ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 1, pp. 25–28. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © T.I. Markovich, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 1, pp. 33–36.
CHEMICAL KINETICS
AND CATALYSIS
The Special Features of the Kinetics of Oxidation of Divalent Iron
during Sulfuric Acid Leaching of Pyrrhotine
with the Participation of Nitrous Acid
T. I. Markovich
Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
e-mail: mark@uiggm.nsc.ru
Received September 10, 2007
Abstract—The kinetics of oxidation of divalent iron ions with molecular oxygen during pyrrhotine leaching in
sulfuric acid solutions with the participation of nitrous acid as an activator was studied. The oxidation of Fe2+
to Fe3+ only occurred at the second slow stage of the process, when the degree of solid phase stripping reached
almost 80% of the value maximum under given conditions. The order of the reaction with respect to sulfuric
acid was negative (–1.1 0.1), as is characteristic of sulfate media. A substantial change in effective activation
energies from 7 5 kJ/mol at cHo SO = 0.25 mol/l to 31 5 kJ/mol at cHo SO = 1.02 mol/l could be caused by
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different contents of the pyrrhotine solid phase in the system at the beginning of the second leaching stage. It
was assumed that the oxidation of Fe2+ to Fe3+ occurred with much lower energy expenditures in the presence
of a sulfide solid phase.
DOI: 10.1134/S0036024409010063
INTRODUCTION
EXPERIMENTAL
Studies of the kinetics of oxidation of Fe2+ in sulfu-
ric acid solutions containing nitrous acid were per-
formed for disperse selective pyrrhotine concentrate
samples from Noril’sk deposit. The samples contained
93.2% Fe0.88S, 1.5% CuFeS2, and 4.5% SiO2. A
weighed amount of sulfide was leached under tempera-
ture-controlled conditions with continuous stirring (an
upper drive) in a closed reaction vessel equipped with a
sampler with a finely porous filter, a feeder for intro-
ducing activator solutions, and a system for supplying
and removing gases, which could be used for the intro-
duction of oxygen into the reaction zone at a pressure
of 1 atm.
The oxidation of divalent iron salts with molecular
oxygen in acid media is the limiting factor of many nat-
ural processes and hydrometallurgical technologies.
One of the methods for increasing the rate of this reac-
tion under laboratory and industrial conditions is the
use of various nitrogen compounds (oxides, nitrates,
nitrites, etc.) as activators [1–5]. The mechanism of
catalysis is very complex and has not been determined
in detail. During oxidation, the system contains quite a
number of nitrogen compounds (NO, N2O3, NO2, N2O4,
HNO2, NO2– , HNO3, NO3– , NO+, and NO2+ ) having
various catalytic activities. According to many experi-
mental studies, the form most reactive kinetically is
nitrous acid and the products of its dissociation. At low
concentrations (<0.1 M), the activating action of HNO2
exceeds the catalytic activity of all the other oxygen
compounds of nitrogen severalfold [2, 4, 5].
Nitrous acid was obtained by the exchange reaction
2NaNO2 + H2SO4 = 2HNO2 + Na2SO4.
A solution of sodium nitrite was dosed directly into the
reaction slurry in certain time intervals. The conditions
for feeding the catalyst were developed taking into
account the occurrence of parallel processes in the oxi-
dation system and allowed the concentration of nitrous
acid to be maintained as close as possible to the
required value 0.001 M.
Another approach to solving the problem of the
acceleration of Fe2+ oxidation is based on the use of a
heterogeneous rather than homogeneous mechanism of
molecular é2 activation. This approach takes the
advantage of the ability of certain solids to sorb dis-
solved oxygen on active centers of their surface [6].
The experimental conditions were changed over the
intervals cHo SO = 0.25–1.02 å and T = 293–333 K.
A study of the behavior of divalent iron in the
Fe0.88S + H2O + H2SO4 + O2 + HNO2 system during
the stripping of the sulfide solid phase capable of
chemisorbing dissolved oxygen was therefore of cer-
tain interest.
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The concentration of nitrous acid (0.001 M), the rate of
stirrer rotations (168 s–1), and oxygen pressure (1.01 ×
105 Pa) were maintained constant in all experiments.
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