Preparation Of Molecular Sieves From By-product Of Zirconium Oxychloride

Zeolite molecular sieve is widely used in industrial adsorption, catalysis, ion-exchange, separation, and high-tech fields because of their unique crystal structure and physico-chemical performances.Silica residue including zirconium, a kind of strong acid emissions, is formed as a by-product of zircon desilication. The main composition of silica residue including zirconium is a kind of high moisture content of mSiO2· nH2O gel polymer. Pile up or landfill disposal was as mainly method to deal with the emissions in the past, these practices would cause serious pollution to water and soil. Recycling of silica residue including zirconium has been concerned. Aimed at the key issues such as pretreatment of silica residue including zirconium, route and law of synthesizing molecular sieve, mechanism of molecular sieve transformation orientation and so on, the technique of preparing molecular sieve using silica residue including zirconium as the silicon source was investigated intensively to recycle silica residue including zirconium.Neutral SiO2 gel was obtained by balancing with waste alkali liquor and washing with deionized water. Physical and chemical methods were used to determine the composition and phase of neutral SiO2 gel, most of which is in the form of amorphous SiO2 with highly reactive suitable for synthesizing molecular sieves. Zeolite NaA is synthesized by one-step hydrothermal synthesis, while zeolite NaX is obtained from two-step hydrothermal synthesis.The preparation of NaA by one-step hydrothermal synthesis:The mixture of sodium meta-aluminate,SiO2 gel, NaOH solution and deionized water in a molar ratio of n(Na20):n(SiO2):n(Al203):n(H20)=3.42:1.8:1:137; the precursor of molecular sieves was obtained after aging 1.5 hours at 60℃ and stirring rate at 200-300r/min; Then zeolite NaA was synthesized from the precursor by hydrothermal reaction at 100℃ for 6 hours and stirring rate at 50-60r/min. It is shown that the conversion rate of molecular sieve was 88.7wt%, and its static water absorption reached 24.38wt%.The preparation of NaX by two-step hydrothermal synthesis:First step, experimental parameters for preparing sodium silicate by hydrothermal synthesis with SiO2 gel and NaOH was obtained. The optimal mixture of SiO2 gel and NaOH solution in a molar ratio of n(Na2O):n(SiO2)=1:2.5, sodium silicate with the modulus of 2.21 was obtained from the mixture by reaction for 3 hours at 180℃. Second step, the mixture of sodium metaaluminate,sodium silicate NaOH solution and deionized water in a molar ratio of n(Na2O):n(SiO2):n(Al2O3):n(H2O)=5.6:4:1:280, the precursor of molecular sieves was obtained after aging for 3.Oh at 40℃and stirring rate at 200～300r/min; Then, zeolite NaX was synthesized from the precursor by hydrothermal reaction for 8h at 90℃ and stirring rate at 50～60r/min. It is shown that the conversion rate of molecular sieve was 92.0wt%, and its static water absorption reached 32.21wt%.Intermediates and products were characterized by XRD, XRF, FT-IR, SEM, and TG-DTA, and the key influencing factors of molecular sieve transformation and product properties were discussed. It turned out that one-step hydrothermal synthesis was suitable for zeolite NaA, while zeolite NaP and NaX would be found in products due to change of synthesis conditions; Two-step hydrothermal synthesis was suitable for zeolite NaX with a narrow range of raw materal ratio, there were zeolite NaA and NaP in products by changing synthesis conditions. Pure zeolite was obtained due to rational control of synthesis conditions.It was showed that synthesis orientation of molecular sieve was affected by ratio of starting material, medium alkalinity, synthesis conditions and so on from the study of and phase diagram. Perfect zeolite NaA was obtained from ratio of raw material at n(SiO2)/n(Al2O3)=1.2 to<3.0, n(Na2O)/n(SiO2)=1.2～2.0, water content in 88.5%, and reaction temperature at 100℃; Zeolite NaA with a small quantity of NaP was synthesized from n(SiO2)/n(Al2O3)=3.0; Zeolite NaP was obtained from changing ratio to n(SiO2)/n(Al2O3)> 3.0,n(Na2O)/n(SiO2)=0.4～2.0. Zeolite NaX could be synthesized from a high ratio of Si/Al due to the Si/Al=1～1.5 of its framework, and pure zeolite NaX was obtained from n(SiO2)/n(Al2O3)=4.0, n(Na2O)/n(SiO2)≥1.4; The change of n(SiO2)/n(Al2O3) caused decrease of conversion rate, and NaA would be found in product when n(SiO2)/n(Al2O3)<4.0.