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Hydrometallurgical Treatment for Valuable Metals Recovery from Spent CoMo/Al2O3 Catalyst. 1. Improvement of Soda Leaching of an Industrially Roasted Catalyst Vincent Ruiz,†,|| Eric Meux,*,† Sebastien Diliberto,† and Michel Schneider‡ †
)
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 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 paper describes the recovery of molybdenum from a spent CoMo/Al2O3 by soda leaching after a roasting step. First, the leaching was performed on a catalyst industrially roasted at 800 °C during 20 min. In order to reach a molybdenum leaching yield higher than 85%, the process was optimized using a 23 design of experiments with parameters such as NaOH amount, leaching time, and temperature. Unfortunately, in the best leaching conditions defined by the factorial design, the leaching yield of molybdenum never exceeded 60% because of an incomplete roasting of the catalyst proved by the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis of the residue. In a second part, the roasting was investigated on an unroasted catalyst containing 23.9% Al, 11% Mo, 2.9% Co, 7.9% S, and 6.3% C using a central composite design. By changing the thermal treatment conditions (700 °C, 20 min) and performing the best soda leaching conditions (an amount of sodium hydroxide equal to the twice of the required stoichiometry and a leaching time of 4 h at 60 °C), it was possible to extract more than 90% of molybdenum initially present in the catalyst.
1. INTRODUCTION In the European Union, the maximum sulfur content in petrol and diesel fuels has been limited to 10 ppm since January 2009.1 To respect this regulation, refiners have to perform the wellknown hydrodesulphurisation process (HDS) which is carried out using Co (or Ni) Mo catalysts whose Mo content ranges between 8 and 12 wt % with a molar ratio M/(Mo þ M) (M = Co or Ni) of about 1/3.28 The active phase of the catalyst is constituted by MoS2 and Co3S4 (or Ni2S3) supported by porous alumina (Al2O3).9 In addition to Co or Ni, phosphorus is frequently added as an activity promoter.8,1012 During the crude process, the catalyst is submitted to various phenomena leading to its deactivation. The loss of activity is due to (i) active phase sintering, (ii) coke deposition, and (iii) deposition of petroleum contaminant, mainly V and Ni.13 Even if carbon deposits can be removed by thermal treatment, the deactivation induced by metals is irreversible.14 As a consequence, about 150 000 t of spent hydrotreating catalysts are generated worldwide each year.15 Considering hazardousness of these wastes and the non-negligible content of expensive metals such as Co, Ni, Mo, and V, metal reclamation can be cost-effective and environmentally suitable. To achieve the spent catalyst beneficiation, several leads have been investigated using pyro- or hydrometallurgy. Hydrometallurgical processes can be classified as follows: oxidative roasting followed by alkaline and/or acidic leaching of the resulting oxides,1622 salt roasting and water leaching of the soluble salts obtained,2327 and direct oxidative leaching of sulphides.28,29 In the best operating conditions, more than 90% r 2011 American Chemical Society
of the targeted metals can be leached.30,31 These processes can be selective or not. Several techniques are available to process the obtained leach liquors.32 Depending on the composition of these solutions, molybdenum can be recovered by ammonium salt precipitation,26,27,33 solvent extraction,21,25,34,35 electrochemical processes,16 or ion-exchange.36 If the leaching is not selective, the recovery of valuable metals will involve several steps and the cost of the industrial process will dramatically increase. In the case of spent catalyst which does not contain vanadium, oxidative roasting followed by caustic leaching is widely used by industrials3741 to perform selective dissolution of molybdenum. This work is divided in two parts. In the first one, the soda leaching of molybdenum from an industrially roasted spent CoMo/Al2O3 catalyst was studied in order to recover at least 85% of molybdenum initially present in roasted catalyst avoiding a too important dissolution of alumina matrix. After this leaching step, the residue was returned to the industrial that performed the roasting to recover cobalt and remaining molybdenum by pyrometallurgical way.42,43 The soda leaching step was investigated using design of experiments (DOE) methodology. The second part of this work concerns the influence of the roasting conditions on molybdenum recovery yield. This study was
Received: July 16, 2010 Accepted: March 15, 2011 Revised: March 11, 2011 Published: April 04, 2011 5295
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Industrial & Engineering Chemistry Research Table 1. Chemical Analysis of Two Spent CoMo/Al2O3 Catalysts (wt %) and Results of the Water Leaching of the Roasted Spent Catalyst CoMo1
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Table 2. Encoded Values of the Three Factors for Spent Catalyst CoMo1 Soda Leaching X1
CoMo1 (roasted) element Al
chemical analysis 37.3 ( 0.3
soluble fraction 1.6 ( 0.9%
CoMo2 (unroasted)
26.7 ( 0.3
C
1.8
Co
3.5 ( 0.1
Mo
10.5 ( 0.2
33.1 ( 3.6%
11.0 ( 0.5
Ni P
0.9 ( 0.1 1.9 ( 0.1
17.5 ( 1.2%
97%, for analysis) and checked by pH-metric titration. The experiments of the DOE were performed in a few days avoiding the carbonation of the sodium hydroxide. At the end of the leaching, the pH value of the pulp was measured (PHM 210 Radiometer pHmeter) and the cell content was filtered through a 90 mm diameter B€uchner vessel. The leaching residue was then washed on the filter with 2 50 mL of deionized water. The leachate volume was adjusted to 200 mL in a volumetric flask. The leaching residue was dried at 105 °C. The Mo and Al contents of the leach liquors were measured by AAS. The chemical composition of residues was determined by quantitative and semiquantitative XRF. To take into account the possible sample heterogeneity, leaching yields were calculated using both leachates and leaching residues composition. Responses. The effects of the different factors investigated were measured for the three following responses: • The molybdenum leaching yield (YMo) which represents the efficiency of the leaching. A maximum value should be reached. • The aluminum leaching yield (YAl) corresponding to the dissolution of the catalyst alumina support. It can be considered as an indicator of the leaching selectivity and must be minimized. • The final pH value (YpH). 2.3. Effect of Roasting Conditions on Molybdenum Leachability. The aim was to determine the influence of the roasting conditions on the molybdenum leaching yield using sodium hydroxide. The effect of the roasting temperature and the roasting duration were studied on an unroasted spent CoMo/ Al2O3 catalyst noted CoMo2. In a previous work, Mohapatra and Park22 studied the effect of the calcination temperature on a spent CoMo/Al2O3 catalyst but between 200 and 600 °C. In our work, we studied the roasting with conditions close to those of industrial flash-roasting performed on the CoMo1 sample.
ð4Þ
The final weight loss (upper 900 °C) is due to MoO3 sublimation. Consequently, the influence of the roasting conditions on the molybdenum leaching yield was studied between 700 and 900 °C. Concerning the roasting duration, the shortest time studied was 20 min corresponding to that used for industrial flashroasting. The longest roasting duration studied was 2 h. The effect of temperature and roasting duration was first studied using a 22 DOE with four center points (8 runs). The average responses of the four runs of the DOE and the average responses of the center points were different. In these conditions, a linear model cannot be accepted. So, a central composite design (CCD) was performed.46 Four axial runs were added to the original 22 DOE, and two center points were performed to check the absence of drift. Table 3 presents the relation between coded and real values of the both studied parameters. Roasting and Leaching Procedures. Roasting experiments were performed in a NABERTHERM L9 muffle furnace under air. A 10 g portion of the CoMo2 sample was placed on a steel grid with a bed depth of about 5 mm. When the required temperature was reached, the catalyst was introduced in the furnace. During the experiment, the grid permits a natural air circulation through the sample. After the roasting step, the catalyst was cooled down to room temperature out of the furnace and weighed. The sample was then leached with sodium hydroxide during 2 h at 60 °C with an amount of NaOH equal to the twice of the stoichiometry needed. The leaching procedure was the same as described above. If we suppose that these operating conditions are the best to ensure the total dissolution of MoO3, the variations of the molybdenum leaching yield will be related to the roasting conditions. Responses. Four responses were measured: • The molybdenum leaching yield (Y1). It was determined measuring molybdenum concentration in leach liquors by AAS. A maximum value should be reached. • The fraction of the initial molybdenum that remains in the insoluble part of the roasted catalyst after sodium hydroxide leaching (Y2). This response must be minimal to ensure a good molybdenum recovery yield. It was determined by quantitative XRF analysis of leaching residues. • The fraction of the initial molybdenum sublimated during the roasting step (Y3). This response was deduced from the both previous responses assuming that the part of 5297
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Table 4. Experimental Matrix in Coded Values and Obtained Responses for CoMo1 Soda Leaching run
leaching reagent quantity X1
temperature X2
leaching duration X3
YMo (%)
YAl (%)
YpH
1 (8)
1
1
1
45.3
0.0
9.6
2 (3)
1
1
1
51.1
4.2
12.4
3 (5)
1
1
1
48.4
0.0
6.8
1
1
1
56.2
3.5
11.6
5 (7)
1
1
1
52.7
0.0
7.8
6 (4)
1
1
1
56.6
4.2
12.1
7 (9)
4 (10)
1
1
1
44.9
0.0
6.6
8 (2) CP1 (1)
1 0
1 0
1 0
61.4 54.4
3.6 0.9
11.7 11.0
CP2 (6)
0
0
0
54.0
0.9
10.8
CP3 (11)
0
0
0
53.0
1.0
10.4
Table 5. Calculated Effects and Interactions with Their Standard Deviations for CoMo1 Leaching effect or interaction
YMo (%)
YAl (%)
YpH 10.7
CP average response
53.8
0.93
average on runs
52.1
1.94
9.8
E1
8.5
3.88
4.3
E2 E3
1.3 3.7
0.33 0.03
1.3 0.6 0.7
E12
3.7
0.33
E13
1.7
0.03
0.4
E23
2.8
0.03
0.5
E123
2.7
0.03
0.3
standard deviation estimated
0.5
0.04
0.2
from CP dispersion (sX)
the molybdenum that was not in the leach liquor or in the leaching residue was lost by sublimation. Y1, Y2, and Y3 were expressed as a percentage of the initial molybdenum content. • The aluminum leaching yield (Y4), expressed as a percentage of the initial aluminum content, represents the selectivity of the leaching step. The alumina support has to be dissolved as little as possible.
3. RESULTS AND DISCUSSION 3.1. Study of Molybdenum Leaching. The experimental matrix (Yates order) and the related results are presented in Table 4. The experiments were performed in random order, except the three center points. The execution order is noted between brackets. The obtained results for the three center points demonstrate the reproducibility of the experiments. The variance of the center points can be considered as an estimate of the experimental variance. This value was used to assess the standard deviation (sX) of each effect and interaction.44 The results obtained for average responses of the eight runs of the DOE, average responses of center points, the effects, the interactions, and their respective standard deviation are presented in Table 5. The examination of the values of the effects and the interactions allows finding the influencing factors. The noninfluencing effects and interactions are those verifying |EXi| < 2sX. Most significant effects and interactions are noted in bold in this table.
Figure 2. Graphical Representation of Results for YMo (between brackets the run number).
Examination of the Results for YMo. The average response of the eight runs and the average response of the three center points are in good agreement. So, the linear model can be accepted. All the molybdenum leaching yields obtained are greater than the water-soluble fraction (33.1%). The most influencing factor is the amount of leaching reactant (X1). The leaching duration (X3) and the interaction X1X2 have influence too. The study of this interaction shows that the maximum leaching molybdenum yields are obtained with twice the stoichiometric quantity of leaching reactant at 60 °C (X1 = X2 = þ1). Nevertheless, two interactions values around (3 (E23 = 2.8 and E123 = 2.7) seem to be a little high. The DOE used being a full factorial design, all the combinations of the three factors were investigated. The eight runs can be shown at the corners of a cube (Figure 2). On this graphic representation, the responses are reported following the factors levels. The figure shows that amount of sodium hydroxide greatly influences the molybdenum leaching yield. Twice the stoichiometric amount of leaching reagent (X1= þ1) is required to solubilize a maximum of the initial molybdenum. Both other factors have to be at their þ1 level (run 8). Nevertheless, the examination of the results of runs 4 and 6 shows that, if X1 = þ1, setting X2 or X3 at their respective þ1 level allows to obtain a molybdenum leaching yield of about 56%. Examination of the results for YAl and YpH. For these responses, the average response of the runs is different from the average response of the center points. An excess of NaOH (X1 = þ1) is necessary to leach a maximum of molybdenum but 5298
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Figure 3. Diffraction pattern (CuKR1) of the roasted spent CoMo/Al2O3 catalyst CoMo1.
Figure 4. SEM image (backscattered electrons) of the longitudinal section of a catalyst stick: (a) embedding resin, (b) roasted part, (c) molybdenum reach interface, and (d) unroasted part.
leads to the dissolution of a part of the alumina support and the increase of the final pH value of the pulp (runs 2, 4, 6, and 8). With X1 = 1, the catalyst support is not leached but the molybdenum leaching yield is lower (runs 1, 3, 5, and 7). Factor X2 is also statistically influencing YpH (effect of 1.3). Except for run 7, a temperature increase permits higher molybdenum leaching yields. Considering the whole experimental domain, final pH value varies between 6.6 and 12.4. The best molybdenum leaching conditions correspond to all factors at the plus level (run 8), i.e. an amount of sodium hydroxide equal to the twice of the stoichiometry, a temperature of 60 °C and a leaching time of 4 h. In order to validate this experimental procedure, the leaching was replicated three times
in these conditions. Results obtained are the following (confidence interval with a 95% probability): YMo = 60.6 ( 1.8%; YAl = 3.6 ( 0.2%; YpH = 11 ( 1. The results demonstrated an incomplete leaching of molybdenum from this industrially roasted spent catalyst with a yield which is about the twice of the molybdenum soluble in water (33.1 ( 3.6%). The reasons limiting molybdenum leaching will be discussed in the following paragraph. Molybdenum Leaching Yield Limitation. In this study, with the best operating conditions, the molybdenum leaching yield from an industrially roasted spent catalyst was limited at about 60%. If the roasting step was fully controlled, all the molybdenum should be in the MoO3 form. This oxide is fully soluble in alkaline medium as MoO42 (Na2MoO4 solubility equals 650 g 3 L1 at 20 °C47). In our case, the highest concentration of Na2MoO4 observed was equal to 12.6 g 3 L1 (run 8). So, a leaching yield below 100% indicates the presence of nonsoluble molybdenum species. The examination of the XRD pattern obtained from the spent catalyst before leaching is presented in the Figure 3. According to the peaks detected, it was possible to identify 3 compounds containing molybdenum: MoS2, MoO2, and CoMoO4. The presence of MoS2 and MoO2 demonstrates that the industrial flash-roasting at 800 °C during 20 min was incomplete. MoO2 is a transition product from the oxidation of MoS2 into MoO3 resulting from the following reaction between sulphide and oxide (eq 5): MoS2ðsÞ þ 6MoO3ðsÞ f 7MoO2ðsÞ þ 2SO2ðsÞ
ð5Þ
CoMoO4 is the product of the reaction between molybdenum and cobalt oxides formed during the roasting of sulphides. The SEM image (backscattered electrons) of the longitudinal section of a catalyst stick is presented in Figure 4. This image shows two parts in a same catalyst stick: the external part (b) was totally 5299
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Figure 5. Diffraction pattern (CuKR1) of leaching residue of run 8.
Table 6. Composition of the Leaching Residue of Run 8 (wt %) element
Al
Co
Mo
Ni
P
S
V
concentration (wt %)
41.2
4.2
4.5
1.0
0.9
1.4
0.1
roasted and no sulfur was detected by EDS analysis. The inner part (d) was not roasted: molybdenum seems to be in a sulphide form. The two parts of the sticks are separated by a molybdenum rich layer (c) (more than 50 wt % of Mo), probably due to MoO3 diffusion during the roasting. The XRD pattern performed on the leaching residue of run 8 is given in the Figure 5. MoS2, MoO2, and CoMoO4 appear in this leaching residue. Indeed, these compounds are insoluble in alkaline solutions. The chemical analysis (XRF) of this solid residue is presented in Table 6. The leaching residue still contains 4.5 wt % of molybdenum. About 40% of the initial molybdenum is present in the alumina matrix. This example underlines the incidence of roasting conditions on molybdenum recovery. In the case of the sample CoMo1, the molybdenum recovery yield seems to be limited by an incomplete oxidation of MoS2. The determination of the optimal roasting conditions allowing a recovery of molybdenum higher than 85% is presented in the following paragraph. 3.2. Influence of Oxidative Roasting Conditions on Molybdenum Recovery. Table 7 shows the results obtained for the runs of the CCD. With two parameters, the second degree model is eq 6: Ycalc ¼ a0 þ aT X T þ aD X D þ aTD X T X D þ aTT X T 2 þ aDD X D 2 ð6Þ with • Ycalc: calculated response • a0: constant term
• XT: roasting temperature (in coded value) • XD: roasting duration (in coded value) • aT and aD: coefficients of the linear terms corresponding to, respectively, XT and XD variables • aTD: coefficient of the interaction term between XT and XD • aTT and aDD: coefficient of the square terms. This model was fitted by multiple linear regression (MLR) to the experimental results to obtain a model for each response. Initial molybdenum is distributed into the three responses Y1, Y2, and Y3 with Y1 þ Y2 þ Y3 = 100%. Only Y1 and Y2 are measured whereas Y3 is deduced. Consequently, models for the three responses are not independent (∑ a0 = 100 and ∑ ai = 0). For a better comprehension, the four models Y1, Y2, Y3, and Y4 are discussed and their respective coefficients are presented in Table 8. Model Validation. The correlation coefficients (R) are good for Y1, Y2, and Y3 (respective values of 0.972, 0.916, and 0.964). For Y4, R = 0.894, but the standard error of estimation is low (sreg = 0.37) because the aluminum leaching yields are low. The four obtained models are tested by ANOVA (analysis of variance).44,46 The lack of fit was tested by means of the P-value48 for each model. The P-value is the smallest level of confidence that would lead to rejection of the null hypothesis. For a confidence level of 95% (R = 0.05), the lack of fit is rejected if P > 0.05 and the model is accepted. All the models are accepted with P-values of 0.349 for Y1, 0.277 for Y2, 0.449 for Y3, and 0.249 for Y4. Graphical representations of the models (response surface) are presented in Figure 6 for the four responses studied. Examination of Model Parameters. The model coefficients are considered as not significant if P > 0.05. The significant parameters are noted in bold in Table 8. In this table, sX is the standard deviation on the coefficients given by MLR. Influence of the Roasting Conditions on the Molybdenum Recoverable Fractions (Y1 and Y3). Both studied parameters 5300
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Table 7. Experimental Matrix in Coded Values and Obtained Responses for CoMo2 Roasting Study run
roasting
roasting
Mo leaching
insoluble Mo
sublimed Mo
Al leaching
temperature XT
duration XD
rate (%) Y1
fraction (%) Y2
fraction (%) Y3
rate (%) Y4
1 (5)
1
1
68.3
21.3
10.4
6.7
2 (3)
þ1
1
48.5
8.9
42.6
6.7
3 (6)
1
þ1
62.5
20.1
17.4
7.1
4 (4)
þ1
þ1
36.7
11.7
51.6
6.0
5 (11)
2
0
77.5
16.7
5.8
5.3
6 (12)
þ2
0
27.2
4.0
68.8
5.3
7 (10) 8 (9)
0 0
2 þ2
61.2 38.7
18.0 11.2
20.8 50.1
7.0 5.8
CP1 (1)
0
0
49.7
18.7
31.6
7.1
CP2 (2)
0
0
55.0
19.1
25.9
7.2
CP3 (7)
0
0
46.6
16.2
37.2
6.8
CP4 (8)
0
0
44.1
13.2
42.7
6.8
CP5 (13)
0
0
47.0
15.7
37.3
6.7
CP6 (14)
0
0
48.4
17.6
34.0
6.3
Table 8. Coefficients of Quadratic Model for the Four Models (R Correlation Coefficient; sreg Standard Error of Estimation) Mo leaching rate (%) Y1 sX
value a0
1.52
49.45
insoluble Mo fraction (%) Y2 P-value
8.8 10
10 6
value
sX
P-value
16.96
0.95
9.8 108
3.85 1.00
0.71 0.71
6.5 104 0.199
aT aD
12.18 5.22
1.15 1.15
5.4 10 1.9 103
aTD
1.50
1.99
0.472
1.00
1.24
0.442
aTT
1.09
0.81
0.213
1.57
0.50
0.014
aDD
0.49
0.558
0.51
0.50
0.339
0.81 R = 0.972
R = 0.916
sreg = 3.97
P-value ANOVA (lack of fit)
0.349
Al leaching rate (%) Y4
sublimed Mo (%) Y3 sX
value
sreg = 2.47 0.277
P-value 7
value
sX
P-value
6.89 0.09
0.14 0.11
3.5 1011 0.414
a0 aT
33.59 16.03
2.22 1.67
3.6 10 1.2 105
aD
6.22
1.67
0.006
0.23
0.11
0.067
aTD
0.50
2.89
0.874
0.28
0.18
0.174
aTT
0.48
1.18
0.684
-0.37
0.08
1.1 103
aDD
0.02
0.986
0.10
1.18 R = 0.964
P-value ANOVA (lack of fit)
sreg = 5.79 0.449
(roasting temperature and roasting duration) are influencing on the molybdenum soda leaching yield. The roasting temperature has a huge effect on this response (aT = 12.20). The roasting duration coefficient (aD) equals 5.20, and the parameters do not interact. The highest molybdenum leaching yield is predicted by roasting the catalyst at 700 °C for 20 min (XT = 2, XD = 2) (Figure 6a). Both roasting temperature and roasting duration influence the molybdenum sublimation yield. Temperature (aT = 16.03) has a higher effect than duration (aD = 6.22). The model predicts that more than 80% of the initial molybdenum is sublimated at 900 °C for 120 min (XT = XD = þ2) (Figure 6c). By contrast, no molybdenum sublimation is predicted with XT = XD = 2. The negative value calculated is due to the mathematical constraint Y1 þ Y2 þ Y3 = 100%.
0.08 R = 0.894
0.238 sreg = 0.37
0.249
Influence of the Roasting Conditions on Leaching Residue Molybdenum Content (Y2). The fraction of the initial molybdenum remaining in the leaching residue is influenced only by the roasting temperature. First (aT) and second degree (aTT) coefficients are both statistically significant. The model predicts that 22.3% of the initial molybdenum is in an insoluble form after a roasting at 700 °C for 20 min (XT = XD = 2). The predicted molybdenum insoluble part with XT = XD = þ2 is only 2.9% (Figure 6b). The analysis of Mo, Co, and S (pelletized samples) was performed on all the residues resulting from the CCD roasting study (Table 9). For sulfur, the measured amounts are less than 0.1 wt % demonstrating the roasting effectiveness. The molybdenum level varies from 0.74 and 3.47 wt %. All the 5301
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Figure 6. Response surface representation of the influence of roasting conditions on (a) molybdenum soda leaching rate, (b) fraction of initial molybdenum in leaching residue, (c) fraction of molybdenum sublimated during roasting process, and (d) aluminum leaching yield.
Table 9. Chemical Analysis of the 14 Leaching Residues from the CCD (wt %) run
insoluble Mo fraction Y2 (%)
% Mo
% Co
%S
molar ratio nMo/nCo
1 2 3 4 5 6 7 8 PC1 PC2 PC3 PC4 PC5 PC6
21.3 8.9 20.1 11.7 16.7 4.0 18.0 11.2 18.7 19.1 16.2 13.2 15.7 17.6
3.47 1.61 3.41 2.10 2.81 0.74 3.06 1.93 3.37 3.38 2.79 2.38 2.79 3.10
4.19 4.76 4.47 4.79 4.34 4.62 4.39 4.44 4.55 4.65 4.49 4.66 4.56 4.57
