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 anal...

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Bibliographic Details
Published in:	Renewable energy 2021-01, Vol.163, p.105-122
Main Authors:	Sarlaki, Ehsan, Sharif Paghaleh, Ali, Kianmehr, Mohammad Hossein, Asefpour Vakilian, Keyvan
Format:	Article
Language:	eng
Subjects:	Energy efficiency Humic acid Lignite Optimization Reactor Structural features
Online Access:	Get full text
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Summary:	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 °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 E4/E6 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. [Display omitted] •HA extraction process are optimized by CCD-RSM and GA-SVM modeling approaches.•Optimum HA extraction paremeters were 0.5 M NaOH, 70 °C, 850 rpm and 4 h.•The developed MA tank reactor improved the extraction efficiency of HA up to 26.91%.•The energy efficiency of HA extraction process can be improved up to 24.87%.•Both proposed models showed acceptable prediction performance.
ISSN:	0960-1481 1879-0682