Study on organic acid leaching method to eliminate impurities in rice husk

Abstract

The aim of this thesis is to investigate the effect of citric acid leaching method to eliminate impurities in rice husk (RH) in order to obtain high purity silica (SiO₂) for the production of rice husk glass (RH-glass). The process parameters involved are citric acid solution concentration, leaching time, and stirring time. The production of RH-glass from high purity SiO₂ was prepared by introducing acid leaching treatment by using citric acid (C₆H₈O₇) followed by combustion. The purity of silica in rice husk were analyzed by using X-ray Fluorescence (XRF) analysis and proven to be high purity by percentage of ˃99 wt% SiO₂. With increasing of citric acid solution concentration, leaching time, and stirring time, the percentage of SiO₂ was increased to >99 wt%. High purity of SiO₂ was achieved even when the RH was leached at minimal parameter of 0.1 M citric acid solution, temperature of 40 ºC for 1 h. The elimination of iron (Fe) element impurities was analyzed by using Inductively Coupled Plasma Mass Spectrometry (ICP). It was found that content of Fe elements discharged into the citric acid solution during the acid leaching process gradually increases with increasing molarity of citric acid solution. Phase of silica in rice husk ash (RHA) was obtained after heated at 1000 ºC for 5 hours. X-ray Diffraction (XRD) shows that the silica exist in amorphous phase and was detected at 2θ=22º. However, the phase structure of the RH-glass was observed to be crystalline due to the high melting temperature of silica. The color of rice husk ashes completely changed from black to white ashes. The microstructure of silica as observed using Scanning Electron Microscope (SEM) and the micrograph shows the homogenous phases and irregular shape of particles. A clear RHglass was produced by heating the high purity SiO₂ at 1560 ºC for 12h.