GB793921A - A process for working up products containing the alkali metal salts of terephthalic acid - Google Patents

Abstract

Products containing an alkali metal salt of terephthalic acid are worked up by reacting said salt, in the presence of water, with an alkaline earth metal salt of an aromatic carboxylic acid, whose salts are capable of thermal re-arrangement into salts of terephthalic acid and so obtaining an alkaline earth metal salt of terephthalic acid. The alkaline earth metal salt may be prepared in situ from the alkaline earth metal hydroxide or carbonate in admixture with a free aromatic carboxylic acid or its anhydride. Suitable aromatic carboxylic acids are benzoic acid, isophthalic acid, orthophthalic acid or tri- and hexabenzene carboxylic acids. The process is preferably used for solutions of sodium or potassium terephthalate and the alkaline earth metal salt used is preferably the calcium salt although the corresponding salts of barium and strontium with aromatic carboxylic acids may also be used. The products containing alkali metal salts of terephthalic acid should contain not more than 20 per cent by weight and preferably below 10 per cent by weighr of aromatic carboxylic acids, other than those of the unreacted starting materials. The concentration of the alkali metal salts of terephthalic acid in solution, should be at least that in which the limit of solubility of the alkaline earth metal salt of terephthalic acid is exceeded at the selected operating temperature. Excess alkaline earth metal salts of aromatic carboxylic acids may be used. Suitable reaction temperatures are from about 10 DEG C. up to 150 DEG C., preferably 70-120 DEG C. The precipitate of alkaline earth metal salts of terephthalic acid in a solution of alkali metal salts of the remaining aromatic carboxylic acids, may be cooled and separated in a known manner, and the salt obtained from the filtrate re-arranged to terephthalic acid. Terephthalic acid may be obtained from the alkaline earth metal salt, by treating the latter with an acid which forms soluble alkaline earth metal salts, e.g. hydrochloric acid, nitric acid, carbonic acid, formic acid or acetic acid. This step may be carried out directly on the filter, or by suspending the alkaline earth metal terephthalate in water and adding the acid, or by adding the terephthalate to the acid. The latter two steps may take place between 10 DEG and 120 DEG C. and preferably at such concentrations that the alkaline earth metal salts which are formed, dissolve completely in the water present in the reaction mixture. When using carbonic acid it is preferable to work at excess pressure, e.g. 50 kg/cm2 pressure of carbon dioxide. The free terephthalic acid so produced may be further purified by known methods. Examples are given of the treatment of crude dipotassium terephthalate with (1) commercial calcium hydroxide and phthalic anhydride; (2) calcium hydroxide and isophthalic acid; (3) calcium benzoate.