Sustainable recovery of nickel, molybdenum, and vanadium from spent hydroprocessing catalysts by an integrated selective route

Author links open overlay panelDi Zhang ^a, Yunqing Liu ^a, Qizhao Hu ^a Xinquan Ke ^a, Shenghua Yuan ^b, Shijun Liu ^a, Xiaobo Ji ^a, Jiugang Hu ^acShow moreAdd to MendeleyShareCite

https://doi.org/10.1016/j.jclepro.2019.119763 Get rights and content

Highlights Ni, Mo, and V were selectively extracted from the spent hydroprocessing catalysts.An integrated ammoniacal leaching and selective separation process was proposed.Vanadium leaching can be greatly improved by adding H₂O₂ at pH 9 and pH 12. Ni(OH)₂, BaMoO₄, and V₂O₅ compounds can be obtained from the proposed recovery route.

Abstract:

In this work, nickel, molybdenum, and vanadium were selectively extracted from the spent hydroprocessing catalysts (Ni–Mo–V/Al₂O₃) by using an integrated ammoniacal leaching and selective separation process. After roasted at 400 °C, molybdenum and nickel were selectively extracted by ammoniacal liquors at pH 9 in the first step and the leaching efficiency of nickel and molybdenum was 88.74% and 97.92%, respectively.

In the second leaching step, vanadium was selectively extracted from the residues by ammoniacal liquors containing 6% (v/v) H₂O₂ at pH 12 and the leaching efficiency of vanadium was above 97.13%. The selective separation of nickel in the leachate was achieved at pH of 6.86 by solvent extraction with 2 mol/L neo-decanoic acid (Versatic10) in sulfonated kerosene, where the extraction efficiency was about 96% and the complete stripping of Ni was achieved with 3 mol/L NaOH to directly obtain Ni(OH)₂ precipitates. The molybdenum in extraction raffinate can be recovered by barium molybdate precipitate method.

The vanadium-bearing leachate was further treated to obtain ammonium metavanadate precursor by adding HNO₃ to adjust the solution pH to 7.5–9 and then flower-like V₂O₅ was obtained after calcination at 500 °C. Based on the above highly-selective treatment process, nickel, molybdenum, and vanadium in the spent hydroprocessing catalysts were recovered as the corresponding metal compounds, respectively.

1. Introduction

The hydroprocessing process is one of the major processes for producing clean fuels and other petroleum products from crude oils. However, since the extension of the service time of the catalyst leads to the changes of its composition and structure, the activity of the catalyst will eventually decline or fail, thus a large amount of spent catalysts are formed (Pradhan et al., 2013, Szymczycha-Madeja, 2011a, Yang et al., 2018). The spent hydroprocessing catalysts usually consist of 10–30% molybdenum, 1–12% vanadium, 0.5–6% nickel, and 1–6% cobalt, which make it serve as an economical secondary source of valuable metals (Kar et al., 2004, Yang et al., 2016b). So recovery of spent hydroprocessing catalysts becomes an inevitable task for lowering the catalyst cost and reducing the environmental pollution of catalyst waste (Marafi and Stanislaus, 2008a, Wang et al., 2019).

Several methods have been proposed to recover the metal values in the spent hydroprocessing catalysts, including chlorination, acid leaching, alkali leaching, bioleaching, roasting with alkali compounds, and so on (Marafi and Stanislaus, 2008b). Alkaline leaching can avoid the interference of metals such as Fe and Ni in the precipitation process (Chen et al., 2006), but the subsequent non-selective precipitation process will affect the purity of the product. Kim et al. (2009a) explored the direct H₂SO₄ leaching of the spent Ni–Mo/Al₂O₃ catalyst after roasted at 400 °C. The extraction efficiency of Mo, Al, and Ni were above 90%. They also found that more than 90% Co and Mo, and 93% Al can be leached at 95 °C when the pre-oxidized Co–Mo/Al₂O₃ spent catalyst at 250–300 °C was treated with H₂SO₄ (Kim et al., 2009b). Although the acid leaching can achieve the high recovery efficiency of metal values, the simultaneous leaching of several metals means that the worse selectivity will complicate the subsequent separation process of metal ions in the leachates. Hence, the selective extraction strategies have been pursued to efficiently recover the metal values from spent hydroprocessing catalysts.

Because of its relatively mild condition and the specific coordination role, ammonia leaching has been widely used to selectively recover metal values from the low grade resources, complex multi-metal ores, spent lithium ions batteries (Wang et al., 2017, Yang et al., 2016a), and so on. Marafi et al. compared the effect of several ammoniacal media on metal leaching of spent hydroprocessing catalysts and found that most of V, Mo, and Ni can be leached when aqueous ammonia and ammonium persulphate were used (Marafi and Rana, 2018). Generally, the aluminum as the matrix component is difficultly dissolved in the ammonia media and the surface components including nickel, molybdenum, and vanadium can combine with ammonia to selectively dissolve into ammonia media under the specific pH conditions. The selective leaching process can reduce the subsequent treatment process and lessen the pressure of subsequent extraction and precipitation procedures (Zhao et al., 2015b). Moreover, the molybdenum and vanadium species can be selectively precipitated or stripped as the metal’s ammonium salts by pH regulation and ammonia stripping, and the corresponding metal oxides can be obtained by direct roasting. For instance, Kim et al. (2014) obtained the roasted vanadium pentoxide from the ammonium-meta-vanadate products, which were produced from the loaded organic phase containing Alamine-336 and TBP by precipitating-stripping with ammonium hydroxide.

In this work, a sustainable recovery strategy of nickel, molybdenum, and vanadium from spent hydroprocessing catalysts (Ni–Mo–V/Al₂O₃) was proposed by an integrated selective route. Briefly, the primary focus was to selectively extract Ni and Mo with ammonia solutions of pH 9 in the meantime. And vanadium in the residues was extracted by ammoniacal liquors containing H₂O₂ at pH 12. The effects of roasting temperature, NH₃–NH₄Cl concentration, leaching temperature, the pH value of leachate, concentration of H₂O₂, and the solid to liquid ratio on the recovery of Ni, Mo, and V were examined in detail. In addition, Ni(OH)₂ and BaMoO₄ were prepared by using solvent extraction and precipitation method, respectively. The flower-like V₂O₅ was produced by direct precipitation and roasting method from the leachate. Hence, the Ni, Mo, and V from the Ni–Mo–V/Al₂O₃ spent catalysts can be selectively recovered as the corresponding compounds.

2. Materials and methods

2.1. Materials and pretreatment

Spent hydroprocessing catalyst (Ni–Mo–V/Al₂O₃) was provided by Dalian oil refining chemical plant. The obtained spent catalyst was burned at 400 °C in air atmosphere for 2 h in a muffle furnace to achieve deoiling and decoking, resulting in the black spent catalysts turning yellow in color. The XRF information of the spent hydroprocessing catalyst roasted at 400 °C was shown in Table 1. The data indicate that the roasted spent catalyst mainly contains 8.79% Ni, 4.13% Mo, 10.63% V, and 28.01% Al. Because the XRF is less sensitive to light elements in C-containing components on catalysts, the result is easy to be influenced by mutual element interference and the superimposed peaks. So the spent catalyst roasted at 400 °C (less than 100 mesh after grinding) was further digested by ST-60 automatic digester and the contents of main metal elements were further determined by ICP-AES. Chemical reagents including NH₃·H₂O, NH₄Cl, H₂O₂, HCl, BaCl₂^.2H₂O, and HNO₃ were of analytical grade and used as received from Sinopharm Chemical Reagent Co., Ltd. The leaching solutions with different pH values contained 6 mol/L total ammonia concentration and the varied mole ratio of NH₃·H₂O and NH₄Cl. All the aqueous solutions were prepared using redistilled water.

Table 1. Chemical compositions of the spent hydroprocessing catalyst roasted at 400 °C.

Constituent	O	Al	Si	P	Ca	V	Fe	S	Ni	Mo
(wt.%)	26.14	28.01	0.39	0.69	0.98	10.63	0.76	2.21	8.79	4.13

2.2. Experimental procedures

The roasted spent hydroprocessing catalyst of 2 g was immersed in 100 mL conical flask in a DF-101S thermostat water bath at a desired temperature. Unless specially specified, the leaching temperature was 75 °C and the solid to liquid ratio was 5% (g/ml). The stirring speed was 200 rpm. After leaching, the liquid/solid separation was performed in the TGL-20 centrifuge with 6500 r/min for 3 min. Then, the desired amount of leaching solution was withdrawn for ICP analysis.

Nickel ions in the leachates were extracted by 2.0 mol/L Versatic10 and the nickel-loaded organic phase was stripped by 3 mol/L sodium hydroxide to form nickel hydroxide precipitate. And barium molybdate precipitate was produced by adding barium chloride into the raffinate. The leaching residue in the first step was further leached with the ammoniacal liquors containing H₂O₂. After solid/liquid separation, the nitric acid was added into the leachates to adjust pH to 7.5–9 until the yellowish precipitates were formed. After rinsed three times by using water and ethanol, the yellowish precipitates were calcined in a muffle furnace at 500 °C for 1 h in ambient air to get V₂O₅. Fig. 1 illustrates the general flow sheet of the sustainable recovery of Ni, Mo, and V from the spent hydroprocessing catalysts.

2.3. Analytical and characterization methods

The contents of metal elementals in the leachate were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, PerkinElmer5300DV). Chemical compositions of the spent hydroprocessing catalyst were measured by X-ray fluorescence (XRF-1800, Japan). Thermogravimetry analysis (TGA) of spent catalyst from 20 to 800 °C was performed by using NETZSCH STA 2500 STA2500A-0215-N at 10 K/min under a gas stream of 5% O₂ in N₂ with a flow rate of 100 mL/min. Textural properties including specific surface area (SSA, m²/g), total pore volume (TPV, cm³/g), and average pore size (APD, nm) of spent catalysts and the roasted spent catalysts were determined by nitrogen physisorption at 150 °C (Quantachrome Nova 4000). For comparison, the textural properties of unroasted spent catalyst was also determined after washed with toluene under reflux to remove adsorbed residual oil. Raman spectra of the obtained compounds were obtained on an inVia Raman spectrometer (Reneshaw, England) with a 532 nm laser. Crystalline phases of roasted spent catalysts before and after leaching were characterized by X-ray diffraction (XRD, Cu Kα, Rigaku, D/max-7500, 10°/min). And the morphology and energy dispersive analysis were performed on a JEOL JSM-6360-LV microscope. The pH of the leaching solution was determined with a pH meter (PHSJ-3F, Thunder magnetic).

3. Results and discussion

3.1. Effect of roasting temperature

The results of the thermogravimetric analysis for spent catalyst after washed with toluene was shown in Fig. 2a. TG curve shows three noticeable weight losses for carbon and sulfur burning of the spent catalyst. The weight loss in the range of 20–300 °C is attributed to the oxidation of metal sulfides (Prajapati et al., 2017, Zeuthen et al., 1991). The weight loss in the range of 300–550 °C is attributed to the evaporation of the oxidation of carbon (Torres-Mancera et al., 2018). The weight loss at temperatures higher than 550 °C is due to the presence of hard coke on the spent catalyst (Marafi and Rana, 2018). The optical images of the samples before and after roasting are shown in Fig. 2b and c. It can be seen that the sticky oils on the surface of the spent hydroprocessing catalysts are cleaned after roasted at 400 °C and the color changes from black to yellow. And the shape of the catalyst particles is clover columnar. The corresponding SEM image in Fig. 2d indicates the coarse and fluffy surface of the roasted spent catalyst, which could benefit for the leaching of metal values. The XRD patterns of the spent Ni–Mo–V/Al₂O₃ catalysts roasted at different temperatures are shown in Fig. 2e. The main crystal phases of the spent hydroprocessing catalyst are Al₂O₃, MoO₃, NiV₂O₆, NiO, NiAl₂₆O₄₀, V₁₂O₂₆, and V₂O₅. As the roasting temperature increases, the crystallinity of various species has been enhanced and the NiV₂O₆ and NiAl₂₆O₄₀ species are formed at the higher of 600 °C. In addition, the weight loss rate of spent catalyst unwashed with toluene is about 31.2%, 36.9%, and 39.2% at 400 °C, 600 °C, and 800 °C, separately, which indicates that the higher temperature will benefit for the removal of carbonaceous matters. Moreover, the sublimation of V and Mo species may be occurred with the roasted temperature beyond 600 °C, thus resulting in the loss of metal values. Table 2 presents the textural properties including SSA, TPV, and APD values of the spent catalysts roasted at 400 °C and washed with toluene. It was found that the values of SSA, TPV and APD were increased 26.81%, 78.95% and 29.74% separately after roasted at 400 °C, indicating that roasting in air atmosphere can improve the specific surface area, total pore volume, and average pore size of spent catalysts. On one hand, the oil and carbon in the spent catalysts can be more effectively removed by calcination. Meanwhile, the improvement of these structural parameters was potentially beneficial to the diffusion of leaching agent and reaction products.

Table 2. Textural properties of washed and roasted spent catalysts.

Spent catalysts	SSA (m²/g)	TPV(mL/g)	APD (nm)
Washed with toluene	79.67	0.19	9.57
Roasted at 400 °C	101.03	0.34	13.62

The leaching behaviors of metal values from the spent catalysts roasted at different temperatures were investigated at both pH 9 and pH 12. The results in Fig. 3 show that the leaching efficiency of aluminum is less than 2% at each roasting temperature and pH condition. The leaching efficiency of nickel fast decreases from 75.42% to 17.29% at pH 9 and from 56.43% to 10.97% at pH 12 as increasing the roasting temperature from 400 °C to 800 °C. As discussed above, the complex nickel-bearing species are formed at the higher temperature, such as NiAl₂₆O₄₀, which could result in the low leaching efficiency of nickel in the mild ammoniacal media (Pinto and Soares, 2012, Zhou et al., 2015). At pH 9, the leaching efficiency of vanadium is less 2% when the roasting temperature is less than 600 °C but it can be leached with a leaching efficiency of 41.17% after roasted at 800 °C, indicating that the high roasting temperature can improve the formation of dissolvable vanadic oxide species, which are verified by XRD analysis in Fig. 2e. The leaching efficiency of molybdenum almost keeps at about 83.9% for each temperature. At pH 12, the leaching efficiency of molybdenum is improved by increasing the roasting temperature, indicating that increasing pH of ammoniacal liquor benefits for the dissolution of molybdenum-bearing species formed at the higher temperature. Meanwhile, the vanadium and molybdenum species are easy to be oxidized into acidic vanadium and molybdenum oxides, which can be effectively leached into the basic media. The leaching efficiency of vanadium and molybdenum can increase from 66.85% to 100% and from 79.27% to 100% with the increase of roasting temperature, separately. Therefore, the high roasting temperature and high pH conditions are beneficial for leaching of both molybdenum and vanadium whereas the low temperature and low pH conditions are beneficial to nickel leaching. Aluminum leaching is hardly affected by pH and roasting temperature. By considering the comprehensive recovery and utilization of nickel, molybdenum, and vanadium, it can be seen that high temperature is not conducive to the separation and recovery of the spent hydroprocessing catalysts.

3.2. Effect of pH and total ammonia concentration

In ammoniacal solutions, the main reactions of Mo, V, and Ni species can be expressed as Eqs. (1), (2), (3) (Zhao et al., 2015a).(1)NiO+4NH3⋅H2O+2NH4+=NiNH362++5H2O(2)MoO3+2NH3⋅H2O=(NH4)2MoO4+H2O(3)V2O5+2NH3⋅H2O=2NH4VO3↓+H2O

Thus, the species distribution diagrams of the Mo, Ni, V, and Al in ammoniacal media are established using MEDUSA® software (Puigdomenech, 1999) and shown in Fig. 4, where the total ammonia concentration is 6 mol/L and the ICP-determined metal concentration is 0.0215 mol/L Mo, 0.0503 mol/L Ni, 0.0667 mol/L V, and 0.541 mol/L Al, respectively. It can be seen that Ni, Mo, V, and Al have diverse species as increasing the pH value. When pH > 4, aluminum ions will be precipitated as the form of Al(OH)₃, which means that the aluminum species in the spent catalysts are indissoluble in the alkaline range of ammoniacal media. Due to the strong coordination role of ammonia, the nickel-bearing species in the spent catalysts could be selectively dissolved at pH > 7 as the form of nickel ammonia complexes. Based on the alkaline environment of the ammonia/ammonium buffer system, the molybdenum-bearing species in the spent catalysts are dissoluble at pH > 6 as the form of MoO₄²⁻. However, the vanadium-bearing species are difficultly dissoluble at pH < 11 due to the formation of ammonium metavanadate precipitate. Although the leaching behaviors of spent catalysts are also dependent on their phase composition and crystallinity, the species distribution diagrams indicate that the valuable Ni, Mo, and V can be selectively extracted by the adjustment of solution pH and ammonia concentration. Alternatively, nickel and molybdenum will be simultaneously leached at pH < 10, and then vanadium will be selectively leached at pH > 10 from the residues.

The effect of different pH and total ammonia concentration on leaching behaviors of the spent catalyst roasted at 600 °C was shown in Fig. 5. Obviously, aluminum is difficultly leached under each condition, which benefits for the subsequent separation procedure of leachates. The vanadium is hardly leached at pH < 9 but the leaching efficiency of vanadium rapidly increases from 2.37% to 68.26% as increasing pH from 9.2 to 12.1. However, the increase of pH has no evident influence on the leaching efficiency of nickel and molybdenum. The leaching efficiency of nickel increases from 31.75% to 38.75% when increasing pH from 8.34 to 9.2 and then almost keeps stable. And the leaching efficiency of molybdenum slightly increases to 80.05% at pH 10.3. When the pH value is fixed at 12, the effect of total ammonia concentration on the leaching of various metals is shown in Fig. 5b. When total ammonia concentration is 1 mol/L, the leaching efficiency of Ni, Mo, and V is 3.64%, 74.99%, and 42.76%, separately. The increase of total ammonia concentration benefits for the leaching of Ni, Mo, and V but has no influence on the leaching of Al. The corresponding leaching efficiency can reach 34.21%, 84.66%, and 62.46% when the concentration of total ammonia is 6 mol/L. It is worth noting that the leaching efficiency of V is more dependent on the solution pH. Even under the condition of 1 mol/L total ammonia concentration, the leaching efficiency of V still can reach 42.76% at pH 12. Therefore, nickel and molybdenum can be firstly leached at pH < 9, while vanadium and aluminum remained in the residues. And then vanadium can be selectively leached from the residues at pH > 12.

3.3. Effect of leaching temperature and time

The leaching behaviors of spent catalysts at different leaching temperature were further explored. Based on the discussion above, because the low roasting temperature is favor for the leaching of nickel but has no influence on the leaching of molybdenum. The roasted spent catalysts at 400 °C will be used to simultaneously recover nickel and molybdenum at pH 9, where there is no vanadium to be leached. As shown in Fig. 6a, the leaching efficiency of molybdenum and nickel increases from 76.91% to 96.73% and 40.3%–84.73% with the increase of temperature from 30 °C to 75 °C, respectively, whereas both vanadium and aluminum cannot be leached. Therefore, the high leaching temperature benefits for improving the recovery rate of molybdenum and nickel. The effect of reaction time on the leaching efficiency of the roasted spent catalysts at 400 °C is shown in Fig. 6b. It can be clearly seen that the leaching efficiency of nickel and molybdenum increase from 81.31% to 93.72%–85.84% and 97.18% respectively, when the leaching time increases from 20 min to 60 min. And then the leaching efficiencies of nickel and molybdenum decrease slightly as increasing leaching time. Both vanadium and aluminum are not dissolved throughout the leaching process. Because the Al₂O₃ support is difficultly dissolved, thus the leaching kinetics of metal values could mainly depend on the surface chemical reaction of surface species and diffusion of reactants and products (Abdel-Aal and Rashad, 2004, Szymczycha-Madeja, 2011b). However, it is notable that vanadium species may involve a dissolution-precipitation process due to the formation of indissoluble ammonium metavanadate, thus resulting in the low recovery efficiency of V. To avoid the evaporation loss of ammonia at high temperature, the leaching temperature of molybdenum and nickel can be alternatively fixed at 75 °C. Moreover, the leaching time of 60 min can efficiently recover the molybdenum and nickel from spent catalysts at pH 9.

3.4. Effect of H₂O₂ concentration on leaching of vanadium

Because the sulphide species in the spent catalysts could not be completely oxidized in the roasting process or some complex metal species could be formed, the oxidants were usually used to further improve the leaching of metal values (Marafi and Rana, 2018). Hydrogen peroxide is a conventional and effective oxidant during the leaching process of molybdenum and vanadium (Zhao et al., 2015b). Thus, hydrogen peroxide of different concentrations was added into the ammoniacal solution to enhance the recovery of valuable metals in the spent catalysts. It can be seen from Fig. 7a that the addition of hydrogen peroxide only affects the leaching of vanadium, but has no obvious effect on the leaching of Ni, Al, and Mo metals. Without hydrogen peroxide, the leaching efficiency of vanadium is negligible. With the increase of hydrogen peroxide concentration, the leaching efficiency of vanadium rapidly increases. Similar with the effect of roasting temperature, this phenomenon could be attributed to the reaction of the indissoluble vanadium species under the oxidation condition of hydrogen peroxide. When 6% (v/v) hydrogen peroxide was added, the leaching efficiency of vanadium can reach about 76.25%, and then it almost no longer increases.

Although the addition of hydrogen peroxide improves the leaching of vanadium, it is unfavorable for the subsequent separation of leachates containing Ni, Mo, and V. The reasonable choose is still the selective leaching of Ni and Mo without oxidants at pH 9 and the subsequent leaching of V at pH 12 from the residues in the first leaching step. Thus, the effect of H₂O₂ concentration on leaching of vanadium for the leaching residues at the first step was further explored at pH 12. As shown in Fig. 7b, the leaching efficiency of vanadium increases as increasing hydrogen peroxide concentration. Obviously, the hydrogen peroxide can improve the recovery of vanadium at both pH 9 and pH 12. The leaching efficiency of vanadium without hydrogen peroxide is about 68.28% at pH 12, and it can be increased to 89.11% when adding 6% (v/v) H₂O₂. Because of the selective leaching in the first step at pH 9, the leaching efficiency of Ni, Mo, and Al is below 3% in the second step, even in the presence of 6% (v/v) H₂O₂, indicating that V can be selectively leached at pH 12 from the leaching residues by the assistance of hydrogen peroxide.

3.5. Effect of solid to liquid ratio

For the leaching of nickel and molybdenum at pH 9, it can be seen from Fig. 8a that the leaching efficiency of both metals increases as the solid to liquid ratio decreases from 1:5 to 1:15, and then it slightly decreases beyond 1:15. In the absence of without hydrogen peroxide, the leaching efficiency of nickel and molybdenum is 60.52% and 70.94% at the S/L ratio of 1:5, but the leaching efficiency of nickel and molybdenum can increase to 88.74% and 97.92% at the S/L ratio of 1:15, separately. Aluminum cannot be leached even at the S/L ratio of 1:40. Although the leaching efficiency of vanadium is negligible at the higher than 1:20, the leaching efficiency of vanadium can increase to 13.33%, which could result from the relative high ammonia content at 1:40 for vanadium. Therefore, the solid/liquid ratio of 1:15 can be used to selectively leach nickel and molybdenum at pH 9. For the leaching of vanadium from the residues after leaching nickel and molybdenum, as shown in Fig. 8b, the leaching efficiency of vanadium at pH 12 increases from 61.13% to 97.13% with the decrease of solid/liquid ratio from 1:5 to 1:40, but which has no influence on the leaching of nickel, molybdenum, and aluminum. The XRD patterns of residues in two leaching steps were shown in Fig. 9. It can be seen that the residue in the first step mainly contains Al₂O₃, NH₄VO₃, and VO₂ phases, which means that vanadium in the first leaching residue mainly exists in the form of ammonium metavanadate and vanadium oxide when the spent hydroprocessing catalyst is leached with ammoniacal solution of pH 9. When the residue in the first step is again leached by using the ammoniacal solution of pH 12 in the presence of 6% (v/v) H₂O₂, the residue mainly includes Al₂O₃ phase. Therefore, the Ni, Mo, and V in the spent catalysts can be selectively extracted by the integrated two-step ammoniacal leaching.

3.6. Separation and recovery of Ni and Mo from the first leachate

Although the precipitation method is simple for metal ions separation, other co-existed metal ions in leachate may reduce the purity of precipitates. In ammoniacal liquors, because nickel ammonia species are the cationic form and molybdenum mainly presents as the form of anionic molybdate, the selective solvent extraction is a reasonable choice to separate nickel and molybdenum in the leachates in the first step. The initial pH of the first leachate is 8.8. The concentrations of Ni and Mo in the leachate are 0.05 mol/L and 0.023 mol/L, separately. Based on the previous reports (Flett, 2004, Inoue et al., 1984), neo-decanoic acid (Versatic10) can be potentially used to selectively extract Ni from the ammoniacal solution. In order to disclose the effect of acidity on the extraction of nickel, the pH of the first leachate was adjusted with hydrochloric acid. It can be seen from Fig. 10a that the extraction efficiency of Ni increases as increasing pH when pH is less than 6.98. The maximum extraction efficiency of Ni at pH 6.98 reaches 96%, and then it sharply decreases as increasing pH. The extraction efficiency of Ni at pH 8.8 is only 32.38%. Interestingly, the opposite phenomenon can be observed for Mo extraction. The extraction efficiency of Mo decreases as increasing pH when the pH is less than 6.98. Although Versatic10 is a cationic extractant, the high extraction efficiency of Mo at pH < 6 should result from the species change of molybdenum at the given pH condition. However, at pH of 6.98, molybdenum is hardly extracted, indicating that the selective separation of nickel over molybdenum in the leachate can be achieved at pH 6.98 by using Versatic10 as the extractant.

In order to recover nickel as a compound, 3 mol/L NaOH was used as stripping agent to directly precipitate Ni from the Ni-loaded organic phase. The ICP determination results indicate that nickel in the loaded organic phase is completely recovered as a green precipitate. After dried at 80 °C, the XRD pattern of the obtained precipitate is shown in Fig. 10b. It can be seen that the diffraction peak positions of the precipitate are consistent with those of Ni(OH)₂ (PDF card No.14–0117) (Kirubasankar et al., 2019). The well-resolved diffraction peaks indicate relative good crystallinity of the obtained nickel hydroxide precipitate. Because Raman spectroscopy is sensitive to phase and bonding information of materials, it is used as a supplementary method to analyze the structure of the obtained products (Fontané et al., 2011). Fig. 10c shows that the peak at 1079 cm⁻¹ is the character peak –OH of Ni(OH)₂. A weak feature peak at 1448 cm⁻¹ may correspond to a two-magnon transition of Ni–O, indicating the obtained precipitate is Ni(OH)₂. Two sharp bands at 3583 cm⁻¹ and 3601 cm⁻¹ can be assigned to the adsorbed H₂O, surface –OH groups, and the O–H stretch for the obtained Ni(OH)₂ (Hall et al., 2012, Wu et al., 2018). Moreover, there is no any Raman peak of organic substances, indicating that the obtained Ni(OH)₂ compound wasn’t contaminated by extractant molecules. After completely removing nickel in the leachate by twice extractions, the white precipitate can be obtained when BaCl₂ solution is added into the raffinate only containing Mo. The XRD pattern of the obtained precipitate is shown in Fig. 10d. The diffraction peak positions of the precipitate are consistent with those of tetragonal scheelite-type phase of BaMoO₄ (PDF card No.29–0193) (Xiao and Schmidt, 2017) and no extra impurity phases are detected, indicating that the molybdate in the raffinate can be recovered as a barium molybdate precipitate.

3.7. Separation and recovery of V from the second leachate

Based on the species distribution in Fig. 4c, vanadium in the ammoniacal media could be selectively recovered as the form of NH₄VO₃ precipitate in the desired pH condition. The concentration of V in the second leachate is 0.0648 mol/L. When the concentrated nitric acid is added drop by drop into the leachate in the second step until the pH is at between 7.5 and 9, the yellowish precipitate can be formed immediately. After reacting 20 min at 25 °C, the precipitate is centrifuged and cleaned with water and ethanol. Finally, the crystalline product is calcined in a muffle furnace at 500 °C in ambient air to get yellow powders. Fig. 11a is XRD patterns of the obtained precipitate. All the diffraction peaks are fully consistent with the phase of orthorhombic V₂O₅ (JCPDS No.72–0433) (Li et al., 2019), indicating that the product is well-crystallized V₂O₅. In Fig. 11b, the Raman vibration peaks can be indexed to orthorhombic V₂O₅ (Wang et al., 2018). The morphology of V₂O₅ is shown in Fig. 11c and d. It can be seen the vanadate species can self-assemble to form the flower-like crystals (Livage, 1998, Zheng et al., 2018). And the atomic weight of Mo in the product is only about 1.25%, indicating the obtained V₂O₅ has a good purity (Fig. 11e).

4. Conclusions

Sustainable recovery of Mo, Ni and V from the spent hydroprocessing catalysts was achieved by using an integrated selective process. After roasted at 400 °C, molybdenum and nickel are selectively extracted by ammoniacal liquors at pH 9 in the first step and the leaching efficiency of nickel and molybdenum is 88.74% and 97.92%, respectively. In the second leaching step, vanadium is selectively extracted from the residues by ammoniacal liquors containing 6% (v/v) H₂O₂ at pH 12 and the leaching efficiency of vanadium is above 97.13%. Moreover, nickel can be selectively separated at pH 6.86 by solvent extraction, where Versatic10 is used as extractant. The extraction efficiency of Ni can reach 96% and Mo is hardly extracted. Ni(OH)₂ can be directly obtained from loaded organic phase by using NaOH as the stripping agent. The barium molybdate precipitate can be further obtained from the lean Ni raffinate when BaCl₂ solution is added. Vanadium can be recovered as the form of NH₄VO₃ by adding concentrated nitric acid into the second-step leaching solution. And the flower-like V₂O₅ can be obtained after washed with ethanol and roasted at 500 °C.

Author contribution

Di Zhang: Data curation, Investigation, Roles/Writing – original draft. Yunqing Liu: Investigation, Methodology. Qizhao Hu: Data curation. Xinquan Ke: Data curation. Shenghua Yuan: Resources, Investigation. Shijun Liu: Supervision, Methodology. Xiaobo Ji: Investigation, Writing – review & editing. Jiugang Hu: Conceptualization, Investigation, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the National Basic Research Program of China (No.2014CB643401), Hunan Provincial Science and Technology Plan Project (Nos. 2016TP1007 and 2019JJ30031), Changsha Science and Technology Project (No. kq1801069), and Major Scientific Research Projects of China Petrochemical Corporation (119014-2).

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