Method for deep iron removal of high-iron gypsum and preparation of high-value gypsum products

Abstract

The present application relates to low-cost high-value comprehensive utilization of high-iron gypsum, and relates to a method for deep iron removal of high-iron gypsum, in particular to a method for deep iron removal of high-iron gypsum and preparation of high-quality high-value gypsum at the same time. The present application firstly removes part of iron in titanium gypsum through acid leaching treatment, then fully releases iron impurities inside gypsum inclusions and intergranular gaps through a dissolution process of gypsum phase regulation, simultaneously regulates the existing form of iron in the solution to avoid its entering into the gypsum lattice in the process of recrystallization, and adjusts the morphology and particle size of the recrystallization product by adding a crystal form regulator to weaken the adsorption of iron on the surface of the gypsum product, finally realizes the efficient separation of iron and gypsum product by releasing iron and weakening the adsorption effect of iron, and forms high-purity high-strength alpha-hemihydrate gypsum. The present application realizes the effective combination of deep iron removal in high-iron gypsum and the preparation process of high-quality high-value gypsum product.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of chemical solid waste treatment and disposal, and particularly relates to a method for deeply and efficiently removing iron impurities in high-iron gypsum and converting the iron impurities into high-value alpha-hemihydrate gypsum. BACKGROUND

[0002] High-iron gypsum mainly refers to titanium gypsum, which is an industrial solid waste discharged in the production of titanium dioxide by the sulfuric acid method. The main component is calcium sulfate dihydrate, and about 5-6 tons of titanium gypsum are generated per ton of titanium dioxide produced. At present, the discharge amount of titanium gypsum in China is about 18 million tons per year, and the utilization rate is only 10%. A large amount of idle titanium gypsum is piled up, which causes serious pollution to the surrounding soil, water and air, and ultimately affects human health. In the process of producing titanium dioxide by the sulfuric acid method of titanium concentrate, iron in the raw material will enter the titanium white waste acid with the liquid phase, and the titanium white waste acid is converted into titanium gypsum containing iron after lime neutralization treatment. The existence of iron impurities makes the titanium gypsum appear dark green or red after iron oxidation, which affects the appearance and performance of the titanium gypsum, greatly increasing the difficulty of resource utilization of the titanium gypsum. Therefore, the main use of titanium gypsum is still limited to cement retarder, wall material, composite cementitious material, etc., with narrow application range and low product added value. Therefore, deep removal of iron impurities in titanium gypsum can greatly broaden and deepen the range and level of its resource utilization.

[0003] Patent CN116655270A discloses a method for preparing building gypsum from titanium gypsum pretreated by sulfuric acid method acid wastewater. After acid washing treatment of the titanium gypsum, building gypsum is obtained through calcination, aging and modification of external agents, which improves the resource utilization rate of titanium gypsum to some extent, but calcination brings higher energy consumption, and the obtained building gypsum mainly contains β-hemihydrate gypsum with low value.

[0004] Patent CN114349039A discloses a method for regulating the morphology and particle size of alpha-hemihydrate gypsum in a solution system. The method regulates the preparation of alpha-hemihydrate gypsum with high purity and high strength by adding crystal seeds, but the patent does not involve step-by-step removal of iron and control of the appropriate range of iron during the removal process. At the same time, it is reported in the prior art (such as patent CN117626398A) that titanium gypsum is different from desulfurization gypsum. The impurities such as iron hydroxide, magnesium hydroxide, metatitanic acid and aluminum hydroxide in desulfurization gypsum are very few, while a large amount of impurities such as iron, silicon, titanium, magnesium and aluminum exist in titanium gypsum. These impurities do not exist in the form of free ions, but exist in the form of a large amount of hydroxide, oxide solid particles and / or colloid. These impurities will have a strong common inhibition effect on the preparation of hemihydrate gypsum whiskers from titanium gypsum.

[0005] Patent CN117626398A discloses a method for preparing hemihydrate gypsum whiskers from titanium gypsum in a glycol-water system. In this patent, the titanium gypsum is first purified by sulfuric acid to remove impurities, and the content of iron, silicon, titanium, magnesium and aluminum is controlled to be 1-5 mg / g, 0.5-2 mg / g, 0.5-1 mg / g, 0.1-0.5 mg / g and 0-0.05 mg / g, respectively. Then, the purified titanium gypsum is mixed with a salt solution to form a suspension with a content of 1.5wt%-3wt%. Finally, the suspension is reacted at 90-100℃ for a long time (e.g. 5-8 hours as exemplified in the examples) to obtain hemihydrate gypsum whiskers.

[0006] The present application is based on the discovery that when the raw material high-iron gypsum (titanium gypsum) is pretreated in an inorganic acid solution to a iron content of 1.0wt%-5.0wt%, the formation of hemihydrate gypsum can be accelerated in a salt solution system with a pH of 1.0-2.5. Based on this discovery, the present application is formed.

[0007] The present application provides a method for deep removal of iron impurities from high-iron gypsum and preparation of high-quality high-value gypsum products. The method is simple, low-cost, and can effectively remove iron impurities from the interior of gypsum inclusions and between particles. It also overcomes some technical biases in the field. SUMMARY

[0008] Therefore, the present application provides a method for deep removal of iron impurities from high-iron gypsum and preparation of high-quality high-value gypsum products.

[0009] As a preferred embodiment, the present application provides a method for deep removal of iron from high-iron gypsum and preparation of high-value gypsum products, comprising the following steps:

[0010] (1) The raw material high-iron gypsum is pretreated in an inorganic acid solution, and after sufficient stirring for a period of time, the solid-liquid separation is performed. The separated solid phase is the pretreated high-iron gypsum, and the iron content in the pretreated high-iron gypsum is 1.0wt%-5.0wt%. The iron content in the raw material high-iron gypsum is >5wt% in step (1);

[0011] (2) The pretreated high-iron gypsum is added to a salt solution system, and after mixing, a gypsum suspension slurry is formed. In the salt solution system, the salt is selected from at least one of soluble sulfate, hydrochloride and nitrate, and the preferred salt is sulfate. The concentration of the salt in the salt solution system is 10wt%-20wt%. The solid content in the obtained gypsum suspension slurry is 5%-30%, and the pH value of the salt solution is 1.0-2.5;

[0012] (3) heating the gypsum suspension slurry to 90℃ or above, adding a crystal form regulator, and stirring the reaction at this temperature, the crystal form regulator being short columnar α-hemihydrate gypsum crystals, the particle size of the crystal form regulator being 12-65μm;

[0013] (4) after a period of time, separating the suspension slurry while hot, washing the separated solid phase with boiling water and drying to obtain a high-purity high-value gypsum product.

[0014] The separated liquid phase is returned to the water solution system after treatment.

[0015] As a preferred embodiment, the iron content in the raw high-iron gypsum is <25wt% and >5wt%, and the calcium sulfate dihydrate (CaSO4·2H2O) content is >70%.

[0016] As a preferred embodiment, the raw high-iron gypsum can be titanium gypsum.

[0017] As a preferred embodiment, the inorganic acid solution in step (1) is one or several of sulfuric acid, hydrochloric acid or nitric acid, preferably sulfuric acid, the pH value of the inorganic acid solution being 1.0-5.0, preferably 1.0-4.0, further preferably 1.0-3.0, and more further preferably 1.5-2.5.

[0018] As a preferred embodiment, the pre-treatment temperature in step (1) is ≤40℃, preferably 20-40℃, the pre-treatment solid-liquid ratio (g / mL) is 1 / 5-1 / 20, preferably 1 / 7-1 / 20, and the pre-treatment time is 15-60min.

[0019] In the present application, the temperature during pre-treatment should not be too high, otherwise it is not conducive to the control of the iron content in the pre-treated high-iron gypsum. The hydrogen ion concentration in the inorganic acid solution during pre-treatment should not be too high, otherwise it is also not conducive to the control of the iron content in the pre-treated high-iron gypsum.

[0020] In step (1) of the present application, a large amount of inorganic acid with low concentration is used to controllably pre-treat the raw high-iron gypsum (titanium gypsum), to preliminarily remove part of the iron impurities in the high-iron gypsum, and to keep the iron content in the high-iron gypsum within a certain range. The advantages of this are that too high iron content in the high-iron gypsum will hinder the subsequent phase change reaction, making the process of converting dihydrate gypsum into α-hemihydrate gypsum impossible, and too low iron content in the high-iron gypsum will affect the phase change rate, causing the phase change time to be prolonged, or even the phase change to be impossible.

[0021] The iron content in the pretreated high-iron gypsum in step (2) is greater than or equal to 1.0 wt%, preferably between 1.0 wt% and 5.0 wt%, the pH value of the salt solution is 1.0-2.5, and the solid content (mass percentage) in the gypsum suspension slurry is 5%-30%, preferably 15-30%, and further preferably 20-25%.

[0022] As a preferred solution, the liquid phase separated in step (1) can be reused in the method for recovering titanium in high-iron gypsum and preparing high-value gypsum products.

[0023] The iron content in the pretreated high-iron gypsum obtained in step (1) is 1.0 wt%-3.0 wt%.

[0024] The solid content (mass percentage) in the gypsum suspension slurry in step (2) is 5%-30%, preferably 15-30%, and further preferably 20-25%.

[0025] In step (2) of the present application, the concentration of the salt solution system is controlled to be 10 wt%-20 wt%, and the solid content (mass percentage) in the obtained gypsum suspension slurry is controlled to be 5%-30%, preferably 15-30%, and further preferably 20-25%, in order to ensure that the phase transition process of dihydrate gypsum to α-hemihydrate gypsum proceeds normally and high-quality low-iron content gypsum products can be obtained, and the treatment process cost is controlled. Too low salt concentration will result in the failure of the phase transition process of dihydrate gypsum to α-hemihydrate gypsum, and too high salt concentration will result in excessive impurity salt ions entering the final gypsum product, reducing the quality of the gypsum product. Too high solid content will reduce the removal efficiency of iron impurities, and it is impossible to achieve deep iron removal, and too low solid content will result in excessive acidic liquid, increasing the treatment cost.

[0026] In step (3), the reaction temperature is 90-105°C, preferably 95-100°C, and the stirring rate is 100-300 rpm.

[0027] In step (3), the addition amount of the crystal form regulator is 9.5%-30% of the mass of the pretreated high-iron gypsum, preferably 10%-20%, and further preferably 11.5-17%.

[0028] In step (4), the reaction time is 30 min-60 min.

[0029] As a preferred scheme, the method for deep iron removal of high-iron gypsum and preparation of high-value gypsum product simultaneously according to the application, the crystal form regulator is short columnar alpha-hemihydrate gypsum crystal with regular morphology, the average length-diameter ratio is less than or equal to 4.0, the adding amount is 9.5%-30% of the mass of the pretreated high-iron gypsum, and the particle size of the crystal form regulator is preferably 20-50 μm. The main reason for controlling the morphology and particle size of the crystal form regulator is that the morphology and size of the crystal form regulator directly affect the morphology and size of the gypsum product. The ideal gypsum product is a short columnar crystal with large particle size. On the one hand, the specific surface area is small, and the adsorption is weak, which inhibits the surface adsorption of iron impurities on the gypsum product. On the other hand, the standard consistency water demand is low, and the mechanical strength is high, which is a high-value high-strength gypsum product. The crystal form regulator with small length-diameter ratio can induce short columnar gypsum product with small length-diameter ratio, and the crystal form regulator with large particle size can induce gypsum product with large particle size. However, the large particle size of the crystal form regulator requires a large amount of addition, which is not reasonable in terms of cost. Therefore, it is necessary to control the morphology and size of the crystal form regulator. In the application, if the crystal form regulator is added too little, it is difficult to play a synergistic role with the reserved iron. If the adding amount is too large, the residual iron in the product will increase, thereby affecting the performance of the final product.

[0030] As a preferred scheme, the method for deep iron removal of high-iron gypsum and preparation of high-value gypsum product simultaneously according to the application, if the gypsum product generated in the process is short columnar alpha-hemihydrate gypsum crystal with regular morphology, the average length-diameter ratio is less than or equal to 4.0, and the particle size is 20-50 μm, the gypsum product can also be used as a crystal form regulator.

[0031] The iron content in the gypsum product obtained by the application is less than 5 mg / g, and the iron content is less than 2 mg / g after optimization.

[0032] The drying compressive strength of the gypsum product obtained by the application is greater than 20 MPa, and the drying compressive strength can be greater than 40 MPa after optimization.

[0033] The application first removes part of the iron impurities in the high-iron gypsum by acid leaching pretreatment, then fully releases the iron impurities in the gypsum inclusions and the crystal lattice by the dissolution process of gypsum phase regulation, controls the existence form of iron in the solution to avoid the iron from entering the gypsum crystal lattice again in the recrystallization process, adjusts the morphology and particle size of the recrystallization product by adding the crystal form regulator to weaken the adsorption of iron on the surface of the gypsum product, and finally realizes the efficient separation of iron impurities and gypsum product by releasing the iron impurities and weakening the eutectic and adsorption effects of iron, and forms high-purity high-strength alpha-hemihydrate gypsum. The application realizes the deep iron removal of high-iron gypsum and the preparation of high-value gypsum product simultaneously.

[0034] Basic principle

[0035] The present application creatively combines sulfuric acid leaching and phase reconstruction, first uses sulfuric acid leaching to pretreat high-iron gypsum, reduces the iron content to a certain range, and then removes the iron impurities inside the gypsum inclusions through phase reconstruction, finally realizes rapid and deep removal of iron impurities in high-iron gypsum.

[0036] In the sulfuric acid leaching pretreatment process, the iron content in high-iron gypsum is reduced to ensure the normal progress of the phase transition of dihydrate gypsum to alpha-hemihydrate gypsum, and at the same time, the iron impurities in high-iron gypsum are ensured to have a certain content (such as 1.0wt%-5.0wt%), so that the iron content cannot be too low. Because the existence of iron impurities can provide nucleation sites to a certain extent, it is beneficial to the formation of hemihydrate gypsum crystal nucleus, promotes the progress of the phase transition reaction, improves the phase transition rate, and shortens the phase transition time.

[0037] Under normal pressure conditions, dihydrate gypsum can be phase transitioned into alpha-hemihydrate gypsum in a salt solution at a certain temperature (above 90℃). This process is a dissolution-recrystallization process. The dissolution process releases the iron impurities in the dihydrate gypsum inclusions into the liquid phase. In the recrystallization process, the particle size and morphology of the gypsum crystal product are controlled by adding a crystal type regulator to make it form a large particle size crystalline product with regular morphology, thereby effectively weakening the adsorption of iron in the liquid phase on the surface of the recrystallized gypsum product. On the other hand, the addition of acid can effectively dissolve the iron impurities, and on the other hand, it can control the existence form of iron in the solution. Iron mainly exists in the form of FeSO4 + and Fe(SO4)2 － in the solution, which makes it difficult to enter the crystal lattice during the recrystallization of alpha-hemihydrate gypsum, thereby promoting the migration of iron impurities to the solution and further improving the removal rate of iron. Based on the complete release of iron impurities by phase transformation, the weakening of iron surface adsorption by crystal type regulation, and the inhibition of iron entering the crystal lattice by iron existence form regulation, the deep removal of iron impurities in high-iron gypsum is realized, and at the same time, high-value alpha-hemihydrate gypsum product is produced.

[0038] Advantages of the present application

[0039] The present application can realize the deep removal of iron in high-iron gypsum, and the iron content in the obtained gypsum product is <5mg / g, and the iron content is <2mg / g after optimization.

[0040] The product obtained by the present application is a high-value gypsum product, and the drying compressive strength is >20MPa, and the drying compressive strength can be >40MPa after optimization.

[0041] The present application realizes the unity of deep iron removal and high-value gypsum product preparation of high-iron gypsum, and the conversion rate of alpha-hemihydrate gypsum is greater than or equal to 99%. BRIEF DESCRIPTION OF DRAWINGS

[0042] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application, but are not intended to limit the application.

[0043] Figure 1 A flow chart of the process of the application;

[0044] Figure 2 Microscopic morphology of the crystal habit modifier used in Examples 1, 2 and Comparative Examples 1, 4, 5;

[0045] Figure 3 Microscopic morphology of the gypsum product obtained in Example 1;

[0046] Figure 4 Microscopic morphology of the gypsum product obtained in Example 2;

[0047] Figure 5 Microscopic morphology of the gypsum product obtained in Comparative Example 1;

[0048] Figure 6 Microscopic morphology of the gypsum product obtained in Comparative Example 2;

[0049] Figure 7 Microscopic morphology of the gypsum product obtained in Comparative Example 3;

[0050] Figure 8 Microscopic morphology of the gypsum product obtained in Comparative Example 4;

[0051] Figure 9 Microscopic morphology of the gypsum product obtained in Comparative Example 5;

[0052] Figure 10 Microscopic morphology of the crystal habit modifier used in Examples 3, 4 and Comparative Examples 6, 7, 8;

[0053] Figure 11 Microscopic morphology of the gypsum product obtained in Example 3;

[0054] Figure 12 Microscopic morphology of the gypsum product obtained in Comparative Example 6;

[0055] Figure 13 Microscopic morphology of the gypsum product obtained in Example 4;

[0056] Figure 14 Microscopic morphology of the gypsum product obtained in Comparative Example 7;

[0057] Figure 15 Microscopic morphology of the gypsum product obtained in Comparative Example 8.

[0058] From Figure 1 The basic flow of the application can be seen.

[0059] The micro-morphology of the crystal form regulator can be seen from Figure 2 The gypsum product obtained in Example 1 is a short columnar crystal with uniform distribution.

[0060] The micro-morphology of the crystal form regulator can be seen from Figure 3 The gypsum product obtained in Example 1 is a short columnar crystal with uniform distribution.

[0061] The micro-morphology of the crystal form regulator can be seen from Figure 4 The gypsum product obtained in Example 2 is a short columnar crystal with uniform distribution.

[0062] The micro-morphology of the crystal form regulator can be seen from Figure 5 The gypsum product obtained in Comparative Example 1 is a fine plate-shaped or massive crystal.

[0063] The micro-morphology of the crystal form regulator can be seen from Figure 6 The gypsum product obtained in Comparative Example 2 is a long rod-shaped crystal.

[0064] The micro-morphology of the crystal form regulator can be seen from Figure 7 The gypsum product obtained in Comparative Example 3 is a long rod-shaped crystal.

[0065] The micro-morphology of the crystal form regulator can be seen from Figure 8 The gypsum product obtained in Comparative Example 4 is mainly a plate-shaped crystal, and occasionally a columnar crystal, indicating that most of the crystals have not undergone phase transition.

[0066] The micro-morphology of the crystal form regulator can be seen from Figure 9 The gypsum product obtained in Comparative Example 5 is a long rod-shaped crystal.

[0067] The micro-morphology of the crystal form regulator can be seen from Figure 10 The gypsum product obtained in Example 3 is a short columnar crystal with uniform distribution.

[0068] The micro-morphology of the crystal form regulator can be seen from Figure 11 The gypsum product obtained in Example 3 is a short columnar crystal with uniform distribution.

[0069] The micro-morphology of the crystal form regulator can be seen from Figure 12 The gypsum product obtained in Comparative Example 6 is a long rod-shaped and short columnar crystal with extremely uneven distribution.

[0070] The micro-morphology of the crystal form regulator can be seen from Figure 13 The gypsum product obtained in Example 4 is a short columnar crystal with uniform distribution.

[0071] The micro-morphology of the crystal form regulator can be seen from Figure 14 The gypsum product obtained in Comparative Example 7 is mainly a plate-shaped crystal, and occasionally a columnar crystal, indicating that most of the crystals have not undergone phase transition.

[0072] The micro-morphology of the crystal form regulator can be seen from Figure 15 The gypsum product obtained in Comparative Example 8 is a short columnar and fine needle-shaped crystal interlaced with each other. Detailed Implementation

[0073] The technical solution of this application will be further described below with reference to specific implementation examples, but the scope of protection of this invention is not limited thereto.

[0074] Example 1

[0075] Table 1. Main elemental composition and proportion in raw material high-iron gypsum (titanium gypsum)

[0076]

[0077] according to Figure 1 The process flow shown involves preparing 600 mL of a sulfuric acid solution with pH=2 and adding it to a pretreatment tank. Then, 60 g of ferrous gypsum (elemental composition as shown in Table 1) is added, and the mixture is stirred and leached at room temperature for 30 min. After solid-liquid separation, pretreated ferrous gypsum (iron content 1.30 wt%) is obtained. A 12 wt% sodium sulfate solution is prepared, and its pH is adjusted to 1 with sulfuric acid. 50 g of pretreated ferrous gypsum is added to the sodium sulfate solution and mixed thoroughly to form a suspension with a solid mass fraction of 20%. The suspension is added to a three-necked round-bottom flask, and the reaction temperature is set to 95℃ with a stirring speed of 120 rpm. Once the temperature of the suspension reaches the set temperature, 8 g of a crystal form regulator (e.g., ...) is added. Figure 2 The crystal form regulator was a regularly shaped α-hemihydrate gypsum crystal with an average aspect ratio of 0.6 and a particle size of 35 μm. After reacting for 45 min, solid-liquid separation was immediately performed while still hot. The separated solid phase was washed with boiling water and transferred to a 60℃ oven to dry to constant weight, yielding the gypsum product with the following microstructure: Figure 3 As shown, its iron content is 1.73 mg / g, its dry compressive strength is 40.36 MPa, and its conversion rate of α-hemihydrate gypsum is as high as 99.11%.

[0078] Conversion rate calculation method:

[0079]

[0080] Where, m a m is the theoretical mass of the raw material when it is completely converted into α-hemihydrate gypsum. 原 M represents the weight of the raw materials. g M is the relative molecular mass of gypsum dihydrate. b is the relative molecular mass of α-hemihydrate gypsum.

[0081]

[0082] Where x is the conversion rate, m a m is the theoretical mass of the raw material when it is completely converted into α-hemihydrate gypsum. b The mass of α-hemihydrate gypsum in the final product.

[0083] m 原 = 60 g, M g = 172 g / mol, M b = 145 g / mol, m b = 58.06 g Substituting into the above equation, the conversion rate x is 99.11%.

[0084] Example 2

[0085] According to the process shown in Figure 1 , 600 mL of a sulfuric acid solution with pH = 3 is added to a pretreatment tank, 100 g of high-iron gypsum (elemental composition as shown in Table 1) is added thereto, and stirring leaching is performed at room temperature for 30 min. After solid-liquid separation, pretreated high-iron gypsum (iron content of 1.56 wt%) is obtained. A 16 wt% potassium sulfate solution is prepared, and its pH is adjusted to 2 with sulfuric acid. The pretreated high-iron gypsum 50 g is added to the potassium sulfate solution to form a suspension slurry with a solid mass fraction of 25%. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 97°C, and the stirring rate is 100 rpm. After the temperature of the suspension slurry reaches the set temperature, 6 g of a crystal habit modifier (as shown in Figure 2 , the crystal habit modifier is a regular α-hemihydrate gypsum crystal with an average aspect ratio of 0.6 and a particle size of 35 μm) is added. After 50 min of reaction, solid-liquid separation is immediately performed while hot. The separated solid phase is washed with boiling water and transferred to a 60°C oven for drying to constant weight. The micro-morphology of the obtained gypsum product is shown in Figure 4 , the iron content is 3.46 mg / g, and the oven-dried compressive strength is 37.83 MPa.

[0086] Comparative Example 1

[0087] According to the process shown in Figure 1 , 600 mL of deionized water is added to a pretreatment tank, 60 g of high-iron gypsum (elemental composition as shown in Table 1) is added thereto, and stirring leaching is performed at room temperature for 30 min. After solid-liquid separation, pretreated high-iron gypsum (iron content of 6.67 wt%) is obtained. A 12 wt% sodium sulfate solution is prepared, and its pH is adjusted to 1 with sulfuric acid. The pretreated high-iron gypsum 50 g is added to the sodium sulfate solution to form a suspension slurry with a solid mass fraction of 20%. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95°C, and the stirring rate is 120 rpm. After the temperature of the suspension slurry reaches the set temperature, 8 g of a crystal habit modifier (as shown in Figure 2 , the crystal habit modifier is a regular α-hemihydrate gypsum crystal with an average aspect ratio of 0.6 and a particle size of 35 μm) is added. After 45 min of reaction, solid-liquid separation is immediately performed while hot. The separated solid phase is washed with boiling water and transferred to a 60°C oven for drying to constant weight. The micro-morphology of the obtained gypsum product is shown inFigure 5 As shown, its iron content is 36.37 mg / g, its main component is gypsum dihydrate, it has no gelling properties and no compressive strength.

[0088] Comparative Example 2

[0089] according to Figure 1 The process flow is as follows: 600 mL of a sulfuric acid solution with pH=2 is added to a pretreatment tank. 60 g of high-ferrous gypsum (elemental composition as shown in Table 1) is then added, and the mixture is stirred and leached at room temperature for 30 min. After solid-liquid separation, pretreated high-ferrous gypsum (iron content 1.30 wt%) is obtained. A 12 wt% sodium sulfate solution is prepared, and its pH is adjusted to 1 with sulfuric acid. 50 g of pretreated high-ferrous gypsum is added to the sodium sulfate solution and mixed thoroughly to form a suspension with a solid mass fraction of 20%. The suspension is added to a three-necked round-bottom flask, and the reaction temperature is set at 95℃. The stirring speed is 120 rpm, and no crystal form regulator is added. After reacting for 45 min, solid-liquid separation is immediately performed while hot. The separated solid phase is washed with boiling water and transferred to a 60℃ oven to dry to constant weight, yielding the microstructure of the gypsum product as shown in Table 1. Figure 6 As shown, its iron content is 8.66 mg / g, and its dry compressive strength is 8.62 MPa.

[0090] Comparative Example 3

[0091] according to Figure 1 The process flow is as follows: 600 mL of a sulfuric acid solution with pH=2 is added to a pretreatment tank. 60 g of high-ferrous gypsum (elemental composition as shown in Table 1) is then added, and the mixture is stirred and leached for 30 min at room temperature. After solid-liquid separation, pretreated high-ferrous gypsum (iron content 1.30 wt%) is obtained. A 12 wt% sodium sulfate solution is prepared, and its pH is adjusted to 3 with sulfuric acid. 50 g of pretreated high-ferrous gypsum is added to the sodium sulfate solution and mixed thoroughly to form a suspension with a solid mass fraction of 20%. The suspension is added to a three-necked round-bottom flask, and the reaction temperature is set at 95℃. The stirring speed is 120 rpm, and no crystal form regulator is added. After reacting for 45 min, solid-liquid separation is immediately performed while hot. The separated solid phase is washed with boiling water and transferred to a 60℃ oven to dry to constant weight, yielding the microstructure of the gypsum product as shown in Table 1. Figure 7 As shown, its iron content is 10.39 mg / g, and its dry compressive strength is 5.50 MPa.

[0092] Comparative Example 4

[0093] according to Figure 1The process flow shown, preparation of hydrogen ion concentration of 0.4 mol / L of sulfuric acid solution 600 mL into the pretreatment tank, to which 60 g of high iron gypsum (elemental composition as shown in Table 1), stirring leaching at room temperature for 30 min, solid-liquid separation after obtaining pretreated high iron gypsum (iron content of 0.52wt%). Preparation of 12wt% sodium sulfate solution, and adjust its pH with sulfuric acid to 1, to the sodium sulfate solution pretreated high iron gypsum 50 g mixed evenly to form a solid mass fraction of 20% suspension slurry. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95℃, the stirring rate is 120 rpm, when the temperature of the suspension slurry reaches the set temperature, add 8g of crystal form regulator (such as Figure 2 , the crystal form regulator is regular morphology of α-hemihydrate gypsum crystal, the average aspect ratio is 0.6, and the particle size is 35 μm), after 45 min of reaction, immediately hot solid-liquid separation, the separated solid phase is washed with boiling water and transferred to a 60℃ oven to dry to constant weight, the micro-morphology of the gypsum product is shown in Figure 8 , the iron content is 4.76 mg / g, and the main component is dihydrate gypsum, no cementation, no compressive strength.

[0094] Comparative example 5

[0095] According to the process flow shown in Figure 1 , 600 mL of pH = 2 sulfuric acid solution is added to the pretreatment tank, 60 g of high iron gypsum (elemental composition as shown in Table 1) is added, stirring leaching at room temperature for 30 min, solid-liquid separation after obtaining pretreated high iron gypsum (iron content of 1.30wt%). Preparation of 12wt% sodium sulfate solution, and adjust its hydrogen ion concentration to 0.4 mol / L with sulfuric acid, to the sodium sulfate solution pretreated high iron gypsum 50 g mixed evenly to form a solid mass fraction of 20% suspension slurry. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95℃, the stirring rate is 120 rpm, when the temperature of the suspension slurry reaches the set temperature, add 8g of crystal form regulator (such as Figure 2 , the crystal form regulator is regular morphology of α-hemihydrate gypsum crystal, the average aspect ratio is 0.6, and the particle size is 35 μm), after 45 min of reaction, immediately hot solid-liquid separation, the separated solid phase is washed with boiling water and transferred to a 60℃ oven to dry to constant weight, the micro-morphology of the gypsum product is shown in Figure 9 , the iron content is 7.79 mg / g, and the oven-dried compressive strength is 9.13 MPa.

[0096] Example 3

[0097] According to the process flow shown in Figure 1 , 600 mL of pH = 5 hydrochloric acid solution is added to the pretreatment tank, 60 g of high iron gypsum (elemental composition as shown in Table 2),

[0098] Table 2. Main elemental composition and proportion of raw material high-speed iron gypsum (titanium gypsum)

[0099]

[0100] The mixture was stirred and leached at room temperature for 20 minutes. After solid-liquid separation, pretreated ferrous gypsum (iron content 2.96 wt%) was obtained. A 12 wt% sodium chloride solution was prepared, and its pH was adjusted to 2 with hydrochloric acid. 50 g of pretreated ferrous gypsum was added to the sodium chloride solution and mixed well to form a suspension with a solid mass fraction of 15%. The suspension was added to a three-necked round-bottom flask, and the reaction temperature was set to 97℃ with a stirring speed of 150 rpm. When the temperature of the suspension reached the set temperature, 10 g of crystal form regulator (e.g., ...) was added. Figure 10 The crystal form regulator was a regularly shaped α-hemihydrate gypsum crystal with an average aspect ratio of 1.1 and a particle size of 30 μm. Immediately after a 60-minute reaction, solid-liquid separation was performed while still hot. The separated solid phase was washed with boiling water and transferred to a 60°C oven to dry to constant weight, yielding the gypsum product with the following microstructure: Figure 11 As shown, its iron content is 6.58 mg / g, and its dry compressive strength is 31.44 MPa.

[0101] Comparative Example 6

[0102] according to Figure 1 The process flow shown involves preparing 600 mL of hydrochloric acid solution with pH=5 and adding it to a pretreatment tank. Then, 60 g of ferrous gypsum (elemental composition as shown in Table 2) is added, and the mixture is stirred and leached for 20 min at room temperature. After solid-liquid separation, pretreated ferrous gypsum (iron content 2.96 wt%) is obtained. A 12 wt% sodium chloride solution is prepared, and its pH is adjusted to 2 with hydrochloric acid. 50 g of pretreated ferrous gypsum is added to the sodium chloride solution and mixed thoroughly to form a suspension with a solid mass fraction of 15%. The suspension is added to a three-necked round-bottom flask, and the reaction temperature is set to 97℃ with a stirring speed of 150 rpm. Once the temperature of the suspension reaches the set temperature, 4.5 g of a crystal form regulator (e.g., ...) is added. Figure 10 The crystal form regulator was a regularly shaped α-hemihydrate gypsum crystal with an average aspect ratio of 1.1 and a particle size of 30 μm. Immediately after a 60-minute reaction, solid-liquid separation was performed while still hot. The separated solid phase was washed with boiling water and transferred to a 60°C oven to dry to constant weight, yielding the gypsum product with the following microstructure: Figure 12 As shown, its iron content is 12.06 mg / g, and its dry compressive strength is 11.86 MPa.

[0103] Example 4

[0104] according to Figure 1The process flow shown, preparation of pH = 4 hydrochloric acid solution 600 mL into the pretreatment tank, to which 80 g of high iron gypsum (elemental composition as shown in Table 2), stirring leaching at room temperature for 20 min, solid-liquid separation after obtaining pretreated high iron gypsum (iron content of 2.52wt%). Preparation of 15% sodium nitrate solution, and adjust its pH to 3 with nitric acid, to the sodium nitrate solution 50 g of pretreated high iron gypsum mixed evenly to form a solid mass fraction of 20% suspension slurry. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95℃, the stirring rate is 120 rpm, when the temperature of the suspension slurry reaches the set temperature, 7 g of crystal form regulator (such as Figure 10 , the crystal form regulator is regular α-hemihydrate gypsum crystal with an average aspect ratio of 1.1 and a particle size of 30 μm), after 60 min of reaction, immediately hot solid-liquid separation, the separated solid phase is washed with boiling water and transferred to a 60℃ oven for drying to constant weight, the micro-morphology of the gypsum product is shown in Figure 13 , the iron content is 3.51 mg / g, and the oven-dried compressive strength is 35.69 MPa.

[0105] Comparative Example 7

[0106] According to the process flow shown in Figure 1 , 600 mL of pH = 1 hydrochloric acid solution is added to the pretreatment tank, 60 g of high iron gypsum (elemental composition as shown in Table 2) is added to the pretreatment tank, stirring leaching at room temperature for 30 min, solid-liquid separation after obtaining pretreated high iron gypsum (iron content of 0.66wt%). Preparation of 15% sodium nitrate solution, and adjust its pH to 3 with nitric acid, to the sodium nitrate solution 50 g of pretreated high iron gypsum mixed evenly to form a solid mass fraction of 20% suspension slurry. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95℃, the stirring rate is 120 rpm, when the temperature of the suspension slurry reaches the set temperature, 7 g of crystal form regulator (such as Figure 10 , the crystal form regulator is regular α-hemihydrate gypsum crystal with an average aspect ratio of 1.1 and a particle size of 30 μm), after 90 min of reaction, immediately hot solid-liquid separation, the separated solid phase is washed with boiling water and transferred to a 60℃ oven for drying to constant weight, the micro-morphology of the gypsum product is shown in Figure 14 , the iron content is 5.70 mg / g, and the main component is dihydrate gypsum, which has no gelling property and no compressive strength.

[0107] Comparative Example 8

[0108] According to the process flow shown in Figure 1The process flow shown, preparation of pH = 4 hydrochloric acid solution 600 mL into the pretreatment tank, to which 80 g of high iron gypsum (elemental composition as shown in Table 2), stirring at room temperature leaching 20 min, after solid-liquid separation to obtain pretreated high iron gypsum (iron content of 2.52 wt%). Preparation of 15% sodium nitrate solution, and adjust its pH to 3 with nitric acid, to the sodium nitrate solution was added pretreated high iron gypsum 50 g mixed to form a solid mass fraction of 20% suspension slurry. The suspension slurry is added to a three-necked round-bottom flask, the reaction temperature is set to 95 ℃, the stirring rate is 120 rpm, when the temperature of the suspension slurry reaches the set temperature, add crystal form regulator 7 g (such as Figure 10 , the crystal form regulator is regular α-hemihydrate gypsum crystal, the average length-diameter ratio is 1.1, and the particle size is 30 μm), after reaction for 80 min, immediately hot solid-liquid separation, the separated solid phase is washed with boiling water and transferred to a 60 ℃ oven to dry to constant weight, the micro-morphology of the gypsum product is shown in Figure 15 , after the phase transition time is extended, the crystal will continue to develop into fine needle-like crystals, the iron content is 10.09 mg / g, and the oven-drying compression strength is 14.85 MPa.

Claims

1. A method for deep iron removal from high-iron gypsum in conjunction with the preparation of high-value gypsum products, characterized in that, The method comprises the following steps: (1) adding raw high-iron gypsum into an inorganic acid solution for pretreatment, and separating the solid phase from the liquid phase after fully stirring for a period of time, wherein the iron content in the pretreated high-iron gypsum is 1.0 wt%-5.0 wt%; the iron content in the raw high-iron gypsum in step (1) is >5 wt%; the inorganic acid solution in step (1) is one or more of sulfuric acid, hydrochloric acid or nitric acid, and the pH value of the inorganic acid solution is 1.0-5.0; (2) adding the pretreated high-iron gypsum into a salt solution system, and mixing to form a gypsum suspension slurry; in the salt solution system, the salt is at least one of soluble sulfate, hydrochloride and nitrate, and the concentration of the salt in the salt solution system is 10 wt%-20 wt%; the solid content in the obtained gypsum suspension slurry is 5%-30%; and the pH value of the salt solution is 1.0-2.5; (3) heating the gypsum suspension slurry to above 90℃, adding a crystal form regulator, and stirring and reacting at the temperature, wherein the crystal form regulator is short-columnar α-hemihydrate gypsum crystals, and the particle size of the crystal form regulator is 12-65 μm; the addition amount of the crystal form regulator in step (3) is 9.5%-30% of the mass of the pretreated high-iron gypsum; (4) separating the suspension slurry while hot after reacting for a period of time, washing and drying the separated solid phase to obtain a high-purity high-value gypsum product, and the reaction time in step (4) is 30 min-60 min; the iron content in the obtained gypsum product is <5 mg / g; the oven-dried compressive strength of the obtained gypsum product is >20 MPa.

2. The method for deep iron removal from high-iron gypsum and for the simultaneous preparation of high-value gypsum products according to claim 1, characterized in that: The iron content in the raw high-iron gypsum in step (1) is <25 wt% and >5 wt%, and the content of calcium sulfate dihydrate is >70%.

3. The method for deep iron removal from high-iron gypsum and for the simultaneous preparation of high-value gypsum products according to claim 1, characterized in that: The raw high-iron gypsum comprises titanium gypsum.

4. The method for deep iron removal from high-iron gypsum and for the simultaneous preparation of high-value gypsum products according to claim 1, characterized in that: The inorganic acid solution in step (1) is sulfuric acid, and the pH value of the inorganic acid solution is 1.0-4.

0.

5. The method for deep iron removal from high-iron gypsum and for the simultaneous preparation of high-value gypsum products according to claim 4, characterized in that: The pH value of the inorganic acid solution in step (1) is 1.0-3.

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6. The method for deep iron removal from high-iron gypsum and for the simultaneous preparation of high-value gypsum products according to claim 5, characterized in that: The pH value of the inorganic acid solution in step (1) is 1.5-2.

5.

7. The method for deep iron removal from high-iron gypsum and for the simultaneous production of high-value gypsum products according to claim 1, characterized in that: The pretreatment temperature in step (1) is ≤40℃, the pretreatment solid-liquid ratio is 1 / 5-1 / 20 (g / mL), and the pretreatment time is 15-60 min.

8. The method for deep iron removal from high-iron gypsum and for the simultaneous production of high-value gypsum products according to claim 1, characterized in that: The solid content in the gypsum suspension slurry in step (2) is 15-30%.

9. The method for deep iron removal from high-iron gypsum and for the simultaneous production of high-value gypsum products according to claim 1, characterized in that: The reaction temperature in step (3) is 90-105℃, and the stirring rate is 100-300 rpm.

10. The method for deep iron removal from high-iron gypsum and for the simultaneous production of high-value gypsum products according to claim 1, characterized in that: The addition amount of the crystal form regulator in step (3) is 10%-20% of the mass of the pretreated high-iron gypsum.

11. The method for deep iron removal from high-iron gypsum and for the simultaneous production of high-value gypsum products according to claim 1, characterized in that: If the obtained gypsum product is short-columnar α-hemihydrate gypsum crystals with regular morphology, the average length-diameter ratio of which is ≤4.0, and the particle size of which is 20-50 μm, the gypsum product can also be used as a crystal form regulator.

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