| High-purity sodium chloride(99.99%)could be extensively applied on food industry,medicine,the medical field,etc.Very similar properties for Na+ and K+ results in difficulty for separation K+,which is mixed from the preparation of sodium chloride solution.It is impossible to obtain a high-purity sodium chloride though conventional methods,such as precipitation,ion exchange,and extraction,due to the low selectivity for K+.In order to obtain high-purity sodium chloride,more attention is focused on the ion-sieve compound with high selectivity for K+ in recent decades.Ion-sieve compound is a metal oxide porous crystals(MOPCS),which is usually synthesized through the template method.And the ion-sieve compound with the unique structure can be obtained by topotactically removing the template ions from inorganic compound with the template ions,which is calcined at the high temperature.The ion-sieve compound show high selectivity for specific ion depending on the template ion,so it can be used for the removal of specific ion in the presence of an excess of other competitive species,and the systhesis of the potassium adsorbent can provide a new method for the preparation of the high-purity sodium chloride.According to the similar properties of Na+ and K+,The adsorbent H8Nb22O59·8H2O was prepared by extracting Rb+ from precursor Rb8Nb22O59 in HNO3 solution,which was synthesized by a solid state reaction at high temperature.Prepared H8Nb22O59·8H2O and Rb8Nb22O59 were characterized by XRD,TG-DSC and SEM.The exchange capacity of adsorbent for Na+ and K+ was investigated and the optimal preparation conditions for precursor Rb8Nb22O59 were proposed as well:calcinating the mixture of Rb2CO3 and Nb2O5 at 1200 0C for 12 hours.Results indicate that the structure of precursor Rb8Nb22O59 under above conditions was similar to that of prepared adsorbent H8Nb22O59·8H2O after acid treatment.Around 97%of Rb+ of precursor could be extracted by acid elution,while the lost amount of niobium ions was negligible(less than 0.04%),showing a good capability of regeneration.In addition,the absorption capacity of H8Nb22O59·8H2O for K+ and Na+ was evaluated in high concentrated sodium chloride solution(mole ratio Na+:K+= 500:1)at pH ranging from 8 to 13.The adsorbent H8Nb22O59·8H2O had a higher distribution coefficient for K+ with separation factor of 43 rather than for Na+.Meanwhile,it was found that distribution coefficients of H8Nb22O59·8H2O for Na+ and K+ were stable when calcination time reached over 6 hours. |
