1. Introduction

A catalyst is a chemical compound used on a wide range of reactions, such as those in the petrochemical industry, medicine, new energy, and the environment. Its reaction performance gradually reduces with the passage of time as it is used.¹⁾ Solid waste is created when a catalyst cannot be regenerated, and its activity cannot be recovered. In 1992, waste catalysts were listed with water wastes, oily sludge, waste chemicals, contaminated soil, and “other wastes” as refinery wastes by American Petroleum Institute (API).²⁾ The disposal and treatment of waste catalyst from refineries are governed by the Resource Conservation and Recovery Act (RCRA) and the Hazardous and Solid Waste Amendment (HSWA).^2,3)

Landfills are the usual disposal method for waste catalysts, but they must meet the harmless landfill standard for disposal.^2,3) Meanwhile, companies have discovered that waste catalysts can be used for metal recovery not only as a cheap source of precious metals, but also to reduce the environmental impact of their disposal.^1,3,4,5) By transferring metals from waste catalysts to solutions by leaching, such as acid leaching, alkaline leaching, ammonia and ammonium salt leaching, bioleaching, chlorination leaching, oxidative leaching, metals can be dissolved in solution.³⁾ The recovery of metals from solution can be achieved using a variety of techniques, including precipitation, adsorption, organic solvent extraction, and ion exchange.^4,5)

Inorganic acids are preferred in the treatment of waste catalysts because of their superior metal dissolution ability. HCl, HNO₃, and H₂SO₄ are three primary inorganic acids frequently utilized in metal recovery leaching processes.^4,5) Banda et al.⁶⁾ dissolved used hydrodesulfurization (HDS) catalysts with 3 M HCl. Research by Siemens et al.¹⁾ showed that 10 % (1.8 M) H₂SO₄ was capable of extracting more than 90 % of Mo and Ni from a spent NiMo/Al₂O₃ catalyst when the solution reached its boiling point. Sheik et al.⁷⁾ utilize HNO₃ to dissolve the spent catalyst. Additionally, acids are mixed to dissolve metals in waste catalysts, as demonstrated by Rabah et al.,⁸⁾ who used a 3 : 1 volume ratio of concentrated H₂SO₄ to HNO₃. Lai et al.⁹⁾ and Szymczycha-Madeja and Mulak¹⁰⁾ documented the effective application of acid solutions like HCl/HNO₃/H₂SO₄, HCl/HNO₃/H₂O₂, HNO₃/HF, and HF/HClO₄/HCl/H₃BO₃ in the leaching of metals from used HDS catalysts. In the process of recovering metals from waste NiMo/Al₂O₃ catalysts, oxidation roasting regeneration results in the NiMoAl mixture being represented as NiO, MoO₃, and Al₂O₃. The metal oxides are first dissolved by acid leaching and transformed into Ni, Al, and Mo ion species in the NiMoAl solution. After separating these metal species, pure metal compounds can be recovered chemically. The dissolution of MoO₃ in waste catalysts can result in H₂MoO₄ or Mo⁶⁺ species, such as MoO_xSO₄ and MoCl_y, depending on the leaching acid utilized.

Recently, the process of reclaiming molybdenum (Mo) from waste catalysts has attracted more interest.^{5,11, 12, 13, 14, 15)} Biswas et al.¹⁶⁾ employed chloride and water vapor roasting of the calcined catalyst for 2 h at 850 °C, followed by leaching with water, resulting in a recovery of 81.78 wt% Mo. Kar et al.¹⁷⁾ achieved a 92 wt% recovery rate of Mo by incorporating 12 wt% soda ash catalyst, calcining at 600 °C for 30 min, and gradually leaching in water. Tran¹⁸⁾ employs ionic liquid as both lixiviant and extractant for target metal ions, utilizing H₂O₂ as an oxidizing agent. Park et al.¹⁹⁾ employs an H₂SO₄ baking-leaching-solvent extraction method to extract metals from Mo-Ni/Al₂O₃. Valverde et al.²⁰⁾ employed a highly concentrated 88 % (16 M) H₂SO₄ solution to extract over 99 % of Mo, Ni, and Al from a NiMo/Al₂O₃ catalyst using a magnetic stirrer at 90 °C.

Due to Mo’s multivalence, researchers are also trying to separate its metal oxygen anion or oxygen anion polymer from other metals.^14,21) In the past, the method of using NiAl precipitation and the solubility of Mo oxygen anion polymer salts in an alkaline environment was applied to extract Mo from metal-containing solutions.^1,2) It was later discovered by researchers that Mo oxygen anions can be generated in an acidic environment, with Ni and Al ions remaining as cations. Certain organic compounds can form H⁺ combining species with acid, selectively interacting with Mo oxygen anions rather than Ni and Al cations. Organic solvent extraction minimizes the environmental impact of chemical reactions compared to traditional acid-base reactions.^21,22) Saberyan et al.¹¹⁾ and Trujillo and Freiser²³⁾ employ the commercial extractants Cyanex 301, P507, and LIX 63 to remove Mo(VI) from acidic solution.^11,21,23) Banda et al.⁶⁾ used hydrochloric acid to leach metals from waste catalysts and employed tri-n-octyl phosphorus oxide (TOPO) to extract Mo. Conversely, some studies found that tributyl phosphate (TBP) might be more effective at stripping than TOPO. This is due to TOPO’s electron-donating groups, in contrast to TBP’s electron-withdrawing groups, which can interact with anionic metal species solutions.^4,24)

Recently, China environmental protection departments have begun paying more attention to waste catalyst disposal. As well as easing the burden on importing metals from abroad every year, recovering metals from waste catalysts can help stabilize mining of domestic non-ferrous metals mines, resulting in environmental and energy savings. In order to replace the backward acid-base treatment with a more environmentally friendly method, our research group developed a more efficient process to extract metal Mo. An acidic dissolution medium was used to dissolve the waste catalyst to obtain NiMoAl solution. TBP, an organic solvent, was chosen as an extractant for Mo metal ions from the acidic dissolution medium. Ammonia was used to strip the organic extraction solution. Recyclable organic extractant of TBP is used as soon as the organic solution becomes colorless.

2. Experimental Procedure

2.1. Metal composition of waste hydrotreating catalyst

As depicted in Fig. 1, the waste hydrotreating (WHT) catalyst is a black solid, whereas the calcined WHT (CWHT) catalyst and its powder are slightly yellow-green, and the acid-leached sediment of CWHT is a gray solid. Table 1 presents the mass fractions of Ni, Mo, and Al, determined by X-ray fluorescence spectrometer (XRF) analysis of a CWHT catalyst from Panjin Xin’anyuan Chemical Industry Co., Ltd.

Fig. 1.

Images of (a) WHT catalyst, (b) CWHT catalyst, (c) CWHT catalyst powder, and (d) acid sediment of CWHT catalyst.

2.2. Metal recovery experiment

2.2.1. Sulfuric acid leaching

A 100 mL three-necked round-bottomed flask with a reflux condenser was placed in a water bath and stirred with a magnetic stirrer at 300 rpm during acid leaching experiments. In each test, 60 mL sulfuric acid solution was mixed with 6.0 gram of 120 mesh CWHT powder. Ni, Mo, and Al contents in filtrate were determined by ICP.

2.2.2. Organic extraction

In a conical flask, a certain volume organic solution was mixed with filtrate, and then the mixture was placed in a water bath. The mixture is decanted in a separating funnel, and after a 20-min settling period, the metal content in the filtrate is measured using ICP. The organic extracted metal content is calculated by subtracting the raffinate metal content from the filtrate metal content. In this study, TBP was used as an extractant to separate Mo anions from solutions containing Ni ions, Mo ions, and Al ions.

2.2.3. Stripping experiment

Mo was stripped from the organic solution after the organic extraction. A Mo-containing organic solution was mixed with 5 wt% (V/V) ammonia, shaken, and allowed to stand for 30 min. Phase transfer occurs when Mo is converted from organic to aqueous phase, resulting in lighter, colorless solutions. The content of Mo in basic inorganic phase solutions can be determined by an ICP technique.

2.3. Analysis and characterization

Ni, Mo and Al contents were determined using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) from Jena, Germany PQ9000. X-ray diffraction (XRD) patterns for both the CWHT catalyst and its leaching sediment were acquired using a Bruker D8 Advanced Power Diffraction System with Cu Ka radiation at 40 kV and 30 mA, covering a 2θ range of 10 to 80°. A Bruker L8TLGER XRF (Germany) was used to analyze the CWHT catalyst and CWHT catalyst leaching sediment. In each ICP analysis, a group of standard samples was utilized to ascertain the linear relationship between the peak area and the metal content, and the sample was interpolated to determine the precise metal concentration. To ensure the accuracy of metal content, each sample’s concentration was measured at least twice. Multiple measurements at the deviation point are needed to accurately determine the concentration when the measured concentration diverges from the curve.

3. Results and Discussion

3.1. Acid leaching

3.1.1. Effect of pH on metal ions in solution

WHT catalyst may result from the operation and regeneration of a hydrotreating catalyst for a long period of time. After calcination, the primary metals in CWHT catalysts are Ni, Mo, and Al from NiO, MoO₃, and Al₂O₃ supports, respectively. To recover valuable metal from CWHT catalyst, it is primarily executed by dissolving metals, separating metal ions, and purifying metal. Dissolving CWHT powder catalyst involves transferring metal compounds from solids to liquids and separating metal species involves removing designed metal ions from liquid mixtures. In purification, metals are reclaimed through phase transfers, such as precipitation from solutions to solids, or by first transferring metals from organic to inorganic solutions, and then transferring them from inorganic solutions to solids by evaporation or precipitation. It is necessary to understand how metal species behave under different pH conditions prior to dissolving CWHT powder catalyst in an acidic solution.

C_Ni⁰ represents the amount of Ni ion content per gram of waste catalyst to the volume of acid solution. CNiNiO represents the amount of Ni ion content from NiO precipitate. K_sp value for NiO is 1.6 × 10^-16. At low pH, where K < K_sp, NiO dissolves in acid, resulting in a Ni²⁺ concentration of C_Ni²⁺ near C_Ni⁰ and a Ni²⁺ species fraction approaching 1. Conversely, at high pH, where K > K_sp, NiO remains insoluble, leading to a C_Ni²⁺ near 0 and a Ni²⁺ species fraction close to 0.^25,26)

Fig. 2(a, b) show the change of Ni species fraction at different pH in the solution. According to Fig. 2(a), acid dissolution of NiO produces Ni²⁺ and Ni species derived from NiO(s), where

Fig. 2.

Change of Ni species fraction at different pH (a) NiO and Ni²⁺ in dissolving NiO (b) Ni(OH)₂ and Ni²⁺ in equilibrium of Ni(OH)₂ precipitation.

According to Fig. 2(b), equilibrium Ni(OH)₂ precipitation produces Ni²⁺ and Ni species derived from Ni(OH)₂(s), where

C_Ni⁰ represents the amount of Ni ion content per gram of waste catalyst to the volume of acid solution. C_Ni(OH)₂ represents the amount of Ni ion content from Ni(OH)₂ precipitate. K_sp value for Ni(OH)₂ is 2.0 × 10^-15. At low pH, where K < K_sp, Ni(OH)₂ precipitate is dissolved resulting in a Ni²⁺ concentration of C_Ni²⁺ near C_Ni⁰ and a Ni²⁺ species fraction approaching 1. Conversely, at high pH, where K > K_sp, Ni(OH)₂ precipitate remains insoluble, leading to a C_Ni²⁺ near 0 and a Ni²⁺ species fraction close to 0. As far as nickel species distributions are concerned, the results are in good agreement with the literature.^25,26)

Since the concentration of acid used in this experiment was usually greater than 2 M, the Ni²⁺ species fraction was close to 1, meaning the waste catalyst could be completely dissolved. Adjusting the pH did not lead to the reformation of NiO following catalyst dissolution, as the processes of NiO dissolution and formation are irreversible. NiO is generally produced by calcining nickel hydroxide rather than increasing the pH of the Ni ion solution. NiO may dissolve in the catalyst by the following reactions:

H₂SO₄ = 2H⁺ + SO₄^2-

NiO + H₂SO₄ = NiSO₄ + H₂O

NiSO₄ = Ni²⁺ + SO₄^2-

Fig. 3(a, b) show the effect of pH on the fraction of Al species in the solution. Acid dissolution of Al₂O₃ produces Al³⁺ and Al species derived from Al₂O₃, where

Fig. 3.

Calculation of the change of Al species ratio with pH at different pH (a) Al₂O₃ and Al³⁺ system (b) Al₂O₃ and Al³⁺ system.

C_Al⁰ represents the amount of Al ion content per gram of CWHT catalyst to the volume of acid solution. C_Al₂O₃ represents the amount of Al ion content from Al₂O₃ in CWHT catalyst. K_sp value for Al₂O₃ is 1.0 × 10^-10. At low pH, where K < K_sp, Al₂O₃ dissolves in acid, resulting in a Al³⁺ concentration of C_Al³⁺ near C_Al⁰ and a Al³⁺ species fraction approaching 1. Conversely, at high pH, where K > K_sp, Al₂O₃ remains insoluble, leading to a C_Al³⁺ near 0 and a Al³⁺ species fraction close to 0. For Al species fraction from Al(OH)₃ precipitation, where

C_Al⁰ represents the amount of Al ion content per gram of CWHT catalyst to the volume of acid solution. C_Al(OH)₃ represents the amount of Al ion content from Al(OH)₃ precipitate. K_sp value for Al(OH)₃ precipitate is 3.0 × 10^-34. When pH is low, K < K_spAl(OH)₃ and Al(OH)₃ precipitate is dissolved by solution. C_Al³⁺ is close to C_Al⁰ and fraction of Al³⁺ species is close to 1. When pH is high, K > K_spAl(OH)₃, Al(OH)₃ precipitate is insoluble in solution. C_Al³⁺ is close to 0 and fraction of Al³⁺ species is close to 0. Fig. 3(b) illustrates that Al(OH)₃ precipitate dissolves within a pH range of 2 to 4, aligning with previous studies. Similarly, Al₂O₃ can be formed by calcining its hydrogen oxide of Al(OH)₃ precipitate.^26,27) Aluminum in CWHT powder catalyst may dissolve through the following reactions:

H₂SO₄ = 2H⁺ + SO₄^2-

Al₂O₃ + H₂SO₄ = Al₂(SO₄)₃ + H₂O

Al₂(SO₄)₃ = 2Al³⁺ + 3SO₄^2-

Al³⁺ + 3OH^- = Al(OH)₃

As demonstrated in Fig. 3, Al(OH)₃ starts precipitating when the pH is over 3. This was further validated by our other experiments; hence extraction experiments were not performance at pH > 3.

Table 2 lists the possible Mo species present in solution. These include MoO₄^-, H₃MoO₄⁺, H₂MoO₄, HMoO₄^- and MoO₄^2-, Mo₇O₂₄^6-, Mo₈O₂₆^4-, Mo₇O₂₄^6-, H₂Mo₇O₂₄^6-, HMo₇O₂₄^6-, MoO₃ ‧ 2H₂O, and MoO₂²⁺ species. At high pH, MoO₄^- is the predominant Mo species.^{13,25,26,28,29,30)} H₃MoO₄⁺, H₂MoO₄, HMoO₄^- and MoO₄^2- are included in calculations of the H₂MoO₄ balance when pH changes.^13,28,29,30) Mo₇O₂₄^6-, Mo₈O₂₆^4- H₂Mo₇O₂₄^6-and HMo₇O₂₄^6- are frequently observed poly-Mo species, and their behavior is thus taken into consideration by researchers.^13,28) Some special Poly-Mo species Mo₇O₂₁(OH)₃^3-, Mo₇O₂₂(OH)₂^4-, and Mo₇O₂₃(OH)^5- with OH groups in the molecule may be stable at elevated pH levels and have been calculated by some researchers.^13,28) In some cases, MoO₃ and corresponding Mo species, MoO₃ and MoO₂⁺, have been included in calculations. NiMoO₄ (S) and AlMo₆O₂₁⁺ have been considered in calculations involving potential metal ion interactions among Ni, Mo, and Al in CWHT catalysts.^25,30) The real Mo species must be verified experimentally in different environmental conditions.

3.1.2. Effect of sulfuric acid concentration on metal leaching

Fig. 4 shows the effect of sulfuric acid concentration in solution on metal leaching. The dissolution time was set at 5 h, and sulfuric acid dissolution was conducted directly in 80 °C water baths. A condenser reflux was used to prevent water loss from the sulfuric acid solution, resulting in stable Ni and Mo concentrations. In the experiment, the sulfuric acid concentration varied from 2 mol/L to 18 mol/L. As shown in Fig. 2(a), NiO in CWHT catalyst is dissolved in acidic solutions and forms Ni²⁺ at low pH. Therefore, as can be seen in Fig. 3, the concentrations of Ni, Mo, and Al in the solution in the sulfuric acid solution are almost unchanged between 4.5 M and 9 M. As predicted in Fig. 1(a), NiO remains partially undissolved with increasing pH. The concentrations of Ni, Mo, and Al notably decrease when the sulfuric acid concentration is reduced from 3 M to 2 M. Furthermore, it was found that 18 M sulfuric acid did not produce the best acid solubility for the waste catalyst. This may be due to the fact that the dissociate hydrogen ions may not be sufficient in concerted sulfuric acid solutions without water.¹⁹⁾ Since the goal of this study is to investigate the proper sulfuric acid concentration for dissolving spent HT catalyst, higher concentrations like 12 M or 15 M were not attempted. Hence, the relationship between metal content (analyzed by ICP) in solution and the concentration of sulfuric acid is depicted with distinct bar graphs. In this study, a 6 M concentration of H₂SO₄ was selected in following dissolution experiments to ensure sufficient dissolution of Ni, Mo, and Al in sulfuric acid. Siemens et al.¹⁾ and Marafi and Stanislaus⁴⁾ found that H₂SO₄ leaching could recover more than 90 % of Mo. Valverde et al.²⁰⁾ employed a highly concentrated 88 % (16 M) H₂SO₄ solution to extract over 99 % of Mo, Ni, and Al from a NiMo/Al₂O₃ catalyst using a magnetic stirrer at 90 °C. This indicates that our experimental results are in line with both the theoretical calculations of the pH effect and previous studies.

Fig. 4.

Effect of sulfuric acid concentration on metal leaching.

3.1.3. Effect of dissolve time on metal leaching

Fig. 5 illustrates the impact of leaching time on metal composition, while keeping sulfuric acid concentration constant at 6 M and leaching temperature constant at 80 °C. As long as the acid dissolution time is longer than 2 h, the leaching concentration of metals in the waste catalyst will remain relatively stable. During the subsequent dissolution experiment, the metal leaching time was kept at 3 h to maintain a good metal leaching effect. Numerous studies have documented the condition of Mo ions and the extent of Mo’s transition from solid to liquid at various pH levels, although the leaching process typically takes at least 2 h.^{1,4,13,31,32)}

Fig. 5.

Effect of acid dissolution time on catalyst metal leaching.

3.1.4. Effect of XRD profiles on acid leaching sediment

Fig. 6 shows the XRD spectra of the CWHT catalyst and the CWHT catalyst leaching sediment. The XRD peaks of CWHT catalyst sediment at 2θ = 6.3, 10.3, 12.1, 15.9, 19.0, 20.7, 24.1, 27.5, 31.3, and 31.9 are consistent with characteristic peaks at SiO₂ (PDF-45-0112), indicating that the CWHT catalyst leaching sediment is dominated by SiO₂. SiO₂ content in CWHT catalyst is 11.2 wt% by XRF, which is close to the weight of CWHT catalyst sediment (about 14 wt%). For CWHT catalyst, alongside the characteristic peaks of SiO₂, the catalyst also exhibit peaks at 2θ = 19.5, 37.6, 45.9, 60.9, and 67.0, aligning with Al₂O₃ (PDF-10-0425). CWHT catalyst’s XRD profile presents a distinct peak at 2θ = 26.9 that does not match SiO₂ or Al₂O₃, and is likely to be due to aluminum silicate (PDF-29-0086). This means that acid dissolution remove γ-Al₂O₃ and aluminum silicate from CWHT catalyst and leave SiO₂ in CWHT catalyst leaching sediment. No peaks corresponding to Ni or Mo on CWHT catalysts were found in XRD characterization of CWHT catalyst, indicating that both species have not accumulated on CWHT catalysts. This is comparable to the hydrotreating catalyst that was somewhat spent and had undergone prior treatment.^19,27,33)

Fig. 6.

XRD profiles of CWHT catalyst and CWHT catalyst leaching sediment.

3.2. Organic extraction

3.2.1. Effect of pH in aqueous filtrate on organic extraction

Based on Fig. 7, Ni, Mo, and Al concentrations in aqueous raffine are affected by pH in aqueous filtrate. There was a 1 : 1 ratio of organic to aqueous (O/A) extraction volume, a 25 °C extraction temperature, and a 30 min extraction duration. In the presence of a change in pH from 1 to 3, the Ni and Al ions remain nearly unchanged during and after extraction. At pH 1.5, Mo ions are at a minimum concentration, approximately 95.7 wt% of Mo ions in the raffinate are transferred to the organic solution. The highest fraction of HMoO₄^- anion in Mo species may occur at this time, resulting in the lowest Mo concentration in aqueous raffinate. By this way, TBP can highly selectively extract Mo ions from Ni and Al species. This closely resembles the method of separating metal ions based on the solubility of various metals in organic solvents as described in the literature.^{4,13,21,27,31)}

Fig. 7.

Effect of pH on concentration of metal ions in filtrate.

As listed above, the dissolution of sulfuric acid in CWHT catalysts efficiently transfers NiMoAl metals from solid to liquid phase, resulting in a filtrate containing Ni, Mo, and Al.^4,14) Literature indicates that organic solvents selectively extract Mo because Mo in an inorganic solution reacts with these solvents to form Mo-containing organic compounds, which can be separated from the inorganic solution. Sato et al.³⁴⁾ discovered that Mo can be extracted as solvated complexes like MoO₂Cl₂ in a 6 N HCl solution. Mishra et al.³¹⁾ and Le and Lee³²⁾ have developed methods for selectively separating Mo species in hydrochloric acid leach solutions using TBP extractants. Mishra et al.³¹⁾ suggests that organic Mo species, specifically organic MoO₂L₂, can be formed when aqueous MoO₂²⁺ species combine with organic HL. Le and Lee³²⁾ indicated that the aqueous MoO₂⁺ species could combine with Cl- and an organic acid solution (HA) to produce organic MoO₂ Cl₂(HA) species. In their research, the extraction of Mo by extractants is attributed to the cation exchange reaction and lowering the pH value should enhance the extraction of Mo species.^14,23,31) However, this does not align with our findings in this study.

According to Sahu et al.,³⁵⁾ the adsorption of Mo species with organic solution extracts will reach an optimal pH when the Mo ion is in an anionic form. The strong ionic interaction was thought as anion HMoO₄^- combine with cationic R₃NH⁺ solution. The optimum pH of Mo extraction effect is because anionic HMoO₄^- species could have a maximum amount when pH is changed.¹³⁾ At lower or higher pH levels, H₂MoO₄ and Poly-Mo anions like Mo₇O₂₄^6- and Mo₈O₂₆^4- form in the solution, reducing the concentration of HMoO₄^- species as the pH changes.¹³⁾

Unlike organic amine solutions, TBP is an organic ester that can break down into an H₃PO₄ and organic alcohols in acidic or basic environments. Therefore, TBP ‧ H⁺ is not as stable as R₃NH⁺ or R₄N⁺ under acidic conditions. Felt suggested that acid anions and phosphorus-containing organic solvents can produce (HS_x)⁺(MXⁿ⁺¹)^- in acidic conditions, enabling inorganic hydrated acid radical anions to penetrate organic solvents.²²⁾ The related molecular expressions found in the literature are:

(HMX_n+1)_a + x (S)₀ = (HS_x)⁺(MXⁿ⁺¹)^-

where M is an n-valent cation, X is a monovalent anion, S is the trialkyl sulphide and the subscripts o and a denote the organic and aqueous phases. The negative acid anion in this study might be HMoO₄^-, and it is represented as follows:

H₂MoO₄ = H⁺ + HMoO₄^-

H₂MoO₄ + xTBP = (HTBP)⁺(HMoO₄^-)

Mo anions in an acidic environment can effectively bond with the solvent, allowing them to transition from the aqueous phase to the organic phase, thus facilitating the selective separation of Mo anions from Ni and Al cations in the aqueous phase. Based on the subsequent results, Ni and Al were not detected in the TBP extraction solution. At a certain pH, almost all Mo species can be removed from the solution. High purity Mo can still be separated from Ni and Al even when pH conditions are not optimal.

The diversity of Mo species reported in the literature is largely theoretical, which makes it challenging to confirm the actual number of MoO₂⁺ cations and MoO₄^- anions in the solution. According to previous report, in acidic conditions, both HMoO₄^- cation and MoO₂²⁺ anion can all exist, in this research, the presence Mo ion as HMoO₄^- anions can align well with the changes in Mo extraction rate and pH. The possible function associated with Mo species are listed as below:

For Mo cation in sulfate acid^14,19,34,36)

MoO₃ + 3H₂SO₄ → MoO₂SO₄ + 3H₂O + 2H₂MoO₄

MoO₂SO₄(aq) + 2TBP(org) → MoO₂SO₄ ‧ 2TBP(org)

For Mo anion in sulfate acid Mo species¹¹⁾

H₂SO₄ = 2H⁺+SO₄^2-

MoO₃ + 2H⁺+H₂O = H₂MoO₄

H₂MoO₄(aq) + 3TBP(org) → H₂MoO₄ ‧ 3TBP(org)

H₂MoO₄ = H⁺ + HMoO₄^-

HMoO₄^-(aq) + TBP(org) = [TBP ‧ H⁺] ∙ [HMoO₄^-]

3.2.2. Effect of TBP amount in organic phase on organic extraction

Due to the fact that an insufficient amount of TBP will result in incomplete extraction, Fig. 8 compares organic TBP to filtrate by volume with Mo extraction. N-Heptane without extraction ability was used to maintain the total volume of organic solution. A 1 : 1 ratio of organic to aqueous volume is maintained, the extraction temperature is 25 °C, the extraction time is 30 min, and the pH of the filtrate is 1.5. The ratio of TBP to filtrate is achieved by changing the volume of TBP in the organic solution. An extraction ratio greater than 0.8 between TBP and filtrate can extract more than 95 wt% of Mo. Research indicates that only when organic solvents are used in sufficient quantities can satisfactory extraction outcomes be achieved. Some non-extractable organic component, was employed to study the amount of organic solvents used.^14,37)

Fig. 8.

Effect of TBP amount in organic phase on organic extraction.

3.2.3. Effect of extraction temperature

Fig. 9 depicts the influence of extraction temperature on the metals content in raffinate, with conditions set at a 30-min extraction duration and a filtrate pH of 1.5. Increased temperature to more than 40 °C enhances the extraction of Mo ions from filtrate. The concentration of Mo ions left in raffinate is below 0.50 mg/L, and the extraction rate exceeds 98.8 wt%. The extraction selectivity varies due to the differing attraction levels of the extractant for various metal ions under different extraction conditions.^{4,5,22,23,31)}

Fig. 9.

Effect of extract temperature on metals content in raffinate.

3.2.4. Metal distribution isotherms

Theoretical insights suggest that an isotherm study can accomplish maximum metal extraction and minimal steps for a certain extractant under isotherm conditions. Fig. 10 shows the MC-Cabe-Thiele plot of Mo extraction from TBP. Pure TBP was used for the organic phase, while filtrate solution was used for the aqueous phase. The extraction reaction time was 30 min, and the extraction temperature was 25 °C. Mo species in organic phase can be determined by subtracting Mo species in aqueous raffinate from Mo species in aqueous filtrate solution by ICP. Through three stages of countercurrent extraction, the Mo concentration in leachate can be reduced to less than 0.24 mg/L, and the extraction rate can reach 99.4 wt%. The change in MC-Cabe-Thiele results follows a trend similar to that documented in the literature.^21,31,32,37)

Fig. 10.

Extraction MC-Cabe-Thiele plot of tributyl phosphate extracted Mo.

3.3. Stripping Mo species and regenerating TBP

Fig. 11 illustrates the impact of the ammonia to organic solvent ratio on the Mo content in the stripped aqueous solution. According to reports as pH increased, Mo species were considered as MoO₄^-. In this case, the binding ability of basic ions to Mo species is stronger than that of TBP, and Mo species can be transferred from organic phase to aqueous phase by stripping. A variety of stripping solutions are available, such as ammonia, ammonium carbonate, and sodium hydroxide. In this study, 5 wt% (V/V) aqueous ammonia was used as the stripping solution because it is relatively noncorrosive and the product is easy to be used in novel catalyst. As shown in Fig. 11, when the volume ratio of ammonia water to organic phase is 4 : 1, the stripping effect of ammonia water on Mo is relatively stable. Increasing the ammonia-to-water volume ratio slowly back-extracted Mo ions into the hydration liquid phase. The color of the organic phase extract gradually changed from yellow to colorless. As TBP is the only organic compound in this process that is water-insoluble, the colorless organic solution extracted from the aqueous ammonia solution is cycled back to the TBP extraction steps. Organic extraction frequently takes advantage of the different forms of metal ions at varying pH levels to selectively extract metals through their combination with extractants in acidic conditions. Facilitating the removal of metals from organic phases through alkaline conditions and promoting the reuse of organic solvents.^{4,5,11,13,32)}

Fig. 11.

Effect of different volume ratios on stripping metal ion concentration.

3.4. Process flow for extraction of Mo from waste catalyst using TBP

Fig. 12 is a flow diagram of the process of recovering Mo from WHT catalyst. The process involves calcination of WHT, dissolving the metal in CWHT catalyst with acid, extracting Mo with TBP, and stripping Mo species with ammonia. The catalyst is crushed before being exposed to a strong acid, resulting in a solid powder with a diameter under 125 microns. The CWHT catalyst powder was treated with 6 M sulfuric acid at a solid-to-liquid volume ratio of 1 : 10 at 80 °C for 3 h. Using filtration to distinguish CWHT catalyst leaching sediment with analogous SiO₂ content from the soluble NiMoAl filtrate. To optimize the separation of Ni and Al from Mo, the filtrate’s pH was adjusted to 1.5 to maximize HMoO₄^- species concentration, utilizing TBP’s electron-withdrawing properties to bind with Mo ions. A study was performed to identify the minimum TBP necessary for effective extraction and the optimal reaction temperature range. Utilizing the MC-Cabe-Thiele curve, a three-stage countercurrent extraction at 25 °C achieved a 99.6 wt% extraction efficiency, reducing the Mo concentration from 40 mg/L in filtrate to 0.24 mg/L in raffinate. With 5 % (V/V) ammonia, more than 98 wt% of Mo can be returned to aqueous solution in 4 : 1 proportion. During recycling, a colorless organic solution devoid of Mo can be reused in the organic TBP solution.

Fig. 12.

Process flow for extraction of Mo from waste catalyst.

Based on Table 3, sulfuric acid leaching process can transfer 94 wt% of Mo species from CWHT catalyst to the solution. Three times of countercurrent organic extraction with TBP can remove 99.6 wt% of Mo species from NiMoAl filtrate. 5 % (V/V) ammonia can recovery 98.30 wt% Mo from raffinate solution by stripping. And more than 90 wt% Mo in CWHT catalyst can be reclaimed through this process. In this study, sulfuric acid leaching and TBP extraction were applied to retrieve Mo from used catalysts. Considering the specific environment of use, process requirements, cost, and other factors, the types of reaction vessels, such as stainless steel, ceramics, special plastics, and steel lined with Teflon material that resist strong acids should be considered for future industrial scale development. Additionally, TBP serve as an organic ester, will face repeated acidic extraction and alkaline stripping environments. If TBP’s chemical stability is decreased by prolonged extraction and stripping, it will result in increased operational expenses and require a change of the extraction reagent.

4. Conclusion

A method for recovering Mo from CWHT catalysts was developed, involving sulfuric acid leaching, tributyl phosphate extraction, and ammonia stripping. The leaching of Mo at a temperature of 80 °C and a concentration of 6 M sulfuric acid results in 94.27 wt% of the metal being released. By utilizing the electron-withdrawing properties of TBP under acidic conditions as well as the effect of pH on Mo species, 98.8 wt% of the acid leaching residue was extracted for 30 min at 40 °C and pH 1.5. The MC-Cabe-Thiele theory predicts that Mo can be reduced to less than 0.2 wt% by three-stage countercurrent extraction. Stripping moves approximately 98 wt% of the Mo from the organic to the inorganic phases. Thus, re-created colorless organic TBP can be used in the recycled extraction process.