Patent GB-809755-A (GB)
Title: A process for the working up of crude alkali metal terephthalates
IPC: C07C51/42
Filed: 1956-11-28
Abstract: Crude alkali metal terephthalates are worked up by preparing a solution of the alkali metal salt concentrating and/or cooling said solution and so separating the corresponding dialkali metal terephthalate in solid form and then converting the alkali metal compounds remaining in the mother liquor into the corresponding salt of a benzenecarboxylic acid which is capable of thermal conversion into a salt of terephthalic acid by addition of the corresponding benzenecarboxylic acid.  Suitable acids are e.g. benzoic, orthophthalic, isophthalic or benzenecarboxylic acids containing 3 or more carboxyl groups.  The crude alkali metal terephthalates obtained by heating alkali metal salts of benzenecarboxylic acids are dissolved in water, organic water-soluble solvents, or mixtures thereof, and the undissolved impurities present in the solutions are filtered off.  Suitable organic solvents are those which are completely miscible with water and boil at temperatures up to 150 DEG C. at normal pressure. The dissolving and filtration may be carried out under pressure at temperatures up to 150 DEG  C. or up to the critical temperature of the solvent, in order to obtain a concentrated solution.  If dilute solutions are used these should be concentrated, e.g. by evaporation.  The dialkali metal terephthalate solution obtained should be a saturated one at, at least 0 DEG  C., preferably at 70 DEG  or 100 DEG  C. or at its critical temperature. The concentration may be carried out until the dialkali metal terephthalate is precipitated in the evaporator.  The solution is preferably cooled to e.g. 50-10 DEG  C. before the separation of the precipitated dialkali metal terephthalate, by various filters or centrifuges, and then washed with a little solvent.  Alternatively the terephthalate may be dissolved in water or a water/solvent mixture, and then precipitated by the addition of an organic solvent, in certain cases after concentration of the solution. The precipitation may also be carried out fractionally.  The mother liquor of concentrated alkali metal carbonate solution and unreacted starting materials and by-products may be used to dissolve the carboxylic acid whose salts can be used for thermal conversion into terephthalic acid, without previous purification. Various methods of obtaining the terephthalic acid from the purified dialkali metal salt are described.  The salt may also be directly converted into the acid chloride or esterified with dimethyl sulphate.  An example is given of the purification of crude dipotassium terephthalate obtained by the thermal re-arrangement of potassium benzoate and the liberation of terephthalic acid from the pure terephthalate, by dissolving the same in water and converting it into the monopotassium terephthalate with carbon dioxide under pressure.  The precipitate so obtained is then treated with benzoic acid.  Potassium benzoate is formed as a by-product, and small amounts of the potassium salts of humic acids and trimesic acids.  The concentrated potash solution of the remaining mother liquor may then be reacted with benzoic acid also to form potassium benzoate.  Alternatively the monopotassium terephthalate may be disproportionated with water at elevated temperature and free terephthalic acid and dipotassium terephthalate formed.  Specifications 744,721, 747,204, 762,448, 762,644, 777,497 and 777,609 are referred to.