A pyridine-based chelating solvent extraction system for selective extraction of nickel and cobalt

hydrometallurgy ELSEVIER

Hydrometallurgy46 (1997) 37-53

A pyridine-based chelating solvent extraction system for selective extraction of nickel and cobalt Taili Zhou *, Batric Pesic Department of Metallurgy, College of Mines, Universi~ of ldaho, Moscow, ID 83843, USA

Received 23 May 1996; accepted 9 December 1996

Abstract

A novel pyridine-based chelating extractant, 2,6-bis-[5-n-nonylpyrazol-3-yl] pyridine (BNPP), has been developed and characterized. The solvent extraction of Ni and Co by a mixed system of BNPP and dinonyl naphthalene sulfonic acid (DNNSA) was studied as a function of pH, diluent, temperature, and DNNSA concentration. Stripping of Ni and Co was examined as a function of HCI and HzSO 4 concentration. The novel system can extract Ni and/or Co selectively against Fe, Mn, Ca, Mg, and A1 from acidic sulfate solutions at a pH as low as 0.5. Separation of Ni and Co can be achieved either during loading, or during stripping stages of solvent extraction. The extractant system is stable and can be regenerated with acid. The synergistic extraction distribution diagrams and the slope analyses methods {log D vs. Iog[BNPP]} showed that both BNPP and DNNSA are present in the extracted Co and Ni complexes, and that two molecules of BNPP are coordinated with one metal cation.

I. Introduction

The separation and recovery of cobalt and nickel from acidic sulfate solutions has always been a great challenge to hydrometallurgists. The difficulty stems from the fact that Co 2÷ and Ni 2+, as transition metals, form relatively weak complexes with most of the currently available industrial extractants, whereas Fe 3÷, Cu 2+ and some other transition metals usually form stronger complexes. Additional difficulty lies in very small differences in chemical behavior between nickel and cobalt. As a result, in

* Corresponding author. Present address: The Shepherd Chemical Co. 4900 Beech Street, Cincinnati, Ohio 45212, U.S.A.

0304-386X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0304-386X(96)00092-8

38

T. Zhou, B. Pesic / Hydrometallurgy 46 (1997)37-53

hydrometallurgical practice, the recovery of cobalt and nickel can be performed only after complete removal of iron, copper and other metals. Selectivity against iron is of particular importance. The presence of iron in the leaching solution is unavoidable, since iron, as one of the naturally most abundant elements, is present in most ore minerals. Nickel and cobalt ores are no exception, and they are particularly closely associated with the iron minerals (in laterites, pyrrhotite, pentlandite, etc.). On the other hand, iron in ferric form can be intentionally used as a lixiviant for the dissolution of metals, presenting another source of iron in the leach solution. An extraction system for nickel and cobalt must also be simultaneously selective against other metals such as manganese, calcium, magnesium, zinc, and aluminum, as well as iron. There have been many studies on solvent extraction of nickel and cobalt The key functional groups of extractants were phosphorus-based acids [1-3], oximes [4-13], chelating amines [14,15], and heterocyclic nitrogen [16-18]. Two acid organic phosphorus compounds [1-3], substituted phosphinic and thiophosphinic acids, represent major progress in the separation of cobalt from nickel. However, the results of selective extraction of nickel from cobalt have not been so satisfactory. Among all nickel/cobalt extractants, oxime-based reagents in combination with various modifiers or with other extractants have been most extensively studied. Among the modifiers, several different organic acids have been examined, such as bromolauric acid [4], Versatic 9-11 [5], and dinonyl naphthalene sulfonic acid (DNNSA). The use of sulfonic acid as a modifier for Lix 63 was first proposed by the AMAX researchers [6], and this extraction system was extensively studied later on by Osseo-Assare and his co-workers [7-10]. However, it was found that degradation of hydroxyoximes by sulfonic acid makes these extraction systems impractical [11]. Non-chelating aliphatic aldoximes, mixed with carboxylic acid or D2EHPA (di(2-ethylhexyl) phosphoric acid), were suggested by Preston and Fleming [12]. Selectivity of nickel over cobalt was achieved during extraction, but the use of dilute acids in the stripping stage made the overall systems unstable. Moreover, the non-chelating oximes cannot even reject iron, manganese, and aluminum, which have to be removed prior to nickel extraction. With respect to hydrolytic stability, the systems based on 2-octyl 3-pyridinecarboxylate [16-18] are more suitable than the non-chelating oximes. Better systems yet are n-octyl 3-pyridinecarboxylate with 3,5-diisopropylsalicylic acid [17], and pyridinecarboxylate esters in a mixture with organophosphorus acids. In this mixture [18], nickel is extracted under acidic conditions and stripped by dilute sulfuric acid; however, iron has to be removed prior to solvent extraction. The chelating amine-based nickel extraction is represented by the N-alkylated bis-picolyamine + DNNSA system [14]. The system exhibits preferential extraction of nickel over cobalt but, in order to prevent emulsion formation, higher temperatures or additional modifiers are required [14,15]. Aware of the lack of efficient N i / C o solvent extraction systems, we began this project with the objective of developing a new and better organic extractant. The conceptual approach was to develop a tridentate chelating agent containing N,N,N donor atoms. In a multi-year effort, six novel organic extractants were designed and synthesized [19]. Among these, a pyridine-based chelating extractant, 2,6-bis-[5-n-nonylpyra-

T. Zhou, B. Pesic /Hydrometallurgy 46 (1997)37-53

39

zol-3-yl] pyridine, abbreviated as BNPP, proved to be the most efficient extractant. This paper is organized into two parts. Part 1 describes the procedures for synthesis of the BNPP molecule, its chemical characteristics, and solvent extraction properties. Part 2 presents the results from solvent extraction of Ni and Co from model aqueous solutions. The solvent extraction system consisted of BNPP in a mixture with DNNSA, both dissolved in kerosene.

2. Part 1: Synthesis and characterization of BNPP

2.1. Synthesis procedure 120,21] A 33.4 g (0.20 M) quantity of 2,6-pyridinedicarboxylic acid was transferred into a 500 ml 3-neck round-bottom flask, to which 100 ml of thionyl chloride was slowly added and refluxed for 10 h. The thionyl chloride was removed by distillation to yield 2,6-pyridinedicarboxylic acid chloride. Absolute ethanol, 120 ml, was added slowly to the 2,6-pyridinedicarboxylic acid chloride, to which 20.1 g (0.372 M) of sodium methoxide was also added. The solution was heated and refluxed for 30 min. The ethanol was removed to yield the diethyl ester of 2,6-pyridinecarboxylic acid as a solid (39.2 g). To a stirred dispersion of 21.6 g (0.40 M) of NaOMe in 200 ml dry methylene chloride, a solution of 39.2 g of diethyl ester of 2,6-pyridinedicarboxylic acid in 50 ml dry methylene chloride was added dropwise at room temperature. Next, a solution of 59 g (0.35 mol) of 2-undecanone in 25 ml dry methylene chloride was added and the reaction mixture was refluxed for 5 h at near boiling temperature. After cooling, 5 ml of acetic acid was added, and the precipitate dissolved in 100 ml water. The two layers were separated, and the aqueous layer was extracted with methylene chloride (2 × 30 ml). The combined organic phases were washed with water and dried over MgSO 4. Tetraketone (71.7 g) was obtained after removal of the methylene chloride. The tetraketone (71.7 g) was dissolved in 250 ml of methanol, and 2 drops of HCI and 20.1 g (0.40 M) of hydrazine monohydrate were added. The mixture was refluxed for 2 h, and the methanol was then removed. A sticky, pale yellow liquid was obtained, which solidified after cooling. The solid BNPP was stored in a vial for chemical characterization and solvent extraction studies.

2.2. Structural characterization of BNPP The chemical structure of BNPP was determined from nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and mass spectroscopy studies. IH NMR and 13C NMR spectra: 1H NMR and 13C NMR studies were performed. IH NMR spectra were obtained by using CDC13 (Bruker AC 300 FT-NMR spectrometer), with the following chemical shift results: 6 0.81-0.88 (t, 6H), 1.23-1.31 (m, 24H), 1.66 (s, 4H), 2.69-2.77 (t, 4H), 6.49 (s, 2H, pz), 7.43-7.70 (m, 3H, py). 13C NMR spectra (IBM NR300 spectrometer)were: 6 14.1, 22.7-31.9, 101.7, 118.4, 137.4, 145.3, 148.9, 151.5. The above results confirm the existence of both pyridine and pyrazole moieties in

T. Zhou, B. Pesic / Hydrometallurgy 46 (I 997) 37-53

40

Compound

Pyrazole

Pyridine

C2'

C3"

C4"

C3

148.9

118.4

137.4

C4

Cs

151.5 101.7 145.3

BNPP Side Chain

C1"

C='-Cg"

14.1

22.7-31.9 4

~ "

HN ~.N ~ - ~ N

"

~<~

5 2"-9'

1"

N INH

Fig. 1. 13C NMR chemical shifts (in ppm) for the BNPP molecule and its chemical structure.

the BNPP molecule, and the direct link between these moieties through carbon atoms at the hetero rings, Fig. 1. Mass spectrum: Mass spectra were recorded by the electron impact method (Varian VG 7070 HS mass spectrometer). The following peaks were found: m / e 463 (M+), 434, 420, 406, 392, 364, 351, 252, 203. According to the molecular ion peaks, the molecular weight of BNPP is 463. Infrared spectrum: Infrared spectra of the BNPP molecule (Perkin Elmer Model 1710 FTIR spectrometer) are given in Fig. 2. The bands (KBr, cm -z) at 1600, 1580, 1570,

1.4-

0 0

1.2-

R b s

1.0-

o

bO.8n

c0.6e 0.40.2-

I

i

I

I

2000

1800

1600

I

140~

I

I

121~1

1000

W~venum~el-~

Fig. 2. IR spectrum (cm -1 ) of the BNPP molecule.

I

800

T. Zhou, B. Pesic/ Hydrometallurgy 46 (1997) 37-53

41

1499, 1475, 1466, 1457 are assigned to the coupled C = N and C = C stretching vibrations of the molecule. UV-Visible spectrum: U V - V i s spectra of the BNPP molecule were recorded on the Perkin Elmer Lambda 4B spectrophotometer with 1.0 cm quartz cell. The adsorption bands found were: 243 nm, 307 nm (in ethanol), 244 nm, 300 nm (acetonitrile). 2.3. Other properties Chemical composition: According to carbon, hydrogen, and nitrogen analysis the chemical composition of BNPP is: 76.36% C, 9.5% H, and 14.14% N. This composition corresponds well with the proposed theoretical structure, Fig. 1:75.12% C, 9.78% H and 15.10% N. Solubility: The solubility was determined by incremental additions of 1.0 mg of BNPP in 1.0 1 of agitated water at 25°C. When dissolution of the final increment ceased (inspection under projected light; after 48 h) the solubility was determined from the total addition. BNPP is a tan-yellow solid material at room temperature. The melting point is 80.0-80.5°C. Its solubility in water and in acidic solution is very low, 3 m g / l at pH 5.8 (25°C, water), and 5 mg/1 in 4 N HCI (25°C), On the other hand, it can be easily dissolved into most of the organic solvents (aliphatic and aromatic) used as diluents in solvent extraction. Stability: The BNPP molecule is very stable. Its stability is determined indirectly by its performance during solvent extraction of Ni and Co. It was found that 20 cycles of consecutive loading and stripping at room temperature and at 50°C did not have any detrimental effect on solvent extraction. Even heating BNPP at 200°C for several hours was not detrimental to subsequent solvent extraction.

3. Part 2: Solvent extraction

3.1. Experimental procedure, analysis, and materials The aqueous and organic phases were mixed in a 25 ml vial held in a wrist-action shaker. In some experiments the vial was placed into a heating mantle to maintain the

Table 1 Standard experimentalconditions during solvent extraction studies Organic phase 0.05 M BNPP+ 0.05 M DNNSA Diluent Kermac470B (kerosene) Aqueous phase Metal concentration 1 g/I 0.45 pH Ratio of O/A 1 Temperature 25°C Time 60 min Mixing wrist-action shaker

T. Zhou, B. Pesic /Hydrometallurgy 46 (1997) 37-53

42

required temperature with a temperature controller. The standard experimental conditions during extraction (loading) are listed in Table 1. After mixing of the organic and the aqueous phases, the two phases were separated. The concentration of the metals in the aqueous raffinate was analyzed by atomic absorption (Varian, Spectra AA-20). The concentrations of metals in the organic phase were determined by the difference of the aqueous feed and aqueous raffinate. The percentage uptake for each metal was then calculated. Dinonyl naphthalene sulfonic acid was supplied by Pfaltz and Bauer Inc., Kermac 470B was obtained from Triangle Refineries Co., Aromatic 150 was provided by Exxon Co. All of these solvents were used as received without further purification.

4. Results and discussion

4.1. Percent E-pH equilibrium diagrams for metal ions The equilibrium diagram, percentage of metal extraction versus pH, for the extractant system of 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B is presented in Fig. 3. The results were obtained by contacting the organic phase with the aqueous phase containing a single metal ion for 1 h at 25°C. According to Fig. 3, Cu 2+ is the most strongly extracted metal. Extractions of Ni 2÷ and Co ~+ are also very strong. The extractions of both Fe 2+ and Fe 3+ are weak. The extractions of Mn 2÷, Ca 2+, A13+ and Mg 2+ are very weak. Therefore, the extraction sequence of the metals is: Cu > Ni > Co > Zn > Fe(II) > Fe(III) > > Mn > Ca > A1 > Mg. Examination of the pHs0 (a pH value corresponding to 50% of metal extraction) quantifies the preferential extraction of Ni and Co over Mn, Ca, A1, and Mg: PHs0(Ni)

100 -

.7~,~'~ ,x,,'~,,,--~,~

"" Co o Ni o Fe~+

/.~-- //" ~/ #

80

' AI I ~, Ca ° Mg

0/A=1/1 T= 2 5 * 0

~,

.F~+ vcul /

l@l 6O ID

U.l 40

20

0

•

0

I

0.5

,

I

1.0

,

I

1.5

'

I

I

2.0

2.5

3.0

3.5

pH(equil) Fig. 3. Percent E-pH equillibrium plot for BNPP/DNNSA/Kermac 470B at 25°C. Organic phase: 0.05 M BNPP + 0.05 M DNNSA+ Kermac470B. Aqueous phase: 1.0 g/l of the single metal, as sulfate.

i2 Zhou, B. Pesic / 14ydrometallurgy 46 (1997) 37-53

100

c~

43

n Ni Co Fe2+

"o

X 40

o

I pH= 0.45 T= 25"C Ratio O / A = 1

0

i

0

20

40 TIME, rain

60

80

Fig. 4. Rates of extraction of Ni, Co and Fe 2+. Each metal extracted from a single metal solution. Organic phase: 0.05 M B N P P + 0 . 0 5 M DNNSA +Kermac 470B. Aqueous phase: 1.0 g/1 of the single metal, as sulfate.

< 0.0; pHs0(Co) < 0.0; pHs0(Mn) = 1.62; pHs0(Ca) = 2.47; pHso(Al) = 3.22; PHso(Mg) = 4.50. These values clearly show that in the novel extraction system there is no interference of Mn, Ca, Mg, and AI with the extraction of Ni and Co. 4.2. Extraction rates of individual metals The rate of extraction of Ni, Co and Fe(II) was determined by contacting the organic phase with the aqueous phase containing a single metal. The rate experiments were performed at PHi. = 0.45 and T = 25°C, Fig. 4. The extraction rates of Co 2+ and Fe 2+ are fast, while the extraction rate of Ni 2÷ is relatively slow. The slow extraction rate of nickel is probably due to its slow rate of water substitution by the BNPP molecule in the inner coordination sphere [22]. 4.3. Effect of pH on extraction rates The effect of initial pH on the solvent extraction of Co z+, Ni 2+ and Fe 2+ with 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B was studied at pH 0.0, 0.45, 0.83 and 1.99. All three metals were simultaneously present as metal sulfates in the solution mixture, each at a concentration of 1.0 g / l . The effect of initial pH on the rate of Ni extraction is presented in Fig. 5a, where it can be seen that the rate of Ni extraction was generally slow. The rate increased with and increase in the pH from 0.0 (1 N H2SO 4) to 0.83. The rate of Ni extraction at pH 1.99 was lower than the rate of Ni extraction at pH 0.83. It is believed that the reason for lower rate of Ni extraction at pH 1.99 was the competitive extraction of ferrous iron and cobalt, which had the highest rates at this pH. The rate of cobalt extraction as a function of initial pH is presented in Fig. 5b. Cobalt extraction was very fast compared to nickel extraction. For example, the extent of cobalt

44

7~ Zhou, B. Pesic / Hydrome tallurgy 46 (1997) 37-53 100

(a)

T = 25"C O/A=1/1

8O uJ I.-

0 60 <: Jr" I.X I.u 40 Z 2O

o []

0 0 100

i

i

20

40

A O t

pH = 0.0 pH= 0.45 pH= 0.83 pH= 1.99 80

6O

T= 25"c O/A=I/t

o [] A O

80 £3 LU I".O 60 CC I-X tU 40

pH= pHpH = pH=

0.0 0.45 0.83 1.99

O

O 2O

(b) 0 0 100

i 20

i 40

i 60

T = 25"C O/A'1/1

80

O pH= 0.0 r'= pH= 0.45

8O

0 LU I-0< 60

A

p H == 0 . 8 3

O

pH'= 1.99

CC IX UJ 40 o

u. 20

"-

o

0

20

40

TIME,

60

(c) 80

rain

Fig. 5. Effect of pH on extraction rate of: (a) Ni, (b) Co and (c) Fe from solution containing a mixture of Ni, Co and Fe. Organic phase: 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B, Aqueous phase: 1.0 g / l each of Ni2+, Co2+ and Fe2÷, as sulfates. extraction within the first minute were 42.6%, 54.1%, 66.1%, and 63.1% at pH values of 0.0, 0.45, 0.83, and 1.99, respectively. Thus, within the first minute of extraction, the increase in pH increased the rate of cobalt extraction. The shape of cobalt extraction

T. Zhou, B. Pesic /Hydrometallurgy 46 (1997)37~53

45

curves clearly indicates, however, that, as extraction continues, cobalt is released back into aqueous solution, a phenomenon not seen when cobalt was extracted in the absence of iron and nickel. By examining the extraction curves for cobalt and nickel (Fig. 5) it appears that the extraction of nickel is responsible for the observed release o f cobalt. Additional support for the role of nickel can be found in Fig. 3, according to which BNPP + D N N S A is a stronger extraction system for nickel than for cobalt. Therefore, with time, the more slowly but more strongly extracted nickel displaces the more quickly, but less strongly, extracted cobalt. The same conclusion can be reached by examining the shapes of extraction curves for nickel and cobalt in Fig. 5. Accordingly, the time regions where the rate of nickel extraction levels off coincide with the time regions when back release of cobalt levels off. The rate o f iron extraction as a function of initial pH is presented in Fig. 5c. At pH 0, only 3.6% of the iron was extracted within the first minute. Iron extraction diminished to zero with time. There was no measurable iron extraction at pH 0.45. At pH 0.83, iron extraction at the end of the first minute was 12.3%, which diminished to 5.0% after 60 min. At pH 1.99, iron extraction at the end of the first minute was the highest, 21.6%, but it also diminished with time to only 6.0% at the end of 60 min. Generally, it can be observed that the shapes of the iron extraction curves are similar to the shapes of cobalt extraction curves; that is, iron was released back into aqueous solution with time. As in the case of cobalt discussed above, the stronger, but slower, extraction of nickel is responsible for the observed behavior of iron extraction. 4.4. Effect o f diluents The effect of two organic solvents, the diluents Kermac 470B and Aromatic 150, on the extraction rate of nickel and cobalt by BNPP modified with D N N S A was examined (Table 2). It can be seen that the rate of nickel extraction (or the rate o f cobalt displacement) with Kermac 470B as a diluent is faster than with Aromatic 150. However, with regard to solvency for metal complexes, Aromatic 150 is better. Both diluents can be successfully used for solvent extraction with BNPP modified with DNNSA.

Table 2 Effect of the diluents Kermac 470B and Aromatic 150, on solvent extraction of Co, Ni and Fe by 0.05 M BNPP + 0.05 M DNNSA Metal Ni Ni Co Co Fe Fe

Diluent Kermac 470B Aromatic 150 Kermac 470B Aromatic 150 Kermac 470B Aromatic 150

Metal extracted (%) 1 min

5 min

10 rain

30 min

60 min

1.03 2.92 54.1 58.5 3.6 0.6

22.0 16.5 49.8 54.6 0.0 0.0

39.7 29.4 39.6 48.4 0.0 0.0

84.2 58.9 12.4 29.6 0.0 0.0

91.4 78.5 10.6 15.9 0.0 0.0

Aqueous phase: 1.0 g/I each Ni 2+, Co2+ and Fe2+, as mixed sulfates; pHin = 0.45; O/A = 1/1; T = 25°C.

T. Zhou, B. Pesic/ Hydrometallurgy 46 (1997) 37-53

46

Table 3 Effect of DNNSA concentration on extraction of Ni, Co and Fe(II) with 0.05 M BNPP in Kermac 470B Metal

Ni Ni Ni Co Co Co Fe Fe Fe

DNNSA

Metal extracted (%)

(M)

1 min

5 rain

10 min

30 min

60 min

0.025 0.05 0.10 0.025 0.05 0.10 0.025 0.05 0.10

1.07 1.03 27.3 41.7 54.1 25.5 1.3 0.0 12.6

3.2 22.0 63.4 48.0 49.8 27.2 0.0 0.0 11.7

22.1 39.7 82.2 38.4 39.6 22.4 0.0 0.0 7.6

38.1 84.2 89.1 27.4 12.4 22.9 0.0 0.0 7.6

61.7 91.4 89.5 7.6 10.6 22.1 0.0 0.0 7.6

Aqueous phase: 1.0 g / l each Ni 2+, Co 2+ and Fe z+, as mixed sulfates; pHi. = 0.45; O / A = 1/1; T = 25°C.

4.5. Effect o f DNNSA concentration

T h e e f f e c t o f D N N S A c o n c e n t r a t i o n ( T a b l e 3) o n the e x t r a c t i o n r a t e s o f cobalt, nickel, a n d iron w a s s t u d i e d w i t h t h r e e d i f f e r e n t D N N S A c o n c e n t r a t i o n s : 0 . 0 2 5 M , 0.05 M , a n d 0.10 M . T h e B N P P c o n c e n t r a t i o n w a s c o n s t a n t at 0.05 M . T h e i n c r e a s e in D N N S A c o n c e n t r a t i o n h a d a s i g n i f i c a n t p o s i t i v e e f f e c t on the rate o f n i c k e l e x t r a c t i o n . S i m u l t a n e o u s l y , the rate o f c o b a l t e x t r a c t i o n w a s s u p p r e s s e d w i t h the i n c r e a s e in D N N S A c o n c e n t r a t i o n . W i t h r e s p e c t to iron, s o m e e x t r a c t i o n was o b s e r v e d o n l y for the h i g h e s t D N N S A c o n c e n t r a t i o n studied. T h e final e x t r a c t i o n results, at 6 0 m i n in T a b l e 3, w e r e f u r t h e r p l o t t e d v e r s u s D N N S A c o n c e n t r a t i o n (Fig. 6). T h e s i g n i f i c a n c e o f this f i g u r e is t h a t it d e m o n s t r a t e s the h i g h

100 r,, 80 iii I-¢.) ,~ 60

N//~"

BNPP= O.05M

fie

IX Ill o

40

20 0

C°~ -'~'~ o

( pH= 0.45

~

,

0.05

0.10

O/A = 1/1 0.15

Concentration of DNNSA, M Fig. 6. Effect of DNNSA concentration on Ni and Co extraction by BNPP/DNNSA mixture. Data from kinetic studies at 60 min. Organic phase: 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B. Aqueous phase: 1.0 g / i each of Ni 2+, Co 2+ and Fe 2+, as sulfates.

T Zhou, B. Pesic/Hydrometallurgy 46 (1997) 37-53

47

selectivity of the B N P P / D N N S A extraction system for nickel over cobalt and iron. The best separation of nickel from cobalt and iron was achieved at 0.05 M DNNSA concentration. At lower than 0.05 M DNNSA concentrations, the extraction rate of nickel would be too slow. At higher than 0.05 M DNNSA, the selectivity of nickel against iron would be decreased. An important point is that high DNNSA concentration had no negative effect on phase separation in the given extraction system.

4.6. Effect of temperature The effect of temperature on the extraction rates of nickel and cobalt with the system consisting of 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B was studied in the range 25-50°C (Fig. 7a-c). As expected, it was found that temperature had a significant effect on extraction rate of nickel (Fig. 7a). For example, the equilibrium for nickel extraction was reached after 60 min, 30 min, 10 min, and about 8 min, at 25°C, 30°C, 40°C, and 50°C, respectively. For the cobalt extraction rate (Fig. 7b) temperature was not a significant parameter. No iron extraction (Fig, 7c) was observed in all of these experiments.

4. 7. Stripping of organic phase Sulfuric and hydrochloric acids were used for stripping the loaded organic phase. Because hydrochloric acid produced better results (i.e., a higher percentage of stripping and better separation of phases) most of the stripping study was performed with this acid. The effect of HCI concentration on the stripping of Ni and Co from the loaded 0.05 M BNPP + 0.05 M DNNSA + Kermac 470B organic phase is presented in Table 4. The organic phase was previously loaded under the following conditions: the aqueous phase contained 1.0 g/1 each of Ni 2÷, Co 2÷, and Fe 2+ as mixed sulfates; pHin 0.45; ratio of O / A = 1/1; and T = 25°C. These data indicate that protonation of the heterocyclic donor nitrogen atom in the ligand (BNPP) is an effective method for decomposition of the loaded cobalt/nickel complexes (i,e., stripping of nickel and cobalt). The cobalt stripping rate was fast, unlike the slow nickel stripping rate. For instance, when 3 N HC1 was used as the stripping reagent, more than 70% of loaded cobalt was stripped in about 2 min at room temperature, while in the same time only 3% of the Ni was stripped. The kinetic difference of stripping with 3 N HC1 can be utilized for efficient separation of cobalt from nickel. Stripping of nickel can be achieved with a higher (5 N) concentration of HC1. The stripping rate of nickel can be increased by raising the temperature, as well as by higher HC1 concentration. Table 5 shows the effect of temperature on the rate of Ni and Co stripping with 5 N HCI. Temperature had a significant effect on the rate of nickel stripping. For instance, at 50°C, more than 90% of loaded nickel was stripped in less than 2 min. Temperature had little effect on the stripping of cobalt.

T. Zhou, B. Pesic / Hydrornetallurgy 46 (1997)37-53

48

100

~,

(a)

I ~ 80

o 5o , x 40

20 pH= 0.45 0/A-1/1 0

1 20

0

1 4O

.• 60

T= T= T= T"

25 *C 30 *C 40'C 50 *C 80

IO0

01A=111 a uJ I.-

T - 30 °C T = 40 *C T = 50 =C

80


-

0 100

R

1

T

T

20

40

60

(C)

80

pH = 0.45

T - 25 *C

0/A~1/1

T= 30 =C T = 40 =C T = 50 *C

80 1:3 ILl I',<: 60 ¢X: I-" X tU 4 0

o 14. 2O

0

20

40

60

80

TIME, rain Fig. 7. Effect of temperature on extraction rate of (a) Ni, (b) Co and (c) Fe from a solution containing a mixture of Ni, Co and Fe. Organic phase: 0.05 M B N P P + 0,05 M DNNSA + K e r m a c 470B. Aqueous phase: 1.0 g / I each of Ni 2+, Co 2+, and Fe 2+, as sulfates.

Because of the phase separation problems only a limited number of experiments were performed for stripping by sulfuric acid. As an example, Table 6 shows the effect of 4 N sulfuric acid on the rate of stripping of nickel and cobalt.

T. Zhou, B. Pesic/ Hydrometallurgy 46 (1997)37-53

49

Table 4 Effect of HC1 concentrationon Ni and Co stripping Me2÷ Ni Ni Ni Co Co Co

HCI

MetM stripped(%)

(N)

l min

5 min

10 min

30 min

3 4 5 3 4 5

1.8 3.8 9.9 62.0 64.8 71.8

6.8 15.6 43.7 70.1 77.5 79.0

9.8 20.6 55.5 77.5 77.5 81.7

23.0 51.8 80.8 78.9 81.7 80.3

Loaded organic phase (0.05 M BNPP+0.05 M DNNSA+Kermac 470B) contained 0.962 g/l Ni and 0.153 g/l Co. Stripping was performed at room temperature with O/A = 1/1.

4.8. Preliminary extraction stoichiometry study Because the primary objective of the present work was to develop a novel extractant for Ni and Co, detailed extraction mechanism studies were not performed. However, in order to understand the novel extractant better, some insight into the possible extraction mechanisms was provided by the already available data. The reaction stoichiometry is important information for determining the extraction mechanism. For the present solvent extraction system, BNPP and DNNSA in a reaction mixture, hydrogen bonding between BNPP and D N N S A occurs, and some DNNSA is probably present as micelle aggregates. The extraction of Ni and Co reactions can generally be written as: Mec~~- + nL,o ) + m ( H D ) p , o ) = [ ( M e L . ) ' Hmp_,2_i)" Dmp]i~o +) + (2 - 0 H [ a )

(1)

where the subscripts a and 0 = aqueous and organic phases, respectively; Me 2 ÷ = Ni 2÷ or C02+; L = BNPP; and HD --- DNNSA; the subscript p = the aggregation number of the D N N S A micelles.

Table 5 Effect of temperature on Ni and Co stripping Me2+ Ni Ni Co Co

Temp.

Metal stripped (%)

(°C)

1 rain

5 min

10 min

30 min

25 50 25 50

9.9 85.1 71.8 87.0

43.7 92.1 79.0 87.0

55.5 91.0 81.7 87.0

80.8 92.3 80.3 89.9

Loaded organic phase (0.05 M BNPP+ 0.05 M DNNSA+ Kermac 470B) contained 0.962 g/l Ni and 0.153 g/l Co. Strippingwas performed with 5 N HCI at O/A = 1/1.

50

T. Zhou. B. Pesic / Hydrometallurgy 46 (1997) 37-53

Table 6 Effect of 4 N H2SO4 on stripping of Ni and Co Me 2+

Metal stripped (%)

Ni Co

1 min

5 min

10 min

30 min

6.1 31.4

24.8 52.7

30.6 55.8

53.8 71.3

Loaded organic phase (0.05 M BNPP + 0.10 M DNNSA + Kermac 470B) contained 0.814 g/l Ni and 0.264 g/l Co. Stripping was performed at room temperature. The O / A ratio was 2/1.

The e q u i l i b r i u m of extraction reaction (1) can be characterized by: K=

i+ H+ 2 - i 2+ n m [ ( M e L n ) ' H m p - ( 2 - i ) ' Dmp](o)[ ](a) / [ M e ](a)[L](o)[HD]p(o)

(2)

(3)

l o g D = l o g K + n l o g [ L ] + m l o g [ H D ] p + (2 - i ) p H

where K = the concentration e q u i l i b r i u m constant and D = the distribution ratio of cobalt and nickel. Plots of log D versus log[BNPP], along with the experimental conditions for nickel and cobalt extraction, are shown in Fig. 8. The experimental data for extraction of both metals fell on straight lines. The slope of the lines contains the information on the stoichiometry of extraction, n. It was found that n = 2 for both nickel and cobalt; that is, each of these metals is c o m p l e x e d with two m o l e c u l e s of BNPP. B e c a u s e D N N S A can also be an extractant for nickel and cobalt (DMe increased with an increase in D N N S A concentration, see Fig. 6), the same type of plot, log D vs.

1.0

o

0.0

0s//

2 /

-1.0

....

-2.5

, -2.0 Iog[BNPP],

[]

Nickel

a

Cobalt

-1.5 M/L

Fig. 8. Determination of stoichiometry plot for reactions of BNPP with cobalt and nickel. Organic phase: BNPP+0.01 M DNNSA in hexane. Aqueous phase: 5.0 mM CoSO4 or NiSO4 in 1.0 M Na2SO4; pH 1.0; O / A = 1/1; T = 25°C.

T. Zhou, B. Pesic / Hydrometallurgy 46 (I 997) 37--53 500

400

Cr = CL+CHo= 0.04 M Cco= 0.005 M as sulfate

Time- 30 mirl snaking T= 25 °C

Na2S04= 1.0 M

pH= 1.77

5.0

4.0

300

51

[ OT= CL÷C~O= 004 M

Time= 150 rnin shaking

~ Cco= 0.005 M as suJfata

T= 25 °C

t Na2SO~= 1.0 M

pH= LO

3.0

t:3 <

a < 2O0

2.0

100

1.0

lal ^

0

-,

0

0,2

,

,

,

0.4

0.6

o.a

~

1.0

X L, Mole Fraction of BNPP

0.0

1.2

0

,

,

,

,

0.2

0.4

0.6

0.8

1.0

1.2

XL, Mole Fraction of BNPP

Fig. 9, Synergistic extraction distribution enhancement of (a) cobalt and (b) nickel in sulfate solutions and a mixture of BNPP+ DNNSA in Kermac 470B.

iog[HD], was constructed to determine the stoichiometry of extraction with DNNSA. The relationship was non-linear, however; that is, a curve for each metal had sections with positive and negative slopes. The non-linear relationship could be caused by reactions of metals with both pure DNNSA and mixed B N P P / D N N S A micelles (with increased DNNSA concentration), a phenomenon similar to that discussed in the L i x 6 3 / D N N S A mixed extractant system [10]. The stoichiometry of extraction was further supported from the synergistic extraction diagrams. The diagrams represent the plots of the distribution ratio, AD, versus the molar fraction of a ligand, X L. The AD is calculated from AD = D L + H D -- D L - DHD , where D E , DHD and D E + r l D represent the distribution ratios of a metal in BNPP, DNNSA and their mixtures, respectively. The molar fraction of a ligand, X L, is calculated from X L = C L / C T, and Cv = C L + CHD, where C L, CHD and Cv represent the molar concentrations of BNPP, DNSSA and the total molar concentration of BNPP + DNNSA, respectively. The results are presented in Fig. 9. It can be seen that AD maxima for cobalt and nickel occur at X L = 0.667, indicating an optimum ratio of BNPP:DNNSA = 2:1 in each extracted complex. As BNPP is a rigid molecule, two molecules of BNPP can present their donor nitrogen atoms at a position very close to the octahedral sites of the nickel or cobalt atoms. This provides a high degree of preorganization prior to nickel and cobalt binding. At the same time, due to complexation at low pH, the non-coordinating nitrogen atoms in the pyrazole moieties of the BNPP molecule retain the protons. In the meantime, hydrogen bonding between BNPP and DNNSA destroys the DNNSA micelle aggregates, as long as the concentration of DNNSA is not higher than the concentration of BNPP. It is thus expected that the monomeric DNNSA anion associates with a metal ionic complex to form an ion pair in the organic phase. However, because one DNNSA anion cannot counter-balance the two positively charged nickel and cobalt cations, other anions, such as HSO 4, may also be involved, probably occupying one coordination

52

T Zhou, B. Pesic /Hydrometallurgy 46 (1997) 37-53

position in the cationic complex. That ion-pair formation extraction mechanisms were present could be inferred from the fact that no obvious color change occurred after addition of DNNSA to the nickel or cobalt-BNPP in hexane solution.

5. Summary The findings above can be summarized as follows: (1) A novel solvent extractant for nickel and cobalt has been developed. Its chemical structure is 2,6-bis-[5-n-nonylpyrazol-3-yl] pyridine, abbreviated as BNPP in this paper. Its molecular weight is 463. BNPP has very good chemical and thermal stability, and a very low solubility in aqueous solutions. (2) BNPP was used in a mixture with DNNSA (as a modifier) to study the solvent extraction of nickel and cobalt from sulfate solutions. (3) The BNPP + DNNSA extraction system (with kerosene, Kermac 470B, as a diluent) selectively extracts nickel and cobalt from sulfate solutions containing iron, manganese, calcium, magnesium, and aluminum. (4) The extraction of nickel and cobalt is possible at a pH as low as 0.5. (5) The novel extraction system can also be used for separation of nickel from cobalt. The separation of these two metals is based upon the higher affinity of BNPP for nickel. The slow extraction rate of nickel can be accelerated by increasing temperature and concentration of DNNSA. (6) Back extraction, or stripping, of nickel and cobalt can be achieved with strong acid solutions, hydrochloric acid being preferred. (7) Nickel can also be separated from cobalt during stripping, due to the fast stripping rate of cobalt with hydrochloric acid, even at room temperature. The stripping rate of nickel is relatively slow and requires higher temperatures (40-50°C), and a higher concentration of acid. (8) BNPP forms octahedral complexes with nickel and cobalt which are strong enough to allow extraction from highly acidic solutions. The extraction occurs most probably via an ion pair formation mechanism.

Acknowledgements This research was supported by the United States Department of Interior, Bureau of Mines, and Department of Energy under Grant No. DE-AC07-76ID01570. The assistance of V.C. Storhok with atomic absorption analysis is also gratefully acknowledged.

References [1] Rickelton, W.A., Flea, D.S. and West, D.W., Cobalt-nickel separation by solvent extraction with bis(2,4,4-trimethylpentyl) phosphinic acid. Solvent Extr. Ion Exch., 2 0984): 815-838. [2] Rickelton, W.A. and Robertson, A.J., Metal recovery with monothiophosphinic acids. U.S. Pat. 5,028,403 (July 2, 1991).

T Zhou, B. Pesic/Hydrometallurgy 46 (1997)37-53

53

[3] Sole, K.C. and Hiskey, J.B., The potential for recovery of cobalt from copper leach solutions. In: W.C Cooper, D.J. Kamp, G.E. Lagos and K.G. Tan (Editors), Proc. Copper 91-Cobre 91 Vol. I11, Hydrometallury and Electrometallurgy of Copper. Pergamon, Oxford (1991), pp. 229-243. [4] Flett, D.S., Cox, M. and Heels, J.D., Extraction of nickel by o~-hydroxy oxime/carboxylic acid mixtures. In: Proc. Int. Solvent Extr. Conf., '74. SCI, London (1974), Vol. 3, pp. 2559-2575. [5] Nyman, B.G. and Hummelstedt, L., Use of liquid cation exchange for separation of nickel(lI) and cobalt(ll) with simultaneous concentration of nickel sulfate. In: Proc. Int. Solvent Extr. Conf., 74', SCI London, (1974) Vol. 3, pp. 669-684. [6] Fekete, S.O., Meyer, G.A. and Wicker, G.R., The selective extraction of nickel and cobalt from acid leach solutions using a mixed solvent system. TMS-AIME Preprint No. A77-95. [7] Osseo-Assare, K. and Keeney, M.E., Aspects of the interfacial chemistry of nickel extraction with Lix 63-HDNNS mixture. Metall. Trans. B, l i b (1980): 63-67. [8] Osseo-Assare, K., Leaver, H.S. and Laferty, J.M., Extraction of nickel and cobalt from acidic solutions using Lix 63-DNNS mixtures. In: M.C. Kuhn (Editor), Process Fundamental Considerations of Selected Hydrometallurgical Systems. AIME, New York (1981), pp. 196-207. [9] Osseo-Assare, K. and Keeney, M.E., Sulfonic acids: catalysis for the liquid-liquid extraction of metals. Sep. Sci. Technol., 15 (1980): 999-1011. [10] Osseo-Asare, K. and Renninger, D.R., Synergic extraction of nickel and cobalt by Lix 63-dinonylnaphthalene sulfonic acid mixtures. Hydrometallurgy, 13 (1984): 45-62. [11] Oliver, A.J. and Ettel, V.A., Lix 65N and Dowfax 2AO interaction in copper solvent extraction and electrolysis. Can. Metall. Quart., 15 (1976): 383-388. [12] Preston, J.S. and Fleming, C.A., The recovery of nickel by solvent extraction from acidic sulphate solutions. In: K. Osseo-Asare and J.D. Miller (Editors), Hydrometallurgy Research, Development and Plant Practice. AIME, New York (1982), pp. 475-489. [13] Redden, LD. and Groves, R.D., The extraction of nickel with aliphatic oximes. Sep. Sci. Technol., 28 (1993): 201-225, [14] Grinstead, R.R. and Tsang, A,L., A selective metallurgical extractant system to recover nickel and cobalt from acid solutions. In: Int, Solvent Extr. Conf. (Denver, Colo.). AIChE, New York (1983), pp. 230-231. [15] Beckstend, L.W. and Chou, E.C., Purification of cobalt sulfate solutions by solvent extraction, ll5th AIME TMS Annu. Meet. (1985) pap. A86-48. [16] Preston, J.S. and du Preez, A.C., Synergistic effects in the solvent extraction of some divalent metals by mixtures of Versatic 10 acid and pyridinecarboxylate esters, J. Chem. Tech. Biotechnol., 61 (1994): 159-165. [17] Preston, J.S. and du Preez, A.C., The solvent extraction of nickel and cobalt by mixtures of carboxylic acids and pyridinecarboxylate esters. Solvent Extr. Ion Exch., 13 (1995): 465-494. [18] Preston, J.S. and du Preez, A.C., Solvent extraction of nickel from acidic solutions using synergistic mixtures containing pyridinecarboxylate esters. Part 1. Systems based on organophosphorus acids. J. Chem. Tech. Biotechnol., 66 (1996): 86-94. [19] Pesic, B. and Zhou, T., Novel solvent extractants for cobalt and nickel. Final Rep. submitted to INEL/LITCO-U,S. Bur. Mines (Jan. 22, 1995). [20] Newcomb, M., Timko, J.M., Walba, D.M. and Cram, D.J., Host-guest complexation 3. Organization of pyridyl binding sites. J. Am. Chem. Soc., 99 (1977): 6392-6398. [21] Levine, R. and Sneed, J.K., The relative reactivities of the isomeric methyl pyridinecarboxylates in the acylation of certain ketones, The synthesis of [3-diketones containing pyridine rings. J. Am. Chem. Soc., 73 (1951): 5614-5616. [22] Eigen, M., Fast elementary steps in chemical reaction mechanisms. Pure Appl. Chem., June (1963): 97-115.