Efficient method for wet briquetting of alkali metal cyanide, in particular sodium cyanide

Abstract

The invention relates to a method for producing alkali metal cyanide briquettes, in particular sodium cyanide briquettes, the method comprising: i) providing an aqueous suspension containing alkali metal cyanide crystals; ii) separating the alkali metal cyanide crystals from the suspension as a moist mass by means of a discontinuously operating centrifuge; iii) temporarily storing the separated moist mass in a buffer container; and iv) feeding the separated moist mass from the buffer container into a continuously operating briquetting machine, in which the moist mass is pressed to form the alkali metal cyanide briquettes. The invention also relates to a device for producing alkali metal cyanide briquettes, in particular sodium cyanide briquettes, in accordance with the method according to the invention.

Description

[0001] Efficient process for wet briquetting of alkali metal cyanide, especially sodium cyanide

[0002] Description

[0003] The present invention relates to a method for producing alkali metal cyanide briquettes, in particular sodium cyanide briquettes, and to an apparatus for carrying out this method.

[0004] Alkali metal cyanides have numerous applications. One particularly important application is their use in the extraction of gold, silver, and other metals through cyanide leaching. Other applications include electroplating and steel hardening. Alkali metal cyanides also serve as starting materials in chemical synthesis. One example is their use in the synthesis of nitriles, which have diverse applications in the chemical industry. Sodium cyanide is a particularly important alkali metal cyanide in industrial applications.

[0005] Typically, metal cyanides are used in the form of briquettes, as this avoids the formation of harmful metal cyanide-containing dust, thus simplifying the use of the metal cyanides.

[0006] Alkali and alkaline earth metal cyanides have long been produced by the neutralization of hydrogen cyanide (HCN), also known as prussic acid. The hydrogen cyanide is used in highly concentrated liquid or gaseous form (e.g., > 99 wt.% HCN) or in dilute gaseous form (e.g., 6-8 wt.% HCN), depending on the production method. In any case, a cyanide solution is formed in which the metal cyanides are still dissolved.

[0007] By concentrating the cyanide solution, usually in a continuous or batch crystallizer under vacuum, a metal cyanide suspension is produced in which metal cyanide crystals form. The crystals are removed from the suspension by various solid-liquid separation devices, such as centrifuges or drum filters. The resulting mother liquor (essentially without crystals) is usually recycled and reused in the process.

[0008] The separated crystals are obtained as a moist mass still containing significant amounts of water. This moist mass is then placed in a steam- or gas-heated drying system and dried to a residual moisture content of 0.1–0.5% by mass.

[0009] The dried metal cyanide powder is then formed into briquettes, e.g. in tablet form, using briquetting machines or tablet presses and subsequently packaged.

[0010] A major problem with existing, state-of-the-art methods for producing alkali metal cyanide briquettes, especially sodium cyanide briquettes, is their high energy consumption. Typically, such a process consumes, for example, 250 to 500 kg of heating steam per ton of HCN, as well as 30 to 70 kW of electrical energy per ton of HCN. This high energy consumption is usually necessary to form sufficiently stable briquettes from the moist mass, briquettes that meet market requirements in terms of strength and chemical resistance. Drying usually requires expensive and maintenance-intensive equipment. Furthermore, the installation of these drying systems typically incurs increased costs, as well as specific building requirements, which also result in higher costs for the user. In addition, the exhaust air generated during the drying process usually requires complex environmental treatment processes.

[0011] Against this background, the present invention was based on the objective of providing a process for the production of alkali metal cyanide briquettes, in particular sodium cyanide briquettes, which avoids the disadvantages described above.

[0012] The present invention was specifically designed to provide a process for producing alkali metal cyanide briquettes, particularly sodium cyanide briquettes, while minimizing energy consumption. The use of costly and maintenance-intensive drying equipment, along with the associated increased building requirements and installation costs, as well as the treatment of the exhaust air generated during drying, should be avoided as far as possible. Furthermore, the briquettes obtained by the process should exhibit good strength and chemical resistance that meet standard market requirements.

[0013] It has been shown that this was achieved by using a batch centrifuge to separate the alkali metal cyanide crystals, especially the sodium cyanide crystals, in conjunction with a continuous briquetting machine, wherein the separated alkali metal cyanide crystals, especially sodium cyanide crystals, are temporarily stored in a buffer tank before being fed to the briquetting machine.

[0014] The invention therefore relates to a process for the production of alkali metal cyanide briquettes, which comprises the following steps: i) providing an aqueous suspension containing alkali metal cyanide crystals, ii) separating the alkali metal cyanide crystals from the suspension as a moist mass using a batch centrifuge, iii) temporarily storing the separated moist mass in a buffer container, and iv) feeding the separated moist mass from the buffer container into a continuously operating briquetting machine, in which the moist mass is compressed to form alkali metal cyanide briquettes.

[0015] The invention relates in particular to a process for the production of alkali metal cyanide briquettes, comprising the following steps: i) providing an aqueous suspension containing alkali metal cyanide crystals, ii) separating the alkali metal cyanide crystals from the suspension as a moist mass using a batch centrifuge, iii) temporarily storing the separated moist mass in a buffer container, and iv) feeding the separated moist mass from the buffer container into a continuously operating briquetting machine, in which the moist mass is compressed into alkali metal cyanide briquettes, wherein the buffer container is provided with a mixing element and a continuous discharge system, and wherein the moist mass separated by the batch centrifuge is temporarily stored in the buffer container and continuously fed from there to the continuously operating briquetting machine.The invention relates in particular to a process for the production of sodium cyanide briquettes, comprising the following steps: i) providing an aqueous suspension containing sodium cyanide crystals, ii) separating the sodium cyanide crystals from the suspension as a moist mass using a batch centrifuge, iii) temporarily storing the separated moist mass in a buffer container, and iv) feeding the separated moist mass from the buffer container into a continuously operating briquetting machine, in which the moist mass is compressed into sodium cyanide briquettes, wherein it is preferred that the buffer container is equipped with a mixing element and a continuous discharge system, and that the moist mass separated by the batch centrifuge is temporarily stored in the buffer container and continuously fed from there to the continuously operating briquetting machine.

[0016] By separating the moist mass containing alkali metal cyanide crystals, particularly sodium cyanide crystals, from the aqueous suspension containing alkali metal cyanide crystals, particularly sodium cyanide crystals, using a batch centrifuge, the separated moist mass has a sufficiently low moisture content so that, after intermediate storage in a buffer tank, it can be pressed into alkali metal cyanide briquettes, particularly sodium cyanide briquettes, in a continuous briquetting machine, preferably a continuous roller press. These briquettes exhibit good strength and chemical resistance and meet standard market requirements. Drying prior to pressing is therefore unnecessary.The inventive method also enables flexible handling of the process, for example by adjusting the process parameters for discontinuous separation in order to set a suitable residual moisture content of the moist mass.

[0017] The following potential savings can be generated by the method according to the invention:

[0018] (i) A drying step involving numerous large, costly, and maintenance-intensive pieces of equipment is eliminated. (ii) Treatment of the exhaust air generated during the drying process is eliminated.

[0019] (iii) Building and installation costs are reduced.

[0020] (iv) The operating costs for electricity and heating steam or heating gas are reduced.

[0021] The invention is explained in detail below.

[0022] An alkali metal cyanide is a cyanide of an alkali metal, such as sodium, potassium, rubidium, or cesium. Alkali metal cyanides are also called alkali cyanides. Alkali metal hydroxides are also called alkali hydroxides.

[0023] The alkali metal cyanide is preferably sodium cyanide or potassium cyanide, with sodium cyanide being particularly preferred.

[0024] It is understood that the alkali metal of the alkali metal cyanide and the alkali metal of the respective alkali metal materials are identical. If the alkali metal cyanide is, for example, sodium cyanide, then the alkali metal cyanide briquette is a sodium cyanide briquette, the alkali metal cyanide crystals are sodium cyanide crystals, the aqueous alkali metal hydroxide solution is a sodium hydroxide solution, the aqueous alkali metal cyanide suspension is a sodium cyanide suspension, and the alkali formate is sodium formate. If the alkali metal cyanide is, for example, potassium cyanide, then the alkali metal cyanide briquette is a potassium cyanide briquette, the alkali metal cyanide crystals are potassium cyanide crystals, the aqueous alkali metal hydroxide suspension is a potassium hydroxide suspension, the aqueous alkali metal cyanide suspension is a potassium cyanide suspension, and the alkali formate is potassium formate.All the following information regarding the production of alkali metal cyanide briquettes applies regardless of the type of alkali metal cyanide, such as sodium cyanide or potassium cyanide.

[0025] The process according to the invention comprises step i) of providing an aqueous suspension containing alkali metal cyanide crystals, in particular sodium cyanide crystals. Providing an aqueous suspension containing alkali metal cyanide crystals, in particular sodium cyanide crystals, typically comprises an absorption step and a crystallization step, in particular ia) an absorption step in the form of absorption of hydrogen cyanide into an aqueous alkali metal hydroxide solution or alkali metal hydroxide suspension, in particular aqueous sodium hydroxide solution or

[0026] sodium hydroxide suspension, forming an aqueous alkali metal cyanide solution, in particular aqueous sodium cyanide solution, and ib) a crystallization step in the form of introducing the aqueous alkali metal cyanide solution, in particular aqueous sodium cyanide solution, into an evaporative crystallizer which is heated and in which a vacuum is present.

[0027] It is understood that the alkali metal in alkali metal cyanide and aqueous alkali metal hydroxide solution or alkali metal hydroxide suspension is identical. An aqueous sodium hydroxide solution or sodium hydroxide suspension is used to produce sodium cyanide briquettes. An aqueous potassium hydroxide solution or potassium hydroxide suspension is used to produce potassium cyanide briquettes.

[0028] The hydrogen cyanide is preferably absorbed in the form of liquid hydrogen cyanide, gaseous hydrogen cyanide or a hydrogen cyanide-containing gas in the aqueous alkali metal hydroxide solution or alkali metal hydroxide suspension, in particular aqueous sodium hydroxide solution or sodium hydroxide suspension.

[0029] The liquid or gaseous hydrogen cyanide can be highly concentrated. For example, the liquid or gaseous hydrogen cyanide can have a hydrogen cyanide content of more than 95% by mass, preferably more than 99% by mass. The liquid or gaseous hydrogen cyanide can be produced, for example, by a conventional synthesis process, such as the Andrussow process, or it can be a waste product from upstream processes.

[0030] The hydrogen cyanide-containing gas can be a hydrogen cyanide-containing synthesis gas, in particular a hydrogen cyanide-containing synthesis gas from the Andrussow process. The proportion of hydrogen cyanide in the hydrogen cyanide-containing gas can vary widely and, for example, be in the range of 4 to 20 wt%, preferably 6 to 8 wt%.

[0031] Several processes are known for the industrial production of HCN as a starting material for the synthesis of alkali metal cyanides. One frequently used process is the so-called Andrussov process. In the Andrussov process, HCN is produced by the catalyzed reaction of methane, ammonia, and atmospheric oxygen. Typically, a mixture of ammonia and methane is reacted on platinum gauzes under the influence of oxygen at high temperatures. The direct synthesis product obtained by the Andrussov process is a mixture of several components, including, in addition to the desired reaction product HCN, unreacted ammonia, hydrogen, nitrogen, and carbon oxides. Typically, the HCN-containing synthesis gas is liquefied by cooling after the reaction, and the hydrogen cyanide is then purified.

[0032] The hydrogen cyanide or the hydrogen cyanide-containing synthesis gas from the Andrussow process can advantageously be used as hydrogen cyanide (starting material) in the process according to the invention.

[0033] The preparation of an aqueous suspension containing alkali metal cyanide crystals, particularly sodium cyanide crystals, typically includes a crystallization step in the form of introducing the prepared aqueous alkali metal cyanide solution, particularly aqueous sodium cyanide solution, into an evaporative crystallizer, which is heated and operated under vacuum. The evaporative crystallizer is a crystallizer in which a portion of the solvent, in this case water, is evaporated. The evaporative crystallizer can be operated continuously or batchwise.

[0034] The crystallization step preferably involves introducing the aqueous alkali metal cyanide solution, particularly aqueous sodium cyanide solution, into an evaporative crystallizer. The crystallizer is heated such that the temperature at the contact surface between the heating element and the alkali metal cyanide solution, particularly sodium cyanide solution, is approximately 60–100°C, preferably approximately 70–90°C, and the pressure is approximately 30–100 mbar, preferably 30–70 mbar. Typically, the evaporative crystallizer is heated by steam. It is generally accepted that increasing the temperature during the crystallization step in the evaporative crystallizer promotes the formation of byproducts, particularly alkali metal formates, especially sodium formates. Therefore, it is advisable to operate at the lowest possible temperature.However, a problem arises here: at low temperatures, very small alkali metal cyanide crystals, especially sodium cyanide crystals, form, which can complicate separation in separation step ii). By combining heating during the crystallization step, such that the contact surface of the heating element with the alkali metal cyanide solution, especially sodium cyanide solution, is maintained at a temperature of approximately 60–100°C, preferably approximately 70–90°C, with the creation of a vacuum during the crystallization step, resulting in a pressure of approximately 30–100 mbar, preferably 30–70 mbar, it is particularly successful in sufficiently suppressing the formation of byproducts, especially formates, while simultaneously achieving good crystal sizes.

[0035] The crystallization solution or suspension is typically heated by a shell-and-tube heat exchanger heated with steam on the shell side. Vacuum steam is preferably used, allowing temperatures of approximately 60 to 100°C, typically around 70 to 90°C, to be reached. These low temperatures largely suppress the formation of byproducts, particularly formates.

[0036] The interplay of these process parameters, specifically the heating and vacuum generation at a pressure of approximately 30 to 100 mbar, preferably approximately 30 to 70 mbar, results in sufficiently large crystal sizes. A grain size distribution with crystal sizes d50 of approximately 50 to 200 pm, particularly approximately 100 to 120 pm, is desired. This enables the almost complete separation of the crystallized alkali metal cyanide crystals, especially sodium cyanide crystals, from the resulting suspension.

[0037] The process according to the invention further comprises, in step ii), the separation of the alkali metal cyanide crystals, in particular sodium cyanide crystals, from the suspension as a moist mass using a batch centrifuge. The separated moist mass contains, in addition to the separated alkali metal cyanide crystals, in particular sodium cyanide crystals, water, which remains as moisture, and optionally impurities. These impurities may be, for example, formates, carbonates, or unreacted alkali metal hydroxide, in particular sodium hydroxide. The remaining mother liquor can be reused, for example, for the aqueous alkali metal cyanide solution, in particular aqueous sodium cyanide solution.

[0038] Preferably, the discontinuous centrifuge is a peeling centrifuge, more preferably a vertical or horizontal peeling centrifuge.

[0039] To adjust the moisture content of the moist mass to a suitable level, the discontinuous centrifuge, in particular a discontinuous peeling centrifuge, is preferably operated at a speed of 10 to 3000 revolutions per minute, preferably 10 to 2000 revolutions per minute, more preferably 30 to 1200 revolutions per minute.

[0040] Preferably, the centrifuge has a capacity such that 100 kg to 2000 kg, preferably 200 kg to 1500 kg, and more preferably 400 kg to 900 kg of moist mass can be obtained in one pass.

[0041] Preferably, the moisture content of the separated moist mass or the moisture content of the moist mass fed to the briquetting machine is between 1.0 and 30% by mass, more preferably between 1.0 and 20% by mass, and more preferably between 2.5 and 12% by mass. The moisture content indicates the water content based on the total weight of the moist mass.

[0042] The inventive method further comprises in step iii) the intermediate storage of the separated moist mass in a buffer container.

[0043] In particular, after separation by means of the discontinuous centrifuge, the moist mass is not fed directly to the continuously operating briquetting machine, but is temporarily stored in a buffer container and fed from there to the briquetting machine, preferably continuously.

[0044] By a method according to the invention, in which the moist mass separated by means of the discontinuously operating centrifuge is temporarily stored in the buffer container and continuously fed from there to the continuously operating briquetting machine, the entire process can be handled more flexibly in order to, for example, make desired adjustments to the process.

[0045] For example, the use of a batch centrifuge, especially a batch peeler centrifuge, allows for flexible adjustment of process parameters. This enables the separation of alkali metal cyanide crystals of various sizes and shapes, as well as the adjustment of different water contents and thus residual contents of alkali metal hydroxide, formate, and carbonate in the separated wet mass (alkali metal cyanide powder). This impacts the safe and high-quality operation of the crystallizer, as the mother liquor produced during centrifugation is recycled back to the crystallizer. The residual content of alkali metal hydroxide, formate, and carbonate in the mother liquor affects the quality of the suspension in the crystallizer and thus the purity of the alkali metal cyanide crystals.The use of a discontinuous centrifuge, in particular a discontinuous peeling centrifuge, thus enables in particular a high and at any time adjustable range of residual moisture in the moist mass and the associated advantages described above.

[0046] The buffer tank serves in particular to homogenize the moist mass separated discontinuously by means of the centrifuge with the moist mass flow, especially the continuous moist mass flow, supplied to the briquetting machine.

[0047] The buffer container can be a simple storage container, e.g., a tank. Advantageously, the buffer container has a sufficient volume to temporarily store a quantity of moist material that allows for simple, continuous operation of the briquetting machine. Preferably, the buffer container is equipped with a mixing element and a discharge system, in particular a continuous discharge system.

[0048] The mixing element in the buffer tank serves to prevent the formation of bridges between the alkali metal cyanide crystals, especially sodium cyanide crystals, in the moist mass, which could lead to clumping. Suitable mixing elements include, for example, conventional mixers or stirring devices. Bridging in the moist mass within the buffer tank must be avoided, as it can obstruct the flow of the moist mass to the briquetting machine.

[0049] The discharge system is used to remove the moist mass from the buffer container, which is then fed to the briquetting machine.

[0050] In step iv), the separated moist mass is fed from the buffer tank to a continuously operating briquetting machine and compressed there into alkali metal cyanide briquettes, in particular sodium cyanide briquettes. Specifically, the moist mass separated by the centrifuge, which is temporarily stored in the buffer tank, is transported from the buffer tank to the briquetting machine. The compression of the moist mass causes further moisture or water to be pressed out, thus removing some of the moisture, and forms it into the desired briquette shape.

[0051] A continuously operating briquetting machine is preferably a continuously operating roller press. Examples of roller presses are twin roller presses and ring roller presses.

[0052] Preferably, the continuously operating briquetting machine is a roller press comprising a feed screw for generating pre-pressure, rollers (preferably two rollers), and a pressing force control system. Preferably, the pressing force control is hydraulic. The pressing pressure between the two rollers is preferably regulated by the rotational speed of the feed screw. The briquettes fall out after passing between the rollers due to the decreasing pressing pressure.

[0053] The rollers have a surface featuring a shape also known as a die. This shape, or die, serves to compress the moist mass into briquettes. The die can be a coating or screwed-on segments. Preferably, the die is a stainless steel die.

[0054] Preferably, the rollers and / or the die mold located on the surface of the roller have a hard surface, e.g., a stainless steel surface. Compared to the commonly used rollers or die molds with elastic surfaces, e.g., made of rubber, a hard surface is advantageous because it allows for good moisture removal and thus a reduction in water content.

[0055] The rollers of the briquetting machine or roller press are preferably steel rollers or cast steel rollers. Steel rollers or cast steel rollers with a stainless steel coating and / or a stainless steel die on the surface are particularly preferred.

[0056] In a preferred embodiment, the continuously operating briquetting machine or roller press has a feed screw for generating pre-pressure, steel rollers or cast steel rollers with a stainless steel coating and / or a stainless steel die on the surface and a hydraulic pressing force control.

[0057] In a preferred embodiment of the roller press, two rollers are operated in opposite directions. The surface of both rollers is preferably provided with a stainless steel coating and / or a stainless steel die.

[0058] The briquetting machine can be adjusted, particularly with regard to compaction force, to control the moisture content of the formed alkali metal cyanide briquettes, especially sodium cyanide briquettes. During briquetting of the moist mass, water is pressed out. The pressed-out water contains dissolved alkali metal cyanide, especially dissolved sodium cyanide, and is removed and can be returned to the crystallizer.

[0059] The moist mass in the briquetting machine is preferably pressed at a temperature of 50 °C to 100 °C, more preferably from 50 °C to 90 °C, and in particular from 60 °C to 80 °C.

[0060] The moist mass in the briquetting machine is preferably compressed with a pressure of 10000 kN / m². 2 up to 50000 kN / m 2 , in particular of 19000 kN / m 2 up to 23000 kN / m 2 , compressed.

[0061] The resulting alkali metal cyanide briquettes, in particular sodium cyanide briquettes, can have any shape. Possible shapes include, for example, briquettes in the form of tablets or pillow-shaped pieces ("pillows"). By briquetting the moist mass containing alkali metal cyanide crystals, in particular sodium cyanide crystals, into alkali metal cyanide briquettes, in particular sodium cyanide briquettes, in the briquetting machine used according to the invention, the moisture content of the moist mass is reduced; that is, the moisture content of the alkali metal cyanide briquettes, in particular the sodium cyanide briquettes, is lower than the moisture content of the moist mass.

[0062] Preferably, the resulting alkali metal cyanide briquettes, in particular sodium cyanide briquettes, have a moisture content of 0.1 to 8 wt%, more preferably 0.1 to 7 wt%, and even more preferably 0.1 to 6 wt%. The moisture content is the water content based on the total mass of the alkali metal cyanide briquettes, in particular sodium cyanide briquettes. Alkali metal cyanide briquettes, in particular sodium cyanide briquettes, with good strength and chemical resistance are obtained.

[0063] An alkali metal cyanide briquette, in particular a sodium cyanide briquette, with such a moisture content has the advantage that, compared to conventionally dried alkali metal cyanide briquettes, in particular sodium cyanide briquettes, significantly lower pressures are required for briquetting in order to form sufficiently firm and therefore marketable and dust-free alkali metal cyanide briquettes, in particular sodium cyanide briquettes.

[0064] An alkali metal cyanide briquette, in particular a sodium cyanide briquette, produced according to the invention has the further advantage that the alkali metal cyanide briquette, in particular a sodium cyanide briquette, can be quickly dissolved again in polar solvent, in particular water.

[0065] Preferably, in addition to the moisture content, no binder is present in the alkali metal cyanide briquettes, especially sodium cyanide briquettes. They are therefore preferably binder-free briquettes.

[0066] Preferably, the proportion of alkali metal cyanide in the alkali metal cyanide briquettes is greater than 86 wt% and / or less than 99 wt%. In particular, the proportion of alkali metal cyanide in the alkali metal cyanide briquettes is in the range of 86 wt% to 98.5 wt%, preferably 90 wt% to 98 wt%, and more preferably 93 wt% to 96 wt%, based on the total mass of the alkali metal cyanide briquette. Similarly, for the preferred sodium cyanide, the proportion of sodium cyanide in the sodium cyanide briquettes is preferably greater than 86 wt% and / or less than 99 wt%. In particular, the proportion of sodium cyanide in the sodium cyanide briquettes is in the range of 86 wt% to 98.5 wt%, preferably 90 wt% to 98 wt%, more preferably 93 wt% to 96 wt%, based on the total mass of the sodium cyanide briquette.In addition to residual moisture (water) and alkali metal cyanide, especially sodium cyanide, the alkali metal cyanide briquettes, especially sodium cyanide briquettes, may contain the aforementioned impurities.

[0067] A particular advantage of the method according to the invention is that the moist mass separated from the suspension does not need to be dried before being fed into the briquetting machine. Drying is understood to be a separate step using a drying device in which the moisture content of the separated moist mass is reduced, e.g., by heating and / or applying a vacuum.

[0068] According to the inventive method, a combination of a batch centrifuge, a buffer tank for intermediate storage of the moist mass (which preferably has a mixing element to prevent bridging of the moist mass and a suitable discharge system), and a continuous briquetting machine, in particular a roller press (which preferably has a feed screw for generating pre-pressure, rollers with stainless steel coating and / or stainless steel dies, and a pressing force control, preferably hydraulic), is used. This combination makes it possible to produce alkali metal cyanide briquettes, in particular sodium cyanide briquettes, which exhibit particularly good strength and chemical resistance.

[0069] The combination described above also makes it particularly feasible to separate a moist mass containing alkali metal cyanide crystals, especially sodium cyanide crystals, from an aqueous suspension containing alkali metal cyanide crystals, preferably formed in a continuously or batchwise operated evaporative crystallizer, using a batchwise centrifuge. The batchwise inflow of moist alkali metal cyanide crystals, especially moist sodium cyanide crystals, (moist mass) is then equalized in a buffer tank, and alkali metal cyanide briquettes, especially sodium cyanide briquettes, are produced continuously. For this purpose, the moisture content of the moist mass can be reduced in the briquetting machine by compression, especially hydraulic compression, to such an extent that the briquettes retain their shape.The extracted liquid corresponds in composition to the aqueous suspension containing alkali metal cyanide crystals, particularly sodium cyanide crystals. This aqueous suspension containing alkali metal cyanide crystals, particularly sodium cyanide crystals, can be collected in the briquetting machine by suitable means and recycled back into the process for further use.

[0070] The present invention further relates to an apparatus for the production of alkali metal cyanide briquettes, in particular sodium cyanide briquettes, according to the method described above according to the invention, wherein the apparatus comprises a) a batch centrifuge for separating alkali metal cyanide crystals, in particular sodium cyanide crystals, from an aqueous suspension containing alkali metal cyanide crystals, in particular sodium cyanide crystals, as a moist mass, b) a buffer container for the intermediate storage of the separated moist mass and c) a continuously operating briquetting machine for compressing the moist mass supplied from the buffer container to form the alkali metal cyanide briquettes, in particular sodium cyanide briquettes.

[0071] The foregoing details, definitions, and preferred embodiments of the process according to the invention apply accordingly to the apparatus according to the invention, in particular with regard to the container for producing the aqueous alkali metal cyanide solution, especially sodium cyanide solution, the evaporative crystallizer, the batch centrifuge, the buffer container, and the briquetting machine. Reference is therefore made to these.

[0072] In the device according to the invention, the buffer tank is preferably provided with a mixing element and with a continuous discharge system.

[0073] The continuously operating briquetting machine is preferably a continuously operating roller press. The device according to the invention further preferably comprises d) a container for absorbing hydrogen cyanide into an aqueous alkali metal hydroxide solution or alkali metal hydroxide suspension, in particular aqueous sodium hydroxide solution or sodium hydroxide suspension, in order to obtain an aqueous alkali metal cyanide solution, in particular aqueous sodium cyanide solution, e) an evaporative crystallizer, which is heatable and to which a vacuum can be applied, for crystallizing the obtained aqueous alkali metal cyanide solution, in particular aqueous sodium cyanide solution, in order to obtain an aqueous suspension containing the alkali metal cyanide crystals, in particular sodium cyanide crystals.

[0074] The invention also relates in particular to the following points:

[0075] Item 1. Method for the production of sodium cyanide briquettes, comprising i) providing an aqueous suspension containing sodium cyanide crystals, ii) separating the sodium cyanide crystals from the suspension by means of a discontinuous centrifuge as a moist mass, iii) temporarily storing the separated moist mass in a buffer tank, and iv) feeding the separated moist mass from the buffer tank into a continuously operating briquetting machine in which the moist mass is compressed to form sodium cyanide briquettes.

[0076] Point 2. Method according to point 1, characterized in that the provision of the aqueous suspension containing sodium cyanide crystals comprises: ia) an absorption step in the form of absorption of hydrogen cyanide into an aqueous sodium hydroxide solution or sodium hydroxide suspension to form an aqueous sodium cyanide solution, wherein the hydrogen cyanide is preferably absorbed in the form of liquid hydrogen cyanide, gaseous hydrogen cyanide or a gas containing hydrogen cyanide, and ib) a crystallization step in the form of introducing the aqueous sodium cyanide solution into an evaporative crystallizer which is heated and in which a vacuum is present.

[0077] Point 3. Method according to point 2, characterized in that the evaporative crystallizer is heated such that the temperature at the contact surface of the heating element to the alkali metal cyanide solution is approximately 60 to 100 °C, preferably approximately 70 to 90 °C, and the pressure is approximately 30 to 100 mbar, preferably approximately 30 to 70 mbar.

[0078] Point 4. Method according to one of the preceding points, characterized in that the discontinuously operating centrifuge is a peeling centrifuge, preferably a horizontal or a vertical peeling centrifuge.

[0079] Point 5. Method according to one of the preceding points, characterized in that the moist mass supplied to the continuously operating briquetting machine, containing the separated sodium cyanide crystals, has a moisture content of 2.5 to 20% by mass, based on the total weight of the moist mass.

[0080] Point 6. Method according to one of the preceding points, characterized in that the continuously operating briquetting machine is a roller press.

[0081] Point 7. Method according to point 6, characterized in that the roller press has a feed screw for generating pre-pressure, rollers, preferably two rollers, and a press force control.

[0082] Point 8. Method according to one of the preceding points, characterized in that the rollers have a die shape on the surface, wherein the die shape is preferably a stainless steel die.

[0083] Point 9. Method according to point 7 or 8, characterized in that the pressing force control is designed as a hydraulic pressing force control, and / or the rollers and / or the die shape have a hard surface.

[0084] Point 10. Method according to points 6 to 9, characterized in that the rollers are steel rollers or cast steel rollers.

[0085] Point 11. A method according to any one of points 7 to 10, characterized in that the continuously operating briquetting machine is a roller press comprising a feed screw for generating pre-pressure, steel rollers or cast steel rollers with a stainless steel coating and / or a stainless steel die on the surface, and a hydraulic pressing force control. Point 12. A method according to any one of the preceding points, characterized in that the moist mass is compressed in the briquetting machine at a temperature in the range of 50 to 100°C, preferably 60 to 80°C, and / or the moist mass is compressed with a pressing force of 10,000 to 50,000 kN / m². 2 preferably 19,000 to 23,000 kN / m 2 , is compressed.

[0086] Point 13. Method according to one of the preceding points, characterized in that the moist mass separated by means of the discontinuously operating centrifuge is temporarily stored in the buffer container and from there continuously fed to the continuously operating briquetting machine.

[0087] Point 14. Method according to one of the preceding points, characterized in that the buffer tank is provided with a mixing device and with a continuous discharge system.

[0088] Point 15. Method according to one of the preceding points, characterized in that the separated moist mass is fed to the continuously operating briquetting machine without prior drying.

[0089] Item 16. Apparatus for the production of sodium cyanide briquettes according to a method according to any one of items 1 to 15, comprising a) a batch centrifuge for separating sodium cyanide crystals from an aqueous suspension containing sodium cyanide crystals as a moist mass, b) a buffer tank for the intermediate storage of the separated moist mass, c) a continuous briquetting machine for compressing the moist mass supplied from the buffer tank to form the sodium cyanide briquettes.

[0090] Point 17. Device according to point 16, characterized in that the buffer container is provided with a mixing element and with a continuous discharge system, and / or the continuously operating briquetting machine is a roller press.

[0091] Item 18. Device according to item 16 or item 17, characterized in that it further comprises d) a container for absorbing hydrogen cyanide into an aqueous sodium hydroxide solution or sodium hydroxide suspension to obtain an aqueous sodium cyanide solution, e) an evaporative crystallizer which is heatable and in which a vacuum can be applied, for crystallizing the aqueous sodium cyanide solution obtained to obtain an aqueous suspension containing the sodium cyanide crystals.

Claims

Patent claims 1. A process for the production of alkali metal cyanide briquettes, in particular sodium cyanide briquettes, comprising: i) providing an aqueous suspension containing alkali metal cyanide crystals; ii) separating the alkali metal cyanide crystals from the suspension as a moist mass by means of a batch centrifuge; iii) temporarily storing the separated moist mass in a buffer tank; and iv) feeding the separated moist mass from the buffer tank into a continuous briquetting machine in which the moist mass is compressed into alkali metal cyanide briquettes, wherein the buffer tank is equipped with a mixing device and a continuous discharge system, and wherein the moist mass separated by means of the batch centrifuge is temporarily stored in the buffer tank and continuously fed from there to the continuous briquetting machine.

2. The method of claim 1, wherein the alkali metal cyanide is sodium cyanide.

3. The method of claim 1, wherein the alkali metal cyanide is potassium cyanide.

4. A method according to any of the preceding claims, characterized in that the provision of the aqueous suspension containing the alkali metal cyanide crystals comprises: ia) an absorption step in the form of absorption of hydrogen cyanide into an aqueous alkali metal hydroxide solution or alkali metal hydroxide suspension to form an aqueous alkali metal cyanide solution, wherein the hydrogen cyanide is preferably absorbed in the form of liquid hydrogen cyanide, gaseous hydrogen cyanide or a gas containing hydrogen cyanide, and ib) a crystallization step in the form of introducing the aqueous alkali metal cyanide solution into an evaporative crystallizer which is heated and in which a vacuum is present.

5. Method according to claim 4, characterized in that the evaporative crystallizer is heated such that at the contact surface of the heating element to the The alkali metal cyanide solution is present at a temperature of about 60 to 100 °C, preferably about 70 to 90 °C, and at a pressure of about 30 to 100 mbar, preferably about 30 to 70 mbar.

6. Method according to one of the preceding claims, characterized in that the discontinuously operating centrifuge is a peeling centrifuge, preferably a horizontal or a vertical peeling centrifuge.

7. Method according to one of the preceding claims, characterized in that the moist mass supplied to the continuously operating briquetting machine with the separated alkali metal cyanide crystals has a moisture content of 2.5 to 20% by mass, based on the total weight of the moist mass.

8. Method according to one of the preceding claims, characterized in that the continuously operating briquetting machine is a roller press.

9. Method according to claim 8, characterized in that the roller press has a feed screw for generating pre-pressure, rollers, preferably two rollers, and a pressing force control.

10. Method according to one of the preceding claims, characterized in that the rollers have a die shape on the surface, wherein the die shape is preferably a stainless steel die.

11. Method according to claim 9 or 10, characterized in that the pressing force control is designed as a hydraulic pressing force control, and / or the rollers and / or the die shape have a hard surface.

12. Method according to claims 8 to 11, characterized in that the rollers are steel rollers or cast steel rollers.

13. Method according to one of claims 8 to 12, characterized in that the continuously operating briquetting machine is a roller press which has a feed screw for generating pre-pressure, steel rollers or cast steel rollers with features a stainless steel coating and / or a stainless steel die on the surface and a hydraulic pressing force control.

14. Method according to one of the preceding claims, characterized in that the moist mass is compressed in the briquetting machine at a temperature in the range of 50 to 100°C, preferably 60 to 80°C, and / or the moist mass is compressed with a pressing pressure of 10,000 to 50,000 kN / m² 2 preferably 19,000 to 23,000 kN / m 2 , is compressed.

15. Method according to one of the preceding claims, characterized in that the separated moist mass is fed to the continuously operating briquetting machine without prior drying.

16. Apparatus for the production of alkali metal cyanide briquettes, in particular sodium cyanide briquettes, according to a method according to any one of claims 1 to 15, comprising a) a batch centrifuge for separating alkali metal cyanide crystals from an aqueous suspension containing alkali metal cyanide crystals as a moist mass, b) a buffer tank for intermediate storage of the separated moist mass, c) a continuous briquetting machine for compressing the moist mass supplied from the buffer tank to form the alkali metal cyanide briquettes.

17. Device according to claim 16, characterized in that the buffer container is provided with a mixing element and with a continuous discharge system, and / or the continuously operating briquetting machine is a roller press.

18. Device according to claim 16 or claim 17, characterized in that it further comprises d) a container for absorbing hydrogen cyanide into an aqueous alkali metal cyanide solution or alkali metal hydroxide suspension to obtain an aqueous alkali metal cyanide solution, e) an evaporative crystallizer which is heatable and in which a vacuum can be applied, for crystallizing the aqueous solution obtained Alkali metal cyanide solution to obtain aqueous suspension containing alkali metal cyanide crystals.