Method for removing fluorine from white fertilizer by dry method and preparation of fluosilicic acid

Abstract

The application discloses a method for removing fluorine from white fertilizer by dry method and preparing fluorosilicic acid, which comprises the following steps: adding dried white fertilizer into a reaction container, mixing the white fertilizer with silicon powder and sulfuric acid under stirring, wherein the mixing ratio is that the molar ratio of SiO2 / F is 0.3-0.8 and the molar ratio of H2SO4 / F is 1-4; then conveying the mixed material to a calcining furnace for calcination; introducing fluorine-containing gas discharged from the calcining into a fluorosilicic acid preparation system, and obtaining fluorosilicic acid products with a purity of greater than or equal to 12 % through two-stage spray absorption; and continuing to use the fluorine-removing calcined slag as white fertilizer. The application can significantly improve the fluorine resource utilization rate and reduce resource waste; meanwhile, by adding sulfuric acid which has the dual functions of raw material and catalyst, the reaction temperature is effectively reduced, the energy consumption and equipment cost are reduced, and the reaction efficiency and process economy are further improved.

Description

Technical Field

[0001] This invention relates to the field of chemical product preparation technology, specifically to a method for defluorinating white fertilizer and using the separated fluorine to prepare fluorosilicic acid. Background Technology

[0002] Fluorosilicic acid has a wide range of applications in industrial production, commonly used to manufacture fluorides such as sodium fluorosilicate and sodium fluoroaluminate, and also has important uses in electroplating, pharmaceuticals, and other fields. However, traditional methods for preparing fluorosilicic acid have many drawbacks, such as low fluorine recovery rates, low product purity, and complex processes. This not only leads to high production costs but also wastes fluorine resources.

[0003] White fertilizer (fertilizer-grade dicalcium phosphate), a fluorine-rich byproduct of the phosphate chemical industry, suffers from excessively high fluorine content due to the increasing scarcity of high-grade phosphate rock. Direct application not only pollutes the soil and inhibits crop growth but also harms human health through the food chain; it further exacerbates water pollution, threatening agricultural production and ecosystem stability. Therefore, promoting the efficient recovery of fluorine resources from white fertilizer is not only a crucial measure to break the pollution chain at its source but also a strategic necessity for achieving the recycling of phosphate and fluorine resources and supporting the development of green chemical industry.

[0004] Traditional methods for fluorine recovery from white fertilizer (fertilizer-grade dicalcium phosphate) mainly include high-temperature calcination-gas absorption and chemical leaching. While high-temperature calcination-gas absorption can recover some fluorine, it suffers from high energy consumption and demanding equipment requirements. It needs to operate in a high-temperature environment, placing stringent demands on the equipment's high-temperature and corrosion resistance, increasing production costs and equipment maintenance difficulty. Chemical leaching has a limited fluorine extraction rate, making it difficult to fully extract fluorine from white fertilizer, and the leaching process may introduce many impurities, affecting product quality. Therefore, developing a highly efficient, environmentally friendly method for preparing fluorosilicic acid with a high fluorine recovery rate is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of low fluorine recovery rate and unstable product quality in the prior art, and to provide a dry defluorination method for white fertilizer with high fluorine recovery rate and high quality fluorosilicic acid preparation, as well as a method for preparing fluorosilicic acid using separated fluorine, so as to realize the efficient utilization of fluorine resources and high-quality production of fluorosilicic acid.

[0006] The technical solution adopted in this invention is as follows: The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid includes the following steps: S1. Raw material mixing: Add the dried white fertilizer to the reaction vessel, and add silicon powder and sulfuric acid under stirring conditions to mix the materials. The mixing ratio is SiO2 / F molar ratio of 0.3-0.8 and H2SO4 / F molar ratio of 1-4. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace for calcination; S3. Gas absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system. After two-stage spray absorption, a fluorosilicic acid product with a purity ≥12% is obtained. S4. The defluorinated calcined residue discharged in step S2 is used as white fertilizer.

[0007] Further, in step S1, the white fertilizer has a P2O5 content of 25-30 wt%, a sulfuric acid concentration of 93-98%, a SiO2 content of ≥90% in the silicon powder, and the mixing time of the raw materials is 20-40 min.

[0008] Furthermore, the calcination temperature in step S2 is 250-300 ℃, and the calcination time is 2-3 h.

[0009] Further, in step S3, the two-stage spray absorption involves introducing fluorine-containing waste gas (mainly composed of SiF4) into the first-stage absorption tower, where it reacts to generate fluorosilicic acid; the tail gas discharged from the first-stage absorption tower enters the second-stage absorption tower, where residual SiF4 is further captured by spraying with fresh water, and the resulting dilute fluorosilicic acid solution is returned to the first-stage absorption tower as washing liquid.

[0010] The present invention has the following advantages: By optimizing the raw material ratio, precisely controlling the calcination conditions, and adopting a highly efficient two-stage spray washing process, the fluorine recovery rate of this invention can reach over 90%, significantly improving the utilization rate of fluorine resources and reducing resource waste.

[0011] By adding sulfuric acid, the reaction temperature was effectively lowered, overcoming the high energy consumption limitation of traditional methods. Sulfuric acid serves both as a raw material and a catalyst in the reaction, promoting its progress, reducing energy consumption and equipment costs, and improving reaction efficiency and economy. Detailed Implementation

[0012] The present invention will be further described below with reference to the embodiments. Example 1

[0013] The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid includes the following steps: S1. Raw material mixing: Weigh out white fertilizer with a P2O5 mass concentration of 25.5% after drying, and transport it to the reaction tank via a pump. Under mechanical stirring conditions, add sulfuric acid with a mass concentration of 93.5% and silicon powder with a SiO2 mass content of 90% to the reaction tank while stirring. The mixing ratio is SiO2 / F molar ratio of 0.3 and H2SO4 / F molar ratio of 1. Stir for 30 min to obtain a homogeneous material. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace, heated to 280 ℃, and calcined at this temperature for 2.5 h; calcination produces fluorine-containing gas and calcined slag; S3. Gas Absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system. After two-stage spray absorption, fluorosilicic acid product is obtained. The fluorosilicic acid preparation system is an existing technology production system. The fluorine-containing waste gas (mainly composed of SiF4) is introduced into the first-stage absorption tower and reacts to generate fluorosilicic acid. The tail gas discharged from the first-stage absorption tower enters the second-stage absorption tower, where residual SiF4 is further captured by fresh water spraying. The resulting dilute fluorosilicic acid solution is returned to the first-stage absorption tower as washing liquid.

[0014] S4. The defluorinated calcined residue discharged in step S2 is crushed and then used as white fertilizer; The obtained fluorosilicic acid product was tested and analyzed according to "HG / T2832-2020", and its purity was 12.45%, with a fluorine recovery rate of 68.45% in the white fertilizer. Example 2

[0015] The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid includes the following steps: S1. Raw material mixing: Weigh out the dried white fertilizer with a P2O5 mass concentration of 27.8% and transport it to the reaction tank. Under mechanical stirring, add sulfuric acid with a mass concentration of 95.5% and silicon powder with a SiO2 mass content of 92% while stirring. The mixing ratio is SiO2 / F molar ratio of 0.5 and H2SO4 / F molar ratio of 3. Stir for 20 min to obtain a homogeneous material. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace, heated to 250 ℃, and calcined at this temperature for 3 h; S3. Gas absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system and absorbed through two stages of spraying to obtain the fluorosilicic acid product. S4. The defluorinated calcined residue discharged in step S2 is crushed and then used as white fertilizer.

[0016] The obtained fluorosilicic acid product was tested and analyzed according to "HG / T2832-2020", and its purity was 12.55%, with a fluorine recovery rate of 80.21% in the white fertilizer. Example 3

[0017] The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid includes the following steps: S1. Raw material mixing: Weigh out the dried white fertilizer with a P2O5 mass concentration of 29.8% and transport it to the reaction tank. Under mechanical stirring, add sulfuric acid with a mass concentration of 93.25% and silicon powder with a SiO2 mass content of 90.58% while stirring. The mixing ratio is SiO2 / F molar ratio of 0.6 and H2SO4 / F molar ratio of 3.5. Stir for 35 min to obtain a homogeneous material. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace, heated to 300 ℃, and calcined at this temperature for 2 h; S3. Gas absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system and absorbed through two stages of spraying to obtain the fluorosilicic acid product. S4. The defluorinated calcined residue discharged in step S2 is crushed and then used as white fertilizer.

[0018] The obtained fluorosilicic acid product was tested and analyzed according to "HG / T2832-2020", and its purity was 12.46%, with a fluorine recovery rate of 89.98% in the white fertilizer. Example 4

[0019] The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid includes the following steps: S1. Raw material mixing: Weigh out the dried white fertilizer with a P2O5 mass concentration of 26.5% and transport it to the reaction tank. Under mechanical stirring, add sulfuric acid with a mass concentration of 98% and silicon powder with a SiO2 mass content of 91% while stirring. The mixing ratio is SiO2 / F molar ratio of 0.8 and H2SO4 / F molar ratio of 4. Stir for 40 min to obtain a homogeneous material. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace, heated to 300 ℃, and calcined at this temperature for 2 h; S3. Gas absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system and absorbed through two stages of spraying to obtain the fluorosilicic acid product. S4. The defluorinated calcined residue discharged in step S2 is crushed and then used as white fertilizer.

[0020] The obtained fluorosilicic acid product was tested and analyzed according to "HG / T2832-2020", and its purity was 12.43%, with a fluorine recovery rate of 90.23% in the white fertilizer.

[0021] Unless otherwise stated, all percentages mentioned in this invention are mass percentages.

Claims

1. A method for dry defluorination of white fertilizer and preparation of fluorosilicic acid, characterized in that, Includes the following steps: S1. Raw material mixing: Add the dried white fertilizer to the reaction vessel, and add silicon powder and sulfuric acid under stirring conditions to mix the materials. The mixing ratio is SiO2 / F molar ratio of 0.3-0.8 and H2SO4 / F molar ratio of 1-4. S2. Calcination: The material that was mixed evenly in step S1 is conveyed to a calcining furnace for calcination; S3. Gas absorption: The fluorine-containing gas discharged from the calcination in step S2 is introduced into the fluorosilicic acid preparation system. After two-stage spray absorption, a fluorosilicic acid product with a purity ≥12% is obtained. S4. The defluorinated calcined residue discharged in step S2 is used as white fertilizer.

2. The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid as described in claim 1, characterized in that, The white fertilizer in step S1 has a P2O5 content of 25-30%, a sulfuric acid concentration of 93-98%, and a SiO2 content of ≥90% in the silicon powder. The mixing time for the raw materials is 20-40 min.

3. The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid as described in claim 1 or 2, characterized in that, The calcination temperature in step S2 is 250-300 ℃, and the calcination time is 2-3 h.

4. The method for dry defluorination of white fertilizer and preparation of fluorosilicic acid as described in claim 1, characterized in that, In step S3, the two-stage spray absorption involves introducing fluorine-containing waste gas into the first-stage absorption tower, where it reacts to generate fluorosilicic acid. The exhaust gas from the first-stage absorption tower enters the second-stage absorption tower, where residual SiF4 is further captured by spraying with fresh water. The resulting dilute fluorosilicic acid solution is returned to the first-stage absorption tower as a washing liquid.