US Patent 7754661 Recycling of oil-based drilling muds

A method of recovering hydrocarbon or synthetic base oils from used oil-based drilling mud utilizes acid to deactivate emulsifiers in the drilling mud for altering the emulsion stability and permitting removal of the solids contained in the drilling mud using conventional separation such as centrifugation. A surfactant is typically added to prevent water-wetting as a result of the addition of the acid. The recovered base oil contains substantially all of the deactivated emulsifiers, which can be reactivated through the addition of lime, allowing new drilling fluids to be prepared without the need to add significant amounts of emulsifier to the recycled base oil.