Organic carboxylic acids of complex molecular structure (aromatic and phenolic) that comprise 10 to 90% of lignite. Humic acids in lignite react with caustic ingredients (NaOH and KOH) in mud. The water solubility of lignite depends on its humic acid content. Decarboxylation of humic acid groups by hydrolysis in alkaline muds is a major source of carbonate and bicarbonate anions in water muds.