Adsorption onto activated carbon for molybdenum recovery from leach liquors of exhausted hydrotreating catalysts

Hydrometallurgy 110 (2011) 67–72

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Adsorption onto activated carbon for molybdenum recovery from leach liquors of exhausted hydrotreating catalysts Francesca Pagnanelli a,⁎, Francesco Ferella b, Ida De Michelis b, Francesco Vegliò b a b

Department of Chemistry, Sapienza University of Rome P. le Aldo Moro 5, I-00185 Rome, Italy Department of Chemistry, Chemical Engineering and Materials, University of L'Aquila, Monteluco di Roio, I-67040 L'Aquila, Italy

a r t i c l e

i n f o

Article history: Received 3 March 2011 Received in revised form 25 August 2011 Accepted 25 August 2011 Available online 1 September 2011 Keywords: Molybdenum Activated carbon Adsorption Hydrotreating catalysts

a b s t r a c t This paper investigated molybdenum recovery from acid leach liquors of exhausted hydrotreating catalysts. Adsorption onto activated carbon was used to separate molybdenum from other metals contained in these leach liquors, namely nickel, cobalt and vanadium. Kinetic tests using Mo-bearing solutions denoted that the rate of adsorption depends on the amount of Mo in solution with an estimated order of reaction of 0.9 ± 0.3. Equilibrium sorption tests showed that metal accumulation presents a bell-shaped behaviour as pH changes with a maximum sorption capacity around pH 5. Preliminary test in column reactor fed with Mo-bearing solution confirmed the sorption capacity estimated in batch tests (0.230 g/g). Sorption tests in batch reactors using leach liquor ([Mo] = 3.06 g/L, [V] = 5.84 g/L, [Ni]= 4.48 g/L, [Al]=1.77 g/L) denoted that Mo can be quantitatively and selectively removed from solution (99% removal for Mo, 24± 2% for Al, 19% for V and 0% for Ni) with no significant reduction of sorption capacity towards Mo (0.250 g/g). Preliminary sorption–desorption cycles denoted that Mo removal was larger than 90% in each cycle, whilst the other metals were minimally removed (Al and V) or completely rejected (Ni). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Hydrotreating catalysts (HTC) are widely used in different oil refinery operations such as demetalisation and desulphurisation of feedstock, and hydrotreating of heavy fractions (Marafi and Stanislaus, 2008). About 150–170 Kt of exhausted catalysts are produced every year in the world (Marafi and Stanislaus, 2008) and this amount will increase due to the growing energy consumption and the increasing importance of heavy fractions and low quality oil requiring significant catalytic pre-treatments. HTC are generally made up of alumina supporting Mo, Ni and Co as catalytic centres. Thermal regeneration can be performed, but once catalytic activity decreased under critical values, HTC were disposed off as wastes. End of life HTC are generally classified as dangerous special wastes due to the high content of toxic metals (Mo, Ni, Co, V). Nevertheless this same characteristic can be favourably exploited using HTC as secondary raw material for the recovery of such metals. Different pyrometallurgical and hydrometallurgical processes were reported in the literature to recover metals from HTC (Marafi and Stanislaus, 2008; Zeng and Cheng, 2009a). Developed processes generally start with thermal pre-treatments in which sulphur and coke are removed and Mo, Ni, Co and V compounds are transformed

⁎ Corresponding author. Tel.: + 39 06 49913333; fax: +39 06 490631. E-mail address: [email protected] (F. Pagnanelli). 0304-386X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2011.08.008

in oxides. Then roasting with alkali or basic leaching can be performed to recover Na2MoO4 and NaVO3 which are soluble in basic solutions, whilst Ni and Co can be extracted by acid leaching. Alternatively acid leaching can be performed directly on exhausted HTC obtaining leach liquors which contain simultaneously all metals (Mo, Ni, Co, V, Al). Most relevant aspects regarding process development for HTC valorisation then address the selective metal recovery from leach liquors containing the different metals (Zeng and Cheng, 2009b). Generally selective precipitation (Chen et al., 2006a; Kar et al., 2004; Navarro et al., 2007), solvent extraction (Busnardo et al., 2007; Chen et al., 2006b; Hirai and Komasawa, 1993; Navarro et al., 2007; Zhang et al., 1996), synthetic resins (Zhang and Zhao, 1989; Zipperian and Raghavan, 1985) and activated carbon (Kar et al., 2004; Park et al., 2006) can be used for the recovery of the different metals from leach liquors. Metal purification and recovery can be difficult especially for acid leach liquors in which all extractable metals are present simultaneously in solution. The problematic is then developing a route for the selective separation of metals from acid leach liquors. In this work activated carbon was tested for the selective recovery of molybdenum from leach liquor solutions obtained by acid leaching of Lc-Finer catalysts, HTC catalysts used in hydrotreating sections of petrochemical plants. Leach liquor composition can have variable metal concentration depending both on the catalysts and on the chemicals used for leaching (Ferella et al., 2011). Qualitative composition

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of leach liquor is: aluminium 8–20 g/L, molybdenum 1–5 g/L, nickel 1–5 g/L, vanadium 3–10 g/L. Aluminium was precipitated as ammonium alum ((NH4)Al(SO4)2(H2O)12) with a residual concentration of 1.2–1.5 g/L. This pre-treated leach liquor containing Mo, V, Ni and Al was treated by activated carbon to separate selectively Mo from the other metals. Novelty aspect of this work is the use of activated carbon to treat acid leach liquors containing all metals, whilst other literature works about HTC processing used activated carbon to recover Mo from basic leach liquors containing only Mo and Al (Park et al., 2006) or for increasing the purity of Mo previously separated by precipitation (Kar et al., 2004).

to pH 3 using NaOH solutions. After other 24 h stirring, a second sample was collected for each of the 15 reactors, pH and residual Mo concentrations were determined, and the suspension pH adjusted at pH 5. The same procedure of sorption and sample collection was repeated as before, and then a final level of pH (8) was investigated. 2.4. Column tests

2. Materials and methods

Column test was performed in a column reactor in PlexiglasTM (internal diameter 3 cm, height 30 cm) filled with 75.3 g of GAC (22.5 cm of height) and two glass wool discs on the top and the bottom of the column (pore volume = 0.2). A Mo-bearing solution (5 g/L) at pH 2.20 was fed upflow by a pump (flow rate 0.16 mL/min). Column effluent was analysed for residual Mo concentration and pH.

2.1. Materials

2.5. Sorption isotherm using leach liquor

Granular activated charcoal (GAC) was Norit® type produced by Fluka (Norit GAC 1240 W): it is a granular steam activated carbon obtained from coal and generally used for potable water processing. Synthetic solutions were obtained by dissolving in distilled water analytical grade salts (Sigma Aldrich) and in particular (NH4)6Mo7O24·4H2O, NiSO4·6H2O, and NH4VO3. Real leach liquor samples were obtained by treating exhausted catalysts collected from hydrotreating sections of petrochemical plants in acid conditions (H2SO4–H2O2) after Al precipitation (Ferella et al., 2011).

Sorption isotherm was obtained at pH 2.20 and room temperature using 50 mL samples of leach liquor (composition [Mo] = 3.060 g/L, [V] = 5.840 g/L, [Ni] = 4.480 g/L, [Al] = 1.770 g/L) added with different amounts of GAC. After 24 h stirring at room temperature, liquid samples were analysed for residual Mo concentration.

2.2. Batch kinetic tests Mo-bearing solutions were prepared using ammonium molybdate salt in order to obtain three initial concentrations of about 2, 5, and 10 g/L of Mo (real initial concentrations were determined by an Atomic Absorption Spectrophotometer, AAS). 100 mL samples of these solutions (initial pH 2.20) were put in contact with 2 g of GAC and kept under magnetic stirring at 25 °C. During time (10′, 20′, 30′, 45′, 60′ 120′, 240′, 650′, 1050′ and 24 h) solution samples were collected and analysed for pH and residual metal concentration by AAS. 2.3. pH edge tests 15 pH edge tests were carried out in 15 distinct reactors using solutions with initial Mo concentrations and amounts of GAC as detailed in Table 1. For all 15 tests initial pH was adjusted to pH 1 with sulphuric acid and, after 24 h stirring at room temperature, liquid samples were collected for the determination of equilibrium pH and residual Mo in solution, whilst the 15 Mo-bearing suspensions were adjusted

2.6. Stripping tests GAC performances were tested in three cycles of sorption and stripping. Sorption tests were performed using 100 mL leach liquor samples put in contact with 2 g of GAC under stirring at room temperature for 24 h. After solid–liquid separation, liquid samples were measured for residual metals, whilst solid samples were used for stripping tests. Stripping tests were performed adding 100 mL of a solution of NH4OH (15%) to GAC samples coming from sorption tests. After 3 h stirring solid–liquid separation was performed, liquid samples were analysed for extracted metals, whilst solid GAC was used in the following sorption-stripping cycles. 3. Results and discussion 3.1. Experimental tests with synthetic solutions 3.1.1. Batch kinetic tests During kinetic tests molybdate solution, initially colourless, gradually changed colour becoming first blue and then, after 24 h stirring, dark green tending to brown. After addition of GAC gas development was observed and, at the end of the experiments, pH values increased (Table 2). All these observations can be explained considering both Mo speciation in solution and acid–base properties of GAC functional groups.

Table 1 Operating conditions in pH edge tests. Test

[GAC] (g/L)

[Mo]0 (g/L)

pHf(1)

pHf(2)

pHf(3)

pHf(4)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

20 20 20 20 20 1 1 1 1 1 5 5 5 5 5

5 10 15 17 20 0.2 0.5 1 2 3 2 4 6 8 10

1.26 1.43 1.33 1.39 1.52 1.22 1.27 1.34 1.40 1.35 1.37 1.38 1.28 1.38 1.41

2.73 2.74 2.70 2.80 2.77 3.11 3.03 3.12 3.06 3.04 2.99 3.02 3.01 3.01 2.97

4.60 4.65 4.63 4.71 4.65 5.06 5.22 5.20 5.16 5.05 5.04 5.10 4.97 5.09 4.97

6.07 6.27 6.50 6.58 6.67 8.01 7.84 8.00 7.86 7.84 7.68 7.57 7.06 7.14 6.78

F. Pagnanelli et al. / Hydrometallurgy 110 (2011) 67–72

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Table 2 Kinetic tests: initial Mo concentrations ([Mo]0), equilibrium pH (pHf), equilibrium concentration in solid phase after 24 h (qe), estimated rate of sorption (− dC/dt), regressed parameters for pseudo-first order kinetic model (qer and k1), and determination coefficient. [Mo]0 (g/L)

pHf

qe (g/g)

−dC/dt (g/min)

qer (g/g)

k1 (min− 1)

R2

2 5 10

3.76 3.21 2.94

0.062 ± 0.005 0.15 ± 0.01 0.33 ± 0.02

0.039 ± 0.004 0.014 ± 0.003 0.009 ± 0.001

0.061 ± 0.002 0.150 ± 0.006 0.321 ± 0.005

(7.8 ± 0.8)10− 3 (5.0 ± 0.6)10− 3 (6.2 ± 0.3)10− 3

0.9866 0.9861 0.9971

As for Mo speciation, the oxidation state of this metal can change from −2 to +6 depending on the pH and electric potential (Eh) in solution (Ullmann, 1996). The predominance diagram Eh versus pH (Fig. 1) shows that Mo(VI) species are stable in oxidising conditions and, according to solution pH, different Mo(VI) species can be present. In the central region of Eh–pH diagram the predominant species is MoO2, whilst for reducing potential metallic Mo predominates. Mo blues (polyoxometalates containing both Mo(V) and Mo(VI) species) can also form for partial reduction of Mo(VI) species giving the formation of solid species such as Mo3O8 (dark blue), Mo2O5 (black–purple), MoO2 (dark violet) (Ullmann, 1996). In presence of diluted acids (HCl and H2SO4) Mo(VI) reduction can occur giving the following soluble complexes: [Mo2O4(H2O)6] 2+ (brown), ([Mo2(OH)2(H2O)6] 4+) (green), and [Mo(H2O)6] 3+ (yellow) (Ullmann, 1996). According to colour change observed during kinetic tests it is reasonable that two molybdenum species can form with brown [Mo2O4 (H2O)6] 2+ and green ([Mo2(OH)2(H2O)6] 4+) colour and with OH − release in solution according to the following reactions assuming C oxidation: −

2þ

4H2 MoO4 þ C↔2Mo2 O4 þ CO2 þ 4OH þ 2H2 O

ð1Þ

−

ð2Þ

2þ

4þ

Mo2 O4 þ C þ 2H2 O↔Mo2 ðOHÞ2 þ CO2 þ 2OH :

The observed trend in final pH values showing an increase which is inversely proportional to Mo initial concentration (from 2.20 to 3.76, 3.21 and 2.94 for initial concentrations 2, 5, 10 g/L, respectively) suggested that also other redox reactions not producing OH − ions occurred in solution and preferentially for large Mo concentrations (pH increase is lower for larger initial Mo concentrations). In particular considering Mo speciation in solution also MoO2 species

can form according to the following reaction without production of OH − 2H2 MoO4 þ C↔2MoO2 þ CO2 þ 2H2 O:

ð3Þ

This reaction producing a solid species (MoO2) not participating to equilibrium is probably favoured for large Mo concentration thus explaining the observed trend of pH. As for the blue transient it can be due to acid–base properties of GAC. In fact carbon surface is characterised by a variety of O-containing functionalities that can explain the acidic and basic character of carbon materials. Acidic functionalities include carboxylic, lactonic, carbonyl and phenolic groups, whilst basic sites can be ketones, pyrones and chromenes (Boehm, 1994; Chun et al., 2004; Fuente et al., 2003). Acidic groups dissociate assuming negative charge, whilst basic sites can undergo protonation and acquire positive charge. As a consequence according to pH conditions, carbon surface can result positivelycharged (acidic pH) or negatively-charged (basic pH), and then binds preferentially negative or positive ions in solution, respectively. Natural pH of GAC used in this work was 10 units denoting the strong basic character of its sites. The addition of GAC to the metal solution determines a dramatic local increase of pH which can cause the reversible formation of Mo blues, typically observed for pH larger than 7 when MoO42− ions are present in solution. When mixing occurred, bulk pH stabilised at acidic pH (around 3) and the blue colour disappeared. As for the quantitative analysis of kinetic tests experimental data were reported in Fig. 2. According to a quite general behaviour the kinetic trend of metal removal is characterised by an initial rapid and quantitatively predominant diminution of metal concentration in liquid phase followed by a second slower and less significant decrease. This trend can be explained by considering the initial abundance of available active sites on the sorbent that becoming gradually

1.0

0.35 0.8

0.30

0.4

0.25

0.2

0.20

q (g/g)

Eh (V)

0.6

0.0

2 g/L 5 g/L 10 g/L

0.15

-0.2

0.10 -0.4 -0.6

0.05

-0.8

0.00 0

-1.0

1

2

3

4

5

6

7

8

9

pH Fig. 1. Molybdenum speciation in solution.

10

11

12

500

1000

1500

Time (min) Fig. 2. Kinetic tests of Mo sorption: Mo concentration in solid phase as function of time for three different initial Mo concentrations (experimental data and pseudo first order model).

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occupied makes sorption less efficient in the slower stage, but even mass transfer effects due to penetration in the solid matrix should be taken in consideration. Kinetic tests also showed that the rate of adsorption depends on the amount of Mo in solution. Equilibrium conditions were reached for the lowest concentration (2 g L − 1) after 250 min (98% of the final removal at 24 h), whilst at the same time only the 70% and the 65% of the final removal at 24 h were obtained for 5 g/L and 10 g/L, respectively. This is mainly due to the gradual filling of sorption sites and then to the decrease of the Mo gradient at the interface. For the range of Mo concentrations here considered 24 h seems to be a time sufficient to reach equilibrium conditions. The rate of Mo sorption (evaluated as the slope of the data collected within the first 60 min) was reported in Table 2 showing that it increases for increasing Mo concentration. Estimated order of reaction by linearised kinetic relation was α = 0.9 ± 0.3 (R 2 = 0.9949). Experimental data also denoted that equilibrium metal uptake (qe, metal concentration in solid phase after 24 h) increased for increasing initial concentration: 0.060 g/g of adsorbed Mo for 2 g/L GAC suspension, 0.150 g/g for 5 g/L, 0.320 g/g for 10 g/L. These data denoted that for the range of concentrations of Mo here investigated GAC capacity is not saturated because metal sorption still increase passing from the median to the highest level of Mo concentration. Kinetic tests can be adequately represented by a pseudo-first order model (Febrianto et al., 2009) assuming that the instant rate of metal removal is proportional to the gradient from equilibrium concentration in solid phase (qer) and the concentration in solid phase at each time (q) dq ¼ k1 ðqer −qÞ: dt

ð4Þ

The estimated parameters qer and k1 for the different initial concentrations were reported in Table 2 showing a good agreement between regressed qer and experimental data of equilibrium sorption capacity qe. 3.1.2. Batch sorption tests: effect of pH Experimental data from pH edge tests were grouped according to the four levels of equilibrium pH (Fig. 3).

0.50 0.45 0.40 0.35 pH =1

qe (g/g)

0.30

pH=3 pH=5

0.25

pH=8 Langmuir (pH=1)

0.20

Langmuir (pH=3)

0.15

Langmuir (pH=5) Langmuir (pH=8)

0.10

Experimental data showed that metal accumulation presents a bell-shaped behaviour as pH changes with a maximum sorption capacity around pH 5 (Fig. 3). This non-monotonic trend can be explained by considering both Mo speciation and acid–base properties of sites present on activated carbon. For pH 1 Mo species in solution are positively-charged ([Mo2O4 (H2O)6] 2+), ([Mo2(OH)2(H2O)6] 4+, [Mo(H2O)6] 3+) just like the basic pyronic sites onto carbon surface þ KH

þ

S þ H ↔ SH :

ð5Þ

At pH around 1 then electrostatic forces disfavour site–metal interactions. As pH increases (3–6 range) positive charge concentration onto carbon surface decreases according to the protonation constant of pyronic sites (KH = 5) (Contescu et al., 1998; Pagnanelli et al., 2008) and also Mo species in solution become negatively charged 6− 5− 4− , HMo7O24 , H2Mo7O24 , and MoO42−). (Mo7O24 For larger pH values (pH N 7) carbon surface becomes negatively charged (due to the dissociation of weakly acidic sites such as carboxylic ones) (Pagnanelli et al., 2008) and electrostatic forces disfavour interactions with negatively-charged species of Mo causing a decrease of metal removal. Each iso-pH set of data was represented by Langmuir isotherm in order to estimate maximum sorption capacity (qmax) in the different operating conditions (Table 3). qe ¼

q max bC e 1 þ bC e

ð6Þ

where qmax and b are the adjustable parameters of this model generally associated to the maximum sorption capacity of the sorbent and the sorbent-metal affinity, respectively. Regressed parameters for each pH level were reported in Table 3 confirming that maximum sorption capacity toward Mo presents a maximum for pH 5. 3.1.3. Preliminary test in column reactor Breakthrough curve for Mo-bearing solution was reported in Fig. 4 along with pH data as a function of the volume of treated effluent measured as bed volume (1 BV = 31.8 mL). Even though this column test can be considered only preliminary (possible leakage or channelling of feed solution could have been because at the very beginning of the test Mo concentration in the effluent is not zero) some interesting observations can be made. After treating, the colourless Mo solution used as feed became first yellow, then light blue (after 15 BV) and finally blue (after 35 BV) with an increasing colour intensity leading to dark blue−black (after 50 BV). pH of feed solution (pH = 2.20) after treating dramatically increased (pH = 8.40) and then as the column saturated, returned to the feed value. These experimental findings denoted that activated carbon acts as a reducing agent towards Mo(VI) giving Mo(V) species with the typical blue colour. Column breakthrough (C/C0 N 0.05) was reached within 5 BV (after the treatment of 160 mL), whilst column saturation (C/C0 N 0.95) occurred after 211 BV (6720 mL). Table 3 Equilibrium modelling of pH edge data: regressed parameters for Langmuir isotherm (qmax and b) and determination coefficient.

0.05 0.00 0

2

4

6

8

10 12 14 16 18

Ce (g/L) Fig. 3. Equilibrium sorption tests: iso-pH sets of data from pH edge experiments and Langmuir model.

Equilibrium pH

1.25 ± 0.09

3.11 ± 0.05

5.09 ± 0.05

8.05 ± 0.05

qmax (g/g) b (L/g) R2

0.22 ± 0.02 0.28 ± 0.07 0.9819

0.27 ± 0.01 4±1 0.9824

0.47 ± 0.02 2.2 ± 0.5 0.9888

0.125 ± 0.004 1.4 ± 0.2 0.9936

F. Pagnanelli et al. / Hydrometallurgy 110 (2011) 67–72

1.0

0.30

9 C/C0

0.9

8

0.25

Adams-Bohart

0.8

7

pH

0.20

0.7

qe (g/g)

6

0.6 5

pH

C/C0

71

0.5 4

0.15

0.10

0.4 3

0.3

0.05

2

0.2

0.00 0

1

0.1

1

2

3

Ce (g/L)

0.0 0

50

100

150

0 200

Fig. 5. Mo sorption isotherm using leach liquor: experimental data and Langmuir model.

Bed Volumes Fig. 4. Mo sorption in column reactor: breakthrough curve and pH profile; Adams– Bohart simulations.

3.2. Experimental tests with real leach liquor Sorption capacity was estimated by column experiment as

qi ¼

∫ðC 0 −C Þdt m

ð7Þ

where C0 is Mo concentration in the feed solution, C is the concentration measured in the effluent at any time until saturation, and m is the total amount of sorbent in the column. This estimate of sorption capacity was about 0.230 g/g which is in the range of values of sorption capacities observed in pH-edge tests (Table 3). Amongst approximate dynamic models the Adams–Bohart model was used to represent breakthrough curve (Salamatinia et al., 2008)

ln

C H ¼ kAB C 0 t−kAB qm C0 u

ð8Þ

where C is the metal concentration in the liquid, kAB is the kinetic constant, H is the height of the column, and u is the superficial velocity. This model was derived under the assumptions of low concentration field (Cb 0.15 C0, generally valid in the initial part of the breakthrough), and that for t → ∞, q → qm where qm is the saturation concentration. Nevertheless regression of data (kAB = 6.4 ∗ 10 − 8 L/(mg ∗ min)) gave Adams–Bohart simulations able to represent breakthrough data up to C = 0.5 C0 for BV = 150. Then without taking any conclusion about the limiting step in sorption process due to the approximate nature of this model, it can be concluded that the Adams–Bohart model can give a good approximation of breakthrough data in the operative range of a fixed column reactor. The height of mass transfer zone (HMTZ: the region inside the column in which the solute concentration in the liquid phase change from 95% to 5% of its inlet value) can be evaluated according to the following equation 

H MTZ

 V E −V B ¼Z V E −0:5ðV E −V B Þ

ð9Þ

where Z is the length of the column, VE is the volume treated up to the saturation, VB is the volume treated up to breakthrough. HMTZ was estimated at 43 cm which is two times the column length.

3.2.1. Sorption isotherm Sorption isotherm of Mo using leach liquor was reported in Fig. 5. Sorption tests using leach liquor denoted that Mo can be quantitatively and selectively removed from solution. In fact (99 ± 1)% removal of Mo was obtained, whilst Ni remained in solution. Only V and Al were partially removed, 19 ± 2% and 24± 2%, respectively. Complete rejection of Ni can be explained by considering that this metal in the operating conditions used here (acid pH) is mainly present as positively charged species not reacting with protonated pyrone sites of GAC. On the other hand V removal can be explained considering the similar chemical properties with respect to Mo. Both these metals are present as anions in the pH range of sorption tests and this plays a crucial role in sorption phenomenon. In particular V can be present in solution as anionic species (such as 6− ) which can interact with posiV10O27OH 5−, V10O26(OH)24−, V10O28 tively charged pyronic sites just like anionic species of Mo. Maximum concentration of Mo in solid phase using leach liquor was about 0.250 g/g denoting that no significant reduction was determined by the presence of other competing metals. 3.2.2. Stripping tests Activated carbon performances were tested in three cycles of sorption and desorption (stripping). In Table 4 for each cycle the % of sorption and desorption were reported for each metal (% of metal recovered during stripping was calculated as % on the base of sorbed metal in the previous sorption step). Mo removal during the three sorptions remained larger than 90%, even though only 40% of sorbed Mo was desorbed in each cycle during stripping. As for the other metals only a low percentage of them were sorbed in each cycle and released during stripping. Table 4 Subsequent sorption and desorption cycles: % of sorbed and desorbed metals during each cycle and final concentration of metal in solid phase. Step

Mo

Al

V

Ni

I Adsorption I Stripping II Adsorption II Stripping III Adsorption III Stripping Final concentration in solid phase (g/g)

94 40 97 38 97 45 0.260

12 6 9 4 10 9 0.093

21 58 17 34 16 45 0.084

3 95 4 64 2 73 0.005

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Metal concentrations in solid phase after three cycles were reported in the last line of Table 4: it is possible to note that the largest removal is observed for Mo, whilst Ni is completely rejected by this sorbent. V and Al are also partially removed according to previous data of adsorption isotherm. Even these preliminary results about sorption-stripping cycles confirmed the selectivity of activated carbon towards Mo using leach liquors and operating conditions (acid pH) typical of acid leaching of end of life HTC (Ferella et al., 2011). About the selectivity of the separation obtained with activated carbon we can perform only preliminary comparison with the data reported in a work addressing Mo recovery by solvent extraction using leach liquor containing metals with similar concentrations to the ones we used (Zhang et al., 1996). In the work of Zhang et al. about 3 g/L of Mo and 0.8 g/L of V were contained in the leach liquor and the extractions yields were 99% for Mo and 70% for V. In our case we have a larger amount of V (about 6 g/L) whilst Mo concentration is the same: we obtained 94% Mo removal and 20% V removal. Then a preliminary comparison puts in evidence that the selectivity of sorption onto activated carbon is comparable with that obtained by solvent extraction using similar leach liquor composition. 4. Conclusions Experimental results reported in this work evidenced the possibility of using activated carbon for the direct recovery of Mo from acid leach liquors of HTC also containing Ni, V, and Al. Activated carbon sorption capacity towards Mo was not affected by the presence of these metals in solution as resulted by the comparison between sorption capacity estimated using Mo-bearing solutions and real leach liquors. Sorption– desorption cycles confirmed this selectivity towards Mo. These preliminary results open new opportunities for the treatment of exhausted HTC are secondary resources for the recovery of metals such a Mo, V, and Ni. In particular a novel process route will be further addressed including acid leaching extracting simultaneously all metals, Al precipitation, and sorption for the selective separation of Mo. References Boehm, H.P., 1994. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 32 (5), 759–769. Busnardo, R.G., Busnardo, N.G., Salvato, G.N., Afonso, J.C., 2007. Processing of spent Ni–Mo and Co–Mo/Al2O3 catalysts via fusion with KHSO4. J. Hazard. Mater. 139, 391–398.

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