Extraction of alumina from kaolin deposits in gem mines of Sri Lanka

Abstract

Aluminum oxide, commonly known as alumina, exhibits multiple polymorphic phases. Most of these phases possess high economic value owing to their versatile industrial applications. The γ phase of alumina finds applications in industries as a catalyst, catalyst support, adsorbent, abrasive, and structural composites. Bauxite, the principal ore for alumina, has become scarce due to extensive exploitation. Due to the limited availability of bauxite ores, there is growing interest in the production of alumina from non-bauxite minerals. Kaolin is a promising alternative for alumina production owing to its abundance, low cost, and high alumina content. Kaolinitic clay deposits, esteemed as valuable mineral resources in Sri Lanka, are widely distributed throughout the country. Some of these clay deposits contain up to 35% alumina by weight. Kaolin deposits in the Ratnapura District are often unearthed during gem mining operations. These excavated deposits are frequently disposed of openly on the ground without undergoing any value-addition process. This practice alters the soil condition in the vicinity, as the clay soil blocks the gravitational flow of rainwater. This environmental impact could be mitigated by adding commercial value to these kaolinitic clay deposits. The present study aims to demonstrate the feasibility of extracting alumina from kaolinitic clay found in gem mining sites. Kaolinitic clay samples were collected from a gem mining site in the Ratnapura District. Kaolin was first transformed into metakaolin by calcining at 600 ºC for 2 hours. Then alumina was extracted from the metakaolin via acid leaching using 6 M HCl as the leaching agent. The Aluminum ions were separated from the leaching solution as Al(OH)3 using 5M NaOH as the precipitating agent. The precipitated Al(OH)3 was transformed into alumina by calcining at 600 ºC. Kaolin, calcined kaolin, and extracted alumina samples were characterized using XRD and XRF analysis. XRD analysis confirmed that the collected clay samples were mineralogically kaolinite. XRF analysis revealed that the kaolin samples were primarily composed of 63.63% silica and 29.11% alumina by weight. Accordingly, the silica/alumina ratio in the sample was 2.18. The successful formation of amorphous metakaolin under the selected calcination conditions was confirmed through XRD analysis. The structure of kaolin shields the aluminum ions. The transformation of kaolin into the amorphous metakaolin phase is necessary for the effective leaching of aluminum ions at low temperatures and short retention times. The extracted alumina samples were found to contain 90.3% alumina and 6.64% silica by weight, as determined by XRF analysis. The silica/alumina ratio in the extracted alumina samples was 0.07. The observed reduction in the silica/alumina ratio signifies the efficient removal of silica during the extraction process. The crystalline phase of the extracted alumina was identified as the γ-phase through XRD analysis. Accordingly, γ-phase alumina with a purity exceeding 90% can be synthesized from the kaolinitic clay sourced from the gem mining site.