Composite pellets suitable for kiln process of kiln method phosphoric acid and preparation method thereof

Abstract

The application discloses a kind of composite pellets suitable for kiln process of kiln method phosphoric acid rotary kiln process and preparation method thereof, and the composite pellets are core-shell structure of shell covering inner ball, the inner ball includes inner ball material and binder, and the shell includes shell material and binder;The inner ball material includes phosphate rock, carbonaceous reducing agent and adjusting agent, and the addition amount of binder in the inner ball is 1-15% of the mass of inner ball material;The shell material includes phosphate rock, carbonaceous reducing agent and adjusting agent, and the addition amount of binder in the shell is 1-15% of the mass of shell material;The inner ball and shell are compounded into core-shell structure by binder. By changing the composition of the material content in the inner and outer ball material, without changing the material composition of the inner and outer ball material, on the one hand, the problem of impurities introduced by non-similar materials can be avoided, and the problem of high-strength composite pellets difficult to be obtained by mutual bonding of non-similar materials.

Description

Technical Field

[0001] This invention relates primarily to the field of phosphoric acid production technology, and in particular to a composite pellet formulation and preparation method suitable for rotary kiln phosphoric acid production. Background Technology

[0002] In the 1960s, Lapple et al. in the United States first proposed a new process for producing phosphoric acid using a rotary kiln. Based on their research results, Occidental Petroleum Corporation (ORC) conducted semi-industrial trials in the 1980s, forming the prototype of the kiln-based phosphoric acid process, also known as the KPA process. This process combines the reduction of phosphate rock and the oxidation of yellow phosphorus in the same rotary kiln. Because the heat released by the oxidation of yellow phosphorus and carbon is higher than the heat absorbed by the reduction of phosphate rock, the energy consumption of phosphate rock reduction is greatly reduced. At the same time, this process can also process low-grade phosphate rock.

[0003] Currently, there are two raw material formulations for kiln-process phosphoric acid production. The first is a high-calcium formulation (CaO / SiO2 molar ratio 2–8), which is well-suited to the characteristics of my country's phosphate rock, requires less silica, has a high P2O5 content in the kiln feed, and yields a large output per unit volume. However, this formulation requires a reduction temperature exceeding 1450℃, making production difficult. The second is a high-silicon formulation (CaO / SiO2 molar ratio 0.2–0.5), which has a lower reduction temperature (only 1300–1350℃) and a high phosphorus volatilization rate. This has been a focus of research and semi-industrial trials have been conducted. However, this formulation typically requires the addition of large amounts of silica, resulting in a low P2O5 content in the kiln feed and reduced output per unit volume. Furthermore, the rotary kiln is prone to forming rings composed primarily of Si and P, preventing the kiln-process phosphoric acid production from operating stably for extended periods and hindering its industrialization.

[0004] Solving the ring formation problem in the kiln-process phosphoric acid production is key to its industrial application. To this end, many improvement measures have been proposed, including: high-temperature consolidation pretreatment, improved rotary kiln method, composite pelletizing process, and catalyst reduction method.

[0005] The high-temperature consolidation pretreatment method can be seen in Chinese patent CN112320775A, which discloses a rotary kiln phosphoric acid production system and method with external kiln preheating and high-temperature consolidation. By preheating and consolidating externally, most of the impurities in the pellets are removed in advance, which solves the problem of rotary kiln ring formation caused by various factors when low-temperature consolidation pellets enter the kiln. However, the additional high-temperature consolidation process inevitably increases the energy consumption of the overall process.

[0006] An improved rotary kiln method can be seen in Chinese patent CN104211032A, which discloses an improved rotary kiln for reducing phosphate rock in the kiln-based phosphoric acid process and a method for solving ring formation at the kiln tail. Specifically, it aims to prevent significant deviation of the kiln flue gas as it enters the outlet flue, thereby preventing centrifugal physical settling at the kiln tail and allowing the metaphosphoric acid in the kiln gas to directly enter the hydration tower with the exiting flue gas. However, this process only alleviates the ring formation problem in rotary kilns and cannot fundamentally prevent it.

[0007] The catalytic reduction method, disclosed in Chinese patent CN101850953A, involves a method to reduce the reaction temperature and accelerate the reaction of a high-calcium phosphate formulation produced in a kiln process. This invention adds a mineral activator at 0.5%-12% of the total material weight percentage to the high-calcium phosphate formulation produced in a kiln process, reducing the calcium oxide content and silica content by approximately three times, and lowering the reaction temperature from 1450℃-1520℃ to 1260℃-1350℃. However, the activators added include substances such as fluorides, phosphoric acid, chlorides, sulfur, sulfides, sulfuric acid, and sulfates. This introduces other impurities into the phosphate rock, resulting in impurities in the subsequent phosphoric acid products that are unpredictable in conventional processes. It also causes the reduction slag to become hazardous solid waste, difficult to dispose of safely.

[0008] The composite pelletizing process is disclosed in Chinese patent CN104211028A, which describes a composite pellet raw material and its forming method for the kiln-process phosphoric acid production. It employs a core-shell structure where an inner sphere is encased in a shell. The inner sphere consists of an inner pellet material and a binder, while the outer shell consists of a coating material and a binder. The inner pellet material mainly consists of carbonaceous reducing agent powder, phosphate powder, and silica powder, while the coating material mainly consists of carbonaceous reducing agent powder and silica powder. The inner and outer shells are bonded together using a binder to form a core-shell structure. However, this process, by simply coating silica powder (which does not contain phosphorus), significantly reduces the phosphorus content of the entire pellet, thus lowering process efficiency. Furthermore, it fails to consider the low softening temperature of SiO2, and under experimental conditions, cannot completely solve the ring formation problem in the rotary kiln. Additionally, Chinese patent application CN104531984A discloses a method for preparing cold-bonded pellets suitable for the rotary kiln process of kiln-process phosphoric acid production, which also uses silica powder as a coating and suffers from the same problems. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a reaction pellet formulation and its preparation method that has low auxiliary raw material consumption, more stable quality, higher strength, higher P2O5 grade of raw material fed into the furnace and effectively inhibits ring formation in the rotary kiln, so as to solve the problem that the current kiln-based phosphoric acid process cannot achieve long-term stable industrial operation.

[0010] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0011] A composite pellet suitable for rotary kiln process of phosphoric acid production, wherein the composite pellet has a core-shell structure with an outer shell covering an inner pellet. The inner pellet comprises inner pellet material and a binder, and the outer shell comprises outer shell material and a binder. The inner pellet material comprises phosphate rock, carbonaceous reducing agent and modifier, and the amount of binder added to the inner pellet is 1%-15% of the mass of the inner pellet material. The outer shell material comprises phosphate rock, carbonaceous reducing agent and modifier, and the amount of binder added to the outer shell is 1%-15% of the mass of the outer shell material. The inner pellet and the outer shell are combined to form a core-shell structure by a binder.

[0012] The aforementioned composite pellet formulation suitable for rotary kiln phosphoric acid production not only considers the key indicators CaO and SiO2 content in traditional formulations, but more importantly, it is based on our latest research on the phase transformation law of the P2O5-CaO-MgO-SiO2-Al2O3-F multi-component system involved in the reduction of phosphate rock in a non-molten system and the phosphorus reduction / fluorine volatilization behavior therein. It also considers the influence of Mg and Al content and fluorine migration behavior in the formulation on the melting point / strength of the pellets. On this basis, a new composite pellet formulation for rotary kiln phosphoric acid production is creatively proposed, which can effectively solve the problem of easy ring formation in traditional high-silicon batching rotary kilns, and avoid the problem of excessively high reduction temperature required for high-calcium batching.

[0013] Preferably, the carbonaceous reducing agent powder includes one or more of anthracite, bituminous coal, coke, or petroleum coke.

[0014] Preferably, the binder is one or more of the following: sodium carboxymethyl starch, bentonite, sodium humate solution, asphalt, ammonium humate, water glass, sulfite pulp waste liquor, syrup, or lignin sulfonate.

[0015] Preferably, the modifier contains one or more of the elements Ca, Si, Al, or Mg.

[0016] Preferably, the modifier includes one or more of quartz, river sand, dolomite, limestone, alumina, magnesium oxide, or magnesium carbonate.

[0017] Preferably, the inner ball material comprises CaO, SiO2, MgO and Al2O3, the molar ratio of CaO / SiO2 in the inner ball is controlled between 0.35 and 2.0, and the total mass of (MgO+Al2O3) accounts for 2% to 15% of the total mass of the inner ball material. The amount of carbonaceous reducing agent powder, measured in effective carbon, is 6 to 8 times the amount of phosphate rock powder, measured in P2O5.

[0018] Preferably, the shell material comprises CaO, SiO2, MgO and Al2O3, wherein the molar ratio of CaO / SiO2 in the shell material is controlled between 2.0 and 7.0, and the total mass of (MgO+Al2O3) accounts for 10-50% of the total mass of the shell material. The amount of carbonaceous reducing agent powder, measured in effective carbon, is 7-9 times the amount of phosphate rock powder, measured in P2O5.

[0019] This application adjusts the formulation ratios of inner and outer pellets (or shell pellets) to different proportions. The molar ratio of CaO / SiO2 in the inner pellet formulation is controlled between 0.35 and 2.0, and the total mass of (MgO+Al2O3) accounts for 2% to 15% of the total inner pellet mass. This ensures a very high reduction and volatilization rate of phosphorus (P) in the inner pellets and promotes the volatilization of phosphorus (F) into the shell as much as possible in the form of SiF4, thus adjusting the physicochemical properties of the shell. The shell formulation can also fully absorb some of the SiF4 volatilized from the inner pellets. Combined with the control of the CaO / SiO2 molar ratio and the total mass percentage of (MgO+Al2O3), this results in a melting temperature of the outer pellets above 1300℃, making them less prone to melting under rotary kiln conditions and effectively suppressing the problem of ring formation in rotary kilns. Furthermore, this method, which only changes the material content ratio of the inner and outer pellets without altering their material composition, effectively solves the structural strength problem of the entire pellet assembly. It also avoids introducing other impurities that could lead to unforeseen impurity components in the later stages of phosphoric acid production.

[0020] Preferably, the mass percentage of P in the composite pellets is ≥15%.

[0021] Under the same technical concept, this application also provides a method for preparing composite pellets suitable for rotary kiln process of phosphoric acid, including the following steps:

[0022] (1) Core formation: Mix carbonaceous reducing agent, phosphate rock and modifier, add binder, mix thoroughly and then pelletize; during pelletizing, add the binder in the form of drop and / or mist, the amount of addition is 1%-15% of the mass of the mixture, and inner spheres are obtained after pelletizing;

[0023] (2) Shell material preparation: Mix carbonaceous reducing agent, phosphate rock and modifier, and add binder at the same time. Mix thoroughly to obtain shell material;

[0024] (3) Composite molding: The inner balls obtained in step (1) are screened and fed into a pelletizing machine. At the same time, the outer shell material obtained in step (2) is added to the pelletizing machine. During the coating process, the binder is added in the form of droplets and / or mist, and the amount added is 1%-15% of the mass of the mixture. The composite green balls are obtained after the coating process is completed.

[0025] (4) Drying and consolidation: The composite green pellets obtained after step (3) are dried and consolidated to finally form composite pellets.

[0026] Preferably, the drying temperature of the pellets in step (4) is ≤600℃ and the drying time is ≤600min.

[0027] Preferably, the drying process in step (4) is divided into three drying stages: low temperature, medium temperature, and high temperature. The low temperature is 100℃-200℃, the medium temperature is 150℃-250℃, and the high temperature is 200℃-350℃.

[0028] Preferably, in the drying and consolidation step, the composite green pellets are fed into a dryer, which is a slab dryer. This slab dryer is divided into three drying sections along the conveying direction of the composite green pellets: low temperature, medium temperature, and high temperature. In the low temperature drying section, low temperature hot air at 100℃-200℃ is drawn from top to bottom or forced from bottom to top, allowing the low temperature hot air to pass vertically through the material layer and perform flow drying on the composite green pellets. The low temperature hot air originates from the exhaust gas discharged from the high temperature hot air outlet of the high temperature drying section. In the medium temperature drying section, medium temperature hot air at 150℃-250℃ is drawn from top to bottom or forced from bottom to top, allowing the medium temperature hot air to pass vertically through the material layer and perform flow drying on the composite green pellets. In the high temperature drying section, high temperature hot air at 200℃-350℃ is drawn from top to bottom or forced from bottom to top, allowing the high temperature hot air to pass vertically through the material layer and perform flow drying on the composite green pellets.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] (1) The effects of elements such as fluorine, aluminum, and magnesium in phosphate rock, modifiers, and carbonaceous reducing powder on the physicochemical properties of pellets and phosphorus reduction volatilization behavior were fully considered, which effectively reduced the quality requirements of phosphate rock powder, expanded the sources of raw materials, and improved the applicability of the process.

[0031] (2) By simply changing the composition of the material content in the inner and outer pellets without changing the material composition of the inner and outer pellets, we can avoid the problems of impurities introduced by different materials and the problem of different materials being difficult to bond together to obtain high-strength composite pellets.

[0032] (3) Both the inner and outer layers of the composite pellets are made of phosphate rock powder, which works together to form a pellet with a high P content in the total pellets, effectively avoiding the problem of excessive reduction of P grade in the pellets during the preparation of composite pellets. Detailed Implementation

[0033] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0034] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0035] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0036] Example 1

[0037] The method will be further described below with reference to the embodiments, but this does not limit the scope of protection of the present invention.

[0038] This embodiment provides a composite pellet formulation and preparation method suitable for rotary kiln phosphoric acid production. The composite pellet has a core-shell structure with an outer shell covering an inner pellet. The inner pellet is mainly composed of inner pellet material and binder, and the outer shell is mainly composed of outer shell material and binder. The inner pellet material is composed of phosphate rock powder, carbonaceous reducing agent powder, and modifier powder. The amount of binder added to the inner pellet is 6% of the mass of the inner pellet material. The outer shell material is also composed of phosphate rock powder, carbonaceous reducing agent powder, and modifier. The amount of binder added to the outer shell is 8% of the mass of the outer shell material. The inner pellet and the outer shell are combined to form a core-shell structure through the binder.

[0039] In this embodiment, the carbonaceous reducing agent powder in the composite pellets is coke powder, the binder is sodium humate solution, and the modifiers are river sand, limestone, and alumina powder. The molar ratio of CaO / SiO2 in the inner pellets is controlled at 0.38, and the total mass percentage of (MgO + Al2O3) is 4%. The amount of carbonaceous reducing agent powder, measured in effective carbon, is 7 times the amount of phosphate rock powder, measured in P2O5. The molar ratio of CaO / SiO2 in the outer shell is controlled at 2.6, and the total mass percentage of (MgO + Al2O3) is 25%. The amount of carbonaceous reducing agent powder, measured in effective carbon, is 9 times the amount of phosphate rock powder, measured in P2O5. The mass percentage of phosphorus (P) in the composite pellets is 18%.

[0040] The molding method of the above-mentioned composite pellets in this embodiment specifically includes the following steps:

[0041] (1) Preparation of inner spheres: Carbonaceous reducing agent powder, phosphate rock powder and modifier are added to a high-power mixer or grinding mill according to the specified ratio. At the same time, binder is added according to the specified amount. The mixture after thorough mixing is fed into a mixing silo and sent to a pelletizing machine through a metering feeder for pelletizing. During pelletizing, the binder is added in the form of droplets and / or mist, and the amount added is 1%-15% of the mass of the mixture. After pelletizing, inner spheres are obtained.

[0042] (2) Preparation of shell material: Add carbonaceous reducing agent powder, phosphate rock powder and modifier to a high-power mixer or grinding mill according to the specified ratio, and add binder according to the specified amount. After thorough mixing, the shell material is sent to the shell material silo.

[0043] (3) Forming of composite green pellets: The inner pellets obtained in step (1) are subjected to double-layer roller screening to screen out the inner pellets that meet the process requirements and send them to another pelletizing machine for coating. At the same time, the outer shell material obtained in step (2) is fed into the pelletizing machine (by an electronic feeding device in a ratio corresponding to the inner pellet material). During the coating process, the binder is added in the form of droplets and / or mist, and the amount added is 1%-15% of the mass of the mixture. The composite green pellets are obtained after the coating process is completed.

[0044] (4) Drying and Consolidation: The composite green pellets obtained after step (3) are fed into a dryer for drying and consolidation. The dryer used is a slab dryer, which is divided into three drying sections along the conveying direction of the composite green pellets: low temperature, medium temperature, and high temperature. The low temperature drying section is vented with low temperature hot air at 100℃-200℃, which is either drawn from top to bottom or blown from bottom to top, so that the low temperature hot air passes vertically through the material layer and performs flow drying on the composite green pellets. The low temperature hot air originates from the high temperature section. The waste gas discharged from the high-temperature hot air outlet of the drying section; the medium-temperature hot air (150℃-250℃) introduced into the medium-temperature drying section is drawn from top to bottom or forced from bottom to top, allowing the medium-temperature hot air to vertically pass through the material layer and perform flow-through drying of the composite green pellets; the high-temperature hot air (200℃-350℃) introduced into the high-temperature drying section is drawn from top to bottom or forced from bottom to top, allowing the high-temperature hot air to vertically pass through the material layer and perform flow-through drying of the composite green pellets. The final product is a composite pellet with a compressive strength ≥250KN / pellet and a drop strength ≥20 times / 1 meter, fully meeting the requirements of the kiln-process phosphoric acid production process.

Claims

1. A composite pellet suitable for rotary kiln process of phosphoric acid production, characterized in that, The composite pellets have a core-shell structure with an outer shell enclosing an inner sphere. The inner sphere comprises an inner sphere material and a binder, and the outer shell comprises an outer shell material and a binder. The inner sphere material comprises phosphate rock, a carbonaceous reducing agent, and a modifier, with the binder added to the inner sphere at 1%-15% of the inner sphere material's mass. The outer shell material comprises phosphate rock, a carbonaceous reducing agent, and a modifier, with the binder added to the outer shell at 1%-15% of the outer shell material's mass. The inner spheres and outer shell are bonded together to form a core-shell structure. The modifier comprises one or more of the following: quartz, river sand, dolomite, limestone, alumina, magnesium oxide, or magnesium carbonate. The inner pellets contain CaO, SiO2, MgO, and Al2O3. The molar ratio of CaO / SiO2 in the inner pellets is controlled between 0.35 and 2.0, and the total mass of (MgO + Al2O3) accounts for 2% to 15% of the total mass of the inner pellets. The outer shell contains CaO, SiO2, MgO, and Al2O3. The molar ratio of CaO / SiO2 in the outer shell is controlled between 2.0 and 7.0, and the total mass of (MgO + Al2O3) accounts for 10% to 50% of the total mass of the outer shell. The mass percentage of phosphorus (P) in the composite pellets is ≥15%.

2. The composite pellets suitable for the rotary kiln process of phosphoric acid production as described in claim 1, characterized in that, The carbonaceous reducing agent powder includes one or more of anthracite, bituminous coal, coke, or petroleum coke.

3. The composite pellets suitable for the rotary kiln process of phosphoric acid production as described in claim 1, characterized in that, The binder is one or more of the following: sodium carboxymethyl starch, bentonite, sodium humate solution, asphalt, ammonium humate, water glass, sulfite pulp waste liquor, syrup, or lignin sulfonate.

4. The composite pellets suitable for the rotary kiln process of phosphoric acid production as described in claim 1, characterized in that, The amount of carbonaceous reducing agent powder in the inner ball material, measured in effective carbon, is 6 to 8 times the amount of phosphate rock powder, measured in P2O5.

5. The composite pellets suitable for the rotary kiln process of phosphoric acid production as described in claim 1, characterized in that, The amount of carbonaceous reducing agent powder in the outer shell material, measured in effective carbon, is 7 to 9 times the amount of phosphate rock powder, measured in P2O5.

6. A method for preparing composite pellets suitable for rotary kiln phosphoric acid processing as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Core formation: The carbonaceous reducing agent, phosphate rock and modifier are thoroughly mixed to obtain a mixture, and then pelletized. During pelletizing, the binder is added in the form of drop and / or mist, and the amount added is 1%-15% of the mass of the mixture. After pelletizing, inner spheres are obtained. (2) Shell material preparation: Mix carbonaceous reducing agent, phosphate rock and modifier, mix thoroughly to obtain shell material; (3) Composite molding: The inner balls obtained in step (1) are screened and fed into a pelletizing machine. At the same time, the outer shell material obtained in step (2) is added to the pelletizing machine. During the coating process, the binder is added in the form of droplets and / or mist, and the amount added is 1%-15% of the mass of the outer shell material. The composite green balls are obtained after the coating process is completed. (4) Drying and consolidation: The composite green pellets obtained after step (3) are dried and consolidated. The drying temperature of the pellets is ≤600℃ and the drying time is ≤600min. The composite pellets are finally formed.

7. The method for preparing composite pellets suitable for rotary kiln phosphoric acid processing as described in claim 6, characterized in that, The drying process in step (4) is divided into three drying stages: low temperature, medium temperature, and high temperature. The low temperature is 100℃-200℃, the medium temperature is 150℃-250℃, and the high temperature is 200℃-350℃.