Indian Journal of Chemical Technology Vol. 8, March 2001, pp. 112-119

Crystallization of silver nitrate from saturated silver nitrate solution in nitric acid

Cengiz Ozmetin, Mehmet <;opur, M Muhtar Kocakerim & Sinan Yapici

AtatUrk University, Chemical Engineering Department, 25240, Erzurum, Turkey

Received 20 October 1999; accepted 18 December 2000

The aim of the present work is to submit an investigation on the optimum conditions of crystallization of silver nitrate salt from saturated si lver nitrate solution in nitric acid. The saturated solutions of AgN03 at 70°C obtained by evaporation of dilute silver nitrate solutions with nitric acid were crystallyzied in a jacketed glass vessel with controlled cooling. The parameters for the crystal li zation operation were chosen as stirring speed and crystallization period. For each experiment, crystals obtained from operations were filtered, washed, dried, and their photographs were taken before particle size analysis. The differential and cumulative analysis and product yield calculations showed that the optimum values of the parameter for the crystallization of silver nitrate salt from saturated si lver nitrate solutions with nitric acid were determined as 500 rpm stirring speed and 240 min crystal li zation period , with the hi ghest yield.

A crystal can be defined as a solid composed of atoms, ions, or molecules, which are an·anged in an orderly and repetitive manner. It is a highly organized matter in which the atoms, ions, or molecules are located in three dimensional arrays or space lattices. Crystallization is a sol id- liquid separation process in which mass transfer of a solute occurs from the liquid phase to a sol id crystalline phase where solid particles are formed from a homogeneous phase. It can be observed in the formation of solid particles in a vapour as in snow, in solidification from a liquid melt, as in the freezing of water or in crystallization from liquid solutions. From the chemical engineering point of view, the last situation is commercially the most important one and is commonly encountered in industrial applications. In crystallization the solution is concentrated by the evaporation of the solvent and then cooled until the solute concentration becomes greater than its solubility at that temperature. As a result, the solute comes out of the solution forming crystals of almost pure solute. In commercial app lication of the crystalli zation, the high yield and purity of crystals are not only important but also appearance, size range and shape of the crystalline product. To minimize caking in the package, for ease of panning, washing and filtering, and for uniform behavior in use, reasonable size and uniformity is often desirable 1.2_

Crystallization may be treated from the standpoint of purity, yield, energy requirement, and rates of nucleation and crystal growth. An understanding of

the mechanisms by which crystals form and then grow is helpful in designing and operating crystall izers. Much experimental theoretical work has been done to help the understanding of crystallization. For an effective crystallization process, crystallizer type and working conditions must be chosen carefully, depending upon the substance to be crystal! ized.

The aim of the present work is to investigate the effect of the experimental conditions on the crystal quality of silver nitrate and crystallization yield in the crystallization from its solutions including nitric acid. AgN03 is most widely used silver salt, mainly in photography, silver electroplating and in the production of other silver salts. AgN03 is produced by the dissolution of metallic silver followed by crystallization process. Solid AgN03 and its solutions are not sensitive to the light in the case that they do not contain any organic impurity. When it is heated over its melting temperature, it is first decomposed to AgN02 and 0 2, and then to metallic silver and nitrogen oxide compounds3

. High purity of AgN03 is desired, specifically for chemical use. The dissolution of metallic silver in HN03 solutions has been investigated from the standpoint of mechanisms of the reaction and of the reaction products4

-12

• Although there are some studies on the purification of AgN03

solutions 13-

14 and crystallization of silver nitrate in magneti c field 15 and crystallization of AgN03 by spray drying 16

, no study was spotted by the authors about the effect of experimental conditions on the

OZMETIN eta/.: CRYSTALLIZATION OF SILVER NITRATE 113

400

360 Temperature : 40 -c 320

,.......280 .J

E 240

§200 .......... 01

........ ,60 X

120

80

40

0 0 2 4 6 8 10 12 14 16

HN03 conc.{M)

Fig. 1-The di ssolution of si lver nitrate in nitric acid

w;--,-,--,-,--.-,--.-,--,-,,-,-~

0 40 110 1W 180 200 240 llme (mln )

Fig. 2-The cooling curves for different crystallization periods

crystal quality in the crystallization of AgN03 from nitric acid sol utions. Therefore, there is a necessity of determining the effect of experimental conditions on the AgN03 crystal quality when it is crystallized out of HN03 solutions.

Experimental Procedure Si lver nitrate solut ion was obtai ned by dissolving

metallic si lver which have a composition of 99.87% Ag, 0.07% Cu and 0.06% other impurities, measured by AAS, in HN03 solution 17

. Ag+ content of the solution was analyzed by Mohr method and AAS 18

•

~ c: 4)

~ 0 (/)

c 0

1J Q) c

·c; ... ~ Q)

> :.;:; 0 :; E :I

(,)

100 ~eo min

80 a::IJ:D 120 min a:D:D 180 min -2-40 min

eo Stirring spud : 500 rpm

70

eo

:K)

40

30

20

10

o;-.,,.-,,-,,~.-~rT~,-~~~ , 00 200 300 400 1100 800 700 800 800 1000

Screen opening(f.Ull)

Fig. 3-The cumulative analysis curves for different crystallization periods

Table 1-The crystallization yields for different crystallization periods

Crystallization Periods Stirring Speed Yield (mi n) (rpm) %

60 9 1.233

120 500 93.570

180 94.978

240 96. 11 0

The HN03 and AgN03 concentration of the solu tion were 6.24 mol. L- 1 and 22.35 g. L-1

, respectively. The concentrations of HN03 and AgN03 of this solut ion saturated at 70°C in a rotating vacuum evaporator were 10.31 mol. L- 1 and 96.92 g.L' 1

, respectively. It was aimed to obtai n optimum crystallization

conditions in a batch crystalli zation process using the parameters of crystallization period and stirri ng speed. The batch controlled-cooling was applied in the experiments, cooling the saturated solution of AgN03

down to 25 °C. For controlled-cooling curve, the following equation is suggested by Mullin and Nyvlt 19

' 20 for the crystallization of materials of all

kind

... (I)

The crystalli zation operat ion was can·ied out in a jacketed-glass vessel of 500 mL equ ipped with a

ll4 INDIAN J. CHEM. TECHNOL., M ARCH 2001

Stirring speed :500 rpm a :50 Crystallization period :60 min Stirring speed :500 rpm

!50.0 Crystall ization period :120 min

40 40.0

JO .. Jll.O ... 20

20.0

to tQ.O

0 0 200 <400 1100 1100 1000 1200 1<400 OJl

0 200 400 1100 IIlii 1000 1200 t400

Davcrage( ~m) Davcrogc(~m)

Stirring speed :500 rpm c :500 rpm d

Stirring speed !>0 Crystallization period :180 min !>0 Crystallization period :240 min

•o •o

JO JO

~ )t

20 20

10 10

0 0 i 0 200 •oo 600 1!00 1000 1200 uoo 0 200 •oo 600 800 1000 1200 1400

D average (11m) Daverage (Jln1)

Fig. 4-The differential analysis curves for different crystalli zation periods

marine type mechanical stirrer and a digital thermometer to read the temperature of the content of vessel within ±0.1 oc. The cooling water was supplied by a constant temperature circulator to the jacket of the vessel .

The agreement between the experimental and calculated temperature values are very good with a relative mean square of errors of 0.00335 calculated by the Eq . 17.21.22,

ER = - ~ ....:...__--:--::---'--[

J N (Tprd -Tcxp )2lt /2

N ~ (Tr'd )z .. . (2)

where Tprct is the calculated va lue by Eq.(l), rxr is the experimental value and N is the number of experimental data.

Jn the crystalli zation, the decrease of the solubility of AgN03 with the increase of the concentration o f

OZMETIN eta!.: CRYSTALUZATION OF SILVER NITRATE 115

Fig. 5-Crysta ls photographs for different crystallization periods (min) : (a) 60, (b) 120, (c) 180, and (d) 240

HN03 gave the facility of crystallizing AgN03. The change of AgN03 solubility with HN03 concentration is given in Fig. l. For all crystallization periods and stirring speed values, the cooling curves g iven by Eq. (I) was followed in the experiments. When the solutions containing silver nitrate and nitric acid were cooled to 25°C, while the concentration of HN03

Table 2-The crystalli zation yields for diffe rent stirring speed

Stirring Speed Crystallization Period

(m in )

Yield

%

~ c t) () .... u (I)

c 0

1J Cll c ·a .... Cll ....

" > ~ 0 3 E :I

(.)

(rp m)

100

300

400

500

700

100

GO

80

70

80

~

40

30

20

10

240

99.275

98.3 12

97.820

96. 11 0

87.255

_ , 00 rpm tuU300 rpm QIUIIUI 400 rpm tuU500 rpm U6U 700 rpm

Cryet. period : 240 min.

~ 1000 1 ~ 2000 2500 3000 3500 4000 ~

Screen opening(J..lm)

Fig. 6-The cumulative anal ysis curves for different stirring speeds

remained almost the same, the concentration of AgN03 reduced to 39.9 1 g. L-1 due to crystal lization.

In the crystallization operation, 250 mL of the

solution saturated at 70°C was put into the crystallizer

and then subjected to cooling to 25°C, follow ing the curve given by Eq . (I) . During the experiments, the temperature of the crystallizer content was checked carefully to fit the temperature values obtained from Eq. (1) , using the thermostat. It was possib le to cool below 25°C, but in this case the control of the temperature according to Eq. (I) became difficult. As

reached at 25°C, the contents of the crystallizer were filtered off, silver nitrate crystals was first washed with cold water in small amount, and then dried in an

incubator at 105°C fo r approx imately 2 h unt il a constant weight was reached. This sample was put into a desiccator containing NaOH pellets for about 24 h23

, and the crystals were later on used for analysis .

116

JO

lit

20

10

40

JO lit

20

10

Stirring speed Crystallization period

:100 rpm :240 min

Daveragc{~lm)

Stirring speed :400 rpm Crystall ization period :240 min

INDIAN J. CHEM. TECHNOL., MARCH 2001

a

c

40

JO lit

20

10

~

40

JO lit

20

10

0

Stirring speed :300 rpm Crystallization period :240 min

Stirring speed :500 rpm Crystallization period :240 min

0i-.-~r-roro~~-.-.~,-.-~~ 0 200 400 eoo eoo o 200 400 eoo eoo ~ 1200 1400 teoo

D,,,g,(pm)

40

20

10

Stirring speed :700 rpm Crystallization period :240 min

e

o4-~rT~~rT~~rT~~rT~~rT~ 1 oo 1~ 200 2M JOO .wJ 400 460 eoo !ISO eoo

D overage(~~ m)

D"""S,().Im)

Fig. 7-The differential analysis curves for different st irring speeds

1000 1200 1400

OZMETIN eta/.: CRYSTALLIZATION OF SILVER NITRATE 117

Fig. 8-Crystals photographs for different stirring speeds (rpm): (a) I 00, (b) 300, (c) 400, (d) 500 and (e) 700

The particle size analyses of the crystals were carried out by ASTM standard sieves, and cumulative and differential distribution of crystals were deter­mined1-24. The photograph was taken by a microscope equipped with a camera. Crystallization yields were calculated by the following equation.

Y =N /(G -G ) 0 70° C 25°C

... (3)

Results and Discussions In the crystallization experiments, the effect of the

crystallization period and stirring speed on the crystallization yield and shape was investi gated . Because used metallic silver was almost pure, the

purity obtained crystals was not taken into consideration.

The effect of the crystallization period The effect of the crystallization period on the

crystal quality was investigated employing 60, 120, 180, 240 min cooling periods, and keeping the stirring speed at 500 rpm. The curves followed in crystalli­zation were shown in Fig. 2 for all periods used in the experiments. The crystallization yields with the change in crystallization period were given in Table I , which showed that the yield increased with increasing period. The cumulative and differential distribution of particles were given in Figs 3 and 4, respectively. The

118 INDIAN J. CHEM. TECHNOL., MARCH 200 1

photographs were also presented in Fig. 5. As seen from the Figs 3 and 4, the increase in the period increased the fraction of large particles. For a good crystallization, the amount of the colloidal particle must be as small as possible such a crystallization can be successful in the crystallization out of the solution with a lesser degree of over saturation. Since increasing cooling rate increases the degree of over saturation, the numbers of nuclei increases. These nuclei reduce the over saturation degree of the solution. Due to over numbered nuclei in the medium, the crystals in small size or in colloidal are formed . Decreasing cooling rate decreases the level of over­saturation and relatively small number of nuclei formation in the medium 1

, therefore a better growth is obtained in thi s crystallization. As a result, decreasing cooling rate resulted in an increase in the fraction of large particles as seen clearly from the photograph taken for crystallization for a cooling period of 240 min in Fig. 5.

The effect of the stirring speed

The effect of the stirring speed on the crystalli­zation was investigated applying 100, 300, 400, 500, 700 rpm stirring speeds, keeping the cooling period constant at 240 min. The curve followed in the crystallization period 240 min was given in Fig. 2. The change in the crystallization yields with stirring speed were given in Table 2. Increasing stirring speed decreased the yield.

The cumulative and differential analysis of the particle size were given in Figs 6 and 7, respectively. The particle size distribution changed substantially with stirring speed, and generally the fraction of the large particles decreases with increasing stirring speed (Figs 6 and 7). The photographs of the crystals at various stirring speeds were given in Fig. 8. As seen from these photographs, in lower speeds, the crystals stacked each other and formed masses while in higher speeds, the crystals cracked and were divided into smaller size 1

• Due to increasing surface area per weight with decreasing crystals size, this cracking increased the losses in filtering and washing. Therefore, the yield decreased with increasing stirring speed. When examining the differential particle size distribution and photographs, it can be said that the good shaped particles and desired particle size distribution were obtained at the stirring speed of 500 rpm despite of the decrease in the yield.

Conclusion Silver nitrate solutions obtained from kinetic

studies were saturated at 70°C by evaporation in vacuum, and silver nitrate was then crystallized by cooling in a batch vessel. In the experiments, controlled-cooling operations were performed. The effect of stirring speed and crystallization period on the crystal quality and crystallization yield were investigated. The photographs of obtained crystals showed that the most homogeneous and most regular crystals in shape were obtained at a stirring speed of 500 rpm for a cooling period of 240 min, with the highest yield. Because used metallic silver was almost pure, the purity of obtained crystals wasn't taken into consideration.

Nomenclature T = temperature at t period, °C T0 = initi al temperature, °C Ts = temperature at the end of operation, °C

= any period, min. t1 = crystallization period, min . Yo = yield of AgN03

N =amount of AgN03 obtained from crystalli zat ion (g) G

70.c =amount of AgN03 in saturated solution at 70°C (g)

G25

.c =amount of AgN03 in saturated solution at 25 °C (g)

D =particle size (~lm)

References I . Warren L M, Julian C S, Unit Operations of Chemical

Engineering, 3'd Edition (Me Graw Hill ), 1976,8 18. 2 Geankoplis C J, Transport Process and Unit Operations, 2"d

Edition (Allyn and Bacon Inc. London), 1978, 726. 3 Kirk R E, Othmer D F, Encyclopedia of Chemical

Technology, Volume IX (The lnterscience Encycloped ia Inc. New York) , 1952,426.

4 Stansbie J H, J Soc Chem lnd, 32 ( 1906) 311. 5 Urmanczy A, ZAnorg Chem, 235 (1938) 363. 6 Bancroft W D, J Phys Chem Soc 28 (1924) 973. 7 Hedges E S, J Chem Soc ( 1930) 561. 8 Batten J J , Australian J Appl Sci, 12 (I 961) 358. 9 Batten] J, AustralianJChem, 15 (1962) 18 1. 10 Martinez L L, Segarra M, Fernandez M & Espiel F,

Metallurgical Transactions (B), 24 (I 993) 827. II Pannain E, Gazz Chim Ita/, 39 (I 907) I I ,234. 12 Que Z, Wrang Y, Xie C & Pan C, Guijinshu, 12(4) (199 1) 9. 13 Asai T, Endo A, Ger offen 2,730,995 (Cl. CO! G5l00). Jan

12 (1978) 22. 14 Jerome AM, US Pat 2,614, 029 (1952) Oct. 14. 15 Mayr G, Rend 1st Lombardo Sci, 82 ( 1949) 406. 16 Othmer D F, US Pat/ , 879, 445, Sept 27. 17 Ozmetin C, <;:opur M, Yarta~i A & Kocakeri m M M, Indian

Eng Chem Res, Vol 37(12)(1998) 464 1 18 Skoog D A & West D M, Fundamentals of Analytical

Chemistry. 3'd ed (Holt, Rinehart and Winston, New York). 1976.

OZMETIN eta/. : CRYSTALLIZATION OF SILVER NITRATE 119

19 Mullin J V, Crystal Growth (B rain R. Ramplin, Pergamon Press, Oxford and New York), 1974,289.

20 Mullin J V & Nyv lt J, Chem Eng Sci, 26 ( 1971) 369. 21 c;:opur M, c;:olak S & Yapici S, Indian Eng Chem Res, 34

(1995) 3995.

22 Ozmetin C, c;:opur M, Yarta~i A, Kocakerirn M M, Chem Eng Techno/, Vol 23(8) (2000) 707.

23 Meyer A, Chem Ztg, 49 ( 1925) 222. 24 Lapple C E, Fluid and Particle Mechanics, Chap 12

(University of Delaware, Newark), 1951 .