Effect of thermal pretreatment on the two-stage extraction of aluminum, sulfur and potassium from a fine-quartz bearing alunite

Abstract

The extraction of aluminum, sulfur, and potassium from an alunite ore (Ghazvin, Iran) was studied by roasting followed by two-stage alkaline leaching. In the roasting stage, the effects of several key parameters including temperature (520 to 620 °C), reaction time (30–150 min), particle size (0.075 to 10 mm), and furnace medium (with and without diesel fuel and with charcoal) were investigated on the leachability of sulfur and potassium by sodium carbonate and of aluminum by sodium hydroxide. The first leaching stage was carried out with sodium carbonate at a pulp density of 20% (w/w), a reaction time of 4 h, a particle size of less than 75 µm, a temperature of 95 °C and a ratio of sodium carbonate to alunite of 1.5:1. The highest extractions of sulfur and potassium (99.9 % for both elements) were obtained with the ore roasted at 620 °C, a particle size of 3 mm, and a roasting time of 90 min. The extractions were significantly increased by increasing the temperature and roasting time. The second stage of leaching was conducted at the temperature of 95 °C, leaching time of 6 h and the sodium hydroxide concentration of 5 M at 15 % pulp density. The highest aluminum extraction (95.3 %) was obtained by reductive roasting with charcoal at a temperature of 570 °C, a roasting time of 30 min, and a particle size of less than 3 mm. For potassium and sulfur extraction, increasing the roasting temperature, reaction time and diesel fuel addition (above 3%) resulted in a decrease in aluminum extraction, which was attributed to the conversion of the alumina phase to a stable phase. The results of this study show that alunite ore could be an interesting alternative for alumina production.