Valorization of lignite wastes into humic acids: Process optimization, energy efficiency and structural features analysis

The present work investigates the optimization of humic acids (HAs) production from lignite processing wastes using a developed mechanical agitated (MA) tank reactor based on the results of response surface methodology (RSM). The structures of the obtained HAs were characterized using elemental analysis, spectroscopy, scanning electron microscopy (SEM), particle size distribution (PSD), acidic functional groups, and ash content. The highest extraction efficiency of HA was 54.2%, which was obtained from the MA tank reactor under optimal conditions of using 0.5 M NaOH, the reaction time of 4 h, stirring speed of 850 rpm, and temperature of 70 degrees C. The developed MA tank reactor was capable of improving the HA extraction and energy efficiency of the process up to 26.9% and 24.8%, respectively, compared to the conventional magnetic stirred (MS) glass reactor. The extracted HA from the MA tank reactor exerted better quality indices, including higher oxygen content (37.02%), higher C/N and O/C ratios (61.26 and 0.47, respectively), higher total acidity and phenolic-OH contents (8.87 and 5.6 meq.g(-1), respectively), higher E-4/E-6 ratio (3.765), more homogeneously porous structure, higher specific surface area, lower H/C ratio (0.64), and lower ash content (2.2%), than the MS method, which indicate its suitability in agricultural applications. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study focuses on optimizing the production of humic acids from lignite processing wastes using a developed mechanical agitated tank reactor. The highest extraction efficiency of 54.2% was achieved under optimal conditions. The developed reactor showed significant improvements in HA extraction and energy efficiency compared to conventional methods. The extracted HA exhibited better quality indices and is suitable for agricultural applications.