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Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability
Hydrometallurgy, 34 (1993) 187-201
187
Elsevier Science Publishers B.V., Amsterdam
Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability J.W.
van Put
a, G.J. Witkamp b,. and G.M.
van Rosmalen
b
a Union Mini~re, Proces Research andDevelopment, Fabrieksstraat 144, B-3900 Overpelt, Belgium b Delft University of Technology, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands (Received September 25, 1992; revised version accepted February 25, 1993 )
ABSTRACT The crystallization of ammonium paratungstate tetrahydrate, ((NH4)IoH2W12042"4H20, APT-4H20) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. A literature survey showed that ammonium paratungstate hexahydrate (APT-6H20) can form under conditions where APT. 4H20 forms as well. Based on this information it was postulated that APT. 6H20 is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO3 and from approximately 90 ° to 96°C. APT-6H20 is not formed when APT-4H20 seeds are added prior to crystallization. The experimental data are presented in a three-dimensionalternary phase diagram. It was found that the recrystallization of APT" 6H20 into APT. 4H20 is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT. 6H20 will not form during industrial crystallization.
INTRODUCTION
The crystallization of ammonium paratungstate tetrahydrate, (NH4) 10 [ H2W12042 ]° 4H20, (APT" 4H20 ) from aqueous ammonium tungstate solutions is a key unit operation in the current production of tungsten powder. The crystal size, crystal size distribution and degree of agglomeration of APT. 4H20 are all likely to be of importance for its processing to tungsten powder [ 1 ]. Both batch and continuous crystallizers are used in industrial practice. The start-up procedure for continuous or batchwise crystallization of APT. 4H20 is to heat an a m m o n i u m tungstate solution to its boiling point, the actual value of which depends on the composition of the solution. The boiling point *Author for correspondence.
0304-386X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
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of the solution increases during the isobaric crystallization of APT.4H20 from approximately 90 ° to 100°C. The crystallization mechanism of APT is generally assumed [2-4] to be represented by the following equations. When free ammonia is driven off, as in evaporative crystallization, the pH drops and a polytungstate begins to form: 6WO 2- + 7H + ~ - H W 6 0 2 5 i The paratungstate A ion,
+
3H20
H W 6 0 2 1es- ,
( 1) dimerizes, as represented by:
2HW6 O251 ~H2W12012°-
(2)
Once the paratungstate B ion, H2W~20~ °- has been formed, APT crystallizes according to:
10NH~ +H2W120410-+xH20-~ (NH4)10[H2WI2042]'xH20$
(3)
The value ofx is determined by the crystallization conditions, and is either 4, 6 or 10 [ 5-10 ]. The monoclinic tetrahydrate [ 5 ] is the modification which is generally produced on an industrial scale, although it has been reported [10] that the triclinic hexahydrate (APT. 6H20) is produced commercially by Hard Metals Limited in the Republic of South Africa. The system WO3-NH3-H20 has three components. The Gibbs phase rule shows that, under isobaric or isothermal conditions, two solid phases can only coexist given one particular solution composition. Given this constraint, it is clear that either the hexahydrate or the tetrahydrate form is the stable phase. The present study aims to identify the crystallization parameters which affect the formation of APT hydrates and to determine whether metastable hydrates are initially formed during crystallization. If metastable hydrates are formed they will ultimately convert into a stable phase, which proceeds via a solvent-mediated process. During this solvent-mediated recrystallization the metastable phase dissolves and the stable phase crystallizes. A literature survey of the crystallization conditions for the formation of the tetra-, hexa- and deca-hydrate phases is given. Linking the data available in the literature with the Gibbs phase rule leads to the postulation that the hexahydrate modification is metastable with respect to the tetrahydrate modification under conditions representative for industrial practice. Experiments were carried out to determine whether the hexahydrate modification is indeed metastable and, if so, under what conditions. REVIEW OF THE LITERATURE
ON THE FORMATIONOF VARIOUS AMMONIUM
PARATUNGSTATEHYDRATES The orthorhombic needle [ 5 ] or lath-like [9 ] deca-hydrate crystals (APT. 10H20) crystallize at room temperature, according to H~ihnert [ 7 ] and Basu and Sale [ 9 ]. Neither of these authors provided data on the corre-
FORMATION OF AMMONIUM PARATUNGSTATE TETRA- AND HEXA-HYDRATE I
189
sponding composition of the solution. Dawihl [6] attempted to dissolve APT. 10H20 in aqueous ammonia. He reported that this hydrate is stable at 19.0 _+0.2 °C and at ammonia concentrations lower than 4 wt% NH 3. Above this concentration a residue with a high ammonia concentration was found. Hempel and Saradshow [ 8 ] dissolved APT. 10H20 in water at temperatures varying from room temperature to 83 ° C. They found that APT- 10H20 is the only stable hydrate in water up to 50°C. It is metastable at higher temperatures. In addition, they reported that the system ammonium paratungstatewater does not exist at temperatures above 83 ° C. In the literature, controversy exists with regard to the formation of the triclinic APT. 6H20 plates [ 5 ]. According to Hiihnert [ 5 ], APT. 6H20 crystallizes at temperatures "higher than 50°C '' when the initial WO3 concentrations are high; that is, almost 30 wt% WO3. No information on the NH3 concentration was given. It was reported that APT. 6H20 recrystallized into APT. 4H:O. No detailed data on the process kinetics nor on the solution composition as a function of time were presented. Lutz [ 10] prepared APT. 6H20 at 93°C at moderate ( 127-167 g/l, i.e. 11-15 wt%) WO3 concentrations after batch and continuous crystallization. The batch experiments lasted 4-24 min. Lutz measured the pH of the solution, but does not provide the ammonia concentrations used. According to H~ihnert [ 5 ], APT. 6H20 also crystallizes at temperatures below 50 ° C. No details on the NH3 concentrations are given. Hempel and Saradshow [ 8 ] crystallized APT. 6H20 at room temperature, from a solution containing 24.0 wt% WO3 and 5.8 wt% NH3, by the addition of concentrated HC1. After dissolving this phase at temperatures from 20 ° C to 83 °C, they concluded that APT. 6H20 is always metastable in water, with either APT. 4H20 or APT. 10H20 being the corresponding stable phases. According to H~ihnert [ 5 ], the monoclinic APT.4H20 cuboids [ 5,9 ] form at temperatures higher than 50°C as long as the initial WO3 concentrations are not too high; that is, between 5 and 15 wt% WO3. Hempel and Saradshow [ 8 ] prepared APT. 4H20 by the evaporation of ammonia and water at 65 ° C from an ammonium tungstate solution containing 24 wt% WO3. APT-4H20 was found to be the stable hydrate in water at temperatures above 50°C, whereas it was found to be metastable in water at lower temperatures. Crystallization at temperatures "of the order of 70 °C and above" results, according to Basu and Sale [9], in the crystallization of APT. 4H20. No details on the solution composition were presented by these authors. In summary, based on the above review, it may be concluded that at high initial WO3 solution concentrations, the formation of APT-6H20, although thermodynamically metastable, is kinetically favoured. Once formed, APT. 6H20 slowly converts into APT. 4H20. At lower initial WO3 solution concentrations, APT. 4H20 and APT. 6H20 were found simultaneously. Thus, APT- 6H20 and APT. 4H20 seem to co-exist over a range of solution concentrations. In applying the phase rule to this system, it was shown that two solid
J.W. VANPUT ET AL.
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hydrates can only co-exist at one particular solution composition. This sustains the reasoning that one of the solid phases is metastable under the conditions discussed. As it was also reported that APT. 6H20 recrystallizes into APT-4H20 at the prevailing temperatures (90-100°C), it is obvious that APT. 6H20 is metastable. C A L C U L A T I O N O F THE WATER ACTIVITY
The activity of water in the solid phase should, at equilibrium, be equal to the activity of water in the liquid phase. This constraint determines the formation of the APT hydrates related to the water activity during crystallization. The equations required to calculate the water activity are presented below. Evaluation of these equations indicates which variables should be measured in order to calculate the water activity. The water activity at time t can be calculated from: a:mo,t = Yri2o,t• [ H 2 0 ]t
(4)
The concentration of water, [H20]t , can be calculated from the WO3 and NH3 concentrations in solution at time t. The activity coefficient at time t, YH20,t, can be calculated from the vapour composition:
YH20,/"P ~H20,t -- XH20,t ° eH20,/sat
(5)
The mole fraction of H20 in the vapour, YH2o, can be calculated. In order to accomplish this, the total amount of vapour, the solution composition, the amount of APT and the solution temperature must be determined. In a previous paper it was shown how these experimental data were used to calculate YH20 [ 12 ]. EXPERIMENTAL
Chemicals Ammonium tungstate solutions were prepared by dissolving industrially produced H2WO4 in concentrated ammonia, followed by dilution with distilled water. The clear solution was syphoned into a stock tank after standing for 24 h. The residue was disposed of.
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191
Experimentalprocedures Experiments were performed to determine whether APT-6H20 forms and if so, at which solution composition (s) and temperature (s) it is metastable. Both isothermal and isobaric experiments were carried out. The isothermal experiments were carried out using solutions ranging in temperature from 30 °C to 80 ° C. Solutions with WO3 concentrations approximately in the range 130-305 g/kg solution and NH3 concentrations of 28-67 g/kg solution were prepared. Experiments were carried out in a 300 ml jacketed glass vessel. A total of 200 ml of an ammonium tungstate solution were heated by pumping oil from a constant temperature bath through the jacket. A sample was drawn off after the solution became opalescent. The vessel was then sealed for 1 h, after which another sample was taken. The isobaric experiments were carried out to gather data for the construction of a phase diagram for the industrial crystallization conditions of APT, where a boiling temperature of approximately 90-100 ° C is used. The system pressure is then 1 bar. Two series of isobaric experiments were performed. In the first series, the set-up of the isothermal experiments was used for various solution compositions. The oil temperature was 120°C, which was sufficiently high to maintain boiling solutions. Samples were taken immediately after the solution became opalescent. In the second series, isobaric experiments were carried out starting from an ammonium tungstate solution with solution compositions comparable to those used in industry; that is, 270 g/kg WO3 and 55 g/kg NH3. The oil temperature was 125°C, which was sufficiently high to keep all solutions boiling. A 2 1 jacketed glass vessel was used as the crystallizer in these experiments. The experimental apparatus is shown in Fig. 1. A negative pressure of 4 cm water column was maintained in the crystallizer to prevent leakage of the water and ammonia vapour to the surroundings. The vapour was condensed and captured in a dilute nitric acid solution. This solution was weighed continuously. The vessel was equipped with four baffles and an axial flow turbine. A Philips
G{.~ 3
H20 out
H2o In 1
Fig. 1. Experimentalset-up. 1= paraffin oil heater; 2 = baffledcrystallizer(jacketed);3= agitator; 4=condensor; 5=flask; 6=weight balance; 7=gas pump (suction).
192
J.W. VAN PUTETAL.
high temperature electrode set was used to monitor the pH of the solution during the experiments. The initial volume in the crystallizer was 1.6 1; that is, 2.1 kg ammonium tungstate solution. Approximately 1.2 kg ofvapour was produced in 5 h. The results of the second series of isobaric experiments were used for the construction of the phase diagram and for the calculation of the water activity as a function of time.
Analyses The WO3 concentration of the solution and of the solid phase were determined using a gravimetric method. The ammonia concentration was determined using the Kjeldahl distillation method. The water concentration was calculated from these data. The crystals were examined with scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). RESULTS A N D D I S C U S S I O N
Isothermal experiments APT.6H20 crystallized in all isothermal experiments between 30°C and 80 °C. The solutions did not boil in these experiments. Sealing the vessel, thus stopping the evaporation, resulted in recrystallization of this hydrate into APT. 4H20. This indicates that APT. 6H20 is metastable under these conditions. No APT. 10H20 was detected in any of the experiments.
Isobaric experiments In the first series of isobaric experiments, the solution temperature rose from room temperature to approximately 100 ° C. It could be determined visually for all experiments that the solution boiled when nucleation started. Nucleation occurred at a solution temperature of 88.5 °C for a solution with an initial concentration of 304.4 g/kg WO3 and 67.4 g/kg NH3. APT. 6H20 crystals were detected at this temperature. The sample solution contained 307.4 g/kg WO3 and 54.4 g/kg NH3. It was found that the APT. 6H20 phase forms at high WO3 concentrations, in this case between 304.4 and 297.9 g/kg solution, while no APT-4H20 crystals were present at these WO3 concentrations. APT. 6H20 crystals are shown in Fig. 2. Lowering the initial WO3 concentration caused the nucleation to take place at higher temperatures. This resulted in a mixture of APT.6H20 and APT-4H20 crystals. A decrease in the WO3 concentration resulted in an increase in the fraction of APT.4H20 crystals. Only a small amount of APT-6H20 crystals were present at approximately 230 g/kg WO3, 40 g/kg
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
193
Fig. 2. Formation of A P T . 6H20. Initial WO3 = 290.8 g / k g ; sample WO3 = 297.9 g/kg; NH3 = 54.3 g/kg; T = 8 9 . 9 ° C .
NH3 and T=96°C. This is shown in Fig. 3, where only a minority of the crystals show the triclinic APT'6H20 habits, in contrast to the majority of the crystals showing the APT. 4H20 cubic habits. APT. 6H20 was no longer observed at a WO3 concentration of approximately 165 g/kg and a solution temperature of 98 ° C. Figure 4 only shows APT. 4H20 crystals. Experiments previously performed, where solutions with an initial WO3 concentration of approximately 300 g/kg and an initial NH3 concentration of 67 g/kg were seeded with 6 g APT.4H20 crystals and the seeds were added when the solution reached 88 °C (i.e. before crystallization started), demonstrated that the initial formation of APT. 6H20 is prevented by the addition of seeds of the stable APT- 4H20 [ 13 ]. The initial formation of APT. 6H20 was observed in unseeded crystallization experiments between 90°C and 96°C. APT.6H20 recrystallizes within approximately 40 min into APT. 4H20, while -the NH3 and WO3 concentrations decrease and the solution temperature increases with time. Experiments with varying amounts of seeds showed that APT. 6H20 is never formed in the presence of APT. 4H20
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J.W. VANPUT ET AL.
Fig. 3. Formation of APT. 6H20 and APT. 4H20. Initial WOa = 214.1 g/kg; sample WO3 = 228.8 g/kg WO3; sample NH3 = 48.4 g/kg; T = 95.7 ° C.
seeds. Addition of less than 1 g of seeds proved to be sufficient to prevent the formation of APT. 6H20. Two kinetic processes govern the solvent-mediated recrystallization: namely, dissolution of the metastable phase and growth of the stable phase [ 14 ]. The recrystallization is determined by both nucleation and growth rate when the nucleation and growth rate of the stable phase are much smaller than the dissolution rate of the metastable phase. The concentration of the solute will, in this case, be relatively high, that is, close to the solubility of the metastable phase. The recrystallization process is dissolution-limited when the dissolution of the metastable phase proceeds slowly compared to the nucleation and growth of the stable phase. The solute concentration will be relatively low in this situation, that is, close to the solubility of the stable phase. No difference in WO3 concentrations was found in unseeded and seeded experiments. At this point it is not clear whether the solubilities of APT. 6H20 and APT. 4H20 differ only slightly or whether the concentration approaches the solubility of the APT. 4H20 phase. In the latter case the conversion rate
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
Fig,. 4. Formation of APT-4H20. Initial WO3 = NH3=22 g/kg; T = 97.8°C.
147.4
g/kg; sample WO3 =
195
163.7
g/kg; sample
of APT'6H20 into APT'4H20 is controlled by the dissolution rate of APT.6H20. CONSTRUCTION OF THE PHASE DIAGRAM
Isothermal ternary solubility diagrams for APT. 6H20 and APT. 4H20 were constructed with the data obtained and using the solubility data of APT. 4H20 in water from literature [15]. Each isotherm contains two data points: one provided by literature and one experimentally determined. The curvature of the liquidus line is chosen arbitrarily. The 90 °C isotherm is shown in Fig. 5. The liquidus line is relatively small. In order to get a better view of the system the concentrations, in moles per kilogram of solution, of WO3 and NH3 were multiplied by 50 and 20, respectively. After 'magnifying' the diagram for WO3 and NH3, the resulting coordinates of APT-4H20 and APT. 6H20 no longer differ significantly. Their composition is therefore represented by the same point in the isotherm ternary solubility diagrams presented below.
196
J.W. VANPUT ETAL. H20
WO3
NH 3
Fig. 5. Ternary solubility diagram for APT. 6H20 and APT-4H20. H20
7
NH 3
E
W03
Fig. 6. Ternary solubility diagram for APT.6H20 (metastable) and APT.4H20 (stable) in aqueous ammonia at 90 ° C.
Ternary solubility diagrams, with multiplied concentrations for APT in aqueous ammonia at 90 °, 98.3 ° and 99.6 °C are presented in Figs. 6, 7 and 8. At point "P" the solution boils and the pressure of the system is 1 bar. A three dimensional phase diagram for APT crystallization, based upon numerous experiments, was constructed with the help of a computer program [ 16 ]. The phase diagram is presented in Fig. 9. The composition of APT.4H20 and APT. 6H20 were represented by the same point in the isothermal ternary diagrams: this results in a line parallel to the pure component axes in the threedimensional ternary diagram. The solubility of APT in water increases slowly with temperature, resulting in the slightly inclined line, AB. The measured solution compositions at 1 bar
FORMATION OF AMMONIUM PARATUNGSTATE TETRA- AND HEXA-HYDRATE I
~
197
H20
5
\
ooV
/
/ .<\\\ 2 \\\\\
\o.o15
NH 3
WO3
Fig. 7. Ternary solubility diagram for APT. 4H20 in aqueous ammonia at 98.3 ° C. H20
~ / NH3
05
\
\ WO 3
Fig. 8. Ternary solubility diagram for APT. 4H20 in aqueous ammonia at 9 9.6 ° C.
are presented by the curved line, BC, which contains all points "P". A top view, which is actually the isobaric projection of all points "P", of the phase diagram is presented in Fig. 10. APT. 4H20 is the stable phase during crystallization. APT-6H20 is metastable in the dashed trajectory from approximately 88.5 °C to 96 ° C. The solution composition at 96 °C is approximately 220 g/kg WO3 and 40 g/kg NH3. APT. 6H20 is unstable at temperatures higher than approximately 96 ° C. This implies that only from solutions with initially higher WO3 concentrations will APT start to nucleate at relatively low temperatures, resulting in crystallization of the metastable APT. 6H20. Lowering the initial WO3 concentrations will increase the boiling point of the solution and the stable APT-4H20 will
198
J.W. VAN PUT ET AL.
100"C B
98.3" 97.3"( 94.9"[
"e-
A
o.ol
302
F--
/WO 3
ll 0 002 00~
NH3
Fig. 9. Ternary phase diagram for APT. 4H20 and APT-6H20.
~
H20
00y
NH3
A\
5
\
WO]
Fig. 10. Ternary phase diagram for APT.4H20 and APT"6H20 crystallization under isobaric conditions (P= 1 bar). Shaded area indicates initial crystallization of metastable APT. 6H20. crystallize. Nucleation o f A P T . 4 H 2 0 apparently proceeds m o r e easily at higher temperatures. According to the Ostwald law of stages, the metastable phase m a y crystallize first from a solution with a concentration higher than the solubility of both phases. The stable phase, with a solubility lower than the metastable
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
96"C~ 90"C
199
APT.6H20 _APT.¢H20
Fig. 11. Schematic view of the solubilities of A P T . 4 H 2 0 and A P T . 6 H 2 0 .
phase formed, will crystallize after a certain time. The solution will become undersaturated with respect to the metastable phase once the stable phase has been formed. The metastable phase starts to dissolve at this point. The crystallization of the stable phase will proceed until the solution reaches its saturation concentration. The metastable phase will be completely dissolved at this point. A schematic view of the solubilities of the APT-6H20 and APT. 4H20 phases is shown in Fig. 11 for clarity. APT. 4H20 has the lowest solubility and is thermodynamically the more stable. The solubility of APT. 6H20 is always higher than the solubility of the APT. 4H20 phase in the temperature range from 90 ° to 96 °C at a pressure of 1 bar. The solubility of the APT'6H20 phase could, unfortunately, not be measured with sufficient accuracy to reveal the difference between the solubilities of the two phases.
CALCULATION OF THE WATER ACTIVITY
The water activity was calculated from the vapour composition to determine whether it can be related to the recrystaUization of APT.6H20 into APT. 4H20. The calculated vapour composition [ 12 ] as a function of time is shown in Fig. 12. The water activity as a function of time is shown in Fig. 13. PH~Osa, was obtained from Perry [ 11 ]. It is obvious that the activity of water decreases during the first 50 min. The relative error in the calculation of the activity is approximately 10%. Although this error is relatively high, the same trend for the water activity was observed for several experiments. It may, therefore, be concluded that the recrystallization of the APT. 6H20 phase into the APT. 4H20 phase is prompted by a decrease in water activity. It should, however, be mentioned that the increasing solution temperature during crystallization also favours the recrystallization of APT.6H20 into APT-4H20: due to the increasing temperature en2o., becomes larger, thus resulting in a lower activity coefficient for water. In addition, it was found that the nucleation of the APT. 4H20 proceeds more easily at higher temperatures.
200
J.W. VANPUTETAL. Concentration (wf%)
40 L
0
60
180
300
Time (min.) Fig. 12. Vapour composition as a function of time for a typical experiment.
H20 activify (mo[/kg) 70
60
s°I
x exp. ~* I o exp. # 2 • exp. * 3
~0 0
I
I
1;o
2so
Time (min.) Fig. 13. Activity of water as a function of time for several experiments. CONCLUSIONS
APT. 6H20 is metastable under isobaric conditions of 1 bar between 90 ° C and 96°C. This hydrate is unstable at temperatures higher than 96°C. APT. 4H20 is the stable hydrate under these conditions. Since APT. 6H20 is metastable, it is concluded that the solubility of APT. 6H20 is higher than that of APT. 4H20. The recrystallization of APT. 6H20 into APT-4H20 is prompted by decreasing water activity. The increasing temperature during isobaric crystallization probably accelerates the conversion. The initial formation of APT-6H20 can be prevented by the addition of a small amount of APT-4H20
FORMATION OF AMMONIUM PARATUNGSTATE TETRA- AND HEXA-HYDRATE I
201
seeds. In i n d u s t r i a l p r a c t i c e s o m e crystals will always r e m a i n p r e s e n t after a b a t c h o p e r a t i o n . T h e r e f o r e , A P T . 6 H 2 0 will n o t be f o r m e d d u r i n g i n d u s t r i a l A P T crystallization. ACKNOWLEDGEMENTS Philips L i g h t i n g B.V. is gratefully t h a n k e d for financially s u p p o r t i n g this research. T h e a u t h o r s are i n d e b t e d to J.C.J. Schellekens for his help with the c o m p u t e r c o n s t r u c t i o n o f the 3-D p h a s e d i a g r a m a n d to F. E l g e r s m a for his careful r e a d i n g o f the m a n u s c r i p t .
REFERENCES 1 Van Put, J.W., Ammonium paratungstate as a raw material for the production of lamp filament tungsten wire. Ph.D. Thesis, Delft Univ. Technol., Delft Univ. Press, Delft ( 1991 ). 2 Kepert, D.L., Isopolytungstates. Prog. Inorg. Chem., 4 (1962): 199-274. 3 Cordis, V., Tytko, K.H. and Glemser, O., Uber Isopolywolframatfeststoffeund deren Beziehung zu Isopolywolframationen in w~il]rigerL/Ssung. Z. Naturforsch., 306 ( 1975 ): 834841. 4 Tytko, K.H. and Glemser, O., Isopolymolybdates and isopolytungstates. Adv. Inorg. Chem., 19 (1977): 239-315. 5 H~ihnert,M., Kristallographische Untersuchung der Ammoniumparawolframate. Z. Kristall., 120 (1964): 216-228. 6 Dawihl, W., Uber die L/Sslichkeit von Wolframs~iure und Ammoniumparawolframat in w~iBrigem Ammoniak. Z. Anorg. Allg. Chem., 244 (1940): 1-12. 7 H~ihnert,M., Uber Ammoniumparawolframate. Z. Anorg. Allg. Chem., 318 (1962): 222232. 8 Hempel, K. and Saradshow, M., L/Sslichkeit und stabile Kristallhydrate im System Ammoniumparawolframat-Wasser. Krist. Tech., 3 (1967): 437-445. 9 Basu, A.K. and Sale, F.R., Characterization of various commercial forms of ammonium paratungstate powder. J. Mater. Sci., 10 ( 1975 ): 571-577. 10 Lutz, W.F., An investigation into the crystallization of ammonium paratungstate. Diss., Univ. Witwatersrand, S. Africa ( 1973 ). 11 Perry, R.H. and Green, D. (Editors), Perry's Chemical Engineers' Handbook. McGraw Hill, New York, 6th ed. (1984). 12 Van Put, J.W., Zegers, T.W., Sandwijk, A. and Van der Straten, P.J.M., The crystallization of APT, the system WOa/NH3/H20. In: M.H.I. Baird and S. Vizjayan (Editors), Proc. 2nd Int. Conf. Separation Science and Technology. Can. Soc. Chem. Eng., Ottawa, Ont., (1989), pp. 387-394. 13 Van Put, J.W., De Koning, P.M., Van Sandwijk, A. and Witkamp G.J., Seeded and unseeded batch crystallization of ammonium paratungstate. In: A. Mersmann (Editor), Proc. l lth Symp. Industrial Crystallization. VDI-Verfahrenstechnik, Diisseldorf, Germany (1990), pp. 547-552. 14 Cardew, P.T. and Davey, R.J., The kinetics of solvent-mediated phase transformations. Proc. R. Soc. London Ser. A., 398 ( 1985): 415-428. 15 Yih, S.W.H. and Wang, C.T., Tungsten. Plenum, New York, (1979). 16 Structural Dynamics Research Corporation, I-DEAS, Level 4. Structural Dynamics Research Corp., Milford, Ohio ( 1988 ).
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Elsevier Science Publishers B.V., Amsterdam
Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability J.W.
van Put
a, G.J. Witkamp b,. and G.M.
van Rosmalen
b
a Union Mini~re, Proces Research andDevelopment, Fabrieksstraat 144, B-3900 Overpelt, Belgium b Delft University of Technology, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands (Received September 25, 1992; revised version accepted February 25, 1993 )
ABSTRACT The crystallization of ammonium paratungstate tetrahydrate, ((NH4)IoH2W12042"4H20, APT-4H20) from aqueous ammonium tungstate solutions is a key unit operation in current tungsten powder production. A literature survey showed that ammonium paratungstate hexahydrate (APT-6H20) can form under conditions where APT. 4H20 forms as well. Based on this information it was postulated that APT. 6H20 is metastable. Experiments were carried out to determine whether and under what conditions the hexahydrate is metastable. It was found that the hexahydrate is metastable at a solution concentration of approximately 300-230 g/kg WO3 and from approximately 90 ° to 96°C. APT-6H20 is not formed when APT-4H20 seeds are added prior to crystallization. The experimental data are presented in a three-dimensionalternary phase diagram. It was found that the recrystallization of APT" 6H20 into APT. 4H20 is prompted by a decreasing water activity in solution during crystallization. The increase in solution temperature during isobaric crystallization also stimulates the recrystallization process. In industrial practice some residual crystals are always present in the crystallizer. Therefore, APT. 6H20 will not form during industrial crystallization.
INTRODUCTION
The crystallization of ammonium paratungstate tetrahydrate, (NH4) 10 [ H2W12042 ]° 4H20, (APT" 4H20 ) from aqueous ammonium tungstate solutions is a key unit operation in the current production of tungsten powder. The crystal size, crystal size distribution and degree of agglomeration of APT. 4H20 are all likely to be of importance for its processing to tungsten powder [ 1 ]. Both batch and continuous crystallizers are used in industrial practice. The start-up procedure for continuous or batchwise crystallization of APT. 4H20 is to heat an a m m o n i u m tungstate solution to its boiling point, the actual value of which depends on the composition of the solution. The boiling point *Author for correspondence.
0304-386X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
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of the solution increases during the isobaric crystallization of APT.4H20 from approximately 90 ° to 100°C. The crystallization mechanism of APT is generally assumed [2-4] to be represented by the following equations. When free ammonia is driven off, as in evaporative crystallization, the pH drops and a polytungstate begins to form: 6WO 2- + 7H + ~ - H W 6 0 2 5 i The paratungstate A ion,
+
3H20
H W 6 0 2 1es- ,
( 1) dimerizes, as represented by:
2HW6 O251 ~H2W12012°-
(2)
Once the paratungstate B ion, H2W~20~ °- has been formed, APT crystallizes according to:
10NH~ +H2W120410-+xH20-~ (NH4)10[H2WI2042]'xH20$
(3)
The value ofx is determined by the crystallization conditions, and is either 4, 6 or 10 [ 5-10 ]. The monoclinic tetrahydrate [ 5 ] is the modification which is generally produced on an industrial scale, although it has been reported [10] that the triclinic hexahydrate (APT. 6H20) is produced commercially by Hard Metals Limited in the Republic of South Africa. The system WO3-NH3-H20 has three components. The Gibbs phase rule shows that, under isobaric or isothermal conditions, two solid phases can only coexist given one particular solution composition. Given this constraint, it is clear that either the hexahydrate or the tetrahydrate form is the stable phase. The present study aims to identify the crystallization parameters which affect the formation of APT hydrates and to determine whether metastable hydrates are initially formed during crystallization. If metastable hydrates are formed they will ultimately convert into a stable phase, which proceeds via a solvent-mediated process. During this solvent-mediated recrystallization the metastable phase dissolves and the stable phase crystallizes. A literature survey of the crystallization conditions for the formation of the tetra-, hexa- and deca-hydrate phases is given. Linking the data available in the literature with the Gibbs phase rule leads to the postulation that the hexahydrate modification is metastable with respect to the tetrahydrate modification under conditions representative for industrial practice. Experiments were carried out to determine whether the hexahydrate modification is indeed metastable and, if so, under what conditions. REVIEW OF THE LITERATURE
ON THE FORMATIONOF VARIOUS AMMONIUM
PARATUNGSTATEHYDRATES The orthorhombic needle [ 5 ] or lath-like [9 ] deca-hydrate crystals (APT. 10H20) crystallize at room temperature, according to H~ihnert [ 7 ] and Basu and Sale [ 9 ]. Neither of these authors provided data on the corre-
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189
sponding composition of the solution. Dawihl [6] attempted to dissolve APT. 10H20 in aqueous ammonia. He reported that this hydrate is stable at 19.0 _+0.2 °C and at ammonia concentrations lower than 4 wt% NH 3. Above this concentration a residue with a high ammonia concentration was found. Hempel and Saradshow [ 8 ] dissolved APT. 10H20 in water at temperatures varying from room temperature to 83 ° C. They found that APT- 10H20 is the only stable hydrate in water up to 50°C. It is metastable at higher temperatures. In addition, they reported that the system ammonium paratungstatewater does not exist at temperatures above 83 ° C. In the literature, controversy exists with regard to the formation of the triclinic APT. 6H20 plates [ 5 ]. According to Hiihnert [ 5 ], APT. 6H20 crystallizes at temperatures "higher than 50°C '' when the initial WO3 concentrations are high; that is, almost 30 wt% WO3. No information on the NH3 concentration was given. It was reported that APT. 6H20 recrystallized into APT. 4H:O. No detailed data on the process kinetics nor on the solution composition as a function of time were presented. Lutz [ 10] prepared APT. 6H20 at 93°C at moderate ( 127-167 g/l, i.e. 11-15 wt%) WO3 concentrations after batch and continuous crystallization. The batch experiments lasted 4-24 min. Lutz measured the pH of the solution, but does not provide the ammonia concentrations used. According to H~ihnert [ 5 ], APT. 6H20 also crystallizes at temperatures below 50 ° C. No details on the NH3 concentrations are given. Hempel and Saradshow [ 8 ] crystallized APT. 6H20 at room temperature, from a solution containing 24.0 wt% WO3 and 5.8 wt% NH3, by the addition of concentrated HC1. After dissolving this phase at temperatures from 20 ° C to 83 °C, they concluded that APT. 6H20 is always metastable in water, with either APT. 4H20 or APT. 10H20 being the corresponding stable phases. According to H~ihnert [ 5 ], the monoclinic APT.4H20 cuboids [ 5,9 ] form at temperatures higher than 50°C as long as the initial WO3 concentrations are not too high; that is, between 5 and 15 wt% WO3. Hempel and Saradshow [ 8 ] prepared APT. 4H20 by the evaporation of ammonia and water at 65 ° C from an ammonium tungstate solution containing 24 wt% WO3. APT-4H20 was found to be the stable hydrate in water at temperatures above 50°C, whereas it was found to be metastable in water at lower temperatures. Crystallization at temperatures "of the order of 70 °C and above" results, according to Basu and Sale [9], in the crystallization of APT. 4H20. No details on the solution composition were presented by these authors. In summary, based on the above review, it may be concluded that at high initial WO3 solution concentrations, the formation of APT-6H20, although thermodynamically metastable, is kinetically favoured. Once formed, APT. 6H20 slowly converts into APT. 4H20. At lower initial WO3 solution concentrations, APT. 4H20 and APT. 6H20 were found simultaneously. Thus, APT- 6H20 and APT. 4H20 seem to co-exist over a range of solution concentrations. In applying the phase rule to this system, it was shown that two solid
J.W. VANPUT ET AL.
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hydrates can only co-exist at one particular solution composition. This sustains the reasoning that one of the solid phases is metastable under the conditions discussed. As it was also reported that APT. 6H20 recrystallizes into APT-4H20 at the prevailing temperatures (90-100°C), it is obvious that APT. 6H20 is metastable. C A L C U L A T I O N O F THE WATER ACTIVITY
The activity of water in the solid phase should, at equilibrium, be equal to the activity of water in the liquid phase. This constraint determines the formation of the APT hydrates related to the water activity during crystallization. The equations required to calculate the water activity are presented below. Evaluation of these equations indicates which variables should be measured in order to calculate the water activity. The water activity at time t can be calculated from: a:mo,t = Yri2o,t• [ H 2 0 ]t
(4)
The concentration of water, [H20]t , can be calculated from the WO3 and NH3 concentrations in solution at time t. The activity coefficient at time t, YH20,t, can be calculated from the vapour composition:
YH20,/"P ~H20,t -- XH20,t ° eH20,/sat
(5)
The mole fraction of H20 in the vapour, YH2o, can be calculated. In order to accomplish this, the total amount of vapour, the solution composition, the amount of APT and the solution temperature must be determined. In a previous paper it was shown how these experimental data were used to calculate YH20 [ 12 ]. EXPERIMENTAL
Chemicals Ammonium tungstate solutions were prepared by dissolving industrially produced H2WO4 in concentrated ammonia, followed by dilution with distilled water. The clear solution was syphoned into a stock tank after standing for 24 h. The residue was disposed of.
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Experimentalprocedures Experiments were performed to determine whether APT-6H20 forms and if so, at which solution composition (s) and temperature (s) it is metastable. Both isothermal and isobaric experiments were carried out. The isothermal experiments were carried out using solutions ranging in temperature from 30 °C to 80 ° C. Solutions with WO3 concentrations approximately in the range 130-305 g/kg solution and NH3 concentrations of 28-67 g/kg solution were prepared. Experiments were carried out in a 300 ml jacketed glass vessel. A total of 200 ml of an ammonium tungstate solution were heated by pumping oil from a constant temperature bath through the jacket. A sample was drawn off after the solution became opalescent. The vessel was then sealed for 1 h, after which another sample was taken. The isobaric experiments were carried out to gather data for the construction of a phase diagram for the industrial crystallization conditions of APT, where a boiling temperature of approximately 90-100 ° C is used. The system pressure is then 1 bar. Two series of isobaric experiments were performed. In the first series, the set-up of the isothermal experiments was used for various solution compositions. The oil temperature was 120°C, which was sufficiently high to maintain boiling solutions. Samples were taken immediately after the solution became opalescent. In the second series, isobaric experiments were carried out starting from an ammonium tungstate solution with solution compositions comparable to those used in industry; that is, 270 g/kg WO3 and 55 g/kg NH3. The oil temperature was 125°C, which was sufficiently high to keep all solutions boiling. A 2 1 jacketed glass vessel was used as the crystallizer in these experiments. The experimental apparatus is shown in Fig. 1. A negative pressure of 4 cm water column was maintained in the crystallizer to prevent leakage of the water and ammonia vapour to the surroundings. The vapour was condensed and captured in a dilute nitric acid solution. This solution was weighed continuously. The vessel was equipped with four baffles and an axial flow turbine. A Philips
G{.~ 3
H20 out
H2o In 1
Fig. 1. Experimentalset-up. 1= paraffin oil heater; 2 = baffledcrystallizer(jacketed);3= agitator; 4=condensor; 5=flask; 6=weight balance; 7=gas pump (suction).
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J.W. VAN PUTETAL.
high temperature electrode set was used to monitor the pH of the solution during the experiments. The initial volume in the crystallizer was 1.6 1; that is, 2.1 kg ammonium tungstate solution. Approximately 1.2 kg ofvapour was produced in 5 h. The results of the second series of isobaric experiments were used for the construction of the phase diagram and for the calculation of the water activity as a function of time.
Analyses The WO3 concentration of the solution and of the solid phase were determined using a gravimetric method. The ammonia concentration was determined using the Kjeldahl distillation method. The water concentration was calculated from these data. The crystals were examined with scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). RESULTS A N D D I S C U S S I O N
Isothermal experiments APT.6H20 crystallized in all isothermal experiments between 30°C and 80 °C. The solutions did not boil in these experiments. Sealing the vessel, thus stopping the evaporation, resulted in recrystallization of this hydrate into APT. 4H20. This indicates that APT. 6H20 is metastable under these conditions. No APT. 10H20 was detected in any of the experiments.
Isobaric experiments In the first series of isobaric experiments, the solution temperature rose from room temperature to approximately 100 ° C. It could be determined visually for all experiments that the solution boiled when nucleation started. Nucleation occurred at a solution temperature of 88.5 °C for a solution with an initial concentration of 304.4 g/kg WO3 and 67.4 g/kg NH3. APT. 6H20 crystals were detected at this temperature. The sample solution contained 307.4 g/kg WO3 and 54.4 g/kg NH3. It was found that the APT. 6H20 phase forms at high WO3 concentrations, in this case between 304.4 and 297.9 g/kg solution, while no APT-4H20 crystals were present at these WO3 concentrations. APT. 6H20 crystals are shown in Fig. 2. Lowering the initial WO3 concentration caused the nucleation to take place at higher temperatures. This resulted in a mixture of APT.6H20 and APT-4H20 crystals. A decrease in the WO3 concentration resulted in an increase in the fraction of APT.4H20 crystals. Only a small amount of APT-6H20 crystals were present at approximately 230 g/kg WO3, 40 g/kg
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
193
Fig. 2. Formation of A P T . 6H20. Initial WO3 = 290.8 g / k g ; sample WO3 = 297.9 g/kg; NH3 = 54.3 g/kg; T = 8 9 . 9 ° C .
NH3 and T=96°C. This is shown in Fig. 3, where only a minority of the crystals show the triclinic APT'6H20 habits, in contrast to the majority of the crystals showing the APT. 4H20 cubic habits. APT. 6H20 was no longer observed at a WO3 concentration of approximately 165 g/kg and a solution temperature of 98 ° C. Figure 4 only shows APT. 4H20 crystals. Experiments previously performed, where solutions with an initial WO3 concentration of approximately 300 g/kg and an initial NH3 concentration of 67 g/kg were seeded with 6 g APT.4H20 crystals and the seeds were added when the solution reached 88 °C (i.e. before crystallization started), demonstrated that the initial formation of APT. 6H20 is prevented by the addition of seeds of the stable APT- 4H20 [ 13 ]. The initial formation of APT. 6H20 was observed in unseeded crystallization experiments between 90°C and 96°C. APT.6H20 recrystallizes within approximately 40 min into APT. 4H20, while -the NH3 and WO3 concentrations decrease and the solution temperature increases with time. Experiments with varying amounts of seeds showed that APT. 6H20 is never formed in the presence of APT. 4H20
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Fig. 3. Formation of APT. 6H20 and APT. 4H20. Initial WOa = 214.1 g/kg; sample WO3 = 228.8 g/kg WO3; sample NH3 = 48.4 g/kg; T = 95.7 ° C.
seeds. Addition of less than 1 g of seeds proved to be sufficient to prevent the formation of APT. 6H20. Two kinetic processes govern the solvent-mediated recrystallization: namely, dissolution of the metastable phase and growth of the stable phase [ 14 ]. The recrystallization is determined by both nucleation and growth rate when the nucleation and growth rate of the stable phase are much smaller than the dissolution rate of the metastable phase. The concentration of the solute will, in this case, be relatively high, that is, close to the solubility of the metastable phase. The recrystallization process is dissolution-limited when the dissolution of the metastable phase proceeds slowly compared to the nucleation and growth of the stable phase. The solute concentration will be relatively low in this situation, that is, close to the solubility of the stable phase. No difference in WO3 concentrations was found in unseeded and seeded experiments. At this point it is not clear whether the solubilities of APT. 6H20 and APT. 4H20 differ only slightly or whether the concentration approaches the solubility of the APT. 4H20 phase. In the latter case the conversion rate
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
Fig,. 4. Formation of APT-4H20. Initial WO3 = NH3=22 g/kg; T = 97.8°C.
147.4
g/kg; sample WO3 =
195
163.7
g/kg; sample
of APT'6H20 into APT'4H20 is controlled by the dissolution rate of APT.6H20. CONSTRUCTION OF THE PHASE DIAGRAM
Isothermal ternary solubility diagrams for APT. 6H20 and APT. 4H20 were constructed with the data obtained and using the solubility data of APT. 4H20 in water from literature [15]. Each isotherm contains two data points: one provided by literature and one experimentally determined. The curvature of the liquidus line is chosen arbitrarily. The 90 °C isotherm is shown in Fig. 5. The liquidus line is relatively small. In order to get a better view of the system the concentrations, in moles per kilogram of solution, of WO3 and NH3 were multiplied by 50 and 20, respectively. After 'magnifying' the diagram for WO3 and NH3, the resulting coordinates of APT-4H20 and APT. 6H20 no longer differ significantly. Their composition is therefore represented by the same point in the isotherm ternary solubility diagrams presented below.
196
J.W. VANPUT ETAL. H20
WO3
NH 3
Fig. 5. Ternary solubility diagram for APT. 6H20 and APT-4H20. H20
7
NH 3
E
W03
Fig. 6. Ternary solubility diagram for APT.6H20 (metastable) and APT.4H20 (stable) in aqueous ammonia at 90 ° C.
Ternary solubility diagrams, with multiplied concentrations for APT in aqueous ammonia at 90 °, 98.3 ° and 99.6 °C are presented in Figs. 6, 7 and 8. At point "P" the solution boils and the pressure of the system is 1 bar. A three dimensional phase diagram for APT crystallization, based upon numerous experiments, was constructed with the help of a computer program [ 16 ]. The phase diagram is presented in Fig. 9. The composition of APT.4H20 and APT. 6H20 were represented by the same point in the isothermal ternary diagrams: this results in a line parallel to the pure component axes in the threedimensional ternary diagram. The solubility of APT in water increases slowly with temperature, resulting in the slightly inclined line, AB. The measured solution compositions at 1 bar
FORMATION OF AMMONIUM PARATUNGSTATE TETRA- AND HEXA-HYDRATE I
~
197
H20
5
\
ooV
/
/ .<\\\ 2 \\\\\
\o.o15
NH 3
WO3
Fig. 7. Ternary solubility diagram for APT. 4H20 in aqueous ammonia at 98.3 ° C. H20
~ / NH3
05
\
\ WO 3
Fig. 8. Ternary solubility diagram for APT. 4H20 in aqueous ammonia at 9 9.6 ° C.
are presented by the curved line, BC, which contains all points "P". A top view, which is actually the isobaric projection of all points "P", of the phase diagram is presented in Fig. 10. APT. 4H20 is the stable phase during crystallization. APT-6H20 is metastable in the dashed trajectory from approximately 88.5 °C to 96 ° C. The solution composition at 96 °C is approximately 220 g/kg WO3 and 40 g/kg NH3. APT. 6H20 is unstable at temperatures higher than approximately 96 ° C. This implies that only from solutions with initially higher WO3 concentrations will APT start to nucleate at relatively low temperatures, resulting in crystallization of the metastable APT. 6H20. Lowering the initial WO3 concentrations will increase the boiling point of the solution and the stable APT-4H20 will
198
J.W. VAN PUT ET AL.
100"C B
98.3" 97.3"( 94.9"[
"e-
A
o.ol
302
F--
/WO 3
ll 0 002 00~
NH3
Fig. 9. Ternary phase diagram for APT. 4H20 and APT-6H20.
~
H20
00y
NH3
A\
5
\
WO]
Fig. 10. Ternary phase diagram for APT.4H20 and APT"6H20 crystallization under isobaric conditions (P= 1 bar). Shaded area indicates initial crystallization of metastable APT. 6H20. crystallize. Nucleation o f A P T . 4 H 2 0 apparently proceeds m o r e easily at higher temperatures. According to the Ostwald law of stages, the metastable phase m a y crystallize first from a solution with a concentration higher than the solubility of both phases. The stable phase, with a solubility lower than the metastable
FORMATIONOF AMMONIUMPARATUNGSTATETETRA-AND HEXA-HYDRATEI
96"C~ 90"C
199
APT.6H20 _APT.¢H20
Fig. 11. Schematic view of the solubilities of A P T . 4 H 2 0 and A P T . 6 H 2 0 .
phase formed, will crystallize after a certain time. The solution will become undersaturated with respect to the metastable phase once the stable phase has been formed. The metastable phase starts to dissolve at this point. The crystallization of the stable phase will proceed until the solution reaches its saturation concentration. The metastable phase will be completely dissolved at this point. A schematic view of the solubilities of the APT-6H20 and APT. 4H20 phases is shown in Fig. 11 for clarity. APT. 4H20 has the lowest solubility and is thermodynamically the more stable. The solubility of APT. 6H20 is always higher than the solubility of the APT. 4H20 phase in the temperature range from 90 ° to 96 °C at a pressure of 1 bar. The solubility of the APT'6H20 phase could, unfortunately, not be measured with sufficient accuracy to reveal the difference between the solubilities of the two phases.
CALCULATION OF THE WATER ACTIVITY
The water activity was calculated from the vapour composition to determine whether it can be related to the recrystaUization of APT.6H20 into APT. 4H20. The calculated vapour composition [ 12 ] as a function of time is shown in Fig. 12. The water activity as a function of time is shown in Fig. 13. PH~Osa, was obtained from Perry [ 11 ]. It is obvious that the activity of water decreases during the first 50 min. The relative error in the calculation of the activity is approximately 10%. Although this error is relatively high, the same trend for the water activity was observed for several experiments. It may, therefore, be concluded that the recrystallization of the APT. 6H20 phase into the APT. 4H20 phase is prompted by a decrease in water activity. It should, however, be mentioned that the increasing solution temperature during crystallization also favours the recrystallization of APT.6H20 into APT-4H20: due to the increasing temperature en2o., becomes larger, thus resulting in a lower activity coefficient for water. In addition, it was found that the nucleation of the APT. 4H20 proceeds more easily at higher temperatures.
200
J.W. VANPUTETAL. Concentration (wf%)
40 L
0
60
180
300
Time (min.) Fig. 12. Vapour composition as a function of time for a typical experiment.
H20 activify (mo[/kg) 70
60
s°I
x exp. ~* I o exp. # 2 • exp. * 3
~0 0
I
I
1;o
2so
Time (min.) Fig. 13. Activity of water as a function of time for several experiments. CONCLUSIONS
APT. 6H20 is metastable under isobaric conditions of 1 bar between 90 ° C and 96°C. This hydrate is unstable at temperatures higher than 96°C. APT. 4H20 is the stable hydrate under these conditions. Since APT. 6H20 is metastable, it is concluded that the solubility of APT. 6H20 is higher than that of APT. 4H20. The recrystallization of APT. 6H20 into APT-4H20 is prompted by decreasing water activity. The increasing temperature during isobaric crystallization probably accelerates the conversion. The initial formation of APT-6H20 can be prevented by the addition of a small amount of APT-4H20
FORMATION OF AMMONIUM PARATUNGSTATE TETRA- AND HEXA-HYDRATE I
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seeds. In i n d u s t r i a l p r a c t i c e s o m e crystals will always r e m a i n p r e s e n t after a b a t c h o p e r a t i o n . T h e r e f o r e , A P T . 6 H 2 0 will n o t be f o r m e d d u r i n g i n d u s t r i a l A P T crystallization. ACKNOWLEDGEMENTS Philips L i g h t i n g B.V. is gratefully t h a n k e d for financially s u p p o r t i n g this research. T h e a u t h o r s are i n d e b t e d to J.C.J. Schellekens for his help with the c o m p u t e r c o n s t r u c t i o n o f the 3-D p h a s e d i a g r a m a n d to F. E l g e r s m a for his careful r e a d i n g o f the m a n u s c r i p t .
REFERENCES 1 Van Put, J.W., Ammonium paratungstate as a raw material for the production of lamp filament tungsten wire. Ph.D. Thesis, Delft Univ. Technol., Delft Univ. Press, Delft ( 1991 ). 2 Kepert, D.L., Isopolytungstates. Prog. Inorg. Chem., 4 (1962): 199-274. 3 Cordis, V., Tytko, K.H. and Glemser, O., Uber Isopolywolframatfeststoffeund deren Beziehung zu Isopolywolframationen in w~il]rigerL/Ssung. Z. Naturforsch., 306 ( 1975 ): 834841. 4 Tytko, K.H. and Glemser, O., Isopolymolybdates and isopolytungstates. Adv. Inorg. Chem., 19 (1977): 239-315. 5 H~ihnert,M., Kristallographische Untersuchung der Ammoniumparawolframate. Z. Kristall., 120 (1964): 216-228. 6 Dawihl, W., Uber die L/Sslichkeit von Wolframs~iure und Ammoniumparawolframat in w~iBrigem Ammoniak. Z. Anorg. Allg. Chem., 244 (1940): 1-12. 7 H~ihnert,M., Uber Ammoniumparawolframate. Z. Anorg. Allg. Chem., 318 (1962): 222232. 8 Hempel, K. and Saradshow, M., L/Sslichkeit und stabile Kristallhydrate im System Ammoniumparawolframat-Wasser. Krist. Tech., 3 (1967): 437-445. 9 Basu, A.K. and Sale, F.R., Characterization of various commercial forms of ammonium paratungstate powder. J. Mater. Sci., 10 ( 1975 ): 571-577. 10 Lutz, W.F., An investigation into the crystallization of ammonium paratungstate. Diss., Univ. Witwatersrand, S. Africa ( 1973 ). 11 Perry, R.H. and Green, D. (Editors), Perry's Chemical Engineers' Handbook. McGraw Hill, New York, 6th ed. (1984). 12 Van Put, J.W., Zegers, T.W., Sandwijk, A. and Van der Straten, P.J.M., The crystallization of APT, the system WOa/NH3/H20. In: M.H.I. Baird and S. Vizjayan (Editors), Proc. 2nd Int. Conf. Separation Science and Technology. Can. Soc. Chem. Eng., Ottawa, Ont., (1989), pp. 387-394. 13 Van Put, J.W., De Koning, P.M., Van Sandwijk, A. and Witkamp G.J., Seeded and unseeded batch crystallization of ammonium paratungstate. In: A. Mersmann (Editor), Proc. l lth Symp. Industrial Crystallization. VDI-Verfahrenstechnik, Diisseldorf, Germany (1990), pp. 547-552. 14 Cardew, P.T. and Davey, R.J., The kinetics of solvent-mediated phase transformations. Proc. R. Soc. London Ser. A., 398 ( 1985): 415-428. 15 Yih, S.W.H. and Wang, C.T., Tungsten. Plenum, New York, (1979). 16 Structural Dynamics Research Corporation, I-DEAS, Level 4. Structural Dynamics Research Corp., Milford, Ohio ( 1988 ).