Method for preparing anhydrous calcium sulfate from industrial by-product gypsum

Abstract

The application discloses a method for preparing anhydrous calcium sulfate from industrial by-product gypsum, which comprises the following steps: heating and drying the industrial by-product gypsum, mixing the heated industrial by-product gypsum with dilute sulfuric acid solution at room temperature, and then reacting, filtering, washing and drying the reactants in sequence to obtain the anhydrous calcium sulfate. The application has the characteristics of less wastewater, lower wastewater treatment cost, slower equipment corrosion rate, lower equipment investment cost, lower heating energy consumption and high production efficiency.

Description

Technical Field

[0001] This invention relates to a method for preparing anhydrous calcium sulfate, and more particularly to a method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum. Background Technology

[0002] Industrial by-product gypsum mainly originates from industries such as wet-process phosphoric acid production, desulfurization, titanium dioxide production, and salt production. It is a solid waste residue generated during industrial production. Globally, the cumulative emissions of phosphogypsum, a by-product of the phosphorus chemical industry alone, are approximately 6 billion tons, and are currently increasing at a rate of 150 million tons per year. Due to its large emissions, complex impurity composition, and difficulty in treatment, its utilization rate is very low, and the accumulation of large amounts of industrial by-product gypsum has caused significant environmental pollution. Therefore, there is an urgent need for the effective treatment and utilization of industrial by-product gypsum.

[0003] Industrial by-product gypsum, based on its water content, is generally classified into dihydrate gypsum (CaSO4·2H2O), hemihydrate gypsum (CaSO4·0.5H2O), and anhydrous gypsum (CaSO4). However, currently, industrial by-product gypsum produced in industrialized production consists mainly of dihydrate and hemihydrate gypsum, with dihydrate gypsum being the primary product. Due to their high water content and complex composition, industrial by-product gypsum is currently difficult to utilize effectively.

[0004] Anhydrous calcium sulfate possesses advantages such as being non-toxic, environmentally friendly, heat-resistant, corrosion-resistant, high-strength, and structurally stable. Compared to dihydrate and hemihydrate gypsum, which are industrial byproducts, it has better economic value and higher utilization value. Currently, anhydrous calcium sulfate has been found to be used as a polishing powder, a paper filler, and also in metallurgy and agriculture. Furthermore, cement plants use anhydrous calcium sulfate to adjust the setting time of cement. In addition, the application of anhydrous calcium sulfate as a polymer filler has also received extensive research.

[0005] However, traditional anhydrous calcium sulfate is mainly obtained through the extraction and processing of natural gypsum. The extraction of natural minerals causes irreversible environmental damage, and with extensive human mining, the reserves of high-quality natural gypsum are rapidly declining. Therefore, processing industrial by-product gypsum, a form of industrial waste, into anhydrous calcium sulfate, thus replacing the extraction of natural gypsum, not only solves the environmental damage caused by natural gypsum extraction and the decline in high-quality ore reserves, but also alleviates the environmental problems caused by the stockpiling of industrial by-product gypsum. This is a multi-benefit industrial innovation with significant economic and social value.

[0006] Traditional methods for preparing anhydrous calcium sulfate from industrial by-product gypsum via crystallization include solvothermal, calcination, and hydrothermal methods. However, these methods share a common drawback: demanding reaction conditions, high energy consumption, and low yield. Therefore, an acidification crystallization method later emerged. For example, patent CN114162845A discloses a method for preparing micron-sized anhydrous calcium sulfate from industrial by-product phosphogypsum. This method involves mixing industrial by-product phosphogypsum with a sulfuric acid solution and heating the mixture to remove the water of crystallization from the phosphogypsum, resulting in anhydrous calcium sulfate after crystallization. This method offers advantages such as simple process, low energy consumption, and high yield. However, it has many drawbacks in actual production: 1. Phosphogypsum has a high water content and contains impurities such as phosphorus (P) and sulfuric acid (F). After acid treatment, the crystal water and impurities such as P and F are discharged together and enter the wastewater. The factory needs to remove the impurities such as P and F from the wastewater before it can be discharged. Since the discharged crystal water mixes with the water in the sulfuric acid solution, the wastewater volume is large, resulting in high wastewater treatment costs. 2. After mixing phosphogypsum and sulfuric acid solution, it needs to be heated to a certain temperature to react. Due to the presence of sulfuric acid solution, the heating equipment is severely corroded during the heating process, which greatly shortens the equipment life and increases the cost of equipment investment. 3. When phosphogypsum and sulfuric acid solution are heated together, the heating time required is long, and the energy consumption required to raise the temperature to the set temperature is large. At the same time, the long heating time results in low equipment capacity. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for preparing anhydrous calcium sulfate through pretreatment of industrial by-product gypsum. This invention achieves water equilibrium through liquid-phase crystallization of industrial by-product gypsum, eliminates wastewater discharge, exhibits slow equipment corrosion rates, lower equipment investment costs, and low heating energy consumption; furthermore, it also boasts high production efficiency.

[0008] The technical solution of the present invention is as follows: A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum, wherein the industrial by-product gypsum is first heated and dried, and then mixed with dilute sulfuric acid solution while hot and reacted. The reactants are then filtered, washed and dried in sequence to obtain anhydrous calcium sulfate.

[0009] By first drying industrial by-product gypsum, most of the moisture in the gypsum can be removed and evaporated. During the subsequent crystallization process, the crystal water of the industrial by-product gypsum becomes free water. This free water is essentially equivalent to the free water adsorbed by anhydrous calcium sulfate after crystallization. This free water evaporates directly during the drying process of the calcium sulfate (this evaporation is the same as the evaporation of traditional hydrated calcium sulfate). The filtrate is recycled for crystallization, so the system does not drain water, achieving water balance in the crystallization system. Furthermore, the heat carried by the dried industrial by-product gypsum is used to react with dilute sulfuric acid solution while still hot, and the heat released from the reaction is used to further maintain the temperature conditions required for the reaction. Therefore, the reaction process does not require an external heat source and can be carried out in some corrosion-resistant, non-heated equipment, greatly simplifying the structure of the crystallization equipment, reducing the frequency of equipment maintenance and replacement, and lowering equipment costs. Furthermore, since the preheating of industrial by-product gypsum can utilize waste heat, the equipment is simple and efficient. The heated industrial by-product gypsum, when mixed with dilute sulfuric acid, can undergo crystal transformation, thus requiring less energy to reach the same set temperature. This significantly improves crystal transformation efficiency and greatly reduces energy costs. In summary, this invention is a more energy-efficient and environmentally friendly method.

[0010] The aforementioned method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum includes the following specific steps:

[0011] 1) Take industrial by-product gypsum and dry it until the moisture content is 5%-25% (total of free water and water of crystallization);

[0012] 2) While still hot, mix and react the industrial by-product gypsum dried in step 1) with a dilute sulfuric acid solution;

[0013] 3) Filter the reaction product from step 2), wash and dry the solid portion to obtain anhydrous calcium sulfate. Compared with the existing method of directly heating industrial by-product gypsum and acid mixture, the preheating of industrial by-product gypsum in this invention significantly shortens the material residence time in the crystallization reactor, greatly reducing energy consumption. Furthermore, it has been verified that adding dilute sulfuric acid solution while the industrial by-product gypsum is still hot after drying can successfully achieve the dehydration reaction of industrial by-product dihydrate and hemihydrate gypsum, ultimately producing anhydrous calcium sulfate.

[0014] In the aforementioned method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum, the industrial by-product gypsum refers to phosphogypsum, desulfurized gypsum, titanium gypsum, salt gypsum, and fluorogypsum.

[0015] In order to achieve the purpose of dehydration and crystallization of industrial by-product gypsum, in the aforementioned method for pretreatment of industrial by-product gypsum to prepare anhydrous calcium sulfate, the concentration of sulfuric acid in the dilute sulfuric acid solution in step 2) is 30-70%. When the concentration of sulfuric acid is 30-70%, the dehydration and crystallization of industrial by-product gypsum can be achieved; if the concentration is too low, it cannot be achieved.

[0016] In order to achieve the purpose of dehydration and crystallization of industrial by-product gypsum, in the aforementioned method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum, the liquid-solid ratio of the industrial by-product gypsum and dilute sulfuric acid solution in step 2) is 0.6-2.0.

[0017] To achieve the dehydration and crystallization of industrial by-product gypsum, the aforementioned method for pretreating industrial by-product gypsum to prepare anhydrous calcium sulfate, in step 2), requires a reaction time of at least 15 minutes. A reaction time of at least 15 minutes is sufficient. Compared to traditional methods that involve heating for several hours, this invention employs preheating, eliminating the need for slow temperature increases in the slurry. This significantly reduces the actual dehydration and crystallization reaction time, shortening the overall processing time and improving crystallization efficiency.

[0018] To conserve resources, the aforementioned method for preparing anhydrous calcium sulfate from industrial by-product gypsum through pretreatment is characterized in that: the filtrate from step 3) can be directly or prepared into a dilute sulfuric acid solution by adding sulfuric acid, and then reused in step 2). Using the filtrate (or the filtrate after adding sulfuric acid) as the dilute sulfuric acid reaction solution avoids wastewater generation, reduces wastewater treatment costs, and simultaneously conserves water resources and sulfuric acid.

[0019] To save energy, in the aforementioned method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum, the waste gas generated after the solid drying in step 3) is recycled for drying industrial by-product gypsum in step 1).

[0020] Beneficial effects of the present invention

[0021] In summary, the method of the present invention has the advantages of generating less wastewater, lower wastewater treatment cost, slower equipment corrosion rate, lower equipment investment cost, and lower heating energy consumption; in addition, it also has the advantage of high production efficiency.

[0022] Wastewater treatment costs

[0023] The current wastewater treatment prices are shown in the table below:

[0024] source Wastewater treatment price Acidic wastewater treatment at a chemical plant in Henan 8.61 yuan / ton Resource-based treatment of acidic wastewater from mines 8.1 yuan / ton

[0025] Taking the annual processing of 1 million tons of industrial by-product phosphogypsum as an example, if the industrial by-product phosphogypsum contains 20% water of crystallization and 15%-18% water adhering to it, without drying to remove the 15%-18% water, then 1 million tons of industrial by-product phosphogypsum will generate 100 × 0.15 = 150,000-180,000 tons of water. If 150,000 tons of water were to be processed, the cost would be 1.292 million yuan and 1.215 million yuan respectively, according to the prices in the table above. Therefore, this method can save an average of 1.254 million yuan per year.

[0026] The above-mentioned aspect alone can save a significant amount of costs annually. In addition, other aspects (such as equipment maintenance, corrosion damage, water consumption, etc.) have not been estimated. Therefore, this method saves economic costs to a certain extent and has practical production significance. Attached Figure Description

[0027] Appendix Figure 1 The data results are based on the determination of the water of crystallization content of the calcium sulfate samples prepared in Examples 1-7 of this invention. As can be seen from the figure, the water of crystallization content is all below 1%, which proves that the prepared calcium sulfate samples are anhydrous calcium sulfate. Detailed Implementation

[0028] The present invention will be further described below with reference to embodiments, but these embodiments are not intended to limit the scope of the invention.

[0029] Example 1: A method for preparing anhydrous calcium sulfate from industrial by-product gypsum (phosphogypsum) through pretreatment, the specific steps of which are as follows:

[0030] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the water content is 22% (total of free water and water of crystallization);

[0031] 2) While the industrial by-product gypsum dried in step 1) is still hot, it is mixed with a dilute sulfuric acid solution (sulfuric acid concentration of 30%) and reacted for 30 minutes. The liquid-solid ratio of the industrial by-product gypsum and the dilute sulfuric acid solution is 2.

[0032] 3) Following conventional processes, the reaction product of step 2) is filtered, the solid part is washed and dried to obtain anhydrous calcium sulfate; the filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum; the waste gas generated during drying is recovered to the processing area of ​​step 1) through a heat recovery system for the heating and drying of industrial by-product gypsum, thereby realizing heat recovery.

[0033] Example 2: A method for preparing anhydrous calcium sulfate from industrial by-product gypsum (phosphogypsum) through pretreatment, the specific steps of which are as follows:

[0034] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the moisture content is 10% (total of free water and crystal water);

[0035] 2) Immediately while hot, the industrial by-product gypsum dried in step 1) is mixed with a dilute sulfuric acid solution (the dilute sulfuric acid solution at this time is prepared by adding sulfuric acid to the filtrate in Example 1, and the sulfuric acid concentration is 50%) and reacted for 30 minutes. The liquid-solid ratio of the industrial by-product gypsum and the dilute sulfuric acid solution is 1.2.

[0036] 3) Following conventional processes, the reaction product of step 2) is filtered, the solid part is washed and dried to obtain anhydrous calcium sulfate; the filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum; the waste gas generated during drying is recovered to the processing area of ​​step 1) through a heat recovery system for the heating and drying of industrial by-product gypsum, thereby realizing heat recovery.

[0037] Example 3: A method for preparing anhydrous calcium sulfate from industrial by-product gypsum (salt gypsum) through pretreatment, the specific steps of which are as follows:

[0038] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the water content is 15% (total of free water and water of crystallization);

[0039] 2) Immediately while hot, mix the dried industrial by-product gypsum from step 1) with a dilute sulfuric acid solution (60% sulfuric acid concentration) and react for 15 minutes. The liquid-solid ratio of the industrial by-product gypsum to the dilute sulfuric acid solution is 0.8.

[0040] 3) Following conventional processes, the reaction product of step 2) is filtered, the solid part is washed and dried to obtain anhydrous calcium sulfate; the filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum; the waste gas generated during drying is recovered to the processing area of ​​step 1) through a heat recovery system for the heating and drying of industrial by-product gypsum, thereby realizing heat recovery.

[0041] Example 4: A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum (desulfurized gypsum), the specific steps of which are as follows:

[0042] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the moisture content is 10% (total of free water and crystal water);

[0043] 2) Immediately while hot, the industrial by-product gypsum dried in step 1) is mixed with a dilute sulfuric acid solution (at this time, the dilute sulfuric acid solution is the filtrate from Example 3 directly used as the dilute sulfuric acid solution) and reacted for 20 minutes. The liquid-solid ratio of the industrial by-product gypsum and the dilute sulfuric acid solution is 1.

[0044] 3) Following conventional processes, the reaction product of step 2) is filtered, the solid part is washed and dried to obtain anhydrous calcium sulfate; the filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum; the waste gas generated during drying is recovered to the processing area of ​​step 1) through a heat recovery system for the heating and drying of industrial by-product gypsum, thereby realizing heat recovery.

[0045] Example 5: A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum (desulfurized gypsum), the specific steps of which are as follows:

[0046] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the water content is 10% (total of free water and water of crystallization);

[0047] 2) Immediately while hot, mix the dried industrial by-product gypsum from step 1) with a dilute sulfuric acid solution (40% sulfuric acid concentration) and react for 25 minutes. The liquid-solid ratio of the industrial by-product gypsum to the dilute sulfuric acid solution is 0.7.

[0048] 3) Following conventional processes, the reaction product of step 2) is filtered, the solid part is washed and dried to obtain anhydrous calcium sulfate; the filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum; the waste gas generated during drying is recovered to the processing area of ​​step 1) through a heat recovery system for the heating and drying of industrial by-product gypsum, thereby realizing heat recovery.

[0049] Example 6: A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum (fluorogypsum), the specific steps of which are as follows:

[0050] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the water content is 5% (total of free water and water of crystallization);

[0051] 2) Immediately while hot, mix the dried industrial by-product gypsum from step 1) with a dilute sulfuric acid solution (50% sulfuric acid concentration) and react for 15 minutes. The liquid-solid ratio of the industrial by-product gypsum to the dilute sulfuric acid solution is 0.6.

[0052] 3) Following conventional processes, the reaction product from step 2) is filtered, and the solid portion is washed and dried to obtain anhydrous calcium sulfate. The filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum. The waste gas generated during drying is recovered to the processing area in step 1) via a heat recovery system for heating and drying of industrial by-product gypsum, thus achieving heat recovery.

[0053] Example 7: A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum (titanium gypsum), the specific steps of which are as follows:

[0054] 1) Take an appropriate amount of industrial by-product gypsum and dry it until the water content is 20% (total of free water and water of crystallization);

[0055] 2) Immediately while hot, mix the dried industrial by-product gypsum from step 1) with a dilute sulfuric acid solution (70% sulfuric acid concentration) and react for 25 minutes. The liquid-solid ratio of the industrial by-product gypsum to the dilute sulfuric acid solution is 1.5.

[0056] 3) Following conventional processes, the reaction product from step 2) is filtered, and the solid portion is washed and dried to obtain anhydrous calcium sulfate. The filtrate is recycled back to step 2) for the crystallization of industrial by-product gypsum. The waste gas generated during drying is recovered to the processing area in step 1) via a heat recovery system for heating and drying of industrial by-product gypsum, thus achieving heat recovery.

[0057] (The above washing should be performed once or twice with clean water, and the washing water should be recycled. The amount of washing water should be 1-4 times that of anhydrous calcium sulfate. When the pH of the washing water drops to 4, it should be neutralized with lime or sodium hydroxide to a pH of 7-8 before being recycled.)

[0058] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum, characterized in that: The steps are as follows: 1) Take industrial by-product gypsum and dry it until the moisture content is 5%-25%; 2) Immediately while still hot, mix and react the industrial by-product gypsum dried in step 1) with dilute sulfuric acid solution; 3) Filter the reaction product from step 2), wash and dry the solid part to obtain anhydrous calcium sulfate; In step 2), the concentration of sulfuric acid in the dilute sulfuric acid solution is 30%-70%. The reaction time in step 2) is more than 15 minutes.

2. The method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum according to claim 1, characterized in that: The industrial by-product gypsum mentioned refers to phosphogypsum, desulfurized gypsum, titanium gypsum, salt gypsum, and fluorogypsum.

3. The method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum according to claim 1, characterized in that: In step 2), the liquid-to-solid ratio of the industrial by-product gypsum and dilute sulfuric acid solution is 0.6-2.

0.

4. The method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum according to claim 1, characterized in that: In step 3), when the concentration of the dilute sulfuric acid solution in step 2 is 50% or higher, the filtrate can be directly used as a dilute sulfuric acid solution in step 2); however, when the concentration of the dilute sulfuric acid solution in step 2 is between 30% and 50% but not exceeding 50%, the filtrate needs to be supplemented with some sulfuric acid to prepare a dilute sulfuric acid solution, which is then reused in step 2).

5. The method for preparing anhydrous calcium sulfate by pretreatment of industrial by-product gypsum according to claim 1, characterized in that: The waste gas generated after the solid drying in step 3) is recycled for drying industrial by-product gypsum in step 1).

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