Recovery method of desulfurized gypsum

Abstract

The invention relates to a flue gas desulfurization gypsum recovery method which comprises the following steps: roasting flue gas desulfurization gypsum and overhaul slag at a set temperature, so that the flue gas desulfurization gypsum and the overhaul slag are subjected to a chemical reaction to generate calcium fluoride and sodium sulfide, and a roasted material is obtained; and the roasted material is leached, and a leaching solution is obtained. Carrying out solid-liquid separation on the leaching solution to obtain material slag and a sodium sulfide-containing solution; and crystallizing the sodium sulfide-containing solution to obtain sodium sulfide crystals. According to the method, sulfur resources in the desulfurized gypsum are effectively recovered, the recovery rate of the sodium sulfide crystals is 70%-80%, and the purity is 92%-95%.

Description

Technical Field

[0001] The present application relates to the technical field of desulfurization gypsum recovery, and in particular to a desulfurization gypsum recovery method. Background Art

[0002] As of 2023, my country's thermal power installed capacity accounts for 69.9% of the total power generation, and the main mode of power supply in my country is still coal-fired power generation. In order to meet the flue gas sulfur dioxide emission standards, more than 90% of my country's coal-fired power plants use the limestone-gypsum method for flue gas desulfurization. Desulfurized gypsum is a solid byproduct of the reaction of limestone slurry absorbing sulfur dioxide, and its main components are dihydrate and calcium sulfate. In 2023 alone, the output of desulfurized gypsum will reach 110 million tons.

[0003] However, desulfurized gypsum contains a lot of impurities and its comprehensive utilization rate is not high. At present, desulfurized gypsum is mainly used in building materials production, road construction, soil treatment, etc. Desulfurized gypsum contains a lot of sulfur resources, which is of great recycling value, but there are relatively few studies in this area. Summary of the invention

[0004] The present application provides a method for recovering desulfurized gypsum to solve the following technical problem: how to effectively recover sulfur resources in desulfurized gypsum.

[0005] In a first aspect, the present application provides a method for recovering desulfurized gypsum, the method comprising:

[0006] Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted so that the desulfurized gypsum and the overhaul slag undergo a chemical reaction to generate calcium fluoride and sodium sulfide, thereby obtaining a roasted material;

[0007] The roasted material is leached to obtain a leaching solution.

[0008] Optionally, the set temperature is 900°C to 1200°C.

[0009] Optionally, the calcination time is 1.5 h to 3 h.

[0010] Optionally, the weight ratio of the desulfurization gypsum to the overhaul slag is (0.5-1.5):1.

[0011] Optionally, the weight ratio of the desulfurization gypsum to the overhaul slag is 1:1.

[0012] Optionally, the leaching is performed using a leaching agent, the leaching agent comprising water.

[0013] Optionally, the ratio V:M of the volume V of the leaching agent to the weight M of the roasted material is (1-5):1; wherein the unit of V is L and the unit of M is kg.

[0014] Optionally, the leaching process parameters include: a leaching temperature of 55°C to 85°C, and a leaching time of 2.5h to 4h.

[0015] Optionally, the particle sizes of the desulfurization gypsum and the overhaul slag are 100 mesh to 250 mesh respectively.

[0016] Optionally, the method further includes:

[0017] The leaching solution is subjected to solid-liquid separation to obtain slag and a sodium sulfide-containing solution;

[0018] The sodium sulfide solution is crystallized to obtain sodium sulfide crystals.

[0019] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0020] The desulfurized gypsum recovery method provided in the embodiment of the present application comprises: roasting the desulfurized gypsum and the overhaul slag under the condition of a set temperature, so that the desulfurized gypsum and the overhaul slag undergo a chemical reaction to generate calcium fluoride and sodium sulfide, and obtain a roasted material; leaching the roasted material to obtain a leaching solution; mixing and crushing the desulfurized gypsum and the overhaul slag to obtain a mixed material; roasting the mixed material under the condition of a set temperature, so that the mixed material undergoes a chemical reaction to generate calcium fluoride and sodium sulfide, and obtain a roasted material; leaching the roasted material to obtain a leaching solution. The main components of desulfurized gypsum are CaSO4, CaCO3, etc., and the main components of overhaul slag are C, NaF, Na3AlF6, SiO2, soluble cyanide, etc. Under the condition of set temperature, calcium sulfate in the roasted desulfurization gypsum reacts with carbon in the overhaul slag to form calcium sulfide, and then calcium sulfide reacts with sodium fluoride in the overhaul slag to form calcium fluoride that is insoluble in water and sodium sulfide that is easily soluble in water; the roasted material is leached to recover sodium sulfide, thereby effectively recovering sulfur resources in the desulfurization gypsum. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0022] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0023] Figure 1A schematic flow chart of a method for recovering desulfurized gypsum provided in an embodiment of the present application. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0025] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0026] In this application, unless otherwise stated, directional words such as "upper" and "lower" refer specifically to the directions of the drawings in the accompanying drawings. In addition, in the description of this application specification, the terms "include", "comprise", etc. mean "including but not limited to".

[0027] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0028] At present, the output of desulfurized gypsum is large. Due to the large amount of impurities it contains, the comprehensive utilization rate is not high. At present, desulfurized gypsum is mainly used in building materials production, road construction, soil treatment and other aspects. Desulfurized gypsum contains a large amount of sulfur resources, which is of great recycling value, but relatively few studies in this area. Overhaul slag is the waste slag produced by the maintenance and replacement of the cathode lining of the electrolytic cell during the aluminum electrolysis production process. It mainly includes waste cathode and waste refractory insulation materials, etc., because it contains excessive toxic substances such as inorganic fluoride. It is calculated that for an electrolytic cell with a cell life of 5-8 years, 10-20kg of overhaul slag is produced per ton of aluminum, which is extremely harmful to the environment. For hazardous waste overhaul slag with leaching toxicity, it must be harmlessly treated before it can be discharged. At present, the main methods for treating overhaul slag are landfill, acid leaching and alkaline leaching. my country mainly uses landfill, but the treatment cost of this method is high and it is also easy to cause secondary pollution. Therefore, based on the fluorine-fixing properties of calcium and the reducing properties of overhaul slag, desulfurization gypsum and overhaul slag are smelted at high temperature, so that calcium sulfate is reduced to calcium sulfide, and reacts with sodium fluoride to generate insoluble calcium fluoride and water-soluble sodium sulfide. The smelting product is then used to recover sodium sulfide through a leaching process. This method not only realizes the recovery of sulfur resources but also achieves the harmlessness of overhaul slag.

[0029] The present application provides a method for recovering desulfurized gypsum. Figure 1 A schematic diagram of a desulfurized gypsum recovery method provided in an embodiment of the present application; see Figure 1 , the method comprising:

[0030] S1. roasting the desulfurized gypsum and the overhaul slag at a set temperature so that the desulfurized gypsum and the overhaul slag react chemically to generate calcium fluoride and sodium sulfide to obtain a roasted material;

[0031] In the embodiment of the present application, the main components of the desulfurization gypsum are CaSO4, CaCO3, etc., and the main components of the overhaul slag are C, NaF, Na3AlF6, SiO2, soluble cyanide, etc. During the roasting process, calcium sulfate in the desulfurized gypsum reacts with carbon in the overhaul slag to form calcium sulfide, and then calcium sulfide reacts with sodium fluoride in the overhaul slag to form calcium fluoride that is insoluble in water and sodium sulfide that is easily soluble in water. The main reactions include: CaSO4.2H2O=CaSO4+2H2O(g); CaSO4+4C=CaS+4CO (g); CaSO4+4CO(g)=CaS+4CO2 (g); CaCO3=CaO+CO2 (g); Na3AlF6=3NaF+AlF3; CaS+2NaF=CaF2+Na2S; CaO+2NaF+SiO2=CaF2+Na2O.SiO2; In addition, cyanide in the overhaul slag decomposes under high temperature roasting conditions to form non-toxic and harmless C and N2. The roasting is carried out under an inert atmosphere or vacuum conditions to ensure an oxygen-free environment to prevent the reduced sulfide from being oxidized.

[0032] In some embodiments, the set temperature is 900°C to 1200°C.

[0033] In the embodiment of the present application, the "set temperature" is the roasting temperature, which can be 900°C to 1200°C, so as to fully carry out the above chemical reaction between the desulfurized gypsum and the overhaul slag and improve the recovery rate of sodium sulfide. Exemplarily, the roasting temperature can be 900°C, 930°C, 950°C, 980°C, 1000°C, 1030°C, 1050°C, 1080°C, 1100°C, 1130°C, 1150°C, 1180, 1200°C, etc.

[0034] In some embodiments, the calcination time is 1.5 h to 3 h.

[0035] In the embodiment of the present application, the roasting time can be 1.5h to 3h, so as to cooperate with the above roasting temperature to achieve the above chemical reaction between the desulfurized gypsum and the overhaul slag and improve the recovery rate of sodium sulfide. Exemplarily, the above roasting time can be 1.5h, 1.7h, 1.9h, 2.0h, 2.3h, 2.5h, 2.7h, 2.9h, 3.0h, etc.

[0036] In some embodiments, the particle sizes of the desulfurization gypsum and the overhaul slag are 100 mesh to 250 mesh respectively.

[0037] In an embodiment of the present application, before the desulfurization gypsum and the overhaul slag are roasted, the desulfurization gypsum and the overhaul slag can be crushed, and they can be crushed separately and then mixed, or they can be mixed and crushed. The particle size of the desulfurization gypsum and the overhaul slag can be 100 mesh to 250 mesh, respectively, so as to increase the contact area between the desulfurization gypsum and the overhaul slag, effectively promote the reaction, and improve the recovery rate of sodium sulfide. Exemplarily, the particle size of the desulfurization gypsum and the overhaul slag can be 100 mesh, 110 mesh, 120 mesh, 130 mesh, 140 mesh, 150 mesh, 160 mesh, 170 mesh, 180 mesh, 190 mesh, 200 mesh, etc., respectively. In addition, a jaw crusher can be used to crush and grind the desulfurization gypsum and the overhaul slag to a particle size of 100 mesh to 250 mesh.

[0038] In some embodiments, the weight ratio of the desulfurization gypsum to the overhaul slag is (0.5-1.5):1.

[0039] In some embodiments, the weight ratio of the desulfurization gypsum to the overhaul slag is 1:1.

[0040] In the embodiment of the present application, the weight ratio of desulfurized gypsum to overhaul slag is (0.5-1.5):1, which can fully realize the fluorine-fixing property of calcium in desulfurized gypsum and the reducing property of overhaul slag, and fully convert desulfurized gypsum into sodium sulfide during the above-mentioned roasting process. Exemplarily, the weight ratio of the above-mentioned desulfurized gypsum to overhaul slag can be 0.5:1, 0.7:1, 0.9:1, 1.1:1, 1.3:1, 1.5:1, etc. Further, the weight ratio of desulfurized gypsum to overhaul slag can be 1:1.

[0041] S2, leaching the roasted material to obtain a leaching solution.

[0042] In some embodiments, the leaching is performed using a leaching agent, the leaching agent comprising water.

[0043] In some embodiments, the ratio V:M of the volume V of the leaching agent to the weight M of the roasted material is (1-5):1; wherein the unit of V is L, and the unit of M is kg.

[0044] In the embodiment of the present application, calcium fluoride which is insoluble in water and sodium sulfide which is easily soluble in water are generated during the above-mentioned roasting process. Therefore, by utilizing the solubility characteristics of the two, water can be used to leach the roasted material. The ratio V:M of the volume V of the leaching agent to the weight M of the roasted material can be (1-5):1, which can fully achieve the separation of calcium fluoride and sodium sulfide and ensure the recovery effect of sodium sulfide. Exemplarily, the ratio V:M of the volume V of the above-mentioned leaching agent to the weight M of the roasted material can be 1:1, 2:1, 3:1, 4:1, 5:1, etc.

[0045] In some embodiments, the leaching process parameters include: a leaching temperature of 55° C. to 85° C., and a leaching time of 2.5 h to 4 h.

[0046] In the embodiment of the present application, the leaching temperature can be 55°C to 85°C, and the leaching time can be 2.5h to 4h. The appropriate leaching temperature and leaching time can ensure the leaching effect of sodium sulfide while taking into account the production cost. Exemplarily, the above leaching temperature can be 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, etc.; the above leaching time can be 2.5h, 2.9h, 3.1h, 3.3h, 3.5h, 3.7h, 3.9h, 4h, etc. In addition, in order to further promote the leaching of sodium sulfide, the roasting can be further ground in an impact mill.

[0047] In some embodiments, the method further comprises:

[0048] S3, separating the leaching solution into solid and liquid to obtain slag and sodium sulfide-containing solution;

[0049] S4, crystallizing the sodium sulfide solution to obtain sodium sulfide crystals.

[0050] In the embodiment of the present application, the leached solution can be subjected to solid-liquid separation to obtain slag and a sodium sulfide solution; the sodium sulfide solution can be subjected to evaporation and crystallization to obtain sodium sulfide crystals and evaporated condensed water, which can be returned to the above leaching step to achieve recycling; in order to further improve the purity of the sodium sulfide crystals, the leached solution can be Ph-adjusted using sodium hydroxide. The recovery rate of the sodium sulfide crystals is 70% to 80%, and the purity is 92% to 95%.

[0051] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0052] The chemical compositions of desulfurization gypsum and overhaul slag produced by a power plant and an aluminum electrolysis plant are shown in Table 1 and Table 2 respectively.

[0053] Table 1 Chemical composition and content of desulfurized gypsum (w%)

[0054] CaO <![CDATA[SO3]]> <![CDATA[Fe2O3]]> <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[H2O]]> other 31.86 42.95 1.98 0.63 1.09 17.9 3.59

[0055] Table 2 Chemical composition and content of overhaul slag (w%)

[0056] <![CDATA[Na3AlF6]]> NaF C <![CDATA[CaF2]]> <![CDATA[SiO2]]> <![CDATA[Al2O3]]> other 5.4 23.7 45.5 2.11 13.6 2.9 6.79

[0057] Example 1

[0058] A method for recovering desulfurized gypsum, comprising:

[0059] S11. Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted (under argon conditions) to make the desulfurized gypsum and the overhaul slag react chemically to generate calcium fluoride and sodium sulfide to obtain a roasted material; wherein, the desulfurized gypsum in Table 1 and the overhaul slag in Table 2 are used; the desulfurized gypsum and the overhaul slag are crushed and ground to 100-150 mesh; the weight ratio of the desulfurized gypsum to the overhaul slag is 0.5:1; the set temperature (roasting temperature) is 950°C, and the roasting time is 1.5h;

[0060] S21, leaching the roasted material to obtain a leaching solution; wherein the particle size of the roasted material is 150 mesh, water is used for leaching, and the ratio V:M of the volume V of water to the weight M of the roasted material is 2:1; wherein the unit of V is L, and the unit of M is kg; the leaching temperature is 65°C, and the leaching time is 3h;

[0061] S31, separating the leached solution into solid and liquid to obtain slag and a sodium sulfide-containing solution;

[0062] S41, evaporating and crystallizing the sodium sulfide solution to obtain sodium sulfide crystals.

[0063] Example 2

[0064] A method for recovering desulfurized gypsum, comprising:

[0065] S12. Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted (under argon conditions) to make the desulfurized gypsum and the overhaul slag react chemically to generate calcium fluoride and sodium sulfide to obtain a roasted material; wherein, the desulfurized gypsum in Table 1 and the overhaul slag in Table 2 are used; the desulfurized gypsum and the overhaul slag are crushed and ground to 150 mesh to 200 mesh; the weight ratio of the desulfurized gypsum to the overhaul slag is 1:1; the set temperature (roasting temperature) is 1000° C., and the roasting time is 1.5 h;

[0066] S22, leaching the roasted material to obtain a leaching solution; wherein the particle size of the roasted material is 150 mesh, water is used for leaching, and the ratio V:M of the volume V of water to the weight M of the roasted material is 3:1; wherein the unit of V is L, and the unit of M is kg; the leaching temperature is 75° C., and the leaching time is 3 h;

[0067] S32, separating the leached solution into solid and liquid to obtain slag and a sodium sulfide-containing solution;

[0068] S42, evaporating and crystallizing the sodium sulfide solution to obtain sodium sulfide crystals.

[0069] Example 3

[0070] A method for recovering desulfurized gypsum, comprising:

[0071] S13. Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted (under argon conditions) to make the desulfurized gypsum and the overhaul slag react chemically to generate calcium fluoride and sodium sulfide to obtain a roasted material; wherein, the desulfurized gypsum in Table 1 and the overhaul slag in Table 2 are used; the desulfurized gypsum and the overhaul slag are crushed and ground to 200-250 mesh; the weight ratio of the desulfurized gypsum to the overhaul slag is 1.5:1; the set temperature (roasting temperature) is 1100° C., and the roasting time is 2.5 h;

[0072] S23, leaching the roasted material to obtain a leaching solution; wherein the particle size of the roasted material is 200 mesh, and water is used for leaching, and the ratio V:M of the volume V of water to the weight M of the roasted material is 4:1; wherein the unit of V is L, and the unit of M is kg; the leaching temperature is 75° C., and the leaching time is 4 hours;

[0073] S33, separating the leached solution into solid and liquid to obtain slag and a sodium sulfide-containing solution;

[0074] S43, evaporating and crystallizing the sodium sulfide solution to obtain sodium sulfide crystals.

[0075] Example 4

[0076] A method for recovering desulfurized gypsum, comprising:

[0077] S14. Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted (under argon conditions) to make the desulfurized gypsum and the overhaul slag react chemically to generate calcium fluoride and sodium sulfide to obtain a roasted material; wherein, the desulfurized gypsum in Table 1 and the overhaul slag in Table 2 are used; the desulfurized gypsum and the overhaul slag are crushed and ground to 200 mesh; the weight ratio of the desulfurized gypsum to the overhaul slag is 1:1; the set temperature (roasting temperature) is 1200° C., and the roasting time is 3 hours;

[0078] S24, leaching the roasted material to obtain a leaching solution; wherein the particle size of the roasted material is 200 mesh, and water is used for leaching, and the ratio V:M of the volume V of water to the weight M of the roasted material is 5:1; wherein the unit of V is L, and the unit of M is kg; the leaching temperature is 85°C, and the leaching time is 5h;

[0079] S34, separating the leached solution into solid and liquid to obtain slag and a sodium sulfide-containing solution;

[0080] S44, evaporating and crystallizing the sodium sulfide solution to obtain sodium sulfide crystals.

[0081] Comparative Example 1

[0082] Based on the disclosure of Example 1, the difference between Comparative Example 1 and Example 1 is that the set temperature (calcination temperature) is 850°C.

[0083] Comparative Example 2

[0084] Based on the disclosure of Example 1, the difference between Comparative Example 1 and Example 1 is that the set temperature (calcination temperature) is 1230°C.

[0085] Comparative Example 3

[0086] Based on the disclosure of Example 1, the difference between Comparative Example 1 and Example 1 is that the leaching temperature is 50°C.

[0087] The sodium sulfide crystals obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were evaluated by indicators. Please refer to Table 3 for the results.

[0088] Table 3 Index evaluation of sodium sulfide crystals

[0089]

[0090]

[0091] As shown in Table 3, the recovery rate of sodium sulfide crystals is 70% to 80% and the purity is 92% to 95% by roasting and leaching desulfurized gypsum and overhaul slag at high temperature. However, the roasting temperature in Comparative Example 1 is too low, resulting in insufficient reaction between desulfurized gypsum and overhaul slag, low sodium sulfide generation rate, low sulfur recovery rate, and low purity of sodium sulfide. The roasting temperature in Comparative Example 2 is too high, which has almost no effect on sulfur recovery, but increases production costs. The leaching temperature in Comparative Example 3 is too low, resulting in insufficient leaching of roasted materials and low sulfur recovery rate.

[0092] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:

[0093] (1) Through the high-temperature roasting-water leaching process of desulfurized gypsum and overhaul slag, on the one hand, the sulfur resources in the desulfurized gypsum are recovered; on the other hand, during the roasting process, calcium ions react with soluble fluorides to form insoluble calcium fluoride, while the toxic cyanide in the overhaul slag is decomposed into non-toxic and harmless C and N2, thereby achieving the harmless treatment of the overhaul slag.

[0094] (2) The roasted material is water-leached without the need to add acid, and the leaching liquid is evaporated and crystallized to obtain the sodium sulfide product, thereby achieving efficient recovery of sulfur resources.

[0095] (3) This method has the advantages of short process flow, low cost, good sulfur resource recovery effect, and achieves the purpose of treating waste with waste.

[0096] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for recovering desulfurized gypsum, characterized in that: The method comprises: Under the condition of a set temperature, the desulfurized gypsum and the overhaul slag are roasted so that the desulfurized gypsum and the overhaul slag undergo a chemical reaction to generate calcium fluoride and sodium sulfide, thereby obtaining a roasted material; The roasted material is leached to obtain a leaching solution.

2. The method according to claim 1, characterized in that The set temperature is 900°C to 1200°C.

3. The method according to claim 1 or 2, characterized in that: The calcination time is 1.5h to 3h.

4. The method according to claim 1, characterized in that: The weight ratio of the desulfurization gypsum to the overhaul slag is (0.5-1.5):

1.

5. The method according to claim 4, characterized in that The weight ratio of the desulfurization gypsum to the overhaul slag is 1:

1.

6. The method according to claim 1, characterized in that The leaching is performed using a leaching agent, which includes water.

7. The method according to claim 6, characterized in that The ratio V:M of the volume V of the leaching agent to the weight M of the roasted material is (1-5):1; wherein the unit of V is L, and the unit of M is kg.

8. The method according to claim 1, 6 or 7, characterized in that: The leaching process parameters include: leaching temperature of 55° C. to 85° C., and leaching time of 2.5 h to 4 h.

9. The method according to claim 1, characterized in that: The particle sizes of the desulfurized gypsum and the overhaul slag are 100-250 meshes respectively.

10. The method according to claim 1, characterized in that The method further comprises: The leaching solution is subjected to solid-liquid separation to obtain slag and a sodium sulfide-containing solution; The sodium sulfide solution is crystallized to obtain sodium sulfide crystals.

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