Coal gasification slag-derived highly reactive silica for high modulus sodium silicate synthesis: Process and mechanism

Coal gasification slag (CGS) was used to synthesize highly reactive silica, which can be rapidly dissolved in dilute alkali solution at ambient temperature. High modulus sodium silicate was further synthesized by alkali dissolution at ambient temperature instead of the conventional high temperature melting method. During the acid activation process, the inert Si-O-M (M = Al, Ca, Fe) bonds are broken, resulting in the formation of Q3 and Q2 coordination structures with active Si-O-H bonds. The active Si-O-H bonds are further spontaneously condensed to generate highly reactive amorphous silica with a low Si-O-Si bond strength. Under the optimum reaction temperature of 25 degrees C and reaction time of 0.5 h, the leaching efficiency of Al, Ca and Fe are 86.20 %, 97.61 % and 97.83 % respectively, and the solubility of silica can reach 97.85 %. During the alkali dissolution of silica process, with the decrease of NaOH concentration and liquid-solid ratio, the breaking of Si-O-Si bridging oxygen bond and the formation of Si-O-Na non-bridging oxygen bond can be decreased, resulting in the direct dissociation of the Si-O groups into the liquid phase in the form of polysilicate, thus synthesizing high modulus sodium silicate. Under the optimum reaction temperature of 25 degrees C and reaction time of 0.5 h, the leaching efficiency of silica is 76.35 % and the modulus of sodium silicate can reach 3.28. The findings of this study provide valuable insights for enriching the theoretical basis of alkali solution-based silica dissolution and for improving the energy-intensive status of the silica industry.