The use of ceramic sludge and recycled glass to obtain engobes for manufacturing ceramic tiles

Abstract

The aim of this work was the valorization of two wastes that are classified as Class I according to the Brazilian Standard ABNT NBR 10004:2004 in order to obtain a ceramic engobe for the production of single-fire ceramic tiles. Ceramic sludge from a wastewater treatment plant of a ceramic tile manufacturer and recycled glass were used to obtain a frit in order to make the sludge and glass inert in a vitreous matrix. Thus, a representative amount of the ceramic sludge was collected, dried, and characterized by chemical analysis, X-ray diffractometry, and thermal analyses (differential thermal analysis, thermogravimetric analysis, and dilatometric analysis). Furthermore, the ceramic sludge was assessed according to the ABNT NBR 10004:2004. Seven formulations containing ceramic sludge contents varying from 20 to 80 wt% and recycled glass contents varying from 12 to 48 wt% and using calcite, dolomite, ulexite, and saltpeter as raw materials were prepared. Each formulation was melted in a laboratory kiln using alumina crucibles at 1420 °C for 1 h, and the melts were poured into water at room temperature to produce frits. Each frit was milled, and the coefficient of thermal expansion was determined by dilatometry. The results show that the studied ceramic sludge presented high contents of barium and fluoride. The main identified crystalline phases were α-quartz, zircon, illite, and barium silicate. The coefficient of thermal expansion after heat treatment was 5.6 × 10−6 °C−1. Once characterized, the feasibility of applying these wastes as raw materials for the production of ceramic frits was evaluated. The results of the dilatometric analysis, glaze fluidity flow test, and visual analysis showed that the incorporation of these wastes would allow for the obtainment of ceramic frits for producing ceramic engobes. Thus, three formulations of engobe were prepared and heat-treated at 1150, 1165, and 1175 °C. The engobes were characterized (XRD, determination of brightness, colorimetry, and microstructural analysis by optical microscopy) and showed suitable properties for use in the production of ceramic tiles. Engobe E3, which presented the highest ceramic sludge content, was selected for an industrial test. The results of a visual analysis and water spot test showed that the investigated engobe, compared with a commercial engobe (engobe reference), could be used in the production of glazed ceramic tiles.