Specific resistance increase with increasing filtration velocity at constant dust concentration. Specific resistance of filter media is independent of pressure drop limit while that of the cake decreases. Specific resistance of filter media increases linearly and that of filer cake decreases on aging. ► Specific cake resistance depends on velocity, pressure drop limit and aging. ► Pressure drop limit influences the cake formation at higher levels only. ► Dust settling decreases at higher velocity and vice versa. ► Filter media resistance is independent of upper pressure drop limit. ► Filter media resistance increases linearly on aging.

This study deals with the extraction and characterization of humic acids from low rank coals of Thar, Chakwal, and Quetta. Three respective coal samples were initially oxidized with 2% HNO₃ and then treated with 2.5, 3, and 3.5% KOH solutions for the extraction of humic acid. The 3.5% concentration of KOH was found to be the optimum for the extraction of humic acid. The extracted humic acid was investigated for its quantitative assessment through gravimetric and UV spectrophotometer procedures. The gravimetric method showed 24.6, 13.6, and 18.0% of humic acid extraction from Thar, Chakwal, and Quetta coals, respectively, whereas the humic acid determined by UV spectrophotometry was 29.67, 20.1, and 22.6% for the same sample sequence. FTIR analyses of extracted humic acid samples have shown the presence of carboxylic, phenols, alcoholic, and amines functional groups. The ¹³C NMR spectra of three humic acids have exhibited the broad diversity of aliphatic and aromatic compounds such as carboxylates, carbohydrates, ethylene, phenols, and amines. The results of the study could be useful for selection of suitable coals for humic acid extraction.