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Liquidus relations in the system lithium metaphosphate-lithium pyrophosphate
J. Inolg. Nucl ('hem.. 1961. Vol. 22, pp. 293 to 296. Pergamon Press ltd. Pr:nted in Northern heland
LIQUIDUS RELATIONS IN THE SYSTEM LITHIUM M ETAPHOSPHATE-LITHI UM PYROPHOSPHATE M. M. MARKOWITZ, R. F. HARRIS and W. N. HAWLEY l o o t e Mineral Co., Research and Engineering Center, P.O. Box 513, West Chester, Pa.
(Receired 10 May 1961, in rel,ised[brm 22 June 1961 )
Abstract--Lithium metaphosphate and pyrophosphate are suggested as possible components tot fused salt nuclear breeder blankets because of the relatively high thermal stability of these compounds and of the very low absorption of thermal neutrons exhibited by phosphorus. The phase system LiPO3-Li4P~O: was studied by means of high temperature microscopy and differential thermal analysis. At liquidus temperatures, the system was determined to be of the simple eutectic type with the eutectic at 84.5 mole % LiPO3 and 6 0 3 . An arrest occurring at 569 ' might indicate the existence of a polyphosphate higher than the pyrophosphate in the solidus region; however, the origin of this halt has not been identified. The'LiPO3-Li4P20r system in contrast to other metaphosphate-pyrophosphate systems appears to be unique due to the absence of higher polyphosphates in the liquidus region.
GRU~N ~ has pointed out some of those factors signiticant to the application of a lithiumbased fused salt system as a breeder blanket for the deuterium-tritium fusion reactorJ z~ The relatively thermally unstable compounds lithium nitrite and nitrate have been considered as possible components, et.z~ Further thought indicates that molten mixtures of lithium metaphosphate and pyrophosphate might be of value as blanket materials because ofthe high intrinsic thermal stability of the component salts and of the low nuclear reactivity to thermal neutrons of the phosphorus-containing oxyanionsJ 4~ Accordingly, a phase study of the system lithium metaphosphate-lithium pyrophosphate at liquidus temperatures was undertaken as a preliminary to further characterization of mixtures of these salts. EXPERIMENTAL
PROCEDURES
AND
RESULTS
Lithium metaphosphate was prepared by dehydration of lithium dihydrogen orthophosphateJ 5~ The latter compound was partially dehydrated at 200 ° for 4 hr to yield substantially lithium dihydrogen pyrophosphate; after grinding, this salt was heated at 300 ~ for 4 hr and then for 4 hr at 800 ° in a platinum dish. Rapid cooling of the melt resulted in a clear glass which was crystallized after grinding by heating at 500 ° for 2 hr. This crystalline material was used in the preparation of all the samples for the phase studies. (Phosphorus: "~ Found, 36.g6; Calcd. for LiPO:~, 36.(36°/,;). Lithium pyrophosphate was prepared by reaction of anhydrous lithium hydroxide with pyrophosphoric acid. A mixture of 139.1 g (0.781 mole) pyrophosphoric acid/v~ ~t~ D. M. GRt:ES, Report ANL-5840 (1958L ca~ R. F. Posr, Ret'. Mad. Phys. 28, 338 (19561. ~a, A. S. BisltoP, Proiect Sherwood: The U.S. Program in Controlled I"usiolr, pp. 42 43, Anchor Books. Doubleday, New York (1960). i1~ R. STEPHE>~SO>;, Introduction of Nuclear Engineering, p. 375. McGraw-Ilill, New York (1954). ~'~ R. K. OsxrRHtLt) and M. M. MARKOWIIZ,J. Phy.s. Chem. 60, 863 (1956). fnJ L. T. Jo~Es, Industr. Engng. Chem., Anal.yr. Ed. 14, 536 (1942L tr~j. F. MALOW'AN, Inorganic 5~vntheses, Vol. 3, pp. 96-97. (Edited by L. F. AL;t)RE~IH) McGraw-Hill Ne~ York (1950). 293
294
M.M. MARKOWiTZ,R. F. HARRISand W. N. HAWLEY
68"0 g (2.839 mole) anhydrous lithium hydroxide, and 1000 ml absolute ethanol was refluxed with vigorous stirring for three days. The insoluble material was filtered, washed with ethanol and dried for 5 hr at 160 ° under vacuum. This solid was then heated in air at 800 ° for 12 hr. (Phosphorus :~6~ Found, 30.36; Calcd. for LiaP20 7, 30"72~o. Pyrophosphate group: ~S~ Found, 84.44°; Calcd. for Li4PzOv 86.24~o). Lithium pyrophosphate has also been prepared in this laboratory by thermal treatment 900
°tO)'m80C " ~
o
~'~"E: 700
n
solution
"~
oo
ooc
~
0 iO Li4PzO z t
I
r~
2
~
r"
o
~--~
a- Li4P2Oz+LiPO~
I0
310 40 50 60 Mote % LIP03 I
~
,
',
•
i
;
I
7'0 '
eSOlutlon
~ - ; , . ~ ~
;
81
0
A
90 I00 LiPO s ,
FIG. l . - - T h e system lithium metaphosphate-lithium pyrophosphate
of the reaction mixtures: 2LiH2PO4 + Li2COa, 2LiPOa + LizCO3, and LizCO3, + (NH4)2HPO 4. The small weight losses engendered during heating the pure metaphosphate and pyrophosphate salts in platinum dishes in air for two days at 900 ° are indicative of the considerable thermal stability of these compounds. Thus, 10.0992 g LiPO a lost 0.0581 g and 9.5452 g Li4P20 7 lost 0.0011 g under these conditions. The phase relationships of the components and of mixtures thereof were studied by the techniques of high temperature microscopy ~ and of differential thermal analysis (DTA). The sample compositions are indicated by the expeiimental points in Fig. 1. A high temperature hot stage was used for the microscopic investigations (1000°C Microscope Heating Stage, No. HATII, E. Leitz, Inc., 468 Fourth Ave., N.Y. 16, N.Y.). In this manner, the first formation ofliquid in all samples was found to occur at about 603 ° over the entire range of sample compositions. The temperatures corresponding to the last disappearance of solid were determined to be unique functions of sample composition. Three separate determinations were made on each sample comsposition of intimately mixed crystalline materials. These combined observations resulted in the liquidus and eutectic portions of the curves of Fig. 1. The DTA runs were performed using the instrumentation described elsewhere~1°~ in conjunction with bare platinum/10 per cent rhodium-platinum thermocouples. A ts~ R. N. BELL, Analyt. Chem. 19, 97 (1947).
t,~ W. C. MCCRoNE, JR., Fusion Methods in Chemical Microscopy, pp. 22-24, 142-171. lnterscience, New York (1957). tt0~ M. M. MARKOWlTZ, d. Phys. Chem. 62, 827 (1958).
liquidus relations in the system lithium metaphosphate-lithium pyrophosphate
295
smaller version of the furnace arrangement of GORDON and C,X.~tI'BEt.t.was used.~ll~ A sample size of 1.5 g contained in a Vycor test-tube was adopted with a heating rate of l0 t` per min; alumina was used as the reference standard. Each sample was subjected to three to five heating and cooling cycles during the DTA experiments. Dissolution of the Vycor container by the phosphate melts was judged to be slight. Thus, a loss in weight of about 10 mg was found for a typical Vycor test-tube (0.5 in. diam .... 3-6 in. length, 8.81 g) containing 1.5 g of a mixture of 50 wt. ,%/oLiPO~-50 w..,t°/,, Li4P,~O7 mainrained at 920 :~ for 5 hr. Samples rich in lithium metaphosphate were found to supercool to glassy materials which recrystallized exothermally upon reheating. Tile behaviour of pure lithium metaphosphate during DTA has already been detailed ;~21 melting of the crystalline material occurs at 658 ". Lithium pyrophosphate was found to exhibit a reversible crystallographic transition at 646 ~ and to melt at 878 e'. Those salt mi×tures displaying this characteristic transition, as determined by DTA, are indicated in Fig. 1. The DTA curves for mixtures of the components showed the presence of an endothermic break at about 569 °. This break was only found to appear clearly after the tirst heating cycle and would thus indicate that intimate mixing of the components. through solution formation, facilitates the occurrence of the process giving rise to the thermal effect. No consistent order was found for the intensity of the break at 569' either as a function of sample composition or during successive DTA runs on the same sample. Apparently, the phenomenon occurring at 569 ~ is of some incompletely reproducible type. This arrest is shown by the dashed line in Fig. 1. Some thermal interaction between the endotherm at 569 ° and the endothermic eutectic break at 603' was also found. This resulted in a somewhat irregular variation of the magnitude of the eutectic break. The liquidus temperatures determined by DTA were in close agreement with those found by microscopic observations. Inspection of Fig. 1 shows the system to be of the simple eutectic type at liquidus temperatures, with the eutectic at 84.5 mole% LiPO:~ and 603"'. DISCUSSION Attempts at identifying the origin of the DTA break at 569 ¢ were unsuccessful. Mixtures of lithium metaphosphate and lithium pyrophosphate in 10 mole~/o increments were fused at 900 °, cooled rapidly, powdered, and tempered at 450 ° for three weeks. This was done to determine if a new crystalline polyphosphate phase would form. X-ray powder patterns of these annealed mixtures evidenced lines attributable only to the original crystalline components. This might indicate that if a crystalline polyphosphate (M,I~ ~,P,,O~,,. 0 higher than the pyrophosphatc Itsl (n ..... 2) is capable of formation in this system, its rate of formation must be very low at temperatures appreciably below its decomposition temperature (569°). The formation and decomposition of the postulated polyphosphate can probably be represented by the equation : Li4P20 7 -? (n - 2)LiPO:3-,Li,,_zP,O:~,,. ~, n : - 3. Lithium metaphosphatc is expressed simply as LiPO:~ for ease of representation ; like the other water-insoluble alkali metal metaphosphatcs, the lithium compound is undoubtedly a salt of highly polymeric. long-chain structure. ( ~ tll) S. GORDON anti C. CAMPBELL, Analyt. Chem. 27, 1102 (19551. ,v.,~ M. M. MARKOwrrz a n d D. A. BORYrA, Analyt. Chem. 32, 1588 (1960). ~u, M. M. MARKOWITZ, J. Chem. Educ. 33, 36 (1956). cll~ j. R. VAN WAZER, Phosphorus and Its Compounds, Vol. 1, pp. 6 7 2 - 6 7 8 . Intcrscience, N e w Y o r k (1958).
296
M.M.
MARKOWITZ, R.
F. HARRISand W. N. HAWLEY
It is of interest to note that in each of the phase systems for which extensive studies have been made between the metaphosphate and pyrophosphate compositions, polyphosphates (n > 3 existing within the liquidus region have been found. This includes the systems NaPOa-Na4P20 r (n = 3), (15) KPO3-K4P207 (n 3), (16'17) :-
and
Ca(PO3)2-Ca2P207 (n = 5),(18,19) Pb(PO3)2-PbzP20r (n ----4). (2°,2')
On the basis of the present studies, no polyphosphate higher than the pyrophosphate has been found to exist within the liquidus region of the lithium metaphosphatelithium pyrophosphate system. ,ls~ E. P. PARTRIDGE,V. HICKS and G. V. SMITH,J. ,4mer. Chem. Soc. 63,454 (1941). c16~ R. K. OSTERHELDand L. F. AUDRIETH,J. Phys. Chem. 56, 38 (1952). tl:~ G. W. MOREV,J. Amer. Chem. Soc. 76, 474 (1954). tls~ W. L. HILL, G. T. FAUSTand D. S. REYNOLDS, Amer. J. Sci. 242,457 542 (1944). c19>j. R. VA,WWAZER and S. OHASHI,J. Amer. Chem. Soc. 80, 1010 (1958). tz0~ R. K. OSTERHELDand R. P. LANOGUTH,J. Phys. Chem. 59, 76 (1955). tzl~ R. P. LANGGUTH, R. K. OSTERrlELD and E. F. KARL-KRouPA, J. Phys. Chem. 60, 1335 (1956).
LIQUIDUS RELATIONS IN THE SYSTEM LITHIUM M ETAPHOSPHATE-LITHI UM PYROPHOSPHATE M. M. MARKOWITZ, R. F. HARRIS and W. N. HAWLEY l o o t e Mineral Co., Research and Engineering Center, P.O. Box 513, West Chester, Pa.
(Receired 10 May 1961, in rel,ised[brm 22 June 1961 )
Abstract--Lithium metaphosphate and pyrophosphate are suggested as possible components tot fused salt nuclear breeder blankets because of the relatively high thermal stability of these compounds and of the very low absorption of thermal neutrons exhibited by phosphorus. The phase system LiPO3-Li4P~O: was studied by means of high temperature microscopy and differential thermal analysis. At liquidus temperatures, the system was determined to be of the simple eutectic type with the eutectic at 84.5 mole % LiPO3 and 6 0 3 . An arrest occurring at 569 ' might indicate the existence of a polyphosphate higher than the pyrophosphate in the solidus region; however, the origin of this halt has not been identified. The'LiPO3-Li4P20r system in contrast to other metaphosphate-pyrophosphate systems appears to be unique due to the absence of higher polyphosphates in the liquidus region.
GRU~N ~ has pointed out some of those factors signiticant to the application of a lithiumbased fused salt system as a breeder blanket for the deuterium-tritium fusion reactorJ z~ The relatively thermally unstable compounds lithium nitrite and nitrate have been considered as possible components, et.z~ Further thought indicates that molten mixtures of lithium metaphosphate and pyrophosphate might be of value as blanket materials because ofthe high intrinsic thermal stability of the component salts and of the low nuclear reactivity to thermal neutrons of the phosphorus-containing oxyanionsJ 4~ Accordingly, a phase study of the system lithium metaphosphate-lithium pyrophosphate at liquidus temperatures was undertaken as a preliminary to further characterization of mixtures of these salts. EXPERIMENTAL
PROCEDURES
AND
RESULTS
Lithium metaphosphate was prepared by dehydration of lithium dihydrogen orthophosphateJ 5~ The latter compound was partially dehydrated at 200 ° for 4 hr to yield substantially lithium dihydrogen pyrophosphate; after grinding, this salt was heated at 300 ~ for 4 hr and then for 4 hr at 800 ° in a platinum dish. Rapid cooling of the melt resulted in a clear glass which was crystallized after grinding by heating at 500 ° for 2 hr. This crystalline material was used in the preparation of all the samples for the phase studies. (Phosphorus: "~ Found, 36.g6; Calcd. for LiPO:~, 36.(36°/,;). Lithium pyrophosphate was prepared by reaction of anhydrous lithium hydroxide with pyrophosphoric acid. A mixture of 139.1 g (0.781 mole) pyrophosphoric acid/v~ ~t~ D. M. GRt:ES, Report ANL-5840 (1958L ca~ R. F. Posr, Ret'. Mad. Phys. 28, 338 (19561. ~a, A. S. BisltoP, Proiect Sherwood: The U.S. Program in Controlled I"usiolr, pp. 42 43, Anchor Books. Doubleday, New York (1960). i1~ R. STEPHE>~SO>;, Introduction of Nuclear Engineering, p. 375. McGraw-Ilill, New York (1954). ~'~ R. K. OsxrRHtLt) and M. M. MARKOWIIZ,J. Phy.s. Chem. 60, 863 (1956). fnJ L. T. Jo~Es, Industr. Engng. Chem., Anal.yr. Ed. 14, 536 (1942L tr~j. F. MALOW'AN, Inorganic 5~vntheses, Vol. 3, pp. 96-97. (Edited by L. F. AL;t)RE~IH) McGraw-Hill Ne~ York (1950). 293
294
M.M. MARKOWiTZ,R. F. HARRISand W. N. HAWLEY
68"0 g (2.839 mole) anhydrous lithium hydroxide, and 1000 ml absolute ethanol was refluxed with vigorous stirring for three days. The insoluble material was filtered, washed with ethanol and dried for 5 hr at 160 ° under vacuum. This solid was then heated in air at 800 ° for 12 hr. (Phosphorus :~6~ Found, 30.36; Calcd. for LiaP20 7, 30"72~o. Pyrophosphate group: ~S~ Found, 84.44°; Calcd. for Li4PzOv 86.24~o). Lithium pyrophosphate has also been prepared in this laboratory by thermal treatment 900
°tO)'m80C " ~
o
~'~"E: 700
n
solution
"~
oo
ooc
~
0 iO Li4PzO z t
I
r~
2
~
r"
o
~--~
a- Li4P2Oz+LiPO~
I0
310 40 50 60 Mote % LIP03 I
~
,
',
•
i
;
I
7'0 '
eSOlutlon
~ - ; , . ~ ~
;
81
0
A
90 I00 LiPO s ,
FIG. l . - - T h e system lithium metaphosphate-lithium pyrophosphate
of the reaction mixtures: 2LiH2PO4 + Li2COa, 2LiPOa + LizCO3, and LizCO3, + (NH4)2HPO 4. The small weight losses engendered during heating the pure metaphosphate and pyrophosphate salts in platinum dishes in air for two days at 900 ° are indicative of the considerable thermal stability of these compounds. Thus, 10.0992 g LiPO a lost 0.0581 g and 9.5452 g Li4P20 7 lost 0.0011 g under these conditions. The phase relationships of the components and of mixtures thereof were studied by the techniques of high temperature microscopy ~ and of differential thermal analysis (DTA). The sample compositions are indicated by the expeiimental points in Fig. 1. A high temperature hot stage was used for the microscopic investigations (1000°C Microscope Heating Stage, No. HATII, E. Leitz, Inc., 468 Fourth Ave., N.Y. 16, N.Y.). In this manner, the first formation ofliquid in all samples was found to occur at about 603 ° over the entire range of sample compositions. The temperatures corresponding to the last disappearance of solid were determined to be unique functions of sample composition. Three separate determinations were made on each sample comsposition of intimately mixed crystalline materials. These combined observations resulted in the liquidus and eutectic portions of the curves of Fig. 1. The DTA runs were performed using the instrumentation described elsewhere~1°~ in conjunction with bare platinum/10 per cent rhodium-platinum thermocouples. A ts~ R. N. BELL, Analyt. Chem. 19, 97 (1947).
t,~ W. C. MCCRoNE, JR., Fusion Methods in Chemical Microscopy, pp. 22-24, 142-171. lnterscience, New York (1957). tt0~ M. M. MARKOWlTZ, d. Phys. Chem. 62, 827 (1958).
liquidus relations in the system lithium metaphosphate-lithium pyrophosphate
295
smaller version of the furnace arrangement of GORDON and C,X.~tI'BEt.t.was used.~ll~ A sample size of 1.5 g contained in a Vycor test-tube was adopted with a heating rate of l0 t` per min; alumina was used as the reference standard. Each sample was subjected to three to five heating and cooling cycles during the DTA experiments. Dissolution of the Vycor container by the phosphate melts was judged to be slight. Thus, a loss in weight of about 10 mg was found for a typical Vycor test-tube (0.5 in. diam .... 3-6 in. length, 8.81 g) containing 1.5 g of a mixture of 50 wt. ,%/oLiPO~-50 w..,t°/,, Li4P,~O7 mainrained at 920 :~ for 5 hr. Samples rich in lithium metaphosphate were found to supercool to glassy materials which recrystallized exothermally upon reheating. Tile behaviour of pure lithium metaphosphate during DTA has already been detailed ;~21 melting of the crystalline material occurs at 658 ". Lithium pyrophosphate was found to exhibit a reversible crystallographic transition at 646 ~ and to melt at 878 e'. Those salt mi×tures displaying this characteristic transition, as determined by DTA, are indicated in Fig. 1. The DTA curves for mixtures of the components showed the presence of an endothermic break at about 569 °. This break was only found to appear clearly after the tirst heating cycle and would thus indicate that intimate mixing of the components. through solution formation, facilitates the occurrence of the process giving rise to the thermal effect. No consistent order was found for the intensity of the break at 569' either as a function of sample composition or during successive DTA runs on the same sample. Apparently, the phenomenon occurring at 569 ~ is of some incompletely reproducible type. This arrest is shown by the dashed line in Fig. 1. Some thermal interaction between the endotherm at 569 ° and the endothermic eutectic break at 603' was also found. This resulted in a somewhat irregular variation of the magnitude of the eutectic break. The liquidus temperatures determined by DTA were in close agreement with those found by microscopic observations. Inspection of Fig. 1 shows the system to be of the simple eutectic type at liquidus temperatures, with the eutectic at 84.5 mole% LiPO:~ and 603"'. DISCUSSION Attempts at identifying the origin of the DTA break at 569 ¢ were unsuccessful. Mixtures of lithium metaphosphate and lithium pyrophosphate in 10 mole~/o increments were fused at 900 °, cooled rapidly, powdered, and tempered at 450 ° for three weeks. This was done to determine if a new crystalline polyphosphate phase would form. X-ray powder patterns of these annealed mixtures evidenced lines attributable only to the original crystalline components. This might indicate that if a crystalline polyphosphate (M,I~ ~,P,,O~,,. 0 higher than the pyrophosphatc Itsl (n ..... 2) is capable of formation in this system, its rate of formation must be very low at temperatures appreciably below its decomposition temperature (569°). The formation and decomposition of the postulated polyphosphate can probably be represented by the equation : Li4P20 7 -? (n - 2)LiPO:3-,Li,,_zP,O:~,,. ~, n : - 3. Lithium metaphosphatc is expressed simply as LiPO:~ for ease of representation ; like the other water-insoluble alkali metal metaphosphatcs, the lithium compound is undoubtedly a salt of highly polymeric. long-chain structure. ( ~ tll) S. GORDON anti C. CAMPBELL, Analyt. Chem. 27, 1102 (19551. ,v.,~ M. M. MARKOwrrz a n d D. A. BORYrA, Analyt. Chem. 32, 1588 (1960). ~u, M. M. MARKOWITZ, J. Chem. Educ. 33, 36 (1956). cll~ j. R. VAN WAZER, Phosphorus and Its Compounds, Vol. 1, pp. 6 7 2 - 6 7 8 . Intcrscience, N e w Y o r k (1958).
296
M.M.
MARKOWITZ, R.
F. HARRISand W. N. HAWLEY
It is of interest to note that in each of the phase systems for which extensive studies have been made between the metaphosphate and pyrophosphate compositions, polyphosphates (n > 3 existing within the liquidus region have been found. This includes the systems NaPOa-Na4P20 r (n = 3), (15) KPO3-K4P207 (n 3), (16'17) :-
and
Ca(PO3)2-Ca2P207 (n = 5),(18,19) Pb(PO3)2-PbzP20r (n ----4). (2°,2')
On the basis of the present studies, no polyphosphate higher than the pyrophosphate has been found to exist within the liquidus region of the lithium metaphosphatelithium pyrophosphate system. ,ls~ E. P. PARTRIDGE,V. HICKS and G. V. SMITH,J. ,4mer. Chem. Soc. 63,454 (1941). c16~ R. K. OSTERHELDand L. F. AUDRIETH,J. Phys. Chem. 56, 38 (1952). tl:~ G. W. MOREV,J. Amer. Chem. Soc. 76, 474 (1954). tls~ W. L. HILL, G. T. FAUSTand D. S. REYNOLDS, Amer. J. Sci. 242,457 542 (1944). c19>j. R. VA,WWAZER and S. OHASHI,J. Amer. Chem. Soc. 80, 1010 (1958). tz0~ R. K. OSTERHELDand R. P. LANOGUTH,J. Phys. Chem. 59, 76 (1955). tzl~ R. P. LANGGUTH, R. K. OSTERrlELD and E. F. KARL-KRouPA, J. Phys. Chem. 60, 1335 (1956).