Valorisation of Metallurgical Waste for Inorganic Pigments Production

Four industrial wastes, namely, tionite (T), iron grit (IG), electroplating sludge (ES), and mill scale (MS), are typically disposed of in controlled hazardous landfills because of their toxic content, posing potential harm to human health and to the environment. At the same time, the chemical composition of these wastes, specifically the nature and content of transition metals, makes them potentially attractive for reuse in pigments manufacturing. This work details the study of these residues for producing coloured perovskites to be tested as inorganic pigments. The residues were mixed, in different proportions, and subsequently calcined to produce the required structures. The colouring potential was then assessed in a ceramic paste and in a transparent glaze. Leaching tests were carried out to verify the effective immobilisation of the hazardous species. Dark pigments were successfully obtained from the mixtures of T: ES: Co3O4, T: MS and T: IG. The crystalline phases present in T: ES: Co3O4 are nickel–chromium iron oxide spinel–Fe1.5Cr0.5NiO4 (without Co) or trevorite–Fe2NiO4 (with Co), titanium nickel oxide–TiNiO3 and titanite–CaTiSiO5. The mixtures T: MS and T: IG presented hematite (Fe2O3) and pseudobrookite (Fe2TiO5). Leaching tests confirmed the non-hazardous or inert character of the synthesized pigments. Products showed brownish or greyish hues, depending on the pigment added. T:75ES_1100, T:73ES:2Co_1100, T:75MS_1000, T:75MS_1100 and T:75IG_1000 pigments can effectively and safely be used to colour ceramic paste replacing partially or totally the commercial pigments.