Method for reducing sulfur content of ferric oxide prepared from ferrous sulfate

Abstract

The invention provides a method for reducing the sulfur content of ferric oxide prepared from ferrous sulfate, and relates to the technical field of inorganic chemical material preparation, and the method comprises the following steps: S1, adopting a two-stage crushing process of mechanical grinding and airflow crushing to preliminarily refine the particle size of a raw material from 50-100 microns to 20-30 microns (D50); s2, an external heating type rotary kiln is adopted for heating and calcining, the temperature in the kiln is monitored and controlled in real time, the temperature of a calcining area is controlled to be 820 + / -10 DEG C, and it is ensured that the decomposition rate of ferrous sulfate is larger than or equal to 99.5%; s3, 0.4 MP superheated steam is introduced into the rotary kiln through a steam generator, the steam reacts with hydroxyl groups on the surfaces of the FeSO4 particles, active sites are formed, the sulfur escape rate is increased, and the interior of the kiln is kept in a micro-negative pressure state through a kiln tail fan; and S4, primarily cooling and collecting the calcined product, and collecting most ferric oxide particles. According to the method, the sulfur content in the product is greatly reduced, the product purity is improved, the production period is shortened, and the comprehensive energy consumption is reduced.

Description

Technical Field

[0001] This invention relates to the field of inorganic chemical material preparation technology, and in particular to a method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate. The process optimization method for reducing the sulfur content of the finished product is applicable to non-ferrous metal smelting and the production of high-quality iron-based pigments and magnetic raw materials. Background Technology

[0002] Ferric oxide (Fe2O3) is a reddish-brown powder with wide applications in industry, such as ceramic glaze colorants, capacitor composite materials, and magnetic materials. In industrial production, ferrous sulfate (FeSO4) is often used as an important raw material for the preparation of ferric oxide. Currently, there are two main processes for preparing ferric oxide from ferrous sulfate: wet process and dry process.

[0003] (I) Wet process

[0004] The wet process typically involves rapidly reacting a 5% ferrous sulfate solution with an excess of caustic soda solution (0.04-0.08 g / ml excess alkali), while air is introduced at room temperature to form a reddish-brown ferric hydroxide colloidal solution. Under the catalysis of metallic iron, the ferrous sulfate in the solution is oxidized by the air to ferric oxide and deposited on the crystal nuclei. Sulfate ions react with iron to form ferrous sulfate, and the cycle continues until the reaction is complete. This process relies on the recycling of crystal nuclei, which not only has problems such as a complex production process, long reaction time (8-12 hours per batch), severe equipment corrosion, and high wastewater generation (5-8 tons of alkaline wastewater per ton of product), but also makes it difficult to meet the purity and particle uniformity requirements of high-end electronics and ceramics industries.

[0005] (ii) Dry process

[0006] Dry processes typically involve simple pretreatment of ferrous sulfate followed by calcination at temperatures above 650°C to prepare ferric oxide. The main reaction mechanism is as follows:

[0007] 2FeSO4 === Fe2O3 + SO2↑ + SO3↑ (at high temperature)

[0008] Existing dry processes also include preparing nano-sized iron oxide red products by preparing iron oxide black and then calcining it (e.g., calcining at 600℃ for 0.5h), calcining ferrous sulfate or ferrous nitrate to decompose it; for example, CN105236897A discloses a method for preparing nano-sized iron oxide red by controlling the calcination temperature, but it does not involve the key technology of sulfur content control.

[0009] (I) Core Issues of Dry Process

[0010] In existing dry preparation methods, the sulfur content of the finished product after high-temperature calcination of ferrous sulfate is poorly controlled, with the sulfur content generally ranging from 0.25% to 0.35% by mass, far exceeding the standard of ≤0.08% for electronic-grade materials. The fundamental reason for this is:

[0011] Lack of particle size control: Conventional processes do not take into account the impact of ferrous sulfate particle size on sulfur release. If the raw material particles (average particle size: 50-100μm) are too large, the sulfur elements inside cannot fully contact oxygen during calcination, resulting in a sulfur residue rate of 30%-40%, which seriously affects product quality.

[0012] The calcination conditions are crude: the temperature of the traditional rotary kiln fluctuates by ±50℃ and no auxiliary gas is introduced for regulation. The sulfur oxide (SO2 / SO3) escape efficiency is only 60%-70%, which requires multiple subsequent calcinations (increasing energy consumption by 40%) or lime to neutralize the tail gas (producing 0.8-1.2 tons of desulfurization slag per ton of product, causing secondary pollution).

[0013] (II) Limitations of wet process

[0014] Although the wet preparation method offers mild reaction conditions (room temperature to 80℃), the process is complex, involving 8-10 reaction steps such as neutralization, oxidation, filtration, and washing, with a production cycle exceeding 24 hours. Furthermore, the washing process involves Fe... 2+ The loss rate is 5%-8%, resulting in waste of raw materials and high wastewater treatment costs (wastewater treatment costs account for 15%-20%). Summary of the Invention

[0015] To address the aforementioned problems, this invention provides a method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate. This invention significantly reduces the sulfur content in the product, improves product purity, shortens the production cycle, and reduces overall energy consumption.

[0016] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0017] A method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate includes the following steps: S1, using a two-stage grinding process of mechanical grinding and air jet milling to initially refine the raw material particle size from 50-100μm to 20-30μm (D50); S2, using an externally heated rotary kiln for heating and calcination, monitoring and controlling the kiln temperature in real time, maintaining the calcination zone temperature at 820±10℃ to ensure a ferrous sulfate decomposition rate ≥99.5%; S3, introducing 0.4MPa superheated steam into the rotary kiln through a steam generator, where the steam reacts with the hydroxyl groups on the surface of FeSO4 particles to form active sites, increasing the sulfur escape rate, and maintaining a slightly negative pressure state inside the kiln through a kiln tail fan; S4, initially cooling and collecting the calcined product, collecting most of the ferric oxide particles; and using a bag filter for secondary collection of the remaining tail gas containing a small amount of ferric oxide particles to ensure a product collection rate ≥99%.

[0018] Preferably, in step S1, the raw material is industrial-grade ferrous sulfate with a purity ≥98%, and the impurity content is controlled: Fe 3+ ≤0.5%, total heavy metal ion content ≤0.1%.

[0019] Preferably, the air jet milling adopts a fluidized bed air jet milling machine, using compressed air as the medium, maintaining the nozzle pressure at 0.7MPa, and the feed rate at 8kg / h for secondary refinement to ensure that more than 90% of the ferrous sulfate particles have a particle size ≤10μm.

[0020] Preferably, the temperature of the superheated steam is 150°C, and the hourly rate of introduction is 5% of the feed amount.

[0021] Preferably, in step S3, the negative pressure is -500Pa±50Pa and the gas flow rate inside the kiln is 1.2m / s, ensuring that the residence time of sulfur oxides is ≤10s.

[0022] Preferably, the calcined product enters the water-cooled jacket via a screw conveyor with a speed of 20 r / min, cooling the product to ≤80℃, and achieving a product collection rate of ≥95%.

[0023] Preferably, the exhaust gas is treated by a bag filter with a filtration accuracy of 1μm and a final product collection rate of ≥99.2%.

[0024] Preferably, the product collected in step S4 is placed in a 5%-10% dilute sulfuric acid solution and soaked and stirred at 55℃±5℃ for 45 minutes.

[0025] The beneficial effects of this invention are as follows:

[0026] Compared with existing technologies, this invention provides a physical basis for the efficient release of sulfur by refining the raw material particles, improves the calcination reaction conditions and uses steam to assist in calcination to promote desulfurization, thereby greatly reducing the sulfur content in the product, improving product purity, shortening the production cycle, and reducing overall energy consumption. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] This invention aims to solve the problems of high sulfur content, high energy consumption, and complex wet process flow in existing dry processes; this invention achieves breakthroughs through the following technical approaches:

[0030] 1. Precise particle size control: Research has found that the particle size of ferrous sulfate is closely related to the sulfur content of the finished product. The smaller the particles, the lower the sulfur content of the finished product (smaller ferrous sulfate particles increase the specific surface area to improve reaction activity, shorten the heat transfer path to achieve uniform reaction, and reduce mass transfer resistance to promote sulfur overflow through three major mechanisms, allowing sulfur to be more fully converted from solid particles into gas and escape, thereby significantly reducing the sulfur residue in the finished product). When the particle size of ferrous sulfate is ≤10μm, the sulfur overflow rate can be increased to over 95%. Therefore, this invention will achieve controllable preparation of particles with a particle size of 5-10μm through a composite pulverization process.

[0031] 2. Optimization of calcination conditions: Construct a synergistic system of "high-temperature calcination + steam activation + micro-negative pressure drainage" to promote the efficient discharge of sulfur in the form of SO2 / SO3 (escape rate ≥98%) and avoid secondary pollution.

[0032] 3. High-efficiency post-processing: Residual sulfur oxides are selectively removed using dilute sulfuric acid, ensuring that the sulfur content of the final product is ≤0.05%, while simplifying the process, reducing costs, and meeting the demand for high-purity Fe2O3 in high-end fields.

[0033] This invention optimizes and upgrades the traditional dry process by improving raw material pretreatment, enhancing the control of calcination reaction conditions, and improving product collection and processing efficiency. The specific implementation scheme is as follows:

[0034] Step 1: Ferrous sulfate pretreatment

[0035] 1. Raw material selection: Industrial-grade ferrous sulfate (FeSO4·7H2O) with a purity ≥98% is selected, and the impurity content is controlled: Fe 3 + ≤0.5%, heavy metal ions (Cu 2+ Zn 2+The total content of impurities (etc.) is ≤0.1%, ensuring that impurities have no significant impact on the calcination reaction.

[0036] 2. Particle refinement process: A two-stage grinding process of "mechanical grinding + air jet milling" is adopted.

[0037] (1) Primary mechanical grinding: High-energy ball mill (model: QM-3SP2, ball-to-material ratio 10:1), grinding time 1.5h, rotation speed 350 r / min, to initially refine the raw material particle size from 50-100μm to 20-30μm (D50).

[0038] The parameter settings for primary mechanical grinding (equipment model, ball-to-material ratio, grinding time, and rotation speed) need to be determined comprehensively based on the characteristics of the raw materials, the desired refinement, and the requirements of the secondary process. Specific details are as follows:

[0039] High-energy ball mill model selection (QM-3SP2): QM-3SP2 is a commonly used high-energy planetary ball mill in laboratories and pilot-scale operations. Its advantages are: structural adaptability: It has the ability to grind multiple tanks simultaneously, and the volume of a single tank is suitable for the raw material processing volume at this stage (without excessive scaling up of equipment, which would lead to energy waste); stable energy output: Its planetary motion trajectory can provide continuous and uniform impact and shear force to the material through the superposition of revolution and rotation, which is suitable for the initial refinement of medium-hardness brittle materials such as ferrous sulfate.

[0040] The ball-to-material ratio of 10:1 is a key parameter affecting grinding efficiency. The reason for choosing 10:1 instead of the conventional 8:1 or 12:1 is that the initial state of the raw materials is suitable: the initial particle size of the raw materials is 50-100μm, which belongs to medium and coarse particles, and sufficient grinding media (balls) are required to provide impact energy.

[0041] When the ball-to-material ratio is too low (e.g., 8:1), the gaps between the balls are large, the probability of the material being impacted is low, and it is difficult to crush quickly; when it is too high (e.g., 12:1), the friction between the balls intensifies, energy consumption increases, and the material may be adsorbed and wrapped by the balls, which will reduce the refining efficiency. At the same time, a ball-to-material ratio of 10:1 can balance the impact force of the balls on the material and the grinding shear force, and efficiently crush coarse particles to 20-30μm within 1.5h, providing an ideal intermediate particle size for secondary air jet milling (avoiding excessively large initial particles that lead to excessively high air jet milling load); this parameter is an optimized design based on the characteristics of the raw materials.

[0042] The grinding time of 1.5 hours is set based on the principle of balancing "refinement effect - energy consumption": Target particle size guidance: It is necessary to refine 50-100μm to 20-30μm (D50). Experimental verification shows that when the grinding time is <1.5 hours, the particle refinement is incomplete (D50 is still >30μm), which cannot meet the feeding requirements of secondary grinding; when the time is >1.5 hours, the particle size does not decrease significantly (it is close to the stage limit of mechanical grinding), but the energy consumption increases by more than 30%, and the particles may agglomerate due to excessive grinding (especially ferrous sulfate is easy to absorb moisture and is prone to agglomeration after long-term grinding).

[0043] Meanwhile, the 1.5-hour processing time ensures batch processing efficiency, matches the capacity of subsequent air jet milling, and avoids material accumulation between processes.

[0044] The rotational speed of 350 r / min directly affects the kinetic energy of the grinding balls and the grinding intensity. Therefore, 350 r / min is chosen over lower speeds (e.g., 300 r / min) or higher speeds (e.g., 400 r / min). The critical speed of the QM-3SP2 ball mill is approximately 450 r / min, with 350 r / min falling within the effective impact range. At this speed, the balls undergo a throwing-impact motion under the combined effects of centrifugal force and gravity, rather than sliding against the wall (excessive speed easily leads to wall adhesion). This maximizes the transfer of kinetic energy to the material, improving crushing efficiency. At too low a speed (300 r / min), the impact force of the balls is insufficient, resulting in slow particle refinement and a wide particle size distribution. At too high a speed (400 r / min), localized temperatures rise (ferrous sulfate is easily oxidized at high temperatures), and fine particles are easily carried out of the grinding jar by the airflow, leading to a decrease in yield. 350 r / min ensures uniform particle refinement (D50 fluctuation < 5 μm) while avoiding side reactions.

[0045] (2) Secondary air jet milling: A fluidized bed air jet mill (model: AFG-200) is used with compressed air as the medium. The nozzle pressure is maintained at 0.7MPa and the feed rate is 8kg / h. Secondary refining is carried out to ensure that more than 90% of the ferrous sulfate particles reach a particle size of ≤10μm (detected by a laser particle size analyzer, model: Malvern Mastersizer 3000).

[0046] The parameters for secondary air jet milling (equipment model, media conditions, pressure, feed rate, and detection methods) need to be designed around three main objectives: "precisely controlling particle size to 5-10μm," "ensuring particle dispersibility," and "adapting to the products of primary grinding." The specific reasons are as follows:

[0047] 1. Fluidized Bed Air Jet Mill Model (AFG-200) The AFG-200 is a typical model of small and medium-sized fluidized bed air jet mills. Its core advantages are: fluidized bed structure adaptability: the material is suspended in the grinding chamber by airflow ("fluidized bed"), and uniform collisions can occur between particles and between particles and the chamber wall, avoiding local over-grinding or under-grinding. It is especially suitable for further refining the primary product of 20-30μm to ≤10μm (requiring narrow particle size distribution).

[0048] Processing capacity matching: Its rated capacity matches the discharge rate of the primary mechanical grinding (calculated based on a 1.5-hour grinding cycle), enabling continuous production and avoiding moisture absorption or agglomeration caused by material accumulation; this model is specifically designed to ensure the stability and efficiency of secondary grinding; the nozzle pressure is 0.7 MPa. The power for airflow grinding comes from high-speed airflow (accelerated to supersonic speed through the nozzle), and the pressure directly affects the grinding energy.

[0049] Energy matching: At 0.7MPa, the airflow velocity can reach 300-500m / s, which is sufficient to break 20-30μm particles to ≤10μm in collisions; when the pressure is too low (e.g., <0.5MPa), the airflow kinetic energy is insufficient, and the particles cannot be fully broken, with a compliance rate of <70%; when the pressure is too high (e.g., >0.9MPa), energy consumption surges (compressed air energy consumption is exponentially related to pressure), and excessive collisions will cause some particles to be <5μm, which will increase the difficulty of subsequent separation.

[0050] The nozzle of the AFG-200 is designed to withstand pressures of 0.6-0.8 MPa, with 0.7 MPa being within a safe and efficient range, which can reduce equipment wear. This pressure is an optimized value for balancing energy consumption and crushing efficiency, and is crucial to the economic efficiency of the process.

[0051] The feed rate should be 8 kg / h. The feed rate must be matched with the "processing capacity - residence time" of the grinding chamber. If the rate is too high (e.g., >10 kg / h), the material will not have enough residence time in the grinding chamber and will be carried out before being fully ground, resulting in a sharp drop in the particle size compliance rate. If the rate is too slow (e.g., <5 kg / h), the capacity will be too low and the particles will circulate excessively within the chamber, easily causing localized temperature increases due to collision-induced heat generation (although airflow grinding generates less heat, excessive residence can still trigger Fe...). 2+ Slight oxidation).

[0052] 8 kg / h is the optimal value verified by experiments, which can maintain the production capacity connection with the first-stage grinding while ensuring that more than 90% of the particles are ≤10μm.

[0053] In summary, the above-mentioned methods enable precise control of particle size. The fluidized bed structure ensures uniform particle stress, and combined with a pressure of 0.7 MPa and a feed rate of 8 kg / h, the target of "90% or more ≤10 μm" can be stably achieved, solving the agglomeration problem in the fine grinding stage of mechanical grinding. It avoids media contamination in mechanical grinding and the introduction of impurities in wet grinding, making it particularly suitable for downstream applications of ferrous sulfate with high purity requirements (such as battery materials and catalyst precursors). Dry operation can be directly connected to primary mechanical grinding without additional processing (such as pulping and drying), reducing energy consumption and production costs. Furthermore, the low temperature and low humidity environment effectively inhibits the oxidation and hydrolysis of ferrous sulfate, ensuring the stability of the product's chemical properties.

[0054] Step 2: Control of calcination reaction conditions

[0055] 1. Calcination equipment and temperature: An externally heated rotary kiln (1.2m in diameter, 15m in length, lined with corundum bricks) is used. The temperature inside the kiln is monitored and controlled in real time by thermocouples. The temperature of the calcination zone is controlled at 820±10℃. This equipment can maintain a stable heating temperature and gas flow to ensure complete decomposition of ferrous sulfate (decomposition rate ≥99.5%).

[0056] The matching logic of 1.2m diameter and 15m length for the calcination decomposition of ferrous sulfate is a typical "heat-driven + mass transfer-dependent" reaction (reaction formula: 2FeSO4→Fe2O3+SO2↑+SO3↑), requiring two core conditions: sufficient heating time and uniform heat transfer; the 15m length design ensures sufficient residence time for the material within the kiln; the decomposition of ferrous sulfate involves two steps: removal of crystal water (low-temperature stage) and decomposition of anhydrous FeSO4 (high-temperature stage). A longer residence time avoids localized incomplete decomposition due to excessively high material flow rate, ensuring... The key to achieving a decomposition rate of ≥99.5% lies in the 1.2m diameter design: matching the length to form a reasonable length-to-diameter ratio (approximately 12.5:1). This avoids both "insufficient heating of the central material" caused by an excessively large diameter (in external heating systems, heat is transferred from the kiln wall to the center, and an excessively large diameter would exacerbate the radial temperature difference) and "excessively thick material accumulation and impeded mass transfer" caused by an excessively small diameter (SO2 / SO3 generated during decomposition needs to be discharged in a timely manner, and an excessively small diameter can easily lead to local gas stagnation, inhibiting the forward reaction). At the same time, this diameter is suitable for the material handling capacity of conventional production, balancing efficiency and stability.

[0057] The core advantages of corundum bricks (mainly composed of α-Al2O3) are high temperature resistance, corrosion resistance, and strong wear resistance: High temperature resistance: The decomposition temperature of ferrous sulfate reaches 820℃, while the refractoriness of corundum bricks is >1700℃, allowing them to withstand this temperature stably for a long time without softening or deformation; The SO2 and SO3 gases produced during decomposition are highly corrosive at high temperatures, but corundum bricks have excellent chemical stability and do not react with acidic gases, which can prevent impurities from being mixed into the product after the lining is corroded (such as traditional clay bricks, which may release silicate impurities due to corrosion, contaminating the Fe2O3 finished product).

[0058] Wear resistance: During the operation of the rotary kiln, the material and the inner lining rub against each other continuously. The corundum brick has high hardness (Mohs hardness 9), which can extend the service life of the equipment and reduce the frequency of maintenance. Second, the reason for selecting the calcination temperature (820±10℃): Temperature is the core control parameter for the complete decomposition of ferrous sulfate. It is necessary to balance "decomposition efficiency" and "product quality": 1. The core basis of 820℃: to ensure complete decomposition and no side reactions.

[0059] The decomposition reaction of FeSO4 is an endothermic reaction. If the temperature is too low, the reaction rate will be slow and the decomposition will be incomplete (residual FeSO4 will lead to an increase in the sulfur content of the finished product). The temperature needs to reach the decomposition threshold (usually FeSO4 begins to decompose significantly above 700℃, and the decomposition rate increases significantly above 800℃). 820℃ is the "high-efficiency decomposition temperature point" verified by experiments, which can guarantee a decomposition rate of ≥99.5%.

[0060] If the temperature is too high (e.g., >850℃), the generated Fe2O3 may sinter (the particles agglomerate and become larger, affecting the activity of subsequent products), or even some Fe2O3 may be reduced to Fe3O4 (if there is local oxygen deficiency in the kiln), resulting in a decrease in product purity; while if the temperature is below 800℃, the decomposition rate drops sharply, making it difficult to achieve the target decomposition rate.

[0061] The necessity of ±10℃ precision control: Excessive temperature fluctuations can damage the stability of decomposition. If the local temperature is <810℃, a "cold zone" may appear, resulting in FeSO4 residue. If the local temperature is >830℃, local overheating may occur, leading to Fe2O3 sintering or impurity generation. The ±10℃ control precision is achieved through real-time monitoring by thermocouples, which can ensure a uniform temperature field inside the kiln and guarantee the consistency of the decomposition reaction.

[0062] 2. Gas atmosphere control:

[0063] (1) Steam activation: 0.4 MPa of superheated steam (temperature 150℃) is introduced into the rotary kiln through a steam generator. The hourly rate is 5% of the feed amount (Wt%, for example, 50 kg / h of steam is introduced for 1 ton of raw material). The steam reacts with the hydroxyl groups on the surface of FeSO4 particles to form active sites, which accelerates the breaking of SO bonds and increases the sulfur release rate by 30%.

[0064] I. Reasons for choosing superheated steam pressure (0.4MPa): The core of setting the pressure to 0.4MPa is to balance the "gas stability" and "mass transfer efficiency" of steam, and avoid negative impacts on the calcination reaction. It ensures the state of superheated steam: Under standard atmospheric pressure, the boiling point of water is 100℃, while the saturation temperature corresponding to 0.4MPa is about 143℃ (according to the steam characteristic curve).

[0065] In this process, the steam temperature is set at 150℃, slightly higher than the saturation temperature at this pressure. Therefore, 0.4MPa ensures that the steam is superheated steam (gaseous) before entering the kiln, avoiding condensation of the steam during transportation or introduction due to excessively low pressure (if the pressure is <0.1MPa, 150℃ steam is prone to becoming saturated steam due to heat dissipation, or even precipitating liquid water); if liquid water comes into contact with high-temperature materials, it will cause localized rapid cooling or material agglomeration, destroying the uniformity of calcination, and at the same time hindering the escape of sulfur gas.

[0066] Furthermore, it can adapt to the pressure balance inside the kiln. During calcination, there is a certain amount of gas flow inside the externally heated rotary kiln (SO2 and SO3 produced by decomposition need to be discharged). The 0.4MPa inlet pressure can ensure that water vapor can diffuse evenly to the material layer inside the kiln and fully contact the FeSO4 particles. If the pressure is too high (e.g., >0.6MPa), it may cause a sudden increase in local pressure inside the kiln, inhibiting the escape of SO2 and SO3 (according to Le Chatelier's principle, excessively high partial pressure of product gas will inversely inhibit the decomposition reaction of FeSO4). If the pressure is too low (e.g., <0.2MPa), water vapor will have difficulty penetrating the material layer, resulting in insufficient generation of active sites and failing to achieve the goal of increasing the sulfur escape rate.

[0067] The reason for choosing a superheated steam temperature (150℃) is to focus on "steam activity" and "avoiding side reactions": it can maintain gaseous activity. As mentioned earlier, 150℃ matches 0.4MPa, ensuring that the steam is superheated steam with high diffusivity and reactivity, and can quickly combine with the hydroxyl groups (-OH) on the surface of FeSO4 particles to form active sites (reaction mechanism: H2O(g) + surface-O-Fe- → surface-O-Fe-OH + H). + (This promotes SO bond polarization breakage).

[0068] If the temperature is too low (e.g., <140℃), the water vapor is likely to approach saturation and may condense on the particle surface to form a liquid film, which in turn hinders the escape of sulfur gas. If the temperature is too high (e.g., >200℃), although the water vapor is more active, it will increase the energy consumption of the steam generator. Furthermore, water vapor at excessively high temperatures may expand excessively after contacting the high-temperature zone (820℃) inside the kiln, leading to local airflow turbulence and affecting the uniformity of material heating.

[0069] It is also compatible with calcination systems. When 150°C steam is introduced into a high-temperature kiln, it will be rapidly heated to near the kiln temperature (around 820°C). However, the initial temperature setting can avoid a sudden drop in local temperature caused by the introduction of low-temperature steam (if room-temperature steam is introduced directly, a "cold zone" may be formed on the material surface, reducing the local decomposition rate). The 150°C preheating state can reduce interference with the kiln temperature field.

[0070] The reason for choosing 5% wt% of feed is that the 5% ratio is the "optimal value" verified by experiments. The core balance is between "active site generation" and "mass transfer inhibition risk": After being refined (5-10μm), ferrous sulfate particles have a large specific surface area, which requires sufficient water vapor molecules to cover the surface and react to generate active sites (the surface hydroxyl density is positively correlated with the amount of water vapor). If the feed rate is <3%, there are not enough water vapor molecules, the surface active site coverage is low, and it is not possible to effectively accelerate SO bond breaking. The effect of improving the sulfur escape rate is not obvious (experimental data show that when the feed rate increases from 1% to 5%, the sulfur escape rate increases linearly, and the increase slows down after exceeding 5%).

[0071] It can also avoid mass transfer inhibition. Excessive water vapor (such as >8%) will lead to excessively high water vapor partial pressure in the kiln. On the one hand, it may dilute the SO2 and SO3 gases produced by decomposition and reduce their escape power. On the other hand, excessive water molecules may form an adsorption layer on the surface of Fe2O3 particles, which will hinder the diffusion of sulfur gas from the inside of the particles to the outside ("increased gas film resistance"). A 5% introduction rate can ensure sufficient active sites without significantly increasing the mass transfer resistance, thus achieving the goal of increasing the sulfur escape rate by 30%.

[0072] Meanwhile, the gas discharge capacity of the externally heated rotary kiln is limited. The 5% intake volume matches the amount of sulfur gas produced by decomposition and can be smoothly discharged through the kiln tail negative pressure system, avoiding material "back-mixing" or equipment sealing problems caused by excessive pressure inside the kiln.

[0073] Compared to other activation methods, and compared to not introducing water vapor or introducing other gases (such as air or nitrogen), superheated steam activation has unique advantages: it can target and promote sulfur escape. The polar water molecules in the water vapor can form hydrogen bonds with the SO bonds in FeSO4, reducing the bond energy (theoretical calculations show that the SO bond energy can be reduced by about 15 kJ / mol), while non-polar gases such as air and nitrogen do not have this effect and cannot accelerate the breaking of sulfur bonds; and no impurities are introduced: the superheated steam is eventually discharged in the form of H2O, without reacting with the material. The reaction (such as introducing air may introduce O2, but O2 does not promote the decomposition of FeSO4 at high temperatures; introducing inert gas only has a dilution effect and does not contribute to activity) avoids the finished product being contaminated by impurities; it can also synergize the particle refinement effect: the refined particles (5-10μm) have a large contact area with water vapor, and 5% of the introduction amount can make full use of the high specific surface area to maximize the density of active sites, forming a synergistic effect of "1+1>2" with the previous particle size control (the finer the particles + the more suitable the water vapor, the more significant the increase in sulfur escape rate).

[0074] (2) Micro negative pressure flow: The kiln tail fan keeps the kiln under a micro negative pressure (-500Pa±50Pa). The airflow distribution in the kiln is optimized by aerodynamic simulation, so that ferrous sulfate is calcined in a hydroxyl-rich environment, which accelerates the reaction rate and ensures that the residence time of sulfur oxides (SO2 / SO3) is ≤10s, thus avoiding secondary adsorption.

[0075] Step 3: Product Collection and Processing

[0076] 1. Multi-stage collection system: The calcined product is initially cooled and collected by a cooling discharge screw, which collects most of the ferric oxide particles; then, the remaining tail gas containing a small amount of ferric oxide particles is collected a second time by a bag filter to ensure that the product collection rate is ≥99%.

[0077] (1) Primary cooling and collection: The calcined product enters the water-cooled jacket via a screw conveyor (speed 20r / min) and is cooled to ≤80℃, with a product collection rate ≥95%.

[0078] (2) Secondary dust collection: The exhaust gas is treated by a bag filter (filtration accuracy 1μm), and the final product collection rate is ≥99.2%, reducing material loss.

[0079] 2. Chemical desulfurization treatment: The collected product is placed in a 5%-10% dilute sulfuric acid solution (H2SO4 concentration gradient adjustable, preferably 8%), and soaked and stirred at 55℃±5℃ for 45 minutes. The reaction mechanism is as follows:

[0080] FeSO4(residual)+H2SO4→Fe2(SO4)3(dissolved)+H2↑

[0081] The Fe2O3 product with a sulfur content of ≤0.05% was finally obtained by filtration (pore size 0.22μm), washing with deionized water (conductivity ≤10μs / cm), and drying at 120℃ for 4h.

[0082] The concentration of dilute sulfuric acid is 5%-10% (preferably 8%); the residual FeSO4 needs to react with H2SO4 to generate water-soluble Fe2(SO4)3 (reaction formula: 2FeSO4+H2SO4=Fe2(SO4)3+H2↑), and sulfur is removed by dissolution and filtration.

[0083] Too low a concentration (<5%): Insufficient H2SO4 will lead to incomplete reaction of FeSO4, and residual sulfur cannot be completely converted, making it easy for the sulfur content of the final product to exceed the standard; Too high a concentration (>10%): Excessive H2SO4 will increase the burden of subsequent washing (more deionized water is needed to remove residual sulfate ions), and may cause slight dissolution of Fe2O3 due to local high acid environment (although Fe2O3 is resistant to dilute acid, there is still a risk of slight dissolution at high concentrations), resulting in product loss.

[0084] The optimal concentration is 8%, and experiments have verified that this concentration yields the highest FeSO4 conversion rate (>99%), while achieving the best balance between subsequent washing costs and product stability.

[0085] The reaction temperature is 55℃±5℃. Increasing the temperature can accelerate the reaction rate of FeSO4 and H2SO4 (the reaction rate increases by about 1-2 times for every 10℃ increase in temperature), shortening the time to reach complete reaction. If the temperature is >60℃, the water evaporates too quickly, which will cause fluctuations in solution concentration and even local crystallization, affecting the uniformity of the reaction. If the temperature is <50℃, the reaction rate will decrease significantly, and the stirring time needs to be extended (e.g., more than 60 minutes), reducing production efficiency. ±5℃ fluctuation range: In actual production, there is a small error in the temperature control equipment. This range can ensure stable reaction efficiency and avoid a decrease in desulfurization effect due to temperature fluctuations.

[0086] Soaking and stirring for 45 minutes is sufficient for a complete reaction. At 55°C and 8% sulfuric acid concentration, 45 minutes can ensure that residual FeSO4 (usually accounting for 1%-3% of the product) is completely converted into Fe2(SO4)3 and dissolved (experimental data show that the conversion rate is about 90% after 30 minutes and more than 99.5% after 45 minutes).

[0087] Too short a reaction time (<30 minutes) will result in incomplete reaction and excessive sulfur residue; too long a reaction time (>60 minutes) will increase equipment energy consumption and production cycle, and reduce the output per unit time. 45 minutes is the optimal value for "complete reaction + efficient production".

[0088] Separation requirements with a filtration pore size of 0.22 μm: After pretreatment, the particle size of the target product Fe2O3 is mostly between 1-10 μm, while soluble Fe2(SO4)3 needs to be separated from solid Fe2O3 by filtration; a pore size of 0.22 μm is a commonly used pore size for precision filtration, which can effectively intercept all Fe2O3 particles (avoiding the loss of fine particles) while allowing the Fe2(SO4)3 solution to pass through smoothly.

[0089] If the pore size is >0.22μm (e.g., 1μm), submicron Fe2O3 particles may be lost with the filtrate, reducing the yield; if the pore size is <0.22μm (e.g., 0.1μm), the filtration resistance will increase, the flow rate will slow down, and the production time will increase.

[0090] Washing until conductivity ≤10μs / cm. Impurity removal target: The core of washing is to remove residual sulfate ions (SO4). 2- Excessive sulfur content and soluble impurities such as H2SO4 are key factors leading to excessive sulfur content in products; conductivity directly reflects the concentration of ions in the solution (mainly SO42-). 2- and H + A conductivity of ≤10μs / cm indicates that the residual electrolyte is extremely low (corresponding to a sulfate concentration of <5ppm), which can ensure that the sulfur content of the final product is ≤0.05%. If the conductivity is >10μs / cm, it indicates that there is still a lot of sulfate residue, which will lead to the sulfur content of the product exceeding the standard.

[0091] The product is dried at 120℃ for 4 hours to remove moisture. After washing, the product contains surface adsorbed water and free water, and needs to be dried to a moisture content of ≤0.5% (to avoid clumping or secondary oxidation during storage). 120℃ can quickly evaporate moisture (far above the boiling point of water) and will not cause a phase change in Fe2O3 (Fe2O3 stability temperature >1000℃), thus avoiding changes in product properties. If the temperature is <100℃, the drying time needs to be extended (e.g., more than 6 hours), resulting in low efficiency. If the temperature is >150℃, it will increase energy consumption without any practical benefit. Time selection: 4 hours can ensure complete evaporation of moisture at 120℃ (experimental verification: approximately 1% moisture residue after 2 hours, <0.3% residue after 4 hours), meeting the product drying requirements.

[0092] The key points of this invention are as follows:

[0093] 1. Particle Size Control – Engineering Application of Quantitative Relationship of Sulfur Content: For the first time, experiments have verified that when the particle size D90 of ferrous sulfate is ≤10μm, the residual sulfur content after calcination has an exponential relationship with the particle size (R0). 2 =0.98), meaning that for every 1μm decrease in particle size, the sulfur content decreases by 0.02%; by precisely controlling the particle size of ferrous sulfate particles to 5-10μm through a two-stage pulverization process, a physical basis is provided for the efficient release of sulfur, which is the key to reducing the sulfur content of the finished product.

[0094] 2. Optimization of Calcination Reaction Conditions – Synergistic Mechanism of Steam-Assisted Calcination: An externally heated rotary kiln is used, which can precisely control the temperature at 800-850℃, providing a stable high-temperature environment for the reaction. Simultaneously, steam with specific parameters is introduced at a rate of 5% (wt%) of the feed amount per hour. The steam not only acts as a heat transfer medium (improving heat transfer efficiency by 20%), but also promotes desulfurization through the following mechanisms.

[0095] (1) It forms hydrogen bonds with the hydroxyl groups on the FeSO4 surface, weakening the Fe-OS bond energy.

[0096] (2) Increase the gas flow rate inside the kiln (from 0.5 m / s to 1.2 m / s) to enhance mass transfer.

[0097] (3) Suppress the excessive growth of Fe2O3 grains (average grain size ≤ 50 nm) and avoid sulfur encapsulation at grain boundaries.

[0098] 3. Product desulfurization treatment - a systematic design of multi-stage purification: through three-stage control of "physical crushing, chemical activation and wet desorption", a complete path of "exposure, discharge and removal" of sulfur elements is formed; compared with the traditional single calcination, the desulfurization effect is improved by 40%, and the energy waste of multiple calcinations is avoided.

[0099] Comparison of key performance indicators with traditional dry and wet processes

[0100] index Traditional dry method wet process Dry method of the present invention Sulfur content (%) 0.25-0.35 0.15-0.20 ≤0.05 Production cycle 6-8h 24h+ 4-5h Energy consumption (kWh / ton) 800-1000 500-600 600-700 Wastewater generation none 5-8 tons / ton small amount

[0101] Practical application advantages

[0102] (1) Electronics industry: When used in electronic composite materials, the dielectric loss is reduced from 0.015 to 0.008, and the conductivity stability is improved by 50%.

[0103] (2) Ceramic industry: As a glaze colorant, the standard deviation of color uniformity is reduced from 1.2 to 0.5, and the high-temperature sintering color change rate is reduced from 8% to 2%.

[0104] (3) Environmental benefits: Compared with wet process, the amount of wastewater generated is reduced by more than 90%, and no desulfurization slag is generated; compared with traditional dry process, the cost of tail gas treatment is reduced by 60%.

[0105] This invention has been experimentally verified, and the results are consistent with the design expectations. To quantify the advantages of this invention, a comparison with traditional preparation methods was conducted using the controlled variable method. Ferrous sulfate raw materials of the same batch and quality were selected, and ferric oxide was prepared using both the method of this invention and the traditional method. Under the same testing environment, the sulfur content of the finished product was detected using a high-frequency infrared carbon-sulfur analyzer; the purity of the finished product was analyzed using an X-ray diffractometer. The experiment was repeated 10 times, and the average value was taken to ensure data accuracy.

[0106] (I) Experimental Design

[0107] Using L9(3) 4 Orthogonal experiments were conducted to investigate the effects of particle size (A: 5-10μm, 10-15μm, 15-20μm), calcination temperature (B: 800℃, 820℃, 850℃), steam flow rate (C: 3%, 5%, 7%), and desulfurization time (D: 30min, 45min, 60min) on sulfur content. Sulfur content was detected using a high-frequency infrared carbon-sulfur analyzer (model: CS-8820), and phase purity was analyzed using an X-ray diffractometer (XRD, model: Bruker D8 Advance).

[0108] (II) Typical Implementation Examples

[0109] Example 1 (Optimal Process):

[0110] Raw material: FeSO4·7H2O (purity 98.5%), particle size D90=8μm.

[0111] Calcination: 816-820℃, steam flow rate 5%, slight negative pressure -450-500Pa, calcination time 2h.

[0112] Post-treatment: 8% dilute sulfuric acid, stir at 52-55℃ for 45 minutes.

[0113] Results: Sulfur content 0.042%, purity 99.85%, particle size distribution 5-12μm (D50=8μm).

[0114] Comparative Example 1 (Traditional Method):

[0115] Raw material: Uncrushed FeSO4 (particle size D90=50μm), calcined at 650℃ for 3h, with no water vapor.

[0116] Results: Sulfur content 0.32%, purity 97.5%, severe particle agglomeration.

[0117] In summary, compared with existing technologies, this invention provides a physical basis for the efficient release of sulfur by refining raw material particles, improves calcination reaction conditions by using steam-assisted calcination to promote desulfurization, greatly reduces the sulfur content in the product, improves product purity, shortens the production cycle, and reduces overall energy consumption.

[0118] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate, characterized in that, Includes the following steps: S1. A two-stage grinding process of mechanical grinding and air jet milling is adopted to initially refine the particle size of the raw material from 50-100μm to 20-30μm (D50). S2. An externally heated rotary kiln is used for heating and calcination, and the temperature inside the kiln is monitored and controlled in real time. The temperature of the calcination zone is controlled at 820±10℃ to ensure that the decomposition rate of ferrous sulfate is ≥99.5%. S3. 0.4MP superheated steam is introduced into the rotary kiln through a steam generator. The steam reacts with the hydroxyl groups on the surface of FeSO4 particles to form active sites, thereby increasing the sulfur release rate. The kiln tail fan keeps the kiln under a slight negative pressure. S4. The calcined product is initially cooled and collected, and most of the ferric oxide particles are collected. The remaining exhaust gas containing a small amount of ferric oxide particles is collected a second time using a bag filter to ensure a product collection rate of ≥99%.

2. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, In step S1, industrial-grade ferrous sulfate with a purity ≥98% is selected as the raw material, and the impurity content is controlled: Fe 3+ ≤0.5%, total heavy metal ion content ≤0.1%.

3. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, The air jet mill uses a fluidized bed air jet mill with compressed air as the medium. The nozzle pressure is maintained at 0.7 MPa and the feed rate is 8 kg / h. Secondary refining is performed to ensure that more than 90% of the ferrous sulfate particles have a particle size ≤10 μm.

4. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, The temperature of the superheated steam is 150°C, and the hourly rate of introduction is 5% of the feed rate.

5. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, In step S3, the negative pressure is -500Pa±50Pa and the gas flow rate inside the kiln is 1.2m / s to ensure that the residence time of sulfur oxides is ≤10s.

6. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, The calcined product is fed into the water-cooled jacket via a screw conveyor at a speed of 20 r / min, cooling the product to ≤80℃, with a product collection rate of ≥95%.

7. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, The exhaust gas is treated by a bag filter dust collector with a filtration accuracy of 1μm and a final product collection rate of ≥99.2%.

8. The method for reducing the sulfur content in the preparation of ferric oxide using ferrous sulfate according to claim 1, characterized in that, The product collected in step S4 was placed in a 5%-10% dilute sulfuric acid solution and soaked and stirred at 55℃±5℃ for 45 minutes.

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