Title of Invention: PROCESS FOR PREPARING AN ISOCYANATE Abstract: The industrial scale preparation of isocyanates gives rise to distillation bottom streams that are in need of further workup. These distillation bottom streams still also contain, as well as a distillation residue consisting of compounds that can be evaporated without decomposition only with great difficulty if at all (also referred to as residue for short), proportions of the desired target product (i.e. isocyanate to be prepared). The present invention relates to a process that enables recovery, in an efficient manner, of this proportion of isocyanate to be prepared in a distillation bottom stream which is obtained in the workup of the crude liquid process product which contains the isocyanate to be prepared and is formed in an isocyanate preparation process. More particularly, the present invention relates to a drying step in which the isocyanate to be prepared is recovered to form a solid material that has been largely to completely freed of this isocyanate. This drying step is characterized by a minimum content of compounds containing carbodiimide groups of 15% by mass, preferably 20% by mass, more preferably 30% by mass, based on the total mass of the distillation residue fed to the drying apparatus used, where this minimum content can also be adjusted by an in situ carbodiimidization. Detailed Description: The industrial scale preparation of isocyanates affords distillation bottom streams that are in need of further workup. These distillation bottom streams contain not only a distillation residue consisting of compounds evaporable without decomposition only with great difficulty, if at all (also referred to as residue for short), but also proportions of the desired target product (i.e. the isocyanate to be prepared). The present invention relates to a process that enables recovery, in an efficient manner, of this proportion of isocyanate to be prepared in a distillation bottom stream obtained in the workup of the crude liquid process product containing the isocyanate to be prepared which is formed in an isocyanate preparation process. More particularly, the present invention relates to a drying step in which the isocyanate to be prepared is recovered to form a solid that has been freed largely to completely of this isocyanate. This drying step is characterized by a minimum content of compounds containing carbodiimide groups of 15% by mass, preferably 20% by mass, more preferably 30% by mass, based on the total mass of the distillation residue supplied to the drying apparatus used, where this minimum content can also be adjusted by in situ carbodiimidization. The prior art for treatment of distillation bottom streams from isocyanate preparation that comprise the distillation residues mentioned describes various processes. General aims of what is called “residue treatment” are the maximization of the isocyanate yield, the minimization of the amount of residue obtained that has been largely to entirely freed of isocyanate, operationally reliable configuration of the residue treatment and the best possible inexpensive and simple utilization of the residue volume no longer utilizable for the isocyanate preparation process. The following processes are known in principle: In principle, a distillate bottom stream comprising the distillation residue can be combusted continuously or batchwise. The process is technically simple and can be used to raise useful steam if a plant for thermal utilization which is suitable for the purpose exists in the vicinity of the isocyanate production plant, in order to assure disposal via a pipeline connection. However, the great disadvantage of this process is the loss of yield, caused by the fact that the distillation bottom stream comprising the distillation residue always also contains proportions of the isocyanate product of value which is burnt as well. If the distillation of the isocyanate were to be conducted in such a way that the isocyanate were to be removed completely or virtually completely from the distillation bottoms, what would remain would be a solid distillation bottoms phase that could be processed only with great difficulty and consisted almost exclusively of residue. In order to avoid this, distillation conditions are typically chosen such that the bottom product from the distillation column remains liquid, but this is only possible if it still contains a substantial proportion of the isocyanate to be prepared, which is thus inevitably sent to the combustion as well. To minimize the isocyanate yield losses, the distillation bottom stream can be transferred into a stirred and heated vessel and mixed with high-boiling hydrocarbons, preferably bitumen, that are inert under the distillation conditions to distill off a maximum amount of the free isocyanate still present in the residue (EP 0 548 685 A2). The remaining residue that has been largely freed of isocyanate can be discharged as a free-flowing solid and sent to incineration. Disadvantages of this process include not only the use of a substance foreign to the process (bitumen) but also yield losses through polymerization of the isocyanate since the process includes high residence times at high temperature. A further process for residue treatment, described in EP 0 626 368 A1, is characterized by the use of heated, product-agitating vacuum driers with a horizontal shaft. The use of bitumen, for example, has the effect that, as in the abovementioned example of the process according to EP 0 548 685 A2, the remaining residue is obtained as a free-flowing solid that can be used as a fuel in cement works for example. The advantage of this process over that mentioned above is an increase in yield. However, the use of mechanical moving parts harbors the risk of elevated wear and associated maintenance expenditure. This is especially true when the residue to be worked up has a very high viscosity. The use of drying apparatuses of this kind with mechanical moving parts, even though it is basically tried and tested, is therefore not always free of challenges in everyday operation. The present invention is concerned with such challenges inter alia. The patent literature also describes processes in which isocyanate distillation bottom streams comprising distillation residues are chemically converted in order to obtain industrially utilizable materials of value, for example the reaction of a distillation bottom stream comprising residue from the preparation of tolylene diisocyanate with alkanolamine (U.S. Pat. No. 5,902,459) or with isocyanates of the diphenylmethane series (DE 42 11 774 A1, U.S. Pat. No. 3,694,323). The hydrolysis of isocyanate distillation bottom streams with water to achieve recovery of the starting amine, especially in the preparation of tolylene diisocyanate (TDI hereinafter), is a field that has been worked on for some long time already and is described in U.S. Pat. Nos. 3,128,310, 3,331,876, GB 795,639, DE 27 03 313 A1 and EP 1 935 877 A1 for example. What is unsatisfactory in these processes is that a portion of the isocyanate product of value has to be hydrolyzed back to the starting material and phosgenated again. This does send the isocyanate present in the distillation bottom stream to a viable physical use, but it would be desirable to be able to recover the isocyanate as such from the distillation bottom streams. EP 1 413 571 A1 and EP 1 371 633 A1 are concerned with the optimization of the workup of TDI by using a dividing-wall column in the distillation which results, inter alia, in a reduction in the TDI content in the bottom product. But here too, it is not possible to completely prevent an isocyanate-containing distillation bottom stream from being obtained. EP 0 017 972 A1 describes a process for separating TDI and/or higher-boiling solvents from distillation bottom streams that arise in the preparation of TDI by phosgenation of tolylenediamine by evaporative concentration in a fluidized bed at temperatures of 140 to 280° C. In this process, the droplets of the distillation bottom stream that have been introduced into the fluidized bed vessel by means of the introduction device are sprayed onto the surface of the initially charged particles, where they spread, which leads to evaporation of the product of value (TDI and/or solvent) and to formation of dish-shaped pellets of residue that are free of material of value. Such a granulation process generally does not have cycle times of any great length and has to be run down for the purpose of intermediate cleaning after particular time intervals. This is disadvantageous for the present case of workup of isocyanate-containing distillation bottom streams owing to the required inertization of the reaction space and the high temperatures, and the running-up problems. WO 522.893.4831 A1 is concerned with a spray drying process for obtaining monomeric isocyanate (i.e. the isocyanate to be prepared, which is desired as the target product, as opposed to unwanted high molecular weight polymers containing isocyanate groups) from distillation residue-containing bottom streams. The spray drying described affords a dried residue that has been largely to completely freed of monomeric isocyanate and a stream comprising monomeric isocyanate. Performance of this process requires a special reactor, and so it generally cannot be integrated into an existing isocyanate production plant without major refitting. Further improvements in the field of workup of isocyanate distillation bottom streams comprising not only distillation residues but also proportions of the isocyanate to be prepared are therefore desirable. More particularly, it would be desirable to separate the desired target product, i.e. the isocyanate to be prepared, from the distillation bottom stream in a simple, operationally stable and economic manner in order to minimize yield losses and maintenance expenditure in continuous operation on the industrial scale as well, specifically also when the composition of the distillation bottom stream containing the residue is subject to production-related variations. It would also be desirable if such an improved process could be integrated in a simple manner into an already existing process that uses prior art drying apparatus (e.g. the heated, product-agitating vacuum driers with a horizontal shaft that have been mentioned). Taking account of the above, the present invention relates to the workup of a distillation bottom stream consisting of the isocyanate to be prepared, optionally low boilers (especially solvent) and distillation residue, wherein said distillation bottom stream comes from the workup of a liquid crude process product which comprises the isocyanate to be prepared and is obtained by phosgenating the primary amine corresponding to the isocyanate to be prepared, wherein the workup of the distillation bottom stream comprises the following steps: 1) optionally pre-concentrating the distillation bottom stream in an evaporator by partially evaporating the isocyanate to be prepared which is present in the distillation bottom stream to obtain a pre-concentrated liquid stream depleted of isocyanate to be prepared; 2) drying the distillation bottom stream or the pre-concentrated liquid stream depleted of isocyanate to be prepared which has been obtained in step 1) in a drying apparatus at a temperature in the range from 150° C. to 500° C., preferably in the range from 185° C. to 300° C., more preferably in the range from 200° C. to 270° C., with evaporation and recovery of isocyanate to be prepared to form a solid process product, where the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue supplied to the drying apparatus is adjusted to a value of at least 15%, preferably at least 20%, more preferably at least 30%. Completely surprisingly, it has been found that the procedure of the invention of establishing a minimum content of compounds containing carbodiimide groups in drying step 2) can reduce or even prevent increased mechanical stress from the drying apparatus (e.g. rise in torque in drying apparatuses with a shaft) which is associated—as a result of the unavoidable rise in viscosity—with the thorough drying of the distillation residue, and this has a positive effect on the operational stability of the drying apparatus and hence on the operational stability of the overall process. More particularly, the present invention provides a process for preparing an isocyanate, comprising the following steps: a) phosgenating the primary amine corresponding to the isocyanate to be prepared to obtain a liquid crude process product comprising the isocyanate to be prepared and a hydrogen chloride-comprising gaseous crude process product, with optional use of a solvent; b) working up the liquid crude process product obtained in step a), comprising the steps of: b.1) optionally separating dissolved phosgene and dissolved hydrogen chloride from the liquid crude process product obtained in step a) to obtain a phosgene- and hydrogen chloride-depleted liquid process product; b.2) optionally separating solvent from the liquid crude process product obtained in step a) or from the phosgene- and hydrogen chloride-depleted liquid process product obtained in step b) to obtain a phosgene-, hydrogen chloride- and solvent-depleted liquid process product; b.3) distilling the liquid crude process product obtained in step a) or the phosgene- and hydrogen chloride-depleted liquid process product obtained in step b.1) or the phosgene-, hydrogen chloride- and solvent-depleted liquid process product obtained in step b.2) to obtain a distillate stream comprising a first portion of the isocyanate to be prepared and a distillate bottom stream consisting of distillation residue, a second portion of the isocyanate to be prepared and optionally low boilers, especially solvent; c) working up the distillation bottom stream obtained in step b.3), comprising the steps of: c.1) optionally pre-concentrating the distillation bottom stream obtained in step b) in an evaporator by partially evaporating the second portion of the isocyanate to be prepared which is present in the distillation bottom stream obtained in step b) to obtain a pre-concentrated liquid stream depleted of isocyanate to be prepared; c2) drying the distillation bottom stream obtained in step b) or the pre-concentrated liquid stream depleted of isocyanate to be prepared which has been obtained in step c.1) in a drying apparatus at a temperature in the range from 150° C. to 500° C., preferably in the range from 185° C. to 300° C., more preferably in the range from 200° C. to 270° C., with evaporation and recovery of isocyanate to be prepared to form a solid process product, where the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue supplied to the drying apparatus is adjusted to a value of at least 15%, preferably at least 20%, more preferably at least 30%. Step c) of the process for preparing an isocyanate corresponds here to the above-described workup of a distillation bottom stream, and steps c.1) and c.2) of the process for preparing an isocyanate correspond to steps 1) and 2) of said workup of a distillation bottom stream. In addition, step b) of the process for preparing an isocyanate corresponds to the workup of the liquid crude process product which comprises the isocyanate to be prepared and is obtained by phosgenating the primary amine corresponding to the isocyanate to be prepared, from which the distillation bottom stream originates. The invention is elucidated more particularly hereinafter with reference to the overall process comprising steps a) to c). This is done for the sake of simplicity and should not be understood such that these elucidations have an automatic limiting effect on the overall process. In the context of the present invention, “primary amine corresponding to the isocyanate to be prepared” is considered to mean that amine that gives the isocyanate to be prepared on conversion of all NH2 groups present to NCO groups. According to the invention, the “distillation bottom stream” consists of at least the isocyanate to be prepared and the distillation residue defined hereinafter. In addition, the distillation bottom stream may also comprise the low boilers defined hereinafter. According to the invention, the distillation bottom stream accordingly consists of two constituents (isocyanate to be prepared and distillation residue) or three constituents (isocyanate to be prepared, low boilers and distillation residue). In the context of the present invention, “distillation residue” is understood to mean that proportion of the distillation bottom stream obtained in the distillation from step b.3) that can be assigned neither to the isocyanate to be prepared nor—if any are present at all—to low boilers, “low boilers” being understood to mean all substances or azeotropically boiling mixtures of substances having a lower boiling point than the isocyanate to be prepared, or, if this isocyanate is in the form of an isomer mixture, lower than the lowest-boiling isomer of the isocyanate to be prepared (i.e., in the case of TDI, 2,6-TDI). Low boilers in this context are especially any solvent used from step a). Low boilers in the sense of low-boiling secondary components are generally either not present at all in the distillation bottom stream from step b.3) or are present at most in trace contents of up to 0.10% by mass, preferably up to 0.010% by mass, more preferably 0.0010% by mass, based on the total mass of the distillation bottom stream obtained in step b.3). The distillation residue contains compounds that do not evaporate under the pressure and temperature conditions chosen for step b.3) or that do not evaporate without chansamantha@example.org. In order that the distillation bottom stream retains good processibility with regard to its flow properties (i.e. does not become too viscous or even firm), the isocyanate to be prepared is not distilled off completely in step b.3), and so a portion thereof remains in the distillation bottom stream. It is likewise possible not to completely distill off any solvent still present (from step a)) in step b.3), and to permit a residual solvent content in the distillation bottom stream of up to 10% by mass, preferably of up to 1.0% by mass, based on the total mass of the distillation bottom stream obtained in step b.3). The compounds in the distillation residue that are evaporable with difficulty, if at all—if they are not impurities from the primary amines used that run through the phosgenation process unchanged—are phosgenation products of high molecular weight, the structure of which is still not exactly known. For instance, they may be compounds that can be derived (in a formal sense) from polymerization products of the amine used by replacement of the unpolymerized amine groups by isocyanate groups. Phosgenation products of higher molecular weight may also partly form (by further reaction) in step b). According to the invention, in this context, phosgenation products of higher molecular weight are also considered to mean “compounds containing carbodiimide groups” which likewise accumulate in the distillation bottom stream and should be considered as a constituent of the distillation residue in the terminology of the present invention. As is known to the person skilled in the art, carbodiimides are a substance group of organic compounds characterized by the structural feature R—N═C═N—R′ where R and R′ denote organic radicals. The phosgenation of primary amines can give rise to such compounds from the isocyanate to be prepared via elimination of carbon dioxide. In the case of a diamine, the simplest conceivable carbodiimide forms via the formation of an “—N═C═N—” bridge between two diisocyanate molecules. In this case, the two R and R′ radicals in turn still contain free NCO functions. If these react further with isocyanate to be prepared or further carbodiimides having free NCO functions with elimination of CO2, structures are formed with multiple “—N═C═N—” bridges and high molar masses. The “proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue supplied to the drying apparatus” in step c.2) is thus formed by addition of up to three contributions, namely from (i) the mass of compounds containing carbodiimide groups present in the starting material from step c.2) (i.e. the distillation bottom stream obtained in step b) or the pre-concentrated liquid stream obtained in step c.1)), (ii) the mass of any added compounds containing carbodiimide groups and (iii) the mass of any compounds containing carbodiimide groups that are formed in situ in step c.2). According to the invention, care should be taken that the sum total of (i), (ii) and (iii) divided by the total mass of the distillation residue supplied to the drying apparatus from step c.2) is at least 0.15, preferably at least 0.20, more preferably at least 0.30 (corresponding to at least 15% by mass, preferably at least 20% by mass, more preferably at least 30% by mass). In the case of a continuous process regime, the corresponding hourly mass flow rates form the basis. It is sufficient when just one of the three components (i) to (iii) is non-zero and is sufficiently large that the requirements of the invention are fulfilled. It is possible, for example, to keep the proportion of compounds containing carbodiimide groups to be attributed to (i) and (iii) at zero (or close to zero) by suitable choice of process bonds (see further details further down), and to fulfill the requirements of the invention solely through addition of compounds containing carbodiimide groups from an external source. But it is also possible to dispense with such an external addition according to (ii) and to fulfill compliance with the requirements of the invention on the proportion by mass of compounds containing carbodiimide groups solely via the proportion according to (i) and/or (iii). In principle, it is accordingly possible to distinguish between three cases that will be elucidated in more detail further down: A. establishing process conditions that promote the formation of compounds containing carbodiimide groups in step a) and/or in step b) and/or—if conducted—in step c.1); B. preparing compounds containing carbodiimide groups from the isocyanate to be prepared in a separate operation (e.g. by thermal or catalytically induced carbodiimidization of the isocyanate to be prepared) and supplying the compounds containing carbodiimide groups thus prepared to the drying apparatus from step c.2); C. forming compounds containing carbodiimide groups in situ (for example by heating prior to the actual drying or performance of the drying at relatively high pressure) in step c.2). Cases A, B and C can occur either individually or in conjunction with one another. It is possible, in case A, to choose the process conditions in step a) and/or in step b) and/or—if conducted—in step c.1) such that the mass of compounds containing carbodiimide groups introduced into this step from step c.2) via the starting material is sufficiently great that the requirements of the invention on the minimum proportion by mass of containing carbodiimide groups in the distillation residue are fulfilled. In this case, external preparation or in situ preparation of compounds containing carbodiimide groups is then unnecessary. It is likewise possible, in case B, to prepare compounds containing carbodiimide groups in a separate operation from the isocyanate to be prepared, and to supply them to step c.2), in such an amount that the requirements of the invention on the minimum proportion by mass of containing carbodiimide groups in the distillation residue are fulfilled. In this case, the establishment of process conditions according to case A or in situ preparation of compounds containing carbodiimide groups is then unnecessary. It is likewise possible, in case C, to form the entire amount of compounds containing carbodiimide groups required to fulfill the requirements of the invention on the minimum proportion by mass of compounds containing carbodiimide groups in the distillation residue in situ in step c.2). In this case, the establishment of process conditions according to case A or external preparation of compounds containing carbodiimide groups is then unnecessary. However, it is also possible to establish the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue that is required in accordance with the invention by bringing about a combination of two or three of cases A, B and C mentioned. Suitable process regimes for case A are elucidated in detail further down. To verify whether the conditions chosen for step a) and/or for step b) and/or—if conducted—for step c.1) are suitable, a determination of the proportion by mass of the compounds containing carbodiimide groups in the starting material to be dried from step c.2) is required. For this purpose, the person skilled in the art is familiar with various analytical methods, all of which can be employed in principle. A crucial factor for assessing whether the requirements of the invention on the minimum proportion by mass of compounds containing carbodiimide groups in the distillation residue are fulfilled, however, is the procedure outlined hereinafter (“Measurement methods I and II”): First of all, the proportion by mass of compounds containing carbodiimide groups in the starting material to be dried which is sent to step c.2) is determined. This is accomplished by means of IR spectroscopy as outlined hereinafter (“Measurement method I”). Measurement Method I: A sample of the starting material to be examined from step c.2) that has been cooled down to ambient temperature is dissolved in ortho-dichlorobenzene (ODB hereinafter) (the proportion by mass of the sample of the starting material from step c.2), based on the total mass of the solution in ODB to be prepared, is set here to 20%) and divided in a ratio of 1:1. A portion of the dissolved sample is boiled under reflux at ambient pressure for 15 min (heating temperature 190° C.) and then cooled indirectly in a water-ice bath. The boiling results in cleavage of uretonimine, the form in which carbodiimide is present at room temperature, back to carbodiimide and isocyanate. The other portion of the dissolved sample remains unchanged. The heat-treated sample is introduced into a cuvette (path length of the sample in the cuvette 150 to 200 μm) and placed into the measurement beam path of an IR spectrometer. The non-heat-treated solution is introduced into a cuvette with the same layer thickness as the measurement cuvette and placed into the comparative beam path of the IR spectrometer. An IR spectrum of the range from 2600 cm−1 to 1900 cm−1 is recorded. The extinction E at 2140 cm−1 (height of the signal) is determined in relation to the baseline in the range from 2500 cm−1 to 1900 cm−1. The uretonimine content ω(UI) in % by mass (calculated for the molar mass M(UI) [in g/mol] of the uretonimine which is formed from the addition of one mole of the isocyanate to be prepared onto one mole of the carbodiimide formed by carbodiimidization of two moles of the isocyanate to be prepared) is calculated by the following equation: ω(UI)=[E(2140 cm−1)·M(UI)·100%]/[d·ε·a]. where M(UI)=molar mass of the uretonimine as defined above in g/mol (478 in the case of TDI as the isocyanate to be prepared); d=path length of the sample in the cuvette in cm; ε=extinction coefficient in L/mol·cm; a=concentration of the solution analyzed in g/L. Since 1 mol of the uretonimine is formed from 1 mol of the carbodiimide and one mole of the isocyanate be prepared, the proportion by mass of uretonimine thus ascertained can easily be converted to the proportion by mass of the carbodiimide ω(CD): ω(CD)=ω(UI)·M(CD)/M(UI); where M(CD) corresponds for the molar mass in g/mol of the carbodiimide formed from 2 mol of the isocyanate to be prepared (304 in the case of TDI as isocyanate to be prepared). The value for ω(CD) thus ascertained corresponds to the proportion by mass of compounds containing carbodiimide groups in the starting material supplied to step c.2). According to the invention, the reference parameter for the quantification of the proportion by mass of compounds containing carbodiimide groups is the total mass of the distillation residue supplied to the drying apparatus from step c.2). To determine the proportion by mass of compounds containing carbodiimide groups as thus defined, quantification of the proportion by mass of the distillation residue in the starting material supplied to step c.2) is required. The procedure for this purpose is as follows (“Measurement method II”): Measurement Method II: A weighed amount of a sample of the starting material to be supplied to step c.2) is initially charged in a laboratory distillation apparatus equipped with a droplet trap (Reitmayer distillation attachment) and distilled at a pressure of 1.0 mbar and at a heating temperature of 250° C. After 30 minutes (measured from the juncture at which pressure and temperature have attained the aforementioned target values), the heating is switched off and the apparatus is cooled down. The distillate obtained and the undistilled proportion of the sample remaining in the charge flask of the distillation apparatus are weighed. In the terminology of the present invention, the undistilled proportion of the sample is considered to be the distillation residue in the context of the invention. By comparison with the known mass of the sample initially charged in the distillation apparatus, the proportion by mass of distillation residue is determined quantitatively in the starting material to be supplied to step c.2). With this result and the result for ω(CD) from measurement method I, it is easily possible to calculate the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue. Once suitable conditions have been fixed in this way for step a) and/or for step b) and/or—if conducted—for step c.1), it is possible in principle to dispense with analysis of the starting material from step c.2) in continuous operation. However, it is advisable to conduct control tests to verify the result once ascertained from time to time, in continuous mode especially at least 1× every 720 h, preferably at least 1× every 360 h, more preferably at least 1× every 180 h, most preferably at least 1× every 90 h. If case A applies and the process conditions in step a) and/or in step b) and/or—if conducted—in step c.1) are chosen such that the mass of compounds containing carbodiimide groups introduced into this step via the starting material from step c.2) is already sufficiently high to meet the requirements of the invention on the minimum proportion by mass of containing carbodiimide groups in the distillation residue, it is possible to dispense with any measurement of the proportion of compounds containing carbodiimide groups that originate from (iii) since it does not matter whether or not further compounds containing carbodiimide groups are formed in situ in step c.2) (no reduction in the proportion of compounds containing carbodiimide groups is to be expected chemically in step c.2)). In this case, any addition of compounds containing carbodiimide groups as per (ii) can of course also be dispensed with. If this case does not apply, in accordance with the invention, it has to be ensured in some other way that compounds containing carbodiimide groups are present in a sufficient amount in step c.2). In case B, this can be accomplished by adding compounds containing carbodiimide groups from an external source. The crucial proportion by mass of compounds containing carbodiimide groups in the added material from an external source is that determined by measurement method I. According to the amount of material added and the proportion by mass of compounds containing carbodiimide groups therein, it is clear even without quantification of the distillation residue whether the demands of the invention are fulfilled (namely when the minimum proportion by mass of compounds containing carbodiimide groups required in accordance with the invention, based on the total mass of the distillation residue, has been attained even assuming that the distillation bottoms consist entirely of residue). Should a quantification of the distillation residue be necessary, the determination by measurement method II is crucial. It is likewise possible, via the choice of suitable process conditions, to ensure that compounds containing carbodiimide groups are formed in situ in step c.2). Suitable process regimes for this case C are elucidated in detail further down. To verify whether the conditions chosen for step c.2) are suitable for this purpose, the mass balance of the drying from step c.2) is monitored as follows (“measurement method III”): Measurement Method III: The drying apparatus from step c.2) is supplied with a starting material to be dried with a known proportion by mass (determined by measurement method II) of distillation residue. The distillate obtained by measurement method II in the distillation described consists of the isocyanate to be prepared and optionally low boilers (especially solvent originating from step a)). The composition of the distillate (ratio of low boilers to isocyanate to be prepared) is quantified by gas chromatography (HP-5, 30 m*320 μm*0.25 μm, 40-250° C., 1-20° C./min, TCD detector). Column: Agilent 19091J-413: 1300.52926, HP-5, 5% phenyl methyl siloxane, 325° C.: 30 m×320 μm×0.25 μm. Heating ramp: starting temperature 40° C., then 1° C./min to 45° C., then 20° C./min to 250° C., hold at 250° C. for 5 min. Elution time: 20.25 min. Detector: TCD (thermal conductivity detector). In this way, it is ascertained how much isocyanate to be prepared can be distilled out of the starting material to be dried in step c.2). In step c.2), isocyanate to be prepared is evaporated to form a solid process product and recovered. The mass (the hourly mass flow rate in a continuous process regime) of the recovered portion of the isocyanate to be prepared is ascertained and compared with the mass (with the hourly mass flow rate in a continuous process regime) of the isocyanate to be prepared which is supplied by the starting material, ascertained as described above. According to the invention, the mass differential ascertained (the differential in the hourly mass flow rate in a continuous process regime) is considered to be attributable to the formation of compounds containing carbodiimide groups, meaning that this mass differential divided by the mass of the distillation residue supplied to the drying apparatus from step c.2) gives the value essential to the invention of the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue supplied to the drying apparatus. If the amount of the compounds containing carbodiimide groups formed in situ is large enough to meet the requirements of the invention on the minimum content in the distillation residue, it is possible to dispense with the analytical determination by measurement method I described for case A. Once suitable conditions for step c.2) have been fixed in this way, it is possible in principle to dispense with constant monitoring of the content of compounds containing carbodiimide groups formed in situ during the operation of the drying apparatus used in step c.2). However, it is advisable to conduct control tests to verify the result once ascertained from time to time, in continuous mode especially at least 1× every 720 h, preferably at least 1× every 360 h, more preferably at least 1× every 180 h, most preferably at least 1× every 90 h. There follows firstly a brief summary of various possible embodiments of the invention: In a first embodiment of the invention, the phosgenation is conducted in the liquid phase in the presence of a solvent. In a second embodiment of the invention, the phosgenation is conducted in the gas phase, the phosgenation comprising a quench in which the gaseous process product formed, comprising the isocyanate to be prepared, is cooled by contacting with a quench liquid selected from the group consisting of solvent, the isocyanate to be prepared and mixtures of the isocyanate to be prepared and solvent, and the isocyanate to be prepared is liquefied. In a third embodiment of the invention, which is a particular configuration of the second embodiment, the quench liquid is selected from the group consisting of solvent and mixtures of the isocyanate to be prepared and solvent. In a fourth embodiment of the invention, which is a particular configuration of all embodiments in which the use of a solvent is envisaged, this solvent is selected from the group consisting of chlorobenzene, ortho-dichlorobenzene, para-dichlorobenzene, the isomers of trichlorobenzene, toluene, the isomers of xylene and mixtures of the aforementioned solvents. In a fifth embodiment of the invention, which is likewise a particular configuration of all embodiments in which the use of a solvent is envisaged, any low boilers present consist of solvent and secondary components, where the proportion by mass of secondary components, based on the total mass of the distillation bottom stream, is not more than 0.10% by mass. A sixth embodiment of the invention, which can be combined with all other embodiments, encompasses step 1), wherein the pre-concentration is effected at a temperature in the range from 120° C. to 180° C. and a pressure in the range from 20 mbar(abs.) to 60 mbar(abs.). In a seventh embodiment of the invention, which can be combined with all other embodiments, the drying apparatus used in step 2) is an apparatus selected from the group consisting of heated, product-agitating vacuum driers with a horizontal shaft, rotary tubes, disk driers, belt driers and pelletizing screws. In an eighth embodiment of the invention, which can be combined with all other embodiments, the process of the invention comprises the following steps: a) phosgenating the primary amine corresponding to the isocyanate to be prepared to obtain a liquid crude process product comprising the isocyanate to be prepared and a hydrogen chloride-comprising gaseous crude process product, with optional use of a solvent; b) working up the liquid crude process product obtained in step a), comprising the steps of: b.1) optionally separating dissolved phosgene and dissolved hydrogen chloride from the liquid crude process product obtained in step a) to obtain a phosgene- and hydrogen chloride-depleted liquid process product; b.2) optionally separating solvent from the liquid crude process product obtained in step a) or from the phosgene- and hydrogen chloride-depleted liquid process product obtained in step b) to obtain a phosgene-, hydrogen chloride- and solvent-depleted liquid process product; b.3) distilling the liquid crude process product obtained in step a) or the phosgene- and hydrogen chloride-depleted liquid process product obtained in step b.1) or the phosgene-, hydrogen chloride- and solvent-depleted liquid process product obtained in step b.2) to obtain a distillate stream comprising a first portion of the isocyanate to be prepared and a distillate bottom stream consisting of distillation residue, a second portion of the isocyanate to be prepared and optionally low boilers, especially solvent; wherein the distillation bottom stream obtained in step b.3) is the distillation bottom stream to be worked up in step 2) or in step 1) and step 2). In a ninth embodiment of the invention, which is especially used from particular configuration of the eighth embodiment, the adjustment of the proportion by mass of compounds containing carbodiimide groups based on the total mass of the distillation residue supplied to the drying apparatus is brought about via exclusively one, two or all of the following measures: A. establishing process conditions that promote the formation of compounds containing carbodiimide groups in step a) and/or in step b) and/or—if conducted—in step 1); B. preparing compounds containing carbodiimide groups from the isocyanate to be prepared in a separate operation and supplying the compounds containing carbodiimide groups thus prepared to the drying apparatus from step 2); C. forming compounds containing carbodiimide groups in situ in step 2). A tenth embodiment of the invention, which is a particular configuration of the ninth embodiment, includes measure A, wherein the adjustment of process conditions that favor the formation of compounds containing carbodiimide groups in step a) and/or in step b) and/or—if conducted—in step 1) is brought about by an increase in the temperature in the respective step and/or by an increase in the residence time of the process product that passes through the respective step. An eleventh embodiment of the invention, which is likewise a particular configuration of the ninth embodiment, but can also be employed together with the tenth embodiment and/or the fourteenth embodiment outlined further down, includes measure B, wherein the preparation of compounds containing carbodiimide groups from the isocyanate to be prepared in a separate operation comprises the following: heating a process product containing the isocyanate to be prepared from the preparation process of the invention in the absence of catalysts at a temperature in the range from 200° C. to 270° C. or in the presence of catalysts of the phospholene oxide at a temperature in the range from 50° C. to 150° C. In a twelfth embodiment of the invention, which is a particular configuration of the tenth and/or eleventh embodiment, especially when measure C) is dispensed with, step 2) is conducted at a pressure in the range from 10 mbar(abs.) to 250 mbar(abs.), preferably in the range from 20 mbar(abs.) to 200 mbar(abs.), more preferably in the range from 30 mbar(abs.) to 100 mbar(abs.). A thirteenth embodiment of the invention, which is likewise a particular configuration of the ninth embodiment, but can also be employed together with the tenth embodiment and/or the eleventh embodiment, includes measure C, wherein the drying in step 2) is conducted first in a first partial step 2.1) at a pressure in the range from >750 mbar(abs.) to 1013 mbar(abs.) and at a temperature in the range from 200° C. to 270° C. and then in a second partial step 2.2) at a pressure in the range from 10 mbar(abs.) 250 mbar(abs.), preferably 20 mbar(abs.) to 200 mbar(abs.), more preferably 30 mbar(abs.) to 100 mbar(abs.), and at a temperature in the range of 200° C. to 270° C., or at a pressure in the range from >250 mbar(abs.) to 750 mbar(abs.), preferably 300 mbar(abs.) to 650 mbar(abs.), more preferably 450 mbar(abs.) to 550 mbar(abs.), and a temperature in the range from 200° C. to 270° C. In a fourteenth embodiment of the invention, which can be combined with all other embodiments, the isocyanate to be prepared is selected from the group consisting of tolylene diisocyanate, naphthyl diisocyanate, pentane 1,5-diisocyanate, hexamethylene 1,6-diisocyanate, isophorone diisocyanate, xylylene diisocyanate and diisocyanatodicyclohexylmethane. The embodiments briefly outlined above and further possible configurations of the invention are elucidated in detail hereinafter. For the sake of simplicity, reference is made to the overall process comprising steps a) to c) (“eighth embodiment of the invention”), which does not mean that all possible configurations of the invention outlined hereinafter are restricted to this eighth embodiment. Various embodiments are combinable with one another as desired unless the opposite is apparent to the person skilled in the art from the context. In step a), the isocyanate to be prepared is first obtained by phosgenating the corresponding primary amine in crude form (i.e. as “the liquid crude process product comprising the isocyanate to be prepared”). The isocyanate to be prepared is preferably a diisocyanate, more preferably a diisocyanate selected from the group consisting of tolylene diisocyanate (TDI hereinafter), naphthyl diisocyanate (NDI hereinafter), pentane 1,5-diisocyanate (PDI hereinafter), hexamethylene 1,6-diisocyanate (HDI hereinafter), isophorone diisocyanate (IPDI hereinafter), xylylene diisocyanate (XDI hereinafter) and diisocyanatodicyclohexylmethane (H12-MDI hereinafter). The primary amines corresponding to the aforementioned isocyanates are tolylenediamine (TDA hereinafter), naphthylenediamine (NDA hereinafter), pentane-1,5-diamine (PDA hereinafter), hexamethylene-1,6-diamine (HDA hereinafter), isophoronediamine (IPDA hereinafter), xylylenediamine (XDA hereinafter) and diaminodicyclohexylmethane (H12-MDA hereinafter). If the amines mentioned may be present in different isomeric forms without explicit specification thereof, all isomer distributions are encompassed in accordance with the invention. If the corresponding primary amine is present as a mixture of various isomers, the isomer distribution of the isocyanate to be prepared, corresponds essentially or entirely to that of the starting amine. In principle, it is also possible to convert mixtures of the aforementioned amines, although this is generally not preferred. More preferably, the process of the invention is used for preparation of TDI by phosgenation of TDA. The exact isomer composition of the TDA used present in each case is unimportant for the process of the invention. Typically, TDA which is used with preference comprises 78.0% by mass to 82.0% by mass of 2,4-TDA and 18.0% by mass to 22.0% by mass of 2,6-TDA, based on the total mass of the 2,4- and 2,6-TDA isomers. Based on the total mass of the TDA, the 2,4- and 2,6-TDA isomers preferably account for a sum total of 95.0% by mass to 100% by mass, more preferably of 98.0% by mass to 100% by mass. The phosgenation of a primary amine to give the corresponding isocyanate is known in principle and can be conducted by any of the methods known in the prior art. Examples include the processes described in the following literature references: Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol. A 19, S. 390 ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991, G. Oertel (Ed.) Polyurethane Handbook, 2nd Edition, Hanser Verlag, Munich, 1993, p. 60 ff., G. Wegener et. al. Applied Catalysis A: General 221 (2001), p. 303 to 335, Elsevier Science B.V., EP 1 369 412 A1, EP 1 754 698 B1 and EP 0 289 840 B1. The reaction of primary amine and phosgene in step a) preferably takes place as follows: Phosgene is used in a stoichiometric excess, based on the primary amine. The phosgenation can be conducted in the liquid phase and in the gas phase, especially in the gas phase. Examples of liquid phase phosgenations are described in DE 37 44 001 C1, EP 0 314 985 A1, EP 1369 412 A1, DE-A-102 60 027, DE-A-102 60 093, DE-A 103 10 888, DE-A-10 2006 022 448, US-A 2007/0299279 and the literature cited in each of these. In a preferred embodiment of step a) as liquid phase phosgenation, the procedure is as follows: The primary amine and phosgene coreactants are dissolved separately in a solvent. Preferred solvents for this purpose are chlorobenzene, ortho-dichlorobenzene, para-dichlorobenzene, the isomers of trichlorobenzene, toluene and/or the isomers of xylene. Particularly preferred solvents are chlorobenzene and dichlorobenzene; ortho-dichlorobenzene is very exceptionally preferred, especially in conjunction with TDI as the isocyanate to be prepared. The primary amine is preferably used in a concentration of 10% by mass to 40% by mass, preferably of 10% by mass to 20% by mass, based on the total mass of the solution. Phosgene is preferably used in a concentration of 10% by mass to 40% by mass, preferably of 25% by mass to 35% by mass, based on the total mass of the solution. Efficient mixing of primary amine and phosgene is of high significance in the liquid phase process. For this purpose, static mixing devices (preferably nozzles) and dynamic mixing devices (containing mechanically moving parts) are used in the prior art. After the mixing, the mixed coreactants pass through a reaction zone to complete the conversion. Mixing device and reaction zone may also be disposed in a common reactor. Phosgene is preferably used in a stoichiometric excess over the primary amino groups of the amine, especially in a molar ratio of phosgene to primary amino groups in the range from 4:1 to 1:1, more preferably in the range from 3:1 to 1:1, most preferably in the range from 2:1 to 1:1. The liquid phase phosgenation can be conducted at various temperature and pressure levels. For example, it is possible to conduct the liquid phase phosgenation at a temperature in the range from 0° C. to 240° C., preferably from 20° C. to 240° C., and at a pressure in the range from 1.0 bar(abs.) to 70 bar(abs.), preferably from 1.0 bar(abs.) to 50 bar(abs.). The hydrogen chloride formed as a coproduct in the reaction is partly dissolved in the liquid phase and partly outgases. The size of the proportion of the dissolved hydrogen chloride compared to that in gaseous form depends on the chosen temperature and pressure level. In each case, in step a), a liquid stream comprising the isocyanate to be prepared and solvent and a gaseous stream comprising hydrogen chloride and any evaporated solvent are obtained. If phosgene is used in a superstoichiometric amount, both streams additionally contain phosgene. Both streams can be taken directly from the reaction zone. It is also possible to take a biphasic process product (comprising a liquid phase and a gas phase) from the reaction zone and transfer it to an apparatus for phase separation. This phase separation can be effected all apparatuses known to those skilled in the art that are suitable for separation of gas and liquid phases. Preference is given to using gas and liquid separators, for example cyclone separators, deflection separators and/or gravitational separators with or without static separation aid. It is likewise possible to assist the phase separation by reducing the pressure compared to the pressure that exists in the reaction zone owing to enhanced outgassing of hydrogen chloride (and of any other gaseous constituents). The liquid stream taken from the reaction zone or—if present—the liquid stream taken from the apparatus connected downstream of the reaction zone for phase separation, in this embodiment, is the starting material for the workup to be conducted in step b), meaning that this liquid phase, in the terminology of the present invention, is “the liquid crude process product comprising isocyanate to be prepared”. Using the example of the primary amine TDA, the liquid phase phosgenation is outlined in detail hereinafter: In the liquid phase process, the TDA, optionally already dissolved in an inert solvent defined further up, is supplied to the mixing with phosgene at a temperature in the range from −10° C. to 220° C., preferably from 0° C. to 200° C., more preferably from 20° C. to 180° C. The phosgene can be supplied to the mixing with TDA either without solvent or likewise dissolved in one of the solvents defined further up at a temperature in the range from −40° C. to 200° C., preferably from −30° C. to 170° C., more preferably from −20° C. to 150° C. The mixing of TDA and phosgene, optionally already dissolved in one of the inert solvents defined further
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