【正文】 on techniques. These include cryogenic processes such as partial condensation or washing with liquid methane。 synthesis proper at 8, from the synthesis mixture output by step 6, pressed at 9。 treatment of the hydrogen sulphide output by step 2, at 11, by a Claus process to produce sulphur, and various heat exchangers 12 to 14. In the prior art, the residual gas from the nitrogenwashing step 6 is burned. A process for production of carbon monoxide, which makes it possible in the particular case above to utilize the residual gas rich in carbon monoxide is known from document EPA0,092,770. The residual gas is then used as feed mixture for the carbon monoxide production process. One of the processes described in that patent application includes a step of washing with methane in order to separate the hydrogen from a mixture also prising methane, carbon monoxide and nitrogen. The condensate thus produced is distilled in a first distillation column in order to separate methane from the rest of the mixture. The gas from the head of this first column, mainly containing carbon monoxide, is distilled in a second distillation column in order to extract the nitrogen and the hydrogen remaining, the pure carbon monoxide being drawn off from the base of this column. The carbon monoxide is then used as a coolant for the head condensers of the first and second columns. A process for production of carbon monoxide from the residual gas, rich in carbon monoxide, output by a nitrogenwashing unit is also described in Patent Application EPA0 676 373. To date, it has thus been possible to produce carbon monoxide by using the residual gas from a nitrogenwashing unit as the feed mixture for a separate carbon monoxide production unit, thus utilizing this residue which in the past was simply burned. SUMMARY OF THE INVENTION The object of the present invention is to provide a process for the production of carbon monoxide with reduced energy consumption and lower investment cost. Another object is to provide carbon monoxide having high purity suitable for other production processes. A further object is to provide such a process which can be integrated with the purification of the hydrogen intended for the production of the ammonia synthesis mixture. An additional object of the invention is to provide a plant for implementing such a process. To this end, the present invention relates to a process for bined production of an ammonia synthesis mixture and carbon monoxide, of the type prising the production of an ammonia synthesis mixture ( +) from, on the one hand, hydrogen under high pressure purified, in a final step, by washing with nitrogen in a washing column and, on the other hand, a stream of nitrogen at high pressure, characterized by the steps consisting in: drawing off a first liquid fraction from the base of the nitrogenwashing column which is rich in carbon monoxide and lean in residual constituents including nitrogen, hydrogen and methane, and a second liquid fraction at an intermediate position between the base and head of the washing column, which is rich in nitrogen and lean in carbon monoxide, purifying, cryogenically, the carbon monoxide from the first fraction by a first separation under medium pressure in a stripping column separating the residual hydrogen at the head, and a second distillation under low pressure in a distillation column separating the carbon monoxide and the residual nitrogen at the head from the base mixture resulting from the first separation in the stripping column. The invention also relates to a plant for the bined production of an ammonia synthesis mixture and carbon monoxide, of the type prising cryogenic means for separating the constituents of a gas mixture containing essentially hydrogen and carbon monoxide as well as other constituents including nitrogen and methane, including a heat exchanger, a source capable of providing a stream of nitrogen at high pressure and a column for washing with liquid nitrogen, characterized in that: the nitrogenwashing column prises means for drawing off liquid from a position intermediate between the base and the head of the column, and the plant furthermore prises a mediumpressure stripping column provided with boiling means, producing a liquid fraction rich in carbon monoxide at the base, and hydrogen at the head, a line provided with expansion means, for conveying the base liquid fraction from the nitrogenwashing column to the stripping column, a lowpressure distillation column provided with boiling means and with a head condenser, producing a fraction rich in methane at the base and a fraction rich in carbon monoxide at the head, a line provided with expansion means, for conveying the base liquid fraction from the stripping column to an intermediate position on the distillation column. The inventors have demonstrated that it is possible to produce carbon monoxide with suitable purity not by utilizing the residual gas output by the conventional final step of purifying hydrogen with a view to the production of an ammonia synthesis mixture, but in parallel with the production of this ammonia synthesis mixture by directly treating the liquid obtained at the base in the nitrogenwashing step. They have thus demonstrated that it is possible to produce carbon monoxide, during the cryogenic purification of hydrogen intended for the production of ammonia, from the residue output by the nitrogen washing, without it being necessary to press and warm it, as was the case in the prior art. The inventors have shown, unexpectedly, that by degrading the (yield) efficiency of carbon monoxide at the initial step of its purification, carbon monoxide with purity suitabl