INVESTIGATION OF SECONDARY SOURCES FOR EXTRACTION OF ALUMINUM VALUES

Abstract

India has abundant bauxite deposits for the extraction of aluminium via well-established Bayer process. However, the growing demand for aluminium metal and estimated available global bauxite reserves suggest that the primary ore deposits may last only for the next 50 years. The Bayer process is limited to processing high Al2O3/SiO2 (~7-10) feed due to the generation of red mud. The current thesis investigates the silicate rocks as a potential dual source for extracting aluminium and potassium values. The secondary sources investigated in this work are mica, sericite, diaspore, feldspar, and fly ash with SiO2-to-Al2O3 ratio in the range of 0.77-3.77. The mica and sericite contain muscovite and illite phases; diaspore contains fine-grained muscovite in addition to diaspore; feldspar contains microcline and albite, and fly ash contains ‘cenospheric’ mullite phase. The Al2O3 content in the samples were 17-35%, except diaspore (50%) and K2O content in the range of 6.3-10% except fly ash. The optical microscopic study reveals the mica contains layered sheets of muscovite in the mica sample, and sericite contains fine-grained illite-muscovite identified by cross-hatched twinning. The microcline and albite in the feldspar are distinguished by cross-hatched and lamellar twinning, respectively. Diaspore contains muscovite and pyrophyllite with a lathe structure. Direct acid leaching and calcination – leaching processes commonly investigated in the literature were found futile. The mechanical activation of the samples resulted in phase amorphization, particle size reduction, enhanced specific surface area, and increased reactivity. The thermal activation using sodium hydroxide in conventional and microwave furnaces dissociate the aluminosilicate phases and form unstable acid-soluble phases such as nepheline, kalsinite, sodalite and sodium silicate. The dissolved Si values in the solution can be separated by gelation at pH ~3, and Al ions in the solution can be precipitated as bayerite (Al(OH)3) in the pH range of 6-7. The γ-alumina concentrate is obtained as product meanwhile; the KCl product is obtained by evaporating solution post alumina precipitation. The muscovite and illite bearing silicate rocks (mica and sericite) are found amenable to the mechanical activation. The statistical process optimization suggested that the milling time and acid concentration strongly influences the Al-K dissolution rate. Refractory or stable minerals (microcline, mullite, diaspore) are found unreactive to milling due to high hardness, which leads to poor Al and K dissolution from diaspore (45%), feldspar (41%), and fly ash (20%). The consolidated parameters for the milling route for maximum aluminium dissolution were 6 h milling at 300 RPM followed by 1.5-2 M HCl leaching at 50-75℃ for 1 h with 4% pulp density. The obtained γ-alumina precipitate had Al2O3 purity of 90-95% with a product yield of 20-25% and a surface area of 90-105 m2/g. The aluminosilicate minerals such as muscovite, illite, microcline, albite, and diaspore were amenable to thermal activation and led to high Al dissolution during HCl leaching in 1 h. The alkali-treated product contains Na2SiO3, NaAlSiO4, and NaAlO2 phases, majorly with sodalite and kalsilite. Microwave-assisted heating resulted in a rapid heating rate (90-120 C/min) in the mixture and also reduced processing duration as well as energy consumption. The optimal processing parameters were ~900 W for 10 min with NaOH: feed ratio of 1:1 followed by water and HCl leaching (0.5 M, 1 h, 50 °C), resulting in more than 80% Al dissolution. The removal of dissolved Si values from the solution improved the alumina purity of the product as direct precipitation yielded alumina purity of 35-68%, whereas silica aged product yielded alumina purity of 89-98% with ~15-30% yield of the alumina precipitate. The techno-economic calculations revealed that milling treatment (29-38 kWh/kg) is more energy-intensive than alkali thermal treatment (8.1-12.6 kWh/kg). Specific HCl consumption is minimum in mica (1.85 kg/kg) followed by diaspore (2.51 kg/kg), sericite (5.42 kg/kg), and feldspar (6.39 kg/kg) for Al dissolution of 80-98%. Finally, it can be concluded that mica is an ideal silicate source, and microwave-assisted alkali thermal treatment was found to be the optimal route.