Process for the preparation of type ii anhydrite and its activation

Abstract

The application belongs to the technical field of comprehensive utilization of solid waste and building materials, and particularly relates to a method for preparing and activating type II anhydrite, and specific steps are as follows: after the gypsum and a compound conversion agent are measured according to a mass ratio and uniformly mixed, type II anhydrite is obtained by dehydration conversion at 120-200 DEG C for 30-60 min, the type II anhydrite is ground to a specific surface area of 400+ / -20 m 2 / kg, calcium active material and an activator are added and uniformly mixed to obtain type II anhydrite products. The water requirement of the type II anhydrite according to the application at a standard consistency is 40-45% of the mass of the mixture, the initial setting time is 30-60 min, the final setting time is 120-180 min, the 28-day flexural strength is greater than 6 MPa, the compressive strength is greater than 25 MPa, and the softening coefficient is greater than 0.8. Compared with the existing calcination method, the dehydration temperature of the application is low and more energy-saving, and compared with the liquid phase method, the application has a simple process, large capacity, does not need drying, and does not have the problem of water balance in the production process. The application provides a new technology for preparing water-resistant and high-strength gypsum products from gypsum, especially chemical gypsum such as phosphogypsum, and has great significance for promoting the resource utilization of phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology for comprehensive utilization of solid waste, specifically a method for preparing type II anhydrous gypsum and activating its activity. Background Technology

[0002] Gypsum can be divided into natural gypsum and chemical gypsum. Chemical gypsum refers to slag or by-products produced in the chemical industry or other industrial production processes, with calcium sulfate as the main component, such as phosphogypsum, sulfur gypsum, water treatment gypsum, etc., which are produced in the form of dihydrates. Gypsum produced from different origins and using different processes varies considerably. For example, natural gypsum exhibits significant differences in gypsum content and impurities depending on its origin. Phosphogypsum, in addition to calcium sulfate, contains incompletely decomposed phosphate rock, residual phosphoric acid, fluorides, acid-insoluble substances, and organic matter. Fluorine can harm human health, primarily causing yellowing of teeth and bone deformities. Phosphorus, when it enters the soil, can cause soil compaction and alkalization; when it enters water bodies, it can cause eutrophication, leading to rapid algal growth and oxygen depletion, resulting in fish deaths. Therefore, the presence of fluorine and phosphorus directly affects their resource utilization. Desulfurization gypsum is a byproduct of desulfurization of flue gas from coal-fired boilers and the metallurgical industry. In addition to its main component, calcium sulfate dihydrate, it also contains fly ash, organic carbon, calcium carbonate, calcium sulfite, and various soluble salts. Its color can be white, gray, or yellow, depending on the conditions during the desulfurization process. These components have a certain impact on the performance of gypsum products.

[0003] The main applications of gypsum are in the building materials industry, such as cement retarders and the production of architectural gypsum. There is significant room for development in the architectural gypsum market. Currently, gypsum accounts for less than 20% of building materials in my country, far lower than the 80% share in developed countries. This is partly due to the influence of traditional brick and tile customs, and partly due to the low softening coefficient of gypsum, which prevents its use in exterior walls. To date, the main processed gypsum products include β-powder (architectural gypsum powder), α-high-strength gypsum, and anhydrous Type II gypsum. Among these three types of products, building gypsum has low production costs but poor physical and mechanical properties; α-high-strength gypsum has superior physical and mechanical properties but high production costs, and due to its complex process, it is generally produced intermittently with small processing capacity; anhydrous type II gypsum has physical and mechanical properties between building gypsum and α-high-strength gypsum, and can be produced on a large scale, but its high energy consumption leads to high production costs and no market competitive advantage. Therefore, developing a low-cost anhydrous type II gypsum production technology, while solving the problem of low softening coefficient of gypsum products, is expected to replace cement-based products in many building materials. At the same time, by utilizing its easy molding and adjustable initial and final setting characteristics, it will surely become the leading product in the field of prefabricated buildings.

[0004] To date, there are two main processes for the treatment and utilization of anhydrous type II gypsum: calcination and hydrothermal methods. In the calcination method, the crystal water of gypsum dihydrate evaporates rapidly through the crystal surface at temperatures of 400-800℃, without altering the original microstructure of the gypsum dihydrate; the resulting anhydrous gypsum typically exhibits a rhomboid microstructure. In the hydrothermal method, under the action of additives such as compounding agents, the gypsum dihydrate first dissolves to release Ca. 2+ With SO4 2- When the concentration of these ions reaches the supersaturation of anhydrous gypsum, anhydrous gypsum crystal nuclei will form. As the dihydrate gypsum continues to dissolve, the anhydrous gypsum crystals gradually grow, which is the process of dissolution-recrystallization. The microstructure of the resulting anhydrous gypsum is different from that of the original dihydrate gypsum crystals, and it usually appears as flakes, needles, or long fibers.

[0005] Chinese invention patent ZL202110646751.0 discloses an apparatus and method for producing type II anhydrous gypsum using industrial by-product gypsum. The apparatus includes a feeding unit, a drying unit, a calcination unit, a crystal control unit, and a product collection unit connected in sequence. The calcination unit can effectively remove the water of crystallization from the dihydrate gypsum to obtain crude type II anhydrous gypsum. Then, by controlling the temperature and residence time through a crystal form controller, precise step-by-step crystal control can be achieved to obtain a type II anhydrous gypsum product with an AⅡ-U crystal form ratio of ≥98wt%.

[0006] Chinese invention patent ZL202010181986.2 discloses a type II anhydrous gypsum calcination process, including the following steps: (1) preparing phosphogypsum raw material with a water content of 10%; (2) sending the phosphogypsum raw material into a sieve to remove large particles of impurities; (3) sending the phosphogypsum raw material with large particles of impurities removed into a flash dryer, using bidirectional convective hot air for convective drying to remove adhering water; (4) using a cyclone dust collector and a bag dust collector to collect the phosphogypsum raw material with adhering water removed, and sending it into an internal heat pipe rotary kiln through a lifting and conveying device; (5) in the internal heat pipe rotary kiln, the phosphogypsum is peristaltic and stir-fried through the internal heat pipe, so that the crystal water in the phosphogypsum generates secondary steam, and the rotation speed of the internal heat pipe rotary kiln is adjusted so that the temperature of the phosphogypsum raw material rises to 420℃-480℃ to obtain slow-dissolving anhydrous gypsum.

[0007] Chinese invention patent ZL202210206416.3 discloses a system for producing high-purity gypsum using industrial by-product gypsum, characterized in that the system includes a pretreatment subsystem, a calcination subsystem, a cooling subsystem, and a flue gas treatment subsystem.

[0008] Chinese invention patent ZL202410662144.7 discloses a method for preparing type II anhydrous gypsum: A mixed acid is obtained by mixing sulfuric acid with a mass concentration of 30-80% and an acid etching agent accounting for 0-5% of the weight of the sulfuric acid; industrial gypsum is added to the mixed acid to carry out a dehydration reaction; the dehydrated product is then filtered, washed, and dried to obtain the final product. This invention eliminates the need for a "dissolution-recrystallization" step, resulting in a short processing time, high efficiency, simple process, mild conditions, energy saving, and emission reduction. The obtained type II anhydrous gypsum product has high purity and low impurities.

[0009] Chinese invention patent ZL202211624297.X discloses a method for preparing type II anhydrous gypsum from high-silica phosphogypsum, relating to the field of phosphogypsum treatment technology. A wet-process phosphoric acid solution with an acid concentration of 30%–40% P₂O₅ is added to hydrofluoric acid to obtain a mixed solution. Phosphogypsum is added to the mixed solution, and sulfuric acid solution is added dropwise. The reaction temperature is controlled at 90–100℃, and the reaction is stirred for 4–8 hours. The reaction slurry is filtered, washed, and dried to obtain type II anhydrous gypsum. By using a mixed acid system of wet-process phosphoric acid, hydrofluoric acid, and sulfuric acid, phosphogypsum is recrystallized at a certain temperature to obtain type II anhydrous gypsum. During the process, hydrofluoric acid reacts with silicon dioxide to produce fluorosilicic acid, which adsorbs calcium ions, promoting the formation of large-particle crystals in the anhydrous gypsum. This facilitates filtration and washing, removing organic matter and other impurities, further improving the quality of the anhydrous gypsum. This method not only achieves the transformation of phosphogypsum into anhydrous gypsum but also purifies the quality of the anhydrous gypsum.

[0010] In summary, current production methods for Type II anhydrous gypsum mainly focus on calcination and liquid-phase methods. The main reason for the lack of large-scale application is the high energy consumption of the calcination method and the complexity, long crystallization time, and stringent washing conditions of the liquid-phase method. While the treatment liquid can be reused as a major production factor compared to phosphogypsum, other types of gypsum do not meet the conditions for widespread application. Therefore, this invention aims to achieve directional crystallization by adding a composite conversion agent, thereby reducing the dehydration activity of gypsum and solving the problems of high energy consumption in the traditional Type II anhydrous gypsum calcination method, low production capacity in the liquid-phase method, and secondary pollution. Summary of the Invention

[0011] The purpose of this invention is to provide a method for preparing type II anhydrous gypsum products using an activity-activated method to solve the problems of high energy consumption and difficulty in large-scale production in the prior art.

[0012] To achieve the above objectives, the technical solution of this invention provides a method for preparing type II anhydrous gypsum and activating its activity. One innovation is the addition of a compound conversion agent to the gypsum to reduce the dehydration activation energy of dihydrate gypsum, while simultaneously adding a surfactant to reduce the surface tension of the gypsum, increasing the contact and wetting between the compound conversion agent and the gypsum, thus promoting the conversion of dihydrate gypsum to type II anhydrous gypsum. Another innovation is that type II anhydrous gypsum has low activity and requires the addition of an activator for dissolution and recrystallization. Since the type II anhydrous gypsum prepared by this invention contains a small amount of free acid, traditional sulfate activation would result in a highly acidic final product, limiting its application environment. Therefore, this invention uses calcium-based active materials as the main activator, in conjunction with a sulfate activator, to neutralize its acidity while obtaining a hydraulic composition, improving the product's strength and softening coefficient, and expanding its application pathways.

[0013] The specific contents of this invention are as follows:

[0014] A method for preparing type II anhydrous gypsum, characterized by comprising the following steps:

[0015] S1. Mixing: After adding the composite conversion agent and surfactant to the gypsum, mix them evenly to obtain a mixture;

[0016] The compound conversion agent is a mixture of pure strong acid, citric acid and aluminum sulfate, and the amounts used are 1-3%, 0.5-1.0% and 0-0.5% of the dry weight of gypsum, respectively.

[0017] The amount of the surfactant used is 0.03-0.05% of the dry weight of the gypsum.

[0018] S2, Phase transformation: The mixture obtained in step S1 is heated with hot air at a temperature of 120-200℃ for 20-60 minutes to remove water, thus obtaining type II anhydrous gypsum.

[0019] The gypsum mentioned in step S1 can be natural gypsum or chemical gypsum, wherein the attached water content is 10-20% or adjusted to the corresponding water content by adding water. The chemical gypsum is phosphogypsum or desulfurized gypsum.

[0020] The strong acid mentioned in step S1 is one of sulfuric acid, hydrochloric acid, and nitric acid.

[0021] The surfactant mentioned in step S1 is one or more of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, and polyethylene glycol.

[0022] In step S2, the heating and dehydration are carried out inside a rotary kiln.

[0023] This invention also discloses a method for activating the obtained type II anhydrous gypsum, characterized by comprising the following steps:

[0024] s1. Mechanical activation: Type II anhydrous gypsum powder is ground to a specific surface area of ​​400±20m². 2 / kg, which means mechanical activation is complete;

[0025] s2. Add active calcium activator, salt activator and modifier to the mechanically activated Type II anhydrous gypsum in step s1, and mix evenly to obtain Type II anhydrous gypsum product; the active calcium activator is quicklime and cement clinker, and their dosages are 10-18% and 2-10% of the mass of Type II anhydrous gypsum, respectively.

[0026] The salt activator is a sulfate, and its dosage is 0.5-1.0% of the mass of type II anhydrous gypsum;

[0027] The amount of the modifier used is 0.05-0.1% of the mass of type II anhydrous gypsum.

[0028] The sulfates mentioned in step s2 are sodium sulfate or alunite.

[0029] The modifier mentioned in step s2 is one of calcium lignosulfonate and sodium lignosulfonate.

[0030] The standard consistency water requirement of the Type II anhydrous gypsum product obtained in step s2 is 40-45%, the initial setting time is 30-60 min, the final setting time is 120-180 min, the 28-day flexural strength is >6 MPa, the softening coefficient is >0.8, and the compressive strength is >25 MPa.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] (1) Compared with the existing mature calcination method, the method of the present invention has a lower conversion temperature and lower cost. Compared with the liquid phase method, the production process is simpler, the production capacity is larger, the conversion time is shorter, and there is no wastewater treatment problem. In summary, the present invention has good prospects for promotion and application.

[0033] (2) The dehydration and conversion process of the present invention is similar to that of β building gypsum, but the present invention is less sensitive to temperature, has a wide temperature operating range, and the dehydration and conversion process can be directly heated, which has fast heat transfer and higher heat utilization rate and larger production capacity than the β building gypsum production method.

[0034] (3) The active activation of the type II anhydrous gypsum prepared by the method of the present invention is mainly based on calcium active materials. Under the synergistic effect of salt activator and surface modifier, the initial and final setting times can be adjusted. The solidified body has both air hardening and water hardening characteristics. The flexural strength at 28 days is >6MPa, the compressive strength is >25MPa, and the softening coefficient is >0.8. Compared with cement-based materials, it is more energy-saving and environmentally friendly and belongs to low-carbon products. Compared with traditional gypsum products, it has a high softening coefficient, breaking through the limitation that traditional gypsum cannot be used as an exterior wall material, and has a wider range of applications. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the present invention;

[0036] Figure 2 The XRD patterns of the raw materials and heat-treated materials in Example 1 of this invention are shown below.

[0037] Figure 3 These are SEM images of specimens at different ages in Example 1 of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited by the embodiments.

[0039] Example 1

[0040] The raw material in this embodiment is phosphogypsum, and its main components are shown in Table 1:

[0041] Table 1. Main chemical components (wt%) of phosphogypsum raw material

[0042] name CaO <![CDATA[SO3]]> <![CDATA[SiO2]]> MgO <![CDATA[Al2O3]]> <![CDATA[Fe2O3]]> <![CDATA[P2O5]]> F water of crystallization <![CDATA[CaSO4.H2O]]> Adhering water content 29.71 41.99 5.67 0.65 0.50 0.47 1.77 0.13 18.78 90.38 20

[0043] By weight, 100 parts of the above-mentioned phosphogypsum raw material (dry basis) were taken, and 3 parts of pure hydrochloric acid, 1.0 part of citric acid, and 0.05 parts of sodium dodecylbenzenesulfonate were added and thoroughly mixed. The mixture was then fed into a rotary kiln via a belt conveyor, and hot air at 120°C was introduced and the mixture was held for 60 minutes to obtain type II anhydrous gypsum. Theoretically, the content of type II anhydrous gypsum formed after complete dehydration and conversion of phosphogypsum is 88.16%. The XRD patterns of the raw material and the type II anhydrous gypsum obtained after heat treatment in this embodiment are shown below. Figure 2 As shown.

[0044] This type II anhydrous gypsum powder is ground to a specific surface area of ​​380 μm. 2 / kg, the phase composition of type II anhydrous gypsum obtained after analysis is shown in Table 2 below:

[0045] Table 2. Phase composition analysis of anhydrous gypsum in Example 1 (Type II).

[0046] composition AⅢ HH DH AⅡ Adhering water Content (wt%) 4.23 2.80 0.85 79.60 0.5

[0047] Calculations show that the conversion rate of type II anhydrous gypsum in this case is 90.3%.

[0048] Add 10 parts of active calcium activator (including quicklime and cement clinker), 1.0 part of alum stone, and 0.1 part of calcium lignosulfonate to every 100 parts of ground Type II anhydrous gypsum. After mixing evenly, the product is Type II anhydrous gypsum.

[0049] Testing revealed that the product has a standard consistency water requirement of 40%, an initial setting time of 30 minutes, a final setting time of 120 minutes, a 28-day flexural strength of 7.55 MPa, and a compressive strength of 30.72 MPa. The softening coefficient is 0.89. SEM images of the specimens at different ages in this embodiment are shown below. Figure 3 As shown.

[0050] Example 2

[0051] The raw material in this case is natural gypsum, and its main components are shown in Table 3:

[0052] Table 3. Main chemical components (wt%) of natural gypsum raw materials

[0053] name CaO <![CDATA[SO3]]> <![CDATA[SiO2]]> MgO <![CDATA[Al2O3]]> <![CDATA[Fe2O3]]> <![CDATA[P2O5]]> F water of crystallization <![CDATA[CaSO4.H2O]]> Adhering water content 31.14 44.56 2.67 0.34 0.20 0.12 0.05 0.01 20.03 95.42 6.8

[0054] The above-mentioned natural gypsum powder was adjusted with water to a water content of 10%. 100 parts by weight of the dry basis of the above-mentioned natural gypsum were added, along with 3 parts of pure sulfuric acid, 0.5 parts of citric acid, 0.5 parts of aluminum sulfate, and 0.03 parts of hexadecyltrimethylammonium bromide. The mixture was thoroughly mixed and fed into a rotary kiln via a belt conveyor. Hot air at 200°C was introduced and the mixture was held for 30 minutes to obtain Type II anhydrous gypsum. Theoretically, the content of Type II anhydrous gypsum formed after complete dehydration and conversion of natural gypsum is 94.39%. This Type II anhydrous gypsum powder was then ground to a specific surface area of ​​420 μm. 2 / kg, the phase composition of type II anhydrous gypsum obtained after analysis is shown in Table 4 below:

[0055] Table 4. Phase composition analysis of anhydrous gypsum in Example 2 (Type II).

[0056] composition AⅢ HH DH AⅡ Adhering water Content (wt%) 1.23 2.42 1.05 89.86 0.0

[0057] Calculations show that the conversion rate of type II anhydrous gypsum in this case is 95.2%.

[0058] Add an active calcium activator to every 100 parts of the ground Type II anhydrous gypsum, including 18 parts of quicklime, 2 parts of cement clinker, 0.5 parts of sodium sulfate, and 0.05 parts of calcium lignosulfonate. After mixing evenly, the Type II anhydrous gypsum product is obtained.

[0059] Tests showed that the product has a standard consistency water requirement of 45%, an initial setting time of 60 minutes, a final setting time of 180 minutes, a 28-day flexural strength of 6.20 MPa, a compressive strength of 26.72 MPa, and a softening coefficient of 0.82.

[0060] Example 3

[0061] The main components of desulfurized gypsum, as shown in Table 5, are as follows:

[0062] Table 5. Main chemical components (wt%) of desulfurized gypsum raw materials

[0063] name CaO <![CDATA[SO3]]> <![CDATA[SiO2]]> MgO <![CDATA[Al2O3]]> <![CDATA[Fe2O3]]> <![CDATA[P2O5]]> F water of crystallization <![CDATA[CaSO4.H2O]]> Adhering water content 27.73 39.72 7.87 3.65 1.65 0.78 0.12 0.01 17.81 85.26 15

[0064] By weight, 100 parts of the above-mentioned desulfurized gypsum raw material were taken, and 3 parts of pure nitric acid, 0.7 parts of citric acid, and 0.3 parts of aluminum sulfate were added. Sodium dodecylbenzenesulfonate and polyethylene glycol were used as surfactants, each accounting for 50%. The total amount of surfactants was 0.04% of the dry weight of the chemical gypsum. After thorough mixing, the mixture was fed into a rotary kiln via a belt conveyor, and hot air at 170°C was introduced and the mixture was held for 40 minutes to obtain type II anhydrous gypsum. Theoretically, the content of type II anhydrous gypsum formed after complete dehydration and conversion of the desulfurized gypsum was 82.03%. This type II anhydrous gypsum was then ground to a specific surface area of ​​400 μm. 2 / kg, the phase composition of type II anhydrous gypsum obtained after analysis is shown in Table 6 below:

[0065] Table 6. Phase composition analysis of anhydrous gypsum in Example 3 (Type II).

[0066] composition AⅢ HH DH AⅡ Adhering water Content (wt%) 3.46 1.40 1.35 75.87 0.2

[0067] Calculations show that the conversion rate of type II anhydrous gypsum in this case is 92.5%.

[0068] Add 15 parts of active calcium activator, 5 parts of cement clinker, 0.5 parts of sodium sulfate, 1.0 part of alum stone, and 0.1 parts of calcium lignosulfonate to every 100 parts of ground Type II anhydrous gypsum, and mix evenly to obtain the Type II anhydrous gypsum product.

[0069] Tests showed that the product has a standard consistency water requirement of 45%, an initial setting time of 45 minutes, a final setting time of 123 minutes, a 28-day flexural strength of 5.85 MPa, a compressive strength of 25.92 MPa, and a softening coefficient of 0.85.

Claims

1. A method for preparing type II anhydrous gypsum, characterized in that, Includes the following steps: S1. Mixing: Add compound conversion agent and surfactant to gypsum and mix evenly to obtain a mixture; The compound conversion agent is a mixture of pure strong acid, citric acid and aluminum sulfate, and the amounts used are 1-3%, 0.5-1.0% and 0-0.5% of the dry weight of gypsum, respectively. The surfactant is one or more selected from sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide, and polyethylene glycol; the amount of the surfactant used is 0.03-0.05% of the dry weight of gypsum. The strong acid mentioned is one of sulfuric acid, hydrochloric acid, and nitric acid; S2, Phase transformation: The mixture obtained in step S1 is heated with hot air at a temperature of 120-200℃ for 20-60 minutes to remove water, thus obtaining type II anhydrous gypsum.

2. The method for preparing type II anhydrous gypsum according to claim 1, characterized in that, The gypsum mentioned in step S1 can be natural gypsum or chemical gypsum, wherein the attached water content is 10-20% or adjusted to the corresponding water content by adding water.

3. The method for preparing type II anhydrous gypsum according to claim 2, characterized in that, The chemical gypsum mentioned is phosphogypsum or desulfurized gypsum.

4. The method for preparing type II anhydrous gypsum according to claim 1, characterized in that, Its features are, In step S2, the heating and dehydration are carried out inside a rotary kiln.

5. A method for activating the activity of type II anhydrous gypsum obtained by the preparation method of type II anhydrous gypsum according to any one of claims 1-4, characterized in that, It includes the following steps: s1, mechanical activation: Type II anhydrite powder is ground to a specific surface area of 400 ± 20 m2 / g, i.e. mechanical activation is completed; 2 / kg, i.e. mechanical activation is completed; s2. Add active calcium activator, salt activator and modifier to the mechanically activated Type II anhydrous gypsum in step s1, mix evenly and you will get the Type II anhydrous gypsum product. The active calcium activator is quicklime and cement clinker, with dosages of 10-18% and 2-10% of the mass of type II anhydrous gypsum, respectively. The salt activator is a sulfate, and its dosage is 0.5-1.0% of the mass of type II anhydrous gypsum. The amount of the modifier used is 0.05-0.1% of the mass of type II anhydrous gypsum.

6. The method for activating type II anhydrous gypsum according to claim 5, characterized in that, The sulfates mentioned in step s2 are sodium sulfate or alunite.

7. The method for activating type II anhydrous gypsum according to claim 5, characterized in that, The modifier mentioned in step s2 is one of calcium lignosulfonate and sodium lignosulfonate.

8. The method for activating type II anhydrous gypsum according to claim 1, characterized in that, The standard consistency water requirement of the Type II anhydrous gypsum product obtained in step s2 is 40-45%, the initial setting time is 30-60 min, the final setting time is 120-180 min, the 28-day flexural strength is >6 MPa, the softening coefficient is >0.8, and the compressive strength is >25 MPa.

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