acfr

i a,

Keywords:Solvent extractionCobalt(II)Nickel(II)Octyl(phenyl)phosphinic acid

ioncanthat

pH. Cobalt was preferentially extracted over nickel in all pH range, DpH value of 1.8 indicates the pos-

Co Ni

nickel increases and reduction in separation factor was observed. The composition of extracted species

d theiom va

tion [8,9]. In sulphate solutions cobalt and nickel exist as divalenthydrated ions and extraction takes place through cation exchange,therefore, organo-phosphorus acids as cation exchangers areemployed for their extraction and separation. Much of the begin-ning work has been reported using di(2-ethylhexyl) phosphoric

process [14]. A process for separation of cobalt and nickel from asolution containing 30 g/L each of cobalt and nickel at pH 5.5 byusing 20 vol.% PC88A in kerosene has been patented by Fujimotoet al. [15]. Di(2,4,4-trimethylpentyl) phosphinic acid, a product ofCytec chemicals with trade name Cyanex 272 has a much strongerability for separation of cobalt from nickel in sulphate solutionthan PC 88-A. Rickelton et al. [16] have shown higher separationfactor between cobalt and nickel for Cyanex 272 compared to theanalogous phosphoric and phosphonic acid and obtained purecobalt metal from a mini-plant scale continuous counter-current

Corresponding author. Address: Hydrometallurgy Section, MPD, MG, BhabhaAtomic Research Centre, Trombay, Mumbai 400085, India. Tel./fax: +91 02225592959.

Separation and Purication Technology 89 (2012) 6670

Contents lists available at

Separation and Puri

.e lE-mail address: jnsharma@barc.gov.in (J.N. Sharma).however, owing to their similar chemical properties in aqueoussolutions, development of separation routes to obtain pure cobaltand nickel compounds from different primary and secondaryresources is difcult using classical methods of separations suchas precipitation, oxidation and crystallization. Solvent extractionusing solvents selective for one of the metal ions has been success-fully employed for this purpose [1,2]. Literatures on this separationindicate extensive use of organo-phosphorus based extractants ofthe class phosphoric, phosphonic and phosphinic acids [37]. Someof the reports also use tributyl phosphate, thiophosphinic acid, LIX860 (5-dodecylsalicylaldoxime) and trioctyl amine for this separa-

composition of extracted complex with D2EHPA was determinedby Grimm and Kolarik [11,12] as Co(HA2)2 and Ni(HA2)2. Organ-ophosphonates, mainly 2-ethylhexyl phosphonic acid mono-2-eth-ylhexyl ester supplied by Daihachi Chemical Industry Limited withthe trade name of PC 88-A has shown a Co/Ni separation factor ofabout 200 times greater than that of D2EHPA under similar condi-tion of extraction. In 1978, Nippon Mining Corporation started theuse of PC 88-A to extract cobalt in their Hitachi Renery [13]. Sim-ilar reagent of the trade name P507 (2-ethylhexyl 2-ethylhexylphosphonate) has been used in China for separation of cobalt froma hydroxide precipitate produced in the nickel anolyte purication1. Introduction

Extraction of cobalt and nickel anextensively studied and reviewed fr1383-5866/$ - see front matter 2012 Elsevier B.V. Adoi:10.1016/j.seppur.2012.01.015determined by slope analysis of the plots of logD vs. log[OPPA]2 and logD vs. equilibrium pH indicatesformation of the species MA2 (HA)2 for both the metal ions. Temperature dependence studies of theextraction constant Kex, showed that the extraction is enthalpy driven with unfavorable entropy.OPPA/dodecane was employed for cobalt/nickel separation from rafnate solution obtained during sol-vent extraction process of spent ammonia cracker catalyst leach solution. 1.0 M OPPA/dodecane in coun-ter-current extraction at O/A of 1.0 was efciently used to separate cobalt from nickel.

2012 Elsevier B.V. All rights reserved.

r separation have beenrious aqueous streams,

acid (D2EHPA). Ritcey et al. [10] have reported the rst processto separate cobalt from nickel by using D2EHPA. In China, severallaboratories and pilot plants have extensively studied separationof cobalt and nickel using D2EHPA in the early seventies. TheAvailable online 14 January 2012 0.5sible separation of both the metal ions from each other. Almost complete extraction of cobalt is observedat pH 4.5 where separation factor (b = D /D ) is approximately 1400, above this pH co-extraction ofEvaluation of n-octyl(phenyl)phosphinicfor separation of cobalt(II) and nickel(II)

S. Mondal a, Vikas Kumar a, J.N. Sharma a,, R.C. HublaHydrometallurgy Section, Bhabha Atomic Research Centre, Trombay, Mumbai, IndiabMaterials Group, Bhabha Atomic Research Centre, Trombay, Mumbai, India

a r t i c l e i n f o

Article history:Received 8 November 2011Received in revised form 29 December 2011Accepted 8 January 2012

a b s t r a c t

Solvent extraction separatOPPA dissolved in n-dodedetermined. It was found

journal homepage: wwwll rights reserved.id (OPPA) as an extractantom sulphate solutions

A.K. Suri b

of divalent cobalt and nickel from sulphate solutions was studied usinge and the optimum conditions for separation of cobalt and nickel wereextraction of both the metal ions increases with increase in equilibrium

SciVerse ScienceDirect

cation Technology

sevier .com/locate /seppur

extraction using Cyanex 272. Outokumphu Harjavalta Metals haveused Cyanex 272 to recover cobalt from nickel matte leaching solu-tion containing about 1.0 g/L of cobalt and 130 g/L of nickel. It em-ploys four stages of extraction, ve stages of scrubbing and fourstages of stripping with the vertical smooth ow mixer-settlers,to obtain strip solution containing 110 g/L cobalt and 0.02 g/L nick-

aration from more acidic aqueous solutions, thus, solving manyproblems associated with operation of extraction process of cobalt

with buffer solutions and the possible error in measurements was

showed the increase in extraction of both the metal ions with in-

S. Mondal et al. / Separation and Puricand nickel at pH range of 6.27.3, required for effective separationusing Cyanex 272. In the present studies, OPPA has been tested forextraction and separation of cobalt and nickel from synthetic solu-tions as well as from the rafnate solution of spent ammoniacracker catalyst process having pH in the range of 4.44.7.

2. Experimental

2.1. Reagents

OPPA was synthesized according to the procedure reportedelsewhere [19]. The product obtained on reuxing ethanolic solu-tion of phenyl phosphinic acid, 1-octene and benzoyl peroxidefor 24 h. was extracted in toluene and washed with 1.0 M HCland successively with water to remove any unreacted phenyl phos-phinic acid. The product was then concentrated by degassing at75 C and 0.01 mm Hg pressure. Potentiometric titration of theproduct in 75:25 volume ratios of ethanol to water showed a purityof more than 99% with no dibasic acid and less than 1.0% of neu-trals impurities. The structural formula of OPPA is shown in Fig. 1.

Phenyl phosphinic acid was synthesized from FriedelCraftsreaction of benzene, phosphorus trichloride and anhydrous alumi-num trichloride [20]. n-Dodecane was used as diluent and obtainedfrom M/s. Numex Chemical Corporation, India. Demineralisedwater was used for preparation of all the aqueous solutions. Thestock solutions of cobalt(II) and nickel(II) sulphate each of 0.17 Mmetal ions were prepared by dissolving analytical grade chemicals

P

OH

OC8H17el [17]. Devi et al. [6] have indicated that a high separation factor ofabout 3000 is obtained between cobalt and nickel at pH 7.2 byusing Cyanex 272. Danesi et al. [18] have also used the sameextractant for cobaltnickel separation and showed that the sepa-ration factor is enhanced by decreasing the unbound concentrationof Cyanex 272 in the organic phase.

Extraction processes for cobalt and nickel separation withCyanex 272 are effectively used in the pH range of 6.27.3. ThispH is difcult to adjust and maintain as it is very close to hydroly-sis pH of cobalt and nickel. Therefore, there is a need for an alter-nate extractant which can work at lower pH, so as to eliminate theneed for stringent pH control as required in the case of Cyanex 272.

In the present work, the extraction and separation of cobalt andnickel from sulphate solution is carried out using octyl(phenyl)-phosphinic acid (OPPA). OPPA is more acidic than its dialkyl coun-terpart as phenyl group has stronger drawing ability for electronsthan alkyl groups, resulting in the OH group more acidic in thecase of phenyl substituted phosphinic acid. The pKa of OPPA is re-ported to be 4.63 which is less than pKa of dialkyl phosphinic acids(Cyanex 272 and di-n-octylphosphinic acid) by approximately0.8 units [19]. The more acidic OPPA can therefore, be used for sep-Fig. 1. Structural formula of octyl(phenyl)phosphinic acid.crease in equilibrium pH. The pH0.5 (at logD = 0) values of cobaltand nickel are 3.0 and 4.8, respectively, which indicate that cobaltis extracted at lower pH value than nickel. These pH values forextraction by OPPA are lower than the corresponding dialkyl phos-phinic acid, Cyanex 272, by a pH unit of approximately 22.5 [7], asOPPA being more acidic than Cyanex 272 it can, therefore, be usedin extraction processes at lower aqueous pH. The DpH0.5 value ofcobalt and nickel extraction in the present experimental conditionsless than 0.2 pH unit. Aqueous solutions (10 ml) containing0.017 M Co2+ and 0.017 M Ni2+ ions separately, were equilibratedwith equal volume of 1.0 M OPPA in n-dodecane for 15 min. Afterphase separation, equilibrium pH of the rafnate solutions wasmeasured. The solutions were diluted as required with dilute HCl(1 M) and analysis of metal values was carried out with GBCmake Model Avante 1.31 atomic absorption spectrometer. Themetal values in the organic solutions were calculated by materialbalance. The error in the analysis was within 2.0% and the detec-tion limit for cobalt and nickel was 0.5 lg/mL. The distributionratio (D) for the metal ions is calculated as the ratio of concentra-tion of metal ion in the organic phase to the aqueous phase. Thestripping studies were carried out with dilute sulphuric acidsolutions prepared from a standard 1.5 M sulphuric acid solution.For measuring the effect of temperature on distribution ratio,experiments were conducted in a thermostat (0.5 C) withmechanical shaking for 30 min. Temperature was varied in therange of 300333 K. Agitated samples were then left in the ther-mostat for half an hour to allow complete separation of twophases. The aqueous phases were separated and analyzed for metalions.

The extraction data generated from synthetic solutions wereused for separation of both the metals from each other present inthe rafnate solution of spent ammonia cracker catalyst processobtained from solvent extraction of spent ammonia cracker cata-lyst leach liquor.

3. Results and discussion

3.1. Effect of equilibrium pH

The extraction of cobalt and nickel from sulphate solution wasstudied using sodium salt of 1.0 M OPPA in the initial pH rangeof 1.06.5, corresponding to the change in the equilibrium pH from1.0 to 7.0, respectively. The plots of logD vs. equilibrium pH (Fig. 2)in demineralised water. 0.6 M sodium sulphate as inert non-complexing electrolyte was also added to the stock solution, withthe assumption to keep ionic strength of the medium constantduring extraction. The sulphate solution mentioned in this paperis contra-anion of cobalt, nickel and sodium ions, the concentrationof which in the stock solution is approximately 0.94 M. pH adjust-ment of the solutions was done by adding concentrated solutionsof NaOH and H2SO4.

2.2. Extraction procedure

Extraction experiments were carried out by the conventionalbatch method using separating funnels. Organic solutions wereprepared by dissolving appropriate amount of extractant inn-dodecane. The initial pH of the aqueous solutions was adjustedto desired value before the equilibration was carried out. pH mea-surements were done by a combination glass electrodes calibrated

ation Technology 89 (2012) 6670 67is obtained as 1.8. The extraction of both the metal ions increaseswith increase in equilibrium pH, almost complete extraction ofcobalt was obtained between pH 4.2 and 4.5 where separation

therefore largely associated with two dimers or four formula unitsof OPPA in organic phase. Extraction of cobalt and nickel from sul-phate medium under these conditions thus can be represented as:

M2aq 2HA2 orgMA2HA2 org 2Haq; 1where (HA)2 represents dimer molecule of OPPA.

The conditional extraction equilibrium concentration constant,

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

-2

-1

0

1

2

3 Cobalt Nickel

Log

D

Equilibrium pH

Fig. 2. The dependence of D on equilibrium pH. Organic phase: 1.0 M OPPA,

68 S. Mondal et al. / Separation and Purication Technology 89 (2012) 6670factor (b = DCo/DNi) obtained is approximately 1400, above this pHco-extraction of nickel was also higher and separation factor re-duces. The plots of Fig. 2 were linear with slope of 1.94 for cobaltand 1.91 for nickel indicating the exchange of approximately twomoles of H+ with one mole of extracted metal species.

3.2. Effect of extractant concentration

The effect of OPPA concentration on the extraction of cobalt andnickel each at 0.017 M was studied in the range of 0.11.0 M OPPA/n-dodecane solution at initial aqueous pH of 4.5. Mason et al. [19]have reported that OPPA exists as dimer in non-polar organic dilu-ents, at low metal ion concentration and assuming the OPPAmonomer concentration as negligible, the analytical concentrationof OPPA can be expressed as [OPPA] = 2 [(OPPA)]2. The logD vs. log[OPPA]2 plots for both the metal ions are shown in Fig. 3. The plotswere straight lines with slopes of 2.2 and 1.66 for cobalt and nickel,respectively, indicating the association of two moles of extractantin the extracted metal species. LogD vs. log[OPPA]2 plots for boththe metal ions were also drawn at different metal ion concentra-tions (0.008 and 0.034 M), the plots were straight lines and nochange in the slopes was observed. This ascertains that the stoichi-

aqueous phase: cobalt sulphate (0.017 M Co) and nickel sulphate (0.017 M Ni)solution, O/A = 1. T = 303 K.ometry of metalOPPA complexes does not change under the stud-ied experimental conditions. As OPPA exists as dimer in non-polarorganic diluents, the doubly charged cobalt and nickel ions are

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2-2

-1

0

1

2

3 Nickel Cobalt

log

D

log [OPPA] 2

Slope = 2.2

Slope = 1.66

Fig. 3. The dependence of D on initial concentration of (OPPA)2. Organic phase:OPPA in dodecane, aqueous phase: cobalt sulphate (0.017 M Co) and nickel sulphate(0.017 M Ni) solutions at initial pH of 4.5, O/A = 1. T = 303 K.Kex, in reaction (1) is described as:

Kex MA2HA2 orgHaq2=M2aq HA2 org2; 2D is distribution coefcient and dened as:

D MA2HA2 org=M2aq 3By substituting Eq. (3) into Eq. (2) and converting into logarith-

mic form the following equations are obtained.

Kex DHaq2=HA2 org2 4log Kex log D 2 log Haq 2 log HA2 org 5

The log Kex values for cobalt and nickel were calculated at pH4.5 and at different concentrations of OPPA, also at constantextractant concentration and at different pH. The log Kex valuesare summarized in Tables 1 and 2. The values obtained are nearlyconstant with an average value of 5.50 for cobalt and 8.48 fornickel. LogKex values calculated at constant extractant concentra-tion and at different pH, are also constant with the average valuesof5.42 for cobalt and8.52 for nickel. This indicates that the pro-posed extraction equilibrium, as dened by Eq. (5), is valid underthe investigated extraction conditions.

3.3. Loading capacity determination

Loading capacity of OPPA was determined by contacting variousconcentrations of OPPA/n-dodecane repeatedly with aqueous solu-tions containing 0.17 M cobalt at initial pH of 4.5. The phases wereseparated after equilibration and cobalt content in the aqueousphase was determined. The amount of cobalt transferred into theorganic phase in each contact was calculated by mass balanceand the cumulative concentration in the organic phase after eachcontact was determined. Loading capacity of 1.0 M and 0.5 MOPPA/n-dodecane solution was found to be 0.315 and 0.166 M,respectively. The loading capacity of cobalt in the organic phasecorresponds to approximately 1:3 stoichiometry between cobaltto OPPA, thus at higher metal loadings, decrease in the content ofextractant associated with the metal ion was observed.

3.4. Effect of temperature on extraction

The effect of temperature on the extraction of cobalt and nickelwas studied in the temperature range of 303333 K from the aque-ous solutions of cobalt and nickel (0.017 M each) at initial aqueouspH of 4.5 using 1.0 M OPPA/n-dodecane solution. It was observedthat extraction of both the metal ions is exothermic in nature asextraction decreases with increase in temperature. This decreasein extraction with increase in temperature may be due to decrease

Table 1Extraction constant (logKex) of cobalt and nickel at different concentrations of OPPA,T = 303 K, pH = 4.5.

[OPPA] M logKex (Co) Average log Kex (Ni) Average

0.1 5.60 8.160.25 5.53 8.36

0.5 5.47 5.50 8.50 8.371.0 5.40 8.48

in stability of the extracted complex with increase in temperature.These results are contrary to the endothermic nature of extractionobtained in the case of Cyanex 272 based solvents [9,21], wherethe extraction is carried out at elevated temperature for improvedseparation of cobalt and nickel. The results of temperature effecton extraction are shown in Fig. 4 as a function of log Kex vs.

3.5. Separation of cobalt and nickel from spent ammonia crackercatalyst

Spent ammonia cracker catalyst generated by heavy waterplants has been an important secondary source for cobalt produc-tion. Different methods have been used for recovery of cobalt fromthis spent catalyst [22,23]. The catalyst containing about 22% wt/wt cobalt and 0.21% wt/wt nickel on Al2O3/Fe2O3 support was lea-

Table 2Extraction constant (logKex) of cobalt and nickel at different pH, [OPPA] = 1.0 M,T = 303 K.

pH logKex (Co) Average logKex (Ni) Average

2.0 5.29 3.0 5.43 3.5 5.42 8.404.0 5.56 4.5 5.40 8.485.5 8.60 8.526.0 8.60

0 2 4 6 8 10 12 14 16 184

6

8

10

12

14

16

18

[Co] o

rg , g/

L

[Co]aq, g/L

Fig. 6. McCabeThiele plot for extraction of cobalt from rafnate solution of spentammonia cracker catalyst solvent extraction process. Organic: 1.0 M OPPA, aqueousphase: rafnate solution containing 17 g/L Co and 0.5 g/L Ni as sulphate at pH of 4.6.T = 303 K.

S. Mondal et al. / Separation and Puric1000/T, K1 for cobalt and nickel. Enthalpy and entropy of extrac-tion were calculated from slopes (DH0/2.303 R) and intercepts(DS0/2.303 R) of the Vant Hoff equation (Eq. (6)).

log Kex DH0=2:303 RT DS0=R 6

The plots of log Kex vs. 1000/T for cobalt and nickel are straightlines, the slopes and intercepts for cobalt and nickel are 4443, 3260and 20.055, 19.277, respectively. The values of enthalpy andentropy of extraction for cobalt and nickel are 85.89 kJ mol1,63.63 kJ mol1 and 387.0 J mol1K1, 374.5 J mol1K1,respectively.

Most of the liquid cation exchange extraction processes are exo-thermic and driven by large and negative enthalpy and opposed bythe negative entropy. The extraction of cobalt and nickel by OPPAresults in a negative enthalpy (DH < 0) and entropy change(DS < 0). The negative enthalpy change is due to the coordinationof cobalt and nickel by organic ligand OPPA which replaces the rel-atively weak hydrogen bonds with stronger metalligand coordi-nation bonds and negative entropy change is due to the increaseof order caused by the formation of new bonds and also to hydra-tion of the exchanged proton over the disorder generated by metalion dehydration. The enthalpy change of cobalt extraction is morenegative than that of nickel, thus, indicating a more favorableextraction of cobalt over nickel.

-6.0

-5.5

-5.0 Cobalt Nickel3.00 3.05 3.10 3.15 3.20 3.25 3.30

-9.5

-9.0

-8.5

-8.0

-7.5

-7.0

-6.5

Log

K ex

1000/T, K -1

Fig. 4. Effect of temperature on the equilibrium constant of cobalt and nickel.Organic phase: 1.0 M OPPA, aqueous phase: cobalt sulphate (0.017 M Co) and nickelsulphate (0.017 M Ni) solutions at initial pH of 4.5, O/A = 1.1 2 3 4 5 6

0

20

40

60

80

100 CobaltNickel

% E

xtra

ctio

n

Equilibrium pH

Fig. 5. Effect of equilibrium pH on the extraction of cobalt and nickel from therafnate solution containing 17 g/L Co and 0.5 g/L Ni as sulphate at pH of 4.6.Organic: 1.0 M OPPA, O/A = 1. T = 303 K.

ation Technology 89 (2012) 6670 69ched with sulphuric acid in oxidizing medium for complete disso-lution of all the metal ions. Iron and aluminum were extracted by85% saponied 0.5 M HDEHP/0.7 M TBP-dodecane solvent leavingcobalt and nickel in the rafnate. The rafnate containing 17 g/LCo and 0.5 g/L Ni as sulphate solution at pH of 4.6 was taken forCo/Ni separation and referred as feed for further separation stud-ies. 1.0 M OPPA/dodecane was used for cobalt/nickel separationfrom feed solution. The initial pH of feed solution was varied andthe percentage extraction of cobalt and nickel was determined.The plot of % E vs. equilibrium pH (Fig. 5) shows the increase inpercentage extraction of both the metal ions with increase in equi-librium pH. However, the % E of nickel in this case was much lesscompared to extraction of nickel as shown in Fig. 2, where nickelwas taken alone in the solution. This reduction in nickel extractionfrom the solution containing higher concentration of cobalt com-pared to nickel is because of crowding effect produced due to high

extraction of cobalt. From the graph it can be seen that almostcomplete extraction of cobalt with co-extraction of about 25% nick-el was observed at pH 4.5. To evaluate the number of stages re-quired for the extraction of cobalt from the feed solution at achosen O/A ratio, the McCabeThiele plot (Fig. 6) was constructedat O/A ratio varying from 1:7 to 5:1. The results of the plot sug-gested that a three-stage extraction at equal phase ratio will besufcient to load maximum cobalt from the feed solution. Athree-stage counter-current extraction study at equal phase ratio

solution. Parallel efforts are also being made to produce OPPA atlarger quantities to study continuous counter-current mixer-set-tler extraction process at pilot scale.

References

[1] G.M. Ritcey, A.W. Ashbrook, Solvent Extraction, Elsevier, Amsterdam, 1979.[2] M. Meretukov, Process of liquid extraction in colour metallurgy, Metallurgia,

Moscow, 1985.

70 S. Mondal et al. / Separation and Purication Technology 89 (2012) 6670(50 ml each) was carried out in separating funnel. Analysis of co-balt and nickel concentrations in aqueous phase indicates com-plete extraction of cobalt with co-extraction of nickel of about0.1 g/L. The co-extracted nickel was scrubbed in a single contactusing 2.0 g/L cobalt sulphate solution at pH of 4.8 at O/A ratio of1:2. Complete stripping was obtained with 1.0 M H2SO4. The prod-uct solution of cobalt sulphate was analyzed and found to containless than 1.0 mg/L nickel.

4. Conclusions

Solvent extraction of cobalt and nickel from sulphate solutionhas been carried out using OPPA diluted with n-dodecane. Percent-age extraction of both the metal ions increases with increase inextractant concentration. The extraction data have shown that co-balt is completely extracted and effectively separated from nickelwith high separation factor of about 1400 at pH of 4.5.

As OPPA is more acidic than Cyanex 272, it therefore can beused for separation of cobalt from nickel from more acidic aqueoussolutions, thus, in our present studies we have seen that OPPAeffectively separates cobalt from nickel at a lower pH than Cyanex272. Use of OPPA at lower pH solves many problems associatedwith hydrolysis of cobalt and nickel when Cyanex 272 is employedfor their separation at higher pH of 6.27.3. However, availabilityof Cyanex 272 at commercial scale makes the separation processeconomically viable, which is yet to be established for OPPA/n-dodecane solvent.

The composition of extracted species determined by slope anal-ysis of the plot of logD vs. log [OPPA] at initial pH of 4.5 indicatesthe association of two moles of extractant for both the metal ions.Thermodynamic data on extraction have shown that extraction ofcobalt is more exothermic compared to nickel. The extraction datahave been used for cobalt and nickel separation from rafnatesolution of solvent extraction process of spent ammonia crackercatalyst leach solution, containing 17 g/L Co and 0.5 g/L Ni as sul-phate at pH of 4.6. Optimum conditions for separation were deter-mined and observed that 1.0 M OPPA/dodecane in counter-currentextraction could load complete cobalt along with co-extraction ofabout 25% of nickel without adjusting the pH of the feed at O/A ra-tio of 1.0. Co-extracted nickel was scrubbed using 2.0 g/L CoSO4solution at pH of 4.8 and O/A ratio of 1:2. Complete strippingwas observed with 1.0 M H2SO4. The analysis of product solutionshowed less than 1.0 mg/L of nickel. The promising results ob-tained in the present studies indicate the possibilities of usingOPPA/n-dodecane solvent for Co and Ni separation from sulphate[3] J.S. Preston, Solvent extraction of cobalt and nickel by organophosphorus acid,Hydrometallurgy 9 (1982) 115133.

[4] J.S. Preston, Solvent extraction of base metals by mixtures oforganophosphorus acids and non-chelating oximes, Hydrometallurgy 10(1983) 187204.

[5] B.K. Tait, Cobalt-nickel separation: the extraction of cobalt(II) and nickel(II) byCyanex 301, Cyanex 302 and Cyanex 272, Hydrometallurgy 32 (1993) 365372.

[6] N.B. Devi, K.C. Nathsarma, V. Chakravortty, Separation and recovery ofcobalt(II) and nickel(II) from sulphate solutions using sodium salts ofD2EHPA, PC 88A and Cyanex 272, Hydrometallurgy 49 (1998) 4761.

[7] K. Sarangi, B.R. Reddy, R.P. Das, Extraction studies of cobalt(II) and nickel(II)from chloride solutions using Na-Cyanex 272, Hydrometallurgy 52 (1999)253265.

[8] P. Zhang, T. Yokoyama, T.M. Suzuki, K. Inoue, The synergistic extraction ofnickel and cobalt with a mixture of di(2-ethylhexyl)phosphoric acid and 5-dodecylsalicylaldoxime, Hydrometallurgy 61 (2001) 223227.

[9] D. Darvishi, D.F. Haghshenas, Alamdari, E. Keshavarz, S.K. Sadrnezhaad, M.Halali, Synergistic effect of Cyanex 272 and Cyanex 302 on separation of cobaltand nickel by D2EHPA, Hydrometallurgy 77 (2005) 227238.

[10] G.M. Ritcey, A.W. Ashbrook, B.H. Lucas, AIME Annual Meeting, San Francisco,Feb, 1972.

[11] R. Grimm, Z. Kolarik, Acidic organophosphorus extractants XIX: extraction ofCu(II), Ni(II), Zn(II) and Cd(II) by di(2-ethylhexyl) phosphoric acid, J. Inorg.Nucl. Chem. 36 (1974) 189192.

[12] R. Grimm, Z. Kolarik, Acidic organophosphorus extractantsXXV: properties ofcomplexes formed by Cu(II), Co(II), Ni(II), Zn(II) and Cd(II) with di(2-ethylhexyl) phosphoric acid in organic solvents, J. Inorg. Nucl. Chem. 38(1976) 14931500.

[13] T. Kasi, H. Nakayama, K. Motoba, E. Itoh, Paper presented to the Joint Meetingof AIME/MMIJ, Tokyo, Nov 1980.

[14] Beijing General Research Institute of Mining and Metallurgy, BeijingUniversity, Institute of Organic Chemistry and Jinchuan Nonferrous MetalComplex, Youse Jinshu, Non Ferrous Metals 1 (1981) 2529.

[15] A. Fujimoto, I. Miura, K. Noquchi, Belg. 87196301, March, 1979.[16] W.A. Rickelton, D.S. Flett, D.W. West, Cobalt nickel separation by solvent

extraction with Bis(2,4,4 Trimethylpentyl) Phosphinic Acid, Sol. Extr. Ion Exch.2 (1984) 815838.

[17] B. Nyman, A. Aaltonen, S.E. Hultholm, K. Karpale, Application of newhydrometallurgical developments in the Outokumpu HIKO process,Hydrometallurgy 29 (1992) 461478.

[18] P.R. Danesi, L. Reichely-Yinger, C. Cianetti, P.G. Rickert, Separation of cobaltand nickel by liquidliquid extraction and supported liquid membranes withDi(2,4,4,-trimethylpentyl)phosphinic acid [Cyanex272], Sol. Extr. Ion Exch. 2(1984) 781814.

[19] G.W. Mason, N.L. Schoffer, D.F. Peppard, Two Octyl Phenyl Phosphinic Acids,(R)(C6H5)PO(OH), as extractants for selected M(III) and M(VI) metallic cations,J. Inorg. Nucl. Chem. 32 (1970) 33753386.

[20] G.M. Kosolapoff, Organophosphorus Compounds, John Wiley & Sons Inc, NewYork, 1950.

[21] Basudev Swain, Jinki Jeong, Jae-Chun Lee, Gae-Ho Lee, Separation of cobalt andlithium from mixed sulphate solution using Na-Cyanex 272, Hydrometallurgy84 (2006) 130138.

[22] R. Sadanandam, M.F. Fonseca, K. Srikant, A.K. Sharma, S.K. Tangri, A.K. Suri,Production of high purity cobalt oxalate from spent ammonia cracker catalyst,Hydrometallurgy 91 (2008) 2834.

[23] M.V. Rane, V.H. Bafna, R. Sadanandam, A.K. Sharma, K. Ramadevi, N.K. Menon,M.F. Fonseca, S.K. Tangri, A.K. Suri, Recovery of high purity cobalt from spentammonia cracker catalyst, Hydrometallurgy 77 (2005) 247251.

Evaluation of n-octyl(phenyl)phosphinic acid (OPPA) as an extractant for separation of cobalt(II) and nickel(II) from sulphate solutions1 Introduction2 Experimental2.1 Reagents2.2 Extraction procedure

3 Results and discussion3.1 Effect of equilibrium pH3.2 Effect of extractant concentration3.3 Loading capacity determination3.4 Effect of temperature on extraction3.5 Separation of cobalt and nickel from spent ammonia cracker catalyst

4 ConclusionsReferences