Ammonium sulfate: properties, production and applications

Ammonium sulfate (ammonium sulphate, (NH₄)₂SO₄) is an inorganic salt that occupies a unique position among mineral fertilizers and technical products. It is a colourless crystalline substance or white powder, readily soluble in water. Unlike most nitrogen fertilizers, ammonium sulfate contains two nutrients simultaneously — nitrogen in ammonium form (20.5–21.5%) and sulfur in sulfate form (about 24%). This combination makes it indispensable for crops experiencing sulfur deficiency and for soils with high pH, as well as a valuable raw material for the chemical, food and pulp and paper industries.

Chemical Nature and Physicochemical Properties

The chemical formula of ammonium sulfate is (NH₄)₂SO₄, with a molecular mass of 132.14 g/mol. The substance consists of colourless transparent orthorhombic crystals or a white crystalline powder without odour. Its density is about 1.77 g/cm³. Ammonium sulfate is readily soluble in water: solubility increases with temperature — from 70.6 g per 100 g of water at 0 °C to 103.8 g per 100 g at 100 °C. Aqueous solutions have a weakly acidic reaction. When heated above 280 °C, the salt decomposes with the release of ammonia and sulfur dioxide.

The key agronomic feature of ammonium sulfate is its physiologically acidic reaction. When applied to the soil, ammonium nitrogen is absorbed by plants and the soil complex, while the sulfate ion partially remains in solution, leading to gradual soil acidification. On neutral and slightly alkaline soils, this is often an advantage, especially for crops sensitive to chlorine and preferring a slightly acidic environment (potatoes, tea, rice, some vegetables). On strongly acidic soils, ammonium sulfate is used after liming or in mixtures with neutralizing additives.

Ammonium sulfate is slightly hygroscopic, does not cake when stored properly, and has excellent flowability and stability, ensuring ease of transportation and application accuracy. These physical properties favourably distinguish it from many other nitrogen fertilizers.

Regulatory Framework and Grades

Requirements for technical ammonium sulfate in Russia and CIS countries are regulated by GOST 9097-82 "Ammonium sulfate. Specifications". The standard applies to ammonium sulfate intended for agriculture, industry, retail trade and export. According to the standard, the product must meet the following indicators:

• Appearance: white or transparent crystals.

• Mass fraction of nitrogen calculated on a dry basis — not less than 21%.

• Mass fraction of water — not more than 0.2%.

• Mass fraction of free sulfuric acid — not more than 0.03%.

• Fractional composition: mass fraction of the fraction larger than 0.5 mm — not less than 80%, less than 6 mm — 100%.

• Flowability — 100%.

• Mass fraction of water-insoluble residue — not more than 0.02%.

For purified ammonium sulfate used in food, pharmaceutical and laboratory practice, GOST 10873-73 "Ammonium sulfate purified. Specifications" applies. This standard establishes stricter requirements for product purity, content of heavy metal impurities and other regulated substances.

Ammonium sulfate is produced in crystalline (fine-crystalline) and granular form. The granular form ensures better flowability, uniform spreading and reduced dusting during application. The product is supplied in bulk, in 50 kg bags and in big bags of 500–1000 kg. The guaranteed shelf life is 6 months from the date of manufacture.

Production Technology

Ammonium sulfate is produced by several industrial methods, the choice of which depends on the availability of raw materials and the structure of a particular production facility.

Neutralisation of sulfuric acid with ammonia. The most common and direct method: gaseous or liquid ammonia reacts with dilute sulfuric acid to form an ammonium sulfate solution, which is then evaporated, crystallised, centrifuged and dried. The reaction is exothermic and proceeds according to the equation: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄. This method ensures high product purity and is widely used in industry.

By-products of chemical production. Significant volumes of ammonium sulfate are formed as a by-product in caprolactam production, in coke-chemical production during the purification of coke oven gas from ammonia, and in flue gas desulfurization processes. It is by-product ammonium sulfate that constitutes a substantial share of global supply and is often the most economically accessible.

Processing of gypsum and phosphogypsum. The method is based on the interaction of gypsum (CaSO₄·2H₂O) or phosphogypsum with solutions of ammonia and carbon dioxide to form ammonium sulfate and calcium carbonate. This route allows large-tonnage waste to be utilized and a valuable fertilizer to be obtained.

Applications

Nitrogen-sulfur fertilizer. Ammonium sulfate is one of the best fertilizers for crops requiring both nitrogen and sulfur. Sulfur plays a key role in the synthesis of proteins, vitamins and enzymes, and its deficiency in soils of many regions of the world is becoming increasingly noticeable. Ammonium sulfate is especially effective for cereals (wheat, barley, corn), oilseeds (rapeseed, sunflower), legumes, potatoes, tea, rice, vegetables and fruit crops. It is used as a basic (pre-sowing) fertilizer, for pre-sowing application and for top dressing. On alkaline soils, ammonium sulfate acidifies the root zone, improving the availability of phosphorus and trace elements.

Production of complex fertilizers and fertilizer blends. Ammonium sulfate serves as a component for preparing blended fertilizers and complex NPK fertilizers with sulfur. Its granular form mixes well with other granular fertilizers and does not cake in blends.

Chemical industry. Ammonium sulfate is used to produce ammonia, sulfuric acid, ammonium salts, as well as a reagent in organic synthesis and in the production of various chemicals.

Food industry. Purified ammonium sulfate is used as food additive E517 (stabiliser, acidity regulator, flour and bread improver), as well as in yeast production and winemaking.

Pulp and paper and textile industries. Ammonium sulfate is used in viscose fibre production as a component of the spinning bath, as well as an additive to glass batch to improve its fusibility.

Water treatment and other applications. Ammonium sulfate is used in water treatment processes, in the production of fire-extinguishing powders, in metalworking, in the leather industry, and as a source of nitrogen and sulfur in microbiological processes.

Safety and Storage

Ammonium sulfate belongs to substances with low toxicity (hazard class 4). When working with the fertilizer, personal protective equipment (respirator, safety goggles, protective clothing) is recommended to prevent irritation of mucous membranes by dust. The product is non-flammable and explosion-proof.

Storage is carried out in closed warehouse premises protecting the product from moisture. Containers with ammonium sulfate may be stored in open areas. The product is transported in bulk and packaged by all modes of transport except air, in covered vehicles in accordance with the rules for the carriage of goods. Ammonium sulfate is slightly hygroscopic; however, during prolonged storage in a humid environment, caking and loss of flowability are possible, so moistening must be prevented.

Market and Trends

The global ammonium sulfate market demonstrates steady growth, driven by several factors. The growing sulfur deficiency in soils of many regions of the world makes nitrogen-sulfur fertilizers increasingly in demand. Tightening environmental requirements stimulate the development of flue gas desulfurization technologies, the by-product of which is ammonium sulfate, as well as the involvement of phosphogypsum and other waste in processing. In addition, demand for ammonium sulfate is growing from producers of complex fertilizers and fertilizer blends, as well as from industrial consumers.

The Asia-Pacific region is the largest producer and consumer of ammonium sulfate, due to large-scale caprolactam production, coke chemistry and intensive agriculture. Significant volumes of by-product ammonium sulfate are produced in China, India, Russia, the Middle East and North America. Key industry trends include the transition to granular forms with improved physical properties, the development of technologies for utilizing waste and by-products, and growing attention to balanced plant nutrition taking into account sulfur requirements.

Conclusion

Ammonium sulfate remains one of the key products of the global chemical industry and agriculture, combining the functions of a highly effective nitrogen-sulfur fertilizer and a multifunctional technical raw material. Its unique properties — physiologically acidic reaction, low hygroscopicity, excellent flowability and the presence of two nutrients — make it indispensable for a wide range of crops and industrial processes. Organising supplies of ammonium sulfate, including grade selection (technical, purified, granular), quality confirmation and logistics, requires an individual approach that takes into account specific technological requirements and operating conditions.