Research On Process Optimization And Mechanism Of Vacuum Carbothermic Reduction Of Low-grade Phosphate Ore

Phosphorus mineral resources in China have the characteristics of"less rich,more poor",and the distribution is extremely uneven.At present,the utilization of low-grade phosphate ore is mostly enriched by dressing process,which will reduce the availability of phosphorus and produce a large amount of tailings.It has been proved that the vacuum carbothermal reduction can effectively recover phosphorus in low-grade mines.Therefore,the process optimization and mechanism of vacuum carbothermic reduction of low-grade phosphate ore was studied.The main conclusions are as follows:The calculation results indicated that:the initial temperature of reactions occurred during the reduction of fluorapatite decreased significantly with the decrease of pressure.The optimum temperature of phosphate ore was 1250?.When Si O₂/Ca O molar ratio were 0.4 and 0.8,the mass of phosphorus in the gas phase increased compared with that without Si O₂ addition.Continued addition of Si O₂,will inhibit the volatilization of phosphorus.The reduction effect of three different carbon sources was:graphite>pulverized coal>coke,which was obtained by using 14%carbon dosage.The calculation results showed that B₂O₃ had little effect on the carbothermal reduction reaction of phosphate rock under vacuum.Experimental results showed that:the reduction ratio of phosphate rock and the volatilization ratio of phosphorus increased at 1100??1400?,and the reaction began to be carried out in large quantities at 1300?.In the experiment of carbon sources,the reduction effect of coke was the worst,and the reduction effects of graphite and pulverized coal were close.Si O₂ addition can significantly increase the reduction ratio and the volatilization ratio of P,when Si O₂/Ca O molar ratio was 0.8,the promotion effect on phosphorus reduction was the most obvious.When Si O₂ was excessive,it will indirectly reduce the grade of P₂O₅ in the ore,resulting in a decrease in the driving force of reduction,at the same time,the rising temperature of liquidus and the reduction in liquid phase quantity lead to the decrease of internal heat rate and mass transfer rate.Thus the volatilization ratio of P was reduced.The reduction of P can be promoted by 2%B₂O₃ at low temperature of 1250?,and the increase of temperature and B₂O₃ addition can reduce the volatilization ratio of P.By comparing thermodynamic calculation with experimental results,the optimum process parameters for the treatment of low-grade phosphate rock by vacuum carbothermic reduction are as follows:under the condition of 1 Pa?14%carbon dosage,the reaction initiation temperature of vacuum carbothermic reduction of low-grade phosphate ore was 1300?;Considering the cost control,the optimal carbon source was pulverized coal during the actual production process;The optimum Si O₂/Ca O molar ratio was 0.8.The reduction process and mechanism of phosphate ore during vacuum carbothermic reduction were obtained by non-isothermal kinetic analysis and the results of vacuum experiment.When the reaction conditions reached,Ca₅?PO₄?₃F first diffused to the surface of Si O₂ to undergo defluorination reaction to form Ca₃?PO₄?₂?when Si O₂ was less,the self-decomposition reaction of Ca₅?PO₄?₃F also occured?.Subsequently,the reduction began at the two-phase interface of Ca₃?PO₄?₂ and C.As the reaction proceeds,there was a shrinking core of unreactants inside the ore.The externally attached solid-state product layers reduces the contact area between Ca₃?PO₄?₂ and carbon,limiting the progress.Gaseous yellow phosphorus continuously diffused to the surface of the sample and escaped with CO.The apparent activation energy of the whole reduction process was 74.54±10.39 k J/mol.And the apparent activation energy of the volatilization process of yellow phosphorus was416.43±1.81 k J/mol.