Both lignin appeared as a fine brown powder with

Both the products were separated from paddy straw
powder by alkali dissolution, followed by acid based precipitation. Silica and
lignin were solublized by reacting with sodium hydroxide (NaOH, strong base) to
form water soluble sodium silicate (Eq. I) as depicted in the following
reaction. A schematic separation process of silica and
lignin from agro-residue waste is outlined in Fig. 1. The filtered sodium
silicate was tan coloured liquid while the residue left behind was light brown
in colour. The obtained sodium silicate was then neutralized slowly with
sulphuric acid solution to pH 6.5, where silica was precipitated (Eq. II). This
process of slow titration was performed under mild stirring to allow proper diffusion
of sodium sulfate (Na2SO4) and to avert the formation of
large aggregates. Upon reaching pH 6.5, silica was gradually precipitated after
18 h of incubation. The resultant silica precipitates were washed repetitively
with ultrapure H2O until no Na2SO4 was
detected in the wash and the recovered silica was dried in an oven at 50°C
overnight. The dried silica appeared as a fine pale white powder at an overall
yield of 9.26%. Simultaneously, lignin was also precipitated by further
lowering down the pH of the sodium silicate solution to pH 4.0 with sulphuric
acid and the precipitates thus obtained were washed and dried. The dried lignin
appeared as a fine brown powder with an overall yield of 2.30%. Minu
et al. (2012) have reported the extraction of silica and lignin from the black
liquor generated during the production of bio-ethanol from agro-residues. Lu
and Hsieh, (2012) has reported the extraction of silica as nano-disks from rice
straw. Preparation of silica nanoparticles from semi-burned rice straw ash has
also been documented (Zaky and co-workers, 2008). However, in this study a
direct process has been developed for nano-silica and lignin separation from
paddy residue.