阅读说明：本技术 一种硝酸分解磷矿酸解液除钙副产α高强石膏的方法 (Method for removing calcium and byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid ) 是由 曹建新 杨林 李玉录 于 2020-04-21 设计创作，主要内容包括：本发明公开了一种硝酸分解磷矿酸解液除钙副产α高强石膏的方法。利用硝酸分解磷矿后,将酸不溶物滤除,首先对酸解液进行部分中和,调控酸解液中钙离子浓度后,将酸解液添加到盛有外加磷酸和硫酸铵的混合溶液的结晶槽中,控制反应温度为50-70℃、反应时间为0.5-2小时先生成二水石膏,然后将反应料浆流入转晶槽中,升温至95-110℃并反应一段时间后,即可副产出α型高强石膏产品,本发明具有使酸解液得以用于生产精细磷酸盐和高水溶性磷肥等产品,同时使酸解液中的钙资源转化为α高强石膏产品,变“固废”磷石膏为“产品”石膏,消除磷铵行业副产大量固废磷石膏带来的环境风险,促进磷复肥行业调整产品结构、减少固废排放,推动磷化工产业持续发展的有益效果。(The invention discloses a method for removing calcium and producing a byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid. Decomposing phosphorite by using nitric acid, filtering out acid insoluble substances, firstly carrying out partial neutralization on acidolysis solution, regulating the calcium ion concentration in the acidolysis solution, adding the acidolysis solution into a crystallization tank containing a mixed solution of additional phosphoric acid and ammonium sulfate, controlling the reaction temperature to be 50-70 ℃ and the reaction time to be 0.5-2 hours, firstly generating dihydrate gypsum, then flowing reaction slurry into a crystal conversion tank, heating to 95-110 ℃ and reacting for a period of time, thus obtaining the alpha-type high-strength gypsum product as a side product Reduce the solid waste discharge and promote the continuous development of the phosphorus chemical industry.)

1. A method for removing calcium and preparing a byproduct of alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid is characterized by comprising the following steps: the method specifically comprises the following steps:

(1) acid hydrolysis: carrying out acidolysis reaction on the phosphate concentrate and nitric acid to obtain a crude acidolysis solution;

(2) and (3) filtering: filtering the crude acidolysis solution obtained in the step (1) to remove acid insoluble substances and organic matters, so as to obtain acidolysis solution;

(3) neutralizing: performing ammonia neutralization on the acidolysis solution obtained in the step (2) to obtain neutralized acidolysis solution;

(4) diluting the neutralized acidolysis solution: diluting the calcium ion concentration in the neutralized acidolysis solution obtained in the step (3) to obtain a diluted acidolysis solution;

(5) diluting acidolysis solution to remove calcium: preheating the diluted acidolysis solution obtained in the step (4), mixing the diluted acidolysis solution with a preheated mixed solution of phosphoric acid and ammonium sulfate, and reacting at constant temperature to obtain reaction slurry;

(6) regulating and controlling a crystal form; sending the reaction slurry obtained in the step (5) into a crystal conversion tank, heating, reacting at constant temperature, and filtering; the filtrate is calcium-removed acidolysis solution, and filter residue is dried to obtain alpha high-strength gypsum;

(7) partial returning slurry of calcium-removing acidolysis solution: and (4) returning part of the calcium-removed acidolysis solution obtained in the step (6) to slurry for preparing a mixed solution of phosphoric acid and ammonium sulfate in the step (5) and for diluting the neutralized acidolysis solution in the step (4).

2. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (1), the mass concentration of the nitric acid is 40-70%, the acid-mineral ratio of the nitric acid to the phosphate concentrate is 1.15-1.35:1, and the acid excess coefficient of the nitric acid is 105-125%; the temperature of the acidolysis reaction is 50-70 ℃, and the time of the acidolysis reaction is 1-3 hours.

3. The method for removing calcium and byproducts α high strength gypsum from acidolysis solution of phosphate rock by nitric acid according to claim 1, wherein in the step (2), the mass concentration of nitric acid in acidolysis solution is 5-30%, and P is₂O₅The mass concentration is 6-20%; the concentration of calcium oxide is 8-25%.

4. The method for removing calcium and byproduct α high-strength gypsum from acidolysis solution of phosphorite by nitric acid as claimed in claim 1, wherein in the step (3), the acidolysis solution of the step (2) is neutralized by ammonia to regulate NH₃:HNO₃In a molar ratio of 0.4-1.6: 1.

5. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (4), the neutralized acidolysis solution obtained in the step (3) is diluted by clear water or calcium-removed acidolysis solution, and the mass concentration of calcium oxide in the diluted acidolysis solution is regulated to be 4-12%.

6. The method for removing calcium and byproduct α high-strength gypsum from acidolysis solution of phosphorite by nitric acid as claimed in claim 1, wherein in the step (5), the content of calcium ion in the acidolysis solution is diluted according to the content of P₂O₅Content, control of Ca²⁺：SO₄ ^2-The molar ratio of (A) to (B) is 1: 1.3-1.9; regulating P in calcium-removing reaction system₂O₅The mass concentration of (A) is 2-15%.

7. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (5), the mixed solution of phosphoric acid and ammonium sulfate is prepared by using: respectively preparing 25-40% ammonium sulfate solution and 60-85% phosphoric acid solution with clear water or calcium-removed acidolysis solution, and mixing to obtain mixed solution of phosphoric acid and ammonium sulfate.

8. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (5), the diluted acidolysis solution, the mixed solution of phosphoric acid and ammonium sulfate are respectively preheated to 50-70 ℃, then mixed to remove calcium, and the constant temperature reaction time is controlled to be 0.5-2 hours.

9. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (6), the temperature is raised to 95-110 ℃, and the reaction is carried out for 8-16 hours at constant temperature.

10. The method for removing calcium and producing high-strength alpha gypsum as a byproduct in acid hydrolysis of phosphorite by nitric acid decomposition according to claim 1, which is characterized in that: in the step (7), the mass percentage of the calcium-removing acidolysis solution for preparing the mixed solution of phosphoric acid and ammonium sulfate in the step (5) is less than or equal to 30%, and the mass percentage of the calcium-removing acidolysis solution for diluting the neutralized acidolysis solution in the step (4) is less than or equal to 15%.

Technical Field

The invention relates to a method for removing calcium and byproduct alpha high-strength gypsum from phosphate rock acidolysis solution, in particular to a method for removing calcium and byproduct alpha high-strength gypsum from phosphate rock acidolysis solution by nitric acid decomposition.

Background

The mineral composition of the alpha-type high-strength gypsum is alpha-type semi-hydrated gypsum. The different crystalline forms of alpha hemihydrate have different gel strengths, with the acicular alpha hemihydrate having the lowest strength and even less than the flaky beta hemihydrate. The short columnar alpha-type semi-hydrated gypsum has the characteristics of small water-paste ratio, short setting time and high breaking and compression strength under the standard consistency, and is widely applied to the fields of gypsum ceramic molds, precision casting, building decoration materials and the like. Therefore, the production of short column alpha hemihydrate gypsum is critical to the high strength of gypsum products.

The phosphoric acid produced by wet method can be decomposed by sulfuric acid or nitric acid. Whether sulfuric acid or nitric acid is used to decompose phosphate ore, the economic and environment-friendly separation of calcium from acid solution is the key to the production of wet-process phosphoric acid and its downstream products.

The sulfuric acid process phosphoric acid technology becomes a complete modern phosphoric acid industrial production system. One of the obvious characteristics is that calcium sulfate precipitate is directly generated in the acidolysis process, the liquid-solid phase separation is easy to realize, and the production method of phosphoric acid is greatly simplified. However, when the sulfuric acid is only used for decomposing phosphorite to produce the nitrogen-phosphorus compound fertilizer, ammonia needs to be additionally introduced, the fertilizer usually only contains ammonium nitrogen, and the generated calcium sulfate is waste residue.

When treating phosphate ore with nitric acid, the nitric acid is used not only to decompose the phosphate ore but also to make the compound fertilizer contain nitrate nitrogen which is easier to be absorbed by plants, and there are various methods for removing calcium from the acidolysis solution, among which, the effective and industrialized methods are the freezing method and the sulfate method. Limited by economic feasibility, the calcium removal rate of the freezing method can only be controlled to be 70-80%, calcium ions in the acid solution can generate a large amount of citrate-soluble or insoluble calcium phosphate salt or double salt in the neutralization process, the phosphorus utilization rate is reduced, and products such as fine phosphate, water-soluble fertilizer and the like are difficult to produce; the sulfate method has high calcium removal rate, is beneficial to improving the water solubility of the nitrophosphate fertilizer product, and the mother liquor can be used for producing ammonium phosphate and other phosphate products, but the generated calcium sulfate can still become waste residues.

The technology for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid is urgently needed to be developed in the field, so that the acidolysis solution can be used for producing products such as fine phosphate, water-soluble fertilizer and the like, and simultaneously, "solid waste" phosphogypsum is changed into "product" gypsum, so that the environmental risk caused by a large amount of solid waste phosphogypsum as a byproduct in the ammonium phosphate industry is eliminated, the product structure adjustment of the phosphorus compound fertilizer industry is promoted, the solid waste discharge is reduced, and the sustainable development of the phosphorus chemical industry is promoted.

The production technology of compound fertilizer by using nitric acid to decompose phosphate rock is mainly freezing method. An enterprise for producing the nitro-compound fertilizer by using a freezing method process in China has Tianqian coal chemical industry group member company Limited (original Shanxi chemical fertilizer plant), and is a NorskHydro freezing method nitrophosphate fertilizer device introduced from Norway. The device uses domestic middle-low grade phosphorite to produce the nitrophosphate fertilizer, and has a plurality of technical problems in the process. The Guizhou Botian ecological engineering corporation successfully finishes the trial run and commissioning of 30 ten thousand tons of domestic nitrophosphate fertilizer devices produced in 2015 in the first set of years, and the byproduct calcium nitrate is mainly used for producing calcium ammonium nitrate fertilizer. But the freezing decalcification rate is 70-80%, and the decalcification mother liquor can not produce fine phosphate and high water-soluble phosphate fertilizer products.

Disclosure of Invention

The invention aims to provide a method for removing calcium and by-producing alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid. The invention has the characteristics that the acidolysis solution can be used for producing products such as fine phosphate, high water-soluble phosphate fertilizer and the like, simultaneously calcium resources in the acidolysis solution are converted into alpha high-strength gypsum products, solid waste phosphogypsum is changed into product gypsum, the environmental risk caused by a large amount of solid waste phosphogypsum as a byproduct in the ammonium phosphate industry is eliminated, the product structure adjustment of the phosphorus compound fertilizer industry is promoted, the solid waste discharge is reduced, and the continuous development of the phosphorus chemical industry is promoted.

The technical scheme of the invention is as follows: a method for removing calcium and preparing a byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid specifically comprises the following steps:

(1) acid hydrolysis: carrying out acidolysis reaction on the phosphate concentrate and nitric acid to obtain a crude acidolysis solution;

(2) and (3) filtering: filtering the crude acidolysis solution obtained in the step (1) to remove acid insoluble substances and organic matters, so as to obtain acidolysis solution;

(3) neutralizing: performing ammonia neutralization on the acidolysis solution obtained in the step (2) to obtain neutralized acidolysis solution;

(4) diluting the neutralized acidolysis solution: diluting the calcium ion concentration in the neutralized acidolysis solution obtained in the step (3) to obtain a diluted acidolysis solution;

(5) diluting acidolysis solution to remove calcium: preheating the diluted acidolysis solution obtained in the step (4), mixing the diluted acidolysis solution with a preheated mixed solution of phosphoric acid and ammonium sulfate, and reacting at constant temperature to obtain reaction slurry;

(6) regulating and controlling a crystal form; sending the reaction slurry obtained in the step (5) into a crystal conversion tank, heating, reacting at constant temperature, and filtering; the filtrate is calcium-removed acidolysis solution, and filter residue is dried to obtain alpha high-strength gypsum;

(7) partial returning slurry of calcium-removing acidolysis solution: and (4) returning part of the calcium-removed acidolysis solution obtained in the step (6) to slurry for preparing a mixed solution of phosphoric acid and ammonium sulfate in the step (5) and for diluting the neutralized acidolysis solution in the step (4).

In the method for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing the phosphate rock acidolysis solution with the nitric acid, in the step (1), the mass concentration of the nitric acid is 40-70%, the acid-mineral ratio of the nitric acid to the phosphate concentrate is 1.15-1.35:1, and the acid excess coefficient of the nitric acid is 105-125%; the temperature of the acidolysis reaction is 50-70 ℃, and the time of the acidolysis reaction is 1-3 hours.

In the method for removing calcium and producing α high-strength gypsum as a byproduct in acid hydrolysis of phosphate rock by nitric acid, in the step (2), the mass concentration of nitric acid in the acid hydrolysis solution is 5-30 percent, and P is₂O₅The mass concentration is 6-20%; the concentration of calcium oxide is 8-25%.

In the method for removing calcium and byproduct α high-strength gypsum from acid hydrolysis solution of phosphate rock decomposed by nitric acid, in the step (3), the acid hydrolysis solution of the step (2) is subjected to ammonia neutralization to regulate and control NH₃:HNO₃In a molar ratio of 0.4-1.6: 1.

In the method for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid, in the step (4), the neutralized acidolysis solution in the step (3) is diluted with clear water or the calcium-removing acidolysis solution, and the mass concentration of calcium oxide in the diluted acidolysis solution is regulated to be 4-12%.

In the method for removing calcium and byproduct α high-strength gypsum from acid hydrolysis solution of phosphorite decomposed by nitric acid, in the step (5), according to the content of calcium ions in diluted acid hydrolysis solution and P₂O₅Content, control of Ca²⁺：SO₄ ^2-The molar ratio of (A) to (B) is 1: 1.3-1.9;regulating P in calcium-removing reaction system₂O₅The mass concentration of (A) is 2-15%.

In the method for removing calcium and producing high-strength alpha gypsum as a byproduct by decomposing phosphate rock acidolysis solution with nitric acid, in the step (5), the mixed solution of phosphoric acid and ammonium sulfate is prepared by the following steps: respectively preparing 25-40% ammonium sulfate solution and 60-85% phosphoric acid solution with clear water or calcium-removed acidolysis solution, and mixing to obtain mixed solution of phosphoric acid and ammonium sulfate.

In the method for removing calcium from the acid hydrolysis solution of phosphorite by nitric acid and producing the byproduct alpha high-strength gypsum, in the step (5), the diluted acid hydrolysis solution, the mixed solution of phosphoric acid and ammonium sulfate are respectively preheated to 50-70 ℃, mixed and decalcified, and the constant-temperature reaction time is controlled to be 0.5-2 hours.

In the method for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing the acid hydrolysis solution of phosphorite with nitric acid, in the step (6), the temperature is increased to 95-110 ℃, and the reaction is carried out for 8-16 hours at constant temperature.

In the method for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing phosphate rock acidolysis solution with nitric acid, in the step (7), the mass percentage of the calcium-removing acidolysis solution for preparing the mixed solution of phosphoric acid and ammonium sulfate in the step (5) is less than or equal to 30%, and the mass percentage of the calcium-removing acidolysis solution for diluting the neutralized acidolysis solution in the step (4) is less than or equal to 15%.

Compared with the prior art, the invention has the beneficial effects that:

1. the Chinese patent of document 1, a process for producing high-concentration nitrophosphate fertilizer (application number: 20091009223.3), comprises the following steps: adding nitric acid into phosphate ore for acidolysis, precipitating and separating acid insoluble substances, freezing and crystallizing calcium nitrate, filtering the calcium nitrate, neutralizing mother liquor, evaporating, granulating and drying, wherein a mother liquor deep calcium removal step is arranged between the calcium nitrate filtration and the mother liquor neutralization, and the method comprises the following steps: deeply removing calcium from mother liquor, adding sulfuric acid or ammonium sulfate into the mother liquor, and generating calcium sulfate dihydrate crystals by calcium ions and sulfate ions of the mother liquor; (II) carrying out graded treatment on the reaction slurry, adopting grading equipment to carry out graded treatment on the reaction slurry, returning the reaction slurry with small-particle calcium sulfate to mother liquor to deeply remove calcium, and sending the reaction slurry with large-particle calcium sulfate to calcium sulfate filtration; and (III) filtering the calcium sulfate, filtering and washing the reaction slurry by using a filter, conveying the primary filtrate and the primary washing liquid to a neutralization process, and returning the secondary washing liquid to the filter.

In document 2, a chinese patent "a method for producing by-product gypsum of a nitro-sulfur-based compound fertilizer by decomposing powdered rock phosphate with nitric acid" (application number: 201110022207.5), the method is a method for producing by-product gypsum of a nitro-sulfur-based compound fertilizer by decomposing powdered rock phosphate with nitric acid, and the method comprises the following steps: (1) grinding phosphorite powder and then carrying out acidolysis reaction on the ground phosphorite powder and nitric acid; (2) filtering the crude acid solution obtained in the step (1) to remove acid insoluble substances and organic suspended matters in the crude acid solution, wherein the filtrate is refined acid hydrolysis solution; (3) adding ammonium sulfate or sulfuric acid into the refined acidolysis solution obtained in the step (2) to carry out double decomposition reaction for decalcification, filtering the reaction solution to obtain solid dihydrate gypsum, washing and drying the solid dihydrate gypsum to obtain high-purity dihydrate gypsum, wherein the filtrate is the decalcification acidolysis solution; (4) and (4) performing neutralization reaction on the decalcified acidolysis solution obtained in the step (3) and gaseous ammonia to obtain nitrophosphate fertilizer slurry containing ammonium nitrate and monoammonium phosphate, evaporating the slurry, adding powdery potassium sulfate, and performing granulation and drying to obtain the nitro-sulfur-based compound fertilizer.

Document 3 "New Process for producing Nitro-based Fertilizer by decomposing phosphorite with nitric acid through semi-water-dihydrate decalcification", which introduces that nitric acid and phosphorite react in an extraction tank to generate extract containing filter residue, the filter residue (containing a large amount of organic matters, nitrate nitrogen and phosphorus) is separated by a filter press, and the filter residue is neutralized by introducing ammonia to produce organic-inorganic compound fertilizer; the filtrate enters a crystallization tank, ammonium sulfate (firstly semi-hydrated gypsum is generated) is added into the crystallization tank, and then mother liquor containing dihydrate gypsum crystal seeds and water are added; filtering and washing the dihydrate gypsum to produce a gypsum product with high added value; and further neutralizing the filtrate to remove part of impurities and then producing the high-quality nitro compound fertilizer.

The main method points of the invention are as follows: after decomposing phosphorite by nitric acid, filtering out acid insoluble substances to obtain acidolysis mother liquor, neutralizing the acidolysis mother liquor, regulating and controlling the calcium ion concentration and the phosphoric acid concentration, adding a mixed solution of ammonium sulfate and phosphoric acid, controlling calcium sulfate crystallization to produce calcium-removed acidolysis liquor and by-producing an alpha-type high-strength gypsum product.

The method of document 1 differs from the present invention in that: in the method disclosed in the document 1, most of calcium in acidolysis mother liquor is removed by freezing crystallization, and then ammonium sulfate (or sulfuric acid) is added for secondary calcium removal to obtain a byproduct calcium sulfate dihydrate. Although the application patent and the document 1 both add ammonium sulfate to generate calcium sulfate dihydrate, the application patent extends the transformation of the generated calcium sulfate dihydrate into alpha high-strength gypsum, which needs to fully consider the crystal form and size formed by the calcium sulfate dihydrate to prepare the alpha high-strength gypsum, and needs multiple tests to creatively discover;

the method of document 2 differs from the present invention in that: the method of the document 2 directly adds ammonium sulfate (or sulfuric acid) to acidolysis mother liquor to remove calcium and obtain a high-purity dihydrate gypsum as a byproduct, and applies for patent to obtain a high-strength alpha gypsum as a byproduct;

the method of document 3 differs from the present invention in that: in the document 3, ammonium sulfate (or sulfuric acid) is directly added to acidolysis mother liquor to remove calcium, calcium sulfate hemihydrate is generated in the reaction process, the temperature is reduced to a specified temperature, and then acidolysis solution containing dihydrate gypsum seed crystals is added to produce dihydrate gypsum with coarse crystal grains. The gypsum crystallization route of the application patent not only needs to change the process conditions such as reaction temperature and the like. More importantly, under the acidic condition of medium strong acid and the ammonium nitrate salt solution environment, nitrate ions are adsorbed on the (111) crystal face of the calcium sulfate hemihydrate to inhibit the growth of the calcium sulfate hemihydrate to the C axis direction, so that the crystal form of the gypsum grows towards the direction of a hexagonal short column, and a short-column alpha high-strength gypsum product is produced.

2. The technological process of the method for removing calcium and producing the byproduct alpha high-strength gypsum by decomposing phosphorite with nitric acid is shown in figure 1, acid insoluble substances are filtered after phosphorite is decomposed with nitric acid, the acidolysis solution is partially neutralized (no precipitation is generated), calcium ion concentration in the acidolysis solution is regulated, the acidolysis solution is added into a crystallization tank containing a mixed solution of additional phosphoric acid and ammonium sulfate, dihydrate gypsum is generated firstly when reaction temperature is controlled to be 50-70 ℃ and reaction time is controlled to be 0.5-2 hours, then reaction slurry is sent into a crystal conversion tank, and after reaction is carried out for a period of time, an alpha high-strength gypsum product can be produced by-product (the strength meets the alpha 30 grade of the national alpha high-strength gypsum standard of JCT-2038 and 2010 alpha high-strength gypsum). And (3) filtering the filtrate obtained in the step (6) to obtain a calcium-removed acidolysis solution, wherein the mass percent of the returned slurry used for preparing the mixed solution of phosphoric acid and ammonium sulfate in the step (5) is less than or equal to 30%, the mass percent of the calcium-removed acidolysis solution used for diluting the neutralized acidolysis solution in the step (4) is less than or equal to 15%, and partial returned slurry of the calcium-removed acidolysis solution can be seen, so that the aim of reducing the production cost can be achieved.

3. The invention filters acid insoluble and organic floating substances before removing calcium from acidolysis solution of nitric acid decomposed phosphorite, the gypsum of byproduct of calcium removal of acidolysis solution is alpha high-strength gypsum, the purity of calcium sulfate can reach more than 99 percent, and the gypsum can be used as high-purity gypsum such as precision casting mold gypsum, medical bone material filling gypsum, filler, high-end building decoration and the like. Solves the problem that the phosphogypsum generated by calcium removal of the acid hydrolysis liquid of phosphorite decomposed by nitric acid influences the environment, converts the calcium resource in the acid hydrolysis liquid into an alpha high-strength gypsum product, and realizes the full utilization of the calcium resource in the phosphorite.

4. The calcium removal rate of the phosphate rock acidolysis solution is up to more than 95%; the method has simple process and simplified operation, can convert calcium resources in the acidolysis solution into alpha high-strength gypsum products while the nitric acid decomposed phosphorite acidolysis solution is used for producing products such as fine phosphate, high-water-solubility phosphate fertilizer and the like, changes 'solid waste' phosphogypsum into 'product' gypsum, and eliminates the environmental risk brought by a large amount of solid waste phosphogypsum as a byproduct in the ammonium phosphate industry; and the part of the calcium-removing acidolysis solution returns to the slurry, so that the use amount of the added phosphoric acid can be reduced, and the production cost of the process is reduced.

5. The invention relates to a chemical reaction formula:

Ca₅F(PO₄)₃+10HNO₃＝5Ca(NO₃)₂+3H₃PO₄+HF↑

(NH₄)2SO₄+Ca(NO₃)2+2H₂O＝CaSO₄·2H₂O↓+2NH₄NO₃

CaSO₄·2H₂O＝CaSO₄·1/2H₂O+1/2H₂O

the crystal transformation condition of the invention is that in the mixed solution of acid and salt existing in phosphoric acid and ammonium nitrate, crystal transformation is firstly carried out to calcium sulfate dihydrate under the conditions that the temperature is controlled to be 50-70 ℃ and the reaction time is 0.5-2 hours (step 5); then the temperature is raised to 95-110 ℃ to dissolve the calcium sulfate dihydrate, water molecules are discharged in the recombination process to form new crystal nuclei, and fine crystal nuclei can be dissolved, and large crystal nuclei grow again, which is the general property of crystal growth. Under the acidic condition of medium strong acid and the ammonium nitrate salt solution environment, nitrate ions are adsorbed on the (111) crystal face of the calcium sulfate hemihydrate to inhibit the growth of the nitrate ions towards the C axis direction, so that the crystal form of the gypsum grows towards the hexagonal short column direction, and a short column alpha high-strength gypsum product can be produced in the system (step 6).

Experiments prove that:

the shape graph of the alpha high-strength gypsum prepared by the embodiment of the invention is shown in fig. 2, the alpha high-strength gypsum prepared by the method shown in fig. 2 has very good crystallization shape, and the length-diameter ratio of the gypsum is about (2.5-3): 1, the sample has good gelling performance after hydration, and the absolute dry compressive strength can reach 38 MPa.

In conclusion, the method has the beneficial effects that the acidolysis solution can be used for producing products such as fine phosphate, high water-soluble fertilizer and the like, calcium resources in the acidolysis solution are converted into alpha high-strength gypsum products, solid waste phosphogypsum is changed into product gypsum, the environmental risk caused by a large amount of solid waste phosphogypsum as a byproduct in the ammonium phosphate industry is eliminated, the product structure in the phosphorus compound fertilizer industry is promoted to be adjusted, the solid waste discharge is reduced, and the sustainable development of the phosphorus chemical industry is promoted.

Drawings

FIG. 1 is a process flow diagram of the present invention;

fig. 2 is a topography of the alpha high strength gypsum produced by the present invention.

Detailed Description

The invention is further illustrated by the following figures and examples, which are not to be construed as limiting the invention.