Ind. Eng. Chem. Res. 2010, 49, 11645–11651
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Recovery of Trace Rare Earths from High-Level Fe3+ and Al3+ Waste of Oil Shale Ash (Fe-Al-OSA) Hualing Yang,†,‡ Wei Wang,†,‡ Dongli Zhang,†,‡ Yuefeng Deng,† Hongming Cui,†,‡ Ji Chen,*,† and Deqian Li† State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P.R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P.R. China
An experimental investigation was undertaken to study the high-efficient and clean enrichment of trace rare earths from high-level Fe3+ and Al3+ waste of oil shale ash (Fe-Al-OSA). The optimum leaching temperature, ratio of solid to liquid, acidity, and reaction time for Fe-Al-OSA were 30 °C, 1:7, 50% (v/v), and 1 h by the Taguchi method, respectively, and the leaching rate of rare earths has been reached up to 96.24%. The optimal extraction conditions for removing Fe3+ from leaching liquor of Fe-Al-OSA were as follows: the organic phase was 30% N235 + 10% isooctyl alcohol (ROH) + 60% n-heptane, acidity of aqueous phase was about 3.00 mol/L, and phase ratio (Vo:Vw) was 8:5. More than 92.09% of Fe3+ was recovered by using countercurrent extraction process with 4-5 stages. The 99.76% of high pure Fe byproduct was obtained by stripping the loaded N235 organic phase, and it can be used as chemistry pure reagent directly. After adjusting the pH of the raffinate to 6.00 with MgO and saturated Na2CO3 solution, all of the Al3+ and rare earths were transformed to hydroxide precipitation and separated from the solution, together with other small amounts of coexisting metal ions such as Ca2+ and Mg2+. Then, the precipitation was washed, collected, and dissolved by HNO3. Rare earths can be separated from the solution by solvent extraction with 30% tributyl phosphate (TBP) +70% n-heptane at the phase ratio (Vo:Vw) 3:2. The recovery rate of rare earths from Fe-Al-OSA has reached 86.30% in the whole separation process. The residual Al3+ in solution was recycled. This work shows that such a treatment route is one kind of highly efficient and clean method for separating Fe3+ and Al3+, and concentrating rare earths from Fe-Al-OSA. The solid waste (Fe-Al-OSA) from the refinery can also be utilized effectively to solve the ecological and environmental problems caused by the waste heap. 1. Introduction Rare earth elements are finding increasing use in various industries as catalysts for petroleum cracking, metallurgy, glass and ceramics, electronics, chemicals, magnets, and nuclear industries, and in the manufacture of various types of phosphors, lamps, mantles, and many others.1 Rare earth minerals have been considered to be an important strategic resource. Rare earth ore deposit types are varied, such as Bayan Obo deposit, Mountain Pass carbonatite deposit, Mount Weld carbonatite deposit, Placer deposits, and ion adsorption clay.2 Rare earth minerals have some common characteristics: comprehensive rare earth elements (REE) pattern, high content, and easy extraction. So, traditional methods can easily obtain highly pure rare earth products from bastnaesite by means of mechanochemical treatment3,4 or by solvent extraction with mono(2-ethylhexyl)2ethylhexyl phosphonate (HEH(EHP)),5 from ion-absorbed type rare earth mineral by HAB two-solvent extraction system on the basis of HA (CA-12) and HB (another extractant),6 and from monazite by a solvent impregnated resin.7 To improve the comprehensive utilization and raise the additional value of rare earth mineral resources, Li has developed a new integrated hydrometallurgical technology for clean separation of cerium(IV) and fluorin from Panxi bastnaesite concentrate,8 and at the same time recovery of the radioactive element Th(IV).9 Many researchers have also been devoted to developing a more clean and efficient separation process for rare earths with ionic * To whom correspondence should be addressed. Tel.: +86-4318526-2646. E-mail:
[email protected] (J.C.). † Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.
liquids.10–13 Recently, the increasing demands of rare earths will exhaust resources in the near future. Exploitation of rare earth resources also involves environmental problems.14 Considering this situation, all governments have strengthened the regulation of import and export of rare earth products and especially emphasized safety and environmental issues of chemical engineering in recent years.15 Recovering rare earths from associated and secondary resources and waste is also an efficient method to release resource pressure.16 Therefore, the phosphorites associated with trace rare earths have been regarded as a kind of potential supplementary rare earth resource, and some new hydrometallurgical processes for extracting rare earths from apatite using solvent extraction were reported.17,18 Pietrelli developed a hydrometallurgical process to obtain mixed rare earths from spent NiMH batteries.19 Satio reported how to recover rare earths from the La-Ni alloys by the glass slag method or from sludges containing rare earth elements.20,21 Oil shale ash (OSA), an inorganic waste obtained after the direct combustion of the oil shale, is another exploitable secondary resource associated with several valuable metals, such as high content of Al, Fe, and trace rare earths.22 The tremendous amount of OSA solid waste is discarded occupying land and causing environmental problems which caused widespread concern;23,24 in addition, the cost of its disposal is also high. Therefore, a highly efficient and clean residual disposal technology with economical benefit is needed to treat dumped solid waste.25 Gan and Liu have discussed the recovery of nonferrous metal Al and element Si from OSA through acidic leaching and alkali precipitation methods.26 However, after the above pretreatment process for OSA, the remaining product is universally known as high-level Fe3+ and Al3+ waste of oil shale ash
10.1021/ie101115e 2010 American Chemical Society Published on Web 10/08/2010
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Scheme 1. Structure of Major Trialkyl Amine N235 Components
Table 1. Typical Metal Elements of Fe-OSA
content (%)
(Fe-Al-OSA), which contained 0.14% rare earth elements. From a distribution model of Fe-Al-OSA, the total rare earth storage resources in oil shale ore are very considerable in quantity, compared with the available rare earth deposits in the world. Therefore, highly efficient enrichment of rare earths and other coexisting valuable elements from OSA are a beneficial supplement for rare earth resources and comprehensive utilization of OSA solid wastes to reduce environmental pollution in oil shale sustainable development. Our work is to separate and recover rare earths from Fe-Al-OSA. When most of the Al3+ and SiO2 were removed by H2SO4 leaching and NaOH precipitation from the initial OSA samples, the Fe-Al-OSA still contained 41.32% Fe3+ and 4.42% Al3+, which were believed to be two main obstacles for trace of rare earth enrichment and were also valuable recyclable byproduct. The rare earth enrichment from Fe-Al-OSA was challenged due to the complex system of about 301 times of Fe/rare earths and 32 times of Al/rare earths. The unique component is different from ordinary rare earth ores, which contain high content of rare earths and trace of Al3+ and Fe3+.27 The traditional method for extracting rare earths from ores is not suitable for the Fe-Al-OSA system. In this paper, the study of highly efficient and clean enrichment of trace rare earths from Fe-Al-OSA was investigated. The main steps are the following: (1) leaching, (2) separation of Fe3+ from leaching solution with tertiary amine N235, (3) precipitation of Al3+ and trace rare earths with MgO and Na2CO3, and (4) separation of rare earths by solvent extraction with TBP and recovery of Al3+. 2. Experimental Section 2.1. Materials and Reagents. The Fe-Al-OSA samples used in this study were obtained from Professor Gan’s lab in Jilin University. The industrial grade trialkyl amine N235 was obtained from Dalian chemical plant without further purification. The active component of N235 is a mixture of saturated and straight chain trialkylamines with carbon chains C8 and C10, in which the proportion of the carbon chain C8 to C10 is about 2 to 1. It has a pKa of at least about 9.4.28 Commercial grade N235 (mass fraction >98%) is a light yellow liquid with an average molecular weight of 392 g/mol, a density of 0.815 g/cm3, and a viscosity of 10.4 × 10-3 Pa s. The solubility in water is less than 0.01 g/L at 25 °C, and the LD(50) is about 442 mg/kg. The chemical structure is shown as follows (Scheme 1): Other solvents used as diluent and phase modifier are n-heptane, tributyl phosphate (TBP), naphthenic acid (HA), and isooctyl alcohol (ROH), and inorganic salts were of analytical reagent grade. 2.2. Analysis. All RE3+ and Al3+ were analyzed by inductively coupled plasma (ICP-AES/MS) with an ion electrode. The acidity of the solution was detected by a PHS-3C pH meter (Rex Instrument Factory, Shanghai, China). The concentration of Fe3+ was determined by Shimadzu UVmini-1240 UV-vis spectrophotometer (Kyoto, Japan) with orthophenanthroline as chromogenic reagent. 2.3. Leaching Solution of Fe-Al-OSA. This industrial OSA was dissolved in concentrated H2SO4 at 80-105 °C after a preliminary treatment, and then the oil shale ash was used to prepare Al2O3 and SiO2 by adjusting the pH value.29 After Al2O3 and SiO2 were removed from the initial samples, Fe-Al-OSA mainly contains Fe(OH)3, Na2SO4, a small amount of Al3+, and
Fe
Ca
Al
Mg
Mn
Cu
RE
41.32
10.30
4.42
0.11
0.05
0.04
0.14
Table 2. Content of Rare Earths in Fe-OSA (ω/10-6)
content
content
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
272.7
416.6
55.70
223.9
45.11
9.023
42.95
6.136
Dy
Ho
Er
Tm
Yb
Lu
Y
34.41
6.858
20.21
2.767
1.925
2.887
207.4
4
Table 3. L9(3 ) Randomized Experimental Plan Table parameters and their levels experiment number 1 2 3 4 5 6 7 8 9
T (°C)
ratio of solid to liquid (g:mL)
acid conc (%)
reaction time (h)
RE yield (%)
20 20 20 30 30 30 40 40 40
1:6 1:7 1:8 1:6 1:7 1:8 1:6 1:7 1:8
30 40 50 40 50 30 50 30 40
0.5 1 1.5 1.5 0.5 1 1 1.5 0.5
58.22 89.00 89.54 84.25 94.80 87.53 87.74 90.77 88.75
a trace of RE(OH)3. Tables 1 and 2 show the contents of typical metal elements and rare earths in Fe-Al-OSA, respectively. The Fe-Al-OSA samples were washed several times to remove most of the Na2SO4 entrainment and then dried at 60 °C to obtain dry solid Fe-Al-OSA. The leaching experiment was carried out in Teflon vessels which attached to condenser tube to collect the volatile gases. In order to determine the influence of different leaching temperature, solid to liquid ratio, acid concentration and reaction time on the leaching rate, the Taguchi method was designed to obtain optimum leaching conditions. The Taguchi method is an effective method for improving the productivity at low cost.30 In the Taguchi method, factors which are thought likely to affect the process are arranged into an orthogonal array. An orthogonal array, L9(34), which denotes four parameters, all with three levels, is chosen, and every experiment is repeated twice under the same conditions to ensure the reliability of results. Table 3 represents the selected orthogonal array for this research. It was found that the maximum rate occurred under the following conditions: leaching temperature 30 °C, reaction time about 1 h, acidity 50% (v/v), and solid to liquid ratio value 1:7. To verify the predicted results, the experiment was carried out according to the optimal process conditions, and the leaching rate of rare earths has been reached at 96.24%. The total amount of rare earths in leaching solution was about 100-200 µg/mL, while the content of Fe3+ and Al3+ could reach 30.00-50.00 g/L and 5.00-10.00 g/L, respectively. 2.4. Experiment Procedure. The organic phase was composed of 30% N235 + 10% isooctyl alcohol (ROH) + 60% n-heptane, and the aqueous phase was the leaching solution of Fe-Al-OSA. Equilibrium distributions of Fe3+ between organic and aqueous phases under different conditions were determined by transferring the two phases into a 60 mL separating funnel and mixing with a mechanical shaker for 15 min at room temperature. After the two phases were separated completely, the concentration of Fe3+ in aqueous phase was analyzed by UV-vis spectrophotometer using an orthophenanthroline Fe2+ oxidation-reduction method, and the pH of the aqueous samples
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was measured by PHS-3C pH meter. The metal concentration in the organic phase for each sample was deduced by subtracting the raffinate concentration from the initial metal concentration in the feed solution. The E% of Fe3+ ion was defined as follows E% )
[M]t - [M]a × 100 [M]t
where [M]t and [M]a separately represent initial and final concentrations of Fe3+ ion in the aqueous phase After Fe3+ was extracted from the leaching solution, the pH of raffinate was adjusted to about 6.00 by adding MgO and saturated Na2CO3 to precipitate Al3+ and RE3+. The mixture was filtered, and the remains were washed by distilled water and then dried. The collected precipitation was dissolved by HNO3(aq), and trace RE3+ was recovered by solvent extraction with tributyl phosphate (TBP). At the same time, Al3+ was still retained in the solution and then was sent back to the initial material OSA for preparation of Al2O3.
Figure 1. Eeffect of different modifiers on Fe3+ extraction in 30% HCl(aq) (Vo:Vw ) 1:1; [N235] ) 30%(v/v); [Fe3+] ) 40.91 g/L; time ) 15 min).
3. Results and Discussion 3.1. Selection of Extractants for Separating the HighLevel Fe3+, Al3+, and Trace RE3+ from Leaching Solution of Fe-Al-OSA. In the leaching solution of Fe-AlOSA, Fe3+ and Al3+ contents are much higher than RE3+. The precipitation process is not suitable for the leaching solution, mainly because Fe(OH)3 colloid has strong adsorption along with other coexisting metal ions including RE3+. Solvent extraction is found to be a quite good choice to solve this problem with high selectivity, low energy consumption and less pollution, etc. Several types of extractants have been used to extract Fe3+ from hydrochloric acid solution, including carboxylic acid, amine, and acidic organophosphorus extractants. Carboxylic acid can be only used to remove Fe3+ at the very low concentration (below 4.00 g/L). Though acidic organophosphorus extractants (such as P204, P507, Cyanex272, etc.) can efficiently extract Fe3+ from a hydrochloric acid system,31 they can simultaneously extract RE3+ and lead to poor selectivity. Additionally, the loaded organic phase containing acidic organophosphorus extractant is not easy to strip. Amine extractants, especially tertiary amine, are a kind of efficient extractant for Fe3+ in a hydrochloric acid system. When Fe3+ is preferentially extracted by tertiary amine at lower concentration of chloride system, the coexisting RE3+ and Al3+ can not be extracted and still remained in the raffinate. Moreover, the loaded organic phase of tertiary amine is more easily stripped. Tertiary amine N235 was chosen to separate Fe3+ from the leaching solution of Fe-Al-OSA due to its high efficiency and good physical phenomena. The equations in N235 system can be explained as follows:32 R3N + HCI h R3NH+CI-
(1)
Fe3+ + 4CI- h FeCI4
(2)
+ R3NH+CI- + FeCI4 h R3NH FeCI4 + CI
(3)
On the basis of eq 1, N235 in the organic phase needs to be acidified with hydrochloric acid before it is used to extract Fe3+ from the feed solution. At the same time, the metal ions in the form of an anionic species with halide ion, like FeCl4- (the stability constant of Fe3+ and chloride ions are the following: log β1 ) 1.48, log β2 ) 2.13, log β3 ) 1.99, and log β4 ) 0.01) in eq 2, can be extracted into the protonated N235
Figure 2. Effect of N235 concentration on Fe3+ extraction in 30% HCl(aq) (Vo:Vw ) 1:1; [ROH] ) 10%(v/v); [Fe3+] ) 40.91 g/L; time ) 15 min).
(R3N · HCl) as in eq 3. At the low chloride ion concentration (10% v/v). Table 4 shows that the extraction efficiency of Fe3+ has increased to some extent with the concentration of HCl increasing in the first contact. In the second contact, the extraction efficiency is cut down also with the decreasing of acidity of the extract liquid. The main reason is that HCl can be extracted by N235. Therefore, the feed solution must choose high acidity to ensure highly efficient extraction even at the last stage. On the basis of the results, the acidity of
phase ratio (Vo:Vw)
distribution coefficient
extraction efficiency (%)
5:2 2:1 3:2 1:1 1:2
1.80 1.39 1.18 1.10 0.63
81.84 73.52 63.90 50.22 30.51
leaching liquor must stay about 2.00-3.00 mol/L after leaching Fe-Al-OSA in HCl(aq). 3.5. Effect of Phase Ratio (Vo:Vw) on Fe3+ Extraction. The phase ratio had a significant effect on distribution coefficient and extraction efficiency. Table 5 shows that the extraction efficiency and distribution coefficient of Fe3+ have been gradually reduced with the decrease of the phase ratio (Vo:Vw). Therefore, with the larger phase ratio (Vo:Vw), the Fe3+ extraction effect will be better. In the countercurrent extraction process, in order to ensure utilization rate of extractants, we usually choose the best phase ratio in which the extraction efficiency is above 50%. 3.6. Effect of Temperature. To study the effect of temperature on the extraction efficiency of Fe3+, the experiments were performed in the range 293.15-313.15 K. The result in Figure 4 shows that the extraction efficiency of Fe3+ increases with the temperature. From the straight line of log DFe versus 1000/T for the extraction of Fe3+ by N235 from 30% HCl(aq), the ∆H value is calculated using the van’t Hoff equation (eq 4), about 4.212 kJ/mol. log DFe )
-∆H +C 2.303RT
(4)
DFe is the distribution coefficient, ∆H is the enthalpy change for the extraction reaction, R is the universal gas constant (8.314 J/mol), and C is a constant for the system. On the basis of the value of ∆H, the corresponding free energy ∆G and entropy ∆S are 7.065 kJ/mol and -9.560J/mol K at 298.15 K using eqs 5 and 6, respectively. ∆G ) -2.303 RT log DFe
(5)
∆G ) ∆H - T∆S
(6)
The value of ∆H indicates that the extraction of Fe3+ in this system is an endothermic reaction. 3.7. Extraction of Al3+ and RE3+ with N235 in HCl (aq). Fe3+ has a strong tendency to form a large number of anionic metal complexes with chloride ion in HCl(aq), such as FeCl3
Figure 4. Relationship between DFe and temperature in the extraction process (organic phase: 30% N235 + 10% ROH + 60% n-heptane; Vo:Vw ) 1:1; C(HCl) ) 30%(v/v); [Fe3+] ) 40.91 g/L; time ) 15 min).
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 Table 6. Effect of Phase Ratio on the Fe
3+
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Stripping Efficiency
phase ratio first second third fourth total (Vo:Vw) stripping (%) stripping (%) stripping (%) stripping (%) stripping (%) 4:1 4:2 4:3 4:4
35.13 57.51 76.65 81.51
38.61 21.33 19.24 17.99
13.12 19.24
6.24
93.10 98.08 96.07 99.50
Table 7. Analysis Results of FeCl3 Crystals
content (%)
Fe3+
Al3+
Ca2+
Cu2+
Mg2+
SO42-
99.76
0.02
0.01
0.02
0.03
0.16
Table 8. Concentration of the Typical Component in Solution Figure 5. Extraction of AlCl3 and RECl3 with N235 in HCl(aq) (organic phase: 30% N235 + 10% isooctyl alcohol + 60% n-heptane; Vo:Vw ) 1:1; [Fe3+] ) 40.91 g/L, [Al3+] ) 4.80 g/L, [La3+] ) 65 µg/mL; [Y3+] ) 48 µg/mL; time ) 15 min).
concentration (mg/mL)
or FeCl4-,32 but the coexisting Al3+ and RE3+ still exist in the free state. So Fe3+ is easily extracted into the organic phase by protonated N235, while the coexisting Al3+ and RE3+ are still left in the raffinate. Figure 5 shows that the extraction efficiency of RE3+ with N235 is about 1% or even lower. It was considered that the content of RE3+ was changed a little, while Fe3+ was almost completely extracted. Therefore, it achieved the goal of separating Fe3+ from coexisting ions. In addition, a certain amount of coexisting AlCl3 in the aqueous phase could promote the increase of extraction efficiency of Fe3+ (Figure 5). The main reason would be that coexisting massive AlCl3 also offered massive chloride ions for Fe3+ to form FeCl4-. So, the coexisting AlCl3 in leaching solution of Fe-Al-OSA can improve extraction efficiency of Fe3+. 3.8. Saturated Extraction Capacity of N235 in 30% HCl (aq). The isothermal of Fe3+ extraction was measured by various liquid-liquid phase ratios. The saturation loading capacity of 30% N235 + 10% ROH + 60% n-heptane was also detected in 30% (v/v) HCl(aq) by using multiple contacts (Figure 6). The phase ratio (Vo:Vw) was established on the basis of the content of Fe3+ in leaching solution of Fe-Al-OSA, and the value of saturation extraction capacity was between 3:2 and 2:1. Through the McCabe-Thiele diagram, the theoretical stages of countercurrent extraction were calculated to be 4-6. In industrial operations, the extraction stages were usually increased by one or two stages on the basis of theoretical value. The results of extraction experiments using leaching solution of Fe-Al-OSA as aqueous phase show that the content of Fe3+ decreases from 45.60 g/L to 0.01 g/L after five-stage semicontinuous countercurrent extraction, when the extraction phase ratio is 8:5 (Vo: Vw). The extraction efficiency of Fe3+ is more than 98.91% from leaching solution of Fe-Al-OSA.
Ca
Fe
Mg
Mn
Cu
RE
56.43
0.542
0.196
0.648
1.056
0.078
0.706
3.9. Stripping of Fe3+ from Loaded Organic Phase. After Fe is extracted into the organic phase by N235, the loaded organic phase is usually stripped with distilled water. For the N235 system, it has some other advantages, such as no emulsion and easy stripping, besides high extraction capacity. The dependence of Fe3+ stripping efficiency on phase ratio is shown in Table 6, and the stripping efficiency gradually increases with the aqueous volume. However, the increase of the strip liquor will lead to a heavy burden on industrial equipment, as well as an increase in the water consumption, and will reduce the concentration of FeCl3 product. So, a phase ratio (Vo:Vw) of 4:1 should be the best choice in the industrial process. When the loaded organic phase is stripped at phase ratio (Vo: Vw) 4:1, the total stripping percentage has been reached at 93.10% after four stripping stages and the FeCl3 strip liquor is reclaimed as highly concentrated byproduct, while regenerating the organic phase for recycling. 3.10. Analysis Result of FeCl3 Byproduct. Under optimized conditions, over 92.09% of Fe3+ has been reclaimed from the leaching solution of Fe-Al-OSA using countercurrent extraction. The strip liquor was evaporated with vacuum distillation and received highly concentrated FeCl3 crystals. Table 7 shows the analysis results of FeCl3 crystals: the purity reaches 99.76%. 3.11. Enrichment of Trace RE3+ and Recovery of Al3+. After Fe3+ in the leaching solution of Fe-Al-OSA was extracted by N235, Al3+ and RE3+ were still left in raffinate, and HCl in the leaching solution of Fe-Al-OSA was also extracted in 4-5 stages of the countercurrent extraction process. Then, by adjusting the pH of the raffinate to about 6.00 and adding relatively cheap MgO and saturated Na2CO3, we transformed all of Al3+ and RE3+ to hydroxide precipitate together with the other small amounts of coexisting metal ions such as Ca2+ and Mg2+. The collected precipitation was washed with deionized water several times to remove entrainment Ca2+ and Mg2+. The mass fraction of hydroxide precipitation only accounted for 16.31% of the raw Fe-Al-OSA material, to achieve preliminary enrichment. Then, the precipitate was dissolved by HNO3(aq); the concentration of the typical components in solution is shown in Table 8. As an organophosphorus ester, TBP was chosen to extract trace RE3+ from high Al(NO3)3 of the Fe-Al-OSA/HNO3 system by one-step extraction. The presence of high Al(NO3)3 in the Fe-Al-OSA/HNO3 system could be used as a saltingout agent to enhance RE3+ extractability, and other impurities, such as Al3+, Ca2+, and Fe3+, could not be extracted by TBP. This can guarantee the purity of rare earth products. Due to the competitive extraction between HNO3 and RE3+ by TBP,37 the influence of HNO3 concentration on extraction 3+
Figure 6. McCable-Thiele diagram of Fe3+ extraction in 30% HCl(aq) with 30% N235 + 10% isooctyl alcohol + 60% n-heptane.
Al
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Figure 7. Effect of acidity on RE3+ extraction (Vo:Vw )1:1, organic phase: 30% TBP + 70% n-heptane, [RE3+] ) 0.71 mg/L; time ) 15 min). Table 9. Analysis Results of REO Elements
RE
Fe
Al
Ca
Mg
Mn
Cu
Content(%)
94.21
0.08
0.73
1.41
0.50
0.41
0.78
efficiency is complicated. The extraction percentage increased with the decrease of feed acidity, as shown in Figure 7. So the acidity of feed should be controlled above pH 0.50. At the phase ratio (Vo:Vw) 3:2, 30% TBP + 70% n-heptane extracted RE3+ from the feed and other coexisting metal ions were still left in the raffinate. Therefore, Al3+ and trace RE3+ in HNO3 feed can be almost completely separated, to achieve preliminary enrichment. The RE3+ was enriched by stripping load organic phase with deionized water, after the load organic phase was scrubbing. A recovery of 86.30% was obtained for rare earths in the entire recycling process. Table 9 shows the analysis results of REO; the purity of REO can be 94.21%. Al3+ in the solution can be returned to initial material OSA for preparation of Al2O3. 4. Conclusions The highly efficient clean process develops for the separation and recovery of rare earths, from Fe-Al-OSA which consists of the following four major unit operations: first, leaching of Fe-Al-OSA with HCl (aq); second, separation of higher content of Fe3+ by solvent extraction with N235; third, selective precipitation of rare earths and Al3+ as hydroxide and separation from impurities; finally, separation of RE3+ by solvent extraction with TBP. A total flow diagram of highly efficient clean enrichment of trace rare earths process is proposed (Figure 8). The best conditions for leaching were found to be 50% (v/v) HCl at a temperature of 30 °C and a solid:liquid ratio of 1:7 for 1 h. Under these conditions, over 96.24% of rare earths were leached from Fe-Al-OSA. Fe3+ was extracted from the leach liquor with 30% N235 + 10% ROH + 60% n-heptane at phase ratio (Vo:Vw) 8:5; Al3+ and trace of rare earths were remaining in the raffinate. More than 92.09% of Fe3+ was recovered by using a countercurrent extraction process with 4-5 stages. Then, Al3+ and rare earths could coprecipitate from the raffinate by adjusting the pH to 6.00 with MgO and saturated Na2CO3 solution. After the mixture of Al and rare earth hydroxide precipitates were dissolved by HNO3(aq), a trace of RE3+ was recycled from solution by solvent extraction with TBP, and the rate was 86.30%; finally, the residual Al3+ was recycled to prepare Al2O3. On the basis of this method, the solid waste (Fe-Al-OSA) was also recycled effectively; it was helpful to solve all the ecological and environmental problems caused by the waste heap.
Figure 8. Flow sheet for extraction and separation of Fe3+, Al3+, and rare earths.
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ReceiVed for reView May 18, 2010 ReVised manuscript receiVed September 13, 2010 Accepted September 15, 2010 IE101115E
