Hydrometallurgical Treatment for Valuable Metals ... - ACS Publications

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Hydrometallurgical Treatment for Valuable Metals Recovery from Spent CoMo/Al2O3 Catalyst. 2. Oxidative Leaching of an Unroasted Catalyst Using H2O2 Vincent Ruiz,†,|| Eric Meux,*,† Michel Schneider,‡ and Vincent Georgeaud|| †

)

Laboratoire d'Electrochimie des Materiaux, Institut Jean Lamour UMR 7198 CNRS-INPL-UHP-UPVM, 1 Boulevard Arago, CP 87811, 57078 Metz cedex 3, France ‡ Laboratoire de Chimie et Methodologie pour l’Environnement, Universite Paul Verlaine—Metz, 1 Boulevard Arago, CP 87811, 57078 Metz cedex 3, France Veolia Environnement Recherche et Innovation, Centre de Maisons-Laffitte Chemin de la Digue, BP 76, 78603 Maisons-Laffitte Cedex, France ABSTRACT: This study deals with the feasibility of the oxidative leaching of molybdenum and cobalt sulphides contained in a spent hydrodesulphurisation catalyst using H2O2 in acidic medium. The study was first carried out with MoS2 in order to define the important parameters of the leaching procedure and then applied to an unroasted CoMo/Al2O3 spent catalyst containing 11 wt % of Mo and 2.7 wt % of Co. In both cases, a 23 factorial design was used. The results obtained with MoS2 highlighted the necessity to limit the increase in temperature of the medium to minimize the decomposition of H2O2 and the necessity to keep pH constant to avoid an important acidification of the medium which could cause dissolution of alumina matrix when leaching will be applied to spent catalysts. The oxidative leaching of the unroasted CoMo/Al2O3 spent catalyst was performed without previous grinding. In a single-step, at pH = 1.3 with a stoichiometric factor equal to 2.4, an L/S ratio equal to 7.5, and an H2O2 concentration of 3.75 mol 3 L1, it was possible to recover molybdenum and cobalt simultaneously with leaching yields of 90% and 83%, respectively, without dissolving more than 8% Al. In these conditions, the maximal temperature recorded during the experiment was about 60 °C.

1. INTRODUCTION CoMo/Al 2 O 3 catalysts are used in petroleum refining of to produce ultra low sulfur fuels. This refining step is called hydrodesulphurisation (HDS). The active phase of the catalyst is constituted by MoS 2 and Co 3 S 4 supported by porous alumina Al 2 O3 . 1 During processing the crude oil, catalyst is submitted to various phenomena leading to its deactivation: active phase sintering, coke deposition due to the presence of asphaltens, and deposition of petroleum contaminants mainly vanadium and nickel. 2,3 Coke deposition is the primary cause of CoMo/Al2 O 3 catalyst deactivation; these catalysts have a life varying from 3 months to 6 years depending on the feed. Then, catalysts become waste which can either be stored or processed to recover molybdenum and cobalt. Spent CoMo/Al2 O3 catalysts generally contain about 412% molybdenum, 1530% aluminum, and 04% cobalt, which makes them economically viable for metal recovery. The literature revealed that most of the works concerning HDS spent catalysts are based on roastingleaching methods, being either oxidative roasting followed by alkaline and/or acidic leaching of the oxides obtained 411 or salt roasting and water leaching of the soluble salts obtained.1215 In the present study, a hydrometallurgical process based on leaching of unroasted spent CoMo/Al 2 O 3 catalyst with hydrogen peroxide H2 O2 has been proposed. The oxidative leaching of sulphides by H 2 O2 (or Na 2 O 2 ) was already investigated for the extraction of various metals from ores such as molybdenite MoS2 or r 2011 American Chemical Society

sphalerite ZnS.1620 The use of this strong oxidative reagent for the treatment of spent catalysts was recently investigated in alkaline media21,22 or in acidic medium23 with samples previously ground before performing the leaching. There are two advantages to perform oxidative leaching with H2 O2 in acidic media. First, no SO2 emission occurs because the spent catalyst is directly leached without roasting. Second, simultaneous leaching of molybdenum and cobalt can be performed in a single step. In the classical roastingleaching methods, molybdenum is leached by NaOH in a first step. The residue is then leached by H2SO4 in order to recover cobalt which is insoluble in alkaline medium. High leaching yields of cobalt are obtained but with a very important dissolution of alumina matrix.8,24 In this work, we have studied the oxidative leaching of an unroasted CoMo/Al 2 O 3 spent catalyst in order to recover molybdenum and cobalt together. The sample was not ground and no pretreatment was applied to this catalyst before leaching. Leaching was first investigated with commercial MoS2 in order to define the important parameters to control for spent catalyst management. Leaching of both MoS2 and of the spent catalyst was investigated using a factorial design of experiments. Received: July 16, 2010 Accepted: March 15, 2011 Revised: March 11, 2011 Published: April 04, 2011 5307

dx.doi.org/10.1021/ie102428r | Ind. Eng. Chem. Res. 2011, 50, 5307–5315

Industrial & Engineering Chemistry Research

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Table 1. Encoded Values of the Three Factors for MoS2 Leaching factor level 1 0 þ1

factor X1

factor X2

factor X3

molar ratio nH2O2/nMoS2

L/S ratio

leaching time (h)

9

10

1

13.5

20

2

18

30

3

2. MATERIALS AND METHODS The most important metal to be recovered here is molybdenum because of its content and its cost. Moreover, spent catalysts from oil industry are the most important secondary sources of molybdenum.10 So, oxidative leaching was first investigated with commercial MoS2 in order to define important parameters to be considered for studying spent catalysts. MoS2 leaching was investigated using a two level-factorial design of experiments (DOE) with three factors (nH2O2/nMoS2 ratio, liquid to solid ratio L/S, and leaching time) and three center points. Spent CoMo/Al2O3 catalysts contain up to 4% of cobalt under sulphide form. The behavior of CoS provided by JohnsonMatthey was investigated performing only qualitative tests. When a small amount of commercial CoS is added to a H2O2 solution, we observed a rapid dissolution of the compound whatever the molar ratio nH2O2/nCoS and the L/S ratio. This dissolution was accompanied by a sudden increasing in temperature and an important effervescence leading to an overflow in the cell. As a matter of fact, it is well-known that Co2þ catalyzes the dispropornation of hydrogen peroxide.25 Therefore, the study of cobalt sulphide leaching was not further investigated. 2.1. Study of MoS2 Leaching. Leaching Procedure. Leaching experiments were carried out by introducing into the cell the amount of H2O2 and pure water to reach the required nH2O2/ nMoS2 and L/S ratios. The initial pH in the cell varied between 3.7 and 4.4 depending on H2O2 concentration. Before MoS2 addition, the cell was at room temperature. In each experiment, 2 g of commercial pure MoS2 (Fluka, particle size ranged from 2.0 to 3.5 μm) is added in two 1 g fractions at the beginning and after 10 min, in order to limit the temperature rise. The content of the cell was stirred with a magnetic stirrer at 300 rpm throughout the leaching procedure. At the end of each experiment, the final pH was measured, the solution was filtered through a 0.22 μm pore size filter, and the solid residue was washed with pure water, vacuum-dried at 80 °C, and weighed. Filtrate and washing water were collected in the same volumetric flask. Molybdenum was measured by atomic absorption spectrometry (AAS) using a VARIAN AA240FS spectrometer. The excess of H2O2 was determined by cerimetric titration performed with Ce(NH4)2(NO3)6 0.02 mol 3 L1 (PROLABO) in H2SO4 1 mol 3 L1.26 The titration was followed by potentiometry using platinum and SCE electrodes. Factors and Domain. Each factor varies between a low level (1 in encoded value) and a high level (þ1 in encoded value). These levels define the domain. The center of this domain is used to define the center point (0 in encoded value). Several replications of this intermediate point are performed in order to obtain an estimate of error, to check for interactions (crossproduct terms) in the model, and to check for quadratic effects (curvature).

The three retained factors were the following: Factor X1. The molar ratio between H2O2 and MoS2 according to the reaction MoS2 þ 9H2 O2 f MoO2 2þ þ 2SO4 2 þ 2Hþ þ 8H2 O ð1Þ This parameter has to be at least equal to 9 (nH2O2/nMoS2) corresponding to the reaction above. The highest level was fixed at 18 (because of H2O2 cost of for a viable industrial process). Factor X2. L/S ratio ranged from 10 to 30 mL 3 g1 in order to limit the increase in the medium temperature due to the exothermicity of the chemical reaction involving MoS2 and H2O2. For each experiment, the amount of MoS2 was equal to 2 g. For such conditions, the ratio factor X1/factor X2 is proportional to the H2O2 concentration. This concentration varied from 1.87 to 11.2 mol 3 L1, this last value corresponding to 35% H2O2 solution. Factor X3. This is the leaching time, which varyies from 1 to 3 h. The three factors, their coded name, and their three corresponding encoded levels are given in Table 1. Responses. The effects of the different factors are measured for the five following responses: • The percentage of molybdenum leached (noted YMo), calculated after the measurement of MoVI concentration in the leachate by AAS • The amount of residual H2O2 in the leachate (noted YH2O2) determined by cerimetric titration and expressed as a percentage of the initial amount of hydrogen peroxide introduced in the cell • The pH of the leachate at the end of the experiments (noted YpH). Reaction 1 indeed indicates acidification of the medium which can cause dissolution of alumina matrix in the case of spent catalysts. • The highest temperature recorded during the experiment (noted YTmax) • The time necessary to reach this temperature (Yt). The two last responses were measured in order to assess the industrial-scale process because hydrogen peroxide has to be handled with care. 2.2. Study of the Spent Catalyst. Characterization of Catalyst Sample. The spent CoMo/Al2O3 catalyst used in this work was supplied by SARP Industries. Catalyst particles were rodshaped elements with a trilobe profile with a diameter of about 1 mm and a length ranging from 0.5 and 1.5 cm. Chemical analysis of the sample was performed by X-ray fluorescence spectrometry (XRF) using a BRUKER AXS S4 Explorer wavelength-dispersive spectrometer. Quantitative analysis of Al, Co, and Mo was performed on beaded samples (Li2B4O7 flux with a flux to sample ratio equal to 8). Sulfur was determined using a semiquantitative method on pelletized samples. Each analysis was replicated three times. Carbon was estimated in an external laboratory using elementary analysis (single measurement). The composition of the spent catalyst is given in the Table 2 (wt %). The radiocrystallographic study of ground samples was performed with a BRUKER D8 Advance diffractometer (Lynxeye θθ detector; incident radiation: Cu KR1). Leaching Procedure. As for pure MoS2, the oxidative leaching by H2O2 was investigated using a two-level factorial design with three factors (molar ratio between H2O2 and MoS2 þ Co3S4, pH, L/S ratio) and four center points. The experimental setup used for the study of oxidative leaching of MoS2 was modified including a system to keep pH constant 5308

dx.doi.org/10.1021/ie102428r |Ind. Eng. Chem. Res. 2011, 50, 5307–5315


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Table 2. Composition of the Studied CoMo/Al2O3 Spent Catalyst (wt %)

Table 3. Encoded Values of the Three Factors for Spent Catalyst Leaching factor X1

factor X2

factor X3

factor level

stoichiometric factor

pH

L/S ratio

1

1.1

2

5

1.65

3

2.2

4

compound

average value

standard deviation

Al

26.7

0.40

Mo Co

11.0 2.9

0.50 0.04

S

7.9

0.11

þ1

C

6.3

0

composed of a pH-probe, a TM 800 Radiometer (pH-stat function) and a buret Radiometer ABU 901 filled with 1 mol 3 L1 NaOH. A 5 g portion of spent catalyst (without preliminary grinding) was added to the H2O2/H2O mixture at room temperature, 1 g every 6 min. The leaching time was fixed at 1 h, from the first gram added. Preliminary experiments indeed showed that after an hour, there was no more hydrogen peroxide in the medium. The kinetics of the oxidation is fast, and the H2O2 in excess is removed instantaneously by Co2þ. The cell was stirred at 300 rpm. After leaching, the content of the cell was filtered through a 0.22 μm pore size filter, and the solid residue was washed with 100 mL pure water. Filtrate and washing water were collected in a 200 mL volumetric flask for determination of molybdenum, cobalt, and aluminum content. Solid residue was dried at 105 °C and weighed. Factors and Domain. The factors and their domain were chosen taking into account: • the results obtained with the first design of experiments performed on MoS2 • the nature of the studied spent catalyst which contains cobalt sulphide, oxidizable by H2O2, and alumina which can be dissolved at low pH.The three factors are the following: Factor X1. This is the molar ratio between H2O2 and MoS2 þ Co3S4 according to reactions 1 and 2 Co3 S4 þ 15H2 O2 f 3Co2þ þ 4SO4 2 þ 14H2 O þ 2Hþ ð2Þ The necessary mole number of hydrogen peroxide is calculated using the weight percentages of molybdenum and total sulfur given in Table 2. The low level of this factor corresponds to the stoichiometric amount of H2O2 increased by 10%. The high level is twice larger than the lower level. This parameter was called the stoichiometric factor. Factor X2: pH. This parameter has a dual influence, i.e. on the redox reaction between MoS2 and H2O2 and on the possible dissolution of the alumina matrix. The range of pH investigated was from 2 to 4. Factor X3. L/S ratio ranged from 5 to 10 mL 3 g1. These ratios, lower than for the first DOE, take into account the amounts of molybdenum and cobalt sulphides in the alumina matrix (Table 2). The three factors, their coded names, and their three corresponding encoded levels are given in Table 3. Responses. Four responses were taken into account: • the percentage of molybdenum leached (noted YMo) • the percentage of cobalt leached (noted YCo) • the percentage of aluminum leached (noted YAl) • the highest temperature recorded during the experiment (noted YTmax). The leaching yields were calculated after measurement of MoVI, Co2þ, and Al3þ concentrations in the leachate by AAS. The temperature was recorded during all the experiments.

7.5 10

The objectives were to determine the best experimental conditions to obtain the following: • the highest leaching yields for molybdenum and cobalt • the lowest amount of aluminum dissolved to facilitate a subsequent industrial purification of the leachate • a temperature as low as possible in view to considering possible treatment at the industrial-scale. 2.3. Calculation of Effects and Interactions for a 23 DOE. If the results obtained for the center points show no drift, these values can be used to evaluate the experimental standard deviation and to estimate the standard deviation of the effects and interactions. For each response, the main effect of factor Xi is the difference between two averages:27 main effect Ei ¼ yiþ yi

ð3Þ

where yiþ is the average response corresponding to the plus level of this factor and yiis the average response to its minus level. Two factors which do not behave additively are said to “interact”. The influence of a factor is depending on the other factor level. Like main effects, interaction effects are defined as the difference between two averages. For example, the interaction between factors X1 and X2 (noted E12) is given by E12 ¼ y12þ y12

ð4Þ

where • y12þ is the average response corresponding to the plus level of the product of columns of factors X1 and X2 • y12 is the average response corresponding to the minus level of the product of the same columns The estimate of the three-factor interaction is again a difference between two averages according to the relation: E123 ¼ y123þ y123

ð5Þ

3. RESULTS AND DISCUSSION 3.1. MoS2 Leaching. The experiments were performed in a random order (except the three center points CP1, CP2, and CP3). Such order permits ensuring that uncontrolled factors do not affect the results. Table 4 shows the experimental matrix (Yates order) and the five corresponding responses for each experiment. In the first column, the order in which runs were performed is indicated in brackets. For each experiment, the calculated mass of MoS2 unleached corresponds to the residual mass of MoS2 recovered after filtration. Molybdate solutions are colorless whatever the pH. In our experiments, all the filtrates are yellow-orange except for the run 4 where the solution has a yellow appearance. These colors are due to the presence of peroxomolybdates which are yellow, orange, or red-brown depending on their concentration. These 5309



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Table 4. Experimental Matrix in Coded Values and Obtained Responses for MoS2 leaching molar ratio nH2O2/nMoS2 (X1)

L/S ratio (X2)

leaching time (h) (X3)

YMo (%)

YH2O2 (%)

YpH

YTmax (°C)

Yt (min)

1 (4)

1

1

1

44.6

5.5

0.6

93.0

12

2 (7)

1

1

1

62.0

4.1

0.4

98.0

7

1

1

1

49.2

5.1

0.6

67.0

15

run

3 (10) 4 (2)

1

1

1

81.9

16.8

0.0

75.0

27

5 (9)

1

1

1

48.1

3.4

0.5

92.0

6

6 (8)

1

1

1

61.0

3.3

0.5

97.5

6

7 (5)

1

1

1

53.7

5.0

0.8

67.0

20

8 (3) CP1 (1)

1 0

1 0

1 0

84.3 66.2

10.8 7.1

0.0 0.6

82.0 92.0

30 12

CP2 (6)

0

0

0

62.7

4.4

0.6

90.0

17

CP3 (11)

0

0

0

63.9

5.1

0.7

91.0

17

Table 5. Calculated Effects and Interactions with Their Standard Deviations for MoS2 Leaching YMo (%)

YTmax (°C)

Yt (min)

average on runs

60.60

83.94

E1

23.40

8.38

15.38 4.25

E2 E3

13.35 2.35

22.38 1.38

15.25 0.25

E12

8.25

3.13

6.75

E13

1.65

1.88

0.75

E23

1.10

2.13

3.75

E123

0.60

1.63

1.75

s: standard deviation

1.30

0.70

2.05

compounds result from the exchange of the oxo ligands present in MoO42 by peroxo ligands and giving dianionic complexes with the general formula Mo(O2)O4n2.28 Even for the runs performed with a large excess of hydrogen peroxide, incomplete leaching of MoS2 can be observed. However, the residual amount of H2O2 is low. This can be explained by two facts: • Under 60 °C, the disproponation of H2O2 is slow whereas this decomposition is self-accelerated for higher temperatures. For all the runs, the recorded temperature was higher than 67 °C • The disproponation of hydrogen peroxide is catalyzed by molybdate ions.29 Whatever the operating conditions, we can observe an acidification of the medium caused by the chemical reaction between MoS2 and H2O2. The pH variations measured at the end of the leaching are small and, in all cases, below 0.8. For the three other responses, the values obtained for effects and interactions are presented in Table 5. The standard deviations are estimated using CP1, CP2, and CP3 values. Since effects and interactions are being calculated by a difference between two averages, they have the same standard deviation given in the last line of Table 5. The noninfluencing effects or interactions are those verifying the constraint |Ei| < 2si. The most significant effects and interactions are indicated in bold in this Table. Examination of the Results for Response YMo. The analysis of the effects in Table 5 shows that: • The leaching time (factor X3) has a weak influence on this response in the studied domain.

Figure 1. Two-way table of interacting factor X1 and factor X2 for MoS2 leaching.

• Factors X1 and X2 are significant as well as interaction 12. This interaction can be interpreted by means of a two-way table (Figure 1) built with the data from the original eight runs. At the corner of the squares, the average response YMo of the runs with the specified settings of the interacting factors are shown. A leaching yield of 83% is obtained only if the factors X1 and X2 are together at their highest level (runs 4 and 8). If one of these factors is at the lowest level, the yield decreases by at least 20%. Examination of the Results for Response YTmax and Yt. As expected, the reaction between MoS2 and H2O2 causes a strong temperature increase in the cell. The highest recorded temperatures are varying between 67 and 98 °C. From an industrial point of view, it is very important to limit this increase to allow safety of the process. Factor X3 (leaching time) is not influencing for these two responses in the studied domain. Factor X2 (L/S ratio) is the most influencing factor whereas factor X1 and interaction 12 are less influencing. The effects of factor X2 have opposite signs for these two responses. The lowest level of this factor leads to a maximum temperature reached within the shortest time (runs 1, 2, 5, and 6). In these conditions, temperature exceeds 90 °C after a time lapse ranging from 6 to 12 min. For runs 2, 5, and 6, the peak temperature was reached before the addition of the second gram of MoS2. This sudden increasing in temperature leads to the rapid decomposition of H2O2 which is no more available for sulphide oxidation. The maximum temperature was reached later if factors X1 and X2 are at the highest level (27 min for run 4 and 30 min for run 8). These levels correspond therefore to the best operating conditions to obtain the highest molybdenum leaching yield. Influence of H2O2 Concentration. The study of the four responses YMo, YpH, YTmax, and Yt shows that the most significant factor is the hydrogen peroxide concentration which is 5310



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Table 6. Average Values of YMo, YpH, YTmax, and Yt Responses versus [H2O2] for MoS2 Leaching [H2O2] mol 3 L1

average YMo(%)

average YpH

average YTmax (°C)

average Yt (min)

3 and 7

1.87

51.45

0.7

67.0

17.5

4 and 8

3.73

83.10

0.0

78.5

28.5

CP1, CP2 and CP3

4.22

64.27

0.6

91.0

15.3

1 and 5

5.6

46.35

0.6

92.5

9.0

2 and 6

11.2

61.50

0.5

98.0

6.5

runs

Figure 4. XRD diagram of the unroasted CoMo/Al2O3 spent catalyst. Figure 2. Variation of YMo and YpH responses vs H2O2 concentration for MoS2 leaching.

Figure 3. Variation of YTmax and Yt responses vs H2O2 concentration for MoS2 leaching.

proportional to the ratio between factor X1 and factor X2. Because of the levels chosen for factors X1 and X2, there is a ratio of 6 between the highest and lowest H2O2 concentrations (11.2 and 1.87 mol 3 L1, respectively). Table 6 gives the four average responses calculated for a same H2O2 concentration. The variation of YMo and YpH are presented in Figure 2, whereas YTmax and Yt responses are gathered in Figure 3.

It clearly appears that the best H2O2 concentration to perform MoS2 leaching is equal to 3.73 mol 3 L1 (runs 4 and 8). For this concentration, the molybdenum leaching yield is maximal and consequently the final pH of the leachate is minimal. The highest temperature recorded during the leaching remains moderate and was reached within approx 30 min. Higher H2O2 concentrations lead to a rapid temperature increase which causes the dispropornation of hydrogen peroxide. In this case, lower molybdenum leaching yields are observed. 3.2. Leaching of the CoMo/Al2O3 Spent Catalyst. The diffraction pattern of the CoMo/Al2O3 spent catalyst is presented in Figure 4. No crystallized phases can be found on this pattern which is fairly similar to those of γ -Al2O3 support. Calculation of Main Effects and Interactions. Table 7 gives the experimental matrix and the four corresponding responses for each experiment. As in the previous DOE, the results obtained for the four center points were used to evaluate the standard deviation of the effects and interactions. For the four responses, the values obtained for effects and interactions are presented in Table 8 with their standard deviation. The most influencing factors and interactions are indicated in bold. Analysis of results Examination of the Results for Response YMo. According to the calculated effects, two factors exhibit a strong influence on this response. The most influencing factor is the pH. As a matter of fact, when this factor is varied from its low level (1) to its high level (þ1), the YMo response decreases in average of 27%. The second one is the stoichiometry with a YMo response increasing in average of 16% from low level to high level. The L/S ratio has a small influence on molybdenum leaching in the studied domain. The best leaching yields of molybdenum were obtained for runs 2 and 6 corresponding to factor X1 at its highest 5311



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Table 7. Experimental Matrix in Coded Values and Experimental Results for Spent Catalyst Leaching stoichiometric factor [1]

pH [2]

L/S ratio [3]

YMo (%)

YCo (%)

YAl (%)

YTmax (°C)

1

1

1

31.3

1

1

1

52.8

64.1

8.6

49

75.8

10.0

3 (6)

1

1

1

65

11.8

37.5

3.8

4 (3)

1

1

55

1

19.3

42.1

1.0

5 (5)

1

68

1

1

35.1

65.6

8.4

6 (2)

46

1

1

1

56.9

81.4

13.0

7 (9)

55

1

1

1

10.9

41.6

0.4

39

8 (11) CP1 (1)

1 0

1 0

1 0

26.3 39.0

55.9 65.4

0.3 4.8

53 55

CP2 (4)

0

0

0

32.6

65.6

4.8

55

CP3 (8)

0

0

0

30.8

64.0

4.6

52

CP4 (12)

0

0

0

35.8

65.6

7.9

50

run 1 (7) 2 (10)

Table 8. Calculated Effects with Their Standard Deviations for Spent Leaching Catalyst effects average on runs E1

YMo (%)

YCo (%)

YAl (%)

YTmax (°C)

30.6 16.6

58.0 11.6

5.7 0.8

53.7 12.9

E2

27.0

27.5

8.6

0.1

E3

3.5

6.3

0.3

11.1

E12

5.1

2.2

2.2

0.4

E13

2.1

3.5

1.5

1.6

E23

0.5

2.7

1.7

4.6

E123

1.9

1.4

0.1

1.9


2.6

0.6

1.2

1.8

level and factor X2 at its lowest. However, these yields are significantly lower than those obtained with pure MoS2. Examination of the Results for response YCo. Figure 5 gives, for each run of the DOE, cobalt leaching yields vs molybdenum leaching yields. We can note a strong correlation between the both yields (r = 0.962). The conditions favorable for MoS2 oxidation will be also favorable for the oxidation of cobalt sulphide. The percentage of cobalt leached is always significantly higher than that of molybdenum one. The average of the four center points (YCo = 65.15%) compared with the average of the eight runs (YCo = 58.0%) shows a significant difference. A simple linear model is not suitable. However, the results show that the three factors have an influence on YCo. The pH (factor X2) is the most influencing. The favorable levels of factors are those used for run 6 (factor X1 at level þ, factor X2 at level , and factor X3 at level þ) where the yield of cobalt leached attains 81.4%. Examination of the Results for Response YAl. The average value of the four center points (YAl = 5.53%) is in good agreement with the average of the eight runs (YAl = 5.69%). So, it is possible to model the variation of this response as a function of the three factors. The factor X2 (pH) is the most influencing factor. When the pH is varied from 2 (low level) to 4 (high level), the amount of aluminum leached decreases by 8.6%. This result is in agreement with the solubility curve of alumina versus pH. Therefore, complete recovery of molybdenum and cobalt cannot be reached without dissolving the alumina matrix.

Figure 5. Correlation between the yields of cobalt and molybdenum leached from spent catalyst.

This dissolution liberates sulphides trapped in catalyst pores and increases consequently the molybdenum and cobalt leaching yields. Figure 6 shows that leaching yields of molybdenum and cobalt increase when the percentage of aluminum leached increases (dissolution of alumina matrix). We can note that the YCo/YMo ratio is close to 2, the both linear regressions being almost parallel with slopes equal to 2.79 and 2.93 for cobalt and molybdenum, respectively. Examination of the Results for Response YTmax. Factors X1 and X2 are influencing whereas the influence of the interaction 23 is moderate (Table 8). The highest temperature recorded corresponds to the highest level of factors X1 and X2 and to the lowest level of factor X3 (run 4, 68 °C). In fact, the most influencing factor is H2O2 concentration which is proportional to ratio factor X1/factor X3. The coefficient of correlation between Tmax and [H2O2] is equal to 0.931 (Figure 7). There is no significant correlation between H2O2 concentration and the other responses. The rise of temperature during spent catalyst leaching (from 39 to 68 °C) is more moderate than during the first DOE performed with pure MoS2 (up to 98 °C) due to the 5312



Figure 6. Correlation between the yields of cobalt, molybdenum, and aluminum leached from spent catalyst.

buffering of alumina matrix which makes sulphides leaching more gradual. 3.3. Improvement of Molybdenum Leaching. The best runs of the DOE (2 and 6) allowed leaching of 79% of cobalt in average but only 55% of molybdenum. For pure MoS2 leaching, the best operating conditions led to an average leaching yield of 83.1% for molybdenum, a value obtained for an H2O2 concentration of about 3.75 mol 3 L1 and a stoichiometric factor equal to 2.2. In order to improve the leaching of molybdenum present in the spent catalyst, several experiments have been performed with this concentration. For [H2O2] = 3.75 mol 3 L1, the highest temperature reached estimated from Figure 7 is about 61 °C. This increasing of temperature is acceptable from an industrial point of view. An increasing in factor X1 is favorable to molybdenum leaching. The stoichiometric factor was fixed at 2.4 (1.36 in coded value), and the L/S ratio was equal to 7.5 corresponding to the value of the center point for factor X3. This factor value allows a reasonable volume of leachate with high concentrations of cobalt and molybdenum to be obtained. A low pH value increases the leaching yield of molybdenum. So, pH was fixed to 1.3 (1.7 in coded value). For this pH value and for factor X1 = 2.4 and factor X3 = 7.5, the linear model predicts less than 15% of aluminum leached. The results obtained are presented in Table 9. The average percentage of molybdenum leached is equal to 88.9%. This represents an increase of 32% compared with the best trial of the DOE (run 6). For cobalt, the average leaching yield 82.6% is comparable to the results obtained in the DOE. The average percentage of aluminum leached (8.1%) is significantly lower than in run 6 (13.0%). The highest recorded temperature has an average value of 61.5 °C very close to the predicted value determined from Figure 6. The composition of the leachate is the following: [MoVI] = 1.17 101 mol 3 L1 (11.2 g 3 L1), [Co2þ] = 4.67 102 mol 3 L1 (2.75 g 3 L1), and [Al3þ] = 1.07 101 mol 3 L1 (2.9 g 3 L1). 3.4. Comparison with the Results Reported in the Literature. Table 10 gathers the results reported in various papers and related to experiments performed either by oxidative roasting, either using salt roasting or by oxidative leaching on spent

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Figure 7. Correlation between Tmax and H2O2 concentration for spent catalyst.

Table 9. Results Obtained for Spent Catalyst Leaching with the Best Operating Conditions molybdenum leached (%)

cobalt

aluminum

leached (%) leached (%) Tmax (°C)

1

89.7

83.0

8.2

60.0

2 3

84.6 92.4

81.5 83.4

8.6 7.6

65.0 59.5

average

88.9

82.6

8.1

61.5

4.0

1.0

0.5

3.1


CoMo/Al2O3 and NiMo/Al2O3 catalysts. The best results obtained in the present work are given in the last line of the table. If we compare our results with those resulting from oxidative leaching of NiMo/Al2O3 catalysts,21,22 we obtain a better leaching yield of molybdenum but an amount of dissolved aluminum higher due to the acidity of the medium. But, contrary to these two works, our spent catalyst sample was not ground before leaching. All the other procedures require a roasting at temperatures ranged between 450 and 900 °C for periods of time varying from 30 mn to 3 h. For oxidative roasting,5,7,8,11 two leaching steps are necessary to recover metals with leaching yields ranged between 87 and 98% for molybdenum and between 77 and 93% for cobalt. The second leaching step, performed with H2SO4, leads to a very important dissolution of alumina matrix (percentage of aluminum leached ranged between 24 and 68%). Salt roasting using Na2CO314 or NaCl15 permits only the recovery of molybdenum with yields higher than 92%. Salt roasting performed with Na2O13 and KHSO412 leads to the almost complete dissolution of alumina. In this last case, we can consider this leaching as a mineralization of the sample which causes the dissolution of the matrix with leaching yields of molybdenum and cobalt equal to 99 and 91%, respectively. 5313



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Table 10. Comparison between the Data Reported in the Literature and the Results of the Present Work first step spent catalyst

roasting conditions

leaching conditions

second step

extraction yields

leaching conditions

extraction yields

overall results

H2SO4 10 g 3 L1

Co 91%

Mo 96%

Al 54%

Co 91%

oxidative roasting CoMo5

T = 450 °C t=2h

NaOH 10 g 3 L1

T = 100 °C

Mo 96% Al 14%

t=2h L/S = 20 CoMo7

CoMo11

t=2h L/S = 20

T = 500 °C

NH3 conc

t=2h

room temp L/S = 25

T = 500 °C

Na2CO3 30 g 3 L1

t=3h

Mo 83%

Mo 97%

T = 30 °C t=1h

CoMo

T = 500 °C t=3h

H2SO4 10% vol

H2SO4 6 mol 3 L1 T = 30 °C t=1h

L/S = 10 23

T = 100 °C

Al 68% Mo 4%

Mo 87%

Co 77%

Co 77%

Co 87%

Mo 97%

Al 38%

Co 87% Al 38%

Co 93% Al 21%

Mo 98% Co 93%

L/S = 10 1

Na2CO3 30 g 3 L T = 90 °C

Mo 98% Al 2.6%

H2SO4 6 mol 3 L1 T = 90 °C

t=1h

t=1h

L/S = 10

L/S = 10

Al 24%

salt roasting CoMo12

13

NiMo

CoMo14

T = 500 °C

deionized water

Mo 99%

Mo 99%

t=3h

T = 90100 °C

Co 91%

Co 91%

KHSO4 10 wt %

t = 40 min

Al 96%

Al 96%

T = 750 °C

Deionized water

Mo 99%

Mo 99%

t = 30 mn

T = 80 °C

Al 97%

Al 97%

Na2O

t=2h Mo 92%

Mo 92%

Mo 99%

Mo 99%

nNa2O/nAl2O3 = 1.2

L/S = 10

T = 600 °C

Deionized water

t = 30 mn

T = 80 °C

Na2CO3 12 wt %

t=2h L/S = 10

15

NiMo

T = 900 °C

Deionized water

t=1h

T = 7090 °C

NaCl 20 wt %

t=1h L/S = 5 oxidative leaching

NiMo21

No roasting

Ground to 100 μm

Mo 85%

Mo 85%

H2O2 6% vol

Ni 65%

Ni 65%

Na2CO3 40 g 3 L1

Al 3%

Al 3%

Ground to 100 μm

Mo 84%

Mo 84%

H2O2 10%

Ni 0.3%

Ni 0.3%

Na2CO3 80 g 3 L1 t=1h

Al 1.5%

Al 1.5%

not ground H2O2

Mo 90% Co 83%

Mo 90% Co 83%

pH = 1.3

Al 8%

Al 8%

t=1h L/S = 5 22

NiMo

no roasting

L/S = 5 CoMo [this work]

no roasting

t=1h L/S = 7.5

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4. CONCLUSION This work studies the oxidative leaching by H2O2 of MoS2 and CoS contained in a spent CoMo/Al2O3 catalyst. The leaching was first investigated on pure MoS2 to define the important parameters to consider for investigating the leaching of the spent catalyst sample. This preliminary study showed the necessity to control the pH in order to avoid the dissolution of alumina and the importance of the L/S ratio which permits limiting the increase of the temperature and consequently the rapid dispropornation of H2O2. The oxidative leaching was then investigated with an unroasted spent CoMo/Al2O3 catalyst without preliminary grinding. At pH = 3, with a stoichiometric factor equal to 2.4 and a L/S ratio of 7.5 (corresponding to H2O2 concentration of equal to 3.75 mol 3 L1), we obtain an average molybdenum leaching yield of 88.9%. In these conditions, the leaching yield of cobalt reaches 82.6% while the dissolution of alumina matrix remains limited (8% of aluminum leached). The highest temperature recorded during the leaching of spent catalyst in these best operating conditions is equal to 61 °C, a temperature compatible with an industrial development. This oxidative leaching of spent catalyst with H2O2 allows a simultaneous recovery of molybdenum and cobalt in a single step without grinding with high leaching yields. ’ AUTHOR INFORMATION Corresponding Author

*Phone number: 33-387-315-438. Fax number: 33-387-315-460. E-mail: [email protected].

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