Synthesis of ceramic pigments from industrial wastes: Red mud and electroplating sludge

The perception of industrial waste has changed, in the last decades, from an economic and environmental problem to a potential raw material. In this work, the synthesis of a stable pigment was prepared by combining two hazardous and unexplored wastes: red mud (RM, rich in Fe) and electroplating sludge (ES, rich in Ni/Cr). The wastes were mixed in different proportions and calcined at 1200 °C. Black and brown pigments were obtained from the mixture of RM/ES in 1:3 and 1:1 wt proportions, respectively. The color was given by the chrome-iron-nickel spinels, based on Ni^{2+ 3}T_1g(³F) transitions (≈13,000 cm^-1), Cr^{3+ 4}A_2g → ⁴T_1g(⁴F) transitions (≈24,400 cm^-1) and Fe³⁺ (≈15,000-≈22,500 cm^-1). The achieved coloring strength and thermal stability on various ceramic glazes render excellent prospects for the industrial application of such waste-based pigments.