Reduction Kinetics and Microscopic Properties Transformation of Boron-Bearing Iron Concentrate-Carbon-Mixed Pellets

Selective reduction followed by magnetic or melting separation is an innovative and effective method for the comprehensive utilization of boron-bearing iron concentrate. As the raw material of boron products, the reserve of szaibelyite ore is significantly depleted. Therefore, the comprehensive utilization of boron-bearing iron ore resources becomes more and more important. To study the reduction kinetics in depth, a boron-bearing iron concentrate containing 52.18% total iron and 5.50% B2O3 was reduced isothermally with coke as the reductant. The results showed that both the reduction degree and the maximum reduction rate increased with temperature and C/O molar ratio (fixed carbon in the coke to oxygen in iron oxides in the ore). 98.00% reduction degree and 0.00414 s(-1) maximum reduction rate were achieved at 1523K of reduction temperature and 1.4 of C/O molar ratio. The reduction kinetics can be best described by Avrami-Erofeev model, g(alpha) = [-ln(1-alpha)](3/2). The apparent activation energy and pre-exponential factor were 264.565 kJ center dot mol(-1) and 1.87 x 10(7) s(-1), respectively. The phase transformation observed by XRD agreed well with the results of reduction degree analysis. The micro-structure evolution behavior of the pellets was analyzed by SEM and EDS. As the reduction time increased from 0 to 600 s, most of the iron oxides were reduced to metallic iron, leaving other major elements in the slag phase. The slag phase mainly consisted of the solid solution of Mg2SiO4 and Mg3B2O6. With further increase to 1200 s, the metallic iron particles migrated and flocculated together forming bigger particles.