and bisulfate same a (H") indicates the importance of anion adsorption. It
ions in the bulk solution at pH 0 to be 0.22 mol 9 dm 3 and should be noted that the dependency of the leaching rate
0.58 tool. dm -3, respectively. The substitution of these upon a(H +) appears to be controlled by adsorbed anions
values into Eq. [4] yields nl = 0.001n2. This calculation next to the surface in the double layer.
indicates that the concentration of sulfate ion next to the
hematite surface is far greater than that of bisulfate ion.
ACKNOWLEDGMENTS
Although Warren and Devuyst proposed the leaching
mechanism of oxides in sulfuric acid by introducing the The authors wish to express their thanks to Dr. Shinnosuke
adsorption of both bisulfate and sulfate ions, the calcula- Usui, Professor of the Research Institute of Mineral Dressing
tion results shown above do not support their mechanism. and Metallurgy, Tohoku University, for his helpful sugges-
Based on these findings, we may assume that chloride and tions. The authors also acknowledge receipt of financial
perchlorate ions are adsorbed as counter ions next to the support from the Ministry of Education of Japan in the form
surface of hematite in hydrochloric acid and perchloric acid of a Scientific Research Grant (Grant No. 56470051),
solutions, respectively, while sulfate ions instead of bi- which enabled this work to be performed.

METALLURGICALTRANSACTIONS B VOLUME 16B. MARCH 1985 29

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30-- VOLUME 16B, MARCH 1985 METALLURGICALTRANSACTIONS B