Investigation of decomposition of dolomite and distribution of iodine migration during the calcination-digestion process of phosphate ore

Abstract

In this work, the thermal decomposition and digestion of dolomite during the calcination of medium and low grade phosphate ore were investigated. Moreover, the migration distribution of co-associated iodine in gas, liquid and solid phases was also examined. The effects of calcination temperature and time, digestion temperature and time, and liquid-solid ratio on the P2O5 grade, MgO removal rate and iodine migration distribution of phosphate ore were systematically studied. The results showed that the main phase Ca5(PO4)3F remained almost no change during calcination-digestion process of phosphate ore. The impurity dolomite was decomposed into magnesium and calcium oxides, and carbon dioxide was precipitated. With the thermal decomposition of dolomite, most of the accompanying iodine adsorbed in phosphate ore was oxidized to iodine and sublimated to the gas phase. A small amount of iodine continued to exist as an isomorph in the fluoroapatite lattice. Increasing the temperature and prolonging the time accelerated the decomposition rate of dolomite in phosphate ore. However, excessively, high calcination temperature and prolonged calcination time affected the decomposition reaction and led to the reduction of effective CaO and MgO content. This was not conducive to the removal of calcium and magnesium, or to the improvement of phosphate grade. Appropriate digestion temperature, time and water volume facilitated the digestion reaction and increased the MgO removal rate. During calcination and digestion progress, 80%~90% of the associated iodine migrated to the gas phase in the form of elemental I2. Under the optimized calcination conditions, the removal rate of MgO reached (85.0 ± 0.1)%, the grade of phosphate concentrate increased up to (32.5 ± 0.1)%, and the content of MgO was (1.02 ± 0.1)%.