Hydrothermal Synthesis of Hydroxyapatite and Pollucite from Calcium Phosphate
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Hydrothermal Synthesis of Hydroxyapatite and Pollucite from Calcium Phosphate Masaaki KANEKO, Yuichi SHOJI, Hiroyuki MATSUURA, and Nakamichi YAMASAKI' Chemical Technology Group Nuclear Engineering Laboratory, Toshiba Corporation, 4-1, UKISHIMA-CHO, KAWASAKI-KU, KAWASAKI, 210, JAPAN *Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University Abstract Hydroxyapatite (HAP : Calo(P04)6(OH)2) and aluminosilicate were easily synthesized using a hydrothermal process with mixing ratios of Ca/P=l.67 (Ca(P03)2 and Ca(OH)2) and Si/Al=4 (SiO0 and AI(OH) 3 ). The temperature was kept constant at 573 K for 60 minutes..Cesium (Cs) and Cerium (Ce) contained in Ca(P03)2 , were found to take on a stable form in the pollucite (CsAlSi2O6) structure and the monazite (CeP04) structure. Introduction One of the methods of recovering uranium and plutonium in reprocessing plants is solvent extraction. Usually, TBP (tributyle phosphate : 2(C4H9O)3PO)/n-dodecane mixture has been used in such a solvent extraction process. Spent solvent premixed with calcium hydroxide (Ca(OH)2) is supplied to the pyrolysis reactor, where the TBP thermally decomposes and reacts with Ca(OH)2 to form a powder of calcium phosphate 1). This powder consists of calcium pyrophosphate (Ca2P2O7) and calcium metaphosphate (Ca(P03)2). It is desirable that these phosphates should be converted into the stable structure of hydroxyapatite (HAP) and the radioactive materials should be immobilized into a stable compound which is highly durable, in order to maintain long-term radioactive waste stability. HAP occurs naturally in bone and tooth, and has a stable structure of the hexagonal system. It is formed from calcium phosphates under hydrothermal conditions 2). Alkaline earth metals and rare earth elements are easily immobilized in HAP crystals, but alkali metals only with difficulty. Alkali metals are easily immobilized in aluminosilicate crystals, however. In particular, pollucite (CsAlSi2O6) is a desirable phase for the immobilization of radioactive Cs 3). Previous studies revealed that HAP and pollucite were formed by hydrothermal reaction individually 4)5), but it has not been found whether HAP and pollucite can be synthesized simultaneously. We have studied the formation regions of HAP and pollucite in systems of Ca(P03)2-Ca(OH)2-Si02-Al(OH) 3 containing Cs and Ce, that is similar to transuranic elements (TRUs) in chemical properties. Experimental The starting material Ca(P03)2 was produced by thermal decomposition of Ca(H2P04)2 at 673 K for 5 hours. The microautoclave used in these experiment is shown in Fig.l (a). Mat. Res. Soc. Symp. Proc. Vol. 353 * 1995 Materials Research Society
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Fig. 1 (a)micro-autoclave 1.Packing nut 2.Cone packing 3.Reaction chamber 4.Hastelloy C 5. Thermocouple well (b)Shaking heater 1.Motor 2.Bearing 3.Heater 4.Micro-autoclave This autoclave ( internal volume : 20cm 3 ) was lined with Hastelloy C. The starting powder mixture (Ca(P03)2, Ca(OH)2, Si0 2 and Al(OH) 3 ) was put into the autoclav
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