Phosphoric acid is one of the most widely used chemicals in industry. This article provides a brief overview of the various applications of this substance in both edible and industrial forms, in different purity percentages, as well as the common methods used today for its production. Phosphoric acid is the second most produced acid by quantity after sulfuric acid. Its primary use is in the production of chemical fertilizers such as monoammonium phosphate, diammonium phosphate, and liquid phosphate fertilizers. It is also used in other chemical industries for the production of mono-, di-, and tri-sodium phosphates, as well as mono-, di-, and tri-calcium phosphates.
Phosphoric acid is produced on a commercial scale primarily through two methods: the wet process and the thermal process. The phosphoric acid obtained from the thermal method, which comes from elemental phosphorus, is very expensive and has fewer impurities compared to the phosphoric acid produced by the wet method. In the wet process, phosphate ores are used to produce the acid. The phosphoric acid produced from the wet method is used in the manufacture of phosphate fertilizers and animal feed, and it can be purified to make it suitable for technical or food grade. Industrial phosphoric acid has a black color and a specific odor, and due to the presence of impurities such as iron, copper, arsenic, mercury, and other toxic and dangerous ions, it can harm the environment. If the concentration of impurities does not fall below standard levels, these substances can accumulate in plants and animal organs, and ultimately enter the human body.
This substance is not flammable or explosive; however, it reacts with most metals, producing flammable and explosive hydrogen gas. Phosphoric acid is corrosive and may cause irritation, redness, and in more severe cases, burning of the skin or permanent eye damage upon contact with skin and eyes.
Phosphate rock, as the only economic source of phosphate carbonate, plays a vital and irreplaceable role in the development of both the agricultural and industrial sectors. Given the vital role of this substance in the ecosystem, the need for it has significantly increased with the growing global population. The importance of phosphates and their role as fertilizers in agriculture became evident by the end of the 18th century. Familiarity with natural phosphates and understanding their potential applications as fertilizers occurred in the early 19th century, and the first phosphate fertilizer, consisting of powdered bones and bird droppings, was used around that time. However, systematic and primary use began around 1850, and until the end of the 19th century, the only commercial application of phosphate was for fertilizer and sulfur. The first mining operation was in Europe, followed by the discovery and exploitation of vast reserves of this rock in the United States, North Africa, and Oceania. Since then, phosphate consumption has steadily increased due to its crucial role in sustaining life, with diverse applications emerging. Today, phosphates are used in a wide range of industries and agriculture.
As mentioned, phosphate rocks are the only source of phosphorus and its compounds, and they are essential for producing various phosphorus compounds in different industries. Two methods are used for this extraction: 1) The thermal method, where phosphate rocks are heated, causing phosphorus to be released as gas, which is then recovered as elemental phosphorus. 2) The wet method, where phosphate rocks are treated with sulfuric acid, nitric acid, or hydrochloric acid, recovering phosphorus in the form of phosphoric acid. Phosphoric acid is the second most produced acid in the world after sulfuric acid. This acid is used as a raw material in the production of cleaning agents, food products, pharmaceuticals, animal feed supplements, chemical fertilizers, and some pesticides. Phosphoric acid is produced through both thermal and wet methods. In the thermal method, pure phosphorus is used as the raw material. The concentration of phosphoric acid produced from the thermal process typically ranges from 75% to 85%. The product from this method has high purity and is only used in the production of cleaning agents, pharmaceuticals, and food products due to its high cost. More than 90% of the phosphoric acid produced in the world is obtained by the wet method, which involves the direct reaction of phosphate rock with sulfuric acid.
In this method, impurities such as cadmium, iron, magnesium, lead, fluoride, and uranium present in the rock also react with the acid, entering the phosphoric acid as impurities. Phosphoric acid produced by the wet method is also referred to as industrial phosphoric acid. This acid is used to produce chemical fertilizers (such as diammonium phosphate) and nutritional supplements for livestock and poultry (such as dicalcium phosphate). If the concentration of impurities does not fall below standard levels, these substances can accumulate in plants and the meat of livestock and poultry, subsequently entering the human body upon consumption. Thus, reducing the concentration of toxic impurities such as magnesium, fluoride, lead, and cadmium is essential. For example, the standard levels of impurities allowed in phosphoric acid used for producing chemical fertilizers are a maximum of 1000 ppm for fluoride ions, 5 ppm for cadmium and lead, and 100 ppm for magnesium.
Phosphoric acid has a wide range of applications. It is available in both industrial and edible forms. Industrial phosphoric acid is used to produce chemical fertilizers, particularly diammonium phosphate, which is used in a 1:1 ratio for production. It is also used to produce another type of chemical fertilizer known as triple super phosphate, which has a higher percentage of P2O5. This fertilizer is preferred over diammonium phosphate due to its higher P2O5 content and other advantages. This acid is of industrial grade purity and contains fewer impurities obtained from the incomplete purification of crude phosphoric acid. It is used in industries such as the production of phosphating agents, decolorizers, and degreasers, the manufacture of non-stick cookware, textile industries, the production of animal feed supplements, the manufacture of phosphate salts, and cleaning products like sodium tripolyphosphate. Industrial phosphoric acid is used in the textile industry as an auxiliary material and additive in catalyst preparation and the production of special anti-static (spark-proof) textiles. It is also used in the production of non-stick cookware to clean oil, paraffin, and grease contaminants from the surface of rolled aluminum, improving the adherence of the non-stick coating. Industrial phosphoric acid is also used in the production of mono- and dicalcium phosphate as nutritional supplements for livestock and poultry, as well as in toothpaste. Other applications of phosphoric acid include the production of monofluorophosphate (a raw material for toothpaste and a fluoride supplier) and zinc phosphate (a raw material for phosphating agents and coloring pigments).
Edible phosphoric acid: Edible phosphoric acid, which has no harmful effects on living organisms’ health, is used in the food industry, such as in vegetable oil, soft drinks, and baking powder. It should be noted that in many of the mentioned applications for industrial phosphoric acid, efforts are made to use edible phosphoric acid with higher purity due to its lack of impurities and the presence of some disruptive elements.
In the vegetable oil industry, phosphoric acid is used at a concentration of 0.15% for refining and separating gums in crude oil, while in the soft drink industry, it is used as a flavor enhancer and modifier and as a preservative at a concentration of 0.019%. Solvent extraction is a very common method for removing most impurities from phosphoric acid, utilizing organic solvent extraction. In this method, a solvent that is completely or partially immiscible with water is mixed with phosphoric acid in several stages, dissolving it. This way, most impurities remain in the aqueous phase and are separated from the acid.
Source: Decolorization and purification of phosphoric acid using solvent extraction and examining the efficiency of different solvents in the process, Mohammad Mohammadi Pour, Rajab Gholi Gholinia, Bahram Vahabzadeh, Mehdi Adeli Nasab * Hamidreza Ahmadzadeh, Central Quality, Gmail: hamidrezaiut@gmail.com, Ministry of Defense and Armed Forces Logistics, Defense Industries Organization, Chemical Group
