Chemistry for Sustainable Development 20 (2012) 391–396
391
UDC 669.2.046:541.11
Kinetic Ñharacteristics of Silver Dissolution in Nitric Acid
Solutions in the Presence of Ammonium Nitrate
A. B. LEBED′,D. YU. SKOPIN a nd G. I. MALTSEV
Uralelectromed′ JSC,
Ul. Lenina 1, Verkhnyaya Pyshma 624091 (Russia)
E-mail: mgi@elem.ru
(Received November 1, 2011; revised November 18, 2001)
Abstract
Kinetic characteristics and mechanism of silver dissolution in the course of leaching the gold-silver
alloy by nitric acid solutions in the presence of ammonium nitrate under a pressure of gas phase reaction
products were investigated by means of the method of rotating disk. Parameters were revealed for the
process occurring in the extern al diffusion, kinetic and transition zones. Factors have been found those
determine the dissolution rate of the metallic silver.
Key words: silver, nitric acid, ammonium nitrate, kinetic characteristics
INTRODUCTION
In the refining technology of gold and silver alloy (93.5–95.0 % Ag, 3.5–4.4 % Au, 1–
1.6 % Cu, 0.4–0.9 % Te, 0.1–0.2 % Pd, 0.03–
0.06 % Pt) processing, the silver uses to be dissolved in nitric acid solutions in the presence
of ammonium nitrate [1]. The latter interacts
with the evolution of nitrogen (II, IV) oxides
with releasing elemental nitrogen according to the
reactions presented in a molecular form [2, 3]:
2NO + 3NH4NO3 → 3N2 + 2HNO3 + 5H2O
2NO2 + NH4NO3 → N2 + 2HNO3 + H2O
Earlier, it was found [4] that the interaction
in the system NO (g)–N2 (g)–H+ (aq)– NH4+ (aq)–
NO–3 (aq)–Ag+(aq)–Ag–H2O represents a mul-
tistage process that occurs in several routes (serial, parallel, conjugate) with an ambiguous
mathematical relationship between quantitative
changing the initial and the intermediate components of the heterogeneous system involving
nitrogen with varying oxidation state. For the
stoichiometric descri ption of the entire totality
of the processes occurring in a non-equilibri Lebed′ A. B., Skopin D. Yu. and Maltsev G. I.
um system communicating with the external environment, only three linearly independent reactions in the ionic form are quite sufficient [4]:
3NH4+ (aq) + NO–3 (aq) + 2NO (g)
→ 2H+ (aq) + 3N2 (g) + 5H2O
+
4
(1)
–
3
8NH (aq) + 4NO (aq) + 2NO (g)
→ 4H+ (aq) + 7N2 (g) + 14H2O
0
–
3
3Ag + NO (aq) + 4H
(2)
+
→ 3Ag+ (aq) + NO (g) + 2H2O
(3)
those are characterized by negative values for
changing the Gibbs free energy and could proceed up to a complete consumption of initial
reagents.
The equations of a conjugating (1) and conjugated (2) reactions occurring in the system
simultaneously, but in different directions, reflect different stoichiometric ratio between the
starting reactants and the reaction products of
the between nitric oxide (II) and ammonium nitrate in nitric acid, which interaction is accompanied by releasing elemental nitrogen gas N2 [2].
For the equilibrium system isolated from the
external environment, the resulting interactions
392
A. B. LEBED′ et al.
therein could be described by a single stoichiometric equation of the following reaction:
2H+ (aq) + NH4+ (aq) + NO–3 (aq) + 2Ag0
→ 2Ag+ (aq) + N2 (g) + 3H2O
(4)
In order to calculate changing the content of
all the components of the system it is enough to
monitor the amount of NH4+ , H+, Ag+ ions [4].
The aim of this work consisted in kinetic
studies concerning the dissolution of metallic
silver from gold-silver alloys in nitric acid solutions in the presence of ammonium nitrate under a pressure of the gas phase under formation. The data obtained allow to optimize the
process of preparing the silver-containing electrolyte for the subsequent electrowinning process in order to obtain cathodic silver [5].
EXPERIMENTAL
The investigation of kinetic laws in the nonequilibrium system involving NO (g)–N2 (g)–
H+ (aq)– NH4+ (aq)– NO–3 (aq)–Ag+ (aq)–Ag–H2O
was carried out under the following conditions.
To an autoclave beaker (1.5 dm3) was put a silver plate (SrA-1 grade, according to the State
Standard GOST 28595–90) of 100 × 100 × 8 mm
in size, poured 1 dm3 of a solution with the
following composition (mol/dm3): HNO3 2.22,
NH4NO3 0.8. The system was sealed, evacuated
and heated up to 378 K. As the excess pressure
in the system reached the value of 0.05 MPa
controlled by a hydraulic lock, the initial sample was taken. Subsequent sampling procedures
nitric oxide (II) [4, 9], into the gas phase.
Ammonium ions react with NO according to the
reactions of releasing the bound nitrogen to give
nitrogen gas (1), (2) with the regeneration of
hydrogen ions, and with increasing the amount
of dissolved silver according to reaction (3).
Changing the concentration of ammonium ions
and hence the rate of the nitrogen release become close to zero after 105 min from the beginning of the experiment with the ongoing process of silver dissolution (Fig. 1, Table 1).
For a non-equilibrium system, the ratio values such as ∆nH+/∆n NH+ and ∆nAg+/∆n NH + in4
4
creased with time within the ranges of 2.041–
2.720 and 2.063–2.834, respectively, whereas
the minimum ratio ∆nH+/∆nAg+ changing within
150 min within the range of 0.99–0.96, after
105 min from the beginning of the process
amounted to 0.929.
For the equilibrium system, the absolute
values of the ratios remained almost constant
during 90 min: ∆nH+(Ag+)/∆n NH + ≈ 2; ∆nH+/∆nAg+ ≈ 1,
4
according to equation (4) (see Table1). Under
the conditions a relative deficiency of ammonium ions consumed in the releasing of bound
nitrogen, with the further removal of the gaseous reaction products such as NO and N2, for
the non-equilibrium system there occurs increasing the absolute values of the ratios: both
∆nH+/∆n NH+ and ∆nAg+/∆n NH + > 2.
4
4
The effect exerted by ammonium nitrate on
dissolving the silver was studied using the rotating disk technique [10] for the system of nitric acid – ammonium nitrate – metallic silver
for the analysis of NH4+ , HNO3, Ag+ [6–8] were
performed at the time intervals of 5–15 min.
In order to the equilibrium system we used
a solution containing 0.52 mol/dm3 of HNO3 and
0.99 mol/dm3 of NH4NO3.
RESULTS AND DISCUSSION
As the reactions (1)–(3) occur, the concentration of ammonium ions and nitric acid in the
system decreases, whereas that of silver ions
increases due to the consumption of nitric acid
for the dissolution of metallic silver. This process is accompanied by the evolution of the nitrate ion reduction products, predomin antly
+
Fig. 1. Concentration of HNO 3 (1), NH4 (2), Ag+ (3)
depending on of the duration of silver dissolution.
393
SILVER DISSOLUTION IN NITRIC ACID SOLUTIONS IN THE PRESENCE OF AMMONIUM NITRATE
TABLE 1
Parameters of silver dissolution process
Duration,
min
Changing the increment in the
amount of components, mol
∆nNH +
∆nH+
∆nAg+
4
Ratio of increments
∆nH+
∆nH+
∆n +
∆nAg+
NH 4
∆nAg+
∆n NH +4
Presssure,
MPa
30
–0.333
–0.679
Non-equilibrium system
0.686
2.041
0.990
2.063
0.05
60
–0.632
–1.341
1.366
2.122
0.982
2.161
0.05
90
–0.754
–1.756
1.896
2.340
0.931
2.513
0.05
105
–0.786
–2.000
2.152
2.545
0.929
2.738
0.05
120
–0.786
–2.013
2.164
2.562
0.930
2.755
0.05
150
–0.786
–2.137
2.226
2.720
0.960
2.834
0.05
Equilibrium system
30
–0.538
–0.267
0.541
2.019
0.996
2.028
3.12
60
–0.786
–0.390
0.790
2.013
0.995
2.024
4.56
90
–0.910
–0.452
0.919
2.011
0.990
2.031
5.28
under the following conditions. To an autoclave
beaker was poured 0.8 dm3 of a solution containing 0.5–1.25 Ì of HNO 3 + 0.1–1 M of
NH 4 NO 3 and heated to a temperature of
353–370 K. After reaching a preset temperature, a fixture ring holding a silver disk sample with a working area of 1.18 and 2.36 cm2
was set, and then the system was sealed and
the disk rotation frequency was preset within
the range of 8.33–18.33 s–1. After 1 min of the
experiment we took the initial sample, and the
subsequent sampling was performed at the time
intervals amounting to 15 min.
The proportional dependence revealed for the
dissolution rate of silver on the number of disk
revolutions raised to the 0.5 power within the
range rotation frequency values of 8.33–11.67 s–1
demonstrates that the process is rate-determined
by the mass transfer of the initial reagents and
reaction products within the bulk of the solution.
In this regard, the further studies were performed
at the rotation frequency 16.67 s–1, which excluded
external diffusion hindrance (Fig. 2, a).
The silver dissolution rate obtained depending on the initial concentration of the acid obey
an exponential law (see Fig. 2, b) both in the
absence of ammonium nitrate (curves 1–4), and
in the presence thereof within the concentration range 0.21 mol/dm3 under investigation
(curve 5). For curves 1–5 (see Fig. 2, b) ratio of
increments ∆log V/∆log [HNO3] amounts to 3
(curves 1–4) and to 1 (curve 5).
Fig. 2. Dissolution rate of silver depending on the rate of
stirring (a), on the initial concentration of HNO3 (b) and
on the initial concentration of NH4NO3 (c): a – temperature,
K: 353 (1), 363 (2), 373 (3, 5), 383 (4); initial concentration
of HNO3 being of 0.5 M; b – initial concentration of
NH4NO3, M: 0 (1–4) 0.5 (5); c – initial concentration of
HNO3 being of 0.5 M.
394
A. B. LEBED′ et al.
Fig. 3. Dissolution rate of silver depending on temperature
at the concentration of HNO3 equal to 0.5 (1, 2) and
1 mol/dm3 (3); concentration of NH4NO3 being of 0.5 mol/
dm3 (2).
It has been found that at a low concentration of ammonium nitrate (≤0.15 mol/dm3) the
rate of silver dissolution remains constant and
increases in proportion within the concentration range of NH4+ equal to 0.2–1 mol/dm3 (see
Fig. 2, c).The reaction order with respect to
ammonium nitrate is equal to 0.64 at the initial
concentration of nitric acid amounting to
0.5 mol/dm3. Studying the effect of the silver
disk area demonstrated that the dissolution rate
of the metal in nitric acid solutions is not dependent on the presence of ammonium nitrate
in the system, whereas it is proportion al to the
area of the working surface (S) being in contact with the solution.
The experimental values of the activation
energy (Fig. 3) were obtained under the following process conditions: the temperature
ranging within 363–393 K, the concentration
of nitric acid ranging within 0.5–1 mol/dm3,
the absence/presence of NH4NO3 (0.2–1 mol/
dm3) in the solution. These values amounted to
(126.4±2.3) and (68.9±1.1) kJ/mol (see Fig. 3,
curves 1–3), respectively. The temperature coefficients (γ) were determined via the equation
γ = VT + 10i/VT
where VÒ is the silver dissolution rate of at a
temperature T, mol/(s ⋅ dm2); i = 1, ...., n. In
the conditions under investigation, these values
amounted to 3.07–2.77 and 1.81–1.79 for
experiments with the presence and the absence
of NH4NO3 in the solution, respectively (see
Fig. 3, curves 1, 3 and curve 2).
It has been found that at the temperature
values above 373 K, the reaction between ammonium ions and nitric oxide proceeds in a
mixed kinetic mode with the evolution of hydrogen ions, and further the dissolution of silver occurs according to equation (3).
An experimental kinetic equation was obtained for the dissolution of silver in the solution of 0.5–1.2 mol/dm3 HNO3 at the temperature ranging within 363–393 K, with the reaction order n = 3 [11]:
V1 = k1[HNO3]3Sexp (–15246.1T–1)
where k1 is the experimental dissolution rate
constant in the absence of silver nitrate, k1 =
7.859 ⋅ 1010 dm5/(s ⋅ mol2); [HNO3] is the acid
concentration expressed, mol/dm3; S is the area
of the disk, dm2; T is temperature, K.
The experimental kinetic equation of silver
dissolution without extern al diffusion restrictions for the solutions containing 0.2–1 mol/dm3
of nitric acid and ammonium nitrate at the temperature ranging within 363–393 K with the
reaction order n = 1 with respect to the acid
has the following form:
V2 = k2[HNO3][NH4NO3]0.64Sexp (–8289.3T–1)
where k2 is the experimental dissolution rate
constant in the presence of silver nitrate, k2 =
2.035 ⋅ 103 dm0.92/(s ⋅ mol0.64); [NH4NO3] is the initial
concentration of ammonium nitrate, mol/dm3.
An assessment was performed concerning a
possible diffusion hindrance in the course of
the dissolution of metallic silver caused by
arising of a saturated concentration film (~3
mol/dm3) of silver nitrate at the interface
between the solid and liquid phases. In this case,
the oxidizing reagent could partici pate in the
reaction as the diffusion occurs onto the surface
of the metal from the solution with the
following composition (mol/dm3): Ag 2.5 ⋅ 10–5,
H+ 0.5, NH4+ 0.5, NO–3 1.0.
TABLE 2
Calculated values for diffusion coefficients (DÒ), 10–3 cm2/s
Ò, K
298*
H+
9.34
+
–
Ag+
NH4
NO3
1.65
1.97
1.9
373
36.7
6.47
7.72
7.45
383
41.7
7.36
8.78
8.48
393
41.7
8.31
9.91
9.56
*According to [12].
SILVER DISSOLUTION IN NITRIC ACID SOLUTIONS IN THE PRESENCE OF AMMONIUM NITRATE
TABLE 3
Calculated values for the diffusion rate Vi, 10–5 mol/(cm2 ⋅ s)
Ò, K
H+
Ag+
NH4
+
NO3
373
1.83
1.94
0.386
0.745
383
2.09
2.21
0.439
0.848
393
2.35
2.49
0.495
0.956
–
The diffusion coefficients (DT) of the system
components at 373–393 K (Table 2) were
calculated from equation [12]
kT = [6.21 ⋅ 10–4(2π)0.5(DÒ)0.667]m–1ν–1
DÒ = D298(Òµ298/298µÒ)
where m is the stoichiometric ratio; ν is the
kinetic viscosity of the solution, cm2/s; µ is
the dyn amic viscosity coefficient for the solution, MPa ⋅ s. For the silver ion: D298 = 1.698 ⋅ 10–2;
D373 = 6.71 ⋅ 10–5.
The absolute values of the diffusion coefficients (DT) for ammonium ions, silver ions and
nitrate ions are close to each other being nearly
four times lower than those for the hydrogen
ions. Thus, the oxidation of metallic silver is
determined by the transport of nitrate ions into
the reaction zone, whereas ammonium ions those
are not involved in the dissolution of silver could
cause an additional diffusion hindrance.
The rate of diffusion was calculated for ions
in the solution (Vi) [13]:
Vi = ±dm/dτ = D(Cn – C0)δ–1
where δ = 1.61µ0.167D0.333ω–0.5 = 2.3 ⋅ 10–2 is the
thickness of the Levich diffusion layer accepted to be of the same value for all the system
components, cm; Cn, C0 are ion concentrations
at the boundary of the solid and liquid phases
and in the bulk of the solution, respectively,
mol/dm3; m is the amount of substance passing through a unit area, mol/cm2; τ is the duration of passing, s; D is the diffusion coefficient, cm/s2.
The diffusion rate values for silver and hydrogen ions are close or exceed the value for
TABLE 4
Calculated values for the reagent delivery rate (Vp),
10–5 mol/(cm2 ⋅ s)
+
–
Ò, K
H+
Ag+
NH4
NO3
373
4.58
6.47
3.86
7.45
383
5.22
7.36
4.39
8.48
393
5.88
8.31
4.95
9.56
395
ammonium ions and nitrate ions (Table 3). The
ratio between the diffusion rate values for the
components of the system and the diffusion
rate of silver ions (Vi /VAg) amounts to 0.94 for
H+, 0.2 for NH4+ , 0.38 for NO–3 . Consequently,
the diffusion restrictions could not be caused
by silver ions and hydrogen.
Taking into account the stoichiometric coefficients of silver dissolution reaction, we calculated the rate of reactant delivery into the
reaction zone (see Table 4) the ratio thereof to
the delivery rate for the silver ions (Vð/VAg) is
equal to 0.71 (H+), 0.6 ( NH4+ ), 1.15 ( NO–3 ).
The delivering rate of NO–3 ions is higher
than the delivering rate values for H+ and NH4+ ;
these appeared comparable with the rate of
Ag+ removal from the reaction zone. For nitric
acid solution without ammonium nitrate the
ratio of Vð/VAg = 0.87 for the nitrate ions. Consequently, increasing the concentration of NO–3
ions results in increasing the rate values for
delivering the oxidizing reagent and for dissolving the silver.
In the case when the concentration of
NH4NO3 ≥0.5 mol/dm3, basing on the competition between the rate values close to the delivery rate inherent in hydrogen and ammonium
ions there could appear diffusion hindrance in
delivering the hydrogen ions into the reaction
zone, which determine the silver dissolution rate.
CONCLUSION
1. The kinetic laws of dissolving the metallic silver in nitric acid solutions without additives and in the presence of ammonium nitrate
were investigated by means of rotating disk
technique, under appropriate conditions determined by the following regimes:
i) the extern al diffusion transfer of the
source reagents and reaction products toward
the interface between the solid and liquid phases
is observed at the speed of rotation ≤11.67 s–1,
the concentration of nitric acid being less than
0.5 mol/dm3, that of ammonium nitrate being
less than 0.1 mol/dm3;
ii) the kinetic mode (in the absence of extern al diffusion hindrance) is observed in the
396
A. B. LEBED′ et al.
solutions of nitric acid with the concentration
less than 0.5–1.0 mol/dm3 without ammonium
nitrate at the temperature ranging within 363–
393 K, the activation energy being equal to 126.7
kJ/mol, the reaction order amounting to 3 with
respect the acid;
iii) the transient mode is observed in the
presence of 0.2–1.0 mol/dm3 of ammonium nitrate in nitric acid solutions: the activation energy being of 68.9 kJ/mol, the reaction order
amounting to 1 (with respect to nitric acid) and
0.64 (with respect to ammonium nitrate).
2. The maximum rate of dissolving the metallic silver is inherent in a mixed solvent that
contains 0.5–1.2 mol/dm3 of HNO3 and 0.5 mol/L
of NH4NO3, at the reaction order equal to 1
(with respect to acid) and 0.64 (with respect to
ammonium nitrate), at the disc rotation frequency >11.67 s–1 and the solvent temperature
equal to 393 K.
REFERENCES
1 Lebed A. B., Skorokhodov V. I., Plekhanov K. A.,
Mastyugin S. A., Naboichenko S. S., 2 Mezhdun ar. Konf.
“Blagorodnye i Redkiye Metally” (Proceedings),
Donetsk, 1997, part 1, p. 163.
2 Tereshchenko A. B., Pozina M. B., Bashlacheva N. N.,
Zh. Prikl. Khim., 42, 12 (1969) 2678.
3 Author’s Certification No. 1447907 USSR, 1988.
4 Skopin D. Yu., Sovershenstvovaniye Podgotovki
Azotnokislykh Rastvorov v Tekhnologii Affinaga Serebra
(Candidate’s Dissertation in Engineering), Yekaterinburg, 2002.
5 RU Pat. No. 2100484, 1997.
6 Klygin A. E., Smirnova E. D., Zavrazhnova D. M., Zh.
Neorg. Khim., 24, 15 (1979) 79.
7 Sharlo G., Methods of An alytical Chemistry [in
Russian], Khimiya, Moscow, 1969.
8 Pyatnitskiy I. V., An alytical Chemistry of Silver [in
Russian], Nauka, Moscow, 1975.
9 Akhmetov N. S., General and Inorganic Chemistry,
Vysshaya Shkola, Moscow, 1988.
10 Kakovskiy I. A., Potashnikov Yu. M., Kinetics of Dissolution Processes [in Russian], Metallurgiya, Moscow, 1975.
11 Kineticheskiye uravneniya. Kineticheskiye Krivye.
URL: http://revolution.allbest.ru/chemistry/c00054058.
html (äàòà îáðàù. 02.10.2011).
12 Robinson R. A., Stocks R. H. (Eds.), Electrolyte Solutions,
Academic Press, Inc., New York, 1959.
13 Delfino M. R., Fusco A. J., Rev. Latinoam. Quim., 11, 2–
3 (1981) 897.