Journal of Minerals & Materials Characterization & Engineering, Vol. 5, No.2, pp 167-177, 2006
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167
Leaching of Sphalerite with Hydrogen Peroxide and Nitric Acid
Solutions
*A.O.Adebayo, K.O.Ipinmoroti, O.O. Ajayi
Department of Chemistry, Federal University of Technology, Akure, Ondo State Nigeria.
Abstract
The leaching of powdered sphalerite using hydrogen peroxide and nitric acid has been investigated.
The important variables such as concentrations of hydrogen peroxide and nitric acid as well as
stirring speed and particle size were examined. The effect of reaction temperature was also examined
on the leaching reaction process. The hydrogen peroxide and nitric acid concentrations have
significant effects on the leaching of sphalerite. The leaching of sphalerite is dependent on
temperature and stirring speed and inversely proportional on the ore particle size. The apparent
activation energy is found to be 28.7kJmol
-1
suggesting that the reaction is chemical - control at the
surface of the particles.
INTRODUCTION
Sphalerite is a polymorph with wurtzite and marttite. The three are called polymorphs (meaning many
shapes) because although they have the same chemical composition, (Zn, Fe, and S as the major
composition), they have different structures and therefore different shapes. Sphalerite is by far the
more common mineral of the three [1]. Zinc is the fourth most widely used metal after iron,
aluminium, and copper. Zinc is used as corrosion-protection coatings on steel (galvanized metal), as
diecastings, as an alloying metal with copper to make brass, and as chemical compounds in rubber,
ceramics, paints, and agriculture. It is also an essential element for proper growth and development of
humans, animals, and plants [2].
A variety of problems such as high energy cost, shortage of high grade ores, processing of lean and
complex ores and exploitation of smaller deposits have prompted the development of low temperature

168 Adebayo, Ipinmoroti and Ajayi Vol. 5, No. 2
hydrometallurgical processes for the extraction of base metals from their sulphide ores and
concentrates [3]. The conventional hydrometallurgical processes for the extraction of a base metal
from a sulphide concentrate consist of a catalytic sulphating, roast, leaching of the metallic values,
solvent extraction and selective stripping. Although this method has been used for many years,
leaching of the concentrate eliminates the roasting steps with elemental sulphur being produced
instead of sulphur dioxide. High rate of extraction of the metals are also achievable as insoluble low
entropy phases are not always formed [4]. However, a high oxidation potential is required for the
dissolution of sulphides [5].
Hydrogen peroxide is a strong oxidizing agent with a standard redox potential of 1.77V in acidic
medium [6]. The redox potential of sulphur / metal sulphide pair is less than that of hydrogen
peroxide, so that the oxidation of sulphide to sulphur is possible [7].
MATERIALS AND METHOD
Sphalerite samples were obtained from an ore deposit at Ebonyi State, Nigeria. The ore was crushed,
dried, ground and sieved using ASTM standard sieves. The chemical analysis was carried out using
gravimetric method to determine the composition of the major elements [8]. The percentage chemical
composition of the major elements are given in table1.
Table1: Percentage elemental composition of the sample.
Metal % composition
Zn 16.40
Fe 7.12
Pb 0.78
S 29.20
SiO
2
24.20
Nd 22.30
Nd -Not determined

Vol. 5, No. 2 Leaching of Sphalerite 169
The leaching experiments were carried out in an all glass reactor (250ml) equipped with a mechanical
stirrer. It was heated at constant temperature in a water bath within a temperature in the range of
±1
o
C. A mixture of 50cm
3
of hydrogen peroxide and 50cm
3
of the nitric acid was charged into the
reactor. At desired temperature, 1.0g of the sphalerite sample was introduced while the content of the
reactor was stirred at selected speeds. At specific time intervals 2ml aliquot was withdrawn and diluted
to 25ml in a standard flask and analysed for zinc using atomic absorption spectrophotometer.
The experimental conditions are summarised in Table 2. These involved varying one condition while
keeping others constant simultaneously. The conditions are temperature, concentrations of hydrogen
peroxide and nitric acid, stirring speed and particle size.
Table 2: Experimental conditions for the leaching of zinc from sphalerite with hydrogen peroxide in
nitric acid solution
Experimental conditions Values
Temperature,
0
C 30*,35, 40,45, 50
Concentration of H
2
O
2
, mol/L 0.50, 1.0, 2.10, 2.95, 5.20*
Nitric acid, mol/L 1.0, 2.5, 5.0, 7.0, 11.4*
Speed of agitation, rpm 300*, 400,500,700
Particle size (mm) 0.15 - 0.1, 0.1 - 0.075*, 0.075 - 0.005
* The values kept constant
RESULTS AND DISCUSSION
Leaching of sphalerite
The leaching of sulphide in oxidizing medium has been investigated by other researchers [3,5,9,10]
and it has been established that the sulphide is oxidized to elemental sulphur and eventually to
sulphate depending on the redox potential of the oxidant. The quantity elemental sulphur increases as
the oxidant concentration and temperature increase. Temperatures above 180
o
C increase the oxidation
to higher state such as sulphate [5].
The leaching of pyrite, a typical sulphide mineral by hydrogen peroxide in acid medium has been
reported by Dimitrijevic et al [10] and the reactions are as follows:

170 Adebayo, Ipinmoroti and Ajayi Vol. 5, No. 2
S
2-
+ H
2
O
2
+ 2H
+
S + 2H
2
O (1)
At higher temperatures the sulphur is oxidised to higher oxidation state such as sulphate.
Analysis of the sample shows that it contains some pyrite. The pyrite is oxidised to form soluble
species (Fe
3+
and SO
4
2-
). The Fe
3+
formed also helps in the dissolution reactions according to Equation
(2).
2Fe
3+
+ ZnS
-
S
o
+ 2Fe
2+
+ Zn
2+
(2)
Copur [9] has earlier reported the dissolution of zinc sulphide in nitric acid and the possible reactions
are written as follows:
OHNOSNOZnHNOZnS
g
o
aqaqaqs2)()(3
2
)()(3)(
4236383++++→+
−+
(3)
OHNOSNOZnHNOZnS
g
o
aqaqaqs2
)(
2)(3
2
)()(3)(
2224++++→+
−+
(4)
OHNOSOZnHNOZnS
gaqaqs2)(
2
4
2
)()(3)(
483383+++→+
−+
(5)
OHNOSOZnHNOZnS
g
aqaqs2
)(
2
2
4
2
)()(3)(
4838+++→+
−+
(6)
+−
++→+HNOSOHNOS
gaqaq
222
)(
2
)(4)(3
(7)
During the leaching process in nitric acid Equations 1,3 and 4 can be proposed for the reaction under
the present investigation. Because after the leaching process elemental sulphur were recovered from
the leached solution. It could be assumed that both hydrogen peroxide and nitric acid mixture actually
responsible for the oxidation of the sulphide group in the mineral leading to the leaching of zinc into
the aqueous solution. From these reactions the leaching process chemistry is sometimes believe to be
complicated and complex.
Effects of Parameters
Zinc constitutes a major component of sphalerite and was therefore used to access the efficiency and
effectiveness of the leaching process. The quantities presented in table 2 are measured against the
conversion fraction of zinc, which is defined as follows:
Leaching efficiency, (X) = the amount of Zn leached
the amount of Zn in the ore
The variations of leaching efficiency (X) with temperature, hydrogen peroxide and nitric acid
concentrations, particle size and stirring speed are shown in Figures 1 – 5.

Vol. 5, No. 2 Leaching of Sphalerite 171
The effect of temperature was carried out at temperature range of 30 - 50
0
C and hydrogen peroxide
and nitric acid concentrations of 5.20 and 11.4 mol/L, respectively. Figure 1 shows that an increase in
Fig. 1-Effect of temperature on the leaching of zinc from sphalerite
in hydrogen peroxide and nitric acid solution
0
0.2
0.4
0.6
0.8
1
1.2
020406080100
Time,t (min)
Fraction of Zn leache
d
300K
308K
313K
318K
temperature increases the leaching efficiency. The effect of hydrogen peroxide concentration was
examined by varying its initial concentration in the range of 0.50 – 5.20 mol/L at 30
0
C in 11.4 mol/L
nitric acid. The effect of hydrogen peroxide concentration (Figure 2) shows that the leaching
efficiency increased with increase in hydrogen peroxide concentration.
The deviation form linearity of the curves at some time during the leaching may be attributed the
emerging ions such as ferric ions which decomposes the hydrogen peroxide and thereby reduce the
local concentration [11]. Examination of the effect of nitric acid concentrations on the leaching
efficiency of sphalerite with hydrogen peroxide (5.20 mol/L) was also carried out at nitric acid
concentrations in the range of 1.0 – 11.4 mol/L. Figure 3 shows that an increase in concentration of
nitric acid also increases the efficiency of sphalerite leaching.

172 Adebayo, Ipinmoroti and Ajayi Vol. 5, No. 2
Fig.3-Effect of nitric acid concentration on the leaching o
f
zinc from sphalerite in hydrogen peroxide solution
0
0.2
0.4
0.6
0.8
1
1.2
020406080100120140
time,t (min)
Fraction of Zn leached
1.0mol/L
2.5mol/L
5.0mol/L
7.0mol/L
11.4mol/L
Fig.2-Effect of hydrogen peroxide concentration on the
leaching on of sphalerite with hydrogen peroxide and nitric
acid mixture
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
050100150200
Time,t (min)
Fractction of Zn leached
0.5mol/L
1.0mol/L
2.10mol/L
2.95mol/L
5.20mol/L

Vol. 5, No. 2 Leaching of Sphalerite 173
Figure 4 shows that the rate of leaching is inversely proportional to ore particle size; this is due to
large surface area as the particle size is reduced. The effect of stirring speed on the leaching of
sphalerite shown in figure 5 shows that the leaching rate is dependent on the speed of agitation.
Kinetic Analysis
According to the unreacted-core model, if a reaction such as
aA (fluid) + bB(solid) products -------------(8)
is controlled by diffusion through product layer, the integrated rate equation is given as
t
RC
bDC
XX
B
A
2
3/2
6
)1(2)1(31=−+−−
…………(9)
If the reaction given in equation (8) is chemically controlled then the integral rate expression becomes
[10]
)10.(..................................................)1(1
3/1
Kt
RC
bkC
X
B
A
=−−
where X is the reacted fraction, t, the time(s),
C
B
, the average apparent concentration of the ore (mol m
–3
), R,
the radius of the solid particle(µm), b, the stochiometric coefficient of the solid reacting with 1 mol of hydrogen
peroxide, k, the rate constant for surface reaction (ms
–1
), D, diffusion coefficient (m
2
s
–1
), and C
A
, the
concentration of the hydrogen peroxide and nitric acid (mol m
–3
).
The effect of temperature was used to determine the rate-controlling step. The experimental data were
analysed using rate equations (9) and (10) to determine the rate controlling step according to the
method of Levespiel [12]. From the analysis equation (10) gave straight lines, which could be
concluded that the leaching process is chemical control. The rate constant, k of the reaction were
determined and plotted against
1
/
T
shown figure 6. The slope of this plot was used to determine the
apparent activation energy with value to be 28.7kJmol
–1
. Based on the value of the activation energy it
may be concluded that chemical reaction is the rate - controlling step for the leaching of sphalerite by
hydrogen peroxide and nitric acid solutions. However the chemical reaction controlled kinetics is
characterized by high energy of activation [13]. For instance in aluminum metallurgy, alkali digestion
of gibbsite has a large (99.8kJmol
–1
) activation energy [14], necessitating autoclave leaching. High
value of activation energy (68kJ/mol) has been reported for the dissolution of pyrite in H
2
O
2
in H
2
SO
4

174 Adebayo, Ipinmoroti and Ajayi Vol. 5, No. 2
Fig.4-Effect of particle size on dissolution of
sphalerite with hydogen peroxide and nitric acid
mixture
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
020406080
Time,t (min)
Fraction of Zn leache
d
0.15-0.1mm
0.1-0.075mm
0.075-0.005mm
Fig. 5: Effect of stirring speed on leaching of zinc from
sphalerite with nitric acid and hydrogen peroxide
mixture
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
01020304050
Time, t(min)
Fraction of Zn leache
d
300rpm
400rpm
500rpm
700rpm

Vol. 5, No. 2 Leaching of Sphalerite 175
Fig.6-Arrhenius plot for the leaching of zinc from
sphalerite with hydrogen peroxide and nitric acid mixture
-20
-19.6
-19.2
-18.8
-18.4
3.13.153.23.253.33.35
1000/T(K
-1
)
Lnk
in which the rate controlling step was concluded to be chemical control [10]. But low value of
activation energy is possible particularly if the interaction between the reacting particles and the
reactants has physical adsorption only and no real chemical bond is formed between them [13].
McKibben [15]
determined activation energy of 33.5kJ/mol for pyrite dissolution by millimolar
concentrations of H
2
O
2
in acidic solutions and reported the reaction to chemical control. A low value
of activation energy as 25.5kJmol
–1
was reported as chemically controlled reaction by Jena and
Brooch [3] in extraction of copper from chalcopyrite by dry and wet chlorination. Harvey et al, [16]
also reported for pressure oxidation of sphalerite a value of 16.0kJ/mol as chemical control. Low
value of activation energy of 27kJ/mol for leaching of CuS-H
2
SO
4
in which porous sulphur, a reaction

176 Adebayo, Ipinmoroti and Ajayi Vol. 5, No. 2
product, constitutes the growing film around the reacting particles has been analysed to be mass
transfer control rate mechanism [17].
It has earlier reported that a reaction rate mechanism may not be accurately determined by the value of
the activation energy but may just be a guide to predict it [13, 18]. Olanipekun has ascertained that the
rate controlling mechanism of heterogeneous leaching reactions is sometimes better predicted from
kinetic equations rather than from the activation energy values. In some instances, the same
mechanistic information is derivable from both variables [19].
CONCLUSION
The dissolution of sphalerite with hydrogen peroxide in nitric acid has been studied. It can be
concluded that increases in temperature has positive effect on the leaching of sphalerite but particle
size has inverse effect on the process. The hydrogen peroxide and nitric acid solutions have
considerable positive effects on the dissolution process, while changes in speed of agitation have a
mild effect on the leaching reaction. The activation energy was found to be 28.7kJmol
–1
in the
temperature range 30 – 50
o
C, suggesting a chemical reaction control.
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