JP7803018B2 - Modified fluorine-containing gypsum, modified fluorine-containing gypsum product and its manufacturing method - Google Patents

Description

The present invention belongs to the field of building materials technology, and specifically relates to modified fluorine-containing gypsum, modified fluorine-containing gypsum products, and methods for producing the same.

Fluorine-containing gypsum is a slag generated when a fluoride salt factory uses fluorite and concentrated sulfuric acid to produce hydrofluoric acid. Its main chemical composition is anhydrous calcium sulfate (type 2), and it also contains a certain amount of residual calcium fluoride, sulfuric acid, hydrofluoric acid, a small amount of silica, and trace amounts of impurities such as Na ⁺ and K ⁺ .

Fluorine-containing gypsum has problems such as low grinding efficiency, strong acidity, and poor activity, which restrict its direct use as a gelling material and therefore its large-scale utilization. Current fluorine-containing gypsum processing still mainly relies on stacking, which not only occupies a large amount of land but also represents a significant waste of resources. Furthermore, toxic and harmful substances such as acidic and soluble ^F- contained in fluorine-containing gypsum that are stacked for a long period of time gradually leach out, causing serious pollution of the surrounding soil and groundwater. Fluorine-containing gypsum has a low density, and wind-blown dust also has a significant impact on the air and the environment. While standard stacking has been implemented in various places in recent years, the high maintenance costs of stacking sites not only reduce the economic benefits of companies but also become a major factor restricting their sustainable development.

Currently, fluorine-containing gypsum is mainly used as an auxiliary gelling material or as a cement gelling inhibitor in place of natural gypsum dihydrate, and its resource utilization rate is relatively low.

Therefore, there is a need to provide a technical solution that improves upon the shortcomings of the above-mentioned conventional technology.

The object of the present invention is to provide modified fluorine-containing gypsum, a modified fluorine-containing gypsum product, and a method for producing the same in order to solve or alleviate the problems present in the prior art described above.

To achieve the above objectives, the present invention provides the following technical solutions.

A step of co-grinding quicklime and fluorine-containing gypsum;
further adding a naphthalene-based water reducing agent;
A method for producing a modified fluorine-containing gypsum, comprising:
a mass ratio of the quicklime to the fluorine-containing gypsum is (0.5 to 1.0):100, and a mass ratio of the naphthalene-based water-reducing agent to the fluorine-containing gypsum is (0.3 to 0.9):100,
The method for producing modified fluorine-containing gypsum, wherein the mixing and grinding time is 15 to 45 minutes.

Modified fluorine-containing gypsum manufactured using the above manufacturing method.

A modified fluorine-containing gypsum product manufactured using the above-mentioned modified fluorine-containing gypsum.

A method for producing the above-mentioned modified fluorine-containing gypsum product, comprising: step (1) of uniformly mixing the modified fluorine-containing gypsum with water by stirring to obtain a modified fluorine-containing gypsum slurry; and step (2) of placing the modified fluorine-containing gypsum slurry in a mold and curing it to obtain the modified fluorine-containing gypsum product.

Beneficial effects

In response to the problems inherent in using fluorine-containing gypsum directly as a gelling material, the present invention proposes a method for modifying it using quicklime and a naphthalene-based water-reducing agent, which is low cost, easy to implement, suitable for industrial production, and requires only a single raw material with a limited variety.

By mixing fluorine-containing gypsum with a certain proportion of quicklime and a naphthalene-based water-reducing agent and then grinding (polishing), not only is the grinding efficiency increased, but the hydration activity of the fluorine-containing gypsum is also enhanced, and the elution of fluoride ions is effectively suppressed. The modified fluorine-containing gypsum of the present invention can be used directly as a gelling material, realizing large-scale resource utilization of fluorine-containing gypsum.

FIG. 2 is a diagram showing the effect of the amount of quicklime mixed in on the pH value of fluorine-containing gypsum provided by an embodiment of the present invention. 1 is a diagram of the effect of grinding time on particle size distribution of modified fluorine-containing gypsum particles, where (a) is particle size distribution and (b) is cumulative volume. 1A and 1B are SEM images of fluorine-containing gypsum after 15 minutes of grinding, where (a) is without quicklime mixed in and (b) is with 0.75% quicklime mixed in. FIG. 1 is a comparative XRD diagram of modified fluorine-containing gypsum obtained by pulverization for 15 minutes in Example 2 and pulverized fluorine-containing gypsum in Comparative Example 1. 1 is a diagram showing the effect of the amount of water-reducing agent mixed in on the particle size distribution of modified fluorine-containing gypsum, where (a) is the particle size distribution and (b) is the cumulative volume. FIG. 1 is a graph showing the setting time and water consumption results for standard consistency of modified fluorine-containing gypsum with different amounts of water-reducing agent mixed in. FIG. 1 is a diagram of the transverse strength of modified fluorine-containing gypsum products with different hydration times. FIG. 1 is a diagram of compressive strength of modified fluorine-containing gypsum products with different hydration times. 1 shows images of modified fluorine-containing gypsum products with different water-reducing agent loadings after 3 days of hydration. FIG. 1 is a graph showing the results of hydration rates of modified fluorine-containing gypsum with different amounts of water-reducing agent mixed in.

The present invention provides a method for producing modified fluorine-containing gypsum, which addresses the problems that currently exist with fluorine-containing gypsum, such as its inappropriate use as a direct gelling material and the difficulty of large-scale resource utilization. The method for producing modified fluorine-containing gypsum includes a step of co-grinding quicklime and fluorine-containing gypsum.

In the process of co-grinding quicklime and fluorine-containing gypsum, quicklime can play a grinding auxiliary role, which helps to improve the grinding effect of the fluorine-containing gypsum, reduce the particle size of the fluorine-containing gypsum, and make the particle size of the resulting modified fluorine-containing gypsum more uniform. Quicklime also plays a role in adjusting pH and hardening free ^F ions, which helps to improve the mechanical properties and hydration rate of the modified fluorine-containing gypsum product.

The method further includes adding a naphthalene-based water-reducing agent. The naphthalene-based water-reducing agent can also act as a grinding aid in the grinding process of the fluorine-containing gypsum, shortening the setting time of the modified fluorine-containing gypsum and reducing the amount of water required for standard consistency, thereby improving the mechanical properties of the modified fluorine-containing gypsum product and increasing the hydration rate.

The mass ratio of quicklime to fluorine-containing gypsum is (0.5-1.0):100 (e.g., 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, or 1.0:100). The mass ratio of naphthalene-based water-reducing agent to fluorine-containing gypsum is (0.3-1.5):100 (e.g., 0.3:100, 0.5:100, 0.7:100, 0.9:100, 1.1:100, 1.2:100, 1.3:100, 1.4:100, or 1.5:100).

The mass ratio of the naphthalene-based water-reducing agent to the fluorine-containing gypsum is (0.3-0.9):100 (for example, 0.4:100, 0.6:100, or 0.8:100). When the mass ratio of the naphthalene-based water-reducing agent to the fluorine-containing gypsum is (0.3-0.9):100, the modified fluorine-containing gypsum product has a flat surface, no obvious cracks, and good mechanical properties and volume stability.

The mass ratio of quicklime to fluorine-containing gypsum is 0.75:100, and the mass ratio of naphthalene-based water-reducing agent to fluorine-containing gypsum is 0.9:100.

Fluorine-containing gypsum is a by-product generated during the production of hydrogen fluoride. The particle size of fluorine-containing gypsum is 0.3 to 1.5 cm, the pH value is 2.3, and the density is 2.59 g/ ^cm3 . The main mineral composition of fluorine-containing gypsum is type 2 anhydrite, and fluorite is also contained in fluorine-containing gypsum.

The effective calcium oxide content of the quicklime is 80 wt. % or more, and the naphthalene-based water-reducing agent is a commercially available naphthalene-based water-reducing agent.

The mixing and grinding is carried out in a ball mill, and the mixing and grinding time is 15 to 45 minutes (e.g., 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, or 45 minutes).

The ball mill is a Φ50cm x 50cm ball mill.

Modified fluorine-containing gypsum manufactured using the above manufacturing method.

A modified fluorine-containing gypsum product manufactured using the above-mentioned modified fluorine-containing gypsum.

A method for producing a modified fluorine-containing gypsum product, comprising: step (1) of uniformly mixing modified fluorine-containing gypsum with water by stirring to obtain a modified fluorine-containing gypsum slurry; and step (2) of placing the modified fluorine-containing gypsum slurry in a mold and curing it to obtain a modified fluorine-containing gypsum product.

Curing is performed at a temperature of 20±5°C (e.g., 15°C, 18°C, 20°C, 22°C, 24°C, or 25°C) and a relative humidity of 70%±4% (e.g., 66%, 68%, 70%, 72%, or 74%).

The modified fluorine-containing gypsum and its manufacturing method, as well as the modified fluorine-containing gypsum product and its manufacturing method of the present invention will be described in detail below using specific examples.

In the following examples, the fluorine-containing gypsum used is a by-product generated during the production of hydrogen fluoride. The grayish-white spherical particles have a particle size of 0.3 to 1.5 cm, a pH value of ^2.3 , and a density of 2.59 g/cm3. Their main mineral composition is type 2 anhydrite, and they also contain a small amount of residual fluorite.

Quicklime is quicklime powder, and its technical requirements meet the requirements of the industry standard "Quicklime Powder for Construction" (JC/T480-92) and above.

Naphthalene-based water reducers are commercially available powders that are brown in color.

Example 1
The method for producing the modified fluorine-containing gypsum of this example includes the steps of mixing 100 parts by mass of fluorine-containing gypsum with 0.5 parts, 0.75 parts, 1.0 parts, and 1.5 parts by mass of quicklime, placing the mixture in a Φ50 cm×50 cm ball mill, and pulverizing the mixture for 15 minutes to obtain the modified fluorine-containing gypsum of this example.

The method for producing the modified fluorine-containing gypsum product of this example is as follows:
Step (1), in which water and powdered modified fluorine-containing gypsum of this embodiment are sequentially charged into a stirring kettle at a predetermined water-to-material ratio, and then uniformly mixed and stirred to obtain a modified fluorine-containing gypsum slurry, the amount of water used in the manufacturing process of the modified fluorine-containing gypsum slurry is determined based on the amount of water used for standard consistency, and when the amount of quicklime used is 0.5 parts, 0.75 parts, 1.0 parts, and 1.5 parts, the corresponding water-to-material ratios are 0.28, 0.31, 0.32, and 0.327, respectively;
Step (2) of placing the modified fluorine-containing gypsum slurry in step (1) into a mold having a predetermined specification, and performing a test on the setting time and compressive strength, respectively;
Step (3) includes testing the setting time and compressive strength of the gypsum slurry using a Vicat apparatus based on the national standard "PLASTERER GYPSUM" (GB/T 28627-2012), with the mechanical performance test specimen of the hardened slurry measuring 40 mm x 40 mm x 160 mm and curing conditions being a temperature of 20±5°C and a relative humidity (RH) of 70%±4%. The flexural strength and compressive strength are tested at 3 days, 7 days, and 28 days, respectively, with three specimens tested for each group, and the average value is used as the final result.

The pH of the fluorine-containing gypsum and the modified fluorine-containing gypsum produced in this example were measured to determine the effect of quicklime content on the pH of the fluorine-containing gypsum, and the experimental results are shown in Figure 1.

As is clear from Figure 1, quicklime was added from the outside in amounts of 0.5%, 0.75%, 1.0%, and 1.5% of the mass of the fluorine-containing gypsum, respectively. The pH of the modified fluorine-containing gypsum produced increased as the amount of quicklime added increased, and when the amount added was 0.75%, the pH value reached neutral, at 7.3.

Example 2
The method for producing the modified fluorine-containing gypsum of this example includes the steps of mixing 100 parts by mass of fluorine-containing gypsum with 0.75 parts by mass of quicklime, placing the mixture in a Φ50 cm×50 cm ball mill, and pulverizing the mixture for 15 minutes, 30 minutes, and 45 minutes, respectively, to obtain the modified fluorine-containing gypsum of this example.

The method for producing the modified fluorine-containing gypsum product of this example is as follows:
Step (1): adding water and powdered modified fluorine-containing gypsum of this embodiment into a stirring kettle sequentially at a set water/material ratio, and uniformly mixing and stirring to obtain a modified fluorine-containing gypsum slurry; the amount of water used in the manufacturing process of the modified fluorine-containing gypsum slurry is determined based on the amount of water used for standard consistency; and the water/material ratios corresponding to the grinding times of 15 minutes, 30 minutes, and 45 minutes are 0.31, 0.35, and 0.38, respectively;
Step (2) of placing the modified fluorine-containing gypsum slurry in step (1) into a mold having a predetermined specification, and performing a test on the setting time and compressive strength, respectively;
Step (3) includes testing the setting time and compressive strength of the gypsum slurry using a Vicat device based on the national standard "Plastering Gypsum" (GB/T 28627-2012). The mechanical performance test specimen of the hardened slurry has dimensions of 40mm x 40mm x 160mm, and is cured under the conditions of a temperature of 20±5°C and a relative humidity (RH) of 70%±4%. The bending strength and compressive strength are tested after 3 days, 7 days, and 28 days, respectively. Three specimens are tested for each group, and the average value is used as the final result.

Example 3
The method for producing the modified fluorine-containing gypsum of this example includes the steps of mixing 100 parts by mass of fluorine-containing gypsum with 0.75 parts by mass of quicklime and 0.3 parts, 0.6 parts, 0.9 parts, 1.2 parts, and 1.5 parts by mass of a naphthalene-based water-reducing agent, placing the mixture in a Φ50 cm×50 cm ball mill, and pulverizing the mixture for 15 minutes to obtain the modified fluorine-containing gypsum of this example.

The method for producing the modified fluorine-containing gypsum product of this example is as follows:
Step (1), in which water and powdered modified fluorine-containing gypsum of this embodiment are sequentially charged into a stirring kettle at a set water/material ratio, and uniformly mixed and stirred to obtain a modified fluorine-containing gypsum slurry, the amount of water used in the modified fluorine-containing gypsum slurry production process is determined based on the amount of water used for standard consistency, and when the amount of naphthalene-based water reducer used is 0.3 parts, 0.6 parts, 0.9 parts, 1.2 parts, and 1.5 parts, the corresponding water/material ratios are 0.396, 0.382, 0.37, 0.312, and 0.232, respectively;
Step (2) of placing the modified fluorine-containing gypsum slurry in step (1) into a mold having a predetermined specification, and performing a test on the setting time and compressive strength, respectively;
Step (3) includes testing the setting time and compressive strength of the gypsum slurry using a Vicat device based on the national standard "Plastering Gypsum" (GB/T 28627-2012). The mechanical performance test specimen of the hardened slurry has dimensions of 40mm x 40mm x 160mm, and is cured under the conditions of a temperature of 20±5°C and a relative humidity (RH) of 70%±4%. The bending strength and compressive strength are tested after 3 days, 7 days, and 28 days, respectively. Three specimens are tested for each group, and the average value is used as the final result.

Comparative Example 1
The method for producing the ground fluorine-containing gypsum of this comparative example includes a step of placing 100 parts by mass of fluorine-containing gypsum in a Φ50 cm × 50 cm ball mill and grinding it for 15 minutes to obtain the ground fluorine-containing gypsum of this comparative example.

The method for producing the pulverized fluorine-containing gypsum product of this comparative example is as follows:
Step (1): water and powdered fluorine-containing gypsum of this comparative example are sequentially charged into a stirring vessel at a set water/material ratio (0.2), and then uniformly mixed and stirred to obtain a pulverized fluorine-containing gypsum slurry;
Step (2) of placing the ground fluorine-containing gypsum slurry in step (1) into a mold with a predetermined specification, and performing a test on the setting time and compressive strength, respectively;
Step (3) includes testing the setting time and compressive strength of the gypsum slurry using a Vicat device based on the national standard "Plastering Gypsum" (GB/T 28627-2012). The mechanical performance test specimen of the hardened slurry has dimensions of 40mm x 40mm x 160mm, and is cured under the conditions of a temperature of 20±5°C and a relative humidity (RH) of 70%±4%. The bending strength and compressive strength are tested after 3 days, 7 days, and 28 days, respectively. Three specimens are tested for each group, and the average value is used as the final result.

Comparative Example 2
The comparative example includes a step of modifying fluorine-containing gypsum with a polycarboxylic acid-based water reducing agent instead of a naphthalene-based water reducing agent, mixing 100 parts by mass of fluorine-containing gypsum and 0.75 parts by mass of quicklime with 0.3 parts by mass of the polycarboxylic acid-based water reducing agent, and pulverizing the mixture in a Φ50 cm×50 cm ball mill for 15 minutes to obtain modified fluorine-containing gypsum of the comparative example.

The method for producing the modified fluorine-containing gypsum product of this comparative example is as follows:
Step (1): adding water and powdered modified fluorine-containing gypsum of this comparative example to a stirring vessel in a set water/material ratio (0.26, determined based on the amount of water used for standard consistency), and uniformly mixing and stirring them to obtain a modified fluorine-containing gypsum slurry;
Step (2) of placing the modified fluorine-containing gypsum slurry in step (1) into a mold having a predetermined specification, and performing a test on the setting time and compressive strength, respectively;
Step (3) includes testing the setting time and compressive strength of the gypsum slurry using a Vicat device based on the national standard "Plastering Gypsum" (GB/T 28627-2012). The mechanical performance test specimen of the hardened slurry has dimensions of 40mm x 40mm x 160mm, and is cured under the conditions of a temperature of 20±5°C and a relative humidity (RH) of 70%±4%. The bending strength and compressive strength are tested after 3 days, 7 days, and 28 days, respectively. Three specimens are tested for each group, and the average value is used as the final result.

Experimental Example 1: The particle size distribution of the modified fluorine-containing gypsum produced in Example 2 and the pulverized fluorine-containing gypsum produced in Comparative Example 1 was measured.
The particle size distribution diagram is shown in FIG. 2 and the particle size characterization parameters are shown in Table 1 below.

Table 1: Particle size characterization of modified fluorine-containing gypsum and ground fluorine-containing gypsum

As can be seen from Figure 2 and Table 1, the addition of quicklime during 15-minute grinding significantly reduced the particle size of the fluorine-containing gypsum, reducing the _Dv (90) of the fluorine-containing gypsum from 176 μm to 60.2 μm and improving the grinding efficiency. Quicklime exerted a good grinding-aid dispersion effect. However, with increasing grinding time, small particles gradually agglomerated due to the collision and extrusion of the grinding body, and the particle size gradually increased.

2. The micromorphologies of the modified fluorine-containing gypsum (containing 0.75% of quicklime) produced by pulverization for 15 minutes in Example 2 and the pulverized fluorine-containing gypsum (containing no quicklime) produced by pulverization for 15 minutes in Comparative Example 1 were observed, and the XRD patterns, main chemical compositions, and soluble F ^- contents of both were measured.

The SEM images of the modified fluorine-containing gypsum (containing 0.75% quicklime) produced by pulverization for 15 minutes in Example 2 and the pulverized fluorine-containing gypsum (containing no quicklime) produced by pulverization for 15 minutes in Comparative Example 1 are shown in FIG. 3(b) and FIG. 3(a), respectively, and the XRD patterns are shown in FIG. 4. The chemical compositions and soluble F ^- contents are shown in Table 3 below.

Table 3: Main chemical composition/wt. % of the modified fluorine-containing gypsum ground for 15 min in Example 2 and the ground fluorine-containing gypsum ground for 15 min in Comparative Example 1

As can be seen from Figure 3, the addition of quicklime made the particle size of the fluorine-containing gypsum after grinding more uniform, the particle surface rougher, and the shape more spherical.

As can be seen from Figure 4, the diffraction peaks characterizing the fluorine-containing gypsum after grinding were clearly enhanced, and its crystal grain size and structural regularity were also significantly improved due to the grinding-assisting effect of quicklime. Furthermore, the diffraction peaks of fluorite were significantly enhanced, with the content increasing from 3.63% to 8.48% of the fluorine-containing gypsum. The increase in fluorite content was also beneficial for grinding the fluorine-containing gypsum.

From Table 3, the soluble F ion content of the modified fluorine ^- containing gypsum was reduced by 80.7%. The following reactions may occur during the grinding process:
CaO+2H ⁺ +2F ^- →CaF ₂ ↓+H ₂ O (1)

3. The effect of quicklime on the performance of fluoride-containing gypsum was considered.
(1) The setting and hardening performance of the modified fluorine-containing gypsum in Example 2, which was prepared by pulverizing for 15 minutes and contained 0.75% quicklime, was compared with that of the pulverized fluorine-containing gypsum in Comparative Example 1, which was prepared by pulverizing for 15 minutes and did not contain quicklime. The test results are shown in Table 4 below.

Table 4: Comparison of physical properties of fluorine-containing gypsum products obtained with and without quicklime mixing during grinding

Table 4 shows the effect of quicklime on the setting and hardening performance of fluorine-containing gypsum. The grinding-assisting effect of quicklime significantly reduced the particle size of the fluorine-containing gypsum, resulting in an increase in the amount of standard consistency water used. Smaller particle size meant that the specific surface area of the fluorine-containing gypsum was larger, which was beneficial for the progress of the hydration reaction, increasing the hydration and hardening rate and shortening the setting time. The mechanical performance of fluorine-containing gypsum products also improved, and the improving effect of quicklime on the strength of fluorine-containing gypsum products became more apparent at 28 days, preventing the occurrence of later strength regression in fluorine-containing gypsum products.

(2) The hydration rates of the modified fluorine-containing gypsum produced by pulverizing for 15 minutes in Example 2, in which the amount of quicklime mixed was 0.75%, and the pulverized fluorine-containing gypsum produced by pulverizing for 15 minutes in Comparative Example 1, in which no quicklime was mixed, were measured.
The hydration rate was measured using a muffler furnace in accordance with the national standard "Building Gypsum - Determination of Crystal Water Content" (GB/T17669.2-1999), and the dihydrate gypsum content and the hydration rate of fluorine-containing gypsum were calculated. The test results are shown in Table 5 below.

Table 5: Comparison of hydration rates of fluorine-containing gypsum with and without quicklime mixing during grinding

The hydration rate of fluorine-containing gypsum also increased after adding quicklime grinding aid, as shown in Table 5. However, the increase in hydration rate was small within 1 day, and the hydration rate increased significantly after 1 day of solidification. This law was consistent with the mechanical performance.

4. Water reducers are a commonly used additive in the application of gelling materials. Although the particle size distribution of fluorine-containing gypsum after grinding is smaller, this results in an increase in the amount of standard consistency water used, which is detrimental to performance development. Grinding aids not only improve the grinding efficiency of materials during the grinding process, but also reduce energy consumption and improve powder performance. Therefore, we explored the effect of naphthalene-based water reducers as grinding aids on the performance of fluorine-containing gypsum and clarified the grinding aid modification mechanism.

The particle size distribution of the modified fluorine-containing gypsum of Example 3 was measured, and the effect of different water-reducing agent contents on the particle size distribution of the fluorine-containing gypsum after grinding was shown in Figure 5 and Table 6 below.

Table 6: Effect of water-reducing agents on the grinding aid effect of fluorine-containing gypsum

The results demonstrated that naphthalene-based water-reducing agents can play a "grinding aid" role in the grinding process of fluorine-containing gypsum. The particle size distribution of the fluorine-containing gypsum first gradually became optimized to finer particles and more concentrated as the amount of water-reducing agent increased. When the amount of water-reducing agent was 0.6%, the particle size distribution reached a minimum, after which the particle size increased as the excess particles of water-reducing agent gradually aggregated. The specific surface area of the fluorine-containing gypsum particles also first increased and then decreased as the amount of water-reducing agent increased.

According to the results in Table 6, when the amount of water-reducing agent mixed was 0.6%, the median diameter D _v (90) of the fluorine-containing gypsum particles decreased to 37.8 μm, D _v (50) decreased to 3.76 μm, and D _v (10) decreased to 0.394 μm, and the specific surface area increased to 472.1 m ² /kg.

The presence of an appropriate amount of water-reducing agent increases the mutual dispersion of fluorine-containing gypsum particles during the grinding process, allowing the water-reducing agent to gradually adhere to the particle surface, reducing mutual extrusion collisions between particles and reducing the occurrence of particle agglomeration and grinding. However, if too much water-reducing agent is used within the same grinding time, the surface roughness increases, the specific surface area increases, and the surface energy rises, causing mechanical interlocking between particles and inducing agglomeration, which is detrimental to grinding efficiency.

5. The effect of naphthalene-based water-reducing agents on the solidification and hardening performance of modified fluorine-containing gypsum was examined.
(1) The standard consistency water requirement and setting time of the modified fluorine-containing gypsum with different amounts of water-reducing agent in Example 3 were measured;
The grinding-assisting effect of the water-reducing agent increased the grinding efficiency of the fluorine-containing gypsum, and therefore the standard consistency water consumption of the fluorine-containing gypsum slurry first increased and then decreased with increasing water-reducing agent content, as shown in Figure 6. When the water-reducing agent content was low, the fluorine-containing gypsum particles were further fragmented, and the water-reducing agent adhered to the particle surface, increasing the charge repulsion between particles, but the increase in specific surface area, which increased the water consumption, was the main effect. As the water-reducing agent content gradually increased, the degree of dispersion between particles increased, and the water consumption decreased.

The setting time of fluorine-containing gypsum decreased as the amount of water-reducing agent increased. When the amount of water-reducing agent was low, the particle size of fluorine-containing gypsum increased, thereby increasing the dissolution rate and the nucleation rate of gypsum dihydrate, accelerating the precipitation and growth of gypsum dihydrate crystals, and accelerating the hydration process, resulting in a shorter setting time. As the amount of water-reducing agent increased and the amount of standard consistency water used decreased, the spacing between slurry particles became smaller, making it easier for the gypsum dihydrate to form a hardened structure after the hydration reaction, shortening the setting time. The initial and final setting times of the slurry decreased from 421 min and 554 min in the control group to 104 min and 237 min, respectively, when the amount of water-reducing agent was 1.5%.

Water requirements and setting times were measured in accordance with the national standard "Plastering Plaster" (GB/T 28627-2012).

(2) Measuring the mechanical properties of the hardened fluorine-containing gypsum (modified fluorine-containing gypsum) for different times in Example 3;
The mechanical properties of hardened fluorine-containing gypsum at different times tended to increase first and then decrease as the amount of water-reducing agent increased, and both reached a maximum when the amount of water-reducing agent was 0.9%.

In Figure 7, the flexural strength of the fluorine-containing gypsum with a mixing amount of 0.9% increased from 1.9 MPa for the control group (sample with 0% naphthalene-based water-reducing agent mixed in, i.e., sample number 1 in Table 6) to 5.4 MPa after 3 days of hydration, an increase of 184.2%. The flexural strength after 7 days of hydration increased from 5.2 MPa for the control group to 9.1 MPa, an increase of 75.0%. The flexural strength after 28 days of hydration increased from 7.9 MPa for the control group to 11.2 MPa, an increase of 41.8%.

In Figure 8, the compressive strength of the control group after 3 days, 7 days, and 28 days of hydration was 3.5 MPa, 7.6 MPa, and 10.0 MPa, respectively. When the amount of water-reducing agent mixed in was 0.9%, the compressive strength of the fluorine-containing gypsum after 3 days, 7 days, and 28 days of hydration increased to 12.0 MPa, 21.1 MPa, and 28.9 MPa, respectively, an increase of 243%, 178%, and 189%, respectively.

In Figure 9, for samples containing 0 to 0.9% water-reducing agent, the modified fluorine-containing gypsum products had a flat surface with no obvious cracks after 3 days of hydration. For samples containing 1.2% and 1.5% water-reducing agent, obvious cracks appeared on the surface of the modified fluorine-containing gypsum products after 3 days of hydration. The higher the water-reducing agent content, the shorter the setting time of the fluorine-containing gypsum slurry and the easier it was for the fluorine-containing gypsum to form a hardened structure. However, because the volume of the dihydrate gypsum crystals generated by hydration is much larger than that of the fluorine-containing gypsum, after forming a stable hardened structure, as hydration continues, the dihydrate gypsum crystals continue to grow, causing their internal volume to constantly expand, leading to the destruction of the hardened structure, a decrease in volume stability, and a significant decrease in the mechanical properties of the samples, until obvious cracks appeared on the sample surface. This was the main reason for the shorter setting time and decreased mechanical properties of the fluorine-containing gypsum after increasing the amount of water-reducing agent.

6. The effect of different hydration times of naphthalene-based water reducers on the hydration rate of fluorine-containing gypsum was investigated.
As shown in Figure 10, the effect of water-reducing admixture on the hydration rate of fluorine-containing gypsum can be divided into two stages depending on the hydration time. Before 3 days of hydration, the hydration rate increased with increasing amounts of water-reducing admixture, rising from 37.68% in the control group to 66.81% when the amount of water-reducing admixture was 0.9%, and reaching a maximum of 69.28% when the amount of water-reducing admixture was 1.5%, which also conformed to the law of set time. The increase in the hydration rate was mainly due to the increase in the dissolution rate of fluorine-containing gypsum and the nucleation rate of gypsum dihydrate.

After 3 days of hydration, the increase in the hydration rate of fluorine-containing gypsum with a water-reducing agent content exceeding 0.9% slowed, while the hydration rate of fluorine-containing gypsum with a water-reducing agent content below 0.9% steadily increased. At 28 days, the hydration rate reached a maximum of 84.21% when the water-reducing agent content was 0.9%. The hydration rates for the control and 1.5% water-reducing gypsum samples were 45.37% and 75.71%, respectively. This result was consistent with the mechanical properties. After the water-reducing agent content exceeded 0.9%, cracks in the fluorine-containing gypsum samples led to the dissipation of moisture from within the samples, disrupting the initial liquid phase environment and inhibiting the sustainability of the hydration reaction, which was detrimental to improving the hydration rate.

7. The particle size, mechanical properties, and hydration rate of the product obtained by modifying fluorine-containing gypsum with a polycarboxylic acid water-reducing agent in Comparative Example 2 were tested, and the test results were as shown in Tables 7 to 9 below.

Table 7: Particle size characterization of modified fluorine-containing gypsum when the polycarboxylic acid water-reducing agent loading was 0.3%

Table 8: Performance of modified fluorine-containing gypsum when the amount of polycarboxylic acid water-reducing agent mixed in is 0.3%

Table 9: Hydration rate of modified fluorine-containing gypsum when the amount of polycarboxylic acid water-reducing agent mixed is 0.3%

From Tables 7 to 9 above, it can be seen that polycarboxylic acids have a high water-reducing efficiency and can clearly play a role in the grinding-assisted modification of fluorine-containing gypsum, but they also inhibit the mechanical properties of the product obtained by modification and result in a low hydration rate.

Claims (4)

A method for producing modified fluorine-containing gypsum, comprising a step of mixing and grinding quicklime, a naphthalene-based water-reducing agent, and fluorine-containing gypsum,
a mass ratio of the quicklime to the fluorine-containing gypsum is 0.75:100, and a mass ratio of the naphthalene-based water-reducing agent to the fluorine-containing gypsum is (0.3 to 0.9):100,
The fluorine-containing gypsum is a by-product generated during the production of hydrogen fluoride, the particle size of the fluorine-containing gypsum is 0.3 to 1.5 cm, the pH value is 2.3, and the density is 2.59 g/cm3, the main mineral composition of the fluorine-containing gypsum is type 2 anhydrite, and the fluorine-containing gypsum also contains fluorite,
The effective content of calcium oxide in the quicklime is 80 wt. % or more,
The method for producing modified fluorine-containing gypsum, wherein the mixing and grinding time is 15 minutes. The method for producing modified fluorine-containing gypsum according to claim 1, characterized in that the naphthalene-based water reducing agent is a commercially available naphthalene-based water reducing agent. A method for producing a modified fluorine-containing gypsum product using the modified fluorine-containing gypsum produced by the production method according to claim 1 or 2,
Step (1) of uniformly mixing the modified fluorine-containing gypsum and water by stirring to obtain a modified fluorine-containing gypsum slurry;
Step (2) of placing the modified fluorine-containing gypsum slurry into a mold and curing it to obtain the modified fluorine-containing gypsum product;
A method for producing a modified fluorine-containing gypsum product, comprising: 4. The method for producing a modified fluorine-containing gypsum product according to claim 3, wherein the curing is carried out at a temperature of 20±5°C and a relative humidity of 70%±4%.