Preparation And Properties Of Spinel-type Manganese Lithium-ion Sieve

With the increasing application of lithium and its compounds, their demand is growing fast. Now researchers are more concerned with seawater and salt lake brine containing rich lithium resource, because of the gradually lack of lithium ore. It difficult to extract by traditional methods due to the lithium concentration in brine is very low. Only solvent extraction or adsorption methods can be used for the extraction of lithium from brine. Manganese oxide lithium ion-sieve is a new typical absorbent. It has high selectivity, high capacity of adsorption for Li+ and friendly to environment. Thus it is considered to be one of the most promising green methods for lithium discovery.The spinel-type structure LiMn2O4 for lithium ion-sieve precursor was synthesized with Manganese Acetate and LiOH. The optimum conditions for the synthesis of LiMn2O4, determined by means of TG-DSC and XRD, were heating the mixtures of Manganese Acetate and LiOH with Li/Mn molar ratio 0.5 at 550℃in air for 10h. Lithium ion-sieve ofλ-MnO2 was prepared by leaching precursor LiMn2O4 in the 0.5mol/L HCl solution to remove Li+. In this concentration of hydrochloride acid, the elution-rate of Li+ from LiMn2O4 was 97%. The dissolving rate of manganese of the ion-sieve precursor was increasing with the concentration of HCl and the time of elution.They were studied that the Li+ exchange capacity of lithium ion-sieve under different pH values and its adsorption kinetics in the thesis. Some samples were characterized by XRD and SEM. The results indicated that some nuance appeared in the microstructure of the samples after adsorbing lithium ion, but it was still similar to LiMn2O4. The exchange capacity of the ion-sieve enhanced with the increasing of pH value in the process of adsorption and its balance adsorption capacity was 23.75mg/g at pH equal to 12.95 in LiOH solution. The adsorption process was found to fit pseudo-second-order kinetic equation and the Langmuir isotherm equation, so the adsorption process was determined to be chemical adsorption. Three times of recycling experiments were done and showed that the adsorptive capacity was slightly decreased because of the dissolve of ion-sieve. The adsorptive capacity was kept 21mg/g after three times of recycling. To a certain extent, this ion-sieve has a good recycling performance. Temperature had significant impact in the process of ion-sieve adsorption Li+ from brine. The adsorption capacity of ion-sieve was increasing with the increasing of temperature, but time and stirring on the adsorption capacity were relatively small.Theε-pH diagrams of Li-Mn-H2O systems at 25℃was used for study the stability of LiMn2O4 in aqueous system. LiMn2O4 is fairly stable in aqueous system since it occupies whole or part of the predominant area of manganese compounds. MnO2 is a very nice absorbent for extracting lithium from solution theoretically. Theε-pH diagrams and experimental results showed thatΔGθof the Li+ extraction/insertion reactions were less than 0. So the extraction and insertion reactions were spontaneous in the standard state. In addition, they also meet the redox mechanism and the ion exchange mechanism.The dissolving rate of manganese could be decreased when doping magnesium on the ion-sieve precursor, but the elution rate of Li+ from LiMn2O4 decreased. Besides, the doped magnesium showed a great loss in elution process, and the adsorptive capacity was lower thanλ-MnO2. Therefore, doping magnesium on the ion-sieve precursor in this experiment failed to improve the performance of ion-sieve.