In-Depth Exploration of Phosphoric Acid Production and Applications: A Comprehensive Analysis of Thermal Phosphoric Acid and Wet-Process Phosphoric Acid

Phosphoric Acid in Glass

Phosphoric acid is an indispensable base chemical in industrial production, widely used in agriculture, food, pharmaceuticals, and chemical industries. This article delves into the two primary methods of phosphoric acid production—thermal phosphoric acid and wet-process phosphoric acid—and provides a detailed analysis of their production processes, scientific principles, application scenarios, and environmental impacts. By comparing the advantages and disadvantages of these two methods, we can better understand the multifaceted effects of phosphoric acid production on the economy, technology, and environment.

1. Phosphoric Acid Production Processes and Technical Details

Phosphate rock is the basic raw material for phosphoric acid production. Through chemical or physical treatment, phosphate rock can be converted into various forms of phosphate fertilizers and other phosphate products. Phosphoric acid production is mainly achieved through two processes: thermal process and wet process.

1.1 Thermal Phosphoric Acid Production Process

The production of thermal phosphoric acid involves the use of elemental phosphorus (yellow phosphorus) as an intermediate, which is extracted and oxidized into phosphoric acid through a high-temperature process. Below are the specific steps and technical analysis of this process.

• Yellow Phosphorus Preparation and Its Physicochemical Properties:

Yellow phosphorus is the key intermediate in the production of thermal phosphoric acid, and its preparation requires a high level of technical expertise. Phosphate rock is first mixed with a certain amount of flux (such as silica) and a reducing agent (such as coke) and heated in an electric furnace to above 1450°C. At this temperature, phosphorus is reduced from the rock and released in a gaseous form. The cooled phosphorus vapor condenses into yellow phosphorus, a waxy solid with high reactivity.

The chemical properties of yellow phosphorus make its storage and handling complex. Yellow phosphorus is highly flammable at room temperature, so it must be stored in water to isolate it from air and prevent oxidation. In the presence of oxygen, yellow phosphorus rapidly oxidizes to form phosphorus pentoxide (P₂O₅), which then reacts with water to produce phosphoric acid. This reaction is not only exothermic but also yields a product of extremely high purity.

• Industrial Production Equipment for Thermal Phosphoric Acid:

The production of thermal phosphoric acid requires specially designed reaction towers. In the reaction tower, yellow phosphorus burns in the presence of oxygen to form phosphorus pentoxide, which is then absorbed by introduced pure water to produce phosphoric acid solution. The design of the reaction tower must consider temperature control, uniform mixing of gases, and rapid cooling and absorption of products to ensure high purity and concentration of the final product. Typically, the concentration of thermal phosphoric acid ranges between 80-85%.

• Scientific Principles and Thermodynamic Analysis:

The oxidation of yellow phosphorus is a typical exothermic reaction. From a thermodynamic perspective, the enthalpy change of this reaction is significant, and it almost entirely converts to phosphorus pentoxide. The efficiency and completeness of this reaction are key reasons for the extremely high purity of thermal phosphoric acid. Furthermore, since no other elements are involved in the reaction process, the likelihood of impurities being introduced is significantly reduced, which is why thermal phosphoric acid is particularly suitable for high-demand applications.

However, the energy consumption during the thermal phosphoric acid production process is considerable. The use of electric furnaces means that the entire production process requires a substantial amount of electricity, which not only increases production costs but also results in significant carbon emissions. As global attention to energy efficiency and environmental protection grows, researchers are exploring ways to reduce energy consumption in thermal phosphoric acid production by improving electric furnace design, using more efficient electrode materials, and developing renewable energy sources for power supply.

1.2 Wet-Process Phosphoric Acid Production Process

Unlike thermal phosphoric acid, wet-process phosphoric acid production is based on acid dissolution reactions and does not require high temperatures, making it relatively less energy-intensive. Wet-process phosphoric acid is the mainstream process for global phosphoric acid production, especially dominating the fertilizer industry.

• Basic Wet-Process Phosphoric Acid Technology:

Wet-process phosphoric acid is typically produced by mixing phosphate rock with strong acids (such as sulfuric acid, nitric acid, or hydrochloric acid) to undergo an acid dissolution reaction. The most common method involves sulfuric acid. Sulfuric acid reacts with the phosphate salts in the phosphate rock, generating phosphoric acid and a by-product known as dihydrate gypsum (CaSO₄·2H₂O). The resulting slurry is filtered to separate the gypsum, and the obtained phosphoric acid generally contains a high level of impurities such as iron, magnesium, and aluminum, requiring further purification.

Depending on the reaction conditions and the type of gypsum by-product, wet-process phosphoric acid production processes can be divided into three types: dihydrate process, hemihydrate-dihydrate process, and hemihydrate process. The dihydrate process is advantageous in terms of production efficiency, with larger gypsum crystals that are easier to filter and separate. The hemihydrate process, though offering a higher concentration of phosphoric acid, has a lower reaction efficiency and is less commonly used in industrial production.

• Purification and Impurity Control in Wet-Process Phosphoric Acid:

Due to the significant amount of impurities in wet-process phosphoric acid, such as heavy metals and other non-metallic contaminants, it must be purified for many industrial applications. The purification process typically includes acid residue reaction, decolorization, and solvent extraction steps. Tributyl phosphate extraction is one of the most commonly used purification technologies. It selectively dissolves impurities through organic solvents, effectively removing them to produce a higher purity phosphoric acid product.

Additionally, activated carbon decolorization is a common physical-chemical treatment method mainly used to remove organic impurities and color from phosphoric acid. In the purification process, controlling the amount and regeneration of solvents and improving purification efficiency are key technical challenges in wet-process phosphoric acid technology.

• Environmental Impact and Waste Management:

The treatment of gypsum by-products in wet-process phosphoric acid production is a major environmental issue. The storage of gypsum occupies large areas of land and can lead to the pollution of groundwater by acidic runoff. In recent years, significant progress has been made in research on the resource utilization of gypsum, focusing on its use in building materials, road construction, and soil conditioners. However, how to scale up these technologies for large-scale application remains a pressing challenge in wet-process phosphoric acid production.

2. Applications of Phosphoric Acid in Various Fields and Market Analysis

As a basic chemical, phosphoric acid is widely used in multiple fields. The application range and market positioning of different purity levels and production processes of phosphoric acid products vary.

2.1 High-End Applications of Thermal Phosphoric Acid

Due to its extremely high purity, thermal phosphoric acid is primarily used in fields with strict quality requirements.

• Food and Pharmaceutical Industries:

In the food industry, phosphoric acid is widely used as an acidity regulator, preservative, and nutrient enhancer in beverages, dairy products, canned foods, and other products. The high purity of phosphoric acid is essential to ensure that it is free from harmful impurities, especially heavy metals. Therefore, thermal phosphoric acid is the preferred raw material for food-grade phosphate salts.

In the pharmaceutical industry, phosphoric acid and its salts are used to produce various medications and injections, requiring highly pure and chemically stable raw materials. Due to its high purity and low impurity levels, thermal phosphoric acid is particularly suitable for the preparation of pharmaceutical-grade phosphate salts, such as monopotassium phosphate and monoammonium phosphate. These compounds are used as buffers, stabilizers, and nutritional supplements in pharmaceutical formulations.

• High-End Chemical and Electronics Industries:

In the high-end chemical industry, thermal phosphoric acid is used as a key raw material for the production of high-purity phosphate salts, flame retardants, and catalysts. For example, phosphate ester compounds are important intermediates in organic synthesis and plasticizers, widely used in the polymer industry. In the electronics industry, phosphoric acid is used to prepare high-purity etching solutions for the manufacturing of semiconductors and integrated circuits. These processes require phosphoric acid with extremely low metal impurity levels to avoid contaminating electronic components.

2.2 Extensive Industrial Applications of Wet-Process Phosphoric Acid

Due to its low production cost and high output, wet-process phosphoric acid is the most widely used type of phosphoric acid in industrial and agricultural applications.

• Agricultural Sector: Production of Phosphate Fertilizers:

Monoammonium phosphate (MAP) and diammonium phosphate (DAP) are the most commonly used phosphate fertilizers globally. They are produced by reacting wet-process phosphoric acid with ammonia, providing a high nutrient content that supplies essential phosphorus to crops, promoting root development, and increasing yields. Wet-process phosphoric acid is the primary raw material for these fertilizers due to its low cost and high output. Additionally, wet-process phosphoric acid is also used to produce compound fertilizers, which combine nitrogen, phosphorus, and potassium nutrients to enhance soil fertility and crop yields.

• Industrial and Feed Sectors:

The refined products of wet-process phosphoric acid are widely used in industrial production and animal feed. In industry, wet-process phosphoric acid is used to manufacture phosphate chemical products such as sodium tripolyphosphate and sodium pyrophosphate, which are widely used in detergents, water treatment, ceramics manufacturing, and other industries. In the feed industry, phosphate salts are used as a phosphorus source in animal feed, ensuring that livestock and poultry obtain sufficient phosphorus to support healthy bone development and metabolic functions.

• Market Analysis and Future Trends:

In the global phosphoric acid market, wet-process phosphoric acid occupies 85-90% of the share, mainly due to its low production cost and mature technology. Especially in developing countries, wet-process phosphoric acid and its derivatives are crucial to agricultural and industrial development. However, as environmental regulations tighten, the waste management issues in wet-process phosphoric acid production are becoming a significant challenge for the industry. Future technological innovations will focus on improving the purification processes of wet-process phosphoric acid, reducing the environmental impact of the production process, and developing resource utilization technologies for gypsum by-products.

3. Identification and Quality Control of Phosphoric Acid Products

In the practical application of phosphoric acid, identifying its source and purity is crucial, especially in fields that require high-purity products.

3.1 Simple Identification of Thermal Phosphoric Acid and Wet-Process Phosphoric Acid

Due to the differences in production processes, thermal phosphoric acid and wet-process phosphoric acid exhibit some significant differences in their physical and chemical properties.

• Crystal Color and Shape:

Phosphate products made from thermal phosphoric acid usually appear as pure white crystals, with the crystals being quadrangular or rectangular and relatively large. In contrast, phosphate products made from wet-process phosphoric acid tend to be pale gray, with the crystals being slender or needle-shaped and relatively small. These physical differences stem from the varying impurity levels in the production processes.

• Transparency and Suspended Particles in Aqueous Solutions:

Solubility testing is a simple and effective method for identification. Products made from thermal phosphoric acid typically dissolve in water to form a colorless, transparent solution without suspended particles. In contrast, solutions of products made from wet-process phosphoric acid may contain trace amounts of white, filamentous suspended particles, mainly due to unremoved impurities such as iron phosphate, magnesium phosphate, and aluminum sulfate.

3.2 Application of Modern Analytical Techniques in Phosphoric Acid Identification

With the advancement of analytical techniques, modern instrumental analysis methods provide more accurate and reliable means for identifying phosphoric acid products.

• X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR):

X-ray diffraction can accurately determine the crystal structure of phosphate products, helping to identify their source and purity. Fourier transform infrared spectroscopy can detect functional groups and chemical bonds in phosphate products, further confirming the chemical composition and impurity content.

• Mass Spectrometry (MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES):

Mass spectrometry can detect trace elements and heavy metal content in phosphate products, ensuring that phosphoric acid products meet the stringent standards required in food, pharmaceutical, and other high-standard industries. Inductively coupled plasma optical emission spectrometry can quickly and accurately measure the metal content in phosphoric acid, helping to control product quality.

• Importance of Quality Control:

In the production and application of phosphoric acid products, quality control is not only a crucial means of ensuring product performance and safety but also a key factor in enhancing the market competitiveness of enterprises. By using advanced analytical instruments and techniques, companies can ensure that their phosphoric acid products meet industry standards and customer requirements while reducing the occurrence of non-compliant products and lowering production costs.

Conclusion and Future Outlook

The production of phosphoric acid involves complex chemical reactions and stringent process control. Different production methods directly determine the purity and application fields of phosphoric acid products. Although the thermal phosphoric acid process is energy-intensive and costly, its high purity gives it irreplaceable advantages in the food, pharmaceutical, and high-end chemical industries. Wet-process phosphoric acid, on the other hand, dominates the agricultural and industrial production sectors due to its low cost and high output. However, as environmental regulations become stricter, the phosphorus chemical industry must find new balances between production efficiency, environmental protection, and resource utilization.

Future phosphoric acid production technologies may evolve toward greener and more sustainable development. By introducing new catalyst technologies, improving energy efficiency in the production process, and developing gypsum resource utilization, the phosphorus chemical industry will be better equipped to address environmental challenges and meet the growing market demand. Meanwhile, as global demand for food increases, the demand for phosphate fertilizers is expected to remain high, driving further innovation and optimization in phosphoric acid production processes.