1. n. 
A type of coal. Lignite is found in surface deposits worldwide. Lignite is mined and put into piles where it can oxidize in the air before it is dried, ground and bagged for use in drilling fluids. The humic acid content of lignite, which varies widely, controls its solubility. The soluble and colloidal lignite components both help in fluid-loss control. Soluble components serve as clay deflocculants and improve filter cake quality. Colloidal lignite helps plug off the permeable parts of filter cake. When straight lignite is added to a mud, caustic soda is also needed to make it dissolve. Precaustisized lignite is available, which contains NaOH or KOH already mixed. Adding chromium salts improves high-temperature performance, but their use is limited by HSE concerns. Organophilic lignite is a straight lignite that has been treated with quaternary amine compounds to make it oil dispersible in oil- and synthetic-base muds.
See: buffered mud, carbonate ion, chromate salt, chrome lignite, chrome-free, conventional mud, deflocculant, deflocculated mud, drilling fluid, hydrolysis, leonardite, mud additive, neutralization, organophilic lignite, polymer, redox, seawater mud, synthetic-base mud