Method for producing high-quality dihydrate gypsum and fluorite using lime sludge and sulfuric acid

Abstract

The present invention provides: a method for producing dihydrate gypsum, the method comprising a) a step for preparing a lime sludge containing 15-40 wt% of CaF2, b) a step for adding sulfuric acid to the lime sludge and heating to a reaction temperature of 210-350 °C while stirring, and c) a step for separating hydrogen fluoride gas generated in step b); and a method for preparing fluorite using the hydrogen fluoride gas.

Description

Method for producing high-quality gypsum and fluorite using lime sludge and sulfuric acid

[0001] This application claims the benefit of priority to Korean Patent Application No. 10-2024-0054917, filed April 24, 2024, and Korean Patent Application No. 10-2025-0052346, filed April 22, 2025, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for producing high-quality gypsum (CaSO4ㆍ2H2O) and fluorite (CaF2, also called 'synthetic calcium fluoride') having an SO3 content of 38% or more using lime sludge and sulfuric acid, particularly lime sludge and waste sulfuric acid generated during semiconductor manufacturing.

[0003] During the cleaning stage of the semiconductor manufacturing process, a large amount of lime sludge and waste sulfuric acid are generated.

[0004] In Korea, approximately 1 million tons of lime sludge is generated annually during semiconductor manufacturing, and a significant amount of waste sulfuric acid is also produced. Most of this lime sludge is landfilled as waste, while the waste sulfuric acid is sold to other companies for water treatment or mixed with limestone (CaCO3) to produce gypsum and supply it to cement manufacturers.

[0005] Under these circumstances, the development of a method for recycling the lime sludge is required. However, lime sludge generated during semiconductor manufacturing is known to be difficult to recycle because it contains a large amount of calcium fluoride (CaF2). For example, if gypsum is manufactured using the lime sludge using a conventional method, the resulting gypsum has a high fluorine (F) content, making it difficult to use in cement manufacturing. That is, gypsum manufactured using the above method contains a large amount of calcium fluoride (CaF2), and this calcium fluoride easily adheres to cement mixing equipment, reducing work efficiency or causing equipment damage. Furthermore, gypsum functions to delay the hardening of cement, but if the fluorine content exceeds 1,000 ppm, the cement does not harden. Therefore, gypsum containing 1,000 ppm or more of the fluorine (F) component contained in calcium fluoride (CaF2) is known to be unsuitable for use as a cement material.

[0006] Meanwhile, gypsum manufactured using the above method has a low SO3 content, making it difficult to use in cement manufacturing. Specifically, gypsum used in cement manufacturing must have an SO3 content of at least 38%, but conventional methods have had difficulty meeting this SO3 content requirement.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] (Patent Document 1) Republic of Korea Patent Publication No. 10-1839251

[0010]

[0011] The present invention has been devised to solve the problems of the prior art as described above.

[0012] The purpose of the present invention is to provide a method for producing high-quality gypsum having a fluorine content of 1,000 ppm or less and an SO3 content of 38 wt% or more using lime sludge containing 15 to 40 wt% of CaF2.

[0013] In addition, the purpose is to provide a method for manufacturing fluorite using hydrogen fluoride generated in the manufacturing process of the above-mentioned gypsum.

[0014]

[0015] In order to achieve the above purpose, the present invention

[0016] a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2;

[0017] b) a step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C; and

[0018] c) a step of separating hydrogen fluoride gas generated in step b); a method for producing gypsum is provided.

[0019]

[0020] In addition, the present invention

[0021] a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2;

[0022] b) A step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C;

[0023] c) a step of separating the hydrogen fluoride gas generated in step b) and reacting it with NaOH to produce NaF; and

[0024] d) A method for producing fluorite (CaF2) is provided, comprising the step of reacting the above NaF with Ca(OH)2.

[0025]

[0026] The method for producing gypsum of the present invention provides the effect of providing high-quality gypsum having a fluorine content of 1,000 ppm or less and an SO3 content of 38 wt% or more, despite using lime sludge containing 15 to 40 wt% of CaF2.

[0027] In addition, the method for producing fluorite (CaF2) of the present invention provides a method for producing high-quality fluorite using hydrogen fluoride gas generated during the production of gypsum.

[0028]

[0029] FIG. 1 is a drawing showing a facility for producing high-quality gypsum and fluorite using lime sludge and waste sulfuric acid according to the present invention.

[0030] Figure 2 is a photograph of lime sludge generated in the semiconductor manufacturing processes of Company A and Company B.

[0031] Figure 3 is a photograph of gypsum obtained by reacting lime sludge containing 15 to 40 wt% of CaF2 with waste sulfuric acid.

[0032] Figure 4 is a photograph of a fluorite manufactured by the present invention.

[0033]

[0034] Hereinafter, with reference to the attached drawings, a method for producing high-quality gypsum and fluorite using lime sludge and waste sulfuric acid according to the present invention will be described in detail as an embodiment. However, these embodiments are provided to enable those of ordinary skill in the art to sufficiently understand the present invention, and may be modified in various other forms and are not intended to limit the present invention.

[0035] The method for manufacturing the gypsum of the present invention is

[0036] a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2;

[0037] b) a step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C; and

[0038] c) a step of separating hydrogen fluoride gas generated in step b);

[0039] According to the above steps a) to c), not only gypsum dihydrate but also hemihydrate gypsum or anhydrite can be produced. Specifically, according to the above method, gypsum mixed with gypsum dihydrate, hemihydrate gypsum and anhydrite can be produced, and these gypsum mixtures can be processed into a single form of gypsum dihydrate, hemihydrate gypsum or anhydrite, respectively, by a known method, for example, a method of drying or mixing with water.

[0040]

[0041] In the above step a), the lime sludge may be used in a dried state, or lime sludge containing moisture may be used after being dried through a drying process. The drying process may be performed, for example, by heating the lime sludge containing moisture to 100 to 300°C while stirring it at 10 to 60 Hz.

[0042] The lime sludge containing 15 to 40 wt% of CaF2 in step a) above may be lime sludge (lime sludge containing F) generated in a semiconductor manufacturing process using hydrogen fluoride gas. In the lime sludge containing F, F may exist in the form of CaF2.

[0043] The above lime sludge may include organic sludge and inorganic sludge generated in a semiconductor manufacturing process.

[0044] The main components of lime sludge generated in the semiconductor manufacturing process are analyzed using XRF (X-ray fluorescence analyzer) and are shown in Table 1 below.

[0045] Analysis results of the classification Sludge① (weight%) Lime Sludge② (weight%) F19.499.49Al2O324.7248.11SiO24.473.59P2O56.412.55SO34.784.25CaO39.1427.54

[0046] Trace elements other than the above major components may include Na2O, MgO, P2O5, Cl, K2O, TiO2, MnO, Fe2O3, CuO, ZnO, SrO, ZrO2, CeO2, WO3, etc., and metals and rare earth elements such as Ce, W, Sn, Sr, Zn, Cu, Fe, Mn, etc. may be further included. In Table 1 above, lime sludge ① was obtained from Company A, and lime sludge ② was obtained from Company B. As can be seen in Table 1 above, lime sludge ① (see the left photo of Fig. 2) and lime sludge ② (see the right photo of Fig. 2) have the same composition and only the component contents are different. Among the above major components, a large amount of Ca and F contained in CaO exist in the form of CaF2.

[0047] In one embodiment of the present invention, the lime sludge containing 15 to 40 wt% of CaF2 may preferably be dried lime sludge, but is not limited thereto.

[0048]

[0049] In step b), the sulfuric acid may be waste sulfuric acid. However, it is not limited thereto. The waste sulfuric acid may contain fluorine or hydrogen fluoride.

[0050] In one embodiment of the present invention, the reaction of step b) may be carried out at, for example, 210 to 350°C, or 240 to 350°C, or 250 to 320°C, or 280 to 320°C. Among these temperature ranges, fluorine removal can be most efficiently carried out at a temperature range of 280 to 320°C.

[0051] If the above reaction temperature is lower than 210℃, the reaction between calcium fluoride (CaF2) and sulfuric acid does not occur sufficiently, so the gypsum produced contains an excessive amount of fluorine components that are not suitable for use in cement. If it exceeds 350℃, the crystal properties of the gypsum may change, and energy consumption may increase excessively compared to the effect obtained, which is not desirable.

[0052] The reaction in step b) may be carried out for, for example, 10 to 200 minutes, 50 to 170 minutes, 80 to 150 minutes, or 90 to 140 minutes, but is not limited thereto. In addition, the stirring speed of the reactants in step b) may be performed at 10 to 60 Hz.

[0053] In the reaction of step b), sulfuric acid as a reaction raw material may be used in a ratio of, for example, 100 to 600 parts by weight, 200 to 600 parts by weight, 300 to 600 parts by weight, or 400 to 600 parts by weight based on 100 parts by weight of dry lime sludge. If the amount of sulfuric acid is less than 100 parts by weight, the reaction does not proceed sufficiently, and if it exceeds 600 parts by weight, unreacted sulfuric acid remains, which is not preferable.

[0054] In the reaction of step b), lime sludge and sulfuric acid as reaction raw materials can be used in appropriate amounts calculated stoichiometrically.

[0055]

[0056] In the present invention, the reaction between calcium fluoride (CaF2) contained in lime sludge and sulfuric acid can be carried out as shown in the following reaction formula (1).

[0057] CaF2(s) + H2SO4(aq) → CaSO4(s) + 2HF(g) (1)

[0058] The reaction of the above reaction scheme 1 can be carried out, for example, at a temperature of 210 to 350°C, or 240 to 350°C, or 250 to 320°C, or 280 to 320°C. Among these temperature ranges, fluorine removal can be most efficiently carried out at a temperature range of 280 to 320°C.

[0059]

[0060] In the above step c), the separation of hydrogen fluoride gas can be performed, for example, using a scrubber. For example, it can be performed by passing it through a primary scrubber and / or a secondary scrubber. Specifically, in the above step b), when lime sludge and sulfuric acid react, fume is generated, and the fine particles contained in the fume are separated by passing it through a primary cyclone and / or a secondary cyclone and then a filter, and the remaining gas is passed through a primary scrubber and / or a secondary scrubber to separate hydrogen fluoride gas.

[0061] In the above method, the gypsum manufactured in steps a) to c) may include dihydrate gypsum (CaSO4·2H2O), hemihydrate gypsum (CaSO4·1 / 2H2O), and anhydrite (CaSO4). In this case, the manufacturing method of the present invention may further include a step of mixing the manufactured gypsum with water to manufacture dihydrate gypsum. That is, the gypsum may be hydrated by reacting with water so that it has two crystal waters, thereby converting hemihydrate gypsum and anhydrite into dihydrate gypsum. A known method may be used for the hydration step.

[0062]

[0063] In one embodiment of the present invention,

[0064] In the above step a), lime sludge can be prepared by adding and mixing at least one selected from among slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) as a calcium agent to lime sludge.

[0065] In addition, the step b) above can be performed by adding at least one selected from among slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) as a calcium agent together with lime sludge and allowing them to participate in the reaction.

[0066] It may be desirable to add the above calcium agent in a crushed form.

[0067] An example of the reaction by adding more quicklime (CaO) is as follows:

[0068] Sludge + H2O + CaO + 2H2SO4→ 2CaSO4+ 2HF + H2O (2)

[0069] In the above reaction formula 2, H2O represents water contained in the sludge.

[0070] In this case, first, powdered CaO is added to lime sludge containing water, then crushed and mixed to obtain a granular mixture for use. However, it is also possible to dry the lime sludge in advance and then use it.

[0071] When adding at least one selected from the above-mentioned slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3), the SO3 content of the gypsum produced can be increased, and when added to the subsequent line, it can be useful for pH control.

[0072] At least one selected from the above-mentioned slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) may be added in an amount of 10 to 350 parts by weight, preferably 50 to 300 parts by weight, based on 100 parts by weight of dry lime sludge. If the amount of quicklime added is less than 10 parts by weight, the effect of increasing the SO3 content may be minimal, and if it exceeds 350 parts by weight, unreacted quicklime is generated, which is not preferable.

[0073] As described above, when adding one or more kinds of calcium agent selected from among slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) to lime sludge as a calcium agent, sulfuric acid can be used 1 to 2 times or 1 to 1.5 times as much as when no calcium agent is added.

[0074]

[0075] In one embodiment of the present invention, the method for producing gypsum of the present invention may further include adding sulfuric acid during or after step b) to cause the sulfuric acid to participate in the reaction, and when adding sulfuric acid after step b) is completed, the method may further include a step of heating the reaction temperature to be 210 to 350°C.

[0076] When performing an additional reaction by adding sulfuric acid after the above step b) is completed, the sulfuric acid may be administered in an amount of 10 to 300 parts by weight based on 100 parts by weight of dry lime sludge.

[0077] Additionally, the sulfuric acid may be added together with the quicklime.

[0078]

[0079] The gypsum produced by the method of the present invention may have a characteristic of having a fluorine content of less than 1,000 ppm, preferably less than 500 ppm, more preferably less than 100 ppm, and even more preferably less than 10 ppm, based on dry weight.

[0080] In addition, the gypsum produced by the method of the present invention may have an SO3 content of 38 wt% or more, 40 wt% or more, or 50 wt% or more based on dry weight.

[0081] Since the gypsum produced by the method of the present invention has the above-described characteristics, it can be usefully used as a cement additive. The gypsum can be dried and used in the form of semi-hydrated gypsum or anhydrite.

[0082]

[0083] The above step c) may further include a process of separating separated hydrogen fluoride and reacting it with NaOH to produce NaF, and reacting the NaF with Ca(OH)2 to produce fluorite (CaF2).

[0084]

[0085] The present invention also provides:

[0086] a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2;

[0087] b) A step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C;

[0088] c) a step of separating the hydrogen fluoride gas generated in step b) and reacting it with NaOH to produce NaF; and

[0089] d) A method for producing fluorite (CaF2), including a step of reacting the above NaF with Ca(OH)2.

[0090] Since the above description can be equally applied to steps a) to c) (hydrogen fluoride gas separation), the description is omitted in this section.

[0091] The above steps c) and d) are processes for producing NaF by reacting separated hydrogen fluoride gas with NaOH, and producing fluorite (CaF2) by reacting the NaF with Ca(OH)2.

[0092] The above fluorite (CaF2) manufacturing process can be represented by the following reaction formula.

[0093]

[0094] 2HF + NaOH → NaF + H2O (3)

[0095] 2NaF + Ca(OH)2→ 2NaOH + CaF2(4)

[0096] As shown in the reaction formula above, fluorite (CaF2) is produced by reacting hydrogen fluoride (HF) and NaOH to obtain NaF, and then reacting this NaF with Ca(OH)2 for 5 to 200 minutes.

[0097] The fluorite (CaF2) manufactured by the above method may contain moisture, in which case the moisture can be removed using a filter press.

[0098] If the above reaction time is less than 5 minutes, the reaction may not occur sufficiently and a large amount of Ca(OH)2 components may remain. If it exceeds 200 minutes, the problem of the reaction occurring unevenly may occur.

[0099] The reaction temperature of the above step c) may be 20 to 300°C, 100 to 250°C, 150 to 250°C, 170°C to 230°C, or 190 to 210°C, and the reaction temperature of the above step d) may be 20 to 100°C, 40 to 80°C, or 50 to 70°C.

[0100] In the above step c), the separation of hydrogen fluoride gas can be performed, for example, using a scrubber. For example, it can be performed by passing it through a primary scrubber and / or a secondary scrubber. Specifically, in the above step b), when lime sludge and sulfuric acid react, fume is generated, and the fine particles contained in the fume are separated by passing it through a primary cyclone and / or a secondary cyclone and then a filter, and the remaining gas is passed through a primary scrubber and / or a secondary scrubber to separate hydrogen fluoride gas.

[0101] The NaF produced above can be stored in a precipitation tank and then transferred to a reaction tank to react with Ca(OH)2.

[0102]

[0103] Hereinafter, the present invention will be described in detail through examples. However, the following examples are intended to further illustrate the present invention, and the scope of the present invention is not limited by these examples. Those skilled in the art may appropriately modify or alter the following examples within the scope of the present invention.

[0104]

[0105] Example 1: Preparation of gypsum

[0106] 100 g of dry lime sludge① and 478 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel, stirred and simultaneously heated at 300°C for 130 minutes to obtain 665 g of gypsum.

[0107] As a result of measuring the F value of the gypsum obtained above by XRF, it can be seen that the F value is 0%, indicating that F has completely disappeared by gasification into HF.

[0108]

[0109] Example 2: Preparation of gypsum

[0110] 100 g of pre-dried lime sludge② and 239 g of 70% pure waste sulfuric acid were introduced into a reaction vessel and heated at 300°C for 130 minutes while stirring to obtain 333 g of gypsum.

[0111] As a result of measuring the F value of the gypsum obtained above by XRF, the F value was 0%, indicating that F was completely vaporized into HF.

[0112]

[0113] Comparative Example 1: Production of gypsum

[0114] 100 g of dried lime sludge① and 78 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel, stirred and heated at 100°C for 120 minutes to obtain 665 g of gypsum.

[0115] As a result of measuring the F value of the gypsum obtained above by XRF, the F value was confirmed to be 18.83% by weight.

[0116]

[0117] Comparative Example 2: Production of gypsum

[0118] 100 g of dried lime sludge① and 478 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel, and heated while stirring at 200°C for 120 minutes to obtain 665 g of gypsum.

[0119] As a result of measuring the F value of the gypsum obtained above by XRF, the F value was confirmed to be 7.53 wt%.

[0120]

[0121] Example 3: Preparation of gypsum

[0122] 100 g of dry lime sludge① and 250 g of powdered quicklime were placed in a crusher and crushed and mixed until granulated to obtain 350 g of a mixture. 350 g of the above mixture and 619 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel and heated at 300°C for 130 minutes while stirring to obtain 671 g of gypsum.

[0123] As a result of measuring the F value of the gypsum obtained above by XRF, it can be seen that the F value is 0%, indicating that F has completely disappeared by gasification into HF.

[0124]

[0125] Example 4: Preparation of gypsum

[0126] Before drying, 100 g of lime sludge② and 250 g of powdered quicklime were placed in a crusher and crushed and mixed until granulated to obtain 350 g of a mixture. 350 g of the above mixture and 591 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel and heated at 300°C for 130 minutes while stirring to obtain 641 g of gypsum.

[0127] As a result of measuring the F value of the gypsum obtained above by XRF, the F value was 0%, indicating that F was completely vaporized into HF.

[0128]

[0129] Comparative Example 2: Production of gypsum

[0130] 100 g of dry lime sludge① and 200 g of powdered quicklime were placed in a crusher and crushed and mixed until granulated to obtain 300 g of a mixture. 200 g of the above mixture and 600 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel and heated at 100°C while stirring for 120 minutes.

[0131] As a result of measuring the F value of the gypsum obtained above by XRF, the F value was high at 16.63 wt%.

[0132]

[0133] Example 5: Preparation of fluorite

[0134] Before drying, 100 g of lime sludge① and 250 g of powdered quicklime were placed in a crusher and crushed and mixed until granulated to obtain 350 g of a mixture. 350 g of the above mixture and 619 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel and heated at 300°C for 130 minutes while stirring to obtain 671 g of gypsum.

[0135] The hydrogen fluoride gas generated during the above gypsum manufacturing process was separated by passing it through a scrubber. The hydrogen fluoride gas was reacted with NaOH at 200°C to produce NaF and stored in a precipitation tank. The NaF and Ca(OH)2,360g were added to the reaction tank and reacted at 60°C to obtain 380g of a CaF2 mixture. Moisture was removed from the CaF2 mixture using a filter press and stored in a storage tank.

[0136] As a result of measuring the F value of the CaF2 mixture obtained above by XRF, the F value was 42 wt%, and when this value is converted to CaF2, it can be seen that the CaF2 mixture contains 86 wt% of CaF2.

[0137]

[0138] Example 6: Preparation of fluorite

[0139] Before drying, 100 g of lime sludge② and 250 g of powdered quicklime were placed in a crusher and crushed and mixed until granulated to obtain 350 g of a mixture. 350 g of the above mixture and 591 g of waste sulfuric acid with a purity of 70% were introduced into a reaction vessel and heated at 300°C for 130 minutes while stirring to obtain 641 g of gypsum.

[0140] The hydrogen fluoride gas generated during the above gypsum manufacturing process was separated by passing it through a scrubber. The hydrogen fluoride gas was reacted with NaOH at 200°C to produce NaF and stored in a precipitation tank. 2,200 g of the NaF and Ca(OH) were added to the reaction tank and reacted at 60°C to obtain 210 g of a CaF2 mixture. Moisture was removed from the CaF2 mixture using a filter press and stored in a storage tank.

[0141] As a result of measuring the F value of the CaF2 mixture obtained above by XRF, the F value was 38 wt%, and when this value is converted to CaF2, it can be seen that the CaF2 mixture contains 78 wt% of CaF2.

[0142]

[0143] Test Example 1: XRF Analysis of Gypsum

[0144] The components of the gypsum obtained in Examples 3 and 4 above were analyzed by XRF, and the results are shown in Tables 2 and 3 below.

[0145] Classification Analysis Results Analysis Sludge① (weight%) Example 3 Manufacturing Gypsum (weight%) F19.490Al2O324.722.53SiO24.470.62P2O56.410.14SO34.7857.57CaO39.1438.32Other0.990.82Total100100

[0146] Classification Analysis Results Analysis Sludge② (weight%) Example 4 Manufacturing Gypsum (weight%) F9.490Al2O348.112.19SiO23.590.55P2O52.550.50SO34.2556.08CaO27.5439.76Others4.470.92Total 100100

[0147] Although the present invention has been illustrated and described with respect to specific embodiments in the above description, it will be readily apparent to anyone skilled in the art that various modifications and changes are possible without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2; b) a step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C; and c) A method for producing gypsum, comprising a step of separating hydrogen fluoride gas generated in step b).

2. In paragraph 1, A method for producing gypsum, characterized in that the lime sludge containing 15 to 40 wt% of CaF2 is lime sludge generated in a semiconductor manufacturing process using hydrogen fluoride gas.

3. In paragraph 1, A method for producing gypsum, characterized in that the lime sludge containing 15 to 40 wt% of CaF2 is a dried lime sludge.

4. In paragraph 1, A method for producing gypsum, characterized in that the sulfuric acid is waste sulfuric acid.

5. In paragraph 1, A method for producing gypsum, characterized in that the step c) further includes a process of producing NaF by separating separated hydrogen fluoride and reacting it with NaOH, and producing fluorite (CaF2) by reacting the NaF with Ca(OH)2.

6. In paragraph 1, In the above step a), at least one selected from among slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) is added to the lime sludge as a calcium agent and mixed, or A method for producing gypsum, characterized in that it further includes a step of adding and reacting at least one selected from slaked lime (Ca(OH)2), quicklime (CaO), and calcium carbonate (CaCO3) as a calcium agent in the step b).

7. In paragraph 1, During step b) or after step b) is completed, sulfuric acid is additionally added to participate in the reaction, b) A method for producing gypsum, characterized in that it further includes a process of heating so that the reaction temperature becomes 210 to 350°C when sulfuric acid is additionally added after the step is completed. 8.a) A step of preparing lime sludge containing 15 to 40 wt% of CaF2; b) a step of adding sulfuric acid to the lime sludge and heating it while stirring so that the reaction temperature becomes 210 to 350°C; and c) a step of separating the hydrogen fluoride gas generated in step b) and reacting it with NaOH to produce NaF; and d) A method for producing fluorite (CaF2), comprising the step of reacting the above NaF with Ca(OH)2.

9. In paragraph 8, A method for producing fluorite (CaF2), characterized in that the lime sludge containing 15 to 40 wt% of CaF2 is lime sludge generated in a semiconductor manufacturing process using hydrogen fluoride gas.

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