Production of clays for white porcelains from porcelain stone using eco-friendly iron impurity leaching with citric acid

Porcelain stone, composed of quartz, feldspar, kaolin, and mica, is used to produce white porcelain clay. Therefore, many countries, such as Arita, Japan, renowned for porcelain stone, are making significant efforts to reduce iron impurities like α-FeOOH and FeS₂ in order to increase whiteness. In this study, the removal of iron impurities from Arita porcelain stone was investigated using citric acid (C₆H₈O₇) as an eco-friendly iron-leaching agent. Stone samples were treated with 0.05–1 M citric acid at 50 and 100 °C to analyze the iron leaching rate and whiteness of fired stones by measuring the residual Fe₂O₃ content. The effectiveness of iron leaching with citric acid was evaluated and compared to conventional 10 % HCl treatment. Specifically, α-FeOOH and FeS₂ iron residues were investigated; α-FeOOH was found to dissolve in acid, while FeS₂ remained insoluble. To modulate the temperature conditions, treatment time and citric acid concentration were effective in enhancing leaching and whiteness. In the results, the leaching rate of citric acid increased to 67.2 % at 50 °C over 54 h. It is novel that citric acid presents a more sustainable and economically viable option for iron impurity removal in porcelain stone processing compared to HCl and other leaching agents.