Chromium-bearing phases in the system MgO-SiO2-Al2O3-Cr2O3, and their relevance on chromium inertization in ceramic systems

Several industrial processes produce large amounts of heavy metals-rich wastes (Cr, Pb, Cd, Ni), which could be considered as "trash-can raw materials". Synthesis in refractory ceramic systems can be regarded as a key process to permanently incorporate dangerous heavy metals in a stable crystalline assemblage, The aim of this work is to insert waste chromium in crystalline phases such as spinel, mullite, corundum and sapphirine, forming in the system MgO-Al2O3-SiO2-Cr2O3. High temperature experiments were performed at 1 atm in the system MgO-Al2O3-SiO2-Cr2O3 (MASCr) in order to study phase relations and chromium partitioning, at temperatures ranging from 1250 degreesC to 1560 degreesC. Bulk compositions were prepared as oxide mixture pellets, with SiO2:Al2O3 ranging from 1:1 to 1:4.5 wt.%, and Cr2O3:Al2O3 ranging from 1:9 mol toward Al-free compositions. Chromium is strongly incorporated in crystals, coexisting with very chromium-poor glasses. Sapphirine presents a surprisingly high chromia content, up to 29 wt%, and it appears to be the only quaternary compound in the system. The MASCr system shows peculiar and complex topologies. Proper selection of bulk compositions and thermal treatments permits to maximize the best host phases for a complete incorporation of Cr in a refractory ceramic assemblage.