Effects of phosphorus pentoxide on viscosity-temperature characteristics of sewage sludge ash slags under mild reducing atmosphere

Abstract

Gasification is one of the promising ways for resourceful utilization of municipal sewage sludge (MSS). For the application of MSS to slagging gasification technology, the effects of phosphorus pentoxide (P2O5) content on slag viscosity-temperature characteristics needs to be investigated. In this study, two series of representative slags containing 0–20 wt% of P2O5 with Al2O3/CaO ratio of 2:8 and 8:2 were prepared synthetically. The viscosity-temperature curves of samples were measured, and the structures and the crystalline behavior of slags were characterized to explore the role of P2O5 in the slags. The results show that the addition of phosphorus slightly increased the high-temperature viscosity of Ca-rich slags but significantly raised their temperatures of critical viscosity (TCVs). When P2O5 content exceeded 10 %, the slag type changed from glassy to crystalline. As for the Al-rich slags, although their viscosity and TCVs decreased significantly with increasing phosphorus content, it was not suitable for use in the slagging gasifier due to their high viscosity. According to Raman and NMR spectra, the addition of phosphorus induced the repolymerization of silica-oxygen tetrahedra in both kinds of slags. However, partial Al3+ ions in the Al-rich slag shifted from tetra- to penta-coordination and extensive [AlPO4]-like units were formed, which weakened the effect of silicon-oxygen network repolymerization in increasing the slag viscosity. SEM-EDS map-scanning results revealed that the precipitation of apatite crystals was an important factor of the rapid increase in viscosity of the Ca-rich phosphorus-containing slag during cooling. The elevated phosphorus content, on the other hand, changed the type of crystals precipitated in the Al-rich slag from large-particle sillimanite to small-particle berlinite, which further moderated the increase in viscosity.