Fine particulate crystalline aluminum orthophosphate and method for preparing same

#The present invention relates to a new method for preparing a crystalline aluminum orthophosphate (AlPO.sub.4), particularly a fine particulate crystalline aluminum orthophosphate.

#Crystalline AlPO.sub.4 has been used as the raw material of optical glasses and various ceramics, as a catalyst, etc. Further, since AlPO.sub.4 has a crystal structure similar to that of rock crystal (SiO.sub.2), it has recently been considered promising as a vibrator material (piezoelectric material) like LiTaO.sub.3 and rock crystal.

#Conventional crystalline AlPO.sub.4 has been produced by the following methods:

#78 g. of aluminum hydroxide (gibbsite type), 145 g. of an aqueous phosphoric acid solution (P.sub.2 O.sub.5 concentration: 54%) and 250 g. of benzene were charged into a flask and the temperature was raised under stirring, then after heating for about 1 hour at a temperature not higher than the reflux temperature (73.degree. C.), the water in the reaction system was removed azeotropically with benzene and dehydration was performed while adding benzene into the system in an amount corresponding to the distilled-off benzene and water. As the dehydration proceeded, the internal temperature rose, and when the internal temperature exceeded 80.degree. C., the heating was discontinued to stop dehydration. After cooling, the product (precipitate) was separated from the solvent by filtration and then dried to give a finely divided white powder. According to X-ray diffraction pattern of this powder, main peaks were observed at diffraction angles of 2.theta.=26.4.degree., 20.8.degree., 49.7.degree., which coincided with the characteristic diffraction peaks of berlinite type AlPO.sub.4 described in ASTM card 10-423. Its average particle diameter was about 50 microns. Its analytical results are shown in Table 2.

#Aluminum hydroxide (gibbsite type), aluminum oxide (Al.sub.2 O.sub.3, percent alpharization: 85%), an aqueous phosphoric acid solution (P.sub.2 O.sub.5 54%) and organic solvents were fed under the conditions set out in Table 1, and as to the other conditions than those shown in Table 1, the conditions described in Example 1 were followed, to obtain crystalline AlPO.sub.4 's, whose yields and analytical results are as shown in Table 2.

#Reference to Tables 1 and 2 clearly shows that according to the present invention there are obtained crystalline AlPO.sub.4 's nearly quantitatively at relatively low temperatures below 200.degree. C. and that in comparison with the conventional AlPO.sub.4 they are of high purity having a content of such impurities as Fe.sub.2 O.sub.3 and Na.sub.2 O fairly lower than that estimated from the analysis of the aluminum compounds used.