Process for the preparation of calcium cyanamide with high nitrogen content

Abstract

The present application relates to a kind of preparation methods of high nitrogen content calcium cyanamide, comprising: solid calcium source, solid carbon source, additive are mixed to form reactant raw material, the active carbon calcium intermediate of metastable state is produced by heating pre-reaction to reactant raw material;Carbon calcium intermediate in situ with nitrogen nitrogenization reaction preparation calcium cyanamide.The calcium cyanamide prepared in the present application does not need to use calcium carbide as raw material, safe and simple.In addition, the calcium cyanamide produced in the present application has high effective nitrogen content, and the nitrogen content of lime nitrogen produced under optimized conditions can be as high as more than 28%, higher than the purpose industrial level.Third, the lime nitrogen produced in the present application is solid particles, which can be used directly as lime nitrogen fertilizer without granulation.Fourth, when preparing dicyandiamide, the calcium cyanamide produced in the present application does not need to be finely ground into fine powder, and the cyanamide waste residue produced after hydrolysis can be reused as raw material for producing calcium cyanamide, and the nitrogen content can exceed 22%, reaching the standard of superior product.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a method for preparing calcium cyanamide with high nitrogen content. Background Technology

[0002] Calcium cyanamide, commonly known as lime nitrogen, is an important raw material for the synthesis of cyanamide, dicyandiamide, melamine, thiourea, and cyanide melts. It is also an important alkaline nitrogen fertilizer that can be directly used for soil improvement. Industrially, calcium cyanamide is typically produced using the calcium carbide (CaC2) nitridation method. However, calcium carbide has a high density and a very compact structure, and its crystal structure is a very stable cubic CaC2-IV crystal form, resulting in low chemical reactivity and a slow reaction rate with nitrogen. To improve the nitridation efficiency of calcium carbide, two measures are taken in industrial production: firstly, large pieces of calcium carbide from the calcium carbide plant are crushed and finely ground into calcium carbide powder with a particle size of less than 100 mesh before reacting with nitrogen to improve its reactivity; secondly, catalysts such as fluorite are added to increase the nitridation rate. US Patent 4849197 proposes a method for preparing calcium cyanamide by reacting calcium carbide with nitrogen in a rotary kiln with a particle size of less than 0.1 mm under the condition of adding about 1% calcium fluoride as a catalyst. The patent states that due to the relatively complete nitriding reaction, the residual calcium carbide in the nitriding product is only 0.12%, and the nitrogen content can reach 24.55%. However, based on the effective content of the calcium carbide raw material being 62.6%, its theoretical maximum nitrogen content should be lower than 20%. CN202211701812.X discloses a production process for 100,000 tons / year of industrial calcium cyanamide. This process uses a rotary kiln nitriding method. After crushing and removing iron from calcium carbide, 1-3% fluorite and recycled calcium cyanamide are uniformly added. The mixture is then ground into calcium carbide powder (fineness ≥200 mesh) using a ball mill and fed into a nitriding furnace. The powder reacts with nitrogen at a high temperature of 1100-1200℃ and is then calcined to produce calcium cyanamide. The molten block is then removed, crushed, and ground into powder using a ball mill to obtain a finished calcium cyanamide (calcium cyanamide) product with a total nitrogen content ≥23.5% and an effective nitrogen content ≥21.5%.

[0003] The nitrogen content of industrial calcium cyanamide in my country is generally between 20% and 22%. Compared with the theoretical maximum effective nitrogen content of 30.4% for calcium cyanamide produced by the calcium carbide process, there is still considerable room for improvement in the nitrogen content of calcium cyanamide, which also indicates that the current production process of calcium cyanamide needs to be improved.

[0004] Figure 1 A schematic diagram of the calcium carbide method for producing calcium cyanamide in my country is provided. Figure 1It can be seen that calcium carbide requires a series of complex processes before it can be used as a raw material for calcium cyanamide. During this long pretreatment process, small calcium carbide particles readily react with moisture in the air to generate acetylene. This not only poses an explosion hazard but also significantly reduces the content of effective components in the calcium carbide, greatly affecting the nitriding effect. Therefore, in actual production, the crushing, screening, batching, grinding, and conveying of calcium carbide raw materials are usually carried out under nitrogen protection. This not only greatly increases the complexity of the process and production costs but also makes it difficult to obtain calcium cyanamide products with high nitrogen content.

[0005] To avoid using calcium carbide as a raw material, researchers have proposed using more chemically reactive nitrogen-containing compounds such as urea, ammonium bicarbonate, and ammonia to replace chemically inert nitrogen gas as a nitrogen source for the synthesis of calcium cyanamide. For example, KR970701673A, "PROCESS FOR PREPARING CALCIUM CYANAMIDE FROM UREA," proposes a method for synthesizing calcium cyanamide using calcium oxide and urea as raw materials. CN201810665529.3 proposes a method for synthesizing calcium cyanamide using ammonium bicarbonate and calcium oxide as raw materials, while CN202011337739.3 proposes a technical route for the co-production of calcium cyanamide and sulfuric acid using calcium carbonate and ammonium sulfate as raw materials. Additionally, some researchers have proposed using plasma heating technology to ionize nitrogen gas and increase its activity for the synthesis of calcium cyanamide, as in CN201911224260.6. In this technical approach, the inventors use carbon from methane cracking as the carbon source for synthesizing calcium cyanamide. They utilize dielectric barrier discharge to ionize nitrogen and (cracked) methane to generate plasma, which then reacts with calcium oxide under the action of a catalyst to produce a small amount of calcium cyanamide. This method, using methane as a carbon source, is not only costly and inefficient (the carbon from methane cracking accumulates on the outer surface of the calcium oxide solid in the reactor, making it difficult to fully contact and react with the calcium oxide), but also involves high industrial-scale costs for the plasma reactor used. Furthermore, water is generated in the system, and the water vapor readily reacts with the carbon from methane cracking, consuming the carbon source. In conclusion, these new technologies provide a novel approach to the synthesis of calcium cyanamide, but their economic viability and feasibility for scale-up require further research and verification.

[0006] On the other hand, calcium cyanamide synthesized by the calcium carbide nitridation method is usually a dense solid after cooling from a molten state (as described in CN202211701812.X), which requires further grinding and sieving before it can be used as a calcium cyanamide fertilizer product or as a raw material for the production of downstream dicyandiamide, thus increasing costs and losses. JP2015147710(A) proposes a method for blending and granulating calcium cyanamide powder with other additives to improve the fertilization effect of calcium cyanamide as a nitrogen fertilizer.

[0007] Furthermore, it is particularly important to point out that the hydrolysis of calcium cyanamide to prepare dicyandiamide generates a large amount of cyanamide waste residue, the main component of which is calcium carbonate. This waste residue is currently difficult to reuse and is mostly disposed of temporarily through landfill, causing serious environmental pollution. CN113277544A proposes a method for preparing calcium carbonate from dicyandiamide waste residue as a filler in masterbatch; CN106012012B proposes a method for preparing nano-calcium carbonate from dicyandiamide waste residue. However, the economic viability of these methods requires further verification.

[0008] In summary, the current calcium carbide nitriding method for producing calcium cyanamide has a long production process, low nitrogen content, and serious environmental pollution; while the economic viability and technological maturity of new technologies such as the urea method and plasma method for synthesizing calcium cyanamide still need further verification. Summary of the Invention

[0009] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a production method for directly preparing high-nitrogen-content calcium cyanamide using carbon-rich solid carbon, solid calcium source, and nitrogen as raw materials. By controlling the operating conditions and raw material formulation, a metastable activated carbon-calcium intermediate with a porous structure is first obtained. Then, the key intermediate is nitrided in situ with nitrogen to prepare a high-nitrogen-content calcium cyanamide product with a porous structure. Compared with the original technology, this new technology does not require calcium carbide as a raw material, the raw materials are inexpensive, the operation is simple, and the process is short. The prepared porous calcium cyanamide product has high activity and can be directly reacted with water to prepare dicyandiamide or used as fertilizer. The by-product cyanamide waste residue after the hydrolysis of calcium cyanamide to prepare cyanamide can be recycled as a raw material to prepare calcium cyanamide. Therefore, this technical route can realize the green transformation of the industrial calcium cyanamide (-dicyandiamide) production process.

[0010] The technical solution for implementing the present invention is as follows:

[0011] Powdered solid calcium source, solid carbon source, and additives are mixed in a certain proportion to obtain reactant raw materials. Under specific operating conditions and with the combined use of additives, the reactant raw materials are heated for pre-reaction to generate a porous, metastable, and reactive calcium carbon intermediate. The calcium carbon intermediate is then subjected to an in-situ nitriding reaction with nitrogen to convert it into porous calcium cyanamide, providing the calcium cyanamide product. The calcium cyanamide product can be in granular or block form, etc.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] Compared to traditional processes, this method directly uses inexpensive and widely available solid carbon and solid calcium as raw materials, eliminating the need for calcium carbide to prepare calcium cyanamide and avoiding raw material crushing, ball milling, screening, and conveying under nitrogen protection. The process is simpler, production costs are lower, and operation is safer and easier. Furthermore, the calcium cyanamide produced using this invention has a high effective nitrogen content; under optimized conditions, the nitrogen content can reach over 28%, exceeding the target industrial level and other technological levels. The cyanamide waste residue generated after the hydrolysis of calcium cyanamide can be reused as a raw material for calcium cyanamide production, reducing the overall emission of cyanamide waste residue, and achieving a nitrogen content that meets the superior grade standard. The granular calcium cyanamide of this invention has high activity and can directly react with water to prepare dicyandiamide or be used as fertilizer without crushing and ball milling. Compared to other reported new processes, such as the urea method and the plasma methane method, the raw materials are cheaper, the production cost is lower, and it is easier to scale up. Attached Figure Description

[0014] Figure 1 A schematic diagram of the process flow for producing calcium cyanamide and dicyandiamide using the calcium carbide nitridation method in the prior art;

[0015] Figure 2 This is a schematic diagram of the synthetic process for preparing calcium cyanamide according to the present invention.

[0016] Figure 3 The images show physical photos of the raw materials, physical and microscopic structural diagrams of the intermediates, physical and microscopic structural diagrams of the products, and XRD patterns of the products. Detailed Implementation

[0017] The following detailed description discloses an embodiment of the method for preparing high-nitrogen-content calcium cyanamide according to this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0018] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0019] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0020] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0021] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0022] Calcium cyanamide, as a raw material for the preparation of downstream products such as dicyandiamide, not only requires a high nitrogen content but also typically desires a large specific surface area to provide more contact sites with other reactant molecules, thereby exhibiting better reactivity. When used as a nitrogen fertilizer, calcium cyanamide is expected to release nitrogen slowly for better absorption by plants. However, calcium cyanamide in my country is produced via the calcium carbide nitridation method, resulting in a dense, molten solid with extremely low reactivity. When used as a raw material for downstream products like dicyandiamide, it needs to be ground to millimeter or even micrometer scale before use; when used as a nitrogen fertilizer, this pulverized calcium cyanamide needs further shaping to obtain a nitrogen fertilizer with a slow-release effect. Through extensive experimental research, the inventors of this invention have rationally and ingeniously designed the raw material system, operating process, and procedures. By controlling the operating conditions and using additives, the carbon and calcium source materials first generate a chemically active metastable carbon-calcium intermediate. Simultaneously, a porous physical structure is obtained through gas release and in-situ pore formation via carbon etching. Therefore, during nitriding, it can react very efficiently with nitrogen gas in situ, resulting in high-nitrogen-content calcium cyanamide with a porous structure. This calcium cyanamide can not only be used directly as a raw material for the synthesis of downstream products such as dicyandiamide, but also directly as a nitrogen fertilizer.

[0023] This invention utilizes solid carbon sources, solid calcium sources, and nitrogen as raw materials to directly prepare high-nitrogen-content calcium cyanamide in a one-step process. Because it eliminates the need for calcium carbide, it avoids the need for raw material crushing, ball milling, sieving, and conveying under nitrogen protection, making it safe, simple, and low-cost. Furthermore, the calcium cyanamide produced using this invention has a high effective nitrogen content; under optimized conditions, the nitrogen content can reach over 28%, far exceeding the target industrial level. It is particularly noteworthy that the inventors, through extensive research, discovered that unlike calcium cyanamide synthesized using traditional techniques, the cyanamide waste generated during the preparation of downstream products such as dicyandiamide using this invention's technology mainly consists of micro / nano-sized light calcium carbonate and a large amount of micron-sized amorphous carbon. This calcium carbonate exhibits good chemical activity and porosity in the reaction system, and this amorphous carbon, rather than graphite carbon, has high reactivity and can serve as a high-quality carbon source for the synthesis of calcium cyanamide. Therefore, this cyanamide waste can be directly reused as a raw material for the production of calcium cyanamide. For the reasons mentioned above, the technology of this invention not only has good economic benefits, but also greatly reduces the emission of cyanamide waste residue, thus having good social benefits and enabling the green transformation of the traditional calcium cyanamide production process. Therefore, this application was completed based on these considerations.

[0024] Specifically, the present invention provides a method for preparing calcium cyanamide with high nitrogen content, the preparation method comprising the following steps:

[0025] Powdered solid calcium source, solid carbon source, and additives are mixed to obtain reactant raw materials. These reactant raw materials are then subjected to a pre-reaction under heating to produce a metastable, reactive calcium carbide intermediate. This calcium carbide intermediate is then subjected to an in-situ nitriding reaction with nitrogen to provide porous calcium cyanamide. Its porous structure can be derived from… Figure 3 This can be seen from the text.

[0026] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, the solid calcium source is one or more calcium-containing compounds such as limestone, calcite, quicklime, hydrated lime, carbide slag, and cyanamide waste. Preferably, the solid calcium source is a calcium-containing compound such as limestone, calcite, or hydrated lime that readily reacts with carbon to release gas. Quicklime is difficult to react with carbon under relatively mild conditions to release gas, and its reaction effect is poor when used alone as a raw material for synthesizing calcium cyanamide.

[0027] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, the solid carbon source is selected from one or more substances such as coke, coal, semi-coke, semi-coke, biomass carbon, pitch, graphite, activated carbon, starch, tar, and sugar. Preferably, the solid carbon source is semi-coke or semi-coke with high volatile content, as well as biomass carbon and activated carbon with well-developed pore structures, and carbon-based mixtures composed of these and other carbon sources such as coke and anthracite. Pitch and graphite have more stable crystal structures and relatively poor reactivity when used alone.

[0028] In the method for preparing high-nitrogen-content porous calcium cyanamide provided by this invention, additives can be used to assist in etching carbon source materials, act as catalysts and stabilizers for forming metastable calcium carbonate intermediates, or act as catalysts for nitriding reactions to improve the overall reaction effect. The additives are selected from one or more of metal halides, metal phosphates, metal carbonates, elemental metals, and polyvinylpyrrolidone (PVP). Further, the metal halides are selected from halides of manganese, barium, calcium, sodium, potassium, iron, copper, or zinc. The metal phosphates are selected from phosphates of calcium, manganese, cobalt, magnesium, iron, copper, or zinc. The metal carbonates are selected from carbonates of calcium, barium, manganese, cobalt, potassium, iron, copper, or zinc. The elemental metals are selected from iron, copper, or zinc. Preferably, the additives are calcium fluoride, calcium phosphate, potassium fluoride, barium carbonate, potassium chloride, copper powder, polyvinylpyrrolidone (PVP), etc. These additives can be used in combination.

[0029] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, powdered solid calcium source, solid carbon source, and additives are mixed and used as reactant raw materials; preferably, the solid calcium source, solid carbon source, and additives are mixed and granulated into particles of 0.2–1.5 cm before being used as reactant raw materials. While directly using powder as raw material can produce high-nitrogen-content calcium cyanamide, it is prone to adhesion to the reactor wall during continuous production, causing scaling and other problems. Furthermore, excessively large particle sizes hinder heat transfer and gas-phase diffusion, thus reducing the reaction efficiency.

[0030] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, the atomic molar ratio of the solid calcium source to the solid carbon source is 1:(2.5-4.5). Optionally, the atomic molar ratio of the solid calcium source to the solid carbon source can be, for example, 1:(2.5-3.5) or 1:(3.5-4.5). The calcium-carbon ratio not only needs to meet the requirements of the chemical reaction, but also affects the molding strength of the raw materials and the activity of the product. When the calcium-carbon ratio is too high, the utilization rate of calcium is low and the material is prone to caking, making nitriding difficult and failing to obtain a qualified calcium cyanamide product; when the calcium-carbon ratio is too low, the raw material cost increases, and the mechanical strength of the molding raw material is poor, which is not conducive to the scale-up of the process.

[0031] In the method for preparing porous calcium cyanamide with high nitrogen content provided by this invention, when using additives, the mass of the additives is 0.01% to 10% of the total mass of the reactant raw materials. Optionally, the mass of the additives can be, for example, 0.01% to 0.5%, 0.5% to 10%, 0.5% to 5%, 5% to 10%, 0.5% to 1%, 1% to 3%, 3% to 5%, 5% to 8%, or 8% to 10% of the total mass of the reactant raw materials. Excessive additives, especially certain inorganic additives, can form dense calcium carbonate intermediates, thereby reducing the activity of subsequent nitriding reactions and hindering the synthesis of calcium cyanamide, while also reducing the content of available nitrogen.

[0032] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, the reactant raw materials are heated and reacted under a non-oxidizing atmosphere. During the heating stage, a non-oxidizing atmosphere can be obtained by sealing and evacuating the gas or by introducing a non-oxidizing protective gas. Optionally, the non-oxidizing gas is selected from inert gases or reducing gases; wherein the inert gas is selected from one or more of argon, nitrogen, etc. The reducing gas is selected from one or more of hydrogen, methane, natural gas, etc.

[0033] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, the reactants first need to be heated for a pre-reaction to obtain a chemically active metastable calcium carbide intermediate. The pre-reaction stage requires maintaining a reaction temperature of 1500℃ to 1900℃ for 0.5 hours to 6 hours. Optionally, the reaction temperature can be, for example, 1500℃ to 1750℃, 1750℃ to 1900℃, 1500℃ to 1600℃, 1600℃ to 1700℃, 1700℃ to 1800℃, 1800℃ to 1900℃, 1500℃ to 1820℃, 1820℃ to 1900℃, 1500℃ to 1880℃, or 1880℃ to 1900℃, etc. Pre-reaction times can be, for example, 0.5–5.5 hours, 5.5–6 hours, 0.5–3.5 hours, 3.5–5.5 hours, 0.5–1 hour, 1–2.5 hours, 0.5–1.5 hours, 1.5–2.5 hours, 2.5–3.5 hours, 3.5–4.5 hours, 4.5–5.5 hours, or 5.5–6 hours. Excessively high temperatures tend to generate chemically inert calcium carbide, thus increasing the difficulty of nitriding; while excessively low reaction temperatures make it difficult to generate effective intermediates. Furthermore, the pre-reaction temperature, atmosphere, and type and amount of additives all interact to influence the formation of calcium carbide intermediates.

[0034] In the method for preparing high-nitrogen-content calcium cyanamide provided by this invention, after the pre-reaction is completed, the calcium carbonate intermediate is cooled to 1300–850°C in a non-oxidizing atmosphere, and then nitrogen gas is introduced to carry out a nitriding reaction. The nitriding time is 0.5–6 hours. Optionally, the temperature can be lowered to 1300–1100°C, 1100–850°C, 1300–1200°C, 1200–850°C, 1300–1000°C, 1000–850°C, 1300–1150°C, 1150–1000°C, 1300–1200°C, 1200–1100°C, 1100–1000°C, 1000–900°C, or 900–850°C, 1100–1200°C, 1150–1200°C, 1200–1250°C, etc. The nitriding time can be, for example, 0.5-1 hour, 1-3 hours, 3-6 hours, 0.5-4 hours, 4-6 hours, 0.5-5 hours, 5-6 hours, etc.

[0035] In the method for preparing high-nitrogen-content calcium cyanamide provided by the present invention, the two operation processes of pre-reaction and nitridation reaction can be carried out in the same reactor using either a batch feed-discharge process or a continuous feed-discharge process.

[0036] 1) When using intermittent feeding and discharging operation, the pre-reaction and nitriding reaction are completed in the same reactor by adjusting the temperature and atmosphere through a program; that is, in the intermittent process, the material can be heated and nitrided in the same reactor through batch operation to obtain calcium cyanamide.

[0037] 2) When a continuous feed-discharge operation is adopted, the material continuously passes through different functional zones of the reactor to complete the reaction. That is, in a continuous process, the material is continuously transported to different functional reaction zones (different temperature zones and atmosphere zones of the reactor) to complete the heating and nitriding steps respectively, and then becomes the calcium cyanamide output reaction system.

[0038] In the method for preparing high-nitrogen calcium cyanamide provided by this invention, the calcium cyanamide is hydrolyzed to provide dicyandiamide and cyanamide waste residue, which is recycled as a raw material (solid calcium source) for the synthesis of calcium cyanamide. The cyanamide waste residue contains calcium carbonate, carbon, magnesium oxide, aluminum oxide, iron oxide, silicon oxide, etc.

[0039] This invention also provides calcium cyanamide with high nitrogen content prepared by the preparation method described herein. The physical image and microstructure diagram of calcium cyanamide are shown below. Figure 3 Calcium cyanamide has a porous structure. This invention can produce granular or blocky calcium cyanamide, with granular calcium cyanamide being preferred.

[0040] It should be noted that, as can be easily understood from the above description, the types and amounts of additives, as well as operating conditions such as atmosphere and temperature, have an interactive effect on the synthesis of intermediates and the formation of calcium cyanamide, requiring rational design. The following examples further illustrate the beneficial effects of the present invention.

[0041] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention is further described in detail below with reference to embodiments. However, it should be understood that the embodiments of this invention are merely for illustrative purposes and not for limiting the invention, and the embodiments are not limited to those given in the specification. Unless otherwise specified, specific experimental or operational conditions in the embodiments were prepared under conventional conditions or according to the conditions recommended by the material supplier.

[0042] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0043] Unless otherwise specified, all reagents, materials and instruments used in the following embodiments are commercially available.

[0044] Example 1:

[0045] Limestone (calcium carbonate purity 98.6%) of 100-120 mesh, coconut shell carbon (fixed carbon content 97.2%), PVP-K30, calcium fluoride (particle size around 100 mesh, content 99%), barium carbonate (particle size around 100 mesh, content 98.7%), and water are mixed evenly in a mass ratio of 100:49:2:1.5:8:40 and granulated in a granulator to form particles with a particle size of 0.5 cm. These particles are then dried until the water content does not exceed 2% and used as raw materials for the synthesis of calcium cyanamide.

[0046] The above-mentioned particulate raw materials were heated to 1750℃ in a hydrogen atmosphere and maintained for 1.5 hours. The material temperature was then lowered to 1200-1250℃, and nitrogen gas was introduced to initiate a nitriding reaction. The product obtained after nitriding for 3 hours is calcium cyanamide. The product was analyzed using the national standard method HG / T 5922-2021, and the calcium carbide content was 0.03%, while the available nitrogen content was 28.7%.

[0047] Example 2:

[0048] Semi-coke powder (80-100 mesh), with a fixed carbon content of 87.1% and an ash content of 8.5%, limestone powder (calcium carbonate content 98.6%), and calcium fluoride (approximately 100 mesh, 99% purity) were mixed uniformly at a mass ratio of 55:100:1.2 and used directly as raw material. The mixture was fed into a fixed-bed reactor and pre-reacted at 1880℃ under a hydrogen atmosphere for 1 hour. The material temperature was then lowered to 1150-1200℃, and nitrogen gas was introduced for nitriding. After nitriding for 4 hours, the nitrogen gas supply was stopped, and the resulting product was porous, blocky calcium cyanamide. Analysis of the product using the national standard method HG / T 5922-2021 showed a calcium carbide content of 0.1% and an available nitrogen content of 25.6%.

[0049] Example 3:

[0050] Coke powder (80-100 mesh, fixed carbon content 89.6%, ash content 5.7%), semi-coke powder (fixed carbon content 87.1%, ash content 8.5%), starch, limestone powder (calcium carbonate content 98.6%), potassium fluoride (approximately 100 mesh, 99% purity), and water were mixed evenly in a mass ratio of 30:25:1.5:1.5:100:25 and granulated in a granulator to form particles approximately 1 cm in size. These particles were then dried until the water content did not exceed 2% and used as raw material for the synthesis of calcium cyanamide.

[0051] The above-mentioned particulate raw materials were fed into a fixed-bed reactor and heated to 1820°C in an argon atmosphere. After maintaining this temperature for 1.5 hours, the material temperature was lowered to 1100-1200°C, and nitrogen gas was introduced to initiate a nitriding reaction. After nitriding for 4 hours, the nitrogen gas supply was stopped, and the temperature was lowered. The resulting product was calcium cyanamide. The product was analyzed using the national standard method HG / T 5922-2021, and the calcium carbide content was 0.1%, and the available nitrogen content was 24.8%.

[0052] Example 4:

[0053] The particulate raw material from Example 3 was heated to 1800°C and held for 2 hours in a mixed atmosphere of nitrogen and argon (nitrogen to argon volume ratio of 1:1). The temperature was then reduced to 1100°C at a rate of 2°C / min and held for another 2 hours to complete the nitriding reaction. The resulting product was analyzed using the national standard method HG / T 5922-2021. Calcium carbide was undetectable, and its effective nitrogen content was 26.9%.

[0054] Example 5:

[0055] Limestone powder (80-100 mesh, 98.6% calcium carbonate content), semi-coke powder (87.1% fixed carbon content, 8.5% ash content), calcium fluoride (approximately 100 mesh, 99% content), starch, copper powder (120 mesh, 99.4% content), and water are mixed evenly in a mass ratio of 100:54:2:1.5:0.7:25 and granulated in a granulator to form particles with a particle size of approximately 0.5 cm. These particles are then dried until the water content does not exceed 2% and used as raw material for the synthesis of calcium cyanamide.

[0056] The above-mentioned particulate raw materials were heated to 1750℃ in a nitrogen and hydrogen atmosphere (nitrogen to hydrogen volume ratio of 1:1) and maintained for 1.5 hours. The material temperature was then lowered to 1150-1200℃ for nitriding. The product obtained after nitriding for 5 hours is calcium cyanamide. Analysis of the product using the national standard method HG / T 5922-2021 showed that calcium carbide was not detected, and its effective nitrogen content was 26.1%.

[0057] Example 6:

[0058] Cyanamide waste residue with a moisture content not exceeding 2% (composition shown in Table 1) was mixed evenly with semi-coke powder (fixed carbon content of 87.1%, ash content of 8.5%), sucrose, potassium chloride (purity of 98.7%), quicklime (purity of 98.8%), and water in a mass ratio of 100:18:1:1.5:10:40 and granulated into particles with a particle size of 0.5 cm in a granulator. The granules were then dried until the moisture content did not exceed 2% and used as raw material for the synthesis of calcium cyanamide.

[0059] The above-mentioned particulate raw materials were heated to 1900℃ in an argon atmosphere and maintained for 1.5 hours. The temperature was then lowered to 1200℃ for nitriding. The product obtained after nitriding for 3 hours was calcium cyanamide. Analysis of the product using the national standard method HG / T 5922-2021 showed a calcium carbide content of 0.12% and an available nitrogen content of 22.2%.

[0060] Table 1 Chemical composition of dicyandiamide waste residue

[0061] Content (w / %) 64.3 16.4 0.3 6.5 1.7 10.8

[0062] Comparative Example 1: Calcium cyanamide was synthesized using methane as a carbon source, nitrogen as a nitrogen source, and calcium oxide as a calcium source.

[0063] Following a similar method to Example 2, limestone powder with a particle size of 80-100 mesh (calcium carbonate content 98.6%) and calcium fluoride with a particle size of approximately 100 mesh and a content of 99% were mixed uniformly at a mass ratio of 100:1.2 as raw materials. The mixture was fed into a fixed-bed reactor, and methane gas was continuously introduced. After a pre-reaction at 1880°C for 1 hour under a methane atmosphere, the material temperature was lowered to 1150-1200°C, and nitrogen gas was introduced for nitriding. After nitriding for 4 hours, the product was analyzed using the national standard method HG / T 5922-2021. The product had a calcium carbide content of 0 and a nitrogen content of 0.6%. XRD analysis of the product showed that the main components were calcium oxide and carbon black, indicating that this method cannot effectively synthesize calcium cyanamide.

[0064] Comparative Example 2: Using the raw materials in Example 1, the process flow was changed to a two-step synthesis of calcium cyanamide.

[0065] The particulate raw material from Example 1 was heated to 1750°C in a hydrogen atmosphere and maintained for 1.5 hours. The material was then cooled to room temperature and removed for later use. The cooled material was placed in a reactor, and the temperature was raised to 1200-1250°C in a nitrogen atmosphere and held for nitriding for 3 hours. The nitrided solid product was analyzed using the national standard method HG / T 5922-2021. The calcium carbide content in the product was 0.03%, and the available nitrogen content was 11.7%.

[0066] Comparative Example 3: Calcium cyanamide was synthesized using quicklime and anthracite as raw materials.

[0067] Quicklime (99.9% calcium oxide content, 100-120 mesh or larger), anthracite (94.1% fixed carbon content, 1.6% volatile matter content, 1.9% ash content, and 2.4% other components), PVP-K30, calcium fluoride (approximately 100 mesh particle size, 99% content), barium carbonate (approximately 100 mesh particle size, 98.7% content), and water are mixed evenly in a mass ratio of 100:93:3.5:2.5:12:40 and granulated in a granulator to form particles with a particle size of 0.5 cm. These particles are then dried until the water content does not exceed 2% and used as raw materials for the synthesis of calcium cyanamide.

[0068] The above-mentioned particulate raw materials were heated to 1750℃ in a hydrogen atmosphere and maintained for 1.5 hours. The material temperature was then lowered to 1200-1250℃, and nitrogen gas was introduced to carry out a nitriding reaction. The product obtained after nitriding for 3 hours was analyzed using the national standard method HG / T 5922-2021. The calcium carbide content in the product was 2.9%, and the available nitrogen content was 16.9%.

[0069] Comparative Example 4: Calcium cyanamide was synthesized by changing the atmosphere using the raw materials from Example 1.

[0070] The particulate raw material from Example 1 was heated to 1750°C in a nitrogen atmosphere and maintained for 1.5 hours. The material temperature was then lowered to 1200-1250°C and held for 3 hours. The product obtained under these conditions was analyzed using the national standard method HG / T 5922-2021. The calcium carbide content in the product was 0.03%, and the available nitrogen content was 7.2%.

[0071] As can be seen from the above examples and comparative examples, the properties and proportions of reactant raw materials, the types and amounts of additives, as well as the operating atmosphere and operating conditions of the reaction interact and have a significant impact on the reaction process and the quality of the reaction products.

[0072] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing calcium cyanamide with high nitrogen content, characterized in that, The preparation method consists of the following steps: Powdered solid calcium source, solid carbon source and additives are mixed to obtain reactant raw materials. The reactant raw materials are heated and pre-reacted to obtain metastable active calcium carbon intermediates. The calcium carbon intermediates are nitrided in situ with nitrogen to obtain porous calcium cyanamide. The reaction temperature of the pre-reaction is 1500℃~1900℃, and the reaction time of the pre-reaction is 0.5 hours~6 hours. After the pre-reaction is completed, the calcium carbonate intermediate is cooled to 1300~850℃ in a non-oxidizing atmosphere and then nitrogen is introduced to carry out the nitriding reaction. The solid calcium source is selected from one or more of limestone and cyanamide waste residue; the solid carbon source is selected from one or more of coke, semi-coke, biomass carbon, starch, and sugar; the atomic molar ratio of the solid calcium source and the solid carbon source is 1:(2.5~4.5); the reactant raw materials are heated and reacted under a non-oxidizing atmosphere; the non-oxidizing atmosphere is obtained by sealing and evacuating or introducing a non-oxidizing protective gas, and the non-oxidizing protective gas is selected from argon and hydrogen.

2. The method for preparing high-nitrogen-content calcium cyanamide as described in claim 1, characterized in that, The two operation processes of pre-reaction and nitriding reaction can be carried out in the same reactor using either a batch feed process or a continuous feed process, including any of the following: 1) When using intermittent feed and discharge operation, the pre-reaction and nitriding reactions are completed in the same reactor by adjusting the temperature and atmosphere through a program. 2) When a continuous feeding and discharging operation is adopted, the material continuously passes through different functional zones of the reactor to complete the reaction.

3. The method for preparing high-nitrogen-content calcium cyanamide according to claim 1, characterized in that, It also includes any of the following features: a1) Mix powdered solid calcium source, solid carbon source and additives and use them as reactant raw materials; or mix solid calcium source, solid carbon source and additives and granulate them into particles of 0.2~1.5cm and use them as reactant raw materials; a2) The mass of the additive is 0.01% to 10% of the total mass of the reactant raw materials.

4. The method for preparing high-nitrogen-content calcium cyanamide according to claim 1, characterized in that, The additive is selected from one or more of the following: metal halides, metal phosphates, metal carbonates, elemental metals, and polyvinylpyrrolidone.

5. The method for preparing high-nitrogen-content calcium cyanamide according to claim 4, characterized in that, It also includes one or more of the following features: b1) The metal halide is selected from halides of manganese, barium, calcium, sodium, potassium, iron, copper or zinc; b2) The metal phosphate is selected from calcium, manganese, cobalt, magnesium, iron, copper or zinc phosphates; b3) The metal carbonate is selected from carbonates of calcium, barium, manganese, cobalt, potassium, iron, copper or zinc; b4) The metallic element is selected from iron, copper or zinc.

6. The method for preparing high-nitrogen-content calcium cyanamide according to claim 1, characterized in that, The nitriding time is 0.5-6 hours.

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