Pressure leaching of aluminum from kaolin by HCl: Experimental and DFT study

Abstract

Kaolin is an important industrial mineral which has many industrial applications. It is also considered as an important source of aluminum; however, great technical challenges are required to be overcome to extract Al from kaolin. In this work, the extraction of aluminum from kaolin using HCl, as a leaching agent, was studied. Kaolin samples were collected from Wadi Al-Hafira, south of Jordan; these were crushed and ground to a particle size of 100% <75 μm. The ground samples were characterized in terms of chemical and mineral composition using X-ray fluorescence (XRF) and X-ray diffraction analysis (XRD) techniques. Kaolin was found to contain kaolinite, muscovite, and quartz as main mineral phases, in addition to anatase and hematite as trace mineral phases, while the chemical analysis revealed that kaolin contains about 36.1% Al2O3, 54% SiO2, 2.5% CaO, 1.96% Fe2O3 and 1.2% TiO2 as major oxides. The minor oxides were K2O, Na2O, and SO3. A microwave digestion vessel was used to provide a suitable environment for high pressure and temperature leaching. The effect of several leaching parameters was studied in this work including, leaching time (30 to 120 min), leaching temperature (80 to 200 °C), and molarity of leaching reagent (1, 2, 3 and 4 M). Additionally, two sources of HCl were used, including analytical grade HCl and HCl collected from the potassium sulphate industry. The extraction of four main elements were monitored during the leaching process employing ICP-OES multi element analysis technique, namely, aluminum, iron, titanium and silicon. The findings of this work suggest that the recovery of these elements increases with the increase of temperature and leaching time. The recovery of silicon decreased with the increase of acid molarity. Complete extraction of Al from kaolinite was obtained at temperatures above 180 °C and leaching time of 120 min, while Al present in muscovite was not amenable to leaching. However, traces of H2SO4 and chloride salts in waste HCl, from potassium sulphate industry, provided complete destruction of muscovite as evident from the XRD analysis of leach residues. Lower energy requirements (computed by DFT simulation) for chlorination of Al sites in reference to Si sites explains a higher recovery percentage of the former.