Preparation method of calcite-based modified anhydrous calcium sulfate whisker

Abstract

The invention discloses a calcite-based modified anhydrous calcium sulfate whisker preparation method, which comprises: 1, adding concentrated sulfuric acid to a calcite suspension, and completely reacting to obtain calcium sulfate; step 2, aging and crystallizing the calcium sulfate obtained in the step 1; 3, after dewatering and filtering, drying and calcining at the calcining temperature of 600-750 DEG C; and 4, fully mixing the calcium sulfate whiskers obtained in the step 3 with a mixed solution composed of a siloxane coupling agent and ethanol under a stirring condition, and reacting to obtain the required anhydrous modified calcium sulfate whiskers. Calcite is directly used as a raw material, and a series of problems existing in a traditional hydrothermal method and an acid fracturing method are avoided. Compared with the calcium sulfate whisker obtained by the existing method, the obtained anhydrous calcium sulfate whisker has better reinforcing and toughening effects.

Description

Technical Field

[0001] This invention relates to the field of materials preparation technology, and specifically discloses a method for preparing modified anhydrous calcium sulfate whiskers based on calcite. Background Technology

[0002] Calcium sulfate whiskers are fibrous or needle-like single crystals with a perfect internal structure, high strength, good toughness, high temperature resistance, and good chemical stability. They are a high-performance, low-cost, and environmentally friendly reinforcing material, widely used as a reinforcing material in industries such as plastics, rubber, polymer materials, friction materials, building materials, and papermaking.

[0003] Currently, the raw materials for preparing calcium sulfate whiskers are mostly natural gypsum (calcium sulfate dihydrate) or various industrial by-product gypsum (such as phosphogypsum, desulfurized gypsum, citric acid gypsum, etc.). Although these methods are relatively mature, industrial by-product gypsum has a complex composition, contains many impurities, and has high pretreatment costs, affecting the purity and performance of the final product.

[0004] In addition, traditional methods for preparing calcium sulfate whiskers, such as hydrothermal method, atmospheric pressure acidification method, ion exchange method, and emulsification method, have the following problems: low and unstable quality, high cost, large equipment investment, unsafe use and short life of aging and crystallization equipment, long process flow, difficult to operate, large labor input, not easy to scale up industrial production, and environmentally unfriendly with wastewater discharge.

[0005] Calcite, primarily composed of calcium carbonate, is one of the most widely distributed and abundant calcium sources in nature, and it is also inexpensive. If high-value-added calcium sulfate whiskers could be prepared directly from calcite powder (primarily calcium carbonate) through a simple chemical transformation, production costs could be significantly reduced, enabling deep processing and high-value utilization of mineral resources, resulting in significant economic and environmental benefits. However, the direct reaction of calcium carbonate with sulfuric acid readily produces granular or short columnar calcium sulfate precipitates, making it difficult to control their crystallization into fibrous or needle-like whiskers. Summary of the Invention

[0006] This invention addresses the problems existing in the prior art by providing a method for preparing modified anhydrous calcium sulfate whiskers based on calcite.

[0007] The technical solution adopted in this invention is: a method for preparing modified anhydrous calcium sulfate whiskers based on calcite, comprising the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; Step 2: Aging and crystallizing the calcium sulfate obtained in Step 1; Step 3: After dehydration and filtration, dry the product and then calcine it at a temperature of 600–750 °C. Step 4: The calcium sulfate whiskers obtained in Step 3 are thoroughly mixed with a mixture of siloxane coupling agent and ethanol under stirring conditions. After the reaction, the desired anhydrous modified calcium sulfate whiskers can be obtained.

[0008] Furthermore, in step 1, the calcite suspension is an aqueous suspension of calcite, and concentrated sulfuric acid is added under stirring conditions; the stirring frequency is 40 Hz; after adding concentrated sulfuric acid to the calcite suspension in step 1, the reaction is continuously stirred for 150 min, and the pH value is 4 after the reaction is completed.

[0009] Furthermore, the mass ratio of calcite to concentrated sulfuric acid is 1:1.

[0010] Furthermore, the aging and crystallization process in step 2 is as follows: The calcium sulfate obtained in step 1 was heated to 60 °C, and sodium hydroxide, titanium dioxide and oxalic acid were added in sequence. After the addition was completed, the temperature was raised to the reaction temperature. Under stirring conditions, the reaction temperature was 100-120 °C, the reaction pressure was 0.15-0.25 MPa, and the aging was completed in 30 min.

[0011] Furthermore, in step 3, the dehydration filtration is performed using an ultrasonic filter, and the moisture content of the filtered material is <45%; in step 1, the concentrated sulfuric acid is added for 80 minutes.

[0012] Furthermore, the mass ratio of sodium hydroxide, titanium dioxide, and oxalic acid is 42:43:28.

[0013] Furthermore, the drying temperature in step 3 is 180–250 °C.

[0014] Furthermore, the volume ratio of siloxane coupling agent to ethanol in the mixture of step 4 is 1:1 to 5.

[0015] Furthermore, in step 4, the stirring speed is 200–1000 rpm.

[0016] Furthermore, in step 4, the siloxane coupling agent is one of KH550, KH560, and KH570.

[0017] The beneficial effects of this invention are: This invention uses calcite directly as raw material, avoiding a series of problems associated with traditional hydrothermal and acid fracturing methods. The resulting anhydrous calcium sulfate whiskers exhibit better reinforcing and toughening effects compared to calcium sulfate whiskers obtained by existing methods. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0020] like Figure 1 As shown, a method for preparing calcite-modified anhydrous calcium sulfate whiskers includes the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; The calcite suspension was an aqueous suspension of calcite. Concentrated sulfuric acid was added under stirring conditions at a frequency of 40 Hz. After adding concentrated sulfuric acid to the calcite suspension, the mixture was stirred continuously for 150 min. The pH value after the reaction was completed was 4. The mass ratio of calcite to concentrated sulfuric acid was 1:1.

[0021] Step 2: Aging and crystallizing the calcium sulfate obtained in Step 1; The aging and crystallization process is as follows: The calcium sulfate obtained in step 1 was heated to 60 °C, and sodium hydroxide, titanium dioxide, and oxalic acid were added sequentially. After the addition was complete, the temperature was raised to the reaction temperature. Under stirring conditions, the reaction temperature was maintained at 100–120 °C, the reaction pressure at 0.15–0.25 MPa, and aging was completed in 30 min. The mass ratio of sodium hydroxide, titanium dioxide, and oxalic acid was 42:43:28.

[0022] Step 3: After dehydration and filtration, the material is dried and then calcined at a temperature of 600–750 °C. Dehydration and filtration are performed using an ultrasonic filter, and the moisture content of the filtered material is <45%. The concentrated sulfuric acid added in Step 1 is for 80 min. The drying temperature is 180–250 °C.

[0023] Step 4: The calcium sulfate whiskers obtained in Step 3 are mixed at high speed with a mixture of siloxane coupling agent and ethanol in a high-speed mixer. After the reaction, the desired anhydrous modified calcium sulfate whiskers are obtained. The volume ratio of siloxane coupling agent to ethanol in the mixture is 1:1 to 5. The stirring speed is 200 to 1000 rpm. The siloxane coupling agent is one of KH550, KH560, and KH570.

[0024] Example 1 A method for preparing calcite-modified anhydrous calcium sulfate whiskers includes the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; The raw materials are 325-mesh calcite powder and 98% concentrated sulfuric acid. The calcite powder has a calcium content of 98.5%, a whiteness of 95%, a purity of 98.5%, and a moisture content of ≤2%.

[0025] 1000 kg of calcite, 1000 kg of concentrated sulfuric acid, and 25 t of tap water or circulating water.

[0026] Finely ground calcite powder is added to 25 t of tap water or circulating water. 98% concentrated sulfuric acid is slowly added under stirring at a frequency of 40 Hz. The acid addition time is 80 min. After the acid is added, the reaction is stirred for another 150 min until calcium carbonate is converted to calcium sulfate. The pH of the emulsion is about 4 and the temperature of the emulsion is room temperature. After the reaction is completed, the emulsion is pumped to the next stage.

[0027] Step 2: Calcium sulfate undergoes aging and crystallization. The calcium sulfate emulsion from the previous section was transported in batches to 10 m. 3 In the reaction vessel, the rated feed rate per batch is 8 m³. 3 Stirring was performed at 15 Hz, and the reactor was heated. When the material temperature reached 60 ℃, 4.2 kg of sodium hydroxide, 4.3 kg of titanium dioxide, and 2.8 kg of oxalic acid were added sequentially. The feeding valve was immediately closed after the additives were added. The material inside the reactor was monitored as the temperature rose to 110 ℃. When the material began to thicken, the stirring speed was increased to 40 Hz. After it had completely thinned, the stirring speed was reduced to 20 Hz. The temperature inside the reactor was maintained at 110 ℃, and the pressure inside the reactor was controlled at 0.2 MPa. The material was discharged after aging for 30 minutes.

[0028] Step 3: After dehydration and filtration, dry the product and then calcine it. An ultrasonic filter (model ZDG-120 / 900, filter capacity 600 kg / h) is used to control the moisture content of the material after filtration to <45%.

[0029] The filtration process is divided into three stages: The vacuum level is 0.01–0.02 MPa in the first stage; 0.03–0.06 MPa in the second stage; and 0.01–0.02 MPa in the third stage. The material thickness is 3–4 cm, and the filter cloth operating frequency is 15 Hz.

[0030] Using natural gas as fuel, a belt calciner was used to dehydrate and dry calcium sulfate whiskers: The temperature of the drying layer conveyor belt is 180 / 250 / 250 / 250 / 250 ℃. The operating speed of the drying layer conveyor belt is 850 mm / min. The calcination layer temperature is preheated to 600 / 750 / 750 / 750 / 750 ℃; the calcination layer steel belt running speed is 750 mm / min; The material dispersion particle size is 1cm*1cm; the thickness of the drying layer is 3.5~4cm; the calcination product is 600kg / h.

[0031] Step 4: Modify to obtain anhydrous modified calcium sulfate whiskers.

[0032] Calcined calcium sulfate whiskers were added to a high-speed mixer at 800 rpm, along with a mixture of KH550 and ethanol in a 1:3 volume ratio. The relative proportions of the mixture and the calcium sulfate whiskers were determined as needed. After the mixture was added, stirring was continued for 5 minutes to obtain anhydrous modified calcium sulfate whiskers.

[0033] After anhydrous calcium sulfate whiskers are prepared, they are metered and packaged.

[0034] Example 2 A method for preparing calcite-modified anhydrous calcium sulfate whiskers includes the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; The raw materials are 325-mesh calcite powder and 98% concentrated sulfuric acid. The calcite powder has a calcium content of 98.5%, a whiteness of 95%, a purity of 98.5%, and a moisture content of ≤2%.

[0035] 1000 kg of calcite, 1000 kg of concentrated sulfuric acid, and 25 t of tap water or circulating water.

[0036] Finely ground calcite powder is added to 25 t of tap water or circulating water. 98% concentrated sulfuric acid is slowly added under stirring at a frequency of 40 Hz. The acid addition time is 80 min. After the acid is added, the reaction is stirred for another 150 min until calcium carbonate is converted to calcium sulfate. The pH of the emulsion is about 4 and the temperature of the emulsion is room temperature. After the reaction is completed, the emulsion is pumped to the next stage.

[0037] Step 2: Calcium sulfate undergoes aging and crystallization. The calcium sulfate emulsion from the previous section was transported in batches to 10 m. 3 In the reaction vessel, the rated feed rate per batch is 8 m³. 3 Stirring was performed at 15 Hz, and the reactor was heated. When the material temperature reached 60 ℃, 4.2 kg of sodium hydroxide, 4.3 kg of titanium dioxide, and 2.8 kg of oxalic acid were added sequentially. The feeding valve was immediately closed after the additives were added. The material inside the reactor was monitored as the temperature reached 100 ℃. When the material began to thicken, the stirring speed was increased to 40 Hz. After it had completely thinned, the stirring speed was reduced to 20 Hz. The temperature inside the reactor was maintained at 100 ℃, and the pressure inside the reactor was controlled at 0.25 MPa. The material was discharged after aging for 30 minutes.

[0038] Step 3: After dehydration and filtration, dry the product and then calcine it. An ultrasonic filter (model ZDG-120 / 900, filter capacity 600 kg / h) is used to control the moisture content of the material after filtration to <45%.

[0039] The filtration process is divided into three stages: The vacuum level is 0.01–0.02 MPa in the first stage; 0.03–0.06 MPa in the second stage; and 0.01–0.02 MPa in the third stage. The material thickness is 3–4 cm, and the filter cloth operating frequency is 15 Hz.

[0040] Using natural gas as fuel, a belt calciner was used to dehydrate and dry calcium sulfate whiskers: The temperature of the drying layer conveyor belt is 180 / 250 / 250 / 250 / 250 ℃. The operating speed of the drying layer conveyor belt is 850 mm / min. The calcination layer temperature is preheated to 600 / 750 / 750 / 750 / 750 ℃; the calcination layer steel belt running speed is 750 mm / min; The material dispersion particle size is 1cm*1cm; the thickness of the drying layer is 3.5~4cm; the calcination product is 600kg / h.

[0041] Step 4: Modify to obtain anhydrous modified calcium sulfate whiskers.

[0042] Calcined calcium sulfate whiskers were added to a high-speed mixer. At 1000 rpm, a mixture of KH570 and ethanol in a 1:5 volume ratio was added. The relative proportions of the mixture and the calcium sulfate whiskers were determined as needed. After the mixture was added, stirring was continued for 5 minutes to obtain anhydrous modified calcium sulfate whiskers.

[0043] After anhydrous calcium sulfate whiskers are prepared, they are metered and packaged.

[0044] Example 3 A method for preparing calcite-modified anhydrous calcium sulfate whiskers includes the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; The raw materials are 325-mesh calcite powder and 98% concentrated sulfuric acid. The calcite powder has a calcium content of 98.5%, a whiteness of 95%, a purity of 98.5%, and a moisture content of ≤2%.

[0045] 1000 kg of calcite, 1000 kg of concentrated sulfuric acid, and 25 t of tap water or circulating water.

[0046] Finely ground calcite powder is added to 25 t of tap water or circulating water. 98% concentrated sulfuric acid is slowly added under stirring at a frequency of 40 Hz. The acid addition time is 80 min. After the acid is added, the reaction is stirred for another 150 min until calcium carbonate is converted to calcium sulfate. The pH of the emulsion is about 4 and the temperature of the emulsion is room temperature. After the reaction is completed, the emulsion is pumped to the next stage.

[0047] Step 2: Calcium sulfate undergoes aging and crystallization. The calcium sulfate emulsion from the previous section was transported in batches to 10 m. 3 In the reaction vessel, the rated feed rate per batch is 8 m³. 3 Stirring was performed at 15 Hz, and the reactor was heated. When the material temperature reached 60 ℃, 4.2 kg of sodium hydroxide, 4.3 kg of titanium dioxide, and 2.8 kg of oxalic acid were added sequentially. The feeding valve was immediately closed after the additives were added. The material inside the reactor was monitored as the temperature reached 120 ℃. When the material began to thicken, the stirring speed was increased to 40 Hz. After it had completely thinned, the stirring speed was reduced to 20 Hz. The temperature inside the reactor was maintained at 120 ℃, and the pressure inside the reactor was controlled at 0.15 MPa. The material was discharged after aging for 30 minutes.

[0048] Step 3: After dehydration and filtration, dry the product and then calcine it. An ultrasonic filter (model ZDG-120 / 900, filter capacity 600 kg / h) is used to control the moisture content of the material after filtration to <45%.

[0049] The filtration process is divided into three stages: The vacuum level is 0.01–0.02 MPa in the first stage; 0.03–0.06 MPa in the second stage; and 0.01–0.02 MPa in the third stage. The material thickness is 3–4 cm, and the filter cloth operating frequency is 15 Hz.

[0050] Using natural gas as fuel, a belt calciner was used to dehydrate and dry calcium sulfate whiskers: The temperature of the drying layer conveyor belt is 180 / 250 / 250 / 250 / 250 ℃. The operating speed of the drying layer conveyor belt is 850 mm / min. The calcination layer temperature is preheated to 600 / 750 / 750 / 750 / 750 ℃; the calcination layer steel belt running speed is 750 mm / min; The material dispersion particle size is 1cm*1cm; the thickness of the drying layer is 3.5~4cm; the calcination product is 600kg / h.

[0051] Step 4: Modify to obtain anhydrous modified calcium sulfate whiskers.

[0052] Calcined calcium sulfate whiskers were added to a high-speed mixer at 200 rpm, along with a 1:1 volume ratio of KH560 and ethanol. The relative proportions of the mixture and the calcium sulfate whiskers were determined as needed. After the mixture was added, stirring was continued for 5 minutes to obtain anhydrous modified calcium sulfate whiskers.

[0053] After anhydrous calcium sulfate whiskers are prepared, they are metered and packaged.

[0054] The performance indicators of the calcium sulfate whiskers obtained in steps 3 and 4 of Example 1 of this invention are shown in Table 1: Table 1. Physical properties of modified and unmodified calcium sulfate whiskers

[0055] As can be seen from Table 1, the activation index (hydrophobicity) of the modified anhydrous calcium sulfate whiskers is significantly improved, and their compatibility with polymers is also better.

[0056] The calcium sulfate whiskers obtained in steps 3 and 4 were added to the polypropylene matrix at a mass fraction of 20 wt.% for blending. The mechanical properties of the resulting composite materials are shown in Table 2.

[0057] Table 2. Comparison of Mechanical Properties

[0058] As can be seen from Table 2, compared with unmodified calcium sulfate whiskers, modified calcium sulfate whiskers improve the impact toughness and rigidity of polypropylene composites through interfacial reinforcement.

[0059] This invention uses calcite directly as raw material, avoiding a series of problems associated with traditional hydrothermal and acid fracturing methods. The resulting anhydrous calcium sulfate whiskers exhibit better reinforcing and toughening effects compared to calcium sulfate whiskers obtained by existing methods.

Claims

1. A method for preparing calcite-modified anhydrous calcium sulfate whiskers, characterized in that, Includes the following steps: Step 1: Add concentrated sulfuric acid to the calcite suspension and allow it to react completely to obtain calcium sulfate; Step 2: Aging and crystallizing the calcium sulfate obtained in Step 1; Step 3: After dehydration and filtration, dry the product and then calcine it at a temperature of 600–750 °C. Step 4: The calcium sulfate whiskers obtained in Step 3 are thoroughly mixed with a mixture of siloxane coupling agent and ethanol under stirring conditions. After the reaction, the desired anhydrous modified calcium sulfate whiskers can be obtained.

2. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, In step 1, the calcite suspension is an aqueous suspension of calcite, and concentrated sulfuric acid is added under stirring conditions; the stirring frequency is 40 Hz; after adding concentrated sulfuric acid to the calcite suspension in step 1, the reaction is continuously stirred for 150 min, and the pH value is 4 after the reaction is completed.

3. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, The mass ratio of calcite to concentrated sulfuric acid is 1:

1.

4. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, The aging and crystallization process in step 2 is as follows: The calcium sulfate obtained in step 1 was heated to 60 °C, and sodium hydroxide, titanium dioxide and oxalic acid were added in sequence. After the addition was completed, the temperature was raised to the reaction temperature. Under stirring conditions, the reaction temperature was 100-120 °C, the reaction pressure was 0.15-0.25 MPa, and the aging was completed in 30 min.

5. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, In step 3, the dehydration filtration is performed using an ultrasonic filter, and the moisture content of the filtered material is <45%; in step 1, the concentrated sulfuric acid is added for 80 minutes.

6. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 4, characterized in that, The mass ratio of sodium hydroxide, titanium dioxide, and oxalic acid is 42:43:

28.

7. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, The drying temperature in step 3 is 180–250 °C.

8. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, The volume ratio of siloxane coupling agent to ethanol in the mixture of step 4 is 1:1 to 5.

9. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, The stirring speed in step 4 is 200–1000 rpm.

10. The method for preparing calcite-modified anhydrous calcium sulfate whiskers according to claim 1, characterized in that, In step 4, the siloxane coupling agent is one of KH550, KH560, and KH570.

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