A method of preventing the sublimation of ammonium chloride during calcination

Abstract

The application discloses a method for preventing ammonium chloride from sublimating and escaping in a roasting process, and belongs to the field of chemical metallurgy. Alkaline chemical metallurgical materials and ammonium chloride are sequentially subjected to mixing, grinding and compression molding to obtain dense ore agglomerates or ore grains, and the dense ore agglomerates or ore grains are subjected to roasting. In the heating and roasting process, the HCl generated by the thermal decomposition of the ammonium chloride cannot escape, which promotes the reaction of the HCl with alkaline substances in the alkaline chemical metallurgical materials to convert into chlorides, the chlorides are converted into ammine chlorides by absorbing ammonia, and the ammine chlorides on the surface layer of the dense ore agglomerates or ore grains are decomposed by heat to release NH3, so that the probability of the ammonium chloride from sublimating and escaping in the roasting process is greatly reduced. The method has the advantages of high economic efficiency, clean and environmental protection and the like, and is suitable for large-scale industrial production of the roasting of the ammonium chloride in the alkaline chemical metallurgical materials.

Description

Technical Field

[0001] The invention relates to a method for roasting ammonium chloride of an alkaline chemical metallurgical material, in particular to a method for preventing sublimation and escape of ammonium chloride during the roasting process of ammonium chloride of an alkaline chemical metallurgical material, and belongs to the technical field of chemical metallurgy. Background Art

[0002] Ammonium chloride is an inexpensive chlorinating agent used in the chlorination and roasting of chemical and metallurgical materials. During the roasting process, ammonium chloride converts oxides, hydroxides, and carbonates of alkali metals, alkaline earth metals, and rare earth elements into soluble chlorides. However, SiO₂, Al₂O₃, and Fe₂O₃ in the materials do not react with ammonium chloride. Therefore, roasting the materials with ammonium chloride followed by water leaching easily yields a chloride solution low in Si, Al, and Fe. However, ammonium chloride readily decomposes upon heating into ammonia and hydrogen chloride gas. Upon cooling, the ammonia and hydrogen chloride gases condense into fine ammonium chloride particles, a process known as the sublimation of ammonium chloride. When the roasting materials are heated above 100°C, ammonium chloride begins to sublime, producing white smoke and causing large amounts of ammonium chloride to escape. For example, when calcium carbonate is roasted with ammonium chloride, where calcium carbonate powder and ammonium chloride crystals are mixed according to the stoichiometric ratio for the roasting reaction and roasted at 450°C for 2 hours, the resulting calcium carbonate conversion rate is less than 25%. To ensure the conversion rate of calcium carbonate, the amount of ammonium chloride added during mixing must be 2-3 times excessive. During the roasting process, the excess ammonium chloride sublimes under the heat and re-condenses in the flue gas duct, blocking the duct and causing the roasting equipment to not operate normally.

[0003] It has recently been discovered that briquettes can reduce the sublimation and escape of ammonium chloride during the roasting process. For example, calcium carbonate powder and ammonium chloride crystals are mixed with water and then roasted in a conventional disc pelletizer to produce mechanical briquettes similar to glutinous rice balls. Specifically, calcium carbonate powder and ammonium chloride crystals are first mixed with water according to the stoichiometric ratio of the roasting reaction to form briquettes. After the briquettes are dried, they are then roasted in a crucible. This can increase the calcium carbonate conversion rate to approximately 85%. If the briquettes are placed in countercurrent contact with the roasting flue gas within the roasting equipment, HCl gas in the flue gas can be completely absorbed, further improving the calcium carbonate conversion rate. A tubular shaft furnace is the ideal roasting equipment for countercurrent contact with the roasting flue gas. When the briquettes are at least 3 meters thick, the calcium carbonate conversion rate can reach over 95%. A roasting layer height of more than 3 meters places high demands on the strength of the briquettes, otherwise they will collapse, preventing the roasting flue gas from being discharged. Therefore, calcium carbonate powder is mixed with ammonium chloride and then roasted into lumps. A binder is added during the lumping process to increase the strength of the lumps. Furthermore, the lumps need to be air-dried, which adds another step and requires space and equipment. Another drawback of using ammonium chloride lumps in a shaft furnace is that, due to the limited properties of the ammonium chloride roasting product, the production capacity of shaft furnace roasting is generally far less than that of rotary kilns or tunnel kilns. Furthermore, the conversion rate of calcium carbonate roasted in rotary kilns or tunnel kilns is not as high as that of shaft furnace roasting. Summary of the Invention

[0004] In view of the problems existing in the prior art, the object of the present invention is to provide a method for preventing ammonium chloride from sublimating and escaping during a roasting process. The method presses alkaline chemical metallurgical materials and ammonium chloride into dense ore masses or ore particles, which can effectively inhibit the decomposition and escape of ammonium chloride during high-temperature roasting, thereby improving the effective utilization rate of ammonium chloride and reducing the cost of the ammonium chloride roasting process.

[0005] In order to achieve the above technical objectives, the present invention provides a method for preventing ammonium chloride from sublimating and escaping during a roasting process. The method comprises sequentially mixing, grinding, and pressing alkaline chemical metallurgical materials and ammonium chloride to obtain dense ore agglomerates or ore particles, and roasting the dense ore agglomerates or ore particles. The pressing conditions are as follows: pressing the dense ore agglomerates or ore particles with a particle size of 0.5 to 5 cm under a pressure of 0.5 to 50 MPa.

[0006] The key to the technical solution of the present invention lies in compressing ammonium chloride and alkaline chemical and metallurgical materials into airtight, dense ore agglomerates or ore pellets. As is well known, when solid powder materials are compressed, not only does the contact surface between the particles increase, enhancing the interlocking force, but the diameter φ of the channels formed between the particles also decreases. When the channel diameter φ is less than or equal to the mean free path λ of gas molecules at the calcination temperature (i.e., φ≤λ), the gas molecules generated by the calcination are unable to diffuse through the channels and escape. Therefore, during the heating and calcining process of the dense ore agglomerates or ore pellets, the HCl gas generated by the thermal decomposition of ammonium chloride cannot diffuse and escape, but is rapidly absorbed by the alkaline substances within the dense ore agglomerates or ore pellets and converted into chloride. The chloride then absorbs the NH3 generated by the decomposition of the ammonium chloride to form ammonia chloride. The ammonia chloride further decomposes upon thermal decomposition to release ammonia gas. This results in staggered chlorine absorption and ammonia release during the high-temperature calcination process, effectively preventing the sublimation and escape of ammonium chloride. Taking alkaline chemical metallurgical materials containing calcium oxide as an example, the basic principle of roasting calcium oxide with ammonium chloride in dense agglomerates or ore particles is as follows:

[0007] NH4Cl =Δ= HCl + NH3 (1)

[0008] CaO + 2HCl = CaCl2 + H2O (2)

[0009] CaCl2 + xNH3 = CaCl2 • xNH3, x = 1~8 (3)

[0010] CaCl2 + yH2O = CaCl2 • yH2O, y = 1~2 (4)

[0011] CaCl2 • xNH3 =Δ= CaCl2 + xNH3↑ (5)

[0012] CaCl2 • yH2O =Δ= CaCl2 + yH2O↑ (6)

[0013] Among them, the decomposition of ammonium chloride is an endothermic reaction, and the absorption of ammonia by chloride is also an endothermic reaction. During the roasting process, as the surface temperature of the dense ore agglomerate or ore particle increases, reactions (1) to (4) take precedence and continue to penetrate into the interior of the dense ore agglomerate or ore particle. Only after reactions (1) to (4) are completed and the surface temperature of the material further increases, reactions (5) and (6) can proceed. This results in the formation of a special temperature field with cold inside and hot outside in the ore agglomerate or ore particle during the roasting process. In addition, due to the sudden increase in the pressure of ammonia and water vapor generated by the heat on the surface of the dense ore agglomerate or ore particle, the surface will crack and expand, and the core will shrink. The reaction in the dense ore agglomerate or ore particle is equivalent to being carried out in a pressurized closed system. However, pressurization is not conducive to the expansion reaction. Reactions (5) and (6) are both expansion reactions, so reactions (5) and (6) are difficult to proceed in a closed system. Only after the surface of the dense ore agglomerate or ore particle is completely converted into anhydrous chloride and cracked and powdered, can they continue to proceed inside. Therefore, during the heating process, the temperature inside the ore agglomerate or ore pellet will be lower than that of its surface. Only after the hydrates and ammonia compounds on the surface decompose and transform into anhydrous chloride can the temperature inside the ore agglomerate or ore pellet gradually increase. This results in that during the initial heating of the material, although ammonium chloride is heated to decompose into NH3 and HCl gas, the HCl is quickly absorbed by the alkaline material and converted into chloride, and the NH3 is in turn absorbed by the chloride. The resulting ammonia chloride can only decompose and release ammonia gas upon further heating. This causes the two processes of "chlorine absorption" and "ammonia release" of the alkaline chemical metallurgical material during the roasting process to occur in staggered layers. As a result, the HCl gas produced by the decomposition of ammonium chloride cannot volatilize and escape along with the NH3 in the dense ore agglomerate or ore pellet. Based on this principle, pressing ammonium chloride and alkaline chemical metallurgical materials into dense ore agglomerates or ore pellets for chlorination roasting can significantly reduce the sublimation of ammonium chloride and improve its effective utilization rate.

[0014] As a preferred embodiment, the alkaline chemical metallurgical material contains an alkaline compound; the alkaline compound includes at least one of an oxide of M, a hydroxide of M, a carbonate of M, and a basic carbonate of M; and M includes at least one of an alkali metal, an alkaline earth metal, and a rare earth metal. Alkaline compounds include alkali metals (e.g., potassium, sodium, etc.), alkali metal salts (e.g., carbonates, basic carbonates, etc.), alkali metal oxides (e.g., sodium oxide, etc.), alkali metal hydroxides (e.g., sodium hydroxide, etc.), and alkaline earth metals (e.g., calcium, magnesium, etc.), alkaline earth metal salts (e.g., calcium carbonate, magnesium carbonate, basic magnesium carbonate, etc.), alkaline earth metal oxides (e.g., calcium oxide, magnesium oxide, etc.), alkali metal hydroxides (e.g., calcium hydroxide, etc.). Rare earth oxides (e.g., lanthanum oxide, etc.) and rare earth hydroxides (e.g., lanthanum hydroxide, etc.) are also included. The water content of the alkaline chemical metallurgical material is <1%. The alkaline compounds in the alkaline chemical metallurgical material of the present invention play an important role in dense ore clusters or ore particles: the alkaline compounds preferentially absorb hydrogen chloride produced by the decomposition of ammonium chloride. The generated chloride serves as both a binder for the material and an absorbent for ammonia. For example, anhydrous calcium chloride absorbs ammonia to form octaammine calcium chloride, anhydrous magnesium chloride absorbs ammonia to form hexaammine magnesium chloride, and brine absorbs ammonia to form ammoniacal brine.

[0015] As a preferred embodiment, the amount of ammonium chloride added is measured at 0.9 to 1.1 times the theoretical molar amount of ammonium chloride required to convert all alkaline compounds in the alkaline chemical metallurgical material into chlorides. Since the alkaline chemical metallurgical material and ammonium chloride are pressed to form dense ore clusters or ore particles, the utilization rate of ammonium chloride is greatly improved. Therefore, the present invention only needs to add ammonium chloride according to the stoichiometric ratio to achieve a good chlorination roasting effect.

[0016] As a preferred solution, the grinding is to control the material particle size to meet -250 mesh. Grinding the material to an appropriate particle size is beneficial to improving the contact between the materials and improving the reaction efficiency.

[0017] The pressing conditions of the present invention are: pressing into dense ore agglomerates or ore particles with a particle size of 0.5 to 5 cm under a pressure of 0.5 to 50 MPa. Under the pressing conditions, dense ore agglomerates or ore particles that meet the density requirements can be obtained. The particle size of the dense ore agglomerates or ore particles should not be too small or too large. If the particle size is too small, the contact surface between the roasting material and the wall of the roasting container will increase, and the roasting process may cause the material to stick to the wall and form a kiln. If the particle size is too large, the roasting time needs to be extended, otherwise an incompletely roasted kernel will be left. A further preferred pressure is 2 to 20 MPa. By controlling the pressing conditions, the channel diameter φ between the material particles in the dense ore agglomerates or ore particles can be regulated. Under the preferred pressing pressure conditions, the channel diameter φ between the material particles in the dense ore agglomerates or ore particles is less than or equal to the mean free path λ of the gas molecules at the roasting temperature (250 to 650°C).

[0018] As a preferred solution, the calcination conditions are: 250-650° C. for 0.5-3.5 hours. The calcination temperature is determined according to the specific components of the basic chemical metallurgical material.

[0019] Compared with the prior art, the beneficial technical effects brought about by the technical solution of the present invention are:

[0020] 1) The present invention presses alkaline chemical metallurgical materials and ammonium chloride into dense ore masses or ore particles, preventing the escape of hydrogen chloride during the heating process. As a result, the HCl produced by the decomposition of ammonium chloride is forced to react only with alkaline substances to be converted into anhydrous metal chloride. The metal chloride is then cleverly utilized to absorb ammonia, while the ammoniated chloride further decomposes upon heating to release ammonia gas. As a result, the chlorine absorption and ammonia release of the materials during the roasting process are staggered, and hydrogen chloride cannot volatilize and escape along with NH3. Compared with traditional processes, the effective utilization rate of ammonium chloride in the roasting process is greatly improved.

[0021] 2) The present invention cleverly utilizes the metal chloride produced by the reaction of ammonium chloride with alkaline substances in alkaline chemical metallurgical materials as a binder for the roasted material, which easily forms dense ore clusters or ore particles during the extrusion and temperature-raising roasting process. The resulting dense ore clusters or ore particles have strong toughness at temperatures below 260°C due to the presence of hydrated chloride, and will not burst even when subjected to high internal pressure.

[0022] 3) The present invention uses pressure to compress ammonium chloride and mineral powder into a compacted form. No water is required during the agglomeration or granulation process. This not only eliminates the need for drying the agglomerates, the space and equipment required for drying the agglomerates, but also reduces the energy required for water evaporation during the roasting process. Furthermore, large amounts of water evaporated easily carry ammonium chloride with them through sublimation and escape. For every one percentage point increase in water content during the roasting process, the effective utilization rate of ammonium chloride decreases by approximately one percentage point. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with examples. The following examples are intended to illustrate the present invention rather than to further limit the present invention.

[0024] Example 1

[0025] 100g of limestone powder with a CaCO3 content of 96.31% and 102.95g of analytically pure ammonium chloride crystals were weighed and mixed and ground. Ammonium chloride was added to the mixture to obtain 0.97 times the theoretical molar amount of anhydrous calcium chloride required for the conversion of CaCO3 into the mixture. The mixture was then powdered, pressed, and demoulded to obtain a 1.5cm thick, 2.5cm diameter round cake at a pressure of 5MPa. The resulting round cake was placed in a crucible and calcined in a muffle furnace at 600°C for 1h to obtain calcined sand. Even if the muffle furnace door was opened during the calcination process, no fumes from the sublimation of the ammonium chloride were observed. The calcined sand was cooled to room temperature, dissolved in water, and filtered to obtain a filtrate and a filter residue. The filter residue was washed, dried, and sampled for analysis. The slag showed that the effective utilization rate of the ammonium chloride during the calcination process reached 97.15%.

[0026] Example 2

[0027] Weigh 80.33g of basic magnesium carbonate (Mg(OH)CO) and 1112.33g of ammonium chloride, mix and grind to -250 mesh. Add ammonium chloride at 1.05 times the theoretical molar amount required to prepare anhydrous magnesium chloride from basic magnesium carbonate, mix and grind, then powder, press, and demold to obtain a round cake with a thickness of 1cm and a diameter of 2cm. The pressing pressure of the round cake is 2.5MPa. The resulting round cake is placed in a crucible and sent to a muffle furnace for roasting at 450°C for 2h to obtain calcine. The calcine is cooled to room temperature, dissolved in water, and filtered to obtain a filtrate and a filter residue. The filter residue is washed, dried, and sampled for analysis. The effective utilization rate of ammonium chloride during the roasting process is 94.73%.

[0028] Example 3

[0029] Weigh 505g of lanthanum oxide (La2O3) and 50g of ammonium chloride, mix and grind to -500 mesh. Add ammonium chloride (1.02 times the theoretical molar amount of lanthanum oxide required to prepare anhydrous lanthanum chloride) and mix and grind. Then, fill, press, and demold the mixture to form a round cake with a thickness of 1cm and a diameter of 2cm. Press the cake at a pressure of 10MPa. Place the resulting cake in a crucible and calcine in a muffle furnace at 330°C for 2.5 hours to obtain calcine. Cool the calcine to room temperature, dissolve it in water, and filter to obtain a filtrate and residue. The residue is washed, dried, and sampled for analysis. The slag indicates that the effective utilization rate of ammonium chloride during the calcination process is 96.48%.

[0030] Comparative Example 1

[0031] Weigh 50g of -500-mesh limestone powder with a 96.31% CaCO3 content and 50g of analytically pure ammonium chloride crystals and mix thoroughly. Add ammonium chloride, equivalent to 1x the theoretical amount required to convert the CaCO3 into anhydrous calcium chloride. Place the mixture in a crucible and roast in a muffle furnace at 600°C for 1 hour to produce calcine. During the roasting process, a large amount of white smoke is emitted. Cool the calcine to room temperature, dissolve it in water, and filter to obtain a filtrate and residue. The residue is washed, dried, and sampled for analysis. The residue indicates that the effective utilization rate of ammonium chloride during the roasting process is 62.13%.

[0032] Comparative Example 2

[0033] Weigh 50g of -500-mesh limestone powder with a 96.31% CaCO3 content and 50g of analytically pure ammonium chloride crystals and mix them evenly. Add ammonium chloride, equivalent to 100% of the theoretical amount required to convert the CaCO3 into anhydrous calcium chloride, and mix thoroughly. Add water as a binder and knead into small pellets with a particle size of 1-1.5cm. Place the pellets in a crucible and roast them in a muffle furnace at 600°C for 1 hour to obtain calcine. The roasting process produces less white smoke than when the mixture is roasted. Cool the calcine to room temperature, dissolve it in water, and filter to obtain a filtrate and residue. The residue is washed, dried, and sampled for analysis. The residue indicates that the effective utilization rate of ammonium chloride during the roasting process is 85.46%.

[0034] According to Comparative Examples 1 and 2, it can be seen that when calcium carbonate powder and ammonium chloride crystals are directly mixed and roasted, ammonium chloride is lost due to volatilization, and the utilization rate of ammonium chloride is low, not exceeding 65%. However, when calcium carbonate powder and ammonium chloride crystals are prepared into briquette by a conventional pelletizing method, the briquette can reduce the volatilization of ammonium chloride, so that the utilization rate of ammonium chloride can be increased to about 85%. Compared with Comparative Examples 1 and 2, Example 1, in which calcium carbonate powder and ammonium chloride crystals are pressed into dense briquette, almost no ammonium chloride is lost due to volatilization, and the effective utilization rate of ammonium chloride is more than 95%.

Claims

1. A method for preventing ammonium chloride from sublimating and escaping during a roasting process, characterized in that: Alkaline chemical metallurgical materials and ammonium chloride are sequentially mixed, ground, and pressed to obtain dense ore agglomerates or ore particles, which are then roasted. The pressing conditions are: pressing at a pressure of 0.5 to 50 MPa to obtain dense ore agglomerates or ore particles with a particle size of 0.5 to 5 cm; and the roasting conditions are: roasting at 250 to 650° C. for 0.5 to 3.5 hours. The alkaline chemical metallurgical materials are alkaline compounds. The alkaline compound is at least one of an oxide of M, a hydroxide of M, a carbonate of M and a basic carbonate of M; and the M is at least one of an alkaline earth metal and a rare earth.

2. A method for preventing ammonium chloride from escaping during sublimation during a roasting process according to claim 1, characterized in that: The amount of ammonium chloride added is measured according to 0.9 to 1.1 times the theoretical molar amount of ammonium chloride required to convert all alkaline compounds in the alkaline chemical metallurgical material into chlorides.

3. A method for preventing ammonium chloride from escaping during sublimation in a roasting process according to claim 1, characterized in that: The grinding is to control the material particle size to meet -250 mesh.