# Phase separation of borosilicate glass containing phosphorus

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

benino@cc.okayama-u.ac.jp

## Abstract
Sodium borosilicate glasses containing $ P_{2}O_{5} $ and $ Al_{2}O_{3} $ were prepared, and the behavior of phosphorus associated with the phase separation and the effect of $ Al_{2}O_{3} $ addition have been investigated. After the heat treatment, phase separation by spinodal decomposition was observed in the all samples. In the $ Al_{2}O_{3} $ -free glasses, phosphorus was preferentially distributed into $ B_{2}O_{3} $ -rich glass phase after the phase separation. With increasing $ Al_{2}O_{3} $ content, the amount of phosphorus distributed into $ SiO_{2} $ -rich glass phase increased. According to $ {}^{31}P $ MAS NMR measurements, in the $ Al_{2}O_{3} $ -containing glasses, $ Q^{4} $ units free from terminal oxygen atoms were formed. It was suggested that the distribution of phosphorus depend on the $ PO_{4} $ units.

### 1. Introduction

Phosphorus is a vital nutrient for all living organisms, and it is therefore indispensable particularly in the fertilizer manufacturing. Recently producing countries of phosphate resource such as China and USA tend to limit their exports, and hence phosphorus resources must be secured also in Japan.

Iron- and steel-making slags consist of chemical components, such as $ SiO_{2} $ , CaO and $ Al_{2}O_{3} $ , which are also included in widely-used commercial glasses. We focused on these components and came up with the idea that phase separation of glass, which has been used for the production of porous glasses in glass industry, could be applied to the recovery of phosphorus from iron- and steel-making slags. However, little is known concerning the distribution of phosphorus after phase separation. Then, in this study, behavior of phosphorus associated with phase separation and the effect of $ Al_{2}O_{3} $ addition have been investigated in the ternary $ Na_{2}O-B_{2}O_{3}-SiO_{2} $ system.

The composition of base glass, $ 10Na_{2}O \cdot 40B_{2}O_{3} \cdot 50SiO_{2} $ (mol%), was chosen because the glass was soluble in hot water [1]. Chemical composition was determined with inductively-coupled plasma (ICP) emission spectroscopy. Glass structure was examined with using $ {}^{11}B $ , $ {}^{27}Al $ , $ {}^{29}Si $ and $ {}^{31}P $ MAS-NMR spectroscopies. Microstructure after phase separation was observed with scanning electron microscope (SEM). From these analyses, changes in glass structure caused by phase separation were studied, from which the distribution of phosphorus was discussed.

### 2. Experimental

The glasses with compositions given in Table 1 were prepared by a conventional melt-quenching method. The raw materials of reagent grade $ Na_{3}PO_{4}\cdot12H_{2}O $ , $ NH_{4}H_{2}PO_{4} $ , $ Na_{2}CO_{3} $ , $ B_{2}O_{3} $ , $ SiO_{2} $ and $ Al_{2}O_{3} $ were mixed thoroughly and heated at $ 600^{\circ}C $ for 30 min to remove $ H_{2}O $ , $ CO_{2} $ and $ NH_{3} $ . Then, the mixture was melted in a Pt crucible at $ 1400^{\circ}C $ for 1 h. The melts were poured onto an iron plate and quenched by being pressed with another iron plate to form glass plates. Differential thermal analyses (DTA) were carried out with a heating rate of 10 K/min in order to determine glass transition and crystallization temperatures, $ T_{g} $ and $ T_{x} $ .

Heat treatments for phase separation were performed at $ 600\;^{\circ}C $ , which was the immiscible temperature of the corresponding glasses. After the heat treatment, the glasses were immersed in hot water or nitric acid of $ 1.0\;N $ for $ 24\;h $ at $ 90\;^{\circ}C $ , and the insoluble residues were recovered with vacuum filtration. After being rinsed in water and dried in an oven, silica-rich glasses were obtained.

X-ray diffraction (XRD) measurement was carried out to confirm the crystallization. Chemical compositions of the specimens, such as the glasses before phase separation, the solutions and insoluble residues after immersed in hot water or nitric acid were determined by ICP emission spectroscopy.