Extraction of sulphate (SO4-2) from Balikesir Sindirgi alunitic kaolins using NaOH

The aim of this study is, investigating the removal of the sulphate content via alkaline leaching from alunitic kaolins which are not suitable to use in ceramic industry. Characterization of the studied sample showed that it is generally ore contains abundant quartz and opal, apart from kaolinite, and alunite, which are exists in opal inclusions, as a SO4-2 source. The mineral liberation changings indicate that liberation of minerals increased with the size reduction and while kaolinite and alunite liberations are similar their liberation ratio is higher than the others. The experiments continued with alkaline leaching studies due to the size distribution studies showed no effect on separation of the alunite, kaolinite, quartz, and opal minerals from each other. -20 µm sized samples used on leaching experiments, according to the characterization and size distribution studies. The effect of NaOH concentration, temperature, time, and solid content was investigated during the leaching experiments. Direct leaching studies showed high (100%) alunite removal under 1.5-2M NaOH concentration, 95°C leaching temperature and 1-4 hours leaching times conditions. However, dissolution of kaolinite beside alunite caused high mass losses (17-45%). To prevent mass losses caused from kaolinite dissolvation during direct leaching, calcined kaolin samples leached in low concentration NaOH solutions. Results showed that highest SO4-2 removal can be obtained with calcined samples for 3 hours at 525°C following NaOH leaching, designed with central composite design, under ~0,2M NaOH concentration, 80°C leaching temperature, 2 hours leaching time conditions. The sulphate content was removed from the sample with only 13% mass loss under these conditions.