Extraction of Humic Acid from Lignite Coals using Stirred Tank Reactors (STRs): Assessment of Process Parameters and Final Product Charaterization

Extraction of humic acid (HA) from lignite coals that are rich in humic compounds has been highlighted in recent years as a superior and prominent research issue. The conventional technique used to extract HA is based on their solubility in alkaline and acidic mediums. Most litertures have reported the extraction time and separation of HA to be 24 hours and the extraction efficiency between 10 to 80%. In this research, the intensification of HA extraction from lignite coals has been studied. For this purpose, a batch stirred tank reactor (BSTR) was first designed and then fabricated. The processing parameters such as temperature at three levels (50, 70 and 90° C), process time at three levels (1, 2 and 4 hours) and stirring speed at three levels (250, 550 and 850 rpm) were considered as independent variables and extraction yield of HA was considered as dependent variable and the performance of the constructed reactor was compared with a conventional method (Hot Plate device). The statistical results of variance analysis (ANOVA) showed a significant difference between the yield of HA extraction with the reactor and Hot Plate method. Under the same conditions of temperature, stirred speed and process time, the yield of HA extraction with the reactor was 30% higher than the Hot Plate device. The optimal conditions for extraction of HA by the reactor were obtained at temperature of 90° C, stirring speed of 850 rpm and 4 hour process time. In these conditions, 56.8% of HA was extracted and separated from lignite coals. The qualitative results of FT-IR spectroscopy showed a predominant aromatic and rich in carboxylic, phenolic and hydroxylic functional groups with aliphatic side chains in the HA structure. The results of quantitative analysis of the CHNOS main elements and atomic ratios (C/N, O/C and H/C), confirms the quality of the used coal and also matches up the quality indices of produced HA with the IHSS standard and commercial Aldrich HA samples.