696 J. Mater. Sci. Technol., Vol.22 No.5, 2006

Kinetics of Silver Dissolution in Nitric Acid from Ag-Au0.04-Cu0.10

and Ag-Cu0.23 Scraps

S.K.Sadrnezhaad†, E.Ahmadi and M.MozammelCenter of Excellence for Advanced Processes of Production of Materials, Department of Materials Science

and Engineering, Sharif University of Technology, Tehran, Iran

[Manuscript received September 27, 2005, in revised form January 30, 2006]

Kinetics of dissolution of silver present in precious metal scraps in HNO3 was studied in temperature range of26∼85◦C. Dissolution rate of silver was much faster than that of copper at all temperatures. Effects of particlesize, stirring speed, acid concentration and temperature on the rate of dissolving of silver were evaluated.Dissolution rate decreases with particle size and increases with temperature. Dissolving was accelerated withacid concentrations less than 10 mol/L. Concentrations greater than 10 mol/L resulted in slowing down of thedissolution rate. Shrinking core model with internal diffusion equation t/τ=1−3(1−x)2/3+2(1−x) could beused to explain the mechanism of the reaction. Silver extraction resulted in activation energies of 33.95 kJ/molfor Ag-Au0.04-Cu0.10 and 68.87 kJ/mol for Ag-Cu0.23 particles. Inter-diffusion of silver and nitrate ions throughthe porous region of the insoluble alloying layer was the main resistance to the dissolving process. Resultswere tangible for applications in recycling of the material from electronic silver-bearing scraps, dental alloys,jewelry, silverware and anodic slime precious metal recovery.

KEY WORDS: Silver; Copper; Kinetics, Dissolution; Shrinking core model; Internal diffusion

1. Introduction

Silver is used as an alloying element, coating mate-rial, ornament, catalyst and biosensor[1,2] in medicine,battery and film industry[2,3]. Silver nitrate has awide application in electroplating, painting, xerogra-phy and chemical processing. Silver is produced di-rectly from its ores or as a by-product of zinc, cop-per and lead[3∼5]. The world consumption of silverexceeds its mine production rate. Silver containingarticles must, therefore, be recycled in order to partlycover its consumption[6]. Recovery of the scrappedproducts plays an important role in fulfillment of theincreasing silver demand in a foreseeable future.

A common process used in precious metals in-dustry to separate silver from gold is nitric aciddissolution[7∼12]. Alloys with high silver content canbe directly treated with HNO3 to dissolve their sil-ver content and to leave the undissolved elements asa separable solid phase[12]. This method is prefer-ably used in small-scale processes like alloy refinementwhen aqua regia dissolution is not a suitable treat-ment.

Many investigations on silver/copper reactionwith nitric acid solutions can be found in litera-ture [7∼10]. Schack and Clemmons[11] illustrated ahigh temperature treatment of silver scrap via smelt-ing and cupellation. Martinez et al.[12] report on sil-ver molar fraction that substantially affect on the rateof silver nitrate formation. With both high nitric acidactivity and temperature, Martinez et al.[12] showedan outward diffusion of nitrate molecules within theundissolved gold containing less than 0.65 mol frac-tion of silver[12] to control the dissolution rate. At 0.7and more silver mole fraction, a solid-surface chemi-cal mechanism with activation energy of 54.3 kJ/molprevails. Jiang et al.[13] illustrated the leaching kine-

† Prof., Ph.D., to whom correspondence should be addressed,E-mail: sadrnezh@sharif.edu; sadrnezh@yahoo.com.

tics of pyrolusite of Mn-Ag ores in presence of hy-drogen peroxide. The activation energy obtained bythem is 4.45±0.3 kJ/mol at 30∼60◦C in nitrate[13].

Traditional recovery of precious metals from anodeslime consists of fusion of sludge in presence of flux,cupellation, melting with-lead in a cupel and parting.Lead is oxidized by atmospheric oxygen and absorbedby the magnesite in the cupel. This leaves gold, sil-ver and platinum group elements as a button, whichcan be weighed to determine the total content of theprecious metals. This usually contains too much goldto be separated by nitric acid treatment.

Kunda[14] applied an alternative hydrometallurgi-cal silver extraction method, which consisted of (a)sulfuric acid leaching of the silver bearing materials,(b) dilution in water of the product, (c) dissolvingin dilute ammonia of the precipitate and (d) reduc-tion with hydrogen of the precipitate into silver[14].Similar processes can be applied to the solid wastematerials containing silver bearing substances. Ultra-sound can help to increase the rate of the leachingprocess[15].

The precious metals such as gold, silver, seleniumand tellurium can be recovered from the anodic slimby (a) nitric acid leaching, (b) liquid-liquid extractionand (c) direct reduction[16,17]. In comparison withthe conventional pyrometallurgical processes, Wanget al.[16] claimed hydrometallurgical process to be eco-nomic, energy-saving and pollution-free.

Demir et al.[18] determined a semi-empirical ki-netic model for dissolution of metallic copper parti-cles in HNO3. Their results showed 18.81 kJ/molof activation energy. Kinetics of reaction be-tween metallic silver and nitric acid was investi-gated by Ozmetin et al.[19] in the concentration rangeof 7.22∼14.44 mol/L. Their results have shown asurface reaction control with activation energy of57.66 kJ/mol.

Literature lacks essential information on kinetics

J. Mater. Sci. Technol., Vol.22 No.5, 2006 697

of silver recovery from silver bearing waste materialssuch as electronic wastes, photographic films, spentcatalysts and jewelry scraps. Significant amounts ofthese materials are created these days throughout theworld[20,21]. This paper aims to elucidate the kinet-ics and mechanism of dissolution of silver present inAg-Au0.04-Cu0.10 and Ag-Cu0.23 scraps in nitric acid.The results are useable to improve recovery of pre-cious metals from silver bearing scraps and wastes[22].It is therefore of value to the newly growing materialsrecycling technologies[20∼22].

2. Experimental

Ag-Cu0.23 ingots were produced by melting andcasting of analytical grade raw materials in aluminacrucibles. Chemical composition of the samples is85 wt pct Ag and 15 wt pct Cu. Pieces of metals (sil-ver and copper) were weighted, charged and heated upto 1050◦C for 15 min. Alloy filings were made of theingot by hack sawing of the specimens. The empiricaldata obtained by Rubcuminintara and Tasaso[6] forAg-Au0.04-Cu0.10 jewelry scraps having 86.91 wt pctAg and 5.89 wt pct Cu were also used for data veri-fication and comparison purposes. The experimentalconditions used in this research are similar to those.

Dissolving experiments were carried out in a one-liter cylindrical container immersed into a thermostat-ically controlled water bath. Test temperatures wereselected in the range of 26 to 85◦C. After the containerwas warmed up to the required temperature, it wasfed with 250 ml of aqueous HNO3 being stirred witha specified rate. The stirring rate remained constantduring each test and one gram of filings was added tothe container. At selected time intervals, 5 ml solu-tion was sampled. The samples were diluted to thedesired volume in a volumetric flask. The analysis ofthe diluted samples was made with an atomic absorp-tion spectrophotometer (AAS).

Dissolution kinetics was determined by analyzingthe dissolved silver. Assuming that there exists aspontaneous reaction, fractional extraction can be de-fined as the ratio of the concentration of the metal-lic species present in the solution at time t dividedby its total amount in the alloy. The apparatus wascalibrated with a series of well known standard solu-tions. The uncertainties of the measurements were es-timated with regression analysis procedure. The cor-relation coefficient (R2) was found to be in the rangeof 0.98∼0.99.

3. Results and Discussion

Different mixing speeds were used to monitor theeffect of stirring on the rate of dissolution of the sil-ver. The rate of dissolution did not change with thespeed of mixing. This finding was consistent with thedata reported by previous authors[6]. Therefore, ex-ternal transfer was not the controlling step for silverdissolution.

Other influential parameters were concentration,alloy particle size and temperature. Figure 1 showsthe effect of nitric acid concentration on fractional dis-solution of silver. It can be seen from Fig.1 that thedissolution rate increases with acid concentrations upto ∼10 mol/L HNO3 and then decreases beyond this

Fig.1 Effect of acid concentration on fractional dissolu-tion of silver of Ag-Au0.04-Cu0.10 alloy

Fig.2 Effect of particle size on fractional extraction ofsilver and copper at 26◦C. Part of the data is ob-tained from literature [6]

concentration. A maximum is therefore observable at∼10 mol/L nitric acid concentration.

Martinez and Espiel[12] investigated the effect ofnitric acid concentration on dissolution rate of sil-ver in Ag-Au alloys and found that the reaction wasfirst order with 51.1 and 75.12 pct silver at 80◦C and3.8∼7.10 mol/L nitric acid concentrations. For alloyswith 51.1 pct silver and acid concentrations exceeding6 mol/L, the reaction order dropped to zero. HNO3

reduced generally to NO2 at these concentrations.For leaching silver from copper anode slime, it was

reported that there is an optimum nitric acid concen-tration of about 7 mol/L[23]. This value was lowerthan the acid concentrations corresponding to themaximum rate of extraction of silver in this investiga-tion. According to Butts and Coxe[24], the solubilityof silver nitrate in water decreases considerably withthe nitric acid concentration. The decrease in dis-solution rate of the silver beyond ∼10 mol/L HNO3

concentration seems therefore to be due to this ef-fect. The concentration for dissolution experimentswas chosen 10.4 mol/L.

Effect of particle size on fractional extraction ofsilver in the nitric acid solution is depicted in Fig.2.As is seen, the dissolution rate decreases with aver-age size of the particles. The silver dissolution rateseems, therefore, to be sensitive to the internal (pore)mass diffusion step. This contradicts with the conclu-sion mentioned by previous researchers, who claimeda single-step chemical control mechanism[6].

Figure 3 shows that the amount of copper extrac-tion is much less than that of silver, especially at lowtemperatures. However, both rates increase dramat-ically with temperature. A much faster dissolutionprocess was therefore observed at 85◦C than at roomtemperature. A comparison of the extraction data forAg-Cu0.23 and Ag-Au0.04-Cu0.10 alloys given in Fig.4

698 J. Mater. Sci. Technol., Vol.22 No.5, 2006

Fig.3 Effect of temperature on extraction: (a) silver and(b) copper from Ag-Cu0.23 alloy having a particlesize of 75∼150 µm with 10.4 mol/L HNO3 and astirring rate of 250 r/min

Fig.4 Effect of temperature on fractional extraction ofsilver from Ag-Cu0.23 and Ag-Au0.04-Cu0.10 alloysleached with 10.4 mol/L HNO3 at 250 r/min. Thedata on ternary alloys are from literature [6]

Table 1 Thermodynamic properties of substances at298 K and 105 Pa[27]

Substance ∆H◦f /(kJ/mol) S◦/(J/mol·K)

Ag(s) 0 42.68Cu(s) 0 33.14AgNO3(l) −124.52 140.92Cu(NO3)2(aq) −350 193HNO2(aq) −119 46.1HNO3(aq) −207 53.5H2O(l) −286 69.95NO(g) 90.2 211NO2(g) 33.2 240

indicates that the presence of gold in the scrap hasan increasing effect on the rate of extraction of sil-ver especially at lower temperatures. This implies apositive interaction parameter of gold on silver in Ag-Au0.04-Cu0.10 alloys employed in this study[25].

The closest kinetic mechanism indicates the op-timal condition for maximum recovery of silver fromsilver containing metallic objects. From thermody-namics, the spontaneity of the dissolution process canbe predicted. The dissolution reaction of the impurealloys in nitric acid can be expressed by several typical

reactions[6,12].

3Ag(s)+4HNO3(aq) → 3AgNO3(l)+NO(g)+2H2O(l)(1)

Ag(s) + 2HNO3(aq) → AgNO3(l) + NO2(g) + H2O(l)(2)

With alloy compositions and nitric acid concentra-tions used in this investigation, Reactions (1) and (2)are spontaneous at room temperature. Sum of thetwo reactions is as follows:

4Ag(s) + 6HNO3(aq) → 4AgNO3(l) +

NO(g)+NO2(g)+3H2O(l) (3)

At high nitric acid concentrations, the following reac-tions may also become feasible[7∼9]:

2Cu(s)+4HNO3(aq) → Cu(NO3)2(aq)+

Cu(NO2)2(aq) + 2H2O(l) (4)

Cu(NO2)2(aq) + 2HNO3(aq) →Cu(NO3)2(aq) + 2HNO2(aq) (5)

Cu(s) + HNO3(aq) + HNO2(aq) →Cu(NO2)2(aq) + H2O(l) (6)

Cu(NO2)2(aq) + 2HNO3(aq) →Cu(NO3)2(aq) + 2HNO2(aq) (7)

HNO2(aq) + HNO3(aq) → 2NO2(g) + H2O(l) (8)

Cu(s)+4HNO3(aq) → Cu(NO3)2(aq)+

2NO2(g) + 2H2O(l) (9)

Dissolution reaction of silver depends on the ni-tric acid concentration. With a dilute acid, Eq.(1)is applicable. With a concentrated one, Eq.(2) pro-ceeds further. For simplicity, we chose Eq.(3), whichis a combination of two reactions. With nitric acid,HNO2 has a catalytic effect on dissolution of copper.Nitrogen dioxide can react with water to form nitricacid (compare Eqs.(8) and (9)). We have, therefore,chosen Eq.(9) for our evaluations.

The standard Gibbs free energy change (∆G◦) inReactions (1)∼(9) can be determined from ∆H◦

f andS◦ data of compounds and elements (see Table 1).Standard Gibbs free energies in Reactions (3) and (9)are, thus, obtained to be−209.06 and−196.20 kJ/molat 298 K and 105 Pa, respectively. If all activities areconsidered unity, the reactions seem spontaneous atroom conditions. Figure 3 shows that Reaction (3) ismore favorable than Reaction (9) under the experi-mental conditions used in this research. Therefore, itis logically reasonable to assume the activity of silvergreater than that of copper and gold.

Reactions (1)∼(3) require NO−3 /Ag+ ion trans-port between the reaction-site and the bulk-liquidphase. The experimental data obtained in this re-search were used to check the different kinetic mod-els available for solid-fluid reactions[26]. Since copperand gold do not favor reaction with HNO3 at roomtemperature, a porous metallic layer composed of Cu(and Au) remains on the surface of the un-reacted Ag-Cu0.23 (or Ag-Au0.04-Cu0.10) core. The radius of thescrap grains does not, therefore, considerably changewith advancement of the reaction.

Pore diffusion control with constant particle-sizegave the best answer with correlation coefficients

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greater than 0.96. For spherical particles having theaverage initial radius of R0, the total conversion timeτ is defined by the following equation.

τ =ρaR

20

6bDecA

(10)

where ρa is the molar density of the alloy; b is the stoi-chiometric coefficient of the reaction (3/4, 1/2 and 2/3for Eqs.(1), (2) and (3), respectively); De is the coeffi-cient of inter-diffusion of Ag+ and NO−3 ions throughthe porous metallic layer and cA is the concentrationof HNO3 in the bulk solution. Reactant and productions, i.e. NO−3 and Ag+ ions, must diffuse through-out the pores in order to obtain an on-going reaction.De is the inter-diffusion coefficient of NO−3 and Ag+

indicating ion transfer within the porous region.The reaction between silver and nitric acid is of

electrochemical nature. Because of considerable con-ductivity of the metallic layer, there is no resistanceto charge transfer through the solid region. The onlyremaining resistance is thus related to the NO−3 andAg+ ion transfer through the aqueous electrolyte thatfills the porous region of the solid phase. This resis-tance is considered by the inter-diffusion coefficientDe in the present system.

The dimensionless conversion time relevant to thepore diffusion mechanism can be obtained by the fol-lowing equation:

t

τ= 1− 3(1− x)2/3 + 2(1− x) (11)

Silver dissolution plot on the right side of Eq.(11) isdepicted against time in Fig.5. Regression analysesof the data indicated correlation values of 0.99 at dif-ferent temperatures. Other model correlations wereplotted against time. No cases gave better straightlines than those in Fig.5.

The data plotted in Fig.2 confirms the above con-clusion. As it is seen, the size of the alloy particleshas a substantial effect on the extraction rate of thesilver. In order to assess the quantitative relationshipbetween the total conversion time and the averagesize of the particles, the slopes of the curves shown inFig.5 were plotted vs R−2

0 , as shown in Fig.6. Thestraight line regression equation was found to have acorrelation factor of 0.98. It was, therefore, assessedonce more that the silver dissolution from the alloysinto the nitric acid was controlled by diffusion.

The effective internal diffusion coefficient De wasestimated from Eq.(10). The density of the Ag-Au0.04-Cu0.10 scrap is nearly 11.03 g/cm3 while thatof Ag-Cu0.23 is about 10.25 g/cm3 (estimated fromdensities of pure elements with an ideal solutionmodel). The average initial size of the filings was usedfrom the sieve analyses data. The results obtainedwas De=7.59×10−7 cm2/s for Ag-Au0.04-Cu0.10 and5.76×10−7 cm2/s for Ag-Cu0.23 scrap at room tem-perature.

The radius of Ag+ ions (0.113 nm) is smaller thanthat of Ag atoms (0.144 nm). On the contrary, theradius of NO−3 ions (0.200 nm) is larger. Consider-ing geometry of the system, it may be concluded thatthe pore diffusion for Ag+/NO−3 ions is not easy fortheir transfer within the bulk phase. Porosity and

Fig.5 Internal diffusion plot for silver dissolution:(a) Ag-Cu0.23 alloy at different temperatures and(b) Ag-Au0.04-Cu0.10 particles at different acidconcentrations

Fig.6 Effect of initial radius on the total extraction timeof silver from Ag-Au0.04-Cu0.10 alloy

tortuosity are two important factors that affect theamount of the diffusion coefficient in porous regions.

De = D(Ag+/NO−3 )εθ (12)

where DAg+/NO−3is bulk inter-diffusion coefficient; ε

is porosity and θ is the tortuosity coefficient of theporous region.

Bulk diffusion coefficient of AgNO3 in aqueousHNO3 media is reported to be 1.71×10−5 cm2/s atroom temperature[28]. The inter-diffusion coefficientsobtained in this research are mush smaller than this.Considerable difference between these coefficients in-dicates significant influence of the electrochemical andgeometrical characteristics of the present porous sys-tem.

AgNO3 has lower solubility at higher acid con-centrations (cHNO3

≥10 mol/L). Solid precipitatemay thus partially block the reaction front atcHNO3

≥10 mol/L. Rate controlling mechanism canhence locally change from pore diffusion into transferin solid phase. The reaction rate is therefore reduceddue to lowering of De inside the porous layer.

The frequency factor and the activation energy ofdissolution of silver are thus determined using the Ar-rhenius law.

De = D0e−Q/RT (13)

700 J. Mater. Sci. Technol., Vol.22 No.5, 2006

Table 2 Activation energies and frequency factorsevaluated from application of the silver dis-solution data to the mathematical modeldeveloped for internal transfer of silver intonitric acid solution

Alloy Q/(kJ/mol) D0/(cm2/s)Ag-Au0.04-Cu0.10 33.95 0.73Ag-Cu0.23 68.87 2.27×106

where D0 is the frequency factor and Q is the acti-vation energy of the diffusion. R is the universal gasconstant and T is the absolute temperature of the so-lution. The results is illustrated in Table 2. The val-ues evaluated for the activation energies of the silversupported the porous metallic layer diffusion. Con-siderable reduction in frequency factor of silver due topresence of gold in the alloy implies the significant in-teraction between the two species of the system. Thiscould be the matter of further investigation.

4. Conclusions

(1) Based on the data presented in this paper, therate of reaction with nitric acid of the silver containedin jewelry scrap, silver-copper and silver-copper-goldalloys are influenced by particle size, acid concen-tration and temperature. Insertion of the empiricaldata into different solid-fluid kinetic models indicateda shrinking core internal diffusion equation explain-ing the mechanism of the reaction. The kinetic datashowed a linear relation of 1−3(1−x)2/3+2(1−x) withtime. The apparent activation energies obtained de-pend on the chemical species present in the scrapphase.

(2) An effective inter-diffusion coefficient, De,of 7.59×10−7 cm2/s for Ag-Au0.04-Cu0.10 and5.76×10−7 cm2/s for Ag-Cu0.23 scrap was obtainedfor ion transfer through the porous silver-less layerformed at exterior of the particles because of disso-lution of silver at room temperature. The maximumrate of dissolution occurs between 7.9 and 10.4 mol/Lacid concentration. This is due to lower water solu-bility for AgNO3 in presence of nitric acid at 26◦C.

Previous investigators have recognized the Ag-Au0.04-Cu0.10 grains size effects on the rate of silverdissolution[6]. This finding is also backed-up by ourfurther measurements (Fig.2). The extraction ratedecreases, for example, by increasing of the size ofthe particles dissolving in the nitric acid solution asshown in Fig.2. This result is in obvious contradic-tion with chemical reaction mechanism reported byprevious authors[6].

Acknowledgements

The authors would like to express their appreciation toMs. A.Ruhani Mashhadi for AAS tests, Mr. P.Abdollahifor technical assistance and Mr. S.M.M.Sadrnezhad forhis help in mathematical calculations.

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Nomenclatures

b: stoichiometric coefficient (dimensionless)cA : concentration of H+ ion in the bulk solution (mol/L)De: effective diffusion coefficient of ions in porous medium

(cm2/s)ρa: molar density of the alloy

R0: initial radius of particles (microns)t: time (s)τ : time for complete conversion (s)T : temperature (K)x: fraction extracted