Synthesis of cristobalite from silica sands of Tuban and Tanah Laut

Synthesis of SiO₂ cristobalite powders has been successfully carried out by a coprecipitation method by making use of local silica sands from districts of Tuban and Tanah Laut, Indonesia. Cristobalite is a phase of SiO₂ polymorphs which can be used as a composite filler, a coating material, a surface finishing media, and structural ceramics. In the first stage of the synthesis, the as-received sands were processed by a magnetic separation, grinding, and soaking with HCl to increase the purity of silica content. X-ray fluorescence (XRF) spectroscopy showed that the atomic content of Si (excluding oxygen) in both powders reached 95.3 and 97.4%. A coprecipitation process was then performed by dissolving the silica powders in a 7M NaOH solution followed by a titration with 2M HCl to achieve a normal pH and to form a gel. Furthermore, the silica gel is washed, dried and then calcined at a temperature of between 950-1200 °C with a variation of holding time for 1, 4 dan 10 hrs to produce white powders. X-ray diffraction (XRD) data analyses showed that the powder with calcination temperature of 1150 °C for 4 hrs exhibited the highest cristobalite content of up to 95wt%. Its scanning electron microscopy (SEM) image showed that its grain morphology was relatively homogeneous.