Research On The Preparation Process Of Nano-iron Oxide Yellow From NdFeB Secondary Waste

NdFeB magnetic material is a kind of permanent magnetic material with excellent performance.In the production process,a large amount of secondary waste with high iron content will be produced,which will not only cause waste of iron resources,but also cause harm to the environment if it is treated as solid waste.In this study,NdFeB secondary waste was used as raw material,and the leaching system formed by sulphuric acid plus additives was used to recover iron from the scrap by wet process,and the leachate formed was then reduced by iron powder and purified by crystallisation to produce high-purity Fe SO₄·7H₂O,and then nano-iron oxide yellow was prepared by air oxidation method.In the process,the influence of different operating conditions on product performance was studied,the reaction kinetics and thermodynamics in the process were revealed,the reaction system was optimized,and a reasonable preparation process was constructed.It provides a theoretical basis for the optimization of nano-iron oxide yellow process,and is also one of the effective ways for the high-value utilization of NdFeB secondary waste.The main research contents are as follows:（1）In the iron leaching experiments,the effects of parameters such as sulfuric acid concentration,auxiliary concentration,temperature,time,liquid-solid ratio（acid volume/waste mass,m L/g）and leaching times on the leaching rate was investigated by using single-factor experiment and response surface optimization experimental methods.Under the optimal conditions of sulfuric acid concentration of 11.0 mol/L,additive concentration of 3.5mol/L,temperature of 80℃,time of 120 min,liquid-solid ratio of 4:1 and leaching twice,the iron leaching rate reached 98.25%.The results of XRF and XRD analysis showed that a large amount of iron was leached.The kinetics of iron leaching process were discussed by condensation model,and the results showed that the iron leaching process was controlled by chemical reaction and diffusion process,with linear correlation R² was greater than 0.94,and the diffusion process was the mainstay.（2）In the reduction experiments,the effect of temperature,time and excess coefficient of reduced iron powder on the reduction rate of Fe³⁺were investigated by single-factor and orthogonal experimental methods.Under the optimal conditions of temperature 80°C,time120 min and excess coefficient of reduced iron powder of 1.8,the Fe³⁺reduction rate was98.37%.In the crystallization purification experiments of Fe SO₄·7H₂O,the effects of initial ferrous sulfate concentration,temperature,time and crystallization number on the crystallinization rate and purity of Fe SO₄·7H₂O were investigated by single-factor experimental method.Under the optimal conditions of initial ferrous sulfate concentration of150 g/L,temperature of 10°C,time of 60 min and tertiary crystal purification,the purity of the prepared product was 99.94%,which was better than the pure standard of GB/T664-2011chemical reagent grade.（3）In the preparation stage of iron yellow seed crystal,the effects of alkali ratio（molar ratio of 2NH₃·H₂O to Fe SO₄·7H₂O）,initial Fe²⁺concentration,temperature and air flow rate on production,productivity and color of crystal seed were investigated by using single-factor experimental method.The reaction conditions were optimized to give an yellow colloidal shape,the production was 8.25 g,the productivity was 1.47 g/h when the alkali ratio was 0.4,the initial Fe²⁺concentration was 38.00 g/100 m L,the temperature was 25°C and the air flow rate was 3.5 L/min.The physical phase of the crystalline seeds wasα-Fe OOH,the morphology was roughly rod-shaped,the particle size were fine and uniform distribution.（4）In secendary oxidation stage of iron oxide yellow,the effects of seed content,p H value,initial Fe²⁺concentration,temperature and air flow rate on production,productivity,color and iron content of iron oxide yellow were investigated by using single-factor experimental method.When the crystalline seed content was 45%,the p H value was 3.5～4,the temperature was 80°C,the initial Fe²⁺concentration was 34.00 g/100 m L and the air flow rate was 3.5 L/min,the product had a bright yellow colour,the production was 25.08 g,the productivity was 3.22 g/h and the iron content was 95.52%,which was better than the requirements of GB/T1863-2008 first grade product.Through XRD,SEM,particle size and TGA proved:the product phase composition wasα-Fe OOH,the particles have a needle-shaped structure with a length to diameter ratio of 8:1,the particle size was mainly concentrated in the range of 10～200 nm,the average particle size was 48 nm,The product removes one crystal water from 200°C to 300°C.The whole process has good economy through economic benefit accounting.The research on the preparation of nano iron oxide yellow from NdFeB secondary waste has achieved the recycling of solid waste secondary resources.The whole process conditions are mild,green and socially beneficial,which is important for industrial production.