Liquid Precipitation - Thermal Decomposition Reduction Method To Prepare Ultrafine Iron

For its excellent physical and chemical performances, ultra-fine iron powder has found extensive applications in powder metallurgy and electromagnetic shielding materials. This thesis carries on the study of preparing claviform ultra-fine iron powder by liquid precipitation-thermal reduction.Firstly, the process of liquid precipitation-thermal reduction is researched in theory. There are the results as follows: the math model of Fe(Ⅱ)-NH₃-SO₄^2--CO₃^2--H₂O system and Fe(Ⅱ)-NH₃-SO₄^2--C₂O₄^2--H₂O system thermodynamic equilibrium system was built, and the corresponding logarithms metallic ions concentration-pH diagrams of the systems were drawn based on the calculation results. The effects of total concentration of ammonia and carbonate and oxalate as well as pH value to iron ion concentration were investigated. The research results based on the thermodynamics and kinetics analysis in thermal reduction showed that it was feasible that reducing FeO by H₂ under 300K-1000K in thermodynamics.The ferrous carbonate and ferrous oxalate were prepared by liquid precipitation with NH₄HCO₃ and H₂C₂O₄·2H₂O as precipitator, FeSO₄·7H₂O as raw material, ammonia as coordinator and pH value adjusted agent. The influences of technological conditions on the morphology, grain degree and dispersion were systematically studied by means of SEM. Through systematic study of the optimum conditions to prepare the precursors were determined as follws: 1.0mol/L for Fe concentration, n(NH₄⁺):n(Fe²⁺)=4.0, 8.8 for pH value, 40℃for reaction temperature, 25 min for feeding time, 20 min for ageing time, 6h for drying time, 45℃for drying temperature, reverse feeding. The precursors prepared under the optimum conditions in Fe(Ⅱ)-NH₃-SO₄^2--CO₃^2--H₂O system were claviform and well-dispersed.In Fe(Ⅱ)-NH₃-SO₄^2--C₂O₄^2--H₂O system ,0.6mol/L for iron concentration, 2-3 for pH value, 40℃for reaction temperature, 20min for feeding time, reverse feeding. The precursors prepared under the optimum conditions were short claviform.The effects of operation conditions during thermal reduction process to morphology and grain degree were investigated. Through systematic study, it was found that well-disperse, pure claviform metallic iron powder would be obtained with well-dispersed claviform FeCO₃ precursors as raw material, 350-400℃for reaction temperature, H₂:N₂=3:5-4:5 mixture for atmosphere and 50-60min for duration and that well-disperse, pure short-claviform metallic iron powder would be obtained with well-dispersed claviform FeC₂O₄ precursors as raw material, 420℃for reaction temperature, H₂:N₂=3:5 mixture for atmosphere and 60min for reaction.