Process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt slurry
阅读说明：本技术 一种利用盐泥连续制备半水和二水硫酸钙晶须的工艺 (Process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt slurry ) 是由 郭方元 陈留平 崔耀星 苏志俊 李直 于 2021-07-28 设计创作，主要内容包括：本发明涉及一种利用盐泥连续制备半水和二水硫酸钙晶须的工艺,首先以盐泥为原料,加入稀盐酸溶解过滤得到盐泥酸浸液。然后进行稀释,与等摩尔等体积的硫酸钠一起加入厚壁耐压瓶中加热,待晶须成长完成后过滤。分别收集滤液和固体。固体洗涤干燥,得到半水硫酸钙晶须；滤液再次加入厚壁耐压瓶中,放入保温箱陈化,冷却后,过滤洗涤干燥,得到二水硫酸钙晶须。本发明无需先制备硫酸钙前驱体,无需添加分散剂,无需搅拌,有效利用滤液和余热,提高晶须总体收率,充分利用能源,降低能耗。并有效利用了盐泥中的镁离子和氯化钠,提高二水硫酸钙晶须的品质,采用的原料为氯碱行业废物盐泥,其产量巨大,盐泥的利用具有很高的经济价值和环保意义。(The invention relates to a process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt slurry. Then diluting, adding the diluted solution and sodium sulfate with equal molar volume into a thick-wall pressure-resistant bottle, heating, and filtering after the whisker growth is finished. The filtrate and solid were collected separately. Washing and drying the solid to obtain calcium sulfate hemihydrate crystal whiskers; and adding the filtrate into a thick-wall pressure-resistant bottle again, putting the thick-wall pressure-resistant bottle into a heat preservation box for aging, cooling, filtering, washing and drying to obtain the calcium sulfate dihydrate crystal whisker. According to the invention, the calcium sulfate precursor is not required to be prepared, the dispersing agent is not required to be added, the stirring is not required, the filtrate and the waste heat are effectively utilized, the overall yield of the crystal whisker is improved, the energy is fully utilized, and the energy consumption is reduced. The magnesium ions and sodium chloride in the salty mud are effectively utilized, the quality of the calcium sulfate dihydrate crystal whiskers is improved, the adopted raw material is the salty mud which is the waste of the chlor-alkali industry, the yield is high, and the utilization of the salty mud has high economic value and environmental protection significance.)
1. A process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt mud takes the salt mud as a raw material, and is characterized in that: the preparation process comprises the following steps of,
(1) adding 12% of dilute hydrochloric acid into the salt slurry for dissolving, and filtering insoluble substances to obtain a salt slurry pickle liquor;
(2) detecting the concentration of calcium chloride in the pickle liquor, diluting the pickle liquor to the concentration required by the reaction, and dropwise adding 35% concentrated hydrochloric acid to adjust the pH value;
(3) and mixing the calcium chloride solution with a mixture with the concentration and the volume of 1: 1, adding the sodium sulfate solution into a thick-wall pressure-resistant bottle together, and heating for reaction;
(4) filtering while the crystal whisker is hot after the crystal whisker growth is completed, and respectively collecting filtrate and solid;
(5) washing and drying the solid obtained by filtering in the step (4) to obtain calcium sulfate hemihydrate crystal whiskers;
(6) pouring the filtrate obtained in the step (4) into a thick-wall pressure-resistant bottle, putting the thick-wall pressure-resistant bottle into a heat preservation box for aging, then cooling, filtering, washing and drying to obtain the calcium sulfate dihydrate crystal whisker.
2. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 1, characterized by: the salt mud is chlor-alkali industrial waste and comprises the following components in parts by weight: CaCO3：60.5％～67.5％、H2O：20.5％～25.5％、、CaSO4：5.5％～10.5％、Mg(OH)2: 0.5% -2.5%, NaCl: 0.1 to 1.5 percent, and the balance of SS.
3. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 1, characterized by: and (3) adjusting the pH value of the calcium chloride solution in the step (2) to be 0.2-2.
4. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 3, characterized by: the concentration of the calcium chloride solution in the step (2) is 0.3-0.7 mol/L.
5. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 1, characterized by: the heating reaction in the step (3) is carried out at the temperature of 100-130 ℃ for 1-4 h.
6. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 1, characterized by: in the step (5), hot water with the temperature of more than 90 ℃ is used for washing, the drying temperature is 80-100 ℃, and the time is 1-2 hours.
7. A process for the continuous preparation of calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 1, characterized by: and (6) the cooling rate of the solution in the heat preservation box is not more than 0.2-0.4 ℃/min, and the aging time is 1-3 h.
8. The method for continuously preparing calcium sulfate hemihydrate and dihydrate whiskers as claimed in claim 7, characterized by: and (3) in the step (6), normal-temperature tap water is used for washing, the drying temperature is 80-85 ℃, and the time is 1-2 hours.
Technical Field
The invention relates to the technical field of environmental protection and solid waste recycling, in particular to a process for preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt slurry in a hydrothermal mode.
Background
The calcium sulfate crystal whisker is a calcium sulfate fibrous single crystal, has uniform cross section, complete appearance and complete internal structure, and has the characteristics of high strength, high modulus, high toughness, wear resistance, high temperature resistance and the like. Calcium sulfate whiskers can be divided into dihydrate, hemihydrate and anhydrous calcium sulfate whiskers according to the content of crystal water in crystals, and whiskers with different crystal water contents can be applied to different industrial fields, wherein the dihydrate calcium sulfate whiskers are widely applied to the papermaking industry, and the anhydrous calcium sulfate whiskers are mostly applied to the industries such as coatings, paints, automobile brake pads and the like. Calcium sulfate hemihydrate whiskers are generally converted to dihydrate or anhydrous whiskers for use due to their poor stability.
The main synthesis methods of the calcium sulfate whisker at present comprise a normal pressure acidification method, a hydrothermal synthesis method, an ion exchange method and a microemulsion method, wherein the hydrothermal synthesis method is the main method used in the current industrial production, and the main process flow for preparing the calcium sulfate whisker by the hydrothermal method comprises the following steps: grinding calcium sulfate dihydrate, preparing slurry, carrying out hydrothermal reaction, filtering and drying.
Chinese patent CN110205671A discloses a method for preparing calcium sulfate dihydrate whisker and calcium sulfate hemihydrate whisker. The method comprises the steps of taking semi-hydrated gypsum or III type anhydrous gypsum as a raw material, mixing the semi-hydrated gypsum or III type anhydrous gypsum in a solution containing a crystal modifier, carrying out hydration crystallization to obtain calcium sulfate dihydrate crystal whiskers, and then heating the calcium sulfate dihydrate crystal whiskers to obtain the calcium sulfate hemihydrate crystal whiskers.
Chinese patent CN112342606A discloses a method for preparing calcium sulfate whiskers by using salt slurry, which comprises the steps of grinding the salt slurry, adding water, stirring, standing twice to obtain a solution, and evaporating to dryness to obtain hydrated salt slurry. Then adding calcium chloride and hydrochloric acid to carry out hydrothermal synthesis to prepare the calcium sulfate whisker, wherein a constant stirring speed is required to be kept in the synthesis process, otherwise, the obtained crystal is uneven in thickness or easy to break.
Chinese patent CN108950674A discloses a method for preparing calcium sulfate hemihydrate and anhydrous crystal whiskers by a hydrothermal method in one step, which takes a calcium nitrate solution, a calcium chloride solution or a mixed solution of calcium nitrate and calcium chloride produced in the smelting industry and the electroplating treatment process as raw materials, uses sulfuric acid as a calcium ion precipitator, and prepares the calcium sulfate hemihydrate and anhydrous crystal whiskers by hydrothermal reaction in a hydrothermal kettle by controlling reaction conditions.
In summary, most of the prior art schemes are complicated in steps, time is consumed for pretreatment, and a crystal modifier and a dispersant are often required to be added in a hydrothermal reaction, or strong acid is used as a reaction raw material. In addition, the hydrothermal preparation of the calcium sulfate whisker usually needs to be carried out while the solution is hot, the content of calcium sulfate in high-temperature filtrate is high, and the filtrate is directly returned to the pretreatment, so that the solution heat is wasted, and the yield of the whisker is reduced. Therefore, it is meaningful to use a simple process and mild raw materials, and to fully utilize energy and raw materials in the process.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: in order to overcome the defects in the prior art, the invention provides a process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt mud, which adopts industrial waste salt mud as a raw material, fully utilizes impurity metal elements in the salt mud and improves the quality of the whiskers. In addition, reaction waste heat is fully utilized to prepare calcium sulfate dihydrate crystal whiskers, the yield of the crystal whiskers is increased, and energy consumption is reduced.
The technical scheme adopted by the invention for solving the technical problems is as follows: a process for continuously preparing calcium sulfate hemihydrate crystal whisker and calcium sulfate dihydrate crystal whisker from salt mud includes such steps as adding diluted hydrochloric acid to salt mud, stirring at ordinary temp for reaction, filtering, diluting filtrate to regulate pH value, adding it to the pressure-resistant bottle, hydrothermal reaction to obtain calcium sulfate hemihydrate crystal whisker, collecting filtrate, loading it in insulating box, slowly cooling and ageing.
The preparation method comprises the following specific steps:
(1) adding 12% dilute hydrochloric acid into the salt slurry, and stirring until no bubbles appear; the original composition of the salt mud is calculated by weightThe parts are calculated as: CaCO3：60.5％～67.5％、H2O：20.5％～25.5％、CaSO4：5.5％～10.5％、Mg(OH)2: 0.5% -2.5%, NaCl: 0.1 to 1.5 percent, and the balance of SS.
(2) And (2) filtering the reaction system obtained in the step (1), and measuring the metal ion concentration in the reaction system.
(3) Diluting the filtrate obtained in the step (2) to enable the concentration of calcium ions to be 0.3-0.7 mol/L, and then adding concentrated hydrochloric acid to adjust the pH value to be 0.2-2.
(4) And (3) adding the solution obtained in the step (3) and an equimolar equal volume of sodium sulfate solution into a thick-wall pressure-resistant bottle, heating to 100-130 ℃, and reacting for 1-4 hours.
(5) And (5) filtering the mixture while the mixture is hot after the reaction in the step (4) is finished, and respectively collecting filtrate and solid.
(6) And (3) washing the solid obtained by filtering in the step (5) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol, and heating and drying by blowing to obtain the calcium sulfate hemihydrate crystal whisker.
The drying temperature is 80-100 ℃, and the drying time is 1-2 h.
(7) And (3) pouring the filtrate obtained by filtering in the step (5) into a thick-wall pressure-resistant bottle, quickly putting the thick-wall pressure-resistant bottle into a heat preservation box, slowly cooling the thick-wall pressure-resistant bottle to precipitate crystals, and aging the crystals.
The solution cooling rate in the heat preservation box is not more than 0.2-0.4 ℃/min, and the aging time is 1-3 h.
(8) And (7) after the ageing is finished, cooling to room temperature, filtering, washing a filter cake with tap water, then washing with absolute ethyl alcohol, and heating and drying by blowing to obtain the calcium sulfate dihydrate crystal whisker.
The drying temperature is 80-85 ℃, and the drying time is 1-2 h.
During the hydrothermal synthesis, the reaction system undergoes the following changes: calcium ions in the solution react with sulfate ions to generate calcium sulfate dihydrate solid; dissolving the calcium sulfate dihydrate solid in the high-temperature water solution again; calcium sulfate hemihydrate solids precipitate at high temperatures and gradually grow into needle-like crystals. In this process, there are many times involved precipitation and dissolution of calcium sulfate solids, which are related to the solubility of calcium sulfate solids in water for different crystal waters. Solubility is related to the concentration of ions in the solution, the temperature of the solution, and the pH. Therefore, the invention adjusts the generation rate of crystal nucleus and the precipitation rate of solid by regulating and controlling the ion concentration, the temperature and the pH value of the reaction system, thereby obtaining the high-quality calcium sulfate hemihydrate crystal whisker.
In the process of preparing calcium sulfate dihydrate crystal whiskers by aging filtrate, the appearance of the crystal is mainly influenced by the number of crystal nuclei and the growth rate of different crystal faces. Therefore, the invention strictly controls the cooling rate of the filtrate, thereby controlling the number of crystal nuclei. In addition, studies have shown small amounts of Mg2+And NaCl, the axial growth of the crystals being promoted by the fact that the salt slurry itself contains Mg (OH)2And NaCl, Mg is retained during the acid treatment of the salt slurry2+And NaCl exert their synergistic effect.
The invention has the beneficial effects that: the invention provides a process for continuously preparing semi-hydrated and dihydrate calcium sulfate whiskers by using salt slurry, which can be used as a raw material of a hydrothermal reaction by simply carrying out acid treatment on industrial waste salt slurry in the chlor-alkali industry. The salt mud has huge yield, and has high economic value and environmental protection significance. In the hydrothermal reaction process, no extra crystal transformation agent and modifier are needed to be added, concentrated sulfuric acid is not adopted, sodium sulfate is used as a raw material, stirring is not needed, the existing process is greatly simplified, the reaction conditions are mild, and the requirements on equipment are reduced.
The invention effectively utilizes the filtrate of the hydrothermal reaction, fully utilizes the waste heat, and slowly cools and ages the filtrate to prepare the calcium sulfate dihydrate crystal whisker. The crystal whisker growth process has high requirements on the cooling rate of the solution, the cooling rate is not more than 0.2-0.4 ℃/min, the cooling is too fast, and the amount of crystal nuclei generated in the moment is too much, so that flaky calcium sulfate dihydrate crystals can be generated. In addition, the salt mud component contains proper amount of Mg (OH)2And NaCl, which has a synergistic effect on whisker growth. Therefore, the invention provides higher requirements on the quality of the salt slurry and the reaction temperature control aspect.
Drawings
The invention is further illustrated with reference to the following figures and examples.
Figure 1 is a polarization microscope photograph of a calcium sulfate hemihydrate whisker sample 1 prepared in example 1 of the present invention.
Fig. 2 is a polarization microscope photograph of calcium sulfate dihydrate whisker sample 2 prepared in example 1 of the present invention.
Figure 3 is a polarization microscope photograph of comparative sample 1 of calcium sulfate hemihydrate whiskers made in comparative example 1.
Fig. 4 is a polarization microscope photograph of comparative calcium sulfate dihydrate whisker sample 2 prepared in comparative example 1.
Figure 5 is a polarization micrograph of the calcium sulfate hemihydrate whiskers comparative sample 3 made in comparative example 2.
Fig. 6 is a polarization microscope photograph of comparative calcium sulfate dihydrate whisker sample 4 prepared in comparative example 2.
Figure 7 is a polarization micrograph of comparative calcium sulfate hemihydrate whisker sample 5 made in comparative example 3.
Figure 8 is a polarization micrograph of the calcium sulfate hemihydrate whisker comparative sample 6 made in comparative example 4.
Fig. 9 is an XRD pattern of sample 1, comparative samples 1, 3, 5, 6.
Figure 10 is an XRD pattern of sample 2, comparative samples 2, 4.
Detailed Description
The present invention will now be described in further detail with reference to examples.
Example 1
The composition of the salty mud used in this example was calculated in parts by weight as follows: CaCO3：64.8％、H2O：23.5％、CaSO4：9.2％、Mg(OH)2: 1.4%, NaCl: 0.5% and the balance SS.
(1) Dropwise adding hydrochloric acid (the mass fraction is 21%) into the 100g of the salt slurry until no bubbles appear in a reaction system;
(2) and (2) filtering the reaction system obtained in the step (1), and measuring the metal ion concentration in the reaction system.
(3) And (3) diluting the filtrate obtained in the step (2) until the concentration of calcium ions is 0.5mol/L and the concentration of magnesium ions is 0.015mol/L, and then adding concentrated hydrochloric acid to adjust the pH value to 0.2.
(4) And (3) adding the solution obtained in the step (3) and an equimolar equal volume of sodium sulfate solution into a thick-wall pressure-resistant bottle, heating to 130 ℃, and reacting for 3 hours.
(5) And (5) filtering while the reaction is hot in the step (4) and respectively collecting filtrate and solid.
(6) And (3) washing the solid obtained by filtering in the step (5) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol (removing free moisture by using ethyl alcohol), and carrying out forced air heating and drying at the temperature of 80 ℃ for 2 hours to obtain the calcium sulfate hemihydrate crystal whisker, namely the sample 1.
(7) Pouring the filtrate obtained by filtering in the step (5) into a thick-wall pressure-resistant bottle, wherein the filtrate is 91.5 ℃, quickly putting the thick-wall pressure-resistant bottle into a heat preservation box, slowly cooling the thick-wall pressure-resistant bottle to separate out crystals, and aging the thick-wall pressure-resistant bottle for 3 hours, wherein the temperature of the filtrate is 66.2 ℃ after the aging is finished.
(8) And (3) after the aging in the step (7) is finished, cooling the system to room temperature, filtering, washing a filter cake with tap water, then washing with absolute ethyl alcohol, and heating and drying for 2 hours by blowing at 80 ℃ to obtain the calcium sulfate dihydrate crystal whisker, which is a sample 2.
Comparative example 1
The composition of the salt mud used in this comparative example was determined to be, in parts by weight: CaCO 3: 64.8%, H2O: 23.5%, CaSO 4: 9.2%, Mg (OH) 2: 1.4%, NaCl: 0.5% and the balance SS.
(1) Dropwise adding hydrochloric acid (the mass fraction is 21%) into the 100g of the salt slurry until no bubbles appear in a reaction system;
(2) and (2) filtering the reaction system obtained in the step (1), and measuring the metal ion concentration in the reaction system.
(3) Diluting the filtrate obtained in the step (2) until the concentration of calcium ions is 1mol/L and the concentration of magnesium ions is 0.03mol/L, and then adding sodium hydroxide to adjust the pH value to 0.2.
(4) And (3) adding the solution obtained in the step (3) and an equimolar equal volume of sodium sulfate solution into a thick-wall pressure-resistant bottle, heating to 130 ℃, and reacting for 3 hours.
(5) And (4) filtering while the reaction is hot after the reaction is finished, and respectively collecting filtrate and solid.
(6) And (3) washing the solid obtained by filtering in the step (5) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol (removing free moisture by using ethyl alcohol), and carrying out forced air heating and drying at the temperature of 80 ℃ for 2 hours to obtain the calcium sulfate hemihydrate crystal whisker, namely a comparative sample 1.
(7) And (5) pouring the filtrate obtained by filtering in the step (5) into a thick-wall pressure-resistant bottle, directly cooling, crystallizing and aging for 3 hours.
(8) And (7) after ageing, filtering, washing a filter cake with tap water, then washing with absolute ethyl alcohol, and heating and drying for 2 hours by blowing at 80 ℃ to obtain the calcium sulfate dihydrate crystal whisker, which is a comparative sample 2.
Comparative example 2
This comparative example used chemically pure CaCl2And 98% concentrated sulfuric acid as a raw material.
(1) Respectively preparing 0.5mol/L CaCl2Solution and 0.5mol/L of H2SO4And (3) solution.
(2) The same volume of CaCl is added2Solution and H2SO4The solution is added into a thick-wall pressure-resistant bottle together, heated to 130 ℃ and reacted for 3 h.
(3) And filtering the mixture while the mixture is hot after the reaction is finished, and respectively collecting filtrate and solid.
(4) And (4) washing the solid obtained by filtering in the step (3) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol (removing free moisture by using ethyl alcohol), and carrying out forced air heating and drying at the temperature of 80 ℃ for 2 hours to obtain the calcium sulfate hemihydrate crystal whisker, namely a comparative sample 3.
(5) And (4) pouring the filtrate obtained by filtering in the step (3) into a thick-wall pressure-resistant bottle, wherein the filtrate is 89 ℃, slowly cooling the filtrate to separate out crystals, and aging the crystals for 3 hours, wherein the temperature of the filtrate is 60.7 ℃ after the aging is finished.
(6) And (5) after the aging is finished, cooling the system to room temperature, filtering, washing a filter cake with tap water, then washing with absolute ethyl alcohol, and heating and drying for 2 hours by blowing at 80 ℃ to obtain the calcium sulfate dihydrate crystal whisker which is a comparative sample 4.
Comparative example 3
This comparative example used chemically pure CaCl2，Na2SO4As a starting material.
(1) Respectively preparing 0.5mol/L CaCl2Solution and 0.5mol/L of Na2SO4And (3) solution.
(2) The same volume of CaCl is added2Solution and Na2SO4The solution is added into a thick-wall pressure-resistant bottle together, heated to 130 ℃ and reacted for 3 h.
(3) And filtering the mixture while the mixture is hot after the reaction is finished, and respectively collecting filtrate and solid.
(4) And (4) washing the solid obtained by filtering in the step (3) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol (removing free moisture by using ethyl alcohol), and carrying out forced air heating and drying at the temperature of 80 ℃ for 2 hours to obtain the calcium sulfate hemihydrate crystal whisker, namely a comparative sample 5.
Comparative example 4
This comparative example uses calcium sulfate dihydrate as the starting material.
(1) 2.15g of calcium sulfate dihydrate is weighed, added with 50mL of water and stirred to prepare slurry.
(2) Adding the slurry into a thick-wall pressure-resistant bottle, heating to 130 ℃, and reacting for 3 h.
(3) And filtering while the solution is hot after the reaction is finished.
(4) And (3) washing the solid obtained by filtering in the step (3) with hot water at the temperature of over 90 ℃, then washing with absolute ethyl alcohol (removing free moisture by using ethyl alcohol), and carrying out forced air heating and drying at the temperature of 80 ℃ for 2 hours to obtain the calcium sulfate hemihydrate crystal whisker, namely a comparative sample 6.
Detection experiment 1
Detecting the length-diameter ratio of the crystal whisker:
the prepared whisker sample is placed in a dispersion medium (ethanol) by using an electron polarization microscope, ultrasonic dispersion is carried out at room temperature, then the sample is dripped on a glass slide, the shape of the whisker is observed, the average length and the average diameter of the whisker are measured simultaneously, the length-diameter ratio of the whisker is calculated, and the results are shown in table 1, and fig. 1-8 show the shape (100X) of the calcium sulfate whisker of the comparative example and the calcium sulfate whisker of the example.
TABLE 1
Detection experiment 2
Whisker X-ray diffraction (XRD) detection:
adopting a step-by-step scanning method, X-ray (Cu), the scanning range is 10.000-35.000, the scanning mode is continuous scanning, the scanning speed is 2.0000deg/min, and the sample inclination is 0.0500 deg; the adjustment time was 1.50 sec.
According to XRD spectrum analysis, H represents a diffraction peak of calcium sulfate hemihydrate, D represents a diffraction peak of calcium sulfate dihydrate, sample 1 and comparative samples 1, 3, 5 and 6 are respectively calcium sulfate hemihydrate whiskers prepared in examples and comparative examples, and by combining data in a table 1 and a whisker morphology graph, the calcium sulfate hemihydrate prepared in the conditions required by the invention is uniform acicular crystals, and the average length-diameter ratio of the calcium sulfate hemihydrate is 259.9; in comparative example 1, the concentration of the reactant is increased, and the concentration exceeds the implementation requirement of the invention, so that the obtained short rod-shaped crystal does not meet the requirement of the whisker; comparative examples 2, 3 use chemically pure CaCl2The raw material is adopted, the sulfuric acid is directly used as a sulfate radical source in the comparative example 2, the diameter of the obtained crystal comparative sample 2 is larger, the length diameter is smaller, the reaction pH value is not adjusted to be within the implementation requirement of the invention in the comparative example 3, and the obtained crystal comparative sample 5 has uneven crystal thickness, rough surface and poor quality; in comparative example 4, calcium sulfate dihydrate was directly used as the raw material, and the obtained whiskers were the shortest without stirring, and it can be seen from the XRD pattern that comparative sample 6 was not completely converted into calcium sulfate hemihydrate whiskers.
Sample 2, comparative samples 2, 4 are calcium sulfate dihydrate whiskers prepared in examples and comparative examples, respectively. In the embodiment 1, the calcium sulfate dihydrate crystal whisker with higher length-diameter ratio and more uniform thickness is successfully prepared by utilizing the residual heat of the filtrate. In comparative example 1, since the temperature reduction rate of the filtrate was not controlled, more fragments were present in the crystals precipitated by direct cooling. In comparative example 2, the filtrate contained no magnesium ions and sodium chloride in appropriate amounts, the crystal thickness was not uniform, and a large number of plate-like crystals were present.
In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.
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