Study On Process Optimization And Mechanism Of Phosphate Ore Smelting Reduction

Phosphorus plays an important role in the human life, and it is an indispensablefactor in the origin of life. In nature, it is a kind of non-renewable resources and it existsmainly in the form of phosphate rock in the mineral. Phosphate ore reserves in Chinashows the overall features of “high quantity but low grade”. With the continuousdevelopment and utilization of phosphate rock resources in China, more and moreproblems emerge, such as high grade ore become less and low grade ore is difficult toutilize. How to utilize middle and low grade phosphate rock efficiently becomes one ofthe hotspots of the phosphorous chemical industry in China.In this paper, thermodynamic calculation of Gibbs free energy of the reactions thatmay exist in the phosphate ore reduction were completed by the software HSC Chemistry5.11. The results indicated that fluorapatite and silicon dioxide cannot react with eachother directly under the atmospheric pressure within the temperature range of0-1900℃,which means, the defluorization reaction cannot occur. It is speculated that fluorapatiteundergo carbothermic reduction reaction first to produce calcium oxide, and silicondioxide combined with calcium oxide to create calcium silicate, which contribute to thereduction reaction. The higher temperature facilitates the reduction of fluorapatite. Anexcessive amount of SiO2can reduce the reaction temperature of phosphate ore reduction,and different amount of SiO2leads to different forms of calcium silicate. Throughthermodynamic analysis, the priority of various generated calcium silicates wasdetermined as CaSiO3, Ca3Si2O7, Ca2SiO4and Ca3SiO5.The factors influencing reduction ratio of phosphate ore were investigated in anelectric furnace, such as reaction temperature, reaction time, the mole ratio of silica tolime, carbon type and its content, reactant particle size and inorganic additives. Theexperimental results indicated that the higher the reaction temperature, the higherreduction ratio of phosphate ore. Prolonging the reaction time enables to increasereduction ratio of phosphate ore to some extent. The reductive role of different reductantsis graphite powder, coke and activated carbon. When the phosphate ore particle size is over150meshes, the reduction ratio of phosphate ore reached99%. Therefore, theoptimal process conditions for smelting reduction of phosphate ore are the following:reduction temperature1450℃, reaction time60min, the mole ratio of silica to lime2.4,excess coefficient of graphite powder1.5, and the particle size150meshes plus. Basedon the optimal process conditions, adding suitable amount of inorganic additives, such asKCl, Na2CO3and Li2CO3, can strengthen the smelting reduction process. Withininorganic additives, the reduction ratio of phosphate ore can reach96%when thetemperature is1350℃.The mechanism of smelting reduction and the reinforcement of additives forphosphate ore smelting reduction process were studied. It is described as follows: firstly,fluorapatite spread to the surface of carbon, the reduction and defluorization reactedslowly, then CaO, CO, CaF2and P2generated. Secondly, silicon dioxide combined withcalcium oxide to produce calcium silicate, which contributes to the reduction. Theadditive will form molten liquid at first, which will greatly promote the mass transferbetween the reactants. Caculated by the software HSC Chemistry5.11, the reactionequilibrium constant increased from1.02to1.06. Through the smelting reductionkinetics analysis, the order of reaction is between first-order and second-order at thetemperature range of1300℃-1450℃. With the temperature increases, the rate constantincreases and the reaction order gradually tends to be the first-order. The apparentactivation energy was derived as184.644kJ/mol.