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Recovery of metal values from spent petroleum catalyst using leaching-solvent extraction technique
Hydrometallurgy 101 (2010) 35–40
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / h y d r o m e t
Recovery of metal values from spent petroleum catalyst using leaching-solvent extraction technique Debaraj Mishra a, G. Roy Chaudhury b, Dong J. Kim a,⁎, Jong G. Ahn a a b
Minerals and Material Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea Department of Environment and Sustainability, Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 5 August 2009 Received in revised form 22 November 2009 Accepted 22 November 2009 Available online 3 December 2009 Keywords: Leaching Spent petroleum catalyst Carbon disulfide Solvent extraction
a b s t r a c t Leaching-solvent extraction process was developed to recover metal values from spent petroleum catalyst containing mainly Ni, V and Mo. The acid leaching studies using acetone-washed spent petroleum catalyst showed good metal recovery except for Mo. The low Mo recovery was due to formation of an impervious sulfur layer over the Mo matrix. The acid leaching efficiency improved using acetone–carbon disulfide (CS2) washed spent petroleum catalyst. More than 90% of Mo–Ni–V was recovered using 1 M H2SO4 followed by (NH4)2CO3 washing. The acidic leach liquor was processed through solvent extraction technique using LIX84I. Mo and V in the leach liquor were separated by carrying out the solvent extraction studies at different pHs. A tentative process flow-sheet was developed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum (Mo) and vanadium (V) are strategic metals which are widely used for preparation of special types of steel. These metals are also used as catalyst in petrochemical, chemical and aerospace industries. Molybdenum is usually produced from molybdenite ore/concentrate. Molybdenum is also produced from other minerals such as wulfenite and powellite (Wang et al., 2009). Vanadium is produced either from caronite ore or as a byproduct in uranium recovery (Moskalyk and Alfantazi, 2003). The gradual depletion of Mo and V containing ores, coupled with increasing demand, has encouraged the search for alternative resources. Alternative resources include secondary sources such as waste materials and byproducts. Of the secondary resources, spent petroleum catalyst is the most important one. Both Mo and V are widely used as a catalyst in petroleum refinery. The catalyst is alumina based over which various metals are impregnated. These catalysts are used for converting crude oil into different fractions. It is reported (Park et al., 2006) that the life of a catalyst varies from 3 to 6 years depending upon the impurities in the feed and number of cycles used. The deactivated catalysts are activated and reused. After a number of deactivation–activation cycles, the catalysts are discarded as a waste (Furimsky, 1996). The waste catalysts are regarded as hazardous materials (Rapaport, 2000) and are therefore, either stored properly or further treated to recover the metal values. The second alternative is more lucrative as in the process valuable metals can be recovered from a hazardous material. Generally, the spent petroleum catalyst from petroleum industry contained metals such as Mo, V, Ni, Co, Al, S etc., and the concentration ⁎ Corresponding author. Tel.: + 82 42 8683592; fax: +82 42 8683415. E-mail address: [email protected] (D.J. Kim). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.11.016
varies depending on the nature of catalyst and crude oil (Zeng and Cheng, 2009). There are various processes reported (Park et al., 2007; Kar et al., 2004; Lee et al., 1992; Ikeyama 1987; Siemens et al., 1986; Hyatt, 1987; Howard and Barnes, 1991; Medvedev and Malochkina, 2007; Gaballah and Diona, 1994; Parkinson and Isho, 1987) to recover metal values from waste petroleum catalyst. The metal values can either be recovered through pyro or hydrometallurgical route. In the hydrometallurgical route, initially the waste material is roasted followed by either acid or alkali leaching (Park et al., 2007; Kar et al., 2004; Lee et al., 1992; Ikeyama 1987). Direct leaching of spent catalyst is also carried out at an elevated pressure (Siemens et al., 1986; Hyatt, 1987). In the pyro technique various techniques are followed such as direct smelting (Howard and Barnes, 1991), calcinations and smelting (Medvedev and Malochkina, 2007), chlorination (Gaballah and Diona, 1994) and salt roasting (Parkinson and Isho, 1987). Both routes suffer from several drawbacks such as slow Mo kinetics and non-recovery of sulfur values. Keeping in view the importance of the above, the main objectives of the present work are as follows: a. enhancement of Mo kinetics b. recovery of sulfur values c. processing of leach liquor to recover metal values. 2. Experimental methods 2.1. Acetone-washed spent petroleum catalyst The spent petroleum catalyst was collected from SK Petroleum Company, Korea. The organic content of the spent catalyst was removed by washing in a soxhlet with acetone. The deoiled mass was dried,
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D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
Table 1 XRD phases of the acetone-washed spent catalyst with respective ‘d’ values. XRD phases
“d” values (major peaks)
Sulfur ε-MoO3 Mo3S4 Al2O3 V4O9 Ni3 − xS2 η-Fe2O3
4.04 3.92 6.46 4.24 4.12 2.96 3.60
3.23 3.60 1.94 8.04 3.22 1.82 4.36
Card no (JCPDS) 2.90 3.39 2.63 7.21 3.18 4.13 6.01
23-0562 09-0209 27-0319 31-0026 23-0720 14-0358 21-0920
ground and sieved to b100 μm size. The metal contents in the waste were analyzed by ICP-AES and the typical values were as follows (wt.%): Al — 19.5, S — 11.5, Ni — 2.0, V — 9.0, Mo — 1.4 and Fe — 0.3.
2.2. Leaching experiments Leaching tests were conducted in a 500 mL volume reactor (stirred reactor) containing 250 mL of solution. All leaching tests were carried out at 10% pulp density and 1 M H2SO4 as a lixiviant unless otherwise specified. A heating tape was coiled around the reactor and connected to a thermostat to control the temperature. Oxygen was supplied into the lixiviant from an oxygen cylinder. A weighed amount of acetonewashed spent petroleum catalyst was added to the reactor and periodically 1.5 mL of samples was withdrawn for metal analysis. Prior to analysis, aliquot of sample was centrifuged then acidified with dil. HCl (10%) and finally analyzed by ICP-AES after proper dilution (JOBIN-YVON JY 38). All the experiments were carried out in duplicates and the average deviation among the replicates was observed to be within ±5%.
2.3. Solvent extraction studies Solvent extraction studies were carried out using LIX-84I as an organic extractant. The organic extractant was used as such without any purification. Kerosene was used as a diluent. Metal salts used in this investigation were of analytical grade reagents. Initial aqueous solutions of known concentration were prepared by dissolving an appropriate amount of salt in deionized water. The pH was adjusted to the required levels by the addition of concentrated H2SO4 or NaOH solution. The actual leach liquor was obtained after sulfuric acid leaching of the spent petroleum catalyst. From the series of leaching experiments it was confirmed that vanadium exists in the + 4 oxidation and molybdenum in +6 state in leach solution. Therefore, in all solvent extraction tests concerning molybdenum and vanadium in the present study, ammonium molybdate and vanadyl sulfate were used.
Fig. 1. Effect of contact time (acetone-washed material). (Conditions: H2SO4 - 1 M, pulp density — 10%, temp — 30 °C).
3. Results and discussion 3.1. Leaching studies 3.1.1. Effect of contact time Initial leaching studies were carried out using acetone-washed materials. The XRD studies (Table 1) showed that V in the oxide form (V4O9), Ni in the sulfide form (Ni3 − xS2), whereas Mo both in oxide (εMoO3) and sulfide (Mo3S4) forms, and sulfur in elemental state. The dissolution reactions for oxides and sulfides are different as shown below: MoS2 þ 4:5O2 þ 3H2 O→H2 MoO4 þ 2H2 SO4 þ
ð1Þ
MoO3 þ 2H þ H2 O→H2 MoO4
ð2Þ
V2 O5 þ H2 SO4 →ðVO2 Þ2 SO4 þ H2 O
ð3Þ
Al2 O3 þ 3H2 SO4 →Al2 ðSO4 Þ3 þ 3H2 O
ð4Þ
NiS þ 2O2 →NiSO4
ð5Þ
The dissolution of sulfide requires an oxidant whereas oxide requires acid. In the present case the oxidant and acid used were oxygen and dilute H2SO4, respectively. The leaching kinetics for all five metals followed dual kinetics i.e., initially fast then a slow period as shown in Fig. 1. The faster and slower kinetics may be due to surface and pore diffusion processes, respectively as reported previously (Mishra et al., 2009; Kim et al., 2009).
2.4. SEM-EDX and XRD analysis The acetone-washed and bioleached spent catalysts were subjected to semi quantitative analysis using a JEOL-JSM6380LA scanning electron microscope-energy dispersive X-ray analysis. The samples were sputter-coated with platinum after spreading the sample on metallic studs with carbon tape prior to observation under the electron microscope. To examine the detail mineral phases, X-ray diffractogram study was conducted by using Rigaku RTC 300. Required amount of sample was taken and mixed with grease prior to fix the same on a glass holder. The XRD analysis was carried out by continuous scanning method at 30 mV and 40 mA.
Fig. 2. Effect of pulp density on leaching of acetone-washed spent petroleum catalyst. (H2SO4 conc. — 1 M, time — 60 min, temp — 30 °C).
D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
37
Fig. 3. SEM-EDAX image of spent petroleum catalyst showing Mo embedded sulfur.
3.1.2. Effect of pulp density Leaching studies were carried out by varying the pulp density from 10 to 40%. It was observed that the kinetics of dissolution increased with increase of pulp density in all cases except Mo, as shown in Fig. 2. For Mo the leaching efficiency decreased at pulp density greater than 10%. The decrease of Mo leaching efficiency may be either due to improper mixing between air and waste catalysts, or depletion of oxidant concentration or both with the increase of pulp density. 3.1.3. Effect of temperature The temperature was varied from 30 to 50 °C in order to evaluate the effect on leaching efficiency. The leaching efficiency increased with increase of temperature indicating the endothermic nature of the reactions. The activation energy was calculated from Arrhenius equation: −E = RT
k = Ae
ð6Þ
where, k A E T
reaction rate constant, frequency factor, activation energy, and absolute temperature.
By plotting log (reaction rate constant) versus 1/Temperature, the activation energy was calculated to be 5.1, 6.9, 8.5 kJ/mole for Mo, V and Ni, respectively. The obtained correlation coefficient values for Mo, V and Ni were 0.86, 0.97, and 0.94, respectively. From the magnitude of the activation energy it can be concluded that the dissolution process followed a mixed kinetics model (Sohn and Wadsworth, 1979). By varying the contact time, pulp density and temperature it can be concluded that the dissolution kinetics of Ni and V were much higher than Mo. The low dissolution efficiency may be either due to the refractoriness of MoS2 or formation of a product layer, or both. From XRD analysis (Table 1) it was found that sulfur was present in elemental form and in order to establish its nature SEM studies were carried out on the raw material. The results are shown in Fig. 3. It was
observed that sulfur formed a layer over the Mo moiety. The sulfur layer may be non-porous thereby preventing the penetration of attacking species. If the sulfur layer is impervious, then leaching would be controlled by diffusion and the dissolution reaction can be presented as (Sohn and Wadsworth, 1979):
kp t = 1−2 = 3 x− ð1− xÞ
2=3
ð7Þ
where, kp
the parabolic rate constant,
t
time, h, and
x
fraction reacted.
So, a plot between 1 − 2/3x − (1 − x)2/3 versus ‘t’ will give a straight line (Fig. 4). From the correlation of determination value of the figure, it can be concluded that the sulfur formed an impervious layer thereby decreasing the leaching efficiency.
Fig. 4. Diffusion controlled model for Mo at different pulp densities.
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D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
Table 2 Both acid and alkali leachings of CS2 washed spent petroleum catalyst. (Conditions: time — 1 h, temperature — 30 °C, and pulp density — 10%). Leaching conditions
H2SO4 washing — 1 M
% of metal leaching
Total metal leaching, %
% of metal leaching (NH4)2CO3, g/L
Mo
V
Ni
Mo
V
Ni
Mo
V
Ni
10 20 30 40
50 54 56 60
21 34 36 38
1 2 3 5
12 14 20 24
52 55 60 61
90 91 93 92
62 68 76 84
73 89 96 99
91 93 96 97
Leaching conditions
% of metal leaching
H2SO4 1M
Mo 28
(NH4)2CO3 washing — 1 M
Total metal leaching, %
% of metal leaching V 94
Ni 97
Mo 70
V 3
Ni 2
Mo 98
V 97
Ni 99
Leach liquor composition for SX-studies Metal ions
Mo
V
Ni
Al
Fe
Concentration, g/L pH of leach liquor
0.3 0.55
8.5
2.0
2.5
0.28
3.2. Leaching studies using sulfur-free material In order to remove the diffusion barrier, (i.e., the sulfur layer) the acetone-washed material was washed with carbon disulfide (CS2) in a soxhlet apparatus. The obtained sample was then dried in oven and again characterized by XRD to observe if there would be presence of any sulfur phase. Absence of sulfur peak depicted its complete removal (data not shown) from the spent petroleum catalyst by CS2. Later, sulfur was recovered in elemental form by the distillation of carbon disulfide. After removal of elemental sulfur, the dried material was used for leaching studies using both alkali and acid media. Leaching studies were conducted using the same reactor size and same solution volume as explained in case of acetone-washed sample leaching. 3.2.1. Leaching with (NH4)2CO3 Experiments were carried out varying the (NH4)2CO3 concentration (Table 2). Ni, Mo, and V leaching efficiencies varied within 1–5%, 50–60% and 21–38%, respectively, depending on the concentration of (NH4)2CO3. The residue was washed with 1 M H2SO4. During H2SO4 washing the Ni, Mo, and V recoveries varied within 90–93%, 12–25% and 50–61%, respectively. The details of leaching along with washing results are shown in Table 2. The Ni and V extraction was more than 90% in the total process, whereas the total Mo leaching efficiency never crossed 85% in either of the cases. This value was, as expected, higher than for catalyst washed only with acetone. Since Mo leaching efficiency did not show higher recovery (≥90%), therefore further studies were carried out in acid media.
Fig. 5. Extraction of metal ions depending on pH by 10% LIX 84I (v/v) dissolved in kerosene.
3.2.2. Leaching with H2SO4 1 M sulfuric acid was used to carry out the leaching of CS2 washed material. It was observed that more than 95% of Ni and V were recovered within 1 h of reaction time. The Mo recovered was only 28%. When the leached residue was washed with 1 M (NH4)2CO3 the remaining Mo came into the solution. In conclusion, almost all the important metal values (Ni, V, and Mo) were recovered within 1 h (Table 2). 3.3. Solvent extraction studies Solvent extraction studies were carried out using leach liquor obtained by using H2SO4 as a lixiviant. The typical leach liquor composition is shown in Table 2. 3.3.1. Effect of pH A stock solution of 2 g/L each of Al, Mo, V, Fe and Ni was prepared. 10% of LIX-84I dissolved in kerosene was used to extract individual metal ions at an organic to aqueous ratio of 1:1 in a separatory funnel. The results are shown in Fig. 5. It was observed that Mo extraction efficiency is higher than V at low pH. The extraction efficiency of V increased with increase of equilibrium pH. At equilibrium pH more than 2.0, the solvent extracted both V and Mo. Up to equilibrium at pH 3.5, there was no extraction of Al, Ni and Fe. Therefore, LIX-84I extractant was selected for treating the leach liquor. From Fig. 5, it can
Fig. 6. McCabe diagram for Mo extraction using 10% (v/v) LIX84I solution dissolved in kerosene.
D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
39
extracted species MoO2L2, the following equilibrium has been proposed: +2
MoO2
þ
ðaqÞ + 2HLðorgÞ ↔MoO2 L2ðorgÞ + 2H
ð8Þ
and the corresponding relation is þ
log D = log K – 2 log½H aq + 2 log½HLorg where, D and K are the distribution ratio and equilibrium constant, respectively. Thus, MoO2L2 seems to be the extracted species.
Fig. 7. McCabe diagram for V extraction using 40% (v/v) LIX84I solution dissolved in kerosene.
be concluded that Mo and V can be co-extracted at higher pH or individually extracted at low and high pH, respectively. We have selected separation at low pH to achieve good separation factor and also because the obtained leach liquor has a low pH, as shown in Table 2. 3.3.2. Extraction of molybdenum The stoichiometry of the extracted species was determined by analyzing the experimental data using the conventional slope analysis method. The plot of logD versus pH is linear having a slope of 1.89 which is close to 2.0, suggesting that 2 mol of the extractant reacted with Mo. In order to explain slope analyses and the nature of the
3.3.3. Extraction isotherms To determine the number of stages required the extraction isotherm was obtained by contacting the leach solution with 10% LIX-84I by varying the A:O phase ratio from 1:5 to 5:1. The pH of the leach liquor was 0.5. The results are shown in Fig 6. From the extraction isotherm curve it is apparent that about 98% of Mo can be extracted at A:O ratio of 5:1. The co-extraction of V was only 100 ppm at the same phase ratio. 3.3.4. Stripping of molybdenum Stripping studies were carried out by using three different solutions such as; NH4OH (20%), NH4OH (20%) + (NH4)2CO3 (2 M) and 10% H2SO4. It was observed that the efficiency of 20% NH4OH was better compared to the other two. Further studies were carried out using 20% NH4OH by varying the A:O ratio from 1:5 to 5:1 as was done in the case of the extraction studies. From stripping studies it can be concluded that Mo quantitatively stripped by 20% NH4OH at A:O ratio of 5:1.
Fig. 8. Tentative process flow-sheet for leaching-solvent extraction of spent petroleum catalyst.
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D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
3.3.5. Extraction of vanadium After extraction of Mo by LIX-84I (10%) the pH of the raffinate increased to 2.6 by addition of NaOH. During pH adjustment, there was no loss of V, Ni or Fe. Extraction isotherm studies were carried out on Mo-barren leach liquor after pH adjustment using 40% LIX-84I solution by varying the A:O ratio from 1:20 to 20:1. The results are shown in Fig. 7. The McCabe–Thiele plot showed that more than 80% of V can be extracted at A:O ratio of 1:5 in 2 stages. 3.3.6. Stripping of vanadium Stripping studies were carried out using various stripping agents: NH4OH (20%), (NH4)2CO3 (2 M), and NH4OH (20%) + (NH4)2CO3 (2 M). Among the three, the last showed the best stripping activities (N99%) for V. The other two stripping agents showed less than 10% stripping result. 4. Conclusion A combination of acid leaching followed by solvent extraction route has been developed to recover selectively the metal values from spent petroleum catalyst. Acid leaching (1 M H2SO4) studies showed more than 95% recovery of V, Ni and Fe in a single stage within 1 h of reaction period. Diffusion controlled model proved the presence of sulfur product layer over Mo matrix in acetone-washed sample. Therefore, removal of sulfur layer by carbon disulfide reflux increased the leaching efficiency of all the metals. The dissolved sulfur was also recovered in elemental form by distillation of carbon disulfide. Nearly 100% of Mo was recovered, about 30% in the acid leaching and the remaining 70% by washing the acid leached residues with (NH4)2CO3. The Al extraction efficiency was less compared to Mo, V, Ni and Fe. A solvent extraction flow-sheet was developed to preferentially extract Mo and V from leach liquor. Mo could be extracted (98%) from leach liquor (pH = 0.5) using 10% LIX-84I at A:O ratio of 5:1. The coextraction of V was very marginal. Mo loaded organic could be stripped with 20% NH4OH at A:O ratio of 5:1. Vanadium was extracted by 40% LIX-84I from Mo-barren raffinate. The stripping was carried out using a mixture of NH4OH–(NH4)2CO3. The detailed flow-sheet is shown in Fig. 8. Acknowledgments This work was supported by the Korea Foundation for International Cooperation of Science and Technology (KICOS) through a grant
provided by the Korean Ministry of Science and Technology (MOST) in 2007 (No. K20602000004-07E0200-00410). One of the authors GRC, is thankful to Director, Institute of Minerals and Material Technology, for the sanction of sabbatical leave and also to Korea Federation of Science and Technology (KOFST) for the award of brain pool program.
References Furimsky, E., 1996. Spent refinery catalysts: environmental safety and utilization. Catal. Tod. 30, 223–286. Gaballah, I., Diona, M., 1994. Valuable metals recovery from spent catalysts by selective chlorination. Res. Cons. Recycl. 10, 87–96. Howard, R.A., Barnes, W.R., 1991. Process for recovering valuable metals from spent catalysts. US Patent No, 5013533. Hyatt, D.E., 1987. Value recovery from spent aluminium-base catalyst. US Patent No, 62102834. Ikeyama, T., 1987. Recovery of high purity V and Mo components from oxidationcalcined spent catalyst. Japanese Patent no, 62102834. Kar, B.B., Dutta, P., Misra, V.N., 2004. Spent catalyst: secondary source for Mo recovery. Hydrometallurgy 72, 87–92. Kim, D.J., Mishra, D., Ahn, J.G., Roy Chaudhury, G., Ralph, D.E., 2009. A novel process to treat spent petroleumcatalyst using sulfur oxidizing lithotrophs. J. Environ. Sci. and Heal. Part-A 44, 1585–1591. Lee, F.M., Knudsen, R.D., Kidd, D.R., 1992. Reforming catalyst made from the metals recovered from spent atmospheric resid desulfurization catalyst. Ind. Eng. Chem. Res. 31, 487–490. Medvedev, A.S., Malochkina, N.V., 2007. Sublimation of molybdenum trioxide from exhausted catalysts employed for the purification of oil products. Russ. J. Non-Ferr. Met. 48, 114–117. Mishra, D., Ahn, J.G., Kim, D.J., Roy Chaudhury, G., Ralph, D.E., 2009. Dissolution kinetics of spent petroleum catalyst using sulfur oxidizing acidophilic microorganisms. J. Hazard. Mater. 167, 1231–1236. Moskalyk, R.R., Alfantazi, A.M., 2003. Processing of vanadium: a review. Miner. Eng. 16, 793–805. Park, K.H., Mohapatra, D., Reddy, B.R., 2006. Selective recovery of Mo from spent HDS catalyst using oxidative soda ash leach/carbon adsorption method. J. Hazard. Mater. B138, 311–316. Park, K.H., Mohapatra, D., Nam, C.W., 2007. Two stage leaching of activated spent HDS catalyst and solvent extraction of Al using organo-phosphinic extractant, cyanex272. J. Hazard. Mater. 148, 287–295. Parkinson, G., Isho, S., 1987. Recyclers try new ways to process spent catalyst. Chem. Eng. 94, 25–31. Rapaport, D., 2000. Are spent hydrocracking catalysts listed hazardous wastes? Hydrocarbon Process. 79, 49–53. Siemens, R.E., Jong, B.W., Russel, J.H., 1986. Potential of spent catalysts as a source of critical metals. Conserv. Recycl. 9, 189–196. Sohn, H.Y., Wadsworth, M.E., 1979. Rate process of extractive metallurgy. Plenium, New York. Wang, M., Wang, X., Liu, W., 2009. A novel technology of molybdenum extraction from low grade Ni–Mo ore. Hydrometallurgy 97, 126–130. Zeng, L., Cheng, C.Y., 2009. A literature review of the recovery of Mo and V from spent hydrodesulphurization catalysts: Part –I: metallurgical processes. Hydrometallurgy 98, 1–9.
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / h y d r o m e t
Recovery of metal values from spent petroleum catalyst using leaching-solvent extraction technique Debaraj Mishra a, G. Roy Chaudhury b, Dong J. Kim a,⁎, Jong G. Ahn a a b
Minerals and Material Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea Department of Environment and Sustainability, Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 5 August 2009 Received in revised form 22 November 2009 Accepted 22 November 2009 Available online 3 December 2009 Keywords: Leaching Spent petroleum catalyst Carbon disulfide Solvent extraction
a b s t r a c t Leaching-solvent extraction process was developed to recover metal values from spent petroleum catalyst containing mainly Ni, V and Mo. The acid leaching studies using acetone-washed spent petroleum catalyst showed good metal recovery except for Mo. The low Mo recovery was due to formation of an impervious sulfur layer over the Mo matrix. The acid leaching efficiency improved using acetone–carbon disulfide (CS2) washed spent petroleum catalyst. More than 90% of Mo–Ni–V was recovered using 1 M H2SO4 followed by (NH4)2CO3 washing. The acidic leach liquor was processed through solvent extraction technique using LIX84I. Mo and V in the leach liquor were separated by carrying out the solvent extraction studies at different pHs. A tentative process flow-sheet was developed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum (Mo) and vanadium (V) are strategic metals which are widely used for preparation of special types of steel. These metals are also used as catalyst in petrochemical, chemical and aerospace industries. Molybdenum is usually produced from molybdenite ore/concentrate. Molybdenum is also produced from other minerals such as wulfenite and powellite (Wang et al., 2009). Vanadium is produced either from caronite ore or as a byproduct in uranium recovery (Moskalyk and Alfantazi, 2003). The gradual depletion of Mo and V containing ores, coupled with increasing demand, has encouraged the search for alternative resources. Alternative resources include secondary sources such as waste materials and byproducts. Of the secondary resources, spent petroleum catalyst is the most important one. Both Mo and V are widely used as a catalyst in petroleum refinery. The catalyst is alumina based over which various metals are impregnated. These catalysts are used for converting crude oil into different fractions. It is reported (Park et al., 2006) that the life of a catalyst varies from 3 to 6 years depending upon the impurities in the feed and number of cycles used. The deactivated catalysts are activated and reused. After a number of deactivation–activation cycles, the catalysts are discarded as a waste (Furimsky, 1996). The waste catalysts are regarded as hazardous materials (Rapaport, 2000) and are therefore, either stored properly or further treated to recover the metal values. The second alternative is more lucrative as in the process valuable metals can be recovered from a hazardous material. Generally, the spent petroleum catalyst from petroleum industry contained metals such as Mo, V, Ni, Co, Al, S etc., and the concentration ⁎ Corresponding author. Tel.: + 82 42 8683592; fax: +82 42 8683415. E-mail address: [email protected] (D.J. Kim). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.11.016
varies depending on the nature of catalyst and crude oil (Zeng and Cheng, 2009). There are various processes reported (Park et al., 2007; Kar et al., 2004; Lee et al., 1992; Ikeyama 1987; Siemens et al., 1986; Hyatt, 1987; Howard and Barnes, 1991; Medvedev and Malochkina, 2007; Gaballah and Diona, 1994; Parkinson and Isho, 1987) to recover metal values from waste petroleum catalyst. The metal values can either be recovered through pyro or hydrometallurgical route. In the hydrometallurgical route, initially the waste material is roasted followed by either acid or alkali leaching (Park et al., 2007; Kar et al., 2004; Lee et al., 1992; Ikeyama 1987). Direct leaching of spent catalyst is also carried out at an elevated pressure (Siemens et al., 1986; Hyatt, 1987). In the pyro technique various techniques are followed such as direct smelting (Howard and Barnes, 1991), calcinations and smelting (Medvedev and Malochkina, 2007), chlorination (Gaballah and Diona, 1994) and salt roasting (Parkinson and Isho, 1987). Both routes suffer from several drawbacks such as slow Mo kinetics and non-recovery of sulfur values. Keeping in view the importance of the above, the main objectives of the present work are as follows: a. enhancement of Mo kinetics b. recovery of sulfur values c. processing of leach liquor to recover metal values. 2. Experimental methods 2.1. Acetone-washed spent petroleum catalyst The spent petroleum catalyst was collected from SK Petroleum Company, Korea. The organic content of the spent catalyst was removed by washing in a soxhlet with acetone. The deoiled mass was dried,
36
D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
Table 1 XRD phases of the acetone-washed spent catalyst with respective ‘d’ values. XRD phases
“d” values (major peaks)
Sulfur ε-MoO3 Mo3S4 Al2O3 V4O9 Ni3 − xS2 η-Fe2O3
4.04 3.92 6.46 4.24 4.12 2.96 3.60
3.23 3.60 1.94 8.04 3.22 1.82 4.36
Card no (JCPDS) 2.90 3.39 2.63 7.21 3.18 4.13 6.01
23-0562 09-0209 27-0319 31-0026 23-0720 14-0358 21-0920
ground and sieved to b100 μm size. The metal contents in the waste were analyzed by ICP-AES and the typical values were as follows (wt.%): Al — 19.5, S — 11.5, Ni — 2.0, V — 9.0, Mo — 1.4 and Fe — 0.3.
2.2. Leaching experiments Leaching tests were conducted in a 500 mL volume reactor (stirred reactor) containing 250 mL of solution. All leaching tests were carried out at 10% pulp density and 1 M H2SO4 as a lixiviant unless otherwise specified. A heating tape was coiled around the reactor and connected to a thermostat to control the temperature. Oxygen was supplied into the lixiviant from an oxygen cylinder. A weighed amount of acetonewashed spent petroleum catalyst was added to the reactor and periodically 1.5 mL of samples was withdrawn for metal analysis. Prior to analysis, aliquot of sample was centrifuged then acidified with dil. HCl (10%) and finally analyzed by ICP-AES after proper dilution (JOBIN-YVON JY 38). All the experiments were carried out in duplicates and the average deviation among the replicates was observed to be within ±5%.
2.3. Solvent extraction studies Solvent extraction studies were carried out using LIX-84I as an organic extractant. The organic extractant was used as such without any purification. Kerosene was used as a diluent. Metal salts used in this investigation were of analytical grade reagents. Initial aqueous solutions of known concentration were prepared by dissolving an appropriate amount of salt in deionized water. The pH was adjusted to the required levels by the addition of concentrated H2SO4 or NaOH solution. The actual leach liquor was obtained after sulfuric acid leaching of the spent petroleum catalyst. From the series of leaching experiments it was confirmed that vanadium exists in the + 4 oxidation and molybdenum in +6 state in leach solution. Therefore, in all solvent extraction tests concerning molybdenum and vanadium in the present study, ammonium molybdate and vanadyl sulfate were used.
Fig. 1. Effect of contact time (acetone-washed material). (Conditions: H2SO4 - 1 M, pulp density — 10%, temp — 30 °C).
3. Results and discussion 3.1. Leaching studies 3.1.1. Effect of contact time Initial leaching studies were carried out using acetone-washed materials. The XRD studies (Table 1) showed that V in the oxide form (V4O9), Ni in the sulfide form (Ni3 − xS2), whereas Mo both in oxide (εMoO3) and sulfide (Mo3S4) forms, and sulfur in elemental state. The dissolution reactions for oxides and sulfides are different as shown below: MoS2 þ 4:5O2 þ 3H2 O→H2 MoO4 þ 2H2 SO4 þ
ð1Þ
MoO3 þ 2H þ H2 O→H2 MoO4
ð2Þ
V2 O5 þ H2 SO4 →ðVO2 Þ2 SO4 þ H2 O
ð3Þ
Al2 O3 þ 3H2 SO4 →Al2 ðSO4 Þ3 þ 3H2 O
ð4Þ
NiS þ 2O2 →NiSO4
ð5Þ
The dissolution of sulfide requires an oxidant whereas oxide requires acid. In the present case the oxidant and acid used were oxygen and dilute H2SO4, respectively. The leaching kinetics for all five metals followed dual kinetics i.e., initially fast then a slow period as shown in Fig. 1. The faster and slower kinetics may be due to surface and pore diffusion processes, respectively as reported previously (Mishra et al., 2009; Kim et al., 2009).
2.4. SEM-EDX and XRD analysis The acetone-washed and bioleached spent catalysts were subjected to semi quantitative analysis using a JEOL-JSM6380LA scanning electron microscope-energy dispersive X-ray analysis. The samples were sputter-coated with platinum after spreading the sample on metallic studs with carbon tape prior to observation under the electron microscope. To examine the detail mineral phases, X-ray diffractogram study was conducted by using Rigaku RTC 300. Required amount of sample was taken and mixed with grease prior to fix the same on a glass holder. The XRD analysis was carried out by continuous scanning method at 30 mV and 40 mA.
Fig. 2. Effect of pulp density on leaching of acetone-washed spent petroleum catalyst. (H2SO4 conc. — 1 M, time — 60 min, temp — 30 °C).
D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
37
Fig. 3. SEM-EDAX image of spent petroleum catalyst showing Mo embedded sulfur.
3.1.2. Effect of pulp density Leaching studies were carried out by varying the pulp density from 10 to 40%. It was observed that the kinetics of dissolution increased with increase of pulp density in all cases except Mo, as shown in Fig. 2. For Mo the leaching efficiency decreased at pulp density greater than 10%. The decrease of Mo leaching efficiency may be either due to improper mixing between air and waste catalysts, or depletion of oxidant concentration or both with the increase of pulp density. 3.1.3. Effect of temperature The temperature was varied from 30 to 50 °C in order to evaluate the effect on leaching efficiency. The leaching efficiency increased with increase of temperature indicating the endothermic nature of the reactions. The activation energy was calculated from Arrhenius equation: −E = RT
k = Ae
ð6Þ
where, k A E T
reaction rate constant, frequency factor, activation energy, and absolute temperature.
By plotting log (reaction rate constant) versus 1/Temperature, the activation energy was calculated to be 5.1, 6.9, 8.5 kJ/mole for Mo, V and Ni, respectively. The obtained correlation coefficient values for Mo, V and Ni were 0.86, 0.97, and 0.94, respectively. From the magnitude of the activation energy it can be concluded that the dissolution process followed a mixed kinetics model (Sohn and Wadsworth, 1979). By varying the contact time, pulp density and temperature it can be concluded that the dissolution kinetics of Ni and V were much higher than Mo. The low dissolution efficiency may be either due to the refractoriness of MoS2 or formation of a product layer, or both. From XRD analysis (Table 1) it was found that sulfur was present in elemental form and in order to establish its nature SEM studies were carried out on the raw material. The results are shown in Fig. 3. It was
observed that sulfur formed a layer over the Mo moiety. The sulfur layer may be non-porous thereby preventing the penetration of attacking species. If the sulfur layer is impervious, then leaching would be controlled by diffusion and the dissolution reaction can be presented as (Sohn and Wadsworth, 1979):
kp t = 1−2 = 3 x− ð1− xÞ
2=3
ð7Þ
where, kp
the parabolic rate constant,
t
time, h, and
x
fraction reacted.
So, a plot between 1 − 2/3x − (1 − x)2/3 versus ‘t’ will give a straight line (Fig. 4). From the correlation of determination value of the figure, it can be concluded that the sulfur formed an impervious layer thereby decreasing the leaching efficiency.
Fig. 4. Diffusion controlled model for Mo at different pulp densities.
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D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
Table 2 Both acid and alkali leachings of CS2 washed spent petroleum catalyst. (Conditions: time — 1 h, temperature — 30 °C, and pulp density — 10%). Leaching conditions
H2SO4 washing — 1 M
% of metal leaching
Total metal leaching, %
% of metal leaching (NH4)2CO3, g/L
Mo
V
Ni
Mo
V
Ni
Mo
V
Ni
10 20 30 40
50 54 56 60
21 34 36 38
1 2 3 5
12 14 20 24
52 55 60 61
90 91 93 92
62 68 76 84
73 89 96 99
91 93 96 97
Leaching conditions
% of metal leaching
H2SO4 1M
Mo 28
(NH4)2CO3 washing — 1 M
Total metal leaching, %
% of metal leaching V 94
Ni 97
Mo 70
V 3
Ni 2
Mo 98
V 97
Ni 99
Leach liquor composition for SX-studies Metal ions
Mo
V
Ni
Al
Fe
Concentration, g/L pH of leach liquor
0.3 0.55
8.5
2.0
2.5
0.28
3.2. Leaching studies using sulfur-free material In order to remove the diffusion barrier, (i.e., the sulfur layer) the acetone-washed material was washed with carbon disulfide (CS2) in a soxhlet apparatus. The obtained sample was then dried in oven and again characterized by XRD to observe if there would be presence of any sulfur phase. Absence of sulfur peak depicted its complete removal (data not shown) from the spent petroleum catalyst by CS2. Later, sulfur was recovered in elemental form by the distillation of carbon disulfide. After removal of elemental sulfur, the dried material was used for leaching studies using both alkali and acid media. Leaching studies were conducted using the same reactor size and same solution volume as explained in case of acetone-washed sample leaching. 3.2.1. Leaching with (NH4)2CO3 Experiments were carried out varying the (NH4)2CO3 concentration (Table 2). Ni, Mo, and V leaching efficiencies varied within 1–5%, 50–60% and 21–38%, respectively, depending on the concentration of (NH4)2CO3. The residue was washed with 1 M H2SO4. During H2SO4 washing the Ni, Mo, and V recoveries varied within 90–93%, 12–25% and 50–61%, respectively. The details of leaching along with washing results are shown in Table 2. The Ni and V extraction was more than 90% in the total process, whereas the total Mo leaching efficiency never crossed 85% in either of the cases. This value was, as expected, higher than for catalyst washed only with acetone. Since Mo leaching efficiency did not show higher recovery (≥90%), therefore further studies were carried out in acid media.
Fig. 5. Extraction of metal ions depending on pH by 10% LIX 84I (v/v) dissolved in kerosene.
3.2.2. Leaching with H2SO4 1 M sulfuric acid was used to carry out the leaching of CS2 washed material. It was observed that more than 95% of Ni and V were recovered within 1 h of reaction time. The Mo recovered was only 28%. When the leached residue was washed with 1 M (NH4)2CO3 the remaining Mo came into the solution. In conclusion, almost all the important metal values (Ni, V, and Mo) were recovered within 1 h (Table 2). 3.3. Solvent extraction studies Solvent extraction studies were carried out using leach liquor obtained by using H2SO4 as a lixiviant. The typical leach liquor composition is shown in Table 2. 3.3.1. Effect of pH A stock solution of 2 g/L each of Al, Mo, V, Fe and Ni was prepared. 10% of LIX-84I dissolved in kerosene was used to extract individual metal ions at an organic to aqueous ratio of 1:1 in a separatory funnel. The results are shown in Fig. 5. It was observed that Mo extraction efficiency is higher than V at low pH. The extraction efficiency of V increased with increase of equilibrium pH. At equilibrium pH more than 2.0, the solvent extracted both V and Mo. Up to equilibrium at pH 3.5, there was no extraction of Al, Ni and Fe. Therefore, LIX-84I extractant was selected for treating the leach liquor. From Fig. 5, it can
Fig. 6. McCabe diagram for Mo extraction using 10% (v/v) LIX84I solution dissolved in kerosene.
D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
39
extracted species MoO2L2, the following equilibrium has been proposed: +2
MoO2
þ
ðaqÞ + 2HLðorgÞ ↔MoO2 L2ðorgÞ + 2H
ð8Þ
and the corresponding relation is þ
log D = log K – 2 log½H aq + 2 log½HLorg where, D and K are the distribution ratio and equilibrium constant, respectively. Thus, MoO2L2 seems to be the extracted species.
Fig. 7. McCabe diagram for V extraction using 40% (v/v) LIX84I solution dissolved in kerosene.
be concluded that Mo and V can be co-extracted at higher pH or individually extracted at low and high pH, respectively. We have selected separation at low pH to achieve good separation factor and also because the obtained leach liquor has a low pH, as shown in Table 2. 3.3.2. Extraction of molybdenum The stoichiometry of the extracted species was determined by analyzing the experimental data using the conventional slope analysis method. The plot of logD versus pH is linear having a slope of 1.89 which is close to 2.0, suggesting that 2 mol of the extractant reacted with Mo. In order to explain slope analyses and the nature of the
3.3.3. Extraction isotherms To determine the number of stages required the extraction isotherm was obtained by contacting the leach solution with 10% LIX-84I by varying the A:O phase ratio from 1:5 to 5:1. The pH of the leach liquor was 0.5. The results are shown in Fig 6. From the extraction isotherm curve it is apparent that about 98% of Mo can be extracted at A:O ratio of 5:1. The co-extraction of V was only 100 ppm at the same phase ratio. 3.3.4. Stripping of molybdenum Stripping studies were carried out by using three different solutions such as; NH4OH (20%), NH4OH (20%) + (NH4)2CO3 (2 M) and 10% H2SO4. It was observed that the efficiency of 20% NH4OH was better compared to the other two. Further studies were carried out using 20% NH4OH by varying the A:O ratio from 1:5 to 5:1 as was done in the case of the extraction studies. From stripping studies it can be concluded that Mo quantitatively stripped by 20% NH4OH at A:O ratio of 5:1.
Fig. 8. Tentative process flow-sheet for leaching-solvent extraction of spent petroleum catalyst.
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D. Mishra et al. / Hydrometallurgy 101 (2010) 35–40
3.3.5. Extraction of vanadium After extraction of Mo by LIX-84I (10%) the pH of the raffinate increased to 2.6 by addition of NaOH. During pH adjustment, there was no loss of V, Ni or Fe. Extraction isotherm studies were carried out on Mo-barren leach liquor after pH adjustment using 40% LIX-84I solution by varying the A:O ratio from 1:20 to 20:1. The results are shown in Fig. 7. The McCabe–Thiele plot showed that more than 80% of V can be extracted at A:O ratio of 1:5 in 2 stages. 3.3.6. Stripping of vanadium Stripping studies were carried out using various stripping agents: NH4OH (20%), (NH4)2CO3 (2 M), and NH4OH (20%) + (NH4)2CO3 (2 M). Among the three, the last showed the best stripping activities (N99%) for V. The other two stripping agents showed less than 10% stripping result. 4. Conclusion A combination of acid leaching followed by solvent extraction route has been developed to recover selectively the metal values from spent petroleum catalyst. Acid leaching (1 M H2SO4) studies showed more than 95% recovery of V, Ni and Fe in a single stage within 1 h of reaction period. Diffusion controlled model proved the presence of sulfur product layer over Mo matrix in acetone-washed sample. Therefore, removal of sulfur layer by carbon disulfide reflux increased the leaching efficiency of all the metals. The dissolved sulfur was also recovered in elemental form by distillation of carbon disulfide. Nearly 100% of Mo was recovered, about 30% in the acid leaching and the remaining 70% by washing the acid leached residues with (NH4)2CO3. The Al extraction efficiency was less compared to Mo, V, Ni and Fe. A solvent extraction flow-sheet was developed to preferentially extract Mo and V from leach liquor. Mo could be extracted (98%) from leach liquor (pH = 0.5) using 10% LIX-84I at A:O ratio of 5:1. The coextraction of V was very marginal. Mo loaded organic could be stripped with 20% NH4OH at A:O ratio of 5:1. Vanadium was extracted by 40% LIX-84I from Mo-barren raffinate. The stripping was carried out using a mixture of NH4OH–(NH4)2CO3. The detailed flow-sheet is shown in Fig. 8. Acknowledgments This work was supported by the Korea Foundation for International Cooperation of Science and Technology (KICOS) through a grant
provided by the Korean Ministry of Science and Technology (MOST) in 2007 (No. K20602000004-07E0200-00410). One of the authors GRC, is thankful to Director, Institute of Minerals and Material Technology, for the sanction of sabbatical leave and also to Korea Federation of Science and Technology (KOFST) for the award of brain pool program.
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