Method for preparing water-soluble fertilizer and saline-alkali soil modifier by catalyzing phosphogypsum and biomass solid waste

Abstract

The invention discloses a method for preparing a water-soluble fertilizer and a saline-alkali soil modifier through catalysis of ardealite and biomass solid waste. Firstly, ardealite and ammonium carbonate react to prepare an ammonium sulfate solution and high-activity calcium carbonate solid slurry, and the ammonium sulfate solution can be used as a liquid nitrogen fertilizer after being subjected to post-treatment such as dilution and pH adjustment; carrying out a hydrothermal reaction on a biomass material, a catalyst, an activator and the high-activity calcium carbonate solid slurry, carrying out solid-liquid separation to obtain a residual reaction solution and solid residues containing calcium humate and fulvic acid, adding hydrochloric acid into the solid residues, and carrying out a reaction to obtain a solution rich in fulvic acid and calcium chloride and a humic acid solid precipitate; the former can be used as a saline-alkali soil conditioner; the latter is subjected to water washing, KOH extraction and filtration to obtain a liquid fertilizer containing potassium humate, and the residual residue can be used as a base fertilizer. The process for preparing ammonium sulfate from ardealite in combination with biomass humification provided by the invention has sufficient feasibility, and the yield of humic acid is increased by 80% compared with that before combination.

Description

Technical Field

[0001] The invention belongs to the technical field of resource utilization of bulk solid wastes, and specifically relates to a method for preparing water-soluble fertilizers and saline-alkali land improvers by catalyzing phosphogypsum and biomass solid wastes. Background Art

[0002] Phosphogypsum is the waste residue produced by the wet process of producing phosphoric acid. Its main component is calcium sulfate, in addition to impurities such as incompletely decomposed phosphate rock, residual phosphoric acid, fluoride, organic matter, etc. The pH is about 1.5-5, and the crystal water content is 20-25%. The wet process of producing phosphoric acid will produce a large amount of phosphogypsum. The stacking of phosphogypsum not only occupies land resources, but also the phosphorus, fluoride, and free acid it contains will pollute the soil and groundwater environment through rainfall infiltration. Therefore, it is urgent to develop comprehensive utilization methods for phosphogypsum. At present, the main utilization methods of phosphogypsum include making cement additives, road filling, making gypsum boards, making building gypsum powder and blocks, etc. In agriculture, using phosphogypsum to replace natural gypsum to produce ammonium sulfate fertilizer has become a major utilization method. However, the calcium carbonate byproduct produced by the process of producing ammonium sulfate using phosphogypsum can only be discharged as general solid waste or sold as low-value-added industrial raw materials due to its low grade and high reprocessing cost. Its economic and environmental benefits are relatively low. Existing researchers use hydrochloric acid to extract calcium carbonate residue to prepare calcium chloride, and then use ammonium bicarbonate and ammonia water as carbonizing agents to further process calcium chloride to prepare calcium carbonate. The product quality reaches first-class or above, and can be used as a filler in industries such as rubber, plastics, papermaking and coatings. However, although the calcium carbonate prepared by this method has a high grade, the cost of its processing process is relatively high, which does not match its economic value and is difficult to realize the industrial process. Therefore, it is of great significance to further utilize its by-product calcium carbonate on the basis of using phosphogypsum to prepare ammonium sulfate in a simpler and more economical way to prepare it into a high value-added industrial product.

[0003] Biomass solid waste can be divided into three categories: agriculture and forestry (crop waste, livestock and poultry manure, forestry processing waste, etc.), light industry (vintners' grains, pressing byproducts, pharmaceutical residues, etc.), and urban solid waste (kitchen waste, municipal garden waste, domestic waste, etc.). The main components include cellulose, lignin, protein, inorganic salts, etc., and its utilization methods include energy, feed, fertilizer and material, among which the fertilizer utilization method is still in its infancy. In recent years, the research on the use of various biomass solid wastes to humify them by composting, hydrothermal and pyrolysis to prepare high value-added products has gradually increased. Fang Deng et al. used the biogas slurry impregnation method to hydrothermally humify the acid hydrolysis residue of corn stalks and prepared products that met the national standards for water-soluble humic acid fertilizers; Xiongxin Peng et al. explored the hydrothermal humification mechanism of corn stalks as raw materials; Xintong Gao et al. explored the humification mechanism of different biomass composting processes such as livestock manure and rice straw. CN 115611683B has developed a method for preparing water-soluble fertilizer containing super-mineral humic acid from agricultural and forestry biomass solid waste, which is subjected to gradient roasting and carbonization under the joint action of a catalyst and an activator to promote the reorganization of the carbon chain of organic matter to obtain a weathered coal precursor, and then enrich its functional groups under the action of an oxidant to convert the organic matter into super-mineral humic acid. CN 114768760A has invented a method for extracting potassium humate from tail vegetable leaves and preparing an iron-carbon composite material, which realizes the full quantitative utilization of tail vegetable leaves. CN113277492B has developed a method for preparing potassium humate and biochar by using agricultural and forestry wastes to be alkali-heated and then mixed with potassium ferrate additives for roasting and quenching. CN 117623822A has developed a method for removing antibiotics from antibiotic fermentation residues and synergistically recovering plant active organic matter. The hydrothermal method is combined with birnessite catalytic oxidation technology to deeply detoxify the antibiotic fermentation residues while promoting the caramelization and Maillard reaction of the organic matter to gradually generate humic acid active substances. The above research and patents all provide methods for preparing water-soluble fertilizers containing humic acid using biomass solid waste as raw materials. On this basis, the present invention hopes to combine the process for preparing ammonium sulfate from phosphogypsum with the method for preparing water-soluble fertilizers from biomass solid waste, reduce production costs, simplify production processes, and improve product value, so as to achieve the full quantitative utilization of phosphogypsum and biomass solid waste, provide a new idea for synergistic efficiency of different types of solid waste resource utilization, and achieve the solid waste resource utilization goal of reducing pollution and carbon emissions. Summary of the invention

[0004] In order to solve the shortcomings and deficiencies of the prior art, the object of the present invention is to provide a method for preparing water-soluble fertilizers and saline-alkali land improvers by catalyzing phosphogypsum and biomass solid waste.

[0005] The process of preparing ammonium sulfate from phosphogypsum is combined with the process of preparing humic acid from biomass solid waste by hydrothermal treatment. First, different pretreatment steps are carried out according to the original sample quality of phosphogypsum, and then it is reacted with ammonia water, ammonium bicarbonate / CO2 and filtered to obtain ammonium sulfate solution and highly active calcium carbonate solid slurry. The ammonium sulfate solution can be used as liquid nitrogen fertilizer after dilution, pH adjustment and other post-treatments; the calcium carbonate solid slurry is stored for standby use. Straw, sawdust, rice bran and other biomass solid wastes are successively cleaned, mechanically ground and other pretreatments to obtain biomass powder. After adding activators, catalysts and highly active calcium carbonate solid slurry produced in the previous process, hydrothermal treatment is carried out under certain conditions. After solid-liquid separation, the remaining reaction liquid and solid residue containing calcium humate and humic acid are obtained. The former can be refluxed into the hydrothermal reactor to continue the reaction. The latter is added with hydrochloric acid and ultrasonically cleaned, and after filtration, a solution rich in humic acid and calcium chloride and a humic acid solid precipitate are obtained. The former can be used as a soil conditioner for saline-alkali land; the latter is washed with water, extracted with KOH, and filtered to obtain a liquid fertilizer containing potassium humate, and the remaining residue can be used as a base fertilizer.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A method for preparing water-soluble fertilizer and saline-alkali land improver by catalyzing phosphogypsum and biomass solid waste, comprising the following steps:

[0008] (a) Ammonium sulfate reaction step: adding ammonia water and ammonium bicarbonate, or ammonia water and CO2 into a reactor, reacting for a period of time, raising the reactor temperature to 40-44°C, then adding pretreated phosphogypsum, polymer cationic dispersant and water into the reactor, and continuing the reaction for a period of time;

[0009] The technical solution in which ammonia water and ammonium bicarbonate or ammonia water and CO2 react for a period of time can be replaced by ammonium carbonate;

[0010] (b) ammonium sulfate post-reaction treatment process: after the reaction in step (a) is completed, the mixed product is allowed to stand for cooling, filtered, and solid-liquid separation is performed to obtain calcium carbonate solid slurry and ammonium sulfate solution, the former is stored for later use, and the latter can be used as a liquid fertilizer after adjusting the pH;

[0011] (c) biomass humification: adding biomass powder, activator, catalyst, calcium carbonate solid slurry obtained in step (b) and water into a hydrothermal reactor for hydrothermal reaction, standing and cooling after the reaction, and solid-liquid separation, the obtained liquid can be refluxed to the reactor for repeated use, and the solid product containing calcium humate and humic acid is stored for processing;

[0012] (d) Processing step of the solid product after the hydrothermal reaction: adding hydrochloric acid to the solid product containing calcium humate and humic acid to adjust the pH and react, and then standing until the humic acid is completely precipitated, filtering to obtain a calcium chloride solution rich in humic acid and a solid precipitate of humic acid, the former of which can be used as an improver for saline-alkali land; washing the solid precipitate of humic acid with water, extracting with KOH, and filtering to obtain a liquid potassium humate fertilizer, and the remaining solid residue can be used as a base fertilizer product.

[0013] Preferably, the molar ratio of the aqueous ammonia to ammonium bicarbonate in step (a) is ≥1:1, more preferably (1.2-1):1; the molar ratio of the aqueous ammonia to CO2 is ≥2:1, more preferably (2.2-2):1, and the aqueous ammonia here refers to pure NH3·H2O.

[0014] Preferably, the ammonia water (pure NH3·H2O) in step (a) is added in the form of an aqueous ammonia solution (a mixture of pure NH3·H2O and water), wherein the mass concentration of the aqueous ammonia solution is 26-28%.

[0015] Preferably, the mass ratio of ammonium carbonate generated by the reaction of ammonia water and ammonium bicarbonate, or ammonia water and CO2, to phosphogypsum in step (a) is ≥8:15, more preferably (9-8):15.

[0016] Preferably, the addition rate of the pretreated phosphogypsum in step (a) is 30-40 g / min; the dosage of the polymer cationic dispersant is 1100-1500 ppm; and the liquid-to-solid ratio of the system is 1:(1-1.1).

[0017] Preferably, the reaction time of the ammonia water and ammonium bicarbonate, or the ammonia water and CO2 in step (a) is 30 to 60 minutes.

[0018] Preferably, the reaction time after the pretreated phosphogypsum and the polymer cationic dispersant are added in step (a) is 50 to 75 minutes.

[0019] Preferably, the pretreated phosphogypsum in step (a) is obtained by the following method: directly drying and sieving the phosphogypsum; or washing the phosphogypsum with water and then drying and sieving it.

[0020] More preferably, the drying refers to drying until the free water content is ≤45%; and the sieving refers to sieving through a 100-mesh standard sieve.

[0021] Preferably, the polymer cationic dispersant in step (a) is cationic polyacrylamide.

[0022] Preferably, the stirring speed of the reaction in step (a) is 150-300 rpm.

[0023] Preferably, the biomass in step (c) is biomass rich in lignin, more preferably at least one of straw, sawdust and vegetable leaves.

[0024] Preferably, the biomass powder in step (c) is obtained by washing, drying, and crushing lignin-rich biomass and then passing through an 80-mesh standard sieve.

[0025] Preferably, the mass ratio of the biomass powder and the calcium carbonate in the calcium carbonate solid slurry in step (c) is (50-70):100.

[0026] Preferably, the concentration of the activator in the system in step (c) is 0.55-0.65 mol / L, more preferably 0.6 mol / L, and the concentration of the catalyst in the system is 8-12 mg / L; the solid-liquid ratio of the reaction system is (4-6):1, more preferably (4.5-5.5):1.

[0027] Preferably, the activator in step (c) is urea, and the catalyst is at least one of MnO2 and nano-iron oxide; more preferably MnO2.

[0028] Preferably, the temperature of the hydrothermal reaction in step (c) is 80-90°C and the time is 90-120 min. Preferably, the solid-liquid separation method in step (c) is centrifugation, filtration or static precipitation.

[0029] Preferably, the pH in step (d) is adjusted to 1.5-2.

[0030] Preferably, the reaction in step (d) is an ultrasonic reaction, and the reaction time is 20 to 30 minutes.

[0031] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0032] The process of preparing ammonium sulfate from phosphogypsum and combining biomass humification proposed in the present invention is fully feasible, and its humic acid yield is greatly improved to 80% compared with that before the combined use. The present invention makes full use of byproducts such as highly active calcium carbonate produced by preparing ammonium sulfate from phosphogypsum and residual reaction liquid produced by alkali heat treatment of biomass solid waste to prepare humic acid, and produces two products with high added value, realizing full quantification, pollution reduction and carbon reduction, and low-pollution resource utilization of the two solid wastes. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the technical roadmap of the present invention.

[0034] Figure 2 The humic acid solid and the FA-rich CaCl2 solution obtained in step (d) of Case 1.

[0035] Figure 3This is the humic acid solution obtained by extracting the solid precipitate with KOH in step (d) of Case 1.

[0036] Figure 4 This is the plant hydroponic experiment of Case 3 (left: pure water group; right: humic acid water-soluble fertilizer group). DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with examples and drawings, but the embodiments of the present invention are not limited thereto.

[0038] If no specific conditions are specified in the examples of the present invention, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. All raw materials, reagents, etc., whose manufacturers are not specified, are conventional products that can be purchased commercially.

[0039] Case 1: Effect of preparing ammonium sulfate from phosphogypsum and combining it with biomass humification process

[0040] Phosphogypsum is taken from the phosphogypsum waste residue produced by a phosphoric acid plant. Its main chemical components are shown in Table 1. Generally speaking, it has a high grade and contains less impurities. Chemical grade ammonium bicarbonate is used, with a nitrogen content of ≥95%; the NH3 content in the ammonia solution used is 26-28%.

[0041] Table 1 Phosphogypsum composition

[0042]

[0043] Step (a), pre-treat the phosphogypsum, dry it at 45°C to make its free water content ≤45%, pass it through a 100-mesh standard sieve, and obtain a sample of the pre-treated phosphogypsum. 90g of ammonia solution and 100g of ammonium bicarbonate are added to the reactor, and the reactor temperature is raised to 44°C using a constant temperature water bath. When the solution in the reactor reaches the preset temperature, turn on the agitator at a stirring speed of 200rpm, add 200g of the pre-treated phosphogypsum sample to the reactor at a speed of 35g / min, add water to control the liquid-to-solid ratio to 1:1.05, and add a cationic polyacrylamide dispersant at a dosage of 1300ppm, and the reaction residence time is 60min.

[0044] Step (b), after the reaction is completed, the mixed product is allowed to stand for cooling and precipitation, and solid-liquid separation is performed to obtain byproduct high-activity calcium carbonate solid slurry and ammonium sulfate solution, wherein the theoretical mass of calcium carbonate is 104g. The sulfate ion content in the ammonium sulfate solution and phosphogypsum is measured by barium sulfate precipitation method, and the conversion rate of ammonium sulfate prepared from phosphogypsum is 94.05%, the concentration of ammonium sulfate solution is 3.3mol / L, and the nitrogen content is 92.4g / L. After adjusting the pH, it can be used as a liquid fertilizer.

[0045] Step (c), wash, dry and crush 60g of dry straw and sawdust (mass ratio of straw: sawdust = 1:1), pass through a 80-mesh standard sieve and dry for standby use. Using urea as an activator and MnO2 powder as a catalyst, add 60g of dry biomass powder, 36g of urea, 10mg of MnO2 powder, and the high-activity calcium carbonate slurry produced in step (b) (theoretical CaCO3 mass is 104g) to the reactor, add 1000mL of water, and hydrothermally treat at 300rpm and 85℃ for 100min. After the reactants are cooled to room temperature, centrifuge at 6000rpm for 10min. The supernatant can be refluxed to the reactor to continue to participate in the reaction during the production process, and precipitate into a solid residue rich in calcium humate and humic acid, which is stored for standby use.

[0046] Step (d), add an appropriate amount of hydrochloric acid to the solid residue, place it in an ultrasonic cleaner for reaction for 30 minutes, and centrifuge to obtain Figure 2 The reaction product shown in the figure is a calcium chloride solution rich in humic acid, which can be used as a saline-alkali land improver; the black precipitate is a mixture of humic acid and biomass residue. The solid residue is washed with water, and a 1 mol / L KOH solution is added and stirred for extraction, and a black humic acid solution is obtained by filtration ( Figure 3 ), the humic acid concentration was determined to be 42.67 g / L, reaching the NY 1106-2006 standard for water-soluble fertilizers containing humic acid. The humic acid yield of this process reached 32.15%. The remaining biomass solid residue can be returned to the hydrothermal reactor to continue to participate in the reaction, or it can be used as agricultural base fertilizer after certain treatment.

[0047] The biomass solid waste raw material used in step (c) is changed to discarded radish leaves, with an amount of 50 g. Other treatment processes and additives remain unchanged. The above steps are repeated to finally obtain a calcium chloride solution and a humic acid solution rich in humic acid, wherein the concentration of the humic acid solution is 47.2 g / L and the yield is 28.33%.

[0048] The catalyst used in step (c) was changed to nano-iron oxide, and the dosage and other treatment processes and additives remained unchanged. The above steps were repeated to finally obtain a calcium chloride solution and a humic acid solution rich in humic acid, wherein the concentration of the humic acid solution was 39.7 g / L and the yield was 29.75%.

[0049] For step (c), the high-activity calcium carbonate solid slurry produced in step (b) is not added, and other process parameters and additives remain unchanged. The above steps are repeated to finally obtain a humic acid acid solution and a humic acid solution that do not contain calcium chloride, wherein the concentration of the humic acid solution is reduced to 23.8 g / L, and the yield is 17.85%, which is about 44.48% lower than that of the combined process.

[0050] The high-activity calcium carbonate solid slurry added in step (c) was changed to industrial-grade calcium carbonate (300 mesh, purity 99.1%), and other process parameters and additives remained unchanged. The above steps were repeated to finally obtain a humic acid solution and a humic acid solution containing calcium chloride, wherein the concentration of the humic acid solution was reduced to 26.2 g / L, and the yield was 21.8%, which was about 32.16% lower than that of the combined process. Using other alternative raw materials will not only reduce the yield of humic acid, but also increase costs, reduce benefits, and waste available resources.

[0051] The process of preparing ammonium sulfate from phosphogypsum and combining biomass humification proposed in the present invention is fully feasible, and its humic acid yield is greatly improved to 80% compared with that before the combined use, and the by-products of the two processes can be fully utilized to achieve full quantitative utilization of solid waste. In addition, by using different biomass solid waste raw materials, catalysts and activators on the basic process route, higher-grade products can be obtained, indicating that the process route has high application value and industrialization potential.

[0052] Case 2: Effect of FA-enriched CaCl2 solution as a soil conditioner

[0053] The FA-rich CaCl2 solution obtained in step (d) of case 1 was used for soil improvement test. The obtained solution was diluted 100, 300, and 500 times and applied regularly in different test plots. The soil was turned over after each application. The test lasted for 180 days. After the test, various properties of the surface soil in the test plot and the untreated area were measured. The results are shown in Table 2.

[0054] Table 2 Soil improvement effect

[0055]

[0056] The results show that the FA-rich CaCl2 solution prepared by the present invention has a good improvement effect on saline-alkali soil, can promote the formation of soil water-stable aggregates, and is beneficial to the improvement and stabilization of soil structure; it can regulate physical and chemical properties, reduce soil pH and salt content, increase soil moisture content and organic matter content, and increase soil fertility.

[0057] Case 3: Effect of Humic Acid Water-soluble Fertilizer

[0058] The humic acid solution obtained by KOH extraction from the solid precipitate in step (d) of Case 1 was used for plant hydroponic experiments. The filter paper pad moistened with deionized water was placed on the bottom of a culture dish with a diameter of 75 mm. The seeds were sterilized and washed and evenly placed on the culture dish. They were cultured in the dark at 25°C for 72 hours. The pH value of the humic acid solution was first adjusted to 7.0±0.2, and then diluted 300 times. Rice seedlings of similar length were divided into two parts, one was placed in 300 mL of the treated humic acid solution, and the other was placed in 300 mL of ultrapure water and cultured at 25°C for 9 days (day: night = 8:16h). Figure 4 As shown, the stem and leaf length of rice seedlings cultured in humic acid solution is significantly longer than that in pure water group, and the plant growth can be promoted under the same conditions, indicating that the humic acid liquid fertilizer prepared by the present invention has good fertilizer effect and can be used as high-quality fertilizer for crops.

[0059] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for preparing water-soluble fertilizers and saline-alkali land improvers by catalyzing phosphogypsum and biomass solid waste, characterized in that: The following steps are involved: (a) Ammonium sulfate reaction step: adding ammonia water and ammonium bicarbonate, or ammonia water and CO2 into a reactor, reacting for a period of time, raising the reactor temperature to 40-44°C, then adding pretreated phosphogypsum, polymer cationic dispersant and water into the reactor, and continuing the reaction for a period of time; The technical solution in which ammonia water and ammonium bicarbonate or ammonia water and CO2 react for a period of time can be replaced by ammonium carbonate; (b) ammonium sulfate post-reaction treatment process: after the reaction in step (a) is completed, the mixed product is allowed to stand for cooling, precipitation, and solid-liquid separation to obtain a calcium carbonate solid slurry and an ammonium sulfate solution, the former of which is stored for later use, and the latter can be used as a liquid fertilizer after adjusting the pH; (c) biomass humification: adding biomass powder, activator, catalyst, calcium carbonate solid slurry obtained in step (b) and water into a hydrothermal reactor for hydrothermal reaction, standing and cooling after the reaction, and solid-liquid separation, the obtained liquid can be refluxed to the reactor for repeated use, and the solid product containing calcium humate and humic acid is stored for processing; (d) Processing step of the solid product after the hydrothermal reaction: adding hydrochloric acid to the solid product containing calcium humate and humic acid to adjust the pH and react, and then standing until the humic acid is completely precipitated, filtering to obtain a calcium chloride solution rich in humic acid and a solid precipitate of humic acid, the former of which can be used as an improver for saline-alkali land; washing the solid precipitate of humic acid with water, extracting with KOH, and filtering to obtain a liquid potassium humate fertilizer, and the remaining solid residue can be used as a base fertilizer product.

2. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalyzing phosphogypsum and biomass solid waste according to claim 1, characterized in that: The mass ratio of the biomass powder and the calcium carbonate in the calcium carbonate solid slurry in step (c) is (50-70):

100.

3. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalyzing phosphogypsum and biomass solid waste according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step (c) is 80-90° C. and the time is 90-120 min.

4. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalyzing phosphogypsum and biomass solid waste according to claim 1, characterized in that: The biomass in step (c) is a biomass rich in lignin; In step (c), the concentration of the activator in the system is 0.55-0.65 mol / L, the concentration of the catalyst in the system is 8-12 mg / L; and the solid-liquid ratio of the reaction system is (4-6):

1.

5. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalyzing phosphogypsum and biomass solid waste according to claim 1, characterized in that: The biomass in step (c) is at least one of straw, wood chips and vegetable leaves; In step (c), the activator is urea, and the catalyst is at least one of MnO2 and nano-iron oxide.

6. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalysis of phosphogypsum and biomass solid waste according to claim 1, characterized in that: The molar ratio of the aqueous ammonia to ammonium bicarbonate in step (a) is ≥ 1:1, and the molar ratio of the aqueous ammonia to CO2 is ≥ 2:1, wherein the aqueous ammonia refers to pure NH3·H2O; The ammonia water in step (a) is added in the form of an aqueous ammonia solution, wherein the mass concentration of the aqueous ammonia solution is 26-28%; The reaction time of the ammonia water and ammonium bicarbonate or the ammonia water and CO2 in step (a) is 30 to 60 minutes.

7. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalysis of phosphogypsum and biomass solid waste according to claim 1, characterized in that: The mass ratio of ammonium carbonate generated by the reaction of ammonia water and ammonium bicarbonate, or ammonia water and CO2, to phosphogypsum is ≥ 8:15 in step (a). The addition rate of the pretreated phosphogypsum in step (a) is 30-40 g / min; The liquid-to-solid ratio of the system is 1:(1-1.1); The reaction time after the pretreated phosphogypsum and the polymer cationic dispersant are added in step (a) is 50 to 75 minutes.

8. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalysis of phosphogypsum and biomass solid waste according to claim 1, characterized in that: The dosage of the polymer cationic dispersant in step (a) is 1100-1500 ppm; the polymer cationic dispersant is cationic polyacrylamide.

9. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalysis of phosphogypsum and biomass solid waste according to claim 6, characterized in that: The molar ratio of the ammonia water to ammonium bicarbonate in step (a) is (1.2-1):1; the molar ratio of the ammonia water to CO2 is (2.2-2):1; The mass ratio of ammonium carbonate generated by the reaction of ammonia water and ammonium bicarbonate, or ammonia water and CO2, to phosphogypsum is (9-8):

15.

10. The method for preparing water-soluble fertilizer and saline-alkali land improver by catalysis of phosphogypsum and biomass solid waste according to claim 1, characterized in that: The pretreated phosphogypsum in step (a) is obtained by the following method: directly drying and sieving the phosphogypsum; or washing the phosphogypsum with water and then drying and sieving it; The drying refers to drying until the free water content is ≤45%; the sieving refers to sieving through a 100-mesh standard sieve; In step (d), the pH is adjusted to 1.5-2; the reaction is an ultrasonic reaction, and the reaction time is 20-30 minutes.

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