Investigation of activation conditions in alumina production from pyrophyllite ore by acid leaching method

In this Ph.D. thesis, conditions for the activation method applied as a pre-treatment method to facilitate higher aluminum recoveries in the hydrochloric acid (HCl) leaching of pyrophyllite (Al2Si4O10(OH)2) ore to be utilized for alumina (Al2O3) production was investigated. Possible usage of the produced alumina in the manufacture of high-tech ceramics was also examined. An ore supplied from the Pütürge (Malatya) pyrophyllite deposits, which has average 23.60% Al2O3 grade was used. Experimental studies were designed to dissolve aluminum by HCl leaching from this ore which contains mainly alumina silicate clay minerals such as pyrophyllite, kaolinite and muscovite, as well as quartz. The ore, products and residues were chemically characterized by XRF and ICP, and physically characterized by XRD, TG-DSC, SEM methods together with particle size and specific surface area measurements. Effectiveness of both of calcination for thermal activation and intensive milling for mechanical activation was examined for the raw ore where the aluminum recovery in the solution with HCl leaching was low. According to the results of the experimental studies, it was found that the aluminum recovery in the solution was 10.57%, 33.63% and 86.53%, for raw ore, thermally activated ore by calcination and mechanically activated ore by intensive milling. It was found that the aluminum recovery in the solution which starting to increase with increase in the calcination temperature from room temperature to about 600 °C dramatically decreases when the calcination is performed at temperatures higher than almost 900 °C due to the formation of new mineralogic structures resistant to dissolution of minerals. Besides, it was found that by means of a short time intensive milling as much as 20 min, mechanical activation is provided as a result of amorphization of pyrophyllite, kaolinite and muscovite almost completely and of quartz slightly. An alumina powder having Al2O3 purity of 97.40% was produced by roasting the aluminum chloride salt at 1150 °C, which was precipitated by evaporation from the pregnant solution obtained by HCl leaching of intensively milled pyrophyllite ore. Aluminum titanate (Al2TiO5) was manufactured by heat treatment of a mixture of Al2O3 produced in the experimental studies in this thesis and commercially available TiO2 at 1370 °C to demonstrate the usefulness of the alumina produced from pyrophyllite ore in the manufacture of high-tech ceramic materials with high added value. Consequently, it was concluded that clay minerals such as pyrophyllite, kaolinite and muscovite in the ore are activated more effectively by intensive milling than calcination, aluminum can be solved from mechanically activated ore at higher recovery values, and it was shown that alumina recovered with hydrometallurgical processes is able to be used in the manufacture of the aluminum titanate.