Indian Journal of Chemistry Vol. 42A, December 2003, pp. 2954-2958

Recovery of cobalt from secondary sector using extraction in Cyanex 923

Bina Gupta, Akash Deep & Shiv N Tandon

Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667 (Uttranchal), India

Email: tandon_sn@yahoo.com

Received 10 January 2003

A process for the recovery of pure Co(II) from spent Co-Mn and Co-Mo catalysts by its extraction in Cyanex 923 is described. The metal ion is partitioned in the organic phase as H2CoC14.2 Cyanex 923 and stripped with a solution of I x 10-3

moiL- I H2S04, The extractant is stable towards prolonged contact with HCI and shows negligible loss in extraction capacity even after recycling for twenty cycles. The partition data have been utilized fo r developing conditions for the feparation of Co(II) from other metal ions, viz., Mo(VI), AI(III), Cr(fII), Fe(III), Mn(H), Ni(II), Cu(ll), Zn(II) and Cd(II). The separation technique has been extended for the recovery of - 95% Co(II) from spent catalysts. The purity of the metal ions thus obtained approximates to 99%.

Cobalt is one of the most useful non-ferrous metals and employed for diverse industrial uses. Some of the cobalt bearing compounds play a significant role in catalyzing a number of important chemical reactions. Because of an ever-increasing demand for the metal, its primary sources are depleting very fast and consequently the focus is now on the development of viable procedures for the recovery of pure metal from secondary sectors l

-3

• Liquid-liquid extraction is one of the most attractive alternatives for this purpose. The extractants used earlier for the separation and recovery of cobalt mainly include high molecular weight amines, hydroxyoximes and organophosphorus compounds. Amongst these reagents the use of organophosphorus extractants offers advantages such as better stability and quicker phase separation over amines and oximes. The extractants, namely, di(2-ethylhexyl) phosphoric acid (DEHPA)4,5 mono(2-ethylhexyl) phosphoric acid (H2MEHPA)6 and bis (2,4,4-trimethylpentyl) phosphinic ac id (Cyanex 272)7.8 have been reported fo r the extraction and recovery of cobalt However, most of these extractants suffer from the drawback of interference from the commonly assoc iated metal ions or strict control of extraction conditions required for the separation of cobalt.

The authors observed that Cyanex 923 (a mixture of four trialkylphosphine oxides, viz. , R3P=0, R2R'P==0, RR'2P=0, '3=0 where R and R' are /1 -

u':l); . Jl . -hc:yl d ai i:-' , rc:-, e ' ti vely) ofk'!> sekcti v~

COllJ ' iull~ for the separaliol of Co(H) from mo!>t of the ol1monly asso iated netal ions. The inherent

advantages of Cyanex 923 such as quicker phase separation and better recycling capacity add to the level of commercial viability of the extractant for the recovery of cobalt Keeping this in mind, the present study was planned to investigate the extraction behaviour of Co(ll) and some other associated metals ions from hydrochloric acid medium in a toluene solution of Cyanex 923. The partition data have been used for identifying the composition of the extracting species. The loading and regeneration capacity of the extractant have been assessed and the stability of the extractant solution towards its long-term acid contact has been checked. Based on the distribution data, the separations of Co(ll) from metal ions such as Mo(VI) , AI(III), Cr(III), Fe(III), Mn(II), Ni(IJ) . Cu(II) , Zn(II) and Cd(II) have been carried out The developed conditions of separation have been applied for the recovery of pure cobalt from spent Co-M n and Co­Mo catalysts.

Materials and Methods Cyanex 923 (Av. mol. wt 348, purity 93%) was

obtained from Cytec Inc. , Canada and used without further purification. All other chemicals were AR grade, reagents from E. MerckfT. Baker (Indi a). The metal ion solutions were prepared by dissolving their suitable salts in a known volume of doubly distilled water containing a minimum concentration of the corresponding mineral acid . The solu tions were standardized by the usual complexometric titrJ ions.

The spent Co-Mn catalyst was obtained from a petroleum refining un it si tuated in Assam (India),

GUPTA et al.: EXTRACTION OF Co(I1) WlTH CY ANEX 923 2955

whereas spent Co-Mo catalyst was procured from Project and Development Indian Ltd, Sindri (India).

Quantification of the metal ions in the aqueous phase was done on an Atomic Absorption Spectrometer (GBC A vanta, Australia).

Procedure The aqueous and organic solutions having (NO)

ratio of 1: 1 (v/v) were shaken at room temperature (25±3°C) for two minutes to ensure complete equilibration. The two layers were separated and the metal concentration in the aqueous phase was assayed by AAS. The minimum equil ibration time required for achieving the li miting extraction of Co(l1) has been found to be one minute and it was observed that prolonged shaking does not affect the results. In all the subsequent studies the aqueous and organic phases were contacted for two minutes.

About 5 g of the powdered sample of spent catalyst was heated with 20 mL of 10 moIL- I HCI and filtered. The solution was made up to 100 mL keeping the overall acidity to 4 moIL- I HC\. The stock solutions of spent Co-Mn and Co-Mo catalysts were labelled as (A-I) and (B -1), respectively . To get a representative value three samples of each of these catalysts were processed by the proposed procedure.

The experimental conditions of different studies are mentioned along with the corresponding results .

(A) 0.3 mol L-1cyanex (toluene)

2

80

~ e..... 60

" .!2 U e x w

40

20

o 2 4 6 10

Results and Discussion

Effect of the nature of diluent To investigate the effect of nature of diluent on the

extraction of Co (II) in 0.6 molL- I Cyanex 923, the following organic solvents were studied (dielectric constants in paranthesis); n-hexane(2 .0), kerosene (160-200°C fraction, 2.0), xylene (2.2), toluene (2.4) , chloroform (4.9) and cyc\ohexanone (18 .3) . The results indicate that the extraction of Co (II) decreases with an increasing dielectric constant of the diluent. The percentage extraction in diluents like n-hexane (75 %), kerosene (74%), xylene (72%) and toluene «74%) with more or less equal dielectric constants is of about the same order. Chloroform and cyclohexanone showed percentage extraction of 63 and 48 respectively. In all further studies toluene was used as diluent. Kerosene can be used for commercial purposes without any changes in the results.

Extraction behaviour of metal ions from HCI in Cyanex 923

In order to identify the optimum conditions for the separations, the extraction behaviour of the metal ions has been investigated in two different molarity of Cyanex 923. Figure 1 (A&B) shows the extraction of Co(ll) along with that of Mo(VI), AI(IIT), Cr(lII) ,

(8) 0.6 mol L-1cyanex 923 (toluene) 4

7

12 0 2 4 6 10 12

fl e l cone. (mol L - I )

Fig. I- Extrac tion behaviour of metal· ions (l>d O-3 moiL- I) from 1-10 molL- I HCl using Cyanex 923 (toluene); (A): 0.3 moiL- I and (B): 0.6 molL- I. [I. Mo(VI); 2, Fe(Hl ); 3, Cu(Il); 4, Zn(Il); 5, Cd(n ); 6, Co(I1); 7, Mn(II )].

2956 INDIAN J CHEM, SEC A, DECEMBER 2003

Fe(III) , Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II) from 1-10 molL - I HCI in 0.3 molL - I and 0.6 molL- I

Cyanex 923, respectively. Co(ll) shows an increase in its extraction with increasing acidity and in 0.6 molL- I Cyanex 923 a quantitative extraction (>95%) is attained at higher acidity . As indicated by the plots the extraction of Mo(VI), Fe(III) , Zn(II) and Cd (II) remains quantitative in 0.6 molL- I Cyanex 923 over a wide range of acidity. However, in 0.3 molL- I Cyanex 923 beyond 6 moiL - I HCI the extraction of Mo(VI), Zn(II) and Cd(II) decreases to around 60% at 10 moiL - I HCI. The extraction of Cu(II) exhibits a maximum around 5 molL- I HCI. Mn(II) is extracted only in small percentage beyond 5 moiL - I Hel whereas AI(III), Cr(lll) and Ni(lI) show negligible extraction « 3%) over the entire investigated range of acid molarity.

Stripping For the stripping of Co(IJ), H2S04 (lxlO-3 molL- I)

was used. One minute contact time was found sufficient for maximum recovery. The variation in the percentage recovery was studied at different A10 ratio (0.1, 0.2, 0.5, 1, 2) . Results suggest that the volume of the stripping reagent required for the extraction is one-tenth the volume of the organic phase. Two washings of the organic phase are required for quantitative stripping of Co(ll).

Effect of cr ion concentration Apparently the extraction of Co(ll) in Cyanex 923

is by solvation of the chloro species . In order to identify the extracting Co(ll) species, the effect of CI­ions on the extraction has been investigated. The acidity of the aqueous phase was kept at 4 molL- I

HCI while CI- ion concentration was varied (0.5 to 2 moiL-I) by using NaCI. Linear log-log plot of [CI-] vs . D with a slope of around four suggests the extraction due to solvation of neutral H2CoCI4 . It is important to point out that the extraction of Co(ll) from a mixture of HCI and NaCI follows more or less the same pattern as that from HCI singly . However, a consistent increase of 5-10% in the extraction is observed from the mixture of HCI and NaCI compared to that from HCI singly . This may be attributed to the salting out effect due to the presence of Na+ ions. The substitution of NaCI for HCI to attain the des ired molarity of CI- ions is useful in avoiding the use of a higher concentration of acid for obtaining the separations of Co(lI).

Effect of the extractant and metal ion concentration The variat~on in the partitioning of Co (II) with

increase in concentration of the extractant [3x lO-2 -0.6 molL - I Cyanex 923] has been studied. Linear log-log plot with a slope of around two suggests the involvement of two molecules of the extractant in the formation of the extracting species. The extracted Co(ll) species can thus be formulated as H2CoCI4.2R (R = Cyanex 923) .

To determine the loading capacity of Cyanex 923 for the extraction of Co(II) the metal ion solutions of different concentrations were subjected to extraction. Results (Fig. 2) show that the change in the distribution of the metal ion is insignificant in the concentration range lxlO-5 -0.3 molL- I indicating that the extracting species does not change in this range of metal ion concentration. Beyond this specified upper limit of metal ion concentration, the extractant gets saturated and the loading condition sets in. From the data it is concluded that Cyanex 923 can hold Co(ll) equivalent to half of its molar concentration. This observation supports the stoichiometry of metal to extractant as proposed above.

Or--------------------------------.

-\

a -2

J ~ "0 E -3

6' '0' ~ 0/)

.2 -4

-5 •

-6+----.----.---.----.----.---~--~

-6 -5 -4 -3 -2 -1 o

log [Co(II), molL·I]A quoous

Fig. 2-Effect of concentration of metal ion (lxlO-5-0 8 moiL- I) on the di stribution of Co(ll) from 4 mol L- I I-ICI+2 moiL - I NaCI using 0.6 moIL- I Cyanex 923

GUPTA et ai.: EXTRACTION OF Co(lI) WITH CY ANEX 923 2957

Stability and recycling capacity of the extractants The stability and recycling capacity of an extractant

help in assessing its utility for commercial purposes. In the present study the said characteristics of Cyanex 923 have been investigated in terms of change in the extraction of Co(Il) at 4 moIL- ' HCl+2 moLl NaCl using 0.6 moIL- ' Cyanex 923. The stability of the extractant towards prolonged use in acidic medium was checked by keeping its solutions in contact with 10 moIL- ' HCl and observing the extraction of Co(JI) periodically after five days. No appreciable decrease in the percent extraction was observed even after a contact of fifty days.

The regeneration capacity of Cyanex 923 was determined by carrying out successive extraction and stripping cycles. The organic phase after the stripping was regenerated by washing with water until the washings were neutral. The recovery of metal ion of each step was calculated from the amount that gets extracted in the organic phase in a particular cycle. It was observed that there is very little change in the percent extraction/stripping of Co(1l) even up to twenty cycles.

Binary sepa rations of Cor II) The partition data of the metal ions in Cyanex 923

provide useful guidelines for achieving the separations of Co(ll) from the associated metal ions. Under the above experimental conditions Co (II) was separated from Mo(VI)/Fe(III)/Cu(II)/Zn(II)/Cd(II) by extraction of the latter metal ion from 4 molL - I HCl using 0.3 moIL- ' extractant. In all the cases, .95 % Co(J1) remained in the aqueous layer and the extracted metal ion was recovered using a suitable strippant. For the separation of Co(II) from Al(III), Cr(III), Mn(II) and Ni(II), the cr ion concentration of the aqueous phase was adjusted to 4 molL- ' HCl + 2 moIL- ' NaCI. Co(Il) was selectively extracted in 0.6 molL - I HClleaving the other metal ion in the aqueous phase. In all cases except Mn(II) , 98-99% Co(Il) was recovered from the organic phase using IxlO-3 molL- 1 H2S04• However, in the case of Co(II)-Mn(IJ) pair, a part of Mn(II) was co-extracted in the organic phase. Prior to the stripping of Co(II), Mn(JI) was stripped by scrubbing repeatedly (five times) with 8 mol L- 1 HCI.

Recovery of cobalt (ll) from spent catalysts A 50 mL aliquot of (A-I) or (B-1) was equilibrated

with 4x25 mL of 0.3 molL- 1 Cyanex 923. This resulted in the quantitative extraction of Mo(VI) , Fe(III), Cu(lI) and Zn(U) leaving other metal ions including Co(Il) in

the aqueous phase. The concentration of Cl- ions in this aqueous phase was then raised to 6 molL - I by the addition of ca. 5.85 g of NaCI and adjusting the final volume to 50 mL. This was contacted with 4x25 mL of 0.6 moIL- ' Cyanex 923. After washing the organic layer with 5x lO mL of 8 moIL- ' HCl to remove Mn(H), Co(ll) was stripped using 2xlO mL of IxlO-3

molL- 1 H2S04 and the solution thus obtained was made up to 50 mL. Co(Jl) solutions recovered from (A-I) and (B-1) were marked as (A-F) and (B-F), respectively . The organic phase containing 0.3 molL- 1

extractant was regenerated by washing subsequently with 2xl0 mL 5% NH4Cl in 75% NH3, 2x lO mL water, 2x lO mL 0 .1 molL- 1 H2S04 and 2x l0 mL water whereas that containing 0.6 molL- 1 extractant was regenerated by washing with 2x lO mL water. In order to get representative data three samples of spent catalysts were processed by the proposed procedure. To assess the recovery and purity of the metal the composition of (A-I) , (B -1), (A-F) and (B-F) is given in Table l. It is evident from the data that more than 95% of cobalt is recovered with a purity around 99%.

Conclusions The foregoing efforts have resulted in the

deve lopment of a simple and convenient method for the recovery of pure cobalt from spent catalysts. The method does not require any strict control of the phase parameters. The stripping reagents used are simple and not likely to pose any problem in further processing of the recovered metal solutions. The extractant has low aqueous solubility and can be

Table I- Recovery of Co(II) from spent catalysts solutions

Metal ions (A-I)/ (A-F)/

(B-1) (B -F)

Spen.t Co-Mil catalyst

Co l.Ox 10~ 9.6x 102 Mn 8.3x 103 5.2 Al 1.6x 102 <0.01 Fe 98 0 .57 Ni 69 <0.01 Cr 33 <O.Ol Cu 29 1.9

Spellt Co-Mo catalyst

Co 7.9x 102 7.67x 102 Mo I.7 x 103 0.67 AI· I.S x 103 0. 12 Fe l. 8x I 02 2.3 Zn 47 0 .58

Recovery (%)

96±1

96±1

Purity (%)

99.S±0.2

99.6±0.2

2958 INDIAN J CHEM, SEC A, DECEMBER 2003

conveniently recycled thus scoring over many of the extractants used earlier for the recovery of cobalt. The extent of recovery of the metal and its purity combined with advantages like the good loading and regeneration capacity of the extractant favour the economic viability of the process. The chemistry of the proposed process has been tested on a bench level scale but apparently there seems to be no difficulty in scaling up the methodology .

Acknowledgement The fin anci al support to the work by the University

Grants Commission and the Council of Scientific and Industrial Research, New Delhi (India) is gratefully

acknowledged. The authors are thankful to Cytec Inc. Canada for the gift sample of Cyanex 923.

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Hydrometallllrgy, 54 (2000) 11 7. 6 Yadav S K, Singh 0 Y & Tandon S N, Indian J Chem, 36A

( 1994) 53. ' 7 Daxing L, KlIongye (Beijing), 5 ( 1996) 5 1,

8 Sarangi K, Reddy B R & Das 'R P, Hydrolll etallurgy, 52 (1998) 253.