Central composite design modelling and developing a method for removing sulphate from an alunitic kaolin ore

Abstract

The main mineral in kaolin ore, kaolinite, is well-known for its many uses in the paint, paper, and ceramics industries. The necessity of getting pure kaolinite is discussed in this work, with an emphasis on the extraction of impurities from kaolin. Specifically, alunitic kaolin, which is common in northwest Turkey, has sulfur concentrations higher than 3 % SO3, which means that it can't be used for many industrial purposes. Leaching is a popular method for removing sulfur, especially when done directly and through pre-treatment techniques like roasting. High-temperature roasting, however, presents difficulties since it causes phase transformation on kaolinite to metakaolin and mullite and on alunite to alum. The temperature of phase transformation is important on high temperature studies which affects negatively the thixotropic qualities that are essential for the production of ceramics. This study introduces a novel approach by optimizing roasting conditions and employing a central composite design methodology for leaching. The optimal parameters for the roasting studies were determined to be 525 °C for 3 h. For leaching, the ideal conditions were identified using the leaching recovery equation, which specifies 0.195 M NaOH, 80 °C, 127 min, and a 2.55 % solid content. These characteristics were confirmed by follow-up experiments, which showed a change in SO42− concentration from 3.03 % to 2.97 %, along with some structural changes in the roasted sample. The subsequent leaching process resulted in a significant drop in SO42− concentration to 0.46 %. By highlighting the effectiveness of lower-temperature roasting and optimizing experimental methodology, this research presents a novel thermochemical method for sulfur removal in alunitic kaolin. © 2024 Elsevier Ltd