Kinetic and processing studies on a novel technology of producing high purity nano-silicon dioxide from an alumina rich coal fly ash with carbon dioxide

To further enrich aluminum content in coal fly ash so as to reduce the cost of it as a resource for industrial aluminum extraction, a novel process was developed to separate silicon from the rest of fly ash. The process was conducted by treating the coal fly ash with high concentration sodium hydroxide solution to dissolve silicon as sodium silicate at room temperature under atmosphere. The sodium silicate solution was separated from the high aluminum residue by filtration and was then subjected to treatment with carbon dioxide. When pH of the solution was reached at value A, carbon dioxide ventilation was interrupted and the step was called first carbonization. The filtrate obtained by filtration of the mixture from the first carbonization was subjected to the second carbonization at the same conditions. When pH of the filtrate was reached at value B, carbon dioxide ventilation was terminated and the residual coal fly ash was obtained by filtration. The whole procedure was named as two-step carbonization process. A highly purified nano-silicon dioxide product, in size of 50 nm and with purity of 96%, was obtained after washing the coal fly ash residue with water and acid. The processing samples were characterized by transmission electronic microscopy (SED), X-ray diffraction (XRD), and infrared spectroscopy (IR) to illustrate the mechanism of the two-step carbonization process. Major reaction in the first carbonization was taken place between sodium hydroxide and carbon dioxide. Significant amount of heat was produced by this reaction. At the same time only a small portion of sodium silicate reacted with carbon dioxide. Most impurities were removed as a result of their adsorption on or reaction with the precipitate produced. The major reaction for the second carbonization was the reaction between sodium silicate and carbon dioxide. Silicon dioxide was precipitated in this step but no significant heat produced. Concentration changes of sodium hydroxide and sodium ca- rbonate in the first carbonization and sodium silicate in the second carbonization were monitored to reveal the kinetic characteristics of the two-step carbonization process. It was found that the reaction was controlled by the mass transferring resistance on the liquid membrane. The first carbonization was a pseudo-first order rapid reaction between carbon dioxide and sodium hydroxide. The second carbonization, a medium fast multi-phase reaction between carbon dioxide and sodium silicate, was classified as second order reaction. The expressions of mass transfer rate were derived, and the criterion M value was obtained.