Patent ID: 8765991

Claim:
A process for preparing an isocyanate comprising the steps of (i) reacting a stream E 1 comprising a primary amine with a phosgene-comprising stream P 16 to form a product stream P 1 , which comprises said isocyanate, hydrogen chloride, unreacted phosgene, and carbon oxides; (ii) separating the product stream P 1 into a liquid product stream P 2 comprising said isocyanate and a gaseous product stream P 3 comprising hydrogen chloride, unreacted phosgene, and carbon oxides; (iii) mixing the product stream P 3 with a gaseous product stream P 4 comprising oxygen to form a gaseous mixed stream P 5 ; (iv) oxidizing the gaseous mixed stream P 5 on a catalyst to form a gaseous product stream P 6 comprising hydrogen chloride, carbon dioxide, excess oxygen, chlorine, and water; (v) partially to completely removing the hydrogen chloride and water from stream P 6 as a stream P 7 comprising hydrochloric acid to form a product stream P 8 depleted in hydrogen chloride and water; (vi) separating the product stream P 8 into a liquid chlorine-rich product stream P 9 , which comprises chlorine, carbon dioxide, and oxygen, and a gaseous, low-chlorine product stream P 10 , which comprises the residual amount of chlorine from P 8 not contained in P 9 , carbon dioxide, and oxygen; (vii) dividing the gaseous product stream P 10 into a gaseous purge stream P 11 and a gaseous product stream P 12 ; (viii) treating the purge stream P 11 with an aqueous base E 3 to form a gaseous purge stream P 13 and a liquid waste water stream P 14 ; (ix) mixing the gaseous stream P 12 with a gaseous stream E 4 comprising oxygen to form the gaseous product stream P 4 which comprises oxygen and is employed in step (iii); (x) evaporating the liquid chlorine-rich product stream P 9 to form a gaseous chlorine-rich product stream and mixing the gaseous chlorine-rich product stream with carbon monoxide E 5 and with chlorine E 6 to give a mixture, and reacting said mixture to give a gaseous product stream P 15 comprising phosgene and carbon oxides; (xi) separating the gaseous product stream P 15 in a separating device I into a phosgene-rich product stream P 16 , which is employed in step (i) and comprises phosgene and carbon oxides, and into a low-phosgene purge stream P 17 which comprises the residual amount of the phosgene from P 15 which is not contained in P 16 and carbon oxides; wherein in step (vi) the separation of the product stream P 8 into a liquid chlorine-rich product stream P 9 and into a gaseous low-chlorine product stream P 10 is carried out by cooling P 8 to a temperature of from −10° C. to −80° C. under an absolute pressure of from 1 bar to 30 bar without subsequent distillation.