Microwave-assisted lime treatment and recovery of lignin from hydrothermally treated sweet sorghum bagasse

Microwave (MW)-assisted lime treatment was applied for the extraction of lignin from sweet sorghum bagasse (SSB) which was previously subjected to a MW-assisted hydrothermal treatment for the extraction of hemicellulose. Under optimum conditions, lime dosage at a concentration of 10% by weight of the substrate and 78 min of treatment residence time resulted in 68.27 (+/- 1.19) % by weight of the lignin extracted from the substrate. The residual SSB was composed of 69.41 (+/- 1.83) % cellulose, 12.14 (+/- 0.71) % hemicellulose, 12.59 (+/- 0.31) % lignin and 4.03 (+/- 0.21) % ash in it. The bulk density of the two-step treated SSB was 3.6 times that of the untreated SSB. The yellow liquor was treated with CO2 and 60.26 (+/- 2.11) % by weight of the dissolved lignin was precipitated from it. The physicochemical properties of the treated-SSB and the lignin samples were analysed using Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and scanning electron microscope (SEM). The mass balance of the MW-assisted fractionation of SSB was conducted and the prospect of the process for on-farm fractionation of lignocellulosic materials was considered.