Method for purifying white mud gypsum and white mud gypsum

Abstract

The invention provides a method for purifying white mud gypsum and the white mud gypsum. The method comprises the following steps: S1, slurry mixing and dispersing: mixing the white mud gypsum with water, and stirring to form homogeneous slurry; s2, acidification and oxidation: adding concentrated sulfuric acid into the slurry for acidification and stirring; then adding a hydrogen peroxide solution for oxidation, and continuously stirring; s3, solid-liquid separation: after the reaction is completed, carrying out solid-liquid separation, collecting a solid product, and washing and drying the solid product to obtain purified white mud gypsum; wherein the white mud gypsum comprises dihydrate gypsum, magnesium, iron and organic impurities. Through the synergistic effect of acidification of concentrated sulfuric acid and oxidation of hydrogen peroxide, and by utilizing endogenous iron ions of a white mud gypsum system, catalytic circulation is constructed, dissolution of magnesium and degradation of organic matters are synchronously realized under a low-temperature condition, and finally, a white mud gypsum product with high purity and high whiteness is obtained.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, and relates to a method for purifying white mud gypsum and white mud gypsum. Background Technology

[0002] White mud gypsum, a byproduct of rare earth smelting, is mainly composed of dihydrate gypsum (CaSO4·2H2O). However, due to limitations in processing technology and raw materials, compared to common industrial byproducts like desulfurization gypsum, it contains a certain amount of magnesium and organic impurities. These impurities are incorporated into the gypsum system through co-precipitation, adsorption, or encapsulation, becoming a key source of pollution affecting its subsequent utilization. Furthermore, the current large-scale production of white mud gypsum, primarily handled through traditional landfill methods, not only results in high processing costs and waste of calcium and sulfur resources but also poses a risk of pollution leakage. While there are reports in literature and patents regarding white mud gypsum, there are no reports on the resource utilization of white mud generated in the rare earth industry. Existing methods for removing gypsum impurities have limitations and are not entirely applicable to the purification and impurity removal of rare earth white mud gypsum.

[0003] Patent CN120023022A proposes a method for purifying phosphogypsum. The specific purification steps include: adjusting the sphericity of the phosphogypsum through ball milling, further adjusting the moisture content, and then completely freezing and pressing it. The resulting pressed material is subjected to ultrasonic stirring and flotation. The remaining slurry in the flotation tank is dehydrated and dried to obtain the purified phosphogypsum. However, this patent mainly targets impurities such as organic matter, quartz, soluble fluorine, and phosphorus in phosphogypsum, while the impurities in rare earth white clay gypsum are mainly magnesium and a small amount of organic matter. The process conditions are not suitable for rare earth white clay gypsum, and the process flow is relatively complex. Patent CN113289768A proposes a highly efficient flotation purification process for mirabilite gypsum. First, calcium mirabilite is prepared into mirabilite gypsum slurry, and then it undergoes roughing, scavenging, and three cleaning processes sequentially. The final foam is then dehydrated to obtain the final concentrate (gypsum product). While this method can remove impurities from gypsum, primarily silica-containing gangue (mica) with small amounts of Mg, Fe, and Al, it doesn't optimize for organic matter removal and has a lengthy process. Patent CN111020201A provides a method for purifying and removing impurities from gypsum used in smelting flue gas desulfurization, using gypsum from tin-containing copper and lead smelting flue gas as raw material. The method includes acid leaching, filtration, neutralization, and separation. However, it also fails to effectively remove residual organic impurities from rare earth white mud gypsum, offering little reference value for impurity removal in rare earth white mud gypsum. Patent CN116621201A discloses a new process for preparing whiskers and recovering magnesium salts using white mud. This includes sulfuric acid leaching to obtain high-whiteness gypsum and a magnesium sulfate solution containing calcium ions. The magnesium sulfate solution is then purified through decalcification, thermal crystallization, carbonization, and calcination to prepare magnesium oxide. The high-whiteness gypsum is then used to prepare high-whiteness nano-sized calcium sulfate fibers and high-strength gypsum products via a hydrothermal method. However, the "sulfuric acid leaching" step in the patent only achieves solid-liquid separation and does not involve the removal of organic matter. In particular, the effect of degradation and removal of low-concentration organic matter is unpredictable.

[0004] Therefore, there is an urgent need to develop a purification method for white clay gypsum that can specifically remove magnesium and organic impurities from it, providing a reliable path for the high-value-added resource utilization of rare earth smelting white clay gypsum. Summary of the Invention

[0005] To address the aforementioned shortcomings in existing technologies, the present invention aims to provide a method for purifying white clay gypsum and white clay gypsum itself. Specifically targeting the challenge of coexisting magnesium and organic impurities in white clay gypsum, a byproduct of rare earth smelting, the method utilizes the synergistic effect of concentrated sulfuric acid acidification and hydrogen peroxide oxidation, along with the catalytic cycle constructed using endogenous iron ions within the white clay gypsum system. This allows for the simultaneous dissolution of magnesium and degradation of organic matter under low-temperature conditions, ultimately yielding a high-purity, high-whiteness white clay gypsum product. This provides a reliable pathway for its high-value-added resource utilization, while overcoming the problems of incomplete impurity removal or ineffective degradation of organic matter in existing technologies.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for purifying white clay gypsum, comprising the following steps: S1. Slurry preparation and dispersion: Mix the white clay gypsum with water and stir to form a homogeneous slurry; S2. Acidification and oxidation: Concentrated sulfuric acid is added to the slurry for acidification and stirring; then hydrogen peroxide solution is added for oxidation, and stirring continues. S3. Solid-liquid separation: After the reaction is completed, solid-liquid separation is performed, the solid product is collected, and after washing and drying, the purified white clay gypsum is obtained. The white clay gypsum includes dihydrate gypsum, magnesium, iron, and organic impurities.

[0007] The method for purifying white clay gypsum provided by this invention achieves efficient and simultaneous deep removal of magnesium and low-concentration organic matter. This method creatively combines the two steps of "acidification with concentrated sulfuric acid" and "hydrogen peroxide oxidation." The addition of concentrated sulfuric acid not only effectively acidifies and dissolves inorganic impurities such as magnesium, but more importantly, it dissolves iron oxides in the white clay, providing endogenous Fe for subsequent reactions. 3+ Catalyst. Hydrogen peroxide (H₂O₂) was subsequently added, along with Fe. 3+ / Fe 2+ This system constitutes a highly efficient Fenton-like catalytic oxidation system, generating highly oxidizing hydroxyl radicals (·OH) that deeply degrade organic matter in the mineralization system. This allows for the simultaneous dissolution of inorganic matter and oxidation of organic matter in a single process, reducing the final product's organic carbon percentage to below 0.25% and significantly improving whiteness, thus solving the problem of incomplete removal of low-concentration organic matter in existing technologies.

[0008] Furthermore, the iron in the white clay gypsum exists in the form of Fe2O3.

[0009] Furthermore, the Fe2O3 has a mass percentage content greater than 0.1%.

[0010] Furthermore, in step S1, the volume ratio of water to the mass of the white clay gypsum is (5~10):1 (mL / g).

[0011] Furthermore, in step S1, the mixing time is 10-20 minutes and the rotation speed is 500-800 rpm.

[0012] Furthermore, in step S2, concentrated sulfuric acid is added to the slurry for acidification, and the pH value of the system is adjusted to 2-5.

[0013] Furthermore, in step S2, the reaction temperature of the acidification reaction is 50-80℃.

[0014] Furthermore, in step S2, the concentrated sulfuric acid is added dropwise, and its mass concentration is 95-98%.

[0015] Furthermore, in step S2, the mass of the concentrated sulfuric acid is 14%-20% of the dry weight of the white clay gypsum.

[0016] Furthermore, in step S2, the reaction temperature of the oxidation reaction is 50-80℃.

[0017] Furthermore, in step S2, the hydrogen peroxide solution is added dropwise, and its mass concentration is 25-35%.

[0018] Furthermore, in step S2, the mass of the hydrogen peroxide is 2.25-4% of the dry basis mass of the white clay gypsum.

[0019] Furthermore, in step S2, the stirring speed of the acidification reaction is 600-900 rpm, the concentrated sulfuric acid is added dropwise over 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is completed.

[0020] Furthermore, in step S2, the stirring speed of the oxidation reaction is 600-900 rpm, the hydrogen peroxide solution is added dropwise over 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is completed.

[0021] Furthermore, in step S2, the hydrogen peroxide solution is added in two parts. The first part is 60% to 80% of the total mass, and after reacting for 15 to 25 minutes, the remaining part is added.

[0022] Furthermore, the first addition of the hydrogen peroxide solution is completed within 2-4 minutes, and the reaction continues for 15-25 minutes after the addition is completed; the remaining hydrogen peroxide solution is completed within 1-3 minutes, and the reaction continues for 10-20 minutes after the addition is completed.

[0023] Furthermore, in step S2, while adding hydrogen peroxide solution, ultrasonic treatment is also performed, with an ultrasonic power of 300-500W.

[0024] Furthermore, in step S3, the solid-liquid separation is performed by a combination of decantation and filtration.

[0025] Furthermore, in step S3, the decantation settling time is 40-80 minutes.

[0026] Furthermore, in step S3, the washing process involves rinsing the filter cake with deionized water.

[0027] Furthermore, in step S3, the drying process involves drying at 50-70°C to a constant weight.

[0028] Secondly, the present invention provides a white clay gypsum obtained by purification using the method described in the first aspect, wherein the white clay gypsum has a mass percentage of magnesium oxide (MgO) ≤0.6%, a mass percentage of carbon (C) ≤0.25%, and a whiteness ≥85%.

[0029] Furthermore, the crystal structure of the white clay gypsum is calcium sulfate dihydrate, without phase transformation or structural damage; the white clay gypsum is suitable for use in building materials, mold preparation, or medical gypsum applications.

[0030] Compared with the prior art, the beneficial effects of the present invention include at least one of the following: 1) Achieved efficient and simultaneous deep removal of magnesium and low-concentration organic matter: This scheme creatively combines the two steps of "concentrated sulfuric acid acidification" and "hydrogen peroxide oxidation". The addition of concentrated sulfuric acid not only effectively acidifies and dissolves inorganic impurities such as magnesium, but more importantly, it dissolves iron oxides in the white mud, providing endogenous Fe for subsequent reactions. 3+ Catalyst. Hydrogen peroxide (H₂O₂) was subsequently added, along with Fe. 3+ / Fe 2+ This system constitutes a highly efficient Fenton-like catalytic oxidation system, generating highly oxidizing hydroxyl radicals (·OH) that deeply degrade organic matter in the mineralization system. This allows for the simultaneous dissolution of inorganic matter and oxidation of organic matter in a single process, reducing the final product's organic carbon percentage to below 0.25% and significantly improving whiteness, thus solving the problem of incomplete removal of low-concentration organic matter in existing technologies.

[0031] 2) Stable acquisition of high-purity white clay gypsum at low temperatures: The entire reaction process is strictly controlled within a low range of 50-80℃, far below the phase transition temperature of white clay gypsum. This effectively prevents the white clay gypsum from dehydrating and transforming into other crystal forms, ensuring the purity and stability of the final product phase. The final product is high-purity white clay gypsum (MgO mass percentage ≤ 0.6%), which can be directly used as a high-quality building material raw material or medical gypsum.

[0032] 3) Utilizing endogenous catalysts for green and efficient processing: This solution cleverly utilizes the iron element contained in the white clay gypsum itself as a catalyst source, achieving "waste treatment with waste." No additional expensive catalysts or reducing agents are required, avoiding the introduction of new impurities and further reducing processing costs, embodying the concepts of green environmental protection and efficient resource utilization.

[0033] 4) Significantly simplified process flow, reduced costs and energy consumption: This invention eliminates many complex processes in traditional processes. The shortened process directly leads to reduced equipment investment, smaller footprint, lower operator requirements, and a significant reduction in energy and material consumption (such as steam, electricity, and other chemicals), making the entire purification process more economical and easier to achieve industrialized continuous production. Attached Figure Description

[0034] Figure 1 The image shows the XRD pattern of the purified white clay gypsum from Example 1 of this invention. Figure 2 The image shows the XRD pattern of the purified white clay gypsum from Comparative Example 3 of this invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0036] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0037] In a first aspect, the present invention provides a method for purifying white clay gypsum, comprising the following steps: S1. Slurry preparation and dispersion: Mix the white clay gypsum with water and stir to form a homogeneous slurry; S2. Acidification and oxidation: Concentrated sulfuric acid is added to the slurry for acidification and stirring; then hydrogen peroxide solution is added for oxidation, and stirring continues. S3. Solid-liquid separation: After the reaction is completed, solid-liquid separation is performed, the solid product is collected, and after washing and drying, the purified white clay gypsum is obtained. The white clay gypsum includes dihydrate gypsum, magnesium, iron, and organic impurities.

[0038] This invention provides a simple, efficient, and low-temperature purification method for bleached gypsum. By combining two core reactions—acidification and oxidation—the dissolution of inorganic impurities (such as magnesium) and the degradation of organic impurities are simultaneously achieved in a single reaction system. The method operates at a low reaction temperature (50-80℃), effectively avoiding the dehydration phase transition of the bleached gypsum and ensuring the stability of the product structure.

[0039] In this invention, the localized over-acidification and exothermic effect generated by the high concentration of concentrated sulfuric acid not only effectively acidifies and dissolves inorganic impurities such as magnesium, but more importantly, it dissolves iron oxides in the white mud, providing an endogenous source of Fe for subsequent reactions. 3+ Catalyst. Meanwhile, concentrated sulfuric acid itself possesses strong oxidizing properties and can directly act on organic matter under heating conditions, achieving preliminary degradation through reactions such as sulfonation and dehydration, effectively reducing the load on subsequent oxidation stages. These effects lay a solid foundation for the subsequent construction of a highly efficient Fenton-like reaction system using hydrogen peroxide, which is key to achieving deep degradation of organic matter and simultaneous removal of inorganic impurities. The subsequently added hydrogen peroxide (H2O2) and Fe... 3+ / Fe 2+ This system constitutes a highly efficient Fenton-like catalytic oxidation system, generating highly oxidizing hydroxyl radicals (·OH) that deeply degrade organic matter in the mineralization system. This allows for the simultaneous dissolution of inorganic matter and oxidation of organic matter in a single process, reducing the final product's organic carbon percentage to below 0.25% and significantly improving whiteness, thus solving the problem of incomplete organic matter removal in existing technologies.

[0040] This invention addresses three major pain points of traditional gypsum impurity removal methods: 1) Complex and lengthy processes involving multiple separation and conversion steps, resulting in high costs and difficulty in industrialization; 2) Ineffective or incomplete removal of low-concentration organic impurities, affecting product whiteness and performance; 3) The stable crystal form of white clay gypsum is easily destroyed under high-temperature and high-pressure conditions (such as hydrothermal methods), leading to product phase transformation and limiting its application. The entire process of this invention requires no complex equipment or multiple separation steps, significantly reducing investment and operating costs, and providing a reliable path for large-scale industrial processing of white clay gypsum.

[0041] As an alternative implementation, the iron in the white clay gypsum exists in the form of Fe2O3.

[0042] As an optional implementation, the Fe2O3 has a mass percentage content greater than 0.1%.

[0043] This invention ensures that endogenous iron is fully dissolved into Fe during the acidification process by limiting the iron to exist in the form of Fe2O3 and having a mass percentage greater than 0.1%. 3+It provides a highly efficient catalyst for hydrogen peroxide, promoting the generation of hydroxyl radicals and enhancing the degradation capacity of organic matter. Insufficient iron content (less than 0.1% by mass) may lead to a lack of catalyst in subsequent oxidation steps, affecting the degradation efficiency of organic matter.

[0044] As an optional implementation, in step S1, the volume ratio of water to the mass of the white clay gypsum is (5~10):1 (mL / g), for example, it can be 5:1 (mL / g), 6:1 (mL / g), 7:1 (mL / g), 8:1 (mL / g), 9:1 (mL / g) or 10:1 (mL / g).

[0045] As an optional implementation, in step S1, the mixing time is 10-20 minutes, for example, 10 minutes, 15 minutes or 20 minutes, and the rotation speed is 500-800 rpm, for example, 500 rpm, 600 rpm, 700 rpm or 800 rpm.

[0046] This invention ensures that the gypsum particles are fully dispersed by limiting the optimal water volume to the mass of the gypsum (5~10:1 (mL / g), forming a homogeneous and stable reaction slurry, providing a sufficiently large solid-liquid contact area for subsequent steps. Insufficient water (liquid-to-solid ratio <5:1) will result in an overly thick slurry, hindered mass transfer, reduced impurity removal rate, and operational difficulties; excessive water (liquid-to-solid ratio >10:1) will dilute the reactants, reduce the reaction rate, and increase acid consumption, energy consumption, and wastewater treatment costs.

[0047] As an optional implementation, in step S2, concentrated sulfuric acid is added to the slurry for acidification to adjust the pH value of the system to 2-5, for example, 2, 3, 4 or 5.

[0048] As an optional implementation, in step S2, the reaction temperature of the acidification reaction is 50-80℃, for example, 50℃, 60℃, 70℃, or 80℃. Too low an acidification temperature will significantly reduce the dissolution rate of magnesium compounds and iron oxides by concentrated sulfuric acid, prolong the reaction time, reduce efficiency, and may lead to incomplete dissolution of impurities. While too high an acidification temperature can accelerate the dissolution rate, it easily causes localized supersaturation of the slurry, leading to calcium sulfate recrystallization and encapsulation of impurities, which is detrimental to deep purification and increases the risk of phase transformation in dihydrate gypsum.

[0049] As an optional implementation, in step S2, the concentrated sulfuric acid is added dropwise, and its mass concentration is 95-98%, for example, it can be 95%, 96%, 97% or 98%.

[0050] As an optional implementation, in step S2, the mass of the concentrated sulfuric acid is 14%-20% of the dry basis mass of the white clay gypsum, for example, it can be 14%, 15%, 16%, 18% or 20%.

[0051] As an optional implementation, the oxidation reaction temperature is 50-80℃, for example, 50℃, 60℃, 70℃, or 80℃. Too low an oxidation temperature will result in insufficient H2O2 activity, significantly reducing the Fenton-like reaction rate, incomplete degradation of organic matter, and affecting product quality; too high an oxidation temperature will cause H2O2 to preferentially undergo ineffective decomposition, resulting in wasted oxidant and increased energy consumption, and severely damaging the stability of gypsum dihydrate crystals, triggering a phase transition risk.

[0052] As an optional implementation, in step S2, the hydrogen peroxide solution is added dropwise, and its mass concentration is 25-35%, for example, 25%, 30% or 35%.

[0053] This invention employs a "droplet addition" method, enabling slow and controllable adjustment of the system's acidity and introduction of the oxidant. This avoids violent exothermic reactions and side reactions, and by limiting the concentration range (90-98% concentrated sulfuric acid, 25-35% H₂O₂), sufficient H₂ can be provided. + The oxidation potential ensures the effectiveness of the reaction reagents.

[0054] As an optional implementation, in step S2, the mass of the hydrogen peroxide is 2.25-4% of the dry basis mass of the white clay gypsum, for example, it can be 2.25%, 2.5%, 2.7%, 3%, 3.6% or 4%.

[0055] This invention further limits the amount of hydrogen peroxide added (based on the dry weight of white clay gypsum). Insufficient hydrogen peroxide leads to inadequate oxidation, incomplete degradation of organic matter, and poor product quality; excessive hydrogen peroxide causes ineffective decomposition, reducing oxidation efficiency and increasing costs. This invention achieves superior impurity removal with lower chemical reagent consumption by precisely controlling the oxidant dosage, significantly reducing production costs and making the process stable and controllable. It avoids product quality fluctuations and cumbersome post-processing caused by improper material feeding.

[0056] As an optional implementation, in step S2, the stirring speed of the acidification reaction is 600-900 rpm, for example, 600 rpm, 700 rpm, 800 rpm or 900 rpm, the concentrated sulfuric acid is added dropwise over 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is completed, for example, 20 minutes, 30 minutes or 40 minutes.

[0057] As an optional implementation, in step S2, the stirring speed of the oxidation reaction is 600-900 rpm, for example, 600 rpm, 700 rpm, 800 rpm or 900 rpm, the hydrogen peroxide solution is added dropwise within 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is completed, for example, 20 minutes, 30 minutes or 40 minutes.

[0058] This invention ensures the uniformity of the reaction system and the efficient mass transfer process by limiting the stirring speed, guaranteeing that concentrated sulfuric acid and hydrogen peroxide can react with impurities efficiently and uniformly, thereby improving reaction efficiency and impurity removal effect. Furthermore, by further limiting the reaction time, it ensures that each step of the reaction is carried out fully, thereby stably producing high-quality, high-purity white clay gypsum.

[0059] As an optional implementation, in step S2, the hydrogen peroxide solution is added in two parts. The first part is 60% to 80% of the total mass, for example, 60%, 70% or 80%. After reacting for 15 to 25 minutes, for example, 15 minutes, 20 minutes or 25 minutes, the remaining part is added.

[0060] As an optional implementation, the hydrogen peroxide solution is added dropwise over 2-4 minutes, and the reaction continues for 15-25 minutes after the addition is completed, for example, 15 minutes, 20 minutes or 25 minutes; the remaining hydrogen peroxide solution is added dropwise over 1-3 minutes, and the reaction continues for 10-20 minutes after the addition is completed, for example, 10 minutes, 15 minutes or 20 minutes.

[0061] The hydrogen peroxide solution of this invention employs a two-stage addition strategy. The first addition of most of the H2O2 (60%–80%) is used to degrade most easily oxidized organic matter. After a period of reaction, the remaining portion is added to further remove residual, recalcitrant organic matter. The inventors have found that this multiple-addition method significantly improves the utilization efficiency of the oxidant and the final degradation effect, ensuring an effective reduction in organic carbon content and further enhancing the product's whiteness. This is likely because multiple additions of hydrogen peroxide help maintain its concentration and inhibit ineffective decomposition.

[0062] As an optional implementation, in step S2, while adding hydrogen peroxide solution, ultrasonic treatment is also performed. The ultrasonic power is 300-500W, for example, 300W, 400W or 500W.

[0063] This invention addresses the issue of impurities in white clay gypsum being incorporated into the gypsum system through co-precipitation, adsorption, or encapsulation. Ultrasonic treatment leverages the localized high temperature and pressure, strong shock waves, and microjets generated by ultrasonic cavitation to effectively break down particles, erode surface coatings, and enhance liquid-solid mass transfer, allowing the oxidant to more effectively attack organic molecules. By limiting the ultrasonic treatment power, degradation efficiency and treatment depth are significantly enhanced, making it particularly suitable for treating stubborn and encapsulated organic matter, thus providing an effective guarantee for achieving deep decolorization and efficient impurity removal.

[0064] As an optional implementation, in step S3, the solid-liquid separation is performed by a combination of decantation and filtration.

[0065] As an optional implementation, the settling time is 40-80 minutes, for example, 40 minutes, 50 minutes, 60 minutes, 70 minutes or 80 minutes.

[0066] As an optional implementation, the washing process involves rinsing the filter cake with deionized water.

[0067] As an optional implementation, the drying is carried out at 50-70°C, for example, at 50°C, 60°C or 70°C until constant weight is achieved.

[0068] This invention first removes most of the waste liquid by decantation, reducing the load on subsequent filtration and improving separation efficiency; water washing of the filter cake effectively removes residual soluble salts and acids entrained in the filter cake, greatly improving the purity of the product; low-temperature drying (50-70℃) thoroughly removes free water while strictly protecting the crystal water of dihydrate gypsum, preventing it from dehydrating and transforming into hemihydrate or anhydrous gypsum, thus ensuring the stability of the product phase and its application performance.

[0069] In a second aspect, the present invention provides a white clay gypsum obtained by the method described in the first aspect, wherein the white clay gypsum has a mass percentage of magnesium oxide (MgO) ≤0.6%, a mass percentage of carbon (C) ≤0.25%, and a whiteness ≥85%.

[0070] As an optional implementation, the white clay gypsum has a crystal structure of calcium sulfate dihydrate, without phase transformation or structural damage; the white clay gypsum is suitable for use in building materials, mold preparation, or medical gypsum.

[0071] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0072] Example 1 The purification process of white clay gypsum raw material (whose main component is dihydrate gypsum, and contains magnesium, iron and organic impurities, with iron existing in the form of Fe2O3, and the Fe2O3 mass percentage being 0.24%) produced by a rare earth smelter includes the following steps: (1) Slurry preparation and dispersion: 100g of the above-mentioned white clay gypsum (dry basis) (the white clay gypsum raw material produced by a rare earth smelter was dried at a low temperature of 60℃ and then crushed by a crusher until there were no obvious large particles to obtain rare earth white clay gypsum (dry basis)) and 500mL of deionized water were placed in a beaker, and the volume ratio of water to white clay gypsum was controlled to be 5:1 (mL / g). The mixture was stirred continuously at a stirring speed of 500rpm for 10 min to completely disperse the white clay gypsum and form a homogeneous slurry.

[0073] (2) Acidification and oxidation: The obtained slurry was heated to 50°C and maintained at a constant temperature. 20g of 98% concentrated sulfuric acid solution was slowly added dropwise while stirring until the pH of the system dropped to 2.0. The stirring speed for the acidification reaction was 600 rpm, with the concentrated sulfuric acid added over 3 minutes. After the addition, stirring was continued for 30 minutes to allow magnesium, iron, etc., to fully dissolve. Subsequently, 7.5g of 30% hydrogen peroxide solution (equivalent to 2.25% of the dry weight of the white clay gypsum) was slowly added dropwise while stirring. Ultrasonic treatment (300W power) was applied simultaneously with the addition of hydrogen peroxide. The stirring speed for the oxidation reaction was 600 rpm, with the hydrogen peroxide solution added over 3 minutes. After the addition, stirring was continued for 30 minutes. The ferric ions dissolved in the system reacted with H2O2 to form ferrous ions, catalyzing the degradation of organic matter.

[0074] (3) Solid-liquid separation and post-treatment: After the reaction was completed, stirring was stopped, and the mixture was allowed to stand for 60 minutes to allow for complete solid-liquid separation. The supernatant was discarded, and the remaining slurry was separated by suction filtration using a Buchner funnel. The filter cake was rinsed with deionized water until neutral, and then dried in a 60℃ oven to constant weight to obtain the purified white clay gypsum product. The MgO and C contents in the white clay gypsum product were determined by X-ray fluorescence combined with chemical analysis. The mass percentage of MgO decreased from 5.69% to 0.31%, and the mass percentage of C decreased from 1.09% to 0.218%. The whiteness (measured using a whiteness meter (Shanghai Yuefeng SBDY-3) under a D65 standard light source) increased from 78.63% to 87.23%. The XRD analysis results are shown in [reference needed]. Figure 1 ,Depend on Figure 1 It can be seen that the crystal structure of the purified white clay gypsum in Example 1 is calcium sulfate dihydrate, and no phase transition has occurred.

[0075] Effect Analysis: Example 1 achieved efficient and simultaneous removal of magnesium impurities (MgO mass percentage decreased from 5.69% to 0.31%) and organic carbon impurities (C mass percentage decreased from 1.09% to 0.218%) from white clay gypsum through synergistic treatment of concentrated sulfuric acid acidification and hydrogen peroxide oxidation. The entire process was carried out at low temperature, with a short process flow and simple operation. The purity of the obtained product was significantly improved, making it suitable for high-value-added resource utilization. After acidification and oxidation, the whiteness of the white clay gypsum was significantly improved, increasing from 78.63% to 87.23%, and its crystal structure remained as calcium sulfate dihydrate, without undergoing a phase transition.

[0076] Example 2 The purification process of white clay gypsum raw materials includes the following steps: (1) Slurry preparation and dispersion: 100g (dry basis) of white clay gypsum (same as in Example 1) and 750 mL of deionized water were placed in a beaker, and the volume ratio of water to white clay gypsum was controlled to be 7.5:1 (mL / g). The mixture was stirred continuously at 600 rpm for 15 min to completely disperse the white clay gypsum and form a homogeneous slurry.

[0077] (2) Acidification and Oxidation: The obtained slurry was heated to 60℃ and maintained at a constant temperature. 18g of 98% concentrated sulfuric acid solution was slowly added dropwise under stirring until the pH of the system dropped to 3.5. The stirring speed for the acidification reaction was 800 rpm, and the concentrated sulfuric acid was added dropwise over 3 minutes. After the addition was completed, stirring was continued for 30 minutes to allow magnesium, iron, etc., to dissolve completely. Subsequently, 9.0g of 30% hydrogen peroxide solution (equivalent to 2.7% of the dry weight of the white clay gypsum) was slowly added dropwise under stirring. Ultrasonic treatment (300W power) was applied simultaneously with the addition of hydrogen peroxide. The stirring speed for the oxidation reaction was 800 rpm, and the hydrogen peroxide solution was added dropwise over 3 minutes. After the addition was completed, stirring was continued for 30 minutes. The ferric ions dissolved in the system reacted with H2O2 to transform into ferrous ions, which catalyzed the degradation of organic matter.

[0078] (3) Solid-liquid separation and post-treatment: After the reaction is completed, stop stirring and decanting, allowing the solid and liquid to separate completely for 60 minutes. Discard the supernatant, and separate the remaining slurry by suction filtration using a Buchner funnel. Wash the filter cake with deionized water until neutral, and then dry it in a 60℃ oven to constant weight to obtain the purified white clay gypsum product. In the white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.37%, the mass percentage of C decreased from 1.09% to 0.209%, and the whiteness increased from 78.63% to 88.45%.

[0079] Effect Analysis: Example 2 achieved efficient and simultaneous removal of magnesium impurities (MgO mass percentage decreased from 5.69% to 0.37%) and organic carbon impurities (C mass percentage decreased from 1.09% to 0.209%) from white clay gypsum through synergistic treatment of concentrated sulfuric acid acidification and hydrogen peroxide oxidation. Increased pH and liquid-solid ratio slightly reduced the MgO removal rate, but by appropriately increasing the H2O2 dosage, excellent organic matter removal was still achieved. The entire process was carried out at low temperature, with a short and simple procedure, and the resulting product had significantly improved purity, making it suitable for high-value-added resource utilization. The whiteness of the white clay gypsum significantly improved after acidification and oxidation, increasing from 78.63% to 88.45%.

[0080] Example 3 The purification process of white clay gypsum raw materials includes the following steps: (1) Slurry preparation and dispersion: 100g (dry basis) of white clay gypsum (same as in Example 1) and 1000mL of deionized water were placed in a beaker, and the ratio of water volume to white clay gypsum mass was controlled to be 10:1 (mL / g). The mixture was stirred continuously at 700 rpm for 20 min to completely disperse the white clay gypsum and form a homogeneous slurry.

[0081] (2) Acidification and oxidation: The obtained slurry was heated to 75°C and maintained at a constant temperature. 15g of 98% concentrated sulfuric acid was slowly added dropwise under stirring until the pH of the system dropped to 5. The stirring speed for the acidification reaction was 900 rpm, with the concentrated sulfuric acid added over 3 minutes. After the addition, stirring was continued for 30 minutes to allow magnesium, iron, etc., to fully dissolve. Subsequently, 12.0g of 30% hydrogen peroxide solution (equivalent to 3.6% of the dry weight of the white clay gypsum) was slowly added dropwise under stirring, with ultrasonic treatment (300W power) applied simultaneously. The stirring speed for the oxidation reaction was 900 rpm, with the hydrogen peroxide solution added over 3 minutes. After the addition, stirring was continued for 30 minutes. The ferric ions dissolved in the system reacted with H2O2 to transform into ferrous ions, catalyzing the degradation of organic matter.

[0082] (3) Solid-liquid separation and post-treatment: After the reaction is completed, stirring is stopped, and the mixture is decanted and allowed to stand for 60 minutes to allow for complete solid-liquid separation. The supernatant is discarded, and the remaining slurry is separated by suction filtration using a Buchner funnel. The filter cake is washed with deionized water until neutral, and then dried in a 60℃ oven to constant weight to obtain the purified white clay gypsum product. In the white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.51%, the mass percentage of C decreased from 1.09% to 0.196%, and the whiteness increased from 78.63% to 89.12%.

[0083] Effect Analysis: Example 3 achieved efficient and simultaneous removal of magnesium impurities (MgO mass percentage decreased from 5.69% to 0.51%) and organic carbon impurities (C mass percentage decreased from 1.09% to 0.196%) from bleached gypsum through synergistic treatment of concentrated sulfuric acid acidification and hydrogen peroxide oxidation. At higher pH and lower acid concentrations, the MgO dissolution efficiency decreased. To compensate for the reduced oxidation efficiency due to increased pH, the amount of H2O2 was increased, thus still ensuring excellent organic matter removal. The entire process was carried out at low temperature, with a short and simple procedure, and the resulting product had significantly improved purity, making it suitable for high-value-added resource utilization. The whiteness of the bleached gypsum improved after acidification and oxidation, increasing from 78.63% to 89.12%.

[0084] Example 4 The purification process in Example 4 was basically the same as in Example 1, except that 7.5 g of a 30% hydrogen peroxide solution (equivalent to 2.25% of the dry weight of the white clay gypsum) was added in two separate additions: First, 70% (5.25 g) of the total mass was added, accompanied by ultrasonic treatment (300 W power) while adding H2O2. The hydrogen peroxide solution was added dropwise over 2 minutes, and the reaction was continued with stirring for 20 minutes after the addition was complete. Second, the remaining 30% (2.25 g) was added, again with ultrasonic assistance (300 W power), the hydrogen peroxide solution was added dropwise over 1 minute, and the reaction was continued with stirring for 10 minutes after the addition was complete. The total oxidation reaction time was 30 minutes.

[0085] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.32%, the mass percentage of C decreased from 1.09% to 0.200%, and the whiteness increased from 78.63% to 88.96%.

[0086] Effect Analysis: Example 4 was conducted under the exact same reaction conditions as Example 1 (liquid-to-solid ratio, temperature, pH, rotation speed, total reagent volume), except that hydrogen peroxide was added in two stages (70% + 30%) instead of all at once. The mass percentage of organic carbon (C) decreased from 0.218% in Example 1 to 0.200%, demonstrating that the staged addition strategy utilizes the oxidant more effectively, possibly achieving more thorough oxidative degradation by first degrading easily oxidizable organic matter and then attacking difficult-to-degrade organic matter. The mass percentage of MgO was 0.32%, at the same excellent level as Example 1 (0.31%), indicating that the acidification step was stable and that the staged addition of the oxidant did not affect the dissolution of inorganic impurities. The whiteness of the product was further improved, increasing from 87.23% in Example 1 to 88.96%.

[0087] Example 5 The purification process in Example 5 is basically the same as that in Example 1, except that ultrasound is not applied when adding H2O2, while the other conditions are the same.

[0088] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.41%, the mass percentage of C decreased from 1.09% to 0.248%, and the whiteness increased from 78.63% to 86.87%.

[0089] Effect analysis: Without the use of ultrasound assistance, the removal efficiency of magnesium oxide decreased, with the MgO mass percentage being 0.41%, higher than 0.31% in Example 1. The degradation efficiency of organic matter also decreased, with the C mass percentage being 0.248%, higher than 0.218% in Example 1. This indicates that ultrasound can enhance the treatment effect of oxidants on impurities mixed into the gypsum system through co-precipitation, adsorption, or encapsulation.

[0090] Comparative Example 1 The purification process of Comparative Example 1 is basically the same as that of Example 1, except that oxidation is not performed, hydrogen peroxide solution is not added in step S2, and solid-liquid separation is performed directly after acidification reaction.

[0091] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.59%, the mass percentage of C decreased from 1.09% to 0.76%, and the whiteness decreased from 78.63% to 73.15%.

[0092] Effect Analysis: Comparative Example 1 omitted the hydrogen peroxide oxidation step. The results showed that the removal of magnesium impurities (MgO) mainly relied on the acidification step, and its mass percentage decreased from 5.69% to 0.59%, but it was still higher than 0.31% in Example 1 of this invention, indicating that the oxidation step also had a certain auxiliary effect on the complete dissolution of inorganic impurities. Most importantly, the removal effect of organic matter was extremely poor. Even with the local oxidation effect of concentrated sulfuric acid, the mass percentage of organic carbon (C) was still as high as 0.76%, which was much higher than that of the product of this invention (≤0.25%). Moreover, the whiteness of the white clay gypsum that was only acidified actually decreased to 73.15%, which was caused by the coloring of residual organic matter, the coloring of ferric ions, and the coloring of their complex.

[0093] Comparative Example 2 The purification process of Comparative Example 2 was basically the same as that of Example 1, except that the order of step S2 was changed: the obtained slurry was heated to 50°C and maintained at a constant temperature. Without first adjusting the pH with concentrated sulfuric acid, 7.5 g of a 30% hydrogen peroxide solution was directly added dropwise, accompanied by ultrasonic treatment (300W power). The stirring speed for the oxidation reaction was 600 rpm. The hydrogen peroxide solution was added dropwise over 3 minutes. After the addition was complete, the reaction was stirred for another 30 minutes. Then, concentrated sulfuric acid was added dropwise over another 3 minutes. The pH was adjusted to 2.0, and the reaction was stirred for another 30 minutes after the addition was complete.

[0094] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.45%, the mass percentage of C decreased from 1.09% to 0.47%, and the whiteness increased from 78.63% to 83.55%.

[0095] Effect Analysis: Comparative Example 2 changed the order of the core processes. When H2O2 was added first, the system pH was high (usually a neutral or weakly alkaline environment), resulting in a low oxidation potential of hydrogen peroxide and a lack of Fe dissolved from the acidification step. 3+ The catalyst cannot form an effective Fenton-like reaction system, resulting in limited oxidation capacity. Most of the H₂O₂ may undergo ineffective decomposition, making it difficult to effectively attack and degrade organic matter. Although the subsequent addition of concentrated sulfuric acid can lower the pH of the system and dissolve Fe, 3+ However, by this time, a large amount of the previously underutilized H2O2 has been consumed, and the remaining oxidant is insufficient to support a complete Fenton-like cycle, resulting in incomplete degradation of organic matter (C mass percentage: 0.47%). Simultaneously, because the organic matter has not been effectively oxidized and removed, some magnesium impurities may still be encapsulated within the intact organic matter or gypsum lattice, affecting their dissolution efficiency in the subsequent acidification process (MgO mass percentage: 0.45%). Therefore, the sequence of oxidation followed by acidification cannot fully leverage the synergistic impurity removal advantages of "acidification-oxidation".

[0096] Comparative Example 3 The purification process of Comparative Example 3 is basically the same as that of Example 1, except that the acidification and oxidation reaction temperatures in step S2 are increased to 95°C (higher than the phase transition temperature of gypsum dihydrate).

[0097] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 0.25%, the mass percentage of C decreased from 1.09% to 0.18%, and the whiteness increased from 78.63% to 90.11%.

[0098] Effect Analysis: Although Comparative Example 3 increased the reaction rate through high temperature, resulting in a magnesium and organic matter removal effect comparable to or even slightly better than Example 1 (MgO 0.25%, C 0.18%), ... Figure 2XRD phase analysis showed that the final product had undergone a severe phase transition. The dihydrate gypsum (CaSO4·2H2O) had partially transformed into hemihydrate gypsum, and was no longer pure dihydrate gypsum. This disrupted the crystal structure, resulting in poor product stability, easy moisture absorption or curing, and rendering it unsuitable for applications with strict requirements on phase composition, such as medical gypsum and high-precision molds.

[0099] Comparative Example 4 The purification process of Comparative Example 4 is basically the same as that of Example 1, except that the concentrated sulfuric acid with a mass concentration of 98% in the acidification and oxidation reactions of step S2 is replaced with dilute sulfuric acid with a mass concentration of 30%.

[0100] In the purified white clay gypsum product, the mass percentage of MgO decreased from 5.69% to 1.14%, the mass percentage of C decreased from 1.09% to 0.62%, and the whiteness increased from 78.63% to 81.23%.

[0101] Effect Analysis: In Comparative Example 4, the removal efficiency of magnesium oxide and the degradation efficiency of organic matter decreased significantly when dilute sulfuric acid was used instead of concentrated sulfuric acid. The reason for using concentrated sulfuric acid is that its multiple synergistic effects in the system far exceed its simple pH adjustment function. The high concentration of concentrated sulfuric acid produces localized over-acidification and exothermic effects, which not only efficiently dissolve iron oxides in the white mud (providing the necessary Fe for subsequent reactions)... 3+ As a catalyst, concentrated sulfuric acid can significantly promote the dissolution and transformation of inorganic impurities such as magnesium. Simultaneously, concentrated sulfuric acid itself possesses strong oxidizing properties and can directly act on organic matter under heating conditions, achieving preliminary degradation through reactions such as sulfonation and dehydration, effectively reducing the load on subsequent oxidation stages. These effects lay a solid foundation for the subsequent construction of a highly efficient Fenton-like reaction system using hydrogen peroxide, which is crucial for achieving deep degradation of organic matter and simultaneous removal of inorganic impurities. If dilute sulfuric acid is used instead, although the pH can be adjusted by increasing the dosage, it cannot provide an effective localized superacidic environment to dissolve key catalysts and impurities, nor does it possess significant direct oxidizing capacity. Furthermore, it introduces a large amount of water, leading to excessive dilution of the reaction system, ultimately resulting in incomplete removal of organic matter, low reaction efficiency, and a significant increase in treatment costs.

[0102] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for purifying white clay gypsum, characterized in that, Includes the following steps: S1. Slurry preparation and dispersion: Mix the white clay gypsum with water and stir to form a homogeneous slurry; S2. Acidification and oxidation: Concentrated sulfuric acid is added to the slurry for acidification and stirring; then hydrogen peroxide solution is added for oxidation, and stirring continues. S3. Solid-liquid separation: After the reaction is completed, solid-liquid separation is performed, the solid product is collected, and after washing and drying, the purified white clay gypsum is obtained. The white clay gypsum includes dihydrate gypsum, magnesium, iron, and organic impurities.

2. The method according to claim 1, characterized in that, The iron in the white clay gypsum exists in the form of Fe2O3; preferably, the mass percentage of Fe2O3 is greater than 0.1%.

3. The method according to claim 1, characterized in that, In step S1, the volume ratio of water to the mass of the white clay gypsum is (5~10):1 (mL / g); and / or, The mixing time is 10-20 minutes, and the speed is 500-800 rpm.

4. The method according to claim 1, characterized in that, In step S2, Concentrated sulfuric acid is added to the slurry for acidification to adjust the pH of the system to 2-5; and / or, The acidification reaction is carried out at a temperature of 50-80°C; and / or, The concentrated sulfuric acid is added dropwise, with a mass concentration of 95-98%; and / or, The concentrated sulfuric acid is 14%-20% of the dry weight of the white clay gypsum; and / or, The oxidation reaction is carried out at a temperature of 50-80℃; and / or, The hydrogen peroxide solution is added dropwise, with a mass concentration of 25-35%; and / or, The mass of the hydrogen peroxide is 2.25-4% of the dry weight of the white clay gypsum.

5. The method according to claim 1, characterized in that, In step S2, the stirring speed for the acidification reaction is 600-900 rpm, the concentrated sulfuric acid is added dropwise over 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is complete; and / or, The stirring speed for the oxidation reaction is 600-900 rpm. The hydrogen peroxide solution is added dropwise over 3-5 minutes, and the reaction continues for 20-40 minutes after the addition is complete.

6. The method according to claim 1, characterized in that, In step S2, the hydrogen peroxide solution is added in two parts. The first part is 60% to 80% of the total mass, and after reacting for 15 to 25 minutes, the remaining part is added. Preferably, the first addition of hydrogen peroxide solution is completed within 2-4 minutes, and the reaction continues for 15-25 minutes after the addition is completed; the remaining hydrogen peroxide solution is completed within 1-3 minutes, and the reaction continues for 10-20 minutes after the addition is completed.

7. The method according to claim 1, characterized in that, In step S2, while adding hydrogen peroxide solution, ultrasonic treatment is also performed, with an ultrasonic power of 300-500W.

8. The method according to claim 1, characterized in that, In step S3, the solid-liquid separation is performed by a combination of decantation and filtration. Preferably, the decantation settling time is 40-80 min; and / or, The washing process involves rinsing the filter cake with deionized water; and / or, The drying process involves drying at 50-70°C to a constant weight.

9. A white clay gypsum purified by any one of claims 1-8, characterized in that, The white clay gypsum contains magnesium oxide (MgO) at a mass percentage of ≤0.6%, carbon (C) at a mass percentage of ≤0.25%, and whiteness ≥85%.

10. The white clay gypsum according to claim 9, characterized in that, The bleached gypsum has a crystal structure of calcium sulfate dihydrate, without undergoing phase transformation or structural damage; the bleached gypsum is suitable for use in building materials, mold preparation, or medical gypsum applications.

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