Kinetic Ñharacteristics of Silver Dissolution in Nitric Acid Solutions in the Presence of Ammonium Nitrate

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1 Chemistry for Sustainable Development 20 (2012) UDC :51.11 Kinetic Ñharacteristics of Silver Dissolution in Nitric Acid Solutions in the Presence of Ammonium Nitrate A. B. LEBED,D. YU. SKOPIN and G. I. MALTSEV Uralelectromed JSC, Ul. Lenina 1, Verkhnyaya Pyshma (Russia) (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 ( % Ag, 3.5. % Au, % Cu, % Te, % Pd, % 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 + 3NH NO 3 3N 2 + 2HNO 3 + 5H 2 O 2NO 2 + NH NO 3 N 2 + 2HNO 3 + H 2 O Earlier, it was found [] that the interaction in the system NO (g)n 2 (g)h + (aq) NH + (aq) NO 3 (aq)ag + (aq)agh 2 O represents a multistage 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-equilibrium system communicating with the external environment, only three linearly independent reactions in the ionic form are quite sufficient []: 3NH + (aq) + NO 3 (aq) + 2NO (g) 2H + (aq) + 3N 2 (g) + 5H 2 O (1) 8NH + (aq) + NO 3 (aq) + 2NO (g) H + (aq) + 7N 2 (g) + 1H 2 O (2) 3Ag 0 + NO 3 (aq) + H + 3Ag + (aq) + NO (g) + 2H 2 O (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 N 2 [2]. For the equilibrium system isolated from the external environment, the resulting interactions Lebed A. B., Skopin D. Yu. and Maltsev G. I.

2 392 A. B. LEBED et al. therein could be described by a single stoichiometric equation of the following reaction: 2H + (aq) + NH + (aq) + NO 3 (aq) + 2Ag 0 2Ag + (aq) + N 2 (g) + 3H 2 O () In order to calculate changing the content of all the components of the system it is enough to monitor the amount of NH +, H +, Ag + ions []. 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)n 2 (g) H + (aq) NH + (aq) NO 3 (aq)ag + (aq)agh 2 O was carried out under the following conditions. To an autoclave beaker (1.5 dm 3 ) was put a silver plate (SrA-1 grade, according to the State Standard GOST ) of mm in size, poured 1 dm 3 of a solution with the following composition (mol/dm 3 ): HNO , NH NO 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 for the analysis of NH +, HNO 3, Ag + [68] were performed at the time intervals of 515 min. In order to the equilibrium system we used a solution containing 0.52 mol/dm 3 of HNO 3 and 0.99 mol/dm 3 of NH NO 3. nitric oxide (II) [, 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 n H +/ n NH + and n Ag +/ n NH + increased with time within the ranges of and , respectively, whereas the minimum ratio n H +/ n Ag + changing within 150 min within the range of , after 105 min from the beginning of the process amounted to For the equilibrium system, the absolute values of the ratios remained almost constant during 90 min: n H + (Ag + ) / n 2; n H +/ n Ag + NH + 1, according to equation () (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 N 2, for the non-equilibrium system there occurs increasing the absolute values of the ratios: both n H +/ n NH + and n Ag +/ n NH + > 2. 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 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), NH + (2), Ag + (3) depending on of the duration of silver dissolution.

3 SILVER DISSOLUTION IN NITRIC ACID SOLUTIONS IN THE PRESENCE OF AMMONIUM NITRATE 393 TABLE 1 Parameters of silver dissolution process Duration, Changing the increment in the Ratio of increments Presssure, min amount of components, mol n + H n + H n + Ag MPa n n H + n Ag + n n Ag + n NH + NH + NH + Non-equilibrium system Equilibrium system under the following conditions. To an autoclave beaker was poured 0.8 dm 3 of a solution containing Ì of HNO M of NH NO 3 and heated to a temperature of K. After reaching a preset temperature, a fixture ring holding a silver disk sample with a working area of 1.18 and 2.36 cm 2 was set, and then the system was sealed and the disk rotation frequency was preset within the range of 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 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 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), and in the presence thereof within the concentration range 0.21 mol/dm 3 under investigation (curve 5). For curves 15 (see Fig. 2, b) ratio of increments log V/ log [HNO 3 ] amounts to 3 (curves 1) and to 1 (curve 5). Fig. 2. Dissolution rate of silver depending on the rate of stirring (a), on the initial concentration of HNO 3 (b) and on the initial concentration of NH NO 3 (c): a temperature, K: 353 (1), 363 (2), 373 (3, 5), 383 (); initial concentration of HNO 3 being of 0.5 M; b initial concentration of NH NO 3, M: 0 (1) 0.5 (5); c initial concentration of HNO 3 being of 0.5 M.

4 39 A. B. LEBED et al. Fig. 3. Dissolution rate of silver depending on temperature at the concentration of HNO 3 equal to 0.5 (1, 2) and 1 mol/dm 3 (3); concentration of NH NO 3 being of 0.5 mol/ dm 3 (2). It has been found that at a low concentration of ammonium nitrate ( 0.15 mol/dm 3 ) the rate of silver dissolution remains constant and increases in proportion within the concentration range of NH + equal to 0.21 mol/dm 3 (see Fig. 2, c).the reaction order with respect to ammonium nitrate is equal to 0.6 at the initial concentration of nitric acid amounting to 0.5 mol/dm 3. 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 proportional 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 K, the concentration of nitric acid ranging within 0.51 mol/dm 3, the absence/presence of NH NO 3 (0.21 mol/ dm 3 ) in the solution. These values amounted to (126.±2.3) and (68.9±1.1) kj/mol (see Fig. 3, curves 13), respectively. The temperature coefficients (γ) were determined via the equation γ = V T + 10i /V T where V Ò is the silver dissolution rate of at a temperature T, mol/(s dm 2 ); i = 1,..., n. In the conditions under investigation, these values amounted to and for experiments with the presence and the absence of NH NO 3 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 mol/dm 3 HNO 3 at the temperature ranging within K, with the reaction order n = 3 [11]: V 1 = k 1 [HNO 3 ] 3 Sexp (1526.1T 1 ) where k 1 is the experimental dissolution rate constant in the absence of silver nitrate, k 1 = dm 5 /(s mol 2 ); [HNO 3 ] is the acid concentration expressed, mol/dm 3 ; S is the area of the disk, dm 2 ; T is temperature, K. The experimental kinetic equation of silver dissolution without extern al diffusion restrictions for the solutions containing 0.21 mol/dm 3 of nitric acid and ammonium nitrate at the temperature ranging within K with the reaction order n = 1 with respect to the acid has the following form: V 2 = k 2 [HNO 3 ][NH NO 3 ] 0.6 Sexp (8289.3T 1 ) where k 2 is the experimental dissolution rate constant in the presence of silver nitrate, k 2 = dm 0.92 /(s mol 0.6 ); [NH NO 3 ] is the initial concentration of ammonium nitrate, mol/dm 3. 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/dm 3 ) 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/dm 3 ): Ag , H + 0.5, NH + 0.5, NO TABLE 2 Calculated values for diffusion coefficients (D Ò ), 10 3 cm 2 /s Ò, K H + Ag + NH + NO 3 298* *According to [12].

5 SILVER DISSOLUTION IN NITRIC ACID SOLUTIONS IN THE PRESENCE OF AMMONIUM NITRATE 395 TABLE 3 Calculated values for the diffusion rate V i, 10 5 mol/(cm 2 s) Ò, K H + Ag + NH + NO The diffusion coefficients (D T ) of the system components at K (Table 2) were calculated from equation [12] k T = [ (2π) 0.5 (D Ò ) ]m 1 ν 1 D Ò = D 298 (Òµ 298 /298µ Ò ) where m is the stoichiometric ratio; ν is the kinetic viscosity of the solution, cm 2 /s; µ is the dyn amic viscosity coefficient for the solution, MPa s. For the silver ion: D 298 = ; D 373 = The absolute values of the diffusion coefficients (D T ) 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 (V i ) [13]: V i = ±dm/dτ = D(C n C 0 )δ 1 where δ = 1.61µ D ω 0.5 = is the thickness of the Levich diffusion layer accepted to be of the same value for all the system components, cm; C n, C 0 are ion concentrations at the boundary of the solid and liquid phases and in the bulk of the solution, respectively, mol/dm 3 ; m is the amount of substance passing through a unit area, mol/cm 2 ; τ is the duration of passing, s; D is the diffusion coefficient, cm/s 2. The diffusion rate values for silver and hydrogen ions are close or exceed the value for TABLE Calculated values for the reagent delivery rate (V p ), 10 5 mol/(cm 2 s) Ò, K H + Ag + NH + NO 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 (V i /V Ag ) amounts to 0.9 for H +, 0.2 for NH +, 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 ) the ratio thereof to the delivery rate for the silver ions (V ð /V Ag ) is equal to 0.71 (H + ), 0.6 ( NH + ), 1.15 ( NO 3 ). The delivering rate of NO 3 ions is higher than the delivering rate values for H + and NH + ; these appeared comparable with the rate of Ag + removal from the reaction zone. For nitric acid solution without ammonium nitrate the ratio of V ð /V Ag = 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 NH NO mol/dm 3, 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 external 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 s 1, the concentration of nitric acid being less than 0.5 mol/dm 3, that of ammonium nitrate being less than 0.1 mol/dm 3 ; ii) the kinetic mode (in the absence of extern al diffusion hindrance) is observed in the

6 396 A. B. LEBED et al. solutions of nitric acid with the concentration less than mol/dm 3 without ammonium nitrate at the temperature ranging within K, the activation energy being equal to kj/mol, the reaction order amounting to 3 with respect the acid; iii) the transient mode is observed in the presence of mol/dm 3 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 0.6 (with respect to ammonium nitrate). 2. The maximum rate of dissolving the metallic silver is inherent in a mixed solvent that contains mol/dm 3 of HNO 3 and 0.5 mol/l of NH NO 3, at the reaction order equal to 1 (with respect to acid) and 0.6 (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 Mezhdunar. Konf. Blagorodnye i Redkiye Metally (Proceedings), Donetsk, 1997, part 1, p Tereshchenko A. B., Pozina M. B., Bashlacheva N. N., Zh. Prikl. Khim., 2, 12 (1969) Author s Certification No USSR, Skopin D. Yu., Sovershenstvovaniye Podgotovki Azotnokislykh Rastvorov v Tekhnologii Affinaga Serebra (Candidate s Dissertation in Engineering), Yekaterinburg, RU Pat. No , Klygin A. E., Smirnova E. D., Zavrazhnova D. M., Zh. Neorg. Khim., 2, 15 (1979) Sharlo G., Methods of Analytical Chemistry [in Russian], Khimiya, Moscow, Pyatnitskiy I. V., Analytical Chemistry of Silver [in Russian], Nauka, Moscow, Akhmetov N. S., General and Inorganic Chemistry, Vysshaya Shkola, Moscow, Kakovskiy I. A., Potashnikov Yu. M., Kinetics of Dissolution Processes [in Russian], Metallurgiya, Moscow, Kineticheskiye uravneniya. Kineticheskiye Krivye. URL: html (äàòà îáðàù ). 12 Robinson R. A., Stocks R. H. (Eds.), Electrolyte Solutions, Academic Press, Inc., New York, Delfino M. R., Fusco A. J., Rev. Latinoam. Quim., 11, 2 3 (1981) 897.