Ind. Eng. Chem. Res. 1993,32,733-737
733
Cerium Concentrate and Mixed Rare Earth Chloride by the Oxidative Decomposition of Bastnaesite in Molten Sodium Hydroxide Toshio IijimaJ Kazuhiro Kato, Toyohiko Kuno, Akitsugu Okuwaki,’ Yoshiaki Umetsu,’ and Taijiro Okabes Department of Molecular Chemistry and Engineering, Faculty of Engineering, and Institute for Advanced Materials and Processing, Tohoku University, Sendai 980,Japan
Bastnaesite was treated in molten NaOH at 623-777 K for 10-60 min under atmosphere. Cerium(111)in the ore was easily oxidized 95% or more within 30 min to give an oxidation product composed of solid solutions of CeOz-rich and CeOz-lean phases and Ce-free rare earth oxide phase. Simultaneously fluoride ion was removed 97 % or more. Cerium concentrate was prepared from the oxidation product by leaching with 0.1-3 M HC1 solution. The yield of cerium concentrate and the CeOz content reached 55457% and 70-72 76, respectively. Mixed rare earth chloride is composed of about 90% rare earth chloride and 10%alkaline earth chloride, and the contents of CeCl3, LaC13, NdCl3, and PrC13 are 11.5,58.5,14.4, and 5.4773, respectively. The particle size of resulting cerium concentrate was fairly uniform and about 0.1 pm. In the processing of rare earth ores in alkaline medium, pressure leaching of monazite (LnP04; Ln, rare earth) is well-known (Ullmann, 1982). Decomposition of bastnaesite (LnC03F) at the boiling point of a 45% NaOH solutionwas studied to produce Ce(OH)3(Bauer and Shaw, 1964). Recently, bastanesite has been decomposed in a supercritical NaOH solution (Toyoda et al., 1991). On the other hand, irradiation treatments with high-frequency waves (Inoue, 1983a), plasma (Inoue, 1983b), and laser (Inoue, 1983c) of a Chinese mixed ore of monazite and bastnaesite in alkaline solutions and of a mixture of NaOH flake and the mixed ore (Matsumoto et al., 1986) have been proposed. In all these processes cerium(II1) in ores is left as Ce(OH)3 in the residue, leached with acids, and then precipitated from the leaching solution as CeOz by oxidation (Shiokawa and Adachi, 1984). We have found that Ce(II1) in bastnaesite was oxidized to CeOz in the treatment of molten NaOH at atmospheric pressure and simultaneously fluoride ion was completely removed. Consequently,cerium concentrate was prepared by leaching of the oxidation product with dilute HC1. Mixed rare earth chloride can be obtained by concentration of the resulting leaching solution. In the present paper, the basic principle and effect of reaction conditions on the degree of oxidation of Ce(II1) and the removal of F- have been studied, and the compositions of cerium concentrate and mixed rare earth chloride were also determined in HCl leaching of the resulting oxidation product in order to establish a new process treating bastnaesite. Cerium concentrate was observed by scanning electron microscopy (SEMI.
Table I. Composition of Bastnaesite and Rare Earth Oxide (90) ~~
bastnaesite LnzOf SrO CaO BaO
F Si02 Fez03 PZOS MgO, NazO, P z O ~ ThOz
rare earth oxide
68-72 1.0 0.4 1.8 5.0 0.4 0.5 1.0
48-50 32-34 13-14 4-5 0.5 0.2 0.1 0.2
C0.5 CO.1
20.0 COZ a Total rare earth oxide.
with a 17 G-4 glass filter, and dried at 353 K or lower. Two grams of the resulting oxidation product was leached with 60 cm3 of HC1 solution in a 100-cm3Erlenmeyer flask at 333 K by stirring with a magnetic bar. Analyses. The degree of oxidation of Ce(II1) to Ce(IV) was determined by titration with a KMn04 solution (JIS, 1976), and the removal of P was calculated by determination of Pin the filtrates by ion chromatography. The contents of Ce, La, Nd, and Pr in cerium concentrate and mixed rare earth chloride were determined by inductively coupled plasma spectrophotometry. Various solid products were identified by X-ray diffractometry (XRD) with Cu Ka and IR spectrophotometry, and observed by SEM.
Results and Discussion Experimental Section Ore. Bastanesite concentrate prepared by Moly Corp. was used, and the composition and rare earth content as received are shown in Table I. Method. Nickel crucibles and a 300-cm3 SUS 304 stainless steel reactor with a stirrer were used (Narita, 1975). The stirring speed was set at 500 rpm. The cooled reaction products were dissolved in distilled water, filtered + Present address: Japan Chem. Ind. Co. Ltd., 136 KoutouKu, Kameido, Japan. Institute for Advanced Materials and Processing. Present address: Ishinomaki Senshu University, 986 Ishinomaki, Japan.
*
1. Theoretical Background for the Oxidation of Ce203 to CeO2. The &,-pHno diagram (Tremillon, 1974) is suitable for a thermodynamic consideration on the reactions in molten hydroxides, where E h is the standard potential (in volts) and pHzO is the negative logarithm of the water concentration in molten hydroxide. However, thermodynamic data on the molten hydroxide system are quite limited and we were forced to use an &-pH diagram at elevated temperatures instead of an Eh-pHzo diagram. Figure 1is the Eh-pH diagram for the Ce-Hz0 system at 298 and 523 K depicted using the data in Schumm et al. (1973) and Wagman et al. (1968) and the entropy correspondence principle for simple ions (Criss and Cobble, 1964). Each line shows the change of standard potential
0888-588519312632-0733$04.00/0 0 1993 American Chemical Society
734 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 I 2
1
> \
x
0
? a
W
-1
-2
E2& Ce2O:
-4
-2
0
2
4
6
8
10
12
PH Figure 1. The Eh-pH diagram for the Ce-HzO system at 298 and 523 K. (-) 298 K,(-) 523 K,activity 1,Pg(partial pressure of gas) 101 kPa.
between two species indicated. Two dotted lines indicate the standard potentials of H2 and 0 2 evolution, respectively, depending on pH. The standard potential for the reaction (1/2)C12+ e = C1-, was added for the explanation of reduction of CeO2 by HCl in section 3.1. At 523 K, the stable area of CeO2 becomes larger than that at 298 K and the pH of precipitation of Ce203 decreases remarkably. However, the relative oxidation potential of oxygen for Cez03 increases a little. The &-pH diagram for the C e H2O system at 298 K has already been constructed by Pourbaix (1966), but a serious difference is observed between Pourbaix's diagram and Figure 1 since the thermodynamic data of cerium compounds differ very much between them. For example, Ce203 cannot be oxidized by oxygen under ordinary pressures in Pourbaix's diagram. On the contrary, this reaction should proceed as shown in Figure 1. It is well-known that Ce(II1) ions in solution are oxidized in a wide pH range by many oxidizing agents to precipitate CeO2. This fact is explained well in Figure 1. In a treatment of bastnaesite in a 50% NaOH solution a t 523 K and oxygen pressure of 5 MPa for 2 h, only Ce(OH)3 was formed and CeO2 has not been obtained. This may indicate that molecular oxygen does not react with Ce(OH)3 or Ce203. However, CeO2 was easily obtained in a treatment of bastnaesite in molten NaOH at atmospheric pressure. This indicates that the superoxide ion acts as an oxidizing agent since it is formed even in an acidic melt of hydroxide, e.g., pH20 = 0-1 (Tremillon, 1974). 2. Oxidative Decomposition of Bastnaesite in Molten Sodium Hydroxide. 2.1. Oxidative Decomposition Process. Figure 2 shows the XRD patterns of raw bastnaesite, intermediate products, and reagent CeO2, respectively. As the oxidative decomposition proceeds, the height of peaks assigned to bastnaesite (A) lowers and that of CeO2 (D) grows. This shows that the oxidation of Ce(II1) in bastnaesite to Ce(1V) took place successfully in molten NaOH at atmospheric pressure. In (C) the four peaks (0.564,0.325,0.227, and 0.186 nm) assigned to Ce(0H)s (ASTM 19-284) can be observed. In (D) the observed peaks can all be assigned to those of calcium fluoride-type CeO2 and no peak is identified corresponding to those for both La203 and Nd2O3 which should be
Figuret. XRD patterns of bastnaesite, intermediates, and oxidation product: (A) bastnaesite; (B)44% F removed; (C)95% Ce oxidized; (D)oxidation product; (E) reagent CeOz.
contained in the oxidation product. Additionally, the peaks for the oxidation product (D) shift to lower angles than those of the reagent CeO2. McCullough(l950) has already reported that, in the two-component oxide systems of Ce(1V)-La(II1) and Ce(IV)-Pr(III), the interplanar spacings of CeO2 are elongated with an increase in the concentrations of La(II1) and Pr(III), respectively. Practically, the elongation of interplanar spacings of CeO2 in the oxidation product was observed and caused by replacement of Ce(1V) with the other trivalent rare earth ions whose ionic radii are larger than that of Ce(1V) ion (Kiriyamaand Kiriyama, 1964). These facts indicate that the trivalent rare earth ions in the oxidation product are mainly contained as solid solution. Thus, the oxidative decomposition of bastnaesite in molten NaOH at atmospheric pressure takes place by two steps: LnC0,F
+ 3NaOH = Ln(OH), + NaF + Na2C03 Ce(OH), + 0,
-
(Ln = Ce, La) (1)
+
Ce02 H20
(2)
Figure 3 shows the IR spectrum of the samples corresponding to the XRD patterns in Figure 2. The strength of absorption at 800-900 and 1400-1500 cm-' assigned to carbonate ion decreased with proceeding of the oxidative decomposition. These peaks, however, were still remained in the completely oxidized sample (D). The peak height did not lower after calcination of the sample (D) at 1073 K for 3 h. Consequently, these peaks can be assigned to carbonate ion of metal carbonates other than bastnaesite since the carbonate ion in bastnaesite is decomposed to evolve CO2 a t as low a temperature as 723 K (Tanaka and Tayama, 1983). Naturally, these peaks disappeared after HC1 leaching of the sample. These results may indicate that bastnaesite contains alkaline earth carbonates. Additionally, in spectrum C a sharp peak assigned to hydroxide is observed a t 3600 cm-'. These results are consistent with those in XRD and support the two-step oxidative decomposition. 2.2. Effect of Reaction Conditions on the Degree of Oxidation of Ce(II1) to Ce(1V) and Removal of Fluoride Ion. The degree of Ce oxidized and the removal
Ind. Eng. Chem. Res., Vol. 32,No. 4, 1993 735 Table 11. Effect of Time on the Oxidation of Ce(II1) and Removal* of F t/min
Ce oxidized1 %
P removed/%
10 20 30
99 98 96
99 99 98
NaOH treatment: 623 K,NaOH 300 g; alkali ratio 10,stirring speed 500 rpm. (I
Table 111. Effect of Temperature and Alkali Ratio on the Oxidation of Ce(III)*
r I
I
I
I
I
om,xxx,mIxx,m wmfu* (rmi)
v
I
400
Figure 8. IR spectra of bastnaesite, intermediates and oxidation product: (A) bastnaesite; (B)44%P removed; (C) 95% Ce oxidized; (D)oxidation product; (E) reagent CeO2.
T/K 623 623 673 673
alkali ratio 5 10 5 10
Ce oxidized/ % 96 100 95 100
NaOH treatment: NaOH 200 g; 1 h; stirring speed 500 rpm. Table IV. Composition of Rare Earth Oxides in the Oxidation Products NaOH treatment TIK alkaliratio 623 5 623 10 673 5 673 10 723 5 723 10
rare earth oxide composition/ 5% CeO2 La203 NdzOs PrsOll Ln20sa 44.2 29.6 11.4 4.0 89.2 29.2 10.9 3.9 43.5 87.5 29.6 11.2 46.0 4.0 90.8 46.9 27.6 10.7 3.8 89.0 30.7 45.6 11.4 4.1 91.8 30.9 11.1 44.0 4.0 90.0
av
45.0
29.6
11.1
4.0
89.7
Sum of these four rare earth oxides.
Figure 4. Effect of alkali ratio on degreeof Ce oxidized and removal of P: (0)Ce oxidized; ( 0 )P removed.
of F are shown in Figure 4. Both values reached 90% or higher at an alkali ratio of 2 (weight ratio of NaOH/ bastnaesite) and 97% or higher at the alkali ratio of 5.In addition to the result shown in Figures 2 and 3,this result also supports the oxidative decomposition taking place easily at 623 K. When the alkali ratio was smaller than 4,the viscosity of the reaction melts was high, any agitation of them was practically impossible, and the values of Ce oxidized and Fremoved were dispersed to some extent. Consequently, it is favorable to operate a continuous process holding the alkali ratio higher than 5. Thus, the following oxidative decomposition was carried out at the alkali ratios of 5 and 10with the stainless steel reactor fitted with a stirrer, and the samples for analysis were withdrawn at certain time intervals using a small stainless steel dipper. Time. Both the degree of oxidation and the removal of F reached 99% within 10 min at 623 K and the alkali ratio of 10as shown in Table 11. Immediately after addition of bastnaesite powder, C02 evolved and the temperature of melt was raised about 7 K. This shows that the decomposition of bastnaesite occurs rapidly and the released C02 cannot be absorbed completely as shown in eq 1. The ratio of COz to Na2C03, however, was not determined. By contrast, fluoride ion in bastnaesite was fiied quantitatively in the melt as shown in Table 11.
Temperature. The effect of temperature on the degree of oxidation is shown in Table I11 at the alkali ratios of 5 and 10. There is no difference in the degrees of oxidation at 623and 673 K. At 773 K, the stainless steel reactor was corroded and chromate was formed which interfered the determination of Ce with KMn04 titration. The oxidative decomposition reaction itself takes place even at as low a temperature as 463 K in NaOH-KOH eutectic mixtures (Kato et al., 1993), and the rate of the reaction is considerably higher at higher temperature than the melting point of NaOH of 590 K. 2.3. Composition of the Oxidation Product. The rare earth content in the oxidation products prepared is shown in Table IV. The rare earth content for each changed only slightly depending on the reaction conditions in the oxidative decomposition such as the temperature and the alkali ratio. In addition, the ratios among the rare earth contents coincided well with those in the raw bastnaesite shown in Table I. This is reasonable since no rare earth oxide dissolves under these alkaline conditions in both the oxidative decomposition and the washing of oxidation product. 3. Preparation of Cerium Concentrate and Mixed Rare Earth Chloride from the Oxidation Product with Dilute HCl Leaching. 3.1. Behavior of the Oxidation Products in HCl Leaching. Figure 5 shows the two typical effects of the HC1 concentration and leaching time on the yield of cerium concentrate and CeOz content in the HC1leaching of oxidation produds prepared at 623 K, time 1 h, stirring speed 500 rpm, and alkali ratio 5 (a), and 673 K, 1 h, and alkali ratio of 5 (b), respectively. The six kinds of oxidation products shown in Table IV were leached with HC1 solutions. The results for the other four oxidation products were not shown here due to the similarity to those shown in Figure 5b. The charactsristics in the HC1 leaching of oxidation product are as follows. 1. The yield of cerium concentrate decreases with an increase in the HC1concentration though the CeOz content increases.
736 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993
. s3 .p
I
W L
a
S
I
I
\
P
!
5.
0
J 0L d20- -40- l60 ‘
80 100 Ce Extracted I %
Figure 6. Relationship among the Ce extracted and La, Nd, and Pr extracted. La, 0;Nd, 0; Pr, 0 .
up to 10-15 % . The valence of the Ce should be IV since the degree of oxidation is high. If the Ce in this phase is contained as Ce(II1)such as Ce(OH)3and Cez03, it should easily dissolve even in dilute HC1. In addition, the slope of La, Nd, and Pr extracted to Ce extracted is significantly higher compared to that in Ce extracted greater than 20 % as shown in Figure 6. Clearly, this phase is different from the CeO2-rich phase. A small amount of Ce(1V) making a solid solution with the other Ln203 may react with dilute HC1. 3. CeOz-rich phase: this is a ceria phase containing the other rare earth oxides as solid solution. Thus, the leaching of La, Nd, and Pr proceeds linearly with dissolution of Ce(1V). Obviously, cerium concentrate produced by this process is formed from the phase. The Ce extracted for the remaining four oxidation products prepared at 673 and 723 K was 5 % or lower even by 3 M HC1 and was entirely different from that for the oxidation product prepared at 623 K. Thus, at 623 K and the alkali ratio of 5, the crystallization of CeO2 in the oxidation product may be insufficient and the reactivity to HC1 is high as described above. The reason why these phases are formed in the oxidative decomposition can be explained by the limit of solid solution of CeO2 with the other trivalent rare earths as follows. The upper limits of solid solutions in Ce(1V)-Nd(II1) and Ce(1V)-Pr(II1) systems are about 70 and 50 atom % , respectively (McCullough, 1950). The limit of solid solution in this complex system is not clear but, perhaps, not so far from these values as shown below. The atomic percent of the total content of La, Nd, and Pr to the Ce content in the oxidation product is 104, which can be calculated from the data shown in Table W. Consequently, all the trivalent rare earth in the oxidation products cannot make a solid solution with the CeOz. The atomic percent was 74 in the cerium concentrate prepared by 0.1 M HCl leaching of the oxidation product obtained at 623 K and alkali ratio of 5. This value is almost equal to the upper limit of solid solution of 70% for the Ce(1V)-Nd(II1) system. Thus, the excess 30 % of trivalent rare earth makes the other two phases soluble in dilute HC1 solutions. 3.3. Shapes of the Oxidation Product and Cerium Concentrate. Figure 7 shows the raw bastnaesite, an oxidation product, and the cerium concentrate. The large bastnaesite particles are completely decomposed to form the oxidation product, coagulated fine oxide powder. In the HC1 leaching of the oxidation product, the external shape changes little; particle size of the resulting cerium concentrate is fairly uniform and about 0.1 pm, and the concentrate is applicable for low-speed abrasives. 3.4. Yield of Cerium Concentrate, CeOz Content, and Composition of Mixed Rare Earth Chloride. Table V shows the compositions of mixed rare earth
2 OO
t lmin
Figure 5. (a, top) Effect of HCl concentration on yield of cerium concentrate and CeOz content. 3 mol dm-3 HC1: 0,CeOz, content; A,yield. 1mol d m 3 HCl: 0 , CeOz content; A,yield. 0.1 mol dm3 HC1: 0 , CeOz content; A, yield. (b, bottom) Effect of HCl concentration on yield of cerium concentrate and CeOz content. 3 mol dm-3 HC1: 0,CeOz content; A,yield. 1mol dm3 HC1: 0 ,CeOz content; A, yield. 0.1 mol dm3 HC1: 0 , CeO2 content; A, yield.
2. In the leaching with 0.10 and 1M HCl solutions the yield of cerium concentrate becomes constant though free HCl remains enough as shown in Figure 5. 3. In 3 M HC1, the leaching continues slowly with time as shown in Figure 5b. This is caused by the reduction of ceria in the oxidation product by HC1 as shown in eq 3, since a smell of chlorine gas was detected. ThermoCeO,
+ 4HC1= CeCl, + (1/2)C12+ 2H20
(3)
dynamically, this reaction should proceed at a pH lower than -0.5 or more concentrated than 3 M HC1 at 298 K as shown in Figure 1. The effects of the oxidative reaction conditions on the characteristics of oxidation product in the HC1 leaching are as follows. Especially, temperatures higher than 623 K reduced the reactivity of oxidation product with HC1. In HCl leaching only the oxidation product prepared at 623 K and the alkali ratio of 5 showed an extraordinary behavior and dissolved completely in 3 M HC1 and no cerium concentrate was obtained as shown in Figure 5a. This may be caused by the insufficient crystallization of CeO2 in the oxidation product. 3.2. Characterization of Phases in the Oxidation Product. In order to analyze the phases in the oxidation product by dissolution behavior of Ce, La, Nd, and Pr the relationships among the Ce extracted and the La, Nd, and Pr extracted are calculated from the results shown in Figure 5 and shown in Figure 6. This result seems to show that the oxidation product consists of a mixture of the following three kinds of rare earth oxides. 1. Ce-free phase: this is indicated by the fact that the oxidation product dissolves easily in very dilute HCl without dissolution of Ce. 2. CeOz-lean phase: this is composed of Ln203 containing Ce. This is indicated by the fact that the La, Nd, and Pr extracted increase steeply with dissolution of Ce
Ind. Eng. Chem. Res., Vol. 32, No.4,1993 737
Figure 'I. Scanning electron microphotographs of (A) baatnaesite, (B) oxidation product, and (C) cerium concentrate
Table V. Typical Composition of Mixed Rare Earth Chloride HCI leaching content of LnCI$/% eoncn/moldm3 tlmin CeCh Lac4 NdCls PrC4 (a) Conditions: 773 K; Alkali Ratio 5: Stirring Speed 5W rpm; 1 h 3.0 15 11.4 68.6 14.5 5.5 3.0 30 14.5 71.5 14.6 5.5 3.0 €8 15.4 68.3 14.3 5.4 5.8 1.0 15 5.1 79.5 15.4 1.0 30 6.8 73.0 14.6 5.4 60 6.2 73.2 15.2 5.4 1.0 1.0 180 6.8 72.8 14.7 5.6 0.1 15 0 81.0 13.6 5.3 60 0 81.5 13.2 5.2 0.1 0 82.7 12.4 4.8 0.1 180 Conditions: 623 K, Alkali h t i a 5: Stirring Speed 500 rpm; 30 min 3.0 15 39.6 41.3 14.0 5.0 3.0 30 48.0 34.6 12.8 4.6 3.0 60 48.9 33.9 12.6 4.5 6.0 1.0 15 12.8 65.1 16.1 1.0 30 15.8 62.2 16.0 5.9 60 18.7 59.4 16.0 5.8 1.0 1.0 180 24.3 54.2 15.8 5.7 0.1 15 5.8 74.9 14.0 5.3 60 5.7 74.9 13.8 5.6 0.1 0.1 180 5.4 76.9 12.8 4.8
' Percentage of each rare earth chloride in these four chlorides.
chlorides calculated from the concentrations of corresponding leaching solutions. The characteristic in these data is that the Ce content in the mixed rare earth chloride can be completely suppressed in 0.1 M HC1 leaching of the oxidation products prepared a t temperatures higher than 673 K as shown in Table Va, where oxidation product was prepared at 773 K, alkali ratio 5, stirring speed 500 rpm for 1 h. In this case, however, the CeOz content in the corresponding cerium concentrate is as low as 55% or so as shown in Figure 5. On the other hand, Table Vb shows that cerium concentrates containing 70-72% CeOz can be obtained in 55-57% yield from the resulting oxidation product in the oxidative decomposition of bastnaesite at 623 K, alkali ratio 5, reaction time of 30 min, and successive 1 M HC1 leaching of the oxidation product at 333 K as shown in Figure 5a. The correspondingmixed rare earth chloride is composed of about 90% rare earth chloride and 10% alkaline earth chlorides. CeCl3, Lack, NdC13, and PrCb are present in the amounts of 11.5,58.5,14.4,and 5.4%, respectively. Finally, the CeOz content in the cerium concentrate and the composition of mixed rare earth chloride can be adjusted to some extent by the combination of the conditions in the oxidative decomposition and in the "21 leaching of the resulting oxidation product.
Literature Cited Beuer, D. J.; Shaw, V. E. Metathesis of Baatnaeaite and Solvent Extraction of Cerium. Bur. Mines Rep. Inuest. 1964. No. 6381, 1-15. Cobble, J. W. Thermodynamic Properties of High Temperature Aqueous Solutions. VI. ApplicationsofEntropy Correspondence to Thermodynamicn and Kinetiea. J . Am. Chem. Soc. 1964,86, 5394-5401. Criss, C. M.; Cobble, J. W. The Thermodynamic Properties of High Temperature Aqueous Solutions. N. Entropiesof Ions up to200 C and the Correspondence Principle. J.Am. Chem. Soc. 1964a. 86,5385-5390. C r b , C. M.; Cobble, J. W. The Thermodynamic Properties of High Temperature Aqueous Solutions. V. The Calculation of Ionic Heat CapaeitiesuptoZWC. EntropiesandHeatCapacitiesabove 2W C. J . Am. Chem. SOC. 1964b, 86.5390-5394. Inoue, K. Jpn. Kokai 58-6947,1983a. Inoue, K. Jpn. Kokai 59176126,1983b. Inoue, K. Jpn. Kokai 59177424,1983~. JIS M 8404. Methods for Determination of Rare-Earths in Or-; 1976. Kato, K.; O d d , Y.; Okuwnki, A. Mechanism of the Oxidative DeeompositionofBastnaesiteinaNaOH-KOHEutecticMixture. Nippon Kogaku Koishi 1993, No. 5, in press. Kiriyama,R.; Kiriyama, H. InorganicStructural Chemistry;Kyoribu Press: Tokyo, 1964; p 283. Mateumoto, T.; Hayaahi, M.; ShinLai, Y.;Ohtaki, N. Jpn. Kokai fil-2RUI)Rl. .._ . . . . 1RM. . .. .. McCullough,J. D.AnX-rayStudyoftheRare-earthOlideSystems: CeN-Ndm, CeN-Pr"', CeN-Pr'" and PP-Ndm. J. Am. Chem. Soc. 1950, 72, 13861390. Narita, E.; Mita, M.; Ohbe, T. Oxidation of Pyrolusite by Molten Potassium Sdte. Nippon Kagaku Koishi 1976,281-288. Pourbair, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Presn: Oxford, 1966; p 194. Sebumm, R H.; Wagman, D. D.; Bailey, S.;Evans, W. H.; Parker, V. B. Selected Values of Chemical Thermodynamic Properties. NBS Tech. Note ( U S . ) 1973, No. 27W7. Shiohwa, J.; Adachi, G. Kidorui Genso no Kagaku; Kagaku Dojm Kyoto, 1984; p 26. Translated into Japanese from: Topp, N.E., Chemistry of the Rare-Earth Elements; Elsevier: Amsterdam,
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1965.
Tanaka, A,; Tayama, K. Jpn. Kokai 5967836,1983, Toyoda, S.; Ito, K.; Tokuda, M.; Kitagawa, K. Extraction of Rare EartbElements by AlkalineHydrothedPmxees. J.Min. Mater. Process. Inst. Jpn. 1991, 107, 231-237. Remillon, B. Chemistry in Non-aqueoua Soluents; Reidel: Dordrecht, 1974; p 178. UIlmcrnns Eneykbpaedie der Teehnischen Chemie; VerLug Chemie: Weinheim, 1982; Vol. 21, p 245. Wagman,D.D.;Evans, W.H.;Parker,V.B.;Hdow.I.;Bailey,S.M.; Scbumm. R. H. Selectad Values of Chemical Thermodynamic Properties. NBS Tech. Note (U.S.) 1968, No. 27W3. Received for review July 30, 1992 Reuised manuscript receiued November 28,1992 Accepted January 4,1993
