What are the routes for producing anhydrous hydrogen fluoride using fluorosilicic acid as raw material?


Anhydrous hydrogen fluoride (AHF) is a key raw material in industries such as aluminum, pharmaceuticals, and petroleum, traditionally derived from fluorite (a non renewable mineral). The unsustainable dependence on fluorite has prompted people to seek alternative AHF production methods. One promising alternative is fluorosilicic acid, which was previously considered a byproduct of the phosphate fertilizer industry as waste.

Converting fluorosilicic acid into AHF can not only generate valuable resources, but also address environmental and economic challenges related to waste management. The innovative practice of producing anhydrous hydrogen fluoride using fluorosilicic acid marks a shift towards sustainable chemical production through waste utilization, potentially reducing reliance on fluorite and reducing the industry's impact on the environment.

This review thoroughly analyzes the anhydrous hydrogen fluoride synthesis process of fluorosilicic acid. Although the importance of fluorinated compounds in numerous industrial applications has been recognized, research on the synthesis of fluorinated compounds from fluorosilicic acid is still limited and dispersed. The purpose of this review is to organize and merge these scattered information by carefully examining different industrial processing methods.

1、 Introduction

Industrial chemistry has made significant progress, guiding people to develop various compounds with multiple applications. Among them, anhydrous hydrogen fluoride (AHF) has received special attention due to its expanding use in different industrial fields.


Figure 1 Application of AHF

As a compound composed of hydrogen and fluorine, anhydrous hydrogen fluoride is the fundamental material for various chemical processes and products, an indispensable catalyst and reactant, spanning fields such as pharmaceuticals, petrochemicals, polymers, and electronics, and becoming a substance with global scientific and industrial interests (Figure 1).

One of the main uses of AHF is to produce fluorocarbons, which are used in refrigeration, air conditioning systems, and aerosol propellants. These fluorocarbons, especially hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), are crucial for replacing chlorofluorocarbons (CFCs) due to their low ozone depletion potential.

In addition, it is also widely used in the production of fluoroplastics, fluororubber, fluorinated drugs, and pesticides. In the petrochemical industry, AHF acts as a catalyst in the alkylation process, promoting the binding of olefins and isobutane to generate the necessary high octane components of gasoline.

In addition, the electronics industry heavily relies on electronic grade hydrofluoric acid (products processed with anhydrous hydrogen fluoride) to clean and etch silicon wafers in semiconductor and integrated circuit manufacturing processes. The precision of its fine etching lines allows for the production of smaller and more powerful electronic devices, which is the foundation of technological progress in the computer and mobile phone industries.

In the nuclear industry, AHF is used for uranium enrichment. The use of AHF produced uranium hexafluoride for gas centrifuges and gas diffusion processes to produce enriched uranium is a crucial step in civilian nuclear power generation and military applications, including the production of weapon grade materials.

In the aerospace field, AHF is a component of the propellant used in synthetic liquid rocket engines, which helps to produce high-energy compounds such as oxidizers for rocket fuels.

In addition, AHF derived compounds can be used as additives to improve the stability and performance of these propellants.

1. Classic production method of anhydrous hydrogen fluoride

Fluorite, also known as fluorite, is a colored mineral composed of calcium and fluorine, commonly found in various geological environments such as hydrothermal veins, sedimentary rocks, and as a vein mineral in mineral deposits. Fluorite has various uses and can be used as a flux in glass and enamel manufacturing, as well as a fluorine source for producing fluorinated compounds.

Due to its unique properties and extensive applications, fluorite plays a crucial role as a mineral resource in various industries. Among them, the reaction between fluorite and acid is one of the most important methods for producing anhydrous hydrogen fluoride (AHF).

This process first combines concentrated sulfuric acid with fluorite to form hydrogen fluoride and calcium sulfate, which are then converted into gas and distilled to obtain AHF. However, this traditional method has some drawbacks: it is very time-consuming, relies on expensive materials, and produces harmful gases.

In addition, the scarcity of fluorite ore also limits the production of AHF and hinders the expansion of related industries. The purification of AHF in this method is also very laborious, requiring multiple distillation and sometimes recrystallization to remove impurities.

Importantly, fluorite is an unsustainable mineral resource, raising concerns about its long-term availability and the industries affected by it. The extraction of fluorite can also have adverse effects on the environment, including habitat destruction, soil erosion, and water pollution. In addition, refining fluorite typically involves the use of chemicals and energy intensive processes, leading to greenhouse gas emissions and further environmental degradation.


Figure 2 Method for producing AHF between sulfuric acid and fluorite ore

To address these challenges, people are exploring new methods, especially those that utilize existing industrial by-products. These methods demonstrate the prospects of industry research and practical applications. By adopting such methods, limitations related to traditional technologies, including high costs and environmental issues, can be overcome. This shift towards innovative processes not only enhances the sustainability of AHF production, but also reduces the environmental impact associated with fluorite extraction and refining.

2. Introduction to fluorosilicic acid

The phosphate fertilizer industry is crucial for modern agriculture as it provides essential nutrients to promote crop growth and food production. However, the process of extracting phosphorus from phosphate ore produces various by-products, including fluorosilicic acid.

Fluorosilicic acid is produced by the reaction of fluorinated minerals with sulfuric acid, which is used to release phosphoric acid from the original phosphate rock. The harm of fluorosilicic acid to the environment is multifaceted, including its obvious toxicity to aquatic organisms. When fluorosilicic acid is released into water through direct discharge or leaching from waste piles, it can cause a rapid decrease in pH value. The increase in acidity may be harmful to fish and other aquatic organisms, disrupting their reproductive cycle, affecting growth rate, and in severe cases, leading to large-scale mortality.

In addition, fluoride components can bioaccumulate in aquatic organisms and may reach toxic levels. This biological amplification effect may have far-reaching effects, extending the food chain to predatory animals, including humans.

Although silicon is less toxic, it can lead to water turbidity and precipitation problems, affecting the transparency and quality of water, and subsequently affecting aquatic plants and organisms that rely on them for photosynthesis.

The release of fluorosilicic acid into soil systems poses another environmental challenge. Fluoride ions can bind with soil particles, thereby reducing their utilization by plants. However, over time, these ions will be released back into the soil solution and absorbed by plant roots, leading to plant fluorosis characterized by leaf tip burns, slow growth, and in extreme cases, even plant death.

The silicon in fluorosilicic acid can also alter the physical structure of soil, which may affect its ventilation and water holding capacity.

The disposal of fluorosilicic acid is another area of concern. Traditionally, this substance is neutralized with lime and then stored in large waste tanks or sold as a water fluorinating agent. However, there is also a risk of pond leakage in the storage scheme, which may contaminate groundwater resources.

Although the use of fluorosilicic acid for water fluorination is considered a beneficial recovery method, there is controversy due to the potential for excessive exposure to fluoride and related health risks. A promising solution is to convert fluorosilicic acid into useful chemical materials. Researchers have been studying how to convert this by-product into AHF. Such applications not only provide recycling methods, but also increase economic value, helping to reduce waste emissions.

3. Fluorosilicic acid as a potential source of AHF production

In the continuous search for sustainable, efficient, and economically feasible chemical production processes, fluorosilicic acid has become a potential important source for producing anhydrous hydrogen fluoride. Its potential application in AHF synthesis has changed people's perspectives, as it not only provides a rich source of fluorine resources, but also an environmentally friendly disposal method.

The process of preparing hydrofluoric acid from fluorosilicic acid can be divided into two methods: direct method and indirect method. The direct method involves the direct thermal decomposition of fluorosilicic acid or the use of concentrated sulfuric acid to produce hydrofluoric acid.

On the contrary, the indirect method does not directly use fluorosilicic acid to prepare hydrofluoric acid. It requires the conversion of fluorosilicic acid into various fluorides, such as calcium fluoride, calcium fluorosilicate, sodium fluorosilicate, magnesium fluorosilicate, potassium hydrogen fluoride/sodium fluoride, and ammonium fluoride. These fluorides are then thermally decomposed or decomposed with concentrated sulfuric acid to obtain hydrofluoric acid.

Figure 3 Production of anhydrous hydrogen fluoride using fluorosilicic acid as raw material

This review aims to analyze the production of AHF from fluorosilicic acid, examine relevant research, and outline the methods, mechanisms, and efficiency of the reactions involved. In addition, a comparative analysis will be conducted between the synthesis of AHF from fluorosilicic acid and traditional methods to quantitatively evaluate the environmental impact of each method and determine the most sustainable option. The purpose of this review is to promote further research on the production of AHF from fluorosilicic acid by identifying potential challenges and areas for improvement, ultimately driving the method towards large-scale industrial implementation.

2、 Method for producing AHF using fluorosilicic acid

1. Direct method

(1) Direct pyrolysis of fluorosilicic acid

Booth Chemical Technology Co., Ltd. (BCT) has developed a thermal decomposition technology for fluorosilicic acid: fluorosilicic acid is directly decomposed into hydrogen fluoride and silicon tetrafluoride at a temperature of 150 ° C. The chemical reaction equation for this process is shown in the following figure:



This process uses polyether and polyethylene glycol as solvents to absorb hydrogen fluoride, and water to absorb tetrafluorosilane gas to obtain recovered fluorosilicic acid as raw material. Subsequently, organic absorbents are separated by fractionation to produce hydrogen fluoride. This process has multiple advantages, including simple program, reduced range of raw materials, and the ability to recover organic solvents within the system. However, it requires a certain level of technical complexity, such as the need to evaporate a large amount of water, resulting in high energy consumption. In addition, strict control over process conditions and equipment material specifications also poses significant challenges.

Another method is to heat fluorosilicic acid to decompose it into silica and dilute hydrogen fluoride. Anhydrous hydrogen fluoride can be produced by treating with sulfuric acid. But the disadvantage of this technology is that the purity of the generated HF is limited and a large amount of concentrated sulfuric acid is required. Mani et al. purified the mixed solution obtained from the thermal decomposition of fluorosilicic acid solution using electrodialysis. However, this method has drawbacks such as complex process, low technological maturity, and high energy consumption.

(2) Fluorosilicic acid is decomposed by sulfuric acid

Wellman Power Gas in the United States has developed a process for sulfuric acid decomposition of fluorosilicic acid to obtain hydrofluoric acid. Fluorosilicic acid is dehydrated and pretreated with concentrated sulfuric acid at 125 ℃. The solution reacts with concentrated sulfuric acid to produce hydrogen fluoride, which is then absorbed by sulfuric acid to form fluorosulfonic acid. The escaping silicon tetrafluoride gas is absorbed and treated with a dilute fluorosilicic acid solution before being returned for use. The chemical reaction shown in this process is as follows:


This process route is short, the operation steps are simple, the equipment requirements are low, the product added value is high, and it is conducive to improving comprehensive economic benefits. A significant drawback of this process is the excessive consumption of concentrated sulfuric acid, which generates a large amount of dilute sulfuric acid with a concentration of about 70% and high concentration of fluoride ions, resulting in a significant loss of fluoride elements in the system. Therefore, it is necessary to properly treat the generated sulfuric acid, which inevitably increases production costs.

Oakley and Moore et al. made significant progress in improving the process flow by implementing a silicon tetrafluoride solution circulation system. They can effectively produce fluorosilicic acid by circulating the solution to the fluorosilicic acid concentration step. However, a challenge encountered in this process is the hydrolysis of silicon tetrafluoride during the generation of fluorosilicic acid. This hydrolysis reaction can lead to the formation of a large amount of silica gel. The presence of silicone can complicate the filtration process and make it difficult to proceed. Due to the tendency of silicone to form colloidal particles, which can easily block or block filters, filtration becomes challenging. Therefore, the filtering process may become slower or less efficient.


Figure 4 Preparation of AHF by fluorosilicic acid sulfuric acid method

BUSS ChemTech AG (BCT) has also conducted systematic research on the process of direct decomposition of fluorosilicic acid with concentrated sulfuric acid, forming a relatively mature BUSS process that has been implemented in industry. However, the by-product silica has not been effectively utilized yet.

Figure 5 BUSS Process

Swiss company Kvaemer AG has conducted research on similar production processes and constructed a trial production facility. Wengfu Company has further improved the efficiency of hydrogen fluoride production process by modifying the design based on this technology. In this process, as shown in Figure 5, the diluted fluorosilicic acid obtained from the phosphate fertilizer plant is first introduced into the concentration system for further concentration. After filtration and separation, concentrated fluorosilicic acid reacts with concentrated sulfuric acid to generate a mixture of gases such as silicon tetrafluoride and hydrogen fluoride. The mixed gas is absorbed by sulfuric acid, and the hydrogen fluoride gas is absorbed and retained by concentrated sulfuric acid, while the remaining silicon tetrafluoride gas is recycled back to the concentration system for continued use.

To separate hydrogen fluoride gas, concentrated sulfuric acid that has absorbed hydrogen fluoride gas can be distilled. The hydrogen fluoride gas is purified and distilled to remove high and low boiling impurities, resulting in anhydrous hydrogen fluoride. The remaining dilute sulfuric acid can be returned to the phosphoric acid reactor for phosphoric acid production. This method provides a relatively simple process. However, in theory, the unidirectional conversion rate of fluorosilicic acid to hydrogen fluoride is only 33.3%.

Figure 6 Production method of anhydrous hydrofluoric acid by Wengfu Group

2. Indirect method

(1) Fluorosilicic acid forms fluorinated precipitates with metal cations

When fluorosilicic acid reacts with metal cations (such as calcium ions or sodium ions), it forms fluorinated salts, such as sodium hexafluorosilicate or calcium hexafluorosilicate. These salts are commonly used for different purposes in different industries. In order to produce hydrogen fluoride, the fluoride salt precipitate is further reacted with concentrated sulfuric acid at a specific temperature. This reaction is commonly referred to as the "fluorosilicic acid method".

1) Calcium salt

The United States Bureau of Mines uses ammoniation of hexafluorosilicic acid to form ammonium fluoride and silica. During this reaction, the pH remains around 9. After filtration, add calcium hydroxide as a precipitant to the filtrate to promote the formation of calcium fluoride by reacting with ammonium fluoride. Separate and dry the obtained calcium fluoride product.

Subsequently, the generated calcium fluoride will be used for the traditional fluorite process to produce hydrogen fluoride. It is worth noting that the ammonia generated during the precipitation process can be recycled and reused. The total fluorine recovery rate of this process is as high as 97.3%, and the ammonia recovery rate is as high as 88.8%. In addition, it does not require any modification of HF production equipment. However, it should be acknowledged that the characteristic of this process is its long duration and increased complexity.


Figure 7 Preparation of calcium fluoride using ammonium fluoride as a precipitant

Yunnan Yuntianhua has developed a comprehensive utilization method for by-products of phosphate fertilizer production, including fluorogypsum and fluorosilicic acid. This method involves the ammonification reaction between fluorosilicic acid and ammonia water, followed by filtration to remove the solid precipitate (white carbon black), resulting in a 5%~22% ammonium fluoride solution. Then mix the solution with fluorogypsum powder at a temperature of 60 ℃~80 ℃ to produce calcium fluoride and ammonium sulfate mother liquor with a purity greater than 92%. Calcium fluoride is further reacted with concentrated sulfuric acid using the fluorite method to obtain hydrogen fluoride products that meet national standards, and the by-product fluorogypsum is reused. The entire development process has mild reaction conditions, high fluoride recovery rate, and easy control of operating conditions. In addition, this process can also produce carbon black and agricultural ammonium sulfate products. But its process flow is relatively long.


Figure 8 Preparation of calcium fluoride using fluorogypsum powder as a precipitant

Different researchers and companies have explored various other routes and processes. For example, Bayer/Kalichemie used a method of reacting calcium carbonate with fluorosilicic acid to generate calcium fluoride and silicon dioxide, which were then separated based on density differences.

Xue et al. also studied a similar process route, setting the molar ratio of hexafluorosilicic acid to limestone to 1:3, and conducting the reaction at a temperature between 70 ℃ and 80 ℃ for 2 hours. Under these conditions, the reaction rate of limestone reaches 93%, and the yield of calcium fluoride exceeds 95%.


Figure 9 Preparation of calcium fluoride using calcium carbonate as a precipitant

Shandong Lubei Enterprise Group Corporation uses sodium fluorosilicate as raw material to co produce anhydrous hydrogen fluoride and zeolite molecular sieve. This process involves the reaction of sodium fluorosilicate and sodium hydroxide solution in the temperature range of 60 ℃ to 90 ℃, solid-liquid separation, and the formation of sodium fluoride solid and sodium silicate solution. Subsequently, sodium fluoride reacts with lime slurry at temperatures ranging from 60 ℃ to 95 ℃ to produce calcium fluoride with a main content exceeding 95%.

The production of anhydrous hydrogen fluoride follows the traditional fluorite process. Then mix the sodium silicate solution with the sodium aluminate solution in sequence, pulping, and crystallization to obtain the zeolite molecular sieve product. This innovative method not only benefits the efficient separation and preparation of anhydrous hydrogen fluoride from sodium fluorosilicate, but also maximizes the utilization of the separated silicon and sodium to produce high-value zeolite molecular sieve products. The characteristic of this process is that the workflow is lengthy, and the main challenge lies in the separation of sodium fluoride and sodium silicate, as well as the efficient conversion and separation of sodium fluoride and calcium fluoride.

In addition, Picardi Aluminum, a French company, successfully produced calcium fluorosilicate by reacting anhydrous calcium chloride with impure hydrogen hexafluorosilicate. This process can accurately precipitate calcium fluorosilicate dihydrate under low temperature conditions by adjusting the concentration of hexafluorosilicic acid and the molar ratio of calcium chloride to fluorosilicic acid. Then, the obtained dihydrate is filtered, washed, and dried to obtain anhydrous calcium fluorosilicate. Calcium fluorosilicate is easily decomposed into calcium fluoride and silicon tetrafluoride at high temperatures, and calcium fluoride is suitable for producing hydrofluoric acid.

Under conditions where the mass concentration of fluorosilicic acid exceeds 25% and the molar concentration ratio of calcium chloride to fluorosilicic acid is 2-5, the yield of anhydrous calcium fluorosilicate can reach over 94%. The solution reacts with hydrogen hexafluorosilicate solution to obtain calcium fluorosilicate. Calcium fluorosilicate further thermally decomposes within the temperature range of 300 ° C-400 ° C to form calcium fluoride. The main challenge of this technology lies in the preparation of calcium fluorosilicate, mainly in the filtration of calcium fluorosilicate, the selection of calcium sources, and the yield of calcium fluorosilicate. Calcium fluorosilicate can be completely decomposed at 400 ℃ for 1 hour, and the resulting products are calcium fluoride ≥ 96.5% and silicon tetrafluoride ≥ 87%. The process flow is too long, making it difficult to filter calcium fluorosilicate. In addition, the reuse of wastewater and waste residue also faces challenges.


Figure 10 Preparation of calcium fluoride by reaction of fluorosilicic acid and calcium chloride

2) Magnesium salt

Hao Jie reported a process route for precipitation of fluorosilicic acid using lightly burned magnesium oxide. Concentrate and dry the magnesium fluorosilicate solution to obtain solid magnesium fluorosilicate. Subsequently, the solid is calcined to produce magnesium fluoride and silicon tetrafluoride gas. Silicon tetrafluoride gas can be continuously recycled and reused after being absorbed by water. Similar to the fluorite method, magnesium fluoride is mixed with concentrated sulfuric acid to produce hydrogen fluoride gas and magnesium sulfate. Hydrogen fluoride gas can be refined to obtain anhydrous hydrogen fluoride, and purified magnesium sulfate can be sold as a by-product.

Multifluoro uses a multi-step process to produce hydrogen fluoride. The first step is to react magnesium oxide with a solution of fluorosilicate to form a solution of magnesium fluorosilicate. Subsequently, impurities were separated from the solution through filtration to obtain a purified magnesium fluorosilicate solution. Then, the purified solution is concentrated and crystallized to produce magnesium fluorosilicate hexahydrate. The obtained magnesium fluorosilicate hexahydrate is further treated and dried to decompose into magnesium fluoride at temperatures ranging from 250 ℃ to 300 ℃.

In order to obtain hydrogen fluoride, it is necessary to use concentrated sulfuric acid to decompose magnesium fluoride. The production process adopted by Duofuduo ensures the efficient and controllable synthesis of hydrogen fluoride from magnesium oxide and fluorosilicate solutions. The characteristic of this process is low production cost, no need for ammonia recovery process, significantly reducing environmental pollution. The purity of the generated hydrogen fluoride is not less than 99.99%, while the purity of the by-product magnesium sulfate is at least 99.65%.

Figure 11 Preparation of AHF using fluorosilicic acid and magnesium oxide

(2) Sodium fluoride/potassium process

White carbon black, also known as silica, is similar to the natural mineral quartz. Due to its white powder like appearance and ideal performance, it is commonly used as a reinforcing filler in various applications. With its high surface area and excellent dispersibility, it can be used as an additive in industries such as rubber, plastics, coatings, adhesives, and sealants, improving its mechanical properties such as tensile strength, tear resistance, and wear resistance. In addition, white carbon black has anti adhesion and anti slip properties, making it suitable for films and coatings. Its low refractive index makes it suitable for optical applications such as optical fibers, glass, and specialty paper. In addition, it also acts as a stabilizer to protect the material from UV damage caused by sunlight exposure.

ISC Chemical Company in the UK has collaborated with Dublin Chemical Company to successfully industrialize the process of producing hydrofluoric acid (HF) and silica from sodium hydrogen fluoride. The process is shown in the following figure, involving several steps of separation and obtaining the desired compound.

Initially, silicon fluoride hydrolyzed with ammonia, causing the separation of silica and the formation of ammonium fluoride solution. Then the solution reacts with potassium fluoride to produce potassium hydrofluoride, which further reacts with NaF to produce sodium hydrogen fluoride. Then solid sodium fluoride is calcined at 300 ℃ to decompose into the main product HF and the by-product sodium fluoride. It is worth noting that this technology faces challenges such as high energy consumption, specific material requirements, and difficulties in selecting suitable process equipment.

Researchers and engineers continue to explore methods to enhance the sodium hydrofluoride process, develop better separation techniques, and minimize the loss of sodium fluoride and potassium fluoride.

For example, Hanover, Germany, improved the process flow of sodium hydrofluorate by adding an equal amount of potassium fluoride as sodium fluoride to produce potassium hydrogen fluoride. This modification allows sodium fluoride and potassium fluoride to circulate in the system without any loss. However, in practical industrialization, achieving 100% recovery of potassium fluoride or sodium fluoride is difficult.

In addition, controlling the precise amount of sodium fluoride and potassium fluoride during operation may be a complex task. Industrial environments often have practical limitations, which make it challenging to strictly control the quantity of each compound.

Another similar process studied by Jishou University and East China Research Institute follows similar steps, but with a key difference. During this process, potassium fluoride is directly thermally decomposed to produce anhydrous hydrogen fluoride, eliminating the conversion steps involving sodium or potassium salts. In theory, this cyclic process using potassium fluoride as a carrier will not cause any losses. However, due to the low melting point of potassium hydrogen fluoride, this direct calcination process also poses challenges, making the treatment and packaging during high-temperature pyrolysis more complex. Therefore, the overall energy consumption of the entire process is relatively high, and the improvement in economic benefits is not significant.

Polyfluoro uses the process of neutralizing the by-product of phosphate fertilizer, fluorosilicic acid. Fluorosilicate reacts with potassium carbonate at a temperature of 90 ° C and then ammoniates within the temperature range of 20 ° C-40 ° C until the pH reaches 8-8.5. Filter the obtained mixture to extract white carbon black, followed by the production of ammonium fluoride solution. Then, potassium fluoride is added to the ammonium fluoride solution, concentrated, crystallized, and high-temperature dried to generate potassium hydrogen fluoride.

Potassium hydrogen fluoride is pre decomposed at temperatures ranging from 150 ℃ to 450 ℃ to obtain a paste, which is then calcined at temperatures ranging from 500 ℃ to 550 ℃ to obtain crude hydrogen fluoride. The by-product potassium fluoride is recovered, and crude hydrogen fluoride is separated and purified to obtain the final product. The electronic grade hydrofluoric acid produced through this process meets the UPSS standard of SEMI (Semiconductor Manufacturing Facility Utility Performance Standards).

This process involves specific technical aspects, including the concentration of potassium hydrogen fluoride, control of pyrolysis energy consumption, and selection of pyrolysis equipment and materials. It should be noted that the melting point of potassium hydrogen fluoride is 238.17 ℃, and its decomposition temperature is higher than its melting point, leading to melting phenomenon during the pyrolysis process. This will cause the potassium fluoride produced during the continuous high-temperature decomposition process to wrap, adhere to walls, and clump, thereby reducing the decomposition efficiency. To solve this problem, a pre decomposition device was used for polyfluoroethylene, which first produces the slurry and then decomposes it through high-temperature calcination. This alleviates the problem of decreased heat transfer caused by the melting and packaging of potassium hydrogen fluoride, thereby reducing energy consumption.

Figure 12 Preparation of Potassium Fluorohydride from Fluorosilicic Acid and AHF

(3) Ammonium fluoride/ammonium hydrogen fluoride process

Guizhou Kaiphosphorus Group and Guizhou Chemical Research Institute have successfully developed an efficient method for obtaining hydrogen fluoride through the decomposition of ammonium fluoride. This process has been industrialized and provides various practical applications in industries that require HF as a raw material or reagent. As shown in the following figure, this process involves several steps.


Initially, fluorosilicic acid reacts with ammonia to form solid ammonium fluorosilicate, which then reacts with ammonia to form ammonium fluoride. Finally, use concentrated sulfuric acid to convert ammonium fluoride into HF. The large-scale successful implementation of this method demonstrates its potential for cost-effectiveness and efficient industrial production.

Similarly, Yunnan Yuntianhua and Tianjin Chemical Design and Research Institute are collaborating to focus on researching the ammonium hydrogen fluoride process.

Firstly, fluorosilicic acid undergoes mild amination at temperatures of 45 ° C and 35 ° C to form a solution containing ammonium fluoride. The solution is then concentrated and reacted with concentrated sulfuric acid to produce hydrogen fluoride (HF) and ammonium sulfate. This process can produce white carbon black and ammonium sulfate as by-products.

A significant advantage of this process is the ability to recover ammonia, thereby reducing its consumption as a reagent. However, due to the specific requirements of the production unit, this process is considered complex. Effectively utilizing ammonia in the actual production process may pose challenges, leading to increased formation of by-products and higher production costs.

Figure 13 Decomposition of ammonium bifluoride by sulfuric acid to produce AHF

3、 Conclusion

In summary, this review rigorously examines the methods for producing anhydrous hydrogen fluoride (AHF) from fluorosilicic acid, which is a rich by-product and industrial waste in the phosphate fertilizer industry. The impact of the reviewed conversion process is far-reaching, demonstrating the possibility of efficient, economical, and more sustainable anhydrous hydrogen fluoride (AHF) production methods. The application of this method not only achieves resource efficiency, but also provides positive solutions for waste management issues, especially for the large amount of by-product waste fluorosilicic acid in the phosphate fertilizer industry.

There are many potential benefits to the transformation process studied. Firstly, using fluorosilicic acid as the starting material for the thermal hydrolysis process of AHF synthesis is beneficial in reducing dependence on mineral fluorite. This reduction can significantly save costs and reduce environmental degradation caused by mining activities.

Secondly, this process redefines fluorosilicic acid from waste to a valuable resource for AHF production. Dealing with this type of industrial waste does not incur costs or potential environmental hazards, but can be utilized in a meaningful manner, contributing to sustainable industrial practices. Another advantage is that it provides excellent opportunities for comprehensive industrial symbiosis, where waste from one industrial process becomes a raw material for another, symbolizing a comprehensive industrial ecosystem.

Finally, this hydrothermal method has shown potential in terms of reaction efficiency, indicating that the process can be applied in industry. This indicates that the method can be further scaled up and optimized to meet the growing demand for AHF in various industries, thereby having a positive impact on industrial growth while adhering to the principles of green chemistry and sustainable development.

Therefore, the impact and potential benefits of the reviewed transformation process mark a significant step forward in more sustainable and greener AHF production, contributing to the overall goals of circular economy and sustainable industrial development. We strongly encourage further exploration and optimization research on this method to fully unleash its potential.





Created on:2024-03-06 13:11
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