Process method for producing calcium magnesium phosphate fertilizer by utilizing flotation phosphate tailings

Abstract

The invention discloses a process method for producing a calcium magnesium phosphate fertilizer by utilizing flotation phosphate tailings, and belongs to the technical field of phosphate fertilizer chemical industry. The method is used for solving the technical problem that the utilization rate of the prepared calcium magnesium phosphate fertilizer is low due to the fact that the calcium magnesium phosphate fertilizer prepared by high-temperature melting of flotation phosphate tailings in the prior art is easily influenced by external environment. A process method for producing a calcium magnesium phosphate fertilizer by utilizing flotation phosphate tailings comprises the following steps: B1, melting the flotation phosphate tailings at high temperature, crushing and screening to prepare tailing powder; b2, blending the tailing powder, citric acid powder, phosphoric acid and a polyester auxiliary material to obtain efficient powder; and efficiently balling the powder to obtain the nearly spherical calcium magnesium phosphate fertilizer. According to the invention, polycondensation of methylacrolein and formaldehyde and polycondensation of terephthalic acid and ethylene glycol are carried out synchronously, and the prepared polyester auxiliary material has the advantages of slow release and good mechanical properties. The synthesized calcium magnesium phosphate fertilizer can be subjected to organic slow release and has good heavy metal adsorption performance.

Description

Technical Field

[0001] This invention relates to the field of phosphate fertilizer chemical technology, specifically to a process for producing calcium magnesium phosphate fertilizer using flotation tailings. Background Technology

[0002] With the increasing demand for phosphate rock resources, low-grade phosphate rock can be used on a large scale through beneficiation, impurity removal, and purification. However, with the proliferation of flotation beneficiation equipment, the byproduct, flotation tailings, is increasing exponentially. The indiscriminate dumping of flotation tailings not only poses environmental risks, but tailings ponds with large quantities of tailings also represent a significant safety hazard. Therefore, the comprehensive utilization of flotation tailings has become an urgent need for mineral processing enterprises.

[0003] Phosphorus is an essential element for promoting crop growth and supporting crop life activities; it is also a component of cell nucleoproteins. Magnesium participates in biological processes such as enzyme activation and is an indispensable element for plant growth. If phosphorus tailings from flotation can be processed into phosphorus-magnesium compound fertilizer products, it will not only generate certain economic benefits and develop a circular economy, but also have significant social implications for implementing a sustainable development strategy for phosphate resources.

[0004] Patent application CN101747090A discloses a method for preparing calcium magnesium phosphate fertilizer using a blast furnace method with flotation tailings of phosphorus tailings. The method uses flotation phosphorus tailings slurry as raw material, which is processed to obtain tailings filter cake. The tailings filter cake is granulated and then pulverized and blended with other primary phosphate rock, silica, and coke to obtain sintered raw material. Calcium magnesium phosphate fertilizer is then prepared using a blast furnace method from the sintered raw material. However, when calcium magnesium phosphate fertilizer prepared from phosphorus tailings is applied to soil contaminated with heavy metals, the organic elements such as calcium and phosphorus readily react chemically with the heavy metals. Furthermore, the synthesized calcium magnesium phosphate fertilizer lacks slow-release properties, both of which lead to a decrease in the utilization rate of the prepared calcium magnesium phosphate fertilizer.

[0005] To address this technical deficiency, a solution is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a process for producing calcium magnesium phosphate fertilizer from flotation tailings, which solves the technical problem in the prior art that calcium magnesium phosphate fertilizer prepared by high-temperature melting of flotation tailings is easily affected by the external environment, resulting in low utilization rate of the prepared calcium magnesium phosphate fertilizer.

[0007] The objective of this invention can be achieved through the following technical solutions: A process for producing calcium magnesium phosphate fertilizer from flotation tailings includes the following steps: B1. High-temperature melting, cooling, crushing and screening of flotation phosphorus tailings to prepare tailings powder; B2. Tailings powder, citric acid powder, phosphoric acid and polyester additives are blended to obtain high-efficiency powder; the high-efficiency powder is pelletized to obtain near-spherical calcium magnesium phosphate fertilizer.

[0008] Furthermore, in step B1, the high-temperature melting temperature is 1600-1700℃ and the high-temperature melting time is 2-4h; the mesh size of the tailings powder after crushing and screening is 200-400 mesh.

[0009] Furthermore, in step B2, the method for preparing the polyester auxiliary material includes the following steps: A stereocomposite crystal was prepared by melt blending of Al, PLLA and PDLA. A2. The slurry preparation tank is connected to the esterification reactor. One esterification reactor and two parallel polycondensation reactors are connected to form a batch unit. Terephthalic acid and ethylene glycol are added to the slurry preparation tank for blending to obtain a blended monomer. Ramie cellulose and antimony acetate catalyst are then added to the slurry preparation tank to obtain a reaction system. The reaction system is fed into the esterification reactor from the top of the slurry preparation tank for esterification to obtain a prepolymer. Polyesters were prepared by intermittent esterification of terephthalic acid and ethylene glycol through esterification, primary polycondensation, and secondary polycondensation; wherein, ramie cellulose rich in hydroxyl groups was used to modify the synthetic polyester.

[0010] A3. The prepolymer is transferred to the first polycondensation reactor, where methacrolein, formaldehyde, and KOH are added to the first polycondensation reactor to undergo primary polycondensation and obtain the primary polymer. The primary polymer is then transferred to the second polycondensation reactor for secondary polycondensation to prepare the composite polyester. The prepolymer undergoes polycondensation in a first and a second polycondensation reactor sequentially. During this process, methacrolein and formaldehyde undergo aldol condensation, primarily forming a 2-methyl-2-propenal-formaldehyde condensate. Excess hydroxyl groups in the ethylene glycol and ramie cellulose in the prepolymer react with methacrolein to prepare the primary polymer. The primary polymer is then transferred to the second polycondensation reactor for secondary polycondensation. Here, the 2-methyl-2-propenal-formaldehyde condensate further polycondenses, as does ethylene glycol terephthalate, yielding a composite polyester. Because the polycondensation reactions of the two polymers occur simultaneously, a composite polyester in which the two polymers are physically intertwined can be prepared.

[0011] A4. The composite polyester and the prepared stereocomposite crystal are blended, melt-extruded, and cooled and solidified to obtain the polyester auxiliary material.

[0012] Furthermore, in step A1, the mass ratio of PLLA to PDLA is 1:1-2; the melt blending temperature is 190-200℃ and the melt blending time is 2-5 min.

[0013] Further, in step A2, the molar ratio of terephthalic acid to ethylene glycol is 1:1.15-1.25, the weight ratio of the blended monomer, ramie cellulose and catalyst is 100:5-15:0.3-0.5; the esterification reaction temperature is 245-265℃, the esterification reaction pressure is 0.1-0.3MPa, and the esterification reaction time is 2-3h.

[0014] Further, in step A3, the weight ratio of prepolymer, methacrylic acid, formaldehyde and catalyst is 100:7:3:0.01-0.03; the temperature of the first polycondensation is 240-250℃ and the duration of the first polycondensation is 1-2 hours; the temperature of the second polycondensation is 260-280℃ and the melt viscosity is 380-400 Pa·s, which is considered as the completion of the second polycondensation.

[0015] Furthermore, in step A4, the weight ratio of composite polyester to stereocomposite crystal is 100:10-20; the melt extrusion temperature is 80-100℃ and the melt extrusion time is 5-10 min.

[0016] Furthermore, in step B2, the weight ratio of tailings powder, citric acid powder, phosphoric acid and polyester auxiliary material is 70-80:3-5:10-15:10-15; the pelletizing speed is 20-40 r / min, the pelletizing time is 10-20 min, and the pelletizing temperature is 100-105℃.

[0017] The present invention has the following beneficial effects: 1. Polyester is prepared from terephthalic acid and ethylene glycol through esterification, primary polycondensation, and secondary polycondensation processes. During the esterification process, doping with ramie cellulose modifies the synthesized polyester, improving its biodegradability. Furthermore, during the primary and secondary polycondensation reactions of terephthalic acid and ethylene glycol, the addition of methacrolein and formaldehyde monomers further polycondenses to generate methyl methacrylate and polymethyl methacrylate (PMMA), thereby improving the chemical resistance of the synthesized composite polyester. PMMA can undergo depolymerization at high temperatures, allowing for the recovery of PMMA monomers. Therefore, although PMMA is a petroleum-based, non-degradable polymer, it can be recycled, contributing to environmental protection and sustainable development.

[0018] 2. The stereocomposite crystals prepared by transesterification of PLLA and PDLA can be used as nucleating agents to further optimize the crystal form of the prepared polyester auxiliary materials, thereby improving the overall performance of the synthesized polyester auxiliary materials. This invention uses the synthesized polyester auxiliary materials as film-forming agents and binders, which can form a dense film on the surface of tailings powder, preventing the loss of nutrients in the synthesized calcium magnesium phosphate fertilizer; the polyester auxiliary materials themselves can also act as slow-release carriers, regulating the release rate of various nutrients and extending the fertilizer's effective period.

[0019] 3. Phosphate tailings can be further enriched with phosphorus, calcium, magnesium, and silicon through flotation. The flotation tailings are then subjected to high-temperature melting to activate the insoluble phosphorus minerals. Furthermore, the high-temperature melting process can fix heavy metals such as lead and cadmium in the flotation tailings in a glassy or crystalline structure, reducing their leaching toxicity. The tailings are then pulverized to obtain a tailings powder of a certain fineness. To avoid chemical reactions between calcium magnesium phosphate fertilizer and heavy metals such as iron, aluminum, calcium, and magnesium in the soil, which would result in a large amount of phosphate fertilizer being fixed in the soil and reducing its utilization rate, this invention blends the tailings powder with organic citric acid powder and phosphoric acid to form spheres. The prepared calcium magnesium phosphate fertilizer is easily absorbed by the organic citric acid powder and phosphoric acid, which can reduce the chemical fixation of water-soluble phosphorus, further improving the utilization rate of the phosphate fertilizer. In addition, the spherical tailings powder coated with polyester additives can act as an adsorbent material, achieving efficient adsorption of heavy metals in contaminated soil and improving the quality of the contaminated soil. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a photograph of the calcium magnesium phosphate fertilizer prepared in Example 4 of the present invention. Figure 2 This is a photograph of the calcium magnesium phosphate fertilizer prepared in Example 6 of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] The PLLA used in Examples 1-3 of this invention was purchased from Shenzhen Lvbao Biotechnology Co., Ltd., product model PLLA-30, with an intrinsic viscosity of 0.3 dL / g and a weight-average molecular weight (10,000) of 1.5 Mw; the PDLA used in Examples 1-3 of this invention was model PDLA-05, with an intrinsic viscosity of 0.3 dL / g and a weight-average molecular weight (10,000) of 1.5 Mw; the ramie cellulose used in Examples 1-3 of this invention was purchased from Jingde County Jinwei Hemp Industry Co., Ltd., product number 008; the phosphate rock flotation tailings used in Examples 4-6 of this invention were purchased from Yunnan Phosphate Group Co., Ltd.; the polymethyl methacrylate used in Comparative Example 2 of this invention was purchased from Sigma-Aldrich Trading Co., Ltd., product number 043982, CAS number 9011-14-7.

[0024] Example 1 This embodiment provides a process for preparing polyester auxiliary materials for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: A1. PLLA and PDLA are added to a mixer in a mass ratio of 1:1 for melt blending. The melt blending temperature is 190℃ and the melt blending time is 2min to prepare a stereocomposite crystal.

[0025] A2. A slurry preparation tank is connected to an esterification reactor. One esterification reactor is connected to two parallel polycondensation reactors, forming a batch reactor. Terephthalic acid and ethylene glycol are added to the slurry preparation tank in a molar ratio of 1:1.15 to obtain a blended monomer. Ramie cellulose and antimony acetate catalyst are then added to the slurry preparation tank to obtain a reaction system; the weight ratio of the blended monomer, ramie cellulose, and catalyst is 100:5:0.3. The reaction system is fed into the esterification reactor from the top of the slurry preparation tank. The esterification reaction temperature is set to 245℃, the esterification reaction pressure to 0.1MPa, and the esterification reaction time to 2 hours, yielding a prepolymer.

[0026] A3. The synthesized prepolymer is transferred to the first polycondensation reactor, where methacrolein, formaldehyde, and KOH catalyst are added to initiate the first polycondensation, yielding the primary polymer. The first polycondensation is carried out at 240°C for 1 hour, with a weight ratio of prepolymer, methacrylic acid, formaldehyde, and catalyst of 100:7:3:0.01. The primary polymer is then transferred to the second polycondensation reactor for a second polycondensation at 260°C. During this process, the melt viscosity is measured to be 380 Pa·s, indicating the completion of the second polycondensation and the preparation of the composite polyester.

[0027] A4. The composite polyester and the prepared stereocomposite crystal are blended at a weight ratio of 100:10, melt-extruded at 80°C for 5 minutes, and then naturally cooled and solidified at room temperature to obtain the prepared polyester auxiliary material.

[0028] Example 2 This embodiment provides a process for preparing polyester auxiliary materials for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: A1. PLLA and PDLA were added to a mixer at a mass ratio of 1:2 for melt blending. The melt blending temperature was 195℃ and the melt blending time was 3 minutes to prepare a stereocomposite crystal.

[0029] A2. A slurry preparation tank is connected to an esterification reactor. One esterification reactor is connected to two parallel polycondensation reactors, forming a batch reactor. Terephthalic acid and ethylene glycol are added to the slurry preparation tank in a molar ratio of 1:1.2 to obtain a blended monomer. Ramie cellulose and antimony acetate catalyst are then added to the slurry preparation tank to obtain a reaction system; the weight ratio of the blended monomer, ramie cellulose, and catalyst is 100:10:0.4. The reaction system is fed into the esterification reactor from the top of the slurry preparation tank. The esterification reaction temperature is set to 255℃, the esterification reaction pressure to 0.2MPa, and the esterification reaction time to 2.2h, yielding a prepolymer.

[0030] A3. The synthesized prepolymer is transferred to the first polycondensation reactor, where methacrolein, formaldehyde, and KOH catalyst are added to induce the first polycondensation, yielding the primary polymer. The first polycondensation is carried out at 245°C for 1.3 hours, with a weight ratio of prepolymer, methacrylic acid, formaldehyde, and catalyst of 100:7:3:0.02. The primary polymer is then transferred to the second polycondensation reactor for a second polycondensation at 270°C. During this process, the melt viscosity is measured to be 390 Pa·s, indicating the completion of the second polycondensation and the preparation of the composite polyester.

[0031] A4. The composite polyester and the prepared stereocomposite crystal are blended at a weight ratio of 100:16, melt-extruded at 90°C for 6 minutes, and then naturally cooled and solidified at room temperature to obtain the prepared polyester auxiliary material.

[0032] Example 3 This embodiment provides a process for preparing polyester auxiliary materials for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: A1. PLLA and PDLA are added to a mixer at a mass ratio of 1:2 for melt blending. The melt blending temperature is 200℃ and the melt blending time is 5min to prepare a stereocomposite crystal.

[0033] A2. A slurry preparation tank is connected to an esterification reactor. One esterification reactor is connected to two parallel polycondensation reactors, forming a batch reactor. Terephthalic acid and ethylene glycol are added to the slurry preparation tank in a molar ratio of 1:1.25 to obtain a blended monomer. Ramie cellulose and antimony acetate catalyst are then added to the slurry preparation tank to obtain the reaction system; the weight ratio of the blended monomer, ramie cellulose, and catalyst is 100:15:0.5. The reaction system is fed into the esterification reactor from the top of the slurry preparation tank. The esterification reaction temperature is set to 265℃, the esterification reaction pressure to 0.3MPa, and the esterification reaction time to 3 hours, yielding a prepolymer.

[0034] A3. The synthesized prepolymer is transferred to the first polycondensation reactor, where methacrolein, formaldehyde, and KOH catalyst are added to initiate the first polycondensation, yielding the primary polymer. The first polycondensation is carried out at 250°C for 2 hours, with a weight ratio of prepolymer, methacrylic acid, formaldehyde, and catalyst of 100:7:3:0.03. The primary polymer is then transferred to the second polycondensation reactor for a second polycondensation at 280°C. During this process, the melt viscosity is measured to be 400 Pa·s, indicating the completion of the second polycondensation and the preparation of the composite polyester.

[0035] A4. The composite polyester and the prepared stereocomposite crystal are blended at a weight ratio of 100:20, melt-extruded at 100°C for 10 minutes, and then naturally cooled and solidified at room temperature to obtain the prepared polyester auxiliary material.

[0036] Example 4 This embodiment provides a preparation process for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: B1. Weigh 5g of flotation phosphate tailings and add it to a high-purity graphite crucible. Using XPS analysis, the typical chemical composition of the flotation phosphate tailings used in this invention is: 57.5%wt CaO, 19.5%wt MgO, 6.8%wt SiO2, 6.0%wt P5O2, 1.90%wt Fe2O3, 1.3%wt Al2O3, 1.1%wt SO3, and 0.73%wt... The mixture contained F, 0.72%wt K₂O, 0.29%wt MnO, 0.2%wt Na₂O, 0.18%wt TiO₂, 0.05%wt BaO, 0.05%wt SrO, 0.05%wt ZnO, 0.011%wt V₂O₅, 0.011%wt ZrO₂, and trace elements (including Mn, Na, Ti, Ba, Sr, Zn, and V). The high-purity graphite crucible was then transferred to a vacuum atmosphere lifting furnace for high-temperature melting at 1600℃ for 2 hours. The molten phosphate tailings were then removed and allowed to cool naturally to room temperature. The tailings were then crushed, sieved, and collected as 100-mesh tailings powder.

[0037] B2. By weight, 70 parts of tailings powder, 3 parts of citric acid powder, 10 parts of phosphoric acid, and 10 parts of the polyester auxiliary material prepared in Example 1 were added to the temporary storage hopper of a mixer and mixed. The mixer speed was 3000 r / min, and the mixing time was 10 min to obtain a high-efficiency powder. The high-efficiency powder was pelletized using a special pelletizing machine at a speed of 20 r / min, a pelletizing time of 10 min, and a pelletizing temperature of 100℃ to obtain a near-spherical calcium magnesium phosphate fertilizer.

[0038] Example 5 This embodiment provides a preparation process for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: B1. Weigh 8g of flotation phosphate tailings and add it to a high-purity graphite crucible. Using XPF analysis, the typical chemical composition of the flotation phosphate tailings used in this invention is: 57.8%wt CaO, 20.3%wt MgO, 7%wt SiO2, 6.05%wt P5O2, 1.95%wt Fe2O3, 1.36%wt Al2O3, 1.15%wt SO3, and 0.735%wt F. The mixture contained 0.727%wt K₂O, 0.296%wt MnO, 0.205%wt Na₂O, 0.183%wt TiO₂, 0.055%wt BaO, 0.051%wt SrO, 0.05%wt ZnO, 0.012%wt V₂O₅, 0.011%wt ZrO₂, and the remaining trace elements (including Mn, Na, Ti, Ba, Sr, Zn, and V). The high-purity graphite crucible was then transferred to a vacuum atmosphere lifting furnace for high-temperature melting at 1680℃ for 3 hours. The molten phosphate tailings were then removed and allowed to cool naturally to room temperature. The tailings were then crushed, sieved, and collected as 200-mesh tailings powder.

[0039] B2. By weight, 77 parts of tailings powder, 4 parts of citric acid powder, 12 parts of phosphoric acid, and 12 parts of the polyester auxiliary material prepared in Example 2 were added to the temporary storage hopper of a mixer for blending. The mixer speed was 4000 r / min, and the mixing time was 15 min, resulting in a high-efficiency powder. The high-efficiency powder was pelletized using a dedicated pelletizing machine at a speed of 30 r / min, a pelletizing time of 16 min, and a pelletizing temperature of 102℃, resulting in a near-spherical calcium magnesium phosphate fertilizer.

[0040] Example 6 This embodiment provides a preparation process for producing calcium magnesium phosphate fertilizer using flotation phosphate tailings, including the following steps: B1. Weigh 10g of flotation phosphate tailings and add it to a high-purity graphite crucible. Using XPF analysis, the typical chemical composition of the flotation phosphate tailings used in this invention is: 58.1%wt CaO, 20.9%wt MgO, 7.2%wt SiO2, 6.1%wt P5O2, 2.0%wt Fe2O3, 1.4%wt Al2O3, 1.2%wt SO3, and 0.74%wt... The high-purity graphite crucible was composed of F, 0.73%wt K₂O, 0.3%wt MnO, 0.21%wt Na₂O, 0.19%wt TiO₂, 0.06%wt BaO, 0.052%wt SrO, 0.051%wt ZnO, 0.013%wt V₂O₅, 0.012%wt ZrO₂, and trace elements (including Mn, Na, Ti, Ba, Sr, Zn, and V). The crucible was then transferred to a vacuum atmosphere lifting furnace for high-temperature melting at 1700℃ for 4 hours. The melted phosphate tailings were then removed and allowed to cool naturally to room temperature. The tailings were then crushed, sieved, and collected as 100-mesh tailings powder.

[0041] B2. By weight, 80 parts of tailings powder, 5 parts of citric acid powder, 15 parts of phosphoric acid, and 15 parts of the polyester auxiliary material prepared in Example 3 were added to the temporary storage hopper of a mixer for blending. The mixer speed was 5000 r / min, and the mixing time was 20 min, resulting in a high-efficiency powder. The high-efficiency powder was pelletized using a dedicated pelletizing machine at a speed of 40 r / min, a pelletizing time of 20 min, and a pelletizing temperature of 105℃, resulting in a near-spherical calcium magnesium phosphate fertilizer.

[0042] Comparative Example 1 The difference between this comparative example and Example 6 is that step A1 is omitted; in step A4, the stereocomposite crystal is replaced with an equal mass of PLLA.

[0043] Comparative Example 2 The difference between this comparative example and Example 6 is that in step A3, no methacrolein and formaldehyde monomers were added to the first polycondensation reactor; in step A4, the composite polyester, stereocomposite crystals and polymethyl methacrylate were blended in a weight ratio of 100:20:10, melt-extruded at 100°C for 10 minutes, and then naturally cooled and solidified at room temperature to obtain the prepared polyester auxiliary material.

[0044] Comparative Example 3 The difference between this comparative example and Example 6 is that, in step B2, citric acid powder was not added when preparing the high-efficiency powder.

[0045] Performance testing: 1. Prepare a 0.01 mL / L NaNO3 solution. Then, using the NaNO3 solution as a solvent, prepare a 200 mg / L Cd(NO3)2 solution, a Pb(NO3)2 solution, and a Cu(NO3)2 solution with a pH of 7. Weigh 0.005 g of the calcium magnesium phosphate fertilizer prepared in Examples 4-6 into a 10 mL centrifuge tube, add 5 mL of Cd(NO3)2 solution, mix thoroughly, and place in a constant-temperature shaker. React at 25°C and 200 rpm for 48 h. After removing the sample, filter it through a 0.22 μm aqueous filter membrane. Dilute the filtrate with 0.5% nitric acid solution. Determine the concentration of heavy metal ions in the filtrate using inductively coupled plasma optical emission spectrometry (ICP-OES). Calculate the removal rate of heavy metals.

[0046] Qe=(C0-Ce)V / m In the formula: Qe is the equilibrium adsorption capacity (mg·g) of the adsorbent at the equilibrium concentration Ce. -1 C0 is the initial concentration of the adsorbate in the solution (mg·L). -1 Ce is the concentration of the adsorbate in the solution (mg·L) when adsorption equilibrium is reached. -1 V is the volume of the adsorption solution (mL); m is the mass of the adsorbent (g).

[0047] 2. The content of available phosphorus pentoxide in the calcium magnesium phosphate fertilizers prepared in Examples 4-6 was determined by the quinoline phosphomolybdate gravimetric method; the content of available magnesium in the calcium magnesium phosphate fertilizers prepared in Examples 4-6 was determined by the disodium ethylenediaminetetraacetate volumetric method.

[0048] 3. The calcium magnesium phosphate fertilizers prepared in Examples 4-6 and Comparative Examples 1-3 were added to a corundum crucible and heated to 200℃ at a heating rate of 20K / min. The weight of the calcium magnesium phosphate fertilizer before and after heating was measured, and its weight loss rate was calculated. Specific test results are shown in the table below: Table 1. Sample performance test data Data Analysis: By comparing and analyzing the data in the table above, the calcium magnesium phosphate fertilizer prepared in Examples 4-6 of this invention can be used as an adsorbent material, exhibiting high adsorption performance for heavy metal elements Cd, Pb, and Cu, and can improve and remediate heavy metal contaminated soil. However, in Comparative Example 2, the composite polyester and polymethyl methacrylate were directly blended; compared to the polyester auxiliary material prepared by mixing the monomers of the composite polyester with the monomers of the polymethyl methacrylate and then undergoing two polycondensation reactions, a polymer alloy was not formed, resulting in a decrease in its adsorption rate for Cd, Pb, and Cu.

[0049] The calcium magnesium phosphate fertilizers prepared in Examples 4-6 of this invention all meet the usage standards for Type II fertilizers, with an effective P2O5 content of approximately 15-18%wt and an effective magnesium content of approximately 5-6%wt. Type II fertilizers can control the nutrient delivery rate to synchronize with crop needs and reduce nutrient loss. The calcium magnesium phosphate fertilizers prepared in Examples 4-6 of this invention all exhibit low weight loss rates under high-temperature conditions (200℃), demonstrating excellent thermal stability. However, in Comparative Example 1, an equal mass of PLLA was used instead of stereocomplex crystals; compared to conventional PLLA, stereocomplex crystals act as nucleating agents, resulting in higher thermal stability in the prepared calcium magnesium phosphate fertilizer. This is manifested in the higher weight loss rate of the calcium magnesium phosphate fertilizer prepared in Comparative Example 1 at 200℃.

[0050] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

[0051] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0052] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A process for producing calcium magnesium phosphate fertilizer from flotation tailings, characterized in that, Includes the following steps: B1. High-temperature melting, cooling, crushing and screening of flotation phosphorus tailings to prepare tailings powder; B2. Tailings powder, citric acid powder, phosphoric acid and polyester auxiliary materials are blended to obtain high-efficiency powder; High-efficiency powder is pelletized to obtain near-spherical calcium magnesium phosphate fertilizer.

2. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 1, characterized in that, In step B1, the high-temperature melting temperature is 1600-1700℃ and the high-temperature melting time is 2-4h; the mesh size of the tailings powder after crushing and screening is 200-400 mesh.

3. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 1, characterized in that, Step B2, the preparation method of the polyester auxiliary material, includes the following steps: A stereocomposite crystal was prepared by melt blending of Al, PLLA and PDLA. A2. The slurry preparation tank is connected to the esterification reactor. One esterification reactor and two parallel polycondensation reactors are connected to form a batch unit. Terephthalic acid and ethylene glycol are added to the slurry preparation tank for blending to obtain a blended monomer. Ramie cellulose and antimony acetate catalyst are then added to the slurry preparation tank to obtain a reaction system. The reaction system is fed into the esterification reactor from the top of the slurry preparation tank for esterification to obtain a prepolymer. A3. The prepolymer is transferred to the first polycondensation reactor, where methacrolein, formaldehyde, and KOH are added to the first polycondensation reactor to conduct the first polycondensation reaction and obtain the primary polymer. The primary polymer is then transferred to the second polycondensation reactor for secondary polycondensation to prepare the composite polyester. A4. The composite polyester and the prepared stereocomposite crystal are blended, melt-extruded, and cooled and solidified to obtain the polyester auxiliary material.

4. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 3, characterized in that, In step A1, the mass ratio of PLLA to PDLA is 1:1-2; the melt blending temperature is 190-200℃ and the melt blending time is 2-5 min.

5. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 3, characterized in that, In step A2, the molar ratio of terephthalic acid to ethylene glycol is 1:1.15-1.25, and the weight ratio of the blended monomer, ramie cellulose, and catalyst is 100:5-15:0.3-0.5; the esterification reaction temperature is 245-265℃, the esterification reaction pressure is 0.1-0.3MPa, and the esterification reaction time is 2-3h.

6. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 3, characterized in that, In step A3, the weight ratio of prepolymer, methacrylic acid, formaldehyde and catalyst is 100:7:3:0.01-0.03; the temperature of the first polycondensation is 240-250℃ and the duration of the first polycondensation is 1-2 hours; the temperature of the second polycondensation is 260-280℃ and the melt viscosity is 380-400 Pa·s, which is considered as the completion of the second polycondensation.

7. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 3, characterized in that, In step A4, the weight ratio of composite polyester to stereocomposite crystal is 100:10-20; the melt extrusion temperature is 80-100℃ and the melt extrusion time is 5-10 min.

8. The process for producing calcium magnesium phosphate fertilizer from flotation tailings according to claim 1, characterized in that, In step B2, the weight ratio of tailings powder, citric acid powder, phosphoric acid and polyester auxiliary material is 70-80:3-5:10-15:10-15; the pelletizing speed is 20-40 r / min, the pelletizing time is 10-20 min, and the pelletizing temperature is 100-105℃.

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