Synthesis Of Tobermorite Powder From Potassium Feldspar: Reactive Mechanism Research

This research is a part of the project of the integrative research of distillingpotassium from potassium feldspar ore of Lushi county, Henan Province. Potassiumfeldspar was hydrothermally decomposed with CaO as an additive, and thentobermorite nano-scale powder was synthesized. The relation between the tobermoritemicrostructure and their properties was analyzed through the characterization of thetobermorite synthesized at various conditions. The reaction mechanism of thepotassium feldspar dissolution and the tobermorite formation was studied on the basisof the former investigations.The potassium feldspar ore from Lushi county is muscovite syenite. The XRD datamanifested that the main phase is microcline with the content of 85.5%,subsidiaryphase is muscovite with the content of 7.6%, other phases are magnetite and sphene.Their content are 2.4% and 1.1% respectively. The wet chemical analyse resultmanifested the main composition of the ore powder are SiO₂,Al2O3 and K₂O. Thecontent of K₂O reach 14.69%.SiO₂ and Al2O3 are available for synthesizingtobermorite;K₂O was for preparation of KOH.Potassium feldspar was hydrothermally decomposed with CaO as an additive,and then tobermorite nano-scale powder was synthesized. The effective factors on thesynthesizing reaction was the mole ratio of CaO/(SiO₂+AlO_1.5),reaction temperatureand reaction time. The experimental results showed the tobermorite was crystallizedwell with the mole ratio of CaO/(SiO₂+AlO_1.5) in the range of 0.75¹.00, at thereaction temperature of 230²50℃ for 5⁸ hours. The dissolving rate of K₂O frompotassium feldspar was higher than 84.8% and the solid product was well-crystallizedfibrous tobermorite.Al3+ have effect on the formation of tobermorite. It was researched that the effectof various mole ratio of AlO_1.5/(SiO₂+AlO_1.5) on synthesized tobermorite powder. Theexperimental results showed the pure tobermorite was synthesized when the mole ofAlO_1.5/(SiO₂+AlO_1.5) is less 0.13. The thermal stability of tobermorite powder wasstudied. The experimental result manifested that Al-substituted tobermorite was betterthan the tobermorite which did not contain aluminum, and suitable for preparingcalcium silicate insulation materials.The structure and performance of tobermorite powder was characterized. Wetchemical analysis, XRD, IR, SEM, and TA-DTA methods were used to characterizethe synthesized tobermorite powder. The synthesized samples are hollow sphericalagglomerates formed with fibrous tobermorite crystals, which are 3¹0μm in length,0.5¹.0μm in width and less 100nm in thickness.The mechanism of the potassium feldspar's dissolution and the tobermoriteformation was studied. The process of tobermorite formation with increasing time wasstudied with various reaction time at the optimum condition of reactiontemperature:250℃, CaO/(SiO2+AlO1.5) molar ratios: 0.83, liquid/solid weight ratios:20 and stirring speed: 400r/min. The research data obtained in this study demonstratethat the reactive mechanism of the potassium feldspar's dissolution can not beexplained solely on the basis of ion exchange or aluminosilicate depolymerizationreaction. The dissolution of potassium feldspar and the crystallization of tobermoritein hydrothermal system is considered possibly as a three step process. At the first, thepotassium feldspar decomposes and gives rise to Si-rich and Al-deficient surfaceprecursor complexes. Secondly, the precursor complexes(SiO2·nH2O)decompose andfurther to form C-S-H gel and hydrogarnet phases. And at the last, the C-S-H gel andhydrogarnet are converted into the tobermorite.There were lanthanon which were La,Ce,Nd in the muscovite syenite. The contentof lanthanon was 800¹000μg/g. It reached cut-off grade, so it has some valuable. Thesphene in the muscovite syenite contains lanthanon, but the content of sphene is low, andthe grain is small. It is difficult that the lanthanon is separated with gravityconcentration method.The LCA technique was used to evaluate the influence of the procedure to theenvironment in the course of the synthesis of the tobermorite. The developedtechnique using the potassium feldspar to prepare tobermorite and distill potassium,was 100% in availability of the insoluble potassium resource, and has no waste water,exhaust gas and solid residue. The energy cost of the technique was low. Thereforethis technique accords with the request of the environment and was available.