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Europium separation from a middle rare earths concentrate derived from Egyptian black sand monazite
Hydrometallurgy 86 (2007) 121 – 130 www.elsevier.com/locate/hydromet
Europium separation from a middle rare earths concentrate derived from Egyptian black sand monazite K.A. Rabie a , S.A. Sayed b , T.A. Lasheen a , I.E. Salama a,⁎ a
Nuclear Materials Authority, Rare Earth Elements Separation Project, P.O.Box 530 El-Maadi, Cairo, Egypt Chemistry Department, Faculty of Science, Helwan University, P.O.Box 11795 Ain Helwan, Cairo, Egypt
b
Received 1 June 2006; received in revised form 2 October 2006; accepted 29 October 2006 Available online 22 February 2007
Abstract Europium separation from a middle rare earth concentrate has been achieved successfully by using the combined chemical reduction– precipitation technique. The separation method depends on the reduction of europium by metallic zinc to its bivalent oxidation state followed by selective precipitation of the sparingly soluble europium (II) sulphate, while leaving the other rare earth sulphates in solution. This process consists of two steps; the first is reduction, which involves passing europium (III) chloride solution through a column packed with zinc particles. The other step involves the precipitation of the produced europium (II) chloride using a sulphate salt under inert atmosphere. Variables such as column dimensions, acidity, europium concentration in feed solution, contact time, ageing time, and concentration of the precipitating agent, have been evaluated with a pure europium synthetic solution. Based on the obtained results, a separation process was suggested for the separation of europium from a middle rare earth concentrate extracted from an Egyptian beach sand monazite. The result of this working-up is a europium yield of about 91% of the amount employed with a purity of about 97%. © 2006 Elsevier B.V. All rights reserved. Keywords: Europium separation; Monazite; Zinc reductor; Reduction; Precipitation
1. Introduction Europium oxide has gained recently considerable technical and commercial interest. Europium is very difficult to isolate and is separated from monazite ore found in India, Brazil, Australia and Africa, bastnasite ore found in China and North America and the ionic clays of Southern China, all of which contain slight amounts of europium. Difficulty in separation and rarity make europium one of the most expensive rare earth elements, though it is used in many common consumer
⁎ Corresponding author. Tel.: +20 27585835; fax: +20 2758532. E-mail address: [email protected] (I.E. Salama). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.10.007
goods. Europium is utilized primarily for its unique luminescent behavior. Excitation of the europium atom by absorption of ultra violet radiation can result in specific energy level transitions within the atom creating an emission of visible radiation. The discovery in 1964 of a europium–yttrium based red phosphor for the production of the cathode ray tube of commercial TV sets represented a major technological breakthrough and a turning point for the rare earth industry (Anon, 1998). Europium has found its greatest use as a phosphor activator and is widely used in europium-activated yttrium compounds to produce red color in television, computer monitor and LED displays. This application accounts for the vast majority of Europium consumption worldwide. Europium is also used to dope plastics in lasers.
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All or most of the europium separation methods make use of the fact that; europium is unique among the rare earth elements in being most easily reduced to a stable bivalent state. The compounds of the latter state show actually a different behavior from those of the trivalent rare earths. On the other hand, the most important significant property of europium in the bivalent state is that, it becomes quite similar to group IIA elements in chemical properties (McCoy, 1936). Europium can be used in its elemental form for petrogenetic tracing of igneous rocks. A Europium alloy can be used, similarly to samarium, as flint. It can also be used for infrared absorbing and petroleum cracking catalysis. Once reduction is achieved, separation of europium (II) from the remaining trivalent rare earths can be done by precipitation of its sparingly soluble sulphate, which resembles those of barium and strontium. Saturated aqueous solutions at 298 °K contain about (6.7± 4.6) × 10− 5 mol L− l of EuSO4 (Rard, 1985). Several combinations and repetitions of such reduction and separation procedures are likely to be necessary to achieve high-grade europium oxide. Several techniques have been used for the separation of europium from other rare earth elements. Chemical reduction using zinc (McCoy, 1935, 1936, 1937; Minkova, 1995) and zinc amalgam (Czegledi et al., 1971; Melnik and Filimonov, 1992; Morais and Ciminelli, 1998, 2001; Preston and Du Preez, 1996; Stone and Hume, 1939) is the traditional method. Other techniques based on electrochemical reduction (Dumousseau et al., 1990; Farah et al., 1972; Girgin and Yorukoglu, 2002; McCoy, 1941a,b; Preston et al., 1996), photochemical reduction (Donohue, 1977, 1979; Hirai and Komasawa, 1995; Hirai et al., 1993; Qiu et al., 1991), and radiochemical reductions have been used (Malan and Muenzel, 1966; Selin et al., 1990; Shastri et al., 1986). There is no indication that photochemical and radiochemical methods are applied on an industrial scale. The separation of europium using the combined chemical reduction-ion exchange methods have been employed extensively to obtain pure europium oxide (Elbanowski and Baranowska, 1985; Jingjun et al., 1996; Minkova and Todorovsky, 1997). The method based on a reduction–precipitation enrichment of the initial europium concentration followed by an ion exchange separation of Eu (II) after Eu (III) reduction. The reduction was done either with zinc or with zinc amalgam. Minkova and Todorovsky (1997) proposed a method for the separation of europium from a concentrate of middle and heavy rare earths obtained from phosphogypsum, a by-product of the apatite processing,
using the combined reduction–precipitation and reduction-ion exchange methods. Europium oxide with an assay of 99.8% and a yield of 27% was prepared. Elbanowski and Baranowska (1985) suggested the use of high pressure ion exchange for final purification. Application of the combined chemical reductionsolvent extraction techniques have been widely investigated for the separation of europium (Lanping et al., 1994; Peppard et al., 1962a,b). After reduction of Eu (III) to Eu (II) by zinc powder or through zinc column, the other rare earths were extracted to the organic phase while Europium (II) remains in the aqueous phase, resulting in a pure europium oxide. D2EHPA (Di-2ethylhexyl phosphoric acid) or P507 (Phosphonic acid (2-ethylhexyl)-mono(2-ethlhexyl) ester) as an acidic phosphonate reagent was employed as the extractant. Peppard et al. (1962a), used a combined reductionsolvent extraction technique to separate divalent europium from the other rare earths. It was first shown by Peppard's group that the solvent extraction of europium (II) by organophosphoric acids is much weaker than that of europium (III), and a process was subsequently patented for the removal of trivalent rare earths by solvent extraction from europium (II) solution prepared by reduction with amalgamated zinc (Peppard et al., 1962b). Two different techniques could be employed to bring about the reduction–precipitation process using zinc namely; in situ-zinc and zinc column methods. In the in situ-zinc process, the reduction and precipitation steps are carried out in the same vessel, while in zinc column, the reduction and precipitation processes are arranged to be made separately. In a previous work (Sayed et al., 2005), a separation process based on in situ-zinc technique was proposed and applied to separate europium from a middle rare earth concentrate derived from Egyptian black sand monazite. The proposed separation process suffers from a significant shortcoming, which lies in the contamination of the raffinate containing Sm and Gd with a large proportion of dissolved zinc. The reason for this contamination is attributed to the dissolution of zinc during both the reduction and precipitation step because of its reaction with the hydrochloric and sulphuric acid respectively. This work focuses on solving this problem by avoiding the acid–Zn metal contact by placing Zn metal in a specific designed column. This is the other worked technique for carrying out the separation process in which the reduction process occurs by the passage in a zinc column, wherein its outlet is dipped in a precipitation kettle. This configuration guarantees the
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occurring of the precipitation reaction without any contact between the reductant and precipitating agent. 2. Experimental 2.1. Reagents and solutions Unless otherwise stated, all reagents were of analytical reagent grade and all solutions were prepared in calibrated flasks with double distilled water. Pure Eu (III) synthetic solutions were prepared by dissolving europium oxide (99.9 wt.% Eu2O3 supplied by Strem, USA) in a hot concentrated hydrochloric acid. The natural middle rare earth concentrate was prepared from the rare earth hydrous oxide cake derived from acid digestion of monazite. The aforementioned rare earth hydrous oxide cake was subjected to many chemical processes, which involves oxidation followed by selective precipitation of cerium (Rabie, 1996; Rabie et al., 1996). The resulted cerium-free rare earths raffinate was then subjected to a solvent extraction process for the preparation of middle rare earth concentrate. Our trials reach to middle rare earth concentrates containing 10–15% Eu of the total REEs contents using laboratory bench scale mixer settler (Salama, 2005). Zinc granular of 20–30 mesh size (99.8% purity, Aldrich, Germany) was utilized as the reductant.
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2.2. Reduction and precipitation experiments The reduction reaction was carried out in a glass column, 60 cm long and 1.2 cm wide internal containing 200 g of 20–30 mesh granular zinc (Zinc Reductor). The outlet of the reductor is dipped into the precipitation flask. The precipitation reaction was carried out in a 250 mL diameter, filtering flask. It has one outlet for the exit of inert gas and two inlets for entrance of the reduced europium solution, and inert gas respectively. Nitrogen gas was sparged into the flask through a Teflon tube. The set-up is illustrated in Fig. 1. In each experiment, the europium solution, under specified concentration and acidity, was added to the column at a constant flow rate. After completion, the column was washed with 100 mL of 0.01 mol/L HCl for the entire removal of europium and soluble zinc. Both the precipitation and wash solution were collected into the Buchner flask, containing the sulphate solution, where the precipitation of europium sulphate took place. The flask was maintained under N2 atmosphere, in order to avoid reoxidation of the reduced europium species. For the natural solutions, about 1 g of the natural middle rare earth concentrate containing 2.5% Nd, 10.13% Sm, 11.35% Eu, 2.6% Gd, and 2.7 wt.%. Tb was dissolved in the least amount of hot concentrated (1:1) hydrochloric acid, and then, boiled and stirred well
Fig. 1. A schematic diagram for employed zinc column reduction precipitation apparatus.
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until clear solution is obtained. The reduction–precipitation procedure was carried out as previously described. The precipitated europium (II) sulphate was allowed to settle, and the residual europium content in the supernatant solution was determined spectrophotometrically by Arzenazo(III) method (Kolthoff et al., 1961) using the absorption band at 650 nm on a double beam of high-resolution power UNICAM–UV spectrophotometer. Europium was determined in the obtained europous sulphate spectrophotometrically using the absorption band at 394 nm (Abdel Hamied et al., 2004; Kostyuk et al., 2004). On working with the natural middle rare earth concentrate, the analysis of the individual rare earth elements has been performed by combined IC–UV–VIS instrument (DX-500) manufactured by Dionex Corp., USA. 3. Results and discussion The recovery of europium by the combined reduction–precipitation technique comprises two phenomena, namely, the reduction of Eu (III) to Eu (II) by metallic zinc, followed by precipitation of Eu (II) as europium (II) sulphate in an inert atmosphere. This can be demonstrated by the reduction potential associated with Eq. (1) (Morais and Ciminelli, 2001): 2þ 2þ 2Eu3þ ðaqÞ þ ZnðsÞ ¼ 2EuðaqÞ þ ZnðaqÞ
0 DE298K ¼ 0:33V:
ð1Þ Europium reduction and precipitation is carried out in an inert atmosphere, since Eu (II) is easily oxidized by oxygen as shown in Eq. (2): þ 3þ 4Eu2þ ðaqÞ þ O2ðgÞ þ 4HðaqÞ ¼ 4EuðaqÞ 0 þ 2H2 OðlÞ DE298K ¼ 1:66V:
ð2Þ
of 60 cm and a diameter of 1.2 cm. The second, possesses a length of 100 cm and its diameter is 2 cm. The third one has a diameter of 1.6 cm and a length of 120 cm. Another column, made of PVC, was tried. It has a length of 60 cm and a large diameter of 2.5 cm. The separation efficiencies of these columns were comparable to each other. The glass column with a length of 60 cm and a diameter of about 1.2 cm gives higher separation efficiency as compared to the others. Previous authors have used different column characteristics in their work. Morais and Ciminelli (2001) carried out the reduction process in a 60 cm long and 1.6 cm wide (internal diameter) glass column. Cooley and Yost (1946), employed a similar apparatus while McCoy (1935) used a 30 cm column in length and 1.7 cm in diameter. It must be mentioned here that, the choice of the appropriate column dimensions is directly related to the employed flow rate and to the europium concentration in the feed solution besides any impurities that may be encountered. 3.2. Effect of acidity of the feeding europium solution The acidity of the europium feed solution is one of the most important factors that directly affects the reduction–precipitation efficiency. Different europium solutions having pH values of 1.0, 2.0, 2.5, 3.0, 3.5, and 4.0 were utilized as the feed solution to inspect the effect of pH on the separation efficiency. The percent recoveries for each solution were represented graphically in Fig. 2. It is clear from Fig. 2 that, the efficiency of the reduction–precipitation process increases with the increase in pH value of europium feed solution. This behavior may be attributed to the competition process between europium and hydrogen reduction reactions as
The precipitation of Eu (II)with sulphuric acid is indicated by Eqs. (3) and (4) (Morais and Ciminelli, 2001): − þ Eu2þ ðaqÞ þ HSO4ðaqÞ ¼ EuSO4 þ HðaqÞ for pHb1:9
ð3Þ
2− Eu2þ ðaqÞ þ SO4ðaqÞ ¼ EuSO4 for pHN1:9:
ð4Þ
3.1. Effect of column dimensions Many trials have been made in order to get the best column dimensions that are capable of providing a simple, efficient, easy handled and economic configuration. Three different glass columns of different lengths and diameter were investigated. The first with a length
Fig. 2. Effect of europium feed pH on percent europium recovery and zinc dissolution.
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shown from Eqs. (1) and (5). 2þ 2Hþ ðaqÞ þ ZnðsÞ ⇔ZnðaqÞ þ H2ðgÞ
Eh-298K ¼ 0:76V: ð5Þ
Eq. (5) shows that, the decrease in pH value is accompanied by an increase in the amount of dissolved zinc which in turn decreases the amount of undissolved zinc reductant available for completion of the reduction process. This explains the decrease in the percent europium recovery with the decrease of the pH of europium feed solution. This explanation is also confirmed by the experimental results shown in Fig. 2 which gives the relation between pH of europium solution and dissolution of zinc reductant. The results obtained show the decrease in zinc dissolution rate with the rise in pH value of europium feed solution. It should be noted that, the increase in dissolution of zinc reductant causes a decrease in reduction–precipitation efficiency and contaminate the resulted europium (II) sulphate as well as the raffinate left behind. Morais and Ciminelli (1998, 2001) used zinc amalgam instead of zinc in order to solve the problem concerning the dissolution of zinc reductant during the reduction process. The zinc metal was covered by a thin layer of Hg and the resulting Zn amalgam was placed in a column forming the Jones reductor. It should be mentioned here that, Hg does not reduce the trivalent europium species. This can be demonstrated by the reduction potential associated with Eq. (6): 2þ 2þ 2Eu3þ ðaqÞ þ HgðlÞ ⇔EuðaqÞ þ HgðaqÞ
DE298K ¼ −1:28V:
ð6Þ Morais and Ciminelli (1998, 2001), studied the effect of Hg concentration in the amalgam from 0.5–1 wt.%., and no difference on europium recovery and purity were observed. Later on, they investigated the europium reduction with pure zinc and no improvement in the overall europium recovery by the use of amalgam was obtained. In addition, the concentration of soluble zinc found in solution after europium reduction by both amalgam and pure Zn were identical. These results obtained by Morais and Ciminelli may be attributed to the low acidity of the feed solution, which determines the final pH in the reduction column and therefore zinc solubility. Upon working with pure zinc, they reported that, a dark layer was formed on zinc surface, which in turn reduce its efficiency after a number of cycles. This layer was identified by X-ray as a mixture of ZnO and Zn(OH)2. In order to overcome
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this problem it was suggested in this work to wash the column periodically with 2 mol/L HCl solution to remove any layer that may be formed on zinc surface. From the experimental results obtained and the above discussion, we can conclude that, a feed europium solution having a pH value 2.5–3.0 is very satisfactory to be used in the reduction–precipitation process. It was noticed that, the percent europium recovery becomes nearly independent of pH values higher than 3. In addition, feed solutions having pH values higher than 3 suffer from the low solubility of europium and rare earth oxides. In the early work employed by McCoy (1935), he did not specify a definite pH value for the feed solution, while Stone and Hume (1939) utilized a pH value of 0.8. Preston and Du Preez (1996), carried out screening experiment on the reduction of pure europium (III) solutions having pH values in the range 2.8–3.2 using zinc metal as the reductant. Recently, Morais and Ciminelli (1998, 2001), carried out the reduction experiments with a feed europium solution with a pH value of 2.5. 3.3. Effect of feed europium solution concentration The europium concentration in feed solution plays an important role in increasing the recovery of europium (II) sulphate. The results shown in Fig. 3 declare that, the more concentrated europium feed solution gives rise to higher percent europium recovery compared to the less concentrated ones. However, this factor cannot be utilized effectively in controlling the efficiency of the separation process because we are governed by the europium and other rare earths concentration in the naturally occurring concentrates. This is because any attempts to increase europium concentration in feed
Fig. 3. Effect of europium concentration in feed solution on percent europium recovery.
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solution may result in incomplete solubility of the middle rare earth concentrate. Preston and Du Preez (1996) utilized europium chloride solution having a concentration of 0.2 mol/L (corresponding to 5 g/L) in the screening experiments for the reduction of pure europium solutions. They carried out reduction–precipitation experiments on a middle rare earth strip solution produced during continuous counter-current solvent extraction trials. The solution contained 7.9 g/L europium. McCoy (1936), used a material feed consisted essentially of the oxalates of Nd, Eu, Sm, Gd. Tb, Dy, Ho, Y, and Er in which the salts of Sm and Gd predominated. The europium oxalate content amounted to about 0.5%. The starting europium raw material utilized by Hugh (1964) was having about 0.4% europium oxide. Morais and Ciminelli (2001), carried out the reduction experiments on a pure europium solution having a concentration of about 2% europium oxide. Upon working with a commercial europium–gadolinium carbonate mixture, the europium content was limited to a maximum of 2.4 g/L europium oxide in the feed solution. Attempts to further increase in europium concentration were found to be detrimental to the complete dissolution of the oxides. The difficulty in increasing the europium concentration in feed solution in this case may be due to the high total rare earth concentration (187.9 g/L) in the worked concentrate. This situation is not present in this work since the prepared middle rare earth concentrate has a much lower rare earth concentration. 3.4. Effect of contact time A study for the effect of contact time between europium solution and zinc was necessary to account for applicability of the proposed separation process on
Fig. 4. Effect of contact time on percent europium recovery.
Fig. 5. Effect of sulphuric acid concentration on percent europium recovery.
industrial scale. The obtained results in Fig. 4 show that, the increase in contact time between europium feed solution and zinc reductant, is accompanied by a similar increase in europium percent recovery. The change in contact time between 15 and 120 min is corresponding to a change in flow rate between 6.5 to 0.8 mL/min. It is clear from Fig. 4 that, europium recovery becomes independent of contact time at times higher than 60 min. The difference in percent europium recovery from contact time range 60–120 min was about 1%. As a result, a contact time of 60 min was chosen to be adequate for the reduction process. This contact time corresponds to a flow rate of about 1.5 mL/min. It should be pointed out here that, the selection of an adequate flow rate is deeply reliant on the column dimensions and europium concentration in the feed solution. Morais and Ciminelli (2001), using a glass column 60 cm long and 1.6 cm wide, have applied a flow rate of 3 mL/min. Cooley and Yost (1946), using a similar apparatus employed a flow rate of 2 mL/min. McCoy (1935), and Hillbrand et al. (1953), studied flow rate up to 30 mL/min.
Fig. 6. Effect of precipitation time on percent europium recovery.
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3.5. Effect of sulphuric acid concentration The effect of sulphuric acid concentration on the reduction–precipitation process was evaluated by changing the acid concentration in the precipitation vessel from 0.25 to 5 mol/L. The volume was set at 20 mL, the minimum to cover inert gas inlet. The europium feed solution (100 mL) had a pH of 3 and a europium content of 0.1 g. Reduction–precipitation experiments were carried out using a flow rate of 1.5 mL/min and precipitation time of 1 h. The results were represented graphically in Fig. 5. The results shown in Fig. 5 indicate that europium recovery did not increase significantly at sulphate concentrations higher than 2 mol/L. By making a comparison with
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the results obtained in case of in-situ zinc method, it was established that there was no difference between the results of the two methods. It should be mentioned that, the reduction–precipitation experiments in case of in-situ zinc method were carried out on a naturally occurring middle rare earth concentrate. As a result, we can make use of the experimental results discussed and evaluated in the pervious work (Sayed et al., 2005). It was found that sulphuric acid concentration has a marked effect upon both the recovery and purity of the separated europium (II) sulphate. The most suitable sulphuric acid concentration to be used in the precipitation of europium (II) sulphate from the prepared middle rare earth concentrate was found to be 3 mol/L which corresponds to an SO42−/Eu molar ratio of 46.
Fig. 7. A proposed flowsheet for the suggested europium separation process via zinc column method.
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Morais and Ciminelli (2001) found that, both recovery and grade of resulted europium (II) sulphate are significantly affected by sulphate concentration. They utilized 30 mL of 3.5 mol/L H2SO4 solution to precipitate 0.4 g Eu, which corresponded to 0.105 mol H2SO4 in the system (sulphate/europium = 39). Cooley and Yost (1946), employed 50 mL of 8 mol/L sulphuric acid solution to precipitate 0.6 g Eu, which corresponded to an excess superior to 7000%. Girgin and Yorukoglu (2002) utilized (sulphate/europium) molar ratio amounting to 25 to precipitate 0.005 mol/L Eu from a synthetic chloride solution. 3.6. Effect of aging time Another important parameter that directly affects the europium recovery is the aging or precipitation time. The effect of precipitation time was investigated utilizing a 100 mL solution of 0.1 g europium. The pH of the feed solution was 3.0 and the reduction was carried out using a flow rate of 1.5 mL/min. The precipitation time was varied from 30 min to 300 min. Generally, it is can be observed from Fig. 6 that, the percent europium recovery increases with the increase in precipitation time. However, the europium recovery does not improve significantly for precipitation times above 120 min. As a result, a precipitation time of 120 min was selected to be used in the reduction– precipitation experiments in this study. Morais and Ciminelli (1998), studied the effect of ageing time on europium recovery in the range 3–5 h. For the pure europium solutions, 3 h was selected as the best condition. Later (Morais and Ciminelli, 2001), they indicated that, this time could be reduced to 2 h. Hugh (1964) employed a precipitation time of 12 h to achieve a good precipitation of europium (II) sulphate. It should be mentioned that, the precipitation time depends mainly on the concentration and type of the other rare earth elements that may accompany europium in feed solutions. According to Preston and Du Preez (1996), the precipitation of europium (II) sulphate proceeds considerably more slowly in the authentic strip liquors than in pure solutions, due to the presence of impurities in their liquor. Morais and Ciminelli (1998, 2001) do not confirm this situation in their work, may be owing to the low impurity concentration in the employed raw material.
a middle rare earth concentrate containing 2.5% Nd, 10.13% Sm, 11.35% Eu, 2.6% Gd, and 2.7 wt.% Tb. Fig. 7 shows a flowsheet for the proposed separation process. The research results, used to introduce this flowsheet, are based on the experiments carried out in the column and in-situ zinc reduction studies. These results could be summarized as follows: i. Reduction–precipitation method: column zinc method ii. Column dimensions: 60 cm in length and 1.2 cm in diameter iii. Reductant: zinc metal its particle size is 20– 30 mesh iv. Europium feed concentration: as concentrated as possible v. pH of feed solution: 2.5–3.0 vi. Precipitating agent: sulphuric acid vii. Sulphuric acid concentration: 3.0 mol/L viii. Aging time: 2 h ix. Non-oxidizing gas: nitrogen x. Flow rate: 1.5 mL/min. Fig. 8 shows a comparison between the absorption spectra of the middle rare feed solution before the separation process and that of the resulted europium oxide after reduction–precipitation process. This figure establishes the effectiveness of the proposed separation process. After the reduction, the samarium (402 nm) absorption peak disappears completely while the absorption peak belonging to europium (394 nm) is still present. The result of this flowsheet is a europium yield of about 91% of the amount employed with a purity of about 97% in a single run. A comparison with the results obtained in case of in-situ zinc reduction technique (purity of 92% and 95% recovery and in the second reduction–
3.7. Europium separation from the prepared middle rare earth concentrate Based on the research results above, a separation process was suggested for the separation of europium from
Fig. 8. Comparison between absorption spectra of middle rare earth feed solution and that of separated europium oxide mixtures.
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precipitation run the purity was increased to 97% with a final recovery of 91%) indicates that, zinc column method gives more pure compounds. In addition, the zinc column method guarantees the occurrence of the precipitation reaction without any contact between the reductant and precipitating agent which in turn decrease the dissolved zinc amount and produce less zinc contaminated products. 4. Conclusion Europium separation efficiency using the zinc column method was found to be affected by column dimensions, acidity, europium concentration in feed solution, contact time, ageing time, and concentration of the precipitating agent. These parameters have been studied and evaluated with a pure europium synthetic solution. The effective conditions were: column dimensions: 60 cm in length and 1.2 cm in diameter; europium feed concentration: as concentrated as possible; pH of feed solution: 2.5–3.0; precipitating agent: 3.0 mol/L sulphuric acid or to a final molar ratio SO42−/Eu of 46; aging time: 2 h; flow rate: 1.5 mL/min. Based on the research results above, a separation process was suggested and applied with success for the separation of europium from a middle rare earth concentrate containing 10–15% Eu of the total REEs contents. The result of this working-up is a europium yield of about 91% of the amount employed with a purity of about 97%. The zinc column method was found to be favorable over the in-situ zinc method owing to its simplicity and lower dissolution of zinc reductant during the reduction–precipitation process. References Abdel Hamied, M.A., Rabie, K.A., Sayed, S.A., Salama, I.E., 2004. Modified spectrophotometric procedures for europium analysis in presence of samarium and gadolinium. Al-Azhar Bull. Sci. 15 (1), 311–324. Anon, K., 1998. The Economics of Rare Earths and Yttrium. Roskill Information Services, London. 359–364 pp. Cooley, R.A., Yost, D.M., 1946. Europium (II) salts. In: Fernelius, W.C. (Ed.), Inorganic Synthesis. McGraw Hill, New York, pp. 69–73. Czegledi, B., Csovari, M., Sarkadi-Nagy, B., 1971. Methods of preparation of pure rare earth compounds: separation of europium from other rare earth elements. In: Buzas, I. (Ed.), Proceedings of the 2nd Conference on Applied Physical Chemistry, Budapest, Hungary, pp. 421–427. Donohue, T., 1977. Photochemical separation of europium from lanthanide mixtures in aqueous solution. J. Chem. Phys. 67 (11), 5402–5404. Donohue, T., 1979. Laser purification of the rare earths. Opt. Eng. 18 (2), 181–186. Dumousseau, J.U., Rollat, A., Sabot, J.L., 1990. Recovery of europium (II) values by electrolysis. US Patent 4,938,852, USA.
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Elbanowski, M., Baranowska, J., 1985. Preparation of high purity europium oxide using combined reduction-ion exchange method. J. Less-Common Met. 112, 267–270. Farah, M.Y., Saleh, F.A., Moustafa, M., 1972. Separation of samarium and europium from Egyptian monazite sands by sodium amalgam. Part I. Z. Anorg. Allg. Chem. 392 (1), 69–80. Girgin, I., Yorukoglu, A., 2002. Recovery of europium by electrochemical reduction from sulphate solutions. Hydrometallurgy 63, 85–91. Hillbrand, W.F., Lundell, E.F., Hoffman, J.I., Bright, H.A., 1953. Applied Inorganic Analysis. John Wiely and Sons Inc, New York. 108 pp. Hirai, T., Komasawa, I., 1995. Separation of Eu from Sm/Eu/Gd mixture by photoreductive stripping in solvent extraction process. Ind. Eng. Chem. Res. 34, 237. Hirai, T., Onoe, N., Komasawa, I., 1993. Separation of europium from samarium and gadolinium by combination of photochemical reduction and solvent extraction. J. Chem. Eng. Jpn. 26, 64. Hugh, B.J., 1964. Purification of europium. US Patent 3,153,571, USA. Jingjun, Z., Shubin, W., Guifen, G., 1996. Method for removing europium oxide from high pure gadolinium oxide. China Patent 1,125,260. Kolthoff, I.M., Elving, P.J., Sandell, E.B., 1961. Treatise on Analytical Chemistry. Interscience Publishers, New York, pp. 97–121. Kostyuk, N.N., Dik, T.A., Trebnikov, A.G., 2004. Estimation of the optimum concentration interval of the constants of praseodymium, neodymium, europium, holmium, and erbium in solution. J. Appl. Spectrosc. 71 (1), 147–150. Lanping, L., Fangchun, X., Zhi, Q., 1994. Process for extracting highpurity europium oxide. China Patent 1087126. Malan, H.P., Muenzel, H., 1966. Rapid radiochemical separations. II. Separation of europium from other lanthanides. Radiochim. Acta 15, 20–23. McCoy, H.N., 1935. The separation of europium from other rare earths. J. Am. Chem. Soc. 57, 1756. McCoy, H.N., 1936. The extraction of europium from monazite residues and the preparation of pure europium compounds. J. Am. Chem. Soc. 58, 2279–2280. McCoy, H.N., 1937. An improved method of purifying europium. J. Am. Chem. Soc. 59, 1131–1134. McCoy, H.N., 1941a. The electrolysis of rare earth acetates and the separation of europium as amalgam from other rare earths. J. Am. Chem. Soc. 63, 3432–3433. McCoy, H.N., 1941b. Europium and ytterbium amalgams. J. Am. Chem. Soc. 63, 1622–1624. Melnik, M.I., Filimonov, V.G., 1992. Reduction of europium (III) by zinc amalgam in acetate medium. Radiokhimiya 34 (6), 69–72. Minkova, N., 1995. Separation of europium from a rare earth concentrate, a by product of the apatite processing. Khim. Fak. 88, 25–30. Minkova, N., Todorovsky, S., 1997. Influence of the eluent concentration on the ion exchange separation of europium (II) from rare earth concentrate. Bulg. Chem. Commun. 29 (1), 84–89. Morais, C.A., Ciminelli, V.S.T., 1998. Recovery of europium from a rare earth chloride solution. Hydrometallurgy 49, 167–177. Morais, C.A., Ciminelli, V.S.T., 2001. Recovery of europium by chemical reduction of a commercial solution of europium and gadolinium chlorides. Hydrometallurgy 60, 247–253. Peppard, D.F., Horwitz, E.P., Masson, G.W., 1962a. Comparative liquid–liquid extraction behaviour of europium (II) and europium (III). J. Inorg. Nucl. Chem. 24, 429–439.
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Peppard, D.F., Horwitz, E.P., Masson, G.W., 1962b. Separation of europium from other lanthanides by solvent extraction. Belgian Patent 620 084. Preston, J.S., Du Preez, A.C., 1996. The separation of europium from a middle rare earth concentrate by combined chemical reduction, precipitation, and solvent extraction methods. J. Chem. Tech. Biotechnol. 65, 93–101. Preston, J.S., Du Preez, A.C., Cole, M.H., Fox, P.M., 1996. The recovery of rare earth oxides rom a phosphoric acid by-product. Part 3. The separation of middle and light rare earth fraction and the preparation of pure europium oxide. Hydrometallurgy 42, 131–149. Qiu, L., Kang, X., Wang, T., 1991. A study on photochemical separation of rare earths: the separation of europium from an industrial concentrate material of samarium, europium, and gadolinium. Sep. Sci. Technol. 26 (2), 199–221. Rabie, K.A., 1996. Studies on The Separation of some rare earth elements from Egyptian monazite sand. Ph. D Thesis, Cairo University, Faculty of Science, Egypt, Cairo.
Rabie, K.A., et al., 1996. Separation of cerium (III) from Egyptian monazite by oxidation to cerium (IV). Third Conf. On Peaceful Uses of Atomic Energy, Damascus, pp. 355–360. Rard, J.A., 1985. Chemistry and thermodynamics of europium and some of its simpler inorganic compounds and aqueous species. Chem. Rev. 85, 555–582. Salama, I.E., 2005. Studies on the separation of europium middle rare earth concentrate derived from Egyptian black sand monazite. M. Sc. Thesis, Helwan University, Helwan. Sayed, S.A., Rabie, K.A., Salama, I.E., 2005. Studies on europium separation from a middle rare earth concentrate by in-situ zinc reduction technique. Sep. Purif. Technol. 46, 145–154. Selin, D.L., Malkov, A.V., Tarasova, N.P., 1990. Europium separation from sulphuric acid solutions. USSR Patent 1,604,741, USSR. Shastri, L.V., Lalitha, J., Mittal, J., Mittal, P., 1986. Radiation chemical separation of europium from aqueous lanthanide mixtures. Radiat. Phys. Chem. 28 (4), 359–361. Stone, H.W., Hume, D.N., 1939. Reduction by amalgamated zinc: significant factors in efficiency. Ind. Eng. Chem. 11 (11), 598–603.
Europium separation from a middle rare earths concentrate derived from Egyptian black sand monazite K.A. Rabie a , S.A. Sayed b , T.A. Lasheen a , I.E. Salama a,⁎ a
Nuclear Materials Authority, Rare Earth Elements Separation Project, P.O.Box 530 El-Maadi, Cairo, Egypt Chemistry Department, Faculty of Science, Helwan University, P.O.Box 11795 Ain Helwan, Cairo, Egypt
b
Received 1 June 2006; received in revised form 2 October 2006; accepted 29 October 2006 Available online 22 February 2007
Abstract Europium separation from a middle rare earth concentrate has been achieved successfully by using the combined chemical reduction– precipitation technique. The separation method depends on the reduction of europium by metallic zinc to its bivalent oxidation state followed by selective precipitation of the sparingly soluble europium (II) sulphate, while leaving the other rare earth sulphates in solution. This process consists of two steps; the first is reduction, which involves passing europium (III) chloride solution through a column packed with zinc particles. The other step involves the precipitation of the produced europium (II) chloride using a sulphate salt under inert atmosphere. Variables such as column dimensions, acidity, europium concentration in feed solution, contact time, ageing time, and concentration of the precipitating agent, have been evaluated with a pure europium synthetic solution. Based on the obtained results, a separation process was suggested for the separation of europium from a middle rare earth concentrate extracted from an Egyptian beach sand monazite. The result of this working-up is a europium yield of about 91% of the amount employed with a purity of about 97%. © 2006 Elsevier B.V. All rights reserved. Keywords: Europium separation; Monazite; Zinc reductor; Reduction; Precipitation
1. Introduction Europium oxide has gained recently considerable technical and commercial interest. Europium is very difficult to isolate and is separated from monazite ore found in India, Brazil, Australia and Africa, bastnasite ore found in China and North America and the ionic clays of Southern China, all of which contain slight amounts of europium. Difficulty in separation and rarity make europium one of the most expensive rare earth elements, though it is used in many common consumer
⁎ Corresponding author. Tel.: +20 27585835; fax: +20 2758532. E-mail address: [email protected] (I.E. Salama). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.10.007
goods. Europium is utilized primarily for its unique luminescent behavior. Excitation of the europium atom by absorption of ultra violet radiation can result in specific energy level transitions within the atom creating an emission of visible radiation. The discovery in 1964 of a europium–yttrium based red phosphor for the production of the cathode ray tube of commercial TV sets represented a major technological breakthrough and a turning point for the rare earth industry (Anon, 1998). Europium has found its greatest use as a phosphor activator and is widely used in europium-activated yttrium compounds to produce red color in television, computer monitor and LED displays. This application accounts for the vast majority of Europium consumption worldwide. Europium is also used to dope plastics in lasers.
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All or most of the europium separation methods make use of the fact that; europium is unique among the rare earth elements in being most easily reduced to a stable bivalent state. The compounds of the latter state show actually a different behavior from those of the trivalent rare earths. On the other hand, the most important significant property of europium in the bivalent state is that, it becomes quite similar to group IIA elements in chemical properties (McCoy, 1936). Europium can be used in its elemental form for petrogenetic tracing of igneous rocks. A Europium alloy can be used, similarly to samarium, as flint. It can also be used for infrared absorbing and petroleum cracking catalysis. Once reduction is achieved, separation of europium (II) from the remaining trivalent rare earths can be done by precipitation of its sparingly soluble sulphate, which resembles those of barium and strontium. Saturated aqueous solutions at 298 °K contain about (6.7± 4.6) × 10− 5 mol L− l of EuSO4 (Rard, 1985). Several combinations and repetitions of such reduction and separation procedures are likely to be necessary to achieve high-grade europium oxide. Several techniques have been used for the separation of europium from other rare earth elements. Chemical reduction using zinc (McCoy, 1935, 1936, 1937; Minkova, 1995) and zinc amalgam (Czegledi et al., 1971; Melnik and Filimonov, 1992; Morais and Ciminelli, 1998, 2001; Preston and Du Preez, 1996; Stone and Hume, 1939) is the traditional method. Other techniques based on electrochemical reduction (Dumousseau et al., 1990; Farah et al., 1972; Girgin and Yorukoglu, 2002; McCoy, 1941a,b; Preston et al., 1996), photochemical reduction (Donohue, 1977, 1979; Hirai and Komasawa, 1995; Hirai et al., 1993; Qiu et al., 1991), and radiochemical reductions have been used (Malan and Muenzel, 1966; Selin et al., 1990; Shastri et al., 1986). There is no indication that photochemical and radiochemical methods are applied on an industrial scale. The separation of europium using the combined chemical reduction-ion exchange methods have been employed extensively to obtain pure europium oxide (Elbanowski and Baranowska, 1985; Jingjun et al., 1996; Minkova and Todorovsky, 1997). The method based on a reduction–precipitation enrichment of the initial europium concentration followed by an ion exchange separation of Eu (II) after Eu (III) reduction. The reduction was done either with zinc or with zinc amalgam. Minkova and Todorovsky (1997) proposed a method for the separation of europium from a concentrate of middle and heavy rare earths obtained from phosphogypsum, a by-product of the apatite processing,
using the combined reduction–precipitation and reduction-ion exchange methods. Europium oxide with an assay of 99.8% and a yield of 27% was prepared. Elbanowski and Baranowska (1985) suggested the use of high pressure ion exchange for final purification. Application of the combined chemical reductionsolvent extraction techniques have been widely investigated for the separation of europium (Lanping et al., 1994; Peppard et al., 1962a,b). After reduction of Eu (III) to Eu (II) by zinc powder or through zinc column, the other rare earths were extracted to the organic phase while Europium (II) remains in the aqueous phase, resulting in a pure europium oxide. D2EHPA (Di-2ethylhexyl phosphoric acid) or P507 (Phosphonic acid (2-ethylhexyl)-mono(2-ethlhexyl) ester) as an acidic phosphonate reagent was employed as the extractant. Peppard et al. (1962a), used a combined reductionsolvent extraction technique to separate divalent europium from the other rare earths. It was first shown by Peppard's group that the solvent extraction of europium (II) by organophosphoric acids is much weaker than that of europium (III), and a process was subsequently patented for the removal of trivalent rare earths by solvent extraction from europium (II) solution prepared by reduction with amalgamated zinc (Peppard et al., 1962b). Two different techniques could be employed to bring about the reduction–precipitation process using zinc namely; in situ-zinc and zinc column methods. In the in situ-zinc process, the reduction and precipitation steps are carried out in the same vessel, while in zinc column, the reduction and precipitation processes are arranged to be made separately. In a previous work (Sayed et al., 2005), a separation process based on in situ-zinc technique was proposed and applied to separate europium from a middle rare earth concentrate derived from Egyptian black sand monazite. The proposed separation process suffers from a significant shortcoming, which lies in the contamination of the raffinate containing Sm and Gd with a large proportion of dissolved zinc. The reason for this contamination is attributed to the dissolution of zinc during both the reduction and precipitation step because of its reaction with the hydrochloric and sulphuric acid respectively. This work focuses on solving this problem by avoiding the acid–Zn metal contact by placing Zn metal in a specific designed column. This is the other worked technique for carrying out the separation process in which the reduction process occurs by the passage in a zinc column, wherein its outlet is dipped in a precipitation kettle. This configuration guarantees the
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occurring of the precipitation reaction without any contact between the reductant and precipitating agent. 2. Experimental 2.1. Reagents and solutions Unless otherwise stated, all reagents were of analytical reagent grade and all solutions were prepared in calibrated flasks with double distilled water. Pure Eu (III) synthetic solutions were prepared by dissolving europium oxide (99.9 wt.% Eu2O3 supplied by Strem, USA) in a hot concentrated hydrochloric acid. The natural middle rare earth concentrate was prepared from the rare earth hydrous oxide cake derived from acid digestion of monazite. The aforementioned rare earth hydrous oxide cake was subjected to many chemical processes, which involves oxidation followed by selective precipitation of cerium (Rabie, 1996; Rabie et al., 1996). The resulted cerium-free rare earths raffinate was then subjected to a solvent extraction process for the preparation of middle rare earth concentrate. Our trials reach to middle rare earth concentrates containing 10–15% Eu of the total REEs contents using laboratory bench scale mixer settler (Salama, 2005). Zinc granular of 20–30 mesh size (99.8% purity, Aldrich, Germany) was utilized as the reductant.
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2.2. Reduction and precipitation experiments The reduction reaction was carried out in a glass column, 60 cm long and 1.2 cm wide internal containing 200 g of 20–30 mesh granular zinc (Zinc Reductor). The outlet of the reductor is dipped into the precipitation flask. The precipitation reaction was carried out in a 250 mL diameter, filtering flask. It has one outlet for the exit of inert gas and two inlets for entrance of the reduced europium solution, and inert gas respectively. Nitrogen gas was sparged into the flask through a Teflon tube. The set-up is illustrated in Fig. 1. In each experiment, the europium solution, under specified concentration and acidity, was added to the column at a constant flow rate. After completion, the column was washed with 100 mL of 0.01 mol/L HCl for the entire removal of europium and soluble zinc. Both the precipitation and wash solution were collected into the Buchner flask, containing the sulphate solution, where the precipitation of europium sulphate took place. The flask was maintained under N2 atmosphere, in order to avoid reoxidation of the reduced europium species. For the natural solutions, about 1 g of the natural middle rare earth concentrate containing 2.5% Nd, 10.13% Sm, 11.35% Eu, 2.6% Gd, and 2.7 wt.%. Tb was dissolved in the least amount of hot concentrated (1:1) hydrochloric acid, and then, boiled and stirred well
Fig. 1. A schematic diagram for employed zinc column reduction precipitation apparatus.
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until clear solution is obtained. The reduction–precipitation procedure was carried out as previously described. The precipitated europium (II) sulphate was allowed to settle, and the residual europium content in the supernatant solution was determined spectrophotometrically by Arzenazo(III) method (Kolthoff et al., 1961) using the absorption band at 650 nm on a double beam of high-resolution power UNICAM–UV spectrophotometer. Europium was determined in the obtained europous sulphate spectrophotometrically using the absorption band at 394 nm (Abdel Hamied et al., 2004; Kostyuk et al., 2004). On working with the natural middle rare earth concentrate, the analysis of the individual rare earth elements has been performed by combined IC–UV–VIS instrument (DX-500) manufactured by Dionex Corp., USA. 3. Results and discussion The recovery of europium by the combined reduction–precipitation technique comprises two phenomena, namely, the reduction of Eu (III) to Eu (II) by metallic zinc, followed by precipitation of Eu (II) as europium (II) sulphate in an inert atmosphere. This can be demonstrated by the reduction potential associated with Eq. (1) (Morais and Ciminelli, 2001): 2þ 2þ 2Eu3þ ðaqÞ þ ZnðsÞ ¼ 2EuðaqÞ þ ZnðaqÞ
0 DE298K ¼ 0:33V:
ð1Þ Europium reduction and precipitation is carried out in an inert atmosphere, since Eu (II) is easily oxidized by oxygen as shown in Eq. (2): þ 3þ 4Eu2þ ðaqÞ þ O2ðgÞ þ 4HðaqÞ ¼ 4EuðaqÞ 0 þ 2H2 OðlÞ DE298K ¼ 1:66V:
ð2Þ
of 60 cm and a diameter of 1.2 cm. The second, possesses a length of 100 cm and its diameter is 2 cm. The third one has a diameter of 1.6 cm and a length of 120 cm. Another column, made of PVC, was tried. It has a length of 60 cm and a large diameter of 2.5 cm. The separation efficiencies of these columns were comparable to each other. The glass column with a length of 60 cm and a diameter of about 1.2 cm gives higher separation efficiency as compared to the others. Previous authors have used different column characteristics in their work. Morais and Ciminelli (2001) carried out the reduction process in a 60 cm long and 1.6 cm wide (internal diameter) glass column. Cooley and Yost (1946), employed a similar apparatus while McCoy (1935) used a 30 cm column in length and 1.7 cm in diameter. It must be mentioned here that, the choice of the appropriate column dimensions is directly related to the employed flow rate and to the europium concentration in the feed solution besides any impurities that may be encountered. 3.2. Effect of acidity of the feeding europium solution The acidity of the europium feed solution is one of the most important factors that directly affects the reduction–precipitation efficiency. Different europium solutions having pH values of 1.0, 2.0, 2.5, 3.0, 3.5, and 4.0 were utilized as the feed solution to inspect the effect of pH on the separation efficiency. The percent recoveries for each solution were represented graphically in Fig. 2. It is clear from Fig. 2 that, the efficiency of the reduction–precipitation process increases with the increase in pH value of europium feed solution. This behavior may be attributed to the competition process between europium and hydrogen reduction reactions as
The precipitation of Eu (II)with sulphuric acid is indicated by Eqs. (3) and (4) (Morais and Ciminelli, 2001): − þ Eu2þ ðaqÞ þ HSO4ðaqÞ ¼ EuSO4 þ HðaqÞ for pHb1:9
ð3Þ
2− Eu2þ ðaqÞ þ SO4ðaqÞ ¼ EuSO4 for pHN1:9:
ð4Þ
3.1. Effect of column dimensions Many trials have been made in order to get the best column dimensions that are capable of providing a simple, efficient, easy handled and economic configuration. Three different glass columns of different lengths and diameter were investigated. The first with a length
Fig. 2. Effect of europium feed pH on percent europium recovery and zinc dissolution.
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shown from Eqs. (1) and (5). 2þ 2Hþ ðaqÞ þ ZnðsÞ ⇔ZnðaqÞ þ H2ðgÞ
Eh-298K ¼ 0:76V: ð5Þ
Eq. (5) shows that, the decrease in pH value is accompanied by an increase in the amount of dissolved zinc which in turn decreases the amount of undissolved zinc reductant available for completion of the reduction process. This explains the decrease in the percent europium recovery with the decrease of the pH of europium feed solution. This explanation is also confirmed by the experimental results shown in Fig. 2 which gives the relation between pH of europium solution and dissolution of zinc reductant. The results obtained show the decrease in zinc dissolution rate with the rise in pH value of europium feed solution. It should be noted that, the increase in dissolution of zinc reductant causes a decrease in reduction–precipitation efficiency and contaminate the resulted europium (II) sulphate as well as the raffinate left behind. Morais and Ciminelli (1998, 2001) used zinc amalgam instead of zinc in order to solve the problem concerning the dissolution of zinc reductant during the reduction process. The zinc metal was covered by a thin layer of Hg and the resulting Zn amalgam was placed in a column forming the Jones reductor. It should be mentioned here that, Hg does not reduce the trivalent europium species. This can be demonstrated by the reduction potential associated with Eq. (6): 2þ 2þ 2Eu3þ ðaqÞ þ HgðlÞ ⇔EuðaqÞ þ HgðaqÞ
DE298K ¼ −1:28V:
ð6Þ Morais and Ciminelli (1998, 2001), studied the effect of Hg concentration in the amalgam from 0.5–1 wt.%., and no difference on europium recovery and purity were observed. Later on, they investigated the europium reduction with pure zinc and no improvement in the overall europium recovery by the use of amalgam was obtained. In addition, the concentration of soluble zinc found in solution after europium reduction by both amalgam and pure Zn were identical. These results obtained by Morais and Ciminelli may be attributed to the low acidity of the feed solution, which determines the final pH in the reduction column and therefore zinc solubility. Upon working with pure zinc, they reported that, a dark layer was formed on zinc surface, which in turn reduce its efficiency after a number of cycles. This layer was identified by X-ray as a mixture of ZnO and Zn(OH)2. In order to overcome
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this problem it was suggested in this work to wash the column periodically with 2 mol/L HCl solution to remove any layer that may be formed on zinc surface. From the experimental results obtained and the above discussion, we can conclude that, a feed europium solution having a pH value 2.5–3.0 is very satisfactory to be used in the reduction–precipitation process. It was noticed that, the percent europium recovery becomes nearly independent of pH values higher than 3. In addition, feed solutions having pH values higher than 3 suffer from the low solubility of europium and rare earth oxides. In the early work employed by McCoy (1935), he did not specify a definite pH value for the feed solution, while Stone and Hume (1939) utilized a pH value of 0.8. Preston and Du Preez (1996), carried out screening experiment on the reduction of pure europium (III) solutions having pH values in the range 2.8–3.2 using zinc metal as the reductant. Recently, Morais and Ciminelli (1998, 2001), carried out the reduction experiments with a feed europium solution with a pH value of 2.5. 3.3. Effect of feed europium solution concentration The europium concentration in feed solution plays an important role in increasing the recovery of europium (II) sulphate. The results shown in Fig. 3 declare that, the more concentrated europium feed solution gives rise to higher percent europium recovery compared to the less concentrated ones. However, this factor cannot be utilized effectively in controlling the efficiency of the separation process because we are governed by the europium and other rare earths concentration in the naturally occurring concentrates. This is because any attempts to increase europium concentration in feed
Fig. 3. Effect of europium concentration in feed solution on percent europium recovery.
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solution may result in incomplete solubility of the middle rare earth concentrate. Preston and Du Preez (1996) utilized europium chloride solution having a concentration of 0.2 mol/L (corresponding to 5 g/L) in the screening experiments for the reduction of pure europium solutions. They carried out reduction–precipitation experiments on a middle rare earth strip solution produced during continuous counter-current solvent extraction trials. The solution contained 7.9 g/L europium. McCoy (1936), used a material feed consisted essentially of the oxalates of Nd, Eu, Sm, Gd. Tb, Dy, Ho, Y, and Er in which the salts of Sm and Gd predominated. The europium oxalate content amounted to about 0.5%. The starting europium raw material utilized by Hugh (1964) was having about 0.4% europium oxide. Morais and Ciminelli (2001), carried out the reduction experiments on a pure europium solution having a concentration of about 2% europium oxide. Upon working with a commercial europium–gadolinium carbonate mixture, the europium content was limited to a maximum of 2.4 g/L europium oxide in the feed solution. Attempts to further increase in europium concentration were found to be detrimental to the complete dissolution of the oxides. The difficulty in increasing the europium concentration in feed solution in this case may be due to the high total rare earth concentration (187.9 g/L) in the worked concentrate. This situation is not present in this work since the prepared middle rare earth concentrate has a much lower rare earth concentration. 3.4. Effect of contact time A study for the effect of contact time between europium solution and zinc was necessary to account for applicability of the proposed separation process on
Fig. 4. Effect of contact time on percent europium recovery.
Fig. 5. Effect of sulphuric acid concentration on percent europium recovery.
industrial scale. The obtained results in Fig. 4 show that, the increase in contact time between europium feed solution and zinc reductant, is accompanied by a similar increase in europium percent recovery. The change in contact time between 15 and 120 min is corresponding to a change in flow rate between 6.5 to 0.8 mL/min. It is clear from Fig. 4 that, europium recovery becomes independent of contact time at times higher than 60 min. The difference in percent europium recovery from contact time range 60–120 min was about 1%. As a result, a contact time of 60 min was chosen to be adequate for the reduction process. This contact time corresponds to a flow rate of about 1.5 mL/min. It should be pointed out here that, the selection of an adequate flow rate is deeply reliant on the column dimensions and europium concentration in the feed solution. Morais and Ciminelli (2001), using a glass column 60 cm long and 1.6 cm wide, have applied a flow rate of 3 mL/min. Cooley and Yost (1946), using a similar apparatus employed a flow rate of 2 mL/min. McCoy (1935), and Hillbrand et al. (1953), studied flow rate up to 30 mL/min.
Fig. 6. Effect of precipitation time on percent europium recovery.
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3.5. Effect of sulphuric acid concentration The effect of sulphuric acid concentration on the reduction–precipitation process was evaluated by changing the acid concentration in the precipitation vessel from 0.25 to 5 mol/L. The volume was set at 20 mL, the minimum to cover inert gas inlet. The europium feed solution (100 mL) had a pH of 3 and a europium content of 0.1 g. Reduction–precipitation experiments were carried out using a flow rate of 1.5 mL/min and precipitation time of 1 h. The results were represented graphically in Fig. 5. The results shown in Fig. 5 indicate that europium recovery did not increase significantly at sulphate concentrations higher than 2 mol/L. By making a comparison with
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the results obtained in case of in-situ zinc method, it was established that there was no difference between the results of the two methods. It should be mentioned that, the reduction–precipitation experiments in case of in-situ zinc method were carried out on a naturally occurring middle rare earth concentrate. As a result, we can make use of the experimental results discussed and evaluated in the pervious work (Sayed et al., 2005). It was found that sulphuric acid concentration has a marked effect upon both the recovery and purity of the separated europium (II) sulphate. The most suitable sulphuric acid concentration to be used in the precipitation of europium (II) sulphate from the prepared middle rare earth concentrate was found to be 3 mol/L which corresponds to an SO42−/Eu molar ratio of 46.
Fig. 7. A proposed flowsheet for the suggested europium separation process via zinc column method.
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Morais and Ciminelli (2001) found that, both recovery and grade of resulted europium (II) sulphate are significantly affected by sulphate concentration. They utilized 30 mL of 3.5 mol/L H2SO4 solution to precipitate 0.4 g Eu, which corresponded to 0.105 mol H2SO4 in the system (sulphate/europium = 39). Cooley and Yost (1946), employed 50 mL of 8 mol/L sulphuric acid solution to precipitate 0.6 g Eu, which corresponded to an excess superior to 7000%. Girgin and Yorukoglu (2002) utilized (sulphate/europium) molar ratio amounting to 25 to precipitate 0.005 mol/L Eu from a synthetic chloride solution. 3.6. Effect of aging time Another important parameter that directly affects the europium recovery is the aging or precipitation time. The effect of precipitation time was investigated utilizing a 100 mL solution of 0.1 g europium. The pH of the feed solution was 3.0 and the reduction was carried out using a flow rate of 1.5 mL/min. The precipitation time was varied from 30 min to 300 min. Generally, it is can be observed from Fig. 6 that, the percent europium recovery increases with the increase in precipitation time. However, the europium recovery does not improve significantly for precipitation times above 120 min. As a result, a precipitation time of 120 min was selected to be used in the reduction– precipitation experiments in this study. Morais and Ciminelli (1998), studied the effect of ageing time on europium recovery in the range 3–5 h. For the pure europium solutions, 3 h was selected as the best condition. Later (Morais and Ciminelli, 2001), they indicated that, this time could be reduced to 2 h. Hugh (1964) employed a precipitation time of 12 h to achieve a good precipitation of europium (II) sulphate. It should be mentioned that, the precipitation time depends mainly on the concentration and type of the other rare earth elements that may accompany europium in feed solutions. According to Preston and Du Preez (1996), the precipitation of europium (II) sulphate proceeds considerably more slowly in the authentic strip liquors than in pure solutions, due to the presence of impurities in their liquor. Morais and Ciminelli (1998, 2001) do not confirm this situation in their work, may be owing to the low impurity concentration in the employed raw material.
a middle rare earth concentrate containing 2.5% Nd, 10.13% Sm, 11.35% Eu, 2.6% Gd, and 2.7 wt.% Tb. Fig. 7 shows a flowsheet for the proposed separation process. The research results, used to introduce this flowsheet, are based on the experiments carried out in the column and in-situ zinc reduction studies. These results could be summarized as follows: i. Reduction–precipitation method: column zinc method ii. Column dimensions: 60 cm in length and 1.2 cm in diameter iii. Reductant: zinc metal its particle size is 20– 30 mesh iv. Europium feed concentration: as concentrated as possible v. pH of feed solution: 2.5–3.0 vi. Precipitating agent: sulphuric acid vii. Sulphuric acid concentration: 3.0 mol/L viii. Aging time: 2 h ix. Non-oxidizing gas: nitrogen x. Flow rate: 1.5 mL/min. Fig. 8 shows a comparison between the absorption spectra of the middle rare feed solution before the separation process and that of the resulted europium oxide after reduction–precipitation process. This figure establishes the effectiveness of the proposed separation process. After the reduction, the samarium (402 nm) absorption peak disappears completely while the absorption peak belonging to europium (394 nm) is still present. The result of this flowsheet is a europium yield of about 91% of the amount employed with a purity of about 97% in a single run. A comparison with the results obtained in case of in-situ zinc reduction technique (purity of 92% and 95% recovery and in the second reduction–
3.7. Europium separation from the prepared middle rare earth concentrate Based on the research results above, a separation process was suggested for the separation of europium from
Fig. 8. Comparison between absorption spectra of middle rare earth feed solution and that of separated europium oxide mixtures.
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precipitation run the purity was increased to 97% with a final recovery of 91%) indicates that, zinc column method gives more pure compounds. In addition, the zinc column method guarantees the occurrence of the precipitation reaction without any contact between the reductant and precipitating agent which in turn decrease the dissolved zinc amount and produce less zinc contaminated products. 4. Conclusion Europium separation efficiency using the zinc column method was found to be affected by column dimensions, acidity, europium concentration in feed solution, contact time, ageing time, and concentration of the precipitating agent. These parameters have been studied and evaluated with a pure europium synthetic solution. The effective conditions were: column dimensions: 60 cm in length and 1.2 cm in diameter; europium feed concentration: as concentrated as possible; pH of feed solution: 2.5–3.0; precipitating agent: 3.0 mol/L sulphuric acid or to a final molar ratio SO42−/Eu of 46; aging time: 2 h; flow rate: 1.5 mL/min. Based on the research results above, a separation process was suggested and applied with success for the separation of europium from a middle rare earth concentrate containing 10–15% Eu of the total REEs contents. The result of this working-up is a europium yield of about 91% of the amount employed with a purity of about 97%. The zinc column method was found to be favorable over the in-situ zinc method owing to its simplicity and lower dissolution of zinc reductant during the reduction–precipitation process. References Abdel Hamied, M.A., Rabie, K.A., Sayed, S.A., Salama, I.E., 2004. Modified spectrophotometric procedures for europium analysis in presence of samarium and gadolinium. Al-Azhar Bull. Sci. 15 (1), 311–324. Anon, K., 1998. The Economics of Rare Earths and Yttrium. Roskill Information Services, London. 359–364 pp. Cooley, R.A., Yost, D.M., 1946. Europium (II) salts. In: Fernelius, W.C. (Ed.), Inorganic Synthesis. McGraw Hill, New York, pp. 69–73. Czegledi, B., Csovari, M., Sarkadi-Nagy, B., 1971. Methods of preparation of pure rare earth compounds: separation of europium from other rare earth elements. In: Buzas, I. (Ed.), Proceedings of the 2nd Conference on Applied Physical Chemistry, Budapest, Hungary, pp. 421–427. Donohue, T., 1977. Photochemical separation of europium from lanthanide mixtures in aqueous solution. J. Chem. Phys. 67 (11), 5402–5404. Donohue, T., 1979. Laser purification of the rare earths. Opt. Eng. 18 (2), 181–186. Dumousseau, J.U., Rollat, A., Sabot, J.L., 1990. Recovery of europium (II) values by electrolysis. US Patent 4,938,852, USA.
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