Phosphoric Acid
Phosphoric acid (H₃PO₄) is an important inorganic moderately strong acid, widely used in fertilizer production, food additives, pharmaceutical intermediates, electronic materials, water treatment, and many other fields. The sophistication and stability of its production equipment directly determine product quality, production efficiency, and environmental performance. Tianli has deep expertise in phosphoric acid production, building complete standardized phosphoric acid production facilities. We employ mature and advanced wet-process phosphoric acid production technology, covering raw material pretreatment, acidolysis reaction, purification, and finished product refining. Leveraging our comprehensive technological advantages, we provide customers with efficient, low-consumption, environmentally friendly, and intelligent phosphoric acid production solutions, helping enterprises achieve large-scale, green, and refined production.
I. Properties of Core Materials
Phosphoric acid is colorless, transparent, viscous liquid with the molecular formula H₃PO₄, relative molecular mass of 98.00, density of 1.87 g/cm³, and melting point of 42.35℃. It is hygroscopic, corrosive, readily soluble in water, and non-volatile. It has good thermal stability; when heated to 213℃, it dehydrates to form pyrophosphoric acid, which further dehydrates and polymerizes into trimer or metaphosphoric acid. It is not easily volatile or decomposed, and possesses both the general properties of acids and the ability to coordinate and complex.
II. Core Process Flow Introduction
The production process of sulfuric acid wet-process phosphoric acid typically includes key unit operations such as raw material pretreatment, reactive leaching, solid-liquid separation, phosphoric acid purification, and concentration. Based on different crystal forms, it is mainly divided into dihydrate process, hemihydrate process, and their derivative processes such as hemihydrate-dihydrate process. Currently, the dihydrate process dominates due to its strong adaptability to ore, while the hemihydrate-dihydrate process has become the mainstream improvement direction because it can produce higher concentrations of phosphoric acid.
1. Phosphate Ore Preparation and Pretreatment
Crushing and Grinding. The raw ore is crushed and screened to control the particle size to meet the requirements of reaction kinetics. It usually needs to be ground into fine powder to increase the specific surface area and accelerate the reaction rate.
Washing and Beneficiation. Some high-grade or specific-requirement ores need to be washed to remove surface impurities, or subjected to physicochemical methods such as flotation to improve the phosphorus grade and reduce the content of harmful impurities such as magnesium, iron, and aluminum.
2. Reactive Leaching (Core Process)
This is the core step in wet-process phosphoric acid production, belonging to a liquid-solid phase heterogeneous reaction process.
Ca5F(PO4)3+5H2SO4+nH2O→3H3PO4+5CaSO4⋅nH2O↓+HF↑
2.1 Reaction Principle: phosphate rock powder reacts chemically with sulfuric acid in a stirred tank, generating phosphoric acid solution and calcium sulfate precipitate. The basic reaction equation is as follows:
Ca5F(PO4)3 + 5H2SO4 + nH2O → 3H3PO4 + 5CaSO4⋅nH2O↓ + HF↑
the value of n depends on the reaction temperature and phosphoric acid concentration, corresponding to anhydrous calcium sulfate (n=0), hemihydrate calcium sulfate (n=0.5), and dihydrate calcium sulfate (n=2), respectively.
2.2 Circulating Slurry Mechanism
To avoid newly generated calcium sulfate coating the surface of unreacted phosphate rock particles and hindering the reaction, the industrial process uses a "circulating phosphoric acid slurry" process. This involves mixing the already generated dilute phosphoric acid with added sulfuric acid to form a mixed acid, which is then brought into contact with the phosphate rock powder for decomposition. This mechanism ensures the renewal of the reaction interface and the thoroughness of the reaction.
2.3 Crystallization Control
Dihydrate Method operates at a relatively low temperature (approximately 70-80℃) and low phosphoric acid concentration to produce calcium sulfate dihydrate crystals. This method is the most mature and stable, but it has relatively high energy consumption and produces a lower concentration of phosphoric acid.
Hemihydrate-Dihydrate Method first produces calcium sulfate hemihydrate at a higher temperature and concentration, then converts it to calcium sulfate dihydrate through dilution and hydrolysis. This method achieves crystal transformation, produces a higher concentration of phosphoric acid, and significantly reduces the impurity content in phosphogypsum.
3. Solid-Liquid Separation
The slurry after the reaction contains phosphoric acid solution and a large amount of solid calcium sulfate (phosphogypsum), requiring efficient separation.
Filtration: Primarily using a vacuum drum filter or belt filter. The filter cake (phosphogypsum) is washed to recover residual phosphoric acid and discharged as a byproduct; the filtrate is crude phosphoric acid.
Demisting and Gas Treatment: Fluorine-containing gases (HF, SiF4, etc.) generated during the reaction process need to be absorbed by a scrubbing tower and prepared into fluorosilicic acid or hydrofluoric acid, achieving resource recovery and preventing environmental pollution.
4. Phosphoric Acid Purification
Crude phosphoric acid contains iron, aluminum, magnesium, arsenic, heavy metals, and organic impurities, which need to be purified according to the end use.
4.1 Chemical Precipitation
Adding agents such as lime milk and iron salts causes some metal ions to form insoluble precipitates for removal.
4.2 Solvent Extraction
For high purity requirements, organic solvents (such as alcohols, ketones, and esters) are often used to extract phosphoric acid and separate impurities. China has made breakthroughs in solvent extraction technology, enabling wet-process phosphoric acid to reach industrial-grade purity standards, comparable to thermal-process phosphoric acid.
4.3 Ion Exchange and Adsorption
Ion exchange resins are used to remove specific metal ions such as calcium and magnesium, or activated carbon and diatomaceous earth are used to adsorb organic impurities.
5. Concentration
Purified dilute phosphoric acid (typically with a concentration of about 20%-30%) is concentrated using a multi-effect evaporator to obtain commercially available concentrated phosphoric acid with a concentration of 50%-54%. The concentration process requires strict control of temperature and vacuum to prevent the phosphoric acid from turning red or scaling.
III. Core Technological Advantages
Compared to thermal phosphoric acid and other wet processes (such as nitric acid and hydrochloric acid processes), the sulfuric acid wet phosphoric acid process has significant technological and economic advantages:
1. Low cost and significant economic benefits
Low Energy Consumption: the energy consumption of wet phosphoric acid production is only about 1/3 that of thermal phosphoric acid. Thermal phosphoric acid requires high-temperature electric furnaces to reduce phosphate rock, consuming a huge amount of electricity; while the wet process mainly involves chemical reactions at room temperature or low temperature, significantly reducing energy consumption.
Investment and Operating Costs: the cost of wet phosphoric acid is 20%~30% lower than that of thermal phosphoric acid. Sulfuric acid and phosphate rock are both readily available and inexpensive raw materials, making this process highly competitive economically.
2. Mature technology and Strong Adaptability
Process Stability: the dihydrate process has been industrialized for over a century, making the technology extremely mature, stable and reliable in operation, and easy to automate on a large scale.
Wide Ore Adaptability: this process has good adaptability to medium and low-grade phosphate rock, fully utilizing resources and reducing dependence on high-grade ore.
3. Significant potential for improving product purity
Purification Potential: although traditional wet-process phosphoric acid contains many impurities, with advancements in purification technologies such as solvent extraction and ion exchange, modern wet-process phosphoric acid, after appropriate purification, can achieve a purity comparable to thermal-process phosphoric acid, meeting the needs of high-value-added fields such as food-grade and electronic-grade phosphoric acid.