Recent advances in the recovery of transition metals from spent hydrodesulfurization catalysts Tungsten

(1)

Tungsten (2021) 3:305–328

https://doi.org/10.1007/s42864-021-00095-5 REVIEW PAPER

Recent advances in the recovery of transition metals from spent hydrodesulfurization catalysts

Jian‑Zhang Wang^1,2 · Hao Du^2,3 · Afolabi Olayiwola^2,3 · Biao Liu^2,3 · Feng Gao^1,2 · Mei‑Li Jia² · Ming‑Hua Wang¹ · Ming‑Lei Gao⁴ · Xin‑Dong Wang⁴ · Shao‑Na Wang^2,3

Received: 31 December 2020 / Revised: 15 February 2021 / Accepted: 20 March 2021 / Published online: 22 June 2021

Abstract

Hydrodesulfurization (HDS) catalysts are widely used in petrochemical industries, playing a crucial role in desulfurization process to get high-quality oil. The generation of Al-based spent HDS catalyst is estimated to be 1.2×10⁵ tons per year around the world. The spent HDS catalysts have been regarded as an important secondary resource due to their abundant output, considerable metal value, and regeneration potential; however, if improperly handled, it would severely pollute the environment due to high content of heavy metals. Thus, the recovery of valuable metals from spent HDS catalysts is of great importance from both resource utilization and environmental protection points of view. In this work, recent advances in the spent HDS catalyst treatment technologies have been reviewed, focusing on the recovery of valuable transition metals and environmental impacts. Finally, typical commercial processes have been discussed, providing in-depth information for peer researchers to facilitate their future research work in designing more effective and environmentally friendly recycling processes.

Keywords Spent hydrodesulfurization catalyst · Recycling · Hydrometallurgical and pyrometallurgical processes · Valuable metals

1 Introduction

Hydrodesulfurization (HDS) catalysts are widely used in petroleum refining and chemical industry and play a crucial role in desulfurization process to get high-quality petroleum products [1–3]. Conventionally, HDS catalysts are based on Mo sulfide and advanced by Ni or Co as active components with γ-Al₂O₃, SiO₂, or TiO₂ as support. In addition, the combination of some main group elements such as N, C, P, and transition metals as active phases can improve the catalytic performance. In general, the most widely used HDS catalysts

in industry are Co-Mo/γ-Al₂O₃ and Ni-Mo/γ-Al₂O₃ catalysts [4–6]. During the high-temperature desulfurization process, sulfur compounds are adsorbed by Mo and removed by H₂, whereas V as well as Ni presented in heavy oil is depos- ited on the catalyst surface. The deposition of V, Ni, and C causes deactivation of the catalysts [7]. The service life of Al- based catalyst for light sweet crude oil treatment is generally 1–3 years, while that for heavy high-sulfur crude oil treatment is merely about 0.5–1 year. Accordingly, the generation of Al- based spent HDS catalyst is estimated to be 1.2×10⁵ tons per year around the world and that is about 2×10⁴ tons in China annually [8, 9]. The contents of V, Mo, Al, Ni, and Co along with other metals in the spent HDS catalyst are relatively high, especially the V content, exceeding most V containing raw materials. It is well recognized that V, Mo, and Ni are important strategic metals [10, 11], and therefore the spent HDS catalysts have been regarded as important secondary resources due to abundant output, considerable metal value, and regeneration potential. On the other hand, environmental regulations on spent catalysts have become increasingly stringent. Several types of catalysts utilized by refinery processes, for instance, spent hydrotreating, hydrocracking, reforming, isomerization, alkylation, and fluid catalytic cracking which contain lots of

Tungsten

www.springer.com/42864

* Shao-Na Wang shnwang@ipe.ac.cn

1 School of Metallurgy, Northeastern University, Shenyang 110819, China

2 CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

3 University of Chinese Academy of Sciences, Beijing 100049, China

4 Chengde V and Ti New Material Co., Ltd., HBIS Group Co., Ltd., Shijiazhuang 050023, China

(2)

organic sulfides and toxic heavy metals such as Cd, Pb, Ni, and Cr, have been categorized as hazardous wastes by the Envi- ronmental Protection Agency in the USA [12, 13]. Improper handling of these wastes severely pollutes the environment due to the high content of heavy metals [14]. Thus, the recovery of valuable metals from spent HDS catalysts is of great importance from both resource utilization and environmental protection points of view.

In recent years, substantial amount of studies have been reported with respect to the recovery of valuable metals from the spent HDS catalysts by hydrometallurgy, pyro-metallurgy, or joint processes [15–19]. These processes cover roasting followed by leaching, direct leaching, bioleaching, etc., and were dependent on the treatment processes.

The strategy for recovering different metals differs significantly. For instance, V and Mo can be recovered sequen- tially through soda roasting-water leaching followed by ammonium precipitation method. Although this extensively applied process is excellent due to its high efficiency and economic benefits, it also contains the shortcomings about the emission of toxic fume and generation of high salinity waste water during roasting and precipitation. For each ton of vanadium production, 30–50 m³ of wastewater will be generated when using soda roasting method [20]. To treat such large amount of waste water, it will cost a lot and, in addition, bring heavy burden to environment. Acid leaching is generally applied to recover all metals from spent catalyst.

However, the following processes to separate products of high purity from leaching solution are quite complex. Many chemical reagents are needed to separate various metal ions in leaching solution and finally large quantities of hazardous waste water are produced [13]. Further, due to the ever- increasing amount of spent HDS catalysts annually and stricter environmental regulations in all the countries, proper handling of these hazardous wastes has become a much more challenging task. In this regard, the goal of this review was to elaborate the typical spent HDS catalyst treatment processes, and discuss their advantages as well as limitations, particularly on their environmental impacts. Further, recent research progresses with respect to trends and developments of the recovery technologies have been addressed in detail to

provide in-depth information for peer researchers to facilitate their future research work.

2 Recovery of transition metals from spent HDS catalysts

The composition of typical inferior diesel and the content of valuable metals in the spent HDS catalysts are summarized in Table 1 and Fig. 1 [1]. It can be seen from Table 1 and Fig. 1 that the content of polycyclic aromatic hydrocarbons (PAHs), S as well as N are extremely high in diesel which leads to serious environmental problems if improperly handled before utilizing. Besides, when the diesel or heavy oil are treated by the HDS catalysts, substantial amount of transition metals are enriched in the deactivated spent HDS catalysts, (Mo ranges from 4 to 12 wt.%, V from 0.5 to 15 wt.%, Ni from 1 to 5 wt.%, and Co from 0.5 to 3 wt.%), which are much richer than most pristine ores, and thus exhibit very high recycling value. Table 2 summarizes the composition of typical spent HDS catalysts around the world, clearly suggesting that significant amount of transition metals is enriched in the catalysts. Therefore, the spent HDS catalysts are typically recognized as mineral concentrates for metal extraction, regardless of being categorized as hazardous solid wastes in most countries. Generally, after decommis- sioning, the spent catalysts are treated via two approaches:

regeneration of catalysts and recovery of valuable metals [19]. When C and S deposition is the main reason for deactivation, the spent catalysts are usually regenerated by simply burning off the C and S so that the catalysts can be reused to save cost. On the other hand, if the catalyst deactivation is caused by deposition of V and Ni compounds, the spent catalysts suffer from permanent deactivation and cannot be regenerated, and consequently, recovery of valuable metals has become a better approach [39].

The recovery processes commonly contain pretreatment, roasting, leaching, and separation as displayed in Fig. 2.

Through the combination of these processes, different metals can be extracted efficiently and selectively. In the following section, typical recycling processes will be discussed in detail, addressing their advantages and disadvantages from

Table 1 Composition of typical inferior diesel

w (S)/(μg·g⁻¹) w (N)/(μg·g⁻¹) Density (20 °C)/(g·cm⁻³) Cetane number Ref.

Inferior diesel 6300 1134 0.9000 28.9 [21]

Coking and catalytic

diesel 1190 912.07 0.8493 46.2 [22]

Fluid catalytic cracking

diesel (FCC) diesel 4400 458.7 0.9594 20.4 [23]

FCC diesel 8150 1762 0.8846 36 [24]

FCC diesel 7428 668 0.9666 19 [25]

(3)

both metal recovery efficiency and environmental impact point of view.

2.1 Pretreatment

The HDS catalysts are generally utilized to remove the organic sulfides from heavy petroleum. During hydrodesulfurization process, the organic sulfides react with H₂ at the surface of the catalysts at about 250–400 °C, and these catalysts suffer from deactivation due to sintering, poisoning by heavy metals, deposition of coke, sulfide, and residual oil, or attrition and loss of active factors [7]. The hydrogenation process of major organic sulfides and metalloporphyrin is presented in Fig. 3 [40]. The organic sulfides such as thiophene, benzothiophene, and dibenzothiophene are difficult to remove by conventional desulfurization approach, and when using HDS catalyst, the coordination unsaturated active factors such as Mo will combine with S in the thiophene and

derivatives. Subsequently, the C–S bond is opened by active hydrogen and S is removed in the form of H₂S, realizing desulfurization [41]. In addition, during the decomposition of metal contaminants (V and Ni porphyrins), the N–V or N–Ni covalent bonds are opened and combined with H radical, generating metal sulfides at the surface of the HDS catalysts [42]. The process will repeat until the catalyst deactivates.

Therefore, the surface and pores of the spent HDS catalysts are covered by sulfur, metal sulfides, coke, and residual oil.

In particular, residual oil generally accounts for about 20 wt.% creating significant challenges for downstream metal recovery via hydrometallurgical methods [43]. Thus, many studies have been made with respect to the pretreatment of the spent catalysts to remove sulfur, coke, and residual oil.

2.1.1 Thermal treatment

Thermal treatment is a facile and low-cost approach, which has been extensively applied in industrial practice.

Most volatile organic components, coke, sulfur or metal sulfide can be removed using this technique and generally the spent catalysts are roasted between 300 and 600 °C in air atmosphere [44, 45, 46]. Nevertheless, under such low temperatures, only the volatile components can be efficiently removed, leaving the nonvolatile components in the catalysts. In addition, deflagration of extremely flammable oil may result in uncontrollable temperature variations, forming undesirable transition metal compounds (CoMoO₄ and NiMoO₄), which are difficult to dissolve in subsequent leaching processes [47]. In order to avoid this phenomenon, microwave-assisted thermal treatments have been adopted due to its unique features including rapid heating and internal heating. As displayed in Fig. 4, under microwave irradiation, the microwave

Fig. 1 Content and component in spent HDS catalysts [1]

Table 2 Composition of typical spent HDS catalysts around the world (wt.%)

Active components Elemental composition/wt.% Country Ref.

Ni/Mo/Al₂O₃ 12.1Mo, 30Al, 2.6Ni, 0.04V Portugal [26]

Co/Ni/Mo/Al₂O₃ 5.4Mo, 3.2Co, 14.9V, 3.1Ni Kuwait [27]

Ni/Mo/Al₂O₃ 2.93 Nii, 17.24 Al, 1.57 Mo, 15.33 V South Korea [28]

Ni/Mo/Al₂O₃ 19.5 Al, 2 Ni

9V, 1.4 Mo South Korea [29]

Ni/Mo/Al₂O₃ 13.1 Mo, 27.4 Al, 2.5 Ni, 0.008 Co, 0.013 V South Korea [30]

Ni/Mo/Al₂O₃ 9 V, 14 Mo, 2 Ni, 19.5 Al South Korea [31]

Co/Ni/Mo/Al₂O₃ 15 V, 5.62 Mo, 2.40 Ni, 2.06 Co, 19.47 Al Japan [32]

Co/Mo/Al₂O₃ 3.12 Mo, 12.69 V, 2.86 Ni, 0.27 Co, 25.63 Al China [33]

Ni/Mo/Al₂O₃ 24.8 Al, 1.72 Co, 3.32 Ni, 5.99 Mo,1.06 V China [9]

Ni/Mo/Al₂O₃ 0.48 Mo, 4.77 Ni, 0.35 Co, 0.55 V, 37.56 Al China [34]

Co/Ni/Mo/Al₂O₃ 12.47 Mo, 2.94 Ni, 0.24 Co, 32.27 Al China [35]

Ni/Mo/Al₂O₃ 8.35 V, 2.94 Ni, 1.73 Mo, 15.19 Al China [36]

Ni/Mo/Al₂O₃ 16 V, 1.2 Mo, 5.1 Ni, 1.5 Fe, 17 Al China [37]

Ni/Mo/Al₂O₃ 20.238 Mo, 5.267 Ni, 27.692 Al Turkey [38]

(4)

Fig. 2 The schematic process flow of the spent HDS catalyst recovery processes

(5)

energy is directly absorbed by materials, promoting the atomic activity and accelerating the internal reaction rate [9]. When the material undergoes internal heating, a temperature gradient from inside to outside will be generated, which can intensify the destruction and collapse of mineral structure substantially, increasing the specific surface area of the samples and promoting the removal of oil and coke significantly. Despite the above-mentioned advantages, hazardous gases and fumes stemming from roasting are inevitable, creating significant challenges for subsequent exhaust gas treatments.

2.1.2 Solvent extraction

Another method is using solvent extraction to desorb residual oil from the spent catalyst. Nonpolar solvents are employed to dissolve the organic components such as oil on the surface or in the pores of the spent HDS catalysts. The residual oil is usually extracted by organic solvent such as acetone [24], naphtha [39], toluene [48] and ethanol [17].

Besides the above-mentioned solvent, aqueous surfactant solution can also be utilized to remove oil via emulsifica- tion. During oil removal process, the surfactant disperses in the interfase of water and oil. The lipophilic groups combine with oil and the hydrophilic groups bind to aqueous solution. Under mechanical stirring or thermal treatment, the residual oil can be removed from the surface and pores, forming oil-in-water (O/W) emulsion. The merits of solvent extraction include convenient operation, high oil removal efficiency, and moreover, facile regeneration of solvents as well as oil. Nonetheless, the inevitable limitation of this approach is that most organic agents are volatile and may cause uncontrolled emission, and further, the operation cost

using solvent extraction is relatively high in comparison with thermal treatment method.

2.1.3 Physical treatment

Further, some studies have examined physical treatment methods to desorb residual oil from the spent HDS catalysts. These methods include mechanical washing, cyclonic gas stripping, and ultrasound-assisted stripping. Mechanical washing is considered as an effective and low-cost method and has been widely studied. It is found that during washing the particle motion in flow field is the main reason for oil removal and the oil removal efficiency can be greatly increased by controlling the motion behavior of particles under turbulence conditions [48, 49]. Hydrocyclone, which can provide sufficient shear strain force to change the motion behavior of particles in turbulent flow field, had been extensively applied to treat the contaminated catalyst due to its high efficiency of oil removal [50]. When particles are treated with hydrocyclones, there will be a large velocity gradient in swirling flow field and the fluid velocity between the two ends of particles is uneven, resulting in the high- speed self-rotation of particle and enhancing the oil removal efficiency. For example, Li et al. [51] proposed a hydrother- mal-hydrocyclone process to clean oily spent catalysts. Their study suggested that the utilization of hydrocyclone could significantly reinforce the release of residual oil from the spent catalysts owing to the self-rotation of particles within the hydrocyclone. Similarly, a novel cyclonic gas stripping method was reported to remove the oil from spent catalysts [52]. This method also utilized the particle high-speed self- rotation to enhance the oil removal effect, and the de-oiling efficiency reached 96.7% and nearly 30% of the catalysts after de-oiling were regenerated as highly active catalysts.

Fig. 4 Schematic diagram presenting the mechanism of microwave heating of the spent HDS catalysts [9]

(6)

In addition, ultrasound-assisted oil removal has been investigated, and organic solvent such as ethanol has been utilized to improve the extraction performance [17].

A summary of different pretreatment methods is displayed in Table 3, and it is obvious that the main function of pretreatment is to remove residual oil, coke, and sulfur from the spent HDS catalysts. Thermal treatment can burn off the contaminants completely; however, the treatment of toxic fume and gas generated is a major challenge. Solvent extraction can dissolve the residual oil effectively; however, it cannot remove coke and sulfur efficiently. Overall, as environmentally friendly, convenient, and economical approaches, physical treatment such as mechanical washing and cyclonic gas stripping appear to be quite attractive in practical applications.

2.2 Roasting

The spent HDS catalysts contain substantial amount of transition metals such as V, Ni, and Mo existing as low-valance sulfides or oxides, which are quite refractory for direct leaching; therefore, high-temperature roasting process has been practiced to covert the metal sulfides to high-valance oxides or corresponding salts to facilitate the subsequent leaching and remove the contaminants such as coke, volatile organics simultaneously [53, 54]. There are two approaches for roasting: oxidizing roasting and salt roasting.

2.2.1 Oxidizing roasting

In comparison with thermal pretreatment, the aim of oxidizing roasting is mainly to oxidize the metal sulfides to corresponding high-valence oxides, and the conventional roasting temperature commonly varies from 650 to 1000 °C, much higher than thermal pretreatment temperature, which is between 300 and 600 °C. However, it is noticed that if the target metal is Ni, the roasting temperature is usually lower than 600 °C in order to avoid the formation of stable spinel NiAl₂O₄ phase, which is very stable and difficult to dissolve [55, 56]. Moreover, as shown in Table 2, if microwave-assisted roasting is employed, the roasting temperature is usually lower than that of conventional roasting due to internal heating and the destruction of

mineral structure originated from the microwave intensifica- tion effect.

Some of the typical reactions during roasting are listed as follows:

The oxidation of MoS₂ usually initiates at 450–500 °C and completes at 600–700 °C [57]. The oxidation of NiS shows thermal stability at 300 °C and NiO formation is observed from 350 to 600 °C. Further, V₅S₈ and V₃S₄ are mainly oxidized in the range of 375 to 525 °C.

V₂S₃ is considered to be oxidized at 240 °C, and at above 700 °C, completely converted to V₂O₅, which can be dissolved in alkaline or acid solution [58, 59, 60]. CoS starts oxidation at around 380 °C, completely converting to CoSO₄ at 540 °C, and CoSO₄ decomposes to Co₃O₄ at 700 to 750 °C. A phase transformation from Co₃O₄ to CoO occurs when temperature is higher than 910 °C. It is worth mentioning that Al₂O₃ in the catalyst will experi- ence a sequential phase transformation with increases in the roasting temperature as depicted in Fig. 5 [61]. The fig- ure indicates that when the spent catalyst is roasted under 1100–1200 °C, the Al₂O₃ carrier will transform from γ, δ, η, θ, and κ phases to α-Al₂O₃ which cannot dissolve in both acid and alkaline solution.

In recent years, microwave-assisted roasting technology has been extensively investigated due to its high heating efficiency, uniform heat transfer, and other benefits [62].

For example, Zhang et al. [63] have reported a technique utilizing microwave roasting to limit the dissolution of Al from spent HDS catalyst. In this process, γ-Al₂O₃ was converted to stable α-Al₂O₃ at low temperature and the (1) 2MoS₂+ 7O₂= 2MoO₃+ 4SO₂↑

(2) MoS₂+ 6MoO₃=7MoO₂+2SO₂↑

(3) 2Ni₃S₄+ 11O₂= 6NiO + 8SO₂↑

(4) 4V₅S₈+ 57O₂= 10V₂O₅+ 32SO₂↑

(5) Co₉S₈+ 14O₂= 3Co₃O₄+ 8SO₂↑

Table 3 A summary of the common methods of pretreatment of spent HDS catalyst

Pretreatment Condition Effect Challenge Ref.

Thermal treatment Roasting at 300–600 °C Remove residue oil, coke and sulfur

effectively Treatment of toxic gas

and fume [44–46]

Solvent extraction Extract by acetone, naphtha, toluene,

or ethanol etc Remove residue oil High operation cost [17, 24, 39, 48]

Physical treatment Mechanical washing, cyclonic gas stripping, and ultrasound-assisted stripping

Remove residue oil, coke and sulfur Low maturity [49–51]

(7)

dissolution of Al could be well controlled, realizing selective leaching other valuable metals such as V, Mo, Co, and Ni. Moreover, a process featuring microwave-assisted roasting was proposed, where at a relatively low roasting temperature of 500 °C, an excellent recovery ratio of target metals was achieved [17]. In comparison to conventional roasting processes, the leaching ratio of valuable metals including V, Mo, Co, or Ni can be improved by microwave-assisted roasting and the energy consumption can be reduced. In addition, the generation of undesirable compounds such as CoMoO₄ can be restrained due to the much-shortened roasting time and, therefore, this process appears to be environmentally friendly with great potential for large-scale industrial application.

2.2.2 Salt roasting

Salt roasting is generally applied to convert insoluble metal sulfides and oxides to water-soluble metal salts, and the most commonly used salts include sodium and calcium salts. The principle of soda and calcification roasting is discussed as follows.

2.2.2.1 Soda roasting In soda roasting process, the spent catalyst is roasted with sodium salt such as NaCl, Na₂CO_3, and NaOH [64, 65]. Among them, Na₂CO₃ is the most commonly used salt. During roasting, V and Mo sulfides are converted to soluble NaVO₃, Na₃VO₄, and Na₂MoO₄, dissolving in water subsequently [66], and meanwhile the metal sulfides or deposition of S is converted to Na₂SO₄, avoiding the emission of toxic fume in contrast with oxidizing roasting. Al, Ni, and Co remaining in the solid residue can be used for regeneration of fresh catalysts. The main chemical reactions during roasting are as follows:

(6) 4V₃S₄+ 34Na₂CO₃+39O₂=12Na₃VO₄+16Na₂SO₄+17CO₂↑

(7) V₂S₃+4Na₂CO₃+7O₂=2NaVO₃+ 3Na₂SO₄+4CO₂↑

(8) V₂O₅+Na₂CO₃ =2NaVO₃+CO₂↑

There are large number of researches focused on recovering of valuable metals (V, Mo) from the spent catalysts using sodium roasting process. For instance, the optimum conditions for extracting Mo using Na₂CO₃ roasting were proposed by Dash et al. [44] and He et al. [67], respectively. A Mo recovery of 99% was reported by Dash et al.

[44] after roasting at 850 °C, and by contrast, the Mo recovery ratio was only 92.5% after roasting at 771 °C by He et al. [67], which indicates that increasing roasting temperature may promote the dissolution ratio of Mo.

Moreover, recovering of V and Mo simultaneously from spent catalyst by soda roasting was investigated [66]. The mixture of spent catalyst and Na₂CO₃ was roasted under 750 °C and about 91.3% of Mo and 90.1% of V could be extracted. The results demonstrate that high roasting temperature is beneficial for the Mo leaching efficiency, but unfavorable in terms of energy consumption. In addition to Na₂CO₃ roasting, other research has investigated the effect of NaCl or NaOH salts and found out that the recovery ratio of valuable metals was even higher than Na₂CO₃ roasting [33, 66, 68]. Moreover, in order to save the energy consumption during roasting, microwave- assisted soda roasting has been tested due to its low roasting temperature and high heating efficiency. Therefore, a method featuring microwave-assisted Na₂CO₃ roasting at 600 °C was adopted and excellent recovery effect of Mo achieved, providing an attractive alternative of sodium salt roasting technology [62]. Despite the prominent recovery efficiency of Mo and V using soda roasting, the generation of high salinity wastewater, Na₂SO₄ liquor, creates significant environmental issues, and thus, alternative additives such as calcium salt have been utilized and investigated.

2.2.2.2 Calcification roasting process Calcification roasting process is used to convert valuable metals to corresponding calcium salts. In this process, CaCO₃ is usually used

(9) 2MoS₂+6Na₂CO₃+9O₂=2Na₂MoO₄+4Na₂SO₄+6CO₂↑ (10) MoO₃+Na₂CO₃=Na₂MoO₄+CO₂↑

Fig. 5 Phase transformation of Al₂O₃ during roasting [61]

(8)

as calcium salt. During calcification roasting process, V sulfides convert to oxides and CaCO₃ is decomposed to CaO and CO₂, and then CaO reacts with V oxides to generate Ca(VO₃)₂ [69]. MoS₂ is preliminarily oxidized to MoO₃ and then reacts with CaCO₃ to produce CaMoO₄ [70]. The main reactions during calcification roasting can be represented as follows (reactions (11) to (15)):

Owing to the relatively low alkalinity of calcium salts, the temperature for calcification roasting is quite high, at about 1000 °C, which is much higher than that of soda roasting [71]. That may result in extra energy consumption and sintering on the surface of material, producing insoluble salts.

The forms of calcium vanadates mainly depend on the Ca to V ratio, and all the calcium vanadates are insoluble in water;

thus, alkaline solutions are generally applied to extract V and Mo from roasted material [72]. The conditions, phase transformation, and products of different roasting processes are summarized in Table 4. According to the products, various leaching methods can be designed to extract target metals from the roasted spent catalysts.

2.3 Leaching

After roasting, most transition metals have been converted to corresponding high-valance oxides or salts and can be subsequently leached out. The design of leaching strategy is dependent on the properties of the roasting products, and some of the typical studies have been summarized in Table 3. In general, water leaching is only effective to treat (11) 2V₂S₃+ 11O₂= 2V₂O₅+ 6SO₂↑

(12) 4VS + 9O₂= 2V₂O₅+ 4SO₂↑

(13) V₂O₅+ CaO = Ca(

VO₃)

2

(14) 2MoS₂+ 7O₂= 2MoO₃+ 4SO₂↑

(15) MoO₃+ CaCO₃= CaMoO₄+ CO₂↑

the products obtained through sodium salt roasting. On the other hand, acid or alkali leaching can be applied directly to unroasted spent catalysts if strong oxidative environment is provided during leaching. The difference is that acidic lixiviant is less selective and can dissolve indistinguishably all transition metals from the spent HDS catalysts, while alkali can be applied to extract and separate V, Mo, and Al from Ni or Co. In addition, bioleaching has been considered as an attractive alternative to recover the spent HDS catalyst due to the low capital cost and less environmental pollution in comparison with the conventional leaching process.

The composition changes during leaching are worth investigating. For example, NiO and CoO generally dissolve in acid as divalent cations (Ni²⁺, Co²⁺) and are insoluble in alkali. Al₂O₃ is in the form of Al³⁺ in acidic system and converted to Al (OH)₄⁻ in alkaline solutions. Furthermore, the ionic composition of V and Mo in leaching liquor is especially complicated, and, therefore, it is necessary to study the specific forms of V, Mo ions. Both V (V) and Mo (VI) have several species [73, 74]. When the pH is lower than 2, the main existing forms of V and Mo are cationic metal complexes such as VO₂⁺, H₃Mo₂O₈²⁺, and H₃MoO₄⁺; with pH increasing, V and Mo generally exist as anionic metal complexes. When the pH is from 2 to 6, the existing forms of V and Mo are V₁₀O₂₇(OH)⁵⁻, V₁₀O₂₆(OH)₂⁴⁻, V₁₀O₂₈⁶⁻, Mo₇O₂₁(OH)₃³⁻, Mo₇O₂₃(OH)⁵⁻, Mo₇O₂₂(OH)₂⁴⁻ and Mo₈O₂₆⁶⁻. When the pH is from 6 to 9, the main existing forms of V and Mo are V₃O₉³⁻, V₄O₁₂⁴⁻, VO₂(OH)₂,⁻ and MoO₄²⁻. When the pH is higher than 10, the main existing forms of V are VO₃(OH)²⁻ and VO₄³⁻.

2.3.1 Water leaching

Typically, direct water leaching is ineffective to dissolve valuable metals from the spent HDS catalysts. Therefore, the samples are usually roasted with sodium salts to convert the metal sulfides or low-valence oxides to water-soluble salts before leaching. For instance, soda-roasting followed by water leaching is an extensively applied approach in industrial production. A study about Na₂CO₃ roasting–water leaching to recover V, Mo, and Al from the spent HDS

Table 4 The conditions and phases of transformation of different roasting processes

Process Roasting condition Phases transformation and product Ref.

Oxidizing roasting Microwave roasting at 500 °C or conventional roasting at

650–1000 °C MoS₂ → MoO₃, NiS → NiO

V₅S₈, V₃S₄, V₂S₃ → V₂O₅ CoS, Co₉S₈ → Co₃O₄, CoO

[57–60]

Salt roasting Conventional roasting at 850 °C or microwave roasting at 600 °C with Na₂CO₃, NaCl, NaOH, etc.

Roasting at 1000–1050 °C with CaCO₃, Ca (OH)₂, CaO

V₅S₈, V₃S₄, V₂S₃→ NaVO₃, Na₃VO₄ MoS₂→ Na₂MoO₄

V₂O₅→ Ca (VO₃)₂ MoS₂→ CaMoO₄

[64–66, 71]

(9)

catalyst was reported and the recovery ratio of V, Mo, and Al was as high as 97.22%, 99.68%, and 95.56%, respectively [75]. Moreover, Kar et al. [68] proposed that using NaCl roasting–water leaching could selectively extract Mo with the leaching efficiency of about 90%.

The advantages of water leaching are facile operation and easy equipment set up. However, the leaching solution usually contains multiple components including transition metals, sodium salts as well as substantial amount of impurities such as Si and P, creating significant difficulty in subsequent separation and purification. In practice, V will be separated via ammonium salt precipitation in acidic solution while Mo is to be further separated via solvent extraction. The residue solution needs to be concentrated to remove ammonium and sodium sulfate salts before recycling, which is not only energy-consuming but also equipment demanding, due to the severe corrosion of normal steel during high-temperature acidic solution vaporization. Normally, titanium-based materials have to be utilized for such operation. In this regard, designing of processes to realize selective leaching of transition metals or avoiding the generation of high salinity acidic wastewater has drawn much research attention.

2.3.2 Alkali Leaching

Due to apparent solubility difference, selective leaching of transition metals can be realized using alkaline solutions.

Particularly, V₂O₅ and MoO₃ will form oxyanion and be dissolved in alkaline solution and in contrast, NiO and CoO are insoluble, realizing selective separation of V and Mo with Ni and Co [66, 76]. Generally, alkali leaching agents include NaOH, NH₃·H₂O, and sodium salts such as Na₂CO₃ [77], and the metal recoveries using different alkaline solutions are summarized in Table 5.

2.3.2.1 NaOH Leaching Due to high leaching efficiency, NaOH solution is extensively utilized to dissolve target metals. The reactions of different metal oxides in NaOH solutions are as follows:

Digestion using NaOH solution is proven to be a practical and efficient method to dissolve V and Mo. For instance, studies with respect to blank roasting–NaOH leaching [33], air oxidation-NaOH leaching [79], and NaOH-H₂O₂ oxidizing leaching [84]. All indicate that NaOH leaching is very efficient. However, it is worth mentioning that Al₂O₃ can also be dissolved in concentrated NaOH solutions, creating significant difficulties in the subsequent separation and purification processes, especially the separation of AlO₂⁻ and MoO₄²⁻. In order to avoid such trouble, leaching using low- concentration NaOH solution was investigated. For example, Huang et al. [45] have reported that selective recovery of Mo (96%) could be realized, with only 0.2% of Al being leached out due to the low solubility of Al₂O₃ in diluted NaOH solutions. On the other hand, it usually takes much longer time to leach Mo in diluted NaOH solutions, and therefore, different methods have been tested to intensify the leaching process. For example, the influence of ultrasound and microwave on the leaching behavior was investigated by Pinto and Soares [26]. Their results suggested that the microwave-assisted approach was favorable for the selective leaching of Mo, and about 90% of Mo could be leached out in just a few minutes while the dissolution of Al was only 5.7–8.8%. Similarly, Ma et al. [36] have also examined a microwave-assisted NaOH leaching process and reported that this approach could realize efficient leaching of both V and Mo in very short leaching time, with the V and Mo leaching ratio of 94.4% and 96.2%, respectively.

The microstructure analysis of the microwave treated spent catalyst suggests that there are apparent cracks on the surface, which is caused by the thermal stress due to the different microwave absorption characteristics of each mineral phase. Under the microwave radiation, the surface area of the (16) MoO₃+ 2OH⁻ = MoO²⁻₄ + H₂O

(17) Al₂O₃+ 2OH⁻ = 2AlO⁻₂ + H₂O

(18) V₂O₅+2OH⁻= 2VO⁻₃ + H₂O

Table 5 Leaching conditions and ratio of valuable metals by alkaline leaching

Process Condition Leaching ratio (%) Ref.

NaOH leaching leaching with NaOH at 100 °C Mo: 96 [45]

Leaching with NaOH first step at 90 °C Mo: 94, Al: 97 [78]

Leaching with NaOH at 85 °C Mo: 99.8 [79]

Microwave leaching with NaOH at 90 °C V: 94, Mo: 96 [36]

Microwave-assisted leaching with NaOH Mo: 90 [26]

Na₂CO₃ leaching leaching with Na₂CO₃ and H₂O₂ at room temperature Mo: 85 [80]

Leaching with Na₂CO₃ at 30 °C Mo: 97 [81]

Ammonium leaching Leaching with NH₃·H₂O at 75 °C Mo: 98, Ni: 88, V: 97 [82]

Leaching with (NH₄)₂SO₄ Mo: 85 [83]

(10)

spent catalysts is increased, and thus the leaching efficiency has been improved remarkably. In summary, leaching using NaOH solution can selectively dissolve V and Mo with high efficiency, facilitating the downstream separation and purification processes.

2.3.2.2 Na₂CO₃ leaching It is known that Al₂O₃ can only be dissolved in strong acid or alkaline solutions; therefore, in order to selectively leach out Mo and V, Na₂CO₃ solutions have been utilized to treat the roasted calcine, and the leaching reactions are shown as follows:

Mohapatra et al. [81] have concluded that over 97% of Mo with negligible amount of Co and Al could be extracted from the spent Co/Mo/γ-Al₂O₃ catalyst using roasting-Na₂CO₃ solution leaching method. Further, direct oxidative leaching using Na₂CO₃ solution has also been proven to be effective, and the commonly used oxidant is H₂O₂ [30, 80]. In such operation, V₂S₃ and MoS₂ in the spent catalysts are converted to soluble compounds, while Co and Ni remain in the residue, which can be further treated using acid leaching to recover Co and Ni [85]. The major reactions are as follows:

Direct oxidative leaching using Na₂CO₃ solution exhib- its obvious advantages in terms of energy consumption and fume emission, however, facing the challenges of high operation cost due to the utilization of expensive oxidants. New methods based on advanced oxidation technologies are the future for such process from both academia and industrial application point of view.

2.3.2.3 Ammonium leaching Ammonium leaching has also been utilized to selectively extract Ni, V, and Mo from spent catalysts [86, 87, 88, 89], and the reactions can be described as follows:

(19) MoO₃+ CO²⁻₃ = MoO²⁻₄ + CO₂↑

(20) V₂O₅ + CO²⁻₃ = 2VO⁻₃ + CO₂↑

(21) MoS₂+ 9H₂O₂+ 3Na₂CO₃

= Na₂MoO₄+ 2Na₂SO₄+ 9H₂O + 3CO₂↑

(22) V₂S₃+ 14H₂O₂+ 4Na₂CO₃

= 2NaVO₃+ 3Na₂SO₄+ 14H₂O + 4CO₂↑

Zhang et al. [82] have proposed an oxidizing roasting- ammonia leaching process. The spent catalysts were first roasted at 400 ^oC and then leached in ammonium solution to dissolve Ni as well as Mo. The leaching ratio of Ni and Mo was reported to be 88.74% and 97.92%, respectively. After filtration, the leaching residue was leached with ammoniacal liquor in the presence of H₂O₂ to dissolve V and the leaching ratio of V was as high as 97.13%. In addition to ammonia, other ammoniacal salts such as (NH₄)₂SO₄ have been used to recover Mo and Ni from the spent HDS catalysts [83]. The ammonium leaching process proposes an integrated selective recovery strategy of Ni, Mo, and V, whereas the shortcoming of this process is the generation of high ammonia–nitrogen wastewater, difficult for subsequent treatment. For comparison, the features of the alkaline leaching processes are summarized in Table 6.

2.3.3 Acid leaching

Due to high leaching efficiency, different acids including mineral acids such as H₂SO₄, HCl, and HNO₃, organic acids such as oxalic acid, citric acids, tartaric acid, succinic acid, malonic acid, glycolic acid, salicylic acid, lactic acid, and phthalic acid [77], and mixtures of acid solutions for instance HNO₃/HF, HCl/HNO₃/H₂O₂, and HF/HClO₄/HCl/H₃BO₃ [48] have been employed as lixiviants. The metal recovery efficiency using different acids is summarized in Table 7.

2.3.3.1 Mineral acids Sulfuric acid: As previously discussed, after roasting, most metal sulfides in the spent catalysts are converted to corresponding oxides, which can be easily leached out in strong acidic solutions, among which H₂SO₄ is most commonly utilized due to the high efficiency and low costs. The major reactions are illustrated as follows:

(23) NiO + 4NH₃⋅H₂O + 2NH⁺

4 = Ni( NH₃)²⁺

6 + 5H₂O (24) MoO₃+ 2NH₃⋅H₂O =(

NH₄)

2MoO₄+ H₂O

(25) V₂O₅+ 2NH₃⋅H₂O = 2NH₄VO₃+ H₂O

(26) Al₂O₃+ 3H₂SO₄= Al₂(

SO₄)

3+ 3H₂O

Table 6 A summary of different alkaline leaching processes

Leaching agent Leaching efficiency Selectivity Environmental impact Ref.

NaOH V, Mo ≥ 95%, with a proportion of

Al Low selectivity due to the

presence of Al³⁺ High alkaline leaching raffinate is

hard to recycle [33, 45, 79, 84]

Na₂CO₃ V, Mo ≥ 95% High selectivity Wastewater is easy to be treated [80, 81, 85]

Ammonium V, Mo nearly 90%, Ni ≥ 95% High selectivity High ammonia–nitrogen wastewater

may result in secondary pollution [86–89]

(11)

Nearly all valuable metals as well as Al₂O₃ carrier can be extracted from spent HDS catalyst through roasting-H₂SO₄ leaching, and the leaching efficiency of V, Mo, Ni, and Co are relatively high, nearly 95% [46, 76]. Further, in order to avoid high temperature roasting operation, direct H₂SO₄ leaching has been investigated. However, due to the presence of insoluble metal sulfides, sulfur, coke, and oil layer on the spent catalysts, the overall metal fleaching ratio was found to be low [95]. In this regard, the spent catalysts need to be treated by organic solvents or de-ionized water to remove the surface contaminants before leaching, and further it is necessary to use oxidizing agents during leaching to enhance the leaching efficiency [96]. The main reactions are listed as follows:

(27) CoO + H₂SO₄= CoSO₄+ H₂O

(28) NiO + H₂SO₄= NiSO₄+ H₂O

(29) V₂O₅+ H₂SO₄=(

VO₂)

2SO₄+ H₂O

(30) MoO₃+ H₂SO₄= MoO₂SO₄+ H₂O

(31) MoS₂+ 9H₂O₂= H₂MoO₄+ 2H₂SO₄+ 6H₂O

(32) 2V₅S₈+ 73H₂O₂=5(

VO₂)

2SO₄+ 11H₂SO₄+ 62H₂O (33) NiS + 4H₂O₂= NiSO₄+ 4H₂O

(34) CoS + 4H₂O₂= CoSO₄+ 4H₂O

Barik et al. [90] have examined the influence of five oxidants including H₂O₂, HNO₃, NaClO₃, FeCl₃, and NaOCl on the leaching ratio of metals. Their results suggested that when H₂O₂ was used as oxidant, the recovery ratio of Mo and Co could reach 99% and 96%, respectively, which was better than the other oxidants. Similarly, Pradhan et al. [29] also used H₂SO₄-H₂O₂ solutions to recover V and Ni, and the leaching efficiencies of V and Ni were more than 90%, respectively.

However, leaching with H₂O₂ is quite expensive, and thus, Mishra et al. [31] have proposed a method using oxygen as the oxidant, and more than 95% of V, Ni, and Mo are reported to be recovered under the optimal conditions. Due to the high leaching efficiency and less energy consumption, direct oxidative leaching appears to be an attractive approach to treat the spent HDS catalysts, on the condition that cheap oxidants can be utilized.

Hydrochloric acid: Alternatively, HCl has also been uti- lized to extract valuable metals due to its strong complexing capability to metal ions [92, 97], and the formation of chlorine complexes will facilitate the separation of metals. It is reported that Mo (VI) and V (V) in the HCl solution exist as anionic complexes such as MoO₂Cl₃⁻ and VO₂Cl [74]. Meanwhile, Ni (II), Co (II) and Al (III) ions in HCl solution exist as cationic complexes such as Ni (H₂O)₆²⁺, Ni (H₂O)₅Cl⁺, CoCl₄²⁻, and [Al (H₂O)₆]³⁺ [98, 99]. The main reactions during HCl leaching can be represented as reactions (35) to (39).

(35) 2H⁺+ 3Cl⁻+ MoO₃= MoO₂Cl⁻₃ + H₂O

(36) 2H⁺+ 2Cl⁻+ V₂O₅ = 2VO₂Cl + H₂O

Table 7 Leaching conditions and valuable metal recovery by acid leaching

Process Condition Leaching ratio (%) Ref.

H₂SO₄ leaching Leaching with H₂SO₄ at 80 ^oC Co:98, Mo: 98, Al: 85 [46]

Leaching with H₂SO₄ at 90 ^oC Co: 93, Ni: 90 [76]

Leaching with H₂SO₄&H₂O₂ at 50 °C Mo: 99.87

Co: 96.25 [90]

Leaching with H₂SO₄&H₂O₂ at 30 °C Ni: 90

V: 90 [29]

Leaching with H₂SO₄ at room temperature V: 95, Ni: 95

Mo: 99 [31]

HCl leaching Leaching with HCl at 90 °C Mo: 97

Co: 94 [91]

Leaching with HCl at 90 °C Mo: 96

Co: 93 [92]

Mixed acid leaching Roasting at 300 °C, leaching with HNO₃/H₂SO₄/HCl at 70 ^oC Mo: 90, Ni: 99

V: 99 [37]

Organic acid leaching Roasting at 500 °C, microwave leaching with EDTA Ni: 80 [93]

Leaching with (COOH)₂ at 50 °C V: 91 [94]

(12)

HCl leaching can realize comprehensive recovery of valuable metals from the spent catalysts, and under optimum conditions, about 95% of metals including V, Mo, Co, and Ni can be dissolved [91, 92, 100]. Nevertheless, in comparison of H₂SO₄, HCl solution is more corrosive, and the leaching raffinate is much more difficult to recirculate. In addition, the emission of chlorine gas during the treatment of wastewater is quite challenging in industrial practice.

Mixed acid: Furthermore, mixed acid solutions have also been tested [101]. For example, an acidic solution consisting of concentrated HNO₃/H₂SO₄/HCl (ratio 2:1:1, vol%) was adopted as lixiviant to recover Mo, V, and Ni and leaching efficiency of Mo, V, and Ni reached 90%, 99%, and 99%, respectively [37]. In this experiment, spent catalyst was roasted at 300 °C, only to remove residual oil, sulfur, and carbon contaminants. During leaching, HNO₃ functions as oxidant, and moreover the introduction of HCl leads to the formation of ion complexes and improves the leaching efficiencies. Mixed acids leaching integrates the advantages of different mineral acids, whereas introduces substantial amounts of anions, creating significant difficulties to the wastewater treatment. Undoubt- edly, mineral acids leaching is capable of recovering nearly all valuable metals with high efficiency, but this technique has the major drawback of using large quantities of reagents during leaching, resulting in significant burden for wastewater treatments

2.3.3.2 Organic acids It is clear that mineral acids are efficient in leaching valuable metals from the spent catalysts, however, incapable of realizing selectively leaching. Further, strong acids solutions are corrosive, and costly in equipment investment and maintenance. Thus, organic acids such as oxalic acid, citric acid, salicylic acid, lactic acid, and phthalic acid, which can extract metals selectively under much milder (37) 6H⁺+ Al₂O₃+ 9H₂O = 2[

Al( H₂O)

6

]3+

(38) 2H⁺+ Cl⁻+ NiO + 4H₂O = Ni(

H₂O)

5Cl⁺

(39) 2H⁺+ 4Cl⁻+ CoO = CoCl²⁻₄ + H₂O

conditions have been investigated [102]. The main reactions using oxalic acid leaching are depicted as follows:

For example, 2 wt.% oxalic acid solution was chosen as lixiviant to recover V and the leaching efficiency was reported to be 91% [94]. In order to enhance the leaching efficiency using organic acid, oxidants such as Fe (NO₃)₃ and H₂O₂ have been utilized [27, 103]. It is reported that H₂O₂ can convert low-valence metal sulfides to high-valence ions in acidic environments, facilitating the formation of soluble complexes in the oxalate ions leaching solution, and the synergy of complexing and oxidizing reactions intensi- fies metals leaching [104]. In another study, Le et al. [105]

compared the efficiency of five organic acids for recovering valuable metals from a spent petroleum catalyst, and the results suggested that V and Mo could be extracted efficiently and selectively using oxalic acid. The power of organic acids is in the order of oxalic acid, tartaric acid, citric acid, maleic acid, and ascorbic acid. Besides, microwave-assisted leaching using ethylenediaminetetraacetic acid (EDTA) has been investigated, and the results indicated that a superior selectivity of Ni over Al and Mo could be achieved by employing EDTA solution [8, 93].

2.3.4 Bioleaching

Study on bioleaching of spent catalysts commences in 1993.

In comparison with the conventional pyrometallurgy and hydrometallurgy process, this method has the advantages of lower capital cost and with low-energy consumption, higher heavy metal removal efficiency, and emission safety [106–111]. The typical microorganism used in bioleaching is summarized in Table 8.

(40) MoO₃+ H₂C₂O₄= MoO₂C₂O₄+ H₂O

(41) V₂O₅+ 2H₂C₂O₄= 2HVO₂C₂O₄+ H₂O

(42) Al₂O₃+ 4H₂C₂O₄= 2HAl(

C₂O₄)

2+ 3H₂O

(43) NiO + H₂C₂O₄= NiC₂O₄↓ + H₂O

Table 8 The typical bacteria and fungi used in bioleaching of the spent HDS catalysts

Microorganism Leaching efficiency (%) Time of incubation Ref.

Al/Mo/Ni adapted aspergillus niger Al: 68.1, Mo: 86.2, Ni: 78.2 30 days [112]

Penicillium simplicium Al: 16.6, Mo: 92.7, Ni: 66.43 30 days [113]

Acidithiobacillus ferrooxidans Al: 63, Mo: 84, Ni: 99, Co: 84 30 days [114]

Acidithiobacillus thiooxidans Al: 2.4, Mo: 95, Ni: 16, Co: 83 30 days [114]

Chemolithotrophic sulfur oxidizing bacteria V: 95.8, Mo: 21.5, Ni: 89.5 7 days [115]

Adapted bacteria culture V: 95, Ni: 95 10 days [116]