Recovery Of Valuable Metals And High Value Utilization Of Iron From Acid Dissolved Slag Of NdFeB Waste

With the rapid development of NdFeB permanent magnetic materials,the output of NdFeB waste has increased,generating around 83000 tons of waste in 2021.The content of rare earth in NdFeB waste materials reaches over 20%.In industry,hydrochloric acid preferential dissolution method is often used to extract and recover rare earth,and a large amount of iron contained in the waste materials will enter the acid soluble slag.The main component in the acid soluble residue of NdFeB waste are Fe₂O₃and Fe（OH）₃,and there are about 0.5%residual rare earth elements and metallic elements such as cobalt.Currently,in industry,adding acid soluble slag to the iron and steel metallurgical process to recover iron from it has low economic benefits;At the same time,valuable metals such as rare earth and cobalt in the slag are not recovered,resulting in a waste of resources.In order to improve the comprehensive utilization value of acid soluble residue from NdFeB waste,this paper developed a process for comprehensively recovering valuable metals from the acid soluble residue of NdFeB waste.Firstly,valuable metals such as iron,cobalt,and rare earth in the acid soluble residue are leached by hydrochloric acid leaching,followed by the separation and enrichment of cobalt in the leaching solution by precipitation,and finally,iron and rare earth are separated by air oxidation precipitation,At the same time,high-purity iron oxide products with high economic value are obtained.The research content and results are as follows:1.The leaching kinetics of iron from acid leached residue by hydrochloric acid was studied.The results showed that the leaching activation energy of iron was 43.60 kJ/mol,and the leaching reaction was controlled by chemical reactions;Under the reaction conditions of hydrochloric acid concentration of 8 mol/L,reaction temperature of 70℃,liquid-solid ratio of 6:1,and reaction time of 4 hours,the leaching rate of Co was99.64%,and the leaching rates of Pr and Nd were 97.40%and 99.49%,respectively;At this time,the leaching rates of Fe and Ce are relatively low,respectively 92.22%and45.40%.XRD,SEM,EDS,and chemical analysis of the residue before and after leaching showed that most elements such as iron,rare earth,and cobalt were transferred into the leaching solution,while relatively insoluble substances such as SiO₂ andα-Al₂O₃remained in the residue;At the same time,there is a phenomenon of SiO₂wrapping Fe₂O₃ in the acid soluble slag to a certain extent,which affects the leaching of iron.2.The hydrochloric acid leaching solution was pretreated by neutralization and reduction to obtain a ferrous chloride solution,followed by a study on the precipitation and enrichment of valuable metals in the solution.The results show that using sodium sulfide alone for precipitation and enrichment is better.At reaction temperature of 50℃,sodium sulfide concentration of 1.2 mol/L,sodium sulfide volume 23 mL,and sodium sulfide flow rate of 0.5 mL/min,the precipitation rate of cobalt is 92.82%,rare earth precipitation rate is 8.52%,and iron precipitation rate is about 1.50%.The phase of the precipitate areγ-FeOOH and S,the content of Fe in the precipitate is 22.99%,the content of aluminum is 6.08%,and the content of cobalt is 7.06%;Compared to raw materials,the content of cobalt has increased by ten times,and the cobalt containing precipitate can be directly sent as cobalt concentrate to a cobalt smelter for cobalt recovery and utilization.3.High purity iron oxide is prepared by air oxidation precipitation,while achieving the separation of rare earth and iron.In an acidic system,the oxidation precipitation reaction is a first order reaction with an apparent activation energy of Ea=29.67 kJ/mol.The reaction is controlled by a mixture of mass transfer diffusion and chemical reaction.In the single factor experiment,the reaction conditions have an impact on the purity and phase of iron oxide.The results show that the reaction pH has a significant impact on the purity and phase of the precipitate;In the pH experiment,when the pH of the system is 4.0 and 4.5,the product is pureγ-FeOOH,and the morphology of the product is lamellar aggregation particles.Under the conditions of reaction temperature of 70℃,stable system pH of 4.0,Fe²⁺concentration of 0.5 mol/L,and air flow rate of 1.6 L/min,well crystallizedγ-FeOOH can be obtained;High purity iron oxide products that meet the national standard（GB/T 24244-2009）are obtained by roasting at 700℃for 2 hours;At this time,the rare earth loss rate is only about 1%,and the vast majority of the rare earth remains in the filtrate.It can be returned to the NdFeB waste recovery process to achieve rare earth recovery.