MXPA98000911A - Process for preparing alkali metal cyanide and alkaline earth metal cyanide granules and the high purity alkali metal cyanide granules obtainable thereby[sg69265 ] - Google Patents

Abstract

The present invention relates to a process for the preparation of an alkaline cyanide or alkaline earth cyanide granulate, which includes a reaction of hydrocyanuric acid with an alkaline or alkaline earth metal hydroxide, in which the alkali hydroxide is sprayed in a reactor as an aqueous solution and the alkaline earth metal hydroxide in the form of an aqueous suspension, and in the reactor it is reacted with gaseous hydrocyanuric acid, and evaporate the water added and formed in the reaction

Description

PROCEDURE FOR THE PREPARATION OF ALKALINE AND ALKALINE CERAMIC GRANULATES AND THE HIGH PURITY ALKALINE CYANIDE GRANULATES OBTAINED BY THE PROCEDURE DESCRIPTION OF THE INVENTION The invention relates to a process for the preparation of alkali and alkaline earth cyanide granules, especially granules of essentially sodium cyanide, potassium cyanide and calcium cyanide. The process is based on the reaction of hydrocyanuric acid with an alkaline or alkaline earth metal hydroxide in a turbulent layer and will be referred to hereinbelow as turbulent layer reaction [RWG]. The invention also relates to the spherical granules of NaCN and KCN obtainable by means of this process which are characterized by a high purity. Sodium and potassium cyanide have wide application for the preparation of galvanic baths and salt baths for tempering, as well as for the synthesis of organic compounds; sodium cyanide as well as calcium, are used in large quantities to obtain gold by means of leaching with cyanide of minerals. For the application in the galvanic baths, the alkaline cyanides additionally must present a high purity. While a reduced alkali metal hydroxide content in alkaline cyanide serves for stabilization, the alkaline carbonate and formate content REF: 26687 alkaline should be as small as possible. To achieve safe handling, cyanides should mostly be in the form of powder-free granules. It is known to prepare alkali cyanides by neutralizing hydrocyanuric acid (HCN) and alkali hydroxide in aqueous solutions with subsequent crystallization, solid-liquid separation and subsequent mechanical molding. These types of procedures are very expensive, and the products tend to form dust and therefore are very difficult to handle. Especially disadvantageous is the need for a part of the mother liquor to be disposed of in order to counteract the enrichment of by-products. Usually between 10 and 30% of alkaline cyanide is discarded with the mother liquor. The technically very complicated process so far for modeling alkali cyanides can be improved essentially by means of so-called turbulent layer spray granulation, as described in patent document EP-A 0 600 282. In the aforementioned process the solution of Alkaline cyanide is sprayed in a turbulent layer on alkaline cyanide seeds and the water used is evaporated by means of a gas-drying stream flowing through the turbulent layer. A granulate consisting of essentially spherical particles is obtained, which have a very reduced wear and a reduced agglomeration index. As for carrying out the process according to EP-A 0 600 282 an aqueous solution of alkali cyanide is required and this is obtained in a known manner by means of the neutralization of HCN with alkali cyanide in aqueous solution, that solution also contains the known by-products, among others especially the corresponding carbonates and formates. Due to the use of alkaline cyanide solution containing by-products, the prepared alkaline cyanide granules can not have a higher purity than the solution. By the reaction of the carbon oxide obtained in the fluidization / drying gas with the excessive alkaline hydroxide contained in the alkali cyanide solution, alkali carbonate is additionally formed, such that the content of alkali carbonate in the cyanide granulate alkaline, in general, is greater than that of alkali cyanides prepared by means of a crystallization process with subsequent modeling. A completely different process for the preparation of alkali cyanides in the form of solid particles is also known from the patent application DE-A 38 32 883, in this process a gas containing hydrocyanuric acid is continuously reacted with finely distributed droplets of an aqueous solution of alkali hydroxide, while simultaneously the used and formed water is evaporated and the solid particles separated, after being separated they are subjected to an modeling and / or subsequent drying. This process also deals with a spray drying, which is combined with a gas-liquid reaction, which in this case is a neutralization. With this procedure, only products with a high content of alkaline cyanide are obtained, when HCN is used in large excess and alkali hydroxide solution in a reduced concentration. The combination of spray drying and neutralization has the disadvantage that fine droplets starting from the outside are dried where the alkaline cyanide has formed. The diffusion of HCN gases in the inert core of the droplets where there is still a liquid solution of alkali hydroxide, worsens as the solid layer in the outer zone increases. To achieve a higher yield, the driving force of the diffusion must be high, which is achieved by means of a large excess of HCN and / or a reduction of the drying time. However, here the space-time performance of the procedure is reduced. Another disadvantage of this process is that the product is obtained as a fine powder and in additional steps of the process it must be formed as a manageable granulate. From the aforementioned document DE-A 38 32 883 it is also concluded that under the temperatures suitable for the process, almost no reaction occurs between the finely distributed solid alkali hydroxide and the gaseous hydrocyanuric acid; the reaction promotes the presence of water and dilute alkaline hydroxide. According to this teaching, this process can not be considered for the preparation of alkaline earth cyanides, especially calcium cyanide, using an aqueous suspension of an alkaline earth metal hydroxide. Trojosky et al. describe in Chem. Ing. echnik 9/95, page 184, a process for the semi-dry desulfurization of smoke gases by means of drying by absorption in the turbulent layer. In this process in the turbulent layer apparatus an aqueous suspension of calcium hydroxide is applied by means of binary nozzles on the surface of a fluidized collector. In which, with the addition of the absorber, a satisfactory level of flue gas desulphurisation can be obtained, however the proportion of gypsum in the final product in granulated form obtained is very low, since in addition to the gypsum, it contains in one volume important calcium oxide or calcium hydroxide. Due to the incomplete reaction and with this the insufficient purity of the obtained granulate, it is not recommended to use the procedure described in that document for the preparation of alkaline and alkaline earth cyanides, especially of high purity alkaline cyanides. The task of the invention is to present an improved process for the preparation of alkali metal and alkaline earth metal granules, which makes a separate preparation of an aqueous solution of an alkaline or alkaline earth cyanide superfluous and thereby avoids the disadvantages of the previously known process which is base in a combination of spray drying and neutralization. Another task of the invention is to present a process for the preparation of granules of sodium and potassium cyanide, which in addition to good granulation properties, is characterized by a high purity. Finally, the process according to the invention should make it possible to use gases containing hydrocyanuric acid from different origins and with this from different gas compositions. A process was found for the preparation of an alkaline cyanide or alkaline earth cyanide granulate, which includes the reaction of hydrocyanuric acid with an alkaline or alkaline earth metal hydroxide, in which alkali hydroxide in the form of an aqueous solution is sprayed in a reactor and the alkaline earth metal hydroxide in the form of an aqueous suspension, and in the reactor it is reacted with gaseous hydrocyanuric acid, and evaporation of the water added and formed in the reactor, which is characterized in that the reaction is carried out in a reactor for the granulation by dew in Turbulent layer at a temperature of the turbulent layer in the range of 100 to 250 ° C, the aforementioned solution or suspension is sprayed onto a turbulent layer of germs of essentially the alkali or alkaline earth cyanide to be prepared, simultaneously being conducted in the reactor a gas containing hydrocyanuric acid in an amount of at least one mole of HCN per equ metallic ivalent and the water is evaporated by means of a fluidizing gas that flows in the turbulent layer, whose inlet temperature is between 110 and 500 ° C. The dependent claims of that claim relate to the preferred embodiments of the method according to the invention. Although cyanide granules of all alkali and alkaline earth metals can be prepared according to the process according to the invention, the process is suitable in particular for the preparation of sodium cyanide (NaCN) - granules, potassium cyanide (KCN) ) and calcium cyanide (CA (CN) 2). The process is especially suitable for the preparation of NaCN and KCN granules, especially at that high purity. The process according to the invention is carried out in the turbulent layer of an individual grain in the following way: the HCN gas introduced into the reactor diffuses into the alkali hydroxide solution or the alkaline earth hydroxide suspension, with which the germs are moistened and react there to give the corresponding cyanide in diluted form. That solution dries from the inside out on the surface of the particles and is associated with the growth of germs. This procedure differs from the known structural granulation, in which the solutions or suspensions containing the material forming the granulate are used, so that a neutralization reaction first takes place on the surface of the particle, forming the required material for the growth of the particles. In this process, the drying process no longer prevents the diffusion of HCN gas, so highly concentrated liquids and HCN can be used in almost stoichiometric proportions. The water formed during the reaction as well as the water added with the solution or suspension is evaporated by using the enthalpy of the neutralization reaction and the latent heat of the hot fluidization gas. The process according to the invention is therefore a turbulent layer reaction granulation (R G). The process according to the invention can be carried out continuously or discontinuously in reactors of any construction, which are suitable for turbulent layer spray granulation. The princi of turbulent layer spray granulation, reactors of different construction are known in their imentation and variants for technicians (as an examH. Uhlemann is recommended in Chem. Ing. Tech. 62 (1990) No. 10, pages 822-834). For exam the reactor may be formed with a round container or as a flow tray provided with floors with streams (turbulent layer floors). One or more spray nozzles are placed in the reactor, the opening of the nozzle being advantageous for the purpose being pursued inside the turbulent layer. The alignment of the nozzles can allow a spray from bottom to top and / or from top to bottom or essentially parallel to the current floor. For continuous operation, suitable reactors in general also have a device for continuous or periodic discharge of the granulate, this device advantageously being formed in such a way that it allows a classified discharge of the granulate. To carry out the process, a gas containing hydrocyanuric acid is introduced into the reactor. That gas can be introduced into the reactor mixed with the fluidizing gas stream. Alternatively or in addition to the previously mentioned introduction forms, the gas containing hydrocyanuric acid can be introduced into the reactor above the air stream floor, in particular by means of the nozzles placed inside the turbulent layer. As long as the gas containing hydrocyanuric acid is added to the fluidizing gas stream, this can be done immediately below the air stream floor or immediately before, for exambefore heating the fluidization gases. The introduction of the alkali hydroxide solution or the alkali hydroxide suspension into the reactor is carried out by means of conventional spray devices, for examby means of pressure nozzles for a material or by means of nozzles for various materials. When using binary nozzles, one of the components can be the alkaline hydroxide solution or the alkaline earth hydroxide suspension and the second component a usual propellant gas, such as N2, or a gas containing hydrocyanuric acid. The alkaline hydroxide solution or to be sprayed may contain a desired concentration of alkali hydroxide. In general, the concentration of alkaline hydroxide in the solution is in the range between 10 and 70% by weight, preferably between 20 and 50% by weight and especially preferred between 30 and 50% by weight. For the preparation of alkaline earth cyanides, an alkaline earth metal hydroxide suspension with a content of at least 50% by weight is advantageously used., especially 5 to 30% by weight. The gas containing hydrocyanuric acid to be used in the process according to the invention can be hydrocyanuric acid or a gas containing hydrocyanuric acid, for example a reaction gas which is obtained in the usual procedure for the preparation of HCN or as in the framework of other processes, especially that which is obtained as a secondary product in usual processes for the preparation of acrylonitrile. The usual procedures for the preparation of hydrocyanuric acid are the Andrusso process, the BMA process and the Shaigan process (See Ullman's Encyclopedia of Industrial Chemistry, 5th edition (1987) vol.A8, 161-163). The reaction gas (crude gas) formed from an ammonium-oxidation of methane in the Andrussow process, is typically formed as follows (in% by weight): N2 53.7%, H20 31.4%, HCN 8.4%, C02 3.6%, H2 1.1%, NH3 1.0%, CO 0.7% and CH4 0.1%. The raw gas of the BMA process contains, as a main component, HCN (approximately 23% by volume) and H2 (approximately 72% by volume).; In addition, the BMA gas still contains unreacted ammonia and methane residues and some nitrogen. Also the raw gas from the Shawinigan process contains HCN (approximately 25% by volume) and H2 (approximately 72% by volume) as the main component. The method according to the invention uses HCN in stoichiometric or superestequimetric proportions, in relation to alkaline hydroxide or alkaline earth metal hydroxide. The molar ratio of HCN to the equivalent metal hydroxide is usually in the range from 1 to 5. For the preparation of NaCN and KCN granules a molar ratio of HCN to alkali hydroxide is used from 1 to 5, preferably 1 to 1.5, and particularly preferred is 1 to 1.1. The temperature of the turbulent layer is usually in the range of about 100 to 250 ° C. In principle, the process can also be carried out at a temperature above 250 ° C, however the cyanide content is increasingly reduced. In principle, the temperature of the turbulent layer can also be below 100 ° C, for example 90 to 95 ° C, when working under reduced pressure. Preferably the temperature of the turbulent layer is between 105 and 180 ° C. A turbulent layer temperature between 105 and 150 ° C is especially preferred. The fluidizing gas, which, as mentioned above, can also contain the gas containing HCN, advantageously has its own temperature between 110 and 500 ° C, preferably between 120 and 400 ° C. Fluidization gases suitable for the process according to the invention are inert gases such as nitrogen, especially superheated steam. The air is not suitable as a fluidizing gas, since the HCN-air mixtures that would be considered for a technical installation could be found in the explosive zone. (In the aforementioned procedure of DE-A 38 32883 for this reason it was inerted with nitrogen). According to the invention if dry water steam is used, if dry water steam is used, at a temperature of the waste gases above 100 ° C, the polymerization of the dry polymer is not obtained in the dry vapor range. Hydrocyanuric acid, in such a way that it is possible to change the apparatus with a gas supply of the circuit. In this type of embodiment, the gas stream leaving the turbulent layer is separated into two partial streams: the first partial stream (circuit gas stream) is used, after re-heating, as the fluidizing gas; the second partial current (excess gas) deviates from the course of the cycle. The steam portion of the gas in the circuit is conducted to the process by means of the reaction water and the drying of the solution. By means of the conduction of the gas of the circuit, the expense for the handling of the waste gases is considerably reduced, because the excess of steam can be completely condensed, and only the volatile constituent parts of the gas must undergo a subsequent treatment. Other embodiments of the process according to the invention are illustrated with the aid of FIGS. 1 to 3. FIG. 1 shows a process diagram for the preparation of an alkali cyanide granulate using a BMA reaction gas after granulation. by turbulent layer reaction (RWG) with circuit gas conduction and burning of excess gas. Figure 2 shows a process scheme for an alternative embodiment of the RWG process, in which the powder separator is integrated into the reactor, the excess vapor is condensed and the waste gas is washed. Figure 3 shows a process diagram of a laboratory apparatus for carrying out the process according to the invention, in which the Andrussow process is simulated, the Andrussow reaction gas was rapidly cooled by the injection of water and the obtained gas mixture is conducted to the reactor of the RWG process as a fluidizing gas containing HCN and the waste gas is extracted through a scrubber. Figure 1 covers the reference numbers 1 to 14, Figure 2 the reference numbers 21 to 35 and Figure 3 the reference numbers 41 to 56. In the system the incoming and outgoing material streams are characterized with letters: To alkaline bleach, B gas containing HCN, C propellant gas for binary or two-material nozzles, D gas classifier, E alkaline cyanide granulate, F excess vapor, G water for steam production, H combustion gas (for start-up) , I combustion air, J waste gas, K waste steam, L rinse gas, M waste water, N alkaline cyanide solution, Or germs, P water vapor. The embodiment shown in FIG. 1 is particularly suitable when gases containing HCN with a high proportion of fuel are to be used, especially gases that also contain HCN and hydrogen, such as those obtained in the BMA and Shawinigan process. In this case the waste gas can be conducted directly to a burner, which at the same time produces the heat to heat the gas in the circuit. In the case of granulation by continuous turbulent layer reaction, an alkaline liquor (A) is sprayed by means of nozzles for one or more materials (2) in the turbulent layer of the RWG reactor (1). As long as a nozzle is used for two materials, the propellant gas (C) can be an inert gas, for example nitrogen or a gas containing HCN. The germs necessary for the operation are placed at the beginning in the reactor; when working, the germs form the turbulent layer (in the figure they are represented as small spheres). During operation, the number of germs can be controlled by means of a screwdriver (5). When the granulate has grown to the desired grain size, it is extracted (E) from the reactor by means of a sorting channel (4); a sorting gas (G) is conducted in the classifying device not shown in detail. The addition of the HCN raw gas from a BMA process (B) is carried out below the turbulent layer floor (3), however, the HCN-containing gas can also be alternatively or additionally used as propellant gas (C) or directly in the turbulent layer (not shown). The waste gas loaded with solids is purified in a separator (6); the separated solids are recycled to the reactor. The waste gas stream released from solids is divided into two partial streams, and this is in a first partial stream, also called circuit gas stream and a second partial stream, which contains the excess gas. The circulating gas stream is again returned to the reactor RWG (1) by means of a gas fan of the circuit (7) through a superheater (9) placed in a combustion chamber (8). The excess gas is extracted by means of a fan (13), operating the installation variant under reduced pressure. The fuel-rich excess gas extracted by the fan (13) is conducted to the combustion chamber (8); by means of combustion, the heat is produced for the superheating of the gas in the circuit (9). For the start-up and / or as a supplement, a fuel gas (H) can additionally be conducted to the combustion chamber; combustion air (I) is conducted through a fan (12) to the combustion chamber. The combustion chamber additionally has a catalyst (10) to extract the waste gas (J). Depending on the power of the reactor and the combustion value of the gas containing HCN there is an excess of energy, which can be used, for example, for the additional production of steam - introduction of water (G) into the combustion chamber and evaporation thereof in the superheater (11). The additionally formed steam can be used to drive a steam turbine (14), which drives the gas fans of the circuit (7); the waste steam (K) is extracted from the steam turbine. In addition, the additionally formed steam can be used as a classifier gas or for other purposes (F). The device shown in FIG. 2 for granulation by turbulent layer reaction is suitable in particular for the preparation of fine-grained granules but without dust. The reactor (21) comprises one or more spray nozzles (22), the turbulent layer floor (23), a device for the graded extraction of the granulate (E) ((24) connected to the gas inlet classifier (D)) , a screw jack (25) for crushing the granulate, filter hoses (26) integrated to the reactor and a jet device (JET) (27) not shown in detail, by means of which, together with a rinsing gas feed (L) ), the filter hoses are cleaned. A gas containing HCN (B) is conducted below the floor of the turbulent layer; an aqueous solution of alkali hydroxide (A) is sprayed onto the turbulent layer using a propellant gas (C) by means of nozzles for two materials (22) (not shown in Figure 2). The dust-free gas leaving the reactor is divided again into two partial streams. The gas stream of the circuit (first partial stream) is transported with the fan (28) through a heater (29) back to the reactor (21). Excessive vapors expelled from the gas circulation (second partial stream) are led to a heat exchanger (33), where the water vapor of the excess vapors condenses. The waste gases of hydrocyanuric acid unreacted are neutralized in the scrubber (30) with alkaline lye (A) and are conducted in the waste water (M) to the subsequent treatment. The scrubber circulation comprises another heat exchanger (31) as well as a circulation pump (32). The volatile constituents of the scrubber are led through a droplet separator (34) and conducted through a waste gas fan (35) to a combustion plant. The waste gas fan (35) produces a pressure in the installation. The alkali cyanide granulate is expelled from the reactor through a lock (36). The process represented in FIG. 3 is coupled to the Andrussow process and the granulation by turbulent layer reaction through the cooling of the Andrussow crude gas. In the Andrussow process, a mixture of methane-ammonia and air in the reactor is conducted in a known manner through a network of catalysts and transformed into hydrocyanuric acid and water at a temperature higher than 1000 ° C which is formed after the ignition. A typical composition of the reaction gas mixture was given above. To avoid the decomposition of HCN the reaction gas should be cooled rapidly to less than 400 ° C. This is achieved, if immediately after the reaction is injected into the quench water of the reactor and / or the heat is conducted to a boiler for residual heat. While energy is usually lost during rapid cooling, it was surprisingly found that the operating parameters of the RWG process and the Andrussow process can be determined from each other, so that a coupling of the process through the reaction gas is possible. For this purpose, the cooled Andrussow reaction gas is used directly as a fluidizing gas in the RWG reactor. The hydrocyanuric acid is already in the fluidization gas stream and determines the amount of alkaline hydroxide solution stream to be sprayed into the turbulent layer. By means of the type and manner of cooling of the reaction gas, the heat and material quantities of the turbulent reaction layer can be determined and regulated. As shown above, the Andrussow gas is cooled by rapid cooling and / or direct heat exchange. In the coupling according to the invention of the Andrussow process with the RWG process, the cooling process is used to adjust the process conditions in the turbulent reaction layer. To adjust an inlet temperature of the gas in the RWG reactor of preferably less than 400 ° C, it can be influenced by the amount of fast cooling water of the mass flow of the fluidizing gas and with this the fluidization rate in the layer turbulent The greater the cooling rate by rapid cooling, the lower the indirect heat conduction through the heat exchanger and vice versa. Thus, simple regulation of the process conditions in the turbulent layer is possible. It is also possible to omit completely from an indirect heat exchanger and adjust the desired fluidization temperature of the Andrussow reaction gases by means of rapid cooling. Because on the one hand the energy supply to the RWG reactor can be adjusted by means of the cooling type of the reaction gas of the Andrussow reactor, while for example the mass current is varied by means of the amount of fast cooling water and the temperature of the gas is kept constant by the boiler for waste heat and on the other hand the power of the RWG reactor is a measure of the temperature difference between the gas inlet temperature and the temperature of the waste gases, with a transformation of given HCN and the corresponding injection of alkaline liquor, a temperature of the waste gases is adjusted. It was found that the RWG process can be performed over a wide temperature range of waste gases; preferably the temperature of the waste gas, which is a little higher than the temperature of the turbulent layer, is in the range between 100 and 200 ° C. If the cooling of the Andrussow reaction gas is mainly or completely obtained by means of rapid cooling, then a high temperature of the waste gases is established, as a consequence of the high mass current in the gas. The power reserve thus obtained from the RWG reactor can be used so that, in addition to the alkali hydroxide solution, alkali metal cyanide liquor is also added. This alkali cyanide bleach can be obtained, for example, from a waste gas scrubbing of a process gas containing HCN. In another embodiment (not shown in FIG. 3) according to the invention, the coupling between the Andrussow process and the RWG process consists in that a part of the waste gas of RWG after the hot gas filtering is recycled. in the Andrussow reactor in order to separate the powder. By means of this recycling an improvement in the performance of the process is obtained. With this embodiment of the process, the concentration of HCN in the reaction gas is precisely reduced, however this is disadvantageous since the injection of water into the reaction stream can then be omitted, because the circulating partial gas stream of the stream mass increases for fluidization and for the introduction of heat in the RWG reactor. The necessary cooling of the reaction gas of the Andrussow reactor can then be carried out only or predominantly by means of an indirect heat exchanger. The reaction gas of the Andrussow process can be divided into a parallel production of bleaches and solids, by driving a partial current through the RWG reactor and the waste stream is directed directly to the production of bleaches. Advantageously, the waste gas from the RWG reactor, which may contain hydrocyanuric acid residues and alkali cyanide powder, can be mixed with the partial gas stream that leads to the production of bleaches. In this way, the treatment of the waste gases from the RWG reactor can be minimized or completely omitted. By means of this separation of the gases a high flexibility is obtained in the mixture of the bleach product and the granulate. By using the Andrussow gas as the HCN source in the RWG process, due to the CO2 content of the Andrussow gas, alkali carbonate is also formed. Before the concurrence reaction itself, the HCN and C02 gases must diffuse into the alkaline hydroxide solution, with which the germs have been wetted; HCN diffuses much better than C02, so that only part of the C02 used before the grain dries has reached the reaction. The content of alkali carbonate in the alkali cyanide granulate produced according to the invention is therefore substantially lower than in the cases of preparation of the granulate by absorption of the Andrussow gases in an alkaline liquor with the subsequent granulation by sprayed in a turbulent layer of the alkali cyanide bleach, which would obtain the same amount of CO2 from the Andrussow gas as alkaline carbonate without additional purification measures. In the laboratory apparatus shown in FIG. 3, no gas was used from an Andrussow reactor, but a typical Andrussow gas composition was synthetically generated by mixing the individual constituent parts in a mixing section (48) and heating the gas mixture in a heating aggregate (49), except that due to the test conditions carbon dioxide was replaced by nitrogen and additionally a larger amount of water vapor (P) was used, so as not to have to heat too much gas mixture. In the gas mixture heated to 650 ° C the water (G) is injected and evaporates in the mixing section (51); the quantity of water (50) is regulated by means of measuring the volume of the reaction gas. Through an indirect heat exchanger (52) a constant temperature of the reaction gas is set between 350 and 380 ° C. The gas is introduced into the lower part of the RWG reactor (41) and flows through a perforated sheet (43) to the turbulent layer (44). The alkaline hydroxide solution (A) is sprayed to the turbulent layer from the bottom upwards through a nozzle for two materials introduced into the perforated sheet using nitrogen as the propellant gas (C). In the reactor whose height-to-diameter ratio is about 30, a second nozzle for two materials (46) was placed, with which alkali hydroxide or alkali cyanide bleach could alternatively be injected from top to bottom in the turbulent layer. At the beginning of the tests the germs (O) forming the turbulent layer are placed in the reactor, the granulate (E) when the test is interrupted is extracted by means of a tube (45). In the upper part of the reactor the waste gas after passing through a filter (47), is extracted through a scrubber (53). The wash circulation also includes the scrubber, the pump (55) and a heat exchanger (54). After a droplet separator (56) the waste gas (J) is ejected.

By means of the process according to the invention, using gases containing low C02 HCN, preferably free of C02, such as for example BMA crude gas, alkali cyanides can be prepared with a combination of especially advantageous properties of the granulates as well as a purity out of the ordinary Thus, an alkaline cyanide granulate based on sodium or potassium cyanide was found, having the following characteristics: (i) essentially spherical particles with flat surface structure or granulated as raspberry, (ii) diameter of the particles in the range of 0.1 to 20mm, preferably 1 to 20mm, for 99% by weight of the granulate, (iii) bulk weight of at least 600g / dm3, preferably greater than 650g / dm3. (iv) wear less than 1% by weight, measured in a roller wear test (TAR wear tester from Erweka with sample of 20 g, 60 minutes, 20 rpm) (v) maximum agglomeration index 4, preferably 3 and less than 3, measured after a load of lOOg during 14 days in a cylinder with a reduced thickness of . 5cm with 10 kg. which is characterized in that the alkali carbonate content is less than 0.5% by weight and the alkaline formate content is less than 0.3% by weight, with the alkali metal in the named secondary products being identical to that of the alkali cyanide. Preferred granulates contain alkali carbonate and alkali formate in a joint amount less than 0.4% by weight. Especially preferred granulates of NaCN and KCN contain less than 0.1% by weight of alkali carbonate and less than 0.1% by weight of alkali formate. The alkali cyanide granules with the very low content of by-products of the alkali metal carbonate and alkali metal formate family can be prepared in particular by means of an embodiment of the process according to the invention, in which a reaction gas is used. containing HCN, which is essentially free of carbon hydroxide and is used as water vapor of the fluidizing gas. Alkaline cyanide granules with the characteristics (i) to (v), as known from EP-A 0600 282, can be prepared by direct turbulent layer spray granulation of an alkaline cyanide solution. In relation to the definition and methods of determination of these characteristics, they refer expressly to the mentioned document. In contrast to the process according to the invention using a gas containing HCN essentially free of C02 in the processes known above, there are obtained products which have a high content of alkali carbonate and can therefore be used very little in the electroplating industry. This problem is solved with the invention.

In addition to the accessibility of the alkali and alkaline earth cyanide granules with a particularly advantageous combination of properties, the process according to the invention is characterized by other advantages: the process is characterized by an especially high space-time efficiency and a low energy expenditure . By means of the process it has become possible to avoid the partially costly separate preparation which leads to side products of an alkaline cyanide or alkaline earth cyanide solution. This leads to a clearly reduced investment in a technical installation. The procedure allows the use of different sources of gases containing HCN, including BMA and Andrussow reaction gas. Depending on the gas containing HCN used, different variants for the process are presented, which lead to both high flexibility and reduced energy use. Other advantages are that both a BMA gas and an Andrussow reaction gas, as well as that obtained in the corresponding reactor with a temperature higher than 1000 ° C, can be used in the process according to the invention, being able to obtain the required cooling of the reaction gas either by feeding it into the gas circulation and / or by rapid cooling with water. By direct coupling of the Andrussow process with the RWG the economy of the preparation of the alkyl cyanide granules is improved. The energy of the reaction gases of the Andrussow reactor is directly used for the evaporation of water from the introduced water with the alkaline hydroxide solution as well as the water formed during the neutralization, without having to be transported through indirect heat exchangers. For the fluidization gas of the turbulent layer separate extractors or heating systems are not required. The surplus energy can be additionally used in parallel to the process according to the invention to spray an alkaline cyanide solution to the turbulent layer reaction granulator for the purpose of granulation formation. Example 1 and 2 NaCN granules are prepared from sodium hydroxide and raw BMA gas according to the RWG method, according to the invention, in a device and under the conditions as described above in figure 1. Diameter of the RWG reactor floor: 250 mm Fluidizing gas: H-vapor circuit gas, 0 Temperature: 270 C Quantity: 10 m3 / h BMA gas supply: below the inflow floor Temperature: 150 C Quantity: 1.09 kg / h HCN Bleach NaOH (35% by weight): 2.33 kg / h Gas temperature of 105 ° C - example 1 waste *): 110 ° C - example 2 *) The temperature in the turbulent layer was approximately 10 ° C above the temperature of the waste gases In both examples, essentially spherical granules having a flat surface with a partial diameter in the range of 4 to 5 mm, a bulk weight in the range of 700 to 750 g / 1 were obtained , wear less than 0.1% by weight and an agglomeration index ion less than 3. The chemical composition is shown in the table: Example 1 Example 2 Gas temperature 105 110 waste (° C) NaCN 98.30 98.07 NaOH 1.01 1.12 Na2C03 0.18 0.08 Na Formate 0.18 0.11 H20 0.33 0.62 The sodium formate content was essential; of the feeding of germs; The NaCN germs used had a mean grain diameter of 2 mm and contained 0.72% Na2C03 and 0.70% HCOONa. Example 3 In a device according to Figure 2, KCN granulate was prepared from alkaline liquor (40% by weight) and HCN gas from a BMA reactor in a typical BMA composition.

Turbulent layer temperature: 107 ° C Circuit gas quantity: 10 m3 / h Circulating gas inlet temperature: 150 ° C HCN / KOH molar ratio: 1.08 Spherical granulate with a bulk weight of 700 to 750 was obtained g / 1, completely free of dust and resistant to wear and not agglomerated. The analysis showed: KCN 99. 1% KOH 0. 2% K2C03 0. 1% HCOOK 0. 1% H20 0. 5% Example 4 The process according to the invention was carried out in the laboratory facilities represented in figure 3 under the conditions described below. Example 4 as well as examples 5 and 6 encompass the coupling of the Andrussow process with the RWG process. The gaseous composition of Andrussow was simulated by mixing HCN, H2, NH3, C02, H20 and N2. For the inert portion of CO of an Andrussow gas, the nitrogen fraction was increased. Additionally, through the stream P, approximately 3 to 4 kg / h of water vapor were mixed, so that the total current was 18 to 19 kg / h. The gas mixture was heated in the heating aggregate (49) to approximately 650 ° C and then to simulate the rapid cooling it was cooled again by spraying 3 to 4 kg / h of water (G), so that the current The mixture entering the RWG reactor was 21 to 23 kg / h. Through the indirect heat exchanger (52) a gas temperature of about 350 ° C was regulated and at this level it was conducted in the RWG reactor. The nozzle (42) was fed with preheated 50% sodium hydroxide solution; the spray was performed from the bottom up. Preheated nitrogen was used as the propellant. 1.5 kg of NaCN granules with a granulation of 0.2 to 0.3 mm were placed in the reactor. The granulate was preheated and then the gas mixture was started. Then the addition of sodium hydroxide in an almost stoichiometric proportion to the HCN portion was performed. Temperatures between 105 and 125 ° C were measured in the waste gas stream at a steady state. Approximately 4 kg of granules were extracted from the reactor. The product contained 92.3% NaCN. The rest was mostly NaOH, sodium carbonate and residual moisture. Example 5 On the gas side the same conditions as in example 4 were set. The heat exchanger (52) was not used and therefore the amount of fast cooling water (G) was increased to 4 to 4.5 kg, such that there was an inlet temperature of the gases in the RWG apparatus of approximately 380 ° C. With comparable amounts of injected sodium liquor, a temperature between 105 and 145 ° C was set in the waste gas. Example 6 Test conditions were adjusted as in example 4. In addition, a nozzle (46) was sprayed through a nozzle (46). 20% sodium cyanide bleach. The feed in the nozzle (46) was increased slowly, until the temperature of the waste gases was reduced to approximately 110 ° C. In this state, up to 0.4 kg / h of 20% cyanide bleach was additionally sprayed. The cyanide content of the product was 95%. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (16)

1. CLAIMS 1. Process for the preparation of a granulate of alkaline cyanide or alkaline earth cyanide, which includes a reaction of hydrocyanuric acid with an alkaline or alkaline earth metal hydroxide, in which the alkali hydroxide is sprayed in a reactor in the form of an aqueous solution and the alkaline earth metal hydroxide in the form of an aqueous suspension, and in the reactor is reacted with gaseous hydrocyanuric acid, and evaporate the water added and formed in the reactor, the process is characterized in that, the reaction is carried out in a reactor for the granulation by Turbulent layer spray at a temperature of the turbulent layer in the range of 100 to 250 ° C, said solution or suspension is sprayed on a turbulent layer of germs of essentially the alkali or alkaline earth cyanide to be prepared, simultaneously conducting in the reactor a gas containing hydrocyanuric acid in an amount of at least one mole of HCN per metallic equivalent and the water is evaporated by means of a fluidizing gas flowing in the turbulent layer whose inlet temperature is between 110 and 500 ° C.
2. 2. - Process according to claim 1, characterized in that, as gas containing hydrocyanuric acid, a reaction gas is used that contains hydrocianuric acid and hydrogen as a main part, coming from a BMA or Shawinigan process.
3. 3. - Process according to claim 1, characterized in that, as a gas containing hydrocyanuric acid, a reaction gas is used that contains as main parts HCN, N2, H2, H20 and CO, coming from an Andrussow process.
4. 4. Method according to one of claims 3, characterized in that a granulate of alkali metal cyanide of the family of granules of NaCN and KCN is prepared, the molar ratio of HCN to alkaline hydroxide being in the range between 1 and 5 and the alkaline hydroxide solution to be sprayed has an alkali hydroxide concentration in the range of 10 to 70% by weight.
5. 5. Method according to one of claims 1 to 4, characterized in that the gas containing hydrocyanuric acid is mixed with the fluidization gas stream below the turbulent layer floor and / or is introduced into the reactor at the zone of the turbulent layer. Method according to one of claims 1 to 5, characterized in that the reaction is carried out at a turbulent layer temperature in the range of 105 to 180 ° C and an inlet temperature of the fluidization gas in the range of 120. at 400 ° C. Method according to one of claims 1 to 6, characterized in that superheated steam or a mixture of gases containing superheated steam is used as the fluidizing gas. 8. Method according to one of claims 1 to 7, characterized in that the gas stream leaving the turbulent layer is divided into two partial streams, the first partial stream is heated again and reused as gas of fluidization and the second partial flow (excess gas) is lled from the circulation. 9. Process according to claim 8, characterized in that the first partial stream before or after its heating is mixed with a gas containing hydrocyanuric acid and the gas mixture is used as a fluidizing gas. 10. Process according to claim 9, characterized in that the first partial stream is added to a reaction gas containing essentially HCN and H2 of a BMA or Shawinigan process with a temperature in the range of 1000 to 1500 ° C, cooling the reaction gas and heating the first partial stream, and the obtained gas mixture is used as the fluidizing gas. 11. Process according to claim 3, characterized in that a reaction gas containing HCN leaving the Andrussow process reactor with a temperature above 1000 ° C., it is rapidly cooled by the injection of water and is thus cooled to a temperature in the range of 150 to 500 ° C and the gas mixture obtained is used as a fluidizing gas. 12. Process according to claim 11, characterized in that a granulation of NaCN or KCN is prepared, in which in addition to the NaOH or KOH solution, an aqueous solution containing NaCN or KCN is sprayed on the germs, and can be obtained the NaCN or KCN solution is a waste gas stream leaving the turbulent layer spray granulation reactor. 13. Process according to claim 11 or 12, characterized in that the gas stream leaving the reactor for the spray granulation in turbulent layer is divided into two partial streams, the first partial stream after the separation of the powder is recycled to the Andrussow reactor and the second partial stream is expelled from the circulation. 14. Alkaline cyanide granules based on sodium or potassium cyanide, having the following characteristics: (i) essentially spherical particles with flat or granulated surface structure such as raspberry, (ii) diameter of the particles in the range of 0.1 to 20mm, for 99% by weight of the granulate, (iii) bulk weight of at least 600 g / dm3, (iv) wear less than 1% by weight, measured in a roller wear test (wear tester TAR of the firm Erweka with sample of 20 g, 60 minutes, 20 rpm) (v) maximum agglomeration index 4, measured after a load of lOOg for 14 days in a cylinder with a reduced thickness of 5.5cm with 10 kg. which is characterized in that the alkali carbonate content is less than 0.5% by weight and the alkaline formate content is less than 0.3% by weight, with the alkali metal in the named secondary products being identical to that of the alkali cyanide. 15. Alkaline cyanide granulate according to claim 14, characterized in that the sum of alkali metal carbonate and alkali metal formate is less than 0.4% by weight. 1
6. 6. Alkaline cyanide granulate according to claim 14 or 15, characterized in that it is obtained by means of a process according to one of claims 1, 2, 4 to 10, using the reaction gas of a BMA process and superheated steam as a fluidizing gas.