A novel method for processing of Bangladeshi zircon: Part I: Baking, and fusion with NaOH

Abstract Dry ground Bangladeshi zircon (44–53 µm) is found completely inert in 6 M HCl, 6 M HNO 3 , 6 M HClO 4 , 3 M H 2 SO 4 and 6 M HCl + 2 M HNO 3 during leaching at their boiling point temperatures under reflux for 4 h. In the 6 M HF and 6 M NaOH solution, only ∼ 5% zircon is dissolved under similar conditions. The mechanical activation of zircon with NaOH at ambient temperature, by grinding in a ball mill for 300 h, is also ineffective with only 2% Zr(IV) dissolved. However, up to 86% zircon can be dissolved on leaching a sodium carbonate baked mass with 6 M HCl under reflux for 1 h. The optimum conditions of baking were found to be 1273 °K for 2.5 h with a Na 2 CO 3 :zircon wt. ratio of 0.6. Higher Na 2 CO 3 :zircon wt. ratios and longer baking times decreased the recovery percentage. The investigation has found for the first time that zircon is completely and rapidly decomposed by fusion with 8 mol NaOH to give a semi-crystalline fused mass. The rate of fusion was monitored by leaching Zr(IV) from the fused mass with 5 M HCl under reflux for 30 min. The optimized fusion conditions were 973 °K for 15 min with a NaOH:zircon wt. ratio of 1.8, or mole ratio of 8:1, which gives the following reaction for fusion: ZrSiO 4  + 8NaOH → Na 4 ZrO 4  + Na 4 SiO 4  + 4H 2 O. The rate of dissolution of zircon in molten NaOH was found to follow the law: 1 − (1 −  R ) 1/3  = ( k  /  ρr o ) [NaOH/zircon] a t . The calculated value of ‘ k ’ was 10 − 4.00 ± 0.04  g min − 1  cm − 2 (for ρ  = 4.7 g cm − 3 and r o  = 26.5 µm) at 973 °K and a  = 1.0. The calculated activation energy for fusion is 125 kJ mol − 1 which is indicative of a chemically controlled reaction with the slow step being the first reaction of NaOH with zircon.