DE1051844B - Process for the production of dicyandiamide from calcium cyanamide and gases containing CO2 - Google Patents

Description

The production of dicyandiamide from calcium cyanamide and _{gases containing CO a} is known. This is done in a closed circuit, in which in particular the mother liquor still containing cyanamide from the dicyandiamide crystallization is used to dissolve new amounts of calcium cyanamide. This final liquor also contains considerable amounts of dicyandiamide. Since dicyandiamide is thus present in the cycle from the start, there is also no need to adhere to such conditions before the dimerization that dicyandiamide formation is avoided.

In-depth investigations have shown that this dicyandiamide content in the starting liquors is based on considerable inconveniences. These dicyandiamide-containing lyes excrete solid dicyandiamide, which, together with the lime-containing lyes, encrusts all vessel walls and especially the cooled walls. The - consequences are poor heat transfer and frequent cleaning and therefore \ ^r erbundenes stopping the system. Furthermore, the precipitated lime sludge contains considerable amounts of dicyandiamide, so that it has to be washed out in a laborious manner with large amounts of water without the dicyandiamide being removed satisfactorily. This means a sensitive loss of nitrogen and also the creation of large amounts of washing water which are difficult or impossible to use, which in turn is associated with a loss of nitrogen.

These drawbacks are overcome by the invention. Here, in the process for the production of dicyandiamide from calcium cyanamide and CO ₂ -Iialtigen gases, the production of the cyanamide liquor in a first process stage is kept completely separate from its dimerization, which is only carried out in a second process stage, by If calcium cyanamide is treated with excess cyanamide lye, which contains less than 10 g of dicyandiamide per liter, at temperatures below 32 ° C, the calcium bicyanamide lye (including the solids contained in it) is immediately _{precipitated using CO 2} and this is filtered off as carbonate together with the solids , washing out the filter cake with water and the remaining Cyanamidlauge partly used for reaction with further calcium cyanamide, partly for the dimerization in dicyandiamide and they together with the arising of the Dicyandiamidkristallisation mother liquor at known temperatures and p _H values under full Ausnutzu ng of the heat of dimerization is converted at the same time and continuously evaporated at a vacuum corresponding to the reaction temperature, whereby a process for the preparation
of dicyandiamide from calcium cyanamide
and CO ₂ -old gases

Applicant:

Dr.-Ing. Rolf Neubner,
° Berlin-Zehlendorf, Schützallee 2

Dr.-Ing. Rolf Neubner, Berlin-Zehlendorf,
has been named as the inventor

a uniformly high dicyandiamide concentration through additional heating of the dimerizing lye is maintained so that the liquor crystallizes on cooling from 78 to 60 ° C., for example begins.

To this end, the following must be stated in detail:

The basic idea of the invention is that the production of the cyanamide in a first Process stage completely from their dimerization to dicyandiamide, which only takes place in a second process stage is used, is kept separate. In particular, it is essential that the processing of the Dicyandiamide mother liquor not when converting calcium cyanamide into cyanamide liquor, but always only takes place when the cyanamide liquor is converted into dicyandiamide. It is also essential that in the whole first stage of the process, namely the production of the cyanamide solution, by avoiding a Dicyandiamidbildung, attention is paid to the generation of a dicyandiamide-free cyanamide liquor, which no solid dicyandiamide precipitates even at low temperatures. For this it is necessary that during the generation of the cyanamide liquor, the temperature rises above that for the formation of dicyandiamide required level and the formation of an alkaline, which favors the formation of dicyandiamide pj-j value is avoided.

According to the invention, the calcium cyanamide is converted and the contained therein is precipitated Carbonated lime as calcium carbonate including all further work steps up to filtration of the solids in the absence of dicyandiamide or in the presence of less than 10 g of dicyandiamide in the liter of the cyanamide liquor. To achieve this goal, the calcium cyanamide has to be dissolved no dicyandiamide-containing alkalis, especially

809 768/543

which do not use mother liquors from dicyandiamide crystallization or excess dicyandiamide-containing wash liquors from filtration. Rather dicyandiamidfreie Cyanamidlaugen only be used in the circuit and the implementation of the Kalkstickstoffes including the precipitation of the calcium carbonate to separate the solids under such known temperature, p _H -value and hold time conditions carried out such that undesirable Dicyandiamidbildung can not occur.

Excess cyanamide lye is used to dissolve or convert the calcium cyanamide, which both calcium cyanamide and the free lime of calcium cyanamide as calcium bicyanamide able to dissolve. The temperature of the lye containing calcium bicyanamide can be changed by any Measures, e.g. B. by evaporation in a vacuum while utilizing the heat of reaction with or without the use of a circulating liquor, be kept below 32 ° C.

The calcium bicyanamide lye is immediately reacted with the solids contained therein, such as carbon and other impurities of calcium cyanamide, with C0 ₂ -containing gases. This liquor does not need to be particularly cooled because the temperature during the conversion of the calcium cyanamide can be kept below 32 ° C. by using unfiltered cyanamide liquor.

After the reaction with the C 0 ₂ -containing gases, a solid, granular lime residue that can be filtered very easily is obtained, the utilization or elimination of which does not present any difficulties.

According to the invention, the heats of reaction generated in the reaction of the calcium cyanamide with excess cyanamide liquor and during the precipitation of calcium carbonate from the calcium cyanamide liquors become free, together or separately in any way, e.g. B. by evaporation in a vacuum, be discharged.

Furthermore, the solids-containing cyanamide liquors can be put in a sludge thickener before filtration into a solids-free cyanamide liquor and into a cyanamide liquor enriched with solids up to 360 g / 1 are divided, with so much of the solid-free cyanamide liquor for further processing on dicyandiamide is taken that the remaining cyanamide liquor together with that from the filtration Cyanamide liquor diluted with washing water in the circuit for the conversion of the calcium cyanamide enough.

The cyanamide liquor removed in this way for the dicyandiamide production is now, as already mentioned, together with the mother liquor resulting from the dicyandiamide crystallization in per se known temperatures and pjj values are implemented simultaneously with full utilization of the heat of dimerization and continuously evaporated at a vacuum corresponding to the reaction temperature, wherein a uniformly high dicyandiamide concentration through additional heating of the dimerizing lye is maintained so that the liquor crystallizes on cooling from 78 to 60 ° C., for example begins.

According to a further concept of the invention, the procedure is such that in this dimerization the still Concentrated dicyandiamide liquors containing free cyanamide are not dimerized to the end, but with a cyanamide content of 50 g / 1, freed from the remaining solids by filtration, the dicyandiamide crystallization are supplied, the

Amount of heat released through cooling and crystallization, e.g. through evaporation in the Vacuum removed and the obtained after separation of the solid dicyandiamide, the unreacted Cyanamide-containing mother liquor is recycled to the dimerization.

The superiority of the new method over the previous mode of operation results from the fact that through the invention after washing with water the nitrogen content in the sludge from 1.34% to 0.21%, i.e. the loss of yield from 11% nitrogen to 1.9% nitrogen when starting from calcium cyanamide with 20.25% cyanamide nitrogen and an accumulation of 6.4 kg of insoluble solids per kilo of cyanamide nitrogen is decreased.

In the German patent specification 804 805 a process is described in which the isolation of the graphite from the solution of technical calcium cyanamide in the form of free lime and calcium cyanamide in the cyanamide liquor requires that this liquor for a relatively long time in the at least weakly alkaline liquor to cope with the filtration process Area is present before _{it is neutralized again by introducing CO 2} (flue gas) and thus precipitation of the lime. In the alkaline range, however, it is known that the dimerization of the cyanamide to dicyandiamide begins, with the result that the lime precipitated and separated off later contains dicyandiamide crystals which, in contrast to the cyanamide carried along from the solution, cannot be washed out with a little cold water. So the lime contains nitrogen.

In this known method, an attempt is now made to eliminate this disadvantage in that two other procedural disadvantages are accepted, namely working with only very dilute cyanamide solutions with, for example, less than 60 g / 1 cyanamide nitrogen and also working at relatively low temperatures of below 20 ° C, which requires that the before Dimerization to dicyandiamide branched off portion of the cyanamide solution, which in the circuit to further Processing of calcium cyanamide is used, must be cooled considerably beforehand, for which at least in summer expensive cold brines are necessary instead of cheap river water. Both of these disadvantages are more economical Nature and are therefore very important.

In contrast, in the method of the present invention is to dispense with the isolation of the graphite share in technical calcium cyanamide, which is then obtained, together with the precipitated lime, which as a gray lime just as well to the usual ¹ as fertilizer is used as a pure white lime.

On the other hand, however, the reaction can now be carried out at significantly higher concentrations of the cyanamide liquors and take place at higher temperatures up to 32 ° C. This increases the space-time yield not inconsiderably increased, and on the other hand, any kind of additional cooling can be dispensed with will. The most important economic advantage, however, is the saving in energy that is otherwise noticeable when working with dilute solutions for water evaporation in the second process stage in the Dicyandiamidbildung is necessary.

According to the process of the present invention, without graphite extraction, the process is carried out in such a way that immediately following the entry of the technical calcium cyanamide into the circulating cyanamide liquor, which takes place in the continuous flow of current, the neutralization by CO _{2 is carried out.}

Claims (1)

Precipitation of lime takes place, which is filtered off together with the graphite and freed from cyanamide by washing out, while dicyandiamide crystals are never present and the lime is thus obtained almost completely free of nitrogen. Even at high concentrations of the cyanamide solutions and at temperatures up to about 32 ° C, which can easily be maintained, no noticeable dicyandiamide formation takes place in the procedure described due to lack of time, which is completely suppressed in the first process stage, so that the upper limit of the dicyandiamide content is slightly can always be kept below about 10 g of dicyandiamide per liter of cyanamide liquor, but then no crystallization of this small proportion of dicyandiamide occurs in the solution and the lime obtained is accordingly free of dicyandiamide. In contrast, according to German patent specification 804 805, a significantly higher concentration of 8.25 g or 12.0 g dicyandiamide nitrogen per liter, corresponding to about 12.4 g or 18 g dicyandiamide per liter, is present even at dilute solutions and at low temperatures. The limit of 10 g / l mentioned according to the present invention is thus far exceeded in the previously known method. Example 4.94 kg of calcium cyanamide fed in with 20.25 percent by weight of cyanamide nitrogen and 21.5 percent by weight of CaO are mixed with 49.14 kg of a clear solution, which is obtained as a filtrate from the rotary filter and as a clear solution in the sludge thickener (see below) and which 73, 1 g of cyanamide-N per kg of solution, and with 217.4 kg of a solution which is branched off from the absorber circuit and which contains 100 g of cyanamide-N and 141 g of substances insoluble in cyanamide solution, such as CaCO3, carbon and other oxidic impurities, per kg of solution, reacted at a temperature of 27 ° C., the temperature increasing by 4.3 to 31.3 ° C. due to the heat of reaction of calcium bicyanamide formation. Immediately after the reaction, 266.54 kg of the calcium bicyanamide-containing solution are mixed with 518 kg of a solution which contains 100 g of cyanamide-N and 141 g of solids (as above) per kg of solution and the temperature of which is 27 ° C., at 784.54 kg of a solution, the temperature of which is 28.5 ° C. The solution containing calcium bicyanamide is fed to the CO2 absorber for immediate neutralization. It can be cooled beforehand in a vacuum at 18 mm Hg (abs.) From 28.5 to 26 ° C. by means of water evaporation, which takes only a little time. However, this precooling can also be dispensed with. The solution contains 3.06 kg of CaO, dissolved as calcium bicyanamide, which is converted in the CO2 absorber with 2.4 kg of CO2 to 5.46 kg of CaCO3 without cooling, with the solution heating up by 1 ° C from 26 ° C to 27 ° C . From this solution, 217.4 kg of solution with the solids contained therein are branched off for the renewed conversion of the supplied calcium cyanamide (see above). To separate the solids, 46 kg of this solution with 100 g of cyanamide-N and 141 g of solids per kg of solution are fed to the sludge thickener, with 27.6 kg of a clear solution with 100 g of cyanamide-N per kg of solution and 18.4 kg of a Solution with 6.48 kg of solids are obtained, which corresponds to a solids content of 352 g per kg of solution. The latter solution containing solids is fed to a rotary filter on which 6.48 kg of solids are washed out with 14.39 kg of cold water. After leaching, the solids contain 1.45 kg of water and form 7.93 kg of a crumbly mass with 18.3% moisture and 0.21% cyanamide-N, which results in a loss of 16.65 g of cyanamide-N or 1.66% of the cyanamide nitrogen content in the calcium cyanamide originally supplied. The filtrate from the rotary filter is 31.34 kg of solution with 58.1 g of cyanamide-N per kg of solution, which together with 17.8 kg of the clear solution from the sludge thickener (see above) 49.14 kg of solution with 73.1 g Cyanamide-N per kg of solution and can be recycled to the initial reaction with calcium cyanamide. 9.8 kg of this remaining solution of the sludge thickener with 97.4 g of cyanamide-N and 2.6 g of dicyandiamide-N per kg of solution together with 9.87 kg of a mother liquor from the dicyandiamide crystallization, which contains 49 g of cyanamide-N and 28, Contains 1 g of dicyandiamide-N per kg of solution, one dimerizing at 80 ° C and at a pressure of 0.35 at abs. Added evaporating solution, from which solid dicyandiamide begins to crystallize after cooling from 80 to 60 ° C. From this solution, 11.3 kg of an alkali with 42.9 g of cyanamide-N and 109 g of dicyandiamide-N per kg of solution and, by cooling in a vacuum from 18 mm Hg (abs.) To 28 ° C., 1434 g of solid dicyandiamide are obtained. The mother liquor obtained after isolation of the dicyandiamide consists of 9.87 kg of solution which contains 49 g of cyanamide-N and 28.1 g of dicyandiamide-N per kg and which is returned to the dimerization together with the remaining solution from the sludge thickener. During the dimerization, 8.33 kg of water are partly due to the heat of dimerization and partly due to the supply of live steam at 0.35 at abs. evaporated, whereby, as stated above, a solution with 109 g of dicyandiamide-N and 42.9 g of cyanamide-N per kg of solution is formed. Patent claims: 1. A process for the production of dicyandiamide from calcium cyanamide and CO ₂ -containing gases, characterized in that the production of the cyanamide liquor which takes place in a first process stage is kept completely separate from its dimerization _1, which is only carried out in a second process stage, by adding calcium cyanamide treated with excess cyanamide lye, which contains less than 10 g of dicyandiamide per liter, at temperatures below 32 ° C, from the calcium bicyanamide lye (including the solids it contains) immediately by means of CO, the lime precipitates, this is filtered off as carbonate together with the solids, the filter cake is washed out with water and the remaining Cyanamidlauge partly used for reaction with further calcium cyanamide, partly for the dimerization in dicyandiamide and in the latter case, together with the obtained from the mother liquor at Dicyandiamidkristallisation to known temperatures and p _H values under full Chamfering tion of the heat of dimerization is implemented at the same time and at one of the reaction temperature