Synthesis Of Novel Cryptomelane-type Ion-sieve For Potassium And Its Application

Synthesizing of a novel K-adsorbent and its adsorptive properties were studied, which mainly include:synthesizing the precursor of the K-adsorbent by high temperature solid-state method and the studying on the thermodynamics and the granulation of the ion-sieve.On the synthesis process, the cryptomelane-type manganese dioxide (CRYMO) was prepared by heating a mixture consisting of potassium methoxide and manganese carbonate in the atmosphere. A novel potassium ion-sieve (CRYMO-H) was obtained by acid-modification of CRYMO. The characterizations of the synthesized CRYMO-H were carried out by AAS, TG-DTA,BET, XRD, FT-IR and SEM. The effects of the Mn/K ratios of raw materials, calcined temperature and calcined time on crystal phase were investigated. The results show that the chemical formula of the sample is K0.95Mn7.96O16.00 and its crystal phase belongs to a-MnO2 when prepared at the Mn/K ratio of 4:1, calcined temperature of 620℃and calcined time of 12h. The sample is stable up to 650℃and its morphology is virgulate-like. In the washing process, the effects of some factors such as different kinds of acid, various concentration of HNO3 and different time of washing on the ratio of K+ migration and the ratio of Mn2+ losing were investigated. So it was considered appropriately that the precursor was washed using 0.5M HNO3 for 50 minutes or so. At the same time, it was found that CRYMO-H preserves the same structure as the precursor CRYMO by XRD and TEM.In the study on adsorptive property process, the effects of pH on the exchange capacities were tested And then the exchange capacities to K+, Ca2+, Mg2+, Na+, Li+ in single salt solution and the selectivity of CRYMO-H were measured. The exchange capacities of CRYMO-H to K+ in pure potassium solution and seawater are 126.78 and 29.11mg/g respectively, and its selectivity coefficients to K+ in the solution of K+/Na+ and K+/Li+ are 115.68 and 134.91 respectively, whereas it has no exchange capacity for Ca2+ and Mg2+ In the study on the ion exchange thermodynamics of CRYMO-H to K+, the exchange capabilities to K+ in pure potassium and the isotherms of ion exchange equilibrium of the ion sieve at different temperature by static method were determined; Further more Kielland diagrams and the expression of ion exchange equilibrium constant were obtained and△1Hmθ,△rGmθ,△rSmθof the ion exchange processes were figured out. It was proved that the ion exchange process is unprompted and the exothermic reaction, and the low temperature is advantageous to K+ absorption.Using PVC as the granulating reagent, the granulation of the ion sieve was studied. The results show granules' losing rate was low in the water when the ratio of PVC is 20%, and its adsorptive capacities in pure potassium solution and seawater are 67.56 and 22.17 mg/g respectively. Therefore, the ion-sieve can be used to extract potash selectively from seawater.