A method for producing calcium oxide by low-temperature decomposition of phosphogypsum

Abstract

This invention discloses a method for producing calcium oxide by reducing the decomposition temperature of phosphogypsum under hydrogen and steam conditions. In this method, after removing silica impurities from the phosphogypsum by acid washing with sulfuric acid solution, the phosphogypsum is reduced and decomposed under a hydrogen atmosphere to obtain calcium sulfide solid product. Subsequently, the hydrogen is turned off and steam is introduced, and the reaction continues under these conditions to produce high-purity calcium oxide. The exhaust gas generated during this process can be dried and used as feed gas for acid production. This method uses pretreated phosphogypsum as raw material to prepare high-purity calcium oxide, simultaneously achieving phosphogypsum reduction and sulfur resource recycling. It lowers the temperature for producing calcium oxide from phosphogypsum, significantly reducing energy consumption and eliminating carbon dioxide emissions compared to existing processes. This is a key technology for the green and sustainable development of phosphogypsum. Under these process conditions, phosphogypsum can be decomposed at a low temperature of 800℃ to obtain high-purity calcium oxide, achieving a calcium hydroxide yield of over 97.22%.

Description

Technical Field

[0001] This invention relates to a method for producing calcium oxide by low-temperature decomposition of phosphogypsum, belonging to the field of environmental protection and solid waste resource utilization. Background Technology

[0002] Phosphogypsum is a large-scale industrial solid waste generated during the wet process of phosphoric acid production. Its main component is calcium sulfate dihydrate (CaSO4·2H2O), and it contains abundant sulfur and calcium resources. my country's annual phosphogypsum production is enormous. Although the comprehensive utilization rate has been increasing year by year, there is still a significant imbalance between production scale and disposal capacity. Large quantities of phosphogypsum are disposed of through open-air stockpiling, which not only occupies valuable land resources but also easily leads to a series of environmental problems such as soil acidification and water pollution. This seriously restricts the green and sustainable development of the phosphorus chemical industry and has become a global problem that urgently needs to be solved. To achieve high-value utilization of phosphogypsum, decomposing it and converting it into high-value products such as calcium oxide is a promising direction for resource recovery.

[0003] Existing phosphogypsum decomposition technologies are mainly divided into two categories: high-temperature decomposition and low-temperature reduction. High-temperature decomposition (>1200℃) consumes a huge amount of energy and is prone to equipment corrosion and product contamination due to impurities. To reduce energy consumption, low-temperature reduction decomposition technology has become a research focus. This mainly involves converting calcium sulfate into intermediate products at lower temperatures using reducing gases, followed by further processing. However, existing technical routes all face significant bottlenecks. Patent CN 102556978 A discloses a method for hydrogen reduction decomposition of phosphogypsum, using low-concentration hydrogen at 950–1050℃ to reduce phosphogypsum. Although this method reduces some energy consumption and avoids carbon emissions, the reduction temperature is still relatively high, and its process focuses on sulfur resource recovery. It does not provide a complete solution on how to efficiently and directionally convert the reduced intermediate into high-purity calcium oxide.

[0004] In the two-step reduction-oxidation process for producing calcium oxide, existing technologies typically face several key challenges. First, while using carbonaceous reducing agents in the reduction step is less expensive, it generates a large amount of carbon dioxide and introduces impurities such as ash, contaminating the product. Second, a more common bottleneck lies in the second step, the oxidation process. Patent CN 114229877 A discloses a method for low-temperature decomposition of phosphogypsum, which uses carbon monoxide reduction followed by oxygen oxidation. However, this process is difficult to control precisely: insufficient oxygen leads to incomplete oxidation of intermediate products (such as calcium sulfide), resulting in high sulfur content; excessive oxygen easily causes the target product, calcium oxide, to be over-oxidized, regenerating calcium sulfate, leading to poor process stability and large fluctuations in product purity. Finally, inherent impurities in phosphogypsum, such as soluble salts, eutectic phosphorus and fluorine, and siliceous components that affect reactivity, are often not deeply removed in existing pretreatment processes. These impurities can not only encapsulate reactants and inhibit decomposition efficiency, but also enter the product phase during high-temperature reactions, seriously affecting the chemical purity and physical properties of the final calcium oxide, making it difficult to meet the requirements of high-end applications.

[0005] In summary, current technologies for decomposing phosphogypsum to produce calcium oxide still have significant shortcomings in terms of deep impurity removal, greening of the reduction process, and controllability of the oxidation steps. There is an urgent need for an innovative method that can systematically solve these problems in order to achieve efficient, clean, and high-value conversion of phosphogypsum. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a method for producing calcium oxide from phosphogypsum through low-temperature decomposition, overcoming the shortcomings of existing technologies such as high energy consumption, low efficiency, and impure products. This method systematically reduces energy consumption at different stages through acid washing for impurity removal, hydrogen reduction, and steam oxidation reactions, at temperatures far lower than those of traditional processes. This results in a significant increase in decomposition and conversion rates under low-temperature conditions, and the acquisition of high-purity calcium oxide products.

[0007] The technical solution of this invention is as follows:

[0008] A method for producing calcium oxide by low-temperature decomposition of phosphogypsum includes the following steps:

[0009] Step A1, raw material pretreatment: Dry the phosphogypsum raw material at 35~45℃ for 24~48h to constant weight to ensure that the attached water is completely removed. Then, pass the dried phosphogypsum through a 300-mesh sieve and collect the material on the sieve for later use.

[0010] Step A2: Mix the phosphogypsum treated in step A1 with a sulfuric acid solution of 20-30% by mass, wherein the mass ratio of phosphogypsum to sulfuric acid solution is 1:4-8, and react for 2 hours under constant temperature and stirring at 60-80℃; then add an extractant for extraction, collect the lower layer product after separation, wash with deionized water until neutral, and then dry at 105℃ for 6-10 hours for later use.

[0011] Step A3: Place the phosphogypsum treated in step A2 into the heating zone of a two-stage fixed-bed reactor. Purge the reactor with nitrogen to remove air from the reactor. Then heat it to 800°C at a heating rate of 10°C / min. Introduce hydrogen gas with a purity of 99.999% for reduction reaction. Control the temperature inside the fixed bed at 800°C and the reaction time at 60–90 min. The tail gas generated during the reduction process is dried and used as raw material gas for the preparation of sulfuric acid.

[0012] Step A4, Water Vapor Oxidation Stage: After the reaction in Step A3 is completed, stop the hydrogen supply and purge the entire reactor with nitrogen at a rate of 200 mL / min. Then, at a reaction temperature of 500°C, water vapor is introduced into the reactor to carry out the oxidation reaction, which lasts for 1 to 13 hours. After the reaction is completed, stop the water vapor supply and heating, and purge the residual water vapor with nitrogen. After the reactor cools to room temperature, collect the obtained calcium oxide-containing solid product.

[0013] Further, in step A1, the phosphogypsum raw material has a water of crystallization wet basis content of 13-21%, an initial particle size of phosphogypsum of 100-400 mesh, and the selected raw material has a particle size of phosphogypsum of 300 mesh or more.

[0014] Furthermore, in step A2, the extractant is tributyl phosphate; in the step of acid washing phosphogypsum with sulfuric acid solution, the acid washing temperature needs to be maintained at 60-80℃ to ensure that impurities such as silica react completely. The tributyl phosphate used as the extractant can effectively separate the silicon-containing impurity phase from the phosphogypsum. After separation by the separatory funnel, it can be reused after standing.

[0015] Furthermore, in step A3, the flow rate of the 99.999% hydrogen is 50–200 mL / min.

[0016] Furthermore, in step A4, the flow rate of the water vapor is 0.1~2 mL / min (referring to the volumetric flow rate of liquid water under standard conditions).

[0017] The mechanism of this invention includes:

[0018] After silicon removal, phosphogypsum is reduced by hydrogen coupled with water vapor. This process alters the valence state and form of sulfur, achieving calcium-sulfur separation and thus producing calcium oxide from phosphogypsum. Hydrogen molecules diffuse to the surface of CaSO4 solid particles and are adsorbed. The activation energy required for the reaction is reached below 800℃, causing active hydrogen to attack the sulfur-oxygen bonds, ultimately stabilizing and producing CaS. When water vapor is introduced into the system, the high temperature provides sufficient energy, and water molecules are chemically adsorbed onto the active sites on the CaS crystal surface (negatively charged sulfur atoms). 2- (Near the ion). Due to the transformation of S to S... 2- S 2- The ion is an extremely strong proton acceptor; it can attack the hydrogen in a water molecule, causing the H₂O bond in the water molecule to break and transferring a proton to the S₂O₃ ion. 2- Subsequently, due to the extreme instability of hydroxide ions at high temperatures, another proton is rapidly released, and gaseous hydrogen sulfide is formed through the gradual transfer of protons, leaving O on the solid surface. 2- It then combines with calcium ions and embeds itself into the rearranged lattice points to form stable CaO. The reaction equation can be expressed as: CaS + H₂O = CaO + H₂S. This reaction is reversible. Continuous water vapor can also be introduced as a carrier gas to remove the gaseous product H₂S, continuously removing the product and driving the reaction forward. Therefore, although this reaction is reversible, the timely removal of the product creates a "non-equilibrium" condition, enabling the reaction to proceed.

[0019] The beneficial effects of this invention include:

[0020] (1) The present invention adopts a three-step synergistic conversion chain of "acid washing to remove impurities - low temperature hydrogen reduction - mild water vapor oxidation", which completely bypasses the high temperature bottleneck of direct decomposition of phosphogypsum and realizes the precise directional conversion of phosphogypsum into high-purity calcium oxide.

[0021] (2) The present invention constructs a stepped energy utilization chain, and achieves a cliff-like reduction in the total energy consumption of the system by allocating energy consumption through raw material activation before reaction - hydrogen reduction - low temperature water oxidation.

[0022] (3) Under the fixed-bed low-temperature reaction conditions, the present invention achieves a calcium sulfate decomposition rate of over 99.5% in phosphogypsum and a calcium oxide content of over 97.22% in the solid product through deep impurity removal and process synergy. Attached Figure Description

[0023] Figure 1 This is a flowchart of the experiment.

[0024] Figure 2 XRD pattern of phosphogypsum raw material.

[0025] Figure 3 XRD patterns of calcium oxide produced by the reduction decomposition of phosphogypsum under different conditions. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] Example 1

[0028] This embodiment provides a method for producing calcium oxide by low-temperature decomposition of phosphogypsum, including:

[0029] (1) Preparation of reaction raw materials: The phosphogypsum raw material was dried at 40℃ for 24 h to constant weight to ensure complete removal of adhering water. Then, the dried phosphogypsum was passed through a 300-mesh sieve, and the pretreated phosphogypsum was used for later use. The content of crystal water was 17.96%, and the content of CaSO4·2H2O was 85.82%. Then, 50g of the treated phosphogypsum was mixed with 400mL of 30% sulfuric acid solution and reacted at 80℃ under constant temperature stirring for 2h. Then, tributyl phosphate was added for extraction. After separation, the lower layer product was collected and washed with deionized water until neutral. Then, it was dried at 105℃ for 6h for later use.

[0030] (2) Place the 8g of phosphogypsum treated above in the heating zone of the two-stage fixed-bed reactor, purge the reactor with nitrogen to remove the air from the reactor, and then heat it to 800°C at a heating rate of 10°C / min. Then, introduce hydrogen with a purity of 99.999% for the reduction reaction. Set the hydrogen flow rate to 200 mL / min, control the temperature in the fixed bed to 800°C, and the reaction time to 60 min. After the reaction is completed, stop heating, turn off the hydrogen, and purge with nitrogen.

[0031] (3) Water vapor oxidation stage: After the hydrogen reduction stage, nitrogen flow rate is set to 200 mL / min to purge for 20 min to remove residual hydrogen; then, at a reaction temperature of 500℃, water vapor is introduced into the reactor at a flow rate of 0.1 mL / min for 7 h to oxidize; after the reaction is completed, water vapor is stopped and heating is stopped, while nitrogen is introduced to purge residual water vapor. After the reactor is cooled to room temperature, the obtained calcium oxide solid product is collected.

[0032] (4) Product detection: In the hydrogen reduction stage, the barium sulfate gravimetric method is used to detect CaSO4. A separate phosphogypsum decomposition experiment is required to ensure that calcium sulfate is completely decomposed before proceeding to the next stage. In the steam oxidation stage, the calcium oxide content can be detected by titration, and XRD is added for auxiliary verification. Under these conditions, after drying the hydrolyzed solid product, the calcium oxide yield was 58.84% after titration and XRD detection.

[0033] The decomposition rate (φ) of phosphogypsum is calculated based on the difference in CaSO4 content before and after the reaction.

[0034]

[0035] Where φ is the decomposition rate of phosphogypsum (%); CaSO4 (%) is the CaSO4 content in the product (%); ω is the CaSO4 content in phosphogypsum (%).

[0036] Example 2

[0037] This embodiment provides a method for producing calcium oxide by low-temperature decomposition of phosphogypsum, including:

[0038] (1) Preparation of reaction raw materials: The phosphogypsum raw material was dried at 40℃ for 48h to constant weight to ensure complete removal of adhering water. Then the dried phosphogypsum was passed through a 300-mesh sieve, and the pretreated phosphogypsum was used for later use. The content of crystal water was 17.96%, and the content of CaSO4·2H2O was 85.82%. Then 50g of the treated phosphogypsum was mixed with 400mL of 30% sulfuric acid solution and reacted at 80℃ under constant temperature stirring for 2h. Then tributyl phosphate was added for extraction. After separation, the lower layer product was collected and washed with deionized water until neutral. Then it was dried at 105℃ for 6h for later use.

[0039] (2) Place the 6g of phosphogypsum treated above in the heating zone of the two-stage fixed-bed reactor, purge the reactor with nitrogen to remove the air from the reactor, and then heat it to 800°C at a heating rate of 10°C / min. Then, introduce hydrogen with a purity of 99.999% for the reduction reaction. Set the hydrogen flow rate to 60 mL / min, control the temperature in the fixed bed to 800°C, and the reaction time to 90 min. After the reaction is completed, stop heating, turn off the hydrogen, and purge with nitrogen.

[0040] (3) Water vapor oxidation stage: After the hydrogen reduction stage, nitrogen flow rate is set to 200 mL / min to purge for 20 min to remove residual hydrogen; then, at a reaction temperature of 500℃, water vapor is introduced into the reactor at a flow rate of 0.5 mL / min for 7 h to oxidize; after the reaction is completed, water vapor is stopped and heating is stopped, while nitrogen is introduced to purge residual water vapor. After the reactor is cooled to room temperature, the obtained calcium oxide solid product is collected.

[0041] (4) Product detection: In the hydrogen reduction stage, the gravimetric method of barium sulfate was used to detect CaSO4. The calcium sulfate must be completely decomposed before proceeding to the next stage. In the steam oxidation stage, the calcium oxide content can be detected by titration, and XRD is used for auxiliary verification. Under these conditions, after drying the hydrolyzed solid product, the calcium oxide yield was 71.38% after titration and XRD detection.

[0042] Example 3

[0043] This embodiment provides a method for producing calcium oxide by low-temperature decomposition of phosphogypsum, including:

[0044] (1) Preparation of reaction raw materials: The phosphogypsum raw material was dried at 45℃ for 36h to constant weight to ensure complete removal of adhering water. Then the dried phosphogypsum was passed through a 300-mesh sieve, and the pretreated phosphogypsum was used for later use. The content of crystal water was 17.96%, and the content of CaSO4·2H2O was 85.82%. Then 50g of the treated phosphogypsum was mixed with 200mL of 30% sulfuric acid solution and reacted at 80℃ under constant temperature stirring for 2h. Then tributyl phosphate was added for extraction. After separation, the lower layer product was collected and washed with deionized water until neutral. Then it was dried at 105℃ for 6h for later use.

[0045] (2) Place the 6g of phosphogypsum treated above in the heating zone of the two-stage fixed-bed reactor, purge the reactor with nitrogen to remove the air from the reactor, and then heat it to 800°C at a heating rate of 10°C / min. Then, introduce hydrogen with a purity of 99.999% for the reduction reaction. Set the hydrogen flow rate to 60 mL / min, control the temperature in the fixed bed to 800°C, and the reaction time to 60 min. After the reaction is completed, stop heating, turn off the hydrogen, and purge with nitrogen.

[0046] (3) Water vapor oxidation stage: After the hydrogen reduction stage, nitrogen flow rate is set to 200 mL / min to purge for 20 min to remove residual hydrogen; then, at a reaction temperature of 500℃, water vapor is introduced into the reactor at a flow rate of 1.0 mL / min for 11 h to oxidize; after the reaction is completed, water vapor is stopped and heating is stopped, while nitrogen is introduced to purge residual water vapor. After the reactor is cooled to room temperature, the obtained calcium oxide solid product is collected.

[0047] (4) Product detection: In the hydrogen reduction stage, the barium sulfate gravimetric method was used to detect CaSO4. The calcium sulfate must be completely decomposed before proceeding to the next stage. In the steam oxidation stage, the calcium oxide content can be detected by titration, and XRD is used for auxiliary verification. Under these conditions, after drying the hydrolyzed solid product, the calcium oxide yield was 82.41% after titration and XRD detection.

[0048] Example 4

[0049] This embodiment provides a method for producing calcium oxide by low-temperature decomposition of phosphogypsum, including:

[0050] (1) Preparation of reaction raw materials: The phosphogypsum raw material was dried at 42℃ for 36h to constant weight to ensure complete removal of adhering water. Then the dried phosphogypsum was passed through a 300-mesh sieve, and the pretreated phosphogypsum was used for later use. The content of crystal water was 17.96%, and the content of CaSO4·2H2O was 85.82%. Then 50g of the treated phosphogypsum was mixed with 400mL of 20% sulfuric acid solution and reacted at 80℃ under constant temperature stirring for 2h. Then tributyl phosphate was added for extraction. After separation, the lower layer product was collected and washed with deionized water until neutral. Then it was dried at 105℃ for 6h for later use.

[0051] (2) Place the 6g of phosphogypsum treated above in the heating zone of the two-stage fixed-bed reactor, purge the reactor with nitrogen to remove the air from the reactor, and then heat it to 800°C at a heating rate of 10°C / min. Then, introduce hydrogen with a purity of 99.999% for the reduction reaction. Set the hydrogen flow rate to 100 mL / min, control the temperature in the fixed bed to 800°C, and the reaction time to 60 min. After the reaction is completed, stop heating, turn off the hydrogen, and purge with nitrogen.

[0052] (3) Water vapor oxidation stage: After the hydrogen reduction stage, nitrogen flow rate is set to 200 mL / min to purge for 20 min to remove residual hydrogen; then, at a reaction temperature of 500℃, water vapor is introduced into the reactor at a flow rate of 1.0 mL / min for 13 h to oxidize; after the reaction is completed, water vapor is stopped and heating is stopped, while nitrogen is introduced to purge residual water vapor. After the reactor is cooled to room temperature, the obtained calcium oxide solid product is collected.

[0053] (4) Product detection: In the hydrogen reduction stage, the gravimetric method of barium sulfate was used to detect CaSO4. The calcium sulfate must be completely decomposed before proceeding to the next stage. In the steam oxidation stage, the calcium oxide content can be detected by titration, and XRD is used for auxiliary verification. Under these conditions, after drying the hydrolyzed solid product, the calcium oxide yield was 97.22% after titration and XRD detection.

[0054] Since the hydrolysis stage requires the complete decomposition of calcium sulfate, only the hydrolysis experimental conditions and evaluation results of Examples 1-4 are listed below, as shown in Table 1:

[0055] Table 1. Yields of calcium hydroxide in Examples 1-5

[0056] Example condition Calcium oxide yield (%) 1 Pretreatment phosphogypsum dosage: 8 g, reaction time: 7 h, water flow rate: 0.1 mL / min 58.84% 2 Pretreatment phosphogypsum dosage: 6 g, reaction time: 7 h, water flow rate: 0.5 mL / min 71.38% 3 Pretreatment phosphogypsum dosage: 6 g, reaction time: 11 h, water flow rate: 1 mL / min 82.41% 4 Pretreatment phosphogypsum dosage: 6 g, reaction time: 13 h, water flow rate: 1 mL / min 97.22%

[0057] As can be clearly seen from Table 1,

[0058] To ensure the complete reduction of CaSO4, the main component of phosphogypsum, to CaS under a hydrogen-reducing atmosphere, thus laying the foundation for subsequent conversion reactions, this study uniformly set the reaction conditions to be isothermal reduction at 800℃ for 60–90 min. Based on the above hydrogen reduction conditions, the directional conversion of CaS to CaO was achieved by continuously introducing water vapor. By controlling key variables such as the water vapor introduction rate and reaction time, the efficient conversion of phosphogypsum to the target product CaO at 800℃ can be ensured.

[0059] Compared to existing literature reports on phosphogypsum decomposition processes, the method proposed in this patent achieves lower reaction temperatures in both the hydrogen reduction and CaS steam conversion stages. Furthermore, compared to traditional carbothermal reduction processes, this process not only avoids the combustion of carbon-based reducing agents (such as coal) or the emission of carbon-containing tail gases (such as CO2 and CO) during the reaction, achieving near-zero pollution emissions, but also significantly reduces overall energy consumption due to the lower reaction temperature and the application of clean reducing agents. It should be noted that in traditional carbothermal reduction processes, coal, as a reducing agent, is often accompanied by mineral impurities such as silicon and aluminum. Additionally, the introduction of catalysts and additives in some processes can further enrich silicates, sulfoaluminates, and other impurities in the product, ultimately leading to low CaO product purity. This process, however, eliminates the need to introduce these impurity sources, effectively avoiding impurity interference and significantly improving CaO product purity.

[0060] In summary, this process has achieved the expected goals in improving the purity of CaO products and optimizing energy-saving and emission-reduction performance, providing precise process parameter support and key technology references for basic research and industrial scale-up applications in the field of phosphogypsum resource utilization.

Claims

1. A method for producing calcium oxide by low-temperature decomposition of phosphogypsum, characterized in that, Includes the following steps: Step A1, Pretreatment of phosphogypsum: Dry the phosphogypsum raw material at 35~45℃ for 24~48h, then pass it through a 300-mesh sieve and collect the material on the sieve for later use; Step A2: Mix the phosphogypsum treated in step A1 with sulfuric acid solution and react for 2 hours under constant temperature and stirring at 60-80℃; then add extractant for extraction, collect the lower layer product after separation, wash with deionized water until neutral, and then dry at 105℃ for 6-10 hours for later use. Step A3, Hydrogen Reduction Stage: The phosphogypsum treated in Step A2 is placed in the heating zone of a two-stage fixed-bed reactor. The reactor is purged with nitrogen to remove air. Then, the temperature is increased to 800°C at a rate of 10°C / min. Hydrogen gas with a purity of 99.999% is introduced and the reaction is carried out at a constant temperature for 60–90 min. The tail gas is dried during the reaction and used as the raw material gas for the production of sulfuric acid in the acid production process. Step A4, Water Vapor Oxidation Stage: After the reaction in Step A3 is completed, stop the hydrogen supply and purge the entire reactor with nitrogen at a rate of 200 mL / min. Then, at a reaction temperature of 500°C, water vapor is introduced into the reactor to carry out the oxidation reaction, which lasts for 1 to 13 hours. After the reaction is completed, stop the water vapor supply and heating, and purge the residual water vapor with nitrogen. After the reactor cools to room temperature, collect the obtained solid product.

2. The method for producing calcium oxide by low-temperature decomposition of phosphogypsum according to claim 1, characterized in that, In step A1, the phosphogypsum raw material has a water of crystallization wet basis content of 13-22%, an initial particle size of 100-400 mesh, and the prepared phosphogypsum has a particle size of 300 mesh or larger.

3. The method for producing calcium oxide by low-temperature decomposition of phosphogypsum according to claim 1, characterized in that, In step A2, the sulfuric acid solution has a mass fraction of 20-30%, and the mass ratio of phosphogypsum to sulfuric acid solution is 1:4-8; the extractant is tributyl phosphate.

4. The method for producing calcium oxide by low-temperature decomposition of phosphogypsum according to claim 1, characterized in that, In step A3, the flow rate of the 99.999% hydrogen is 50–200 mL / min.

5. The method for producing calcium oxide by low-temperature decomposition of phosphogypsum according to claim 1, characterized in that, In step A4, the flow rate of the water vapor is 0.1~2 mL / min.

Images