Ind. Eng. Chem. Res. 1995,34, 3963-3969
3963
Mutual Separation of Mixed Praseodymium and Neodymium Oxides via Metal Halide Gaseous Complexes K u n i a k i Murase, Teruaki Fukami, Ken-ichi Machida, and Gin-ya Adachi* Department of Applied Chemistry, Faeulty of Engineering, Osaka Uniuersity, 2-1 Yamadaoka, Suita, Osaka 565, Japan
A dry process for mutual separation of rare earths was investigated on a mixed oxide of praseodymium and neodymium using a flow-type chemical vapor transport reaction mediated by metal halide vapor complexes, KFtCldg) (R = Pr, Nd). Some potassium salts, KzCO3, KzS04, moa, KF, and KAl(S04)2,were used as a precursor of a complex former, KCl, and the yields of RC13 transported were 55,37,32,18,and 26%, respectively, while the yield when the KC1 was directly used as the complex former was 25%. The transport reactions using KzCO3 were carried out under various kinds of temperature gradients with a constant-temperature plateau zone, and the decrease of the temperature for such a plateau zone resulted in an increase of the separation factor for Nd. Empirical vapor pressures of KPrCL(g) and KNdCla(g) as a function of temperature were evaluated by altering the plateau temperature. The empirical vapor pressures were about 1 order smaller than those expected from the existing thermodynamical functions probably due to a nonequilibrium flow-type reaction in this system. Finally, a heat necessary for the chemical vapor transport of Ndz03using KzCO3 as the precursor was calculated as 191.2 kJ mol-’ of Nd.
Introduction
I
Halogen-bridged vapor complexes are known t o form in a number of metal halide binary systems (Hilpert, 1990;Papatheodorou, 1982 and references cited therein). The apparent volatility of rare-earth chlorides (RC13) increases in the presence of aluminum chloride, iron chloride, or alkaline metal chlorides (AC1) due to the formation of vapor complexes, RAlnC13+3,(g),RFe,Cl3+3,(e)(n = 1-4). and ARClde). We have reDorted recently tgat a chemical vapor &&port (CVT)reaction medi”ated by the vapor complexes is a promising dry process alternative t o a wet method whereby purified rare earths for commercial use are a t present produced (Adachi et al., 1991,1992;Murase et al., 1992,1993, 1994). The raw material so far used for the CVT process has been a mixed rare-earth chloride, which is difficult t o handle due to high hygroscopicity. In the present work, the separation characteristics of rare earths were studied using a mixed rare-earth oxide as a raw material for the CVT reaction. The resulting separation efficiency was discussed in terms of the chlorination and transport conditions and mechanism, and, further, a simulation of the transport phenomena was carried out by a calculation based on known thermodynamic data. Experimental Section Materials. The equimolar mixture of praseodymium and neodymium oxides (total 6.0 x mol) was used for the CVT reactions. Praseodymium and neodymium are one of the neighboring rare-earth pairs and the most difficult system for mutual separation. In order to obtain a homogeneously mixed raw material, the mixed oxide was prepared from a corresponding aqueous solution of the mixed praseodymium and neodymium chlorides by precipitation with a saturated (C0ONHa)z solution in pH = 2. The obtained Rz(C00)3 was then filtered off, washed with deionized water, and calcined
* Author t o whom correspondence should be addressed.
a1anric furnace
I
Figure 1. b s e m b l y of electric furnaces for cVT Numbers in the electric furnace denote fraction number (FN) of separation.
with a methane flame on a platinum crucible for 1 day to give the mixed oxide, &Os. The oxide was analyzed on an X-ray fluorometer to make sure the composition was Prmd = 111. Potassium chloride (6.0x 10-3 mol) equimolar with a R3+ ion in the &OS was used as the complex former (transporting agent) for the CVT reaction. As well as KCl, some potassium salts, KzCO3, KzSO4, moa, KF, and KAl(S04)2, were also used as a precursor of the complex former, KC1, in order to avoid a deviation of the composition of the raw melt where a mole ratio FUK of l/1 is desirable as mentioned below. These potassium salts were reagent grade (>99.0%)and used without any further purification except for KAl(SO4)z which was prepared from a corresponding hydrate by heating a t 423 K. A powdery active carbon (-0.5 g) as a deoxidant was also mixed with the raw material without any pretreatment. Operation. The instrumentation to obtain a temperature gradient for CVT reaction (Figure 1)has been described in detail elsewhere by Murase et al. (1993). A raw mixture of the mixed oxide, potassium salt, and active carbon was put in a mullite boat (length, ca. 9 em). The boat was then placed in a quartz inner tube (outer diameter, 22 mm; inner diameter, 19 mm; length, 14 em) and loaded in a quartz reactor tube (inner diameter, 25 mm; length -1.0 m) with a stream of Nz (30 cm3 min-1) and Clz (5 cm3 min-’) gases. Upon heating by electric furnaces up to 1273 K, the rare-earth mixed oxide and potassium salts were chlorinated by Clz to RC13 and KCl. These reactions are expressed as
0888-5885/95/2634-3963$09.00/00 1995 American Chemical Society
3964 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
follows:
+
-.
+
+ 3CO(g) (1) + C12(g)+ 2C(s) - 2KCl(s,l) + 3CO(g)
R203(s) 3c12(g) ~ C ( S ) 2RC13(s,l) &CO,(s)
(2) and
Table 1. Transported Amounts and Yield of Rare Earths When Various Kinds of Potassium Salts Were Used as a Precursor of KCP transported a m o ~ n t / l O mol -~ potassium salt Kzco3
&so4 mo3 KF
~(so4)z KCl KClb
where (s), (11, and (g)represent solid, liquid, and gaseous states, respectively. The mixture was finally heated at 1273 K, and the resulting RC13 and KC1 were converted to the vapor complexes via reaction as
+
RC13(s,l) KCl(s,l) = KRC14(g)
(4)
The complexes, KPrC14(g)and KNdCWg), were driven with the N2-Cl2 gas stream in the reactor along a temperature gradient and decomposed according to the reverse process of eq 4, and rare-earth chlorides were regenerated. After the CVT reaction lasted for 6-82 h, the resulting deposits along the temperature gradient were collected from the inner tubes (see Figure 1). The deposits in the inner tubes and the residual mixture on the boat were dissolved individually in dilute hydrochloric acid to determine the composition of Pr, Nd, and K for each inner tube (FN, fraction number) on an X-ray fluorometer (Rigaku System 3270A1.
Results and Discussion 1. CVT Reaction Using Various Potassium Salts. In the previous study (Murase et al., 1992), where an equimolar mixture of anhydrous chlorides, RCl3 (1.65 x mol; Pr/Nd = 1/1) and KC1 (1.65 x mol), was used as a starting material for the CVT reaction, 82% of initially loaded RCl3 was transported after reaction for 6 h; the yield was calculated as
yield (%) = lOO(N,, - N&V0
(5)
where NOand N B are the molar quantity values of the rare-earth contents of raw material initially loaded and residue in the boat after the CVT reaction, respectively. However, in the present study where not RC13 but Rz03 was used as the starting materials, the direct addition of KC1 to the mixed R203 resulted in a low yield of rareearths, 25%, even if the same reaction temperature and time were employed. In this case, a part of the KC1 may have vaporized before the chlorination of R2O3 to RC13, and, as a result, the composition of the raw material deviates from the mole ratio R/K of 1/1. This mole ratio is of importance for the effective transport of RCb via the vapor complex, mc14(g), since the vapor pressure of KRC14(g) above a melt with this composition is the highest (Murase et al., 1992). In order to avoid the deviation of composition, some potassium salts other than KC1 were tried as a precursor for KC1. The potassium salt charged as the precursor of KC1 was gradually chlorinated simultaneously with the mixed rare-earth oxide, and, therefore, the mole ratio (FUK = 1/11 was expected to be kept during chlorination. Table 1summarizes the amounts of transported rareearth chlorides and the yield for five precursors, K2CO3, KzSO4, KN03, KF, and KAl(S04)2, and also for KC1. Of all the five precursors tested, KzCO3, KzS04, and KNO3
Pr
Nd
1.57 1.03 0.93 0.48 0.64 0.69 0.67
1.72 1.22
1.01 0.58 0.76 0.82 0.69
yield/% 55 37 32 18 9.8 25 82
a Raw material was R203 (6.0 x mol; Pr/Nd = 1\11. Mixed mol; PdNd = 1/11was used as anhydrous chloride (1.65 x the raw material.
improved the transportation efficiency, that is the yield, of RC13. Among them the yield obtained by using K2COS was the highest, and 55% of rare-earths initially loaded was transported. On the other hand, KF and KAl(SO4)zprovided negative effects on the yield of RC13. As for the KF, the decrease in the yield can be explained from a thermodynamical (Barin and Kanacke, 1973) aspect that the chlorination of KF' takes place with difficulty. Even for the most plausible chlorination reaction of KF, KF'(1)
+ C12(g)= KCl(1) + ClF(g)
(6)
lies far to the left-hand side, AGs(1300 K) = +63 k J mol-l, while both the eqs 2 and 3 are leaned toward the right-hand side, AGz(1300 K) = -533 k J mol-l and AG3(1300 K) = -603 k J mol-'. On the contrary, the chlorination of KAl(SO4)2 took place easily, and, furthermore, the chlorination of KAl(S04)2 gives KC1 and at the same time, both of which function as the complex formers against RCl3. Therefore, the amount of RC13 using KAl(SO4)z was expected to be increased compared with that for K2SO4. However, the yield for K A l ( S 0 4 ) 2 was almost on the same level with that when KC1 was directly used. This can be interpreted as the generation of a more stable vapor species KAlCk(g)from KC1 and (Millman and Kusch, 1939). Hence, the effective amount of KC1 for the vapor complexation with RCl3 was reduced. Consequently, the use of some potassium salts as a precursor of KC1 renders the effective CVT reaction of rare-earth oxides possible, and KzCO3 is the most appropriate from a viewpoint of the transport efficiency. Equation 3 shows that CO gas generates during the process. Though the CO generation seems disadvantageous, it is usual for a high-temperature chlorination process and oxidation processes of CO to C02 to have been established. 2. CVT Reaction Using Stepwise Temperature Gradients. Introduction of Stepwise Temperature Gradients. The vapor complex KNdCUg) is more stable than KPrC4(g), and the KPrCL(g) tends to decompose a t higher temperatures than KNdCl4(g) (Murase et al., 1992). Therefore, Pr- and Nd-rich deposits are obtained from high- and low-temperature fractions in the temperature gradients, respectively. In the present section temperature gradients with a plateau zone (see Figure 2a) with a temperature of Tconst-stepwise temperature gradients-were employed. Figure 2b shows the amount of rare earths condensed over 13 fractions when a typical stepwise gradient with the constant temperature of 1183 K was employed. Here, KzCO3 was added to the R203 raw mixture as a
Ind. Eng. Chem. Res., Vol. 34, No. 11,1995 3965 1273
2.00
I
1173 1073
1.80
L
.
-3 2
1.60 c .P 1.40 -
2 2 973 9
g
1
873
5
+: 773
g
8
673 573
1 2 3 4
5 6 7
8
9101112
1.20 1.00 -
L
0.80 1050
Fraction number
1100
1150
1200
1250
Constant temperature I K 5 2.50 E
,
0 .
9
" c 2
0
5
2.00 1.50
1.00
?. 1 2 3 4
5
$ 0.50
6 7 8 9 10111213
Fraction number Figure 2. Typical profile of (a) a stepwise temperature gradient with the plateau zone maintained at T,,., = 1183 K and (b) distribution of PrC4 and NdC13 when reacted a t 1273 K. The raw material was equimolar mixed praseodymium and neodymium oxide (6.0 x mol); the complex former was &COS (3.0 x mol) as a precursor of KCI; an active carbon powder (0.5 g ) was added to the raw material as a deoxidant: a mixed N2 and Clz gas (N2, 30 cm3 min-'; Cln, 5 em3 min-'1 was flowed as the carrier; reaction time was 82 h.
precursor for KC1. At the fraction number of 8 (FN = 81, which corresponds to the constant-temperature zone, no deposition took place and, hence, the Pr- and Ndrich deposits were separately obtained from hightemperature fractions (FN = 3-7; T > T,,,,,)and lowtemperature fractions (FN = 9-13; T < T,,,,J, respectively. In other words, the Pr- and Nd-rich fractions, that is the high- and low-temperature fractions, became clear by use of the stepwise temperature gradients. Separation factors, P h N d and P N ~ . were , defined as follows for the Pr- and Nd-rich portions, respectively: &jNd
= (lhH)/(l/'O)
(7)
=L'"O
(8)
c
2 0.w .. 1050
1100
1150
1200
1250
Constant temperature I K
Figure 3. Relationship between the constant temperature of the stepwise temperature gradient (T,..t) and separation characteristics: (a) separation factors for the Pr-rich portion (BRM) and the Nd-rich portion ( S N ~ )(h) ; the transported amounts of PrCl3 a t the Pr-rich portion (NHR) and of NdC13 a t the Nd-rich portion ( N L N ~The ) . reaction times were 48 and 82 h.
and low-temperature regions, NHP~, "Nd, NLP~, and NLN~, were calculated on the basis of the separation factors, PPrmd and b~d/pr, as
"Nd
= NONd
- NLNd
(10)
and
and PNmr
where rH, m,and ro are N d P r mole ratios for the deposits of the high- and low-temperature fractions and the raw material, respectively. In the present work ro was kept at 1.00 for all runs. For the deposition profile of Figure Zb, for example, PPmmd and PN~IR were calculated as 1.19 and 1.25. Since the transported RC13 was condensed not only on the surface of the inner tubes (see Figure 1)but also on the inner wall of the quartz reactor tube, all RC13 deposits along the temperature gradient were unable to be recovered. We can recover the deposits only on the inner tubes. Thus, total amounts of RC13 detected from the inner tubes are always less than real transported amounts of RCl3, which are expressed as NONg in eq 5. On the other hand, the separation factors are invariant values whether the deposits are recovered thoroughly or not. In the following sections, the real transported amounts of fic13 and NdC13 a t the high-
where NORand Nom are the molar quantities of P13+ and Nd3+ions in the initially loaded mixed oxide, R& (NO= NOR Nod, and Npr and are the total transported amounts of PrCh and NdC4 (NO- NB= NB N~:), respectively. Effects of the Constant TemDeratures,. Tmnat,on Separation Efficiency. The '2i.r reactions using the stepwise temperature gradients with various constant temperature, TcOnst, were carried out. The reaction temperature was kept a t 1273 K, and the reaction time was altered from 48 to 82 h. Figure 3 shows the relationship between the TmnStand the separation factors (B~dlprand &Nd) and the transported amounts ("pr and NLNd) when reacted for 48 and 82 h. As TcoDst is increased, Bpr~.nvdincreases and P N decreases ~ ~ (Figure 3a), whereas N H P~ decreases and the N L N d increases. In other words, the CVT reaction using a temperature gradient with low T,,, gives a high-purity NdC13whose yield is, however, very low and vice versa. These are explained as follows. Since the KNdCldg)
+
+
3966 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
. 5
1.5
0
5
1.0
E0
ee 0.5
l-
0
20
40
60
80
Time I h
Figure 4. Schematic representation of deposition profiles of PrCl3 and NdC13 as a function of temperature and definition of Pr-rich and Nd-rich portions when the constant temperature of the is relatively (a) low and (b) stepwise temperature gradient (Tconst) high.
complex is more stable than KPrC14(g) (Murase et al., 1992), the temperature for the region where NdC13 deposits is lower than that for PrCl3, as represented schematically in Figure 4. If the constant temperature is relatively low (Tconst = Ta; see Figure 4a), then the /&&‘pr value becomes large since most of the deposit at a T Ta region is NdCl3, while the amount of NdC13 deposit at this region, N L N d , is small. On the contrary, if the constant temperature is high (Tconst = Tb; see Figure 4b), becomes small, whereas N L N d increases. Consequently, it is difficult to raise the separation efficiency and the yield at the same time. One should select an appropriate Tconstvalue according to the situations such as composition of the raw material and demand on the market. 3. Simulation of the CVT Reaction. Since the CVT reaction takes place at high temperatures, the reaction mechanism is too complicated to be described as a simple reaction route. Therefore, it seems difficult to predict theoretically the separation efficiency based on some existing thermodynamic data. Furthermore, characteristics of this system, where the CVT reaction goes under thermodynamically nonequilibrium conditions due to a flow-type reactor, also make the theoretical interpretation of the reaction difficult. On the contrary, the fact that no deposition of RC13 was observed at around the constant-temperature region (see Figure 2b) suggests that the gas phase in the constant temperature region is apparently in an “equilibrium” condition. Hence, the transported amounts at the low-temperature portion, N~prand N L N d , represent this “quasi equilibrium” composition of the gas phase at the constant temperature, Tconst.In this section, on the basis of empirical vapor pressures calculated by changing the constant temperature and the reaction time, an estimate of the separation efficiency has been attempted. First, the relationship between the total transported amounts, Npr and “d, and the reaction time during the CVT reaction at 1273 K was obtained both for PrCl3 and NdCb. Partial vapor pressures of KPrC4(g)and KNdC4(g) above the raw mixture after the elapse of time t
Figure 5. Total transported amounts of PrC13 (Np,) and NdC13 (“d) as a function of the reaction time when reacted at 1273 K. The raw material was equimolar mixed praseodymium and neodymium oxide (6.0 x mol); the complex former was Kzcos (3.0 x mol) as a precursor of KC1; an active carbon powder (0.5 g) was added to the raw material as a deoxidant; a mixed Nz and Cl2 gas (N2, 30 cm3 min-l; Clz, 5 cm3 min-’) was flowed as the carrier.
can be determined from the slope of the NPr and N N d vs t curves given in Figure 5 and the equation of state for an ideal gas. Though the Npr(t) and N N d ( t ) values increase with time, the rate of increase gradually decreases as the time passes, indicating that the amounts of generating vapor complexes were changing through the CVT reaction. From the analysis of the residue on the boat, it turned out that this change of the vapor pressure is due to the deviation of composition of the melt, where the K/R ratio gradually decreases during the reaction, since KCl(g)vaporizes simultaneously with nc14(g). At the early stage of the CVT reaction (t = 0-12 h) the rate of KNdCL(g) generation is larger than that of KPrC14(g),owing to the difference in stability of these complexes. On the contrary, the Npr(t) and N N d ( t )vs t curves become parallel at t > 20 h, suggesting that the generating rates become almost equal. Empirical vapor pressures of the vapor complexes were, then, calculated for KPrC14(g) and KNdCl4(g) according to the following procedures: (a) Saturated vapor pressures of KRCUg) (PwrC14(T) and f h d C 1 4 ( T ) ) at a temperature T were calculated from the vapor pressures of RC13(g)(Novikov and Baev, 1962) and KCl(g) (Zimm and Mayer, 1944) by assuming the equilibrium constants, Kp,(T) and K N d ( T ) , of the equation KRC14(g)= KCl(g)
+ RC13(g)
(R = Pr, Nd)
(13)
whose enthalpy and entropy changes have been reported = 247 k J mol-l and by Novikov and Baev (1964): @13 = 136 J mol-l K-l for both Pr and Nd. Furthermore, empirical factors, fpr(T) and fNd(T), were introduced to evaluate apparent equilibrium constants, f p r ( T ) K P r ( T ) and fNd(T) K N d ( T ) , i.e.
The vapor pressures of RC13, PRCls(T), and KC1, PKC1(T),represented in atmospheres, were given as log PKcl(T) = -10710T1 - 3.0 log T
+ 16.03
(15)
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3967
+
Table 2. Empirical Factors, fk(n, for Apparent Equilibrium Constants of Reaction KRC4(g) = KCl(g) RC13(g) and Apparent Vapor Pressures of KRc14(g),P ~ C ~ , ~ for ~ ~the ( T Flow-Type ) , Chemical Vapor Transport Reaction (R = Pr and NdP reaction time/h 48 48 48 48 82 82 82 82 82 82 82
TconstbK 1203 1183 1163 1123 1223 1203 1183 1163 1123 1103 1073
f~r(Tconst1
9.0 7.4 8.6 9.4 9.3 8.2 6.9 9.4 6.0 6.9 3.4
KPrCldg) P~~~l,.app(Tconst)/atm 4.6 x 10-4 4.0 x 10-4 2.5 10-4 1.1x 10-4 6.1 10-4 5.1 x 10-4 4.3 x 10-4 2.3 x 10-4 1.8 x 10-4 1.1x 10-4 1.2 x 10-4
fNd(Tconst1
11.0 9.4 9.8 9.6 9.9 10.5 9.7 11.5 6.9 7.8 4.2
KNdClAg) PKNdci4,app(TConst)latm 6.8 x 5.9 x 10-4 4.2 10-4 2.2 10-4 1.0 x 10-3 7.2 10-4 5.8 x 10-4 3.6 10-4 3.1 10-4 2.0 10-4 2.1 10-4
a Raw materials were (1.65 x mol), K2C03 (8.25 mol), and an active carbon powder; a mixed N2 (30 cm3 min-l) and Cl2 (5 cm3 min-') gas was passed through the reactor. The reaction temperature was 1273 K. Constant temperature in the stepwise temperature gradient; see the text.
log Pprc13(T) = -13810Tl
+ 7.563
(16)
log P,dCl3(T)
+ 7.089
(17)
and = -12930T1
(b) The reaction period was divided into short time intervals, Ati (i = 1, 2, 3, ...), and the amounts of generated vapor complexes, A i V i , m c 1 4 (R = Pr, Nd), during each Ati were obtained from Figure 5. For the sake of convenience, the Npr and vs t curves (Figure 5) were approximated by some adequate polynomial functions though there is no theoretical background. The amounts, A i V i , m C l 4 , were then converted to vapor pressures, (R = Pr, Nd), at the temperature of Tconst using the relation Pi,KRCl,
=
mi,mCl,RTconst
SvAti
(R = Pr, Nd;i = 1,2,3,...) (18)
where S, v , and R are the cross section of the reactor, the velocity of the N2-Cl2 current, and the ideal gas constant, respectively. (c) If P m L , a p p ( T c o n s t ) IP i , m 4 , then all of the m 1 4 (g) complex was considered t o deposit at the lowtemperature region below Tconst, and if Tconst) P i , m 1 4 , then the complexes corresponding to the difference P i , m r C 1 4 - PmrCl4,app(Tconst) were assumed to deposit a t the high-temperature region above Tconst, and the rest Pmrc14,app(Tconst) was condensed to the lowertemperature region. For the KNdC14(g) complex, a similar calculation was carried out. (d) For each Ati the calculations b-c were done, and, then, a pair of calculated "Pr, " N d , N L P r , and N m d values was obtained as a summation over all 2s. (e) The calculations b-d were repeated by altering the fPr(T) and fNd(T) factors from 1 t o larger values to fit the calculated N H p r , "Nd, NLP~, and N L N d with the experimental ones obtained from eqs 9-12. Table 2 summarizes the empirical factors together with the apparent vapor pressures, PmC14,app(T), calculated from eq 14. The empirical factors lie around 6-11, suggesting the apparent vapor pressures are lowered compared with the equilibrium vapor pressures, P m c 1 4 (27, which are predicted from eqs 13 and 15-17 ( P m c 1 4 ( T ) = P R c i 3 ( T ) PKC~(T)IKR(T)). f d T ) is generally larger than the correspondent &(T) value. Several factors can
be stipulated for the lowering of vapor pressures: (i) RC13 and KC1 are thermally stabilized by forming the molten mixture and, therefore, the vapor pressures of RCldg) and KCl(g) are reduced compared to those expected from eqs 15-17; (ii) the vapor complexation usually has a slow reaction rate (0ye and Gruen, 1969), resulting in an apparently low vapor pressure, since the CVT reaction takes place on a flow-type reactor where eq 13 is not thoroughly thermodynamicallyequilibrated; (iii) there are some interactions between Pr- and Ndvapor species, though these are not taken into account in the above calculations. Although the extent of contribution of these factors is ambiguous, it is noteworthy that the apparent vapor pressures are more or less affected by equilibrium vapor pressures, P m r C 1 4 ( T ) and PKNdC14(T), because the log P K R C ~ , ~ ~ ~ (vs T ) 1/T plots roughly show a linear relationship and the PKNdCl4,app(T) is always larger than P m r c l r 4 , a p p ( T ) in analogy with Pmc4(T)where h N d C l , ( T ) is also larger than PmrC14(T). Based on the apparent vapor pressures, the relationship between the transported amounts, N H P r and N L N d , and the separation factors, PPrlNd and PNW,., was predicted for 48 and 82 h reaction. Here, the apparent vapor pressures were approximated as
log P ~ ~ 1 4 , a p p ( T = ) -7519T1
+ 29134
(19)
log 'KNdCl,,app (2'') = -9537T1
+ 21850
(20)
and
by the least-squares method on the basis of the log vs 1/T plots given in Figure 6. Together with the calculated relationship (Figure 7) assuming reaction for 48 and 82 h, experimental values for the same reaction time were plotted. When N L N d is at a low level, the separation factor, P N W r , is expected to reach around 1.8-1.9, which exceeds the factor for conventional = 1.7) (Pierce and Peck, 1963) solvent extraction (PNw~ where bis(2-methylhexyl)phosphoric acid (DBEHPA) was used for an extractant and, actually, the factor of 1.7-1.9 was obtained. 4. Necessary Heat for the CVT Reaction. The chlorination and vapor complexation are so complicated that we cannot write down a simple reaction scheme as mentioned above. However, it is important to calculate the necessary energy for the CVT reaction for the sake of comparingthe CVT process with the conventional wet methods. So, the necessary heat for formation of a vapor complex, KNdC14(g),was calculated by assuming some steps using existing thermodynamic functions
PmCl4,app
3968 Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995
'% 0
. $
9
-3.0
'.-
1
KNdCI, KPrCI, KNdCI,
-4.0 -4.5 8.0
=r I
1
.g
4
1
"
8.4
'
'
8.8
'
"
I
1
0.8 0.10
'
9.2
1.2
v)
i
KPrCI,
1.4
em 8
9.6
1.7
i o 4 r-li K"
Transported amount / 1O 3 mol
Figure 6. Vapor pressures of the KRC4 complex calculated from existing thermodynamic functions (solid line) and apparent vapor pressures of KRC4 observed in a flow-type CVT reaction (dashed line).
(Barin and Kanacke, 1973; Novikov and Baev, 1964; Ciach et al., 1973). All starting materials, Nd203, KZCOS,C, and Clz, were supposed to be introduced into a reactor at room temperature (298 K) and heated up to 1300 K. Then, the NdzO3 and K2CO3 are chlorinated, yielding NdCl3, KC1, and CO gas, and the resulting NdCl3 and KC1 vaporize and subsequently form the vapor complex, KNdCldg). The enthalpy change for each step is summarized in Figure 8. The heat necessary for the overall reaction '/,Nd,O,(s)
+ '/zl(jC03(~)+ 5/2C(s)+ 2C12(g)[298 KlKNdCl,(g) + 3CO(g) [1300 Kl (21)
was calculated as 190.1 kJ mol-' which corresponds to 1.12 x lo6 k J ton-' of Nd203 oxide treatment. For the formation of the KPrClr(g) complex, almost the same amount of heat is expected due to the similarity in chemical properties between Pr and Nd. Since the efficiency of the apparatus is less than loo%, a practical necessary heat for the CVT process is higher than the above calculated value. However, the heat necessary for the CVT process can be reduced by combining the process with a conventional direct chlorination method for extracting rare-earths from the ores, because the heating and chlorinating steps of Nd203
'/,Nd,O,(s)
+ 3/,C(~)+ 3/2C12(g)1298 Kl NdC13(1) + 3/2CO(g)[1300 Kl
1.30
0.90
0.50
2.0 I
0.8
I
I
0.25
I
I
1
I
0.75
I
I
I
1
1.25
I
I
I
I
1.75
I
2.25
Transported amount / 1O 3 mol
Figure 7. Relationship between the transported amount of PrC13 ) the separation factor (Pprmd) and at the Pr-rich portion ( N ~ p rand between that of NdCl3 at the Nd-rich portion (NLNd) and the ) reacted for (a) 48 and (b) 82 h. Solid separation factor ( P N ~when and dashed lines are calculated, and plots are experimentally observed values. (298K)
(1300K)
(1300K)
(1300K)
(1300K)
'r
- 248.1 KNdCI&)
(22)
will be omitted in the combination process. We cannot compare the necessary heat for the CVT process with that of other conventional wet methods. However, it is no doubt that the dry CVT processes need less energy than the wet methods, since the CVT process is quite simple whereas the wet ones always require a series of complicated treatments such as dissolution of raw material, precipitation of filtrates, and drying and calcination of precipitates.
Conclusions The mutual separation of praseodymium and neodymium oxides was conducted effectively using the CVT process mediated by a metal halide vapor complex, KRc14(g), where KzC03 was used as a precursor of KC1, the complex former. The separation characteristics of the complexes for the flow-type CVT reaction using the stepwise temperature gradients with various constant
Figure 8. Change of enthalpy for each step for calculation of the necessary heat of CVT reaction represented in kJ mol-l of Nd3+ or Kf except for C(s) and Clz(g) which are in kJ mol-l.
temperature zones strongly depended on the temperature of the plateau, Tconst. When Tconst is low, the amount of recovered NdC13 was small whereas the separation factor is high, and vice versa. By employing apparent vapor pressure curves of the complexes for the flow-type CVT reaction, this alternative feature of the recoveries was simulated.
Acknowledgment The present work is supported by Grants-in-Aid for Scientific Research 06241106,06241107 (Priority Areas "New Development of Rare Earth Complexes"), and 051298 from the Ministry of Education, Science, Sports, and Culture, Japan. K.M. is in receipt of a Fellowship
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3969 of the Japan Society for the Promotion of Science for Japanese Junior Scientists.
Nomenclature A = alkaline metals CVT = chemical vapor transport fpr(T), fNd(T) = empirical factors for vapor pressures of KPrC14(g) and KNdC14(g) F N = fraction number for separation AG2, AG3, AGg = free energy changes of corresponding reactions, kJ mol-l AH13 = enthalpy change of reaction 13,kJ mol-1 KR(T)= equilibrium constant of eq 13,atm NO= amount of initially loaded rare earths (=NOR. NONd), mol NOR,NoNd = amounts of initially loaded praseodymium and neodymium oxides, mol NB= amount of residual rare earths on the boat, mol Np,, NNd = total transported amounts of PrCl3 (=NHR. NLPr) and NdCb (="Nd -k NLNd), mol NHR.,NHNd = transported amounts of PrCb and NdC4 at the high-temperature region, mol NLP~, NLNd = transported amounts of PrCl3 and NdCl3 at the low-temperature region, mol ANt,mcl4= amount of generated KRCk(g)vapor during Ati, mol P d T ) , Pprci3(T), PNdC13(T) = vapor pressures of KC1, fic13, and NdCl3, atm P-L,(T), PKNdCL(T) = saturated vapor pressures of KPrC4(g) and KNdCldg), atm &rC4,app(T), PKNdC14,app(T) = apparent vapor-pressures of KPrCldg) and KNdCldg), atm P L , ~ e Pg,KNdC14 4, = mean vapor pressures of KPrC4(g) and KNdCl*(g)during Ati, atm ro, r H , r L = mole ratio (Nd/Pr)for the raw material, for the deposit of the high-temperature region, and for the deposit of the low-temperature region R = rare earths: praseodymium and/or neodymium R = ideal gas constant S = cross section of the reactor tube A S 1 3 = entropy change of reaction 13,J mol-I K-' t = reaction time of CVT AtL= time intervals for calculation of empirical vapor pressures T = temperature, K Teonst= constant temperature of the stepwise temperature gradient, K u = velocity of the Nz-Cl2 current
+
+
Greek Symbols &y"&P N ~ /=P separation ~ factor for Pr-rich and Nd-rich portions
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Received for review November 10, 1994 Accepted J u n e 12, 1995@ IE940656Z
@
Abstract published in Advance ACS Abstracts, September
15, 1995.
