A method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate

Abstract

The present invention provides a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate. The method mainly includes: 1) Dissolving sodium carbonate in water, then adding phosphogypsum, stirring at a constant temperature to carry out a decomposition reaction. After the reaction ends, calcium carbonate residue and sodium sulfate mother liquor are separated; 2) Adding the calcium carbonate residue to acetic acid solution to react to obtain calcium acetate mother liquor and solid residue; 3) Adding the calcium acetate mother liquor to the sodium sulfate mother liquor to react to obtain calcium sulfate dihydrate precipitate and sodium acetate trihydrate mother liquor. After adjusting the pH of the mother liquor, it is concentrated and crystallized to obtain sodium acetate trihydrate product. The present invention uses a chemical method to treat the bulk solid waste phosphogypsum to prepare high-purity gypsum, and at the same time by-produces sodium acetate trihydrate, and develops a low-cost and high-efficiency process for the high-value utilization of phosphogypsum.

Description

Technical Field

[0001] The present invention belongs to the field of treatment and comprehensive utilization of chemical industrial wastes, and particularly relates to a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate. Background Art

[0002] Phosphogypsum is a large amount of solid waste in phosphate fertilizer production, mainly composed of calcium sulfate dihydrate and containing many impurities, with an appearance of grayish-black or grayish-white powder. The impurities in phosphogypsum can be divided into inorganic types (such as silicon, phosphorus, fluorine, iron, etc.) and organic types (such as humus, flotation reagents, etc.). Silicon impurities mainly exist as free silicon dioxide. When its content is relatively high, it will reduce the grade of phosphogypsum and cause wear to production equipment. Soluble phosphorus and soluble fluorine impurities will prolong the setting time of phosphogypsum products, make the structure loose and reduce the strength, and will also cause harm to the environment. Organic impurities are mainly attached to the surface of calcium sulfate crystals, which will reduce the whiteness of phosphogypsum, weaken the binding force between gypsum crystals, and make the structure of the hardened body loose.

[0003] In the wet-process phosphoric acid process, 4.5 - 5 tons of phosphogypsum will be by-produced for every 1 ton of phosphoric acid produced. The annual output of phosphogypsum in China is about 75 million tons, and the comprehensive utilization rate is about 45%, and the cumulative stockpile has exceeded 830 million tons. The global comprehensive utilization rate of phosphogypsum is only about 25%, and the cumulative stockpile has exceeded 6 billion tons. The stacking of phosphogypsum has caused a huge waste of sulfur and calcium resources, occupied a large amount of land resources, and at the same time polluted the groundwater, soil and vegetation near the stockyard, etc. At present, the treatment processes of phosphogypsum mainly include water washing, alkali neutralization, calcination and flotation, etc., which have problems such as incomplete impurity removal, high cost and secondary pollution.

[0004] The Chinese gypsum product market is broad, and the exploitation volume of natural gypsum reaches 90 million tons / year. If the harmful substances in phosphogypsum can be removed efficiently and at low cost, making it replace natural gypsum and reducing the exploitation and damage to nature, it can provide a new way for the industrial application of phosphogypsum. Summary of the Invention

[0005] The main purpose of the present invention is to provide a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate. This method can significantly improve the purity and whiteness of gypsum. While preparing high-purity gypsum, it by-produces sodium acetate trihydrate, and has the advantages of simple operation, low requirements for reaction equipment, low process cost and high efficiency.

[0006] In order to achieve the above purpose, the present invention adopts the following technical scheme:

[0007] Provide a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate, comprising the following steps:

[0008] 1) Dissolve sodium carbonate in water, then add phosphogypsum, stir at a constant temperature to carry out a decomposition reaction. After the reaction ends, separate to obtain calcium carbonate residue and sodium sulfate mother liquor;

[0009] 2) Add the calcium carbonate residue obtained in step 1) to acetic acid solution, and react to obtain calcium acetate mother liquor and solid residue;

[0010] 3) Add the calcium acetate mother liquor obtained in step 2) to the sodium sulfate mother liquor obtained in step 1), react to obtain calcium sulfate dihydrate precipitate and sodium acetate trihydrate mother liquor, adjust the pH of the mother liquor and then concentrate and crystallize to obtain sodium acetate trihydrate product.

[0011] According to the above scheme, in step 1), the phosphogypsum is ground to pass through a 200-mesh sieve completely.

[0012] According to the above scheme, in step 1), the molar ratio of sodium carbonate to calcium sulfate dihydrate in phosphogypsum is 0.92:1 - 1.12:1; the volume-mass ratio of water to phosphogypsum is 2:1 - 5:1 mL / g.

[0013] According to the above scheme, in step 1), the decomposition and transformation process is: reaction time 20 - 120 min, reaction temperature 20 - 50 °C, stirring speed 500 - 700 rpm.

[0014] According to the above scheme, in step 2), the concentration of acetic acid solution is 2.0 - 4.0 mol / L, the stirring speed is 60 - 400 rpm, and the theoretical molar ratio of calcium carbonate to acetic acid obtained is 1:2 - 1:4.

[0015] According to the above scheme, in step 2), the CO2 obtained from the reaction is collected and used to prepare Na2CO3.

[0016] According to the above scheme, in step 3), the sodium sulfate mother liquor is pre-concentrated to a concentration of 1.16 - 2.09 mol / L.

[0017] According to the above scheme, in step 3), the reagent for adjusting the pH of the sodium acetate trihydrate mother liquor is sodium hydroxide, sodium carbonate, sulfuric acid or acetic acid.

[0018] According to the above scheme, in step 3), the pH of the sodium acetate trihydrate mother liquor is adjusted to 6.3 - 10.0, and the concentration temperature of the mother liquor is 45 - 100 °C.

[0019] According to the above scheme, in step 3), the purity of the prepared calcium sulfate dihydrate is ≥97%, the whiteness is ≥90%, and the purity of the by-product sodium acetate trihydrate is ≥95%, and the whiteness is ≥90%.

[0020] The present invention provides a method for preparing high-purity gypsum from phosphogypsum as a raw material and by-producing sodium acetate trihydrate. First, use sodium carbonate to decompose and transform phosphogypsum to convert SO4 in the gypsum 2-Leached into the solution, calcium carbonate residue and sodium sulfate solution are obtained through transformation, and the impurities in phosphogypsum mainly precipitate together with the calcium carbonate residue; by adding phosphogypsum into sodium carbonate solution and controlling reasonable material ratio, liquid-solid ratio, temperature, rotation speed and time, a high decomposition conversion rate of phosphogypsum is achieved. Secondly, the calcium carbonate residue is further treated with acetic acid solution. By controlling appropriate acetic acid concentration and reaction material ratio, calcium element is efficiently leached into the solution to obtain calcium acetate mother liquor and solid residue. Finally, the above-mentioned calcium acetate solution is added into the concentrated sodium sulfate solution, and a double decomposition reaction occurs to precipitate calcium sulfate dihydrate, while sodium acetate trihydrate is generated; by concentrating the sodium sulfate solution to an appropriate concentration, an efficient reaction is achieved, and the reaction yield and purity are improved. This method is simple and efficient. While preparing high-purity gypsum, sodium acetate trihydrate product is by-produced, and impurities such as silicon, phosphorus, fluorine, and humus in phosphogypsum are solidified in the solid residue. Silicon impurities in the solid residue mainly exist in the form of silicon dioxide, and phosphorus and fluorine impurities are converted into Ca3(PO4)2 and CaF2 with low solubility during alkali leaching. These two substances are insoluble in acetic acid and stably exist in the solid residue. Organic impurities mainly undergo desorption and re-adsorption processes and mainly adhere to the surface of the solid residue. The CO2 produced during acetic acid leaching has a high purity, is easy to collect and purify, and can be used to prepare Na2CO3.

[0021] The beneficial effects of the present invention are as follows:

[0022] 1. The present invention provides a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate. While preparing high-purity gypsum, sodium acetate trihydrate product is by-produced. During the preparation process, no waste liquid is discharged, soluble impurities are converted into solids with low solubility, and the yields and purities of the obtained calcium sulfate dihydrate and by-produced sodium acetate trihydrate are high.

[0023] 2. The process of the present invention is simple and efficient, has low requirements for reaction equipment, and can provide an effective way for the high-value utilization of phosphogypsum. Description of the Drawings

[0024] Figure 1 It is the process flow chart of the embodiment of the present invention. Detailed Embodiments

[0025] To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the present invention will be further described in detail below.

[0026] The chemical compositions of the phosphogypsum used in the examples and comparative examples are shown in Table 1.

[0027] Table 1 Chemical Compositions of Phosphogypsum

[0028]

[0029] Example 1

[0030] A method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate includes the following steps:

[0031] (1) Weigh 11.20 g of anhydrous sodium carbonate and add it to 70 mL of deionized water, stir to dissolve. Weigh 20 g of ball-milled phosphogypsum and add it to the sodium carbonate solution. After adding the materials, react at a stirring rate of 650 rpm for 60 min at 30 °C. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 95.19%.

[0032] (2) Add the calcium carbonate residue in step (1) to 65 mL of acetic acid solution with a concentration of 3.5 mol / L, react at a stirring rate of 60 rpm at 15 °C for 20 min, stir until the solid no longer dissolves, filter and wash, and combine the filtrate and washing solution. Obtain calcium acetate solution and solid residue. The solid residue is mainly impurities migrated from phosphogypsum and unreacted calcium carbonate. At this time, the leaching rate of calcium element is 82.16%.

[0033] (3) Concentrate the sodium sulfate solution in step (1) to 1.16 mol / L, take the calcium acetate solution in step (2) and add it to the sodium sulfate solution, react at a stirring rate of 400 rpm at 25 °C for 30 min. After the reaction, filter and wash to obtain calcium sulfate dihydrate solid and sodium acetate trihydrate solution. Use acetic acid and sodium carbonate solution to adjust the pH of the sodium acetate trihydrate solution to 7.2 - 7.4, heat up and concentrate until solids appear in the mixed solution, filter while hot and keep warm, cool and crystallize to obtain sodium acetate trihydrate. At this time, the purity of calcium sulfate dihydrate is 97.78%, the whiteness is 93.1%, and the yield is 70.04%. The purity of sodium acetate trihydrate is 95.02%, the whiteness is 91.6%, and the yield is 99.53%.

[0034] Example 2

[0035] A method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate includes the following steps:

[0036] (1) Weigh 11.43 g of anhydrous sodium carbonate and add it to 70 mL of deionized water, stir to dissolve. Weigh 20 g of ball-milled phosphogypsum and add it to the sodium carbonate solution. After adding the materials, react at a stirring rate of 650 rpm for 55 min at 40 °C. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 99.20%.

[0037] (2) Add the calcium carbonate residue in step (1) to 80 mL of acetic acid solution with a concentration of 3.5 mol / L, react at a stirring rate of 120 rpm at 15 °C for 20 min, stir until the solid no longer dissolves, filter, wash, and combine the filtrate and washing solution. Obtain a calcium acetate solution and a solid residue, where the solid residue is mainly impurities migrated from phosphogypsum and unreacted calcium carbonate. At this time, the calcium element leaching rate is 87.24%.

[0038] (3) Concentrate the sodium sulfate solution in step (1) to 2.09 mol / L, take the calcium acetate solution in step (2) and add it to the sodium sulfate solution, react at a stirring rate of 400 rpm at 25 °C for 30 min. After the reaction, filter and wash to obtain calcium sulfate dihydrate solid and sodium acetate trihydrate solution. Use sulfuric acid and sodium hydroxide solution to adjust the pH of the sodium acetate trihydrate solution to 7.2 - 7.4, heat up and concentrate until solids appear in the mixed solution, filter while hot and keep warm, cool and crystallize to obtain sodium acetate trihydrate. At this time, the purity of calcium sulfate dihydrate is 97.83%, the whiteness is 93.0%, and the yield is 80.01%. The purity of sodium acetate trihydrate is 95.08%, the whiteness is 91.7%, and the yield is 99.28%.

[0039] Example 3

[0040] Provide a method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate, including the following steps:

[0041] (1) Weigh 11.20 g of anhydrous sodium carbonate, add it to 70 mL of deionized water and stir to dissolve. Weigh 20 g of ball-milled phosphogypsum, add it to the sodium carbonate solution. After adding the materials, react at a stirring rate of 600 rpm at 40 °C for 70 min. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 97.64%.

[0042] (2) Add the calcium carbonate residue in step (1) to 75 mL of acetic acid solution with a concentration of 3.5 mol / L, react at a stirring rate of 180 rpm at 15 °C for 20 min, stir until the solid no longer dissolves, filter, wash, and combine the filtrate and washing solution. Obtain a calcium acetate solution and a solid residue, where the solid residue is mainly impurities migrated from phosphogypsum and unreacted calcium carbonate. At this time, the calcium element leaching rate is 83.34%.

[0043] (3) Concentrate the sodium sulfate solution in step (1) to 1.31 mol / L. Take the calcium acetate solution in step (2) and add it to the sodium sulfate solution. React at a stirring rate of 400 rpm at 25 °C for 30 min. After the reaction, filter and wash to obtain calcium sulfate dihydrate solid and sodium acetate trihydrate solution. Adjust the pH of the sodium acetate trihydrate solution to 7.2 - 7.4 using acetic acid and sodium hydroxide solution. Heat up and concentrate until solids appear in the mixed solution, then filter while keeping it warm. Cool and crystallize to obtain sodium acetate trihydrate. At this time, the purity of calcium sulfate dihydrate is 97.73%, the whiteness is 93.0%, and the yield is 72.84%. The purity of sodium acetate trihydrate is 95.09%, the whiteness is 91.8%, and the yield is 99.60%.

[0044] Example 4

[0045] Provide a method for preparing high-purity gypsum from phosphogypsum as raw material and by-producing sodium acetate trihydrate, which includes the following steps:

[0046] (1) Weigh 11.65 g of anhydrous sodium carbonate and add it to 60 mL of deionized water, stir to dissolve. Weigh 20 g of ball-milled phosphogypsum and add it to the sodium carbonate solution. After adding the materials, react at a stirring rate of 650 rpm at 40 °C for 65 min. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 97.09%.

[0047] (2) Add the calcium carbonate residue in step (1) to 100 mL of acetic acid solution with a concentration of 3.0 mol / L. React at a stirring rate of 240 rpm at 15 °C for 20 min, stir until the solid no longer dissolves, filter, wash, and combine the filtrate and washing solution. Obtain calcium acetate solution and solid residue. The solid residue is mainly impurities migrated from phosphogypsum and unreacted calcium carbonate. At this time, the calcium element leaching rate is 85.34%.

[0048] (3) Concentrate the sodium sulfate solution in step (1) to 1.74 mol / L. Take the calcium acetate solution in step (2) and add it to the sodium sulfate solution. React at a stirring rate of 400 rpm at 25 °C for 30 min. After the reaction, filter and wash to obtain calcium sulfate dihydrate solid and sodium acetate trihydrate solution. Adjust the pH of the sodium acetate trihydrate solution to 7.2 - 7.4 using sulfuric acid and sodium carbonate solution. Heat up and concentrate until solids appear in the mixed solution, then filter while keeping it warm. Cool and crystallize to obtain sodium acetate trihydrate. At this time, the purity of calcium sulfate dihydrate is 97.81%, the whiteness is 93.0%, and the yield is 74.79%. The purity of sodium acetate trihydrate is 95.12%, the whiteness is 91.7%, and the yield is 99.73%.

[0049] Example 5

[0050] A method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate includes the following steps:

[0051] (1) Weigh 11.20 g of anhydrous sodium carbonate and add it to 80 mL of deionized water to stir and dissolve. Weigh 20 g of ball-milled phosphogypsum and add it to the sodium carbonate solution. After the feeding is completed, react at a stirring rate of 550 rpm for 70 min at 40 °C. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 96.11%.

[0052] (2) Add the calcium carbonate residue in step (1) to 90 mL of acetic acid solution with a concentration of 3.5 mol / L, and react at a stirring rate of 120 rpm at 15 °C for 20 min. Stir until the solid no longer dissolves, filter and wash, and combine the filtrate and washing liquid. Obtain calcium acetate solution and solid residue. The solid residue is mainly impurities migrated from phosphogypsum and unreacted calcium carbonate. At this time, the calcium element leaching rate is 85.69%.

[0053] (3) Concentrate the sodium sulfate solution in step (1) to 1.50 mol / L, take the calcium acetate solution in step (2) and add it to the sodium sulfate solution, and react at a stirring rate of 400 rpm at 25 °C for 30 min. After the reaction, filter and wash to obtain calcium sulfate dihydrate solid and sodium acetate trihydrate solution. Use acetic acid and sodium carbonate solution to adjust the pH of the sodium acetate trihydrate solution to 7.2 - 7.4, heat up and concentrate until solids appear in the mixed solution, filter while hot, and cool and crystallize to obtain sodium acetate trihydrate. At this time, the purity of calcium sulfate dihydrate is 97.81%, the whiteness is 93.1%, and the yield is 72.13%. The purity of sodium acetate trihydrate is 95.04%, the whiteness is 91.8%, and the yield is 99.48%.

[0054] Comparative Example 1

[0055] Weigh 20 g of ball-milled phosphogypsum and add it to 40 mL of deionized water to prepare a gypsum slurry. Weigh 11.20 g of anhydrous sodium carbonate and add it to 40 mL of deionized water to stir and dissolve. Under stirring, add the sodium carbonate solution to the gypsum slurry. After the feeding is completed, react at a stirring rate of 550 rpm at 40 °C for 70 min. After the reaction, filter and wash to obtain calcium carbonate residue and sodium sulfate solution. At this time, the decomposition conversion rate of phosphogypsum is 91.20%.

[0056] The above are only specific embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing high-purity gypsum from phosphogypsum and by-producing sodium acetate trihydrate, characterized in that, It includes the following steps: 1) Dissolve sodium carbonate in water, then add phosphogypsum, stir at a constant temperature to carry out a decomposition reaction. After the reaction ends, separate to obtain calcium carbonate residue and sodium sulfate mother liquor. Among them: the molar ratio of sodium carbonate to calcium sulfate dihydrate in phosphogypsum is 0.92:1 - 1.12:1; the volume-mass ratio of water to phosphogypsum is 2:1 - 5:1 mL / g; the decomposition and conversion process is: reaction time 20 - 120 min, reaction temperature 20 - 50 °C, stirring speed 500 - 700 rpm; 2) Add the calcium carbonate residue obtained in step 1) to acetic acid solution to react to obtain calcium acetate mother liquor and solid residue. Among them: the concentration of acetic acid solution is 2.0 - 4.0 mol / L, and the theoretical molar ratio of calcium carbonate to acetic acid is 1:2 - 1:4; 3) Add the calcium acetate mother liquor obtained in step 2) to the sodium sulfate mother liquor obtained in step 1). Among them, the sodium sulfate mother liquor is previously concentrated to a concentration of 1.16 - 2.09 mol / L, react to obtain calcium sulfate dihydrate precipitate and sodium acetate trihydrate mother liquor, adjust the pH of the mother liquor to 6.3 - 10.0, and then concentrate and crystallize to obtain sodium acetate trihydrate product.

2. The method according to claim 1, wherein In step 1), the phosphogypsum is ground to pass through a 200-mesh sieve completely.

3. The method according to claim 1, wherein In step 2), the stirring speed is 60 - 400 rpm.

4. The method according to claim 1, wherein In step 3), the reagent for adjusting the pH of the sodium acetate trihydrate mother liquor is sodium hydroxide, sodium carbonate, sulfuric acid or acetic acid.

5. The method according to claim 1, wherein In step 3), the concentration temperature of the sodium acetate trihydrate mother liquor is 45 - 100 °C.

6. The method according to claim 1, wherein In step 3), the purity of the prepared calcium sulfate dihydrate is ≥97%, and the whiteness is ≥90%; the purity of the by-product sodium acetate trihydrate is ≥95%, and the whiteness is ≥90%.

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