A process for preparing an isocyanate comprising the steps of (i) reacting a stream E1 comprising a primary amine with a phosgene-comprising stream P16 to form a product stream P1, which comprises said isocyanate, hydrogen chloride, unreacted phosgene, and carbon oxides; (ii) separating the product stream P1 into a liquid product stream P2 comprising said isocyanate and a gaseous product stream P3 comprising hydrogen chloride, unreacted phosgene, and carbon oxides; (iii) mixing the product stream P3 comprising hydrogen chloride, unreacted phosgene, and carbon oxides with a gaseous product stream P4 comprising oxygen to form a gaseous mixed stream P5; (iv) oxidizing the gaseous mixed stream P5 on a catalyst to form a gaseous product stream P6 comprising hydrogen chloride, carbon dioxide, excess oxygen, chlorine, and water; (v) partially to completely removing the hydrogen chloride and water from P6 as a stream P7 comprising hydrochloric acid to form a product stream P8 depleted in hydrogen chloride and water; (vi) separating the product stream P8 into a liquid chlorine-rich product stream P9, which comprises chlorine, carbon dioxide, and oxygen, and a gaseous, low-chlorine product stream P10, which comprises the residual amount of chlorine from P8 not contained in P9, carbon dioxide, and oxygen; (vii) dividing the gaseous product stream P10 into a gaseous purge stream P11 and a gaseous product stream P12; (viii) treating the purge stream P11 with an aqueous base E3 to form a gaseous purge stream P13 and a liquid waste water stream P14; (ix) mixing the gaseous stream P12 with a gaseous stream E4 comprising oxygen to form the gaseous product stream P4 which comprises oxygen and is employed in step (iii); (x) evaporating the liquid chlorine-rich product stream P9, mixing the gaseous chlorine-rich product stream obtained in this way with carbon monoxide E5 and with chlorine E6, and reacting said mixture obtained in this way to give a gaseous product stream P15 comprising phosgene and carbon oxides; (xi) separating the gaseous product stream P15 in a separating device I into a phosgene-rich product stream P16, which is employed in step (i) and comprises phosgene and carbon oxides, and into a low-phosgene purge stream P17 which comprises the residual amount of the phosgene from P15 which is not contained in P16 and carbon oxides; wherein in step (vi) the separation of the product stream P8 into a liquid chlorine-rich product stream P9 and into a gaseous low-chlorine product stream P10 is carried out by cooling P8 to a temperature of from -10 °C to -80 °C under an absolute pressure of from 1 bar to 30 bar without subsequent distillation. (i) reacting a stream E1 comprising a primary amine with a phosgene-comprising stream P16 to form a product stream P1, which comprises said isocyanate, hydrogen chloride, unreacted phosgene, and carbon oxides; (ii) separating the product stream P1 into a liquid product stream P2 comprising said isocyanate and a gaseous product stream P3 comprising hydrogen chloride, unreacted phosgene, and carbon oxides; (iii) mixing the product stream P3 comprising hydrogen chloride, unreacted phosgene, and carbon oxides with a gaseous product stream P4 comprising oxygen to form a gaseous mixed stream P5; (iv) oxidizing the gaseous mixed stream P5 on a catalyst to form a gaseous product stream P6 comprising hydrogen chloride, carbon dioxide, excess oxygen, chlorine, and water; (v) partially to completely removing the hydrogen chloride and water from P6 as a stream P7 comprising hydrochloric acid to form a product stream P8 depleted in hydrogen chloride and water; (vi) separating the product stream P8 into a liquid chlorine-rich product stream P9, which comprises chlorine, carbon dioxide, and oxygen, and a gaseous, low-chlorine product stream P10, which comprises the residual amount of chlorine from P8 not contained in P9, carbon dioxide, and oxygen; (vii) dividing the gaseous product stream P10 into a gaseous purge stream P11 and a gaseous product stream P12; (viii) treating the purge stream P11 with an aqueous base E3 to form a gaseous purge stream P13 and a liquid waste water stream P14; (ix) mixing the gaseous stream P12 with a gaseous stream E4 comprising oxygen to form the gaseous product stream P4 which comprises oxygen and is employed in step (iii); (x) evaporating the liquid chlorine-rich product stream P9, mixing the gaseous chlorine-rich product stream obtained in this way with carbon monoxide E5 and with chlorine E6, and reacting said mixture obtained in this way to give a gaseous product stream P15 comprising phosgene and carbon oxides; (xi) separating the gaseous product stream P15 in a separating device I into a phosgene-rich product stream P16, which is employed in step (i) and comprises phosgene and carbon oxides, and into a low-phosgene purge stream P17 which comprises the residual amount of the phosgene from P15 which is not contained in P16 and carbon oxides;