Wet Grinding of Calcified Slag to Improve Alumina Extraction from Red Mud by the Calcification–Carbonization Method

Wet grinding of calcified slag was conducted to improve alumina extraction from red mud by the calcification–carbonization method. The grinding operation conditions and mechanism were investigated. The results showed that wet grinding destroyed the compact structure of spherical hydrogarnet and broke down agglomerates. The rough surface of ground hydrogarnet promoted the formation of aragonite-type calcium carbonate, and then inhibited the formation of a dense production layer on the unreacted particles, which consequently improved carbonization efficiency and alumina extraction. Alumina extraction increased with increased rotation speed within the low speed range, and decreased dramatically over a critical speed. Prolonged grinding time increased alumina extraction until large particles comminuted. Both lower and higher liquid-to-solid ratios decreased alumina extraction due to the formation of adhered layers and the decrease of ball-to-powder collision probability. A smaller ball-to-powder ratio reduced ball-to-powder collisions, whereas excess balls caused energy loss during ball-to-ball collisions, which decreased alumina extraction.