Phosphoric acid, chemically known as H3PO4, is a colorless and odorless inorganic compound. It is frequently encountered as a viscous, non-volatile aqueous solution, typically at an 85% concentration. This compound is a significant industrial chemical, playing a foundational role in many manufacturing processes. Its widespread use across various industries underscores the importance of understanding its production methods.
Key Ingredients for Production
The primary raw material for producing phosphoric acid is phosphate rock, also known as phosphorite. This sedimentary rock contains high concentrations of phosphate minerals, predominantly fluorapatite. Two other essential ingredients are used depending on the production method. Sulfuric acid (H2SO4) serves as a key reactant in the wet process, while elemental phosphorus (P4) is the starting material for the thermal process.
The Wet Process Method
The wet process is the most widely adopted method for phosphoric acid production, accounting for over 80% of global output. This method begins by crushing and drying phosphate rock before it reacts with sulfuric acid in a reactor. The chemical reaction converts the phosphate minerals, such as fluorapatite, into phosphoric acid while also forming calcium sulfate, commonly known as gypsum.
Following this reaction, the insoluble gypsum is separated from the phosphoric acid solution through filtration. This gypsum byproduct can contain trace levels of naturally occurring radioactive elements. The resulting phosphoric acid, typically containing 26-52% P2O5, is then concentrated to a higher strength, often around 54% P2O5, through evaporation to meet commercial specifications.
The Thermal Process Method
The thermal process yields phosphoric acid of a higher purity compared to the wet process, although it is more energy-intensive. This method first involves producing elemental phosphorus. Phosphate rock, along with silica and coke, is smelted in an electric arc furnace at high temperatures, typically between 1400 and 1500°C, to yield gaseous elemental phosphorus.
The elemental phosphorus vapor is then condensed into a liquid. Subsequently, this liquid elemental phosphorus is burned (oxidized) in a combustion chamber at extremely high temperatures, ranging from 1650 to 2760°C, forming phosphorus pentoxide (P2O5). The final step involves hydrating the phosphorus pentoxide by dissolving it in water, which produces phosphoric acid. This multi-step thermal process results in a phosphoric acid product with a very high P2O5 concentration, often around 85%.
Refining and Grades of Phosphoric Acid
After initial production, phosphoric acid undergoes purification steps, particularly for the wet process acid. Wet process acid often contains various impurities, including sulfates, fluorides, and certain metals originating from the phosphate rock. These contaminants are removed through methods such as liquid-liquid extraction or by inducing their precipitation.
The extent of purification determines the acid’s grade. For instance, “food-grade” phosphoric acid demands a much higher purity than “industrial-grade” or “fertilizer-grade” acid. Wet process acid needs rigorous treatment to meet stringent purity standards for specific applications like food and pharmaceuticals. The final concentration of the acid is also adjusted to suit its intended use.