Preparation And Lithium Adsorption Performance Of Layered Manganese-based Ion Sieves With Low Dissolution

With the rapid development of the industry of new energy,there is an urgent demand for lithium resources in the market.The lithium extraction by the absorption method from aqueous lithium resources is one of important ways to get lithium resources.Manganese-based ion sieves have low cost of preparation,high efficiency of extraction,and high selectivity of lithium,which is one of lithium adsorbents with great prospects of industrial application.However,there are some problems in the application process of manganese-based ion sieves,such as poor performance of lithium-ion cyclic absorption/desorption and high manganese dissolution.The key to solve these problems is to clarify mechanisms of lithium desorption and manganese dissolution,then to guide the preparation of new manganese-based ion sieves with low dissolution.In this dissertation,the typical precursors of manganese-based ion sieves are taken as research objects.Mechanisms of lithium desorption and manganese dissolution are clarified.A new layered precursor of the manganese-based ion sieve,Li1.90Mn1.52O4,is orientedly prepared.The preparation mechanism of Li1.90Mn1.52O4 is clarified.The performance of lithium extraction is systematically investigated.Then a technical route of selectively adsorbing lithium by layered manganese-based ion sieve with high adsorption and ultra-low dissolution is formed.The main research contents and conclusions are as follows:(1)Mechanisms of lithium desorption and manganese dissolution of three typical precursors of manganese-based ion sieves are researched.Precisely controlled acid treatment experiments and instant acid treatment experiments are orientedly designed,and intermediate products of lithium desorption and manganese dissolution of LiMn2O4,Li1.33Mn1.67O4,and Lii.6Mn1.6O4 are firstly prepared.Characterization and DFT calculations are conducted on precursors and their intermediate products.It is found that both the lithium desorption mechanism and the manganese dissolution mechanism of LiMn2O4 are the same disproportionation reaction of a large number of trivalent manganese ions.The mechanism of lithium desorption of Li1.33Mn1.67O4 and Li1.6Mn1.6O4 is the ion exchange reaction between lithium ions and hydrogen ions.The lithium ions at 8 a sites are preferentially desorbed,and the migration path of the desorbed lithium ions in the crystal tends to the nearby lithium holes of 8a sites.The mechanism of manganese dissolution of Li1.33Mn1.67O4 and Li1.6Mn1.6O4 is the disproportionation reaction of a trace amount of trivalent manganese ions in the bulk phase of precursors.(2)The high temperature hydrothermal preparation process and characterization of a new layered precursor of the manganese-based ion sieve are researched.The layered precursor of the manganese-based ion sieve is firstly prepared by high temperature hydrothermal preparation process.The optimized preparation conditions(the mole ratio of lithium to manganese in the reaction system:1.2,the concentration of LiOH:1.2 mol/L,the hydrothermal temperature:280?,the hydrothermal time:24 h,the roast temperature:350?,the roast time:24 h)are obtained;Characterization such as XRD with Rietveld method and MAS-NMR are performed on the precursor prepared under the optimized preparation conditions.It is found that the structural formula of the precursor is Li1.90Mn1.52O4,which is of monoclinic structure,of layered morphology,and of C2/m space group.The layered structure is formed alternately by lithium-manganese layer and lithium layer.The valence of manganese is tetravalent.The capacity of lithium adsorption is 60-69 mg/g in the adsorption process with 0.00%of the manganese dissolution ratio.The manganese dissolution ratio is 1.00%-1.24%in the desorption process.(3)The preparation mechanism of Li1.90Mn1.52O4 is researched.Characterization such as XRD with Rietveld method and in-situ FT-IR are conducted on the precursor and its intermediate products of preparation reactions.Combined with DFT calculations,the preparation mechanism of Li1.90Mn1.52O4 is clarified.In the hydrothermal reaction stage,?-MnO2 or ?-MnO2 dissolves into[MnO6]groups in the hydrothermal environment.[MnO6]groups self-assembly form metastable and thin-layer ?-MnO2.?-MnO2 keeps its structure stable under the influence of enough lithium ions in the hydrothermal environment.Then layered hydrothermal products are formed under the influence of hydroxide ions at once with lithium ions embeded between layers.In the roast reaction stage,a small amount of lithium ions of layered hydrothermal products migrate from the lithium layer to the lithium-manganese layer.The basic composition groups remain unchanged.And the crystallinity increases slightly.(4)The performance of lithium extraction of the layered manganese-based ion sieve is researched.Experiments of adsorption kinetics,adsorption isotherm,comparative adsorption of anion system,selective adsorption of cation,and cyclic adsorption/desorption in real solutions are carried on systematically.As a result,the adsorption kinetics model is the pseudo-second order model.The adsorption isotherm model is the Langmuir model.The standard enthalpy and entropy of adsorption process are 11.942 kJ/mol and 78.332 J/(mol·K),respectively.The standard Gibbs free energy is negative at 20-50?,and decreases with the increase of the reaction temperature.The order of adsorption selectivity of cation is Li+>>Na+>K+>Ca2+>Mg2+.The reason of this order is that cations in the adsorption process are synergetically controlled by the process of dehydration outside the crystal and the process of migration inside the crystal.Adopting desorption conditions with additives,the capacity of lithium adsorption of the layered manganese-based ion sieve is kept at 29-34 mg/g in LiOH simulated solutions and at 17-19 mg/g in the industrial wastewater containing lithium stably with 0.00%of manganese dissolution ratio of both systems in cyclic adsorption/desorption experiments.