Method for preparing high-strength alpha-semi-hydrated gypsum from phosphogypsum

Abstract

The invention belongs to the technical field of ardealite treatment, and particularly relates to a method for preparing high-strength alpha-semi-hydrated gypsum from ardealite. The method comprises the following steps: fully dissolving an organic acid salt and a crystal modifier in deionized water by adopting a normal-pressure organic acid salt solution method to obtain an organic acid salt solution system; heating the organic acid salt solution system to a dehydration conversion reaction temperature, and then adding ardealite for reaction; and filtering the reaction liquid, and cleaning and drying a filter cake to obtain the high-strength alpha-semi-hydrated gypsum. By controlling various factors such as the variety and concentration of the organic acid salt, the dosage of the crystal modifier, the reaction temperature and the addition amount of the phosphogypsum, the conversion of the phosphogypsum to the alpha-semi-hydrated gypsum is realized in multiple ways, and the crystal morphology of the alpha-semi-hydrated gypsum is effectively regulated and controlled, so that the alpha-semi-hydrated gypsum forms a short columnar crystal of which the length-diameter ratio is close to 2.2; therefore, the high-strength gypsum with the compressive strength reaching 25MPa is prepared.

Description

Technical Field

[0001] The present invention belongs to the technical field of phosphogypsum treatment, and particularly relates to a method for preparing high-strength α-hemihydrate gypsum from phosphogypsum. Background Art

[0002] Phosphogypsum (PG) is an industrial waste discharged from the wet-process phosphoric acid of phosphate chemical enterprises. Approximately 5t of PG is generated for every 1t of phosphoric acid produced. Currently, the globally discharged PG is mainly disposed of by stacking and landfilling. The harmful impurities it contains infiltrate into the soil and water bodies with rainwater, causing serious environmental impacts and occupying a large amount of land. The main component of PG is dihydrate gypsum (CaSO4·2H2O), with a content usually above 85%. However, globally, only about 15% of PG is used in the production of building materials, CO2 mineral sequestrants, soil stabilizers, cement materials, road backfill materials, etc. Therefore, it is very important to seek efficient and green utilization methods and technologies for phosphogypsum.

[0003] α-Hemihydrate gypsum can be used in building materials, ceramics, precision investment casting, bone cement, pharmaceutical carriers, etc. Under the regulation of specific thermodynamic conditions and crystal conversion agents, phosphogypsum can be dehydrated and transformed into short-columnar high-strength α-hemihydrate gypsum (α-HH), which is one of the ways to realize the high-value utilization of PG. It can not only reduce the environmental harm of phosphogypsum but also promote the recycling of phosphogypsum resources.

[0004] The current production processes of α-hemihydrate gypsum mainly include the autoclave method, the pressurized hydrothermal method, and the atmospheric pressure salt solution method. The autoclave method is usually used for massive gypsum and is not applicable to powdered phosphogypsum. The pressurized hydrothermal method requires high-temperature and high-pressure reaction conditions, with high requirements for reaction equipment and energy consumption. As an emerging method, the atmospheric pressure salt solution method mainly solubilizes phosphogypsum through salt ions. Due to the controllable reaction process, mild reaction conditions, low energy consumption, and high product quality, it has received attention. Currently, the atmospheric pressure salt solution method is mainly carried out in inorganic acid and chloride salt solutions. In order to improve the conversion efficiency, the concentration of the salt medium solution used is generally high. However, this method not only leaves halogens on the α-hemihydrate gypsum product but also severely corrodes the reaction vessel in the reaction system, making wastewater treatment difficult and costly, resulting in problems such as difficult overall crystal conversion process and poor finished product quality. Summary of the Invention

[0005] The purpose of the present invention is to provide a new method for preparing high-strength α-hemihydrate gypsum from phosphogypsum, which can solve the problems of large salt consumption, difficult treatment of filtrate, or poor finished product quality in the preparation of high-strength α-hemihydrate gypsum by the atmospheric pressure salt solution method in the prior art.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing high-strength α-hemihydrate gypsum from phosphogypsum, said method adopting the atmospheric-pressure organic acid salt solution method.

[0008] In the present invention, high-strength α-hemihydrate gypsum is prepared from phosphogypsum by using an organic acid salt solution system under atmospheric pressure. This method uses an organic acid salt as the salt medium to prepare high-strength gypsum based on phosphogypsum, which can solve the problems of high salt medium consumption, high solution corrosivity, difficult waste liquid treatment, and low conversion efficiency in the existing atmospheric-pressure salt solution method.

[0009] Furthermore, the method for preparing high-strength α-hemihydrate gypsum from phosphogypsum is as follows:

[0010] 1) Fully dissolve the organic acid salt and the crystal conversion agent in deionized water to obtain an organic acid salt solution system;

[0011] 2) Heat the organic acid salt solution system to the dehydration conversion reaction temperature, and then add phosphogypsum for reaction;

[0012] 3) After the dehydration conversion reaction ends, filter the reaction solution, and wash and dry the filter cake to obtain the high-strength α-hemihydrate gypsum.

[0013] Among them, the organic acid salt is one or any mixture of sodium glycolate, sodium lactate, sodium acetate, and sodium propionate. The present invention selects the sodium salts of the above organic acid salts, mainly considering solubility and cost, and other organic acid salts with the same properties are also feasible.

[0014] The crystal conversion agent is a composite crystal conversion agent, which includes polyols and polybasic organic acids.

[0015] The polyols are preferably one or a mixture of two or more of ethylene glycol, propylene glycol, and glycerol, and the polybasic organic acids are preferably one or a mixture of two or more of succinic acid, malic acid, and maleic acid.

[0016] Preferably, in the organic acid salt solution system, based on the aqueous solution of the organic acid salt, the mass percentage content of the organic acid salt in the aqueous solution is 10-20 wt%, the addition amount of polyols is 20-60 wt% of the mass of the aqueous solution of the organic acid salt, and the addition amount of polybasic organic acids is 0.05-0.20 wt% of the mass of the aqueous solution of the organic acid salt.

[0017] In step 2), the addition amount of phosphogypsum is 15-25% of the mass of the aqueous solution of the organic acid salt.

[0018] Furthermore, the dehydration conversion reaction temperature is 85-110 °C.

[0019] The dehydration conversion reaction time is 4-6 h.

[0020] The kinetic driving force for the conversion of phosphogypsum to α - hemihydrate gypsum in the present invention is the solubility difference between the two phases of CaSO4·2H2O and CaSO4·0.5H2O. However, how to increase the driving force while solving the problems existing in the prior art is an important consideration in the present invention.

[0021] In the present invention, better conversion effects are obtained through one or more of the following aspects:

[0022] 1) The organic acid salt medium added in the present invention, as an electrolyte, can reduce the water activity. And the hydroxyl and carboxyl groups in the organic acid root can form hydrogen bond interactions with the hydrogen atoms in water molecules, generating a significant salt effect, which helps the efficient conversion of phosphogypsum. Moreover, when the salt medium is selected as an organic acid salt, it can effectively alleviate the corrosion problem of the chloride salt system and the interference of the common ion effect of sulfates, can effectively reduce the water activity, increase the solubility of CaSO4·2H2O, and decrease the solubility of CaSO4·0.5H2O, which is conducive to the precipitation of α - hemihydrate gypsum.

[0023] 2) In the salt solution, a low salt concentration has an insignificant effect on reducing the water activity, resulting in the inability to precipitate α - hemihydrate gypsum; while too high a salt concentration not only causes waste of the reagent, but also makes the dehydration conversion rate of phosphogypsum to α - hemihydrate gypsum too fast, resulting in many defects in the growth of α - hemihydrate gypsum crystals and poor comprehensive performance. Therefore, the present invention adds alcohols to the salt solution to reduce the salt dosage, which helps to reduce the corrosion of the reaction vessel and the residue of salt ions in the product; further, the introduction of polyols can not only effectively reduce the water activity, but also has hydrogen bond interactions with the hydroxyl, carboxyl and other groups in the organic acid salt, and can alleviate the limitation of the relatively large anion volume of the organic acid salt on the conversion of CaSO4·2H2O to CaSO4·0.5H2O. This enables the concentration of the organic acid salt in the dehydration conversion reaction system to be controlled at a relatively low level.

[0024] 3) The binary organic acid crystal modifier generally selectively adsorbs the carboxylate ions ionized from it on the end face of α - hemihydrate gypsum, and forms a stable complex inhibition layer through the complexation of the carboxylate group with Ca 2+ Ultimately, the growth of α - hemihydrate gypsum along the c - axis end face is restricted, and finally the aspect ratio of α - hemihydrate gypsum crystals is reduced. And the atomic spacing between Ca 2+ on the end face of α - hemihydrate gypsum is fixed. Therefore, a binary organic acid with a shorter carbon chain length and a carboxyl spacing match is preferably selected as the crystal form control agent.

[0025] 4) The addition amount of phosphogypsum, that is, the concentration of the solid phase in the slurry, affects the conversion rate of CaSO4·2H2O to CaSO4·0.5H2O; when the solid phase ratio is too low, the Ca 2+ dissolved from CaSO4·2H2O in phosphogypsum and SO4 2-They are relatively less distributed in the solution, increasing the difficulty of the collision between the two to form α - hemihydrate gypsum crystal nuclei, resulting in difficulty in the crystallization and precipitation of α - hemihydrate gypsum; while if the solid phase proportion is too high, it is difficult to form a stable crystallization environment for the slurry near the container wall during the stirring process, leading to incomplete dehydration conversion of some phosphogypsum solid phase.

[0026] 5) The temperature of the dehydration conversion reaction has the following effects on the thermodynamic driving force for the conversion of CaSO4·2H2O to CaSO4·0.5H2O: If the reaction temperature is too low, the solubility difference between the two phases is small, and it is difficult to precipitate α - hemihydrate gypsum, making it difficult to maintain the conversion of phosphogypsum; if the reaction temperature is too high, the precipitation rate of α - hemihydrate gypsum increases, and there are too many nucleation sites, resulting in imperfect crystal growth and poor crystal morphology.

[0027] In summary, by controlling various factors such as the type and concentration of organic acid salts, the dosage of crystal conversion agents, reaction temperature, and the addition amount of phosphogypsum, the present invention is conducive to the conversion of phosphogypsum to α - hemihydrate gypsum, and effectively regulates the crystal morphology of α - hemihydrate gypsum, making it form short columnar crystals with an aspect ratio close to 2.2, thereby preparing high - strength gypsum with a compressive strength reaching 25 MPa.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1) The present invention adds organic acid salts. On the one hand, as a salt medium, it can increase the solubility of phosphogypsum through the salt effect, promoting the conversion of phosphogypsum to α - hemihydrate gypsum; on the other hand, compared with chloride salts, it has environmental protection and degradability, little harm to the filtrate, and high treatability.

[0030] 2) The present invention can effectively regulate the microstructure of high - strength gypsum with less usage of organic acid salts and polyols, as well as trace amounts of dibasic organic acids, reducing the chemical agent cost required for production.

[0031] 3) The present invention selects a reaction system of organic acid salt + polyol + dibasic organic acid. During the preparation of high - strength gypsum by the atmospheric - pressure organic acid salt solution method, the weak alkalinity of the organic acid salt neutralizes the acidity of phosphogypsum itself, without the need to regulate the pH of the solution, simplifying the production steps.

[0032] 4) The α - hemihydrate gypsum crystals prepared by the present invention are short columnar crystals with an aspect ratio as low as 2.19, and their dried compressive strength is close to 25 MPa, belonging to high - strength gypsum. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the XRD pattern of the products of Examples 1 - 3;

[0034] Figure 2 It is the XRD pattern of the products of Examples 4 - 6;

[0035] Figure 3 XRD patterns of the products of Comparative Examples 1-3;

[0036] Figure 4 XRD patterns of the products of Comparative Example 1 and Comparative Examples 4-6;

[0037] Figure 5 SEM images of the products obtained in Examples 1-3;

[0038] Figure 6 SEM images of the products obtained in Examples 4-6;

[0039] Figure 7 SEM images of the products obtained in Comparative Examples 1-3;

[0040] Figure 8 SEM images of the products obtained in Comparative Examples 4-6. Detailed implementation manners

[0041] The following specific examples are used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto:

[0042] Example 1

[0043] A method for efficiently preparing phosphogypsum-based high-strength gypsum by an atmospheric-pressure organic acid salt solution method, the method comprising:

[0044] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a mass concentration (the same hereinafter) of 12.5 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 20% of the mass of the sodium lactate aqueous solution (the same hereinafter), and the dosage of malic acid is 0.10% of the mass of the sodium lactate aqueous solution (the same hereinafter). After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum (the addition amount of dry-based phosphogypsum is 20% of the mass of the sodium lactate aqueous solution), and carry out stirring reaction at a constant temperature of 95 °C in an oil bath. The dehydration conversion reaction is carried out for 4 h. After the reaction is completed, immediately filter the slurry, and wash the obtained filter cake 2 times with boiling water and 1 time with absolute ethanol, and then put it into a blast drying oven at 60 °C for drying treatment.

[0045] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and simultaneously use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of dehydration conversion reaction of phosphogypsum shows completely the peaks of hemihydrate gypsum; the sample morphology is long columnar, its average length is 13.07 μm, the average aspect ratio is 3.70, and the drying compressive strength is 17.94 MPa.

[0046] Example 2

[0047] A method for efficiently preparing high-strength phosphogypsum-based gypsum by an organic acid salt solution method under normal pressure, the method comprising:

[0048] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 20% of the sodium lactate aqueous solution, and the dosage of malic acid is 0.15% of the sodium lactate aqueous solution. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed 2 times with boiling water and 1 time with absolute ethanol, and then dried in a blast drying oven at 60 °C.

[0049] The dried sample is ground, and then an X-ray diffractometer is used to determine the crystal form change process, and at the same time, a SEM scanning electron microscope is used to observe the crystal morphology. The results show that the XRD of the sample after 4 h of dehydration conversion reaction of phosphogypsum shows completely hemihydrate gypsum peaks; the sample morphology is close to short columnar, with an average length of 14.74 μm, an average aspect ratio of 2.85, and a drying compressive strength of 21.79 MPa.

[0050] Example 3

[0051] A method for efficiently preparing high-strength phosphogypsum-based gypsum by an organic acid salt solution method under normal pressure, the method comprising:

[0052] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 20 wt% of the sodium lactate aqueous solution, and the dosage of malic acid is 0.20 wt% of the sodium lactate aqueous solution. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed 2 times with boiling water and 1 time with absolute ethanol, and then dried in a blast drying oven at 60 °C.

[0053] The dried sample is ground, and then an X-ray diffractometer is used to determine the crystal form change process, and at the same time, a SEM scanning electron microscope is used to observe the crystal morphology. The results show that the XRD of the sample after 4 h of dehydration conversion reaction of phosphogypsum shows completely hemihydrate gypsum peaks; the sample morphology is short columnar, with an average length of 15.86 μm, an average aspect ratio of 2.19, and a drying compressive strength of 25.65 MPa.

[0054] Example 4

[0055] A method for efficiently preparing phosphogypsum-based high-strength gypsum by an organic acid salt solution method under normal pressure, the method comprising:

[0056] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 20.0 wt%, then add malic acid, and the dosage of malic acid is 0.20 wt% of the sodium lactate aqueous solution, but do not add ethylene glycol. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C, and the dehydration conversion reaction is carried out for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed 2 times with boiling water and 1 time with absolute ethanol, and then placed in a blast drying oven at 60 °C for drying treatment.

[0057] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of phosphogypsum dehydration conversion reaction shows completely hemihydrate gypsum peaks; the sample morphology is short columnar, its average length is 18.52 μm, the average length-to-diameter ratio is 4.93, and the drying compressive strength is 14.69 MPa.

[0058] Example 5

[0059] A method for efficiently preparing phosphogypsum-based high-strength gypsum by an organic acid salt solution method under normal pressure, the method comprising:

[0060] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 20.0 wt%, then add ethylene glycol and malic acid, the dosage of ethylene glycol is 20 wt% of the sodium lactate aqueous solution, and the dosage of malic acid is 0.20 wt% of the sodium lactate aqueous solution. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C, and the dehydration conversion reaction is carried out for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed 2 times with boiling water and 1 time with absolute ethanol, and then placed in a blast drying oven at 60 °C for drying treatment.

[0061] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of phosphogypsum dehydration conversion reaction shows completely hemihydrate gypsum peaks; the sample morphology is short columnar, its average length is 15.44 μm, the average length-to-diameter ratio is 1.96, and the drying compressive strength is 25.07 MPa.

[0062] Example 6

[0063] A method for efficiently preparing phosphogypsum-based high-strength gypsum by an organic acid salt solution method under normal pressure, the method comprising:

[0064] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 20.0 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 60 wt% of the aqueous sodium lactate solution, and the dosage of malic acid is 0.20 wt% of the aqueous sodium lactate solution. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then dried in a blast drying oven at 60 °C.

[0065] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a scanning electron microscope (SEM) to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the dehydration conversion reaction of phosphogypsum shows completely the peaks of hemihydrate gypsum; the sample morphology is short columnar, with an average length of 17.61 μm, an average aspect ratio of 3.20, and a drying compressive strength of 23.91 MPa.

[0066] Comparative Example 1

[0067] A method for preparing phosphogypsum-based gypsum, the steps include:

[0068] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then without adding polyols and dibasic organic acids, directly add 20 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then dried in a blast drying oven at 60 °C.

[0069] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a scanning electron microscope (SEM) to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the dehydration conversion reaction of phosphogypsum shows basically the peaks of dihydrate gypsum, and the phosphogypsum is not completely dehydrated and converted; the sample morphology is rhombic flake dihydrate gypsum crystals.

[0070] Comparative Example 2

[0071] A method for preparing phosphogypsum-based gypsum, the steps include:

[0072] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 15.0 wt%, then without adding polyol and dibasic organic acid, directly add 20 g of dry-based phosphogypsum, and carry out a stirring reaction under the constant temperature condition of 95 °C in an oil bath, and the dehydration conversion reaction is carried out for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then it is placed in a forced-air oven at 60 °C for drying treatment.

[0073] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a scanning electron microscope (SEM) to observe the crystal morphology. The results show that the XRD of the sample after 4 h of dehydration conversion reaction of phosphogypsum shows basically the peaks of gypsum dihydrate, and the phosphogypsum is not completely dehydrated and converted; the sample morphology is a mixed phase of rhombic flake gypsum dihydrate crystals and long rod-shaped hemihydrate gypsum crystals, and the crystal surface is rough and attached with fine particles.

[0074] Comparative Example 3

[0075] A method for preparing phosphogypsum-based gypsum, the steps include:

[0076] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 17.5 wt%, then without adding polyol and dibasic organic acid, directly add 20 g of dry-based phosphogypsum, and carry out a stirring reaction under the constant temperature condition of 95 °C in an oil bath, and the dehydration conversion reaction is carried out for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then it is placed in a forced-air oven at 60 °C for drying treatment.

[0077] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a scanning electron microscope (SEM) to observe the crystal morphology. The results show that the XRD of the sample after 4 h of dehydration conversion reaction of phosphogypsum shows basically the peaks of hemihydrate gypsum; and the sample morphology is long rod-shaped hemihydrate gypsum crystals, and the crystal surface is relatively rough and covered with fine particles.

[0078] Comparative Example 4

[0079] A method for preparing phosphogypsum-based gypsum, the steps include:

[0080] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol. The dosage of ethylene glycol is 20 wt% of the sodium lactate aqueous solution, but do not add dibasic organic acid. After stirring evenly until completely dissolved, add 20 g of dry basis phosphogypsum, and then carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then dried in a forced-air oven at 60 °C.

[0081] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the phosphogypsum dehydration conversion reaction shows completely the peaks of hemihydrate gypsum; the sample morphology is long rod-shaped, with an average length of 25.30 μm and an average aspect ratio of 6.82.

[0082] Comparative Example 5

[0083] A method for preparing phosphogypsum-based gypsum, the steps include:

[0084] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol. The dosage of ethylene glycol is 40 wt% of the sodium lactate aqueous solution, but do not add dibasic organic acid. After stirring evenly until completely dissolved, add 20 g of dry basis phosphogypsum, and then carry out a stirring reaction in an oil bath at a constant temperature of 95 °C. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then dried in a forced-air oven at 60 °C.

[0085] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the phosphogypsum dehydration conversion reaction shows completely the peaks of hemihydrate gypsum; the sample morphology is long rod-shaped, with an average length of 25.89 μm and an average aspect ratio of 7.06.

[0086] Comparative Example 6

[0087] A method for preparing phosphogypsum-based gypsum, the steps include:

[0088] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol with a dosage of 60 wt% of the sodium lactate aqueous solution, but do not add dibasic organic acid. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and then carry out a stirring reaction in an oil bath at a constant temperature of 95 °C for 4 h for the dehydration conversion reaction. After the reaction is completed, immediately filter the slurry, wash the obtained filter cake twice with boiling water and once with absolute ethanol, and then place it in a blast drying oven at 60 °C for drying treatment.

[0089] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the phosphogypsum dehydration conversion reaction shows completely the peaks of hemihydrate gypsum; the sample morphology is fine needle-like, with an average length of 37.97 μm and an average aspect ratio of 24.34.

[0090] Comparative Example 7

[0091] A method for preparing phosphogypsum-based gypsum, the steps include:

[0092] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 20 wt% of the sodium lactate aqueous solution, and the dosage of malic acid is 0.30 wt% of the sodium lactate aqueous solution. After stirring evenly until completely dissolved, add 30 g of dry-based phosphogypsum, and carry out a stirring reaction in an oil bath at a constant temperature of 95 °C for 4 h for the dehydration conversion reaction. After the reaction is completed, immediately filter the slurry, wash the obtained filter cake twice with boiling water and once with absolute ethanol, and then place it in a blast drying oven at 60 °C for drying treatment.

[0093] Grind the dried sample, then use an X-ray diffractometer to determine the crystal form change process, and at the same time use a SEM scanning electron microscope to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the phosphogypsum dehydration conversion reaction shows the coexistence of hemihydrate gypsum phase and dihydrate gypsum phase; the sample morphology is the intermingling of long rod-shaped hemihydrate gypsum and rhombic flake-shaped phosphogypsum.

[0094] Comparative Example 8

[0095] A method for preparing phosphogypsum-based gypsum, the steps include:

[0096] Mix sodium lactate with deionized water, stir evenly until completely dissolved to obtain a 100 mL sodium lactate solution with a concentration of 12.5 wt%, then add ethylene glycol and malic acid. The dosage of ethylene glycol is 20 wt% of the sodium lactate aqueous solution, and the dosage of malic acid is 0.20 wt% of the sodium lactate aqueous solution. After stirring evenly until completely dissolved, add 20 g of dry-based phosphogypsum, and carry out a stirring reaction at a constant temperature of 70 °C in an oil bath. The dehydration conversion reaction proceeds for 4 h. After the reaction is completed, immediately filter the slurry, and the obtained filter cake is washed twice with boiling water and once with absolute ethanol, and then dried in a blast drying oven at 60 °C.

[0097] The dried sample is ground, and then an X-ray diffractometer is used to determine the crystal form change process, and at the same time, a scanning electron microscope (SEM) is used to observe the crystal morphology. The results show that the XRD of the sample after 4 h of the phosphogypsum dehydration conversion reaction shows that it is completely the gypsum dihydrate phase; the sample morphology is rhombic flake-shaped phosphogypsum, indicating that phosphogypsum cannot undergo dehydration conversion at this temperature.

[0098] Table 1 Preparation parameters and sample properties of hemihydrate gypsum in examples and comparative examples

[0099]

[0100] Combined with the data in Table 1, it can be seen that in Examples 1-3, in the organic acid salt solution, when the proportion of polyol is fixed, the dosage of malic acid, a binary organic acid, increases, resulting in a gradual decrease in the aspect ratio of α-hemihydrate gypsum crystals, reaching a minimum of 2.19. This shows that an appropriate dosage of binary organic acid can regulate the morphology of α-hemihydrate gypsum crystals.

[0101] In Examples 4-6, in a higher concentration organic acid salt solution, when the dosage of the binary organic acid is fixed, increasing the proportion of polyol ethylene glycol will make the surface of the product α-hemihydrate gypsum crystals gradually smooth, but its aspect ratio decreases and then increases, indicating that an appropriate concentration of ethylene glycol and dicarboxylic acid can better synergistically regulate the morphology of α-hemihydrate gypsum crystals.

[0102] In Comparative Examples 1-3, in the case of not adding polyol and binary organic acid, the organic acid salt solution at a lower concentration is not sufficient to completely dehydrate and convert phosphogypsum into α-hemihydrate gypsum. When the concentration reaches 17.5 wt%, the salt effect of the organic acid salt is sufficient to promote the reaction. And in the absence of alcohol and binary organic acid to regulate the crystal morphology, the organic acid root makes the surface of the generated hemihydrate gypsum crystals relatively rough.

[0103] As can be seen from Comparative Examples 4 to 6, after adding polyols and without adding dibasic organic acids, a 12.5 wt% organic acid salt solution can promote the complete dehydration conversion of phosphogypsum into α-hemihydrate gypsum at this time. However, the lack of dibasic organic acids adsorbed on the end faces of α-hemihydrate gypsum crystals inhibits their growth along the c-axis. This results in an increase in the ethylene glycol concentration, making the overall α-hemihydrate gypsum crystals longer and thinner, with a higher aspect ratio of crystal length to diameter.

[0104] As can be seen from Comparative Examples 7 to 8, both a lower reaction temperature and a higher addition amount of phosphogypsum will hinder the dehydration conversion of phosphogypsum into α-hemihydrate gypsum.

[0105] In summary, the present invention utilizes the salt effect of organic acid salts, further reduces the water activity by adding polyols, and controls the salt medium concentration within a lower range to achieve the dehydration conversion of phosphogypsum at a low salt concentration, saving the reagent cost and the filtrate treatment cost; uses polyols and dibasic organic acids together to regulate the morphology of hemihydrate gypsum, enabling the addition of a small amount of dibasic organic acids to achieve an excellent regulation effect on the morphology of hemihydrate gypsum, with a simple process and a relatively mild reaction environment; the high-strength gypsum based on phosphogypsum prepared by the present invention has a good crystal morphology, large and short-columnar grains, the aspect ratio of crystal length to diameter is between 2.19 and 3.70, and the drying compressive strength reaches 25 MPa.

[0106] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing high-strength α-hemihydrate gypsum from phosphogypsum, characterized in that, The method adopts the atmospheric pressure organic acid salt solution method.

2. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 1, characterized in that, The steps are as follows: 1) Sufficiently dissolve the organic acid salt and the crystal conversion agent in deionized water to obtain an organic acid salt solution system; 2) Heat the organic acid salt solution system to the dehydration conversion reaction temperature, and then add phosphogypsum for reaction; 3) After the dehydration conversion reaction is completed, filter the reaction solution, and wash and dry the filter cake to obtain the high-strength α-hemihydrate gypsum.

3. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 1, wherein, The organic acid salt is one or any mixture of sodium glycolate, sodium lactate, sodium acetate and sodium propionate.

4. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 1, characterized in that, The crystal conversion agent is a composite crystal conversion agent, comprising polyhydric alcohols and polybasic organic acids.

5. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 4, characterized in that, The polyhydric alcohols are one or a mixture of two or more of ethylene glycol, propylene glycol and glycerol, and the polybasic organic acids are one or a mixture of two or more of succinic acid, malic acid and maleic acid.

6. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 5, characterized in that, In the organic acid salt solution system, based on the aqueous solution of the organic acid salt, the mass percentage content of the organic acid salt is 10-20 wt%, the addition amount of the polyhydric alcohols is 20-60 wt% of the mass of the aqueous solution of the organic acid salt, and the addition amount of the polybasic organic acids is 0.05-0.20 wt% of the mass of the aqueous solution of the organic acid salt.

7. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 6, characterized in that, In step 2), the addition amount of phosphogypsum is 15-25% of the mass of the aqueous solution of the organic acid salt.

8. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 2, characterized in that, In step 2), the dehydration conversion reaction temperature is 85-110 °C.

9. The method for preparing high-strength α-hemihydrate gypsum from phosphogypsum according to claim 8, characterized in that, In step 2), the dehydration conversion reaction time is 4-6 h.

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