Recovery of rare earths from spent FCC catalysts by solvent extraction using saponified 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA)

JOURNAL OF RARE EARTHS, Vol. 35, No. 7, Jul. 2017, P. 716

Recovery of rare earths from spent FCC catalysts by solvent extraction using saponified 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) YE Sishi (叶思施), JING Yu (靖 宇), WANG Yundong (王运东)*, FEI Weiyang (费维扬) (The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China) Received 25 September 2016; revised 24 January 2017

Abstract: A process to recover rare earth (RE) metals from spent fluid catalytic cracking (FCC) catalysts by solvent extraction was studied, using saponified 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA or P507). The recovery process involved three steps: (1) leaching REs (mainly lanthanum and cerium); (2) solvent extraction by applying saponified P507-kerosene system; (3) stripping. Experiments to assure optimal operating conditions were conducted. Results indicated that RE metals could be recovered effectively from spent catalyst with saponified P507-kerosene-HCl system. At room temperature of 25 ºC, 10 g spent catalyst with 110 mL of HCl (1 mol/L) could achieve a leaching efficiency of 85%. For extraction, initial pH value of 3.17, organic/aqueous ratio (O/A ratio) of 2:1 with an extractants’ saponification rate of 20% could obtain 100% efficiency. In the stripping process, 1 mol/L HCl with O/A ratio of 1:1 led to a stripping efficiency of 96%. In the present study, RE metals from spent FCC catalysts were effectively recovered, which avoided wasting a large amount of RE resources. It provides a theoretical support for commercial recycling of RE resources.  Keywords: recovery; rare earths; fluid catalytic cracking catalysts; saponified P507-kerosene system

In the early 1960s, the zeolite-based cracking catalysts set off a boom in the petroleum refining industry with significant breakthrough in fluid catalytic cracking (FCC) process[1]. FCC process refers to industrial conversion of crude oil into other more valuable fuel products in the presence of catalysts[2,3]. There are mainly two kinds of catalysts, one is amorphous aluminum silicate, the other one is molecular sieve. Zeolites are typical molecular sieves, which can be divided into A type, X type, Y type zeolite, according to different ratios of silicon and aluminum. Among those zeolites, Y type zeolites are one of the most prominent catalysts[4,5], due to its relatively high activity, selectivity and stability, as well as high poison resistance[6,7]. To improve their performance to a larger extent, explorations of additives modifications on Y zeolites have been made frequently in recent years[8–11]. Researchers have found that a small amount of rare earth elements can significantly stabilize the structure of the catalysts and enhance its thermal stability and regeneration performance[1,12]. Contemporarily, the enhancement behavior of different rare earth elements is investigated in various situations by developing deep understandings into the intrinsic transformations of structures and the influence of active acidity sites in FCC process[1,13–19].

However, the catalytic activity and efficiency would be lowered due to the accumulation of many hazardous substances, such as heavy metals and carbon residuals. The worldwide supply of FCC catalysts is estimated at about 840000 t with an anticipated annual increase of 5%[3,20], and ~200000 t/year of spent catalysts residues will emerge within a few years. With the abundant applications of rare earths containing FCC catalysts, disposal of spent catalysts and recycle of rare earth metals from spent catalysts[2,21–25] have been confronted with great challenges, especially severe environmental issues. Among lots of separation and disposal approaches, solvent extraction is continuously favored and appreciated for its high capacity, rapid reaction time and high efficiency[26–28]. As one of the most popular extractants, 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA or P507) has been widely adopted in rare earths solvent extraction for its good separation performance[29]. The hydrometallurgical reclaiming procedures of rare earths from spentmaterials have been evaluated and described many times in literatures and patents. Emphasis was placed on the reclamation of rare earths by solvent extraction using P507 as extractant. Niu et al.[3] compared the performance of P507 with saponified P507 through the process of separating erbium (Er) from Ca

Foundation item: Project supported by the National Key Basic Research Program of China (2012CBA01203) and the Specialized Research Fund for Doctoral Programme of Higher Education of MOE of China (20130002110018) in the State Key Laboratory of Chemical Engineering of Tsinghua University * Corresponding author: WANG Yundong (E-mail: [email protected]; Tel.: +86-10-62794448) DOI: 10.1016/S1002-0721(17)60968-2

YE Sishi et al., Recovery of rare earths from spent FCC catalysts by solvent extraction using saponified 2-ethylhexyl …

and Fe in HNO3 solution[30], and Liu et al.[31] applied saponified P507-HCl-kerosene system to separate praseodymium (Pr) and neodymium (Nd) in membrane dispersion micro extractor. Fontana et al.[32] studied the extraction of middle rare earths from chloride media by P507-kerosene system. Fu and Tanaka[33] investigated the extraction equilibrium of yttrium (Y) and europium (Eu), which was extracted by P507 in solvent Shellsol D70 under non-ideal condition. Huang et al.[34] added P507 into di-(2-ethylhexyl)phosphoric acid (P204) to estimate the isolation between neodymium (Nd) and samarium (Sm), taking advantages of the synergistic extraction system. Moreover, there are plenty of other literatures describing the solvent extraction process that separated rare earths via adopting P507 or synergistic system containing P507 as extractant[35–39]. Meanwhile, preliminary explorations have been carried out to collect rare earths from spent fluid cracking catalysts. Yuan et al.[40] inspected the recycling performance of P507 when it extracted rare earths from spent FCC catalysts, and explored optimal leaching conditions. With respect to the optimal extraction conditions, He and Meng[41] found that adjusting pH of leaching liquid, increasing O/A ratio, reducing the concentration of P507 and prolonging extraction time were efficient ways to ameliorate extraction efficiency. Nevertheless, the hydrometallurgical investigations for recovering rare earths from scrapped FCC catalysts by P507 were quite insufficient. Moreover, saponified P507 system has not yet been explored to recover rare earths from spent FCC catalysts sufficiently. With regard to the operation conditions, less optimization was undertaken for the recovery process, such as generally high level of acidic concentration. The aim of this paper was to employ saponified P507 to recover rare earths (mainly lanthanum and cerium) from spent FCC catalysts by solvent extraction. Subsequently, the research focused on probing the optimal operating conditions. Attentions would be drawn to our study, which was concentrated on the performance of the saponified P507-kerosene-hydrochloric acid system and maximizing the recovery efficiency.

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1 Experimental 1.1 Reagents and apparatus The spent FCC catalysts were provided by Yongping refinery which attached to Yanchang Petroleum (group) Refining & Petrochemical Company. Hydrochloric acid solution (36 wt.%–38 wt.%) was purchased from Beijing Chemical Works. Ammonia solution (25.0 wt.%–28.0 wt.%) was purchased from Beijing Tongguang Fine Chemicals Company. The diluent, sulphonated kerosene was purchased from Hubei Prosperity Galaxy Chemical Co., Ltd., and the acidic extractant, EHEHPA (>95%) was purchased from Luoyang Aoda Chemical Co. Ltd. All the reagents were used without any further purification. The aqueous phase was obtained by dissolving the spent catalysts inhydrochloric acid solution. The organic phase was prepared by mixing saponified EHEHPA with sulphonated kerosene. EHEHPA was saponified with aqueous ammonia. Organic and aqueous phase mixtures were stirred and equilibrated in a thermostatic incubator shaker (HZ-9212S) at temperature of 25±0.1 ºC, which was purchased from Taicang Science and Education Instrument Plant. The composition of spent catalysts was determined by the X-ray fluorescence (XRF-1800, Shimadzu, Japan). The concentrations of metal ions, especially Fe, Na, Ca, La, Ce and Al, were detected by Inductively Coupled Plasma (IRISIntrepidII, Thermo, China). The pH value in aqueous phase was measured by pH meter (Five Easy, Metler) with a Sanxin electrode (200-C). 1.2 Recovery process Schematic diagram for the recovery process is illustrated in Fig. 1. Three important steps were involved: (1) leaching REs (mainly lanthanum and cerium); (2) solvent extraction by applying saponified P507-kerosene system; (3) stripping. 1.2.1 Leaching procedure Selecting appropriate acid system as leaching agent was crucial. Strong oxidizing acid, such as nitric acid, was excluded because cerium (Ce) could be easily oxidized, resulting in hydrolysis and precipitation. Sulfuric acid, oxalic acid and carbonic acid were dismissed on

Fig. 1 Schematic diagram for recovery of rare earths from spent FCC catalysts

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account of generating precipitation, which would require secondary leaching and roasting to get aqueous solution. Therefore, hydrochloric acid was applied in this study as leaching agent. Leaching experiments were carried out as follows: an appropriate amount of spent FCC catalysts ranging from 5 to 10 g was impregnated in hydrochloric acid solution with stirring for 2–10 h. The aqueous phase, which contained rare earths, was collected. Further adjustment of pH by ammonia was necessary for extraction. Metal ions concentration was measured by using ICP. According to the conservation of mass, the leaching efficiency (L) was defined as follows: CL,ele × Vacid L= × 100% (1) mFCC × ωele where CL,ele refers to the concentration of different ions in the aqueous phase (mg/L), Vacid denotes volume of the acid consumed in leaching section (L), mFCC denotes the mass of spent FCC catalysts (mg), and ωele denotes the mass fraction of specified element (%). 1.2.2 Extraction equilibrium procedure In the extraction process, aqueous solution containing RE metals was mixed with the organic solution containing mixed extractants in a 250 mL conical flask, which was immersed in a thermostatic incubator shaker at temperature of 25±0.1 ºC for 1–2 h. The mixture was then settled for phase separation. Subsequently, the organic and aqueous phases were collected for analysis, respectively. Attention should be paid to control the operation conditions, specifically equilibrium time, saponification rate, aqueous solution pH and O/A ratio. The extraction efficiency (E) was defined as follows: E=

CO,ele × VO C A,ele × VA

× 100%

(2)

where CO,ele and CA,ele refer to the concentrations of different elements in the organic phase and aqueous phase (mg/L), respectively. VO and VA are volume of the extract phase and the corresponding aqueous phase (L), respectively. Concentration of the rare earth elements in organic phase was obtained through a conservation equation as follows: CO,ele × VO =CL,ele × VL − CR,ele × VR (3) where CR,ele refers to the concentration of different elements in residual aqueous liquid (mg/L), VL and VR are volumes of the leaching solution and the corresponding residual aqueous liquid (L), respectively. 1.2.3 Stripping equilibrium procedure To strip rare earths from organic phase, aqueous hydrochloric acid solution and organic phase were kept shaking for 1–2 h at temperature of 25±0.1 ºC. Effects of equilibrium time, O/A ratio and acidity on the stripping system were evaluated. Similarly, the stripping efficiency

(S) was calculated as follows: S=

CS,ele × VS CO,ele × VO

× 100%

(4)

where CS,ele represent the concentration of different elements in aqueous phase; VS represents volume of the aqueous liquid.

2 Results and discussion 2.1 Spent FCC catalysts The composition of the spent FCC catalyst from Yongping Refinery is shown in Table 1. Its main components were Al (23.55%), RE (rare earth elements) (3.27%), Fe (0.55%), Ca (0.28%) and Na (0.11%). The remaining 69% content was silicon. Though La and Ce were discussed separately, the total data (RE) were given as a whole to offer an intuitive understanding for recovery efficiency. 2.2 Preliminary leaching of the spent FCC catalysts with hydrochloric acid 2.2.1 Capacity and acidity Since the acidity and the capacity would affect the leaching equilibrium, experiments were first conducted to determine the appropriate capacity of the raw material and optimal acidity within sufficient leaching time of 24 h, by comparing the leaching efficiency in different cases. In Table 2, 10 g catalysts with 110 mL 1 mol/L HCl could obtain the highest leaching efficiency. The leaching efficiency was slightly raised from 80.52% to 85.71% when the capacity increased from 5 to 10 g in 110 mL 1 mol/L HCl solution. When treating 10 g catalysts with 110 mL 2 mol/L HCl, the leaching efficiency was stable, ranging from 84% to 85%, which indicated that high concentration of acid was not necessary. High consumption and high concentration of acid would require higher requirements of equipment and operation, and more importantly, disposal of waste acid might heighten the already grave environmental issues. 2.2.2 Equilibrium time for leaching As the capacity and acidity were determined, the equilibrium time for leaching was easy to identify. Fig. 2 provides the leaching concentrations of ions, which varied along with time, helping to obtain the equilibrium time for leaching. The concentrations of La and Ce increased rapidly in the first 9 h, reaching to the maximum concentration of 11.79 and 13.67 mg/L, respectively. While, concentrations of other ions turned out to be stable after 2 h. The spent FCC catalysts should be Table 1 Chemical composition (wt.%) of the spent FCC catalyst Element

Fe

Na

Ca

La

Ce

Re

Al

Content

0.55

0.11

0.28

1.69

1.57

3.27

23.55

YE Sishi et al., Recovery of rare earths from spent FCC catalysts by solvent extraction using saponified 2-ethylhexyl …

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Table 2 Leaching efficiency of different ions in several operation conditions 10 g catalysts with 110 mL 1 mol/L HCl *

C/(mg/L)

**

10 g catalysts with 110 mL 2 mol/L HCl

5 g catalysts with 110 mL 1 mol/L HCl

L/%

C/(mg/L)

L/%

C/(mg/L)

L/%

16.1±0.32

0.76±0.038

15.22±0.76

0.41±0.07

16.29±2.78

1.32±0.045

100±3.41

1.31±0.073

100±5.57

1.47±0.03

100±2.04

3.8±0.2

100±5.26

4.09±0.17

100±4.16

4.72±0.23

100±4.87

Fe

0.8±0.016

Na Ca La

11.79±0.63

76.62±3.70

11.67±1.02

75.84±6.63

5.49±0.14

71.38±1.82

Ce

13.67±0.83

95.49±4.47

13.53±1.24

94.51±8.66

6.47±0.43

90.34±6.00

RE

25.46±0.73

85.71±2.05

25.2±1.13

84.84±3.80

11.96±0.28

80.52±1.88

Al

50.64±1.43

23.65±0.67

47.53±1.88

22.2±0.88

21.3±1.51

19.89±1.41

*

The volume of hydrochloric acid solution was fixed as 110 mL. C stands for concentration. **L stands for leaching efficiency

Fig. 2 Variation of metal concentrations with leaching time

treated with hydrochloric acid more than 9 h to make sure that rare earths were solubilized completely. Here, the leaching efficiency of rare earths was 85.71%, and leaching efficiency was the premise of recycling. 2.3 Recovery of rare earths by solvent extraction 2.3.1 Effect of leaching solution pH on extraction efficiency Influence of different pH of leaching solution on extraction efficiency was evaluated. Noteworthy, the extraction efficiencies of Al and Ce were higher than the other 4 kinds of ions in any pH values. As the value of pH increased from 2.17 to 3.17, the extraction efficiency of rare earths was enhanced distinctly (as shown in Fig. 3). The extraction efficiency of Al kept rising slightly from 85% to 100%, and remained stable when the value of pH was higher than 2.7. Apparently, Na was totally separated in organic phase. For Fe and Ca, their extraction efficiencies were ascended as the value of pH increased. The dependence of extraction efficiency of rare earths on the value of pH may be attributed to the changes of H+ concentration that released by the acidic extraction[42]. According to Eq. (5), the concentration of H+ was increased as the extraction proceeded, which would lower pH value of the system, as well as the extraction efficiency of RE. Appropriate adjustments of pH value would ease the reverse effect caused by increasing concentration of H+.

Fig. 3 Effect of leaching solution pH values on extraction efficiency for different ions

However, excess adjustments would result in precipitation, which might hinder the recovery process. M + nHR → MR n + nH n+

+

(5)

2.3.2 Effect of saponification rate on extraction efficiency To determine the important factors associated with saponification rate, experiments were carried out to identify the appropriate value, which would be varied in different systems. Table 3 shows that unsaponified extractants can hardly extract the desired rare earths. As the saponification rate increased from 0 to 20%, the extraction efficiency of rare earths was enhanced greatly from 9.67% to 100%. While, the extraction efficiency decreased to 47.67% when the saponification rate continued to increase to 30%. For Al, the tendency of its extraction efficiency behaved similarly to rare earths, Table 3 Effect of saponification rate on extraction efficiency (%) Metal

SR=0*

SR=10%

SR =20%

SR =30% 100±1.8

*

Fe

94.4±1.13

92.91±0.97

100±1.21

Na

0±1.33

0±1.03

0±2.31

0±2.31

Ca

3.95±0.91

0±1.24

66.03±1.24

15.54±1.35

La

0±1.34

11.61±0.069

100±0.071

24.6±1.69

Ce

18.34±0.037

47.11±0.13

100±0.046

68.12±0.7

RE

9.67±0.69

30.41±0.10

100±0.058

47.67±1.20

Al

17.48±0.42

49.0±0.91

100±1.01

81.06±1.17

SR stands for saponification rate

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JOURNAL OF RARE EARTHS, Vol. 35, No. 7, Jul. 2017

which reached to 100% when the value of saponification rate was 20%, especially similar to the value of Ce. Relatively, saponification rate showed little influence on Fe, whose extraction efficiency stayed higher than 90%. Satisfactorily, Na was absolutely not extracted. Otherwise, there was no regular pattern for Ca because of its irregular fluctuation with different saponification rates. To explain the necessity of saponification, the possible mechanism for solvent extraction of rare earths was discussed[42]. On account of the dimer form of P507, its molecular formula could be abbreviated as (HL)2. The most likely mechanism for solvent extraction of rare earths using saponified P507 with low acidity was cation exchange. Eqs. (6) and (7) illustrate the equilibrium reaction and the extraction equilibrium constant, respectively. According to Eq. (7), high concentration of H+ had adverse influence on solvent extraction that more H+ would obstruct the forward reaction. As shown in Eq. (8), after being saponified by ammonia, H+ in P507 was replaced by NH4+, the extraction process would not be impeded as the reaction proceeded. Whereas, high saponification rate would cause emulsification, which might disable extractant. Hence, the appropriate saponification rate could result in an excellent extraction process. 1 RE +3(HL) 2 ←⎯ → RE((HL) 2 )3 +3H

K

3+

+

(6)

+ 3

K1 =

[RE((HL) 2 ) 3 ][H ] 3+

2.3.4 Equilibrium time for extraction Experiment of extraction equilibrium was conducted with O/A ratio of 2:1, saponification rate of 20% and initial leaching solution pH value of 3.17. Fig. 4 shows the extraction efficiency of different ions as a function of time. The extraction efficiency of Al, La and Ce tended to be steady after 1 h, reaching to 100%. After 2 h, the extraction efficiencies of Fe and Ca were stabilized mainly on 100% and 77%, respectively. Fortunately, Na was completely separated as impurity. The kinetics for the extraction remain to be studied further. To achieve complete extraction, an equilibrium time for more than 1 h was needed. 2.4 Stripping procedure to enrich rare earths 2.4.1 Selection of acidity (concentration and O/A ratio) Considering the industrial application, it was necessary and important to select acidity, including its concentration and dosage. Clearly, Table 5 gives clues of suitable acidity. With O/A ratio of 0.5:1, 1 mol/L HCl was enough to recycle the rare earths by 91.97%. However, dispose of the excessive aqueous solution would be another big problem. For another case with O/A ratio of 1:1, the recovery efficiencies of La and Ce were slightly higher than the former case, which could reach to 96.57% and accompanied with less Al. When the concentration of HCl was increased from 1 to 2 mol/L with-

(7)

3

[RE ][(HL) 2 ]

NH 4 OH+HL → NH 4 L+H 2 O

(8)

2.3.3 Effect of O/A ratio on extraction efficiency Subsequently, influence of organic/aqueous (O/A) ratio on extraction capability was examined. Generally, O/A ratio should be selected on the premise that rare earths can be extracted totally, with low extraction rate of other impurities. As demonstrated in Table 4, the extraction efficiency of rare earth was improved from 12.27% to 100% as the O/A ratio kept rising from 0.5:1 to 2:1. Additionally, most ions (except Fe) had similar tendency with rare earths, and their extraction capability was enhanced as the ratio increased. In contrast, the extraction process for Fe seemed to decrease, with 91.19%, 89.65% and 85.95% through the rising of O/A ratio. Hence, O/A ratio of 2:1 was chosen as the optimal ratio.

Fig. 4 Effect of extraction time on extraction efficiency for different ions Table 5 Stripping efficiency of different ions with various acidity (%) S*/%

Table 4 Effect of O/A ratio on extraction efficiency (%)

1 mol/L HCl

1 mol/L HCl

2 mol/L HCl

1 mol/L HCl

(O/A ratio=0.5:1) (O/A ratio=1:1) (O/A ratio=1:1) (O/A ratio=2:1)

Metal

O/A ratio = 0.5:1

O/A ratio = 1:1

O/A ratio = 2:1

Fe

4.24±1.76

10.45±1.04

40.20±1.73

Fe

91.19±7.68

89.65±1.18

85.95±1.8

Na

0±1.01

0±1.37

0±2.46

0±1.29

Na

0±1.26

0±2.31

0±1.49

Ca

100±2.48

100±3.29

100±4.51

100±2.89

Ca

0±1.24

5.47±1.04

58.33±1.35

La

91.25±1.59

95.25±1.74

91.52±2.08

46.21±1.97

La

4.57±0.071

9.13±0.69

100±0.034

Ce

92.57±1.42

97.74±1.31

88.55±1.94

45.73±1.56

Ce

18.75±0.028

41.17±0.70

100±0.13

Re

91.97±1.51

96.57±1.52

89.91±2.01

45.95±1.76

RE

12.27±0.050

26.54±0.70

100±0.082

Al

89.74±1.17

48.31±0.89

88.56±1.39

25.32±0.88

Al

56.22±0.23

85.62±0.42

100±1.17

*

S stands for stripping efficiency

43.63±2.15

YE Sishi et al., Recovery of rare earths from spent FCC catalysts by solvent extraction using saponified 2-ethylhexyl …

out changing O/A ratio, the stripping efficiency was lowered to 89.91%, and the selectivity for Fe was obviously improved. While under O/A ratio of 2:1, the stripping efficiency of rare earths was sharply decreased to 45.95%, which might be caused by insufficient volume of aqueous phase. According to the simplified model raised by Wu et al.[43], it seemed to be reasonable that the stripping rate decreased with high HCl concentration. The simplified model is expressed as Eq. (9), where m and n are constant, xH and y are the concentrations of H+ and rare earths in aqueous phase, respectively. While strong acid and big dosage proposed a great challenge to post-processing, 1 mol/L HCl with O/A ration of 1:1 was favored as a balance. y = xH + mxH − n 3

(9)

2.4.2 Equilibrium time for stripping The optimal operational conditions were investigated to explore the kinetics for stripping process. Herein, Fig. 5 shows the equilibrium time for stripping. La and Ca reach to equilibrium after 1 h, giving recovery efficiencies of 95% and 100%, respectively. Soon after 2 h, the stripping efficiency of Ce was also stabilized at 97%. For Na was not extracted in the anterior section, there was no doubt that its recovery efficiency would be zero. Despite its high extraction efficiency, Fe was back-extracted with lower average recovery selectivity of 16% in stripping part. Moreover, Al seemed to have the similar stripping rule with Fe. The kinetics was complex; more investigations need to be adopted.

Fig. 5 Effect of stripping time on stripping efficiency for different ions

3 Conclusions In this paper, a study was conducted on the technology of recycling rare earths from used FCC catalysts, emphasizing on adopting saponified phosphoric acid. The investigation probed optimal operation conditions for leaching, extraction and stripping, respectively. Considering the preliminary leaching, a rational capacity of 10 g catalysts with 110 mL 1 mol/L HCl could receive a

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leaching efficiency of 85.71%. For the extraction, initial pH value of 3.17, O/A ratio of 2:1 with saponification rate of 20% were confirmed as the optimal conditions. Unsaponified P507 could hardly extract the desired rare earths (with extraction efficiency of nearly 10%), while 20% saponification of P507 could enhance the extraction efficiency greatly, resulting in extraction efficiency of 100%. The reason is that H+ dissociated by P507 had adverse effect on solvent extraction, while it could be replaced by NH4+ via saponification. Yet high saponification rate would cause emulsification, optimal saponification rate must be determined, where the value was 20% for our system. In the stripping section, the effect of O/A ratio on the recovery efficiency was studied. When the acid system was 1 mol/L HCl and the O/A ratio was 1:1, the stripping efficiency could be larger than 95%. Our experiments based on actual system gave clues for a deep understanding of rare earths extraction mechanism and approaches to dispose spent FCC catalyst.

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