A method for producing calcium hydroxide by low-temperature decomposition of phosphogypsum coupled with hydrolysis

Abstract

This invention provides a method for producing calcium hydroxide by low-temperature decomposition of phosphogypsum coupled with hydrolysis. In this method, phosphogypsum is used as raw material. After simple drying and sieving pretreatment, it is reduced with hydrogen to obtain a solid product mainly composed of calcium sulfide. This product is then used as raw material in a next stage for hydrolysis in a reactor to obtain high-purity calcium hydroxide. This method uses pretreated phosphogypsum as raw material, hydrogen as the reducing gas, and a low-temperature hydrolysis reaction (60~150 °C). Both stages lower the temperature for producing calcium hydroxide from phosphogypsum, significantly reducing the reaction temperature and energy consumption. This process produces no carbon dioxide emissions and further reduces energy consumption, enabling energy conservation and emission reduction in the phosphogypsum decomposition process. Under these conditions, phosphogypsum can be decomposed below 800 °C and then hydrolyzed at 150 °C, achieving a calcium hydroxide yield of over 98%.

Description

Technical Field

[0001] This invention relates to a method for preparing calcium hydroxide by hydrogen reduction at low temperature decomposition of phosphogypsum coupled with hydrolysis reaction, belonging to the field of environmental protection and solid waste resource utilization. Background Technology

[0002] Phosphogypsum, a major byproduct of wet-process phosphoric acid production, generates approximately 4.5 to 5 tons of phosphogypsum for every ton of phosphoric acid produced, resulting in annual emissions of up to about 80 million tons. Due to the high energy consumption of its treatment process, open-air storage remains the primary disposal method. This method not only occupies a large amount of land resources, but the soluble phosphorus and fluoride impurities it contains can also be leached and seeped in by rainwater, causing serious pollution to groundwater and soil, becoming a key bottleneck restricting the green development of the phosphoric acid chemical industry. Since the main component of phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O), which contains abundant sulfur and calcium resources, how to separate sulfur and calcium and achieve resource utilization is a global challenge. Phosphogypsum can be used to produce sulfuric acid and co-produce cement clinker. Although the existing process of using coke to reduce phosphogypsum to produce sulfuric acid and co-produce cement clinker is relatively mature, it still has several significant drawbacks, including excessive energy consumption, poor cost-effectiveness, low economic feasibility, and problems such as corrosion and agglomeration within the reactor.

[0003] The biggest challenge in the decomposition of phosphogypsum is its high energy consumption. Therefore, many researchers have focused on low-temperature decomposition methods for phosphogypsum. Patent CN 101357773 B discloses a method for reducing the decomposition temperature of phosphogypsum during the sulfuric acid production process. This method uses a gaseous reducing agent and a solid additive to synergistically reduce and decompose phosphogypsum. The method involves mixing dried phosphogypsum, coal, and specific additives (the additives are a solid formulation composed of fly ash, coal gangue, metal oxides, and chlorides, or combined with gaseous reducing agents such as CO and H2S) in a specific mass ratio, heating the mixture to 700–950°C, reacting for 1–4 hours, and then cooling the solid residue before using it as cement clinker.

[0004] Patent CN 102556978 A discloses a method for reducing the decomposition temperature of phosphogypsum. This method involves mixing pretreated phosphogypsum, additive A, and additive B in different mass ratios, heating the mixture to 950–1050°C under an Ar atmosphere, and then introducing H2 with a volume concentration of 1–5% into a fluidized bed reactor. The reactor temperature is controlled at 950–1050°C, and the reaction time is 10–60 minutes. This method reduces material agglomeration and charring, results in a high average SO2 volume concentration in the exhaust gas, and increases the decomposition and desulfurization rates, but the decomposition temperature remains relatively high.

[0005] In the current research field of phosphogypsum resource utilization technology, the mainstream thermochemical reduction route still faces several key challenges. While the traditional carbothermal reduction method is relatively mature, its reaction temperature is typically high (>1000℃), resulting in high energy consumption and significant CO2 emissions, which significantly contradicts current low-carbon process requirements. The sulfur reduction route shows some advantages in lowering the reaction temperature; however, its industrial application is severely constrained by the stability of sulfur raw material supply and market price fluctuations, especially given my country's heavy reliance on imported sulfur resources, making long-term stable operation difficult to guarantee. Although additive-assisted decomposition methods can effectively promote the low-temperature conversion of phosphogypsum, the introduction of additives often triggers side reactions, resulting in impurities existing in complex phases in the product, making subsequent separation and purification difficult and costly. The calcium component in the decomposition products obtained by existing processes is generally insufficient in purity, and is mostly used to prepare cement clinker with higher impurity tolerance, rather than being economically and efficiently used for the direct synthesis of high-purity calcium hydroxide.

[0006] Calcium hydroxide, as an important industrial alkali source, is widely used in flue gas desulfurization, wastewater treatment, chemical synthesis, and new building materials. Its purity directly affects the efficiency and economics of downstream applications. Therefore, developing a low-temperature, low-carbon, and low-impurity phosphogypsum conversion technology to directly obtain high-purity calcium hydroxide has become an important research direction in the context of high-value utilization of solid waste and the circular economy. However, to date, no systematic and feasible technical solution has been formed in this direction, and relevant public reports are relatively scarce. There is still room for improvement in how to produce high-purity calcium hydroxide, and no reports have yet presented this information. Summary of the Invention

[0007] This invention aims to overcome the technical challenges of low-temperature processing, high-value utilization, and complete resource recovery in phosphogypsum treatment, and provides a process for the low-temperature decomposition of phosphogypsum coupled with hydrolysis to produce calcium hydroxide. This method can reduce energy consumption in the production of calcium hydroxide from phosphogypsum at different stages and achieve a significant increase in decomposition and conversion rates under low-temperature conditions.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] Step A1, raw material pretreatment: Dry the phosphogypsum raw material at 35~45℃ for 24~48h, sieve it using a 300-mesh sieve, and collect the material on the sieve for later use;

[0010] The 300-400 mesh particle size ensures that phosphogypsum forms a stable and porous reaction bed in the fixed bed, providing a smooth diffusion channel for hydrogen and ensuring sufficient specific surface area for reaction, thus avoiding increased mass transfer resistance or local overheating caused by fine powder.

[0011] Step A2, Hydrogen Reduction Stage: Place the phosphogypsum treated in Step A1 into a two-stage fixed-bed reactor. Adjust the position of the raw material to place it in the heating zone of the reactor. Purge the reactor with nitrogen at a rate of 200 mL / min to remove air from the reactor. Set the heating rate to 10℃ / min to raise the temperature to 600-800℃. Then, introduce hydrogen with a purity of 99.999%. Control the temperature inside the fixed bed at 600-800℃ and react for 30-60 minutes. The tail gas during the reaction can be dried and used as raw material gas for the production of sulfuric acid in the acid production process.

[0012] Step A3, Hydrolysis Stage: The solid product obtained in Step A2 is added to a reactor equipped with a stirrer at a mass ratio of 1:5~20 for hydrolysis. The stirring speed of the reactor is set to 150~350 rpm, the reaction time to 0.5~4 h, and the reaction temperature to 60~150℃. The main reaction occurring during the reaction is CaS + 2H2O → Ca(OH)2 + H2S. The reactor heating and stirring devices are turned on. The gas generated during the reaction can be directly used as raw material for sulfuric acid production. The solid product is washed with deionized water to remove sulfur-containing substances (mainly sulfur produced from the hydrogen reduction phase), and then dried in an oven at 105℃ to remove moisture.

[0013] Furthermore, in step A1, the phosphogypsum raw material has a water of crystallization wet basis content of 13-21% and a phosphogypsum particle size of 100-400 mesh.

[0014] Furthermore, in step A2, the flow rate of 99.999% hydrogen is 50–200 mL / min; and the reactor is purged with nitrogen at a flow rate of 200 mL / min before and after the reaction.

[0015] Furthermore, in steps A2 and A3, the heating rate is 10℃ / min, and the timing begins when the reactor reaches the target temperature.

[0016] Furthermore, in step A3, the amount of deionized water used for washing is 25 to 50 times the mass of the solid product obtained in step A2.

[0017] The mechanism of this invention includes:

[0018] The normal decomposition temperature for phosphogypsum is above 1300℃. Experiments have shown that introducing hydrogen gas can effectively lower this decomposition temperature to 800℃. Hydrogen molecules diffuse onto the surface of CaSO4 solid particles and are adsorbed, reducing the sulfur content from +6 (S₆) to... 6+ Restored to -2 valence (S) 2-The process involves the formation of solid calcium sulfide (CaS), with active hydrogen attacking the sulfide-oxygen bond to ultimately produce CaS. The hydrolysis of calcium sulfide can be represented as: CaS + 2H₂O → Ca(OH)₂ + H₂S. CaS is a slightly soluble ionic crystal, and its dissolution and dissociation process (CaS(s) → Ca)₂... 2+ +S 2- The solubility of CaS crystals is significantly affected by temperature; increasing the temperature breaks the ionic bonds on the surface of the CaS crystals, thus increasing their solubility. As S... 2- The increase of S 2- H-binding of water molecules + As the ability gradually increases, the OH produced - It also increases significantly. This is because the OH groups generated by hydrolysis under these temperature conditions... - It can exist stably and react with Ca in solution. 2+ They combine to form slightly soluble Ca(OH)₂. 2- H-binding of water molecules + Even after formation, the resulting HS- group remains highly alkaline and can continue to draw H+ from water molecules. + H₂S is formed. Since the hydrolysis products are hydrogen sulfide gas and calcium hydroxide, and calcium hydroxide is only slightly soluble, the concentration of calcium in the system can be further reduced. 2+ and S 2- The concentration of [amount] promotes the forward reaction, eventually leading to complete hydrolysis and the formation of calcium hydroxide.

[0019] Compared with existing technologies, the beneficial effects of this invention are as follows:

[0020] (1) This invention uses high-purity hydrogen as a clean reducing agent and couples low-temperature reduction and controllable hydrolysis into an integrated process path. At the same time, it overcomes the problems of low carbonization, low temperature reduction and high value of phosphogypsum treatment, effectively controls the flow of impurities, and realizes a qualitative leap from solid waste to high-purity chemical products.

[0021] (2) The present invention constructs a clean cascade reaction chain of gas-solid reduction-liquid-solid hydrolysis, realizing a high degree of synergistic coupling between materials and elements, forming a resource-based closed loop with complete internal material circulation.

[0022] (3) No solid additives are introduced throughout the process of this invention, thus avoiding impurity pollution; the reducing agent is hydrogen, so there is no carbon emission; sulfur is efficiently recovered in gaseous form to produce acid, so there is no secondary solid waste; and finally, high-purity calcium hydroxide that can be directly used in fine chemicals is obtained, rather than low-value building materials.

[0023] (4) The calcium sulfide hydrolysis process of the present invention has a lower temperature and shorter time, and can achieve a calcium sulfate decomposition rate of over 99.5% and a calcium hydroxide yield of over 98% in phosphogypsum. Attached Figure Description

[0024] Figure 1This is a flowchart of the experiment.

[0025] Figure 2 The XRD pattern of phosphogypsum raw material.

[0026] Figure 3 XRD pattern of calcium hydroxide prepared by the reduction decomposition of phosphogypsum. Detailed Implementation

[0027] To enable researchers to better understand the technical solutions and advantages of this invention, the invention will be described in detail below with reference to embodiments. It should be noted that the embodiments are only used to clearly and completely describe this invention and should not be considered as limiting the scope of this invention.

[0028] Example 1:

[0029] The specific implementation process in this embodiment is as follows:

[0030] Step A1, Preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 40℃ for 24h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 17.96%, a CaSO4·2H2O content of 85.82%, and a particle size in the range of 100-400 mesh.

[0031] Step A2, Hydrogen Reduction Stage: 8 g of the pretreated phosphogypsum from Step A1 was placed in a quartz tube and placed in the heating zone of a fixed-bed reactor. Under nitrogen atmosphere protection, the temperature was raised to 800℃, and then 99.999% hydrogen was introduced at a rate of 50 mL / min, maintaining the temperature in the fixed bed at 800℃. The reaction was allowed to proceed for 60 min. After the reaction was complete, heating was stopped, the hydrogen supply was shut off, and nitrogen was introduced for purging. Once the reaction tube temperature had cooled to room temperature, the sample was removed and its decomposition rate was measured. Under these conditions, calcium sulfate was completely decomposed for use in the hydrolysis stage. The decomposition rate of the phosphogypsum in this stage was 99.58%.

[0032] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:20. The stirring speed of the reactor is set to 350 rpm, the reaction time to 4 hours, and the reaction temperature to 60℃. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 50g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105℃ to remove moisture.

[0033] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 53.47% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0034] The decomposition rate (φ) of phosphogypsum is calculated based on the difference in CaSO4 content before and after the reaction.

[0035]

[0036] Where φ is the decomposition rate of phosphogypsum (%); CaSO4 (%) is the CaSO4 content in the product (%); ω is the CaSO4 content in phosphogypsum (%).

[0037] Example 2:

[0038] The specific implementation process in this embodiment is as follows:

[0039] Step A1, preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 45℃ for 48h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 18.02%, a CaSO4·2H2O content of 86.17%, and a particle size in the range of 100 to 400 mesh.

[0040] Step A2, Hydrogen Reduction Stage: Take 8 g of the pretreated phosphogypsum from Step A1 and place it in a quartz tube. Place the tube in the heating zone of a fixed-bed reactor and heat it to 800℃ under a nitrogen atmosphere. Then, introduce 99.999% hydrogen sulfate at a rate of 50 mL / min, maintaining the temperature in the fixed bed at 800℃. React for 60 min. After the reaction is complete, stop heating, turn off the hydrogen supply, and purge with nitrogen. After the reaction tube temperature drops to room temperature, remove the sample and test its decomposition rate. Under these conditions, calcium sulfate can be completely decomposed for use in the hydrolysis stage. The decomposition rate of phosphogypsum in this stage is 99.46%.

[0041] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:20. The stirring speed of the reactor is set to 350 rpm, the reaction time to 4 hours, and the reaction temperature to 90℃. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 100g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105℃ to remove moisture.

[0042] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 66.75% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0043] Example 3:

[0044] The specific implementation process in this embodiment is as follows:

[0045] Step A1, Preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 42℃ for 36h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 17.96%, a CaSO4·2H2O content of 85.82%, and a particle size in the range of 100-400 mesh.

[0046] Step A2, Hydrogen Reduction Stage: Take 8g of the pretreated phosphogypsum from Step A1 and place it in a quartz tube. Place the tube in the heating zone of a fixed-bed reactor and heat it to 800℃ under a nitrogen atmosphere. Then, introduce 99.999% hydrogen at a rate of 200mL / min, maintaining the temperature in the fixed bed at 800℃. React for 60 minutes. After the reaction is complete, stop heating, turn off the hydrogen supply, and purge with nitrogen. After the reaction tube temperature drops to room temperature, remove the sample and test its decomposition rate. Under these conditions, calcium sulfate can be completely decomposed for use in the hydrolysis stage. The decomposition rate of phosphogypsum in this stage is 99.58%.

[0047] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:20. The stirring speed of the reactor is set to 350 rpm, the reaction time to 3 hours, and the reaction temperature to 110°C. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 80g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105°C to remove moisture.

[0048] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 98.24% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0049] Example 4:

[0050] The specific implementation process in this embodiment is as follows:

[0051] Step A1, Preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 40℃ for 48h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 17.96%, a CaSO4·2H2O content of 85.82%, and a particle size in the range of 100-400 mesh.

[0052] Step A2, Hydrogen Reduction Stage: 8g of the pretreated phosphogypsum from Step A1 was placed in a quartz tube and placed in the heating zone of a fixed-bed reactor. Under a nitrogen atmosphere, the temperature was raised to 800℃, and then 99.999% hydrogen was introduced at a rate of 100mL / min. The temperature inside the fixed bed was controlled at 800℃, and the reaction was allowed to proceed for 60 minutes. After the reaction was complete, heating was stopped, the hydrogen supply was turned off, and nitrogen was introduced for purging. After the reaction tube temperature cooled to room temperature, the sample was removed and its decomposition rate was measured. Under these conditions, calcium sulfate was completely decomposed for use in the hydrolysis stage. The decomposition rate of phosphogypsum in this stage was 99.58%.

[0053] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:10. The stirring speed of the reactor is set to 350 rpm, the reaction time to 3 hours, and the reaction temperature to 130°C. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 80g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105°C to remove moisture.

[0054] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 98.86% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0055] Example 5:

[0056] The specific implementation process in this embodiment is as follows:

[0057] Step A1, Preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 40℃ for 48h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 17.96%, a CaSO4·2H2O content of 85.82%, and a particle size in the range of 100-400 mesh.

[0058] Step A2, Hydrogen Reduction Stage: 8g of the pretreated phosphogypsum from Step A1 was placed in a quartz tube and placed in the heating zone of a fixed-bed reactor. Under nitrogen atmosphere protection, the temperature was raised to 800℃, and then 99.999% hydrogen was introduced at a rate of 50mL / min, maintaining the temperature in the fixed bed at 800℃. The reaction was allowed to proceed for 60 min. After the reaction was complete, heating was stopped, the hydrogen supply was shut off, and nitrogen was introduced for purging. Once the reaction tube temperature had cooled to room temperature, the sample was removed and its decomposition rate was measured. Under these conditions, calcium sulfate was completely decomposed for use in the hydrolysis stage. The decomposition rate of the phosphogypsum in this stage was 99.58%.

[0059] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:10. The stirring speed of the reactor is set to 200 rpm, the reaction time to 2 hours, and the reaction temperature to 150°C. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 80g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105°C to remove moisture.

[0060] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 99.34% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0061] Example 6:

[0062] The specific implementation process in this embodiment is as follows:

[0063] Step A1, Preparation of reaction raw materials: Take phosphogypsum raw material and dry it at 40℃ for 48h to constant weight, pass it through a 300-mesh sieve, and the resulting pretreated phosphogypsum is ready for use; wherein the phosphogypsum raw material has a crystal water content of 17.96%, a CaSO4·2H2O content of 85.82%, and a particle size in the range of 100-400 mesh.

[0064] Step A2, Hydrogen Reduction Stage: Take 8 g of the pretreated phosphogypsum from Step A1 and place it in a quartz tube. Place the tube in the heating zone of a fixed-bed reactor and heat it to 800℃ under a nitrogen atmosphere. Then, introduce 99.999% hydrogen at a rate of 150 mL / min, maintaining the temperature in the fixed bed at 800℃. React for 60 min. After the reaction is complete, stop heating, turn off the hydrogen supply, and purge with nitrogen. After the reaction tube temperature drops to room temperature, remove the sample and test its decomposition rate. Under these conditions, calcium sulfate can be completely decomposed for use in the hydrolysis stage. The decomposition rate of phosphogypsum in this stage is 99.58%.

[0065] Step A3, Hydrolysis Reaction: The product from the reduction stage of phosphogypsum is used as the raw material for the hydrolysis stage. 2g of the solid obtained in Step A2 (calcium sulfide solid) and deionized water are added to a reactor equipped with a stirrer at a mass ratio of 1:5. The stirring speed of the reactor is set to 200 rpm, the reaction time to 0.5 h, and the reaction temperature to 150℃. The reactor heating and stirring devices are turned on. After the reaction is complete, the product is washed with 80g of deionized water to remove sulfur-containing substances. The solid product is then dried in an oven at 105℃ to remove moisture.

[0066] Product detection: During the hydrogen reduction stage, the gravimetric method for CaSO4 was used to ensure complete decomposition of calcium sulfate for the next stage. During the hydrolysis stage, the calcium hydroxide content was determined by titration, with XRD used for further verification. Under these conditions, the calcium hydroxide yield was 99.02% after drying the hydrolysis solid product and subsequent titration and XRD analysis.

[0067] Since the hydrolysis stage requires the complete decomposition of calcium sulfate, only the hydrolysis experimental conditions and evaluation results of Examples 1-6 are listed below, as shown in Table 1:

[0068] Table 1. Yields of calcium hydroxide in Examples 1-6

[0069] Example Hydrolysis reaction conditions Calcium hydroxide yield (%) 1 Liquid-to-solid mass ratio 20:1, rotation speed 350 rpm, reaction time 4 h, reaction temperature 60℃ 53.47% 2 Liquid-to-solid mass ratio 20:1, rotation speed 350 rpm, reaction time 4 h, reaction temperature 90℃ 66.75% 3 Liquid-to-solid mass ratio 20:1, rotation speed 350 rpm, reaction time 3 h, reaction temperature 110℃ 98.24% 4 Liquid-to-solid mass ratio 10:1, rotation speed 350 rpm, reaction time 3 h, reaction temperature 130℃ 98.86% 5 Liquid-to-solid mass ratio 10:1, rotation speed 200 rpm, reaction time 2 h, reaction temperature 150℃ 99.34% 6 Liquid-to-solid mass ratio 5:1, rotation speed 200 rpm, reaction time 0.5 h, reaction temperature 150℃ 99.02%

[0070] As can be clearly seen from Table 1,

[0071] To ensure complete reduction of phosphogypsum under hydrogen conditions, decomposing it into calcium sulfide for subsequent reactions, the reaction conditions selected here were all at 800℃ for 60 minutes. Hydrolysis case studies under different temperatures, liquid-to-solid ratios, and reaction temperatures showed that complete hydrolysis in a short time could be achieved by lowering the hydrolysis temperature to below 110℃. Compared with the literature, this represents a temperature reduction of approximately 180℃, and the hydrolysis time was also significantly reduced. The reaction achieved the desired progress and effect under these specific conditions. The data clearly present this significant phenomenon and result, providing precise and crucial evidence and reference for related research and process applications.

[0072] The above description of the embodiments is intended to facilitate understanding and use of the present invention by those skilled in the art. Those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without inventive effort. Therefore, the above descriptions are merely embodiments of the present invention and should not be construed as limiting the scope of the invention. Simple equivalent changes and modifications made according to the present invention are still within the scope of patent protection of this invention.

Claims

1. A method for low-temperature decomposition of phosphogypsum coupled with hydrolysis to produce calcium hydroxide, characterized in that, Includes the following steps: Step A1, raw material pretreatment: Dry the phosphogypsum raw material at 35~45℃ for 24~48h, sieve it using a 300-mesh sieve, and collect the material on the sieve for later use; Step A2, hydrogen reduction stage: The phosphogypsum treated in step A1 is placed in the heating zone of a two-stage fixed-bed reactor. The reactor is purged with nitrogen to remove the air. Then, the temperature is increased to 600-800°C at a rate of 10°C / min. Hydrogen with a purity of 99.999% is then introduced. The temperature is maintained for 30-60 min. The tail gas generated during the reaction is dried and used as the raw material gas for the production of sulfuric acid in the acid production process. Step A3, hydrolysis stage: The solid product obtained in step A2 and deionized water are sequentially added to a reaction vessel equipped with a stirrer. The hydrolysis reaction is carried out at a predetermined stirring rate, reaction time, and reaction temperature, and the gas produced by the reaction is collected. After the reaction is completed, the remaining solid product in the vessel is washed with deionized water to remove sulfur-containing substances, and then dried at 105°C to obtain calcium hydroxide solid product.

2. The method for low-temperature decomposition of phosphogypsum coupled with hydrolysis for hydrogen and calcium hydroxide production 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% and a particle size range of 100-400 mesh.

3. The method for low-temperature decomposition of phosphogypsum coupled with hydrolysis for hydrogen and calcium hydroxide production according to claim 1, characterized in that, In step A2, the flow rate of the 99.999% hydrogen is 50–200 mL / min; and the reactor is purged with nitrogen at a flow rate of 200 mL / min before the hydrogen is introduced and after the reaction is completed.

4. The method for low-temperature decomposition of phosphogypsum coupled with hydrolysis for hydrogen and calcium hydroxide production according to claim 1, characterized in that, In step A3, the mass ratio of the solid product obtained in step A2 to deionized water is 1:5~20; the predetermined stirring rate is 150~350 rpm, the reaction time is 0.5~4 h, and the reaction temperature is 60~150℃; the amount of deionized water used for washing is 25~50 times the mass of the solid product obtained in step A2.

5. The method for low-temperature decomposition of phosphogypsum coupled with hydrolysis for hydrogen and calcium hydroxide production according to claim 1, characterized in that, In step A3, the gas produced by the collected reaction can be directly used as raw material for the production of sulfuric acid; the sulfur-containing substance is mainly sulfur produced by the hydrogen reduction part; the main reaction that occurs during the reaction is CaS + 2H2O → Ca(OH)2 + H2S.

Images