Isolation and purification of lignin and silica from the black liquor generated during the production of bioethanol from rice straw

Bioethanol production from lignocellulosic agricultural residues is an attractive future biofuel option from the standpoints of food security and environment friendliness. The safe disposal of the black liquor produced during the process of lignocellulosic ethanol production however remains a challenge. In this study, a method has been developed for the efficient recovery of lignin and silica from black liquor. Rice straw was pre-treated in two different steps, with dilute acid hydrolysis followed by alkaline peroxide delignification. Lignin and silica were isolated from the black liquor by precipitation method using dilute sulphuric acid for reducing the pH of the black liquor. The pH values required for the recovery of lignin and silica were optimised separately. Two different precipitation and separation methods were followed for product recovery from black liquor. Chemical characterisation of isolated lignin was done by FTIR and compared with commercial lignin to evaluate its potential industrial applications. The quality of the filtrate after precipitate separation was also monitored in each step by COD and TDS analyses. Two-stage treatment of black liquor, which involves isolation of silica and lignin separately, was found to provide superior quality products and cleaner effluent than the direct precipitation of lignin without the separation of silica from black liquor.

► The precipitation of lignin and silica from the black liquor of rice straw.
► Effect of pH on the precipitation of lignin from the black liquor of rice straw.
► Effect of temperature, type of acid, precipitate removal method on the precipitate.
► Two-step precipitation of silica and lignin from the black liquor of rice straw.
► The precipitated lignin from the black liquor of rice straw has good qualities.