transition metal ions should be recovered by immersing the phase-separated wastes into aqueous acids, such as hydrochloric acid. Akai et al. $ ^{[21]} $ have applied phase separation to a recycling process of waste glasses. Then, the author's research group have tried to apply the process to inorganic wastes, such as granulated blast furnace slag $ ^{[22]} $ and municipal waste slag. $ ^{[23]} $ The results for a municipal waste slag are shown in Figs. 9 and 10. As shown in Fig. 9, the recovered solids are colorless and transparent in visible region. As shown in Fig. 10, $ SiO_{2} $ content in the recovered solids is higher than 80 mass%, and coloring agents of $ Fe_{2}O_{3} $ and S are almost completely removed. Boron is not detectable with the X-ray fluorescence apparatus used in the present study, and thereby $ B_{2}O_{3} $ is not listed in Fig. 10.

(a)

(b)

Fig. 9. Photographs of (a) the ash-melted slag and (b) an end product recovered from the slag (s:b = 9:1).$^{23)}$

It is also noted in Fig. 10 that comparatively large amount of $ P_{2}O_{5} $ and $ TiO_{2} $ are remaining in the recovered solids, suggesting that phase separation is available for the recycling of valuable elements, such as P and Ti. In particular, Japan completely depends on imports for phosphorous resources, and hence phase separation is expected to be used as a phosphorous recycling method. The author's research group is currently investigating the distribution of phosphorous during the phase separation of borosilicate glasses, and it is revealed that phosphorous distribution is greatly influenced by $ Al^{24} $ and Ti. $ ^{25} $ It is consequently suggested that the P, Ti and Al distributions shown in Fig. 10 are dependent on the coexisting elements. By clarifying the distribution mechanism, selective recovery of valuable elements should be possible.

Fig. 10. X-ray fluorescence-analytical composition of the end products recovered from a municipal waste slag. $ ^{23)} $ The ratio s:b indicates the weight ratio between the slag and $ B_{2}O_{3} $ in the slag glasses. The composition of the untreated slag is also shown for comparison.

### 5. Concluding remarks

In 1992 the XPS apparatus was installed in Okayama University, which was almost 20 years ago. After that, many findings were obtained; not only O1s but also Si2p shifts chemically, and furthermore, they shift to the same direction. BO1s and NBO1s components also shift chemically, and the magnitude of the shifts are almost the same with those of the core level components of cations with strongly-interacted with respective oxide ions. O1s binding energy can be used as a measure of basicity. Heavy metal oxides such as $ TeO_{2} $ and $ Bi_{2}O_{3} $ seem to be basic in experimental O1s binding energy but acidic in calculated optical basicity. To interpret these findings, MO calculations have been introduced, from which a number of know-how have been accumulated.

Much knowledge was also obtained from borosilicate glasses. Structural changes in borosilicate glasses are quite complicated; coordination change in boron and NBO formation. However, the changes are successfully explained on the basis of basicity. BO that looks like NBO was also found in borosilicate glasses. MO calculations were helpful to understand unique electronic states of borosilicate glasses. The knowledge obtained from borosilicate glasses and basicity has been applied to interpret the distribution of elements during phase separation of borosilicate glasses and vitrified wastes. There remain a number of unclear phenomena in borosilicate glasses and phase separation, and hence the author will pursue the unsolved issues further.

Acknowledgment The research outcomes shown in