Study On The Synthesis Of Calcium Silicate-based Mineral Materials From Alkaline Sodium Silicate Solution

In recent years, some of the high silica ores were processed by the use of chemical processing method to leach of silicon with alkali, which resulted in a large number of alkaline sodium silicate solution(AS).The comprehensive utilization of sodium and silicon included by the AS will generate significant economic value. But there are no effective processes and technology for the utilization of this ore. In this paper, the usage of the AS was researched by exploring the feasibility of synthesizing wollastonite at atmospheric pressure, and carrying out the method of dynamic hydrothermal synthesizing tobermorite whisker. Some of the major conclusions were obtained:1) Under normal pressure, hydrate calcium silicate, obtained by the reaction between sodium silicate solution and milk of lime, calcined in a muffle furnace to get products which contain wollastonite, dicalcium silicate and silica. The caustic promoted the formation of dicalcium silicate in the calcined product. The addition of aluminum hydroxide increased the stability of the sodium silicate solution and reduced the reaction rate of calcium and silica. When using the AS to synthesize wollastonite, the results are similar with the above synthesis results.2) The results of using the dynamic hydrothermal synthesize tobermorite are as follows:When the caustic concentration was low, the main phase of product is pectolite. Along with the increasing of the caustic concentration, the content of tobermorite was gradually increased. The joining of Al(OH)3benefits the formation of1.1nm tobermorite, and the synthesis product is all the whiskers of1.1nm tobermorite. The addition of NaCl can improve the purity of tobermorite, and the crystalline surface of whiskers is more complete. The tobermorite whiskers are length of5～10μm,diameter of0.3～0.5μm under the condition of Ca/Cl molar ratio of5and Ca/Al molar ratio of5.In the best experimental conditions, the tobermorite whiskers which are length of2～10μm,diameter of0.2～1.0μm,were successfully synthesized by using the AS. 3) The results of thermodynamics calculations about the synthesis of calcium silicate hydrate are as follows:For clinotobermorite and1.1nm tobermorite,220℃is the boundary of stable temperature. When higher than220℃, clinotobermorite generated easier than1.1nm tobermorite. The1.1nm tobermorite was obtained under the condition of adding the aluminum ion, which indicated that addition of aluminum ion changed the environment of the formation of clinotobermorite, prompt the formation of1.1nm tobermorite.4) Combined with the synthetic results and related information, the role of behavior of various ions in the process of synthesizing tobermorite was investigated. Na+would occupy the position of H in the Si-OH groups, form Si-ONa groups, and eventually make the formation of pectolite. With increasing of the OH-concentration, the contact opportunities between OH-and Si-ONa raised, which transformed Si-ONa groups to Si-OH groups, and prompted the formation of the bridge chain double. By the role of the chloride ion, the salt effect promoted the dissociation of Ca(OH)2,increased the content of Ca2+in solution, and made the formation of tobermorite more easier. What’s more, chloride ions can make Ca2+in the surface of tobermorite parse or continue to combine with the silicon-oxygen tetrahedron chain, which ensured the whisker the integrity of the crystal surface. Aluminum ion can replace the bridging silicon of the silicon-oxygen tetrahedron, and impeded the bonding between the bridging silicon-oxygen tetrahedron. Ultimately the formation of1.1nm tobermorite occurred.