GB955673A - Purification of aromatic polycarboxylic acids - Google Patents

Abstract

Crude aromatic polycarboxylic acids, containing at least 95% by weight of a single aromatic polycarboxylic acid, the remainder being essentially oxidation by-products including colour-imparting organic compounds, are purified by a process which comprises forming an aqueous solution of an alkali metal or ammonium salt of the crude acid and passing the aqueous solution through a bed of activated carbon at a temperature above 20 DEG C., the salt having from 90 to 99.9% of the carboxy hydrogen atoms of the acid replaced by alkali metal or ammonium ions and being present in solution in a concentration from 25 to 100% of the saturation concentration at the temperature at which it is passed through the carbon and recovering the aromatic polycarboxylic acid from the carbon-treated solution. The acid is recovered from the salt solution by acidification e.g. with sulphuric, hydrochloric, sulphurous or carbonic acid. The preferred salts are the sodium, potassium and ammonium salts. The above process may be preceded by a chemical oxidation at a pH above 6 of the salt solution e.g. with alkali metal permanganate or hypochlorite. In examples the purification of terephthalic, isophthalic and trimellitic acids is described. Other acids specified are trimeric, tertiary butyl-tere- and iso-phthalic, di- and tetra-chloro-terephthalic, biphenyl-p-dicarboxylic and naphthalic acids.