Plant and process for producing a solid-state alkali metal cyanide product containing alkali metal hydroxide

Abstract

The present invention relates to a plant and a method for the production of an alkali metal cyanide solid product. To obtain an alkali metal cyanide solid product with a high alkali metal hydroxide weight ratio and to ensure its stability during the operation of the plant, it is proposed to increase the alkali metal hydroxide weight ratio of a suspension taken from the crystallization vessel and fed into the solid-liquid separation device before it enters the solid-liquid separation device.

Description

[0001] The present invention relates to a plant and a process for producing an alkali metal hydroxide-containing alkali metal cyanide solid product.

[0002] In this case, the term "solid product" means that at least 50% by weight of the solid product consists of solids. The solid product may contain a liquid phase in addition to a solid phase.

[0003] When alkali metal cyanide comes into contact with an acid, it decomposes, releasing hydrogen cyanide. Contact with slightly carbonated water, which can occur, for example, when carbon dioxide from the air dissolves in water, is sufficient for this to happen.

[0004] To counteract the unwanted release of hydrogen cyanide during storage, handling, and use of alkali metal cyanide, alkali metal cyanide products contain a small amount of alkali metal hydroxide. The alkali metal hydroxide neutralizes acids, such as hydrochloric acid, and thus prevents the decomposition of the alkali metal cyanide. However, an excessively high proportion of alkali metal hydroxide is problematic due to its hygroscopic properties, especially for relatively dry solid alkali metal cyanide products, which can clump together due to moisture absorbed from the air.

[0005] To produce alkali metal hydroxide-containing alkali metal cyanide solid products, systems with a crystallization vessel can be used, into which aqueous alkali metal hydroxide solution is fed, having been consumed in the formation of aqueous alkali metal cyanide solution. These systems may exhibit unstable behavior with a clearly audible knocking and thumping sound.

[0006] Such a system is known from GB 858,844 A. The system comprises an evaporative crystallizer, which has a crystallization vessel operating under negative pressure and containing a suspension collection chamber. The suspension collection chamber contains a first suspension, which includes a liquid phase with aqueous sodium cyanide and aqueous sodium hydroxide, as well as a solid phase with sodium cyanide crystals. A suspension outlet line directs the first suspension from the suspension collection chamber into a rotary filter, which filters out the sodium cyanide crystals. This process separates the first suspension into a sodium cyanide solid product and a second suspension, which has a lower weight fraction of sodium cyanide crystals than the first suspension. The second suspension is then returned to the suspension collection chamber of the crystallization vessel.

[0007] Upstream of the rotary filter, a reaction loop branches off from the suspension intake line. In this loop, a reactant gas, essentially pure gaseous hydrogen cyanide, is added to the first suspension. This creates a reaction mixture with a third suspension, which, compared to the first suspension, contains a higher proportion of aqueous sodium cyanide and a lower proportion of aqueous sodium hydroxide. The reaction mixture is then directed into the suspension collection chamber of the crystallization vessel.

[0008] Vapors rise from the suspension collection chamber. If the reaction mixture contains a gas phase, this separates and enriches the vapors.

[0009] The vapors undergo a gas scrubbing process. A sieve tray is located in the upper section of the crystallization vessel. This tray is filled with an aqueous sodium hydroxide solution, from which sodium hydroxide solution rains down. The vapors rising towards the sieve tray encounter this rain. The vapors must also pass through the sieve tray. If the vapors contain hydrogen cyanide, further aqueous sodium cyanide is formed upon contact with the sodium hydroxide solution. The remaining sodium hydroxide from the rain is used to replenish the sodium hydroxide reacted in the reaction loop.

[0010] GB 858,844 A states that a sodium hydroxide concentration of more than approximately 4% in the mother liquor in the suspension collection chamber leads, on the one hand, to serious rumbling and knocking as well as strong vibrations in the circulation system, and on the other hand, to an undesirable product with an excessively high sodium hydroxide content. Therefore, it is proposed to keep the sodium hydroxide concentration of the mother liquor in the suspension collection chamber in the range of 0.2 to 3%, preferably below 1%.

[0011] German patent application DE 1064934 A describes how, when using a strong alkaline solution, the reaction with hydrogen cyanide can start very violently. When the hydrogen cyanide enters the alkaline solution, collisions can occur due to the heating of the reaction mixture by the heat released during the reaction. To solve this problem, it is proposed to use an alkaline solution already containing alkali cyanide and to rotate the liquid in the reaction vessel so strongly that a distinct vortex forms.

[0012] Even from a suspension with a comparatively low alkali metal hydroxide concentration, a relatively dry alkali metal cyanide solid product with a comparatively high alkali metal hydroxide concentration can be produced. However, this requires a correspondingly higher residual moisture content in the solid product, which then needs to be dried further. Higher residual moisture has several disadvantages. For example, more energy is required for further drying than if the separation had been performed with lower residual moisture. The more intensive the drying process, the more the decomposition of alkali metal cyanide is promoted, leading to the formation of undesirable alkali metal formate and ammonia. Higher residual moisture also results in a greater tendency for clumping.In a manufacturing plant, caking and build-up can lead to problems, especially blockages and poorer heat transfer to heating and cooling surfaces.

[0013] US Patent 4,847,062 A discloses in Figure 3 an apparatus and a process for producing a sodium cyanide solid product. The apparatus includes a vessel in which absorption and crystallization take place and in which a first suspension is prepared. The first suspension contains a liquid phase with aqueous sodium cyanide and aqueous sodium hydroxide, as well as a solid phase with sodium cyanide crystals. First suspension is drawn from the vessel through a suspension extraction line. A portion of it is fed into a recirculation loop. The remainder is fed into a solid-liquid separation unit, in which the introduced first suspension is separated into a sodium cyanide solid product and a second suspension. The second suspension has a lower weight fraction of sodium cyanide crystals than the first suspension.The second suspension is fed into the recirculation loop and mixed with the first suspension, creating a third suspension. A sodium hydroxide solution is added to the third suspension, creating a fourth suspension. The fourth suspension is introduced into the top of the container. A synthesis gas containing hydrogen cyanide is introduced into the bottom of the container.

[0014] German patent application DE 1235873 A describes a system for producing an alkali metal cyanide solid, comprising a feedstock containing a first suspension. The first suspension consists of a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals. The first suspension is drawn from the feedstock and passed through a filter system where the alkali metal cyanide crystals are separated. This produces a second suspension with a lower weight fraction of alkali metal cyanide crystals compared to the first suspension. An alkali metal hydroxide solution is added to the second suspension, resulting in a third suspension. This third suspension is passed into an absorption tube, down the inner wall of which it flows. Simultaneously, a hydrogen cyanide-containing gas mixture is introduced into the absorption tube.At the lower end of the absorption tube are a fourth suspension and a reaction gas mixture, which flow into the receiving vessel. The reaction gas mixture fills an upper section of the receiving vessel and flows out of it through a pipe.

[0015] US Patent 3,079,239 A discloses a system and a process for the batch production of an aqueous sodium cyanide solution. The system comprises a reaction vessel in which a first sodium cyanide solution, containing aqueous sodium cyanide and aqueous sodium hydroxide, is produced. An aqueous sodium hydroxide solution and liquid hydrogen cyanide are introduced into the vessel. The supply of liquid hydrogen cyanide is controlled according to the sodium hydroxide weight fraction of the first sodium cyanide solution. To determine the sodium hydroxide weight fraction of the first sodium cyanide solution, the first sodium cyanide solution is transferred from the reaction vessel to a measuring loop, where pure water is added for dilution, resulting in a second sodium cyanide solution. The second sodium cyanide solution is then transferred to a measuring unit that measures the electrical conductivity of the second sodium cyanide solution.From this, the sodium hydroxide weight fraction of the first sodium cyanide solution is deduced. The second sodium cyanide solution is introduced into the reaction vessel.

[0016] The present invention is based on the objective of providing a system with a crystallization vessel with which an alkali metal cyanide solid product can be produced which has a good weight fraction of alkali metal hydroxide, whereby it remains as stable as possible in the system.

[0017] The problem is solved according to the invention with a system having the features of claim 1. In the system according to the invention, the alkali mixing device is located downstream of the crystallization vessel and upstream of the solid-liquid separation device. The alkali mixing device allows the reactant liquid containing aqueous solution of alkali metal hydroxide to be added to the first suspension. This allows a suspension (second suspension) to be supplied to the solid-liquid separation device, the liquid phase of which has a higher alkali metal hydroxide content by weight than the liquid phase of the suspension (first suspension) located in the suspension collection chamber of the crystallization vessel.In this way, an alkali metal cyanide solid product can be obtained which, for the sake of greater safety against unintentional release of hydrogen cyanide, has a higher alkali metal hydroxide fraction by weight than an alkali metal cyanide solid product that would otherwise be obtained by solid-liquid separation of the suspension (first suspension) located in the suspension collection chamber of the crystallization vessel.At the same time, the alkali metal hydroxide weight fraction of the liquid phase of the suspension (first suspension) in the suspension collection chamber of the crystallization vessel can be chosen lower for the benefit of a more stable process in the plant than would be possible if the suspension (first suspension) in the suspension collection chamber of the crystallization vessel already had to contain the entire alkali metal hydroxide content required for the desired alkali metal cyanide solid product with greater safety against unwanted release of hydrogen cyanide.

[0018] Preferably, water from the liquid phase of the first suspension can be evaporated in the crystallization vessel. This promotes the formation and growth of alkali metal cyanide crystals.

[0019] Advantageously, the second separation unit discharge line can fluidically connect the solid-liquid separation unit and the crystallization vessel of the suspension production unit. This second discharge line allows the third suspension to be drawn from the solid-liquid separation unit and fed into the crystallization vessel. This third suspension can then be used to produce further alkali metal cyanide solid product. Simultaneously, alkali metal hydroxide, which was consumed during the formation of alkali metal cyanide, can also be added to the crystallization vessel with the third suspension. Less alkali metal hydroxide enters the crystallization vessel with the third suspension than if the reactant liquid containing aqueous solution of alkali metal hydroxide were introduced directly into the crystallization vessel instead of being added to the first suspension via the alkali mixing device.Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension, more alkali metal hydroxide ends up in the alkali metal cyanide solid product. This excess alkali metal hydroxide therefore cannot enter the crystallization vessel.

[0020] Advantageously, the second separation device discharge line can include a pressure drop limiter through which the third suspension can flow, thus maintaining an operating pressure differential between the solid-liquid separation device and the crystallization vessel of the suspension production device while the third suspension flows through the pressure drop limiter. This allows the flow of the third suspension from the solid-liquid separation device into the crystallization vessel to be pressure-neutral with respect to the operating pressure in the crystallization vessel.

[0021] Advantageously, a reaction loop can be provided for the preparation of a fourth suspension, which contains a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, as well as a solid phase with alkali metal cyanide crystals, wherein The first suspension can be introduced into the reaction loop, the reaction loop has a hydrogen cyanide mixing device with which the fourth suspension, which has a lower weight fraction of aqueous dissolved alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension, can be produced by adding a liquid reactant containing hydrogen cyanide to the first suspension, and the fourth suspension can be introduced into the crystallization vessel.

[0022] This allows the formation of alkali metal cyanide to occur outside the crystallization vessel, which further contributes to quieter processes.

[0023] Preferably The reaction loop is fluidically connected to the suspension extraction line, allowing the first suspension to be drawn from the suspension extraction line and directed into the reaction loop, and the reaction loop is fluidically connected to the crystallization vessel, allowing the fourth suspension to be directed from the reaction loop into the crystallization vessel.

[0024] In this way, the reaction loop can be fed from the suspension extraction line and the suspension collection chamber only needs to be equipped with one outlet for the discharge of the first suspension instead of two.

[0025] A particularly preferred feature is the provision of a lye supply line, which is fluidically connected to the crystallization vessel and through which a reactant liquid containing aqueous solution of alkali metal hydroxide can be introduced into the crystallization vessel. This provides an additional means of introducing aqueous solution of alkali metal hydroxide into the crystallization vessel, particularly for replenishing consumed alkali metal hydroxide during the formation of alkali metal cyanide. Furthermore, this allows for a smaller quantity of alkali metal hydroxide to be added to the first suspension via the lye mixing device.

[0026] It is particularly advantageous for the alkali mixing device to be fluidically connected to the alkali supply line, allowing aqueous solution of alkali metal hydroxide-containing reactant liquid to be fed from the alkali supply line into the alkali mixing device. In this way, both the alkali mixing device and the crystallization vessel can be supplied from a single source of aqueous solution of alkali metal hydroxide-containing reactant liquid.

[0027] The present invention further aims to provide a process by which an alkali metal cyanide solid product can be produced which has a good weight fraction of alkali metal hydroxide, and which remains as stable as possible in the equipment used.

[0028] This problem is solved according to the invention by a method with the features of claim 9. By mixing the first suspension with the reactant liquid containing aqueous solution of alkali metal hydroxide, a suspension (second suspension) can be supplied to the solid-liquid separation device, the liquid phase of which has a higher alkali metal hydroxide fraction by weight than the liquid phase of the suspension (first suspension) located in the suspension collection chamber of the crystallization vessel. In this way, an alkali metal cyanide solid product can be obtained which, for greater safety against unintentional release of hydrogen cyanide, has a higher alkali metal hydroxide fraction by weight than an alkali metal cyanide solid product that would otherwise be obtained by solid-liquid separation of the suspension (first suspension) located in the suspension collection chamber of the crystallization vessel.At the same time, the alkali metal hydroxide weight fraction of the liquid phase of the suspension (first suspension) in the suspension collection chamber of the crystallization vessel can be chosen lower for the benefit of a more stable process in the plant than would be possible if the suspension (first suspension) in the suspension collection chamber of the crystallization vessel already had to contain the entire alkali metal hydroxide content required for the desired alkali metal cyanide solid product with greater safety against unwanted release of hydrogen cyanide.

[0029] Preferably, the third suspension can be fed from the solid-liquid separation unit into the crystallization vessel of the suspension production unit. This allows the third suspension to be used to produce further alkali metal cyanide solid product. In this process, alkali metal hydroxide, which was consumed during the formation of aqueous alkali metal cyanide, can also be added to the crystallization vessel with the third suspension. Less alkali metal hydroxide enters the crystallization vessel with the third suspension than if the reactant liquid containing aqueous alkali metal hydroxide were introduced directly into the crystallization vessel instead of being added to the first suspension via the alkali mixing device.Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension, more alkali metal hydroxide ends up in the alkali metal cyanide solid product. This excess alkali metal hydroxide therefore cannot enter the crystallization vessel.

[0030] Advantageously, an operating pressure differential can be maintained between the solid-liquid separation unit and the crystallization vessel while the third suspension is fed from the solid-liquid separation unit into the crystallization vessel. In this way, the third suspension can flow from the solid-liquid separation unit into the crystallization vessel at a pressure neutrality with respect to the operating pressure in the crystallization vessel.

[0031] Especially preferred can The first suspension is directed into a reaction loop, in which a fourth suspension is produced by adding a liquid reactant containing hydrogen cyanide to the first suspension. This fourth suspension contains a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, as well as a solid phase with alkali metal cyanide crystals. Compared to the first suspension, the fourth suspension has a lower weight fraction of aqueous alkali metal hydroxide and a higher weight fraction of alkali metal cyanide. The fourth suspension is then directed into the crystallization vessel.

[0032] This allows the formation of aqueous alkali metal cyanide outside the crystallization vessel, which further contributes to quieter processes.

[0033] Preferably, a reactant liquid containing aqueous solution of alkali metal hydroxide can be introduced into the crystallization vessel via an alkali supply line. This provides an additional method for introducing aqueous solution of alkali metal hydroxide into the crystallization vessel, particularly for replenishing consumed alkali metal hydroxide during the formation of alkali metal cyanide. Furthermore, this method allows for a smaller quantity of alkali metal hydroxide to be added to the first suspension to form the second suspension.

[0034] Particularly advantageous is the removal of aqueous solution containing alkali metal hydroxide from the alkali supply line and its addition to the first suspension in step (c).

[0035] In this way, both the reactant liquid used in step (c) and the reactant liquid that is (directly) fed into the crystallization vessel can be supplied from one source.

[0036] In Figure 1 An embodiment of the invention is illustrated by means of a schematic flowchart and is described below.

[0037] Figure 1A schematic flow diagram illustrates plant 1 for the production of an alkali metal cyanide solid product containing alkali metal hydroxide. Plant 1 comprises a suspension production unit 2 with a crystallization vessel 3, inside of which 4 a suspension collection chamber 5 is provided for collecting the suspension. The suspension collection chamber 5 contains a first suspension, which includes a liquid phase with aqueous alkali metal hydroxide and aqueous alkali metal cyanide, as well as a solid phase with alkali metal cyanide crystals. The first suspension has a surface area 6, which is Figure 1 is also indicated by a surface symbol 7.

[0038] At the lower end 8 of the crystallization vessel 3, the crystallization vessel 3 has a first outlet 9 to which a suspension extraction line 10 is fluidically connected. First suspension can be discharged from the suspension collection chamber 5 through the suspension extraction line 10. The suspension extraction line 10 includes a first pump 11.

[0039] The system 1 includes a lye mixing device 12, with which a reactant liquid containing aqueous alkali metal hydroxide can be added to the first suspension for the production of a second suspension, which contains a liquid phase with aqueous dissolved alkali metal cyanide and aqueous dissolved alkali metal hydroxide as well as a solid phase with alkali metal cyanide crystals, the alkali metal hydroxide weight fraction of which is higher than the alkali metal hydroxide weight fraction of the liquid phase of the first suspension.

[0040] The alkali mixing device 12 has a first inlet 13, a second inlet 14, and an outlet 15. The suspension extraction line 10 is fluidically connected to the first inlet 13. The suspension extraction line 10 thus fluidically connects the suspension collection chamber 5 of the crystallization vessel 3 and the alkali mixing device 12. Initial suspension can be fed from the suspension collection chamber 5 into the alkali mixing device 12 via the suspension extraction line 10.

[0041] A first alkali supply line 16 is fluidically connected to the second inlet 14. The reactant liquid containing aqueous solution of alkali metal hydroxide can be fed into the alkali mixing device 12 through the first alkali supply line 16.

[0042] A discharge line 17 for the caustic mixing device is fluidically connected to outlet 15. Second suspension from the caustic mixing device 12 can be discharged through the discharge line 17.

[0043] The system 1 includes a solid-liquid separation device 18, which, by separating the solid phase of the second suspension, can separate the second suspension into an alkali metal cyanide solid product and a third suspension, wherein the third suspension contains a liquid phase with aqueous alkali metal hydroxide and aqueous alkali metal cyanide, as well as a solid phase with alkali metal cyanide crystals. The alkali metal cyanide crystal fraction by weight of the third suspension is lower compared to the second suspension.

[0044] The solid-liquid separator 18 has an inlet 19, a first outlet 20, and a second outlet 21. The alkaline mixing device discharge line 17 is fluidically connected to the inlet 19. The alkaline mixing device discharge line 17 thus fluidically connects the alkaline mixing device 12 and the solid-liquid separator 18. Second suspension can be introduced into the solid-liquid separator 18 through the alkaline mixing device discharge line 17.

[0045] A first separation device discharge line 22 is fluidically connected to the first outlet 20. Alkali metal cyanide solid product can be discharged from the solid-liquid separation device 18 through the first separation device discharge line 22.

[0046] A second separation device discharge line 23 is fluidically connected to the second outlet 21. Third suspension can be discharged from the solid-liquid separation device 18 through the second separation device discharge line 23.

[0047] The second separation device discharge line 23 is fluidically connected to a first inlet 24 of the crystallization vessel 3. That is, the second separation device discharge line 23 fluidically connects the solid-liquid separation device 18 and the crystallization vessel 3. Third suspension can be discharged from the solid-liquid separation device 18 and introduced into the crystallization vessel 3 via the second separation device discharge line 23. The first inlet 24 of the crystallization vessel 3 is located below the liquid level 6 of the first suspension in the suspension collection chamber 5 of the crystallization vessel 3.

[0048] The second separation device discharge line 23 has a pressure drop limiting device 25 through which the third suspension flows, and which maintains an operating pressure difference between the interior of the solid-liquid separation device 18 and the interior 4 of the crystallization vessel while the third suspension flows through the pressure drop limiting device 25. The flow of the third suspension from the solid-liquid separation device 18 into the crystallization vessel 3 can therefore be pressure-neutral with respect to the operating pressure in the crystallization vessel 3.

[0049] Preferably, the pressure drop limiting device 25 can include a liquid lock. To implement the liquid lock function, a U-shaped pipe section can be provided, for example, which operates on the principle of a siphon. Such a liquid lock could also be referred to as a siphon-type liquid lock. This type of pressure drop limiting device 25 can be designed as a so-called feed well.

[0050] The suspension production device 2 has a reaction loop 26 with which a fourth suspension can be produced, which contains a liquid phase with aqueous dissolved alkali metal hydroxide and aqueous dissolved alkali metal cyanide, and a solid phase with alkali metal cyanide crystals, wherein the fourth suspension has a lower weight fraction of alkali metal hydroxide and a higher weight fraction of alkali metal cyanide than the first suspension.

[0051] The reaction loop 26 includes a hydrogen cyanide mixing device 27, by which a liquid reactant containing hydrogen cyanide can be added to the first suspension. The hydrogen cyanide mixing device 27 has a first inlet 28, a second inlet 29, and an outlet 30. A reaction feed line 31 is fluidically connected to the first inlet 28. In the present embodiment of the invention, the reaction feed line 31 is also fluidically connected to the suspension withdrawal line 10. It thus fluidly connects the suspension withdrawal line 10 to the hydrogen cyanide mixing device 27. Through the reaction feed line 31, first suspension can be fed from the suspension withdrawal line 10 into the hydrogen cyanide mixing device 27. The reaction feed line 31 includes a second pump 32.

[0052] A hydrogen cyanide supply line 33 is fluidically connected to the second inlet 29. The liquid reactant containing hydrogen cyanide can be introduced into the hydrogen cyanide mixing device via the hydrogen cyanide supply line 33.

[0053] A reaction discharge line 34 is fluidically connected to the outlet 30 of the hydrogen cyanide mixing device 27. Fourth suspension can be discharged from the hydrogen cyanide mixing device 27 through the reaction discharge line 34.

[0054] The reaction discharge line 34 is also fluidically connected to a second inlet 35 of the crystallization vessel 3. It thus fluidically connects the hydrogen cyanide mixing device 27 and the crystallization vessel 3. Fourth suspension can be introduced into the crystallization vessel 3 via the reaction discharge line 34.

[0055] The second inlet 35 of the crystallization vessel 3 is provided at a point below the liquid level 6 of the first suspension located in the suspension collection chamber 5 of the crystallization vessel 3.

[0056] The reaction discharge line 34 has a heat exchanger 36, with which heat can be supplied to the fourth suspension.

[0057] The system 1 has a second alkali supply line 37, which is fluidically connected to a third inlet 38 of the crystallization vessel 3. An aqueous solution of alkali metal hydroxide-containing reactant liquid can be introduced into the crystallization vessel 3 via the alkali supply line 37 and the third inlet 38. The third inlet 38 is located at a point above the liquid level 6 of the first suspension in the suspension collection chamber 5 of the crystallization vessel 3. By means of a Figure 1In the distribution device (not shown), the reactant liquid is distributed over a circumferential area of ​​the crystallization vessel 3 and directed to an inner surface 39 of a wall 40 of the crystallization vessel 3, and then runs down the inner surface 39. Where the reactant liquid runs down the inner surface 39, it counteracts the formation of alkali metal cyanide deposits on the inner surface 39.

[0058] In the present embodiment of the invention, the first alkali supply line 16 is fluidically connected to the second alkali supply line 37 and is fed by the latter with the reactant liquid containing aqueous dissolved alkali metal hydroxide.

[0059] Above the distribution device, a gas scrubbing module 41 is provided inside the crystallization vessel 3. The space between the suspension collection chamber 5 and the gas scrubbing module 41 is a first gas chamber 42, and the space above the gas scrubbing module 41 is a second gas chamber 43. The gas scrubbing module 41 can remove hydrogen cyanide contained in vapors rising through the first gas chamber 42. This allows gas scrubbing to take place.

[0060] The crystallization vessel 3 has a fourth inlet 44 for supplying the gas scrubbing module 41 with an aqueous solution of alkali metal hydroxide. This inlet is located at the level of an upper region 45 of the gas scrubbing module 41. It also has a second outlet 46 for discharging gas scrubbing liquid from the gas scrubbing module 41. This second outlet is located at the level of a lower region 47 of the gas scrubbing module 41. A scrubbing module supply line 48 is fluidically connected to the fourth inlet 44 for supplying gas scrubbing liquid, and a scrubbing module discharge line 49 is fluidically connected to the second outlet 46 for discharging gas scrubbing liquid.

[0061] The removed gas scrubbing liquid remains separate from the suspensions and the solid product. It therefore does not flow into any of the first four suspensions or into the solid product.

[0062] A third outlet 51 is provided at the upper end 50 of the crystallization vessel 3, to which a vapor extraction line 52 is fluidically connected. Vapors that have passed through the gas scrubbing module 41 can be discharged from the second gas chamber 43 of the crystallization vessel 3 via the vapor extraction line 52.

[0063] The system 1 comprises a vacuum unit 53 with an inlet 54, a first outlet 55, and a second outlet 56. The vapor extraction line 52 fluidically connects the second gas chamber 43 of the crystallization vessel 3 to the inlet 54 of the vacuum unit 53. The vacuum unit 53 can be used to draw in the vapors and generate the required vacuum for operating the crystallization vessel 3.

[0064] Condensate formed in the vacuum unit 53 can be discharged from the vacuum unit 53 via a first vacuum unit discharge line 57, which is fluidically connected to the first outlet 55 of the vacuum unit 53. The remaining material flow can be discharged from the vacuum unit 53 via a second vacuum unit discharge line 58, which is fluidically connected to the second outlet 56 of the vacuum unit 53.

[0065] In the manufacturing process carried out with Annex 1, the first suspension is taken from the suspension extraction line 10 and fed into the reaction loop 26. In the present embodiment of the manufacturing process, a reactant liquid containing hydrogen cyanide is added to the first suspension in the hydrogen cyanide mixing device 27. Alkali metal hydroxide from the liquid phase of the first suspension and hydrogen cyanide from the reactant liquid are reacted to form alkali metal cyanide and water. Accordingly, the fourth suspension has a lower weight fraction of alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension.

[0066] Alkali metal cyanide crystals are newly formed and existing alkali metal cyanide crystals grow when the saturation limit for alkali metal cyanide is reached in the liquid phase and further alkali metal cyanide is formed in the liquid phase and / or water is removed from the liquid phase, for example by evaporation. In the present embodiment of the manufacturing process, both occur. In reaction loop 26, alkali metal cyanide is formed in the liquid phase, and water is evaporated in crystallization vessel 3. To enable the water to evaporate at low temperatures, the crystallization vessel 3 is operated under reduced pressure. The heat required for the evaporation of water is partly derived from the exothermic reaction of alkali metal hydroxide and hydrogen cyanide to form alkali metal cyanide and water. A further portion of the heat is supplied by further heating the fourth suspension using the heat exchanger 36.The heat exchanger 36 therefore functions as a reboiler. For example, the heat exchanger 36 can heat the fourth suspension to a temperature of approximately 65 °C.

[0067] When water evaporates, alkali metal cyanide can decompose, releasing hydrogen cyanide. This means that hydrogen cyanide-containing vapors can form in crystallization vessel 3.

[0068] The vapors flow through the gas scrubbing unit 41, where hydrogen cyanide is scrubbed from the vapors. The now purified vapors flow into the second gas chamber 43 and exit the crystallization vessel 3 through the outlet 51, being discharged through the vapor extraction line 52. The desired operating vacuum in the crystallization vessel 3 is maintained by the conveying action of the vacuum unit 53 – which draws the vapors from the second gas chamber 43 through the vapor extraction line 52.

[0069] The portion of the first suspension that flows past the connection point of reaction loop 26 and through the suspension extraction line 10 is mixed in the alkali mixing device 12, i.e., downstream of the connection point of reaction loop 26, with aqueous solution of reactant liquid containing alkali metal hydroxide, the weight fraction of which is higher than that of the liquid phase of the first suspension. This produces the second suspension, whose liquid phase has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension.

[0070] The reactant liquid containing aqueous solution of alkali metal hydroxide is supplied to the alkali mixing device 12 through the first alkali supply line 16.

[0071] The second suspension is discharged from the alkali mixing device 12 via the alkali mixing device discharge line 17 and introduced into the solid-liquid separation device 18. In the solid-liquid separation device 18, the second suspension is separated into an alkali metal cyanide solid product and a third suspension during the separation of the solid phase. The third suspension has a lower weight fraction of alkali metal cyanide crystals compared to the second suspension.

[0072] The alkali metal cyanide solid product is discharged from the solid-liquid separation device 18 through the first separation device discharge line 22, and the third suspension is discharged through the second separation device discharge line 23.

[0073] The solid-liquid separation device 18 is a type of solid-liquid separation device that does not completely separate the liquid phase from the solid phase. It can be designed, for example, as a filter or centrifuge. Since the solid-liquid separation device 18 does not completely separate the liquid phase from the solid phase, the alkali metal cyanide solid product also contains a liquid phase. Its weight fraction depends in particular on the efficiency with which the liquid phase is separated from the solid phase in the solid-liquid separation device 18.

[0074] The alkali metal cyanide solid product can be, for example, a crystalline slurry or simply a wet or moist crystalline product. If an even drier alkali metal cyanide solid product is desired, the alkali metal cyanide solid product discharged from the solid-liquid separation unit 18 can be subjected to further drying.

[0075] Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than that of the first suspension, the alkali metal cyanide solid product has a higher weight fraction of alkali metal hydroxide than if the first suspension had been introduced into the solid-liquid separation device 18 instead of the second suspension.

[0076] The third suspension is fed into the crystallization vessel 3 via the second separation device discharge line 23. Since the liquid phase of the third suspension has a higher weight fraction of alkali metal hydroxide than that of the first suspension, introducing the third suspension into the crystallization vessel 3 replenishes a further portion of the alkali metal hydroxide consumed in the reaction loop 26 during the formation of alkali metal cyanide.

[0077] The solid-liquid separation unit 18 operates at a higher absolute operating pressure than the crystallization vessel 3. The crystallization vessel 3 is operated under negative pressure, which lowers the boiling point of water within it. The pressure drop limiting device 25 of the second separation unit discharge line 23 ensures that the flow of the third suspension from the solid-liquid separation unit 18 into the crystallization vessel 3 is pressure-neutral with respect to the operating pressure in the crystallization vessel 3.

[0078] The alkali metal hydroxide used can be, for example, sodium hydroxide, in which case the resulting alkali metal cyanide solid product would be a sodium cyanide solid product, or potassium hydroxide, in which case the resulting alkali metal cyanide solid product would be a potassium cyanide solid product.

Claims

1. Plant (1) for producing a solid-state alkali metal cyanide product containing alkali metal hydroxide, wherein the plant (1) comprises: (i) a suspension production unit (2) for producing a first suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, wherein the suspension production unit (2) has a crystallization vessel (3) in which a suspension collection space (5) for collecting first suspension is provided; (ii) a suspension withdrawal conduit (10) for removing first suspension from the suspension collection space (5) of the crystallization vessel (3); (iii) an alkali admixing unit (12) for producing a second suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, by mixing into the first suspension a reactant liquid containing aqueously dissolved alkali metal hydroxide, wherein the proportion by weight of alkali metal hydroxide of the reactant liquid is higher than the proportion by weight of alkali metal hydroxide of the liquid phase of the first suspension, wherein the suspension withdrawal conduit (10) fluidically interconnects the suspension collection space (5) of the crystallization vessel (3) of the suspension production unit (2) and the alkali admixing unit (12), and first suspension can be led from the suspension collection space (5) into the alkali admixing unit (12) through the suspension withdrawal conduit (10); (iv) an alkali admixing unit removal conduit (17) for removing second suspension from the alkali admixing unit (12); (v) a solid-liquid separation unit (18) with which the second suspension can be separated by separating solid-phase of the second suspension into a solid-state alkali metal cyanide product and a third suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, wherein the alkali admixing unit removal conduit (17) fluidically interconnects the alkali admixing unit (12) and the solid-liquid separation unit (18), and second suspension can be led from the alkali admixing unit (12) into the solid-liquid separation unit (18) through the alkali admixing unit removal conduit (17); (vi) a first separation unit removal conduit (22) for removing solid-state alkali metal cyanide product from the solid-liquid separation unit (18); (vii) a second separation unit removal conduit (23) for removing third suspension from the solid-liquid separation unit (18).

2. Plant according to Claim 1, characterized in that water in the liquid phase of the first suspension can be evaporated in the crystallization vessel (3).

3. Plant according to Claim 1 or 2, characterized in that the second separation unit removal conduit (23) fluidically interconnects the solid-liquid separation unit (18) and the crystallization vessel (3) of the suspension production unit (2), and third suspension can be removed from the solid-liquid separation unit (18) and led into the crystallization vessel (3) through the second separation unit removal conduit (23).

4. Plant according to Claim 3, characterized in that the second separation unit removal conduit (23) has a pressure drop limiting unit (25) through which third suspension can flow and with which an operating pressure difference between the solid-liquid separation unit (18) and the crystallization vessel (3) of the suspension production unit (2) can be maintained while third suspension is flowing through the pressure drop limiting unit (25).

5. Plant according to any of the preceding claims, characterized in that a reaction loop (26) for producing a fourth suspension, containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, is provided, wherein • first suspension can be introduced into the reaction loop (26), • the reaction loop (26) has a hydrogen cyanide admixing unit (27) with which, by mixing a liquid hydrogen cyanide-containing reactant liquid into the first suspension, the fourth suspension having a lower proportion by weight of aqueously dissolved alkali metal hydroxide and a higher proportion by weight of alkali metal cyanide compared to the first suspension can be produced, and • fourth suspension can be introduced into the crystallization vessel (3).

6. Plant according to Claim 5, characterized in that • the reaction loop (26) is fluidically connected to the suspension withdrawal conduit (10), wherein first suspension can be removed from the suspension withdrawal conduit (10) and led into the reaction loop (26), and • the reaction loop (26) is fluidically connected to the crystallization vessel (3), and fourth suspension can be led from the reaction loop (26) into the crystallization vessel (3).

7. Plant according to any of the preceding claims, characterized in that an alkali feed conduit (37) is provided, which is fluidically connected to the crystallization vessel (3) and through which a reactant liquid containing aqueously dissolved alkali metal hydroxide can be led into the crystallization vessel (3).

8. Plant according to Claim 7, characterized in that the alkali admixing unit (12) is fluidically connected to the alkali feed conduit (37), and reactant liquid containing aqueously dissolved alkali metal hydroxide can be led from the alkali feed conduit (37) into the alkali admixing unit (12).

9. Process for producing a solid-state alkali metal cyanide product containing alkali metal hydroxide, wherein the process comprises the following steps: (a) producing a first suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, in a suspension production unit (2); (b) removing first suspension from a crystallization vessel (3) of the suspension production unit (2); (c) producing a second suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, by mixing first suspension removed from the crystallization vessel (3) with a reactant liquid containing aqueously dissolved alkali metal hydroxide, wherein the proportion by weight of alkali metal hydroxide of the reactant liquid is higher than the proportion by weight of alkali metal hydroxide of the liquid phase of the first suspension; (d) introducing the second suspension into a solid-liquid separation unit (18) and separating the second suspension into a solid-state alkali metal cyanide product and a third suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals; (e) removing solid-state alkali metal cyanide product from the solid-liquid separation unit (18); (f) removing third suspension from the solid-liquid separation unit (18).

10. Process according to Claim 9, characterized in that third suspension is led from the solid-liquid separation unit (18) into the crystallization vessel (3) of the suspension production unit (2).

11. Process according to Claim 10, characterized in that an operating pressure difference between the solid-liquid separation unit (18) and the crystallization vessel (3) is maintained while third suspension is being led from the solid-liquid separation unit (18) into the crystallization vessel (3).

12. Process according to any of Claims 9 to 11, characterized in that • first suspension is led into a reaction loop (26), • in the reaction loop (26), a fourth suspension is produced by mixing a liquid hydrogen cyanide-containing reactant liquid into the first suspension and contains a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, wherein the fourth suspension has a lower proportion by weight of aqueously dissolved alkali metal hydroxide and a higher proportion by weight of alkali metal cyanide compared to the first suspension, and • the fourth suspension is led into the crystallization vessel (3).

13. Process according to any of Claims 9 to 12, characterized in that a reactant liquid containing aqueously dissolved alkali metal hydroxide is led into the crystallization vessel (3) through an alkali feed conduit (37).

14. Process according to Claim 13, characterized in that reactant liquid containing aqueously dissolved alkali metal hydroxide is removed from the alkali feed conduit (37) and, in step (c), mixed into the first suspension.