Study On Leaching And Recovery Of Thorium From Rare Earth Waste Residue

As a typical radioactive element associated in rare earth ore, thorium makes it difficult to the treatment and disposal of rare earth slag. So in order to achieve the reduction and harmlessness of rare earth waste, we need to seek a processing method to enrich, separate, recover thorium from rare earth waste residue. This paper mainly studied the leaching and separation process conditions of thorium from rare earth waste residue, and specific activity level changes of the residue was investigated. Moreover, the synthetic temperature sensitive urea formaldehyde resin was used to remove thorium from the lixivium as an adsorption material, which realized the goal of avoiding secondary pollution. Meanwhile, the recovery rate of thorium and the regeneration performance of the resin was explored under the condition of extraction. Here are the conclusions:(1) Nitric acid system has a good leaching effect to rare earth waste residue. The maximum leaching rate of rare earth waste residue is 15.12%, which obtained at conditions of 4 mol/L HNO3,50g/L ionic strength,10:1mL/g liquid-solid ratio after leaching 5 hours at 80℃, and alpha decay counts of the lixivium is 69.742 (cpm). The radioactive level of rare earth waste residue was measured by HPGe gamma spectrometer. After leaching treatment, the specific activity of thorium series can be lowered from original 53.63Bq/g to 2.45Bq/g-5.83Bq/g, which are decreased by almost 90%. Obviously, radition hazards of the waste residue are greatly reduced.(2) Urea formaldehyde resin is synthetised by alkaline-acid two step method. The results show that the resin has great temperature sensitive characteristic, which can be reversibly dissolved and precipitated under the condition of heating and cooling. The molecular structure of temperature sensitive urea formaldehyde resin contains abundant of active groups that can be coordinated with Th4+ such as -OH,-NH2. By scanning electron microscope (SEM) observation, we found that the resin is presented as particle structure and uniformly dispersed with the particles size about 1.5～2.5μm. So the huge surface area of the resin is propitious to the adsorption.(3) Under the condition of constant temperature oscillation, the highest Th4+ uptake values are obtained in 6 hours at pH 4.5 and Th4+ adsorption capacity of temperature sensitive urea formaldehyde resin is up to 20.90mg/g. In addition, the kinetic adsorption of Th4+ on the resin can be fitted well by pseudo-second-order kinetic model. Besides, the adsorption isotherm can be described by Langmuir model. From thermodynamics analysis, the adsorption of Th4+ on the resin is an endothermic and spontaneous process. Under the condition of strong shear, temperature sensitive urea formaldehyde resin for the removal efficiency of Th4+ is better, and the actual adsorption capacity towards Th4+ is 34.62mg/g. The adsorption equilibrium of Th4+can be easily reached within almost 12 minutes.(4) Th4+ can be efficiently removed from the temperature sensitive urea formaldehyde resin by TBP-benzene extracted system. The extraction rate of Th4+ can reach 95.13% at 60℃, with the condition of 2mol/L TBP in organic phase,0.1mol/L NaNO3 (salting-out agent) in aqueous phase, and 2:1 O/A ratio. The distribution ratio of Th4+ in organic phase to aqueous phase is 9.77. The concentrated resin retains in aqueous phase is regenerated after cooled, and the removal rate of Th4+ can still be calculated as 90.12% after four times cyclic operation. The reverse extraction rate of Th4+ in organic phase is up to 83.76% by utilizing dilute nitric acid, and organic phase can be recycled.