Sulfuric Acid Dealumination of Metakaolin as a Scalable Precursor Route for the Synthesis of High-Surface-Area γ-Alumina

Abstract

The growing demand for high-purity γ-alumina and the declining availability of high-grade bauxite have stimulated interest in alternative alumina feedstocks derived from abundant clay minerals. This study investigates the suitability of Ikere-Ekiti kaolin, Nigeria, as a low-cost precursor for the synthesis of high-surface-area γ-alumina through beneficiation, thermal activation, and sulfuric acid dealumination. Raw kaolin was subjected to mechanical scrubbing, particle size classification, and gravitational sedimentation to enhance alumina content and remove siliceous and metallic impurities. The beneficiated kaolin was thermally activated at temperatures between 550 and 850°C for 15 and 120 minutes to produce reactive metakaolin. Structural transformation was evaluated using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), while chemical evolution during beneficiation and dealumination was monitored using X-ray fluorescence (XRF). Complete dehydroxylation and amorphization of kaolinite were achieved at 550°C after 120 min, yielding highly reactive metakaolin suitable for acid extraction. Sulfuric acid leaching conducted at 3, 5, and 7 M H₂SO₄ resulted in dealumination efficiencies of 76.8%, 86.7%, and 89.6%, respectively. The progressive increase in residue Si/Al molar ratio from 1.0 to 17.0 confirmed selective aluminum dissolution and the formation of an aluminum-rich, silicon-depleted leachate favorable for γ-alumina precursor recovery. Beneficiation increased the Al₂O₃ content from 24.9 wt.% in the raw clay to 47.6 wt.% in the refined fraction, demonstrating the effectiveness of the upgrading strategy. Among the investigated conditions, 5 M H₂SO₄ provided the most favorable balance between dealumination efficiency, precursor purity, impurity control, and reagent economy. The results establish sulfuric acid dealumination of metakaolin as a technically viable and scalable route for producing high-purity γ-alumina from indigenous Nigerian kaolin resources, thereby promoting sustainable mineral beneficiation and value addition.