Recovery of silver from nitrate solution by means of ...

JOUF~

of Membrane Science, 68 (1994) 137-143

137

Elsevier Science B.V., Amsterdam

Recovery of silver from nitrate solution by means of rotating film pertraction L. Boyadzhie? and K. Dimitrov h..stitute of Chemicd Engineering, &dgariun Academy of Sciences, 1113 Sofia (&&aria)

(Received December 23,1992; accepted in revised form August. 16,1993)

Abstract Facilitated transport of silver across a bulk liquid membrane was studied in a rotating film (RF) pertraction device. A 0.06 M solution of tri-isobutylphosphine sulfide (TIBPS) (Cyanex” 471x) in noctane was used as a carrier and an aqueous ammonia solution as a stripping phase. The influence of the main process parameters such as temperature, disc rotation velocity and nitric acid concentration was investigated as well as the separation selectivity with respect to other metals such as Cu, Zn and Ni present in the feed solution. It was shown that the RF-pertraction using TIBPS as a carrier was a successful method for selective silver recovery from ita dilute nitrate solutions. Key

words:

liquid membranes; silver extraction; pertraction; tri-isobutylphosphine

Introduction Silver recovery from various solutions and industrial wastes is of great practical significance, since this metal finds broad application in medicine, photography,electronic industry, jewelry,etc. Among the variousmethods of silver recovery, emphasis was laid recently on membrane methods and especially on liquid membranes,due to their selectivityand low operationalcost. Solutetransportacross a liquid membraneis a combinationof extractionand strippingin an integralprocess called pertraction [ 11. Usually two aqueous solutions - the donor D and the acceptor A - are separatedby an organic liquid - the membranephase M, practicallyinsoluble in both aqueous solutions. Due to the favourable thermodynamicconditions created at the D/M interface,the solute is extractedfrom the ‘To whom correspondence should be addressed. 0376-7388/94/$07.00

0 1994 Elsevier Science B.V. All rights

SSDZ0376-7388(93)E0135-7

sulfide

donor solution and transferredinto the membrane liquid.The conditions existingat the M/ A interface enable the stripping of the solute from the membrane liquid and its accumulation in the acceptor solution A. Both emulsionliquid membrane (ELM) and supportedliquidmembrane(SLM) techniques have been used for silverrecoveryfrom various solutions. As carriers, several macrocyclic ethers (DC18C6 [2-41, DKPlW6 [2], 18C6 [5], pyridone and triazole macrocycles [5], Kriptofix@ 22 DD [ 7,8] ) as well as other compounds (tri-isobutylphosphinesulfide [6,9], 8[p- (l’, l’, 3’, 3’-tetramethylbutyl)phenoxy]3,6-dioxaoctanol-1 [8], 16,19-dioxa-13,22-dithiohexatriacontane [ 81 and 2,2’ ,2V-tris(benzothiophenyl) triethylamine [lo] ) have been used. In the last few years, the high selectivityof tri-isobutylphosphine sulfide (TIBPS ) towards silver [ 111 called forth the interest for this reagent.TIBPS dissolvedin isopropylben-

reserved.

138

L. Boyadzhiev, K. Dimitrov / J. Membrane Sci. 86 (1994) 137-143

zene has been used to extract silver from sulfate, chloride or nitrate solutions. In these studies the SLM technique was applied and the solid porous support was PTFE [6] or poly (vinylidene fluoride) [ 9 1. As stripping agent, sodium thiosulphate or ammonia was used. The results obtained reveal very good separation of Ag from Cu, Zn, Pb and Fe. Some drawbacks of the supported liquid membranes limit, however, their application on industrial scale: - the low but appreciable solubility of the membrane liquid in both aqueous solutions leads to its washing out from the support; - the high velocities of the liquids on both sides of the membrane may create pressure differences and therewith pushing out the liquid from the pores of the support. The latter effect becomes more pronounced with the ageing of the polymer matrices and the related structural changes, which are stronger in corrosive media. The above-mentioned drawbacks are avoided in the new rotating film pertraction (RFP) technique, where the membrane liquid is in contact with the A and D solution films adhering to the surfaces of rotating discs. According to the latest studies, the RFP offers a stable operation without side effects, under both continuous and batch regimes [ 12,131. The purpose of the present work was to study the silver recovery from nitrate solutions by means of the RFP technique using TIBPS as a carrier. Experimental Reagents and analytical methods used

Tri-isobutylphosphine sulfide - Cyanex@ 471x (kindly supplied by Cyanamid Canada Inc., Niagara Falls, Ont. ) - with no further purification, was dissolved in n-octane (Fluka, Buchs); ammonia and nitric acid (Riedel-de Haen, Seelze-Hannover ) , &POa,

CU(NO,)~-~H,O andNi(N0,),*6Hz0 (Fluka, Buchs) and Zn(N0,)z*2Hz0 (Reachim, Warsaw), p.a. grade reagents, were used as received. The concentrations of Ag, Cu, Zn and Ni in the aqueous solution were determined by an atomic absorption spectrometer (Perkin-Elmer 3030). The hydrogen ion concentration in the donor phase was measured with a pH-meter (Radelkis OP-211/l). The experiments were carried out at a temperature of 293 K, unless otherwise stated. Equipment and procedure used

Prior to the study of the liquid membrane process, the equilibrium distribution of silver between the aqueous and organic phase was determined, using 100 ml separating funnels for this purpose. The water phase D was an aqueous solution of silver nitrate and the organic phase M a solution of TIBPS in n-octane. When necessary, the pH of the aqueous phase was adjusted by addition of nitric acid. The aqueous and organic phases (each 20 ml) were intensively shaken for 15 min. The initial concentration of silver in the aqueous phase, [ Ag+ I$, was determined prior to phase contact and the equilibrium concentration of silver in the aqueous phase, [Ag+ I,, after complete phase separation. As the initial concentration of silver in the organic phase was zero, the equilibrium concentration in this phase, [Aglorg, was found through the mass balance. Pertraction studies were carried out in a laboratory pertraction device, the general scheme of which is shown in Fig. 1. The apparatus consists of a body (1 ), two sets of discs (4) and separating walls (2 ) . The longitudinal half-wall (3) divides the bottom part of the device into two chambers for the donor and acceptor solution, respectively. In order to prevent intermixing of the D and A solutions, their levels were maintained - 10 mm below the top edge of this

L. Boyadzhieu, K. Dimitrov / J. Membrane Sci. 86 (1994) 137-143

139

Fig. 2. Experiment flow diagram: (1) RF-contactor; (2) peristaltic pumps; (3) heat exchangers; (4) electric drive; (5) speed controller and gearbox; and (6) sampling probles. Fig. 1. Principal scheme of contactor: ( 1) contactor body; (2 ) stage wall, separating donor and acceptor solutions; ( 3 ) longitudinal half-wall separating donor and acceptor solutions; and (4) rotating discs.

half-wall. The two sets of discs, 120 mm in diameter, mounted at a distance of 10 mm on two independent rotating shafts were partially (up to l/3 of the disc diameter) immersed in the D or A aqueous solutions, respectively. The rest of their hydrophilic surfaces were in contact with the membrane liquid M, filling the upper part of the device. The transversal walls (2) divided each chamber in 6 compartments, corresponding to the 6 pairs of discs. The membrane liquid entrained by the rotating discs, circulated in each compartment without leaving it. Holes at the bottom of the walls allowed the aqueous solutions to flow through all six compartments in counter- or co-current mode. The body and the walls were made of Plexiglas which permitted to observe the level of the solutions. The experimental setup arranged for a batch pertraction mode with a recycling of both aqueous phases is shown in Fig. 2. Along with the pertraction device (1)) the scheme also involves two peristaltic pumps (2) for aqueous solutions recycling and two heat exchangers (3) for maintaining constant temperature in the

contactor. The discs were set into motion by a motor (4) supplied with an electronically stabilized rotation controller (5). In this operational mode, the composition of each aqueous solution was practically identical throughout the corresponding flow circuit. Sampling was made by means of the sampling probes (6). The transfer of silver was studied using the following solutions: - donor phase D: 0.005 M aqueous solution of silver nitrate, pH 1.2,290 ml; -membrane phase M: 0.06 M solution of TIBPS in n-octane, 1800 ml; - acceptor phase A: 1.0 M aqueous solution of ammonia, 290 ml. Liquid recycling also assisted in avoiding the dead zones in the device and in homogenizing the aqueous solution D and A, and thus averaging the aliquots taken for analysis. It should be mentioned that the low but appreciable solubility of ammonia in the membrane liquid caused a noticeable transfer of NH, into the opposite direction. This was, however, of no significance for the process, since the pH of the donor solution was continuously controlled and adjusted to the initial value, if necessary. The silver concentration in the aqueous phases was directly measured and the organic phase from the silver mass balance.

L. Boyadzhiev, K. Dimitrov /J. Membrane Sci. 86 (1994) 137-143

140

Results and discussion

5

Equilibrium studies

&---T---

The distribution coefficient of silver, mAg, is defined as: mAg

=

silver in membrane phase silver in aqueous phase

(1)

Along with the Ag+ cations, the reagent also extracts NO, anions - so ion exchange does not occur. The formation of the following complexes is assumed [ 91: (Ag+ ), +n(TIBPS),,+

(NO, ),

= (Ag(TIBPS),NO,),,,

P3 E 2 2-

l0 0

5

10 [Ag’]:

15 , mmol

20

25

3(

II

Fig. 3. Effect of solute concentration on silver distribution coefficient (m&). [HNO,],=O.O63M, [TIBPS],=0.15 M.

(2)

The value of n can be experimentally evaluated from the equilibrium constant: K=

[MTIBW,NW,,/(

k+ lw

- [NO, lw-[TIBPSI:,,)

(3)

By inserting: mh=

[Ag(TIBPS),NO,l.,,/[Ag+l,

(4)

into eqn. (3 ) one obtains: lgrnA,=lgK+lg

10-6 10-5 10-4 la3

162 I$ I

lHN&1,, mall

[NO;],

+ nalg [TIBPS].,,

01

(5)

n can be determined from the slope of this straight line. Figure 3 shows lg mAe as a function of the silver concentration in the aqueous phase. As it is seen, the distribution coefficient mAg is slightly affected by this concentration. The effect of nitric acid concentration on the equilibrium is shown in Fig. 4. The distribution coefficient mAg increases at higher nitric acid concentrations, which is one of the reasons for carrying out the pertraction process at low pH. Figure 5 illustrates the effect of TIBPS concentration in the organic phase on the distribution coefficient of silver. The slopes n of the

Fig. 4. Effect of nitric acid concentration on silver dietribution coefficient (m& at [TIBPS],,=O.lB (O), 0.06 (0) and0.03 M (a).

straight lines are close to 2, namely n = 1.82 at pH 1.2 and n=2.27 at pH 2.2. Therefore, eqn. (2 ) can be rewritten as: (Ag+),+2(TIBPS),,+(NO,), = (Ag(TIBPS)zNOs).,

(2a)

as suggested earlier by Baba et al. [ 151. From the data obtained one can calculate the value of the equilibrium constant K. In this case it was found that K=5.1*10-6 (13/mol)3. Since the solubility of the silver complex Ag( TIBPS)zN03 in the organic phase used is very low, several recommended [ 141 modifiers

141

L. Boyadzhiev, K. Dimitrov /J. Membrane Sci. 86 (1994) 137-143

0.5

-L

-1

lg [TIBPSI,,.rmlll

1.0

1.5

2.5 2.0 t.h

3.0

3.5

Fig. 6. Variation of dimensionless silver content (G/Go ) in donor (0 ), membrane (0) and acceptor (0 ) solutions versus time (t). [HN0,],=0.063 M; T=291 K; ~=20 rpm.

Fig. 5. Silver distribution coefficient ( mk) versus carrier concentration in the organic membrane ( [TIBPS],) at pH 1.2 (0) and2.2 (0).

were tested. The best results were obtained when 20% p-nonylphenol (with respect to the n-octane) was added. In our mass transfer studies, however,we did not use modifiers becausethe silverconcentrationsin the donor solution were low and the rate of the extraction step was ratherhigh.

008

00

20.6 5

,o b 5

0.4

Pertraction studies The pertraction study results are shown in Figs. 6-8. Figure6 illustratesthe typical changesin silver content in all threephases versustime. It is evident,that the extraction rate, includingthe formationrateof the complexAg ( TIE3PS)zN03 is very high: more than 95% of the silver is transferredinto the membraneduringthe first 15 min. Fast recovery of silver can be effected with low carrier concentrations in the mem-

0.2

0 0

0.5

1.0

t.h

1.5

2,o

2.5

Fig. 7. Effect of temperature (‘I’) on the rates of silver extraction (open symbols) and membrane phase stripping (closed symbols) at 291 (0), 301 (A) and 311 K (Cl). [HNOs],=0.063 M, w=20 rpm.

142

L. Boyadzhiev, K. Dimitrov /J. Membrane Sci. 86 (1994) 137-143

is a slower process. The effect of temperature on the rate of silver pertraction is shown in Fig. 7. Higher temperature is found to favor the process, which could be attributed to the lower liquid viscosities and therefore to higher mass transfer coefficients or/and to the higher rate constants of the chemical reactions. The explanation of this effect will be an aim of our further studies. Figure 8 presents silver recovery as a function of the velocity of disc rotation. The rate of mass transfer increases with the rotation speed of the discs. To avoid aqueous drop formation on the disc periphery, which could deteriorate the process efficiency, the rotation velocity was limited up to 25 rpm. At these conditions the process was stable with no noticeable changes of disc wettability or pertraction efficiency deterioration.

0.8

02

0 0

0.5

1.5

1.0

2.0

2:5

th

Separation of silver from other metals

Fig. 8. Influence of disc rotation speed (w ) on the rates of silver extraction (open symbols) and silver back extraction(closedsymbols)atw=15 (0),2O(A)and25rpm (0). [HNO,],=O.O63M; T=297K.

Of practical interest is the case when the donor solution contains other metal ions along with silver. Due to the expressed selectivity of TIBPS towards silver [ 111, the latter is extracted into the membrane phase, the other metal ions remaining in the donor solution. The complex Ag (TIBPS ) pNOs reacts with ammonia, according eqn. (6)) in the acceptor solution, forming an silver-ammonia complex insoluble in the membrane M.

brane phase, which permits to use minimum amounts of the reagent. Silver stripping into the acceptor phase due to a silver-ammonia complex formation, Ag(TIBPS)2N03

+3NH3 +H20

-+Ag(NHB)20H+2TIBPS+NH4NOB

(6)

TABLE 1 Separation of silver from other metals Metal

Ag cu Zn Ni

Donor solution (mw

Acceptor solution (m&f)

t=O (h)

t=O (h)

t=2 (h)

t=4 (h)

12.0 107.0 88.7 78.3

0 0 0 0

3.06 0.10 0.17 0.04

7.18 0.17 0.31 0.99

143

L. Boyadzhiev, K. Dimitrov /J. Membrane Sci. 86 (1994) 137-143

The experiments were carried out according to the scheme shown in Fig. 2. The donor solution of pH 1.2 contained silver, copper, zinc and nickel cations and nitrate anions. The results are summarized in Table 1. It is seen that silver is much faster extracted, whereas the pertraction of other metal ions is insignificant. Hence, RF-pertraction could be successfully applied to the separation of silver from the accompanying metals copper, zinc and nickel.

2

3

4

5

Conclusions The obtained results support the assumption silver concentrations low that at ,,,=0.005-0.02 M), its reaction with ( [Ag+ I” TIBPS takes place according to eqn. (2a). The distribution coefficient mAs increases with nitric acid concentration in the donor solution and the carrier concentrations in the membrane liquid. Higher temperature and higher disc rotation velocities favor the silver pertraction. It is shown that silver can be successfully separated from the accompanying metals as copper, zinc and nickel by means of RF-pertraction using as a membrane phase a dilute (0.06 M) solution of TIBPS in n-octane.

6

7

8

9

10 11

Acknowledgement This work was partially supported by the Ministry of Education and Science under research contract No. X-11. References 1

L. Boyadzhiev, Liquid pertraction or liquid membranes - state of the art, Sep. Sci. Technol., 25 (1990) 187.

12

13

14 15

R.M. Izatt, D.V. Dearden, P.R. Brown, J.S. Bradshow, J.D. Lamb and J.J. Christensen, Cation fluxes from binary Ag+-Mn+ mixtures in a HzO-CHCl,Hz0 liquid membrane system containing a series of macrocyclic ligand carriers, J. Am. Chem. Sot., 105 (1983) 1785. R.M. Izatt, G.A. Clark and J.J. Christensen, Transport of AgB?-, in an emulsion liquid membrane using M”+-DC18C6 carriers (M”+=Li+, Na+, K+, or Mg?+), J. Membrane Sci., 24 (1985) 1. R.M. Izatt, G.A. Clark and J.J. Christensen, Transport of AgBr; , PdB