System for recovering sulfur and calcium from low-temperature reduced gypsum

Abstract

The present application belongs to the field of environmental protection technology, and particularly relates to a system for recovering sulfur and calcium from low-temperature reduced gypsum, comprising a low-temperature reduction unit, an interactive reaction unit, a catalytic reduction unit, a sulfur condensation unit and a post-treatment process; gypsum, a reducing agent and a promoter are reacted in the low-temperature reduction unit to generate calcium sulfide which is input into the interactive reaction unit, and tail gas generated is input into the post-treatment process for incineration and purification; calcium sulfide and gypsum are reacted in the interactive reaction unit to generate calcium oxide which is discharged from the system, and sulfur vapor generated is mostly input into the sulfur condensation unit, and the remaining part is input into the catalytic reduction unit; the sulfur vapor input into the sulfur condensation unit is discharged from the system after being liquefied, and the remaining non-condensable gas is returned to the low-temperature reduction unit; the sulfur vapor input into the catalytic reduction unit is reacted with the reducing agent to generate sulfide gas which is returned to the low-temperature reduction unit as the promoter. Compared with the prior art, the present application realizes a process for low-temperature reduction of calcium sulfate which is suitable for various reducing agents.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, specifically relating to a system for recovering sulfur and calcium from low-temperature reduced gypsum. Background Technology

[0002] Sulfur dioxide (SO2) is a gaseous pollutant widely distributed in industries such as steel, power, non-ferrous metals, petrochemicals, chemicals, and building materials. Wet desulfurization processes generate large amounts of gypsum waste, wasting valuable sulfur resources and causing environmental pollution. Currently, the reuse of waste gypsum includes its use as an auxiliary material in building materials, as a sulfur and calcium fertilizer for crop growth, and as a soil conditioner. However, the consumption rate of waste gypsum through these reuse methods is far slower than its generation rate. Landfill disposal is no longer sufficient to handle the massive production of waste gypsum and results in significant loss of calcium and sulfur resources. Therefore, developing new gypsum reuse technologies is crucial for the non-ferrous metals industry to achieve green and low-carbon production.

[0003] Compared to traditional gypsum recycling methods, the CaSO4 reduction method not only solves the environmental problems caused by gypsum stockpiling but also recovers high-value-added sulfur (S) and calcium (Ca) resources. However, the reduction temperature of CaSO4 is generally above 1000℃, limiting the widespread adoption of this method. Using sulfides as inducing agents can significantly lower the initiation temperature of the reaction between the reducing agent and CaSO4. Simultaneously, the CaS produced by CaSO4 reduction can further react with CaSO4 to produce sulfur and calcium oxide. Furthermore, sulfur vapor can be catalytically reduced to sulfides such as H2S, CS2, and COS, continuing to function as an inducing agent for CaSO4 reduction.

[0004] The reduction and utilization of waste gypsum is an important way to realize the resource utilization of sulfur and calcium. However, the high starting temperature of the reduction reaction involving CaSO4 has limited the promotion of this method. Chinese patent CN200910216326.7 proposes a method for preparing calcium sulfide by reducing and decomposing gypsum with sulfur, which uses gaseous sulfur and preheated gypsum to react to prepare CaS, and the generated SO2 tail gas is used to prepare sulfuric acid; patent CN200910216325.2 proposes a method for reducing and decomposing phosphogypsum with sulfur, which uses gaseous sulfur and phosphogypsum to react, and the generated CaS is ground and mixed with phosphogypsum to produce CaO; both schemes require additional sulfur to be provided to the reaction to take place. Patent CN201210522386.3 proposes a method for preparing calcium sulfide by reducing and decomposing phosphogypsum in a fluidized bed. The method involves mixing coal powder and phosphogypsum in a certain proportion and reacting them at 800-950℃ to prepare CaS, achieving a high CaSO4 reduction rate. However, the high reaction temperature leads to high energy consumption, and there is a lack of methods to reduce the reaction temperature to reduce energy consumption.

[0005] In summary, existing recycling technologies still require additional sulfur input or high reaction temperatures, making it difficult for these technologies to be practically applied and promoted. Therefore, it is necessary to develop a new gypsum recycling technology to recover sulfur and calcium from gypsum and achieve green and low-carbon production. Summary of the Invention

[0006] The purpose of this invention is to provide a system for recovering sulfur and calcium from gypsum by low-temperature reduction, in order to solve at least one of the above-mentioned problems. This system addresses the shortcomings of existing technologies where the recovery of Ca and S from CaSO4 is energy-intensive or requires the addition of additional S, and realizes a process for the low-temperature reduction of calcium sulfate applicable to a variety of reducing agents.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A system for recovering sulfur and calcium from low-temperature reduced gypsum includes a low-temperature reduction unit, an interactive reaction unit, a catalytic reduction unit, a sulfur coagulation unit, and a post-treatment process.

[0009] Gypsum, reducing agent and accelerator react in the low-temperature reduction unit, the generated calcium sulfide is input into the interactive reaction unit, and the generated tail gas is input into the post-treatment process for incineration and purification.

[0010] Calcium sulfide and gypsum react in the interactive reaction unit, the generated calcium oxide is discharged from the system, most of the generated sulfur vapor is input into the sulfur condensation unit, and the remainder is input into the catalytic reduction unit.

[0011] The sulfur vapor input to the sulfur condensation unit is liquefied and then discharged from the system, while the remaining non-condensable gas is returned to the low-temperature reduction unit.

[0012] The sulfur vapor input to the catalytic reduction unit reacts with the reducing agent, and the generated sulfide gas is returned to the low-temperature reduction unit as a promoter.

[0013] Preferably, the gypsum is anhydrous calcium sulfate or calcium sulfate containing water of crystallization; the accelerator is one or more of hydrogen sulfide, carbon disulfide, and COS; the reducing agent participating in the reaction in the low-temperature reduction unit is a solid reducing agent or a gaseous reducing agent, wherein the solid reducing agent is one or two of pulverized coal and biomass, and the gaseous reducing agent is one or more of coal gas, natural gas, and hydrogen.

[0014] Preferably, in the low-temperature reduction unit, the molar ratio of the reducing agent to gypsum participating in the reaction is 0.95-1.05 times the stoichiometric ratio, the amount of accelerator is 2-15 mol% of gypsum, the reaction temperature is 600-900℃, and the reaction time is 15-90 minutes.

[0015] Preferably, in the interactive reaction unit, the molar ratio of calcium sulfide to gypsum participating in the reaction is 3:1-1.1, and the reaction temperature is 800-1100℃.

[0016] Preferably, the system further includes a dust removal unit connected after the interactive reaction unit, and the sulfur vapor is filtered and removed by the dust removal unit before being fed into the sulfur condensation unit and the catalytic reduction unit respectively.

[0017] Preferably, the dust removal unit uses a ceramic fiber membrane or a metal sintered membrane for filtration and dust removal.

[0018] Preferably, the sulfur vapor input to the catalytic reduction unit is 10-20 vol% of the total amount produced in the reaction. The sulfur vapor reacts with the reducing agent in a stoichiometric ratio. The reducing agent is one or more of coal gas, natural gas, and hydrogen. The reaction catalyst is alumina supported on 2-5 wt% transition metal oxides. The reaction temperature is 600-750°C, and the space velocity is 50-500 h⁻¹. -1 The generated sulfide gas is one or more of hydrogen sulfide, CS2, and COS.

[0019] Preferably, the sulfur condensation unit cools the sulfur vapor to 200-250°C.

[0020] Preferably, the post-treatment process includes an incineration and waste heat recovery unit, a preheating unit, and a tail gas purification unit.

[0021] The exhaust gas generated in the low-temperature reduction unit is fed into the incineration and waste heat recovery unit for incineration treatment, and the heat generated is used to preheat the reducing agent.

[0022] The exhaust gas after heat exchange with the reducing agent is input into the preheating unit for preheating the gypsum.

[0023] The exhaust gas after heat exchange with gypsum is input into the exhaust gas purification unit for dust removal and desulfurization treatment. The CO2 in the exhaust gas is captured, and the remaining exhaust gas is discharged.

[0024] Preferably, 40-70% of the exhaust gas generated in the low-temperature reduction unit is returned to the low-temperature reduction unit.

[0025] The working principle of this invention is as follows:

[0026] Using on-site generated gaseous sulfides as an inducing component, gypsum is converted into an activated state, allowing it to be rapidly reduced to calcium sulfide at a lower temperature in a low-temperature reduction unit. Subsequently, the incandescent calcium sulfide is mixed with gypsum, and sulfur vapor and calcium oxide products are generated through an interactive reaction of sulfur elements in an interactive reaction unit. The flue gas containing sulfur vapor is diverted for treatment; most of the sulfur is recovered as liquid sulfur through a sulfur condensation unit, while the remaining sulfur vapor is converted into gaseous sulfides through a reduction reaction in a catalytic reduction unit, which are then used as an inducing agent for gypsum reduction.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The reaction method of this invention can significantly reduce the reduction temperature of gypsum, shorten the reaction time, and increase the conversion rate of CaS. The residual heat of CaS can be used to initiate the interaction reaction between CaS and gypsum, and release heat that can sustain the reaction, thereby increasing the conversion rates of calcium and sulfur to calcium oxide and sulfur, respectively, and promoting their resource recovery. The preheated gypsum participates in both the gypsum reduction and interaction reactions, making full use of residual heat while accelerating the reaction rate.

[0029] 1) The induction of a small amount of gaseous sulfide can greatly reduce the reaction initiation temperature of the reducing agent on gypsum. The operation is simple and quick, and the CaSO4 reduction rate is over 90%.

[0030] 2) The sulfur resources in gypsum are eventually converted into solid sulfur, which has high utilization value, and the calcium resources are converted into calcium sulfide, which has broad application prospects.

[0031] 3) The sulfur production tail gas is converted into gaseous sulfides through a reduction process, which can be recycled as an inducing agent. The demand for external sulfur resources is small, and the reducing agents used are widely available and flexible in selection, which can be adapted to different industrial needs.

[0032] 4) The entire process of this invention is simple, producing only sulfur and calcium oxide, without generating waste residue or wastewater, making it a clean production process. Attached Figure Description

[0033] Figure 1 This is a flowchart of the system.

[0034] In the diagram: 1-Low-temperature reduction unit; 2-Interactive reaction unit; 3-Dust removal unit; 4-Catalytic reduction unit; 5-Sulfur condensation unit; 6-Incineration and waste heat recovery unit; 7-Preheating unit; 8-Tail gas purification unit. Detailed Implementation

[0035] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0036] First, the structure of the system will be explained.

[0037] A system for recovering sulfur and calcium from low-temperature reduced gypsum, such as Figure 1 As shown, it includes a low-temperature reduction unit 1, an interactive reaction unit 2, a catalytic reduction unit 4, a sulfur-condensing unit 5, and a post-treatment process;

[0038] Gypsum, reducing agent and accelerator react in low-temperature reduction unit 1, and the generated calcium sulfide is input into interactive reaction unit 2. The generated tail gas is input into the post-treatment process for incineration and purification.

[0039] Calcium sulfide and gypsum react in interactive reaction unit 2. The generated calcium oxide is discharged from the system, most of the generated sulfur vapor is input into sulfur condensation unit 5, and the remainder is input into catalytic reduction unit 4.

[0040] The sulfur vapor input to sulfur condensation unit 5 is liquefied and discharged from the system, while the remaining non-condensable gas is returned to low-temperature reduction unit 1.

[0041] The sulfur vapor input to the catalytic reduction unit 4 reacts with the reducing agent, and the generated sulfide gas is returned to the low-temperature reduction unit 1 as a promoter.

[0042] This system proposes a low-temperature reduction process for calcium sulfate applicable to various reducing agents. Specifically, it is a method for reducing CaSO4 to CaS at low temperature using gaseous sulfides as inducing agents, thereby achieving the recycling of sulfur resources. The process is mainly accomplished through the following steps: 1) Dehydrated gypsum (calcium sulfate or its hydrate) is preheated and then enters the low-temperature reduction unit 1, where it reacts with preheated reducing gas and sulfide gas; 2) Under the promotion of sulfide gas, the reaction temperature of the reducing gas and gypsum is significantly reduced, rapidly converting them into calcium sulfide; The aforementioned calcium sulfide solid is discharged at the tail end of the low-temperature reduction unit 1 and mixed with the preheated gypsum during the transfer to the interactive reaction unit 2. After entering the interactive reaction unit 2, the two undergo an interactive reaction of sulfur elements at a certain temperature, forming sulfur vapor and calcium oxide. The calcium oxide is discharged from the tail end of the interactive reaction unit 2 and can be used as a lime product resource after cooling. After the gas flow containing sulfur vapor passes through the high-temperature dust removal unit 3, most of the gas is cooled by the sulfur condensation unit 5 to liquefy and collect the sulfur. The non-condensable tail gas is returned to the low-temperature reduction unit 1. Another part of the sulfur vapor enters the sulfur catalytic reduction unit 4. With the addition of a certain amount of reducing gas, the sulfur vapor is reduced to sulfide gases such as hydrogen sulfide, CS2, or COS under the action of a catalyst. It then enters the low-temperature reduction unit 1 together with other reducing gases as an inducing agent for gypsum reduction. The tail gas discharged from the low-temperature reduction unit 1 contains reducing agent and sulfide residues. Some of the high-temperature tail gas is mixed with the added reducing gas and returned to the low-temperature reduction unit 1 or enters the catalytic reduction unit 4. The excess tail gas is incinerated in the incineration and waste heat recovery unit 6 and then preheated by the added gas reducing agent through a heat exchanger. Subsequently, the exhaust gas comes into contact with gypsum in the gypsum preheating unit 7 to preheat it. The exhaust gas exiting the gypsum preheating unit 7 is then purified by dust removal and desulfurization in the exhaust gas purification unit 8 before being directly discharged. The preheated gypsum participates in gypsum reduction and interactive reactions, fully utilizing waste heat while accelerating the reaction rate. Utilizing these reactions can significantly reduce the reduction temperature of gypsum, shorten the reaction time, and increase the conversion rate of CaS. The waste heat of CaS can be used to initiate its interactive reaction with gypsum and release heat sufficient to sustain the reaction, increasing the conversion rates of calcium and sulfur to calcium oxide and sulfur, respectively, and promoting their resource recovery.

[0043] Gypsum is anhydrous calcium sulfate or calcium sulfate containing different amounts of water of crystallization. It mainly comes from solid waste generated in fields such as flue gas desulfurization, waste acid neutralization, and phosphate fertilizer manufacturing.

[0044] Low-temperature reduction unit 1 is a reduction furnace, specifically a rotary kiln or fluidized bed furnace, with a reaction temperature of 600-900℃ and a gypsum reduction time of 15-90 minutes. The reducing agent used is divided into two categories: solid reducing agent and reducing gas. When using a solid reducing agent, one or more of coal powder or biomass needs to be mixed with gypsum beforehand and added to the reduction furnace. When using a reducing gas, gypsum is added to the reduction furnace first, followed by the introduction of one or more of coal gas, natural gas, or hydrogen. The dosage ratio of the reducing agent to gypsum is 0.95-1.05 times the theoretical value (stoichiometric ratio). Preferably, the reducing agent is a reducing gas, which can be consistent with the reducing agent in catalytic reduction unit 4.

[0045] The reduction furnace uses sulfide gas as a gypsum reduction accelerator. The sulfides are mainly a mixture of hydrogen sulfide, carbon disulfide, or a small amount of COS produced on-site. Using a small amount of gaseous sulfides can effectively reduce the starting temperature of gypsum reduction. The total amount of reduction accelerator added is 2-15% (molar amount) of the amount of gypsum to be reduced. Finally, the gypsum in the reduction furnace is reduced to calcium sulfide.

[0046] Interactive reaction unit 2 is an interactive reactor, specifically a rotary kiln or a fluidized bed furnace. Its feed is a mixture of calcium sulfide produced in the reduction furnace and preheated gypsum in a molar ratio of 3:1-1.1, and the reaction temperature is 800-1100℃. The interactive reaction is exothermic, and the products are sulfur vapor and CaO. The heat of reaction and sulfur vapor are carried out of the interactive reactor by an externally input reducing gas or inert gas.

[0047] After exchanging heat with the solid feed, the temperature of the sulfur vapor-containing gas drops from 800-1100℃ to 600-800℃ and flows out of the interactive reactor. It then undergoes high-temperature filtration and dust removal through dust removal unit 3 (using ceramic fiber membrane or metal sintered membrane). After the sulfur vapor is diverted, part of it is used for catalytic reduction to prepare gaseous sulfides, and most of it is recovered after condensation.

[0048] After dust removal, 10-20% of the sulfur-containing vapor gas stream is extracted and a corresponding amount of reducing agent is added before it enters catalytic reduction unit 4. The reducing agent used here is one or more of coal gas, natural gas, or hydrogen. The catalytic reduction reaction converts the sulfur vapor into a mixed gas of sulfides such as hydrogen sulfide, CS2, or COS, which is then sent to the reduction furnace. The catalyst used is alumina supported on at least one of transition metal oxides such as Co, Mo, and Ni, with a loading of 2-5%. The sulfur reduction temperature is 600-750℃, and the space velocity is 50-500 h⁻¹. -1 .

[0049] The remaining 80-90% of the sulfur vapor is cooled to 200-250°C through sulfur condensation unit 5 (sulfur condenser), so that the sulfur in it is cooled into liquid and recovered. The uncondensed gas (mainly the reducing gas remaining from the reaction) is sent to the gypsum reduction furnace.

[0050] 40-70% of the gas discharged from the reduction furnace is returned to the reduction furnace along with the replenished reducing agent. The remaining exhaust gas is sent to the incineration and waste heat recovery unit 6 for incineration treatment. The heat generated is first used for waste heat recovery of the reducing gas through a heat exchanger, and then enters the gypsum preheating unit 7 as a heat source.

[0051] The gypsum preheating unit 7 used is a countercurrent contact rotary furnace or fluidized bed. The temperature of the gypsum after preheating is 200-400℃. The cooled exhaust gas is discharged after dust removal and desulfurization treatment by the exhaust gas purification unit 8, and the CO2 in it is captured.

[0052] The relevant reaction equations in this invention are as follows, taking CH4 as the main reducing agent as an example:

[0053] CaSO4 + CH4 → CaS + CO2 + 2H2O (1)

[0054] CaSO4 + 3CaS → 4CaO + 4S (2)

[0055] 4S + 2H₂O + CH₄ → 4H₂S + CO₂ (3)

[0056] H₂S + CaSO₄ → CaSO₃ + H₂O + S (4)

[0057] The aforementioned sulfur condensation unit 5 and preheating unit 7 can both use conventional heat exchangers.

[0058] The following are laboratory and factory case studies for this system.

[0059] Example 1

[0060] Laboratory-scale low-temperature reduction of CaSO4 with CH4 to prepare CaS and experimental verification of sulfur resource recycling.

[0061] In the low-temperature reduction of CaSO4 to CaS stage, CaSO4, calcined and dehydrated at 400℃, was pre-packed into a quartz tube. 20 vol% CH4 gas was passed through the tube at 100 mL / min. The reaction was carried out at 850℃ for 1 hour, during which approximately 50% of the CaSO4 was reduced. However, at 800℃, the reaction between CaSO4 and CH4 was minimal. At 800℃, H2S gas was injected at a CaSO4 / H2S molar ratio of 1:0.2. After 1 hour of reaction, 90% of the CaSO4 was reduced to CaS by CH4. This demonstrates that the addition of a promoter can effectively lower the initial reaction temperature.

[0062] In the interactive reaction stage of CaS and CaSO4, the mixture was mixed at a molar ratio of CaSO4 / CaS of 1:3 and reacted at 1000℃ for 2 hours. The purity of CaO in the product was 93%, and the purity of sulfur collected by condensation after dust removal was 99%.

[0063] In the catalytic reduction stage of sulfur, CH4 is used as the reducing agent to reduce sulfur to produce hydrogen sulfide, where the stoichiometric ratio of CH4 to S is 1:4. Water vapor is introduced via bubbling, and the stoichiometric ratio of CH4 to H2O is 1:2.5. N2 is used as the carrier gas. A tubular furnace reactor is used, and the catalyst is a commercially available cobalt-molybdenum-alumina catalyst, with a space velocity set at 1000 h⁻¹. -1 The reaction temperature was 650℃, and the yield of hydrogen sulfide was 97%.

[0064] Example 2

[0065] In the non-ferrous smelting industry, 90% of the waste gypsum used on-site is CaSO4, and natural gas is used as a reducing agent (CH4, 80 vol%).

[0066] In the CaS preparation stage of natural gas reduction from CaSO4, desulfurized gypsum was packed into a rotary kiln with a CH4 / CaSO4 molar ratio of 2:1, and CH4 gas was introduced. The rotary kiln was operated at 800℃ and a rotation speed of 300 r / h for 2 hours, with a CaS yield of approximately 10%. However, under the same conditions, when H2S gas was introduced with a CaSO4 / H2S molar ratio of 1:0.2, the CaS yield increased to 93%. This indicates that the addition of a promoter can effectively lower the reaction initiation temperature.

[0067] In the interactive reaction stage of CaS and CaSO4, the mixture of CaSO4 and CaS in a molar ratio of 1:3 was loaded into a rotary kiln and reacted at 1000℃ and 300r / h for 2 hours. The purity of CaO in the product was 90%, and the purity of sulfur collected by condensation after dust removal was 99%.

[0068] In the catalytic reduction stage of sulfur, natural gas is used as the reducing agent to reduce sulfur to produce hydrogen sulfide, with a CH4 to S stoichiometric ratio of 1:4. Water vapor is introduced via bubbling, with a CH4 to H2O stoichiometric ratio of 1:2.5, and N2 is used as the carrier gas. The catalyst is a nickel-molybdenum-alumina catalyst, and the space velocity is set to 800 h⁻¹. -1 The reaction temperature was 650℃, and the yield of hydrogen sulfide was 99%.

[0069] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A system for recovering sulfur and calcium from low-temperature reduced gypsum, characterized in that, It includes a low-temperature reduction unit (1), an interactive reaction unit (2), a catalytic reduction unit (4), a sulfur condensation unit (5), and a post-treatment process; Gypsum, reducing agent and accelerator react in the low-temperature reduction unit (1), and the generated calcium sulfide is input into the interactive reaction unit (2), and the generated tail gas is input into the post-treatment process for incineration and purification; the accelerator is one or more of hydrogen sulfide, carbon disulfide and COS, and the accelerator is used as an inducing agent for gypsum reduction to reduce the reaction initiation temperature. Calcium sulfide and gypsum react in the interactive reaction unit (2), the generated calcium oxide is discharged from the system, most of the generated sulfur vapor is input into the sulfur condensation unit (5), and the remainder is input into the catalytic reduction unit (4); in the interactive reaction unit (2), the molar ratio of calcium sulfide to gypsum participating in the reaction is 3:1-1.1, and the reaction temperature is 800-1100℃. The sulfur vapor input to the sulfur condensation unit (5) is liquefied and discharged from the system, while the remaining non-condensable gas is returned to the low-temperature reduction unit (1). The sulfur vapor input to the catalytic reduction unit (4) reacts with the reducing agent, and the generated sulfide gas is returned to the low-temperature reduction unit (1) as a promoter. The sulfur vapor input to the catalytic reduction unit (4) is 10-20 vol% of the total amount generated in the reaction. The sulfur vapor reacts with the reducing agent in stoichiometric ratio. The reducing agent is one or more of coal gas, natural gas and hydrogen. The reaction catalyst is alumina loaded with 2-5 wt% transition metal oxides. The reaction temperature is 600-750℃ and the space velocity is 50-500 h⁻¹. -1 The generated sulfide gas is one or more of hydrogen sulfide, CS2, and COS.

2. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 1, characterized in that, The gypsum is anhydrous calcium sulfate or calcium sulfate containing water of crystallization; the reducing agent participating in the reaction in the low-temperature reduction unit (1) is a solid reducing agent or a gaseous reducing agent. The solid reducing agent is one or two of coal powder and biomass, and the gaseous reducing agent is one or more of coal gas, natural gas and hydrogen.

3. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 2, characterized in that, In the low-temperature reduction unit (1), the molar ratio of the reducing agent to gypsum participating in the reaction is 0.95-1.05 times the stoichiometric ratio, the amount of accelerator is 2-15 mol% of gypsum, the reaction temperature is 600-900℃, and the reaction time is 15-90 minutes.

4. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 1, characterized in that, The system also includes a dust removal unit (3) connected to the interactive reaction unit (2). The sulfur vapor is filtered and dust removed by the dust removal unit (3) and then fed into the sulfur condensation unit (5) and the catalytic reduction unit (4).

5. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 4, characterized in that, The dust removal unit (3) uses ceramic fiber membrane or metal sintered membrane for filtration and dust removal.

6. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 1, characterized in that, The sulfur condensation unit (5) cools the sulfur vapor to 200-250°C.

7. The system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 1, characterized in that, The post-treatment process includes an incineration and waste heat recovery unit (6), a preheating unit (7), and a tail gas purification unit (8). The tail gas generated in the reaction in the low-temperature reduction unit (1) is fed into the incineration and waste heat recovery unit (6) for incineration treatment, and the heat generated is used for preheating of the reducing agent. The exhaust gas after heat exchange with the reducing agent is input into the preheating unit (7) for preheating the gypsum. The exhaust gas after heat exchange with gypsum is input into the exhaust gas purification unit (8) for dust removal and desulfurization treatment. The CO2 in the exhaust gas is captured and the remaining exhaust gas is discharged.

8. A system for recovering sulfur and calcium from low-temperature reduced gypsum according to claim 1 or 7, characterized in that, 40-70 vol% of the tail gas produced in the reaction in the low-temperature reduction unit (1) is returned to the low-temperature reduction unit (1).

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