Waste recycling and hazardous elements inertization for a sustainable ceramic process

A strong contribution to the sustainability of the ceramic manufacturing process can derive from the recycling of waste, especially if it contains hazardous elements (HEs). The range of secondary raw materials would be broadened to elements that, however, must undergo a total inertization into the crystalline lattice of the residual phases or in the glass matrix of ceramic bodies through the so-called ceramization process. In fact, prior any possible resorting to HEs (e.g. Ba, Co, Cr, Cu, Mo, Pb, Sb, Sn, Sr, V, Zn), their degree of inertization within different ceramic matrices have to be quantitatively assessed. To this end, in this contribution four ceramic batches were formulated to obtain: three largely vitrified ceramic products (i.e., two different porcelain stoneware bodies and a red stoneware) and a largely unreacted body (low fired brick). These formulations, taken as ceramic standards, were then added with 10 wt% of two different type of waste: bottom ashes (BA) from MWI and an artificial waste (an aluminosilicate matrix containing all the HEs of above to simulate the BA composition). Fired products from all twelve ceramic formulations, experimented at the laboratory scale simulating the industrial tile-making process, were characterized through a combined analytical approach (QPA-XRPD, SEM, water absorption, and bulk density), and then subjected to a leaching test (EN 12457-2:2002) to evaluate the HEs mobility. Obtained results highlight a different mobilization mechanism depending on both specific hazardous element and ceramic formulation, with significant criticisms related to Mo, Cr and Sr, mostly in the largely unreacted body.