Method for preparing high-strength alpha-calcium sulfate hemihydrate from salt gypsum under normal pressure

Abstract

The invention relates to a method for preparing high-strength alpha-calcium sulfate hemihydrate from salt gypsum under normal pressure, which comprises the following steps: respectively adding sodium chloride salt and a small amount of crystal modifier into deionized water to form a salt solution-crystal modifier system, uniformly mixing, adding a proper amount of salt gypsum, putting the mixed material into an oil bath device, preheating to a specified temperature, and reacting for a certain time at the temperature; and after the reaction is finished, rapidly washing and filtering the product, and drying to obtain the alpha-calcium sulfate hemihydrate. According to the method, the problems of high requirements on reaction equipment, high energy consumption and complicated operation of a hydrothermal method and an autoclaved method are solved, the alpha-calcium sulfate hemihydrate with regular morphology can be prepared in an electrolyte solution under a normal pressure condition, and the prepared alpha-calcium sulfate hemihydrate is excellent in mechanical property, meets the strength requirement of alpha-type high-strength gypsum, and is suitable for industrial production. A new thought is provided for high-added-value upgrading and recycling of the industrial byproduct gypsum, the application way of salt gypsum can be widened, the comprehensive utilization rate can be increased, and green and sustainable development of the salt chemical industry is facilitated.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of inorganic material preparation, and particularly relates to a green process for preparing alpha-hemihydrate calcium sulfate (alpha-HH) in a sodium chloride solution as a reaction medium under normal pressure, which is suitable for high-value utilization of industrial by-product gypsum (such as salt gypsum and phosphogypsum). BACKGROUND

[0002] Phosphogypsum accounts for nearly half of the industrial by-product gypsum, and its stockpiling poses a serious threat to the ecological environment and human health: long-term stockpiling not only occupies a large amount of land resources, but also produces strong acid (pH 1.5-4.5), high-fluorine phosphorus and heavy metal exceeding standard leachate through rainwater leaching, polluting groundwater and surrounding water bodies; the radioactive radon gas (up to 1500 Bq / kg) and dust (influence radius 3-5 km) generated by dry stockpiling can cause air pollution; the unstable stockpile can easily cause dam break accidents, and the pollutants can be enriched through the food chain, causing the heavy metals in surrounding crops to exceed the standard by 2-10 times and the biodiversity to decrease by 40-60%. According to the monitoring data, the environmental pollution problem of the old stockpile with a stockpiling period of more than 10 years is particularly prominent. Therefore, the resource utilization of industrial by-product gypsum is of great significance to environmental protection, circular economy and industrial upgrading.

[0003] Alpha-hemihydrate gypsum, also known as high-strength gypsum, is a new type of gypsum-based material with excellent performance. It is widely used in the fields of construction, medical treatment and mold manufacturing due to its excellent mechanical properties and multifunctionality. Compared with ordinary building gypsum (beta-hemihydrate gypsum), the crystal structure of alpha-HH is more compact, and the crystal grains are short columnar or prismatic, which significantly improves the compressive strength, usually up to 25-50 MPa, which is 2-3 times that of ordinary building gypsum (10-20 MPa). Alpha-HH has a lower water requirement (usually 30%-40%) and low porosity after hardening, so it has higher compactness and durability, and is suitable for engineering scenarios with high strength requirements. Alpha-HH also exhibits good workability, making it easy to pour, shape and process, and is suitable for precision casting, 3D printing building materials and other fine manufacturing fields. As an inorganic cementitious material, alpha-HH meets the requirements of green building and sustainable development.

[0004] Currently, the main preparation methods of alpha-hemihydrate calcium sulfate include saturated steam pressurization method, pressurized hydrothermal method, normal pressure salt solution method, alcohol-water solution method and microwave heating method. The saturated steam pressurization method treats gypsum under high temperature and high pressure, while the pressurized hydrothermal method is usually carried out in an electrolyte solution under normal pressure or near normal pressure. Although these two methods are effective, they require a large initial investment, have high energy consumption, and are heavily dependent on the quality of gypsum, which is usually beneficial to natural gypsum.

[0005] In contrast, the normal pressure salt solution method reduces the water activity by introducing electrolyte salt into the aqueous solution involved in the reaction, so that the reaction temperature required for the conversion of DH into α-hemihydrate calcium sulfate is reduced to the boiling point of the solution, thereby allowing the reaction to be carried out at normal pressure, and this method is easy to control the reaction process. Patent CN107973329A discloses a preparation method of short prism-shaped α-hemihydrate calcium sulfate powder, which utilizes calcium salt-ethanol solution and sulfate-water solution to form calcium sulfate dihydrate precursor, and adopts external circulation rotating packing technology to prepare short prism-shaped α-hemihydrate calcium sulfate, but the constant temperature reaction time is relatively long (4-12h) and the reaction solution is acidic (pH is 2-8). SUMMARY

[0006] In order to overcome the above-mentioned shortcomings of the prior art, the purpose of the present application is to provide a method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum at normal pressure, so as to solve the problems of high equipment requirement, high energy consumption and complex operation of the hydrothermal method and autoclave method, and to prepare high-strength α-hemihydrate calcium sulfate in sodium chloride solution (neutral solution) at normal pressure.

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is:

[0008] A method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum at normal pressure, sodium chloride salt and a small amount of crystal modifier are added to deionized water to form a salt solution-crystal modifier system, and after being uniformly mixed, a proper amount of salt gypsum is added, and under normal pressure, the mixture is first preheated to a specified temperature (typically 100-130℃), and then reacted at this temperature for a certain time (typically 1-3h), after the reaction is completed, the product is quickly washed and filtered, and dried to obtain α-hemihydrate calcium sulfate.

[0009] In one embodiment, the sodium chloride is added to deionized water, and the concentration of the sodium chloride solution is 15wt%-25wt%.

[0010] In one embodiment, the crystal modifier is one or two of inorganic acids such as succinic acid, sodium succinate and ethylenediaminetetraacetic acid, and the total addition amount of the crystal modifier accounts for 0.1%-0.3% of the dry basis mass of the salt gypsum.

[0011] In one embodiment, the solid-liquid mass ratio of the addition of salt gypsum in the sodium chloride-crystal modifier system is 1:4.

[0012] In one embodiment, the reaction is carried out in an oil bath device, and the stirring speed is maintained at 100-300r / min during the reaction, and the quick washing and filtering include: washing with boiling water for 3 times, washing with anhydrous ethanol for 2 times to terminate hydration, and finally performing vacuum suction filtration.

[0013] In one embodiment, the drying temperature is 40-60℃, and the drying time is 24h.

[0014] In one embodiment, the α-hemihydrate calcium sulfate is hexagonal prism shape, short columnar.

[0015] In one embodiment, the α-hemihydrate calcium sulfate has a crystal length reaching micron level, the length being 20-120 μm, and the crystal length-diameter ratio being controlled at 1-3.

[0016] In one embodiment, the α-hemihydrate calcium sulfate has a dry compressive strength reaching 25 MPa or above.

[0017] Compared with the prior art, the present application has the following beneficial effects:

[0018] The following are three beneficial effects of the present application compared with the prior art:

[0019] (1) Preparation under normal pressure condition, simple process and low cost: The prior art usually needs to prepare α-hemihydrate calcium sulfate under high pressure or high temperature and high pressure environment, while the present application only needs to react under normal pressure oil bath condition, without the need for complex equipment (such as autoclave), which reduces the equipment requirements and production cost, and improves the operation safety.

[0020] (2) Optimization of crystal modifier, high product strength and controllable crystal morphology: The present application effectively controls the crystal growth of α-hemihydrate calcium sulfate by introducing a small amount of crystal modifier and combining with a salt solution system, so that the short columnar (hexagonal prism) crystal structure with low length-diameter ratio (1-3) is formed, and the dry compressive strength can reach 25 MPa or above, which is significantly better than the loose or rod-shaped crystals prepared by traditional methods.

[0021] (3) Resource utilization of salt gypsum, environmental protection and high efficiency: Using industrial by-product salt gypsum as raw material and combining with a rapid washing process, not only realizes high-value utilization of waste residue, but also reduces the traditional acid washing or complex purification steps, reduces wastewater discharge, avoids the high pressure equipment and high energy consumption demand of traditional hydrothermal method or autoclaving method, is environmentally friendly and suitable for large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The present application is a preparation method flow chart.

[0023] Figure 2 The scanning electron microscope (SEM) photo of the α-hemihydrate calcium sulfate prepared in Example 1.

[0024] Figure 3 The X-ray diffraction (XRD) spectrum of the α-hemihydrate calcium sulfate prepared in Example 1. DETAILED DESCRIPTION

[0025] The embodiments of the present application will be described in detail below with reference to the drawings and examples.

[0026] The application is a method for preparing high-strength alpha-hemihydrate calcium sulfate from salt gypsum at normal pressure, referring to Figure 1 as shown, mainly comprising the following steps:

[0027] Step 1, add soluble sodium chloride salt to a certain amount of deionized water.

[0028] In the application, the concentration of sodium chloride solution is 15wt%-25wt%.

[0029] The purpose of this step is to prepare a soluble sodium chloride solution.

[0030] Step 2, add a certain amount of crystal modifier to the above solution.

[0031] The purpose of this step is to configure the components of the salt solution-crystal modifier system. By introducing the specific crystal modifier of the application into the high-concentration sodium chloride solution, the activation energy of the transformation of calcium sulfate dihydrate (DH) to alpha-hemihydrate calcium sulfate (alpha-HH) is significantly reduced, and the growth habit of alpha-HH crystals is effectively controlled, so that it can finally be rapidly transformed into high-strength alpha-HH with low aspect ratio and hexagonal prism morphology under the conditions of normal pressure and oil bath temperature of 100-130℃. At the same time, the sodium chloride solution reduces the activity of water, and the crystal modifier selectively adsorbs on the specific crystal face of alpha-HH crystal, inhibiting its preferential growth along the c-axis direction, thereby promoting the formation of short columnar crystals, providing a new idea for the high-value utilization of industrial by-product gypsum (such as salt gypsum, phosphogypsum).

[0032] Step 3, place the uniformly mixed salt solution-crystal modifier system in an oil bath device and preheat to the reaction temperature (100-130℃) set in step 5.

[0033] The purpose of this step is to allow the reaction system to reach the set reaction temperature (100-130℃) before adding salt gypsum, avoiding the temperature drop after adding the raw material, which leads to unstable reaction conditions.

[0034] Step 4, add an appropriate amount of salt gypsum to the above preheated salt solution-crystal modifier system.

[0035] The purpose of this step is to put in the raw material.

[0036]

[0036] Step 5, uniformly stir the mixed solution for reaction.

[0037] The purpose of this step is to provide a stable reaction environment for the crystal transformation reaction, and uniform stirring can ensure sufficient contact between solid and liquid, so that the crystal growth is in an ideal environment. Generally, to achieve this purpose, the reaction temperature can be set to 100-130℃, the reaction time can be set to 1-3h, and the stirring rate can be set to 100-300r / min.

[0038] Step 6, after the reaction is completed, the reaction mixture is immediately subjected to vacuum filtration to separate the solid product.

[0039] The purpose of this step is to wash the prepared calcium sulfate hemihydrate, separate it from the salt solution by rapid filtration, wash off the excess salt by boiling water for 3 times, and then wash it with anhydrous ethanol for 2 times to stop the reaction and vacuum filtration.

[0040] Step 6, after the reaction is completed, the reaction mixture is immediately subjected to vacuum filtration to separate the solid product.

[0041] The purpose of this step is to dry the residual solvent on the surface of the α-hemihydrate calcium sulfate. Generally, to achieve this purpose, the drying temperature can be set to 40-60℃.

[0042] It is worth noting that there is no public report on the preparation of high-strength α-hemihydrate calcium sulfate with good performance (such as dry compressive strength ≥ 25 MPa) and good morphology (such as short hexagonal prism) using salt gypsum in a sodium chloride-crystal modifier system at normal pressure. The above steps of the present application are all carried out at normal pressure and in a neutral solution environment. Therefore, compared with the prior art, the advantages of the present application mainly lie in the neutral environment (avoiding equipment corrosion and subsequent treatment), shorter reaction time, and the use of specific salt gypsum raw materials and crystal modifier combination to obtain high-strength products with specific morphology (short hexagonal prism).

[0043] To further illustrate the technical solutions of the present application, the present application will be further described below in conjunction with specific examples.

[0044] Example 1

[0045] Take 80g of sodium chloride and add it to 320g of deionized water to prepare a sodium chloride solution. Then, take 0.2g of sodium succinate crystal modifier and add it to the sodium chloride solution. Stir the solution with a glass rod and place it in an oil bath device to heat to 100℃. Then add 100g of salt gypsum and mix it evenly with the solution. Use a magnetic rotor to stir at a speed of 200r / min at 115℃ for 2h. After the reaction is completed, wash it with boiling water and anhydrous ethanol, and then perform vacuum filtration. Place the prepared α-hemihydrate calcium sulfate in a 40℃ oven for 24h to obtain the product. Use a scanning electron microscope to observe the obtained sample. The generated α-hemihydrate calcium sulfate is in the form of a short hexagonal prism, not the common fibrous or rod-shaped, and has better mechanical properties, which can be applied to medical biomaterials and building materials. The sample obtained in this example is shown in Figures 1 and 2. Figure 2 (SEM) and Figure 3As shown in XRD, the generated calcium sulfate crystal morphology is short hexagonal prism, the average length of the crystal is 20.90 μm, the average diameter is 20.44 μm, and the average length-diameter ratio is 1.12.

[0046] The prepared sample was characterized by X-ray diffractometer (XRD). The X-ray diffraction of the obtained sample is shown in Figure 3 As shown in the figure, the prepared sample is CaSO4·0.5H2O.

[0047] The dry compressive strength of the α-hemihydrate calcium sulfate test block was measured by YAW-300C automatic pressure testing machine. The dry compressive strength of the test block is 31.93 MPa, which meets the high-strength gypsum industry standard α30 level and has high added value.

[0048] Example 2

[0049] With salt gypsum as raw material, sodium chloride salt solution and sodium succinate crystal modifier as reaction system, 80g of sodium chloride was weighed and added to 320g of deionized water to prepare sodium chloride solution, and 0.2g of succinic acid crystal modifier was added to the sodium chloride solution. The solution was stirred uniformly with a glass rod and placed in an oil bath device to heat to 100℃. Then 100g of salt gypsum was added and mixed uniformly with the solution, and stirred at a speed of 200r / min by using a magnetic rotor, and stirred uniformly at 115℃ for 2h. After the reaction was completed, the product was washed with boiling water and anhydrous ethanol, and then filtered, and the prepared α-hemihydrate calcium sulfate was placed in a 40℃ oven for 24h to obtain the product. The obtained sample was observed by scanning electron microscope, and the generated α-hemihydrate calcium sulfate was also short hexagonal prism with smooth surface.

[0050] Example 3

[0051] With salt gypsum as raw material, sodium chloride salt solution and sodium succinate crystal modifier as reaction system, 80g of sodium chloride was weighed and added to 320g of deionized water to prepare sodium chloride solution, and 0.2g of succinic acid crystal modifier was added to the sodium chloride solution. The solution was stirred uniformly with a glass rod and placed in an oil bath device to heat to 100℃. Then 100g of salt gypsum was added and mixed uniformly with the solution, and stirred at a speed of 200r / min by using a magnetic rotor, and stirred uniformly at 115℃ for 2h. After the reaction was completed, the product was washed with boiling water and anhydrous ethanol, and then filtered, and the prepared α-hemihydrate calcium sulfate was placed in a 40℃ oven for 24h to obtain the product. The obtained sample was observed by scanning electron microscope, and the generated α-hemihydrate calcium sulfate was also short hexagonal prism with smooth surface.

[0052] Example 4

[0053] With salt gypsum as raw material, sodium chloride salt solution and sodium succinate crystal modifier as reaction system, 80g of sodium chloride is weighed and added into 320g of deionized water to configure into sodium chloride solution, and 0.2g of sodium succinate crystal modifier is further added into the sodium chloride solution, the solution is stirred uniformly with a glass rod and placed in an oil bath device and heated to 100 DEG C. Then 100g of salt gypsum is added and mixed uniformly with the solution, and is stirred at 120 DEG C with a magnetic rotor at a speed of 200r / min for 2h. After the reaction is completed, the product is obtained by washing with boiling water and anhydrous ethanol, filtering, and placing in a 40 DEG C oven for 24h. The obtained sample is observed by a scanning electron microscope, and the generated alpha-hemihydrate calcium sulfate is also a short hexagonal prism with smooth surface.

[0054] According to the above embodiment, the present application solves the problems of high requirement of reaction equipment, high energy consumption and complex operation of hydrothermal method and autoclave method, and can prepare alpha-hemihydrate calcium sulfate with regular morphology in electrolyte solution under normal pressure condition, and the prepared alpha-hemihydrate calcium sulfate has excellent mechanical properties and meets the strength requirement of alpha-type high-strength gypsum, which provides a new idea for high-value-added upgrading and recycling of industrial by-product gypsum, can widen the application way of salt gypsum, improve the comprehensive utilization rate, and is helpful for the green and sustainable development of salt chemical industry.

Claims

1. A method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure, characterized in that, Sodium chloride and a crystallizing agent are added to deionized water to form a salt solution-crystallizing agent system. After mixing evenly, an appropriate amount of salt gypsum is added. Under normal pressure, the resulting mixture is preheated to 100-130℃ and reacted for 1-3 hours. After the reaction is completed, the product is quickly washed, filtered, and dried to obtain α-hemihydrate calcium sulfate.

2. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The sodium chloride is added to deionized water, and the concentration of the sodium chloride solution is 15 wt.%-25 wt.%.

3. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The crystallization agent is selected from one or two of succinic acid, sodium succinate, and ethylenediaminetetraacetic acid.

4. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The total amount of the crystallization agent added accounts for 0.1%-0.3% of the dry weight of the salt gypsum.

5. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The mass ratio of the salt gypsum to the salt solution-transforming agent system solution is 1:

4.

6. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The reaction is carried out in an oil bath apparatus, with a stirring speed of 100-300 r / min maintained during the reaction. The rapid washing and filtration includes washing three times with boiling water, then washing twice with anhydrous ethanol to terminate hydration, and finally vacuum filtration.

7. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The drying temperature is 40-60℃, and the drying time is 24 hours.

8. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The α-calcium sulfate hemihydrate is hexagonal prism in shape, and is short columnar.

9. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 8, characterized in that, The α-hemihydrate calcium sulfate has a crystal length that reaches the micrometer level, with a length of 20-120 μm, and its crystal aspect ratio is controlled between 1 and 3.

10. The method for preparing high-strength α-hemihydrate calcium sulfate from salt gypsum under normal pressure according to claim 1, characterized in that, The α-hemihydrate calcium sulfate has a dry compressive strength of over 25 MPa.

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