【文章內(nèi)容簡(jiǎn)介】 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 suitable for its conventional applications can in the end be obtained with savings in energy and investment cost. BRIEF DESCRIPTION OF THE DRAWINGS An illustrative embodiment of the invention will now be described with reference to the appended drawings, wherein: FIG. 1 is a conventional process of producing carbon monoxide。 FIG. 2, which schematically represents a plant for bined production of an ammonia synthesis mixture and carbon dioxide according to the invention。 and FIG. 3 represents an alternative embodiment of a plant for bined production of an ammonia synthesis mixture and carbon dioxide according to the invention. DETAILED DESCRIPTION OF THE INVENTION The plant represented in FIG. 2 replaces block 6 in FIG. 1. It is intended, on the one hand, for the purification of hydrogen from a feed gas mixture 50 containing essentially hydrogen and carbon monoxide as well as other constituents including methane and nitrogen, in order to obtain a synthesis mixture that can be used for the production of ammonia and, on the other hand, for the production of carbon monoxide of high purity (at least 98%) directly from the residue output by the preceeding hydrogen purification. The position of the feed mixture of the plant 6 may vary depending on the type of process employed for generating the ammonia synthesis gas (steam reforming, partial oxidation, etc.). The pressures referred to below are absolute pressures. The plant 6 essentially prises a heat exchanger 60 of the indirect countercurrent type, a liquidnitrogen washing column 61, a stripping c