Fundamental Research On Comprehensive Utilization Of Boron-bearing Iron Concentrate By Coal-based Reduction And Melting Separation

In order to make full use of low grade ludwigite resource in China and guarantee the strategy supply of boron products, the rational use of boron-bearing iron concentrate was conducted in the present work. Based on the principle of selective reduction-melting separation-boron extraction of boron rich slag, the new process for the comprehensive utilization of boron-bearing iron concentrate based on rotary hearth furnace coal-based direct reduction technology was proposed. Related basic research has been carried out, which could provide guidance for the comprehensive utilization of ludwigite as well as other Mnds of complex iron ore.Firstly, the carbothermal reduction property and behaviors of the boron-bearing iron concentrate were examined. The carbothermal reduction experiments of 2 kinds of boron-bearing iron concentrates and 4 kinds of ordinary magnetite iron concentrates were performed using TG under the same experimental conditions. It was found that the boron-bearing iron concentrate had the lowest starting reduction temperature and highest final reduction degree compared with the other magnetites. The effects of temperature, particle size of reducing agent, carbon content, species of reducing agent, pelletizing pressure and size of pellet on the reduction of boron-bearing iron concentrate/coal composite pellet were investigated. The formation process of off-gas, evolution of microslmcture of reduced pellet (such as metallic iron, gangue and pulverized coal) and volume shrinkage of reduced pellet during reduction were revealed at the same time. The isothermal reduction kinetics analysis of the boron-bearing iron concentrate was performed by using high purity graphite as reducing agent and the integral method. The results showed that the reduction rate was determined both by temperature and reduction degree. The effects of strengthening measures, such as mechanical milling, Na2CO3 and high reactive reducing agent, on the reduction behaviors were also investigated and the mechanical milling of ore/coal mixture was found to be the most effective method.Secondly, the experiments of reduction-melting separation of boion-bearing iron concentrate/coal composite pellet and melting separation of reduced pellet were performed. The composite pellet could realize fast reduction and melting separation at 1400℃. The melting phenomenon began to appear at 8min and realized clean separation at 11min. The molten slag played a key role during the contacting carburization between the reduced metallic iron and coal particle. The experiment result showed that the melting separation of prereduced pellet is almost the same as the reduction-melting separation of composite in nature. The viscocity of the molten boron-rich slag increased obviously at the temperature of 1475℃, which is bad for the separation of pig iron and molten slag. The species of crucible heavily affected the composition and properties of separaed slag and iron. The chemical composition of boron-bearing iron concentrate is the most important factor determining the melting separation result.Thirdly, the microstructure, crystallization and extraction efficiency of boron (EEB) were investigated based on above experiment. The crystalline phases of the slow cooled boron-rich slag are suanite, kotoite and olivine, and the disseminated relation between minerals is simple. The mineral phase composition and the property of the mineral changed during the cooling process, which would influence the EEB of the boron-rich slag. At the temperature of 1100℃, the suanite and kotoite occurred in large amount and the water quenched slag had the highest EEB in the present study. The smelting temperature, chemical composition and cooling rate would affect the microstructure and EEB of the boron-rich slag obviously.Finally, the effects of leaching temperature, the amount of acid and liquid to solid ratio on the B extraction of boron-rich slag in the sulfuric acid leaching process were studied. The results showed that the raising temperature, increasing acid amount and decreasing liquid to solid ratio would improve the extraction of B. The elements Mg, Si, Fe and U would be leached out during the leaching. The leaching of Si would affect the further utilization of lixivium. The comparison leaching experiment between boron-rich slag and natural szaibelyite ore was performed, which indicated that the extraction properties of B, Mg and Si elements of the boron-rich slag were much better than the szaibelyite ore and the appropriate leaching temperature decreased obviously.