Method for preparing slow-release medium element fertilizer by utilizing low-quality phosphoric acid phosphate acidolysis tailings

Abstract

The invention discloses a method for preparing a slow-release medium element fertilizer by using low-quality phosphoric acid acidolysis phosphate tailings, which comprises the following steps: 1) drying phosphate tailings, crushing and sieving to obtain phosphate tailings powder for later use; 2) uniformly stirring low-quality phosphoric acid for later use; and (3) stirring and mixing the phosphate tailing powder in the step (1) and the low-quality phosphoric acid in the step (2) according to the mass ratio of 1: (0.25-1.50), fully reacting, drying and crushing to obtain the phosphate tailing-based slow-release medium element fertilizer. According to the technology provided by the invention, the influence of an acidolysis process on phosphate tailing decomposition is explored, the optimal process condition of the medium element fertilizer is provided under the condition of ensuring the highest conversion rate, the process efficiency and the conversion rate of calcium and magnesium are improved, and multi-target collaborative optimization in the activation process is realized; the medium element product prepared by the invention is rich in medium nutrient elements such as calcium, magnesium and the like required by crops and soil, is lower in preparation cost, is more in line with actual agricultural requirements, and has higher economic value and environmental value.

Description

Technical Field

[0001] The invention relates to the technical field of plant nutrition fertilizers, and in particular to a method for preparing a slow-release medium-element fertilizer by utilizing low-quality phosphoric acid to acidify phosphate tailings. Background Art

[0002] Phosphorus is widely used in agricultural production, industrial manufacturing, food hygiene and other fields. With the rapid development of my country's economy and society, phosphate resources continue to be consumed, and the development and utilization of phosphate resources in industry has entered a bottleneck period. In pursuit of profits, enterprises often mine rich and abandon poor, mine easy and abandon difficult, and waste phosphate resources seriously. Realizing the sustainable development and rational utilization of phosphate resources has become the primary issue of development. The utilization rate of elements other than phosphorus in phosphorus chemical production is low, and the direct utilization of medium and low-grade phosphate ores and the resource utilization of phosphate tailings have not been promoted and applied.

[0003] Phosphate tailings are low-grade phosphate ore discharged after phosphate ore dressing. The trace nutrients such as calcium, magnesium, and silicon contained in them play an irreplaceable role in promoting crop yield, quality, and efficiency, and protecting the health of animals and humans. Therefore, strengthening the comprehensive utilization of phosphate tailings and preparing them into fertilizer products for application in the soil can effectively supplement calcium and magnesium nutrients, improve acidic soil, and increase crop yield and quality. Some studies have proposed to use chemical fertilizers prepared from phosphate tailings as agricultural inputs for agricultural production, but they have not been popularized due to immature technology, low market demand, and high price costs.

[0004] Phosphate tailings fertilizer utilization refers to the use of phosphate tailings as the core raw material, using certain process technologies to prepare it into fertilizer products for use in agricultural production. It mainly includes phosphate magnesium fertilizers, calcium magnesium phosphate fertilizers, phosphorus-containing organic fertilizers and compound fertilizers.

[0005] Phosphorus-magnesium fertilizer generally refers to fertilizer products with phosphorus and magnesium as the main nutrients. Studies have been conducted to prepare phosphate-magnesium fertilizer by decomposing phosphate tailings in three ways: sulfuric acid method, sulfur-phosphorus mixed acid method and raffinate acid method. Huang Lihua et al. used sulfuric acid method to decompose phosphate tailings, then filtered, then added magnesium oxide powder and phosphoric acid to the filtrate to control the pH, and finally matured and dried to obtain phosphate-magnesium fertilizer products. In order to further improve the utilization rate of phosphorus and magnesium, Hu Hong et al. used sulfur-phosphorus mixed acid method to decompose tailings under the conditions of 110g phosphoric acid, 15g sulfuric acid, 20min reaction time and 50°C reaction temperature to produce phosphate-magnesium fertilizer products with a magnesium content of 10%. Jie Tian et al. used raffinate acid, a byproduct of wet phosphoric acid purification with a wide source and low cost, as an acidolysis agent to react with phosphate tailings, and added surfactants to produce phosphate-magnesium fertilizers with an effective phosphorus-magnesium content of 43%. The preparation of phosphate-magnesium fertilizers using phosphate tailings as raw materials does not require high production equipment, and the products have certain economic value, which provides ideas for the utilization of phosphorus and magnesium resources in phosphate tailings. However, the experiment affected P 2 O 5The factors affecting the conversion rate of magnesium and phosphate fertilizer are still under investigation, and there is a lack of mechanistic understanding. The specific process parameters and technical conditions still need to be explored, and the agronomic effect of magnesium phosphate fertilizer needs to be further verified.

[0006] Calcium magnesium phosphate fertilizer is a glassy phosphate fertilizer with phosphorus, calcium, magnesium and silicon as its main components. 2 O 5 Phosphate ore with a content greater than 16% is prepared by melting it with a flux at a temperature above 1400°C. Zheng Jianguo and others mixed phosphate tailings and phosphoric acid in proportion and calcined them to produce a polymerized calcium magnesium phosphate fertilizer containing polyphosphate. This fertilizer product converts phosphorus into polyphosphate, making up for the disadvantage that calcium magnesium phosphate fertilizer only contains citrate-soluble phosphorus, and makes phosphorus, calcium and magnesium all have slow release properties, which can effectively reduce nutrient leaching and fixed losses. However, the existence forms of calcium and magnesium in this fertilizer and the mechanism of their influence on phosphate dehydration condensation need further study.

[0007] Phosphorus-containing organic fertilizers generally refer to natural fertilizers containing phosphorus that are made from various sources of animal and plant residues or human and livestock excrement, and are accumulated on site or directly buried for application. They are also customarily called farmyard manure. The application of microbial phosphate decomposition technology to activate the insoluble phosphorus in phosphate tailings powder can be used to produce phosphorus-containing organic fertilizers, which play a key role in improving soil structure and promoting the growth of crop roots. The application of microbial phosphate decomposition technology to activate the insoluble phosphorus in phosphate tailings powder can be used to produce phosphorus-containing organic fertilizers, which play a key role in improving soil structure and promoting the growth of crop roots.

[0008] Phosphate tailings are a kind of sandy material, and most of the nutrients are insoluble or insoluble, making it difficult to use directly. Exploring efficient activation technology is the key to preparing compound fertilizers. Tang Jianwei and others reacted phosphate tailings with sulfuric acid to convert calcium, magnesium, phosphorus and other elements in the phosphate tailings, and added nitrogen, phosphorus and potassium sources to spray granulation to successfully prepare compound fertilizers. Making full use of the characteristics of phosphate tailings resources, achieving synergistic effects of trace elements such as calcium and magnesium with large elements, exploring efficient activation technology, and industrial production technology that cannot be achieved under existing equipment, and new granulation process technology are the research focuses of realizing the preparation of compound fertilizers from phosphate tailings.

[0009] At present, phosphate resources are becoming increasingly depleted, and it is increasingly difficult to meet the quality requirements of wet phosphoric acid production. Phosphate ore has to be floated. With the continuous expansion of the scale of the phosphorus chemical industry, the accumulation of phosphorus tailings has continued to increase. At present, the annual output of phosphorus tailings has reached 7 million tons, but its utilization rate is only 20%. The phosphorus tailings produced by flotation are piled up in large quantities, occupying land, causing serious environmental pollution and waste of resources. Therefore, it is necessary to explore scientific, effective and economic ways to comprehensively utilize phosphorus tailings, realize the resource utilization of phosphorus chemical by-products, turn waste into treasure, and promote the sustainable development of phosphate resources.

[0010] Although there are some methods for further utilizing phosphate tailings, such as the method of preparing phosphate-magnesium compound fertilizer using raffinate acid and high-magnesium phosphate tailings slag in Chinese patent CN102161602A, there are still problems such as low utilization rate of low-grade phosphate tailings, high production cost, and low conversion rate of calcium and magnesium. Therefore, it is urgent to develop a method for preparing slow-release medium-element fertilizer using low-quality phosphoric acid acid hydrolyzed phosphate tailings to solve the above-described problems. Summary of the invention

[0011] The object of the present invention is to provide a method for preparing a slow-release medium-element fertilizer by acidulating phosphate tailings using low-quality phosphoric acid, so as to solve the problems of low utilization rate of phosphate tailings and low conversion rate of calcium and magnesium therein.

[0012] The scheme of the present invention is:

[0013] A method for preparing slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid comprises the following steps:

[0014] 1) drying the phosphate tailings, crushing and sieving them to obtain phosphate tailings powder for standby use;

[0015] 2) Stir low-quality phosphoric acid evenly for use;

[0016] 3) mixing 1) phosphate tailing powder and 2) low-quality phosphoric acid at a mass ratio of 1:0.25-1.50, drying and crushing after sufficient reaction, to obtain a phosphate tailing-based slow-release medium-element fertilizer.

[0017] As a preferred technical solution, the method further includes 4) mixing the base fertilizer with the binder, and subjecting the mixture to disc granulation, pellet screening, drying and index testing to obtain a phosphate tailings-based slow-release medium-element fertilizer. The binder is one of water, attapulgite and sodium lignin sulfonate.

[0018] As a preferred technical solution, the phosphate tailings are by-products of the production of phosphate fertilizers in the phosphate chemical industry, and the by-products contain 1-6.7% effective phosphorus, 15-34.2% effective calcium, and 8-16.3% effective magnesium.

[0019] As a preferred technical solution, the drying temperature of the phosphate tailings is 40-80°C, the drying time is 3-4h, and the phosphate tailings powder is obtained by pulverizing and passing through a 60-100 mesh sieve.

[0020] As a preferred technical solution, the low-quality phosphoric acid is purified wet phosphoric acid raffinate, with a solid content of 6.22%, a pH value of less than 1, an effective magnesium content of 2.0-5.4%, and an effective phosphorus content of 25-45%.

[0021] As a preferred technical solution, the stirring time of the low-quality phosphoric acid in 2) is 5 to 15 minutes, and the stirring rate is 50 to 150 r / min.

[0022] As a preferred technical solution, the mass ratio of the phosphate tailings powder to the low-quality phosphoric acid in 3) is 1:0.75.

[0023] As a preferred technical solution, the reaction time of the low-quality phosphoric acid and the phosphate tailings powder in 3) is 20 to 100 minutes, the reaction temperature is 40° C. to 90° C., and the reaction stirring rate is 100 to 900 r / min.

[0024] As a preferred technical solution, the reaction time of the low-quality phosphoric acid and the phosphate tailings powder in 3) is 40 minutes.

[0025] As a preferred technical solution, the drying temperature of the reaction product of the low-quality phosphoric acid and the phosphate tailings powder in the above 3) is 70-80°C for 12-15 hours. After drying, it is cooled to room temperature and then crushed through a 60-mesh sieve to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0026] As a preferred technical solution, the reaction temperature in 3) is 60°C.

[0027] As a preferred technical solution, the reaction stirring rate in 3) is 500 r / min.

[0028] The invention also discloses an element fertilizer containing a phosphorus tailings-based slow-release medium-element fertilizer.

[0029] A method for preparing a slow-release medium-element fertilizer by acidulating phosphate tailings with low-quality phosphoric acid is adopted, which comprises the following steps: 1) drying the phosphate tailings, crushing and sieving them to obtain phosphate tailings powder for standby use; 2) stirring the low-quality phosphoric acid evenly for standby use; 3) stirring and mixing 1) the phosphate tailings powder and 2) the low-quality phosphoric acid at a mass ratio of 1:0.25-1.50, drying and crushing them after sufficient reaction, to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0030] Advantages of the present invention:

[0031] The invention directly uses phosphate tailings and raffinate acid, which are byproducts in the phosphorus chemical production process, as main raw materials, thus avoiding resource waste and environmental pollution caused by the storage of phosphate tailings and raffinate acid, and having no purchase cost.

[0032] The phosphate tailings product activated by raffinate acid under different acid hydrolysis process conditions in the present invention can meet the industry standard for non-water-soluble medium-element fertilizers (NYT 3830-2021), meet the actual needs of my country's soil, and realize the efficient utilization of phosphate tailings nutrients in the agricultural end.

[0033] The slow-release medium-element fertilizer prepared by the present invention has low raw material cost and can achieve high-value recycling of waste resources. The adding method is simple and efficient. It can be truly applied to agricultural production while solving the problem of industrial solid waste, realizing the integration of industry and agriculture.

[0034] Under the condition of ensuring the highest conversion rate, the present invention improves the process efficiency and the conversion rate of calcium and magnesium by preparing the process conditions of the slow-release medium-element fertilizer, and realizes the multi-objective coordinated optimization in the activation process; the slow-release medium-element product prepared by the present invention is rich in medium-amount nutrients required by crops and soil, such as calcium and magnesium, and has a lower preparation cost, is more in line with the actual needs of agriculture, and has higher economic value and environmental value. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0036] Figure 1 The present invention is a process flow for preparing the phosphate tailings-based slow-release medium-element fertilizer of Test Example 1;

[0037] Figure 2 This is a comparison chart of the cumulative release of calcium nutrients in Test Example 2 of the present invention;

[0038] Figure 3 This is a comparison chart of the cumulative release of magnesium nutrients in Test Example 2 of the present invention;

[0039] Figure 4 This is a comparison chart of the conversion rates of calcium and magnesium under different conditions. DETAILED DESCRIPTION

[0040] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below in conjunction with specific embodiments.

[0041] The phosphate tailings used in the following examples were sourced from Yunnan Phosphate Chemical Group Co., Ltd., and the low-quality phosphoric acid was sourced from Yunnan Tianan Chemical Co., Ltd.

[0042] Example 1

[0043] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26%; P 2 O 5 :6.67%.

[0044] 2. The low-quality phosphoric acid was heated at 100 r / min for 10 min at 25°C until it was uniform and then put into use. The proportions of the components of the low-quality phosphoric acid were MgO: 5.40%; P 2 O 5 : 44.96%;

[0045] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0046] Example 2

[0047] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0048] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0049] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.25. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0050] Example 3

[0051] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0052] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0053] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.50. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0054] Example 4

[0055] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0056] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0057] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:1.00. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0058] Example 5

[0059] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0060] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0061] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:1.25. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0062] Example 6

[0063] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0064] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0065] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:1.50. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0066] The effects of fertilizers with different mineral acid ratios in Example 1 and Examples 2 to 6 on the conversion rates of calcium and magnesium were tested. It was found that when the mineral acid ratio was 1:0.75, the conversion rates of calcium and magnesium were the highest, which were 86.16% and 65.25%, respectively. At this time, the effective calcium and magnesium accounted for 29.3% and 10.4%, respectively.

[0067] Example 7

[0068] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0069] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0070] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 20 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0071] Example 8

[0072] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0073] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0074] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 40 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0075] Example 9

[0076] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0077] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0078] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 80 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0079] Example 10

[0080] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0081] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0082] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 100 minutes; the reaction temperature is 60°C; the reaction stirring rate is 500r / min; after sufficient reaction, it is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0083] The effects of different reaction times of fertilizers in Example 1 and Examples 7 to 10 on the conversion rates of calcium and magnesium were tested. It was found that when the reaction time was 60 minutes, the conversion rates of calcium and magnesium were the highest, which were 86.54% and 65.78%, respectively. At this time, the effective calcium and magnesium accounted for 29.4% and 10.5%, respectively.

[0084] Embodiment 11

[0085] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0086] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0087] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 100r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0088] Example 12

[0089] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0090] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0091] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 300r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0092] Example 13

[0093] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0094] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0095] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 700r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0096] Embodiment 14

[0097] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0098] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0099] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 60°C; the reaction stirring rate is 900r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0100] The effects of fertilizers with different stirring rates in Example 1 and Examples 11 to 14 on the conversion rates of calcium and magnesium were tested. It was found that when the stirring rate was 500 r / min, the conversion rates of calcium and magnesium were the highest, which were 87.26% and 66.53%, respectively. At this time, the effective calcium and magnesium accounted for 29.6% and 10.6%, respectively.

[0101] Embodiment 15

[0102] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0103] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0104] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 40°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0105] Example 16

[0106] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0107] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0108] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 50°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0109] Embodiment 17

[0110] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0111] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0112] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 70°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0113] Embodiment 18

[0114] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0115] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0116] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 80°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0117] Embodiment 19

[0118] 1. The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder. The nutrient ratios of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 :6.67%.

[0119] 2. Low-quality phosphoric acid is heated at 100 r / min for 10 minutes at 25°C until it is uniform and ready for use. The proportions of the components of low-quality phosphoric acid are MgO: 5.40; P 2 O 5 : 44.96%;

[0120] 3. The phosphate tailings powder and low-quality phosphoric acid are stirred and mixed in a mass ratio of 1:0.75. The reaction time of the low-quality phosphoric acid and the phosphate tailings powder is 60 minutes; the reaction temperature is 90°C; the reaction stirring rate is 500r / min; after sufficient reaction, the mixture is dried at 80°C for 3 hours and then crushed to obtain a phosphate tailings-based slow-release medium-element fertilizer.

[0121] The effects of different reaction temperatures on the conversion rates of calcium and magnesium in Example 1 and Examples 15 to 19 were compared. It was found that when the reaction temperature was 60°C, the conversion rates of calcium and magnesium were the highest, which were 89.23% and 67.88%, respectively. At this time, the effective calcium and magnesium accounted for 30.3% and 10.8%, respectively.

[0122] The effective calcium and magnesium indexes of the intermediate element fertilizers prepared in the above examples were measured, and the conversion rates of calcium and magnesium were measured. The results are shown in Figure 4 . Figure 4 It shows that under different conditions, the conversion rate of calcium and magnesium in fertilizer is the highest when the mineral acid ratio is 1:0.75, the reaction time is 60 minutes, the reaction temperature is 60℃, and the stirring speed is 500r / min.

[0123] Comparative Example 1

[0124] The phosphate tailings were dried in an oven at 60°C for 3 hours and then crushed through a 60-mesh sieve to obtain phosphate tailings powder, wherein the proportions of the components of the phosphate tailings powder were as follows: CaO: 34.14%; MgO: 16.26; P 2 O 5 : 6.67%, as a medium element fertilizer.

[0125] Test Example 1

[0126] Experimental design: According to Examples 1-19, according to a single factor experiment, the mass ratio of phosphate tailings and raffinate was 1:0.75, the reaction time was 60 min, the reaction temperature was 60 ° C, and the reaction stirring rate was 500 r / min. The process parameters were determined to obtain a phosphate tailings-based slow-release medium-element fertilizer (ie, Example 1), and water, attapulgite, and sodium lignin sulfonate were selected as binders, and a disc granulator was used to prepare a phosphate tailings-based slow-release medium-element fertilizer (PSN). Phosphate tailings + raffinate + deionized water (PSN-1), phosphate tailings + raffinate + sodium lignin sulfonate (PSN-2), and phosphate tailings + raffinate + attapulgite (PSN-3), a total of 3 phosphate tailings-based slow-release medium-element fertilizers.

[0127] Test steps:

[0128] (1) Preparation of acid hydrolysis products

[0129] The phosphate tailings and the raffinate acid are mixed according to a certain ratio, and the optimal mineral acid ratio, reaction time, reaction temperature and stirring rate (mineral acid ratio 1:0.75, reaction time 60 min, reaction temperature 60°C, stirring rate 500 r / min) are selected. The mixture is placed in a reactor to synthesize the acidolysis reaction product. The reaction product is crushed by a pulverizer and passed through a 60-mesh test sieve as a raw material for preparing phosphate tailings-based slow-release medium-element fertilizer.

[0130] (2) Disc granulation

[0131] The acid hydrolysis product and different binder materials are fully mixed at a ratio of 1.5%. 200g of the mixture is taken and granulated in a disc granulator. At the same time, a proper amount of water is sprayed on the mixture with a spray pot. It is decided whether to add water or dry the material according to the observed granulation situation. This is repeated until a large amount of granular material is formed (process as shown in FIG. Figure 1 shown).

[0132] (3) Fertilizer screening

[0133] The granulated material in the disc is taken out and sieved. The particles that pass the 5mm test sieve but cannot pass the 2mm test sieve are qualified. The remaining residual material is put into the disc again for granulation.

[0134] (4) Fertilizer drying

[0135] After screening, the qualified fertilizer particles of 2 to 5 mm are placed in a forced air drying oven and dried at 75°C for 9.5 hours. After drying, they are taken out and cooled, and relevant indicators are measured.

[0136] (5) Index determination

[0137] Particle strength (30 fertilizer granules were randomly selected and hardness was measured using a semi-automatic particle strength tester), pH value (measured using a Leici PHS-25), and calcium and magnesium content (measured using ICP-MS).

[0138] result:

[0139] The basic physical and chemical indicators of the prepared phosphate tailings-based slow-release medium-element fertilizers are shown in Table 1. Particle strength is an important physical indicator of granular fertilizers, which affects the storage, transportation and application of fertilizer products. The particle strengths of the phosphate tailings-based slow-release medium-element fertilizers granulated with attapulgite and sodium lignin sulfonate as binders are 27.1N and 20.2N respectively, which can meet the industry standard requirements of granular medium and trace element fertilizers for particle strength ≥20N. The fertilizer particle strength granulated with water binder is 15.8N, which cannot meet the above industry standard requirements. The price of attapulgite as a binder is the most economical among the three binders, so it is the best choice of binder material. The three phosphate tailings-based slow-release medium-element fertilizers are not easily soluble in water. The elemental calcium and magnesium content (≥20%), pH (6-11), and moisture (≤3%) of the three phosphate tailings-based slow-release medium-element fertilizers can meet the industry standard requirements of non-water-soluble medium-element fertilizers.

[0140] Table 1 Phosphorus tailings-based slow-release medium-element fertilizer physical and chemical indicators

[0141]

[0142] Test Example 2

[0143] Experimental design: The soil column nutrient release simulation technology was used to evaluate the nutrient release of slow-release medium-element fertilizers based on phosphate tailings in acidic soil. A total of 6 treatments were set up: Treatment 1, CK (only NPK fertilizer was applied); Treatment 2, PT (phosphate tailings were applied); Treatment 3, SQM (commercially available medium-element water-soluble fertilizers were applied); Treatment 4, PSN-1 (No. 1 phosphate tailings-based slow-release medium-element fertilizer was applied); Treatment 5, PSN-2 (No. 2 phosphate tailings-based slow-release medium-element fertilizer was applied); Treatment 6, PSN-3 (No. 3 phosphate tailings-based slow-release medium-element fertilizer was applied). The application rate of medium-element fertilizers in treatments 2 to 6 was 600 mg / kg soil, and the total amount of application was calculated based on the total mass of soil in the soil column. Each treatment was repeated 3 times.

[0144] Test steps:

[0145] (1) Device installation

[0146] Fix the polyvinyl chloride tube on the iron frame, fill it with 800g of soil, compact and flatten it, mix the fertilizer with another 200g of soil and then put it in, fill the upper and lower interfaces of the soil column with 2cm thick quartz sand, and place gauze and filter paper, place a large glass funnel at the bottom, and use a plastic bottle or beaker at the bottom to collect the leachate.

[0147] (2) Nutrient leaching

[0148] An infusion bag was used to simulate leaching precipitation. 443 mL of pure water was leached each time, and the leaching was done once every 5 days. The soil column was slowly injected through an infusion tube to ensure a uniform flow rate. The leaching was done 10 times in total for 50 days. The simulated rainfall was about 1000 mm, which is close to the lower limit of the average annual rainfall in southern my country.

[0149] (3) Sample collection. During the nutrient release test, the leached liquid was collected every 5 days, and its volume (V) was accurately measured, and about 100 mL of sample liquid was retained for testing. After the nutrient release test, the leached soil sample was collected, air-dried, and passed through a 20-mesh test sieve for testing.

[0150] (4) Data measurement

[0151] The calcium and magnesium contents of the leachate were determined; the cumulative release of calcium and magnesium was the product of the volume of the leachate and the calcium and magnesium concentrations of the leachate.

[0152] Test results:

[0153] like Figure 2 As shown in the figure, during the entire 50-day nutrient release period, the cumulative calcium release of the three phosphate tailings-based slow-release medium-element fertilizers, PSN-1, PSN-2, and PSN-3, was higher than that of the CK (NPK fertilizer only) and PT (phosphate tailings) treatments, but lower than that of the SQM (commercially available medium-element water-soluble fertilizer) treatment. The cumulative calcium release trends of the PSN-1, PSN-2, and PSN-3 treatments were similar, with the average daily releases of PSN-1, PSN-2, and PSN-3 being 7.3 mg, 6.6 mg, and 6.0 mg, respectively. After the 15th day of nutrient release, the cumulative calcium release of the three phosphate tailings-based slow-release medium-element fertilizers decreased with the leaching time, and the increase rate gradually slowed down, with the average daily release basically maintained at 0.5 mg, 0.5 mg, and 0.6 mg. At the end of 50 days of nutrient release, the cumulative calcium releases of PSN-1, PSN-2, and PSN-3 were 126.7 mg, 116.1 mg, and 112.2 mg, respectively.

[0154] like Figure 3As shown in the figure, the cumulative release trend of magnesium nutrients in PSN-1, PSN-2, and PSN-3 treatments was similar, and they continued to release during the entire 50-day test period, with the average daily release maintained at 0.3 mg. At the end of 50 days of nutrient release, the cumulative release of magnesium in PSN-1, PSN-2, and PSN-3 treatments was 14.0 mg, 14.5 mg, and 15.4 mg, respectively. In the first 15 days, the magnesium nutrient release in SQM treatment was fast, with an average daily release of 10.6 mg. After 15 days of nutrient release, the cumulative release of magnesium gradually increased with the leaching time. Within 15 to 50 days of nutrient release, the average daily release of magnesium in SQM treatment remained at 0.12 mg. At the end of 50 days of nutrient release, the cumulative release of magnesium was 20.2 mg.

[0155] Test Example 3

[0156] Raw materials: The fertilizers used are preferably phosphate tailings-based slow-release medium-element fertilizers (combined with attapulgite binder in Example 1), commercially available medium-element water-soluble fertilizers, calcium magnesium phosphate fertilizers, urea (nitrogen content 46%), diammonium phosphate (15-46-0), potassium sulfate (potassium content 50%). The test soil pH is 6.5. The test crop is pakchoi.

[0157] Test time and location: The test was conducted in Shangsuan Town, Jinning District, Kunming City, Yunnan Province. The relevant index tests were conducted in the shared laboratory of the public innovation platform of the National Agricultural Innovation Center in Nanjing, Jiangsu Province from March to May 2023.

[0158] Experimental design: A single-factor experiment was designed with a total of 5 treatments, each with 3 replicates. Treatment 1: CK (no application of medium-element fertilizer); Treatment 2: PT (direct application of phosphate tailings); Treatment 3: FMP (application of calcium-magnesium phosphate fertilizer); Treatment 4: SQM (application of commercially available medium-element water-soluble fertilizer); Treatment 5: PSN (application of preferred phosphate tailings-based slow-release medium-element fertilizer). The fertilization method is basal application, and each treatment applies the same nitrogen, phosphorus, potassium base fertilizer and trace element fertilizer. Tillage, irrigation, and late topdressing measures are the same. Sowing on November 8, 2022, the seeds need to be soaked and disinfected before sowing, and thinning begins when 2 to 4 true leaves grow after germination. Keep the cultivation and management conditions of each treatment consistent during the experiment. Harvest on December 28, 2022, and collect plant samples and soil samples.

[0159] Yield, sample collection and processing: After the pakchoy is harvested, it is placed in a nylon mesh bag and weighed on an electronic scale. The value is recorded as the plot yield.

[0160] Test results:

[0161] As can be seen from Table 2, under the conditions of the field test, the application of the preferred phosphate tailings-based slow-release medium-element fertilizer has a significant effect on the yield traits of Chinese cabbage. The yield of the plot without medium-element fertilizer was 25.7kg, the yield of the plot treated with phosphate tailings was 29.0kg, the yield of the plot treated with calcium magnesium phosphate fertilizer was 28.8kg, the yield of the plot treated with commercial medium-element water-soluble fertilizer was 31.4kg, and the yield of the plot treated with the preferred phosphate tailings-based slow-release medium-element fertilizer was 30.9kg.

[0162] The net weight of a single plant treated without medium-element fertilizer was 0.3kg, the net weight of a single plant treated with phosphate tailings was 0.3kg, the net weight of a single plant treated with calcium-magnesium phosphate fertilizer was 0.3kg, the net weight of a single plant treated with commercially available medium-element water-soluble fertilizer was 0.4kg, and the net weight of a single plant of pakchoy treated with phosphate tailings-based slow-release medium-element fertilizer was 0.3kg.

[0163] According to the planting density of 8000 plants / mu, the theoretical yield per mu of the treatment without medium-element fertilizer is 2853.3 kg, the theoretical yield per mu of the treatment with phosphate tailings is 3222.2 kg, the theoretical yield per mu of the treatment with calcium magnesium phosphate fertilizer is 3200.0 kg, the theoretical yield per mu of the treatment with commercial medium-element water-soluble fertilizer is 3492.2 kg, and the theoretical yield per mu of the treatment with phosphate tailings-based slow-release medium-element fertilizer is 3432.2 kg.

[0164] Compared with the treatment without applying medium-element fertilizer, the yield increase rate of phosphate tailings treatment was 12.9%, the yield increase rate of calcium magnesium phosphate fertilizer treatment was 12.1%, the yield increase rate of commercially available medium-element water-soluble fertilizer treatment was 22.3%, and the yield increase rate of the preferred phosphate tailings-based slow-release medium-element fertilizer treatment was 20.2%.

[0165] Table 2 Effects of slow-release medium-element fertilizers based on phosphate tailings on the yield of Chinese cabbage

[0166] deal with Plot yield (kg) Net weight per plant (kg) Theoretical yield per mu (kg) Yield increase rate (%) CK 25.7 0.3 2853.3 - PT 29.0 0.3 3222.2 12.9 FMP 28.8 0.3 3200.0 12.2 SQM 31.4 0.4 3492.2 22.4 PSN 30.9 0.3 3432.2 20.3

[0167] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid, characterized in that: The following steps are involved: 1) drying the phosphate tailings, crushing and sieving them to obtain phosphate tailings powder for standby use; 2) Stir low-quality phosphoric acid evenly for use; 3) mixing 1) phosphate tailing powder and 2) low-quality phosphoric acid at a mass ratio of 1:0.25-1.50, drying and crushing after sufficient reaction, to obtain a phosphate tailing-based slow-release medium-element fertilizer.

2. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The method also includes 4) mixing the base fertilizer and the binder, and subjecting the mixture to disc granulation, granule screening, drying and index testing to obtain a phosphate tailings-based slow-release medium-element fertilizer.

3. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The phosphate tailings are byproducts in the process of producing phosphate fertilizers in the phosphate chemical industry. The byproducts contain 1-6.7% effective phosphorus, 15-34.2% effective calcium and 8-16.3% effective magnesium.

4. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The drying temperature of the phosphorus tailings is 40-80°C, the drying time is 3-4h, and the phosphorus tailings powder is obtained by pulverizing and passing through a 60-100 mesh sieve.

5. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The low-quality phosphoric acid is purified wet-process phosphoric acid raffinate, has a solid content of 6.22%, a pH value of less than 1, an effective magnesium content of 2.0-5.4%, and an effective phosphorus content of 25-45%.

6. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The stirring time of the medium and low quality phosphoric acid in 2) is 5 to 15 minutes, and the stirring speed is 50 to 150 r / min.

7. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The mass ratio of the phosphate tailings powder to the low-quality phosphoric acid in 3) is 1:0.

75.

8. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The reaction time of the medium- and low-quality phosphoric acid and the phosphorus tailings powder in 3) is 20 to 100 minutes, the reaction temperature is 40° C. to 90° C., and the reaction stirring rate is 100 to 900 r / min.

9. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 1, characterized in that: The drying temperature of the reaction product of the low-quality phosphoric acid and the phosphate tailing powder in the above 3) is 70-80° C. and the time is 12-15 hours. After the drying is completed, it is cooled to room temperature and then crushed to obtain a phosphate tailing-based slow-release medium-element fertilizer.

10. A method for preparing a slow-release medium-element fertilizer by acidifying phosphate tailings with low-quality phosphoric acid as claimed in claim 2, characterized in that: The binder is one of water, attapulgite and sodium lignin sulfonate, and the added amount is 1.5%.

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