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The silica extraction from palm oil mill fly ash (POMFA) using sodium hydroxide as one of viable processes for obtaining silica from agricultural waste was investigated. The effects of extraction time and temperature were closely examined to study the kinetics of the process. The fixed variables used in the present work were mass of POMFA 468.2 gram; the POMFA mass to NaOH volume ratio 0.2341 g/cm3; the concentration of NaOH 1.4 N and the stirring speed of 1065 RPM. The levels of temperature employed were 348 K, 358 K, 368 K and 378 K for different time durations up to 60 min. The mechanical fragmentation process was applied to obtain precipitated silica from the extracted silica. The precipitation conditions were: stirring speed of 1160 RPM, pH of 8.75, temperature of 303 K and precipitation time of 100 min. The shrinking core model (SCM) with intra-particle diffusion controlled mechanism and the Jander equation can satisfactorily represent the extraction process. The activation energy for silica extraction was 58.20 kJ/mol for the SCM with intra-particle diffusion and 62.22 kJ/mol for the Jander equation respectively. The precipitated silica agglomerate obtained at the time of 100 min has the median-weighed volume particle size distribution of 114.07 μm. The chemical composition and physical characteristic of precipitated silica which were analyzed with LPSA, XRF, XRD, FTIR and SEM are similar to the precipitated silica from the references.

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In this research, sol-gel precipitation using CO 2 impregnation and mechanical fragmentation method was applied to produce precipitated silica from Palm Oil Mill Fly Ash (POMFA). Carbon dioxide (CO 2) was used in order to reduce the cost of the process and to enable sodium hydroxide recovery. The precipitation process was done in a stirred temperature-controlled baffled glass precipitator. The response surface method with the central composite design was applied to optimize the stirring speed and the CO 2 flow rate. The pH and the temperature of the precipitation process were varied for tailoring the specific surface area of the precipitated silica. The mechanical fragmentation and wet crushing process were applied to control the agglomerate particle size of the precipitated silica obtained. The results show that precipitated silica with a specific surface area in the range of 50-140 m 2 /g can be obtained.

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