Phosphoric acid (\(\text{H}_3\text{PO}_4\)) is one of the world’s most widely utilized industrial chemicals. It is typically encountered as a colorless, odorless, non-volatile syrupy liquid in an 85% aqueous solution. This compound is a fundamental component in numerous manufacturing processes, ranging from food production to agriculture. The immense demand for this acid necessitates two distinct manufacturing methods, chosen based entirely on the required purity level of the final product.
Sourcing Phosphate Rock
The production of nearly all phosphoric acid begins with the extraction of a naturally occurring mineral known as phosphate rock. This raw material is primarily composed of calcium phosphate minerals, most commonly fluoroapatite (\(\text{Ca}_5(\text{PO}_4)_3\text{F}\)), which contains the necessary phosphorus atoms. Large reserves of this rock are found globally, with significant deposits located in regions like North Africa.
Once mined, the raw phosphate rock must undergo initial preparation before it can enter the chemical reaction stage. This preparation typically involves crushing and grinding the rock into a fine powder to maximize the surface area for subsequent chemical treatment. Additional steps, referred to as beneficiation, may be employed to remove impurities and concentrate the phosphate content.
Manufacturing via the Wet Process
The wet process is the most common and cost-effective method for producing phosphoric acid, accounting for over 90% of global output. This method involves the direct chemical reaction of the finely ground phosphate rock with concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)). The reaction takes place in large, agitated reactors, where the sulfuric acid digests the phosphate rock to release the phosphorus content.
The main chemical reaction produces a slurry containing both phosphoric acid and an insoluble byproduct, calcium sulfate (\(\text{CaSO}_4\)). This calcium sulfate is better known as gypsum, which precipitates out of the solution during the reaction. The process is designed to ensure the gypsum forms crystals that are easily filterable, which is a key technical challenge in maintaining high efficiency.
Following the reaction, the slurry is moved to a filtration system, where the liquid phosphoric acid is separated from the solid gypsum. The resulting crude acid is relatively dilute, containing approximately 26% to 32% phosphorus pentoxide (\(\text{P}_2\text{O}_5\)). To create the concentrated “merchant grade” acid used in commerce, this dilute product is then sent to evaporators to remove excess water. The resulting acid retains significant impurities, making it unsuitable for human consumption.
Manufacturing via the Thermal Process
The thermal process is an alternative, more energy-intensive method specifically designed to produce highly purified phosphoric acid. This process begins by feeding the phosphate rock, coke, and silica into a high-temperature electric furnace. Inside the furnace, the raw materials are subjected to extreme heat, which reduces the phosphate content to its elemental form: phosphorus vapor (\(\text{P}_4\)).
This elemental phosphorus is collected and burned in a specialized combustion chamber in the presence of air. The burning of the phosphorus vapor creates phosphorus pentoxide (\(\text{P}_2\text{O}_5\)). This step effectively separates the phosphorus from all the impurities contained in the original phosphate rock, as only the pure phosphorus is volatilized.
In the final stage, the gaseous phosphorus pentoxide is cooled and hydrated in water. This reaction yields high-purity phosphoric acid, often reaching concentrations of 75% to 85% \(\text{H}_3\text{PO}_4\). Although significantly more expensive due to the high electrical demand, the thermal process consistently produces acid that meets the strict purity standards required for food and technical applications.
Common Uses of Phosphoric Acid Grades
The two distinct manufacturing processes directly determine the applications for the phosphoric acid produced. The lower-purity acid derived from the wet process, often called fertilizer grade, is utilized in the agricultural sector. This grade serves as the primary feedstock for creating phosphate-based fertilizers, such as diammonium phosphate (DAP) and monoammonium phosphate (MAP).
Conversely, the high-purity acid from the thermal process is reserved for applications requiring minimal contaminants. This purer grade, often labeled as food-grade, is used as an acidulant in the food and beverage industry, providing the tart flavor found in soft drinks. It is also a key ingredient in pharmaceuticals, dental cements, and cosmetics. Technical uses include metal treatment for rust removal and to create phosphate coatings that increase corrosion resistance.