Investigations On The Potassium Ion Exchange Of Molecular Sieves

As one of largest agriculture countries, China’s annual requirement for potassiumfertilizer exceeds more than10million tons (measured in KCl). However, China isseverely deficient and unevenly distributed in potassium-bearing resource. China’sin-land potassium resource roughly just accounts for2.2%of total potassium resourcein the world. Hence, efficient and conservative exploitation of potassium resourcematters very much to China’s safety in agriculture products. Therefore, potassiumextraction from seemingly endless seawater has attracted intensive research attentionand various technology have been developed, such as precipitation method with aid ofdipicrylamine, exchange with phosphate salts and natural molecular sieves andmembrane separation technology. Among these methods, ion exchange method withmolecular sieves is regarded as most promising technology due to their high exchangecapacity for potassium ion. Needless to say, exchange capacity and selectivity forpotassium ions of molecular sieves are most strong determinants in practicalapplication, naemly potassium extraction from seawater. In this work, we investigatedthe potassium ion exchange and selectivity behavior of9different molecular sieves(ZSM-5，NaY，USY，13X，SAPO-34、β、mordenite、4A and clinoptilolite). First,the structures of9molecular sieves were characterized by using XRD,TEM andNitrogen adsorption isotherm analysis and lattice parameters change induced bypotassium ions exchange was analyzed. Position of monovalent cation in Molecularsieves lattice was simulated with Material Studio. Saturated exchange quantity andselectivity of potassium ions were correlated to the acquired molecular sievesstructure. As a result, a suitable zeolite structure for potassium exchange wasobtained.In KCl solution, experimental results show that potassium exchange quantitydecreases with increasing Si ratio to Al of molecular sieves.13X、NaY and4Amolecular sieves hold higher potassium exchange quantity than other molecularsieves. As one of aluminium phosphate molecular sieves, SAPO4-34’s potassiumexchange quantity is low. Saturated potassium exchange quantity fells in the order ofNaY>13X>4A>clinoptilolite. Meanwhile, the selectivity is in the order of clinoptilolite>4A>13X> NaY, suggesting clinoptilolite might be a highly selectivemolecular sieves in potassium extraction.In practical potassium extraction from seawater, when seawater was brought intocontact with molecular sieves for120mins, clinoptilolite exhibits a potassiumexchange quantity of0.15×10-3mol/g, which is much higher than exchange capacityfor calcium and magnesium ions, confirming its specific high selectivity of potassiumto other cations in seawater. However,13X, NaY and4A show no selectivity topotassium ion in seawater.In FAU-type molecular sieves, cations are distributed in the larger cages in FAUstructure and close to bridging O atom between2Al atoms, according to simulationresult by Material Studio. In LTA-type molecular sieves, Na ions are located in a8-member ring but impartial to the center and close to neighboring3O atoms inframework. In HEU-type molecular sieves, cations are in the same plate of3bridgingO atoms of Al atom in framework and vibrated between equivalent position in thisplate. Lattice parameters of NaY and clinoptilolite decreased caused by introductionof potassium ions to their pores.The resultant optimum zeolite structure is as follows: low Si ratio to Al (2-4),pore size of0.60nm. Hence the MFI-type, HEU-type and BEA-type molecular sieveswith lower Si ratio to Al are expected to be suitable for potassium extraction.