Adsorption,Migration And Leaching Mechanism Of Rare Earth Ions On The Surface Of Clay Minerals

Ion adsorption rare earth?IARE? resources which are rich in heavy rare earth elements play a very important role in the development of high-tech. But there still exist many problems in the mining process, such as the extraction efficiency is not high and the environmental impact has still not been completely eliminated. The solution of these problems depend on our understanding the leaching mechanism and the behavior of leaching agents. Therefore, this study aimed at the adsorption, migration and leaching characteristics of IARE on the surface of clay minerals for seeking new ideas and methods to solve the above problems. The concrete content includes:The adsorption isotherms of rare earth ions on clay minerals with grain sizes ranging from-100 to +200 mesh, and under-800 mesh sieving were comparatively investigated with respecting to their mineral composition and particle size. The results indicate that the adsorption isotherm of rare earth ions on the surface of clay mineral are all of optimal, however, the adsorption capacity and ability increase with the decrease of the particle size. Therefore, the migration of rare earth ions among the clay minerals with different particle sizes has been demonstrated, and the preferring migration direction is from minerals with large particle to that with small particle.The leaching percentage of IARE by solutions of eight salts( including cation NH₄⁺, Na⁺, K`+ and anions Cl^-, NO₃^-, SO₄^2-) with different concentrations were determined, and related to the Zeta Potential??? values during leaching and soaking stages. Based on the electrical double layer model of interface between clay mineral and water, the cation and anion distribution in electric double layer and their contribution to the rare earth leaching are discussed in detail. It was found that rare earth leaching rate shows a positive correlation with the increase of absolute ? value, and the leaching efficiency increases with the decreasing of hydrated radius of the cation and anion. These results prove that both cation and anion are all attributed to the leaching of rare earth ions, however, the cation is the dominating factor which influences on the rare earth leaching rate. More specifically, based on the electric double layer theory, the cations and anions are all in hydrated form during the leaching process and can be adsorbed in the stationary layer?the stern layer? in Gouy-Chapman model; The promotion of anions for leaching rare earth ions is arising from their penetrating stationary layer which can attract more cations entering stationary layer to exchange rare earth ions. At the same time, the evacuation ability of anion and cation in the stern-layer reflected by the falling speed of absolute ? value in the water leaching process is proportional to their hydrated ionic radius. The anions show faster evacuation speed, only need one or two times soaking, and is also associated with the withdrawal speed of cation. Among the 8 kinds of inorganic salts tested in this study, ammonium sulfate is a leaching reagent with highest efficient for leaching rare earth and with slowest evacuating speed in water soaking process.The leaching Kinetics of IARE by low concentration of ammonium sulfate solution was investigated using column elution manner within a longer period. It was found that the leaching Kinetics curves plotting 1-2/3?-?1-??^2/3 via time show a two stages linear relationship when leaching with low concentration of ammonium sulphate solution, indicating that the leaching process of IARE can be described by a successively two stage of inner diffusion controlled kinetics models. As the concentration of ammonium sulfate solution increased, the difference of the exchange rate constant between the two stages increases. The leaching rate for the first stage increases sharply, and that for the second stage increases first and then decreases. Combined with the leaching kinetics study results of samples with different particle size and re-adsorption rare earth, it is believed that the two stages of leaching processes are respectively corresponded to rare earth ions adsorbed on the clay surface and entered into the inner layer of clay mineral particles, their proportion changes with the sample size, leaching agent concentration and the formation history of IARE. Increasing the leaching agent concentration, the share of rare earth ions on the surface which belongs to the first stage of leaching process increases.