Solid-state synthesis of mullite from spent catalysts for manufacturing refractory brick coatings

This paper shows the results of the solid-state synthesis of mullite from spent catalysts discarded from fluid catalytic cracking (FCC); the catalysts are mainly composed of silica and alumina but are polluted with SOX, forming a non-crystalline network. The synthesized mullite was used as a feedstock to thermally spray a coating onto a silica-alumina refractory brick, and its chemical resistance at high temperature was subsequently evaluated by contact with K2CO3 at 950 °C. Initially, the spent catalyst was thermally treated for 2 h at 600, 900, and 1200 °C to eliminate the SOX pollutant. The heat treatment at 1200 °C completely removed the SOX in the sample. Additionally, four thermal processes were performed by heating the spent FCC catalyst in an electrical furnace to 1500 and 1600 °C and by using an oxyacetylene flame to synthesize mullite. Thermal treatments at 1500 °C were performed with and without alumina added to the spent FCC catalyst, whereas those conducted at 1600 °C and using a flame were performed using only added alumina. In the powders thermally treated at 1500 °C, silica-rich mullite (3Al2O3.2SiO2) accompanied by an excess of alumina or silica was obtained with or without alumina added, respectively. In contrast, the materials treated at 1600 °C formed alumina-rich mullite (2Al2O3.SiO2), which was accompanied by an excess of alumina. Mullite was not synthesized in the flame-heated powder. The silica-rich mullite accompanied by an excess of alumina was used as feedstock powder to modify the surface of a refractory brick, improving its resistance to chemical attack by K2CO3 at high temperature.