Ind. Eng. Chem. Res. 1998, 37, 4641-4645
4641
Kinetic Investigation of Reaction between Metallic Silver and Nitric Acid Solutions in the Range 7.22-14.44 M Cengiz O 2 zmetin,* Mehmet C ¸ opur, Ahmet Yartasi, and M. Muhtar Kocakerim Engineering Faculty, Chemical Engineering Department, University of Atatu¨ rk, 25240 Erzurum, Turkey
In this study, the reaction kinetics between metallic silver and nitric acid solutions was investigated by taking into consideration the parameters of temperature, solid-to-liquid ratio, particle size, stirring speed, nitric acid concentration, and addition of sodium nitrite. It was determined that the dissolution rate of the process increased with decreasing particle size, solidto-liquid ratio, and acid concentration, and increasing reaction temperature and stirring speed. It was found that the amount of sodium nitrite in the solution has no significant effect on the dissolution rate. In the present study, the examination of shrinking core models of fluid-solid systems showed that the dissolution of metallic silver in the range of 7.22-14.44 M nitric acid solutions was controlled by the film diffusion. The following semiempirical model, which represented well the process, was developed by statistical methods: 1 - (1 - X)1/2 (5271.13)(D)-0.548(S/L)-0.3(W)0.851(C)-3.41e-18.81/RTt, where D is the particle size, C the acid concentration, S/L the solid-to-liquid ratio, T the reaction temperature, W the stirring speed, and t the reaction time. The activation energy of the process was found to be 18.81 kJ mol-1. Introduction Silver nitrate, which holds a very important place in industry, is obtained by solving metallic silver and then evaporating the solution. Silver nitrate crystallized from the solution is used in photography and plating and in the production of other silver salts. Silver nitrate used, except that used in photography and technical AgNO3, must have very high purity.1 Up to the present date, a lot of studies have been carried out on the dissolution and the dissolution kinetics of Ag in HNO3 solutions. In the earlier studies, it was expressed that, in the reactions of metallic alloys with HNO3, the physical and chemical conditions of the metal, its purity, reaction temperature, and the product formed in the solutions should be considered. It was thought that the reaction between silver and nitric acid solutions took place through a complex mechanism, but the mechanism was studied.2 It was claimed that rotating silver disks were not dissolved in 2.59-3.95 M HNO3, but dissolved in 4.49-5.17 M HNO3 and the dissolution rate decreased with increasing rotation speed.3 It was stated that the catalytic effect of nitrous acid decreases with increasing stirring speed and this negative effect could be eliminated by the addition of sodium nitride into the medium.4 Nitrous acid might be formed in the medium under the catalytic effects of light-solid catalyses, and HNO2 acted catalytically to activate HNO3, possibly through the dynamic equilibrium.5
HNO3 + HNO2 S 2NO2 + H2O
(1)
In another study performed at 25 °C with 2.72 M HNO3, it was found that the reaction is significantly affected, depending upon whether the metal was placed on the base of reactor, dispersed in the acid, or immersed vertically into the acid. It was stated that this effect might have resulted from the reaction being very complex and autocatalytic or the intense interface at the base of the reactor.6 At 65 °C and within a 3.8-7.1 M acid concentration range, it was observed that the reaction was realized
through two different stoichometry and kinetics. Within the range of 3.8-4.9 M, it was found that the reaction was second-order and the stoichometry was as follows:
4HNO3 + 3Ag f 3AgNO3 + NO + 2H2O
(2)
but within the 4.9-7.1 M range the reaction was firstorder and its stoichometry was as follows:
2HNO3 + Ag f AgNO3 + NO2 + H2O
(3)
The activation energy of the dissolution was 12.1 kcal mol-1 within the range 4.9-7.1 M. It was stated that silver and copper and their alloys did not dissolve in 60 Be0 H2SO4 and the dissolution started only after HNO3 addition.7 In H2SO4-HNO3 mixtures, the effect of acid ratios, temperature, and time on the weight decrease in the metal was studied and it was found that the dissolution of silver within a 10% HNO3 concentration range at fairly high temperatures was first-order and its activation energy was 42.08 kJ mol-1.8 A silver nitrate solution with a specific weight of 1.27 was obtained by heating the granular pure silver to 65-100 °C in a reaction tower made of stainless steel and spraying HNO3 solution on it (under 1 atm pressure and in the presence of oxygen).9 No reference was found by the authors in the literature related to the dissolution of metallic silver using concentrated nitric acid solutions. In this study on the production of AgNO3 by the dissolution of metallic silver in the range of 7.22-14.44 M nitric acid solutions, the dissolution reaction mechanisms were studied and the best conditions for the reaction were determined. Deriving a mathematical expression representing the dissolution rate from the experimental results was attempted. Experimental Section The spherical metallic silver supplied from the market was sieved to obtain the particle size fractions of -2800
10.1021/ie980121w CCC: $15.00 © 1998 American Chemical Society Published on Web 10/30/1998
4642 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 1. Effect of particle size on the dissolution of metallic silver.
Figure 2. Effect of solid-to-liquid ratio on the dissolution of metallic silver.
+2360, -2360 +1700, -1700 +1400, -1400 +1000, -1000 +850, -850 +600 µm. The chemical analysis of the silver by A.A.S. gave the following results: 99.87% Ag, 0.07% Cu, and 0.06% others. The dissolution experiments were carried out in a 500-cm3 glass reactor, equipped with a digital controlled mechanical stirrer, a thermostat, and a back cooler, at atmospheric pressure. Mixing was provided by a marine-type blade and the reactor was fitted with a fin in order to prevent a vortex being formed.10 The temperature of the reaction medium could be controlled within (0.5 °C. First, 250 cm3 of HNO3 solution at a given concentration was put into the reactor, and after reaching a desired temperature of the reactor contents, a certain amount of the metal was added into the solution while stirring the contents of the vessel at a certain stirring speed during the reaction. Samples were collected from the reaction medium at different times and analyzed for their Ag content.11 At a given condition, each experiment was repeated twice and the arithmetic average was used for the kinetic analysis. The Effects of Parameters The data obtained from the experiments were plotted in time versus conversion fraction, described as
X ) (the amount of dissolved silver)/ (the amount of initial silver) As seen in Figure 1, the decreasing particle size increases the dissolution rate, which can be attributed to the increase of the contact surface on which the main dissolution reaction occurs and to the increase of the dispersion degree of suspension. The behavior of the dissolution for various solid-to-liquid ratios is shown in Figure 2. This figure shows that decreasing solid-toliquid ratios are in favor of the dissolution rate, which can be explained by the decrease of the solid amount per amount of the reagent in the suspension. It can
Figure 3. Effect of stirring speed on the dissolution of metallic silver.
clearly be seen from Figure 3 that the dissolution rate increased with an increasing stirring speed, which can be attributed to the reduction of the liquid film thickness around the solid particle due to the increase of the velocity of the solid relative to the liquid velocity. The experiments to observe the effect of the HNO3 concentration on the dissolution process were carried out in the concentration range 7.22-14.44 M. It is thought that the dissolution reaction of metallic Ag in nitric acid solutions having different concentrations takes place through two different mechanisms. In lowconcentration nitric acid solutions, the following reac-
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4643
tions take place:5,7,12
6Ag + 6HNO3 h 3AgNO3 + 3AgNO2 + 3H2O 3AgNO2 + 3HNO3 h 3AgNO3 + 3HNO2
(4) (5)
6Ag + 3HNO3 + 3HNO2 h 6AgNO2 + 3H2O (6) 6AgNO2 + 6HNO3 h 6AgNO3 + 6HNO2
(7)
6HNO2 + 6HNO3 h 12NO2 + 6H2O
(8)
12NO2 + 4H2O f 8HNO3 + 4NO
(9)
3Ag + 4HNO3 f 3AgNO3 + 2H2O + NO
(2)
and in high-concentration nitric acid solutions, the following reactions occurs.5,7,12
6Ag + 6HNO3 h 3AgNO3 + 3AgNO2 + 3H2O (4) 3AgNO2 + 3HNO3 h 3AgNO3 + 3HNO2
(5)
6Ag + 3HNO3 + 3HNO2 h 6AgNO2 + 3H2O (6) 6AgNO2 + 6HNO3 h 6AgNO3 + 6HNO2
(7)
6HNO2 + 6HNO3 h 12NO2 + 6H2O
(8)
Ag + 2HNO3 h AgNO3 + NO2 + H2O
(3)
Figure 4. Effect of acid concentration on the dissolution of metallic silver.
There is a catalytic effect of HNO2 on the dissolution reaction of metallic Ag in nitric acid solutions.4 According to eq 8, HNO2 decreases with increasing HNO3 concentration. It is thought that the concentration of the shared ion of AgNO3 and HNO3 increases with an increase in the acid concentration. Therefore, a saturated liquid film is formed on the surface.13 Both a decrease of HNO2 and the saturated liquid film cause a decrease of the dissolution rate with increasing acid concentration (Figure 4). The NaNO2 addition for 11.55 M HNO3 concentration did not effect the dissolution rate. This can be explained with the decomposition of nitrous acid at a high nitric acid concentration.4,5
NaNO2 + HNO3 f NaNO3 + HNO2
(10)
HNO2 + HNO3 h 2NO2 + H2O
(1)
Figure 5 shows that increasing temperature has an increasing effect on the dissolution rate as expected because of the exponential dependence of the rate constant in the Arrhenius form. Dissolution Kinetics of Metallic Silver in HNO3 Solutions In the determination of the dissolution kinetics of metallic silver in nitric acid solutions, integral rate expressions derived for fluid-solid reaction models14 were considered. The fit of all the experimental data into the integral rate was tested by using a computer program, and the multiple regression coefficients obtained for the integral rate expression are given in Table 1. To confirm the results of these statistical
Figure 5. Effect of temperature on the dissolution of metallic silver.
analyses, the experimental data for each parameter was analyzed by graphical methods. As a result, it was found that the integral rate expression for the dissolution of silver in HNO3 solution may be given by the following equation:
1 - (1 - X)1/2 ) kt
(11)
From the results of the statistical analysis it was found that the dissolution of Ag in HNO3 solutions is controlled by film diffusion. To express the effect of the parameters, it can be assumed that the rate constant of the dissolution process depends on the parameters
4644 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 Table 1. Multiple Regression Coefficients of Rate Control Models for (7.22-14.44 M) High HNO3 Concentrations models of rate control
rate expression
coefficients multiple regression
film diffusion control for shrinking sphere (large particle) film diffusion control for shrinking sphere (small particle) reaction controls film diffusion control for constant size particles pseudohomogeneous first-order reaction model homogeneous second-order reaction model ash (or products) diffusion control for constant size particles
1 - (1 ) kt 1 - (1 - X)2/3 ) kt 1 - (1 - X)1/3 ) kt X ) kt -ln(1 - X) ) kt X/1 - X ) kt 1 - 3(1 - X)2/3 + 2(1 - X) ) kt
0.9770 0.9628 0.9607 0.9322 0.9203 0.7021 0.6116
X)1/2
Table 2. Values of Chosen Parameters and Reaction Periods Calculated from Equation 14 for These Parameters dissolution (%)
D (µm)
S/L (g cm-3)
W (rpm)
C (mol dm-3)
T (°C)
t (min)
100 100 100
725 725 725
0.01 0.1 0.1
1200 1200 800
7.22 7.22 7.22
50 50 50
3.94 7.86 11.09
errors of 0.0097 calculated by the equation
ER )
Figure 6. Agreement between experimental conversion values and predicted values from one semiempiric expression.
as follows:
k ) k0(D)a(S/L)b(W)c(C)de-E/RT
(12)
Then, eq 11 can be written as
1 - (1 - X)1/2 ) k0(D)a(S/L)b(W)c(C)de-E/RTt
(13)
Statistical calculation by simultaneous multiple regression gave the results of k0 ) 5271.13, a ) -0.548, b ) -0.3, c ) 0.851, and d ) -3.41 for the constants in eq 13, and inserting the estimated values into this equation resulted in the following kinetic model:
1 - (1 - X)1/2 ) (5271.13)(D)-0.548(S/L)-0.3(W)0.851(C)-3.41e-18.81/RTt (14) As seen from the kinetic model for the dissolution process in eq 14 the most effective parameter is the acid concentration and then the stirring speed follows it. The significant effect of the stirring speed and an activation energy being lower than 20 kJ mol-1 confirms that the dissolution is controlled by film diffusion.15 Moreover, since no ash film was observed around the particle during dissolution, the diffusion-controlled process through ash film was discarded. To test the agreement between the experimental conversion values and the values calculated from the empirical equation, the plot of Xexp versus Xprd was drawn as seen in Figure 6. The agreement between the experimental and calculated values is very good, with a relative mean square of
[
1
N
∑
Ni)1
]
(Xprd - Xexp)2 (Xprd)2
1/2
(15)
where Xprd is the calculated value, Xexp the experimental value, and N the number of experimental data, which is 0.0097 for the present case. It should be emphasized that this empirical model can be applicable for the experimental condition ranges and reactor geometry used in the present study. The reaction periods for 100% dissolution from the empirical equation were given in Table 2 for some parameters. When the solidto-liquid ratio increased from 0.01 to 0.1, the reaction period also increased from 3.94 to 7.86 min. Even though the period increases 2-fold, a 10-fold amount of the metal can be handled. Because of the advantages of obtaining the solution with a higher concentration and of handling more metal, studying with a 0.1 solidto-liquid ratio is more advantageous than with a 0.01 solid-to-liquid ratio. The working of higher temperatures has two advantages: one to spend less energy for saturating AgNO3 solution and two to obtain a higher dissolution rate. For these reasons, a reaction temperature of 50 °C can be recommended for a 7.22 M acid concentration, 1200 rpm stirring speed, and 725 µm mean particle size, the parameters giving the maximum dissolution rate. The study can be extended to a plot scale work with higher solid-to-liquid ratios and to the production of silver nitrate from the silver-containing solution. Conclusions A kinetic study of the dissolution of metallic silver with HNO3 solutions was performed in a stirred reactor. The parameters were chosen to be temperature, acid concentration, solid-to-liquid ratio, particle size, stirring speed, and NaNO2 concentration. Acid concentration and stirring speed are found to be the most effective parameters. However, at the HNO2 concentrations being studied, the addition of NaNO2 has an insignificant effect on the dissolution. The kinetic analysis using fluid-solid reaction models and homogeneous reaction models incorporating statistical methods proved that the kinetic model best representing the process was diffusion through a fluid film model. An empirical kinetic expression including the parameters used in the study was developed, and this expression can estimate the
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4645
conversion fraction with a relative mean square of errors of 0.0097. From an empirical kinetic expression, the activation energy for the process was obtained as 18.81 kJ mol-1. Nomenclature X ) conversion fraction t ) reaction time (min) k ) rate constant for surface reaction(min-1) a ) a constant in eq 12 b ) a constant in eq 12 c ) a constant in eq 12 d ) a constant in eq 12 k0 ) a constant in eq 12 C ) concentration of HNO3 solution (mol dm-3) S ) amount of solid (g) L ) amount of liquid (cm3) D ) particle size (µm) W ) stirring speed (rpm) E ) activation energy (kJ mol-1) R ) universal gas constant (J K-1 mol-1) T ) temperature (K)
Literature Cited (1) Turkish Standard Institute: Silver Nitrate; Ankara, 1973; TS 1151. (2) Stansbie, J. H. The reaction of metals and alloys with nitric acid. J. Soc. Chem. Ind. 1906, 32, 311-9. (3) Urmanczy, A. Corrosion of rotating metal disks. Z. Anorg. Chem. 1938, 235, 363-8. (4) Hedges, E. S. The action of nitric acid on some metals. J. Chem. Soc. 1930, 561-9.
(5) Bancroft, W. D. Catalytic action of nitrous acid. J. Phys. Chem. Soc. 1924, 28, 973-83. (6) Batten, J. J. Positional effects in the rate of reaction between silver and nitric acid. Aust. J. Appl. Sci. 1961, 12, 358-60. (7) Martinez, L. L.; Segarra, M.; Fernandez, M.; Espiel, F.; Kinetic of the dissolution of pure silver-gold alloys in nitric acid solution. Metall. Trans. B 1993, 24, 827-837. (8) Que, Z.; Wrang, Y.; Xie, C.; Pan, C. Kinetics of the reaction of silver and copper with mixed nitric and sulphuric acids. Guijinshu 1991, 12 (4), 9-15. (9) Critchley, T. Silver and Bismuth Nitrates; Johnson & Sons’ Smelting Works Ltd.: U.K., 1950. (10) Perry, R. H. Perry’s Chemical Engineers Handbook, 6th ed.; McGraw-Hill Inc.: New York, 1984; Chapters 5 and 19. (11) Skoog, D. A.; West, D. M. Fundamentals of Analytical Chemistry, 3rd ed.; Holt, Rinehart, and Winston: New York, 1976. (12) Mellor, J. W. A Comprehensive Treatise On Inorganic and Theoretical Chemistry; Longsman, Grun and Co.: London, 1961; Vol. 3. (13) O ¨ zmetin, C. Elementel gu¨mu¨s¸ ve Nitrik asitten gu¨mu¨s¸ nitrat olus¸ umunun kinetigi ve elde edilen c¸ o¨zeltiden gu¨mu¨s¸ nitratın kristalizasyonu. Atatu¨rk U ¨ niversitesi Fen Bilimleri Enstitu¨su¨, Yu¨ksek lisans Tezi, Erzurum-Turkey, 1996. (14) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons Inc.: New York, 1972. (15) Jackson, E. Hydrometallurgical Extraction and Reclamation; Ellis Horwood Ltd.: Chichester, 1986.
Received for review February 24, 1998 Revised manuscript received August 25, 1998 Accepted August 31, 1998 IE980121W
