Method for regulating and controlling length-diameter ratio of calcium sulfate hemihydrate whiskers

Abstract

The invention discloses a method for regulating the length-diameter ratio of calcium sulfate hemihydrate whiskers prepared from ardealite, and belongs to the field of ardealite resource utilization. According to the invention, carbohydrates, alcohols and other substances containing hydroxyl groups are used as the crystal modifier, and the calcium sulfate hemihydrate whisker is prepared by regulating and controlling phosphogypsum through a normal-pressure hydrothermal method, so that the length-diameter ratio of the whisker is regulated and controlled within a certain range. The method solves the problems that the length-diameter ratio of the whisker in the calcium sulfate hemihydrate whisker product prepared from ardealite is not uniform and cannot be regulated and controlled, the preparation process is tedious, the corrosivity is strong due to excessive salt solution in the reaction system, and the waste liquid has great harm to the environment, and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of comprehensive utilization of industrial solid waste phosphogypsum resources, and particularly to a method for regulating the aspect ratio of hemihydrate calcium sulfate whiskers prepared from phosphogypsum. Background Art

[0002] Phosphogypsum is a solid waste residue generated during the production of wet-process phosphoric acid, with the main component being calcium sulfate dihydrate (CaSO 4 ·2H 2 O), containing a small amount of phosphorus, fluorine, and heavy metal impurities. The global annual emissions exceed 300 million tons. Its stacking occupies land and is prone to causing water pollution. However, due to its rich calcium and sulfur resources, it has become a key object for the resource utilization of industrial solid waste in recent years. Currently, the resource utilization of phosphogypsum can be mainly used in fields such as building materials, soil improvement, and sulfur resource recovery. The development of high-efficiency purification and high-value utilization technologies is the key to breaking through the bottleneck of its large-scale application.

[0003] Hemihydrate calcium sulfate whiskers (CaSO 4 ·0.5H 2 O) is a single-crystal fiber material based on hemihydrate calcium sulfate, with characteristics such as high strength, high temperature resistance, and good interfacial compatibility. It is prepared by regulating the crystallization process through a hydrothermal method or an atmospheric pressure acidification method, and is widely used in the field of enhancing composite materials such as plastics, rubbers, and coatings. It can also be used as an environmental protection adsorbent or paper filler. Its low cost and degradable characteristics conform to the development trend of green materials. However, the regulation of the aspect ratio of whiskers and surface modification are still the core of technology optimization.

[0004] At present, the preparation of calcium sulfate hemihydrate whiskers mainly includes the salt solution method, the alcohol-water method, and the hydrothermal method. The hydrothermal method is an earlier method used to prepare calcium sulfate hemihydrate whiskers. The process is to dissolve and recrystallize phosphogypsum in an aqueous solvent into calcium sulfate hemihydrate whiskers under high temperature and high pressure. The hydrothermal method needs to be completed at a relatively high temperature or pressure, with high energy consumption, and has high requirements for aspects such as the airtightness of the reaction device, and it is difficult to control the aspect ratio of the whiskers. These have certain limitations for the popularization and application of the hydrothermal method. The atmospheric pressure salt solution method is a method for preparing calcium sulfate hemihydrate whiskers developed on the basis of the hydrothermal method. It mainly realizes the preparation of calcium sulfate hemihydrate whiskers at atmospheric pressure and relatively low temperature by adding some salts to the aqueous solvent to form an electrolyte solution and reducing the activity of water. Although the atmospheric pressure salt solution method does not need to be carried out under high-pressure equipment, the preparation process is relatively complex, the process is difficult to control, and the salts mainly selected are chlorides and sulfates, which will cause great corrosion to the reaction equipment, and its filtrate will also cause certain harm to the environment, and it is difficult to control the aspect ratio of the whiskers. The alcohol-water method is a method of adding a certain proportion of alcohol to the aqueous solvent to form a non-electrolyte reaction solvent, which can realize the dehydration conversion of phosphogypsum to calcium sulfate hemihydrate whiskers at a relatively low temperature. The addition of alcohol can reduce the activity of water, realize the conversion of phosphogypsum to calcium sulfate hemihydrate whiskers at a relatively low temperature, and the reaction conditions are mild. Compared with the atmospheric pressure salt solution method, the alcohol-water method does not introduce other metal ions, has no corrosion or less corrosion to the equipment, and the alcohol can be recycled. However, the aspect ratio of the whiskers prepared by the alcohol-water method is relatively small, the aspect ratio of the whiskers is not uniform, and there is a problem of incomplete conversion in part.

[0005] In summary, there are some problems in the currently adopted control methods, including corrosiveness, environmental unfriendliness, difficulty in controlling the aspect ratio of whiskers, etc. Therefore, it is urgent to develop a new control method. Summary of the Invention

[0006] In view of the above technical problems, the present invention proposes a method for controlling the aspect ratio of calcium sulfate hemihydrate whiskers to solve the problems such as small and uneven distribution of the aspect ratio of calcium sulfate hemihydrate whiskers prepared from phosphogypsum at present, large energy consumption and low efficiency caused by high autoclaving temperature and long reaction time in the preparation process.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows: A method for controlling the aspect ratio of calcium sulfate hemihydrate whiskers, comprising the following steps: Step S1: Wash the original phosphogypsum with sulfuric acid, hydrogen peroxide, and ethanol in stages, and dry the washed phosphogypsum to obtain pretreated phosphogypsum; Step S2: Ultrasonically mix the crystal conversion agent with deionized water to obtain a mixed slurry, heat it to the crystal conversion temperature, then add the pretreated phosphogypsum for crystal conversion, filter by suction after the reaction is completed, wash the filter cake, and dry it to obtain long rod-shaped calcium sulfate hemihydrate whiskers.

[0008] Further, the method of stepwise water washing in step S1 is to fully mix the as-received phosphogypsum with 20% concentrated sulfuric acid, fully wash it with deionized water, then fully mix it with 30% hydrogen peroxide, and then wash it with deionized water and finally fully mix and wash it with 50% ethanol. Finally, wash it clean with deionized water and place it in a constant temperature oven at 65 °C to dry to a constant weight. Further, in step S2, the aspect ratio of the calcium sulfate hemihydrate whiskers is controlled by regulating the number of hydroxyl groups in the crystal conversion agent, specifically by mixing crystal conversion agents with different numbers of hydroxyl groups.

[0009] Further, in step S1, the water washing time for each water washing in the stepwise water washing is 2 h.

[0010] Further, in step S2, the crystal conversion agent is one or several of sugars or alcohols containing hydroxyl groups, such as glycerol, xylitol, sorbitol, glucose, and fructose, which are mixed in a certain proportion.

[0011] Further, in step S2, the water-to-material ratio of the crystal conversion agent to deionized water is 1:15 - 1:18, preferably 1:17. If the dosage of the crystal conversion agent is too small, the driving force for crystal conversion will be insufficient, affecting the crystal conversion process. If the dosage of the crystal conversion agent is too large, the viscosity of the mixed slurry will increase, affecting the movement between ions and thus hindering the crystal conversion process.

[0012] Further, in step S2, the mass ratio of the pretreated phosphogypsum to the mixed slurry is 1:100 - 1:110, preferably 1:103. If the dosage of the pretreated phosphogypsum is too small, the dissolution will be incomplete, and it cannot be fully converted to calcium sulfate hemihydrate whiskers. If the dosage is too large, the crystal conversion will be incomplete, forming a precipitation layer and affecting the crystal conversion process.

[0013] Further, in step S2, the crystal conversion temperature is 140 °C - 160 °C, preferably 155 °C. If the crystal conversion temperature is too low, the dissolution of phosphogypsum will be incomplete, and the driving force for crystal conversion will be insufficient. If the temperature is too high, the calcium sulfate hemihydrate whiskers formed will dehydrate to form anhydrous calcium sulfate.

[0014] Further, in step S2, the drying temperature is 55 - 65 °C, and the drying time is 24 h, preferably the drying temperature is 65 °C.

[0015] The beneficial effects of the present invention are as follows: (1) The present invention uses the method of oil bath heating for temperature rise treatment, which requires lower requirements for the reaction vessel and has a simpler process compared to other high-temperature and high-pressure temperature rise methods, making it easy for industrial scale-up.

[0016] (2) The present invention uses alcohols and sugars as the crystal conversion agent, which has a lower cost, is not affected by the solution pH, and the hydroxyl groups contained therein can reduce the water activity, thereby promoting the crystal conversion process of phosphogypsum to calcium sulfate hemihydrate whiskers.

[0017] (3) By using saccharides and alcohol crystal conversion agents containing different hydroxyl groups, the present invention prepares calcium sulfate hemihydrate whiskers with different aspect ratios. Furthermore, the aspect ratio of the whiskers can be adjusted by mixing them in a certain proportion to control the hydroxyl groups, thereby realizing the adjustment of the aspect ratio of the whiskers within a certain range.

[0018] (4) The calcium sulfate hemihydrate whiskers prepared by the present invention have a uniform aspect ratio and meet the national standard of "Whiskers for Friction Materials" (GB / T 35471-2017). Description of the Drawings

[0019] Figure 1 It is a flow chart for preparing calcium sulfate hemihydrate whiskers from phosphogypsum.

[0020] Figure 2 It is a list of aspect ratios of whiskers prepared with different crystal conversion agents under different molar ratios. The results show that in the present invention, as the number of hydroxyl groups increases, the aspect ratio of the whiskers gradually increases, and the mixing of crystal conversion agents with different numbers of hydroxyl groups can be adjusted within the range of aspect ratios of whiskers prepared by the two crystal conversion agents.

[0021] Figure 3 It is a scanning electron microscope (SEM) image of the calcium sulfate hemihydrate whiskers obtained in Example 8. The results show that the whiskers have a uniform aspect ratio, a smooth and complete surface, and no defects.

[0022] Figure 4 It is an optical microscope image of the phosphogypsum raw material.

[0023] Figure 5 It is an optical microscope image of the calcium sulfate hemihydrate whiskers obtained in Example 8. Detailed Embodiments

[0024] To better illustrate the technical solution of the present invention, the following further describes it in detail with specific examples and drawings. However, the protection scope of the present invention is not limited to the following description.

[0025] The principle of the conversion of phosphogypsum to calcium sulfate hemihydrate whiskers is to utilize the solubility difference between the main component of phosphogypsum, calcium sulfate dihydrate (CaSO 4 ·2H 2 O), and calcium sulfate hemihydrate whiskers (CaSO 4 ·0.5H 2 O). By adding a certain proportion of alcohol or sugar to form a non-electrolyte reaction solvent in an aqueous solvent, the dehydration conversion of phosphogypsum to calcium sulfate hemihydrate whiskers can be achieved at a lower temperature. Among them, the crystal conversion agent, sugar or alcohol, contains hydroxyl groups, and the hydroxyl group (-OH) can combine with water (H + ) to reduce the water activity and promote the conversion of calcium sulfate dihydrate (CaSO 4 ·2H 2O) dissolve, and because the hydroxyl group can bind to the side active sites of the hemihydrate calcium sulfate crystal nucleus (CaSO 4 ·0.5H 2 O) to inhibit the lateral growth of the crystal nucleus and thus promote the growth of the crystal nucleus along the C-axis, that is, long whisker-shaped whiskers are obtained.

[0026] The amount of hydroxyl groups in the crystal modifier has different effects on the conversion process of calcium sulfate (CaSO 4 ·2H 2 O) to hemihydrate calcium sulfate whiskers (CaSO 4 ·0.5H 2 O). Therefore, by utilizing the characteristics of the difference in the amount of hydroxyl groups, the aspect ratio of the whiskers can be significantly adjusted within a certain range.

[0027] Example 1 A method for regulating the aspect ratio of hemihydrate calcium sulfate whiskers, comprising the following steps: Step S1: Wash the as-received phosphogypsum with sulfuric acid, hydrogen peroxide, and ethanol in a graded manner, dry the washed phosphogypsum, and obtain the treated phosphogypsum (purity > 95.4%) after drying. The specific method of graded water washing is to take 50 g of as-received phosphogypsum and mix it with 20% concentrated sulfuric acid for 2 h, wash it thoroughly with deionized water (three times), then mix it with 30% hydrogen peroxide for 2 h, then wash it with deionized water (three times), and then mix it with 50% ethanol for 2 h for washing, and finally wash it clean with deionized water and place it in a constant temperature oven at 65 °C to dry to constant weight.

[0028] Step S2: Mix 16.5 g of the crystal modifier glycerol with deionized water at a volume ratio of 1:17 to obtain a mixed slurry, heat it in an oil bath to 155 °C, then add the pretreated phosphogypsum. The mass ratio of phosphogypsum to the mixed slurry is 1:103. After the reaction, filter the product, wash the filter cake, and dry it at 65 °C for 24 h to obtain long rod-shaped hemihydrate calcium sulfate whiskers, and the aspect ratio of the hemihydrate calcium sulfate whiskers is 38.38.

[0029] Example 2 S1: The step of pretreating phosphogypsum is the same as that in Example 1; S2: Mix 16.5 g of xylitol with 97.4 g of deionized water to obtain a mixed slurry. After placing it in an oil bath and heating to 155 °C, add 1.1 g of pretreated phosphogypsum to the above-mentioned mixed slurry to obtain a whisker preparation mixed slurry. Among them, in the above-mentioned whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the rest is deionized water. Then, under the monitoring of the thermometer in the three-necked flask, keep heating at a constant temperature of 155 °C for 2 h for the crystal conversion reaction. After the crystal conversion is completed, filter to obtain a filter cake, wash the filter cake with water no less than three times, and then place it in a constant temperature drying oven at 65 °C for drying for 24 h. After the sample reaches a constant weight, conduct a microscopic analysis on it, and measure the aspect ratio of the hemihydrate calcium sulfate whiskers to be 58.03.

[0030] Example 3 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: Mix 16.5 g of fructose with 97.4 g of deionized water to obtain a mixed slurry. Place it in an oil bath and heat to 155 °C, and add 1.1 g of pretreated phosphogypsum to the above-mentioned mixed slurry to obtain a whisker preparation mixed slurry. Among them, in the above-mentioned whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the rest is deionized water. Then, under the monitoring of the thermometer in the three-necked flask, keep heating at a constant temperature of 155 °C for 2 h for the crystal conversion reaction. After the crystal conversion is completed, filter to obtain a filter cake, wash the filter cake with water no less than three times, and then place it in a constant temperature drying oven at 65 °C for drying for 24 h. After the sample reaches a constant weight, conduct a microscopic analysis on it, and measure the aspect ratio of the hemihydrate calcium sulfate whiskers to be 62.80.

[0031] Example 4 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: Mix 16.5 g of glucose with 97.4 g of deionized water to obtain a mixed slurry. Place it in an oil bath and heat to 155 °C, and add 1.1 g of pretreated phosphogypsum to the mixed slurry to obtain a whisker preparation mixed slurry. Among them, in the above-mentioned whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the rest is deionized water. Then, under the monitoring of the thermometer in the three-necked flask, keep heating at a constant temperature of 155 °C for 2 h for the crystal conversion reaction. After the crystal conversion is completed, filter to obtain a filter cake, wash the filter cake with water no less than three times, and then place it in a constant temperature drying oven at 65 °C for drying for 24 h. After the sample reaches a constant weight, conduct a microscopic analysis on it, and measure the aspect ratio of the hemihydrate calcium sulfate whiskers to be 66.01.

[0032] Example 5 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: 16.5 g of sorbitol is mixed with 97.4 g of deionized water to obtain a mixed slurry, which is placed in an oil bath and heated to 155 °C. 1.1 g of pretreated phosphogypsum is added to the mixed slurry to obtain a whisker preparation mixed slurry. Among the above whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the remaining components are deionized water. Then, under the monitoring of the thermometer in the three-necked flask, the temperature is kept constant at 155 °C and heating is maintained for 2 h for the crystal conversion reaction. After the crystal conversion ends, suction filtration is carried out to obtain a filter cake, and the filter cake is washed with water no less than three times. After washing, it is placed in a constant temperature drying oven at 65 °C and dried for 24 h. After the sample reaches a constant weight, it is subjected to microscopic analysis, and the aspect ratio of the hemihydrate calcium sulfate whiskers is measured to be 79.56.

[0033] Example 6 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: 16.5 g of a crystal conversion agent (molar ratio of sorbitol: glycerol = 1:3) is mixed with 97.4 g of deionized water to obtain a mixed slurry, which is placed in an oil bath and heated to 155 °C. 1.1 g of pretreated phosphogypsum is added to the mixed slurry to obtain a whisker preparation mixed slurry. Among the above whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the remaining components are deionized water. Then, under the monitoring of the thermometer in the three-necked flask, the temperature is kept constant at 155 °C and heating is maintained for 2 h for the crystal conversion reaction. After the crystal conversion ends, suction filtration is carried out to obtain a filter cake, and the filter cake is washed with water no less than three times. After washing, it is placed in a constant temperature drying oven at 65 °C and dried for 24 h. After the sample reaches a constant weight, it is subjected to microscopic analysis, and the aspect ratio of the hemihydrate calcium sulfate whiskers is measured to be 44.73.

[0034] Example 7 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: 16.5 g of a crystal conversion agent (molar ratio of sorbitol: glycerol = 2:2) is mixed with 97.4 g of deionized water to obtain a mixed slurry, which is placed in an oil bath and heated to 155 °C. 1.1 g of pretreated phosphogypsum is added to the mixed slurry to obtain a whisker preparation mixed slurry. Among the above whisker preparation mixed slurry, the mass ratio of pretreated phosphogypsum is 9.6%, the mass ratio of the crystal conversion agent is 14.3%, and the remaining components are deionized water. Then, under the monitoring of the thermometer in the three-necked flask, the temperature is kept constant at 155 °C and heating is maintained for 2 h for the crystal conversion reaction. After the crystal conversion ends, suction filtration is carried out to obtain a filter cake, and the filter cake is washed with water no less than three times. After washing, it is placed in a constant temperature drying oven at 65 °C and dried for 24 h. After the sample reaches a constant weight, it is subjected to microscopic analysis, and the aspect ratio of the hemihydrate calcium sulfate whiskers is measured to be 54.87.

[0035] Example 8 S1: The steps for pretreating phosphogypsum are the same as those in Example 1; S2: 16.5 g of the crystal modifier (molar ratio of sorbitol to glycerol = 3:1) was mixed with 97.4 g of deionized water to obtain a mixed slurry, which was placed in an oil bath and heated to 155 °C. 1.1 g of pretreated phosphogypsum was added to the mixed slurry to obtain a whisker preparation mixed slurry. In the above whisker preparation mixed slurry, the mass ratio of the pretreated phosphogypsum was 9.6%, the mass ratio of the crystal modifier was 14.3%, and the remaining components were deionized water. Then, the trans-crystallization reaction was carried out by maintaining the heating at a constant temperature of 155 °C for 2 h under the monitoring of a thermometer in a three-necked flask. After the trans-crystallization was completed, the filter cake was obtained by suction filtration, washed with water no less than three times, and then dried in a constant temperature drying oven at 65 °C for 24 h. After the sample reached a constant weight, it was subjected to microscopic analysis, and the aspect ratio of the hemihydrate calcium sulfate whiskers was measured to be 63.28.

[0036] In summary, the present invention prepared hemihydrate calcium sulfate whiskers with different aspect ratios by heat-treating a crystal modifier containing different numbers of hydroxyl groups and phosphogypsum under certain conditions. The hydroxyl groups can promote the axial growth of hemihydrate calcium sulfate crystal nuclei to obtain a higher aspect ratio. The number of hydroxyl groups can affect the aspect ratio of the prepared whiskers under certain parameter conditions. It can be seen from Examples 1-5 that under the condition of the same other parameters and different crystal modifiers, as the number of hydroxyl groups contained in the crystal modifier increases, the aspect ratio of the hemihydrate calcium sulfate whiskers prepared therefrom shows an increasing trend. Based on this, it can be seen from Example 1 (glycerol), Example 5 (sorbitol), and Examples 6-8 that by mixing glycerol (straight carbon chain) and sorbitol (straight carbon chain) in a certain gradient molar ratio to prepare hemihydrate calcium sulfate whiskers, the aspect ratio of the prepared whiskers also shows an increasing trend as the proportion of sorbitol increases. Thus, the aspect ratio of the whiskers can be further regulated by controlling the number of hydroxyl groups in the crystal modifier.

[0037] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. A method for regulating the aspect ratio of calcium sulfate hemihydrate whiskers prepared by phosphogypsum, characterized in that: The following steps are involved: Step S1: washing the original phosphogypsum with sulfuric acid, hydrogen peroxide and ethanol in a graded manner, drying the washed phosphogypsum, and performing drying treatment to obtain pretreated phosphogypsum; Step S2: Ultrasonic mixing of the crystal conversion agent and deionized water to obtain a mixed slurry, heating to the crystal conversion temperature and then adding the pretreated phosphogypsum to perform crystal conversion, filtering after the reaction is completed, washing the filter cake and drying it to obtain long rod-shaped hemihydrate calcium sulfate whiskers.

2. The method according to claim 1, characterized in that: The method of graded water washing in step S1 is to take the original phosphogypsum and mix it with 20% concentrated sulfuric acid, wash it with deionized water, and then mix it with 30% hydrogen peroxide, then wash it with deionized water and then mix it with 50% ethanol, finally wash it with deionized water, and place it in a constant temperature oven at 55~65℃ to dry to constant weight.

3. The preparation method according to claim 1, characterized in that: In step S2, the whisker ratio of calcium sulfate hemihydrate is regulated by regulating the number of hydroxyl groups in the crystal-transforming agent, specifically, by mixing crystal-transforming agents containing different numbers of hydroxyl groups.

4. The method according to claim 1, characterized in that: The time for graded water washing in step S1 is 2 hours.

5. The method according to claim 1, characterized in that: The crystal-changing agent in step S2 is a sugar or alcohol containing a hydroxyl group, including glycerol, xylitol, sorbitol, glucose, fructose, or a mixture of several thereof.

6. The method according to claim 1, characterized in that: In step S2, the water-to-material ratio of the crystallization agent to deionized water is 1:15-1:

18.

7. The method according to claim 1, characterized in that: The mass ratio of the pretreated phosphogypsum to the mixed slurry in step S2 is 1:100-1:

110.

8. The method according to claim 1, characterized in that: The crystal transformation temperature in step S2 is 140° C.-160° C., and the crystal transformation time is 2 hours.

9. The method according to claim 1, characterized in that: The drying temperature in step S2 is 55-65° C., and the drying time is 24 hours.

10. Calcium sulfate hemihydrate whisker prepared according to any one of claims 1 to 9, characterized in that: The aspect ratio of the calcium sulfate hemihydrate whisker is 38-80.

Images