# Effect of $ TiO_{2} $ addition on the distribution of phosphorus associated with phase separation of borosilicate glasses

Y. Ohtsuki $ ^{1} $ , S. Sakida $ ^{2} $ , Y. Benino $ ^{1} $ and T. Nanba $ ^{1} $

1 Graduate School of Environmental Science Okayama University, Japan

2 Environmental Management Center Okayama University, Japan

E mail: tokuro_n@cc.okayama-u.ac.jp

## Abstract

Behavior of phosphorus associated with phase separation and the effect of $ TiO_{2} $ addition on the phosphorus distribution were investigated in $ Na_{2}O-B_{2}O_{3}-SiO_{2} $ ternary glass system. After heat treatment, phase separation by spinodal decomposition was commonly observed. In the phase separation of $ TiO_{2} $ -free glasses, phosphorus was preferentially distributed into $ B_{2}O_{3} $ -rich phase, and after $ TiO_{2} $ addition, phosphorus distribution into $ SiO_{2} $ -rich phase was also observed. With increasing $ TiO_{2} $ content, the amount of phosphorus distributed into $ SiO_{2} $ -rich phases increased up to 90%. $ {}^{31} $ P MAS NMR indicated that phosphorus in the glasses was mainly present in negatively-charged $ Q^{2} $ units with terminal oxygen atoms, and in the $ TiO_{2} $ -containing glasses, $ Q^{1} $ or $ Q^{0} $ units containing P-O-Ti bond were confirmed. Despite the formation of negatively-charged $ Q^{1} $ or $ Q^{0} $ units, phosphorus was distributed into $ SiO_{2} $ -rich phase, which was due to the distribution of $ Ti^{4+} $ ions with higher coordination numbers acting as network modifiers.

Keywords: borosilicate glass, phase separation, phosphorus, glass structure

### 1. INTRODUCTION

Phosphorus has been widely used in various applications, such as fertilizer, agrichemicals, pesticides, cement, toothpaste, food additives, and so on. However, Japan has long ceased to the production of phosphorus natural resources and is almost totally dependent on imports of phosphate ores for the phosphorus resources. Recently, phosphorus producing countries such as China and USA limit the exports of phosphorus resources, and hence stable supply of phosphorus resources becomes a critical issue in Japan.

The authors’ research group has developed a chemical recycling process of inorganic wastes by using phase separation of glass $ [1,2] $ . In inorganic wastes such as granulated blast furnace slags and high-temperature molten slags of municipal wastes, $ SiO_{2} $ , CaO and $ Al_{2}O_{3} $ are present as major constituents. When $ B_{2}O_{3} $ is added, the wastes are easily vitrified, and phase separation is induced by heat-treatments. In the phase separation, coloring species such as Fe ions as well as alkali and alkaline earth ions such as Na and Ca ions are preferentially introduced to $ B_{2}O_{3} $ -rich phase, and hence $ SiO_{2} $ -rich colorless and transparent glass is recovered by immersing the phase-separated slag glass into acidic solutions. In the municipal waste slags $ [2] $ , it was found that phosphorus was distributed to $ SiO_{2} $ -phase at a high rate of 60 - 70%, suggesting that phase separation of wastes was also available to the recovery of phosphorus, and phosphorus recovered from wastes was used as a phosphorus resource.

Then, the authors have examined the distribution of phosphorus due to phase separation. In the previous study $ [3] $ , a simple $ Na_{2}O-B_{2}O_{3}-SiO_{2} $ system was chosen to reveal fundamental mechanisms of phosphorus distribution, in which it was found that phosphorus was completely distributed to $ B_{2}O_{3} $ -rich phase without additives other than $ P_{2}O_{5} $ , and addition of $ Al_{2}O_{3} $ initiated the phosphorus distribution to $ SiO_{2} $ -rich phase. In the $ Na_{2}O-B_{2}O_{3}-SiO_{2} $ glass, aluminum was selectively distributed to $ SiO_{2} $ -rich phase, and in the municipal waste slag glass, however, aluminum was oppositely distributed to $ B_{2}O_{3} $ -rich phase. In the municipal waste slag glass, simultaneous distribution of titanium to $ SiO_{2} $ -rich phase was observed $ [2] $ . Implying that titanium also had a promoting effect of phosphorus distribution to $ SiO_{2} $ -rich phase. Then, in this study, the effect of $ TiO_{2} $ addition on the phosphorus distribution has been investigated in the phase separation of ternary $ Na_{2}O-B_{2}O_{3}-SiO_{2} $ system, and the mechanism of phosphorus distribution was also discussed based on the structural analyses.