Patent No. U.S. 6,942,803
Title or Subject: Process for Purifying a Organic Acid
The patent is aimed at the purification of lactic acid (LA) produced by fermentation. LA is used to produce polylactic acid (PLA) that is a biodegradable plastic with a wide variety of uses. As with many plastics, clarity and colour are important, and the presence of impurities impairs the quality of the final material. When LA is produced by fermentation it contains other organic acids such as pyruvic acid (PA) that give rise to colour problems in PLA. The patent discloses a process for removing acids such as PA by a multistep procedure. This process removes PA and other acids using a basic extractant that is immiscible with the fermentation mixture. The patent describes examples in which weakly basic ion-exchange resins (IER) are used or primary amines with >8 C atoms. The amine or IER is chosen so that it preferentially complexes with the PA. The choice is based on pKa values, and the separation of the complexed acid from the solution of LA is simple because it is insoluble. Acidification of the complex releases the PA, and the amine or IER can be regenerated and reused. The LA can then be further purified by being removed itself by complexation using a different amine or IER. The LA is then released by treating the complex with an acid, and the released aqueous LA is concentrated prior to crystallisation.