The Core Divide in Technical Routes: Process Logic of Wet and Thermal Methods
The selection of technical routes for phosphoric acid production is essentially a dynamic balance among resource endowments, market demands, and environmental constraints. Currently, the world's mainstream technologies are divided into two major systems: Wet Process Phosphoric Acid (WPA) and Thermal Process Phosphoric Acid (TPA). Centered on the decomposition of phosphate rock by sulfuric acid, the wet process obtains crude phosphoric acid through solid-liquid separation, accounting for over 85% of the global phosphoric acid production capacity. Its economic advantage lies in compatibility with medium-and low-grade phosphate rock (requiring only ≥28% P₂O₅ content) and large-scale production capacity. In contrast, the thermal process produces phosphoric acid via the combustion and hydration of yellow phosphorus, yielding electronic-grade products (with impurities <1ppm). However, its unit energy consumption is as high as 13,000-15,000 kWh/ton, 5-8 times that of the wet process, and it mainly serves high-end markets such as food additives and electronic etchants.
The technical divergence between the two is particularly prominent in raw material selection: the wet process consumes 4.5-5.5 tons of sulfuric acid and 4-5 tons of phosphogypsum per ton of product. Meanwhile, the thermal process requires 1.2-1.5 tons of yellow phosphorus per ton of phosphoric acid, and the production of yellow phosphorus itself consumes 14,000-15,000 kWh of electricity and 6-8 tons of phosphate rock. This difference in resource dependence directly leads to the wet process dominating the fertilizer sector (accounting for over 90%), while the thermal process establishes a technical barrier in the high-end chemical market.
Technological Iteration of Wet Process Phosphoric Acid: From Extensive Production to Fine Purification
A key advantage of the hemihydrate-dihydrate process lies in its flexible adaptation to variable phosphate rock quality-even when processing ores with fluctuating P₂O₅ content (ranging from 25% to 35%) or high impurity levels (such as magnesium and aluminum oxides), it maintains stable phosphorus recovery. For instance, in a 500,000-ton/year wet phosphoric acid project in Brazil, China National Chemical Wuhuan Engineering Co., Ltd. optimized the process by adjusting the hemihydrate crystallization temperature (controlled at 82-88℃) and the dihydrate washing ratio (1:3.5), which not only kept the phosphorus recovery rate above 98.5% but also reduced the magnesium content in the final phosphoric acid to less than 0.8%-a critical improvement for downstream diammonium phosphate (DAP) production, as excessive magnesium would otherwise cause fertilizer caking. Additionally, the α-type high-strength gypsum produced as a by-product has a compressive strength of over 25 MPa after hydration, meeting the European standard EN 13279-1 for gypsum plasterboards.
In the solvent extraction process, recent innovations have focused on enhancing solvent stability and reducing environmental risks. Traditional TBP-based solvents are prone to degradation under high temperatures (above 60℃) or acidic conditions, generating acidic by-products that corrode equipment and increase solvent loss. To address this, Sichuan University has modified the extraction system by adding 5-8% trioctylamine (TOA) as a stabilizer, which forms a protective complex with TBP and extends the solvent's service life from 12 months to over 24 months. In a 300,000-ton/year food-grade phosphoric acid project in Thailand, this modified solvent system achieved a fluoride removal rate of 99.2%, reducing fluoride content in the final product to less than 5ppm-well below the U.S. FDA's limit of 10ppm for food additives. For the kiln process, its applicability in resource-poor regions is further enhanced by its compatibility with low-cost coal gasification technology. In a pilot project in Ethiopia (where local phosphate rock has a P₂O₅ content of only 16-18%), the kiln process uses coal gas produced from low-rank lignite (locally available at $30/ton) to reduce phosphate rock at 1250-1300℃, producing crude phosphoric acid with a P₂O₅ concentration of 28-30%. Compared to importing high-grade phosphoric acid (which costs $800/ton), the local production cost is reduced to $420/ton, significantly supporting Ethiopia's domestic fertilizer industry development.
Technological Breakthroughs in Thermal Process Phosphoric Acid: From High Energy Consumption to Heat Recovery
Extended Content for Paragraph 1 (Combustion Heat Recovery & Equipment Corrosion Prevention)
To further enhance corrosion resistance in the two-stage process, modern equipment design incorporates specialized materials: membrane heat exchangers are typically fabricated from Hastelloy C-276 or silicon carbide (SiC), which resist oxidation and acid erosion even at flue gas temperatures of 800-900℃. For example, in a 100,000-ton/year thermal phosphoric acid plant in South Korea, replacing traditional carbon steel heat exchangers with SiC membrane units reduced equipment maintenance frequency from once every 6 months to once every 24 months, cutting annual maintenance costs by $300,000. Additionally, the co-produced 0.8MPa steam is often integrated into the plant's internal energy system-used to preheat combustion air or melt solid yellow phosphorus-creating an energy loop that further slashes external steam purchases by 30-40% for some facilities.
Extended Content for Paragraph 2 (High-Purity Phosphoric Acid Technologies)
While crystallization technology shows promise, its commercialization requires precise control of operating parameters: for instance, cooling the phosphoric acid solution at a rate of 0.5-1℃/hour and maintaining a pH of 1.2-1.5 ensures that impurities like iron, aluminum, and calcium form large, easily separable crystals, while phosphoric acid remains in the mother liquor. A pilot project by a Japanese electronics material firm demonstrated that this method can reduce metal ion content in electronic-grade phosphoric acid to <0.05ppb, exceeding the requirements of advanced 7nm semiconductor processes. For the POCl₃ distillation process, efforts to mitigate environmental impact have led to the adoption of closed-loop chlorine recovery systems-capturing unreacted chlorine gas from the chlorination step and reusing it in yellow phosphorus chlorination, which reduces chlorine consumption by 15% and cuts chlorine-containing wastewater generation to 0.8-1.2 tons per ton of product at leading facilities.
Multi-Dimensional Game in Technology Selection: Linkage of Cost, Environmental Protection, and Market
The selection of technical routes requires comprehensive consideration of resource endowments, policy constraints, and market demands. In regions with abundant phosphate rock resources and low electricity prices (such as Yunnan, China, and Morocco), wet process phosphoric acid remains the first choice. Taking an enterprise in Yunnan as an example, adopting the hemihydrate-dihydrate process to produce phosphoric acid, combined with phosphogypsum acid production and cement co-production, reduces the cost per ton of acid to 2,800 yuan, a 15% decrease compared to traditional processes. In regions where electricity costs are below 0.3 yuan/kWh (such as Norway and Canada), thermal process phosphoric acid maintains competitiveness in the food additive market due to its high-purity advantage.
Environmental policies have become a key variable. China's "Regulations on the Prevention and Control of Phosphogypsum Pollution in Hubei Province" require the comprehensive utilization rate of phosphogypsum to reach 65% by 2025, forcing enterprises to adopt the hemihydrate-dihydrate process or phosphogypsum acid production technology. The EU REACH Regulation limits the fluoride content in phosphoric acid to below 10ppm, compelling export-oriented enterprises to upgrade purification processes. In the new energy sector, the surging demand for lithium iron phosphate has driven the expansion of battery-grade refined phosphoric acid production capacity. Liuguo Chemical invested 1.194 billion yuan in a 280,000-ton/year plant, adopting the "wet purification + crystallization" process, with product iron content <5ppm, directly supplying battery manufacturers such as CATL.
Future Trends: Greenization, High-Value Utilization, and Intellectualization
Phosphoric acid production is undergoing technological restructuring and industrial integration. In terms of green technology, the hydronium ion method synthesizes proton sources through non-metallic composite materials, completely replacing sulfuric acid for phosphate rock decomposition, achieving "zero phosphogypsum" emissions, and its carbon emissions are only 1/5 of traditional processes. This technology has entered the pilot scale stage and is expected to disrupt the existing production model. In the direction of high-value utilization, the resource recycling of phosphogypsum is expanding from building materials to the agricultural sector. The modified spherical phosphogypsum developed by Xinyangfeng, after acid-base neutralization treatment, can be used as a soil amendment to improve acidic soil, with an application rate of 2-3 tons per mu, opening up a new path for solid waste disposal.
The application of intelligent technologies accelerates process optimization. The real-time phosphate rock grade monitoring system based on the Internet of Things (IoT) can dynamically adjust the amount of sulfuric acid added, increasing phosphate rock utilization by 3-5%. The AI-driven extraction process control model optimizes the number of extraction stages and solvent ratio through machine learning, improving purification efficiency by 10-15%. In the field of energy management, the coupled operation of waste heat power generation systems and phosphoric acid plants can meet 30% of the plant's electricity demand, reducing reliance on the power grid.
