Recovery of metals from an acid leachate of spent hydrodesulphurization catalyst using molecular recognition technology

Chemical Engineering Science 138 (2015) 353–362

Contents lists available at ScienceDirect

Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces

Recovery of metals from an acid leachate of spent hydrodesulphurization catalyst using molecular recognition technology Isabel S.S. Pinto a, S. Maryam Sadeghi a, Neil E. Izatt b, Helena M.V.M. Soares a,n a b

REQUIMTE/LAQV, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Portugal IBC Advanced Technologies, Inc., American Fork, UT 84003, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T


Recovery of Ni, Al and Mo from an H2SO4 leaching solution of spent catalyst.
Use of SuperLigs 167 based on molecular recognition technology.
Ni and Mo retained by the resin and separated from Al efficiently.
Two-step elution separates Ni from Mo with high purity and yield.

art ic l e i nf o

a b s t r a c t

Article history: Received 10 March 2015 Received in revised form 23 June 2015 Accepted 4 August 2015 Available online 25 August 2015

Acid leaching of spent hydrodesulphurization (HDS) catalysts solubilizes efficiently all metals present in the catalyst; however, this methodology poses subsequent difficulties in achieving metals separation with high purity. In the present work, molecular recognition technology, using SuperLigs 167 resin, was applied to recover Ni, Mo and Al from H2SO4 leachate of spent HDS catalyst. Batch tests showed a good resin affinity to Ni, in acid, with a maximum capacity of 1.35 mmol/g. The separation of Al, which was in excess, was efficient, but Mo was also retained by the resin. Column studies, using 6 g of resin and performed at three flow rates (1.0, 1.5 and 2.1 mL/min), showed that the lower flow rate was better to treat a higher volume of solution. At 1.0 mL/min, each gram of resin treated almost 40 mL of solution with 97% and 100% of Ni and Mo retention, respectively. Elution with NaOH 0.25 mol/L, followed by HNO3 4 mol/L, both at 2.1 mL/min, led to efficient separation of Mo and Ni. The use of SuperLigs 167 resin to treat the leaching solution enabled an easy and efficient separation of the metals present in the acid leachate into mono-metal solutions (Al in the raffinate, Mo and Ni in the alkaline and acid eluates, respectively) with high yield (99.7%, 100% and 87%, respectively) and purity (99.3%, 99.8% and 98.7%, respectively). & 2015 Elsevier Ltd. All rights reserved.

Keywords: Spent catalyst Nickel Molybdenum Molecular recognition technology

1. Introduction

n

Correspondence to:Departamento de Engenharia Química, Faculdade de Engenharia do Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal. Tel.: þ 351 225081650; fax: þ 351 225081449. E-mail address: [email protected] (H.M.V.M. Soares). http://dx.doi.org/10.1016/j.ces.2015.08.018 0009-2509/& 2015 Elsevier Ltd. All rights reserved.

Catalysts are widely used in the petroleum refining industry for hydrodesulphurization (HDS) operations; after the end of their life-time, which occurs after continuous activity and regenerations, they become hazardous wastes that cannot be disposed of. About 120,000 tons (dry basis) of spent HDS catalysts are

354

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

produced every year (Dufresne, 2007). These catalysts have valuable metals, such as Mo, Co and Ni supported in an alumina base, that make the recycling of used catalysts an economically and environmentally attractive option. Hydrometallurgical processes, based on the extraction of metals through aqueous solutions, have some advantages over traditional pyrometallurgical operations (Ghosh and Ray, 1991; Kim et al., 2009). Dissolution of metals from spent HDS catalysts through hydrometallurgical operations usually consists of an acid or an alkaline leaching or even a combination of both at atmospheric pressure and below 100 °C. The extraction of metals performed under concentrated acid solutions solubilizes all the metals present in the catalyst (Idris et al., 2010; Kim et al., 2009; Pinto and Soares, 2013a; Valverde et al., 2008). This procedure requires further separation of the metals to obtain pure products suitable for being used in other industries. The separation and recovery of metals from the spent HDS catalyst leachate is frequently based on a combination of solvent extraction and precipitation methods. In the case of the recovery of Ni from an Al solution, several examples can be found in the literature (Kim and Cho, 1997; Park et al., 2012, 2007; Rao et al., 2012; Zhang et al., 1995). Extraction using Cyanex 272 proved to be more effective than precipitation at pH 12.5, as the second led to a solid of Ni with significant amount of Al (Orive et al., 1992). In the case of precipitation of Al at pH around 5, some Ni was lost to the solid due to its co-precipitation; so, this method is only suitable if Ni is in great excess (Lee et al., 2010). The synergy between solvent extraction and precipitation allows great recoveries of the metal salts with high purity; however, several stages and reagents are involved, including organic extractant, solvent and strong acid stripping solution. The application of adsorption with activated carbon to separate Mo from Ni and Al from the leaching solutions of spent HDS catalysts is possible and efficient under very acidic conditions; however, both Al and Ni remain in solution (Mohapatra and Park, 2007; Pagnanelli et al., 2011; Park et al., 2006). A chelating resin was studied to retain Ni in the presence of a large quantity of Al from a solution obtained after H2SO4 leaching of HDS catalyst (Nagib et al., 1999). Molecular recognition technology (MRT) has been gaining a lot of attention during the last decades. This technology started with the synthesis of crown ethers in the late of 1960s; this research evidenced promising results on the selective complexation of cations and culminated with the award of the Nobel Prize in chemistry in 1987 to Pedersen, Lehn and Cram, three of the main contributing scientists (Glennon, 2000; Izatt, 1997). Pedersen synthesized a large number of cyclic polyethers and observed that some of them had remarkable selectivity for specific alkali metal ions (Pedersen, 1967). This work eventually led to the development of SuperLigs resin systems in which a metal-selective ligand is attached by a chemical linker arm to a solid support particle, such as silica gel. An example of such a SuperLigs system is given in Fig. 1. The concept of this technology is basically “host-guest” chemistry: the design of macrocycle molecules with determined

Fig. 1. Representative SuperLigs system consisting of a solid support particle, silica gel, to which a metal-selective ligand, 18-crown-6, is attached by a tether, which is chemically bound to both the ligand and the silica gel.

cavity sizes and shapes that can suit the selected cation diameter and thus influence complexing properties (Bradshaw et al., 2000; Glennon, 2000; Izatt et al., 2000). The binding of such ligands to silica or polymeric supports for solid phase extraction (SPE) has the main advantages of forming a permanent bond; thus, the system can be used multiple times without loss of the macrocycle and allows the concentration of ions by eluting them with small solution volumes (Izatt, 1997). In addition, the product can be packed in fixed-bed columns for continuous operation at full-scale. Comparison with other traditional separation processes, like precipitation, ion-exchange and solvent extractions, SPE-MRT can overcome problems like low selectivity in the recovery and low purity of the products, organic contamination of streams, large space requirements, slow kinetics and treatment of low concentrated streams (Izatt et al., 2015, 2011). The high selectivity of MRT compared to other methods can be explained by the multiple parameters that affect the binding (ion radius, coordination chemistry, geometry, charge). Thanks to this specificity, several MRT products are commercially available that are adequate for different cations depending on the purpose of the process. MRT can be useful for analytical purposes to concentrate solutions and eliminate interferences (Devol et al., 2009; O'Hara et al., 2009). Quite a few studies can be found in the literature and there are already implemented full-scale processes that use SPEMRT to separate, recover or refine metal ions from different streams. Recovery of precious and toxic metals from electronic wastes, refining of platinum, rhodium and gold, uranium removal from mining solutions, cesium and technetium removal from wastewaters, removal of nickel, lead, bismuth or cobalt in the presence of other metal cations are examples of applications (with interest or already implemented) that use SPE-MRT (Adu-Wusu et al., 2006; Belanger et al., 2008; Hassan et al., 2002; Izatt et al., 2012, 2011; King et al., 2005; Navarro et al., 2012). Since MRT has been showing encouraging results in the separation of metals from aqueous streams, the possibility of using such technology in the treatment of spent catalysts containing Ni, Mo and Al was assessed for the first time. In the present work, the separation and recovery of metals, obtained after H2SO4 leaching of a spent HDS catalyst, was studied using MRT product SuperLigs 167 from IBC Advanced Technologies.

2. Experimental 2.1. Reagents and materials Synthetic solutions of Ni, Al and Mo were prepared from Ni(NO3)2  6H2O, Al2(SO4)3  16H2O and Na2MoO4  2H2O, respectively. The initial pH was controlled through the addition of concentrated H2SO4. Aqueous solutions prepared from analytical grade HNO3 (65%) and NaOH were used in the elution studies. The real solution was obtained after the second leaching stage of a HDS spent Ni–Mo catalyst. Briefly, the catalyst was roasted in air, at 500 °C, and leached with NaOH 0.25 mol/L (Pinto and Soares, 2012); the remaining solid was then separated, dried and treated with H2SO4 0.8 mol/L in a thermostatic bath, at 80 °C, for 4 h (Pinto and Soares, 2013a). The solution, previously separated by filtration, had a final pH of 1.2 and contained 2.3  10  2 mol/L of Ni, 4.2  10  1 mol/L of Al and 4.0  10  3 mol/L of Mo. The MRT product chosen was SuperLigs 167 from IBC Advanced Technologies, Inc. (American Fork, Utah, United States). The product is in the form of dark brown beads, 0.5 mm, made of polyacrylate, to which the ligand has been chemically attached. Chemical composition of the product is a trade secret, but a generic example of the ligand molecules is shown in Fig. 1.

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

2.2. Batch experiments

log ( qe − qt ) = log qe −

For all the experiments performed in batch, synthetic solutions were used. Samples of 0.1 g of SuperLigs 167 resin, used as received, were rigorously weighted to 50 mL flasks and a synthetic solution, with a defined pH, was added to perform a solid–liquid ratio (s/l) of 10 g/L. The flasks were shaken in an incubator at 25 °C. To ensure that the metals concentration near the resin surface was approximately equal to that of the bulk phase and retention reaches maximum values, agitation was performed at 150 rpm. After 24 h, the solution was separated from the resin, by filtration, and final pH was verified. Initial and final concentrations were analyzed by atomic absorption spectroscopy with flame atomization (AAS-FA) in a Perkin Elmer AAnalyst 400 spectrometer (Norwalk, CT, USA). The quantity of metal adsorbed per mass of resin (qe) was calculated according to Eq. (1), where C0 and Ce are the initial and equilibrium metal concentrations (mmol/L), m is the mass of resin (g), V is the volume of solution (L) and qe is the equilibrium adsorption capacity (mmol/g)

qe =

(C0 − Ce ) V m

(1)

To study the elution of metals, the resin obtained after filtration was carefully washed three times with deionized water. After separation of water, solutions of HNO3 or NaOH at various concentrations were added to make s/l ¼20 g/L and the flasks were shaken for 24 h. The solution was then separated by filtration and the volume was made up to 25 mL. Using this procedure, it was possible to calculate the exact mass of metals eluted using AAS-FA analysis. 2.2.1. Adsorption isotherms modeling For adsorption isotherms, Ni concentration was varied between 1 and 20 mmol/L at 25 °C. Langmuir and Freundlich isotherm models were tested in the experimental data obtained. Langmuir isotherm assumes adsorption on homogeneous surface (monolayer) with negligible interaction between adsorbed molecules; it allows the estimation of the maximum metal uptake (qmax) and the Langmuir constant (kL). The linear form of the Langmuir isotherm is shown in Eq. (2). Based on this equation, it is possible to represent the experimental points in a plot Ce/qe vs. Ce and calculate the model parameters by the linear method

Ce 1 C = + e qe qmax kL qmax

(2)

Freundlich isotherm assumes adsorption on heterogeneous surfaces. Freundlich constant (kF) and n are Freundich parameters that can be estimated from experimental data by adjusting the linear form of Eq. (3). Values of n greater than 1 indicate favorable conditions for adsorption

log qe = log kF +

1 log Ce n

(3)

2.2.2. Adsorption kinetics experiments and modeling Kinetic studies were conducted in 500 mL glass flasks. About 2.2 g of resin was rigorously weighed into the flask and 200 mL of 20 mmol/L Ni solution was added. Samples of 1 mL were taken at different intervals of time and the respective Ni concentration was measured by AAS-FA. Langergren pseudo-first order and type I pseudo-second order models were fitted into the experimental points using the linear regression equations (Eqs. (4) and (5), respectively) and from the values of slope and interception obtained, qe, k1 and k2 was calculated

k1 t 2. 303

t 1 t = + qt qe k2 qe2

355

(4)

(5)

2.3. Column studies The potentiality of using the process in continuous mode was assessed through studies performed in a glass column with adjusting the top screw (di ¼15 mm, h¼250 mm). A rigorous amount of resin (6 g) was weighted and transferred to the column with water. Diluted HNO3 was passed through the column and left during 24 h to allow expansion; maximum bed volume was 23 cm3. Solutions were fed from the top of the column at defined flow rates that were varied between 1.00 and 2.10 mL/min, using a peristaltic pump MS-Reglo (Ismatec, Switzerland). From the outlet solution, samples were collected at different intervals during a defined period of time (3–6 min depending on the flow rate) to measure metals concentration, by AAS-FA, and pH. Elution of the retained metals was conducted in down-flow mode with either NaOH or HNO3; in order to calculate total recovery, samples collected during the experiment and the total effluent were analyzed by AAS-FA. All the experiments were performed, in triplicate, at room temperature. Each experiment was executed according to the following sequence: 1. Load: before passing the leaching solution, the column must be acid; otherwise, Al will precipitate. When necessary, a volume of 20 mL of a diluted H2SO4 solution (0.05 mol/L) was passed through the resin to lower the pH. The solution (synthetic or real) was fed to the column. 2. Pre-elution wash: firstly, 20 mL of diluted H2SO4 was used in order to avoid precipitation of Al present in the column. Then, 80 mL of water was passed through the column. 3. First elution þ wash: the acid or alkaline solution was fed to the top of the column; 80 mL of water was used to wash the column after the elution stage. 4. Second elution (when applicable) þwash: similar to first elution. The resin, present in the column, was not changed throughout all the work performed; so, after elution, the column was washed and reused for the next experiment. A total of 12 experiments were conducted with the same material. 2.3.1. Thomas model The equation used to describe the experimental data was based on the Thomas model (Eq. (6)), where C0 and C are the inlet and outlet metal concentrations (mmol/L), respectively, kT is the Thomas rate constant [L/(mmol min)], Q is the volumetric flow rate (L/ min), qmax is the exchange capacity of the bed (mmol/g), m is the amount of resin in the column (g), and V is the total volume treated at each time (L).

C = C0 1 + exp

1

(

kT Q

( qmax m − C0 V ) )

(6)

This model is empirical and one of the most ones used to describe column processes. It assumes Langmuir kinetics of an exchange/elution process, no axial dispersion and that the rate driving force obeys second-order reaction kinetics (Thomas, 1944). Linearization of the equation facilitates the calculation of the kinetic parameter kT and the qmax from a plot of ln[(C0/C)  1] vs.

356

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

metal ion and the solid surface. Hence, theoretical models are typically adjusted to experimental data to predict the extent of adsorption/exchange in the system. The solution obtained after leaching the spent catalyst with H2SO4 has a pH close to 1 and contains a high concentration of Al. For that reason, pH should not be increased to values above 2–2.5 because Al will precipitate and drag Ni with it, as it was verified experimentally. In agreement to that, the pH of the solution was varied between 0.5 and 2. No significant changes of the capacity of the resin to retain Ni were observed between the different experiments (Fig. 2A, white bars); so, pH ¼ 1 was chosen to perform the equilibrium studies. The variation of qe, after 24 h, with Ni concentration (s/l ¼ 10 g/ L, pH ¼1) is shown in Fig. 3. Langmuir and Freundlich isotherms were adjusted to the experimental results. While Langmuir model assumes a monolayer adsorption, Freundlich is interpreted as sorption to heterogeneous surfaces supporting sites of different affinities. The parameters of each model were calculated by the respective linear plots (Eqs. (2) and (3)) and the obtained values are presented in Table 1. The maximum capacity of the resin, calculated by Langmuir model, was 1.43 mmol/g; however, experimental points seem to stabilize around 1.1–1.2 mmol/g. The fact that the value for n calculated from Freundlich model was higher than 1 indicates favorable conditions for adsorption. From the analysis of the correlation coefficient of the linear regression to the data points for both models and observation of Fig. 3, it can be concluded that Langmuir model adjusts better to the experimental data than Freundlich one. Comparison of the obtained qmax with values present in the literature for other types of solid phases can give an idea of the efficiency of SuperLigs 167 to retain Ni. For example, the capacity of the chelant resin studied by Li et al. (2012) to retain Ni was affected by the pH, with significant decrease for pH below 3 and maximum capacity of 1.13 mmol/g at pH 5. In Juang et al. (2006), a strong-acid cation exchange resin Purolite NRW-100, tested to remove Ni from synthetic waste water, showed an increasing capacity to retain Ni when pH values were increased, with qmax 1.06 and 1.65 mmol/g for pH 0.5 and 3, respectively. In terms of

Ni + Al + Mo

1.4

Ni

1.2

q Ni, mmol/g

1 0.8 0.6 0.4 0.2 0 0.8

1.3 pH

1.9

Mo

2 1.8

Ni

1.6

q, mmol/g

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

0.004 0.01 [Mo], mol/L

0.02

1.8

Fig. 2. Capacity of the resin to retain: (A) Ni (20 mmol/L) at different pH values in the absence and presence of Mo (4 mmol/L) and Al (20 mmol/L); (B) Ni (20 mmol/ L) and Mo at different Mo concentrations. Each bar represents the average of three independent experiments.

⎛C ⎞ k T qmax m k C ln ⎜ 0 − 1⎟ = − T 0 Vtot ⎝ C ⎠ Q Q

(7)

1.4

qe, mmol/g

t at a given flow rate and initial concentration.

1.6

3. Results and discussion For a solution containing Ni, Al and Mo, precipitation by changing the pH is not an option as a primary method of separation since Al and Ni would precipitate together. In the case of solvent extraction, it is used to separate Al from Ni with high purities and yield, but in a solution also containing Mo, two extractants would be necessary. 3.1. Batch studies

1.2 1.0 0.8 0.6

Exp

0.4

Langmuir

0.2

Freundlich

0.0 0

2

4

6

8

12

14

Fig. 3. Comparison between experimental points (pH ¼ 1) and calculated isotherms for Ni-resin system. Each point represents a single experiment.

Table 1 Langmuir and Freundlich isotherm parameters for Ni-resin system. Langmuir

3.1.1. Adsorption isotherms The analysis of equilibrium data is important on the design of adsorption and ion-exchange processes because it allows developing an equation that represents the interaction between the

10

Nie, mmol/L

Freundlich

qmax (mmol/g)

kL (L/mmol)

Linear r

1.429

0.339

0.977

2

kF (mmol/g)

n

Linear r2

0.319

1.72

0.954

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

3.1.2. Effect of Mo and Al Although the resin has proven to fulfill the requirement to retain Ni, the goal was to treat the solution that also contained Mo and Al. For a solution containing simultaneously Ni, Al and Mo at concentrations close to the ones expected in the leaching solution (see Section 2.1), the capacity of the resin to retain Ni was not affected (analysis of variance with 95% confidence) when compared to the absence of these metals (Fig. 2A). The concentration of Al in solution remained unaltered, which indicates that its retention by the resin was null; however, Mo was bound to the resin together with Ni. At pH ¼1, it was confirmed that the retention of Mo increased continuously with the Mo concentration both in the presence (Fig. 2B) and in the absence (data not shown) of Ni; however, even for higher concentrations of Mo, the capacity of the resin for Ni remains unaltered. These results support a different mechanism of retention for Ni and Mo. The actual mechanism by which Ni is being held cannot be discussed in detail due to the trade secret composition of the resin. Nonetheless, it is known that this product was designed to bind selectively with some metal cations due to the presence of ligand(s), chosen based on their properties, namely the ability to form complexes with metal ions. For example, in Fig. 1, the SuperLigs system presented is selective for K þ ion, which fits into the cavity of the 18-crown-6 molecule due to the good match of the K þ ionic radius with the 18-crown-6 cavity radius. Na þ and Cs þ ions have radii too small or too large, respectively, for a fit, so they do not bind as well as the K þ ion. The fit of the guest metal ions in the host cavity is supported by log K values (Christensen et al., 1974). Similar principles apply to the selective separations described here in this paper. With the SuperLig s 167, the ligand bonded covalently to the solid support of the resin binds Ni2 þ and has no affinity for Al3 þ proven by the absence of retention in the experiments. Molybdenum has a more complex chemistry because it is commonly in the form of Mo(VI) and, in solution, it forms the molybdate ion (MoO42  ). A possibility for the retention of Mo can be the binding of molybdate to the Ni complex; however, Mo retention by the resin was observed experimentally even in the absence of other metals in solution. In acid solution, such as is the case of the leachate, Mo is present in the form of polymolybdates negatively charged (Aveston et al., 1964) that can be retained by the resin through anion exchange and not because it has affinity for the ligand and is “stealing” Ni sites. This is in agreement with the results obtained and can be an explanation about what is occurring during the experiments. 3.1.3. Kinetics The evolution of Ni retention through time was studied in four experiments from two independent solutions. Experimental points are presented in Fig. 4 and show that after 24 h the retention process was already stabilised. Nevertheless, the rate of Ni retention does not seem very fast. The kinetics of an adsorption/

1.2 1

q, mmol/g

quantity of Ni adsorbed, the magnitude was similar to that obtained in the present work. Priya et al. (2009) applied a cationexchange resin to remove Ni from a synthetic electroplating rinse water and obtained a qmax of 0.848 mmol/g at pH 5.8, after adjusting the Langmuir model. Solvent impregnated resins, studied by Guo et al. (2012), did not achieve such capacities of Ni adsorption from aqueous chloride media, being able to retain around 0.4–0.5 mmol/g at pH 4/6.8, depending on the solvent. The paper authored by Dizge et al. (2009) presents a table that compares the sorption of Ni by different resins. The results described in the literature depend on the pH and matrix of the solution and they show that the SuperLigs 167 capacity to retain Ni is good, especially considering the low pH requirement.

357

0.8 0.6

Exp 0.4

1st order

0.2

2nd order

0 0

500

1000

1500

2000

t, min Fig. 4. Comparison between experimental points (pH¼ 1) and calculated kinetic models for the Ni-resin system (C0 ¼20 mmol/L); typical example of an experiment performed four times.

Table 2 Reaction-based kinetic parameters for Ni-resin system. First-order

Second-order

qe (mmol/g) k1 (min 1.037

1

)

2

Linear r

2.42  10  3 0.960

qe (mmol/g) k2 (g/ mmol min) Linear r2 1.275

1.95  10  3

0.958

exchange process is affected by (1) the mass transfer of the ions through the solution to the liquid thin layer that surrounds the particle; (2) mass transfer of the ions through the thin layer to the solid surface; (3) internal diffusion of the ions; and (4) rate of the reaction. Pseudo-first order and type I pseudo-second order kinetic models were adjusted to the experimental points through the respective linear plots; both models assume that the reaction rate is the limiting step in the mechanism. The calculated parameters, obtained by adjusting Eqs. (4) and (5) to data from four independent experiments, are presented in Table 2. Both models represent well the experimental data as it can be seen by the similarity of the values of r2 and observation of Fig. 4 (example of one experiment). The value for qe, predicted by the pseudo-second order model, is higher, which is more in agreement with the value calculated from Langmuir isotherm model. The influence of the presence of Mo on the kinetics of Ni retention was evaluated as well (data not shown) but the comparison with the already obtained results did not show significant differences, which supports the hypothesis of different binding mechanisms. 3.1.4. Elution The fact that both Ni and Mo were retained by the resin is a drawback in the recovery of Ni with high purity. Mo is soluble in alkaline solution and, even when in the presence of strong complexants, it is present in the free form, as MoO42  (Kula, 1966). On the other hand, at high pH, the Ni binding to the resin might have two fates: (i) it is stable and Ni remains complexed with the ligand or (ii) Ni is removed and precipitates due to its insolubility at pH above 8. In acid solution, both metals are soluble; still, they might need different acidities to be eluted. Therefore, alkaline and acid elution at different concentrations was studied to assess a possible separation of both metals by performing the elution in two stages. Firstly, HNO3 elution was studied by varying its concentration (Fig. 5A). Mo was more likely to solubilize in lower acid concentration (1–2 mol/L) than Ni; however, the separation was not

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

Eluted Metal

358

100%

100%

80%

80%

60%

60%

40%

40%

Ni Mo

20%

20% 0%

0

1

2 HNO3, mol/L

3

4

0% 0.00

0.10

0.20 0.30 NaOH, mol/L

0.40

0.50

Fig. 5. Recovery of Mo and Ni (C0 ¼4 and 20 mmol/L, respectively) after elution with (A) HNO3; (B) NaOH. Each point represents the average of three independent experiments.

1

0.8 Ni 1.0 mL/min 0.6

Ni 2.1 mL/min Al 1.0 mL/min Al 1.5 mL/min Al 2.1 mL/min

0.2

real solution 0 0

200

400

600

800

600

800

t, min 1

3.2. Column studies 3.2.1. Retention of metals Synthetic solutions were prepared to match the metal concentrations predicted after acid leaching of the catalyst ([Ni] ¼2.3  10  2, [Al] ¼4.2  10  1 and [Mo] ¼4.0  10  3 mol/L) and the pH was adjusted to 1. The flow rates studied were 1.0, 1.5 and 2.1 mL/min, which correspond to 0.17, 0.25 and 0.35 ml/ min/g of resin. For each flow rate, three experiments were executed, but only one of each is presented in Fig. 6, as an example. Fig. 6A presents the variation of the ratio between metal concentration in the outlet (C) and inlet (C0) with time and Fig. 6B was normalized to the volume of solution passed through the column. The observation of Fig. 6 shows that the higher flow rate was not efficient because the concentration of Ni in the outlet started to increase just after the beginning of the experiment. The lower flow rate 1.00 mL/min permitted an efficient treatment of a larger volume of solution due to the more accentuated breakthrough curve. The passage of the liquid in a slower mode allowed a better retention in the beginning of the experiment, which decreased the loss of Ni to the outlet solution. At the end of the experiment, the amount of Ni retained by the resin was calculated from the concentration measurements (Table 3) by estimating the area above the breakthrough curve; qexp values, around 1.3 mmol/g, are in agreement with the values predicted by isotherm and kinetic models, calculated from batch experiments. After all, the adsorption capacity was the same for the three flow rates tested. When using 2.1 mL/min, the saturation of the binding sites occurred after more volume of solution was treated, but higher speed of the

Ni 1.5 mL/min

0.4

C/C0

effective as Ni was also partially solubilized. When the acid concentration was higher than 3 mol/L, both metals were efficiently solubilized with recoveries above 95%; although these recoveries correspond to good results, separation of Ni and Mo is not feasible under the present conditions. The second approach consisted on varying the concentration of NaOH solutions (Fig. 5B). It was verified that Mo was removed efficiently for concentrations higher than 0.1 mol/L, while Ni remained in the resin (less than 1% lost). For that reason, a twostage elution process, where the first step happens in alkaline medium and the second one occurs under acidic conditions, was tested. A second elution with HNO3 4 mol/L was able to remove Ni and the remaining Mo that, in the cases when the first elution was performed with [NaOH]Z0.25 mol/L, was below 5% of the total retained.

0.8 0.6 0.4 0.2 0 0

200

400

V, mL Fig. 6. Ratio between the concentrations of Ni and Al in the outlet, C, and inlet, C0 (2.3  10  2, 4.2  10  1 and 4.0  10  3 mol/L of Ni, Al and Mo, respectively) (A) vs. time (B) vs. volume of solution passing through the column.

Table 3 Capacity of the resin, Thomas parameters, volume treated corresponding to [Ni]o/[Ni]i ¼ 0.1 and respective percentage of retained Ni. Values with standard deviation from three independent experiments. Q (mL/ qexp (mmol/g) Thomas parameters min) qmax kT (L/ (mmol/g) (mmol min)) 1.00 1.50 2.10

1.38 7 0.04 1.277 0.04 1.28 7 0.06

1.36 1.31 1.32

8.02  10  4 7.80  10  4 7.02  10  4

V (mL)

Retained Ni (%)

2317 25 977 2 1417 21 967 3 617 9 947 2

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

3.2.2. Elution During column experiments, both Ni and Mo were retained by the resin. An acid elution, using HNO3 4 mol/L, removed efficiently both metals (Fig. 7A); however, the solution containing Ni and Mo must be treated to recover each metal separately, as the concentration peaks are coincident. Batch studies (Fig. 5) indicated that Mo and Ni retained in the column could be separated by two-step elution: the first step using an alkaline solution and the second one using an acid solution. Elution with NaOH was studied at two different concentrations, 0.10 and 0.25 mol/L with a flow rate of 2.1 mL/min. Results (Fig. 7B) show that, although both led to good recovery of Mo (99–100%), the higher NaOH concentration allows a more defined peak, which results in more concentrated Mo solution, as well as faster processing. Elution with NaOH proved to be effective in removing Mo, with no trace of Al found in the solution, while leaving Ni still bound to the resin. During the second step (acid elution), HNO3 solution of 4 mol/L removed Ni efficiently. Both flow rates tested were efficient regarding the Ni recovery; however, 2.1 mL/min led to a more defined concentration peak (Fig. 7C), which has advantages in which concerns the operation time and further recovery of the metal. Considering the results described above, a process comprising the treatment of the leaching solution at 1.0 mL/min followed by a first elution with NaOH 0.25 mol/L and a second one with HNO3 4 mol/L, both using a flow rate of 2.1 mL/min, is proposed.

Concentration, mol/L

0.14 0.12 0.1 0.08

Ni

0.06

Mo

0.04

Al

0.02 0 0

20

40

60

80

100

t, min

0.08 0.07

Mo, mol/L

0.06 0.05 0.04

0.10 mol/L

0.03

0.25 mol/L

0.02 0.01 0 0

50

100 t, min

150

200

0.18 0.16 0.14

Ni, mol/L

liquid was a disadvantage in the process of metal retention, because Ni concentration in the outlet started to increase earlier in the experiment. The solid lines in Fig. 6A are the representation of Thomas model after adjusting it to the experimental data. The qmax estimated from linear regression was very close to the value calculated experimentally (Table 3). The kinetic parameter, kT, decreased with the increasing flow rate, especially for 2.1 mL/min. This lower value obtained for 2.1 mL/min can be explained by the rapid increase in the outlet concentration in the beginning of the experiments that translates into slower kinetics of exchange. For higher kT, the breakthrough curve is more accentuated and this is why the estimated rate parameter had a higher value for the lowest flow rate (Table 3). These results show that limitations of Ni removal from solution are mainly due to kinetics of reaction and not mass transfer since, theoretically, with increase in fluid velocity, the turbulence would do the mixing in fluid phase, reducing mass transfer limitations and increasing retention. The influence of the pore diffusion was not studied since the particle size of the SuperLigs 167 resin could not be altered. Comparison of the volume of solution treated, by establishing a goal of [Ni]/[Ni]0 ¼0.1, is presented in Table 3. The flow rate of 1.0 mL/min was able to treat almost 4 and 1.6 times more solution than 2.1 and 1.5 mL/min, respectively. For Al, the concentration in the outlet increased from the beginning of the experiment and C/C0 rapidly reached 1. Al was not retained by the resin, which is in agreement with the results obtained in batch experiments and fulfills the requirement of Ni and Al separation. The results for Mo are not presented in Fig. 6 because the concentration of Mo in the outlet was zero throughout the experiment, which translates into 100% of retention. Depending on the volume of solution treated, the amount of Mo retained varied between 0.45 and 0.55 mmol/g of resin, a value that is close to that obtained in batch (0.4 mmol/g) for [Mo]¼ 4.0  10  3 mol/L (Fig. 2B). Although the retention of Mo was anticipated by the batch results, it forces the separation of both Mo and Ni to obtain independent solutions containing each pure metal.

359

0.12 1.5 mL/min

0.1

2.1 mL/min

0.08 0.06 0.04 0.02 0 0

50

100 t, min

150

200

Fig. 7. Elution profiles of (A) metals with HNO3 4 mol/L at 2.1 mL/min; (B) Mo with different concentrations of NaOH at 2.1 mL/min; (C) Ni with HNO3 4 mol/L at different flow-rates. These are typical examples of experiments performed two times.

3.2.3. Real solution and final process To validate the results, the solution obtained from the second leaching of a spent Ni–Mo catalyst (H2SO4 0.8 mol/L, 80 °C, 4 h) was passed through the column at 1.00 mL/min. The results are in agreement with those obtained using synthetic solutions (Fig. 6). The ratio [Ni]/[Ni]0 ¼ 0.1 was achieved after treating 230 mL of the leaching solution with Ni and Mo retentions of 97% and 100%, respectively. Mo was recovered through NaOH elution. Due to the acidity of the leaching solution, and even though the resin was washed with water after the experiment, the column was still

360

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

acid; so, the first 30 min of elution with NaOH 0.25 mol/L was used for its neutralization (Fig. 8A). From that point, the pH increases as well as the solubilization of Mo. After 80 min, recovery of Mo from the resin achieves 100 72%, with a purity of 99.8%. The second elution step, with a solution of HNO3 4 mol/L at 2.1 mL/min, led to 907 4% recovery of Ni, after 60 min, with a purity of 98.7% (Fig. 8B). A flow sheet of the final proposed process is presented in Fig. 9. Spent catalyst was treated with a H2SO4 solution of 0.8 mol/L, at 80 °C for 4 h, that dissolves 95% of Ni, 60% of Al and 34% of Mo (Pinto and Soares, 2013a). The solution was then passed through a column packed with SuperLigs 167 that was able to treat 38.4 mL of leachate per gram of resin at a flow rate of 0.17 ml/min/g of resin. The volume was chosen to avoid excessive loss of Ni into the outlet solution and thus maximizing the Ni recovery. Since all Mo and 97% of Ni were retained, the outlet solution was mostly Al that can be recovered through precipitation as aluminum hydroxide, by increasing the pH to 4.

14

0.050

12 0.040 10 Mo

8

Ni 0.020

Al

6

pH

4

Concentration, mol/L

0.010

pH

0.030

2

0.000 0

20

40

60

80

0 100

t, min 0.20

0.15

Mo

0.10

Ni Al

0.05

0.00 0

20

40

60

80

t, min Fig. 8. Elution profiles of metals using two sequential steps (2.1 mL/min): (A) first step with NaOH 0.25 mol/L; (B) second step with HNO3 4 mol/L. This are typical examples of experiments performed four times.

H2SO4 0.8 mol/L leaching spent HDS catalyst S Alumina residue

L

Mo was entirely recovered by alkaline elution with practically no co-extraction of Ni. From this solution, Mo can be recovered as a solid product by precipitation (Chen et al., 2006; Evans et al., 1998; Pinto and Soares, 2013b; Swinkels et al., 2004) or deposition of molybdenum oxides (Banica et al., 2009; Kusnetsov et al., 2004; Sinkeviciute et al., 2011). The second elution with HNO3 managed to recover 90 74% of the retained Ni, which corresponds to a total recovery of 83% of the Ni present in the spent catalyst. Recovery, as solid hydroxide, is possible by increasing the pH to 8. All the column experiments were performed without changing the resin, with no consequence in the efficiency of retention. The product life of SuperLigs 167 has been tested (Belanger et al., 2008) and showed promising results for application in a commercial process. Pilot and industrial scale experiments of similar processes have been studied with good results that show applicability of the proposed process to recover metals from spent HDS catalysts (Belanger et al., 2008; Izatt et al., 2011). The use of concentrated mineral acids might be seen as a disadvantage in the scale-up of the process; the main drawback is related with the fact of being corrosive and thus requires special handling, storage and expensive materials for the equipment. However, the leaching of spent catalysts using H2SO4 and treatment with SuperLigs 167 showed advantages in purity and efficiency when recovering Ni, Al and Mo from the solution (Pinto et al., 2015; Pinto and Soares, 2013a). 3.3. Application to Co–Mo catalysts The process presented in this paper was developed based on the recovery of metals from a spent Ni–Mo catalyst acid leaching solution (Pinto and Soares, 2013a). HDS catalysts often contain cobalt (Co), combined with Ni and Mo, or as an alternative to Ni. Therefore some experimental results and considerations are here presented in order to analyze the possibility of applying the suggested process to a catalyst containing Co. In a similar way as it was done for Ni, adsorption experiments with solutions containing only Co were firstly performed (Section 2.2.1). When Langmuir isotherm model (Eqs. (2) and (3)) was adjusted to the experimental points, the obtained parameters were qmax ¼1.45 mmol/g, kL ¼ 0.119 L/mmol. The lower kL value achieved when compared to Ni indicates that for the same values of metal concentration in equilibrium, the quantity of Co retained by the resin was lower than for Ni. Nevertheless, if the metal concentration was high enough, the capacity seemed to achieve similar qmax values between Ni and Co. When the two metals were put simultaneously in contact with the resin, with [Co]0 ¼[Ni]0, both were retained, with a slight preference of Ni binding over Co. For each experiment, it was verified that the sum of q(Co) and q (Ni) was always between 1 and 1.3 mmol/g, which is basically the maximum experimental capacity of the resin observed for Ni and Co separately. This fact confirms that Ni and Co are being bonded to the resin by the same mechanism (see Section 3.1.2).

Column SuperLig® 167 0.17 ml/min/g V ≈ 40 mL/g

Elution NaOH 0.25 mol/L

Mo solution

Ni solution

Al solution Purity = 99.3 % η from solution=99.7 %

Purity = 99.8 % η elution=100 % η from solution=100 %

Purity = 98.7 % η elution=90 % η from solution=87 % η from catalyst = 83 %

Elution HNO3 4 mol/L

Fig. 9. Flow sheet of the proposed process to recover Ni, Mo and Al from spent Ni–Mo catalyst.

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

Regarding the elution step, Co and Ni need similar conditions, which makes difficult their separation. Although Co seemed to be eluted slightly faster from the resin, there was no experimental conditions tested (variation of HNO3 concentration and flow-rate) where they could be eluted separately. Such behavior was expected due to the physico-chemical similarities of both metals. These experiments indicate that when treating a solution obtained from the acid leaching of a catalyst containing only Mo, Co and Al, a similar process to that developed in the present paper might be applied since Co will have an analogous performance as Ni. On the other hand, the simultaneous presence of Co and Ni in the leaching solution does not make the application of SuperLigs 167 efficient because the separation of both metal ions cannot be achieved and other separation processes would be required.

4. Conclusions The separation and recovery of Ni, Mo and Al from H2SO4 leaching solution of spent Ni–Mo catalyst was evaluated. The SuperLigs 167 resin was applied with the goal of binding Ni selectively. However, Mo was held by the resin, even though it did not affect the capacity of the resin for Ni due to different exchange mechanisms. Langmuir isotherm and pseudo-second order kinetics proved to be adequate models to predict the capacity and kinetics of the resin when it comes to Ni retention. The binding Ni capacity of SuperLigs 167 resin at pH 1, calculated from equilibrium and kinetic tests, was 1.35 mmol/g. The capacity of the resin to treat the leaching solution in continuous mode was tested in a column at different flow rates. It was found that lower flow rates separate Ni from Al more efficiently; however, due to co-binding of Mo, a two step-elution was necessary. In a continuous mode, each gram of resin treated 38.4 mL of the leaching solution at a flow rate of 0.17 ml/min/g, which resulted in a: 1. removal of 97% and 100% of Ni and Mo, respectively, leaving the solution mostly with Al; 2. recovery of 100% of Mo by elution with NaOH 0.25 mol/L; 3. recovery of 90% of retained Ni by elution with HNO3 4 mol/L. In conclusion, SuperLigs 167 resin seems to be a suitable product to treat acid leachates from HDS catalysts.

Conflict of interest The authors, Isabel S.S. Pinto and Helena M.V.M. Soares, declare that this article content has no conflicts of interest. The author Neil E. Izatt declares that he is employed by IBC Advanced Technologies, Inc., a producer of the SuperLigRs 167 resin.

Acknowledgments This work has been supported by Fundação para a Ciência e a Tecnologia (FCT), from the Portuguese Government, through grant UID/QUI/50006/2013. I. Pinto acknowledges a grant scholarship (SFRH/BD/70450/2010) financed by FCT.

References Adu-Wusu, K., Hassan, N.M., Nash, C.A., Marra, J.C., 2006. Sorption of cesium from aqueous waste solution on a SuperLig (R) 644 resin. J. Radioanal. Nucl. Chem. 267, 381–388.

361

Aveston, J., Johnson, J.S., Anacker, E.W., 1964. Hydrolysis of Molybdenum(VI). Ultracentrifugation acidity measurements þraman spectra of polymolybdates. Inorg. Chem. 3, 735–746. Banica, R., Barvinschi, P., Vaszilcsin, N., Nyari, T., 2009. A comparative study of the electrochemical deposition of molybdenum oxides thin films on copper and platinum. J. Alloy. Compd. 483, 402–405. Belanger, S.M., Izatt, N.E., Izatt, S.R., Dale, J.B., Bruening, R.L., 2008. Selective removal of nickel from cadmium and zinc-rich sulphate electrolyte in the zinc industry. In: Young, C.A., et al. (Eds.), Proceedings of the 6th International Symposium of Hydrometallurgy. Society for Mining, Metallurgy, and Exploration, Inc. (SME), Phoenix, AZ, USA, pp. 243–347. Bradshaw, J.S., Izatt, R.M., Savage, P.B., Bruening, R.L., Krakowiak, K.E., 2000. The design of ion selective macrocycles and the solid-phase extraction of ions using molecular recognition technology: a synopsis. Supramol. Chem. 12, 23–26. Chen, Y., Feng, Q.M., Shao, Y.H., Zhang, G.F., Ou, L.M., Lu, Y.P., 2006. Research on the recycling of valuable metals in spent Al2O3-based catalyst. Miner. Eng. 19, 94–97. Christensen, J.J., Eatough, D.J., Izatt, R.M., 1974. The synthesis and ion binding of synthetic multidentate macrocyclic compounds. Chem. Rev. 74, 351–384. Devol, T.A., Clements, J.P., Farawila, A., O'Hara, M.J., Egorov, O.B., Grate, J.W., 2009. Characterization and application of SuperLig(R) 620 solid phase extraction resin for automated process monitoring of Sr-90. J. Radioanal. Nucl. Chem. 282, 623–628. Dizge, N., Keskinler, B., Barlas, H., 2009. Sorption of Ni(II) ions from aqueous solution by Lewatit cation-exchange resin. J. Hazard. Mater. 167, 915–926. Dufresne, P., 2007. Hydroprocessing catalysts regeneration and recycling. Appl. Catal. A—Gen. 322, 67–75. Evans, T.I., Cannon, R.L., Evans, T.J., 1998. Separation of Molybdenum Epoxidation Catalyst. US Patent No. 5,776,848. Ghosh, A., Ray, H.S., 1991. Principles of Extractive Metallurgy, 2nd ed. Wiley, New York. Glennon, J.D., 2000. Molecular Recognition Technology—Inorganic Extraction. In: Wilson, I.A., Adlard, T.R., Poole, C.F., Cook, M. (Eds.), Academic Press, San Diego. Guo, F.Q., Nishihama, S., Yoshizuka, K., 2012. Selective recovery of copper, cobalt, and nickel from aqueous chloride media using solvent impregnated resins. Solvent Extr. Ion Exch. 30, 579–592. Hassan, N.M., King, W.D., McCabe, D.J., Hamm, L.L., Johnson, M.E., 2002. SuperLig (R) 639 equilibrium sorption data for technetium from Hanford tank waste supernates. Solvent Extr. Ion Exch. 20, 211–226. Idris, J., Musa, M., Yin, C.Y., Hamid, K.H.K., 2010. Recovery of nickel from spent catalyst from palm oil hydrogenation process using acidic solutions. J. Ind. Eng. Chem. 16, 251–255. Izatt, N.E., Bruening, R.L., Krakowiak, K.E., Izatt, S.R., 2000. Contributions of professor Reed M. Izatt to molecular recognition technology: from laboratory to commercial application. Ind. Eng. Chem. Res. 39, 3405–3411. Izatt, N.E., Izatt, S.R., Bruening, R.L., 2012. Green Procedure for the Selective Recovery of Precious, Specialty, and Toxic Metals from Electronic Wastes. Berlin, Germany. Izatt, R.M., 1997. Review of selective ion separations at BYU using liquid membrane and solid phase extraction procedures. J. Incl. Phenom. Mol. Recognit. Chem. 29, 197–220. Izatt, R.M., Izatt, S.R., Izatt, N.E., Krakowiak, K.E., Bruening, R.L., Navarro, L., 2015. Industrial applications of molecular recognition technology to separations of platinum group metals and selective removal of metal impurities from process streams. Green Chem. 17, 2236–2245. http://dx.doi.org/10.1039/c4gc02188f. Izatt, S.R., Izatt, N.E., Bruening, R.L., 2011. Selective separations of cobalt, uranium, zinc, nickel, and associated contaminants from various process streams. In: Proceedings of the 6th Southern African Base Metals Conference. The Southern African Institute of Mining and Metallurgy, Phaloborwa, South Africa. Juang, R.S., Kao, H.C., Chen, W., 2006. Column removal of Ni(II) from synthetic electroplating waste water using a strong-acid resin. Sep. Purif. Technol. 49, 36–42. Kim, H.I., Park, K.H., Mishra, D., 2009. Influence of sulfuric acid baking on leaching of spent Ni–Mo/Al2O3 hydro-processing catalyst. Hydrometallurgy 98, 192–195. Kim, K., Cho, J.W., 1997. Selective recovery of metals from spent desulfurization catalyst. Korean J. Chem. Eng. 14, 162–167. King, W.D., Spencer, W.A., Hamm, L.L., Aleman, S.E., Duffey, C.E., Pettis, M.A., 2005. Column performance testing of SuperLig (R) 639 resin with simulated hanford waste supernates: identification of the primary sorbing species and detailed characterization of their desorption profiles. Sep. Sci. Technol. 40, 383–394. Kula, R.J., 1966. Solution equilibria and structures of molybdenum(VI) chelates(ethylenedinitrilo)tetraacetic acid. Anal. Chem. 38, 1581–1584. Kusnetsov, V.V., Pavlov, M.R., Zimakov, D.I., Chepeleva, S.A., Kudryavtsev, V.N., 2004. Electroreduction of molybdate ions in solutions containing ammonium ions. Russ. J. Electrochem. 40, 711–715. Lee, J.Y., Rao, S.V., Kumar, B.N., Kang, D.J., Reddy, B.R., 2010. Nickel recovery from spent raneynickel catalyst through dilute sulfuric acid leaching and soda ash precipitation. J. Hazard. Mater. 176, 1122–1125. Li, B.H., Liu, F.Q., Wang, J.F., Ling, C., Li, L.J., Hou, P., Li, A.M., Bai, Z.P., 2012. Efficient separation and high selectivity for nickel from cobalt-solution by a novel chelating resin: batch, column and competition investigation. Chem. Eng. J. 195, 31–39. Mohapatra, D., Park, K.H., 2007. Selective recovery of Mo, Co and Al from spent Co/Mo/gamma-Al2O3 catalyst: effect of calcination temperature. J. Environ. Sci. Health A—Toxic/Hazard. Subst. Environ. Eng. 42, 507–515.

362

I.S.S. Pinto et al. / Chemical Engineering Science 138 (2015) 353–362

Nagib, S., Inoue, K., Yamaguchi, T., Tamaru, T., 1999. Recovery of Ni from a large excess of Al generated from spent hydrodesulfurization catalyst using picolylamine type chelating resin and complexane types of chemically modified chitosan. Hydrometallurgy 51, 73–85. Navarro, L., Morris, T., Read, W., 2012. Copper refining electrolyte purification by the use of molecular recognition technology (MRT) for bismuth removal. In: Proceedings of the T.T. Chen Honorary Symposium on Hydrometallurgy, Electrometallurgy and Materials Characterization, pp. 141–149. O'Hara, M.J., Burge, S.R., Grate, J.W., 2009. Automated radioanalytical system for the determination of Sr-90 in environmental water samples by Y-90 Cherenkov radiation counting. Anal. Chem. 81, 1228–1237. Orive, M.M., Olazabal, M.A., Fernandez, L.A., Madariaga, J.M., 1992. The recovery of cobalt and nickel from acidic sulfate-solutions in the presence of aluminum. Solvent Extr. Ion Exch. 10, 787–797. Pagnanelli, F., Ferella, F., De Michelis, I., Veglio, F., 2011. Adsorption onto activated carbon for molybdenum recovery from leach liquors of exhausted hydrotreating catalysts. Hydrometallurgy 110, 67–72. Park, K.H., Kim, H.I., Parhi, P.K., Mishra, D., Nam, C.W., Park, J.T., Kim, D.J., 2012. Extraction of metals from Mo–Ni/Al2O3 spent catalyst using H2SO4 baking– leaching-solvent extraction technique. J. Ind. Eng. Chem. 18, 2036–2045. Park, K.H., Mohapatra, D., Nam, C.W., 2007. Two stage leaching of activated spent HDS catalyst and solvent extraction of aluminium using organo-phosphinic extractant, Cyanex 272. J. Hazard. Mater. 148, 287–295. Park, K.H., Mohapatra, D., Reddy, B.R., 2006. Selective recovery of molybdenum from spent HDS catalyst using oxidative soda ash leach/carbon adsorption method. J. Hazard. Mater. 138, 311–316. Pedersen, C.J., 1967. Cyclic polyethers and their complexes with metal salts. J. Am. Chem. Soc. 89, 7017. Pinto, I.S.S., Sadeghi, S.M., Soares, H.M.V.M., 2015. Separation and recovery of nickel, as a salt, from an EDTA leachate of spent hydrodesulphurization catalyst using precipitation methods. Chem. Eng. Sci. 122, 130–137.

Pinto, I.S.S., Soares, H.M.V.M., 2012. Selective leaching of molybdenum from spent hydrodesulphurisation catalysts using ultrasound and microwave methods. Hydrometallurgy 129–130, 19–25. Pinto, I.S.S., Soares, H.M.V.M., 2013a. Microwave-assisted selective leaching of nickel from spent hydrodesulphurization catalyst: a comparative study between sulphuric and organic acids. Hydrometallurgy 140, 20–27. Pinto, I.S.S., Soares, H.M.V.M., 2013b. Recovery of molybdates from an alkaline leachate of spent hydrodesulphurisation catalyst-proposal of a nearly-closed process. J. Clean. Prod. 52, 481–487. Priya, R.G., Basha, C.A., Ramamurthi, V., Begum, S.N., 2009. Recovery and reuse of Ni (II) from rinsewater of electroplating industries. J. Hazard. Mater. 163, 899–909. Rao, S.V., Yang, D.H., Sohn, J.S., Kim, S.K., 2012. Purification of sulphate leach liquor of spent Raneynickel catalyst containing Al and Ni by solvent extraction with organophosphorus-based extractants. Sci. World J., Article ID 286494. Sinkeviciute, D., Baltrusaitis, J., Dukstiene, N., 2011. Layered molybdenum oxide thin films electrodeposited from sodium citrate electrolyte solution. J. Solid State Electrochem. 15, 711–723. Swinkels, P.L.J., van der Weijden, R.D., Ajah, A.N., Arifin, Y., Loe, H.L., Manik, M.H., Siriski, I., Reuter, M.A., 2004. Conceptual process design as a prerequisite for solving environmental problems; a case study of molybdenum removal and recovery from wastewater. Miner. Eng. 17, 205–215. Thomas, H.C., 1944. Heterogeneous ion exchange in a flowing system. J. Am. Chem. Soc. 66, 1664–1666. Valverde, I.M., Paulino, J.F., Afonso, J.C., 2008. Hydrometallurgical route to recover molybdenum, nickel, cobalt and aluminum from spent hydrotreating catalysts in sulphuric acid medium. J. Hazard. Mater. 160, 310–317. Zhang, P.W., Inoue, K., Tsuyama, H., 1995. Recovery of metal values from spent hydrodesulfurization catalysts by liquid–liquid-extraction. Energy Fuels 9, 231–239.