Separation of molybdenum and cobalt from spent catalyst using Cyanex 272 and Cyanex 301

International Journal of Mineral Processing 127 (2014) 52–61

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Separation of molybdenum and cobalt from spent catalyst using Cyanex 272 and Cyanex 301 E. Padhan ⁎, K. Sarangi Academy of Scientific and Innovative Research, India CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India

a r t i c l e

i n f o

Article history: Received 18 October 2013 Received in revised form 10 January 2014 Accepted 17 January 2014 Available online 24 January 2014 Keywords: Molybdenum Cobalt Solvent extraction Cyanex 272 Cyanex 301

a b s t r a c t Separation of molybdenum and cobalt from a spent catalyst leach liquor bearing 12.52 g/l Mo, 1.74 g/l Co and 9.98 g/l Al was investigated using solvent extraction technique followed by preparation of metal oxides. Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid) and Cyanex 301 (bis(2,4,4-trimethylpentyl)dithiophosphinic acid) were used as extractants for molybdenum and cobalt, respectively. The effects of various parameters such as pH, concentration of extractant, A/O ratio and temperature on extraction of Mo and Co were studied. The number of stages required for extraction and stripping were determined from the McCabe–Thiele diagram and confirmed by counter current simulation study. The extracted species for Mo and Co were found to be MoO2A2.H2A2 and CoA2.H2A2, respectively. The thermodynamic parameters such as ΔH, ΔS and ΔG were calculated for molybdenum and cobalt. The enthalpy change (ΔH) values for extraction of molybdenum and cobalt were positive indicating the extraction processes were endothermic. The oxides of molybdenum and cobalt were prepared from the strip solutions of molybdenum and cobalt by crystallization followed by thermal decomposition and the products were characterized by XRD and RAMAN spectra. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Molybdenum has been used in different industries because of its high melting point, high tensile strength and has properties such as corrosion and abrasion resistance. The compounds of molybdenum are used as catalysts in petroleum-refining industries, petrochemicals and coal-derived liquids. The molybdenum catalysts are extensively used in petroleum refining industry for hydrodesulfurization and during the process, the catalysts are deactivated and contaminated with toxic material. Due to contamination of toxic material, these are dumped as hazardous waste after some time. But these spent catalysts can be used as secondary resource for metal recovery and valuable products because of increasing environmental concerns and rapid depletion of primary sources. A wide variety of experiments and studies have been carried out to recover molybdenum from spent catalyst using pyro and hydrometallurgical processes. The requirement of large amount of heat energy and evolution of harmful gases in pyrometallurgical processes attracted the attention of researchers towards hydrometallurgical process. Recovery of molybdenum has been studied using different techniques such as ion exchange (Kononova et al., 2003; Liansheng et al., 2001), adsorption (Pagnanelli et al., 2011; Xiong et al., 2011), supported liquid membrane (Basualto et al., 2003; Marchesea et al., 2004; Valenzuela et al., 2000), ⁎ Corresponding author at: CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India. Tel.: +91 7873111893 (Mobile). E-mail address: [email protected] (E. Padhan). 0301-7516/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2014.01.003

precipitation (Zhao et al., 2011a, 2011b) and solvent extraction (Brassier-Lecarme et al., 1997; Saberyan et al., 2003). Although precipitation and adsorption processes are simple and low cost methods for separation of molybdenum and cobalt, it is very difficult to obtain high pure product. Among all these methods, ion exchange and solvent extraction are two processes which can suitably and effectively be used for separation of metal ions. Solvent extraction is a proven technology having high mass transfer rate and can be used for molybdenum and cobalt extraction in commercial scale. This technique has been used for recovery and separation of molybdenum and copper from an acid leaching residual solution of Chilean molybdenite concentrate (Valenzuela et al., 1995). After leaching, molybdenum was extracted with PC-88A. There was no co-extraction of other metals at pH 0.8. Separation and recovery of molybdenum with Alamine 304-1 from the leach liquor of sea nodule containing 0.505 kg/m3 Mo along with Fe, Cu, Co and Ni have been studied (Parhi et al., 2011). The parameters such as contact time, pH, extractant concentration etc. were optimized for extraction of Mo. The number of stages and A:O phase ratio for extraction and stripping were determined from the McCabe–Thiele diagram. Solvent extraction of Mo(VI) has also been studied by diisododecylamine DIDA from sulphuric acid solutions (0.25–0.3 M) (Palant et al., 1998). The possibility of using the extractant without a modifier in the DIDA-diluent system is the advantages of DIDA over TOA tri-n-octyl amine. The influences of aqueous phase acidity, type of diluent, molybdenum concentration, and DIDA concentration on the extraction of molybdenum VI have been examined. Extraction of Mo from the leach liquor of Ni–Mo ore using the mixture of tertiary amine

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

(N-235) and secondary caprylic alcohol dissolved in kerosene has been investigated (Zhao et al., 2011a,b,c) and the effects of several process parameters such as extractant concentration, feed solution pH, O/A ratio, temperature of extraction, and contact time were studied. The extraction efficiency of Mo was 99.4% at pH 3, time 2 min and O/A ratio of 1:4 with 15 v% N-235. The stripping of Mo with a 15% ammonia solution was essentially completed (99.9%) in a single stage at an O/A ratio of 3. Also works have been carried out on Mo extraction using chelating (Sastre and Alguacil, 2001; Zeng and Cheng, 2010) and solvating extractants (Cruywagen and McKay, 1970). Banda et al. (2012) studied the separation and recovery of Mo and Co from a synthetic chloride leach liquor of petroleum refining catalyst by employing TOPO and Alamine 308 as extractants. The synthetic leach liquor contained Mo 394 mg/L, Al 1782 mg/L and Co 119 mg/L in 3 M HCl. After separation of Mo, the concentration of chloride ion was adjusted to 5 M and Co was extracted with Alamine 308. A review paper (Zeng and Cheng, 2009) described different methods for separation/recovery of molybdenum from the leach solutions of spent catalysts. The phosphinic and dithiophosphinic acid extractants (Cyanex 272 and Cyanex 301) have been used for separation and recovery of different metals (Tsakiridis and Agatzini-Leonardou, 2004; Devi et al., 1998; Sarangi et al., 1999; Panda et al., 2013; Gharabaghi et al., 2013; Staszak et al., 2011; Ocio and Elizalde, 2011; Madaeni and Islami, 2013; Knyaz'kina et al., 2010.). But studies on extraction of Mo using phosphinic acid (Cyanex 272) are scanty. In one paper Wu et al. (Wu et al, 2012) described the recovery of Mo from the leach liquor of a low grade black shale Ni–Mo ore. From this leach liquor Mo was separated from Fe, As and V in five stages with an A:O ratio of 1:1 and stripping was achieved in three stages at O:A phase ratio of 5.0. But in the present paper, separation of Mo was carried out from a leach liquor containing Mo, Co and Al. In addition to the flow sheet development for separation of Mo and Co, the synergistic effect of Cyanex 272 and MIBK, thermodynamic parameters such as ΔH, ΔS, ΔG, preparation of molybdenum oxides and tricobalt tetroxide etc. were also reported. For this study, the leaching of spent catalyst was carried out with H2SO4. The leaching condition used was: 20% pulp density, 3% H2SO4, 80 °C temperature, −90 + 75 μm particle size and 3 h leaching time. The leach liquor thus obtained contained 12.52 g/l Mo, 1.74 g/l Co and 9.98 g/l Al. 2. Experimental 2.1. Materials and reagents The spent catalyst containing molybdenum and cobalt was leached with H2SO4 acid and the leach liquor obtained was analyzed for metal ion concentration. The composition of leach liquor was found to be: 12.52 g/l Mo, 1.74 g/l Co and 9.98 g/l Al. The pH of the leach liquor was 2.68. The commercial extractants bis (2,4,4-trimethyl pentyl) phosphinic acid (Cyanex 272) and bis (2,4,4-trimethyl pentyl) dithiophosphinic acid (Cyanex 301) were supplied by Cytec Inc., USA and were used as such without any purification. Distilled kerosene (b.p.180–240 °C) was used as the diluent. In each dilution of Cyanex 272, 5 vol.% TBP and 20 vol.% MIBK were used as phase modifier. The pH of aqueous phase was adjusted with dilute/concentrated H2SO4/NaOH. All other reagents used for the experiment were obtained from Qualigens Fine Chemicals, India. 2.2. Apparatus The pH of the aqueous solution was measured with a digital pH meter (ELICO, model L1 120) provided with a combined glass electrode. The metal ion concentration in the aqueous phase was measured by Atomic Absorption Spectrophotometer (AAS) (Perkin Elmer, Model A Analyst-200). The crystal structure of the product was studied by X-Ray Diffraction, Model X'pert pro-3040/60 using Cu-Kα target with λ = 1.54 Å. The Raman Spectra were obtained with Renishaw via

53

micro Raman spectrometer (Renishaw plc. Gloucestershire, UK) equipped with a 514 nm green laser having 1 cm−1 spectral resolution of Raman shift, X–Y step resolution of 0.1 μm and confocal resolution of 2.5 μm. The laser output power on the sample was set at 5.1 mw with spectrum curve fitting employing Lorentzian peaks with FWHM. 2.3. General experimental procedure For the experiment equal volume of leach liquor containing metal ions and the extractant were equilibrated manually for 5 min. After the phase disengagement the raffinate was separated and equilibrium pH was measured. After required dilution with 10% HCl, the raffinate was analyzed for metal ion concentration with AAS. The metal ion concentration in the organic phase was calculated from the difference between the concentration in aqueous phase before and after extraction. All the experiments were carried out at room temperature (30 ± 1 °C) except for the temperature effect, where the temperature was varied from 10 to 50 °C. As and when required, the metal concentration in the organic phase was determined after filtrating the loaded organic through Whatman 1PS phase separation paper and stripping with dil. NH4OH/dil. H2SO4 followed by analysis. 3. Results and discussion 3.1. Extraction of molybdenum with Cyanex 272 The leach liquor of the spent catalyst contains 12.52 g/l molybdenum, 1.74 g/l cobalt and 9.98 g/l aluminum. From some preliminary experiments it was observed that Cyanex 272 extracts molybdenum at lower pH than cobalt. So studies were carried out to extract molybdenum from the leach liquor prior to cobalt. When equilibrium studies were carried out with only Cyanex 272, there was no phase separation. So 5% TBP was added to each dilution as phase modifier. So the combination of Cyanex 272 + 5% TBP may be considered as the extractant. But TBP also extracts some amount of Mo. The percentage extraction of Mo with 5% TBP at pH 1.25 was 4.34%. Also with increase of Mo concentration in the organic phase, some black particles were observed at the interface of both the phases for which MIBK was used. By adding MIBK it was dissolved and formed two clear phases. These black particles may be due to various heptamolybdate and octamolybdate anions which are exracted at higher concentration of Mo (Bal et al., 2004). Equilibrium study was also carried out with 20% MIBK which showed 13.13% extraction of Mo. So the percentage extraction of Mo obtained in various studies as mentioned below is the combined effect of Cyanex 272, TBP and MIBK. 3.1.1. Effect of equilibrium pH To study the effect of pH on the extraction of molybdenum, experiments were carried out with different initial pH ranging from 0.52 to 3.0. The corresponding equilibrium pH was varied from 0.43 to 2.88. The concentration of Cyanex 272 and A:O phase ratio were kept constant at 0.1 M and 1:1, respectively. The percentage extraction of molybdenum was plotted against equilibrium pH in Fig. 1 which indicated an increase in percentage extraction of molybdenum from 35.3 to 88.34 with increase of equilibrium pH from 0.43 to 0.99. With further increase of equilibrium pH from 0.99 to 2.88, the percentage extraction of molybdenum decreased and reached a value of 1.06 at equilibrium pH of 2.88. This decrease in percentage extraction may be due to decrease in availability of MoO2+ 2 species in feed solution which is extracted by Cyanex 272. The existence of molybdenum species in aqueous 2+ solution increases in the order MoO24 −, HMoO− 4 , H2MoO4 and MoO2 as the acidity of aqueous solution increases (Marchesea et al., 2004). So with decreasing acidity the MoO22 + species decreases. The coextraction of cobalt and aluminum was nil within the entire pH range studied.

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100

100

80

% of extraction

% Extraction

80

60

40

60

40

20

0

20

0

0.1

0.2

0.3

0.4

0.5

[Cyanex 272], M 0 0

0.5

1

1.5

2

2.5

3

Equilibrium pH Fig. 1. Effect of equilibrium pH on extraction of molybdenum. Experimental condition: 0.1 M Cyanex 272, A:O ratio = 1:1.

The plot of log D versus equilibrium pH up to 0.99 (Fig. 2) was a straight line with a slope value of 2.04 indicating the release of two H+ ion during extraction.

3.1.2. Effect of Cyanex 272 concentration To know the effect of Cyanex 272 concentration on extraction of molybdenum, the concentration of Cyanex 272 was varied in the range of 0.01 to 0.4 M keeping the pH of the solution and the A:O phase ratio constant at 1.25 and 1:1, respectively. The plot of percentage extraction of molybdenum versus extractant concentration is shown in Fig. 3. The percentage extraction of molybdenum increased sharply from 4.57 to 88.44 with increase of the extractant concentration from 0.01 to 0.2 M. After 0.2 M, the plot forms a plateau and reached a value of 91.69 at 0.4 M. The plot of log DMo versus log [extractant] was a straight line with a slope value of 1.95 (Fig. 4) indicating the association of two moles of extractant with one mole of molybdenum in the extracted species.

Fig. 3. Effect of [extractant] on extraction of Mo. Experimental condition: pH:1.25, A:O ratio = 1:1.

The extracted species of molybdenum was found to be MoO2·A2·H2A2. 3.1.3. McCabe–Thiele plot for Mo extraction To determine the number of stages and A:O phase ratio required for quantitative extraction of molybdenum, the extraction isotherm was plotted. For extraction isotherm, the leach liquor containing molybdenum was contacted with the organic phase (0.2 M Cyanex 272 + 5% TBP + 20% MIBK) at pH 1.25. The A: O phase ratio was varied from 1:5 to 5:1 while keeping the total volume constant. The aqueous and organic phases were analyzed and the McCabe–Thiele plot was constructed as shown in Fig. 5. The plot indicates three stages at A: O phase ratio of 2:1 for quantitative extraction of molybdenum. To confirm the above data, a 3-stage counter current simulation study was carried out with 0.2 M Cyanex 272 (with 5% TBP + 20% MIBK) at A:O phase ratio of 2:1. The raffinates and the loaded organics were collected and analyzed. The 1st, 2nd and 3rd stage raffinates contained 5.085 g/L, 1.868 g/L and 1.03 g/L molybdenum indicating 59.37%, 85.17% and 91.77% extraction. The concentration of molybdenum in loaded organic was 22.98 g/L. Sufficient amount of molybdenum loaded organic was generated for stripping study. 1.5

1.6 1.0

0.5

y = 1.95x + 2.5 R2 = 0.99

log DMo

log DMo

0.8

y = 2.04x - 1.12 R 2 = 0.99

0.0

0.0

-0.5

-1.0

-0.8 0.4

0.6

0.8

1

1.2

Equilibrium pH Fig. 2. Plot of log DMo vs. equilibrium pH. Experimental condition: 0.1 M Cyanex 272, A:O ratio = 1:1.

-1.5 -2.5

-2

-1.5

-1

-0.5

log [Cyanex 272] Fig. 4. Plot of logDMo vs. log [Cyanex 272]. Experimental condition: pH:1.25, A:O ratio = 1:1.

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

35

The loaded organic contained 22.98 g/l molybdenum and to find out the number of stages and A/O phase ratio for stripping of molybdenum, the McCabe–Thiele diagram was constructed for stripping with 0.5 M NH4OH + 0.5 M (NH4)2CO3. From the McCabe–Thiele plot (Fig. 7) it was evident that quantitative stripping of molybdenum was possible in two counter-current stages at A: O ratio of 1:2. To confirm this, a 2-stage counter current simulation study for stripping was carried out with 0.5 M NH4OH + 0.5 M (NH4)2CO3. The strip solution and the spent organic were analyzed and found to contain 41.92 g/l and 2.01 g/l molybdenum leading to 91.25% stripping.

28

[Mo]Org, g/l

55

21

14

3.2. Extraction of Co with Cyanex 301 7

0

0

2

4

6

8

10

12

[Mo]Aq, g/l Fig. 5. Extraction isotherm of Mo with Cyanex 272. Experimental condition: 0.2 M Cyanex 272 with 5% TBP and 20% MIBK, pH:1.25.

3.1.4. Stripping of molybdenum loaded organic The stripping of molybdenum loaded organic was carried out with NH4OH and to know the stripping efficiency of NH4OH, its concentration was varied from 0.1 M to 2.5 M. The stripping efficiency of NH4OH was shown in Fig. 6 which indicated an increase in percentage of stripping from 56.2 to 81.47 with increase of NH4OH concentration from 0.1 M to 0.4 M and with further increase of concentration, the percentage stripping decreased. With increasing ammonia concentration, there is a possibility of salt formation of ammonia with the free Cyanex 272 in the loaded organic phase. This salt formation may reduce the ammonia concentration in the strip solution which may be the reason for decrease of stripping efficiency at higher concentration. Similar trend was also observed for Mo extraction by solvent extraction and hollow fiber membrane technique using LIX 84I (Rout and Sarangi, 2013). So NH4OH was tested for 2-stage stripping and it was observed that after first stage, the strip solution was not clear and it took more time for phase separation. To avoid this problem, the mixture of (NH4)2CO3 and NH4OH was used as the stripping agent. By adding 0.5 M (NH4)2CO3 to 0.5 M NH4OH the percentage stripping increased to 83.01 and the strip solution was clear.

After extraction of molybdenum, the raffinate contained 1.74 g/l cobalt, 9.98 g/l Al and 1.03 g/l Mo. So to obtain pure cobalt solution, it was necessary to extract cobalt with a suitable extractant. There are many commercial extractants which extract cobalt at higher pH (Cheng, 2006). Cyanex 272 extracts cobalt at pH 4.5–6.0 (Parhi et al., 2008). But while increasing the pH of the above raffinate, the precipitation of aluminum started beyond pH 3.5. So Cyanex 301 (bis(2,4,4trimethylpentyl) dithiophosphinic acid) which extracts cobalt at lower pH (0.5–2.5) was used. 3.2.1. Effect of equilibrium pH on extraction of Co The extraction of cobalt from the raffinate was studied using 0.1 M Cyanex 301 in kerosene within the initial pH range of 0.54–2.5 at 1:1 phase ratio. The equilibrium pH was varied from 0.47–2.21. The percentage of cobalt extraction at different equilibrium pH was plotted in Fig. 8 which showed an increase of cobalt extraction from 0 to 47.88% with increase of equilibrium pH from 0.47 to 2.21. The molybdenum which was left in the raffinate after extraction with Cyanex 272 was co-extracted with cobalt and the percentage of co-extraction was 69.22% at equilibrium pH 2.21. The co-extraction of aluminum with cobalt was nil. The plot of log D versus equilibrium pH (Fig. 8) was a straight line with a slope value of 1.8 indicating the release of two moles of H+ ion per extraction of one mole of cobalt ion. 3.2.2. Effect of Cyanex 301 concentration Various concentrations (0.05–0.3 M) of Cyanex 301 were used to study the effect of extractant concentration on the extraction of cobalt at 1:1 phase ratio. The initial pH of the solution was kept constant at 2.5 by using NaOH. The percentage extraction of cobalt increased 80

60

[Mo]aq, g/l

% Stripping

80

40

60 20

0

40 0

0.5

1

1.5

2

2.5

[NH4OH] in M Fig. 6. Effect of [NH4OH] on stripping of Mo. Experimental condition: [Loaded organic]: 22.98 g/l Mo, A:O ratio = 1:1.

0

4

8

12

16

20

24

[Mo]org, g/l Fig. 7. Stripping isotherm of Mo. Experimental condition: [Loaded organic]: 22.98 g/l Mo 0.5 M NH4OH + 0.5 M (NH4)2CO3.

56

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

60

0.5

0.8

0.4

40

log DCo

-0.5

log DCo

Cobalt extraction, %

y = 1.8x - 2.9 R2 = 0.99

-1.5

20

y = 1.9x + 1.9 R2 = 0.99

0

-0.4

%Extn. logD of Co

-0.8 -1.5

0

-1.3

0

1

2

-1.1

-0.9

-0.7

log [Cyanex 301]

-2.5 3

Fig. 10. Plot of log DCo vs. log [Cyanex 301]. Experimental condition: pH:2.5, A:O ratio = 1:1.

Equilibrium pH Fig. 8. Effect of equilibrium pH on extraction of Co and log DCo vs. Equil.pH plot. Experimental condition: 0.1 M Cyanex 301, A:O ratio = 1:1.

linearly from 21.02 to 90.42 with increase of Cyanex 301 concentration from 0.05 to 0.2 M (Fig. 9). Beyond 0.2 M, the percentage extraction of cobalt increased slowly and reached 99.37 at 0.3 M. Molybdenum was also co-extracted along with cobalt. As shown in Fig. 9, the extraction of Mo increased from 12.45 to 97.45% with increasing Cyanex 301 concentration from 0.05 to 0.3 M. To extract both the metals into the organic phase, 0.25 M Cyanex301 was suitable as 95.22% of Co and 89.63% of Mo was extracted at this concentration. Both the metals were separated by selective stripping. The plot of log DCo versus log [Extractant] (Fig. 10) was linear with a slope value of 1.95 indicating the association of two moles of extractant with the extracted species. The extracted species of cobalt was found to be CoA2.H2A2.

3.2.4. Stripping of cobalt The loaded organic contained 1.72 g/l cobalt and 0.98 g/l molybdenum which was stripped with H2SO4. The stripping studies were carried out with different concentrations of H2SO4 (0.2–5%) at A:O ratio of 1:1. The stripping efficiency as a function of H2SO4 concentration was plotted in Fig. 12 which showed an increase in percentage of stripping from 26.39 to 87.85 with increasing H2SO4 concentration from 0.2 to 5 vol.%. The McCabe–Thiele plot for stripping of cobalt loaded organic was constructed with 2% H2SO4 at A: O ratios within 1:5 to 5:1 and the diagram indicated 3-stages at A:O phase ratio of 1:2 (Fig. 13) for quantitative stripping of cobalt. A 3-stage counter-current simulation

100

2

80

1.6

60

1.2

[Co]org, g/l

% of Extraction

3.2.3. Extraction isotherm for cobalt To extract cobalt from the molybdenum-free solution, it was required to determine the number of stages at the chosen phase ratio for which the McCabe–Thiele plot (Fig. 11) was constructed at pH 2.5 with the feed solution and 0.25 M Cyanex 301 within the A: O phase

ratios of 1:5 to 5:1. Fig. 11 predicted 2 stages of extraction at A:O phase ratio of 1:1. To confirm the above data, a 2-stage counter current simulation study for extraction of cobalt was carried out with the above conditions which showed 0.02 g/l cobalt and 0.05 g/l Mo in the raffinate indicating 98.85% and 95.15% extraction of cobalt and Mo respectively. The loaded organic contained 1.72 g/l cobalt and 0.98 g/l molybdenum.

40

0.8

Co Mo

0.4

20

0

0 0

0.1

0.2

0.3

0.4

[Cyanex 301] Fig. 9. Effect of [Cyanex 301] on extraction of Co. Experimental condition: pH:2.5, A:O ratio = 1:1.

0

0.5

1

1.5

2

[Co]aq, g/l Fig. 11. McCabe–Thiele diagram for extraction of cobalt. Experimental condition: 0.2 M Cyanex 301, pH:2.5.

100

-5.2

80

-5.3

ln kext.

% Stripping

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

60

57

3.00 2.50 y = -3.77x + 14.2 R2 = 0.99

-5.4

2.00 1.50

-5.5

40

y = -0.58x - 3.5 R2 = 0.99

Mo

1.00

Co

-5.6

0.50

20 -5.7 2.9

3.0

3.1

3.2

0 0

1

2

3

4

5

3.3

3.4

3.5

0.00 3.6

1000/T, K

6

[H2SO4], % Fig. 12. Effect of [H2SO4] on stripping of cobalt loaded organic. Experimental condition: [Loaded organic]: 1.72 g/l Co and 0.71 g/l Mo.

Fig. 14. Effect of temperature on extraction of molybdenum and cobalt. Experimental condition: Mo extraction: 0.08 M Cyanex 272, A:O = 1:1. Co extraction: 0.05 M Cyanex 301, A:O = 1:1.

the equilibrium constant K, is written by study for stripping of cobalt loaded Cyanex-301 was carried out with 2% H2SO4 at A: O phase ratio of 1:2 to confirm the above data. The analysis of 3rd stage spent organic showed 0.5 mg/l cobalt indicating 99.71% stripping. There was no stripping of molybdenum with cobalt. Molybdenum was stripped out with 0.5 M (NH4)2CO3 + 0.5 M NH4OH at A:O = 1:1 and was found to be 96.94%.

4. Extraction mechanism for molybdenum and cobalt

K¼D

ð2Þ

 þ 2 H aq

ð3Þ

½H 2 A2 2org

where, D is the distribution coefficient and is the ratio of metal ion concentration between organic and aqueous phases.

4.1. Extraction mechanism for molybdenum Molybdenum exhibits different ionic forms depending on the pH value of the aqueous solution. With increasing the pH value, molybdenum changes from cationic to anionic species. Within the pH range of 1 to 2, it exists in the form of MoO2+ (Fujiia et al., 2001) which can be 2 extracted by acidic extractant like Cyanex 272 and the extraction mechanism can be described by Eq. (1) þ

2þ

 2 ½MoO2 A2 :H2 A2 Org Hþ Aq  K¼  2 MoO2þ 2 Aq :½H 2 A2 org

½MoO2 Aq þ 2½H 2 A2 Org ⇔½MoO2: A2 :H 2 A2 Org þ 2H Aq

ð1Þ

D¼

½MoO2 A2: ½H 2 A2 org ½MoO2 2þ aq

:

ð4Þ

Taking logarithm of Eq. (3) and rearranging Eq. (5) is obtained logD ¼ logK þ 2pH þ 2 log½H2 A2 org

ð5Þ

the slopes for the plots of log D vs. equilibrium pH (Fig. 2) and log D vs. log [H2A2] (Fig.4) for Mo were 2.04 and 1.95, respectively support the above extraction mechanism. 4.2. Extraction mechanism for cobalt

5

The extraction mechanism of cobalt with Cyanex 301 can be written as:

4

þ

2þ

[Co]Aq., g/L

CoAq þ 2H 2 A2Org ⇔CoA2 :H 2 A2Org þ 2H Aq

ð6Þ

3 Table 1 Thermodynamic parameters for extraction of Mo with Cyanex 272 and extraction of Co with Cyanex 301.

2

Mo-Cyanex 272

1

0

0

0.3

0.6

0.9

1.2

1.5

1.8

[Co]Org., g/L Fig. 13. Stripping isotherm for loaded organic of cobalt. Experimental condition: [Loaded organic]:1.72 g/l Co and 0.71 g/l Mo.

Co-Cyanex 301

Temp, K

ΔH, kJ mol−1

ΔG, kJ mol−1

283 293 303 313 323 283 293 303 313 323

31.4

−1.8 −3.3 −4.4 −5.5 −6.5 13.1 13.4 13.7 14.0 14.3

4.8

ΔS, J mol−1 K−1 117.7

−29.3

58

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

Table 2 Distribution coefficient values of molybdenum using Cyanex 272 (0.15 M) as extractant and MIBK as synergist. Sl. no.

[MIBK] in M

DE, (0.15 M Cyanex 272)

DS, MIBK

DE + DS

DE + S

ΔD = D(E + S) − (DE + DS)

S.C.

1 2 3 4 5

0.5 0.75 1.0 1.25 1.5

5.84 5.84 5.84 5.84 5.84

0.22 0.26 0.29 0.36 0.41

6.06 6.10 6.13 6.20 6.25

6.17 6.24 6.30 6.39 6.45

0.11 0.14 0.17 0.19 0.20

0.008 0.009 0.011 0.013 0.013

Table 3 Distribution coefficient values of molybdenum using MIBK (1.5 M) as extractant and Cyanex 272 as synergist. Sl. no.

[Cyanex 272] in M

DE, (1.5 M MIBK)

DS, Cyanex 272

DE + DS

DE + S

ΔD = D(E + S) − (DE + DS)

S.C.

1 2 3 4 5

0.05 0.075 0.1 0.125 0.15

0.41 0.41 0.41 0.41 0.41

0.45 1.50 4.81 5.61 5.84

0.86 1.91 5.22 6.01 6.25

0.87 1.95 5.34 6.17 6.45

0.01 0.04 0.12 0.16 0.20

0.008 0.009 0.01 0.011 0.013

the equilibrium constant K can be written as Eq. (7)  2 ½CoA2 H 2 A2 Org H þ Aq K ¼  2þ  Co Aq ½H2 A2 2Org or K ¼ D

ð7Þ

 þ 2 H Aq

ð8Þ

½H 2 A2 2Org

where R is the gas constant, T is temperature and ΔS is the entropy change. The ln k values for molybdenum and cobalt extraction with Cyanex 272 and Cyanex 301 were calculated from Eqs. (5) and (9) and were plotted against 1000/T in Fig. 14. The plots were straight lines having slope values of − 3.77 and − 0.58 and intercept values of 14.2 and −3.5 for molybdenum and cobalt, respectively. From these slopes and intercepts values, the values of ΔH and ΔS were calculated for Mo

½CoA H A  where D ¼  22þ 2 2 Co Aq 15000

ð9Þ

Eq. (9) is supported by the slope values of Fig. 8 and 10. 5. Effect of temperature

12000

Intensity (a.u.)

logD ¼ logK þ 2 log½H2 A2  þ 2pH

The effect of temperature on extraction of molybdenum and cobalt was studied in the range of 283 to 323 K. The concentrations of Cyanex 272 and Cyanex 301 for Mo and Co extraction were kept constant at 0.08 and 0.05 M, respectively. The Gibbs–Helmholtz equation for enthalpy and entropy changes can be written as: lnk ¼ −

ΔH ΔS þ RT R

Raman Spectrum of MoO3

816

taking logarithm and rearranging Eqs. (8), (9) was obtained

991

9000 6000

659

3000 0 500

600

700

800

900

1000

1100

1200

Raman Shift (cm-1)

ð10Þ

Fig. 16. Raman spectrum of MoO3.

350 250

(210)

XRD of MoO 3

(113)

XRD Pattern of Co 3 O 4

200

250

Intensity (a.u.)

Intensity (a.u.)

300

200 (210)

150 100

(400) (101)

(309) (111) (011) (211)

(600)

150 (115)

100

(111)

(044)

(004) (022)

50

50

0

0 10

20

30

40

2θ (Degree) Fig. 15. XRD of MoO3.

50

60

70

30

40

50

2θ (Degree) Fig. 17. XRD of Co3O4.

60

70

80

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

3000

6. Synergistic effect of Cyanex 272 and MIBK

667

Intensity (a.u.)

2800

Raman spectrum of Co3O4

The percentage extraction of molybdenum with 20% (1.59 M)MIBK was 13.13%. So the experiments were carried out to know whether there is some synergistic effect with Cyanex 272 or not. In each dilution of Cyanex272, 5% TBP was added to inhibit third phase formation. So the solution of Cyanex 272 + 5% TBP was taken as the solvent. The pH of the solution was 1.25 and the extraction ratio of aqueous and organic phase was 1:1. The concentration of MIBK was varied from 0.5 to 1.5 M while keeping the concentration of Cyanex 272 constant at 0.15 M. In another set of experiment the concentration of MIBK was kept constant at 1.5 M and the concentration of Cyanex 272 was varied from 0.05 to 0.15 M. The synergistic coefficient (S.C.) was calculated by using Eq. (12)

2600 2400

457 504 2200 2000 200

400

59

600

800



Raman Shift (cm-1)

S:C: ¼ log



DðEþSÞ

ð12Þ

DE þ DS

Fig. 18. Raman spectrum of Co3O4.

and Co. The free energy changes (ΔG) were calculated from the relationship as given in Eq. (11). ΔG ¼ −RTln kext: :

ð11Þ

The values of ΔG, ΔH and ΔS obtained for both molybdenum and cobalt extractions using Cyanex 272 and Cyanex 301, respectively were given in Table 1. The enthalpy change (ΔH) values were positive for both the extractions indicating the extraction processes were endothermic.

DE, DS and DE + S refer to the distribution ratio of extractant, synergist and their mixture respectively. Table 2 shows the experimental data where Cyanex 272 and MIBK were used as extractant and synergist, respectively. The difference of DE + S and DE + DS was very less and varied between 0.11 and 0.2. The synergistic coefficient increased from 0.008 to 0.013 with increasing MIBK concentration from 0.5 to 1.5 M. Table 3 represents the data for the experiments where MIBK was used as the extractant and Cyanex 272 was used as the synergist. In this case also the same result was obtained as above. The difference of DE + S and DE + DS was very less and the synergistic coefficient increased from 0.008 to 0.013 with increasing Cyanex 272 concentration from 0.05 to 0.15 M.

Mo spent catalyst leach liquor, [Mo] =12.52 g/l, [Co] =1.74g/l, [Al] =9.98g/l

0.2 M Cyanex 272

0.5M NH4 OH + 0.5M (NH4)2CO3 Mo LO 22.98 g/l

Mo Extraction, 3- stages, A:O=2:1

Crystallisation

Raffinate [Mo]=1.03 g/l, [Co]=1.74g/l,[Al]=9.98g/l 0.25 M Cyanex301

Mo SS 41.92g/l

Mo Stripping, 2-stages, A:O=1:2

Thermal decomposition at 400oC for 2 hours LO, 1.72g/l Co, 0.98 g/l Mo

Co extraction, 2- stages, A:O=1:1

2% H2SO4

Co Stripping, 3-stages, A:O=1:2

Co SS 3.44 g/l 1M NaOH

SO

Precipitation

Mo Stripping

Thermal decomposition at 400o C for 3 hours

0.5M NH4OH + 0.5M (NH4)2CO3

Mo SS 0.95 g/l

Co3O4

Fig. 19. Flow sheet for preparation of MoO3 and Co3O4 from spent Mo catalyst.

MoO3

60

E. Padhan, K. Sarangi / International Journal of Mineral Processing 127 (2014) 52–61

7. Preparation of metal oxide 7.1. Molybdenum oxide The molybdenum loaded Cyanex 272 was stripped with 0.5 M NH4OH + 0.5 M (NH4)2CO3. From the strip solution, molybdenum was obtained as ammonium molybdate with some unreacted NH4OH and (NH4)2CO3. Experiments were carried out to obtain pure MoO3 from this solution. In this study the strip solution was first crystallized by evaporation. During this process the strip solution was free from NH3, H2O and CO2, and a grayish solid substance was obtained. The second step after crystallization was the thermal decomposition of the solid substance at 400 °C for 2 h. The compound obtained after thermal decomposition was characterized by X-Ray diffraction (XRD). The XRD pattern of the thermal decomposed product was shown Fig. 15. All the major peaks of Fig. 15 correspond to MoO3. Approximately 99.9% pure product (MoO3) was obtained in this process. Similar study was also done and reported by Parhi et al., 2011. The Raman absorption spectra of Cyanex 272 were recorded to know the interaction between the extractant and the metal ion. The Raman spectra of extractants and their complexes with molybdenum were shown in Fig. 16. The spectra had a strong peak at 991.345 cm−1 which is the characteristic of the terminal double Mo_O bonds. Seguin et al., 1995 reported that terminal double Mo_O bonds are characterized by narrow Raman bands occurring in the range of 920–1000 cm−1 and the peak at 659.908 cm−1 refers to the stretching vibrations of bridging oxygen atoms linked to three metal atoms in orthorhombic MoO3. The peak at 816.6 cm−1 could be due to the vibration in orthorhombic MoO3 (Ivanova et al., 2002). 7.2. Cobalt oxide The strip solution contained 3.44 g/l cobalt which was precipitated with NaOH. For precipitation, 1 M NaOH was added to the strip solution drop wise with continuous stirring by a mechanical stirrer till the pH of the solution was raised to 9.0 (Oustadakis et al., 2006). After complete precipitation, the solution was filtered and the residue was washed repeatedly with distilled water. The residue was dried in an oven at 110 °C and the final product was obtained after thermal decomposition of the dried residue at 400 °C for three hours. To confirm the formation of cobalt oxide the sample was characterized by XRD and Raman spectroscopy. The XRD pattern of cobalt oxide was shown in Fig. 17. The data was matched with JCPDS files which indicated the compound to be tricobalt tetraoxide (Co3O4). This was confirmed by Raman spectrum as shown in Fig. 18. The spectrum showed bands at 467, 511 and 671 cm−1 which is assigned to Co3O4 (Tang et al., 2008). [30]. A flow-sheet for preparation of MoO3 and Co3O4 from molybdenum catalyst was shown in Fig. 19. 8. Conclusion The recovery of molybdenum and cobalt from the leach liquor of molybdenum–cobalt spent catalyst bearing 12.52 g/l Mo, 1.74 g/l Co and 9.98 g/l Al was studied using Cyanex 272 and Cyanex 301. Extraction of molybdenum was carried out using 0.2 M Cyanex 272. With increase in equilibrium pH from 0.43 to 0.99, the percentage extraction of molybdenum increased from 35.3 to 88.34 and after that it decreased. The McCabe–Thiele diagram showed three stages at A: O ratio of 2:1 at pH 1.25. The loaded organic of molybdenum was stripped using a strip solution containing 0.5 M NH4OH and 0.5 M (NH4)2CO3. The quantitative stripping of molybdenum was obtained in 2-stages at O:A ratio of 2:1. After molybdenum extraction, cobalt was extracted with Cyanex 301 in two stages at A:O ratio of 1:1 and was stripped with H2SO4 in three stages at A:O ratio of 1:2. The extracted species of Mo and Co were found to be MoO2·A2·H2A2 and CoA2·H2A2, respectively. From the strip solution of molybdenum and cobalt, MoO3 and Co3O4 were

prepared and were characterized by XRD and Raman spectra. The thermodynamic parameters such as ΔH, ΔS and ΔG values for molybdenum and cobalt extraction using Cyanex 272 and Cyanex 301 were calculated. The enthalpy change (ΔH) values were positive for both the extractions indicating the extraction processes were endothermic.

Acknowledgment The authors wish to thank Dr. I. N. Bhattacharya, HOD, Hydro & Electrometallurgy Department for the encouragement and Prof. B.K. Mishra, Director, CSIR-IMMT, Bhubaneswar for the kind permission to publish this paper. The authors are also thankful to the Council of Scientific & Industrial Research, New Delhi, India for the financial support.

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