Nitrate-induced cancrinite precipitation in alkaline solution at low temperature and relevance to the Bayer alumina process

Abstract

Cancrinite-type zeolites were proposed as functional materials which exhibits promising photocatalytic properties, enables carbon capture and can even be used in the storage of liquid radioactive waste. Conventional cancrinite is precipitated by reacting Na2CO3 or CaCO3 with Al- and Si- rich alkaline solutions at an elevated temperature of 150 °C in Bayer Alumina process. Here we report the effect of nitrate on cancrinite formation at 90 °C. At low alkalinity, nitrate accelerates transformation of zeolite LTA to sodalite, followed by minor cancrinite precipitation at a later stage. In highly alkaline solution, nitrate addition shortens the induction time required for homogenous cancrinite precipitation. In highly alkaline solutions with nitrate, it was also found that zeolite LTN precipitates as an intermediate phase which transforms into sodalite/cancrinite. Raman spectroscopy and thermogravimetric analysis confirmed the incorporation of nitrate into the structure of cancrinite and its thermal stability, at room temperature. These findings may help to optimize the Bayer process, mitigate the potential environmental risks of bauxite residue, and aid in the capture of the greenhouse gases from the air.