AUTOCLAVED MONOLITHS INCORPORATING LARGE CONTENTS OF RESIDUES FROM THE ALUMINA AND ALUMINIUM INDUSTRIES

This paper presents a process to valorize large amounts of bauxite residue (BR) as well as other residues from the aluminium industry. The process comprises the alkali-activation of solids, followed by press-shaping and autoclave curing of the resulting monoliths, delivering products that resemble terracotta bricks or tiles. In this work, the precursor consisted of a mix containing 75 wt% of residues/ by-products from the alumina and aluminium industries (BR (70 wt%) and CHAC (5%), a calcium-rich by-product from the SO2 scrubber system of the coke calciner plant), 10 wt% of construction quartz sand and 15 wt% of vitrified BR (as a binder, produced at pilot scale). The solids were activated using an alkaline solution recovered from the Spent Pot Lining (SPL) treatment plant. After mixing the materials, cubic specimens of 4 cm3 were press-shaped at 47 MPa and autoclaved at 200 °C and steam pressure of 15 bars for 24 hours. The specimens were tested on compressive strength, hydrolytic stability by water boiling test, and X-ray diffraction (XRD) analysis. The results revealed that the hydrothermal curing promoted the dissolution of quartz, the formation of katoite and sodalite, and the partial crystallization of the amorphous iron into hematite and goethite. The autoclaved monoliths were hydrolytically stable, and both the reference and water-boiled samples showed compressive strengths around 32 MPa. Acknowledging that autoclave curing is more energy-intensive than oven curing, a significantly higher mechanical performance was reached in comparison to specimens cured in an oven at 95 °C or 200 °C (< 22 MPa in both cases). An upscaling of the process seems feasible, even within the premises of the alumina plants, as most of the raw materials are available in-house whereas the knowledge about such processing is available within the industry.