art%3A10.1007%2FBF02654031.pdf

Application of Ultrasound in Extractive Metallurgy: Sonochemical Extraction of Nickel

BATRIC PESIC and TAILI ZHOU

The importance of ultrasound in solvent extraction was examined by studying solvent extraction of nickel with Lix 65N and Lix 70 extractants. The studied parameters were ultrasonic energy and frequency, pH, temperature, and organic and aqueous solution composition. The stability of extractants under the influence of ultrasound was also examined. It was found that ultrasound had a significant importance, because it increased the extraction rates four- to sevenfold. The effect of ultrasound was physical, i .e. , to increase the surface area. Ultrasonic energy consumption was also evaluated. The consumption was high, but it can be significantly reduced by the appropriate method of application. A novel solvent extraction method, extraction-in-pipe, was also proposed.

I . I N T R O D U C T I O N

U L T R A S O U N D assists heterogenous processes such as flotation, r~l leaching, ~zj electrometallurgy, t3~ filter- ing, I41 drying, ~51 and others. It is believed that cavitation (bubble formation and implosion), microstreaming, ra- diation pressure, acoustic pressure, and degassing ac- count for ultrasound's advantages.

Solvent extraction is another typical heterogenous system that involves two distinct and immiscible phases. In such a system, ultrasound, because of its compression and expansion, can tear the liquid droplets, increasing the contact area between the immiscible organic and aqueous phases through emulsion and dramatically increasing the rate of extraction. In addition to its emul- sifying effect, ultrasonic energy further increases solvent extraction through (1) enhanced local agitation, (2) in- duced-circulation currents, (3) prolate-oblate oscillations inside the liquid drop, and (4) the removal of stagnant liquid near the interface between the organic and aqueous phases. [6~

Most of the studies on solvent extraction with ultrasound have been done on nonmetallic liquid-liquid extraction systems, those controlled by mass transfer. Only a few studies have been done on the solvent extraction of metals, f6,7,8] However, many systems for solvent extraction of metal are controlled by chemical reaction mechanisms that may also benefit from ultrasound.

To study the role of ultrasonic energy in solvent extraction, two typical chelating solvent extraction systems were selected, one acidic and the other strongly basic. The basic system was used in the extraction of gallium with Kelex 100, as reported earlier by Pesic and Z h o u . [9]

The present work studies the role of ultrasound on solvent extraction of metals from acidic solutions. The solvent extraction of nickel with hydroxyoxime extractants

BATRIC PESIC, Associate Professor of Metallurgy, and TAILI ZHOU, Research Associate, are with the College of Mines, University of Idaho, Moscow, ID 83843-4199.

Manuscript submitted March 11, 1991.

was selected for study because of nickel 's slow rate of extraction.

The hydroxyoxime-carboxylic acid system for the se- lective extraction of nickel from cobalt in acidic sulfate solutions was first recommended by Flett and West. ~t~ They found that when carboxylic acid was added to c~-hydroxyoxime (Lix 63), a synergistic effect occurred, increasing the yield of nickel significantly. However, the rate of extraction was so slow that it took about 3 hours to reach equilibrium, a phenomenon that may be linked to the presence of carboxylic acid at the interface, as indicated by the interracial tension determination.[~ L,~2,13] However, because of the complicated extraction mech- anism, the slow extraction rate of nickel was not fully explained. Several ways to increase the extraction rate have been suggested. Nyman and Hummelstedt ~41 reported that the type of diluent and its temperature con- siderably influenced the extraction rate. They suggested the addition of a DNNSA (dinonylnaphthalene sulfonic acid) surfactant into the systems Lix 65N + Versatic 911 or Lix 70 + Versatic 911 to accelerate the rate of nickel extraction. Hummetstedt et al.,f~5~ who studied the synergistic phenomena between Versatic 911 and Kelex 100, recommended raising the temperature to increase the extraction rate. Flett et a / . [161 proved that adding Versatic 911 to Kelex 100 can induce an interaction between these two extractants that prevents the oxidation and extraction of Co(lI) and increases the separation of nickel from cobalt.

II. EXPERIMENTAL

A. Reagents

The following reagents were used: Lix 65N and Lix 70 (Henkel Corporation, lot no. 82B265010 and 8J16029, respectively), lauric acid (Eastman Kodak Co.), and dinonylnaphthalene sulfonic acid (Pfaltz and Bauer Inc.). Other chemicals used were Kermac 470B (Triangle Refineries Co.), Escaid 200 and Aromatic 150 (Exxon Co.), and NiSOa '6H20 and KNO3 (analytical reagent grade).

METALLURGICAL TRANSACTIONS B VOLUME 23B, FEBRUARY 1992--13

B. Apparatus and Procedure

A baffled reactor (inner diameter, 71 mm; height, 85 mm) was specially designed to withstand vigorous stirring by a turbine-type TEFLON* stirrer (diameter,

*TEFLON is a trademark of E.I. Du Pont de Nemours & Co., Inc., Wilmington, DE.

25 mm; height, 5.5 mm). Two TEFLON baffles, 13 x 80 mm, were mounted near the wall of the reactor.

The reactor cover was designed to accept an ultrasonic horn, a mechanical stirrer, and a sampler. Ultrasound was produced with a Branson Model 184V ultrasonic generator and a Branson Model 102 convertor. A 1-in. titanium horn was used to deliver ultrasonic energy at a frequency of 20 KHz into the solvent extraction system. The standard ultrasonic energy output was 47 W. Con- stant temperature conditions were maintained by circu- lating water cooled by an immersion cooler. The schematic representation of the experimental apparatus is given in Figure 1.

The (nickel-free) organic and aqueous phases were transferred into the reactor, and the ultrasound was turned on. After the equilibrium temperature was reached, 1 ml of nickel stock solution was added. Samples of the solution were taken at fixed intervals and immediately cen- trifuged, and the aqueous phase was analyzed for nickel by atomic absorption. The rate of nickel solvent extraction was determined from the rate of change of nickel concentration in the aqueous phase.

The standard experimental conditions were as follows. The organic phase consisted of 100 ml of 10 pct (v /v) Lix 65N (or Lix 70) + 0.1 M lauric acid dissolved in Kermac 470B. The aqueous phase consisted of 100 ml

of 30 mg Ni2+/1 and 1 M KNO3. The standard temperature was 25 ~ and the stirring speed was 2000 min -1.

III. RESULTS AND DISCUSSION

A. Effect of Stirring Speed

To simplify the study, it was essential to run experiments in a kinetically limited region. Figure 2 indicates that when the stirring speed was higher than 2000 rain-1 the extraction rate for both the Lix 65N and the Lix 7lJ systems was independent of the stirring speed. There- fore, a stirring speed of 2000 min- ~ was used in all other experiments, both with and without ultrasound.

When ultrasound was used, the sonic energy formed a tremendous number of tiny droplets, whose extreme turbulence and direction of propagation were visible. Extraction rates with ultrasound were much greater than those produced by mechanical stirring. Even though the contribution of mechanical stirring in sonochemical extraction was small, it was still used to ensure a uniform distribution of phases in the reactor by bringing the organic and aqueous phases under the ultrasonic horn.

B. Effect of pH

Figures 3 and 4 illustrate the effects of pH on nickel extraction by the Lix 65N and Lix 70 systems, respectively, both with and without ultrasound. The pH rep- resents initial values, ranging from 4 to 7. The extraction rate increased with the increase of initial pH until pH = 6.02, above which pH did not have further effect.

The apparent forward reaction rate constant (kf) was calculated from the slope of the line in the plot [(Co - Ce)/Co] In [(Co - C~)/(C, - C~)] vs time, the method

Fig. 1 - - S c h e m a t i c representation of the experimental apparatus for solvent extraction with ultrasound.

14--VOLUME 23B, FEBRUARY 1992 METALLURGICAL TRANSACTIONS B

100 I RPM I [] �9 1000 o �9 1500 / o �9 2 0 0 0 & �9 2 5 0 0

8 0 -1 * * with ultrasound / (at 2000 rpm)

x~ 40

U.I

20

0 " solid, I.IxT0 s y s t e m

0 20 40 60 80 100 120 140 160 180

TIME, rain

Fig. 2 - - S t i r r i n g rate effect on solvent extraction of nickel with Lix 65N-lauric acid-Kermac 470B and Lix 70-1auric acid-Kermac 470B. Organic phase: 10 pct (v/v) Lix 65N or Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 rag/1 NiZ+; 1 M KNO3; pH~: = 5.05. Ratio of O /A: 100 ml /100 ml; T = 25 ~ ultrasound: 20 KHz, 47 W.

developed by Flett et al. ,[12] where C = the nickel concentration in the aqueous phase, inferior t = the extraction time, inferior 0 = the initial state, and inferior e = the equilibrium state. The solvent extraction rate constants, kl, for each extraction system, are presented in tables in corresponding figures (Figures 3 and 4).

A comparison of the rates of conventional and ultrasound extractions shows that ultrasound increases the extraction rate of nickel by as much as 4 to 7 times. The extraction rates in the Lix 65N system were faster than those in the Lix 70 system, which required about 1 hour to reach equilibrium even with ultrasound. However, the uptake of nickel was higher with the Lix 70 system than with the Lix 65 system. The slower kinetic performance of the Lix 70 system was caused by steric effects. The

100 LIX70

I@I 8O

Iii }- 6O r Q zx

I-- 4O X ILl no ultrasound with ultrasound �9 -- pH kf p H kf und

Z 20 o 4.02 1.43 �9 4.02 7.83 zx 5.05 2.67 �9 5.05 10.1 [] 6.02 2.85 �9 6.02 12.0Z0 o 7.03 2.79 �9 7.03 13.0

0 0 20 40 60 80 100 120 140 160 180 200

TIME, rain Fig. 4 - - E f f e c t of pH on nickel extraction with Lix 70-1auric acid-Kermac 470B. Organic phase: 10 pct (v /v) Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 mg/l Ni2"; 1 M KNO3. Ratio of O /A: 100 ml /100 nal; T = 25 ~ stirring rate = 2000 rpm; ultrasound: 20 KHz, 47 W.

bulky chlorine atom at the ortho position of the phenolic hydroxyl group in the molecule of Lix 70 diminished the possibility of a complexation reaction, causing the slower extraction rates. On the other hand, the inductive effect of the chlorine atom, because of its electronegativity, increased the dissociation of the hydrogen ion in the phenolic hydroxyl group and increased the extraction capacity of the extractant.

C. Effect of Temperature

The effect of temperature on the solvent extraction of nickel was studied in the range of 25 ~ to 50 ~ Both extraction systems, Lix 65N (Figure 5) and Lix 70 (Figure 6) were examined. According to the results presented in Figures 5 and 6, temperature had a positive

50

t LIX65N

p. k, -i/7,/?' o 402 322 ~ f f zx 5.05 3.46

[] 6.02 3.79 o 7.02 4.27

0 20 40 60

TIME, rain

~4~ 4 0

d LU !-- 30 O

I--" 20 X till ~

Z 10

0

with ultrasound

pH kf �9 4.02 13.7 �9 5.05 16.5 �9 6.02 19.8 �9 7.02 33.4

80 100 120

Fig. 3 - - E f f e c t of pH on nickel extraction with Lix 65N-lauric acid-Kermac 470B. Organic phase: 10 pct (v/v) Lix 65N; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 rng/l Ni-~+; 1 M KNO3. Ratio of O/A: 100 ml /100 ml; T = 25 ~ stirring rate: 2000 rpm; ultrasound: 20 KHz, 47W.

60 1 LIX65N

/

40

o o

20 tU nd

I t / / 6 T c/ k, r l ~ k, I~r o 25 3.46 �9 25 16.5 ~ / zx 40 5.78 �9 40 27.1 ,L F" m 50 13.70 �9 50 37.1

O~ T J

0 20 40 60 80 100

TIME, rnin Fig. 5 - - T e m p e r a t u r e effect on nickel extraction with Lix 65N-lauric acid-Kermac 470B. Organic phase: 10 pct (v/v) Lix 65N; 0.1 M lauric acid; Kermac 470]3. Aqueous phase: 30 m g / l Ni2+; 1 M KNO3; pH~.~, = 5.05. Ratio of O /A: 100 ml /100 ml; stirring rate = 2000 rpm; ultrasound: 20 KHz, 47 W.

METALLURGICAL TRANSACTIONS B VOLUME 23B, FEBRUARY 1992-- 15

100

1~ 80

60

40

LII

@ / o 25 2.67 �9 25 10.1 t, 40 7.92 �9 40 23.6 t3 50 13.30 �9 50 49,3

O l r T l T t t I I

0 20 40 60 80 100 120 140 160

TIME, rain

Fig. 6--Temperature effect on nickel extraction with Lix 70-1auric acid-Kermac 470B. Organic phase: 10 pct (v/v) Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 mg/l Ni2§ 1 M KNO3; pH~, = 5.05. Ratio of O/A: 100 ml/100 ml; stirring rate = 2000 rpm; ultrasound: 20 KHz, 47 W.

effect on the solvent extract ion rates o f nickel in both extract ion systems and under both condi t ions , with and without ul t rasound. This conclus ion can also be made f rom the ca lcula ted extract ion rate constants , ks, which are presented in tables in the cor responding figures (Figures 5 and 6). These extract ion rate constants were used to calculate the act ivat ion energies of extract ion. For extract ion with Lix 65N, the act ivat ion energy without ul trasound was 42.4 k J / m o l e and the act ivat ion energy with ul t rasound was 35.8 k J / m o l e . For extract ion with Lix 70 without and with ul trasound, the act ivat ion energies were 52.1 k J / m o l e and 50.1 k J / m o l e , respec- t ively. These values indica ted that the solvent extract ion of nickel by both Lix 65N and Lix 70 was cont ro l led by a chemical react ion.

The high act ivat ion energy for the sonochemica l extract ion o f nickel with the Lix reagents presents a strik- ing contrast to the zero act ivat ion energy obta ined during the sonochemical extraction of gallium with Kelex 100.r91 This phenomenon can be expla ined by the difference in the species and the compos i t ion o f the two extract ion systems. Gal l ium, in a strong basic medium, is present as a gal late ion, Ga(OH)4- , which is surrounded with a shell of water molecules. According to Zhou and Pesic, I~7~ the solvent extract ion of ga l l ium with Kelex 100 is con-

trol led by the rate of dehydrat ion. Ultrasound can over- ride the contr ibut ion of thermal energy to dehydra t ion and the di f fus ion of gal late ions. On the other hand, the rate at which water is r emoved from the shell of molecules surrounding the nickel ion is several thousand times faster, but this step is unimportant during the solvent extract ion of nickel . The in termediate complex of nickel and carboxyl ic acid reacts s lowly with the molecule of hyd roxyox ime , and this react ion rate controls the overal l solvent extract ion ra teJ 12]

The overal l react ion of nickel extract ion can be rep- resented by Eq. [1]:

Ni 2~ + 2HOXorg

+ 2RHorg Kq NiOX2(RH)2org + 2H + [1]

for which the concentra t ion equi l ibr ium constant , Kex, can be ca lcula ted from the mass action law, Eq. [2]:

K~x = [H+]2[NiOX2(RH)i]org/[Ni 2+] [HOX]2org[HR]2org

[2]

where [NiOX2(RH)2]org is the concentra t ion of nickel in the organic phase, [HOX]o~g is the concentrat ion o f hy- d roxyox ime , and [RH]o,g is the concentrat ion of lauric acid.

The value o f K~., can be ca lcula ted from the measured pH values and concentrat ions of nickel in the organic and aqueous phases. The concentrat ions of oximes and lauric acid were assumed to be constant , because they were much higher than the concentra t ion of nickel. The equi l ibr ium constants as a function o f tempera ture are given in Table I.

The Rex values in Table I were used to calculate the enthalpy change o f nickel extract ion by the V a n ' t Hof f ' s equat ion, In Kex = -AH~ + C. Table II shows the calcula ted A H ~ values o f nickel extract ion.

The posi t ive enthalpy changes character ized the nickel extract ion react ions with the hyd roxyox ime extractants as endothermic .

D. Effect of Diluents

Kermac 470B was used as a s tandard di luent in this study. Because it contains about 17.3 pct aromat ic compounds , the contr ibut ion of such compounds to the solvent extract ion o f nickel was examined . The use of toluene, a representat ive aromatic compound , as a diluent during the solvent extract ion of nickel with Lix 70

Table I. Effect of Temperature on the Equilibrium Constant of Nickel Extraction, K,~, Calculated from Data in Figures 5 and 6

Temperature (~

Lix 65N/lauric acid/Kermac 470B

Without Ultrasound

With Ultrasound

Lix 70/lauric acid/Kermac 470B

Without Ultrasound

With Ultrasound

25 40 50

1.16 • 10 -5 2.40 x 10 -5 5.53 • t0 -5

1.12 x 10 -s 3.03 x 10 -5 5.47 • 10 -5

2.73 • 10 -5 6.63 x 10 -5 1.05 x 10 -4

3.44 x 10 5 8.47 x 10 -5 1.10 x 10 -4


Table II. Enthalpy Change for Nickel 0.6 Extraction With and Without Ultrasound

&H, KJ/mole

Lix 65N/lauric acid/Kermac 470B

Lix 70/lauric acid/Kennac 470B

Without With Without With Ultrasound Ultrasound Ultrasound Ultrasound

48.8 50.8 43.4 38.1

decreased extraction rates (Figure 7). The uptake, however, was significantly higher. For example, with ultrasound, the equilibrium uptake of nickel with toluene was 90 pct, as compared to the equilibrium uptake of 63 pct with Kermac 470B.

The slower extraction rates are explained by the strong solvency of the aromatic compound. The aromatic diluent also causes the extractant molecule (Lix 70) to be predominantly inside the droplets of organic phase instead of at the interface. The increased uptake is explained by the increased solubility of the chelated complex in the aromatic solvent.

open: No ultrasound solid: with ultrasound

. 0.5 L ix70 t i t O3

-1- 0.4

o 0.3 z

0.2

0 65N ,,, ._= 0.1 a -

Z O I I I I I

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Ni in AQUEOUS PHASE, g/I

Fig. 8 - -Ext rac t ion isotherms of nickel extraction with Lix 65N-lauric acid-Kermac 470B and Lix 70-1auric acid-Kermac 470B. Organic phase: 10 pct (v /v) Lix 65N or Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: Ni2+; 1 M KNO3; pHoq = 3.2 to 3.3 (for Lix 65N); 2.9 to 3.1 (for Lix 70). T = 25 ~ stirring rate = 2000 rpm; ultrasound: 20 KHz, 32.5 W.

E. Effect of Nickel Concentration

The effect of nickel concentration was examined by comparing the results at two different initial nickel con- centrations, 30 and 300 mg Ni /L . In the absence of ultrasound, the extraction rate, with Lix 70, from solution containing 300 m g / L of nickel was about 2.5 times lower than the rate from 30 mg Ni /L. Application of ultrasound to the system with 300 m g / L of nickel increased the rate of extraction by about 3 times. Ultra- sound did not enhance the capacity of the extractant in the system studied.

F. Effect of Ultrasound on Extraction Isotherms

The equilibrium loading isotherms for solvent extraction of nickel with Lix 65N and Lix 70 are shown in

Figure 8. The open symbols represent the solvent extraction results without ultrasound, and the closed symbols represent the sonochemical extraction results. No effect of ultrasound on the equilibrium of extraction was demonstrated.

G. Extraction of Cobalt and Separation of Cobalt and Nickel

The effect of ultrasound on the solvent extraction of cobalt was studied in the Lix 70 + lauric acid + Kermac 470B system, and the results from pH variation are presented on Figure 9. In this system, the extraction rate of cobalt, like that of nickel, was slow, failing to

100

I - L,• ]

8O 8o

Soo o [] , ,

~x 4o ,~ 40 u.I Ill

~ 20

20 ~ open: no ultrasound i ~ �9 Kermac470Btl ~[ solld', wlth ultrasound In �9 Toluene II o

0 l I I I I I

0 60 120 180 240 300

TIME, rain

Fig. 7 - - D i l u e n t effect on solvent extraction of nickel with Lix 70-1auric acid. Organic phase: 10 pct (v /v) Lix 70; 0.1 M lauric acid. Aqueous phase: 30 mg/1 Ni 2§ 1 M KNO3, pHinit = 5.05. Ratio of O /A: 100 ml /100 ml; stirring rate = 2000 rpm; ultrasound: 20 KHz, 47 W.

pHin a open: no ultrasound LIX70 o �9 4 0 4 solid: with ultrasound

t, �9 5105

I I I

0 60 120 180 240

TIME, rain

Fig. 9 - - E f f e c t of pH on cobalt extraction with Lix 70-1auric acid-Kermac 470B. Organic phase: 10 pct (v /v) Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 mg/1 Co2+; 1 M KNO3; pHi,, = 5.05, Ratio of O /A: 100 ml /100 ml; T = 25 ~ stirring rate = 2000 rpm; ultrasound: 20KHz, 47 W.


reach equilibrium even after 3 hours. Ultrasound substantially increased the cobalt extraction rate, so that an increase in extraction uptake from about 30 pct in the regular system to about 70 pct with ultrasound was achieved in 3 hours. However, cobalt stripping from the organic phase loaded with ultrasound was difficult, es- pecially when loading was performed at a higher initial pH. Cobalt stripping was possible only with the addition of a reductant, such as sodium sulfite. In extraction systems without ultrasound, the cobalt(II) extracted with substituted hydroxyoxime alone can be oxidized to a Co(Ill) state. The organic phase in which cobalt(II) is converted to Co(Ill) becomes very difficult to strip. 118) The addition of a carboxylic acid as a modifier dimin- ishes the oxidation of cobalt in the organic phase. The interfacial activity of lauric acid slows down the oxidation of cobalt by minimizing the interfacial oxidation reaction.

During solvent extraction from solutions containing a mixture of Ni and Co, the presence of nickel suppressed cobalt extraction. Figure 10 shows that without ultrasound, the equilibrium of cobalt extraction, 12.8 pct, was quickly achieved (after 10 minutes), while the extraction of nickel continued throughout the experiment, increasing the separation of nickel from cobalt. With ultrasound, the situation was reversed (Figure 11). Cobalt extraction continued throughout the experiment, while nickel extraction reached equilibrium, 61.4 pct, after only 30 minutes. Comparison of the cobalt extraction results presented in Figures 9 through 11 indicates that without ultrasound the reaction equilibrium state for cobalt may not be completely reached and that the obtained plateau is only the result of an extremely slow reaction. When ultrasound was added, the extraction of cobalt corresponding to the plateau in Figure 11 continued toward the true equilibrium position. Unfortunately, the short duration of the experiment prevented the true equilibrium position of cobalt from being reached. The cobalt extraction results without ultrasound slowly approach the corresponding results with ultrasound.

H. Effect of Surfactants Surfactants play an important role in the kinetics of

solvent extraction of metals. The addition of DNNSA surfactant, even at a low concentration, increased the extraction rate of nickel in the hydroxyoxime/lauric acid/ Kermac 470B system through enhancement of the interfacial area between the two immiscible phases during their mixing. Also, the active sulfonic acid molecule has a preferential position against other extractant molecules at the interface and can itself take up nickel. [19]

Table III, which compares the effects of DNNSA surfactant and ultrasound in increasing extraction rate by the formation of microemulsions, shows that ultrasound had a stronger effect than the addition of DNNSA. In general, oil and water systems require an appropriate surfactant and cosurfactant to form an emulsion.

Bauer and Komornicki [2~ used sodium laurylsulfate and n-pentanol instead of DNNSA to produce a microemul- sion in the Lix 70/Versatic 911/kerosene system, ob- taining a very fast nickel extraction kinetic. These surfactants were not compared with the ultrasound in this study.

100

- 80

u.I I - 0 60 < n" I - X U.I 40

0

0 . 20

L,X

0 20 40 60 80 100 120 140

TIME. rain

10 r./) i 'n "lg

8 z

-irl

6 0 --I 0

4 Z B.

2 o 3 0

o o 160

Fig. 10 - -So lven t extraction of Ni and Co with Lix 70-1auric acid-Kermac 470B in the absence of ultrasound. Organic phase: 10 pct (v/v) Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 m g / l Co 2§ and 30 mg/1 Ni2"; 1 M KNO3; pHio~, = 5.05. Ratio of O / A = 100 ml /100 ml, T = 25 ~ stirring rate = 2000 rpm. SF = {[Ni]0/[Ni]a}/{[Co]o/[Co]a}.

100 1 ux70 s

," / d8~ i .z

60 2

40 Z

" - 0 , . . . . . . 1;0 ~176 0 20 40 60 80 100 120 1

TIME, rain

Fig. l l - - S o l v e n t extraction of Ni and Co with Lix70-1auric acid-Kermac 470B in the presence of ultrasound. Organic phase: 10 pct (v/v) Lix 70; 0.1 M lauric acid; Kermac 470B. Aqueous phase: 30 m g / l Co 2+ and 30 m g / l Ni2+; 1 M KNO3; pHi., = 5.05. Ratio of O /A: 100 rnl/100 ml; T = 25 ~ stirring rate = 2000 rprn; ultrasound: 20 KHz, 47 W.

Table III. C o m p a r i s o n o f the Effects o f D N N S A as a Sur fac tant and U l t r a s o u n d on Nicke l Extrac t ion

Time to Equilibrium

(Min)

Ultrasound Surfactant Lix 65N Lix 70 (47 W/cm 2) (mole/l) System System

No No 60 150 Yes No 10 60 No 0.005M DNNSA 30 140 Yes 0.005M DNNSA - - 60

Organic: 10 pct (vol.) hydroxyoxime/0.1 M lauric ac id/Kermac 470B. Aqueous: 30 mg Ni2"/l; 1 M KNO3; pH~n~ = 5.05. O / A ratio = 100 ml /100 ml; T = 25 ~ stirring = 2000 m i n i .


I. Stability of the Extractants

Two tests were performed to detemine whether the powerful cavitation effects produced by ultrasound can degrade the extractant molecules. In the first test of the stability of the extractant molecules, a dried and undi- luted sample of Lix 65N was continuously irradiated for 6 hours at 60 ~ with ultrasound at a 47 W intensity. Samples of Lix 65N taken before and after irradiation were inspected with infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. Identical IR and NMR spectra were obtained, indicating that Lix 65N did not undergo any change.

In the second test, multiple cycles of loading (under standard conditions) and stripping (2N H 2 8 0 4 , T = 25 ~ time = 15 minutes) of the organic phase were performed. If ultrasound degraded the stability of the extractant molecule, then the extraction of nickel would have deteriorated with the number of cycles. The almost identical results (about 27.8 pct extraction) in each of five multiple-cycle tests performed with the Lix 65N/lauric acid/Kermac 470B system indicated that the Lix 65N molecule did not undergo any change.

J. Effect of Ultrasonic Frequency

The frequency of ultrasound is an important parameter but is also the most difficult to study because of the lack of equipment with various frequencies. Most of the available equipment has a low, 20 to 40 KHz, frequency, because its intended use requires strong cavitation effects.

To study the effect of high-frequency ultrasound on solvent extraction, a separate reactor was designed. A 2 MHz ultrasonic transducer and the electronic compo- nents were obtained by dismantling an ultrasonic humid- ifier. The 1-in. transducer was mounted on the bottom of a 200-ml reactor made of an acrylic plastic. Experi- ments were performed by placing 100 ml of organic phase and 100 ml of aqueous phase into the reactor and then turning on the ultrasonic power.

High-frequency ultrasound did not have enough power to cause any mixing of organic and aqueous phases, re- sulting in very poor extraction of nickel with both Lix 65N and Lix 70. The absence of cavitation was responsible for the poor performance of high-frequency ultrasound, since cavitation effects can be produced only at low frequencies, most intensely in the 20 to 40 KHz range.

K. Energy Consumption

All of the described experiments took place in the continuous presence of ultrasound. In such batch applica- tions, the total energy consumed was prohibitively high and could not be offset even by the substantially lower extraction time. However, it was found that continuous ultrasound was not required for the extraction of nickel. When ultrasound was briefly applied, then briefly dis- continued, and these cycles repeated, no deterioration of solvent extraction was found. This phenomenon can be explained by the microdroplets formed in the presence of ultrasound, which did not disappear immediately after the ultrasound was turned off. The interfacial area produced allowed extraction to continue even in the absence

of ultrasound. When a significant number of droplets co- alesced, ultrasound was again required to form a new batch of droplets and to continue extraction.

These observations prompted an examination of the effect of intermittent ultrasound on solvent extraction of nickel. The intermittent study was characterized by the time when the ultrasound was "on" (T) and "off" (To). The total on plus off time was fixed at 5 seconds; thus, the frequency of on and off cycles was fixed to 12 per minute. The ultrasound generator was turned on and off by a specially built electronic controller. The total energy consumed by the ultrasonic apparatus during extraction was measured by a watt meter (the watt meter was placed in line between the wall outlet and ultrasonic generator). The intensity of introduced ultrasound was also varied. The results (Table IV) show that energy consumption was lowest when the on time was 0.25 seconds, the off time was 4.75 seconds, and the ultrasonic input energy was 61 W. Under these conditions, the total consumed energy was 2.66 W per hour. It is important to note that the on time can be substantially decreased by increasing the intensity of ultrasound, with an overall decrease in energy consumption. Even though a higher intensity ultrasound was used, the shorter on time reduced energy consumption. The higher intensity ultrasound apparently forms more droplets, and probably smaller droplets, which take a longer time to coalesce, lengthening the off time.

When a similar study was performed with Lix 70, it was found that intermittent ultrasound was not as effec- tive as with Lix 65N. The stability of droplets with Lix 70 was much lower than with Lix 65N, causing them to coalesce faster and reducing their interracial area. Thus, to achieve the same level of extraction, longer on times were required with Lix 70.

The encouraging results obtained from the intermittent application of ultrasound suggested that continuous-flow solvent extraction might also reduce energy consumption. The objective of using a continuous flow was to simulate the addition of a variable amount of ultrasonic energy (of the same intensity) by varying the flow rate. For this study, a separate reactor was designed (Figure 12), consisting of two holding containers, one for the organic phase and the other for the aqueous phase, an ultrasonic

Table IV. Ultrasonic Energy Consumption during Nickel Extraction

with Lix 65N (ultrasound was used intermittently)

Ultrasonic Equipment Setup Energy Equilibrium Input On Off Consumed Achieved (W) Time Time (Wh) After (Min)

21 0.25 4.75 9,56 90 21 0.50 4.50 9.41 60 21 1.00 4.00 7.76 30 21 2.00 3.00 6.71 15 41 0.25 4.75 3.81 30 61 0.25 4.75 2.66 15

Organic phase: 10 pct (vol.) Lix 65N/0.1 M lauric acid/Kermac 470B. Aqueous phase: 30 nag Ni2+/l; 1 M KNO3, pH~o~ = 5.05. O / A ratio: 100 ml/100 ml; stirring = 600 min -~. Ultrasonic pulsa- tion cycles (on and off) 12 rain J.

METALLURGICAL TRANSACTIONS B VOLUME 23B, FEBRUARY 1992-- 19

Fig. 12--Schematic representation of the apparatus for continuous solvent extraction in the presence of ultrasound.

chamber , and a rece iv ing container . The ul trasonic chamber , which was water - jacke ted to control the temperature o f extract ion, rece ived the separate organic and aqueous phases and mixed them by insonation. The mixed phases were then p u m p e d into a receiv ing container . The res idence t ime in the ul trasonic chamber was var ied by the pumping rate o f the organic and aqueous phases , and the energy consumpt ion was moni to red by a wat t -hour meter.

The results (Table V) show that the energy consump- t ion, expressed in k i lowat t -hour per cubic meters o f solution, can be reduced by decreas ing res idence t ime and increasing the temperature at which solvent extract ion

takes place. Under the condi t ions studied, the lowest en- e rgy consumpt ion was achieved at 26 ~ and at the high- est flow rates, 160 ml /min . About 33 pet Ni was extracted under these condi t ions. Higher f low rates were not examined because o f the l imitat ions o f the exper imenta l setup. Despi te this l imita t ion, which could affect the ac- curacy of the ca lcula ted consumpt ion o f energy, the exper iment still c lear ly indicated how energy consumpt ion could be reduced.

The lowest energy consumption obtained, 11 k w h / m 3, is still too high to compete with solvent extract ion performed by convent ional mixing , where the upper ac- ceptable energy consumpt ion l imit is about 1 k w h / m 3.

Table V. Ultrasonic Energy Consumption during Solvent Extraction of Nickel by Using a Continuous Flow of Organic and Aqueous Phases

Flow rate Flow rate Organic Aqueous Residence Ni Extracted Energy consumed

(ml/min) (ml/min) T (~ Time (Min) (Pet) (KWh/m 3)

10 10 15 10.0 15 176 20 20 15 5.0 15 88 40 40 15 2.5 15 88 40 40 26 2.5 33 44 80 80 26 1.25 33 22

160 160 26 0.625 33 11

The experimental conditions were as follows. Organic: 10 pet (vol.) Lix 65N/0.1 M lauric acid/Kermac 470B. Aqueous: 30 mg Ni:§ 0.5 M KNO3; pHini, = 5.05. Ultrasound: frequency = 20 KHz; intensity = 47 W.


Fig. 13--Schematic representation of the proposed new method of solvent extraction: solvent extraction-in-pipe, possible because the extraction rates are significantly enhanced with ultrasound.

However, the results from the continuous-flow and intermittent ultrasound experiments clearly suggest that optimization studies may show how to further reduce energy consumption. The most promising solvent extraction technique may be the one that utilizes a pipe for mixing the organic and aqueous phases, with properly spaced ultrasonic horns inserted through the pipe wall (Figure 13). In this kind of a plant, in addition to the composition of aqueous and organic phases, the other important process parameters would be the flow rate in the pipe, the size of ultrasonic horns, the spacing of ultrasonic horns, and the ultrasonic input energy. The pipe length between a leaching reactor and a settler would serve as a mixer, thus avoiding the need for a mixer as in the traditional mixer/settler design.

IV. C O N C L U S I O N S

1. Ultrasound dramatically improved solvent extraction of nickel with Lix 65N and Lix 70, increasing the extraction rate four- to sevenfold.

2. Ultrasound had no effect on extraction equilibrium. 3. Continuous ultrasound was not needed for solvent ex-

traction of nickel. Intermittent application of ultrasound could produce the same results as continuous application, thus reducing energy consumption.

4. Because intermittent ultrasound produced the same results as continuous ultrasound and because ultrasound did not affect the extraction equilibrium, it can be concluded that ultrasound did not affect chemical reaction mechanisms. That is, the effect of ultrasound was physical, i . e . , to increase the surface area.

5. The extraction rate of divalent cobalt and its uptake by the organic phase were increased by ultrasound. However, the sonochemical extraction of cobalt en- tailed more difficult stripping from the loaded organic phase. Because the stripping of trivalent cobalt is always more difficult than the stripping of divalent cobalt, it was concluded that ultrasound caused oxidation of cobalt during sonochemical extraction.

6. Further research is needed to develop the application of ultrasound to the solvent extraction of metals. Spe- cial attention should be given to reactor design with respect to sonochemical extraction under atmospheric vs pressurized conditions, intermittent application of ultrasound, application of ultrasound in a pipe, and other parameters.

7. It should also be noted that because of much faster solvent extraction rates, ultrasound can also be used to shorten the equilibrium studies of very slow extraction systems, which are usually enhanced by the addition of surfactants. Ultrasound provides a better method of studying the equilibrium reactions, because it does not introduce additional chemical com- ponents into the studied systems (a noncontaminating tool for study).

A C K N O W L E D G M E N T

This work was supported by the United States Department of the Interior, Bureau of Mines, under Contract No. J0134035 through Department of Energy Contract No. DE-AC07-76IDO1570.


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